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First-principles studies on structural and electronic properties of GaN-AlN heterostructure nanowires
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The structural and electronic properties of core-shell, eutectic, biaxial and superlattice GaN-AlN nanowires were studied through density functional theory computations. Due to more surface dangling bonds, nanowires with smaller diameters are energetically unfavorable. For the GaN-AlN heterostructure nanowires, their electronic properties highly depend on the GaN content, axial strain, configuration, and size. The valence bands are less affected by the GaN content, while the conduction bands depend on it. Hydrogen-passivated nanowires have much larger band gaps than their counterparts, since the surface states are removed by saturating the dangling bonds with hydrogen atoms. Moreover, due to multiple quantum-well structures, the confined electrons (holes) of superlattice nanowires become more localized and the difference of the mobility between the electron and hole becomes less apparent if the width of the barrier is larger. These findings are of value for better understanding heterostructure nanowires and their potential utilization.
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... Recently, the core-shell NWs constructed by different wurtzite components have been successfully synthesised, which includes III-V [37,38] and II-VI [39,40] core-shell NWs. Compared to their single-component counterparts, the core-shell NWs not only facilitate the design and fabrication of nanodevices by requiring no further assembly, but also offer unprecedented properties for a variety of novel applications [41,42]. For instance, it is reported that the piezopotential generated in the wurtzite III-V core-shell NWs is much larger than that in their single-component counterparts [38]. ...
... This result suggests that the wurtzite core-shell NWs are good candidates for serving as the building blocks of high performance piezotronic nanodevices. Although the piezopotential [38,[42][43][44] and semiconducting properties [41,45] of the wurtzite coreshell NWs have been individually investigated in the previous studies, their piezotronic behaviours (the concerted interplay of piezopotential and semiconducting properties) are still not clear. Thus, to promote the further applications of wurtzite core-shell NWs in the piezotronic devices, a comprehensive understanding of their piezotronic behaviours becomes very important and necessary. ...
The piezotronic behaviours of wurtzite core-shell nanowires (NWs) are studied in this paper by using a multiscale modelling technique. A difference between piezopotentials obtained from molecular dynamics simulations and finite element calculations indicates that due to the influence of small-scale effects the widely used conventional electromechanical theory is not accurate in describing the piezopotential properties of the present core-shell NWs. Although the residual strains intrinsically existing in core-shell NWs and the structural reconstruction at their surface and interface both account for these small-scale effects, the latter is found to play the dominate role, which makes the material properties of core-shell NWs significantly depend on their geometric size. A novel core-interface-shell-surface model is proposed here to analytically describe the size dependence of the material properties and thus the small-scale effects on the piezopotential of core-shell NWs. Besides possessing a good piezoelectric performance, our density functional theory calculations also show that the core-shell NWs under external loading can retain the semiconducting properties, which confirms the existence of piezotronic effects in them. However, owing to the intrinsic asymmetric Schottky barriers at the source and drain contacts induced by residual piezopotentials in core-shell NWs, the piezotronic effects of core-shell NWs are different to those of their conventional single-component counterparts. The superb piezopotential properties and unique piezotronic behaviours observed in wurtzite core-shell NWs make them good candidates for high performance components in novel piezotronic nanodevices.
... Previous theoretical studies have been focusing on electron transport [39][40][41][42][43][44], optical characteristics [45], pressure, and magnetism of GaN/AlN NWs [46,47], while their interfacial thermal transport has been barely investigated. Polanco and Lindsay [48] performed the density functional theory (DFT) calculations to describe the phonon thermal conductivity (G) of the GaN-AlN interface by using the non-equilibrium Green's function (NEGF) method. ...
The ability to tune the interfacial thermal conductance of GaN/AlN heterojunction nanowires with a core/shell structure is shown using molecular dynamics and nonequilibrium Green’s functions method. In particular, an increase in the shell thickness leads to a significant improvement of interfacial thermal conductance of GaN/AlN core/shell nanowires. At room temperature (300 K), the interfacial thermal conductance of nanowires with specific core/shell ratio can reach 0.608 nW/K, which is about twice that of GaN/AlN heterojunction nanowires due to the weak phonon scattering and phonon localization. Moreover, changing the core/shell type enables one to vary interfacial thermal conductance relative to that of GaN/AlN heterojunction nanowires. The results of the study provide an important guidance for solving the thermal management problems of GaN-based devices.
... 8 Additionally, heterostructure nanowires combining GaN with another important component of III−V semiconductor AlN offer more possibilities for the utilization of GaN. 9 In consequence of these excellent properties, GaN has been used extensively to fabricate electronic and optoelectronic devices, 10 such as light emitting diodes, 11 transistors, 3,12 solar cells, 13,14 photodetectors, 15 and so on. Moreover, GaN has attracted great attention as a field emission electron source because it has a lower electron affinity than Si and a lot of metals. ...
