Academic literature on the topic 'Chemical Vapour Depostion'

High-quality single-walled carbon nanotubes (SWNTs) are a key aspect in the emerging field nanotechnology. Although many approaches have been developed, the research on the synthesis of SWNTs is still needed. In this study, we report the synthesis of high-quality SWNTs by floating catalyst chemical vapor deposition, which employs ferrocene as the catalyst precursors. We obtained massive deposits at low temperature region. The deposits were characterized by scanning electron microscopy, transmission electron microscopy, and visual laser Raman spectroscopy. The Raman spectrum obtained from raw deposits shows clear radial breathing mode at the range from 180cm-1 to 300cm-1 and high-intensity graphite mode at 1577.7cm-1 with a shoulder at 1550.5cm-1, and almost no detectable peak around at 1545cm-1, which is induced by defects, is observed. These results indicate that the deposits are high-quality SWNTs.

Carbon nanotubes (CNT) are a material with unique properties (mechanical, electrical, electrochemical, etc) allied with low density and high specific area. The present paper studied the electrochemical properties of carbon nanotubes growth by Chemical Vapor Depostion (CVD) technique. The samples were characterized by SEM, Raman Spectroscopy and the double layer capacitance of the powders was evaluated in a Teflon capacitor system with a Ag/AgCl (3M) as reference electrode. The catalyst remotion is provided in Hydrochloric acid washing and the wet oxidative treatments promotes the CNT oxidation and increase the pseudocapacitive response.

Well-aligned carbon nanotubes were synthesized in a horizontal furnace using acetylene as the carbon source and argon as the carrier gas through by AAO templates. To cut the upper parts of CNTs out of the template, the temperature of the furnace was first cooled to 350 °C under an argon atmosphere, and the temperature of 350 °C was remained for 5 min under an air atmosphere, then the furnace was cooled to room temperature in air. The as-prepared aligned CNTs were also carefully scratched from the substrate and some of them were placed on a copper grid for high resolution transmission electron microscopy (HRTEM) observations. The morphology of CNTs was examined by scanning electron microscopy (SEM). The results show that well-aligned carbon nanotubes were obtained and the carbon nanotubes on the surface of AAO templates were cut off successfully.

High-quality Ta2O5 thin films for high-density memory devices were prepared at low temperatures by electron cyclotron resonance plasma-enhanced chemical vapor depostion (ECR-PECVD) without postannealing treatment. The effects of deposition temperature on the microstructure, composition, and electrical properties of the dielectric films were studied. The increase in deposition temperature from 145 °C to 205 °C improved the stoichiometry of the Ta2O5 thin films. As a consequence, EBD increased from 3.3 MV/cm to 4.4 MV/cm, and ∊Ta2O5, increased from 14 to 25. Interfacial SiO2 layer was observed by cross-sectional TEM, and its effects on the electrical properties of the overall dielectric film were also studied. The incubation period in which interfacial SiO2 grows was discussed with regard to reactivity between oxygen and Si substrate.

The present work focuses on the changing of the structural characteristics of the grown materials through different material characterization methods. Semiconductor CdSxSe 1-x nano crystallines have been synthesized by chemical vapor depostion. (X- ray Diffraction; XRD), (Field Emission Scanning Electron Microscopy; FESEM), measured the characterization of Semiconductor CdSxSe1-x nano crystallines. The optical properties of semiconductor CdSxSe1-x nanocrystallines have been studied by the photoluminescence (PL) (He-Cd pulsed ultraviolet laser at 325nm excitation wavelength) at room temperature. The results showed the change rule of photoluminsence peak at different S/Se ratios according to the photoluminsence spectral analysis technology. The photuminscence peak can be continuously modulated between (500- 650) nm, so the tunable emission of the materials in the present work have novle applications in the area of bioscience and spectroscopy, etc.

