Oxygen-Controlled Photoconductivity in ZnO Nanowires

Article pubs.acs.org/JPCC

Oxygen-Controlled Photoconductivity in ZnO Nanowires Functionalized with Colloidal CdSe Quantum Dots Dongchao Hou,† Apurba Dev,†,‡ Kristian Frank,† Andreas Rosenauer,† and Tobias Voss*,†,§ †

Institute of Solid State Physics, University of Bremen, D-28359 Bremen, Germany School of Information and Communication Technology, Royal Institute of Technology (KTH), Electrum 229, S-16440, Sweden § Institute of Microsystems Engineering, University of Freiburg, Freiburg, Germany ‡

ABSTRACT: ZnO nanowire arrays were functionalized with colloidal CdSe quantum dots stabilized by 3-mercaptopropionic acid to form hybrid devices. The photoconductivity of the nanowire/quantum-dot devices was studied under selective photoexcitation of the quantum dots, and it was found that the dynamics strongly depend on the gas environment. Desorption of surface oxygen from both the ZnO nanowires and the CdSe quantum dots, activated by electron tunnelling between the nanowires and the quantum dots, is found to be the dominating process that determines the dynamics of the photoconductivity in the hybrid nanowire/quantum-dot devices.

1. INTRODUCTION Modern nanotechnology has great interest in the assembly and study of hybrid structures composed of different materials that offer enhanced properties or achieve new functions through the interactions between different constituents.1−3 Benefitting from the size effect, the small volume, and therefore the large surfaceto-volume ratio, nanostructured devices can obtain improved performance compared with their conventional bulk counterparts.1,4,5 One instructive example is the nanostructured dyesensitized solar cell (DSSC), which has attracted tremendous attention since its first report in 1991 by O’Regan and Grätzel.6 The TiO2 nanoparticle film greatly increases the effective optical thickness for the light-absorbing sensitizers, suggesting a potential innovation in solar energy conversion devices. The approach of DSSC technologies toward commercial application relies on an improved understanding of their operating principles and a continuous introduction and testing of new materials. In the search for next-generation sensitizers, inorganic colloidal semiconductor nanocrystals, commonly known as quantum dots (QDs), have been suggested and tested with expected advantages over organic dyes, such as tunable energy gaps by controlling their particle sizes during synthesis, high absorption coefficients, and the potential to generate multiple electron−hole pairs by the absorption of a single photon.7−9 ZnO nanowires (NWs) have been widely investigated as photoelectrodes in nanostructured photovoltaic devices.10−12 Compared with the previously developed mesoporous TiO2 substrates, NW arrays can provide direct electrical pathways for the charge carriers and a more open morphology that, in some cases, can allow sensitizers to more easily reach the NW surface.11,13 Many research works have been performed about NW/QD hybrid photovoltaic devices. However, some practical problems, such as the low coverage of the NWs with QDs as well as the limited understanding of the charge-transfer mechanisms between the constituents, still © 2012 American Chemical Society

hinder the breakthrough in the energy conversion efficiency.2,10,11,14,15 In the present work, hybrid structures consisting of ZnO NW arrays grown on FTO (fluorine-doped tin oxide)-covered glass substrates and colloidal CdSe QDs stabilized with 3mercaptopropionic acid (MPA) were synthesized. The QDs were anchored onto the NW surface by the chemical bonds formed between the outgoing carboxyl groups of the QD stabilizers and the zinc ions on the surface of NWs, which can facilitate the electron transfer between these two constituents.11 Using an Ar laser with a photon energy smaller than the band gap of ZnO but larger than the fundamental transition of the QDs, the photoconductivity of the NW/QD devices was studied in multiple gas environments, through which the dynamics of the electron transfer between the NWs and the QDs as well as the desorption and readsorption of the surface oxygen were investigated.

