Room-temperature ionic liquids (RTIL) are important alternatives to aqueous electrolytes in electrocatalytic reactions, batteries, and fuel cells. They are known to reduce existing high overpotentials and increase CO2 solubility as well as product selectivity in CO2 reduction reactions (CO2RR). In our work, we have studied the activity for CO2RR of Au(111), Cu(111), and Cu-modified Au(111) electrodes with 1/3, 2/3, and 3/3 Cu monolayers, as well as of AuCu and AuCu3 intermetallics in contact with 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][NTf2] electrolytes with 1.5 M H2O. Using offline gas chromatography (GC), we demonstrate the formation of H-2 and mainly CO as the only reduction products at Au(111), while exclusively H-2 is formed at Cu(111). Synergistic electronic and geometric effects lead to higher levels of CO formation at Cu-modified Au(111) electrodes in comparison to neat Au(111). Operando IR absorption spectroscopy (IRAS) of the bulk electrolyte shows the formation of a 2-imidazolium carboxylic acid intermediate that can lower the overpotential for CO2 reduction and does not require stabilization of a CO2- radical anion as an alternative intermediate at the interface. Systematic variation of the copper content at the catalysts' surfaces enables us to adjust the H-2/CO syngas ratio to a maximum of 1.8 for Cu-modified Au(111) electrodes and similar to 3.2 for AuCu3 catalysts at electrolysis times of 20 min, demonstrating a large tunability of the syngas ratio with electrode potential. The observed range of H-2/CO ratios includes the ideal ratio of 2 for the Fischer-Tropsch process to produce hydrocarbons and the ratio of 3 needed for methanation.

We study the origin and formation of antiphase domains (APDs) and related defects in 7 nm thin, lattice-matched GaP buffer layers deposited by metal-organic chemical vapor deposition (MOCVD) on well-defined, nearly single-domain, double layer stepped, low-miscut Si(100) substrates obtained by specific treatment with arsenic. Using dark-field imaging modes in low energy electron microscopy (LEEM), the minority reconstruction domains of Si(100):As and the APDs of the deposited GaP epilayer are identified, quantified, and compared. We show that residual (2 x 1)-reconstructed terraces of the minority domain on the Si substrate cause the formation of APDs and that the fraction of the minority domain of the substrate ( approximately equal to 0.07) entails a comparable fraction of APDs in thin GaP epilayers. The topographies of APDs are revealed by atomic force microscopy (AFM) and by scanning tunneling microscopy (STM). We observe two very different APD-related defects in the GaP epilayer, both pinned to residual monolayer steps of the substrate. GaP growth on minority domain terraces with widths in the range of 40-100 nm gives rise to APDs of comparable lateral dimensions. Minority domain terraces of the substrate with widths <20 nm cause the formation of 7- 20 nm wide trenches in the GaP layer with rampart-like mounds along their rims. Using nanoscale Auger electron spectroscopy (AES), we provide evidence that these trenches extend through the GaP layer down to the exposed, uncovered Si substrate. We conclude that nucleation of GaP on small minority domain terraces is largely inhibited as most Ga and P atoms deposited on these terraces diffuse across the domain boundary and side walls of emerging trenches to adjacent majority domain terraces where they form the observed mounds. Nucleation of GaP does take place on minority domain terraces with widths >= 40 nm and leads to the growth of APDs.

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 (~10 7 wires/cm 2 ), 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).

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.

The presence of a surrounding medium strongly affects the spectral properties of localized surface plasmons at metallic nanoparticles. Vice versa, plasmonic resonances have large impact on the electric polarization in a surrounding or supporting material. For applications, e.g., in light-converting devices, the coupling of localized surface plasmons with polarizations in semiconducting substrates is of particular importance. Using photoemission electron microscopy with tunable laser excitation, we perform single-particle spectroscopy of silver nanoclusters directly grown on Si(100). Two distinct localized surface plasmon modes are observed as resonances in the two-photon photoemission signals from individual silver clusters. The strengths of these resonances strongly depend on the polarization of the exciting electric field, which allows us to assign them to plasmon modes with polarizations parallel and perpendicular, respectively, to the supporting silicon substrate. Our mode assignment is supported by simulations which provide insight into the mutual interaction of charge oscillations at the particle surface with electric polarizations at the silver/silicon interface.

