The Effect of Annealed Au Thin Film on the Surface Plasmon Resonance Measurements

Transcription

1 Indiana University of Pennsylvania Knowledge IUP Theses and Dissertations (All) The Effect of Annealed Au Thin Film on the Surface Plasmon Resonance Measurements Waseem Abdullah Al Luhaybi Indiana University of Pennsylvania Follow this and additional works at: Recommended Citation Al Luhaybi, Waseem Abdullah, "The Effect of Annealed Au Thin Film on the Surface Plasmon Resonance Measurements" (2014). Theses and Dissertations (All) This Thesis is brought to you for free and open access by Knowledge IUP. It has been accepted for inclusion in Theses and Dissertations (All) by an authorized administrator of Knowledge IUP. For more information, please contact cclouser@iup.edu, sara.parme@iup.edu.

2 THE EFFECT OF ANNEALED AU THIN FILM ON THE SURFACE PLASMON RESONANCE MEASUREMENTS A Thesis Submitted to the School of Graduate Studies and Research in Partial Fulfillment of the Requirements for the Degree Master of Science Waseem Abdullah AL Luhaybi Indiana University of Pennsylvania August 2014

3 2014 Waseem Abdullah AL Luhaybi All Rights Reserved ii

4 Indiana University of Pennsylvania School of Graduate Studies and Research Department of Physics We hereby approve the thesis of Waseem Abdullah AL Luhaybi Candidate for the degree of Master of Science Andy Zhou, Ph.D. Professor of Physics, Advisor Muhammad Numan, Ph.D. Professor of Physics Ajawad Haija, Ph.D. Professor of Physics ACCEPTED Timothy P. Mack, Ph.D. Dean School of Graduate Studies and Research iii

5 Title: The Effect of Annealed Au Thin Film on the Surface Plasmon Resonance Measurements Author: Waseem Abdullah AL Luhaybi Thesis Chair: Dr. Andy Zhou Thesis Committee Members: Dr. Muhammad Numan Dr. Ajawad Haija Surface Plasmon Resonance (SPR) is an optical phenomenon which detects the change of the refractive index in the dielectric/metal interface. When a p-polarized light strikes a thin-film metal under the condition of total internal reflection, plasmons propagate through a dielectric/metal interface. As a result, there is a sharp dip in the curve that indicates the minimum reflectance of the light. In this thesis, the SPR measurement of distilled water solution is investigated experimentally using the NanoSPR6 device. The Win Spall 3.2 software is used to do the theoretical simulation. The SPR angle of distilled water was observed at. The Surface Plasmon Resonance of sugar solutions was measured with various concentrations (5%, 10%, 15%, and 20%). It shows that the SPR angle has a linear relationship with the refractive index. The purpose of this thesis is to investigate the effect of the annealing of the Au thin film on Surface Plasmon Resonance. The samples were annealed at C, C, C, and C for various times. At C, the gold film evaporated from the glass slide. The value of the dielectric functions of Au thin film changes with annealing due to the change of the morphology of the Au thing film. Therefore, the Surface Plasmon Resonance will change. The values of the dielectric functions of the Au thin films annealed at different temperatures were estimated by curve fittings. iv

6 Finally, the Surface Plasmon Resonance of Au-PCBM polymer was studied with various sugar solutions. The Surface Plasmon Resonance measurement was done for the sample before and after 24 hours. It is shown that the Surface Plasmon Resonance was shifted slightly when the sample left for 24 hours. The refractive index of PCBM was estimated by curve fitting and found to be The Surface Plasmon Resonance angle difference of PCBM-distilled water before and after 24 hours was 0.6 degree. v

7 ACKNOWLEDGMENTS I would like to show my sincerest gratitude and appreciation to my advisor Dr. Andy Zhou for his help, encouragement, and guidance throughout my thesis. I would like to thank him for providing me with an outstanding atmosphere for doing research. Without his continuous support this thesis would not have been accomplished. I would like to thank my thesis committee members Dr. Muhammad Numan and Dr. Ajawad Haija for serving on my committee and for the valuable and helpful suggestions. I also would like to thank my graduate coordinators Dr. Gregory Kenning, Dr. Majid Karimi, Dr. V.J. Wijekumar, and the entire faculty and the staff for their support and assistance during my study in the program. I would like to thank Dr. Guangyong Li at the University of Pittsburgh who prepared the Au-PCBM samples. A special thanks to my parents, my mother, Jumma, and my father, Abdullah, for all of the sacrifices that they have made on my behalf. Without their love and support this thesis would not have been possible. The endless love and encouragement of the rest of my family, my grandmother, my grandfather, my brothers, my sisters, my uncles, my aunts, and my cousins, especially Tarik, was crucial in the completion of this program. I also would like to thank my dear friends Fathi Mubaraki, Ahmed Sultan, Saif Al Manaseer, Ahmad Al Manaseer, Salman Alhenaki, Mubarak Alkhulifi, Mohammed Alameer, Abdullah Aldawood, and Chuck McCutcheon for their encouragement. You guys have been amazing. My gratitude goes to my colleagues, Talal Alshammari, Ibunkunoluwa Korede, Megan Agosti, Jared, Alissa Richard, Richard Casselberry, and Brian Ford for all their understanding and collaboration. vi

8 program. Finally, I am grateful for everyone who helped and supported me during my studies in the vii

