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Following the Drude model for dielectric constant, e(E) = (E) + i(E), the plasmonic resonance energy Eres in a semiconductor depends on four parameters: () (or ) , effective mass m*, optical mobility µopt, and optical carrier concentration nopt. By solving the Drude equation at e(Eres) = 0, we obtain a relationship between µopt and nopt at constant Eres [or wavelength res(µm) = 1.2395/Eres(eV)]. A family of µopt vs nopt curves covering a range of res values (including the limiting wavelength res = ) constitute a plasmonic resonance phase diagram (PRPD) for a semiconductor defined by only e and m*. The PRPD is a convenient instrument that allows an immediate prediction of res from Hall-effect measurements of µH and nH. Furthermore, if the µH/nH point falls outside the family of µopt vs nopt curves, it shows that no resonance at all is possible. We apply the PRPD analysis to a series of ten ZnO samples grown by pulsed laser deposition at 200 C in an ambient of 33%H2:67%Ar and annealed in 25-C steps for 10 min in air at various temperatures from 400 C to 600 C. The unannealed sample had a resistivity  = 1.67 x 10-4 -cm, nmore » = 1.39 x 1021 cm-3, and µ = 26.8 cm2/V-s, and the 600-C sample,  = 1.52 -cm, n = 5.03 x 1017 cm-3, and µ = 8.2 cm2/V-s. Each of the ten µH/nH points was plotted on the PRPD. For the samples that were annealed at 550 C or lower, the µH/nH points yielded predicted values of res that ranged from 1.07 to 2.80 µm, respectively; however, the 575-C and 600-C samples were predicted to have no resonances at all. Reflectance curves for the eight samples annealed up to 550 C de-creased slowly from 6 eV down to about 1 eV, and then increased rapidly at an energy evidently close to Eres. In contrast, there was no such increase for the 575-C and 600-C samples, and thus presumably, no resonance. Satisfactory agreement is found between the reflectance and Hall-effect-predicted values of res.« less

Nonuniform and highly localized Li dendrites are known to cause deleterious and, in many cases, catastrophic effects on the performance of rechargeable Li batteries. However, the mechanisms of cathode failures upon contact with Li metal are far from clear. In this study, using in situ transmission electron microscopy, the interaction of Li metal with well-defined, epitaxial thin films of LiCoO2 , the most widely used cathode material, is directly visualized at an atomic scale. It is shown that a spontaneous and prompt chemical reaction is triggered once Li contact is made, leading to expansion and pulverization of LiCoO2 and ending with the final reaction products of Li2 O and Co metal. A topotactic phase transition is identified close to the reaction front, resulting in the formation of CoO as a metastable intermediate. Dynamic structural and chemical imaging, in combination with ab initio simulations, reveal that a high density of grain and antiphase boundaries is formed at the reaction front, which are critical for enabling the short-range topotactic reactions and long-range Li propagation. The fundamental insights are of general importance in mitigating Li dendrites related issues and guiding the design principle for more robust energy materials.

We describe the synthesis and characterization of high-performance, superparamagnetic, iron oxide nanoparticle-based, heavy metal sorbents, which demonstrate excellent affinity for the separation of heavy metals in contaminated water systems (i.e., spiked Columbia River water). The magnetic nanoparticle sorbents were prepared from an easy-to-synthesize iron oxide precursor, followed by a simple, one-step ligand exchange reaction to introduce an affinity ligand to the nanoparticle surface that is specific to a heavy metal or class of heavy metal contaminants. The engineered magnetic nanoparticle sorbents have inherently high active surface areas, allowing for increased binding capacities. To demonstrate the performance of the nanoparticle sorbents, river water was spiked with specific metals and exposed to low concentrations of the functionalized nanoparticles. In almost all cases, the nanoparticles were found to be superior to commercially available sorbent materials as well as the unfunctionalized iron oxide nanoparticles.

