Memristors: 'Computer synapse' analyzed at the nanoscale

Researchers at Hewlett Packard and the University of California, Santa Barbara, have analysed in unprecedented detail the physical and chemical properties of an electronic device that computer engineers hope will transform computing.

Memristors, short for memory resistors, are a newly understood circuit element for the development of electronics and have inspired experts to seek ways of mimicking the behaviour of our own brains' activity inside a computer.

Research, published today, Monday, 16 May, in IOP Publishing's Nanotechnology, explains how the researchers have used highly focused x-rays to map out the nanoscale physical and chemical properties of these electronic devices.

It is thought memristors, with the ability to 'remember' the total electronic charge that passes through them, will be of greatest benefit when they can act like synapses within electronic circuits, mimicking the complex network of neurons present in the brain, enabling our own ability to perceive, think and remember.

Mimicking biological synapses - the junctions between two neurons where information is transmitted in our brains ? could lead to a wide range of novel applications, including semi-autonomous robots, if complex networks of neurons can be reproduced in an artificial system.

In order for the huge potential of memristors to be utilised, researchers first need to understand the physical processes that occur within the memristors at a very small scale.

Memristors have a very simple structure ? often just a thin film made of titanium dioxide between two metal electrodes ? and have been extensively studied in terms of their electrical properties.

For the first time, researchers have been able to non-destructively study the physical properties of memristors allowing for a more detailed insight into the chemistry and structure changes that occur when the device is operating.

The researchers were able to study the exact channel where the resistance switching of memristors occurs by using a combination of techniques.

They used highly focused x-rays to locate and image the approximately one hundred nanometer wide channel where the switching of resistance takes place, which could then be fed into a mathematical model of how the memristor heats up.

John Paul Strachan of the nanoElectronics Research Group, Hewlett-Packard Labs, California, said: "One of the biggest hurdles in using these devices is understanding how they work: the microscopic picture for how they undergo such tremendous and reversible change in resistance.

"We now have a direct picture for the thermal profile that is highly localized around this channel during electrical operation, and is likely to play a large role in accelerating the physics driving the memristive behavior."

This research appears as part of a special issue on non-volatile memory based on nanostructures.

Pairing quantum dots with fullerenes for nanoscale photovoltaics

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Left: Photoinduced electron transfer occurring in quantum dot-bridge-fullerene hererodimers and observed with single molecule microscopy. Right: Control of electron transfer (ET) rate by variation of interparticle distance (R, upper panel) and quantum dot size (D, lower panel).

In a step toward engineering ever-smaller electronic devices, scientists at the U.S. Department of Energy's (DOE) Brookhaven National Laboratory have assembled nanoscale pairings of particles that show promise as miniaturized power sources. Composed of light-absorbing, colloidal quantum dots linked to carbon-based fullerene nanoparticles, these tiny two-particle systems can convert light to electricity in a precisely controlled way.

"This is the first demonstration of a hybrid inorganic/organic, dimeric (two-particle) material that acts as an electron donor-bridge-acceptor system for converting light to electrical current," said Brookhaven physical chemist Mircea Cotlet, lead author of a paper describing the dimers and their assembly method in Angewandte Chemie.

By varying the length of the linker molecules and the size of the quantum dots, the scientists can control the rate and the magnitude of fluctuations in light-induced electron transfer at the level of the individual dimer. "This control makes these dimers promising power-generating units for molecular electronics or more efficient photovoltaic solar cells," said Cotlet, who conducted this research with materials scientist Zhihua Xu at Brookhaven's Center for Functional Nanomaterials.

Scientists seeking to develop molecular electronics have been very interested in organic donor-bridge-acceptor systems because they have a wide range of charge transport mechanisms and because their charge-transfer properties can be controlled by varying their chemistry. Recently, quantum dots have been combined with electron-accepting materials such as dyes, fullerenes, and titanium oxide to produce dye-sensitized and hybrid solar cells in the hope that the light-absorbing and size-dependent emission properties of quantum dots would boost the efficiency of such devices. But so far, the power conversion rates of these systems have remained quite low.

"Efforts to understand the processes involved so as to engineer improved systems have generally looked at averaged behavior in blended or layer-by-layer structures rather than the response of individual, well-controlled hybrid donor-acceptor architectures," said Xu.

The precision fabrication method developed by the Brookhaven scientists allows them to carefully control particle size and interparticle distance so they can explore conditions for light-induced electron transfer between individual quantum dots and electron-accepting fullerenes at the single molecule level.

