Transition metal dichalcogenides (TMDs), as a typical class of two-dimensional (2D) materials similar to graphene and black phosphorus, have attracted much attention because of their physical properties and promising potential applications in the field of electronics and optoelectronics.^[1–7] Due to the quasi-2D nature of their lattice, they possess unique electronic band structure that can be effectively tuned by methods such as layer-dependence controlling, intercalating a variety of organic molecules, alkali or transition metals between the layered structures, applying pressure, mechanical strain or external electric fields on these materials.^[6,8–18] For example, Mak et al. showed that decreasing the thickness of MoS₂ induces a crossover of the bandgap from indirect one in bulk material to a direct-gap in monolayer, which causes a strong enhancement of light emission as well as an increase of luminescence quantum efficiency by more than 4 orders of magnitude.^[8] In addition, the intercalation of C₃H₅NH₂ molecules between layers of NbSe₂ behaves as electron donors and increases interlayer separations, resulting in profound changes in the electronic structure of the host lattice to induce an obvious increase of optical absorption.^[11]

A computational study revealed that the electron mobilities of PtS₂ can be as high as ∼ 4000 cm²/V·s, which is among the highest in the selected TMDs.^[4,5] Besides, PtS₂ has a large tunable bandgap, ranging from 1.6 eV in monolayer, 1.2 eV in the bilayer, and ultimately to 0.25 eV in the bulk PtS₂.^[6] Recently, it was also shown that PtS₂ exhibits excellent optoelectronic performance with high responsivity up to 1.56 × 10³ A/W and high detectivity of 2.9 × 10¹¹ Jones.^[9] Density functional theory (DFT) calculations predicted that, when under a mechanical strain, the indirect bandgap of monolayer PtS₂ becomes quasi-direct band-gap, which would be further in favor of the efficiency of light absorption and emission.^[16]

In this paper, we employ high pressure means to manipulate the optoelectronic properties of PtS₂. Experimentally, it has been shown that the high pressure indeed plays an effective role in tuning the electronic properties of TMDs.^[19–21] We find that the photocurrent shows weak pressure dependence below 3 GPa but increases rapidly until a maximum at ∼ 4 GPa, of ∼ 6 times higher than that at ambient pressure. Further increase of pressure results in a metallization and hence a sharp drop of photocurrent at 5.6 GPa–6 GPa. X-ray diffraction patterns reveal that the lattice structure of PtS₂ is ultra-stable under pressure, without observation of structural phase transition up to 26.8 GPa.

High-quality PtS₂ single crystals were grown by chemical vapor transport method using phosphorus as transporting agent.^[9] The mixture of Pt, S, and P powers in stoichiometric ratio 1:3:1 was placed inside an evacuated and sealed quartz tube. The tube was slowly heated up to 85 °C on one end while the other end was kept at 75 °C. After 20 days of growth, plate-like crystals were obtained on the cold end. High-pressure photocurrent and resistance were performed in a screw-pressure-type diamond anvil cell made of BeCu alloy which was measured in High Magnetic Field Laboratory of the Chinese Academy of Sciences. Diamond anvils of 300-μm culets and a T301 stainless-steel gasket covered with a mixture of epoxy and fine cubic boron nitride (c-BN) powder were used for high-pressure transport measurements. The two-probe method was applied in the ab plane of single crystals with typical dimensions of 70 μm × 40 μm × 10 μm. The high-pressure photocurrent was measured under the illumination of a 532-nm excitation laser with power of 8.5 mW. The in situ high-pressure synchrotron radiation x-ray diffraction (XRD) (λ = 0.3100 Å) experiments were carried out at 16-BM-D station of High-Pressure Collaborative Access at Advanced Photo Source of Argonne National Laboratory.^[22] The DIOPTAS and RIETICA programs were employed for the image integrations and the XRD profile refinements, respectively.^[23,24] The LeBail method was used to extract the lattice parameters. High-pressure Raman scattering experiments were carried out using a Raman spectrometer with a 532-nm excitation laser. The ruby fluorescence method was used to determine the pressure for all of the above experiments.^[25]

A PtS₂-based device was fabricated for investigating the optoelectronic properties and transport properties under pressures. The sample configuration is illustrated in Fig. 1(a), where Au/Ti top electrodes were patterned via thermal evaporation. Figure 1(b) shows that all the I–V curves measured under different pressures exhibit linear behaviors, indicative of good Ohmic contacts between the sample and Au/Ti electrodes. Room-temperature electrical resistance was extracted from the slopes of the I–V curves and displayed as a function of applied pressure in Fig. 1(c). One can see that the resistance decreases monotonically with increasing pressure, which was well reproducible for compression and decompression procedures. The decrease of resistance under high pressure can be attributed to a gradually enhanced overlap of the electron wave functions during the compression of the lattice. When increasing the pressure to 6 GPa, the temperature-dependent resistance of PtS₂ device shows a metallic conduction behavior at 6 GPa, in contrast to that of the semiconducting behavior under ambient pressure condition as shown in Fig. 1(d).

