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
Research Objectives and Themes
The primary research objective of this thesis is to investigate, fabricate, and optimize periodically ordered two-dimensional (2D) metallo-dielectric structures, specifically noble metal-coated polystyrene microsphere arrays, for their application in sensitive optical spectroscopy and nanophotonics. The study seeks to address how these nanostructures can be engineered to enhance sensitivity in spectroscopic techniques, specifically Surface Enhanced Raman Scattering (SERS), Metal Enhanced Fluorescence (MEF), and Surface Plasmon Resonance (SPR) sensing.
- Fabrication methods for regular 2D polystyrene microsphere arrays (drop-coating, sonication, convective self-assembly).
- Development of hybrid nanofabrication techniques combining Nanosphere Lithography (NSL) and Nanoimprint Lithography (NIL).
- Fundamental study of photonic and plasmonic properties of these structures via experimental transmission/reflectivity and FDTD simulations.
- Optimization of SERS and fluorescence enhancement efficiency based on geometric parameters like sphere diameter and metal film thickness.
- Demonstration of molecular sensing capabilities using metal-coated microsphere arrays.
Excerpt from the book
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 [2-4]. 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 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 modeling [5] 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.
Summary of Chapters
CHAPTER 1: This chapter provides an overview of periodically structured metals and dielectrics, introducing key concepts such as photonic band gaps and surface plasmon excitations.
CHAPTER 2: This chapter covers experimental results on the preparation of regular 2D polystyrene microsphere arrays using different techniques and discusses an original nanofabrication approach combining Nanosphere and Nanoimprint Lithography.
CHAPTER 3: This chapter investigates the optical properties of the fabricated nanostructures, correlating experimental transmission and reflectivity data with theoretical FDTD simulations to understand the photonic and plasmonic effects.
CHAPTER 4: This chapter focuses on employing metal-coated microsphere arrays as active substrates for enhanced optical spectroscopies, specifically SERS, MEF, and SPR sensing, and optimizes their efficiency.
Keywords
Nanotechnology, Nanophotonics, Plasmonics, Surface Enhanced Raman Scattering, SERS, Metal Enhanced Fluorescence, MEF, Surface Plasmon Resonance, SPR, colloidal crystals, microsphere arrays, Nanosphere Lithography, Nanoimprint Lithography, FDTD simulations.
Frequently Asked Questions
What is the fundamental goal of this research?
The fundamental goal is to explore, design, and fabricate noble metal-coated 2D microsphere arrays to create multifunctional substrates capable of significantly enhancing the sensitivity of optical spectroscopic techniques like SERS and fluorescence.
What are the primary thematic areas covered?
The core areas include nanofabrication techniques for creating periodic arrays, characterization of these ordered structures, investigation of their unique photonic and plasmonic optical properties, and their subsequent application in molecular sensing.
What is the primary research question being addressed?
The research asks how the geometric and physical parameters of noble metal-coated 2D microsphere arrays—such as particle size and metal film thickness—can be optimized to maximize field confinement and sensitivity for spectroscopic sensing applications.
Which scientific methods are utilized?
The work employs a combination of experimental methods including drop-coating, convective self-assembly, Nanosphere Lithography (NSL), Nanoimprint Lithography (NIL), optical spectroscopy, and Scanning Electron Microscopy (SEM). These are complemented by advanced numerical modeling using Finite-Difference Time-Domain (FDTD) simulations.
What is treated in the main part of the thesis?
The main body details the preparation of the colloidal crystal templates, the development of nanolithographic methods, the detailed optical study of the photonic and plasmonic effects in these systems, and the experimental assessment of their performance as substrates for SERS, fluorescence enhancement, and SPR.
Which keywords best characterize the work?
Key terms include Nanophotonics, Plasmonics, colloidal crystals, SERS, MEF, SPR, Nanosphere Lithography, and FDTD modeling.
What is the specific role of the "hot-spots" mentioned in the research?
Hot-spots are regions of extremely high electromagnetic field intensity localized at the junctions between metallic caps in the structure; they are critical because they enable the detection of individual molecules through SERS.
How does the author determine the number of colloidal layers on the samples?
The author uses simple optical measurements; by analyzing the transmission and reflectivity spectra (specifically the Fabry-Perot interference fringes and the spectral position of the Bragg peak), the number of layers can be determined nondestructively.
Why is the "convective self-assembly" method considered superior?
It is favored for being a more practical and controlled technique that allows for the creation of large-area ordered colloidal crystals with controllable layer thickness in a relatively short time.
- Quote paper
- Cosmin Farcau (Author), 2008, Ordered Plasmonic Nanostructures: from Fabrication to Relevant Applications in Optical Spectroscopy and Sensing, Munich, GRIN Verlag, https://www.grin.com/document/293519
