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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Straintronics with single-layer MoS 2 : A quantum Monte Carlo study

Using state-of-the-art quantum Monte Carlo (QMC) methods, we study straintronic properties of a single MoS 2 monolayer. 2D MoS 2 is a quintessential straintronic material for which many experiments have been performed. First, we determine the equilibrium atomic structure which is not known experimentally and is strictly needed to correctly determine the straintronic properties. That enables us to precisely analyze the quasiparticle band gaps for any applied biaxial strain, which we describe by a bivariate paraboloid function of lattice constant and internal structural parameter. Using the fixed-node QMC calculations fitted by analytical formulas, we localize the following excited state crossings between the direct, K → K , and indirect Γ → X and K → K /2 excitations. Based on this highly accurate many-body treatment, we predict a gauge factor of 136 meV/% for the K → K transition and a fairly narrow window of ≈ 2.8 % from compressive to mildly tensile strains, accounting for only ≈ 0.3 eV band gap change maintaining the direct character of the gap. Consequently, we suggest that, compared to other 2D materials, such as phosphorene, there is only a limited straintronic tuneability in this material often studied for its straintronic properties. QMC results are compared to results of standard DFT modeling, which reveal insights into the corresponding inaccuracies and therefore open a window for educated use of rapid DFT approaches. Published by the American Physical Society 2024

Huang, Y.↗
DOE OSTI2023

Anodic Dissolution Rates Accelerate with Decreasing MoS 2 Nanoflake Thickness

Electrochemical gating of 2D transition metal dichalcogenide (TMD) electrodes is an emerging frontier in the field of semiconductor electrochemistry. In this approach, an applied bias modifies the charge carrier concentration of the 2D TMD, causing band edge shifts and drastic changes in charge transfer rates. However, leveraging this effect for (photo)electrocatalysis is practically limited by the stability of the TMD material under gating conditions. Gerischer showed anodic dissolution of bulk TMD electrodes can occur in the dark and hypothesized that the reaction proceeds via an electron tunneling mechanism from surface states to the TMD conduction band [H. Gerischer, D. Ross, and M. Lubke, Z. Physickalische Chem., 139, 1 (1984)]. Here we investigate this possibility in single MoS 2 nanoflakes using in situ optical microscopy and explore whether Gerischer’s electron tunneling mechanism can explain anodic dissolution rates of thin 2D semiconductors. Here, spatially resolved measurements show anodic dissolution initiates at perimeter edge sites and accelerates exponentially with decreasing layer thickness, consistent with Gerischer’s tunneling mechanism. Interestingly, single layer MoS 2 is impervious to anodic dissolution at applied potentials >200 mV more positive than those required to drive dissolution in bulk and multilayer-thick nanoflakes.

2D semiconductor↗
DOE OSTI2020

A Physical Model for Understanding the Activation of MoS 2 Basal-plane Sulfur Atoms for the Hydrogen Evolution Reaction

All the DFT calculations are done with the Vienna Ab Initio Simulation Package (VASP) using the projector augmented wave method. The Bayesian error estimation exchange-correlation functionals (BEEF) with van der Waals interactions are employed. A plane-wave cutoff energy of 400 eV is used together with PAW-PBE potentials where semi core p states are treated as valence. All the calculations allow for spin-polarization. The structures are relaxed until the force is converged to < 0.01 eV/Å. The lattice parameter of MoS 2 unit cell, optimized with this functional, is 3.19 Å. A (4×4) supercell is used to model all the transition metal doped MoS 2 systems studied here, including those with S vacancies. For calculations in the initial dopant structure exploration, the Brillouin zone is sampled with a 3x3x1 Monkhorst-Pack k-point mesh. A 6×6×1 Monkhorst-Pack k-point mesh is used for the H binding energy and density of states calculations. In all calculations, the vacuum layer is set as 15 Å to eliminate periodic interaction perpendicular to the basal plane.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2023

MoS 2 Nanoplatelets on Hybrid Core-Shell (HyCoS) AuPd NPs for Hybrid SERS Platform for Detection of R6G

In this work, a novel hybrid SERS platform incorporating hybrid core-shell (HyCoS) AuPd nanoparticles (NPs) and MoS 2 nanoplatelets has been successfully demonstrated for strong surface-enhanced Raman spectroscopy (SERS) enhancement of Rhodamine 6G (R6G). A significantly improved SERS signal of R6G is observed on the hybrid SERS platform by adapting both electromagnetic mechanism (EM) and chemical mechanism (CM) in a single platform. The EM enhancement originates from the unique plasmonic HyCoS AuPd NP template fabricated by the modified droplet epitaxy, which exhibits strong plasmon excitation of hotspots at the nanogaps of metallic NPs and abundant generation of electric fields by localized surface plasmon resonance (LSPR). Superior LSPR results from the coupling of distinctive AuPd core-shell NP and high-density background Au NPs. The CM enhancement is associated with the charge transfer from the MoS 2 nanoplatelets to the R6G. The direct contact via mixing approach with optimal mixing ratio can effectively facilitate the charges transfer to the HOMO and LUMO of R6G, leading to the orders of Raman signal amplification. The enhancement factor (EF) for the proposed hybrid platform reaches ~10 10 for R6G on the hybrid SERS platform.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Focused Helium Ion Beam for Direct Patterning of Monolayer MoS 2 Nanoribbon Field Effect Devices