ABSTRACTSupercapacitors are promising candidates for alternative energy storage applications since they can store and deliver energy at relatively high rates. In this work, we integrated large area chemical vapor deposition (CVD) grown three dimensional graphene heterostructures with high capacitance metal oxides (MnO2) to fabricate highly conductive, large surface-area composite thin films. Uniform, large area 3D graphene heterostructures layers were produced by a one-step CVD on nickel foams. MnO2 nanowires were deposited on the as-obtained 3D graphene heterostructures film by a simple chemical bath depostion process. The oxide loading of the 3D graphene/MWNTs/MnO2 nanowires (GMM) composite films can be simply controlled by deposition time and nanowire solution concentration. The surface morphology was investigated by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM), and Energy-dispersive X-ray spectroscopy (EDS) was performed to characterize the MnO2on the surface of the film. By introducing the fast surface redox reactions into the graphene heterostructures film via integrating pseudocapacitive material like MnO2, the capacitive ability of the system enhanced dramatically. Supercapacitor was fabricated based on the 3D graphene heterostructures /MnO2 hybrid film electrodes; the measurements of cyclic voltammetry, and electrochemical impedance spectroscopy (EIS) are conducted to determine its performance for the electrodes of supercapacitors.

In order to understand the influence of various growth and processing procedures on the quality of silicon grown on sapphire (SOS), one needs a way to measure microtwin density as a function of distance from the interface. In previous studies, however, the technique best suited for determining such a density profile, transmission electron microscopy (TEM), has been applied to this problem in a relatively qualitative manner. We have chosen to apply TEM in a more quantitative fashion to this problem by keying our observations to microtwin morphology as determined by trace analysis and stereomicroscopy (Fig.1). These observations indicate that the microtwins occuring in SOS grown by chemical vapor depostition (CVD) are faceted and grow contiguously from the interface. The volume fraction of silicon occuring as microtwins does not decrease away from the interface at as large a rate as indicated by previous TEM studies (Fig.2). Instead, the decrease in the number of twins away from the interface appears to be balanced by an increase in twin size.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2009.
Cataloged from PDF version of thesis.
Includes bibliographical references.
Photoinitiated chemical vapor deposition (piCVD) is developed as a simple, solventless, and rapid method for the deposition of swellable hydrogels and functional hydrogel copolymers. Mechanistic experiments show that piCVD is predominantly a surface reaction, allowing it to coat non-planar geometries such as particles. The process is gentle enough to coat delicate optical sensors without degrading their function. Chemically functional hydrogels can be synthesized by incorporating a comonomer, and this functionality can be nanoconfined to the near surface region. Random amphiphilic copolymer films deposited via piCVD represent a novel polymer film system, and these surfaces present molecular-scale compositional heterogeneities that interfere with protein adsorption events. Also described is the mechanism by which thin films form on non-planar geometries via initiated chemical vapor deposition (iCVD) . The conformality of these films in microtrenches is assessed and an analytical model is developed in order to quantify the sticking probability of the initiating radical. Mechanistic insight from these experiments is used to predict the conformality based on the fractional saturation of the monomer vapor.
by Salmaan Husain Baxamusa.
Ph.D.

A rapid, mask-less deposition technique for writing metal tracks has been developed. The technique was based on laser-induced chemical vapour deposition. The novelty in the technique was the usage of pulsed ultrafast lasers instead of continuous wave lasers in pyrolytic dissociation of the chemical precursor. The motivation of the study was that (1) ultrafast laser pulses have smaller heat affected zones thus the deposition resolution would be higher, (2) the ultrashort pulses are absorbed in most materials (including those transparent to the continuous wave light at the same wavelength) thus the deposition would be compatible with a large range of materials, and (3) the development of higher frequency repetition rate ultrafast lasers would enable higher deposition rates. A deposition system was set-up for the study to investigate the ultrafast laser deposition of tungsten from tungsten hexacarbonyl chemical vapour precursors. A 