Biaxial strain tuned upconversion photoluminescence of monolayer WS2

Monolayer tungsten disulfide (1L-WS2) is a direct bandgap atomic-layered semiconductor material with strain tunable optical and optoelectronic properties among the monolayer transition metal dichalcogenides (1L-TMDs). Here, we demonstrate biaxial strain tuned upconversion photoluminescence (UPL) from exfoliated 1L-WS2 flakes transferred on a flexible polycarbonate cruciform substrate. When the biaxial strain applied to 1L-WS2 increases from 0 to 0.51%, it is observed that the UPL peak position is redshifted by up to 60 nm/% strain, while the UPL intensity exhibits exponential growth with the upconversion energy difference varying from − 303 to − 120 meV. The measured power dependence of UPL from 1L-WS2 under biaxial strain reveals the one photon involved multiphonon-mediated upconversion mechanism. The demonstrated results provide new opportunities in advancing TMD-based optical upconversion devices for future flexible photonics and optoelectronics.


Results and discussion
The optical reflection microscope image of the mechanically exfoliated 1L-WS 2 flake transferred on flexible PC cruciform substrate is displayed in Fig. 1a. Figure 1b shows the schematic diagram of the biaxial strain apparatus with the bending and indentation method 37,41 , where the biaxial strain is applied to 1L-WS 2 flake transferred at the center area of PC cruciform substrate pinned at its four edges by metal rods.The PC cruciform substrate is inserted between the rod assembly and the central sphere indenter.As the central sphere indenter deflects the substrate upwards, the equi-biaxial tensile strain is applied on the top surface of the substrate.The biaxial strain applied to the 1L-WS 2 flake is described by ε = 3Dt/L 2 , where L is the distance between the two opposite pin edges (L = 25.4 mm), t is the thickness of the PC cruciform substrate (t = 0.25 mm), and D is the displacement of the central sphere indenter.
The biaxial strain tuned PL spectra excited at 532 nm and UPL spectra excited at 685 nm and 725 nm for the 1L-WS 2 sample at room temperature are presented in Fig. 2. In Fig. 2a, the PL peak position is observed at around 619 nm without strain, which matches with the typical PL spectrum of 1L-WS 2 [1][2][3][4][5] .The PL peak position continuously gets redshifted as the biaxial strain increases from 0 to 0.51%, with approximately 4 times    enhancement in the PL intensity.In Fig. 2b,c, the UPL peak positions without strain under the excitation wavelengths of 685 nm and 725 nm are around 616 nm, which is consistent with the PL peak position.It is observed that the UPL peak positions are also redshifted under the influence of increasing levels of applied biaxial strain, while the enhancement of UPL intensity is nearly 15 and 5 times under the excitation wavelengths of 685 nm and 725 nm, respectively.It is noted that the UPL intensity at 0.51% strain under 725 nm excitation is slightly lower compared to that at 0.435% strain, which may be resulted from the change in sample condition after several cycles of applied strain.Figure 3a summarizes the PL and UPL peak positions as a function of the biaxial strain applied to 1L-WS 2 under different excitation wavelengths, showing a linear dependence of the peak position shift on the strain level.The PL peak position redshift of 55 nm/% strain is obtained at the excitation of 532 nm with the strain tuning gauge factor of 163 meV/% strain, which is similar to the results reported in the previous work 37,42 .The UPL peak position redshifts at the excitations of 685 nm and 725 nm are 54 nm/% strain and 60 nm/% strain, corresponding to the strain tuning gauge factors of 163 meV/% strain and 179 meV/% strain, respectively.The slight difference between the strain tuning gauge factors at different excitation wavelengths is due to different strain transfer conditions between 1L-WS 2 and substrate during each cycle of applied strain 30 .For the biaxial strain varied from 0 to 0.51%, the upconversion energy difference between the excitation photon energy and the UPL emission energy ΔE = ℏω ex.− ℏω UPL is tuned from − 205 to − 120 meV at the excitation of 685 nm, and from − 303 to − 209 meV at the excitation of 725 nm. Figure 3b shows the dependence of the integrated UPL intensity from 1L-WS 2 on the strain tuned upconversion energy difference in the wide range of − 303 to − 120 meV under the excitations of 685 nm and 725 nm at room temperature.It shows that the UPL intensity follows an exponential growth function with the upconversion energy difference, with the enhancement of UPL intensity in nearly 75 times.The UPL intensity can be described by the Boltzmann function as I UPL ∝ exp(−|�E|/k B T) , where | E| is the upconversion energy gain, k B is the Boltzmann constant, and T is the room temperature at 298 K.The effec- tive phonon number involved in the multiphonon-assisted upconversion process in 1L-WS 2 is approximately estimated from 6 to 2, based on the ratio between |ΔE| from 303 to 120 meV and the phonon of 52 meV for the A 1g transverse optical phonon in 1L-WS 2 .
The power dependent PL and UPL intensities from 1L-WS 2 excited at 532 nm, 685 nm, and 725 nm are plotted in a log-log scale in Fig. 4a,b at the biaxial strain of 0% and 0.51%, respectively.The power law of I = αP β is used to fit the PL and UPL intensities, where P is the excitation power, α is the fitting parameter, and β is the exponent.It is observed that the β values for PL emission at both strain levels show a sublinear power dependence, which is due to the presence of nonradiative exciton-exciton annihilation at high excitation intensity.The β values for UPL emission at 0% strain also exhibit a sublinear power dependence at the excitations of 685 nm and 725 nm.At 0.51% strain, the β values increase to around 0.9, which is close to linear power dependence.The sublinear power dependence for the UPL emission may be related to the change of densities of phonons and exciton complexes 9 .The power dependence of UPL emission from 1L-WS 2 at both strain levels further indicates the one photon involved upconversion photon emission process assisted by multiphonon absorption, rather than any nonlinear optical generation of the upconversion like exciton Auger scattering or two-photon excitation-induced emission 9,16 .
The biaxial strain dependent Raman spectra of 1L-WS 2 excited at 532 nm are presented in Fig. 5a.The Raman spectrum of 1L-WS 2 at 0% strain (black curve) clearly shows two characteristic peaks that correspond to the inplane longitudinal optical phonon mode (E 1 2g ) and the second-order acoustic mode [2LA(M)] at the Raman shift of ~ 352 cm −1 , and the out-of-plane transverse optical phonon mode (A 1g ) at 419 cm −1 , which are consistent with the previously reported Raman modes of 1L-WS 2 [1][2][3][4][5]43 . It s noteworthy that the observed multiphonon-assisted UPL process is mediated by the out-of-plane A 1g transverse optical phonon mode, rather than the in-plane E 1 2g longitudinal optical phonon mode 9,16 .When the biaxial strain gradually increases from 0 to 0.51%, the Raman www.nature.com/scientificreports/peak positions of A 1g and 2LA(M) + E 1 2g vibrational modes continuously get redshifted by approximately 1.8 cm −1 and 5 cm −1 respectively, which is resulted from the strain induced crystal symmetry breaking and vibration softening as well as the high strain sensitivity of E 1 2g mode 31,[44][45][46] .Figure 5b plots the linear dependence of the Raman peak positions of A 1g and 2LA(M) + E 1 2g modes on the biaxial strain, showing the redshifts of 3.5 cm −1 /% strain and 9.99 cm −1 /% strain, respectively.The Raman peak position of A 1g mode is nearly three times less sensitive to the applied strain compared to 2LA(M) + E 1 2g mode, giving only slight change in the optical phonon energy around 52 meV during the strain tuned UPL emission from 1L-WS 2 .It is noted that instead of its low strain sensitivity, the A 1g mode of 1L-WS 2 is highly sensitive to the electronic changes like electrostatic doping 45,46 .The overall Raman intensity is increased about 3.5 times at 0.51% strain compared to that at 0% strain.