9 TABLE OF CONTENTS Chapter Page I INTRODUCTION Introduction Motivation of the Study History of SPR Outline of the Thesis...3 II THEORY OF SURFACE PLASMON RESONANCE Introduction Maxwell s Equation and the Wave Equation The Dielectric Constant and the Appearance of Refractive Index Total Internal Reflection and the Evanescent Wave Total Internal Reflection Evanescent Waves Polarization of Light Waves Dielectric Function of Surface Plasmon based on Drude- Sommerfeld Model The Dispersion Relation of Surface Plasmon Polaritons The Excitation of Surface Plasmon Polariton Prism Coupling Surface Plasmon Resonance The Reflection Intensity The Feature of Surface Plasmon Resonance Curve...26 III SURFACE PLASMON RESONANCE AND MEASUREMENT OF SUGAR SOLUTIONS Introduction Equipment and Devices NanoSPR 6 Device Calibration The Process of Taking Measurements Win Spall Software Introduction Simulation Curves Surface Plasmon Resonance for Sugar Solutions...34 viii

10 IV THE EFFECT OF THE ANNEALING OF AU THIN FILM ON SPR CURVES AND THE SPR CURVE OF PCBM-AU Introduction Experimental Procedure The Annealing Process of Au Thin Film Polymer Measurements Results and Discussion The Annealing Process of Au Thin Film SPR Curves for Au-PCBM Sample...51 V CONCLUSION...55 REFERENCES...57 ix

11 LIST OF TABLES Table Page 3.1 Some Features of NanoSPR6 Device The Values of the Thickness and Refractive Index of Different Layers Provided by NanoSPR Amounts of Sugars and Concentrations The Values of the Real and the Imaginary Part of the Dielectric Function of Au Thin Film for Various Annealing Time The Estimated Values of the Real and the Imaginary Part of the Dielectric Function of Au Annealed at C for Various Times The Estimated Values of the Real and the Imaginary Part of the Dielectric Function of Au Annealed at C...49 x

12 LIST OF FIGURES Figures Page 1.1 Number of articles in which the title contains Surface Plasmon Resonance Propagation of light along z direction The reflection of a wave incident. The left shows the normal refraction. The right shows the total internal refraction An evanescent wave Schematic of p-polarized electromagnetic field -or TM mode- on between two Media Real and imaginary part of the dielectric function for gold based on Drude- Sommerfeld model The electromagnetic field of SPs propagating on the x direction of the surface Interface between two dielectric mediums The dispersion relation for surface plasmons, light in a vacuum, light in a prism The distribution of electric field intensity to metal and dielectric mediums Prism coupling using Otto configuration Prism coupling using Kretschmann configuration The Surface Plasmon Resonance curve The three layers reflections between two parallel surfaces The Surface Plasmon Resonance curve of gold and silver films NanoSPR6 device A close view of NanoSPR The single measurement of Surface Plasmon Resonance Simulation of SPR curve using the WinSpall software...33 xi

13 3.5 The total internal reflection curve at the glass/ air interface Experimental and theoretical SPR curves of the gold/ distilled water interface Experimental and theoretical SPR curves for different concentrations: (a) 5% sugar solution, (b) 10% sugar solution, (c) 15% sugar solution, and (d) 20% sugar solution SPR dip angle versus refractive index of various sugar solutions Refractive index versus concentration of sugar solution An Au slide with a thickness of 50 nm A gold slide coated with PCBM SPR curves of 10% sugar solution annealed at C SPR curve fitting for 10% sugar solution annealed for 120 min at C SPR angle versus annealing time at C SPR curves of 10% sugar solution annealed at C for different times Curve fitting of 10% sugar solution annealed for 120 minutes at C The SPR angle versus the annealing time for samples annealed at C SPR curves of 10% sugar solution annealed at C SPR curve for 10% sugar solution annealed for 120 minutes at C SPR angle versus annealing time at C SPR curves of distilled water annealed at various temperatures for 120 min SPR curves of Au-PCBM for various concentrations SPR curves of Au-PCBM for various concentrations after 24 hours The SPR angles versus concentrations for three samples The minimum reflectance versus concentrations for three samples The SPR curve for PCBM in an air medium...54 xii

14 CHAPTER I INTRODUCTION 1.1 Introduction Surface Plasmon Resonance (SPR) is an optical phenomenon that is exploited to detect the change of the refractive index at the dielectric/metal interface. When a p-polarized light strikes a thin-film metal under the condition of total internal reflection, plasmons propagate through a dielectric/metal interface and it is denoted by a sharp dip in the curve that indicates a minimum reflectance of light. When the wave vector of incident light and the wave vector of surface plasmon are equaled, resonance occurs. Therefore, the intensity of the reflected light reaches its minimum value at a particular angle or wavelength, and it is demonstrated by a sharp dip in the curve that is called the SPR curve. In this phenomenon, approximately a 50 nm gold thin-film is located between a prism and a dielectric medium (could be a gaseous or liquid medium). Several configurations have been demonstrated to create the SPR technique: Kretschmann-Raether configuration, Otto configuration, grating configuration, and waveguides [1, 2]. Moreover, the first two configurations are called prism coupling because a prism is used to excite the plasmons. In this thesis, it is Kretschmann-Raether configuration that is being studied. 1.2 Motivation of the Study Surface Plasmon Resonance has been exploited in many applications such as cancer detection and treatment, measuring bimolecular interactions and interaction between DNAprotein, and chemical and biological sensors [1]. Furthermore, SPR phenomenon is exploited in measuring the optical constants of metals and surface-enhanced Raman spectroscopy. Because of the various applications that the SPR phenomenon has, the interests in SPR and publications 1

15 Number of articles relating to it have been increasing over the last decade. As seen in Figure 1.1, the number of articles on Surface Plasmon Resonance increased rapidly in Thus, Surface Plasmon Resonance is a fertile technique to be applied in many fields Number of Articles Publication year Figure 1.1 Number of articles in which the title contains Surface Plasmon Resonance since 1995 based on data provided on ( 1.3 History of SPR The first observation of Surface Plasmon Resonance was done by R.M. Wood at John Hopkins University in He observed how a pattern of anomalous dark and light bands popped up in the reflected light when a polarized light hit a metal. However, a clarification of those anomalies was not provided until 1907 when Lord Rayleigh interpreted those anomalies. Subsequently, more elaboration to the phenomenon was added by Fano [3, 4, and 5]. In 1968, the breakthrough in the phenomenon was ushered in by Otto and Kretschmann- Raether. They are considered the modern pioneers of SPR, because they succeeded in 2

