Optical Properties of Colloidal Nanocrystals
When irradiated by an electromagnetic wave, metallic nanocrystals exhibit a very large absorption band that stem from the free electrons at the metal surface resonatibng with the incident photons. This effect is known as a Surface Plasmon Resonnance (SPR).
Plasmonics is related to the localization, guiding, and manipulation of electromagnetic waves down to the nanometer-length scale.6, 7, 61, 62 Metals are key component in plasmonics because they supports surface plasmon polariton modes. Surface plasmons have been known for more than 160 years since Micheal Faraday demonstrated their existence for the first time in 1857.27 Two types of plasmons can be differenciated, that are two common types of plasmonic modes: localized surface plasmons (LSPs) and propagating surface plasmons (PSPs).7 When metal nanoparticles are irradiated by light at wavelengths much larger than the size of the metal nanoparticles, the surface electron cloud is displaced with respect to the metal nuclei, generating a restoring force arising from Coulomb attractions between electrons and nuclei. This kind of attraction leads to the oscillation of the surface electron. At a special frequency the surface electrons are oscillating coherently with the incident light, resulting in the plasmonic resonance, which is commonly known as a localized surface plasmon resonance (LSPR) mode.66 (Figure 1.5-a) There are several factors influencing the oscillation frequency: (1) the density of surface electrons; (2) the effective electron mass (3) the shape and size of the charge distribution.
In contrast, PSPs are supported by structures that have at least one dimension that approaches the excitation wavelength, such as nanowires, as shown in Figure 1.5-b.6 In this case, the electron field is not uniform across the structure and other effects must be considered.
SPR of silver nanoparticles
As mentioned above, noble metallic nanoparticles, especially silver nanoparticles are a very important material in plasmonics. Ag has the largest quality factor across most of the spectrum from 300 to 1200 nm.76 In physical and engineering the quality factor is a dimensional parameter that describes how under-damped an oscillation or resonators, are as well as characterized a resonators’ bandwith relative to its center frequency. Compared to Au and Cu that exhibit a rather broad and anisomerous plasmonic resonance due to damping effect, the interband transitions of silver, that is the special area where electrons are excited from the conduction band to higher energy levels, take place at much higher frequencies than that of the LSPRs. That is why silver plasmon spectra show very narrow resonance band and the shape of the band is much more symmetrical compared to other methods.
It is worth noting that the most traditional approaches used to characterize the plasmonic response of metal nanostructures is to measure their extinction spectrum using UV-vis-NIR spectroscopies.77 Several calculations and experimental works reported that various factors affect the plasmonic properties of silver nanostructures. First of all, the morphology of Ag nanocrystals is an important factor. For example, Xia’s group showed UV-visible extinction spectra of three Ag nanostructures in aqueous media and containing silver spheres, cubes, and triangular thin plates with almost the same sizes. (Figure 1.6) The silver nanospheres exhibit one symmetric extinction peak centered at 430 nm due to the isotropic symmetry of sphetical structures. However, cubic nanoparticles displayed three SPR peaks located at 350, 400, and 470 nm, respectively. The spectra of triangular nanoplates displayed three peaks at around 335, 470 and 690 nm. This phenomenon can be explained by theoretical calculation carried out by Schatz’s group: these peaks correspond to the out-of-plane quadrupole, in-plane quadrupole, and in-plane dipolar plasmon resonance modes, respectively.
Two-dimensional (2D) nanocrystal superlattices
Over the last 20 years, a number of groups have succeeded in fabricating self-ordered nanocrystals in two-dimensional closed-packed superlattices with a small number of defects and at very large scale. These 2D superlattices self-assembled by inorganic nanoparticles provide new possibilities of fabricating new solid-state materials and devices with novel physical properties because the interactions between nanopariticles can generate new collective phenomenon.
There are several dominating strategies to produce 2D self-assembled superlattices: (1) Langmuir-Blodgett methods are well known procedures to generate 2D hexagonal closed- packed superlattices.95 This type of strategies was developped in the early 20th century by Irving Langmuir and Katherine Blodgett. In this method, a Langmuir monolayer is held at constant surface pressure while transferring it onto a solid substrate. Currently, this approach has already been explored to fabricate closed-packed 2D films composed of nanomaterials with different shapes. Tao et al. used Langmuire-Blodgett methods to fabricate 2D superlattices assembled of silver nanowires and then subjected the sample to SERS measurements.96 (2) There is another convenient way to prepare 2D superlattices: one-drop colloidal solution of known concentration is deposited on the substrate and the evaporation process occurs at the substrate surface.47 This method is easier and more flexible than LB based procedures, but the superlattices obtained only exhibit local ordering, with some areas containing multilayers. Other parameters that must also be considered are related to the material itself, the particle-particle and the particle-substrate interactions.86, 91 For instance, Pileni’s group successfully prepared 2D Cobalt self-assembled arrays with this method.86 (3) Finally, liquid-air interfacial assembly approaches have become a popular method in recent years to produce 2D superlattices.97 This method is a combination of modified LB and evaporating method. Murray et al. used this method to prepare 2D superlattices with different kind of nanocrystals. 2D superlattices films obtained in the centimeter-scale and transferable for example.
