Structural evolution and bandgap modulation of layered β-GeSe2 single crystal under high pressure

Hengli Xie(謝恒立)， Jiaxiang Wang(王家祥)， Lingrui Wang(王玲瑞)， Yong Yan(閆勇)， Juan Guo(郭娟)，Qilong Gao(高其龍)， Mingju Chao(晁明舉)， Erjun Liang(梁二軍)， and Xiao Ren(任霄)，?

1Key Laboratory of Materials Physics of Ministry of Education，School of Physics and Microelectronics，Zhengzhou University，Zhengzhou 450052，China

2College of Physics and Materials Science，Henan Normal University，Xinxiang 453007，China

Keywords: high pressure，structural phase transition，Raman spectroscopy scattering，layered material

1. Introduction

Recently， the exploration of the relationship between structure and properties of two-dimensional (2D) materials has become the focus of research.[1–5]This is attributed to the promising applications of 2D materials in field-effect transistors，[6–8]photodetectors，[9，10]and electrocatalysts，[11]etc.To further develop the applications of 2D materials， it is necessary to break through the intrinsic structure of the materials and to discover， characterize， and control the crystal structure and property changes of 2D materials.[12–14]For example， relying on the structural phase transition of metal dichalcogenides(TMDs)materials(MoS2，[12]MoTe2，[15]and WS2[16]) from the semiconductor phase (2H) to the metallic phase (1T， 1T′)， 2D electronic devices with low contact resistance could be fabricated accompanying high carrier mobility，high switching rate，and superior electrocatalytic performance.[17，18]Therefore， how to effectively regulate the structure of 2D materials to develop more potential applications remains attractive.

As a powerful experimental method for revealing the effects of atomic and electronic structure changes on the electronic and optoelectronic properties， pressure is the best candidate to modulate the structure in 2D materials without introducing impurities and damages.[19–22]For example， the exciton binding energy of 2D perovskites is reduced and the photocurrent is significantly enhanced under pressure.[23]Furthermore，TMDs exhibit specific physical phenomena such as metallization and superconductivity under pressure.[24–28]Therefore，it is significant to explore the tunability of the lattice and electronic structures of 2D materials by pressure to get more insight into their intrinsic structural properties.

Germanium diselenide(β-GeSe2)plays an essential role in the family of 2D Ge-based binary materials.[29]Relying on its direct wide bandgap of 2.74 eV，[30，31]weak interlayer interaction，[32]and good air stability，[31]β-GeSe2could be used in the fields of ultraviolet photoelectric detection，[33]infrared waveguides，[34]polarization-sensitive photodetection，[30，31]and storage units.[35]Since it performs well in many applications at atmospheric pressure，it is necessary to reveal the behavior of lattice and electronic structures under high pressure and to explore its more potential applications under extreme conditions. Actually， many efforts have been made to uncover the lattice and electronic structure evolution ofβ-GeSe2under high pressure，but many controversies still remain.

In experiments，Popovi′cet al.utilized high-pressure Raman scattering to demonstrate thatβ-GeSe2exhibited amorphization at 6.20 GPa and transformed toα-GeSe2upon further compression to 7.00 GPa， then back to amorphization when the pressure continuously increased to 8.00 GPa.[36]However， another pressure-dependent Raman experiment by Andrzej Grzechniket al.speculated thatβ-GeSe2partially broke at 7.00 GPa and the bilaterally shared Ge2Se8tetrahedra began to break locally， forming a local HgI2-type structure. Furthermore，their high-pressure energy dispersive x-ray diffraction supported thatβ-GeSe2at 11.00 GPa was completely amorphous and no structural phase transition existed during pressuring to 14.00 GPa.[37]In addition to the experiments，the local density approximation(LDA)and generalized gradient approximation(GGA)calculations supported theoretically that the structure ofβ-GeSe2suffered Tl-cristobalitetype at 0.52 GPa， HgI2-type at 2.30 GPa， and CdI2-type at 3.56 GPa.[38]While anotherab initiocalculation depicted the onset of CdI2-type as 18.50 GPa.[39]Accordingly，as outlined above，there are still a lot of controversies on the lattice evolution ofβ-GeSe2in the experimental and theoretical results.

