Characterization of Pure and dopantTiO2thin films for gas sensorsapplicationsA thesis submitted ByKhaled Z.YahyaA thesis submitted to the University of Technology, department ofApplied Sciences as a partial fulfillment for the requirement ofDoctor of Philosophy degreeIn Laser and Opto-electronic TechniqueSupervised byProf.Dr. Adawiya J.Haider Prof.Dr. Raad M.S.Al-HaddadJune 2010Ministry of Higher Education andScientific ResearchUniversity of TechnologyApplied Sciences Department 2010CONTENTCHAPTER ONE Introduction and Historical Review Page1- Introduction...... 11-1 Historical Review ...... 21-2 Aim of the work.. 10CHAPTER TWO Fundamental Properties of Laser Deposition and TiO2Thin Films2. Introduction...................................................... 112.1 Laser ablation mechanisms ... 112.1.1 Laser Target interaction ...................................................................... 112.1.2 Laser plasma interaction . 142.1.3 Plasma plume expansion 152.1.4 Plume- Substrate Interaction . 172.2 Lasers for PLD............... 182.3 Advantages of PLD...... 192.4 Titanium Dioxide ... 202.4.1 Crystalline Structure of TiO2 ..... 212.4.1.1 TiO2in Anatas Metastable Phase. 212.4.1.2 TiO2 in Rutile stable Phase .. 222.4.1.3 TiO2in Brookite Structure ....... 232.4.1.4 X-ray diffraction ....... 252.5 Surface Morphology . 292.6 Optical Properties ... .... 322.7 Optical Absorption and Absorption Edge . 362.8 Optical Constants ... 372.9 Doping TiO2.... 382.10 Noble Metal Diffusion inside TiO2 Bulk.. 412.11 Gas sensors . 432.12 Operation principles of the semiconductor gas sensor. 432.13 Thin film resistive gas sensors... 442.14 Chemical Sensors 452.15 General Approach to Semiconductor Gas Sensors.. 482.16 Semiconductor Sensing Materials for TiO2. 542-17 Bulk Conductance Effects . 542.18 Carbon Monoxide ..... 552.19 Other applications of TiO2.. 56CHAPTER THREE Experimental Work and Techniques3. Introduction ... 583.1 Deposition Equipment .......... 583.1.1 Nd: YAG Laser Source. 583.2 Vacuum System...... 593-2-1 Vacuum pump. 593-2-2 Pressure monitoring.. 593-2-3 Gas Supply System. 593.2.4 Target holder ......... 603.2.5 Substrate heater ... 613.3 Target preparation... 613.4 Preparation of Substrate Surface for Thin Film Deposition 623.5 Procedure of Thin Film Deposition by PLD.. 633.6 Characterization measurements .... 643.6.1 Electrodes Deposition.. 643.6.2 Thickness measurement .................. 643.6.3 Structural and morphological measurements. 65A) X-Ray Diffraction spectra.. 65B) Scanning electron microscopy (SEM). ...... 67C) Atomic Force Microscopy (AFM) .. 68D) X-ray Fluorescence (XRF) .............. 683.6.4 Optical measurement ... 69A)Transmission measurement.. 69B) Photoluminescence measurements. 693.6.5 Gas sensors measurement.. 70CHAPTER FOURE Results and Discussion4 Introduction 714.1 Effect of Deposition Conditions on the Characteristic of Thin filmsGrown Using PLD.714.1.1 substrates Temperatures effect ....... ..... 714.1.2 Oxygen pressur effect ..... 754.1.3 Laser Fluence effect ........................... 774.1.4 The doping effect of noble metal (Ag, Pt, Pd and Ni).. 784-2 X - ray Florescence .... 844.3 Surface Morphology by SEM.... 864.3.1 Effect of substrate Temperatures.. 864.3. 2 Oxygen Pressure effect . 864.3. 3 The doping effect of noble metal (Ag, Pt, Pd and Ni).. 874.4 Atomic Force Microscopy (AFM).... 924.4.1 Substrate Temperatures effect . 924.4.2 Oxygen Pressure effect .... 934.4.3 The doping effect of noble metal (Ag, Pt, Pd and Ni).. 934-5 Film thickness measurement. 984.6 Optical Properties... 1004.6.1 Transmission 1004.6.2 Absorption 1034.6.3 Optical Energy Gap. 1084.6.4 Refractive index 1134-6-5 Extinction Coefficient. 1154.6.6 Dielectric Constant.. 1174.6.7 Photoluminescence (PL)...... 1204.6.7.1 Substrate Temperatures effect ..... 1204.6.7.2 The doping effect of noble metal (Ag, Pt, Pd and Ni)... 1224.7 Sensing properties: Chemical Sensing Measurements 1254.7.1 Operation time Effect on sensing properties.. 1264.7.2 Operation time Effect on resistance properties.. 1274.7.3 Operation time Effect on current properties 1284.8.4 Operation Substrate temperature Effect on sensing properties 130CHAPTER FIVE Conclusion and Future work5. Conclusion .. 1375.1 Future Work.. 138References 139Khaled Z.Yahya . Characterization of Pure and dopant TiO2 thin filmsfor gas sensors applications" University of TechnologyDepartment of Applied Science .PH.D Supervisors : Dr. AdawiyaJ.Haider and Dr. Raad M.S.Al-Haddad. 