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Highly selective GaN-nanowire/TiO2 -nanocluster hybrid sensors for detection of benzene and related environment pollutants View the table of contents for this issue, or go to the journal homepage for more 2011 Nanotechnology 22 295503 (http://iopscience.iop.org/0957-4484/22/29/295503) Home Search Collections Journals About Contact us My IOPscience IOP PUBLISHING NANOTECHNOLOGY Nanotechnology 22 (2011) 295503 (11pp) doi:10.1088/0957-4484/22/29/295503 Highly selective GaN-nanowire/TiO2- nanocluster hybrid sensors for detection of benzene and related environment pollutants Geetha S Aluri1,2, Abhishek Motayed1,3,5 , Albert V Davydov1, Vladimir P Oleshko1 , Kris A Bertness4 , Norman A Sanford4 and Mulpuri V Rao2 1 Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA 2 Department of Electrical and Computer Engineering, George Mason University, Fairfax, VA 22030, USA 3 Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, MD 20742, USA 4 Physical Measurement Laboratory, National Institute of Standards and Technology, Boulder, CO 80305, USA E-mail: amotayed@nist.gov Received 13 April 2011, in final form 25 May 2011 Published 15 June 2011 Online at stacks.iop.org/Nano/22/295503 Abstract Nanowire–nanocluster hybrid chemical sensors were realized by functionalizing gallium nitride (GaN) nanowires (NWs) with titanium dioxide (TiO2) nanoclusters for selectively sensing benzene and other related aromatic compounds. Hybrid sensor devices were developed by fabricating two-terminal devices using individual GaN NWs followed by the deposition of TiO2 nanoclusters using RF magnetron sputtering. The sensor fabrication process employed standard microfabrication techniques. X-ray diffraction and high-resolution analytical transmission electron microscopy using energy-dispersive x-ray and electron energy-loss spectroscopies confirmed the presence of the anatase phase in TiO2 clusters after post-deposition anneal at 700 ?C. A change of current was observed for these hybrid sensors when exposed to the vapors of aromatic compounds (benzene, toluene, ethylbenzene, xylene and chlorobenzene mixed with air) under UV excitation, while they had no response to non-aromatic organic compounds such as methanol, ethanol, isopropanol, chloroform, acetone and 1,3-hexadiene. The sensitivity range for the noted aromatic compounds except chlorobenzene were from 1% down to 50 parts per billion (ppb) at room temperature. By combining the enhanced catalytic properties of the TiO2 nanoclusters with the sensitive transduction capability of the nanowires, an ultra-sensitive and selective chemical sensing architecture is demonstrated. We have proposed a mechanism that could qualitatively explain the observed sensing behavior. (Some figures in this article are in colour only in the electronic version) 1. Introduction Detection of chemical species in air such as industrial pollutants, poisonous gases, chemical fumes and volatile 5 Author to whom any correspondence should be addressed. organic compounds (VOCs) is vital for the health and safety of communities around the world [1]. Due to their small size, ease of deployment and low-power operation, solid-state thin film sensors are often favored over analytical techniques like optical and mass spectroscopy, and gas chromatography when it comes to real-time environmental monitoring [2–4]. 0957-4484/11/295503+11$33.00 1 © 2011 IOP Publishing Ltd Printed in the UK & the USA Nanotechnology 22 (2011) 295503 G S Aluri et al Selectivity, which is a sensor’s ability to discriminate between the components of a gas mixture and provide a detection signal for the component of interest, is crucial for its real-life applicability. Metal-oxide-based thin film sensors, despite their commercial success and decades of research and development [5–7], still lack selectivity for different species, and often require high working temperatures [8–10]. This severely limits their usability and poses long-term reliability problems. For a chemical sensor, the active surface area is one of the important factors determining its detection limits or sensitivity. It is well known that the electrical properties of NWs change significantly in response to their environments due to their high surface to volume ratio [11–14]. Nanowires are well suited for direct measurement of changes in their electrical properties (e.g. conductance/resistance, impedance) when exposed to various analytes. Substantial research has demonstrated the enhanced sensitivity, reactivity and catalytic efficiency of the nanoscale structures [11, 