Atomic Force Microscopy of Electrodeposited Aluminum Oxide Thin Films

Transcription

1 Atomic Force Microscopy of Electrodeposited Aluminum Oxide Thin Films Farzana. Majid, Saira. Riaz, and Shahzad. Naseem Abstract Electrodeposition method is used for preparing alumina Al 2 O 3 thin films for their applications as resistive coatings. DC voltage in the 1 5volts was used for film preparation. Reaction time is varied from 30mins to 90mins in order to observe the changes in structural and morphology of thin films. Presence of (222), (111) and (100) planes in XRD confirm the formation of FCC α-al 2 O 3 phase. The films are oriented preferentially along (222) plane. AFM results indicate the presence of uniformly distributed grains with smooth surface. Increase in deposition time from 30min to 90min increases the surface roughness, ~ 9 nm, of the films but the films are still quite smooth and have the potential to be utilized as thermal control coatings. Keywords Aluminum oxide, Atomic Force Microscope, Electrodeposition, Thin films. A I. INTRODUCTION LUMINUM has been used in aerospace sectors for more than seventy years. Recent advances in aluminum alloying made it best candidate for aircraft structures. Alloying adjustment and microstructure control improve the alloy s performance. The series Al alloys are extensively used to increase pay load and fuel efficiency along with cost and weight requirement objectives due to high specific strength, low density and other valuable properties [1,2].The high strength of aluminum 7075 is due to the fine and uniformly distributed precipitates and main strengthening precipitates are MgZn 2 [3,4]. Al 2 O 3 offer unique advantages as high melting and boiling points, high dielectric constant, it s chemically stability and its high strength. In addition to these properties its ability of being radiation resistive makes it an important candidate for in nuclear reactor as optic window [1]. Moreover, alumina exists in seven different polymorphs (α, β, γ, θ, η, κ, χ) [3] and it crystallizes in rhombohedral unit cell (D 3d space group). However the most thermodynamically stable phase is α-al 2 O 3 and all other are transitional phases obtained during calcinations of hydroxides of aluminum [2]. Farzana. Majid is with Centre of Excellence in Solid State Physics, University of the Punjab, Lahore, Pakistan. Saira. Riaz is with Centre of Excellence in Solid State Physics, University of the Punjab, Lahore, Pakistan (phone: ; fax: ; saira_cssp@yahoo.com). Shahzad. Naseem is with with Centre of Excellence in Solid State Physics, University of the Punjab, Lahore, Pakistan ( The particular structural requirements for space vehicles working in arduous space environment are its capability to survive in high vacuum conditions, very low outgassing to prevent surface degradation, resistance to extremely high ultraviolet light, impeding against orbit charge particle radiation, having ability to withstand erosion by atomic oxygen and tenacity to withstand high range of temperature [5-7].These all requirements should be fulfilled simultaneously. Aluminum alloys being soft in nature cannot attain all the above mentioned features. In order to achieve all these characteristics, the wide spread method adopted is, the surface coating. The importance of surface coating can be depicted by the fact that world s economy bear enormous losses reaching milliards dollar that is due to damage of surface [8]. This is crucial challenge to find efficient and cost effective way to protect the aerospace structures from unforgiving and complex environment [9]. Complete knowledge about the microstructure and chemistry of aluminum alloy, play vital role in formulating surface coating and structural bonding system. The purpose of the present work is formulation of thermal control coatings by using electro-deposition technique to minimize the synergistic effect of space environmental factors. The application of electro-deposited aluminum alloy has become more and wider due to many excellent properties of coatings such as low solar absorbance, high bonding strength with aluminum alloy and durable in severe space vacuum [10]. In this research work alumina thin films are prepared by electrodeposition method using neutral electrolyte at room temperature. Effect of variation of deposition time on structural and surface morphology is discussed in detail. II. EXPERIMENTAL DETAILS All the reagents, used in this research work, were of analytical grade. Fresh neutral electrolytes were prepared for depositing aluminum oxide thin films. Electrolyte was deaerated before each experiment. Electrodeposition was carried out in a three-electrode cell with control of the temperature and at a constant potential of 3V. However the electrodeposition time was varied as 30min, 60min and 90min. Vertical arrangement of the working electrode was used for the growth of anodized aluminum oxide thin films. A microcomputer-controlled Versa stat 4 58