UV-light emitting diodes (395–278 nm) were used to investigate the gas sensing attributes of planar and nanostructured ZnO/AlN thin films on Si substrate towards NO2 at room temperature. A significant increased sensitivity ((Rg − Ra)/Ra = 65.3 ppm NO2 in air) and a strong reduction in recovery time (Trec = 14 min) were already observed for the planar ZnO/AlN thin films under UV-B (305 nm) irradiation compared to the other UV wavelengths, while the device showed no obvious response in dark. By enlarging the surface-to-volume ratio of the sensors (i.e., creating nanostructured ZnO/AlN thin films), an increased response time is expected to be observed.

A detailed understanding of charge transfer mechanisms at the electrode/electrolyte interface is important for the development of electrodes for efficient photoelectrochemical (PEC) processes. We have studied the charge transfer processes across the n-GaP(100) photoanode/electrolyte interface. In a 0.02 M HCl electrolyte, high photoanodic currents from the n-GaP(100) photoanodes related to photolytic water splitting were measured at low anodic potentials but these photocurrents diminished at cathodic potentials and high anodic potentials. Electrochemical impedance spectroscopy (EIS) was carried out for n-GaP(100) photoanodes at different potentials to analyze the relevant charge transfer processes. Our EIS results suggest that the adsorption of hydroxide on metal-like surface Ga and their subsequent oxidative transformations – formation of these surface species is most favorable at low anodic potentials – is the driving force for high photoanodic currents at low anodic potentials. III-V semiconductors are prone to corrosion during PEC water splitting processes with corrosion-related decrease of efficiency. Our n-GaP(100) photoanodes were surface-conditioned via oxidizing at 0.8 V vs RHE and subsequently hydrogenated to passivate the defects in the oxide film. After this preparation, water splitting was observed at potentials between 0 and 0.3 V. The Nyquist plots derived from our EIS measurements for the n-GaP(100) photoanode consist of three semicircles indicating three distinct charge transfer mechanisms. The plots were analyzed with two different equivalent electrical circuits for the electrode/electrolyte system. These equivalent electrical circuits allowed to evaluate the relevance of different possible charge transfer pathways from the n-GaP(100) photoanode to the electrolyte. For each equivalent electrical circuit, the potential dependence of the resistances and capacitances including constant phase elements were determined from the fit and then compared with the potential variation of the current in the cyclic voltammogram. In a theoretical study Bertoluzzi et al. discussed the PEC implications of charge transfer from an n-doped semiconductor to the electrolyte that takes place via two different pathways1: by a direct charge transfer from the valence band of the semiconductor to the electrolyte, and by an indirect charge transfer from the valence band to the electrolyte via surface states (or defect states at the surface). The latter gives rise to a pronounced maximum of the interface- or defect-related capacitance as a function of potential. Our EIS results suggest that at low anodic potentials, the effective charge transfer process occurs from the valence band of the n-GaP(100) photoanode to defect states present in a thin Ga2O3-like layer at the surface, and subsequently, from these defect states to the electrolyte. This indirect charge transfer pathway is derived from a pronounced maximum of the capacitance as a function of applied potential. Our experimental results also suggest that subsequent charge transfer from these defect states leads to the formation of a molecular hydroxide layer the surface of metal-like Ga regions at the n-GaP(100) photoanode. Metal-like Ga at the surface of the photoanode was identified by XPS. A tafel slop of 63 mV/decade measured at the onset potential of the photoanodic current strongly points to the presence of adsorbed metal hydroxide (GaOH) species and subsequent oxidation products GaO and GaOOH as surface intermediates for the oxygen evolution which are finally catalyzed to molecular oxygen2. Our electrochemical impedance spectroscopy results also suggest that at low anodic potentials, overpotentials for the formation of hydroxide absorbates and their subsequent oxidation are lowest whereas at higher anodic potentials these hydroxide absorbates are stable and do not oxidize further to produce oxygen. At cathodic potentials, on the other hand, these surface hydroxides are reduced. As a consequence, large PEC currents associated with oxygen formation are possible only at low anodic potentials. By analyzing and understanding the charge transfer processes and the effect of different electrochemical parameters on these processes, we were able to optimize the PEC conditions which finally led to photolytic water splitting without applied voltage or addition of catalysts. Bertoluzzi, L; Lopez-Varo, P; Tejada, J. A. J; Bisquert, J., J. Mater. Chem. A., 2016, 4, 2873. Antoine, O; Butel, Y; Durand, R., J. Electroanal. Chem. 2001, 499, 85.

