Semiconductor electrochemistry

Predicting or manipulating charge-transfer at semiconductor interfaces, from molecular electronics to energy conversion, relies on knowledge generated from a kinetic analysis of the electrode process, as provided by cyclic voltammetry. Scientists and engineers encountering non-ideal shapes and positions in voltammograms are inclined to reject these as flaws. Here we show that non-idealities of redox probes confined at silicon electrodes, namely full width at half maximum <90.6 mV and anti-thermodynamic inverted peak positions, can be reproduced and are not flawed data. These are the manifestation of electrostatic interactions between dynamic molecular charges and the semiconductor's space-charge barrier. We highlight the interplay between dynamic charges and semiconductor by developing a model to decouple effects on barrier from changes to activities of surface-bound molecules. These findings have immediate general implications for a correct kinetic analysis of charge-transfe...

Spray deposition followed by sintering of nickel oxide (NiO x) nanoparticles (average diameter: 40 nm) has been chosen as method of deposition of mesoporous NiO x coatings onto indium tin oxide (ITO) glass substrates. This procedure allows the scalable preparation of NiO x samples with large surface area (~ 10 3 times the geometrical area) useful for applications like electrocatalysis or electrochemical solar energy conversion that require high electroactivity in confined systems. The potential of these NiO x films as semiconducting cathodes for dye-sensitized solar cells (DSCs) purposes has been evaluated for 0.3-3 m thick films of NiO x sensitized with erythrosine B (ERY). The electrochemical processes involving the NiO x coatings in the pristine and sensitized states result surface-confined as proved by the linear dependence of the current densities with the scan rate of the cyclic voltammetry. Cathodic polarization of NiO x on ITO can also lead to the irreversible reduction of the underlying ITO substrate due to the mesoporous nature of sintered NiO x film that allows the shunting of ITO to the electrolyte. ITO-based reduction processes alter irreversibly, the properties of charge transfer through the ITO/NiO x interface and limit the range of potential to NiO x coatings sintered for DSCs purposes.

Initial stages of electrochemical oxidation of UHV-prepared n-GaAs(ll0) surfaces, possessing either a long-range order or an extensively damaged surface layer, were compared using an apparatus coupling ultra-high vacuum and electrochemical practices. Not only the sputtered but also the annealed, stoichiometric surface is initially significantly more active than a surface prepared by Br2/Me etching. Anodic treatment results in a rapid loss of surface order and reactivity. A subsequent XPS analysis reveals that, irrespective of the extent of the initial surface order, an overlayer with a great enrichment of As is produced in acidic solution but not in an alkaline environment. A model is derived explaining the influence of the pH on the reactivity.

We present a comprehensive study of epitaxially grown and As-coated GaAs(100) surfaces as a function of As desorption temperature and background pressure. We have used low-energy electron diffraction to determine surface reconstruction, and core-level and valence-band soft-x-ray photoemission spectroscopy to perform chemical and electronic characterization of these surfaces. We find gradual changes in surface geometry and composition, and a limited (-120 meV) Fermi-level movement over numerous reconstructions in the 250-650'C annealing temperature range. The surface ionization potential and work function exhibit large changes between different surface reconstructions. In conjunction with other techniques, work-function measurements present evidence of surface inhomogeneity for many of the desorption temperatures and surface reconstructions. This inhomogeneity appears related to the existence of differently reconstructed patches on the surface. Our results emphasize the complexity of reconstructed GaAs(100) surfaces and the advantages of a multiple-technique approach for their characterization.

Conductive polymer electrodes have exceptional promise for next-generation bioelectronics and energy conversion devices due to inherent mechanical flexibility, printability, biocompatibility, and low cost. Conductive polymers uniquely exhibit hybrid electronic-ionic transport properties that enable novel electrochemical device architectures, an advantage over inorganic counterparts. Yet critical structure-property relationships to control the potential-dependent rates of charge transfer at polymer/electrolyte interfaces remain poorly understood. Herein, we evaluate the kinetics of charge transfer between electrodeposited poly-(3-hexylthiophene) films and a model redox-active molecule, ferrocenedimethanol. We show that the kinetics directly follow the potential-dependent occupancy of electronic states in the polymer. The rate increases then decreases with potential (both normal and inverted kinetic regimes), a phenomenon distinct from inorganic semiconductors. This insight can be inv...