The helium ion microscope (HIM) focused ion beam (FIB) has emerged as a powerful tool to directly pattern nanostructures below 10 nm due to its high-resolution capabilities and the inert nature of the ion source. These attributes make HIM FIB particularly interesting for patterning 2D materials such as transition metal dichalcogenides (TMDs) to investigate transport phenomena at the nanoscale. Reported here is the fabrication of MoS 2 nanoribbon devices using HIM FIB-induced etching (FIBIE) with XeF 2 , allowing for reduced ion dose compared to direct sputtering. While patterning is efficacious, the devices exhibit performance degradation with decreasing nanoribbon width due to damage up to 150 nm beyond the patterned edge. Incorporating an hBN encapsulation improves device performance by one order of magnitude, although the lateral extent of damage remains unchanged. The spatial distribution of damage is shown to be determined by the forward- and backscattered ions and electrons, while the hBN encapsulation layer substantially reduces damage from XeF 2 interactions in unexposed regions. Raman and photoluminescence (PL) measurements corroborate these findings, while ion/solid interaction simulations further elucidate the resolution limits imposed by substrate interactions. In conclusion, this work provides critical insights and a practical pathway for utilizing HIM FIBIE in 2D TMD functional device patterning.

MoS 2↗
DOE OSTI2020

Materials Data on Sb(MoS)2 by Materials Project

Sb(MoS)2 crystallizes in the monoclinic P2_1/m space group. The structure is three-dimensional. there are two inequivalent Mo+3.50+ sites. In the first Mo+3.50+ site, Mo+3.50+ is bonded in a 6-coordinate geometry to two equivalent Sb3- and four S2- atoms. Both Mo–Sb bond lengths are 2.91 Å. There are a spread of Mo–S bond distances ranging from 2.37–2.61 Å. In the second Mo+3.50+ site, Mo+3.50+ is bonded in a 3-coordinate geometry to three equivalent Sb3- and three equivalent S2- atoms. There are one shorter (2.88 Å) and two longer (2.92 Å) Mo–Sb bond lengths. There are two shorter (2.37 Å) and one longer (2.39 Å) Mo–S bond lengths. Sb3- is bonded in a 5-coordinate geometry to five Mo+3.50+ atoms. There are two inequivalent S2- sites. In the first S2- site, S2- is bonded in a 3-coordinate geometry to three equivalent Mo+3.50+ atoms. In the second S2- site, S2- is bonded in a 4-coordinate geometry to four Mo+3.50+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Cs(MoS)3 by Materials Project

Cs(MoS)3 crystallizes in the hexagonal P6_3/m space group. The structure is three-dimensional. Cs is bonded in a 9-coordinate geometry to nine equivalent S atoms. There are three shorter (3.64 Å) and six longer (3.78 Å) Cs–S bond lengths. Mo is bonded in a distorted see-saw-like geometry to four equivalent S atoms. There are a spread of Mo–S bond distances ranging from 2.49–2.60 Å. S is bonded in a 7-coordinate geometry to three equivalent Cs and four equivalent Mo atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K(MoS)3 by Materials Project

K(MoS)3 crystallizes in the hexagonal P6_3/m space group. The structure is three-dimensional. K is bonded in a 9-coordinate geometry to nine equivalent S atoms. There are six shorter (3.48 Å) and three longer (3.52 Å) K–S bond lengths. Mo is bonded in a distorted see-saw-like geometry to four equivalent S atoms. There are a spread of Mo–S bond distances ranging from 2.49–2.60 Å. S is bonded in a 7-coordinate geometry to three equivalent K and four equivalent Mo atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Rb(MoS)3 by Materials Project

Rb(MoS)3 crystallizes in the hexagonal P6_3/m space group. The structure is three-dimensional. Rb is bonded in a 9-coordinate geometry to nine equivalent S atoms. There are three shorter (3.57 Å) and six longer (3.61 Å) Rb–S bond lengths. Mo is bonded in a distorted see-saw-like geometry to four equivalent S atoms. There are a spread of Mo–S bond distances ranging from 2.49–2.60 Å. S is bonded in a 7-coordinate geometry to three equivalent Rb and four equivalent Mo atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K(MoS)3 by Materials Project

