Scientists from Manchester University, the Ulsan National Institute of Science & Technology and the Korea Institute of Science and Technology have developed a novel technology, which combines the fabrication procedures of planar and vertical heterostructures in order to assemble graphene-based single-electron transistors.
In the study, it was demonstrated that high-qualitygraphene quantum dots (GQDs), regardless of whether they were ordered or randomly distributed, could be successfully synthesized in a matrix of monolayer hexagonal boron nitride (hBN).
Here, the growth of GQDs within the layer of hBN was shown to be catalytically supported by the platinum (Pt) nanoparticles distributed in-between the hBN and supporting oxidised silicon (SiO2) wafer, when the whole structure was treated by the heat in the methane gas (CH4). It was also shown, that due to the same lattice structure (hexagonal) and small lattice mismatch (~1.5%) of graphene and hBN, graphene islands grow in the hBN with passivated edge states, thereby giving rise to the formation of defect-less quantum dots embedded in the hBN monolayer.
Such planar heterostructures incorporated by means of standard dry-transfer as mid-layers into the regular structure of vertical tunnelling transistors (Si/SiO2/hBN/Gr/hBN/GQDs/hBN/Gr/hBN; here Gr refers to monolayer graphene and GQDs refers to the layer of hBN with the embedded graphene quantum dots) were studied through tunnel spectroscopy at low temperatures (3He, 250mK).
The study demonstrated where the manifestation of well-established phenomena of the Coulomb blockade for each graphene quantum dot as a separate single electron transmission channel occurs.
‘Although the outstanding quality of our single electron transistors could be used for the development of future electronics, “This work is most valuable from a technological standpoint as it suggests a new platform for the investigation of physical properties of various materials through a combination of planar and van der Waals heterostructures.” as explained study co-author Davit Ghazaryan, Associate Professor at the HSE Faculty of Physics, and Research Fellow at the Institute of Solid State Physics (RAS)
A team of researchers from Denmark has solved one of the biggest challenges in making effective nanoelectronics based on graphene. The new results have just been published in Nature Nanotechnology.
For 15 years, scientists have tried to exploit the “miracle material” graphene to produce nanoscale electronics. On paper, graphene should be great for just that: it is ultra-thin – only one atom thick and therefore two-dimensional, it is excellent for conducting electrical current, and holds great promise for future forms of electronics that are faster and more energy efficient. In addition, graphene consists of carbon atoms – of which we have an unlimited supply.
In theory, graphene can be altered to perform many different tasks within e.g. electronics, photonics or sensorssimply by cutting tiny patterns in it, as this fundamentally alters its quantum properties. One “simple” task, which has turned out to be surprisingly difficult, is to induce a band gap – which is crucial for making transistors and optoelectronic devices. However, since graphene is only an atom thick all of the atoms are important and even tiny irregularities in the pattern can destroy its properties.
“Graphene is a fantastic material, which I think will play a crucial role in making new nanoscale electronics. The problem is that it is extremely difficult to engineer the electrical properties,” says Peter Bøggild, professor at DTU Physics.
The Center for Nanostructured Graphene at DTU and Aalborg University was established in 2012 specifically to study how the electrical properties of graphene can be tailored by changing its shape on an extremely small scale. When actually patterning graphene, the team of researchers from DTU and Aalborg experienced the same as other researchers worldwide: it didn’t work.
“When you make patterns in a material like graphene, you do so in order to change its properties in a controlled way – to match your design. However, what we have seen throughout the years is that we can make the holes, but not without introducing so much disorder and contamination that it no longer behaves like graphene. It is a bit similar to making a water pipe that is partly blocked because of poor manufacturing. On the outside, it might look fine, but water cannot flow freely. For electronics, that is obviously disastrous,” says Peter Bøggild.
Now, the team of scientists have solved the problem. The results are published in Nature Nanotechnology. Two postdocs from DTU Physics, Bjarke Jessen and Lene Gammelgaard, first encapsulated graphene inside another two-dimensional material – hexagonal boron nitride, a non-conductive material that is often used for protecting graphene’s properties.
Next, they used a technique called electron beam lithography to carefully pattern the protective layer of boron nitride and graphene below with a dense array of ultra small holes. The holes have a diameter of approx. 20 nanometers, with just 12 nanometers between them – however, the roughness at the edge of the holes is less than 1 nanometer, or a billionth of a meter. This allows 1000 times more electrical current to flow than had been reported in such small graphene structures. And not just that.
“We have shown that we can control graphene’s band structure and design how it should behave. When we control the band structure, we have access to all of graphene’s properties – and we found to our surprise that some of the most subtle quantum electronic effects survive the dense patterning – that is extremely encouraging. Our work suggests that we can sit in front of the computer and design components and devices – or dream up something entirely new – and then go to the laboratory and realise them in practice,” says Peter Bøggild. He continues:
“Many scientists had long since abandoned attempting nanolithography in graphene on this scale, and it is quite a pity, since nanostructuring is a crucial tool for exploiting the most exciting features of graphene electronics and photonics. Now we have figured out how it can be done; one could say that the curse is lifted. There are other challenges, but the fact that we can tailor electronic properties of graphene is a big step towards creating new electronics with extremely small dimensions,” says Peter Bøggild.
