Catalyst-Free Growth of ZnO Nanowires Based on Topographical

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Catalyst-Free Growth of ZnO Nanowires Based on Topographical Confinement and Preferential Chemisorption and Their Use for Room Temperature CO Detection Seul Ki Youn,† Niranjan Ramgir,*,† Chunyu Wang,‡ Kittitat Subannajui,† Volker Cimalla,‡ and Margit Zacharias† Nanotechnology, Institute of Microsystems Engineering (IMTEK, TF), Albert-Ludwigs-UniVersity Freiburg, 79110 Freiburg, Germany, and Fraunhofer-Institut fu¨r Angewandte Festko¨rperphysik (IAF), 79108 Freiburg, Germany ReceiVed: January 17, 2010; ReVised Manuscript ReceiVed: March 30, 2010

Two novel approaches for patterned growth of ZnO nanowires (NWs) based on the selective deposition of zinc acetate (ZA) solution precursors are presented and compared. The first is using the topographical confinement within a photoresist pattern on Si/SiO2 substrates (type I), and the second is using preferential chemisorption on self-assembled monolayer modified Au electrodes on Si/SiO2 substrates (type II). In both approaches, the ZnO seeds from the ZA solution form crystallites without severe defects over a large area via annealing at 350 °C. These seed layers were used to grow ZnO NWs via a catalyst-free vapor-phase deposition method using temperatures of up to 900 °C. The presented method is effective for realizing NW growth on both Si and Au electrodes by preserving the electrode configuration even at such high temperatures used for NW growth, which is a novelty and crucial for future sensor applications. As a result NWs connected through metal contact pads or electrodes have been realized in very simple and effective way. As a test of principle the resulting configurations were used to demonstrate a highly sensitive room temperature CO sensor. A CO concentration as low as 120 ppb was detected using both types of sensors. The type II sensor exhibited enhanced sensing properties compared to that of type I. 1. Introduction Because of their inherently large surface-to-volume ratio and excellent optoelectronic properties, ZnO nanowires (NWs) offer the promise of enhancement in device performance, with particularly high sensitivity and selectivity for sensor applications.1,2 To realize their potential in respective applications, cheap and simple techniques are necessary that enable site-selective growth, allow vertical alignment, and have the potential for large-scale fabrication. Templates based on spatially arranged gold nanodot arrays are normally used for a controlled growth of pattern-arranged NW arrays.3 However, such metal catalysts pose serious concerns since they can potentially degrade device performance and increase the complexity of the fabrication process.4 Therefore, catalyst-free growth offering the advantage of vertical alignment with potential for large-scale fabrication has generated interest and accordingly has been widely attempted. For catalystfree growth, many groups have utilized different seed layers,5,6 namely, a submonolayer of ZnO nanoparticles7 and ultrathin film of textured ZnO nuclei.8,9 Both approaches are two-step methods consisting of seeding and subsequent NW growth. In the latter case, thermal decomposition of zinc acetate (ZA) is carried out to create ZnO nanocrystals directly on an arbitrary substrate as nucleating seeds. ZnO nuclei can be used further to grow NWs employing either the low-temperature solution approaches or high-temperature vapor-phase (VP) deposition. Accordingly, patterned growth of ZnO NWs has been demonstrated using poly(dimethylsiloxane) (PDMS) microfluidic chan* Corresponding author. E-mail: [email protected]/zacharias@ imtek.de. † Albert-Ludwigs-University Freiburg. ‡ Fraunhofer-Institut fu¨r Angewandte Festko¨rperphysik (IAF).

