Ordered plasmonic nanostructures are currently the subject of numerous scientific studies, due to their potential applications, from optical communications to chemical analyses and biomedicine. This thesis is focused on a special type of periodically ordered two-dimensional (2D) metallo-dielectric structure, noble metal films over microsphere arrays: from their fabrication and characterization, to spectroscopic applications. Prepared structures exhibit remarkable optical properties (including an unusually high transmittance, resembling the extraordinary optical transmission phenomenon), resulting from the excitation of surface plasmons. It is demonstrated that these plasmo-photonic structures are very promising multifunctional active-substrates for Surface Enhanced Raman Scattering, Metal Enhanced Fluorescence, and Surface Plasmon Resonance Spectroscopy.
Chapter 1 is devoted to giving an overview of the interesting aspects related to the optical properties and applications of periodically structured metals and dielectrics. We also briefly describe some of the currently developed nanofabrication techniques. A few concepts, like the photonic band and surface plasmon are introduced, being useful for the discussions in the upcoming chapters. We pay a special attention to colloidal photonic crystal films and noble metal structures obtained by templating on two-dimensional colloidal crystals. Chapter 2 presents own experimental results concerning the preparation, characterization and lithographic applications of two-dimensional (2D) colloidal crystal films. Results of the optical properties investigations are presented in Chapter 3, and divided in two main categories: i) photonic properties of bare microsphere arrays, and ii) plasmonic properties of noble metal nanostructured films deposited over microsphere arrays. In Chapter 4 we explore the capabilities of noble metal coated microsphere arrays to improve the sensitivity of optical spectroscopic methods: Surface Enhanced Raman Scattering, Metal Enhanced Fluorescence, and Surface Plasmon Resonance Spectroscopy.
Contents
Introduction
Chapter 1 Periodically structured Metals and Dielectrics with Enhanced Optical Properties
1.1 Photonic Materials
1.1.1 Photonic band gap
1.1.2 Applications of photonic crystals
1.1.3 Photonic crystals fabrication methods
1.1.4 Photonic materials based on colloidal crystals
1.2 Plasmonic Materials
1.2.1 Surface plasmons
1.2.1.1 Localized surface plasmons (LSP)
1.2.1.2 Surface plasmon polaritons (SPP)
1.2.2 Applications of surface plasmons
1.2.2.1 Surface plasmon-enhanced spectroscopy
1.2.3 Fabrication of plasmonic nanostructures
1.2.4 Plasmonic materials based on colloidal crystals
1.3 References
Chapter 2 Preparation of 2D Microsphere Arrays and Use in Nanolithography
2.1 Introduction
2.2 Preparation of 2D Polystyrene Microsphere Arrays
2.2.1 Drop-coating
2.2.2 Improving drop-coating by sonication
2.2.3 Convective self-assembly
2.3 Morphological characterization
2.3.1 Microsphere arrays prepared by drop-coating
2.3.2 Microsphere arrays prepared by ultrasound assisted self-assembly
2.3.3 Microsphere arrays prepared via convective self-assembly
2.4 Lithographic Applications of 2D Microsphere Arrays
2.4.1 Metal Nanostructures Obtained by Nanosphere Lithography
2.4.1.1 Nanosphere Lithography with a colloidal mono-layer
2.4.1.2 Nanosphere Lithography with a colloidal double-layer
2.4.2 Polymer Nanostructured Surface by Combining Nanoimprint Lithography and Nanosphere Lithography
2.4.2.1 Polymer surface with arrayed nano-bumps
2.4.2.2 Polymer surface with arrayed nano-dimples
2.5 Conclusions
2.6 References
Chapter 3 Optical Properties of Fabricated Ordered Nanostructures
3.1 Introduction
3.2 Methods and Instrumentation
3.3. Photonic Properties of 2D Microsphere Arrays
3.3.1 Transmission and reflectivity of colloidal monolayers
3.3.2 FDTD simulations of transmission and reflectivity
3.3.3 Angle-resolved transmission measurements
3.3.4 Assessment of the layers number and stacking pattern by micro-spectroscopy
3.4 Plasmonic Properties of Metal Films over Microsphere Arrays
3.4.1 Unusual optical transmission
3.4.1.1 Dependence on the sphere diameter
3.4.1.2 Angle-resolved transmission
3.4.2 Reflectivity of metal coated microsphere arrays
3.5 Conclusions
3.6 References
Chapter 4 Metal Coated Microsphere Arrays as Substrates for Enhanced Optical Spectroscopies
4.1 Metal Coated Microsphere Arrays as Surface Enhanced Raman Scattering Substrates
4.1.1 Introduction
4.1.2 Experimental
4.1.3 Results and Discussion
4.1.3.1 SERS of Nanostructured Ag film
4.1.3.2 Optimization of SERS efficiency
Optimization by sphere diameter
Optimization by metal film thickness
4.1.3.3 Confocal Imaging of local SERS enhancements distribution
Single-molecule SERS hot-spots
FDTD simulations of EM fields distribution
4.2 Metal Coated Microsphere Arrays as Substrates for Fluorescence Enhancement
4.2.1 Introduction
4.2.2 Experimental
4.2.3 Results and Discussion
4.3 Plasmonic Molecular Sensing with Silver Coated Microsphere Arrays
4.4 Conclusions
4.5 References
General Conclusions
Appendix
List of own publications
Acknowledgement
Abstract
Ordered plasmonic nanostructures are currently the subject of numerous scientific studies, due to their potential applications, from optical communications to chemical analyses and biomedicine. This thesis is focused on a special type of periodically ordered two-dimensional (2D) metallo-dielectric structure, noble metal films over microsphere arrays: from their fabrication and characterization, to spectroscopic applications. Prepared structures exhibit remarkable optical properties (including an unusually high transmittance, resembling the extraordinary optical transmission phenomenon), resulting from the excitation of surface plasmons. Thus our structures are very promising multifunctional active-substrates for Surface Enhanced Raman Scattering, Metal Enhanced Fluorescence, and Surface Plasmon Resonance Spectroscopy.
