Room-temperature growth of carbon nanofibers on plastic substrates

A very simple method to synthesize densely distributed carbon nanofibers (CNFs) on flexible plastic substrates at room temperature with no catalyst is demonstrated. Carbon film was deposited onto polyimide, poly-ethylene-terephthalate (PET) films, Si plates and a Ni mesh, which were then sputtered with obliquely incident Ar⁺ ions at 3 keV at room temperature. Linear-shaped CNFs oriented in the incidence direction of the ion beam grew on the sputtered substrates, as confirmed by scanning (SEM) and transmission electron microscopes (TEM). CNF growth on a PET substrate, which is a non-heat-tolerant plastic, has never been reported so far. CNFs thus grown were characterized as amorphous without a hollow structure. The diameter of CNFs was almost identical (20-30 nm) despite a large difference in CNF length (0.1-4 μm). In addition, the CNF-tipped cones were demonstrated to act successfully as a template to fabricate one dimensional (1-D) zinc oxide (ZnO) nanostructures on a PET substrate. Thus, it was believed that the ion-irradiation technique would open up a new approach to fabricate any kinds of 1-D nanomaterials on flexible substrates at room temperature.     

Structural phase transition induced by ion irradiation in the equilibrium immiscible Ag–Co system

In the equilibrium immiscible Ag–Co system characterized by a large positive heat of formation (28 kJ/mol), interesting structural phase transitions were observed in Ag–Co multilayered films upon 200-keV xenon-ion irradiation at 300 K within a dose range of 5×10¹⁴ to 9×10¹⁵ Xe⁺/cm². The formation of a new metastable HCP phase and observation of other structural phase transitions upon ion irradiation were quite in accordance with first principles and thermodynamic calculations. It was observed that in the as-deposited Ag–Co multilayered films, the stress from the Co layers resulted in a condensation effect on the Ag lattice, and that after irradiation to a dose of 5×10¹⁴ Xe⁺/cm², the Ag lattice recovered to its original size along with the hcp Co transforming into its high-temperature-stabilized fcc structure. Besides, the mechanism of the metastable phase formation as well as the observed structural phase transitions induced by ion irradiation is discussed in terms of thermodynamics and growth kinetics. PACS 61.82.Bg; 64.60.My; 64.70.Kb     

A new structure of In-based ohmic contacts ton-type GaAs

Electrical, structural and reaction characteristics of In-based ohmic contacts ton-GaAs were studied. Attempts were made to form a low-band-gap interfacial phase of InGaAs to reduce the barrier height at the metal/semiconductor junction, thus yielding low-resistance, highly reliable contacts. The contacts were fabricated bye-beam sputtering Ni, NiIn and Ge targets on VPE-grownn ⁺-GaAs film (1 m, 2 × 10¹⁸ cm^–3) in ultrahigh vacuum as the structure of Ni(200 Å)/ NiIn(100 Å)/Ge(40 Å)/n ⁺-GaAs/SI-GaAs, followed by rapid thermal annealing at various temperatures (500–900°C). In this structure, a very thin layer of Ge was employed to play the role of heavily doping donors and diffusion limiters between In and the GaAs substrate. Indium was deposited by sputtering NiIn alloy instead of pure In in order to ensure In atoms to be distributed uniformly in the substrate; nickel was chosen to consume the excess indium and form a high-temperature alloy of Ni₃ln. The lowest specific contact resistivity ( _c) of (1.5 ± 0.5) × 10^–6 cm² measured by the Transmission Line Method (TLM) was obtained after annealing at 700°C for 10 s. Auger sputtering depth profile and Transmission Electron Microscopy (TEM) were used to analyze the interfacial microstructure. By correlating the interfacial microstructure to the electronical properties, In _x Ga_{1– x}As phases with a large fractional area grown epitaxially on GaAs were found to be essential for reduction of the contact resistance.This work was supported by the National Natural Sciences Foundation of China (NSFC)     

I–V curves of GdBaCuO/n-Si contact: Revealing of the structural phase transition in the GdBaCuO film at 210–220 K

TheI–V curves of GdBaCuO/n-Si structures have been studied as a function of the doping of silicon and as a function of temperature. The GdBaCuO films were deposited by a magnetron sputtering without using any buffer layer. A transition of the dependence fromI~V ² toI~V ³ has been observed in the forward biasI–V characteristics in the temperature region 210–220 K and at doping levels of the silicon substrate 10¹⁷–10¹⁹ cm^–3. The nature of this transition is discussed. A model of the structure was proposed on the basis of the analysis of the reverse-biasedI–V curves.     

