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
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
To join NDL as a graduate student or postdoctoral research visit
the web-page HERE.
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Devices and Circuits visit the web-page HERE.