Temperature- and Pressure-Dependent Photoluminescence Emission of Bulk GaSe_0.5Te_0.5 Alloy
GaSe_（_0.5）Te__（0.5）合金块体材料的光致发光随温度和压力的变化

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This article references 63 other publications.

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   A review. Recent years have witnessed many breakthroughs in research on graphene (the first 2D at. crystal) as well as a significant advance in the mass prodn. of this material. This one-atom-thick fabric of C uniquely combines extreme mech. strength, exceptionally high electronic and thermal conductivities, impermeability to gases, as well as many other supreme properties, all of which make it highly attractive for numerous applications. Here we review recent progress in graphene research and in the development of prodn. methods, and critically analyze the feasibility of various graphene applications.

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   Wang, Yan; Kim, Jong Chan; Wu, Ryan J.; Martinez, Jenny; Song, Xiuju; Yang, Jieun; Zhao, Fang; Mkhoyan, Andre; Jeong, Hu Young; Chhowalla, Manish

   Nature (London, United Kingdom) (2019), 568 (7750), 70-74CODEN: NATUAS; ISSN:0028-0836. (Nature Research)

   As the dimensions of the semiconducting channels in field-effect transistors decrease, the contact resistance of the metal-semiconductor interface at the source and drain electrodes increases, dominating the performance of devices. Two-dimensional (2D) transition-metal dichalcogenides such as molybdenum disulfide (MoS2) have been demonstrated to be excellent semiconductors for ultrathin field-effect transistors. However, unusually high contact resistance has been obsd. across the interface between the metal and the 2D transition-metal dichalcogenide. Recent studies have shown that van der Waals contacts formed by transferred graphene10,11 and metals12 on few-layered transition-metal dichalcogenides produce good contact properties. However, van der Waals contacts between a three-dimensional metal and a monolayer 2D transition-metal dichalcogenide have yet to be demonstrated. Here we report the realization of ultraclean van der Waals contacts between 10-nm-thick indium metal capped with 100-nm-thick gold electrodes and monolayer MoS2. Using scanning transmission electron microscopy imaging, we show that the indium and gold layers form a solid soln. after annealing at 200 °C and that the interface between the gold-capped indium and the MoS2 is atomically sharp with no detectable chem. interaction between the metal and the 2D transition-metal dichalcogenide, suggesting van-der-Waals-type bonding between the gold-capped indium and monolayer MoS2. The contact resistance of the indium/gold electrodes is 3,000 ± 300 Ω micrometres for monolayer MoS2 and 800 ± 200 Ω micrometres for few-layered MoS2. These values are among the lowest obsd. for three-dimensional metal electrodes evapd. onto MoS2, enabling high-performance field-effect transistors with a mobility of 167 ± 20 square centimetres per V per s. We also demonstrate a low contact resistance of 220 ± 50 Ω micrometres on ultrathin niobium disulfide (NbS2) and near-ideal band offsets, indicative of defect-free interfaces, in tungsten disulfide (WS2) and tungsten diselenide (WSe2) contacted with indium alloy. Our work provides a simple method of making ultraclean van der Waals contacts using std. lab. technol. on monolayer 2D semiconductors.

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   Transition Metal Chalcogenides: Ultrathin Inorganic Materials with Tunable Electronic Properties

   Heine, Thomas

   Accounts of Chemical Research (2015), 48 (1), 65-72CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)

   A review. After the discovery of graphene and the development of powerful exfoliation techniques, exptl. prepn. of two-dimensional (2D) crystals can be expected for any layered material that is known to chem. Besides graphene and hexagonal boron nitride (h-BN), transition metal chalcogenides (TMC) are among the most studied ultrathin materials. In particular, single-layer MoS2, a direct band gap semiconductor with ∼1.9 eV energy gap, is popular in physics and nanoelectronics, because it nicely complements semimetallic graphene and insulating h-BN monolayer as a construction component for flexible 2D electronics and because it was already successfully applied in the lab. as basis material for transistors and other electronic and optoelectronic devices. Two-dimensional crystals are subject to significant quantum confinement: compared with their parent layered 3D material, they show different structural, electronic, and optical properties, such as spontaneous rippling as free-standing monolayer, significant changes of the electronic band structure, giant spin-orbit splitting, and enhanced photoluminescence. Most of those properties are intrinsic for the monolayer and already absent for two-layer stacks of the same 2D crystal. For example, single-layer MoS2 is a direct band gap semiconductor with spin-orbit splitting of 150 meV in the valence band, while the bilayer of the same material is an indirect band gap semiconductor without observable spin-orbit splitting. All these properties have been obsd. exptl. and are in excellent agreement with calcns. based on d.-functional theory. This Account reports theor. studies of a subgroup of transition metal dichalcogenides with the compn. MX2, with M = Mo, or W and X = Se or S, also referred to as "MoWSeS materials". Results on the electronic structure, quantum confinement, spin-orbit coupling, spontaneous monolayer rippling, and change of electronic properties in the presence of an external elec. field are reported. While all materials of the MoWSeS family share the same qual. properties, their individual values can differ strongly, for example, the spin-orbit splitting in WSe2 reaches the value of 428 meV, nearly three times that of MoS2. Further, we discuss the effect of strain on the electronic properties (straintronics). While MoWSeS single layers are very robust against external elec. fields, bilayers show a linear redn. of the band gap, even reaching a semiconductor-metal phase transition, and an increase of the spin-orbit splitting from zero to the monolayer value at rather small fields. Strain is yet another possibility to control the band gap in a linear way, and MoWSeS monolayers become metallic at strain values of ∼10%. The d.-functional based tight-binding model is a useful tool to investigate the electronic and structural properties, including electron conductance, of large MoS2 structures, which show spontaneous rippling in finite-temp. mol. dynamics simulations. Structural defects in MoS2 result in anisotropy of the elec. cond. Finally, DFT predictions on the properties of noble metal dichalcogenides are presented. Most strikingly, 1T PdS2 is an indirect band gap semiconductor in its monolayer form but becomes metallic as a bilayer.

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   A review, with 28 refs. In this contribution the authors discuss a group of layered tellurides with modular structures (tinker toy phases) and possible low temp. routes for the synthesis of chalcogenides with porous structures. Layer compds. [(M2Te2)(ATe2)](MTe2)n and related phases have been synthesized as main group counterparts of layered metal-rich early transition metal tellurides. Special features of these compds. are (i) the unusual square planar Te coordination of the main group atoms Ga, Si, and Ge and (ii) their modular structure based on four building blocks. Similarly to layered metal-rich early transition metal tellurides these phases are electronically stabilized by extensive bonding between early transition metals and main group heteroatoms. Size effects are important for the structural stability of the tinker toy phases; attempts to substitute A = Ga by the group homologs B or In give alternate phases such as Ta4BTe8 contg. metal clusters with interstitial atoms or intercalate phases such as InxNb3Te4. Reactions in thiophosphate fluxes and under solvothermal conditions have been explored to synthesize materials with microporous structures. The formation of (poly)thiophosphates such as K2MP2S7 (M = V, Cr) and K4Ti2P6S25 shows that reactions in molten thiophosphates are controlled by the redox equil. and the basicity of the flux. The reaction conditions prevent the use of templates which are needed for the formation of compds. with porous structures. Reaction of telluroarsenates with Cr(CO)6 under solvothermal condition, however, gave a unique polytelluride [Cr(en)3][Te6] with a microporous structure. Solvated Cr(en)33+ cations serve as templates in the synthesis of this material.

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   Jung, Yeonwoong; Zhou, Yu; Cha, Judy J.

   Inorganic Chemistry Frontiers (2016), 3 (4), 452-463CODEN: ICFNAW; ISSN:2052-1553. (Royal Society of Chemistry)

   Intercalation is a reversible insertion process of foreign species into crystal gaps. Layered materials are good host materials for various intercalant species ranging from small ions to atoms to mols. Given the recent intense interest in two-dimensional (2D) layered materials in thin limits, this review highlights the opportunities that intercalation chem. can provide for nanoscale layered materials. Novel heterostructures or emergent elec. properties not found in the intrinsic host materials are possible with intercalation. In particular, we review various exfoliation methods developed for 2D layered nanomaterials based on intercalation chem. and extensive tuning of the elec., optical, and magnetic properties of 2D layered materials due to intercalation.

