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First-principles calculation of electronic properties of SiC-based bilayer and trilayer heterostructures
Abstract
Recently, van der Waals (vdW) two-dimensional heterostructures have attracted great attention. The combination structures demonstrate unique properties that individual layer does not possess, which foretell promising future applications. Here, we investigate the structural and electronic properties of SiC/Graphene, SiC/MoS2, and Graphene/SiC/MoS2 vdW heterostructures using first-principles calculations. The SiC/Graphene interface forms a p-type Schottky contact, which can be turn into an n-type Schottky contact by applying an external electric field. Moreover, a transition from a Schottky to Ohmic contact at the interface can be triggered by varying the interlayer distance or applying an external electric field. The SiC/MoS2 interface forms a type-II heterostructure, in which the recombination of photoexcited charges will be greatly suppressed. The transition from type-II to type-III band alignment can be realized in SiC/MoS2 heterostructure by applying a biaxial strain. This heterostructure also shows excellent optical absorption ability in visible and far-infrared range, which is the merit for photocatalyst. The trilayer heterostructure exhibits an n-type Schottky contact that assembled graphene could act as protective encapsulating on SiC/MoS2. The results shows the graphene and MoS2 can tune and improve the electronic performance of SiC and demonstrate promising application of SiC-based heterostructure for nanoelectronic and nanophotonics.
... They are close with reported values [8,33,36,37]. We have calculated the binding energy − 39.84meV/Ǻ 2 of G/h-BN, it is well with reported value of 2D materials [38,39]. Hence, G/h-BN HS is a stable material. ...
... The stability of defected (G/h-BN_1 N, G/h-BN_nBN & G/h-BN_aBN) materials is determined by their binding energy which is found to be − 36.32meV/Ǻ 2 , -21.75meV/Ǻ 2 and -26.79meV/Ǻ 2 respectively. These values are also comparable with other 2D materials [9,39]. We have also calculated interlayer distance between graphene and h-BN supercells of G/h-BN_1 N, G/h-BN_nBN and G/h-BN_aBN, they are found to be 3.31 Å, 3.35 Å and 3.34 Å respectively. ...
... Many other ultrathin vdW heterostructures have been fabricated experimentally and investigated theoretically. To name a few of these two-dimensional heterostructures, the following can be mentioned (graphene is denoted as G): G/phosphorene [13], G/blue phosphorene [14], G/GaSe [15], G/GaS [16], G/g-GaN [17], G/SiC [18], G/arsenene [19], G/graphene-like GeC [20], G/ZrS 2 [21], Ga 2 SeTe/InS [22] and G/MoS x Se 2−x [23]. The graphene layer used as the contact easily outperforms traditional metallic substances, the cause of which is its superior qualities mentioned above. ...
... Among the many effects that this substrate can have on the heterostructure, strain cannot be ignored. Earlier work suggests strain as an instrument for changing an ohmic contact to one of Schottky nature, with the other way around also a possibility [18,21]. In order to see how G/HfS 2 changes when put under strain, biaxial strain is applied to the system based on equation (5). ...
Using density functional theory calculations and the addition of van der Waals correction, the graphene/HfS 2 heterojunction is constructed, and its electronic properties are examined thoroughly. This interface is determined as n -type ohmic, and the impacts of different amounts of interlayer distance and strain on the contact are shown using Schottky barrier height and electron injection efficiency. Dipole moment and work function of the interface are also altered when subjected to change in these two categories. The effects of an applied electric field on transforming the ohmic contact to Schottky is also investigated. The conclusions given can assist in the design and modeling of HfS 2 based devices in the future.
... To realize more controllable device functions, dynamic modulation of band alignments is a desirable method which can flexibly design devices with multi-functional applications [25][26][27]. Two types of band arrangement can be observed in the two-dimensional perovskite heterostructures, including straddling type I and staggered type II band alignments [28][29][30]. ...
... Li et al investigated the structural and electronic properties of SiC/ graphene, SiC/MoS 2 , and graphene/SiC/MoS 2 vdW heterostructures by first-principles calculations. The transition from type-II to type-III band alignment can be realized in the SiC/MoS 2 heterostructure under a biaxial strain [26]. Liao et al researched the effects of composition modulation on the type of band alignments for Pd 2 Se 3 /CsSnBr 3 and achieved the transformation of type I Pd 2 Se 3 /CsSnBr 3 vdWH to type II by partly replacing the Pd atom with Ni atom in Pd 2 Se 3 monolayer [27]. ...
Perovskite-based van der Waals heterostructures with high carrier mobility and perfect photoresponse performance can be used to design transistors and photovoltaic devices. However, the function of a single heterojunction is relatively limited because of its specific energy band type. To achieve more controllable device functions, in this work, the band structure, effective mass, optical absorption spectrum, differential charge density and other electronic and optical properties for the novel heterostructure system composed of emerging two-dimensional material Pd2Se3 with a high electron mobility and all-inorganic halide perovskite Cs2PbI2(1+x)Cl2(1-x) is systematically studied by using density functional theory. The results demonstrate that the band alignments in Cs2PbI2Cl2/Pd2Se3 heterostructure realize the transition from type-I to type-II by changing the concentration of I and Cl atoms in the all-inorganic two-dimensional Ruddlesen-Popper perovskite Cs2PbI2Cl2, which can be applied to different functional devices. With the increase of iodine atoms, the valence band maximum (VBM) contributed by Pd2Se3 in the heterostructure moves downward, and the valence band of Cs2PbI2Cl2 becomes the new VBM. Moreover, the transport and optical properties for the Cs2PbI2Cl2/Pd2Se3 heterostructure can be improved, due to the reductions in the band gap and effective mass, which are caused by the transition of band alignment. Our works indicate that a controllable band alignment of Cs2PbI2Cl2/Pd2Se3 heterostructure can provide theoretical guidance for the flexible designs of devices with multi-functional applications.
... Since the successful preparation of graphene [1], there has been a surge in research into two-dimensional (2D) materials, including 2D WS 2 [2,3], GaN [4][5][6], BN [7,8], black phosphorus [9,10], ZnO [11,12], SiC [13,14], etc. SiC is a third-generation semiconductor material with a wide band gap, high electron saturation drift rate, high breakdown field strength, high thermal conductivity, high radiation resistance, etc. It has a wide range of applications in solar cells, high-frequency high-power devices, and high-temperature electronic devices. ...
The electronic, magnetic, and optical behaviors of metals (M = Ag, Al, Au, Bi, Ca, Co, Cr, Cu, Fe, Ga, K, Li, Mn, Na, Ni) adsorbed on the SiC monolayer have been calculated based on density functional theory (DFT). The binding energy results show that all the M-adsorbed SiC systems are stable. All the M-adsorbed SiC systems are magnetic with magnetic moments of 1.00 μB (Ag), 1.00 μB (Al), 1.00 μB (Au), 1.01 μB (Bi), 1.95 μB (Ca), 1.00 μB (Co), 4.26 μB (Cr), 1.00 μB (Cu), 2.00 μB (Fe), 1.00 μB (Ga), 0.99 μB (K), 1.00 μB (Li), 3.00 μB (Mn), and 1.00 μB (Na), respectively, except for the Ni-adsorbed SiC system. The Ag, Al, Au, Cr, Cu, Fe, Ga, Mn, and Na-adsorbed SiC systems become magnetic semiconductors, while Bi, Ca, Co, K, and Li-adsorbed SiC systems become semimetals. The Bader charge results show that there is a charge transfer between the metal atom and the SiC monolayer. The work function of the K-adsorbed SiC system is 2.43 eV, which is 47.9% lower than that of pristine SiC and can be used in electron-emitter devices. The Bi, Ca, Ga, and Mn-adsorbed SiC systems show new absorption peaks in the visible light range. These results indicate that M-adsorbed SiC systems have potential applications in the field of spintronic devices and solar energy conversion photovoltaic devices.
... Additionally, due to their excellent properties, silicon carbide (SiC) and germanium carbide (GeC) have garnered a lot of interest. [22,23] SiC possesses a large bandgap of about 3.354 eV, [24] a high saturation electron drift velocity (3 × 10 7 cm/s), a strong electric breakdown field (3 × 10 6 V/cm), and is used in high-temperature devices suitable for DC to microwave frequencies. [25] SiC is also a potential electromag-netic shielding material and it can be used for electronic packaging of highly integrated circuits, wireless communication, electronic base stations and other electronic equipment. ...
Two-dimensional materials with novel mechanical and thermal properties are available for sensors, photodetectors, thermoelectric, crystal diode and flexible nanodevices. In this investigation, the mechanical and thermal properties of pristine SiC and GeC are explored by molecular dynamics simulations. First, the fracture strength and fracture strain behaviors are addressed in the zigzag and armchair directions at 300 K. The excellent toughness of the SiC and GeC is obtained by the maximal fracture strain as 0.43 and 0.47 in the zigzag direction, respectively. Interestingly, the temperature-tunable tensile strength of the SiC and GeC is also investigated. Then, using non-equilibrium molecular dynamics (NEMD) calculations, the thermal performances of the SiC and GeC are explored. In particular, the thermal conductivity of the SiC and GeC present a pronounced size-dependent thermal conductivity, which can be enhanced up to 85.67 Wm–1K–1 and 34.37 Wm–1K–1, respectively. The goal of our work is to provide a theoretical framework that can be used in the near future. This will enable us to design an efficient thermal management scheme for 2D materials in electronics and optoelectronics.
... The former is from groups 4 to 10 of the periodic table of elements and the latter is from group 16, i.e. S, Se, Te. In recent years, two-dimensional (2D) molybdenum disulfide (MoS2) has become a well-known and the most studied compound from this group of materials [184][185][186][187][188][189][190]. This is perhaps due to its potential integration in electronic technologies due to its intrinsic properties such as tunable band gap (1.1-1.9 eV) via controlling the number of its layers and relatively easy stacking to create new structures [191][192][193][194]. ...
Molybdenum disulfide (MoS2) nanoflakes, a two-dimensional crystal with tunable bandgap depends on number of layers, is a promising candidate for future nanoelectronic devices. In the present research, flower shaped MoS2 nanoflakes have been synthesized via hydrothermal method. As part of the research, the growth of triangular MoS2 monolayer up to 10 micrometers has optimized by caring out several chemical vapor deposition experiments with varying deposition parameters. The prepared samples were characterized using optical, scanning electron, and atomic force microscopes, x-ray diffraction analysis, and Raman spectroscopy. Based on the experimental results, the growth mechanism has been suggested. Chemiresistor gas sensor based on flower shaped MoS2 nanoflakes which were prepared by a facile hydrothermal method were fabricated. Gas sensing characteristics of these sensors were evaluated upon exposure to 200 to 400 ppm of xylene and methanol vapors. As working temperature increased, the sensitivity was improved from 25 to 55 for 200 °C and the response time decreased from about 800 s to 120 s. The results showed higher sensitivity and shorter response times for methanol at 200 °C. The gas sensing mechanisms for both vapors were discussed. Furthermore, the vertical MoS2 nanoflake films grown by CVD method, were used as n-channel for back-gated FET gas sensors, for which the contacts were made by sputtering of 50 nm titanium and then 100 nm gold. Dynamic gas sensing behavior of the produced transistors were investigated at room temperature and 50 °C in the presence of different concentrations (1 ppm, 10 ppm) of ethanol and methanol vapors. As the temperature increased to 50 ºC, the sensor response was enhanced from about 2.7 to almost 9.7 for 1 ppm ethanol, while for methanol the response increased from about 2.2 to 4.6. The gas sensing mechanism for both vapors at room temperature and 50 ºC were suggested. For better understanding the gas sensing mechanism, the adsorption of methanol and xylene gas molecules were investigated by means of density functional computations. The adsorption parameters such as adsorption energy and energy band gap, density of states, and Mulliken charge transfer were studied. The adsorption energy and charge transfer for xylene adsorption shows a significant improvement after nickel decoration on monolayer MoS2. Therefore, surface modification of monolayer MoS2 can improve the adsorption of xylene vapor comparing to methanol molecules.
