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Tuning Electronic and Optical Properties of MoS2 Monolayer via Molecular Charge Transfer
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Density functional theory computations were performed to investigate the adsorption of four organic molecules, including tetracyanoquinodimethane (TCNQ), tetracyanoethylene (TCNE), tetrathiafulvalene (TTF) and benzyl viologen (BV) on the basal plane of MoS2 monolayer (MoS2ML). There mainly exist non-covalent weak interactions between these organic molecules and MoS2ML with considerable charge transfer. Due to the adsorption of organic molecules, the band gap of MoS2ML can be efficiently reduced as the flat molecular levels lie in the band gap region of MoS2ML. Moreover, the adsorption of TNCQ can significantly enhance the optical absorption of MoS2ML in the infrared region of solar spectrum, whereas the adsorption of other molecules has negligible effect on the optical properties of MoS2ML. Our computations provide a flexible approach towards tuning the electronic and optical properties of MoS2ML.
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... TTF/fluorographene and DMPD/fluorographene have the ionization energy of 0.082 eV and 0.086 eV, suggesting the effective n-type doping. The ionization energy values are much smaller than those for two-dimensional materials (table S3), such as 0.17 eV for TCNE/MoS 2 , 0.23 eV for BV/MoS 2 [43], 0.12 eV of TCNQ/phosphorene, 0.73 eV of TTF/phosphorene [26], 0.37 eV of TCNE/arsenene, 0.82 eV of TTF/arsenene [27], 0.73 eV of TTF/antimonene [44]. A typical p-doped (n-doped) semiconductor requires the enough small ionization energy, which means lower activation energy for the holes (electrons) localized at impurity to become charge carriers. ...
In this work, we investigate the structural and electronic properties of molecular acceptors and donors adsorbed on H/F-functionalized graphene and h-BN by using first principles calculation. Graphane adsorbed with acceptors show p-type doping features, and fluorographene with donors are n-type doping. On the other hand, both H- and F-functionalized BN adsorbed systems exhibit n-type doping. The different doping characteristics depend on the relative energy level alignment of the molecules and substrates, which determines the electron transfer direction. In addition, the bands of adatoms are close to the band edges of substrates near Fermi level (0.0004–0.113 eV), denoting the efficient doping for H/F-functionalized graphene and h-BN. These results provide important indications for designing novel two-dimensional materials with suitable doped characteristics for opto-electronics applications.
... To spin-coat a WS 2 monolayer on a PDMS/glass substrate, we pipette 20 µL of TCNQ solution followed by spinning for 1 minute at 500 rpm. At this speed, the deposition produces a thin, flat, and homogenous film [50][51][52] with minimal damage to the monolayer semiconductor. The TCNQ molecules are expected to lie flat on the monolayer, which facilitates their use as a controlled spacer. ...
Optical absorption plays a central role in optoelectronic and photonic technologies. Strongly absorbing materials are thus needed for efficient and miniaturized devices. There exists, however, a fundamental limit of 50% absorptance for any ultrathin film in a symmetric environment. Although deviating from these conditions allows higher absorption, finding the thinnest possible material with the highest intrinsic absorption is still desirable. Here, we demonstrate strong absorption approaching the fundamental limit by artificially stacking WS2 monolayers into superlattices. We compare three simple approaches based on different spacer materials to surpass the record peak absorptance of WS2 monolayers, which stands at 16% on ideal substrates. Through direct monolayer stacking without an intentional spacer, we reach a transmittance contrast of 30% for an artificial bilayer, although with limited control over interlayer distance. Using a molecular spacer via spin coating, we demonstrate controllable spacer thickness in a bilayer, reaching 28% transmittance contrast while increasing photoluminescence thanks to doping. Finally, we exploit atomic layer deposition of alumina spacers to boost the transmittance contrast to 36% for a 4-monolayer superlattice. Our results demonstrate that monolayer superlattices are a powerful platform directly applicable to improve exciton-polariton phenomena such as strong light-matter coupling and nanophotonic devices such as modulators and photodetectors.
... A 4 × 4 × 1 2H-MoS 2 monolayer supercell is modeled for the singleatom decoration and gas adsorption. The optimized lattice constant 3.18 Å and the S-Mo bond length 2.42 Å are consistent with the previous results for the pristine MoS 2 monolayer [79]. The adsorption energy (E ads ) of one transition metal atom (Co/Pd/Pt) on MoS 2 monolayer was defined as the equation (1): ...
