Record conductivity achieved in strained lattice organic semiconductor

By packing molecules closer together, chemical engineers at Stanford have dramatically improved the electrical conductivity of organic semiconductors. The advance could herald flexible electronics, more efficient solar panels, and perhaps even better television screens.

Organic semiconductors could usher in an era of foldable smartphones, better high-definition television screens and clothing made of materials that can harvest energy from the sun needed to charge your iPad, but there is one serious drawback: Organic semiconductors do not conduct electricity very well.

In a paper recently published in the journal Nature, researchers at Stanford led by chemical engineer Zhenan Bao have changed that equation by improving the ability of the electrons to move through organic semiconductors. The secret is in packing the molecules closer together as the semiconductor crystals form, a technique engineers describe as straining the lattice.

Bao and her colleagues have more than doubled the record for electrical conductivity of an organic semiconductor and shown an eleven-fold improvement over unstrained lattices of the same semiconductor.

"Strained lattices are no secret. We've known about their favorable electrical properties for decades and they are in use in today's silicon computer chips, but no one has been successful in creating a stable strained lattice organic semiconductor with a very short distance between molecules, until now," said Bao.

In the past, engineers have tried to compress the lattices in these materials by synthetically growing the crystals under great pressure. "But, as soon as you release the pressure, the crystal just goes back to its natural, unstrained state," said Bao. "We've been able to stabilize these crystals in tighter formations than ever before."

Fine-tuning

Bao's team used a solution shearing technique similar to a coating process well known in the semiconductor industry. Solution shearing involves a thin liquid layer of the semiconductor sandwiched between two metal plates. The lower plate is heated and the upper plate floats atop the liquid, gliding across it like a barge. As the top plate moves, the trailing edge exposes the solution to a vaporized solvent and, heated by the lower plate, the crystals form into a thin film.

"Using a process so similar to current industry technology is important, as it could speed these new semiconductors to market," said Bao.

The engineers can then "tune" the speed at which the top plate moves, the thickness of the solution layer, the temperature of the lower plate, and other engineering factors to achieve optimal results.

The crystals form in differing structures based on the speed at which the top plate moves. These differences are clearly evident in photographs. At slow speeds, the crystals form in long, straight structures, in line with the direction the top plate is moving. At higher speeds, the crystals form wildly irregular patterns, and in other speeds the patterns resemble tiny snowflakes.

The engineers next tested the various crystalline patterns for their electrical properties. They found that optimal electrical conductivity was achieved when the top plate moved at 2.8 millimeters per second, a speed in the middle of the range they tested.

"In comparing the photographs of the crystals, it is not the longest, straightest structures that result in the best electrical characteristics," said Bao, "but the one with a shorter, yet highly consistent pattern."

New structures, new analyses

Bao's new semiconductor proved challenging in at least one other regard: Measurement and visualization of the lattices to understand how and why they work. To gain this understanding, she turned to Stefan Mannsfeld, PhD, a staff scientist and expert in x-ray scattering at Stanford Synchrotron Radiation Lightsource, a co-author of the paper.

"We have been able to improve how we analyze the relative brightness of the peaks we can see in x-ray diffraction images," said Mannsfeld. "Previously this was only possible when analyzing relatively big single crystals, but we have for the first time been able to duplicate this for very thin films of these crystals."

With improved analysis, the team was able to understand the physics behind the improvement. "Our analysis made it possible not only to see the impact of the strain on the lattice geometry, but also to determine the exact way in which the molecules pack in the lattice. As a result we obtained a better understanding of why such structures improve the molecule-to-molecule electrical coupling that improves the electrical efficiency," said Mannsfeld.

In the paper, Bao describes her new technique as general enough as to be applicable to other materials that might someday yield even better electrical characteristics in in a wide range of organic semiconductors.

Stanford doctoral candidates Guarav Giri and Eric Verploegen, former post-doctoral scholar Hector Becerril, PhD, in Bao's lab and Michael F. Toney, PhD, of the Stanford Synchrotron Radiation Lightsource, contributed to this research.

Story Source:

The above story is reprinted from materials provided by Stanford School of Engineering. The original article was written by Andrew Myers.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Gaurav Giri, Eric Verploegen, Stefan C. B. Mannsfeld, Sule Atahan-Evrenk, Do Hwan Kim, Sang Yoon Lee, Hector A. Becerril, Alán Aspuru-Guzik, Michael F. Toney, Zhenan Bao. Tuning charge transport in solution-sheared organic semiconductors using lattice strain. Nature, 2011; 480 (7378): 504 DOI: 10.1038/nature10683

Perspective article examines conductivity at the LaAlO3 and SrTiO3 (001) interface

Complex oxides have the potential to inject new functionalities into technologies that require semiconductors.  The correlated behavior of itinerant electrons in these materials sets complex oxides apart from traditional semiconductors such as Si and GaAs. Potential applications abound, but the fundamental properties of these materials, particularly when combined to make interfaces, must be understood.  In an invited Perspective article in Surface Science, Dr. Scott Chambers of PNNL examines conductivity at the interface of polar and nonpolar complex oxides from outside the reigning paradigm and considers how unintentional dopants and defects, resulting from interfacial mixing, might affect the electronic properties.

