A general method for transferring graphene onto soft surfaces

• Jie Song,
• Fong-Yu Kam

Recent advances in chemical vapour deposition have led to the fabrication of large graphene sheets on metal foils for use in research and development.

However, further breakthroughs are required in the way these graphenes are transferred from their growth substrates onto the final substrate because they normally break or conjugate in this process. Although various methods have been developed, as yet there is no general way to reliably transfer graphene onto arbitrary surfaces, such as ‘soft’ ones, like polymers or textiles. Here, they reported a method that allows the graphene to be transferred with high fidelity at the desired location on almost all surfaces, including fragile polymer thin films and hydrophobic surfaces.

The method consisted on a sacrificial ‘self-releasing’ polymer layer placed between a conventional polydimethylsiloxane elastomer stamp and the graphene that is to be transferred. This self-releasing layer provides a low work of adhesion on the stamp, which facilitates delamination of the graphene and its placement on the new substrate regardless of its nature. This method synthesized the graphene by CVD over a metal (Cu or Ni) as it has been reported previously. This makes easier to change the normal process that has been used to adapted to this one.

 To demonstrate that the method can be done with any surface, they fabricate high field-strength polymer capacitors using graphene as the top contact over a polymer dielectric thin film.

These capacitors showed superior dielectric breakdown characteristics compared with those made with evaporated metal top contactsby CVD.

 So they take it to a new level by fabricating low-operation-voltage organic field-effect transistors using graphene as the gate electrode placed over a thin polymer gate dielectric layer.

So they finally made artificial graphite by alternating layers of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ). This compound, which comprises graphene sheets p-doped by partial hole transfer from the F4TCNQ, shows a high and remarkably stable hole conductivity, even when heated in the presence of moisture.

Large tunable image-charge effects in single-molecule junctions

Researchers at Leiden University and Delft determined what makes electron transport in a single molecule so difficult thanks to a new measurement technique. One of the leaders of the team comments that the expectations they have are for large surfaces, like displays where there is a great advantage to having a single layer of molecules that can be p

ut together simply and cheaply.

This study offers insight into fundamental physical behavior of individual molecules. A molecule can act as a very sensitive sensor or nanotransistor between two electrodes, but the problem with the development of this type of molecular electronics is that it is really difficult to make electrical contact with a single molecule.

Researchers were able to create a new method for measuring conductivity in a molecule which is based on the mechanically driven break junction technique developed by Prof. Jan van Ruitenbeek. The paper explains that a freely suspended bridge in a metal conductor is subjected to mechanical pressure so it bends and breaks. Then, the molecule attaches itself to the two clean break surfaces. If they vary the distance between the electrodes, the image charge is impacted and researches can control the energy levels of the molecule, determining the role of image charge in numerical terms.

Reference:

Perrin M.L., Verzijl C.J.O., Martin C.A. et al. Large tunable image-charge effects in single-molecule junctions. Nature Nanotechnology, 2013. DOI: 10.1038/nnano.2013

Biological transistor

Researchers have already made impressive strides over the last decade in mimicking the behavior of electronic circuitry using DNA, RNA, and proteins. But often these devices tend to work only in the precise setting for which they were designed, such as turning up the expression of one particular gene in response to detecting a specific input signal.

Drew Endy, a synthetic biologist at Stanford University in Palo Alto, California, and his colleagues have been looking for an approach that could be more broadly applicable, in the same manner that an electronic transistor can be wired into numerous different circuit patterns to carry out myriad different functions. So they decided to use biology to mimic the most common type of electronic transistor. A transistor is essentially an electronic switch with three terminals or electrodes. A relatively small input of electrons flowing into a control electrode, called the gate, opens an electronic doorway, allowing a larger electrical current to flow between two other electrodes, known as the source and drain. This signal amplification can feed numerous downstream transistors enabling further logical operations to occur. Endy and his colleagues reasoned that amplification could help future genetic circuitry designs as well, because biological signals often quickly die out as weak molecular signals often get swamped by other molecular "noise" inside a cell.

At the heart of the new biological transistors, which Endy's team calls "transcriptor," are three components: an engineered DNA strand; RNA polymerase (RNA-P), an enzyme that travels along DNA and copies it into RNA; and proteins called integrases that are capable of cutting and pasting DNA. The DNA acts like a wire, Endy says. But instead of controlling how electrons flow down the wire, the team uses the integrases (the gate) to control how many molecules of RNA-P (the electrons) travel down the DNA strand, they report online today in Science.

