Enhanced photocurrent gain by CdTe quantum dot modified ZnO nanowire

Sensors and Actuators A 232 (2015) 292–298

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Enhanced photocurrent gain by CdTe quantum dot modified ZnO nanowire Lei Li a,b , Shuming Yang a,b,∗ , Weixuan Jing a,b , Zhuangde Jiang a,b , Feng Han a,b a b

Collaborative Innovation Center of High-End Manufacturing Equipment, Xi’an Jiaotong University, Xi’an 710049, PR China State Key Laboratory for Manufacturing Systems Engineering, Xi’an Jiaotong University, Xi’an 710049, PR China

a r t i c l e

i n f o

Article history: Received 2 November 2014 Received in revised form 17 June 2015 Accepted 17 June 2015 Available online 22 June 2015 Keywords: ZnO nanowire CdTe quantum dots Photodetector Charge transfer

a b s t r a c t In this paper, a zinc oxide (ZnO) nanowire (NW) photodetector was fabricated using dielectrophoresis technique. The ZnO NW was synthesized by chemical vapor deposition (CVD) method, and characterized by SEM, XRD and photoluminescence (PL) spectrum. The photodetector showed obvious photoresponse to 365 nm UV light. By decorating the ZnO NWs with CdTe quantum dots (QDs), the photocurrent (PC) gain was enhanced from 199 to 2896, when the light intensity was 5.3 mW/cm2 . The response spectrum was also extended to the visible light region. The underlying mechanism of the enhancement was assigned to the high-efficiency charge transfer caused by the type-II band structure between ZnO NWs and CdTe QDs. The greatly quenched PL intensity of CdTe QDs/ZnO NWs composites provided further evidence for the high efficiency charge transfer. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Zinc oxide (ZnO) nanowire (NW) has been considered as ideal building blocks for the nano-scale ultraviolet (UV) optoelectronic devices due to its wide direct band gap, large excitation banding energy and surface-to-volume ratio etc. [1]. ZnO NWs reported so far have been used for lasers [1], solar cells [2,3] and photodetectors etc. [4–8]. Since Kind et al. [6] reported the single ZnO NW UV photodetector in 2002, there have been many reports about NW photodetectors [7,8]. Due to the large surface-to-volume ratio, the surface of the NW influences the photodetector performance significantly [9,10], and many methods based on the NW surface modification have been introduced to improve the performance of the ZnO NW photodetector. For example, some metal nanoparticles, such as Au, Ag and Ti modified ZnO NWs have been used to enhance the photocurrent because of the surface plasmon resonance or nano heterojunction effect [11–13]. Organic polymer was also introduced to passivate the NW surface states and enhance the photocurrent [14,15]. Compared with the aforementioned methods, quantum dot (QD) modified NWs is a promising method for the QD’s unique opti-

∗ Corresponding author. Fax: +86 29 8266 8616. E-mail address: [email protected] (S. Yang). http://dx.doi.org/10.1016/j.sna.2015.06.020 0924-4247/© 2015 Elsevier B.V. All rights reserved.

cal properties, such as tunable emission and absorption band, good photostability, and high light absorption coefficient etc. [16]. However, most of the QDs/NWs composites were used in photovoltaic cells devices to enhance the light-converting efficiency, there were rare reports using QDs/NW composites to fabricate the photodetectors. Recently, CdSe and I–III–VI (CuInS2 , CuInSe2 ) QDs have been proved to enhance the photocurrent gain of SnO2 NW, and an enhancement factor of up to 700% was realized [16,17]. For ZnO NW based photodetector, only Aga et al. [28] reported the cadmium telluride (CdTe) decorated ZnO nanowire for enhancing the photocurrent gain. But the photocurrent gain was still very low, even enhanced after CdTe QDs decoration. In this paper, a ZnO NW photodetector with large photocurrent gain was fabricated using alternating current (AC) dielectrophoresis (DEP) technique, which was much easier than electron beam lithography (EBL) and focused ion beam (FIB) techniques [4,5]. CdTe QDs were used to increase the photocurrent gain of the photodetector. The CdTe QDs were deposited on the ZnO NWs surface by a simple drop casting method [17]. After deposition of CdTe QDs, the photocurrent (PC) gain was enhanced more than 10 times. The large enhancement was attributed to the high-efficiency charge transfer between QDs and NWs caused by the type-II band structure. The photoluminescence results also proved the high efficiency charge transfer at the interface of NWs and QDs.

