The Phase Transition Study Of Binary Metal Sulfides/oxides Nanomaterials Under High Pressure

Comparing with the corresponding bulk sample, the nanosacled semiconductor materials have good optical and electrical properties, and are the important materials in the electrical industries with wide use in the optical and electrical filed. The study of properties for the different nanomaterils with different sizes and morphologies is the important and interesting topic in the fileds of condensed physics and nanomaterils. High pressure is a unique method for us to understand the structures, properties and their relationship further, and can change the physical properties by changes the structure of nanomaterials.The relative size changes greatly at different directions for the nanomaterials with different morphologies. Because the quantum confinement effect and the surface effect is related to the size of the nanomaterials, thus the quantum confinement effect and surface effect are different at different directions for the nanomaterials with large difference in size at different directions, and therefore the surface energe at different directions are also different. The different size of the quantum confinement effect and surface effect at different directions will influence the pressure induced behavior abundantly. Up to now, there is few reports for the CdSe and ZnS nanomaterials with special morphologies under high pressure; especially it is unknown how the different morphology of the CdSe and ZnS nanomaterials influences the pressure induced phase transition pressure and transition behavior and what is the induced mechanism. In this work, we studied the influences on the pressure behavior of CdSe core for the CdSe/ZnS core/shell quantum dots and ZnS by the ZnS shell and the different morphologies of ZnS nanomaterials.We carried out the pressure induced phase transition behavior of one dimensional ZnS nanorods firstly, and find that the wurtzite structure of the ZnS nanorods transforms to rock salt structure at 19.6GPa without undergoing the phase transition to zinc blende structure like the behavior of ZnS bulk sample and nanoparticles. There is no report about the"jump of phase transition"for ZnS, and this is the first report. As the pressure increases up to 32.7GPa, the ZnS nanorods are in rock salt phase. By fitting the experimental data of using Birch-Murnaghan equation, we get the bulk modulus of 98.6(3.5)GPa and 188.5(9.1)GPa respectively for the two structures, and the bulk modulus are larger than that of bulk ZnS sample and ZnS nanomaterials reported in the litereatures.After analyzing the lattice parameters with the pressure of the ZnS nanorods, we find that the linear coefficient of lattice parameter c with the pressure is bigger than that of lattice parameter a, and the values are -0.0093 ?/GPa and -0.01396?/GPa respectively. We also observe that the strain of ZnS nanorods along the c axis is larger than that of a axis in the very close pressure range comparing with the strain of ZnS bulk sample, by investigating the varation of ZnS nanorods in the a and c axises. This indicates that the change in the direction of c axis of ZnS nanorods in the wurtztite structure is larger than that of a axis under high pressure. There is no report about the phenomeno previously in the reports of ZnS bulk sample, nanoparticles and nanobelts in the literatures, and we observed it for the first time.Because the quantum confinement effect and the surface effect is rlated to the size of the nanomaterials, the different sizes of a and c axises of ZnS nanorods bring different quantum confinement effect and surface effect which causes different surface energy in the both directions. Under high pressure, the nanorods where the surface energy is larger has the weaker compression. This is the reseaon that we find that the strain of ZnS nanorods in the a and c axises are different obviously and this is determined by the special morphology of ZnS nanorods.According to the detailed atomic arrangement of both wurtzite and zinc blended structure, we know that the rearrangement of the atoms in the a axis is influenced much that in the c axis due to the different surface energy, and this results that the possibility of rearrangement of atoms becomes smaller that in the c axis. On the other hand, the larger possibility of atomic rearrangement in the c direction makes it easy for the nanorods transforming from wurtzite structure to rock salt structure without undergoing the zinc blended structure. This also indicates that there are hexagonal or h-MgO metastable or intermediate structure in the transformation process from wurtzite phase to rock salt phase very possibly like the hexagonal process reported by Cai et al.We carried out the study of pressure induced behavior of ZnS nanosheets for the first time, and find that the ZnS nanosheet transforms from zinc blended structure to rock salt structure at 13.1GPa. When the pressure is up to 15.1GPa, the ZnS nanosheets transform into rock salt phase completely, and the critical pressure is close to that of ZnS bulk samle and ZnS nanocrystals. By fitting the experimental data of ZnS nanosheets using Birch-Murnaghan equation, we get the bulk modulus 73.9GPa and 102GPa of ZnS nanosheets in the two structure of zinc blended phase and rock salt phase. When the pressure is up to 20.3GPa, the ZnS nanosheets begin transforming from rock salt phase to Cmcm structure, and until the highest pressure in the experiment, the Cmcm structure exists all along. Compare with the transition pressure from rock salt phase to Cmcm phase of bulk sample reported in the literature, the critical pressure we got is lower much than that of bulk sample. This