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   Nanosheet materials such as graphene, boron nitride and transition metal dichalcogenides have gathered a lot of interest in recent years thanks to their outstanding properties and promises for future technol., energy generation and post-CMOS device concepts. Amongst this class of materials transition metal dichalcogenides based on molybdenum, tungsten, sulfur and selenium gathered a lot attention because of their semiconducting properties and the possibility to be synthesized by bottom up techniques. Vapor phase processes such as chem. vapor deposition permit to produce high quality layers and to precisely control their thickness. In order to target industrial applications of transition metal dichalcogenides it is important to develop synthesis methods that allow to scale up wafer size, and eventually integrate them with other technol. important materials. This review will cover all the currently proposed methods for the bottom up synthesis of transition metal dichalcogenides from the vapor phase, with particular emphasis on the precursors available and on the most common semiconductor techniques like metal org. chem. vapor deposition and at. layer epitaxy. A summary of the most common characterization technique is included and an overview of the growth issues that still limit the application of TMD is given.

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   The electronic and thermoelec. properties of one to four monolayers of MoS2, MoSe2, WS2, and WSe2 are calcd. For few layer thicknesses, the near degeneracies of the conduction band Κ and Σ valleys and the valence band Γ and Κ valleys enhance the n-type and p-type thermoelec. performance. The interlayer hybridization and energy level splitting det. how the no. of modes within kBT of a valley min. changes with layer thickness. In all cases, the max. ZT coincides with the greatest near-degeneracy within kBT of the band edge that results in the sharpest turn-on of the d. of modes. The thickness at which this max. occurs is, in general, not a monolayer. The transition from few layers to bulk is discussed. Effective masses, energy gaps, power-factors, and ZT values are tabulated for all materials and layer thicknesses. (c) 2014 American Institute of Physics.

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    Jie, W.; Chen, X.; Li, D.; Xie, L.; Hui, Y. Y.; Lau, S. P.; Cui, X.; Hao, J. Layer-dependent nonlinear optical properties and stability of non-centrosymmetric modification in few-layer GaSe sheets. Angew. Chem., Int. Ed. 2015, 54, 1185– 1189,  DOI: 10.1002/anie.201409837

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    Layer-Dependent Nonlinear Optical Properties and Stability of Non-Centrosymmetric Modification in Few-Layer GaSe Sheets

    Jie, Wenjing; Chen, Xi; Li, Dian; Xie, Lu; Hui, Yeung Yu; Lau, Shu Ping; Cui, Xiaodong; Hao, Jianhua

    Angewandte Chemie, International Edition (2015), 54 (4), 1185-1189CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Ga selenide, an important 2nd-order nonlinear semiconductor, has received much scientific interest. However, the nonlinear properties in its 2-dimensional (2D) form are still unknown. A strong 2nd harmonic generation (SHG) in bilayer and multilayer GaSe sheets is reported. This is also the 1st observation of SHG on 2-dimensional GaSe thin layers. The SHG of multilayer GaSe >5 layers shows a quadratic dependence on the thickness; while that of a sheet thinner than 5 layers shows a cubic dependence. The discrepancy between the 2 SHG responses is attributed to the weakened stability of noncentrosym. GaSe in the atomically thin flakes where a layer-layer stacking order tends to favor centrosym. modification. Importantly, 2-photon excited fluorescence also was obsd. in the GaSe sheets. Free-energy calcns. based on 1st-principles methods support the obsd. nonlinear optical phenomena of the atomically thin layers.

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    Effective nonlinear GaSe crystal. Optical properties and applications

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    Laser Physics (2009), 19 (5), 1092-1104CODEN: LAPHEJ; ISSN:1054-660X. (Pleiades Publishing, Ltd.)

    A review. We present an overview of the current state of the literature and research performed by the authors of the present paper on the exptl. and theor. results on the structural-, optical-, nonlinear optical (NLO)-properties (including two-photon absorption (TPA) and the terahertz (THz) range of spectra) and practical applications of a highly anisotropic Gallium Selenide (GaSe) semiconductor with emphasis on the .vepsiln.-GaSe. Phys. properties of .vepsiln.-GaSe are important to researchers and designers developing different devices by using this material. This crystal possesses an outstanding NLO properties: high optical birefringence Δn ∼ 0.3 at 700 nm; high transparency range (0.7-18.0 μm) with low absorption coeff. (α ≤ 0.3 cm-1); very high nonlinear susceptibility χ(2) (d 22 ≈ 86 ± 17 pm/V, corresponding to (2.0 ± 0.4) × 10-7 esu) that is used for phase matched second harmonic generation (SHG) in a wide transparency range; high power threshold for optical damage; possibility to perform optical frequency conversion under phase-matching conditions in the near- to mid-IR and THz range of spectra, etc. The domain structure of crystal in connection with the NLO properties is discussed as studied by confocal Raman microscopy expts. Perspectives for future research of GaSe are considered in the present article, which does not pretend to be one reflecting all existing papers on GaSe crystal and discussed subjects.

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    Hu, P.; Zhang, J.; Yoon, M.; Qiao, X. F.; Zhang, X.; Feng, W.; Tan, P.; Zheng, W.; Liu, J.; Wang, X. Highly sensitive phototransistors based on two-dimensional GaTe nanosheets with direct bandgap. Nano Res. 2014, 7, 694– 703,  DOI: 10.1007/s12274-014-0430-2

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    Hu, Pingan; Zhang, Jia; Yoon, Mina; Qiao, Xiao-Fen; Zhang, Xin; Feng, Wei; Tan, Pingheng; Zheng, Wei; Liu, Jingjing; Wang, Xiaona; Idrobo, Juan C.; Geohegan, David B.; Xiao, Kai

    Nano Research (2014), 7 (5), 694-703CODEN: NRAEB5; ISSN:1998-0000. (Springer GmbH)

    Highly sensitive phototransistors based on two-dimensional (2D) GaTe nanosheet have been demonstrated. The performance (photoresponsivity, detectivity) of the GaTe nanosheet phototransistor can be efficiently adjusted by using the applied gate voltage. The devices exhibit an ultrahigh photoresponsivity of 274.3 AW-1. The detectivity of 2D GaTe devices is ∼1012 Jones, which surpasses that of currently-exploited InGaAs photodetectors (1011-1012 Jones). To reveal the origin of the enhanced photocurrent in GaTe nanosheets, theor. modeling of the electronic structures was performed to show that GaTe nanosheets also have a direct bandgap structure, which contributes to the promotion of photon absorption and generation of excitons. This work shows that GaTe nanosheets are promising materials for high performance photodetectors. [Figure not available: see fulltext.].

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    Li, X.-L.; Han, W.-P.; Wu, J.-B.; Qiao, X.-F.; Zhang, J.; Tan, P.-H. Layer-number dependent optical properties of 2D materials and their application for thickness determination. Adv. Funct. Mater. 2017, 27, 1604468  DOI: 10.1002/adfm.201604468

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    Rajabpour, S.; Vera, A.; He, W.; Katz, B. N.; Koch, R. J.; Lassaunière, M.; Chen, X.; Li, C.; Nisi, K.; El-Sherif, H. Tunable 2D Group-III Metal Alloys. Adv. Mater. 2021, 33, 2104265  DOI: 10.1002/adma.202104265

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    Tunable 2D Group-III Metal Alloys

    Rajabpour, Siavash; Vera, Alexander; He, Wen; Katz, Benjamin N.; Koch, Roland J.; Lassauniere, Margaux; Chen, Xuegang; Li, Cequn; Nisi, Katharina; El-Sherif, Hesham; Wetherington, Maxwell T.; Dong, Chengye; Bostwick, Aaron; Jozwiak, Chris; van Duin, Adri C. T.; Bassim, Nabil; Zhu, Jun; Wang, Gwo-Ching; Wurstbauer, Ursula; Rotenberg, Eli; Crespi, Vincent; Quek, Su Ying; Robinson, Joshua A.

    Advanced Materials (Weinheim, Germany) (2021), 33 (44), 2104265CODEN: ADVMEW; ISSN:0935-9648. (Wiley-VCH Verlag GmbH & Co. KGaA)

    Chem. stable quantum-confined 2D metals are of interest in next-generation nanoscale quantum devices. Bottom-up design and synthesis of such metals could enable the creation of materials with tailored, on-demand, electronic and optical properties for applications that utilize tunable plasmonic coupling, optical nonlinearity, epsilon-near-zero behavior, or wavelength-specific light trapping. In this work, it is demonstrated that the electronic, superconducting, and optical properties of air-stable 2D metals can be controllably tuned by the formation of alloys. Environmentally robust large-area 2D-InxGa1-x alloys are synthesized byConfinement Heteroepitaxy (CHet). Near-complete solid soly. is achieved with no evidence of phase segregation, and the compn. is tunable over the full range of x by changing the relative elemental compn. of the precursor. The optical and electronic properties directly correlate with alloy compn., wherein the dielec. function, band structure, supercond., and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer.