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2. Experiments 2.1. Synthesis of CdTe QDs The Mercaptosuccinic acid (MSA, 99.0%) capped CdTe QDs were synthesized via a modified procedure [18]. Typically, CdCl2 (0.04 mol/L, 4 mL) was diluted to 50 mL in a three-necked flask, and tri-sodium citrate dihydrate (400 mg). Na2 TeO3 (0.01 mol/L, 4 mL), MSA (100 mg) and NaBH4 (50 mg) were added successively, under vigorous stirring. When the solution became transparent and the color changed to green, the flask was attached to a condenser and refluxed at 100 ◦ C for 9 h in N2 atmosphere, and then the CdTe QDs were synthesized. 2.2. ZnO NW growth The ZnO NW growth was conducted using a horizontal tube furnace by chemical vapor deposition (CVD) process [19]. Before NW growth, 8 nm Au thin film was deposited on the Si substrate by magnetron sputtering, served as the NW growth catalyst. The mixture of ZnO and C powder (1:1 weight ratio) was used as the source material for NW growth, and placed at the center of the heating zone. The Au coated Si substrate was placed downstream about 3 cm away from the source material. 200 sccm Argon/Oxygen (Ar:O2 = 9:1, volume ratio) was introduced into the tube furnace as carrier gas, and the pressure was maintained at 2000 Pa. The furnace was heated from room temperature to 950 ◦ C at a rate of 40 ◦ C/min, and held at 950 ◦ C for 40 min. Then the furnace was cooled down to room temperature naturally, and the ZnO NW was obtained. Before the dielectrophoresis (DEP) experiments, the ZnO NW was immersed into 5 mL ethanol solution and ultrasonicated for 15 min to detach and disperse the ZnO NWs into the ethanol and form ZnO NW-ethanol suspension. 2.3. Preparation of photodetector

Fig. 1. The absorption and photoluminescence spectra of the as synthesized CdTe QDs.

tion spectrum of the CdTe QDs solution was measured by UV–vis spectrophotometer (UV3600, Shimadzu, Japan). The QDs and the ZnO NW/CdTe QDs composite were characterized by TEM (JEM2100F, JEOL, Japan) at 200 kV. For photoresponse measurement, the sample was illuminated by a 365 nm UV light. The photocurrent was recorded by a Source Meter system (Keithley 2611A). 3. Results and discussion 3.1. CdTe QDs The QDs showed obviously green fluorescence light under the UV lamp. The photoluminescence (PL) and UV/Vis absorption spectra of CdTe QDs solution were shown in Fig. 1. Compared with the bulk materials, the absorption and the emission peaks of the CdTe QDs were blue shifted to 500 nm and 540 nm, respectively, because of the quantum size effect [20]. The QDs exhibited very strong PL intensity, indicating that the QDs had high quantum yield [21].

The metal-ZnO NW-metal (MSM) photodetector was fabricated by DEP method [4]. The DEP electrode was prepared on the Si substrate coated with 300 nm SiO2 using UV photolithography and lift-off techniques. The Pd/Ti (200 nm/50 nm) electrode was deposited using DC magnetron sputtering technique. The electrode interval was 10 ␮m and the length was 110 ␮m. Before DEP experiment, a drop of the NW solution was dripped on the electrode interval. Then an AC electric field (f = 20 MHz, Vpp = 20 V) was applied on the electrodes. The DEP experiment was executed for 3 min. After DEP assembling, the sample was rinsed several times to remove the NWs that did not steadily contact with the electrode. Finally, the sample was dried on a hot plate at 100 ◦ C for 5 min to remove organic residue. For CdTe QDs decoration experiment, the CdTe QDs were firstly diluted to 20% of its initial concentration. And then the diluted CdTe QDs was deposited on the NW surface by drop casting method [16]. Finally, the sample was baked at 100 ◦ C for 5 min to evaporate the residual solution in order to achieve high coupling strength between the CdTe QDs and ZnO NWs.

