A team of researchers at the Indian Institute of Technology (IIT) Gandhinagar has stumbled upon an unexpected phenomenon that would have significant implications on the existing protocols followed to synthesize graphene and other two dimensional (2D) nanomaterials.
Graphene is the thinnest material that has led to several developments in fundamental and applied science since it was first discovered 15 years ago. This new knowledge has also led to the development of a range of new 2D nanomaterials, which are like graphene but made from other elements.
One of the most popular methods to synthesize graphene is liquid-phase exfoliation, in which the graphite powder is mixed in a suitable liquid medium and exposed to bursts of high-intensity sound energy (ultrasonication). This ultrasonic energy delaminates the layered parent crystals into daughter nanosheets that suspend and swim in the organic solvents to form a stable dispersion of 2D nanomaterials.
In this method, it has always been presumed that the ultrasonic energy bursts would not affect the organic solvent. However, researchers at IIT Gandhinagar came across a surprise. They were extending this method to synthesize dispersions of borophene, a 2D material that is like graphene but made from boron. While conducting the control experiments, doctoral student Saroj Kumar Das made an unexpected observation that the organic solvent itself was transforming into quantum dots − extremely tiny fluorescent nanostructures just 2 nm in diameter.
" This was a surprise finding because scientists till now believed that that the liquid medium or organic solvent remains stable during exposure to sound waves and nothing happens to it. "
Das made this observation when he was shining the dispersion with laser and it exhibited beautiful fluorescent colours, a behaviour that is characteristic to quantum dots. Such a concurrent formation of quantum dots, along with the formation of nanosheets, has not been seen before. So he reported this unexpected result to his advisor Dr. Kabeer Jasuja.
Initially, the team could not accept the outcome, thinking that these quantum dots could have come from possible contamination. However, after conducting several experiments in different setups and verifying their results, the team was able to validate that the organic solvent used during the process itself is transforming into carbon quantum dots.
“This was a surprise finding because scientists till now believed that that the liquid medium or organic solvent remains stable during exposure to sound waves and nothing happens to it. That is the main reason these are used as a dispersing medium for such experiments.
No one has ever suspected that the molecules of organic solvent can transform into carbon quantum dots by the sound energy. This new physical insight would form an important addition to the protocols followed to synthesize nanosheets,” Jasuja said.
To demonstrate the relevance of these results, the research team also revisited protocols that utilize ultrasonication to synthesize other 2D materials. They found that in these protocols, one ends up getting a mixture of carbon quantum dots along with the 2D nanosheets that are originally intended. The findings imply that before drawing inferences about nanosheets formed by such methods, one needs to acknowledge the presence of these quantum dots.
New technological devices are prioritizing non-invasive tracking of vital signs not only for fitness monitoring, but also for the prevention of common health problems such as heart failure, hypertension, and stress related complications, among others. Wearables based on optical detection mechanisms are proving an invaluable approach for reporting on our bodies inner workings and have experienced a large penetration into the consumer market in recent years.
Current wearable technologies, based on non-flexible components, do not deliver the desired accuracy and can only monitor a limited number of vital signs. To tackle this problem, conformable non-invasive optical-based sensors that can measure a broader set of vital signs are at the top of the end-users’ wish list.
In a recent study published in Science Advances ("Flexible graphene photodetectors for wearable fitness monitoring"), ICFO researchers have demonstrated a new class of flexible and transparent wearable devices that are conformable to the skin and can provide continuous and accurate measurements of multiple human vital signs. These devices can measure heart rate, respiration rate and blood pulse oxygenation, as well as exposure to UV radiation from the sun.
While the device measures the different parameters, the read-out is visualized and stored on a mobile phone interface connected to the wearable via Bluetooth. In addition, the device can operate battery-free since it is charged wirelessly through the phone.
“It was very important for us to demonstrate the wide range of potential applications for our advanced light sensing technology through the creation of various prototypes, including the flexible and transparent bracelet, the health patch integrated on a mobile phone and the UV monitoring patch for sun exposure. They have shown to be versatile and efficient due to these unique features”, reports Dr. Emre Ozan Polat, first author of this publication.
The bracelet was fabricated in such a way that it adapts to the skin surface and provides continuous measurement during activity (see Figure 1). The bracelet incorporates a flexible light sensor that can optically record the change in volume of blood vessels, due to the cardiac cycle, and then extract different vital signs such as heart rate, respiration rate and blood pulse oxygenation.
