Automated and scalable inline two-stage synthesis process for high quality colloidal quantum dots

Oct28

(Nanowerk Spotlight) Colloidal quantum dot (CQD) nanocrystals are attractive materials for optoelectronics, sensing devices and third generation photovoltaics, due to their low cost, tunable bandgap – i.e. their optical absorption can be controlled by changing the size of the CQD nanocrystal – and solution processability. This makes them attractive candidate materials for cheap and scalable roll-to-roll printable device fabrication technologies.

One key impediment that currently prevents CQDs from fulfilling their tremendous promise is that all reports of high efficiency devices were from CQDs synthesized using manual batch synthesis methods (in classical reaction flasks).

Researchers have known that chemically producing nanocrystals of controlled and narrow size-distributions requires stringent control over the reaction conditions – e.g. temperature and reactant concentration – which is only practical for small-scale reactions.

Such a synthesis is extremely difficult to scale up, hence very costly to mass produce without severely compromising quality. The reason for this is that, just like rain droplets, nanocrystals form sequentially by ‘nucleation’ and ‘growth’. Both these phenomena are highly sensitive to temperature and reagent concentration. Moreover, nucleation and growth must occur at substantially different temperatures and, in fact, to obtain nanocrystals of uniform sizes, one must be able to rapidly cool down the reaction from the nucleation temperature to the growth temperature.

Hence, the quality of the product is contingent upon how well and fast one can homogenize the reactor, both chemically and thermally. Unfortunately, the only way to scale up batch reactors is by increasing their volume, whereupon it becomes difficult to homogenize the reactor and impractical to rapidly cool. The end result is nanocrystals of low-quality and broad size distributions, which are not useful for fabricating devices.

Some researchers have sought to circumvent this limitation by conducting the reactions in narrow fluidic channels (less than a 1 mm in diameter) while the reactants are continuously pumped through the channels, so called ‘continuous-flow reactors’.

Conceptually, this scheme has several advantages. Narrow-width channels afford uniform heating and mixing of the reaction, while the reaction is scalable by simply increasing channel length and pump rate of the reagents. This sort of scaling does not effect the quality of the product, because the channel width, and hence the effective reaction volume, remains the same. Despite these advantages, most attempts to use continuous-flow reactors in the past have resulted in nanocrystals with a much lower quality than the batch produced ones.

“We have analyzed the nucleation and growth of CQDs in continuous-flow reactors and realized that, in order to achieve controllable size and narrow size-distributions, one must employ two temperature stages in the reactor: one for nucleation, and another for growth,” Osman Bakr, an assistant professor in the Solar & Photovoltaics Engineering Research Center at King Abdullah University of Science and Technology (KAUST), tells Nanowerk.

“By separating these two crucial steps in the formation of the CQDs in time, temperature, and space, we were able to obtain very high quality nanocrystals, as good as the best batch synthesis, by a process that is low-cost, mass-producible, and automated.”

Schematic of a conventional batch synthesis setup and a dual-stage continuous flow reactor setup

Schematic of (a) a conventional batch synthesis setup and (b) a dual-stage continuous flow reactor setup with precursor A (Pb-oleate, octadecene) and precursor B (bis(trimethylsilyl) sulfide in octadecene). (Reprinted with permission from American Chemical Society)

Reporting their findings in ACS Nano (“Automated Synthesis of Photovoltaic-Quality Colloidal Quantum Dots Using Separate Nucleation and Growth Stages”), Bakr and his team demonstrated the quality of the CQDs produced by their method by using them to make CQD-based solar cells that showed very high efficiencies.

“In this paper, we developed an automated, scalable, in-line synthesis methodology of high-quality CQDs based on a flow-reactor with two temperature-stages of narrow channel coils,” says Professor Ted Sargent from the University of Toronto who, together with Bakr, led this work. “The flow-reactor methodology not only enables easy scalability and cheap production, but also affords rapid screening of parameters, automation, and low reagent consumption during optimization.

Moreover, the CQDs are as good in quality and device performance as the best CQDs that are produced in the traditional batch methodology.” The team also developed a general theory for how one can use the flow-reactors to finely tune the quality and size distribution of the CQDs, and explained why previous attempts of using flow-reactors based on a single-temperature-stage, as opposed to a dual-temperature-stage, necessarily produce CQDs of low-quality and broad size distribution.