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    Sun, Y.; Fujisawa, K.; Lin, Z.; Lei, Y.; Mondschein, J. S.; Terrones, M.; Schaak, R. E. Low-temperature solution synthesis of transition metal dichalcogenide alloys with tunable optical properties. J. Am. Chem. Soc. 2017, 139, 11096– 11105,  DOI: 10.1021/jacs.7b04443

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    Low-Temperature Solution Synthesis of Transition Metal Dichalcogenide Alloys with Tunable Optical Properties

    Sun, Yifan; Fujisawa, Kazunori; Lin, Zhong; Lei, Yu; Mondschein, Jared S.; Terrones, Mauricio; Schaak, Raymond E.

    Journal of the American Chemical Society (2017), 139 (32), 11096-11105CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)

    Nanostructures of layered transition metal dichalcogenide (TMD) alloys with tunable compns. are promising candidates for a broad scope of applications in electronics, optoelectronics, topol. devices, and catalysis. Most TMD alloy nanostructures are synthesized as films on substrates using gas-phase methods at high temps. Lower temp. soln. routes present an attractive alternative with the potential for larger-scale, higher-yield syntheses of freestanding, higher surface area materials. The direct soln. synthesis is reported of colloidal few-layer TMD alloys, MoxW1-xSe2 and WS2ySe2(1-y), exhibiting fully tunable metal and chalcogen compns. that span the MoSe2-WSe2 and WS2-WSe2 solid solns., resp. Chem. guidelines for achieving the targeted compds. are presented, along with comprehensive structural characterizations (x-ray diffraction, electron microscopy, Raman, and UV-visible spectroscopies). High-resoln. microscopic imaging confirms the formation of TMD alloys and identifies a random distribution of the alloyed elements. Anal. of the tilt-angle dependency of the intensities assocd. with at.-resoln. annular dark field imaging line scans reveals the types of point vacancies present in the samples, thus providing at.-level insights into the structures of colloidal TMD alloy nanostructures that were previously only accessible for substrate-confined films. The A excitonic transition of the TMD alloy nanostructures can be readily adjusted between 1.51 and 1.93 eV through metal and chalcogen alloying, correlating the compositional modulation to the realization of tunable optical properties.

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    Yao, J.; Yang, G. 2D layered material alloys: Synthesis and application in electronic and optoelectronic devices. Adv. Sci. 2022, 9, 2103036  DOI: 10.1002/advs.202103036

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    Wei, C.; Chen, X.; Li, D.; Su, H.; He, H.; Dai, J.-F. Bound exciton and free exciton states in GaSe thin slab. Sci. Rep. 2016, 6, 33890,  DOI: 10.1038/srep33890

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    21

    Bound exciton and free exciton states in GaSe thin slab

    Wei, Chengrong; Chen, Xi; Li, Dian; Su, Huimin; He, Hongtao; Dai, Jun-Feng

    Scientific Reports (2016), 6 (), 33890CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)

    The photoluminescence (PL) and absorption expts. have been performed in GaSe slab with incident light polarized perpendicular to c-axis of sample at 10 K. An obvious energy difference of about 34 meV between exciton absorption peak and PL peak (the highest energy peak) is obsd. By studying the temp. dependence of PL and absorption spectra, we attribute it to energy difference between free exciton and bound exciton states, where main exciton absorption peak comes from free exciton absorption, and PL peak is attributed to recombination of bound exciton at 10 K. This strong bound exciton effect is stable up to 50 K. Moreover, the temp. dependence of integrated PL intensity and PL lifetime reveals that a non-radiative process, with activation energy extd. as 0.5 meV, dominates PL emission.

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22. 22

    Chen, J.; Tan, X.; Lin, P.; Sa, B.; Zhou, J.; Zhang, Y.; Wen, C.; Sun, Z. Comprehensive understanding of intrinsic mobility in the monolayers of III-VI group 2D materials. Phys. Chem. Chem. Phys. 2019, 21, 21898– 21907,  DOI: 10.1039/C9CP04407H

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    22

    Comprehensive understanding of intrinsic mobility in the monolayers of III-VI group 2D materials

    Chen, Jianhui; Tan, Xiaolin; Lin, Peng; Sa, Baisheng; Zhou, Jian; Zhang, Yinggan; Wen, Cuilian; Sun, Zhimei

    Physical Chemistry Chemical Physics (2019), 21 (39), 21898-21907CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)

    Monolayers of III-VI group two-dimensional (2D) materials MX (M = Ga and In and X = S, Se, and Te) have attracted global interest for potential applications in electronic and photoelec. devices due to their attractive phys. and chem. characteristics. However, a comprehensive understanding of the distinguished carrier mobility in MX monolayers is of great importance and not yet clear. Herein, using a Boltzmann transport equation (BTE) solver and first principles calcns., we have precisely revealed that the intrinsic mobility in MX monolayers is significantly limited by phonon scattering. Note that the longitudinal acoustic phonon mode and optic phonon modes A'1 and E'2 were found predominantly coupled with electrons, which strongly restrained the intrinsic mobility in the MX monolayers. Interestingly, apart from a moderate band gap, the GaSe and GaTe monolayers exhibit high electron mobility exceeding 103 cm2 V-1 s-1 and may serve as outstanding electron transport channels. We believe that our findings will shed light on the design and applications of MX monolayers and 2D materials in nanoscale electronic and photoelec. devices.

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23. 23

    Ni, Y.; Wu, H.; Huang, C.; Mao, M.; Wang, Z.; Cheng, X. Growth and quality of gallium selenide (GaSe) crystals. J. Cryst. Growth 2013, 381, 10– 14,  DOI: 10.1016/j.jcrysgro.2013.06.030

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    23

    Growth and quality of gallium selenide (GaSe) crystals

    Ni, Youbao; Wu, Haixin; Huang, Changbao; Mao, Mingsheng; Wang, Zhenyou; Cheng, Xudong

    Journal of Crystal Growth (2013), 381 (), 10-14CODEN: JCRGAE; ISSN:0022-0248. (Elsevier B.V.)

    High quality nonlinear IR crystal material GaSe was grown using a seed aided Bridgman-Stockbarger method having a size of 24-26 mm in diam. and 55-70 mm in length. The crystals were characterized using x-ray diffraction, electron energy scattering, transmission spectrophotometry and IR microscopy. The transmission spectrum showed that the IR transmission is about 67% in an 8 mm thick sample cleaved along (001) face, and mean absorption 0.01-0.08 cm-1 in the range 0.9-15 μm. It may be suitable for the fabrication of IR nonlinear optical devices, photoelec. analyzers of polarized light and so on. Crystals grown by this method with the described procedures may also be useful for other similar layered compds.

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24. 24

    Liu, H.; Antwi, K. A.; Chua, S.; Chi, D. Vapor-phase growth and characterization of Mo 1- x W x S 2 (0 × 1) atomic layers on 2-in. sapphire substrates. Nanoscale 2014, 6, 624– 629,  DOI: 10.1039/C3NR04515C

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    24

    Vapor-phase growth and characterization of Mo1-xWxS2 (0 ≤ x ≤ 1) atomic layers on 2-inch sapphire substrates

    Liu, Hongfei; Antwi, K. K. Ansah; Chua, Soojin; Chi, Dongzhi

    Nanoscale (2014), 6 (1), 624-629CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)

    Atomically thin Mo1-xWxS2 (0 ≤ x ≤ 1) ternary compds. have been grown on 2-in. c-plane sapphire substrates with high uniformity by sulfurizing thin Mo1-xWx layers that were deposited at room temp. using a co-sputtering technique. Atomic force microscopy (AFM), Raman scattering, and optical absorbance spectroscopy (OAS) studies reveal that the Mo1-xWxS2 films consist of crystallites of two-to-four monolayers in thickness. XPS shows that the core levels of Mo3d and W4f shift to lower binding energies while that of S2p shifts to higher ones with the increase in W compns., which can be related to the larger electron affinity of W (0.8163 eV) than that of Mo (0.7473 eV). OAS has also shown that the direct bandgap of Mo1-xWxS2 is tuned from 1.85 to 1.99 eV by increasing x from 0 to 1. Both E12g and A1g phonon modes of the Mo1-xWxS2 films exhibit a two-mode behavior. The bandgap tuning and the two-mode phonon behaviors are typically the same as those recently obsd. in monolayer Mo1-xWxS2 obtained by mech. exfoliation, thus shedding light on the bottom-up growth of large-scale two-dimensional Mo1-xWxS2 ternary alloys.

