IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 22, NOVEMBER 15, 2012
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Solution-Processed Photoconductive UV Detectors Based on ZnO Nanosheets Elham Amini, Mahboubeh Dolatyari, Ali Rostami, Hemayat Shekari, Hamed Baghban, Hassan Rasooli, and Somayeh Miri Abstract— A low-cost and complexity-free demonstration of two relatively fast ultraviolet (UV) detectors based on ZnO nanosheets are reported in this letter for the first time. Highly crystalline ZnO nanosheets have been synthesized by a sonochemical method. The proposed UV detectors were fabricated through coating the synthesized materials on Cu electrodes. Experimentally obtained responsivity, detectivity, absorption, and I–V characteristics along with the quantum efficiency are reported. Comparison of the conventional ZnO photo-sensors with nanoparticle-based ones reveals that the proposed nanosheets photosensors have faster response time in the UV region, which might be attributed to low density of defect states (created during synthesis process) on nanosheets surfaces. Index Terms— Nanomaterials, Ultrasonic, ultraviolet (UV) detectors, ZnO.
I. I NTRODUCTION
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OLLOIDAL semiconductor nanocrystals have attracted significant interest for applications in solutionprocessable devices such as photodetectors [1], light emitting diodes [2] and solar cells [3]. In addition to processability from solutions, nanocrystals are attractive because of a variety of novel shapes and their internal structures. Nowadays, ultraviolet (UV) detectors have a wide range of industrial and environmental applications and have been fabricated based on wide-band gap materials in recent years. As a candidate for UV photo-detection ZnO possesses high photosensitivity which is considerably important for ultraviolet photo-detection [4]. The physical, chemical and mechanical properties of nano-crystalline materials are basically influenced by their defect structures. The surface defects of nanocrystalline materials are affected by synthesis methods [5]. Using ultrasound radiation, during the homogeneous precipitation of the precursor, is expected to reduce the reaction time and to ensure homogeneity of the cations in the precursor [6] which possibly influences the density of crystalline defects in the structure of ZnO
Manuscript received May 12, 2012; revised July 11, 2012; accepted August 29, 2012. Date of publication September 27, 2012; date of current version October 31, 2012. This work was supported in part by the Research Office at the University of Tabriz, Iran. E. Amini, M. Dolatyari, H. Baghban, H. Rasooli, and S. Miri are with the School of Engineering-Emerging Technologies, University of Tabriz, Tabriz 5166614761, Iran (e-mail:
[email protected];
[email protected];
[email protected];
[email protected];
[email protected]). A. Rostami is with the Photonics and Nanocrystal Research Laboratory, Faculty of Electrical and Computer Engineering, University of Tabriz, Tabriz 5166614761, Iran (e-mail:
[email protected]). H. Shekari is with Department of Physical Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz 5166614761, Iran. Digital Object Identifier 10.1109/LPT.2012.2217375
nanomaterials. Herein, we report on synthesis of ZnO nanosheets by a sonochemical method as well as solution process fabrication and characterization of relatively fast photoconductive ZnO metal-semiconductor-metal (MSM) UV detectors using Cu electrodes deposited on fiber glass substrate. This fabrication method offers a simultaneous solution to some challenges: It is processed from solution which allows convenient integration with any substrate and optimal selection of effective band gap for a given application. Finding suitable synthesis method to obtain nanocrystals with low crystalline defects could solve response time problem of ZnO photodetectors. Here we report ZnO-based UV detector with millisecond response time, while, there has been no report on response time faster than 1 second for nano ZnO photoconductors till now to the best of our knowledge. II. E XPERIMENTAL M ETHOD All chemicals were of analytical grade and were used without further purification. In a typical procedure, Zn(NO3 )2 (1.0 mmol) and NaOH (10 mmol) were dissolved in double distilled water in separate beakers. These solutions were then added to 15 ml absolute alcohol. The obtained solution was divided into two parts, one of which was irradiated for 15 min (a) (33 kHz, 50 W) and the other for 30 min (b) (24 kHz, 100 W) at room temperature. The resultant products were centrifuged and washed several times with distilled water and ethanol (to remove unreacted reagents) and were dried in air at 60°C. To fabricate the MSM photoconductive detectors, the synthesized ZnO nanosheets were dispersed in ethanol and coated on interdigitated Copper (Cu) contacts deposited on a Fiber-glass substrate using spin coating method. The fingers have a length of 0.5 cm and a width of 150 μm with a pitch of 150 μm. The thickness of the Cu layer was 500 nm. The crystal structure and phase purity of ZnO nanoparticles were characterized by powder X-ray diffraction (PXRD) on a Siemens D500 using Cu-kα radiation (λ = 1.541 Å). UV–V is absorption spectra were recorded employing a PG instruments Ltd T70 UV/V is spectrophotometer. Photoluminescence measurements were carried out by a Perkin–Elmer LS55 luminescence spectrophotometer. Ultrasound radiation was performed using UP400s Germany (0.3 cm diameter Ti horn, 200 W, 24 kHz) and UW3200 Germany (1.4 cm diameter Ti horn, 50 W, 33 kHz). Surface morphology and distribution of the particles were studied via a Philips model XL30 scanning electron microscope (SEM). Current-voltage (I –V ) characteristics and spectral responsivity of the fabricated devices were measured using A and C mercury lamps as the light sources.
