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Optical detection of CO and H based on surface plasmon resonance with Ag-YSZ, Au and Ag-Cu nanoparticle films
h [electronic resource] /
by Denis Kitenge.
[Tampa, Fla] :
b University of South Florida,
Title from PDF of title page.
Document formatted into pages; contains 60 pages.
Thesis (M.S.M.E.)--University of South Florida, 2009.
Includes bibliographical references.
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ABSTRACT: Silver, gold, and copper metallic nanoparticle films have been utilized in various MEMS devices due to not only their electrical but also their optical properties. The focus of this research is to study the detection at room temperature of carbon monoxide (CO) and hydrogen (H) via Surface Plasmon Resonance (SPR) phenomenon of silver-embedded Yttrium Stabilized Zirconium (Ag-YSZ) nanocomposite film, gold (Au) nanoparticle film, and an alloy film of silver-copper (Ag-Cu) grown by the Pulsed Laser Deposition (PLD). To determine the appropriate film materials for quick and accurate CO and H detection at room temperature with the PLD technique, the growth process was done repeatedly. Optical tools such as X-Ray Diffraction, Alpha Step 200 Profilometer, Atomic Force Microscopy, and Scanning Electron Microscopy were used to characterize thin films.The gas sensing performance was studied by monitoring the SPR band peak behavior via UV/vis spectrophotometer when the films were exposed to CO and H2 and estimating the percent change in wavelength. The metallic nanoparticle films were tested for concentration of CO (100 to 1000 ppm) and H (1 to 10%). Silver based sensors were tested for the cross-selectivity of the gases. Overall the sensors have a detection limit of 100 ppm for CO and show a noticeable signal for H in the concentration range as low as 1%. The metallic films show stable sensing over a one-hour period at room temperature. The SPR change by UV/vis spectrophotometer shows a significant shift of 623 nm wavelength between 100 ppm CO gas and dry air at room temperature for the alloy films of Ag-Cu with a wider curve as compared to silver and gold films upon their exposure to CO and H indicating an improvement in accuracy and quick response.The results indicate that in research of CO and H detection at room temperature, optical gas sensors rather than metal oxide sensors are believed to be effective due to not only the absence of chemical involvement in the process but also the sensitivity improvement and accuracy, much needed characteristics of sensors when dealing with such hazardous gases.
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Advisor: Ashok Kumar, Ph.D.
Pulsed laser deposition
x Mechanical Engineering
t USF Electronic Theses and Dissertations.
Optical Detection of CO and H2 Based on Surface Plasmon Resonance with Ag-YSZ, Au and Ag-Cu Nanoparticle Films by Denis Kitenge A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Mechanical Engineering Department of Mechanical Engineering College of Engineering University of South Florida Major Professor: Ashok Kumar, Ph.D. Frank Pyrtle III, Ph.D. Muhammad Rahman, Ph.D. Date of Approval: November 4, 2009 Keywords: gas sensors, pulsed laser de position, thin film, ab sorption, critical angle Copyright 2009, Denis Kitenge
I would like to dedicate this thesis to: my late mother, Ema Marie-Rosalie, for teaching me how to love the Lord, and to my late father, Daniel Longanga, for teaching me that working hard is the ingredient for success.
ACKNOWLEDGEMENTS I would like to express my d eep gratitude to Dr. Ashok Kumar, my major advisor, and committee members, Dr. Muhammad Rahman and Dr. Frank Pyrtle III, for their valuable insights and suggestions. I am gr ateful to Mr. Bernard Batson (Director, Diversity and Outreach Programs Director who has provided financial support from different grants. I am also thankful to Dr. Rakesh Joshi who provided help and encouragement during this work. A special thanks to all my friends who stand by me in challenging moments of my life, for their help and support throughout this endeavor. The work was supported by NSF through NIRT # ECS 0404137, Graduate Research Supplement Program, and NSF-FGLSAMP Bridge to Doctorate Program.
i TABLE OF CONTENTS LIST OF TABLES iii LIST OF FIGURES iv ABSTRACT vii CHAPTER 1: INTRODUCTION 1 1.1 Background 1 1.2 Surface Plasmon Resonance (SPR) 2 1.3 Parameters of Surface Plasmon Resonance Based Sensors 5 1.3.1 Sensitivity 5 1.3.2 Resolution 6 1.3.3 Selectivity or Cross-reactivity 7 1.4 Surface Plasmon Resonance Materials: Silver, Copper and Gold 7 CHAPTER 2: EXPERIMENTAL METHOD 9 2.1 Pulsed Laser Deposition (PLD) 9 2.1.1 Optimization of Parameters for PLD 12 22.214.171.124 Target-to-Substrate Distance 12 126.96.36.199 Effect of Temperature on Films Properties 13 2.2 Microstructure (X-Ray Diffraction) 14 2.3 Atomic Force Microscopy (AFM) 16 2.4 Scanning Electron Microscopy (SEM) 17 2.5 UV/vis Spectrophotometer 19 CHAPTER 3: STRUCTURAL AND GAS SE NSING CHARACTERISTICS OF FILMS 22 3.1 SPR Based Sensors Composed of Ag-YSZ Nanoparticle Films 22 3.2 SPR Based Sensors Composed of Au Nanoparticle Films 34 3.3 SPR Based Sensors Composed of Ag-Cu Nanoparticle Films 42 CHAPTER 4: CONCLUSION AND FUTURE WORK 49 4.1 Conclusion 49 4.2 Future Work 51
