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1
Optical properties of nanostructured metal films

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj1

Nanostructured metal films of platinum, gold and silver up to 675 nm thick we prepared by electrochemical deposition through templates of 700 nm diameter polystyrene spheres assembled as hexagonal close packed monolayer on an evaporated gold surface followed by removal of the template by dissolution in tetrahydrofuran.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp1

The reflection spectra of the films at normal incidence were recorded as a function of film thickness and the spectra correlated with the local visual appearance of the film and the surface structure from SEM.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp2

For thin films, below one quarter sphere height, the spectra show a single reflectivity dip at a wavelength just below the sphere diameter consistent with surface-plasmon grating-like behaviour.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res1

For the thicker films several reflectivity dips are observed which move towards longer wavelength with increasing film thickness.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res2

This behaviour is shown to be consistent with a model in which light reflected from the top of the structure interferes with light reflected from within the spherical segment cavities in the film.

Type: Conclusion | Advantage: None | Novelty: None | ConceptID: Con1

Introduction

Spherical colloidal particles of polymers (typically polystyrene) or silica with diameters from 20 nm up to 1 µm and larger with low coefficients of variation in their diameter (often ∼1.5% for polystyrene particles) are readily available either from commercial suppliers or by published syntheses.¹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac1

As the volume fraction of particles in solution increases the particles crystallise into close packed arrays driven by maximisation of the vibrational entropy of the system.²

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac2

These close packed arrays of uniform sub-micron particles offer an attractive and, in principle, simple means to template the three dimensional structure of a variety of materials.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac3

Arrays of particles of this type one or two layers thick have been used as masks in lithographic processes where material was either deposited through the layer or the layer was used as an etch resist in a process sometimes referred to as “natural lithography”,³ for examples see the recent review by Burmeister et al.⁴

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

The attraction of this approach is that it allows the use of sub-micron lithographic processes without the associated high cost of using X-ray or electron beam lithographic techniques.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac4

Three dimensional close packed arrays of uniform colloidal spherical particles (artificial opals) have also attracted attention because of their potential as templates for the fabrication of photonic crystals.⁵

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac5

These three dimensional colloidal particle arrays show diffractive properties (they are opalescent) and in transmission possess a stop band due to Bragg diffraction between the layers of particles in the crystal.^6–8

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac6

However for photonic crystal applications the inverse opal structure is more attractive because it has a higher proportion of void space.^5,9,10

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac7

In a three dimensional dielectric photonic crystal it is possible for a photonic band gap to arise corresponding to a frequency range in which light will not propagate within the crystal because of multiple Bragg reflections.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac8

Interest in two and three dimensional photonic crystals is driven by the potential applications of these structures in optoelectronic devices such as integrated waveguide structures,¹¹ optical limiters and switches.¹²

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

These applications require a high degree of regular packing, uniform void feature size and, in many cases, the systematic introduction of defects at particular places within the structure.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac9

There has also been interest in these inverse opal structures as materials for chemical sensors¹³ or filtration,^14,15 although data on these applications is rather limited.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac10

Inverse opals have been prepared by several groups by infiltration of the spaces between the colloidal particles in the template by precursor materials such as Si(OH)₄ solution,^16,17 metal alkoxides,^18–21 phenolic resin,²² polymers^14,15,23 and semiconductor quantum dots²⁴ with varying degrees of success in terms of the quality and regularity of the final nanostructured material.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

In all these approaches the template is removed after infiltration, in the case of the silica template by dissolving in HF, in the case of the polymer template by either dissolving in a suitable organic solvent or by calcination.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac11

Complete removal of the template spheres is possible because where the spheres touch in the template structure circular “windows” between the spherical voids are left in the final inverse structure so that all the voids are interconnected.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac12

The diameter of these interconnecting windows depends on the method used to infiltrate the template and form the final material.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac13

After removal of the template the material is usually consolidated or converted into the final chemical form by calcination.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

This process often leads to significant shrinkage^16,18,19 of the structures (typically by 20 to 35%) and cracking of the sample.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac14

Metallic inverse opal structures show different photonic properties to those fabricated from dielectric materials and are, therefore, of interest in their own right.^25–28