About the Center for Nanostructured Graphene
• Funded by the Danish National Research Foundation with a total budget of 100 million DKK for the ten-year period 2012 – 2022. It focuses on basic research, but all its research projects have long-term perspectives for applications.
• the team is also part of the Graphene Flagship, which with a budget of €1 billion represents a new form of joint, coordinated research on an unprecedented scale, and is Europe’s biggest ever research initiative. It is tasked with bringing together academic and industrial partners to take graphene from the realm of academic laboratories into society in the space of 10 years, thus generating economic growth, new jobs and new opportunities for Europe.
A Columbia-led team has discovered a new method to manipulate the electrical conductivity of this game-changing material, the strongest known to man with applications ranging from nano-electronic devices to clean energy.
Graphene has been heralded as a wonder material. Not only is it the strongest, thinnest material ever discovered, its exceptional ability to conduct heat and electricity paves the way for innovation in areas ranging from electronics to energy to medicine.
Now, a Columbia University-led team has developed a new method to finely tune adjacent layers of graphene—lacy, honeycomb-like sheets of carbon atoms—to induce superconductivity. Their research provides new insights into the physics underlying this two-dimensional material’s intriguing characteristics.
The team’s paper is published in the Jan. 24 issue of Science.
“Our work demonstrates new ways to induce superconductivity in twisted bilayer graphene, in particular, achieved by applying pressure,” said Cory Dean, assistant professor of physics at Columbia and the study’s principal investigator. “It also provides critical first confirmation of last year’s MIT results—that bilayer graphene can exhibit electronic properties when twisted at an angle—and furthers our understanding of the system, which is extremely important for this new field of research.”
In March 2018 researchers at the Massachusetts Institute of Technology reported a groundbreaking discovery that two graphene layers can conduct electricity without resistance when the twist angle between them is 1.1 degrees,referred to as the “magic angle.”
But hitting that magic angle has proven difficult. “The layers must be twisted to within roughly a tenth of a degree around 1.1, which is experimentally challenging,” Dean said. “We found that very small errors in alignment could give entirely different results.”
So Dean and his colleagues, who include scientists from the National Institute for Materials Science and the University of California, Santa Barbara, set out to test whether magic-angle conditions could be achieved at bigger rotations.
“Rather than trying to precisely control the angle, we asked whether we could instead vary the spacing between the layers,” said Matthew Yankowitz, a postdoctoral research scientist in Columbia’s physics department and first author on the study. “In this way any twist angle could, in principle, be turned into a magic angle.”
They studied a sample with twist angle of 1.3 degrees—only slightly larger than the magic angle but still far enough away to preclude superconductivity.
Applying pressure transformed the material from a metal into either an insulator—in which electricity cannot flow—or a superconductor—where electrical current can pass without resistance—depending on the number of electrons in the material.
“Remarkably, by applying pressure of over 10,000 atmospheres we observe the emergence of the insulating and superconducting phases,” Dean said. Additionally, the superconductivity develops at the highest temperature observed in graphene so far, just over 3 degrees above absolute zero.”
To reach the high pressures needed to induce superconductivity the team worked closely with the National High Magnetic Field user facility, known as the Maglab, in Tallahassee, Florida.
“This effort was a huge technical challenge,” said Dean. “After fabricating one of most unique devices we’ve ever worked with, we then had to combine cryogenic temperatures, high magnetic fields, and high pressure—all while measuring electrical response. Putting this all together was a daunting task and our ability to make it work is really a tribute to the fantastic expertise at the Maglab.”
The researchers believe it may be possible to enhance the critical temperature of the superconductivity further at even higher pressures. The ultimate goal is to one day develop a superconductor which can perform under room temperature conditions, and although this may prove challenging in graphene, it could serve as a roadmap for achieving this goal in other materials.
Andrea Young, assistant professor of physics at UC Santa Barbara, a collaborator on the study, said the work clearly demonstrates that squeezing the layers has same effect as twisting them and offers an alternative paradigm for manipulating the electronic properties in graphene.
“Our findings significantly relax the constraints that make it challenging to study the system and gives us new knobs to control it,” Young said.
Dean and Young are now twisting and squeezing a variety of atomically-thin materials in the hopes of finding superconductivity emerging in other two-dimensional systems.
“Understanding ‘why’ any of this is happening is a formidable challenge but critical for eventually harnessing the power of this material—and our work starts unraveling the mystery,'” Dean said.
MIT researchers have devised a way to grow single crystal GaN thin film on a GaN substrate through two-dimensional materials. The GaN thin film is then exfoliated by a flexible substrate, showing the rainbow color that comes from thin film interference. This technology will pave the way to flexible electronics and the reuse of the wafers.
Photo credits: Wei Kong and Kuan Qiao
Cost-effective method produces semiconducting films from materials that outperform silicon.
“In smart cities, where we might want to put small computers everywhere, we would need low power, highly sensitive computing and sensing devices, made from better materials,” Kim says. “This [study] unlocks the pathway to those devices.”