2. EXPERIMENTAL DETAILS The MPA-capped CdSe QDs were synthesized in aqueous phase according to a method adapted from a literature.16 First, a solution of 0.01 M sodium selenosulfate (Na2SeSO3) was prepared by refluxing 20 mg selenium and 124 mg sodium sulfite (Na2SO3) in 25 mL of water at 90 °C under nitrogen for 4 h. The obtained solution was stored under nitrogen for later use. A solution of 0.064 g (0.24 mmol) of cadmium acetate (Cd(CH3COO)2•2H2O) and 50.2 μL of MPA (0.58 mmol) in 48 mL of water was placed in a three-necked flask, and the pH value was adjusted to 10.5 by a 1 M NaOH solution. The mixture was bubbled with nitrogen for 1 h with stirring to replace the dissolved oxygen inside. Under vigorous stirring, 2.4 Received: July 21, 2012 Revised: August 3, 2012 Published: August 15, 2012 19604

dx.doi.org/10.1021/jp307235u | J. Phys. Chem. C 2012, 116, 19604−19610

The Journal of Physical Chemistry C

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source. The laser intensity was ∼10 mW cm−2. QD powder for the PL measurements was prepared by drying the QD solution naturally in air and in darkness and transferring the QDs afterward onto a silicon wafer. The samples were mounted in the same vacuum chamber used for the photoconductivity measurements. The PL spectra were recorded using an AvaSpec-2048TEC fiber spectrometer. The measurements were performed in air, vacuum, and nitrogen flow, respectively. The same conditions of vacuum and nitrogen flow were applied as in the photoconductivity measurements.

mL of the prepared Na2SeSO3 solution was swiftly injected into the mixture. The molar ratio of Cd2+:MPA:Se2− is 1:2.4:0.1. This reaction system was heated to 100 °C and refluxed for several days until QDs with the desired size were obtained. The formation of QDs was monitored by taking aliquots frequently and measuring their absorption using a Jasco V-670 UV−vis spectrophotometer. After synthesis, 2-propanol was added to the prepared solution to make the QDs precipitate, followed by centrifuging at 3500 rpm for 10 min. The supernatant was decanted, and the QDs deposited at the bottom were washed with ethanol twice to remove the residual chemicals, especially MPA. Afterward, the purified QDs were redispersed in water at pH 9.0 adjusted by tetraethylammonium hydroxide and stored in a dark environment. ZnO NW arrays were grown on FTO-coated glass by a hydrothermal method. Zinc acetate dihydrate (1.3 g) was dissolved in 10 mL of ethanol. Diethanolamine was added under stirring until the solution became transparent. The obtained solution was then spin-coated onto the FTO glass and annealed in air at 500 °C for 1 h. This coating-annealing process was repeated twice to obtain a uniform layer of ZnO seeds on the glass sheets for the NW growth. Potassium hydroxide (7.2 g) and 4.5 g zinc nitrate hexahydrate were dissolved in 30 mL of water, respectively, and then mixed together. The FTO substrates with ZnO seeds were put inside the mixture in a beaker, and this reaction system was heated to 80 °C and kept at this temperature for 3 h. After this synthesis, NW arrays were obtained on the FTO glass surface. To assemble the ZnO-NW/CdSe-QD hybrid structures, we first annealed ZnO NWs in air at 500 °C for 1 h to remove the surface hydroxyl and hydrocarbon groups, which are commonly adsorbed on metal oxide surface.17,18 The annealed samples were cooled in air for 5 min and then immersed in a CdSe QD solution at pH 9.0 and kept inside for 1 day to ensure a saturated attachment of the QDs. Afterward, the samples were rinsed thoroughly with Millipore water (18 MΩ) to remove the loosely attached QDs and then put in air to dry. The obtained hybrid samples are referred to as NW/QD assembly. To measure the photoconductivity of the NW/QD assemblies, we mounted the samples in a small chamber and covered them with a piece of FTO glass sheet to form a top contact with the tips of the ZnO NWs. The size of the contact area was ∼0.4 cm2. The chamber is equipped with a quartz window and can be pumped to vacuum and filled with gas. An argon laser emitting at 458 nm (2.71 eV) was used to irradiate the sample vertically through the top FTO glass. The laser has a photon energy below the ZnO band gap (3.37 eV at room temperature19) and is therefore only exciting the CdSe QDs. A Keithley 2400 SourceMeter was used to apply a constant voltage (1.0 V) between the top electrode and the substrate of ZnO NWs and to measure the electric current through the NWs. The laser spot was centered on the contact area of the sample with a diameter of 1.5 cm and an intensity of 1.0 mW cm−2. The photoconductivity of the NW/QD assemblies were investigated in air, vacuum (10−6 mbar), oxygen as well as constant flow of nitrogen and argon (40 mL min−1), respectively. All measurements were performed at room temperature. Several NW/QD hybrid samples were investigated, and all measurements exhibited similar behaviors. The results shown here were all obtained from the same sample. The photoluminescence (PL) of the CdSe QDs was measured with a HeCd laser emitting at 325 nm as excitation

