SYNOPSIS OF STUDIES ON THE PHYSICAL PROPERTIES AND ELECTROCHROMIC PERFORMANCE OF PURE AND TITANIUM DOPED TUNGSTEN OXIDE THIN FILMS PREPARED BY DC MAGNETRON SPUTTERING A THESIS to be submitted by A. KARUPPASAMY for the award of the degree of DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS INDIAN INSTITUTE OF TECHNOLOGY MADRAS CHENNAI 600 036, INDIA March 2007
1. INTRODUCTION In the recent days, several materials that exhibit electrochromism have been extensively investigated for the smart window applications. Electrochromic smart windows are characterized by their ability to modulate the visible light passing through them. In this system, when a small current flows through the cell, the ions stored in the counter electrode diffuse toward the electro reduced electrochromic layer and change its transmittance continuously over a wide spectral range and consequently modify the overall optical transmission of the device. Tungsten oxide (WO 3 ) is the most extensively studied electrochromic material due to its enhanced electrochromic (EC) properties. The potential applications of the electrochromic WO 3 thin films are the electrochromic displays used in cameras and anti-dazzling rear-view mirrors for automobiles etc, [Granqvist,1995]. The ultimate use for electrochromism lies in the area of architecture wherein smart windows and glass facades can regulate the visible light and solar energy so that energy efficiency and improved indoor comfort can be achieved simultaneously. The electrochromic properties like coloration efficiency, cyclic durability and kinetics of colorationbleaching process of tungsten oxide strongly depend on its structural, morphological, and compositional characteristics, and therefore on the deposition techniques and growth parameters [Sivakumar et. al., 2007]. Among the various thin film deposition methods (thermal evaporation, electron beam evaporation, pulsed laser ablation, chemical vapor deposition, sol-gel coating, etc.) sputtering is an industrially viable technique with exact control parameters and therefore can be used to deposit uniform large area coatings. So, a detailed study on the effect of physical properties (structure, 1
morphology, composition) on the electrochromic performance of sputtered tungsten oxide thin films would enhance the smart window based applications. For practical electrochromic devices, improvement in the electrochromic property such as, electrochromic reversibility, stability, etc. are required. To augment the EC performance, mixed tungsten oxide thin films have been investigated. It has been proved that the addition of adequate dopant in WO 3 modifies its structure, leading to better EC performance. Several researchers have shown that the reversibility of EC glass can be improved by adding TiO 2 to the tungsten oxide [Wang et. al., 2000, Zayim, 2005]. The coloration efficiency decreases slightly but the lifetime of WO 3 TiO 2 thin films can be longer than that of pure WO 3 [Göttsche et. al., 1993]. The real time application of electrochromic smart windows as facades of large architectural buildings demands self-cleaning. In general, the pollutants on the surface of a material can be removed by photocatalysis process. TiO 2, ZnO and N doped TiO 2, Ti doped WO 3 are the known photocatalyic materials in the UV and visible range [Fouad et. al., 2006, Asahi et. al., 2001]. The deposition of photocatalytic material on the surface of electrochromic devices gives rise to selfcleaning smart window. 2. OBJECTIVES AND SCOPE OF THE WORK The objectives of the present work are (a) to study the effect of deposition parameters on the structural, optical, morphological, electrical and electrochromic performance of dc magnetron sputtered tungsten oxide and titanium doped tungsten oxide thin films (b) to prepare thin films of titanium dioxide, nitrogen doped titanium dioxide, titanium doped tungsten oxide and zinc oxide and to study their 2
