n-doped sno2 nanocrystals with green emission dependent ... · 2 nanocrystals with green emission...

6
N-doped SnO 2 nanocrystals with green emission dependent upon mutual effects of nitrogen dopant and oxygen vacancy G.X. Zhou a , S.J. Xiong a , X.L. Wu a,b,, L.Z. Liu a , T.H. Li a , Paul K. Chu b a National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China b Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China Received 19 May 2013; received in revised form 16 August 2013; accepted 21 August 2013 Available online 23 September 2013 Abstract A facile and economical chemical route was used to synthesize nitrogen-doped tin oxide (SnO 2 ) nanocrystals (NCs). Infrared and Raman spectral examinations reveal the existence of oxygen vacancies and local disorder. Temperature-dependent photoluminescence (PL) measurements display a broad band at 640 nm at room temperature that shifts to a higher energy at lower measurement temper- ature. Excitation wavelength-dependent PL spectra show that the band blue-shifts and its line width decreases as the excitation wave- length is reduced. The PL peak also blue-shifts when the annealing temperature is increased. Spectral analysis and theoretical calculation suggest that the PL band stems from the mutual effects of oxygen vacancies and nitrogen dopants. This PL investigation on N-doped SnO 2 NCs provides more insights about the optical properties and will promote further applications of SnO 2 NCs. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: SnO 2 nanocrystal; N-dopant; Oxygen vacancy; Photoluminescence 1. Introduction Nanostructured tin oxide (SnO 2 ) has attracted consider- able attention due to its excellent optical and electrical properties and has found applications in gas sensors, solar cells, photocatalysts, photodetectors and lithium-ion bat- teries [1–5]. However, to extend its applications, modifica- tion of the physical properties of this material is necessary. Doping can alter the electrical, optical and magnetic prop- erties, and considerable efforts have recently been devoted to this. SnO 2 doped with extrinsic impurities shows improved conductivity [6,7], gas sensitivity [8,9] and photo- catalytic capability [3]. It has also been reported that SnO 2 doped with a transition metal is a diluted magnetic semi- conductor having a Curie temperature higher than room temperature [10,11]. Compared to other nitrogen-doped (N-doped) metal oxides, N-doped SnO 2 has not been investigated exten- sively [10–14], and the effects of N on the optical and elec- tronic properties of SnO 2 nanostructures are not well understood. N-doped SnO 2 films have been fabricated by magnetron reactive sputtering in N 2 as the nitrogen source [12–14]. Some films exhibit p-type conduction [13] and enhanced photocatalytic activity due to the red shift of the band gap into the partial visible light region [12]. Com- pared to high-temperature synthesis, low-temperature chemical methods are more convenient because of their simplicity and low energy requirements. In this work, a fac- ile and economical technique to synthesize N-doped SnO 2 nanocrystals (NCs) is described and their electronic struc- tures are investigated by photoluminescence (PL). A green emission band which is observed to shift to a high energy with increasing excitation energy is attributed to optical transitions induced by both the oxygen vacancy (OV) levels at different depths and localized states arising from the nitrogen dopants. 1359-6454/$36.00 Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2013.08.040 Corresponding author at: National Laboratory of Solid State Micro- structures, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of China. Tel.: +86 25 83686303; fax: +86 25 83595535. E-mail address: [email protected] (X.L. Wu). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com ScienceDirect Acta Materialia 61 (2013) 7342–7347

Upload: lamhanh

Post on 25-Aug-2018

224 views

Category:

Documents


0 download

TRANSCRIPT

Available online at www.sciencedirect.com

www.elsevier.com/locate/actamat

ScienceDirect

Acta Materialia 61 (2013) 7342–7347

N-doped SnO2 nanocrystals with green emission dependentupon mutual effects of nitrogen dopant and oxygen vacancy

G.X. Zhou a, S.J. Xiong a, X.L. Wu a,b,⇑, L.Z. Liu a, T.H. Li a, Paul K. Chu b

a National Laboratory of Solid State Microstructures, Department of Physics, Nanjing University, Nanjing 210093, People’s Republic of Chinab Department of Physics and Materials Science, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China

Received 19 May 2013; received in revised form 16 August 2013; accepted 21 August 2013Available online 23 September 2013