... The fascinating properties of mentioned pioneering structures have inspired researchers to probe other novel vdWHs, and several structures have been already explored [38][39][40][41][42][43][44][45][46][47]. The most straightforward possible structures within this category are those formed by different binary monolayers. ...
Two-dimensional heterostructures are an emerging class of materials for novel applications because of extensive engineering potential by tailoring intriguing properties of different layers as well as the ones arising from their interface. A systematic investigation of mechanical, electronic, and optical properties of possible heterostructures formed by bilayer structures graphenelike ZnO and MgO monolayers is presented. Different functionality of each layer makes these heterostructures very appealing for device applications. ZnO layer is convenient for electron transport in these structures, while MgO layer improves electron collection. At the outset, all of the four possible stacking configurations across the heterostructure are mechanically stable. In addition, stability analysis using phonon dispersion reveals that the AB stacking formed by placing the Mg atom on top of the O atom of the ZnO layer is also dynamically stable at zero temperature. Henceforth, we have investigated the optical properties of these stable heterostructures by applying many-body perturbation theory within the framework of GW approximation and solving the Bethe-Salpeter equation. It is demonstrated that strong excitonic effects reduce the optical band gap to the visible light spectrum range. These results show that this new two-dimensional form of ZnO/MgO heterostructures open an avenue for novel optoelectronic device applications.
... where Advances in Condensed Matter Physics of system-I, system-IIa, system-IIb, and system-IIc is comparable with the values of other vdW HSs [32,33]. us, considered systems are stable because negative binding energy means that materials are energetically stable at the ground state. ...
First-principle calculations based on the spin-polarized density functional theory (DFT) with vdW corrections by DFT-D2 approach have been carried out to study structural, electronic, and magnetic properties of water-adsorbed graphene/MoS2 heterostructures (system-I), and water-adsorbed graphene/MoS2 heterostructures with vacancy defects in Mo sites (systems-II). We consider vacancy defects in different Mo sites such as centre-1Mo atom vacancy defect (system-IIa), left-1Mo atom vacancy defect (system-IIb), and 2Mo atom vacancy defects (system-IIc). All the systems considered in this study are structurally stable; however, the stability of defected systems decreases with an increase in defect concentrations. The calculated binding energies of HS used in this study agree with the reported work. Electronic properties of system-I and systems-II reveal that they have metallic characteristics. Our investigation shows that system-I is nonmagnetic and systems-II are magnetic. The magnetic moment in the defected systems (system-IIa, system-IIb, and system-IIc) is developed by unpaired up and down-spins of electrons created in the orbitals of atoms due to vacancy defects in Mo atoms.
... Table 2 shows the values of binding energy for three studied hetero-bilayers. The best equilibrium heights (h) are also reported in table 2. Based on the equilibrium inter-layer distances, the interaction between the GDY and borophene in the three hetero-bilayer structures can be considered of the Van der Waals type [39][40][41]. ...
The integration of dissimilar 2D materials is important for nanoelectronic and thermoelectric applications. Among different polymorphs and different bond geometries, borophene and graphdiyne are two promising candidates for these applications. In the present paper, we have studied hetero-bilayers comprising graphdiyne-borophene (GDY-BS) sheets. Three structural models, namely S0, S1 and S2 have been used for borophene sheets. The optimum interlayer distance for the hetero-bilayers was obtained through binding energy calculations. Then, the structure and electronic properties of the monolayers and hetero-bilayers were individually examined and compared. Graphdiyne monolayer was shown to be a semiconductor with a band gap of 0.43 eV, while the borophene monolayers, as well as all studied hetero-bilayers showed metallic behavior. The thermoelectric properties of borophene and graphdiyne monolayers and the graphdiyne-borophene bilayers were calculated on the basis of the semi-classical Boltzmann theory. The results showed signs of improvement in the conductivity behavior of the hetero-bilayers. Furthermore, considering the increase in Seebeck coefficient and the conductivity for all the structures after calculating figure of merit and power factor, a higher power factor and more energy generation were observed for bilayers. These results show that the GDY-BS hetero-bilayers can positively affect the performance of thermoelectric devices
... Besides, as the properties of bilayer heterostructure can be modulated with the change of interlayer distance, in order to investigate whether the properties of AlN/h-BN/MoS 2 was controllable by changing the relative position of h-BN [44], two models were constructed. The first construction(I) way was that the distance between h-BN and AlN was optimized and fixed, while the distance between AlN/h-BN and MoS 2 was changed. ...
The van der Waals(vdW) heterostructures formed by stacking layered two-dimensional materials can improve the performance of materials and provide more applications. In our paper, six configurations of AlN/MoS2 vdW heterostructures were constructed, the most stable structure was obtained by calculating the binding energy. On this basis, the effect of external vertical strain on AlN/MoS2 heterostructure was analyzed, the calculated results show that the optimal interlayer distance was 3.593Å and the band structure was modulated. Then the h-BN intercalation was inserted into the AlN/MoS2 heterostructure, by fixing the distance between h-BN and AlN or MoS2, two kinds of models were obtained. Furthermore, the electronic properties of AlN/MoS2 heterostructure can be regulated by adding h-BN intercalation layer and adjusting its position. Finally, the optical properties show that the absorption coefficient of AlN/MoS2 heterostructure exhibits enhancement characteristic compared with that of the individual monolayers. Meantime, compared with AlN/MoS2, the AlN/h-BN/MoS2 shows a redshift effect and the light absorption peak intensity increased, which indicated that h-BN intercalation layer can be used to regulate the electronic and optical properties of AlN/MoS2 heterostructure.
... This is comparable with the value of other 2D HS materials. 46,47 The negative binding energy value of GBN-I reveals the fact that it is energetically stable at the ground state. In addition, we predicted that van der Waals (vdW) force exists between the graphene and h-BN of GBN-I through the evidence of the calculated value of the binding energy and interlayer distance. ...
Graphene (G) and hexagonal Boron Nitride (h-BN) are structurally similar materials but have very different electronic and magnetic properties. Heterostructures formed by the combination of these materials are of great research interest. To assess the role played by the crystalline defects in such heterostructures is also of crucial importance owing to their novel properties. In the present work, we study the structural, electronic, and magnetic properties of the G/h-BN heterostructure and the different possible point defects of B and N atoms in it by using first-principles calculations based on the spin-polarized density functional theory (DFT) method within the van der Waals correction DFT-D2 approach. The structural analysis of these systems shows that they are stable two dimensional van der Waals heterostructure materials. Band structure calculations of these materials reveal their semimetallic nature. On the basis of density of states and partial density of states calculations, the defective systems are magnetic materials. The magnetic moment obtained in these defective systems is due to the unpaired up-spin and down-spin states in the orbitals of C, B, and N atoms created by the vacancy defects. On the other hand, the G/h-BN heterostructure has an approving condition for ferromagnetism due to the presence of flat bands in the neighborhood of the Fermi energy.
Photocatalytic water splitting is a sustainable and eco-friendly method for renewable energy production. The fabrication of an efficient photocatalyst based on two-dimensional (2D) interfaces with suitable band offsets is at...
The high-performance sodium-ion batteries (SIBs) require anode materials with high capacity and fast kinetics. Based on first-principle calculations, we propose BC3N2 and BC3N2/Graphene (B/G) heterostructure as outstanding SIBs anode materials....
Context:
Two-dimensional semiconductor materials have received much attention in recent years due to their wide variety of applications in the field of nano-optoelectronic devices. In this project, we applied stresses ranging from -6 to +6% to the ZrSe2/HfSe2 heterostructure and systematically investigated its electrical and optical properties. It is discovered that stress can effectively modulate the forbidden bandwidth of the ZrSe2/HfSe2 heterojunction; whereas, under compressive stress, the forbidden bandwidth of the material decreases further until the bandgap is zero, leading to the material's transformation from semiconductor to metal. The forbidden band gap of the ZrSe2/HfSe2 heterojunction increases with increasing horizontal biaxial tensile strain. We discovered that the light absorption performance of this heterostructure is significantly better than that of its similar monomolecular layer and that its light absorption intensity can reach an order of magnitude of 104. Under compressive and tensile stresses, the ZrSe2/HfSe2 heterojunctions exhibit different degrees of red or blue shift. The results indicate that constructing ZrSe2/HfSe2 heterojunctions and applying horizontal biaxial stresses to them can significantly modulate the optoelectronic properties of the materials. ZrSe2/HfSe2 heterojunction is a new type of high-performance photogenerated carrier transport device with a wide range of applications.
Methods:
The calculations in this study are carried out the first principles approach of density functional theory, as implemented in the CASTEP module of Materials-Studio2019. The researchers used an ultrasoft reaction potential to calculate the interactions between the ion core and the electrons and applied the Perdew-Burke-Ernzerhof (PBE) and the generalized gradient approximation (GGA) to perform the calculations. The Monkhorst-Pack technique was employed to create the k-point samples utilized for integration on the Brillouin zone, and the k-point grid was uniformly 6 × 6 × 1. In addition, in order to avoid interactions between the atomic layers affecting the properties and stability of the material, such interactions were prevented by adding a 30 Å vacuum layer. Using a plane-wave energy cutoff of 500 eV and the convergence accuracy of the iterative process was set to 1 × 10-5 eV to ensure the accuracy of the computational results, and in addition. The maximum stress in the lattice was limited to less than 0.05 GPa or the interaction force between neighboring atoms was lower than 0.03 eV/Å. For the calculation of the properties of the optical properties, a k-point grid of 18 × 18 × 1 is used for optimization, and the polarization direction of the material is not taken into account, considering that the material is isotropic. This study proposes to apply the Tkatchenko-Scheffler (TS) dispersion correction method in DFT-D to appropriately represent the interlayer van der Waals interaction forces to solve inaccuracies in the computation of van der Waals interactions via density functional theory.
In recent years, α-In2Se3 has attracted great attention in miniaturizing nonvolatile random memory devices because of its room temperature ferroelectricity and atomic thickness. In this work, we construct two-dimensional (2D)...
Two-dimensional (2D) heterostructures reveal novel physicochemical phenomena at different length scales that are highly desirable for technological applications. We present a comprehensive density functional theory study of van der Waals (vdW) heterostructures constructed by stacking 2D TiO2 and 2D MoSSe monolayers to form the TiO2–MoSSe heterojunction. The heterostructure formation is found to be exothermic, indicating stability. We find that by varying the atomic species at the interfaces, the electronic structure can be considerably altered due to the differences in charge transfer arising from the inherent electronegativity of the atoms. We demonstrate that the heterostructures possess a type II or type III band alignment, depending on the atomic termination of MoSSe at the interface. The observed charge transfer occurs from MoSSe to TiO2. Our results suggest that the Janus interface enables the tuning of electronic properties, providing an understanding of the possible applications of the TiO2–MoSSe heterostructure.
The solution to the issue of energy scarcity lies in the search for an effective photocatalyst. In this study, the monolayers β-AsP and SiC are selected to build the heterostructure...
Silicon carbide has a planar two-dimensional structure; therefore it is a potential material for constructing twisted bilayer systems for applications. In this study, DFT calculations were performed on four models with different twist angles. We chose angles of 21.8°, 17.9°, 13.2°, and 5.1° to estimate the dependence of the electronic and phononic properties on the twist angle. The results show that the band gap of bilayer SiC can be changed proportionally by changing the twist angle. However, there are only small variations in the band gaps, with an increment of 0.24 eV by changing the twist angle from 5.1° to 21.8°. At four considered twist angles, the band gaps decrease significantly when fixing the structure of each layer and pressing the separation distance down to 3.5 Å, 3.0 Å, 2.7 Å, and 2.5 Å. A noteworthy point is that the pressing also makes the band linearly smaller at a certain rate regardless of the twist angles. Meanwhile, the phonon bands are not affected by the value of the twist angle. The optical bands are between 900 cm⁻¹ and 1100 cm⁻¹ and the acoustic bands are between 0 cm⁻¹ and 650 cm⁻¹ at four twist angles.
