Emmanuel Spanakis

Silicon electron emitters fabricated by ultraviolet laser pulses

In this letter we consider the effect of laser pulse duration on the surface morphology and the field emission properties of silicon structured by UV laser pulses. In three different pulse duration regimes ranging from sub-ps to ns, we altered the morphology of the fabricated silicon microspike arrays. The field emission properties of the microspike arrays were influenced by the morphological changes exhibiting a reduction of the emission threshold field to 2.5V∕μm for 15ns laser pulses. The ability of tuning the field emission properties of laser-fabricated silicon microspike arrays makes them excellent candidates for use as field emission cathodes.

Field Emission Properties of Low-Temperature, Hydrothermally Grown Tungsten Oxide

Tungsten oxide layers have been prepared on conductive glass substrates using aqueous chemical growth from a sodium tungstate precursor at low-temperature hydrothermal conditions. The deposits were then tested as cold electron emitters. Traceable layers could be deposited only within a narrow pH range of 1.5-2 at a time length not exceeding 4 h. Transmittance in the visible spectrum was found to decrease with deposition time. The presence of both monoclinic and hexagonal phases was always detected. At the longest deposition times and highest precursor concentrations, morphologies comprise randomly oriented spikes or rods. The overall emission performance is found to improve with growth time and precursor concentration. The role of morphology on the emission properties of the films is discussed.

Photoinduced stress in hydrogenated amorphous silicon films

Photo-induced compressive stress ΔS in hydrogenated amorphous silicon (a-Si:H) has been studied using films deposited by plasma-enhanced or hot-wire chemical vapor deposition on crystalline silicon microcantilevers. The kinetics of ΔS(t) first rises with exposure time as t0.5 and follows a stretched exponential. The saturation values ΔSsat correspond to volume changes of about 10−3, which excludes the possibility that ΔS is a consequence of the light-induced creation of coordination defects. The highest-quality films have large initial stress, small values of the Young’s modulus, and a rapid approach of ΔS(t) towards saturation.

Stress and internal friction associated with light-induced structural changes of a-Si:H deposited on crystalline silicon microcantilevers

The study of light-induced changes of the mechanical properties of a-Si:H should be valuable in the search for the microscopic mechanisms behind the Staebler-Wronski (SW) effect. We have developed a sensitive technique for studying such changes by depositing a-Si:H films onto commercial scanning probe microscope Si microcantilevers. The detection system of the microscope provides for measurements of beam bending, oscillation resonant frequency, and in-resonance damping factor. The internal friction of an a-Si:H film is much larger than that of crystalline Si and is the largest damping factor of the bilayer beam. We observed an increase in relative volume, ΔV/V, with photocarrier generation rate, G, and exposure time, t, following ΔV/V G 0.7 t 0.45 in intrinsic as well as in 1 ppm PH 3 /SiH 4 doped a-Si:H. The volume changes could be reversed by annealing and were the same for CW and pulsed light exposures using 400 μs long square pulses at a rate of 200 s -1 . Based on the magnitude of ΔV/V and the fact that it does not saturate we suggest that the structural changes causing ΔV/V permeate the whole film and are not limited to defect sites.

Imaging dielectric properties of Si nanowire oxide with conductive atomic force microscopy complemented with femtosecond laser illumination.

In most Si nanowire (NW) applications, Si oxide provides insulation or a medium of controlled electron tunneling. This work revealed both similarities and differences in the dielectric properties of NW oxide compared with that grown on wafers. The interface barrier to electron transit from the semiconductor to the dielectric and the threshold electric field for current flow are quite similar to those in the planar geometry. This is not true for the lowest currents measured which are not uniformly distributed, indicating variations of trap density in the gap of NW oxide.

Polymer-nanotube composite mats with improved field emission performance and stability.

The results of electron field emission from single walled carbon nanotubes (SWCNTs) mats deposited on different composite films of SWCNTs and poly(3-octylthiophene) (P3OT) semiconducting polymer are presented. Three different structures were tested: (a) dense and sparse SWCNT mats on n+ -Si; (b) SWCNT mats on composite films with different SWCNT-P3OT ratios; (c) composite films with different SWCNT-P3OT ratios on n+ -Si. The experiments show that there is a critical SWCNT-P3OT concentration in which the field emission stability of SWCNT mats is remarkably improved with a small reduction in the emission threshold compared to the optimum pristine SWCNT film. The contribution of the composite film morphology as well as the role of polymer-nanotube interaction on the emission performance are evaluated. The physical mechanism behind the stability of composite field emitters is also discussed.

