UC Berkeley Labs: A Semiconductor That Can Beat the Heat



Berkeley Lab, UC Berkeley scientists discover unique thermoelectric properties in cesium tin iodide

JULY 31, 2017

A newly discovered collective rattling effect in a type of crystalline semiconductor blocks most heat transfer while preserving high electrical conductivity – a rare pairing that scientists say could reduce heat buildup in electronic devices and turbine engines, among other possible applications.

A team led by scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) discovered these exotic traits in a class of materials known as halide perovskites, which are also considered promising candidates for next-generation solar panels, nanoscale lasers, electronic cooling, and electronic displays.

These interrelated thermal and electrical (or “thermoelectric”) properties were found in nanoscale wires of cesium tin iodide (CsSnI3). The material was observed to have one of the lowest levels of heat conductivity among materials with a continuous crystalline structure.


Image – Rattling structures of halide perovskites: cesium tin iodide (left) and cesium lead iodide (right). (Credit: Berkeley Lab/UC Berkeley)

This so-called single-crystal material can also be more easily produced in large quantities than typical thermoelectric materials, such as silicon-germanium, researchers said.

“Its properties originate from the crystal structure itself. It’s an atomic sort of phenomenon,” said Woochul Lee, a postdoctoral researcher at Berkeley Lab who was the lead author of the study, published the week of July 31 in the Proceedings of the National Academy of Sciences journal. These are the first published results relating to the thermoelectric performance of this single crystal material.

Researchers earlier thought that the material’s thermal properties were the product of “caged” atoms rattling around within the material’s crystalline structure, as had been observed in some other materials. Such rattling can serve to disrupt heat transfer in a material.

“We initially thought it was atoms of cesium, a heavy element, moving around in the material,” said Peidong Yang, a senior faculty scientist at Berkeley Lab’s Materials Sciences Division who led the study.

Jeffrey Grossman, a researcher at the Massachusetts Institute of Technology, then performed some theory work and computerized simulations that helped to explain what the team had observed. 

Researchers also used Berkeley Lab’s Molecular Foundry, which specializes in nanoscale research, in the study.

“We believe there is essentially a rattling mechanism, not just with the cesium. It’s the overall structure that’s rattling; it’s a collective rattling,” Yang said. “The rattling mechanism is associated with the crystal structure itself,” and is not the product of a collection of tiny crystal cages. “It is group atomic motion,” he added.

Within the material’s crystal structure, the distance between atoms is shrinking and growing in a collective way that prevents heat from easily flowing through.

But because the material is composed of an orderly, single-crystal structure, electrical current can still flow through it despite this collective rattling. Picture its electrical conductivity is like a submarine traveling smoothly in calm underwater currents, while its thermal conductivity is like a sailboat tossed about in heavy seas at the surface.

Yang said two major applications for thermoelectric materials are in cooling, and in converting heat into electrical current. For this particular cesium tin iodide material, cooling applications such as a coating to help cool electronic camera sensors may be easier to achieve than heat-to-electrical conversion, he said.

A challenge is that the material is highly reactive to air and water, so it requires a protective coating or encapsulation to function in a device.

Cesium tin iodide was first discovered as a semiconductor material decades ago, and only in recent years has it been rediscovered for its other unique traits, Yang said. “It turns out to be an amazing gold mine of physical properties,” he noted.


SEM images of suspended micro-island devices. Individual AIHP NW is suspended between two membranes. (Credit: Berkeley Lab/UC Berkeley)

To measure the thermal conductivity of the material, researchers bridged two islands of an anchoring material with a cesium tin iodide nanowire. The nanowire was connected at either end to micro-islands that functioned as both a heater and a thermometer. 
Researchers heated one of the islands and precisely measured how the nanowire transported heat to the other island.

They also performed scanning electron microscopy to precisely measure the dimensions of the nanowire. They used these dimensions to provide an exacting measure of the material’s thermal conductivity. The team repeated the experiment with several different nanowire materials and multiple nanowire samples to compare thermoelectric properties and verify the thermal conductivity measurements.

“A next step is to alloy this (cesium tin iodide) material,” Lee said. “This may improve the thermoelectric properties.”