Three-dimensional (3D) Nanocrystal Superlattices
Similarily to 2D superlattices, controlled assembly of long range three-dimensional (3D) superlattices with well-defined structures and desired types of NCs can lead to many unique properties and their subsequent use in different applicatiomns, hence their production has been a long-standing challenge.
The structures of 3D superlattices are similar to atoms in bulk phase metals and in nanocrystals such as body centered cubic (bcc), face centered cubic (fcc) and also the hexonganal close packed (hcp) structures. Amorphous structures of disordered nanoparticles also exist. (Figure 1.8)25 When the fabrication conditions of these superlattices are well controlled, the structures can be varied between the different types.
UV-visible absorption measurements
The UV-visible absorption measurements presented here were performed using a Varian Cary 5000 double monochromator recording spectrophotometer. To perform measurements in transmission geometry at various angles of incidence, the spectrophotometer was equipped with a Harrick Scientific (Pleasantville, NY) variable angle transmission accessory (VATA), which has been specially designed for transmission studies of samples up to 3 mm thick. The VATA allows a given sample and a blank, which should match the former in both thickness and refractive index, to be placed in the light beam at the same angle of incidence, but in the opposite orientation, in order to avoid any misalignment of the detected beam that could affect the measured transmittance. A Glan-Taylor polarizer was also used in all our measurements to polarize the light parallel to the plane of incidence. Under such a configuration, information on the optical anisotropy of the sample can interestingly be achieved by considering the fact that the electric field is then projected in both perpendicular and parallel directions to the 2D assembly of nanocrystals.
Discrete dipole approximation method
The discrete dipole approximation (DDA) method16, 17 is used to simulate the optical response of the NP assemblies. The DDA is a flexible and powerful method widely used to simulate the scattering and absorption by nanometer-sized targets of chosen geometry and dielectric properties.17-28 The principle and accuracy of the DDA method are described elsewhere.29 In this work, the free software ‘DDSCAT 7.0’ is used to carry out the DDA calculations.30 As illustrated in the inset of Figure 3, the 2D nanocrystal assembly is modeled as a planar hexagonal arrangement of 91 regularly spaced and equally sized spheres. The relevance of using this DDA target was already discussed in previous works.11, 12 For these calculations, the refractive index of the surrounding medium is set to be that of dodecanethiol (n = 1.46) which is used as coating agent to passivate the Ag and Au nanospheres. To account for the finite size effects, the bulk dielectric functions of Ag and Au, as taken from Palik’s handbook,31 are size-corrected following the procedure described elsewhere.32, 33 The DDA absorption spectra of Ag and Au nanospheres films are calculated for various incidence angles ranging from 0°to 60°.
Table of contents :
1.1 Nanotechnologies and Nanocrystals, Introduction
1.1.1 General Overview
1.1.2 Silver Nanomaterials
188.8.131.52 History and Prospect of Silver at Different Scales.
184.108.40.206 Applications of Ag at Different Scales
1.2 Synthesis Strategies
1.2.1 General Synthesis Strategies of Nanocrystals
1.2.2 Synthesis Strategies of Silver Nanostructures
220.127.116.11 The need of Stabilization of Ag nanocrystals
18.104.22.168 Synthesis Methods
1.3 Optical Properties of Colloidal Nanocrystals
1.3.1 SPR of silver nanoparticles
1.4 Self-assemblies of Nanocrystals
1.4.1 Application of Superlattices
1.4.2 Two-dimensional (2D) nanocrystal superlattices
1.4.3 Three-dimensional (3D) Nanocrystal Superlattices
22.214.171.124 General View on Formation Mechanism of 3D Superlattices
126.96.36.199 Growth Methods
1.4.4 Binary Nanocrystal Superlattices
188.8.131.52 General View on Formation Mechanism
184.108.40.206 Growth Methods
1.5 Optical Properties of Assemblies
Chapter 2 Ag nanocrystals differing by their coating agents: unexpected behaviors
2.2 Article: Collective Surface Plasmon Resonances in Two-Dimensional
Chapter 3 Assemblies of metal Nanocrystals: experiments and simulations
3.2 Articles: Ag Nanocrystals : Effect of Ligands on Plasmonic Properties
3.3 Supporting Information
4.2 Article: Surface Plasmon Resonance of Silver Nanocrystals Differing by Sizes and Coating Agents Ordered In 3D Supracrystals
4.3 Supporting Information
5.2 Article : Surface Chemistry Controls the Crystal Structures in Binary Nanocrystal Superlattices
5.3 Supporting Information