Both AFM imaging and stress measurements were carried out in-situ during potentiostatic electrodeposition of copper on gold in 0.05 mol dm-3 CuSO4 in 0.1 mol dm-3 H2SO4. In the absence of additives, compressive stress generally developed initially and films subsequently underwent a compressive-to-tensile (C-T) transition. The nucleation density measured by AFM increased from 2.7x107 cm-2 at -75 mV to 2.5x109 cm-2 at -300 mV. Very little coalescence of nuclei was observed at -75 mV but the rate of coalescence increased rapidly with increasing negative potential. The time for the C-T transition correspondingly decreased rapidly until, at -75 mV, none was observed. This is consistent with models that attribute the C-T transition to increasing tensile stress due to coalescence of nuclei. With a combination of Cl-, PEG and MPSA, compressive stress generally developed initially and was greater than in additive-free electrolyte. At less negative potentials, the stress continued to evolve i...

Predicting or manipulating charge-transfer at semiconductor interfaces, from molecular electronics to energy conversion, relies on knowledge generated from a kinetic analysis of the electrode process, as provided by cyclic voltammetry. Scientists and engineers encountering non-ideal shapes and positions in voltammograms are inclined to reject these as flaws. Here we show that non-idealities of redox probes confined at silicon electrodes, namely full width at half maximum <90.6 mV and anti-thermodynamic inverted peak positions, can be reproduced and are not flawed data. These are the manifestation of electrostatic interactions between dynamic molecular charges and the semiconductor's space-charge barrier. We highlight the interplay between dynamic charges and semiconductor by developing a model to decouple effects on barrier from changes to activities of surface-bound molecules. These findings have immediate general implications for a correct kinetic analysis of charge-transfe...

Intramolecular charge transfer processes play an important role in many biological, chemical and physical processes including photosynthesis, redox chemical reactions and electron transfer in molecular electronics. These charge transfer processes are frequently influenced by the dynamics of their molecular or atomic environments, and they are accompanied with energy dissipation into this environment. The detailed understanding of such processes is fundamental for their control and possible exploitation in future technological applications. Most of the experimental studies of the intramolecular charge transfer processes so far have been carried out using time-resolved optical spectroscopies on large molecular ensembles. This hampers detailed understanding of the charge transfer on the single molecular level. Here we build upon the recent progress in scanning probe microscopy, and demonstrate the control of mixed valence state. We report observation of single electron transfer between...

Redox properties of As(III) species have been studied in molten KGaCI,. The plot of equilibrium potential of As vs acidity is intcrpretcd in terms of reaction AsCI, +3 e =As+4 CT. X-Ray analysis of electrodeposits shows that only As is formed under potenliostadc conditions. Conversely, gallium arsenide is observed under galvanosraric conditions. for which rhe elccrrode porenrial becomes very negative.

Layered MoS2 is considered as one of the most promising two-dimensional photocatalytic materials for hydrogen evolution and water splitting; however, the electronic structure at the MoS2-liquid interface is so far insufficiently resolved. Measuring and understanding the band offset at the surfaces of MoS2 are crucial for understanding catalytic reactions and to achieve further improvements in performance. Herein, the heterogeneous charge transfer behavior of MoS2 flakes of various layer numbers and sizes is addressed with high spatial resolution in organic solutions using the ferrocene/ferrocenium (Fc/Fc+) redox pair as a probe in near-field scanning electrochemical microscopy, i.e. in close nm probe-sample proximity. Redox mapping reveals an area and layer dependent reactivity for MoS2 with a detailed insight into the local processes as band offset and confinement of the faradaic current obtained. In combination with additional characterization methods, we deduce a band alignment o...

Retrieval of mercury from aqueous streams has significant environmental and societal importance due to its very high toxicity and mobility. We present here a method to retrieve mercury from aqueous feeds via electrochemical alloy formation on thin platinum films. This application is a green and effective alternative to traditional chemical decontamination techniques. Under applied potential, mercury ions in solution form a stable PtHg 4 alloy with platinum on the cathode. A 100 nanometres platinum film was fully converted to a 750 nanometres thick layer of PtHg 4 . The overall removal capacity is very high, > 88 g mercury per cm 3 . The electrodes can easily be regenerated after use. Efficient and selective decontamination is possible in a wide pH range, allowing processing of industrial, municipal, and natural waters. The method is suited for both high and low concentrations of mercury and can reduce mercury levels far below the limits allowed in drinking water.