ii REFERENCES 52 BIBLIOGRAPHY 55
iii LIST OF TABLES Table 1.1 Dielectric constants and refractiv e indices of materials of interest 4 Table 3.1 Gold nanoparticle films experimental conditions 35 Table 3.2 Ag-Cu alloy films experimental conditions 43 Table 4.1 H2 and CO sensor signals comparisons for Ag, Au and Ag-Cu films 49
iv LIST OF FIGURES Figure 1.1 Visible range of electromagnetic radiation 3 Figure 1.2 Schematic diagram of SPR sensor 5 Figure 2.1 Schematic diagram of a PLD vacuum chamber 10 Figure 2.2 PLD vacuum chamber and apparatus at USF 10 Figure 2.3 Plasma emissions in the vacuum chamber 11 Figure 2.4 Target-to-substrate distance inside PLD vacuum chamber 12 Figure 2.5 Substrate holder assembly 13 Figure 2.6 Temperature displayed inside the vacuum chamber 14 Figure 2.7 Braggs law 15 Figure 2.8 Atomic force microscope block diagram 16 Figure 2.9 Hitachi S-800 18 Figure 2.10 UV/vis spectrophotometer with optical system open 19 Figure 2.11 Gas-sensor ce ll testing assembly 21 Figure 3.1 Thickness of films grown for 5, 10, 15 min. measured by profilometer 23 Figure 3.2.A Typical AFM micrograph of Ag film 24 Figure 3.2.B AFM average size of Ag film (20.6nm) synthesized at RT 25 Figure 3.3 XRD pattern for YSZ films grown on Si substrates from RT400 C 26 Figure 3.4 Typical AFM image of YSZ film at 400 C 27
v Figure 3.5 Absorption spectra of Ag f ilm exposed to dry air at 23 C 28 Figure 3.6 Variation of SPR peak w ith deposition time for Ag film 29 Figure 3.7 Absorption spectra of Ag and YSZ-Ag films 30 Figure 3.8 Absorption spectra of Ag-YSZ films exposed to dry air, H2 and CO 31 Figure 3.9 Change in sensor signal of Ag films with concentration of H2 32 Figure 3.10 Change in sensor signal of Ag films with concentration of CO 33 Figure 3.11 Cross-selectivity of Ag f ilm for CO in presence of 10% H2 34 Figure 3.12 Thickness of Au films fabric ated at 60 m Torr for 10, 15, 20 min. 36 Figure 3.13 SEM image of film fabricated for 10 min. 37 Figure 3.14 SEM image of film fabricated for 15 min. 37 Figure 3.15 SEM image of film fabricated for 20 min. 38 Figure 3.16 Roughness analysis of Au na noparticle film grown for 20 min. 38 Figure 3.17 XRD pattern of Au nanoparticle film grown for 20 min. 39 Figure 3.18 SPR peak for Au nanopartic le film exposed to dry air 40 Figure 3.19 Absorption spectra of Au f ilm exposed to CO 100 and 1000 ppm 40 Figure 3.20 Change in sensor signal of Au film with CO 100 and 1000 ppm 41 Figure 3.21 Au film (57 nm) exposed to dry air and H2 41 Figure 3.22 Picture of silver and copper targets 42 Figure 3.23 Thickness of Ag-Cu films fabricated at 100 m Torr for 20, 25 and 30 min. 44 Figure 3.24 XRD pattern for Ag-Cu alloy film 45 Figure 3.25 Absorption spectra of Ag -Cu film exposed to dry air at 23 C 46
vi Figure 3.26 Absorption spectra: Ag-Cu fi lm exposed to dry air, 10% H2, CO 100 ppm 47 Figure 4.1 H2 sensor signal comparison for Ag, Au and Ag-Cu films 50 Figure 4.2 CO 100 ppm sensor signal comp arison for Ag, Au and Ag-Cu films 50
vii OPTICAL DETECTION OF CO AND H2 BASED ON SURFACE PLASMON RESONANCE WITH Ag-YSZ, Au AND Ag-Cu NANOPARTICLE FILMS Denis Kitenge ABSTRACT Silver, gold, and copper metallic nanoparticle films have been utilized in various MEMS devices due to not only their electrical but also their optical properties. The focus of this research is to study the detecti on at room temperature of carbon monoxide (CO) and hydrogen (H2) via Surface Plasmon Resonan ce (SPR) phenomenon of silverembedded Yttrium Stabilized Zirconium (A g-YSZ) nanocomposite film, gold (Au) nanoparticle film, and an alloy film of silver-copper (Ag-Cu) grown by the Pulsed Laser Deposition (PLD). To determine the appropriate film mate rials for quick and accurate CO and H2 detection at room temperature with the PLD technique, the growth process was done repeatedly. Optical tools such as X-Ra y Diffraction, Alpha Step 200 Profilometer, Atomic Force Microscopy, and Scanning Electr on Microscopy were used to characterize thin films. The gas sensing performance was studied by monitoring the SPR band peak behavior via UV/vis spectrophotometer when the film s were exposed to CO and H2 and estimating the percent change in wavelength. The metallic nanoparticle films were tested for concentration of CO (100 to 1000 ppm) and H2 (1 to 10%). Silver based sensors were
viii tested for the cross-selectivit y of the gases. Overall the se nsors have a detection limit of 100 ppm for CO and show a noticeable signal for H2 in the concentration range as low as 1%. The metallic films show stable sensing over a one-hour period at room temperature. The SPR change by UV/vis spectrophotomet er shows a significant shift of 623 nm wavelength between 100 ppm CO gas and dry ai r at room temperature for the alloy films of Ag-Cu with a wider curve as compared to silver and gold films upon their exposure to CO and H2 indicating an improvement in accuracy and quick response. The results indicate that in research of CO and H2 detection at room temperature, optical gas sensors rather than metal oxide se nsors are believed to be effective due to not only the absence of chemical involvement in the process but also the sensitivity improvement and accuracy, much needed char acteristics of sensors when dealing with such hazardous gases.