Introduction
Current developments of nanotechnology rely on the rational design and controlled fabrication of particles, regular arrays of particles, patterned films with at least one of their dimension parameters in the 1-100 nm size regime 1 2 3. At this scale of matter radically different phenomena are present and dominate the system’s behaviour. Such mechanisms include quantum effects, surface effects (dominant surface interactions), statistical time variations of properties and their scaling with particle size, absence of defects in the volume of nanocrystals. These effects endue nanoscale particles and structures with unique mechanical, electronic, magnetic, optical, chemical properties, opening the door to an enormous new domain of engineered nanostructures and integrated nanodevice designs, with unimaginable applications in every aspect of life.
Thus there have been continuous efforts in the last 1-2 decades, in exploring novel approaches to parallel nanolithography. Electron-beam lithography (EBL), Focused ion beam (FIB), X-ray lithography (XRL) are among the most performant lithographic techniques available nowadays for the fabrication of nanoparticle arrays with well-defined size and shape 4 5. Each of these techniques yield ordered structures, allow for a good control over their periodicity and symmetry, but are time-consuming and expensive. That is why, in order for the potential of nanotechnology to be realized, novel alternative manufacturing methods deviating from scaled-down versions of currently practised technologies, are required at the nanoscale.
Nanophotonics, as a new scientific research field, deals with exploring optical properties of nanostructured materials and new phenomena concerned with light confinement at the nanoscale, which represent subjects of major scientific interest 6 7 8. There are at least two remarkable achievements of materials design in the field of nanophotonics on which emphasis must be put on: a) photonic crystals – artificial structures with a periodic variation of the refractive index, on length scales comparable to the wavelength – which control the way photons propagate inside of them; b) noble metal nanostructures (Au, Ag, Cu, etc.) which localize electromagnetic energy with spatial resolution beyond the diffraction limit (<100 nm).
Periodically nano-micro-structured thin films (dielectric and/or metallic) with their size-dependent properties are receiving great scientific attention because of the continuous strives for device miniaturization, needed to achieve improved performance and reduced costs in optoelectronics, communications, data storage industries [1,2]. Beside intensive research on development of new methods to fabricate periodically ordered nanostructures another key aspect is understanding their complex optical properties, dominated by photonic or plasmonic effects. Therefore periodically nano(micro)structured thin films are interesting and demanding also from a fundamental point of view. Only a good understanding of the physics behind the intricate optical properties of such structures will allow full development of nanophotonic technological or biological applications.
The work in this thesis is focused on a special type of periodically ordered two-dimensional (2D) metallo-dielectric structure, noble metal-coated microsphere arrays: from their fabrication and characterization, to spectroscopic applications.
Chapter 1 is devoted to giving an overview, without having the pretension of being exhaustive, of the interesting aspects related to the optical properties and applications of periodically structured metals and dielectrics. We also briefly describe some of the currently developed fabrication techniques. A few concepts, like the photonic band and surface plasmon are introduced, being useful for the discussions in the upcoming chapters. We pay a special attention, by devoting some separate sections, to colloidal photonic crystal films and noble metal structures obtained by templating on two-dimensional colloidal crystals.
Beginning with Chapter 2 own experimental results are presented and discussed. We firstly describe the methods employed to prepare regular 2D arrays of polystyrene (PS) microspheres (i.e. 2D colloidal crystal). The morphological characterisation of the samples is made by optical microscopy and electron microscopy, allowing us to adjust experimental parameters. We show how we succeeded in improving the drop-coating technique, by applying a random shaking on the drying microsphere solution. In the second part of this chapter we propose an original nanofabrication technique, able to produce periodic nano-patterns on polymer surfaces. This procedure was developed by combining Nanosphere Lithography (NSL) and Nanoimprint Lithography (NIL).
Results of the optical properties investigations are presented in Chapter 3, and divided in two main categories: i) photonic properties of bare microsphere arrays, and ii) plasmonic properties of noble metal nanostructured films deposited over microsphere arrays. We observe minima in the transmission spectra of the bare colloidal monolayers. By performing a series of well-designed experiments (varying the sphere diameter, varying the incident angle), and comparing experimental results with computer simulations we will clarify the origin of these minima. We demonstrate that simple optical measurements can be used to unambiguously determine the number of colloidal crystal layers (e.g. mono-, double-, triple-layer) or to evidence different stacking patterns in multi-layered structures. Noble metal films deposited on 2D colloidal crystals exhibit unusually high transmittance, which resemble the enhanced optical transmission (EOT) of periodic arrays of holes in metal films9. By taking into account both angle-resolved transmission spectra and reflectivity measurements we aim at understanding the mechanism of this unusual transmission.
In Chapter 4 we employ metal-coated microsphere arrays as active substrates to enhance the sensitivity of three optical spectroscopic methods. Firstly we test their efficiency as Surface Enhanced Raman Scattering (SERS)-active substrates. Then we seek to optimize their efficiency by verifying its dependence on the sphere diameter or metal film thickness. We will also establish correlations between the optical properties and SERS activities. Then by scanning confocal SERS and atomic force microscopy (AFM) we search for correlations between SERS activity and structure topography. We investigate the capacity of the microstructures to sustain highly localized and intense electromagnetic fields able to promote the detection of single molecules. FDTD simulations give some hints on the origin of the measured distribution of SERS intensities. In a second independent study we investigate the suitability of silver coated microspheres to amplify molecular fluorescence. We also inquire about the enhancement efficiency dependence on the diameter of the spheres. In the last part we test the sensitivity of silver coated microsphere arrays to molecular adsorption, via surface plasmon resonance (SPR) spectroscopy. Again, the size-dependency of the SPR sensor is tested.