Conclusion
We have demonstrated the biaxial strain tuning of UPL emission from mechanically exfoliated 1L-WS 2 on flexible PC cruciform substrate by utilizing the bending and indentation method.The UPL peak positions get redshifted by 54 nm/% strain and 60 nm/% strain under the excitation wavelengths of 685 nm and 725 nm as the biaxial strain varies from 0% to 0.51%, which provide the strain tuning gauge factors of 163 meV/% strain and 179 meV/% strain, respectively.The strain tuned UPL intensity follows an exponential function with the upconversion energy difference in a range of − 303 to − 120 meV, with the UPL intensity enhancement of nearly 75 times.The power dependence of UPL emission from 1L-WS 2 under biaxial strain indicates the one photon involved upconversion process mediated by multiphonon absorption.The demonstrated results will enable new opportunities in promoting TMD-based tunable photon upconversion devices used for many applications such as infrared sensing, imaging, night vision, and flexible optoelectronics.

Figure 1 .
Figure 1.(a) Optical image of an exfoliated 1L-WS 2 flake transferred on flexible PC cruciform substrate.(b) Schematic diagram for the biaxial strain apparatus with the bending and indentation method.

6Figure 3 .
Figure 3. (a) PL and UPL peak positions as a function of the biaxial strain applied to 1L-WS 2 under different excitation wavelengths.(b) Dependence of the integrated UPL intensity on the strain tuned upconversion energy difference under the excitations of 685 nm and 725 nm.

Figure 4 .Figure 5 .
Figure 4. Power dependent integrated PL and UPL intensities in a log-log scale under the excitations of 532 nm, 685 nm, and 725 nm at the biaxial strain of (a) 0% and (b) 0.51%.