2010147p.AbstractTitanium dioxide (TiO2) has been extensively studied and demonstrated to besuitable to detect toxic gases such as CO and NOx that effect the quality of life .There fore TiO2 thin films are good candidates in the gas sensor industry .Inaddition noble metal dopants to the titanium dioxide materials make them sensitiveto CO gas. In this work, a double frequency Q-switching Nd:YAG laser beam (=532nm, repetition rate 6 Hz and the pulse duration 10ns)has been used, to deposit TiO2 thinfilms pure and doped with (Ag ,Pt ,Pd and Ni ) at various doping percentages (1wt.%, 2 wt.% and 3 wt.%) on glass and Si (111) substrates to be activated by visiblelight irradiation as well as ultraviolet irradiation. Basic material characterizationhas been carried out. Many growth parameters have been considered to specify theoptimum condition, namely substrate temperature (200-500C), oxygen pressure (10 -510-2mbar) and laser fluence energy density (0.8, 1.2 and 1.8)J/cm2. Thestructure properties of TiO2 pure and doped with noble metal were investigated bymeans of x-ray diffraction. As a result, it has been found that film structure andproperties strongly depended on substrate temperature and doping concentration.X-ray diffraction (XRD) showed that at substrate temperatures higher than 300 Cthe structure of the deposited thin films changed from amorphous to crystallinecorresponding to the tetragonal TiO2 anatase phase, and at substrate temperatureTs=500C produced both rutile and anatase phases. It has been found that 3 % wt(Ag ,Pt ,Pd and Ni ) doped TiO2 thin film was the most sensitive element to CO gas.The surface morphology of the deposits materials have been studied by usingscanning electron (SEM) and atomic force microscopes (AFM). The grain size ofthe nanoparticles observed at the surface depended on the substrate temperature,where 500C was the best temperature and partial pressure of oxygen 510-1mbarwas the best pressure during the growth process . TiO2 doped with Pt metal has thesmallest grain size (1nm ) , RMS roughness increased with increasing substratetemperature (Ts) which are (11.2nm) for thin films deposited at (500)C and thesamples are very rough with RMS value of (28 nm) for TiO2 thin films doped with3% (Pt).UV-VIS transmittance measurements have shown that our films are highlytransparent in the visible wavelength region, with an average transmittance of~90% which makes them suitable for sensor applications .Dopants such as Ag andPt shifted the absorption edge of TiO2 into the visible region .The optical band gapof the films has been found to be 3.2 eV for indirect transition and 3.6 eV for directtransition at 400C. The refractive index (n) and extinction coefficient (K) decreased as the substrate temperature increased.In addition the photoluminescence spectrum analyses of all the thin filmsshowed that there were two kinds of luminescence transitions involved when thethin film was excited with energy higher than the band gap value (3.2 eV) . Theintensity of two peaks, A lied in the UV spectrum (375 nm) and B lied in the visiblespectrum (525 nm).The sensitivity toward CO gas has been measured under 50 ppmconcentrations . TiO2 doped with noble metal has a sensitivity higher than pureTiO2 where as TiO2 doped with Pt metal deposited on Si (111) has maximumsensitivity to CO gas with avalue of (23 %) with best annealing operationtemperature at 250C, and resistance decreased reached (109) with increasingdoping concentration due to increase in the sensing current reached (10 nA) of theTiO2 films.Keywords :TiO2 PLD Technique. CO NOx CO . nm 1 , 2 , 3 % ) 200-500 C (10-5*10-2 J/cm2) 0.8 1.8,1.2 ( 300 C 500 C 3% ( .CO ) SEM ( (AFM) . 500 C 10-1) 500 C 3% . 90 % 3.2 eV ( 3.6 eV) 500 C ) n ( ) K ( . 3.2 eV) ( A B . CO ) ( CO ) 23% ( 250 C . Chapter One Introduction and Historical Review11- IntroductionTitanium dioxide TiO2 (titania) is a cheap, non-toxic and one of the mostefficient semiconductor photocatalysts for extensive environmentalapplications because of its strong oxidizing power, high photochemicalcorrosive resistance and cost effectiveness[1]. Due to these inherentproperties, TiO2 is the most suitable candidate for degradation andcomplete mineralization of toxic organic pollutants in water[1,2]. It is wellknown that TiO2 exists in three crystalline structures: rutile, anatase andbrookite [3,4]. The anatase phase is especially adequate for thoseapplications due to its crystal structure and a higher band gap of 3.2 eVcompared to the 3 eV in rutile. Anatase and rutile have properties ofinterest for sensing applications[5]. Ni,Pt, and Ag has been found to be anefficient dopant for improving the gas sensor activity for CO gases [6,7]. Inprinciple, transition metal with proper oxidation state replace some of