Also, just as computer chip manufacturers implant a succession of elements into silicon wafers to improve their electronic properties – a process known as “doping” – scientists hope to use similar techniques to more fully exploit the thermoelectric traits of this semiconductor material. This is relatively unexplored territory for this class of materials, Yang said.

The research team also included other scientists from Berkeley Lab’s Materials Sciences Division and the Molecular Foundry, the Kavli Energy NanoScience Institute at UC Berkeley and Berkeley Lab, and UC Berkeley’s Department of Chemistry.

The Molecular Foundry is a DOE Office of Science User Facility that provides free access to state-of-the-art equipment and multidisciplinary expertise in nanoscale science to visiting scientists from all over the world.

This work was supported by the Department of Energy’s Office of Basic Energy Sciences.
More information about Peidong Yang’s research group: http://nanowires.berkeley.edu/.

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Lawrence Berkeley National Laboratory addresses the world’s most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab’s scientific expertise has been recognized with 13 Nobel Prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy’s Office of Science. For more, visit http://www.lbl.gov.
DOE’s Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

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Nanotechnology Education for the Global World: Training the Leaders of Tomorrow


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Nanoscience is one of the fastest growing and most impactful fields in global scientific research. In order to support the continued development of nanoscience and nanotechnology, it is important that nanoscience education be a top priority to accelerate research excellence. In this Nano Focus, we discuss current approaches to nanoscience training and propose a learning design framework to promote the next generation of nanoscientists. Prominent among these are the abilities to communicate and to work across and between conventional disciplines. While the United States has played leading roles in initiating these developments, the global landscape of nanoscience calls for worldwide attention to this educational need. Recent developments in emerging nanoscience nations are also discussed. Photo credit: Jae Hyeon Park.

Education has long been recognized as an important factor for growing the fields of nanoscience and nanotechnology and solidifying and expanding their roles in the global economy. In many countries, there is growing interest in developing educational programs across the full spectrum of educational levels from K-12 to postgraduate studies.

Various formal and informal educational practices are being designed and tested that promote general awareness of nanoscience and nanotechnology as well as provide advanced learning and skills development, including through group learning and peer assessment”In their article, the authors discuss innovative learning models that are being applied at the undergraduate level in order to train future leaders at the interface of engineering and management.

students running nanoscience experiments

Middle and high school students spend time at the California NanoSystems Institute at UCLA running nanoscience experiments. High school teachers from over 100 schools and 30 school districts are trained, networked to one another, and supplied with kits for their classrooms. Graduate students, postdocs, faculty, and staff run, expand, and improve these fully subscribed outreach events on a continuous basis. (© American Chemical Society)

While thee programs are not strictly focused on nanotechnology, many graduates pursue nanotechnology-focused careers and they provide examples of important factors that should be considered in the nanotechnology field.Moreover, they represent the growing trend of holistic learning, which integrates coursework across disciplines, promotes foreign experiences, and encourages industrial internships.

Here is the set of recommendations they make:

Inspire Students To Envision What Is or Could Be Possible

Possibilities include a greater focus on nanotechnology applications in courses or hands-on laboratory experiences that tie in with class concepts. Even before reaching the classroom, students should have positive views of nanoscience and the potential it holds. Successful learning practices start with capturing the imagination of students. Communicating the remarkable features of nanoscience in a simple and clear way to the mainstream public would go a long way toward achieving this goal.

Promote Role Models Who Impact Society

From an educational perspective, the tech world is a particularly good example because successful entrepreneurs such as Steve Jobs, Elon Musk, Sheryl Sandberg, and Mark Zuckerberg have captured the public audience and inspired countless students to think beyond the classroom. In nanotechnology, similar role models can inspire students with the many opportunities available in the field.

Encourage Global Collaboration

Nanotechnology research and development is truly global. Early exposure to these trends will better inform students about career opportunities and give them ideas about how to work together in teams across disciplines and cultures. A growing number of partnerships already provide international experiences for nanoscience and nanotechnology students.

Support Early Exposure Inside and Outside of the Laboratory

For many students, nanoscience and nanotechnology are about working in a lab doing scientific research. While this activity is common, its generalization could not be farther from the truth. There are many possible ways to get involved in nanotechnology, from instructional education and hands-on training to entrepreneurship and manufacturing.Holistic approaches that integrate these different possibilities, while providing targeted career development, would greatly benefit students and the overall goals of nanotechnology education. Developing a strong workforce infrastructure for nanotechnology

Communication Across Fields

Stressing the importance of communication, the authors conclude:

“Finally, one of the great strengths of the nanoscience and nanotechnology communities is that we have taught each other how to communicate across fields, to look at and to leverage each other’s approaches, and to address the key issues of a multitude of fields.