This work paves the way towards the large-scale and affordable synthesis of high-quality CQD nanocrystals in tunable sizes, enabling photovoltaics, light-emitting diodes, photodetectors, and biological tagging technologies that take advantage of the nanoscale properties of those promising materials.

“Over the last ten years we have seen tremendous advancements in software and computer integration, in items that we use in our everyday lives,” says Bakr. “Flow-reactors as a platform are ideally placed to take advantage of this trend. Software that automates the routines of flow-reactors already exists. In the near future, researchers will be able to run and monitor hundreds of experiments to produce CQDs from home using a mobile app.

Moreover, because flow-reactors contain very few moving parts, essentially just programmable pumps, I expect that it will become an automated research platform that most labs studying nanocrystals can afford.” “Our work has shown that flow-reactors can produce nanocrystals that are as good as the best batch produced reactions, with exquisite control over reaction conditions,” he adds. “We believe that this will encourage the nanomaterials community to take advantage of the enormous productivity gains in R&D afforded by flow-reactors, which other chemical industries, such as pharmaceuticals, are currently utilizing earnestly.”

Nanotechnology Solar Cell Applications – Graphene-Based Materials

Oct18

(Nanowerk Spotlight) Graphene-based nanomaterials have many promising applications in energy-related areas. In particular, there are four major energy-related areas where graphene will have an impact: solar cells, supercapacitors, lithium-ion batteries, and catalysis for fuel cells (read more: “Graphene-based nanotechnology in energy applications”).

The extremely high electron mobility of graphene – under ideal conditions electrons move through it with roughly 100 times the mobility they have in silicon – combined with its superior strength and the fact that it is nearly transparent (2.3 % of light is absorbed; 97.7 % transmitted), make it an ideal candidate for photovoltaic applications. It could be a promising replacement material for indium tin oxide (ITO), the current standard material for transparent electrodes used for electrodes in LCD displays, solar cells, iPad and smart-phone touch screens, and organic light-emitting diode (OLED) displays for televisions and computer monitors.

Just yesterday, for instance, there was a report (“Nanotechnology researchers make major leap towards graphene for solar cells”) that shows that graphene retains its impressive set of properties when it is coated with a thin silicon film. These findings pave the way for entirely new possibilities to use in thin-film photovoltaics.

A new review in Advanced Energy Materials (“Graphene-Based Materials for Solar Cell Applications”) by a team of scientists from Nanyang Technological University, led by Prof. Hua Zhang, provides an overview of the recent research on graphene and its derivatives, with a particular focus on synthesis, properties, and applications in solar cells.

organic solar cell fabricated with graphene as anodic electrode

With the unique properties, i.e., highly optical transparence, highly electrical conduction, and mechanical flexibility, graphene and its derivatives have been investigated extensively in the field of solar cells. The review looks in detail at some of the impressive results that have been reported where graphene was used as the electrodes, i.e.:

–transparent anodes
–non-transparent anodes
–transparent cathodes
–catalytic counter electrodes

as well as where graphene was used as the active layer, i.e.:

–light harvesting material
–Schottky junction
–electron transport layer
–hole transport layer
–both hole and electron transport layer
–and interfacial layer in the tandem configuration.

Summing up their review, the authors conclude that it is promising that graphene, as the transparent electrode material, has exhibited superiority in that it is highly flexible, an abundant carbon source, and has high thermal/chemical stability, compared to the traditional ITO. In particular, the flexible transparent electrodes show applications not only in solar cells, but also in flexible touch screens, displays, printable electronics, flexible transistors, memories, etc.

transfer process of CVD-graphene onto transparent substrate

Schematic illustration of the transfer process of CVD-graphene onto transparent substrate. (©Wiley-VCH Verlag)

“In addition to working as transparent electrodes, graphene, graphene oxide (GO), and their derivatives show many other important applications that include being electron/hole transporters and serving as interfacial layers and Schottky junction layers in photovoltaics devices,” write the authors. “Two-dimensional (2D) graphene oxide is capable of π-π stacking and hydrogen bonding. This makes it possible to use such a 2D scaffold as the template to self-assemble GO-based novel inorganic, organic, and inorganic-organic hybrids with multifunctionalities for applications in photovoltaics.”

We believe that graphene will play more and more important roles in solar cells and other fields, such as energy storage, optoelectronics, electrics and sensing, in the near future.”
Read more: http://www.nanowerk.com/spotlight/spotid=32691.php#ixzz2i7UENfhw

Direct printing of liquid metal 3D microstructures

Oct10

(Nanowerk Spotlight) The ability to pattern materials into arbitrary three-dimensional (3D) microstructures is important for electronics, microfluidic networks, tissue engineering scaffolds, photonic band gap structures, and chemical synthesis.

