Monolayer MoS 2 has long been considered as the most promising candidate for wearable photovoltaic devices. However, its photovoltaic efficiency is restricted by its large band gap (2.0 eV). Though the band gap can be reduced by increasing the number of layers, the indirect band gap nature of the resulting multilayer MoS 2 is unfavorable. Herein, we report a theoretical discovery of the hitherto unknown symmetry-broken phase (denoted as 1T d ) of monolayer MoS 2 through a swarm structure search. The 1T d phase has a distorted octahedral coordinated pattern of Mo, and its direct band gap of 1.27 eV approaches the optimal value of 1.34 eV that gives the Shockley–Queisser limit for photovoltaic efficiency. Importantly, the direct band gap nature persists in thin films with multilayers owing to extremely weak vdW forces between adjacent 1T d layers. The theoretical photovoltaic efficiency at 30 nm thickness reaches ∼33.3%, which is the highest conversion efficiency among all the thin-film solar cell absorbers known thus far. Furthermore, several feasible strategies including appropriate electron injection and annealing methods were proposed to synthesize the 1T d phase. Once synthesized, the superior photovoltaic properties of the 1T d phase may lead to the development of an entirely new line of research for transition metal dichalcogenide solar cells.
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Thickness-dependent phase transition kinetics in lithium-intercalated MoS 2
Abstract The phase transitions of two-dimensional (2D) materials are key to the operation of many devices with applications including energy storage and low power electronics. Nanoscale confinement in the form of reduced thickness can modulate the phase transitions of 2D materials both in their thermodynamics and kinetics. Here, using in situ Raman spectroscopy we demonstrate that reducing the thickness of MoS 2 below five layers slows the kinetics of the phase transition from 2H- to 1T′-MoS 2 induced by the electrochemical intercalation of lithium. We observe that the growth rate of 1T′ domains is suppressed in thin MoS 2 supported by SiO 2 , and attribute this growth suppression to increased interfacial effects as the thickness is reduced below 5 nm. The suppressed kinetics can be reversed by placing MoS 2 on a 2D hexagonal boron nitride ( h BN) support, which readily facilitates the release of strain induced by the phase transition. Additionally, we show that the irreversible conversion of intercalated 1T′-MoS 2 into Li 2 S and Mo is also thickness-dependent and the stability of 1T′-MoS 2 is significantly increased below five layers, requiring a much higher applied electrochemical potential to break down 1T′-MoS 2 into Li 2 S and Mo nanoclusters.
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- Award ID(s):
- 1749742
- NSF-PAR ID:
- 10400844
- Date Published:
- Journal Name:
- 2D Materials
- Volume:
- 9
- Issue:
- 2
- ISSN:
- 2053-1583
- Page Range / eLocation ID:
- 025009
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Layered transition‐metal dichalcogenides (TMDs) have shown promise to replace carbon‐based compounds as suitable anode materials for Lithium‐ion batteries (LIBs) owing to facile intercalation and de‐intercalation of lithium (Li) during charging and discharging, respectively. While the intercalation mechanism of Li in mono‐ and bi‐layer TMDs has’ been thoroughly examined, mechanistic understanding of Li intercalation‐induced phase transformation in bulk or films of TMDs is still largely unexplored. This study investigates possible scenarios during sequential Li intercalation and aims to gain a mechanistic understanding of the phase transformation in bulk MoS2using density functional theory (DFT) calculations. The manuscript examines the role of concentration and distribution of Li‐ions on the formation of dual‐phase 2H‐1T microstructures that have been observed experimentally. The study demonstrates that lithiation would proceed in a systematic layer‐by‐layer manner wherein Li‐ions diffuse into successive interlayer spacings to render local phase transformation of the adjacent MoS2layers from 2H‐to‐1T phase in the multilayered MoS2. This local phase transition is attributed to partial ionization of Li and charge redistribution around the metal atoms and is followed by subsequent lattice straining. In addition, the stability of single‐phase vs. multiphase intercalated microstructures, and the origins of structural changes accompanying Li‐ion insertion are investigated at atomic scales.
