Category: Electronics

A new “electron camera” can capture images of individual moving atoms as they form wrinkles on a three-atom-thick material and in trillionths of a second — one of the world’s fastest. It has been developed by scientists from the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University.

This unprecedented level of detail could guide researchers in developing more efficient solar cells, fast and flexible nanoelectronics, and high-performance chemical catalysts.

The breakthrough, published Aug. 31 in Nano Letters, was made possible by SLAC’s instrument for ultrafast electron diffraction (UED), which uses energetic electrons to take snapshots of atoms and molecules.

SLAC National Accelerator Laboratory | 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 2-D materials

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.

For example, the monolayer form is normally an insulator, but when stretched, it can become electrically conductive. This switching behavior could be used to function like transistors 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.

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.

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Abstract of Dynamic Structural Response and Deformations of Monolayer MoS2 Visualized by Femtosecond Electron Diffraction

Two-dimensional materials are subject to intrinsic and dynamic rippling that modulates their optoelectronic and electromechanical properties. Here, we directly visualize the dynamics of these processes within monolayer transition metal dichalcogenide MoS2 using femtosecond electron scattering techniques as a real-time probe with atomic-scale resolution. We show that optical excitation induces large-amplitude in-plane displacements and ultrafast wrinkling of the monolayer on nanometer length-scales, developing on picosecond time-scales. These deformations are associated with several percent peak strains that are fully reversible over tens of millions of cycles. Direct measurements of electron–phonon coupling times and the subsequent interfacial thermal heat flow between the monolayer and substrate are also obtained. These measurements, coupled with first-principles modeling, provide a new understanding of the dynamic structural processes that underlie the functionality of two-dimensional materials and open up new opportunities for ultrafast strain engineering using all-optical methods.

A research team has created complex 3-D micro- and nanostructures out of silicon and other materials used in advanced technologies by employing a new assembly method that uses a Japanese Kirigami paper-cutting method.

The method builds on the team’s “pop-up” fabrication technique — going from a 2-D material to 3-D in an instant, like a pop-up children’s book — reported in January this year on KurzweilAI and in the journal Science. Those earlier ribbon-like structures yielded open networks, with limited ability to achieve closed-form shapes or to support more complex spatially extended devices.

In their new work, the research team at Northwestern University, University of Illinois and Tsinghua University solved this problem by borrowing ideas from Kirigami, the ancient Japanese technique for forming paper structures by folding and cutting. The Kirigami study was published last week (Sept. 8) in the Proceedings of the National Academy of Sciences (PNAS).

Starting with 2-D structures formed using state-of-the-art methods in semiconductor manufacturing and carefully placed “Kirigami cuts,” the researchers created more than 50 different mostly closed 3-D structures that, in theory, could contain cells or support advanced electronic or optoelectronic devices. The structures also suggest use in tissue engineering and industrial applications, such as biomedical devices, energy storage and microelectromechanical systems.

Creating 3-D pop-ups by cutting at strain points

“The key concept in Kirigami is a cut,” said Yonggang Huang, the Walter P. Murphy Professor of Civil and Environmental Engineering and Mechanical Engineering at Northwestern’s McCormick School of Engineering. “Cuts usually lead to failure, but here we have the opposite: cuts allow us to produce complex 3-D shapes we wouldn’t have otherwise,” he said. “This unique 3-D fabrication technique now can be used by others for their own creations and applications.”

Huang and his team worked with the research group of John A. Rogers, the Swanlund Chair and professor of materials science and engineering at the University of Illinois. Rogers and Huang are co-corresponding authors of the study.

The research team made 3-D structures from materials including silicon, polymers, metals and dielectrics. Some structures combined a number of materials, such as gold and a semiconductor, including patterns that provide useful optical responses.

The Kirigami technique is suitable for mass production, and the breadth of materials that can be manipulated illustrates its usefulness over 3-D printing, which is generally only applied with polymers, the researchers suggest. The Kirigami method also is fast, while 3-D printing is slow.

The researchers started with a flat material adhered at certain places to a stretched substrate. They strategically made “cuts” in the material so that when the stretch is released and the surface “pops up” into three-dimensions, all the physical strain from the new 3-D shape is released through the cuts, keeping the structure from breaking. The cuts are made in just those places where strain normally would be concentrated.

