Wafer-scale transfer route for top-down III-nitride nanowire LED arrays based on the femtosecond laser lift-off technique.

The integration of gallium nitride (GaN) nanowire light-emitting diodes (nanoLEDs) on flexible substrates offers opportunities for applications beyond rigid solid-state lighting (e.g., for wearable optoelectronics and bendable inorganic displays). Here, we report on a fast physical transfer route based on femtosecond laser lift-off (fs-LLO) to realize wafer-scale top-down GaN nanoLED arrays on unconventional platforms. Combined with photolithography and hybrid etching processes, we successfully transferred GaN blue nanoLEDs from a full two-inch sapphire substrate onto a flexible copper (Cu) foil with a high nanowire density (~107 wires/cm2), transfer yield (~99.5%), and reproducibility. Various nanoanalytical measurements were conducted to evaluate the performance and limitations of the fs-LLO technique as well as to gain insights into physical material properties such as strain relaxation and assess the maturity of the transfer process. This work could enable the easy recycling of native growth substrates and inspire the development of large-scale hybrid GaN nanowire optoelectronic devices by solely employing standard epitaxial LED wafers (i.e., customized LED wafers with additional embedded sacrificial materials and a complicated growth process are not required).

Catalytic CO oxidation on Pt under near ambient pressure: A NAP-LEEM study.

A near ambient pressure low-energy electron microscope (NAP-LEEM) has recently been constructed, that allows in situ imaging of surfaces up to a pressure of 10-1 mbar. Here we report on pattern formation in catalytic CO oxidation on a Pt(110) single crystal surface and on a polycrystalline Pt foil in the 10-2 mbar range, operating the microscope in the mirror electron microscopy (MEM) and in the LEEM mode. Excitations localized at structural defects and spiral wave fragments have been observed.

Nanocylindrical confinement imparts highest structural order in molecular self-assembly of organophosphonates on aluminum oxide.

We report the impact of geometrical constraint on intramolecular interactions in self-assembled monolayers (SAMs) of alkylphosphonates grown on anodically oxidized aluminum (AAO). Molecular order in these films was determined by sum frequency generation (SFG) spectroscopy, a more sensitive measure of order than infrared absorption spectroscopy. Using SFG we show that films grown on AAO are, within detection limits, nearly perfectly ordered in an all-trans alkyl chain configuration. In marked contrast, films formed on planar, plasma-oxidized aluminum oxide or α-Al2O3 (0001) are replete with gauche defects. We attribute these differences to the nanocylindrical structure of AAO, which enforces molecular confinement.

Insights into Interfacial Changes and Photoelectrochemical Stability of In(x)Ga(1-x)N (0001) Photoanode Surfaces in Liquid Environments.

The long-term stability of InGaN photoanodes in liquid environments is an essential requirement for their use in photoelectrochemistry. In this paper, we investigate the relationships between the compositional changes at the surface of n-type In(x)Ga(1-x)N (x ∼ 0.10) and its photoelectrochemical stability in phosphate buffer solutions with pH 7.4 and 11.3. Surface analyses reveal that InGaN undergoes oxidation under photoelectrochemical operation conditions (i.e., under solar light illumination and constant bias of 0.5 VRHE), forming a thin amorphous oxide layer having a pH-dependent chemical composition. We found that the formed oxide is mainly composed of Ga-O bonds at pH 7.4, whereas at pH 11.3 the In-O bonds are dominant. The photoelectrical properties of InGaN photoanodes are intimately related to the chemical composition of their surface oxides. For instance, after the formation of the oxide layer (mainly Ga-O bonds) at pH 7.4, no photocurrent flow was observed, whereas the oxide layer (mainly In-O bonds) at pH 11.3 contributes to enhance the photocurrent, possibly because of its reported high photocatalytic activity. Once a critical oxide thickness was reached, especially at pH 7.4, no significant changes in the photoelectrical properties were observed for the rest of the test duration. This study provides new insights into the oxidation processes occurring at the InGaN/liquid interface, which can be exploited to improve InGaN stability and enhance photoanode performance for biosensing and water-splitting applications.

A highly selective and self-powered gas sensor via organic surface functionalization of p-Si/n-ZnO diodes.

Selectivity and low power consumption are major challenges in the development of sophisticated gas sensor devices. A sensor system is presented that unifies selective sensor-gas interactions and energy-harvesting properties, using defined organic-inorganic hybrid materials. Simulations of chemical-binding interactions and the consequent electronic surface modulation give more insight into the complex sensing mechanism of selective gas detection.

Band engineered epitaxial 3D GaN-InGaN core-shell rod arrays as an advanced photoanode for visible-light-driven water splitting.

3D single-crystalline, well-aligned GaN-InGaN rod arrays are fabricated by selective area growth (SAG) metal-organic vapor phase epitaxy (MOVPE) for visible-light water splitting. Epitaxial InGaN layer grows successfully on 3D GaN rods to minimize defects within the GaN-InGaN heterojunctions. The indium concentration (In ∼ 0.30 ± 0.04) is rather homogeneous in InGaN shells along the radial and longitudinal directions. The growing strategy allows us to tune the band gap of the InGaN layer in order to match the visible absorption with the solar spectrum as well as to align the semiconductor bands close to the water redox potentials to achieve high efficiency. The relation between structure, surface, and photoelectrochemical property of GaN-InGaN is explored by transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), Auger electron spectroscopy (AES), current-voltage, and open circuit potential (OCP) measurements. The epitaxial GaN-InGaN interface, pseudomorphic InGaN thin films, homogeneous and suitable indium concentration and defined surface orientation are properties demanded for systematic study and efficient photoanodes based on III-nitride heterojunctions.

Superstructures and order-disorder transition of sulfate adlayers on Pt(111) in sulfuric acid solution.

The surface structure of Pt(111) in a 0.1 M H2SO4 electrolyte was investigated in the potential range of sulfate adsorption with electrochemical scanning tunneling microscopy (STM) and cyclic voltammetry. Two ordered anion structures were observed coexisting in the potential range between 0.49 and 0.79 V (vs RHE): the well-known (radical3xradical7)R19.1 degrees superstructure with an anion coverage of 0.20 monolayer and a new, high-density (3x1) superstructure with a coverage of 0.33 monolayer. Both superstructures undergo a reversible order-disorder transition at 0.8 V. Simultaneous imaging of the adsorbed ions and of topographic details of the Pt substrate lattice allows us to study the local adsorption geometry of the sulfate. In the (radical3xradical7)R19.1 degrees, structure the sulfate ions are adsorbed close to depressions in the STM image of the Pt substrate which may be identified with face-centered cubic (fcc) hollow sites. In addition to the sulfate ions, a coadsorbed species, possibly water molecules, is observed in the unit cell of the (radical3xradical7)R19.1 degrees superstructure. Preliminary potentiodynamic STM data indicate that the transformation of the ordered sulfate adlayer into a disordered structure at 0.8 V is not directly related to adsorption/desorption features in the voltammogram commonly attributed to the adsorption/desorption of OH, and that the sulfate adlayer remains on the surface for potentials well above the adsorption potentials of OH.