However, existing commercial processes to 3D print metals usually require expensive equipment and large temperatures.

In contrast, a novel, relatively simple method developed by researchers at North Carolina State University can print metal structures at room temperature. This makes the technique it compatible with many other materials including plastics. Also, the resulting structures are liquid and are therefore soft and stretchable.

“The key concept is that the liquid metal forms spontaneously a thin oxide layer on its surface,” Michael Dickey, an Associate Professor of chemical and biomolecular engineering at NC State, tells Nanowerk. This oxide layer is solid and allows the metal to be printed into 3D shapes despite being a liquid. When two droplets of water come together, they form a larger droplet. However, this does not happen with the liquid metal due to the oxide ‘skin’.” As the team reports in a recent issue of Advanced Materials (“3D Printing of Free Standing Liquid Metal Microstructures”), they have demonstrated that it is possible to direct write structures composed of a low-viscosity liquid with metallic conductivity at room temperature. The liquid metal is useful for soft, stretchable, or shape reconfigurable electronics.

Direct writing of liquid metal 3D structures of varying sizes. (Image: Dickey Research Group, North Carolina State University) (click image to enlarge)

Metals have unique electrical, optical, and thermal properties. With this novel technique, it is now possible to print metal microstructures directly to creates various parts including electronics. The resulting parts, if designed correctly, can be stretchable. The general approach for printing liquid metal structures involves applying modest gauge pressure to a syringe needle that then extrudes the liquid metal – for this work they used the binary eutectic alloy of gallium and indium but they say that any alloy of gallium will also work – onto a substrate controlled by a motorized translation stage. Upon exposure to air, the metal forms a thin (∼1 nanometer) passivating ‘skin’ composed of gallium oxide. This oxide skin on the surface of the metal stabilizes the liquid metal wire against gravity and surface tension of the liquid. Once detached from the syringe, the wires maintain their shape.

3D printing of liquid metals at room temperature.

“The formation of the wires is remarkable and unexpected” says Dickey. “The process of forming the wires begins by forming a bead of the metal on the tip of the syringe.

Although the metal is under pressure the entire time, it does not flow out of the syringe due to the stabilizing influence of the oxide skin. Without increasing or decreasing the pressure in the syringe, wires form when the metal contacts the substrate and the tip of the syringe withdraws away from the substrate. Because the oxide skin spans from the nozzle of the syringe to the substrate, increasing the distance between the nozzle and substrate generates a tensile force along the axis of the wire that yields the skin and allows the wire to elongate.

The pressure of the liquid metal retards any destabilizing capillary forces long enough for new skin to form and thereby mechanically stabilizes the wire.” Altogether, the researchers describe four different methods to direct write 3D, free standing, liquid metal microstructures by extruding the liquid metal through a capillary: “In addition to extruding wires, it is possible to form free standing liquid metal microstructures using at least three additional methods,” Dickey explains: “1) Expelling rapidly the metal to form a stable liquid metal filament; 2) stacking droplets; and 3) injecting the metal into microchannels and subsequently removing the channels chemically.”

The smallest components that the team fabricated were about 10 µm, but they note that there may be opportunities to create smaller structures through, for example, the use of smaller nozzles. Dickey’s team is currently exploring how to further develop these techniques, as well as how to use them in various electronics applications and in conjunction with established 3-D printing technologies.

Dickey notes that the work by an undergraduate student, Collin Ladd, also the paper’s first author, was indispensable to this project. “He helped develop the concept, and literally created some of this technology out of spare parts he found himself.”

Nanotechnology for solar cell applications – graphene-based materials

Oct10

It could be a promising replacement material for indium tin oxide (ITO), the current standard material for transparent electrodes used for electrodes in LCD displays, solar cells, iPad and smart-phone touch screens, and organic light-emitting diode (OLED) displays for televisions and computer monitors.

organic solar cell fabricated with graphene as anodic electrode

Schematic representation of the energy level alignment (top) and the construction of heterojunction organic solar cell fabricated with graphene as anodic electrode: graphene/PEDOT/CuPc/C60/BCP/Al. (©Wiley-VCH Verlag) With the unique properties, i.e., highly optical transparence, highly electrical conduction, and mechanical flexibility, graphene and its derivatives have been investigated extensively in the field of solar cells.