16 demonstrating a suitable technique for the excitation of surface plasmon, and introduced surface plasmon into modern optics. In 1983, the first application of SPR-based sensors to bimolecular interaction monitoring was demonstrated by Liedberg et al. [3, 4, and 5]. 1.4 Outline of the Thesis This exploration and theory of Surface Plasmon Resonance continues into chapter two. Chapter three will provide a brief explanation of the SPR device, its calibration, and the SPR of sugar solution with various concentrations. Chapter four will discuss the effect of annealing of Au thin film in SPR measurements and the SPR of (Phenyl-C61-butyric acid methyl ester) PCBM. Finally, in chapter five main conclusions of the thesis will be presented. 3

17 CHAPTER II THEORY OF SURFACE PLASMON RESONANCE 2.1 Introduction Surface plasmons are collective oscillations of electron density waves in a metal-dielectric interface with an opposite sign of dielectric constant. Surface Plasmon Resonance can be excited by a p-polarized light using attenuated the total internal reflection method. There are two configurations that excite Surface Plasmons: Otto and Kretschmann-Rarther configurations. A sharp minimum in the intensity of the reflected light appears indicating Surface Plasmon Resonance. Because the interaction between the light incident and the Surface Plasmons is subjected to Maxwell s equations, a brief review of Maxwell s equation is presented in the next section (2.2) of this chapter [1, 4, 6, and 7]. 2.2 Maxwell s Equation and the Wave Equation The interaction between light and matter can be described by Maxwell s equations since the light is an electromagnetic wave that consists of electric and magnetic fields oscillating at right angles to each other and to the direction of wave propagation. Thus, Maxwell s equations in a medium are given by: (2.1) (2.2) (2.3) (2.4) 4

18 These four equations ( ) express Gauss s law for electric field, Gauss s law for magnetic field, Ampere s law, and Faraday s law of induction respectively: where is denotes the dielectric displacement, represents the charge density, denotes the magnetic field, is the electric field, is the magnetic field, and denotes the current density. Figure 2.1 Propagation of light along z direction [8]. In this thesis, the treatment of Maxwell s equations is restricted to linear, nonmagnetic and isotropic media. Therefore, the constitutive relations are expressed as: (2.5) (2.6) (2.7) Where is the relative permittivity, is the relative permeability which is equal to one in the nonmagnetic medium, and is the conductivity. 5

19 However, requiring that the light propagates in homogeneous medium, which means that and do not depend on the direction of the propagation, and and equal to zero, and substituting ( ) into ( ), Maxwell s equations become: (2.8) (2.9) (2.10) (2.11) By taking the curl of eq. (2.10), we will have: ( ) (2.12) Substituting eq. (1.12) into (2.11), gives (2.13) But ( ), (2.14) And since, Then substituting eq. (2.14) into (2.13) gives: (2.15) 6

20 This equation is the wave equation in three dimensions [1, 6, 8, 9, and 10]. 2.3 The Dielectric Constant and the Appearance of Refractive Index The refractive index is a dimensionless number that describes the interaction between material and light. Refractive index describes how light propagates in a material and how light gets reflected by a material on its surface. It is defined as the ratio of the speed of light in vacuum ( ) to the speed of light in a material ( ) as follows: (2.16) However, refractive index can be described in terms of the dielectric constant ( ) and the relative permeability ( ) by finding the solution of the wave equation in the previous section. The wave equation, Eq. (2.15), can be written in one dimension as follows: (2.17) The solution of this equation is a sinusoidal wave: (2.18) where is the wavelength of the wave and is the speed of light. Differentiating equation (2.18) with respect to, results in (2.19) And differentiating equation again (2.18) with respect to, yields 7

21 (2.20) Subtitiuting eq. (2.19) and (2.20) into (2.17) gives (2.21) Simplifying and rearranging eq. (2.21), the speed of light in a medium ( ) can be expressed as:, (2.22) Or (2.23) For non-magnetic materials, the permeability turns out to be, so will be: (2.24) And the refractive index is given by: (2.25) (2.26) The dielectric constant or relative permittivity is given by:, (2.27) where, is the permittivity of a material and is the vacuum permittivity. Therefore: 8

22 (2.28) In the rest of the thesis we will represent the dielectric constant as to make it handy [11]. 2.4 Total Internal Reflection and the Evanescent Wave Total Internal Reflection According to Snell s law when a light beam propagating from a medium with higher refractive index ( ) to a medium with lower refractive index ( ), undergoes refraction. where denotes the incident angle of light, denotes the index of refraction of the first medium, denotes the refraction angle, and denotes the index of refraction of the second medium. Z Z k t X X Figure 2.2 The reflection of a wave incident. The left shows the normal refraction. The right shows the total internal refraction [9]. 9

23 When the angle of the refracted light is equal to 90, the angle of incident light is called critical angle ( ). Total internal reflection occurs when the incident angle ( ) is larger than critical angle ( ). Therefore, all the light will be totally internally reflected and there is no refraction light. This is called Total Internal Reflection (TIR) [8, 12] Evanescent Waves Evanescent means tending to vanish. The continuity condition of the electric field in Maxwell s equations imposes the existence of transmitted waves in the second medium. The intensity of these waves decays exponentially to zero into the dielectric layer -the medium of lower index of refraction- without conveying energy from the interface. They are called Evanescent waves, which derived from the Latin root evanescere, mean to tend to vanish. They form when the sinusoidal waves are under Totally Internal Reflection (TIR) where the angle at the interface is greater than the critical angle. Figure 2.3 An evanescent wave [13]. 10