nels,10 e-beam lithography,11 inkjet printing,12 and conventional photolithography.13 Of these, the low-temperature solution-based methods undesirably result in strong defective emission and possess difficulty for intentional doping.14,15 In addition, VP deposition, because of the high temperatures involved (up to 930 °C), causes degradation of the contact electrodes which is a major problem. Also, a uniform distribution of the ZnO seeds over the target area and an accurate position control have not been realized so far. To take full advantage of these NW-based materials, integration into Si-based microelectronic devices is mandatory. Up to now the “pick and place” approach is the most widely used method for nanomaterial integration. However, it is not considered to be suitable for large-scale manufacturing because of the inherent drawbacks such as random placement, contaminations, and a general incompatibility with Si processing. Another approach is the deposition of the electrode directly on top of the NWs by e-beam evaporation. However, because of the multiple and slow processes involved, this method is also not practical on a large scale. Hence, a more elegant approach would be to selectively grow nanostructures directly onto desired areas of the substrate such as, for instance, directly on electrodes.16 This is particularly significant for sensor applications where a good ohmic contact between the metal electrode and the semiconductor NWs would ensure better performance. In this work, we report the catalyst-free and area-selective vapor-solid (VS) growth of ZnO NW arrays on Si substrates using topographical confinement (type I) and directly on Au electrode using preferential chemisorption on self-assembled monolayers (SAMs) (type II). Self-integration of ZnO NW bridges on Si substrates and Au electrodes has been achieved without severe defects over large area. Obviously, the preformed ZnO seed layer with SAMs in the type II approach acts as a

10.1021/jp100446r  2010 American Chemical Society Published on Web 05/14/2010

Catalyst-Free Growth of ZnO Nanowires

Figure 1. Schematic illustration of patterned growth of NWs based on controlled deposition of ZA solution using (a) type 1, spatial confinement on photoresist-structured substrate, and (b) type 2, selective chemisorptions on SAM-modified Au electrodes.

passivating layer against the deformation of the underlying contact electrode. A first investigation of both respective sensor configurations has been performed and will be discussed. 2. Experimental Section 2.1. Materials. 2-Mercaptopropionic acid (MPA, CH3CHSHCOOH) (99.8 %), methanol (99.9 %), absolute ethanol, and ZA dihydrate (98 %) were purchased from Sigma Aldrich and used without further purification. For the NW growth, ZnO (Sigma Aldrich, 99.9 %) and graphite of ∼200 mesh size (Alfa Aesar, 99.9 %) were used. 2.2. Preparation of Patterned Substrates for ZA Solution Deposition. Patterned growth of NWs on Si substrates and on Au electrodes deposited on Si substrates were obtained by using (a) a topographically patterned template (type I) and (b) a chemically patterned template with SAM of MPA (type II). The overall procedure consists of three main steps: patterning with ZA precursors, oxidation of the ZA precursor to form ZnO crystallites, and NW growth via VP deposition. Figure 1 shows the complete schematic representation of the steps involved in the selective ZnO seed patterning. In brief, for type I patterning, a positive tone photoresist (AZ 5214E, Microchemicals) was uniformly spin-coated on a Si substrate and topographically patterned by conventional photolithography. For type II patterning, Au electrodes were patterned by conventional photolithography using physical vapor deposition (Au: 100 nm, Cr: 5 nm) and lift-off processes. A surface modification of Au was carried out by immersing the substrates into a 1 mM ethanol solution of MPA for 5 h and subsequent washing with ethanol to remove excess MPA.17 2.3. ZnO Seed Layer Formation on the Patterned Substrates. A series of experiments were conducted to optimize the parameters of the solution deposition process by varying the concentration of ZA solution, solvent, and the amount of droplets. In our optimized condition, 5 mM ZA in methanol has been dropped onto type I or type II substrates four times, followed by drying in air at 70 °C and a final annealing in air at 350 °C for 20 min. The annealing results in the thermal

