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Formation Of Nanocoatings By Laser-Assisted Spray Pyrolysis And Laser Ablation On 2d Gold Nanotemplates by Gayan S. Dedigamuwa A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science Department of Physics College of Arts and Sciences University of South Florida Major Professor: Sarath Witanachchi, Ph.D. Pritish Mukherjee, PhD. George Nolas, PhD. Date of Approval: May 24, 2005 Keywords: titanium carbide, thin films, ferrous oxide, drop casting, nanoparticles Copyright 2005 Gayan S. Dedigamuwa
ACKNOWLEDGMENTS I would like to thank Dr. Sarath Witanach chi, my supervisor for his suggestions and unwavering support. Everything I know about research has come from him. His vast experience and creativity helped me greatly along the way, and were essential to completion of this thesis. I would also lik e to thank Dr. P. Mukherjee for agreeing to serve as my faculty advisor, and providing advice when I needed it. Further, I would like to thank Dr. Ma thews and August for providing the AFM facilities where I needed some time in this project. Also I would like to thank all my lab mates specially Ted Wangensteen and Post Do c. Nagaraja Hosakoppa the assistant they gave me throughout the project. And last, but not least, thanks to my parents Premadase and Jayanthi who encouraged me along and helped me through the difficult times.
i TABLE OF CONTENTS LIST OF FIGURES iii ABSTRACT v CHAPTER 1. INTRODUCTION AND BACKGROUND 1 1.1. Introduction 1 1.2. Bottom-up approach to nanostructure growth 4 1.2.1. Molecular Self Assembly 6 1.2.2. Langmuir-Blodgett Technique 7 1.2.3. Drop-Casting (Vapor-Phase Self Assembling) Technique 7 1.3. Top-down approach to nanostructure growth 8 1.4. X-ray diffraction analysis 10 1.5. Atomic Force microscopy 10 1.6. Titanium carbide nano-particle coatings 12 1.7. Ferrous Oxide (Fe3O4) nano-particle coatings 13 CHAPTER 2. SYNTHSIS OF TiC AND Fe3O4 NANOPARTICLES COATING BY LASER-ASSISTED SPRAY PYROLYSIS 15 2.1. Spray Pyrolysis 15 2.2. Ultrasonic Nebulizer 15 2.3. Laser-assisted Spray Pyrolysis 16 2.4. Procedure 17 2.4.1. Synthesis of Tita nium Carbide (TiC) 18 2.4.2. Synthesis of Ferrous ferric oxide (Fe3O4) 19 CHAPTER 3. CHARACTERIZATION OF FILMS DEPOSITED BY SPRAY PYROLYSIS 20 3.1. Crystallinity of nanocoating 20 3.2. Effect of Laser heati ng of nanocoating of TiC 23 3.3. Particle Size Analysis 25 CHAPTER 4. ANALYSIS OF GROWTH PARAMETERS 41 4.1. Effect of Flow rate on Laser Heating 42 4.2. Thermal Model for Laser-Assisted Spray Pyrolysis 47 4.3.1 On-axis Laser heating 50
ii CHAPTER 5. NANOGRAINED FILM S GROWN BY CHEMICAL SELF ASSEMBLY AND VAPOR PHASE GROWTH 57 5.1. Bottom-up approach for Nano-Structured film Growth 57 5.1.1. Laser Ablated film Growth on nanotemplates 59 5.1.2. Pulse Laser Deposition (PLD) 60 5.2. Procedure 61 5.2.1. Substrate Preparati on for Gold Synthesis 61 5.2.2. Synthesis of Gold nanoparticles 2D Coating 61 5.3 Deposition of TiC on Gold nanoparticles 64 CHAPTER 6. CONCLUSION 65 6.1. Laser Assisted Spray Pyrolysis 65 6.2. TiC Growth on Gold nanoparticles 66 REFERENCES 67
iii LIST OF FIGURES Figure 1.1. Yield strength of Fe-Co alloys 1/d1/2 3 Figure 2.1. Schematic diagram of experimental setup. 17 Figure 3.1. XRD pattern of TiC without laser. 20 Figure 3.2. XRD pattern of TiC with a laser. 21 Figure 3.3. XRD pattern of Fe2O3 with a laser. 22 Figure 3.4. AFM image of TiC without laser. 23 Figure 3.5. AFM image of TiC with a laser. 24 Figure 3.6. AFM images of Fe2O3 film (0.1M Fe (CO)5 400 0C, initial growth). 26 Figure 3.7. AFM image of Fe2O3 film (0.25M Fe (CO)5 400 0C). 27 Figure 3.8. AFM image of Fe2O3 film (0.2M Fe (CO)5 400 0C). 28 Figure 3.9. AFM image of Fe2O3 film (0.1M Fe (CO)5 400 0C). 29 Figure 3.10. AFM image of Fe2O3 film (0.05M Fe (CO)5 400 0C). 30 Figure 3.11. Particles size histog ram at concentration 0.25M. 32 Figure 3.12. Particles size hist ogram at concentration 0.2M. 33 Figure 3.13. Particles size hist ogram at concentration 0.1M. 34 Figure 3.14. Particles size hist ogram at concentration 0.05M. 35 Figure 3.15. Particle size dependence on th e concentration of the precursor. 36 Figure 3.16. Comparison of the computed & experimental di ameter of the grain sizes with changing concentration. 39
iv Figure 4.1. Experimental set-ups for study of flow rate effect on Laser heating. 42 Figure 4.2. Graph of Temperature of flow verses Flow rate. 43 Figure 4.3. Graph of absorbed la ser energy verses flow rate. 44 Figure 4.4. Normalize graph of temperatur e and laser energy ve rses flow rate. 45 Figure 4.5. Change in temperature of the flow with diameter of the nozzle. 46 Figure 4.6. Schematic diagram of focussing of collimated beam to the nozzle. 47 Figure 4.7. Graph of temperature verses flow rate when the nozzle is perpendicular to flow. 49 Figure 4.8. Experimental arrangement s for parallel configuration. 50 Figure 4.9. Parallel configuration of the nozzle. 52 Figure 4.10. Graph of temperature verses flow rate when the nozzle is parallel to flow. 54 Figure 4.11. AFM images for parallel configuration. 55 Figure 5.1. TEM image of 2D self-organiz ed particle (done by other group). 59 Figure 5.2. A schematic diagram of a standard PLD system. 60 Figure 5.3. AFM images of Gold nanoparicles. 62 Figure 5.4. AFM images of Gold nanoparicles after heating. 63 Figure 5.5. AFM images of TiC top of Gold nanoparticles. 65
v Formation of Nanocoatings by Laser-Assi sted Spray Pyrolysis and Laser Ablation on 2D Gold Nanotemplates Gayan S. Dedigamuwa ABSTRACT This thesis describes a new Laser-Assis ted Spray Pyrolysis technique developed to grow nanoparticle coatings with controllable particle size s. In this method, droplets of a precursor formed by a nebulizer are in jected into a growth chamber using SF6 carrier gas. An experimental study and a computati onal model to investig ate the particle size dependence on various growth parameters have been carried out. Th e results show that heating of 1.5 m droplets of metalorganic precurs or in a carrier gas using a CO2 laser resulted in the formation of TiC and Fe3O4 particles with diameters in the range of 5060nm. Also the results show that by reducing the concen tration of the metal organic precursor the diameter of the depos ited particles can be reduced. In addition, we have investigated the fo rmation of 2-D super lattices of gold nanoparticles on silicon substr ate by using a technique calle d vapor-phase self-assembly, and have used this monolayer as a base coating for growth of TiC coatings with controlled grain sizes. The initial growth of TiC by laser ablation on heated templates indicates preferential nucleation of TiC on Gold Nanoparticles.
