P. A. Parilla

The first true inorganic fullerenes

Boron nitride and materials of composition MX2, where M is molybdenum or tungsten and X is sulphur or selenium, can form fullerene-like structures such as nested polyhedra or nanotubes. However, the analogy to the carbon fullerene family falls short because no small preferred structure akin to C60(ref. 5) has been found. We have discovered nano-octahedra of MoS2of discrete sizes in soots that we prepared by laser ablation of pressed MoS2targets. These nano-octahedra are much larger than C60structures, having edge lengths of about 4.0 and 5.0 nanometres, and may represent the first ‘inorganic fullerenes’.

Combinatorial studies of Zn-Al-O and Zn-Sn-O transparent conducting oxide thin films

Abstract In this work, we discuss the development of combinatorial deposition and analysis tools for the investigation of and the optimization of transparent conducting oxides. Library deposition by co-sputtering followed by optical analysis is shown to be a facile way to achieve these goals. Initial work focused on Zn-Al-O libraries with low Al contents as a test case. Subsequent work has focused on the ZnO-SnO2 tie line. Local maxima in the composition dependence of the conductivity were found for Zn/Sn ≈2:1 (Zn2SnO4) and Zn/Sn ≈1:1 (ZnSnO3). For these two representative stoichiometries, constant composition films have also been grown by pulsed laser deposition.

High-mobility transparent conducting Mo-doped In2O3 thin films by pulsed laser deposition

Highly conductive and transparent Mo-doped indium oxide (IMO) thin films were grown on glass and (100) yttria-stabilized zirconia (YSZ) single-crystal substrates by pulsed laser deposition. The electrical, optical, and structural properties were measured for films grown from 0, 1, 2, and 4 wt % Mo-doped targets. Films grown from the 2 wt % Mo-doped target had the best overall properties. In particular, for biaxially textured 2 wt % Mo IMO films grown on (100) YSZ, the conductivity was ∼3000 S cm−1 with a mobility greater than 95 cm2 V−1 s−1. In the visible, the optical transmittance normalized to the substrate was greater than 90%.

Nanoengineered Carbon Scaffolds for Hydrogen Storage

Single-walled carbon nanotube (SWCNT) fibers were engineered to become a scaffold for the storage of hydrogen. Carbon nanotube fibers were swollen in oleum (fuming sulfuric acid), and organic spacer groups were covalently linked between the nanotubes using diazonium functionalization chemistry to provide 3-dimensional (3-D) frameworks for the adsorption of hydrogen molecules. These 3-D nanoengineered fibers physisorb twice as much hydrogen per unit surface area as do typical macroporous carbon materials. These fiber-based systems can have high density, and combined with the outstanding thermal conductivity of carbon nanotubes, this points a way toward solving the volumetric and heat-transfer constraints that limit some other hydrogen-storage supports.

Pulsed Laser Deposition and Characterization of Crystalline Lithium Cobalt Dioxide ( LiCoO2 ) Thin Films

The pulsed laser deposition of LiCoO 2 and LiCo 0.5 Al 0.5 O 2 thin films was investigated as a function of deposition conditions. The initial growth process optimization focused on films grown on (200)-textured SnO 2 -coated glass substrates. Film growth was also investigated on ZnO and indium-tin oxide (ITO) coated substrates. For both LiCoO 2 and LiCo 0.5 Al 0.5 O 2 , dense uniaxially textured (003)-oriented films of the layered LiCoO 2 phase were grown on the SnO 2 coated substrates. The grain size increased substantially with increased substrate temperature in the range from 400 to 700°C. For constant current cycling between 3.5 and 4.4 V vs. Li, the best crystalline LiCoO 2 films, grown at T s = 700°C, p O2 = 2000 mTorr, had an initial discharge capacity of ∼118 8 mAh/g (0.43 Li/Co) which decreased ∼0.5% per cycle. For LiCo 0.5 Al 0.5 O 2 , although, as predicted, the films did have a higher cathode potential, the charge capacity was signilicantly lower than that for LiCoO 2 films, Finally, functional LiCoO 2 thin film cathodes were grown on flexible ITO-coated Upilex polymer substrates at T s = 300°C.

