- Email: [email protected]
Photoluminescence investigation on the gas sensing property of ZnO nanorods prepared by plasma-enhanced CVD method
Sensors and Actuators B 145 (2010) 114–119
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Photoluminescence investigation on the gas sensing property of ZnO nanorods prepared by plasma-enhanced CVD method Ning Han a,b , Peng Hu a , Ahui Zuo a,b , Dangwen Zhang a,b , Yajun Tian a , Yunfa Chen a,∗ a b
State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 31 July 2009 Received in revised form 29 October 2009 Accepted 18 November 2009 Available online 24 November 2009 Keywords: ZnO nanorod Intrinsic defects Photoluminescence Peak decomposition Gas sensor
a b s t r a c t Gas sensing property of ZnO nanorods prepared by plasma-enhanced chemical vapor deposition (CVD) method is studied using formaldehyde as the probe gas, and the intrinsic defects are investigated by photoluminescence (PL). The results show that high ratio of visible to ultra-violet luminescence cannot account for high gas response. The PL spectra are Gaussian decomposed to subpeaks according to their origination, which are separated into donor- (DL) and acceptor-related (AL) ones. A conclusion is derived that where the content of DL is high and that of AL is low, the gas response is high. This conclusion is further confirmed by tuning the PL spectra and gas sensing property through annealing in different atmospheres. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Since the first application in gas sensor [1], ZnO has found its way in explosive gas alerting and toxic gas detection for decades, with various morphologies and different dopants contributing much to the development of resistance-based ZnO gas sensors [2,3]. But the key factor determining the gas sensing property of ZnO is still under debate. The most prevalent model is founded using SnO2 by Xu et al. [4] that compares the particle diameter (D) and depth of surface charge layer (L): if D is comparable to or less than 2L, the gas response is expected to be high. However, just comparing D and L leads to dilemma in some studies [5–7], because L is hard either to be measured or to be calculated. As the electronic property of ZnO mainly depends on its intrinsic defects [8], the gas response defined as the ratio of resistance in air and in detectant (Ra /Rg ) is therefore closely correlated to the intrinsic defects [9,10]. Photoluminescence (PL) spectra is the luminescences originated from the photo-induced electron/hole and/or the intrinsic defects in ZnO [8]. Therefore, there are several defectrelated luminescences as well as the photo-induced near band edge excitation in ZnO PL spectra. Thus some researchers used the
intensity ratio of visible luminescence to ultra-violet luminescence (IVL /IUL ) to evaluate the crystallinity of ZnO crystal: the higher the ratio, the more the intrinsic defects [10,11]. However, Shi et al. [12] pointed out that the ratio of IVL /IUL is affected by sample type as well as excitation density, and could not be simply used to assess the crystal defects. Therefore, we tried to use decomposed PL spectra to distinguish donor-related (DL) and acceptor-related luminescences (AL) and further to investigate the relationship between the intrinsic defects and the gas sensing property of ZnO. The ZnO nanorods with different lengths used here are prepared in seconds by plasma-enhanced chemical vapor deposition (CVD) method under high temperature and thus possess lots of intrinsic defects [13,14]. Furthermore, the defects were tailored by annealing in different atmospheres and the relationship between the intrinsic defects measured by PL and the gas sensing property of ZnO was confirmed. It should be noted that the donors and acceptors in ZnO crystal play different roles in electron transport, and they affect the gas sensing property of ZnO in opposite way. 2. Experimental 2.1. ZnO nanorods preparation
∗ Corresponding author at: State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Haidian District, Beijing 100190, PR China. Tel.: +86 10 82627057; fax: +86 10 62542803. E-mail addresses: [email protected] (N. Han), [email protected] (P. Hu), [email protected] (A. Zuo), [email protected] (D. Zhang), [email protected] (Y. Tian), [email protected] (Y. Chen). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.11.042
The ZnO nanorods were prepared by RF thermal plasmaenhanced CVD method as we reported earlier [13]. Briefly, zinc powder and oxygen were introduced into the Argon plasma (30 kW, 4 MHz), after a vapor-solid (VS) growth process and with a twodirectional growth mechanism, the ZnO nanorods were gained. The
N. Han et al. / Sensors and Actuators B 145 (2010) 114–119
115
Fig. 1. SEM images of (a) ZnO200, (b) ZnO500 and (c) ZnO2k.
morphology of final product could be well controlled by adjusting the ratio of oxygen partial pressure (oxygen flow rate). The three samples used herein are: 200 nm, 500 nm and 2000 nm in length respectively, with similar sectional diameter of 50–100 nm (denoted as ZnO200, ZnO500 and ZnO2k), as observed by scanning electron microscopy (SEM, JSM-6700F) in Fig. 1. In the post annealing treatment, the ZnO samples were placed in Al2 O3 boats in a tube furnace (inner diameter 45 mm) at required temperature for 2 h. For air annealing, the tube was open to atmosphere, while for H2 , N2 and O2 annealing, these gases were introduced into the sealed tube at a velocity of 100 ml min−1 .
The formaldehyde gas sensing property of the ZnO nanorods at 300 ◦ C and 400 ◦ C, where the gas sensing property is high according to the literature [2,3,15], are tested as shown in Fig. 3. We can see that ZnO500 performed the highest gas response at both 300 ◦ C and 400 ◦ C, followed by ZnO200 and ZnO2k. This result is hard to explain from the viewpoint of the widely accepted model that compares D and L [4], because the material used herein is not particles
2.2. Characterization The gas sensing property was tested in a home-made instrument as we reported earlier [15,16], and the gas response is defined as Ra /Rg , where Ra and Rg are the resistance of the sensor in air and in detectant. The Raman spectra were measured on Horiba Jobin Yvon LabRAM HR800 Raman Microscope (514 nm Argon laser, 20 mW, France). And the PL spectra were recorded from 350 nm to 800 nm at room temperature by a 325 nm excitation from Xe lamp (Perkin Elmer LS 55 fluorometer). 3. Results and discussion
Fig. 2. Raman spectra of ZnO nanorods.
3.1. As-prepared ZnO nanorods It is reported that nitrogen might be incorporated into the ZnO crystal by high temperature annealing of ZnO in N2 atmosphere [17], and so might do in plasma-CVD system (about thousands degree). In N-doped ZnO, there are additional Raman shift peak at ∼510 cm−1 and 643 cm−1 [18], besides the ZnO peaks of E2L (∼100 cm−1 ), A1T (∼380 cm−1 ), E1T (∼410 cm−1 ), E2H (∼438 cm−1 ), A1L (∼574 cm−1 ) and E1L (∼584 cm−1 ) [19]. But actually, only 3E2L , A1T , E1T , and E2H peaks were observed in the Raman spectra of the three ZnO nanorods, as shown in Fig. 2, with no Raman shift peak related to N-dopant observed. And no XRD peak related to Zn3 N2 was detected in our previous study [13]. So, we can rule out the extrinsic defect (N-dopant), and are concentrated on the gas sensing property investigation of pure ZnO nanorods.