Abstract

A facile and economical chemical route was used to synthesize nitrogen-doped tin oxide (SnO2) nanocrystals (NCs). Infrared andRaman spectral examinations reveal the existence of oxygen vacancies and local disorder. Temperature-dependent photoluminescence(PL) measurements display a broad band at 640 nm at room temperature that shifts to a higher energy at lower measurement temper-ature. Excitation wavelength-dependent PL spectra show that the band blue-shifts and its line width decreases as the excitation wave-length is reduced. The PL peak also blue-shifts when the annealing temperature is increased. Spectral analysis and theoreticalcalculation suggest that the PL band stems from the mutual effects of oxygen vacancies and nitrogen dopants. This PL investigationon N-doped SnO2 NCs provides more insights about the optical properties and will promote further applications of SnO2 NCs.� 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: SnO2 nanocrystal; N-dopant; Oxygen vacancy; Photoluminescence

1. Introduction

Nanostructured tin oxide (SnO2) has attracted consider-able attention due to its excellent optical and electricalproperties and has found applications in gas sensors, solarcells, photocatalysts, photodetectors and lithium-ion bat-teries [1–5]. However, to extend its applications, modifica-tion of the physical properties of this material is necessary.Doping can alter the electrical, optical and magnetic prop-erties, and considerable efforts have recently been devotedto this. SnO2 doped with extrinsic impurities showsimproved conductivity [6,7], gas sensitivity [8,9] and photo-catalytic capability [3]. It has also been reported that SnO2

doped with a transition metal is a diluted magnetic semi-conductor having a Curie temperature higher than roomtemperature [10,11].

1359-6454/$36.00 � 2013 Acta Materialia Inc. Published by Elsevier Ltd. All

http://dx.doi.org/10.1016/j.actamat.2013.08.040

⇑ Corresponding author at: National Laboratory of Solid State Micro-structures, Department of Physics, Nanjing University, Nanjing 210093,People’s Republic of China. Tel.: +86 25 83686303; fax: +86 25 83595535.

E-mail address: [email protected] (X.L. Wu).

Compared to other nitrogen-doped (N-doped) metaloxides, N-doped SnO2 has not been investigated exten-sively [10–14], and the effects of N on the optical and elec-tronic properties of SnO2 nanostructures are not wellunderstood. N-doped SnO2 films have been fabricated bymagnetron reactive sputtering in N2 as the nitrogen source[12–14]. Some films exhibit p-type conduction [13] andenhanced photocatalytic activity due to the red shift ofthe band gap into the partial visible light region [12]. Com-pared to high-temperature synthesis, low-temperaturechemical methods are more convenient because of theirsimplicity and low energy requirements. In this work, a fac-ile and economical technique to synthesize N-doped SnO2

nanocrystals (NCs) is described and their electronic struc-tures are investigated by photoluminescence (PL). A greenemission band which is observed to shift to a high energywith increasing excitation energy is attributed to opticaltransitions induced by both the oxygen vacancy (OV) levelsat different depths and localized states arising from thenitrogen dopants.

rights reserved.

G.X. Zhou et al. / Acta Materialia 61 (2013) 7342–7347 7343

2. Samples and experiments

SnO2 NCs were fabricated at room temperature. Nitricacid and deionized water were mixed 1:2 v/v and tin pow-ders with NC diameters on the order of micrometers wereadded to the solution. The solution was continuously stir-red for 40 h and the white precipitate was collected by cen-trifugation and washed several times with deionized waterand ethanol. The precipitate was dried at 80 �C in air for40 h. After drying, the white powders were annealed inO2 at temperatures of 500, 700, 900 and 1000 �C for30 min.

Scanning electron microscopy (SEM) was performedwith a field emission microscope equipped for energy-dis-persive spectroscopy (EDS) to observe the morphologyand grain size. The variations in the structural and crystal-lite size of the doped SnO2 NCs with annealing tempera-ture (Ta) were investigated by X-ray diffraction (XRD).Temperature-dependent PL spectra were acquired on afluorescence spectrophotometer (Edinburgh FLS-920) witha 450 W xenon lamp as the excitation source. When wemeasured the PL spectra of the samples excited under dif-ferent wavelengths, we used almost same excitation powerby adjusting the incident slit width. This ensures that theobtained PL intensities are comparable. Time-resolvedPL spectra were also acquired on the same EdinburghFLS-920 system and a nanosecond flashlamp (hydrogenlamp with a pulse width of <1 ns) was used as the excita-tion source. Raman scattering was performed on a tripleRaman system (Jobin Yvon T64000) with the 514.5 nmgreen line of an argon ion laser as the excitation source.