J. Phys. Chem. C, Vol. 114, No. 22, 2010 10093 decomposition of ZA, forms the ZnO seeds, and at the same time removes the photoresist. 2.4. Vapor-Solid (VS) Growth of ZnO NW Arrays. Afterward the ZnO NW arrays were grown by VP deposition using the VS growth mechanism. In particular, a source boat loaded with a mixture of powdered ZnO and graphite (∼ 1:1 wt %) is used as source material. The boat is placed at the center of a horizontal 1 in. single zone tubular furnace. The patterned substrates containing the ZnO seed layer (Section 2.3) were placed at an upstream position from the source.18 The tube was then evacuated and purged with a continuous flow of 7 sccm Ar with 60 ppm O2 until the desired pressure of 200 mbar was reached. The furnace was ramped to 930 °C (center temperature), held constant for 20 min, and then allowed to cool down naturally to room temperature (RT). The substrates are placed in an area of reduced temperature (∼ 875 °C). ZnO NWs grown under the same condition will have the same length and the same configuration showing overall a good repeatability of the process. 2.5. Characterization and Sensor Fabrication. The contact angles were determined optically with an OCA20 system from Dataphysics using distilled water. For the dynamic measurements, liquid was pumped into/sucked from the drop with a syringe pump. Each angle was measured multiple times, resulting in an average value with a standard deviation in the range of 2-3°. High resolution scanning electron microscopy (HRSEM) images and energy-dispersive X-ray spectrometry (EDX) data were collected using Nova NanoSEM (FEI) with an EDAX system. RT photoluminescence (PL) measurements were performed using a spectrophotometer with a liquid nitrogen cooled charge-coupled device (CCD) camera and a 325 nm HeCd laser for excitation. X-ray photoelectron spectroscopy (XPS) analysis was performed using XPS (PHI 5600-CI) on a sample with dense ZnO NWs at each step of UV pretreatment and CO exposure and compared to the as-grown NWs. Sensor measurements were carried out at RT in a dynamic setup using a two-probe method. First, the Au contacts were deposited onto the patterned substrate as shown in Figure 2 and then wire-bonded to the measuring electrodes in a ceramic integrated circuit (IC) package.19 An ultraviolet light emitting diode (UV LED) having a photon energy of ∼3.5 eV was mounted above the ZnO NWs. The ceramic IC package containing the sensor material is placed inside the system. The concentration of CO gas inside the system was adjusted using mass flow controllers (MFCs); the data were collected using the computer interface and Labview software. All measurements were performed at RT, and the recovery was achieved by shining the UV light with N2 purge. The response (S) was calculated as

S ) RCO /RLED-on

(1)

where RCO is the resistance maximum after switching off the LED in the presence of CO gas, while RLED-on is the resistance minimum after switching on the LED and purging the system with N2 gas. For the measurements, the sample was first illuminated with an UV LED for 10 min, and the measuring chamber was purged with N2. The resistance of both types of ZnO NWs decreases after UV illumination. After that, the UV LED was switched off, and the N2 purge was stopped followed by an introduction of CO gas with varied concentrations (N2 as carrier gas) into the measuring chamber for a further 20 min by simultaneous monitoring the NW resistance.19

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Figure 2. Schematic representation of type I and type II ZnO NW sensors (a, b) and the nanobridges formed between the adjacent lines (c, d). The corresponding SEM images of the NWs taken at a 45° tilt are shown in (e, f). The inset (e) shows an enlarged image of the ZnO nanobridges.

Figure 3. SEM images of VS-grown ZnO NWs with seed layers preannealed at temperatures ranging from 200 to 500 °C. The images are taken at a 30° tilt.

3. Results and Discussion 3.1. Effect of Annealing Temperature on ZnO NWs Alignment. The alignment of the ZnO NWs grown on Si wafers using ZA derived seeds is strongly dependent on the annealing temperatures. As shown in Figure 3, a temperature below 350 °C yields thinner and poorly aligned NWs, in agreement with

previous studies.16 Higher temperatures above 350 °C result in larger NWs with lower density. Using the same growth conditions, the difference in the NW geometry is attributed to originate from the differences in the size and orientation of the ZnO seed crystallites which mainly depend upon the annealing temperature.8


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Figure 4. Schematic illustration of selective surface modification and subsequent adsorption of Zn species on patterned Au electrode on Si (100) substrates.

