Stuart Pearce

Highly sensitive biosensor based on UV-imprinted layered polymeric–inorganic composite waveguides

An evanescent field sensor utilizing layered polymeric-inorganic composite waveguide configuration was developed in this work. The composite waveguide structure consists of a UV-imprint patterned polymer inverted rib waveguide with a Ta2O5 thin film sputter-deposited on top of the low refractive index polymer layers. The results suggest that the polymer based sensor can achieve a detection limit of 3 × 10(-7) RIU for refractive index sensing and corresponding limit of about 100 fg/mm2 for molecular adsorption detection. Besides enhancing the sensitivity significantly, the inorganic coating on the polymer layer was found to block water absorption effectively into the waveguide resulting in a stabilized sensor operation. The ability to use the developed sensor in specific molecular detection was confirmed by investigating antibody - antigen binding reactions. The results of this work demonstrate that high performance sensing capability can be obtained with the developed composite waveguide sensor.

Area Selective Growth of Titanium Diselenide Thin Films into Micropatterned Substrates by Low-Pressure Chemical Vapor Deposition.

The neutral, distorted octahedral complex [TiCl4(SenBu2)2] (1), prepared from the reaction of TiCl4 with the neutral SenBu2 in a 1:2 ratio and characterized by IR and multinuclear (1H, 13C{1H}, 77Se{1H}) NMR spectroscopy and microanalysis, serves as an efficient single-source precursor for low-pressure chemical vapor deposition (LPCVD) of titanium diselenide, TiSe2, films onto SiO2 and TiN substrates. X-ray diffraction patterns on the deposited films are consistent with single-phase, hexagonal 1T-TiSe2 (P3̅m1), with evidence of some preferred orientation of the crystallites in thicker films. The composition and structural morphology was confirmed by scanning electron microscopy (SEM), energy dispersive X-ray, and Raman spectroscopy. SEM imaging shows hexagonal plate crystallites growing perpendicular to the substrate, but these tend to align parallel to the surface when the quantity of reagent is reduced. The resistivity of the crystalline TiSe2 films is 3.36 ± 0.05 × 10–3 Ω·cm with a carrier density of 1 × 1022 cm–3. Very highly selective film growth from the reagent was observed onto photolithographically patterned substrates, with film growth strongly preferred onto the conducting TiN surfaces of SiO2/TiN patterned substrates. TiSe2 is selectively deposited within the smallest 2 μm diameter TiN holes of the patterned TiN/SiO2 substrates. The variation in crystallite size with different diameter holes is determined by microfocus X-ray diffraction and SEM, revealing that the dimensions increase with the hole size, but that the thickness of the crystals stops increasing above ∼20 μm hole size, whereas their lengths/widths continue to increase.

Non-aqueous electrodeposition of functional semiconducting metal chalcogenides: Ge2Sb2Te5 phase change memory

We report a new method for electrodeposition of device-quality metal chalcogenide semiconductor thin films and nanostructures from a single, highly tuneable, non-aqueous electrolyte. This method opens up the prospect of electrochemical preparation of a wide range of functional semiconducting metal chalcogenide alloys that have applications in various nano-technology areas, ranging from the electronics industry to thermoelectric devices and photovoltaic materials. The functional operation of the new method is demonstrated by means of its application to deposit the technologically important ternary Ge/Sb/Te alloy, GST-225, for fabrication of nanostructured phase change memory (PCM) devices and the quality of the material is confirmed by phase cycling via electrical pulsed switching of both the nano-cells and thin films.

Non-aqueous electrodeposition of p-block metals and metalloids from halometallate salts

A versatile electrochemical system for the non-aqueous electrodeposition of crystalline, oxide free p-block metals and metalloids is described, and it is demonstrated that by combining mixtures of these reagents, this system is suitable for electrodeposition of binary semiconductor alloys. The tetrabutylammonium halometallates, [NnBu4][InCl4], [NnBu4][SbCl4], [NnBu4][BiCl4], [NnBu4]2[SeCl6] and [NnBu4]2[TeCl6], are readily dissolved in CH2Cl2 and form reproducible electrochemical systems with good stability in the presence of a [NnBu4]Cl supporting electrolyte. The prepared electrolytes show a wide potential window and the electrodeposition of indium, antimony, bismuth, tellurium and selenium on glassy carbon and titanium nitride electrodes has been demonstrated. The deposited elements were characterised by scanning electron microscopy, energy dispersive X-ray analysis and powder X-ray diffraction. The compatibility of the reagents permits the preparation of a single electrolyte containing several halometallate species which allows the electrodeposition of binary materials, as is demonstrated for InSb. This room temperature, ‘bottom-up’ electrochemical approach should thus be suitable for the one-pot deposition of a wide range of compound semiconductor materials.