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Fig. 1.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 24, NO. 22, NOVEMBER 15, 2012
XRD pattern of the synthesized ZnO nanoparticles (sample b).
III. R ESULTS AND D ISCUSSION The phase and purity of the samples were determined by powder X-ray diffraction. PXRD patterns of the synthesized materials are given in Fig. 1. The position of the Bragg peaks is in accordance with the JCPDS card (36–1451) for wurtzite hexagonal crystalline phase of ZnO with the lattice constants of a = 3.2498 Å, and c = 5.2066 Å. No peaks attributable to possible impurities are observed. Fig. 2 shows SEM images of the synthesized ZnO nanosheets. Sheets of about 179 × 49 nm and 400 × 70 nm with thicknesses less than 10 nm, respectively for samples a and b were observed. These results indicate that increasing the ultrasonic irradiation time leads to growing of the sheets. Also, increasing the ultrasonic irradiation time increases the uniformity of the sheets. These nanoparticles exhibit a band-to-band absorption peak at 3.35 eV for sample a and 3.3 eV for sample b as depicted in Fig. 3, whereas, bulk ZnO shows an excitonic absorption peak at 3.2 eV. By the effective mass approximation model [7] given by E = E bulk + (π 2 h¯ 2 /2R 2 ) × (1/m ∗e +1/m ∗h ) (where E is the resonant energy of the nanoparticles, E bulk is the band gap of the bulk material, R is the size of particles, m ∗e and m ∗h are effective masses of the electrons (m ∗e = 0.24 m o ) and holes (m ∗h = 0.45m o )), a particle size of 4 and 5 nm was predicted for sample a and b respectively using these spectroscopic data. The corresponding particle sizes agree with the thickness of the sheets obtained from the SEM images. The energy levels and electronic transition bands of ZnO nanosheets were calculated by quantum well approximation. The sufficiently deep rectangular potential wells in the conduction band and the valence band can be approximated as a 1-D infinite potential well in which the particles of mass m ∗ (m ∗e for electrons in conduction band and m ∗h for holes in valence band) are confined inside the potential well region. In the case of ZnO quantum well with width of l = 4 and 5 nm, the allowed energy levels of electrons (by solving 1-D Schrödinger equation) with effective mass of m ∗e = 0.24 m o are 0.097, 0.391, 0.881, 1.567 eV and 0.068, 0.272, 0.612, 1.088, 1.701 eV respectively. So, the electronic transition bands are calculated as 3.35, 3.80, 4.55 and 5.6 eV for l = 4 nm (sample a) and 3.3, 3.617, 4.139, 4.87 and 5.8 eV for l = 5 nm (sample b) which are in agreement with the experimental spectra. Fig. 3 shows the experimental absorption spectra of the ZnO nanosheets. The spectra show well defined excitonic
Fig. 2. SEM images of ZnO nanoparticles synthesized by the sonochemical method with different ultrasonic irradiation times. (a) 15 min. (b) 30 min.
