Nanoparticles molecules

Figure 1.1 displays an idealized metal nanoparticle / molecule system exposed to an incident light wave, Eq. A dipolar SP excitation can lead to large enhancements in the near-fields around the metal nanoparticle sur ces, particularly at the north and south poles along the axis associated with the incident polarization, which is the z-axis in this example (see red-colored regions). Let the typical magnitude of the field due to an SP excitation in such regions be written as... 

Figure 9.1 Schematic diagram of a metal nanoparticle / molecule system with z-polarized incident light. A spherical nanoparticle of radius a (gold-colored) is centered at the origin that, in the small particle limit, is consistent with a oscillating dipole, ps, at the origin. Regions of high near-field intensity are indicated (red). A molecule (small white circle) is assumed to lie along the z-axis a distance d from the metal surface and its induced dipole moment, pm, is also taken to be on the z-axis and centered on the molecule.

There is a very great variety of nanoparticle molecules known and described in the literature, and it is certainly well beyond the scope of this chapter to review them. Good reviews do exist see References 1 to 4. Here I classify these into four major groups. 

Control of the ligand capping can also allow formation of amphiphilic nanopailicle molecules by capping with both hydrophilic and hydrophobic ligands. Such mixed coatings have been reported in the literature (38—40) and phase equUibiia of such nanoparticle molecules have been studied with simulations to yield a potentially rich phase diagram (41). Mixed coatings portend the prospect to have anisotropic interactions between nanoparticles. 

The nanoparticle molecules can be made chemicaUy reactive by placing appropriate functional groups at opposite ends of the capping hgands. For example. 

It is interesting that here we encounter the double identity of nanoparticle molecules. Are they suspended particles in a colloid or dissolved molecules in a solution We contend that if they satisfy the condition of near stoichiometry discussed above to classify them as molecules, then their stable suspensions are more than that, they are solutions as well. We remark here that gravity can now get in the way, for even a monodisperse, hence stoichiometric, system of particles wiU settle out if the thermal energy, kT, is not large enough to keep the monomers suspended. This happens for particles on the order of 50 nm. 

Figure 3.6 Radius of gold nanoparticle molecule clusters formed by temperature quenching stable solutions of the molecules in a mixture of 2-butanone and tert-butyltoluene to various depths below the saturated solution temperature.

Figure 3.8 Various components of the interaction potential between two 5-nm gold nanoparticle molecules with dodecanethiol ligands.

Figure 3.8 shows the interaction potential between two 5-nm gold nanoparticle molecules with dodecanethiol ligands, the sum of Equations (3.3), (3.6), (3.7), and (3.8). 

Perhaps one of the most molecular things nanoparticle molecules do is form two-and three-dimensional crystals in which the nanoparticles sit at lattice sites. Such crystals of nanoparticles, which are often crystals themselves, can form from solution and are called superlattices. The formation of a superlattice is usually called self-assembly, which seems to give the particles some degree of free will. However, if we view the nanoparticles as molecules, we realize that self-assembly is simply crystallization, a process common for atoms and molecules. 

Fundamental aspects of this new class of nanoparticle molecules are their controllable size and their core-shell stmcmre. With controlled size, we can control their properties. The core-shell structure gives us extra latitude in property control because these two aspects can be, for the most part, varied separately. Thus, nanoparticle molecules offer the possibility to new materials. 

Consider a 5.0-nm diameter gold nanoparticle molecule ligated with dodecylthiol. If the thiols stretch out radially from the nanoparticle, how much surface area do they have per thiol at the edge of this ligand shell The data in Equation (3.1) will be useful. Compare this to the fact that each thiol covers 0.124 nm on the surface of the gold nanoparticle. 

A more elegant method is to use the adsorption of aromatic compounds. In general, this strategy involves attaching linker molecules to hydrophobic surfaces of CNTs via n-n stacking. Subsequently, nanoparticles, molecules, or proteins can be selectively attached to the linker molecules. This... 

It should be noted that, so far, these studies have been mostly confined to relatively simple organometallic molecular complexes, and none have involved nanoparticle molecules. Therefore, a fundamental question arises, when redox-active moieties are bound onto a transition-metal nanoparticle surface through conjugated chemical bonds will effective electronic communication take place between these particle-bound functional moieties This issue has been addressed in a recent study in which carbene-functionalized ruthenium nanoparticles are used as the nanoscale structural scaffold, and ferrocene moieties are exploited as the molecular probe because of well-known electrochemical and spectroscopic characteristics. 

Chu, C. W., Na, J. S., Parsons, G. N. Conductivity in alkylamine/gold and alkane-thiol/gold molecular junctions measured in molecule/nanoparticle/molecule bridges and conducting probe structures. J Am Chem Soc 2007,129, 2287-2296. 

These equations have been also obtained by Carminati et al. [17] and directly show a d dependence at large nanoparticle-molecule distances. In the Carminati model the metal nanosphere is considered as a point-like (located in rp) polarizable entity with polarizability (i.e. due to radiative damping, see Sec. 1.6.4). Thus... 

Strictly speaking, the plasmonics nanoparticle-molecule coupling should be always described by such a set of equation. While it can be demonstrated that not all the components of E, B, H, D fields are independent, it is still necessary to solve for at least one vectorial and one scalar field. However, Eqs. (5.1-5.4) greatly simplify when the intrinsic spatial variations of EM fields are smooth on the scale of the studied systems, i.e. when the latter are much smaller that the wavelength of the free propagating light at the relevant frequencies. In this situation we can in fact assume a wavevector k = co/c —> 0. Under this limit, a few terms in the equations can be disregarded, and they simplify to ... 

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