The common paradigm used to explain the observation of conductivity at interfaces of materials such as lanthanum aluminate and strontium titanate is that electrons move across the interface to alleviate the so-called polar catastrophe created by polar/nonpolar interface creation.  Based on a number of different experimental results, Chambers argues that this simple paradigm is inadequate to explain observed conductivity.

"Intermixing occurs, and the resulting cation rearrangement cannot be ignored," said Chambers, a Fellow of the AVS and the American Association for the Advancement of Science. "Moreover, defects and dopants appear to play a role in facilitating, if not enabling conductivity."

Providing insights into the fundamental relationships between composition/structure, and the resulting electronic, magnetic, and surface chemical properties of complex oxides could enable these materials to have an impact on next-generation electronics, chemical sensors, and photocatalysts. These advances could include more energy-efficient field effect transistors and photocatalysts that use visible light from the sun.

Chambers and his colleagues around the world are continuing to make strides in understanding the complex relationships between atom distributions near the interface and conductivity. One upshot is that significantly more insight into the growth process is necessary to characterize and ultimately control defect creation during heterojunction formation.

"Then and only then can interface structures suspected of facilitating conductivity be changed to see if doing so actually reduces or eliminates conductivity," said Chambers.

More information: Chambers SA. 2011. "Understanding the Mechanism of Conductivity at the LaAlO3 and SrTiO3 (001) Interface." Surface Science 605:1133-1140.

Provided by Pacific Northwest National Laboratory (news : web)

Researchers image graphene electron clouds, revealing how folds can harm conductivity

 A research team led by University at Buffalo chemists has used synchrotron light sources to observe the electron clouds on the surface of graphene, producing a series of images that reveal how folds and ripples in the remarkable material can harm its conductivity.

The research, scheduled to appear June 28 in Nature Communications, was conducted by UB, the National Institute of Standards and Technology (NIST), the Molecular Foundry at Lawrence Berkeley National Laboratory (Berkeley Lab), and SEMATECH, a global consortium of semiconductor manufacturers.

Graphene, the thinnest and strongest material known to man, consists of a single layer of carbon atoms linked in a honeycomb-like arrangement.

Graphene's special structure makes it incredibly conductive: Under ideal circumstances, when graphene is completely flat, electric charges speed through it without encountering many obstacles, said Sarbajit Banerjee, one of the UB researchers who led the study in Nature Communications.

But conditions are not always optimal.

The new images that Banerjee and his colleagues captured show that when graphene is folded or bent, the electron cloud lining its surface also becomes warped, making it more difficult for an electric charge to travel through.

"When graphene is flat, things just kind of coast along the cloud. They don't have to hop across anything. It's like a superhighway," said Banerjee, an assistant professor of chemistry. "But if you bend it, now there are some obstacles; imagine the difference between a freshly paved highway and one with construction work along the length forcing lane changes.

"When we imaged the electron cloud, you can imagine this big fluffy pillow, and we saw that the pillow is bent here and there," said Banerjee, whose National Science Foundation CAREER award provided the primary funding for the project.

To create the images and understand the factors perturbing the electron cloud, Banerjee and his partners employed two techniques that required use of a synchrotron: scanning transmission X-ray microscopy and near edge X-ray absorption fine structure (NEXAFS), a type of absorption spectroscopy. The experiments were further supported by computer simulations performed on computing clusters at Berkeley Lab.

"Using simulations, we can better understand the measurements our colleagues made using X-rays, and better predict how subtle changes in the structure of graphene affect its electronic properties," said David Prendergast, a staff scientist in the Theory of Nanostructures Facility at the Molecular Foundry at Berkeley Lab. "We saw that regions of graphene were sloped at different angles, like looking down onto the slanted roofs of many houses packed close together."

Besides documenting how folds in graphene distort its electron cloud, the research team discovered that contaminants that cling to graphene during processing linger in valleys where the material is uneven. Such contaminants uniquely distort the electron cloud, changing the strength with which the cloud is bound to the underlying atoms.

Graphene's unusual properties have generated excitement in industries including computing, energy and defense. Scientists say that graphene's electrical conductivity matches that of copper, and that graphene's thermal conductivity is the best of any known material.

But the new, UB-led study suggests that companies hoping to incorporate graphene into products such as conductive inks, ultrafast transistors and solar panels could benefit from more basic research on the nanomaterial. Improved processes for transferring flat sheets of graphene onto commercial products could greatly increase those products' efficiency.

