From Plasma Deposited Thin Films to the

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From Plasma Deposited Thin Films to the preparation of supported oxide Nanofibers A. Borrás, M. Macías, P R B M M P. Romero-Gómez, J. Cotrino, A. Barranco, A.R. González-Elipe* Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla). Spain *arge@icmse.csic.es http://www.sincaf-icmse.es / Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Outline Introduction and motivation Factors controlling the nanostructure of PECVD oxide thin films Metal seeds and the formation of oxide nanofibers by PECVD Mechanism of f formation of oxide nanofibres Conclusions and perspectives

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Requirements in new field of applications: Control of surface roughness Control of the thin film nanostructure. Processing of porous films Other surface morphologies (e.g., nanofibers) Current methods: Wet chemical or electrochemical routes Vapor deposition methods PVD Plasma etching …… Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Electrochemical methods GLAD Physical Vapour Deposition 500 nm Electrochemically formed TiO 2 thin film for solar cell applications ( Electrochem. Comm. 7 (2005) 1133) PVD-GLAD TiO 2 – SiO 2 oxide thin films prepared by electron evaporation. (L. González-García et al. J. Mater. Chem. 20, 2010, 6408-6412) Plasma etching : Magnetron sputtering plus plasma Surface of polystyrene subjected to plasma etching for increasing periods of time. (Ehret et al. Plasma Process. Polym. 2009, 6, 840–847) SEM micrographs of MS plus plasma deposited Ti- polymer composite thin films. (Choukourov et al. Plasma Proc. Polym. 2010, 7, 25–32)

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Can PECVD compete with wet chemical routes or with PVD to get a well defined control of the thin film and surface nanostructure? Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION Plasma (0.5 m) : Temperature, density, potential, chemical nature of plasma species,… Plasma Sheath (10µm-1mm) : Sheath potential, local inhomogeneities Surface(1 20 nm) : Diffusion processes shadowing effects, sticking probability, surface reactions, etching effects, … PLASMA SUBSTRATE Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Factors controlling the nanostructure of PECVD thin films Plasma properties Nucleation and thin film growth on the surface Angular distribution function of impinging species Plasma phase and bias voltages Surface diffusion vs. shadowing effects Substrate temperature and chemical characteristics of plasma species Crystallization phenomena Substrate temperature