1 CHAPTER 1 INTRODUCTION AND BACKGROUND 1.1 INTRODUCTION Thin film coating technology is now being strongly influenced by nanotechnology. Nano-scale materials, charac terised by grain sizes of less than 100nm are being investigated and si gnificant advances are being made for industrial use. Thinking of small has been a global trend sinc e the first calculator was built and nearly filled a small room. Since then, how to make a product smaller, more portable, and easier to manipulate and maneuver has been on the minds of innovators. But in recent years, small has taken on a whole new dimension. Many developers are now thinking all the way down to one-billionth of a meterthe nanometer. A nanometer is 80,000 times smaller than the width of a human hair. When controlling matter at this scale, different laws of physics come into play. The transition from micro to nano can lead to a number of changes in physical properties. Two of the major factors in this are the in crease in the ratio of surface ar ea to volume, and the size of the particle moving into the realm where quantum effects predominate. The increase in the surface-area to-volume ratio, which is a gradual progression, as the particle gets smaller, leads to an in creasing dominance of th e behavior of atoms on the surface of a particle over which of those in the interior of the pa rticle. This affects both the properties of the particle in isolati on and its interaction w ith other materials.
2 High surface area is a critical factor in perfor mance of catalysis and structures such as electrodes, allowing improvement in performa nce in devices such as fuel cells and batteries. The large surface area of nanoparicles also results in substantial interactions between the intermixed matters in nanocomposite s, leading to special properties such as increased strength and /or heat resistance, ultraviolet (UV) blocking, anti-static, and conductive capabilities. Paint and coating industries were among the first to take advantage of these capabilities. Companies also found that with the incorporation of nanoparticles and thin film nanocoatings have stronger bonds and better flexibility, with little cost differences. These coatings are smoother, stronger, and more durable. When used on products, the results range from scra tch-resistant and self-cleaning surfaces to moisture-absorbing clothing. Also it is well known that when the coa ting is composed of nanocomposites, the hardness of the coating is a ltered according to the Hall-Pe tch relation. The Hall-Petch relation describes the de pendence of hardness ( H) on the grain size in comparison to the conventional material. The relationship is as follows. H =H0 + kd-1/2 . . (1.1) Where H0 and k are constant, d is the average size of the grains. This relationship has been confirmed in both theory and practice in many metallic materials.
3 Figure 1.1: Yield strength of Fe-Co alloys 1/d1/2. (Adapted from C. H. Shang et al ., J. Mater. Res 15, 835, 2000 ) Figure 1.1 shows the measured Yield strength of Fe-Co alloys as a function of d. Assuming that the equation is valid for nanos ized grains, a bulk material having 50nm grain size would have a yield st rength of 4.14 Gpa. The reason for the increase in yield strength with smaller grain size is that mate rials having smaller grains have more grain boundaries, blocking dislocation movement. 1 Additionally, the fact that nanoparticles have dimens ions below the critical wavelength of light makes them transparent, a property which makes them very useful for applications in packaging, cosmetics and clear coatings.
4 These wide ranges of applications with na noparticles and coatings have led to the development of a variety of methods and th e growth techniques of making nanocoating. The growth techniques available in the industr y can be mainly identified as physical and chemical synthesis. It can also be identified as bottom-up and top-down growth approach depending on the way that the coatings are being laid out on the substrate. 1.2 BOTTOM-UP APPROACH TO NANOSTRUCTURE GROWTH The idea of this method is to create the assembly of nanoparticles into welldefined two-dimensional arrays on the substrat e. These 2D nanostructures are important because they have ultimate properties that can be critically determined by the topology and characteristic length scal es of the network. Therefore, the control of the lateral ordering of the particles length scale appears the most challenging issue in this domain. Unlike micron size hard spheres, the organiza tion of nanoparticles on a substrate is not simply entropy driven. Instead, interactions between particles, interaction between particle and substrate, and size distribution of the particles also play an important role in determining the packing morphology of the arra ys. These challenges kept most of the researchers to work on other alternating me thods such as top-down approaches. But recently, significant progress has been made to accomplish this task. The first scientific approach in this se nse has been reported at the beginning of 19th century. The scientists tried to make me tal and gold nanoparticles in particular to cure ailments, although without understanding the exact nature of these particles. More recently though, a plethora of methods have been devised for the synthesis of gold 2,3, silver 4,5, platinum 6 and palladium 7,8 nanoparticles in variable sizes and shapes. In 1951,
5 Turkevich et al developed a method for the pr eparation of gold partic les using citrate as the reducing agent, yieldi ng stable nanoparticel soluti ons of low polydispersity. Hereafter, the solution will be referred to as polydisperse if the standard deviation exceeds 20% of the average value. Since the work of Turkevich, considerable effort has been devoted to developing new synthetic me thods aiming at achieving better control of the size, the shape and the di stance between the particles for the purpose of nanoparticle assembly. Following the basic principle of this method, different groups have successfully synthesized gold nanop articles in colloidal form 9,10. They used reduction of HAuCl4 with trisodium citrate as the nuclea ting and reducing agent. By controlling concentration of reducing agent they have successfully showed that the particle size can be changed. This is not the only method that has been developed in making nano particles with narrow size distribution. The reverse micelle 11 is also a well-known method that one can use. In this method, gold nano particles we re prepared by the re duction of CTAB/octane +1-butanol/H2O reverse micelle system using sodium tetrahydriborate (NaBH4) as a reducing agent. A dodecanethiol (C12H25SH) was used to passivate the gold nanoparticles immediately after the formation of the gold colloid. By re-dispersing in toluene under ultrasonica tion, a supernatant contai ning nearly monodispersed dodecanethiol-capped gold nanoparticles were obtained. Even though this method is straightforward, the preparat ion of these chemicals should be very precise to get successful results.
6 1.2.1 MOLECULAR SELF ASSEMBLY Molecular self-assembly is one of the techniques, which offers dimensional control in the depositi on of nanoparticles of a fixed size on a substrate. Here, in this method the fabrication of a 2D array of na noparticles on a suitable substrate largely depends on the size of the partic les in a colloidal suspension. Since the polydispersity in particle size prevent the construction of well-de fined arrays, the particle size distribution has to be narrow. Solution-based chemical routes, which are discussed in the above section, are ideally suited for producing mono-dispersed hi gh purity nanomaterials with particle size tunability by adjust ment of chemical concentration. The first step towards controlled partic le production is the in itial isolation of species in the solution. Subsequently, a co mpound within the species is induced to decompose to desired material. If local sepa ration is reached, the system spontaneously reaches a state of low energy by crystallizati on. The removal of excess solution prevents the continuous growth of th e particles. The grown pa rticles have a tendency to agglomerate slowly and loose th eir dispersion. Various organic surfactants that attach to surface molecules of the particles have been used to prevent particle agglomeration. These include fatty acids, alkyl thiols, alkyl di sulfides, alkyl nitriles, and alkyl isonitrales. These surfactants posses end groups that attach to the surface of the particle, and steric repulsive methylene chains that are 8 to 12 units long. Under controlled conditions, the combination of Wan der Waals forces between the particles and dispersive forces cause the particles to selfassemble into closedpacked 2D superlattices.
7 1.2.2 LANGMUIR-BLODGETT TECHNIQUE The LB technique for forming nanoparticles arrays is an important approach in which it is easy to control the preparation condition and to obta in uniform long-range order of nanoparticles. Th e LB-technique is one of the most promising techniques for preparing such thin films as it enables (i) the prec ise control of monolayer thickness, (ii) homogeneous deposition of the monolayer over large areas and (iii) the possibility to make multilayer structure w ith varying layer composition 12. An additional advantage of the LB technique is that monolayers can be deposited on almost any kind of solid substrate. Deposition of films using the LB techique involves di pping a substrate through a monolayer of material on a water surf ace. If the substrate is hydrophobic, the hydrophobic ends of the molecules stick to the su bstrate as it is dippe d through water. As the substrate is withdrawn, the hydrophilic e nds of the monolayer stick to the hydrophilic ends of the deposited film. Using this t echnique multilayers can be manufactured. 1.2.3 DROP-CASTING TECHNIQUE (VAPOR-PHASE SELFASSEMBLING) This appears to be the simplest techni que for the formation of 2-dimensional arrays on a substrate. In this process, a drop of the colloid al nanoparticles is laid on the well-cleaned substrate and let the solvent evaporate slowly in a clean ventilated environment. As the solvent evaporates from the droplet, the particles in the solvent are likely to settle down on lowest possible energy on the substrate. The settlement of the particles on the substrate entirely depends on the size of the particles in colloidal form
8 and the physical forces among them. This me thod has proven very effective in creating sliver and gold 2D arrays. 13,14,15,16 1.3 TOP-DOWN APPROACH TO NANOSTRUCTURE GROWTH The ultimate idea of this technique is to get a uniform nanograined thin film on the substrate. The top-down growth tec hnique is most straightforward method to fabricate a nanocoating. Majority of the na nocoating growth techni ques fall into this category. The main efforts have been to upgr ade the quality of th e films including the surface morphology, density and crystallinit y. Among the several top-down growth methods available for the fabrication of thin films, chemical and physical vapor deposition processes are known to produce the best quality films 17. These techniques enable precise control of the growth process, and have therefore been extensively used for growth of films for device applications. For applications that do not require high quality, a number of more economical t echniques are available. These include electroplating 18, plasma spray 19and screenprinting 20. The simplicity, low capital cost of equipment, and the adap tability to large scale pr ocessing make these methods attractive for coating large surface areas. Apart from all, the most important issue in the coating industry is making films with cont rolled particle sizes. Even though above techniques (except CVD) are used in coating industries, still making nanocoatings with a narrow grain size distribution appe ars to be a big challenge. The spray pyrolysis and chemical vapour deposition (CVD) are the popular techniques that have been developed and ar e being used to overcome this challenge.