Light induced stress in a-Si1-xGex:H alloys and its correlation with the Staebler-Wronski effect

Abstract Photoinduced compressional stress ΔS has been studied in hydrogenated amorphous silicon and silicon–germanium alloys, a-Si1−xGex:H with x=0, 0.4 and 0.67. The films were deposited by plasma enhanced chemical vapor deposition onto 4 μm thick crystalline silicon microcantilevers commonly used in scanning probe microscopy. The initial stress S0 of the films was obtained from the initial bending and the Youngs modulus from the cantilever resonance frequency. The kinetics of ΔS(t) follow a stretched exponential. ΔS(t) cannot be a consequence of photoinduced defect creation because ΔS continues to rise when defect creation has saturated and the largest ΔS corresponds to a relative volume change ΔV/V=1×10−3, too large for 4×1017 cm−3 defects. We observe a significant decrease in ΔS between x=0.4 and x=0.67 alloy composition just where defect creation is greatly diminished. We suggest that defect creation is associated with the time dependent large local strains in and around the volume elements of electron–hole recombination. ΔS is the time and spatial average of the local configuration changes.

Atomic force microscopy based, multiphoton, photoelectron emission imaging

Images of photoelectron emission from metallic surfaces were obtained with a modified atomic force microscope operating in air. Illumination of the samples was achieved in the near field of a metal-coated microcantilever tip, placed in the beam of a femtosecond pulsed laser that is incident at a grazing angle with respect to the sample surface. Photoelectron currents were measured through the tip with a prototype amplifier. The power law dependence of average photocurrent on light intensity is compatible with multiphoton photoelectric effect and the work function of the metal covering a particular area on the two-metal patterned samples used.

Study of petal topography of Lysimachia arvensis grown under natural conditions

Abstract The present study revealed that adaxial and abaxial petal epidermises of the blue-flowered Lysimachia arvensis consist of elongated, multi-micro-papillate cells, which may aid the rapid petal expansion. The epidermal cells are covered by a wrinkled relief, which is further ornamented by submicron features that increase in size the surface area of lobes; this may be a well-adapted mechanism of the small-sized flowers of L. arvensis with the short life span. The sculpturally increased surface area of adaxial epidermal cells of petals is expected to contribute to optical and adhesive properties, and wettability of the floral tissues. The adaxial and the abaxial petal surfaces of L. arvensis possess submicron cuticular folds, smaller than the sub-wavelength visible spectrum, which reflect radiation of shorter rather than longer wavelengths, whereas intense absorption was detected in the red spectral region. Also, three-celled capitate trichomes with a pigmented spherical head, which are densely distributed at the corolla margins of L. arvensis, may be involved in adhesive, defensive and functional properties of the floral tissues.

Triethyllead-induced inhibition of proliferation of normal human lymphocytes through decreased expression of the Tac chain of interleukin 2 receptor.

Triethyllead (Et3Pb+) in concentrations 10(-5) to 10(-6) M has been shown to inhibit several key cellular molecular systems. In order to evaluate the effect of Et3Pb+ on the human immune system, the mitogen-induced cell proliferation of peripheral blood mononuclear cells was studied in the presence of Et3Pb+. Preincubation of normal T-lymphocytes with 10(-6) M Et3Pb+ for 1-4 h, which has been shown to be non-cytotoxic, was sufficient to inhibit subsequent mitogenic-induced cell growth. The Et3Pb(+)-induced impairement of the in vitro proliferation of mitogen-activated normal T-cells was due to a dose-dependent decreased expression of the p55 polypeptide chain (Tac molecule) of the interleukin-2 receptor (IL-2-R), which could not be enhanced by exogeneously added recombinant interleukin 2 (rIL-2). Conversely, the same concentrations of Et3Pb+ could not inhibit the mitogen-induced expression of MHC class II molecules and the transferin receptor on activated T-cells. The impaired membrane expression of p55 on T-cells induced by Et3Pb+ was due to a decrease of Tac mRNA transcripts as showed by Northern blot analysis. This effect seems to be specific since in parallel experiments Et3Pb+ could not inhibit both the accumulation of actin mRNA and the production of IL-2 by Et3Pb(+)-treated mitogen-activated cells. The effect of this organolead compound was also associated with a dose-dependent decrease of the Na(+)-K(+)-ATPase activity of normal lymphocytes. These results indicate that Et3Pb+ could affect specifically T-cell proliferative responses through an imbalance of the IL-2/IL-2-R system.