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

Nanoelectronics Engineers Develop Transistor that Overcomes Fundamental Power Limitations

A new atomically-flat transistor developed by UC Santa Barbara engineers overcomes one of the fundamental limitations of conventional transistors and reduces power dissipation by over 90 percent

One of the greatest challenges in the evolution of electronics has been to reduce power consumption during transistor switching operation. In a study recently reported in Nature, engineers at UC Santa Barbara, in collaboration with Rice University, have demonstrated a new transistor that switches at only 0.1 volts and reduces power dissipation by over 90% compared to state-of-the-art silicon transistors (MOSFETs).

MOSFETs have been the building blocks of everyday electronic products since the 1970s. However, to sustain the ever-growing need for increased transistor densities, miniaturization of MOSFETs has given rise to a power dissipation challenge due to the fundamental limitations of their turn-on characteristics.

"The steepness of a transistor's turn-on is characterized by a parameter known as the subthreshold swing, which cannot be lowered below a certain level in MOSFETs," explained Kaustav Banerjee, Professor of Electrical and Computer Engineering at UC Santa Barbara. A minimum gate voltage change of 60 millivolts at room temperature is required to change the current by a factor of ten in MOSFETs. In essence, the existing state of transistor technology limits the energy efficiency potential of digital circuits in general.

The research group of Professor Banerjee at UC Santa Barbara took a new approach to subverting this fundamental limitation. They employed the quantum mechanical phenomenon of band-to-band tunneling to design a tunnel field effect transistor (TFET) with sub-60mV per decade of subthreshold swing.

"We restructured the transistor's source to channel junction to filter out high energy electrons that can diffuse over the source/channel barrier even in the off state, thereby making the off state current negligibly small," explained Banerjee. At UCSB, Banerjee's Nanoelectronics Research Lab includes Deblina Sarkar, Xuejun Xie, Wei Liu, Wei Cao, Jiahao Kang, and Stephan Kraemer, as well as Yongji Gong and Pulickel Ajayan of Rice University.

Banerjee and his colleagues are motivated by a global electronics industry that loses billions of dollars each year to the impact of power dissipation on chip cost and reliability. "This translates into lower battery lifetime in personal devices like cell phones and laptops, and massive power consumption of servers in large data centers," adds Banerjee, pointing out the global scale of this energy demand.

An industry that relies on conventional semiconductors such as silicon or III-V compound semiconductors as the channel material for TFETs, Banerjee explains, "faces limitations because these materials have high density of surface states, which increase leakage current and degrade the subthreshold swing."

The TFET designed by the UCSB team overcame this challenge in a few ways, most significant being the use of a layered two-dimensional (2D) material called molybdenum disulphide (MoS2). As the current-carrying channel placed over a highly doped germanium (Ge) as the source electrode, MoS2 offers an ideal surface and thickness of only 1.3nm. The resulting vertical heterostructure provides a unique source-channel junction that is strain-free, has a low barrier for current-carrying electrons to tunnel through from Ge to MoS2 through an ultra-thin (~0.34nm) van der Waals gap, and a large tunneling area.

"The crux of our idea is to combine 3D and 2D materials in a unique heterostructure, to achieve the best of both worlds. The matured doping technology of 3D structures is married to the ultra-thin nature and pristine interfaces of 2D layers to obtain an efficient quantum-mechanical tunneling barrier, which can be easily tuned by the gate," commented Deblina Sarkar, lead author of the paper and PhD student in Banerjee's lab.

"We have engineered what is, at present, the thinnest-channel subthermionic transistor ever made," said Banerjee. Their atomically-thin and layered semiconducting channel tunnel FET (or ATLAS-TFET) is the only planar architecture TFET to achieve subthermionic subthreshold swing (~30 millivolts/decade at room temperature) over four decades of drain current, and the only one in any architecture to achieve so at an ultra-low drain-source voltage of 0.1V.

Ajayan, co-author and professor of chemical and biomolecular engineering at Rice University, commented, "This is a remarkable example showing the uniqueness of 2D atomic layered materials that enables device performance which conventional materials will not be able to achieve. This is perhaps the first breakthrough in a series of novel devices that people will now aspire to build using 2D materials."

