Micro-Raman investigation of optical phonons in ZnO nanocrystalsKhan A. Alim, Vladimir A. Fonoberov, Manu Shamsa, and Alexander A. Balandina兲Nano-Device Laboratory, Department of Electrical Engineering, University of California-Riverside,Riverside, California 92521共Received 28 February 2005; accepted 5 May 2005; published online 27 June 2005兲We have measured nonresonant and resonant Raman-scattering spectra from ZnO nanocrystals withan average diameter of 20 nm. Based on our experimental data and comparison with the recentlydeveloped theory, we show that the observed shifts of the polar optical-phonon peaks in the resonantRaman spectra are not related to the spatial phonon confinement. The very weak dispersion of thepolar optical phonons in ZnO nanocrystals does not lead to any noticeable redshift of the phononpeaks for 20-nm nanocrystals. The observed phonon shifts have been attributed to the local heatingeffects. We have demonstrated that even the low-power ultraviolet laser excitation, required for theresonant Raman spectroscopy, can lead to the strong local heating of ZnO nanocrystals. The lattercauses significant 共up to 14 cm−1兲 redshift of the optical-phonon peaks compared to their position inbulk crystals. Nonresonant Raman excitation does not produce noticeable local heating. Theobtained results can be used for identification of the phonons in the Raman spectra of ZnOnanostructures. © 2005 American Institute of Physics. 关DOI: 10.1063/1.1944222兴I. INTRODUCTIONZinc oxide 共ZnO兲 presents interesting material systemfor investigation due to its wide band gap of 3.37 eV andsome intriguing optical properties. A prominent feature ofZnO is its large exciton binding energy 共⬃60 meV兲 at roomtemperature, which results in extreme stability of excitons.1Recently, nanostructures made of ZnO have attracted signifi-cant attention due to their proposed applications in the low-voltage and short-wavelength 共368 nm兲 electro-optical de-vices, transparent ultraviolet 共UV兲 protection films, gassensors, and even spintronic devices. Despite practical im-portance, current knowledge of vibrational 共phonon兲 proper-ties of ZnO nanostructures is rather limited. Understandingthe specifics of phonon spectrum 共both optical and acoustic兲of ZnO nanostructures can help in development of ZnO-based optoelectronic devices.Raman spectroscopy is a nondestructive characterizationmethod of choice for many recent studies of the vibrationalproperties of ZnO nanostructures.2–14Due to the Heisenberguncertainty principal, the fundamental q ⬃0 Raman selectionrule is relaxed for a finite-size domain, allowing the partici-pation of phonons away from the Brillouin-zone center. Thephonon uncertainty goes roughly as ⌬q ⬃1/d, where d is thediameter of a nanocrystal or quantum dot. This spatial con-finement inside nanocrystals gives rise to a redshift andasymmetric broadening of the Raman peaks in nanostruc-tures compared to bulk crystals.ZnO is a semiconductor with wurtzite crystal structure.Wurtzite structure belongs to the space group C6␯4with twoformula units per primitive cell, where all atoms occupy C3␯sites. The Raman active zone-center optical phonons pre-dicted by the group theory are A1+2E2+E1. The phonons ofA1and E1symmetry are polar phonons and, hence, exhibitdifferent frequencies for the transverse-optical 共TO兲 andlongitudinal-optical 共LO兲 phonons. Nonpolar phonon modeswith symmetry E2have two frequencies, E2共high兲 is associ-ated with oxygen atoms and E2共low兲 is associated with Znsublattice. All described phonon modes have been reportedin the Raman-scattering spectra of bulk ZnO.15,16Raman fre-quencies of both polar and nonpolar optical phonons areshifted in the spectra obtained from ZnO nanostructurescompared to their positions in the spectra from bulk ZnO.The origin of these phonon frequency shifts is still underdebate. In our recent letter,17we reported considerations,which brought us to the conclusion that the strong redshift ofphonon peaks in Raman spectra from ZnO quantum dots andrelated nanostructures is related to local heating rather thanto the spatial confinement. In order to avoid misinterpretationof the phonon properties of ZnO nanocrystals, it is importantto gain complete understanding of the origin of the phononfrequency shifts. This paper provides details of our measure-ments and analysis in support of the local heating argument.It also outlines a method for distinguishing the confinement-induced shifts from the local heating shifts.There are three possible mechanisms for the phononpeak shifts in Raman spectra of nanostructures. The first oneis spatial confinement within the quantum dot 共nanocrystal兲boundaries. The second one is related to the phonon local-ization by defects. Nanocrystals or quantum dots, producedby chemical methods or by the molecular-beam epitaxy, nor-mally have more defects than corresponding bulk crystals.The spatial confinement of optical phonons was studied byRichter et al.,18who showed that the Raman spectra of nano-crystalline semiconductors are redshifted and broadened dueto the relaxation of the q-vector selection rule in the finite-size nanocrystals. Optical-phonon confinement in wurtzitenanocrystals leads to somewhat different changes in Ramanspectra due to the optical anisotropy of wurtzite lattice. Re-cently, Fonoberov and Balandin19–21derived analytically anexpression for the interface and confined polar optical-phonon modes in spheroidal quantum dots 共QDs兲 witha兲Electronic mail: [email protected] OF APPLIED PHYSICS 97, 124313 共2005兲0021-8979/2005/97共12兲/124313/5/$22.50 © 2005 American Institute of Physics97, 124313-1Downloaded 27 Jun 2005 to 138.23.212.31. Redistribution subject to AIP license or copyright, see http://jap.aip.org/jap/copyright.jspwurtzite crystal structure. Confined optical phonons inwurtzite nanocrystals were shown to have a discrete spec-trum of frequencies different from those of bulk phonons.In this paper we present details of the experimentalstudy, which indicate that the large redshift 共up to 14 cm−1兲in ZnO nanocrystals with the diameter of 20 nm is related tolocal heating rather than to phonon confinement. This con-clusion is in line with several reports22–35of the photolumi-nescence 共PL兲 and Raman peak shifts in nanostructures,which were also attributed due to the local laser heating. InSec. II we describe the investigated ZnO