3. Results and discussion

Fig. 1a shows the XRD patterns of the as-synthesizedand annealed SnO2 NCs. All the peaks can be indexed tothe tetragonal rutile phase of SnO2. In the as-synthesizedsample, the NCs are very small, as indicated by the broadXRD peaks. With increasing Ta from 500 to 1000 �C, thediffraction peak becomes sharper, indicating a gradualincrease in the NC size. According to the Scherer formula,the average NC size is calculated to be �2.2, 5.4, 40, 80 and

b

10 20 30 40 50 60 70 80

211

200

101

as-synthesized

500 oC

700 oC

900 oC

1000 oC

2θ (Degree)

Inte

nsity

(a.

u.)

110 Annealed in O2a

Fig. 1. (a) XRD patterns acquired from the as-synthesized and annealed SnO2

O2. The inset shows a local EDS spectrum with a clear N signal.

102 nm for the as-synthesized sample and samples annealedat 500, 700, 900 and 1000 �C, respectively. The XRD peakintensities also increase with Ta, implying a gradualimprovement in the crystallinity. Fig. 1b depicts a represen-tative SEM image of the SnO2 NC sample annealed at700 �C in O2. The sample consists of many NCs with diam-eters between 10 and 80 nm. Although these NCs agglom-erate, their boundaries are still distinguishable. EDS (inset)shows nitrogen and suggests that nitrogen has been incor-porated into the SnO2 NCs because absorption of nitrogenon the surface should be insignificant at a hightemperature.

Fig. 2 displays the room-temperature Raman spectraobtained from the as-synthesized and annealed SnO2 NCsamples. The spectrum of the as-synthesized SnO2 NCs inFig. 2a shows two strong vibration modes at 578.2 and234.5 cm�1. The shoulder at 622.9 cm�1 corresponds tothe A1g mode but the position down-shifts compared tothe mode at 633 cm�1 for bulk SnO2. This is due to theexistence of bridging oxygen vacancies (OVs) [15]. Thestrong 578.2 cm�1 vibration mode can be assigned to in-plane OVs localized on the NC surface [16]. The234.5 cm�1 mode corresponds to the Eu TO mode attrib-uted to sub-bridging OVs [15]. The Eu TO mode is an infra-red (IR) active mode and commonly inactive in Ramanscattering. Its appearance can be ascribed to the relaxationof the Raman selective rule induced by the high concentra-tion of surface defects such as OVs and lattice disorder [17].

In the Raman spectrum of the sample annealed at Ta of500 �C (Fig. 2b), four peaks at 253.9, 302.1, 556.3 and627.8 cm�1 can be observed but the peak at 578.2 cm�1 dis-appears. As Ta is increased, the NC size increases, causingthe A1g mode at 627.8 cm�1 to return to 633 cm�1 (bulk)and become stronger [15]. This implies a gradual decreasein the in-plane OVs and eventual disappearance. In theannealed samples, there is a broad Raman peak at556.3 cm�1, which has been suggested to have a disorder-induced consequence [18]. The IR mode at 253.9 cm�1

upshifts by �19 cm�1 compared to that of the as-synthe-sized SnO2 NCs. The presence of the IR mode at302.1 cm�1 indicates that the OV density is larger than6% [19]. These IR active modes provide evidence of the

NC samples. (b) SEM image of the SnO2 NC sample annealed at 700 �C in

240 360 480 600 240 360 480 600

622.9

578.2

Raman shift (cm-1)

Inte

nsity

(a.

u.)

a

234.5

627.8556.3

302.1253.9

500 oC

700 oC

900 oC

Raman shift (cm-1)

b

1000 oC

Fig. 2. Raman spectra: (a) as-synthesized SnO2 NC sample and (b) NCsamples annealed at different temperature in O2. These spectra have beenshifted vertically for spectral comparison.