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PLASMA PARTICLE DIRECTIONALITY AND SURF. ROUGHNESS SiO 2 and SiO x C y H z thi fil grown b PECVD by AFM images of the surface topography of SiO 2 thin films deposited for increasing periods of time Simulation of the growing process by assuming different directionalities for the plasma particles (Dynamic scaling principles and Monte Carlo simulations) Phys. Rev. L etters 96, 236101 (2006) Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Accumulated distribution function : probability for a particle to reach the surface with an angle smaller than θ with respect to the normal direction of the surface z=0 ; Maxwellian distribution z=2 ; vertical incidence z, normalized velocity SUBSTRATE SUBSTRATE Monte Carlo simulation (restricted surface diffusion) Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Plasma particle directionality and surface nanostructure Evolution of the simulated surface topography with deposition time (z=0) Surface roughness and film porosity depend on the average incoming angle of plasma species: Shadowing effects Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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PARTICLE DIRECTIONALITY AND SURFACE NANOSTRUCTURE PECVD vs. Magnetron sputtering deposition g TiO 2 thin films grown by PECVD and MS J. Appl. Phys. 108, 064316 2010 Cathode (Ti) θ 150nm a) d-MS ϕ Plasma (Ar + O 2 ) 150nm b) i-MS Ballistic d-MS i-MS Isotropic Deposition Particle Deposited Particle 150nm c) MC simulation Surface topography (SEM) of films prepared by d-MS, i-MS, PECVD and simulated through MC simulations PECVD 150nm MS configurations d) Similar surface topography for i-MS, PECVD and MC films MC Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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PLASMA SPECIES AND SURFACE DIFFUSION PHENOMENA TiO 2 thin films prepared by PECVD: influence of plasma gas composition 50nm 50nm 50nm 50nm 50nm 100% O 2 1μ m 1μ m 1μ m 1μ m 10nm 10nm 10nm 10nm 10% O 2 90% Ar 1μ m 1μ m 1μ m 1μ m Higher roughness for 100% O 2 than 10% O 2 samples Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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PLASMA SPECIES AND SURFACE DIFFUSION PHENOMENA 100 % O 2 n (100% O 2 ): 2.1 n (10% O 2 ): 1.95 10 % O 2 , 90% A SEM micrographs 1.0 0.8 1.0 0.8 0.6 0.4 0.2 0.0 0.0 0.2 0.4 P/P 0 0.6 0.6 0.4 0.2 100% O 2 0.8 1.0 10% O 2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 PORE VOLUME Ar SAMPLE > PORE VOLUME O 2 SAMPLE V(cm 6 0 V(cm 3 P/P 0 Water adsorption isotherms J. Electrochem. Soc. 154 (2007) 152; PRB 76 (2007) 235303; Microporous Mesop. Mater. 118 (2009) 314–324 Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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GROWTH MODEL OF THIN FILMS OES analysis of plasma and Dynamic Scaling description of surface roughness 100 % O 2 , (298 K) 10% O 2 , (298 K) TiOC, O* TiO 100% O 2 samples : shadowing controls the growth mechanism. High sticking coefficient of impinging particles 10% O2 samples : surface diffusion of particles. Lower sticking coefficient, shadowing does no play any significant role Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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SUBSTRATE TEMPERATURE AND CRYSTALLIZATION PHENOMENA DURING PECVD TiO 2 thin films grown by PECVD at 523 K v 1 Anatase crystalline structure Development of crystalline columns. Different crystal habits depending on deposition rate Planar view (left) and cross section (right) SEM micrographs for samples with different thickness grown at v1 and v2 v 2 Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Model for growing crystalline TiO 2 thin films by PECVD Crystalline TiO2 thin films grown by PECVD follow the Kolmogorov model developed to account for the growth of crystals from saturated homogeneous media¡ Crystal Growth Design 9 (2009) 2868. Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Metal seeds and the formation of CNT by PECVD Carbon nanotubes and nanofibers growth by plasma deposition Z.L. Tsakadze, I. Levchenko, K. Ostrikov, S. Xu: Carbon 45 (2007) 2022–2030 SEM micrograph of Ni catalyst film SEM micrographs of carbon nanotubes prepared by PECVD for different periods of time Catalytically driven fiber formation of CNT. Vertically aligned CNT Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Plasma Sheath inhomogeneities: metal/dielectric surfaces Barnat and Hebn er J. Appl. Phys. 96 , 4762 (2004) Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Metal films as substrates: Phenomenology TiO 2 on Ag-NPs/Si(100) a) silver TiO 2 on a substrate partially covered by a silver layer silicon 1 um TiO 2 on Si(100) b) 200 nm c) 200 nm ZnO on Si (left) and on a silver layer (right) ZnO on Si(100) ZnO on Ag Changes in morphology for TiO 2 and ZnO films on a silver layer J. Phys. D.: Appl. Phys. 44 (2011) 174016 Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Metal films as substrates: Phenomenology (2) Cross sections of ZnO deposited on Si (left) and on Au/Si (right) ZnO on Si(100) ∼ 800 nm ∼ 160 nm 500 nm ∼ 990 nm ∼ 55 nm 500 nm ZnO on Au e) Length of nanocolumns is higher and the aspect ratio much smaller on Au/Si J. Phys. D.: Appl. Phys. 44 (2011) 174016 Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Metal particles on flat substrates as seeds: Phenomenology 200 nm TiO 2 on Si (298 K) TiO 2 on Ag particles deposited on Si (K) TiO 2 on Au and Pt particles deposited on Si (298 K) J. Phys. D.: Appl. Phys. 44 (2011) 174016 Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Metal particles on flat substrates as seeds: Phenomenology TiO 2 d on A particles ( Si substrate) it Ag ti l (on b t t ) , Nanotechnology 17 (2006) 3518; Plasma Process. Polym. 4 (2007) 515 Feather-like TiO 2 columnar microstructure Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Deposition on silver (bulk) pre-treated with a plasma of O 2 Ag@TiO 2 (amorphous) 130 C Ag@TiO 2 fibers length ~ 1-3 μ m Ag@TiO 2 fiber diameter ~ 40- 150nm Ag core di ~ 20-30 nm t 20 30 • Process time • Plasma conditions (O 2 /precursor/total pressure) • Plasma treatment of the substrate Nanofiber density Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Deposition on silver (bulk) pre-treated with a plasma of O 2 Ag@TiO 2 (anatase) 250ºC 5 μ m fiber length ~ 1-7 μ m Ag@TiO 2 diameter ~ 40nm- 500nm • Process time • Plasma conditions (O 2 /precursor/pressure) • Plasma treatment of the substrate Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Ag@TiO 2 nanofibers:TEM and TEELS characterization a) Ag core Anatase TiO 2 b) In u.) Ag Plasmon Ag Plasmon 3 6 9 250ºC 12 15 TiO 2 shell 100 nm Low Loss 0 20 40 60 80 Energy Loss (eV) ) Ag fcc single crystal. Growth according to the [110] direction. 100 c) Ag core d) Intensity Ag Plasmon Ag Plasmon 3 6 9 130ºC 12 15 fcc Ag TiO 2 film [110] Ti M 2,3 Edge Low Loss 0 20 40 60 80 Energy Loss (eV) ϕ Ag core ~ 20-30 nm (111) 2 nm 100 Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Growth of Ag@TiO 2 nanofibers by plasma deposition Volcano-like mode of fiber growth t 0 t 3 Role of local inhomogeneities of plasma sheath ?? t 1 Ag 2 O segregate nuclei Ag substrate Ag 2 O TiO x C y fragments t 2 Electric field vector Equipotential lines Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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PECVD Nanofiber formation with other systems TiO 2 /SiO 2 on Ag SiO 2 on Ag TiO 2 on Au ZnO on Ag Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Control of the nanofiber formation on procesable substrates TEM micrograph showing the incorporation of silver in the interior of the fiber a) and b) SE cross section and BSE cross section images respectively of Ag@TiO2 nanofibers deposited on a Si(100) wafer; c) and d) Idem of Ag/ZnO nanowires deposited on Si(100). J. Phys. D.: Appl. Phys. 44 (2011) 174016 Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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CONCLUSIONS: PLASMA ENHANCED CHEMICAL VAPOUR DEPOSITION OF OXIDE NANOSTRUCTURES PECVD can be used for tailoring the nanostructure of oxide thin films. Shadowing effects related with particle directionality , surface diffusion processes and crystallization phenomena are critical for the control of the film nanostructure. Metal layers and metal particles acting as seeds during the PECVD of oxides favor the development of fibers and other complex nanostructures The mechanism of formation of the oxide nanofibers is not catalytic and is controlled by inhomogeneities in the electrical field developed in the plasma sheath . Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)

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Acknowledgments People form the “Surfaces, Interfaces and Thin film” group of the ICMSE (Sevilla-Spain) A.R. González-Elipe J.P. Espinós F. Yubero J. Cotrino A. Barranco A. Palmero R. Alvarez A. Borrás A. Yanguas P. Romero-Gómez V. Rico J.R. Sánchez-Valencia Grants : •National and regional f di •EU project NATAMA (Strep 032583, VI framework programm) Instituto de Ciencia de Materiales de Sevilla (CSIC-Univ. Sevilla)