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Abstract Transition metal dichalcogenides (TMDs) are a class of 2D materials demonstrating promising properties, such as high capacities and cycling stabilities, making them strong candidates to replace graphitic anodes in lithium-ion batteries. However, certain TMDs, for instance, MoS2, undergo a phase transformation from 2H to 1T during intercalation that can affect the mobility of the intercalating ions, the anode voltage, and the reversible capacity. In contrast, select TMDs, for instance, NbS2and VS2, resist this type of phase transformation during Li-ion intercalation. This manuscript uses density functional theory simulations to investigate the phase transformation of TMD heterostructures during Li-, Na-, and K-ion intercalation. The simulations suggest that while stacking MoS2layers with NbS2layers is unable to limit this 2H → 1T transformation in MoS2during Li-ion intercalation, the interfaces effectively stabilize the 2H phase of MoS2during Na- and K-ion intercalation. However, stacking MoS2layers with VS2is able to suppress the 2H → 1T transformation of MoS2during the intercalation of Li, Na, and K-ions. The creation of TMD heterostructures by stacking MoS2with layers of non-transforming TMDs also renders theoretical capacities and electrical conductivities that are higher than that of bulk MoS2.
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Abstract Monolayer molybdenum disulfide (MoS 2 ) is one of the most studied two-dimensional (2D) transition metal dichalcogenides that is being investigated for various optoelectronic properties, such as catalysis, sensors, photovoltaics, and batteries. One such property that makes this material attractive is the ease in which 2D MoS 2 can be converted between the semiconducting (2H) and metallic/semi-metallic (1T/1T′) phases or heavily n-type doped 2H phase with ion intercalation, strain, or excess negative charge. Using n -butyl lithium (BuLi) immersion treatments, we achieve 2H MoS 2 monolayers that are heavily n-type doped with shorter immersion times (10–120 mins) or conversion to the 1T/1T′ phase with longer immersion times (6–24 h); however, these doped/converted monolayers are not stable and promptly revert back to the initial 2H phase upon exposure to air. To overcome this issue and maintain the modification of the monolayer MoS 2 upon air exposure, we use BuLi treatments plus surface functionalization p-(CH 3 CH 2 ) 2 NPh-MoS 2 (Et 2 N-MoS 2 )—to maintain heavily n-type doped 2H phase or the 1T/1T′ phase, which is preserved for over two weeks when on indium tin oxide or sapphire substrates. We also determine that the low sheet resistance and metallic-like properties correlate with the BuLi immersion times. These modified MoS 2 materials are characterized with confocal Raman/photoluminescence, absorption, x-ray photoelectron spectroscopy as well as scanning Kelvin probe microscopy, scanning electrochemical microscopy, and four-point probe sheet resistance measurements to quantify the differences in the monolayer optoelectronic properties. We will demonstrate chemical methodologies to control the modified monolayer MoS 2 that likely extend to other 2D transition metal dichalcogenides, which will greatly expand the uses for these nanomaterials.more » « less
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Two-dimensional materials, such as transition metal dichalcogenides, have generated much interest due to their strain-sensitive electronic, optical, magnetic, superconducting, or topological properties. Harnessing control over their strain state may enable new technologies that operate by controlling these materials’ properties in devices such as straintronic transistors. Piezoelectric oxides have been proposed as one method to control such strain states on the device scale. However, there are few studies of how conformal 2D materials remain on oxide materials with respect to dynamic applications of the strain. Non-conformality may lead to non-optimal strain transfer. In this work, we explore this aspect of oxide-2D adhesion in the nanoscale switching of the substrate structural phase in thin 1T′-MoTe 2 attached to a mixed-phase thin-film BiFeO 3 (BFO), a multiferroic oxide with an electric-field induced structural phase transition that can generate mechanical strains of up to 2%. We observe that flake thickness impacts the conformality of 1T′-MoTe 2 to structural changes in BFO, but below four layers, 1T′-MoTe 2 fully conforms to the nanoscale BFO structural changes. The conformality of few-layer 1T′-MoTe 2 suggests that BFO is an excellent candidate for deterministic, nanoscale strain control for 2D materials.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1088/2053-1583/ac4e9b