In this work， we have synthesized high-quality single crystals ofβ-GeSe2by the chemical vapor transfer(CVT)experimental method. We apply symmetric diamond anvil cell(DAC)high-pressure technique for in situ angle-dispersive xray diffraction (ADXRD)， ultra low frequency (ULF) Raman scattering， and UV-vis absorption experiments to obtain the effect of pressure on the lattice and electronic structure ofβ-GeSe2. Our results approve that no structural phase transition exists as pressure increases to 13.80 GPa but the structure becomes disorder at 6.91 GPa and turns into the amorphous state at 13.80 GPa. Some phonon energies show inconsistency at 6.91 GPa， indicating that the inter-layer interactions play a more significant role for the phonon energy than intra-layer interactions for some certain phonons. Two Raman modes depict softening behaviors over a large pressure range，which could be attributed to the weak intramolecular bonds.This phenomenon is accord with the solid hydrogen and GeP5up to high pressure.[40，41]The electronic bandgap ofβ-GeSe2decreases linearly with increasing pressure signifying that no electronic phase transition appears. The color variation of the sample with pressure was captured in real-time，changing from bright yellow to dark red and finally to black. Our ADXRD and UV-vis absorption experiments prove the existence of an incomplete recrystallization process inβ-GeSe2.

2. Experimental details

2.1. Synthesis of β-GeSe2 single crystals

The bulkβ-GeSe2single crystals were synthesized in a dual-temperature zone tube slide furnace by chemical vapor transfer(CVT)reaction technique. Analytical grade reagents of GeSe(purity 99.999%)and Se(Aladdin in purity 99.999%)powders were used as raw materials based on the chemical stoichiometry ofβ-GeSe2(Fig.1). Firstly，two crucibles separately containing GeSe(0.01 mol)and Se(0.01 mol)powders were placed in the quartz tube， 4 cm and 20 cm from the left side away from the first heating center respectively. After the quartz tube was vacuumized to-0.1 MPa，the high-purity Ar gas was filled into the quartz tube until the pressure in the tube returned to atmospheric pressure. This process of gas washing was repeated three times. Then the quartz tube was purged with Ar gas for 60 min with a flow rate of 200 sccm to ensure that no air existed in the quartz tube before the quartz tube was heated up. The left and right temperature zone of the quartz tube was set as 720°C and 710°C with a rate of 10°C/min， separately. Once the temperature of the left zone reached 720°C，the furnace needs to be moved to the left by 4 cm timely to ensure that the two raw materials could achieve sublimation point simultaneously.Meanwhile，the Ar gas with 200 sccm was replaced by the mixed gas of 90%Ar and 10%H2with a rate of 40 sccm. The whole reaction process was kept for 25 min，then the tube was cooled to room temperature at a cooling rate of 2°C/min. Then， the high-quality single crystalsβ-GeSe2with bright yellow flake were obtained.

Fig.1. Sketch of preparation equipment of β-GeSe2 single crystal by CVT method.

2.2. High-pressure generation

A symmetric DAC was used for high-pressure experiments， and the culet size of the diamond anvils was 400 μm.The sample was loaded into a 150-μm diameter hole of the T301 stainless steel gasket，which was pre-pressed to a thickness of 40 μm. A small ruby ball was placed in the sample compartment for in situ pressure calibration，utilizing the standard ruby fluorescence method. Silicone oil was used as the pressure transmitting medium for high-pressure experiments.

2.3. In situ high-pressure measurements

In situhigh-pressure ADXRD experiments at a wavelength of 0.6199 ?A beam were carried out at beamline Unit BL15U1， Shanghai Synchrotron Radiation Facility (SSRF)，China. CeO2was used as the standard sample to do the calibration. The collected 2D images were integrated based on the FIT2D program to obtain a plot of one-dimensional (1D)intensityversusdiffraction angle 2-theta patterns. The Reflex module of Materials Studio was used to refine the XRD patterns.