To do so, in the middle of the DNA strand, they place a short DNA snippet called a "terminator" that kicks RNA-P molecules off the DNA. The key is that the terminator kicks off RNA-P only when RNA-P is traveling in one direction; say from left to right, but not from right to left. The Stanford team then uses the integrators to cut out the terminator DNA snippet that's in the middle of the longer DNA strand, turn it around, and then reinsert it. That means as the RNA-P travels down the DNA strand from left to right, it no longer recognizes the terminator. So it stays attached to the DNA and continues its transcription to RNA. Thus, the signal is turned on. In this case, if the RNA-Ps make it to the end point, they transcribe the gene for green fluorescent protein, lighting up the cell. Flip the terminator again and the RNA-P is kicked off, and the light turns off.

The Stanford team then showed that they could line up multiple transcriptors to carry out logical functions, creating standard logical circuits called AND gates, OR gates, XOR gates, and so on, which combine signals according to certain rules. (A computer's processor is a vast assemblage of such gates.) They also showed that their novel biological circuit designs were adept at producing signals with large amplification and that they could be used to up the expression of a variety of genes, such as the production of fluorescent signals that made it simple to detect cells that were carrying out their programming.

The Stanford team isn't the first to use integrases, DNA, and RNA-P to build circuitry. In February, a team led by synthetic biologist Timothy Lu of the Massachusetts Institute of Technology in Cambridge reported in Nature Biotechnology that they had used a similar strategy to carry out complex cellular logic and memory functions. But Lu says that "Drew's paper is really exciting" because it demonstrates that the strategy can be used to amplify signals. That could be helpful in designing novel cellular circuitry to detect small molecular signals that might indicate the presence of a disease and create a large output signal that can be easily detected.


Reference
http://news.sciencemag.org/sciencenow/2013/03/a-computer-inside-a-cell.html

Supramolecular thiophene nanosheets

Dr. Taichi Ikeda of the NIMS Electronic Functional Materials Group, Polymer Materials Unit of the National Institute for Materials Science (NIMS), in joint research with Prof. Hans-Jürgen Butt group of the Max Planck Institute for Polymer Research (Germany), developed the world’s first supramolecular thiophene nanosheets, which is a 2-dimensional organic material with a thickness of 3.5 nm.
In recent years, electronic materials with 2-dimensional sheet structures such as “graphene” have attracted considerable attention. However, in the case of graphene, size control is difficult, and chemical functionalization of the graphene surface is impossible. On the other hand, the thiophene derivatives have been actively investigated as electronic materials for field effect transistors (FET), organic solar cells, organic electroluminescence (organic EL) materials, and other applications. However, the manufacturing process of thiophene thin film has many problems. For instance, vacuum vapor deposition requires much energy and expensive equipment. Although thin film fabrication method via simple wet process have been developed using a polymer solution, it is difficult to obtain polymer thin films with high crystallinity. In this research, Ikeda overcame these problems and found a facile manufacturing method of thiophene nanosheets with high crystallinity in the solution.



In this work, Ikeda discovered that an alternating copolymer, in which a thiophene derivative and flexible ethylene glycol chain are alternately connected, is folded in some organic solvents in such a way that the thiophene units are stacked each other, and the folded copolymers self-assemble into a 2-dimensional sheet structure. Although the length of the polymer used in this work is approximately 80 nm, the thickness of the sheet is only 3.5 nm due to the folded conformation of the copolymer. The arrangement of the thiophene units in the nanosheet was confirmed to be the same as that manufactured by vacuum vapor-deposition of low-molecular-weight thiophene compounds. Therefore, our thiophene nanosheets are feasible to the application of organic electronics devices. The lateral size of the nanosheet was controllable by tuning the concentration of the polymer solution. The chemical modification of the nanosheet surface was also possible by introducing the other functional unit at the terminals of the copolymer.

Since it is possible to fabricate monolayers like those manufactured by vacuum deposition by just dissolving a polymer in a solvent, this method will lead to simple, low-cost and energy-efficient device fabrication. The self-assembly process via polymer folding reported herein is also of great scientific interest, as it artificially reproduces the folding and self-assembly of proteins in nature.

Reference
This research achievement was published online on March 26 in the international scientific journal Angewandte Chemie International Edition of the German Chemical Society.

Enhanced Charge Carrier Mobility in 2-D Material for Electronics

This paper is about a new two-dimensional nanomaterial that could revolutionize electronics developed at CSIRO and RMIT University. This material is made of layers of molybdenum oxide and has unique properties which encourage free flow of electrons at ultra-high speeds. The researches adapted graphene to create a new conductive nanomaterial. 

Even though graphene supports high-speed electrons, its physical properties prevent it from being used for high-speed electronics, so this new material was also made up of layered sheets but within these layers, electrons are able to zip through at high speeds with minimal scattering.

