Photoelectrochemistry of Ultrathin, Semitransparent, and Catalytic Gold Films Electrodeposited Epitaxially onto n‑Silicon (111)

An ultrathin, epitaxial Au layer was electrochemically deposited on n-Si(111) to form a Schottky junction that was used as the photoanode in a regenerative photoelectrochemical cell. Au serves as a semitransparent contact that both stabilizes n-Si against photopassivation and catalyzes the oxidation of Fe²⁺ to Fe³⁺. In this cell, Fe²⁺ was oxidized at the n-Si(111)/Au(111) photoanode and Fe³⁺ was reduced at the Au cathode, leading to the conversion of solar energy into electrical energy with no net chemical reaction. The photocurrent was limited to 11.9 mA·cm^–2 because of the absorption of light by the Fe^2+/3+ redox couple. When a transparent solution of sulfite ion was oxidized at the photoanode, photocurrent densities as high as 28.5 mA·cm^–2 were observed with AM 1.5 light of 100 mW·cm^–2 intensity. One goal of the work was to determine the effect of the Au layer on the interfacial energetics as a function of the Au coverage. There was a decrease in the barrier height from 0.81 to 0.73 eV as the gold coverage was increased from island growth with 10% coverage to a dense Au film with a thickness of 11 nm. In all cases, the band-bending in n-Si was induced by the n-Si/Au Schottky junction instead of the energetic mismatch between the Fermi level of n-Si and the redox couple. The dense Au film gave the greatest stability. Although the photocurrent of the n-Si/Au photoanode with 10.2% island coverage dropped nearly to zero within 2 h, the photocurrent of the photoanode with a dense 11 nm thick Au film only decreased to 92% of its initial value after irradiation at open circuit with AM 1.5 light for 16 h. A 2.1 nm thick layer of SiO_<i>x</i> formed between the Au film and n-Si. With further irradiation, the fill factor decreased because of the increase of series resistance as the SiO_<i>x</i> layer thickness exceeded tunneling dimensions

Copper Nanofilament Formation during Unipolar Resistance Switching of Electrodeposited Cuprous Oxide

An emerging nonvolatile, solid-state memory is resistance random access memory (RRAM). RRAM is based on reversible switching of resistance in semiconductor and insulator thin films. Here, unipolar resistance switching is demonstrated in electrodeposited films of [111]-textured cuprous oxide (Cu₂O). The textured Cu₂O is electrodeposited from a highly alkaline bath using tartrate as the complexing agent. The switching is observed in a cell composed of a film of Cu₂O sandwiched between Au and Au–Pd contacts. The switching is attributed to the formation and rupture of a Cu nanofilament in the Cu₂O. The initial resistance of the cell is 6.5 × 10⁶ Ω, and a conducting filament is formed in the film by scanning the applied electric field to 6.8 × 10⁶ V m^–1. The cell is then reversibly cycled between a low resistance state of 16.6 Ω and a high resistance state of 4 × 10⁵ Ω by the SET and RESET processes. In the low resistance state, the resistance decreases linearly with decreasing temperature, consistent with metallic behavior. The resistance temperature coefficient of 1.57 × 10^–3 K^–1 is similar to that of nanoscale metallic Cu. Current–voltage (<i>I</i>–<i>V</i>) data suggests that applying a higher compliance current increases the filament size during the FORMING and the SET process and also causes a higher RESET current. The filament diameter varies from 50 to 147 nm for compliance currents ranging from 10 to 100 mA. At high electric field in the as-deposited state, the conduction behavior follows Poole-Frenkel emission. The filament temperature is estimated from the nonohmic behavior of the cell in the RESET step. The calculated temperature of 798 K before rupture of the Cu filament suggests Joule heating of the filament, resulting in melting, sintering, or thermal oxidation of the Cu filament

Electrodeposition of Epitaxial Lead Iodide and Conversion to Textured Methylammonium Lead Iodide Perovskite

Applications for lead iodide, such as lasing, luminescence, radiation detection, and as a precursor for methylammonium lead iodide perovskite photovoltaic cells, require highly ordered crystalline thin films. Here, an electrochemical synthesis route is introduced that yields textured and epitaxial films of lead iodide at room temperature by reducing molecular iodine to iodide ions in the presence of lead ions. Lead iodide grows with a [0001] fiber texture on polycrystalline substrates such as fluorine-doped tin oxide. On single-crystal Au(100), Au(111), and Au(110) the out-of-plane orientation of lead iodide is also [0001], but the in-plane orientation is controlled by the single-crystal substrate. The epitaxial lead iodide on single-crystal gold is converted to textured methylammonium lead iodide perovskite with a preferred [110] orientation via methylammonium iodide vapor-assisted chemical transformation of the solid