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac15

Structures made with features in the hundreds of nm size range should show photonic properties in the visible region of the spectrum.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac16

The first examples of metallic inverse opal structures where reported by Jiang et al²⁹. and by Velev et al.³⁰

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac17

Jiang et al. used silica spheres as the template and used nanocrystal catalysed electroless deposition to infiltrate the structures with platinum, copper, nickel, gold and silver.^29,31

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac17

Velev et al^30,32. used polystyrene latex templates and infiltrated the structure with colloidal gold particles.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac17

Two other approaches which have been used to prepare metallic nanostructures are the infiltration of the colloidal crystal template by an appropriate metal salt, conversion to the oxalate and then thermal decomposition to give the metal^33–35 and the direct infiltration of the template by the molten metal.³⁶

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac18

The first example of electrochemical deposition through a colloidal template was reported by Braun and Wiltzius who described the electrochemical deposition of CdSe and CdS through templates assembled from 466 nm diameter polystyrene spheres assembled on an indium tin oxide electrode.³⁷

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac19

Subsequent work has extended this approach to the deposition of other semiconductors,^38,39 metals (notably platinum, gold, silver, cobalt, nickel tin/cobalt alloy),^40–45 metal oxides (ZnO and PbO₂),^46,47 and conducting polymers.^48–50

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac20

A recent review by Braun and Wiltzius concentrates on the electrochemical growth of photonic crystals.⁵¹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac21

Electrochemical deposition has a number of significant advantages, particularly for the deposition of thin, supported films of macroporous materials.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac22

First, electrochemical deposition produces a high density of the deposited material within the voids of the template and leads to volume templating of the structure as opposed to templating of material around the surface of the template spheres.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac23

As a result there is no shrinkage of the material when the template is removed and no need for further processing steps or the use of elevated temperatures.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac24

In consequence the resulting metal film is a true cast of the template structure and the size of the spherical voids within the metal is directly determined by the size of template spheres used.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac24

Second, electrodeposition can be used to prepare a wide range of materials from both aqueous and non-aqueous solutions under conditions which are compatible with the template.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac25

Third, electrochemical deposition allows fine control over the thickness of the resulting macroporous film through control of the total charge passed to deposit the film.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac26

This is a unique feature of the approach.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac26

Fourth, electrochemical deposition is ideal for the production of thin supported layers for applications such as photonic mirrors since the surface of the electrochemically deposited film can be very uniform.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac27

It is also important to note that, because the template spheres are assembled onto the flat surface of the electrode and because electrochemical deposition occurs from the electrode surface out through the overlying template, the first layer of templated material, deposited out to a thickness comparable to the diameter of the template spheres used, has a different structure from subsequent layers.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac28

Subsequent growth of the film by electrodeposition out through the template leads to a modulation of the surface topography of the film in a regular manner that will depend on the precise choice of deposition bath and deposition conditions.⁴⁵

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac28

Despite the significant interest in photonic structures there have been very few reports of the optical properties of nanostructured metal films of this type.^{36,45,52–54}

Type: Motivation | Advantage: None | Novelty: None | ConceptID: Mot1

In this paper we report the first results for a detailed study of the reflectance spectra of nanostructured films of platinum, gold and silver recorded at normal incidence as a function of the thickness of the nanostructured metal film.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj2

Experimental

Chemicals

The templates were made of monodisperse polystyrene latex spheres (Duke Scientific Corporation) supplied as a 1 wt.% solution in water (manufacturer's certified mean diameter of 701 nm ± 6 nm, coefficient of variation in diameter 1.3%).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp3

Before use, the suspensions were homogenized by successive, gentle inversions for a couple of minutes followed by a sonication for 30 s.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp4

All solvents and chemicals were of reagent quality and were used without further purification.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp5

The commercial cyanide free gold plating solution (Tech.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

Gold 25, containing 7.07 g dm^–3 gold) was obtained from Technic Inc. (Cranston, RI, USA).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp6

Hexachloroplatinic acid, H₂PtCl₆ (purity 99.99%), isopropanol, tetrahydrofuran (THF), cysteamine and ethanol were obtained from Aldrich.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp7