9 These techniques are popular because they are easy to handle and suitable to be scaled up to industrialization and also they can be applied to wide variety of materials. The main distinction between spray pyrolysis (SP) and CVD 21 techniques is the way the precursor materials reach the substrate surface. In CVD technique, precursor compounds impinge on the substrate in a vapor phase in such a way that a chemical reaction can take place as soon as (or even before) the substrate surface is reached. In the SP technique, usually liquid droplets splash the substrate, the temperat ure of the substrate evaporates the residual solvent leaving a dry pr ecipitate, and a chemical reaction follows. However, whether or not the initial droplets really splash the substrate depends on the thermodynamic properties of the source solution that contains the material to be deposited. Indeed, if the solvent within the droplets is evaporated before they reach the substrate surface, the SP technique may lead to a growth process similar to that of chemical vapor deposition for nanoparticle coatings. Here, in this work, two techniques for nanostructured film growth have been investigated. First, a laser-assisted spray pyrolysis technique was used, which is a topdown method, to deposit TiC and Fe3O4 nanoparticle coatings. The distribution of the particle sizes produced by this method and the ability to control the particle size by controlling the precursor concentr ation have been investigated. Second, gold nanparticles were formed by using sodium citrate as the reducing agent (Turkevich method) and transferred to the silicon substrate by drop casting to form 2D arrays of templates. TiC films were deposited on these 2D templates to study the specific nucleation and subsequent growth.
10 The films deposited by both spray pyrolysi s and laser ablation were characterized by X-ray diffraction and Atomic Force Microscope. 1.4 X-RAY DIFFRACTION ANALYSIS X-ray powder diffraction (XRD) is one of the most powerful techniques for qualitative and quantitative an alysis of crystalline compound s. The information obtained include types and nature of crystalline phases present, structural makeup of phases, degree of crystallinity, amount of amorphous cont ent, size and orientation of crystallites. When a material (sample) is irradiated with a parallel beam of monochromatic Xrays, the atomic lattice of the sample acts as a three dimensional diffraction grating causing the X-ray beam to be diffracted to sp ecific angles. The diffraction pattern, that includes position (angles) and intensities of the diffract ed beam, provides structural information about the sample. 1.5 ATOMIC FORCE MICROSCOPY ANALYSIS Atomic force microscopy ( AFM ) is a method of measuring surface topography on a scale from angstroms to 100 microns. The technique involves imaging a sample through the use of a probe, or tip, with a ra dius of 20 nm. The tip is held several nanometers above the surface using a feedback mechanism that measures surfacetip interactions on the scale of na nonewtons. Variations in tip he ight are recorded while the tip is scanned repeatedly across the samp le, producing a topographic image of the surface. There are three different modes i nvolving AFM. They are contact, tapping and non-contact. The basic modes of AFM operation are outlined below.22
11 Contact mode is one of the widely used scanning probe modes, and operates by rastering a sharp tip across the sample. An extremely low force (~10-9 N, interatomic force range) is maintained on the cantilever, th ereby pushing the tip against the sample as it rasters. Either the repulsive force be tween the tip and sample or the actual tip deflection is recorded relative to spatial variation and then converted into an analogue image of the sample surface. Tapping mode is the next most common mode used in AFM. When operated in air or other gases, the cantilev er is oscillated at its res onant frequency (often hundreds of kilohertz) and positioned above the surface so th at it only taps the surface for a very small fraction of its oscillation period. This is still contact with the sample in the sense defined earlier, but the very short time over which this contact occurs means that lateral forces are dramatically reduced as the tip scans over the surface. When imaging poorly immobilised or soft samples, tapping mode may be a far better choice than contact mode for imaging. Non-contact operation is another method, whic h may be employed when imaging by AFM. The cantilever must be oscillated above the surface of the sample at such a distance that we are no longer in the repulsive regime of the inter-molecular force curve. This is a very difficult mode to operate in ambient conditions with the AFM. The thin layer of water contamination, which exists on the surface on the samp le, will invariably form a small capillary bridge between the ti p and the sample and cause the tip to "jumpto-contact".
12 1.6 TITANIUM CARBIDE NANO -PARTICLE COATINGS Titanium carbide (TiC) has been extensivel y used in many applications due to its interesting properties, such as very high melting point (melting temperature 30000 C), high hardness, and thermal stability with excellent wear resistance, high oxidation resistance and low friction coefficient. 23 TiC coatings are widely used on metal cutting tools. TiC has also been utilized fo r diffusion barriers in semiconductor devices 24 and for thermal barrier coatings in fusion reactors 25. Moreover it also can be used for coating steel bearings to reduce fr iction by exploiting the high yi eld stress of the coating. 26 A TiC coatings used in such applications is gene rally crystalline and, though fairly effective, may exhibit (under extreme chemical, thermal, and mechanical conditions) some of the shortcomings inherent to crystalline mate rials, in particular, grain-boundary corrosion and decohesion 27. Hence, an amorphous, semimetallic chemically inert, low-friction form of titanium carbide could represent a significant advance in materials engineering. Amorphous films of TiC have been gr own by metallorganic chemical vapor deposition (MOCVD) from titanium-coordination compound tris(2,2-bipyridine) titanium by Morancho and Ehrhard.28 This approach was chosen because the molecules of metalorganic compounds dissociate at re latively low temperature, 200 to 500 0C, compared to the temperature of 1100 to 1350 0C that are required for crystalline TiCfilm growth using precursors titanium tetrachloride, methane, and hydrogen 29. We have used the titanium-coordinati on compound cyclopentadiethyltitanium trichloride in the Laser Assisted Spray Pyro lysis (LASP) process to deposit nanoparticle coating of TiC. This compound has a dissociation temperat ure close to 350 0C.
13 1.7 FERROUS OXIDE (Fe3O4) NANO-PARTICLE COATINGS Magnetite, Fe3O4 is an extensively studied material due to its several interesting properties. Magnetite, Fe3O4 is a mixed-valence 3d transiti on metal oxide that has an inverse spinel structure (space group Fd3m ) with a lattice constant of 0.8397 nm. The tetrahedral sites of the spinel structure are entirely occupied by Fe3+, whereas the octahedral sites are occupied half by Fe2+ and half by Fe3+. Fe3O4 undergoes a metal-toinsulator Verwey transition at 120 K and th e Curie temperature of magnetite is 860 K. Spintronic materials are cu rrently receiving a lot of attention due to potential applications in giant magnetoresistive (GMR ) devices such as magnetic field sensors, magnetoresistive random access memories (MRAM), read heads, and galvanic isolators. These devices require a source of spin-pol arized electrons. As the GMR effect is originated from the spin dependent scatte ring near interface between magnetic and nonmagnetic layers, it is possible to utilize half-metallic ferroma gnets, in which the spin of electrons at Fermi surface is comp letely polarized. Magnetite, Fe3O4, is a promising source of spin-polarized carriers, because sp in-resolved density of states calculations have suggested that electrons at the Fe rmi level are 100% spin polarized. Although experiments have not confirmed the predicted complete half-metallicity of Fe3O4, they do show that the number of minority electrons is much larger than th e number of majority electrons at the Fermi level. Recently, GMR effects greater than 500% have been reported at room temperature for Fe3O4 nanocontacts.30 Although the fabrication methods as well as the physical properties of Fe3O4 were extensively studied in 1980s, th e need for high storage density still provides considerable impetus to new fabrication technologies for obtaining Fe3O4 thin films, which serve as a
14 precursor for GMR devices. The response of GMR-based devices depends critically on the physical structure of the films, with para meters such as layer thickness, and chemical abruptness and roughness of the interfaces bei ng crucial. In additi on, transition of grain structure of polycrystalline Fe3O4 films from micro regime to nano regime is expected to alter the spin related magnetic properties. This comes about as a result of approaching single domain characteristics wi th decreasing grain size. There have been many studies on synthesizing high quality Fe3O4 thin film and tunnel junctions using Fe3O4. A variety of techniques in cluding molecular beam epitaxy 31, sputtering, 32 pulsed laser deposition and chemical s ynthesis have been used to deposit Fe3O4 films. We have used laser-assisted sp ray pyrolysis technique to grow nano-grained coatings of iron oxide. The size of the grains in the nano regime can be easily controlled by this method. The details of the growth pr ocess are presented in next chapters.