3. RESULTS AND DISCUSSION Figure 1 shows the absorption and PL spectra of the asprepared QDs. The narrow emission band (fwhm 38 nm) and

Figure 1. Absorption and photoluminescence spectra of the synthesized MPA-capped CdSe QDs after 160 h refluxing. (Photoluminescence was measured using a HeCd laser exciting at 325 nm.)

the absence of defect-related emission in the PL spectra indicate their good crystalline quality and narrow size distribution. The first absorption peak at ∼505 nm corresponds to an average QD diameter of 2.5 nm.20 This is in good agreement with theoretical calculations found in the literature that are based on the finite-depth square-well model and the recently developed potential-morphing method.21,22 Measurements after QD purification did not show obvious change in the spectra. A TEM image of the QDs is shown in Figure 2.

Figure 2. TEM bright-field image of the prepared CdSe quantum dots without postpreparative size separation. The inset is a high-resolution TEM (HRTEM) image of an isolated QD. 19605



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Figure 4 shows the absorption spectrum of a typical ZnO NW sample grown by the hydrothermal method directly

Visibly, the QDs are monodisperse in size with some dark spots attributed to the aggregation and overlap of the QDs during the preparation of the TEM sample. The inset is a high-resolution TEM image of an isolated CdSe QD, indicating a single-crystal structure of the QD core. Figure 3a is an SEM image of the prepared ZnO NWs with typical lengths in the range of 500 nm to 1 μm and diameters of

Figure 4. Absorption spectra of hydrothermally grown ZnO nanowires, MPA-capped CdSe QDs, and ZnO-NW/CdSe-QD hybrid structures. The spectra were rescaled and shifted vertically for easy comparison.

compared with that of the final ZnO-NW/CdSe-QD hybrid structure. The absorption onset of ZnO appears around 400 nm due to the interband absorption.19 An absorption spectrum of the QDs is also plotted for comparison. A strong QD-related signal is visible in the spectrum of the NW/QD hybrid sample, illustrating that their optical properties were well-preserved during the coating process. The photoconductivity of the NW/QD hybrid sample was measured by monitoring the photoresponse of the electric current flowing through the ZnO NWs under laser irradiation. Figure 5 compares the results obtained with the NW/QD

Figure 3. (a) SEM image of ZnO nanowires before QD coating. (b) SEM of a single ZnO nanowire after QD coating. (c) TEM image of ZnO nanowires decorated with CdSe quantum dots. The inset is a magnification of a QD cluster on the nanowire surface. (d) HRTEM image of the CdSe QD cluster. A QD was marked with a circle.

50 to 300 nm. The density is estimated to be ∼109 cm−2. To fabricate the NW/QD assembly, the ZnO NWs were immersed in an aqueous QD dispersion at pH 9.0 for 1 day. When ZnO is placed in an aqueous environment, the surface will be charged due to the interaction with the H+ and OH− groups in the surroundings.23 The net surface charge can be positive or negative depending on the abundance of the H+ in the solution. The pH value of the solution for which the oxide surface carries no net charge is the so-called isoelectric point of the oxide, which is ∼9.5 for ZnO.23,24 Therefore, in the QD suspension with pH 9.0 we used, the ZnO NW surface is positively charged, which electrostatically facilitates the access of the negatively charged carboxyl groups of the QD ligands to form tight chemical bonds.25,26 Additionally, the pH value of the QD dispersion is also below the pKa of the thiol group in MPA (∼ 10.2).27 This can cause a partial removal of the capping MPA molecules from the QD surface by a gradual protonation process of the thiol group.28,29 These partially capped QDs can undergo a kind of agglomeration on the ZnO NW surface and thus increase the attachment efficiency, which is confirmed by the SEM image shown in Figure 3b. A clear change in the surface appearance can be seen after the decoration with QDs. The QDs on the NW surface form dense cluster-like aggregates that can be recognized as dark floccules in the TEM image shown in Figure 3c. The inset is a magnified image of a QD cluster with a typical dimension around 30 nm. A highresolution TEM image is shown in Figure 3d in which the QDs can be recognized by their lattice fringes.