The review looks in detail at some of the impressive results that have been reported where graphene was used as the electrodes, i.e.:

–transparent anodes
–non-transparent anodes
–transparent cathodes
–catalytic counter electrodes

as well as where graphene was used as the active layer, i.e.:

–light harvesting material
–Schottky junction
–electron transport layer
–hole transport layer
–both hole and electron transport layer
–and interfacial layer in the tandem configuration.

. transfer process of CVD-graphene onto transparent substrate

Schematic illustration of the transfer process of CVD-graphene onto transparent substrate. (©Wiley-VCH Verlag)

“On the other hand, to enrich the application of graphene, processes on bandgap opening have always attracted the attention of scientists. To date, many methods have been investigated to engineer the band structure of graphene, including inducing a quantum confinement effect by reduction of graphene lateral size to form nanoribbons or nanomesh introducing foreign elements, and employing a strain effect from the substrate. We believe that graphene will play more and more important roles in solar cells and other fields, such as energy storage, optoelectronics, electrics and sensing, in the near future.”

Making Inorganic Solar Cells with an Airbrush Spray

Sep25

(Nanowerk Spotlight) There is currently a tremendous amount of interest in the solution processing of inorganic materials. Low cost, large area deposition of inorganic materials could revolutionize the fabrication of solar cells, LEDs, and photodetectors. The use of inorganic nanocrystals to form these structures is an attractive route as the ligand shell that surrounds the inorganic core allows them to be manipulated and deposited using organic solvents.

The most common methods currently used for film formation are spin coating and dip coating, which provide uniform thin films but limit the geometry of the substrate used in the process. The same nanocrystal solutions used in these procedures can also be sprayed using an airbrush, enabling larger areas and multiple substrates to be covered much more rapidly.

The trade-off is the roughness and uniformity of the film, both of which can be substantially higher. Reporting their findings in a recent online edition of ACS Applied Materials & Interfaces (“Inorganic Photovoltaic Devices Fabricated Using Nanocrystal Spray Deposition”), researchers have now attempted to quantify these differences for a single-layer solar cell structure, and found the main difference to be a reduction in the open circuit voltage of the device. deposited films of CdTe nanocrystals SEM images of the top surface of the deposited films following deposition and sintering, showing (a) CdTe spin coated and (b) CdTe spray coated. The scale bar in both images represents 200 nm. (Reprinted with permission from American Chemical Society)

“Our work was motivated by a desire to coat larger substrate areas more efficiently,” Edward Foos, a research scientists in the Materials Synthesis and Processing Section of the Chemistry Division at the Naval Research Laboratory, and first author of the paper, tells Nanowerk. “Our initial work indicated that if the layers were thick enough to cover the substrate completely and avoid pinhole formation that would lead to shorting of the device, then the increased surface roughness might be tolerable.”

He adds that this is the first time the impact of this surface roughness on the performance characteristics has been directly compared for these types of devices.

The team prepared single-layer Schottky-barrier solar cells using spray deposition of inorganic (CdTe) nanocrystals with an airbrush. The spray deposition results in a rougher film morphology that manifests itself as a 2 orders of magnitude higher saturation current density compared to spin coating. “We’re currently working to improve the spray coating process to improve the layer uniformity,” says Foos. “If the surface roughness can be reduced, then the overall device performance should increase.” The team is confident that further optimization of the spray process to reduce this surface roughness and limit the Voc suppression should be possible and eventually lead to comparable performances between the two deposition techniques. “Importantly” Foos points out, “the spray-coating process enables larger areas to be covered more efficiently, reducing waste of the active layer components, while enabling deposition on asymmetric substrates.

These advantages should be of substantial interest as inorganic nanocrystal-based solar cells become increasingly competitive as third-generation devices.” The team’s next step will be the fabrication of more complex device architectures that incorporate multiple solution processed layers. These structures will have an even smaller tolerance for variation. In addition, the deposition chemistry used must not interfere with the material applied in the previous step.

Solar paint paves the way for low-cost photovoltaics

Sep2

(Nanowerk Spotlight) Using quantum dots as the basis for solar cells is not a new idea, but attempts to make such devices have not yet achieved sufficiently high efficiency in converting sunlight to power. The latest advances in quantum dots photovoltaics have recently resulted in solar cell power conversion efficiencies exceeding 7% (see for instance: “Graded Doping for Enhanced Colloidal Quantum Dot Photovoltaics”).