Fig. 1.

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Fig. 1. (a) Schematic illustration of the structure of fabricated PtS2 device. (b) The I–V curves of the PtS2 device measured at room temperature under different applied pressures. (c) Pressure dependence of the room-temperature resistance of the PtS2 device. Inset displays a microphotograph of the sample configuration in a DAC. (d) Temperature dependence of resistance measured under pressures of 1 atm (1 atm = 1.01325 × 105 Pa). and 6 GPa.

In order to examine the pressure effect on the optoelectronic properties of the PtS₂ device, we measured the time-dependent current I–t under different applied pressures, by intentionally switching ON/OFF the incident light of 532-nm laser illumination. The current was measured under a bias voltage of 0.1 V, in a linear range of I(V_bias) (see Fig. 2(b)). From Fig. 2(a), which displays the I–t curves under selected pressures, one can clearly see that the current I exhibits instant responses to the switching ON and OFF of the illumination. Zoom-in of a typical I–t pulse in Fig. 2(c) shows that the rise time and fall time, determined by the acquisition rate, are of ∼ 0.2 seconds. This implies a real response time shorter than 0.2 seconds.

Fig. 2.

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	Fig. 2. (a) Photocurrent of the PtS2 device measured under applied pressures of 1.8 GPa, 3.2 GPa, and 4.2 GPa. (b) The photocurrent as a function of bias-voltage under pressures of 1.4 GPa and 3.2 GPa. (c) Time-resolved photoresponse recorded at Vds = 0.1 V. (d) The photocurrent as a function of applied pressure in compression (com.) and decompression (decom.) processes.

The values of photocurrent I_ph = I_illumination – I_dark under different pressures are extracted from the I–t pulses and then represented as a function of applied pressure in Fig. 2(d). It is found that the photocurrent I_ph shows weak pressure dependence below 3 GPa and then increase with pressure rapidly until a maximum at around 4 GPa, where the I_ph is of ∼ 6 times larger than that at ambient pressure. Further increase of the pressure results in a sharp drop of I_ph. We attribute such drop to the metallization of the system under high pressure, which is revealed by the pressure-dependent resistance measurements. The difference of I_ph between compression and decompression processes is ascribed to the hysteresis effect caused by the reduced pressure response of the lattice in decompression process, after the sample experienced an application of high pressure.

In some materials, such as ferroelectric photovoltaics KBiFe₂O₅, organic perovskite CH₃NH₃PbBr₃ and CH₃NH₃SnI₃, the variation of photoresponsivity (R = I_ph/P_eff, where P_eff is the effective illumination power) is originated from pressure-driven structural phase transition as revealed by existing literatures.^[26–28] Therefore, it is of great interest to clarify what is happening to the lattice structure during the compression for understanding the change of the optoelectronic properties and resistance under pressure. The high-pressure in situ synchrotron x-ray diffraction (XRD) study of the PtS₂ sample was performed up to 26.8 GPa. Selected XRD spectra during compression procedures are displayed in Fig. 3(a). With increasing pressure in the whole measured range, no emergence of new diffraction peaks was observed, which excludes the possibility of occurrence of structural phase transition and indicates an ultra-stable lattice structure of PtS₂ under pressure up to 26.8 GPa.

Fig. 3.

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Fig. 3. (a) in situ high-pressure synchrotron x-ray diffraction (XRD) patterns of PtS2 up to 26.8 GPa. As an example, the fitting of the 0.3-GPa XRD data is shown at the bottom, where experimental data in black circle, best fit in solid red curve, allowed reflections in blue tick marks, and the difference between the observed and calculated profiles in green curve (b). Pressure dependence of the lattice constants a, c, and c = a, as well as the cell volume in panel (c).