3.2. Selective Deposition of ZnO Seeds Using Topographical Photoresist Pattern and SAM-Modified Au Electrodes. Figure 1a outlines our novel strategy for the selective deposition of ZA precursors to fabricate micropatterns of ZnO NW arrays using spatial confinement imposed by the side walls of the photoresist patterned by conventional photolithography. When the ZA solution spreads on the prepatterned substrate, the contact line of the droplet remains fixed for a few minutes and retracts slowly as the droplet shrinks because of the uniform evaporation throughout the liquid-gas interface. Along with the solvent, ZA precursors are selectively accumulated into the line-shaped wells confined by the photoresist side walls while the contact area of the droplet decreases. When the receding contact line meets the hollow patterns, it becomes pinned along the periphery of the hollow patterns and pulls back from the hydrophobic photoresist walls. In other words, the physical confinement and hydrophobicity of the photoresist walls induce the ZA solution to flow out of the nonwetting photoresist film and into the spatially confined hollow structure. During the evaporation, the concentration of the ZA solution steadily increases, which in turn increases the chance of the contact line pinning. To achieve defect-free patterns in large area, the contact line pinning of the droplet should be minimized. For this reason, a series of experiments were carried out to find out the optimum concentration of ZA solution, showing no considerable contact line pinning. In the case of concentrations higher than 5 mM, some defects with undesirable NW growth and a ZnO seed layer were observed on the gaps between the patterned NW arrays. In our optimized procedure, the patterned substrate with ZA solution was dried in air and annealed at 350 °C for 20 min to form ZnO islands that serve as nucleation seeds for the NW growth. On the contrary, for the growth of NWs on Au patterned electrodes, it is crucial to understand the role of Au, which is known to act as a preferred site for vapor adsorption via the VS mechanism. We note here that control experiments using Au-patterned substrates without a ZA solution deposition showed significant Au pattern collapse after annealing and a nonuniform NW growth. During annealing and VS growth steps at high temperature, the Au film was reconstructed and dewetted from Si substrate.20 It was reported that dewetted holes and the deformed pattern were randomly observed over a large area, conceivably hindering subsequent NW growth. In our initial experiments, the ZA solution was dropped directly on the Aupatterned substrate without an additional surface modification, expecting that intrinsically polar Au films capture the droplet flow of the solution so that ZA crystallites are selectively located on Au films. However, the NWs were grown with differences in density and alignment of the NWs on Au and Si. It indicates that the ZA crystallites were deposited by the wettability of the ionic solution on the polar Au substrate as well as the spatial confinement within Au walls. SAMs are an elegant and simple way to obtain a chemically bound well-organized structure on Au without a vacuum process. The process offers control over the chemical composition and

structure of the interface between the Au films and the ZA derived seed layers. Thus, to minimize the undesirable deposition of the ZA precursors on Si and to generate a uniform seed layers on Au, a simple surface modification method with SAM was investigated. Figure 1b represents schematically the alternative method for the selective deposition of ZA precursors on MPA-SAM modified Au electrodes. In our approach, SAMs of MPA, which possess two anchor/functional groups, namely, thiol and carboxylic acid, were used to discriminate between Au and Si surface as well as to interact with cationic Zn species in the solution phase. The monolayer was achieved by immersing the patterned substrate in MPA solution, which is instigated by a strong Au-thiol interaction (Figure 4a).21 The wetting behavior of the SAM was probed by means of contact angle measurements using pure water. The water contact angles of the Au films before and after SAM formation were found to be 89.7 ( 2.7° and 39.0 ( 2.9°, respectively, which proves that after the immersion step the hydrophilicity of the Au surface was significantly increased due to the terminal -COOH groups of MPA-SAMs. This change in wettability means that, when a ZA solution of the same concentration is dropped, it forms a thinner liquid layer and thus the contact line can be easily pinned before it flows into the wells. In the following droplet-deposition process of ZA solution, solution-phase Zn species are selectively chemisorbed by interacting with the terminal -COOH groups on SAM-modified substrate (Figure 4b). Since the metal coordination reaction at the surface is a self-limiting process, we expect that the ZA crystallites are uniformly distributed over the Au surface with high surface coverage. In other words, surface modification of patterned Au films enhances not only the adsorption of the Zn species, but also the surface selectivity of the Au surface over the surrounding Si wells. SEM and EDX measurements as shown in Figure 5 were carried out to confirm the selective formation of ZnO seeds after annealing for type I and type II substrates. EDX profiles confirm that the ZnO seeds are preferentially present only in the gap between the adjacent photoresist walls for type I substrate and on the patterned Au surface for type II substrates. This implies that the photoresist-assisted protection successfully blocked the unwanted deposition of ZnO seeds and that the use of SAM assured selective seed formation on Au. Besides, the pattern edges of type I substrates are sharper than that of type II, which can be understood by morphological changes of the underlying Au film at high processing temperatures. However, these minor imperfections did not cause any noticeable errors in the following NW growth process. 3.3. ZnO NW Growth on Patterned Substrates. Having shown the selective patterning of ZA-derived seeds, we next generated various ZnO NW arrays through the VS method. Figure 6 shows the SEM images of VS-ZnO NWs grown on patterned ZnO seed layers with various sizes of the photoresist pattern gaps. It is clearly evident that the ZnO NW arrays grow on the patterned seed layer and not on the surrounding bare Si substrate. The resulting gap distance of the patterned NW arrays