theTi (IV) from lattice producing an impurity state that reduces the band gapof TiO2 . In particular the pd doping was found to increase the sensitivityof CO gas to 3 times from pure TiO2[8].It is already established that material properties depend strongly onprecursors and synthesis methods in correlation with the thermodynamicprocess parameters. For the synthesis of nanoparticle systems thehydrothermal method was intensively utilised in the last decade [9].Titanium dioxide( TiO2) has attracted much attention in recent years due toits great potential for applications in optical elements, electrical insulation,capacitors or gates in microelectronic devices , photovoltaic solar cells, antireflection coatings , optical waveguides, photonic crystals [3], devicesbased on metal etc[10]. TiO2 films with specific crystal structure, orientationor morphology exhibit specific characteristics, which makes it important tocontrol the phase structure of TiO2 films during the growth. The methods ofsolgel spin-coating, anodization, oxygen plasma assisted molecular beamepitaxy and pulsed laser deposition (PLD) have been used to fabricate TiO2Chapter One Introduction and Historical Review2films. Among these methods, PLD technique has been widely used forgrowing oxide films because it allows for stoichiometry of the synthesizedmaterial. And because Si substrate is widely used in semiconductorindustry the growth of TiO2 films on Si substrates using PLD attractedmuch attention. TiO2 is a unique material in view of its versatile propertieswhich comprise high refractive index, wide band gap, and resistance tochemical and physical impacts [11,12].Gas sensors based on semiconductor metal oxide thin films focusednumerous research efforts during the last few years. Among them, titaniumdioxide (TiO2) has been investigated due to its sensing properties in front ofhydrogen [2], carbon monoxide and oxygen, hydrocarbons, or humiditydetectors[13]. Its sensing capability has been proved to improve with theaddition of metal dopants such as Pt , Ni ,Pd ,Ag ,Cr,Fe and Co[14].1. Historical ReviewThe efforts toward using Lasers in depositing thin films started soonafter the invention of reliable high power lasers. Early observations of theease with which the material could be vaporized by the intenseinteraction of high power laser pulses with material surfacedemonstrated that the intense laser radiation could be successfully used todeposit thin films of that material .Titanium Oxide thin film has attractingattention as one of the promising material with wide applications. It hasbeen prepared and characterized by many workers and using differenttechnique, which greatly affected the obtained film characteristic. The firstcomplete study about the electrical and optical properties for crystallinetitanium dioxide type ( rutile ) was prepared as crystal pieces by researcher(D.C.Cronemeger 1952) [18].In recent years, applications to environmental cleanup have been oneof the most active areas in heterogeneous photocatalysis. This isChapter One Introduction and Historical Review3inspired by the potential application of TiO2 based photocatalysts forthe destruction of organic compounds in polluted air andwastewater[16].In [1995] J. Osterwalderb et al [19]Deposited TiO2: Cr grown byplasma-assisted molecular beam epitaxy. They studied the relationshipbetween structural quality and magnetic ordering, using epitaxial Cr-doped anatase TiO2 with excellent structural quality as a model system.Epitaxial films deposited slowly at 0.08 (A/Sec) possess a perfectcrystalline structure, whereas films deposited at 0.2 (A/Sec) are found tohave a highly defected crystalline structure, as characterized by X-raydiffraction (XRD).In [1997] G. Korotcenkov and Sang Do Han[20]prepared (Cu, Fe,Co, Ni)-doped Titanium dioxide films deposited by spray pyrolysis. Theannealing at 850-1030C was carried out in the atmosphere of the air. Forstructural analysis of tested films they have been using X-ray diffraction,Scanning Electron Microscopy (SEM), and Atomic Force Microscopy(AFM) techniques. It was established that the doping did not improvethermal stability of both film morphology and the grain size. It was made aconcluded that the increased contents of the fine dispersion phase ofTitanium dioxide in the doped metal oxide films, and the coalescence ofthis phase during thermal treatment were the main factors, responsible forobserved changes in the morphology of the doped TiO2 films.In [] X. H. XU et al [21]studied the effect of calcinationstemperatures on photocatalytic activity of TiO2 films prepared by anelectrophoretic deposition ( EPD) method. TiO2 films fabricated ontransparent electro-conductive glass substrates and were furthercharacterized by X-ray diffraction (XRD), X-ray photoelectronspectroscopy (XPS), field emission scanning electron microscope(FESEM), UVvis diffuse reflectance spectra and PhotoluminescenceChapter One Introduction and Historical Review4spectra (PL). FESEM images indicated that the TiO2 films had roughnesssurfaces, which consisted of nano-sized particles.In [] Yanan Fu et al [22]studied transparent TiO2 thin filmswith high photocatalytic activity prepared on glass substrates via the sol-gelmethod from tetra isopropyl titanium ethanol solution containingpolyethylene glycol and diethylene glycol . The former was chosen toincrease the surface area of the film and the later to stabilize the dippingsolution. The dipping process with a pull-up speed of 1.5( mm/s) was usedto obtain the thin films. The dipping process was carried out 10-40 times.The thin films were calcined at 450 C for 1 hour after every ten dippings.The thickness of a 40x-dipped film was 0.6 mm and the apparent area ofthe TiO2 thin film was 12.7 cm2. Photocatalytic activity of the thin filmswas studied using the decomposition of gaseous acetaldehyde. Six 10 Wfluorescent black light bulbs provided the irradiation. The concentration ofthe acetaldehyde vapor was 1000 ppm throughout the experiment.In [] E. Gyorgy and E. Axente [23]studied Thecharacterization and CO gas sensing properties of pure and doped TiO2with Pt thin films deposited on glass substrates by (PLD) technique, atlaser energy densities of (1 J/cm2). Pure TiO2 thin film less sensitiveto CO gas compared to the TiO2 thin film doped with 4% Pt.In [2001] B. Farkas et al [24]prepared transparent TiO2:Ni thinfilms with different Ni concentration 0.01, 0.015 and 0.03 at 600 C onquartz substrates by (PLD) technique using Nd: YAG pulsed laser(=532 nm). .Ni doping thin films showed a shift towards the visible in the absorption edge of the UV-Vis absorption spectra of the thin film.The magnitude of this shift was found to increase with the amount ofdopant. The values of band gap values for pure, 0.01, 0.015 and 0.03Chapter One Introduction and Historical Review5Ni concentration were determined to be 3.1, 2.76, 2.62, and 2.23respectively.In [2002] D. Dzibrou et al [25]deposited TiO2 thin films onquartz and silicon wafers, by PLD method using Nd: YAG pulsed laser(=355nm, 10 Hz) with laser energy density of 1.5 J/cm2. The thinfilms were thermally treated at temperatures of 300 C, 400 and 500 Cin air for 1 hour. The coatings obtained were uniform, smooth withvery good optical properties. The sample annealed at lowertemperature had the characteristic appearance of an amorphousmaterial. The samples treated at 400C and 500 C were crystallized.TiO2 had direct and indirect band gaps.The band gap values for bothtransitions were different in comparison to the well-known value of 3.03eV for the indirect band gaps and 3.43 for the direct .In [2002] S.A. Tomas et al [26]carried out an experiment usingradio frequency reactive sputtering technique to prepare transparent,nanocrystalline and photocatalytic TiO2 pure and doping 1 and 3% Ag thinfilm . Spectroscopic techniques have been used to study the optical andstructural properties of the films. A gradual shift of the transmissionspectrum towards longer wavelengths has been observed when TiO2 dopedwith an increased amount of Ag, which indicates a decrease in the band gapvalue of TiO2 upon Ag doping. The photoluminescence (PL) spectrum ofthe films, have been measured which showed a gradual shift of theemission peak towards the longer wavelength region and supported thelowering of the band gap with Ag doping. The band gap energies werecalculated from transmittance and reflectance data.In[2003] H. Shinguu et al [27]studied the structural properties andmorphologies of TiO2 thin films, in which they were deposited on Si(100)and Si(111) substrates by using ArF excimer laser (operating withwavelength 248 nm at 500 C) .The films have been annealed for 10 hoursat the temperature 600C, in oxygen and air flow.The TiO2 film depositedChapter One Introduction and Historical Review6on (111)-oriented silicon exhibited a better anatase crystalline than that on(100)-oriented silicon. Whereas a higher annealing time needed totransform anatase structure into rutile structure for films deposited onSi(111) than on Si(100). The AFM images showed that the substrateorientation had no great effect on the surface morphologies for both anataseas-deposited films and rutile annealed films.In[2004] L.C.Tien et al [28]deposited TiO2 thin films on sapphireby