Formation of Type II Silicon Clathrate with Lithium Guests through Thermal Diffusion

At low guest atom concentrations, Si clathrates can be viewed as semiconductors, with the guest atoms acting as dopants, potentially creating alternatives to diamond Si with exciting optoelectronic and spin properties. Studying Si clathrates with different guest atoms would not only provide insights into the electronic structure of the Si clathrates but also give insights into the unique properties that each guest can bring to the Si clathrate structure. However, the synthesis of Si clathrates with guests other than Na is challenging. In this study, we have developed an alternative approach, using thermal diffusion into type II Si clathrate with an extremely low Na concentration, to create Si clathrate with Li guests. Using time-of-flight secondary-ion mass spectroscopy, X-ray diffraction, and Raman scattering, thermal diffusion of Li into the nearly empty Si clathrate framework is detected and characterized as a function of the diffusion temperature and time. Interestingly, the Si clathrate exhibits reduced structural stability in the presence of Li, converting to polycrystalline or disordered phases for anneals at temperatures where the starting Na guest Si clathrate is quite stable. The Li atoms inserted into the Si clathrate lattice contribute free carriers, which can be detected in Raman scattering through their effect on the strength of Si-Si bonds in the framework. These carriers can also be observed in electron paramagnetic resonance (EPR). EPR shows, however, that Li guests are not simple analogues of Na guests. In particular, our results suggest that Li atoms, with their smaller size, tend to doubly occupy cages, forming "molecular-like" pairs with other Li or Na atoms. Results of this work provide a deeper insight into Li guest atoms in Si clathrate. These findings are also relevant to understanding how Li moves through and interacts with Si clathrate anodes in Li-ion batteries. Additionally, techniques presented in this work demonstrate a new method for filling the Si clathrate cages, enabling studies of a broad range of other guests in Si clathrates.

Tuning the work function of nickel oxide using triethoxysilane functionalized monolayers

The work function of nickel oxide (NiOx) electrodes was tuned by the covalent attachment of commercially available as well as specially synthesized triethoxysilane functionalized molecules with a range of dipole moments. The presence of the silane molecular layers on the NiOx surface was verified using Fourier Transform Infrared (FTIR) spectroscopy and contact angle measurements. While these tests indicated the surface coverage was incomplete, Kelvin probe measurements showed that the coverage was sufficient to change the work function of the NiOx across a range of ∼900 meV. Density functional theory (DFT) calculations of the dipole moments of the isolated molecules correlated well with the measured work function changes.

CO-Bridged H-Cluster Intermediates in the Catalytic Mechanism of [FeFe]-Hydrogenase CaI

The [FeFe]-hydrogenases ([FeFe] H₂ases) catalyze reversible H₂ activation at the H-cluster, which is composed of a [4Fe-4S]_H subsite linked by a cysteine thiolate to a bridged, organometallic [2Fe-2S] ([2Fe]_H) subsite. Profoundly different geometric models of the H-cluster redox states that orchestrate the electron/proton transfer steps of H₂ bond activation have been proposed. We have examined this question in the [FeFe] H₂ase I from Clostridium acetobutylicum (CaI) by Fourier-transform infrared (FTIR) spectroscopy with temperature annealing and H/D isotope exchange to identify the relevant redox states and define catalytic transitions. One-electron reduction of H_ox led to formation of H_redH⁺ ([4Fe-4S]_H²⁺-Fe^I-Fe^I) and H_red' ([4Fe-4S]_H¹⁺-Fe^II-Fe^I), with both states characterized by low frequency μ-CO IR modes consistent with a fully bridged [2Fe]_H. Similar μ-CO IR modes were also identified for H_redH⁺ of the [FeFe] H₂ase from Chlamydomonas reinhardtii (CrHydA1). The CaI proton-transfer variant C298S showed enrichment of an H/D isotope-sensitive μ-CO mode, a component of the hydride bound H-cluster IR signal, H_hyd. Equilibrating CaI with increasing amounts of NaDT, and probed at cryogenic temperatures, showed H_redH⁺ was converted to H_hyd. Over an increasing temperature range from 10 to 260 K catalytic turnover led to loss of H_hyd and appearance of H_ox, consistent with enzymatic turnover and H₂ formation. The results show for CaI that the μ-CO of [2Fe]_H remains bridging for all of the "H_red" states and that H_redH⁺ is on pathway to H_hyd and H₂ evolution in the catalytic mechanism. These results provide a blueprint for designing small molecule catalytic analogs.