Pressure-dependent Raman spectra were collected with Horiba Jobin Yvon LabRAM HR Evolution Raman spectrometer in a backscattering configuration equipped with 1800-g/mm grating.BragGrate notch filters allow for measurements down to about 10 cm-1. A semiconductor refrigeration CCD detector with a focal length of 800 mm after gratings provides a high energy resolution of 0.6 cm-1. The incident laser with a wavelength of 633 nm originating from an He–Ne laser was used for excitation. To reduce the influence of laser heating on the sample，the laser power was lower than 0.15 mW.

In situ high-pressure optical absorption spectra measurement was performed between 250 nm and 1000 nm using a deuterium–halogen light source and recorded with an optical fiber spectrometer (Ocean Optics， QE65000). Each new acquisition was performed after a few minutes as pressure rise to a certain value，which will intrinsically explain any kinetic dependence of the measurement process. The transmission spectrum of silicon oil around the sample was subtracted as the background.

3. Results and discussion

3.1. High precision pressure-dependent Raman spectroscopy revealing structural evolution and pressure limit of β-GeSe2

GeSe2crystallizes in three phases，including orthorhombicα-GeSe2(space groupPmmn)， monoclinicβ-GeSe2(space groupP21/c)， and hexagonalγ-GeSe2(space groupPˉ3m1).[38]All three structures are layered. In particular，β-GeSe2is the most stable form with the lowest energy among the three phases.[31，38]The crystal structure basic unit ofβ-GeSe2is a GeSe4tetrahedron. Every unit cell has two layers and van der Waals forces along thezaxis separate the layers.[32]Each layer is composed of parallel chains of cornersharing GeSe4/2tetrahedra (CST) along theaaxis and interconnected by pairs of edge-sharing Ge2Se8double tetrahedra(EST)along thebaxis(Fig.2).[30]

Fig.2. Crystal structure of β-GeSe2 from side(a)and top(b)view.

A situhigh-pressure Raman scattering is performed to study the modification of structure under the pressure ofβ-GeSe2. The group theory analysis provides the irreducible representations ofβ-GeSe2that could rule the vibrational patterns of phonon atΓpoint of the Brillouin zone as the following equation:

whereAgandBgphonons are Raman-active modes with the symmetric center of vibrations， whileAuandBuare infrared(IR)-active modes with no symmetric vibrational center. Our single crystal sample ofβ-GeSe2for Raman testing is a bulk material and the highest symmetric axis is along theb-axis direction.[36]Thus theZdirection in the phonon selection rule is thebaxis.[42]In order to best detect theAgandBgphonons，the incident and scattered light polarizations to obey the phonon selection rule. For example， part ofAgmodes could be obtained when the incident and scattered light polarizations are parallel to theaaxis. If we put theaaxis along theXdirection，the configuration above could be noted asXX(aa). According to this naming method，Agmodes could be detected underXX(aa)，YY(cc)，XY(ac)，andZZ(bb)polarizations.Bgmodes could be obtained byXZ(ab) andYZ(cb)polarizations. For the texture ofβ-GeSe2single crystal is soft， it is difficult to obtainacorbcplane and only the experimental measurements on theabplane could be available. Therefore， we just obtain partialAgandBgmodes.Figure 3 shows our room temperature and ambient pressure Raman spectra under polarized laser configuration on theabplane. 33 branches ofAgmodes and 31 branches ofBgmodes have been detected successfully (Fig. 3) and the other no detectable branches， 3 branches ofAgmodes and 5 branches ofBgmodes， may originate fromYY(cc) andXY(ac) polarizations inacplane or cannot be explored due to their low intensities.