1 CHAPTER 1: INTRODUCTION 1.1 Background Health and safety issues associated w ith environmental pollutants and hazardous gases such as CO, H2 HS, CO2, NOx and NH3 have always been the subject of priority in materials research. With the rising desire for better manufacturing of goods and the increasing concern about air quality due to pollution, it has become an obligation to develop not only highly sensitive but accurate gas sensors to cut down the number of accidents and the loss of lives, and environmental hazards, due to gas leakages . The main purpose of having such sensors should be the setting of a clea r and distinct alert announcing the presence of a gas in a lo cation. Metal oxide sensors have been a significant part of the gas se nsor technology and remain a widely used choice for a range of gas species -. The working principle of a typical resistive metal oxide gas sensor is based on a shift of state of equilibriu m of the surface oxygen reaction due to the presence of a target analyte. The change in concentration of chemisorbed oxygen is recorded as a change in resistance of the gas-sensing material. Optical gas sensors based on surface plas mon resonance are useful type of gas sensors used along with metal oxide sensors fo r detection of toxic gases in the past. The sensing mechanism based on SPR spectroscopy is related to the measurement of a small
2 change in refractive index that occurs in response to an an alyte binding at or near the surface of a noble metal such as Au and Ag . Surface Plasmon Resonance was first obser ved by Wood  to describe narrow dark bands in the spectrum of the diffracted light when a metallic diffraction grating with polychromatic light. These bands are associat ed with the excitation of electromagnetic surface waves on the surface of the diffracti on grating. More recently, the phenomenon has been studied and applied in bio-sens ing field, biopharmaceutical manufacturing, the spacecraft industry, sound-recording tec hnology, and windows surface coating -. The main advantages of SPR technology incl ude high sensitivity, la bel-free, real-time and rapid detection . This rapid detecti on is due to the fact that SPR of metaldielectric composite films formed by using metal nanoparticles embedded in a dielectric is very sensitive to the changes in the re fractive index induced by physical absorption or chemical reactions on the surf ace of the material -. 1.2 Surface Plasmon Resonance (SPR) Surface Plasmon Resonance is described as the quanta of waves generated by collective effects of a considerable number of electrons in matter. Metals show plasmon effect because they have high density of free electrons. Recently, surface plasmons resonance has become a promising technique of understanding va rious relationships between molecules. Plasmon sensitivity depe nds mostly on particle size and shape, refractive index of the medium and th e dielectric constants of the metal.
3 For most metals the plasmon energy is correlated to ultr aviolet photon energy. For silver and gold the plasmon energy is as low as that of visible photon making it possible to be excited with a light such as laser light. Figure 1.1 shows the visible spectrum where silver and gold plasmon energy is located. Figure 1.1 Visible range of electromagnetic radiation These specific plasmons exist at the surf ace and they have been utilized for many applications. Plasmons are also known as ch arge-density oscillation of free electron density against the fixed positive ions occu rring at the interface of two media with dielectric constants of opposite signs. Norma lly associated with charge density is an electromagnetic wave or the field vectors wh ich reach their maxima at the interface and decay evanescently into both media . The propagation of wave plasma is done in the direction parallel to the metal surface. Given the free space wave number ( k ), the dielectric constant of the metal ( m) and the refractive inde x of the dielectric ( n2 s), the propagation constant of the surface plasma wave ( ) is calculated following this equation:
4 2 2 s m s mn n k (Eqn. 1) The dielectric constant refers to a number relating the ability of a material to carry alternating current to that ability of a vac uum -; for a particular material or medium and wavelength, refractiv e index is the ratio of the ve locity of light in a vacuum to that in the material or medium  Table 1.1 gives the dielectric constants and refractive indices of materials of in terest for this work -. Table 1.1 Dielectric constants and refract ive indices of materials of interest Material Dielectric Constant Refractive Index Silver complex number 1.35 Copper complex number 2.43 Gold complex number 0.47 Hydrogen 1.000284 at 100 C 1.000132 Carbon monoxide (CO) 1.00070 at 23 C 1.000340 Air (dry) 1. 000536 at 20 C 1.0002926
5 In this study, the absorption ba nd exhibited as SPR arises at the resonance of the incident photon frequency with collective oscillation of the conduction electrons in the metal surfaces. This phenomenon is known as the Localized Surface Plasmon Resonance (LSPR) -. 1.3 Parameters of Surface Plasmon Resonance Based Sensors 1.3.1 Sensitivity An optical sensor is a device which converts the quantity being measured (measurand) to another quantity (output). This is typically enc oded into one of the characteristics of a light wa ve. Figure 1.2 diagrams the main components of a SPR sensor aligned in a sequential or der of a normal operation Figure 1.2 Schematic diagram of SPR sensor
6 The measurand is followed by the optical system housing the thin film, then by the output quantified as wavelength. SPR sensor sensitivity is essentially its ability to respond to an analyte. To a concentration of analyte c the sensitivity of an SPR sensor can be written as : c Ysc (Eqn. 2) In the present study, sensor sensitivity is referred to as the change of SPR band peak which mainly depends on the refractiv e index of the medium, size and shape of particles, and the nature of the surface material. SPR sensor sensitivity measurement is estimated as a percent change for SPR peaks following this expression: Technically the SPR sensor sensitivity in our study has two contributors: the wavelength obtained when the film is exposed to dry air and the wavelength when the targeted gas is present. 1.3.2 Resolution Typically a sensor is a device that in volves change and response. However, determining how much change is needed to cr eate a response is the re al issue in sensing technology. Resolution of the SPR sensors de fines the smallest change in the bulk refractive index that produces a detectable ch ange in the sensor output . Such change is measured and determined.