The structure - property relationship provides liberty of tuning the properties of the materials by changing the crystal structures. The materials can be categorized in several classes, yet on the basis of the crystal structure, the materials may be classified in two major types i.e. layered and non-layered materials. The non-layered materials are materials with isotropic bonding throughout while the layered materials are materials with anisotropic bonding i.e. the strong in-plane and weak van der Waals interactions in out-of-plane directions. In this introductory chapter, The synopsis that layeredness, the ability of materials to exist in the form of atomic layers, is an intrinsic property of the materials which has been elaborated in this chapter. The two dimensionality is merely a repetition of unit-cell structure in two-dimensions and each material can be peeled off to make their 2D counter parts whereas the layeredness is an intrinsic material property. The misconception that all 2D materials or monolayers are not essentially layered and vice versa has also been addressed. Further, the methodology to study the layered materials through first principles techniques is also discussed in details.
Abstract Janus nanosystems enable one to achieve complementary properties in a single entity. In the current study, the fundamental properties like structural, electronic, and dynamical of Janus hexagonal boron nitride (h‐BN) by selectively hydrogenating and fluorinating a h‐BN surface are systematically examined, using density functional theory. Functionalization of h‐BN introduces partial sp3 (buckled) character in the predicted materials as compared to planar sp2 h‐BNs. Fully fluorinated and hydrogenated h‐BN have a direct bandgap of 3.42 and 3.37 eV, respectively. All the investigated configurations are predicted to be dynamically stable. Furthermore, optical properties including dielectric function, absorption spectra, refractive index, and reflectivity are evaluated to realize the optical and photocatalytic behavior of considered systems. The dielectric function ɛ2(ω) shows fundamental absorption edge arising at 3.2, 3.9, 2.8, and 3.4 eV for hydrogen on boron and nitrogen, hydrogen on boron and fluorine on nitrogen, fluorine on boron and hydrogen on nitrogen (FBNH) and fluorine on boron and nitrogen which is comparable to the bandgap of respective monolayers. Solar cell parameters of all considered BN structures are calculated using the Shockley–Queisser (SQ) limit. The highest short‐circuit current density (Jsc ) for FBNH is found to be 2.1 mA cm−2 providing the efficiency of 8.27% making FBNH a potential candidate for absorber layer in solar cells.
A hotspot in renewable energy research is visible-light-driven photocatalytic water splitting for hydrogen production. Monolayer stacking in the form of van der Waals heterojunctions (vdWHs) can be used for band gap engineering and the control of exciton dynamics for prospective nano-electronic devices. Using state-of-the-art hybrid dispersion-corrected density functional theory calculations (DFT-D3(BJ)), 2D/2D CS/SiC vdWHs are designed and investigated for the feasibility of using them as a potential photocatalyst for visible-light-driven H2 generation. It has been established that the most favourable stacking pattern of CS/SiC vdWHs is mechanically, dynamically, and energetically stable, paving the way for their experimental synthesis. Furthermore, the charge transfer at the interface area generates a built-in electric field that may be used to prevent electron–hole recombination, which is favourable for achieving increased carrier mobility and prolonged lifetimes. Additionally, the CS/SiC vdWHs show a maximum optical absorption intensity for visible light, reaching 10⁵ cm⁻¹, and a suitable band gap (1.96 eV) that crosses the redox potentials of photocatalytic water splitting at pH = 7. A positive electric field (+0.6 to +0.8 V Å⁻¹) and a negative electric field (−0.2 to −0.6 V Å⁻¹) can be used to tune the band alignment of CS/SiC vdWHs to type I, offering theoretical guidance for their experimental synthesis for applying them in next-generation optoelectronics and solar energy devices. These results provide comprehensive knowledge of the enhanced photocatalytic mechanism of 2D/2D CS/SiC vdWHs, and they also offer a rational method for developing very effective CS/SiC photocatalysts for H2 production.
The optoelectronic and magnetic behaviors of GeC monolayer after transition metals (TMs) adsorption have been systematically discussed using density functional theory. The calculated data illustrates that the optimal adsorption sites of Sc-, Ti-, V-, Cr-, Mn-, Fe-, and Co-GeC systems are all located at [Formula: see text] site, while the Ni- and Cu-GeC systems are situated in [Formula: see text] site. The band structures of Ti-, Fe-, and Ni-GeC systems still remain nonmagnetic semiconductors, while the Sc-, Cr-, and Cu-GeC systems exhibit magnetic semiconductor behaviors, and the band gaps are 0.11 eV (Sc), 0.30 eV (Cr), and 0.57 eV (Cu), respectively. In particular, V- and Mn-GeC systems exhibit half-metallic characteristics, and Co-GeC system exhibits magnetic metal characteristics. And the magnetic moments of Sc-, V-, Cr-, Mn-, Co-, and Cu-GeC structures have been obtained to be 0.08, 1.00, 2.00, 1.00, 0.04, and 1.00 [Formula: see text], respectively. Furthermore, the charge transfer was exhibited between the GeC and TM. Especially, the work function of GeC can decrease greatly after TM adsorption, among them, the work function of Sc-GeC is 37.9% lower than that of GeC. Consequently, it indicates the usefulness of the TM-GeC system for the fabrication of spintronic and nanoelectronic devices.
The goal of this study is to investigate the thermoelectric properties of Graphene/h-BN (G/h-BN), 1B vacancy defect in G/h-BN (G/h-BN_1B), 1C vacancy defect in G/h-BN (G/h-BN_1C) and 2C vacancy defects in G/h-BN (G/h-BN_2C) heterostructures (HS) materials by using first-principles calculations based on spin-polarized DFT-D2 perspective and semi-classical Boltzmann transport theory. We found that all the studied materials are stable. We have computed the Seebeck coefficient (S), thermoelectric power factor (P), electrical conductivity (σ) and electronic contribution of thermal conductivity (K) to study the transport properties of considered materials. The temperature dependent (at constant energy), S of the above materials have positive and negative values at 300 K because the sign of S changes for different types of charge carriers. In addition, it is found that G/h-BN has a symmetry curve but defected materials have slightly asymmetry curves in S verses chemical potential (µ) plot at different constant temperatures. The asymmetry is caused by asymmetric effective mass. We have estimated the P of considered materials by taking constant and found that P of G/h-BN_1B is higher than that of other materials. An expected σ of G/h-BN follows the exponentially increasing nature with an increase in temperature. The σ of defected materials has greater values than that of G/h-BN. In addition, we have calculated the temperature dependent (at constant energy) K of mentioned materials and found them to be increased with the increase in temperatures. K of G/h-BN increases somehow exponentially; however, K of defected materials has a more-less linear nature. Among them, K of G/h-BN_1B retains a higher value at 300 K. By the evaluation of S, P, σ and K, we concluded that defected materials are more promising materials than G/h-BN in the field of thermoelectricity.Graphical abstract
First principles calculations are performed to explore the electronic structure and optical properties of BlueP/X Te2 (X = Mo, W) van der Waals heterostructures after biaxial strain has been applied. The type-II band alignments with indirect band gap are obtained in the most stable BlueP/X Te2 heterostructures, in which the photon-generated carriers can be effectively separated spatially. The BlueP/MoTe2 and BlueP/WTe2 heterostructures both have appreciable absorption of infrared light, while the shielding property is enhanced. The increase of biaxial compressive strain induces indirect-direct band gap transition and semiconductor-metal transition when a certain compressive strain is imposed on the heterostructures, moreover, the band gap of the heterostructures shows approximately linear decrease with the compressive strain increasing, and they undergo a transition from indirect band gap type-II to indirect band gap type-I with the increase of biaxial tensile strain. These characteristics provide an attractive possibility of obtaining novel multifunctional devices. We also find that the optical properties of BlueP/X Te2 heterostructures can be effectively modulated by biaxial strain. With the increase of compression strain, the absorption edge is red-shifted, the response of light absorption extends to the mid-infrared light and the absorption coefficient increases to 10–5 cm–1 for the two heterostructures. The BlueP/MoTe2 shows stronger light absorption response than the BlueP/WTe2 in the mid-infrared to infrared region and the ε1(0) increases significantly. The BlueP/X Te2 heterostructures exhibit modulation of their band alignment and optical properties by applied biaxial strain. The calculation results not only pave the way for experimental research but also indicate the great potential applications of BlueP/XTe2 van der Waals heterostructures in narrow band gap mid-infrared semiconductor materials and photoelectric devices.
Six new SiC phases with direct bandgaps were found by replacing carbon atoms with carbon and silicon atoms with a stoichiometric ratio of 1:1 in the SACADA-Samara Carbon Allotrope Database of 522 pure carbon structures via the global search method. The six newly discovered SiC phases are in the space groups of Pccn, P4/ncc, Pmn21, P63/m, I4¯3m, and Pnma, in which crystal structures, stabilities, elastic anisotropy, electronic and effective mass were investigated in detail based on first-principles calculations. The formation energies of Pmn21-SiC and Pnma-SiC are very close to that of F4¯3m-SiC, revealing their superiority in experimental implementation. Pccn-SiC, P4/ncc-SiC, Pmn21-SiC, and Pnma-SiC have better compression resistance than F4¯3m-SiC. All the proposed structures are direct bandgap semiconductors with a wide bandgap range of 2.86 ∼ 3.72 eV, which have a smaller effective mass than diamond silicon, giving promising applications in high-frequency, high-temperature and high-power electronic devices.
WS2 has become a hot research topic because of its excellent properties in the fields of chemistry, physics, and material science. We investigate the structural and electronic behaviors of organic molecules adsorbed on the WS2 single-layer using density functional theory. The results observed that organic molecules can inject additional carriers into the WS2 single-layer. Tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) doping achieve p-type doping, while tetrathiafulvalene (TTF) doping can introduce n-type doping behavior. Furthermore, the work function of WS2 can be tuned in the range of 4.05 eV to 5.71 eV after organic molecules doping. Importantly, the doping gap of the TTF-WS2 systems is reduced with the decrease of the applied negative electric field. Especially at -0.45 V/Å, the doping gap of the TTF-WS2 system is 0.01 eV. It demonstrated that the applied negative electric field can induce effective n-type doping behavior. Therefore, organic molecular doping can regular the electronic behaviors of WS2, which is useful for the design of nanoelectronic devices.
In this study, C60 coverage was deposited on few-layer MoS2 flakes, the results of which were analyzed by investigating the morphology and performing Raman spectroscopy and photoluminescence (PL) spectroscopy. The surface roughness of 5 nm C60/MoS2 was ±1 nm. When the thickness of the C60 was increased to 9–20 nm, the roughness saturated at ±2 nm and did not advance further. The added presence of C60 on MoS2 did not cause observable changes in the Raman spectra, indicating that the van der Waals contact between C60 and MoS2 is weak and does not induce noticeable microstructural changes. When the C60 coverage increased, the PL peaks occurred at the C60-dominated 1.69 eV rather than at the original MoS2-dominated 1.83 eV. Furthermore, continuous wave (CW) laser-induced local desorption was revealed in the C60/MoS2 heterostructure. A laser power of 10 mW/μm2 was sufficient for 20 nm thick C60 to desorb from a MoS2 flake and could be used to create designed patterns with a resolution of approximately 500 nm. A patterned PL could also be achieved in the C60/MoS2 through CW laser–induced desorption of C60. This method can be applied in the future fabrication of functional nanodevices combining 0-dimensional and 2-dimensional materials.