Fig. 3. Polarized Raman spectrum of β-GeSe2 single crystal with parallel(a) and vertical (b) polarization at room temperature and ambient pressure.The inset in Fig. 3(a) shows the atomic vibration patterns of 210 cm-1 and 216 cm-1，respectively.

We denote the Raman peaks ofAgandBgmodes at atmospheric pressure and room temperature clearly in Fig. 3 and specific phonon energies are performed in Table S1 in supporting information. It is worth noting that most ofAgandBgmodes stand similarly， which is consistent with the previous Raman research.[43]Compared with the previous results， the Raman peaks we obtained at atmospheric pressure and room temperature are more accurate and complete， which provides more information for the structural analysis ofβ-GeSe2. The highest Raman peak at 210 cm-1in Fig.3 is the characteristic peak ofβ-GeSe2and could be distinguishable with that ofα-GeSe2at 200 cm-1.[30，44]The dominant peak located at 210 cm-1is assigned to the symmetric breathing vibration mode of the shared-angle GeSe4tetrahedra and the atomic vibrational pattern of 216 cm-1is also presented (the inset of Fig.3).[36，37]

The specific variable pressure study of the Raman spectrum ofβ-GeSe2is shown in Fig. 4(a)， where the pressure is gradually raised in a stepwise fashion up to～13.80 GPa.We fitted all Raman peaks at different pressures except the peaks that were too broad and weak to be fitted. The fitted peak positions are displayed in Fig. S1 in supporting information. It is macroscopically observable that many Raman peaks become broad and seem to merge above 6.91 GPa.We depicted the pressure-dependent full width at half maximum (FWHM) of six peaks that could be well fitted crossing 6.91 GPa (Figs. 4(b)–4(d) and Fig. S2). In Raman experiment， the FWHM reflects the length of the phonon lifetime which could be modulated by the external factors that disturb the intrinsic vibration of phonons.[45]In Figs. 4(b)–4(d)and Fig.S2， it is noticeable that the significant broadening of Raman peaks happen at 6.91 GPa，which is speculated to be caused by a substantial increase in the disorder of the atomic arrangement in the unit cell ofβ-GeSe2. This is in sync with the previous Raman result that the material shows disorder at 7.00 GPa marked by the indistinguishable peaks at 210 cm-1and 216 cm-1.[37]Resorting to the high resolution of our Raman spectroscopy， as high as 0.6 cm-1， it is obvious that the 210 cm-1(marked by the inverted triangle in Fig.S3)and 216 cm-1(marked by the circle in Fig.S3)peaks always separate in the whole pressure(Fig.S3). Thus，we provide more specific and systematic evidence for the disorder at 7.00 GPa resorting to the abnormal broadening of FWHM at 6.91 GPa than the previous Raman result.[37]In addition， to further demonstrate the effect of the variation of the hydrostatic pressure environment on the experimental results， we also conducted a high-pressure Raman experiment with liquid argon (Ar) as the pressure transfer medium， as shown in Fig.S4. The evolutions of the high-pressure Raman spectra ofβ-GeSe2under the two pressure transfer media remain consistent with each other， which proves that if the hydrostatic pressure environment in the DAC changes， the experimental results are unacted.

Fig.4. (a)Variable pressure Raman spectra of β-GeSe2 single crystal with silicone oil as the pressure transfer medium. The pressure-dependent peak energy and FWHM of 196 cm-1 (b)，210 cm-1 (c)，and 216 cm-1 (d)peaks.

