To facilitate future novel devices incorporating rare earth metal films and III-V semiconductors on Si substrates, this study investigates the mechanisms of growth via molecular beam epitaxy of gadolinium (Gd) on aluminum nitride (AlN) by determining the impact of substrate temperature on microstructure. The Gd films underwent extensive surface analysis via in situ reflective high energy electron diffraction (RHEED) and ex-situ SEM and AFM. Characterization of the surface features of rare earth metal films is important, as surface geometry has been shown to strongly impact magnetic properties. SEM and AFM imaging determined that Gd films grown on AlN (0001) from 80 °C to 400 °C transition from wetting, nodular films to island–trench growth mode to reduce in-plane lattice strain. XRD and Raman spectroscopy of the films revealed that they were primarily comprised of GdN, Gd and Gd2O3. The samples were also analyzed by a vibrating sample magnetometer (VSM) at room temperature. From the room temperature magnetic studies, the thick films showed superparamagnetic behavior, with samples grown between 240 °C and 270 °C showing high magnetic susceptibility. Increasing GdN (111) 2θ peak position and single-crystal growth modes correlated with increasing peak magnetization of the thin films, suggesting that lattice strain in single-crystal films was the primary driver of enhanced magnetic susceptibility.

Plasmon-exciton coupling in hetero-bilayer of WSe2 and WS2 transferred onto Au nanorod arrays is studied. Dark-field scattering measurements reveal that the in-plain dipole moment of excitons in monolayer WS2 allows only the narrow spectral range of 30 nm for the resonant coupling between the localized particle plasmons from Au nanorods and the bright excitons from WS2. We demonstrate that the q-parameter that represents the asymmetry of Fano resonances from plasmon-exciton coupling can be controlled by the polarization states of incident light. Surface lattice resonances in between individual Au nanorods play a role to diminish the damping factor of plasmon-exciton coupling in the arrays.

In this work, we introduce a new design of the interdigitated electrode (IDE) for the measurement of conductivity of ionomer thin film on the planar carbon surface, which is relevant to many electrochemical systems including fuel cells. The design is an advancement over commonly used SiO2 substrate based IDE, previously employed for Nafion thin film conductivities. Electrochemical impedance spectroscopy technique was applied to measure the conductivity of 55 nm thick Nafion film on carbon surface. Through complementary modeling work using equivalent circuit, we identify that the competition between the double layer capacitances at ionomer/carbon and ionomer/electrode interfaces can mask the conventionally observed high-to-medium impedance arc observed for Nafion films on SiO2 substrate. This problem can be circumvented by using electrochemically active Pt electrode in hydrogen environment. Hence, this systematic study describes a method to determine the thin film conductivity on carbon substrate.

A simple two-step, shaking-assisted polydopamine (PDA) coating technique was used to impart polypropylene (PP) mesh with antimicrobial properties. In this modified method, a relatively large concentration of dopamine (20 mg ml−1) was first used to create a stable PDA primer layer, while the second step utilized a significantly lower concentration of dopamine (2 mg ml−1) to promote the formation and deposition of large aggregates of PDA nanoparticles. Gentle shaking (70 rpm) was employed to increase the deposition of PDA nanoparticle aggregates and the formation of a thicker PDA coating with nano-scaled surface roughness (RMS = 110 nm and Ra = 82 nm). Cyclic voltammetry experiment confirmed that the PDA coating remained redox active, despite extensive oxidative cross-linking. When the PDA-coated mesh was hydrated in phosphate saline buffer (pH 7.4), it was activated to generate 200 μM hydrogen peroxide (H2O2) for over 48 h. The sustained release of low doses of H2O2 was antibacterial against both gram-positive (Staphylococcus epidermidis) and gram-negative (Escherichia coli) bacteria. PDA coating achieved 100% reduction (LRV ~3.15) when incubated against E. coli and 98.9% reduction (LRV ~1.97) against S. epi in 24 h.

