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Researchers have created nanoribbons of an emerging class of materials called topological insulators and used a magnetic field to control their semiconductor properties, a step toward harnessing the technology to study exotic physics and building new spintronic devices or quantum computers.

Unlike ordinary materials that are either insulators or conductors, topological insulators are paradoxically both at the same time – they are insulators inside but conduct electricity on the surface, said Yong P. Chen, a Purdue University associate professor of physics and astronomy and electrical and computer engineering who worked with doctoral student Luis A. Jauregui and other researchers.

The materials might be used for “spintronic” devices and practical quantum computers far more powerful than today’s technologies. In the new findings, the researchers used a magnetic field to induce a so-called “helical mode” of electrons, a capability that could make it possible to control the spin state of electrons.

The findings are detailed in a research paper that appeared in the advance online publication of the journal Nature Nanotechnology on Jan. 18 and showed that a magnetic field can be used to induce the nanoribbons to undergo a “topological transition,” switching between a material possessing a band gap on the surface and one that does not.

“Silicon is a semiconductor, meaning it has a band gap, a trait that is needed to switch on and off the conduction, the basis for silicon-based digital transistors to store and process information in binary code,” Chen said. “Copper is a metal, meaning it has no band gap and is always a good conductor. In both cases the presence or absence of a band gap is a fixed property. What is weird about the surface of these materials is that you can control whether it has a band gap or not just by applying a magnetic field, so it’s kind of tunable, and this transition is periodic in the magnetic field, so you can drive it through many ‘gapped’ and ‘gapless’ states.”

The nanoribbons are made of bismuth telluride, the material behind solid-state cooling technologies such as commercial thermoelectric refrigerators.

“Bismuth telluride has been the workhorse material of thermoelectric cooling for decades, but just in the last few years people found this material and related materials have this amazing additional property of being topological insulators,” he said.

The paper was authored by Jauregui; Michael T. Pettes, a former postdoctoral researcher at the University of Texas at Austin and now an assistant professor in the Department of Mechanical Engineering at the University of Connecticut; Leonid P. Rokhinson, a Purdue professor of physics and astronomy and electrical and computer engineering; Li Shi, BF Goodrich Endowed Professor in Materials Engineering at the University of Texas at Austin; and Chen

A key finding was that the researchers documented the use of nanoribbons to measure so-called Aharonov-Bohm oscillations, which is possible by conducting electrons in opposite directions in ring-like paths around the nanoribbons. The structure of the nanoribbon – a nanowire that is topologically the same as a cylinder – is key to the discovery because it allows the study of electrons as they travel in a circular direction around the ribbon. The electrons conduct only on the surface of the nanowires, tracing out a cylindrical circulation.

“If you let electrons travel in two paths around a ring, in left and right paths, and they meet at the other end of the ring then they will interfere either constructively or destructively depending on the phase difference created by a magnetic field, resulting in either high or low conductivity, respectively, showing the quantum nature of electrons behaving as waves,” Jauregui said.

The researchers demonstrated a new variation on this oscillation in topological insulator surfaces by inducing the spin helical mode of the electrons. The result is the ability to flip from constructive to destructive interference and back.

“This provides very definitive evidence that we are measuring the spin helical electrons,” Jauregui said. “We are measuring these topological surface states. This effect really hasn’t been seen very convincingly until recently, so now this experiment really provides clear evidence that we are talking about these spin helical electrons propagating on the cylinder, so this is one aspect of this oscillation.”

Findings also showed this oscillation as a function of “gate voltage,” representing another way to switch conduction from high to low.

“The switch occurs whenever the circumference of the nanoribbon contains just an integer number of the quantum mechanical wavelength, or ‘fermi wavelength,’ which is tuned by the gate voltage of the electrons wrapping around the surface,” Chen said.

It was the first time researchers have seen this kind of gate-dependent oscillation in nanoribbons and further correlates it to the topological insulator band structure of bismuth telluride.

