Photoluminescence

Photoluminescence is the radiation emitted by the recombination process and as such is a direct measure of the radiative transition. Information about non-radiative recombination can often be inferred from the luminescence intensity, which is reduced by the competing processes (Street 1981a). The most useful feature of the luminescence experiment is the ability to measure the emission spectrum to obtain information about the energy levels of the recombination centers. The transition rates are found by measuring the transient response of the luminescence intensity using a pulsed excitation source. Time resolution to about 10 s is relatively easy to obtain and is about the maximum radiative recombination rate. The actual recombination times of a-Si H extend over a wide range, from 10 s up to at least 10- s. 

The absolute luminescence efficiency is difficult to measure precisely and can typically be obtained only within a factor 2. The relative efficiency in two samples can be measured much more accurately, provided that care is taken with the sample configuration. There are strong optical interference effects in the luminescence of thin a-Si H 

Liuninescence measurements are usually made at low temperature because the competing non-radiative transitions are enhanced at elevated temperatures. At room temperature the luminescence intensity is low and almost undetectable. The intensity is almost constant below about 50 K, so that a low temperature of 10-20 K is adequate for the measurements. 

The photoluminescence spectra of SCPs can be greatly tuned through the molecular design of their chemical structures. The photoluminescence can vary from a UV light to a red light. 

Because they have the largest band gap among SCPs, poly(3,6-dibenzosilole) s 5 are expected to show fluorescence at the shortest wavelength. Under 

Highly efficient green photoluminescence has also been realized from SCPs. Copolymers 11 (Fig. 5) derived from 2,7-fluorene and 2,3,4,5-tetraphenylsilole show absolute PL quantum yields up to 84%.28 A well-defined alternating copolymer 12 with a repeating unit made up of ter-(2,7-fluorene) and 2,5-silole possesses an absolute PL quantum yield 80%.29 SCPs 13 with a main chain structure of 3,6-carbazole-2,7-fluorene-2,5-silole also show absolute PL quantum yields up to 86%.30 An energy transfer copolymer 14 of 2,7-dibenzosilole and 

3-benzothiadiazole displays a green emission with an absolute PL quantum yield of 52%.26 

Copolymers 15 (Fig. 6) derived from 2,7-fluorene and 2,5-dithienylsilole show red fluorescence via an energy transfer process.31 The APl could be 591 nm for copolymers with higher contents of 2,5-dithienylsilole. The absolute PL quantum yields ( 30%) of the copolymers are somewhat lower than the green fluorescent SCPs. A copolymer 16 derived from 2,7-dibenzosilole and 4,7-dithienyl-2,l,3-benzothiadiazole show a better red fluorescence.26 The APL of the copolymer is at 629 nm, with an absolute PL quantum yield of 53%. 

Perhaps the most informative variant of the time-resolved techniques is the one based on photoluminescence (91). The time resolution can extend below the picosecond regime. In addition to band-gap emission one can monitor the emission due to specific recombination centers. The disadvantages are that one is confined to materials that emit iight and the sensitivity is such that most of the work is reported under high injection conditionswherethesystemwasdriventofiatband. The one dimensional continuity equation under these conditions was solved and the experimental results with CdS were analyzed to yield the surface recombination velocity that was found to be affected by the choice of electrolyte (91). The time-resolved study of cathodic and anodic electroluminescence of ZnO in the /isec time scale was reported (92). 

The broadening of the absorption tail in Fig. 4 with decreasing layer thickness shows that the layered structure also affects the distribution of 

Although the photon energy of the peak emission in the photoluminescence increases with decreasing layer thickness, the magnitude of the change (0.1 eV) is smaller than the shift in the optical gap (0.4 eV) for the same series of samples, presumably because the photoluminescence is associated with localized states that are relatively insensitive to the layered structure. Since the width of the emission band also increases (from 0.3 to 0.5 eV FWHM) as the layer thickness is reduced, the magnitude of the shift of the 