24 The transmitted wave that occurs due to total internal reflection in the interface between the prism and the dielectric medium (e.g. vacuum) is given by: (2.29) Assuming that the light beam propagates into certain direction such that. And (2.30) Using Snell s laws Eq. (2.30) can be written as:, (2.31) where is the critical angle of total internal reflection. Substituting Eq. (2.31) into Eq. (2.29): (2.32) Defining to make the equation well-ordered, eq. (2.32) becomes: (2.33) (2.34) Eq. (2.34) represents the mathematical expression of evanescent waves. in the equation shows that the evanescent wave is moving alongside direction but its amplitude decreases exponentially into the dielectric medium (the 2 nd medium), according to. 11

25 2.5 Polarization of Light Waves Polarization is a feature of electric vector that oscillates in more than one orientation. Linear polarization is one kind of polarizations where the electric vector oscillates within a plane which is constant over time. Linear polarization can be classified into Transverse Magnetic mode (TM) or p-polarized and Transverse Electric mode (TE) or s- polarized with respect to the normal direction of the surface that is contained by the plane and the ray direction which is the propagation vector. In p-polarized, the incident electromagnetic waves are parallel to the plane of incidence and that is why it s called p-polarized. The magnetic field vector of the electromagnetic radiation has only one component, which is, that is tangential to the interface (Transverse Magnetic or TM mode). However, the electric field vector has two components: which is normal to the interface and which is tangential as illustrated in Figure 2.4. On the other hand, the electric field vector is orthogonal to the plane of incidence in s- polarized which took from German word Senkrecht which means perpendicular or vertical. In this mode, the electric field vector has only one component,, which is tangential to the interface (Transverse Electric or TE mode) and the magnetic field vector has which is normal to the interface and which is tangential. Furthermore, Surface Plasmon Polariton can only be excited by TM mode or p-polarization because of the discontinuity of the normal component of electric field that causes the polarization to change at plane interface [1, 12, and 14]. 12

26 Figure 2.4 Schematic of p-polarized electromagnetic field -or TM mode- on between two media [14]. 2.6 Dielectric Function of Surface Plasmon based on Drude-Sommerfeld Model The Kinetic Theory of gases can be applied to a metal. When atoms gather around to form a metal, the valance electrons become detached and move freely throughout the metal, whereas the metallic ions have an impact on the immobile positive particles. As a result, they experience no restoring force and the resonance frequency is zero. Thus, at the presence of an electromagnetic field, electrons start oscillating and the electric field can be written as: (2.35) where, is the amplitude of the applied electric field, and is the frequency of the applied electric field. The electron s equation of motion is [15]:, (2.36) where is the charge of the free electrons, and is the damping term which is the inverse of the relaxation time of the electrons or the collision time as the following:. 13

27 Figure 2.5 Real and imaginary part of the dielectric function for gold based on Drude-Sommerfeld model [16]. The solution of the equation of motion is:. (2.37) Consequently the susceptibility is given by: (2.38) Note that is the volume plasma frequency of the free electron gas and is defined by. The relative permittivity ) (the dielectric function) can be related to the susceptibility ) as the following: (2.39) Therefore, the relative permittivity of the free electron gas is: (2.40) And then it can be divided into real and imaginary parts as follows: (2.41) 14

28 However, if the damping term ( ) in Eq. (2.41) is neglected, assuming there is no damping in the Surface Plasmon wave, the dielectric function becomes [15, 16, 17 and18]: (2.42) 2.7 The Dispersion Relation of Surface Plasmon Polaritons If the second medium is not magnetic material, then the relative permeability is unity. So there is no discontinuity at the interface. The discontinuity is on dielectric constant. Figure 2.6 The electromagnetic field of SPs propagating on the x direction of the surface [19]. If we take, as shown in Figure 2.7, [x-y] plane to be the interface plane and the positive [z] halfspace as medium (2), then for x direction, the wave propagation is [19]: (2.43) ( ) (2.44) (2.45) 15

29 ( ) (2.46) Figure 2.7 Interface between two dielectric mediums [20]. The previous equations ( ) have to satisfy Maxwell s Equations: (2.47) (2.48) (2.49) (2.50) By applying Eq. (2.49), we find: (2.51) (2.52) By applying Eq. (2.48), we find the relationship between [ ] as follows: (2.53) 16

30 (2.54) We know that tangential and tangential are both continuous by applying the boundary conditions at : (2.55) (2.56) Inserting eq. (2.55) and eq. (2.56) into eq. (2.53) and eq. (2.54): (2.57) In most general case of metal/dielectric interface we have: (2.58) (2.59) Substituting eq. (2.58) and eq. (2.59) into eq. (2.57): (2.60) (2.61) (2.62) (2.63) (2.64) (2.65) 17

31 Eq. (2.65) describes the dispersion relation, which is a relation between the wave vector along the propagation direction and the angular frequency, and is real. Figure 2.8 The dispersion relation for surface plasmons, light in a vacuum, light in a prism [21]. The normal component of the wave vector ( ) is (2.66) where and are both imaginary. There are conditions that have to be satisfied in order for an interface mode to exist hence we have to make some assumptions. First of all, the imaginary parts of the complex dielectric functions can be ignored since they are small with respect to the real parts. Secondly, has to be real in order for the interface waves to propagate along the interface. These two conditions can be fulfilled if the product and the sum of the dielectric functions in eq. (2.65) are either both negative and both positive. Furthermore, the normal components of the wave vector ( ) have to 18

