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Atomic layer deposited (ALD) TiO2 protection layers may allow for the development of both highly efficient and stable photoanodes for solar fuel synthesis; however, the very different conductivities and photovoltages reported for TiO2-protected silicon anodes prepared using similar ALD conditions indicate that mechanisms that set these key properties are, as yet, poorly understood. In this report, we study hydrogen-containing annealing treatments and find that postcatalyst-deposition anneals at intermediate temperatures reproducibly yield decreased oxide/silicon interface trap densities and high photovoltage. A previously reported insulator thickness-dependent photovoltage loss in metal-insulator-semiconductor Schottky junction photoanodes is suppressed. This occurs simultaneously with TiO2 crystallization and an increase in its dielectric constant. At small insulator thickness, a record for a Schottky junction photoanode of 623 mV photovoltage is achieved, yielding a photocurrent turn-on at 0.92 V vs NHE or -0.303 V with respect to the thermodynamic potential for water oxidation.

State-of-the-art silicon water splitting photoelectrochemical cells employ oxide protection layers that exhibit electrical conductance in between that of dielectric insulators and electronic conductors, optimizing both built-in field and conductivity. The SiO2-like layer interposed between a deposited protective oxide film and its Si substrate plays a key role as a tunnel oxide that can dominate the total device impedance. In this report, we investigate the effects of changes in interfacial SiO2 resistance and capacitance in the oxide bilayer through both solid state leakage current and capacitance–voltage measurements and through electrochemical methods applied to water splitting cells. Modelling is performed to describe both types of data in a simple and intuitive way, allowing for insights to be developed into the connections among both the dielectric (charge storage) and conductive (charge transport) properties of bilayer protective oxides and their effects on oxygen evolution performance. Finally, atomic layer deposited (ALD) Al2O3 is studied as an insulator layer with conductivity intermediate between that of tunnel oxide SiO2 and the more conductive ALD-TiO2, to further generalize this understanding.

Metal oxide protection layers for photoanodes may enable the development of large-scale solar fuel and solar chemical synthesis, but the poor photovoltages often reported so far will severely limit their performance. Here we report a novel observation of photovoltage loss associated with a charge extraction barrier imposed by the protection layer, and, by eliminating it, achieve photovoltages as high as 630mV, the maximum reported so far for water-splitting silicon photoanodes. The loss mechanism is systematically probed in metal-insulator-semiconductor Schottky junction cells compared to buried junction p(+) n cells, revealing the need to maintain a characteristic hole density at the semiconductor/insulator interface. A leaky-capacitor model related to the dielectric properties of the protective oxide explains this loss, achieving excellent agreement with the data. From these findings, we formulate design principles for simultaneous optimization of built-in field, interface quality, and hole extraction to maximize the photovoltage of oxide-protected water-splitting anodes.

Atomic layer deposition of ultrathin TiO2 films for metal-insulator-silicon photoelectrochemical applications is accomplished in a custom built ALD chamber using the precursor tetrakis(dimethylamido)titanium (TDMAT) and water vapor co-reactant. This paper discusses the procedures used to achieve high quality, repeatable depositions for this particular application. Characterization of the ALD-TiO2 films can be performed by metal-insulator-semiconductor (MIS) structure capacitance-voltage methods. Electrochemical characterization methods are also used to determine the approximate film resistivity, photovoltage, differential resistances, and stability of an MIS device performing water splitting. This set of electrochemical measurements allows for the rapid characterization of ultrathin ALD films for MIS photoelectrochemical applications.

We synthesized nanoscale TiO2-RuO2 alloys by atomic layer deposition (ALD) that possess a high work function and are highly conductive. As such, they function as good Schottky contacts to extract photogenerated holes from n-type silicon while simultaneously interfacing with water oxidation catalysts. The ratio of TiO2 to RuO2 can be precisely controlled by the number of ALD cycles for each precursor. Increasing the composition above 16% Ru sets the electronic conductivity and the metal work function. No significant Ohmic loss for hole transport is measured as film thickness increases from 3 to 45 nm for alloy compositions >= 16% Ru. Silicon photoanodes with a 2 nm SiO2 layer that are coated by these alloy Schottky contacts having compositions in the range of 13-46% Ru exhibit average photovoltages of 525 mV, with a maximum photovoltage of 570 mV achieved. Depositing TiO2-RuO2 alloys on nSi sets a high effective work function for the Schottky junction with the semiconductor substrate, thus generating a large photovoltage that is isolated from the properties of an overlying oxygen evolution catalyst or protection layer.