The nanoribbons are said to possess “topological protection,” preventing electrons on the surface from back scattering and enabling high conductivity, a quality not found in metals and conventional semiconductors. They were fabricated by researchers at the UT Austin.

The measurements were performed while the nanoribbons were chilled to about minus 273 degrees Celsius (nearly minus 460 degrees Fahrenheit).

“We have to operate at low temperatures to observe the quantum mechanical nature of the electrons,” Chen said.

Future research will include work to further investigate the nanowires as a platform to study the exotic physics needed for topological quantum computations. Researchers will aim to connect the nanowires with superconductors, which conduct electricity with no resistance, for hybrid topological insulator-superconducting devices. By further combining topological insulators with a superconductor, researchers may be able to build a practical quantum computer that is less susceptible to the environmental impurities and perturbations that have presented challenges thus far. Such a technology would perform calculations using the laws of quantum mechanics, making for computers much faster than conventional computers at certain tasks such as database searches and code breaking.

Oxygen is indispensable to animal and plant life, but its presence in the wrong places can feed a fire and cause iron to rust.

In the fabrication of solid state lighting devices, scientists are learning, oxygen also plays a two-edged role. While oxygen can impede the effectiveness of gallium nitride (GaN), an enabling material for LEDs, small amounts of oxygen in some cases are needed to enhance the devices’ optical properties. GaN doped with europium (Eu), which could provide the red color in LEDs and other displays, is one such case.

Last week, an international group of researchers shed light on this seeming contradiction and reported that the quantity and location of oxygen in GaN can be fine-tuned to improve the optical performance of Eu-doped GaN devices. The group includes researchers from Lehigh, Osaka University in Japan, the Instituto Superior Técnico in Portugal, the University of Mount Union in Ohio, and Oak Ridge National Laboratory in Tennessee.

Writing in Scientific Reports, a Nature publication, the group said that small quantities of oxygen promote the uniform incorporation of Eu into the crystal lattices of GaN. The group also demonstrated a method of incorporating Eu uniformly that utilizes only the oxygen levels that are inevitably present in the GaN anyway. Eu, a rare earth (RE) element, is added to GaN as a “dopant” to provide highly efficient red color emission, which is still a challenge for GaN-based optoelectronic devices.

The devices’ ability to emit light is dependent on the relative homogeneity of Eu incorporation, said Volkmar Dierolf, professor and chair of Lehigh’s physics department.

“Some details, such as why the oxygen is needed for Eu incorporation, are still unclear,” said Dierolf, “but we have determined that the amount required is roughly 2 percent of the amount of Eu ions. For every 100 Eu ions, you need two oxygen atoms to facilitate the incorporation of Eu to GaN.

“If the oxygen is not there, the Eu clusters up and does not incorporate. When the oxygen is present at about 2 percent, oxygen passivation takes place, allowing the Eu to incorporate into the GaN without clustering.”

The article is titled “Utilization of native oxygen in Eu(RE)-doped GaN for enabling device compatibility in optoelectronic applications.” The lead author, Brandon Mitchell, received his Ph.D. from Lehigh in 2014 and is now an assistant professor of physics and astronomy at the University of Mount Union and a visiting professor at Osaka University.

 

A comprehensive study

Gallium nitride, a hard and durable semiconductor, is valued in solid state lighting because it emits light in the visible spectrum and because its wide band gap makes GaN electronic devices more powerful and energy-efficient than devices made of silicon and other semiconductors.

The adverse effect of oxygen on GaN’s properties has been much discussed in the scientific literature, the researchers wrote in Scientific Reports, but oxygen’s influence on, and interaction with, RE dopants in GaN is less well understood.

“The presence of oxygen in GaN,” the group wrote in their article, which was published online Jan. 4, “…is normally discussed with a purely negative connotation, where possible positive aspects of its influence are not considered.