Fig. 2 showed the typical SEM images of the as grown ZnO NWs. The diameter of the ZnO NWs ranged from 50 nm to 200 nm, and the length was several tens of micrometers. Fig. 3a was the XRD pattern of ZnO NWs, all of the diffraction peaks can be indexed to the typical hexagonal wurtzite structure of ZnO (JCPDS No. 36-1451), the calculated lattice constants were a = 0.326 nm and c = 0.521 nm which were consistent with the standard values. The sharp diffraction peaks indicated that the ZnO NWs were highly crystallized. Two weak diffraction peaks of cubic phase Au (1 1 1) and (2 0 0) were also detected (JCPDS No. 65-8601), which come from the Au catalyst thin film. The PL spectrum of ZnO NWs was shown in Fig. 2b, the NWs presented strong near band edge (NBE) emission centered at 383 nm. A wide visible light spectrum was also investigated, which was because of the oxygen vacancies defects [22].

2.4. Apparatus

3.3. Device fabrication and CdTe QDs modification

The ZnO NW and the device were investigated by FE-SEM (SU 8010, Hitachi, Japan). The NW crystal structure was characterized by X-ray diffractometer (XRD, XPert Pro, PANalytical, Netherlands). The photoluminescence (PL) spectra of ZnO NW, CdTe QDs and the NW/QD composite were measured by fluorescence spectrometer (PTI-40, PTI, USA) with the excitation light of 325 nm. The absorp-

The results of DEP experiment are shown in Fig. 4a–c. Since most of the NWs were longer than the electrode interval, the NW can directly span across the electrodes. The following current–voltage measurement indicated that the ZnO NWs contacted well with the Pd electrodes. The TEM image of CdTe QDs decorated ZnO NWs was shown in Fig. 4d, the black dots was CdTe QDs, the diameter was

3.2. ZnO NWs

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Fig. 2. The low (a) and high (b) magnification SEM images of the as grown ZnO NWs.

about 2–3 nm. The CdTe QDs attached randomly on the ZnO NW surface without forming extra conductive path.

incident photons in one second during the photoresponse process [24]:

3.4. The photoresponse experimental results

 =

Fig. 5a was the current–voltage (I–V) curves of the device before and after decoration with CdTe QDs. The ZnO NWs lied on the electrodes can be regarded as the parallel resistances. The device showed typical rectifying characteristics because of the formation of back-to-back Schottky diodes between ZnO NWs and Pd electrodes, since the work function of Pd (5.12 eV) was higher than ZnO NWs (4.65 eV) [23]. After decoration with CdTe QDs, the Schottky barrier was reduced obviously, the I–V characteristic became approximately ohmic, the current was also increased from about 60 ␮A to more than 800 ␮A at 5 V. Fig. 5b shows the dark and light I–V curves after decoration with CdTe QDs, the dark current was only 2 ␮A, and increased to more than 800 ␮A when light was on. Fig. 6a shows the time-dependent photoresponse curves with and without CdTe QDs under the light intensity of 28.7 mW/cm2 at the bias voltage of 5 V. The photocurrent was enhanced about 12 times after the decoration with CdTe QDs. Fig. 6b shows the photocurrent as a function of light intensity. The photocurrent increased much quicker after the decoration of CdTe QDs with the light intensity increase. For a photodetector, the PC gain () is an important parameter which is defined as the number of electrons induced by unit

Nelectron i/q 1 = × Nphoton  P/h

(1)

where Nelectron is the number of electrons; Nphoton is the number of incident photons; i ent; q is the electron charge; h is the energy of the incident photon; P is the power of incident light, that can be obtained by P = I × S, where I is the light intensity, S is the area of the illuminated region;  is the quantum efficiency which is set to be 1 for simplicity. Under the illumination of 365 nm light, the calculated PC gain  as a function of light intensity is shown in Fig. 6c. When the light intensity was 5.3 mW/cm2 , the PC gain was 199, and increased up to 2896 after the decoration of CdTe QDs. 3.5. The photoresponse mechanism analysis The photoresponse mechanism of the ZnO NWs related to the absorption and desorption of O2 on the NWs surface was discussed as follows. When ZnO NWs was placed in the dark, the absorbed O2 on the NWs surface could combine with the electrons in NWs, as shown in Eq. (2) [7,25], and cause the electrons accumulation on the surface. Consequently, the high-resistance surface depletion layer referred to space charge region (SCR) was formed because of the existence of surface band bending. The surface depletion

Fig. 3. (a) XRD pattern and (b) photoluminescence spectrum of as-grown ZnO NWs.