The single-atom decoration has attracted significant research interest by virtue of its critical role in modulating the electronic structure of transition metal dichalcogenides, and has been widely used in the fields of single-atom catalysis. Herein, we investigated the structures, electronic properties, and gas adsorption performance of single-atom (Co, Pd, Pt)-decorated 2H-MoS2 monolayer for adsorption of typical toxic gases, NO2 and NH3, using spin-polarized density functional theory calculations. The results indicate that the decorated metal atoms substantially improved the activity and electronic properties of the MoS2 monolayer, and further enhanced the adsorption and sensing performance for NO2 and NH3. The Pd-MoS2 monolayer is found to be a promising highly efficient gas sensing material for both gases in the temperature range of 400–500 K. On the other hand, the Co-MoS2 and Pt-MoS2 monolayers are not suitable for gas sensing applications, however, they can be considered as gas capture materials. These findings privide valuable references for the design and development of single-atom decorated MoS2 monolayer as high-performing gas sensing or scavenging materials for environmental monitoring and other related applications.
Optical absorption plays a central role in optoelectronic and photonic technologies. Strongly absorbing materials are thus needed for efficient and miniaturized devices. A uniform film much thinner than the wavelength can only absorb up to 50% of the incident light when embedded in a symmetric and homogeneous environment. Although deviating from these conditions allows higher absorption, finding the thinnest possible material with the highest intrinsic absorption is still desirable. Here, we demonstrate strong absorption by artificially stacking WS2 monolayers into superlattices. We compare three simple approaches based on different spacer materials to surpass the peak absorptance of a single WS2 monolayer, which stands at 16% on ideal substrates. Through direct monolayer stacking without an intentional spacer, we reach an absorptance of 27% for an artificial bilayer, although with limited control over interlayer distance. Using a molecular spacer via spin coating, we demonstrate controllable spacer thickness in a bilayer with 25% absorptance while increasing photoluminescence thanks to doping. Finally, we exploit the atomic layer deposition of alumina spacers to boost the absorptance to 31% for a 4-monolayer superlattice. Our results demonstrate that monolayer superlattices are a powerful platform directly applicable to improve strong light-matter coupling and enhance the performance of nanophotonic devices such as modulators and photodetectors.
Band gap engineering of atomically thin two-dimensional layered materials (2DLMs) is critical for their applications in nanoelectronics, optoelectronics, and photonics. Here we report, for the first time, a simple one-step chemical vapor deposition approach for the simultaneous growth of alloy MoS2xSe2(1-x) triangular nanosheets with complete composition tunability. Both the Raman and the photoluminescence (PL) studies show tunable optical properties consistent with composition of the alloy nanosheets. Importantly, all samples show a single bandedge emission peak, with the spectral peak position shifting from 668 nm (for pure MoS2) to 795 nm (for pure MoSe2), indicating the high quality for these complete compositionalloy nanosheets. These band gap engineered 2D structures could open up exciting opportunity for probing their fundamental physical properties in 2D and may find diverse applications in functional electronic/optoelectronic devices.
Tuning band energies of semiconductors through strain engineering can significantly enhance their electronic, photonic and spintronic performances. Although low-dimensional nanostructures are relatively flexible, the reported tunability of bandgap is within 100 meV per 1% strain. It is also challenging to control strains in atomically thin semiconductors precisely and monitor the optical and phonon properties simultaneously. Here, we developed an electro-mechanical device that can apply biaxial compressive strain to tri-layer MoS2 supported by a piezoelectric substrate and covered by a transparent graphene electrode. Photoluminescence and Raman characterizations show that the direct bandgap can be blue-shifted for ~300 meV per 1% strain. First-principles investigations confirm the blue-shift of the direct bandgap and reveal a higher tunability of the indirect bandgap than the direct one. The exceptional high strain tunability of electronic structure in MoS2 promising a wide range of applications in functional nanodevices and the developed methodology should be generally applicable for two-dimensional semiconductors.
Catalysts having structures containing the Mo6S8 cluster (crystalline Chevrel phases, amorphous ternary molybdenum sulfides, and ligated molecular clusters) have been discovered to be active for thiol synthesis from methanol and hydrogen sulfide at 250°C and 1 atm. An amorphous lanthanum molybdenum sulfide material was found to be the most selective catalyst (>80% methanethiol) at low conversions. CO2, CO, CS2, and CH4 were the major by-products in these studies.