We reported a new van der Waals (vdW) heterostructure formed by monolayer black phosphorene (BP) and monolayer violet phosphorene, also known as Hittorf's phosphorene (HP). The First-principles calculation showed that the BP/HP heterostructure exhibits a direct band gap (1.43 eV) with distinct type-I band alignment. By applying tensile strains, the BP/HP heterostructure was modified from type-I junction to Z-scheme one, forming the built-in electric field directed from BP to HP which just promotes the separation of photogenerated holes and electrons, whereas compressive strains maintain the type-I band alignment but bring to the bandgap (direct to indirect) and the conductivity (semiconductor to metal) transitions. Moreover, negative electric fields could induce a more steady and robust Z-scheme band alignment over a wide range of the electric field intensities. Our present studies pointed out that this high tunability of the electronic structure in BP/HP vdW heterostructure is suitable for future nanoelectronics and photocatalysts.
In recent years, the van der Waals (vdW) Heterostructures (HTSs) are broadly studied for their capabilities to modulate the performance of two-dimensional (2D) materials. Herein, we constructed four types of vdW HTSs by vertically stacking the α-polytype and δ-polytype of single-layered SnS and SnSe. The constructed HTSs have been designated as HTS-I (SnS(α)/SnS(δ)), HTS-II (SnSe(α)/SnSe(δ)), HTS-III (SnS(α)/SnSe(δ)), and HTS-IV (SnSe(α)/SnS(δ)) and their physical properties are systematically explored by the first-principles approach. The electron density mapping revealed that the monolayers constituting these HTSs are stacked by vdW coupling which persists for interlayer distance (Δy) up to ∼7 Å. However, these tin-chalcogenide-based HTSs demonstrated the highest formation energies (Ef) and binding energies (Eb) for Δy = ∼3.75 Å. The electronic structure calculations revealed them as semiconductors of indirect bandgaps of magnitude 1.22, 1.28, 1.06, and 1.22 eV recorded for HTS-I, HTS-II, HTS-III, and HTS-IV, respectively. They exhibited type-II (staggered) band alignment where the valence band maximum occurs in the δ-type of monolayer and the conduction band minimum is located in α-type of monolayers that causes the splitting of the photo-generated electron-hole pairs at the interface. Therefore, the staggering gap and large density of states observed near the bandgap edges have triggered a significantly improved optical absorption in these HTSs compared to freestanding monolayers. Moreover, the transparent nature of these HTSs has been recognized against incident light of energy less than 5 eV. These predictions illustrate the development of vdW HTSs as an effective approach to improve the functionalities of 2D materials for advanced technological applications.
Introduction of Two-Dimensional (2D) photocatalysts for solar water splitting has attracted significant attention in recent years, due to their considerably large surface areas and short pathways of charge carrier migration compared to their bulk counterparts. Herein, monolayers of Ag2S, Ag2Se, SAg4Se, SAg4Te, SeAg4Te and their heterostructures are predicted as promising overall-water-splitting photocatalysts according to first-principles calculations. Compared with Ag2S and Ag2Se monolayers, the Janus ones not only possess direct band gaps and moderate band edges for overall water splitting but also achieve the Janus-induced built-in electric fields of 0.14, 0.34 and 0.21 eV respectively, which could improve the photocatalytic efficiency via restraining the electron-hole recombination. Further investigations on their heterostructures show that they are also suitable for overall water splitting with enhanced optical absorbances and larger built-in electric fields. Our findings extend the scope of overall-water-splitting photocatalysts to monolayers of Ag2S, Ag2Se, SAg4Se, SAg4Te, SeAg4Te and their heterostructures, which is expected to inspire the experimental fabrications and relative investigations.
Heterostructures (HS), vacancy defects in HS, and molecular adsorption on defected HS of 2D materials are fervently inspected for a profusion of applications because of their aptness to form stacked layers that confer approach to an amalgamation of favorable electronic and magnetic properties. In this context, graphene (Gr), hexagonal boron nitride (h-BN), HS of graphene/h-BN (Gr/h-BN), and molecular adsorption on Gr/h-BN offer promising prospects for electronic, spintonic, and optoelectronic devices. In this study, we investigated the structural, electronic, and magnetic properties of C sites vacancy defects in Gr/h-BN HS and adsorption of water molecule on defected Gr/h-BN HS materials by using first-principles calculations based on spin-polarized density functional theory method within van der Waals (vdW) corrections DFT-D2 approach. We found that these considered materials are stable 2D vdW HS. Based on band structure calculations, they are semimetallic, and on density of states and partial density of states analysis, they are magnetic materials. The magnetic moment developed in these defected systems is due to the unpaired up-spin and down-spin states in the orbitals of atoms present in the materials created by the vacancy defects.
Two-dimensional (2D) van der Waals (vdW) heterostructures are a new class of materials with highly tunable bandgap transition type, bandgap energy and band alignment. Herein, we have designed a novel 2D g-GaN/Sc2CO2 heterostructure as a potential solar-driven photocatalyst for the water splitting process and investigate its catalytic stability, interfacial interactions, and optical and electronic properties, as well as the effects of applying an electric field and biaxial strain using first-principles calculation. The calculated lattice mismatch and binding energy showed that g-GaN and Sc2CO2 are in contact and may form a stable vdW heterostructure. Ab initio molecular dynamics and phonon dispersion simulations show thermal and dynamic stability. g-GaN/Sc2CO2 has an indirect bandgap energy with appropriate type-II band alignment relative to the water redox potentials. Meanwhile, the interfacial charge transfer from g-GaN to Sc2CO2 can effectively separate electron-hole pairs. Moreover, a potential drop of 3.78 eV is observed across the interface, inducing a built-in electric field pointing from g-GaN to Sc2CO2. The heterostructure shows improved visible-light optical absorption compared to the isolated g-GaN and Sc2CO2 monolayers. Our study demonstrates that tunable electronic and structural properties can be realised in the g-GaN/Sc2CO2 heterostructure by varying the electric field and biaxial strain. In particular, the compressive strain and negative electric field are more effective for promoting hydrogen production performance. Since it is challenging to tune the electric field and biaxial strain experimentally, our research provides strategies to boost the performance of MXene-based heterojunction photocatalysts in solar harvesting and optoelectronic devices.
The formation of graphene-based van der Waals heterostructures has shown great potential for designing novel electronic and optoelectronic nanodevices. However, the Schottky barrier generated by the contact between semimetal (graphene)...
In this paper, we stacked WSe2 and BC2P monolayers to form three stable BC2P/WSe2 (-α, -β, -γ) vdW heterostructures, and the BC2P monolayer has been predicted and proved stable by our group. Firstly, we discussed structures and formed possibilities of three vdW heterostructures by calculating binding energies, elastic constants, plane-averaged differential charge densities and performing equilibrium MD simulations at room temperature respectively. Using GGA-PBE (HSE06) functional, BC2P/WSe2-α and BC2P/WSe2-β heterostructures are indirect semiconductors with the band gaps as 1.533 (2.02) eV and 1.521 (2.01) eV, and BC2P/WSe2-γ heterostructure is direct with the band gap as 1.524 (2.00) eV. Moreover, under different in-plane stain ratios, three BC2P/WSe2 vdW heterostructures can transfer from indirect to direct. Finally, three BC2P/WSe2 vdW heterostructures are all transparent materials, and their absorption coefficients can reach up to high values (>20% of incident light) at the visible and near ultraviolet light area. These results make three BC2P/WSe2 vdW heterostructures as good potential materials in the application of electronic, optoelectronic and photovoltaics devices.
The recent emergence of two-dimensional (2D) Janus materials has opened a new avenue for spintronic and optoelectronic applications. However, 2D magnetic Janus materials and Janus monolayer-based magnetic heterostructures are yet to be fully studied. Herein, the stability and electronic structure of 2D Janus V2I3Br3 and V2I3Cl3 monolayers, and the electronic and magnetic properties of 2D graphyne/Janus V2I3Br3 (γ-GY/V2I3Br3) heterostructures are calculated based on the density functional theory. Janus V2I3Br3 and V2I3Cl3 monolayers are ferromagnetic semiconductors with good stability and direct band gap. By combing the graphyne layer, the Janus V2I3Br3 monolayer shows half-metallic characteristics. The electrical conductivity of the Janus V2I3Br3 monolayer in γ-GY/V2I3Br3 heterostructures is further improved, which is very favorable for the applications of the γ-GY/V2I3Br3 heterostructure in battery anodes. Moreover, the Janus V2I3Br3 monolayer possesses a smaller perpendicular magnetic anisotropy (PMA), and the PMA can be effectively enhanced by combing γ-GY. Herein, the enhanced PMA was discovered to depend on the stacking patterns of γ-GY and V2I3Br3 monolayers. Biaxial strains can further affect the PMA of the γ-GY/V2I3Br3 heterostructure. Meanwhile, at a compressive strain, the Janus V2I3Br3 monolayer in the γ-GY/V2I3Br3 heterostructure realizes the transition from the magnetic half-metallic to the magnetic metal state. These results can enrich the applications and designs of γ-GY/V2I3Br3 magnetic heterostructures in spintronic devices and energy fields.
MoSi2N4 is a recently developed 2D material that exhibits remarkable thermal, mechanical, electronic, and optical properties. We used first principles calculations to study the structural and electronic properties of organic molecule doped MoSi2N4 monolayers. Effective p-doping was achieved by molecular doping with tetracyanoquinodimethane and tetracyanoethylene, while n-doping was achieved by molecular doping with tetrathiafulvalene. The doping gap of tetrathiafulvalene-doped MoSi2N4 was successfully modulated by the application of an external electric field, which resulted in effective n-doping. Furthermore, molecular doping injects additional carriers into the host, which is beneficial for enhancing the performance of MoSi2N4 in nanoelectronic devices. Our results demonstrate the importance of molecular doping in tuning the electronic properties of MoSi2N4 and broadening its applications.
Designing van der Waals (vdW) heterostructures of two-dimensional materials is an efficient way to realize amazing properties as well as open up opportunities for applications in solar energy conversion, nanoelectronic and optoelectronic devices. The electronic structures and optical and photocatalytic properties of SiS, P and SiC van der Waals (vdW) heterostructures are investigated by (hybrid) first-principles calculations. Both binding energy and thermal stability spectra calculations confirm the stability of these heterostructures. Similar to the corresponding parent monolayers, SiS-P (SiS-SiC) vdW heterostructures are found to be indirect type-II bandgap semiconductors. Furthermore, absorption spectra are calculated to understand the optical behavior of these systems, where the lowest energy transitions lie in the visible region. The valence and conduction band edges straddle the standard redox potentials of SiS, P and SiC vdW heterostructures, making them promising candidates for water splitting in acidic solution.
Graphene-based van der Waals heterostructures have received tremendous interest from both fundamental and experimental studies because they can enhance the properties and expand the possibility of applications of both graphene and two-dimensional materials. Motivated by the successful synthesis of the graphene/BiI3 heterostructure [Chang et al., Adv. Funct. Mater. 28, 1800179 (2018)]., here, we systematically investigate the electronic structure and interfacial characteristics of this material using first-principles simulations. We find that the structure of the graphene/BiI3 heterostructure is mainly characterized by weak van der Waals interactions, which keeps the heterostructure feasible. In the ground state, the graphene/BiI3 heterostructure forms the n-type Schottky contact with a barrier of 0.53 eV. The barriers of the Schottky contact can be adjusted by various factors, including interlayer coupling and electric gating. Both the interlayer coupling and electric gating lead to the transformation from the n-type Schottky contact to the p-type one or to the n-type Ohmic contact. These findings demonstrate that graphene/BiI3 can be considered a promising building block for high-performance photoresponsive optoelectronic devices.
