Inverse pulsed laser deposition

Since the advent of pulsed laser deposition (PLD), several different target-substrate arrangements have been proposed. Besides the most common on-axis PLD, several off-axis geometries were studied, mainly to protect the substrate from the agglomerated species (clusters, droplets, particulates) of the plasma plume, which are detrimental to the homogeneity of films. Recently we introduced a novel geometry, termed inverse pulsed laser deposition (IPLD), in which the substrate is placed parallel to and slightly above the target plane. In this paper we summarize our results on this new geometry, and show how it can extend the perspectives of pulsed laser deposition, e.g., by improving the surface morphology of the films. Effects of ambient pressure are presented and exemplified on metallic and compound IPLD films, including Ti, CN _x , and Ti-oxides. AFM topographic images are used to prove that under optimized conditions IPLD is capable of growing compact and smooth films that are superior to PLD ones. A special—but easy-to-implement—IPLD arrangement is also introduced that considerably improves the homogeneity of IPLD films. In this geometry, the properties (e.g., deposition rate and roughness) of the films grown in the 1–25 Pa pressure domain are examined.     

Efficient synthesis of ZnO nanoparticles,nanowalls, and nanowires by thermal decomposition of zinc acetate at a low temperature

ZnO nanoparticles, nanowires, and nanowalls were synthesized rapidly on Si via thermal decomposition of zinc acetate by a modified chemical vapor deposition at a low substrate temperature of 200–250°C for the first time. The diameters of the synthesized nanoparticles and nanowires are around 100 and 30 nm, respectively, and the thickness of nanowalls is around 20 nm. High-resolution transmission electron microscopy shows that the nanowires as well as nanowalls are single-crystalline, and the nanoparticles are highly-textured poly-crystalline structures. Room-temperature photoluminescence spectra of the nanostructures show strong ultraviolet emissions centered at 368–383 nm and weak violet emissions at around 425 nm, indicating good crystal quality. The study provides a simple and efficient route to synthesize ZnO diverse nanostructures at low temperature.     

Polycrystalline silicon films obtained by hot-wire chemical vapour deposition

Silicon films were deposited at moderate substrate temperatures (280–500° C) from pure silane and a silane-hydrogen mixture (10% SiH₄, 90% H₂) in a hotwire CVD reactor. The morphology, structure and composition of the samples were studied with scanning electron microscopy, transmission electron microscopy, transmission electron diffraction, X-ray diffraction, Raman spectroscopy and secondary ion mass spectrometry. The sample deposited at 500° C with pure silane has an amorphous structure, whereas the samples obtained from silane diluted in hydrogen have a polycrystalline structure, even that grown at the lowest temperature (280° C). Polycrystalline samples have a columnar structure with 0.3–1 m crystallite sizes with preferential orientation in [220] direction. Deposition rates depend on the filament-substrate distance and range from 9.5 to 37 Å/s for the polycrystalline samples. The high quality of the polycrystalline samples obtained makes the hot-wire technique very promising. Moreover, it is expected to be easily scaled up for applications to large-area optoelectronic devices and to photovoltaic solar cells.     

Zinc oxide nanostructures grown by pulsed laser deposition

We report the use of PLD to grow different ZnO nanostructures. Very different film morphologies have been observed using different laser wavelengths to ablate the target. The influence of substrate temperature and oxygen background pressure on the film morphology has been investigated too. Smooth and rough films, hexagonal pyramids and columns have been obtained by using a KrF excimer laser (248 nm) for the target ablation, while hexagonal hierarchical structures and pencils have been obtained by using ArF (193 nm). Photoluminescence and X-ray diffraction measurements revealed the good quality of the samples, in particular of those deposited using the ArF laser beam.     

Highly oriented electrospun polycaprolactone micro/nanofibers prepared by a field-controllable electrode and rotating collector

Novel ZnO tetrapod-shaped nanostructures with pearl-necklace-shaped arms were successfully synthesized using mixture of Zn, ZnO, and carbon powder as source. The definite supersaturation ratio provided by Zn, ZnO, and carbon powder was considered as the crucial factor of determining the formation of this kind of structure, and a negative feedback growth model combined with octahedral nucleation mechanism was proposed. Two other comparative experiments were also conducted to study the growth behavior of reagent species under different supersaturation ratios. Our experiments provided a beneficial experimental exploration in controlled growth of nanostructures through modulating supersaturation ratio by source, and these obtained novel nanostructures were also expected to have potential application as functional blocks in nanodevices. Furthermore, the study of photoluminescence indicated that the physical properties were strongly dependent on the crystal structure.     