405 nm laser diode was used for continuous wave deposition experiments that were optimized to achieve the lowest track resistivity. These results were used for comparison with the ultrafast laser track deposition. The usage of the 405 nm laser diode was itself novel and beneficial due to the low capital and running cost, high wall plug efficiency, high device lifetime, and shallower optical penetration depth in silicon substrates compared to green argon ion lasers which were commonly used by other investigators. The lowest as-deposited track resistivity achieved in the continuous wave laser experiments on silicon dioxide coated silicon was 93±27 µΩ cm (16.6 times bulk tungsten resistivity). This deposition was done with a laser output power of 350 mW, scan speed of 10 µm/s, deposition pressure of 0.5 mBar, substrate temperature of 100 °C and laser spot size of approximately 7 µm. The laser power, scan speed, deposition pressure and substrate temperature were all optimized in this study. By annealing the deposited track with hydrogen at 650 °C for 30 mins, removal of the deposition outside the laser spot was achieved and the overall track resistivity dropped to 66±7 µΩ cm (11.7 times bulk tungsten resistivity). For ultrafast laser deposition of tungsten, spot dwell experiments showed that a thin film of tungsten was first deposited followed by quasi-periodic structures perpendicular to the linear polarization of the laser beam. The wavelength of the periodic structures was approximately half the laser wavelength (λ/2) and was thought to be formed due to interference between the incident laser and scattered surface waves similar to that in laser-induced surface periodic structures. Deposition of the quasi-periodic structures was possible on stainless steel, silicon dioxide coated silicon wafers, borosilicate glass and polyimide films. The thin-films were deposited when the laser was scanned at higher laser speeds such that the number of pulses per spot was lower (η≤11,000) and using a larger focal spot diameter of 33 µm. The lowest track resistivity for the thin-film tracks on silicon dioxide coated silicon wafers was 37±4 µΩ cm (6.7 times bulk tungsten resistivity). This value was achieved without post-deposition annealing and was lower than the annealed track deposited using the continuous wave laser. The ultrafast tungsten thin-film direct write technique was tested for writing metal contacts to single layer graphene on silicon dioxide coated silicon substrates. Without the precursor, the exposure of the graphene to the laser at the deposition parameters damaged the graphene without removing it. This was evidenced by the increase in the Raman D peak of the exposed graphene compared to pristine. The damage threshold was estimated to be 53±7 mJ/cm2 for a scanning speed of 500 µm/s. The deposition threshold of thin-film tungsten on graphene at that speed was lower at 38±8 mJ/cm2. However, no graphene was found when the deposited thin-film tungsten was dissolved in 30 wt% H2O2 that was tested to have no effect on the graphene for the dissolution time of one hour. The graphene likely reacted with the deposited tungsten to form tungsten carbide which was reported to dissolve in H2O2. Tungsten carbide was also found on the tungsten tracks deposited on reduced graphene oxide samples. The contact resistance between tungsten and graphene was measured by both transfer length and four-point probe method with an average value of 4.3±0.4 kΩ µm. This value was higher than reported values using noble metals such as palladium (2.8±0.4 kΩ µm), but lower than reported values using other metals that creates carbides such as nickel (9.3±1.0 kΩ µm). This study opened many potential paths for future work. The main issue to address in the tungsten ultrafast deposition was the deposition outside the laser spot. This prevented uniform deposition in successive tracks close to one another. The ultrafast deposition technique also needs verification using other precursors to understand the precursor requirements for this process. An interesting future study would be a combination with a sulphur source for the direct write of tungsten disulphide, a transition metal dichalcogenide that has a two-dimensional structure similar to graphene. This material has a bandgap and is sought after for applications in high-end electronics, spintronics, optoelectronics, energy harvesting, flexible electronics, DNA sequencing and personalized medicine. Initial tests using sulphur micro-flakes on silicon and stainless-steel substrates exposed to the tungsten precursor and ultrafast laser pulses produced multilayer tungsten disulphide as verified in Raman measurements.

Thesis (Ph. D.) -- University of Maryland, College Park, 2007.
Thesis research directed by: Chemical Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.

