Photoluminescent materials, spectra

On the other hand, in the excitation spectrum, the emission intensity 4, at the monitored emission band, is plotted as a function of the wavelength A of the excitation light, w hich varies as the extinction coefficient of the absorbing molecules. Therefore, the excitation spectrum exhibits the same spectral appearance as that of the absorption spectrum. The advantage of measuring the excitation spectrum in addition to the emission spectrum is the greater sensitivity even for low concentrations of photoluminescent material compared to standard absorption measurements. 

Interestingly, it has been argued that nanoparticulate formation might be considered as a possibility for obtaining new silicon films [379]. The nanoparticles can be crystalline, and this fact prompted a new line of research [380-383], If the particles that are suspended in the plasma are irradiated with, e.g., an Ar laser (488 nm), photoluminescence is observed when they are crystalline [384]. The broad spectrum shifts to the red, due to quantum confinement. Quantum confinement enhances the bandgap of material when the size of the material becomes smaller than the radius of the Bohr exciton [385, 386]. The broad PL spectrum shows that a size distribution of nanocrystals exists, with sizes lower than 10 nm. 

Figure 11.2. Nanowire electronic and optical properties, (a) Schematic of an NW-FET used to characterize electrical transport properties of individual NWs. (inset) SEM image of an NW-FET two metal electrodes, which correspond to source and drain, are visible at the left and right sides of the image, (b) Current versus voltage for an n-type InP NW-FET. The numbers inside the plot indicate the corresponding gate voltages (Vg). The inset shows current versus Vg for Fsd of 0.1 V. (c) Real-color photoluminescence image of various NWs shows different color emissions, (d) Spectra of individual NW photoluminescence. All NW materials show a clean band-edge emission spectrum with narrow FWHM around 20nm. (See color insert.)...

The zone of recombination can be very small as was shown by Aminaka et al. [225] by doping only a thin layer (5 nm) in the device by a red emission material. By observing the ratio of host and dopant emission, the authors were able to show that the recombination zone of the device was as thin as 10 nm. The emitted light is usually coupled out at the substrate side through the transparent anode. As a rule, the electroluminescence spectrum does not differ much from the photoluminescence spectrum. 

The optical reflectance spectra were dependent on the nanocrystallite structure and dimensions, porosity and the layer thickness (Fig. 9.2). The maximal photosensitivity in the visible wavelength range of the spectra (30-35 mA/Lm) was typical of the sNPS layers with the nanocrystallite dimensions of 15 nm, and it decreased with increasing size of the nanocrystallites. The maximal sensitivity to the ultraviolet irradiation was obtained for sNPS layers with nanocrystalhte dimensions of 20-25 nm. sNPS layers obtained by electrochemical etching as well as by chemical etching showed the photoluminescence typical of this material a broad peak in the visible spectrum with the intensity sufficient for observation of the photoluminescence with a naked eye. sNPS samples obtained by chemical or electrochemical etching had intensive emission with the maximum at A, 640 nm and 700 nm. 

We would like to mention the fascinating triboluminescence239 of uranyl nitrate, where the emitted spectrum looks like photoluminescence, whereas several other triboluminescent materials crushed in nitrogen or neon give the characteristic spectra of electric discharges in the surrounding gas. 

Fig. 12.4 Photoluminescence of host materials and absorption spectrum of DCM2 (a), and absorption coefficients of the host materials (b). The vertical line indicates the pump wavelength.

The absorption and photoluminescence spectra of both materials are shown in Fig. 12.7 (c,d). The luminescence of p-scxiphcnyl is very similar to the spectrum of the spiro-derivative, only the vibronic sidebands are more pronounced. This shows that aside from the improved morphological stability, the spiro-linkage merely leaves the optical properties of the chromophore unaltered. 

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