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Oxygen Vacancy: An Electron-Phonon Interaction Decoupler to Modulate the Near-Band-Edge Emission of ZnO Nanorods Huaiyi Ding, Zhi Zhao, Guanghui Zhang, Yukun Wu, Zhiwei Gao, Junwen Li, Kun Zhang, Nan Pan, and Xiaoping Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp305480e • Publication Date (Web): 28 Jul 2012 Downloaded from http://pubs.acs.org on August 3, 2012

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Figure 1. (a): Schematic of the two-heating-zone ZnO nanorod growth system. (b-g): Top-view SEM images of the samples S1-S6. Scale bar: 5 µm. (h): RT-PL spectra of NBE emission from the samples S1-S6. The intensities have been normalized and the curves are shifted vertically for clarity. 140x56mm (300 x 300 DPI)

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Figure 2. (a-c): The XPS data (hollow squares) and the spectral deconvolution results (blue lines for the individual components and red lines for the sum) of the O 1s peaks for the samples S1 (a), S4 (b) and S6 (c). (d): The correlation between the VO concentration and the measured spectral position of the NBE emission for the samples S1-S6. 79x59mm (300 x 300 DPI)


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Figure 3. VT-PL spectra of the samples S1 (a) and S6 (b). The spectra are displayed logarithmically, normalized and shifted for clarity. For each sample, the curves from bottom to top respectively denote the spectra taken from low to high temperatures (i.e., 8-300 K). The changes in the energies of the FX, BX and their phonon replicas with the temperature are labeled with dot lines. 140x66mm (300 x 300 DPI)



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Figure 4. The temperature-dependent behaviors of the photon energies of the FX (black), FX-TO (red) and FX-TO-LO (green) emissions for the samples S1-S6. The spots are the experimental data, and the solid lines are the corresponding fitting results according to Eq. 1. 80x65mm (300 x 300 DPI)


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Figure 5. (a-f): Deconvolution of the RT-PL spectra of the samples S1-S6, respectively. The spots are the experimental data, while the lines are the total fitting curves (magenta) and their spectral components respectively located at the energies of the FX (black), FX-TO (red), FX-TO-LO (blue) and FX-TO-2LO (green), which are obtained from figure 4. 140x80mm (300 x 300 DPI)



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Figure 6. (a): The correlation between the EPC strength and the VO concentration (as well as the measured spectral position of the NBE emission) for the samples S1-S6. Black solid squares give the coupling strengths estimated by the Huang-Rhys parameter S from the RT-PL spectra. Blue hollow squares show the intensity ratios of the 2LO spectral line to the LO counterpart from RT Raman spectra. The corresponding Raman spectra, normalized and shifted vertically, are given in the inset. (b): Schematic of the role of the VO as a “decoupler” to suppress the EPC. An isolated VO and its decoupling region are labeled by the dashed circle and the large yellow-shaded area, respectively. 79x149mm (300 x 300 DPI)


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Oxygen vacancy: an electron-phonon interaction decoupler to modulate the near-band-edge emission of ZnO nanorods Huaiyi Ding, Zhi Zhao, Guanghui Zhang, Yukun Wu, Zhiwei Gao, Junwen Li, Kun Zhang, Nan Pan*, Xiaoping Wang Hefei National Laboratory for Physical Sciences at the Microscale and Department of Physics, University of Science and Technology of China, Hefei, 230026, P. R. China

Abstract: Through in-situ control of the growth condition in the vapor phase transport and condensation, we intentionally prepared ZnO nanorods with different concentrations of oxygen vacancy (VO), as confirmed by the X-ray photoelectron spectra. A spectral shift in the ultraviolet (UV) emission between these nanorods, as large as 80 meV, has been observed in the room temperature photoluminescence (PL) spectra, showing strong correlation to the VO concentration. With the help of the variable-temperature PL, this spectral shift is clearly attributed to the different spectral contributions of the free exciton emission and its phonon replicas. Furthermore, a remarkable variation in the electron-phonon interaction strength among these samples is unambiguously revealed by the Raman spectra, which is in good consistence with the Huang-Rhys parameters obtained from the PL. This study implies that, VO in ZnO nanostructures can not only 1 ACS Paragon Plus Environment


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modulate the visible emission as intensively investigated previously, it can also significantly suppress the electron-phonon interaction strength and therefore tailor the UV (near-band-edge) emission property. This finding is useful to design and fabricate ZnO-based high-performance short-wavelength photonic and optoelectronic devices at the nanoscale.