15–20]. Unfortunately, the surface/adsorbate interactions of the nanowires are limited and non-specific. Despite being electrically sensitive, nanowires still suffer from the same lack of selectivity as their bulk counterpart devices. The idea of functionalizing or decorating the NW surface with metal or metal oxide nanoparticles or nanoclusters aims at resolving the deficiencies of such NW-based sensors. When carefully selected metal/metal oxide nanoparticles are placed on the surface of a nanowire, significant changes can result in the physical properties of the system. The nanoparticles can increase the adsorption of chemical species by introducing additional adsorption sites, thus increasing the sensitivity of such a system. Also, the metal or metal oxide nanoparticles can be selected to act as catalysts designed to lower the activation energy of a specific reaction, which produces active radicals by dissociating the adsorbed species. These radicals can then spill over to the semiconductor [21, 22] where they can be more effective in charge carrier transfer. Finally, the nanoparticles can modulate the current through the nanowire through the formation of a nanosized depletion region [23], which is in turn a function of the adsorption on the nanoparticles. In the last few years, there have been impressive demonstrations of hybrid gas sensors based on nanotubes/nanowires decorated with nanoparticles of metal and metal oxides. Leghrib et al reported gas sensors based on multi-wall carbon nanotubes (CNTs) decorated with tin oxide (SnO2) nanoclusters for detection of NO and CO [24]. Using mixed SnO2/TiO2 included with CNTs, Duy et al demonstrated ethanol sensing at a working temperature of 250 ?C [25]. Bal´azsi et al fabricated hybrid composites of hexagonal WO3 powder with metal-decorated CNTs for sensing NO2 at room temperature [26]. Kuang et al demonstrated an increase in the sensitivity of SnO2 nanowire sensors to H2S, CO and CH4 by surface functionalization with ZnO or NiO nanoparticles [27]. ZnO NWs decorated with Pt nanoparticles were used by Zhang et al, and they showed that the response of Pt nanoparticle-decorated ZnO NWs to ethanol is three times higher than that of bare ZnO NWs [28]. Chang et al showed that, by adsorption of Au nanoparticles on ZnO NWs, the sensor sensitivity to CO gas could be enhanced significantly [29]. Dobrokhotov et al constructed a chemical sensor from mats of GaN NWs decorated with Au nanoparticles and tested their sensitivity to N2 and CH4 [30]. GaN NWs coated with Pd nanoparticles were employed for the detection of H2 in N2 at 300 K by Lim et al [31]. All these results clearly demonstrate the potential of the nanowire–nanocluster-based hybrid sensors. However, there are still fundamental questions and challenges, which have not been investigated properly. Most of the reports are on mats of nanowires. Although this often increases the sensitivity, the complex nature of inter-wire conduction makes interpreting the results difficult. Also, few reports have actually shown room-temperature operation, and the selectivity is often shown for a very limited number of chemicals. The majority of highly sensitive hybrid nanowire–nanocluster devices developed so far require high operating temperatures (250 ?C) and large response times (more than 5 min). Roomtemperature operation of nanowire–nanocluster devices has been demonstrated by several groups but reported sensitivities were quite low with long response times. Very few research groups have actually demonstrated operation of the sensors with air as the carrier gas. However, the ability of a sensor to detect chemicals in air is what ultimately determines its usability in real life. Our approach described in this paper utilizes n-type (Sidoped) GaN NWs functionalized with TiO2 nanoclusters for selectively sensing benzene and related aromatic environmental pollutants such as toluene, ethylbenzene and xylene. This group of chemicals is commonly referred to as BTEX. GaN is a wide-bandgap semiconductor (3.4 eV), with many unique properties [32]. Its chemical inertness and capability of operating in extreme environments (high temperatures, presence of radiation, extreme pH levels) is highly desirable for sensor design. TiO2 is a photocatalytic semiconductor with a bandgap energy of 3.2 eV (anatase phase). Photocatalytic oxidation of various organic contaminants over titanium dioxide (TiO2) has been studied for decades [33–35]. The TiO2 nanoclusters were selected to act as nanocatalysts to increase the sensitivity, lower the detection time and, most importantly, enable us to tailor the selectivity of these structures to organic analytes. 