Fig. 3. Formaldehyde response of the ZnO nanorods (tested at 300 ◦ C and 400 ◦ C at RH 70%).
116
N. Han et al. / Sensors and Actuators B 145 (2010) 114–119
Fig. 4. Room temperature PL spectra of the ZnO nanorods and relative intensity normalized at 375 nm (inset).
but ZnO nanorods with different length-to-diameter ratios. Actually as mentioned before, L is hard either to be measured or to be calculated, limiting the application of the model. Recently, some researchers attributed the high gas response to the high intensity ratio of visible luminescence to ultra-violet luminescence (IVL /IUL ), which, they say, means much more defects in the crystal [9,10]. Accordingly, the PL spectra of the ZnO nanorods are measured and shown in Fig. 4, from which we can see that the UV peak is stronger for ZnO2k, while the visible peaks are stronger for ZnO500 and ZnO200. The relative intensity normalized at 375 nm, which is reported originated from the conduction band to valence band (CB–VB) combination, is plotted in Fig. 4 inset. However, we find that the intensity ratio of ZnO200 is the highest followed by ZnO500 and ZnO2k, which means the ratio of IVL /IUL cannot explain the different gas sensing property of the ZnO nanorods used herein, either. In fact, the ratio is affected by sample type as well as excitation density, and could not be simply used to assess the crystal defects [12]. Consequently, we should study more insight into the intrinsic defects in the ZnO nanorods. As there are primarily two kinds of defects: the donor and the acceptor, therefore we tried to decompose the PL spectra to distinguish the DL and AL subpeaks. It is generally believed to be five intrinsic defects in ZnO: interstitial zinc (Zni ), zinc vacancy (VZn ), oxygen vacancy (VO ), oxygen interstitial (Oi ) and oxygen antisite (OZn ), of which Zni and VO are donor and VZn , Oi and OZn are acceptor [20,21]. At room temperature, the band gap of ZnO is 3.3–3.4 eV (∼375 nm), therefore, the ultra-violet (UV) subpeak near 380 nm is usually attributed to the CB–VB combination [22]. Srikant and Clarke [23] reported the shallow donor-related UV emission at 3.15 eV (∼395 nm), though the shallow donor is not clear. The peak at ∼2.9 eV (420 nm) attributes to Zni [22,24], and the peak at ∼2.7 eV (460 nm) is related to Zn vacancy (VZn ) [25,26]. The peak at ∼2.53 eV (490 nm) and ∼2.38 eV (520 nm) is the most controversial, and are more preferably attributed to VO [27,28] and OZn [24,29], respectively. The origin of yellow and orange luminescence (>540 nm) is usually ascribed to Oi [30], and the peak at ∼760 nm is the secondary UV diffraction.
Fig. 6. Formaldehyde response of ZnO2kA800, ZnO2kA900 and ZnO2kA1000 (tested at 300 ◦ C and 400 ◦ C at RH 70%).
Then the PL spectra are Gaussian decomposed according to the subpeaks position mentioned above in the range of 350–700 nm regardless of the secondary UV diffraction. As there is no universal guideline for the spectra peak decomposition, the main consideration in this paper is: (1) the PL subpeaks are Gaussian type [31–33]; (2) the peak position is relatively fixed; (3) strong peaks (obvious peaks in PL spectra) first and then weak ones (residue of PL spectra after decomposing obvious peaks) and (4) relatively high correlation coefficient (r2 > 0.999). The results are shown in Fig. 5a–c, together with the contents of the subpeaks in the total PL intensity. Then, if we separate the subpeaks by DL (peaks at ∼395 nm, ∼420 nm and ∼490 nm) and AL (peaks at ∼460 nm, ∼520 nm, ∼560 nm and ∼610 nm) ones, we can gain that for ZnO200, ZnO500 and ZnO2k, the contents are (39.77%, 59.39%), (47.91%, 51.35%) and (38.21%, 59.54%), respectively. The increasing sequence of DL subpeaks content (ZnO2k < ZnO200 < ZnO500) and the decreasing sequence of AL sequence (ZnO2k > ZnO200 > ZnO500) are in accordance with the order of the gas sensing property thereof (ZnO2k < ZnO200 < ZnO500). Then, from Figs. 3 and 5, we can conclude that the higher DL content and lower AL content lead to better gas sensing property. 3.2. Post-annealed ZnO nanorods The defects (measured by PL spectra) in ZnO could be tailored by annealing in different atmospheres, that is annealing in oxidative gas reduces donor and increases acceptor, while annealing in reductive or inert gas reduces acceptor and increases donor as reported elsewhere [14,34,35]. Therefore, to verify whether the relationship between the PL subpeaks contents and gas sensing property is valid, ZnO2k was taken as an example to be annealed in air at 800 ◦ C, 900 ◦ C and 1000 ◦ C for 2 h (denoted as ZnO2kA800, ZnO2kA900 and ZnO2kA1000), and the gas sensing property was tested. As can be seen in Fig. 6, the gas responses are lower than the original one. Furthermore, ZnO2k was also annealed in H2 at 500 ◦ C for 2 h, in N2 at 600 ◦ C for 2 h and in O2 at 800 ◦ C for 2 h (denoted as ZnO2kH, ZnO2kN and ZnO2kO). The gas sensing property in Fig. 7
Fig. 5. Gaussian decomposition of the ZnO nanorods PL: (a) ZnO200, (b) ZnO500 and (c) ZnO2k.
N. Han et al. / Sensors and Actuators B 145 (2010) 114–119
Fig. 7. Formaldehyde response of ZnO2kH, ZnO2kN and ZnO2kO (tested at 300 ◦ C and 400 ◦ C at RH 70%).
117
Fig. 10. Relationship between formaldehyde response of the ZnO nanorods at 300 ◦ C () & 400 ◦ C (䊉) and the DL () & AL () subpeaks contents.
From Fig. 10, we can see where the DL content is high and the AL content is low, the gas sensing property is high, regardless of the sample pretreatment, and vice versa. Extremely, ZnO2kA1000 has the lowest DL content and the highest AL one, which then performs the lowest gas response. While ZnO500, ZnO2kN and ZnO2kH have higher DL content and lower AL one, thus have higher gas sensing property. 3.3. Probable mechanism analysis
Fig. 8. Room temperature PL spectra of ZnO2k annealed in different atmospheres.
shows ZnO2kH and ZnO2kN have higher gas sensing property while ZnO2kO has lower property than the original one. Then the PL spectra were measured as shown in Fig. 8. We can see that, in contrast with the gas response, the ratio of IVL /IUL of ZnO2kH and ZnO2kN are lower while that of ZnO2kA800–1000 and ZnO2kO are higher. However, if we decompose the PL spectra into subpeaks and then separate the DL and AL ones as shown in Fig. 9, and plot the relative content together with the gas response to 205 ppm in Fig. 10, we can find out the tendency.