In recent years, the van der Walls (vdW) stacking of two-dimensional (2D) materials has been widely practiced to improve their physical properties and functionalities. In this work, we constructed vdW homo- and hetero-bilayers by stacking α- and δ-types of SnS and SnSe monolayers from the first-principles approach. The designed vdW bilayers exhibited the highest binding energies for interlayer separation (Δy) equivalent to ∼3.75Å, whereas, the vdW coupling between the constituting monolayers has been realized for Δy as large as 7Å. The weakening of the vdW interactions driven by the increase in Δy has resulted in the widening of the energy bandgaps. The α-type vdW bilayers exhibited indirect bandgap whereas nearly direct bandgap has been realized for δ-type of vdW bilayers. The high probability of photogenerated electronic transitions in electronic structures of these homo- and hetero-bilayers has resulted in substantial optical absorption (of the order of ∼10⁵ cm⁻¹). Moreover, the constructed vdW bilayers exhibited interesting optical properties and transparent behavior in the infrared, visible, and ultraviolet ranges below 5 eV. This study indicates the vdW stacking as an effective route to engineer the physical properties of 2D tin-monochalcogenides for diverse electronic, optoelectronic, and photovoltaic applications.
Structural, electric and optical properties of a new van der Waals heterostructure, C2N/g-ZnO, composed of C2N and g-ZnO monolayers with an intrinsic type-II band alignment and a direct bandgap of 0.89 eV at Γ point, are extensively studied using first-principles density functional theory calculations. Results indicate the special optoelectronic properties of the constructed heterostructure mainly originate from the interlayer coupling and electron transfer between the C2N and g-ZnO monolayers, and the photogenerated electrons and holes are located on the C2N and g-ZnO layers, respectively, which reduces the recombination probability of the electron-hole pairs. According to Bader charge analysis, there are 0.029 electrons transferred from g-ZnO to C2N to form a built-in electric field of ~ 9.5 eV at the interface. Furthermore, the tunability of the electronic properties of the C2N/g-ZnO heterostructure under vertical strain and the electric field is explored. Under different strains, the type-II band alignment property of the heterostructure remains and the vertical compressive strain has a greater influence on the bandgap modulation than the vertical stretching strain. The implemented electric field also does not change the type-II band alignment but changes the bandgap of the heterostructure from 1.30 to 0.58 eV when the electric field strength varies from -0.6 to 0.6 V/Å. In addition, the absorption spectrum of the C2N/g-ZnO heterostructure under solar light is also studied. The absorption range of the heterostructure varies from ultraviolet to near-infrared with the absorption intensity in the order of 105 cm-1. All of these studies indicate that the C2N/g-ZnO heterostructure has excellent electronic and optical properties and promising applications in nanoelectronics and optoelectronics.
Two‐dimensional (2D) materials have attracted much attention for applications in halide perovskite optoelectronic devices due to their high carrier mobility and adjustable work function. Heterogeneous stacking of halide perovskites with 2D materials to form heterostructures is an effective way to improve the performance of perovskite photovoltaic and optoelectronic devices. Here, we report the investigation of the interfacial interactions and electronic properties of graphene/CsPbI3 heterostructures with two types of exposed surfaces of CsPbI3 by first‐principles calculations. The calculated results show that the intrinsic electronic properties of isolated graphene and the CsPbI3 slabs are well preserved in the heterostructures and the interaction between the heterostructures is weak van der Waals (vdW) interaction. The Schottky barriers are built in the vdW heterostructures with the optimized layer‐to‐layer distance. The barrier height can be tuned by doping graphene with boron and nitrogen atoms and by applying an external electric field. Our work provides useful information for designing next‐generation electronic devices based on graphene/CsPbI3 vdW heterostructures.
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Recently, negative differential resistance devices have attracted considerable attention due to their folded current–voltage characteristic, which presents multiple threshold voltage values. Because of this remarkable property, studies associated with the negative differential resistance devices have been explored for realizing multi-valued logic applications. Here we demonstrate a negative differential resistance device based on a phosphorene/rhenium disulfide (BP/ReS2) heterojunction that is formed by type-III broken-gap band alignment, showing high peak-to-valley current ratio values of 4.2 and 6.9 at room temperature and 180 K, respectively. Also, the carrier transport mechanism of the BP/ReS2 negative differential resistance device is investigated in detail by analysing the tunnelling and diffusion currents at various temperatures with the proposed analytic negative differential resistance device model. Finally, we demonstrate a ternary inverter as a multi-valued logic application. This study of a two-dimensional material heterojunction is a step forward toward future multi-valued logic device research.
Making use of the osmotic pressure difference between fresh water and seawater is an attractive, renewable and clean way to generate power and is known as 'blue energy'. Another electrokinetic phenomenon, called the streaming potential, occurs when an electrolyte is driven through narrow pores either by a pressure gradient or by an osmotic potential resulting from a salt concentration gradient. For this task, membranes made of two-dimensional materials are expected to be the most efficient, because water transport through a membrane scales inversely with membrane thickness. Here we demonstrate the use of single-layer molybdenum disulfide (MoS2) nanopores as osmotic nanopower generators. We observe a large, osmotically induced current produced from a salt gradient with an estimated power density of up to 10(6) watts per square metre-a current that can be attributed mainly to the atomically thin membrane of MoS2. Low power requirements for nanoelectronic and optoelectric devices can be provided by a neighbouring nanogenerator that harvests energy from the local environment-for example, a piezoelectric zinc oxide nanowire array or single-layer MoS2 (ref. 12). We use our MoS2 nanopore generator to power a MoS2 transistor, thus demonstrating a self-powered nanosystem.
Band alignment in two-dimensional (2D) lateral heterostructures is fundamentally different from three-dimensional (3D), as Schottky barrier height is at the Schottky-Mott limit and band offset is at the Anderson limit, regardless interfacial conditions. This robustness arises because, in the asymptotic limit, effect of interfacial dipole vanishes. First-principles calculations of graphene/h-BN and MoS2/WS2 show that 2D junction width W is typically an order of magnitude longer than 3D. Therefore, heterostructures with dimension less than W can also be made, leading to tunable band alignment.
Combining atomically-thin van der Waals (vdW) materials into heterostructures provides a powerful path towards the creation of designer electronic devices. Interactions between the individual atomic layers of the heterostructure strongly affect the properties of these systems. Here, we demonstrate unprecedented control over these interactions by locally modifying the interlayer separation between graphene and hexagon boron nitride (hBN), which we achieve by applying pressure with a scanning tunneling microscopy (STM) tip. For the case of nearly aligned graphene on hBN, the graphene lattice can stretch and compress locally to compensate for the slight lattice mismatch between the two materials. We find that modifying the interlayer separation directly tunes the lattice strain and induces commensurate stacking underneath the tip. Our results open a path towards tailored electronic properties of vdW heterostructures controlled through the interlayer separation degree of freedom.
Electronic, optical and transport properties of the MoS2/graphene heterostructure have been investigated as function of applied uniaxial compression normal to the interface plane using first principles calculations and a non-equilibrium Green's function approach. The results show that a small compressive load (~1 GPa) can open up the band gap (~12 meV), reduce the optical absorption coefficient (~7%), redshift the absorption spectrum, and create non-Ohmic I
–V characteristics that depend on the magnitude of applied bias. This suggests that graphene/MoS2 heterostructure can be suitable for electromechanical and photomechanical devices where the electronic, optical and transport properties can be tuned by an appropriate application of bias and mechanical deformations.
This study presents a new ultrathin SiC structure prepared by a catalyst free carbothermal method and post-sonication process. We have found that merging ultra-light 3D graphene foam and SiO together at high temperature leads to the formation of a complex SiC structure consisting of 3D SiC foam covered with traditional 1D nanowires. Upon breaking off, the 3D SiC was confirmed to be made from 2D nanosheets. The resulting novel 2D SiC nanosheets/nanoflakes were thoroughly investigated by using optical microscope, SEM, EDS, TEM, STEM, AFM, and Raman, which verified the highly crystallised structure feature. AFM results revealed an average thickness of 2-3 nm and average size of 2 μm for the flakes. This new SiC structure could not only actualise SiC usage for nano-electronic devices but is also expected to open new applications as well.
The development of two-dimensional (2D) layered materials is driven by fundamental interest and their potential applications. Atomically thin 2D materials provide a wide range of basic building blocks with unique electrical, optical, and thermal properties which do not exist in their bulk counterparts. The van der Waals interlayer interaction enables the possibility to exfoliate and reassemble different 2D materials into arbitrarily and vertically stacked heterostructures. Recently developed vapor phase growth of 2D materials further paves the way of directly synthesizing vertical and lateral heterojunctions. This review provides insights into the layered 2D heterostructures, with a concise introduction to preparative approaches for 2D materials and heterostructures. These unique 2D heterostructures have abundant implications for many potential applications.
Hexagonal boron nitride (h-BN) is a natural hyperbolic material 1 , in which the dielectric constants are the same in the basal plane (ε t ≡ ε x = ε y) but have opposite signs (ε t ε z < 0) in the normal plane (ε z) 1–4. Owing to this property, finite-thickness slabs of h-BN act as multimode waveguides for the propagation of hyperbolic phonon polaritons 1,2,5 —collective modes that originate from the coupling between photons and electric dipoles 6 in phonons. However, control of these hyperbolic phonon polari-tons modes has remained challenging, mostly because their electrodynamic properties are dictated by the crystal lattice of h-BN 1,2,7. Here we show, by direct nano-infrared imaging, that these hyperbolic polaritons can be effectively modulated in a van der Waals heterostructure 8 composed of monolayer graphene on h-BN. Tunability originates from the hybridization of surface plasmon polaritons in graphene 9–13 with hyperbolic phonon polaritons in h-BN 1,2 , so that the eigenmodes of the gra-phene/h-BN heterostructure are hyperbolic plasmon–phonon polaritons. The hyperbolic plasmon–phonon polaritons in gra-phene/h-BN suffer little from ohmic losses, making their propagation length 1.5–2.0 times greater than that of hyperbolic phonon polaritons in h-BN. The hyperbolic plasmon–phonon polaritons possess the combined virtues of surface plasmon polaritons in graphene and hyperbolic phonon polaritons in h-BN. Therefore, graphene/h-BN can be classified as an electromagnetic metamaterial 14 as the resulting properties of these devices are not present in its constituent elements alone. Van der Waals (vdW) heterostructures assembled from mono-layers (one or a few) of graphene, hexagonal boron nitride (h-BN), MoS 2 and other atomic crystals in various combinations are emerging as a new paradigm with which to attain desired electronic 8,15
Two key subjects stand out in the pursuit of semiconductor research: material quality and contact technology. The fledging field of atomically thin transition metal dichalcogenides (TMDCs) faces a number of challenges in both efforts. This work attempts to establish a connection between the two by examining the gate-dependent conductance of few-layer (1-5L) WSe2 field effect devices. Measurements and modeling of the subgap regime reveal Schottky barrier
transistor behavior. We show that transmission through the contact barrier is dominated by thermionic field emission (TFE) at room temperature, despite the lack of intentional doping. The TFE process arises due to a large number of subgap impurity states, the presence of which also leads to high mobility edge carrier densities. The density of states of such impurity states is self-consistently determined to be approximately 1–2 × 1013/cm2/eV in our devices. We demonstrate that substrate is unlikely to be a major source of the impurity states and suspect that lattice defects within the material itself are primarily responsible. Our experiments provide key information to advance the quality and understanding of TMDC materials and electrical devices.