K(MoS)3 crystallizes in the hexagonal P6_3/m space group. The structure is three-dimensional. K is bonded in a trigonal planar geometry to three equivalent S atoms. All K–S bond lengths are 3.11 Å. Mo is bonded in a distorted see-saw-like geometry to four equivalent S atoms. There are a spread of Mo–S bond distances ranging from 2.49–2.60 Å. S is bonded in a 5-coordinate geometry to one K and four equivalent Mo atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on K(MoS)9 by Materials Project

K(MoS)9 crystallizes in the hexagonal P6_3/m space group. The structure is three-dimensional. K is bonded in a 9-coordinate geometry to nine S atoms. There are three shorter (3.42 Å) and six longer (3.44 Å) K–S bond lengths. There are two inequivalent Mo sites. In the first Mo site, Mo is bonded in a distorted see-saw-like geometry to four S atoms. There are a spread of Mo–S bond distances ranging from 2.49–2.55 Å. In the second Mo site, Mo is bonded in a distorted see-saw-like geometry to four S atoms. There are a spread of Mo–S bond distances ranging from 2.49–2.55 Å. There are two inequivalent S sites. In the first S site, S is bonded in a distorted pentagonal planar geometry to one K and four Mo atoms. In the second S site, S is bonded in a distorted pentagonal planar geometry to one K and four Mo atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Rb(MoS)9 by Materials Project

Rb(MoS)9 crystallizes in the hexagonal P6_3/m space group. The structure is three-dimensional. Rb is bonded in a 9-coordinate geometry to nine S atoms. There are three shorter (3.44 Å) and six longer (3.53 Å) Rb–S bond lengths. There are two inequivalent Mo sites. In the first Mo site, Mo is bonded in a distorted see-saw-like geometry to four S atoms. There are a spread of Mo–S bond distances ranging from 2.50–2.55 Å. In the second Mo site, Mo is bonded in a distorted see-saw-like geometry to four S atoms. There are a spread of Mo–S bond distances ranging from 2.50–2.55 Å. There are two inequivalent S sites. In the first S site, S is bonded in a distorted pentagonal planar geometry to one Rb and four Mo atoms. In the second S site, S is bonded in a distorted pentagonal planar geometry to one Rb and four Mo atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Cs(MoS)9 by Materials Project

Cs(MoS)9 crystallizes in the hexagonal P6_3/m space group. The structure is three-dimensional. Cs is bonded in a 9-coordinate geometry to nine S atoms. There are three shorter (3.65 Å) and six longer (3.68 Å) Cs–S bond lengths. There are two inequivalent Mo sites. In the first Mo site, Mo is bonded in a distorted see-saw-like geometry to four S atoms. There are a spread of Mo–S bond distances ranging from 2.50–2.55 Å. In the second Mo site, Mo is bonded in a distorted see-saw-like geometry to four S atoms. There are two shorter (2.50 Å) and two longer (2.54 Å) Mo–S bond lengths. There are two inequivalent S sites. In the first S site, S is bonded in a 5-coordinate geometry to one Cs and four Mo atoms. In the second S site, S is bonded in a distorted pentagonal planar geometry to one Cs and four Mo atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2023

On the Interplay between Size and Disorder in Suppressing Intercalation-Induced Phase Transitions in Pseudocapacitive Nanostructured MoS 2

Pseudocapacitors are an emerging class of energy storage materials that offer an attractive compromise between the energy density of batteries and power density of electric double-layer capacitors. Decreasing particle size and increasing surface area of battery materials is a common approach for introducing pseudocapacitive behavior and increasing power density. However, in many cases, as the crystal size is reduced, lattice disorder of unknown extent is also introduced, making it difficult to characterize the relative contribution of size and disorder to fast-charging performance. Here, in this work, a series of nanostructured MoS 2 materials are synthesized with different crystallite sizes and degrees of crystallinity to decouple the effects of size and disorder on charge/discharge kinetics. The extent and type of disorder in each material is quantified by total X-ray scattering experiments and pair distribution function analyses. Electrochemical characterization, including galvanostatic rate capability, cyclic voltammetry, and various kinetic analyses, are used to demonstrate that both decreasing particle size and introducing lattice disorder are effective strategies for increasing charge storage kinetics, and that the effects are additive. Finally, operando X-ray diffraction measurements show that both size and disorder can be used suppress first-order Li + intercalation-induced phase transitions, a key feature for enabling pseudocapacitive charge storage.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Unraveling the Correlation between Raman and Photoluminescence in Monolayer MoS 2 through Machine‐Learning Models