photocatalytic properties and (c) to fabricate a solid state electrochromic device based on titanium doped tungsten oxide for smart windows applications. Growth of pure tungsten oxide and titanium doped tungsten oxide thin films by dc magnetron sputtering in active arc suppression mode and characterization using X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), UV-Vis spectrophotometer, reflectance measurements, Raman spectroscopy, contact potential measurements (CPD) by Kelvin probe, cyclic voltammetry (CV), and optical dynamic response characteristics. To diagnose the sputtering plasma using optical emission spectroscopy and to analyze the spectrum by a specline data base package Growth of tungsten oxide thin films by using pure oxygen as the sputter gas. Coloration of tungsten oxide thin films by electron bombardment. Growth of TiO 2, ZnO, WO 3 : Ti and TiO 2 : N and evaluation of their photocatalytic property by photo degradation of methylene blue. In summary, the research work is an attempt to fabricate a 'self - cleaning smart window' by studying the effect of morphology, structure and stoichiometry on the electrochromic performance of pure and titanium doped tungsten oxide thin films. 3. SUMMARY OF THE RESEARCH WORK 3.1. Growth and characterization of pure tungsten oxide thin films In the present study, the physical properties of tungsten oxide thin films have been studied by varying the deposition and post treatment conditions. For example, tungsten oxide films of different stoichiometry has been obtained by the room temperature deposition of WO 3 at different oxygen chamber pressures; 1 x 10-3 to 5 x 10-3 mbar. The as-deposited films are amorphous in nature but when they are annealed 3
at 450 C for three hours, they become crystalline with different surface morphology as shown in Fig. 1. In addition, different phases of tungsten oxide are grown by varying the deposition conditions such as oxygen chamber pressures (1-5 x 10-3 mbar) and substrate heating (450 C). The films deposited at an oxygen chamber pressure of 1 x 10-3 mbar and 2 x 10-3 mbar leads to the formation of nanowires as shown in Fig. 2 (a, b). As displayed in the SEM image (Fig 2. a), the nanowires deposited at 1 x 10-3 mbar oxygen pressure have uniform diameter and length of 60-80 nm and 10 µm respectively. The maximum length of these wires extends up to 30 µm. The films deposited at 2 x 10-3 mbar have a non-uniform surface morphology. Fig.1: Scanning electron micrographs of WO 3 deposited at oxygen chamber pressures of (a) 1 x 10-3 mbar (b) 2 x 10-3 (c) 3 x 10-3 mbar (d) 4 x 10-3 mbar. In a matrix of lengthy and horizontally dispersed nanowires, we find an island of quasi-aligned nanowires with larger diameters and shorter lengths. With further increase in oxygen pressure, the films formed are found to be microcrystalline (Fig. 2 c-d). The nanowire formed by this method is catalyst free and so a detailed investigation on the microstructures has been made using transmission electron 4
microscopy (TEM). The SAD pattern of an individual nanorod (Fig. 3 a) shows a single crystalline pattern with a preferential growth direction of (010). The higher magnification TEM image shows individual nanorods of length 150 nm and diameter 40-100 nm (Fig. 3 b). 3.2. Investigation on the properties of oxygen sputtered tungsten oxide thin films Tungsten oxide thin films were deposited at room temperature (300 K) by dc magnetron sputtering with active arc suppression mode from a metallic tungsten target using pure oxygen as sputter gas. The purpose of using pure oxygen as a sputter gas was to ensure the formation of stoichiometric WO 3 throughout the film. Three samples were deposited under different oxygen flow rates corresponding to the chamber pressure of 1.5, 3.1, and 5.2 x 10-2 mbar (these samples are named as S1, S2 and S3 respectively). The XRD spectra of all the as-deposited films reveal a broad featureless peak indicating amorphous nature of the films. The composition of the films studied by Rutherford backscattering spectrometry measurements reveals a WO 3 stoichiometry with oxygen to tungsten ( O /W ) ratio of 2.98, 3.01 and 3.05 respectively. All the as deposited films were found to be highly transparent in the visible range. With intercalation of protons, the transmittance of the films falls considerably to lower values of 1.0 % for S1, 0.6 % for S2 and 0.4 % for S3 at λ = 633nm. The dynamic coloring- bleaching characteristics of WO 3 films at λ = 633 nm is given in Fig 4 (b). The switching time taken by all the films during proton de-intercalation is found to be the same; however, the switching time to get a transmittance of 0.3 % during proton insertion is found to depend on the sputter gas pressure. The difference in the switching time between the colored and bleached state of the films is attributed to the chemical potential changes associated with coloration. 5