7344 G.X. Zhou et al. / Acta Materialia 61 (2013) 7342–7347

existence of OVs and local disorder. Theoretical calcula-tions have also disclosed that N is energetically favorableto substitute O sites in N-doped SnO2 nanostructures dueto the small formation energy [20]. As a result, some OVsare formed to attain charge neutrality [21] and it can beconcluded that there are always OVs in both the as-synthe-sized and annealed NC samples.

The PL properties are interesting because different opto-electronic properties are expected from N-doped SnO2

nanostructures. Fig. 3a shows the measurement-tempera-ture (MT)-dependent PL spectra of the SnO2 NC sampleannealed at Ta of 700 �C in O2 excited by the 320 nm lineof a xenon lamp. Two bands at 400 and 500–650 nm are

400 500 600 700 400 500 600 700

PL I

nten

sity

(a.

u.)

Wavelength (nm)

10 K50 K100 K150 K200 K250 K300 K

aMT (K)

58%

75%

88% 330 nm320 nm

310 nm

Wavelength (nm)

290 nm

λex b

100%

300 350 400 450520nm

290nm

Monitored at

PL

E In

ten

sity

(a.

u.)

Wavelength (nm)

500nm

Fig. 3. (a) Temperature-dependent PL spectra of the SnO2 NC sampleannealed at 700 �C in O2. All the spectra are excited by the 320 nm line ofa xenon lamp at 10 K. (b) Excitation-wavelength-dependent PL spectra ofthe SnO2 NC sample annealed at 700 �C in O2. The inset shows the PLEspectra obtained by monitoring the two emission wavelengths of 500 and520 nm. These spectra have been shifted vertically for spectralcomparison.

observed. The former consists of three sharp peaks asshown in Fig. 4 (circle). They are assigned to the band–acceptor and donor–acceptor pair transitions as well asphonon replica, respectively [22,23]. Here, we focus ourattention on the latter, the broad PL band. The profile ofthis band and its change with MT are completely differentfrom those of the undoped SnO2 NCs [24]. With increasingMT, its intensity decreases and peak position red-shifts. Atroom temperature, the PL band shifts to 640 nm and itsintensity decreases drastically. The PL shift displays a sim-ilar trend with MT in the as-synthesized and other annealedsamples. In the SnO2 nanoribbons, a clear double-peak PLband is observed at 480 and 620 nm, but each subpeakposition hardly changes as MT varies [24]. This implies thatthis PL band should not originate from the mutual effectsof OV and oxygen interstitial postulated previously.Instead, the MT-dependent PL red shift should arise fromthe stress induced by the surface shrinkage effect [25], eventhough the PL origin is unclear.

Fig. 3b presents the excitation-wavelength (kex)-depen-dent PL spectra of the sample annealed at Ta of 700 �Cand measured at 10 K. The PL peak exhibits a monotonicblue shift from 530 to 500 nm and the intensity increasesgradually as kex decreases from 330 to 290 nm. The full-width at half-maximum (FWHM) diminishes as well. Sincethe band gap of the NC sample is �345 nm (3.60 eV), thequantum confinement effect of small size NCs can be ruledout as the origin of the green band. It is known that vari-ations in the OVs with depth can lead to a blue shift inthe PL peak with decreasing kex [26], but the FWHMincreases because more OVs are excited. Here, the FWHMdecreases and so this PL band should not be only related toOVs at different depths. Annealing in O2 can lead to vari-ations in the oxygen interstitials with depth, also causingthe FWHM to increase with decreasing kex. This meansthat oxygen interstitials cannot cause the observed PL.Since our Raman and EDS results reveal the presence ofboth OVs and N dopants in the NC samples, the observed

350 400 450 500 550 600 650 700

as-synthesized

500 oC

700 oC

900 oC

Emission wavelength (nm)

PL I

nten

sity

(a.

u.) 1000 oC

Annealing temperature (Ta)

MT = 10 K

λem = 320 nm

Fig. 4. PL spectra of the as-synthesized and annealed SnO2 NC samplesexcited by the 320 nm line of a xenon lamp at 10 K. These spectra havebeen shifted vertically for spectral comparison.

