Quantum confinement in Volmer–Weber-type self-assembled ZnO nanocrystalsTae-Bong Hur, Yoon-Hwae Hwang, and Hyung-Kook Kim Citation: Applied Physics Letters 86, 193113 (2005); doi: 10.1063/1.1921357 View online: http://dx.doi.org/10.1063/1.1921357 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/86/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Self-assembled formation and transformation of In/CdZnTe(110) nano-rings into camel-humps Appl. Phys. Lett. 100, 213116 (2012); 10.1063/1.4721805 Evolution of self-assembled type-II ZnTe/ZnSe nanostructures: Structural and electronic properties J. Appl. Phys. 111, 093524 (2012); 10.1063/1.4705385 Self-assembled ZnO quantum dots with tunable optical properties Appl. Phys. Lett. 89, 023122 (2006); 10.1063/1.2221892 Quantum confinement in ZnO nanorods Appl. Phys. Lett. 85, 3833 (2004); 10.1063/1.1811797 Self-organized ZnO quantum dots on SiO 2 / Si substrates by metalorganic chemical vapor deposition Appl. Phys. Lett. 81, 5036 (2002); 10.1063/1.1527690

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Quantum confinement in Volmer–Weber-type self-assembledZnO nanocrystals

Tae-Bong Hur, Yoon-Hwae Hwang, and Hyung-Kook Kima!

Department of Physics and Research Center for Dielectric Advanced Matter Physics, Pusan NationalUniversity, Busan 609-735, Korea

sReceived 13 September 2004; accepted 15 March 2005; published online 5 May 2005d

We have studied the quantum confinement effect on Volmer–Weber-type self-assembled ZnOnanocrystals. Volmer–Weber-type self-assembled ZnO nanocrystals were grown on the Pts111dsubstrate by using a rf-magnetron sputtering method and were confirmed by the Auger electronspectroscopy. The free exciton transition energies of 57-, 38-, and 24-nm-size nanocrystals werefound to be roughly 3.298, 3.311, and 3.337 eV, respectively, by photoluminescence measuremnetsat room temperature. The blueshift of the photoluminescence peak energy of ZnO nanocrystals of24 nm in diameter roughly varied by 40 meV compared to bulk ZnO. ©2005 American Institute ofPhysics. fDOI: 10.1063/1.1921357g

Recently, many groups have studied fabrication of ZnOnanocrystals because unique properties such as UV nanolaseractivity have been demonstrated.1–3 The fabrication of ZnOnanostructures such as nanowires, nanorods, andnanotubes4–6 has been reported. Several articles reported onthe weak and strong confinement effects of ZnO nanocrystalsthat depend on nanocrystal size or shape.7–10The strong con-finement effects of colloidal insulating nanocrystals based onthe tight-binding theory were reported by Germeauet al.8

The grain-size-dependent weak confinement effects of nano-crystals fabricated in a partial oxidation process in metallicZn system were revealed to be of the smaller size than 61 nmsRef. 10d or 40 nm,11 and the blueshift of the 20 nm grainsize was roughly 54 meV. The confinement effects of ZnOquantum particles by the quenching method were reportedfor 50 nm radius.12 Park et al. reported the confinementeffects13 in ZnO/Zn0.8Mg0.2O multilayer on the top of ZnOnanorods. The energy shift due to the confinement variedfrom 0 to ,155 meV by changing the well width from11 to 1.1 nm. However, the quantum confinement effects ofVolmer–Weber-type self-assembled ZnO nanocrystal filmhave not been investigated because the deposited atoms onthe substrate were usually grown in the Stranski–Krastanowgrowth mode. In this letter, we present the confinement effectof Volmer–Weber-type ZnO nanocrystals on the Pt substratecorresponding to the weak confinement regime.

Self-assembled ZnO nanocrystals were grown onPts111d /TiO2/SiO2/Si substrates by radio frequency magne-tron sputtering deposition in a gas mixture of argon and oxy-gen. The purity of both gases was 99.999%. The O2/Ar ratiowas varied between 2.57 and 2. The Zn target was 2.5 cm indiameter and its purity was 99.999%. The Pt substrate wasattached at the edge of the heater holder and was rotatedusing a 3.5 cm diameter heater holder. The substrate washeated at 680 °C. The rotation speed and angle of the heaterholder were roughly 36 rpm and 45° for a vortex depositionmotion. The size of the Volmer–Weber-type self-assembledZnO nanocrystals was controlled by varying the sputteringtime. The average diameter of the ZnO nanocrystals wasmeasured by scanning electron microscopy and was analyzed

by the Image-Pro-Plus Program. A surface analysis was ac-complished by Auger electron spectroscopysAESd. The en-ergy and base pressure of Auger electron spectroscopy were10 kV and 1.3310−9 mb. The optical properties of the ZnOnanocrystals were investigated by photoluminescencesPLdwith a He–Cd lasers325 nmd at room temperature.