7 There are times when changes are intr oduced into the sensing device without being desired. The intensity of light, for ex ample, can cause fluctuation and lead to unwanted responses. To avoid fluctuation a st able light source is needed to improve sensor reliability. In sensor resolution is the limit of detection. In this study, the expression limit of detection is adopted when CO and H2 are monitored via the SPR band shifts. 1.3.3 Selectivity or Cross-reactivity In the normal conditions a gas sensor dete cts the presence of a particular gas. However, it is very important to evaluate the ability of a sensor to respond to a specific gas exposure while another gas is present at a low concentration. In this study we introduce H2 while exposing the sensing silver film at the exposure of CO to determine the cross selectivity. The following expressi on was used to do the calculations: 100 ) (2 2H CO H 1.4 Surface Plasmon Resonance Materials: Silver, Copper and Gold Silver (Ag), copper (Cu) and gold (Au) shows the characteristics of surface plasmon resonance. All three have one el ectron in the outer atomic shell. Those materials have ability to e xhibit surface plasmon resonance absorption bands in visible light . These bands undergo some change in the vicinity of gas flow therefore; become a reliable indicator when used as a mode of assessing gas c oncentration in an
8 environment. Silver, Gold and Copper have one single electron (e-) occupying the last energy level. This conduction electron is th e driving force for generating a plasmon. The bands produced by silver nanocry stals are not always stable at high temperatures due to oxidation -. To prevent the Ag film from oxidation we have chosen (YSZ) as the host material , which is chemica lly and mechanically stable .
9 CHAPTER 2: EXPERIMENTAL METHOD 2.1 Pulsed Laser Deposition (PLD) Pulsed Laser Deposition (PLD) was used to grow the thin films for sensing applications. This deposition procedure allo ws a laser pulse to enter through a window into a vacuum chamber and impinges on the mate rial to be deposited . Laser pulse (width 20 30 nanosecond) is focused to an energy density (~1 10 J/cm2) to vaporize a few hundred angstroms of surface material (calle d the plume) in the form of neutral or ionic atoms and molecules with electron-volt ki netic energies which then deposit onto the substrate positioned in front of the target . The distance between the target and the substrate can be adjusted based on the target structure and the type of the film to be fabricated. Excimer laser at lower powe r densities have been used to deposit semiconducting and superconducting thin film from bulk targets. Prior to the deposition the vacuum chamber needs to be evacuated from the atmosphere by using a roughing pump to 30 m Torr. The Turbo molecular pump can be activated if a high vacuum is needed. PLD parameters for thin film fabrica tion have been found to be different from other deposition methods. Pulse laser radiation is used for material vaporization and the deposition of thin film. Figure 2.1 is a sche matic diagram of a PLD vacuum chamber, while Figure 2.2 displays the entire PLD system.
10 Figure 2.1 Schematic diagram of a PLD vacuum chamber Figure 2.2 PLD vacuum chamber and apparatus at USF
11 During this procedure the laser radiati on is absorbed by a solid surface and the electromagnetic energy is converted into elec tronic excitation and then into thermal to cause evaporation, ablation, excitation, plasma formation, and exfoliation . Figure 2.3 shows plasma generated during a depos ition by PLD of YSZ at 400 C. Figure 2.3 Plasma emissions in the vacuum chamber Besides its simplicity and versatility, PLD offers other advantages that make it a phenomenal deposition technique such as its ab ility to preserve the stochiometry of a multicomponent system. PLD is proven to be the appropriate technique for thin film fabrication needed for this study.
12 2.1.1 Optimization of Parameters for PLD 188.8.131.52 Target-to-Substrate Distance Target to substrate distance is a critical pa rameters for growth of films. Substrate should have an optimized distance from the target holder to rece ive the ejected ions from the target. The maximum possible distan ce between target holder assembly and the substrate is 90 cm. Substrate to target distance is useful to control the grain size of the particles in the films. Higher the di stance lowers the particle size. Figure 2.4 Target-to-substrate distance inside PLD vacuum chamber
13 Figure 2.5 Substrate holder assembly 184.108.40.206 Effect of Temperature on Films Properties Optimized Temperature is important in order to grow the film with required properties. Most films used in this st udy were grown at 23 C (room temperature). However the effect of temperature on mor phology of the films was studied and will be discussed in next chapter. In the PLD used for this study a thermo couple is built into the system to record the inside vacuum temper ature and display it in a small window as shown in Figure 2.6
14 Figure 2.6 Temperature displayed inside the vacuum chamber 2.2 Microstructure (X-Ray Diffraction) In this study crystallogr aphic structure of silv er, copper and gold was characterized using X-Ray Diffraction. A t ypical XRD is a technique utilized to determine grain size and preferred orientati on in polychrystalline or powdered solid samples. Due to the fact that the wave lengt hs used in X-rays ar e very small they can penetrate a considerable number of planes in the crystal, and therefore give information about crystal orientati on, material crystallite thickness, the distance between planes lying in atoms, and even residual stress in the film. The spacing between atoms is determine by the use of Braggs equation give n in Figure 2.7 where
15 n = integer representing the order of peak diffraction = wavelength of x-Ray d = inter-planar distance = scattering angle ACB = n Figure 2.7 Braggs law Average grain size can be calculated using the Scherrers equation given as: BB k t cos (Eqn. 4) Diffracted X-rays from thin film sample s pass through the soller slit, which limits divergence of X-rays in verti cal direction. After getting diffracted by a diffracted beam monochromator (graphite plate crystal), the si gnal is fed to a photomultiplier tube (PMT) interfaced to a personal computer which stores intensity vs. 2 data in a file. 2 values for the peaks were used to determine the crys tal structure and lattice parameter (s) of the film materials with the help of the Braggs equation.