Computer simulations

The “cuts” are not made physically in the material, Huang explained. Instead, methods based on manufacturing approaches for computer chips allow these features to be defined in the material with extremely high engineering control. The researchers successfully predicted by computer simulation the 2-D shape and cuts needed to produce the actual 3-D structure. The ability to make predictions eliminates the time and expense of trial-and-error experiments.

The sizes of the 3-D structures range from 100 nanometers square to 3 centimeters square while the cuts themselves are typically between 1 micron and 10 microns wide for silicon 3-D structures — small enough to interface directly with cells or intracellular structures or to manipulate components in microelectronics.

The researchers also can reversibly tune the optical properties of their structures by mechanical stretching, after they are formed. They demonstrated a simple optical shutter based on arrays of rotating microplates, operating much like shutters on a window.

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Abstract of A mechanically driven form of Kirigami as a route to 3D mesostructures in micro/nanomembranes

Assembly of 3D micro/nanostructures in advanced functional materials has important implications across broad areas of technology. Existing approaches are compatible, however, only with narrow classes of materials and/or 3D geometries. This paper introduces ideas for a form of Kirigami that allows precise, mechanically driven assembly of 3D mesostructures of diverse materials from 2D micro/nanomembranes with strategically designed geometries and patterns of cuts. Theoretical and experimental studies demonstrate applicability of the methods across length scales from macro to nano, in materials ranging from monocrystalline silicon to plastic, with levels of topographical complexity that significantly exceed those that can be achieved using other approaches. A broad set of examples includes 3D silicon mesostructures and hybrid nanomembrane–nanoribbon systems, including heterogeneous combinations with polymers and metals, with critical dimensions that range from 100 nm to 30 mm. A 3D mechanically tunable optical transmission window provides an application example of this Kirigami process, enabled by theoretically guided design.

Researchers from Bilkent University, Ankara, Turkey, have shown that twisting straight nanowires into springs can increase the amount of light the wires absorb by up to 23 percent. Absorbing more light is important because one application of nanowires is turning light into electricity, for example, to power tiny sensors instead of requiring batteries.

If nanowires are made from a semiconductor like silicon, light striking the wire will dislodge electrons from the crystal lattice, leaving positively charged “holes” behind. Both the electrons and the holes move through the material to generate electricity. The more light the wire absorbs; the more electricity it generates. (A device that converts light into electricity can function as either a solar cell or a photosensor.)

In 2007, U.S. researchers introduced a single nanowire photosensor that produced enough electricity from sunlight (up to 200 picowatts) to power nanoscale electronic circuits. More recently, a European researcher team built a nanowire solar cell with almost 14 percent efficiency from the compounds of indium and phosphorus. This efficiency is not enough to beat the best crystalline silicon solar cells on the market, but because nanowires can cover more area with less material, the nanowire solar cells could ultimately be cheaper.

“There is huge potential in the area of nanoscale photosensors,” said Mehmet Bayindir, Director, National Nanotechnology Research Center, Bilkent University. “More efficient outputs might induce the emergence of a new generation of photosensor technology and eventual commercialization of these products.”

Mie resonances increase current flow

Bayindir and his colleague Tural Khudiyev, now a postdoctoral associate at The Massachusetts Institute of Technology, have found that adjusting the geometry of the typical nanowire may be one way to realize the desired efficiency enhancement.

Nanowires are usually long, thin and straight. Their tiny dimensions mean they interact with light differently than ordinary materials. Certain wavelengths of light will match up in just the right way with the dimensions of the nanowire, causing the light to “resonate” or bounce around inside the wire.

These “Mie resonances” are especially advantageous at the nanoscale, Khudiyev said. The resonances are named after the early-20th-century German physicist Gustav Mie, who developed equations to describe why tiny metal particles make stained glass windows glow so brightly.

Mie resonances will occur with straight nanowires, but by twisting the nanowire into a helical shape, the researchers found they could take double advantage of the phenomena. “When the nanospring period matches the Mie resonance points, a ‘double resonance’ condition occurs, which boosts light harvesting efficiency,” Khudiyev said.

Additionally, twisting the wire upwards shortened its length, reducing the required area by up to 50 percent.

Nanoscale sensors

The enhanced light harvesting efficiency of nanosprings opens new opportunities to build nanoscale devices that power themselves, such as sensors to detect environmental toxins or to monitor the structural integrity of a bridge.

