Assembly Behavior And Optical Properties Of Typical Indium-based Semiconductor Nanocrystals Under High Pressure

As an important unconventional extreme condition,high pressure is an important means for the research and development of new materials.It can effectively change the crystal structure of materials,and then affect its internal electronic orbital and distribution,thereby realizing the control of the structure and properties of the material.Pressure-induced assembly and piezochromic materials are typical pressure-driven new smart materials:(i)the separation distance between particles can be adjusted accurately and systematically through pressure control until the particles are close and contact each other,and then fuse into new superlattice structures;(ii)by adjusting the distance between atoms,lattice spacing and cell volume by pressure,the valence and conduction band energy levels of semiconductor nanomaterials are adjusted to modulate the luminescence.Previous studies on piezochromic luminescence materials have been focused on organic polymers and/or compounds containing organic molecules due to their excellent pressure sensitivity.However,such materials are also sensitive to temperature,water and oxygen,resulting in instability under ambient conditions.Luminescent semiconductor nanomaterials have attracted considerable interest owing to their unique optical properties,such as high photoluminescence(PL)quantum yield,good photochemical stability and precisely tunable PL and absorption spectra.In particular,I-III-VI(Cu In S2)and III-V(In P)semiconductor nanomaterials are considered environmentally friendly semiconductor nanomaterials with tunable emission covering the visible-to-near-infrared region and high color purity.Therefore,exploring the optical and structural response of these two types of materials to pressure and revealing the intrinsic relationship between the change of macroscopic properties and microstructure under high pressure can provide scientific basis for improving the optical properties of semiconductor nanomaterials and realizing large-scale,continuous and accurate modulation of emission and absorption spectra,thus providing new ideas for obtaining efficient and stable functional nanomaterials.The specific research results were shown as following:(1)We systematically studied the changes in the assembly morphology,optical properties and crystal structure of Cu In S2 nanoparticles(NPs)under high pressure,and discussed the relationship between them.Driven by increasing the external pressure,the zero-dimensional Cu In S2 NPs have been assembled to form one-dimensional nanorods,nanowires and two-dimensional(2D)nanosheets.Combining highresolution transmission electron microscopy(TEM)images and microscopic atomic structure,we found that the tetragonal chalcopyrite(CH)phase Cu In S2 preferentially assembles along the <112> direction to form nanorods and nanowires.Upon 16.0 GPa,the CH phase transforms into a cubic cubic rock-salt(RS)phase,and the high-energy(110)plane of the RS-Cu In S2 was laterally exposed,which accelerated the lateral sintering of nanowires to form nanosheets.Atomic force microscope measurements showed that the thickness of 2D Cu In S2 nanosheets is comparable to the diameter of the initial NPs.The generated 2D lamellar nanostructures exhibit a narrowing of 0.21 e V from the initial 2.03 e V in optical band gap.As a result,the optical band gap of 2D Cu In S2 nanosheets is close to the optimal range of solar spectral.PL measurements showed that the nanowires maintain the same PL intensity as the initial NPs,while the PL of the assembled nanosheets completely disappears.Combined with the assembly process of NPs,we believe that the lattice distortion caused by the assembly of NPs under low pressure will recover with the release of pressure and the PL will be reversible.Upon higher pressure,nanoparticles are assembled to form nanosheets.This weakened quantum confinement effect makes excitons delocalized,PL weakens or even disappears,and the band gap decreases.(2)Combined with in situ high-pressure PL photographs and spectra tests,we found that the In P/Zn S nanocrystals(NCs)exhibit noticeable PL color changes(Orange?yellow?green),with the PL intensity showing slight enhancement below applied pressure of 2.5 GPa.The PL peak position and intensity can recover by decompression,and this abnormal phenomenon can be reproduced in our repeated experiments.With the further increase of pressure above 2.5 GPa,the PL intensity drops markedly and until disappearing at 11.3 GPa.An ultrabroad energy tenability range up to 400 me V has been observed.Time-resolved PL data represent a shortened in the average PL lifetime before 2.5 GPa,implying the depressed defect states.Upon further compression,the PL lifetime of the In P/Zn S NCs begains to increase,indicating a pressure-restrained radiative exciton recombination rate.In addition,first-principles calculations indicate that the lattice mismatch of core/shell In P/Zn S NCs displays a decreasing trend in the low pressure regime.Therefore,the pressure optimizes the core/shell interfacial strain and reduces the interfacial defect states,resulting in stable PL emission of In P/Zn S NCs at the low pressure regime.In addition,in situ highpressure synchrotron radiation X-ray diffraction spectra show that the cell volume and lattice spacing of In P/Zn S NCs shrink rapidly at low pressure,which causing a rapid blue shift in the PL.(3)Compared with the 7.7% lattice mismatch rate between Zn S and In P,the lattice mismatch rate between Zn Se and In P is only 3.2%.This makes In P/Zn Se have a narrow emission spectrum compared to In P/Zn S and ensure the luminescence purity of In P/Zn Se NCs.In addition,the low lattice mismatch also makes it easier for Zn Se to grow epitaxially on the In P core and precisely adjust the size of the core-shell structured NPs.The thick Zn Se shell can effectively suppresses the blinking and auger recombination behavior of excitons.Combined with in situ high pressure measurement and characterization techniques,we explored the optical properties and assembly behavior of In P/Zn Se NCs.By applying pressure from 1 atm to 5.6 GPa,the PL peak position of In P/Zn Se NCs gradually shifted from 619 nm(red)to 546 nm(green).The PL peak position and intensity can recover by decompression.Simultaneously,an ultrabroad tunable bandgap up to 460 me V has been observed,starting from 1.99 e V(1atm)and reaching phase transition at 2.45 e V(14.2 GPa).After releasing the pressure from 14.2 GPa,the PL peak position and absorption edge returned to the initial positions and the PL intensity decreased slightly.Upon the decompression from 21.1GPa,the PL of the In P/Zn Se NCs completely disappeared and the absorption edge exhibited a red-shift in comparison with the initial edge.In situ small-angle synchrotron X-ray scattering and TEM images demonstrate that the core-shell In P/Zn Se NPs were sintered under higher pressure to form 2D nanostructures.Upon low pressure,the In P/Zn Se NPs remain monodispersed,and their optical properties are completely restored with the release of pressure.Further compression makes the lattice seriously distorted,changes the original crystal field and the overlap of wave functions between atoms,increases the defect state and makes the photogenerated carriers trapped,thus weakening or even quenching the PL and the process is irreversible.At about 21.0 GPa,the NPs are sintered to form nanosheets.After the pressure release,this morphology is intercepted and the band gap is reduced from 1.99 e V to 1.67 e V.