Herein, for the first time, the ultra-thin SiC nanoparticles or ultra-thin layers covered graphene nanosheets were successfully prepared via using a facile in-situ vapor-solid reaction. The samples were characterized by X-ray diffraction, UV-visible spectroscopy, photoluminescence spectra analysis, Raman spectra, transient photocurrent responses and transmission electron microscopy. The photocatalytic activities were also evaluated by H2 evolution from pure water or water containing Na2S as an electron donor. The resulting SiC-graphene hybrids show enhanced photocatalytic H2-evolution activities in the presence of an electron donor. Especially, the graphene nanosheet-SiC nanocrystal hybrids show the highest photocatalytic activity in H2 production under visible light, which is about 10 times higher than that of the SiC nanocrystals. The enhanced activities of the SiC-graphene hybrids can be attributed to their 2D nanosheet structures, large surface area, enhanced visible-light absorption and rapid interfacial charge transfer from SiC to graphene. Our results can provide an effective approach to synthesize graphene-based heterogeneous nanocomposites for a wide variety of potential applications in solar energy conversion and storage, separation, and purification processes.
Vertical integration of two-dimensional van der Waals materials is predicted
to lead to novel electronic and optical properties not found in the constituent
layers. Here, we present the direct synthesis of two unique, atomically thin,
multi-junction heterostructures by combining graphene with the monolayer
transition-metal dichalocogenides: MoS2, MoSe2, and WSe2.The realization of
MoS2-WSe2-Graphene and WSe2-MoSe2-Graphene heterostructures leads toresonant
tunneling in an atomically thin stack with spectrally narrow room temperature
negative differential resistance characteristics.
Graphene/hexagonal boron nitride (h-BN) vertical heterostructures have recently revealed unusual physical properties and new phenomena, such as commensurate-incommensurate transition and fractional quantum hall states featured with Hofstadter's butterfly. Graphene-based devices on h-BN substrate also exhibit high performance owing to the atomically flat surface of h-BN and its lack of charged impurities. To have a clean interface between the graphene and h-BN for better device performance, direct growth of large-area graphene/h-BN heterostructures is of great importance. Here we report the direct growth of large-area graphene/h-BN vertical heterostructures by a co-segregation method. By one-step annealing sandwiched growth substrates (Ni(C)/(B, N)-source/Ni) in vacuum, wafer-scale graphene/h-BN films can be directly formed on the metal surface. The as-grown vertically stacked graphene/h-BN structures are demonstrated by various morphology and spectroscopic characterizations. This co-segregation approach opens up a new pathway for large-batch production of graphene/h-BN heterostructures and would also be extended to the synthesis of other van der Waals heterostructures.
In recent years there have been many breakthroughs in two-dimensional (2D) nanomaterials, among which the transition metal dichalcogenides (TMDs) attract significant attention owing to their unusual properties associated with their strictly defined dimensionalities. TMD materials with a generalized formula of MX2, where M is a transition metal and X is a chalcogen, represent a diverse and largely untapped source of 2D systems. Semiconducting TMD monolayers such as MoS2, MoSe2, WSe2 and WS2 have been demonstrated to be feasible for future electronics and optoelectronics. The exotic electronic properties and high specific surface areas of 2D TMDs offer unlimited potential in various fields including sensing, catalysis, and energy storage applications. Very recently, the chemical vapour deposition technique (CVD) has shown great promise to generate high-quality TMD layers with a scalable size, controllable thickness and excellent electronic properties. Wafer-scale deposition of mono to few layer TMD films has been obtained. Despite the initial success in the CVD synthesis of TMDs, substantial research studies on extending the methodology open up a new way for substitution doping, formation of monolayer alloys and producing TMD stacking structures or superlattices. In this tutorial review, we will introduce the latest development of the synthesis of monolayer TMDs by CVD approaches.
We have investigated thermal conductivity of graphene laminate films deposited on polyethylene terephthalate substrates. Two types of graphene laminate were studied - as deposited and compressed - in order to determine the physical parameters affecting the heat conduction the most. The measurements were performed using the optothermal Raman technique and a set of suspended samples with the graphene laminate thickness from 9 µm to 44 µm. The thermal conductivity of graphene laminate was found to be in the range from 40 W/mK to 90 W/mK at room temperature. It was found unexpectedly that the average size and the alignment of graphene flakes are more important parameters defining the heat conduction than the mass density of the graphene laminate. The thermal conductivity scales up linearly with the average graphene flake size in both uncompressed and compressed laminates. The compressed laminates have higher thermal conductivity for the same average flake size owing to better flake alignment. Coating plastic materials with thin graphene laminate films that have up-to-600X higher thermal conductivity than plastics may have important practical implications.
The so-called Klein paradox-unimpeded penetration of relativistic
particles through high and wide potential barriers-is one of the most
exotic and counterintuitive consequences of quantum electrodynamics. The
phenomenon is discussed in many contexts in particle, nuclear and
astro-physics but direct tests of the Klein paradox using elementary
particles have so far proved impossible. Here we show that the effect
can be tested in a conceptually simple condensed-matter experiment using
electrostatic barriers in single- and bi-layer graphene. Owing to the
chiral nature of their quasiparticles, quantum tunnelling in these
materials becomes highly anisotropic, qualitatively different from the
case of normal, non-relativistic electrons. Massless Dirac fermions in
graphene allow a close realization of Klein's gedanken experiment,
whereas massive chiral fermions in bilayer graphene offer an interesting
complementary system that elucidates the basic physics involved.
Based on ab initio calculations of both the ABC- and AB-stacked graphites, interlayer potentials (i.e., graphene-graphene interaction) are obtained as a function of the interlayer spacing using a modified Möbius inversion method, and are used to calculate basic physical properties of graphite. Excellent consistency is observed between the calculated and experimental phonon dispersions of AB-stacked graphite, showing the validity of the interlayer potentials. More importantly, layer-related properties for nonideal structures (e.g., the exfoliation energy, cleave energy, stacking fault energy, surface energy, etc.) can be easily predicted from the interlayer potentials, which promise to be extremely efficient and helpful in studying van der Waals structures.
Two-dimensional (2D) material-based van der Waals (vdW) heterostructures have recently attracted much attention because the combined merits of two 2D monolayers enable many novel applications in nanoelectronic and optoelectronic devices. The recently synthesized graphene/WSe2 vdW heterostructures are especially appealing, and understanding the effect of structural imperfection on the electronic properties of the graphene/WSe2 vdW heterostructures is important for their practical application. In this study, we used first-principles calculations to investigate the structural and electronic properties of a graphene monolayer stacked on a WSe2 monolayer with various structural defects: two Se vacancies (WSe2-Vdi-Se) and W vacancy (WSe2-VW). After the structural defects were introduced, the calculated properties of the graphene/WSe2 vdW heterostructure were significantly changed. Therefore, it is extremely important to prepare defect-free WSe2 for the nanoelectronics applications with dedicated function. On
A monolayer 2D capping layer with high Young's modulus is shown to be able to effectively suppress the dewetting of underlying thin films of small organic semiconductor molecule, polymer, and polycrystalline metal, respectively. To verify the universality of this capping layer approach, the dewetting experiments are performed for single-layer graphene transferred onto polystyrene (PS), semiconducting thienoazacoronene (EH-TAC), gold, and also MoS2 on PS. Thermodynamic modeling indicates that the exceptionally high Young's modulus and surface conformity of 2D capping layers such as graphene and MoS2 substantially suppress surface fluctuations and thus dewetting. As long as the uncovered area is smaller than the fluctuation wavelength of the thin film in a dewetting process via spinodal decomposition, the dewetting should be suppressed. The 2D monolayer-capping approach opens up exciting new possibilities to enhance the thermal stability and expands the processing parameters for thin film materials without significantly altering their physical properties.
Blue phosphorene (BlueP) is a graphene-like phosphorus nanosheet which was synthesized very recently for the first time [Nano Lett., 2016, 16, 4903-4908]. The combination of electronic properties of two different two-dimensional materials in an ultrathin van der Waals (vdW) vertical heterostructure has been proved to be an effective approach to the design of novel electronic and optoelectronic devices. Therefore, we used density functional theory to investigate the structural and electronic properties of two BlueP-based heterostructures - BlueP/graphene (BlueP/G) and BlueP/graphene-like gallium nitride (BlueP/g-GaN). Our results showed that the semiconducting nature of BlueP and the Dirac cone of G are well preserved in the BlueP/G vdW heterostructure. Moreover, by applying a perpendicular electric field, it is possible to tune the position of the Dirac cone of G with respect to the band edge of BlueP, resulting in the ability to control the Schottky barrier height. For the BlueP/g-GaN vdW heterostructure, BlueP forms an interface with g-GaN with a type-II band alignment, which is a promising feature for unipolar electronic device applications. Furthermore, we discovered that both G and g-GaN can be used as an active layer for BlueP to facilitate charge injection and enhance the device performance.
The composition dependence of the natural band alignment at nonpolar AlxGa1-xN/AlyGa1-yN heterojunctions is investigated via hybrid functional based density functional theory. Accurate band-gap data are provided using Heyd-Scuseria-Ernzerhof (HSE) type hybrid functionals with a composition dependent exact-exchange contribution. The unstrained band alignment between zincblende (zb) AlxGa1-xN semiconductor alloys is studied within the entire ternary composition range utilizing the Branch-point technique to align the energy levels related to the bulklike direct Γv→Γc and indirect, pseudodirect, respectively, Γv→Xc type transitions in zb-AlxGa1-xN. While the zb-GaN/AlxGa1-xN band edges consistently show a type-I alignment, the relative position of fundamental band edges changes to a type-II alignment in the Al-rich composition ranges of zb-AlxGa1-xN/AlN and zb-AlxGa1-xN/AlyGa1-yN systems. The presence of a direct-indirect band-gap transition at xc=0.63 in zb-AlxGa1-xN semiconductor alloys gives rise to a notably different composition dependence of band discontinuities in the direct and indirect energy-gap ranges. Below the critical direct-indirect Al/Ga-crossover concentration, the band offsets show a close to linear dependence on the alloy composition. In contrast, notable bowing characteristics of all band discontinuities are observed above the critical crossover composition.
Using first-principles calculations, we systematically investigated the electronic properties of graphene/g-GaN van der Waals (vdW) heterostructures. We discovered that the Dirac cone of graphene could be quite well preserved in the vdW heterostructures. Moreover, a transition from an n-type to p-type Schottky contact at the graphene/g-GaN interface was induced with decreased interlayer distance from 4.5 to 2.5 Å. This relationship is expected to enable effective control of the Schottky barrier, which is an important development in the design of Schottky devices.
The mechanical properties of the SiC/graphene composites under tensile are studied via molecular dynamics (MD) methods. In our study, the SiC/graphene composites with monolayer graphene and multilayer graphene are built and compared. As a result, the composites with different interface structures present different tensile behaviors. The single graphene sheet in contact with Si-terminated SiC(1 1 1) surface brings the higher failure strain and strength. It is also found that the elastic modulus of the composites increase with the increasing graphene volume fraction. As for the composites fabricated with the multilayers graphene, the failure strain of the whole systems would decrease with the increasing graphene layers in the case that graphene serves as single layer. By contrast, when the graphene sheets serve as continuous layers, the failure strain does not change significantly and the strength would increase monotonically with the increasing number of graphene layers.
The structural, electronic, and thermodynamic properties of a monolayer honeycomb Si1-xGexC sheet are analyzed using first-principles calculations based on density functional theory. The dynamical stability of our structures is testified by the analysis of phonon dispersion curves. Deviations of the lattice parameter, Young's modulus, and band gap of relaxed structures derived from Vegard's law are investigated; small bowing coefficients are observed except for Young's modulus. The band gaps are found to decrease as the concentration x increases. The T-x phase diagram, which identifies the stable, metastable, and unstable mixing areas, is also calculated and shows a critical temperature Tc of 187.45 K. The results provide a new method to modify the electronic properties of 2D-SiC, which has great importance in its applications for optoelectronic devices.