A study of the Soret effect in laser-induced chemical vapor deposition

This paper presents an investigation of the modeling of the process of pyrolytic laser-induced chemical vapor deposition (LCVD) applied to study the Soret effect. LCVD is a thermally activated process characterized by strongly coupled mass and energy transport phenomena, together with chemical reactions, which are difficult to investigate experimentally. A physical and numerical model based on a commercial computational fluid dynamics package is developed and used to simulate a reactor operating at conditions of room temperature and pressure. The proposed numerical methodology will allow us to assess and analyze the effect of various factors controlling the process, and in particular the Soret effect. This numerical model is validated by comparison with the measured growth rate of the fiber. While several studies have proposed simulations of the LCVD process, this is among the first attempts at including the Soret effect in the numerical modeling at the micro-scale level. It is expected that the fundamental insights thus obtained will guide experimental investigations which can be applied to establish reactor design and process control guidelines.     

The structure and photoluminescence properties of TiO<Subscript>2</Subscript>-coated ZnS nanowires

The structure and photoluminescence properties of TiO₂-coated ZnS nanowires were investigated. ZnS nanowires were synthesized by thermal evaporation of ZnS powder and then coated with TiO₂ by using the metal organic chemical vapor deposition (MOCVD) technique. We performed scanning electron microscopy, transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy, and photoluminescence (PL) spectroscopy to characterize the as-synthesized and TiO₂-coated ZnS nanowires. TEM and XRD analyses revealed that the ZnS core and the TiO₂ coatings had crystalline zinc blende and crystalline anatase structures, respectively. PL measurement at room temperature showed that the as-synthesized ZnS nanowires had two emissions: a blue emission centered in the range from 430 to 440 nm and a green emission at around 515 nm. The green emission was found to be dominant in the ZnS nanowires coated with TiO₂ by MOCVD at 350°C for one or more hours, while the blue emission was dominant in the as-synthesized ZnS nanowires. Also the mechanisms of the emissions were discussed.     

The importance of non-local shadowing for the topography evolution of a-C:H films grown by toluene based plasma enhanced chemical vapor deposition

The topography evolution of hydrogenated diamond-like carbon coatings deposited through toluene based capacitively coupled plasma enhanced chemical vapor deposition has been studied experimentally and with continuum growth models. The experimentally observed mound formation and surprisingly large growth exponents (β≈ 0.9±0.1) cannot be reproduced by familiar local stochastic differential equations that are successfully used for other thin film deposition techniques. Here we introduce a novel numerical approach to simulate a continuum growth model that takes into account non-local shadowing effects. We show that the major characteristics of the experimentally observed topography evolution can be accurately represented by this model.     

Synthesis of multi-wall carbon nanotubes by copper vapor laser

Synthesis of multi-wall carbon nanotubes in a 1473 K furnace using a copper vapor laser (CVL) is reported. The operating parameters of this laser, i.e. a high fluence at the focal point and an extremely high frequency of 10 kHz, distinguished it from common laser sources in the synthesis of CNTs. Therefore, the unexpected experimental findings, the formation of MWNTs instead of the generally reported SWNTs, would be verified by these two notable parameters. Electron microscopy beside Raman spectroscopy illustrates the presence of multi-wall carbon nanotubes in the resulting product.     

Flow assisted synthesis of highly ordered silica nanowire arrays

Silica nanowires were synthesized through a self-supplied vapor–liquid–solid (VLS) mechanism. Unlike randomly entangled nanowires on the substrate, highly ordered nanowire arrays have been successfully fabricated using a sandwich-like configuration to define and enhance the local gas-flow. As-synthesized nanowires were characterized to be amorphous silicon dioxide with Au as catalysts. The role of the sandwich-like structure and the effect of gas-flow on the alignment of silica nanowires are demonstrated.     