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Figure 5. SEM image of patterned ZnO seeds on type I substrate (a) and the corresponding EDX profile (b); SEM image of patterned ZnO seeds on type II substrates (c) and the corresponding EDX profile (d).

precisely corresponds to the original pattern size of the photoresist walls, indicating that the region of growth was strictly determined by the topographic features on the substrate. Under the same conditions as the type I patterning, the deposited ZA crystallites on patterned Au were transformed into nucleation seeds and used for the NW growth. Figure 7 shows the corresponding SEM images of the ZnO NWs arrays grown only on the patterned Au. Noticeably, no significant deintegration of the line-shaped Au pattern was observed in all experiments. It indicates that the ZnO seed layer formed in advance efficiently minimizes or compensates surface diffusion or splitting up (into small dots) of the underlying Au electrode. The pattern size and gap distance of the resulting NW arrays reflect the original Au pattern. The precise pattern transfer and its preservation even during the high temperature ZnO NW growth is the main advantage of our selective NW growth on SAM-modified Au pattern. 3.4. PL Measurement of ZnO NWs at RT. As shown in Figure 8, ZnO NWs exhibited a similar intensity for both the green and the near band edge (NBE) emission. Please note that here the reported spectra represent the signal averaged over a macroscopic area and hence the emission of a huge number of NWs contributes to the signal. One could compare the spectra with the result from polycrystalline films prepared at high temperatures. Cho et al.21 succeeded in growing high-quality polycrystalline films by oxidizing sputtered Zn films at temperatures ranging from 300 to 1000 °C, resulting in an increase of the grain size from 18 to 61 nm. A full width at halfmaximum (FWHM) of 107 meV was reported for band gap

emission, peaking at around 3.26 eV for an oxidizing temperature of 700 °C, which was reduced to 23 meV for the sample annealed at 1000 °C. They even reported lasing actions for their films which is a hint to the good crystal quality. In our case we observe a strong and sharp UV emission peak at 3.29 eV (379 nm; Figure 8) with a FWHM of 102 meV, similar to the results of Cho et al.21 The broad green emission around 508 nm (2.45 eV) is normally attributed to deep-level or trap-state emission caused by defects of the crystal or at the crystal surface.22 The involvement of oxygen (O) vacancies, interstitial O, Zn vacancies, Zn interstitials, and extrinsic impurities has been discussed.23 3.5. Gas Sensing Properties of Patterned ZnO NWs. Electrodes are deposited at the ends of the pattern which measure the effective resistance arising from the NW bridges.24 The initial resistance of the type I sensor is high in comparison with that of type II. The difference between the effective resistances (type I compared to type II) could be a result of the different ZnO seed layer formation process. The uniform loading, the underneath Au layer, and the NW bridges contribute equally to the effective resistance. In the type I sensor the ZA species distribution is dependent on the rate of evaporation and surface tension of the solution and hence does not promise a uniform and dense packing of the seed layer. On the other hand, in case of the type II sensor, the chemical anchoring of ZA species onto the SAM modified Au ensures a uniform and dense packing of the seed layer. The presence of the gold underneath the ZnO seeds will for sure influence the conductance and transport properties of the system. In case of type I, the system is a kind



Figure 6. SEM images of ZnO NWs on type I substrates with different size of the photoresist wells (line width × gap distance): (a) 1.5 × 1.5 µm, (b) 3 × 2 µm, (c) 3 × 3 µm, (d) 3 × 5 µm. Images are taken at a 45° tilt.