Chapter 1 Periodically Structured Metals and Dielectrics with Enhanced Optical Properties
This first chapter is devoted to giving an overview, without having the pretension of being exhaustive, of currently developed fabrication methods and interesting aspects related to the optical properties and applications of periodically structured metallic/dielectric thin films. Particularly special attention is given to colloidal photonic crystal films and noble metal structures obtained by templating on two-dimensional colloidal crystals.
Periodically structured dielectric and metallic thin films have unique size-dependent optical properties, which are absent in their non-structured equivalents. Understanding and then designing these new optical properties is the key to development of applications, from optical communications to chemical analyses and biomedicine. Moreover, by properly adjusting the period of such structures these new properties can be tuned to the visible spectral range, which is most convenient and accessible to humans.
1.1 Photonic Materials
Photonics is the established term to denominate the technology of photons, by analogy to electronics, the technology of electrons1. Hence photonic materials are materials with the aid of which the propagation of photons inside of them can be controlled and manipulated, similarly to the way semiconductors do this with electronic conduction.
1.1.1 Photonic band gap
Photonic crystals (PhCs) or photonic band gap (PBG) materials are a class of material structures in which the dielectric function undergoes a spatially periodic modulation 2 3 4. This periodic variation of the dielectric function determines the appearance of energy bands for photons in the same way as electrons travelling in the periodic potential of a crystal have energies arranged into bands. The electronic energy bands in a crystal can be separated by gaps, where propagating states are prohibited, as in the case of semiconductors. Analogous photonic band gaps can exist for photons travelling through a periodically structured dielectric material.Electromagnetic (EM) waves with frequencies inside the band gap cannot propagate
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Figure 1.1 (A) Photograph (taken in the Naturhistorisch Museum from Vienna) of butterflies exhibiting intense coloration due to photonic crystal-like structures in their wings. (B) Up: Optical image of a compact disc reflecting different colours under white light illumination; Low: Scheme of white light diffraction on a grating of parallel grooves.
in any direction inside the material. The spectral range of the band gap is determined by the length scale on which the dielectric function is modulated (lattice parameter) in the PhC and the wavelengths at which the effects are felt are comparable to the lattice parameter.
Photonic crystals are a nice example where theoretical modeling5 was successfully used to design dielectric structures with desired optical properties. After it was demonstrated that a periodic arrangement of dielectric spheres with the diamond lattice symmetry would exhibit a full PBG, Yablonovitch et al 6 found an ingenious way of fabricating such a structure. By drilling three tilted cylindrical holes in each position of a hexagonal lattice marked on the surface of a dielectric block they realized the first experimental structure to demonstrate the existence of a PBG. Since the drilled holes had diameters in the millimetre range the obtained PBG was in the microwave regime. The optical properties of PhCs are scalable, thus by reducing the lattice parameters also the wavelengths at which the PhC is operational are reduced.
The iridescent, intense coloration of the butterflies’ wings in Figure 1.1A is due to a periodic photonic crystal-like structure embedded in their wings 7. Periodicity comparable to incident light wavelengths determines constructive interferences between light scattered by each lattice point. The observed phenomenon resembles quite a lot to what happens in diffraction gratings. The tracks of a compact disc for example act as a diffraction grating, producing a separation of the colours
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Figure 1.2 Schematic representations of 1D (left), 2D (center), 3D (right) photonic crystals.
contained in the white light. The periodicity of this grating (1.6 µm) is comparable to visible wavelengths.
If the dielectric function is modulated in one-, two- or three dimensions (1D, 2D or 3D) a PBG can exist for photons propagating in one, two or all three dimensions. Correspondingly the periodic dielectric structures are called 1D, 2D or 3D PhCs, and their schematized representations are given in Figure 1.2. The simplest case is that of 1D PhCs, and it’s most important characteristics will be briefly presented in the following paragraphs.
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Figure 1.3 Typical transmission (left) and reflection (right) spectra for 1D photonic crystals.
A 1D PhC usually consists of alternating planar layers made of two materials with different refractive indices nh, nl and thickness dh, dl (Abbildung in dieser Leseprobe nicht enthalten, the period of the structure), which repeat N times8. The most essential properties of this structure, as probed by continuous wave white light illumination are the following: i) its transmission spectrum shows regions where transmittance drops to zero, i.e. light cannot propagate inside the structure (see Figure 1.3); ii) the transmission spectrum presents oscillations, whose number is determined by the thickness of the structure; the reflections at the ends of the structure build up Fabry-Perot interference fringes; iii) the features of the band gap are determined by the physical parameters of the periodic structure: the amplitude of the gap is dependent on the refractive index contrast nh - nl and the abruptness of its edges depends on the number N of periods; iv) the electric field is distributed in a selected way inside the structure; certain transmitted modes are localized more into the high or low refractive index regions depending on their wavelength.
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Figure 1.4 Dispersion relation for a homogenous material (dashed line) and for a 1D PhC with period a and the same average refractive index.
A brief and intuitive explanation of the physical origin of the PBG can be given as follows. The electric field is in the form of a plane wave Abbildung in dieser Leseprobe nicht enthaltenmodulated by the periodic dielectric function of the lattice Abbildung in dieser Leseprobe nicht enthalten. The Bragg condition is satisfied at the edge of the Brillouin zone, Abbildung in dieser Leseprobe nicht enthaltenfor this structure. The right and left travelling waves create a standing wave of the form Abbildung in dieser Leseprobe nicht enthaltenand Abbildung in dieser Leseprobe nicht enthalten. The energy is proportional to the magnitude square of the electric field. The low order mode (n=1) tends to confine its energy in the high dielectric region while lowering the frequency. At the same time, high order mode (n=2) tends to confine its energy in the high dielectric region while increasing its frequency. The difference of energy confinement in the low and high refractive index causes the energy gap (band gap) as shown in Figure 1.4. Inside the band gap k is imaginary so the field is exponentially decaying and outside of the band gap k is real and the field is propagating. The Bragg condition is a transition state where the field is a standing wave.
