Structure-Property Relationships of 2D Ga/In Chalcogenides (original) (raw)
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An ab initio study of the electronic structure of indium and gallium chalcogenide bilayers
The Journal of Chemical Physics, 2017
Using first principle calculations, we have studied the structural and electronic properties of two dimensional bilayers of indium and gallium chalcogenides. With density functional theory corrected for van der Waals interactions, the different modes of stacking were investigated in a systematic way, and several of them were found to compete in energy. Then, their band structures were obtained with the GW approximation and found to correspond to indirect bandgap semiconductors with a small dependency on the mode of stacking. Finally, by analysing the electron density, it appeared that GaSe–InS is a promising system for electron-hole separation.
Changes of Structure and Bonding with Thickness in Chalcogenide Thin Films
Advanced Materials, 2020
constantly being refined. On the computational side, high-throughput methods have come into play, offering the possibility of systematic screening to predict novel stable 2D materials. [3,4] Among the recently discovered families of 2D compounds, group IV chalcogenides (IV = Ge, Sn; chalcogen = S, Se, Te) are attracting significant interest due to their remarkable electronic properties, which include in-plane ferroelectric polarization tunable by strain engineering. [5-12] A fundamental question arising in the study of 2D and few-layer materials is how bulk properties emerge upon increasing the film thickness. Interestingly, a number of different scenarios appear feasible. van der Waals bonded materials like graphite or multilayer graphene exhibit only a very small interlayer coupling and hence weak film thickness dependence of their properties. A stronger coupling can only be produced if adjacent bilayers are twisted, giving rise to exciting phenomena such as superconductivity in twisted graphene sheets. [13] A weak thickness dependence is expected whenever interlayer coupling is accomplished by weak van der Waals forces. On the other hand, covalently bonded materials like Si show pronounced relaxations or reconstructions at the surface to compensate for the lack of bonding partners. [14] The resulting atomic rearrangements are usually restricted to a few monolayers. Chalcogenides such as transition metal dichalcogenides also Extreme miniaturization is known to be detrimental for certain properties, such as ferroelectricity in perovskite oxide films below a critical thickness. Remarkably, few-layer crystalline films of monochalcogenides display robust in-plane ferroelectricity with potential applications in nanoelectronics. These applications critically depend on the electronic properties and the nature of bonding in the 2D limit. A fundamental open question is thus to what extent bulk properties persist in thin films. Here, this question is addressed by a first-principles study of the structural, electronic, and ferroelectric properties of selected monochalcogenides (GeSe, GeTe, SnSe, and SnTe) as a function of film thickness up to 18 bilayers. While in selenides a few bilayers are sufficient to recover the bulk behavior, the Te-based compounds deviate strongly from the bulk, irrespective of the slab thickness. These results are explained in terms of depolarizing fields in Te-based slabs and the different nature of the chemical bond in selenides and tellurides. It is shown that GeTe and SnTe slabs inherit metavalent bonding of the bulk phase, despite structural and electronic properties being strongly modified in thin films. This understanding of the nature of bonding in few-layers structures offers a powerful tool to tune materials properties for applications in information technology.
AIP Advances, 2023
The electronic, magnetic, and optical properties of NaS and NaSe compounds have been studied by using first-principles calculations based on density-functional theory and full-potential linearized augmented plane-wave method. The Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) and modified Becke-Johnson (mBJ-GGA) have been used to deal with the exchange-correlation potential. The PBE-GGA and mBJ-GGA electronic calculation of the spin-up configuration shows an insulating behavior, while the spin-down shows a metallic behavior. In addition, both PBE-GGA and mBJ-GGA agree that the total magnetic moment per unit cell for these compounds is 1 μB. From optical calculations, we see that ε 1 (0) value in the spin-up channel is positive, which shows an insulating character, while it has a large negative value for the spin-down configuration, which shows a metallic character. The NaS and NaSe refractive index n(ω) indicates a metallic demeanor as the real and imaginary parts of the dielectric constant.
AIP Advances
Chalcogenide crystals are a unique class of materials very different from semiconductors or metallic alloys. They also have many practical applications, especially in relation to their optical properties. However, the fundamental understanding of their electronic structure and physical properties is rather scattered and incomplete. We present a detailed study using first-principles calculations on the electronic structure, interatomic bonding, and optical and mechanical properties for 32 chalcogenide crystals. They consist of 22 binary (AnBm) and 10 ternary (AnA ′ Bm) crystals with A =
Optical properties of two-dimensional gallium chalcogenide films
Gallium chalcogenides are promising building blocks for novel van der Waals heterostructures. We report low-temperature micro-photoluminescence (PL) of GaTe and GaSe films with thickness ranging from from 200 nm to a single unit cell. In both materials, PL shows dramatic decrease by 10 4 -10 5 when film thickness is reduced from 200 to 10 nm. Based on evidence from cw and time-resolved PL, we propose a model explaining the PL decrease as a result of non-radiative carrier escape via surface states.
arXiv: Materials Science, 2020
Defect-free epitaxial growth of 2D materials is one of the holy grails for a successful integration of van der Waals (vdW) materials in the semiconductor industry. The large-area (quasi-)vdW epitaxy of layered 2D chalcogenides is consequently carefully being researched since these materials hold very promising properties for future nanoelectronic applications. The formation of defects such as stacking faults like 60o twins and consequently 60o grain boundaries is still of major concern for the defect-free epitaxial growth of 2D chalcogenides. Although growth strategies to overcome the occurrence of these defects are currently being considered, more fundamental understanding on the origin of these defects at the initial stages of the growth is highly essential. Therefore this work focuses on the understanding of 60o twin formation in (quasi-)vdW epitaxy of 2D chalcogenides relying on systematic molecular beam epitaxy (MBE) experiments supported by density functional theory (DFT) calc...