According to our fitting results， only the Raman peaks at 196 cm-1and 210 cm-1undergo an anomalous redshift at 6.91 GPa(Figs.4(b)and 4(c))，while the energies of the other four peaks show monotonic changes (Fig. 4(d) and Fig. S2)including the 216-cm-1peaks. In the following part，we will study the reason of the anomaly between 210-cm-1and 216-cm-1peaks. According to the vibrational schematic diagrams in Fig.3(a)，the two Raman peaks at 210 cm-1and 216 cm-1are both contributed by the same Se atoms vibrations，but different vibrational directions. One direction is almost parallel to the Se–Se chain and the other is almost perpendicular to the interlayer Se–Se direction. When the sample turns into disordered at 6.91 GPa， some Ge–Se bonds of the GeSe4tetrahedron framework would break.[37，46]Thus， we infer that the softening of 210-cm-1peaks is attributed to the Ge–Se bond breaking. The reason why 210-cm-1phonon， not 216-cm-1phonon，softens above 6.91 GPa is that the 210-cm-1phonon involves the Se–Se vibrations along layers while the 216-cm-1phonon is caused by the Se–Se vibration across layers. Due to the bond length decreasing of Se–Se atoms between interlayer upon compression， to a certain extent， it will compensate for the energy loss induced by the Ge–Se bond breaking for 216-cm-1phonon. The monotone increase of 216-cm-1phonon energy demonstrates that the shortening of interlayer spacing，which is a relatively straightforward result of high pressure，has a more obvious effect on the phonon energy than the Ge–Se bond breaking. Therefore， the partial breaking of the Ge–Se bond would not induce a macroscopically structural deformation. To verify our inference， a high-resolution ADXRD experiment that reflects the intrinsic structure evolution ofβ-GeSe2needs to be done， which will introduce specifically in the following part.

For the peak at 196 cm-1，with the similar abnormal behavior of 210 cm-1， no explicit vibration modes have been given according to previous studies. Our results (Figs. 4(b)and 4(c))put forward that the energies of 196-cm-1and 210-cm-1phonons harden respectively 4 cm-1and 9 cm-1from 1 atm (1 atm = 1.01325×105Pa) to 6.91 GPa and soften 0.7 cm-1and 1.4 cm-1from 6.91 GPa to 10.30 GPa. The ratio of 0.7 cm-1/4 cm-1is similar to 1.4 cm-1/9 cm-1indicating that these two phonons have a similar response to pressure. Thus，we conjecture that the abnormal energy change in the 196-cm-1peak is also caused by the partial breaking of the Ge–Se bond like 210-cm-1peak.What is more，the 210-cm-1peak splits at 2.12 GPa (Fig. S3)， which could be interpreted as the internal vibrational modes like that in the isomorphic GeS2.[47]

Apart from the abnormal changes in 210 cm-1and 196 cm-1， the energies of 151-cm-1and 167-cm-1phonons soften from 1 atm to 6.07 GPa above which the peaks are too broad to be fitted with a large error as shown in Fig. 5 and Fig.S5(The specific fitting diagram is shown in Fig.S6).Different from phonon softening which reflects the structural phase transition at some pressure，[48]our phonon softening behavior upon compression is just like that in solid hydrogen and the layered GeP5at high pressure.[40，41]The continuous softening of the two modes may be caused by the elongation of the length of the intra-layer，resulting in the weakening of the intramolecular bonds.

Fig.5.Pressure-dependent Raman peak positions of 151-cm-1 (a)and 167-cm-1 (b)peaks.

For purpose of investigating the pressure resistance limit of theβ-GeSe2crystal structure，we applied high pressures of 19.20 GPa toβ-GeSe2. According to the Raman spectra in Fig.4(a)，it is clear that the distinguishable Raman peaks exist until 13.80 GPa. Figure 6(a)displays the Raman spectra during decompression from 13.80 GPa， where the Raman characteristic peaks representing the amorphization ofβ-GeSe2appeared around 175 cm-1， 199 cm-1， and 216 cm-1，[36，49]which signaled that the crystal structure ofβ-GeSe2is irreversibly amorphous at 13.80 GPa. The released sample after complete decompression was found to be consistent with that of previously reported glassy GeSe2，[49]implying that the amorphous of GeSe2induced by pressure possesses the same origin as that prepared at atmospheric pressure， which will provide good insights for understanding and regulating the structure of the GeSe2glassy state. As shown in Fig. S7(a)，when the pressure exceeds 13.80 GPa，the Raman spectra only contain peaks that is similar with the that in 13.80 GPa，which demonstrates only amorphous state exists andβ-GeSe2does not form a new long-range ordered structure near 18.50 GPa.Our conclusion is different from the results predicted by the calculation.[39]Moreover，collecting Raman spectra of the decompression process reveals that the structure ofβ-GeSe2does not recover with the decrease of pressure (Fig. S7(b)).If we reduce the maximum pressure to 12.83 GPa， the structure ofβ-GeSe2will be restored completely as the pressure is relieved to the ambient pressure(shown in Fig.6(b)).