Predicting or manipulating charge-transfer at semiconductor interfaces, from molecular electronics to energy conversion, relies on knowledge generated from a kinetic analysis of the electrode process, as provided by cyclic voltammetry. Scientists and engineers encountering non-ideal shapes and positions in voltammograms are inclined to reject these as flaws. Here we show that non-idealities of redox probes confined at silicon electrodes, namely full width at half maximum <90.6 mV and anti-thermodynamic inverted peak positions, can be reproduced and are not flawed data. These are the manifestation of electrostatic interactions between dynamic molecular charges and the semiconductor's space-charge barrier. We highlight the interplay between dynamic charges and semiconductor by developing a model to decouple effects on barrier from changes to activities of surface-bound molecules. These findings have immediate general implications for a correct kinetic analysis of charge-transfer at semiconductors as well as aiding the study of electrostatics on chemical reactivity.

Here we describe a " light projector " system that can address, i.e. " read and write " electrochemical reactions on a non-structured macroscopic semiconducting electrode with spatial and temporal resolution. In our approach the illumination of an amorphous silicon electrode/electrolyte interface is spatially defined by means of a ferroelectric micromirror system that gives total freedom on both the two-dimensional light profile (illumination shapes) as well as on the transient times of the projected images. The device has no moving parts and allows for spatial and temporal control of the illumination stimulus driving local changes to the rate of an electrochemical reaction. The performance of the system is assessed by generating microscale patterns of Cu 2 O on the electrode (" electrochemical writing ") followed by their 2D current mapping (" electrochemical reading ") using methanol electro-oxidation and carbon dioxide electro-reduction. The latter illustrate the electrochemical imaging aspects of the device using two technologically relevant examples.

XPS investigations on emersed electrodes reveal in some cases a systematic shift in the binding energy of the upd metal or other double layer constituents with the electrode potential while in other cases such a dependence lacks. In the first case, there is also valid a 1:1 variation of the electrode work function with the emersion potential but the electrode work function keeps always a constant value in the second one. Such observations yield the tentative conclusion that the electrochemical shifts might reflect actually shifts in the reference level.

The paper presents a detailed reaction model for the corrosion of III-V compounds which quantitatively represents the experimental data (mainly Tafel plots of current vs. band displacement), presented in Part I J. Electroanal. Chem. Such data allow the discrimination between different reaction schemes. From a physical point of view, the model shows clearly that the differences observed between GaAs and InP arise from their different electronic surface properties. From a chemical point of view, a chemical signature of the corrosion states of GaAs is proposed: the states E, -0.98 eV and E, -1.15 eV, previously derived from photocapacitance spectroscopy, Ber. Bunsenges. Phys. Chem. 92 (1988) 895, could be associated with As sites and Ga sites, respectively. An analogous description holds for InP, with different positions of states. Within the framework of these original data, corrosion can be described by a first hole capture process on the cation site which becomes an anion site by the liberation of the cation. A chemical reaction step occurs before the second hole capture proceeds to dissolve the anion-related defect. Such a description is predicted by thermodynamical data and is further sustained by experimental facts such as the well-known As enrichment of GaAs surface in acidic media.

The methods of the layer by layer or "digital" etching of crystals, which makes it possible to control lably remove monolayers of a crystal and to maintain the atomic smoothness of the surface, have been actively developed in the last two decades. The cre ation of heterostructures for nanoelectronic investiga tions requires not only the layer by layer growth but also layer by layer etching of semiconductors. The dry (gas phase) etching methods such as reactive ion and ion beam etchings, which are widely used in the tech nology of the production of semiconducting struc tures, do not provide the control of etching at the atomic level. The layer by layer etching of III-V semiconducting compounds can be implemented by using adsorbates selectively reacting with atoms of group III or V.

Synchrotron photoemission spectroscopy is applied to study the interaction of 2-propanol with the clean GaAs(100) surface to understand the solid-solvent interaction at the semiconductor/electrolyte interface. Adsorption of 2-propanol molecules at liquid-nitrogen temperature and emersion of the semiconductor from liquid 2-propanol at room temperature are performed to gain a more detailed insight into surface reaction steps and processes involved. It is found that on adsorption at liquid-nitrogen temperature a 2-propanol molecule dissociates on a GaAs(100) surface to a hydrogen atom, which is adsorbed on an As atom that is part of an As-As dimer, and a 2-propoxy group, which adsorbs to an adjacent Ga atom. On a GaAs(100) surface emersed from liquid 2-propanol at room temperature, two different states of dissociative adsorption are observed. The first state corresponds to dissociation of a 2-propanol molecule into a hydrogen atom and a 2-propoxy group. The second state corresponds to dissociation into an OH group and a 2-propyl radical. On the GaAs(100) surface neither adsorption of 2-propanol molecules nor emersion from liquid 2-propanol causes destruction of the As-As dimer structure.