"The work is a significant step forward in the search for a low voltage logic transistor. The demonstration of sub-thermal operation over four orders of magnitude is impressive, and the on-current also advances the state-of-the-art. There is still a long ways to go, but this work demonstrates the potential of 2D materials to realize the long-sought, low-voltage device," commented Mark Lundstrom, professor of electrical and computer engineering at Purdue University.

"We have demonstrated how to achieve the most important metric of steep subthreshold swing that meets ITRS requirements. Our transistor can be utilized for a number of low-power applications including arenas where the steep subthreshold swing is the main requirement, such as biosensors or gas sensors. With improved performance, the range of applications of this transistor can be further expanded," explained Wei Cao, a PhD student in Banerjee's group and a co-author of the article.

"This work represents an important step of bringing 2D materials closer to real applications in electronics. The use of 2D materials in tunneling transistors started only recently, and this paper gives the whole field yet another strong boost in improving the characteristics of such devices even further," commented Dr. Konstantin Novoselov, a professor of physics at University of Manchester. Novoselov was co-recipient of the 2010 Nobel Prize in Physics, awarded for the discovery of graphene.

"When I first heard Banerjee's idea of using 2D materials for designing inter-band tunneling transistors in 2012, I recognized its merit and immense potential for ultra-low power electronics. I am pleased to see that his vision has been realized," commented James Hwang, professor of electrical engineering at Lehigh University, who was then the AFOSR program manager responsible for funding this research.

UC Santa Barbara, College of Engineering

http://www.nanotechnologyworld.org/#!Nanoelectronics-Engineers-Develop-Transistor-that-Overcomes-Fundamental-Power-Limitations/c89r/5665e4850cf2a72d69b6fec0

A Different Type of 2D Semiconductor

Berkeley Lab Researchers Produce First Ultrathin Sheets of Perovskite Hybrids

To the growing list of two-dimensional semiconductors, such as graphene, boron nitride, and molybdenum disulfide, whose unique electronic properties make them potential successors to silicon in future devices, you can now add hybrid organic-inorganic perovskites. However, unlike the other contenders, which are covalent semiconductors, these 2D hybrid perovskites are ionic materials, which gives them special properties of their own.

Researchers at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have successfully grown atomically thin 2D sheets of organic-inorganic hybrid perovskites from solution. The ultrathin sheets are of high quality, large in area, and square-shaped. They also exhibited efficient photoluminescence, color-tunability, and a unique structural relaxation not found in covalent semiconductor sheets.

“We believe this is the first example of 2D atomically thin nanostructures made from ionic materials,” says Peidong Yang, a chemist with Berkeley Lab’s Materials Sciences Division and world authority on nanostructures, who first came up with the idea for this research some 20 years ago. “The results of our study open up opportunities for fundamental research on the synthesis and characterization of atomically thin 2D hybrid perovskites and introduces a new family of 2D solution-processed semiconductors for nanoscale optoelectronic devices, such as field effect transistors and photodetectors.”

(From left) Peidong Yang, Letian Dou, Andrew Wong and Yi Yu followed up on research first proposed by Yang in 1994 with “thumbs-up” success. (Photo by Kelly Owen)

Yang, who also holds appointments with the University of California (UC) Berkeley and is a co-director of the Kavli Energy NanoScience Institute (Kavli-ENSI), is the corresponding author of a paper describing this research in the journal Science. The paper is titled “Atomically thin two-dimensional organic-inorganic hybrid perovskites.” The lead authors are Letian Dou, Andrew Wong and Yi Yu, all members of Yang’s research group. Other authors are Minliang Lai, Nikolay Kornienko, Samuel Eaton, Anthony Fu, Connor Bischak, Jie Ma, Tina Ding, Naomi Ginsberg, Lin-Wang Wang and Paul Alivisatos.