“For the continued optimization of this material, the positive and negative roles of critical defects, such as oxygen, need to be explored.”

The group used several imaging techniques, including Rutherford Backscattering, Atomic Probe Tomography and Combined Excitation Emission Spectroscopy, to obtain an atomic-level view of the diffusion and local concentrations of oxygen and Eu in the GaN crystal lattice.

Its investigation, the group wrote, represented the “first comprehensive study of the critical role that oxygen has on Eu in GaN.” The group chose to experiment with Eu-doped GaN (GaN:Eu), said Dierolf, because europium emits bright light in the red portion of the electromagnetic spectrum, a promising quality given the difficulty scientists have encountered in realizing red LED light.

The group said its results “strongly indicate that for single layers of GaN:Eu, significant concentrations of oxygen are required to ensure uniform Eu incorporation and favorable optical properties.

“However, for the high performance and reliability of GaN-based devices, the minimization of oxygen is essential. It is clear that these two requirements are not mutually compatible.”

Preliminary LED devices containing a single 300-nanometer active GaN:Eu layer have been demonstrated in recent years, the group reported, but have not yet achieved commercial viability, in part because of the incompatibility of oxygen with GaN.

To overcome that hurdle, said Dierolf, the researchers decided that instead of growing one thick, homogeneous layer of GaN:Eu they would grow several thinner layers of alternating doped and undoped regions. This approach, they found, utilizes the relatively small amount of oxygen that is naturally present in GaN grown with organo-metallic vapor phase epitaxy (OMVPE), the common method of preparing GaN.

“Instead of growing a thick layer of Eu-doped GaN,” said Dierolf, “we grew a layer that alternated doped and undoped regions. Through the diffusion of the europium ion, oxygen from the undoped regions was utilized to incorporate the Eu into the GaN. The europium then diffused into the undoped regions.”

To determine the optimal amount of oxygen needed to circumvent the oxygen-GaN incompatibility, the researchers also conducted experiments on GaN grown with an Eu “precursor” containing oxygen and on GaN intentionally doped with argon-diluted oxygen.

They found that the OMVPE- grown GaN contained significantly less oxygen than the other samples.

“The concentration of this oxygen [in the OMVPE- grown GaN] is over two orders of magnitude lower than those [concentrations] found in the samples grown with the oxygen-containing Eu…precursor,” the group wrote, “rendering the material compatible with current GaN-based devices.

“We have demonstrated that the oxygen concentration in GaN:Eu materials can be reduced to a device-compatible level. Periodic optimization of the concentration ratio between the normally occurring oxygen found in GaN and the Eu ions resulted in uniform Eu incorporation, without sacrificing emission intensity.

“These results appear to coincide with observations in other RE-doped GaN materials. Adoption of the methods discussed in this article could have a profound influence on the future optimization of these systems as well as GaN:Eu.”

The group plans next to grow GaN quantum well structures and determine if they enable Eu to incorporate even more favorably and effectively into GaN. Toward that end, Dierolf and Nelson Tansu, professor of electrical and computer engineering and director of Lehigh’s Center for Photonics and Nanoelectronics, have been awarded a Collaborative Research Opportunity (CORE) grant from Lehigh.

Stanford University researchers sponsored by Semiconductor Research Corporation (SRC) have developed a new area selective atomic layer deposition (ALD) process that promises to accelerate the manufacturing of higher performing, more energy efficient semiconductors.

It is well known that next-generation electronic, optoelectronic and sensing devices that contain nanoscale dimensions face increasingly difficult materials and fabrication challenges as the downward scaling of these devices continues. Conventional semiconductor manufacturing processes are time-consuming and expensive, in part due to the need for lithographic patterning. The Stanford research leverages simple ALD and etching processes that eliminate this lithography step and improve selective deposition of dielectric materials by more than 10 times in film thickness compared to similar advanced processes.