Superconducting Filaments Formed During Nonvolatile Resistance Switching in Electrodeposited δ‑Bi₂O₃

We show that electrodeposited films of δ-Bi₂O₃ in a Pt/δ-Bi₂O₃/Au cell exhibit unipolar resistance switching. After being formed at a large electric field of 40 MV/m, the cell can be reversibly switched between a low resistance state (156 Ω) and a high resistance state (1.2 GΩ) by simply cycling between SET and RESET voltages of the same polarity. Because the high and low resistance states are persistent, the cell is a candidate for nonvolatile resistance random access memory (RRAM). A Bi nanofilament forms at the SET voltage, and it ruptures to form a 50 nm gap during the RESET step at a current density of 2 × 10⁷ A/cm². The diameter of the Bi filament is a function of the compliance current, and can be tuned from 140 to 260 nm, but the current density in the RESET step is independent of the Bi diameter. An electromigration rupture mechanism is proposed. The Bi nanofilaments in the low resistance state are superconducting, with a <i>T</i>_c of 5.8 K and an <i>H</i>_c of 5 kOe. This is an unexpected result, because bulk Bi is not a superconductor

Epitaxial Electrodeposition of Methylammonium Lead Iodide Perovskites

An electrochemical/chemical route is introduced to deposit both textured and epitaxial films of methylammonium lead iodide (MAPbI₃) perovskites. The perovskite films are produced by chemical conversion of lead dioxide films that have been electrodeposited as either textured or epitaxial films onto [111]-textured Au and [100] and [111] single-crystal Au substrates. The epitaxial relationships for the MAPbI₃ films are MAPbI₃(001)­[010]∥PbO₂(100)⟨001⟩ and MAPbI₃(110)­[111]∥PbO₂(100)⟨001⟩ regardless of the Au substrate orientation, because the in-plane order of the converted film is controlled by the epitaxial PbO₂ precursor film. The textured and epitaxial MAPbI₃ films both have trap densities lower than and photoluminescence intensities higher than those of polycrystalline films produced by spin coating

Electrodeposited Germanium Nanowires

Germanium (Ge) is a group IV semiconductor with superior electronic properties compared with silicon, such as larger carrier mobilities and smaller effective masses. It is also a candidate anode material for lithium-ion batteries. Here, a simple, one-step method is introduced to electrodeposit dense arrays of Ge nanowires onto indium tin oxide (ITO) substrates from aqueous solution. The electrochemical reduction of ITO produces In nanoparticles that act as a reduction site for aqueous Ge(IV) species, and as a solvent for the crystallization of Ge nanowires. Nanowires deposited at 95 °C have an average diameter of 100 nm, whereas those deposited at room temperature have an average diameter of 35 nm. Both optical absorption and Raman spectroscopy suggest that the electrodeposited Ge is degenerate. The material has an indirect bandgap of 0.90–0.92 eV, compared with a value of 0.67 eV for bulk, intrinsic Ge. The blue shift is attributed to the Moss–Burstein effect, because the material is a p-type degenerate semiconductor. On the basis of the magnitude of the blue shift, the hole concentration is estimated to be 8 × 10¹⁹ cm^–3. This corresponds to an In impurity concentration of about 0.2 atom %. The resistivity of the wires is estimated to be 4 × 10^–5 Ω·cm. The high conductivity of the wires should make them ideal for lithium-ion battery applications

Electrochemical Synthesis and Nonvolatile Resistance Switching of Mn₃O₄ Thin Films