Figure 7. SEM images of ZnO NWs on type II substrates with different sizes of the Au electrodes (electrode line width × gap distance): (a) 1.5 × 3 µm, (b) 3 × 1.5 µm, (c) 3 × 5 µm, (d) 3 × 6 µm. Images are taken at a 45° tilt.

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Figure 8. PL measurement of ZnO NWs using ZA-derived seeds.

Figure 9. I-V characteristic of both type I and type II sensors. The inset shows the characteristic of type I sensor which reminds on a MSM device characteristic.

of a metal-semiconductor-metal (MSM) system, where the semiconductor in the middle is represented by millions of ZnO nanobridges in series as well as parallel and the metal contacts are the two outer electrodes. In contrast, the type II device represents a sequence of MSM devices connected in series. Also in this case the inner semiconductor part consists of millions of ZnO nanobridges in parallel. For both cases, a nonlinear behavior was observed, as shown in Figure 9. As evident from the I-V measurements, the nonlinearity for the type II sensor

Youn et al. is negligible, making the sample nearly ohmic. It could indicate that the preformed ZnO seeds layer on Au electrode efficiently acts as a passivating or protecting layer against a severe deformation of the underlying Au electrode at high deposition temperatures. Consequently, the presence of Au underneath the ZnO nuclei seems to help, realizing a good contact between the adjacent NWs and also the bridging NWs. In the case of the type I sensor, the I-V characteristic is reminiscent of the MSM behavior we observed for contacting single wires by a pair of nanomanipulators.25 We expect that thermionic emission over the barriers as well as tunneling assisted by interface states contributes to the observed I-V characteristic. Figure 10 shows the sensor response toward different concentrations of carbon monoxide (CO) at RT. CO is a colorless, odorless gas and is toxic even at small concentrations as low as 100 ppm. The type II sensor exhibited a better response with a high sensitivity (S) of about ∼900 compared to that of the type I sensor which showed S ) 50 toward 40 ppm of CO. The sensitivity increased with the gas concentration with the lowest detectable concentration for type I and type II sensors of ∼1.2 ppm and 120 ppb, respectively. Importantly, even at relatively low CO concentration, the type II sensor exhibited a superior response variation (S ∼ 9 for 120 ppb). The gas-sensing mechanism is commonly understood by the interaction of gas molecules with the adsorbed oxygen species. Oxygen is mainly adsorbed on the surface in ionic forms such as O2- (< 100 °C), O- (100-300 °C), and O2- (> 300 °C). Normally, CO gas is considered to have a reducing effect, and its absorption on the active surface of ZnO NWs causes a decrease in the resistance because of the electron exchange occurring between CO and ionosorbed oxygen species.26 At low operation temperatures (< 100 °C), CO molecules do not possess sufficient thermal energy to react with ionosorbed oxygen species. However, in the present case, we observed an increase in the resistance, and hence we believe that CO interaction with the ionosorbed oxygen species is not the governing reaction for the sensing. We anticipate that the CO is mainly reacting with oxygen vacancies or hydroxyl groups induced on the photostimulated surface of the ZnO NWs at RT, which is apparently an oxidizing behavior. It is worthy to note that the

Figure 10. Response curve of the type I sensor (a) and type II sensor (c) toward CO and the corresponding dependence of the sensor response on CO concentration (b, d).