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I. INTRODUCTION Room temperature (RT) spectral shift in the near-band-edge (NBE) emission of ZnO nanostructures has often been observed, which could be attributed to quantum size confinement,1 surface effect,2 lattice strain,3 equivalent doping4 or electron-phonon coupling (EPC),5-8 respectively. Among these possible origins, the EPC picture has been extensively studied. In general, the imperfection of the ZnO crystals, such as the nonstoichiometry5 and the surface defects (caused by surface roughness6-7 or large surface-to-volume ratio8), was used to considered as the main reason for the enhancement of the EPC; besides, Lee et al.9 proposed that the nonradiative process could also raise the EPC strength. However, the crystal imperfection was only inferred from the morphology or the growth condition, there is still lack of evidence to establish direct correlations between the specific crystal defects and the EPC strength. Although it is well accepted that a decreased (increased) EPC can lead to a blue (red) shift in the UV emission of ZnO,5-9 the intrinsic reason for the variation of this interaction is rarely demonstrated. In this paper, we show that the oxygen vacancy (VO), one of the most important native defects in ZnO that was usually considered as an origin of the visible emission,10 can significantly modulate the NBE (UV) emission property through controlling the EPC strength. Intentionally preparing ZnO nanorods at a series of growth conditions through the vapor phase transport and condensation (VPTC) allows us to in-situ control the relative content (i.e., the concentration) of VO in the samples. Both the photoluminescence 3 ACS Paragon Plus Environment


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(PL) and the Raman results clearly reveal that the VO concentration is negatively correlated with the EPC strength, which directly results in the observed UV spectral shift. Such a result not only expands the understanding on the role of VO in ZnO’s luminescent property, but also provides a straightforward method to improve the performance of various ZnO nanodevices.

II. EXPERIMENTAL SECTION The ZnO nanorods were synthesized by VPTC method in a two-heating-zone tube furnace. High-purity ZnO powder uniformly mixed with spectrograde graphite powder at a molar ratio of 1:1 was used as the precursor, while clean and flat Si (001) wafers covered by a 50 nm-thick ZnO seed-layer were served as the growth substrates. As shown in figure 1a, the substrates were placed in series along the stream direction. For nanorod growth, the source temperature (heater 1 in figure 1a) was set and kept at 1080 C, while the substrate temperature (heater 2) was fixed for each growth but can be varied from 550 to 700 C for different growth. Therefore, the different growth conditions can be realized by altering the substrate temperature and position. During a typical growth, both temperatures were increased to and kept at the set points for 30 min, after which they were then cooled down naturally to RT. A constant flow of Ar (O 2) was kept at 50 sccm (75 sccm) during the growth acting as a carrier (reactive) gas.


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The morphologies of the ZnO nanorods were checked by a JEOL-6700 field-emission scanning electron microscope (FE-SEM). The crystallographic information was revealed by a PHILIPS X'PERT PRO X-ray diffractometer (XRD) with Cu Kα line (λ=1.54184 Å). The X-ray photoelectron spectroscopic (XPS) analysis was performed using an ESCALAB 250 X-ray photoelectron spectrometer with a monochromatic Al Kα radiation (hν=1486.6 eV).The variable-temperature PL (VT-PL) were measured by using 325 nm excitation with a Fluorolog Tau-3 Lifetime System, Jobin Yvon Inc. The Raman spectra were obtained on a LABRAM-HR Raman spectrometer under a 514.5 nm excitation of an Ar+ laser.

III. RESULTS AND DISCUSSION The growth (substrate) temperatures, substrate positions, average diameters, lattice parameters and peak positions of the UV emission for a series of ZnO nanorods (the samples S1-S6) have been summarized in Table 1. The corresponding SEM images for these samples are shown in figure 1b-g, while figure 1h shows the RT UV emission spectra of the samples, from which the spectral shift can be determined as large as ~80 meV from 3.212 eV for S1 to 3.289 eV for S6. The XRD 2θ scan result and the XPS spectral survey (from which the lattice parameters and stoichiometries are determined, respectively) are also provided in figures S1 and S2. From these results, one can clearly see that the observed UV spectral shift is not due to the quantum confinement (since the 5 ACS Paragon Plus Environment


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diameters are all far beyond the quantum region for ZnO), surface (since the shift still exists even for the samples with the same surface-to-volume ratio, such as for S1-S3, and for S4-S6), strain (no obvious difference in their lattice parameters), or nonstoichiometric (the fluctuation of the stoichiometry between the samples is rather small and random, as revealed by the XPS) effects. As we will show later, the UV spectral shift neither could be attributed to the Burstein-Moss (BM) band filling effect.4 Since all of the other possible mechanisms have been ruled out, could the observed NBE shift arise from the different EPC strength in the different samples? What is the intrinsic difference in these samples that may cause the spectral shift?

