Watching Electrons Cool in 30 Quadrillionths of a Second

Two University of California, Riverside assistant professors of physics are among a team of researchers that have developed a new way of seeing electrons cool off in an extremely short time period.

The development could have applications in numerous places where heat management is important, including visual displays, next-generation solar cells and photodetectors for optical communications.

In visual displays, such as those used in cell phones and computer monitors, and photodetectors, which have a wide variety of applications including solar energy harvesting and fiber optic telecommunications, much of the energy of the electrons is wasted by heating the material.

Controlling the flow of heat in the electrons, rather than wasting this energy by heating the material, could potentially increase the efficiency of such devices by converting excess energy into useful power.

The research is outlined in a paper, “Tuning ultrafast electron thermalization pathways in a van der Waals heterostructure,” published online Monday (Jan. 18) in the journal Nature Physics. Nathan Gabor and Joshua C.H. Lui, assistant professors of physics at UC Riverside, are among the co-authors.

In electronic materials, such as those used in semiconductors, electrons can be rapidly heated by pulses of light.  The time it takes for electrons to cool each other off is extremely short, typically less than 1 trillionth of a second.

To understand this behavior, researchers use highly specialized tools that utilize ultra-fast laser techniques. In the two-dimensional material graphene cooling excited electrons occurs even faster, taking only 30 quadrillionths of a second. Previous studies struggled to capture this remarkably fast behavior.

To solve that, the researchers used a completely different approach. They combined single layers of graphene with thin layers of insulating boron nitride to form a sandwich structure, known as a van der Waals heterostructure, which gives electrons two paths to choose from when cooling begins. Either the electrons stay in graphene and cool by bouncing off one another, or they get sucked out of graphene and move through the surrounding layer.

By tuning standard experimental knobs, such as voltage and optical pulse energy, the researchers found they can precisely control where the electrons travel and how long they take to cool off. The work provides new ways of seeing electrons cool off at extremely short time scales, and demonstrates novel devices for nanoscale optoelectronics.

This structure is one of the first in a new class of devices that are synthesized by mechanically stacking atomically thin membranes. By carefully choosing the materials that make up the device, the researchers developed a new type of optoelectronic photodetector that is only 10 nanometers thick. Such devices address the technological drive for ultra-dense, low-power, and ultra-efficient devices for integrated circuits.

The research follows advances made in 2011 Science article, in which the research team discovered the fundamental importance of hot electrons in the optoelectronic response of devices based on graphene.

Other co-authors of the Nature Physics paper are: Qiong Ma, Trond I. Andersen, Nityan L. Nair, Andrea F. Young, Wenjing Fang, Jing Kong, Nuh Gedik and Pablo Jarillo-Herrero, all of the Massachusetts Institute of Technology; Mathieu Massicotte and Frank H. L. Koppens, both of The Institute of Photonic Sciences in Spain; and Kenji Watanabe and Takashi Taniguchi, both of the National Institute for Materials Science in Japan.

University of California, Riverside

Clearing the way for Floquet-Bloch states

Snapshots of the full 3-D band structure of bismuth selenide as it is radiated by light. The coherent interaction between the time-periodic potential of the light pulse and electrons in the Dirac cone results in Floquet-Bloch states, which appear as replicas of the original Dirac cone. Dynamic gaps open up at positions where the replica cones intersect the original cone. Intensity of these replicas is governed by the interference between Floquet-Bloch and Volkov states, free electron-like states coupled with photons. Replica bands disappear once the driving potential shuts off. Wave-like white lines (bottom) represent the laser pulse. Image courtesy of Fahad Mahmood.

Team demonstrates pure quantum-mechanical mixture of electrons and photons in the solid topological insulator bismuth selenide.

In 2013, MIT physicists showed for the first time that shining powerful mid-infrared laser light on solid bismuth selenide produces Floquet-Bloch states, which are characterized by replicas of electronic energy states inside a solid with gaps opening up at crossing points of replica states. The same external light also interacts with free electron states immediately outside the solid producing a competing state, called the Volkov state, which is gapless.

Now, researchers led by Nuh Gedik, the Lawrence C. (1944) and Sarah W. Biedenharn Career Development Associate Professor of Physics, have shown that changing the light’s polarization eliminates competition from Volkov states, yielding pure Floquet-Bloch states.