Smart adhesive hydrogels containing 10 mol% each of dopamine methacrylamide (DMA) and 3-acrylamido phenylboronic acid (APBA) were polymerized in situ onto polydimethylsiloxane (PMDS) micropillars with different aspect ratios (AR = 0.4, 1 and 2). Using Johnson-Kendall-Roberts (JKR) contact mechanics tests, the adhesive-coated pillars demonstrated strong wet adhesion at pH 3 (Wadh = 420 mJ m-2) and can be repeatedly deactivated and reactivated by changing the pH value (pH 9 and 3, respectively). When compared to the bulk adhesive hydrogel of the same composition, the adhesive-coated pillars exhibited a significantly faster rate of transition (1 min) between strong and weak adhesion. This was attributed to an increased surface area to volume ratio of the adhesive hydrogel-coated pillars, which permitted rapid diffusion of ions into the adhesive matrix to form or break the catechol-boronate complex.

Many biological lab-on-a-chip applications require electrical and optical manipulation as well as detection of cells and biomolecules. This provides an intriguing challenge to design robust microdevices that resist adverse electrochemical side reactions yet achieve optical transparency. Physical isolation of biological samples from microelectrodes can prevent contamination, electrode fouling, and electrochemical byproducts; thus this manuscript explores hafnium oxide (HfO2) films - originating from traditional transistor applications - for suitability in electrokinetic microfluidic devices for biological applications. HfO2 films with deposition times of 6.5, 13, and 20 min were sputter deposited onto silicon and glass substrates. The structural, optical, and electrical properties of the HfO2 films were investigated using atomic force microscopy (AFM), X-ray diffraction, energy dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, ellipsometry, and capacitance voltage. Electric potential simulations of the HfO2 films and a biocompatibility study provided additional insights. Film grain size after corrosive Piranha treatment was observed via AFM. The crystalline structure investigated via X-ray diffraction revealed all films exhibited the (111) characteristic peak with thicker films exhibiting multiple peaks indicative of anisotropic structures. Energy dispersive X-ray spectroscopy via field emission scanning electron microscopy and Fourier transform infrared spectroscopy both corroborated the atomic ratio of the films as HfO2. Ellipsometry data from Si yielded thicknesses of 58, 127, and 239 nm and confirmed refractive index and extinction coefficients within the normal range for HfO2 ; glass data yielded unreliable thickness verifications due to film and substrate transparency. Capacitance-voltage results produced an average dielectric constant of 20.32, and the simulations showed that HfO2 dielectric characteristics were sufficient to electrically passivate planar microelectrodes. HfO2 biocompatibility was determined with human red blood cells by quantifying the hemolytic potential of the HfO2 films. Overall results support hafnium oxide as a viable passivation material for biological lab-on-a-chip applications.

Boron-doped silicon nanowires (SiNWs) grown by the vapor-liquid-solid growth mechanism using silicon tetrachloride (SiCl4) as the silicon precursor and trimethylboron (TMB) as the boron source were studied to understand the axial and radial doping uniformity. TMB-doped SiNWs with diameters up to 400 nm and lengths > 7.5 μm were integrated into a global back-gated test structure with multiple electrodes for electrical characterization. From gate modulated measurements, the SiNWs were confirmed to be heavily doped p-type. Multiple four point resistivity measurements across a total length of 7.5 μm were taken on as-grown SiNWs. Resistivity, corrected for surface charge, was determined to be 0.01 +/− 0.002 Ω cm along the entire length of the as-grown boron doped SiNWs. This was also observed in the axial direction for etched SiNWs, with corrected resistivity of 0.01 +/− 0.003 Ω cm, therefore confirming the uniform p-type doping of SiNWs using TMB and SiCl4 as precursors.