A fundamental challenge in developing photoelectrochemical cells for the renewable production of solar chemicals and fuels is the simultaneous requirement of efficient light absorption and robust stability under corrosive conditions. Schemes for corrosion protection of semiconductor photoelectrodes such as silicon using deposited layers were proposed and attempted for several decades, but increased operational lifetimes were either insufficient or the resulting penalties for device efficiency were prohibitive. In recent years, advances in atomic layer deposition (ALD) of thin coatings have made novel materials engineering possible, leading to substantial and simultaneous improvements in stability and efficiency of photoelectrochemical cells. The self-limiting, layer-by-layer growth of ALD makes thin films with low pinhole densities possible and may also provide a path to defect control that can generalize this protection technology to a large set of materials necessary to fully realize photoelectrochemical cell technology for artificial photosynthesis.

Silicon photoanodes protected by atomic layer deposited (ALD) TiO2 show promise as components of water splitting devices that may enable the large-scale production of solar fuels and chemicals. Minimizing the resistance of the oxide corrosion protection layer is essential for fabricating efficient devices with good fill factor. Recent literature reports have shown that the interfacial SiO2 layer, interposed between the protective ALD-TiO2 and the Si anode, acts as a tunnel oxide that limits hole conduction from the photoabsorbing substrate to the surface oxygen evolution catalyst. Herein, we report a significant reduction of bilayer resistance, achieved by forming stable, ultrathin (

Molecularly homogeneous, polynuclear phenolic compounds were esterified with acrylic acid and methacrylic acid chlorides. The pcresyl esters of acrylic and methacrylic acid react in high dilution in boiling benzene with a large excess of radicals derived from AIBN in such manner that the growth step of the free-radical polymerization is prevented; this may be denoted a “suppressed” polymerization. The application of these conditions to the previously mentioned poly-nuclear esters results in the formation of molecularly homogeneous cyclooligomers (ladder oligomers). Alkaline hydrolysis of these oligomers permits recovery of the polynuclear phenols and yields molecularly homogeneous oligoacrylic acid and oligomethacrylic acid, the structures of which were proved by the preparation of a variety of ester derivatives. The presented reaction path seems to be a very simple model of the biosynthesis of proteins. Finally, evidence is presented for the consideration of this reaction sequence as a synthetic matrix reaction.

As the size of electronic devices scales down, series resistance (RS) and gate leakage effects are commonly observed in electrical measurement of metal-oxide-semiconductor gate stacks. As a result of their effects on device characteristics, these phenomena complicate the analysis of border trap density (Nbt) in the gate insulator using capacitance-voltage (C-V) and conductance-voltage (G-V) measurements. In this work, we develop methods to correct for the effects of RS and gate leakage in Al2O3/InGaAs gate stacks to enable reliable fitting of C-V and G-V data to determine Nbt. When tested using data from Pd/Al2O3/InGaAs gate stacks, the RS correction method successfully removes the RS-induced high frequency dispersion in the accumulation region of the C-V curves and provides an accurate extraction of RS and Nbt. The gate leakage correction method is tested on gate stacks with high gate leakage current of ∼25 μA at 2 V bias, and is found to effectively fit capacitance and conductance data, to achieve consi...