15 CHAPTER 2 SYNTHESIS OF TiC AND Fe3O4 NANOPARTICLE COATING BY LASER ASSISTED SPRAY PYROLYSIS 2.1 SPRAY PYROLYSIS The spray pyrolysis technique is particular ly interesting since it can be used to form coatings of variety of different materi als. The main component of spray pyrolysis system is an atomizer that generates microdroplets of precursor so lution dissolved in a relatively volatile solvent. The droplets in th e form of a fine spray are carried out of a nozzle onto a heated substrate by a carrier gas that can be iner t or reactive. The constituents of the droplet decompose and reac t on the hot substrate to form the chemical compound. The substrate temperature should be high enough to evaporate the volatile solvents. The spray nozzle is usually scanne d continuously during the growth to coat a large area of the substrate. 2.2 ULTRASONIC NEBULIZER The sizes of the produced droplets depend on the technique used to atomize the solution. The simplest way to generate an ae rosol spray is by a pneumatic process. In this method, the pressure drop at the orifice of a nozzle due to high flow gas causes the atomization of the solution. However, contro l of the particle si ze distribution produced by this method is very difficult. On the other hand, ultrasonic nebulizers are known to
16 produce a fairly uniform dist ribution of micrometer size droplets. Generally, the nebulizer is operated at a fr equency of 2.4MHz, where the precursor solution is converted into a mist of particle s in the range of 1-2 m in diameter. The diameter of the particle has a dependency on the operating frequency of the nebulizer. As produced these particles lack sufficient inertia and thus, have to be transported by carrier gas. This gas flow is usually about 3 to 4 cc/s. Since the aerosol-generating rate is independent of the flow rate, the tran sport of the droplets to the substrate can be controlled without affecting th e function of the nebulizer. 2.3 LASER ASSISTED SPRAY PYROLYSIS This is a new technique th at one can use to get finer and more concentrated droplets of precursor compared to regular spray pyrolysis. The only difference is, the droplets interact with a continuous wave (CW) CO2 beam as they come out of the nozzle. If the molecules of the precursor have a strong resonance absorption band at the wavelength of the laser beam, the molecule is dissociated. This method has been successfully used to form am orphous nanopartcles of Si/N/C 33. However, use of this method is restricted to compounds that have resonance absorption ba nds at an available laser wavelength. This restrict ion is avoidable by introducing a carrier gas that has a high absorption in the CO2 wavelength. In this way, the la ser energy is transfered to the molecule in the form of heat.
17 2.4 PROCEDURE Figure 2.1 Schematic diagram of the experimental setup. The schematic diagram of the experiment al apparatus is shown in fig 2.1. A deposition chamber was made of Glass. A CW CO2 laser of 3W was focused to a point just above the funnel tube orifice. Since th e proposed precursor doe s not have resonance absorption with CO2 wavelength, sulfurhexafluoride (SF6) is used as the carrier gas for aerosol transport. A CO2 laser beam of wavelength 10.6 m was resonantly absorbed into the SF6 molecules through vibrational excitation. Therefore, the carrier gas was heated by the CO2 laser as the aerosol/gas mixture was inj ected into the chamber. The flow rate and thus the speed of the aeros ol into the chamber were cont rolled by a gas flow meter. The pressure inside the chamber was kept very close to atmospheric. Also at the same time, the substrate was heated to promote f ilm growth. When the droplets impinge upon SF6 CO2 Laser Substrate heater Vacuum N2 Precurso r Nebulize r
18 the heated substrate the subsequent film fo rmation is dependant on the velocity of the drop, rate of reaction and the rate of evapora tion of the solvent. At high velocities the droplets will flatten on the substr ate leading to large particle sizes. On the other hand, if most of the solvent is evaporated when the dr op gets to the substrate, the solid particles will stick to the substrate to form a crystallit e smaller than the initial droplet. Therefore by adjusting the concentration of the solvent, the size of the part icles depositing on the substrate can be controlled. 2.4.1 SYNTHESIS OF TITANIUM CARBIDE (TiC) TiC nanoparticle coatings were synthesized by the laser assist ed spray pyrolysis method. The precursor was prepared by di ssolving the TiC containing organometallic compound, cyclopentadiethyl titanium trichloride (C5H5TiCl3 crystalline from Alfa Aeser) in toluene with different concentrations. Th e Si(100) substrates were cleaned with an ultrasonic agitator in repeated baths of ethanol and acetone, then rinsed in high-purity deionized water and dried with nitrogen flow prior to loading into the chamber. Next, the prepared substrate was set up on the subs trate holder 4cm away from the nozzle and the holder was heated to 3500C. The nebulizer filled with precursor was setup as shown in fig 2.1. The chamber was continually pumped by a vacuum pump while nitrogen gas was injected to maintain the ambient pressure about 760mmHg. When the holder temperature reached 350 0C, the nebulizer was switch on. The process was run for 20min to get a good layer of TiC. Several sa mples were synthesized by changing the concentration.
19 2.4.2 SYNTHESIS OF FERROUS OXIDE (Fe3O4) Fe3O4 nanoparticle coatings were synthesized by laser assisted spray pyrolysis method. The pentacarbonyliron (Fe (CO)5 99.5% from Alfa Aesar) in toluene based solution was used. All the parameters were kept as same as for titanium carbide synthesis. The process was run for 15min. Several sa mples were synthesized with precursor concentrations 0.05M, 0.1M, 0.2M and 0.25M.
20 CHAPTER 3 CHARACTERIZATION OF FILMS DE POSITED BY SPRAY PYROLYSIS 3.1 CRYSTALLINITY OF NANOCOATINGS X-ray diffraction (XRD) wa s performed on TiC and Fe2O3 samples. Since prepared samples were typically 800-2500Ao, a thin film attachment was used. The Scans were performed over 2 = 20-80o for each sample. Fig 3.1, 3.2 and 3.3 respectively show TiC and Fe2O3 representative XRD patterns. Figure 3.1: X-ray diffraction pattern of a TiC nanograined film deposited by spray pyrolysis without laser heating
21 Figure 3.2: X-ray diffraction pattern of a TiC nanograined film deposited by spray pyrolysis with laser heating. Fig 3.1 and 3.2 show the comparison of the XRD patterns of the TiC deposited on Si(100), which were made with and without a laser. Both indicate crystalline peaks of TiC (111), (200) and (220). Also it i ndicates the crystall ine peaks of TiO2 (112) and (211). The system currently in use for film growth is not high vacuum compatible, and thus, the deposited films are s ubjected to partial oxidation.
22 Figure 3.3: X-ray diffrac tion pattern of a Fe3O4 nanograined film deposited by spray pyrolysis with laser heating. Fig 3.3 shows the XRD patterns of the deposited Fe2O3 on Si(100), which were made by using a laser. It indi cates a crystalline peak of Fe3O4 (220). Also it indicates the crystalline peaks of -Fe2O3 (112). The intensity of the pe aks is relatively low. Also it shows a XRD pattern of an amor phous structure. This peak can be eliminated and a pure film of Fe3O4 can be obtained by sealing the chamber for the oxygen.