Fig. 6. The decompressed Raman spectra of β-GeSe2 from 13.80 GPa (a)and 12.83 GPa(b)at room temperature.

3.2. ADXRD under high pressure demonstrates no structural phase transition in β-GeSe2 single crystal

Figure 7(a) shows representative ADXRD data ofβ-GeSe2during compression to 13.20 GPa and decompression.The Reflex module of Materials Studio was used to refine the XRD patterns. The refinement of ADXRD data at ambient pressure indicates that the intrinsic structure ofβ-GeSe2belongs to the monoclinic phase withP21/cspace group(Fig. S8(a))， which is consistent with previous reports.[32，33]Most diffraction peaks broaden and disappear when the pressure exceeds 7.19 GPa， implying some disorder existing in the structure， which is consistent with our Raman result. Beyond 7.19 GPa， the diffraction peaks still move uniformly with upon to 13.22 GPa implying no obvious amorphization at 13.22 GPa. This could be verified by the ADXRD pattern of the decompression sample at 1 atm. The ADXRD pattern of the decompression sample owns the same ADXRD positions with the initial sample but lower intensity， which is in sync with the incomplete recrystallization process (Fig. 7(a)and Fig.S9). The representative refinement results are shown in Fig. S8. The corresponding refinement results， including cell volume and lattice parameters， are shown in Table S2. Cell parameters and volume ofβ-GeSe2are decreasing monotonously before amorphization(Figs.7(b)and 7(c)).Compared with previous reports，[37]we have a high precision and more systematic XRD measurement to obtain the accurate structural parameters ofβ-GeSe2at different pressures，etc.，providing crucial evidence to reveal the crystal lattice structure evolution ofβ-GeSe2under high pressure. In addition，the experimental pressure–volume(P–V)relationship is fitted using the third-order Birch-Murnaghan equation of state[21，50，51]

whereV0is the atmospheric pressure volume andB0is the bulk modulus at ambient pressure. The fitted bulk modulus(B0)is 22.15 GPa，which is smaller than that of other 2D layered materials such as MoSSe (31.30 GPa) and MoS2(47.65 GPa)，suggesting a large variation in the bulk compressibility of theβ-GeSe2cell under pressure.[52，53]

Fig. 7. (a) Representative ADXRD patterns at different pressures for β-GeSe2. Pressure dependence of lattice parameters (b) and the unit cell volume(c)of β-GeSe2.

3.3. Pressure regulates linearly the bandgap of β-GeSe2

Our Raman and ADXRD experiments support that no structural phase transition upon compression from 1 atm to 13.80 GPa.To uncover the electronic behavior under pressure，high-pressure in situ UV-vis absorption spectroscopy was performed to understand the evolution of the electronic bandgap ofβ-GeSe2. At atmospheric pressure，the absorption edge at 478 nm is observed，then the absorption edge varies monotonously as pressure(Fig.8(b)). Tauc plots ofβ-GeSe2at different pressures are shown in Fig.S10. Figure 8(c)shows the fitted bandgap ofβ-GeSe2. The bandgap value of our bulkβ-GeSe2under atmospheric pressure conditions is 2.59 eV， smaller than the previously reported 2.74 eV ofβ-GeSe2.[31]This divergence could be attributed to the thickness of the sample and is in accord with the calculation result.[32，33]In Fig.8(c)，the bandgap decreases linearly to 1.65 eV when the pressure is increased to 12.16 GPa with a 0.94-eV reduction across 12.16 GPa，illustrating that the bandgap ofβ-GeSe2is highly tunable under pressure， which will provide new insights into the fabrication ofβ-GeSe2-based pressure sensors and switching devices. The linear drop of the bandgap also suggests thatβ-GeSe2has no detectable electronic phase transition upon pressure，[21]which is in agreement with the lattice structure according to our Raman and ADXRD experimental results.

