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Letter
Correlatively Dependent Lattice and Electronic Structural Evolutions in Compressed Monolayer Tungsten Disulfide Bo Han, Fangfei Li, Liang Li, Xiaoli Huang, Yuan-Bo Gong, Xinpeng Fu, Hanxue Gao, Qiang Zhou, and Tian Cui J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00133 • Publication Date (Web): 08 Feb 2017 Downloaded from http://pubs.acs.org on February 11, 2017
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The Journal of Physical Chemistry Letters
Correlatively Dependent Lattice and Electronic Structural Evolutions in Compressed Monolayer Tungsten Disulfide Bo Han, Fangfei Li*, Liang Li, Xiaoli Huang, Yuanbo Gong, Xinpeng Fu, Hanxue Gao, Qiang Zhou* and Tian Cui
State Key Laboratory of Superhard Materials, College of Physics, Jilin University, Changchun 130012, China
Contributions
Bo Han and Fangfei Li finished the experiments and analyzed data. Xiaoli Huang and Yuanbo Gong assisted in designing experiments. Liang Li, Xinpeng Fu and Hanxue Gao assisted in sample preparation. Bo Han, Fangfei Li, Qiang Zhou and Tian Cui wrote this manuscript. Corresponding Author *To
whom
correspondence
should
be
addressed.
Fangfei
[email protected]. Qiang Zhou, E-mail:
[email protected].
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Li,
E-mail:
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ABSTRACT Transition metal dichalcogenides (TMDs) are promising materials for optoelectronic devices. Their lattice and electronic structural evolutions under high strain conditions and their relations remain open questions. We exert pressure on WS2 monolayers on different substrates, namely Si/SiO2 substrate and diamond anvil surface up to ~25 GPa. Structural distortions in various degree are disclosed based on the emergence of Raman-inactive B mode. Splits of out-of-plane B and A1’ modes are only observed on Si/SiO2 substrate, due to extra strain imported from volume decrease of Si and corrugation of SiO2 surface, and its PL quenches quickly because of decreased K-K transition by conspicuous distortion of Brillouin zone. While diamond anvil surface provides better hydrostatic environment, combined analysis of PL and absorption proves that pressure effectively tunes PL emission energy and enhances Coulomb interactions. Knowledge of these distinct pressure tunable characteristics of monolayer WS2 improves further understanding of structural and optical properties of TMDs. TOC Graphic
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2H phase layered transition metal dichalcogenides (2H-TMDs) have attracted considerable attentions during the last decade due to their attributes in mechanics and optoelectronics. Unlike the zero gap graphene, they embrace unique band structure that can transform from indirect band gap in bulk to direct band gap in monolayer TMDs.1-4 Recently, based on spin-orbital coupling (SOC) and inversion symmetry breaking of 2H-TMD monolayers, selective excitation by circularly polarized light can be realized at K (or K’) valleys in the Brillouin zone (BZ), initiating a new field of valleytronics and spintronics.5-7 Owing to potential optoelectronic applications from solar cells and light emitting diodes to optical interconnects and quantum logical devices, the behaviors of neutral exciton as well as trion (charged exciton) are also investigated.8-9 Of the TMDs monolayer, MoS2 is mostly investigated based on aforementioned distinct characteristics. Similar to MoS2, the monolayer WS2 also exhibits high in-plane carrier mobility, electrostatic modulation of conductance and even greater SOC splitting due to the larger size of tungsten atoms. Besides, the WS2 monolayer possesses the highest quantum yield and the largest photoluminescence (PL) emission energy (640 nm) compared to others including MoS2(677 nm)1, WSe2(750 nm)10 and MoSe2(840 nm)11, rendering it a promising material for optoelectronic device. In order to modify or tune the electronic properties of WS2, several methods such as intercalation, layer confinement, chemical vapor growth, dual gating and strain have been explored experimentally.12-16 Appling hydrostatic pressure may effectively change the structural, vibrational
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and electrical properties in bulk TMDs17-18, as well as tune the lattice structure to tailor the optoelectronic properties in few-layered TMDs. Compared to the extensive researches of pressure induced structural and electronic properties transition in MoS2 and MoSe2, there are few investigations reported on WS2. The bulk WS2 evolves from semiconductor to metallic phase at 280 K around 22 GPa evidenced by pressure modulation of conductivity, carrier density and mobility, whereas no structural transition is found in bulk WS2 up to 36 GPa either from X-ray diffraction or lattice vibration measurements.19-21 For monolayer 2H-WS2, it has a direct band gap around 2.7 eV with a SOC splitting around 0.4 eV on the top of valance band where A and B excitons generate.2,
22-23
The pressure tunable lattice and electronic structural
evolutions in monolayer 2H-WS2 require systemic and in-depth investigation but still not disclosed so far. Keeping these questions in mind and in order to explore the correlation between lattice and electronic structural changes, we exert hydrostatic pressure on monolayer 2H-WS2 in a diamond anvil cell (DAC) and perform in situ Raman and PL spectrum measurements. Through the comparative pressure response of monolayer 2H-WS2 on different substrate i.e. Si/SiO2 (300 nm) and diamond anvil surface, we found the disorder-induced structural distortion by the appearance of Raman-inactive B mode, enlargement of PL emission energy and non-radiative exciton to trion transition resulting from enhanced coulomb interactions under pressure.
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Figure 1. a) PL of few-layered WS2 marked by layer numbers on sample photo; inset: Lorentz fittings centering at 1.95 eV and 1.915 eV for the PL of A exciton and A- trion in monolayer WS2 (area.1). b) Raman spectra normalized to own 2LA(M) under parallel polarization; the left histogram is intensity ratio
for few-layered WS2, which is used here as criteria to
determine the layer number. c) Lorentzian fitting of Raman spectrum for the monolayer (area.1) taken under perpendicular polarization. A new mode around 310 cm-1 appears; insets: Raman-active modes E2g1, A1g and Raman-inactive mode B1u in bilayer WS2 and corresponding Raman-active modes E’ and A1’ in WS2 monolayer.
WS2 monolayers were firstly synthesized on a Si/SiO2 (300 nm) substrate by chemical vapor deposition (CVD) method (Supporting Information, Section 1 for details). Figure 1 shows the Raman and PL spectra of CVD grown few-layered WS2 at ambient conditions. The numbers marked on the sample photo indicate layer number of synthesized WS2, which is determined based on the intensity ratio between 2LA(M) and A1g(Г) modes from Raman spectra.
is around 5 for monolayer, 2 for
bilayers and 1 for quadlayer (Figure 1b). It drops to the minimum (