23 3.2 EFFECT OF LASER HEATING ON NANOCOATINGS OF TiC The laser heating of the droplets in spra y pyrolysis technique can largely affect the surface morphology of the film The laser heating in this process evaporates the solvent from the droplets and at the same time it decomposes the organomatallic precursor into the metal basics. As a resu lt, the droplets shrink and become denser. Therefore the resultant coating consists of well-defined particles. The thin TiC films were analyzed by atomic force microscopy (AFM). An AFM image of a film deposited without laser heating is show n in (Fig 3.4). The average particle size is about 150nm. Irregular particle shapes and size are visibl e in the three dimensional image. In comparison, a film deposited with CO2 laser heating of the carrier gas shows a distribution of well-defined part icles (Fig 3.5). The average particle size is about 50nm. Figure 3.4: 2D and 3D AFM image of a TiC film deposited by spray pyrolysis without the laser heating.
24 Figure 3.5: 2D and 3D AFM image of a TiC film deposited by spray pyrolysis with the laser heating. In fig 3.4 and 3.5, one can clearly see that in the presence of laser heating the grain sizes are much smaller than in absence of laser heating. Wit hout laser heating the average size of the grains is about 150nm in radius, while with laser heating, the grain size is reduced to about 50nm in radius. The reason for the change in particles size, with and without a laser, can be expl ained in terms of evaporation of the solvent. As a droplet comes out of the nozzle, the laser heating of SF6 results in the evaporation of toluene from the droplet. Then the droplet becomes de nser as compared to the base composition, which is used in the precursor. The hi gh density and smaller si ze reduce the possibility of droplet flattening when colliding with the substrate. This causes the deposited film to be consisting of well-defined part icles as shows in the figure 3.5.
25 3.3 PARTICLE SIZE ANALYSIS With decreasing concentration of the precursor the percentage of the compound present in a 1.5 m droplet also decreases. Since laser heating evaporates the solvent, the solid particle produced will become smaller with decreasing concentration. The films deposited with precursor concentrations of 0.05M, 0.1M, 0.2M, and 0.25M have been analyzed by AFM to study the concentration-particle size re lation in the spray pyrolysis process. Toluene was used as the solvent for all solution. To investigate the initial gr owth of the particles precursor droplets were deposited for 30 second on silicon substrate. The substrate was heated to 400 0C. AFM images of individual particles formed at the early stag e of growth with lase r heating are shown in Fig 3.6. The AFM images show the initial pa rticle sizes to be of about 50-60 nm. (a)
26 (b) Figure 3.6: (a) 2D and 3D AFM image of a Fe3O4 film deposited by spray pyrolysis with the laser heating (1minute deposition) (b) Enlargement of figure (a). The boiling point of toluene is 93 0C. Therefore, if the laser heating is sufficient to heat the carrier gas SF6 above this temperature, complete evaporation of the solvent can be expected.
27 Figure 3.7: 2D AFM image of a Fe3O4 (0.25M Fe (CO)5 ,substrate Temp 400 0C) film deposited by spray pyrolysis with the laser heating.
28 Figure 3.8: 2D AFM image of a Fe3O4 (0.2M Fe (CO)5 substrate Temp 400 0C ) film deposited by spray pyrolysis with the laser heating
29 Figure 3.9: 2D AFM image of a Fe3O4 (0.1M Fe (CO)5 substrate Temp 400 0C ) film deposited by spray pyrolysis with the laser heating
30 Figure 3.10: 2D AFM image of a Fe3O4 (0.05M Fe (CO)5 substrate Temp 400 0C ) film deposited by spray pyrolysis with the laser heating
31 The above figures (from fig 3.7 to 3.10) show the cross sectional AFM analysis of samples with different concentrations. They s how the cross sectional diameter of grains in each samples, which represent the mean di ameters of their particle distributions. To select the mean particle from the distribut ion, a statistical method was used as follows; This method is based on collecting random pa rticles (about 50 or more) from each sample. To determine this change, a stat istical study was conducted on each sample. First, the diameter measurements of 50 ra ndomly selected particle s in each particle distribution were recorded followed by the c onstruction of a histogram of number of particles counted verses diameter for each samp le. The results were fitted into a standard Gussian distribution to determine the mean va lue. Particle distribution for films deposited with the concentration of 0.25M, 0.2M, 0.1M and 0.05M are shown in Fig 3.11, 3.12, 3.13 and 3.14.
32 20406080100120140 0 5 10 15 20 25 30 Data: Data1_B M odel: Gauss Chi^2/DoF= 12.26106 R^2= 0.89415 y01.924551.52302 xc80.751921.72017 w25.253924.10478 A769.05508136.71038percentage (%) out of 50particle size (nm) Gauss fit of Data1_B Figure 3.11: Percentage of particle diameter s counted for each set of 10nm blocks when concentration is 0.25M.
33 020406080100120 0 5 10 15 20 25 30 Gauss fit of Data1_B Data: Data1_B Model: Gauss Chi^2/DoF= 8.42206 R^2= 0.93946 y00.550161.30665 xc74.653841.31949 w26.717393.20314 A904.21293120.63885percentage(%) out of 50particle size (nm) Figure 3.12: Percentage of pa rticle diameters counted for each set of 10nm blocks when concentration is 0.20M.
34 020406080 0 5 10 15 20 25 30 35 Data: Data1_B Model: Gauss Chi^2/DoF= 4.20308 R^2= 0.98519 y0-0.672141.21779 xc57.815450.67958 w24.110471.76411 A1063.6871792.58197pecentage (%) out of 50particle size (nm) Gauss fit of Data1_B Figure 3.13: Percentage of particle diameter s counted for each set of 10nm blocks when concentration is 0.10M.
35 01020304050607080 0 5 10 15 20 25 30 35 40 45 Data: Data1_B Model: Gauss Chi^2/DoF= 16.90307 R^2= 0.96365 y0-0.350772.38838 xc48.572520.98963 w18.943992.50886 A1018.32756152.0197percentage (%) out of 50particle size (nm) Gussian fit of Data 1_B Figure 3.14: Percentage of particle diameter s counted for each set of 10nm blocks when concentration is 0.05M.
36 Figure 3.15: Particle size dependence on the concentration of the precursor. The graph above shows the particle size va riation with the concentration for the sample. According to the graph, it is clea r that the size of the particles have been decreasing with the con centration. Also this variation can be perfectly fitted with a 3rd order polynomial ( (x) = -1000x 3 + 240.67x 2 + 166.2x + 39.783). The AFM analysis of the different samples shows a clear change in grain size in different concentrations. A si mple model can be used to co mpute the expected particle size for a given concentration. In this anal ysis, it is assumed that the droplet is 1.5 m in 48.57 57.81 74.65 80.7530 40 50 60 70 80 90 100 00.050.10.150.20.250.3concentration in molediameter of the particles (nm)
37 diameter and the solvent is completely ev aporated from the droplet by laser heating before it arrives at the substrate. Also the drop remains a spherical particle on the substrate after the collision. The model is outlined below. The case where the concentration is 0.05M, the number of Fe (CO)5 molecules in a 1.5 m droplet of Fe (CO)5 (formula weight 231.4) in toluene are; = 5.32 10 7 Since 3 molecules of Fe (CO)5 lead to a Fe3O4 molecule after dissociation, the number of Fe3O4 molecules in a 1.5 m droplet are; = 5.32 10 7 = 1.77 10 7 Assuming the final product to be a solid spherical particle of Fe3O4 with a density of 5.7 g/cm3 (bulk density of Fe3O4), the size of the Fe3O4 particle produced by a droplet can then be calculated as follows. (Since V= M / ) 3
38 Table 3.1, shows the computed average particle size for c oncentrations used in the experiment Concentration (M) Size of the particle (nm) 0.05 131.69 0.1 165.92 0.15 189.93 0.2 209.05 0.25 225.52 Table 3.1 the computed average particle size for different concentration.
39 48.57 58.16 74.65 80.75 131.69 165.92 209.05 225.5240 90 140 190 240 00.050.10.150.20.250.3 concentraion in molediameter of the particles (nm) Experimental Calculated Figure 3.16 Comparison of the computed & experi mental diameter of the grain sizes with changing concentration. The computed grain sizes of each concentr ation along with the experimental data are shown in graph 3.16. It is clear that there is a consid arable difference between the calculated and the experimental graphs. The cal culated values became more than twice as the experimental values. This difference can occur due to several reasons. The first reason is, the concentration of the solution that was considered in the calculation is much higher than the concentra tion of prepared precursor. This is because the organomatallic precursor, Pentacarbonyliron, is in liquid form and is highly light sensitive. Exposure to light and air causes iron pentacarbonyl to gradually decompose.
