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. 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." Release Date: January 8, 2016 Source: Tohoku University
Abstract: 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 (Fig. 1a). This configuration enabled researchers to demonstrate that the electronic architecture at the interconnection points between two GNRs (Fig. 1b) 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 (Fig. 1a). 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." About Tohoku University The Advanced Institute for Materials Research (AIMR) at Tohoku University is one of nine World Premier International Research Center Initiative (WPI) Programs established with the support of the Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT). The program aims to develop world-class research bases in Japan. After its establishment in 2007, AIMR has been active in conducting research activities and creating new systems in order to become a global center for materials science. Since 2012, AIMR has also been conducting fundamental research by finding connections between materials science and mathematics. Learn more at www.wpi-aimr.tohoku.ac.jp For more information, please click Contacts: Patrick Han 81-222-176-170 For information on AIMR and all other enquiries: Marie Minagawa Public Relations & Outreach office Advanced Institute for Materials Research, Tohoku University aimr-outreachgrp.tohoku.ac.jp Fax: +81-22-217-6146 If you have a comment, please us. Issuers of news releases, not 7th Wave, Inc. or Nanotechnology Now, are solely responsible for the accuracy of the content.
Silicon photovoltaic devices typically sandwich two semiconductor layers containing positively or negatively charged impurity atoms, or dopants, into a so-called p-n junction. The electric field that forms at the p-n interface is an efficient way to collect charge carriers generated from incident light. However, accurately implanting or diffusing dopants into silicon requires specialized equipment and ultra-clean manufacturing conditions. Goutam Dalapati and co-workers from the A*STAR Institute of Materials Research and Engineering found that metal silicides, substances produced when metal coatings are annealed with silicon wafers, hold new promise for reducing solar cell production costs. Metal silicides are fundamental to the operation of nearly all microelectronic devices, and behave like conductive wires or voltage-dependent switches depending on their contents and preparation conditions—an adaptable nature the team aimed to exploit with iron-based silicides. "We studied iron silicides because they have metallic states as well as semiconducting states," says Dalapati. "We found the metallic states of iron silicide can re-grow the silicon substrate they were deposited on—something that has not been investigated before for solar cell applications." The team used simple sputter deposition to lay down nanometer-thin coatings of aluminum and iron silicide precursors onto an n-type silicon wafer. After a rapid thermal anneal that transforms the coatings into a metallic aluminum alloyed iron silicide state in just one minute, they examined the resulting interfaces with transmission electron microscopy (see image). Sandwiched between the aluminum alloyed iron silicide coating and the n-type silicon was a 5–10 nanometer wide strip of regrown silicon crystals with high densities of aluminum p-type dopants—a spontaneously formed p-n junction. Solar illumination experiments revealed the team's interface engineering had a strong effect on photovoltaic performance: the light harvesting efficiency improved from 0.8 to 5.1 per cent after the rapid thermal anneal. Dalapati explains that the metallic state of iron silicides acts as a dopant reservoir, supplying silicon atoms to re-grow into uniform, aluminum-doped layers in a few short steps. He also notes that optimization of parameters such as aluminum concentrations might enable further control over this regions' thickness. "This approach makes a p-n junction that is very stable, reproducible, and possible to make in large areas" says Dalapati. "With proper anti reflection coatings, we might get to record efficiencies for silicon cells." Explore further: Simple, inexpensive fabrication procedure boosts light-capturing capabilities of tiny holes carved into silicon wafers More information: Goutam Kumar Dalapati et al. Aluminium alloyed iron-silicide/silicon solar cells: A simple approach for low cost environmental-friendly photovoltaic technology, Scientific Reports (2015). DOI: 10.1038/srep17810
Holograms contain complex, three-dimensional image information that makes them difficult—but not impossible—to counterfeit. One way to improve their security is by using sophisticated devices that enhance holographic resolution. Nanophotonic devices deploy arrays of nanoscale light scattering pixels that encode additional layers of information through 'near field' optical interactions between lasers and the pixels. Recently, researchers have shown nanoscale holes carved into thin metal sheets to be effective light scattering pixels. Surprisingly, when these nanoholes are arranged randomly, instead of periodically, the generated hologram becomes more uniform. Designing devices with randomly arranged components, however, is technically challenging, as parameters such as nanohole radius and spacing can vary over a wide range of values. To overcome these obstacles, Jinghua Teng from the A*STAR Institute of Materials Research and Engineering and colleagues devised a theoretical method that deconstructs the complex diffracted field from a single nanohole into simple