2. Experimental details The NWs used in this study were c-axis, n-type, Si-doped GaN grown by catalyst-free molecular beam epitaxy on Si(111) substrates. Details of the NW growth can be found elsewhere [36, 37]. The process of sensor fabrication is schematically illustrated in figure 1. Post-growth device fabrication was done by dielectrophoretically aligning the nanowires on 9 mm × 9 mm sapphire substrates [38]. These device substrates had 12 nm thick Ti alignment electrodes of semicircular geometry with gaps between them ranging from 4 to 8 µm. After the alignment of the nanowires the samples were dried at 75 ?C for 10 min on a hot plate for evaporation of the residual solvent. This was followed by a plasma-enhanced chemical vapor deposition (PECVD) of 50 nm of SiO2 at a deposition temperature of 300 ?C. This passivation layer was 2 Nanotechnology 22 (2011) 295503 G S Aluri et al Figure 1. Schematic representation of the hybrid sensor fabrication process (not drawn to scale). deposited to ensure a higher yield for the fabrication process. After the oxide deposition, photolithography was performed to define openings for the top contact. The oxide in the openings was etched using reactive ion etching (RIE) with CF4/CHF3/O2 (50 sccm/25 sccm/5 sccm) gas chemistry. The top contact metallization was deposited in an electron beam evaporator with base pressure of 10-5 Pa. The deposition sequence was Ti (70 nm)/Al (70 nm)/Ti (40 nm)/Au (40 nm). The oxide layer over the nanowires between the end contacts was then etched in buffered HF etching solution for 15 s. A negative resist is used to protect the end metal contacts from the etching solution. The TiO2 nanoclusters were deposited on the exposed GaN NWs using RF magnetron sputtering. The deposition was done at 325 ?C with 50 sccm of Ar flow and 300 W RF power. The deposition rate was about 0.2 A s ° -1. Thermal annealing of the complete sensor devices (GaN NW with TiO2 nanoclusters) was done at 650 ?C to 700 ?C for 30 s in a rapid thermal processing system with 6 slpm (standard liter per min) flow of ultrahigh purity Ar. A relatively slow ramp rate of 100 ?C min-1 was chosen to reduce the stress in the metal–nanowire contact area during heating. The anneal step was optimized to facilitate ohmic contact formation to the GaN NWs and also to induce crystallization of the TiO2 clusters. Additional lithography was performed to form thick metal bond pads with Ti (40 nm) and Au (160 nm). The crystallinity and phase analysis of the sputtered TiO2 films were assessed by x-ray diffraction (XRD). The XRD scans were collected on a Bruker-AXS D8 scanning x-ray microdiffractometer equipped with a general area detector diffraction system (GADDS) using Cu Ka radiation6. The 6 Certain commercial equipment, instruments or materials are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the National Institute of Standards and two-dimensional 2-? patterns were collected in the 2 = 23?–51? range followed by integration into conventional -2 scans. The microstructure and morphology of the sputtered TiO2 films used for the fabrication of sensors were characterized by high-resolution analytical transmission and scanning transmission electron microscopy (HRTEM/STEM) and cold field emission scanning electron microscopy (FESEM). GaN nanowires with sputtered TiO2 were deposited onto a lacey carbon film supported by Cu-mesh grids and analyzed in a 300 kV TEM/STEM microscope. The instrument was equipped with an x-ray energy-dispersive spectrometer (XEDS) and an electron energy-loss spectrometer (EELS) as well as bright-field (BF) and annular dark-field (ADF) STEM detectors to perform spot and line profile analyses. The device substrates, i.e. the sensor chips, were wirebonded on a 24-pin ceramic package for the gas sensing measurements. The device characterization and the timedependent sensing measurements were done using an Agilent B1500A semiconductor parameter analyzer. Each sensor chip was placed in a custom-designed stainless steel test chamber of volume 0.73 cm3 with separate gas inlet and outlet. The test chamber had a quartz window on top for UV excitation provided by a 25 W deuterium bulb (DH-2000-BAL, Ocean Optics) connected to a 600 µm diameter optical fiber cable with a collimating lens at the end for uniform illumination over the sample surface. The operating wavelength range of the bulb was 215–400 nm. The intensity at 365 nm measured using an optical power meter was 375 nW cm-2. For all the sensing experiments regular breathing air (