Highly Sensitive and Selective Chemical/Gas Sensors Using Hybrid Nanowire and Nanocluster Based Devices

Abstract

The development of reliable, portable gas sensors that can detect harmful gases in real-time with high sensitivity and selectivity, is very important for clean environment and national security. In the last few decades, significant advances have been made in the field of metal oxide based thin-film sensors. Metal oxide sensors have been utilized for several decades for low-cost detection of combustible and toxic gases. However, issues with sensitivity, selectivity, and stability have limited their use, often in favor of more expensive approaches such as IR sensors and gas chromatography. In recent years, there has been a tremendous interest in the development of nano-engineered materials, such as nanowires and nanoclusters, for gas sensing because of their high sensitivity. In most of these nanostructure based sensors, poor selectivity and limited sensitivity are still major obstacles for their commercialization. For real-world applications, selectivity between different classes of compounds (such as between aromatic compounds and alcohols) is highly desirable. In fact, an ideal chemical sensor is one that can distinguish between the individual analytes belonging to a particular class of compounds, e.g. detection of the presence of benzene or toluene in the presence of other aromatic compounds. This is extremely challenging as most semiconductor-based sensors use metal-oxides (such as SnO2, In2O3, ZnO) as the active elements, which have inherent non-selective surface adsorption sites. Recently, a new class of nanowire-based gas sensors have gained interest. The nanowire-nanocluster (NWNC)-based gas sensors represent a way of functionalizing the surfaces of nanowires for selective adsorption and detection of analytes. They also offer the potential of tuning their sensitivity and selectivity by adjusting the composition, size, and density of the nanoparticles which decorate the nanowires. This makes them a good alternative to conventional metal-oxide based thin film sensors. In recent years, researchers have demonstrated the potential of NWNC hybrids for sensing many different chemicals. However, most of the hybrid devices developed so far require elevated working temperatures, have long response/recovery times, and operate in inert atmospheres, which limit their use in environmental, domestic, and industrial applications. Approach of this work utilizes n-type (Si doped) GaN nanowires functionalized with different metal oxide and metal-metal oxide composite nanoclusters for highly selective gas sensing. In this work, it has been demonstrated that the GaN-TiO2 (nanowire-nanocluster) hybrid devices use the photocatalytic properties of TiO2 to sense specific volatile organic compounds mixed in air at room temperature and ambient humidity. The photo-modulated GaN/TiO2 NWNC hybrids showed remarkable selectivity to benzene and related aromatic compounds, with no measureable response for other analytes (like alcohols, ketones, aldehydes, amides etc) at room temperature. Xylene, ethybenzene, benzene, and toluene were detected at concentration levels of 50 ppb in approximately 75 s. These sensor devices were highly stable and able to sense aromatic compounds reliably with concentrations as high as few percents in air. These GaN/TiO2 NWNC hybrids also sensed very low concentrations of explosive nitro-aromatic compounds as well. The hybrid sensor devices were able to detect trinitrotoluene (TNT) concentrations as low as 500 ppt in air and dinitrobenzene concentrations as low as 10 ppb in air in approximately 30 s. It was found that sensors with TiO2-Pt multicomponent NCs on GaN NW were only sensitive to methanol, ethanol, and hydrogen. Higher carbon-containing alcohols (such as n-propanol, iso-propanol, n-butanol) did not produce any sensor response. The GaN/(TiO2-Pt) hybrids were able to detect ethanol and methanol concentrations as low as 100 ppb in air in approximately 100 s, and hydrogen concentrations from 1 ppm to 1% in nitrogen in less than 60 s. These sensors have the highest sensitivity towards hydrogen. Prior to the Pt deposition, the GaN/TiO2 NWNC hybrids did not exhibit any response to alcohols. The GaN/Pt hybrids only showed sensitivity to hydrogen and not to methanol or ethanol. The sensitivity of GaN/Pt hybrids towards hydrogen was lower compared to the GaN/(TiO2-Pt) hybrids. GaN/SnO2 NWNC prototype devices were also developed in this study, which showed selective response to alcohols for a wide range of alcohol vapor concentrations, from 5000 ppm down to 1 ppm in air. It is observed that the sensor response decreases with the increasing carbon chain from methanol to n-butanol. This study indicates the potential of multicomponent NWNC based sensors for developing the next-generation of ultra sensitive and highly selective chemical sensors. Through combinations of metal and metal-oxides available, one can produce a library of sensors, each with precisely tuned selectivity, on a single chip for detecting a wide variety of analytes in many different environments at room temperature. Also, due to the small magnitude of device operating current and sensor activation at low illumination intensity, these sensors have low power consumption which makes them suitable for portable applications.