Silver was deposited from a low cyanide bath composed of 0.3027 mol dm^–3 K₄P₂O₄ (Aldrich), 0.1228 mol dm^–3 KCN (BDH) and 0.0185 mol dm^–3 AgCN (Aldrich).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp8

All solutions were freshly prepared using reagent-grade water (18 MΩ cm) from a Whatman RO80 system coupled to a Whatman “Still Plus” system.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp9

Preparation of gold substrates

Evaporated gold electrodes used as substrates were prepared by evaporating 10 nm of chromium, followed by 200 nm of gold onto 1 mm thick glass microscope slides.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp10

These gold substrates were thoroughly cleaned before use by sonication in deionized water for 30 min followed by sonication in isopropanol for 90 min.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp11

They were then rinsed with deionized water and dried under a gentle stream of argon (BOC Gases).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp12

Cysteamine was self-assembled onto the evaporated gold electrodes by immersing the freshly cleaned gold substrate in a 10 mmol dm^–3 ethanolic solution of cysteamine at room temperature for several days.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp13

Assembly of the colloidal templates

The deposition of the colloidal template layers was carried out in a thin layer cell (2 cm × 1.5 cm) made up of the cysteamine coated gold electrode and a clean, uncoated, microscope cover glass held 100 µm apart by a spacer cut from Parafilm (Pechiney Plastic Packaging, Inc.).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp14

The space between the two plates was filled with the aqueous 1 wt.% suspension of polystyrene latex spheres.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp15

The transparent glass plate allows the filling of cell to be monitored and a very slight argon stream was used to remove any trapped air bubbles.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp16

The filled thin layer cell was held vertically in an incubator (Model LMS series 1) in order to control the rate of evaporation from the cell.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp17

After drying the template appears opalescent with colours from green to red, depending on the angle of observation, clearly visible when illuminated from above with white light.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs1

The templates are robust and adhere well to the gold substrates.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs2

There was no evidence for re-suspension of the latex particles when placed in contact with the electroplating solutions.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs3

Electrochemical deposition of platinum, gold and silver

Electrochemical deposition was performed in a thermostated cell at 25 °C using a conventional three-electrode configuration controlled by an Autolab PGSTAT30.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp18

The template-coated gold substrate was the working electrode with a large area platinum gauze counter electrode and a home-made saturated calomel reference electrode (SCE).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp19

Gold and platinum films were deposited under potentiostatic conditions at –0.95 V or 0.05 V vs. SCE respectively.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp20

Silver was deposited from a low cyanide plating bath using pulse plating.⁵⁵

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met1

This was chosen because other silver plating baths exhibit instability and the resulting deposits are of low quality in terms of adhesion.⁵⁶

Type: Method | Advantage: Yes | Novelty: Old | ConceptID: Met1

The templated silver films were produced using galvanostatic pulse plating with the first pulse to a current density of 20 mA cm^–2 for 100 ms (this is important to ensure good adhesion) followed by a train of pulses of 5 mA cm^–2 for 50 ms separated by a rest time (i.e. zero current) of 1 s.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp21

In some experiments gold, platinum and silver films were grown with a series of steps in thickness.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp22

This was achieved by withdrawing the substrate in a series of steps (each of about 500 µm) from the electrodeposition solution using a microstage.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp22

When the electrochemical deposition was complete the samples were soaked in THF for 2 h to dissolve the polystyrene template.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp23

An environmental scanning electron microscope (Philips XL30 ESEM) was used to study the morphology and microstructure of both the polystyrene templates and the macroporous metal films.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp24

Optical measurements

Simultaneous visual observation and spectral analysis of small sample areas (l µm²–100 µm²) was achieved using an optical microscopy arrangement (BX51TRF Olympus), Fig. 1.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp25

An incoherent white light source was used to illuminate the samples and images were recorded using a CCD camera (DP2 Olympus).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp26

A fibre-coupled spectrometer (Ocean Optics, spectral range 300–1000 nm, resolution l nm) placed in the focal plane of the image was used to acquire the spectral response from the selected area (approximate diameter 50 µm at ×10 magnification).

