Nanoparticle based dielectrics

A key issue in nanostructured materials is the dipole coupling between nanocrystals which will cause the optical properties of a nanocrystal ensemble to become like those of the bulk material. There has been extensive investigation of the interactions between particles embedded within media for a range of boundary conditions. We have found that the effective dielectric function given by Eq. (10), based on the Maxwell-Garnett model [1] is very accurate for quite dense nanocrystal arrays. In practice, one measures the transmittance of a thin film of the dense nanoparticle based film. Conventional solutions are simply... 

Colodrero et al. have studied the effects of photonic crystals on the LHE and photocurrent of DSSCs closely. They have attributed the improvement in current in the region of the stop band of the photonic crystal as due to the dielectric mirror phenomenon, a result of the periodic repeating structure. For a structure such as that shown above, where a photonic crystal is assembled above a dye-sensitised, nanoparticle-based electrode, the current is increased or decreased depending on the direction of illumination. When the light reaches the electrode through... 

Simultaneous evaporation of metal with organic and inorganic substances followed by vapor deposition on a substrate allows the production of composite films containing M nanoparticles stabilized in various dielectric matrices [2, 28]. The use of monomer molecules in this process polymerizing during deposition or as a result of the subsequent reactions yields polymeric nanocomposite films with metal inclusions [2, 3, 28, 37]. The new low-temperature synthesis of polymeric nanocomposite films has been elaborated recently. This synthesis is based on the deposition of M/SC and monomers vapors at temperature 80 K followed by low-temperature solid-state polymerization of obtained films in conditions of frozen thermal movement of molecules (cryochemical synthesis) [2], This synthesis has important features, which will be considered further. 

Based on conceptions of work [105] the specific dielectric relaxation in PPX with M nanoparticles is supposed to be connected with reorientation of dipoles in polymer environment of M nanoparticles that accompanies the electron transfer between M nanoparticles of percolation cluster. Dipole centers in PPX are (Tv-units of polymer chains on a surface of lamellar PPX crystallites. Such centers are characteristic, in particular, for extended polymer defects (dislocations, grain boundaries, interfaces between amorphous and crystalline areas) where, most probably, M nanoparticles are formed. 

Optical properties of metal nanoparticles embedded in dielectric media can be derived from the electrodynamic calculations within solid state theory. A simple model of electrons in metals, based on the gas kinetic theory, was presented by Drude in 1900 [9]. It assumes independent and free electrons with a common relaxation time. The theory was further corrected by Sommerfeld [10], who incorporated corrections originating from the Pauli exclusion principle (Fermi-Dirac velocity distribution). This so-called free-electron model was later modified to include minor corrections from the band structure of matter (effective mass) and termed quasi-free-electron model. Within this simple model electrons in metals are described as... 

These electromagnetic waves are very sensitive to any change in the boundary—for example, to the adsorption of molecules onto the metal surface. SPR has measured the absorption of material onto planar metal surfaces (typically Au, Ag, Cu, Ti, or Cr) or onto metal nanoparticles and is used in many color-based biosensor applications and lab-on-a-chip sensors. To observe SPR, the complex dielectric constants e1 of the metal and s2 of the dielectric (glass or air) must satisfy the conditions Re(ei) < 0 and > e21,... 

Quenching At shorter distances, ranging from few nanometers to the physical contact with the metallic structure, a mechanism tends to increase the total decay rate. This effect, which is responsible for fluorescence quenching, is due to the absorption of fluorescence photons in the metallic structure itself (99). Another effect is based on interactions of the fluorophore with free electrons in the metal, wherein the plasmon absorption leads to lower fluorescent emission efficiency (100). Theoretical study asserts that the optimized distance between the excitation source and the fluorophore is around 2-5 nm (99, 101,102). Nanoparticles coated with a thin shell (e.g. silica, 5nm in thickness) and the dye attached to the dielectric shell could overcome quenching effects (84, 103). The quenching effect can also be found in the quantum dot / GNP system (104). It is noted that as the concentration of fluorophore is high, the self-quenching effect should also be considered. (100)... 