analytical expressions that can be solved exactly. By superimposing the solutions together, they can calculate local, specified electric fields instead of expending significant computational resources to numerically simulate the entire nanophotonic array. The researchers turned to genetic algorithms to efficiently arrange the holes in a photon sieve arrangement. By repeatedly pairing, crossing, and mutating 'chromosomes' containing different 'genes'—labels of different nanohole sizes and positions—an aperiodic pattern evolves that optimizes holographic light control based on the simplified electric field calculations. Next, the team used electron-beam lithography to turn their design into a practical device by etching over 34, 000 aperiodic nanoholes into a thin chromium film (see image). The resulting prototype boosted diffraction efficiency by nearly 50 per cent compared to conventional nanophotonic devices with image resolution hundreds of times better. Common holographic errors or 'artefacts' such as twin images were also eliminated through this technique. "The high quality holographic images are promising for applications like anti counterfeiting, optical encryption and portable information identification system," says Teng. "For example, it could be used in anti counterfeiting in banknotes, with its ultra-compact size, high quality, and even multi level holographs." The researchers demonstrated another application of their approach by designing a 'superfocusing' system that can resolve objects smaller than the wavelength of light. With the nanoholes arranged into concentric rings, the photon sieve lens focuses light down to spots just 200 nanometers wide—scales useful for biological imaging and optical manipulations. More information: Kun Huang et al. Ultrahigh-capacity non-periodic photon sieves operating in visible light, Nature Communications (2015). DOI: 10.1038/ncomms8059
At Biopolis, a sprawling research complex in Singapore, Chi Ching Goh leans over an anaesthetized mouse lying on the table in front of her, and carefully injects it with a bright yellow solution. She then gently positions the mouse's ear underneath a microscope, and flips a switch to bathe the ear in ultraviolet light. Seen through the microscope's eyepiece, the illumination makes the blood underneath the skin glow green, tracing the delicate vessels that carry the solution through the creature's body. Ultimately, Goh, a PhD candidate at the National University of Singapore, hopes that the method will help her to find blood vessels that are leaking owing to inflammation, perhaps helping to detect malaria or predict strokes. Crucial to her technique are the virus-sized particles that give the solution its colour. Just a few tens of nanometres across, they are among a growing array of 'nanolights' that researchers are tailoring to specific types of fluorescence: the ability to absorb light at one wavelength and re-emit it at another. Many naturally occurring compounds can do this, from jellyfish proteins to some rare-earth compounds. But nanolights tend to be much more stable, versatile and easier to prepare — which makes them attractive for users in both industry and academia. The best-established examples are quantum dots: tiny flecks of semiconductor that are prized for their beautiful, crisp colours. Now, however, other types of nanolight are on the rise. Some have a rare ability to absorb lots of low-energy photons and combine the energy into a handful of high-energy photons — a trick that opens up opportunities such as producing multiple colours at once. Others are made from polymers or small organic molecules. These are less toxic than quantum dots and often outshine them — much to the amazement of chemists, who are used to carbon-based compounds simply degrading in the presence of ultraviolet light. “I was kind of surprised to find that we can make organic particles much brighter than inorganic particles,” says Bin Liu, a chemical engineer at the National University of Singapore and the designer of the fluorescent nanoparticles that Goh is using. Nanolights have already begun to find application in areas ranging from flat-screen displays to biochemical tests. And researchers are working towards even more ambitious uses in fields such as solar energy, DNA mapping, motion sensing and even surgery. “The research is certainly fast-paced,” says Daniel Chiu, who studies fluorescent nanoparticles at the University of Washington in Seattle. It is also increasingly wide ranging, adds Paul Alivisatos, a chemist at the University of California, Berkeley, and a co-founder of the first quantum-dot technology companies. “It's so much fun now.” The nanolight era began with the discovery of quantum dots in 1981. Russian physicists were growing tiny crystals of the semiconductor cuprous chloride in silicate glass and observed that the colour of the glass depended on the size of the particles1. The crystals were so small that quantum effects were kicking in and they were behaving somewhat like atoms: they could absorb or emit light only as specific colours, with the exact frequencies depending on the size or shape of the particles (see 'Bridge the gap'). The quantum dots were bright and beautiful, says Yin Thai Chan, who studies them at the National University of Singapore, but “there were no obvious applications”. By the early 2000s, however, the pure colours had begun to attract television manufacturers, as well as biomedical researchers, who saw their potential for labelling specific proteins and DNA segments. “Everything is good about quantum dots,” says Liu — except for one thing: their toxicity. The best-performing