Photoluminescence quantum size effect

Small gold clusters (<100 atoms) have become the subject of interest because of their use as building blocks of nanoscale devices and because of their quantum-size effects and novel properties such as photoluminescence, magnetism, and optical activity [427]. 

Pickering and co-workers observed visible photoluminescence (PL) from PS at 4.2 K in 1984 [Pil], which they interpreted as due to a complex mixture of amorphous phases. The questions of why PS is transparent for visible light and why it is photoluminescent remained unanswered until 1990-91 when a quantum size effect was proposed as an explanation [Cal, Lei]. Two years later PL was also found for oxidized PS [Lel5, It2]. These astonishing optical properties of PS initiated vigorous research and resulted in more than a thousand publications, as well as several books and reviews [Cu2, Th7]. 

Photoluminescence spectra of the films were measured (77 K) and compared with an epitaxial PbSe layer in the same study. Blue shifts in the spectra (greater for the selenourea films) were attributed to quantum size effects (see Chap. 10). The crystal size was reported to be 40-60 nm (the selenourea ones being somewhat smaller than the selenosnlphate ones), growing to 100-150 nm after annealing. 

Studies on luminescence of CdS colloids provide useful knowledge on the energy and nature of recombination sites of charge carriers in the colloidal particles. The regularities of the colloid photoluminescence quenching provide the information on the dynamics of electrons and holes in semiconductor particles as well as on the kinetics of interfacial electron transfer. Of a particular interest are studies on the luminescence of colloidal solutions of the so-called Q-semiconductors, their properties depending on the size of semiconductor particles due to the quantum size effects. 

Photoluminescence could be due to the radiative annihilation (or recombination) of excitons to produce a free exciton peak or due to recombination of an exciton bound to a donor or acceptor impurity (neutral or charged) in the semiconductor. The free exciton spectrum generally represents the product of the polariton distribution function and the transmission coefficient of polaritons at the sample surface. Bound exciton emission involves interaction between bound charges and phonons, leading to the appearance of phonon side bands. The above-mentioned electronic properties exhibit quantum size effect in the nanometric size regime when the crystallite size becomes comparable to the Bohr radius, qb- The basic physics of this effect is contained in the equation for confinement energy,... 

The nanocrystalline semiconductors, PbS and CuS, were prepared by y-irradiation at room temperature in an ethanol system by Qiao et al. (1999). Carbon disulfide was used as the sulfur source lead acetate and copper chloride were used as metal ion sources. The purity and compositions of the products were examined by x-ray photoelectron spectroscopy. The photoluminescence property of as-prepared PbS was further studied. A blue shift observed in the PL spectra indicated the quantum size effect on nanocrystalline PbS. 

In 1990, Canham observed intense visible photoluminescence (PL) from PSi at room temperature. Visible luminescence ranging from green to red in color was soon reported for other PSi samples and ascribed to quantum size effects in wires of width 3 nm (Ossicini et al, 2003). Several models of the origin of PL have been developed, from which we chose two. In the first (the defect model), the luminescence originates from carriers localized at extrinsic centers that are defects in the silicon or silicon oxide that covers the surface (Prokes, 1993). In the second model (Koch et al., 1996), absorption occurs in quantum-confined structures, but radiative recombination involves localized surface states. Either the electron, the hole, both or neither can be localized. Hence, a hierarchy of transitions is possible that explains the various emission bands of PSi. The energy difference between absorption and emission peaks is explained well in this model, because photoexcited carriers relax into surface states. The dependence of the luminescence on external factors or on the variation of the PSi chemistry is naturally accounted for by surface state changes. 

Conjugated polymer superlattices and porphyrin arrays connected with molecular wires are superstructured materials, which exhibit unique optical and photonic functions. The former shows a shift in photoluminescence to higher energy which is interpreted as a quantum size effect. The latter class of materials exhibits photoconductivity by a hole carrier mechanism and photoinformation storage by a localized excitation mechanism. The syntheses of these two classes of materials are described. 

These observed photoluminescence spectral properties of conjugated polymer heterolayers fabricated by the present PPEP method appear to be due to quantum size effects. However, additional studies of these materials will be necessary to establish the true origin of their properties. In any case, these results also suggest that many other novel structures of functional materials and devices can be fabricated by this method. 

As a result, the photoluminescence output rises dramatically and the peak position is blue-shifted toward 780 nm (Fig. 9). Because the removal of Si constitutes an overall decrease in Si skeleton, this experience was considered as an indirect evidence that visible room temperature photoluminescence could be attributed to quantum size effect [6]. 

The demonstration in 1990 that porous silicon could emit efficient tunable visible photoluminescence (PL) at room temperature and attributed to quantum-size effects in crystalline silicon (Canham 1990) has induced considerable worldwide research activities in order to (i) identify the various PL bands and their respective properties and emission mechanisms, (ii) optimize the PL efficiency, (iii) optimize the PL stability, and (iv) tailor the PL spectrum (peak wavelength and FWHM). This chapter reviews briefly the specificities of porous silicon PL measurements, the PL of individual silicon nanocrystals from porous silicon, and the PL of porous silicon layers. 

The Si nanocrystals exhibit photoluminescence upon irradiation with UV light at 230 nm. The MPL spectrum is shown in Figure 10. The spectrum is similar to that reported for 4 nm Si nanocrystals upon excitation with 350 nm at 20 K and also to that PL spectrum of Porous Silicon (49). In these systems the red luminescence is interpreted as a consequence of quantum crystallites which exhibit size-dependent, discrete excited electronic states due to a quantum effect (6,50,51). This quantum confinement shifts the luminescence to higher energy than the bulk crystalline Si (1.1 eV) band gap. This indirect gap transition is dipole forbidden in the infinite preferred crystal due to translational symmetry. By relaxing this symmetry in finite crystallite, the transition can become dipole allowed. As pointed out by Brus (49), the quantum size effect in Si nanocrystals is primarily kinetic mainly due to the isolation of electron-hole pairs from each other. 

Quantum confinement effects of nanocrystals are evidenced most clearly in the optical properties of the system, as the electronic energy levels of the clusters become a function of size (a detailed account of this aspect is provided in Chapter 5). The basic optical characterization of semiconductor nanocrystals provides important information on particle size - from the position of the band gap energy, and the size distribution - from the sharpness of peaks in absorption and luminescence. Figure 3.25 shows the room-temperature absorption spectra for a series of InAs nanocrystal sizes, along with the photoluminescence spectra. The quantum confinement effects are clearly evident from the size-dependent nature of the spectra, with the band gap in all samples being shifted substantially from the bulk InAs gap of 0.42 eV. In all samples, the absorption onset is characterized by a distinct feature at... 

Silicon nanoparticles (Si NPs) with sizes in the order of bulk exciton Bohr radius [1, 2] present interesting optical properties for fluorescent labeling in biological imaging applications with their potential nontoxicity [3-6], However, the origin of their photoluminescence has been subjected to intense debate for almost two decades. This debate has been focused on whether quantumatomic-scale defects at the surface of the nanocrystals are responsible for the light emission [7]. 

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