The vast majority of computing devices today are made from silicon, the second most abundant element on Earth, after oxygen. Silicon can be found in various forms in rocks, clay, sand, and soil. And while it is not the best semiconducting material that exists on the planet, it is by far the most readily available. As such, silicon is the dominant material used in most electronic devices, including sensors, solar cells, and the integrated circuits within our computers and smartphones.
Now MIT engineers have developed a technique to fabricate ultrathin semiconducting films made from a host of exotic materials other than silicon. To demonstrate their technique, the researchers fabricated flexible films made from gallium arsenide, gallium nitride, and lithium fluoride — materials that exhibit better performance than silicon but until now have been prohibitively expensive to produce in functional devices.
The new technique, researchers say, provides a cost-effective method to fabricate flexible electronics made from any combination of semiconducting elements, that could perform better than current silicon-based devices.
“We’ve opened up a way to make flexible electronics with so many different material systems, other than silicon,” says Jeehwan Kim, the Class of 1947 Career Development Associate Professor in the departments of Mechanical Engineering and Materials Science and Engineering. Kim envisions the technique can be used to manufacture low-cost, high-performance devices such as flexible solar cells, and wearable computers and sensors.
Details of the new technique are reported today in Nature Materials. In addition to Kim, the paper’s MIT co-authors include Wei Kong, Huashan Li, Kuan Qiao, Yunjo Kim, Kyusang Lee, Doyoon Lee, Tom Osadchy, Richard Molnar, Yang Yu, Sang-hoon Bae, Yang Shao-Horn, and Jeffrey Grossman, along with researchers from Sun Yat-Sen University, the University of Virginia, the University of Texas at Dallas, the U.S. Naval Research Laboratory, Ohio State University, and Georgia Tech.
Now you see it, now you don’t
In 2017, Kim and his colleagues devised a method to produce “copies” of expensive semiconducting materials using graphene — an atomically thin sheet of carbon atoms arranged in a hexagonal, chicken-wire pattern. They found that when they stacked graphene on top of a pure, expensive wafer of semiconducting material such as gallium arsenide, then flowed atoms of gallium and arsenide over the stack, the atoms appeared to interact in some way with the underlying atomic layer, as if the intermediate graphene were invisible or transparent. As a result, the atoms assembled into the precise, single-crystalline pattern of the underlying semiconducting wafer, forming an exact copy that could then easily be peeled away from the graphene layer.
The technique, which they call “remote epitaxy,” provided an affordable way to fabricate multiple films of gallium arsenide, using just one expensive underlying wafer.
Soon after they reported their first results, the team wondered whether their technique could be used to copy other semiconducting materials. They tried applying remote epitaxy to silicon, and also germanium — two inexpensive semiconductors — but found that when they flowed these atoms over graphene they failed to interact with their respective underlying layers. It was as if graphene, previously transparent, became suddenly opaque, preventing atoms of silicon and germanium from “seeing” the atoms on the other side.
As it happens, silicon and germanium are two elements that exist within the same group of the periodic table of elements. Specifically, the two elements belong in group four, a class of materials that are ionically neutral, meaning they have no polarity.
“This gave us a hint,” says Kim.
Perhaps, the team reasoned, atoms can only interact with each other through graphene if they have some ionic charge. For instance, in the case of gallium arsenide, gallium has a negative charge at the interface, compared with arsenic’s positive charge. This charge difference, or polarity, may have helped the atoms to interact through graphene as if it were transparent, and to copy the underlying atomic pattern.
“We found that the interaction through graphene is determined by the polarity of the atoms. For the strongest ionically bonded materials, they interact even through three layers of graphene,” Kim says. “It’s similar to the way two magnets can attract, even through a thin sheet of paper.”
The researchers tested their hypothesis by using remote epitaxy to copy semiconducting materials with various degrees of polarity, from neutral silicon and germanium, to slightly polarized gallium arsenide, and finally, highly polarized lithium fluoride — a better, more expensive semiconductor than silicon.
They found that the greater the degree of polarity, the stronger the atomic interaction, even, in some cases, through multiple sheets of graphene. Each film they were able to produce was flexible and merely tens to hundreds of nanometers thick.
The material through which the atoms interact also matters, the team found. In addition to graphene, they experimented with an intermediate layer of hexagonal boron nitride (hBN), a material that resembles graphene’s atomic pattern and has a similar Teflon-like quality, enabling overlying materials to easily peel off once they are copied.
However, hBN is made of oppositely charged boron and nitrogen atoms, which generate a polarity within the material itself. In their experiments, the researchers found that any atoms flowing over hBN, even if they were highly polarized themselves, were unable to interact with their underlying wafers completely, suggesting that the polarity of both the atoms of interest and the intermediate material determines whether the atoms will interact and form a copy of the original semiconducting wafer.
“Now we really understand there are rules of atomic interaction through graphene,” Kim says.
With this new understanding, he says, researchers can now simply look at the periodic table and pick two elements of opposite charge. Once they acquire or fabricate a main wafer made from the same elements, they can then apply the team’s remote epitaxy techniques to fabricate multiple, exact copies of the original wafer.
“People have mostly used silicon wafers because they’re cheap,” Kim says. “Now our method opens up a way to use higher-performing, nonsilicon materials. You can just purchase one expensive wafer and copy it over and over again, and keep reusing the wafer. And now the material library for this technique is totally expanded.”
