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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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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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Graphene and CMOS Become One, Offering New Hope in Electronics

Posted By Dexter Johnson, IEEE Spectrum, Friday, June 9, 2017

 

 

Complimentary metal-oxide semiconductors (CMOS) have served as the backbone of the electronics industry for over four decades.  However, the last decade has been marked by increasing concerns that CMOS will not be able to continue to meet the demands of Moore’s Law in which the number of transistors in a dense integrated circuit doubles approximately every two years. If CMOS is going to continue to be a force in electronics, it will become necessary to integrate CMOS with other semiconductor materials other than silicon.

It appears that research out of The Institute of Photonic Sciences in Barcelona (ICFO) and supported by The Graphene Flagship has found a way to integrate graphene into a CMOS integrated circuit

In research described in the journal Nature Photonics, the ICFO researchers combined the graphene-CMOS device with quantum dots to create an array of photodetectors.

While the photodetector arrays could enable digital cameras capable of seeing UV, visible and infrared light simultaneously, the technology could have a wide range of applications, including microelectronics to low-power photonics.

“The development of this monolithic CMOS-based image sensor represents a milestone for low-cost, high-resolution broadband and hyperspectral imaging systems" said, Frank Koppens, a professor at ICFO in a press release.

Koppens, who The Graphene Council interviewed back in 2015believes that "in general, graphene-CMOS technology will enable a vast amount of applications, that range from safety, security, low cost pocket and smartphone cameras, fire control systems, passive night vision and night surveillance cameras, automotive sensor systems, medical imaging applications, food and pharmaceutical inspection to environmental monitoring, to name a few."

The researchers were able to integrate the graphene and quantum dots into a CMOS chip by first depositing the graphene on the CMOS chip. Then this graphene layer is patterned to define the pixel shape. Finally a layer of quantum dots is added.

“No complex material processing or growth processes were required to achieve this graphene-quantum dot CMOS image sensor,” said Stijn Goossens, another researcher from ICFO in Barcelona. “It proved easy and cheap to fabricate at room temperature and under ambient conditions, which signifies a considerable decrease in production costs. Even more, because of its properties, it can be easily integrated on flexible substrates as well as CMOS-type integrated circuits."

The graphene-enabled CMOS chip achieves its photoresponse through something called the photogating effect, which starts as the quantum dot layer absorbs light and transfers it as photo-generated holes or electrons to the graphene. These holes or electrons move through the material because of a bias voltage applied between two pixel contacts. The photo signal triggers a change in the conductivity of the graphene and it is this change that is sensed. Because graphene has such high conductivity, a small change can be quickly detected giving the device extraordinary sensitivity.

Andrea Ferrari, science and Technology offficer of the Graphene Flagship added: "The integration of graphene with CMOS technology is a cornerstone for the future implementation of graphene in consumer electronics. This work is a key first step, clearly demonstrating the feasibility of this approach.”

Tags:  CMOS  digital cameras  graphene  low-power photonics  quantum dots  The Graphene Flagship 

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