Although these performance levels are promising, all high-performing device results to date have relied on a multiple-layer-by-layer strategy for film fabrication rather than employing a single-layer deposition process. The attractiveness of using quantum dots for making solar cells lies in several advantages over other approaches: They can be manufactured in an energy-saving room-temperature process; they can be made from abundant, inexpensive materials that do not require extensive purification, as silicon does; and they can be applied to a variety of inexpensive and even flexible substrate materials, such as lightweight plastics.

In new work, reported in the August 12, 2013 online edition of Advanced Materials (“Directly Deposited Quantum Dot Solids Using a Colloidally Stable Nanoparticle Ink”), a research team from the University of Toronto and King Abdullah University of Science and Technology (KAUST) developed a semiconductor ink with the goal of enabling the coating of large areas of solar cell substrates in a single deposition step and thereby eliminating tens of deposition steps necessary with the previous layer-by-layer method.

“We sought an approach that would achieve highly efficient utilization of CQD materials,” says Professor Ted Sargent from the University of Toronto, who, together with Osman Bakr, an assistant professor in the Solar & Photovoltaics Engineering Research Center at KAUST, led the work. “To achieve this, we made a solar cell ink that can be deposited in a single step which makes it an excellent material for high-throughput commercial fabrication.”

The team’s ‘solar paint’ is composed of semiconductor nanoparticles synthesized in solution – so-called colloidal quantum dots (CQDs). They can be used to harvest electricity from the entire solar spectrum because their energy levels can be tuned by simply changing the size of the particle. Previously, films made from these nanoparticles were built up in a layer-by-layer fashion where each of the thin CQD film deposition steps is followed by curing and washing steps to densify the film and form the final semiconducting material.

These additional steps are required to exchange the long ligands that keep the CQDs stable in solution for short ligands that allow efficient charge transport. However, this means that many steps are required to build a thick enough film to absorb enough sunlight. “We simplified this process by engineering the CQD surfaces with short organic molecules in the solution phase to enable a stable colloidal solution and reduce the film formation to a single step,” Bakr explains to Nanowerk. “At the same time, the post processing steps are reduced significantly, since the semiconducting material is formed in solution. This means that CQD films can be deposited quickly and at low cost, similar to a paint or ink.”

colloidal quantum dot solar cell fabrication methods

a) Schematic of the standard layer-by-layer spin-coating process with active materials usage yield and required total material indicated. b) Schematic of the single-step film process with active materials usage yield and required total material indicated. (Reprinted with permission from Wiley-VCH Verlag)

Besides the reduction in processing steps, the new process is also much more efficient in terms of materials usage. While the layer-by-layer, solid-state treatment approach provides less than 0.1% yield in its application of CQD materials from their solution phase onto the substrate, the new approach achieves almost 100% use of available CQDs.

“This means that for the same amount of CQD material, we could make a thousand-fold larger area of solar cells compared with conventional methods,” Bakr points out. “Our technology paves the way for low-cost photovoltaics that can be fabricated on flexible substrates using roll-to-roll manufacturing, similar to a printing press,” adds Lisa Rollny, a PhD candidate in Sarget’s group and a co-author of the paper. “Our ink is also useful in biological applications, e.g. in biosensors and tracing agents with an infrared response.”

“In previous work, we found new routes of passivating the CQD surface using a combination of organic and inorganic compounds in a solid state approach with large improvements in efficiency,” says Rollny. “We intend to integrate this knowledge with our solar CQD ink to further improve the performance of this material, especially in terms of how much solar energy is converted into usable electrical energy.”

Although the team have developed an effective method for producing a CQD film in a single step, the electronic properties of the resulting films are not optimized yet. This is due to the very small imperfections on the CQD surface that reduce the usable electricity output of a solar cell. Through careful engineering of CQD surfaces in solution, the researchers plan to eliminate these unwanted surface sites in order to make higher quality, higher efficiency CQD solar cells using their single step process.

Inkjet printing of graphene for flexible electronics

Aug26

(Nanowerk Spotlight) Graphene has a unique combination of properties that is ideal for next-generation electronics, including mechanical flexibility, high electrical conductivity, and chemical stability. Combine this with inkjet printing, already extensively demonstrated with conductive metal nanoparticle ink (see for instance: “Low-cost nanotechnology substitute for gold and silver in printable electronics”), and you get an inexpensive and scalable path for exploiting these properties in real-world technologies.