Fig.8. (a)Optical micrograph of β-GeSe2 piezochromic phenomenon upon compression and decompression. (b)and(c)Selected optical absorption spectra and bandgap evolutions of β-GeSe2 as a function of pressure during pressurization. (d) Direct bandgap Tauc plots of β-GeSe2were collected at ambient pressure(red)and decompression(blue).

Figure 8(a) shows the color changing of theβ-GeSe2sample under pressure during the high-pressurein situUVvis absorption experiment. At ambient pressure， the sample is transparent with the color of light yellow. As the pressure keeps increasing， the color of the sample gradually turns to dark red，and finally changes to black and opaque at 12.16 GPa completely. After pressure was released， the sample returned to the transparent state with yellow color instead of light yellow. Meanwhile， the sample recovered to a bandgap of 2.45 eV smaller than the initial 2.59 eV after decompression(Fig. 8(d) and Fig. S11)， which is attributed to the incomplete recrystallization ofβ-GeSe2.[21]When the sample was pressed to 14.01 GPa， it turns into an amorphous state and the color of the decompression sample is dark red not the yellow color of the decompression sample from the no amorphous state (Fig. S12). Although the sample became amorphous， it still owns its unique crystal structure，[42]which is verified by the partial bandgap of 2.13 eV of the decompression sample(Fig.S13).

4. Conclusion

In summary， we have systematically and comprehensively investigated the crystal structure and bandgap evolution of the 2D semiconductorβ-GeSe2under high pressure. According to our high-precise ULF Raman scattering measurement and high-resolution synchrotron radiation x-ray diffraction results，there is no obvious structural phase transition during the pressurization up to 13.22 GPa. Some disorder exists above 6.91 GPa，which could be well supported by the FWHM collective broaden of phonons above 6.91 GPa in Raman spectra. The inconformity in the energy between 210-cm-1and 216-cm-1phonons above 6.91 GPa suggests that interlayer spacing decreasing induced by pressure along thecaxis has a more obvious effect on the phonon energy than Ge–Se bond breaking. This indirectly supports that the partial breaking of the Ge–Se bond would not induce a macroscopically structural deformation. Based on the same response to pressure，the anomaly at 6.91 GPa in 196-cm-1phonon energy is considered with the same origin as the 210-cm-1phonon，both of which are attributed to the Ge–Se bond of tetrahedral breaking. The reason for the continuous softening of 151-cm-1and 167-cm-1Raman peaks with pressure is interpreted by the weak of intramolecular bonds. Apart from lattice information，for electronics，the bandgap could be continuously modulated from 2.59 eV to 1.65 eV as pressure changing from 1 atm to 12.16 GPa，accompanied by the sample color varying from shiny yellow to black. If the sample is not pressed into an amorphous state，the ADXRD patterns and bandgap results both support thatβ-GeSe2shows an incomplete recrystallization process when the sample be completely decompressed.These research results refresh the fundamental understanding of the structure and bandgap ofβ-GeSe2and can provide some insights into the pressure domination of the structural and optoelectronic properties of 2D Ge-based binary materials.

Acknowledgments

Project supported by the National Natural Science Foundation of China (Grant Nos. 12004339， 11874328，11904322，61804047，22071221，and 21905252)，China Postdoctoral Science Foundation (Grant Nos. 2018M640679 and 2019T120629)， and the Zhongyuan Academician Foundation(Grant No.ZYQR201810163).