Due to difficulties of current top-down fabrication processes that contain multiple deposition, lithography and etching steps, along with problems with misalignment in lithography, using an alternative approach in which the desired materials are directly and selectively deposited would significantly facilitate the process, according to the Stanford team.

“Our technology is a promising candidate for overcoming the challenges of top-down processing and misalignment because it greatly improves the ability to perform selective deposition of materials. This research introduces a novel processing method to meet the increasingly difficult materials challenges associated with new devices,” said Dr. Stacey Bent, Department of Chemical Engineering Chair and Jagdeep and Roshni Singh Professor in School of Engineering, Stanford University.

Current approaches utilize lithography for nanoscale patterning. Using lithography and etching for fabrication of 2D or 3D structures often results in misaligned features and causes a risk of shorting or high resistant areas. However, selective deposition using ALD can reduce these risks and reduce the process time and steps.

Bent explained that selective deposition allows layers of material to be added onto a substrate only where desired without the need for additional lithography steps. However, the high level of selectivity needed for a manufacture-worthy process has not yet been achieved in area selective deposition studies. In addition, most methods for area selective deposition require long processing times.

The Stanford research has been focused on selective deposition of dielectric materials on metal/dielectric patterns. These type of structures can be found in interconnects and back-end-of-line (BEOL) processing. With ALD being used in other stages of the device fabrication process as well, the results from the Stanford experiments can potentially be applied to a variety of nanoscale electronic, optoelectronic and sensing devices.

The research developments occurred during the second year of research on the topic, and the Stanford team is continuing to explore new methods for area selective ALD to improve both selectivity and manufacturability.

“The Stanford team’s research has shown for the first time that, by following selective deposition of a dielectric material using pre-treatment by an inhibitory material, they can significantly reduce the process time (from 48 hours to less than 1 hour) and also improve the limits of selective deposition of dielectrics by more than 10 times,” said Kwok Ng, Senior Science Director of Nanomanufacturing Materials and Processes at SRC.

By Arnaud Furnemont, Department Director Memory at imec

Research in memory is really exciting these days: in parallel you have the scaling of classical memories (SRAM, DRAM, Flash) and the emergence of new memories capable of enabling new applications or even new system hierarchies. At imec, we mostly focus on three concepts which all come with different challenges.

First is Flash, and specifically 3D NAND. Here it’s the integration challenge that is keeping us all busy. Before, the focus was on device scaling, but now it’s all about stacking more layers. Last year, we explored new materials for the channel (e.g. III-V channel in 3D NAND) and for the trapping layer (YAlO instead of SiN), in parallel with device reliability characterization and modelling.

Another important memory type is STT-MRAM where a complex magnetic stack makes the scene. Focus here is on choosing the right material combination and developing the perfect stack (with perfect interfaces!). Over the last years, imec made a lot of progress to build a good stack. But even more challenging is the patterning of this multi-layers structure without affecting the magnetic properties of the device. Very recently we were able to demonstrate 45nm devices with good performance. Tool suppliers are improving the etch platforms and I expect STT-MRAM as embedded memory in the foundries by 2017 and as standalone memory by 2020. In the latter case, more scaling is necessary and this implies more etch issues which will have to be solved.

Thirdly, we explore resistive RAM. The challenge for this type of memory is picking the right combination out of the numerous kinds of stacks and materials. And to do this, you need a fundamental understanding of what happens inside each stack. Imec has developed in depth characterization and modelling on OxRAM and CBRAM memories, expected to be used in embedded applications. Globally, RRAM suffers from a trade-off between write energy and stability. VMCO is another RRAM variant developed at imec to break this trade-off. To be competitive in standalone applications, RRAM will also need to be combined with a selector, which requires again material selection and benchmarking. This is a role that imec is willing to take on for its partners.

Finally, there is also a high-level challenge that the memory researchers and developers are facing. It’s the changing landscape in which emerging memories have more and more impact on the system architecture. Before, the system hierarchy was built with the memory technologies that were available. In the future it might be the other way around: the system architects will tell us what to develop. A closer collaboration between the device team and system architects is therefore indispensable. Imec’s memory ‘insite’ activity will tackle this challenge. 

