Chin. Phys. B, 2020, Vol. 29(11): 116802    DOI: 10.1088/1674-1056/abb310
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# Interfaces between MoOx and MoX2 (X = S, Se, and Te)

Fengming Chen(陈凤鸣)1, Jinxin Liu(刘金鑫)1, Xiaoming Zheng(郑晓明)1, Longhui Liu(刘龙慧)1, Haipeng Xie(谢海鹏)1, Fei Song(宋飞)2, Yongli Gao(高永立)3,1, and Han Huang(黄寒)1, †
1 Hunan Key Laboratory of Super-microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha 410083, China
2 Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201204, China
3 Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
 Abstract  In the past decades there have been many breakthroughs in low-dimensional materials, especially in two-dimensional (2D) atomically thin crystals like graphene. As structural analogues of graphene but with a sizeable band gap, monolayers of atomically thin transition metal dichalcogenides (with formula of MX2, M = Mo, W; X = S, Se, Te, etc.) have emerged as the ideal 2D prototypes for exploring fundamentals in physics such as valleytronics due to the quantum confinement effects, and for engineering a wide range of nanoelectronic, optoelectronic, and photocatalytic applications. Transition metal trioxides as promising materials with low evaporation temperature, high work function, and inertness to air have been widely used in the fabrication and modification of MX2. In this review, we reported the fabrications of one-dimensional MoS2 wrapped MoO2 single crystals with varied crystal direction via atmospheric pressure chemical vapor deposition method and of 2D MoOx covered MoX2 by means of exposing MoX2 to ultraviolet ozone. The prototype devices show good performances. The approaches are common to other transition metal dichalcogenides and transition metal oxides. Keywords:  MoOx      epitaxial relationships      MoX2      layer-by-layer oxidation Received:  08 July 2020      Revised:  10 August 2020      Accepted manuscript online:  27 August 2020 Fund: the National Natural Science Foundation of China (Grant No. 11874427), the National Science Foundation DMR-1903962, and the Fundamental Research Funds for the Central Universities of Central South University (Grant No. 2019zzts429). Corresponding Authors:  †Corresponding author. E-mail: physhh@csu.edu.cn

 Fig. 1.  Crystal structures of MoS2 (a) and MoO2 (b). (c) Schematic of transition between MoOx and MoX2. Fig. 2.  Morphologies. Schematic description of the crystalline structure at the surfaces of (a) c-sapphire, (b) m-sapphire, and (c) a-sapphire. Al atoms and O atoms are represented by purple and red balls, respectively. The unit cells and lattice constants are marked. The representative OM images of nanorods on (d) c-sapphire, (e) m-sapphire, and (f) a-sapphire.[80] Fig. 3.  Compositions. (a) Typical Raman spectra of as-made c- and m-MoO2@MoS2 nanorods, nanorods, and MoS2. (b) Raman map of m-nanorods at 207 cm−1. Raman maps of c-nanorods at (c) 201 cm−1 and (d) 400 cm−1.[79,80] Fig. 4.  Global structural properties. (a) XRD patterns of c- (black) and m-nanorods (blue). (b) 2D-GIXRD pattern of c-nanorods.[79] Fig. 5.  Local structural properties. Cross-sectional SEM image of single (a) c-nanorod and (b) m-nanorod. (c) Low magnification image of c-nanorod. The insets show the HRTEM image in [201] zone and the corresponding SAED pattern. (d) Low-magnification TEM image of m-nanorod. Inset: SAED patterns of individual nanorod and enlarged high-resolution TEM image. The EDS pattern of the individual (e) c-nanorod and (f) m-nanorod.[79,80] Fig. 6.  Three-dimension growth models of (a) m-nanorod and (b) c-nanorod.[80] Fig. 7.  Electrical properties. (a) V23–I14 curve of individual c-nanorod. Inset: SEM image of the device. (b) I–V characteristics measured as a function of the ambient temperature. Inset: Resistance of the channel versus ambient temperature. (c) I14 as a function of V23. (d) I–V curve (solid black line) and conductivity curve (dotted blue line) of individual m-nanorod. Inset: The SEM image of the device.[80,81] Fig. 8.  MoS2 nanoribbons by PMMA assisted decoupling. (a) Schematic diagram of the proposed transfer progress. (b) AFM topography of transferred MoS2 flake and nanoribbon. Inset: Height profiles of line scans marked in (b). (c) PL spectra of the MoS2 flake, MoO2@MoS2 nanorod, and transferred MoS2 nanoribbon. (d) Schematic representation of the formation of MoS2 nanoribbons on SiO2/Si.[82] Fig. 9.  CVD grown MoS2 nanoribbons without catalysts. (a) The Raman ${{\rm{E}}}_{2{\rm{g}}}^{1}$ and (b) PL A intensity maps of MoS2 nanoribbons. (c) Low magnification TEM image of transferred MoS2 nanoribbons. (d) ADF-STEM image of MoS2 nanoribbons in the [0001] zone.[83] Fig. 10.  Layer-by-layer oxidation of few-layer MoTe2 using UVO. (a) Raman spectra (laser wavelength 532 nm) of tetralayer MoTe2 upon intermittent UVO exposure from 0 to 50 min. The inset shows the intensity ratio of ${{\rm{B}}}_{2{\rm{g}}}^{1}/{{\rm{E}}}_{2{\rm{g}}}^{1}$. (b) The corresponding Raman mapping of ${{\rm{B}}}_{2{\rm{g}}}^{1}$ and ${{\rm{E}}}_{2{\rm{g}}}^{1}$ modes. The scale bar is 5 μm. (c) Extracted parameters as a function of UVO treatment time: mobility μ (left axis) and hole concentration nh (right axis) in logarithmic coordinates. (d) The corresponding band diagram.[84,86] Fig. 11.  (a)–(b) Extracted effective barrier height φB of pristine and UVO treatment MoTe2 FETs as a function of Vg. Insets in panels (a) and (b): Energy band structures between metal contacts and MoTe2 before and after MoOx doping. EF is the Fermi level energy. EC and EV represent the minimum energy of conduction band and the maximum energy of valance band, respectively. ΦSB and WSB are the Schottky barrier height and width, respectively. The Schottky barrier formed at metal/MoTe2 interface is compressed for hole doped MoTe2 device, facilitating the tunneling transport of hole.[86] Fig. 12.  Lateral MoSe2 p–n junction. (a) The cross section schematic structure of the lateral p–n junction device. (b) The optical photograph of p–n device. (c) Id–Vd curve of the MoSe2 diode in linear (black line) and log (red) scale at Vg = 0 V. The inset shows the transfer characteristic Id–Vg curve of the MoSe2 diode at Vd = 1 V. (d) Dynamic optical response of the device (Vd = 0 V, Vg = 0 V) under illumination of light of 450 nm, 520 nm, 633 nm.[85]