Secondly, the researchers report on the integration of a graphene health patch onto a mobile phone screen, which instantly measures and displays vital signs in real time when a user places one finger on the screen (see Figure 2). A unique feature of this prototype is that the device uses ambient light to operate, promoting low-power-consumption in these integrated wearables and thus, allowing a continuous monitoring of health markers over long periods of time.
ICFO’s advanced light sensing technology has implemented two types of nanomaterials: graphene, a highly flexible and transparent material made of one-atom thick layer of carbon atoms, together with a light absorbing layer made of quantum dots. The demonstrated technology brings a new form factor and design freedom to the wearables’ field, making graphene-quantum-dots-based devices a strong platform for product developers.
Dr. Antonios Oikonomou, business developer at ICFO emphasized this by stating that “The booming wearables industry is eagerly looking to increase fidelity and functionality of its offerings. Our graphene-based technology platform answers this challenge with a unique proposition: a scalable, low-power system capable of measuring multiple parameters while allowing the translation of new form factors into products.”
Dr. Stijn Goossens, co-supervisor of the study, also comments that “we have made a breakthrough by showing a flexible, wearable sensing system based on graphene light sensing components. Key was to pick the best of the rigid and flexible worlds. We used the unique benefits of flexible components for vital sign sensing and combined that with the high performance and miniaturization of conventional rigid electronic components.”
Finally, the researchers have been able to demonstrate a broad wavelength detection range with the technology, extending the functionality of the prototypes beyond the visible range. By using the same core technology, they have fabricated a flexible UV patch prototype (see Figure 3) capable of wirelessly transferring both power and data, and operating battery-free to sense the environmental UV-index. continuous monitoring of health markers over long periods of time.
The patch operates with a low power consumption and has a highly efficient UV detection system that can be attached to clothing or skin, and used for monitoring radiation intake from the sun, alerting the wearer of any possible over-exposure.
“We are excited about the prospects for this technology, pointing to a scalable route for the integration of graphene-quantum-dots into fully flexible wearable circuits to enhance form, feel, durability, and performance”, remarks Prof. Frank Koppens, leader of the Quantum Nano-Optoelectronics group at ICFO. “Such results show that this flexible wearable platform is compatible with scalable fabrication processes, proving mass-production of low-cost devices is within reach in the near future.”
A KAIST team has designed a novel strategy for synthesizing single-crystalline graphene quantum dots, which emit stable blue light. The research team confirmed that a display made of their synthesized graphene quantum dots successfully emitted blue light with stable electric pressure, reportedly resolving the long-standing challenges of blue light emission in manufactured displays. The study, led by Professor O Ok Park in the Department of Chemical and Biological Engineering.
Graphene has gained increased attention as a next-generation material for its heat and electrical conductivity as well as its transparency. However, single and multi-layered graphene have characteristics of a conductor so that it is difficult to apply into semiconductor. Only when downsized to the nanoscale, semiconductor's distinct feature of bandgap will be exhibited to emit the light in the graphene. This illuminating featuring of dot is referred to as a graphene quantum dot.
Conventionally, single-crystalline graphene has been fabricated by chemical vapor deposition (CVD) on copper or nickel thin films, or by peeling graphite physically and chemically. However, graphene made via chemical vapor deposition is mainly used for large-surface transparent electrodes. Meanwhile, graphene made by chemical and physical peeling carries uneven size defects.
The research team explained that their graphene quantum dots exhibited a very stable single-phase reaction when they mixed amine and acetic acid with an aqueous solution of glucose. Then, they synthesized single-crystalline graphene quantum dots from the self-assembly of the reaction intermediate. In the course of fabrication, the team developed a new separation method at a low-temperature precipitation, which led to successfully creating a homogeneous nucleation of graphene quantum dots via a single-phase reaction.
Professor Park and his colleagues have developed solution phase synthesis technology that allows for the creation of the desired crystal size for single nanocrystals down to 100 nano meters. It is reportedly the first synthesis of the homogeneous nucleation of graphene through a single-phase reaction.
Professor Park said, "This solution method will significantly contribute to the grafting of graphene in various fields. The application of this new graphene will expand the scope of its applications such as for flexible displays and varistors."
This research was a joint project with a team from Korea University under Professor Sang Hyuk Im from the Department of Chemical and Biological Engineering, and was supported by the National Research Foundation of Korea, the Nano-Material Technology Development Program from the Electronics and Telecommunications Research Institute (ETRI), KAIST EEWS, and the BK21+ project from the Korean government.
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.
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.
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.
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.”
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.
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 2015, believes 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.”