2012/2013
Il tema fondamentale della mia attività di ricerca di dottorato è stato la produzione e caratterizzazione di interfacce a base di grafene. Negli ultimi dieci anni, il grafene, il singolo strato perfettamente bidimensionale di atomi di carbonio, si è imposto all'attenzione della comunità scientifica come un materiale rivoluzionario con eccezionali proprietà meccaniche, elettroniche e termiche, potenzialmente in grado di superare il silicio nella prossima generazione di dispositivi elettronici. Un problema fondamentale connesso alla produzione del grafene, tuttavia, è la sintesi di film uniformi ed estesi di carbonio, necessaria per espandere la produzione del grafene a livelli adeguati per applicazioni industriali. In questo riguardo, la crescita epitassiale del grafene mediante deposizione in fase vapore (CVD) di molecole idrocarburiche su substrati solidi è al presente uno dei procedimenti più promettenti. La mia attività di ricerca si è incentrata soprattutto sullo studio dell'interazione tra grafene e substrato in sistemi epitassiali grafene/metallo e sui suoi effetti sulla corrugazione dei sistemi con un mismatch reticolare e sulle proprietà elettroniche dello strato di carbonio. A questo scopo, la spettroscopia di fotoemissione dai livelli di core ad alta risoluzione energetica, con la sua eccezionale sensibilità alle condizioni locali degli atomi di superficie e d'interfaccia, è stata la mia tecnica preferenziale. Questa è stata affiancata da altre tecniche sperimentali, in particolare la diffrazione e microscopia a elettroni lenti e la fotoemissione risolta in angolo, così come da calcoli basati sulla teoria del funzionale densità condotti da gruppi collaboratori. L'obiettivo primario è stato conseguire un controllo fine dell'adesione grafene-metallo, fondamentale per capire come le proprietà del grafene cambiano quando l'accoppiamento con il substrato viene variato in modo continuo da interazione forte a quasi-disaccoppiamento. Questo obiettivo è stato perseguito con diverse tecniche. Questa tesi inizia presentando i risultati da noi ottenuti per un sistema grafene/metallo fortemente interagente, grafene/Re(0001), che rappresenta un caso esemplare delle diverse fasi (grafene, carburi, carbonio dissolto nel bulk) che il carbonio può formare per esposizione ad alta temperatura del substrato ad etilene. In questo riguardo, è emerso che le condizioni di temperatura e pressione svolgono un ruolo chiave nel favorire la formazione di una specie di carbonio piuttosto che un'altra. Un'altra scoperta cruciale è che l'instabilità termica ad alta temperatura del grafene/Re(0001) è direttamente correlata alla corrugazione della cella di moiré, in particolare alla presenza di 'avvallamenti', ovvero di regioni fortemente interagenti in cui la rottura del legame C-C è favorita. Oltre alla scelta della specie chimica del substrato, strategie più raffinate per ottenere un controllo accurato dell'interazione grafene-metallo sfruttano, ad esempio, alterazioni geometriche o chimiche del substrato. Utilizzare superfici cristalline con simmetria non-threefold, ad es. superfici vicinali, permette di crescere grafene con una cella di moiré anisotropica, con parametri reticolari non-equivalenti nelle direzioni parallela e ortogonale agli step. Questa opportunità è stata esplorata, durante il mio dottorato, per il caso del grafene su Rh(533). I nostri dati evidenziano un ruolo primario degli step nell'indebolire il legame del grafene con il substrato e nello stabilizzare il legame C-C ad alta temperatura, come finora predetto solo da conti teorici. Un approccio alternativo e versatile per modificare l'interazione grafene-substrato in modo continuo è variando la composizione chimica della sola superficie del substrato. Più precisamente, l'impiego di una lega superficiale di PtRu su Ru(0001), con una concentrazione variabile di atomi di Pt distribuiti omogeneamente nel primo strato, si è dimostrata un'ottima strategia per regolare finemente, in modo controllabile, il livello di accoppiamento tra adesione forte e interazione debole, senza alterare la qualità e periodicità dello strato di carbonio. Un'ulteriore strategia per disaccoppiare il grafene dal substrato è offerta dalla crescita di uno strato di ossido all'interfaccia grafene/metallo, che è di particolare interesse in vista della combinazione del grafene con materiali dielettrici ad alto k in batterie, condensatori e altri dispositivi. Come parte del mio progetto di dottorato, ho partecipato alla caratterizzazione sperimentale del grafene cresciuto epitassialmente su una superficie di Ni3Al e delle modifiche strutturali ed elettroniche indotte dalla formazione di uno spesso strato di ossido all'interfaccia. Questo metodo rappresenta una valida alternativa, a basso costo e non distruttiva, alle convenzionali tecniche di trasferimento finora sviluppate per depositare grafene su supporti