UHV-HREM and diffraction of surfaces

Characterization of the structure of surfaces is very important in order to develop a fundamental understanding of the electronic, mechanical and chemical properties of a material. While transmission electron microscopy imaging (TEM) and diffraction (TED) techniques are capable of providing surface structural information at the atomic level, such data would be suspect if obtained under conventional vacuum conditions (10^-6–10^-8 Torr). Ultrahigh vacuum (UHV) conditions are imperative during both preparation and observation of clean surfaces/interfaces. Conventional TEM techniques are very powerful for UHV-TEM investigations; however, the marriage of surface science and conventional TEM to yield an UHV-TEM is a complex task. These complexities and some of the results obtained using UHV-TEM and UHV-TED techniques for surfaces i.e. solid-vacuum interfaces will be illustrated.     

Magnetic force microscopy signal of flux line above a semi-infinite type II-superconductor

We have examined a single flux line in the semi-infinite type-II superconductor. The stray magnetic field of the flux line has been calculated. We have found that the vertical force exerted on a magnetic force microscopy (MFM) tip from the flux line is measurable by currently existing MFM. Two types of magnetic tips were taken into consideration, solid and thin film tips. For example, with a Cobalt film of the thickness of 100 nm and 30 nm on a tip, we found a vertical force of 4*10^–10 N and 1.5*10^–10 N, respectively. The lateral force exerted on a tip by the flux line was also calculated. The lateral force must be small enough to prevent the flux line from becoming depinned.     

Nature of oxygen in the passive film on stainless steels in 0.1 M NaCl solution

Three different oxygen species, O^2–, OH^–, and H₂O, are introduced into the deconvolution of the oxygen peaks measured in the prior study and their true concentrations are investigated in the depth profile. It is found that H₂O exists mainly in the outermost part of the passive film at the interface of solution and the passive film. OH^– and O^2– are found throughout the passive film; they have a maximum concentration in the outer layer at around 10 to 20 Å depth, with a concentration of 40 at.% for the stainless steel 29-4-2 and 50 at.% for 18-12. After the maximum the concentrations are sharply reduced with increasing sputtering time and remain at a constant concentration of about 10 at.% at 35 Å depth. Both species still exist at this concentration even after long time sputtering.     

Electrophysical properties of zinc-sulfide films obtained by high-frequency magnetron sputtering

The structure and electrophysical properties of zinc-sulfide films obtained by high-frequency magnetron sputtering are investigated. It is shown that the electrical strength of the films is no less than 10⁶ V·cnr^–1, the width of the forbidden band is 3.2–3.3 eV, and the refractive index is 1.8. The films are tested as electroluminescent layers in thin-film electroluminescent emitters. Luminescence with a brightness of 150 cd/m² is obtained.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 29–32, June, 1990.It remains to thank A. A. Miller for perfecting the electron-microscope investigations.     

A study of the infrared spectra of carbon monoxide adsorbed on metals of the copper subgroup and on cobalt,ruthenium, rhodium,and palladium

A study has been made of the infrared spectra of carbon monoxide CO, adsorbed on Cu, Ag, Au, Co, Ru, Rh and Pd at temperatures from –160° C to 200° C and pressures from 10^–5 to 1–10 mm Hg. The spectra of CO adsorbed on Cu, Ag, and Au show one absorption band, characteristic of a surface compound of carbon monoxide, with linear structure. It has been found that in the stable chemosorption of CO on Co, Ru, Rh, and Pd in the range of temperatures and pressures studied, two types of surface compound-linear and bridged-are produced.     