Traditional perovskites are typically metal-oxide materials that display a wide range of fascinating electromagnetic properties, including ferroelectricity and piezoelectricity, superconductivity and colossal magnetoresistance. In the past couple of years, organic-inorganic hybrid perovskites have been solution-processed into thin films or bulk crystals for photovoltaic devices that have reached a 20-percent power conversion efficiency. Separating these hybrid materials into individual, free-standing 2D sheets through such techniques as spin-coating, chemical vapor deposition, and mechanical exfoliation has met with limited success.

In 1994, while a PhD student at Harvard University, Yang proposed a method for preparing 2D hybrid perovskite nanostructures and tuning their electronic properties but never acted upon it. This past year, while preparing to move his office, he came upon the proposal and passed it on to co-lead author Dou, a post-doctoral student in his research group. Dou, working mainly with the other lead authors Wong and Yu, used Yang’s proposal to synthesize free-standing 2D sheets of CH3NH3PbI3, a hybrid perovskite made from a blend of lead, bromine, nitrogen, carbon and hydrogen atoms.

Structural illustration of a single layer of a 2D hybrid perovskite (C4H9NH3)2PbBr4), an ionic material with different properties than 2D covalent semiconductors.

“Unlike exfoliation and chemical vapor deposition methods, which normally produce relatively thick perovskite plates, we were able to grow uniform square-shaped 2D crystals on a flat substrate with high yield and excellent reproducibility,” says Dou. “We characterized the structure and composition of individual 2D crystals using a variety of techniques and found they have a slightly shifted band-edge emission that could be attributed to structural relaxation. A preliminary photoluminescence study indicates a band-edge emission at 453 nanometers, which is red-shifted slightly as compared to bulk crystals. This suggests that color-tuning could be achieved in these 2D hybrid perovskites by changing sheet thickness as well as composition via the synthesis of related materials.”

The well-defined geometry of these square-shaped 2D crystals is the mark of high quality crystallinity, and their large size should facilitate their integration into future devices.

“With our technique, vertical and lateral heterostructures can also be achieved,” Yang says. “This opens up new possibilities for the design of materials/devices on an atomic/molecular scale with distinctive new properties.”

This research was supported by DOE’s Office of Science. The characterization work was carried out at the Molecular Foundry’s National Center for Electron Microscopy, and at beamline 7.3.3 of the Advanced Light Source. Both the Molecular Foundry and the Advanced Light Source are DOE Office of Science User Facilities hosted at Berkeley Lab.

Berkeley Lab

Nanotechnology World Association

Laser spectroscopy of ultrathin semiconductor reveals rise of ‘trion’ quasiparticles

Insight into dynamics may spur improved materials for solar energy, quantum computing

Quasiparticles—excitations that behave collectively like particles—are central to energy applications but can be difficult to detect. Recently, however, researchers have seen evidence of quasiparticles called negative trions forming and fading in a layer of semiconducting material that is 100,000 times thinner than a human hair. Scientists at the Department of Energy’s Oak Ridge National Laboratory used ultrafast laser spectroscopy at the Center for Nanophase Materials Sciences (CNMS) to demystify the dynamics of the negative trions. They explored the behavior of the charged quasiparticle in a two-dimensional (2D) semiconductor that is an excellent absorber of sunlight.

Their insights, published in the journal Physical Review B, may prove important for advancing technologies for solar energy and quantum computing.

“We observed negative trions in a two-dimensional tungsten disulfide monolayer excited by a laser beam,” said ultrafast laser spectroscopist Abdelaziz Boulesbaa, who co-led the study with theorist Bing Huang and consulted with laser spectroscopy expert Alex Puretzky. “This discovery may open new opportunities to optoelectronic applications, including information technology, as well as fundamental research in the physics of low-dimensional materials.”

When a semiconductor absorbs light, electrons can be knocked loose and can participate in an electrical current. However, typically two charges form—one negative (an electron) and one positive (a hole)—and are bound to each other for a short time, traveling through the crystal as a quasiparticle called an “exciton.” When an exciton binds to an additional electron, the complex formed is a negative trion, or if it binds to an additional hole, the resulting quasiparticle is a positive trion.

