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Two-dimensional Materials Are Changing the Landscape for Optoelectronics
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Two-dimensional Materials Are Changing the Landscape for Optoelectronics


Graphene and other 2D materials are offering some big improvements to optoelectronic devices


Optoelectronics may have come a little later than electronics in the list of applications for graphene and other two-dimensional (2D) materials, but their attractive properties in this field have made up for whatever time it may have lost in not being at the top of the list. 


Graphene’s properties as a broadband absorber are off the charts, and make it ideal for enabling photodetection of the visible, infrared and terahertz frequencies. In addition, graphene’s inherent photoresponse speed is very high and when methods were developed to make its photosensivity even higher by combining it with other nanomaterials, its future in optoelectronics looked bright.


In fact, the property that was a mark against it in electronics, namely it lacking an inherent band gap, was thought to be a benefit in optoelectronics.


Two-dimensional Material Combined with Quantum Dots Could Change the Photonics


Now researchers at the University of Rochester have combined the 2D material tungsten diselenide with the other darling of optoelectronics: quantum dots, which could lead to a new era in optoelectronics.


The Rochester team claims that the work it has done with tungsten diselenide represents the first time that 2D materials have produced optically active quantum dots. Quantum dots are nanoscale semiconductor crystals that are sometimes referred to as “artificial atoms” because like atoms when they absorb the right amount of energy they give off that energy as colored light.


This combination of a 2D material and quantum dots could lead to the integration of quantum photonics with solid-state electronics, resulting in a new way to produce so-called integrated photonics, according to the researchers. 


The method for producing this combination involved placing layers of tungsten diselenide on top of each other, which created defects in the layers that led to the fabrication of the quantum dots. These quantum dots—the defects in the tungsten diselenide layers—did not adversely impact the electrical or optical performance of the semiconductor. They also discovered that if they applied an electrical or magnetic field to the material, they could control the electrical and optical properties of the quantum dots.


"What makes tungsten diselenide extremely versatile is that the color of the single photons emitted by the quantum dots is correlated with the quantum dot spin," said Chitraleema Chakraborty, one of the authors of the Nature Nanotechnology paper, in a press release.


Graphene Enables 3D Imagining on a Mobile Device


While it might be difficult to envision what the future of integrated photonics might look like, it’s a lot easier to imagine your mobile device providing you with 3D imaging. That’s what researchers at Swinburne University of Technology in Australia are proposing by using graphene. 


“Our technique can be leveraged to achieve compact and versatile optical components for controlling light,” said Min Gu, director of Swinburne’s Centre for Micro-Photonics, in a press release. “We can create the wide angle display necessary for mobile phones and tablets.”


The Swinburne scientists developed a method of using graphene oxide that has been reduced back towards pure graphene (dubbed “reduced graphene oxide”) that could create wide-angle and full-color 3-D images using a single femtosecond pulsed laser beam.


While previous research has been to get reduced graphene oxide to produce these 3D optical effects, it did not really produce images with optimal contrast since it involved very high temperatures in the thermal treatment process.


In the process developed by the Swinburne researchers, which is described in the journal Nature Communications, it became possible to change the refractive index of the reduced graphene oxide. It’s that change that makes it possible to record individual pixels used in the holograms that can be seen by the naked eye.


“If you can change the refractive index you can create lots of optical effects,” said Gu in the release. “Owing to its atomic layer thickness and high mechanical strength, the use of graphene in mobile display units for flat two-dimensional displays is burgeoning. Our technology could also underpin future flexible and wearable display devices and transform them for 3D display.”


Graphene Pushes Photodetectors Towards the Threshold of Teraherz Speeds


The last edition of our newsletter had an interview with Frank Koppens at the Institute of Photonic Sciences (ICFO) in Barcelona, Spain. Koppens and his team at ICFO are again in the news with their demonstration of graphene-based ultrafast photodetector that can convert absorbed light into an electrical voltage at speeds of less than 50 femtoseconds. It may not illustrate it much better, but a femtosecond is a thousandth of a millionth of a millionth of a second.


To achieve these unimaginable speeds, the researchers addressed the problem that has plagued photothermoelectric devices, namely charge-carrier-cooling times that limit their switching speeds.


To understand what charge-carrier-cooling times are you need to understand a little bit of what happens when light hits graphene layers that have each been chemically altered, called doping. When the light hits these layers, it creates an excitation that generates electron-hole pairs and a photovoltage. After the electron-hole pairs are produced, the electrons start heating up and start scattering between the charge carriers. Finally, the electrons cool and this period of cooling is the charge-carrier-cooling times.


The problem has been that this cooling time for the conversion of light to an electric voltage happens in picoseconds, which means that it limits the switching rates for a photothermoelectric device to a few hundred gigahertz.


In research published in the journal Nature Nanotechnology, the cooling time has been shortened from picoseconds to femtoseconds, opening up the potential for reaching terahertz switching speeds using graphene-based photothermoelectric devices.


Koppens and his colleagues recognized that the key of graphene that allows this faster electron cooling is the efficient interaction between all its conduction band carriers. This property allowed for the rapid creation of an electron distribution in the material that gave its electrons a higher temperature. So when light hits graphene, its electrons already have a higher temperature, which means that when the light hits graphene it is very quickly turn into electron heat. The electron heat is then converted to a voltage and the whole process is nearly instantaneous. 


“Graphene photodetectors show fascinating performance and properties, enabling a wide range of applications,” said Koppens in an article published by the Graphene Flagship. “Ranging from multi-spectral imaging to ultra-fast communications, such applications are being actively developed within the Graphene Flagship programme.”


Graphene Quantum Dots Move to Optoelectronic Applications


We reported on work last year out of Rice University in which graphene was used to produce quantum dots. At the time, the target application was to replace platinum as a catalyst in fuel cells.



While platinum catalysts are an expensive element for fuel cells, they are certainly not the largest obstacle for the wider adoption of fuel cells. 


With this in mind, the Rice researchers looked again at their graphene quantum dots (GQDs) and developed a simple manufacturing technique that can sort out the GQDs according to their size—and therefore, their semiconducting properties.


If you want to make an optoelectronic device based on fluorescence, it is critical that you have a high level of control over the sorting process.


The researchers achieved this control through carefully monitoring the reaction temperature in the oxidation process that turned coal into the quantum dots. By controlling the temperature they could control the size of the dots they produced.


The size of the dot is critical since it determines which wavelength, or color, is absorbed. Smaller dots emit green light, while the larger dots emit light in the orange to red range. The Rice team discovered that the smallest quantum dots, which emit blue light, were the easiest for them to produce from coal.


Based on this work, the Rice researchers are discussing potential applications for GQDs metal or chemical detectors that let engineers tune the fluorescence of the quantum dots so the devices can avoid interference with the target materials’ emissions.