Experimental and Theoretical Comparison of Potential-dependent Methylation on Chemically Exfoliated WS and MoS.

Reductant-activated functionalization is shown to enhance the methylation of chemically exfoliated MoS (MoS) and WS by introducing excess negative charge to facilitate a nucleophilic attack reaction. Relative to methylation in the absence of a reductant, the reaction produces a twofold increase in coverage of WS, from 25 to 52% coverage per WS. However, at every potential, the methyl coverage on WS was ∼20% lower than that on MoS.

Design of robust 2,2'-bipyridine ligand linkers for the stable immobilization of molecular catalysts on silicon(111) surfaces.

The attachment of the 2,2'-bipyridine (bpy) moieties to the surface of planar silicon(111) (photo)electrodes was investigated using ab initio simulations performed on a new cluster model for methyl-terminated silicon. Density functional theory (B3LYP) with implicit solvation techniques indicated that adventitious chlorine atoms, when present in the organic linker backbone, led to instability at very negative potentials of the surface-modified electrode. In prior experimental work, chlorine atoms were present as a trace surface impurity due to required surface processing chemistry, and thus could plausibly result in the observed surface instability of the linker.

Atomic force microscopy: Emerging illuminated and operando techniques for solar fuel research.

Integrated photoelectrochemical devices rely on the synergy between components to efficiently generate sustainable fuels from sunlight. The micro- and/or nanoscale characteristics of the components and their interfaces often control critical processes of the device, such as charge-carrier generation, electron and ion transport, surface potentials, and electrocatalysis. Understanding the spatial properties and structure-property relationships of these components can provide insight into designing scalable and efficient solar fuel components and systems.

Fine-tuning polyoxometalate non-linear optical chromophores: a molecular electronic "Goldilocks" effect.

A new aryl-imido polyoxometalate non-linear optical chromophore (POMophore) with a diphenylamino donor group attains the highest β value (196 × 10 esu by Hyper-Rayleigh Scattering, HRS), and best transparency/non-linearity trade off yet for such materials. Stark spectroscopic and DFT investigation of this compound, plus NMe and carbazole analogues, show that its high performance results from a combination of strongly dipolar electronic transitions, and strong electronic communication across the π-system.

Organoimido-Polyoxometalate Nonlinear Optical Chromophores: A Structural, Spectroscopic, and Computational Study.

Ten organoimido polyoxometalate (POM)-based chromophores have been synthesized and studied by hyper-Rayleigh scattering (HRS), Stark and Resonance Raman spectroscopies, and density functional theory (DFT) calculations. HRS β values for chromophores with resonance electron donors are significant (up to 139 × 10 esu, ∼5 times greater than that of the DAS cation), but systems with no donor, or the -NO acceptor show no activity, in some cases, despite large DFT-predicted β-values. In active systems with short (phenyl) π-bridges, β values comfortably exceed that of the purely organic structural analogue N,N-dimethyl-4-nitroaniline (DMPNA), and intrinsic β-values, β/N (where N is the number of bridge π-electrons) thus appear to break empirical performance limits (β/N vs λ) for planar organic systems.

Photoelectrochemical Behavior of a Molecular Ru-Based Water-Oxidation Catalyst Bound to TiO-Protected Si Photoanodes.

A hybrid photoanode based on a molecular water oxidation precatalyst was prepared from TiO-protected n- or p-Si coated with multiwalled carbon nanotubes (CNT) and the ruthenium-based water oxidation precatalyst [Ru(tda)(py-pyr)(O)], 1(O) (tda is [2,2':6',2″-terpyridine]-6,6″-dicarboxylato and py-pir is 4-(pyren-1-yl)-N-(pyridin-4-ylmethyl)butanamide). The Ru complex was immobilized by π-π stacking onto CNTs that had been deposited by drop casting onto Si electrodes coated with 60 nm of amorphous TiO and 20 nm of a layer of sputtered C. At pH = 7 with 3 Sun illumination, the n-Si/TiO/C/CNT/[1+1(O)] electrodes exhibited current densities of 1 mA cm at 1.