MIT graduate student Fahad Mahmood and postdoc Ching Kit (Chris) Chan, demonstrate experimental proof and offer a mathematical framework for understanding interference between these competing states as a function of electron momentum. The results are published online in Nature Physics.

“Fahad figured out a clever way of quantifying the interference of these two states with each other, and then from this interference, we can deduce selectively, this part is coming from the outside, this part is coming from inside,” says Gedick, who is senior author on the new work. “I think this is a big step because if you eventually want to realize a new state of matter based on periodic excitation, you really need to be able to isolate just the contribution of the electrons inside the solid.”

MIT co-authors of the study are Zhanybek Alpichshev, a postdoc in Gedik’s group; recent physics alumnus Dillon R. Gardner PhD ’15; Professor Young S. Lee; and William and Emma Rogers Professor of Physics Patrick A. Lee.

Proportional bandgap

Floquet-Bloch states, occurring on incredibly fast time scales, are observed using an experimental technique called time-and-angle-resolved photoemission spectroscopy (Tr-ARPES). This consists of using a mid-IR laser pulse, with energy below the bulk band gap of the material, to stimulate electrons in the solid. A second laser pulse, at a lesser intensity, overlaps the first and leads to emission of electrons, which are collected in a time-of-flight analyzer that records their angle of emission and energy. “We study the photoemitted electron intensity as a function of the electron energy and momentum,” Gedik says. “If you’re using this technique, it’s super important to be able to selectively, only, probe the photo-excited state inside the solid, and this paper gives you a way,” Gedik explains.

The researchers also show that the value of the artificially induced bandgap is proportional to the square root of the intensity of the light. “That really means it is proportional to the electric field, rather than the intensity, so this is what is actually expected from all this theory work that this gap would actually scale with the electric field of the light,” Gedik says.

The new work yields higher resolution for experimental demonstrations of replica electronic levels, which are also called sidebands, and also offers a theoretical explanation for distinguishing Floquet states from Volkov states. “It was very helpful to have Patrick Lee and his postdoc [Chan] help with the theory. I think it would have been very challenging without their help,” Mahmood says. “That close collaboration helped us a lot because now we can make predictions of exactly how the interference would behave as a function of this electron momentum and you can see that theory and experiment match really well for different directions of electron momentum.”

The electronic state on the surface of the topological insulator bismuth selenide is graphically represented by a Dirac cone (left), which plots energy versus momentum for electrons. Stimulation by laser pulses results in replica cones (right), which represent hybrid photon-electron states known as Floquet-Bloch states. Dynamic gaps open up at positions where the replica cones intersect the original cone. These states, occurring on incredibly fast time scales, are observed using an experimental technique called time-and-angle-resolved photoemission spectroscopy. Wave-like white lines (center) represent the laser pulse. Image courtesy of Fahad Mahmood.

Hybrid states

Interest in graphene and topological insulators such as bismuth selenide triggered a quest for ways to artificially stimulate a bandgap in them so they could be used for electronic devices. Gedik’s recent work focuses on using periodic light bursts to accomplish this. Although the electrons don’t absorb the light, they react to the electric field by forming a hybrid, or dressed, state between electrons and photons. This effect only occurs when both electrons and photons are present together, and it changes the electronic energy levels inside the material. Changing electronic properties through chemical processes, such as inserting another element into the solid, are usually permanent, but changing the properties of electrons with light excitation is reversible and controllable. “You can just change the intensity of the light, the frequency of the light, or polarization of the light,” Gedik says.

Lee, who contributed to the theoretical work in the new paper, says, “This work represents an important experimental progress in creating novel laser-driven electronic states, called Floquet states, on the surface of materials. We found that the traditional interpretation of the data was not adequate because of extrinsic effects not considered previously. Once this was understood, we found a tunable way to manipulate these Floquet states. The result may aid in the design of light-controllable electronic devices.”

Experimental challenges

To generate the light-driven state, Mahmood used mid-IR light pulses that diverge and lose power rapidly as they propagate. Guiding these pulses onto a small sample in a vacuum chamber while maximizing their power required special optical setups developed in the Gedik Lab. The time-of-flight analyzer is also relatively new requiring careful alignment. “We had to make sure that our beam was focused in such a way to get the best resolution out of our instrument,” Mahmood says.