Fig. 3. Experimental absorption (left) and PL spectra (right) of ZnO nanoparticles (λex = 256 nm) with ultrasonic irradiation times of (a) 15 min and (b) 30 min.
absorption peaks as a characteristics of ZnO nanoparticles. The observed significant blue-shift, compared to the bulk ZnO, indicates that the average particle sizes are in the quantum size regime. As indicated by the red shift in the absorption edges of the obtained materials synthesized by different reaction times, since the ZnO nanosheets grow with increasing the ultrasonic irradiation time, the ultrasonic irradiation strongly affect the particle size and hence, band gap of the ZnO nanoparticles. The PL spectra of the synthesized materials (Fig. 3) shows a strong and dominant UV emission at 372 nm and a very weak and suppressed green emission at 528 nm for sample a and only one dominant and sharp exciton peak at 374 nm with no observable deep level emission for sample b. The UV emission is generally originated from the direct recombination of the free excitons through an exciton–exciton collision process which is called near band edge emission (NBE) [8]. It is believed that the appearance of the visible emission is due to the impurities and structural defects in the ZnO crystals [9]. Vanheusden et al have predicted the PL mechanism responsible for green emission in ZnO and suggested that intrinsic defects, especially oxygen vacancies, play a key role for it. This emission is generated by the recombination of electrons in singly occupied oxygen vacancies with photoexcited holes in the valence band [10]. It has also been reported that increasing the concentration of singly ionized oxygen vacancies would result in non-stoichiometric phase structure formation and leads to the broadened green emission intensity. In comparison, a decrease in the concentration of the oxygen vacancies may cause a decrease in the green emission intensity [11]. Bagnall et al have found that the green emission in the PL spectra is quite dependent on the crystal quality of the synthesized structures; i.e., if the structures have good
AMINI et al.: SOLUTION-PROCESSED PHOTOCONDUCTIVE UV DETECTORS
Fig. 4. I –V characteristics of the fabricated UV detectors (samples a and b) under different irradiances. (IA ) UV-A, (IC ) UV-C, and Id is dark current.
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nano-materials [13], [14]. Comparing the fabricated detectors shows that the photo-responsivity of device b is faster than a which can be attributed to lower defect density in the structure of nanosheets in the sample b. Increasing the bias voltages results in no considerable changes in the response time of the detectors. However, low power illuminations cause to relatively slow responses in the case of the fabricated detectors. IV. C ONCLUSION
Fig. 5. Response time for fabricated detectors. (a) Device a irradiated by power of 30 μW/cm2 . (b) Device a irradiated by power of 16 μW/cm2 . (c) Device b irradiated by power of 30 μW/cm2 . (d) Device b irradiated by power of 16 μW/cm2 (bias voltage: 10 V).
crystal quality (fewer structural defects such as oxygen vacancies and interstitials of zinc), an enhancement of UV emission with a suppressed green emission in the visible region would be observed [12]. In our case, the synthesized nanosheets show a strong near band edge emission and a suppressed and weak green emission for sample a and no green emission for sample b. These results indicate that the ZnO nanosheets have very few structural defects. Also, absence of green emission in these nano sheets can only indicate presence of an undetectable concentration of structural defects. The experimentally obtained responsivity for device a at 220 nm is 3.424 A/W. Also, the obtain responsivity for device b is 0.237 A/W (at 10 V bias) which agrees well with the theoretical results. The detectivity values for devices a and b 12 12 at √ 220 nm are 2.581 × 10 and 1.244 × 10 jones (cm Hz/W ), respectively. Fig. 4. Shows the dark current and photocurrent characteristics as a function of bias voltage for synthesized ZnO photodetectors. Both of these characteristics appear nearly linear with respect to current and voltage, indicating that fine ohmic contacts have formed for bias voltage up to 10 V for both of the fabricated detectors. The dark current values are about 20 and 10 nA, however, the