using ArF excimer laser (operating with wavelength 193 nm, pulsewidth 15 ns, repetition frequency 10 Hz and power 100 mJ ) at a substratetemperature of 500C. The diagnostic of the ablation plume showed theinteraction of the evaporated Ti particles with buffer O2 gas. Thedependence of the buffer O2 gas pressure was studied by spectroscopy ofablation plume, thickness of films, morphology of the surface using SEMand AFM micrographs, XRD patterns and Raman spectra. The morphologyshowed the formation of nanostructure by interactions of evaporated Tiparticles with the buffer O2 gas. The structures of the PLD thin filmsshowed epitaxial growths in the high substrate temperature (500C) and anappearance of anatase at high buffer O2 gas pressure owing to thecontributions of the TiO molecules.In[2004] Yoshiaki Suda et al [29]prepared TiO2 films ondifferent substrate at different temperatures (100-400) C by using KrFExcimer laser ( =532nm, =3.5ns) at about 1 J/cm2laser density. Theyfound that all films showed (101) anatase phase at the optimizedconditions. Photoluminescence (PL) results indicated that the thin filmsfabricated at the optimized conditions showed the intense near band PLemissionsIn[2005] Tamiko and Ohshima [30]prepared TiO2 thin films byPLD method using XeCl excimer laser 308 nm wavelength which was usedto irradiate TiN (purity 99.9%) target and TiO2 (99.99%) target innitrogen/oxygen gas mixture. The color of the film changed fromChapter One Introduction and Historical Review7TiO2(transparent) to TiN(dark brown) with increasing the nitrogenconcentration ratio. The crystalline structure of the films prepared stronglydepends on the nitrogen concentration ratio in the gas mixture and thetarget material. The anatase type TiO2 crystalline structure can be observedto be independent of the nitrogen concentration ratio in nitrogen/oxygengas mixture.In[2005] A. P. Caricato et al [31]studied nanostructured TiO2 thinfilms prepared by (PLD) KrF excimer pulsed laser system (wavelength =248 nm) on indium-doped tin oxide (ITO) substrates under differentsubstrate temperature and pressure conditions (T = 250 ,400,500 and 600C, P = 10-2and 10-1Torr ) . AFM results showed the samples prepared at400 C have much more uniform surfaces and smaller particle size than thatprepared at 600 C. The XPS results indicated that the binding energy of theTi core level system pressure was dependent on substrate temperature .However, under 10-1Torr, only anatase phase was observed even at thetemperature higher than the commonly reported anatase-to-rutile phasetransition range (~ 600 C).In[2006] Narumi Inoue et al [32]prepared thin films of pure andTiO2 doped Pd using Nd: YAG pulsed laser (=355nm, 10 Hz) with laser energy density of 1.8 J/cm2. The gas sensing performance of these films forvarious gases was tested. Both pure and Pd -doped TiO2 based sensorsshowed highest responses to CO gas with poor sensitivity to H2 gas ascompared to later doped Pd ( 3%).TiO2 thin films showed sensitivity to COgas as high as 14% while pure TiO2 thin films showed poorsensitivity toCO gas(4%). The effects of microstructure and additive concentration onthe gas response, selectivity, response time and recovery time of the sensorin the presence of H2 gas were studied and discussed.In[2006] Kishor Karki et al [33]deposited TiO2 thin films on Si(111) substrate by using (PLD) ArF excimer laser (operating withwavelength 193 nm).The sensor detected different concentrations of COChapter One Introduction and Historical Review8gas from alterations in resistances of samples. The operation temperaturevaries from room temperature to 230C. X-ray diffraction (XRD) and(AFM) were applied to characterize the structure and surface morphologyof the deposited TiO2/Si films.In [2007] T. Nambara, K. Yoshida [34]studied the crystalline rutiletype titanium dioxide (TiO2) thin films which were prepared by (PLD) atsubstrate temperature 850 C . The optical properties of the present rutilefilms were different from that of single crystal TiO2. UV-Vis spectra ofPLD films showed a blue shift. The value of the gap was 3.30 eV, whichwas shifted from 3.02 eV as the bulk value. They considered quantum sizeand strain effects of PLD-TiO2 crystalline.They observed sensitivity trendswith respect to thickness 250 nm of TiO2thin film sensors This thickness iscomparable to the depletion length Comparison of the sensitivity of TiO2films toward 250 ppm of CO gas at 550 C.In [2008] Matthew et al [35]examined the growth of TiO2 thin filmsby (PLD) on SrTiO(STO), LaAlO3 (LAO), and fused silica with a KrF(248 nm, 2 Hz pulses). The laser output was maintained at 320 mJ perpulse, substrates at 550 C The films grown on the fused silica substratesshowed very small XRD peaks of the (200) and (110) oriented rutile TiO2,but are much less crystalline and conductivity 1000 ( cm)-1than thefilms grown on STO 2500 ( cm)-1 and LAO 2000 ( cm)-1at thesame conditions. The films on STO were not only crystalline, but appearedto grow epitaxially on the lattice-matched substrate.In [2008] M. Walczak et al [36]studied the effect of oxygen pressureon the structural and morphological characterization of TiO2 thin filmsdeposited on Si (100) by using KrF Excimer laser operated at wavelengthof 248 nm and repetition rate 5Hz . The laser energy density was about 2J/cm2). They found that the decreasing of oxygen pressure from (10 -2Torrto 10 -1Torr) produced highly homogeneous nanostructured morphologyChapter One Introduction and Historical Review9with grain size as small as 40 nm and high quality nanostructure wasobserved at the 10 -1Torr of oxygen .In [2009] Mikel Sanz et al [37]deposited TiO2films on Si (100) byPLD by using three different Nd:YAG laser wavelengths (266nm, 532nmand 355nm). They found that the films grown at =266 nm has smallest nanoparticles (with average diameter 25 nm) and the narrowest sizedistribution was obtained by ablation at 266 nm under 0.05 Pa of oxygen.The effect of temperature on the structural and optical properties of thesefilms have been investigated systematically by XRD, SEM, FTIR, and PLspectra.In [2009], Mohammad Hafizuddin et al [38]prepared TiO2 thinfilms onto SiO2 via sol-gel technique .They studied gas sensingproperties and microstructures of TiO2 thin films. TiO2 thin film exhibit asatisfactory response towards ethanol and methanol vapor. However, theability to select different type of gas remain the main issue as the detectionpattern for both gases are similar where XRD investigation showed that thethin films were amorphous.In [2009], Dang Thi et al [39]prepared Titanium dioxide transparent thinfilms (TiO2) sensors,with different thickness by (PLD) techniques (usingXeCl Excimer Laser 308 nm wavelength )using ceramic targets onto glasssubstrates .Thick films were necessary to increase the sensitivity of TiO2sensors to CO gas. Electrical measurements, x-ray diffraction and scanningelectron microscopy have been used to study the (CO) gas sensitivity,structure and morphology of the sensors.In [2009] J. D. Fergusona et al [40]studied PL spectra of TiO2 thinfilms by PLD using Nd:YAG laser (532nm). Thin films calcined atdifferent temperature with an excitation wavelength of 300 nm are shownin Fig (1-6) . A strong and wide PL signal at about 380420 nm isattributed to the bandband PL phenomenon with the light of energyapproximately equal to the band gap energy of the anatase and rutile. TheChapter One Introduction and Historical Review10band of anatase and rutile are 387.5 and 413.3 nm, due to the fact that theirband gap energies are 3.2 and 3.0 eV, respectively .1-2 Aim of the workThe aim of this work is to reveal specific properties of TiO2 nanostructureprepared by pulsed laser deposition technique .1. Characteristics of structural , microstructural and photoluminescenceproperties of TiO2 .2.Studing the sensitivity and selectivity of thin films pure and doped withdifferent noble metal deposited by PLD to CO gas.Chapter Two Fundamental Properties of Laser Deposition and TiO2Thin Films 2. IntroductionPulsed laser deposition (PLD) is a thin-film deposition method,which uses short and intensive laser pulses to evaporate target material.The ablated particles escape from the target and condense on the substrate.The deposition process occurs in vacuum chamber to minimize thescattering of the particles. In some cases, however, reactive gases are usedto vary the stoichiometry of the deposit [41]. This chapter explains the mainproperties which make TiO2 a good candidate for certain application andhow such properties change with deposition conditions according to what isalready published .2.1 Laser ablation mechanismsIn PLD a pulsed high-energetic laser beam is focused on a targetresulting in ablation of material. At the early stage of the laser pulse adense layer of vapor is formed in front of the target. Energy absorptionduring the remainder of the laser pulse causes, both, pressure andtemperature of this vapor to increase, resulting in partial ionization. Thislayer expands from the target surface due to the high pressure and formsthe so-called plasma plume [2]. During this expansion, internal thermal andionization energies are converted into the kinetic energy (several hundredeV) of the ablated particles. Attenuation of the