How SEM helps understand the behavior of nanowire-based gas sensors

By Karl Kersten - July 13, 2017

Nanowires are widely used in electronic applications. They are typically used for transistors, where they bring benefits in terms of efficiency due to their high aspect ratio that enables good control of the channel potential. Nanowires are also being widely studied when used as sensors for proteins and chemicals. Researchers are exploring new and more efficient nanowire-based gas sensors by improving and developing new fabrication methods. In this blog, we discuss how microscopy helps to characterize nanowires and understanding their gas-sensing behavior. 

Nanowires for gas sensing

An enlightening review paper (X. Chen et al., Sensors and Actuators B: Chemical, 177 (2013): 178-195.) describes in detail the fabrication processes, the possible configurations, and the working principles of gas sensors based on nanowires. They typically present ultra-high sensitivity and fast response times, high selectivity, and stability. Nanowires are light weight and have low power consumption, wireless communication capabilities, and low-temperature operations that make them suitable for a wide range of applications. The synthesis of nanowires comprises several techniques that can be classified into two groups:

  • Top-down approaches, that begin with objects patterned on a large scale and then reduced to the nanoscale;
  • Bottom-up approaches, where the desired nanostructures are built starting from individual atoms and molecules.

SbSI nanosensors: a new frontier in humidity and CO2 sensing

K. Mistewicz et al. (Nanoscale Research Letters (2017) 12:97) researched the suitability of SbSI nanowires as sensors for humidity and CO2 in nitrogen. The nanowires were fabricated sonochemically from xerogel — a substance containing nanocrystals — following the bottom-up approach.

Two kinds of sensors were fabricated and analysed: chaotically oriented nanowires made from a rectangular sample of SbSI xerogel, shown in Figure 1, and a few SbSI nanowires stuck on a substrate, made from diluted xerogel, shown in Figure 2.

In the latter case, the nanowires were aligned perpendicularly to the electrodes by direct current electric field-assisted technique. Microscopy analysis was crucial in determining the structure of the nanowire-based sensors and in understanding their behavior.

Results from the experiments found that the chaotically oriented nanowires performed better in humidity sensing, but that they also showed a slower response. The reason lies in the absorption of water molecules agglomerated on the nanowires’ boundaries and near the contact between nanowires. For the first time, the author presented the electrical response of SbSI nanowires to CO2, namely an increase in electric conductance, where the array of aligned single nanowires performed better than the xerogel. The capability of knowing the shape is crucial for understanding the behavior of nanowire-based gas sensors.


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Figure 1: SEM image of the chaotically oriented SbSI nanowires gas sensor [courtesy of K. Mistewicz]


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Figure 2: SEM image of the oriented single SbSI nanowires gas sensor [courtesy of K. Mistewicz]


A nanowire-based gas sensor is just one of the many products and materials that have been analysed with an electron microscope to determine its structure. We have seen thousands of materials through the lens of a scanning electron microscope (SEM), and have selected the most interesting ones to be part of a short but fun SEM images quiz. Your challenge is to guess what material you’re looking at — can you get all 10 images right? Take the quiz now and find out! New call-to-action

 


About the author

Karl Kersten is head of the Thermo Scientific Phenom Desktop SEM Application Team at Thermo Fisher Scientific. He is passionate about the Phenom Desktop SEM product and likes converting customer requirements into product or feature specifications so customers can achieve their goals.

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