Optical Properties of ZnO Nanomaterials Leah Bergman University of Idaho DMR The objective of this project is to understand the optical and structural properties of ZnO nanomaterials. One of the main advantages of a nanomaterial is the tunability of its light emission energy as a function of size; by the choice of nanoparticle size a semiconductor can emit light at a desirable energy. ZnO nanomaterials are potential building blocks for optical as well as novel opto- magnetic devices such as spin light emitting diodes, and magneto-optical switches. The study of material optical and structural properties is key to viable fabrication of such devices. Nanocrystallites of different morphologies and size distribution are studied via photoluminescence (PL) and Raman scattering in order to determine the role of surface defects and size distribution on the luminescence. As a preliminary study two samples of different morphology and size distribution were investigated: ZnO nanocrystallites synthesized via sonochemical technique (inset to Fig.1) [1], and cobalt-doped ZnO nanoclusters grown via magnetron-sputtering aggregation [MSA] technique (inset to Fig. 2) [2]. Both nano-materials exhibit room- temperature UV luminescence which is blue-shifted relative to that of the bulk (Fig. 1) as expected from the confinement effect. The size distribution of the nanoclusters grown via MSA is smaller than that of the nanocrystallites, and the crystallites are of more uniform shape. However, the PL linewidths of both samples are comparable implying that the PL spectral-lines do not reflect solely the statistical nature of the samples. A feasible explanation is that at this size regime the surface to volume ratio is sufficiently large that surface defects also affect the optical properties. Additionally, our study into the issue of magnetic doping, that in the long run might enable novel opto-magnetic applications at the nanoscale, shows promising results [2]. ZnO nanoclusters of cobalt doping of concentration range ~ 2% - 5% were found to exhibit UV-PL (Fig. 1) and ferromagnetic behavior (Fig. 2) both at room temperature. Future research will study the impact of surface defects, size and distribution, as well as doping concentration on the optical and magnetic properties. These efforts will contribute to the realization of highly functional ZnO nanomaterials. 1. In collaboration with Kumar. Das, Tuskegee University. Abstract accepted to MRS Fall In collaboration with You Qiang, University of Idaho. Abstract accepted to the 49 th conference on Magnetism and Magnetic Materials. Fig. 1. The ultraviolet photoluminescence of ZnO bulk, cobalt-doped nanoclusters, and undoped nanocrystallites. Inset is the TEM of the undoped nanocrystallites [1]. Fig. 2. Room-temperature hysteresis loop of 5% cobalt-doped ZnO nanoclusters. Inset is the AFM of the nanoclusters grown in MSA system at Qiang’s lab, UI [2].

At the bulk or macro-scale ZnO is a direct wide-bandgap semiconductor of deep exciton level ~ 60 meV: these attributes should result in efficient room temperature luminescence. Recent work has indicated that ZnO can also be a host matrix for rare earth and magnetic dopants, both of which make ZnO a desirable candidate for novel optical as well as opto-magnetic applications. At the nanoscale, an additional advantage of ZnO has to do with the ability of changing the PL emission energy via the size of the crystallites, thus achieving new emission lines that otherwise might not be possible. The objective of this project is to understand the underlying mechanisms which determine the optical properties of ZnO nanomaterials and to explore new material functionalities via doping. For our optical studies a state-of-the-art UV-optical system of high spatial and spectral resolutions is used. The spectral resolution is ~ 3 cm-1 at 244 nm, the probing spot diameter is ~ 600 nm, and the CCD camera allows viewing of the laser spot and the sampled area simultaneously. The ZnO crystallites studied here (Fig. 1 and Fig.2) exhibit room temperature luminescence of significant intensity (the actual quantum yield is under investigation); however, the spectral line is broadened relative to that of the bulk (Fig. 1). Ideally the line width needs to be narrow for opto-device applications. In general, PL line-broadening in the above type of crystallites may be a result of structural defects and size distribution effect, both of which are referred to as the inhomogeneous line broadening mechanism. At the nanoscale the large surface to volume ratio comes into play, and surface defects such as dangling bonds, surface impurities, and stress are expected to control the properties of the PL. The above samples have different shape and size distribution: the nanoclusters grown via MSA have spherical symmetry and a better size distribution than the nanocrystallites grown via the sonochemical method, which consist mainly of needles of varying length and thickness as well as a small percentage of circular platelets of average size

~ 30 nm. However, the PL linewidths of both samples are comparable, implying that another broadening mechanism is present along with that due to the variance in the size distribution. We speculate that surface defects contribute to the line broadening. Currently we are addressing the issues of the origins of the inhomogeneous broadening of the PL, and will be looking at issues such as O-H type impurities which are known to be prevalent in ZnO. Additionally we will explore ways to passivate the surface of the clusters in order to achieve high quality luminescent nanomaterial. Another topic that is currently being researched is the issue of finding the right doping concentration that will yield both significant ferromagnetic behavior as well as efficient UV-luminescence. Our preliminary results [2] indicated that UV-PL can be observed in samples of up to ~ 5% cobalt doping; above that level the PL intensity was found to be significantly diminished, although the sample still exhibits ferromagnetic behavior. At present it is speculated that the absence of luminescence at elevated cobalt doping may be a result of the degradation of material quality due to cobalt aggregation or due to PL quenching via complexes that act as nonradiative centers.

Education: The graduate students who are involved and supported via this grant are John L. Morrison and Jesse Huso. Xiang-Bai Chen just graduated with a P.hD (August 04) and stays in the group as a postdoc. The undergraduates are Heather Hoeck and Jennifer Elle, both of whom are supported via this grant. The PI also participates each summer in the NSF-REU as well as in the HOIST programs, the latter which is a program dedicated to involving Native American high school students in active research in the sciences. Outreach: The PI and her undergraduate student, Heather Hoeck, initiated contact with the Inland Empire Girl Scouts of America. This organization will hold an “Expanding Your Horizons Conference” in Spring This yearly conference was developed by the Math/Science Network in The Math/Science Network consisted of women scientists, and its purpose was to educate young women about opportunities in the math and science fields and hopefully improve enrollment of women in these fields. Heather is on the conference organizing committee, which is involved in finding speakers and promoting awareness about the conference activities to local youth. In addition, Heather will participate in the conference as a speaker and will give demonstrations involving optics and light. After the conference she will continue involvement by becoming a mentor to the girls who are interested in physics. Heather and the PI are currently working on designing physics demonstrations suitable for girls ages years. Optical Properties of ZnO Nanomaterials Leah Bergman University of Idaho DMR Heather and Chen working at the UV Raman system