Scientists from Heidelberg and St Andrews work on the basics of new light sources from organic semiconductors
With their research on nanomaterials for optoelectronics, scientists from Heidelberg University and the University of St Andrews (Scotland) have succeeded for the first time to demonstrate a strong interaction of light and matter in semiconducting carbon nanotubes. Such strong light-matter coupling is an important step towards realising new light sources, such as electrically pumped lasers based on organic semiconductors. They would be, amongst other things, important for applications in telecommunications. These results are the outcome of a cooperation between Prof. Dr Jana Zaumseil (Heidelberg) and Prof. Dr Malte Gather (St Andrews), and have been published in “Nature Communications”.
Organic semiconductors based on carbon are a cost and energy-efficient alternative to conventional inorganic semiconductors such as silicon. Light-emitting diodes consisting of these materials are already ubiquitously found in smartphone displays. Further components for application in lighting technology, data transmission and photovoltaics are currently at the prototype stage. So far, however, it has not been possible to produce one important component of optoelectronics with organic materials – the electrically pumped laser. The main reason is that organic semiconductors have only limited capacity for charge transport.
Prof. Zaumseil explains that research over the past few years has increasingly focused on laser-like light emission of organic semiconductors based on light-matter coupling. If photons (light) and excitons (matter) are brought to interact sufficiently, they couple so strongly that they produce so called exciton-polaritons. These are quasi-particles that also emit light. Under certain conditions, such emissions can take on the properties of laser light. Combined with sufficiently fast charge transport, exciton-polaritons could bring the production of an electrically pumped carbon-based laser within reach, according to Jana Zaumseil who is the head of the Nanomaterials for Optoelectronics research group at the Heidelberg University’s Institute for Physical Chemistry.
Thanks to the cooperation between Prof. Zaumseil and Prof. Gather, it was possible for the first time to demonstrate the formation of exciton-polaritons in semiconducting carbon nanotubes. Unlike other organic semiconductors, these microscopically small, tube-shaped carbon structures transport positive and negative charges extremely well. PhD student Arko Graf, the first author of the study, explains that exciton-polaritons also display extraordinary optical properties. The scientists in Heidelberg and St Andrews see their research results as an important step towards realising electrically pumped lasers on the basis of organic semiconductors. Prof. Zaumseil emphasises: “Besides the potential generation of laser light, exciton-polaritons already allow us to vary the wavelength of the light emitted by the carbon nanotubes over a wide range in the near-infrared.”
Researchers at Queen’s University Belfast and ETH Zurich, Switzerland, have created a new theoretical framework which could help physicists and device engineers design better optoelectronics, leading to less heat generation and power consumption in electronic devices which source, detect, and control light.
Speaking about the research, which enables scientists and engineers to quantify how transparent a 2D material is to an electrostatic field, Dr Elton Santos from the Atomistic Simulation Research Centre at Queen’s, said: “In our paper we have developed a theoretical framework that predicts and quantifies the degree of ‘transparency’ up to the limit of one-atom-thick, 2D materials, to an electrostatic field.
“Imagine we can change the transparency of a material just using an electric bias, e.g. get darker or brighter at will. What kind of implications would this have, for instance, in mobile phone technologies? This was the first question we asked ourselves. We realised that this would allow the microscopic control over the distribution of charged carriers in a bulk semiconductor (e.g. traditional Si microchips) in a nonlinear manner. This will help physicists and device engineers to design better quantum capacitors, an array of subatomic power storage components capable to keep high energy densities, for instance, in batteries, and vertical transistors, leading to next-generation optoelectronics with lower power consumption and dissipation of heat (cold devices), and better performance. In other words, smarter smart phones.”
Explaining how the theory could have important implications for future work in the area, Dr Santos added: “Our current model simply considers an interface formed between a layer of 2D material and a bulk semiconductor. In principle, our approach can be readily extended to a stack of multiple 2D materials, or namely, van der Waals heterostructures recently fabricated. This will allow us to design and predict the behaviour of these cutting-edge devices in prior to actual fabrication, which will significantly facilitate developments for a variety of applications. We will have an in silico search for the right combination of different 2D crystals while reducing the need for expensive lab work and test trials.”
Further information on the Atomistic Simulation Research Centre at Queen’s is available online at http://titus.phy.qub.ac.uk/
As an important step towards graphene integration in silicon photonics, researchers from the Graphene Flagship have published a paper which shows how graphene can provide a simple solution for silicon photodetection in the telecommunication wavelengths.
Published in Nano Letters, this exciting research is a collaboration between the University of Cambridge (UK), The Hebrew University (Israel) and Johns Hopkins University (USA).
The mission of the Graphene Flagship is to translate graphene out of the academic laboratory, through industry and into society. This broad and ambitious aim has been at the forefront of the choices made to direct the Flagship; it focuses on real problem areas where it can make a real difference such as in Optical Communications.
Optical Communications are increasingly important because they have the potential to solve one of the biggest problems of our information age: energy consumption. Almost everything we do in everyday life consumes information and all of this information is powered by energy. If we want more and more information, we need more and more energy. In the near future, the major consumers of data traffic will be machine-to-machine communication and the Internet of Things (IoT).
To enable the IoT and the level of information it requires, current silicon photonics has a problem: it needs ten times more energy than we can provide. So, if we want this new, improved internet age, new technological, power-efficient solutions need to be found. This is why the drive to graphene-based optical communication is so important.
