Pulsed laser deposition of multiwalled carbon nanotubes thin films

The physical/chemical properties of multiwalled carbon nanotubes have attracted much interest for applications in different fields, from micro-electronic to coating technology due, in particular, to their peculiar conductivity properties, to their hardness and high resistance to thermal stress. The technology to produce carbon nanotubes thin films with the desired properties, however, is still under development. In this work, we report on multiwalled carbon nanotubes thin films deposited by pulsed laser deposition techniques ablating commercially polystyrene-nanotubes pellets on alumina substrates. MicroRaman spectroscopy and high resolution Transmission Electron Microscopy provide the experimental confirmation that carbon nanotubes-like structures are present on the alumina surface with both minimal morphological damage of the tubes and structural changes induced by laser beam.     

Superconductivity in boron-doped diamond

Superconductivity of boron-doped diamond, reported recently at T(c)=4 K, is investigated exploiting its electronic and vibrational analogies to MgB2. The deformation potential of the hole states arising from the C-C bond-stretch mode is 60% larger than the corresponding quantity in MgB2 that drives its high T(c), leading to very large electron-phonon matrix elements. The calculated coupling strength lambda approximately 0.5 leads to T(c) in the 5-10 K range and makes phonon coupling the likely mechanism. Higher doping should increase T(c) somewhat, but the effects of three dimensionality primarily on the density of states keep doped diamond from having a T(c) closer to that of MgB2.     

Superconductivity in ropes of single-walled carbon nanotubes

We report measurements on ropes of single-walled carbon nanotubes (SWNT) in low-resistance contact to nonsuperconducting (normal) metallic pads, at low voltage and at temperatures down to 70 mK. In one sample, we find a 2 orders of magnitude resistance drop below 0.55 K, which is destroyed by a magnetic field of the order of 1 T, or by a dc current greater than 2.5 microA. These features strongly suggest the existence of superconductivity in ropes of SWNT.     

Superconductivity in sputtered thin films of Tl-Ba-Ca-Cu-O

Films of Tl-Ba-Ca-Cu-O have been made by multi-target magnetron sputtering. The best films show an onset of superconductivity at ≈ 110 K and zero resistance at 96 K. Preliminary X-ray diffraction analysis suggests the films to be predominantly oriented with the c-axis perpendicular to the film surface with the lowest multiple of lattice spacing along the c-axis being ∼ 2.94 nm, consistent with the Tl₂Ba₂CaCu₂O_x (2212) phase.     

Superconductivity in entirely end-bonded multiwalled carbon nanotubes

We report that entirely end-bonded multiwalled carbon nanotubes (MWNTs) can exhibit superconductivity with a transition temperature (T(c)) as high as 12 K, which is approximately 30 times greater than T(c) reported for ropes of single-walled nanotubes. We find that the emergence of this superconductivity is highly sensitive to the junction structures of the Au electrode/MWNTs. This reveals that only MWNTs with optimal numbers of electrically activated shells, which are realized by end bonding, can allow superconductivity due to intershell effects.     

Monitoring structural defects and crystallinity of carbon nanotubes in thin films

We report the influence of catalyst formulation and reaction temperature on the formation of carbon nanotube (CNT) thin films by the chemical vapour deposition (CVD) method. Thin films of CNTs were grown on Fe-Mo/Al₂O₃-coated silicon wafer by thermal decomposition of methane at different temperatures ranging from 800 to 1000°C. The electron microscopic investigations, SEM as well as HRTEM, of the as-grown CNT thin films revealed the growth of uniform multi-walled CNTs in abundance. The intensity ratio of D-band to G-band and FWHM of G-band through Raman measurements clearly indicated the dependency of structural defects and crystallinity of CNTs in thin films on the catalyst formulation and CVD growth temperature. The results suggest that thin films of multi-walled CNTs with negligible amount of defects in the nanotube structure and very high crystallinity can be obtained by thermal CVD process at 925°C.     