10

100

10 20 30 40 50 60 7010

100

Cou

nts

(a.u

.)

a

Β: τ2 = 9.4 ns (39%)

Α: τ1 = 0.9 ns (61%)

Β: τ2 = 16.2 ns (76%)

Time (ns)

Α: τ1 = 1.6 ns (24%)b

Fig. 6. Time-resolved PL decay curves of the SnO2 NC sample annealedat 700 �C in O2 excited by two wavelengths and monitored at two emissionwavelengths at 10 K (blue dots): (a) kex = 290 nm, kem = 500 nm and (b)kex = 330 nm, kem = 530 nm. The decay curves are fitted by two exponen-tial functions (red line). (For interpretation of the references to color inthis figure legend, the reader is referred to the web version of this article.)

G.X. Zhou et al. / Acta Materialia 61 (2013) 7342–7347 7345

PL band may be a mutual effect of both the OVs and Ndopants.

OVs and N dopants at different depths have differentdefect levels in the forbidden band gap. In the NC samplesannealed at different Ta, the depth distributions of the OVsand N dopants are different. The deeper they are, thehigher are the corresponding energy levels. To investigatethis further, the PL spectra of the NC samples annealedat different Ta are shown in Fig. 4. The peak positionblue-shifts and FWHM increases with increasing Ta. Sincesubstitution of O by N creates an OV in the SnO2 NC andhigh-temperature annealing in O2 makes the OV positiondeeper, the PL blue shift is mainly caused by the increaseddepth of the OVs. It can thus be inferred that the observedPL originates from the mutual effects of both the OVs andN dopants. With increasing Ta, the OVs migrate towardsthe interior and the N dopants remain unchanged in thelattice position, except that some N atoms escape fromthe NC surface. Consequently, the PL spectrum blue-shiftswith increasing Ta, as shown in Fig. 4. The inset in Fig. 3bshows two PL excitation (PLE) spectra. When the excita-tion wavelength reaches the strong PLE spectral positionat 290 nm (Fig. 5), the high energy levels (�500 nm emis-sion) will have a higher population of electrons, whichare not only from the defect levels with the 290 nm excita-tion but also from the transition from the conduction bandto the high energy levels with the �500 nm emission. Thus,with decreasing excitation wavelengths, the PL peak inten-sity increases and the FWHM deceases as illustrated inFig. 3b. To obtain experimental confirmation, the time-resolved PL decay curves at 500 and 530 nm excited by290 and 330 nm at 10 K, respectively, are displayed inFig. 6a and b. The decay curves can be fitted by twostretched exponential components (A and B) according tothe following function [27]:

I ¼ I1e�t=s1 þ I2e�t=s2

where I1 and I2 are the PL intensities of the componentswith decay times of s1 and s2, respectively. The PL decaylifetimes are thus calculated to be (0.9, 9.4) and (1.6,16.2) ns, respectively. The standard deviation are obtainedto be (0.1, 0.4) and (0.1, 0.7) ns, respectively. We can seethat good agreement is achieved between the fit result

Fig. 5. Energy level schematic of SnO2 nanocrystals.

and experimental data. As the monitoring wavelength de-creases, the intensity ratio of component B to A increasesfrom 0.64 (39/61) to 3.17 (76/24). As shown in Fig. 5, sincecomponent B with a long lifetime is associated with thetransition between the defect levels (the red arrow from le-vel 1 to 2), the increase in this transition component will in-crease the intensity of the PL peak with the transition fromlevel 2 (high energy defect level) to the valence band(ground state). At the same time, since little electron tran-sition occurs between levels 1 and 3 and the electron pop-ulation in level 3 mainly comes from the conductionband, this reduces the PL intensity on the low energy sidewith the radiative transition from level 3 (low energy defectlevel) to the valence band (ground state). As a result, the500 nm PL peak narrows compared to the 530 nm PLone. Hence, when the excitation wavelength approaches290 nm, the 500 nm PL peak has higher intensity and nar-rower line width than the 530 nm PL one, as shown inFig. 3b.