photocurrent values are about 56.58 and 4.11 μA under a bias voltage of 10 V for detectors a and b respectively, which are obviously higher than the dark currents. Reduction of structural defects (oxygen vacancies and zinc interstitials) in the ZnO crystals reduce the nonradiative recombinations which leads to an decrease in carrier lifetime (τ ) and consequently leads to decreasing of gain (G = τ /ttr , ttr is transit time of carrier through the device). This is according to our results and for the sample b that the density of the defects are low, the gain value is lower than sample a. Fig. 5 shows the rise and fall times of the fabricated detectors upon switched UV light from on level to off level. As shown in Fig. 5, both of the rise and decay times are relatively fast (rise times: 260 ms, 225 ms and decay time: 1656, 366 ms for devices a and b respectively at irradiation of 30 μW/cm2 ) in comparison with other UV detectors based on ZnO
Synthesis of low defect ZnO nanosheets with different sizes using ultrasonic method is reported in this letter. The synthesized nanosheets were used for fabrication of UV photodetectors for the first time. The effect of the sizes of nanoparticles on photo-responsivity of the fabricated detectors was investigated. The experimental characteristics (such as responsivity, detectivity, absorption, photo luminescence and I –V curves) of both fabricated detectors were discussed. The obtained devices show good responsivity and fast response time compared to the previously reported detectors based on ZnO nano materials. Response time improved considerably, although gain decreases to somewhat which is mainly due to the trade-off between gain and temporal response. R EFERENCES [1] G. Konstantatos, et al., “Ultrasensitive solution-cast quantum dot photodetectors,” Nature, vol. 442, pp. 180–183, Jul. 2006. [2] G. Konstantatos, C. Huang, L. Levina, Z. Lu, and E. H. Sargent, “Efficient infrared electroluminescent devices using solution-processed colloidal quantum dots,” Adv. Funct. Mater., vol. 15, no. 11, pp. 1865– 1869, Nov. 2005. [3] E. J. D. Klem, D. D. MacNeil, P. W. Cyr, L. Levina, and E. H. Sargent, “Efficient solution-processed infrared photovoltaic cells: Planarized allinorganic bulk heterojunction devices via inter-quantum-dot bridging during growth from solution,” Appl. Phys. Lett., vol. 90, no. 18, pp. 183113–183115, May 2007. [4] J. H. He, Y. H. Lin, M. E. McConney, V. V. Tsukruk, Z. L. Wang, and G. Bao, “Enhancing UV photoconductivity of ZnO nanobelt by polyacrylonitrile functionalization,” J. Appl. Phys., vol. 102, no. 8, pp. 084303-1–084303-4, 2007. [5] M. Palumbo, S. J. Henley, T. Lutz, V. Stolojan, and S. R. P. Silva, “A fast sonochemical approach for the synthesis of solution processable ZnO rods,” J. Appl. Phys., vol. 104, no. 7, pp. 0749061–0749066, 2008. [6] H. Y. Zheng and M. Z. An, “Electrodeposition of Zn-Ni-Al2 O3 nanocomposite coatings under ultrasound conditions,” J. Alloys Compounds, vol. 459, nos. 1–2, pp. 548–552, 2008. [7] K. D. Sattler, Handbook of Nanophysics. Boca Raton, FL: CRC Press, 2011, pp. 1–6. [8] Y. C. Kong, D. P. Yu, B. Zhang, W. Fang, and S. Q. Feng, “Ultravioletemitting ZnO nanowires synthesized by a physical vapor deposition approach,” Appl. Phys. Lett., vol. 78, no. 4, pp. 407–409, 2001. [9] D. M. Bagnall, Y. F. Chen, M. Y. Shen, Z. Zhu, T. Goto, and T. Yao, “Room temperature excitonic stimulated emission from zinc oxide epilayers grown by plasma-assisted MBE,” J. Cryst. Growth, vols. 184– 185, pp. 605–609, Feb. 1998. [10] K. Vanheusden, C. H. Seager, W. L. Warren, D. R. Tallant, and J. A. Voigt, “Correlation between photoluminescence and oxygen vacancies in ZnO phosphors,” Appl. Phys. Lett., vol. 68, no. 3, pp. 403–405, 1996. [11] Y. Dai, Y. Zhang, Y. Q. Bai, and Z. L. Wang, “Bicrystalline zinc oxide nanowires,” Chem. Phys. Lett., vol. 375, no. 1, pp. 96–101, 2003. [12] D. M. Bagnall, et al., “High temperature excitonic stimulated emission from ZnO epitaxial layers,” Appl. Phys. Lett., vol. 73, no. 8, pp. 1038– 1040, 1998. [13] Y. Li, F. D. Valle, M. Simonnet, I. Yamada, and J. J. Delaunay, “Highperformance UV detector made of ultralong ZnO bridging nanowires,” Nanotechnology, vol. 20, no. 4, pp. 045501–045506, 2009. [14] J. Cheng, Y. Zhang, and R. Guo, “ZnO microtube ultraviolet detectors,” J. Cryst. Growth, vol. 310, no. 1, pp. 57–60, 2008.