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25. 25

    Zhang, W.; Li, X.; Jiang, T.; Song, J.; Lin, Y.; Zhu, L.; Xu, X. CVD synthesis of Mo_(1-x)W_xS₂ and MoS_2(1-x)Se_2x alloy monolayers aimed at tuning the bandgap of molybdenum disulfide. Nanoscale 2015, 7, 13554– 13560,  DOI: 10.1039/C5NR02515J

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    25

    CVD synthesis of Mo(1-x)WxS2 and MoS2(1-x)Se2x alloy monolayers aimed at tuning the bandgap of molybdenum disulfide

    Zhang, Wenting; Li, Xiaodong; Jiang, Tongtong; Song, Jiangluqi; Lin, Yue; Zhu, Lixin; Xu, Xiaoliang

    Nanoscale (2015), 7 (32), 13554-13560CODEN: NANOHL; ISSN:2040-3372. (Royal Society of Chemistry)

    As a rising star in two-dimensional (2D) layered materials, transition metal dichalcogenides (TMDs) have attracted tremendous attention for their potential applications in nanoelectronics, optoelectronics and photonics. Driven by the high stds. of practical devices, alloying theory has been proposed for modulating the electronic structure of TMDs materials as well as their phys. and chem. properties. To date, however, very limited alloy materials can be synthesized by chem. vapor deposition (CVD) and a very limited band gap range can be achieved. Herein, for the first time, we report a one-step CVD strategy for the growth of ternary alloy Mo(1-x)WxS2 monolayers (ML) on SiO2/Si substrates with controllable compn. Both Mo(1-x)WxS2 and MoS2(1-x)Se2x alloy materials with high crystallinity were synthesized in this study. Therefore, the bandgap photoluminescence (PL) can be broaden from 1.97 eV (for ML-WS2) to 1.55 eV (for ML-MoSe2). Furthermore, d. functional theory calcns. were performed to reveal the important role of alloying in tailoring the electronic structure of 2D materials.

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26. 26

    Zheng, S.; Sun, L.; Yin, T.; Dubrovkin, A. M.; Liu, F.; Liu, Z.; Shen, Z. X.; Fan, H. J. Monolayers of W_xMo_1xS₂ alloy heterostructure with in-plane composition variations. Appl. Phys. Lett. 2015, 106, 063113  DOI: 10.1063/1.4908256

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    There is no corresponding record for this reference.
27. 27

    Cai, H.; Chen, B.; Blei, M.; Chang, S. L.; Wu, K.; Zhuang, H.; Tongay, S. Abnormal band bowing effects in phase instability crossover region of GaSe1-x Te x nanomaterials. Nat. Commun. 2018, 9, 1927,  DOI: 10.1038/s41467-018-04328-z

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    There is no corresponding record for this reference.
28. 28

    Fonseca, J. J.; Horton, M. K.; Tom, K.; Yao, J.; Walukiewicz, W.; Dubon, O. D. Structure–Property Relationship of Low-Dimensional Layered GaSe_xTe_1–x Alloys. Chem. Mater. 2018, 30, 4226– 4232,  DOI: 10.1021/acs.chemmater.8b00130

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    28

    Structure-property relationship of low-dimensional layered GaSexTe1-x Alloys

    Fonseca, Jose J.; Horton, Matthew K.; Tom, Kyle; Yao, Jie; Walukiewicz, Wladek; Dubon, Oscar D.

    Chemistry of Materials (2018), 30 (13), 4226-4232CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)

    We report the growth of layered GaSexTe1-x mesostructures across the whole compn. range. For compns. up to x = 0.32 (the Te-rich region), mesocrystals form predominantly in the monoclinic structure, similar to naturally occurring GaTe. However, the hexagonal crystal structure, similar to naturally occurring GaSe, begins growing at the x = 0.28 compn. and grows almost exclusively in the range of x = 0.32 to pure GaSe, establishing a region of compn. where both monoclinic and hexagonal crystals exist. While the optical bandgap of the monoclinic phase increases linearly from 1.65 to 1.77 eV with increasing Se content, the incorporation of Te in the hexagonal phase reduces the optical gap from 2.01 (pure GaSe) to 1.38 eV (x = 0.28). Specifically, a bandgap difference of ∼0.35 eV between monoclinic and hexagonal crystals is obsd. in the compn. range where both crystal structures can be grown. These observations are in good agreement with direct-gap trends calcd. by d. functional theory, which show a linear dependence on compn. for the direct gap of the monoclinic phase and a considerable bowing of the direct gap of the hexagonal phase for Te-rich compns. Our results show that layered semiconductor alloys are remarkably versatile systems in which electronic properties can be controlled by not only thickness but also structural phase and compn.

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29. 29

    Azizi, A.; Antonius, G.; Regan, E.; Eskandari, R.; Kahn, S.; Wang, F.; Louie, S. G.; Zettl, A. Layer-Dependent Electronic Structure of Atomically Resolved Two-Dimensional Gallium Selenide Telluride. Nano Lett. 2019, 19, 1782– 1787,  DOI: 10.1021/acs.nanolett.8b04802

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    29

    Layer-Dependent Electronic Structure of Atomically Resolved Two-Dimensional Gallium Selenide Telluride

    Azizi, Amin; Antonius, Gabriel; Regan, Emma; Eskandari, Rahmatollah; Kahn, Salman; Wang, Feng; Louie, Steven G.; Zettl, Alex

    Nano Letters (2019), 19 (3), 1782-1787CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)

    Alloying two-dimensional (2D) semiconductors provides a powerful method to tune their phys. properties, esp. those relevant to optoelectronic applications. However, as the crystal structure becomes more complex, it becomes increasingly difficult to accurately correlate response characteristics to detailed at. structure. We investigate, via annular dark-field scanning transmission electron microscopy, electron energy loss spectroscopy, and second harmonic generation, the layered III-VI alloy GaSe0.5Te0.5 as a function of layer no. The local at. structure and stacking sequence for different layers is explicitly detd. We complement the measurements with first-principles calcns. of the total energy and electronic band structure of GaSe0.5Te0.5 for different crystal structures and layer no. The electronic band gap as well as the π and π + σ plasmons are found to be sensitive to layer no.

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30. 30

    Zhong, X.; Zhou, W.; Zhou, Y.; Zhou, F.; Liu, C.; Yin, Y.; Peng, Y.; Tang, D. High-performance photodetectors based on bandgap engineered novel layer GaSe_0.5Te_0.5 nanoflakes. RSC Adv. 2016, 6, 60862– 60868,  DOI: 10.1039/C6RA09239J

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31. 31

    Splendiani, A.; Sun, L.; Zhang, Y.; Li, T.; Kim, J.; Chim, C.-Y.; Galli, G.; Wang, F. Emerging Photoluminescence in Monolayer MoS2. Nano Lett. 2010, 10, 1271– 1275,  DOI: 10.1021/nl903868w

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    31

    Emerging Photoluminescence in Monolayer MoS2

    Splendiani, Andrea; Sun, Liang; Zhang, Yuanbo; Li, Tianshu; Kim, Jonghwan; Chim, Chi-Yung; Galli, Giulia; Wang, Feng

    Nano Letters (2010), 10 (4), 1271-1275CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)

    Novel phys. phenomena can emerge in low-dimensional nanomaterials. Bulk MoS2, a prototypical metal dichalcogenide, is an indirect bandgap semiconductor with negligible photoluminescence. When the MoS2 crystal is thinned to monolayer, however, a strong photoluminescence emerges, indicating an indirect to direct bandgap transition in this d-electron system. Quantum confinement in layered d-electron materials like MoS2 provides new opportunities for engineering the electronic structure of matter at the nanoscale.