Quasiparticles like excitons may sound exotic, but getting electrons and holes together is the basis for everyday light-emitting diodes (LEDs). When an electron and hole recombine in an LED, a photon is emitted. That’s the light we see in applications from traffic lights and electronic signage to camera flashes and vehicle headlights.

Whereas LEDs emit light, solar cells absorb light and convert its energy into electricity. To make solar cells work, scientists try to separate the electrons from the holes and collect those charges before they have a chance to recombine. Future materials may make use of negative trions to improve charge collection in solar cells, according to Boulesbaa.

Pump–probe experiment

To harness negative trions for improving solar cells and other optoelectronic technologies, scientists need answers to basic questions: How do negative trions form? How long do they live? Why do they form so efficiently in an ultrathin semiconductor?

To answer these questions, the ORNL scientists needed a “camera” of sorts that could make a super-slow-motion movie to reveal quasiparticle dynamics, akin to the camera techniques photographers employ to capture speeding bullets obliterating apples—only a billion times faster. A split laser beam created that camera.

Employing half the laser beam, they fired laser pulses lasting a mere 40 femtoseconds (million-billionths of a second) to excite an ultrathin crystal of tungsten disulfide. Then, for their super slow-motion movie, they fashioned a strobe using the other half of the laser beam—an ultrafast flash of white light—and passed it through the crystal at different delayed times. By measuring the photon energy wavelengths (colors) the crystals absorbed at each time, the scientists built, frame by frame, a slow-motion “movie” of how trions form and fade.  They probably skipped the popcorn, as their movie lasted only a nanosecond (one billionth of a second).

Their movie revealed trions form only after electron–hole pairs form. Then the holes get trapped, most likely by the substrate in contact with the crystal, leaving extra electrons.  

These extra electrons allow the crystal to absorb another photon to form a negative trion.  Because the ultrathin crystals are all “surface,” they have a lot of opportunity to interact with surroundings and to separate charges that are created, making them great trion generators.

Because the researchers used white light, a mixture of all frequencies of light in the visible spectrum, their observation of light of different colors revealed that two different trions had formed, which had not been seen previously.

Next the scientists will study the role of the substrate in defining optical and electrical properties of 2D semiconducting materials. Some substrates trap electrons, leaving excess holes to carry charges, whereas others trap holes, leaving excess electrons to carry charges. Furthermore, the researchers will isolate the 2D semiconductor from the substrate by introducing, in between, an insulator to prevent holes and electrons from reaching the substrate, allowing excitons to live longer and emit light for a greater duration.

The title of the paper is “Observation of two distinct negative trions in tungsten disulfide monolayers.” This research was conducted at the Center for Nanophase Materials Sciences at ORNL. Computations were performed at the National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory. Both are DOE Office of Science User Facilities.

ORNL

Nanotechnology World Association

SLAC's ultrafast 'electron camera' visualizes ripples in 2-D material

Researchers have used SLAC’s experiment for ultrafast electron diffraction (UED), one of the world’s fastest “electron cameras,” to take snapshots of a three-atom-thick layer of a promising material as it wrinkles in response to a laser pulse. Understanding these dynamic ripples could provide crucial clues for the development of next-generation solar cells, electronics and catalysts. (SLAC National Accelerator Laboratory)

Understanding Motions of Thin Layers May Help Design Solar Cells, Electronics and Catalysts of the Future

New research led by scientists from the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University shows how individual atoms move in trillionths of a second to form wrinkles on a three-atom-thick material. Revealed by a brand new “electron camera,” one of the world’s speediest, this unprecedented level of detail could guide researchers in the development of efficient solar cells, fast and flexible electronics and high-performance chemical catalysts.

The breakthrough, accepted for publication Aug. 31 in Nano Letters, could take materials science to a whole new level. It was made possible with SLAC’s instrument for ultrafast electron diffraction (UED), which uses energetic electrons to take snapshots of atoms and molecules on timescales as fast as 100 quadrillionths of a second.

“This is the first published scientific result with our new instrument,” said scientist Xijie Wang, SLAC’s UED team lead. “It showcases the method’s outstanding combination of atomic resolution, speed and sensitivity.”