Metal-insulator-semiconductor (MIS) structures are promising candidates for integrated solar driven water splitting devices. Often, the metal serves a dual purpose, catalyzing water oxidation while also setting the built-in field for extracting photogenerated carriers from the semiconductor. Recently, incorporation of additional protection layers into the MIS junction has made it possible to use semiconductor materials, such as silicon, that would normally be unstable under the conditions required for water oxidation.1 Although the theoretical maximum photovoltage for silicon photovoltaics is 700-800 mV,2 there is a tradeoff between photocurrent and photovoltage in adventitiously-doped TiO2.3 For example, maximum photovoltages of ~400 mV have been reported for nSi / TiO2 / metal photoanodes protected by highly conductive TiO2 layers,4 with some devices achieving only 200-250 mV.5 Our goal is to develop a more ideal protection layer using transition metal oxide-titania alloys synthesized by ALD. An ideal protection layer should be corrosion resistant, highly conductive, and enable a high photovoltage. For n-type silicon Schottky photoanodes, this means that the protection layer must also possess a high work function. We previously demonstrated that TiO2-RuO2 alloys composed of 13-46% Ru were highly conductive and consistently achieved photovoltages >525 mV without any post-processing anneals.6 The built-in field was set by the TiO2-RuO2 alloy, which possessed a sufficiently high work function of 5.02 eV. Although TiO2-RuO2 alloys were ideal Schottky contacts to nSi, they lacked the long-term corrosion resistance required of a protection layer. RuO2 is known to be unstable at the conditions required for water oxidation. We also report on ALD alloys of TiO2 and IrO2, which exhibit high conductivity and photovoltage > 600 mV on nSi, but with improved stability. Additionally, TiO2-IrO2 alloys are capable of catalyzing oxygen evolution. Thus, TiO2-IrO2alloys have the potential to be an “all-in-one” catalyst, Schottky contact, and corrosion resistant protection layer. References: 1. Chen, Y.C. et al. Atomic Layer-Deposited Tunnel Oxide Stabilizes Silicon Photoanodes for Water Oxidation. Nature Mater. 10, 539-44 (2011). 2. Green, M. A. Limits on the Open-circuit Voltage and Efficiency of Silicon Solar Cells Imposed by Intrinsic Auger Processes. IEEE Trans. Electron Devices ED-31,671–678 (1984). 3. Scheuermann, A.G. et al. Photovoltage Design for Metal Oxide Protected Solar-Water-Splitting Photoanodes. Nature Mater. 15, 99-105 (2016). 4. Hu, S. et al. Amorphous TiO2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation. Science (80-. ). 344,1005–1009 (2014). 5. McDowell, M. T. et al. The Influence of Structure and Processing on the Behavior of TiO 2 Protective Layers for Stabilization of n-Si/TiO 2 /Ni Photoanodes for Water Oxidation. ACS Appl. Mater. Interfaces 7,15189–15199 (2015). 6. Hendricks, O. L. et al. Isolating the Photovoltaic Junction : Atomic Layer Deposited TiO2 − RuO2 Alloy Schottky Contacts for Silicon Photoanodes. (2016). doi:10.1021/acsami.6b08558

The conductive atomic force microscope (CAFM) has become an essential tool for the nanoscale electronic characterization of many materials and devices. When studying photoactive samples, the laser used by the CAFM to detect the deflection of the cantilever can generate photocurrents that perturb the current signals collected, leading to unreliable characterization. In metal-coated semiconductor samples, this problem is further aggravated, and large currents above the nanometer range can be observed even without the application of any bias. Here we present the first characterization of the photocurrents introduced by the laser of the CAFM, and we quantify the amount of light arriving to the surface of the sample. The mechanisms for current collection when placing the CAFM tip on metal-coated photoactive samples are also analyzed in-depth. Finally, we successfully avoided the laser-induced perturbations using a two pass technique: the first scan collects the topography (laser ON) and the second collects the current (laser OFF). We also demonstrate that CAFMs without a laser (using a tuning fork for detecting the deflection of the tip) do not have this problem.

International audience; Scanning tunneling microscopy and dynamic force microscopy in the noncontact mode are used in combination to investigate the reversible switching between two stable states of a copper complex adsorbed on a NaCl bilayer grown on Cu(111). The molecular conformation in these two states is deduced from scanning tunneling microscopy imaging, while their charge is characterized by the direct measurement of the tip-molecule electrostatic force. These measurements demonstrate that the molecular bistability is achieved through a charge-induced rearrangement of the coordination sphere of the metal complex, qualifying this system as a new electromechanical single-molecular switch.