Table 1. Growth conditions and properties of the ZnO nanorods labeled as samples S1-S6. Sample

S1

S2

S3

S4

S5

S6

T (C)

600

600

700

650

600

550

P (cm)

23

22

21

21

21

21

D (nm)

260

260

260

90

90

90

c (Å)

5.196

5.206

5.196

5.196

5.202

5.190

a (Å)

3.263

3.250

3.263

3.263

3.260

3.260

E (eV)

3.212

3.221

3.255

3.263

3.272

3.289

* T: the substrate (growth) temperature; P: the substrate position with respect to the source; D: the average diameter of the ZnO nanorod; c and a: the lattice parameters (wurtzite ZnO) for the sample; and E: the peak energy of the UV emission.


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Figure 1. (a): Schematic of the two-heating-zone ZnO nanorod growth system. (b-g): Top-view SEM images of the samples S1-S6. Scale bar: 5 m. (h): RT-PL spectra of the NBE emission from the samples S1-S6. The intensities have been normalized and the curves are shifted vertically for clarity.

To answer this question, the XPS O 1s peak is carefully revisited to reveal the detailed binding information of the O element, the spectral features of which for the samples S1-S6 are shown in figure 2a-c and figure S3a-c. Generally, the O 1s peak of ZnO can be deconvoluted into three spectral components respectively centered at 532.25±0.30, 531.31±0.30, and 530.15±0.30 eV.11,12 The component with the lowest binding energy is attributed to the O2- ions in perfect ZnO lattice and is known as lattice oxygen (OL), whose contribution represents the relative amount of the O at the perfect lattice positions. The one with the medium binding energy centered at 531.31±0.30 eV corresponds to the O2- ions in the O deficient regions (i.e., those around VO) and is therefore called as 7 ACS Paragon Plus Environment


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deficient oxygen (OD).11,12 In other words, the contribution of OD reflects the relative amount of VO in the ZnO crystal. Besides, the highest binding energy component can be ascribed to the chemisorbed O species on the surface of ZnO, such as H2O, -CO3 and so on. Therefore, the intensity ratio of OD to OL components can well represent the concentration of the intrinsic VO in a ZnO crystal, where a lower I(OD)/I(OL) ratio indicates a lower VO in the sample, and vice versa. Based on the spectral deconvolution results (figure 2a-c and figure S3a-c), the intensity ratios of OD/OL for all of the samples can be readily obtained, as plotted in figure 2d. From the result we can clearly see that the ratio, namely the VO concentration in the nanorods, monotonically increases from S1 to S6. Moreover, a clear positive correlation between the VO concentration and the spectral position of the NBE peak can be obtained. In fact, such a difference in the VO concentration between these samples is caused by the intentionally varied growth conditions, i.e., the substrate position and temperature. In our study, we found that the lower the O2 pressure and/or substrate temperature, the higher VO concentration can be introduced into the ZnO nanorods.13


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Figure 2. (a-c): The XPS data (hollow squares) and the spectral deconvolution results (blue lines for the individual components and red lines for the sum) of the O 1s peaks for the samples S1 (a), S4 (b) and S6 (c). (d): The correlation between the VO concentration and the measured spectral position of the NBE emission for the samples S1-S6.