Structural and optical properties of In0.5Ga0.5As/GaAs quantum dots in an In0.1Ga0.9As well using repeated depositions of InAs/GaAs short-period superlattices for the application of optical communication

We report structural and optical properties of In_0.5Ga_0.5As/GaAs quantum dots (QDs) in a 100 Å-thick In_0.1Ga_0.9As well grown by repeated depositions of InAs/GaAs short-period superlattices with atomic force microscope, transmission electron microscope (TEM) and photoluminescence (PL) measurement. The QDs in an InGaAs well grown at 510 °C were studied as a function of n repeated deposition of 1 monolayer thick InAs and 1 monolayer thick GaAs for n=5–10. The heights, widths and densities of dots are in the range of 6–22.0 nm, 40–85 nm, and 1.6–1.1×10¹⁰/cm², respectively, as n changes from 5 to 10 with strong alignment along [1 −1 0] direction. Flat and pan-cake-like shape of the QDs in a well is found in TEM images. The bottoms of the QDs are located lower than the center of the InGaAs well. This reveals that there was intermixing—interdiffusion—of group III materials between the InGaAs QD and the InGaAs well during growth. All reported dots show strong 300 K-PL spectrum, and 1.276 μm (FWHM: 32.3 meV) of 300 K-PL peak was obtained in case of 7 periods of the QDs in a well, which is useful for the application to optical communications.     

Helical nanocables with SiC core and SiO2 shell

A novel one-dimensional helical nanocable structure has been synthesized through the reaction of carbon fiber with SiO vapor. The crystalline SiC core typically has diameters of 10–20 nm with a helical periodicity of 30–40 nm and is covered by a uniform layer of 10–20 nm thick amorphous SiO₂. A growth mechanism is proposed on the basis of difference in the growth rate between crystalline SiC core and amorphous SiO₂ shell.     

Reactive Ion Beams Sputtering of Vanadium Oxides Films for Uncooled Microbolometer

Vanadium oxides thin films for uncooled bolometric detectors have been fabricated on Si₃N₄-film-coated Si substrates by low temperature reactive ion beam sputtering in a controlled oxygen atmosphere. The typical growth temperature is kept at 200°C during sputtering, which is compatible with the post-CMOS technology. The as-deposited film exhibits sheet resistance and temperature coefficient resistant of 32 k and –0.025 K^–1 at room temperature, respectively.     

Observation of vacuum fluctuation in finite space

New shifts have been investigated for the gamma-ray emitted from nuclei placed between two parallel plates. The theoretical result of the shift is –2.13×10^–8 eV with a plate separationb=1 mm for the transition from the ground state to the first excited state of¹⁹³Ir. The preliminary experimental result is (–0.23±0.03)×10^–8 eV forb=1 mm.     

Edge cathode luminescence of zinc-selenide single crystals at high levels of excitation

Investigating cathode-luminescence spectra over a broad range of temperature at high levels of excitation (G 10²⁷ cm^–3 · sec^–1) permits the establishment of the experimental conditions for the observation of a multiplasmon structure in studying electron-hole plasma of zinc-selenide single crystals.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 69–73, July, 1989.     

Spectral Properties of Single Crystals of Synthetic Diamond

The half-width of the spectrum of Raman scattering (RS) of the first order of a diamond single crystal grown in a nickel-free system containing nitrogen getters is identical to all growth sectors (1.69 ± 0.02 cm^–1). The sectorial inhomogeneity is not reflected in the transmission spectra and birefringence of this crystal. The nitrogen concentration is 4·10¹⁷ cm^–3. For different growth sectors of the diamond crystal grown in the Ni–Fe–C system, the half-width of the Raman line varies from 1.74 to 2.08 cm^–1, differences in the transmission spectra and birefringence are observed, and photoluminescence is revealed. The concentration of nitrogen in the growth sectors {001} is 1.6·10¹⁹ cm^–3, the content of nickel is estimated to be at a level of 10¹⁹ cm^–3, and the content of nitrogen in the {¯111} sectors is 4·10¹⁹ cm^–3.     

Surface strain-hardening of iron alloys with an intense pulsed electron beam

Results are presented from study of surface strain-hardening and measurements of the structure of carbon (St. 45, U7A, 40Kh) and alloy (R6M5, Kh6VF) steels subjected to surface fusion by pulsed electron beams with the following parameters: electron energy 20–250 keV, pulse duration 5·10^–8–3·10^–4 sec, power density 10⁵–10⁹ W/cm³. It is shown that the microhardness of the surface of most alloys increases by a factor of 1.2–1.7 on quenched specimens and by a factor of 2.5–3.5 on unquenched specimens, depending on the regime. Microhardness increases in the surface layer due to quenching from the liquid state. An increase in electron energy from 40 to 250 keV with a pulse duration of 6·-10^–8 sec leads to a severalfold increase in the thickness of the strengthened layers and to a shift of the microhardness peak from the surface to a depth of 70 m. Here, microhardness reaches 2000 kgf/mm². This is due to an increase in the mean free path of the electrons in the metal and displacement of the energy-release maximum of the bundle deeper into the specimen.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, No. 6, pp. 38–43, June, 1985.     