Size-Dependent Exciton Formation Dynamics in Colloidal Silicon Quantum Dots

We report size-dependent exciton formation dynamics within colloidal silicon quantum dots (Si QDs) using time-resolved terahertz (THz) spectroscopy measurements. THz photoconductivity measurements are used to distinguish the initially created hot carriers from excitons that form at later times. At early pump/probe delays, the exciton formation dynamics are revealed by the temporal evolution of the THz transmission. We find an increase in the exciton formation time, from ∼500 to ∼900 fs, as the Si QD diameter is reduced from 7.3 to 3.4 nm and all sizes exhibit slower hot-carrier relaxation times compared to bulk Si. In addition, we determine the THz absorption cross section at early delay times is proportional to the carrier mobility while at later delays is proportional to the exciton polarizability, αX. We extract a size-dependent αX and find an ∼r(4) dependence, consistent with previous reports for quantum-confined excitons in CdSe, InAs, and PbSe QDs. The observed slowing in exciton formation time for smaller Si QDs is attributed to decreased electron-phonon coupling due to increased quantum confinement. These results experimentally verify the modification of hot-carrier relaxation rates by quantum confinement in Si QDs, which likely plays a significant role in the high carrier multiplication efficiency observed in these nanomaterials.

Conjugated phosphonic acid modified zinc oxide electron transport layers for improved performance in organic solar cells

Phosphonic acid modification of zinc oxide (ZnO) electron transport layers in inverted P3HT:ICBA solar cells was studied to determine the effect of conjugated linkages between the aromatic and phosphonic acid attachment groups. For example, zinc oxide treated with 2,6-difluorophenylvinylphosphonic acid, having a conjugated vinyl group connecting the aromatic moiety to the phosphonic acid group, showed a 0.78 eV decrease in the effective work function versus unmodified ZnO, whereas nonconjugated 2,6-difluorophenylethylphosphonic acid resulted in a 0.57 eV decrease, as measured by Kelvin probe. This resulted in an average power conversion efficiency of 5.89% for conjugated 2,6-difluorophenyvinylphosphonic acid modified solar cells, an improvement over unmodified (5.24%) and nonconjugated phosphonic acid modified devices (5.64%), indicating the importance of the conjugated linkage.

Ultrafast Electrical Measurements of Isolated Silicon Nanowires and Nanocrystals

We simultaneously determined the charge carrier mobility and picosecond to nanosecond carrier dynamics of isolated silicon nanowires (Si NWs) and nanocrystals (Si NCs) using time-resolved terahertz spectroscopy. We then compared these results to data measured on bulk c-Si as a function of excitation fluence. We find >1 ns carrier lifetimes in Si NWs that are dominated by surface recombination with surface recombination velocities (SRV) between ∼1100-1700 cm s(-1) depending on process conditions. The Si NCs have markedly different decay dynamics. Initially, free-carriers are produced, but relax within ∼1.5 ps to form bound excitons. Subsequently, the excitons decay with lifetimes >7 ns, similar to free carriers produced in bulk Si. The isolated Si NWs exhibit bulk-like mobilities that decrease with increasing excitation density, while the hot-carrier mobilities in the Si NCs are lower than bulk mobilities and could only be measured within the initial 1.5 ps decay. We discuss the implications of our measurements on the utilization of Si NWs and NCs in macroscopic optoelectronic applications.