Although a direct correlation between the VO concentration and the spectral position of the NBE peak has been clearly observed in the ZnO nanorods, the mechanism how the VO affects the NBE spectral feature is still not clear. In order to reveal the reason of the RT NBE spectral shift, VT-PL of the ZnO nanorods S1-S6 are investigated from 8 to 300 K. The results for the samples S1 and S6 are shown in figure 3 ( the others are shown in figure S4), in which the peaks of free exciton (FX), bound exciton (BX), and their corresponding phonon replicas are clearly identified and labeled on the spectra. 14-19 As seen, the dominant asymmetric peak at ~3.37 eV can be assigned to ionized donor BX (3.367 eV) and FX (3.377 eV), which have just a small energy difference at low 9 ACS Paragon Plus Environment


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temperature but entirely different temperature dependences in their emission intensities.14 According to the energies of ~70 meV for a LO-phonon and ~52 meV for a TO-phonon,16 the other five peaks at 3.296, 3.226, 3.321, 3.245 and 3.173 eV can be attributed one by one to the phonon replicas of the two different excitons as FX-TO (i.e., the energy of FX minus that of one TO-phonon, similar for the rest), BX-LO, FX-TO-LO, BX-2LO and FX-TO-2LO, respectively; which are also consistent with the reported values of intrinsic bulk ZnO.17 As the temperature increases, more and more BXs are thermally activated and convert to FXs; consequently, the relative intensities of the FX emission as well as its phonon replicas increase obviously and eventually become dominant in the RT-PL spectra, as indicated by the green dot lines in figures 3 and S4. It is worth pointing out that, from figures 3 and S4, each of the peaks (FX, BX, or any of their phonon replicas) at 8 K for all of the samples remains at the same position, indicating no band-gap change between the nanorods produced at the different conditions. This result also confirms that neither the strain3 nor BM effect4 could be responsible for the spectral shift between the different nanorods. In other words, the lattice strain or VO-induced doping in the samples is still too trivial to cause any spectral shift.


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Figure 3. VT-PL spectra of the samples S1 (a) and S6 (b). The spectra are displayed logarithmically, normalized and shifted for clarity. For each sample, the curves from bottom to top respectively denote the spectra taken from low to high temperatures (i.e., 8-300 K). The changes in the energies of the FX, BX and their phonon replicas with the temperature are labeled with dot lines.

In principle, the quantitative contributions of the FX and its phonon replicas to the RT-NBE emission can be revealed by performing spectral deconvolution, on condition that the peak position of each component can be defined. However, due to the broad spectral shapes at RT, it is impossible to obtain the peak positions of all components from the RT-PL spectra directly. Therefore, we solve this difficulty with the help of Varshini’s empirical relation, which can well describe the energy variations of FX and its phonon replicas with the temperature,18 11 ACS Paragon Plus Environment


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E FX (T )  E(0) 

E FX TO(T )  E(0) 

E FX TO  LO(T )  E(0) 

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T 2 T 

T 2 1  E(TO )  kT T  2

T 2 1  E(TO )  E(LO )  kT T  2

Eq.1

where  and  are two fitting constants, E(0) is the FX energy at 0 K, while E(TO) and E(LO) are those for a TO- and LO- phonon at 0 K, respectively, and k is the Boltzmann constant.19

Figure 4 shows the experimental data for the samples S1-S6 (extracted from the data in figures 3 and S4) and the corresponding fitting curves based on Eq. 1. As seen, the uncertainties (the error bars in figure 4) of the peak positions are all less than 0.5 nm, clearly demonstrating that the FX emission (and each of its phonon replicas) for all of the samples have the same temperature-dependent behavior. Furthermore, the fitting curves are almost perfectly consistent with the experimental data for the temperatures lower than 200 K, and the fitting parameters of E(0)=3.378 eV, =7.7×10-4 eV/K and =800 K are also in good agreement with the previous report.19 Through extrapolating the fitting curves to RT, we can determine the peak positions of the FX, FX-TO and FX-TO-LO at RT to be 3.315, 3.274 and 3.201 eV, respectively.


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Figure 4. The temperature-dependent behaviors of the photon energies of the FX (black), FX-TO (red) and FX-TO-LO (green) emissions for the samples S1-S6. The spots are the experimental data, and the solid lines are the corresponding fitting results according to Eq. 1.

Given the above peak energies at RT, each RT-PL spectrum of the samples S1-S6 (figure 1h) can be well fitted by four Gaussian curves respectively centered at the FX (3.315 eV), FX-TO (3.274 eV), FX-TO-LO (3.201 eV) and FX-TO-2LO (3.131 eV),20 the results are shown in figure 5. As can be seen, the FX emission of S6 (black line in figure 5f) has a strong contribution to the intensity of its RT-PL; in sharp contrast, the contributions from the FX’s phonon replicas prevail and become dominant for S1 (figure 5a). Therefore, we can conclude that, it is the different contributions of the FX emission and its phonon replicas that results in the apparent redshift in the RT-NBE emission from S6 to S1. 13 ACS Paragon Plus Environment


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Figure 5. (a-f): Deconvolution of the RT-PL spectra of the samples S1-S6, respectively. The spots are the experimental data, while the lines are the total fitting curves (magenta) and their spectral components respectively located at the energies of the FX (black), FX-TO (red), FX-TO-LO (blue) and FX-TO-2LO (green), which are obtained from figure 4.

