ZnO has recently attracted significant attention as a very promising wide-band-gap material (3.37 eV at T=300 K) for a variety of optoelectronic and spintronic applications. Due to unique excitonic properties of ZnO, such as large exciton binding energy (~60 meV) and biexcitonic binding energy (15 meV) many interesting optical effects are observed and expected in nanostructures made of ZnO (see Figure 1). Investigation of ZnO-based nanostructurs is only at its beginning. A number of theoretical and experimental issues have to be addressed. The origin of reported ~ 500 times increase in third-order non-linear optical susceptibility in ZnO nanoparticles compared to bulk ZnO is not clear. The fact that ZnO has very small dielectric constant, and correspondingly large electron - hole Coulomb interaction, leading to very small exciton diameter complicates theoretical treatment for ZnO nanostructures. Professor Balandin's Nano-Device Laboratory (NDL) research group carries out both theoretical and experimental research aimed at understanding physical properties of ZnO quantum dots and development of novel optical, electrical and spintronic devices based on ZnO nanostructures.

Figure 1. TEM of spherical ZnO quantum dot with diameter D=4nm.
Data is after M. Shamsa and A.A. Balandin, NDL, 2005.

NDL researchers have recently demonstrated experimentally that even the low-power ultraviolet laser excitation, required for the resonant Raman spectroscopy, can lead to the strong local heating of ZnO nanocrystals. The latter causes significant (up to 14 cm-1) red shift of the optical phonon peaks of ZnO nanocrystals (diameter D~20 nm) compared to their position in bulk crystals (see Figure 2). The obtained results are used for identification of the phonons in Raman spectra of ZnO nanostructures and for their optimization for optoelectronic applications (phonon bottle-neck effect). Experimental polarization dependant study was carried out using the NDL micro-Raman spectroscopic facilities .

Figure 2. Raman spectrum of ZnO nanocrystals. Results are after
K. Alim, V.A. Foboberov and A.A. Balandin et al., Appl. Phys. Lett., 2004.

NDL member Dr. V.A. Fonoberov and Prof. A.A. Balandin have derived within the dielectric-continuum model an integral equation that defines interface and confined polar optical-phonon modes in nanocrystals with wurtzite crystal structure (ZnO or GaN). It has been demonstrated theoretically, that while the frequency of confined polar optical phonons in zincblende nanocrystals is equal to that of the bulk crystal phonons, the confined polar optical phonons in wurtzite nanocrystals have a discrete spectrum of frequencies different from those of the bulk crystal. The calculated frequencies of confined polar optical phonons in wurtzite ZnO nanocrystals are found to be in an excellent agreement with the experimental resonant Raman scattering spectra of spherical ZnO quantum dots (see Figuire 3).

Figure 3. Calculated optical phonon modes in wurtzire ZnO quantum dots.
Results are after V.A. Fonoberov and A.A. Balandin, Phys. Rev. B (2004).

NDL researchers investigated the origin of ultraviolet photoluminescence (PL) in ZnO quantum dots with diameters from 2 to 6 nm. Two possible sources of ultraviolet PL have been considered: excitons confined in the quantum dot and excitons bound to an ionized impurity located at the quantum-dot surface (see Figure 4). It is found that depending on the fabrication method and surface passivation technique, the ultraviolet PL of ZnO quantum dots can be attributed to either confined excitons or surface-bound ionized acceptor-exciton complexes. The exciton radiative lifetime is shown to be very sensitive to the exciton localization and can be used as a tool to discriminate between these two sources of PL. Details of the study are reported in V.A. Fonoberov and A.A. Balandin, Appl. Phys. Lett., 2004.

Figure 4. Exciton energy levels and oscillator strength in ZnO quantum dots. NDL, 2004.

More information on the projects currently under way in the Nano-Device Laboratory (NDL) can be found HERE. To join NDL as a graduate student or postdoctoral research visit the web-page HERE. To learn more about course offering in the field of Materials, Devices and Circuits visit the web-page HERE.