An example of 2D PhC is a two-dimensional array of rods periodically arranged parallel to one another. In the 3D case an object (a small cube on the right side of Figure 1.2) is arrayed in all three dimensions. All the basic properties presented for the 1D systems are valid also in the 2D and 3D PhCs, although being more complex.
Several methods of computation for photonic band structure have been developed from which I mention: Plane Wave Expansion (PWE) method9 ; Koringa-Kohn-Rostoker (KKR) – spherical waves expansion10 ; Transfer matrix methods (Pendry’s group11, Rigorous Coupled Wave analysis RCWA12 ); finite difference time domain (FDTD) 13 14.
1.1.2 Applications of photonic crystals
1D PhCs are widely used in optical applications under the name of Bragg mirrors or reflectors, which consist of stacks of alternating high and low refractive index transparent materials. They are superior and therefore preferred over metallic mirrors because they work without dissipation.
If a defect is introduced in the middle of a 2D PhC (for example one rod missing from a 2D lattice of rods), this acts as a cavity where EM energy is stored. If this system is stretched in the direction parallel to the rods a PhC optical fibre is obtained, in which the defect constitutes the core through which light travels, while the PhC surrounding it represents the cladding. A great variety of other optical devices (passive or active) can be constructed by introducing point or line defects into photonic crystals.
One goal of photonic band gap engineering is to control the spontaneous decay rate of molecules or other emitters embedded in the PhC structure. The density of states (DOS) is zero within the band gap because the wave vector k is evanescent. Since the spontaneous-emission rates are proportional to the DOS, it becomes possible to inhibit the spontaneous emission. The possibility to control the spontaneous emission with PBGs may lead to more efficient light emitters, such as thresholdless semiconductor lasers15, single-mode light-emitting diodes16 and efficient antennas17. A particularly promising application, in a 21th century world in search for new energetic resources, is the use of PhCs to improve the efficiency of solar cells 18 19]
Other very interesting effects observed in PhCs, and which will lead to development of new applications are also: superprism effect20, negative refraction21, high group velocity dispersion and other nonlinear effects22.
1.1.3 Photonic crystals fabrication methods
A major challenge, even nowadays, is to design 3D structures that exhibit PBG and to develop fabrication techniques for their practical realization at optical wavelengths. However the rapid advancements in nanofabrication techniques in the last years promise to win this challenge and take big steps ahead in this field.
Fabrication of PhCs can be either facile or extremely difficult, depending on the desired wavelength of the bandgap and the level of dimensionality. Since the wavelength at which the PhC is operational scales directly with the lattice constant of the PhC, long wavelength structures that require larger dimensions are easier to fabricate. At microwave frequencies, where the wavelength is on the scale of centimetres, the PhCs can be fabricated by employing simple machining techniques or rapid prototyping methods. At the other extreme, optical wavelength PBGs require crystal lattice constants smaller than 1 µm. Building PBGs in the optical regime requires methods that push current state-of-the-art micro- and nano-fabrication techniques.
As already described above Yablonovitch realized the first PhC by drilling three tilted cylindrical holes in each position of a hexagonal lattice marked on the surface of a dielectric block. Figure 1.5 shows the PhC with diamond lattice symmetry fabricated by the Yablonovitch group in 1991[6].
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Figure 1.5 Schematic representation of the method to fabricate a PhC with fcc lattice structure: a slab of material is covered with a mask consisting of a triangular array of holes; each hole is drilled through 3 times at an angle 35.6? away from normal, and spread out 120? from the azimuth. Reprinted with permission from ref [6] Copyright (1991) by the American Physical Society.
The dimensionality of the PBG has also a big impact on the ease or dif?culty of fabrication. Since 1D PBGs require periodic variation of the dielectric constant in only one direction, they are relatively easy to build at all length scales. Antireflection coatings, notch filters, distributed Bragg reflectors have been produced and widely used in optical applications for a long time. Simple evaporation techniques have been used.
A simple way of preparing silica-air 2D PhC structures is by mechanical drawing. A macroscopic silica tube is drawn and then cut to create many tubes (~1 mm inner diameter, 1 m long). These are then stacked by hand in the desired structure, fused and again drawn in order to reduce their diameter to the desired lattice parameter23.
One well established method, commonly employed for the fabrication of sub-micron structures is photo-lithography24. The processes involved are schematized in summary in Figure 1.6. An ultraviolet (UV) light source illuminates a photosensitive material (photoresist) through a mask, starting a photochemical process only in the illuminated areas of the resist. The minimal feature size is determined by the photoresist sensitivity and the wavelength of the UV light used. X-ray lithography can be seen as a variant of photolithography, which uses shorter wavelengths25.
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Figure 1.6 Schematic steps of photolithography.
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Figure 1.7 Schematic steps of nanoimprint lithography. Reprinted with permission from ref [26]. Copyright 2007, AIP Publishing LLC.
Nanoimprint lithography26 uses a master mould (which is primarily fabricated by other methods) to transfer its pattern onto other resin or polymer moulds. 2D PhCs that can be fabricated by these methods are not the ideal PhC case, since although they will have the periodicity in two dimensions they are of finite length in the third direction. In order to obtain a 3D structure the 2D patterned films can be stacked in a multilayer fashion, thus adding the third dimension, as can be seen in the example in Figure 1.7.
Other novel top-down techniques are electron-beam27, focused ion beam lithography28 which are very similar to photolithography with the advantage of using electrons with associated shorter wavelengths, meaning that smaller features can be fabricated. Dip Pen nanolithography29 is another option, which uses the tip of an atomic force microscope (AFM) to write molecular patterns on atomically flat surfaces. All of these techniques satisfy the request for smaller dimensions, although they are serial techniques, not suitable for the structuring of large areas.