The University of Florida (UF) have recently collaborated with Raith Inc. to modify Raith’s ion beam lithography, nanofabrication and engineering (ionLiNE) station that utilizes only Ga ions, into a multi-ion beam system (MionLiNE) by adding the capabilities to use liquid metal alloy sources (LMAIS) to access a variety of ions and an EXB filter for mass separation. The MionLiNE modifications discussed below provide a wide range of spatial and temporal precision that can be used to investigate ion solid interactions under extended boundary conditions, as well as for ion lithography and nanofabrication. Here we demonstrate the ion beam lithographic capabilities of the MionLiNE for fabricating patterned arrays of Au and Si nanocrystals, with nanoscale dimensions, in SiO2 substrates, by direct implantation; and show that the same directwrite/maskless-implantation features can be used for in situ fabrication of nanoelectronic devices. Additionally, the spatial and temporal capabilities of the MionLiNE are used to explore the effects of dose rate on the long-standing surface morphological transformation that occurs in ion bombarded Ge.

Samarium oxide (Sm2O3) and samarium gallium oxide (SmxGa1-x)2O3 have been proposed as candidate dielectric materials for the development of gallium arsenide (GaAs) Metal Oxide Semiconductor Field Effect Transistor (MOSFET) technology. Growth of thin (20nm-50nm) Sm2O3 and (SmxGa1-x)2O3 layers on GaAs substrates via plasma-assisted molecular beam epitaxy (MBE) has been performed using a range of growth temperatures and samarium cell temperatures. X-ray photoelectron spectroscopy (XPS) of the deposited films showed evidence of unbonded Sm metal in the films which decreased with decreasing Sm cell temperature, but was relatively independent of substrate temperature. Stoichiometry of the oxide was found to be independent of substrate temperature, but increased in oxygen to metal ratio as the Sm cell temperature was decreased. Decreasing the Sm cell temperature also suppressed the formation of the monoclinic phase and promoted the growth of the cubic phase. Films grown at higher (500şC) temperature showed the presence of a crystalline interface, but relatively high surface roughness and the presence of multiple crystalline phases. Current-voltage analysis of one hundred micron diameter MOS diodes showed breakdown fields at 1 mA/cm2 of up to 4.35 MV/cm. Breakdown field was found to decrease with increasing Sm unbonded metal content in the films. The effect of stoichiometry and phase distribution on the interface state density (Dit) and capacitance-voltage behavior of MOS diodes was also investigated.

Metal oxide protection layers for photoanodes may enable the development of large-scale solar fuel and solar chemical synthesis. ALD-TiO2 remains the most widely used material because of its excellent stability under water oxidation conditions and potential for high electrical conductivity both as an ultrathin film and with thicknesses exceeding 100 nm [1-3]. However, the most conductive ALD-TiO2films exhibit poor photovoltages of ~ 400 mV and less [3]. Even assuming near-ideal fill factors, these voltages fall far short of the values needed for viable water splitting devices. Photovoltage optimization is especially difficult in MOS photoanodes due to the range of factors which influence the conductivity and stability of the protection layer as well as the presence of electrically active defect states at the semiconductor/oxide interface. Therefore, understanding how to optimize photovoltage and stability is of utmost concern for the advancement of the field. Here we report a novel observation of photovoltage loss associated with charge transfer in these metal-oxide protected devices, and by eliminating it, achieve photovoltages as high as 630 mV, the maximum reported to date for single-junction water-splitting silicon cells. The loss mechanism is systematically probed in MOS Schottky junction cells compared to buried junction p+n cells, revealing the need to maintain a characteristic hole density at the semiconductor/insulator interface. A capacitor model that predicts this loss is developed, and is related to the dielectric properties of the protective oxide, achieving excellent agreement with the data. From these findings, we extract design principles for simultaneous optimization of charge transfer resistance and interface quality to maximize the photovoltage of metal-oxide protected MOS water splitting devices. [1] Y.W. Chen, et al. Nature Mat. 2011, 10, 539-544. [2] A. G. Scheuermann, et al. Energy Environ. Sci. 2013, 6, 2487–2496. [3] S. Hu, et al. Science 2014, 344, 1005−1009. Figure 1 | Charge transfer in three cell types for water splitting applied to silicon: The Type 0 Semiconductor-Liquid (SL), Type 1 Metal-Insulator-Semicondcutor (MIS), and Type 2 p+n junction. The density of states on either side of the oxides and the excition splitting position with respect to these layers play a crucial role in mediating efficient charge transfer. These effects are so strong that Type 0 protected silicon cells exhibit essentially no photovoltage, Type 1 nSi cells show a linear photovoltage loss with oxide thickness, and Type 2 cells--where the hole concentration on the Si/SiO2 interface is always high--exhibit record photovoltages at all oxide thicknesses and pH values studied. Figure 1

