Nanowire solar cells raise efficiency limit.

At the Nano Science Center at the Niels Bohr Insitute in Denmark, a group of scientists have demonstrated that a single nanowire can concentrate sunlight up to 15 times the normal intensity of sunlight. 

One the left an image of a SEM of the GaAs nanowire crsytal, next to it an image of a TEM of a single nanowire and on the right side a capture of a STEM of the nanowire's surface.

The nanowire naturally concentrates the sun's rays into a very small area in the crystal by up to a factor 15. This happens because the diameter of the nanowire is smaller than the wavelength of the light that comes form the sun and it causes resonances in the intensity of the ray of light in and around the Thus, the resonances can give a concentrated sunlight, where the energy is converted, which can be used to give a higher conversion effeciency of the sun's energy.

Fuente: Nanowerk, (2013). Nanowire solar cells raise efficiency limit. Recuperado el 11 de mayo de 2012, de http://www.nanowerk.com/news2/newsid=29679.php

Interaction of spin and vibrations in transport through single molecule magnets.

Measurement of transport in nanoscale magentic systems show how a few magnetic atoms in such environment respond to electron current. The environment is provided by ligand groups that hold atoms together in a single molecule held in the middle of a break junction. These systems are refered as single molecule magnets, which are consituted by large spinmoment with spin anisotropy.

 Illustration of a molecular junction.

The tunel effect that gives several charge states of the single molecule magnet can exhibit enhanced magnetic properties. When such charge states are only virtually accessible, effective spin–spin exchange interaction arises and inelastic excitation of the spin moment is possible, allowing for time-dependent control.

The longitudinal coupling to the vibration increases the zero field splitting and suppresses the quantum spin tunneling. A vibrationally induced quantum spin tunneling effect can occur at zero bias if transverse coupling is present as well. The transition to virtual vibrational excited states and the transverse spin mixing in these virtual states results in a Kondo effect. The Kondo effect describes the scattering of electrons creating a conductance path in metals due to magnetic impurities. 

Fuente: May F. et al. (2011). Interaction of spin and vibrations in transport through single-molecule magnets. Recuperado el 11 de mayo de 2013, de http://www.beilstein-journals.org/bjnano/content/pdf/2190-4286-2-75.pdf

Circular currents in Molecular Wires.

Circular current in molecules is more characteristic in species that have circular structures, such as rings. When  a current is applied an interesting phenomenon is observed, depending on the molecular structure framework it will allow circular currents. These type of measurementes and observations are limited to computational theoretical analysis. Depending on the characteristics of the system that is being analyzed, the results may differ. 

For example, circular currents often appear in certain voltage regimes in junctions characterized by multiple pathways that may close within a given molecular bridge to give a circular pathway. In other cases, the voltage of the circular current can be larger than the net junction current. Another different result was that a t a strong circular current can appear near conduction thresholds in the current.

In the isolated ring these orbitals are degenerate,and are characterized by equal and opposite orbital angular momentum along the molecular ring. such circular currents are found to be associated with considerable magnetic fields at the center of the ring. Analogous effects in molecules were discussed extensively in the context of molecular magnetic response, in particularas the origin of magnetic shielding phenomena in NMR spectroscopy.

Fuente: Rai, D. et al. (2010). Circular current in Molecular Wires. Recuperado el 11 de mayo de 2013, de 

http://www.tau.ac.il/~odedhod/papers/paper21.pdf

Measuring transport through a single atom in a transistor.

Researchers from Delft University of Technology and the FOM Foundation(Fundamental Research on Matter) have successfully measured transport through a single atom in a transistor. This research offers new insights into the behaviour of so-called dopant atoms in silicon. The researchers are able to measure and manipulate a single dopant atom in a realistic semi-conducting environment. The individual behaviour of dopant atoms is a stumbling block to the further miniaturisation of electronics. The researchers have published their findings in the Physical Review Letters.

The electronic industry uses a semiconducting material, dominantly silicon, that contains dopant atoms. This 'contamination' is necessary for giving the silicon the desired electronic characteristics. Owing to the continuing process of miniaturisation, a situation has arisen in which the characteristics of two chips, despite both being manufactured in a totally identical way, still differ from each other. The number of dopant atoms per transistor has in fact become so small (only a few dozen) that they can no longer be regarded as a continuum. The position and effect of each individual atom influences how the entire transistor works. Effectively, this means that even perfectly manufactured transistors will not behave identically. This is an especially alarming situation for the electronics industry, which has already been feeling the pinch for a number of years.

Researchers Sellier, Lansbergen, Caro and Rogge of the Kavli Institute of Nanoscience Delft and the FOM Foundation have successfully managed to measure a single dopant atom in an actual semi-conducting environment. The researchers, who work in the Photronic Devices, transported a charge through one atom. Moreover, they successfully measured and manipulated the quantum mechanical behaviour of a single dopant atom. They were able for example to place one or two electrons in a particular shell of the atom.

Fuente: Nanowerk (2013). Measuring transport through a single atom in a transistor. Recuperado el 6 de mayo de 2013, de http://www.nanowerk.com/news/newsid=1055.php.

Quantum information: Computing with a single nuclear spin in silicon.