After a master in electro-mechanics, Arnaud Furnémont completed his PhD at IMEC, focusing on characterization of nitride-based memory such as NROM and TANOS. In 2008, he joined Intel in Boise (Idaho) and became responsible for 20nm planar Flash reliability, and later for an emerging memory concept. In 2013 he joined IMEC as memory characterization and integration teams manager. Since end of 2014, Arnaud serves as memory department director, MRAM and Flash program director at imec.

Organic-inorganic perovskite materials are key components of the new generation of solar cells. Understanding properties of these materials is important for improving lifetime and quality of solar cells. Researchers from the Energy Materials and Surface Sciences (EMSS) Unit, led by Prof. Yabing Qi, at the Okinawa Institute of Science and Technology Graduate University (OIST) in collaboration with Prof. Youyong Li’s group from Soochow University (China) and Prof. Nam-Gyu Park’s group from Sungkyunkwan University (Korea) report in the Journal of the American Chemical Societythe first atomic resolution study of organic-inorganic perovskite.

Perovskites are a class of materials with the general chemical formula ABX3. A and B are positive ions bound by negative ions X. Organic-inorganic perovskites used in solar cells are usually methylammonium lead halides (CH3NH3PbX3, where X is bromine, iodine, or chlorine). The OIST scientists used a single crystal of methylammonium lead bromide (CH3NH3PbBr3) to create topographic images of its surface with a scanning tunneling microscope.

This microscope uses a conducting tip that moves across the surface in a manner very similar to a finger moving across a Braille sign. While the bumps in Braille signs are a few millimetres apart, the microscope detects bumps that are more than million times smaller — atoms and molecules. This is achieved by the quantum tunneling effect — the ability of an electron to pass through a barrier. The probability of an electron passing between the material surface and the tip depends on the distance between the two. The resulting atomic-resolution topographic images reveal positions and orientations of atoms and molecules, and also provide a detailed look at structural defects in the surface.

“At room temperature atoms and molecules are quite mobile, so we decided to freeze the crystal to almost absolute zero (-269ºC) to get a good picture of its atomic structure,” says Dr. Robin Ohmann, a member of the EMSS Unit and the first author of the paper. The crystal was cut and studied in a vacuum to avoid contamination of the surface. Dr. Ohmann’s colleagues from Soochow University calculated atomic structures using principles of quantum physics and then compared them with scanning tunneling microscopy data.

The researchers discovered that methylammonium molecules can rotate and that they favour specific orientations that lead to two types of surface structures with distinctly different properties. Apart from rotation, these molecules affect positions of neighbouring bromine ions, further altering the atomic structure. Since the structure dictates the electronic properties of the material, the geometric positions of atoms are essential for understanding of solar cells.

Scanning tunneling microscope images also reveal local imperfections caused by dislocations of molecules and ions and, probably, missing atoms. These imperfections may affect device performance, for example, by changing electrical properties such as conductivity.

The structure of perovskite materials is temperature-sensitive and the observed structure of the frozen crystal might not be fully identical to the structure at room temperature. However, the comprehensive description of perovskite crystals at the atomic level paves the way to better understanding of their behaviour under real-life conditions. The current findings shed light on molecule-ion interplay on the surface of an organic-inorganic crystal and should help to improve designs of future solar cells. The next goal of the researchers is to examine interactions between perovskites and other molecules, for example water molecules that are known to interfere with the performance of solar cells.

An international research team at Tohoku University’s Advanced Institute of Materials Research (AIMR) succeeded in chemically interconnecting chiral-edge graphene nanoribbons (GNRs) with zigzag-edge features by molecular assembly, and demonstrated electronic connection between GNRs. The GNRs were interconnected exclusively end to end, forming elbow structures, identified as interconnection points.