Photoelectrochemical cells are becoming increasingly attractive for the on-site production of clean chemicals and fuels, both as end-use products and as an integrated energy storage solution for mitigating the intermittency of solar radiation. One persistent challenge in this field is the difficulty of simultaneously achieving high photovoltaic efficiency and chemical stability of semiconductor photoelectrodes during water oxidation under wide-ranging pH conditions. Wide band gap semiconducting metal oxides are frequently stable under water oxidation conditions but have inherent photocurrent limitations. We have recently demonstrated that atomic layer deposition (ALD) of a thin and chemically-protective TiO2 layer can avoid oxidation of the surface of a silicon photoanode during long-duration oxygen evolution. Our initial work showed that it was possible to decouple efficient light absorption by the silicon anode from the oxygen evolution reaction (OER) occurring on the surface of a water oxidation catalyst layer by the presence of an interposed ALD-TiO2 layer. This conformal and pinhole-free oxide layer was found to be dense enough to block the solution from oxidizing the underlying semiconductor (greatly increasing its stability), but still thin enough to allow efficient hole transport between an n-type silicon substrate and the iridium OER catalyst [1]. In more recent work, we have showed that ALD-TiO2 could be used with a variety of other catalysts to produce efficient devices with more than 600mV of open circuit voltage, high saturation currents, and similar stability. In addition, by studying thicker ALD-TiO2 films, we found that protection could be increased with a minor increase in anode resistance. The additional resistance is associated with bulk conduction of electronic carriers through states in the TiO2 band-gap between the catalyst and anode [2]. Together, these findings indicate that atomic layer deposited coatings in metal-insulator-semiconductor (MIS) photoanodes serve as a general protection method for a variety of semiconductor-catalyst systems. In addition to summarizing prior reports and the most recent results on such MIS photoelectrodes, this presentation will touch on future directions for efficient artificial photosynthesis using ALD surface protection layers. [1] YW Chen, J.D. Prange, et al. Nat. Mater. 2011, 10, 539-44. [2] A.G. Scheuermann, et al. Energy Environ. Sci. 2013, 6, 2487 – 96.

This paper presents a new algorithm for the calculation of stream surfaces for tetrahedral grids. It propagates the surface through the tetrahedra, one at a time, calculating the intersections with the tetrahedral faces. The method allows us to incorporate topological information from the cells, e.g., critical points. The calculations are based on barycentric coordinates, since this simplifies theory and algorithm. The stream surfaces are ruled surfaces inside each cell, and their construction is started with line segments on the faces. Our method supports the analysis of velocity fields resulting from computational fluid dynamics (CFD) simulations.