Generally, it is believed that the sensing mechanism follows two reactions. In air, oxygen is adsorbed on the ZnO particle surface, which then captures electrons from ZnO to be ion-sorbed state (Oad − ) at 300–400 ◦ C (Eq. (1)) [8]. In sensing process of reductive gases (e.g. formaldehyde), detectants would react with Oad − , whose electron is then given back to ZnO crystal, making the electron density increase (Eq. (2)). Then, oxygen in the two reations is mostly concerned, and efforts are to increase oxygen adsorption and ionization. But the other aspect is usually concerned less, i.e. where are the electrons in Eq. (1) come from? It is also a probable factor that influences gas sensing property. O2 + 2e− → 2Oad − −
HCHO + 2Oad → H2 O + CO2 + 2e
(1) −
Fig. 9. Gaussian decomposition of PL: (a) ZnO2kA800, (b) ZnO2kA900, (c) ZnO2kA1000, (d) ZnO2kH, (e) ZnO2kN and (f) ZnO2kO.
(2)
118
N. Han et al. / Sensors and Actuators B 145 (2010) 114–119
From the solid physics point of view, the donors (D) in ZnO give rise to the free electrons in ZnO, while the acceptors (A) consume free electrons (Eq. (3) and (4)). In equilibrium, the electrons donor released are more than that the acceptors consumed, which makes ZnO an n-type semiconductor. This way, we can see that the electrons donors released are consumed in two competitive ways: if the electrons are more consumed by the intrinsic acceptors, less adsorbed oxygen would be ionized. D → D+ + e−
(3)
−
(4)
A +e → A
−
Then the density ratio of free electrons after and before gas response reaction (Eq. (2)) can be regarded the same as the gas response defined as the resistance ratio as shown in Eq. (5): Gg ng e ng Ra = = = Rg Gs na e ne
(5)
where Gg and Ga are conductance in detectant and in air, respectively, ng and na are electron concentrations in detectant and in air, respectively, and e are mobility and charge of electron, respectively [8,36]. As the amount of oxygen is usually postulated to be several magnitudes larger than that of the free electron in ZnO [8], it is not hard to postulate that if fewer electrons released by donors are consumed by acceptors, and more are captured by adsorbed oxygen to form Oad − , the electron ratio (i.e. the gas sensing property) is high. However, it should be noted that the subpeak content is not the real defect content in ZnO crystal, but a reflection of the radioactive defect (luminescence centre) apart from the non-radioactive ones. But the result of comparing subpeak contents is reasonable for different samples, as the same instrument and the same data processing method are used. 4. Conclusions In summary, ZnO nanorod with the length of 500 nm showed higher gas sensing property than 200 nm and 2000 nm length ones, prepared by plasma-enhanced CVD method. The intrinsic defects were investigated by PL spectra, which were Gaussian decomposed to subpeaks according to the peak position reported in the literature. A conclusion is gained that gas response is enhanced by more donor- and less acceptor-related subpeak contents. Further, this conclusion is verified by annealing the 2000 nm ZnO sample in different ambients to obtain different donorand acceptor-related subpeak contents and gas sensing property. Acknowledgements The authors thank National Natural Science Foundation of China (No. 90406024) and National 863 Program (No. 2007AA061401) for the support. References [1] T. Seiyama, A. Kato, K. Fujiishi, M. Nagatani, A new detector for gaseous components using semiconductive thin films, Anal. Chem. 34 (1962) 1502– 1503. [2] L.J. Bie, X.N. Yan, J. Yin, Y.Q. Duan, Z.H. Yuan, Nanopillar ZnO gas sensor for hydrogen and ethanol, Sens. Actuator B 126 (2007) 604–608. [3] D.R. Patil, L.A. Patil, P.P. Patil, Cr2 O3 -activated ZnO thick film resistors for ammonia gas sensing operable at room temperature, Sens. Actuator B 126 (2007) 368–374. [4] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Grain size effects on gas sensitivity of porous SnO2 -based elements, Sens. Actuator B 3 (1991) 147–155. [5] C.H. Wang, X.F. Chu, M.W. Wu, Detection of H2 S down to ppb levels at room temperature using sensors based on ZnO nanorods, Sens. Actuator B 113 (2006) 320–323.
[6] A. Ahmad, J. Walsh, T.A. Wheat, Effect of processing on the properties of tin oxide-based thick-film gas sensors, Sens. Actuator B 93 (2003) 538–545. [7] J.Q. Xu, Q.Y. Pan, Y.A. Shun, Z.Z. Tian, Grain size control and gas sensing properties of ZnO gas sensor, Sens. Actuator B 66 (2000) 277–279. [8] G. Heiland, E. Mollwo, F. Stockmann, Electronic processes in zinc oxide, Solid State Phys. 8 (1959) 191–323. [9] Y. Zhang, J.Q. Xu, Q. Xiang, H. Li, Q.Y. Pan, P.C. Xu, Brush-like hierarchical ZnO nanostructures: synthesis, photoluminescence and gas sensor properties, J. Phys. Chem. C 113 (2009) 3430–3435. [10] T. Gao, T.H. Wang, Synthesis and properties of multipod-shaped ZnO nanorods for gas-sensor applications, Appl. Phys. A 80 (2005) 1451–1454. [11] T. Sahoo, S.K. Tripathy, Y.T. Yu, H.K. Ahn, D.C. Shin, I.H. Lee, Morphology and crystal quality investigation of hydrothermally synthesized ZnO micro-rods, Mater. Res. Bull. 43 (2008) 2060–2068. [12] W.S. Shi, B. Cheng, L. Zhang, E.T. Samulski, Influence of excitation density on photoluminescence of zinc oxide with different morphologies and dimensions, J. Appl. Phys. 98 (2005) 083502. [13] P. Hu, F.L. Yuan, L.Y. Bai, J.L. Li, Y.F. Chen, Plasma synthesis of large quantities of zinc oxide nanorods, J. Phys. Chem. C 111 (2007) 194–200. [14] X. Liu, X. Wu, H. Cao, R.P.H. Chang, Growth mechanism and properties of ZnO nanorods synthesized by plasma-enhanced chemical vapor deposition, J. Appl. Phys. 95 (2004) 3141–3147. [15] N. Han, Y.J. Tian, X.F. Wu, Y.F. Chen, Improving humidity selectivity in formaldehyde gas sensing by a two-sensor array made of Ga-doped ZnO, Sens. Actuator B 138 (2009) 228–235. [16] N. Han, Y.J. Tian, L.Q. Wei, C. Wang, Y.F. Chen, NiO thin film fabricated by electrophoretic deposition and formaldehyde gas sensing property thereof, J. Nanosci. Nanotechnol. 