32 be purely imaginary to acquire exponentially decaying solutions; this means that the sum of the denominator of eq. (2.66) has to be negative [14]. By taking all these assumptions into consideration, we can say that the conditions for an interface mode to exist are: (2.67) (2.68) This can occur if is negative and greater than ; and this is the case for noble metals (such as gold and silver) that they have large negative real part and small imaginary part of the dielectric function. In a metal, the incident E field causes an electron scattering (Ohmic losses) which leads to the damping of the oscillations. Thus, the imaginary part of the dielectric function of the metal ( ) has to be introduced. (2.69) where both and are real. In order to acquire a complex parallel wave number ( ), we neglect the loss in the second medium (dielectric medium) so that is real. Therefore, is (2.70) where, (2.71) and ; (2.72) is the real part of the parallel wavenumber ( ) and it is responsible for the Surface Plasmon wavelength while is the imaginary part and it is in charge of the damping of the Surface 19

33 Plasmon as it propagates alongside the interface. Furthermore, the Surface Plasmon Polariton wavelength can be written as:, (2.73) where is the wavelength of the excitation light in vacuum and it is given by. Using the same method, the real and imaginary parts of the normal component ( ), can be obtained respectively as: (2.74) and (2.75) Eq. (2.74) and Eq. (2.75) show that the decay towards the metal is shorter than the decay towards the dielectric as shown in Fig (2.7) [16, 21]. Figure 2.9 The distribution of electric field intensity to metal and dielectric mediums [21]. 20

34 2.8 The Excitation of Surface Plasmon Polariton Prism Coupling As we have seen in section (2.7), the wave vector of light in free space is smaller than the wave vector ( ) of Surface Plasmon Polariton. As a result, Light in free space cannot excite Surface Plasmon Polariton directly unless special methods are applied. Scientists have developed several methods to excite the Surface Plasmon Polariton. However, prism coupling is the most widespread method that has been used in that last decade. There are two different geometries of prism coupling: Otto and Kretschmann configurations. In these two geometries, the Plasmon can be excited by p-polarized light undergoing total internal reflection (TIR) on the prism surface, which causes the evanescent wave to be released and decayed exponentially. A brief explanation of these two configurations is shown below: a) Otto configuration: In Otto configuration, the metal surface ( ) is separated from the medium ( ) by an additional dielectric layer such as air, water, or polymer and its dielectric constant ( ) must be less than the dielectric medium ( ). The surface plasmon resonance occurs at metaldielectric interface. The width of the (air, water) slit should be about m in order for the excitation to occur. For this reason, the Otto method has not been used widely in surface plasmon resonance sensing [16, 22]. 21

35 b) Kretschmann configuration: Figure 2.10 Prism coupling using Otto configuration [6]. The experiments that are done in this thesis are based on this configuration. In this configuration, the thin metal film is evaporated on top of a prism which has a high refractive index ( ).The dielectric layer ( ),which has a refractive index smaller than the refractive index of the prism, is deposited on the top of both. In this geometry, the metal thin film acts as the spacer. At an angle that is greater than the critical angle of total internal reflection, photons strike the prism and hence the evanescent waves couple the Surface Plasmon on the opposite metal/dielectric medium interface. This configuration is widely explored [16, 22]. Figure 2.11 Prism coupling using Kretschmann configuration [6]. 22

36 2.8.2 Surface Plasmon Resonance As we have seen in the previous sections light that used to excite Surface Plasmon polariton is p-polarized or TM mode. The component of the incident electromagnetic radiation has wave vector ( ) given by the following expression:, (2.76) where is refractive index of prism, ( ) is the Attenuated total reflection angle (SPR angle) and ( ) is the wavelength of the polarized light. On other hand, the wave vector of plasmon mode ( ) is given by the following expression:, (2.77) where ( ) is the dielectric constant for metal thin film and ( ) is the dielectric constant of dielectric medium. A decrease in the intensity of the reflected light occurs demonstrating the Surface Plasmon Resonance when the incident wave vector ( ) is equal to the wave vector of plasmon ( ) mode. The intensity of reflectance minimum that results from plasmon resonance is caused by the phase difference of the Surface Plasmon mode relative to the incident electromagnetic field, is. As a result, a destructive interference between the reflected photon and the photon that emitted by the excited plasmon occurs. Furthermore, the minimum intensity of the reflected light is shown as a curve which is called the SPR curve (figure2.10) [22, 23]. 23

37 Figure 2.12 The Surface Plasmon Resonance curve [21] The Reflection Intensity The reflectance of the p-polarized light for a three layers (prism, metal, and dielectric medium) can be calculated by Fresnel s equation. The Fresnel reflection and transmission coefficients of the p-polarized light are given respectively by:, (2.78), (2.79) where, is the amplitude of the electric vectors of the wave hitting the surface, is the reflected wave, and is the transmitted wave, are the refractive index of the medium 1 and 2 respectively. Also, and are the reflective angle and refractive angle respectively. 24

38 Figure 2.13 The three layers reflections between two parallel surfaces [24]. From Eq. (2.78) and Eq. (2.79), it can be concluded that: (2.80) (2.81) Thus, according to Fresnel equation, the reflectance of light in the prism (p), metal (m), and dielectric medium (d) system is given as the following: ; (2.82) (2.83) D is the thickness of thin film metal and and are the amplitude reflectance given by Fresnel equations, respectively. (2.84) 25

39 ; (2.85) is the perpendicular component of the wave vector to the interface medium where :, (2.86) where is the angle of incidence. According to Eq. (2.82), we can determine the minimum value of the reflectivity of the light which can lead to the resonance in Surface Plasmon Resonance. Furthermore, the minimum value of the reflectivity depends on the dielectric function of the metal and the thickness of the thin film metal, and other properties as we will see in the next section (2.9) [23-26]. 2.9 The Feature of Surface Plasmon Resonance Curve As it s shown in the section (2.2.8), the intensity of reflected light is shown as a curve that is known as a Surface Plasmon Resonance curve. There are three different areas in the Surface Plasmon Resonance curve that can determine the shape of the curve. The first area is the edge of the total internal reflection. The position of this area depends only on the difference in the dielectric constant of prism and the cover medium which is either water or air. The value is known as critical angle and is fixed for a given system. A second factor that may have great impact on determining the curve is the resonance angle. This is the quantity that is needed to be determined. The angle shift depends on the real part of dielectric constant of the metal and the dielectric constant of the cover medium (air, water, or polymer). However, the Surface Plasmon Resonance dip angle is determined by the thickness of metal layer. The dip must not reach the zero line. The closer this thickness is to 50 nm the deeper the dip angle will be. The last factor 26