Photovoltaic (PV) devices based on arrays of silicon wires with diameters in the nanometer to micron range have sparked considerable interest due to their attractive light trapping characteristics and the prospects to enhance carrier collection in radial junction structures. This chapter reviews fabrication techniques, design and testing considerations, and the PV performance of silicon wire devices. Common bottom-up and top-down wire fabrication methods are initially reviewed including vapor–liquid–solid growth, metal-assisted chemical etching, and deep reactive ion etching. Design considerations for wire array solar cells are discussed such as the impact of wire array geometry on light absorption and the effects of wire diameter and doping on PV performance. Device results for three common wire junction geometries are then reviewed including radial and axial junction wire structures and planar junctions with nanowire surface texturing. Methods to form radial junctions in silicon wires are compared such as the use of an electrolyte, thermal diffusion of dopants, and wire-coating techniques based on chemical vapor deposition. Applications for nanowire texturing of silicon monocrystalline and multicrystalline cells are described. The advances in device design and processing have resulted in continual improvements in the PV performance of silicon wire array devices to the point where efficiencies ranging from ~ 10% for radial junction cells up to as high as 22.1% for nanowire-textured devices have been reported.

Silicon dichalcogenides have an intriguing crystal structure consisting of long tetrahedral chains held together by van der Waals forces but the electronic and optical properties have been less explored. In the present work, bulk SiSe2, SiS2, and Si(SexS1−x)2 were synthesized by the congruent melt growth method and characterized by Raman spectroscopy, X-ray Diffraction and UV/visible/IR transmission measurements supported by first-principles calculations. First-principles calculations reveal a nearly linear decrease of band gap energy in Si(SexS1−x)2 with increasing Se content, i.e., from SiS2 to SiSe2 which corresponds with a blue-shift in the transmission spectra from bulk SiSe2 to Si(Se0.6S0.4)2, and to SiS2. Air stability tests demonstrate the formation of toxic H2Se/H2S gas from sample oxidation at room temperature upon exposure to ambient air, and great care should be paid when handling these materials.

Mixed phase nanocrystalline/amorphous-silicon (nc/a-Si:H) thin films with band-gap higher than bulk silicon are prepared by depositing silicon nanoparticles (SiNPs), prepared in a separate deposition zone, and hydrogenated amorphous silicon (a-Si:H), simultaneously. Since the two deposition phases are well decoupled, optimized parameters for each component can apply to the growth process. Photoluminescence spectroscopy (PL) shows that the embedded SiNPs are small enough to exhibit quantum confinement effects. The low temperature PL measurements on the mixed phase reveal a dominant emission feature, which is associated with SiNPs surrounded by a-Si:H. In addition, we compare time dependent low temperature PL measurements for both a-Si:H and mixed phase material under intensive laser exposure for various times up to two hours. The PL intensity of a-Si:H with embedded SiNPs degrades much less than that of pure a-Si:H. We propose this improvement of photostability occurs because carriers generated in the a-Si:H matrix quickly transfer into SiNPs and recombine there instead of recombining in a-Si:H and creating defect states (Staebler-Wronski Effect).

Developing silicon nanoparticle (SiNP) synthesis techniques that allow for straightforward control of nanoparticle size and associated optical properties is critical to potential applications of these materials. In addition, it is, in general, hard to probe the absorption threshold in these materials due to silicon's low absorption coefficient. In this study, size is controlled through direct introduction of sulfur hexafluoride (SF6) into the dilute silane precursor of plasma synthesized SiNPs. Size reduction by nearly a factor of two with high crystallinity independent of size is demonstrated. The optical absorption spectra of the SiNPs in the vicinity of the bandgap are measured using photothermal deflection spectroscopy. Bandgap as a function of size is extracted taking into account the polydispersity of the samples. A systematic blue shift in absorption edge due to quantum confinement in the SiNPs is observed with increasing flow of SF6. Photoluminescence (PL) spectra show a similar blue shift with size. However, a ∼300 meV difference in energy between emission and absorption for all sizes suggests that PL emission involves a defect related process. This shows that, while PL may allow size-induced shifts in the bandgap of SiNPs to be monitored, it cannot be relied on to give an accurate value for the bandgap as a function of size.