The various contributions of the FX and its phonon replicas to the RT-NBE emission can be attributed to the different coupling strengths of the FXs to the LO-phonons. Such a coupling is dominated by the Frohlich interaction and becomes remarkable in polar materials such as ZnO.21 The relative coupling strength can be described by the Huang-Rhys parameter S,22 defined as the intensity ratio of the first order LO-phonon 14 ACS Paragon Plus Environment

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emission to the zeroth. Considering the FX-TO-LO peak as the first order LO-phonon emission while both the FX and the FX-TO peaks as the zeroth, the corresponding intensity ratios S in the RT-NBE emission for the samples S1-S6 (figure 5) can therefore be calculated, the results are given in figure 6a (black solid squares). As can be seen, the spectral position of the NBE emission can be remarkably tuned by such an electron-phonon interaction, i.e., the stronger the EPC, the larger redshift can be observed in the RT-PL spectra, and vice versa.



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Figure 6. (a): The correlation between the EPC strength and the VO concentration (as well as the measured spectral position of the NBE emission) for the samples S1-S6. Black solid squares give the coupling strengths estimated by the Huang-Rhys parameter S from the RT-PL spectra. Blue hollow squares show the intensity ratios of the 2LO spectral line to the LO counterpart from RT Raman spectra. The corresponding Raman spectra, normalized and shifted vertically, are given in the inset. (b): Schematic of the role of the 16 ACS Paragon Plus Environment

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VO as a “decoupler” to suppress the EPC. An isolated VO and its decoupling region are labeled by the dashed circle and the large yellow-shaded area, respectively.

The strong electron-phonon interaction in the ZnO nanorods and its variation between the samples are further investigated, more straightforwardly, by RT Raman spectroscopy measurement with a Z(XX)Z backscattering configuration. As shown in the inset of figure 6a, the main Raman peaks at ~582 and ~437 cm-1 can be respectively ascribed to the A1(LO) and E2 phonon modes of ZnO, perfectly consistent with the previous report.23 In addition, another peak around 1150 cm-1, originated from the second order scattering of the A1(LO), are clearly observable for all of the samples, indicating the strong EPC in these nanorods. Similar to the previous reports,24-27 we use the Raman intensity ratio of the second to the first order scattering of the A1(LO) mode to estimate the EPC strength in the nanorods. After careful deconvolution of the peaks around the 2A1(LO) and A1(LO), the corresponding ratios for all of the samples are obtained and plotted together in figure 6a (blue hollow squares). As seen, the EPC strength estimated from the Raman spectra shows very similar trend with that derived from the RT-PL. This consistency firmly confirms that the spectral shift in the NBE emission of the ZnO nanorods is indeed resulted from the variation in the EPC.



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It is quite unusual to find that, as shown in figure 6a, the EPC strength in the ZnO nanorods S1-S6 decreases drastically with the increasing VO concentration, suggesting that the VO could act as a “decoupler” for the EPC and suppress the electron-phonon interaction efficiently. This finding is in sharp contrast to the previous observations, where the EPC, due to the Frohlich effect in polar ZnO, is closely dependent on the crystalline quality and becomes extremely weak in a perfect crystal due to the parity conservation.5-9 At this stage, we can not present the exact interpretation for this “anomalous” phenomenon (that imperfections could suppress the EPC). However, because the Frohlich effect is in essence the Coulomb interaction between the free electrons and the longitudinal electric field generated by the collective vibration of cations and anions, such as Zn2+ and O2- in ZnO crystals, we can understand the result phenomenologically as follows. When a number of VO are introduced in the ZnO lattice, due to the deep donor nature of the VO,28-30 they are most likely to form localized electronic states in their surrounding regions, as sketched in figure 6b (yellow-shaded area). This assumption is well consistent with our observation that the free electron concentration in the ZnO nanorods is independent of the VO concentration (figure S5).31,32 Although these localized electrons can not contribute to the free carrier density, they do screen the free electrons outside from coupling with the lattice in the “decoupling” region. As a result, the Frohlich interaction strength around the VO is dramatically weakened by the localized VO states, thus leading to the more significant suppression of the EPC in the samples with higher VO concentrations. That is to say, the VO can act as an 18 ACS Paragon Plus Environment

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efficient EPC-decoupler to straightforwardly modulate the electron-phonon interaction in the ZnO nanorods.