As a field, we are increasingly viewed as problem solvers in science and technology, developing new tools, materials, methods, and opportunities. Bringing this aspect of our field to students (and scientists and engineers at all levels) will have significant impact on the world around us and our ability to make it better.”

By Michael Berger. © Nanowerk

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New Kind of Nanowires Designed For Efficient Water Splitting and Solar Energy Storage


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California is committed to 33 percent energy from renewable resources by 2020. With that deadline fast approaching, researchers across the state are busy exploring options.

Solar energy is attractive but for widespread adoption, it requires transformation into a storable form. This week in ACS Central Science, researchers report that nanowires made from multiple metal oxides could put solar ahead in this race.

One way to harness solar power for broader use is through photoelectrochemical (PEC) water splitting that provides hydrogen for fuel cells. Many materials that can perform the reaction exist, but most of these candidates suffer from issues, ranging from efficiency to stability and cost.

Peidong Yang and colleagues designed a system where nanowires from one of the most commonly used materials (TiO2) acts as a “host” for “guest” nanoparticles from another oxide called BiVO4. BiVO4 is a newly introduced material that is among the best ones for absorbing light and performing the water splitting reaction, but does not carry charge well while TiO2 is stable, cheap and an efficient charge carrier but does not absorb light well.

Together with a unique studded nanowire architecture, the new system works better than either material alone.

The authors state their approach can be used to improve the efficiencies of other photoconversion materials.

Synopsis

We report the use of Ta:TiO2|BiVO4 as a photoanode for use in solar water splitting cells. This host−guest system makes use of the favorable band alignment between the two semiconductors. The nanowire architecture allows for simultaneously high light absorption and carrier collection for efficient solar water oxidation.

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Metal oxides that absorb visible light are attractive for use as photoanodes in photoelectrosynthetic cells. However, their performance is often limited by poor charge carrier transport. We show that this problem can be addressed by using separate materials for light absorption and carrier transport. Here, we report a Ta:TiO2|BiVO4 nanowire photoanode, in which BiVO4 acts as a visible light-absorber and Ta:TiO2 acts as a high surface area electron conductor. Electrochemical and spectroscopic measurements provide experimental evidence for the type II band alignment necessary for favorable electron transfer from BiVO4 to TiO2. The host–guest nanowire architecture presented here allows for simultaneously high light absorption and carrier collection efficiency, with an onset of anodic photocurrent near 0.2 V vs RHE, and a photocurrent density of 2.1 mA/cm2 at 1.23 V vs RHE.