Figure 1 shows SEM images of the Volmer–Weber-typeself-assembled ZnO nanocrystals on the Pts111d substrate.The morphology of the ZnO nanocrystals was hexagonal.The SEM images were shown at 300 nmfsad, sbdg and600 nm fscd, sddg scales. When the zinc and oxygen atomswere deposited on the substrate at a low temperature, below600 °C, the deposited atoms formed the 2D wetted layer ofStranski–Krastanow growth type. Also, when the sputteredatoms were deposited on the fixed substrate at the high sub-strate temperature of 680 °C, they formed the continuousbowls-type layer. However, the Volmer–Weber-type self-assembled ZnO nanocrystals were grown on the rotated sub-strate at high temperature.

adElectronic mail: hkkim@pusan.ac.kr

FIG. 1. SEM images of self-assembled ZnO nanocrystals grown on Pts111dsubstrate. The scalessad andsbd were 300 nm. The scales ofscd andsdd were600 nm. The average diameterssheightd of ZnO nanocrystalssad, sbd, scd,and sdd were 24 nms15 nmd, 35 nm s22 nmd, 42 nm s27 nmd, and 57 nms32 nmd, respectively.

APPLIED PHYSICS LETTERS86, 193113s2005d

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Figure 2 shows the size distribution of ZnO nanocrystalsobtained from the 2D SEM imagessFig. 1d using the Image-Pro-Plus Program and synchrotron x-ray scattering. The av-erage diameters of ZnO nanocrystalssad, sbd, scd, and sddwere 24, 35, 42, and 57 nm, respectively. These diameterswere obtained from the assumed spherical shapes4pr3/3=HCLd calculated by the heightsHd, crosssCd, and lengthsLd of a nonspherical shape like that of a disk. The fullwidths at half maximum of the size distributions of thesenanocrystals were roughly 8, 10, 12, and 17 nm, respec-tively. The inset of Fig. 2 shows on Auger electron spectra ofself-assembled ZnO nanocrystals. An Auger electron spec-trum plots the number of electrons detected as a function ofelectron kinetic energy. The spectra were obtainedsed on thenanocrystal andsfd between the nanocrystals of the singlesample surface by the multipoint AES scanning method. Thespectrum of sed shows the energy levels of Zn LM2s994.87 eVd and Zn LM1 s1018.10 eVd with the referredcarbon levelsC KL1d of 271.47 eV. However, the energylevels of Zn LM1 and Zn LM2 atsfd completely disappeared.This indicates that the ZnO nanocrystals directly grew asonly 3D islands without the 2D wetting layer on the substratesurface even at the initial growth stage. In other words, thefabricated nanocrystals were grown in the Volmer–Webergrowth mode. From synchrotron x-ray scattering measure-ments at the 3C2 beamline at Pohang Light Source, the pre-ferred orientation of nanocrystals was thec-axis of ZnO hex-agonal structure along the surface normal direction. Alongitudinal scan at the ZnOs0002d peak clearly showed theinterference fringes. This indicates that the self-assembledZnO nanocrystals have an atomically well-ordered finite-sizedomain. Therefore, the thicknesses of the ZnO nanocrystalswere calculated from the full with at half maximum and theinterference fringe of the ZnOs0002d peak along the surfacenormal direction. The heights of nanocrystalssad–sdd wereroughly 15, 22, 27, and 32 nm, respectively. As the averagediameters of ZnO nanocrystals increased, the size distribu-tion increased. The size distributions of nanocrystals werefitted into a Gaussian function. The size-distribution width ofthe nanocrystals resulted in the broadening of photolumines-cence peak width.

