4D STEM/EELS Characterization of Au Nanodots in a ZnO Thin Film

Abstract

Nanostructured thin films can be engineered during synthesis [1-2] to yield unique structure-property relationships. In a thin film of zinc oxide (ZnO) with embedded Au nanodots [2], the material properties are anisotropic due to the ZnO wurtzite crystal structure and vary in three-dimensions as dictated by changes in the film's composition and crystallographic orientation. Recently, researchers have used EELS spectroscopy to explore effects of size and crystal orientation on the electronic structure of ZnO nanowires and nanobelts [3][4][5]. Crystallographic effects were reported as systematic differences in low-loss and core-loss structure when spectra were obtained with the electron beam parallel and perpendicular to the c-axis of the ZnO [5]. The authors also show that these differences vary off-axis but do not explore this variation systematically. In this study we used a combination of FIB nanofabrication and STEM/EELS nanoanalysis to determine morphology and arrangement of the Au nanodots in the ZnO film and collect data on optoelectronic properties of this nanomaterial composite. We report the VEELS spectra of ZnO as mapped in three-dimensions by acquiring a series of STEM/EELS spectrum images at regular sampling intervals during a 360° rotation.Nanofabrication with a FIB was used to prepare a pillar of ZnO with final dimensions measuring ~5um in length and ~140nm thickness. Imaging and analysis was performed in a 200kV Hitachi HD-2300A STEM equipped with a Gatan Enfina EELS spectrometer. HAADF and phase contrast imaging were used to document the Au nanodot size, arrangement and morphology. Direct threedimensional probing of the low-loss electronic structure was made possible by utilizing the STEM's high probe-current and an improved EELS spectrum detection speed to achieve read-out speeds of greater than 100 spectra/sec [6]. Gold nanodots embedded in the ZnO thin film matrix, with diameters on the order of ~30nm, are seen as regions of bright contrast in the cross section micrograph of FIG. 1A. The pillar for 360˚ rotation was nanofabricated from the approximate region indicated by the red box in FIG. 1B and is shown at low magnification (FIG. 2A) and higher magnifications (FIG. 2B and 2C) to better resolve the Au nanodots. The 360˚ image data was tiled together in sequential order to make movies allowing simple interpretation of Au nanodot arrangement in the thin film. The systematic variation in EELS low-loss spectra as a function of rotation angle is shown in FIG 3. The spectra taken with the beam on and near the film's c-axis shows a small peak at 13eV labeled "B" in the figure which dissappears when the beam is aligned close to the film's a-axis. The origin of this feature is still under investigation and is believed to be either due to surface plasmon excitation or the anisotropic properties of the nanomaterial. Diffraction patterns obtained during the same analysis (also shown in FIG. 3) clearly illustrate the ability of this 4D STEM-EELS technique to probe the same sample with the beam both parall...