Structural And Physical Properties Of Wurtizite Nano-CuInS₂ Under High Pressure

Among the A^?-B^?-C₂^?series of compounds,CuInS₂ semiconductor materials has excellent properties,such as tunable absorption band gap?1.1-1.5 eV?,high absorption coefficient,photoluminescence and strong local surface plasmon resonance in photovoltaic cells,light emitting diodes,biomacromolecule fluorescent labeling and non-linear optical devices.It has great potential application and attracts much attentions.The chalcopyrite structure of CuInS₂ compound exists in nature and the znic blende and wurtzite structure can be obtained under high temperature conditions through temperature adjustment that they will return to the chalcopyrite structure after cooling.In addition to chemical methods,high pressure can be used as a physical mean to change the interatomic distance and electronic structure,thereby adjusting the physical and chemical properties of the substance,and to obtain new phenomena,new properties and new structures that other means cannot.Under high pressure,chalcopyrite CuInS₂ exhibits rich structural phase transitions.It has been found that pressure can effectively regulate its optical and electrical properties.In recent years,the metastable wurtzite nano-CuInS₂ has been found to be stable under ambient temperature and pressure,but its structural stability and the physical and chemical properties under high pressure are still unclear.In this paper,we successfully prepared the metastable nano-sized wurtzite CuInS₂?P6₃mc?by hot injection method.By using diamond anvil cells?DAC?combined with synchrotron radiation X-ray diffraction,absorption spectrum,resistivity,photocurrent,we systematically studied the structural stability,the changes of the optical properties,electrical properties,optoelectronic properties of the sample under pressure,and obtained the following valuable research results:1.Successfully prepared the metastable nano-wurtzite CuInS₂ by hot injection method.The XRD pattern was refined that the sample had hexagonal structure and the space group was P6₃mc;the grain size was calculated about 20 nm by Scherrer formula.TEM test results showed that the sample morphology is hexagonal flakes with good crystallinity.The EDS test results showed that the element ratio is 27.42%,24.64%and 47.94%.XPS test results showed that the element ratio is 28.90%,23.96%and 47.14%,which is almost consistent with the EDS test results.2.In-situ high-pressure synchrotron radiation X-ray diffraction results showed that the structural transformation from disordered wurtzite CuInS₂?P6₃mc?to disordered rock salt CuInS₂?Fm-3m?occured at10.3 GPa.It was completely transformed into Fm-3m structure at 17.2 GPa,and maintained to 50.2 GPa.During the pressure decompression process,the structure changed to ordered chalcopyrite CuInS₂?I4-2d?,indicating that this structural phase transition is irreversible.The first-principles calculations well explained the structural evolution from disordered wurtzite structure?disordered rock-salt structure?ordered chalcopyrite structure under pressure.3.In-situ high-pressure absorption spectrum test of wurtzite nano-CuInS₂ showed that the sample is a direct band gap semiconductor at ambient pressure,and the band gap value increases with pressure?dE_g/dP=24.2 meV/GPa?in wurtzite phase.The absorption edge changed after 10.1GPa and a direct?indirect band gap transition occurred along with the structural phase transition.The indirect band gap decreased with pressure?dE_g/dP=-30.5 meV/GPa?in rock salt phase.The first-principles calculations showed that the pressure-induced band gap opening can be mainly attributed to the distortion of InS₄ tetrahedral,that the S-3p state and the In-5s state gradually moved toward the high energy level.4.In-situ high-pressure resistivity measurement of wurtzite nano-CuInS₂ found that the resistivity showed discontinuous change around10.5 GPa,at which a phase transition from wurtzite phase to rock salt phase occurred.The resistivity increases with pressure increasing in wurtzite phase but decreases with pressure increasing in rock salt phase.This variation trend is the evolution of the the structural transformation and the bandgap.5.In-situ high-pressure photocurrent test of wurtzite nano-CuInS₂showed that the photocurrent obviously improved by pressure and reaches the maximum value at 6.0 GPa,at which the bandgap was modulated to the optimal value?1.5 eV?for photovoltaic solar energy conversion.When the pressure exceeds 10.3 GPa,,the photocurrent almost disappears,which was related to the direct band gap?indirect band gap transition.Such remarkable enhancement in photocurrent performance of wurtzite CuInS₂ by pressure,as far as we know,is the first report for any A^I-B^?-C₂^? materials.