The results show that the interference between Floquet-Bloch and Volkov states depends strongly on the polarization of the light. This polarization determines the orientation of the electric field with respect to the sample plane. Since the Volkov state is primarily activated by an out-of-plane electric field, switching the light polarization to one with only an in-plane electric field eliminates the Volkov state.

“This is a very challenging experiment, as it requires performing detailed photoelectron spectroscopy measurements on a transient photo-driven state that lasts merely fractions of a picosecond,” says David Hsieh, assistant professor of physics at Caltech. “The Gedik group has succeeded not only in directly visualizing the spectrum of this transient state, but also in quantifying and controlling the electromagnetic coupling between different energy bands of the transient state. This provides an interesting opportunity to use optical excitation as a means to engineer a desired materials response.”

Future possibilities

It may be a bit of science fiction at this point, but one possibility, according to Gedik, is using this technology to spatially pattern the light excitation which could lead to devices with customized geometries. To get there, he says, “We need to do this basic work, which is to understand what these states are and how they interact with other states.” Mahmood continues to study how long it takes for these Floquet states to form and to disappear. “All of this needs to be understood before we can move onto there,” Gedik says.

Work also remains to be done to understand how the electrons relax down from the Floquet states. It is especially important to understand how they interact with other states inside the solid such as vibrations of the lattice called phonons or collective excitations of the electrons called plasmons.

MIT

Nanotechnology World Association

2D Islands in Graphene Hold Promise for Future Device Fabrication

Berkeley Lab Scientists Discovery Could Help Improve Graphene Electronics

 

In what could prove to be a significant advance in the fabrication of graphene-based nanodevices, a team of Berkeley Lab researchers has discovered a new mechanism for assembling two-dimensional (2D) molecular “islands” that could be used to modify graphene at the nanometer scale. These 2D islands are comprised of F4TCNQ molecules that trap electrical charge in ways that are potentially useful for graphene-based electronics.

“We’re reporting a scanning tunneling microscopy and non-contact atomic force microscopy study of F4TCNQ molecules at the surface of graphene in which the molecules coalesce into 2D close-packed islands,” says Michael Crommie, a physicist who holds joint appointments with Berkeley Lab’s Materials Sciences Division and UC Berkeley’s Physics Department. “The resulting islands could be used to control the charge-carrier density in graphene substrates, as well as to modify how electrons move through graphene-based devices. They might also be used to form precise nanoscale patterns that exhibit atomic-scale structural perfection unmatched by conventional fabrication techniques.”

Crommie is one of four corresponding authors of a paper describing this research published byACS Nano. The paper is titled “Molecular Self-Assembly in a Poorly Screened Environment: F4TCNQ on Graphene/BN.” The other corresponding authors are Steven Louie and Marvin Cohen, both with Berkeley Lab and UC Berkeley, and Jiong Lu of the National University of Singapore.

Graphene is a sheet of pure carbon just one atom thick through which electrons speed 100 times faster than they move through silicon. Graphene is also slimmer and stronger than silicon, making it a potential superstar material for the electronics industry. However, graphene must be electrically doped to tune the number of charge carriers it contains in order to be useful in devices, and F4TCNQ has proven to be an effective dopant for transforming graphene into a “p-type” semiconductor.

“F4TCNQ is known to extract electrons from a substrate, thus changing the substrate charge-carrier density,” Crommie says. “Previous studies looked at F4TCNQ adsorbed on graphene supported by a metal substrate, which creates a highly screened environment. F4TCNQ adsorbed on graphene supported by the insulator boron nitride (BN) creates a poorly screened environment. We found that, unlike with metals, F4TCNQ molecules on graphene/BN form 2D islands by a unique self-assembly mechanism that is driven by the long-range Coulomb interactions between the charged molecules. Negatively-charged molecules coalesce into an island, increasing the local work function above the island and causing additional electrons to flow into the island. These additional electrons cause the total energy of the graphene layer to decrease, resulting in island cohesion.”

Crommie and his co-authors believe that this 2D island formation mechanism should also apply to other molecular adsorbate systems that exhibit charge transfer in poorly screened environments, thereby opening the door to tuning the properties of graphene layers for device applications.