Single wire p(+)-n(+) radial junction nanowire solar cell devices were fabricated by low pressure chemical vapor deposition of n(+) silicon shell layers on p(+) silicon nanowires synthesized by vapor-liquid-solid growth. The n(+)-shell layers were deposited at two growth temperatures (650 °C and 950 °C) to study the impact of shell crystallinity on the device properties. The n-type Si shell layers deposited at 650 °C were polycrystalline and resulted in diodes that were not rectifying. A pre-coating anneal at 950 °C in H2 improved the structural quality of the shell layers and yielded diodes with a dark saturation current density of 3 × 10(-5) A cm(-2). Deposition of the n-type Si shell layer at 950 °C resulted in epitaxial growth on the nanowire core, which lowered the dark saturation current density to 3 × 10(-7) A cm(-2) and increased the solar energy conversion efficiency. Temperature-dependent current-voltage measurements demonstrated that the 950 °C coated devices were abrupt junction p(+)-n(+) diodes with band-to-band tunneling at high reverse-bias voltage, while multi-step tunneling degraded the performance of devices fabricated with a 950 °C anneal and 650 °C coating. The higher trap density of the 950 °C annealed 650 °C coated devices is believed to arise from the polycrystalline nature of the shell layer coating, which results in an increased density of dangling bonds at the p(+)-n(+) junction interface.

A dual plasma reactor has been developed for depositing nanocrystalline silicon with quantum confined silicon nanoparticles, either by sequential or concurrent deposition of amorphous silicon and silicon nanoparticles. Sequential deposition allows for complete decoupling of the amorphous and nanoparticle deposition and well-defined layers of silicon nanoparticles between amorphous silicon. The concurrent deposition is similar to conventional deposition of nanocrystalline silicon, which allows for complete mixing of the silicon nanoparticles and amorphous silicon. With the introduction of the silicon nanoparticles into the amorphous silicon, we observe a quenching of the photoluminescence, which has been reported in the literature to be from the transfer of charge from the amorphous silicon to silicon crystallites.

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.

Quantum confined silicon nanoparticles (SiNPs) may allow for the fabrication of an all silicon multiple junction photovoltaic cell, or novel photovoltaic cells that surpasses the single junction limitations. One method of growing SiNPs is by PECVD where the plasma is confined to a quartz tube. To further understand this technique, computational fluid dynamic calculation were coupled with experimental growths, which allows for the development of a phase diagram based around the reactor pressure, and gas flow. SiNP size and crystallinity were evaluated using a combination of Raman scattering, X-ray diffraction, and transmission electron microscopy (TEM), these results were used to further the develop the phase diagram. For residence times less than 1 ms, SiNPs were produced a band gap of >1.6 eV, but very low crystallinity. By maintaining a constant low gas flow (100 sccm) and increasing reactor pressure, crystallinity and size of the SiNPs were improved and increased, respectively. For SiNPs grown with a constant residence time (~2 ms), we observe a reduction in the crystallinity with increased gas flow. Closer inspection by TEM showed a significant amount of a-Si with the SiNPs. Using this growth technique we can reliably grow SiNPs with diameters from 3 nm to 8 nm, relating to a photoluminescence peak position from 1.6 eV to 1.3 eV. This range is ideal for the development of an all silicon tandem photovoltaic cell.

Radial junction Si pillar array solar cells based on the heterojunction with intrinsic thin layer (HIT) structure were fabricated from p-type crystal Si (c-Si) wafers of different doping densities. The HIT structure consisting of intrinsic/n-type hydrogenated amorphous Si (a-Si:H) deposited by plasma-enhanced chemical vapor deposition (PECVD) at low temperature (200°C) was found to effectively passivate the high surface area of the p-type Si pillar arrays resulting in open circuit voltages (V oc >0.5) comparable to that obtained on planar devices. At high c-Si doping densities (>10 18 cm -3 ), the short-circuit current density (J sc ) and energy conversion efficiency of the radial junction devices were higher than those of the planar devices demonstrating improved carrier collection in the radial junction structure.