Absorption and Photoluminescence Spectroscopy

The luminescence properties of nanoscale diamond have as well been subject to extensive study. For nanoscopic particles, as compared to the respective bulk material, deviating characteristics are generally expected due to the large portion of surface atoms and a potentially distorted band structure. Yet for diamond, the bandgap is unaffected by particle dimensions (at least in the relevant range), and the luminescence of the nanomaterial has many features in common with that of the bulk phase. 

Defects and impurities, in general, play a comparably important role for the luminescence properties of nanodiamond like they do for the bulk material. Owing to their existence, there are electronic states situated within the bandgap, which allow for inducing luminescence in nanodiamond samples also with longer wave radiation. Upon excitation with wavelengths between 300 and 365 nm, fluorescence bands are observed at more than 400 nm. They arise from various nitrogen defects. In comparison to bulk diamond, the Ufetime of the excited states is rather short, which possibly is due to the effect of surface states and to the increased density of excitons on the surface. 

The so-called N-V-centers (nitrogen-vacancy centers) constitute very interesting defects of the diamond lattice. As described in Section 5.2.1, they consist of a nitrogen atom incorporated into the lattice and an adjacent vacancy. Fluorescence in the red to infrared range of the spectmm can be induced by excitation with 

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Numerous spectroscopic methods have been applied to examine the physical properties and to elucidate the structure of carbon onions. They include IR- and Raman spectroscopy. X-ray diffraction, electron energy loss spectroscopy (EELS), absorption, and photoluminescence spectroscopy and NMR-spectroscopy. Each of these methods gives account of certain aspects of the geometric and electronic structure, so altogether quite a detailed picture is obtained of the situation in carbon onions and related materials. There is, however, a strong dependency on... 

Nanometric particles of InP were readily prepared by the decomposition of an indium phosphide complex, In(PBu 2)3, at 167 °C in 4-ethylpyridine. The resulting material shows marked quantum confinement effects, and was investigated using optical absorption and photoluminescence spectroscopies, and TEM. " " Similar precursors were used... 

The formed colloidal solutions with nanoparticles were characterized by optical absorption and photoluminescence spectroscopy for monitoring the changes in the plasmon absorption characteristics and luminescence properties, transmission electron microscopy (TEM) and X-ray diffraction (XRD) in order to analyze the final size and structure of nanoparticles. The absorption spectra of the colloids were recorded with a UV-visible spectrophotometer (CARY 500) using a 1-cm-pathlength-quartz cell for the absorption measurements. 

The CD with the larger ring, tj-CD 59 (Fig. 18), was employed to thread SWNTs (MER Co., USA), resulting in solubilizing them in water and separating them with respect to diameters [141]. However, more experimental evidence such as Raman, absorption and photoluminescence spectroscopies is considered to be required to confirm firmly the diameter-based separation in the paper by Dodziuk et al. (2003) [ 141]. 

Two separate experimental approaches, diffuse reflectance and photoluminescence spectroscopy, were then taken both led to similar results. The latter technique is the more sensitive, and well-resolved spectra can often be observed, but only when a radiative decay of the excited state occurs. The diffuse reflectance spectra are broader in scope but the absorption bands appear as shoulders. The reflectance spectra of alkaline earth oxides were examined by Zecchina et al. (77, 78), Garrone et al. (79), and Zecchina and Scarano (80), but an overpressure of a quenching gas (usually oxygen) had to be used to suppress the fluorescence and to allow observ ation of the reflectance absorption bands (Fig. 10). In addition to usual bands in the U V region due to bulk excitations (bulk cxcitons), new absorption bands which correspond to excitations localized on the surface ions are present. 

For these purposes different spectral methods have been widely used, among them the UV-visible absorption and emission spectroscopy proved to be very informative [2, 3], The investigation of the ground-state absorption and photoluminescence emission spectra of adsorbed probe molecules have been successfiil in the study of the electron-and charge-transfer reactions within zeolites. 

In addition to the usual microscopy techniques typically employed in nanotechnology research (e.g. TEM, SEM, AFM) several techniques are particularly suitable for characterizing the geometric and electronic structures of SWNT. Here, we briefly describe the type of structural information that can be obtained firom scanning tunneling microscopy and spectroscopy (STM/STS), Raman spectroscopy, optical absorption, and photoluminescence. 

A series of four platinum acetylide complexes that contain 4-ethynylstilbene (4-ES) ligands have been subjected to a detailed photochemical and photophysical investigation [83]. Using absorption, variable temperature photoluminescence, and transient absorption spectroscopy, UV-vis absorption, and NMR spectroscopy, it was shown that these compounds undergo trans-cis photoisomerization from the triplet excited state. The obtained experimental data indicated that in all of the complexes, excitation led to a high yield of a 3n,n excited state that is localized on... 