An electrodeposition method is introduced to produce crystalline films of Mn₃O₄ from aqueous solution. The films are electrodeposited potentiostatically from a solution of 0.2 M Mn­(II) acetate at a pH of 6 and a deposition temperature of 80 °C. The anodic deposition is performed in the potential range of 0.275–0.350 V vs Ag/AgCl/KCl­(sat.). In this potential range the current efficiency is 100%. Both the stoichiometry and the morphology of the films can be controlled through the applied potential. Films deposited at a low overpotential grow with a [001] preferred orientation, columnar microstructure, and near-stoichiometric Mn­(III)/Mn­(II) ratio of 1.99. Films deposited at higher overpotential deposit with a near-random orientation, porous or powdery microstructure, and Mn­(III)/Mn­(II) ratio of 2.62. At potentials greater than 0.38 V, amorphous films of MnO₂ are produced. The as-deposited Mn₃O₄ has a very high resistivity of 4.4 × 10⁸ Ωcm. The electrodeposited Mn₃O₄ films undergo unipolar, nonvolatile resistance switching. An Au/Mn₃O₄/AuPd cell can be reversibly switched between a high resistance state (30 MΩ) and a low resistance state (14.8 Ω) by applying SET and RESET voltages of the same polarity. SET and RESET times of approximately 2 and 50 ns were observed. Because of the large resistance contrast between the two states and the persistence of each state, Mn₃O₄ is a candidate for future resistive random access memory (RRAM) devices

Nanometer-Thick Gold on Silicon as a Proxy for Single-Crystal Gold for the Electrodeposition of Epitaxial Cuprous Oxide Thin Films

Single-crystal Au is an excellent substrate for electrochemical epitaxial growth due to its chemical inertness, but the high cost of bulk Au single crystals prohibits their use in practical applications. Here, we show that ultrathin epitaxial films of Au electrodeposited onto Si(111), Si(100), and Si(110) wafers can serve as an inexpensive proxy for bulk single-crystal Au for the deposition of epitaxial films of cuprous oxide (Cu₂O). The Au films range in thickness from 7.7 nm for a film deposited for 5 min to 28.3 nm for a film deposited for 30 min. The film thicknesses are measured by low-angle X-ray reflectivity and X-ray Laue oscillations. High-resolution TEM shows that there is not an interfacial SiO_<i>x</i> layer between the Si and Au. The Au films deposited on the Si(111) substrates are smoother and have lower mosaic spread than those deposited onto Si(100) and Si(110). The mosaic spread of the Au(111) layer on Si(111) is only 0.15° for a 28.3 nm thick film. Au films deposited onto degenerate Si(111) exhibit ohmic behavior, whereas Au films deposited onto n-type Si(111) with a resistivity of 1.15 Ω·cm are rectifying with a barrier height of 0.85 eV. The Au and the Cu₂O follow the out-of-plane and in-plane orientations of the Si substrates, as determined by X-ray pole figures. The Au and Cu₂O films deposited on Si(100) and Si(110) are both twinned. The films grown on Si(100) have twins with a [221] orientation, and the films grown on Si(110) have twins with a [411] orientation. An interface model is proposed for all Si orientations, in which the −24.9% mismatch for the Au/Si system is reduced to only +0.13% by a coincident site lattice in which 4 unit meshes of Au coincide with 3 unit meshes of Si. Although this study only considers the deposition of epitaxial Cu₂O films on electrodeposited Au/Si, the thin Au films should serve as high-quality substrates for the deposition of a wide variety of epitaxial materials

Electrodeposition of Co_<i>x</i>Fe_3–<i>x</i>O₄ Epitaxial Films and Superlattices

Spinel ferrites are of interest because of their potential applications in spintronics (spin-based electronics) and solid-state memories. Cobalt ferrite (CoFe₂O₄) is an inverse spinel ferrite. Currently, the utility of CoFe₂O₄ is mainly based on its high coercivity and magnetocrystalline anisotropy. The magnetic and electrical properties of CoFe₂O₄ depend on its Co:Fe ratio. In this paper, a one-step electrodeposition of Co_<i>x</i>Fe_3‑xO₄ (0 < <i>x</i> < 1) thin films from an alkaline Fe³⁺- and Co²⁺-triethanolamine solution is presented. The Co:Fe ratio in the Co_<i>x</i>Fe_3–<i>x</i>O₄ thin films can be tuned by controlling the deposition potential. That is, Co_<i>x</i>Fe_3–<i>x</i>O₄ thin films with tunable chemical and physical properties can be produced from a single solution. Superlattices in the Co_<i>x</i>Fe_3–<i>x</i>O₄ system were also electrodeposited from the same solution by simply pulsing between two potentials. Compared to Co_<i>x</i>Fe_3–<i>x</i>O₄ individual films, superlattices exhibit resistance switching and a more pronounced negative differential resistance (NDR) feature at lower current during perpendicular transport measurements, which could be used in resistive random access memories