Kim envisions that remote epitaxy can now be used to fabricate ultrathin, flexible films from a wide variety of previously exotic, semiconducting materials — as long as the materials are made from atoms with a degree of polarity. Such ultrathin films could potentially be stacked, one on top of the other, to produce tiny, flexible, multifunctional devices, such as wearable sensors, flexible solar cells, and even, in the distant future, “cellphones that attach to your skin.”
“In smart cities, where we might want to put small computers everywhere, we would need low power, highly sensitive computing and sensing devices, made from better materials,” Kim says. “This [study] unlocks the pathway to those devices.”
This research was supported in part by the Defense Advanced Research Projects Agency, the Department of Energy, the Air Force Research Laboratory, LG Electronics, Amore Pacific, LAM Research, and Analog Devices.
Microelectronic devices – from pacemakers to cellphones – have long shaped the course of human health and telecommunications. But, scientists have struggled to navigate the technology gap between microelectronics and the biological world.
For example, today’s consumers cannot tap into their smartphones to uncover information about an infection or illness affecting their body, nor can they use their phones to signal a device to administer an antibiotic or drug.
One of the primary reasons for this disconnect between the body and everyday technology is that microelectronic devices process information using materials such as silicon, gold, or chemicals, and an energy source that provides electrons; but, free electrons do not exist in biology. As such, scientists encounter a major roadblock in their efforts to bridge the gap between biological systems and microelectronics.
But, engineers at the University of Maryland’s A. James Clark School of Engineering, along with researchers from the University of Nebraska-Lincoln and the U.S. Army Research Laboratory, may have found a loophole.
In biological systems, there exists a small class of molecules capable of shuttling electrons. These molecules, known as “redox” molecules, can transport electrons to any location. But, redox molecules must first undergo a series of chemical reactions – oxidation or reduction reactions – to transport electrons to the intended target.
By engineering cells with synthetic biology components, the research team has experimentally demonstrated a proof-of-concept device enabling robust and reliable information exchanges between electrical and biological (molecular) domains.
“Devices that freely exchange information between the electronic and biological worlds would represent a completely new societal paradigm,” said bioengineering professor William E. Bentley, director of the UMD Robert E. Fischell Institute for Biomedical Devices. “It has only been about 60 years since the implantable pacemaker and defibrillator proved what devices could achieve by electronically stimulating ion currents. Imagine what we could do by transferring all the knowledge contained in our molecular space, by tapping into and controlling molecules such as glucose, hormones, DNA, proteins, or polysaccharides in addition to ions.”
Building on their progress, the research team is now working to develop a novel biological memory device that can be written to and read from via either biological and/or electronic means. Such a device would function like a thumb drive or SD card, using molecular signals to store key information, and would require almost no energy. Inside the body, these devices would serve the same purpose – except, instead of merely storing data, they could be used to control biological behaviors.
“For years, microelectronic circuits have had limited capabilities in maximizing their computing and storage capacities, mainly due to the physical constraints that the building-block inorganic materials – such as silicon – imposed upon them,” said UMD professor Reza Ghodssi, who specializes in electrical and computer engineering. “By exploring and utilizing the world of biology through an integrated and robust interface technology with semiconductor processing, we expect to address those limitations by allowing our researchers and students to design and develop first-of-kind innovative and powerful bioelectronic devices and systems.”
The collaborative research team will work to integrate subsystems and create biohybrid circuits to develop an electronically controlled device for the body that interprets molecular information, computes desired outcomes, and electronically actuates cells to signal and control biological populations.
The hope is that such a system could seek out and destroy a bacterial pathogen by recognizing its secreted signaling molecules and synthesizing a pathogen-specific toxin. In this way, the group will, for the first time, explore electronic control of complex biological behaviors.
This year, the group was awarded a $1.5 million National Science Foundation grant through the Semiconductor Synthetic Biology for Information Processing and Storage technologies (SemiSynBio) program. Their earlier related work was published in Nature Communications.
Read about related microbiology research at Maryland.
For quantum dot (QD) materials to perform well in devices such as solar cells, the nanoscale crystals in them need to pack together tightly so that electrons can hop easily from one dot to the next and flow out as current. MIT researchers have now made QD films in which the dots vary by just one atom in diameter and are organized into solid lattices with unprecedented order. Subsequent processing pulls the QDs in the film closer together, further easing the electrons’ pathway. Tests using an ultrafast laser confirm that the energy levels of vacancies in adjacent QDs are so similar that hopping electrons don’t get stuck in low-energy dots along the way.
Taken together, the results suggest a new direction for ongoing efforts to develop these promising materials for high performance in electronic and optical devices.
In recent decades, much research attention has focused on electronic materials made of quantum dots, which are tiny crystals of semiconducting materials a few nanometers in diameter. After three decades of research, QDs are now being used in TV displays, where they emit bright light in vivid colors that can be fine-tuned by changing the sizes of the nanoparticles. But many opportunities remain for taking advantage of these remarkable materials.
“QDs are a really promising underlying materials technology for energy applications,” says William Tisdale, the ARCO Career Development Professor in Energy Studies and an associate professor of chemical engineering.