Perspectives of a new approach for the synchrotron photoemission spectroscopic analysis of chemical processes at solid/liquid interfaces under UHV conditions have been explored. A thin layer of HCl-2-propanol solution was frozen-in on the semiconductor GaAs(100) wafer surface by cooling the substrate to liquid nitrogen temperature after etching off the native oxide layer under N 2 atmosphere. Chemical reactions induced in situ by exposure to synchrotron radiation (SR) and by stepwise heating have been monitored. Right after etching and freezing, the surface is covered by gallium chlorides with 1, 2, 3, and 4 Cl ions attached and lattice back-bonded to As atoms, as well as by elemental arsenic As 0 and 2-propanol. Exposure to SR at low temperature produces surface As chlorides at the expense of As 0 . The GaCl 3 and GaCl 2 emissions diminish while GaCl is enhanced. On the other hand, heating the sample to approximately 130 K just above H 2 O desorption causes the thermodynamically expected reaction of AsCl 3 with the substrate GaAs to form Ga chloride species and As 0 . Heating the sample to room temperature leaves only As 0 on the surface and for gallium the content of all surface chlorides is drastically reduced. By further heating to 400 K elemental arsenic starts to desorb and the Ga chloride surface content is reduced. Using different excitation energies the depth composition of the reaction products has been monitored indicating a tendency of decreasing chlorination numbers and increasing Ga vs As chloride content toward the pristine substrate at each stage of the reaction.

Semiconductor/electrolyte model interfaces are prepared in UHV by coadsorbing the redox active electrolyte species Br2 and the solvent H2O onto chemically inert van der Waals (0001) surfaces of WSe2. Only a small fraction of the adsorbed Br2 is ionosorbed due to charge exchange with the semiconductor bulk; most of it and H2O are molecularly adsorbed. Thus the observed changes of band bending and work function are related to charge transfer processes at the interface to achieve electronic equilibrium. The value of band bending obtained for the model electrolyte is in reasonable agreement to the equivalent Volta potential of the electrolyte contact. For the Br2/H2O coadsorption system it is only determined by Br2 and not affected by coadsorbed H2O. In addition, the relative density of state distribution of the semiconductor and the model electrolyte can directly be determined from the electron distribution curve of the valence band region and compared to theoretical expectations of el...

Synchrotron photoemission spectroscopy is applied to study the interaction of 2-propanol with the clean GaAs(100) surface to understand the solid-solvent interaction at the semiconductor/electrolyte interface. Adsorption of 2-propanol molecules at liquid-nitrogen temperature and emersion of the semiconductor from liquid 2-propanol at room temperature are performed to gain a more detailed insight into surface reaction steps and processes involved. It is found that on adsorption at liquid-nitrogen temperature a 2-propanol molecule dissociates on a GaAs(100) surface to a hydrogen atom, which is adsorbed on an As atom that is part of an As-As dimer, and a 2-propoxy group, which adsorbs to an adjacent Ga atom. On a GaAs(100) surface emersed from liquid 2-propanol at room temperature, two different states of dissociative adsorption are observed. The first state corresponds to dissociation of a 2-propanol molecule into a hydrogen atom and a 2-propoxy group. The second state corresponds to dissociation into an OH group and a 2-propyl radical. On the GaAs(100) surface neither adsorption of 2-propanol molecules nor emersion from liquid 2-propanol causes destruction of the As-As dimer structure.

Perspectives of a new approach for the synchrotron photoemission spectroscopic analysis of chemical processes at solid/liquid interfaces under UHV conditions have been explored. A thin layer of HCl-2-propanol solution was frozen-in on the semiconductor GaAs(100) wafer surface by cooling the substrate to liquid nitrogen temperature after etching off the native oxide layer under N 2 atmosphere. Chemical reactions induced in situ by exposure to synchrotron radiation (SR) and by stepwise heating have been monitored. Right after etching and freezing, the surface is covered by gallium chlorides with 1, 2, 3, and 4 Cl ions attached and lattice back-bonded to As atoms, as well as by elemental arsenic As 0 and 2-propanol. Exposure to SR at low temperature produces surface As chlorides at the expense of As 0 . The GaCl 3 and GaCl 2 emissions diminish while GaCl is enhanced. On the other hand, heating the sample to approximately 130 K just above H 2 O desorption causes the thermodynamically expected reaction of AsCl 3 with the substrate GaAs to form Ga chloride species and As 0 . Heating the sample to room temperature leaves only As 0 on the surface and for gallium the content of all surface chlorides is drastically reduced. By further heating to 400 K elemental arsenic starts to desorb and the Ga chloride surface content is reduced. Using different excitation energies the depth composition of the reaction products has been monitored indicating a tendency of decreasing chlorination numbers and increasing Ga vs As chloride content toward the pristine substrate at each stage of the reaction.