Although liquid-phase graphene dispersions have been demonstrated (see: “Inkjet-printed graphene opens the door to foldable electronics”), researchers are still struggling with sophisticated inkjet printing technologies that allow efficient and reliable mass production of high-quality graphene patterns for practical applications. There are several challenges that need to be overcome:

a good ink should possess proper fluidic properties, in particular the right viscosity and surface tension;
the graphene concentration in these solvents is often quite low so that several tens of print passes are required to obtain functional films, reducing efficiency of the technique;
graphene flakes easily aggregate in inks or during solvent evaporation, which decreases the ink stability and/ or degrades the film/device performance;
ideal solvents for graphene dispersions are toxic so that their corresponding inks cannot be used in an open environment;
most studies published thus far on inkjet printing of graphene are actually based on graphene oxide inks, not graphene inks.

Recent work by researchers at the KTH Royal Institute of Technology in Sweden has addressed these issues and proposes an approach to overcome these problems. Reporting their findings in a recent issue of Advanced Materials (“Efficient Inkjet Printing of Graphene”), a team led by Max Lemme and Mikael Östling, professors at the School of Information and Communication Technology at KTH, demonstrates a mature but simple technology for inkjet printing of high-quality few-layer graphene.

Inkjet printed graphene patterns

Inkjet printed graphene patterns. a–c) Optical images of as-printed patterns on glass slides: a) droplet matrix, b) lines, and c) a film corner. (Reprinted with permission from Wiley-VCH Verlag)

The approach is based on the team’s previously published distillation-assisted solvent exchange technique to prepare high-concentration graphene dispersions (“A simple route towards high-concentration surfactant-free graphene dispersions”). They first exfoliate graphene from graphite flakes in dimethylformamide (DMF), and then DMF is exchanged by terpineol through distillation by virtue of the large difference between their boiling points.

Therefore, graphene can be significantly concentrated if terpineol is of much lower volume than DMF. More importantly, the solvent is changed from low-viscosity and toxic DMF to high-viscosity (about 40 cP at 20° C) and environmentally-friendly terpineol. They write, though, that the disadvantages of the technique in the previous work – a short stable period (the dispersion can only be stable for about 10 hours) and severe flake aggregation during solvent evaporation – prevent the dispersions from being practical inks. “In this work, we have improved the ink formulation mainly through polymer stabilization.

Before distillation, a small amount of polymer (ethyl cellulose) is added into the harvested graphene/DMF dispersion to protect the graphene flakes from agglomeration. After printing, the stabilizing poly mers can be effectively removed through a simple annealing process.” The resulting graphene dispersion had a stable period of at least several weeks.

The researchers point out that the inks provide well-directed and constant jetting out of all nozzles at an even velocity, which is comparable to the performance of commercially available inks. To investigate the quality of the printed graphene, the team fabricated large-area centimeter-scale graphene thin films with between 1 and 6 printing layers on glass slides.

“Printed transparent conductive films attain a sheet resistance around 200 kΩ/sq at a transmittance of about 90%,” they summarize their results. “Printed narrow-line resistors exhibit a resistance range from a few kΩ to several MΩ. Printed few-layer graphene thin film transistors can be modulated by the electric field effect.

Printed micro-supercapacitors achieve a high specific capacitance of 0.59 mF cm-2 and a rapid frequency response time around 13 ms.” The team concludes that the present technology provides an efficient and low-cost method to fabricate a variety of graphene electronic devices with good performance and is a promising alternative for future commercial applications in printed and flexible electronics.

By Michael Berger. Copyright © Nanowerk
Read more: http://www.nanowerk.com/spotlight/spotid=31868.php#ixzz2d5jygcw5

Nano-storage wires

Aug13

(Nanowerk Spotlight) Nanowires are considered a major building block for future nanotechnology devices, with great potential for applications in transistors, solar cells, lasers, sensors, etc. (see for instance: “Nanowires for the electronics and optoelectronics of the future” and “Nanotechnology explained: Nanowires and nanotubes”).

Now, for the first time, nanotechnology researchers have utilized nanowires as a ‘storage’ device for biochemical species such as ATP. Led by Seunghun Hong, a professor of physics, biophysics and chemical biology at Seoul National University, the team developed a new nanowire structure – which they named ‘nano-storage wire’ – which can store and release biomolecules.

