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.