Using first-principles calculations, we investigate the effect of molecular doping and sulfur vacancy on the electronic properties and charge modulation of monolayer MoS2. It is found that tetrathiafulvalene (TTF) and dimethyl-p-phenylene diamine (DMPD) molecules are effective donors, while tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) are effective acceptors, and all these molecules are able to shift the work function of MoS2. For MoS2 containing sulfur vacancies, these molecules are able to change the position of the defect levels within the band gap and modulate the carrier density around the defect center. Charge transfer analysis shows that TCNE and TCNQ induce a free-carrier depletion of the defect states, which is beneficial for the suppression of the non-radiative trionic decay and a higher excitonic efficiency due to a decrease of the screening of excitons. Furthermore, the effects of molecular adsorption on Seebeck coefficient of MoS2 are also explored. Our work suggests that an enhanced excitonic efficiency of MoS2 may be achieved via proper defect engineering and molecular doping arising from the charge density modulation and charge screening.
Multilayer van der Waals (vdWs) heterostructures assembled by diverse atomically thin layers have demonstrated a wide range of fascinating phenomena and novel applications. Understanding the interlayer coupling and its correlation effect is paramount for designing novel vdWs heterostructures with desirable physical properties. Using a detailed theoretical study of 2D MoS2-graphene (GR)-based heterostructures based on state-of-the-art hybrid density functional theory, we reveal that for 2D few-layer heterostructures, vdWs forces between neighboring layers depend on the number of layers. Compared to that in bilayer, the interlayer coupling in trilayer vdW heterostructures can significantly be enhanced by stacking the third layer, directly supported by short interlayer separations and more interfacial charge transfer. The trilayer shows strong light absorption over a wide range (<700 nm), making it very potential for solar energy harvesting and conversion. Moreover, the Dirac point of GR and band gaps of each layer and trilayer can be readily tuned by external electric field, verifying multilayer vdWs heterostructures with unqiue optoelectronic properties found by experiments. These results suggest that tuning the vdWs interaction, as a new design parameter, would be an effective strategy for devising particular 2D multilayer vdWs heterostructures to meet the demands in various applications.
Germanene, a two-dimensional (2D) Dirac semimetal beyond graphene, has been recently synthesized on a nonmetallic substrate, which offers great opportunities for realization of germanene-based electronic devices. Understanding the effects of substrate and chemical modification on the electronic properties of germanene is thus crucial for tailoring this novel 2D material for future applications. Herein we investigate the structure, interlayer interaction and electronic band structure of monolayer germanene supported on various transition metal dichalcogenide (TMD) substrates. A band gap of 38~57 meV can be opened by the TMD substrates due to breaking of lattice symmetry of the germanene sheet. An electron donor molecule – tetrathiafulvalene (TTF) – is exploited to noncovalently functionalize the germanene on MoS2 substrate. The electron transfer from TTF to germanene disturbs the Dirac cone of germanene, and leads to an augment of the band gap up to 180 meV. Meanwhile, the charge carriers of the hybrid system are still mobile possessing small effective masses (≤ 0.16 m0). Applying a vertical electric field can increase the interface dipole of the hybrid system, and further enhance the band gap up to 214 meV. These theoretical results provide an effective and reversible route for engineering the band gap and work function of germanene without severely affecting the transport properties of this material.
We demonstrate near unity, broadband absorbing optoelectronic devices using sub-15 nm thick transition metal dichalcogenides (TMDCs) of molybdenum and tungsten as van der Waals semiconductor active layers. Specifically, we report that near-unity light absorption is possible in extremely thin (< 15 nm) Van der Waals semiconductor structures by coupling to strongly damped optical modes of semiconductor/metal heterostructures. We further fabricate Schottky junction devices using these highly absorbing heterostructures and characterize their optoelectronic performance. Our work addresses one of the key criteria to enable TMDCs as potential candidates to achieve high optoelectronic efficiency.
The dielectric functions of few-layer graphene and the related temperature dependence are investigated
from the atomic scale using first-principles calculations. Compared with ellipsometry experiments
in the spectral range of 190–2500 nm, the normalized optical constants of mono-layer
graphene demonstrate good agreement and further validate first-principles calculations. To interpret
dielectric function of mono-layer graphene, the electronic band structure and density of states are
analyzed. By comparing dielectric functions of mono-, bi-, and tri-layer graphene, it shows that
interlayer screening strengthens intraband transition and greatly enhances the absorption peak
located around 1 eV. The strengthened optical absorption is intrinsically caused by the increasing
electron states near the Fermi level. To investigate temperature effect, the first-principles calculations
and lattice dynamics are combined. The lattice vibration enhances parallel optical absorption
peak around 1 eV and induces redshift. Moreover, it is observed that the van der Waals force plays
a key role in keeping the interlayer distance stable during dynamics simulations.
We present a comprehensive study of the band alignments of two-dimensional (2D) semiconducting materials and highlight the possibilities of forming momentum-matched type I, II, and III heterostructures, an enticing possibility being atomic heterostructures where the constituent monolayers have band edges at the zone center, i.e., Γ valley. Our study, which includes the group IV and III-V compound monolayer materials, group V elemental monolayer materials, transition-metal dichalcogenides, and transition-metal trichalcogenides, reveals that almost half of these materials have conduction and/or valence band edges residing at the zone center. Using first-principles density functional calculations, we present the type of the heterostructure for 903 different possible combinations of these 2D materials which establishes a periodic table of heterostructures.
Graphene and other 2D atomic crystals are of considerable interest in catalysis because of their unique structural and electronic properties. Over the past decade, the materials have been used in a variety of reactions, including the oxygen reduction reaction, water splitting and CO 2 activation, and have been shown to exhibit a range of catalytic mechanisms. Here, we review recent advances in the use of graphene and other 2D materials in catalytic applications, focusing in particular on the catalytic activity of heterogeneous systems such as van der Waals heterostructures (stacks of several 2D crystals). We discuss the advantages of these materials for catalysis and the different routes available to tune their electronic states and active sites. We also explore the future opportunities of these catalytic materials and the challenges they face in terms of both fundamental understanding and the development of industrial applications.
The world of two-dimensional (2D) heterostructures continues to expand at a rate much greater than anyone could have predicted 10 years ago, but if we are to make the leap from science to technology, many materials challenges must still be overcome. Recent advances, such as those by Liu et al. in this issue of ACS Nano, demonstrate that it is possible to grow rotationally commensurate 2D heterostructures, which could pave the way toward single crystal van der Waals solids. In this Perspective, I provide some insight into a few of the challenges associated with growth of heterostructures, and discuss some of the recent works that help us better understand synthetic realization of 2D heterostructures.
Recent findings of group-V nanosheets provide new building units for van der Waals hetero-nanostructures. Based on first-principles calculation, we investigate the structural and electronic properties of bilayer hetero-sheets composed of group-V (arsenene/antimonene) and group-IV (graphene/silicene) layers. These hetero-sheets exhibit typical van der Waals features with small binding energies and soft interlayer elastic constants. In the hetero-sheets, the Dirac characteristics of the group-IV layer and the semiconducting feature of the group-V one are well preserved, which causes a Schottky contact at the metal-semiconductor interface. The Schottky barriers are always p-type in the Si-based hetero-sheets, whereas in the C-based ones, the interfacial feature is sensitive to the interlayer distance. A tensile strain would induce a p-type-to-n-type Schottky barrier transition for the As-C hetero-sheet, while a compressive strain can cause a Schottky-to-ohmic contact transition in the Sb-C one. Moreover, due to the inhomogeneous charge redistribution, a sizeable band gap is opened at the Dirac point of the Sb-Si hetero-sheet, which could be linearly modulated by perpendicular strains around the equilibrium site. The versatile electronic structures and tunable interfacial properties enable the group-V-group-IV hetero-bilayer structures to have many potential applications in nano-devices and nano-electronics.
Using first-principles calculations and deformation potential theory, we investigate the intrinsic carrier
mobility (μ) of monolayer MoS2 sheet and nanoribbons. In contrast to the dramatic deterioration of μ in graphene upon forming nanoribbons, the magnitude of μ in armchair MoS2 nanoribbons is comparable to its sheet counterpart, albeit oscillating with ribbon width. Surprisingly, a room-temperature transport polarity reversal is observed with μ of hole (h) and electron (e) being 200.52 (h) and 72.16 (e) cm2 V−1 s−1 in
sheet, and 49.72 (h) and 190.89 (e) cm2 V−1 s−1 in 4 nm nanoribbon. The high and robust μ and its polarity reversal are attributable to the different characteristics of edge states inherent in MoS2 nanoribbons. Our study suggests that width reduction together with edge engineering provide a promising route for improving the transport properties of MoS2 nanostructures.
A method is given for generating sets of special points in the Brillouin zone which provides an efficient means of integrating periodic functions of the wave vector. The integration can be over the entire Brillouin zone or over specified portions thereof. This method also has applications in spectral and density-of-state calculations. The relationships to the Chadi-Cohen and Gilat-Raubenheimer methods are indicated.
Vertical integration of two-dimensional materials has recently emerged as an
exciting method for the design of novel electronic and optoelectronic devices.
Using density functional theory, we investigatethe structural and electronic
properties of two heterostruc-tures, graphene/phosphorene (G/BP) and hexagonal
boron nitride/phosphorene (BN/BP). We found that the interlayer distance,
binding energy, and charge transfer in G/BP and BN/BP are similar. Interlayer
noncovalentbonding is predicted due to the weak coupling between the pz orbital
of BP and the {\pi} orbital of graphene and BN. A small amount of electron
transfer from graphene and BN, scaling with the vertical strain, renders BP
slightly n-doped for both heterostructures. Several attractive characteristics
of BP, including direct band gap and linear dichroism, are preserved. However,
a large redistribution of electrostatic potential across the interface is
observed, which may significantly renormalize the carrier dynamics and affect
the excitonic behavior of BP. Our work suggests that graphene and BN can be
used not only as an effective capping layer to protect BP from its structural
and chemical degradation while still maintain its major electronic
characteristics, but also as an active layer to tune the carrier dynamics and
optical properties of BP.
We use flash-photolysis time-resolved microwave conductivity experiments (fp-TRMC) and fs-ns pump-probe transient absorption spectroscopy to investigate photoinduced carrier generation and recombination dynamics of a trilayer cascade heterojunction (P3HT/TiOPc/C60) composed of poly(3-hexylthiophene) (P3HT), titanyl phthalocyanine (TiOPc), and fullerene (C60). Carrier generation following selective photoexcitation of TiOPc is independently observed at both the P3HT/TiOPc and TiOPc/C60 interfaces. The transient absorption results indicate that following initial charge generation processes to produce P3HT•+/TiOPc•- and TiOPc•+/C60•- at each interface from (P3HT/TiOPc*/C60), the final charge separated product of (P3HT•+/TiOPc/C60•-) is responsible for the long-lived photoconductance signals in fp-TRMC. And in both P3HT/TiOPc and P3HT/TiOPc/C60 cases, the electron transfer appears to occur only with the crystalline (weakly coupled H-aggregate) phase of the P3HT.
We describe the successful in situ chemical vapor deposition synthesis of a graphene-based heterostructure in which a graphene monolayer is protected by top and bottom boron nitride films. The boron nitride film/graphene monolayer/boron nitride film (BGB) was found to be a mechanically robust and chemically inert heterostructure, from which the deleterious effects of mechanical transfer processes and unwanted chemical doping under air exposure were eliminated. The chemical compositions of each film layer were monitored ex situ using UV-visible absorption spectroscopy and X-ray photoelectron spectroscopy, and the crystalline structures were confirmed using transmission electron microscopy and selected-area electron diffraction measurements. The performance of the devices fabricated using the BGB film were monitored over six months and did not display large changes in the mobility or Dirac point, unlike the conventional graphene devices prepared on a SiO2 substrate. The in situ-grown BGB film properties suggest a novel approach to the fabrication of commercial-grade graphene-based electronic devices.
