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
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Shedding a Light on the Cause for Nanoparticle Size Distribution

QDOTS imagesCAKXSY1K 8(Nanowerk News) When buying shoes it does not matter  how good-looking the shoes might be if the size does not fit. This is similar  with nanoparticles, which are made by the so-called emulsionsolvent evaporation  process. This process allows for the production of nanoparticles with high  purity. Nevertheless they can still be improved: so far, their size distribution  cannot be fully controlled. However, a defined size is of prime importance for  future applications, whether it is for drug delivery or for intelligent  coatings.
An interdisciplinary and international research collaboration at  the Max Planck Institute for Polymer Research in Mainz was able to rule out  coalescence as reason for the borad nanoparticle size distribution. Coalescence  describes the tendency of colloidal droplets to melt together. For the first  time, Daniel Crespy, who is group leader in the department of Katharina  Landfester, was able to prove that the coalescence between droplets during the  process is not significantly responsible for the broad size distribution of the  particles.
“This study elucidates the mechanism of a common process used  for the preparation of nanoparticles,“ says Daniel Crespy about his research  results.
The chemist labeled the original materials prior to the  preparation of the nanoparticles. Some polymers were labeled with red and others  with blue dyes. During the synthesis, the polymers and a solventwere emulsified  in water. After the evaporation of the solvent, solid nanoparticles are  obtained. This is a common method to produce all types of nanoparticles.  Crespy’s trick: Upon adding both red- and blue-labeled polymers to the solvent,  nanoparticles with both colors were obtained. The so-called negative control  shows that if red and blue particles are mixed, no aggregation occurs because  species with both dyes were not detected.
What happens if a red emulsion from polymer and solvent is mixed  with a blue emulsion? Less than every twelfth particle –around 8 percent – were  labeled with both red and blue dyes, which means that coalescence does not play  a significant role in the process.
For the first time, the scientists were able to directly  quantify the occurrence of coalescence. Together with Kaloian Koynov, who is  physicist and expert for spectroscopic methods at the MPI-P, Crespy could  monitor the coalescence of nanometer sized droplets by fluorescence correlation  spectroscopy.
The experimental results were finally confirmed by simulations  based on Monte-Carlo algorithms performed by Davide Donadio, group leader of a  Max Planck Research Group. Thanks to this study (“Particle Formation in the Emulsion-Solvent Evaporation  Process”), the reason for the broad size distribution could be attributed to  the process itself.
Source: Max Planck Institute for Polymer Research

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Polymer Structures Serve as ‘Nanoreactors’ for Nanocrystals

QDOTS imagesCAKXSY1K 8Using star-shaped block co-polymer structures as tiny reaction vessels, researchers have developed an improved technique for producing nanocrystals with consistent sizes, compositions and architectures – including metallic, ferroelectric, magnetic, semiconductor and luminescent nanocrystals. The technique relies on the length of polymer molecules and the ratio of two solvents to control the size and uniformity of colloidal nanocrystals.


The technique could facilitate the use of nanoparticles for optical, electrical, optoelectronic, magnetic, catalysis and other applications in which tight control over size and structure is essential to obtaining desirable properties. The technique produces plain, core-shell and hollow nanoparticles that can be made soluble either in water or in organic solvents.

“We have developed a general strategy for making a large variety of nanoparticles in different size ranges, compositions and architectures,” said Zhiqun Lin, an associate professor in the School of Materials Science and Engineering at the Georgia Institute of Technology. “This very robust technique allows us to craft a wide range of nanoparticles that cannot be easily produced with any other approaches.”

The technique was described in the June issue of the journal Nature Nanotechnology. The research was supported by the Air Force Office of Scientific Research.

Georgia Tech professor Zhiqun Lin examines a gold nanoparticle toluene solution. The work is part of research on using star-shaped block co-polymers to create nanocrystals of uniform size and shape.

(Photo Credit:  Georgia Tech Photo: Gary Meek)

The star-shaped block co-polymer structures consist of a central beta-cyclodextrin core to which multiple “arms” – as many as 21 linear block co-polymers – are covalently bonded. The star-shaped block co-polymers form the unimolecular micelles that serve as a reaction vessel and template for the formation of the nanocrystals.

