91ÉçÇø

Research Highlights:

Dr. Louis Piper's journal article, entitled: Distinction between Intrinsic and X-ray-Induced Oxidized Oxygen States in Li-Rich 3d Layered Oxides and LiAlO2, has made the cover of ACS Publications for May 2019. To read the full article, please click

Researchers used the Advanced Light Source (ALS), in collaboration with Dr. Louis Piper, have quantified a strong, beneficial, and reversible (over hundreds of cycles) chemical reaction involving oxygen ions in the crystal lattice of battery electrode materials. The results open up new ways to explore how to pack more energy into batteries with electrodes made out of low-cost, common materials. For more information, please click

Spotlight Article:

"Exploring the surface of gallium oxide featured". Authors report downward band bending and electron accumulation at the surface of gallium oxide due to the presence of hydroxide. Swallow et al. investigated the surface electron properties of gallium oxide, and observed downward band bending and electron accumulation at the surface of as-received gallium oxide crystals for the first time.
They found that this is not the inherent state of the surface of gallium oxide. When vacuum thermal annealing is used to clean the surface, its natural state is electron depletion. However, before cleaning, the unintentional presence of adsorbed hydrogen provides an extrinsic source of surface donors, which causes an accumulation layer of electrons on the surface. For more information, please click

Dr. Louis Piper journal article entitled,was published as the cover art of the Journal of the American Chemical Society for the November 2018 issue.

Summary of Paper:

Tackling the complex challenge of harvesting solar energy to generate energy-dense fuels such as hydrogen requires the design of photocatalytic nanoarchitectures interfacing components that synergistically mediate a closely interlinked sequence of light-harvesting, charge separation, charge/mass transport, and catalytic processes. The design of such architectures requires careful consideration of both thermodynamic offsets and interfacial charge-transfer kinetics to ensure long-lived charge carriers that can be delivered at low overpotentials to the appropriate catalytic sites while mitigating parasitic reactions such as photocorrosion. Here we detail the theory-guided design and synthesis of nanowire/quantum dot heterostructures with interfacial electronic structure specifically tailored to promote light-induced charge separation and photocatalytic proton reduction. Topochemical synthesis yields a metastable β-Sn0.23V2O5 compound exhibiting Sn 5s-derived midgap states ideally positioned to extract photogenerated holes from interfaced CdSe quantum dots. The existence of these midgap states near the upper edge of the valence band (VB) has been confirmed, and β-Sn0.23V2O5/CdSe heterostructures have been shown to exhibit a 0 eV midgap state-VB offset, which underpins ultrafast subpicosecond hole transfer. The β-Sn0.23V2O5/CdSe heterostructures are further shown to be viable photocatalytic architectures capable of efficacious hydrogen evolution. The results of this study underscore the criticality of precisely tailoring the electronic structure of semiconductor components to effect rapid charge separation necessary for photocatalysis.
 

Dr. Wei-Cheng Lee, Assistant Professor in Physics, was part of a collaborative research project on cuprate superconductors that landed in ,  and

Summary of paper:Cuprate superconductors are well-known for their unusual high critical temperature, but the superconducting mechanism is still under debate. It has been well-established that the two-dimensional CuO2 plane together with the magnetic interaction play the crucial role. Although theorists like A. J Leggett, P. W. Anderson have once suggested that the Coulomb (charge) interaction between CuO2 layers (3D) might have help increase the critical temperature, the experimental evidence for the 3-D charge excitation is still lacking. Scientists at SLAC used the technique of 'Resonance Inelastic X-Ray Spectroscopy (RIXS)' to detect solid evidences for the charge excitations with a 3-D nature in cuprates for the first time, and Prof. Lee, a faculty member in Physics department and IMR at 91ÉçÇø, is part of the team to construct the theoretical model. This work will stimulate more ideas about the key ingredients to find new high-temperature superconductors.

Director, Dr. Louis Piper, et al. publication entitled, Role  of disorder in limiting the true multi-electron redox in 3-  LiVOPO4, made the cover art for the Journal of  Materials Research (Nov. 2018)

Summary of Paper:
Recent advances in materials syntheses have enabled 3-LiVOPO4 to deliver capacities approaching, and in some cases exceeding the theoretical value of 305 mA h g^-1 for 2Li intercalation, despite its poor electronic and ionic conductivity. However, not all of the capacity corresponds to the true electrochemical intercalation/deintercalation reactions as evidenced upon systematic tracking of V valence through combined operando and rate-dependent ex situ X-ray absorption study presented herein. Structural disorder and defects introduced in the material by high-energy ball milling impede kinetics of the highvoltage V5+/V4+ redox more severely than the low-voltage V4+/V3+ redox, promoting significant side reaction contributions in the high-voltage region, irrespective of cycling conditions. The present work emphasizes the need for nanoengineering of active materials without compromising their bulk structural integrity in order to fully utilize high-energy density of multi-electron cathode materials. 

recently published an article on Director Louis Piper. Dr. Piper, along with a team of scientists though his DOD funded MURI work, have developed "neuristor" circuits that behave similarly to biological neurons in the human brain, which can perform complex computations using an incredibly small amount of power. These neuristors are being used to created energy efficient computing. 

Summary of paper:

In 1962, an electronic circuit was theorized that could propagate electrical signals without attenuation. These "neuristor" circuits would behave similarly to biological neurons in the human brain, which can propagate signals and thus perform complex computations using an incredibly small amount of power, equivalent to that of a dim incandescent light bulb (~20 Watts). In comparison, modern desktop PCs using traditional circuits typically use about 10x the power (~200 Watts), but provide only a tiny fraction of the computing power.

More recently, a vital component of this neuristor circuit was created using NbO2, which replicates the switching behavior observed in ion channels within biological neurons. These NbO2 devices are created by applying a large voltage across a non-conductive Nb2O5 film, causing the formation of conductive NbO2 filaments which are responsible for the important switching behavior. Unfortunately, this high-voltage and time-consuming post-fabrication process makes it near impossible to create the dense circuits needed for complex computer processors. In addition, these NbO2 devices require an additional companion capacitor to function properly within the neuristor circuit, making them more complex and unwieldy to implement.

In this study, Nb2O5−x-based devices are created that reproduce the behavior of the combined NbO2/capacitor pair while at the same time requiring no difficult, high-voltage post-fabrication treatments. Instead these devices are found to operate due to defects in the material structure inherent to the Nb2O5−x fabrication process. This finding will lead to more inexpensive, energy-efficient, and high-density neuristor circuits than previously possible, accelerating the way to more energy efficient and adaptable computing.

Summary of paper:
Understanding the structural transformations that materials undergo during the insertion and deinsertion of Li-ions is crucial for designing high-performance intercalation hosts as these deformations can lead to significant capacity fade over time. Distortions are driven by polaronic charge transport (i.e. the electrons and ions moving through the lattice in a coupled way) due to the semiconducting nature of most oxide hosts. Here we present a study of the metallic perovskite ReO3; despite its metallic character multiple phase changes occur with inserting/removing lithium involving the correlated twisting of rigid octahedral subunits. These results suggest that phase transformations during alkali ion intercalation can result from local strains in the lattice and not exclusively due to polaron migration.