"The support we have received has been invaluable and will enable us to produce state-of-the-art products with a much faster time to market"_Victor Higgs, C.E.O. Applied Nanodetectors

Applied Nanodetectors Ltd. develop novel sensor arrays which use chemically modified nanotubes as a gas sensing agent. These arrays are physically smaller and offer the potential of increased sensitivity compared with current commercial sensors.  However, in order to accelerate development of intellectual property (IP) and characterise this innovatory technology, extended access to both state of the art measurement facilities and specialists with expertise in nanotube properties was required.

Victor Higgs, Applied Nanodetectors’ C.E.O, approached CEMMNT to gain open access to a range of techniques to characterise the structure, properties and performance of nanotube gas sensors.  Victor leveraged consultancy and secondment opportunities offered by the DTI’s (now BERR) Measurement for Innovators (MFI) scheme.  Working with Alex Cuenat from the CEMMNT technical team and NPL’s nanomaterials group, Victor gained hands on experience on CEMMNT funded atomic force microscopy (AFM) and transmission electron microscopy (TEM) systems.  AFM was used to determine 3-dimensional nanotube morphology and is being extended to characterise the electrical (electron transfer) properties of carbon nanotubes using metal coated probes to carry out conductivity and surface potential measurements. TEM images provided a larger area overview of nanotube morphology and valuable data on tube structure and crystallinity.

To characterise the sensitivity of the gas sensors to trace element pollutants, Victor worked with Nick Martin of the Analytical Science group at CEMMNT. Most conventional sensors can currently detect 50 – 100 ppb of the pollutant nitrogen dioxide (NO2). However, there is now increasing industrial demand for higher sensitivity miniaturised sensors. The automotive industry is seeking a new generation of high-sensitivity, low-cost miniature sensors to measure pollution levels inside cars.

The team characterised the response of a number of sensors over a large range of NO2 aerosol concentrations. To be commercially viable, gas sensors need to demonstrate linear response as a function of pollutant concentration. A number of sensors responded to exceptionally low levels of NO2 – down to 4ppb in one sensor which is far beyond the sensitivity of any sensor currently on the market.

This understanding of these measurement techniques and their applications has enabled Applied Nanodetectors to benefit from funding opportunities that support the further development of gas sensors using CEMMNT and NPL as the collaborating metrology partners. This example proves the benefits that the CEMMNT partnership and its open access philosophy can bring to SMEs seeking to accelerate commercialisation and develop vital IP.

Carbon nanotubes present unique measurement challenges due to both their physical dimensions and morphology. Atomic Force Microscopy (AFM) can accurately measure the 3-dimensional profile and distribution of nanotubes when prepared in sub-monolayer quantities on flat substrates. AFM can additionally probe both the mechanical, thermal and electrical properties of nanotubes. For example, in Figure 1 (below), current-AFM is used to identify the spatial distribution of carbon nanotubes on a silicon substrate through their enhanced electrical conductivity relative to the semiconductor substrate.

(Fig 1)

High-resolution TEM (HRTEM) imaging gives structural information at the most detailed level (e.g. lattice imaging), and can be coupled with additional methods, such as energy-dispersive X-ray analysis (EDX) or electron energy loss spectroscopy (EELS), to allow nanoscale elemental analysis. TEM provides a wide range of dimensional and structural information:

(Fig 2 - Scanning Electron Microscope image of carbon nanotubes bridging electrodes) 

Raman spectroscopy complements microscopy techniques and can acquire data from multiple sample areas. The technique can detect single nanotubes and analyse structural forms of nanotubes (individual strands, ropes, bundles, aggregates).

A number of prototype novel gas sensors developed by Applied Nanodetectors have been evaluated for the first time using NPL’s environmental testing facilities. The sensors, which are based on carbon nanotube technology, were exposed to known concentrations of some priority gases that are relevant to the power distribution industry. The gases employed were nitrogen dioxide and nitric oxide, together with the potentially cross interfering species sulphur hexafluoride and carbon dioxide. The resistance of each sensor was measured as a means of monitoring the concentration of the selected gases.

NO2 could be detected over a wide range of concentrations from low parts per billion to high parts per million while the sensors were found to be relatively insensitive to high concentrations of the interferents SF6 and CO2. Such performance is a desirable characteristic for the development of a practical in-situ monitor that could be employed in the field at a relatively low cost. If developed further, the technology could potentially provide a wide spatial coverage to detect malfunctions in electrical switchgear. This is not currently possible to achieve with the present technology, which is both complex and expensive to deploy.

(Fig 3 - Typical response curves showing variation of measured resistance with NO concentration for two sensors)