2D aggregate sizing by combining laser-induced incandescence (LII) and elastic light scattering (ELS)

The combination of laser-induced incandescence and elastic light scattering has been further developed to allow for a quantitative two-dimensional determination of characteristic properties of soot aggregates, namely radius of gyration R _g and number N _p of primary particles per aggregate. In demonstrating the principle of the method, we have in a first approach approximated the particle ensemble as monodisperse and used a structure factor with an exponential cut-off function. Nonetheless, experiments performed on a laminar premixed ethene flame demonstrate basically good agreement with observations from literature and data from electron microscopy on thermophoretically obtained samples.     

Structural properties of size-selected FeCo nanoparticles deposited on W(110)

Size-selected iron and iron–cobalt alloy clusters have been studied with high resolution transmission electron microscopy (HRTEM) and scanning tunneling microscopy (STM). The clusters were produced by a continuously working arc cluster ion source and subsequently size-selected by an electrostatic quadrupole deflector. The crystalline structure of pure clusters has been investigated with HRTEM to ensure a reliable determination of the lattice parameter for the alloy clusters. The composition of the alloy clusters was checked with energy dispersive X-ray spectroscopy (EDX). The height of the deposited FeCo clusters on the (110) surface of tungsten was determined via STM. These results were compared with the lateral size distribution being investigated by TEM and allow a conclusion on the shape of the deposited alloy clusters. Furthermore, the behavior of the alloy clusters on the W(110) surface at elevated temperatures has been examined, at which the clusters show anisotropic spreading.     

Effect of temperature on growth and structure of carbon nanotubes by chemical vapor deposition

The effect of temperature on growth and structure of carbon nanotubes (NTs) using chemical vapor deposition (CVD) has been investigated. Iron embedded silica was used to grow NTs in large quantity at various temperatures from 600 to 1050 °C with gas pressure fixed at 0.6 and 760 Torr, respectively. The growth and structure of the NTs are strongly affected by the temperature. At low gas pressure, the NTs are completely hollow at low temperature and bamboo-like structure at high temperature. While at high gas pressure, all the NTs are bamboo-like structure regardless of temperature. The diameter of NTs increases significantly with temperature. At low gas pressure the diameter gets bigger by mainly increasing the number of graphene layers of the wall of NTs, whereas at high gas pressure the diameter gets bigger by increasing both the number of graphene layers of the wall and the inner diameter of the NTs. This result indicates that the growth temperature is crucial in synthesizing NTs with different structures. The findings here are important for realizing controlled growth of NTs for their applications in different fields. Received: 20 November 2001 / Accepted: 21 November 2001 / Published online: 4 March 2002     

SEM and HRTEM analysis of ZnS nanoflakes produced by?a?simple route

In this work, we report the synthesis of ZnS nanostructures by a simple and eco-friendly method that makes possible producing nanoflakes at room temperature. Scanning electron microscopy and transmission electron microscopy methods (mainly bright-field, high resolution and high angle annular dark-field) were used to identify and study the obtained nanostructures. The structure of these nanoflakes consists of nanosized crystalline particles around 1.5 to 3 nm. Domains with different contrast of nanometer-size diameters are formed in the self-assembled nanoflakes as a result of a noncompact arrangement of nanocrystallites during agglomeration and differences in the presence of the organic passivation agent. Agglomeration can be attributed to the amount of crystallites generated at the beginning of the reaction or to an anisotropic interaction between phosphate ions and the surfaces of ZnS clusters, and consequently a bottom-up synthesis is considered, which opens a simple route for the production of nanomaterials with the inclusion of extra elements by a simple way.     

Boundary layer-assisted chemical bath deposition of well-aligned ZnO rods on Si by a one-step method

ZnO seed layers and well-aligned ZnO single-crystalline micro/nanorods were synthesized on bare Si in one step without the assistance of catalysts by chemical bath deposition. Scanning electron microscopy (SEM) images and X-ray diffraction patterns show that the alignment of ZnO rods on Si(100) could be adjusted by varying the substrates’ angles of incline, the reaction temperature, and the precursor concentration. Transmission electron microscopy cross-sectional images demonstrate that a polycrystalline seed layer with (0002) preferred orientation was formed between the well-aligned rods and Si substrate placed vertically while a randomly oriented layer was formed between the randomly aligned rods and Si substrate placed horizontally. The formation of seed layers and alignment of as-synthesized ZnO rods were attributed to the assistance of boundary layers in a chemical bath deposition system.     