16 2.3 Atomic Force Microscopy (AFM) Surface morphology of Ag, Cu, Au and YSZ were studied using AFM (Atomic Force Microscope). AFM is a technique used to analyze the surf ace of materials with magnifications up to 108. AFM provides three-dimensi onal images from conducting or non-conducting samples with extraordinary topographic contrast direct height measurement and unobscured view of surface f eatures without any sample preparation. Figure 2.6 below shows an atomically a shar p tip made of silicon extended down from the end of a cantilever with f eedback mechanisms that enable the piezo-electric scanners to maintain the tip at a constant forc e or height above the sample surface. Figure 2.8 Atomic force microscope block diagram
17 AFM relies on the interatomic van der Waals forces as the factor creating the interaction mechanism between the tip and the sample. Deflection is being done by a laser beam making the system substantially more effec tive. AFM operates in three modes namely, the contact mode, the non contac t mode and the tapping mode. In the contact or repulsive mode, the AFM tip makes contact with the sample, the sample and tip interact by repulsive forces due to quantum mechanical exclusion principle. This mode provides the best resolution but th e tip can deform the surface of the sample due to excessive cantilever force on tip agains t the sample. In the non-cont act mode however the AFM cantilever is vibrated within tens to hundr eds of Angstroms of th e specimen surface; the van der Waals, magnetic or capillary for ces produce images of topography with lower poor resolutions than from the contact mode. In the tapping mode (intermittentcontact mode) the vibrating cantilever tip is brought closer to the samp le so that, at the bottom of its traverse, it just barely hits the sample resulting in smaller damage to the sample than in the contact mode. 2.4. Scanning Electron Microscopy (SEM) In the operation of SEM (Hitachi S-8 00, figure 2.7) a tungsten tip launches a high-energy beam of electrons in a raster scan pattern in vacuum conditions to avoid any possible interference between th e beam and the atmosphere. As electrons interact with the atoms that make up the sample, secondary back-scattered electr ons from the sample are generated and signals are produced, revea ling the arrangement of different elements in the sample, containing information about the sample's surface topography,
18 composition and electrical conductiv ity. In this work SEM was used to reveal the size of particles as they play a crucial role in se nsing mechanism. The range of magnification for the SEM is up to 300,000 times of the actua l size. In addition, its electron beam is used to perform chemical analysis on micr ostructures below 1 micron in size using an Xray spectrometer. Figure 2.9 Hitachi S-800
19 2.2.5 UV/vis Spectrophotometer UV/vis spectrophotometer in absorption mode was used to measure SPR peak behavior of silver, gold and silver -copper films. Essentially, the UV/vis spectrophotometer allows inte raction between the film, a liquid solution or a solid material and a monochromatic light to take pl ace in a cell isolated from the outside light. As result, the variables such as transmittan ce, reflectance, and absorbance are obtained . The main components of the equipm ent are: a source generating a broad band of electromagnetic radiation, a dispersion device selecting a particular wavelength, a chopper dividing the paths of light and a detect or measuring the intensity of radiation. Figure 2.10 shows UV/vis spectrophotometer wi th the optical system displaying the optical system open and the sample area magnified to the right. Figure 2.10 UV/vis spectr ophotometer with optical system open
20 The working principle of a typical spectr ophotometer is based on the use of light that becomes monochromatic passing through a dispersion device and is launched onto the chopper, which switches th e light path between a refe rence optical path and the sample optical path to the detector. The chopper rotates at such speed that alternate measurements of blank and sample occu r several times per second. The degree of interaction of the sample with radiation (transmittance or absorption) is determined by measuring both the intensity of the incident radiation or I0 (without the sample) and the transmitted intensity or I (with the sample). In this respect, Transmittance (T) and Absorbance (A) are defined as follows: T = o or %T = 100o A = log T The UV/vis spectrophotometer was not desi gned to test the gas concentration therefore, an additional component was desi gned and assembled to accommodate the film setting and gas flow for measurement of SPR band behavior. The gas-sensor cell testing assembly is a 4 x 4 x 2 cm, 606 aluminum box with the main opposite sides covered with Plexiglas making a path to laser light as s hown in figure. The inlet and outlet gas tubing and the fittings are made of st ainless steel. In order to avoid gas leak a teflon is used in the fittings. Once integrated in the optical uni t, the film holder is designed to remain
21 steady, to avoid any unnecessary movement th at would introduce random error in the measurement. Figure 2.16 displays the cell assembly and its main parts. Figure 2.11 Gas-sensor cell testing assembly
22 CHAPTER 3: STRUCTURAL AND GAS SENSING CHARACTERISTICS OF FILMS 3.1 SPR Based Sensors Composed of Ag-YSZ Nanoparticle Films In this study we coupled Ag with YSZ to prevent it from oxidizing and keep the SPR bands stable when being monitored in the presence of CO and H2. Silver (Ag) nanoparticle films were deposited on glass substrates using as described in chapter 2. The substrates were cleaned with alcohol and acetone followed by drying in nitrogen. A pure silver target of 99.99% with 40 mm diameter was enclosed in a stainless vacuum chamber at 23 C. The distance between the ro tating target holder, which is the ablation point, and the center of the subs trate was 90 mm. This distance can be varied to change the final particle size in the films an d consequently change the film quality. Krypton fluoride (KrF) Excimer laser 248 nm (Lambda Physik Inc., LPX 201i) was set up to run at a repetition rate of 10H z with a pulse width ~25 ns for 5 minutes, 10 minutes, and 15 minutes. The 15 minutes film is reported to have 117 nm while the 5 minutes film has 37 nm, and the 10 minutes is 76 nm thick. The thicknesses of films as measured by profilometer are presented in Figure 3.1.