Tunable Chiral Second-Order Nonlinear Optical Chromophores Based on Helquat Dications.

Fourteen new dipolar cations have been synthesized, containing methoxy or tertiary amino electron donor groups attached to helquat (Hq) acceptors. These Hq derivatives have been characterized as their TfO salts by using various techniques including NMR and electronic absorption spectroscopies. UV-vis spectra show intense, relatively low energy absorptions with λ ≈ 400-600 nm, attributable to intramolecular charge-transfer (ICT) excitations.

Nanoelectrical and Nanoelectrochemical Imaging of Pt/p-Si and Pt/p -Si Electrodes.

The interfacial properties of electrolessly deposited Pt nanoparticles (Pt-NPs) on p-Si and p -Si electrodes were investigated on the nanometer scale using a combination of scanning probe methods. Atomic force microscopy (AFM) showed highly dispersed Pt-NPs with diameters of 20-150 nm on the Si surface. Conductive AFM measurements showed that only approximately half of the particles exhibited measurable contact currents, with a factor of 10 difference in current observed between particles at a given bias.

Atomic force microscopy with nanoelectrode tips for high resolution electrochemical, nanoadhesion and nanoelectrical imaging.

Multimodal nano-imaging in electrochemical environments is important across many areas of science and technology. Here, scanning electrochemical microscopy (SECM) using an atomic force microscope (AFM) platform with a nanoelectrode probe is reported. In combination with PeakForce tapping AFM mode, the simultaneous characterization of surface topography, quantitative nanomechanics, nanoelectronic properties, and electrochemical activity is demonstrated.

Ferrocenyl helquats: unusual chiral organometallic nonlinear optical chromophores.

Three new dipolar cations have been synthesised, containing ferrocenyl (Fc) electron donor groups attached to helquat (Hq) acceptors. These organometallic Hq derivatives have been characterised as their TfO salts by using various techniques including NMR and electronic absorption spectroscopies and electrochemical measurements. UV-vis spectra show multiple intense low energy absorptions attributable to intramolecular charge-transfer (ICT) excitations.

Immobilization and electrochemical properties of ruthenium and iridium complexes on carbon electrodes.

We report the synthesis and surface immobilization of two new pyrene-appended molecular metal complexes: a ruthenium tris(bipyridyl) complex (1) and a bipyridyl complex of [Cp*Ir] (2) (Cp*  =  pentamethylcyclopentadienyl). X-ray photoelectron spectroscopy confirmed successful immobilization on high surface area carbon electrodes, with the expected elemental ratios for the desired compounds. Electrochemical data collected in acetonitrile solution revealed a reversible reduction of 1 near  -1.

Helquat Dyes: Helicene-like Push-Pull Systems with Large Second-Order Nonlinear Optical Responses.

Helquat dyes combine a cationic hemicyanine with a helicene-like motif to form a new blueprint for chiral systems with large and tunable nonlinear optical (NLO) properties. We report a series of such species with characterization, including determination of static first hyperpolarizabilities β0 via hyper-Rayleigh scattering and Stark spectroscopy. The measured β0 values are similar to or substantially larger than that of the commercial chromophore E-4'-(dimethylamino)-N-methyl-4-stilbazolium.

Comparison of the Performance of CoP-Coated and Pt-Coated Radial Junction n(+)p-Silicon Microwire-Array Photocathodes for the Sunlight-Driven Reduction of Water to H2(g).

The electrocatalytic performance for hydrogen evolution has been evaluated for radial-junction n(+)p-Si microwire (MW) arrays with Pt or cobalt phosphide, CoP, nanoparticulate catalysts in contact with 0.50 M H2SO4(aq). The CoP-coated (2.