kinetic energy due tomultiple collisions occurs during expansion into low- pressure backgroundgas. Usually, the laser ablation process is divided in two stages, separatedin time [19,16]:1. Target evaporation and plasma formation2. Plasma expansion.2.1.1 Laser Target interactionIdeally the plasma plume produced should have the samestoichiometry as the target if we hope to grow a film of the correctChapter Two Fundamental Properties of Laser Deposition and TiO2Thin Films composition. For example, if the target surface was heated slowly, say byabsorbing the light from a CW laser source, and then this would allow asignificant amount of the incident power to be conducted into the bulk ofthe target. The subsequent melting and evaporation of the surface wouldessentially be thermal i.e. the difference between the melting points andvapor pressures of the target constituents would cause them to evaporate atdifferent rates so that the composition of the evaporated material wouldchange with time and would not represent that of the target. Thisincongruent evaporation leads to films with very different stoichiometryfrom the target [45]. To achieve congruent evaporation the energy from thelaser must be dumped into the target surface rapidly, to prevent asignificant transport of heat into the subsurface material, so that the meltingand vapor points of the target constituents are achieved nearsimultaneously. The high laser power density that this implies is mostreadily achieved with a pulsed or Q-switched source focused to a smallspot on the target. If the energy density is below the ablation threshold forthe material then no material will be removed at all, though some elementsmay segregate to the surface [43,44].In general the interaction between the laser radiation and the solid materialtakes place through the absorption of photons by electrons of the atomicsystem. The absorbed energy causes electrons to be in excited states withhigh energy and as a result the material heats up to very high temperaturesin a very short time. Then, the electron subsystem will transfer the energyto the lattice, by means of electron-phonon coupling [45,16].When thefocused laser pulse arrives at the target surface the photons are absorbed bythe surface and its temperature begins to rise. The rate of this surfaceheating, and therefore the actual peak temperature reached, depends onmany factors: most importantly the actual volume of material being heated.This will depend not only upon how tightly the laser is focused but also onChapter Two Fundamental Properties of Laser Deposition and TiO2Thin Films the optical penetration depth of the material. If this depth is small then thelaser energy is absorbed within a much smaller volume. This implies thatwe require a wavelength for which the target is essentially opaque and it isin general true that the absorption depth increases with wavelength. Therate of heating is also determined by the thermal diffusivity of the targetand the laser pulse energy and duration.In a high vacuum chamber, elementary or alloy targets are struck at anangle of 45oby pulsed and focused laser beam. The atoms and ions ablatedfrom the target are deposited on substrate, which is mostly attached withthe surface parallel to the target surface at a target-to-substrate distance oftypically 2-10 cm [21]. In PLD technique, the target materials are firstsputtered (or say ablated) into a plasma plume by a focused laser beam anangle of 45o. The materials ablated then flow (or fly) onto the substratesurface, on which the desired thin films are developed. Therefore, theinteraction of intense laser which matters plays an important role in PLDprocess [38]. The thin film formation process in PLD generally can bedivided into the following four stages (see figure 2-1)1. Laser radiation interaction with the target.2. Dynamics of the ablation materials.3. Deposition of the ablation materials with the substrate.4. Nucleation and growth of a thin film on the substrate surface.Chapter Two Fundamental Properties of Laser Deposition and TiO2Thin Films Figure (2-1): Interaction between laser beam and matters [21].The incident laser pulse induces extremely rapid heating of significantmass/volume of the target material. This may cause phase transition andintroduce high amplitude stress in the solid target. The output of pulsedlaser is focused onto a target material maintained in vacuum or with anambient gas. The target is usually rotated in order to avoid repeatedablation from the same spot on the target.The lasers used in PLD studies range in output wavelengths from theultraviolet (excimer laser which operates