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Fig. 4. (a–c) the SEM images of the DEP manipulation experiment results, (d) the TEM image of the CdTe QDs/ZnO NWs composite.

layer reduced the conductive channel width and made the NW high resistance. O2 + e− → O− 2

(2)

Once the NWs were illuminated by UV light, the photo generated holes drifted to the surface and combined with the absorbed O− 2 and desorbed the O2 , as shown in Eq. (3) [7,25], + O− 2 + h → O2

(3)

This process reduced the thickness of the surface depletion layer, and enhanced the nanowire conductance. Meanwhile, the photo-generated electrons in NWs increased the electron carriers concentration, and contributed to the photoresponse.

According to Garrido et al.’s theory [26], The relationship of SCR related PC gain  and the light intensity I followed an inverse power law  ∝ I−k , and the exponent k was between 0.5 and 0.9. Fig. 6c shows the logarithmic plot of the PC gain  versus light intensity I. The fitted k was 0.63 and decreased to 0.51 after decoration with CdTe QDs, which were both in the range of the theoretical predicted value. The smaller k indicated that the PC gain decayed slower with the increment of light intensity. The results were important to realize a high PC gain under high light intensity. The enhancement of the photocurrent and PC gain after CdTe QDs decoration was most probably because of the type-II band structure of the ZnO NW/CdTe QDs, which induced an built-in electric field pointed from ZnO NW to CdTe QDs at the interface, and resulted in the spatially charge transfer between the QDs and NWs

Fig. 5. (a) The I–V curves of the device with and without CdTe QDs under 365 nm UV light; (b) the dark and light I–V curves of the device after decoration of CdTe QDs.

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Fig. 6. (a) The light response curves of the device with and without CdTe QDs; (b) the relationship between photocurrent and light intensity; (c) the relationship between PC gain and light intensity.

[3,27,28]. The schematic diagram of the band structure of ZnO/CdTe is illustrated in Fig. 7. When the light was on, the photogenerated holes in ZnO were driven into the conduction band of CdTe by the interface electric field, and the electrons stayed in the conduction band of ZnO. This process reduced the recombination rate of electron–hole pairs in ZnO, increased the lifetime of electron carriers, and thus, enhanced the photocurrent. On the CdTe QDs side, the photo generated holes stayed in the CdTe valence band,

Fig. 7. The schematic diagram of the type-II band structure between CdTe QDs and ZnO NWs, SBB: Surface band bending, SCR: Space charge region.

while the electrons moved to the conduction band of ZnO NWs. This process increased the electrons concentration of ZnO NWs, and enhanced the photocurrent. In summary, the charge transfer at the interface increased the density and lifetime of electron carriers in ZnO NWs, and thus the PC gain was enhanced. Another interesting phenomenon was the reduced exponent k after the decoration of CdTe QDs. Due to the photo generated electrons in CdTe QDs can transfer to the conduction band of ZnO NWs, the NW surface band bending was flattened, and the charge transfer across the interface became easier. Therefore, the PC gain decayed slower with light intensity increase, and k was smaller. We also measured the photocurrent of CdTe QDs decorated ZnO NWs using a laser with the wavelength of 425 nm and the current about 50 ␮A was obtained. Because the energy of 425 nm laser was smaller than the band gap energy of ZnO, the bare ZnO NW was not sensitive to the light, the photocurrent under 425 nm light can only be originated from CdTe QDs and transferred to the ZnO NWs. This result also provided further evidence for the charge transfer between CdTe QDs and ZnO NWs. In order to further evaluate the efficiency of the charge transfer between QDs and NWs, we measured the PL spectra of the ZnO NWs/CdTe QDs composite. The results are shown in Fig. 8. The PL intensity of both ZnO NWs and CdTe QDs in the ZnO NWs/CdTe QDs composite were significantly quenched compared with the bare