The entire assembly process takes place on a surface and in a stepwise fashion to limit the interactions of the components (particles), which could otherwise combine in a number of ways if assembled by solution-based methods. This surface-based assembly also achieves controlled, one-to-one nanoparticle pairing.

To identify the optimal architectural arrangement for the particles, the scientists strategically varied the size of the quantum dots - which absorb and emit light at different frequencies according to their size - and the length of the bridge molecules connecting the nanoparticles. For each arrangement, they measured the electron transfer rate using single molecule spectroscopy.

"This method removes ensemble averaging and reveals a system's heterogeneity - for example fluctuating electron transfer rates - which is something that conventional spectroscopic methods cannot always do," Cotlet said.

The scientists found that reducing quantum dot size and the length of the linker molecules led to enhancements in the electron transfer rate and suppression of electron transfer fluctuations.

"This suppression of electron transfer fluctuation in dimers with smaller quantum dot size leads to a stable charge generation rate, which can have a positive impact on the application of these dimers in molecular electronics, including potentially in miniature and large-area photovoltaics," Cotlet said.

"Studying the charge separation and recombination processes in these simplified and well-controlled dimer structures helps us to understand the more complicated photon-to-electron conversion processes in large-area solar cells, and eventually improve their photovoltaic efficiency," Xu added.

A U.S. patent application is pending on the method and the materials resulting from using the technique, and the technology is available for licensing.

More information: Quantum Dot-Bridge-Fullerene Heterodimers with Controlled Photoinduced Electron Transfer, Dr. Zhihua Xu, Dr. Mircea Cotlet, Article first published online: 10 MAY 2011 DOI: 10.1002/anie.201007270

Abstract
A series of donor?bridge?acceptor systems in the form of core/shell CdSe/ZnS quantum dot?bridge?fullerene heterodimers (see picture) with varying bridge length and varying quantum dot size were self-assembled by a surface-based stepwise method to demonstrate control of the rate and of the magnitude of fluctuations of photoinduced electron transfer at the single-molecule level.

Researchers use nanoscale transistors to study single-molecule interactions

Researchers use nanoscale transistors to study single-molecule interactions

An interdisciplinary team from Columbia University that includes electrical engineers from Columbia's Engineering School, together with researchers from the University's departments of Physics and Chemistry, has figured out a way to study single-molecule interactions on very short time scales using nanoscale transistors. In a paper to be published online January 23 in Nature Nanotechnology, they show how, for the first time, transistors can be used to detect the binding of the two halves of the DNA double helix with the DNA tethered to the transistor sensor. The transistors directly detect and amplify the charge of these single biomolecules.

Prior to this work, scientists have largely used fluorescence techniques to look at interactions at the level of single molecules. These studies have yielded fundamental understanding of folding, assembly, dynamics, and function of proteins and other cellular machinery. But these techniques require that the target molecules being studied be labeled with fluorescent reporter molecules, and the bandwidths for detection are limited by the time required to collect the very small number of photons emitted by these reporters.

The Columbia researchers, including Professor of Electrical Engineering Ken Shepard, Professor of Chemistry Colin Nuckolls, and graduate students Sebastian Sorgenfrei and Chien-Yang Chiu, realized that transistors, like those used in modern integrated circuits, have reached the same nanoscale dimensions as single molecules. "So this raised the interesting question," said Sorgenfrei, the lead author on the study, "as to whether these very small transistors could be used to study individual molecules."

They have discovered that the answer is "yes." The transistors employed in this study are fashioned from carbon nanotubes, which are cylindrical tubes made entirely of carbon atoms. While these are still emerging devices for electronics applications, they are exquisitely sensitive because the biomolecule can be directly tethered to the carbon nanotube wall creating enough sensitivity to detect a single DNA molecule.

The Columbia team expects this new technique to be a powerful tool for looking at single molecule interactions and is looking at instrumentation applications that currently rely almost exclusively on fluorescence such as protein assays and DNA sequencing. They also plan to study interactions at time scales several orders of magnitude greater than current techniques based on fluorescence.

"The area of single molecule research is an important one and pushes the envelope on our sensing systems," commented Ken Shepard, Professor of Electrical Engineering at Columbia Engineering. "There is a huge potential for modern nanoelectronics to play an important role in this field. Our work, which has been a terrific collaboration between groups from Electrical Engineering, Chemistry, and Physics, is a great example of how nanoelectronics and biotechnology can be combined to produce new, exciting results."

Shepard hopes that this research, which was funded primarily by the National Science Foundation and the National Institutes of Health, will lead to exciting new applications for nanoscale electronic circuits.