is also first time to observed the phase transition to Cmcm structure in the ZnS nanomaterials.Like the description previously, the ZnS nanosheets has different size at different directions and the quantum confinement effect and surface effect are different in the vertical and parallel directions of the sheets where there are different surface energy. The (001) plane of ZnS nanosheets in rock salt phase is vertical to the sheets. The distortion occurs from rock salt phase to Cmcm phase consists (a) a shearing of alternate (001) planes in the [010] direction, (b) a puckering of the [100] atomic rows of rock salt in the [010] direction. The relative smaller quantum confinement effect and surface effect in the direction of (010) plane induced the phase transition from rock salt to Cmcm structure. This is the reseaon that the transition pressure is lower much than that of bulk sample.The morphology of the ZnS nanosheets is still kept after the high pressure treatment without destroy. The X-ray diffraction pattern shows that the released ZnS nanosheets from 32.7GPa in in mixed strucuture of zinc-blended structure and Cmcm structure. This indicates the Cmcm structure is kept at ambient condition, and this is the first report of occurrence of Cmcm structure for ZnS at ambient condition which testifies the effect of the special morphology of ZnS nanosheets in the pressure induced behavior.The pressure behavior of ZnS nanosheets tell us that we can induce the phase transition that can not be observed for the bulk materials under high pressure by control the morphology of nanomaterials basing on the different quantum confinement effect and surface effect, particularly, for the possible metastable or intermediate structure of materials predicted in the therotical works and this can also help us explore the basal theory and the principle under high pressure.In the study of the pressure behavior of CdSe/ZnS core/shell quantum dots, we find that base on the influence of ZnS shell, the CdSe core transforms from initial wurtztite structure to rock salt structure at 6.3GPa which is larger than the transition pressure of bulk CdSe sample and uncapped CdSe nanocrystals with similar size. When the pressure is 45.1GPa, there is a new and weak diffraction peak at 2.228? in d-spacing value which shifts to the smaller d-spacing value, and this peak corresponds to the (021) peak of Cmcm or distorted Cmcm structure observed in CdSe bulk sample possibly. Combining with the Ramam scattering spectra of CdSe/ZnS core/shell quantum dots under high pressure, we think the transformation from rock salt structure to Cmcm or distorted Cmcm structure occurs for the CdSe core of the CdSe/ZnS core/shell quantum dots. This is the first report that the rock salt phase transforms to Cmcm or distorted Cmcm structure for the CdSe nanomaterials and the transition pressure is also much higher than that of the bulk sample.By fitting the experimental data under high pressure for the CdSe using the Bich-Murnaghan, we get the bulk modulus in 94GPa of CdSe core in rock salt phase which is larger than that of the uncapped CdSe nanocrystals with ZnS shell. When the pressure is released completely, for the first time we find that the CdSe core of the CdSe/ZnS core/shell quantum dots still in rock salt phase for a certain time stead of returning to the initial wurtzite structure, and we also find the same phenomeno in the pressure dependent photoluminescence spectra.We give the explaniation of the influence of the ZnS shell on the pressure to compare the different compression of the core and shell by comparing the different bulk modulus of core and shell. The ZnS shell is harder than the CdSe core, and ZnS shell has the smaller compressibility, and thus the shell has the effect of protection for the CdSe core, and the under this protection, the compression of the core becomes smaller, therefore the transtition pressure of the CdSe core is higher than that of uncapped CdSe nanocrystals with ZnS shell and the bulk modulus of rock salt phase is enhanced. Due to the hysteresis effect of ZnS shell when decompression, there is a residual pressure on the CdSe core under the protection of ZnS shell after the the pressure released. And the the residual pressure keeps the CdSe in rock salt phase for a certain time. This tells us that we can make the structure of nanomaterials with reverible phase trasition kept in the high pressure phase at ambient condition by capping a relative harder shell on the nanomaterials.We find that the variation Y-O bonds of the cubic phase exhibits confusion with the increase of pressure unlike that of Y2O3/Eu3+ bulk sample decreasing linearly with the pressure, and we also observe that the Y2-O(1) bond increases with the pressure. This indicates that the deformation of the YO6 octahedron is outstanding comparing with that of bulk sample. To explore the influence of the Y-O bond variation on the arrangement of YO6 octahedrons for the cubic structure, we investigate the Y-Y distance under pressure. We give the mostly close Y1-Y2 and Y2-Y2 distances after the deformation of the YO6 and we also find that the variations of Y1-Y2 and Y2-Y2 distances are not linear like that of bulk sample. This indicates that the disordered deformation of octahedron results in the disorder of the arrangement of octahedron and destroys the long-distance arrangement of atoms of Y2O3/Eu3+ nanotubes, and finally the nanotubes transform into amorphous phase. Though the nanotubes are in length of 1um, however, the thickness of the wall is very thin in 5-10nm. Thus, the nanoscaled effect in vertical of the thickness is obvious and the deformation is much harder which induces the formation disordered structure.