QD materials pique his interest for several reasons. QDs are easily synthesized in a solvent at low temperatures using standard procedures. The QD-bearing solvent can then be deposited on a surface—small or large, rigid or flexible—and as it dries, the QDs are left behind as a solid. Best of all, the electronic and optical properties of that solid can be controlled by tuning the QDs.
“With QDs, you have all these degrees of freedom,” says Tisdale. “You can change their composition, size, shape, and surface chemistry to fabricate a material that’s tailored for your application.”
The ability to adjust electron behavior to suit specific devices is of particular interest. For example, in solar photovoltaics (PVs), electrons should pick up energy from sunlight and then move rapidly through the material and out as current before they lose their excess energy. In light-emitting diodes (LEDs), high-energy “excited” electrons should relax on cue, emitting their extra energy as light.
With thermoelectric (TE) devices, QD materials could be a game-changer. When TE materials are hotter on one side than the other, they generate electricity. So TE devices could turn waste heat in car engines, industrial equipment, and other sources into power—without combustion or moving parts. The TE effect has been known for a century, but devices using TE materials have remained inefficient. The problem: While those materials conduct electricity well, they also conduct heat well, so the temperatures of the two ends of a device quickly equalize. In most materials, measures to decrease heat flow also decrease electron flow.
“With QDs, we can control those two properties separately,” says Tisdale. “So we can simultaneously engineer our material so it’s good at transferring electrical charge but bad at transporting heat.”
Making good arrays
One challenge in working with QDs has been to make particles that are all the same size and shape. During QD synthesis, quadrillions of nanocrystals are deposited onto a surface, where they self-assemble in an orderly fashion as they dry. If the individual QDs aren’t all exactly the same, they can’t pack together tightly, and electrons won’t move easily from one nanocrystal to the next.
Three years ago, a team in Tisdale’s lab led by Mark Weidman Ph.D. ’16 demonstrated a way to reduce that structural disorder. In a series of experiments with lead-sulfide QDs, team members found that carefully selecting the ratio between the lead and sulfur in the starting materials would produce QDs of uniform size.
“As those nanocrystals dry, they self-assemble into a beautifully ordered arrangement we call a superlattice,” Tisdale says.
Scattering electron microscope images of those superlattices taken from several angles show lined-up, 5-nanometer-diameter nanocrystals throughout the samples and confirm the long-range ordering of the QDs.
For a closer examination of their materials, Weidman performed a series of X-ray scattering experiments at the National Synchrotron Light Source at Brookhaven National Laboratory. Data from those experiments showed both how the QDs are positioned relative to one another and how they’re oriented, that is, whether they’re all facing the same way. The results confirmed that QDs in the superlattices are well ordered and essentially all the same.
“On average, the difference in diameter between one nanocrystal and another was less than the size of one more atom added to the surface,” says Tisdale. “So these QDs have unprecedented monodispersity, and they exhibit structural behavior that we hadn’t seen previously because no one could make QDs this monodisperse.”
Controlling electron hopping
The researchers next focused on how to tailor their monodisperse QD materials for efficient transfer of electrical current. “In a PV or TE device made of QDs, the electrons need to be able to hop effortlessly from one dot to the next and then do that many thousands of times as they make their way to the metal electrode,” Tisdale explains.
One way to influence hopping is by controlling the spacing from one QD to the next. A single QD consists of a core of semiconducting material—in this work, lead sulfide—with chemically bound arms, or ligands, made of organic (carbon-containing) molecules radiating outward. The ligands play a critical role—without them, as the QDs form in solution, they’d stick together and drop out as a solid clump. Once the QD layer is dry, the ligands end up as solid spacers that determine how far apart the nanocrystals are.
A standard ligand material used in QD synthesis is oleic acid. Given the length of an oleic acid ligand, the QDs in the dry superlattice end up about 2.6 nanometers apart—and that’s a problem.
“That may sound like a small distance, but it’s not,” says Tisdale. “It’s way too big for a hopping electron to get across.”
Using shorter ligands in the starting solution would reduce that distance, but they wouldn’t keep the QDs from sticking together when they’re in solution. “So we needed to swap out the long oleic acid ligands in our solid materials for something shorter” after the film formed, Tisdale says.
To achieve that replacement, the researchers use a process called ligand exchange. First, they prepare a mixture of a shorter ligand and an organic solvent that will dissolve oleic acid but not the lead sulfide QDs. They then submerge the QD film in that mixture for 24 hours. During that time, the oleic acid ligands dissolve, and the new, shorter ligands take their place, pulling the QDs closer together. The solvent and oleic acid are then rinsed off.
Tests with various ligands confirmed their impact on interparticle spacing. Depending on the length of the selected ligand, the researchers could reduce that spacing from the original 2.6 nanometers with oleic acid all the way down to 0.4 nanometers. However, while the resulting films have beautifully ordered regions—perfect for fundamental studies—inserting the shorter ligands tends to generate cracks as the overall volume of the QD sample shrinks.
Energetic alignment of nanocrystals
One result of that work came as a surprise: Ligands known to yield high performance in lead-sulfide-based solar cells didn’t produce the shortest interparticle spacing in their tests.
