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Graphene-based Image Sensors Offer New Commerical Avenues

Posted By Dexter Johnson, IEEE Spectrum, Tuesday, March 13, 2018

The Mobile World Congress (MWC) held annually in Barcelona, Spain is one of the largest technology conferences in the world. For the last three years, the MWC has been hosting the Graphene Pavilion that showcases the research institutes and technologies that they have developed under the EU’s Graphene Flagship

The Graphene Council visited the Graphene Pavilion last month in Barcelona and we came back with some videos. One of the anchor institutions at the Pavilion is The Institute of Photonics (ICFO)  located just outside of Barcelona. The Graphene Council has been speaking to Frank Koppens at ICFO since 2015 about how graphene was impacting photonics and optoelectronics. 

In our latest visit with them at MWC this year, we got an update on some of the ways they are applying their technologies to various technologies.

In the one shown in the video below, the researchers have developed ultraviolet (UV) sensors for protecting the wearers from overexposure to the sun.

While specifics of the underlying technology are not discussed in the video, it would appear to be based on the CMOS-based image sensor for UV-visible-infrared light that the ICFO developed based on a combination of graphene and quantum dots.

What the ICFO discovered six years ago was that while graphene generates an electron-hole pair for every single photon the material absorbs generates, it doesn’t really absorb that much light. To overcome this limitation of graphene, they combined it with quantum dots with the hybrid material being capable of absorbing 25 percent of the light falling on it. When you combine this new absorption capability with graphene’s ability to make every photon into an electron-hole pair, the potential for generating current became significant.

The ICFO has been proposing applications like this for this underlying technology for years, and producing working prototypes. At the MWC in 2016, the ICFO was exhibiting a heart rate monitor. In that device, when a finger is placed on the photodetector, the digit acts as an optical modulator, changing the amount of light hitting the photodetector as your heart beats and sends blood through your fingertip. This change in signal is what generates a pulse rate on the screen of the mobile device.

This same basic technology is at the heart of another technology ICFO was exhibiting this year (see video below) in which the graphene-based photodector can determine what kind of milk you are about to drink. This could conceivably be used by someone who has a lactose intolerance that could threaten their lives and by using the detector could determine if it was cow’s milk or soy milk, for instance.

While ICFO goes so far as to discuss prices for the devices, it’s not clear that ICFO is really committed to any of these technologies for its wide-spectrum CMOS graphene image sensor, or not. In the case of the heart monitor, the researchers claimed at the time it was really just intended to demonstrate the capabilities of the technology.

The long-range aim of the technology is to improve the design of these graphene-based image sensors to operate at a higher resolution and in a broader wavelength range. Once the camera is improved, the ICFO expects that will be used inside a smartphone or smart watch. In the meantime, these wearable technologies offer intriguing possibilities and maybe even a real commercial avenue for the technology.

Tags:  CMOS  graphene  ICFO  infrared  Mobile World Congress  photodetectors  quantum dots  ultraviolet 

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Graphene Membranes Enable a Novel Approach to Ubiquitous Photodetectors

Posted By Dexter Johnson, IEEE Spectrum, Monday, March 12, 2018

Image: University of Manchester

Back in January, scientists in Andre Geim’s research team at the University of Manchester reported that light could be used to enhance proton transport through graphene.  What this means is the possibility of an entirely new class of photodetectors, which are used in just about everything from high-speed optical communication networks to the remote control for your TV.

Needless to say, an entirely new class of photodetectors—based on proton transport as opposed to all current photodetectors today that are based on electron transport—is a pretty significant development. You add on to this the fact that the photodetectors made from graphene are 100,000 times more responsive than silicon and you have the basis of a transformative technology.

What regular readers of The Graphene Council may have missed earlier this month in an Executive Q&A with Jeffrey Draa, CEO of Grolltex, was that we got some indications in that interview that the technology being developed in Geim’s lab is ramping up for commercial applications.

Draa said in the interview: “…we’re also starting to get some inquiries for an application that actually Dr. Andre Geim at the University of Manchester, who, of course, was the discoverer of graphene was very passionate about. This is one of the very first applications that he thought futuristically would really make the world a better place, and that third application that we're starting to see on the horizon is graphene as a proton exchange membrane in a hydrogen fuel cell.”

Draa in this interview points to the initial applications that were discussed almost four years ago for this graphene-based proton exchange membrane. At the time, Geim had discovered that contrary to the prevailing wisdom that graphene was impermeable to all gas and liquids  it could, in fact, allow protons to pass through. This made scientists immediately conjure up the proton exchange membranes that are central to the functioning of fuel cells.

While there’s no reason to think that these graphene membranes won’t someday make for excellent proton exchange membranes for fuel cells, the problem is that fuel cells are not exactly ubiquitous. However, photodetectors certainly are ubiquitous, making for a much larger potential market for these graphene membranes.

Of course, it’s a pretty big step to make these graphene membranes go from being used for fuel cells to being used in photodetectors. So how did this application switch occur?