To theoretically confirm our assignment of the PL origin,an ab initio computational study is performed on the elec-tronic properties of the surface of N-doped SnO2 crystalwith OVs and N-dopants at different depths. The calcula-tion is carried out using the density functional theory withthe generalized gradient approximation of the potentialfunctional proposed by Perdew et al. [28] under packageCASTEP [29], in which a plane-wave norm-conservingpseudopotential method [30] is used. The kinetic energycutoff is 450 eV to represent the single-particle wavefunctions and slabs are adopted to investigate the surfaceproperties of the NCs. The surface is represented by a slab1.44 nm thick carved along the h110i direction. Theperiodic structure for the calculation in the CASTEP

7346 G.X. Zhou et al. / Acta Materialia 61 (2013) 7342–7347

package is reconstructed by using a 0.637 � 0.670 nm2

supercell in the on-plane direction and adopting 1 nm thickvacuum slabs to separate the periodically repeating SnO2

slabs. In every supercell, an N atom is the dopant and anoxygen vacancy is created in the initial structure at differentdepths. The geometry of the configuration is optimizedusing the BFGS minimizer in the CASTEP package withdefault convergence tolerances: 2 � 10�5 eV for energy,0.5 eV nm�1 for maximum force and 0.0002 nm formaximum displacement [31]. Fig. 7 shows the densities ofstates (DOSs) of the three configurations: (a) one shallowN-dopant and one shallow OV per supercell, (b) one deepN-dopant and one deep OV per supercell and (c) one shal-low N-dopant and one deep OV. The DFT calculation isknown to underestimate the energy band but the band fea-ture is still available [24,32]. Obviously, these DOSs areinconsistent with those of SnO2 nanoribbons with differentamounts of OVs/oxygen interstitials [24]. The results areindicative of the role of the N dopant. According to thecomparison between the DOS curves (b) and (c) with curve(a), by increasing the depth of the OV, the top of the valenceband (position A) is lowered (the red arrow toward left side)and meanwhile, the energy (peak position) of the impuritylevel B changes only slightly (the dashed green circle) andthe energy of the impurity level C is enhanced (the red arrowtoward right side). This suggests that the transition from Cto A or from B to A has a higher energy if the OV is deeper.This effect persists even though the position of the N dopantis not changed (the blue arrows toward left and right sides).To further confirm the optical transition, we plot the curvesof the imaginary part of the dielectric function in the inset ofFig. 7, which reflects the intensity of the corresponding opti-cal matrix elements. The increase in the OV depth reallycauses the blue shift in the optical transition (the red arrowtoward right side from peak 1 to 2), in spite of the positionof N. Moreover, when the OV depth increases (N dopantposition remains unchanged), the intensity of the transition

-2 -1 0 1 2 3

(a) (b) (c)

Den

sity

of s

tate

s (a

rb.u

nits

)

Energy (eV)

A

BC

1

2

Fig. 7. DOSs of SnO2 surface with N dopants and OVs at different depths:(a) a shallow N-dopant and a shallow OV; (b) a deep N-dopant and a deepOV and (c) a shallow N-dopant and a deep OV. The insets show theimaginary part of dielectric function as a function of energy forconfigurations (a), (b) and (c).

matrix element increases and especially its FWHMdecreases (from peak 1 to 2). This explains very well thePL results in Fig. 3b.

4. Conclusion

In conclusion, N-doped NCs are synthesized andannealed in O2 at different temperatures, and Raman scat-tering and EDS reveal the existence of OVs and N dopantsin the NCs. The synthesized NCs exhibit an optical emis-sion band that varies with measurement temperature, exci-tation wavelength and annealing temperature. Spectralanalyses and theoretical calculations suggest that theobserved PL is closely related to both OVs and N-dopants.The optical transition induced by the OV/N-dopant levelsis proposed to be responsible for the PL band. This PLinvestigation on N-doped SnO2 NCs provides moreinsights into the optical properties of these materials andwill promote further applications of SnO2 NCs.

Acknowledgements

This work was supported by National Basic ResearchProgram of China under Grants (Nos. 2011CB922102and 2013CB932901) and PAPD. Partial support was alsofrom the National Natural Science Foundation of China(No. 61264008) and Hong Kong Research Grants CouncilGeneral Research Funds Nos. CityU 112510 and 112212.