40 that shapes the Surface Plasmon Resonance curve is the full width at half maximum. This depends solely on the imaginary part of the dielectric constant of the metal. The smaller the value, the sharper the curve is. This is why silver plasmons are much narrower, sharper than gold plasmons as illustrated schematically in Figure (2.14) [27]. Figure 2.14 The Surface Plasmon Resonance curve of gold and silver films [14]. 27

41 CHAPTER III SURFACE PLASMON RESONANCE AND MEASUREMENT OF SUGAR SOLUTIONS 3.1 Introduction In this chapter, the SPR plots of sugar solution with various concentrations are investigated experimentally through the NanoSPR6 device and theoretically through the Win Spall 3.2 software. Finally, the NanoSPR6 device, Win Spall 3.2 software, and other equipment are introduced. 3.2 Equipment and Devices NanoSPR 6 Device Nano SPR 6 is a device that is made by the NanoSPR Company to observe the SPR phenomenon (see figure 3.1). Its design is based on the Kertshcmann configuration (see chapter two, section b). Laser source. Sensor chip. Prism. Figure 3.1 NanoSPR6 device. Syringe. 28

42 The main function of the device is the retroreflecting measurement prism, which is located in the rotating table. The intensity of the incident polarized light reflected by the Au layer is studied as a function of angle of light incident on the Au layer. As shown in Figure 3.2, the face of the prism is made to reflect the incident polarized light; the top right angle of the prism is a 90 degree angle. Immersion oil is applied and distributed on the top of the prism and then the sensor chip (a glass slide with deposited thin Au layer) is delicately balanced on the top of the prism. Furthermore, the rubber gasket and the chamber are located on the top of the sensor chip and we can fill the chamber with the solution by using a syringe, Figure 3.1. The refractive index measurement range, the refractive index sensitivity, and other features of the NanoSPR6 device are summarized in the table 3.1 [NanoSPR User-Manuel]. Table 3.1. Some Features of NanoSPR6 Device Refractive index measurement range. 1.0 to 1.5. Refractive index sensitivity Maximum angular scan. 17 degree. Single resonant curve measurement time. 3sec. Light source. GaAs semiconductor laser (. Weight. 4.8 lb (2.2 kg). Computer connection. USB Connection. 29

43 Figure 3.2 A close view of NanoSPR Calibration After we set up the NanoSPR 6 device, we need to perform calibration. Basically, calibration allows us to determine the precise value of angles of incidence. Calibration is made before starting significant measurements [NanoSPR User-Manuel]. It helps us determine the SPR curve in accurate values of light incident angles. After making the calibration, the device is ready for taking measurements Process of Taking Measurements The steps of setup and measurement can be summarized as follows: 1) Install the software that came with the device in the computer. 2) Connect the device with a power supply and the computer. 3) Start the software program in the computer. 30

44 4) Make sure that the device and the computer are connected by choosing the correct COM-port. 5) Choose the type of prism based on the liquid solution, either F1-65 (water) or K8-50 (gas). 6) Start the device calibration by pressing the calibration button. 7) Press the single measurement button. 8) The SPR curve should appear in the center of the window with a minimum position that is denoted by a vertical line as seen in Figure (3.3). Figure 3.3 The single measurement of Surface Plasmon Resonance Win Spall Software Introduction WinSpall is software that is created by the Wolfgang Knoll Group in Max Planck Institute for Polymer Research. The software creates a simulation of surface plasmon resonance curves of a multilayer system based on the Fresnel equations. It is easy and effective to use. 31

45 Simulation Curves The theory of surface plasmon resonance was used to build the SPR curve using the Fresnel equation and it is discussed in chapter two see The reflectance of the light in a three-layer system is given as the following:, (2.82) and (2.83) After that we enter the values of parameters in the table as given from the company NanoSPR, which is listed in the Table 3.2: Table 3.2. The Values of the Thickness and Refractive Index of Different Layers Provided by NanoSPR6 Layers Thickness (nm) Real part of refractive index Imaginary part of refractive index Prism Chromium Gold Water

46 A typical SPR curve is shown Figure 3.4, below. Figure 3.4 Simulation of SPR curve using the WinSpall software. Moreover, the curve of the total internal reflection at a glass/ air interface can be acquired by entering the value of the real part of the dielectric constant of the prism ( ) in the first layer. In the second layer, the value of the real part of the dielectric constant of the air ( ) is entered. After entering the values and clicking okay, a typical curve describing a total internal reflection will appear as shown in figure 3.5. As seen in figure 3.5, light is reflected from the interface separating high index (prism) and low index material (air). All the light passes through the glass/ air interface without any reflection at low angles. On the other hand, at angle the light is reflected completely and the onset of the total internal reflection is reached. 33

47 Figure 3.5 The total internal reflection curve at the glass/ air interface. 3.3 Surface Plasmon Resonance for Sugar Solutions In this section, the surface plasmon resonance curves of sugar solution with various concentrations (0.00%, 5.00%, 10.00%, 15.00%, and 20.00%) are prepared and investigated experimentally. Furthermore, the experimental data are fitted and compared to the simulation data to test the sensitivity and accuracy of the NanoSPR6. The same values that are given in table 3.2 are used and the sugar solutions are prepared with various concentrations and made as a fourth layer in our multilayer system. The weight percent (wt. %) method is used to dissolve the sugar into water to make the sugar solutions with various concentrations. The formula 3.1 is used to prepare the sugar solutions with different concentrations (5.00%, 10.00%, 15.00%, and 20.00%): Grams of Sugar = (3.1) 34