ZnO NWs showed a much higher and faster resistance change upon exposure to CO. The response to all other interfering coexisting residual gases in the detecting system (such as H2O or O2, which is below 1 ppm) is marginal and can be excluded.27 In the present work, the oxidizing behavior of CO was additionally investigated using XPS studies. For this, a fresh sample with dense NWs was scanned at each step of UV pretreatment and CO exposure and subsequently compared to the as-grown NWs. In all of the cases, the symmetrical Zn 2p3/2 spectrum centered at 1022 eV attributed to Zn2+ species in ZnO is observed. The asymmetrical O 1s spectrum is derived from two oxygen species, namely, O2- ions in the wurtzite ZnO (O1, the lower energy peak, 531 eV) and O2- ions in oxygen-deficient regions including surface hydroxyl groups (O2, the higher energy peak, 532-533 eV).28 A relative increase in the O2 peak confirms that the oxygen vacancies and reactive hydroxyl groups on the ZnO NW surface increase upon UV pretreatment. More importantly, after CO exposure, a decrease in the O2 peak intensity was observed. This confirms that the CO interacts mainly with the oxygen defect sites, capturing the electrons on the surface and consequently showing a curve similar to the native oxidized sample. It is well-established that the irradiation on ZnO surfaces with UV photons with energies above the band gap (3.2 eV) results in surface defects which interact with atmospheric water molecules, generating hydroxyl groups on ZnO surface.29 In particular, two types of hydroxyl groups were produced from two kinds of defective sites associated with oxygen vacancy and bond cleavage. These hydroxyl groups further cause hydroxylation of CO molecules (C-atom down being weakly held by electrostatic interactions),30 generating formate species (HCO2-) via the following equation:31

CO(g) + Zn - OH(ads) f Zn+ - HCO2(ads)-

(2)

The formation of formate species occurs by the abstraction of electron from ZnO, thereby leading to an increase in the resistance. In the same manner, sensor recovery at re-exposure to UV illumination is understood by the photoinduced oxidation of formate species on ZnO surface, releasing CO2 and pumping free charge carriers into the conduction band of ZnO.32

HCO2(ads)- + h+(ZnO) f H+ + CO2 + e-

(3)

Additional experiments are underway for better understanding of the exact sensing mechanism and the remarkable high RT sensitivity toward CO. 4. Conclusions We demonstrated two new approaches for surface patterning of ZnO NWs based on selective deposition of ZA-derived seeds. The two approaches, namely, topographical confinement using a photoresist pattern and selective chemisorption on SAMmodified gold electrodes, show high potential toward fabricating NW-based sensor materials with controlled location, size, and distribution. The NWs were grown selectively in the regions defined by designed pattern and features. The realized ZnO seed layers stabilized by strong gold-thiol and carboxylate-zinc species interactions efficiently protect the underneath Au electrodes from disintegration or collapse during high-temperature NW growth. Similar to this method, a number of specific and selective substrate-adsorbate interactions can be envisioned

