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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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Plasmonics Without Light Just Flipped Nanophotonics on its Head

Posted By Dexter Johnson, IEEE Spectrum, Monday, October 23, 2017

The use of graphene in the growing field known as plasmonics—in which the waves of electrons known as surface plasmons that are generated when photons strike a metallic structure—has been transforming the world of photonics and optoelectronics, enabling the possibility of much smaller devices operated by photons rather than electrons.

The Graphene Council has covered the work being performed at one of the leading research institutes in the world in this field of plasmonics, the Institute of Photonic Sciences (ICFO) in Barcelona. 

We had the opportunity to visit ICFO last week and speak to a number of their researchers, which we will be sharing in the coming weeks. In particular, we spoke to F. Javier García de Abajo from the Nanophotonics Theory research group at ICFO,  who has proposed a revolutionary approach of exploiting graphene for plasmonics.

It’s worth providing a bit of background on the field of plasmonics before jumping to this latest research. The use of photons instead of electrons for something like an integrated circuit has the clear benefit that photons travel much faster than electrons, promising much faster devices. However, the use of light in these applications is limited by the relatively large size of wavelengths of light. Light is fast, but their wavelengths are much larger than nanometer-scale dimensions of most integrated circuits.

Plasmonics provides a way to convert that light—photons—into waves of electrons that can be tuned to have much smaller dimensions than those of light. The dimensions of these plasmon waves can be a hundred times smaller than the smallest wavelengths of light. This means that light can serve as the basis of photonic integrated circuits, but many more devices than that.

The field of plasmonics has really taken in off in the last half-decade, and ICFO has been at the forefront of a lot of that work, especially in using graphene to enable the effect. However, what Garcia de Abajo has proposed is a new theoretical approach to generate visible plasmons in graphene not from light but from tunneling electrons.

In research published in the journal ACS Photonics, Garcia de Abajo and his colleague Sandra de Vega have suggested that there are more efficient ways of generating surface plasmons on graphene than using an external light source and have instead shown through models that graphene plasmons can be efficiently excited via electron tunneling in a sandwich structure formed by two graphene monolayers separated by a few atomic layers of hexagonal boron nitride.

As mentioned, it’s possible to tune the size of the plasmon waves, especially graphene plasmons, which can be changed in size according to the amount of doping level (an addition of other materials). While high doping levels can push the wavelength of the graphene plasmons towards the visible range, these grpahene plasmons primarily reside in the mid-infrared region, which translates into a weak coupling between far-field light and graphene.

What de Vega and García de Abajo have proposed is a methodology for visible-plasmon generation in graphene that requires no light at all. Instead, plasmons are generated from tunneling electrons, which are electrons that are able to pass through a material on the quantum level that they could not otherwise pass through.

To achieve this photon-less plasmonics, the researchers propose a graphene–hexagonal boron nitride (hBN)–graphene sandwich structure. In their model, the hBN layer is 1-nm thick that is sandwiched between two graphene monolayers.

When the right amount of voltage (bias) is applied between the two graphene sheets, it produces tunneling electrons through the gap. The researchers discovered a particular voltage window in which the tunneling electrons lose energy through the excitation of a propagating optical plasmon rather than dissipate through coupling with the vibrations of the crystal lattice of hBN that carry heat, which are known as phonons, (low bias) or electron–electron interactions (high bias).

One of the side benefits of plasmonic devices that operate in this way—without the need for photons—can also be used in reverse as sensors. In this way when a change occurs in the graphene plasmon properties, that change could lead to a voltage readout.

Tags:  electrons  graphene  hexagonal boron nitride  ICFO  photonics  photons  plasmonics  sensors 

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Established Optical Society Sees the Light in Graphene

Posted By Dexter Johnson, IEEE Spectrum, Saturday, October 8, 2016
Updated: Thursday, October 6, 2016

SPIE—the international society for optics and photonics—has been a society set up to advance light-based technologies since 1955. In this role, it has offered its members conferences, news services and a range of different avenues for exchanging information on this quickly developing field.

As evidence of its commitment to staying ahead of the latest science and technologies in photonics and optics, SPIE has been offering conferences on the topic of graphene since 2009. SPIE has identified graphene and other two-dimensional materials as a key area of interest for its members because of the properties these new materials are offering in the field.

The Graphene Council certainly shares in SPIE’s interest in how two-dimensional materials, including graphene, will play a key role in optoelectronics and photonics, with our frequent coverage of these two fields. 