The detailed lattice parameters were obtained by refinement of the XRD patterns using the Lebail method. The starting fitting point of structural parameters of the hexagonal (P-3m1) phase were taken from the study of PtS₂ by Furuseth et al., and the fitted parameters were used for the starting point of the next higher-pressure point.^[29] As an example, the lines at the bottom of Fig. 3(a) show the fitting results for 0.3 GPa with the P-3m1 space group (a = b = 3.555 Å, c = 5.0188 Å). The evolutions of the obtained lattice parameters with pressure are depicted in Figs. 3(b) and 3(c). It is found that the PtS₂ shows large anisotropy of axial compressibility: the c parameter decreases monotonically with increasing pressure by ∼ 22.7% at 26.8 GPa, whereas the a parameter shows much weaker decrease of ∼ 1.7%. Such anisotropy, also given by the ratio of c/a, is due to the quasi 2D nature of the lattice structure. The pressure dependence of the cell volume is fitted to the third-order Birch–Murnaghan equation of state^[30] (red line in Fig. 3(c)),

We further carried out a detailed Raman study on PtS₂ under pressures up to 6.4 GPa, to further confirm that the sharp increase of photocurrent between 3 GPa–4 GPa is NOT due to a potential structural phase transition. Figure 4(a) displays the in situ Raman spectra of PtS₂ during compression procedures. Consistent with previous report, two main Raman modes E_g and A_1g were observed at 309.4 cm⁻¹ and 343.2 cm⁻¹ under ambient pressure. As illustrated in the insets of Figs. 4(b) and 4(c), the out-of-plane A_1g mode occurs due to opposing vibrations of the two S atoms with respect to the Pt atom, and the E_g mode is associated with the in-plane-vibration of the two S atoms, in opposite direction with respect to each other.

Fig. 4.

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	Fig. 4. (a) Raman spectra of PtS2 under pressures up to 6.4 GPa in compression procedures. The peak frequencies of Eg and A1g modes of PtS2 as functions of applied pressure are displayed in panel (b) and panel (c), respectively. Insets of panels (b) and (c) depict the schematics of the Eg and A1g vibration modes.

With increasing pressure, the increase of relative intensity ratio of the E_g to A_1g peaks may be attributed to the dominance of the in-plane lattice vibration when the lattice is more strongly compressed along the c direction. The evolutions of the frequencies of the two modes are displayed as functions of pressure in Figs. 4(b) and 4(c). One can see that the frequencies of both E_g and A_1g peaks increase gradually with pressure. The continuous blue shifts of both peaks indicate the strengthened interaction between S and Pt atoms caused by the contraction of the lattice under pressure. In line with the XRD results, no abrupt anomaly is observed, which confirms the stability of the lattice under pressure below 6.4 GPa.

Generally, the ability of a semiconductor to absorb light and to gain photogenerated carriers is directly influenced by the band structure.^[31] For a semiconductor with a direct bandgap, photons with energy higher than the bandgap are readily to be absorbed, but for a system with indirect bandgaps, the efficiency of absorption of photons is much reduced due to the necessity of absorption of an additional phonon to supply the difference in momentum.^[31] For example, benefiting from the direct nature of the bandgap in monolayer MoS₂ in contrast to the indirect bandgap in bulk MoS₂, the photoresponsivity (i.e., photocurrent per illumination power) of the monolayer MoS₂ was significantly enhanced by ∼ 9000 times compared to that in multilayer devices.^[32] For monolayer PtS₂, as mentioned above, the density functional theory (DFT) calculations predict that a transition of the bandgap from an indirect nature to a quasi-direct one will take place when the system is under a mechanical strain,^[16] which would be much in favor of the efficiency of light absorption and the yield of photogenerated carriers. Here in our PtS₂ device, we show that under high pressure of 1 GPa–3 GPa the resistance firstly decreases monotonically without obvious change of the photocurrent observed, indicative of a continuous narrowing of the indirect bandgap. But then in the pressure range of 3 GPa–4 GPa, the photocurrent displays a sharp increase with a maximum enhancement of ∼ 6 times compared to that at ambient pressure. These facts can be attributed to a underlying transition of the bandgap under pressure from an indirect gap to a direct or quasi-direct gap. Further increase of the pressure results in a metallization of the system (as is proved by our temperature-dependent resistance measurements) and hence a sharp drop of the photocurrent down to almost zero. All these results fit together and establish that the high pressure is an effective method to tune the band structure of PtS₂ and to improve the optoelectronic properties.

In summary, we report on the pressure-induced enhancement of optoelectronic properties in PtS₂. It is found that the photocurrent is effectively improved by ∼ 600% at ∼ 4 GPa compared to that at ambient pressure, which is attributed to an underlying bandgap transition from an indirect one to a direct or quasi-direct one. Further increase of the applied pressure causes a metallization of the system at 5.6 GPa–6 GPa. High-pressure XRD results indicate no structural phase transition and an ultra-stability of the lattice under pressures up to 26.8 GPa. Our results reveal that pressure is a simple but effective tool to improve the optoelectronic properties of PtS₂.