a b c d Fig. 2: SEM micrograph of the WO 3 films deposited at O 2 pressures of (a) 1 x 10-3 (b) 2 x 10-3 (c) 3x 10-3 (d) 4 x 10-3 mbar with substrate heating of 450 º C. Fig.3: (a) SAD pattern and (b) TEM image of substrate heated WO 3 films grown at 1 x 10-3 mbar. At a given wavelength, coloration efficiency (CE) is defined as the change in optical density with charges intercalated per unit electrode area. Fig. 4 (a) shows the spectral CEs as a function of oxygen sputter pressure. It may be observed that in all the films the CE is minimum at the fundamental absorption edge of WO 3 thin films, and then it increases monotonically with wavelength and reaches a soft saturation value at λ ~ 633 nm. The stability of an electrochromic device with repeated color / bleach cycles depends on the amount of residual charge left on the film in each cycle after de-intercalation. We have tested the cycling durability of the films by measuring the amount of residual charge left in the sample using Kelvin probe. The contact potential difference (CPD) measurements on the tungsten oxide thin films have been carried out using Kelvin probe equipment designed and fabricated in house [Sureshkumar et. al., 1996]. Fig. 5 shows the topography of CPD for the tungsten oxide electrochromic layer (scanned over an area of 2.0 cm x 1.0 cm) depicting the variation in CPD of selectively colored WO 3 film. 6
100 220 200 180 160 5.2 X 10-2 mbar 3.1 X 10-2 mbar 50 100 1.5 x 10-2 mbar 100 200 CE (cm 2 /C) 140 120 100 80 60 40 1.5 X 10-2 mbar Transmittance (%) 50 100 3.1 x 10-2 mbar 100 200 20 400 500 600 700 800 Wavelength (nm) 50 5.2 x 10-2 mbar 0 0 100 Time (s) Fig. 4: (a) Spectral dependence of the coloration efficiency of oxygen sputtered WO 3 films in the visible range (b) dynamic optical modulation of the WO 3 films subjected to an applied potential of -2.0 V (coloration) and + 1.5 V (decoloration). The CPD values observed for uncolored and colored tungsten oxide thin films respectively are: - 10 mv and + 550 mv; the bleached portion of the film has a CPD of -120 mvolts, which is less than the CPD of the rest of the sample. The dip in the figure clearly indicates that there are no residual charges left in the sample after bleaching; ensuring a better cycling durability of the electrochromic device based on oxygen sputtered WO 3 films. 3.3 Electron beam induced coloration and luminescence in tungsten oxide films From literature it could be found that, once colored, tungsten oxide has an inbuilt memory to retain the coloration information even after the source is removed. This property of tungsten oxide makes it a candidate suitable for possible ultra high density data storage application [Aoki et. al., 2006). In the present study, tungsten oxide thin films have been deposited by dc magnetron sputtering of tungsten in argon and oxygen atmosphere. The as-deposited WO 3 film is amorphous, highly transparent and shows a layered structure along the edges. 7