Structural distortions emerge from nothing at the nanoscale

Structural distortions emerge from nothing at the nanoscale

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Simon Billinge (left) and Emil Bozin (right) at beamline X17A at the National Synchrotron Light Source (NSLS) at Brookhaven Lab, where the scientists will continue their explorations of local atomic structure to identify materials useful for energy applications.

Scientists have discovered that a class of materials known to convert heat to electricity and vice versa behaves quite unexpectedly at the nanoscale in response to changes in temperature. The discovery - described in the December 17, 2010, issue of Science - is a new "opposite-direction" phase transition that helps explain the strong thermoelectric response of these materials. It may also help scientists identify other useful thermoelectrics, and could further their application in capturing energy lost as heat, for example, in automotive and factory exhaust.

The scientists - from the U.S. Department of Energy's Brookhaven National Laboratory, Columbia University, Argonne National Laboratory, Los Alamos National Laboratory, Northwestern University, and the Swiss Federal Institute of Technology - were studying lead chalcogenides (lead paired with tellurium, selenium, or sulfur) using newly available experimental techniques and theoretical approaches that allow them to "see" and model behavior of individual atoms at the nanoscale, or on the order of billionths of a meter. With those tools they were able to observe subtle changes in atomic arrangements invisible to conventional probes of structure.

To understand the phase transition the scientists observed, think of the everyday response of a gas like steam cooling to form liquid water, and then freezing to form solid ice. In each case, the atoms undergo some form of structural rearrangement, explains Simon Billinge, a physicist at Brookhaven Lab and Columbia University's School of Engineering and Applied Science and a lead author on the Science paper.

"Sometimes, further cooling will lead to further structural transitions: Atoms in the crystal rearrange or become displaced to lower the overall symmetry," Billinge says. The development of such localized atomic distortions upon cooling is normal, he says. "What we discovered in lead chalcogenides is the opposite behavior: At the very lowest temperature, there were no atomic displacements, nothing - but on warming, displacements appear!"

The techniques the scientists used to observe this nanoscale atomic action were high-tech versions of x-ray vision, aided by mathematical and computer analysis of the results. First the lead materials were made in a purified powder form at Northwestern University. Then the scientists bombarded the samples with two kinds of beams - x-rays at the Advanced Photon Source at Argonne and neutrons at the Lujan Neutron Scattering Center at Los Alamos. Detectors gather information about how these beams scatter off the sample to produce diffraction patterns that indicate positions and arrangements of the atoms. Further mathematical and computational analysis of the data using computer programs developed at Brookhaven and Columbia allowed the scientists to model and interpret what was happening at the atomic level over a range of temperatures.

Brookhaven physicist Emil Bozin, first author on the paper, was the first to notice the odd behavior in the data, and he worked tenaciously to prove it was something new and not a data artifact. "If we had just looked at the average structure, we never would have observed this effect. Our analysis of atomic pair distribution functions gives us a much more local view - the distance from one particular atom to its nearest neighbors - rather than just the average," Bozin says. The detailed analysis revealed that, as the material got warmer, these distances were changing on a tiny scale - about 0.025 nanometers - indicating that individual atoms were becoming displaced.

The scientists have made an animation to illustrate the emergence of these displacements upon heating. In it, the displacements are represented by arrows to indicate the changing orientations of the atoms as they flip back and forth, or fluctuate, like tiny dipoles.

According to the scientists, it is this random flipping behavior that is key to the materials' ability to convert heat into electricity.

"The randomly flipping dipoles impede the movement of heat through the material in much the same way that it is more difficult to move through a disorderly wood than an orderly apple orchard where the trees are lined up in rows," Billinge says. "This low thermal conductivity allows a large temperature gradient to be maintained across the sample, which is crucial to the thermoelectric properties."

When one side of the material comes in contact with heat - say, in the exhaust system of a car - the gradient will cause charge carriers in the thermoelectric material (e.g., electrons) to diffuse from the hot side to the cold side. Capturing this thermally induced electric current could put the "waste" heat to use.

This research may help scientists search for other thermoelectric materials with exceptional properties, since it links the good thermoelectric response to the existence of fluctuating dipoles.

"Our next step will be searching for new materials that show this novel phase transition, and finding other structural signatures for this behavior," Billinge said. "The new tools that allow us to probe nanoscale structures are essential to this research.

"Such studies of complex materials at the nanoscale hold the key to many of the transformative technological breakthroughs we seek to solve problems in energy, health, and the environment."