Transferable substrate-less InGaN/GaN light-emitting diode (LED) chips have successfully been fabricated in a laser lift-off (LLO) process employing high power ultrashort laser pulses with a wavelength of 520 nm. The irradiation of the sample was conducted in two sequential steps involving high and low pulse energies from the backside of the sapphire substrate, which led to self-detachment of the GaN stack layer without any additional tape release procedure. To guarantee their optoelectrical function and surface quality, the lifted LED chips were assessed in scanning electron microscopy (SEM) and electroluminescence (EL) measurements. Moreover, surface characterizations were done using atomic force microscopy (AFM) and Auger Electron Spectroscopy (AES).

The production of hydrogen via photoelectrochemical (PEC) water splitting using III-V semiconductors as photoelectrodes is a field of current research and challenges in materials science. Under PEC conditions relevant to water splitting, III-V semiconductors are prone to corrosion and suffer from corrosion-related decrease of efficiency, which so far impedes long-term usage of III-V semiconductor-based photoelectrodes. Gallium phosphide (GaP) has an indirect band gap of 2.26 eV which covers both the hydrogen evolution potential (HEP) and the oxygen evolution potential (OEP). Thus, in principal, GaP can be used both as photocathode and photoanode. Notwithstanding the favorable band gap energy, the use of GaP photoelectrodes for the photolysis of water has so far not been successful without applying an additional bias potential. In this contribution we demonstrate that, by specific surface conditioning, we are able to control and optimize the photoelectrochemical surface properties of photoanodes prepared from n-doped GaP(100) for PEC water splitting. We have studied the structural and chemical surface modifications of such photoanodes before and after extended PEC processes by scanning electron microscopy (SEM), atomic force microscopy (AFM), Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). An approximately (2±1) nm thin oxide film is produced at the surface of n-GaP(100) via oxidizing the surface at 0.8 V vs RHE (reversible hydrogen electrode) and subsequent hydrogen evolution at very cathodic potentials. The latter step is crucial for a substantial reduction of the potential for water splitting, presumably because the evolved hydrogen passivates electrically active defects in the oxide film. After appropriate surface conditioning, photolytic water splitting was observed without applied voltage or addition of catalysts, as confirmed by detection of hydrogen gas evolving at the Pt counter cathode with a gas spectrometer. The n-GaP(100) photoanode was exposed to high-intensity illumination (120 mW/cm2) at 0 V vs RHE in 0.02 M HCl electrolyte for a duration of 3 days while yielding stable photocurrents. Ex-situ surface characterization of the n-GaP(100) photoanodes was carried out after extended PEC water splitting. AFM revealed a very flat surface, and no signs of corrosion were observed even after extended periods of water splitting. AES depth profiles showed that phosphorous was depleted near the surface of the n-GaP(100) photoanode. Using AES spectra, the thickness of the oxide film was estimated. XPS measurements revealed that the surface oxide consists mainly of Ga2O3, while small concentrations of GaPO4, P2O5 and also metal-like Ga are also present at the topmost surface layers of the n-GaP(100) photoanode. Electrochemical impedance spectroscopy (EIS) was carried out to understand the charge transfer processes across the semiconductor/electrolyte interface. Our EIS results suggest that defect states in the surface oxide and metal-like Ga at the very surface in contact with the electrolyte play crucial roles in the effective charge transfer across the semiconductor-electrolyte interface. Our surface conditioning processes lead to the formation of a nonporous, thin and stable Ga surface oxide which inhibits PEC surface corrosion and, at the same time, subserves the water splitting process.