2 potentiostat/galvanostat was used for all the depositions. Aluminum oxide thin films were characterized structurally by using Rigaku D/MAX-IIA X-ray Diffractometer (CuKα = A) operating at an accelerating potential of 30kV and current of 22.5mA. Surface morphology was studied by using Bruker CP-II atomic force microscope working in non-contact mode. III. RESULTS AND DISCUSSIONS Fig. 1 shows XRD plot of aluminum oxide thin film electrodeposited at 3V for 90min. XRD plots for rest of the samples electrodeposited for 30min and 60min are similar to the results presented in Fig. 1. Presence of (222), (100) and (111) peak confirms the formation of FCC Al 2 O 3. Films are oriented preferentially along (222) plane. However, the peak intensity of (222) is much higher 90min as compared to films deposited for 30min and 60min. Optical properties of electrodeposited alumina thin films were also studied using Variable Angle Spectroscopic Ellipsometer (VASE). A brief overview is discussed in this paper and details of optical analysis are reported elsewhere [11]. The band gap is found to be varied from 3.93 to 4.02eV with increase in deposition time. The refractive index is 1.78 at a wavelength of 300nm. Both optical and SEM results indicate the electrodeposited films are dense and have low porosity values. Thickness of around one micrometer is obtained using VASE of all the films [11]. Surface topography measurements were done using Atomic Force Microscope (AFM) operating in the non-contact mode. Fig. 2 show three dimensional (3-D) images of electrodeposited alumina thin films deposited at a constant potential of 3V for 30min, 60min and 120min. The films show smooth surface with few large grains. The grains are uniformly packed. No coalescence of grains is observed with change in the deposition time. However it can be seen that surface becomes rough with increase in deposition time from 30min to 90min at an applied potential of 3V. Fig. 3 (a), (b) and (c) show area profile of Al 2 O 3 thin films electrodeposited for 30min, 60min and 90min respectively. The root mean square roughness (R rms ) is 5.066nm, 6.953nm and nm where as the average roughness (R av ) is 3.8nm, 5.637nm and 9.713nm for films deposited at a constant potential of 3V for 30min, 90min and 120min respectively Fig. 2 3-D AFM images of alumina thin films at a constant potential of 3V with variation in electrodeposition time (a) 30min (b) 60min (c) 90min R rms is plotted as a function of deposition time in Fig. 4 and it is seen that as the deposition time is increased from 30min to 90min the roughness of the films increases. These films are further intended to be utilized as thermal control coatings and low values of average and RMS roughness give the evidence of extremely smooth surface and low roughness of electrodeposited alumina thin films. The mean height for the films is 14.81nm and 13.28nm and 39.53nm and median height is 14.11nm, 12.13nm and 37.33nm with electrodeposition time of 30min, 60min and 120min respectively. Fig. 1 XRD of electrodeposited Al 2 O 3 thin films electrodeposited at 3V for 90min 59

3 R rms (nm) 14 R rms Mean height Time (min) Mean height (nm) for some particular positions. Fig. 5 show line profile for FCC Al 2 O 3 thin films with change in deposition time form 30min to 90min. Line profiles images of AFM also indicate extremely smooth surface with very low R rms of 5.276nm, 5.279nm and 9.103nm for 30min, 60min and 120min electrodeposition deposition time respectively. However as seen in three dimensional AFM images (Fig. 2) that there were few large grains present on the surface of the films and these large grains results in high values of peak to valley (R p-v ) but otherwise the films are fairly uniform with uniformly distributed grains. R p-v varies from 24.83nm to 22.44nm and the increases to 51.15nm for 30min, 60min and 90min deposition time respectively. Fig. 4 Root mean square roughness (R rms ) and Mean height as a function of deposition time at a constant potential of 3V Not only the three dimensional and area profile of electrodeposited alumina thin films taken using AFM is reported in this research work but also the films were analyzed Fig. 3 Area profile for electrodeposited Al 2 O 3 thin films (a) 30min (b) 60min (c) 90min. 60

4 2nd International Conference on Biotechnology, Nanotechnology and its applications (ICBNA'2013) June 17-18, 2013 London (UK) Fig. 5: Line profile AFM images of Al 2 O 3 thin films with deposition time of (a) 30min (b) 60min (c) 90min (222) plane and peak intensity corresponding to (222) plane increases as the deposition time increases. Detailed AFM analysis of the films indicates that the films are composed of uniformly distributed grains and increasing the deposition time has a great impact on the topography of the films. Electrodeposited alumina thin films are fairly smooth and the average roughness is 3.8nm, 5.637nm and nm for 30min, 60min and 90min electrodeposited Al 2 O 3 thin films respectively. IV. CONCLUSION Aluminum oxide thin films are prepared using electrodeposition method at room temperature. The films are deposited at a constant DC voltage of 3V. However, the deposition time was varied from 30min to 60min and then to 90min to study the changes in deposition time on structural and morphological properties. Presence of (100), (111) and (222) planes indicate the formation of FCC alumina by electrodeposition. The films are oriented preferentially along 61

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