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


By Michael Berger. Copyright © Nanowerk

longpredicte(Nanowerk Spotlight) Colloidal quantum dot (CQDnanocrystals 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.”

Read more: http://www.nanowerk.com/spotlight/spotid=32945.php#ixzz2j2YbZvu8

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Nanotechnology Solar Cell Applications – Graphene-Based Materials


By Michael Berger. Copyright © Nanowerk

longpredicte(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

 

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.

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.”

“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.”
Read more: http://www.nanowerk.com/spotlight/spotid=32691.php#ixzz2i7UENfhw

Direct printing of liquid metal 3D microstructures


By Michael Berger. Copyright © Nanowerk

Nano Particles for Steel 324x182(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

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.”

Read more: http://www.nanowerk.com/spotlight/spotid=32574.php#ixzz2hLXHYA7S

Nanotechnology for solar cell applications – graphene-based materials


By Michael Berger. Copyright © Nanowerk

longpredicte(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

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.

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.”

“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.”

Read more: http://www.nanowerk.com/spotlight/spotid=32691.php#ixzz2hLLeHzxN

Making Inorganic Solar Cells with an Airbrush Spray


 

Nano Particles for Steel 324x182(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.

By Michael Berger. Copyright © Nanowerk

Read more: http://www.nanowerk.com/spotlight/spotid=32458.php#ixzz2fyNZ5tzG

 

Solar paint paves the way for low-cost photovoltaics


072613solar(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.

 

By Michael Berger. Copyright © Nanowerk

Read more: http://www.nanowerk.com/spotlight/spotid=31922.php#ixzz2dkjBHzZG

Inkjet printing of graphene for flexible electronics


how-nanotechnology-could-change-solar-panels-photovoltaic_66790_600x450(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 /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


(QDOTS imagesCAKXSY1K 8Nanowerk 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.”

nano-storage wire 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)   

“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.

By Michael Berger. Copyright © Nanowerk

Read more: http://www.nanowerk.com/spotlight/spotid=31619.php#ixzz2buibAVQY

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


 

QDOTS imagesCAKXSY1K 8(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.

By Michael Berger. Copyright © Nanowerk

Read more: http://www.nanowerk.com/spotlight/spotid=31796.php#ixzz2buWpLAkI

Read more: http://www.nanowerk.com/spotlight/spotid=31796.php#ixzz2buVppHdn

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


By Michael Berger. Copyright © Nanowerk

201306047919620(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

 

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.”

  nano-storage wire 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)   

“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.

By Michael Berger. Copyright © Nanowerk

Read more: http://www.nanowerk.com/spotlight/spotid=31619.php#ixzz2apWtDi34