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Two Dimensional Materials for Bolometers of the Future

Periodic Reporting for period 1 - FUTBOL (Two Dimensional Materials for Bolometers of the Future)

Reporting period: 2016-04-01 to 2018-03-31

Remote thermal sensing and imaging is currently one of the most prominent technological fields. It is raising a lot of industrial interest as its market value is expected to grow beyond $10 billion/year by 2020. Remote thermal sensing and imaging finds applications in fundamental research, industrial quality control, future communication networks (THz based), military, isolation in housing, smart buildings, etc. The most used approach for the required sensors is bolometers, which are uncooled radiation heat sensors. The route for improved performance is enhanced absorption of thermal radiation, improved thermal isolation, reduced thermal mass, increased sensitivity of thermal transduction and reduced noise. However, even though the route is clear, bolometer technology has stagnated over the last decade.

FUTBOL aims to cause a paradigm shift in bolometer technology by making use of suspended devices made of atomically thin single crystalline sheets of material, 2D materials, to create a novel sensor with unseen resolution and measuring speed. Indeed, 2D materials are an attractive alternative to traditional bolometer materials. Their extreme aspect ratio allows for excellent thermal isolation, low areal mass density results in microscopic thermal mass and electrostatically tunable properties allows active tuning of temperature sensitivity and absorption of thermal radiation. The working principle of the 2D materials based bolometer is as follows: Absorbed thermal radiation heats up the suspended sheet, which can be monitored by measuring the induced change in the resonance frequency or electrical resistance. A schematic of the bolometer concept can be seen in the figure.

Realization of the suggested 2D material based bolometer will contribute to the need for better, faster and cheaper bolometers. Existing applications will benefit from improve technology and new applications will emerge. For instance, thermal imaging sensors complements LIDAR, RADAR and ultrasonic scanners used in self-driving cars.

The main objective of the FUTBOL project was to fabricate and characterize 2D material based bolometers.

At the end of the FUTBOL project, we have successfully fabricated graphene based bolometers. The measured relative frequency change per absorbed microwatt is 0.7% which is comparable to state of the art resonating radiation sensors. The minimum detectable power was estimated to be 1.4 nW. By exchanging graphene with e.g. WSe2 we expected this value can be improved with orders of magnitude and be beyond state-of-the-art.
Progress in the project can be divided into: (i) theoretical work, (ii) device fabrication, (iii) device characterization, (iv) training of the fellow and (v) dissemination of the project.

Theoretical work: We have focused on modelling the temperature fluctuation noise in micro- and nano-mechanical resonators. A topic that has received little attention in the literature. Our model shows that the temperature fluctuation noise should appear in an Allan Deviation plot (frequency stability plot) with slopes that are different from those of the classical noise sources. The estimated magnitude of the noise shows that it is relevant for state-of-the-art micro- and nano-mechanical resonators.

Device fabrication: We have successfully fabricated two types of suspended graphene structures: membranes and clamped-clamped beams. The clamped-clamped beams are suspended above a gate electrode and connected by a source and drain electrode.

Device characterization: We have performed electrical and mechanical characterization of the fabricated devices. Electrostatic doping was demonstrated and the estimated charge carrier mobility is comparable to those listed in the literature. Bolometric sending using electrical readout turned out to be inconclusive. Variation in resistance when irradiated cannot be attributed only to the change in temperature. We have yet not demonstrated integrated transduction of graphene resonators. Hence, characterization of the mechanical properties was done using optical techniques. We have successfully measured the resonance frequency and quality factor of graphene resonators with a diameter ranging from 4 µm up to 100 µm. The responsivity due to absorption of light has been estimated to 0.71 %/μW. A value comparable to state of the art resonating radiation sensors. The minimum detectable power was estimated to be 1.4 nW. By exchanging graphene with e.g. WSe2 we expected this value can be improved with orders of magnitude and then be beyond state-of-the-art.

Training of the fellow: The fellow has been trained on 2D material transferring techniques at DTU Nanotech, Denmark, and various micro-fabrication and characterization techniques at CMi, EPFL, Switzerland. In addition, the fellow has also received training in and practiced project management, resolving conflicts, negotiation skills, project proposal writing etc.

Dissemination of the project: During the project the fellow have contributed to 4 papers, 16 national and international conferences. The fellow has supervised 12 students projects (2 master, 3 bachelor, 5 semester and 2 intern) during the project period. At lab open doors events, the fellow has presented the FUTBOL project for more than 400 high school students from Switzerland, France and Israel. The FUTBOL project has been presented for two companies visiting the lab and at small meetings at DTU Nanotech, Denmark and TU Torino, Italy.
The three major outcomes of the FUTBOL project and their impact are:

1. A theoretical model giving a description of temperature fluctuation noise in micro- and nano-mechanical resonators. We believe that temperature fluctuation noise have been neglected in the literature and that it is more relevant than ever due to recent years progress within the field. A proper description of temperature fluctuation noise is relevant not only for the FUTBOL project but also for micro- and nanomechanical resonators in general. Existing and future use of these resonators could potentially benefit from the development of our model.

2. We have fabricate graphene based mechanical resonators and measured responsivity of 0.7%/μW and sensitivity of 1.4 nW. Results showing that 2D material based resonating sensors are indeed interesting. We used our resonators for radiation sensing. However, they may also find applications in other fields such as gas sensing, pressure sensing, photothermal spectroscopy etc.

3. The FUTBOL project has been presented to a large and diverse group of people. This includes presentations for visiting high school students and companies and at national and international conferences. These presentations have contributed to a larger awareness of bolometers, the used of 2D materials in bolometers and 2D materials in general.
Bolometer design and working principle