... In the same year, Vu Ngoc Tuoc et al. [13] studied GaN/AlN core-shell NWs on the role of the surface reconstruction using first principles. Meanwhile, H. J. Zhang [14] calculated structural and electronic properties of GaN/ AlN heterostructure nanowires using first-principles. In 2016, Pavloudis et al. [8] investigated the structural, thermal, and electronic properties of polar GaN/AlN core/shell nanowires through interatomic potential based molecular dynamics and ab initio calculations. ...
... In the geometry relaxation and self-consistent calculations, 15 × 15 × 1 Monkhorst-Pack k-points mesh was utilized. Considering the underestimated band gaps by PBE functional, hybrid functional proposed by Heyd-Scuseria-Ernzerhof (HSE06) [54], which is proven a reliable method for computing the electronic and optical properties [10,55,56], was employed to calculate the electronic structure and dielectric constants of g-BCN monolayers. In all computations of geometry optimization and chemical potential of elements, spin polarization was considered. ...
By employing particle-swarm optimization (PSO) and first-principles computations, we theoretically predicted five stable phases of graphene-like borocarbonitrides (g-BCN) with the stoichiometric ratio of 1:1:1 and uniformly distributed B, C, N atoms, which are the isoelectronic analogues of graphene. These g-BCN monolayers are effectively stabilized by their relatively high proportion of robust C-C or B-N bonds and strong partial ionic-covalent B-C and C-N bonds within them, leading to pronounced thermal and kinetic stability. The visible-light absorption and high carrier mobility of the investigated g-BCN monolayers indicate their possible applications in high-efficiency photochemical processes and electronic devices. Our computations could provide some guidance for designing the graphene-like materials with earth-abundant elements, as well as some clues for the experimental synthesis and practical applications of ternary BCN nanosheets.
First principles calculations are performed to study the effects of the interlayer distance and biaxial strain on the electronic properties and contact properties of graphene/MoS2 heterostructures. The interlayer interaction is weakened and the charge transfer from the graphene layer to the MoS2 layer is reduced with increasing interlayer distance in graphene/MoS2 heterostructures, resulting in a shift of the Fermi level to a high energy state. The n-type Schottky barrier is formed with ΦSB,N values of 0.647 eV, 0.568 eV, 0.509 eV, and 0.418 eV when the interlayer distances are 3.209 Å, 3.346 Å, 3.482 Å, and 3.755 Å, respectively. The interlayer distance and charge density difference change slightly, but the electronic structure of the graphene/MoS2 heterostructure changes obviously by applying the biaxial strain. For the biaxial strain from −4% to +6%, the ΦSB,P gradually increases for the graphene/MoS2 heterostructure, while the ΦSB,N increases initially and then decreases. Moreover, the ΦSB,N is only 0.080 eV under a biaxial strain of +6%, indicating that the Ohmic contact is nearly formed. The results demonstrate the significant effects of a biaxial strain on the physical properties of 2D heterostructures.
The electronic structure and optical properties of GaN nanowires under external electric fields with different directions and field intensities are studied via first-principles. The results show that the GaN nanowires under an electric field in the (001) direction is the most stable. The external electric field can adjust the band gap of GaN nanowires, and a further increase to the electric field intensity of 0.1 V/Å could suddenly reduce the band gap. Moreover, the GaN nanowires under the 0.2 V/Å field intensity exhibited the Stark effect. The application of an electric field could reduce the charge transfer between Ga atoms and N atoms in GaN nanowires. In addition, larger electric field intensity can reduce the absorption coefficient and reflectivity of GaN nanowires. These findings may provide valuable information for the development of field-assisted GaN nanowire photocathodes.
Consider that a variable composition structure can introduce an internal field, stability, and photoelectric features of Ga1‐xAlxN superlattice nanowires having adjustable composition are studied. Different Al composition intervals and ratios in GaN nanowires are considered to create various superlattice nanowires. Structure and photoelectronic features of Ga1‐xAlxN superlattice nanowires having diameters of 7.4, 9.8, and 12.8 Å are investigated based on first principles. Increasing the content of Al atoms in the corresponding sublayers can reduce the formation energy, and can appear blue shifts in optical properties such as absorption coefficient. The continuously gradual built‐in electric field makes Ga1‐xAlxN superlattice nanowires have a lower work function. The lowest work function can reach 4.27 eV, and the corresponding band gap is 3.812 eV. The direct band gap of Ga1‐xAlxN superlattice nanowires is influenced by the nanowire diameter, length, and sublayer. These studies can provide early guidance for improving the performance of spin‐polarized electron sources.