With MoS 2 as saturable absorber, passive Q -switching and Q -switched mode-locking operations of a Tm-doped calcium lithium niobium gallium garnet (Tm:CLNGG) laser were experimentally demonstrated. The Q -switched laser emitted a maximum average output power of 62 mW and highest pulse energy of 0.72 μJ. Q -switched mode locking was also obtained in the experiment. The research results will open up applications of MoS 2 at the mid-infrared wavelength.
Schottky barriers formed by graphene (monolayer, bilayer, and multilayer) on 2D layered semiconductor tungsten disulfide (WS2) nanosheets are explored for solar energy harvesting. The characteristics of graphene-WS2 junction varies significantly with the number of graphene layers on WS2, resulting in difference in solar cell performance. Compared with monolayer or stacked bilayer, multilayer graphene helps to achieve improved solar cell performance due to superior electrical conductivity. The all-layered-materials Schottky barrier solar cell employing WS2 as a photoactive semiconductor exhibits efficient photon absorption in visible spectral range, yielding 3.3 % photoelectric conversion efficiency with multilayer graphene contact. Carrier transport at graphene/WS2 interface and interfacial recombination process in the Schottky barrier solar cells are examined.
Phosphorene, an elemental 2D material, which is the monolayer of black phosphorus, has been mechanically exfoliated recently. In its bulk form, black phosphorus shows high carrier mobility (~10000 cm2/V•s) and a ~0.3 eV direct bandgap. Well-behaved p-type field-effect transistors with mobilities of up to 1000 cm2/V•s, as well as phototransistors, have been demonstrated on few-layer black phosphorus, showing its promise for electronics and optoelectronics applications due to its high mobility and thickness-dependence direct bandgap. However, p-n junctions, the basic building blocks of modern electronic and optoelectronic devices, have not yet been realized based on black phosphorus. In this letter, we demonstrate a gate tunable p-n diode based on a p-type black phosphorus/n-type monolayer MoS2 van der Waals p-n heterojunction. Upon illumination, these ultra-thin p-n diodes show a maximum photodetection responsivity of 418 mA/W at the wavelength of 633 nm, and photovoltaic energy conversion with an external quantum efficiency of 0.3%. These p-n diodes show promise for broadband photodetection and solar energy harvesting.
Two-dimensional (2D) materials have generated great interest in the last few years as a new toolbox for electronics. This family of materials includes, among others, metallic graphene, semiconducting transition metal dichalcogenides (such as MoS2) and insulating Boron Nitride. These materials and their heterostructures offer excellent mechanical flexibility, optical transparency and favorable transport properties for realizing electronic, sensing and optical systems on arbitrary surfaces. In this paper, we demonstrate a novel technology for constructing large-scale electronic systems based on graphene/molybdenum disulfide (MoS2) heterostructures grown by chemical vapor growth. High-performance devices and circuits based on this heterostructure have been fabricated, where MoS2 is used as transistor channel and graphene as contact electrodes and circuit interconnects. In addition, the graphene/MoS2 heterojunction contact has been systematically compared with more traditional MoS2-metal junctions and studied using density functional theory. The tunability of the graphene work function with electrostatic doping significantly improves the ohmic contact to MoS2. The high-performance large-scale devices and circuits based on this 2D heterostructure paves the way for practical flexible transparent electronics in the future.
Trace chemical detection is important for a wide range of practical applications. Recently emerged two-dimensional (2D) crystals offer unique advantages as potential sensing materials with high sensitivity, owing to their very high surface-to-bulk atom ratios and semiconducting properties. Here, we report the first use of Schottky-contacted chemical vapor deposition grown monolayer MoS2 as high-performance room temperature chemical sensors. The Schottky-contacted MoS2 transistors show current changes by 2-3 orders of magnitude upon exposure to very low concentrations of NO2 and NH3. Specifically, the MoS2 sensors show clear detection of NO2 and NH3 down to 20 ppb and 1 ppm, respectively. We attribute the observed high sensitivity to both well-known charger transfer mechanism and, more importantly, the Schottky barrier modulation upon analyte molecule adsorption, the latter of which is made possible by the Schottky contacts in the transistors and is not reported previously for MoS2 sensors. This study shows the potential of 2D semiconductors as high-performance sensors and also benefits the fundamental studies of interfacial phenomena and interactions between chemical species and monolayer 2D semiconductors.
Transport in ultrathin graphite films grown on single-crystal silicon
carbide is dominated by the electron-doped epitaxial graphene layer at
the interface and shows graphene characteristics. Epitaxial graphene
provides a platform for studying the novel electronic properties of this
2D electron gas in a controlled environment. Shubnikov-de Haas
oscillations in the magnetoresistance data indicate an anomalous Berry's
phase and reveal the Dirac nature of the charge carriers. The system is
highly coherent with phase coherence lengths beyond 1 micrometer at
cryogenic temperatures, and mobilities exceeding 2.5 square meters per
volt-second. In wide structures, evidence is found for weak
anti-localization in agreement with recent graphene weak-localization
theory. Patterned narrow ribbons show quantum confinement of electrons.
Several Hall bar samples reveal anomalous magnetoresistance patterns
consisting of large structured non-periodic oscillations that may be due
to a periodic superlattice potential.
Composite materials: Tungsten disulfide and WS2 /reduced graphene oxide (WS2 /rGO) nanosheets were fabricated by hydrothermal synthesis using tungsten chloride, thioacetamide, and graphene oxide (GO) as starting materials. The WS2 nanosheets are efficiently templated on the rGO layer. The WS2 /rGO hybrid nanosheets show much better electrocatalytic activity for the hydrogen evolution reaction than WS2 nanosheets alone.
The structural and mechanical properties of graphene-like honeycomb monolayer structures of MoS2 (g-MoS2) under various large strains are investigated using density functional theory (DFT). g-MoS2 is mechanically stable and can sustain extra large strains: the ultimate strains are 0.24, 0.37, and 0.26 for armchair, zigzag, and biaxial deformation, respectively. The in-plane stiffness is as high as 120 N m(-1) (184 GPa equivalently). The third, fourth, and fifth order elastic constants are indispensable for accurate modeling of the mechanical properties under strains larger than 0.04, 0.07, and 0.13 respectively. The second order elastic constants, including in-plane stiffness, are predicted to monotonically increase with pressure while the Poisson ratio monotonically decreases with increasing pressure. With the prominent mechanical properties including large ultimate strains and in-plane stiffness, g-MoS2 is a promising candidate of elastic energy storage for clean energy. It possesses a theoretical energy storage capacity as high as 8.8 MJ L(-1) and 1.7 MJ kg(-1), or 476 W h kg(-1), larger than a Li-ion battery and is environmentally friendly.
At the level of a single particle, nanocrystal quantum dots (NQDs) are observed to fluoresce intermittently or "blink." They are also characterized by an efficient non-radiative recombination process known as Auger Recombination (AR). Recently, new approaches to NQD heterostructuring have been developed that directly impact both blinking and AR, resulting in dramatic suppression of these unwanted processes. The three successful hetero-NQD motifs are reviewed here: (1) interfacial alloying, (2) thick or "giant" shells, and (3) specific type-II electronic structures. These approaches, which rely on modifying or tuning internal NQD core/shell structures, are compared with alternative strategies for blinking suppression that rely, instead, on surface modifications or surface-mediated interactions. Finally, in each case, the unique synthetic approaches or challenges addressed that have driven the realization of novel and important functionality are discussed, along with the implications for development of a comprehensive 'materials design' strategy for blinking and AR-suppressed heterostructured NQDs.
Research on graphene and other two-dimensional atomic crystals is intense and is likely to remain one of the leading topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated atomic planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. The first, already remarkably complex, such heterostructures (often referred to as 'van der Waals') have recently been fabricated and investigated, revealing unusual properties and new phenomena. Here we review this emerging research area and identify possible future directions. With steady improvement in fabrication techniques and using graphene's springboard, van der Waals heterostructures should develop into a large field of their own.
Zusammenfassung In der neuen Theorie der unipolaren Vorgänge an der Grenze Metall-Halbleiter spielt der Halbleiter nicht mehr die Rolle der schlecht emittierenden Elektrode einer Diodenanordnung, in der beide Glühelektroden durch einen Potentialberg getrennt sind (Abschnitt 1), sondern erscheint, wie das Vakuum in einer Glühelektronenröhre, als Träger eines zwischen den beiden Metallelektroden in ungleicher Dichte vorhandenen Elektronen- (oder Defektelektronen-) Gases, das allerdings anderen Strömungsgesetzen gehorcht als im Vakuum (Abschnitt 2). Bei größeren Schichtdicken ist auch diese Vakuumanalogie nicht ausreichend; wegen der in ihm vorhandenen thermisch dissoziierbaren Störstellen verhält sich vielmehr der Halbleiter ähnlich wie ein mit Natriumdampf von Glühtemperatur erfüllter Raum zwischen zwei thermisch emittierenden Elektroden. Besondere Übergangswiderstände an der Grenze Metall-Halbleiter entstehen, wenn die thermische Randdichte der Elektronen kleiner ist als die durch den Störstellenghalt bedingte Elektronendichte des neutralen Halbleiterinnern. Es bildet sich dann eine Raumladungsrandschicht aus, die sich bei einer in den Halbleiter hinein gerichteten Elektronenbewegung ausdehnt und dadurch ihren Widerstand vergrößert, während sie sich in umgekehrter Stromrichtung bis auf Null zusammenzieht (Abschnitt 3). Diese Deutung der Gleichrichterwirkung, die schon bei völlig gleichmäßiger Störstellenverteilung zu Übergangswiderständen und unipolarer Leitfähigkeit führt, wird in Abschnitt 4 und 5 durch die Betrachtung des zusätzlichen Einflusses einer ungleichmäßigen Störstellenverteilung erweitert. Besonders wird eine Störstellenverarmung des Halbleiters an der Metallgrenze („chemische Sperrschicht“) betrachtet; allgemein wird darauf hingewiesen, daß jede durch besondere Störstellenverteilung oder durch Randwirkungen hervorgerufene Ausbildung einer Raumladungszone innerhalb des Halbleiters als Ursache nichtlinearer und unipolarer Leitungsvorgänge in schwachen Feldern wirksam ist. Nachdem in Abschnitt 6 die Voraussetzungen dieser Raumladungstheorie noch einmal zusammengestellt sind, wird in den folgenden Paragraphen versucht, eine erste Übersicht über ihre Bedeutung für das ganze vorliegende Beobachtungsmaterial zu geben. In allen Fällen scheinen die sich aus der Raumladungstheorie ergebenden Schichtdicken (etwa 10-6 bis 10-3 cm, je nach der Leitfähigkeit des Störstellenhalbleiters) den tatsächlichen Abmessungen der anomal leitenden Gebiete an der Grenze Metall-Halbleiter zu entsprechen. Für Flächengleichrichter kann das aus Kapazitätsmessungen, für Spitzendetektoren durch eine Analyse der noch als „Spitzen“ wirksamen Berührungsflächen nachgewiesen werden (Abschnitt 7), wobei von der Annahme einer besonderen Sperrschicht an der Berührungsstelle Spitze-Halbleiter im allgemeinen kein Gebrauch gemacht wird. In Abschnitt 8 werden unter ähnlichen Geschichtspunkten „künstliche Sperrschichten“ behandelt, ferner die Vorgänge beim Gegenpressen von Folien, bei der losen Berührung verschiedener Halbleiter untereinander und beim Auftreten von Störzonen im Innern massiver Halbleiter. Abschnitt 9 gibt einen historischen Überblick über die Vorgeschichte der Theorie und ihre Stellung im Rahmen der sonstigen heute vorhandenen Erklärungsversuche.