Researchers working at the Cambridge Graphene Centre have developed a novel graphene-based pyroelectric bolometer that detects infrared (IR) radiation to measure temperature with an ultrahigh level of accuracy.
The work, published in Nature Communications, demonstrates the highest reported temperature sensitivity for graphene-based uncooled thermal detectors, capable of resolving temperature changes down to a few tens of µK. Only a few nano-Watts of IR radiation power are required to produce such a small temperature variation in isolated devices, about 1000 times smaller than the IR power delivered to the detector by a human hand in close proximity.
The work was performed as part of a collaboration within the Graphene Flagship, the European research consortium aiming to bring graphene technologies to commercial markets within ten years. For this project, collaborators included the Institute of Photonic Sciences in Barcelona and the University of Ioannina in Greece, as well as Nokia UK and Emberion, who are local industrial partners of the Cambridge Graphene Centre.
Figure: Cambridge Graphene Centre/Emberion - Graphene pyroelectric bolometer -(a) Scheme of an individual device, where the conductance of a SLG channel is modulated by the pyroelectric substrate and by a floating gate. This is driven by two metallic pads in contact with the substrate, with a total area much larger than the overlap with the SLG channel. Such pads can be either uniform or patterned. (b) Circuit diagram for the device in a. (c) Optical image of a device with lateral pads patterned as electrically connected finger-like structures. Scalebar, 300 μm. (d) Response at 1,100 cm−1 (∼9 μm) over several ON/OFF cycles induced by a manual shutter. The laser spot size is 300 μm. The drain current is measured for a 10 mV drain voltage (Vd).
The graphene-based devices consist of a pyroelectric substrate, with a conductive channel of single-layer graphene and a floating gate electrode placed on top. In pyroelectric materials, changes in temperature lead to a spontaneous electric field inside the material. The floating gate electrode concentrates this field on the graphene, and the field causes changes in the electrical resistance of the graphene, which are measured as the device output.
Typical IR photodetectors operate either via the pyroelectric effect, or as bolometers, which measure changes in resistance due to heating. The graphene-based pyroelectric bolometers combine both effects for excellent performance and could be used as pixels in a high resolution thermal imaging camera.
Graphene acts as a built-in amplifier for the pyroelectric signal, without needing external transistor amplifiers as in typical pyroelectric thermal detectors. This direct integration means that there are no losses and no additional noise from connections to external amplifying circuits.
The use of graphene also offers benefits for further integrating the detector pixels with the external readout integrated circuit (ROIC) used to interface with the detector pixels and the recording device.
Impedance matching is essential to ensure that the signal is transmitted as efficiently as possible. This benefit is unique to graphene due to its combination of high conductivity and strong field effect.
The high sensitivity will be key for spectroscopic applications beyond thermal imaging, such as in security screening. Current IR photodetectors rely on integrated background IR radiation to provide a signal, and are not useful for spectroscopy. With a high-performance graphene IR detector that gives an excellent signal with less incident radiation, it is possible to isolate different parts of the IR spectrum.
According to the researchers these devices are set to make a clear impact in IR spectroscopic imaging.
Dr Alan Colli (Emberion), co-author of the work, said, "We can build the amplifier directly on the pyroelectric material. So, all the charge that it develops goes to the amplifier. There is nothing lost along the way." Alan, added, "To match the input impedance of the ROIC, you need something that is as conductive as possible. The intrinsic conductivity of graphene helps the further integration with silicon." Alan concluded by saying, "With a higher sensitivity detector, then you can restrict the band and still form an image just by using photons in a very narrow spectral range, and you can do multi-spectral IR imaging. For security screening, there are specific signatures that materials emit or absorb in narrow bands. So, you want a detector that’s trained in that narrow band. This can be useful while looking for explosives, hazardous substances, or anything of the sort."
Dr Daniel Neumaier (AMO, Germany) the leader of the Graphene Flagship Electronics and Photonics Integration Division, "The market size of IR detectors has increased dramatically in the last couple of years and these devices are entering more and more application areas. In particular, spectroscopic security screening is becoming more important." Daniel, added, "This requires high sensitivity under room temperature operation. The present work is a huge step forward in meeting these requirements in graphene-based IR detectors."
Prof Andrea Ferrari (University of Cambridge, UK), Science and Technology Officer of the Graphene Flagship, Chair of the Flagship Management Panel and co-author of the research, said "This work is another example of the steady march of graphene on the roadmap towards applications. Emberion is a new company created to produce graphene photonics and electronics for infrared photodetectors and thermal sensors, and this work exemplifies how basic science and technology can lead to swift commercialisation."
Article | OPEN
Graphene-based mid-infrared room-temperature pyroelectric bolometers with ultrahigh temperature coefficient of resistance
U. Sassi | R. Parret | S. Nanot | M. Bruna | S. Borini | D. De Fazio | Z. Zhao | E. Lidorikis | F.H.L. Koppens | A. C. Ferrari | A. Colli
Nature Communications 8 | Article number: 14311 (2017) | doi:10.1038/ncomms14311
Received: 09 September 2016 | Accepted: 16 December 2016 | Published online: 31 January 2017
There is a growing number of applications demanding highly sensitive photodetectors in the mid-infrared. Thermal photodetectors, such as bolometers, have emerged as the technology of choice, because they do not need cooling. The performance of a bolometer is linked to its temperature coefficient of resistance (TCR, ∼2–4% K−1 for state-of-the-art materials). Graphene is ideally suited for optoelectronic applications, with a variety of reported photodetectors ranging from visible to THz frequencies. For the mid-infrared, graphene-based detectors with TCRs ∼4–11% K−1 have been demonstrated. Here we present an uncooled, mid-infrared photodetector, where the pyroelectric response of a LiNbO3 crystal is transduced with high gain (up to 200) into resistivity modulation for graphene. This is achieved by fabricating a floating metallic structure that concentrates the pyroelectric charge on the top-gate capacitor of the graphene channel, leading to TCRs up to 900% K−1, and the ability to resolve temperature variations down to 15 μK.