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32. 32

    Vaquero, D.; Clericò, V.; Salvador-Sánchez, J.; Martín-Ramos, A.; Díaz, E.; Domínguez-Adame, F.; Meziani, Y. M.; Diez, E.; Quereda, J. Excitons, trions and Rydberg states in monolayer MoS2 revealed by low-temperature photocurrent spectroscopy. Commun. Phys. 2020, 3, 194,  DOI: 10.1038/s42005-020-00460-9

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    32

    Excitons, trions and Rydberg states in monolayer MoS2 revealed by low-temperature photocurrent spectroscopy

    Vaquero, Daniel; Clerico, Vito; Salvador-Sanchez, Juan; Martin-Ramos, Adrian; Diaz, Elena; Dominguez-Adame, Francisco; Meziani, Yahya M.; Diez, Enrique; Quereda, Jorge

    Communications Physics (2020), 3 (1), 194CODEN: CPOHDJ; ISSN:2399-3650. (Nature Research)

    Abstr.: Exciton physics in two-dimensional semiconductors are typically studied by photoluminescence spectroscopy. However, this technique does not allow for direct observation of non-radiating excitonic transitions. Here, we use low-temp. photocurrent spectroscopy as an alternative technique to investigate excitonic transitions in a high-quality monolayer MoS2 phototransistor. The resulting spectra presents excitonic peaks with linewidths as low as 8 meV. We identify spectral features corresponding to the ground states of neutral excitons (XA1s and XB1s) and charged trions (TA and TB) as well as up to eight addnl. spectral lines at energies above the XB1s transition, which we attribute to the Rydberg series of excited states of XA and XB. The intensities of the spectral features can be tuned by the gate and drain-source voltages. Using an effective-mass theory for excitons in two-dimensional systems we are able to accurately fit the measured spectral lines and unambiguously assoc. them with their corresponding Rydberg states.

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33. 33

    Mouri, S.; Miyauchi, Y.; Matsuda, K. Tunable Photoluminescence of Monolayer MoS2 via Chemical Doping. Nano Lett. 2013, 13, 5944– 5948,  DOI: 10.1021/nl403036h

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    33

    Tunable Photoluminescence of Monolayer MoS2 via Chemical Doping

    Mouri, Shinichiro; Miyauchi, Yuhei; Matsuda, Kazunari

    Nano Letters (2013), 13 (12), 5944-5948CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)

    The authors demonstrate the tunability of the photoluminescence (PL) properties of monolayer (1L)-MoS2 via chem. doping. The PL intensity of 1L-MoS2 was drastically enhanced by the adsorption of p-type dopants with high electron affinity but reduced by the adsorption of n-type dopants. This PL modulation results from switching between exciton PL and trion PL depending on carrier d. in 1L-MoS2. Achievement of the extn. and injection of carriers in 1L-MoS2 by this soln.-based chem. doping method enables convenient control of optical and elec. properties of atomically thin MoS2.

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34. 34

    Toda, Y.; Moriwaki, O.; Nishioka, M.; Arakawa, Y. Efficient Carrier Relaxation Mechanism in InGaAs/GaAs Self-Assembled Quantum Dots Based on the Existence of Continuum States. Phys. Rev. Lett. 1999, 82, 4114– 4117,  DOI: 10.1103/PhysRevLett.82.4114

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35. 35

    Wang, Y.; Nie, Z.; Wang, F. Modulation of photocarrier relaxation dynamics in two-dimensional semiconductors. Light: Sci. Appl. 2020, 9, 192,  DOI: 10.1038/s41377-020-00430-4

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    35

    Modulation of photocarrier relaxation dynamics in two-dimensional semiconductors

    Wang, Yuhan; Nie, Zhonghui; Wang, Fengqiu

    Light: Science & Applications (2020), 9 (1), 192CODEN: LSAIAZ; ISSN:2047-7538. (Nature Research)

    Abstr.: Due to strong Coulomb interactions, two-dimensional (2D) semiconductors can support excitons with large binding energies and complex many-particle states. Their strong light-matter coupling and emerging excitonic phenomena make them potential candidates for next-generation optoelectronic and valleytronic devices. The relaxation dynamics of optically excited states are a key ingredient of excitonic physics and directly impact the quantum efficiency and operating bandwidth of most photonic devices. Here, we summarize recent efforts in probing and modulating the photocarrier relaxation dynamics in 2D semiconductors. We classify these results according to the relaxation pathways or mechanisms they are assocd. with. The approaches discussed include both tailoring sample properties, such as the defect distribution and band structure, and applying external stimuli such as elec. fields and mech. strain. Particular emphasis is placed on discussing how the unique features of 2D semiconductors, including enhanced Coulomb interactions, sensitivity to the surrounding environment, flexible van der Waals (vdW) heterostructure construction, and non-degenerate valley/spin index of 2D transition metal dichalcogenides (TMDs), manifest themselves during photocarrier relaxation and how they can be manipulated. The extensive phys. mechanisms that can be used to modulate photocarrier relaxation dynamics are instrumental for understanding and utilizing excitonic states in 2D semiconductors.

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36. 36

    Araujo, F. D. V.; Silva, F. W. N.; Zhang, T.; Zhou, C.; Lin, Z.; Perea-Lopez, N.; Rodrigues, S. F.; Terrones, M.; Souza Filho, A. G.; Alencar, R. S.; Viana, B. C. Substrate-Induced Changes on the Optical Properties of Single-Layer WS2. Materials 2023, 16, 2591,  DOI: 10.3390/ma16072591

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    There is no corresponding record for this reference.
37. 37

    Davila, Y. G.; Silva, F. W. N.; Oliveira, M. C. D.; Yu, Z.; Carvalho, T. C. V.; dos Santos, C. C.; Filho, A. G. S.; Terrones, M.; Alencar, R. S.; Viana, B. C. Temperature and power-dependent photoluminescence spectroscopy in suspended WSe2 monolayer. J. Phys. D: Appl. Phys. 2024, 57, 165304  DOI: 10.1088/1361-6463/ad211d

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    There is no corresponding record for this reference.
38. 38

    Li, C.; Liu, Y.; Yang, Q.; Zheng, Q.; Yan, Z.; Han, J.; Lin, J.; Wang, S.; Qi, J.; Liu, Y.; Zhu, J. Tuning of Optical Behavior in Monolayer and Bilayer Molybdenum Disulfide Using Hydrostatic Pressure. J. Phys. Chem. Lett. 2022, 13, 161– 167,  DOI: 10.1021/acs.jpclett.1c03919

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    38

    Tuning of Optical Behavior in Monolayer and Bilayer Molybdenum Disulfide Using Hydrostatic Pressure

    Li, Chenkai; Liu, Yaoyao; Yang, Qishuo; Zheng, Qunfei; Yan, Zhipeng; Han, Jun; Lin, Junhao; Wang, Shanmin; Qi, Jingbo; Liu, Ying; Zhu, Jinlong

    Journal of Physical Chemistry Letters (2022), 13 (1), 161-167CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)

    Researchers have shown great interest in two-dimensional crystals recently, because of their thickness-dependent electronic and optical properties. We have investigated the Raman and photoluminescence spectra of free-standing monolayer and bilayer MoS2, as a function of pressure. As the enforcement of layer interaction, an electronic and a crystal phase transition were revealed at ~ 6 GPa and ~ 16 GPa, resp., in bilayer MoS2, while no phase transition in the monolayer is obsd. The electronic phase transition at ~ 6 GPa is supposed to be a direct interband changing to an indirect Λ-K interband transition, and the new structure shown at ~ 16 GPa is not metalized and supposed to be a transformation from stacking faults due to layer sliding like 2Hc to 2Ha. The different pressure-induced features of monolayer MoS2, compared with bilayer MoS2, can help to get a better understanding about the importance of interlayer interaction on modifying the optical properties of MoS2 and other fundamental understanding of 2D materials.

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39. 39

    Loi, M.; Bongiovanni, G.; Mura, A.; Cai, Q.; Martin, C.; Chandrasekhar, H.; Chandrasekhar, M.; Graupner, W.; Garnier, F. High pressure effects on the photoluminescence intensity of sexithiophene single crystals. Synth. Met. 2001, 116, 311– 315,  DOI: 10.1016/S0379-6779(00)00428-8

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40. 40

    Aguiar Sousa, J. H.; Araújo, B. S.; Ferreira, R. S.; San-Miguel, A.; Alencar, R. S.; Souza Filho, A. G. Pressure Tuning Resonance Raman Scattering in Monolayer, Trilayer, and Many-Layer Molybdenum Disulfide. ACS Appl. Nano Mater. 2022, 5, 14464– 14469,  DOI: 10.1021/acsanm.2c02819

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    40

    Pressure Tuning Resonance Raman Scattering in Monolayer, Trilayer, and Many-Layer Molybdenum Disulfide

    Aguiar Sousa, Jose H.; S. Araujo, Bruno; S. Ferreira, Ramon; San-Miguel, Alfonso; Alencar, Rafael S.; Souza Filho, Antonio G.