Use of Mixed CH3-/HC(O)CH2CH2-Si(111) Functionality to Control Interfacial Chemical and Electronic Properties During the Atomic-Layer Deposition of Ultrathin Oxides on Si(111).

Silicon surfaces terminated with a mixed monolayer containing both a propyl aldehyde functionality and methyl groups were prepared and used to control the interfacial chemical and electronic properties of Si(111) surfaces during atomic-layer deposition (ALD) of Al2O3 or MnO. Si(111) surfaces functionalized only with the aldehyde moiety exhibited surface recombination velocities, S, of 2500 ± 600 cm s(-1) whereas the mixed CH3-/HC(O)CH2CH2-Si(111) surfaces displayed S = 25 ± 7 cm s(-1). During the ALD growth of either Al2O3 or MnO, both the HC(O)CH2CH2-Si(111) and CH3-/HC(O)CH2CH2-Si(111) surfaces produced increased metal oxide deposition at low cycle number, relative to H-Si(111) or CH3-Si(111) surfaces.

Stable Solar-Driven Water Oxidation to O2(g) by Ni-Oxide-Coated Silicon Photoanodes.

Semiconductors with small band gaps (<2 eV) must be stabilized against corrosion or passivation in aqueous electrolytes before such materials can be used as photoelectrodes to directly produce fuels from sunlight. In addition, incorporation of electrocatalysts on the surface of photoelectrodes is required for efficient oxidation of H2O to O2(g) and reduction of H2O or H2O and CO2 to fuels. We report herein the stabilization of np(+)-Si(100) and n-Si(111) photoanodes for over 1200 h of continuous light-driven evolution of O2(g) in 1.

Synthesis and Characterization of Atomically Flat Methyl-Terminated Ge(111) Surfaces.

Atomically flat, terraced H-Ge(111) was prepared by annealing in H2(g) at 850 °C. The formation of monohydride Ge-H bonds oriented normal to the surface was indicated by angle-dependent Fourier-transform infrared (FTIR) spectroscopy. Subsequent reaction in CCl3Br(l) formed Br-terminated Ge(111), as shown by the disappearance of the Ge-H absorption in the FTIR spectra concomitant with the appearance of Br photoelectron peaks in X-ray photoelectron (XP) spectra.

The influence of structure and processing on the behavior of TiO2 protective layers for stabilization of n-Si/TiO2/Ni photoanodes for water oxidation.

Light absorbers with moderate band gaps (1-2 eV) are required for high-efficiency solar fuels devices, but most semiconducting photoanodes undergo photocorrosion or passivation in aqueous solution. Amorphous TiO2 deposited by atomic-layer deposition (ALD) onto various n-type semiconductors (Si, GaAs, GaP, and CdTe) and coated with thin films or islands of Ni produces efficient, stable photoanodes for water oxidation, with the TiO2 films protecting the underlying semiconductor from photocorrosion in pH = 14 KOH(aq). The links between the electronic properties of the TiO2 in these electrodes and the structure and energetic defect states of the material are not yet well-elucidated.

Stable solar-driven oxidation of water by semiconducting photoanodes protected by transparent catalytic nickel oxide films.

Reactively sputtered nickel oxide (NiOx) films provide transparent, antireflective, electrically conductive, chemically stable coatings that also are highly active electrocatalysts for the oxidation of water to O2(g). These NiOx coatings provide protective layers on a variety of technologically important semiconducting photoanodes, including textured crystalline Si passivated by amorphous silicon, crystalline n-type cadmium telluride, and hydrogenated amorphous silicon. Under anodic operation in 1.

Microwave near-field imaging of two-dimensional semiconductors.

Optimizing new generations of two-dimensional devices based on van der Waals materials will require techniques capable of measuring variations in electronic properties in situ and with nanometer spatial resolution. We perform scanning microwave microscopy (SMM) imaging of single layers of MoS2 and n- and p-doped WSe2. By controlling the sample charge carrier concentration through the applied tip bias, we are able to reversibly control and optimize the SMM contrast to image variations in electronic structure and the localized effects of surface contaminants.