Efficient photoelectrochemical water oxidation over cobalt-phosphate (Co-Pi) catalyst modified BiVO4/1D-WO3 heterojunction electrodes

We report the design, synthesis and photoelectrochemical characterization of cobalt phosphate (Co-Pi) oxygen evolution catalyst modified heterojunction photoelectrodes consisting of one-dimensional WO3 nanorods (1D-WO3) and highly porous BiVO4 layers. The 1D-WO3 nanorods were prepared by the decomposition of the tetrabutylammonium decatungstate precursor in the presence of poly(ethylene glycol) as a binding agent. The porous BiVO4 layers were spray deposited using a surfactant assisted metal-organic decomposition method. The Co-Pi oxygen evolution catalyst was deposited onto the BiVO4/1D-WO3/FTO heterojunction electrode using a photoassisted electrodeposition method. The Co-Pi catalyst modified heterojunction electrodes exhibited a sustained enhancement in the photocurrent compared to the unmodified BiVO4/1D-WO3/FTO heterojunction electrodes. The improved photoelectrochemical properties profited from the enhanced charge carrier separation achieved through the integration of highly porous BiVO4 layers on top of 1D-WO3 nanorods and from the superior kinetics due to the presence of the Co-Pi oxygen evolution catalyst on top of BiVO4/1D-WO3/FTO heterojunction electrodes.

BiVO(4)/CuWO(4) heterojunction photoanodes for efficient solar driven water oxidation

BiVO(4)/CuWO(4) heterojunction electrodes were prepared using spray deposition of a highly porous bismuth vanadate film onto the surface of an electrodeposited three dimensional network connected copper tungstate. Bilayer BiVO(4)/CuWO(4)/fluorine doped tin oxide glass (FTO) electrodes demonstrated higher photocurrent magnitudes than either with BiVO(4)/FTO or CuWO(4)/FTO electrodes in 1.0 M Na(2)SO(4) electrolyte buffered at pH 7. The photocurrent is enhanced by the formation of the heterojunction that aids charge carrier collection brought about by the band edge offsets. When the pH 7 buffered electrolytes contained 1.0 M bicarbonate is employed instead of 1.0 M sulfate, the charge transfer resistance was decreased. This led to nearly 1.8 times the photocurrent density at 1.0 V vs. Ag/AgCl. The photocurrent was stable over 24 hours in bicarbonate electrolyte.

Spiral plasmonic nanoantennas as circular polarization transmission filters

We present simulation and experimental results for easily fabricated spiral plasmonic antenna analogues providing circular polarization selectivity. One circular polarization state is concentrated and transmitted through a subwavelength aperture, while the opposite circular state is blocked. The spectral bandwidth, efficiency, and extinction ratios are tunable through geometric parameters. Integration of such structures onto a focal plane array in conjunction with linear micropolarizers enables complete Stokes vector imaging, that, until now, has been difficult to achieve. An array of these structures forms a plasmonic metamaterial that exhibits high circular dichroism.

Ultra-high extinction ratio micropolarizers using plasmonic lenses

The design of a new type of plasmonic ultra-high extinction ratio micropolarizing transmission filter is presented along with an experimental demonstration. A pair of dielectric coated metal gratings couple incident TM polarized light into surface plasmons, which are fed into a central metal-insulator-metal (MIM) waveguide, followed by transmission through a sub-wavelength aperture. Extinction ratios exceeding 10¹¹ are predicted by finite element simulation. Good absolute agreement for both the spectral and polarization response is obtained between measurement and simulations using measured geometric parameters. The filters can be easily fabricated and sized to match the pixel pitch of current focal plane arrays.