Effect of oxygen partial pressure on the structural and optical properties of dc reactive magnetron sputtered molybdenum oxide films

Molybdenum oxide films (MoO₃) were deposited on glass and crystalline silicon substrates by sputtering of molybdenum target under various oxygen partial pressures in the range 8 × 10⁻⁵–8 × 10⁻⁴ mbar and at a fixed substrate temperature of 473 K employing dc magnetron sputtering technique. The influence of oxygen partial pressure on the composition stoichiometry, chemical binding configuration, crystallographic structure and electrical and optical properties was systematically studied. X-ray photoelectron spectra of the films formed at 8 × 10⁻⁵ mbar showed the presence of Mo⁶⁺ and Mo⁵⁺ oxidation states of MoO₃ and MoO_3−x. The films deposited at oxygen partial pressure of 2 × 10⁻⁴ mbar showed Mo⁶⁺ oxidation state indicating the films were nearly stoichiometric. It was also confirmed by the Fourier transform infrared spectroscopic studies. X-ray diffraction studies revealed that the films formed at oxygen partial pressure of 2 × 10⁻⁴ mbar showed the presence of (0 k 0) reflections indicated the layered structure of α-phase MoO₃. The electrical conductivity of the films decreased from 3.6 × 10⁻⁵ to 1.6 × 10⁻⁶ Ω⁻¹ cm⁻¹, the optical band gap of the films increased from 2.93 to 3.26 eV and the refractive index increased from 2.02 to 2.13 with the increase of oxygen partial pressure from 8 × 10⁻⁵ to 8 × 10⁻⁴ mbar, respectively.     

Influence of carbon nanofibers on electrochemical properties of carbon nanofibers/glass fibers composites

Conducting glass fiber fabrics (GFs) with double-scale roughness were fabricated by growing carbon nanofibers (CNFs) on its surfaces. The homogeneous growth of CNFs was achieved by the decomposition of C₂H₂ on glass fibers surfaces coated with Fe-doped mesoporous silica films at different C₂H₂ flow rates. The chemical composition and surface structure of the GFs before and after CNFs growth were examined by electron dispersive X-ray spectrometry (EDX), N₂ full isotherms, and scanning electron microscopy (SEM). The electrical properties of the GFs were examined using a four-probe volume resistivity tester. The CNFs with a mean diameter of 50 nm grew uniformly and densely on the glass fiber surfaces. The CNFs/GFs fabrics surface exhibited excellent electrochemical properties due to the CNFs. The specific capacitance of the GFs ranged from 0.2 to 4 F/g at 1 A/g in a 1 M H₂SO₄ aqueous solution.     

Enhanced soft X-ray emission from carbon nanofibers irradiated with ultra-short laser pulses

A comparative experimental study of the X-ray emission in the water-window spectral region has been performed using carbon nanofibers (CNFs) of different sizes and graphite plate targets, irradiated with ultra-short (Ti:sapphire) laser pulses. More than an order of magnitude enhancement in the X-ray yield is observed from CNFs of 60-nm diameter with respect to graphite targets. The X-ray emission from CNFs of 160-nm diameter was also high. The integrated X-ray yield of these carbon-based targets scales with the laser intensity (I _L) as I_L ^{~ 1.3-1.4}I_{\mathrm{L}}^{\sim 1.3-1.4} in the intensity range of 4×10¹⁶–4×10¹⁷ W/cm². The effect of the laser pulse duration on the X-ray emission from the CNFs was also studied by varying the pulse duration from 45 fs up to 3 ps at a constant fluence of 2×10⁴ J/cm². The optimum laser pulse duration for maximum X-ray emission increases with the diameter of the CNFs used. The results are explained from physical considerations of heating and hydrodynamic expansion of the CNF plasma in which resonance field enhancement takes place while passing through two times the critical density. The results add to the efforts towards achieving an efficient low-cost water-window X-ray source for microscopy.