Figure 5. Photoconductivity of the NW/QD assembly and a bare ZnO NW reference sample in air under Ar laser irradiation at 458 nm (1.0 mW cm−2) as a function of time. The sample area for the measurement was ∼0.4 cm2. The bias voltage was uB = 1.0 V. Point I: laser on, II: laser off.

assembly and an uncoated ZnO NW sample. The measurements were conducted in air with a constant bias voltage uB = 1.0 V. Upon irradiation, the current of the assembly rises rapidly from 28 to 200 μA in the first 6 min. Then, it gradually approaches a steady value around 260 μA. When the laser is blocked after 2 h of illumination, the current reduces sharply (∼ 25 μA min−1 in the first 5 min), followed by a slow decay, and it takes more than 6 h for the current to return to the original 19606



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value obtained in air. After more than 10 h of illumination, the current gets to ∼310 μA without hints for approaching a steady-state value. When the laser is blocked (point II), the current shows a small decay (from 310 μA to 285 μA in 2 h), followed by a sharp decrease when air is filled into the chamber (point III) and gradually gets back to the original value (not shown in the Figure). Attempting to reach the steady state of the photoconductivity in vacuum, another measurement was performed for a longer time (Figure 7b). After 96 h of Ar laser illumination, the electric current gets to ∼1.1 mA (almost 40fold increase with the initial value of 28 μA). However, the steady state is still not reached despite the further slowing down of the current increase during laser illumination. This behavior is quite different from previous reports that used UV light for irradiation, in which the photoconductivity enhanced much faster in vacuum than in air.31,32 Keeping in mind the important role of the CdSe QDs in the photoconductivity enhancement, this unexpected photoconductivity behavior points to the vacuum conditions substantially modifying the optoelectronic properties of the QDs. To verify the above assumption, we performed PL measurements of the NW/QD assembly in air, vacuum, and constant nitrogen flow, respectively. In Figure 8a, the temporal

dark value. In comparison, the uncoated ZnO NWs show a much weaker current increase (from 36 to 56 μA) under the same irradiation conditions, indicating that the CdSe QDs play a crucial role in the strong photoconductivity enhancement of the NW/QD assembly. Previous studies have shown that the gas environment has a significant impact on the photoconductivity behavior of ZnO NWs.30,31 To investigate the effect of the oxygen partial pressure on the photoconductivity of the NW/QD assembly, measurements were performed in pure oxygen with a pressure of 1.07 bar. The other conditions were identical to the measurements in air. The result is shown in Figure 6 compared

Figure 6. Photoconductivity of the NW/QD assembly in air and pure oxygen under Ar laser irradiation (1.0 mW cm−2) with bias voltage uB = 1.0 V. Point I: laser on, II: laser off.

with that recorded in air. Both signals have similar dynamics on the same time scale. However, the saturation current reached in pure oxygen is much smaller than that in air (100 μA compared with 260 μA reached in air). This suggests that the observed photoconductivity crucially depends on the gas environment and is related to surface effects in the ZnO NWs. A quite different photoresponse was observed in vacuum (10−6 mbar), shown in Figure 7a. Upon exposure to the laser, a rapid current increase (like that in air) is observed during the very first minutes. Then, the increase slows down abruptly to a much lower rate (from 27 μA min−1 to 0.4 μA min−1, see inset). It takes ∼8 h for the current to reach the steady-state

Figure 8. Excitonic PL intensity of the CdSe QDs measured with the NW/QD hybrid devices (a) and the pure CdSe QD powder (b) as a function of time after the onset of laser excitation, compared with the evolution of the photoconductive current of NW/QD hybrids measured in vacuum. The initial PL intensities at time t = 0 min and the maximum of the current are normalized to the same value while the initial value of the current is set to zero for easy comparison. A 325 nm HeCd laser was used for the PL measurements.

evolution of the excitonic emission intensity of the QDs is compared with the dynamics of the photoconductivity

Figure 7. (a) Photoconductivity as a function of time of the NW/QD assembly measured in air and vacuum under Ar laser irradiation with bias voltage uB = 1.0 V. The inset is a zoom-in of the initial period of the photoconductivity enhancement. (b) Photoconductivity as a function of time of the NW/QD assembly measured in vacuum for a longer irradiation time under the same conditions. Point I: laser on, II: laser off, III: air in. 19607