IV. CONCLUSION In summary, we have intentionally prepared single-crystal ZnO nanorods with different VO concentrations through the controllable VPTC growth. The VT-PL and Raman results consistently show that the higher VO concentration causes the lower EPC strength, and vice versa. This result clearly demonstrates that the VO, probably the most important native defect in ZnO, can serve as an efficient electron-phonon interaction decoupler to tailor the NBE emission of ZnO. Qualitative description for the phenomenon could be due to the suppression of the Frohlich interaction by the localized electronic states of the VO through screening the free electrons. This work proposes a new viewpoint to understand the impact of the VO defect on the optical emission property and the electron-phonon interaction of the important photonic and optoelectronic material ZnO.

■ ASSOCIATED CONTENT Supporting Information Available. XRD 2 scans, XPS full spectral surveys of S1-S6; XPS spectral deconvolution results of the O 1s peaks for S2, S3 and S5; VT-PL spectra of S2-S5; and Raman A1(LO) phonon modes obtained from S1-S6. This material is available free of charge via the Internet at http://pubs.acs.org. 19 ACS Paragon Plus Environment


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■ AUTHOR INFORMATION Corresponding author *E-mail: [email protected]

■ ACKNOWLEDGEMENT This work is supported by MOST of China (2011CB921403), the National Natural Science Foundation of China (NSFC) under Grant Nos. 90921013, 11074231, 11004179, 91021018 and 21121003 as well as by CAS. N. P. also thanks the support from the Fundamental Research Funds for the Central Universities (FRFCU) under Grant No. WK2340000011.

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(13)When the growth temperature is kept the same, the O2 pressure drops from 23 to 21 cm (labeled in figure 1a) in the growth system, leading to the gradually increased VO concentrations in S1, S2 and S5; when the substrate position is fixed, the VO concentrations in S3, S4, S5 and S6 increases with the decreasing growth temperature because the Zn vapor has a higher deposition rate at a lower temperature. (14)Meyer, B. K.; Alves, H.; Hofmann, D. M.; Kriegseis, W.; Forster, D.; Bertram, F.; Christen, J.; Hoffmann, A.; Strassburg, M.; Dworzak, M.; Haboeck, U.; Rodina, A. V. Phys.stat.sol.(b) 2004, 241, 231-260. (15)Shan, W.; Walukiewicz, W.; Ager, J. W.; Yu, K. M.; Yuan, H. B.; Xin, H. P.; Cantwell, G.; Song, J. J. Appl. Phys. Let.t 2005, 86, 191911. (16)Zhang, Y.; Lin, B. X.; Sun, X. K.; Fu, Z. X. Appl. Phys. Lett. 2005, 86, 131910. (17)Teke, A.; Ozgur, U.; Dogan, S.; Gu, X.; Morkoc, H.; Nemeth, B.; Nause, J.; Everitt, H. O. Phys. Rev. B 2004, 70, 195207. (18)Varshni, Y. P. Physica (Amsterdam) 1967, 34, 149 (19)Wang, L. J.; Giles, N. C. J. Appl. Phys. 2003, 94, 973-978. (20)The spectral position of the FX-TO-2LO peak at RT is estimated by directly subtracting 70 meV from that of the FX-TO-LO peak; due to the intensity of this


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(31)Cheng, A.-J.; Tzeng, Y.; Xu, H.; Alur, S.; Wang, Y.; Park, M.; Wu, T.-H.; Shannon, C.; Kim, D.-J.; Wang, D. J. Appl. Phys. 2009, 105, 073104. (32)Under a Z(XX)Z backscattering Raman configuration, for quasi-vertically-aligned ZnO nanorods, the frequency (wavenumber) of the A1(LO) phonon mode can be used to estimate the electron concentrations of the nanorods, as demonstrated by Ref. 31. Here, an A1(LO) phonon mode at ~582 cm-1 is found for all samples (figure S5), indicating the roughly same electron concentration of ~1017 cm-3 despite the various VO concentrations.

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