The University of Manchester scientists started with monolayer graphene decorated with platinum (Pt) nanoparticles. In operation, photons (light) strike the membrane and excite the electrons in the graphene around the Pt nanoparticles. This makes the electrons in the graphene become highly reactive to protons. This, in turn, induces the electrons to recombine with protons to form hydrogen molecules at the Pt nanoparticles. This process mimics the way in silicon-based photodetectors operate based on electron-hole recombination.

While there are similarities between the semiconductor approach to electron-hole recombination, the photon-proton effect used in this graphene membrane would represent a big departure from the previous approach and nobody is quite sure what the implications might be.

However, it is clear that this graphene membrane that Grolltex is working on with the scientists at Manchester may have a new set of applications that extends far beyond just typical membrane-based technologies.

Tags:  Andre Geim  fuel cells  membranes  photodetectors  University of Manchester 

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How Quantum Dots and Graphene Combined to Change the Landscape for Optoelectronics

Posted By Dexter Johnson, IEEE Spectrum, Wednesday, November 1, 2017

Last June, we covered research that brought graphene, quantum dots and CMOS all together into one to change the future of both optoelectronics and electronics. 

That research was conducted at the Institute of Photonics (ICFO) located just outside of Barcelona, Spain. The Graphene Council has been speaking to Frank Koppens at ICFO since 2015 about how graphene was impacting photonics and optoelectronics.

Now, in a series of in-person interviews with several researchers at ICFO (the first of which you can find here),  we are gaining better insight into how these technologies came to be and where they ultimately may lead.

Gerasimos Konstantatos - group leader at ICFO

The combination of graphene with quantum dots for use in optoelectronics stems in large part from the contributions of Gerasimos Konstantatos, a group leader at ICFO, who worked with Ted Sargent at the University of Toronto, whose research group has been at the forefront of exploiting colloidal quantum dots for use in a range of applications, most notably high-efficiency photovoltaics.

“Our initial expertise and focus was on actually exploiting the properties of solution-process materials particularly colloidal quantum dots as optoelectronic materials for solar cells and photodetectors,” explained Konstantatos. “The uniqueness of these materials is that they give us access to a spectrum that is very rarely reached in the shortwave and infrared and they can do it at a much lower cost than any other technology.”

Konstantatos and his group were able to bring their work with quantum dots to the point of the near-infrared wavelength spectrum, which falls in the wavelength size range of one to five microns. Konstantos is now developing these solution-based quantum dot materials to produce even more sensitive materials capable of getting to 10 microns, putting them squarely in the mid-infrared range.

“My group is now working with Frank Koppens to sensitize graphene and other 2D materials in order to make very sensitive photodetectors at a very low cost that are capable of accessing the entire spectrum, and this cannot be done with any other technology,” said Konstantatos.

What Konstantatos and Koppens have been able to do is to basically eliminate the junction between graphene and the quantum dots and in so doing have developed a way to control the charge transfer in a very efficient way so that they can exploit the very high mobility and transport conductance of graphene.

“We can re-circulate the charges through the materials so that with a single photon we have several billion charges re-circulating through the material and this constitutes the baseline of this material combination,” adds Konstantatos.

With that as their baseline technology, Konstantatos and his colleagues have engineered the quantum dot layer so instead of just having a passive quantum dot layer they have converted it into an electro-diode. In this way they can make much more complex detectors. In the combination of the graphene-based transistor with the quantum dots, it’s not just a collection of quantum dots but is a photodiode made from quantum dots.

“In this way, we kind of get the benefit of both kinds of detectors,” explains Konstantatos. “You have a phototransistor that has a very high sensitivity and a very high gain, but you also get the high quantum efficiency you get in photodiodes. It’s basically a quantum photodiode that activates a transistor.”

In addition to the use of graphene, the ICFO researchers are looking at other 2D materials in this combination, specifically the semiconductor molybdenum disulfide. While this material is a semiconductor and sacrifices somewhat on the electron mobility of graphene, it does make it possible to switch off the material to control the current. As a result, Konstantatos notes that you can have much lower noise in the detector with much lower power consumption.

In continuing research, Konstantatos hinted at yet to be published work on how all of this combination of quantum dots and graphene could be used in solar cell applications.

In the meantime, the work they have been doing with graphene and quantum dots is much further advanced than what they have yet been able to achieve with molybdenum disulfide, mainly because work has advanced much further in making large scale amounts of graphene. But as the processes for producing other 2D materials improves, there will be a real competition between all of the 2D materials to see which provides the best possible performance as well as manufacturability properties.

In any event, Konstantatos sees that the way forward with both quantum dots and 2D materials is using them together.

He adds: “I think we can explore the synergies in between different material platforms. There's no such thing as a perfect material that can do everything right. But there is definitely a group of materials with some unique properties. And if you can actually combine them in a smart way and make hybrid structures, then I think you can have significant added value.”

Tags:  2D materials  graphene  optoelectronics  photodetectors  photonics  photovoltaics  quantum dots 

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