600 500 400 300 WO 3 colored CPD ( mv ) 200 100 0 WO 3 bleached -100-200 -300 2000 400060008000 10000 1200014000 Distance (µm) 0 2000 4000 WO 3 as prepared 10000 8000 6000 Distance (µm) Fig. 5: The contact potential difference scan of the tungsten oxide thin films taken at the 100 th cycle of proton intercalation/deintercalation on selected area. Electron beam irradiation (3.0 KeV) of the as-deposited films results in crystallization and show coloration (deep blue) and luminescence (intense red emission). The changes in physical properties are attributed to the desorption of oxygen and structural modifications induced by electron bombardment. The above method of coloration and luminescence has a potential for fabricating high-density optical data storage device. A detailed investigation on the surface of the electron bombarded sample was done by atomic force microscopy (Fig. 6 a-d). At the point of impingement, the surface is found to be smooth with large flakes separated by fine cracks. The high magnification AFM image of individual flakes shows randomly dispersed nanoparticles within the flake. The surface crystallite size of the nanoparticles determined from the height image was found to be ~ 25 nm. When the samples are bombarded with electron beams of low current (3 ma), the entire film 8
(1.0 cm x 1.0 cm) turns deep blue in color, showing a maximum transmittance of 15 % at λ = 503 nm. Fig. 6: The AFM images of electron bombarded sample taken at different magnifications. Fig. 7: photographs of (a) as-deposited WO 3, and electron bombarded WO 3 at (b) 3 ma (c) 6 ma (d) PL spectrum of bombarded WO 3 (e) Outline But when bombarded with electrons of high current (6 ma), a portion of the film in and around the point of impingement turns deep blue and also gives an intense red emission at λ = 619 nm. The photographs of the as-deposited, colored and light emitting samples are shown in Fig. 7 (a-c). Fig.7 (d) shows the photoluminescence spectrum of the electron bombarded sample excited at 325 nm. The intense red emission is attributed to the transitions due to localized sites induced by the presence of oxygen vacancies and defects in WO 3 film. 3.4 Growth and characterization of titanium doped tungsten oxide thin films The Titanium doped tungsten oxide thin films have been deposited by co-sputtering metallic titanium and tungsten in the presence of argon and oxygen. The oxygen chamber pressure is varied in the range 1 x 10-3 - 5 x 10-3 mbar keeping the sputtering power of titanium and tungsten constant at 2 W/cm 2 and 3 W/cm 2 respectively. In addition, the effects of stoichiometry on the structural, morphological, optical and electrochromic properties of the titanium doped tungsten oxide thin films 9
have been investigated. The XRD spectra of titanium doped tungsten oxide thin films deposited by reactive co-sputtering of metallic titanium and tungsten at various oxygen chamber pressures are shown in Fig.8 (a). The as-deposited samples are amorphous whereas the films annealed at 450 ºC are found to be highly crystalline with prominent peaks corresponding to the hexagonal phase of tungsten oxide. The absence of additional peaks corresponding to metal clusters or mixed metal oxides clearly indicates the homogenous mixing of titanium in the matrix of tungsten oxide. The addition of titanium creates disorder in the tungsten structure and hence leads to delay in crystallization [Patil et. al., 2005]. Fig.8 (b) shows the SEM pictures of titanium doped tungsten oxide thin films deposited at various oxygen pressures. From EDAX results, the titanium doping in WO 3 is found to be in the range of ~ 1 to 1.3 % (atomic percent). The morphology of the films deposited at lower oxygen pressure (1 x 10-3 mbar) is uniform, dense and highly crystalline with particulates of two different sizes; ~ 150 nm and 1µm. With increase in sputtering pressure, the larger particulate on the surface of the film shows a regular pattern, however with reduced dimensions. d c b a Fig. 8: The XRD pattern and SEM micrograph of titanium doped tungsten oxide thin films grown at O 2 chamber pressures of (a) 1 x 10-3 (b) 2 x 10-3 (c) 3 x 10-3 (d) 4 x 10-3 mbar. Furthermore, the optical modulation ( OD), coloration efficiency (CE) and swi tching time (t s ) of the proton based device deposited at an O 2 pressure of 4 x 10-3 mbar was 10