SLAC Director Chi-Chang Kao said, “Together with complementary data from SLAC’s X-ray laser Linac Coherent Light Source, UED creates unprecedented opportunities for ultrafast science in a broad range of disciplines, from materials science to chemistry to the biosciences.” LCLS is a DOE Office of Science User Facility.

This animation explains how researchers use high-energy electrons at SLAC to study faster-than-ever motions of atoms and molecules relevant to important materials properties and chemical processes.

Extraordinary Material Properties in Two Dimensions

Monolayers, or 2-D materials, contain just a single layer of molecules. In this form they can take on new and exciting properties such as superior mechanical strength and an extraordinary ability to conduct electricity and heat. But how do these monolayers acquire their unique characteristics? Until now, researchers only had a limited view of the underlying mechanisms.

“The functionality of 2-D materials critically depends on how their atoms move,” said SLAC and Stanford researcher Aaron Lindenberg, who led the research team. “However, no one has ever been able to study these motions on the atomic level and in real time before. Our results are an important step toward engineering next-generation devices from single-layer materials.” The research team looked at molybdenum disulfide, or MoS₂, which is widely used as a lubricant but takes on a number of interesting behaviors when in single-layer form – more than 150,000 times thinner than a human hair.

For example, the monolayer form is normally an insulator, but when stretched, it can become electrically conductive. This switching behavior could be used in thin, flexible electronics and to encode information in data storage devices. Thin films of MoS₂ are also under study as possible catalysts that facilitate chemical reactions. In addition, they capture light very efficiently and could be used in future solar cells.

Because of this strong interaction with light, researchers also think they may be able to manipulate the material’s properties with light pulses.

“To engineer future devices, control them with light and create new properties through systematic modifications, we first need to understand the structural transformations of monolayers on the atomic level,” said Stanford researcher Ehren Mannebach, the study’s lead author.Visualization of laser-induced motions of atoms (black and yellow spheres) in a molybdenum disulfide monolayer: The laser pulse creates wrinkles with large amplitudes – more than 15 percent of the layer’s thickness – that develop in a trillionth of a second. (K.-A. Duerloo/Stanford)

Electron Camera Reveals Ultrafast Motions

Previous analyses showed that single layers of molybdenum disulfide have a wrinkled surface. However, these studies only provided a static picture. The new study reveals for the first time how surface ripples form and evolve in response to laser light.

Researchers at SLAC placed their monolayer samples, which were prepared by Linyou Cao’s group at North Carolina State University, into a beam of very energetic electrons. The electrons, which come bundled in ultrashort pulses, scatter off the sample’s atoms and produce a signal on a detector that scientists use to determine where atoms are located in the monolayer. This technique is called ultrafast electron diffraction.

The team then used ultrashort laser pulses to excite motions in the material, which cause the scattering pattern to change over time.

“Combined with theoretical calculations, these data show how the light pulses generate wrinkles that have large amplitudes – more than 15 percent of the layer’s thickness – and develop extremely quickly, in about a trillionth of a second. This is the first time someone has visualized these ultrafast atomic motions,” Lindenberg said.

Once scientists better understand monolayers of different materials, they could begin putting them together and engineer mixed materials with completely new optical, mechanical, electronic and chemical properties.

The research was supported by DOE’s Office of Science, the SLAC UED/UEM program development fund, the German National Academy of Sciences, and the U.S. National Science Foundation.

To study ultrafast atomic motions in a single layer of molybdenum disulfide, researchers followed a pump-probe approach: They excited motions with a laser pulse (pump pulse, red) and probed the laser-induced structural changes with a subsequent electron pulse (probe pulse, blue). The electrons of the probe pulse scatter off the monolayer’s atoms (blue and yellow spheres) and form a scattering pattern on the detector – a signal the team used to determine the monolayer structure. By recording patterns at different time delays between the pump and probe pulses, the scientists were able to determine how the atomic structure of the molybdenum disulfide film changed over time. (SLAC National Accelerator Laboratory)

SLAC