A research team, including members from the London Centre for Nanotechnology (LCN), has created the first working quantum bit based on the nuclear spin of a single phosphorus atom in silicon, opening the door for dramatically improved data processing in ultra-powerful quantum computers of the future.

A landmark paper published today in the journal Nature ("High-fidelity readout and control of a nuclear spin qubit in silicon"), describes how to write and read quantum information with record-setting accuracy using the nuclear spin, or magnetic orientation, of a phosphorus atom in a silicon transistor – similar to silicon chips used in modern electronics.

The nucleus of a phosphorus atom is a very, very weak magnet, and can be imagined as a compass needle that can point north or south. These north or south positions are equivalent to the zero and one of binary code, which governs classical computing. In this experiment, the researchers controlled the direction of the nucleus, in effect “writing” an arbitrary value onto its spin, and were then able to “read” the value out. They observed quantum oscillations of the spin between north and south, and all the quantum superpositions of those two directions – where the spin exists in both states simultaneously. Scanning electron micrograph of the active area of the qubit device, showing an implanted donor (donor as blue arrow), the single electron transistor (SET) and the short-circuit termination of the microwave line. The device is mounted in a dilution refrigerator with an electron temperature of,300 mK, and is subjected to static magnetic fields B0 between 1.0T and 1.8 T. B0 is oriented perpendicular to the short-circuit termination of the microwave line (solid orange single-ended arrow), which carries a current (solid double-ended arrow) and produces an oscillating magnetic field B1 (represented by the solid and dashed circles) perpendicular to the surface of the device. TG, top gate; PL, plunger gate; LB, left barrier; RB, right barrier. Fuente: Nanowerk (2013). Quantum information: Computing with a single nuclear spin in silicon. Recuperado el 6 de mayo de 2013, de http://www.nanowerk.com/news2/newsid=30267.php

Quantum nanoelectronics - An introduction to electronic nanotechnology and quantum computing.

The march of Moore's Law takes electronics to the molecular level, and indeed molecules are likely to be incorporated in future hybrid computer chips.This self-contained text guides students as well as professionals to the new possibilities presenting a treatment of Quantum Computing, a promising new approach which is based on Quantum Mechanics. This essential new title also covers topics which connect to alternative energy technology, for example solar cell design, photocatalytic conversion of water to hydrogen, and high performance batteries.

'Quantum Nanoelectronics' is the first textbook to handle important growth areas not covered in existing books, including adiabatic quantum computing, nanoelectronic aspects of ink-printed thin film solar cells, nanostructured electrodes, solar water splitting, and convenient hydrogen storage, thereby suggesting profitable new directions for nanoelectronic technology. Expanded tutorial coverage is provided for aspects of molecular electronics, from the basics of electronic conduction through chemical bonds to a sixteen-bit computing device as shown in the cover illustration. The interested reader, either a student or a professional interested in a new career direction, is encouraged to use simple theoretical models and to return to the entrepreneurial approach of the pioneers in the Moore's Law revolution.

Fuente: Nanowerk (2013). Quantum nanoelectronics - An introduction to electronic nanotechnology and quantum computing. Recuperado el 6 de mayo de 2013.

Improving AFM probe performance with a coat of graphene.

 Most of the research efforts on developing synthesis methods for graphene has focused on flat substrates. Some groups have reported the formation of free-standing 3D graphene-based macroscopic structures (see for instance "Making graphene 'bread' - leavening technique results in freestanding graphene oxide films"). However, direct growth of graphene layers on prepatterned substrates has remained elusive.In previous work, researchers led by James K. Gimzewski at UCLA have already shown that graphene grows continuously over large areas on different copper terraces without the underlying substrate morphology affecting the atomic arrangement of the grown material ("Continuity of Graphene on Polycrystalline Copper"). In new work, just reported in ACS Nano ("Graphene MEMS: AFM Probe Performance Improvement"), the team has applied this technique to grow graphene in prepatterned copper-coated substrates, and they apply this protocol for the fabrication of MEMS devices, in particular, atomic force microscope (AFM) probes."Conformal deposition of graphene on prepatterned substrates opens a multitude of device fabrication possibilities," writes Gimzewski. "One of them is the addition of different layers to the graphene, mostly by deposition, using the prepatterned substrate as a mold. This can result in hybrid 3D structures covered with graphene."In this new work, the team demonstrates the fabrication of polymeric AFM probes covered by monolayer graphene that had been previously deposited on a prepatterned substrate. Furthermore, they show how graphene improves the functionality of the probes by making them conductive and more resistant to wear.The fabrication steps utilized in the process are very standard and highly compatible with current silicon fabrication technology, which allows its application in any clean room or microtechnology laboratory. The entire process is based on the multiple spincoating, exposure, and development of SU-8 photoresist on a prestructured mold.

The team also tested the conductivity of the graphene layer on the AMP probe. The results show a conductive behavior, proving the continuity of the graphene layer on the AFM probe.


Fuente: Nanowerk (2013), Improving AFM probe performance with a coat of graphene. Recuperado el 6 de mayo de 2013, de http://www.nanowerk.com/spotlight/spotid=30346.php

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