Quaternary two-dimensional (2D) transition metal dichalcogenides (TMDs) with tunable bandgap
Sandhya SusarlaAlex KutanaJordan A. HachtelVidya KochatAmey ApteRóbert VajtaiJuan Carlos IdroboBoris I. YakobsonChandra Sekhar TiwaryPulickel M. Ajayan
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Abstract:
Alloying/doping in two-dimensional material has been important due to wide range band gap tunability. Increasing the number of components would increase the degree of freedom which can provide more flexibility in tuning the band gap and also reduced the growth temperature. Here, we report synthesis of quaternary alloys MoxW1-xS2ySe2(1-y) using chemical vapour deposition. The composition of alloys has been tuned by changing the growth temperatures. As a result, we can tune the bandgap which varies from 1.73 eV to 1.84 eV. The detailed theoretical calculation supports the experimental observation and shows a possibility of wide tunability of bandgap.Keywords:
Wide-bandgap semiconductor
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Chemical alloying is a powerful approach to tune the electronic structure of semiconductors and has led to the synthesis of ternary and quaternary two-dimensional (2D) dichalcogenide semiconductor alloys (e.g., MoSSe₂, WSSe₂, etc.). To date, most of the studies have been focused on determining the chemical composition by evaluating the optical properties, primarily via photoluminescence and reflection spectroscopy of these materials in the 2D monolayer limit. However, a comprehensive study of alloying in multilayer films with direct measurement of electronic structure, combined with first-principles theory, is required for a complete understanding of this promising class of semiconductors. We have combined first-principles density functional theory calculations with experimental characterization of MoS₂₍₁₋ₓ₎Se₂ₓ (where x ranges from 0 to 1) alloys using X-ray photoelectron spectroscopy to evaluate the valence and conduction band edge positions in each alloy. Moreover, our observations reveal that the valence band edge energies for molybdenum sulfide/selenide alloys increase as a function of increasing selenium concentration. These experimental results agree well with the results of density functional theory calculations showing a similar trend in calculated valence band edges. Our studies suggest that alloying is an effective technique for tuning the band edges of transition-metal dichalcogenides, with implications for applications such as solar cells and photoelectrochemical devices.
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The two-dimensional (2D) transitional metal dichalcogenides (TMDS) have become an intensive research topic recently. The alloys of these TMDs have offered continuous tunability of the bandstructure and carrier concentration, providing a new opportunity for various device applications. Here the rich variations in optical excitations in RexMo1-xS2alloy at the nanoscale region are shown. The alloy bandgap and charge response are probed by low-loss high-resolution transmission electron energy loss spectroscopy (HR-EELS). Concurrent density functional theory calculations revealed many electronic structures from n-type semiconductors to metallic and p-type semiconducting nature with band bowing effect. The alloying-induced Peierls distortion leads to a change in crystal symmetry and decreased interlayer coupling. These alloys undergo indirect to direct bandgap transition with the function of Re concentration. These unique correlated structural and electronic properties of these 2D alloys can be potentially applicable for various electronic and optoelectronic devices.
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Abstract Two-dimensional van der Waals (vdW) magnetic materials have emerged as possible candidates for future ultrathin spintronic devices, and finding a way to tune their physical properties is desirable for wider applications. Owing to the sensitivity and tunability of the physical properties to the variation of interatomic separations, this class of materials is attractive to explore under pressure. Here, we present the observation of direct to indirect band gap crossover and an insulator–metal transition in the vdW antiferromagnetic insulator CrPS 4 under pressure through in-situ photoluminescence, optical absorption, and resistivity measurements. Raman spectroscopy experiments revealed no changes in the spectral feature during the band gap crossover whereas the insulator–metal transition is possibly driven by the formation of the high-pressure crystal structure. Theoretical calculations suggest that the band gap crossover is driven by the shrinkage and rearrangement of the CrS 6 octahedra under pressure. Such high tunability under pressure demonstrates an interesting interplay between structural, optical and magnetic degrees of freedom in CrPS 4 , and provides further opportunity for the development of devices based on tunable properties of 2D vdW magnetic materials.
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Novel doping alternatives for transition metal dichalcogenides from high-throughput DFT calculations
Successful doping of single layer transition metal dichalcogenides (TMDs)
remains a formidable barrier to their incorporation into a range of
technologies. We use density functional theory within the generalized gradient
approximation to assess the possibility of substitutional doping the metal and
chalcogen sites in molybdenum and tungsten disulfide as well as diselenide
against a large fraction of the periodic table. An automated analysis of the
energetics, atomic and electronic structure of thousands of calculations
results in insightful trends across the periodic table and points out promising
dopants to be pursued experimentally. The automated analysis of the electronic
structure is able to capture and graphically represent subtleties in the
electronic structure of doped TMDs including the presence of gap states and the
results are in good agreement with the limited experimental data available.