We conclude from these data that the distribution of localized states broadens as the layer thickness decreases. At this point, we do not have enough information to know whether this broadening of the localized-state distribution is due to new localized states associated with the interfaces or whether it is simply a consequence of the fact that the band-tail states become progressively more strongly localized farther from the band edges. In the latter picture, the superlattice potential has a relatively small effect on the energy of the strongly localized states that are deep in the band tails and a 

Unlike the solutions of 1-3, the chloroform solution of 4 emits a green light at 523 nm with a Opp of 2.8%. Because its monomer is a weak emitter when molecu-larly dissolved, the green light observed here thus should be associated witii the backbone emission. We also checked the AIE property of 4 by adding menthol into its chloroform solution. The PL intensity can, however, barely be enhanced even when 90 vol % menthol is added. The reason 4 is AIE inactive may be similar to that 

Aggregation normally quenches light emission what is the cause for this abnormal AIE phenomenon To address this question, we designed and carried out more experiments. When a dilute dioxane solution of 3 (lOjiM) is cooled, the intensity of its PL spectrum is progressively increased in a nonlinear fashion (Fig. 6A). When cooled from room temperature to below the melting point (11, Z°C) of the solvent, the liquid solution changes to a solid glass. The intramolecular rotations or 

To separate die cooling effect from the glass effect, we choose dichlo-romethane (DCM), a liquid wifli much lower melting point, for the PL measurement. The PL intensity of die solution increases with a decrease in temperature in a nearly hnear fashion (Fig. 6B). This enhancement in emission must be due to the restricted intramolecular rotation caused by cooling-induced conformation freezing because the melting point of the solvent (-95°C) is lower than the lowest temperature we tested for this solution (-78°C). Similar to 3, polymers 2,4, and 7 also show much stronger emission when their solutions are cooled. 

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Empedocles S A, Norris D J and Bawendi M G 1996 Photoluminescence spectroscopy of single CdSe nanocrystallite quantum dots Phys. Rev. Lett. 77 3873-6... 

Blanton S A, Hines M A and Guyot-Sionnest P 1996 Photoluminescence wandering in single CdSe nanocrystals App/. Phys. Lett. 69 3905-7... 

Experimentally, local vibrational modes associated witli a defect or impurity may appear in infra-red absorjrtion or Raman spectra. The defect centre may also give rise to new photoluminescence bands and otlier experimentally observable signature. Some defect-related energy levels may be visible by deep-level transient spectroscopy (DLTS) [23]. 

Photoluminescent spectra for methyltetrahydrofolate and the enzyme methyltransferase. When methyltetrahydrofolate and methyltransferase are mixed, the enzyme is no longer photoluminescent, but the photoluminescence of methyltetrahydrofolate is enhanced. (Spectra courtesy of Dave Roberts, DePauw University.)... 

The release of a photon following thermal excitation is called emission, and that following the absorption of a photon is called photoluminescence. In chemiluminescence and bioluminescence, excitation results from a chemical or biochemical reaction, respectively. Spectroscopic methods based on photoluminescence are the subject of Section lOG, and atomic emission is covered in Section lOH. 

Standardizing the Method Equations 10.32 and 10.33 show that the intensity of fluorescent or phosphorescent emission is proportional to the concentration of the photoluminescent species, provided that the absorbance of radiation from the excitation source (A = ebC) is less than approximately 0.01. Quantitative methods are usually standardized using a set of external standards. Calibration curves are linear over as much as four to six orders of magnitude for fluorescence and two to four orders of magnitude for phosphorescence. Calibration curves become nonlinear for high concentrations of the photoluminescent species at which the intensity of emission is given by equation 10.31. Nonlinearity also may be observed at low concentrations due to the presence of fluorescent or phosphorescent contaminants. As discussed earlier, the quantum efficiency for emission is sensitive to temperature and sample matrix, both of which must be controlled if external standards are to be used. In addition, emission intensity depends on the molar absorptivity of the photoluminescent species, which is sensitive to the sample matrix. 

Nonradiative Decay. To have technical importance, a luminescent material should have a high efficiency for conversion of the excitation to visible light. Photoluminescent phosphors for use in fluorescent lamps usually have a quantum efficiency of greater than 0.75. AH the exciting quanta would be reemitted as visible light if there were no nonradiative losses. 