9 (2009) 1346–1349. [17] N.Y. Garces, N.C. Giles, L.E. Halliburton, G. Cantwell, D.B. Eason, D.C. Reynolds, D.C. Look, Production of nitrogen acceptors in ZnO by thermal annealing, Appl. Phys. Lett. 80 (2002) 1334–1336. [18] A. Kaschner, U. Haboeck, M. Strassburg, M. Strassburg, G. Kaczmarczyk, A. Hoffmann, C. Thomsen, A. Zeuner, H.R. Alves, D.M. Hofmann, B.K. Meyer, Nitrogen-related local vibrational modes in ZnO:N, Appl. Phys. Lett. 80 (2002) 1909–1911. [19] T.C. Damen, S.P.S. Porto, B. Tell, Raman effect in zinc oxide, Phys. Rev. 142 (1966) 570–574. [20] A. Poppl, G. Volkel, Esr and photo-Esr investigations of zinc vacancies and interstitial oxygen ions in undoped ZnO ceramics, Phys. Status Solidi A 125 (1991) 571–581. [21] F. Tuomisto, K. Saarinen, D.C. Look, G.C. Farlow, Introduction and recovery of point defects in electron-irradiated ZnO, Phys. Rev. B 72 (2005) 085206. [22] M.K. Lee, H.F. Tu, Ultraviolet emission blueshift of ZnO related to Zn, J. Appl. Phys. 101 (2007) 126103. [23] V. Srikant, D.R. Clarke, On the optical band gap of zinc oxide, J. Appl. Phys. 83 (1998) 5447–5451. [24] L. Zhao, J.S. Lian, Y.H. Liu, Q. Jiang, Influence of preparation methods on photoluminescence properties of ZnO films on quartz glass, Trans. Nonferr. Met. Soc. 18 (2008) 145–149. [25] X.Q. Wei, B.Y. Man, M. Liu, C.S. Xue, H.Z. Zhuang, C. Yang, Blue luminescent centers and microstructural evaluation by XPS and Raman in ZnO thin films annealed in vacuum, N2 and O2 , Physica B 388 (2007) 145–152. [26] K.T. Roro, J.K. Dangbegnon, S. Sivaraya, A.W.R. Leitch, J.R. Botha, Influence of metal organic chemical vapor deposition growth parameters on the luminescent properties of ZnO thin films deposited on glass substrates, J. Appl. Phys. 103 (2008) 053516. [27] W. Cheng, P. Wu, X. Zou, T. Xiao, Study on synthesis and blue emission mechanism of ZnO tetrapodlike nanostructures, J. Appl. Phys. 100 (2006) 054311. [28] T.M. Borseth, B.G. Svensson, A.Y. Kuznetsov, P. Klason, Q.X. Zhao, M. Willander, Identification of oxygen and zinc vacancy optical signals in ZnO, Appl. Phys. Lett. 89 (2006) 262112. [29] C.H. Tsai, W.C. Wang, F.L. Jenq, C.C. Liu, C.I. Hung, M.P. Houng, Surface modification of ZnO film by hydrogen peroxide solution, J. Appl. Phys. 104 (2008) 053521. [30] S.A. Studenikin, N. Golego, M. Cocivera, Fabrication of green and orange photoluminescent, undoped ZnO films using spray pyrolysis, J. Appl. Phys. 84 (1998) 2287–2294. [31] Y.Y. Gong, T. Andelman, G.F. Neumark, S. O’Brien, I.L. Kuskovsky, Origin of defect-related green emission from ZnO nanoparticles: effect of surface modification, Nanoscale Res. Lett. 2 (2007) 297–302. [32] Y.J. Zeng, Z.Z. Ye, W.Z. Xu, J.G. Lu, H.P. He, L.P. Zhu, B.H. Zhao, Y. Che, B.S. Zhang, pType behavior in nominally undoped ZnO thin films by oxygen plasma growth, Appl. Phys. Lett. 88 (2006) 262103. [33] M.K. Ryu, S.H. Lee, M.S. Jang, G.N. Panin, T.W. Kang, Postgrowth annealing effect on structural and optical properties of ZnO films grown on GaAs substrates by the radio frequency magnetron sputtering technique, J. Appl. Phys. 92 (2002) 154–158. [34] T.B. Hur, G.S. Jeen, Y.H. Hwang, H.K. Kim, Photoluminescence of polycrystalline ZnO under different annealing conditions, J. Appl. Phys. 94 (2003) 5787– 5790. [35] J.D. Ye, S.L. Gu, S.M. Zhu, F. Qin, S.M. Liu, W. Liu, X. Zhou, L.Q. Hu, R. Zhang, Y. Shi, Y.D. Zheng, Production of high-quality ZnO films by the two-step annealing method, J. Appl. Phys. 96 (2004) 5308–5310. [36] J. Law, J. Thong, Improving the NH3 gas sensitivity of ZnO nanowire sensors by reducing the carrier concentration, Nanotechnology 19 (2008) 205502–205600.
N. Han et al. / Sensors and Actuators B 145 (2010) 114–119
Biographies Ning Han received his master’s degree in Environmental Engineering at Harbin Institute of Technology (HIT), PR China, in 2006. And now he is a doctoral candidate in Institute of Process Engineering, Chinese Academy of Sciences. His research interests are preparation and application of gas sensing materials such as semiconductive metal oxides (NiO, ZnO, etc.), carbon nanotubes and so on. Peng Hu received his PhD in Chemical Engineering in Institute of Process Engineering, Chinese Academy of Sciences, PR China in 2008, and now he is an assistant professor of the Institute of Process Engineering. His current research fields focus on the synthesis, characterization and self-assembly of inorganic nanoparticles with novel structures, such as 1D nanostructure, hollow structure, and property investigation and functionalization of these novel nanostructures. Ahui Zuo is a master student in Institute of Process Engineering, Chinese Academy of Sciences. Her research interests are preparation of functional materials, such as nanostructured ZnO, Al2 O3 and so on. Dangwen Zhang received her bachelor’s degree in Chemical Engineering and Technology at Lanzhou Jiaotong University, Gansu province, China, in 2006. She is now
119
a two-year master candidate of Institute of Process Engineering, Chinese Academy of Sciences. She had committed herself to preparation of a variety of ZnO nanostructrues such as nanobelt, nanorod and so on. Yajun Tian received his PhD in Chemical Engineering at Taiyuan University of Technology, PR China, in 2002. He is an associate professor of the Institute of Process Engineering, Chinese Academy of Sciences. His current research interests are carbon nanotube growth mechanism and applications to gas sensors, etc. He is also interested in II–VI semiconductors synthesis and regulation and control of electrical properties. Yunfa Chen received his PhD in Material Science at University Louis Pasteur Strsbourg (ULP), France in 1993. He is a professor of the Graduate School of Chinese Academy of Sciences, and research professor and vice director of Institute of Process Engineering, Chinese Academy of Sciences. His current research interests are preparation and assembly of nanoparticles, functional materials, organic–inorganic composite materials and layered materials. And he is also interested in industrial application of nanomaterials.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Photoluminescence investigation on the gas sensing property of ZnO nanorods prepared by plasma-enhanced CVD method Ning Han a,b , Peng Hu a , Ahui Zuo a,b , Dangwen Zhang a,b , Yajun Tian a , Yunfa Chen a,∗ a b
State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e
i n f o
Article history: Received 31 July 2009 Received in revised form 29 October 2009 Accepted 18 November 2009 Available online 24 November 2009 Keywords: ZnO nanorod Intrinsic defects Photoluminescence Peak decomposition Gas sensor