Beyond previously studied cases, our predictions suggest promising candidates
for p-type doping and reveal interesting physics behind the doping of the metal
site. Doping with early transition metals (TMs) leads to tensile strain and a
significant reduction in the bandgap. The bandgap increases and strain is
reduced as the d-states are filled into the mid TMs; these trends reverse are
we move into the late TMs. Thus, strain and bandgaps are dominated by the
non-monotonous variation in atomic radius of the series. Additionally, the
Fermi energy increases monotonously as the d-shell is filled from the early to
mid TMs and we observe few to no gap states indicating the possibility of both
n- (early TMs) and p- (mid TMs) type doping.
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The preceding decade have witnessed incredible advances in the research of two-dimensional (2D) materials such as graphene, transition metal carbides and nitrides (MXenes), and transition metal dichalcogenides (TMDCs). Since their discovery, 2D materials have enabled the design of nano-scale devices with unique functionalities that are otherwise unavailable in conventional 3D systems. This dissertation focus on the electrical an thermal properties of these materials and presents the study of (1) contribution of the encapsulating layers and (2) surface parameters to the thermal transport at the van der Waals interfaces, (3) synthesis of quasi-binary TMDC alloys through computationally predicted stability maps, and (4) the phase‐dependent band gap engineering in alloys induced by charge density wave (CDW) phases.
In the first and second project, power dissipation and thermal management in the nanoscale structures are investigated which is of great importance for the design and operation of energy-efficient 2D nano-devices. Energy transport is heavily dependent of the thermal boundary conductance (TBC) at the van der Waals interfaces, particularly coupling at the interface of 2D channels with their underlying 3D substrates. A low TBC with underlying substrates puts an extrinsic limitation on the ability of 2D materials to conduct heat and dissipate the applied power. In this project a novel self-heating/self-sensing electrical thermometry platform based on atomically thin Ti3C2Tz MXene sheets is developed, which enables experimental investigation of the thermal transport at a Ti3C2Tz/SiO2 interface, with and without an encapsulating layer. Furthermore, the hydrophilic nature and variability of MXene surface terminations together with their metallic nature, provide a new platform to study the effect of the surface parameters on the thermal transport through and along the 2D flakes.
In the third project, at theory-guided synthesis approach is employed to achieve 25 unexplored quasi-binary TMDC alloys through computationally predicted stability maps and equilibrium temperature-composition phase diagrams. Compared to other 2D materials, TMDCs exhibit diverse, exciting physical properties, including topological insulator behavior, superconductivity, valley polarization, and superior electrocatalytic activity compared to noble metals. Their properties can be further tuned — or even new properties engineered — by alloying two different elements at either the transition metal or the chalcogen site to form quasi-binary alloys, or by simultaneous alloying at both the sites to form quaternary alloys. The synthesized alloys can be exfoliated into 2D structures, and some of them exhibit: (i) outstanding thermal stability tested up to 1230K, (ii) exceptionally high electrochemical activity for CO2 reduction reaction, (iii) excellent energy efficiency in a high rate Li-air battery, and (iv) high break-down current density for interconnect applications.
In the last project, a novel form of bandgap engineering involving alloying non‐isovalent cations in a 2D transition metal dichalcogenide (TMDC) is presented. By alloying semiconducting MoSe2 with metallic NbSe2, two structural phases of Mo0.5Nb0.5Se2, the 1T and 2H phases, are produced each with emergent electronic structure. At room temperature, it is observed that the 1T and 2H phases are semiconducting and metallic, respectively. For the 1T structure, scanning tunneling microscopy/spectroscopy (STM/STS) is used to measure band gaps. Electron diffraction patterns of the 1T structure obtained at room temperature show the presence of a nearly commensurate charge density wave (NCCDW) phase with periodic lattice distortions.
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A specific structure of doped graphene with substituted silicon impurity is introduced and ab. initio density-functional approach is applied for energy band structure calculation of proposed structure. Using the band structure calculation for different silicon sites in the host graphene, the effect of silicon concentration and unit cell geometry on the bandgap of the proposed structure is also investigated. Chemically silicon doped graphene results in an energy gap as large as 2eV according to DFT calculations. As we will show, in contrast to previous bandgap engineering methods, such structure has significant advantages including wide gap tuning capability and its negligible dependency on lattice geometry.
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