Reporting their findings in the July 16, 2013 online edition of ACS Nano (“Nano-Storage Wires”), Hong’s group demonstrated that their nano-storage wire structure can be deposited onto virtually any substrate to build nanostorage devices for the real-time controlled release of biochemical molecules upon the application of electrical stimuli.

“Our nano-storage wires are multisegmented nanowires comprised of three segments and each segment plays a role in extending the applications of the nanowire,” Hong explains to Nanowerk: “1) the conducting polymer segment stores biomolecules; 2) the nickel segment allows the utilization of magnetic fields to drive the nanowires and place them onto a specific location for device applications; and 3) the gold segment enables a good electrical contact between the deposited nano-storage wires and the electrodes. The polymer segment is utilized for the controlled release of ATP molecules. The nickel segment enables the magnetic localization of nano-storage wires, while the gold segment provides a good electrical contact with electrodes.”

Left: Schematics of a nano-storage wire. Right: SEM image of a single nano-storage wire. The dark, intermediate, and bright regions represent PPy-ATP (conducting polymer with ATP molecules), nickel, and gold segments, respectively. (Images: Dr. Seunghun Hong, Seoul National University) The released biomolecules from such a nanowire-storage system can be used for instance to control the activity of biosystems. As a proof of concept, the researchers stored ATP in their nano-storage wires and released it by electrical stimuli, which activated the motion of motor protein systems. The team also demonstrated flexible nanostorage devices. Here, nano-storage wires were driven by magnetic fields and deposited onto nickel/gold films on a transparent and flexible polyimide film. The device transmitted some light, and it can be easily bent. They also showed that the nanowires could be deposited onto curved surfaces such as the sharp end of a micropipet.

nano-storage wires deposited on tip of a micropipette

SEM image of nano-storage wires deposited on a micropipet. (Reprinted with permission from American Chemical Society)

“Nano-storage wires will allow the fabrication of advanced biochips which can activate or deactivate the activities of biosystems in real time,” Hong points out. “The activation and deactivation of biosystems such as biomotors, are controlled by specific biomolecules. In our method, we can selectively control the biomolecular activities related with ATP or any released chemical species while leaving other biomolecular activities unaltered.”

Having demonstrated the storage of ATP, the team is now planning to store other biomolecules in our nano-storage wires. Examples are drugs to control the activity of cells and tissues, enzymes to activate specific signal pathways in biosystems etc. “Eventually, we would like to build an advanced biochip which can be utilized to control the activities of desired biosystems in real-time,” says Hong.

How squid and octopus might point the way to nanotechnology-based stealth coatings

Aug13

(Nanowerk Spotlight) For a long time, scientists have been fascinated by the dramatic changes in color used by marine creatures like squids and octopuses, but they never quite understood the mechanism responsible for this.

Only recently they found out that a neurotransmitter, acetylcholine, sets in motion a cascade of events that culminate in the addition of phosphate groups to a family of unique proteins called reflectins. This process allows the proteins to condense, driving the animal’s color-changing process. The latest findings revealed that there is a nanoscale mechanism behind cephalopods’ ability to change color. Watch this amazing video of a camouflaging octopus:

Having begun to unravel the natural mechanisms behind these amazing abilities, researchers are trying to use this knowledge to make artificial camouflage coatings. New work from the lab of Alon A. Gorodetsky, Assistant Professor at the Henry Samueli School of Engineering at the University of California, Irvine, addresses the challenge of making something appear and disappear when visualized with standard infrared detection equipment.

In a paper in the July 30, 2013, online edition of Advanced Materials (“Reconfigurable Infrared Camouflage Coatings from a Cephalopod Protein”), the team demonstrates graphene-templated, biomimetic camouflage coatings that possess several important advantages. “We used reflectin, a protein that is important for cephalopod structural coloration, as a functional optical material,” Gorodetsky explains to Nanowerk. “We fabricated thin films from this protein, whose reflectance – and coloration – could be dynamically tuned over a range of over 600 nm and even into the infrared (in the presence of an appropriate stimulus).

Our approach is environmentally friendly and compatible with a wide range of surfaces, potentially allowing many simple objects to acquire camouflage capabilities.” The novelty of these findings lies in the functionality of the team’s thin-films within the infrared region of the electromagnetic spectrum, roughly 700nm to 1200nm, which matches the standard imaging range of infrared visualization equipment. This region is not commonly accessible to biologically derived materials. Gorodetsky notes that reflectin’s tunable optical properties compare favorably to those of artificial polymeric materials.