23 Figure 3.1 Thickness of films grown for 5, 10, 15 min. measured by profilometer Atomic force microscopy (AFM) was performed to examine the surface morphology and the uniformity of the Ag film s. Thin films of YSZ were also grown using PLD technique with a change of de position conditions. The distance between the ablation point and the center substrate was 40 mm at temperatures of 23 C, 200 C and 400 C respectively with background vacuum in 200 mTorr. The deposition time was 30 minutes for each film. The YSZ films we re first deposited on the silicon (Si) substrates and structural properties were studied using X-ray diffraction. YSZ film was deposited on glass substrates following the same procedure used for Ag deposition. The Ag films with different thicknesses were deposited on top of YSZ films. Gases of different concentrations were used. Gas flow of 100 sccm was used with mass flow controllers (MKS Inc., Type 247 controller). To ma ke a selected gas flow through UV/vis
24 spectrophotometer chamber over the film under investigation for absorption change, the aluminum cell described in Chap ter 2 was used effectively. The Ag nanoparticle films were tested for different concentrations of H2 (1%, 5% and 10% in nitrogen) and CO (100 ppm, 500 ppm and 1000 ppm in air). Surface morphology of Ag nanoparticles was studied by using atomic force microscopy. Figure 3.2-A shows the typical AFM micrograph, and Figure 3.2-B the AFM average size of Ag film. Figure 3.2.A Typical AFM micrograph of Ag film
25 Figure 3.2.B AFM average size of Ag film (20.6nm) synthesized at RT Ag nanoparticles with average diameter of 20 nm were synthesized for a deposition time of 10 minutes. The average grain size of the films was observed to increase with increasing the deposition time for the Ag films. X-ray diffraction (XRD) was performed to study the growth of base material YSZ. The XRD pattern for YSZ at room temperature (RT) to 400 C is shown in Figure 3.3.
26 Figure 3.3 XRD pattern for YSZ films grown on Si substrates from RT 400 C It can be seen that at room temperature there is no remarkable YSZ peak, suggesting that the film was amorphous in such conditions. However, at 200 C for 30 minutes, the YSZ peak began to appear, and more noticeably when the substrate temperature reached 400 C, indicating th at there was a sustainable crystalline improvement in the film. With such temp erature increase, there is remarkable enhancement in surface morphology of YSZ film as displayed in Figure 3.4.
27 Figure 3.4 Typical AFM image of YSZ film at 400 C Since only the YSZ film synthesized at 400 C is clearly observed to possess tetragonal structure with a str ong preferential orientation alon g (111), it is believed that a higher substrate temperature could result in the amelioration of film crystallinity. The XRD pattern in Figure 3-4 shows a peak of silicon emerging at approximately 35 along (200) orientation. When made on glass substrate at 400 C, the new YSZ film brought nothing but the ab ility to accommodate silver nanoparticles deposition for absorption monitoring and measurement in UV/vis spectrophotometer. Surface plasmon resonance technique has been used for investigating sensing behavior of Ag films. Surface plasmon res onance on metal particles has been explained by G. Mie . The SPR peak for silver is generally observed at 460 nm in our PLD grown films, as shown in Figure 3.5.
28 Figure 3.5 Absorption spectra of Ag f ilm exposed to dry air at 23 C The broadness of the peak reduces with hi gher grain size in the films, which can be varied by changing the parameters such as deposition temperature and the distance between the ablation point and the center substrate. SPR peak shift was observed by varying the grain size. It is be lieved that the growth of collo id sizes in Ag nanoparticles is the likely cause of the shift toward the l onger wavelengths -[ 37]. The absorption peak grew linearly with time of depo sition, as can be observed in Figure 3.6.
29 Figure 3.6 Variation of SPR peak with deposition time for Ag film Films with short time deposition show a re latively short wavelength as opposed to those with long time deposition. For using the Ag films as gas sensors an optimized growth temperature (room temp) and distance equal to 90 mm (between the ablation point and the center substrate) were selected and all the Ag films were deposited for 5 minutes with average grain size of 20 nm. Measuring the surface plasmon resonan ce in the Ag and Ag-YSZ film and determining the sensing mechanism was the ma in goal of this study. As preliminary stage of the investigation, the SPR pr oprieties of Ag and Ag-YSZ n eeded to be assessed before recording the measurement with a designated gas flowing over the fi lm. As it turned out, at room temperature the SPR peak for Ag was located at ~ 460 nm, and there was no
30 damping in the width of the absorption spectrum. Using Ag-YSZ film for the same measurement, the peak was, as expected, at the same location, suggesting that YSZ brought no negative effect in the SPR of Ag nanoparticles. To clarity the discussion, Figure 3.7 shows the absorption spectra for YSZ-Ag film and Ag film. Figure 3.7 Absorption spectra of Ag and YSZ-Ag films This result backs the idea of using YSZ as base material for Ag nanoparticle to be used as SPR based gas sensors in harsh envi ronment in order to pr event the oxidation of Ag at higher temperature. Figure 3.8 illustrates the absorb ance spectrum of Ag-YSZ nanocomposite for different gas flow at room temperature.