The inner blocks of unimolecular micelles are poly(acrylic) acid (PAA), which is hydrophilic, which allows metal ions to enter them. Once inside the tiny reaction vessels made of PAA, the ions react with the PAA to form nanocrystals, which range in size from a few nanometers up to a few tens of nanometers. The size of the nanoparticles is determined by the length of the PAA chain.

The block co-polymer structures can be made with hydrophilic inner blocks and hydrophobic outer blocks – amphiphilic block co-polymers, with which the resulting nanoparticles can be dissolved in organic solvents. However, if both inner and outer blocks are hydrophilic – all hydrophilic block co-polymers – the resulting nanoparticles will be water-soluble, making them suitable for biomedical applications.

Lin and collaborators Xinchang Pang, Lei Zhao, Wei Han and Xukai Xin found that they could control the uniformity of the nanoparticles by varying the volume ratio of two solvents – dimethlformamide and benzyl alcohol – in which the nanoparticles are formed. For ferroelectric lead titanate (PbTiO3) nanoparticles, for instance, a 9-to-1 solvent ratio produces the most uniform nanoparticles.

The researchers have also made iron oxide, zinc oxide, titanium oxide, cuprous oxide, cadmium selenide, barium titanate, gold, platinum and silver nanocrystals. The technique could be applicable to nearly all transition or main-group metal ions and organometallic ions, Lin said.

“The crystallinity of the nanoparticles we are able to create is the key to a lot of applications,” he added. “We need to make them with good crystalline structures so they will exhibit good physical properties.”

Earlier techniques for producing polymeric micelles with linear block co-polymers have been limited by the stability of the structures and by the consistency of the nanocrystals they produce, Lin said. Current fabrication techniques include organic solution-phase synthesis, thermolysis of organometallic precursors, sol-gel processes, hydrothermal reactions and biomimetic or dendrimer templating. These existing techniques often require stringent conditions, are difficult to generalize, include a complex series of steps, and can’t withstand changes in the environment around them.

Georgia Tech professor Zhiqun Lin (standing) watches research scientist Xinchang Pang tuning the experimental condition in the nanocrystal synthesis.

(Photo Credit:  Georgia Tech Photo: Gary Meek)

By contrast, nanoparticle production technique developed by the Georgia Tech researchers is general and robust. The nanoparticles remain stable and homogeneous for long periods of time – as much as two years so far – with no precipitation. Such flexibility and stability could allow a range of practical applications, Lin said.

“Our star-like block co-polymers can overcome the thermodynamic instabilities of conventional linear block co-polymers,” he said. “The chain length of the inner PAA blocks dictates the size of the nanoparticles, and the uniformity of the nanoparticles is influenced by the solvents used in the system.”

The researchers have used a variety of star-like di-block and tri-block co-polymers as nanoreactors. Among them are poly(acrylic acid)-block-polystyrene (PAA-b-PS) and poly(acrylic acid)-blockpoly(ethylene oxide) (PAA-b-PEO) diblock co-polymers, and poly(4-vinylpyridine)-block-poly(tert-butyl acrylate)-block-polystyrene (P4VP-b-PtBA-b-PS), poly(4-vinylpyridine)-block-poly (tert-butyl acrylate)-block-poly(ethylene oxide) (P4VP-b-PtBA-b-PEO), polystyrene-block-poly(acrylic acid)-block-polystyrene (PS-b-PAA-b-PS) and polystyrene-block-poly(acrylic acid)-block-poly(ethylene oxide) (PS-b-PAA-b-PEO) tri-block co-polymers.

For the future, Lin envisions more complex nanocrystals with multifunctional shells and additional shapes, including nanorods and so-called “Janus” nanoparticles that are composed of biphasic geometry of two dissimilar materials.

Georgia Tech professor Zhiqun Lin (standing) and research scientist Xinchang Pang compare two cadmium selenide (CdSe) nanocrystals made by Pang. The researchers are examining the absorption spectra of the nanocrystals in front of the computer.

(Photo Credit:  Georgia Tech Photo: Gary Meek)