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp27

All spectra were normalised with respect to the reflection off a silver mirror, whose reflectivity over the wavelength range studied is constant.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp28

Polarisation studies and film smoothness measurements were carried out using polarisation selective optics and dark field apertures, respectively.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp29

The domain size of the macroporous regions in a template was measured through observation of diffracted wavelengths from wide-angle white light illumination and are 100–5000 µm across depending on the templating conditions.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp30

In this study the reflection spectra at normal incidence were recorded as a function of the thickness and surface morphology of the films.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp31

To achieve this the films were grown with a series of discrete steps in thickness (see above) and reflection spectra and SEM images were recorded at known points on the sample for each step.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp31

This was achieved by using optical images of the film as a map to pin-point the locations of each measurement relative to particular defects or irregularities in the film.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp32

Replicate measurements of the reflection spectra at different regions on each step were made to confirm that the spectra were representative for each film thickness.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp33

Results

Assembly of the template

Many methods have been proposed to produce colloidal crystalline samples which are free from defects over macroscopic length scales.⁵⁷

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac29

A popular approach is to use gravity sedimentation from a dispersion of the colloid in a suitable solvent.^58–61

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met2

This yields centimetre-scale pieces of polycrystalline material but with numerous defects in the crystal lattice (stacking faults, dislocations, grain boundaries and vacancies).

Type: Method | Advantage: No | Novelty: Old | ConceptID: Met2

In addition, during gravity sedimentation it is difficult to control film thickness.

Type: Method | Advantage: No | Novelty: Old | ConceptID: Met2

Other methods for ordering colloids into arrays have been investigated.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac30

The flow of solvent through micromachined channels has been used to create dense colloidal arrays of multiple layers.^14,15,62

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met3

Recently, the strong capillary forces which develop at a meniscus between a substrate and a colloidal solution have been used to produce three dimensional arrays of controlled thickness.^63–69

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met4

If this meniscus is slowly swept across a vertically placed substrate by, for example solvent evaporation, thin planar opaline films can be deposited.

Type: Method | Advantage: Yes | Novelty: Old | ConceptID: Met4

A modification of this approach was used in the present work using cysteamine modified gold surfaces.

Type: Method | Advantage: None | Novelty: New | ConceptID: Met5

The formation of close packed monolayers or multilayers of colloidal particles on the modified gold surface was achieved by careful control of the evaporation rate of the suspension.

Type: Method | Advantage: Yes | Novelty: New | ConceptID: Met5

The arrangement of the spheres in the template films was investigated using high magnification scanning electron microscopy.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj3

Fig. 2a shows a tilted view of a typical template assembled from 500 nm polystyrene spheres on a cysteamine treated gold surface.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs4

The spheres are close packed in a well ordered hexagonal array.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res3

Electrochemical deposition

Platinum, gold and silver films of controlled thickness were electrochemically deposited from aqueous solution through the pre-assembled templates.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj4

Fig. 2b shows a typical SEM image of a templated platinum film 140 nm thick grown though a template made up of a monolayer of 700 nm diameter polystyrene spheres.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs5

100

The micrograph shows that the spherical segment voids left in the platinum films after the removal of the polystyrene spheres have smooth and uniform mouths and are arranged in a well-ordered, close-packed, array as expected from the structure of the original template.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res4

101

The centre to centre distance measured for the pores in the film shown in Fig. 2b, and for similar SEM images of other films, is the same as the diameter of the polystyrene spheres used to prepare the template; thus the diameter of the spherical segment voids within the platinum film is directly determined by the diameter of the polystyrene spheres used to form the template.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res5

102

Fig. 2c shows scanning electron micrograph of the top surface of a macroporous platinum film deposited through a template made up of multilayers of 700 nm diameter polystyrene spheres.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs6

103

In this case the platinum film was grown to be about 650 nm thick, that is approaching one template sphere in diameter.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs7

104

At this thickness, because of the geometry of the packing of the spheres in the template, the film starts to grow around the spheres in the second layer.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs8

105

As electrochemical deposition of the film proceeds out from the planar substrate, it is hindered by the polystyrene spheres in the upper layers.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs9

