Internal Reflection Spectroscopy at an Optically Transparent Electrode

The basic principle of IRS is that, at an angle ^ greater than or equal to the critical angle, a light beam will be reflected firom an interface between two media of refiractive indexes n, and nj, where n, > n2. This is diagramed schematically in Figure 1.1. The critical angle 0^ can be determined by application of Snell's law;

sinft = ^ (11)

"i

At the reflection, a phenomenon known as the evanescent field is observed, an extension

Evanescent Field Solution

Electrode Layer

IRE Substrate

Figure 1.1. An illustration of internal total reflection in an internal reflection element (IRE). An incident beam of intensity I^Q is reflected at angles greater than a critical angle determined with Snell's law. At the reflection, an evanescent field is created, which can be used to optically probe the region above the IRE.

A

C

B vVVvV

Figure 1.2. As the thickness of an ERE is decreased (A to B), the number of reflections, and hence the absorbance sensitivity, also increases. Below ca. 10 jxm,, discrete reflections disappear, and the IRE becomes known as an integrated optic waveguide (C).

of the electromagnetic field into the rarer medium. Because the evanescent field decays exponentially with distance, IRS selectively probes only the region adjacent to the

internal reflection element (IRE). Typical penetration depths are from 100-1000 nm, and are dependent on parameters such as wavelength, the IRE material, and the angle ((>. The absorbance sensitivity for IRS can be described through the use of the Beer's law

analogue;

A = N^{s6C) (1-2)

where e and C have their usual meanings, 6 is the penetration depth of the evanescent field, and a sensitivity factor dependent on the optical constants of the IRE and the IRE superstrate, the incident reflection angle, and the wavelength and polarization of the light. The product 6 N^\s often given as the equivalent transmission pathlength.

Theoretically, for a single-reflection IRS experiment 6^ can vary from 0.01 A. (where X is the experimental wavelength) at high (j> to infinity near the critical angle'".

The first examples of the combination of IRS and spectroelectochemistry were in the mid-1960's, when transparent metal oxide electrodes were first utilized as IRE's for spectroelectrochemistry in the visible portion of the electromagnetic spectrum by the group of Ted Kuwana" '^. In this initial work, IRS was used to monitor absorbance changes at the electrode-solution interface, with and without Faradaic reactions (such as the reduction of methyl viologen). A value of ca. 10 was obtained for a single reflection; the sensitivity was increased fivefold, however, through the use of a multiple

internal reflection element with five reflections, yielding a net SO"'.

Further increases in sensitivity for a multiple-reflection IRE could be realized by increasing the number of reflections present. Increasing the length past ca. 10 reflections on a 2 mm thick IRE (such as a microscope slide), however, is not experimentally

practical. An altemate approach is to decrease the thickness of the IRE, illustrated in Figure 1.2, as the number of reflections for a given IRE length is inversely proportional to the IRE thickness. At an IRE thickness of ca. 10 txm or less, the nxmiber of equivalent reflections/cm can be upwards of 1000. In this thickness regime, the IRE is known as an integrated optical waveguide (10

1.1^ Electroactive Integrated Optic Waveguides

A fundamental feature of an lOW is that only specific reflection angles <j> are allowed, as compared to thicker IRE's where the only restriction for (j) is that it must be greater than the critical angle. Each allowed reflection angle of an lOW corresponds to a different mode of the waveguide. It should be stated, however, that the concept of a reflection angle in an lOW is a construct of the ray-optics model often used to describe the optical properties of these ultrathin structures. In the lOW regime, discrete reflections disappear, and the bound light is present as a continuous streak down the length of the waveguide. A more fimdamental and general method for determining the optical

properties of an lOW is to use a full-wave description, where numerical methods are used to solve Maxwell's equations imder a given set of boundary conditions (determined by layer thickness, reflective index etc.)^°~'. Each eigenvalue solution that is obtained

corresponds to an allowed mode of the waveguide structure. For most lOW structures (those fabricated of transparent or weakly-absorbing materials), however, the ray-optics approach is simple to implement and quite adequate in its accuracy. Nonetheless, a full-wave model will be used exclusively to describe the properties of the multilayer

electroactive lOW structures that are the topic of this dissertation due to its greater flexibility (the full-wave model will be discussed further in Chapter 2).

The use of non-electroactive lOW's for chemical measurements has been

discussed in several reviews. Chemical sensors have been fabricated by overcoating an lOW with a layer that contains an indicator for the analyte of interest;"-^ these lOW-ATR (attenuated total reflectance) sensors have been demonstrated to be highly sensitive and to possess fast response times relative to other optical geometries such as single-pass transmission absorbance measurements. Other apphcations of lOW spectroscopy include the measurement of protein adsorption,*"*' the excitation of Raman scattering in thin films adsorbed to the lOW surface,^ and the measurement of adsorbance for highly turbid or absorbing solutions*"^. A proven lOW application that promises to become very important for any electroactive lOW technology is the measurement of molecular

orientation distributions of monolayer and submonolayer adsorbed films""'. This will be discussed in more detail in Chapters 4 and 6.