Nitzan and Brus developed an analytical formula for the molecular absorption cross section given the model defined above [14]. Figure 9.2 is taken fi"om Ref. [13] and shows the calculated absorption cross section based on the model associated with the photodissociation of I2. (The I2 formed through the absorption process is very short lived.) Photodissociation predicted to be enhanced as the molecule is placed near a silver metal nanoparticle of radius a - 50 nm near the electronic transition resonance position of cat) 22,200 cm . If e eiai(co) is the dielectric fiinction for the metal, a small metal nanoparticle plasmon in air will have its dipolar surface plasmon resonance at frequency <24 such that [1]... 

LSPs are detected as resonance peaks in the absorption or scattering spectra or as dips in the transmission spectra of the metallic nanoparticles. Nanoparticles of very conductive metals like gold, silver, and copper are ideal materials for excitation of localized surface plasmons due to an extremely high ratio of the modulus of the real (Sr) to the imaginary parts (8i) of its dielectric constant. Silver and copper nanoparticles are prone to oxidation and therefore often require coatings of protective over layers. Gold nanoparticles are chemically stable and are employed for the development of devices based on plasmon resonances of nanoparticles. 

Conducting polymers also can be utilized to form core-shell structures with high dielectric constant particles. Fang et al. used PANl to encapsulate barium titanate via in situ oxidative polymerization. They examined the influence of the fraction of BaTiOs particles on the ER behavior, and found that the PANl/ BaTiOs compo-sites-based ERFs exhibit a better ER effect than does pure PANl, which result might be due to the unique ferroelectric properties as well as the high dielectric constant of BaTiOs nanoparticles. 

Cui, T.H. and Liang, G.R., Dual-gate pentacene organic field-effect transistors based on a nanoassembled SiOj nanoparticle thin film as the gate dielectric layer, Appl. Phys. Lett. 86 (6), 064102, 2005. 

Dielectric elements that are based on nanostructures are of recent interest for the scaling-down of DRAMs (dynamic random access memories) [11.2]. The need to reduce capacitance requires materials with larger dielectric permittivity. One method to achieve this is to disperse conductive particles in a dielectric matrix by using nanoparticles, the dissipation factor is kept low. 

To demonstrate the influence of longitudinal coherent interactions we have investigated the transmission and reflection spectra of ID photonic crystals based on close-packed silver nanosphere monolayers separated by thin solid dielectric films. The strongest spectral manifestation of longitudinal electrodynamic coupling was shown [2] to take place in the case of joint electron and photonic confinements. In order to achieve it we chose intermonolayer film thicknesses Im so that the photonic band gap and the metal nanoparticle surface plasmon band could be realized at close frequencies in the visible. 

In the present work, we consider the two approaches for synthesis of nanoparticles designed for metal particles and being in the progress for ultraflne semiconductors. They allow to fabricate nanocomposites of the type nanoparticles-in-dielectrics with amorphous and crystalline matrices. The first one is based on the sol-gel technique producing dielectric silica films with nanoparticles incorporated within silica matrix [1]. Nanoparticles provide an optical response of the material due to the plasmon resonance [2] with variable spectral position and band shape. In the second approach nanoparticles are produced within the crystalline zeolite matrices which stabilize both the few-atomic clusters (e.g., Agg) and metal particles in the size range of 1-20 nm [3], Chemical routes of their synthesis admit easy control of size and optical properties. The metal nanoparticles in zeolites can be transformed into semiconductors without destroy of the zeolite matrix and with incorporation of zeolite microcrystals into transparent silica films. This construction... 

Optical and photonic properties. The efficiency of charge transfer over nanoscale distances and the quantum confinement of electrical carriers within nanoparticles are two factors that make nanomaterials optically different from bulk crystals. Nanophotonic properties can be linear and nonlinear, and can be finely tailored by controlling material dimensions and surface chemistry. Distinct color indicators may be based on surface plasmons the light output is dictated by the dielectric function of a nanomaterial and the shape of a nanoparticle. Specific needs include reactivity of nanoscale materials to electromagnetic radiation, including photoreactivity. 

---