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Scheme of high-pressure PLD chamber for nanoheterostructures.
Typical SEM images of PLD grown nanowires on sapphire: (a) ZnO nanowire array limited to size of gold nucleation film, (b) needle-like ZnO nanostructures obtained at reduced laser pulse energy, (c) aligned hexagonal ZnO nanocrystallites grown at reduced Ar pressure, (d) microcrystals with round cross section and crystalline cap obtained by successive deposition from ZnO and ZnO:Zn targets, (e) hexagonal ZnO microdots with wires on top grown on buffer layer, and (f) typical nanowire array.
ZnO wires with diameter 100–150 nm (a) and 1000 nm (b) are grown at 30 and 10 mm PLD target-substrate distance, respectively. The wire diameter (labeled ZnO, NiO denotes a NiO nucleation layer instead of Au) and CL peak energies of wires (labeled MgZnO, for both front edge and center position at the sample) clearly correlate to PLD parameters. The Mg content is due to Eq. (1) using . The lines are drawn to guide the eye.
(Color online) Lateral homogeneity of Mg content in a nanowire array: (a) SEM image, the white rectangle indicates the area of the CL scan image of the excitonic peak energy (b). (c) shows a selection of three typical CL spectra as used for (b), together with a spectrum of a ZnO nanowire, and the histogram of CL peak energies with Gaussian fit. (d) shows FWHM of CL spectra at of individual wires vs Mg concentration [obtained from energy of the CL peak maximum and Eq. (1)]. The dashed curve in (d) is theoretical alloy broadening (see Ref. 20) for ZnO exciton parameters (exciton radius 1.8 nm). For the solid line in (d) the exciton radius has been linearly decreased to account for an increase of the dielectric constant by 6% for (see Ref. 21) and a decrease of the exciton binding energy by 20% at (see Ref. 12).
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