Titania nanoparticle-based thin films are highly attractive for a vast range of commercial applications. Although their application on polymer-based substrates is particularly appealing, the requirement of low process temperatures results in low mechanical stability. Highly crystalline anatase nanoparticles were used as the building blocks for coatings through a two-stage process. The main benefits of this method, over the more common sol-gel ones, are the relatively low temperature required for the production of metal oxide coatings, allowing the use of polymer-based substrates, and the defined crystallinity of the resulting thin films. Although in several cases moderate temperatures can be utilized for drying the films, the mechanical stability of the respective coatings remains a critical issue. In this contribution, we present a strategy to achieve network formation between TiO2 nanoparticles in a preformed thin film on the basis of the cross-linking of the functionalized nanoparticles. In the first stage, the nanoparticles were functionalized by dicarboxylic acids, concurrently leading to a stable colloidal dispersion that could be utilized for dip-coating to obtain TiO2 thin films with high homogeneity and optical transparence. During the second stage, the films were immersed in a solution of a diamine as the linker molecule, to achieve cross-linking between the nanoparticles within the film. It is demonstrated that indeed covalent bonding was realized and functional coatings with significantly enhanced mechanical properties were obtained by our strategy.

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.

The feasibility to couple surface plastrons with molecular excitations is an intriguing feature of plasmonic-organic hybrid systems. To date, investigations of plasmonic-excitonic coupling have largely focused on ensembles of nanoparticles and on purely optical methods. Here we present a single-particle approach based on laser-spectroscopic photoemission electron microscopy. Localized surface plasmons give rise to photoemission resonances in laser excitation spectra of individual silver nanoclusters. As a most striking manifestation of plasmon excitor coupling, upon deposition of a thin zinc tetraphenylporphyrin (ZnTPP) film, a second resonance near the ZnTPP Soret band appears in our spectra. In accordance with simulations, spectral repulsion of both resonances as well as intensity reditribution between both modes indicates their plasmonic-excitonic hybrid character.

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 InxGa1-xN (x similar to 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 V-RHE), 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.

Understanding the mechanisms of charge transfer across the semiconductor/liquid interface is crucial to realize efficient photoelectrochemical devices. Here, the interfacial charge transfer characteristics of n-type In0.1Ga0.9N photoanodes are investigated and correlated to their photo-activity properties measured in phosphate buffered saline solution (pH 7) under illumination conditions. Cyclic voltammetry measurements show evident photoactivity changes as the number of cycles increases. In particular, the photocurrent density reaches its maximum value after 49 voltammetric cycles; meanwhile, the photocurrent onset potential shifts toward more negative cathodic potentials. Electrochemical impedance measurements reveal that, first, the hole transfer process occurs mainly via localized states at the surface and the photocurrent onset potential is dependent on the energetic position of those states. Therefore, the observed initial photocurrent increase and cathodic shift of the photocurrent onset potential can be attributed to a decrease of the transfer resistance and partial passivation of the states at the surface. On the other hand, a gradual oxidation and corrosion of the InGaN surface arises, causing a consequential decrease of the photocurrent. At this point, the charge transfer process occurs predominantly from the valence band. This work provides a basic understanding of the charge transfer mechanisms across the InGaN/liquid interface which can be used to improve the overall photoanode efficiency.