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measured in vacuum during the first minutes after starting the laser excitation. The results are normalized for easy comparison. The PL intensity in air shows little decay over time, probably due to a slight degradation of the QDs induced by the UV laser irradiation. In vacuum and nitrogen flow, the PL intensity decreases quickly and distinctly after starting the laser irradiation. During the first 3 min, it is reduced to 10% (vacuum) and 25% (nitrogen) of the initial values, respectively. The time scale of the strong reduction coincides with the abrupt slowing down of the photoconductivity enhancement of the hybrid assembly observed in vacuum, suggesting a clear correlation between these two effects. When air was filled into the chamber, a quick recovery of the PL was observed in both cases, and the intensity returned back to the initial values within 10 min. The experiments were repeated for CdSe QDs powder (Figure 8b), which shows a very similar behavior. These observations demonstrate that the gas environments have a strong influence on the fundamental properties of CdSe QDs under photoexcitation, which may account for the photoconductivity dynamics of the hybrid assembly in vacuum. Combining the results shown above, we now present a model to interpret the operating mechanism of the photoconductivity enhancement of the NW/QD assembly under Ar laser irradiation (Figure 9). It is well known that oxygen molecules

impact the electrical properties with a different mechanism.38 They were suggested to act as donor-like surface states rather than acceptor-like ones as in oxides and carbon materials, which is also consistent with the nature of substitutional oxygen impurities in GaN bulk material.40,41 Absorption of oxygen on GaN NW surfaces was proposed to release surface trapped electrons and hence reduce the surface band bending, which accelerates the electron−hole recombination in the NWs during photoexcitation process.38 In our NW/QD assembly, the CdSe QDs are chemically linked to the ZnO NW surface. Previous studies have shown that oxygen is also adsorbed on the QD surface when the QDs are handled in air, which can passivate surface defects that are commonly present due to the finite size of the crystal structure and the large curvature at the QD surface.42−44 In the photoconductivity measurement, when an electron in a CdSe QD is excited to a higher level, it can tunnel into the conduction band of the ZnO NW that lies at a lower energy level (Figure 9). The hole left inside the QD can recombine with an electron trapped by the oxygen ion species adsorbed on the NW surface. After this cycle is completed, the QD is back in its ground state, an electron previously bound by the surface oxygen has been transferred into the conduction band of the NW, and the oxygen molecule has been released from the NW surface. The transfer of the electron from a localized state at the NW surface into the conduction band additionally reduces the width of the depletion layer of the ZnO NW. As the above process continues, the concentration of the free electrons in the conduction band of the NWs further increases and the surface band bending is further reduced, thus resulting in the continuous rise of the photocurrent. (See Figures 5 and 6.) At the same time, oxygen can also be readsorbed onto the NW surface and capture electrons from the conduction band. Finally, a dynamic equilibrium will be reached and the conductivity of the assembly will maintain a steady level, corresponding to the steady currents reached in Figures 5 and 6. In pure oxygen, due to the larger partial pressure, oxygen is more probable to be readsorbed, inducing a lower extent of the surface oxygen desorption under equilibrium. Therefore, a smaller steady-state current is reached (Figure 6). When the laser is blocked, oxygen attaches to the NW surface and traps electrons again, resulting in the sharp decrease in the photocurrent. We note that a rather weak conductivity enhancement upon irradiation with the Ar laser is also observed in the uncoated ZnO NW sample (Figure 5). However, this reference measurement clearly demonstrates that the contribution of sub-bandgap absorption in ZnO NWs via defect states to the observed photoconductivity enhancement is negligible compared with the much stronger contribution related to the electron transfer from the CdSe QDs to the NWs. The photogenerated holes in the QDs can recombine not only with electrons trapped at the ZnO NW surface but also with the electrons trapped by oxygen species adsorbed on the QD surface. Through such a recombination process, oxygen is released from the QD surface, leaving behind unpassivated QD surface defects.42−44 In an oxygen-rich environment, these active defects can be immediately passivated again by adsorbing oxygen from the surrounding atmosphere. In vacuum, however, the oxygen desorbed from the QD surface will be pumped away and have no chance to be readsorbed. Those activated surface defects can provide additional nonradiative recombination routes for the photoexcited electrons to relax back to the

Figure 9. Diagram of the energy level alignment, the electron transfer, as well as the surface oxygen desorption from the ZnO nanowire surface in the photoconductivity measurement of the NW/QD assembly.