at different UV wavelengths) tothe near- and mid-infrared (Nd-YAG and CO2 lasers) through the visiblewavelengths, with fundamental and SHG laser output [21,47]. Figure (2-2)shows the theory of the Pulsed Laser Deposition (PLD) and Pulsed LaserAblation (PLA).2.1.2 Laser plasma interactionIn the description of the laserplasma interaction, the laser pulseduration plays a crucial role. Whereas in the case of nanosecond (ns) laserFigure (2-2) typical pulsed laser deposition or ablation [47]Chapter Two Fundamental Properties of Laser Deposition and TiO2Thin Films pulse, the forming plasma interacts with the laser beam tail. In the case offemtosecond (fs) laser pulse the previous mechanism doesnt take place.Because of the formation of a plasma in front of the target, the laserbeam will be partially absorbed before it reaches the target i.e. so called(plasma shielding effect) [48]and increases the plume ionization degree,complicating the plume expansion mechanism. Due to the plasma-laserinteraction, the temperatures of the evaporated material increases thereforerapidly to extremely high values and the electrons are further accelerated.The excited particles will emit photons, leading to a bright plasma plume,which is characteristic for the laser ablation process.The main absorption processes are the Inverse Bremsstrahlung (IB)and the direct single-photon processes, IB involves absorption of photonsby free electrons which are accelerated during collision with neutral orionized atoms. The cross-section for IB via electron-neutral collisions ismuch smaller than that via electron ion collisions, but can be important forthe initial plume of a weakly ionized gas. Initially, there may be very few"seed" electrons present, produced by thermal emission from the solid ormulti-photon ionization processes[49,50].The contribution from multi-photon processes increases withdecreasing wavelength, but it particularly important for ultra fast lasers [51].2.1.3 Plasma plume expansionSince the onset of the material removal described in the previoussections takes place within a very short time after the pulse (1-100 ps), onthe time scale of the plasma expansion (s), the lasertarget event can be regarded as a momentary release of energy.The spatial structure of the vapor plasma at the early stage of itsexpansion is well known to be a cloud strongly forwarded in the directionnormal to the ablated target. The reason of this characteristic plasmaelliptic shape, called plume, is in the strong difference in pressure gradientsChapter Two Fundamental Properties of Laser Deposition and TiO2Thin Films in axial and radial directions: the plasma expands in the direction ofmaximum pressure gradient[52,53].Another important characteristic of the ablation plume pertinent toPLD is the angular distribution of the ejected species in the plume orsimply the plume angular distribution .In case of vacuum the plume angular distribution is determined bythe collisions of the plume particles among themselves in the initial stage.When plume is small however in the presence of the ambient gas the plumeangular distribution is modified due to collision between the plume speciesand background gas atoms. These collisions scatter the plume particlesfrom their original trajectories and broaden the angular distribution.It is generally expected that for a given background gas theseadditional collisions will lead to wider angular distribution of lighter plumespecies and similarly a scattering ambient with high mass will moreeffectively disperse the plume species compared to a low mass scatteringambient [54,17].Expansion the plume in vacuum is driven by the energy which isaccumulated as thermal energy and energy which is stored as excitation andionization in the initial layer. This energy is converted to kinetic energy ofthe atoms in the plume, and eventually all atoms will move with anasymptotic, constant velocity distribution. As soon as the laser pulse ends,there is little further transfer of energy and mass to the ablation plume, andthe plume propagation can essentially be considered as an adiabaticexpansion [55,56].The initial expansion of an ablation plume in a background gas doesnot differ much from the expansion in vacuum, since the driving pressure(~ 1 kbar) usually is much higher than that of a low-pressure backgroundgas (< 1 mbar).Chapter Two Fundamental Properties of Laser Deposition and TiO2Thin Films Most of the other existing treatments have been performed for aspecific choice of target and background gas and cannot readily beextended to other target-gas combinations[57, 58]..Laser ablation with ultra short pulses (