One of the few methods capable of producing a 3D structure in a single step is holographic interference lithography30. This method is based on exposing a photosensitive material to the periodic patterns created by interference of multiple coherent laser beams. Actually this method can be employed successfully for the fabrication of 1D, 2D, and 3D periodic structures, depending on the number and directions of the interfering beams.
The tunability of any PhC device would enhance its functionality. Though tuning by changing the physical parameters (period, dimensions) of PhCs during the fabrication process can modify the properties of PCs, it is not practical. Therefore several groups aimed their efforts to fabricating PhCs from materials that are sensitive to external parameters such as electric field, magnetic field, temperature, pressure or humidity 31 32 33.
1.1.4 Photonic materials based on colloidal crystals
A very special class of photonic materials is constituted by periodic arrangements (close or non-close packed) of colloidal particles. Polymer (mostly polystyrene - PS, but also polymethyl methacrylate - PMMA) or silica microspheres are usually the building blocks of such structures, which are generally called colloidal crystals 34. A periodic arrangement of spherical particles has also spaces filled with air in between them, and that makes them a periodic arrangement of two alternating dielectric materials. Therefore they can act as PhCs composed of air and the sphere material. One example of colloidal crystal occurring in nature, and having PhC properties, are opals, which owe their beautifully iridescent colours to the silica colloids (which have no intrinsic colour) being arranged into a 3D periodic structure (see Figure 1.8).
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Figure 1.8 Opal gemstone (left) and scheme showing the periodic arrangement of silica microspheres consisting an opal (right). The periodic arrangement determines reflection of selected light wavelengths and thus the colours as observed by human eye.
One big difference compared to the PhCs described in the previous sections is in the techniques to fabricate colloidal crystals: here, bottom-up techniques are used instead of top-down, meaning that the structure is fabricated starting from the building blocks (the spheres), and arranging them in a regular array. Some methods to obtain regular 2D and 3D arrays of nano(micro)spheres and their interesting properties and applications will be presented in this section. Attention must be paid throughout these pages to the following considerents: a 2D colloidal crystal is a planar structure, a 2D array of nano(micro)spheres, and therefore it does not make a 2D PhC, in the classical way; a 3D colloidal crystal can be a 3D PhC.
Besides their photonic properties, colloidal arrays are employed in a wealth of other appealing applications: as physical masks for evaporation or reactive ion etching (RIE) to fabricate nano and micro structured particles and surfaces 35 36; as arrays of microlenses for lithography37 ; to cast elastomeric stamps for use in lithographic techniques38, as superhydrophobic surfaces39.
Some methods to prepare regular arrays of polymer colloids will be presented, with an emphasis on 2D colloidal crystals.
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Figure 1.9 Two-dimensional ordered clusters of gamboge particles as observed by Perrin in 1909 [40].
First observations that colloidal particles can self-organize in ordered lattices when the solvent in which they are dispersed evaporates, were made back in 1909, by J. Perrin40. He dropped small amounts of solution containing gamboge (gomme-gutte) particles on a glass slide and observed by microscope that clusters of 2D particle arrays form (see Figure 1.9 – the figure from Perrin’s paper). Even nowadays the most widely spread techniques for assembling colloidal particles into regular 2D arrays are based on capillary forces which appear between particles during the evaporation of the solvent in which they are dispersed [34, 41 42 43].
The process of colloidal arrays fabrication starts by purchasing or synthesis of the polymer colloids. Emulsion polymerization is one of the most commonly used methods to prepare polymer colloids[34]. The simplest method to arrange colloidal particles into lattices is drop-coating. A drop of the colloidal solution is put on the surface of a glass, mica, or SiO2 substrate. The glass substrate has to be previously treated to become hydrophilic, so the drop spreads, forming a thin film. The principal players in the mechanism of array formation are capillary forces, which arise between neighbouring particles as the solvent evaporates and its layer thickness reaches the size of the colloids. By evaporation of the solvent (which is very often water) the polymer spheres will self-organize into periodic lattices, which are monolayers or multilayers, ordered, partially ordered, or disordered, depending on the interplay between some experimental conditions, as concentration, temperature, humidity, substrate surface chemistry 44 45. An improved variant of this method is convective self-assembly46, in which two glass slides are used. One is used as a substrate for the crystal growth, and the other one to move the meniscus between the colloidal solution and the substrate across the latter. Capillary forces arising between neighbouring colloids, in the meniscus area, will start the formation of small clusters of few spheres, which will then grow due to particles brought from the solution by hydrodynamic flow.
Another method to assemble colloids into regular arrays is based on hydrodynamic flow and physical confinement 47 48. By imposing physical constrains, the colloids are forced to occupy a certain volume in the most convenient way. Other methods to obtain colloidal crystals employed ink-jet printing49 thermophoresis and convection50, electrophoresis51. By developing a method based on spin-coating even the preparation of non-close packed arrays of polymer spheres was demonstrated52.
Assembly of colloids into 3D colloidal crystals is traditionally achieved by gravitational sedimentation[34]. Silica colloids are more appropriate for this technique, because they have a higher density as compared to polystyrene. Crystallization via repulsive electrostatic interactions is another approach to 3D colloidal crystals53. However convective self-assembly appears to be one of the most convenient techniques, because it lends itself to the production of both 2D and 3D arrays, and also multilayered stacks of photonic crystal films composed of different sized colloids54. Progress has been made also in fabrication of binary and ternary arrays of colloidal particles, by assembly into preformed V-shaped
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Figure 1.10 Collection of representative images of 2D colloidal architectures: (A) monolayer of ordered close packed PS spheres; (B) monolayer of ordered non-close packed PS spheres. Reprinted with permission from ref [52]. Copyright 2006 AIP Publishing LLC; (C) several colloidal lattices obtained by flow through microchannels of different widths (a-f). Reprinted with permission from ref [48]. Copyright (2003) by the American Physical Society; (D) binary particle arrays obtained by first assembling colloids into V-shaped grooves, and then assembling other colloids into the V-shaped arrays of PS spheres. Adapted with permission from ref [55]. Copyright (2003) American Chemical Society.
microchannels55. To visually summarize some recent achievements in the fabrication of 2D colloidal crystals we present in Figure 1.11 a collection of representative images from the literature.