Manipulation of optical field distribution in layered composite polymeric-inorganic waveguides

We discuss the manipulation of optical field distribution in a low-refractive index polymeric waveguide by depositing a thin high refractive index Ta2O5 film on top of the waveguide. According to microstructure studies, the sputtered Ta2O5 thin films deposited on the polymer layer were very smooth with root mean square surface roughness value of 0.58 nm, had amorphous phase, and were optimal for integrated optical devices. Both computational and experimental optical studies suggest that the inorganic-polymeric composite waveguide design greatly increases the intensity distribution of the propagating mode at the surface. Consequently, the interaction of the optical field with the ambient surrounding medium is enhanced by a factor of about 1.7 in order of magnitude.

Three-Mask Polysilicon Thin-Film Transistor Biosensor

Biosensors are commonly produced using a siliconon-insulator (SOI) CMOS process and advanced lithography to define nanowires. In this paper, a simpler and cheaper junctionless three-mask process is investigated, which uses thin-film technology to avoid the use of SOI wafers, in situ doping to avoid the need for ion implantation and direct contact to a low-doped polysilicon film to eliminate the requirement for heavily doped source/drain contacts. Furthermore, TiN is used to contact the biosensor source/drain because it is a hard resilient material that allows the biosensor chip to be directly connected to a printed circuit board without wire bonding. pH sensing experiments, combined with device modeling, are used to investigate the effects of contact and series resistance on the biosensor performance, as this is a key issue when contacting directly to low-doped silicon. It is shown that in situ phosphorus doping concentrations in the range 4 × 1017-3 × 1019 cm-3 can be achieved using 0.1% PH3 flows between 4 and 20 sccm. Furthermore, TiN makes an ohmic contact to the polysilicon even at the bottom end of this doping range. Operation as a biosensor is demonstrated by the detection of C-reactive protein, an inflammatory biomarker for respiratory disease.

Self-modulation of ultra-fast laser pulses with 1550 nm central wavelength in VO2 thin films

The possibility to use an ultra-fast laser operating at 1550 nm wavelength to induce intensity self-modulation in metal-insulator phase transition VO2 thin films was investigated. The results show that a self-modulation value upto 0.55 can be obtained by using z-scan method. In comparison, an externally triggered phase transition induced by heating the sample produced a modulation depth of 0.995 corresponding to almost complete light absorption. The results suggest that significant self-modulation can be produced by fs laser pulses, but the modulation strength is partially suppressed by incomplete transition from a transparent to an absorbing state and potentially time delay in the rise of absorbance.

3D analysis of surface plasmon dispersion for SERS sensor based on inverted pyramid nanostructures

Surface enhanced Raman scattering (SERS) can be used to amplify the Raman cross-section of signals by several orders of magnitude, when a mixed photon-Plasmon mode (surface Plasmon polaritons) couples to molecules on a nano textured metallo-dielectric substrate. In this paper we demonstrate a comprehensive 3D computational model based on Rigorous coupled wave analysis (RCWA) for the purpose of analysing propagating and localised surface Plasmon polaritons supported by planar SERS substrates based on periodic array of metal coated inverted pyramidal nanostructures. Although studies [1, 2] have explored the optical properties of inverted square pyramidal pits using simulation and experimentation, there has yet been no investigation performed on rectangular inverted pyramidal pits. Here we perform 3D modelling and simulation on rectangular pit arrays with aspect ratio 1:1.2 over 400nm thick gold. We investigate the effect of incident polarisation and electric-field density within the pits and show that inverted rectangular pyramidal pit array can be used as highly effective SERS and Plasmonic substrates.

Spectroscopy of ytterbium-doped tantalum pentoxide rib waveguides on silicon

The design, fabrication and spectroscopic characterization of ytterbium-doped Ta2O5 rib waveguide are described. The waveguides are fabricated on silicon substrates and operate in a single mode at wavelengths above 970 nm. The peak absorption cross-section was measured to be 2.75 × 10−20 cm2 at 975 nm. The emission spectrum was found to have a broad fluorescence spanning from 990 nm to 1090 nm with the fluorescence emission peak occurring at a wavelength of 976 nm. The excited-state life time was measured to be approximately 260 µs.

Nanostructured surface enhanced Raman scattering sensor platform with integrated waveguide core

We present a planar waveguide based sensor capable of simultaneous surface enhanced Raman scattering (SERS)/surface plasmon resonance (SPR) sensing methodologies. The sensor consists of a nanostructured area etched into a low loss planar waveguide fabricated from silicon oxynitride. The selective deposition of the 25 nm thick gold film on the nanostructured features was applied to create the SERS/SPR active sites. In this work, we adapt the SPR approach, coupling light propagating along the slab waveguide to the nano-textured area from underneath. The shapes of the nanostructures, thickness, and morphology of the gold coating are chosen to be suitable for SERS and SPR. Effects of geometric parameters associated with the nanostructured features such as diameters, length, and pitch were investigated. Detection of Benzyl Mercaptan was accomplished using a 785 nm laser in a SERS configuration excited from the underlying waveguide core. The detection of the analyte was confirmed by normal incident SERS measure...