are commonly adsorbed on metal oxide surfaces and capture electrons from the conduction band to form negatively charged species (O2−, O2−2), resulting in upward band bending and electron depletion layers in the near-surface region, which decrease the conductivity.33−35 There have been a number of reports about the photoresponse of ZnO films and NWs under UV irradiation.30−32,36,37 The holes generated in the valence band can drift to the ZnO surface, neutralize the adsorbed oxygen species, and set the trapped electrons free, which results in an increased conductivity. The neutralized oxygen can desorb from the oxide surface afterward, and the width of the depletion layer is reduced. Surface oxygen adsorption was also reported to impact dramatically the electrical conductivity of other nanostructured materials, like GaN NWs and carbon nanotubes.38,39 In the latter work, simply changing the environmental conditions from vacuum to oxygen was found to decrease the electrical resistance of the single-walled carbon nanotubes by up to 15%, which was attributed to oxygen adsorption that captures electrons and results in a p-type conductivity in the nanotubes.39 Oxygen molecules on GaN surface, however, 19608



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ground levels (Figure 9). This will decrease the probability for the photoexcited electrons to tunnel into the ZnO NWs and of the NW surface oxygen to be released. As a result, the enhancement of the photoconductivity of the NW/QD assembly slows down, which causes the abrupt change in the current increase rate shown in Figure 7. The higher photoconductivity finally reached in Figure 7 can be expected because the oxygen, once desorbed, has no chance to be readsorbed in vacuum. Therefore, more bound electrons can be transferred from localized states at the NW surface into the conduction band. The PL measurements of the CdSe QDs shown in Figure 8a can also be explained by the above model. The activated surface defects of the QDs in vacuum act as the nonradiative recombination centers for the photoexcited electrons, which therefore induce the significant decrease in the excitonic luminescence intensity of the QDs.43,44 The results shown in Figure 8b demonstrate that nitrogen flow through the chamber has a similar effect as the vacuum conditions to carry away the oxygen released from the surfaces. According to the above analysis, a similar photoconductivity of the NW/QD assembly should be observed in nitrogen flow as in vacuum. Figure 10 compares the photoconductivity of the

The water molecules compete with oxygen molecules for the absorption sites on the ZnO surface (without further illumination of the samples). During photoexcitation, they trap electron−hole pairs, thus decreasing the maximum obtained photoconductivity. However, in the present work, the kinetics observed in oxygen-free and also water-free environments, namely, vacuum, nitrogen, and argon, differ significantly from that obtained in the (also water-free) pure oxygen atmosphere. These results clearly demonstrate that oxygen is dominating the photoconductivity behaviors in our work, and the effect of water vapor should be negligible. In addition, considering the larger work function of the FTO substrate (4.4 to 5.0 eV46) relative to the electron affinity of ZnO (∼4.2 eV47), Schottky-type contacts are actually formed between the FTO glass and the ZnO NWs. Previous works suggested that the larger free-electron density in ZnO under UV illumination can effectively reduce the Schottky barrier width, which therefore increases the conductivity of the device and may even induce a transition from Schottky-type to ohmic contact.35,48 However, this effect seems to be very limited in the present work. First, the increase in the free-electron density in the ZnO NWs is mostly resulting from oxygen desorption and electron transfer from the CdSe QDs. These processes are not as efficient as direct excitation of electron−hole pairs in the ZnO NWs with above-bandgap UV light. Additionally, the I−V characteristics of the hybrid samples under laser irradiation still follow a clear Schottky-type behavior. Therefore, we attribute the observed photoconductivity enhancement mostly to a larger conductivity of the ZnO NWs and not to changes at the NWFTO-contacts. Considering all observations and the detailed analysis given above, a safe conclusion can be reached now. Photogenerated electrons from the excited quantum dot states transfer to the conduction band of the NWs, thereby increasing the freecarrier concentration. Surface oxygen of the ZnO NWs is released by capturing the photogenerated holes from the CdSe QDs. The removal of the surface oxygen further reduces the width of the depletion layer at the NW surface. The combination of these processes eventually induces the strong photoconductivity enhancement of the NW/QD assembly in multiple gas environments. Our results clearly demonstrate that the removal of the surface oxygen by far dominates the maximum enhancement and the dynamics of the photoconductivity. They furthermore show the huge potential of MPA-capped CdSe QDs as photosensitizers in nanostructured photodetectors or photovoltaic devices and the importance of a carefully controlled surface termination of the used nanostructures for optimized performance.

Figure 10. Photoconductivity dynamics of the NW/QD assembly measured in 4 different gas environments under the same excitation conditions.

