Ag nanocrystals differing by their coating agents: unexpected behaviors

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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 closedChapter 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.

General View on Formation Mechanism of 3D Superlattices

During superlattices growth in a solvent, the nanoparticles crystallize from the suspension in order to achieve the thermodynamic equilibrium state, which results in the minimum Gibbs free energy 98, 99 of the system. The Gibbs free energy changes (ΔG) at a given temperature is as follows: ΔG = ΔH-TΔS.
where ΔH is the standard enthalpy of formation, ΔS is the standard entropy formation, and T is the temperature in K. The energetic contribution ΔH accounts for the various types of interactions (van der Walls force, Columbic force, dipolar interaction, etc) between nanoparticles in the suspension, while the entropy changes during the self-assembly processes are generally related to the sum of each nanoparticles’ free volumes in the system.
Both entropy and isotropic van der Walls interactions should favor structures with high packing densities, fcc and hcp. In the hard-sphere system, fcc is favored over hcp due to its higher entropy, although the free energy difference is small. In addition, the solvent flow can direct nanocrystals toward the fcc lattice.

Article: Surface Plasmon Resonance of Silver Nanocrystals Differing by Sizes and Coating Agents Ordered In 3D Supracrystals

Inorganic nanocrystals self-assembled in two-dimensional (2D) or three-dimensional (3D), referred to as supracrystals, have received great interest over the last two decades.1-5 Hence, long-range ordered assemblies of nanocrystals can in some cases exhibit novel optical, electronic, mechanical or magnetic properties, thus providing a route to the manufacture of metamaterials with unique chemical and physical properties.6-9 For example, vibrational coherence in supracrystals was first discovered by Low-Frequency Raman Spectroscopy (LFRS) measurements for fcc supracrystals of 5-nm Ag nanocrystals, and is in good agreement with theoretical calculations carried out on similar system.10 This vibrational coherence was demonstrated by a decrease in the quadrupolar mode bandwidth. Propagative vibrations in supracrystals have also been evidenced by time-resolved pump probe measurement (phonons are associated to long-range order: they can be evidenced not
demonstrated). Long-time scale differential reflectivity dynamics reveal large oscillations for the ordered systems, whereas a monotonous decay was observed in the disordered system.
Furthermore, it was shown that the electron transport properties can be controlled by the degree of ordering in the nanocrystal assemblies.
The bright and fascinating colors of noble-metal nanocrystals (i.e. Au, Ag…) have attracted considerable interest since historical times for their use as decorative pigments in stained glasses and artworks.15-17 Nowadays, the attention is focused on applications ranging from photonics to biomedicine.18, 19 Noble-metal nanocrystals, when dispersed in solvent, exhibit a strong localized surface plasmon resonance (SPR) peaks resulting from the collective resonance oscillation of conduction electrons under the irradiation light.20, 21 The SPR frequency depends not only on the type of nanomaterials, but also on its size and shape,20, 22 and on the dielectric properties of the carrier solvent.23, 24 Colloidal Ag nanocrystals are attractive candidates as nanoscale plasmonic building blocks because of their sharper resonance, greater electromagnetic field enhancement and lower loss, compared to that of Au or Cu for instance.25 Recent studies on the size-dependence SPR absorption band show a strong blue-shift when increasing the nanocrystal size from 2 to 12 nm, which is in opposition with observations made for this type of materials in the size range of tens of nanometers.21,24 When 5-nm Ag nanocrystals are self-assembled in 2D superlattices, it has been demonstrated by reflectivity, 26 photo-induced STM, 27 ellipsometry that splitting of the SPR band takes place. More recently it was also shown that this splitting was due to inter-particle coupling interactions,13 and consequently the optical properties of noble-metal nanocrystals can be modulated efficiently through tailor-made design. The sensitivity of the SPR to the inter-particle interactions has been explored.26,28,29 The discrete dipolar approximation (DDA) method pioneered by Purcell and Pennypacker 30 has been applied to simulate the absorption spectra of metal nanoparticles assembled in a 2D hexagonal network. However, the experimental demonstration of collective SPR of 3D supracrystals made of noble-metal nanocrystals remains to be demonstrated. Near-field-coupled noble-metal nanocrystals have been studied, revealing the coupled SPR in plasmonic dimers composed of octahedral goldnanocrystals within the near-field range.31 Long-range periodic assemblies of noble-metal nanocrystals, are frequently studied through theoretical approaches, but rarely experimentally.

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Article : Surface Chemistry Controls the Crystal Structures in Binary Nanocrystal Superlattices

Self-assembly of micrometer-sized colloids have been intensively studied, and the phase behavior of those colloids is determined by minimizing the free energy F = U – TS or, since those colloids are forbidden to interpenetrate and the thermal energy U is a constant, by maximizing the entropy S.1-5 Coming to the nanoscale, owing to the surface coat of flexible organic molecules, the assembly process is accompanied by effective control over the interactions between the nanocrystals and all entropic forces.6-8 The interaction between nanocrystals can be described by a soft model, which assumes isotropic nanocrystal interaction potentials and, thus predicts the formation of close-packed arrays.9,10 However, the electrical charge induced by ligand and the surface coverage of the ligand introduces important perturbations that can lead to the formation of superlattices with lower packing density.11,12 Although advances have been made using a variety of electrostatic forces, covalent and noncovalent molecular interactions, to control the crystal structure, it remains a challenge to use spontaneous self-assembly to predict the phase structures comprised of two different types of building blocks, namely binary nanocrystal superlattices.
A variety of crystal phases in binary nanocrystal superlattices have been fabricated from nanocrystals of semiconductors, metals and oxides, and the prediction of crystal structures is mainly replies on the space-filling principles. In addition to the well-known crystal structures analogous to NaCl, AlB2, NaZn13 and laves phases that observed in binary microcolloids,14-16 other phases such as CuAu-type, Cu3Au-type, Fe4C-type, CaB6- type as well as quasicrystalline ordering are also discovered in binary nanocrystal superlattices.17-20 The emergence of these binary nanocrystal superlattices that cannot be simply predicted from the hard-sphere models needs to be further studied.

Table of contents :

Chapter 1
1.1 Nanotechnologies and Nanocrystals, Introduction
1.1.1 General Overview
1.1.2 Silver Nanomaterials History and Prospect of Silver at Different Scales. 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 The need of Stabilization of Ag nanocrystals 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 General View on Formation Mechanism of 3D Superlattices Growth Methods
1.4.4 Binary Nanocrystal Superlattices General View on Formation Mechanism Growth Methods
1.5 Optical Properties of Assemblies
Chapter 2 Ag nanocrystals differing by their coating agents: unexpected behaviors
2.1 Abstract
2.2 Article: Collective Surface Plasmon Resonances in Two-Dimensional
 Chapter 3 Assemblies of metal Nanocrystals: experiments and simulations
3.1 Abstract
3.2 Articles: Ag Nanocrystals : Effect of Ligands on Plasmonic Properties
3.3 Supporting Information
Chapter 4
4.1 Abstract
4.2 Article: Surface Plasmon Resonance of Silver Nanocrystals Differing by Sizes and Coating Agents Ordered In 3D Supracrystals
4.3 Supporting Information
Chapter 5
5.1 Abstract
5.2 Article : Surface Chemistry Controls the Crystal Structures in Binary Nanocrystal Superlattices
5.3 Supporting Information


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