dielettrici. L'ultima parte di questo lavoro di tesi è incentrata sull'impiego del grafene epitassiale come 'matrice' per la deposizione ordinata di nanocluster di metalli di transizione, resa possibile dalla corrugazione periodica a lungo raggio del reticolo di moirè. Questo aspetto è stato esplorato, in particolare, nello studio della reattività chimica di nanocluster di Rh supportati da grafene/Ir(111). L'alto grado di cristallinità esibito dai cluster a seguito del loro riscaldamento e la presenza di atomi di Rh non equivalenti con una diversa coordinazione sono stati oggetto di indagine mediante spettroscopia di fotoemissione dai livelli di core ad alta risoluzione energetica. In particolare, mi sono concentrata sull'interazione di questi sistemi con l'ossigeno e il monossido di carbonio, e sul ruolo chiave degli atomi di Rh sotto-coordinati, che sono siti preferenziali di legame nella fase iniziale dell'adsorbimento. Presenterò anche il nuovo metodo che abbiamo recentemente messo a punto per sintetizzare nanoparticelle di Rh altamente ossidate, che potrebbe essere di impatto in relazione alla superiore selettività catalitica mostrata dai film di ossidi metallici verso l'ossidazione del CO e del NO. I grandi progressi degli ultimi anni nel campo dei nanocluster metallici supportati sono molto promettenti per la futura applicazione di questi sistemi nella catalisi e nei dispositivi elettronici di prossima generazione. Oltre alla necessità di produrre reticoli di nanocluster ordinati a lungo raggio, che può essere soddisfatta facendo uso, ad es., di una matrice di grafene, un'altra sfida chiave posta dall'impiego di nanoparticelle su scala sub-nanometrica è rappresentata dal controllo accurato delle loro dimensioni, che ha effetti drammatici sulla morfologia e struttura elettronica di questi sistemi. Per questo motivo, la parte finale della mia tesi è dedicata alla descrizione di un sistema sperimentale avanzato progettato per produrre nanocluster di metalli di transizione selezionati in massa e depositarli a bassa energia su substrati solidi. Questa macchina, che è attualmente in fase di assemblaggio al Laboratorio di Scienza delle Superfici, farà uso di una sorgente di ablazione laser per generare gli ioni e di uno spettrometro di massa a quadrupolo ad alta risoluzione per assicurare una selettività superiore. Una volta operativo, questo sistema, che sarà la prima sorgente di nanocluster selezionati in massa per studi di fisica delle superfici in Italia, metterà a disposizione della comunità scientifica internazionale di utenti di Elettra uno strumento allo stato dell'arte per condurre esperimenti su nanoparticelle supportate e su materiali a base di cluster, in combinazione con tecniche basate sulla radiazione di sincrotrone.
The focus of my PhD research has been on the production and characterisation of graphene-based interfaces. In the last decade, graphene, the perfectly two-dimensional single layer of carbon atoms, has risen to the attention of the scientific community as a revolutionary material with exceptional mechanical, electronic and thermal properties, which could potentially outperform silicon in the next generation of electronic devices. A key issue related to graphene synthesis, however, is the facile production of uniform and extended carbon flakes, which is required to scale up graphene synthesis for industrial applications. In this respect, the epitaxial growth of graphene by chemical vapour deposition of hydrocarbon molecules on a solid substrate is currently one of the most promising routes. My research activity has addressed, in particular, the graphene-substrate interaction in epitaxial graphene/metal systems and its effects on the corrugation of lattice-mismatched systems and on the electronic properties of the carbon sheet. For the purpose of investigating this, high-energy resolution core level photoelectron spectroscopy, with its superior sensitivity to the local environment experienced by surface and interface atoms, has been technique of choice. This has been complemented by other experimental techniques, in particular low energy electron diffraction and microscopy and angle-resolved photoemission spectroscopy, as well as by state-of-the-art density functional theory calculations performed by collaborating groups. The primary objective has been that of achieving fine control of the graphene-metal adhesion, which is key to understanding how the graphene properties evolve as the coupling to the substrate is continuously varied from strong interaction to the quasi free-standing case. This goal has been tackled with several different strategies. This thesis starts by presenting our results for a strongly interacting graphene/metal system, graphene/Re(0001), which represents an exemplary case of the different phases (graphene, carbides and bulk-dissolved carbon) carbon can form upon high temperature ethylene