Over the last few years, optical communications have increased their viability over standard metal-based electronic interconnects. The current silicon-based photodetector used in optical communications has a major issue when it comes to detecting data in the near infrared range, which is the range used for telecommunications. The telecom industry has overcome this problem by integrating germanium absorbers with the standard silicon photonic devices. They have been able to make fully functioning devices on chips using this process. However, this process is complex.
In the new paper, graphene is interfaced with silicon on chip to make high responsivity Schottky barrier photodetectors. These graphene-based photodetectors achieve 0.37A/W responsivity at 1.55μm using avalanche multiplication. This high responsivity is comparable to that of the Silicon Germanium detectors currently used in silicon photonics.
Prof. Andrea Ferrari from the Cambridge Graphene Centre, who is also the Science and Technology Officer and the Chair of the Management Panel for the Graphene Flagship stated; “This is a significant result which proves that graphene can compete with the current state of the art by producing devices that can be made more simply, cheaply and work at different wavelengths. Thus paving the way for graphene integrated silicon photonics.”
In the quest to harvest light for electronics, the focal point is the moment when photons — light particles — encounter electrons, those negatively-charged subatomic particles that form the basis of our modern electronic lives. If conditions are right when electrons and photons meet, an exchange of energy can occur. Maximizing that transfer of energy is the key to making efficient light-captured energetics possible.
“This is the ideal, but finding high efficiency is very difficult,” said University of Washington physics doctoral student Sanfeng Wu. “Researchers have been looking for materials that will let them do this — one way is to make each absorbed photon transfer all of its energy to many electrons, instead of just one electron in traditional devices.”
In traditional light-harvesting methods, energy from one photon only excites one electron or none depending on the absorber’s energy gap, transferring just a small portion of light energy into electricity. The remaining energy is lost as heat. But in a paper released May 13 in Science Advances, Wu, UW associate professor Xiaodong Xu and colleagues at four other institutions describe one promising approach to coax photons into stimulating multiple electrons. Their method exploits some surprising quantum-level interactions to give one photon multiple potential electron partners. Wu and Xu, who has appointments in the UW’s Department of Materials Science & Engineering and the Department of Physics, made this surprising discovery using graphene.
“Graphene is a substance with many exciting properties,” said Wu, the paper’s lead author. “For our purposes, it shows a very efficient interaction with light.”
Graphene is a two-dimensional hexagonal lattice of carbon atoms bonded to one another, and electrons are able to move easily within graphene. The researchers took a single layer of graphene — just one sheet of carbon atoms thick — and sandwiched it between two thin layers of a material called boron-nitride.
“Boron-nitride has a lattice structure that is very similar to graphene, but has very different chemical properties,” said Wu. “Electrons do not flow easily within boron-nitride; it essentially acts as an insulator.”
Xu and Wu discovered that when the graphene layer’s lattice is aligned with the layers of boron-nitride, a type of “superlattice” is created with properties allowing efficient optoelectronics that researchers had sought. These properties rely on quantum mechanics, the occasionally baffling rules that govern interactions between all known particles of matter. Wu and Xu detected unique quantum regions within the superlattice known as Van Hove singularities.
“These are regions of huge electron density of states, and they were not accessed in either the graphene or boron-nitride alone,” said Wu. “We only created these high electron density regions in an accessible way when both layers were aligned together.”
When Xu and Wu directed energetic photons toward the superlattice, they discovered that those Van Hove singularities were sites where one energized photon could transfer its energy to multiple electrons that are subsequently collected by electrodes— not just one electron or none with the remaining energy lost as heat. By a conservative estimate, Xu and Wu report that within this superlattice one photon could “kick” as many as five electrons to flow as current.
With the discovery of collecting multiple electrons upon the absorption of one photon, researchers may be able to create highly efficient devices that could harvest light with a large energy profit. Future work would need to uncover how to organize the excited electrons into electrical current for optimizing the energy-converting efficiency and remove some of the more cumbersome properties of their superlattice, such as the need for a magnetic field. But they believe this efficient process between photons and electrons represents major progress.
“Graphene is a tiger with great potential for optoelectronics, but locked in a cage,” said Wu. “The singularities in this superlattice are a key to unlocking that cage and releasing graphene’s potential for light harvesting application.”
Epitaxy, or growing crystalline film layers that are templated by a crystalline substrate, is a mainstay of manufacturing transistors and semiconductors.
If the material in one deposited layer is the same as the material in the next layer, it can be energetically favorable for strong bonds to form between the highly ordered, perfectly matched layers. In contrast, trying to layer dissimilar materials is a great challenge if the crystal lattices don’t match up easily. Then, weak van der Waals forces create attraction but don’t form strong bonds between unlike layers.
In a study led by the Department of Energy’s Oak Ridge National Laboratory, scientists synthesized a stack of atomically thin monolayers of two lattice-mismatched semiconductors. One, gallium selenide, is a “p-type” semiconductor, rich in charge carriers called “holes.” The other, molybdenum diselenide, is an “n-type” semiconductor, rich in electron charge carriers. Where the two semiconductor layers met, they formed an atomically sharp heterostructure called a p–n junction, which generated a photovoltaic response by separating electron–hole pairs that were generated by light. The achievement of creating this atomically thin solar cell, published in Science Advances,shows the promise of synthesizing mismatched layers to enable new families of functional two-dimensional (2D) materials.