Pressure-induced Optical Properties Of CdSe Nanocrystal And CdSe/CdS Core/Shell Nanocrystal

Optical properties of semiconductor nanocrystals(NCs)have attracted considerable attention in recent years.They have already been utilized for quantum device applications such as lasers and fluorescent biomedical probes.Meanwhile,the core/shell semiconductor NCs are also the main research object in the field of materials.It is found that the epitaxial CdS shell around the CdSe core can significantly improve its photoluminescence(PL)efficiency,as well as chemical and thermal stabilities.Pressure can change the electronic structure and emission behavior of NCs.We use diamond anvil cell(DAC)to investigate the distinct effect between the strain induced by mismatch of the core and shell and the external pressure on core/shell NCs and provides new ideas for further development of advanced semiconductor NCs devices.We performed transmission electron microscopy(TEM),in situ PL,absorption,transient fluorescence and X-ray diffraction spectra to investigate the pressure-induced morphology,optical and structural changes of zinc-blende CdSe NCs and CdSe/CdS NCs with different shell thickness.The PL intensity significantly increased as the pressure increased to 3.5 GPa,followed by a continuous reduction and disappearance at 7.5 GPa.Meanwhile,the band gap energy of zinc-blende CdSe NCs increased first and then decreased under pressure.The structure of CdSe NCs did not change before 7.5 GPa,so we thought that the change of optical properties was related to the increase of the size of CdSe NCs under pressure.Due to the quantum confinement effect,the increasing size of CdSe NCs resulted in the red shift of absorption and fluorescence spectra under pressure.Through TEM,we found that thesize of the samples after decompression was larger than that of the original samples,which proved the ripening process of CdSe NCs under high pressure.We also found that the pressure-dependent PL spectra demonstrate a sudden decrease in PL intensity,because of the enhanced rate of auger recombination of type I structured CdSe/CdS NCs.Likewise,the large decrease in the PL lifetime with pressure confirmed that the electrons were mainly confined in the CdSe core,indicating the decreased separation of electrons and holes in type I band alignment.We proposed that pressure increased the conduction band energy of the CdS shell but hardly changed that of the CdSe core,thus ultimately increasing the conduction band offsets between the CdSe core and CdS shell to form the type I core/shell configuration.Our studies elucidate the significance of external pressure in determining the electronic and optical properties of core/shell NCs.