Now that SPIE has become one of our Corporate Members we took the opportunity to speak to Robert F. Hainsey, Ph.D., the Director of Science and Technology for SPIE to ask him about the role graphene is positioned to play in optics and photonics, how the market is developing and the role of SPIE as these developments evolve.

Q: Graphene has exhibited a number of appealing properties for applications within photonics and optoelectronics, so it’s clear to see why SPIE would become involved with the topic. But could you tell us a little bit about the evolution of how SPIE started getting involved in the topic of graphene? 

A:  SPIE has a long history of supporting the topic of graphene having launched a volunteer-inspired conference at our Optics + Photonics event held annually in San Diego as early as 2009.  The topic appears in a number of other SPIE conferences as well.  In 2014, Frank Koppens of ICFO delivered an excellent plenary talk on the subject at our Photonics Europe event in Brussels, and this led, in turn, to Frank Koppens and Nathalie Vermeulen of the B-PHOT team at Vrije Universiteit Brussel organizing and chairing a full-day workshop at this year’s Photonic Europe event on applications and commercialization of graphene.  We continue to look for methods to enable the community to best share results and exchange ideas in this rapidly evolving field.

Q: How is SPIE now approaching the topic, i.e. what sort of mediums are you using to get the message out about graphene? How do you see this information serving your members? 

A:  The information is disseminated in a number of ways.  Primary among these methods are our conferences which enable researchers to share and discuss the latest findings in the area of graphene and similar materials.  The work shared in those conferences is then packaged into proceedings and made part of the SPIE Digital Library so as to share the results with a wider audience.  We also have our journals where researchers can publish their results in a peer-reviewed medium.  The “SPIE Professional” magazine, the quarterly magazine for our members, has included articles in this area including one written by Frank Koppens earlier this year.  Naturally, we share news about graphene research on our News Room webpage, via Twitter and through our LinkedIn groups.  In terms of serving our members, we hope that this diverse set of methods of sharing information keeps our members informed on the latest work in the field and stimulates discussion among researchers to advance the field.

Q: There are a number of different applications within photonic and optoelectronics in which graphene has exhibited promise. In one of your more recent conferences on graphene, communication applications were identified as the most near-term. Has SPIE begun to get a better feel of how graphene applications within photonics and optoelectronics are developing commercially? And could you give us an outline of that development? 

A:  The workshop you refer to is a positive step towards moving graphene along the commercialization pipeline.  This workshop served to bring together academic and industrial researchers as well as entrepreneurs and start-up companies to discuss what is needed to move graphene from a laboratory to a production setting.  A look at the program for that event illustrates that large enterprises are investing in the research.  In addition, more start-up’s are appearing on the scene at various positions of the value chain.  Progress is being made on the road to full-scale production but there is still work to be done.

Q: Is SPIE involved with any of the standards bodies that are attempting to create industry standards for the material? Whether you are involved or not, does SPIE have a position on the role of materials standards as the material becomes increasingly commercialized?

A:  At this point we are not actively engaged in the work on developing standards outside of the presentations given in our conferences.  That said, one sign of research maturing and preparing to transition to a production environment is the discussion and adoption of standards.  Standards are oftentimes crucial since they provide a baseline for methods and performance by which the industry can determine capability and map progress.  SPIE supports standards development in other areas through methods such as providing meeting space for standards bodies at our events.  We would welcome dialogue with standards bodies in this area to determine if there is a way SPIE can more actively support that work.

Q: How do you see SPIE’s role in graphene education and providing information evolving as the field moves from the lab to the fab? Does the approach to disseminating information on a topic change as it moves from research to commercial interests? 

A:  Certainly the topic will continue to be a vibrant one in our conferences, our proceedings, the SPIE Digital Library, and our social media outlets.  SPIE events also include a set of industry sessions containing presentations, panel discussions, and networking opportunities focused on the commercial aspects of optics and photonics technologies.  This combination of conferences, publications, and industry sessions positions SPIE events to track the migration of the technology as it matures.  The flexibility we have within our events to include unique offerings such as the dedicated workshop on graphene commercialization at the SPIE Photonics Europe event earlier this year allows SPIE to tailor the forum to best serve the community.

Q: How does partnering with groups, such as The Graphene Council, help or contribute to your strategy in education and providing information on the topic of graphene?

A:  SPIE is an organization dedicated to serving the optics and photonics community.  Partnering with other organizations to further the sharing of information and enhancing the discussion around technologies not only helps SPIE meet its charter but, more importantly, enables the advancement of research, science, engineering and practical applications in these technologies.

Tags:  corporate members  optoelectronics  photonics  SPIE 

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