“Given these advantages, our dynamically tunable, infrared-reflective films represent a crucial first step towards the development of reconfigurable and disposable biomimetic camouflage technologies for stealth applications,” says Gorodetsky. ” I can also imagine applications in energy efficient reflective coatings and biologically inspired optics.” The team began their studies by developing a protocol for the production of the histidine-tagged reflectin A1 (RfA1). Experimenting with a variety of substrates and surface treatments for the reliable formation of RfA1 thin films, they achieved best results by spincasting 5 to 10 nm films of graphene oxide on glass substrates. They then spread RfA1 onto the graphene oxide-coated substrates, yielding smooth films over centimeter areas.

appearance of the RfA1 film in the absence and presence of an external stimulus, when visualized with an infrared camera

Illustration depicting the appearance of the RfA1 film in the absence and presence of an external stimulus (acetic acid), when visualized with an infrared camera. (Reprinted with permission from Wiley-VCH Verlag)

These films showed a distinct coloration, depending on their thickness. For instance, a 125 nm-thick film was blue and a 207 nm-thick film was orange. “Inspired by the dynamic optical properties of reflectin nanostructures, we sought to shift the reflectance of our RfA1 films into the infrared region of the electromagnetic spectrum,” says Gorodetsky.

“Given that some squid can dynamically modulate their skin reflectance across the entire visible spectrum and even out to near infrared wavelengths of ∼800 nm, we postulated that it should also be possible to tune the reflectance of our RfA1 thin films across a similar, or even larger, wavelength range. Thus, we sought conditions that would significantly increase the thickness of our RfA1 films and, consequently, shift their reflectance spectra toward the infrared.”

To that end, the researchers explored the response of their RfA1 coatings to a variety of chemical stimuli. They discovered that exposing the films to vapor from a concentrated acetic acid solution induced a large, reversible shift in the reflectance spectra, caused by the acid-induced swelling of the closely packed RfA1 nanoparticles in the film. “With the goal of fabricating dynamically tunable camouflage materials, which will self-reconfigure in response to an external signal, we are currently developing alternative, milder strategies for triggering coloration changes in our material,” Gorodetsky describes the team’s future work plans.

Nanowires: Major Building-block for Nanotechnology Devices: Transistors, Solar Cells, Lasers and More

Aug2

(Nanowerk Spotlight) Nanowires are considered a major building block for future nanotechnology devices, with great potential for applications in transistors, solar cells, lasers, sensors, etc.

*** Read articles explaining how ‘nanowires and nanotubes’ differ from other quantum materials, such as quantum dots, and their potential applications here:

http://www.nanowerk.com/news2/newsid=29945.php

http://www.nanowerk.com/news/newsid=16857.php

The released biomolecules from such a nanowire-storage system can be used for instance to control the activity of biosystems. As a proof of concept, the researchers stored ATP in their nano-storage wires and released it by electrical stimuli, which activated the motion of motor protein systems. The team also demonstrated flexible nanostorage devices. Here, nano-storage wires were driven by magnetic fields and deposited onto nickel/gold films on a transparent and flexible polyimide film. The device transmitted some light, and it can be easily bent. They also showed that the nanowires could be deposited onto curved surfaces such as the sharp end of a micropipet.

nano-storage wires deposited on tip of a micropipette

SEM image of nano-storage wires deposited on a micropipet. (Reprinted with permission from American Chemical Society)

“Such probe-shaped storage devices can be used for the delivery of chemicals to individual cells through a direct injection,” says Hong. “Basically, our results show that nano-storage wires are quite versatile structures and we can deposit them onto virtually any structure to create nanoscale devices for the controlled release of biochemical materials.” “Nano-storage wires will allow the fabrication of advanced biochips which can activate or deactivate the activities of biosystems in real time,” Hong points out. “The activation and deactivation of biosystems such as biomotors, are controlled by specific biomolecules.

In our method, we can selectively control the biomolecular activities related with ATP or any released chemical species while leaving other biomolecular activities unaltered.” Having demonstrated the storage of ATP, the team is now planning to store other biomolecules in our nano-storage wires. Examples are drugs to control the activity of cells and tissues, enzymes to activate specific signal pathways in biosystems etc. “Eventually, we would like to build an advanced biochip which can be utilized to control the activities of desired biosystems in real-time,” says Hong.

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Nanowires: Major Building-block for Nanotechnology Devices: Transistors, Solar Cells, Lasers and More

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