dots contain cadmium, which can poison cells. This limits their usefulness in biology and in applications such as household electronics, because some countries do not allow use of the element in such devices. To some extent, this problem can be overcome by replacing cadmium with zinc or indium, which are considerably less toxic, or by wrapping cadmium-based quantum dots in polymers that are biocompatible. But the toxicity is still a drawback for researchers who are pursuing ambitious applications such as fluoresence-guided surgery, in which nanoparticles are injected into a tumour, for instance, to make it glow and help surgeons to remove all traces of it. Partly in response to this challenge, researchers have begun to develop nanoparticles from materials that fluoresce naturally. Because the light-emitting properties of these nanolights come from their composition rather than their size or shape, they are easier to make with specific colours. “Practically, this is useful because of the difficulties to synthesize everything in the same size,” says Chiu. It also frees up nanolight researchers to explore alternative materials, such as semiconducting polymers. Studied for their potential in electronics since the 1950s, these polymers consist of simple compounds linked into a long chain in which electrons are free to move, but only at certain energies determined by the chain's composition. Light is emitted when electrons are kicked up to higher energy levels by some outside source, such as ultraviolet light, then fall back down to lower levels. The polymers can also be decorated with side groups to give them specific properties — for example, targeting them to cancer cells, or helping them to dissolve in water. And when chains are aggregated into polymer nanoparticles, or 'P-dots', they can be as much as 30 times brighter than a quantum dot of comparable size2. Semiconducting polymers do tend to be less stable than the inorganic semiconductors used in quantum dots. But because they are based on carbon, and contain no metals, they are much more likely to be biocompatible. P-dots have been used to stain and image cells, and also as sensors to detect oxygen, enzymes or metal ions such as copper. In 2013, for example, Chiu and his collaborators reported that a P-dot bound to a terbium ion can detect biomolecules produced by bacterial spores3. Under an ultraviolet lamp, the P-dots glow dark blue and the terbium ions emit a faint neon green colour. But when passing biomolecules attach themselves to terbium, the ions' light strengthens to a bright green. The P-dots' light remains unchanged, so it serves as an internal standard. Unfortunately, P-dots also have a fundamental problem: the polymer molecules are packed together so closely that they can be affected by 'quenching' — a phenomenon in which most of the energy coming from the original light source is quickly dissipated and fails to trigger fluorescence. Quenching has a huge impact on efficiency, says Yang-Hsiang Chan, a chemist at National Sun Yat-Sen University in Kaohsiung, Taiwan. One way to tackle it is to add bulky groups onto the polymer backbone to prevent the polymers from getting too close to each other. But this can be self-defeating: the resulting nanoparticles tend to be too fat to get into cells, say, or too dim to be useful. “It is very hard to get the right balance,” says Chan, who is working to solve the problem by designing new polymers. A more fundamental solution was pioneered in 2001, when Ben Zhong Tang at the Hong Kong University of Science and Technology in Clear Water Bay found that a class of small organic molecules would fluoresce only when they aggregate together4. These molecules are shaped like propellers or pinwheels, and they fluoresce when packed because they can no longer move and waste their energy. Instead, they release their energy as light — a phenomenon Tang has named aggregation-induced emission (AIE). He called the molecules AIE-gens. Over the next few years, Tang and his students changed the side groups and introduced elements such as nitrogen or oxygen, and AIE-gens can now glow in the entire spectrum of colours from ultraviolet to near-infrared. “My students quickly made a lot,” says Tang. “We can change the colour at will.” In 2011, Tang met Liu through a collaboration at the Institute of Materials Research and Engineering in Singapore, part of the government-backed Agency for Science, Technology and Research (A∗STAR). At that time, AIE-gens were performing well, except that they could not dissolve in water, which made them difficult to use in biological applications. Liu was an expert in making things water-soluble, so Tang gave her some of his best AIE-gens to work with. Liu solved the problem by experimenting with polymers that are oil-loving on one end and water-loving on the other. The AIE-gens crowd within the polymer's oil-loving ends, and its water-loving ends point outwards to form a protective shell, resulting in a water-soluble capsule with a dense core full of AIE-gens. Liu designed a protective shell for the resulting nanoparticles, called AIE-dots, such that it could be decorated with various chemical groups that are tailored to specific applications. The shell can easily accommodate a wide variety of AIE-gens, says Liu, “so that we can screen a lot of molecules very quickly to find out which one is the best.” AIE-dots have been used to stain various tissues, from blood vessels to cancer cells to intracellular organelles such as mitochondria. Last year, Liu, Tang and their colleagues reported an AIE-dot that could be useful in a type of light-activated treatment known as