Correlatively Dependent Lattice and Electronic Structural Evolutions in Compressed Monolayer Tungsten Disulfide

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 WS monolayers on different substrates, namely, Si/SiO 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 A' modes are only observed on Si/SiO substrate due to extra strain imported from volume decrease in Si and corrugation of SiO surface, and its photoluminescence (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 WS improves further understanding of structural and optical properties of TMDs.

On the impact of the stress situation on the optical properties of $WSe_2$ monolayers under high pressure

“…Whereas for the hBN/WSe 2 -ML/hBN heterostructure both exciton energies increase linearly with increasing pressure with a coefficient of 3.5-3.8 meV/GPa, for the bare WSe 2 ML the excitons decrease in energy with a slope of -3.1 and -1.3 meV/GPa for the A-and B-exciton, respectively. Comparing with the available literature data, the pressure coefficient of the A-exciton determined here for the encapsulated WSe 2 ML is roughly one order of magnitude smaller than the one reported for a single ML of MoS 2 (20 meV/GPa [14], 30 meV/GPa [15], 40 meV/GPa [16], 50 meV/GPa [17]), WSe 2 (32 meV/GPa [19]) and WS 2 (20 meV/GPa [20]). All these data were obtained for monolayers on Si/SiO 2 substrates, where the oxide layer was fairly thick, ranging between 200 and 300 nm.…”

“…All these data were obtained for monolayers on Si/SiO 2 substrates, where the oxide layer was fairly thick, ranging between 200 and 300 nm. In the work of Han et al [20], the same experiment is reported for a WS 2 ML exfoliated directly onto the diamond and a much smaller pressure coefficient is found (10 meV/GPa). In the next section, we suggest a possible way to explain such a large disparity in the high pressure results concerning the excitonic properties of single MLs on the basis of a close inspection of the stress-strain relations.…”

“…Because the bands at different points of the Brillouin zone shift in energy with different pressure coefficients, high pressure experiments can be used to distinguish between k-space direct and indirect transitions. As far as truly two-dimensional (2D) TMD materials are concerned, there are already several reports on the dependence on hydrostatic pressure of the emission and vibrational properties of monolayers of MoS 2 [14][15][16][17][18], WSe 2 [19] and WS 2 [20]. The results, however, are surprisingly contradictory.…”

“…All studies were performed on MLs transferred or deposited on SiO 2 /Si wafers, except for the work of Ref. [20], in which authors also looked at the pressure behavior of the ML directly on the diamond anvil surface. We conducted high-pressure opticalspectroscopy experiments on WSe 2 MLs on the diamond anvil and encapsulated by hBN layers for comparison.…”

Photoluminescence and Raman Spectra Oscillations Induced by Laser Interference in Annealing‐Created Monolayer WS2 Bubbles

“…Due to these superior properties, monolayer WS 2 has shown promising applications in electronics, spintronics, valleytronics, and optoelectronic, such as field‐effect transistors, photodetectors, magnetoluminescence devices, and valleytronic devices . Monolayer WS 2 crystal, whose bandgap and lattice vibrations are highly sensitive to external conditions like strain, pressure, temperature, and electric field, is also a perfect model material for PL and Raman studies . Although numerous studies and research on atomically thin WS 2 crystals have been carried out, there has been no experimental observation on WS 2 bubble so far, much less its properties.…”