“Our nanospring shape induces more power output both in the broad spectrum range and at some desired single point (which can be engineered easily), and these make powering of more advanced nanosystems possible with a single nanospring-based photovoltaics system,” Khudiyev said.

“Experimental observation of a nanospring-based photosensor design and its integration into a large-scale fiber embedded system would be interesting as the next steps,” Bayindir said.

The group has already developed an easy way to produce nanosprings by first making long nanowire arrays, then heating them to a temperature at which the arrays can be twisted into the nanospring shape. The technique can be varied to control the diameter of the spring and the tightness of the curl.

The results of this research are published in the journal Applied Optics, from The Optical Society (OSA).

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Abstract of Nanosprings harvest light more efficiently

Nanotechnology presents versatile architectural designs for the purpose of utilization as a building block of 1D optoelectronic nanodevices because current nanowire-based schemes require more effective solutions for low absorption capacity of nanoscale volumes. We report on the potential of nanospring absorbers as an alternative light-harvesting platform with significant advantages over conventional nanowires. Absorption capacity of nanospring geometry is found to be superior to cylindrical nanowire shape. Unlike nanowires, they are able to trap a larger amount of light thanks to characteristic periodic behavior that boosts light collection for the points matched with Mie resonances. Moreover, nanospring shape supplies compactness to a resulting device with area preservation as high as twofold. By considering that a nanospring array with optimal periods yields higher absorption than individual arrangements and core-shell designs, which further promote light collection due to unique antireflection features of shell layer, these nanostructures will pave the way for the development of highly efficient self-powered nanosystems.

University of British Columbia (UBC) physicists have created the first single-layer superconducting graphene sample by coating it with lithium atoms.

Although superconductivity has already been observed in layered bulk graphite, inducing superconductivity in single-layer graphene has until now eluded scientists.

“This first experimental realization of superconductivity in graphene promises to usher us in a new era of graphene electronics and nanoscale quantum devices,” says Andrea Damascelli, director of UBC’s Quantum Matter Institute and leading scientist of the Proceedings of the National Academy of Sciences study outlining the discovery. A superconductive wire would have zero resistance at ultra-low temperatures (at a critical temperature* of about 5.9K), so a current flowing through it would generate no heat.

Given the massive scientific and technological interest, the ability to induce superconductivity in single-layer graphene promises to have significant cross-disciplinary impacts, the researchers say.

To achieve this breakthrough, the researchers, which include colleagues at the Max Planck Institute for Solid State Research, prepared the lithium-decorated graphene in ultra-high vacuum conditions.

Scientists eventually hope to make very fast transistors, semiconductors, sensors, and transparent electrodes using graphene, a single layer of carbon atoms arranged in a honeycomb pattern.

* The temperature below which superconductivity appears.

UPDATE Sept. 14, 2015: critical temperature added.

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Abstract of Evidence for superconductivity in Li-decorated monolayer graphene

Monolayer graphene exhibits many spectacular electronic properties, with superconductivity being arguably the most notable exception. It was theoretically proposed that superconductivity might be induced by enhancing the electron–phonon coupling through the decoration of graphene with an alkali adatom superlattice [Profeta G, Calandra M, Mauri F (2012) Nat Phys 8(2):131–134]. Although experiments have shown an adatom-induced enhancement of the electron–phonon coupling, superconductivity has never been observed. Using angle-resolved photoemission spectroscopy (ARPES), we show that lithium deposited on graphene at low temperature strongly modifies the phonon density of states, leading to an enhancement of the electron–phonon coupling of up to λ≃0.58. On part of the graphene-derived π∗-band Fermi surface, we then observe the opening of a Δ≃0.9-meV temperature-dependent pairing gap. This result suggests for the first time, to our knowledge, that Li-decorated monolayer graphene is indeed superconducting, with T_c≃5.9 K.

Northwestern University engineers say that have finally found the key to practical use of carbon nanotubes (CNTs) in integrated circuits. Individual transistors made from CNTs are faster and more energy-efficient and reliable than those made from other materials.

The problem. But making the leap to wafer-scale integrated circuits (a microprocessor typically has a billion transistors) is a challenge. The process is incredibly expensive, often requiring billion-dollar cleanrooms to keep the delicate nano-sized components safe from the potentially damaging effects of air, water, and dust.