40 This leads to a reduction in concentration with time. For example, the actual molar ratio of 0.05M solution could be much lower. The second reason is, in the calculation we assumed that only the solvent in the droplet is evaporated. In addition to the solv ent the precursor will also evaporate in the hot SF6 gas, when the drop travels through the laser spot. If the drop falls on the heated substrate without complete decomposition, further evaporation will make the final particle size much smaller th an the computed valve. Therefore, the number of atoms available in the real situation can be lot less than the calculated amount. The actual concentr ation also can be lot less than in the calculation.
41 CHAPTER 4 ANALYSIS OF GROWTH PARAMETERS As we discussed in the previous chapter, the formation of the film mainly depends on how much energy is absorbed by the dropl et as it passes thr ough the laser beam. There is an optimum condition to maximize th e energy absorbed to the droplet. One of the parameters that determine the maximum ener gy coupling is the flow rate of the carrier gas. The flow rate determines how much tim e the droplets are kept in hot region while it passes through the laser. Wh en the droplet interacts with the hot gas for an extended time, the temperature of the droplet reaches to a higher value and causes solvent evaporation. The optimum condition of the process can be found by studying the energy absorption to the carrier gas and temperature va riation of the carrier gas in different flow rates of carrier gas.
42 4.1 EFFECT OF FLOW RATE ON LASER HEATING The experimental arrangement shown in Fig 4.1 was used to study the effect of the flow rate on laser hea ting of the carrier gas SF6. Figure 4.1 Experimental set-up for study of flow rate effect on Laser heating The temperature of the Laser heated ga s was measured by a K-type thermocouple placed in front of the nozzle ope ning just after the laser focu s. The laser energy absorbed to the flow was measured by placing a en ergy meter on the Laser path, outside the chamber as shown in fig 4.1.The graph 4.2 and 4.3 respectively, show th e variation of the temperature of the flow jet (spray) and the vari ation of the laser energy with respect to the flow rate. Nebulizer SF6 Precursor Flow meter thermocouple Energy meter CO2Laser beam Nebulizer SF6 Precursor Flow meter thermocouple Energy meter CO2Laser beam
43 80 90 100 110 120 130 140 150 00.511.522.533.544.5flow rate (SLPM AIR)Temperature ( C) Figure.4.2: Graph of Temp erature of flow verses Flow rate of SF6 gas. The results obtained in figure 4.1 show th at the temperature is linearly decreasing with increasing of the flow ra te of the carrier gas. This can happen due to two reasons. When the flow rate increases, the droplet has less time in contact with the laser. Therefore, it absorbs less energy from the la ser beam and causes the temperature to drop. The second reason is, as the flow rate is increased, the concentration of SF6 molecule in the chamber also increases. Th en the pumping is not enough to take all the SF6 molecules coming into the chamber. Therefore, SF6 molecules in the chamber absorb some amount of laser energy before the laser interacts with the flow out of the nozzle. This effect leads to a drop in temperature of the flow.
44 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 00.511.522.533.544.5 flow rate (SLPM AIR)absorbed laser energy (W) Figure 4.3: Graph of absorbed laser ener gy verses flow rate of the droplets. The results in figure 4.2 show that the ab sorption of laser energy is linearly increasing with the increasing of flow rate. The reason for this is, as the flow rate increases, the number of SF6 passing through per second incr eases, giving rise to an increase in the amount of energy absorbed.
45 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 00.511.522.533.544.5 Flow rate (SLPM)absorbed laser energy (W)0 20 40 60 80 100 120 140 160temperature C absorb laser energy temperature Figure 4.4: Normalize graph of temperature verses flow rate and laser energy verses flow rate. The figure 4.4 represents the normalized gr aphs of temperature versus flow rate on one scale and absorbed laser energy versus fl ow rate in the other scale. As can be see in the figure, the point where two graphs inte rcept is identified as the optimum point of operation for the experiment. Since the flow ra te is the only varying parameter, the flow rate of the carrier gas has to be fixed at the optimum value, which is 3.0 SLPM AIR. (This Flow rate value depends on the setup. It can be adjusted depending on any changes in the set up such as size of the no zzle and pressure of the chamber).
46 Figure 4.5: Change in temperature of the flow with diameter of the nozzle. The above graph shows the temperature vari ation with respect to flow rates for different nozzle diameter. As shown in th e figure, when the diameter of the nozzle decreases, the temperature of the flow ra te is shifted towards a higher elevation by keeping the gradient almost the same. Ther efore the nozzle size and the focusing of the beam are critical factors to transfer a maximum amount of energy from the laser to the flow. To obtain the maximum amount of en ergy these factors have to be optimized. It can be done using the beam optics. 70 90 110 130 150 17000.511.522.533.544.5Flow rate (SLPM)Temperature ( C ) .66mm 1.7mm 3mm
47 4.2 THERMAL MODEL FOR LASER ASSISTED SPRAY PYROLYSIS As a principle, in beam optics, the beam waist radius is important for maximum energy (power). It is directly related to the intensity of the beam. Within any transverse plane, the beam intensity assumes its peak value on the beam axis, and drops by the factor 1/e2 0.135 at the radial distance W (z). Si nce 86% of power (energy) is carried within a circle of radius W (z), we regard W (z) as the beam radius (also called the beam width). It assumes its minimum value Wo in the plane z=0, called the beam waist. Thus Wo is the waist radius. The waist diameter 2W is called spot size. To absorb maximum amount of energy from the laser beam, the nozzle size has to be the same as the spot size. Since our nozzle size is fixed the only way th is is achievable is by changing the position of the lens so as to match the nozzle size. In our application, for a given (wavelength of CO2 laser), the spot size can be found by using following relation. ( = 10.6 for CO2, Wo =10mm, f = 80mm). 2W = 4 f 2Wo For a CO2 laser, according to the above form ula the minimum spot size is found to be 0.27mm. Figure 4.6 Focusing a collimated beam to matc h with the size of the nozzle diameter. Wo W= 0.27mm f d d nozzle
48 The position of the lens is selected so as to have the required spot size to match the diameter of the nozzle. The experiment was run by keeping the se t up under the configuration shown in fig 4.6. The flow rate versus temperature data for that configuration is shown in fig 4.7. The calculated data is also shown in the same figure. To calculate this data, we used the following mathematical model for this computat ion. We assumed the cross-section of the beam to be a top-hat profile that has a beam width of W equal to the FWHM of the gaussian profile. We assumed this rectangular beam to be interacting with a gas volume of W W d where d is the diameter of the orifice of the tube. R is flow the rate measured. Q = m C T ) 1 ( ) 1 ( ) 1 (2 2 2 2 d d d oe W C R d P T T C d W R Wd e P R Wd v W t But T C d W t W e I Here, = 6.164 kg/m3, C = 598.8 J/Kg.K P = 3 W = 0. 7cm-1 Fl ow I o I W d Intensity change within the absorbing volume I = Io ( 1-e d) = velocityoftheflow
49 A1V1 = flow 0.16 10-4 W=0.5mm. Flow rate (SLPM) Actual Flow Rate (SLMP) Temp (C ) 0.5 0.12 189.64 1.0 0.24 108.32 2.0 0.48 67.66 3.0 0.72 54.01 4.0 0.96 47.33 Table 4.1 Theoretical change of temperat ure with flow rate in perpendicular configuration. Figure 4.7 Graph of Temperature of flow ve rses Flow rate of the droplets when the nozzle is set out of the chamber perpendicular to the flow. 0 50 100 150 200 250 300 350 00.511.522.533.544.5flow rate (SLPM)temp of flow (C) calculated experimental
50 The main drawback for this configuration is when the flow rate is at 3.0 (where is the optimum rate for the experiment) the te mperature of the flow does not reach the decomposition temperature of the chemical. The reason is, the time available for flow to interact with the laser is limited in this configuration. One can increase the laser-gas interaction time by setting the laser parallel to the flow path. In this configuration the laser energy is absorbed by SF6 for a longer time period, and therefore, expect the final temperature of the gas to be much higher. 4.2.1 ON-AXIS LASER HEATING The experimental arrangement for on ax is laser heating is shown in fig 4.8 Figure 4.8 experimental arrangements for parallel configuration. Vacuum SF6 Precursor Nebulizer Flow meter thermocouple N2 substrate Vacuum SF6 Precursor Nebulizer Flow meter thermocouple N2 substrate SF6 Precursor Nebulizer Flow meter SF6 Precursor Nebulizer Flow meter thermocouple N2 substrate
51 The Laser beam was focused to the nozzle opening so that the laser would not hit the glass wall. A nitrogen jet was introduced to change the direction of the flow so that the particles were steered away from the path of the laser. This experiment was run to see the temp erature variation of the flow when the system is outside the chamber. By doing th is, one can avoid the absorption that takes place inside the chamber due to the surrounding SF6 gas. The graph in fig 4.10 shows the temperature variation for diffe rent flow rates. Fig 4. 10 also includes a computed temperature variation with flow rate using a thermal model (fig 4.9). The fig 4.8 shows the situation where the laser beam is directed parallel to the flow. By making the laser beam parallel to the flow, the molecules in the flow can be exposed for a longer length of time to the laser energy. As a result of this, the temperature of the flow went up compared to the perpendicular configuration. The drop in temperature as the flow rate increases is e xpected in both situati ons. At high flow rates the laser-gas interaction time is reduced and the volume of the gas heated per second is increased. Fig 4.9 shows the data for a mathematical model for the configuration where the laser beam is parallel to the flow. The temp erature in this graph for each flow rate was found by integrating a dx cross section at x dist ance through out the path of the particle in a beam to the edge of the nozzle.