Electrical properties of boron-doped diamond-like carbon thin films deposited by femtosecond pulsed laser ablation

We report on electrical measurements and structural characterization performed on boron-doped diamond-like carbon thin films deposited by femtosecond pulsed laser deposition. The resistance has been measured between 77 and 300 K using four probe technique on platinum contacts for different boron doping. Different behaviours of the resistance versus temperature have been evidenced between pure DLC and boron-doped DLC. The a-C:B thin film resistances exhibit Mott variable range hopping signature with temperature. Potential applications of DLC thin films to highly sensitive resistive thermometry is going to be discussed.     

Superconductivity in thin tin films: Fluctuation effects

The excess electrical conductivity in thin tin films due to thermodynamic fluctuations above T_c has been measured as a function of temperature and sheet resistance R_ns of films which varied from 0.2 Ω/? to 120 Ω/?. Results have been compared with the Aslamazov-Larkin, Maki-Thompson and Patton theories. It is found that among three the Maki-Thompson theory gives a better fit to experimental data. The pair breaking parameter a fitting parameter in the M-T theory, is found to behave linearly with R_ns of films and is given by ? ? 9x10^?3 + 1.02x10^?2R_ns for R_ns ≤ 12 Ω/?.     

Superconductivity in disordered thin films: giant mesoscopic fluctuations

We discuss the intrinsic inhomogeneities of superconductive properties of uniformly disordered thin films with a large dimensionless conductance g. It is shown that mesoscopic fluctuations, which usually contain a small factor 1/g, are crucially enhanced near the critical conductance g(cF) > 1 where superconductivity is destroyed at T = 0 due to Coulomb suppression of the Cooper attraction. This leads to strong spatial fluctuations of the local transition temperature and thus to the percolative nature of the thermal superconductive transition.     

Preparation and characteristic of platinum catalyst deposited on boron-doped carbon nanotubes

In this work, boron doped multi-walled carbon nanotubes (BMWNTs) were introduced as a Pt catalyst support due to their unique physicochemical properties. The effect of BMWNTs on methanol oxidation was investigated with different Pt loading contents. The surface and structural properties of the modified MWNT supports were characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), respectively. The Pt loading contents in the catalysts were confirmed by inductive coupled plasma-mass spectrometer (ICP-MS) and the morphological structures of the catalysts were analyzed by transmission electron microscopy (TEM). The electrocatalytic activity of Pt/MWNTs was investigated by cyclic voltammetry measurement. As a result, the boron oxide vapor reacted with MWNTS to form BMWNTs, which led to enhancing the properties, such as graphitization and electrochemical behaviors. Moreover, Pt deposited on BMWNTs exhibited better electrocatalytic activity than on MWNTs for methanol oxidation. Consequently, it was found that partial boron doped MWNTs could influence on the properties of the MWNTs, resulting in enhancing the electrocatalytic activity of the catalysts for DMFCs.     

Optical properties of carbon nanotubes and BaTiO3 composite thin films

Multiwalled carbon nanotubes and BaTiO₃ composite films have been prepared by pulsed-laser deposition technique at room temperature and high temperature of 600℃, separately. The structures of the composite films are investigated by using scanning electron microscopy and x-ray diffraction. The optical behaviours of the samples produced at different temperatures are compared with Raman spectroscopy, and UV-visible absorption. And the observation by Z-scan technique reveals that the composite films have a larger optical nonlinearity, and the samples prepared at high temperatures have better transmittance and opposite sign imaginary part of optical third-order nonlinearity.     

Transparent conducting hybrid thin films fabricated by layer-by-layer assembly of single-wall carbon nanotubes and conducting polymers

The effect of conducting polymers on the performance of transparent conducting SWNT thin films fabricated by the layer-by-layer (LBL) assembly has been investigated. Transparent conducting SWNT thin films were fabricated by the LBL assembly in two ways, one using conducting polymers, PEDOT-PEG, and the other using non-conducting polymers, PAH, and their electrical and optical properties were compared. The sheet resistance of (PSS-SWNT/PEDOT-PEG) _n films is more than thrice lower than that of (PSS-SWNT/PAH) _n films for the same n while the decrease of optical transmittance due to the absorbance of PEDOT-PEG is fairly small (ca. 2?% at n=30). The conductivity ratio of the (PSS-SWNT/PEDOT-PEG)₃₀ is 3.3 times larger than that of the (PSS-SWNT/PAH)₃₀. These figures indicate that the performance of the transparent conducting SWNT thin films fabricated by the LBL assembly is highly improved by using conducting polymers instead of non-conducting ones.     