31 Figure 3.8 Absorption spectra of Ag-YSZ films exposed to dry air, H2 and CO The SPR peak was observed to shift w ith changing the gaseous environment around the sample, in the case of 100 ppm CO in comparison to H2. Without any targeted gas the SPR peak was at 460 nm, while with 10% hydrogen exposure the peak rises at 478 nm, and with 100 ppm CO the SPR peak is obtained at 480 nm. Figure 3.6 shows this peak shift for the discussed gases. The SPR band peak was observed at 492 nm for 500 ppm of CO and at 498 nm for 1000 ppm CO. In this work the gas sensor si gnal is defined as the percent change in SPR peak shift in absence and presence of the target. It is represen ted as sensor signal
32 Here gas and no gasindicate wavelengths where the SPR peak was observed for the Ag nanoparticle films in absence and presence of the target gas respectively. Variations of gas sensor si gnal with concentration of H2 and CO are shown in Figures 3.9 and 3.10. Figure 3.9 Change in sensor signal of Ag films with concentration of H2
33 Figure 3.10 Change in sensor signal of Ag films w ith concentration of CO The Ag nanoparticle films based sensors we re tested for their cross-reactivity or selectivity for CO in presence of H2 gas. For this purpose the absorption spectra with SPR peak at 478 nm was recorded in presence of 10% hydrogen. Later, in the same hydrogen environment, CO was introduced in vari ous concentrations (100, 500 and 1000 ppm). The change in absorption spectra with SPR peak shift was recorded. The sensor signal in this case was determined as: Cross selectivity = 100 ) (2 2H CO H The occurrence of SPR and LSPR depends on morphology and the nature of the sensing materials; the LSPR is consider ed as a predominant phenomenon in our Ag nanoparticle films. Values of 4.3, 6.7 and 8.4 for the cross-selectivity for CO in the presence of 10% H2 are plotted in Figure 3.11.
34 Figure 3.11 Cross-selectivity of Ag f ilm for CO in presence of 10% H2 3.2 SPR Based Sensors Composed of Au Nanoparticle Films Au nanoparticle films were grown on gla ss substrates using PLD. Alcohol and acetone were used to remove impurities from the substrates, while nitrogen gas was to dry the substrates.. The laser beam irradiated a 99.99% pure gold target enclosed in a stainless vacuum chamber. The target-t o substrate distance was 90 mm. During the deposition the background argon pressure was kept 60 m Torr. These conditions were maintained as the deposition time change d three values: 10, 15 and 20 minutes respectively in order to get films with diffe rent thickness. Table 3.1 shows the deposition conditions used for the purpose of optimizing the experiment.
35 Table 3.1 Gold nanoparticle films experimental conditions Fluence 300 m/J Target 99.99% Au Substrate glass Target-to-substrate distance 90 mm Substrate temperature ( Ts) RT to 100 C Argon pressure ( Arp) 60 m Torr Deposition time 10, 15, 20 minutes Figure 3.12 shows the thickness of gold nanoparticle films synthesized at 60 m Torr for three different deposition times. Films thickness of approximately 28, 46 and 57 nm was achieved for the deposition time of 10, 15 and 20 minutes, respectively.
36 Figure 3.12 Thickness of Au f ilms fabricated at 60 m Torr for 10, 15, 20 min. Figures 3.13, 3.14 and 3.15 show SEM images of the three films with thicknesses as given above. The particle size in films obser ved and increase with increase of deposition time as seen from the SEM images.
37 Figure 3.13 SEM image of film fabricated for 10 min. Figure 3.14 SEM image of film fabricated for 15 min.
38 Figure 3.15 SEM image of film fabricated for 20 min. Surface morphology study and roughness measur ement were carried out using AFM. Figure 3.17 shows the AFM image for selected area (2.769 m2) of film. Figure 3.16 Roughness analysis of Au na noparticle film grown for 20 min.
39 The typical XRD pattern (shown in Figure 3.18) fo r the Au film shows that the film has a crystalline Au phase with a strong pref erred orientation along (111) at ~38. Figure 3.17 XRD pattern of Au nanoparticle film grown for 20 min. Similar to the sensing investigation made in silver films, the sensing performance of gold nanoparticle films was accomplished by monitoring the SPR band behavior. It is observed in Figure 3.18 that the SPR peak for the gold film surface with 46 nm thickness exposed to dry air using UV/vis spectrosc opy is located at 525 nm, whereas for a 10 minutes silver film in the same conditions, 460 nm is retained as peak location. Also noticeable here is the broadening of the SPR curve.
40 Figure 3.18 SPR peak for Au nanopart icle film exposed to dry air When exposed to CO at two different c oncentrations, 100 ppm and 1000 ppm, the Au film under test shows SPR peaks at 532 nm and 539 nm respectively, shown in Figure 3.19. Figure 3.20 gives the variation in sens or signal from 10 0 ppm and 1000 ppm. Figure 3.19 Absorption spectra of Au film exposed to CO 100 and 1000 ppm
41 Figure 3.20 Change in sensor signal of Au film with CO 100 and 1000 ppm The film with thickne ss of 57 nm was used for testing hydrogen concentration as shown in figure 3.21 Figure 3.21 Au film (57 nm) exposed to dry air and H2
42 3.3 SPR Based Sensors Composed of Ag-Cu Nanoparticle Films In this section we study the optical properties of Ag-Cu nanocomposite film. Commercially available 99.99% pure silver a nd 99.99% pure copper targets were divided in half. The glass substrate used for the expe rimentation was also prepared in the same manner. In order to be set in the target holde r the two semicircle ta rgets were taped from the back and screwed from the side. The targ et was irradiated with Lambda Physik LPX 201i Excimer-laser lig ht at wavelengths = 248 nm and 300 m/J as energy. The temperature in the chamber was 100 C a nd the background pressure was 100 m Torr. The target-to-substrate distance remained 90 mm as it was for the deposition of Ag and Au nanoparticle films. Since no weight was added to the newly assembled target, the rotation of the carousel drive remained 10 rp m. During the depositi on, the repetition rate was 10 Hz. Figures 3.22 displays the silver and copper targets Figure 3.22 Picture of silv er and copper targets
43 Upon completion of the deposition the film remained in the vacuum chamber for one hour until the temperature came down to 23 C. The experimental conditions are shown in Table 3.2. Table 3.2 Ag-Cu alloy films experimental conditions Fluence 300 m/J Targets 99.99% Ag and Cu Substrate glass Target-to-substrate distance 90 mm Substrate temperature ( Ts) 100 C Argon pressure ( Arp) 100 m Torr Deposition time 20, 25, 30 minutes Figure 3.23 shows film thickness as function of ablation time. As in the previous film thickness investigations, the thickness of Ag-Cu nanoparticle films increased with ablation time. For the 20, 25 and 30 minutes films, 112, 141 and 165 nm were found as thickness measurements using profilometer.