The range of uses of mercuric iodide has increased because of its abiUty to detect nuclear particles. Various metals such as Pd, Cu, Al, Tri, Sn, Ag, and Ta affect the photoluminescence of Hgl2, which is of importance in the preparation of high quaUty photodetectors (qv). Hgl2 has also been mentioned as a catalyst in group transfer polymerization of methacrylates or acrylates (8). 

The dichloride of molybdenum(II) [13478-17-6] M0CI2, contains Mo CF g core units (Fig. 6c) having chloride bridges in its soHd-state stmcture. Similar or identical hexanuclear units are known in soluble species such as Mo3Ch 24 other derivatives containing the Mo CF g core. These compounds have been under investigation because of their photochemical and photoluminescent activity (see Photochemical technology) (36,37). The hexanuclear... 

Turning to non-metallic catalysts, photoluminescence studies of alkaline-earth oxides in dre near-ultra-violet region show excitation of electrons corresponding to duee types of surface sites for the oxide ions which dominate the surface sUmcture. These sites can be described as having different cation co-ordination, which is normally six in the bulk, depending on the surface location. Ions on a flat surface have a co-ordination number of 5 (denoted 5c), those on the edges 4 (4c), and dre kiirk sites have co-ordination number 3 (3c). The latter can be expected to have higher chemical reactivity than 4c and 5c sites, as was postulated for dre evaporation mechanism. 

In photoluminescence one measures physical and chemical properties of materials by using photons to induce excited electronic states in the material system and analyzing the optical emission as these states relax. Typically, light is directed onto the sample for excitation, and the emitted luminescence is collected by a lens and passed through an optical spectrometer onto a photodetector. The spectral distribution and time dependence of the emission are related to electronic transition probabilities within the sample, and can be used to provide qualitative and, sometimes, quantitative information about chemical composition, structure (bonding, disorder, interfaces, quantum wells), impurities, kinetic processes, and energy transfer. 

Photoluminescence finds its greatest strengths as a qualitative and semiquantitative probe. Quantification based on absolute or relative intensities is difficult, although it is useful in applications where the sample and optical configurations may be carefully controlled. The necessary conditions are most easily met for analytical applica-... 

Photoluminescence is a well-established and widely practiced tool for materials analysis. In the context of surface and microanalysis, PL is applied mostly qualitatively or semiquantitatively to exploit the correlation between the structure and composition of a material system and its electronic states and their lifetimes, and to identify the presence and type of trace chemicals, impurities, and defects. 

K. D. Mielenz, ed. Measurement of Photoluminescence, vol. 3 of Optical Ractiation Measurements. (R Grum and C. J. Bardeson, eds.) Academic Press, London, 1982. A thorough treatment of photoluminescence spectrometry for quantitative chemical analysis, oriented toward compounds in solution. 

Band gaps in semiconductors can be investigated by other optical methods, such as photoluminescence, cathodoluminescence, photoluminescence excitation spectroscopy, absorption, spectral ellipsometry, photocurrent spectroscopy, and resonant Raman spectroscopy. Photoluminescence and cathodoluminescence involve an emission process and hence can be used to evaluate only features near the fundamental band gap. The other methods are related to the absorption process or its derivative (resonant Raman scattering). Most of these methods require cryogenic temperatures. 

For applied work, an optical characterization technique should be as simple, rapid, and informative as possible. Other valuable aspects are the ability to perform measurements in a contactless manner at (or even above) room temperature. Modulation Spectroscopy is one of the most usehil techniques for studying the optical proponents of the bulk (semiconductors or metals) and surface (semiconductors) of technologically important materials. It is relatively simple, inexpensive, compact, and easy to use. Although photoluminescence is the most widely used technique for characterizing bulk and thin-film semiconductors. Modulation Spectroscopy is gainii in popularity as new applications are found and the database is increased. There are about 100 laboratories (university, industry, and government) around the world that use Modulation Spectroscopy for semiconductor characterization. 

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