As already mentioned above, 2D colloidal crystals are not 2D photonic crystals, and maybe this is a reason why their optical properties were not as appealing as those of 3D crystals until recent years. However due to the in-plane periodicity they exhibit interesting optical properties, which were the object of very small number of studies 56 57 58. These arrays exhibit a photonic band structure but it is very different than that of ordinary 2D PhCs like rod arrays. By imaging the fluorescence with a scanning near-field optical microscope, it was observed that light of certain wavelengths propagates and dissipates within the array59. The dissipative 2D photonic band structures can be obtained from transmission spectra, since the resonant phenomenon between the electromagnetic eigenmodes of the dielectric sphere array and the incident light appears as dips (minima) in the transmission spectra. The behaviour of these photonic bands can be modified for example by etching of the PS spheres60, or by deposition of dielectric films on top of the array61.
Although some nice results have been obtained, as those from refs 56-58, thorough experimental studies of the optical properties of 2D periodic sphere arrays are lacking in the literature. If the transmission spectra of these periodic structures exhibit dips, what is the behaviour of the light reflected from the structure? How do the optical properties change when we go from a mono-layer to a double-, triple-layer? Is there any non-destructive, practical recipe for rapidly distinguishing between e.g. a double-layer and a triple-layer? Are there some correlations between the optical properties of a monolayer and those of multi-layered, 3D photonic colloidal crystals? Within this thesis, we studied several aspects of the optical properties of polystyrene colloidal crystal films trying to answer some of these questions.
1.2 Plasmonic Materials
Plasmonics is a newly emerging branch of the field of Nanophotonics, which explores how electromagnetic fields can be confined and manipulated over dimensions comparable and even smaller than the wavelength. Its foundations lie on the interactions between photons and conduction electrons at metal/dielectric interfaces or in small metallic nanostructures. The two main ingredients of Plasmonics are therefore surface plasmon polaritons (SPP) and localized surface plasmons (LSP).
1.2.1 Surface plasmons
In solid state physics, a plasmon is a quasiparticle resulting from the quantization of plasma oscillations62. Thus, plasmons are collective oscillations of the free electron gas density. Surface plasmons are those plasmons that are confined to metal/dielectric interfaces. They are the results of strong interactions between photons and free conduction electrons near the metallic surface63. Two main categories of surface plasmons exist and will be discussed in the following pages: localized surface plasmons (LSP), which are confined on metal particles smaller than the wavelength (i.e. nanoparticles), and surface plasmon polaritons (SPP), which propagate along metal/dielectric interfaces.
1.2.1.1 Localized surface plasmons (LSP
It has been known for a long time that very fine particles of noble metals appear very brightly coloured. Noble metal colloids have actually been used, by craftsmen from the Middle Ages, as dyes to colour church windows. Although they probably did not realize it, they were the first to develop applications of localized surface plasmons. The colours of these noble metal fine particles are a result of electromagnetic interactions between incident light and conduction electrons in the metal particle.
If we consider a spherical particle much smaller than the wavelength (few to tens of nanometres) its diameter will be also on the order of the penetration depth of electromagnetic waves in metals64. Therefore the excitation incident light is able to penetrate the particle. The field inside the particle produces a displacement of the conduction electrons relative to the fixed positive charge of the ionic lattice. These displaced electrons will cause charge accumulation on the surface at one side of the particle. The attraction between this negative charge and the positive charge of the fixed lattice ions on the opposite side results in a restoring force. If the frequency of the incident light ?eld is in resonance with the eigenfrequency of this collective oscillation, even a small exciting ?eld can lead to a strong oscillation. The magnitude of the oscillation depends mainly on the damping involved, which can be both radiative and nonradiative65. The strength of the restoring force is the main parameter determining the resonance frequency. This force depends on the separation of the surface charges, i.e. the particle size, the polarizability of the core electrons of the metal particle, and that of the surrounding medium. The alternating surface charges e?ectively form an oscillating dipole, which radiates electromagnetic waves. Localized surface plasmons are thus non-propagating excitations of the conduction electrons of metallic nanostructures coupled to an electromagnetic ?eld.
The consequences of the resonant excitation are to significantly increase the absorption and scattering cross sections for electromagnetic waves, as well as to strongly enhance the near ?elds in the immediate vicinity of the particle surface[63]. It is this resonantly enhanced near ?eld that will give rise to most of the promising applications of metal nanoparticles.
From a theoretical point of view, if we consider a spherical particle of diameter d, its interaction with the electromagnetic ?eld can be analyzed using a simple quasi-static approximation provided that d << ?, i.e. the particle is much smaller than the wavelength of light in the surrounding medium. In this case, the phase of the harmonically oscillating electromagnetic ?eld can be approximated as constant over the particle volume, so that one can calculate the spatial ?eld distribution by assuming the simpli?ed problem of a particle in an electrostatic Field[63]. The harmonic time dependence can then be added to the solution once the ?eld distributions are known. By solving a Laplace equation for the potential, Abbildung in dieser Leseprobe nicht enthalten, and applying the right boundary conditions, one can find the following expressions for the potentials Fin inside and Fout outside the metal sphere:
Abbildung in dieser Leseprobe nicht enthalten (1)
Abbildung in dieser Leseprobe nicht enthalten (2)
where e and em are the dielectric functions of the metal and surrounding medium, E0 is the incident electric field, a is the metal sphere radius, ? is the angle between the position vector r at point P and the z-axis. It can be seen that equation 2 describes the superposition of the applied field and that of a dipole located at the particle center. Its dipole moment p would be given by:
Abbildung in dieser Leseprobe nicht enthalten (3)
From here the polarizability a of a small sphere of sub-wavelength diameter in the electrostatic approximation can be extracted:
Abbildung in dieser Leseprobe nicht enthalten (4)
It appears from this equation that the polarizability experiences a resonant enhancement under the condition that | e + 2 em | is a minimum. Considering that e is a complex value, varying with the frequency in a Drude form, and assuming a slow-varying Im[ e ] this simplifies to:
Abbildung in dieser Leseprobe nicht enthalten (5)
This result constitutes the Fröhlich condition and the associated mode (in an oscillating electric ?eld) is the dipole surface plasmon of the metal nanoparticle.