Nanopolar reorientation in ferroelectric thin films

The influence of varying oxygen pressure P(O2) during the growth of Ba0.4Sr0.6TiO3 thin films is investigated using dielectric and local optical probes. A transition from in-plane to out-of-plane ferroelectricity is observed with increasing P(O2). Signatures of in-plane and out-of-plane ferroelectricity are identified using dielectric response and time-resolved confocal scanning optical microscopy (TRCSOM). At the crossover pressure between in-plane and out-of-plane polarization (Pc=85 mTorr), TRCSOM measurements reveal a soft, highly dispersive out-of-plane polarization that reorients in plane under modest applied electric fields. At higher deposition pressures, the out-of-plane polarization is hardened and is less dispersive at microwave frequencies, and the dielectric tuning is suppressed. Nanopolar reorientation is believed to be responsible for the marked increase in dielectric tuning at P(O2)=Pc.

A Comparison of Single-Wall Carbon Nanotube Production Using Continuous Wave and Pulsed Laser Vaporization

We have produced single-wall carbon nanotubes (SWNTs) by Nd:Yag laser vaporization of porous cobalt-nickel/graphite targets for the first time without applying external heat to the target. Both continuous wave (c.w.) and pulsed laser-techniques were explored. In the pulsed experiments, the energy density per pulse was varied between 0.12 and 0.62 J/cm 2 and pulse rates ranged from 24 kHz down to 3 kHz corresponding to average powers of 25.5 to 16 W. Continuous wave experiments were conducted at average powers which corresponded to those measured for each of the pulsed runs. An additional run at 30 W was also performed. A rigorous method developed for the analysis of transmission electron microscopy (TEM) images was used to estimate SWNT content in the laser-generated carbon samples. The pulsed and continuous wave processes both produced a linear increase in SWNT content with increasing average power. These results are not intuitive for the pulsed laser-production since a higher average power corresponds to a lower peak pulse power. In fact, a pulsed run with the maximum pulse energy of 0.62 J/cm 2 (3 kHz, 16 W), produced no detectable SWNTs. A maximum SWNT content of 78% was observed for c.w. experiments at a power of 30 W. Additional laser experiments performed on dense cobalt-nickel / graphite targets at room temperature enabled a better understanding of the carbon removal mechanisms for the pulsed versus c.w. processes. Cumulatively, these investigations indicate that high energy laser pulses produce particles from highly porous targets by ablation which are too large to readily be incorporated into growing nanotubes. Successful high-yield production of SWNTs relies upon remaining in a vaporization regime during synthesis.

Altering the Nucleation of Thermally Annealed Hydrogenated Amorphous Silicon with Laser Processing

We demonstrate the use of laser processing to affect the nucleation of crystallites in thermally annealed hydrogenated amorphous silicon (a-Si:H) thin films. The influence of film H content and subcrystallization threshold laser fluence are investigated by x-ray diffraction measurements during in situ thermal annealing at 600 °C. All laser-treated films show a reduced incubation time for crystallization compared to as-grown films, with the largest differences exhibited for samples with higher film H and higher laser fluences. These results are consistent with multivacancy annihilation by laser processing, based upon a recently developed model for a nucleation center in a-Si:H.

Performance of ferroelectric based tunable capacitors as a function of electrode geometry

Abstract High quality Ba0.4Sr0.6TiO3 and SrTiO3 films were grown by Pulsed Laser Deposition on single crystal LaAlO3 and MgO substrates. Temperature dependencies of the dielectric constant and loss tangent of the films were studied using planar interdigitated test capacitors with various electrode geometries. The temperature where the maximum or “peak” capacitance occurs (Tp and referred to as the “peak temperature”) is found to depend on the electrode geometry in these films. As much as 40 K difference in Tp was observed between the STO test capacitors with 5 μm and 40 μm gaps between electrodes. Interface built-in electric field and metal-ferroelectric thermal mismatch strain are considered as possible explanation of the effect of electrode geometry on peak temperature of the capacitors.

Charge carrier generation and exciton quenching at M3EH-PPV/small-molecule and M3EH-PPV/oxide interfaces

The need for efficient exciton dissociation is one of the most important factors limiting improved efficiencies in organic photovoltaic devices. Using luminescence as a probe, we studied the quenching of excitons in semiconducting polymers for a variety of quenching materials, including transparent conducting oxides (TCOs) and small molecule perylene diimide thin films. Perylene benzimidazole (PBI) is shown to be the best quencher of those studied. This result is consistent with the improved conversion efficiencies demonstrated when this material is used in a polymer bilayer photovoltaic device.