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

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

Experimental graphene earphones outperform most commercial headsets.

In two previous Nanowerk Spotlights, we reported about work by a group of Chinese scientists that demonstrated that carbon nanotube sheets can act as powerful thermoacoustic loudspeakers ("Nanotechnology that will rock you") and of results by a team from UT Dallas that observed surprisingly high underwater sound generation efficiency using multi-walled carbon nanotube sheets ("Nanotechnology loudspeakers keep on rocking - even underwater").Now, Alex Zettl's group at the University of California at Berkeley has exploited the extraordinary electrical and mechanical properties of graphene to create a very efficient electrical/sound transducer."Our graphene loudspeaker, without any optimized acoustic design, is simple to make and already performs comparably to or better than similar sized commercial counterparts, and with much lower power consumption," Zettl tells Nanowerk.Zettl and Qin Zhou, a postdoc researcher in his group, describe their graphene speaker design in a paper currently posted on arXiv ("Electrostatic Graphene Loudspeaker").Schematics of the electrostatically driven graphene speaker. A graphene diaphragm, biased by a DC source, is suspended midway between two perforated electrodes driven at opposite polarity. The varying electrostatic force drives the graphene diaphragm which in turn disturbs air and emits sound through the electrodes. The light mass and low spring constant of the graphene diaphragm, together with strong air damping, allow for high-fidelity broad-band frequency response. Such a speaker also has extremely high power efficiency. (Image: Dr. Zettl, UC Berkeley)"For human audibility, an ideal speaker or earphone should generate a constant sound pressure level from 20 Hz to 20 kHz, i.e. it should have a flat frequency response," explains Zettl. "Most speakers available today reproduce sound via a mechanical diaphragm, which is displaced oscillatorily during operation. A wide-band audio speaker typically requires significant damping to broaden the response and, unfortunately, 'damping engineering' quickly becomes complex and expensive, with inevitable power inefficiencies."Graphene, though, is an ideal building material for small, efficient, high-quality broad-band audio speakers because it satisfies all the criteria for an ideal audio transduction diaphragm – it should have small mass and a soft spring constant, and be non-perforated to efficiently displace the surrounding air.Graphene's extremely low mass produces flat frequency response. Its high mechanical strength allows construction of relatively large and thin membrane for efficiently generating sound. Finally, good electrical conductivity means the membrane does not need to be "metallized"; it already is."The electrostatic graphene speaker has a fairly flat frequency response in the human audible region, low distortion and very low power consumption," says Zettl. "To our knowledge, it is the first one constructed." (Although graphene has been used previously to construct thermoacoustic speakers and piezoelectric speakers, but those have very poor frequency response and/or very poor power efficiency.)To fabricate their graphene earbuds, the two researchers sandwiched a 30 nm thin graphene film between two supporting frames and attached a 20 µm-diameter gold wire as the electrical contact. A prototype was fabricated simply by sandwiching the graphene diaphragm between two silicon electrodes.Images of (a) 7mm diameter graphene diaphragm suspended across annular support frame, (b) actuating electrodes, and (c) assembled speaker. (Image: Dr. Zettl, UC Berkeley)"The sound generated by the graphene speaker is easily audible by the human ear" says Zettl. "The fidelity is qualitatively excellent when listening to music."This novel, electrostatically driven, high-efficiency, mechanically vibrating graphene-diaphragm based audio speaker exhibits excellent performance: Even without optimization, the speaker is able to produce frequency response across the whole audible region (20 Hz∼20 KHz), comparable or superior to performance of conventional-design commercial counterparts.Applications-wise, the two now plan to demonstrate a larger graphene speaker and a graphene microphone.