found to be better with typical values; optical modulation = 60 %, CE = 54 cm 2 /C (at λ = 550 nm) and t s ~11s (for 1 cm x 1 cm device). 3.5 Growth and characterization of photocatalytic materials In order to achieve self-cleaning mechanism in the electrochromic smart windows, various metal oxide systems like ZnO, TiO 2, nitrogen doped titanium dioxide and titanium doped tungsten oxide have been deposited and characterized for their photocatalytic properties. This part of the work has resulted in two new observations, (i) Rapid thermal annealing of ZnO by electron bombardment and (ii) Room temperature deposition of TiO 2 by oxygen sputtering. Initially, zinc oxide thin films were deposited on to quartz and glass substrates by electron beam evaporation of high pure ZnO pellets and are then irradiated with electron beam of power density 10 6 W/cm 2 for a fixed time of 3 seconds. The cathodoluminescence in e - - bombarded ZnO films deposited on quartz substrates are shown in Fig. 9 (a). The electron bombardment leads to better crystallinity, improved morphology (figure 9. b), enhanced UV emission and photocatalysis (as observed by the degradation of methylene blue). a b c Fig. 9: (a) Electron beam irradiation of ZnO thin film (b) AFM image of electron bombarded ZnO thin film (c) AFM image of oxygen sputtered TiO 2 thin film. These changes in the material property are ascribed to the surface annealing effect of electron beam irradiation. 11
Of the three crystalline structures of TiO 2 (anatase, rutile and brookite), only anatase phase is photocatalytic. But the growth of anatase phase requires either post annealing treatment or substrate heating [Verma et. al., 2005]. Using a method of oxygen sputtering, we have deposited nano anatase phase TiO 2 at room temperature. The particle sizes of these films are in the range 28 43 nm (Fig. 9 c). The photocatalytic property of these films has been tested by methylene blue degradation in the presence of UV light (350 nm). 4. CONCLUSIONS The tungsten oxide thin films deposited under various oxygen pressures of 1x 10-3 to 5 x 10-3 mbar gives rise to different stoichiometric films of O/W ratio 2.6 to 3.0. The post treatment of these films at 450 C leads to significant changes in the morphology of the films. The surface crystallite sizes were found to vary in the range 150 nm to 1.5 µm. Different phases of tungsten oxide have been grown by substrate heating (450 C) at various oxygen chamber pressures. Tungsten oxide nanowires (WO 2.9 ) of diameter 60-80 nm and length 10 µm was formed at a sputtering pressure of 1 x 10-3 mbar. The tetragonal and hexagonal phases of WO 3 have been deposited at elevated oxygen chamber pressures (>2 x 10-3 mbar). The electrochromic performances of the films are found to depend on stoichiometry, morphology and structure. The coloration efficiency of the films is found to vary in the range 43-90 cm 2 /C. Tungsten oxide thin films were deposited using pure oxygen as the sputter gas. The films deposited at an oxygen chamber pressure of 5.2 x 10-2 mbar exhibited better electrochromic properties with coloration efficiency of 182 cm 2 /C, optical 12
modulation of 83 % and faster switching speed of t c ~32s at λ = 633nm for a 1cm x 1cm device. The as-deposited WO 3 films were bombarded with electron beam of energy 3.5 KeV with two different currents of 3.0 ma and 6.0 ma. The electron bombarded films turned deep blue in color and emitted light. This property of the material has been proposed for data storage application. The effects of structure, morphology and stoichiometry on the properties of titanium doped tunsgten oxide thin films have been studied. Photocatalytic materials like TiO 2, ZnO, TiO 2 : N and WO 3 : N have been grown and characterized. Finally, a solid state electrochromic device based on titanium doped tungsten oxide with self -cleaning mechanism has been fabricated. REFERENCES Granqvist C. G., (1995) Ed. Handbook of Inorganic Electrochromic Materials, New York: Elsevier. Sivakumar R., R. Gopalakrishnan, M. Jayachandran, and C. Sanjeeviraja (2007) Preparation and characterization of electron beam evaporated WO 3 thin films, Optical materials 29, 679. Göttsche J., A. Hinsch, and V. Wittwer (1993) Electrochromic mixed WO 3 -TiO 2 thin films produced by sputtering and the sol-gel technique: A comparison, Sol. Energy Mater. Sol. Cells, 31, 415. Wang Z. and X. Hu (2000) Electrochromic properties of TiO 2 -doped WO 3 films spin-coated from Ti-stabilized peroxotungstic acid, Electrochim. Acta, 46, 1951. anti- Zayim E. O. (2005) Optical and electrochromic properties of sol gel made reflective WO 3 TiO 2 films, Sol. Energy Mater. Sol. Cells 87, 695. Asahi. R., T. Morikawa, T. Ohwaki, K. Aoki, and Y. Taga (2001) Visible-Light Photocatalysis in Nitrogen-Doped Titanium Oxides, Science 293, 269. Fouad O. A., A. A. Ismail, Z. I. Zaki, and R. M. Mohamed (2006) Zinc oxide thin films prepared by thermal evaporation deposition and its photocatalytic activity, Applied Catalysis B: Environmental 62, 144. 13