This configuration enabled researchers to demonstrate that the electronic architecture at the interconnection points between two GNRs is the same as that along single GNRs; evidence that GNR electronic properties, such as electron and thermal conductivities, are directly extended through the elbow structures upon chemical GNR interconnection.

This work shows that future development of high-performance, low-power-consumption electronics based on GNRs is possible.

Graphene has long been expected to revolutionize electronics, provided that it can be cut into atomically precise shapes that are connected to desired electrodes. However, while current bottom-up fabrication methods can control graphene’s electronic properties, such as high electron mobility, tailored band gaps and s pin-aligned zigzag edges, the connection aspect of graphene structures has never been directly explored. For example, whether electrons traveling across the interconnection points of two GNRs would encounter higher electric resistance remains an open question. As the answers to this type of questions are crucial towards the realization of future high-speed, low-power-consumption electronics, we use molecular assembly to address this issue here.

“Current molecular assemblies either produce straight GNRs (i.e., without identifiable interconnection points), or randomly interconnected GNRs,” says Dr. Patrick Han, the project leader. “These growth modes have too many intrinsic unknowns for determining whether electrons travel across graphene interconnection points smoothly. The key is to design a molecular assembly that produces GNRs that are systematically interconnected with clearly distinguishable interconnection points.”

To reach this goal, the AIMR team used a Cu substrate, whose reactivity confines the GNR growth to six directions, and used scanning tunneling microscopy (STM) to visualize the GNR electronic structures. By controlling the precursor molecular coverage, this molecular assembly connects GNRs from different growth directions systematically end to end, producing elbow structures–identified as interconnection points. Using STM, the AIMR team revealed that the delocalization of the interconnected GNR π*-states extends the same way both across a single straight GNR, and across the interconnection point of two GNRs (periodic features in Fig. 1b, bottom panel). This result indicates that GNR electronic properties, such as electron and thermal conductivities, should be the same at the termini of single GNRs and that of two connected GNRs.

“The major finding of this work is that interconnected GNRs do not show electronic disruption (e.g., electron localization that increases resistance at the interconnection points),” says Han. “The electronically smooth interconnection demonstrates that GNR properties (including tailored band gaps, or even spin-aligned zigzag edges) can be connected to other graphene structures. These results show that finding a way to connect defect-free GNRs to desired electrodes may be the key strategy toward achieving high-performance, low-power-consumption electronics.”

When people think about Wide Band Gap (WBG) materials for power electronics applications, they usually think of GaN or SiC. This is a not a surprise: indeed SiC and GaN are currently the most advanced WBG technologies for power electronics applications. However, there are materials with an even larger band gap which can further increase power device performance. What is the development status of such innovative technologies? Are there already some products available on the market? What is the added-value of such materials?

Yole Développement (Yole) proposes a comprehensive overview of the whole WBG solutions dedicated to the power electronics industry. This survey is entitled SiC, GaN and other WBG materials for power electronics applications. Including a detailed analysis of the most advanced WBG materials, SiC and GaN, Yole’s report also highlights the added-value of disruptive technologies such as Ga2O3, diamond and AlN. Yole’s analysts detail the status of such new solutions and the related technology roadmap. The “More than Moore” market research and strategy consulting company also presents the technical and market challenges facing WBG players.

wbj materials

As the Si technology is reaching the theoretical limits, new semiconductor materials called wide band gap (WBG) is becoming the new choice for power electronics applications. Different WBG materials are SiC, GaN, Ga2O3, Diamond and AlN. The development status of these WBG materials varies from one to other. Indeed SiC and GaN-on-Si based power devices are commercially available today; the development of GaN-on-GaN power devices is ongoing; Ga2O3, diamond and AlN power devices are just at a primitive stage. And Yole details:

•  Thanks to its high band gap and doping possibility at room temperature, Ga2O3 has been proposed for power electronics applications. Compared to existing SiC and bulk GaN technology, GaSO3 key selling point is the possibility of using melt growth to make large, inexpensive wafers. Under this process, much little energy is used compared to energy-consuming methods employed for GaN and SiC bulk crystals and substrates creation: sublimation, vapor phase epitaxy, and high-pressure synthesis.