48 Where is denotes the concentration of sugar solution and indicate the amount of water in milliliters. For example, in 5.00% concentration, the amount of sugar that should be dissolved in of water is. Table 3.3 summarizes the different amounts of sugars and the concentrations: Table 3.3. Amounts of Sugars and Concentrations Concentrations (wt. %). Amount of sugar (gram) in 50 ml of water. 0.00% % % % % 12.5 After the sugar solutions with different concentrations were prepared, they are injected separately into the chamber through the syringe and the Surface Plasmon Resonance curves for each one are studied individually. Furthermore, the simulation of the SPR curves for each one was done and theoretical fitting curves were obtained. First of all, the Surface Plasmon Resonance of distilled water was measured and the SPR dip angle was observed at. Moreover, the measured SPR curve of distilled water was theoretically fitted as shown in Figure 3.6 which shows a good agreement between the 35

49 experimental and simulation curves where the parameters in the table 3.2 were used in the simulation. Figure 3.6 Experimental and theoretical SPR curves of the gold/ distilled water interface. The determined value of refractive index of distilled water was as reported by Optical Society of America [28]. In the same manner, the SPR curves of various sugar solutions- from 5.00% to 20.00%- were obtained and fitted theatrically throughout the WinSpall software as shown in Figure 3.7. (a) 36

50 (b) (c) 37

51 SPR dip angle (degree) (d) Figure 3.7 Experimental and theoretical SPR curves for different concentrations: (a) 5% sugar solution, (b) 10% sugar solution, (c) 15% sugar solution, and (d) 20% sugar solution. The refractive indices of different sugar solutions were estimated by fitting the measured SPR dip angle. The estimated refractive indices are plotted as a function of measured SPR dip angle in Figure Refractive index Figure 3.8 SPR dip angle versus refractive index of various sugar solutions. 38

52 Refractive index As shown in Figure 3.8, the values of the refractive indices increase linearly with SPR dip angle. Moreover, the values of refractive indices also increase with an increase in the concentrations Figure % 5.00% 10.00% 15.00% 20.00% 25.00% Concentration of sugar solution (wt.%) Figure 3.9 Refractive index versus concentration of sugar solution. To sum up, according to the fitting data that was provided previously, it can be said that the simulations fit well to the experimental data and the NanoSPR6 device works properly. 39

53 CHAPTER IV THE EFFECT OF THE ANNEALING OF AU THIN FILM ON SPR CURVES AND THE SPR CURVE OF PCBM-AU 4.1 Introduction The resonant frequency in SPR phenomenon depends on the morphology characteristics of Au NPs such as particle size, shape, and environment [29]. It has been shown that the annealing of the Au thin film can alter the surface topography of it [30]. Furthermore, the change in the morphology of Au thin film throughout annealing is one of the most effective methods for tuning the optical properties of Au thin film [29]. As a result, Surface Plasmon Resonance depends highly on the nanoparticles layer structure and morphology. The effects of annealing time in various temperatures on the resonance shift and the reflectance minimum in SPR, and the polymer effect on the shape of SPR curves before and after 24 hours are going to be explored in this chapter. 4.2 Experimental Procedure The Annealing Process of Au Thin Film The gold slides were purchased from NanoSPR Company. Each gold slide consists of one Au layer with a thickness of 45 nm and an intermediate Cr layer with a thickness of 5 nm to increase the adhesion of the thin film, (see Figure 4.1). 40

54 Figure 4.1 An Au slide with a thickness of 50 nm. The samples were heated using a tube furnace that is manufactured by Lindberg Company. The samples were heated at various temperatures for different amounts of time. The annealing was conducted in an Argon atmosphere to avoid the oxidation of the thin film and the flow rate of the argon gas was consistent. The first sample was annealed at Celsius for 30, 60, 90,120, and 150 minutes respectively and then the sample was left in the furnace until it reached room temperature. The second sample was annealed at Celsius for the same duration of time as the Celsius sample. Finally, the third sample was annealed at Celsius under the same atmosphere. After each time in every temperature, the Surface Plasmon Resonance was measured and the result was saved. In addition, the SPR of the untreated sample was measured as well to compare its result with the annealing samples. At Celsius, the gold film evaporated from the glass slide. The time duration of the annealing process of every sample (, and Celsius) took about six and a half hours including SPR measurements. 41

55 4.2.2 Polymer Measurements In this work, the 100 nm of Polymer was coated as a substrate above the gold layer and the SPR phenomenon was studied. The polymer was prepared by Dr. Li s group at the University of Pittsburg. The polymer that is coated on the top of the gold layer is PCBM, which is the abbreviation for Phenyl-C61-butyric acid methyl ester, see figure 4.2. Because of the quick oxidation of the polymer layer, the Au- Polymer sample was sealed properly to prevent them from exposing to the air. Furthermore, the SPR measurement was done swiftly to avoid any interaction with air. Figure 4.2 A gold slide coated with PCBM. 4.3 Results and Discussion The Annealing of Au Thin Film 42

56 As it has been mentioned in the introduction, the annealing of Au thin film causes a change in the structure, and therefore, the real part and the imaginary part of the dielectric function of Au changes. The SPR curves of 10% sugar solution with the Au thin film annealed for various times at C is shown in figure As deposited. At 30 min. At 60 min. At 90 min. At 120 min Figure 4.3 SPR curves of 10% sugar solution annealed at C. As noticed in Figure 4.3, the annealing changed the characteristic of SPR curves and the SPR angle. To estimate the new values of the real and the imaginary part of the dielectric function of Au thin film, a curve fitting for SPR of 10% sugar solution annealed for 120 minutes is done in Figure