“Reducing that spacing to get good conductivity is necessary,” says Tisdale. “But there may be other aspects of our QD material that we need to optimize to facilitate electron transfer.”
One possibility is a mismatch between the energy levels of the electrons in adjacent QDs. In any material, electrons exist at only two energy levels—a low ground state and a high excited state. If an electron in a QD film receives extra energy—say, from incoming sunlight—it can jump up to its excited state and move through the material until it finds a low-energy opening left behind by another traveling electron. It then drops down to its ground state, releasing its excess energy as heat or light.
In solid crystals, those two energy levels are a fixed characteristic of the material itself. But in QDs, they vary with particle size. Make a QD smaller and the energy level of its excited electrons increases. Again, variability in QD size can create problems. Once excited, a high-energy electron in a small QD will hop from dot to dot—until it comes to a large, low-energy QD.
“Excited electrons like going downhill more than they like going uphill, so they tend to hang out on the low-energy dots,” says Tisdale. “If there’s then a high-energy dot in the way, it takes them a long time to get past that bottleneck.”
So the greater mismatch between energy levels—called energetic disorder—the worse the electron mobility. To measure the impact of energetic disorder on electron flow in their samples, Rachel Gilmore Ph.D. ’17 and her collaborators used a technique called pump-probe spectroscopy—as far as they know, the first time this method has been used to study electron hopping in QDs.
QDs in an excited state absorb light differently than do those in the ground state, so shining light through a material and taking an absorption spectrum provides a measure of the electronic states in it. But in QD materials, electron hopping events can occur within picoseconds—10-12 of a second—which is faster than any electrical detector can measure.
The researchers therefore set up a special experiment using an ultrafast laser, whose beam is made up of quick pulses occurring at 100,000 per second. Their setup subdivides the laser beam such that a single pulse is split into a pump pulse that excites a sample and—after a delay measured in femtoseconds (10-15 seconds)—a corresponding probe pulse that measures the sample’s energy state after the delay. By gradually increasing the delay between the pump and probe pulses, they gather absorption spectra that show how much electron transfer has occurred and how quickly the excited electrons drop back to their ground state.
Using this technique, they measured electron energy in a QD sample with standard dot-to-dot variability and in one of the monodisperse samples. In the sample with standard variability, the excited electrons lose much of their excess energy within 3 nanoseconds. In the monodisperse sample, little energy is lost in the same time period—an indication that the energy levels of the QDs are all about the same.
By combining their spectroscopy results with computer simulations of the electron transport process, the researchers extracted electron hopping times ranging from 80 picoseconds for their smallest quantum dots to over 1 nanosecond for the largest ones. And they concluded that their QD materials are at the theoretical limit of how little energetic disorder is possible. Indeed, any difference in energy between neighboring QDs isn’t a problem. At room temperature, energy levels are always vibrating a bit, and those fluctuations are larger than the small differences from one QD to the next.
“So at some instant, random kicks in energy from the environment will cause the energy levels of the QDs to line up, and the electron will do a quick hop,” says Tisdale.
The way forward
With energetic disorder no longer a concern, Tisdale concludes that further progress in making commercially viable QD materials will require better ways of dealing with structural disorder. He and his team tested several methods of performing ligand exchange in solid samples, and none produced films with consistent QD size and spacing over large areas without cracks. As a result, he now believes that efforts to optimize that process “may not take us where we need to go.”
What’s needed instead is a way to put short ligands on the QDs when they’re in solution and then let them self-assemble into the desired structure.
“There are some emerging strategies for solution-phase ligand exchange,” he says. “If they’re successfully developed and combined with monodisperse QDs, we should be able to produce beautifully ordered, large-area structures well suited for devices such as solar cells, LEDs, and thermoelectric systems.”
The ability to quickly generate ultra-small, well-ordered nanopatterns over large areas on material surfaces is critical to the fabrication of next-generation technologies in many industries, from electronics and computing to energy and medicine. For example, patterned media, in which data are stored in periodic arrays of magnetic pillars or bars, could significantly improve the storage density of hard disk drives.
Scientists can coax thin films of self-assembling materials called block copolymers—chains of chemically distinct macromolecules (polymer “blocks”) linked together—into desired nanoscale patterns through heating (annealing) them on a substrate. However, defective structures that deviate from the regular pattern emerge early on during self-assembly.
Materials scientist Gregory Doerk preparing a sample for electron microscopy at Brookhaven Lab’s Center for Functional Nanomaterials. The scanning electron microscope image on the computer screen shows a cross-sectional view of line …more
The presence of these defects inhibits the use of block copolymers in the nanopatterning of technologies that require a nearly perfect ordering—such as magnetic media, computer chips, antireflective surfaces, and medical diagnostic devices. With continued annealing, the block copolymer patterns can reconfigure to remove the imperfections, but this process is exceedingly slow. The polymer blocks do not readily mix with each other, so they must overcome an extremely large energy barrier to reconfigure.
