Photoluminescence experiments

EXAMPLE 1.5 The sensitivity of luminescence. Consider a photoluminescence experiment in which the excitation source provides a power of 100 ptW at a wavelength of400 nm. The phosphor sample can absorb light at this wavelength and emit light with a quantum efficiency of r] = O.I. Assuming that kg = 10 fii.e., only one-thousandth of the emitted light reaches the detector) and a minimum detectable intensity of l(f photons per second, determine the minimum optical density that can be detected by luminescence. 

The samples with a nanolayer of 15-18 nm thickness had the maximal photosensitivity. This resnlt is in agreement with the results of the photoluminescence experiments (Fig. 9.4). 

Photoluminescence experiments with ni-V wafers of Ino.5o(Gao.9o A1o.io)o.5oP were conducted 114]. The Lewis basic gaseous analytes ammonia, methylamine, dimethyl amine, and trimethylamine all yielded reversible PL enhancements. The Lewis acid sulfur dioxide, in contrast, caused reversible quenching of the semiconductor s PL intensity. These PL intensity changes were consistent with analyte-induced modifications of the dead-layer thickness. 

Potential-dependent photoluminescence experiments provide information not only on quantum efficiencies but also on radiative or non-radiative recombination centres. Luminescence images of electrodes can be produced with the same set-ups that are used for photocurrent or microwave conductivity imaging, with the additional installation of a photodetector or photomultiplier for the measurement of luminescence. As with other imaging experiments, the observed images may show a luminescence distribution that depends strongly on applied electrode potential (see Section 12.8). 

Formation of big combined biomolecular structures in a buffer solution was verified by photoluminescence experiments during which amyloid specific dye (thioflavin T) was mixed into the colloidal solution. Thioflavin T related increase in the intensity of fluorescence was detected for the solutions with both GDH-Ap40 and Trx-Ap40 hybrid proteins compared to the solutions with single biomolecular components. The intensity increase was obtained after incubation of protein solutions at 310 K in PBS buffer (pH = 7.4) for two days. It was supposed that this period was required for formation of fibrils because the increase in the intensity of fluorescence was comparable with the results obtained for the solution with lysozyme fibrils. 

Here, Mo -O denotes one of the two molybdenyl bonds of a tetrahedrally coordinated Mo ion on the silica surface. ( Mo -0 ) stands for a short-lived charge-transfer excited state which is formed upon absorption of UV-light quantum (hv). The excited state was earlier detected in photoluminescence experiments. I is the light intensity, and is the rate constant for deactivation of the excited state. Reaction (3) is the interaction of CO with the excited state to yield Mo and CO2. Reaction (4) is quenching of the excited state ( Mo -0 ) by NO molecules without formation of reaction products. Reaction (4) is of importance for the kinetic consideration. 

Equations (10)-(12) can be further reduced to equations (13)- 15) assuming that 2K 1, i.e., that NO is a stronger queneher than CO. This assumption was found to be valid from the results of photoluminescence experiments which will be reported elsewhere. 

The effect of a high-temperature thermal treatment has been analyzed from X-ray diffraction, Raman and photoluminescence experiments on polythiophene samples synthesized from a-bis silylated thiophene monomers [20]. It has been shown that treatment at an elevated temperature of polythiophene led to significant... 

In order to characterize poly(3-alkylthiophene) prepared from silylated monomers, room-temperature photoluminescence experiments have been performed on polymers in solution (THF) [132]. Four samples have been studied poly(3-alkylthiophene) synthesized from hydrogenated and silylated monomer, respectively (the polymers are called pATh-1, 1 Table 14.14) and poly(3-alkylthiophene) synthesized from die two isomeric mono silylated monomer pATh-3,4 (Table 14.14). 

Fig. 5.11 Schematic representation of the sample geometry for a photoluminescence experiment on a square cell containing solutions with different absorbance at the excitation wavelength a low absorbance (<0.1) and b high absorbance (>2.0). Legend exc, exerting light em, emitted light C, cell F, slit R, detector...

The samples are usually prepared as thin films deposited on a suitable support (e.g., a glass slide), plates, or powders. However, by utilizing special accessories (e.g., optical fiber probes) and instrumental setups, in principle one can perform photoluminescence experiments on any object. As a consequence of the front-face geometry and the low transparency of the sample, it is often the case that luminescence measurements on solids involve only the outer part of the sample, for a depth on the order of a few micrometers or less. Therefore, particularly in the case of powdered samples, it is important to obtain a homogeneous dispersion of the granules and deposit it as a thin layer on a transparent support, such as glass or quartz, potentially utilizing an inert medium like mineral oil. 

Interesting photoinduced electron-transfer reactions in mixed films of r-conjugated polymers and a homologous series of tetracyano-p-quinodimethane (TCNQ) derivatives have been recently reported [135]. Photoinduced absorption (PIA) and photoluminescence experiments have been carried out to study the photoexcitation of poly[2-methoxy-5-(2 -ethylhexy-loxy)-l,4-phenylenevinylene] (MEH-PPV) [136] and poly[3-(2-(3-methylbutoxy)ethyl)thiophene]... 

In addition to the radiative processes, there are nonradiative processes in semiconductors because of imperfections that act as nonradiative centers. We should mention some defects as radiative recombination centers, which in a photoluminescence experiment can shed light on the energy levels of defect states. For a semiconductor containing nonradiative traps or recombination centers, in an experiment such as time-dependent PL, the decay in the integrated PL intensity versus temperature is related to the low-temperature integrated PL intensity as... 

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