Sureshkumar C., A. Subrahmanyam, and J. Majhi (1996) Automated reed-type Kelvin probe for work function and surface photovoltage studies, Rev. Sci. Instrum. 67, 805. Aoki T., T. Matsushita, A. Suzuki, K. Tanabe and M. Okuda (2006) Write-once optical recording using WO 2 films prepared by pulsed laser deposition, Thin Solid Films 509, 107. Patil P. S, S. H. Mujawar, A. I. Inamdar, and S.B. Sadale (2005) Electrochromic properties of spray deposited TiO 2 -doped WO 3 thin films, Applied Surface Science 250, 117. Verma A., A. Basu, A. K. Bakhshi, and S. A. Agnihotry (2005) Structural, optical and electrochemical properties of sol gel derived TiO 2 films: Annealing effects, Solid State Ionics 176, 2285. Proposed organization of the thesis Chapter 1 Chapter 2 Chapter 3 Chapter 4 Chapter 5 Chapter 6 Introduction Experimental techniques used for characterizing the films Studies on the growth and characterization of pure tungsten oxide thin films for their electrochromic properties Studies on the growth and characterization of titanium doped tungsten oxide thin films for their electrochromic properties Studies on the solid state electrochromic devices and self cleaning mechanism Summary and conclusions VISIBLE OUTPUT BASED ON THE RESEARCH WORK Patent filed 1. A. Subrahmanyam and A. Karuppasamy (2006) Method of inducing chromism and photoluminescence in reactive magnetron sputtered nonstoichiometric tungsten oxide thin films. Applied for an Indian patent Application No. 658 CHE 2006. 14
List of publications (in refereed journals) 1. A. Subrahmanyam, A. Karuppasamy and C. Sureshkumar (2006) Oxygen- Sputtered Tungsten Oxide thin for Enhanced Electrochromic Properties, Electrochem. Solid-State Lett. 9, H11. 2. A. Subrahmanyam and A. Karuppasamy (2007) Optical and electrochromic properties of oxygen sputtered tungsten oxide (WO 3 ) thin films. Sol. Energy Mater. Sol. Cells 91, 266. 3. A. Karuppasamy and A. Subrahmanyam (2007) Effect of electron bombardment on the properties of ZnO thin films, Mater. Lett, 61, 1256. 4. A. Karuppasamy and A. Subrahmanyam (2007) Studies on the room temperature growth of nano anatase phase TiO 2 thin films by pulsed dc magnetron sputtering with oxygen as sputter gas, J. Appl. Phys., (Accepted for publication). 5. A. Karuppasamy and A. Subrahmanyam (2007) Electron beam induced coloration and luminescence in layered structure of WO 3 thin films grown by pulsed dc magnetron sputtering, J. Appl. Phys., (Accepted for publication). 6. A. Karuppasamy and A. Subrahmanyam (2007) Studies on Electrochromic smart windows based on titanium doped WO 3 thin films, Thin Solid Films, (under review). 7. A. Karuppasamy and A. Subrahmanyam (2007) Electrochromism in tungsten oxide nanowires grown by thermally induced strain, Appl. Phys. Lett., (under review). Presentations in conferences 1. A. Karuppasamy and A. Subrahmanyam, Enhanced electrochromism in DC magnetron sputtered Titanium oxynitride thin films a report, paper presented at the MRS Fall Meeting-2005, 28 th Nov - 2 nd Dec, Boston, MA, USA. 2. A. Subrahmanyam and A. Karuppasamy, Studies on DC magnetron sputtered nano anatase Titanium oxide thin films prepared in pure oxygen plasma at room temperature, proceedings of the 49 th SVC Annual Technical Conference 2006, April 22-27, Washington DC, USA. 3. A. Karuppasamy and A. Subrahmanyam, Studies on electrochromic smart windows based on titanium doped WO 3 thin films, Accepted for ICMCTF- 2007, to be held at San Diego, California, USA in April 23-27. 15