In addition to Crommie, Louie,Cohen and Lu, other co-authors of  ACS Nano paper were Hsin-Zon Tsai, Arash Omrani, Sinisa Coh, Hyungju, Sebastian Wickenburg, Young-Woo Son, Dillon Wong, Alexander Riss, Han Sae Jung, Giang Nguyen, Griffin Rodgers, Andrew Aikawa, Takashi Taniguchi, Kenji Watanabe and Alex Zettl.

Berkeley Lab

Graphene nanoribbons get metallic

Researchers at Aalto University have succeeded in experimentally realizing metallic graphene nanoribbons that are only 5 carbon atoms wide.

The results suggest that these extremely narrow and single-atom-thick ribbons could be used as metallic interconnects in future microprocessors. In their article published in Nature Communications, the research team demonstrated fabrication of the graphene nanoribbons (GNR) and measured their electronic structure.

Graphene nanoribbons have been suggested as ideal wires for use in future nanoelectronics: when the size of the wire is reduced to the atomic scale, graphene is expected to outperform copper in terms of conductance and resistance to electromigration, which is the typical breakdown mechanism in thin metallic wires. However, all demonstrated graphene nanoribbons have been semiconducting, which hampers their use as interconnects. Headed by Prof. Peter Liljeroth, researchers from the Atomic Scale Physics and Surface Science groups have now shown experimentally that certain atomically precise graphene nanoribbon widths are nearly metallic, in accordance with earlier predictions based on theoretical calculations.

The team used state-of-the-art scanning tunneling microscopy (STM) that allows them to probe the material’s structure and properties with atomic resolution.

– With this technique, we measured the properties of individual ribbons and showed that ribbons longer than 5 nanometers exhibit metallic behaviour, says Dr Amina Kimouche, the lead author of the study.

The nanoribbon fabrication is based on a chemical reaction on a surface.

– The cool thing about the fabrication procedure is that the precursor molecule exactly determines the width of the ribbon. If you want one-carbon-atom-wide ribbons, you simply have to pick a different molecule, explains Dr Pekka Joensuu, who oversaw the synthesis of the precursor molecules for the ribbons.

The experimental findings were complemented by theoretical calculations by the Quantum Many-Body Physics group headed by Dr Ari Harju. The theory predicts that when the width of the ribbons is increased atom-by-atom, every third width should be (nearly) metallic with a very small band gap.

– According to quantum mechanics, normally when you make your system smaller, it increases the band gap. Graphene can work differently due to its extraordinary electronic properties, says Harju’s doctoral student Mikko Ervasti, who performed the calculations.

These results pave the way for using graphene in future electronic devices, where these ultra-narrow ribbons could replace copper as the interconnect material. Future studies will focus on all-graphene devices combining both metallic and semiconducting graphene nanostructures.

– While we are far from real applications, it is an extremely exciting concept to build useful devices from these tiny structures and to achieve graphene circuits with controlled junctions between GNRs, says Liljeroth.

The research “Ultra-narrow metallic armchair graphene nanoribbons” was published in Nature Communications Science publication.

The study was performed at Aalto University’s Department of Applied Physics and Department of Chemistry. The groups are part of the Academy of Finland’s Centres of Excellence in Low Temperature Quantum Phenomena and Devices (LTQ) and Computational Nanosciences (COMP). The Academy of Finland and the European Research Council ERC funded the research.

Aalto University

Graphene partnership could deliver next generation of aircraft

Graphene composite-based planes could be a step closer to reality as a result of a partnership between The University of Manchester and a leading Chinese aviation company.

 

Graphene composites could deliver the next generation of aeroplanes

Beijing Institute of Aeronautical Materials（BIAM）has become the latest partner of the National Graphene Institute (NGI), based at the University, to promote the research and development in graphene reinforced aluminium matrix composites areas, and accelerate the application of graphene in the aviation industry and other sectors.

The NGI has around 50 industrial partners working collaboratively on a range of potential applications. The Institute has a number of collaborations with Chinese companies, and in October hosted President Xi Jinping as part of his state visit to the UK.

For this project, BIAM and the University, under the leadership of Professor Robert Young, will exchange expertise and cooperate on the structural characterisation and property testing of graphene reinforced aluminium matrix nanocomposites. 

The projects could result in lighter, stronger and conductive parts for aeroplanes, high speed trains and other industrial equipment to replace traditional materials. With graphene added, the strength of aluminium alloy could be highly improved while retaining its ductile quality. 