Interface interaction and wetting of Sc<Subscript>2</Subscript>O<Subscript>3</Subscript> exposed to Cu–Al and Cu–Ti melts

Scandia is a thermodynamically stable oxide and could be used as a structural material for a crucible in order to avoid a melt contamination. In the present study wetting experiments of Cu–Al and Cu–Ti melts on Scandia substrate were preformed at 1423 K by a sessile drop method. It was established that Al and Ti additions lead to the improved wetting and that the final contact angle decreases with increasing the additives concentration. For Al containing melts, the contact angle changes gradually with time, and a relatively thick interaction layer, which consists of Al₂O₃, Sc₂O₃, and metallic channels, was formed at the Sc₂O₃/Cu–Al interface. For Ti containing melts, the final contact angle is achieved already during heating, and an extremely thin layer based on a Ti–Sc–O compound was detected by AES at the Sc₂O₃/Cu–Ti interface. The results of a thermodynamic analysis, which takes into account the formation free energy of the oxides, involved in the systems, and the thermodynamic properties of the liquid solutions are in a good agreement with the experimental observations.     

Laser transfer of diamond nanopowder induced by metal film blistering

Blister-based laser induced forward transfer (BB-LIFT) is a promising technique to produce surface microstructures of various advanced materials including inorganic and organic micro/nanopowders, suspensions and biological micro-objects embedded in life sustaining medium. The transferred material is spread over a thin metal film irradiated from the far side by single laser pulses through a transparent support. Interaction of the laser pulse with the metal–support interface under optimized conditions causes formation of a quickly expanding blister. Fast movement of the free metal surface provides efficient material transfer, which has been investigated for the case of diamond nanopowder and diamond-containing suspension. The unique features of the given technique are universality, simplicity and efficient isolation of the transferred material from the ablation products and laser heating.     

Microstructure and electrical conductivity of YSZ thin films prepared by pulsed laser deposition

Yttria-stabilized zirconia (YSZ) is the most common solid electrolyte material used e.g. in ceramic fuel cells. Thin films of YSZ were deposited on c-cut sapphire single crystals by pulsed laser deposition using a KrF excimer laser focused on a polycrystalline 8 mol% Y₂O₃-stabilized ZrO₂ target. Depending on the substrate temperature and the oxygen background pressure during deposition, different microstructures are obtained. XRD and high-resolution SEM revealed the formation of dense amorphous films at room temperature. At 600°C preferentially (111) oriented polycrystalline films consisting of densely agglomerated nm-sized grains of the cubic phase resulted. Grain size and surface roughness could be controlled by varying the oxygen background pressure. RBS and PIXE evidenced congruent transfer only for a low number of pulses, indicating a dynamical change of the target stoichiometry during laser irradiation. The in-plane ionic conductivity of the as-deposited crystalline films was comparable to bulk YSZ. The conductivity of initially amorphous YSZ passes a maximum during the crystallization process. However, the relative changes remain small, i.e. no significant enhancement of ionic conductivity related to the formation of a nanocrystalline microstructure is found.     

Modulating the diameter of carbon nanotubes in array form via floating catalyst chemical vapor deposition

Based on the analysis of catalyst particle formation and carbon nanotube (CNT) array growth process in floating catalyst chemical vapor deposition (CVD), delicately controlled gaseous carbon sources and catalyst precursors were introduced into the reactor for the controllable growth of CNT array. The low feeding rate of ferrocene was realized through low-temperature sublimation. With less ferrocene introduced into the reactor, the collision among the in situ formed iron atoms decreased, which led to the formation of smaller catalyst particles. The mean diameter of the CNT array, grown at 800^oC, decreased from 41 to 31 nm when the ferrocene-sublimed temperature reduced from 80 to 60^oC. Furthermore, low growth temperature was adopted in synthesis, through the modulation of the CNT diameter, by controlling the sintering of catalyst particles and the collision frequency. When the growth temperature was 600^oC, the as-grown CNTs in the array were with a mean diameter of 10.2 nm. If propylene was used as carbon source, the diameter can be modulated in similar trends. The diameter of CNT can be modulated by the parameter of the operation using the same substrate and catalyst precursor without other equipment or previous treatment. Those results provide the possibility for delicately controllable synthesis of CNT array via simple floating catalyst CVD.