a b s t r a c t Gas sensing property of ZnO nanorods prepared by plasma-enhanced chemical vapor deposition (CVD) method is studied using formaldehyde as the probe gas, and the intrinsic defects are investigated by photoluminescence (PL). The results show that high ratio of visible to ultra-violet luminescence cannot account for high gas response. The PL spectra are Gaussian decomposed to subpeaks according to their origination, which are separated into donor- (DL) and acceptor-related (AL) ones. A conclusion is derived that where the content of DL is high and that of AL is low, the gas response is high. This conclusion is further confirmed by tuning the PL spectra and gas sensing property through annealing in different atmospheres. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Since the first application in gas sensor [1], ZnO has found its way in explosive gas alerting and toxic gas detection for decades, with various morphologies and different dopants contributing much to the development of resistance-based ZnO gas sensors [2,3]. But the key factor determining the gas sensing property of ZnO is still under debate. The most prevalent model is founded using SnO2 by Xu et al. [4] that compares the particle diameter (D) and depth of surface charge layer (L): if D is comparable to or less than 2L, the gas response is expected to be high. However, just comparing D and L leads to dilemma in some studies [5–7], because L is hard either to be measured or to be calculated. As the electronic property of ZnO mainly depends on its intrinsic defects [8], the gas response defined as the ratio of resistance in air and in detectant (Ra /Rg ) is therefore closely correlated to the intrinsic defects [9,10]. Photoluminescence (PL) spectra is the luminescences originated from the photo-induced electron/hole and/or the intrinsic defects in ZnO [8]. Therefore, there are several defectrelated luminescences as well as the photo-induced near band edge excitation in ZnO PL spectra. Thus some researchers used the
intensity ratio of visible luminescence to ultra-violet luminescence (IVL /IUL ) to evaluate the crystallinity of ZnO crystal: the higher the ratio, the more the intrinsic defects [10,11]. However, Shi et al. [12] pointed out that the ratio of IVL /IUL is affected by sample type as well as excitation density, and could not be simply used to assess the crystal defects. Therefore, we tried to use decomposed PL spectra to distinguish donor-related (DL) and acceptor-related luminescences (AL) and further to investigate the relationship between the intrinsic defects and the gas sensing property of ZnO. The ZnO nanorods with different lengths used here are prepared in seconds by plasma-enhanced chemical vapor deposition (CVD) method under high temperature and thus possess lots of intrinsic defects [13,14]. Furthermore, the defects were tailored by annealing in different atmospheres and the relationship between the intrinsic defects measured by PL and the gas sensing property of ZnO was confirmed. It should be noted that the donors and acceptors in ZnO crystal play different roles in electron transport, and they affect the gas sensing property of ZnO in opposite way. 2. Experimental 2.1. ZnO nanorods preparation
∗ Corresponding author at: State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Haidian District, Beijing 100190, PR China. Tel.: +86 10 82627057; fax: +86 10 62542803. E-mail addresses: [email protected] (N. Han), [email protected] (P. Hu), [email protected] (A. Zuo), [email protected] (D. Zhang), [email protected] (Y. Tian), [email protected] (Y. Chen). 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.11.042
The ZnO nanorods were prepared by RF thermal plasmaenhanced CVD method as we reported earlier [13]. Briefly, zinc powder and oxygen were introduced into the Argon plasma (30 kW, 4 MHz), after a vapor-solid (VS) growth process and with a twodirectional growth mechanism, the ZnO nanorods were gained. The
N. Han et al. / Sensors and Actuators B 145 (2010) 114–119
115
Fig. 1. SEM images of (a) ZnO200, (b) ZnO500 and (c) ZnO2k.
morphology of final product could be well controlled by adjusting the ratio of oxygen partial pressure (oxygen flow rate). The three samples used herein are: 200 nm, 500 nm and 2000 nm in length respectively, with similar sectional diameter of 50–100 nm (denoted as ZnO200, ZnO500 and ZnO2k), as observed by scanning electron microscopy (SEM, JSM-6700F) in Fig. 1. In the post annealing treatment, the ZnO samples were placed in Al2 O3 boats in a tube furnace (inner diameter 45 mm) at required temperature for 2 h. For air annealing, the tube was open to atmosphere, while for H2 , N2 and O2 annealing, these gases were introduced into the sealed tube at a velocity of 100 ml min−1 .
The formaldehyde gas sensing property of the ZnO nanorods at 300 ◦ C and 400 ◦ C, where the gas sensing property is high according to the literature [2,3,15], are tested as shown in Fig. 3. We can see that ZnO500 performed the highest gas response at both 300 ◦ C and 400 ◦ C, followed by ZnO200 and ZnO2k. This result is hard to explain from the viewpoint of the widely accepted model that compares D and L [4], because the material used herein is not particles
2.2. Characterization The gas sensing property was tested in a home-made instrument as we reported earlier [15,16], and the gas response is defined as Ra /Rg , where Ra and Rg are the resistance of the sensor in air and in detectant. The Raman spectra were measured on Horiba Jobin Yvon LabRAM HR800 Raman Microscope (514 nm Argon laser, 20 mW, France). And the PL spectra were recorded from 350 nm to 800 nm at room temperature by a 325 nm excitation from Xe lamp (Perkin Elmer LS 55 fluorometer). 3. Results and discussion
Fig. 2. Raman spectra of ZnO nanorods.
3.1. As-prepared ZnO nanorods It is reported that nitrogen might be incorporated into the ZnO crystal by high temperature annealing of ZnO in N2 atmosphere [17], and so might do in plasma-CVD system (about thousands degree). In N-doped ZnO, there are additional Raman shift peak at ∼510 cm−1 and 643 cm−1 [18], besides the ZnO peaks of E2L (∼100 cm−1 ), A1T (∼380 cm−1 ), E1T (∼410 cm−1 ), E2H (∼438 cm−1 ), A1L (∼574 cm−1 ) and E1L (∼584 cm−1 ) [19]. But actually, only 3E2L , A1T , E1T , and E2H peaks were observed in the Raman spectra of the three ZnO nanorods, as shown in Fig. 2, with no Raman shift peak related to N-dopant observed. And no XRD peak related to Zn3 N2 was detected in our previous study [13]. So, we can rule out the extrinsic defect (N-dopant), and are concentrated on the gas sensing property investigation of pure ZnO nanorods.