“It is estimated that the power dissipated per-unit-area of substrate at the time of production is just one-third of that associated with SiC sublimation, due to a lower growth temperature and a higher growth rate,” explained Dr. Hong Lin, Technology & Market Analyst at Yole. “As the same system configuration for sapphire is used, it should be possible to make cheaper Ga2O3 substrates than bulk GaN or SiC. If there is demand, it should be also possible to make 6” Ga2O3 substrates at a low unit cost. However, the demand is quite limited so far and the price remains high.”

•  Diamond is the ideal candidate for power electronic applications, thanks to a combination of unique properties. Electronics applications identified by Yole are Schottky diodes, transistors, etc. They require high-quality single-crystalline CVD diamond.

•  Having initially targeted UV LED applications but finding subpar demand, some AlN suppliers are now targeting the power market in order to diversify their activities. AlN’s key value proposition for power applications is the fact that it has the largest band gap.

Under its WBG materials report, Yole’s analysts reveal the state-of-the-art materials like SiC, GaN, Ga2O3, diamond, and AlN. They define a comprehensive technology roadmap and propose a deep understanding of the WBG materials evolution in the power electronics sector.

How do you get to know a material that you cannot see?

That is a question that researchers studying nanomaterials–objects with features at the sub-micrometer scales such as quantum dots, nanoparticles and nanotubes–are seeking to answer.

Though recent discoveries–including a super-resolution microscopy which won the Nobel Prize in 2014–have greatly enhanced scientists’ capacity to use light to learn about these small-scale objects, the wavelength of the inspecting radiation is always much larger than the scale of the nano-objects being studied. For example, nanotubes and nanowires-the building blocks of next-generation electronic devices-have diameters that are hundreds of times smaller than the light could resolve. Researchers must find ways to circumvent this physical limitation in order to achieve sub-wavelength spatial resolution and explore the nature of these materials for future computers.

Today, a group of scientists–John A. Rogers, Eric Seabron, Scott MacLaren and Xu Xie from the University of Illinois at Urbana-Champaign;  Slava V. Rotkin from Lehigh University; and, William L. Wilson from Harvard University–are reporting on the discovery of an important method for measuring the properties of nanotube materials using a microwave probe. Their findings have been published in ACS Nano in an article called: “Scanning Probe Microwave Reflectivity of Aligned Single-Walled Carbon Nanotubes: Imaging of Electronic Structure and Quantum Behavior at the Nanoscale.”

The researchers studied single-walled carbon nanotubes. These are 1-dimensional, wire-like nanomaterials that have electronic properties that make them excellent candidates for next generation electronics technologies. In fact, the first prototype of a nanotube computer has already been built by researchers at Stanford University. The IBM T.J. Watson Research Center is currently developing nanotube transistors for commercial use.

For this study, scientists grew a series of parallel nanotube lines, similar to the way nanotubes will be used in computer chips. Each nanotube was about 1 nanometer wide–ten times smaller than expected for use in the next generation of electronics. To explore the material’s properties, they then used microwave impedance microscopy (MIM) to image individual nanotubes.

“Although microwave near-field imaging offers an extremely versatile ‘nondestructive’ tool for characterizing materials, it is not an immediately obvious choice,” explained Rotkin, a professor with a dual appointment in Lehigh’s Department of Physics and Department of Materials Science and Engineering. “Indeed, the wavelength of the radiation used in the experiment was even longer than what is typically used in optical microscopy-about 12 inches, which is approximately 100,000,000 times larger than the nanotubes we measured.”

He added: “The nanotube, in this case, is like a very bright needle in a very large haystack.”

