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1. Phase‐Dependent Band Gap Engineering in Alloys of Metal‐Semiconductor Transition Metal Dichalcogenides

   https://doi.org/10.1002/adfm.202004912  

   Wang, Shuxi; Cavin, John; Hemmat, Zahra; Kumar, Khagesh; Ruckel, Alexander; Majidi, Leily; Gholivand, Hamed; Dawood, Radwa; Cabana, Jordi; Guisinger, Nathan; et al (September 2020, Advanced functional materials)

   Bandgap engineering plays a critical role in optimizing the electrical, optical and (photo)‐electrochemical applications of semiconductors. Alloying has been a historically successful way of tuning bandgaps by making solid solutions of two isovalent semiconductors. In this work, 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 in the range of 0.42–0.58 at 77 K. 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 that result in an uncommon 4 × 4 supercell, rotated approximately 4° from the lattice. Density‐functional‐theory calculations confirm that local distortions, such as those in a NCCDW, can open up a band gap in 1T‐Mo0.5Nb0.5Se2, but not in the 2H phase. This work expands the boundaries of alloy‐based bandgap engineering by introducing a novel technique that facilitates CDW phases through alloying. 

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2. Phase-Dependent Band Gap Engineering in Alloys of Metal-Semiconductor Transition Metal Dichalcogenides

   https://doi.org/https://doi.org/10.1002/adfm.202004912  

   Shuxi Wang, John Cavin (January 2020, Advanced functional materials)

   null (Ed.)

   Bandgap engineering plays a critical role in optimizing the electrical, optical and (photo)-electrochemical applications of semiconductors. Alloying has been a historically successful way of tuning bandgaps by making solid solutions of two isovalent semiconductors. In this work, 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 in the range of 0.42–0.58 at 77 K. 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 that result in an uncommon 4 × 4 supercell, rotated approximately 4° from the lattice. Density-functional-theory calculations confirm that local distortions, such as those in a NCCDW, can open up a band gap in 1T-Mo0.5Nb0.5Se2, but not in the 2H phase. This work expands the boundaries of alloy-based bandgap engineering by introducing a novel technique that facilitates CDW phases through alloying. 

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3. Stabilizing the heavily-doped and metallic phase of MoS ₂ monolayers with surface functionalization

   https://doi.org/10.1088/2053-1583/ac3f44  

   Zhang, Hanyu; Koledin, Tamara D; Wang, Xiang; Hao, Ji; Nanayakkara, Sanjini U; Attanayake, Nuwan H; Li, Zhaodong; Mirkin, Michael V; Miller, Elisa M (December 2021, 2D Materials)

   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. 

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4. The dimensional and hydrogenating effect on the electronic properties of ZnSe nanomaterials: a computational investigation

   https://doi.org/10.1039/c8cp04472d  

   Lv, Xiaodong; Li, Fengyu; Gong, Jian; Chen, Zhongfang (January 2018, Physical Chemistry Chemical Physics)

   We performed a comprehensive first-principles study on the structural and electronic properties of ZnSe two-dimensional (2D) nanosheets and their derived one-dimensional (1D) nanoribbons (NRs) and nanotubes (NTs). Both hexagonal and tetragonal phases of ZnSe (h-ZnSe and t-ZnSe) were considered. The tetragonal phase is thermodynamically more favorable for 2D monolayers and 1D pristine ribbons, in contrast, the hexagonal phase is preferred for the edge-hydrogenated 1D NRs and NTs. The 2D h-ZnSe monolayer is a direct-bandgap semiconductor. Both the pristine zigzag nanoribbons (z-hNRs) and the corresponding edge-hydrogenated NRs gradually convert from the direct-bandgap semiconducting phase into a metallic phase as the ribbon width increases; the pristine armchair nanoribbons (a-hNRs) remain as semiconductors with indirect bandgaps with increasing ribbon width, and edge hydrogenating switches the indirect-bandgap feature to the direct-bandgap character or the metallic character with different edge passivation styles. The 1D h-ZnSe single-walled nanotubes in both armchair and zigzag forms keep the direct-bandgap semiconducting property of the 2D counterpart but with smaller band gaps. For the thermodynamically more favorable t-ZnSe monolayer, the intrinsic direct-bandgap semiconducting character is rather robust: the derived 1D nanoribbons with edges unsaturated or hydrogenated fully, and 1D single-walled nanotubes all preserve the direct-bandgap semiconducting feature. Our systemic study provides deep insights into the electronic properties of ZnSe-based nanomaterials and is helpful for experimentalists to design and fabricate ZnSe-based nanoelectronics. 

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5. Monolayer 1T-NbSe ₂ as a 2D-correlated magnetic insulator

   https://doi.org/10.1126/sciadv.abi6339  

   Liu, Mengke; Leveillee, Joshua; Lu, Shuangzan; Yu, Jia; Kim, Hyunsue; Tian, Cheng; Shi, Youguo; Lai, Keji; Zhang, Chendong; Giustino, Feliciano; et al (November 2021, Science Advances)

   Monolayer group V transition metal dichalcogenides in their 1T phase have recently emerged as a platform to investigate rich phases of matter, such as spin liquid and ferromagnetism, resulting from strong electron correlations. Newly emerging 1T-NbSe 2 has inspired theoretical investigations predicting collective phenomena such as charge transfer gap and ferromagnetism in two dimensions; however, the experimental evidence is still lacking. Here, by controlling the molecular beam epitaxy growth parameters, we demonstrate the successful growth of high-quality single-phase 1T-NbSe 2 . By combining scanning tunneling microscopy/spectroscopy and ab initio calculations, we show that this system is a charge transfer insulator with the upper Hubbard band located above the valence band maximum. To demonstrate the electron correlation resulted magnetic property, we create a vertical 1T/2H NbSe 2 heterostructure, and we find unambiguous evidence of exchange interactions between the localized magnetic moments in 1T phase and the metallic/superconducting phase exemplified by Kondo resonances and Yu-Shiba-Rusinov–like bound states. 

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