And researchers have struggled to create a carbon nanotube-based integrated circuit in which the transistors are spatially uniform across the material, which is needed for the overall system to work.

The solution. Now Hersam and his team have found a key to solving all these issues: newly developed encapsulation layers that protect carbon nanotubes from environmental degradation.

Dealing with environmental degradation

“One of the realities of a nanomaterial, such as a carbon nanotube, is that essentially all of its atoms are on the surface,” explained Northwestern Engineering’s Mark Hersam, the Walter P. Murphy Professor of Materials Science and Engineering. “So anything that touches the surface of these materials can influence their properties.

“If we made a series of transistors and left them out in the air, water and oxygen would stick to the surface of the nanotubes, degrading them over time. We thought that adding a protective encapsulation layer could arrest this degradation process to achieve substantially longer lifetimes.”*

To demonstrate proof of concept, Hersam developed nanotube-based static random-access memory (SRAM) circuits. SRAM is a key component of all microprocessors, often making up as much as 85 percent of the transistors in the central-processing unit in a common computer. To create the encapsulated carbon nanotubes, the team first deposited the carbon nanotubes from a solution previously developed in Hersam’s lab. Then they coated the tubes with their encapsulation layers.

Functional CNT-based SRAM memory circuits

Using the encapsulated carbon nanotubes, Hersam’s team successfully designed and fabricated arrays of working SRAM circuits. Not only did the encapsulation layers protect the sensitive device from the environment, but they improved spatial uniformity among individual transistors across the wafer. While Hersam’s integrated circuits demonstrated a long lifetime, transistors that were deposited from the same solution but not coated degraded within hours.

“After we’ve made the devices, we can leave them out in air with no further precautions,” Hersam said. “We don’t need to put them in a vacuum chamber or controlled environment. Other researchers have made similar devices but immediately had to put them in a vacuum chamber or inert environment to keep them stable. That’s obviously not going to work in a real-world situation.”

Implications for portable/wearable electronics

These features, when combined with recent advances in flexible and printed electronics have potentially wide-ranging implications for high-performance portable and wearable electronics, the researchers suggest in the paper.

Flexible carbon nanotube-based transistors could replace rigid silicon to enable wearable electronics, Hersam says. The cheaper manufacturing method also opens doors for smart cards — credit cards embedded with personal information to reduce the likelihood of fraud.

“Smart cards are only realistic if they can be realized using extremely low-cost manufacturing,” he said. “Because our solution-processed carbon nanotubes are compatible with scalable and inexpensive printing methods, our results could enable smart cards and related printed electronics applications.”

The research appeared online in Nature Nanotechology on September 7. It was supported by the Office of Naval Research and the National Science Foundation.

* Hersam compares his solution to one currently used for organic light-emitting diodes (LEDs), which experienced similar problems after they were first realized. Many people assumed that organic LEDs would have no future because they degraded in air. After researchers developed an encapsulation layer for the material, organic LEDs are now used in many commercial applications, including displays for smartphones, car radios, televisions, and digital cameras. Made from polymers and inorganic oxides, Hersam’s encapsulation layer is based on the same idea but tailored for carbon nanotubes.

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Abstract of Solution-processed carbon nanotube thin-film complementary static random access memory

Over the past two decades, extensive research on single-walled carbon nanotubes (SWCNTs) has elucidated their many extraordinary properties, making them one of the most promising candidates for solution-processable, high-performance integrated circuits. In particular, advances in the enrichment of high-purity semiconducting SWCNTs have enabled recent circuit demonstrations including synchronous digital logic, flexible electronics and high-frequency applications. However, due to the stringent requirements of the transistors used in complementary metal–oxide–semiconductor (CMOS) logic as well as the absence of sufficiently stable and spatially homogeneous SWCNT thin-film transistors, the development of large-scale SWCNT CMOS integrated circuits has been limited in both complexity and functionality. Here, we demonstrate the stable and uniform electronic performance of complementary p-type and n-type SWCNT thin-film transistors by controlling adsorbed atmospheric dopants and incorporating robust encapsulation layers. Based on these complementary SWCNT thin-film transistors, we simulate, design and fabricate arrays of low-power static random access memory circuits, achieving large-scale integration for the first time based on solution-processed semiconductors.