Introduction


Harnessing energy from sunlight is a means of meeting the large global energy demand in a cost-effective and environmentally benign manner. However, to provide constant and stable power on demand, it is necessary to convert sunlight into an energy storage medium.(1) An example of such a method is the production of hydrogen by photoelectrochemical (PEC) water splitting. The direct splitting of water can be achieved using a single semiconductor; however, due to the voltage requirement of the water splitting reaction and the associated kinetic overpotentials, only wide-band-gap materials can perform overall water splitting, limiting the efficiency due to insufficient light absorption.(2) To address this issue, a dual-band-gap z-scheme system can be utilized, with a semiconductor photoanode and photocathode to perform the respective oxidation and reduction reactions.(3) This approach allows for the use of lower-band-gap materials that can absorb complementary portions of the solar spectrum and yield higher solar-to-fuel efficiencies.(4, 5) In this integrated system, the charge flux is matched in both light absorbers of the photoelectrochemical cell. Therefore, the overall performance is determined by the limiting component. In most photoelectrosynthetic cells, this limiting component is the semiconductor photoanode.(6)
Metal oxides have been heavily researched as photoanode materials since few conventional light absorber materials are stable at the highly oxidizing conditions required for water oxidation.(7) However, the most commonly studied binary oxide, TiO2, has a band gap that is too large to absorb sunlight efficiently (∼3.0 eV), consequently limiting its achievable photocurrent.(8) While promising work has recently been done on stabilizing conventional light absorbers such as Si,(9) GaAs,(10) and InP,(11) the photovoltage obtained by these materials thus far has been insufficient to match with smaller-band-gap photocathode materials such as Si and InP in a dual absorber photoelectrosynthetic cell.(12, 13) Additionally, these materials have high production and processing costs. Small-band-gap metal oxides that absorb visible light and can be inexpensively synthesized, such as WO3, Fe2O3, and BiVO4, are alternative materials that hold promise to overcome these limitations.(14-16) Among these metal oxides, BiVO4 has emerged as one of the most promising materials due to its relatively small optical band gap of ∼2.5 eV and its negative conduction band edge (∼0 V versus RHE).(17, 18) Under air mass 1.5 global (AM1.5G) solar illumination, the maximum achievable photocurrent for water oxidation using BiVO4 is ∼7 mA/cm2.(16) However, the water oxidation photocurrent obtained in practice for BiVO4 is substantially lower than this value, mainly due to poor carrier transport properties, with electron diffusion lengths shorter than the film thickness necessary to absorb a substantial fraction of light.(17)
One approach for addressing this problem is to use two separate materials for the tasks of light absorption and carrier transport. To maximize performance, a conductive and high surface area support material (“host”) is used, which is coated with a highly dispersed visible light absorber (“guest”). This architecture allows for efficient use of absorbed photons due to the proximity of the semiconductor liquid junction (SCLJ). This strategy has been employed in dye sensitized (DSSC) and quantum dot sensitized solar cells (QDSSC).(19, 20) Using a host–guest scheme can improve the performance of photoabsorbing materials with poor carrier transport but relies upon appropriate band alignment between the host and guest. Namely, the electron affinity of the host should be larger, to favor electron transfer from guest to host without causing a significant loss in open-circuit voltage.(21) Nanowire arrays provide several advantages for use as the host material as they allow high surface area loading of the guest material, enhanced light scattering for improved absorption, and one-dimensional electron transport to the back electrode.(22) Therefore, nanowire arrays have been used as host materials in DSSCs, QDSSCs, and hybrid perovskite solar cells.(23-25) In photoelectrosynthetic cells, host–guest architectures have been utilized for oxide photoanodes such as Fe2O3|TiSi2,(26) Fe2O3|WO3,(27) Fe2O3|SnO2,(28) and Fe2TiO5|TiO2.(29) For BiVO4, it has been studied primarily with WO3|BiVO4,(30-32) ZnO|BiVO4,(33) and anatase TiO2|BiVO4.(34) While attractive for its electronic transport properties, ZnO is unstable in aqueous environments, and WO3 has the disadvantage of having a relatively positive flatband potential (∼0.4 V vs RHE)(14) resulting in potential energy losses for electrons as they are transferred from BiVO4 to WO3, thereby limiting the photovoltage of the combined system. Performance in the low potential region is critical for obtaining high efficiency in photoelectrosynthetic cells when coupled to typical p-type photocathode materials such as Si or InP.(12, 13) TiO2 is stable in a wide range of pH and has a relatively negative flat band potential (∼0.2 V vs RHE)(7) which does not significantly limit the photovoltage obtainable from BiVO4, while still providing a driving force for electron transfer. While TiO2 has intrinsically low mobility, doping TiO2 with donor type defects could increase the carrier concentration and thus the conductivity. Indeed, niobium and tantalum doped TiO2 have recently been investigated as potential transparent conductive oxide (TCO) materials.(35, 36) A host material with high carrier concentration could also ensure low contact resistance with the guest material.(37)
Using a solid state diffusion approach based on atomic layer deposition (ALD), we have previously demonstrated the ability to controllably and uniformly dope TiO2.(38) In this study we demonstrate a host–guest approach using Ta-doped TiO2 (Ta:TiO2) nanowires as a host and BiVO4 as a guest material. This host–guest nanowire architecture allows for simultaneously high light absorption and carrier collection efficiency, with an onset of anodic photocurrent near 0.2 V vs RHE, and a photocurrent of 2.1 mA/cm2 at 1.23 V vs RHE. We show that the synergistic effect of the host–guest structure results in higher performance than either pure TiO2 or BiVO4. We also experimentally demonstrate thermodynamically favorable band alignment between TiO2 and BiVO4 using spectroscopic and electrochemical methods, and study the band edge electronic structure of the TiO2 and BiVO4 using X-ray absorption and emission spectroscopies.