The three‐dimensional growth of GaN structures as a basis for the fabrication of 3D GaN core–shell LEDs has attracted substantial attention in the past few years. GaN nanorods or microrods with high aspect ratios can be grown by selective area epitaxy on a GaN buffer through a SiOx mask. It has been found earlier that silane substantially initiates vertical growth, with the exact underlying mechanisms being still unclear. Here, the influence of silane on the 3D GaN column growth was investigated by performing detailed growth experiments in combination with a thorough surface analysis in order to get insight into these mechanisms. The vertical growth rate is significantly enhanced by high silane fluxes, whereas the saturation of growth rate with the time is reduced. Thus, homogenous GaN columns with an aspect ratio of more than 35 could be achieved. A thin Si‐rich layer on the non‐polar m‐plane facets of the columns has been detected using a combination of transmission electron microscopy, energy dispersive X‐ray spectroscopy and Auger electron spectroscopy. This layer is suggested to be the reason for the increase in growth rate, modifying the effective collection range of the species along the sidewalls, and preventing the lateral growth.

The prevailing design approaches of semiconductor gas sensors struggle to overcome most of their current limitations such as poor selectivity, and high power consumption. Herein, a new sensing concept based on devices that are capable of detecting gases without the need of any external power sources required to activate interaction of gases with sensor or to generate the sensor read out signal. Based on the integration of complementary functionalities (namely; powering and sensing) in a singular nanostructure, self-sustained gas sensors will be demonstrated. Moreover, a rational methodology to design organic surface functionalization that provide high selectivity towards single gas species will also be discussed. Specifically, theoretical results, confirmed experimentally, indicate that precisely tuning of the sterical and electronic structure of sensor material/organic interfaces can lead to unprecedented selectivity values, comparable to those typical of bioselective processes. Finally, an integrated gas sensor that combine both the self-powering and selective detection strategies in one single device will also be presented.

For GaP-on-Si(100) heteroepitaxy, currently considered as a model system for monolithic integration of III–V semiconductors on Si(100), the surface steps of Si(100) have a major impact on the quality of the GaP film. Monoatomic steps cause antiphase domains in GaP with detrimental electronic properties. A viable route is to grow the III–V epilayer on single-domain Si(100) with biatomic steps, but preferably not at the expense of reduced terrace widths introduced by miscut substrates. We have performed in situ investigations of the influence of Ga deposition on the kinetics of surface steps and terraces of Si(100) at substrate temperatures above 600 °C by low-energy electron microscopy. Starting from nearly equally distributed TA and TB terraces of a two-domain Si(100) surface, submonolayer deposition of Ga results in a transformation into a surface dominated by TA terraces and biatomic DA steps. This transformation is reversible, and Si(100) with monoatomic steps is recovered upon termination of the Ga flux. Under conditions of higher coverages (but still below 0.25 monolayer), we observe restructuring into a surface with TB dominance, similar to the findings of Hara et al. [J. Appl. Phys. 98, 083515 (2005)]. The occurrence and mutual transformations of surface structures with different terrace and step structures in a narrow range of temperatures and Ga deposition rates is discussed.

Dye molecules like porphyrins alter their optical properties upon condensation to a solid film. Besides intermolecular interactions, specific substrates can influence their optical characteristics, especially if the molecular film is in contact to plasmonic nanostructures. We apply multiphoton photoemission electron microscopy (nP-PEEM) with tunable laser excitation to the laterally resolved spectroscopy of magnesium-tetraphenylpoiphyrin (MgTPP) films deposited on nanostructured silver substrates. The high molecular specificity of nP-PEEM is demonstrated by the observation of a strong resonance at about 425 nm caused by optical excitation of the S-0 -> S-2 transition (Soret band) of the MgTPP molecules. This molecular excitation gives rise to remarkably strong three-photon photoemission. The spectral position of the Soret resonance in our nP-PEEM laser spectra points to reduced excitonic coupling between the surface molecules of the MgTPP film. By comparison of the photoemission intensities from MgTPP on nanostructured and unstructured silver regions, we conclude noticeable plasmon-mediated enhancement of photoemission at resonant Soret excitation even under off-resonance conditions for the localized surface plasmons (LSPs). Combining molecular specificity with high sensitivity to field enhancements, we demonstrate that multiphoton PEEM is an excellent tool for the investigation of solid dye films and their interaction with plasmonic substrates.

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.

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.