Adding small things with a big impact
Now, scientists from the Center for Functional Nanomaterials (CFN)—a U.S. Department of Energy (DOE) Office of Science User Facility at Brookhaven National Laboratory—have come up with a way to massively speed up the ordering process. They blended a line-forming block copolymer with significantly smaller polymer chains made of only one type of molecule (homopolymers) from each of the two constituent blocks. The electron microscopy images they took after annealing the films for only a few minutes show that the addition of these two smaller homopolymers dramatically increases the size of well-ordered line-pattern areas, or “grains.”
“Without the homopolymers, the same block copolymer cannot produce grains with these sizes,” said CFN materials scientist Gregory Doerk, who led the work, which was published online in an ACS Nano paper on December 1. “Blending in homopolymers that are less than one-tenth of the size of the block copolymer greatly accelerates the ordering process. In the resulting line patterns, there is a constant spacing between each of the lines, and the same directions of line-pattern orientations—for example, vertical or horizontal—persist over longer distances.”
Doerk and coauthor Kevin Yager, leader of the Electronic Nanomaterials Group at CFN, used image analysis software to calculate the grain size and repeat spacing of the line patterns.
While blending different concentrations of homopolymer to determine how much was needed to achieve the accelerated ordering, they discovered that the ordering sped up as more homopolymer was added. But too much homopolymer actually resulted in disordered patterns.
“The homopolymers accelerate the self-assembly process because they are small enough to uniformly distribute throughout their respective polymer blocks,” said Doerk. “Their presence weakens the interface between the two blocks, lowering the energy barrier associated with the block copolymer reconfiguring to remove the defects. But if the interface is weakened too much through the addition of too much homopolymer, then the blocks will mix together, resulting in a completely disordered phase.”
Guiding the self-assembly of useful nanopatterns in minutes
To demonstrate how the rapid ordering in the blended system could accelerate the self-assembly of well-aligned nanopatterns over large areas, Doerk and Yager used line-pattern templates they had previously prepared through photolithography. Used to build almost all of today’s digital devices, photolithography involves projecting light through a mask (a plate containing the desired pattern) that is positioned over a wafer (usually made of silicon) coated with a light-sensitive material. This template can then be used to direct the self-assembly of block copolymers, which fill in the spaces between the template guides. In this case, after only two minutes of annealing, the polymer blend self-assembles into lines that are aligned across these gaps. However, after the same annealing time, the unblended block copolymer self-assembles into a mostly unaligned pattern with many defects between the gaps.
“The width of the gaps is more than 80 times the repeat spacing, so the fact that we got this degree of alignment with our polymer blend is really exciting because it means we can use templates with huge gaps, created with very low-resolution lithography,” said Doerk. “Typically, expensive high-resolution lithography equipment is needed to align block copolymer patterns over this large of an area.”
For these patterns to be useful for many nanopatterning applications, they often need to be transferred to other more robust materials that can withstand harsh manufacturing processes—for example, etching, which removes layers from silicon wafer surfaces to create integrated circuits or make the surfaces antireflective. In this study, the scientists converted the nanopatterns into a metal-oxide replica. Through chemical etching, they then transferred the replica pattern into a silicon dioxide layer on a silicon wafer, achieving clearly defined line patterns.
Doerk suspects that blending homopolymers with other block copolymers will similarly yield accelerated assembly, and he is interested in studying blended polymers that self-assemble into more complicated patterns. The x-ray scattering capabilities at the National Synchrotron Light Source II—another DOE Office of Science User Facility at Brookhaven—could provide the structural information needed to conduct such studies.
“We have introduced a very simple and easily controlled way of immensely accelerating self-assembly,” concluded Doerk. “Our approach should substantially reduce the number of defects, helping to meet the demands of the semiconductor industry. At CFN, it opens up possibilities for us to use block copolymer self-assembly to make some of the new functional materials that we envision.”
Electrons flowing like liquid in graphene start a new wave of physics – University of Manchester
A new understanding of the physics of conductive materials has been uncovered by scientists observing the unusual movement of electrons in graphene.
Graphene is many times more conductive than copper thanks, in part, to its two-dimensional structure. In most metals, conductivity is limited by crystal imperfections which cause electrons to frequently scatter like billiard balls when they move through the material.
Now, observations in experiments at the National Graphene Institute have provided essential understanding as to the peculiar behaviour of electron flows in graphene, which need to be considered in the design of future nanoelectronic circuits.
In some high-quality materials, like graphene, electrons can travel micron distances without scattering, improving the conductivity by orders of magnitude. This so-called ballistic regime, imposes the maximum possible conductance for any normal metal, which is defined by the Landauer-Buttiker formalism.
Appearing today in Nature Physics (“Superballistic flow of viscous electron fluid through graphene constrictions”), researchers at The University of Manchester, in collaboration with theoretical physicists led by Professor Marco Polini and Professor Leonid Levitov, show that Landauer’s fundamental limit can be breached in graphene. Even more fascinating is the mechanism responsible for this.
Last year, a new field in solid-state physics termed ‘electron hydrodynamics’ generated huge scientific interest. Three different experiments, including one performed by The University of Manchester, demonstrated that at certain temperatures, electrons collide with each other so frequently they start to flow collectively like a viscous fluid.
The new research demonstrates that this viscous fluid is even more conductive than ballistic electrons.
The result is rather counter-intuitive, since typically scattering events act to lower the conductivity of a material, because they inhibit movement within the crystal. However, when electrons collide with each other, they start working together and ease current flow.