Fig. 3. Formaldehyde response of the ZnO nanorods (tested at 300 ◦ C and 400 ◦ C at RH 70%).
116
N. Han et al. / Sensors and Actuators B 145 (2010) 114–119
Fig. 4. Room temperature PL spectra of the ZnO nanorods and relative intensity normalized at 375 nm (inset).
but ZnO nanorods with different length-to-diameter ratios. Actually as mentioned before, L is hard either to be measured or to be calculated, limiting the application of the model. Recently, some researchers attributed the high gas response to the high intensity ratio of visible luminescence to ultra-violet luminescence (IVL /IUL ), which, they say, means much more defects in the crystal [9,10]. Accordingly, the PL spectra of the ZnO nanorods are measured and shown in Fig. 4, from which we can see that the UV peak is stronger for ZnO2k, while the visible peaks are stronger for ZnO500 and ZnO200. The relative intensity normalized at 375 nm, which is reported originated from the conduction band to valence band (CB–VB) combination, is plotted in Fig. 4 inset. However, we find that the intensity ratio of ZnO200 is the highest followed by ZnO500 and ZnO2k, which means the ratio of IVL /IUL cannot explain the different gas sensing property of the ZnO nanorods used herein, either. In fact, the ratio is affected by sample type as well as excitation density, and could not be simply used to assess the crystal defects [12]. Consequently, we should study more insight into the intrinsic defects in the ZnO nanorods. As there are primarily two kinds of defects: the donor and the acceptor, therefore we tried to decompose the PL spectra to distinguish the DL and AL subpeaks. It is generally believed to be five intrinsic defects in ZnO: interstitial zinc (Zni ), zinc vacancy (VZn ), oxygen vacancy (VO ), oxygen interstitial (Oi ) and oxygen antisite (OZn ), of which Zni and VO are donor and VZn , Oi and OZn are acceptor [20,21]. At room temperature, the band gap of ZnO is 3.3–3.4 eV (∼375 nm), therefore, the ultra-violet (UV) subpeak near 380 nm is usually attributed to the CB–VB combination [22]. Srikant and Clarke [23] reported the shallow donor-related UV emission at 3.15 eV (∼395 nm), though the shallow donor is not clear. The peak at ∼2.9 eV (420 nm) attributes to Zni [22,24], and the peak at ∼2.7 eV (460 nm) is related to Zn vacancy (VZn ) [25,26]. The peak at ∼2.53 eV (490 nm) and ∼2.38 eV (520 nm) is the most controversial, and are more preferably attributed to VO [27,28] and OZn [24,29], respectively. The origin of yellow and orange luminescence (>540 nm) is usually ascribed to Oi [30], and the peak at ∼760 nm is the secondary UV diffraction.
Fig. 6. Formaldehyde response of ZnO2kA800, ZnO2kA900 and ZnO2kA1000 (tested at 300 ◦ C and 400 ◦ C at RH 70%).
Then the PL spectra are Gaussian decomposed according to the subpeaks position mentioned above in the range of 350–700 nm regardless of the secondary UV diffraction. As there is no universal guideline for the spectra peak decomposition, the main consideration in this paper is: (1) the PL subpeaks are Gaussian type [31–33]; (2) the peak position is relatively fixed; (3) strong peaks (obvious peaks in PL spectra) first and then weak ones (residue of PL spectra after decomposing obvious peaks) and (4) relatively high correlation coefficient (r2 > 0.999). The results are shown in Fig. 5a–c, together with the contents of the subpeaks in the total PL intensity. Then, if we separate the subpeaks by DL (peaks at ∼395 nm, ∼420 nm and ∼490 nm) and AL (peaks at ∼460 nm, ∼520 nm, ∼560 nm and ∼610 nm) ones, we can gain that for ZnO200, ZnO500 and ZnO2k, the contents are (39.77%, 59.39%), (47.91%, 51.35%) and (38.21%, 59.54%), respectively. The increasing sequence of DL subpeaks content (ZnO2k < ZnO200 < ZnO500) and the decreasing sequence of AL sequence (ZnO2k > ZnO200 > ZnO500) are in accordance with the order of the gas sensing property thereof (ZnO2k < ZnO200 < ZnO500). Then, from Figs. 3 and 5, we can conclude that the higher DL content and lower AL content lead to better gas sensing property. 3.2. Post-annealed ZnO nanorods The defects (measured by PL spectra) in ZnO could be tailored by annealing in different atmospheres, that is annealing in oxidative gas reduces donor and increases acceptor, while annealing in reductive or inert gas reduces acceptor and increases donor as reported elsewhere [14,34,35]. Therefore, to verify whether the relationship between the PL subpeaks contents and gas sensing property is valid, ZnO2k was taken as an example to be annealed in air at 800 ◦ C, 900 ◦ C and 1000 ◦ C for 2 h (denoted as ZnO2kA800, ZnO2kA900 and ZnO2kA1000), and the gas sensing property was tested. As can be seen in Fig. 6, the gas responses are lower than the original one. Furthermore, ZnO2k was also annealed in H2 at 500 ◦ C for 2 h, in N2 at 600 ◦ C for 2 h and in O2 at 800 ◦ C for 2 h (denoted as ZnO2kH, ZnO2kN and ZnO2kO). The gas sensing property in Fig. 7
Fig. 5. Gaussian decomposition of the ZnO nanorods PL: (a) ZnO200, (b) ZnO500 and (c) ZnO2k.
N. Han et al. / Sensors and Actuators B 145 (2010) 114–119
Fig. 7. Formaldehyde response of ZnO2kH, ZnO2kN and ZnO2kO (tested at 300 ◦ C and 400 ◦ C at RH 70%).
117
Fig. 10. Relationship between formaldehyde response of the ZnO nanorods at 300 ◦ C () & 400 ◦ C (䊉) and the DL () & AL () subpeaks contents.
From Fig. 10, we can see where the DL content is high and the AL content is low, the gas sensing property is high, regardless of the sample pretreatment, and vice versa. Extremely, ZnO2kA1000 has the lowest DL content and the highest AL one, which then performs the lowest gas response. While ZnO500, ZnO2kN and ZnO2kH have higher DL content and lower AL one, thus have higher gas sensing property. 3.3. Probable mechanism analysis
Fig. 8. Room temperature PL spectra of ZnO2k annealed in different atmospheres.
shows ZnO2kH and ZnO2kN have higher gas sensing property while ZnO2kO has lower property than the original one. Then the PL spectra were measured as shown in Fig. 8. We can see that, in contrast with the gas response, the ratio of IVL /IUL of ZnO2kH and ZnO2kN are lower while that of ZnO2kA800–1000 and ZnO2kO are higher. However, if we decompose the PL spectra into subpeaks and then separate the DL and AL ones as shown in Fig. 9, and plot the relative content together with the gas response to 205 ppm in Fig. 10, we can find out the tendency.