Fuente:  Nanowerk, (2013) Experimental graphene earphones outperform most commercial headsets. Recuperado el 23 de marzo de 2013, de http://www.nanowerk.com/spotlight/spotid=29653.php 

Silicon LEDS are an alternative to toxic quantum dot LEDs

(Nanowerk Spotlight) Quantum dots (QDs) are nanoscale crystals of semiconductor material that glow with bright, rich colors when stimulated by an electric current. First discovered in the 1980s, these materials have been the focus of intense research because of their potential to provide significant advantages in a wide variety of optical applications, among them light-emitting diodes (LEDs). Quantum dots are expected to deliver lower cost, higher energy efficiency and greater wavelength control for a wide range of products, including lamps, displays and photovoltaics.

Unfortunately, the toxicity of the elements used for efficient quantum dot based LEDs – CdS, CdSe, and their Pb containing counterparts – is a severe drawback for many applications. Therefore, light-emitting devices which are based on the non-toxic element silicon are extraordinary promising candidates for future QD-lighting applications.

Researchers in Germany have now demonstrated highly efficient and widely color-tunable silicon light-emitting diodes (SiLEDs). The emission wavelength of the devices can easily be tuned from the deep red (680 nm) down to the orange/yellow (625 nm) spectral region by simply changing the size of the used size-separated silicon nanocrystals.

Reporting their work in the January 15, 2013 online edition of Nano Letters ("Multicolor Silicon Light-Emitting Diodes (SiLEDs)"), a multidisciplinary team at Karlsruhe Institute of Technology, led by Annie K. Powell, Geoffrey A. Ozin and Uli Lemmer, also showed that the size of the silicon nanocrystals has a significant impact on the valence band position of the material.

Size-separated silicon nanocrystals (ncSi) and their corresponding PL spectra. (a) Nanoparticles dispersed in toluene showing intense luminescence from the deep red to the yellow spectral region. (b) PL spectra of the three samples used for SiLED fabrication. Excitation: (a) 365 nm LED and (b) 355 nm Nd:YAG laser. (Reprinted with permission from American Chemical Society)

"Compared to other reports on silicon-based LEDs, we show that the emission color can be tuned down to the yellow/orange spectral region whereas existing reports especially focused on NIR-devices," Florian Maier-Flaig, a researcher in Lemmer's group and first author of the paper, tells Nanowerk. "This is the first report of yellow/orange emitting SiLEDs based on size-selected silicon nanocrystals. In addition, size-separation of the particles leads to significantly increased device operation lifetimes."

The team's silicon nanocrystals are capped with allylbenzene and produced by solid-state synthesis. The resulting nanoparticles are colloidally stable in toluene, feature sizes in the range of 1-3 nm, and exhibit photoluminescence quantum yields of up to 43%.

Potential applications are silicon-based light-emitting diodes. Maier-Flaig notes that the use of the particles as down-converting red-emitting “phosphors” for white light generation is also conceivable. The latter are currently fabricated using spectrally broad emitting phosphors which do not feature a strong contribution in the red spectral region being nevertheless crucial for high color rendering indices and generation “warm white” light.

With regard to practical applications, the team cautions that, in order to be compatible with II-VI semiconductor-based QD-LEDs, external quantum efficiency of the devices, quantum efficiency of the nanoparticles itself, as well as long-term stability of the devices should be further improved.

SiLEDs connected to a 9 V battery in series to an ohmic resistor limiting the voltage to 6 V. The individual photographs are taken at ambient lighting conditions and are not modified with any image processing software. (Reprinted with permission from American Chemical Society.

Referencia: Berger, M. (2013) Silicon LEDS are an alternative to toxic quantum dot LEDs, recuperado el 28 de febrero de 2013, de: http://www.nanowerk.com/spotlight/spotid=28925.php

Organic ferroelectric molecule shows promise for memory chips, sensors .

At the heart of computing are tiny crystals that transmit and store digital information's ones and zeroes. Today these are hard and brittle materials. But cheap, flexible, nontoxic organic molecules may play a role in the future of hardware. A team led by the University of Washington in Seattle and the Southeast University in China discovered a molecule that shows promise as an organic alternative to today's silicon-based semiconductors. The findings, published this week in the journal Science ("Diisopropylammonium Bromide Is a High-Temperature Molecular Ferroelectric Crystal"), display properties that make it well suited to a wide range of applications in memory, sensing and low-cost energy storage.

http://www.nanowerk.com/news2/newsid=28645.php

Electrical response of the newly developed organic crystal.

