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Since its rediscovery by Professors Geim and Novoselov at The University of Manchester, graphene has been recognised as a revolutionary material with countless applications. It has extremely interesting and valuable properties; including, a strength 300 x greater than steel, as well as being the most electrically and thermally conductive material known.
Thermal and Electrical Conductivity
Graphene is recognised as the most efficient conductor of heat at room temperature and is a leading candidate to produce sensitive thermometers. In 2015 a novel graphene thermometer was unveiled at CES 2015 by Linktop. The device can be used to accurately monitor the temperature of children over a 24-hour period and, in conjunction with a Smartphone, alert parents to any anomalies.
Graphene operates like a superconductor at room temperature; it has been used to produce LEDs on a flexible substrate, demonstrating potential application in flexible and almost transparent smart devices; it is predicted to supplant the use of silicon due to its marked improvements in conductance efficiency.
Furthermore, these properties of thermal and electrical conductivity are intrinsically linked, leading to research into graphene in the development of bolometers.
Bolometers
Bolometers are used to measure infrared radiation by employing an absorber whose electric resistance is influenced by its temperature: Exposure to radiation causes an increase in temperature relative to a connected heat reservoir, which results in a measurable change in resistance. In practice, the temperature can be measured using an attached resistive thermometer or the absorber itself.
Samuel Langley invented the bolometer, using a platinum absorber, which had an accuracy of 0.00001 °C. Today, semiconductors and superconductors are routinely used at low temperatures in applications such as astronomy and particle detection.
Bolometer performance is linked to its temperature coefficient of resistance (TCR) which is the percentage change in resistance / Kelvin. The TCR of graphene-based bolometers has ranged between 2.4 and 11% K-1 (compared to 2 - 4% K-1 in current semiconductor systems). Applications for such detectors include fire detection, motion detection, healthcare and astronomy.
By integrating a graphene amplifier with a pyroelectric (LiNbO3) substrate scientists at the Cambridge Graphene Centre constructed a bolometer with a TCR of 900% K-1. This was achieved by fabricating a floating metallic structure that concentrates the pyroelectric charge on the top-gate capacitor of the graphene channel.
The circuit design allows graphene conductivity to be modulated by the electric field from the pyroelectric substrate, as well as from the floating gate, in a dual-gate capacitive structure. The perimeter of the C3 component defines pixel size, from which the source and drain contacts connect with the measurement electronics.
Checking Material Quality by Raman Spectroscopy
Raman Spectroscopy measures the spectra of scattered light to provide information about molecular composition. Materials were checked for quality using Raman Spectroscopy with a Renishaw Inviva™ confocal microscope. Spectra were used to show a defect concentration that was similar before and after transfer, indicating the sound structural integrity of the graphene channel.
The Renishaw Inviva™ offers unsurpassed performance with high spatial resolution, sample flexibility and a user-friendly interface for a multitude of applications.
Graphene-based bolometers: sensitive applications
The device size can be scaled down to decrease pixel size and increase resolution in imaging applications, without affecting the all-important temperature resistance coefficient. With a TCR of up to 900% K-1 and a resolution of 15 µK, the bolometer described has a range of exciting potential applications, including; high-resolution thermal imaging and highly sensitive spectroscopy in the MIR and far-IR.
References
1. Rogalski, A. Recent progress in infrared detector technologies. in Infrared Physics and Technology 54, 136–154 (2011).
2. Withers, F. et al. Light-emitting diodes by band-structure engineering in van der Waals heterostructures. Nat Mater 14, 301–306 (2015).
3. Sassi, U. et al. Graphene-based, mid-infrared, room-temperature pyroelectric bolometers with ultrahigh temperature coefficient of resistance. Nat. Commun. 0, 1–15 (2016).
4. Du, X., Prober, D. E., Vora, H. & Mckitterick, C. B. Graphene-based Bolometers. Graphene 2D Mater. 1, 1–22 (2014).
5. Ferrari, A. C. Raman spectroscopy of graphene and graphite: Disorder, electron-phonon coupling, doping and nonadiabatic effects. Solid State Commun. 143, 47–57 (2007).
6. Graham, M. W., Shi, S.-F., Ralph, D. C., Park, J. & McEuen, P. L. Photocurrent measurements of supercollision cooling in graphene. Nat. Phys. 9, 103–108 (2013).
7. Image Credit: Shutterstock.com/plotplot
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