NW/QD sample measured in multiple environments. For the measurement in nitrogen, highly pure nitrogen gas (99.999%, O2 < 2.0 ppm) was flowing through the sample chamber with a constant speed of ∼40 mL min−1. From the results, the current also undergoes a slowing down in the increase, like in vacuum. However the reduced increase rate is still quite large with a value of 0.8 μA min−1 compared with 0.4 μA min−1 in vacuum. This can be explained as follows: first, in the photoconductivity measurements the sample was covered with another piece of FTO glass, which was unfavorable for the nitrogen to reach the sample surface and drive the desorbed oxygen away. As a result, they were probably readsorbed by the QDs. Besides, the nitrogen gas we used still contains very small amounts of oxygen. Using argon gas with higher purities (99.9999%, O2 < 0.1 ppm, flow rate of ∼40 mL min−1), we got a more approximate photoconductivity behavior to that in vacuum. (See Figure 10.) In previous works, water vapor in air is suggested to influence significantly the photoconductivity of ZnO-based devices.45

4. CONCLUSIONS In summary, we synthesized hybrid nanostructures consisting of ZnO NW arrays functionalized with CdSe quantum dots stabilized by MPA. Via preannealing treatment of the NWs, purification of the prepared QDs, and an intended selection of the pH environment of the QDs dispersion, a dense attachment of the QDs on ZnO NWs in cluster form was obtained, confirmed by electron microscopy characterization and optical measurements. The photoconductivity of the NW/QD hybrid structures was studied using an Ar laser to selectively excite the QDs in different gas environments. Oxygen desorption from the ZnO-NW and the CdSe-quantum dot surfaces, activated by electron tunnelling processes, was demonstrated to dominate 19609



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(30) Keem, K.; Kim, H.; Kim, G.-T.; Lee, J. S.; Min, B.; Cho, K.; Sung, M.-Y.; Kim, S. Appl. Phys. Lett. 2004, 84, 4376. (31) Bao, J.; Shalish, I.; Su, Z.; Gurwitz, R.; Capasso, F.; Wang, X.; Ren, Z. Nanoscale Res. Lett. 2011, 6, 404. (32) Zhang, D.; Brodie, D. E. Thin Solid Films 1995, 261, 334−339. (33) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, U.K., 1996. (34) Zhang, Y.; Kolmakov, A.; Chretien, S.; Metiu, H.; Moskovits, M. Nano Lett. 2004, 4, 403−407. (35) Heo, Y. W.; Tien, L. C.; Norton, D. P.; Kang, B. S.; Ren, F.; Gila, B. P.; Pearton, S. J. Appl. Phys. Lett. 2004, 85, 2002. (36) Collins, R.; Thomas, D. Phys. Rev. 1958, 112, 388−395. (37) Shapira, Y.; Cox, S. Surf. Sci. 1976, 54, 43−59. (38) Chen, R. S.; Lu, C. Y.; Chen, K. H.; Chen, L. C. Appl. Phys. Lett. 2009, 95, 233119. (39) Collins, P.; Bradley, K.; Ishigami, M.; Zettl, A. Science 2000, 287, 1801−1804. (40) Chung, B.-C.; Gershenzon, M. J. Appl. Phys. 1992, 72, 651. (41) Mattila, T.; Nieminen, R. Phys. Rev. B 1996, 54, 16676−16682. (42) Chestnoy, N.; Harris, T. D.; Hull, R.; Brus, L. E. J. Phys. Chem. 1986, 90, 3393−3399. (43) Müller, J.; Lupton, J. M.; Rogach, a. L.; Feldmann, J.; Talapin, D. V.; Weller, H. Appl. Phys. Lett. 2004, 85, 381. (44) Shu, G.-W.; Lee, W.-Z.; Shu, I.-J.; Shen, J.-L.; Lin, J. C.-a.; Chang, W. H.; Ruaan, R.-C.; Chou, W. C. IEEE Trans. Nanotechnol. 2005, 4, 632−636. (45) Li, Y.; Della Valle, F.; Simonnet, M.; Yamada, I.; Delaunay, J.-J. Appl. Phys. Lett. 2009, 94, 023110. (46) Helander, M. G.; Greiner, M. T.; Wang, Z. B.; Tang, W. M.; Lu, Z. H. J. Vac. Sci. Technol., A 2011, 29, 011019. (47) Xu, Y.; Schoonen, M. A. A. Am. Mineral. 2000, 85, 543−556. (48) Zhou, J.; Gu, Y.; Hu, Y.; Mai, W.; Yeh, P.-H.; Bao, G.; Sood, A. K.; Polla, D. L.; Wang, Z. L. Appl. Phys. Lett. 2009, 94, 191103.

the strong photoconductivity enhancement in the hybrid structures.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank D. Wöhrle and M. Bäumer (University of Bremen) for assistance and fruitful discussions in the preparation of the QDs and J. Gutowski (University of Bremen) for valuable advice and discussion of the experimental results. D.H. is grateful to the China Scholarship Council for a scholarship support.