References

[1] Kolmakov A, Klenov DO, Lilach Y, Stemmer S, Moskovits M. NanoLett 2005;5:667.

[2] Shen E, Wang C, Kang Z, Gao L, Hu C, Xu L. Mater Lett2004;58:3761.

[3] Kumar V, Govind A, Nagarajan R. Inorg Chem 2011;50:5637.[4] Wu JM, Kuo CH. Thin Solid Films 2009;517:3870.[5] Ding SJ, Chen JS, Qi GG, Duan XN, Wang ZY, Giannelis EP, et al. J

Am Chem Soc 2011;133:21.[6] Wang Y, Brezesinki T, Antonietti M, Smarly B. ACS Nano

2009;3:1373.[7] Dhage SR, Ravi V. Appl Phys Lett 2003;83:4539.[8] Shukla S, Patil S, Kuiry SC, Rahman Z, Du T, Ludwig L, et al. Sens

Actuat B 2003;96:343.[9] Jia LC, Cai WP, Wang HQ. Appl Phys Lett 2010;96:103115.

[10] Archer PI, Radovanovic PV, Heald SM, Gamelin DR. J Am ChemSoc 2005;127:14479.

[11] Ogale SB, Choudhary RJ, Buban JP, Lofland SE, Shinde SR, KaleSN, et al. Phys Rev Lett 2003;91:077205.

[12] Pan SS, Shen YD, Teng XM, Zhang YX, Li L, Chu ZQ, et al. MaterRes Bull 2009;44:2092.

[13] Pan SS, Li GH, Wang LB, Shen YD, Wang Y, Mei T, et al. ApplPhys Lett 2009;95:222112.

[14] Pan SS, Zhang YX, Teng XM, Li GH, Li L. J Appl Phys2008;103:093103.

[15] Liu LZ, Li TH, Wu XL, Shen JC, Chu PK. J Raman Spectrosc2012;43:1423.

[16] Liu LZ, Wu XL, Gao F, Shen JC, Li TH, Chu PK. Solid StateCommun 2011;151:811.

[17] Abello L, Bochu B, Gaskov A, Koudryavtseva S, Lucazeau G,Roumyantseva M. J Solid State Chem 1998;135:78.

[18] Loridant S. J Phys Chem B 2002;106:13273.

G.X. Zhou et al. / Acta Materialia 61 (2013) 7342–7347 7347

[19] Li TH, Liu LZ, Li XX, Chen HT, Chu PK. Opt Lett 2011;36:4296.[20] Sun XQ, Long R, Cheng XF, Zhao X, Dai Y, Huang BB. J Phys

Chem C 2008;112:9861.[21] Batzill M, Morales EH, Diebold U. Phys Rev Lett 2006;96:026103.[22] Kar A, Stroscio MA, Dutta M, Kumari J, Meyyappan M. Appl Phys

Lett 2009;94:101905.[23] Liu B, Cheng CW, Chen R, Shen ZX, Fan HJ, Sun HD. J Phys Chem

C 2010;114:3047.[24] Chen HT, Xiong SJ, Wu XL, Zhu J, Shen JC, Chu PK. Nano Lett

2009;9:1926.[25] Liu LZ, Xu W, Wu XL, Zhang YY, Chen TH, Chu PK. Appl Phys

Lett 2012;100:121904.

[26] Wu XL, Xiong SJ, Guo JH, Wang LL, Hua CY, Hou Y, et al. J PhysChem C 2012;116:2356.

[27] Suzuno M, Koizumi T, Suemasu T. Appl Phys Lett 2009;94:213509.[28] Perdew JP, Burke K, Ernzerhof M. Phys Rev Lett 1996;77:3865.[29] Clark SJ, Segall MD, Pickard CJ, Hasnip PJ, Probert MJ, Refson K,

et al. J Crystallogr 2005;220:567.[30] Hamann DR, Schluter M, Chiang C. Phys Rev Lett 1979;43:1494.[31] Pfrommer BG, Cote M, Louie SG, Cohen ML. J Comput Phys

1997;131:233.[32] Liu LZ, Wu XL, Xu JQ, Li TH, Shen JC, Chu PK. Appl Phys Lett

2012;100:121903.