Semiconductor device properties based on electrolyte contacts or modified by electrochemical reactions are dominated by the electronic structure of the interface. Electron spectroscopy as e.g. photoemission is the most appropriate surface science techniques to investigate elementary processes at semiconductor/electrolyte interfaces. For such investigations a specific experimental set-up (SoLiAS) has been built-up which allows performing model experiments as well as surface analysis after emersion under different experimental conditions. The experimental approach is presented by a number of experiments performed during the last years with GaAs as substrate material. Model experiments by adsorption and coadsorption of electrolyte species give information on fundamental aspects of semiconductor/electrolyte interactions. Emersion experiments give information on a final composition and the related electronic structure of electrodes after electrochemical reactions. The use of frozen electrolytes will help to bridge the gap between these two approaches. With the combination of the experimental procedures one may expect a detailed analysis of electrolyte (modified) interfaces covering chemical composition, electronic structure of surfaces/interfaces as well as surface/interface potentials. #

Electrochemically-induced oxidation and reduction reactions of UHV-cleaved GaAs(110) surfaces have been studied after emersion under potential control using high resolution synchrotron-induced photoelectron spectroscopy. High quality spectra of the As and Ga core 3d lines and the valence band region have been obtained. The spectra of the anodic oxide show strong emission of bulklike Ga 2 O 3 and some As 2 O 3 with the admixture of suboxides and hydroxides. Ga 2 O 3 and As 2 O 3 are cathodically reduced leaving the GaAs surface covered mostly with elemental As, some As-H and remnants of Ga-suboxides and -hydroxides.

High-resolution synchrotron photoemission spec-troscopy has been applied to detail the electrochemical and photoelectrochemical corrosion reactions at the liquid junction n‑GaAs(100)/1 M aqueous HCl solution. Under anodic polarization of 1.8 eV, the main process initiated by the presence of holes in the Ga−As bonding states of the valence band is the formation of soluble gallium chloride complexes and insoluble elemental arsenic on the surface. In addition, arsenic hydroxide forms, which reacts further to soluble HAsO 2. In toto, the As/Ga atomic ratio increases, which is accompanied by an increase of the work function. The anodic decomposition reaction is enhanced by illumination as more holes reach the n-semiconductor/electrolyte junction. Under cathodic polarization of 1.5 eV, only minor changes are observed in Ga and As core-level spectra, giving no indication of corrosion, but specific adsorption of hydrated HCl molecules and/or Cl − ions considerably modifies valence band spectra.

Theoretical expressions are presented corresponding to the response of adsorbed molecules which exhibit a quasi-reversible or totally irreversible charge transfer in cyclic reciprocal derivative chronopotentiometry with exponential currents (CRDEC). The higher and better defined peaks obtained in CRDCEC compared with those obtained in cyclic voltammetry (CV) indicate that CRDEC could be considered as a powerful tool in the characterization of molecular films. Although reciprocal derivative chronopotentiometry with constant current (RDC) enjoys at present an extensive use in the elucidation of electrode processes, when this technique is applied to adsorbed molecules exhibiting an irreversible charge transfer, peaks are not observed. Under these conditions, RDCEC turns out to be more suitable than RDC. Furthermore, the use of programmed currents makes the selection of an appropriate range of transition times easier than does the use of constant currents. Equations for the peak currents, peak potentials, ratio between peak heights, and difference between peak potentials of successive responses corresponding to two exponential currents of opposite signs have been applied to the determination of the kinetic parameters of the experimental system azobenzene in protic media.

In recent years, the use of Scanning Electrochemical Microscopy in its Alternating Current mode (AC-SECM) has increased due to its capability for detecting and interpreting local variations of electrochemical properties on surfaces with high spatial resolution. In this paper, we intend to extend the applications of AC-SECM for identifying local impedance variations on surfaces of materials with different redox potential, immersed in resistive media such as tequila in presence of supporting electrolyte. Preliminary results are promising since it is possible to visualize boundaries of materials with different redox potential by means of localized impedance measurements, which opens a new perspective for studying composite materials such as minerals.