"This molecule is quite remarkable, with some of the key properties that are comparable with the most popular inorganic crystals," said co-corresponding author Jiangyu Li, a UW associate professor of mechanical engineering.

The carbon-based material could offer even cheaper ways to store digital information; provide a flexible, nontoxic material for medical sensors that would be implanted in the body; and create a less costly, lighter material to harvest energy from natural vibrations.

The new molecule is a ferroelectric, meaning it is positively charged on one side and negatively charged on the other, where the direction can be flipped by applying an electrical field. Synthetic ferroelectrics are now used in some displays, sensors and memory chips.

In the study the authors pitted their molecule against barium titanate, a long-known ferroelectric material that is a standard for performance. Barium titanate is a ceramic crystal and contains titanium; it has largely been replaced in industrial applications by better-performing but lead-containing alternatives.

The new molecule holds its own against the standard-bearer. It has a natural polarization, a measure of how strongly the molecules align to store information, of 23, compared to 26 for barium titanate. To Li's knowledge this is the best organic ferroelectric discovered to date.

A recent study in Nature announced an organic ferroelectric that works at room temperature. By contrast, this molecule retains its properties up to 153 degrees Celsius (307 degrees F), even higher than for barium titanate.

The new molecule also offers a full bag of electric tricks. Its dielectric constant -- a measure of how well it can store energy -- is more than 10 times higher than for other organic ferroelectrics. And it's also a good piezoelectric, meaning it's efficient at converting movement into electricity, which is useful in sensors.

The new molecule is made from bromine, a natural element isolated from sea salt, mixed with carbon, hydrogen and nitrogen (its full name is diisopropylammonium bromide). Researchers dissolved the elements in water and evaporated the liquid to grow the crystal. Because the molecule contains carbon, it is organic, and pivoting chemical bonds allow it to flex.

The molecule would not replace current inorganic materials, Li said, but it could be used in applications where cost, ease of manufacturing, weight, flexibility and toxicity are important.

Li is working on a number of projects relating to ferroelectricity. Last year he and his graduate student found the first evidence for ferroelectricity in soft animal tissue. He was co-author on a 2011 paper in Science that documents nanometer-scale switching in ferroelectric films, showing how such molecules could be used to store digital information.

"Ferroelectrics are pretty remarkable materials," Li said. "It allows you to manipulate mechanical energy, electrical energy, optics and electromagnetics, all in a single package."

He is working to further characterize this new molecule and explore its combined electric and mechanical properties. He also plans to continue the search for more organic ferroelectrics.

Fuente: http://www.nanowerk.com/news2/newsid=28645.php

High electron mobility in nano-composite thin film transistors

Researchers from Cambridge University and the London Centre for Nanotechnology have demonstrated extremely high electron mobility in nano-composite thin film transistors using zinc oxide and organic semiconductors.

Organic semiconductors  are often limited by their lower field-effect mobility, with high-performance n-type devices proving a particular challenge. Professor Arokia Nathan's work uses a technique for combining zinc oxide (ZnO) nanostructures synthesized using vapor phase deposition with organic semiconductors to create nano-composite thin film transistors with the highest reported electron field-effect mobility in solution processed devices.

A dispersion of ZnO nanorods in an n-type organic semiconductor ([6, 6]-phenyl-C61-butyric acid methyl ester, PCBM) are shown to enhance the electron field effect mobility by as much as a factor or 40 from the pristine state. The results although preliminary, show a highly promising enhancement for realization of high-performance solution-processable n-type organic TFTs.

 More information available from:

Li FM, Nathan A, Dalal S, et al. Zinc Oxide Nanostructures and High Electron Mobility Nanocomposite Thin Film Transistor. IEEE Transactions on Electron Devices. 2008:55(11):3001-11. DOI: 10.1109/TED.2008.2005180