This happens because some electrons remain near the crystal edges, where momentum dissipation is highest, and move rather slowly. At the same time, they protect neighbouring electrons from colliding with those regions. Consequently, some electrons become super-ballistic as they are guided through the channel by their friends.
Sir Andre Geim said: “We know from school that additional disorder always creates extra electrical resistance. In our case, disorder induced by electron scattering actually reduces rather than increase resistance. This is unique and quite counterintuitive: Electrons when make up a liquid start propagating faster than if they were free, like in vacuum”.
The researchers measured the resistance of graphene constrictions, and found it decreases upon increasing temperature, in contrast to the usual metallic behaviour expected for doped graphene.
By studying how the resistance across the constrictions changes with temperature, the scientists revealed a new physical quantity which they called the viscous conductance. The measurements allowed them to determine electron viscosity to such a high precision that the extracted values showed remarkable quantitative agreement with theory.
Schematic representation of the folding and anchoring processes needed to obtain π-folded molecular junctions from a representative member of the foldamer family studied in this work. ( Nature) (click on image to enlarge)
The current demand for small-sized electronic devices is calling for fresh approaches in their design.A group of researchers at the Basque Excellence Research Center into Polymers (POLYMAT), the University of the Basque Country (UPV/EHU), the University of Barcelona, the Institute of Bioengineering of Barcelona (IBEC), and the University of Aveiro, and led by Aurelio Mateo-Alonso, the Ikerbasque research professor at POLYMAT, have developed a new suite of molecular wires or nanowires that are opening up new horizons in molecular electronics.The research is being published today in the prestigious journal Nature Communications (“High conductance values in π-folded molecular junctions”).
The growing demand for increasingly smaller electronic devices is prompting the need to produce circuits whose components are also as small as possible, and this is calling for fresh approaches in their design.
Molecular electronics has sparked great interest because the manufacture of electronic circuits using molecules would entail a reduction in their size.
Nanowires are conducting wires on a molecular scale that carry the current inside these circuits. That is why the efficiency of these wires is crucially important.
In fact, one of the main novelties in this new suite of nanowires developed by the group led by Aurelio Mateo lies in their high efficiency, which constitutes a step forward in miniaturizing electronic circuits.
Recently, Researchers in Tsinghua University have proposed a nitrogen-doped graphene matrix with densely and uniformly distributed lithiophilic functional groups for dendrite-free lithium metal anodes, appearing in the journal Angewandte Chemie International Edition.
Since lithium metal possesses an ultrahigh theoretical specific capacity (3860 mAh g-1) and the lowest negative electrochemical potential (-3.040 V vs. the standard hydrogen electrode), lithium metal has been regarded as the most promising electrode material for next-generation high-energy-density batteries. However, the application of lithium metal batteries is still not in sight. “Lithium dendrite growth has hindered the development of lithium metal anodes,” said Dr. Qiang Zhang, the corresponding author, a faculty at Department of Chemical Engineering, Tsinghua University. “Lithium dendrites that form during repeated lithium plating and stripping cycles can not only induce many ‘dead Li’ with irreversible capacity loss, but also cause internal short circuits in batteries and other hazardous issues.”
“We found that a lithiophilic material with good metallic lithium affinity can guide the lithium metal nucleation. Therefore, designing a lithium-plating matrix with a high surface area and lithiophilic surface makes sense for a safe and efficient lithium metal anode,” said Xiao-Ru Chen, an undergraduate student in Tsinghua University. “So we employed a nitrogen-doped graphene matrix with densely and uniformly distributed nitrogen containing functional groups to guide lithium metal nucleation and growth.”
“The nitrogen containing functional groups are lithiophilic sites, confirmed by our experimental and DFT calculation results. Lithium metal can plate with uniform nucleation during the charging process, followed by growth into dendrite-free morphology. While on the normal Cu foil-based anode, the nucleation sites are scattered, which may cause lithium dendrite growth more easily,” said Xiang Chen, a Ph.D. student at Tsinghua University.
With the lithiophilic nitrogen-containing functional groups, the N-doped graphene matrix can regulate the nucleation process of lithium electrodeposition. As a result, dendrite-free lithium metal deposits were obtained. Additionally, this matrix shows impressive electrochemical performance. The Coulombic efficiency of the N-doped graphene-based electrode at a current density of 1.0 mA cm-2 and a cycle capacity of 1.0 mAh cm-2 can reach 98 percent for nearly 200 cycles.
“We have proposed a new strategy based on lithiophilic site-guided nucleation to settle the tough dendrite challenge in this publication,” said Qiang. “Further research is required to investigate and control the lithium nucleation in lithium metal batteries. We believe that the practical application of lithium metal anodes can be finally realized.” The control of the nucleation process of lithium plating with a lithiophilic matrix has shed a new light on all lithium metal-based batteries, such as Li-S, Li-O2 and future Li-ion batteries.
More information: Rui Zhang et al. Lithiophilic Sites in Doped Graphene Guide Uniform Lithium Nucleation for Dendrite-Free Lithium Metal Anodes, Angewandte Chemie International Edition (2017). DOI: 10.1002/anie.201702099