Advanced Theory and Simulations, 2020
Heterostructures built from 2D materials and organic semiconductors offer a unique platform for addressing many fundamental physics and construction of functional devices. Interfaces play a crucial role in tailoring the heterostructure properties. Here, density functional theory computations are performed to explore the interfacial properties of heterostructures made of group VI transition metal dichalcogenides (TMD) and organic molecules such as perylene tetracarboxylic dianhydride (PTCDA) and pentacene. First principle calculations predict that the organic pentacene layer exhibits covalent interfacing with MoSe 2 and WSe 2 , while the interface of other studied TMD/organic heterostructures form van der Waals (vdW) interfaces. Owing to the different molecular geometry of PTCDA and pentacene in their respective heterostructures, the work function can be modulated of the order of 1.0 eV in comparison with pure monolayer MX 2 in MX 2 /pentacene (M = Mo, W; X = S, Se) heterostructures, while the change of work function in MX 2 /PTCDA (M = Mo, W; X = S, Se) is negligible (order of 0.1 eV) in comparison with pure monolayer MX 2. This study will be helpful to design high-performance optoelectronic devices based on TMDs and organic semiconductors.
We report first principles calculations of the electronic structure of monolayer 1H-MX2 (M = Mo, W; X = S, Se, Te), using the pseudopotential and numerical atomic orbital basis sets based methods within the local density approximation. Electronic band structure and density of states calculations found that the states around the Fermi energy are mainly due to metal d states. From partial density of states we find a strong hybridisation between metal d and chalcogen p states below the Fermi energy. All studied compounds in this work have emerged as new direct band gap semiconductors. The electronic band gap is found to decrease as one goes from sulphides to the tellurides of both Mo and W. Reducing the slab thickness systematically from bulk to monolayers causes a blue shift in the band gap energies, resulting in tunability of the electronic band gap. The magnitudes of the blue shift in the band gap energies are found to be 1.14 eV, 1.16 eV, 0.78 eV, 0.64, 0.57 eV and 0.37 eV for MoS2, WS2, MoSe2, WSe2, MoTe2 and WTe2, respectively, as we go from bulk phase (indirect band gap) to monolayer limit (direct band gap). This tunability in the electronic band gap and transitions from indirect to direct band make these materials potential candidates for the fabrication of optoelectronic devices.
Electron Density and Its Relation with Electronic and Optical Properties in 2D Mo/W Dichalcogenides
Nanomaterials, 2020
Two-dimensional MX2 (M = Mo, W; X = S, Se, Te) homo- and heterostructures have attracted extensive attention in electronics and optoelectronics due to their unique structures and properties. In this work, the layer-dependent electronic and optical properties have been studied by varying layer thickness and stacking order. Based on the quantum theory of atoms in molecules, topological analyses on interatomic interactions of layered MX2 and WX2/MoX2, including bond degree (BD), bond length (BL), and bond angle (BA), have been detailed to probe structure-property relationships. Results show that M-X and X-X bonds are strengthened and weakened in layered MX2 compared to the counterparts in bulks. X-X and M-Se/Te are weakened at compressive strain while strengthened at tensile strain and are more responsive to the former than the latter. Discordant BD variation of individual parts of WX2/MoX2 accounts for exclusively distributed electrons and holes, yielding type-II band offsets. X-X BL ...
Strong interlayer coupling in van der Waals heterostructures built from single-layer chalcogenides
Proceedings of the National Academy of Sciences, 2014
Semiconductor heterostructures are the fundamental platform for many important device applications such as lasers, light-emitting diodes, solar cells and high-electron-mobility transistors. Analogous to traditional heterostructures, layered transition metal dichalcogenide (TMDC) heterostructures can be designed and built by assembling individual single-layers into functional multilayer structures, but in principle with atomically sharp interfaces, no interdiffusion of atoms, digitally controlled layered components and no lattice parameter constraints. Nonetheless, the optoelectronic behavior of this new type of van der Waals (vdW) 2 semiconductor heterostructure is unknown at the single-layer limit. Specifically, it is experimentally unknown whether the optical transitions will be spatially direct or indirect in such hetero-bilayers. Here, we investigate artificial semiconductor heterostructures built from single-layer WSe2 and MoS2 building blocks. We observe a large Stokes-like shift of ~100 meV between the photoluminescence peak and the lowest absorption peak that is consistent with a type II band alignment with spatially direct absorption but spatially indirect emission. Notably, the photoluminescence intensity of this spatially indirect transition is strong, suggesting strong interlayer coupling of charge carriers. The coupling at the hetero-interface can be readily tuned by inserting hexagonal BN (h-BN) dielectric layers into the vdW gap. The generic nature of this interlayer coupling consequently provides a new degree of freedom in band engineering and is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties with customized composite layers.