    ACS Applied Nano Materials (2022), 5 (10), 14464-14469CODEN: AANMF6; ISSN:2574-0970. (American Chemical Society)

    We report a pressure-dependent resonance Raman study of mech. exfoliated one-layer (1L), three-layer (3L), and many-layer MoS2 near the A excitonic transitions by using an excitation energy of 1.96 eV. Our results show a linear blue shift for the majority of phonon modes, except for the second-order phonons LA(K) + TA(K) and 2LA, whose pressure-dependent frequency is non-linear. The pressure dependence of these bands is explained by considering the dispersive scattering process that gives rise to these two modes, where both phonon and exciton energies blue-shift as the pressure increases. In addn., the resonance of the A1g mode in 1L-MoS2 is achieved at ~ 7.0 GPa, while for 3L-MoS2 and many-layer MoS2, it occurs at ~ 3.4 GPa. This difference is attributed to the smaller pressure coeff. of the A excitonic transition for 1L-MoS2, as compared with that of 3L-MoS2 and the MoS2 bulk. Our findings constitute an important step toward understanding and controlling the optoelectronic properties of few-layer MoS2 by means of strain/pressure, which are relevant in designing new flexible electronic and light-emitting devices.

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41. 41

    Gontijo, R. N.; Gadelha, A.; Silveira, O. J.; Carvalho, B. R.; Nunes, R. W.; Campos, L. C.; Pimenta, M. A.; Righi, A.; Fantini, C. Temperature dependence of the double-resonance Raman bands in monolayer MoS2. J. Raman Spectrosc. 2019, 50, 1867– 1874,  DOI: 10.1002/jrs.5736

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    41

    Temperature dependence of the double-resonance Raman bands in monolayer MoS2

    Gontijo, Rafael N.; Gadelha, Andreij; Silveira, Orlando J.; Carvalho, Bruno R.; Nunes, Ricardo W.; Campos, Leonardo C.; Pimenta, Marcos A.; Righi, Ariete; Fantini, Cristiano

    Journal of Raman Spectroscopy (2019), 50 (12), 1867-1874CODEN: JRSPAF; ISSN:0377-0486. (John Wiley & Sons Ltd.)

    This work reports a detailed study of the double-resonance (DR) Raman bands of single-layer MoS2 as a function of temp., using many different laser energies in the region of the excitonic transitions and at different temps. between 80 and 300 K. Our measurements show that the DR bands are strongly affected by temp. and the results are explained in terms of the temp. dependence of both the phonon wavenumber and the excitonic energy. In order to distinguish these two effects, the excitonic transitions were directly measured by photoluminescence as a function of temp. It was obsd. from the multiple-excitation results that the dispersion of the DR bands measured with different laser lines depends on temp. The resonance condition was evidenced by considering the difference between energies of the laser excitation and the excitonic transition, at a given temp. Our findings for the temp. dependence of the DR process in single-layer MoS2 can be extended to other classes of transition metal dichalcogenide materials.

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42. 42

    Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys. Rev. 1964, 136, B864– B871,  DOI: 10.1103/PhysRev.136.B864

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    Giannozzi, P. QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials. J. Phys.: Condens. Matter 2009, 21, 395502  DOI: 10.1088/0953-8984/21/39/395502

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    QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials

    Giannozzi Paolo; Baroni Stefano; Bonini Nicola; Calandra Matteo; Car Roberto; Cavazzoni Carlo; Ceresoli Davide; Chiarotti Guido L; Cococcioni Matteo; Dabo Ismaila; Dal Corso Andrea; de Gironcoli Stefano; Fabris Stefano; Fratesi Guido; Gebauer Ralph; Gerstmann Uwe; Gougoussis Christos; Kokalj Anton; Lazzeri Michele; Martin-Samos Layla; Marzari Nicola; Mauri Francesco; Mazzarello Riccardo; Paolini Stefano; Pasquarello Alfredo; Paulatto Lorenzo; Sbraccia Carlo; Scandolo Sandro; Sclauzero Gabriele; Seitsonen Ari P; Smogunov Alexander; Umari Paolo; Wentzcovitch Renata M

    Journal of physics. Condensed matter : an Institute of Physics journal (2009), 21 (39), 395502 ISSN:.

    QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.

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45. 45

    Giannozzi, P. Advanced capabilities for materials modelling with Quantum ESPRESSO. J. Phys.: Condens. Matter 2017, 29, 465901,  DOI: 10.1088/1361-648X/aa8f79

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    Advanced capabilities for materials modelling with QUANTUM ESPRESSO

    Giannozzi, P.; Andreussi, O.; Brumme, T.; Bunau, O.; Buongiorno Nardelli, M.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Cococcioni, M.; Colonna, N.; Carnimeo, I.; Dal Corso, A.; de Gironcoli, S.; Delugas, P.; Di Stasio, R. A., Jr.; Ferretti, A.; Floris, A.; Fratesi, G.; Fugallo, G.; Gebauer, R.; Gerstmann, U.; Giustino, F.; Gorni, T.; Jia, J.; Kawamura, M.; Ko, H.-Y.; Kokalj, A.; Kucukbenli, E.; Lazzeri, M.; Marsili, M.; Marzari, N.; Mauri, F.; Nguyen, N. L.; Nguyen, H.-V.; Otero-de-la-Roza, A.; Paulatto, L.; Ponce, S.; Rocca, D.; Sabatini, R.; Santra, B.; Schlipf, M.; Seitsonen, A. P.; Smogunov, A.; Timrov, I.; Thonhauser, T.; Umari, P.; Vast, N.; Wu, X.; Baroni, S.

    Journal of Physics: Condensed Matter (2017), 29 (46), 465901/1-465901/30CODEN: JCOMEL; ISSN:0953-8984. (IOP Publishing Ltd.)

    QUANTUM ESPRESSO is an integrated suite of open-source computer codes for quantum simulations of materials using state-of-the-art electronic-structure techniques, based on d.-functional theory, d.-functional perturbation theory, and many-body perturbation theory, within the plane-wave pseudopotential and projector-augmented-wave approaches. QUANTUM ESPRESSO owes its popularity to the wide variety of properties and processes it allows to simulate, to its performance on an increasingly broad array of hardware architectures, and to a community of researchers that rely on its capabilities as a core open-source development platform to implement their ideas. In this paper we describe recent extensions and improvements, covering new methodologies and property calculators, improved parallelization, code modularization, and extended interoperability both within the distribution and with external software.

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46. 46

    Monkhorst, H. J.; Pack, J. D. Special points for Brillouin-zone integrations. Phys. Rev. B 1976, 13, 5188– 5192,  DOI: 10.1103/PhysRevB.13.5188

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    Bachelet, G. B.; Hamann, D. R.; Schlüter, M. Pseudopotentials that work: From H to Pu. Phys. Rev. B 1982, 26, 4199– 4228,  DOI: 10.1103/PhysRevB.26.4199

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    Pseudopotentials that work: From hydrogen to plutonium

    Bachelet, G. B.; Hamann, D. R.; Schluter, M.

    Physical Review B: Condensed Matter and Materials Physics (1982), 26 (8), 4199-228CODEN: PRBMDO; ISSN:0163-1829.

    Recent developments have enabled pseudopotential methods to reproduce accurately the results of all-electron calcns. for the self-consistent electronic structure of atoms, mols., and solids. The properties of these potentials are discussed in the context of earlier approaches, and their successful applications are summarized. Although the generation of these pseudopotentials from all-electron atom calcns. is straightforward in principle, detailed consideration of the differences in physics of various groups of atoms is necessary to achieve pseudopotentials with the most desirable attributes. One important attribute developed here is optimum transferability to various systems. Another is the ability to be fitted with a small set of analytic functions useful with a variety of wave-function representations. On the basis of these considerations, a consistent set of pseudopotentials is given for the entire Periodic Table. Relativistic effects are included in a way that enables the potentials to be used in nonrelativistic formulations. The scheme used to generate the numerical potentials, the fitting procedure, and the testing of the fit are discussed. Representative examples of potentials are shown that display attributes spanning the set. A complete tabulation of the fitted potentials is given along with a guide to its use.

