K. Giannakopoulos

Polymer/carbon nanotube composite patterns via laser induced forward transfer

Direct and high spatial resolution printing of polymer/carbon nanotube (CNT) composite layers has been demonstrated by means of laser induced forward transfer (LIFT). Laser irradiation of composite target materials, such as poly(acrylic acid)/CNT and polyvinylpyrrolidone/CNT, enabled dry deposition of well resolved composite pixels onto glass substrates. The dispersion of the CNT into the deposited composite pixels was investigated by transmission electron microscopy. The LIFT technique was also employed for the accurate deposition of polymer/CNT composite pixels onto aluminum microelectrodes for the fabrication of chemical sensors based on polymer/CNT compounds.

Ultra-thin porous anodic alumina films with self-ordered cylindrical vertical pores on a p-type silicon substrate

Ultra-thin alumina films with self-ordered cylindrical vertical pores were fabricated on a p-type silicon substrate by anodization of Al films with thickness in the range of 30–500 nm in sulfuric or oxalic acid aqueous solutions. In both cases the pores were arranged in hexagonal cells in a close-packed structure and their diameter and density depended on the electrochemical solution used. In the case of sulfuric acid both 30 and 500 nm Al films resulted in a similar uniform porous structure using exactly the same anodization conditions for both thicknesses, the pore diameter being in the range of 10–30 nm and their density of the order of 6–8 × 1010 pores cm−2. In the case of oxalic acid the 500 nm thick films resulted in a uniform porous structure with larger pores than in sulfuric acid, of diameter in the range of 20–40 nm and a density of the order of . On the other hand, with oxalic acid it was impossible to form a uniform porous structure from the 30 nm thick Al film at the same conditions as used for the 500 nm thick film. Plan-view and cross-sectional transmission electron microscopy was used to investigate systematically the structure and morphology of the alumina films. Cross-sectional TEM images showed that the alumina/Si interface was sharp, but a void was observed beneath each pore, separated from the pore by a thin alumina layer. The same structure was obtained with both electrolytes. The effect of pre-annealing of the Al films on the anodic alumina layers was also investigated in detail.

High strain sensitivity controlled by the surface density of platinum nanoparticles

We report a controllable strain gauge factor obtained using a two-dimensional nanoparticle layer formed from platinum nanoparticles. A vacuum technique is used for room temperature nanoparticle deposition that allows control of the electrical resistance of the film, exhibiting semiconducting-like behavior when nanoparticle arrays cover the surface below a threshold value while above it a metallic behavior is prevalent. The highest sensitivity is obtained for intermediate density values of the nanoparticle assemblies, which could be explained using a tunneling and hopping current expression. The device, which exhibits more than one order of magnitude higher strain sensitivity than continuous metallic films, is fabricated at room temperature through standard lithographic processing allowing for miniaturization and easy integration in silicon technology or flexible substrates.

Porous hot-wire deposited WO3 films with high optical transmission

Tungsten oxide films were deposited on Si and fused silica substrates by heating metallic filaments at temperatures of 650, 750, and 800 °C at a pressure of 1 Torr of N2. During deposition the substrates remained at or near room temperature. These hot-wire (hwWO3) films were found to be composed by amorphous material and highly transparent within the range 350−1000 nm. Spectroscopic ellipsometry measurements have shown that the real part of refractive index (n) of hwWO3 films exhibited features similar to those of stoichiometric WO3 films indicating that hwWO3 films were also stoichiometric. The values of n were found to depend on deposition time (film thickness) and after 2 s, have fallen below 1.45 within the visible range, while the imaginary part (k) remained near zero. These low values of n and k were attributed to the porosity of hwWO3 films, which as shown by simulations based on the effective medium approximation, after 2 s of deposition saturated near 60%. As shown by scanning electron microscopy...

Simple method for the fabrication of a high dielectric constant metal-oxide-semiconductor capacitor embedded with Pt nanoparticles

We present a simple method for the fabrication of Pt nanoparticles embedded in a high-k dielectric. The nanoparticles are formed during the first deposition stages of a thin Pt layer on a 30A SiO2 tunneling layer, at room temperature, performed with electron-beam (e-beam) evaporation of metallic Pt. Then, the nanoparticles are covered, in situ, by a thicker HfO2 layer, which forms a control oxide. The fabricated nanoparticles have an average diameter of 4.9nm, sheet density of 3.2×1012cm−2 and they present high uniformity in their size. High-frequency capacitance-voltage (C-V) measurements demonstrate that this structure operates as a memory device.