An interesting consequence of relation 5 is that the resonance frequency is dependent on the dielectric constant of the embedding medium. From this have emerged many applications of noble metal nanoparticles as optical sensing platforms66.
1.2.1.2 Surface plasmon polaritons (SPP)
As briefly mentioned in the beginning of Section 1.2.1 surface plasmons polaritons (SPP) are propagative modes, contrary to localized plasmons. As also the term polariton states, they are quasiparticles formed by coupling of photons to surface plasmons[62].
From a historical point of view the studies of SPPs started at the beginning of the 20th century, with R. W. Wood’s observations on metallic gratings. He found that the spectra of light reflected by the grating contained minima, which shifted in frequency as the angle of incidence was varied. He also noticed that this shift occurred only for light polarized perpendicularly to the grooves. It was only in 1973 that these observations were connected with surface plasmons excitation on the gratings67.
The analysis of the interaction between electromagnetic waves and a metal dielectric interface begins with Maxwell’s equations. We assume that the metal/dielectric interface lies in the xy plane at z =0, and the wave propagates in the x direction. It is beyond our scope to detail the mathematical treatment, so we will skip directly to the most important result:
Abbildung in dieser Leseprobe nicht enthalten (6)
This is the dispersion relation for SPPs propagating at the interface between two half spaces. kSPP is the plasmon momentum and Abbildung in dieser Leseprobe nicht enthaltenis that of light in the dielectric medium, while e1 and e2 are the dielectric constants of the metal and the dielectric. A result of the theoretical treatments, which has to be noted, is that for TE polarization no surface modes exist that fulfil the continuity conditions across the interface. Surface plasmon polaritons therefore can exist only for TM polarization. Another consequence of the continuity conditions is that that surface electromagnetic modes can only be excited at interfaces between two media with opposite signs of the real part of their dielectric permittivities, i.e. between a conductor and an insulator.
illustration not visible in this excerpt
Figure 1.12 presents the plot of equation 6 for a metal with negligible damping described by a real Drude dielectric function, for an air (e2 = 1) and a fused silica (e2 = 2.25) interface. The most important observation is that the SPP dispersion curves lie to the right of the corresponding light line. This means that the momentum of a free photon k0 propagating in the dielectric medium is always smaller than the momentum of a surface plasmon mode kSPP, propagating along the interface between that same medium and the metal. To put it even more clearly, SPPs can not be excited by simple irradiation with light of the metallic surface. This limitation can be overcome by the use of some special techniques, such as prism or grating coupling. In the prism configuration photons are not coupled directly to the metal/dielectric interface, but via the evanescent tail of light totally internally re?ected at the base of a high-index prism (with eprism > e1). As for the grating coupling, as the name says, a grating (1D or 2D) is used to provide the additional momentum required to match the momentum of the SPP.
It was this type of grating coupling the physical mechanism that allowed Ebbesen’s discovery of the Extraordinary Optical Transmission (EOT) through subwavelength hole arrays68. The peculiar observation was that light incident on a silver film perforated with arrays of holes (smaller than the wavelength of incident light) ca be transmitted in a percent much larger than the summation of transmittances of the individual holes would yield. The explanation is nowadays accepted to be the following: the coupling of light to SPPs on the incident surface, transmission through the holes to the second surface and then re-emission from the second surface. Probability of transmission through the holes is increased due to the enhanced SPP electromagnetic fields near the holes. The SPP momentum on a 2D patterned metallic surface can be estimated by the following relation:
Abbildung in dieser Leseprobe nicht enthalten (7)
where Gx and Gy are the grating vectors in the two directions, i.e. reciprocal vectors of the lattice, i and j are integers. If we go back to Wood’s observations they can be explained by relation 7: i) the grating provided the condition for SPP excitation, which caused the minimum in reflectivity; and ii) by changing the incident wavelength also the angle ? at which SPP’s are excited was changed.
1.2.2 Applications of surface plasmons
The EOT phenomenon gave rise to a new birth of the scientific interest for SPPs and surface plasmons in general. As for example the 1998 Nature paper was cited more than 1500 time since then. Along with intensive studies of fundamental nature, aimed at understanding the physics of surface plasmons, many applications were naturally developed. They span over a wide range of interests, from information technology (both passive plasmonic devices like filters, waveguides, polarizers, Bragg mirrors, nanoscopic light sources 69 70, and active plasmonic devices like LED-emission amplifiers, switches, modulators 71 72), plasmonic lithography73 to chemical analysis (Surface Enhanced Raman Spectroscopy - SERS, Surface Enhanced IR Absorption – SEIRA 74 75, plasmonic biosensors76 ) and biomedical applications (monitoring of intracellular processes77, tumor photothermal localized therapy78 ).
1.2.2.1 Surface plasmon-enhanced spectroscopy
Perhaps one of the most widespread applications of surface plasmons is Surface Enhanced Raman Spectroscopy (SERS). SERS makes use of the enhanced EM fields associated to surface plasmons excitation, in order to amplify the Raman signal of molecules placed in these enhanced fields [74,79]. Therefore the key ingredients of SERS are noble metal nanostructures able to support highly amplified electromagnetic near fields. These nanostructures can be regular (like arrays of nanoparticles, arrays of holes in metal films) or irregular (self-affine colloidal clusters)80. Although irregular aggregated colloids appear to be the most efficient81 in terms of enhancement factors (the degree to which SERS is enhanced compared to normal Raman of the same molecules), ordered structures are more useful for many applications, due to their good reproducibility and stability.