Forming bilayer or multilayer heterostructures via interlayer van der Waals interactions is a superior preparation strategy for two-dimensional heterojunctions. In this work, by employing density functional theory computations, we investigated heterostructured bilayers of transition-metal dichalcogenides (TMDs) (including MoS2, WS2, MoSe2, and WSe2) and MXene (exemplified by Sc2CF2) monolayer. All TMD and Sc2CF2 materials are hexagonal with little mismatch. Compared with separate TMD and Sc2CF2 monolayers, TMD–Sc2CF2 bilayers can be tuned to indirect semiconductors with the band gaps of 0.13–1.18 eV; more importantly, they are type-II heterostructures with the valence band maximum and conduction band minimum located at Sc2CF2 and TMDs, respectively. Stretching or compressing would reduce or enlarge the band gaps of the heterostructures, respectively. The tunable band structures make TMD–Sc2CF2 bilayers pomising candidates for electronic device applications.
The lack of a band gap has greatly hindered the applications of graphene in electronic devices. By means of dispersion-corrected density functional theory computations, we demonstrated that considerable CH/π interactions exist between graphene and its fully (graphane) or patterned partially (C4H) hydrogenated derivatives. Due to the equivalence breaking of two sublattices of graphene, a 90 meV band gap is opened in the graphene/C4H bilayer. The band gap can be further increased to 270 meV by sandwiching graphene between two C4H layers. By taking advantage of the similar SiH/π interactions, a 120 meV band gap also can be opened for silicene. Interestingly, the high carrier mobility of graphene/silicene can be well-preserved. Our theoretical results suggest a rather practical solution for gap opening of graphene and silicene, which would allow them to serve as field effect transistors and other nanodevices.
By means of density functional theory computations, we systematically investigated the adsorption and diffusion of Li on the 2-D MoS2 nanosheets and 1-D zigzag MoS2 nanoribbons (ZMoS2NRs), in comparison with MoS2 bulk. Although the Li mobility can be significantly facilitated in MoS2 nanosheets, their decreased Li binding energies make them less attractive for cathode applications. Because of the presence of unique edge states, ZMoS2NRs have a remarkably enhanced binding interaction with Li without sacrificing the Li mobility, and thus are promising as cathode materials of Li-ion batteries with a high power density and fast charge/discharge rates.
Air-stable doping of transition metal dichalcogenides is of fundamental importance for enabling a wide range of optoelectronic and electronic devices while exploring their basic material properties. Here, we demonstrate benzyl viologen (BV), which has one of the highest reduction potentials of all electron-donor organic compounds, as a surface charge transfer donor for MoS2 flakes. The n-doped samples exhibit excellent stability in both ambient air and vacuum. Notably, we obtained high electron sheet density of ~ 1.2 × E13 cm-2, which corresponds to the degenerate doping limit for MoS2. The BV dopant molecules can be reversibly removed by immersion in toluene, providing the ability to control the carrier sheet density as well as selective removal of surface dopants on demand. By BV doping of MoS2 at the metal junctions, the contact resistances are shown to be reduced by >3×. As a proof of concept, top-gated field-effect transistors are fabricated with BV-doped n+ source/drain contacts self-aligned in respect to the top-gate. The device architecture, resembling that of the conventional Si transistors, exhibits excellent switching characteristics with a subthreshold swing of ~77 mV/decade.
Due to the recent expanding interest in two-dimensional layered materials, molybdenum disulphide (MoS2) has been receiving much research attention. Having an ultra-thin layered structure and an appreciable direct bandgap of 1.9 eV in the monolayer regime, few-layer MoS2 has good potential applications in nanoelectronics, optoelectronics and flexible devices. In addition, the capability of controlling spin- and valley-degrees-of-freedom makes it a promising material for spintronic and valleytronic devices. In this review we attempt to provide an overview of the research relevant to the structural and physical properties, fabrication methods and electronic devices of few-layer MoS2. Recent developments and advances in studying the material are highlighted.
Molybdenum disulfide has recently emerged as a promising two-dimensional semiconducting material for nanoelectronic, optoelectronic, and spintronic applications. Here, we investigate the field effect transistor behavior of MoS2 with ferromagnetic contacts to explore its potential for spintronics. In such devices, we elucidate that the presence of a large Schottky barrier resistance at the MoS2/ferromagnet interface is a major obstacle for the electrical spin injection and detection. We circumvent this problem by a reduction in the Schottky barrier height with the introduction of a thin TiO2 tunnel barrier between the ferromagnet and MoS2. This results in an enhancement of the transistor on-state current by two orders of magnitude, and an increment in the field effect mobility by a factor of six. Our magnetoresistance calculation reveals that such integration of ferromagnetic tunnel contacts opens up the possibilities for MoS2 based spintronic devices.