Abstract 2D transition metal dichalcogenides (TMDCs) with intense and tunable photoluminescence (PL) have opened up new opportunities for optoelectronic and photonic applications such as light‐emitting diodes, photodetectors, and single‐photon emitters. Among the standard characterization tools for 2D materials, Raman spectroscopy stands out as a fast and non‐destructive technique capable of probing material's crystallinity and perturbations such as doping and strain. However, a comprehensive understanding of the correlation between photoluminescence and Raman spectra in monolayer MoS 2 remains elusive due to its highly nonlinear nature. Here, the connections between PL signatures and Raman modes are systematically explored, providing comprehensive insights into the physical mechanisms correlating PL and Raman features. This study's analysis further disentangles the strain and doping contributions from the Raman spectra through machine‐learning models. First, a dense convolutional network (DenseNet) to predict PL maps by spatial Raman maps is deployed. Moreover, a gradient boosted trees model (XGBoost) with Shapley additive explanation (SHAP) to bridge the impact of individual Raman features in PL features is applied. Last, a support vector machine (SVM) to project PL features on Raman frequencies is adopted. This work may serve as a methodology for applying machine learning to characterizations of 2D materials.

Lu, Ang‐Yu↗
DOE OSTI2022

A comparative analysis of different van der Waals treatments for molecular adsorption on the basal plane of 2H-MoS 2

The binding energy of hydrogen sulfide, ammonia, ethane, ethylene, butadiene, benzene, toluene, pyridine, pyrrole, and thiophene on the basal plane of the semi-conducting 2H-molybdenum sulfide (MoS 2 ) was calculated with the following flavors of Density Functional Theory (DFT): GGA-PW91, PBE-D3, vdW-DF, optPBE, optB86b, optB88, vdW-TS, and BEEF-vdW. The GGA-PW91 binding energies are negligible (<0.07 eV in magnitude) in all cases. The predictions with vdW-DF and PBE-D3 are the closest (error <0.05 eV) to the isosteric heats of adsorption calculated from reported temperature programmed desorption data for thiophene and butadiene. For all dispersion flavors examined here, the magnitude of the dispersion contribution to the binding energy increases linearly with the number of heavy atoms in the adsorbate, with each atom contributing 0.05 eV (BEEF-vdW) – 0.09 eV (optB88-vdW). Further, this implies that the calculated adsorption constants of molecules larger than acridine (i.e., comprising > 14 non-heavy atoms) can vary by more than four orders of magnitude at industrial conditions depending on the chosen method of dispersion correction. Further, dispersion effects fall off rapidly (>0.03 eV/ non-hydrogen atom/Å) as the adsorbate-surface distance increases.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2026

Transition Metal Dichalcogenide MoS 2 : Oxygen and Fluorine Functionalization for Selective Plasma Processing

Low-temperature plasma processing is a promising technique for tailoring transition metal dichalcogenides (TMDs). For chalcogen substitution processing, a key challenge is to identify the ion energy window that enables selective chalcogen removal while preserving the metal lattice. Using ab initio molecular dynamics (AIMD), we demonstrate that oxygen and fluorine functionalization widen the processing window by significantly lowering the sulfur sputtering energy threshold (E sputt,S ) of MoS 2 from ∼30 to ∼10 eV via formation of sputtering products such as SO 2 and SF n . Additionally, we show that experimentally relevant cryogenic temperatures strongly affect E sputt,S (T). The dependence is confirmed via AIMD and also predicted by a mechanistic parameter-free theory, suggesting that E sputt (T) generalizes to other TMDs, functionalizations, and surface impact conditions. Our results highlight oxygen/fluorine functionalization, ionic impact angle, and material temperature to be key control parameters for selective, damage-controlled chalcogen removal in TMD processing.

Polyachenko, Yury [Princeton Plasma Physics Labora↗
DOE OSTI2023

Exciton Lifetime and Optical Line Width Profile via Exciton–Phonon Interactions: Theory and First-Principles Calculations for Monolayer MoS 2

Exciton dynamics dictates the evolution of photoexcited carriers in photovoltaic and optoelectronic devices. However, interpreting their experimental signatures is a challenging theoretical problem due to the presence of both electron–phonon and many-electron interactions. Here, we develop and apply here a first-principles approach to exciton dynamics resulting from exciton–phonon coupling in monolayer MoS 2 and reveal the highly selective nature of exciton–phonon coupling due to the internal spin structure of excitons, which leads to a surprisingly long lifetime of the lowest-energy bright A exciton. Moreover, we show that optical absorption processes rigorously require a second-order perturbation theory approach, with photon and phonon treated on an equal footing, as proposed by Toyozawa and Hopfield. Such a treatment, thus far neglected in first-principles studies, gives rise to off-diagonal exciton–phonon self-energy, which is critical for the description of dephasing mechanisms and yields exciton line widths in excellent agreement with experiment.

77 NANOSCIENCE AND NANOTECHNOLOGY↗