Generally, it is believed that the sensing mechanism follows two reactions. In air, oxygen is adsorbed on the ZnO particle surface, which then captures electrons from ZnO to be ion-sorbed state (Oad − ) at 300–400 ◦ C (Eq. (1)) [8]. In sensing process of reductive gases (e.g. formaldehyde), detectants would react with Oad − , whose electron is then given back to ZnO crystal, making the electron density increase (Eq. (2)). Then, oxygen in the two reations is mostly concerned, and efforts are to increase oxygen adsorption and ionization. But the other aspect is usually concerned less, i.e. where are the electrons in Eq. (1) come from? It is also a probable factor that influences gas sensing property. O2 + 2e− → 2Oad − −
HCHO + 2Oad → H2 O + CO2 + 2e
(1) −
Fig. 9. Gaussian decomposition of PL: (a) ZnO2kA800, (b) ZnO2kA900, (c) ZnO2kA1000, (d) ZnO2kH, (e) ZnO2kN and (f) ZnO2kO.
(2)
118
N. Han et al. / Sensors and Actuators B 145 (2010) 114–119
From the solid physics point of view, the donors (D) in ZnO give rise to the free electrons in ZnO, while the acceptors (A) consume free electrons (Eq. (3) and (4)). In equilibrium, the electrons donor released are more than that the acceptors consumed, which makes ZnO an n-type semiconductor. This way, we can see that the electrons donors released are consumed in two competitive ways: if the electrons are more consumed by the intrinsic acceptors, less adsorbed oxygen would be ionized. D → D+ + e−
(3)
−
(4)
A +e → A
−
Then the density ratio of free electrons after and before gas response reaction (Eq. (2)) can be regarded the same as the gas response defined as the resistance ratio as shown in Eq. (5): Gg ng e ng Ra = = = Rg Gs na e ne
(5)
where Gg and Ga are conductance in detectant and in air, respectively, ng and na are electron concentrations in detectant and in air, respectively, and e are mobility and charge of electron, respectively [8,36]. As the amount of oxygen is usually postulated to be several magnitudes larger than that of the free electron in ZnO [8], it is not hard to postulate that if fewer electrons released by donors are consumed by acceptors, and more are captured by adsorbed oxygen to form Oad − , the electron ratio (i.e. the gas sensing property) is high. However, it should be noted that the subpeak content is not the real defect content in ZnO crystal, but a reflection of the radioactive defect (luminescence centre) apart from the non-radioactive ones. But the result of comparing subpeak contents is reasonable for different samples, as the same instrument and the same data processing method are used. 4. Conclusions In summary, ZnO nanorod with the length of 500 nm showed higher gas sensing property than 200 nm and 2000 nm length ones, prepared by plasma-enhanced CVD method. The intrinsic defects were investigated by PL spectra, which were Gaussian decomposed to subpeaks according to the peak position reported in the literature. A conclusion is gained that gas response is enhanced by more donor- and less acceptor-related subpeak contents. Further, this conclusion is verified by annealing the 2000 nm ZnO sample in different ambients to obtain different donorand acceptor-related subpeak contents and gas sensing property. Acknowledgements The authors thank National Natural Science Foundation of China (No. 90406024) and National 863 Program (No. 2007AA061401) for the support. References [1] T. Seiyama, A. Kato, K. Fujiishi, M. Nagatani, A new detector for gaseous components using semiconductive thin films, Anal. Chem. 34 (1962) 1502– 1503. [2] L.J. Bie, X.N. Yan, J. Yin, Y.Q. Duan, Z.H. Yuan, Nanopillar ZnO gas sensor for hydrogen and ethanol, Sens. Actuator B 126 (2007) 604–608. [3] D.R. Patil, L.A. Patil, P.P. Patil, Cr2 O3 -activated ZnO thick film resistors for ammonia gas sensing operable at room temperature, Sens. Actuator B 126 (2007) 368–374. [4] C. Xu, J. Tamaki, N. Miura, N. Yamazoe, Grain size effects on gas sensitivity of porous SnO2 -based elements, Sens. Actuator B 3 (1991) 147–155. [5] C.H. Wang, X.F. Chu, M.W. Wu, Detection of H2 S down to ppb levels at room temperature using sensors based on ZnO nanorods, Sens. Actuator B 113 (2006) 320–323.
[6] A. Ahmad, J. Walsh, T.A. Wheat, Effect of processing on the properties of tin oxide-based thick-film gas sensors, Sens. Actuator B 93 (2003) 538–545. [7] J.Q. Xu, Q.Y. Pan, Y.A. Shun, Z.Z. Tian, Grain size control and gas sensing properties of ZnO gas sensor, Sens. Actuator B 66 (2000) 277–279. [8] G. Heiland, E. Mollwo, F. Stockmann, Electronic processes in zinc oxide, Solid State Phys. 8 (1959) 191–323. [9] Y. Zhang, J.Q. Xu, Q. Xiang, H. Li, Q.Y. Pan, P.C. Xu, Brush-like hierarchical ZnO nanostructures: synthesis, photoluminescence and gas sensor properties, J. Phys. Chem. C 113 (2009) 3430–3435. [10] T. Gao, T.H. Wang, Synthesis and properties of multipod-shaped ZnO nanorods for gas-sensor applications, Appl. Phys. A 80 (2005) 1451–1454. [11] T. Sahoo, S.K. Tripathy, Y.T. Yu, H.K. Ahn, D.C. Shin, I.H. Lee, Morphology and crystal quality investigation of hydrothermally synthesized ZnO micro-rods, Mater. Res. Bull. 43 (2008) 2060–2068. [12] W.S. Shi, B. Cheng, L. Zhang, E.T. Samulski, Influence of excitation density on photoluminescence of zinc oxide with different morphologies and dimensions, J. Appl. Phys. 98 (2005) 083502. [13] P. Hu, F.L. Yuan, L.Y. Bai, J.L. Li, Y.F. Chen, Plasma synthesis of large quantities of zinc oxide nanorods, J. Phys. Chem. C 111 (2007) 194–200. [14] X. Liu, X. Wu, H. Cao, R.P.H. Chang, Growth mechanism and properties of ZnO nanorods synthesized by plasma-enhanced chemical vapor deposition, J. Appl. Phys. 95 (2004) 3141–3147. [15] N. Han, Y.J. Tian, X.F. Wu, Y.F. Chen, Improving humidity selectivity in formaldehyde gas sensing by a two-sensor array made of Ga-doped ZnO, Sens. Actuator B 138 (2009) 228–235. [16] N. Han, Y.J. Tian, L.Q. Wei, C. Wang, Y.F. Chen, NiO thin film fabricated by electrophoretic deposition and formaldehyde gas sensing property thereof, J. Nanosci. Nanotechnol. 