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REFERENCES

(1) Yan, R.; Gargas, D.; Yang, P. Nat. Photonics 2009, 3, 569−576. (2) Rühle, S.; Shalom, M.; Zaban, A. ChemPhysChem 2010, 11, 2290−2304. (3) Li, J.; Zhang, J. Z. Coord. Chem. Rev. 2009, 253, 3015−3041. (4) Liu, S.; Tang, Z. J. Mater. Chem. 2010, 20, 24−35. (5) Grätzel, M. Nature 2001, 414, 338−344. (6) O’Regan, B.; Grätzel, M. Nature 1991, 353, 737−740. (7) Alivisatos, A. P. Science 1996, 271, 933−937. (8) Ellingson, R. J.; Beard, M. C.; Johnson, J. C.; Yu, P.; Micic, O. I.; Nozik, A. J.; Shabaev, A.; Efros, A. L. Nano Lett. 2005, 5, 865−871. (9) Schaller, R.; Klimov, V. Phys. Rev. Lett. 2004, 92, 186601. (10) Seol, M.; Kim, H.; Tak, Y.; Yong, K. Chem. Commun. 2010, 46, 5521−5523. (11) Leschkies, K. S.; Divakar, R.; Basu, J.; Enache-Pommer, E.; Boercker, J. E.; Carter, C. B.; Kortshagen, U. R.; Norris, D. J.; Aydil, E. S. Nano Lett. 2007, 7, 1793−1798. (12) Briscoe, J.; Gallardo, D. E.; Hatch, S.; Lesnyak, V.; Gaponik, N.; Dunn, S. J. Mater. Chem. 2011, 21, 2517−2523. (13) Tang, Y. B.; Chen, Z. H.; Song, H. S.; Lee, C. S.; Cong, H. T.; Cheng, H. M.; Zhang, W. J.; Bello, I.; Lee, S. T. Nano Lett. 2008, 8, 4191−4195. (14) Kuo, K.-T.; Liu, D.-M.; Chen, S.-Y.; Lin, C.-C. J. Mater. Chem. 2009, 19, 6780−6788. (15) Leschkies, K. S.; Jacobs, A. G.; Norris, D. J.; Aydil, E. S. Appl. Phys. Lett. 2009, 95, 193103. (16) Chen, X.; Hutchison, J. L.; Dobson, P. J.; Wakefield, G. J. Mater. Sci. 2008, 44, 285−292. (17) Nagao, M. J. Phys. Chem. 1971, 75, 3822−3828. (18) Parfitt, G. D. Pure Appl. Chem. 1976, 48, 415−418. (19) Ö zgür, U.; Alivov, Y. I.; Liu, C.; Teke, A.; Reshchikov, M. A.; Doǧan, S.; Avrutin, V.; Cho, S.-J.; Morkoç, H. J. Appl. Phys. 2005, 98, 41301. (20) Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854−2860. (21) Pellegrini, G.; Mattei, G.; Mazzoldi, P. J. Appl. Phys. 2005, 97, 073706. (22) Baskoutas, S.; Terzis, A. F. J. Appl. Phys. 2006, 99, 013708. (23) Degen, A.; Kosec, M. J. Eur. Ceram. Soc. 2000, 20, 667−673. (24) Andeen, D.; Kim, J. H.; Lange, F. F.; Goh, G. K. L.; Tripathy, S. Adv. Funct. Mater. 2006, 16, 799−804. (25) Sakai, N.; Tatsuma, T. Adv. Mater. 2010, 22, 3185−3188. (26) Chen, J.; Zhao, D. W.; Song, J. L.; Sun, X. W.; Deng, W. Q.; Liu, X. W.; Lei, W. Electrochem. Commun. 2009, 11, 2265−2267. (27) Sambur, J. B.; Riha, S. C.; Choi, D.; Parkinson, B. A. Langmuir 2010, 26, 4839−4847. (28) Eychmüller, A.; Rogach, A. L. Pure Appl. Chem. 2000, 72, 179− 188. (29) Aldana, J.; Wang, Y. a; Peng, X. J. Am. Chem. Soc. 2001, 123, 8844−8850. 19610


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