Hybrid plasmon/dielectric waveguide for integrated silicon-on-insulator optical elements

VLSI compatible optical waveguides on silicon are currently of particular interest in order to integrate optical elements onto silicon chips, and for possible replacements of electrical cross-chip/inter-core interconnects. Here we present simulation and experimental verification of a hybrid plasmon/dielectric, single-mode, single-polarization waveguide for silicon-on-insulator wafers. Its fabrication is compatible with VLSI processing techniques, and it possesses desirable properties such as the absence of birefringence and low sensitivity to surface roughness and metallic losses. The waveguide structure naturally forms an MOS capacitor, possibly useful for active device integration. Simulations predict very long propagation lengths of millimeter scale with micron scale confinement, or sub-micron scale confinement with propagation lengths still in excess of 100 microns. The waveguide may be tuned continuously between these states using standard VLSI processing. Extremely long propagation lengths have been simulated: one configuration presented here has a simulated propagation length of 34 cm.

High-sensitivity aminoazobenzene chemisorbed monolayers for photoalignment of liquid crystals

We describe a new type of optically controlled liquid crystal alignment layer that demonstrates unprecedented performance. It consists of an aminoazobenzene-type material with a very simple molecular structure, which is derived from methyl red by a one-step synthesis. We have devised a method of forming covalently attached monolayers of this material on glass by an amine-assisted condensation reaction involving the triethoxysilane end of the molecule. A nematic liquid crystal (LC) cell made with the monolayer and a rubbed polymer layer was switched from a uniform state to a twisted state with a polarized 450 nm control beam having a dose of 5.5 mJ/cm(2). This is equivalent to an average of only one absorbed photon per azobenzene group. Through atomic force microscopy, absorption spectroscopy, spectroscopic ellipsometry, and second harmonic generation experiments, we have confirmed that layers of this type are smooth and uniform with a surface coverage consistent with a monolayer and that the azobenzene groups are tilted, on average, 55 degrees with respect to the surface normal. These characteristics lead to a large interaction energy density between the layer and LC. The monolayer's rapid response in developing anisotropy in this property can be attributed to a large absorption cross section, as well as the favorable tilt angle, which allows for sufficient photoisomerization free volume in a dense layer.

Surface modification of ZnO using triethoxysilane-based molecules

Zinc oxide (ZnO) is an important material for hybrid inorganic-organic devices in which the characteristics of the interface can dominate both the structural and electronic properties of the system. These characteristics can be modified through chemical functionalization of the ZnO surface. One of the possible strategies involves covalent bonding of the modifier using silane chemistry. Whereas a significant body of work has been published regarding silane attachments to glass and SiO2, there is less information about the efficacy of this method for controlling the surface of metal oxides. Here we report our investigation of molecular layers attached to polycrystalline ZnO through silane bonding, controlled by an amine catalyst. The catalyst enables us to use triethoxysilane precursors and thereby avoid undesirable multilayer formation. The polycrystalline surface is a practical material, grown by sol-gel processing, that is under active exploration for device applications. Our study included terminations with alkyl and phenyl groups. We used water contact angles, infrared spectroscopy, and X-ray photoemission spectroscopy to evaluate the modified surfaces. Alkyltriethoxysilane functionalization of ZnO produced molecular layers with submonolayer coverage and evidence of disorder. Nevertheless, a very stable hydrophobic surface with contact angles approaching 106 degrees resulted. Phenyltriethoxysilane was found to deposit in a similar manner. The resulting surface, however, exhibited significantly different wetting as a result of the nature of the end group. Molecular layers of this type, with a variety of surface terminations that use the same molecular attachment scheme, should enable interface engineering that optimizes the chemical selectivity of ZnO biosensors or the charge-transfer properties of ZnO-polymer interfaces found in oxide-organic electronics.

Second-harmonic generation from rubbed ferroelectric liquid crystal mesogenic monolayer surfaces

We have designed self-assembled monolayers of molecules containing a ferroelectric liquid crystal mesogen attached to a glass surface through an alkane chain. After mechanical rubbing these layers induce a single domain in a cell containing a high-polarization achiral liquid crystal in the smectic-C phase. We have used optical second-harmonic generation to demonstrate that this behavior is explained by rubbing-induced in-plane anisotropy of the angular distribution function that describes the ensemble of mesogenic units. The surface order parameter is 0.094, a substantial fraction of what has been observed for rubbed polymeric alignment layers.