Superconductivity in ultrathin films

It is shown that the pair breaking parameter of the Maki-Thompson contributions to the fluctuation conductivity above the superconducting transition temperature has the same thickness dependence as the transition temperature depression in very thin amorphous Be-Al films. Both can be ascribed to an extremely thin surface sheath with suppressed superconductivity.Supported by the Swedish Natural Science Research Board.     

Preparation of Sn films deposited on carbon nanotubes

In this work carbon nanotubes were first grown on copper substrate by chemical vapor deposition method. The Sn deposits were then deposited on the surface of as-grown carbon nanotubes by three different methods: electroplating, electroless plating and displacing methods. The Sn deposits on CNTs surface were characterized by both scanning electron microscope and field emission scanning electron microscope. The compositions of Sn deposits were analyzed by energy dispersive X-ray spectroscope. The results showed that both electroless plating and displacing deposits can but the electroplating deposits cannot cover on the surface of CNTs. Besides C, Sn, Ni and Pd, the electroless deposits also contain element of oxygen and the displacing deposits also contain elements of copper and oxygen.     

Superconductivity in amorphous Cr films

Superconducting transition has been observed for the first time in amorphous Cr films prepared by co-sputtering of Cr and BN. Superconducting transition temperatures (Tc) up to 1.14 K have been observed with an orbital contribution (i.e.(dH_c2⊥/dT)_Tc) as large as 49 kOe K^-1. The electronic specific heat coefficient (γ) has been estimated to be about 1.6 times as large as that of amorphous Mo-BN (or Mo-metalloid) alloys. This finding, i.e. the low Tc with the large γ for amorphous Cr is inconsistent with data reported for amorphous alloys of 4d and 5d transition elements. This inconsistency implies a formation of local magnetic moments or spin fluctuation in the amorphous Cr film.     

Thermolysis of C60 thin films yields Ni-filled tapered nanotubes

60 and Ni deposited on a silica plate at 950 °C. High-resolution transmission electron microscopy (HRTEM) studies reveal that these tapered structures are almost completely filled with Ni and are closed at both ends. The diameters of the needles (2–5 μm in length) range between 10–20 nm at one end and 30–200 nm at the other. A surprisingly high degree of graphitization is observed in the walls of the nanotubes. These unique structures may prove useful as electrodes with various applications such as precision probes in biological systems or scanning tunnelling microscope tips. Received: 13 August 1998/Accepted: 13 August 1998     

Superconductivity and x—ray photoemission study of MgB2 thin films

Highly c-axis oriented MgB2 thin films with Tc^onset of 39.6K were fabricated by magnesium diffusing into pulsedlaser-deposited boron precursors.The estimation of critical current density Jc,using hysteresis loops and the Bean model,has given the value of 10^7A/cm^2(15K,0T),which is one of the highest values ever reported.The x-ray photoemission study of the MgB2 thin films has revealed that the binding energies of Mg 2p and B 1s are at 49.4eV and 186.9eV,which are close to those of metallic Mg and transition-metal diborides,respectively.     

Secondary electron emission from thin carbon films

For the purpose of investigating how secondary electrons are produced in carbon, the correlation between energy-loss events and secondary electrons was studied experimentally by using the coincidence method. If a secondary electron is detected in coincidence with an electron transmitted through a thin film which has lost an amount of energy E, then the process causing this energy loss results in the production of secondary electrons. We established the existence of these coincidences and have taken inelastic and coincidence spectra for films of different thickness. We found that in carbon secondary electrons are predominantly produced as a result of energy losses of about 20 eV, with an efficiency of about 5%. The escape depth of secondary electrons was also estimated to be approximately 30 Å.