The imaging method they developed shows exactly where the nanotubes are on the silicon chip. More importantly, the information delivered by the microwave signal from individual nanotubes revealed which nanotubes were and were not able to conduct electric current. Unexpectedly, they were finally able to measure the nanotube quantum capacitance-a very unique property of an object from the nano-world-under these experimental conditions.

“We began our collaboration seeking to understand the images taken by the microwave microscopy and ended by unveiling the nanotube’s quantum behavior, which can now be measured with atomistic resolution,” said Rotkin.

As an inspection tool or metrology technique, this approach could have a tremendous impact on future technologies, allowing optimization of processing strategies including scalable enriched nanotube growth, post-growth purification, and fabrication of better device contacts. One can now distinguish, in one simple step, between semiconductor nanotubes that are useful for electronics and metallic ones that can cause a computer to failure. Moreover this set of imaging modes sheds light on the quantum properties of these 1D structures.

Whether showing off family photos on smartphones or watching TV shows on laptops, many people look at liquid crystal displays (LCDs) every day. LCDs are continually being improved, but almost all currently use color technology that fades over time. Now, a team reports in ACS Nano that using aluminum nanostructures could provide a vivid, low-cost alternative for producing digital color.

Conventional color technology used in displays is susceptible to photobleaching, or fading. So researchers have looked toward aluminum nanoparticles that can display colors in electronics, thanks to a property called ‘plasmon resonance.’ To create plasmonic color devices, researchers group nanostructures into arrays called pixels. Color is generated by scattering light onto the pixels, with different arrangements creating different colors. Aluminum plasmonic pixels are advantageous for use in electronic displays because they are inexpensive and can be made in an ultrasmall size, which can increase image resolution. But these pixels create muted and dull colors. In a recent publication, Stephan Link and colleagues developed a method that allows the red end of the color spectrum to be more vibrant. Now, the same team reports another approach that makes the blue end of the spectrum much more brilliant, too.

The researchers used a three-step design approach to create aluminum nanostructure pixels that exploit ‘Fano interference’ — an interaction between the plasmon resonance and the pixel’s array structure — to produce vibrant blue-end colors. Combining their previous research with this new development, the team was able to create pixels with extremely vivid colors across the entire visible spectrum. The researchers then incorporated a set of red, green and blue pixels into a liquid crystal display that could be electrically turned on and off, demonstrating this work’s potential use in commercial flat-panel displays.

An international team of researchers including Professor Federico Rosei and members of his group at INRS has developed a new strategy for fabricating atomically controlled carbon nanostructures used in molecular carbon-based electronics. An article just published in the prestigious journal Nature Communications presents their findings: the complete electronic structure of a conjugated organic polymer, and the influence of the substrate on its electronic properties.

The researchers combined two procedures previously developed in Professor Rosei’s lab–molecular self-assembly and chain polymerization–to produce a network of long-range poly(para-phenylene) (PPP) nanowires on a copper (Cu) surface. Using advanced technologies such as scanning tunneling microscopy and photoelectron spectroscopy as well as theoretical models, they were able to describe the morphology and electronic structure of these nanostructures.

“We provide a complete description of the band structure and also highlight the strong interaction between the polymer and the substrate, which explains both the decreased bandgap and the metallic nature of the new chains. Even with this hybridization, the PPP bands display a quasi one-dimensional dispersion in conductive polymeric nanowires,” said Professor Federico Rosei, one of the authors of the study.

Although further research is needed to fully describe the electronic properties of these nanostructures, the polymer’s dispersion provides a spectroscopic record of the polymerization process of certain types of molecules on gold, silver, copper, and other surfaces. It’s a promising approach for similar semiconductor studies–an essential step in the development of actual devices.

The results of the study could be used in designing organic nanostructures, with significant potential applications in nanoelectronics, including photovoltaic devices, field-effect transistors, light-emitting diodes, and sensors.