106

The presence of the template spheres in the upper layers has two effects: the spheres both block the growth of metal out from the substrate and they block diffusion of metal ions from the solution to the growing metal surface.⁴⁵

Type: Result | Advantage: None | Novelty: None | ConceptID: Res6

107

As a result the surface of the metal film is not planar and this accounts for the complex structures seen in Fig. 2c where the mouths of the pores no longer appear circular.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res7

108

By controlling the number of layers of spheres in the template we are able to avoid this effect.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res8

109

Fig. 2d shows an SEM image for a platinum film of the same thickness but this time electrochemically deposited through a template consisting of a monolayer of 700 nm diameter polystyrene spheres.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs10

110

Now the mouths of the pores are nearly circular.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs11

111

This is because for the monolayer of polystyrene spheres there is no blocking effect from the second layer.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res9

112

The samples used here for reflection spectroscopy studies were all prepared using monolayer template films.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp34

Film thickness

113

Film thicknesses were calculated using the radius of the pore mouth measured in the SEM and the known radius of the template sphere.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj5

114

From simple trigonometry the thickness is given by t = r ± (r² – r2pore)^1/2 where t is the film thickness, r the radius of the template sphere and r_pore the radius of the pore mouth.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

115

The choice sign for the ± term depends on whether the film is thicker or thinner than the radius of the template sphere.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod1

116

Despite the apparent accuracy of the SEM images there is evidence from AFM cross-sections that the lips of the spherical cavities are rounded rather than sharp.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs12

117

This might be expected given the coupled processes of diffusion and reaction involved in the electrochemical deposition.⁵²

Type: Result | Advantage: None | Novelty: None | ConceptID: Res10

118

The presence of a rounded lip to the spherical segment cavity complicates the precise measurement of the pore mouth diameter in the SEM and thus the estimation of film thickness.

Type: Object | Advantage: No | Novelty: New | ConceptID: Obj5

119

In addition the nature of the trigonometric relation between thickness and pore mouth diameter magnifies the uncertainty in the thickness for thicknesses around one half the template sphere diameter since at this point the pore mouth diameter changes only slowly with changing thickness.

Type: Object | Advantage: No | Novelty: New | ConceptID: Obj5

120

These uncertainties are reflected in the estimates of film thickness given below through the values given for the estimated errors which are largest around half sphere height.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs13

121

Nevertheless, other methods of determining film thickness have their own drawbacks; cross-sectioning the film has the obvious disadvantage that it damages the sample, AFM measurements are problematic because as the film gets thicker, the pore mouth closes up.

Type: Object | Advantage: Yes | Novelty: None | ConceptID: Obj6

Reflection spectra of nanostructured metal films

122

The reflection spectra at normal incidence of the nanostructured films of platinum, gold and silver, each prepared using a monolayer template of 700 nm polystyrene spheres, were recorded as a function of film thickness.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj7

123

Reflection spectra for the platinum film are shown in Fig. 3 together with corresponding optical and SEM images.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs14

124

The optical images show the change in film colour from grey-green through red, orange and blue and then back to grey-green as the film thickness increases.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs15

125

The corresponding SEM images show the associated change in pore mouth diameter.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs16

126

The individual reflection spectra are plotted on a log-scale (off-set from one another for clarity) with the sample thickness increasing from top to bottom.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp35

127

Dips in reflectivity are observed which shift to progressively longer wavelengths as the film thickness increases, causing the dramatic changes in the observed colour of the sample.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs17

128

Looking at the data in more detail we see that below one quarter sphere height (spectra 1–7) there is a dip in reflectivity around 670 nm which grows steadily stronger as the film thickness increases and shifts towards shorter wavelengths (higher energy).

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs18

129

Reflectivity dips at around the periodicity of the structure suggest a grating like effect.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res11

130

Much work has focused on the behaviour of gratings, notably by Wood who observed anomalies in the spectral intensity of different polarisations of light reflected from a grating.⁷⁰

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac31

131

Further research indicated that a grating, through conservation of momentum, could allow conduction electrons and light to couple together to produce a travelling wave solution along the dielectric/metal boundary, known as a surface plasmon.⁷¹

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac32

132

This condition produces a coupling condition of sin(θ) = [ε₁(ω)/(1 + ε₁(ω)]^1/2 – mλ/d where θ is the angle of incidence, ε₁(ω) the real part of the dielectric function of the metal, λ the incident wavelength, d the grating periodicity and m is an integer.