exposure of the substrate. In this respect, the temperature and pressure conditions are found to play a crucial role in favouring the formation of one carbon species over the others. Another key finding is that the high temperature thermal instability of graphene/Re(0001) is directly linked to the corrugation of the moiré cell, in particular to the presence of buckled, strongly interacting regions where C-C bond breaking is favoured. Besides the elemental choice of the substrate, more refined strategies to achieve accurate control of the graphene-metal interaction exploit, e.g. geometrical or chemical modifications of the substrate. Using crystalline surfaces with non-threefold symmetry, e.g. vicinal surfaces, allows growing graphene with an anisotropic moiré cell, with non-equivalent lattice parameters in the directions parallel and orthogonal to the steps. This possibility has been explored for the case of graphene on Rh(533). The obtained data point out a primary role of the steps in weakening the graphene bonding to the substrate and stabilising the carbon layer against defect--induced C-C bond breaking at high temperature, as so far predicted only by theoretical calculations. An alternative, versatile approach to modify the graphene-substrate interaction in a continuous way is by changing the elemental composition of the first layer of the substrate. More specifically, the use of a PtRu surface alloy on Ru(0001) with a variable concentration of randomly distributed Pt atoms in the first layer has proven to be an excellent strategy for tuning the degree of coupling from strong adhesion to very weak interaction in a controllable way, without affecting the quality and periodicity of the carbon layer. A further strategy for decoupling graphene from the substrate is offered by the growth of an oxide buffer layer at the graphene/metal interface, which is of special interest for the prospective combination of graphene with high-k dielectric materials in batteries, capacitors and other devices. As part of my PhD project, I participated in the experimental investigation of epitaxial graphene on a Ni3Al surface and of the structural and electronic changes induced by the formation of a thick interface oxide layer. This method represents a viable, low cost and non-destructive alternative to the conventional transfer techniques so far developed to deposit graphene on dielectric supports. The last part of this thesis work focuses on the employment of epitaxial graphene as a template for the self-assembly of transition metal nanoclusters, made possible by the long-range periodic corrugation of the moiré superlattice. This topic has been addressed, in particular, in the study of the chemical reactivity of Rh nanoclusters supported on graphene/Ir(111). The high degree of crystallinity exhibited by the clusters after annealing and the presence of non-equivalent Rh atoms with a different coordination number are addressed by means of high-energy resolution core level photoelectron spectroscopy. In particular, I have focussed on the interaction of these systems with oxygen and carbon monoxide, and on the key role played by the highly reactive, undercoordinated Rh atoms, which are preferential bonding sites in the early stage of adsorption. I will also present the new method we recently developed to synthesise highly oxidised Rh nanoparticles, which could be of impact in relation to the enhanced catalytic selectivity towards CO and NO oxidation exhibited by metal oxide films. The great progress achieved in recent years in the field of supported metal nanoclusters is very promising for the prospective application of these systems in catalysis and next-generation electronic devices. In addition to the need to producing long-range ordered superlattices of nanoclusters, which can be met by making use of, e.g a graphene template, another key challenge posed by the use of particles in the sub-nanometre scale is represented by the accurate control of their size, which has dramatic effects on the morphology and electronic structure of these systems. For this reason, the final part of my thesis is dedicated to the description of an advanced experimental setup designed to produce size-selected transition metal nanoclusters and soft-land them on solid substrates. This machine, which is currently being assembled at the Surface Science Laboratory, will make use of a laser ablation source to generate the ions and of a high resolution quadrupole mass spectrometer to ensure a superior mass selectivity. Once operational, this system, which will be the first size-selected cluster source for surface science studies in Italy, will provide the international scientific community working at Elettra with a state-of-the-art tool to conduct experiments on supported nanoparticles and cluster-based materials, in combination with synchrotron radiation-based techniques.
XXVI Ciclo
1986