Light and Graphene
Graphene and other 2D materials for optical sensing.
Graphene — and other two-dimensional materials — has a long list of unique properties that have made it a hot topic for intense scientific research and the development of technological applications. This has led to sky-high expectations that applications exploiting 2D materials will become the next disruptive technology impacting several cornerstones of our society.
However, bringing a completely new material system to market is a huge challenge, as the adoption of a new material requires large-scale production, integration capabilities, reliability, standardization, etc. The big question is which key properties of 2D materials, and not just their wonderful scientific properties, will be the starting point to overcome these challenges.
It may be that the true killer applications will come from an unexpected corner, and the crucial material properties that are the basis of a technological victory may be completely different from the ones that drove the initial scientific interest.
The European Union Graphene Flagship program aims to act as a catalyst for the development of groundbreaking applications by bringing together academia and industry to take graphene into society within 10 years.
One of the largest work packages is centered on optoelectronic applications. Graphene and other 2D materials are expected to offer an all-in-one solution to the challenges of future optoelectronic technologies because of their tunable optical properties, broadband absorption (from UV to THz), high electrical mobility for ultrafast operation, and novel gate-tunable plasmonic properties.
Two-dimensional material-based photodetectors are one of the most mature and promising solutions for this program.
The detection and conversion of light into electrical signals is at the heart of many technologies that affect our daily lives. These include video imaging, optical communications, biomedical imaging, security, night vision, gas sensing, wearable health devices, and motion detection. There is an ever-stronger demand for sensors or imaging systems that are faster and can cover spectral ranges further into infrared and even THz.
Existing technologies are limited, however. For example, different semiconductor materials with different bandgaps had to be developed to facilitate optical sensing from the UV to the infrared. Importantly, technologies covering applications in the UV, IR, and THz range cannot be monolithically integrated with current complementary metal-oxide semiconductor (CMOS) electronics, and that has led to a dramatic increase in costs as well as limitations in the performance of the sensors.
Photodetectors based on 2D materials have a number of distinct, beneficial characteristics. First, graphene is gapless and thus absorbs light in the UV, visible, short-wave infrared, near-IR, mid-IR, far-IR and THz spectral regimes. In addition, 2D material-based photodetectors are extremely fast, with intrinsic limits exceeding 250GHz.
Another important advantage is that 2D materials can be monolithically integrated with silicon electronics, so we can take advantage of the trillions of euros that have been invested in highly advanced Si-CMOS integrated electronics. This will allow graphene photodetector arrays to be monolithically integrated with multi-megapixel read-out electronics for high-resolution imaging or spectroscopy systems.
Being one atom thick is in itself a unique characteristic but what makes graphene useful is its ability to bend, stretch, and roll while maintaining its other properties. The emergence of flexible electronics, wearable electronics, and the “Internet of Things” imposes a strong requirement that components, including photodetectors, be foldable and flexible. This is a piece of cake for graphene photodetectors as they can be readily combined with any type of flexible substrate.
One example of a rather mature and high-performing 2D-material-based photodetector is a hybrid system developed at the Institute of Photonic Sciences (ICFO) that combines 2D materials with semiconductor nanoparticles (quantum dots). This hybrid system, with performance parameters beyond existing technologies, enables very high sensitivities for visible and IR light as well as high photodetection gain of more than a million.
Additional advantages of this system include low-cost production and the potential to integrate on thin, transparent, and flexible substrates. These hybrid phototransistors are fully compatible with silicon and CMOS technologies, offering large cost reductions in the development and production of the imaging systems as well as the electronics.
This technology will be a competitive alternative for applications in health, safety, security, and automotive systems. For example, flexible, wearable, and compact sensors for health applications can enable constant, noninvasive health monitoring of vital parameters.
By transforming visible and IR sensing and imaging technology into low-cost applications, we can introduce graphene-based applications and devices such as pocket cameras and night-vision goggles into a completely new consumer market.
Other tangible examples are high-speed photodetectors for optical communications applications where graphene is clearly superior to existing technologies. In principle, graphene detectors can have a bandwidth up to about 250 GHz. This makes them capable of outperforming other technologies being investigated for optical communications, such as monolithically integrated germanium.
An additional distinct advantage is that graphene is a platform for high-speed optical modulation and detection on the same chip.
Individual elements have already been realized, and the integration with silicon photonics is currently a high priority within the Graphene Flagship program. High-speed photonic integrated circuits with low energy consumption will soon be in high demand due to the rapid growth of the Internet of Things, the emergence of fifth-generation wireless networks, etc.
Adopting new materials is considered an inconvenience for most companies, as they prefer to improve existing technologies rather than accepting completely new concepts that require new production processes. Thus, they explore risk-averse strategies first.
This actually gives graphene and 2D materials an advantage because, apart from its unique properties, 2D materials may be combined with other existing technologies, silicon electronics, semiconductor nanoparticles, plastics, etc. Industries can thus introduce graphene and 2D materials gradually into their products, adding more and more functionality to existing technologies.
The Graphene Flagship is a €1 billion research program in Europe that is bringing together more than 140 academic and industrial research partners in 23 countries to take graphene from the realm of academic labs into European society to generate economic growth and new jobs.
Along with the Human Brain Project, the Graphene Flagship is the first of the European Commission’s Future and Emerging Technology (FET) Flagships, whose mission is to address big scientific and technological challenges of the age through long-term, multidisciplinary research and development efforts.
Launched in 2013, the project is coordinated by Chalmers University of Technology in Sweden and includes associated members who are included in 16 work packages.
–Frank Koppens is a professor at the Institute of Photonic Sciences (ICFO) in Barcelona where he leads the Quantum Nano-Optoelectronics Group. He gave a “Hot Topics” talk on graphene at SPIE Photonics Europe 2014 and is scheduled to cochair a graphene workshop at SPIE Photonics Europe 2016 5 April in Brussels. Koppens is a leader of the optoelectronics activities of the Graphene Flagship program. He has a PhD in nanoscience and quantum computation from the Kavli Institute of Nanoscience Delft (Netherlands). Koppens discusses his research on light and graphene in an SPIE.TV video: spie.org/Koppens2012
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