Embedded nanocrystals

Meltzer et al. (2001) showed direct evidence for long-range interactions between lanthanide impurity ions in embedded nanocrystals with the TLS s of the matrix based on hole burning data. The samples were prepared as follows monoclinic 23 run Y2C>3 0.1% Eu3+ nanocrystals, produced by condensation after laser evaporation, were dispersed in a siloxane polymer. Fig. 13 shows the temperature dependence of the width of spectral holes (khb), which were burned in the 5Do <- 7Fo transition of Eu3+ ions located on the c sites (582.8 nm)innanociys-tals embedded in polymer, in comparison with the case of the free, as-prepared nanocrystals having the same size. The embedded nanocrystals exhibit a drastic increase in hole width and a very different power-law behavior for the temperature dependence of the hole widths relative to similar isolated nanoparticles. The 7 3 temperature dependence of the hole width... 

Fig. 7. TEM picture of iron nanocrystals collected from the chamber soot nanocrystals are embedded in amorphous carbon globules. On the surface of some core crystals, a few fringes with 0.34-0.35 nm spacing suggesting the presence of graphitic layers are observed, as indicated by arrows.

Fig. 6. Electrical characterization of (a) SiGe nanocrystals (b) Ni nanocrystals embedded in HfD2 from MOS capacitors.

The collagen fibers leave small compartments where apatite nanocrystals are deposited during a controlled biomineralization process [20]. The collagen acts as a structural framework in which plate-like nanocrystals of carbonated hydroxyapatite (CHA) are embedded to strengthen the bone. The chemical formula of biological CHA can be represented as follows ... 

Nanocrystals of Si trapped in some matrix form an attractive system for device fabrication when compared with Jt-Si, because of the increased surface stability and material rigidity. Visible EL has been observed, for example, from Si nanocrystals embedded in films of a-Si H78 and from an electrochemically-formed nanocrystalline Si thin film deposited on SnCL.79 In the latter case the p-i-n LED at room temperature emitted orange-red light (1.8 eV) that was readily visible to the eye. The light emission is ascribed variously to near surface states78 and the quantum size effect.79 Also, infrared emission near 1.1 eV has been obtained from a room-temperature EL device comprised of Si nanocrystals embedded within a Si-rich Si02-x matrix.80 The PL from this structure has an external quantum efficiency of 10 3. Substantial progress in the development of such nanocrystalline-Si EL structures can be expected over the next few years. 

Optical absorption in M nanocrystals embedded in dielectric matrix depends on characteristics of matrix and interface between matrix and nanocrystals. In the classical model of Mie only macroscopical dielectric permeability of environment e 2 is taken into account [16]. In this model charges at the M nanocrystal surface are determined by s2 and so frequency coa corresponding to a peak of resonant absorption is defined from a relation [18]. 

According to UV-vis spectra [75,80] and X-ray diffraction [73] data the d value of Ag nanocrystals embedded in PPX and C1PPX films by cryochemi-cal synthesis is 5nm. At the same time X-ray diffraction pattern of a layered Ag-CIPPX system obtained by successive deposition of monomer and Ag at 77 K, followed by cryopolymerization and annealing at 293 K, has shown the value d T2nm [73]. So d of Ag nanocrystals in a polymerized co-condensate is determined mainly, by steric restrictions on the crystal growth in the interior of solid matrix. 

Similar histograms were determined by TEM for Pb-, Zn-, and Cd-containing nanocomposite PPX films prepared by vapor deposition cryochemical synthesis [85]. The value d of metal nanocrystals in these films is also 5nm. The same approximately size d ( 4.5nm) has been evaluated from Ai/2 of X-ray diffraction peak for semiconductor PbS nanocrystals in PbS-PPX nanocomposite [71]. It should be particularly emphasized that d value of M/SC nanocrystals embedded by cryosynthesis in PPX and C1PPX matrices does not depend on M/SC content as for low loading (0.2-2 vol.% for Ag in PPX and C1PPX [75, 80] and 0.01-1 vol.% for Pb in PPX [85]) and for high loading (5-11 vol.% for PbS in PPX [3, 71, 86]) systems. 

Lanthanide-doped nanocrystals embedded in amorphous matrices... 

Although optical spectra of lanthanide-doped insulating nanociystals embedded in amorphous matrices are very similar to the free-standing nanocrystal counterparts, their excited state dynamics behaves very differently from that in simple nanocrystals. Some distinct dynamic properties have recently been found for nanocrystals embedded in polymers or glasses. Simple models for the interaction between lanthanide ions and the matrices were also proposed. However, further studies are needed in order to quantitatively understand the observed size-dependence and dynamic mechanisms. 

Fig. 13. Temperature dependence of the hole width for Eu + in Y2O3 nanocrystals. Solid lines y > T (embedded in polymer) and y > (isolated, as prepared) (reprinted with permission from Meltzer et al. (2001)).

---