Catalysis of proton reduction by a [BO4]-bridged dicobalt glyoxime.

We report the preparation of a dicobalt compound with two singly proton-bridged cobaloxime units linked by a central [BO4] bridge. Reaction of a doubly proton-bridged cobaloxime complex with trimethyl borate afforded the compound in good yield. Single-crystal X-ray diffraction studies confirmed the bridging nature of the [BO4] moiety.

Assembly, characterization, and electrochemical properties of immobilized metal bipyridyl complexes on silicon(111) surfaces.

Silicon(111) surfaces have been functionalized with mixed monolayers consisting of submonolayer coverages of immobilized 4-vinyl-2,2'-bipyridyl (1, vbpy) moieties, with the remaining atop sites of the silicon surface passivated by methyl groups. As the immobilized bipyridyl ligands bind transition metal ions, metal complexes can be assembled on the silicon surface. X-ray photoelectron spectroscopy (XPS) demonstrates that bipyridyl complexes of [Cp*Rh], [Cp*Ir], and [Ru(acac)2] were formed on the surface (Cp* is pentamethylcyclopentadienyl, acac is acetylacetonate).

Amorphous TiO₂ coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation.

Although semiconductors such as silicon (Si), gallium arsenide (GaAs), and gallium phosphide (GaP) have band gaps that make them efficient photoanodes for solar fuel production, these materials are unstable in aqueous media. We show that TiO2 coatings (4 to 143 nanometers thick) grown by atomic layer deposition prevent corrosion, have electronic defects that promote hole conduction, and are sufficiently transparent to reach the light-limited performance of protected semiconductors. In conjunction with a thin layer or islands of Ni oxide electrocatalysts, Si photoanodes exhibited continuous oxidation of 1.

Photoelectrochemical behavior of hierarchically structured Si/WO3 core-shell tandem photoanodes.

WO3 thin films have been deposited in a hierarchically structured core-shell morphology, with the cores consisting of an array of Si microwires and the shells consisting of a controlled morphology WO3 layer. Porosity was introduced into the WO3 outer shell by using a self-assembled microsphere colloidal crystal as a mask during the deposition of the WO3 shell. Compared to conformal, unstructured WO3 shells on Si microwires, the hierarchically structured core-shell photoanodes exhibited enhanced near-visible spectral response behavior, due to increased light absorption and reduced distances over which photogenerated carriers were collected.

Synthesis, structures, and optical properties of ruthenium(II) complexes of the tris(1-pyrazolyl)methane ligand.

Four new complex salts [Ru(II)Cl(Tpm)(L(A))2][PF6]n [Tpm = tris(1-pyrazolyl)methane; n = 1, L(A) = pyridine (py) 1 or ethyl isonicotinate (EIN) 2; n = 3, L(A) = N-methyl-4,4'-bipyridinium (MeQ(+)) 3 or N-phenyl-4,4'-bipyridinium (PhQ(+)) 4] have been prepared and characterized. Electronic absorption spectra show intense d → π* metal-to-ligand charge-transfer (MLCT) absorption bands, while cyclic voltammetry reveals a reversible Ru(III/II) wave, accompanied by quasireversible or irreversible L(A)-based reductions for all except 1. Single crystal X-ray structures have been obtained for 1•Me2CO, 2, and 3•Me2CO.

Noncovalent immobilization of electrocatalysts on carbon electrodes for fuel production.

We show that molecular catalysts for fuel-forming reactions can be immobilized on graphitic carbon electrode surfaces via noncovalent interactions. A pyrene-appended bipyridine ligand (P) serves as the linker between each complex and the surface. Immobilization of a rhodium proton-reduction catalyst, [Cp*Rh(P)Cl]Cl (1), and a rhenium CO2-reduction catalyst, Re(P)(CO)3Cl (2), afford electrocatalytically active assemblies.