Profesor Kourosh Kalantarzadeh from RMIT said the researchers were able to remove the "road blocks" that could obstruct the electrons and he also mentioned that instead of scattering when they hit road blocks, as they would in conventional materials, they can simply pass through this new material and get through the structure faster.

Scientists used a process known as "exfoliation" to create layers 11 nm thick and manipulated the material to convert it into a semiconductor, then nanoscale transistors were created using molybdenum oxide. The mobility values achieved were more than 1000 cm2/Vs, which exceedes the current industry standard for low dimensional silicon.

Reference:

Balendhran S., Deng J., Zhen Ou J et al. Enhanced Charge Carrier Mobility in Two-Dimensional High Dielectric Molybdenum Oxide. Advanced Materials, 2013. DOI: 10.1002/adma.201203346

Nanodevices for Energy-Efficient Electronics

Scientists from the National University of Singapore (NUS) and University College Cork fabricated nanodevices for energy-efficient electronics. Switching efficiency was improved obtaining ten times higher results by changing just one carbon atom in the molecules of the device. The possible applications of this device would be providing new ways to avoid overheating in laptops, tablets and mobile phones and also to aid in electrical stimulation of tissue repair for wound healing.

This paper was featured in the February edition of Nature Nanotechnology and the research team was led by Dr. Damien Thompson and Prof. Chris Nijhus, who created the devices based on molecules that act as electrical valves or diode rectifiers. Their results have shown that by adding one carbon atom, the device's performance increases by a factor of ten and they hope to create a wide range of new components for electronic devices.

One interesting finding by these researchers is that molecules with an odd number of carbon atoms stand straighter than the ones with an even number. A tightly packed assemble of straight molecules packed together, were aligned on metal electrode surfaces and were found to be free of defects.

Dr. Thompson explains that their study shows how Van der Waals effects present in every molecular scale device can be turned to optimize the performance of the device by creating tighter seals between molecules. The molecular device is illustrated in the following image.

Reference:
- Nerngchamning N., Yuan L., Qi D,. Li J., Thompson D., Nijhus C.A. The role of van der Waals forces in the performance of molecular diodes. Nature Nanotechnology, 2013. DOI: 10.1038/nnano.2012.238.

Transparent Active Matrix Organic Light-Emitting Diode Displays Driven by Nanowire Transistor Circuitry

Optically transparent, mechanically flexible displays are attractive for next-generation visual technologies and portable electronics. In principle, current generation organic light-emitting diodes (OLEDs) satisfy key requirements for this application: transparency, lightweight, flexibility, and low-temperature fabrication, among current-generation electroluminescent materials, both small molecules and polymers are promising for full-color active matrix OLED (AMOLED). The fabrication of this kind of circuits also has an important challenge, the need of transparent transistor and circuit integration strategies. Consider that each pixel must have at least one switching transistor, one driver transistor, and a storage capacitor. Silicon technologies made over glass haven proven to be able to deliver the correct amount of power over AMOLED’s. However, conventional poly Si backplanes are optically opaque and not well suited for flexible displays requiring low-temperature processing and transparency. While organic thin-film transistors are compatible with low-temperature processing and some are optically transparent they have relatively low carrier mobility and typically utilize relatively long channel lengths, dictating relatively large transistor areas to provide the required drive current, this makes architectures in which the TFT area becomes comparable to that of the pixel emitter area, leading to unacceptable brightness resolution-power consumption trade-offs.

Nanowire transistors are transistors having one or more semiconductor nanowires as active channel region, potentially offer the performance required for AMOLED circuitry along with desired transparency and processing characteristics. Approaches in which NWs are synthesized on sacrificial substrates and then straightforwardly transferred to the device substrate allow the realization of high-performance channel regions without high-temperature processing. In this paper In₂O₃  nanowires were used as active channel materials, a performance-enhancing hing-k organic self-assembled nanodielectric as the gate unsulator, and ITO as the conducting gate and S-D electrodes.

The full fabrication process can be found in the papper. 

The figure shows the cross-section view of the AMOLED structure.

This figure shows the NW-AMOLED display, where clearly we can see the brightness of the display and the possibility of controlling the brightness individually.  So in general the conclusions are that it’s possible to manufacture an AMOLED based on room temperature process and highly transparent components manufactured by nanowires.  Even do there are great results, it’s necessary to improve the maximum brightness and maximum aperture ratio.

This AMOLED’s can be used for applications such as windshield displays, head mounted displays, transparent screen monitors, mobile phones, PDAs, personal computers, etc.

Source: NanoLetters 2008 Vol. 8, No. 4. 997-1004