The combination of lOW's and spectroelectrochemistry was first reported by Itoh and Fujishima in the late 1980's^"^. In their experiment, they used a gradient-index lOW constructed through an ion-diffusion process in which a glass slide is immersed into a KNOj melt; a thin (ca. 100 nm) antimony-doped tin oxide layer was then applied over

this waveguide to serve as the electrode. The sensitivity factor as measured through the reduction of methylene blue at the tin oxide electrode, was reported to be 30-40 for a multimode structure (ca. 8 ^m thick), and ca. 150 for a 2 fjxa single mode waveguide structure with a distance of 3.3 cm between the input and output grating couplers.

Another gradient-index lOW design that has appeared in the literature is that of Schiffrin et al., who constructed a chlorine sensor based on a thin film of lutetiimi phthalocyanine coated onto an electroactive lOW platform^*^®. Absorbance changes in the phthalocyanine film occurring upon its oxidation by chlorine were monitored using the lOW; the ITO electrode was used to electrochemically reset the sensing film. As the sensing film was extremely thick (10 nm) and highly absorbing, it was not clear that there was any significant sensitivity advantage of using an lOW over another optical geometry in this application. To control the sensitivity and optical loss of the electroactive lOW, a 200 nm silica buffer layer was placed between the ITO electrode and the gradient-index lOW; this optical buffer layer was incorporated into the design of the step-index

electoactive lOW design discussed later in this Chapter.

A third group, that of Naoki Matsuda, has also reported the use of a gradient-index row overcoated with indium tin oxide (ITO), using it to measure the spectrum of an adsorbed heptylviologen film under cathodic bias^®. The waveguide structure utilized for this study was not described in detail; by inference to previous work referenced in this paper, it was most likely ca. 3 ^m thick^^. A value for was not reported.

Gradient-index waveguides are simple to fabricate; the sensitivity of these

structures can be limited, however, by the small refiractive index difference Aw ( <0.01 for

K*-exchanged waveguides) between the index gradient and the substrate^^. An alternate design is a step-index lOW where the waveguide is deposited as a thin film of constant refiactive index by a technique such as RF sputtering or sol-gel dip-coating; in general, the absorbance sensivity for step-index lOW's is greater than that for gradient-index lOW's {vide infrdf*. The structural difference between these two lOW designs is illustrated in Figure 1.3, the refiractive index profiles of an electroactive gradient-index lOW (Figure 1.3A) and an electroactive step-index lOW (Figure 1.3B). Figure 1.4 plots the relative light intensity of the two structures of Figure 1.3 as a function of position across the waveguide (calculated using the full-wave model). It can be clearly seen that in the gradient-index lOW, the mode profile is shifted away from the lOW/solution interface relative to the step-index design, resulting in a lower sensitivity to absorbance by molecules at or near the ITO surface. The absorbance sensitivity of a gradient-index structure could be increased by increasing An through ion-exchange with cations that are more polarizable than K* (such as Xg or Tr)^'but a step-index lOW where the waveguide layer has a refractive index greater than the substrate by A/i will always possess more sensitivity than the corresponding gradient-index lOW with the same A/i, a consequence of the gradient mdex profile.

The research described in this dissertation is concemed with the development and application of a single-mode, step-index electroactive integrated optic waveguide, the EA-IOW*' "*". The structure of the EA-IOW is given in Figure 1.5; it is comprised of three layers that have been sputtered onto a soda lime glass substrate into which integral grating input and output gratings have been etched. The Coming 7059 glass film serves

SubstrateljSuperstrate

X 1.55

® 1.50

- Refractive index gradient

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

Distance from lOW/Solution interface,

Superstrate

1

5 1-55

Q> Coming 7059

® 1.50

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0

Distance from lOW/Solution Interface,

Figure 1.3. A comparison of the refractive index profiles, as a function of position across the waveguide, for (A) a gradient-index electroactive lOW based on the design of Itoh and Fujishima, and (B) a step-index electroactive lOW design, the EA-IOW.

lOW/Superstrate Interface

Distance from the lOW/Soiution Interface, ^m Substrate

Distance from the iOW/Solution Interface, (tm

Figure 1.4. (A) A comparison of the light intensity between a step-index electroactive lOW and a gradient-step-index electroactive lOW, as a function of position across the waveguide structure. (B) A blow-up of the interfacial region in (A). Wavelength; 633 nm. Polarization;

transverse electric (TE).

n=1.46 ^00 nm

Soda lime glass n=1.51

Light Out

Figure 1.5. The structure of the EA-IOW, along with layer thicknesses and approximate refractive indexes.

as the primary waveguiding layer. An indium tin oxide (TTO) film serves as the transparent electrode; in between these two layers is a pure silica optical buffer layer, used to limit optical losses due to light absorption by the ITO. The EA-IOW is much more sensitive than earlier electroactive waveguide designs, exhibiting an up to 4000 at 633 nm as demonstrated by measuring the absorbance of methylene blue

adsorbed at a surface coverage of ca. 4% of a full-packed monolayer''^. A summary of the development and application of the EA-IOW, in order of coverage, follows. First,

however, a brief mention of other techniques that have been utilized to obtain optical data for monolayer films at an electrode surface, and how they compare to the EA-IOW, is necessary for completeness.

1.1.4 Other Optical Techniques for the Measurement of Optical Properties of Thin