Nicholas Alderman

Large surface photovoltages observed at methyl-terminated silicon surfaces synthesised through a two-step chlorination-alkylation method

Methylation of the silicon surface through a chlorination-alkylation method has been used to improve the electronic properties of silicon. Upon alkylation of the surface, an increase in the minority carrier recombination lifetime and the surface photovoltage is observed, in line with an increase in surface charge. A likely explanation of this unusually large band bending is charge accumulation during the removal of chlorine from the surface.

Silicon sensitisation using light harvesting layers

Abstract Langmuir–Blodgett monolayers of a cyanine dye mixed with stearic acid were deposited on glass and silicon substrates with spacer layers of pure stearic acid monolayers or silicon dioxide films deposited by PECVD. By using the time correlated single photon counting technique, time resolved emission spectra (TRES) and decay curves were measured to characterise the dependence of energy transfer rate on the separation between the dye monolayer and the silicon surface and also for the dye concentrations in the monolayers. We observe interlayer energy transfer between monomers, dimers and higher aggregates present in the monolayer deposited on glass but also competing directly with energy transfer to silicon at close distances. We find that the fluorescence lifetime of the dye monolayer is significantly shortened when deposited close to the silicon surface signifying efficient energy transfer. The dissipation of the excitation energy near silicon is explained using the classical theory developed for metals and a deviation is observed for monolayers deposited at distances close to the silicon surface.

Kelvin probe studies of alkyl monolayers on silicon (111) for surface passivation

A chlorination–alkylation procedure has been investigated with a view to improving the surface passivation properties on silicon. We have found that increasing surface coverage of the alkyl monolayer raises the measured SPV and recombination lifetime. A logarithmic trend is observed between the SPV and recombination, showing that monolayer preparation is an important factor in determining the lifetimes observed. The high SPVs measured by Kelvin probe suggest the presence of a silicon surface with a lower concentration of electrons, reducing surface recombination.

Photochemical water splitting mediated by a C1 shuttle

The possibility of performing photochemical water splitting in a two-stage system, separately releasing the H2 and O2 components, has been probed with two separate catalysts and in combination with a formaldehyde/formate shuttling redox couple. In the first stage, formaldehyde releases hydrogen vigorously in the presence of an Na4[Fe(CN)6]·10H2O catalyst, selectively affording the formate anion. In the second stage, the formate anion is hydro-genated back to formaldehyde by water and in the presence of a Bi2WO6 photocatalyst whilst releasing oxygen. Both stages operate at room temperature and under visible light irradiation. The two separate photocatalysts are compatible since water splitting can also be obtained in one-pot experiments with simultaneous H2/O2 evolution.

High pressure polymorphism of beta-TaON

The high pressure behavior of TaON was studied using a combination of Raman scattering, synchrotron X-ray diffraction, and X-ray absorption spectroscopy in diamond anvil cells to 70 GPa at ambient temperature. A Birch-Murnaghan equation of state fit for baddeleyite structured β-TaON indicates a high bulk modulus value Ko = 328 ± 4 GPa with K = 4.3. EXAFS analysis of the high pressure XAS data provides additional information on changes in the Ta-(O,N) and Ta-Ta distances. Changes in the X-ray diffraction patterns and Raman spectra indicate onset of a pressure induced phase transition near 33 GPa. Our analysis indicates that the new phase has an orthorhombic cotunnite-type structure but that the phase transition may not be complete even by 70 GPa. Similar sluggish transformation kinetics are observed for the isostructural ZrO2 phase. Analysis of compressibility data for the new cotunnite-type TaON phase indicate a very high bulk modulus Ko ∼ 370 GPa, close to the theoretically predicted value.

Radical Behavior of CO2 versus its Deoxygenation Promoted by Vanadium Aryloxide Complexes: How the Geometry of Intermediate CO2-Adducts Determines the Reactivity.

The reactivity of carbon dioxide with vanadium(III) aryloxo complexes has been investigated. The formation of either carbon monoxide or incorporation into the ligand system with the ultimate formation of organic ester was observed depending on the overall electron donor ability of the ligand field. DFT calculations were carried out to investigate the proposed mechanism for carbon dioxide coordination and reduction.

Photon tunneling into a single-mode planar silicon waveguide

We demonstrate the direct excitation of a single TE mode in 25 nm thick planar crystalline silicon waveguide by photon tunneling from a layer of fluorescent dye molecules deposited by the Langmuir-Blodgett technique. The observed photon tunneling rate as a function of the dye-silicon separation is well fitted by a theoretical tunneling rate, which is obtained via a novel approach within the framework of quantum mechanics. We suggest that future ultrathin crystalline silicon solar cells can be made efficient by simple light trapping structures consisting of molecules on silicon.

Reaction of CO2 with a Vanadium(II) Aryl Oxide: Synergistic Activation of CO2/Oxo Groups towards H‐Atom Radical Abstraction

Treatment of divalent (ONNO)V(TMEDA) (1; ONNO=[2,4-Me2 -2-(OH)C6 H2 CH2 ]2 N(CH2 )2 NMe2 ) with CO2 afforded [(ONNO)V]2 (μ-OH)(μ-formate) (2). Whereas the bridging hydroxo and formate groups both originated from CO2 , the H atoms present on the two residues were obtained through H-atom radical abstraction from the solvent. DFT calculations revealed an initially linear CO2 bonding mode, followed by deoxygenation, and highlighted a synergistic effect between the so-formed oxo group and an additional bridging CO2 residue in promoting radical behavior.

Novel recombination lifetime mapping technique through Kelvin probe studies

The Kelvin probe is a very powerful and versatile tool, allowing the extraction of data such as diffusion length, surface photovoltage and impurity concentrations. This paper investigates the extraction of surface recombination velocities (and assuming a bulk lifetime, the surface recombination lifetime) from the I-V type dependence of the sample. By using an X-Y stage, the surface recombination lifetime can be imaged for entire wafers, instead of obtaining an average value of lifetime similar to that obtained from the Sinton WCT-120 lifetime tool. This is useful in determining where further improvements in the surface passivation can be obtained, by observing problem areas in the passivation layer.

Determination of surface recombination velocities of organic monolayers on silicon through Kelvin probe

We report the determination of the surface recombination velocity of electron-hole pairs for silicon samples passivated with organic monolayers using the Kelvin probe. The recombination velocity was determined from the surface photovoltage and incident photon flux. By scanning of the Kelvin probe tip over the sample, the change in surface recombination velocity can be measured allowing recombination lifetime mapping. Organic monolayers with different chain lengths and exhibiting various recombination lifetimes were synthesized through a two-step chlorination-alkylation technique. The estimated recombination lifetimes were compared against those obtained from an industrial standard technique and were found to be in good agreement.