We report ambipolar field-effect transistors fabricated from rubrene thin films on SiO2∕Si substrates. The mobilities of both holes and electrons were extremely low, ranging from 2.2×10−6to8.0×10−6cm2∕Vs, due to disorder in the films. Rubrene forms three-dimensional circular islands even at extremely low coverages and x-ray diffraction observations suggest that the film is amorphous. The formation of the conducting channel of the transistor follows the geometric percolation of rubrene islands.

Near edge x-ray absorption fine structure (NEXAFS) spectroscopy is used to study the orientation of pentacene molecules within thin films on SiO2 for thicknesses ranging from monolayers to the bulk (150 nm). The spectra exhibit a strong polarization dependence of the pi* orbitals for all films, which indicates that the pentacene molecules are highly oriented. At all film thicknesses the orientation varies with the rate at which pentacene molecules are deposited, with faster rates favoring a thin film phase with different tilt angles and slower rates leading to a more bulklike orientation. Our NEXAFS results extend previous structural observations to the monolayer regime and to lower deposition rates. The NEXAFS results match crystallographic data if a finite distribution of the molecular orientations is included. Damage to the molecules by hot electrons from soft x-ray irradiation eliminates the splitting between nonequivalent pi* orbitals, indicating a breakup of the pentacene molecule.

We fabricated an additive-free MAX(Ti₃AlC₂) phase Li-ion battery (LIB) electrode using the electrophoretic deposition (EPD) method. In this study, MAX, a precursor of MXene, which has recently been receiving great attention as a negative electrode material for LIBs, was manufactured as a coin cell through EPD rather than the conventional slurry system. We excluded the effect of additives on the electrochemical performance, enabling evaluation of the intrinsic electrochemical properties related to battery charging and discharging. As a result, the battery using MAX as an anode material showed a large specific capacity of 148.2 mAh/g in the first discharge and superior cycle stability. Enhanced cycle stability and reversible electrochemical reactions were attributed to activation of faradaic and non-faradaic behavior, i.e., pseudocapacitive behavior, caused by delamination of the MAX(Ti₃AlC₂) into MXene (Ti₃C₂). This was confirmed by the decrease in the charge transfer resistance and the increase in total capacitance at the interface, using electrochemical impedance spectroscopy and cyclic voltammetry measurements. In addition, the activation of pseudocapacitive behavior was confirmed by the change in kinetic mechanism, as evidenced from a significant increase in the Li ion diffusivity with cycles. These results demonstrate that MAX(Ti<sub>3<sub>AlC<sub>2<sub>) is promising as an anode material for LIBs and at the same time shows potential for tuning electrochemical properties through the electrochemical delamination process.

The lattice-mismatch-induced strain in growth of Ge on Si produces a host of exciting scientific and technological consequences, both in 3D nanostructure formation and, when silicon-on-insulator (SOI) is used as a substrate, in 2D membrane fabrication. One can use the ideas of strain sharing and critical thickness, combined with the ability to release the top layers of SOI, to create freestanding, dislocation-free, elastically strain relieved flexible Si/Ge membranes with nanometer scale thickness, which we call NanoFLEXSi or Si nanomembranes (SiNMs). The membranes can be transferred to new substrates, producing the potential for novel heterogeneous integration.The very interesting, and in some cases surprising, structural and electronic properties of these very thin membranes have been revealed using STM, x-ray diffraction, and electronic transport measurements. For example, STM shows that conduction in very thin Si layers on SOI with bulk-Si mobilities is possible even though the membrane is bulk depleted. Using the effect of elastic strain, we have fabricated two-dimensional electron gases (2DEGs) in membrane structures; we support the transport measurements with calculations suggesting that we are observing a single bound state in the well. We have fabricated thin-film transistors (TFTs) that we have transferred to flexible-polymer hosts that show a very high saturation current and transconductance. Thus very highspeed flexible electronics over large areas become possible.Index Terms -flexible electronics, Si nanomembrane, elastic strain relief, strained Si, 2DEG, thin-film transistor.

In this study, single-crystalline methylammonium lead tribromide (MAPbBr₃, CH₃NH₃PbBr₃) flakes with a higher theoretical specific capacity than methylammonium lead iodide (MAPbI₃, CH₃NH₃PbI₃), the prototype of organic/inorganic hybrid perovskite, were synthesized by the inverse temperature crystallization method. The combination of the additive-free electrophoretic deposition (EPD) system and the single crystal fabrication method provided a fundamental understanding of the electrochemical properties associated with the lithium-ion storage mechanism in which the formation of the lithiated-phase (Li_x:CH₃NH₃PbBr₃) and the conversion reaction causes significant irreversible capacity, reducing battery cycle stability. The conversion reaction in MAPbBr₃ was observed to be the main factor for high irreversible capacity, while the capacity due to the alloying reaction was more significant. The occurrence of the capacity due to the conversion reaction in the slurry system demonstrates that the additives worked as a buffer to relieve the stress associated with the formation of the lithiated phase, which was not observed in the MAPbBr₃ EPD film. As part of an investigation of the active material/electrolyte in terms of cycling stability, the problem of structural instability was addressed by replacing the lithium salt and organic solvent that are components of the electrolyte. Our findings shed light on the intrinsic electrochemical properties of MAPbBr₃ during lithiation/delithiation in the charging/discharging process, eliminating the complex effect caused by the MAPbBr₃/additive mixture. Structural stability at the MAPbBr₃/electrolyte interface was probed by varying the solvent and lithium salt.