Type: Model | Advantage: None | Novelty: None | ConceptID: Mod2

133

For our experiment, with θ of 0° and d of 700 nm and using the appropriate values for ε₁(ω) for platinum,⁷² eqn. (2) predicts a value of the wavelength of 675 nm, close to the experimentally observed values in Fig. 3.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res12

134

A more detailed approach to grating anomalies, involving solution of the electromagnetic field at this condition, reveals a change in the nature of the anomaly with variation in grating depth.⁷³

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

135

Briefly stated, the increase in depth of the groove initially strengthens the reflectivity minima and decreases its bandwidth.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

136

However, further increases in depth weaken and broaden the reflectivity dip.

Type: Method | Advantage: None | Novelty: Old | ConceptID: Met6

137

Although the experimental situation here is more complicated it seems reasonable to suggest that some of the features described by this theory are consistent with our data.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

138

This grating effect appears to decrease in intensity around one quarter sphere height (compare spectra 6–9) and we assume that this is because the area of the flat metal regions between the spherical pores is decreasing so that coupling to excitation of plasmons on the surface of the film decreases.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

139

As the film becomes thicker still (spectra 8–24) dips in reflectivity appear which shift to longer wavelength as the thickness increases (for example compare spectra 11 and 18).

Type: Result | Advantage: None | Novelty: None | ConceptID: Res13

140

Imaging the sample in this region at increasing magnification modifies the measured reflection spectrum; the reflectivity dips broaden and decrease in intensity, suggesting that interference or scattering effects dominate the optical response rather than absorption of light by the metal, Fig. 4.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res14

141

The increase in optical resolution at higher magnification separates individual ray paths, diminishing interference peaks.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs19

142

However this is also associated with a collection cone of greater angle (higher numerical aperture) so that scattering losses are also diminished.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs20

143

Consequently the data in Fig. 4 alone do not allow us to distinguish between interference and scattering effects.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res15

144

In order to separate the two effects we turn to studies on samples with substantially larger spherical cavities for which the scattering due to the periodicity of the structure will be shifted into the infra-red and for which we can study the reflection properties of individual cavities.

Type: Object | Advantage: None | Novelty: New | ConceptID: Obj8

145

Our studies of larger metallic structures formed by electrochemical deposition of gold around 5 µm or larger diameter spheres have shown that the extreme curvature of the cavities produces unusual polarisation properties caused by a symmetric geometric reflection off the sides of each cavity.⁵²

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac33

146

Bright field images of the gold sample were taken through collinear polarisers, Fig. 5(a), and through crossed polarisers, Fig. 5(b), with the incident polarisation set to be vertical with respect to the images.

Type: Experiment | Advantage: None | Novelty: None | ConceptID: Exp36

147

Collinear polarisers only image cavity reflections which preserve the polarisation state, such as that from the single reflection off the bottom of the cavity.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac34

148

Light experiencing a multiple reflection off the sides of the cavity acquires a more complicated polarisation state depending on where on the sphere segment the light hits with respect to its polarisation.

Type: Background | Advantage: None | Novelty: None | ConceptID: Bac35

149

In Fig. 5 the polarisation state is preserved for light hitting the top, bottom, left and right sides of the micro-reflector.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs21

150

At other positions on the sides of the reflector (i.e. along the diagonal orientation) the polarisation is rotated.

Type: Observation | Advantage: None | Novelty: None | ConceptID: Obs22

151

The origin of the polarisation rotation is geometric; at each interface the light picks up a twist in linear polarisation due to the out-of-plane reflection geometry.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res16

152

Polarisation studies of our sub-micron structures have also revealed the presence of these novel two reflection polarisation rotations.

Type: Result | Advantage: None | Novelty: None | ConceptID: Res17

153

Assuming that interference effects dominate, the reflection spectra of the platinum surfaces can be modelled as an interference effect between reflections off the top of the structure and off the curved surfaces within the pores.