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48. 48

    Longuinhos, R.; Ribeiro-Soares, J. Ultra-weak interlayer coupling in two-dimensional gallium selenide. Phys. Chem. Chem. Phys. 2016, 18, 25401– 25408,  DOI: 10.1039/C6CP03806A

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    Ultra-weak interlayer coupling in two-dimensional gallium selenide

    Longuinhos, R.; Ribeiro-Soares, J.

    Physical Chemistry Chemical Physics (2016), 18 (36), 25401-25408CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)

    Beyond-graphene 2-dimensional materials are envisioned as the future technol. for optoelectronics, and the study of group IIIA metal monochalcogenides (GIIIAMMs) in 2-dimensional form is an emerging research field. Bulk gallium selenide (GaSe) is a layered material of this family which is widely used in nonlinear optics and is promising as a lubricant. The interlayer coupling in few-layer GaSe is currently unknown, and the stability of different polytypes is unclear. Here the authors use symmetry arguments and first-principles calcns. to investigate the phase stability, interlayer coupling, and the Raman and IR activity of the low-frequency shear and breathing modes expected in few-layer GaSe. Strategies to distinguish the no. of layers and the β and ε polytypes are discussed. These symmetry results are valid for other isostructural few-layer GIIIAMM materials. Most importantly, by using a linear chain model, the authors show that the shear and breathing force consts. reveal an ultra-weak interlayer coupling at the nanoscale in GaSe. These results suggest that β and ε few-layer GaSe show similar lubricant properties to those obsd. for few-layer graphite. The authors anal. opens new perspectives about the study of interlayer interactions and their role in the mech. and elec. properties of these new 2-dimensional materials.

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49. 49

    Oliveira, V. V.; Leite, F. F.; Silva, F. W.; Oliveira, F. W.; Araujo, F. D.; Menezes, A. S.; Paraguassu, W.; Filho, A. G. S.; Viana, B. C.; Alencar, R. S. Temperature- and pressure-dependent phonon dynamics properties of gallium selenide telluride. J. Raman Spectrosc. 2022, 53, 1275– 1284,  DOI: 10.1002/jrs.6364

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    Temperature- and pressure-dependent phonon dynamics properties of gallium selenide telluride

    Oliveira, Victor V.; Leite, Fabio F.; Silva, Francisco W. N.; Oliveira, Francisco W. C.; Araujo, Francisco D. V.; Menezes, Alan S.; Paraguassu, W.; Souza Filho, Antonio G.; Viana, Bartolomeu C.; Alencar, Rafael S.

    Journal of Raman Spectroscopy (2022), 53 (7), 1275-1284CODEN: JRSPAF; ISSN:0377-0486. (John Wiley & Sons Ltd.)

    Understanding the thermodn. properties of materials is a fundamental issue in physics, and its knowledge is crucial for targeting a specific material for possible applications. In this work, we report a temp.- and pressure-dependent Raman study of bulk GaSe0.5Te0.5 alloy, besides their relevant thermodn. parameters. Our results show a nonlinear red shift for the A1g

    𝐴1𝑔$$A_{1g}$$

    and E2g

    𝐸2𝑔$$E_{2g}$$

    vibrational modes as the temp. increases in the temp. range from 10 to 748 K. Such behavior is well described by considering both thermal expansion and phonon-phonon coupling contributions. By combining d. functional theory (DFT) calcns. and Raman spectroscopy expts., the anharmonic consts. relative to the three- and four-phonon decay processes, mode-Grueneisen parameters, Debye temp., thermal expansion coeff., and bulk modulus were estd. for GaSe0.5Te0.5 alloy. Furthermore, the high-pressure measurements and DFT calcns., performed in the pressure range from 0 to 26.4 GPa, show a quadratic trend for the ωA1g

    𝜔𝐴1𝑔$${\omega }_{A_{1g}}$$

    and ωE2g

    𝜔𝐸2𝑔$${\omega }_{E_{2g}}$$

    modes as a function of pressure, with the A1g

    𝐴1𝑔$$A_{1g}$$

    modes being more compressible than E2g

    𝐸2𝑔$$E_{2g}$$

    one, i.e., .vdelta.ωA1g.vdelta.P>.vdelta.ωE2g.vdelta.P

    ∂𝜔𝐴1𝑔∂𝑃>∂𝜔𝐸2𝑔∂𝑃$$\frac{\mathrm{\partial }{\omega }_{A_{1g}}}{\mathrm{\partial }P}>\frac{\mathrm{\partial }{\omega }_{E_{2g}}}{\mathrm{\partial }P}$$

    . No structural phase transition is obsd. until the max. pressure reached in the expt. This study took a step forward in the understanding of mech. and thermal properties related to GaSe0.5Te0.5 alloy, whose detd. parameters are important for designing new applications.

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    Dai, Y.-H. Convergence Properties of the BFGS Algoritm. SIAM J. Optim. 2002, 13, 693– 701,  DOI: 10.1137/S1052623401383455

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    O’Donnell, K. P.; Chen, X. Temperature dependence of semiconductor band gaps. Appl. Phys. Lett. 1991, 58, 2924– 2926,  DOI: 10.1063/1.104723

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    Temperature dependence of semiconductor band gaps

    O'Donnell, K. P.; Chen, X.

    Applied Physics Letters (1991), 58 (25), 2924-6CODEN: APPLAB; ISSN:0003-6951.

    The application of a simple three-parameter fit to the temp. dependence of semiconductor band gaps is justified on both practical nd theor. grounds. In all trials the fit is numerically better than that obtained using the widely quoted Varshni equation. The formula is shown to be compatible with reasonable assumptions about the influence of phonons on the band-gap energy. Approx. anal. expressions are derived for the entropy and enthalpy of formation of electron-hole pairs in semiconductors.

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52. 52

    Muhimmah, L. C.; Ho, C. H. Dual phase two-color emission observed in van der Waals GaTe planes. Appl. Surf. Sci. 2021, 542, 148593  DOI: 10.1016/j.apsusc.2020.148593

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    Dual phase two-color emission observed in van der Waals GaTe planes

    Muhimmah, Luthviyah Choirotul; Ho, Ching-Hwa

    Applied Surface Science (2021), 542 (), 148593CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)

    After the eagerly study on two-dimensional (2D) electronic devices made by layered semiconductors, light-emitting devices based on 2D semiconductors have also flourished in recent years due to their special characters of flexibility, flatness surface, and thickness-tunable band gap modulation, etc. Among the 2D materials, III-VI layer compds. with hexagonal structure like GaSe and InSe are essentially direct semiconductors that own high band-edge luminescence efficiency and wide thickness tunable range from bulk to thin few-layer scale. In this work, dual-phase two-color emissions of multilayered gallium monotelluride positioned at 1.588 and 1.652 eV are simultaneously detected by micro-photoluminescence (μPL) measurement at 300 K. The lower-energy peak may originate from the hexagonal GaTe (H-GaTe) phase while the higher-energy luminescence might come from monoclinic GaTe (M-GaTe) phase and which are verified by micro-thermoreflectance (μTR) and high-resoln. transmission electron microscopy (HRTEM). Micro-time-resolved photoluminescence (TRPL) and area fluorescence lifetime mapping (AFLM) of the multilayer GaTe indicated that the luminescence decay time const. of the H-GaTe is larger than that of the M-GaTe for verification of the dominant phase in multilayered GaTe is the monoclinic phase. In-plane structural and optical anisotropy of the multilayered GaTe were characterized by using polarized micro-Raman and polarized μPL measurements. The results clearly indicate that the monoclinic-phase GaTe possesses higher polarized extinction ratio with respect to that of the hexagonal GaTe for fully extinction of the Raman vibration mode and band-edge emission in the dual-phase gallium telluride. The existence of dual cryst. phases in multilayer GaTe may render it possible for fabrication of two-color emission device available for near-IR (NIR) optical communication use.

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    Scrmid, P. H. VOITCIXOWKY: Electron-Lattice Interaction in GaSe 249 phys ; 1974.

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    Aydinli, A.; Gasanly, N. M.; Uka, A.; Efeoglu, H. Anharmonicity in GaTe layered crystals. Cryst. Res. Technol. 2002, 37, 1303– 1309,  DOI: 10.1002/crat.200290006

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    Gasanly, N. M.; Aydinli, A.; Özkan, H.; Kocabas, C. Temperature dependence of the first-order Raman scattering in GaS layered crystals. Solid State Commun. 2000, 116, 147– 151,  DOI: 10.1016/S0038-1098(00)00292-1

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    Temperature dependence of the first-order Raman scattering in GaS layered crystals

    Gasanly, N. M.; Aydinli, A.; Ozkan, H.; Kocabas, C.