A particularly interesting newly developed variant of SERS is TERS – Tip Enhanced Raman Scattering82. In this technique a nanoscopically sharp metallic tip is brought in the vicinity of the analyte and laser illuminated. The SERS effect arises due to enhanced EM fields at the tip’s appex. For example by placing the studied molecules in between the tip and a metallic surface a further increase in the Raman intensity can be obtained. But also other optical spectroscopic techniques benefit from the near field enhancement effect of surface plasmons. Surface Enhanced Infrared Absorption has been demonstrated, although the enhancements are much smaller than in SERS[75]. The enhancement of luminescent emission (fluorescence of molecules, emission of semiconductor quantum dots) by plasmon mechanism has also been demonstrated83.
1.2.3 Fabrication of plasmonic nanostructures
Electron beam lithography is probably the most performant lithographic technique, in terms of control over the shape and size of the fabricated metal nanostructure. Controlling these parameters allows the optical/plasmonic properties of the fabricated nanostructures to be controlled. Figure 1.13A presents a selection of different patterns achievable by EBL fabrication84. By employing such arrays as SERS substrates it was demonstrated how direct correlations between SERS signals and the localized surface plasmon local ?eld spectral pro?le of the nanoparticles can be established85.
illustration not visible in this excerpt
Figure 1.13 (A) SEM images of nanostructured ordered arrays fabricated by EBL. Reprinted with permission from ref [84. Copyright (2005) American Chemical Society. (B) Ion-beam assisted deposition and sputtering machined structures by FIB [86].
Focused Ion Beam (FIB) lithography is similar to EBL with the difference that it uses a beam of Ga ions instead of electrons, and can provide more lithographic regimes (direct writing by sputtering or ion beam assisted deposition, as in the example from Figure 1.13B86 ). Metal nanostructures can be obtained also by ingeniously combining several techniques. As an example the silver nanostructured surfaces in Figure 1.14A were fabricated by nanoimprint lithography of a polymer surface and metal film deposition by evaporation87. Again, by controlling the substrate’s nanotopography its plasmon responce can be tuned (Figure 1.14B).
illustration not visible in this excerpt
Figure 1.14 (A) SEM images of nanostructures obtained by imprint lithography and evaporation of metal films. (B) Reflectivity of these samples exhibiting different plasmonic responses. Adapted with permission from ref [87]. Copyright (2007) American Chemical Society.
We chose to discuss only methods to fabricate ordered noble metal nanostructures, which are more relevant in relation with the work presented in this thesis. Irregular nanostructures are usually obtained by chemical synthesis and self-assembly.
1.2.4 Plasmonic materials based on colloidal crystals
Of high practical importance are alternative lithographic methods, because the top-down techniques (like EBL and FIB), although performant, are very expensive and low-throughput, being serial techniques. The most powerful among alternatives are those based on colloidal crystals (2D or 3D) as lithographic masks or templates for noble metal structuring.
One well-known method to produce 2D arrays of nanoparticles on solid substrates is that known as Nanosphere Lithography (NSL)88. A close-packed ordered array of nano-micro-spheres is used as a physical mask for metal evaporation, to obtain ordered arrays of nanoparticles. Several enhancements of this technique were proposed, by etching the initial sphere mask, to obtain nanohole arrays89 upon metal deposition, or tilting and rotating the sphere array90.
Another interesting method, based on 2D colloidal crystals is that developed by Bartlett’s and Baumberg’s groups91. In their approach a 2D array of microspheres is assembled on a thin metallic film, and then the metal film is further grown around the spheres, by electrochemical plating. In this way, by controlling the plating conditions, arrays of nanovoids in a metal film (Figure 1.15A), with tunable plasmonic properties (from localized to propagative plasmons and their mixing), can be obtained.
By co-deposition of polymer microspheres and gold colloids (particles much smaller than the polymer spheres) a periodic structure is obtained upon solvent removal. The large polymer spheres self-organize in a compact ordered lattice and the metallic colloids fill the spaces between the spheres. By removing the polymer template an interconnected periodic metallic network was obtained, asshown in Figure 1.15.
illustration not visible in this excerpt
Figure 1.15 Ordered structure made of assembled gold colloids, obtained by co-deposition with polymer microspheres, followed by removal of polymer spheres.
As we have showed there are several excellent methods to fabricate noble metal plasmonic nanostructures by templating on colloidal crystals. These methods have the advantage of being economic, easy to implement and also reproducible. If applied for SERS the obtained nanostructures are probably more efficient, in terms of enhancement factors, than structures produced by e.g. EBL.
Finally, probably the simplest method to obtain a colloidal crystal-based metal nanostructure is by evaporation of a metallic film over the 2D nano-micro-sphere array. The deposited metal film will follow the geometry imposed by the spheres, a 2D metallic periodic structured film being obtained. It is this type of noble metal plasmonic ordered structure that will be the object of study of main part of this thesis. Such a structure has already been demonstrated to be an effective SERS substrate92. However we embark on the study of its optical properties, which were not explored so far, and as we will show they are particularly unique, challenging, and useful for applications. We further test the ability of noble metal coated sphere arrays to improve the sensitivity of optical spectroscopic methods (enhanced Raman scattering, enhanced fluorescence, surface plasmon resonance spectroscopy).
[...]
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- Quote paper
- Cosmin Farcau (Author), 2008, Ordered Plasmonic Nanostructures: from Fabrication to Relevant Applications in Optical Spectroscopy and Sensing, Munich, GRIN Verlag, https://www.hausarbeiten.de/document/293519
