Christopher J. Patridge

Imaging local electronic corrugations and doped regions in graphene

Electronic structure heterogeneities are ubiquitous in two-dimensional graphene and profoundly impact the transport properties of this material. Here we show the mapping of discrete electronic domains within a single graphene sheet using scanning transmission X-ray microscopy in conjunction with ab initio density functional theory calculations. Scanning transmission X-ray microscopy imaging provides a wealth of detail regarding the extent to which the unoccupied levels of graphene are modified by corrugation, doping and adventitious impurities, as a result of synthesis and processing. Local electronic corrugations, visualized as distortions of the π*cloud, have been imaged alongside inhomogeneously doped regions characterized by distinctive spectral signatures of altered unoccupied density of states. The combination of density functional theory calculations, scanning transmission X-ray microscopy imaging, and in situ near-edge X-ray absorption fine structure spectroscopy experiments also provide resolution of a longstanding debate in the literature regarding the spectral assignments of pre-edge and interlayer states.

Synthesis, Spectroscopic Characterization, and Observation of Massive Metal—Insulator Transitions in Nanowires of a Nonstoichiometric Vanadium Oxide Bronze

Metal-insulator transitions in strongly correlated materials, induced by varying either temperature or dopant concentration, remain a topic of enduring interest in solid-state chemistry and physics owing to their fundamental importance in answering longstanding questions regarding correlation effects. We note here the unprecedented observation of a four-orders-of-magnitude metal-insulator transition in single nanowires of delta-K(x)V(2)O(5), when temperature is varied, which thus represents a rare new addition to the pantheon of materials exhibiting pronounced metal-insulator transitions in proximity to room temperature.

Electrically driven metal-insulator switching in δ-KxV2O5 nanowires

Metal-insulator transition (MIT) in δ-KxV2O5 nanowires is studied via tuning temperature, voltage, and current. In the temperature-driven case, a massive drop in resistance over ∼4 orders of magnitude at ∼380 K is reported [C. J. Patridge et al., Nano Lett. 10, 2448 (2010)]. Our observation of electrically driven MIT results from a systematic study in any δ-MxV2O5 system (M is the intercalation ion). In the voltage-driven case, the threshold voltage follows an exponential relation with temperature. In the current-driven case, a negative differential resistance region is observed. These results suggest that δ-KxV2O5 is an interesting oxide system exhibiting strong electrically driven MIT and will hence be useful in several switching applications.