 

Article adapted from a American Chemical Society news release. To Read the FULL release, please click on the link provided below.

Publication: TiO2/BiVO4 Nanowire Heterostructure Photoanodes Based on Type II Band Alignment. Resasco, J et al. ACS Central Science (3 February, 2016): Click here to view.

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Alivisatos (UC Berkley) Appointed Samsung Distinguished Chair in Nanoscience


 

By Public Affairs, UC Berkeley | August 22, 2013

BERKELEY —201306047919620Chemist Paul Alivisatos, one of the pioneers of nanoscience, has been appointed to the Samsung Distinguished Chair in Nanoscience and Nanotechnology at UC  Berkeley in recognition of his many scientific achievements.

The endowed chair, established through the support of Samsung Electronics Co., will help cement the campus’s leadership in research and innovation in an area that has great implications for many fields ranging from biology to energy, the Office of the Vice-Chancellor for Research announced Friday (Aug. 23). Alivisatos, director of the Lawrence Berkeley National Laboratory and a UC Berkeley professor of chemistry, is known for his research into quantum dot semiconductor nanocrystals, clusters of hundreds to thousands of atoms with novel properties that can be applied to electronic devices and solar cells as well as light-emitting diodes (LEDs).

Paul Alivisatos

Paul Alivisatos, the newly named Samsung Distinguished Chair in Nanoscience and Nanotechnology, in conversation with Dr. Young Hwan Kim of the Samsung Advanced Institute of Technology, Korea, at Alivisatos’s lab on the UC Berkeley campus. A delegation from SAIT visited UC Berkeley Thursday, Aug. 22.  (Photo by Roy Kaltschmidt, Berkeley Lab)

Dr. Youngjoon Gil, executive vice president of the Samsung Advanced Institute of Technology, welcomed the appointment.

“Historically, the invention of a new material can initiate a quantum leap in the development of industry,” said Dr. Gil. “Nanomaterials offer such opportunities for the electronics as well as the biosciences industry, where precise control and manipulation of energy is required. Quantum dot, pioneered by Professor Alivisatos, has established its commercial value by reproducing more realistic colors on displays. Through the establishment of the endowed chair, Samsung anticipates a closer partnership with UC Berkeley, the world’s leader in nanoscience, in exploring the commercial value of nanotechnology.”

Over the past two decades, UC Berkeley has become a brain trust in nanoscience and nanotechnology, with nearly a hundred nanoscience and nanotech researchers in the fields of biology, chemistry, physics and materials science. These researchers have made major advances in understanding the nano-scale molecular motors that move materials around inside cells or manipulate DNA; creating tiny motors, lasers and photonic devices for smaller electronic circuits; creating flexible and inexpensive solar cells from nanorods; and understanding the properties of new materials such as graphene and high-temperature superconductors.

Graham Fleming, UC Berkeley’s vice chancellor for research, lauded Samsung for its initiative in establishing this chair.

“The new chair helps build on our strengths in the conversation and utilization of energy on the nano scale,” said Fleming. “It is a fitting recognition of Paul’s achievements and his world-wide influence on the field of nanoscience. We look forward to continue expanding our relationship with Samsung in this area.”

Alivisatos is widely recognized for his contributions to the study of nanocrystals, ranging from control of their synthesis and fabrication to studies of their optical, electrical, structural, and thermodynamic properties. He demonstrated that semiconductor nanocrystals can be grown into rods as opposed to spheres. This achievement paved the way for a slew of new synthetic advances, developing methods for controlling the shape, connectivity and topology of nanocrystals.

Nanocrystals are typically a few nanometers in diameter — larger than molecules but smaller than bulk solids — and frequently exhibit physical and chemical properties somewhere in between. Given that a nanocrystal is virtually all surface and no interior, its properties can vary considerably as the crystal grows.

Alivisatos’s research has opened the door to a number of potential new applications for nanocrystals. These include their use as fluorescent probes for the study of biological materials and LEDs, and the fabrication of hybrid solar cells that combine nanotechnology with plastic electronics.