for the generation of nanostructures derived from a wide range of seed layer materials in the future. The SAM-modified Au electrode-based sensor type II showed an enhanced sensitivity in comparison with that of the photoresist-based sensor system type I, which can be attributed to a series connection of the multi MSM devices based on millions of nanobridging ZnO NWs and the underneath Au electrodes. Our results clearly demonstrate for the first time that the direct integration of NWs on top of the electrodes is possible, ensures better conductance properties, and is thereby advantageous for fabricating sensor configuration. Further, this work introduces a reliable and efficient CO detection sensor, particularly meeting the requirements of RT operation to solve the power consumption and safety issues and thus enhancing applicability toward portable devices. Acknowledgment. S.K.Y. acknowledges DIP-K.6.1 for financial support. N.R. thanks the Humboldt Foundation for the fellowship. The authors thank Daniel Hiller for PL measurements. The work was supported by Fraunhofer Grant (ATTRACT, CHALLENGE). References and Notes (1) Wang, H. T.; Kang, B. S.; Ren, F.; Tien, L. C.; Sadik, P. W.; Norton, D. P.; Pearton, S. J.; Lin, J. Appl. Phys. Lett. 2005, 86, 243503. (2) Hsueh, T.-J.; Chang, S.-J.; Hsu, C.-L.; Lin, Y.-R.; Chen, I.-C. Appl. Phys. Lett. 2007, 91, 053111. (3) Fan, H. J.; Werner, P.; Zacharias, M. Small 2006, 2, 700. (4) Conley, J. J. F.; Stecker, L.; Ono, Y. Appl. Phys. Lett. 2005, 87, 223114. (5) Greene, L. E.; Law, M.; Goldberger, F.; Kim, F.; Johnson, J. C.; Zhang, Y.; Saykally, R. J.; Yang, P. Angew. Chem., Int. Ed. 2003, 42, 3031. (6) Lee, Y. J.; Sounart, T. L.; Scrymgeour, D. A.; Voigt, J. A.; Hsu, J. W. J. Cryst. Growth 2007, 304, 80. (7) Tian, Z. R.; Voigt, J. A.; Liu, J.; McKenzie, B. B.; McDermott, M. J.; Rodriguez, M. A.; Konishi, H.; Xu, H. Nat. Mater. 2003, 2, 821. (8) Greene, L. E.; Law, M.; Tan, D. H.; Montano, M.; Goldberger, J.; Somorjai, G.; Yang, P. Nano Lett. 2005, 5, 1231. (9) Law, M.; Greene, L. E.; Johnson, J. C.; Saykally, R.; Yang, P. Nat. Mater. 2005, 4, 455. (10) Lee, S. H.; Lee, H. J.; Oh, D. C.; Lee, S. W.; Goto, H.; Buckmaster, R.; Yasukawa, T.; Matsue, T.; Hong, S. K.; Ko, H. C.; Cho, M. W.; Yao, T. J. Phys. Chem. B 2006, 110, 3856. (11) Kim, Y. J.; Lee, C. H.; Hong, Y. J.; Yi, G. C.; Kim, S. S.; Cheong, H. Appl. Phys. Lett. 2006, 89, 163128. (12) Kitsomboonloha, R.; Baruah, S.; Myint, M. T. Z.; Subramanian, V.; Dutta, J. J. Cryst. Growth 2009, 311, 2352. (13) Kang, B. S.; Pearton, S. J.; Ren, F. Appl. Phys. Lett. 2007, 90, 083104. (14) Kim, Y. J.; Lee, C. H.; Hong, Y. J.; Yi, G. C.; Kim, S. S.; Cheong, H. Appl. Phys. Lett. 2006, 89, 163128. (15) Kang, B. S.; Pearton, S. J.; Ren, F. Appl. Phys. Lett. 2007, 90, 083104. (16) Vlad, A.; Ma´te´fi-Tempfli, M.; Antohe, V. A.; Faniel, S.; Reckinger, N.; Olbrechts, B.; Crahay, A.; Bayot, V.; Piraux, L.; Melinte, S.; Ma´te´fiTempfli, S. Small 2008, 4, 557. (17) Chah, S.; Yi, J.; Pettit, C. M.; Roy, D.; Fendler, J. H. Langmuir 2002, 18, 314. (18) Fan, H. J.; Lee, W.; Hauschild, R.; Alexe, M.; Rhun, G. L.; Scholz, R.; Dadgar, A.; Nielsch, K.; Kalt, H.; Krost, A.; Zacharias, M.; Go¨sele, U. Small 2006, 2, 561. (19) Wang, C. Y.; Cimalla, V.; Kups, T.; Ro¨hlig, C. C.; Stauden, T.; Ambacher, O. Appl. Phys. Lett. 2007, 91, 103509. (20) Mougin, K.; Zheng, Z.; Piazzon, N.; Gnecco, E.; Haidara, H. J. Colloid Interface Sci. 2009, 333, 719. (21) Cho, S.; Ma, J.; Kim, Y.; Sun, Y.; Wong, G. K. L.; Ketterson, J. B. Appl. Phys. Lett. 1999, 75, 2761. (22) Hong, W.-K.; Sohn, J. I.; Hwang, D.-K.; Kwon, S.-S.; Jo, G.; Song, S.; Kim, S.-M.; Ko, H.-J.; Park, S.-J.; Welland, M. E.; Lee, T. Nano Lett. 2008, 8, 950. (23) Klingshirn, C.; Hauschild, R.; Fallert, J.; Kalt, H. Phys. ReV. B 2007, 75, 115203. (24) Ahn, M. W.; Park, K. S.; Heo, J. H.; Kim, D. W.; Choi, K. J.; Park, J. G. Sens. Actuators, B 2009, 138, 168. (25) Subannajui, K.; Kim, D. S.; Zacharias, M. J. Appl. Phys. 2008, 104, 014308.

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