photodynamic therapy5. It carries two molecules on its surface: one to get the dot into a cancer cell, and another to make it stick to the mitochondria. Once excited by an external light source, the AIE-dot produces red light that generates oxygen radicals near the mitochondria and kills the cancer cells. The best AIE-dots can be 40 times brighter than quantum dots6. “With AIE, high density in constrained space produces high brightness,” says Guangxue Feng, a research assistant in Liu's lab. That is particularly useful for applications such as visualization of tissues or long-term tracking of cancer cells, which halve the number of nanoparticles per cell every time they divide. But the brightness comes at a cost: AIE-dots produce a much broader, more-muted spectrum than the pure, brilliant colours of quantum dots. But that hasn't kept Liu from starting LuminiCell, a spin-off company in Singapore that produces AIE-dots in three colours and three sizes for research such as Goh's at A∗STAR. Tang is also trying to start a company; both he and Liu are now hoping to gain approval from the US Food and Drug Administration to test AIE-dots for human use in applications such as fluorescence-guided surgery. Another thing that limits the biological use of nanolights is that most of them absorb ultraviolet or visible light, which can penetrate only a few millimetres into tissue. Longer-wavelength near-infrared radiation can penetrate up to three centimetres — a much better depth for uses such as releasing drugs. But infrared light does not have enough energy to break the bonds that hold drugs on the nanoparticle, so many researchers are turning to a process called upconversion. This involves making material that can absorb multiple low-energy infrared photons, accumulate the energy and then re-emit it as higher-energy ultraviolet or visible photons. The group of heavy-metal elements known as lanthanides are particularly good at this trick. In 2011, Xiaogang Liu at the National University of Singapore reported that his laboratory had created a particularly versatile type of nanoparticle7 with a Russian doll-like structure. It consists of a series of concentric shells that each contains a different combination of lanthanides. The energy from infrared light is absorbed by the core, then migrates outwards layer by layer, snowballing from lanthanide to lanthanide before finally emerging as high-energy light near the surface. The 15 lanthanides can be combined in numerous different ways to produce nanoparticles that emit in all colours, sometimes even several at once. In one demonstration, a student in Liu's lab shone an infrared laser through a series of beakers containing clear solutions of the nanoparticles: glowing lines of purple and green light appeared in the beakers where the infrared beam passed through. Liu thinks that these upconversion nanoparticles have tremendous potential in photovoltaics, where they could help to capture near-infrared light, which makes up almost half of the Sun's radiation. This is a long way from being practical, however: the brightest available nanoparticles convert just 10% of the light they absorb. Liu's group is working to build a library of these nanoparticles — no small task considering the number of lanthanides — to systematically study their properties and work on making them brighter. Last December, Marta Cerruti, a biomaterials scientist at McGill University in Montreal, Canada, reported a proof-of-concept system in which a lanthanide-containing nanoparticle is coated with a gel that contains a 'drug' — for testing purposes, a compact, stable protein8. After absorbing near-infrared light, the nanoparticle emits infrared, visible and ultraviolet light simultaneously. The infrared emission allows the researchers to track the nanoparticle's location, and the ultraviolet light cleaves the protein's bond to the gel and releases it — or at least, it has in the laboratory. Cerruti's group is now planning tests in animals. At the end of the day, quantum dots are still the nanolights to beat. “They are the de facto standard,” says Chan. “A lot of the fundamental phenomena concerning light emission are established in quantum dots and it shapes the way others explain what they see.” Quantum dots are also still a research frontier. For example, they are getting a boost from relatively new semiconducting materials such as the perovskites. Unlike conventional semiconductors, which have a fixed ratio of elements, perovskites can have variable ratios, so researchers can tailor the dots' emission by varying their composition as well as their size. “They have two degrees of freedom for tunability,” says Edward Sargent, a materials engineer at the University of Toronto, Canada. Last year, Sargent reported a hybrid material in which quantum dots are held within a perovskite9, yielding the kind of high brightness and good electron mobility that manufacturers might like for use in flat-screen displays. Other researchers are hoping to combine the best properties of each component by pursuing hybrid nanolights. Bin Liu, for example, is trying to blend AIE-dots with quantum dots to produce narrow emissions. And semiconducting polymers paired with AIE-dots can produce much brighter particles than either alone10. Another grand challenge for nanolights is to create versions that emit infrared wavelengths efficiently. That would open up applications in motion sensing, from tiny detectors that tell the screen to turn off when a mobile phone is lifted to the ear to sophisticated devices for self-driving cars and home monitoring for elderly people. “There's so much more we could do,” says Sargent.