52 Figure 4.9 Parallel configuration of the nozzle The change in intensity across an abso rbing laser of thickness dx is given by x dx Wo W= 0.27mm f nozzle A(x) W(x) Io I d(x) x dx Wo W= 0.27mm f nozzle Wo W= 0.27mm f nozzle A(x) W(x) Io I d(x)
53 Carrying out a thermal balance for the laye r of thickness dx at a distance of x, an equation for the temperature can be obtained. Here, D = 6.164 kg/m3, C = 598.8 J/Kg.K P = 3 W = 0. 7cm-1 A1V1 = flow 0.16 10-4 Flow Rate (SLPM) Actual Flow Rate (SLMP) Temperature ( C ) 0.5 0.12 1476.8 1 0.24 756.9 2 0.48 389.4 3 0.72 268.6 4 0.96 208.2 Table 4.1 Theoretical change of temperat ure with flow rate in perpendicular configuration. T dx x x x V DCA P exp 6 03 2 12 1 102 2 1 1 3
54 Figure 4.10 Graph of Temperatur e of flow Vs Flow rate of the droplets when the nozzle is set out of the chamber perpendicular to the flow. Both experimentally measured and com puted temperatures of the gas show a decrease with increase in flow rate. The measured values do not represent the exact temperature of the gas since the gas is significantly cooled by the N2 gas jet when it comes in contact with the thermocouple.However, these measurements can be used to study the trend of temperatur e change with flow rate. A film of iron oxide was deposited on a silicon substrate by using this on-axis heating configuration. The film is deposited only for about 5 minutes to study the initial growth. The following AFM images are taken for the samples, which were made by keeping the nozzle parallel with the beam. 0 200 400 600 800 1000 1200 1400 1600 00.511.522.533.544.5 Flow rate (SLPM)Flow temperature (C) calculated experimental
55 Fig 4.11 AFM images of the samples, which taken for parallel configuration. (a) Away from the center. (b) Enlargement of image (a). (c)Close to the center. In this experiment the films are deposited in parallel configuration as in fig 4.8. Here, as shown in the schematic diagram, th e flow coming out is heated by the laser, (a) (b) (c)
56 deflected by the nitrogen flow, which is then collected on to the subs trate. The center of the flow then deflects and is deposited on the center of the substrate (fig 4.11(c)), while off axis is deposited on the areas away fr om the center on the substrate (fig 4.11 (a)). While images (a) and (b) show nicely distri buted isolated grains of about 50nm in diameter, image (c) shows larg e random grains of about 1000 nm in diameter. The reason for this formation is mainly because of th e variation of the temperature in the flow coming out of the nozzle. When laser is focused onto the flow, the intensity of the Gaussian beam drops by the factor 1/e2 as it goes away from the focused point. As a result, the temperature at the center reaches much higher values compared to off axis. The on axis temperature may be sufficient to melt the formed particle, causing them to agglomerate and form large particles as seen in fig 4.11(c). But on the other hand, comparatively low off axis temperature is sufficient to evaporate the solvent and decompose the precursor to form a solid part icle, but is not sufficient to cause melting and agglomeration. Therefore, it is obvious that the temper ature for decomposition of the chemical can be reached in parallel configurati on. In order to accomplish the complete decomposition one can set the system in such a way that it is decomposed completely as the chemical coming out. Therefore, the paralle l configuration gives us a promising result in making thin nanocoatings.
57 CHAPTER 5 NANO GRAINED FILM GROWTH BY CHEMICAL SELF ASSEMBLY AND VAPOR PHASE GROWTH 5.1 BOTTOM-UP APPROACH FOR NANO -STRUCTURED FILM GROWTH Nanostructured surfaces, films with mor phological features in the nanometer range and ordered assemblies of nanometer-s ized particles are interesting class of nanomaterials with great tec hnological potential. Innovative applications for these new materials include high-density information storage media, biological sensor arrays, magnetic fluids, medical diagnos tics and catalysts. Compar ed to conventional surface science techniques, such as gas-phase s ynthesis and nanostructuring or deposition of nanoparticles under ultrahigh-vacuum condition, the soft-matter approach is a scientifically and economically interesting altern ative. In this approach self-organisation in the bottom-up formation of nanostructured interfaces in liquid environment and selfassembled deposition of nanoparticles from collo idal suspension play a predominant role. The 'bottom up' approach is a new para digm for synthesis in the nanotechnology world that could revolutionize the way the mate rials are made. Inst ead of starting with large materials and chipping away to reveal small materials, the bottom up approach starts with atoms and molecule s and creates larger (but no t too large!) nanostructures. The bottom up approach requires a thorough unders tanding of forces of attraction, which holds the particles together when they were at nano regime. The simplest such bottom up
58 synthesis route is electroplati ng. By inducing an electric fi eld with an applied voltage, the charge particles can attract to the su rface of a substrate where bonding will occur. Most nanostructured metals with high hardness values are created with this approach. It has already been proven that electroplating creates a materi al layer-by-layer, atom-byatom. Chemical Vapor Deposition is anothe r well-known method among chemists. Using a mixture of volatile gases and taking advantage of some simple thermodynamic principles, it is possible to have your source material migrat e its way to th e substrate and then bond to the surface due to high chemical potentials. This is the one proven method for creating nanowires and carbon nanotubes. It is also a method of choice for creating quantum dots. Right now, CVD is the most popular and readily available method for creating nanostructur es of all kinds. Self-assembly promises to be the revo lutionary new way of creating materials from the bottom up. With this method, the nanop article will self-organ ize into 2D arrays while the chemical is being evaporated fr om the surface. One way to achieve selfassembly is through physical attr active forces such as elect ricity, Van der Waals forces, and a variety of other short-range attractiv e forces, which can be used to orient constituent molecules in a re gular array. This method has proven very effective in creating large grids of Silver (Ag), Go ld (Au) in a prove n periodic lattice.32
59 Figure 5.1 TEM images (Philips EM-400, 80 keV) of self-organized 2D arrays of 34 2nm(left) and 87 7 nm(right) Au nanoparticles stabilized by resorcinarene. The arrays were formed at the air-water inte rfaceand transferred onto Formvar-coated Cu grids.32 5.1.1 LASER ABLATED FILM GROWTH ON NANOTEMPLATES The two-dimensional nanotemplates created in the previous step will selectively promote crystalline growth of the plume ma terial (TiC) on the hot nanospheres, with intervening amorphous grain boundaries in the c ooler regions on the substrate. This will result in the transference of the nanotemplate pattern to the nucleation of an ordered array of crystalline regions with grain boundaries defined by initial size of the nanoparticles on the template. Subsequent deposition on this nanocrystalline pattern on the substrate will lead to the vertical, selfaligned growth of nanocolumns of the crystalline material with intervening grain boundaries (see fig 5.2). The lateral dimension of the crystalline grains and their positions will be controllable on the nanoscale, while the coatings could potentially be several microns thick, with large-area two-dimensional coverage on the substrate. This technique will therefore le ad to a manufacturing process for nanograined coatings that integrates nano, micr oand meso-scale features.