Growth of two-dimensional arrays of silicon nanocrystals in thin SiO2 layers by low pressure chemical vapour deposition and high temperature annealing/oxidation. Investigation of their charging properties

Two-dimensional arrays of silicon nanocrystals embedded in ultrathin SiO2 layers for application in silicon nanocrystal memories were fabricated by a three-step process: (a) growth of a tunnelling silicon oxide, (b) low pressure chemical vapour deposition (LPCVD) of a thin layer of amorphous silicon (α-Si), and (c) solid phase crystallization of the α-Si layer in a high temperature furnace under nitrogen flow, followed by thermal oxidation in the same furnace. Transmission electron microscopy (TEM) was used for the structural characterization of the three-layer structure and the determination of layer thicknesses and silicon nanocrystal size, while capacitance–voltage (C–V) and current–voltage (I–V) measurements were used to investigate the charging properties of the silicon nanocrystal layer. In an attempt to increase the silicon nanocrystal density, as suggested in the literature, a dip of the oxidized wafer in diluted HF before LPCVD deposition was used, but this step was found to seriously affect the charging properties of the structure.

Coevaporation of CoPt nanoparticles

Co50Pt50 nanoparticles were codeposited on thermally oxidized Si substrates by electron beam evaporation, at a temperature of the substrate of 700to750°C. The codeposition led to a perfect mixture of the Co and Pt elements within the three-dimensional nanoislands, which exhibit a mean diameter between ∼18 and ∼20nm. The postannealing treatment of the CoPt nanograins resulted in the progressive crystallization of the L10 ordered phase and, consequently, in the progressive magnetic hardening of the samples with a maximum coercivity of ∼5.6kOe.

Synthesis and characterization of Cr–B–N coatings deposited by reactive arc evaporation

Nanocomposite Cr–B–N coatings were deposited from CrB 0.2 compound targets by reactive arc evaporation using an Ar/N 2 discharge at 500 °C and −20 V substrate bias. Elastic recoil detection (ERDA), x-ray photoelectron spectroscopy (XPS), x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and selected-area electron diffraction (SAED) were used to study the effect of the N 2 partial pressure on composition and microstructure of the coatings. Cross-sectional scanning electron microscopy (SEM) showed that the coating morphology changes from a glassy to a columnar structure with increasing N 2 partial pressure, which coincides with the transition from an amorphous to a crystalline growth mode. The saturation of N content in the coating confirms the formation of a thermodynamically stable CrN–BN dual-phase structure at higher N 2 fractions, exhibiting a maximum in hardness of approximately 29 GPa.

Sol–gel synthesized, low-temperature processed, reduced molybdenum peroxides for organic optoelectronics applications

Reduced molybdenum peroxides with varying degrees of reduction were synthesized following a modified sol–gel peroxo method and the respective films were employed as anode interfacial layers in organic optoelectronics applications, such as organic light emitting diodes (OLEDs) and organic photovoltaics (OPVs). The degree of reduction was controlled through both the synthesis route and the thermal treatment protocol of the obtained films. The films were thoroughly investigated with a variety of spectroscopic, diffraction, and electron microscopy methods (UV-Vis, FT-IR, XPS, UPS, Raman, XRD, SEM, and TEM). These films were found to be considerably sub-stoichiometric with a relatively high content of hydrogen. When they were used as anode interfacial layers in OLED and OPV devices, high efficiencies and adequate temporal stability were achieved. The enhanced hole injection/extraction properties of the reduced molybdenum peroxide films were attributed to the improved charge transport facilitated through the gap states present in these materials.

Low temperature growth of single-crystal ZnO nanorods

ZnO nanorods are grown on large-area glass substrates with a ZnO overlayer by an aqueous solution method at temperatures ranging from 65 °C down to room temperature. The structure and morphology of the nanorods is studied with electron microscopy. They are single crystalline and grow preferentially perpendicular to the substrate. Their aspect ratio can be tailored by modifying the growth parameters. Self-assembly and subsequent selective growth of the nanorods is accomplished in pre-patterned substrates. Room temperature photoluminescence measurements reveal a strong band edge emission at 378 nm, while no defect-related visible emission is detected except for the nanorods grown at RT.