Constructing vertical heterostructures by placing Graphene (Gr) on two-dimensional materials have recently emerged as an effective way to enhance the performance of nanoelectronic and optoelectronic devices. In this work, first principles calculations are employed to explore the structural and electronic properties of Gr/GeC and Gr/Functinalized-GeC by H/F/Cl surface functionalizations. Our results imply that the electronic properties of the Gr, GeC and all Functionalized-GeC monolayers are well preserved in Gr/GeC and Gr/Functionalized-GeC heterostructures, and the Gr/GeC heterostructure forms a $p$-type Schottky contact. Interestingly, we find that the $p$-type Schottky contact in Gr/GeC can be converted into the $n$-type one and into $n$-type Ohmic contact by the H/F/Cl surface functionalization to form the Gr/Functionalized-GeC heterostructures. Furthermore, we find that the electric fields and strain engineering can turn both the Schottky barrier heights and the contact types of the Gr/Functionalized-GeC vdWHs. These findings suggest that the Gr/Functionalized-GeC heterostructures can be considered as promising candidate for designing high-performance optoelectronic and nanoelectronic devices.
Searching for new two-dimensional (2D) Dirac cone materials has been popular since the discovery of graphene with a Dirac cone structure. Based on density functional theory (DFT) calculations, we theoretically designed a HfB2 monolayer as a new 2D Dirac material by introducing the transition metal Hf into a graphene-like boron framework. This newly predicted HfB2 monolayer has pronounced thermal and kinetic stabilities along with a Dirac cone with a massless Dirac fermion and Fermi velocities (3.59 × 10⁵ and 6.15 × 10⁵ m s⁻¹) comparable to that of graphene (8.2 × 10⁵ m s⁻¹). This study enriches the diversity and promotes the application of 2D Dirac cone materials.
Highly ordered AlN nanowire arrays were synthesized via a simple physical vapor deposition method on sapphire substrate. The nanowires have an extremely sharp tip <10 nm, with the average length around 3 μm. Raman spectroscopy analysis on the AlN nanowire arrays revealed that the lifetime of the phonons is shorter than that in bulk AlN. The transmission spectra of the AlN nanowires showed a blueshift ∼ 0.27 eV at the absorption edge with that of the bulk AlN, which is closely related to the small size of the nanowires.
ZnO/CdS core/shell nanowire heterostructure arrays are fabricated by a two-step chemical solution method for use in semiconductor-sensitized photoelectrochemical cells (PECs). The successive ion layer adsorption and reaction (SILAR) shows a remarkable controllability of CdS shell thickness, which affects visible-light absorption properties and PEC performances of the heterostructures. The cell has a high short-circuit photocurrent density of 7.23 mA/cm2 with a power conversion efficiency of 3.53% under AM 1.5G illumination at 100 mW/cm2. These results demonstrate that the ZnO/CdS core/shell heterostructure nanowire arrays can provide a facile and compatible frame for the potential applications in nanowire-based solar cells.
Nanowires and nanotubes carry charge and excitons efficiently, and are
therefore potentially ideal building blocks for nanoscale electronics and
optoelectronics1, 2. Carbon nanotubes have already been exploited
in devices such as field-effect3, 4 and single-electron5, 6 transistors, but the practical utility of nanotube components
for building electronic circuits is limited, as it is not yet possible to
selectively grow semiconducting or metallic nanotubes7, 8. Here
we report the assembly of functional nanoscale devices from indium phosphide
nanowires, the electrical properties of which are controlled by selective
doping. Gate-voltage-dependent transport measurements demonstrate that the
nanowires can be predictably synthesized as either n- or p-type. These doped
nanowires function as nanoscale field-effect transistors, and can be assembled
into crossed-wire p–n junctions that exhibit rectifying behaviour. Significantly,
the p–n junctions emit light strongly and are perhaps the smallest light-emitting
diodes that have yet been made. Finally, we show that electric-field-directed
assembly can be used to create highly integrated device arrays from nanowire
building blocks.
By means of first-principles calculations, we investigate the structural and electronic properties in a series of InAs/InP-core–shell nanowires with different sizes and compositions. Analysis of the subband charge density distributions reveals that both electrons and holes are spatially localized at the InAs-region of the core–shell nanowires, in consistent with a type-I band alignment. The size and composition effects on the confinement energies of electron carriers and the band gaps in the core–shell nanowires are investigated detailedly. These results provide an important guidance for the fabrication and applications of the core–shell nanowires.