The collaboration could improve the technology maturity and accelerate the updating of aeroplane structural materials and functional materials.

In the future, the two parties expect to expand the collaboration on graphene materials. Besides the graphene reinforced aluminium matrix composites, both parties will seek collaboration in areas of graphene energy storage materials, environmental purification materials and information materials.

Dai Shenglong, President of BIAM, said: “Graphene is expected to revolutionize the technology sector. Graphene and its products have unique properties and show wide prospects for applications. 

“BIAM has obtained achievements in the areas of graphene modified structural materials and functional materials, the preparation process and large scale production of graphene nanoflakes and graphene films, and has achieved some extensive application experiences in the aviation industry. 

“Now the collaboration with The University of Manchester will give full play to both sides’ favorable conditions, enhance fundamental research level, improve the technology maturity, and promote together the development of graphene and its products.”

James Baker, Business Director at The University of Manchester, said: “We are delighted to be working with BIAM on collaborative graphene applications.

“Aerospace and other transport areas show great potential for graphene activity and The University of Manchester is leading the way in these areas and many others. Partnering with companies like BIAM gives up the opportunity to accelerate products to market and turn fundamental research into applications of the future.” 

BIAM mainly focuses on the engineering application of graphene modified structural materials and functional materials, and the R&D of graphene related new products and exploitation. 

Manchester National Graphene Institute

Molecular nanoribbons as electronic highways

Physicists at Umeå University have, together with researchers at UC Berkeley, USA, developed a method to synthesise a unique and novel type of material which resembles a graphene nanoribbon but in molecular form. This material could be important for the further development of organic solar cells. The results have been published in the scientific journal ACS Nano.

The nanoribbons are comprised of molecules with the chemical formula [6,6]-phenyl-C61-butyric acid methyl ester. For short it is denoted PCBM, and in practice it is a fullerene molecule (a football-shaped carbon molecule) with an attached side arm to increase its solubility. PCBM molecules are commonly used in organic solar cells since they have a very good ability to transport free electrons that are “generated” by solar light.

The researchers at Umeå University and UC Berkeley have now developed a method to arrange such molecules into thin, crystalline nanoribbons that are only four nanometres wide. The nanoribbons are grown in a solution process with quite high efficiency and all nanoribbons have a unique morphology with edges in a zigzag.

“It is a very intriguing material and the method is quite simple. The material resembles the more commonly known graphene nanoribbons, but in our material each carbon atom is ‘replaced’ by a molecule,” says Thomas Wågberg, associate professor at the Department of Physics, who has led the study.

The findings are interesting for several reasons; it is the first time that structures with so small dimensions have been produced with this type of molecule, and the dimensions of the nanoribbons suggest that they should be ideal as “electronic highways” in organic solar cells. An organic solar cell usually consists of two types of material, one that conducts the electrons and one that conducts the “holes” that are left behind when the electron gets an energy boost from the incoming solar light (you can see the transport of “hole” as an empty space in traffic moving backwards in a traffic queue moving forwards).

An electron conductor in organic solar cells should ideally form long pathways to the electrode but concurrently be thinner than 10-15 nanometres (approximately 10,000 times thinner than a normal hair). The newly developed PCBM nanoribbons fulfil all these requirements.

“Together with professor Ludvig Edman’s group at the Department of Physics at Umeå University, we are now investigating this material further as a potential component in organic solar cells in the hope of making such devices more efficient,” says Thomas Wågberg.

Our study is of course also interesting for fundamental reasons since it opens up possibilities to investigate important physical properties of molecular materials with nanoscale dimensions.

About nanoribbons:

 

Carbon nanostructures exist in many different forms. Graphene is a single layer of carbon atoms, which can be “ripped” into nanoribbons under certain circumstances. Due to the decrease in dimensions along one direction, the graphene nanoribbons display many unique properties.

Fullerenes on the other hand are football-shaped molecules also built up from carbon atoms, while PCBM are fullerene-like molecules with several interesting properties and an attached side arm to increase their solubility. In the current study the researchers have been able to construct nanoribbons comprising PCBM molecules instead of carbon atoms, so that the structure strongly resembles a graphene nanoribbon in molecular form.

Umeå University

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