DUV spectroscopy includes several techniques, such as absorption and scattering spectroscopy, vibrational spectroscopy, photoluminescence, and plasmonically enhanced spectroscopy. This spectroscopic technique has been applied to materials that specifically interact with photons with DUV energies. The methods are not confined to component analysis, but have also been extended to microscopy (Chap. 7) and nanoscopy (near-field microscopy. Chap. 8), although the field is still in its infancy. 

The photoelectronic properties of poly(dihexylgermane) were investigated by photoluminescence spectroscopy, after one- and two-photon absorption. The spectra were compared with those of the analogous poly(dihexylsilane)111. 

In addition to the characteristic XRD patterns and photoluminescence, UV-visible and X-ray absorption spectra, another fingerprint thought to indicate lattice substitution of titanium sites was the vibrational band at 960 cm-1, which has been recorded by infrared and Raman spectroscopy (33,34). Although there is some controversy about the origin of this band, its presence is usually characteristic of a good TS-1 catalyst, although it turned out to be experimentally extremely difficult to establish quantitative correlations between the intensity of the 960 cm-1 band and the Ti content of a Ti silicate and/or its catalytic activity. 

While shifts due to size quantization have most commonly been seen in absorption spectroscopy, other spectroscopies, such as photoelectrochemical photocurrent, photovoltage (using a vibrating Kelvin probe), photoluminescence, and photoconductivity spectroscopies have all shown quantum shifts in various CD films. 

The results of infrared absorption measurements on GaAstMn prepared by the solid-state diffusion method are also in good agreement with the A°(d5+h) center model (Linnarsson et al. 1997). According to infrared spectroscopy and photoluminescence (PL) measurements for GaAstMn with a Mn concentration of 1018 cm-3, this acceptor level is located 113 meV above the top of the valence band (Chapman and Hutchinson 1967 Ilegems et al. 1975). Two photoluminescence (PL) lines observed by Liu et al. (1995) in... 

A complete and satisfactory characterization of quantum dots prepared by any of these methods requires many of the same techniques listed for metal nanoparticles described already (see above). In addition to critical electronic properties, photoluminescence spectroscopy is an extremely valuable tool to obtain preliminary information on size and size distribution of quantum dots, which can in many cases (i.e., for larger sizes and quasi-spherical shapes) be estimated from 2max and the full width at half maximum (fwhm) of the absorption or emission peak using approximations such as Bras model or the hyperbolic band model [113]. 

In addition to the IR, Raman and LIBS methods previously discussed, a number of other laser-based methods for explosives detection have been developed over the years. The following section briefly describes the ultraviolet and visible (UV/vis) absorption spectra of EM and discusses the techniques of laser desorption (LD), PF with detection through resonance-enhanced multiphoton ionization (REMPI) or laser-induced fluorescence (LIF), photoacoustic spectroscopy (PAS), variations on the light ranging and detecting (LIDAR) method, and photoluminescence. Table 2 summarizes the LODs of several explosive-related compounds (ERC) and EM obtained by the techniques described in this section. 

Photoluminescence excitation spectroscopy (PLE) is generally used to identify the excited-state structure in quantum wells. For GalnN/GaN quantum wells, Im et al [14] used PLE to study single wells of various widths. Similarly to the results from absorption, electro-absorption, and electroreflectance measurements, a large Stokes shift of the onset of the PLE spectrum with respect to the dominating photoluminescence peak was observed at low temperature [14]. 

Figure 19.5 Schematic diagram showing decomposition of total phosphorescence enhancement of PtOEP on silver films into absorption enhancement E X. ) and emissive rate enhancement E (%.2) based on the photophysical model described in the text and data from steady state and transient spectroscopy of PtOEP films with various thicknesses and excitation wavelengths as labeled. The lines represent the possible combinations that could explain the experimentally observed changes in photoluminescence where each position on the line represents a different choice of fQ, the fraction of the excited states that are quenched nonradiatively by interactions between the molecule and the metallic surface. The blue shaded region on the vertical axis is the range of possibilities allowed by constraints from extinction and excitation spectra as explained in the text. The dotted oval is what we believe to be the most likely decomposition for the 6 nm films characterized in Figure 19.4 as discussed in the text. Reprinted from reference 45 with permission of the American Chemical Society.

When the sample is stimulated hy application of an external electromagnetic radiation source, several processes are possible. For example, the radiation can be scattered or reflected. What is important to us is that some of the incident radiation can be absorbed and thus promote some of the analyte species to an excited state, as shown in Figure 24-5. In absorption spectroscopy, we measure the amount of light absorbed as a function of wavelength. This can give both qualitative and quantitative information about the sample. In photoluminescence spectroscopy (Figure 24-6), the emission of photons is measured after absorption. The most important forms of photoluminescence for analytical purposes are fluorescence and phosphorescence spectroscopy. 

We have shown via absorption, PL, morphology-dependent PL, PDS, photoluminescence excitation, and time-resolved photoluminescence spectroscopy that exciplexes form at the PFB F8BT heterojunction. We note that exciplexes of poly-fluorenes with triphenylamine monomers have been observed recently [34]. 

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