Silicon nanowires can be prepared with single-crystal structures, diameters as small as several nanometers and controllable hole and electron doping, and thus represent powerful building blocks for nanoelectronics devices such as field effect transistors. To explore the potential limits of silicon nanowire transistors, we have examined the influence of source-drain contact thermal annealing and surface passivation on key transistor properties. Thermal annealing and passivation of oxide defects using chemical modification were found to increase the average transconductance from 45 to 800 nS and average mobility from 30 to 560 cm2/V‚s with peak values of 2000 nS and 1350 cm2/V‚s, respectively. The comparison of these results and other key parameters with state-of-the-art planar silicon devices shows substantial advantages for silicon nanowires. The uses of nanowires as building blocks for future nanoelectronics are discussed.
Mechanical properties, atomic and energy band structures of bare and hydrogen-passivated SinGen nanowire superlattices have been investigated by using first-principles pseudopotential plane-wave method. Undoped, tetrahedral Si and Ge nanowire segments join pseudomorphically and can form superlattice with atomically sharp interface. We found that Sin nanowires are stiffer than Gen nanowires. Hydrogen passivation makes these nanowires and SinGen nanowire superlattice even more stiff. Upon heterostructure formation, superlattice electronic states form subbands in momentum space. Band lineups of Si and Ge zones result in multiple quantum wells, where specific states at the band edges and in band continua are confined. The electronic structure of the nanowire superlattice depends on the length and cross section geometry of constituent Si and Ge segments. Since bare Si and Ge nanowires are metallic and the band gaps of hydrogenated ones vary with the diameter, SinGen superlattices offer numerous alternatives for multiple quantum well devices with their leads made from the constituent metallic nanowires.
This review discusses salient features of interfacial electron transfer reactions in colloidal semiconductor solutions and thin films and their application for solar light energy conversion and photocatalytic water purification. This research is interdisciplinary and is situated at the limit between colloid science, and electrochemistry, and semiconductor physics. The section on colloidal semiconductors discusses material science aspects, optical properties, electronic properties, and charge carrier reactions in colloidal semiconductor solutions. The section on nanocrystalline semiconductor electrodes discusses some basic properties and preparation procedures of nanocrystalline TiO[sub 2] films, energetics and operations of the nanoporous solar cell, observations and importance of electronic states in the band gap region, photoinduced processes in nanocrystalline semiconductor electrodes, the electric field in a nanocrystalline electrode, and kinetic rate constants in the photosensitized solar cell. 156 refs.
The electronic structure of Si/Ge core-shell nanowires along the [110] and [111] directions are studied with first-principles calculations. We identify the near-gap electronic states that are spatially separated within the core or the shell region, making it possible for a dopant to generate carriers in a different region. The confinement energies of these core and shell states provide an operational definition of the “band offset,” which is not only size dependent but also component dependent. The optimal doping strategy in Si/Ge core-shell nanowires is proposed based on these energy results.
First principles density functional theory is used to study the composition dependence of the structural and electronic properties of the epitaxial silicon-germanium core-shell [111]-oriented nanowires with diameter ranging from 1.3 to 1.7 nm. An analysis of the results obtained for these nanowires reveals that variations of geometrical and electronic characteristics with composition are strongly type (core/shell) specific. There exist positive and negative deviations from the Vegard’s law for the structural parameters (lattice constants, diameters, and mean bond lengths) of the nanowires with compressive strained Ge core and tensile strained Si core, respectively. Direct-to-indirect transition for the fundamental band gap is found in Ge-core/Si-shell nanowires, while Si-core/Ge-shell nanowires preserve a direct energy gap almost over the whole compositional range. An analysis of dependence of the energy gap on the diameter and shell thickness of the nanowires reveals a strong size dependent redshift, consistent with the well-known spectroscopic data. These results provide an understanding of the interplay between variation of structural characteristics and changes in electronic properties.
First-principles density functional theory has been employed to study the composition dependent quantum size effect in a series of silicon-germanium core-shell structured nanowires with the diameters ranging from 0.5 to 3.2 nm. Analysis of the calculated band gap energies in Si-core/Ge-shell and Ge-core/Si-shell structured nanowires shows a nonlinear composition dependence for nanowires with fixed diameter (fixed total number N=NCore+NShell of Si and Ge atoms in the unit cell). In contrast, for nanowires with fixed core size and varying shell thickness, our calculation results reveal a striking linear blueshift of the direct band gap with composition. The obtained linear composition effect implies an inverse square relation between the energy of the fundamental band gap and the size of nanowire, in agreement with experimental observations. Our results provide useful guidelines for experimental gap engineering in the core-shell structured nanowire heterostructures.