44 Figure 3.23 Thickness of Ag-Cu films fabri cated at 100 m Torr for 20, 25 and 30 min. The 20, 25 and 30 minutes ablation time for Ag-Cu is an increase from the 10, 15, and 20 minutes time used for Au. This provokes thought for possible reasons: melting point could be one. To investigate the microstructure of Ag -Cu film, its deposit ion was made in a silicon wafer following the conditions given a bove. XRD analysis shows, as in Figure 3.24, that the structure of the deposited film contained a cr ystalline silvercopper phase with a strong preferential or ientation along (111) at ~38 for silver. Copper, however, had a (200) orientation at ~44 while si licon orientation was (101) at ~33.
45 Figure 3.24 XRD pattern for Ag-Cu alloy film Figure 3.25 shows absorption spectra of Ag-C u exposed to dry air with 581 nm as peak location. As can be see n, the curve covers almost the entire visible region. The higher conductivity of coppe r, compared to silver, seems to influence the increase in broadness.
46 Figure 3.25 Absorption spectra of Ag -Cu film exposed to dry air at 23 C Ag-Cu alloy nanoparticle films have what it takes to make a more sensitive SPR based sensor. Figure 3.29 shows absorption spectra of Ag-Cu film exposed to dry air, 10 % H2 and CO at 100 ppm with 581, 611 and 623 nm respectively. It is believed, based on the pattern of the previous analysis, that the higher the concen tration the longer the wavelength at which the absorption peak attributed to SPR is located.
47 Figure 3.26 Absorption spectra: Ag-Cu fi lm exposed to dry air, 10% H2, CO 100 ppm The sensing signal calculation was estimated by percentage change in wavelength for SPR shift in the same manner we had done all along. As it turned out, 5.16 was recorded as sensing signal when the Ag-C u nanoparticle film was exposed to 10% H2 and 7.22 at the exposure to CO (100 ppm). This re sult puts Ag-Cu alloy film at the top, when compared with Ag and Au films, for H2 and CO detection at the indicated concentration and under standard conditions. From the preceding analysis and results it might be tempting to conclude that surface plasmon resonance based sensors bring nothing but perfection to sensing technology. A close and careful look, however at the functionality of SPR sensors yields to the conclusion that SPR based sensor s have issues like other types of sensors. The first issue associated with SPR sensors is related to the infiltra tion of random errors
48 which are the statistical fluctuations in the measured data due to the precision limitations of the sensor system. These errors are genera ted by instrument components such as light source and detector capability. Progress has been made to improve light efficiency in the past. Quartz tungsten halogen lamp for exam ple shows high stability but fluctuations in the light source remains an issue in SPR based sensors. To solve this issue, development of detectors of light intensity with a higher signal-tonoise ratio could be the solution -. The same assessment can be made regarding the limit of detection (LOD) in SPR based sensors. It was determined in th e course of this study that the gases in concentration like 1% for H2 and 100 ppm for CO can be sensed. The reasons for these delays could have been the quality of the films or the instability in the cell-testing assembly.
49 CHAPTER 4: CONCLUSI ON AND FUTURE WORK 4.1 Conclusion This study has demonstrated the detection of CO and H2 in low concentrations using SPR phenomenon. The results suggest that Ag-YSZ, Au a nd Ag-Cu nanoparticle films, which served for test sensing devi ces, exhibit relatively high sensitivity with reference to the shift in peak position and the narrow ing of SPR band. The overall results put Ag nanoparticle films at the top wh en compared to that of silver as far as sensitivity is concerned. However, the sens ing performance observed in silver-copper alloy film indicates that promising sens ing material might be coming from the combination of different materials rather th an from single ones, as shown in Table 4.1 and Figures 4.1 and 4.2. Table 4.1 H2 and CO sensor signals comparisons for Ag, Au and Ag-Cu films Sensor sensing element Target gas & concentration Sensor signal Silver nanoparticle film H2 (10%) 3.9 Silver nanoparticle film CO (100 ppm) 4.3 Gold nanoparticle film H2 (10%) 1.3 Gold nanoparticle film CO (100 ppm) 1.6 Silver-copper nonocomposite film H2 (10%) 5.1 Silver-copper nonocomposite film CO (100 ppm) 7.2
50 Figure 4.1 H2 sensor signal comparison for Ag, Au and Ag-Cu films Figure 4.2 CO 100 ppm sensor signal comp arison for Ag, Au and Ag-Cu films
51 It was also demonstrated, in the course of the experimental pr ocedure, that film sensitivity depends for the most part on the refractive index of the medium, which makes it possible to predict the signal range and sensitivity le vel. Other variables, such as particle size and shape, thickness along w ith film morphology, grain size and even the distance particle-to-particle, do play a relevant role in the sensing mechanism as well. The results and discussion yi eld to the following comments. Compared with metal oxide-based sensor s, a promising way of using UV/vis spectrophotometer to detect the presence of CO and H2 in standard conditions is initiated in this study. However, the actual configuration of the sensor requires more research time and resources to find an effective techni que to reduce the noise in the system, which could have been generated from the instability of the film holder or most likely from the systematic control of gas flow over the film under investigation. 4.2 Future Work Further research on the following topics co uld significantly enhance the range of sensitivity observed in the films: 1) Other gases like NH4, and NOx, H2S, Ethanol vapors can be tested for SPR based gas sensing. 2) Use alloy and composite film s to improve the gas sensor signal and selectivity.
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