Ion Beam Synthesis Of Cds, ZnS, And PbS Compound Semiconductor Nanocrystals
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399 Mat. Res. Soc. Symp. Proc. Vol. 504 0 1998 Materials Research Society
RESULTS X-ray diffraction results demonstrating the formation of CdS nanocrystals in Si0 2 and A120 3 are shown in Figs. 1 and 2. In Si0 2 (Fig. 1), diffuse scattering from Si0 2 is observed in addition to strong lines arising from CdS. In A120 3, there are a multitude of diffraction lines arising from CdS, in addition to the expected diffraction from the matrix. The position and relative intensity of x-ray tines characteristic of the hexagonal wurtzite structure of CdS and the two possible cubic structures, zincblende (ZB) and rocksalt (NaCl), are indicated in Figs. 1 and 2 (from powder files). Detailed analysis of the x-ray diffraction results show that most of the CdS nanocrystals in both Si0 2 and A12 0 3 have the hexagonal structure. In addition, the CdS nanocrystals in A120 3 are oriented with the matrix having their (002) planes parallel to the c-planes of A120 3. These nanoparticles exhibit strong in-plane alignment also. Therefore, oriented CdS nanoparticles with hexagonal structure are produced in A120 3 if implantation is carried out at elevated temperatures. By contrast, we find that the cubic structure is produced if implantation is done at low temperature where we form the amorphous phase during implantation [13]. 120
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Fig. 1. X-ray diffraction showing the formation of CdS nanocrystals in Si0 2/Si. Equal concentrations (5.3 x 1021/cm 3) of Cd and S were implanted to a depth of -180 nm.
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Fig. 2. X-ray diffraction from CdS nanocrystals in A120 3 . Equal doses (4.3 x 6 2 10' /cm ) of Cd (450 keV) and S (164 keV) were implanted at 550'C.
Figures 3 and 4 are x-ray diffraction results demonstrating the formation of ZnS (Fig. 3) and PbS (Fig. 4) in Si0 2 and A120 3 as a result of implantation and annealing. ZnS has hexagonal and cubic structures, and the ZnS nanocrystals produced in Si0 2 and A120 3 by ion implantation are a mixture of the two structures. In SiO 2, the nanocrystals are randomly oriented, but in A12 0 3 , they are oriented with respect to the matrix having the (002) planes of the hexagonal ZnS and the (111) planes of cubic ZnS parallel to the c-planes of A120 3. In A12 0 3, the ZnS nanoparticles also exhibit strong in-plane orientation. Figure 4 shows that cubic PbS nanocrystals are formed in both SiO 2 and A120 3 as a result of ion implantation and annealing. In Si0 2, the nanoparticles are randomly oriented, but in A120 3, they are oriented with their (002) planes parallel to the c planes of A120 3. Figures 5 and 6 are cross-section TEM micrographs showing the microstructures in the nearsurface region of Si0 2 and A120 3 samples containing CdS, ZnS, and PbS nano
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