We report on the effects on water oxidation performance of varying (1) the nanoscale TiO2 thickness and (2) the catalyst material in catalyst/TiO2/SiO2/Si anodes. Uniform films of atomic layer deposited TiO2 are prepared in the thickness range ∼1–12 nm on degenerately-doped p+-Si, yielding water oxidation overpotentials at 1 mA cm−2 of 300 mV to 600 mV in aqueous solution (pH 0 to 14). Electron/hole transport through Schottky tunnel junction structures of varying TiO2 thickness was studied using the reversible redox couple ferri/ferrocyanide. The dependence of the water oxidation overpotential on ALD-TiO2 thickness, with all other anode design features unchanged, exhibits a linear trend corresponding to ∼21 mV of added overpotential at 1 mA cm−2 per nanometer of TiO2 for TiO2 thicknesses greater than ∼2 nm. For thinner TiO2 layers, an approximately thickness-independent overpotential is observed. The linear behavior for anodes with thicker TiO2 layers is consistent with the predicted effect of bulk TiO2-limited electronic conduction on the voltage required to sustain the current density across the TiO2/SiO2 insulator stack. Eight different oxygen evolution catalysts of thickness 1–3 nm are studied. For the anodes investigated, 3 nm of Ir or Ru gave the best water oxidation performance, but both thinner layers and other catalysts can be quite effective, suggesting the potential for reduced materials cost. Lastly, a flat band voltage analysis of solid state thin film capacitors was done for five different gate metals on n-Si to probe junction energetics directly relevant to a photoanode. The results are consistent with a Schottky junction in which the Fermi level at the semiconductor surface is unpinned.

The thermal and chemical stability of potential dielectrics for GaN-based devices in a GaN metal organic chemical vapor deposition (MOCVD) environment is investigated, and their suitability for use as a gate dielectric and as a regrowth mask for the selective area growth of GaN is discussed. Thin films of MgO, Sc2O3, and Sc2O3/MgO were grown by molecular beam epitaxy on GaN/sapphire substrates and then annealed in a MOCVD reactor under GaN growth conditions except for the lack of trimethylgallium. All films were processed into metal-oxide-semiconductor diodes and were characterized before and after being annealed using atomic force microscopy, x-ray reflectivity, x-ray photoelectron spectroscopy, current-voltage (I-V), and capacitance-voltage (C-V) measurements. The Gibbs free energy of all possible reactions was calculated, and their probability and possible influence on the characterization results is examined. After being annealed, the atomic force microscopy of the oxide films showed some degree of ro...

A technique is described which produced coatings of refractory compounds on substrates using electron beam evaporation of a metal in a reactive atmosphere. The parameters that control the properties of the film are also presented.

Es wurde aus einer phenolischen Zweikernverbindung eine cyclische Verbindung IV hergestellt, die bei der alkalischen Hydrolyse eine molekulareinheitliche Tetramethacrylsaure (VII) und die phenolische Ausgangsverbindung gab. Die Synthese der gleichen Saure (VII) gelang auch mit strukturell verschiedenen Zweikernverbindungen, wenn nur bei den verwendeten Matrizen der innermolekulare Abstand der beiden phenolischen Hydroxylgruppen fur die Reaktion gunstig war. Alkaline hydrolysis of a cyclic compound (IV) which was prepared out of a dinuclear phenolic compound gave a molecularly homogeneous tetra-methacrylic acid (VII) and the original phenolic compound. The synthesis of the same acid could also be accomplished from structurally different dinuclear phenolic compounds only when the inner molecular distance between phenolic hydroxyl groups remained favourable to the reaction.

In order to fulfill the information storage needs of modern societies, the performance of electronic nonvolatile memories (NVMs) should be continuously improved. In the past few years, resistive random access memories (RRAM) have raised as one of the most promising technologies for future information storage due to their excellent performance and easy fabrication. In this work, a novel strategy is presented to further extend the performance of RRAMs. By using only cheap and industry friendly materials (Ti, TiO2, SiOX, and n++Si), memory cells are developed that show both filamentary and distributed resistive switching simultaneously (i.e., in the same I–V curve). The devices exhibit unprecedented hysteretic I–V characteristics, high current on/off ratios up to ≈5 orders of magnitude, ultra low currents in high resistive state and low resistive state (100 pA and 125 nA at –0.1 V, respectively), sharp switching transitions, good cycle-to-cycle endurance (>1000 cycles), and low device-to-device variability. We are not aware of any other resistive switching memory exhibiting such characteristics, which may open the door for the development of advanced NVMs combining the advantages of filamentary and distributed resistive switching mechanisms.