Photoemission electron microscopy (PEEM) excited by x-ray and UV sources is used to investigate epitaxial anatase thin films embedded with rutile nanocrystals, a model system for the study of heterocatalysis on mixed-phase TiO2. Both excitation sources show distinct contrast between the two TiO2 phases, however, the contrast is reversed. Rutile nanocrystals appear darker than the anatase film in X-ray PEEM images but brighter in UV-PEEM images. Topography-induced contrast is dominant X-ray PEEM imaging, whereas work function contrast, dominates for UV-PEEM. Work function contrast results from the differences in work function and surface defect state densities between the two phases near the Fermi level. This assertion is confirmed by UPS data that shows the rutile work function to be 0.2 eV lower and a greater occupied valence band density-of-states in rutile (100) than in anatase (001). Since the boundaries between rutile nanocrystals and the anatase film are clearly resolved, these results indicate that PEEM studies of excited state dynamics and heterocatalysis are possible at chemically intriguing mixed-phase TiO2 interfaces and grain boundaries.

The thermally-induced martensitic phase transformation in a polycrystalline NiTiCu thin film shape memory alloy was probed by photoelectron emission microscopy (PEEM). In situ PEEM images reveal distinct changes in microstructure and photoemission intensity at the phase transition temperatures. In particular, images of the low temperature, martensite phase are brighter than that of the high temperature, austenite phase, due to the relatively lower work function of the martensite. Ultra-violet photoelectron spectroscopy shows that the effective work function changes by about 0.16 eV during thermal cycling. In situ PEEM images also show that the network of trenches observed on the room temperature film disappear suddenly during heating and reappear suddenly during subsequent cooling. These trenches are also characterized by atomic force microscopy at selected temperatures. We describe implications of these observations with respect to the spatial distribution of phases during thermal cycling in this thin film shape memory alloy.

Interfaces between minerals and water, and minerals and microbes, are chemically complex and traditionally have been considered beyond the capabilities of surface science techniques, except for model systems under controlled laboratory conditions. We report on some advances in soft x-ray spectroscopy and imaging that make it possible to extract meaningful chemical information about interfaces of specimens that have complex histories, involving environmental exposure. These measurements utilize x-ray absorption spectroscopy, in combination with spatial resolution, in a technique called x-ray spectromicroscopy. Examples are drawn from attempts at Mn and Fe speciation of biologically produced minerals, bio-corrosion deposits and clays.

Epitaxial chromium ferrite (Fe2CrO4), prepared by state-of-the-art oxygen plasma assisted molecular beam epitaxy, is shown to exhibit unusual electronic transport properties driven by the crystallographic structure and composition of the material. Replacing 1/3 of the Fe cations with Cr converts the host ferrimagnet from a metal into a semiconductor by virtue of its fixed valence (3+); Cr substitutes for Fe at B sites in the spinel lattice. By contrast, replacing 2/3 of the Fe cations with Cr results in an insulator. Three candidate conductive paths, all involving electron hopping between Fe2+ and Fe3+, are identified in Fe2CrO4. Moreover, Fe2CrO4 is shown to be photoconductive across the visible portion of the electromagnetic spectrum. As a result, this material is of potential interest for important photo-electrochemical processes such as water splitting.

Two-dimensional (2D) ultra-high carrier densities at complex oxide interfaces are of considerable current research interest for novel plasmonic and high charge-gain devices. However, the highest 2D electron density obtained in oxide heterostructures is thus far limited to 3×1014 cm-2 (½ electron/unit cell/interface) at GdTiO3/SrTiO3 interfaces, and is typically an order of magnitude lower at LaAlO3/SrTiO3 interfaces. Here we show that carrier densities much higher than 3×1014 cm-2 can be achieved via band engineering. Transport measurements for 3 nm SrTiO3/t u.c. NdTiO3/3 nm SrTiO3/LSAT (001) show that charge transfer significantly in excess of the value expected from the polar discontinuity model occurs for higher t values. The carrier density remains unchanged, and equivalent to ½ electron/unit cell/interface for t < 6 unit cells. However, above a critical NdTiO3 thickness of 6 u.c., electrons from the valence band of NdTiO3 spill over into the SrTiO3 conduction band as a natural consequence of the band alignment. An atomistic model consistent with first-principle calculations and experimental results is proposed for the charge transfer mechanisms. These results may provide an exceptional route to the realization of the room-temperature oxide electronics.