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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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Effects of Strong Electron Correlations and Van Der Waals Interactions in the Physical Properties of Bulk and 2d Fecl2
We conducted a first-principles study of FeCl2, focusing on the significance of strong electron correlations using the GGA+U approximation and van der Waals (vdW) interactions to enhance its physicochemical properties description. Our results provide an excellent characterization of both the bulk CdCl2-type structure and the 2D phase 1T crystal structure. We found that both phases were elastically and dynamically stable, showing good agreement with the experimental data from IR, Raman, inelastic neutron scattering, and magnetic measurements. The impact of the FeCl2 dimensionality is discussed in detail. Additionally, we investigated the less-explored distorted 1T phase (1T’), where structural distortions introduce anisotropies that notably affect its properties, particularly its semiconducting behavior. Moreover, our analysis of the magnon spectrum aligns with the recently characterized magnetic properties of the FM 1T phase. Simultaneously, magnetic anisotropy calculations revealed that the 1T’ configuration exhibits greater stability in the presence of an external magnetic field.
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- Award ID(s):
- 2117575
- PAR ID:
- 10566319
- Publisher / Repository:
- SSRN
- Date Published:
- Journal Name:
- SSRN
- ISSN:
- 1556-5068.
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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
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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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