"A lot of people know how to grow graphene, but it's not well understood how to transfer it onto something without it folding onto itself," Banerjee said. "It's very hard to keep straight and flat, and our work is really bringing home the point of why that's so important."

"Graphene is going to be very important in electronics," said PhD candidate Brian Schultz, one of three UB graduate students who were lead authors on the Nature Communications paper. "It's going to be one of the most conductive materials ever found, and it has the capability to be used as an ultrahigh-frequency transistor or as a possible replacement for silicon chips, the backbone of current commercial electronics.

"When graphene was discovered, people were just so excited that it was such a good material that people really wanted to go with it and run as fast as possible," Schultz continued. "But what we're showing is that you really have to do some fundamental research before you understand how to process it and how to get it into electronics."

Other research partners offered the following insight into the significance of the findings:

Dan Fischer, NIST Material Measurement Laboratory, leader, Synchrotron Methods Group: "The NEXAFS results indicating that performance-damaging contaminants cling to graphene during processing highlights the importance of chemically sensitive advanced synchrotron measurement method developments for promoting innovation and industrial competiveness in commercial applications of nanotechnology."Pat Lysaght, SEMATECH Front End Processes, senior member technical staff: "We place a premium on the power of collaboration, and this is a great example of the benefits associated with that philosophy. The unique expertise of each of the four collaborative entities has come together to forge a new understanding of subtle functionalization variations of surface graphene atoms. Our findings represent another important step toward potential industrial applications such as low-cost broadband radio frequency (RF) devices, and correlation of NEXAFS with Raman spectroscopy which may enhance monitoring capabilities for graphene as a replacement for large area organic LED displays."

Synchrotron imaging was conducted at the Canadian Light Source in Saskatchewan in Canada and at the National Synchrotron Light Source (NSLS ) at Brookhaven National Laboratory in New York State. NEXAFS was measured at the NIST soft X-ray beamline of the NSLS.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by University at Buffalo, via EurekAlert!, a service of AAAS.

Journal Reference:

Brian J. Schultz, Christopher J. Patridge, Vincent Lee, Cherno Jaye, Patrick S. Lysaght, Casey Smith, Joel Barnett, Daniel A. Fischer, David Prendergast, Sarbajit Banerjee. Imaging local electronic corrugations and doped regions in graphene. Nature Communications, 2011; 2: 372 DOI: 10.1038/ncomms1376

Delving into manganite conductivity

 Chemical compounds called manganites have been studied for many years since the discovery of colossal magnetoresistance, a property that promises important applications in the fields of magnetic sensors, magnetic random access memories and spintronic devices. However, understanding -- and ultimately controlling -- this effect remains a challenge, because much about manganite physics is still not known. A research team lead by Maria Baldini from Stanford University and Carnegie Geophysical Laboratory scientists Viktor Struzhkin and Alexander Goncharov has made an important breakthrough in our understanding of the mysterious ways manganites respond when subjected to intense pressure.

At ambient conditions, manganites have insulating properties, meaning they do not conduct electric charges. When pressure of about 340,000 atmospheres is applied, these compounds change from an insulating state to a metallic state, which easily conducts charges. Scientists have long debated about the trigger for this change in conductivity.

The research team's new evidence, published online Feb. 11 in Physical Review Letters, shows that for the manganite LaMnO3, this insulator-to-metal transition is strongly linked to a phenomenon called the Jahn-Teller effect. This effect actually causes a unique distortion of the compound's structure. The team's measurements were carried out at the Geophysical Laboratory.

Counter to expectations, the Jahn-Teller distortion is observed until LaMnO3 is in a non-conductive insulating state. Therefore, it is reasonable to believe that the switch from insulator to metal occurs when the distortion is suppressed, settling a longstanding debate about the nature of manganite insulating state. The formation of inhomogeneous domains -- some with and some without distortion -- was also observed. This evidence suggests that the manganite becomes metallic when the breakdown of undistorted to distorted molecules hits a critical threshold in favor of the undistorted.

"Separation into domains may be a ubiquitous phenomenon at high pressure and opens up the possibility of inducing colossal magnetoresistance by applying pressure" said Baldini, who was with Stanford at the time the research was conducted, but has now joined Carnegie as a research scientist.

Some of the researchers were supported by various grants from the Department of Energy, Office of Science and National Nuclear Security Administration. Some of the experiments were supported by DOE and Carnegie Canada.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Carnegie Institution.

Journal Reference:

M. Baldini, V. Struzhkin, A. Goncharov, P. Postorino, W. Mao. Persistence of Jahn-Teller Distortion up to the Insulator to Metal Transition in LaMnO3. Physical Review Letters, 2011; 106 (6) DOI: 10.1103/PhysRevLett.106.066402