    Solid State Communications (2000), 116 (3), 147-151CODEN: SSCOA4; ISSN:0038-1098. (Elsevier Science Ltd.)

    The temp. dependence (15-293 K) of the six Raman-active mode frequencies and linewidths in Ga sulfide was measured in the frequency range from 15 to 380 cm-1. The authors obsd. softening and broadening of the optical phonon lines with increasing temp. Comparison between the exptl. data and theories of the shift and broadening of the interlayer and intralayer phonon lines during the heating of the crystal showed that the exptl. dependencies can be explained by the contributions from thermal expansion and lattice anharmonicity. The pure-temp. contribution (phonon-phonon coupling) is due to three- and four-phonon processes.

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56. 56

    Luckert, F.; Yakushev, M. V.; Faugeras, C.; Karotki, A. V.; Mudryi, A. V.; Martin, R. W. Excitation power and temperature dependence of excitons in CuInSe2. J. Appl. Phys. 2012, 111, 093507  DOI: 10.1063/1.4709448

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    Excitation power and temperature dependence of excitons in CuInSe2

    Luckert, F.; Yakushev, M. V.; Faugeras, C.; Karotki, A. V.; Mudryi, A. V.; Martin, R. W.

    Journal of Applied Physics (2012), 111 (9), 093507/1-093507/8CODEN: JAPIAU; ISSN:0021-8979. (American Institute of Physics)

    Excitonic recombination processes in high quality CuInSe2 single crystals were studied by photoluminescence (PL) and reflectance spectroscopy as a function of excitation powers and temp. Excitation power dependent measurements confirm the identification of well-resolved A and B free excitons in the PL spectra and anal. of the temp. quenching of these lines provides values for activation energies. These vary from sample to sample, with values of 12.5 and 18.4 meV for the A and B excitons, resp., in the 1 showing the highest quality spectra. Anal. of the temp. and power dependent PL spectra from the bound excitonic lines, labeled M1, M2, and M3 appearing in multiplets points to a likely assignment of the hole involved in each case. The M1 excitons appear to involve a conduction band electron and a hole from the B valence band hole. But an A valence band hole appears to be involved for the M2 and M3 excitons. The M1 exciton multiplet seems to be due to the radiative recombination of excitons bound to shallow hydrogenic defects, whereas the excitons involved in M2 and M3 are bound to more complex defects. In contrast to the M1 exciton multiplet, the excitonic lines of M2 and M3 sat. at high excitation powers suggesting that the concn. of the defects involved is low. (c) 2012 American Institute of Physics.

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57. 57

    Gupta, J. D.; Jangra, P.; Majee, B. P.; Mishra, A. K. Morphological dependent exciton dynamics and thermal transport in MoSe2 films. Nanoscale Adv. 2023, 5, 2756– 2766,  DOI: 10.1039/D3NA00164D

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    Fang, Y.; Wang, L.; Sun, Q.; Lu, T.; Deng, Z.; Ma, Z.; Jiang, Y.; Jia, H.; Wang, W.; Zhou, J.; Chen, H. Investigation of temperature-dependent photoluminescence in multi-quantum wells. Sci. Rep. 2015, 5, 12718,  DOI: 10.1038/srep12718

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    Investigation of temperature-dependent photoluminescence in multi-quantum wells

    Fang, Yutao; Wang, Lu; Sun, Qingling; Lu, Taiping; Deng, Zhen; Ma, Ziguang; Jiang, Yang; Jia, Haiqiang; Wang, Wenxin; Zhou, Junming; Chen, Hong

    Scientific Reports (2015), 5 (), 12718CODEN: SRCEC3; ISSN:2045-2322. (Nature Publishing Group)

    Photoluminescence (PL) is a nondestructive and powerful method to investigate carrier recombination and transport characteristics in semiconductor materials. In this study, the temp. dependences of photoluminescence of GaAs-AlxGa1-xAs multi-quantum wells samples with and without p-n junction were measured under both resonant and non-resonant excitation modes. An obvious increase of photoluminescence(PL) intensity as the rising of temp. in low temp. range (T < 50 K), is obsd. only for GaAs-AlxGa1-xAs quantum wells sample with p-n junction under non-resonant excitation. The origin of the anomalous increase of integrated PL intensity proved to be assocd. with the enhancement of carrier drifting because of the increase of carrier mobility in the temp. range from 15 K to 100 K. For non-resonant excitation, carriers supplied from the barriers will influence the temp. dependence of integrated PL intensity of quantum wells, which makes the traditional methods to acquire photoluminescence characters from the temp. dependence of integrated PL intensity unavailable. For resonant excitation, carriers are generated only in the wells and the temp. dependence of integrated PL intensity is very suitable to anal. the photoluminescence characters of quantum wells.

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59. 59

    Nayak, A. P.; Pandey, T.; Voiry, D.; Liu, J.; Moran, S. T.; Sharma, A.; Tan, C.; Chen, C.-H.; Li, L.-J.; Chhowalla, M. Pressure-Dependent Optical and Vibrational Properties of Monolayer Molybdenum Disulfide. Nano Lett. 2015, 15, 346– 353,  DOI: 10.1021/nl5036397

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    Pressure-Dependent Optical and Vibrational Properties of Monolayer Molybdenum Disulfide

    Nayak, Avinash P.; Pandey, Tribhuwan; Voiry, Damien; Liu, Jin; Moran, Samuel T.; Sharma, Ankit; Tan, Cheng; Chen, Chang-Hsiao; Li, Lain-Jong; Chhowalla, Manish; Lin, Jung-Fu; Singh, Abhishek K.; Akinwande, Deji

    Nano Letters (2015), 15 (1), 346-353CODEN: NALEFD; ISSN:1530-6984. (American Chemical Society)

    Controlling the band gap by tuning the lattice structure through pressure engineering is a relatively new route for tailoring the optoelectronic properties of 2-dimensional (2D) materials. Here, the authors study the electronic structure and lattice vibrational dynamics of the distorted monolayer 1T-MoS2 (1T') and the monolayer 2H-MoS2 via a diamond anvil cell (DAC) and d. functional theory (DFT) calcns. The direct optical band gap of the monolayer 2H-MoS2 increases by 11.7% from 1.85 to 2.08 eV, which is the highest reported for a 2-dimensional transition metal dichalcogenide (TMD) material. DFT calcns. reveal a subsequent decrease in the band gap with eventual metalization of the monolayer 2H-MoS2, an overall complex structure-property relation due to the rich band structure of MoS2. Remarkably, the metastable 1T'-MoS2 metallic state remains invariant with pressure, with the J2, A1g, and E2g modes becoming dominant at high pressures. This substantial reversible tunability of the electronic and vibrational properties of the MoS2 family can be extended to other 2-dimensional TMDs. These results present an important advance toward controlling the band structure and optoelectronic properties of monolayer MoS2 via pressure, which has vital implications for enhanced device applications.

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60. 60

    Ye, Y.; Dou, X.; Ding, K.; Jiang, D.; Yang, F.; Sun, B. Pressure-induced K- crossing in monolayer WSe2. Nanoscale 2016, 8, 10843– 10848,  DOI: 10.1039/C6NR02690G

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    Segura, A.; Manjón, F. J.; Errandonea, D.; Pellicer-Porres, J.; Muñoz, V.; Tobias, G.; Ordejón, P.; Canadell, E.; San Miguel, A.; Sánchez-Portal, D. Specific features of the electronic structure of III–VI layered semiconductors: recent results on structural and optical measurements under pressure and electronic structure calculations. Phys. Status Solidi B 2003, 235, 267– 276,  DOI: 10.1002/pssb.200301567

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    Pellicer-Porres, J.; Segura, A.; Ferrer, C.; Muñoz, V.; San Miguel, A.; Polian, A.; Itié, J. P.; Gauthier, M.; Pascarelli, S. High-pressure x-ray-absorption study of GaSe. Phys. Rev. B 2002, 65, 174103  DOI: 10.1103/PhysRevB.65.174103

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    Pellicer-Porres, J.; Segura, A.; Muñoz, V.; San Miguel, A. High-pressure x-ray absorption study of GaTe including polarization. Phys. Rev. B 2000, 61, 125– 131,  DOI: 10.1103/PhysRevB.61.125

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