57 Figure 4.4 SPR curve fitting for 10% sugar solution annealed for 120 min at C. The real part of the dielectric function of Au thin film is estimated to be and the imaginary part is. All the values of the real and the imaginary part of the dielectric function of Au are summarized in Table 4.1. Table 4.1. The Values of the Real and the Imaginary Part of the Dielectric Function of Au Thin Film for Various Annealing Time Annealing time. 30 min min min min

58 SPR angle (degree) It is observed that the SPR angle increases linearly with the annealing time, Figure 4.5. The SPR angle is shifted to the right when the time annealing is increased Annealing time (minute) Figure 4.5 SPR angle versus annealing time at C. Furthermore, The SPR curves of 10% sugar solution that is annealed at C for various times are shown in Figure

59 As deposited At 30 min-10% At 60 min-10% At 90 min-10% At 120 min-10% At 150 min-10% Figure 4.6 SPR curves of 10% sugar solution annealed at different times. C for The curve fitting is done for all of the curves to determine the values of the real and the imaginary part of the dielectric function. As an example, Figure 4.7 shows the curve fitting of the sample that is annealed for 120 minutes. Figure 4.7 Curve fitting of 10% sugar solution annealed for 120 minutes at C. 46

60 After the curve fitting was done for all the SPR curves of different times, the values of dielectric function are listed in Table 4.2. Table 4.2. The Estimated Values of the Real and the Imaginary Part of the Dielectric Function of Au Annealed at C for Various Times Annealing time. 30 min min min min min In the same manner, the SPR angle versus the annealing time is plotted for C in Figure 4.8. At C, the SPR angle reaches the maximum value when annealed for 90 minutes and then started to decrease. 47

61 SPR angle(degree) Annealing time( minute) Figure 4.8 The SPR angle versus the annealing time for samples annealed at C. The last sample was annealed at C at the same various times as the two previous samples were annealed. The SPR curves of 10% sugar solution are illustrated in Figure As deposited At 30 min. At 60 min. At 90 min. At 120 min. At 150 min Figure 4.9 SPR curves of 10% sugar solution annealed at C. 48

62 In the same manner, the values of the real and the imaginary part of the dielectric function were estimated by fitting curves. The curve fitting of 10% annealed for 120 minutes is shown as an example in Figure Figure 4.10 SPR curve for 10% sugar solution annealed for 120 minutes at C. The real and the imaginary part of the dielectric function for the sample annealed for 30, 60, 90, 120, and 150 minutes are shown below in Table 4.3. Table 4.3. The Estimated Values of the Real and the Imaginary Part of the Dielectric Function of Au Annealed at C Annealing time. 30 min min min min min

63 SPR angle ( degree) Moreover, the SPR angles change with the changing of the annealing time. Figure 4.11 shows the relation between the angle shifting and the annealing time Annealing time ( minute) Figure 4.11 SPR angle versus annealing time at C. It is observed that when the sample annealed for 120 minutes at C, C, and C, the depth of the SPR curve reaches its maximum. The sample that was annealed at C has more depth than any other temperature, (see Figure 4.12) AS deposited. At 200 C. At 300 C. At 400C Figure 4.12 SPR curves of distilled water annealed at various temperatures for 120 min. 50

64 As shown in Figure 4.12, the SPR angle shifted to the left when the temperature is increased. It can be concluded that the annealing increases the depth of the SPR curve; however, it decreases the value of the SPR angle SPR Curves for Au-PCBM Sample The SPR curves were measured in aqueous and gaseous medium. For aqueous medium, distilled water and sugar solution with various concentrations were used, and an air was used for the gaseous medium. In Figure 4.13, The SPR curves for different concentrations of sugar solutions, 0%, 5%, 10%, 15%, and 20% respectively Distilled water With 5%. With 10%. With 15%. With 20% Figure 4.13 SPR curves of Au-PCBM for various concentrations. 51

65 The thickness of PCBM is WinSpall software and is and the refractive index is estimated by curve fitting using. As seen in figure 4.13, the SPR angle for distilled water is and the resonance angle increases with the increasing of the sugar solution. As observed, the adding of the PCBM layer changes the SPR angle by approximately Furthermore, The sample was left for 24 hours and the SPR measurement was taken to see the change in the resonance angle. The SPR curves of Au- PCBM after 24 hours are shown in Figure Distilled water With 5%. With 10%. With 15%. With 20% Figure 4.14 SPR curves of Au-PCBM for various concentrations after 24 hours. The resonance angle of Au-PCBM seems to change slightly. The SPR angle for distilled water after 24 hours is after 24 hours is.the change difference of the SPR curve of distilled water before and. The SPR angle as a function of concentrations for PCBM samples before the 24 hours, PCBM samples after the 24 hours, and without PCBM samples are plotted in Figure

66 The minimum reflectance SPR angle (degree) SPR angle with PCBM SPR angle with PCBM After 24 hour SPR angle with no polymer) Concentration (wt.%) Figure 4.15 The SPR angles versus concentrations for three samples. As expected, there is a linear relationship between the resonance angle and the sugar concentrations. Moreover, the reflectance minimum of the three samples was plotted as a function of concentrations in Figure Concentration% The minimum reflectance with PCBM The minimum reflectance with PCBM after 24 hours The minimum reflectance with no polymer. Figure 4.16 The minimum reflectance versus concentrations for three samples. 53

67 As seen in Figure 4.16, the minimum reflectance is decreased in the PCBM sample and the sample with no PCBM coating has more depth than the others. Finally, the SPR curve for the PCBM in an air medium was measured and Figure 4.17 shows the SPR curve. Figure 4.17 The SPR curve for PCBM in an air medium. 54