60 5.1.2 PULSE LASER DEPOSITION (PLD) The principle of pulsed laser deposition (P LD) is quite simple. A highly intense UV laser beam is focused on a target where th e high energy density during the laser pulse (about 1 GW within 25 ns) ablates almost a ny material. The ablated material forms plasma, which is deposited on a substrate opposite the target. This method is quite flexible in preparing films under a wide ra nge of deposition conditions such as kinetic energy, deposition rate and ambient gas. The advantages of the PLD method are flexibility, fast respond, energetic evaporants, and congruent ev aporation. This method is particularly suited for the growth of trib ological coatings since films deposited from highly excited and ionized species promote hi gh densities. A schematic diagram of a standard PLD system is shown in fig 5.2. Th e disadvantages are th e limited area of film growth and presence of micron sized particulates. Figure 5.2: A schematic diagram of a standard PLD system. E X C I M E R L A S E R Mirro r Lens Substrate Target Vacuum chamber Plume
61 5.2 PROCEDURE 5.2.1 SUBSTRATE PREPARATION FOR GOLD SYNTHESIS Silicon substrates were cleaned by chemical solvent an outlined in chapter 2. In addition, solvent cleaned substrates were cleaned in piranha solution. The piranha solution is made by 1:3 H2SO4: H2O2 solutions heated up to 600C for 10 minutes. [Caution! This solution contai ns strong oxidizing agents, wh ich cause severe burns in contact with skin and react violently with organic compounds. Storage after use should be avoided and great care and appropriate protective clothi ng must be employed when handling this mixture.] 5.2.2 SYNTHESIS OF GOLD NA NOPARICLE 2D COATING The procedure for the formation of gold nanoparticles is referred to as the Turkevich method. In this method particles we re precipitated in solution by a chemical reaction between HAuCl4 and sodium citrate Tetrachlorauric acid (99.99%, HauCl 4, 3H2O) and Citrate trisodium (95%, C6H5O7Na, 2H2O) was purchased from Alfa Aesar. A solution of 95ml of gold tetrachlorauric acid, with a we ight content of 5mg of gold, was heated until boiling point under vigorous stirring. Then 5ml of a 1% sodium citrate aqueous solution was added. The solution was then stirred and kept at boiling condition for another 45 minutes. After the introduction of the citrate solution, a purple co lor appeared which then turned ruby red. Drop casting technique was used to form the self-assembled nanotemplate. A drop of aqueous gold suspension was placed on a solv ent-cleaned Si substrate. Most of the
62 particles are retained on th e surface of the drop, due to the surface tension. The surfactants prevented agglomeration and forced them to selfassemble into a twodimensional network. As the solvent evapor ated, the particles settled on the substrate forming 2-D template. Typically, it takes ove rnight to evaporate aqueous part of the solvent and produce a light yellow color layer on the substrate. Figure 5.3 2D and 3D AFM images of gold na noparticles on a silicon substrate before annealing.
63 Figure.5.4 2D and 3D AFM images of gold nanoparticles on a silicon substrate after annealing to 500 0C. In the figure 5.3 before annealing the sa mple, the top right end of the images shows that the particles are arranged in such a way as to form a monolayer network. The particles touch each other without any agglom eration. The particles appear to be the same size and shape. The sizes of the pa rticles are in the ra nge of 80 to 90nm in diameter. In the figure 5.4 after the annealing at 500 0C, the particle sizes have decreased. The particle sizes are in the range of 40 to 60nm. This is to be expected since heating removes the surfactants that coat the particle s. Some evaporation of Au may also have taken place.
64 5.3 DEPOSITION OF TiC ON GOLD NANOPARTICLES A sample made by above method was washed in deionizied wate r, and dried with nitrogen, and placed on a heating block in a deposition chamber. A TiC target was ablated by a KrF excimer laser at the wa velength of 248nm to deposit a film on the heated substrate. The process was run for 30 minutes by keeping subs trate temperature at 5000C and a base pressure at 10-6 Torr. Figure.5.5 2D AFM images of TiC film on gold nanoparticles The figure 5.5 shows the images of Gold monolayer after deposition of TiC. The distance between particles and the size of the TiC particles are more defined and most of them are lying in the diameter range of 50 to 60nm. It is clear from this AFM that the TiC crystallites have preferentially nucleated on Au nanopa rticles to produce a nanograined film.
65 CHAPTER 6 CONCLUSION 6.1 LASER ASSISTED SPRAY PYROLYSIS In both TiC and Fe3O4 synthesis, we have incorporated a CO2 laser into a chemical spray pyrolysis system to heat th e droplets of an organometallic precursor dissolved in a volatile organic solvent. The drops with average diameter of 1.5 m in a narrow size distribution were generated by an ultrasonic nebulizer. The experiments have shown that when the droplets were direc tly incident on the subs trate, they initially flatten on the surface, followed by evaporation of the solvent and decomposition Leading to large grains. When the droplets are h eated up prior to incidence on the heated substrate, most of the solvent is evaporat ed, leading to solid pa rticles impinging on the substrate that led to reduced the particle si ze. By controlling the concentration of the precursor, a further reduction in a particle size is observed. Since the laser radiation is absorbed into the carrier gas, resonance absorption by precursor is not important. Therefore this technique can be extended to any organometallic compound that can be dissolved in a volatile solvent.
66 6.2 TiC GROWTH ON GOLD NANOPARTICLES In the Gold nanoparticle synthesis by Turk evich method, the resultant particle size and shape in the aqueous suspension was ma inly dependent on the concentration of sodium citrate. In the presence of sodium c itrate, the gold particle size is smaller and the particle size decreases with an increase in the concentrati on of sodium citrate. The sodium citrate serves not only as a surfactant but also as a stabilizer for nanoparticles, to prevent their further growth. Dropcasting method has been successfully used to form 2D arrays of Au particles. Also it was indicated that the surface treatment of s ubstrate is very important in the self-assembly of gold partic les as a 2D array. Therefor e, Piranha solu tion helps to functionalize the substrate to promote adhesi on of the gold nanoparticles. The adhesion of gold particles is most probably due to inte rfacial electrostatic in teraction between the positive charge, by the silicon lattice and citrate anions attached to the gold nanoparticles. In the sample in PLD ablation, the Ti C nano particles are likely to grow on gold nanoparicles rather than on the naked substrate. Then the sizes of the TiC particles are mainly dependent on the size of the gold na nograin on the substrate. This work also demonstrated the possibility of using the Au nanoparticles that are attached to the substrate as nucleation sites for the growth of TiC. By controlling the size of the Au particles, it may be possible to cont rol the size of the TiC grains.
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Dedigamuwa, Gayan S.
Formation of nanocoatings by laser-assisted spray pyrolysis and laser ablation on 2d gold nanotemplates
h [electronic resource] /
by Gayan S. Dedigamuwa.
[Tampa, Fla.] :
b University of South Florida,
Thesis (M.S.)--University of South Florida, 2005.
Includes bibliographical references.
Text (Electronic thesis) in PDF format.
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ABSTRACT: This thesis describes a new Laser-Assisted Spray Pyrolysis technique developed to grow nanoparticle coatings with controllable particle sizes. In this method, droplets of a precursor formed by a nebulizer are injected into a growth chamber using SF6 carrier gas. An experimental study and a computational model to investigate the particle size dependence on various growth parameters have been carried out. The results show that heating of 1.5and#61549;m droplets of metalorganic precursor in a carrier gas using a CO2 laser resulted in the formation of TiC and Fe3O4 particles with diameters in the range of 50-60nm. Also the results show that by reducing the concentration of the metal organic precursor the diameter of the deposited particles can be reduced.
Adviser: Dr. Sarath Witanachchi.
t USF Electronic Theses and Dissertations.