9 (2009) 1346–1349. [17] N.Y. Garces, N.C. Giles, L.E. Halliburton, G. Cantwell, D.B. Eason, D.C. Reynolds, D.C. Look, Production of nitrogen acceptors in ZnO by thermal annealing, Appl. Phys. Lett. 80 (2002) 1334–1336. [18] A. Kaschner, U. Haboeck, M. Strassburg, M. Strassburg, G. Kaczmarczyk, A. Hoffmann, C. Thomsen, A. Zeuner, H.R. Alves, D.M. Hofmann, B.K. Meyer, Nitrogen-related local vibrational modes in ZnO:N, Appl. Phys. Lett. 80 (2002) 1909–1911. [19] T.C. Damen, S.P.S. Porto, B. Tell, Raman effect in zinc oxide, Phys. Rev. 142 (1966) 570–574. [20] A. Poppl, G. Volkel, Esr and photo-Esr investigations of zinc vacancies and interstitial oxygen ions in undoped ZnO ceramics, Phys. Status Solidi A 125 (1991) 571–581. [21] F. Tuomisto, K. Saarinen, D.C. Look, G.C. Farlow, Introduction and recovery of point defects in electron-irradiated ZnO, Phys. Rev. B 72 (2005) 085206. [22] M.K. Lee, H.F. Tu, Ultraviolet emission blueshift of ZnO related to Zn, J. Appl. Phys. 101 (2007) 126103. [23] V. Srikant, D.R. Clarke, On the optical band gap of zinc oxide, J. Appl. Phys. 83 (1998) 5447–5451. [24] L. Zhao, J.S. Lian, Y.H. Liu, Q. Jiang, Influence of preparation methods on photoluminescence properties of ZnO films on quartz glass, Trans. Nonferr. Met. Soc. 18 (2008) 145–149. [25] X.Q. Wei, B.Y. Man, M. Liu, C.S. Xue, H.Z. Zhuang, C. Yang, Blue luminescent centers and microstructural evaluation by XPS and Raman in ZnO thin films annealed in vacuum, N2 and O2 , Physica B 388 (2007) 145–152. [26] K.T. Roro, J.K. Dangbegnon, S. Sivaraya, A.W.R. Leitch, J.R. Botha, Influence of metal organic chemical vapor deposition growth parameters on the luminescent properties of ZnO thin films deposited on glass substrates, J. Appl. Phys. 103 (2008) 053516. [27] W. Cheng, P. Wu, X. Zou, T. Xiao, Study on synthesis and blue emission mechanism of ZnO tetrapodlike nanostructures, J. Appl. Phys. 100 (2006) 054311. [28] T.M. Borseth, B.G. Svensson, A.Y. Kuznetsov, P. Klason, Q.X. Zhao, M. Willander, Identification of oxygen and zinc vacancy optical signals in ZnO, Appl. Phys. Lett. 89 (2006) 262112. [29] C.H. Tsai, W.C. Wang, F.L. Jenq, C.C. Liu, C.I. Hung, M.P. Houng, Surface modification of ZnO film by hydrogen peroxide solution, J. Appl. Phys. 104 (2008) 053521. [30] S.A. Studenikin, N. Golego, M. Cocivera, Fabrication of green and orange photoluminescent, undoped ZnO films using spray pyrolysis, J. Appl. Phys. 84 (1998) 2287–2294. [31] Y.Y. Gong, T. Andelman, G.F. Neumark, S. O’Brien, I.L. Kuskovsky, Origin of defect-related green emission from ZnO nanoparticles: effect of surface modification, Nanoscale Res. Lett. 2 (2007) 297–302. [32] Y.J. Zeng, Z.Z. Ye, W.Z. Xu, J.G. Lu, H.P. He, L.P. Zhu, B.H. Zhao, Y. Che, B.S. Zhang, pType behavior in nominally undoped ZnO thin films by oxygen plasma growth, Appl. Phys. Lett. 88 (2006) 262103. [33] M.K. Ryu, S.H. Lee, M.S. Jang, G.N. Panin, T.W. Kang, Postgrowth annealing effect on structural and optical properties of ZnO films grown on GaAs substrates by the radio frequency magnetron sputtering technique, J. Appl. Phys. 92 (2002) 154–158. [34] T.B. Hur, G.S. Jeen, Y.H. Hwang, H.K. Kim, Photoluminescence of polycrystalline ZnO under different annealing conditions, J. Appl. Phys. 94 (2003) 5787– 5790. [35] J.D. Ye, S.L. Gu, S.M. Zhu, F. Qin, S.M. Liu, W. Liu, X. Zhou, L.Q. Hu, R. Zhang, Y. Shi, Y.D. Zheng, Production of high-quality ZnO films by the two-step annealing method, J. Appl. Phys. 96 (2004) 5308–5310. [36] J. Law, J. Thong, Improving the NH3 gas sensitivity of ZnO nanowire sensors by reducing the carrier concentration, Nanotechnology 19 (2008) 205502–205600.
N. Han et al. / Sensors and Actuators B 145 (2010) 114–119
Biographies Ning Han received his master’s degree in Environmental Engineering at Harbin Institute of Technology (HIT), PR China, in 2006. And now he is a doctoral candidate in Institute of Process Engineering, Chinese Academy of Sciences. His research interests are preparation and application of gas sensing materials such as semiconductive metal oxides (NiO, ZnO, etc.), carbon nanotubes and so on. Peng Hu received his PhD in Chemical Engineering in Institute of Process Engineering, Chinese Academy of Sciences, PR China in 2008, and now he is an assistant professor of the Institute of Process Engineering. His current research fields focus on the synthesis, characterization and self-assembly of inorganic nanoparticles with novel structures, such as 1D nanostructure, hollow structure, and property investigation and functionalization of these novel nanostructures. Ahui Zuo is a master student in Institute of Process Engineering, Chinese Academy of Sciences. Her research interests are preparation of functional materials, such as nanostructured ZnO, Al2 O3 and so on. Dangwen Zhang received her bachelor’s degree in Chemical Engineering and Technology at Lanzhou Jiaotong University, Gansu province, China, in 2006. She is now
119
a two-year master candidate of Institute of Process Engineering, Chinese Academy of Sciences. She had committed herself to preparation of a variety of ZnO nanostructrues such as nanobelt, nanorod and so on. Yajun Tian received his PhD in Chemical Engineering at Taiyuan University of Technology, PR China, in 2002. He is an associate professor of the Institute of Process Engineering, Chinese Academy of Sciences. His current research interests are carbon nanotube growth mechanism and applications to gas sensors, etc. He is also interested in II–VI semiconductors synthesis and regulation and control of electrical properties. Yunfa Chen received his PhD in Material Science at University Louis Pasteur Strsbourg (ULP), France in 1993. He is a professor of the Graduate School of Chinese Academy of Sciences, and research professor and vice director of Institute of Process Engineering, Chinese Academy of Sciences. His current research interests are preparation and assembly of nanoparticles, functional materials, organic–inorganic composite materials and layered materials. And he is also interested in industrial application of nanomaterials.