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References

Fig. 8. The PL spectra of ZnO NWs, CdTe QDs and the ZnO NWs/CdTe QDs composites.

samples. The NBE emission of ZnO NWs was restrained to about 1/9 of bare ZnO NWs, while the emission of CdTe QDs was nearly completely quenched. According to the charge transfer theory [29,30], the PL process of bare ZnO NWs and CdTe QDs was shown in Eq. (4): ZnO NWs (or CdTe QDs) absorbed the incident light and generated the electron–hole (e–h) pairs. Then the e–h pairs recombined and emitted light. The PL process of the ZnO NWs/CdTe QDs composite was shown in Eq. (5). Because of the electric field at the interface, the photogenerated e–h pairs were separated. The separation of e–h pairs reduced the radiative recombination rate, and restrained the NBE emission. The greatly quenched NBE emission of ZnO and CdTe proved the high-efficiency charge transfer at the ZnO/CdTe interface, which was critical for the large enhancement of the PC gain. CdTe + h → CdTe(h + e) → CdTe + h ZnO + h␯ → ZnO(h + e) → (Radiative recombination and NBSE emmission)

ZnO + h

(4)

CdTe(h + e) + ZnO → CdTe(h) + ZnO(e) ZnO(h + e) + CdTe → ZnO(e) + CdTe(h (Charge transfer and PL quenching)

)

(5)

4. Conclusion A ZnO NWs photodetector was fabricated by DEP technique. After decoration the ZnO NW with CdTe QDs, the PC gain was enhanced from 199 to 2896 under the light intensity of 5.3 mW/cm2 . The large enhancement of PC gain was attributed to the high efficiency photogenerated charge transfer between CdTe QDs and ZnO NWs, raised from the type-II band structure, which increased the concentration and lifetime of electrons in ZnO NWs, and thus enhanced the PC gain. The obvious PL quenching of ZnO and CdTe in ZnO/CdTe composites also provided further evidence for the high efficiency charge transfer between CdTe and ZnO. This research could provide a method for the fabrication of ZnO NW photodetectors with high PC gain. Acknowledgement The authors would like to thank the financial supports by National Natural Science Foundation of China (No. 51175418), National Natural Science Foundation of China Major Research Program on Nanomanufacturing (No. 91323303), and the Fundamental Research Funds for the Central Universities.

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Weixuan Jing: received the Ph. D degree from Xi’an Jiaotong University in 2003. And now, he is a professor in Xi’an Jiaotong University. His fields of interest include the MEMS and Micro&Nano manufacturing, sensor technology and the Micro&Nano detection technology.

Biographies

Lei Li: received the BS degree from Huazhong University of Science and Technology in 2009. He is pursuing the Ph. D degree in Xi’an Jiaotong University. His fields of interest include the synthesis of 1D ZnO nanowire, the optical and optoelectronic properties of the ZnO nanowire and the optoelectronic devices based on the ZnO nanowires.

Shuming Yang: received the Ph. D degree from University of Huddersfield in 2009. And now, he is a professor in Xi’an Jiaotong University. His fields of interest include the nano devices and integrated optics system, nano fabrication and measurement, optical detection and ultra precision manufacturing.

Zhuangde Jiang: received the BS and MS degrees in Xi’an Jiaotong University in 1977, 1988, respectively. In 2011, he received the Ph. D degree in the University of Birmingham. He is now the Academician of the Chinese Academy Engineering. His research interest covers MEMS and Nano manufacturing technology, precise instrument and sensor technology, precise optical electric measurement technology, precise and super-precise manufacturing technology.

Feng Han: received the BS degree from Xi’an Jiaotong University in 2010. She is pursuing the Ph. D degree in Xi’an Jiaotong University. Her fields of interest include the synthesis of grapheme film and its application in surface plasmon effect and related optoelectronic devices.