Figure 3 shows the photoluminescence spectra of theVolmer–Weber-type self-assembled ZnO nanocrystals at

room temperature. The free exciton transition energies of57-, 38-, and 24-nm-size nanocrystals were roughly 3.298,3.311, and 3.337 eV, respectively. The free exciton transitionenergies of smaller-size ZnO nanocrystals were shifted tohigher energy than that of bulk ZnO. The full widths at halfmaximum of the free exciton emission peaks were roughlyvaried from 134 to 125 meV. The FWHM of the free excitonemission peaks decreased as the size distribution of ZnOnanocrystals decreased. Size distributions of single nanocrys-tals will lead to a statistical distribution of the eigenenergiescharacterized by a spectral width.14 Also as the average di-ameter of the Volmer–Weber-type self-assembled ZnO nano-crystals decreased, the free exciton emission energy progres-sively increased. The inset of the Fig. 3 shows thebackground PL signal of the pure Pts111d /TiO2/SiO2/Sisubstrate. The broad emission energy was roughly 2.76 eV;we speculate that it came from defects or naturally formed Sinanocrystal in SiO2.

15,16 The green band emissions,2.38 eVd of self-assembled ZnO nanocrystals did not ap-pear.

Figure 4 shows the variation of the emission energy as afunction of the ZnO nanocrystal diameter. The variation ofthe emission energy of the ZnO nanocrystals was calculatedfrom the free exciton transition energy at room temperature.We observed the blueshift of the emission energy of ZnOsemiconductor with a decrease in the particle size. The varia-tion energy of the ZnO nanocrystals was roughly 40 meV,

FIG. 2. Size distribution obtained from the SEM images of Fig. 1 and thesynchrotron x-ray scattering. The inset shows the AES of self-assembledZnO nanocrystals. The spectra were obtainedsed on the nanocrystal, andsfdbetween nanocrystals.

FIG. 3. PL spectra of Volmer–Weber-type self-assembled ZnO nanocrystalsThe inset shows the background photoluminescence signal of the purePts111d /TiO2/SiO2/Si substrate.

FIG. 4. Variation of the emission energy as a function of the ZnO nanocrys-tals diameter. The dotted and solid lines are curves fitted by the EMA andthe phenomenological expression.

193113-2 Hur, Hwang, and Kim Appl. Phys. Lett. 86, 193113 ~2005!

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and occurred as a result of changing the diameter from bulkto 24 nmsheight: 15 nmd. The first explanation for the sizedependence of the electronic properties of the nanocrystalswas the effective mass approximation in infinite confiningpotential by Efros and Efros.17 In this model,dEg=C/d2,where thed is diameter of nanocrystals, andC is the param-eter related to the effective masses. The size dependence ofelectronic properties in a finite-sized system was proposedby Brus.18 This expression was composed of a kinetic energyterm, a Coulomb attraction term between electron and hole,and a polarization term. The size- dependent function of Bruswas useful in the strong confinement regime and belowroughly 7 nm in radius with ZnO parameters such as effec-tive masses and static dielectric constant. Recently, Sapraand Sarma19 proposed a simple exponential function relatingDEg to any size of nanocrystals,DEg=a1e

−d/b1+a2e−d/b2,

while this expression is entirely phenomenological. It has thecorrect limiting behavior at a large diameterd with the pa-rameter valuesa1, b1, a2, andb2. Therefore, the data on thesize dependence of the emission energy shift in Fig. 4 werefitted by an effective mass approximation and phenomeno-logical simple exponential functions. The quantum confine-ment effects in the Volmer–Weber-type self-assembled ZnOnanocrystals were apparently revealed for the smaller sizesthan 57 nm in diametersheight: 32 nmd. The previousresults10–13 including our result regarding the confinementeffects are different from each other. Therefore, to correct thedifferent values, the detailed studies on the surface polariza-tion effects and geometrical effects of nonspherical-typenanocrystals are necessary.

In summary, Volmer–Weber-type self-assembled ZnOnanocrystals were grown on the Pts111d /TiO2/SiO2/Si sub-strate using the rf-magnetron sputtering method. The quan-tum confinement in the ZnO nanocrystals on 24 nm diametersheight: 15 nmd was roughly 40 meV. The widths of the pho-

toluminescence emission peaks decreased as the size distri-bution decreased. The quantum confinement effect as a func-tion of the nanocrystal size was explained by aphenomenological simple exponential function.

This work was supported by Korea Research FoundationGrant No. KRF 2004-005-C00065.

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193113-3 Hur, Hwang, and Kim Appl. Phys. Lett. 86, 193113 ~2005!

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