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Exciton quantum size effects

Here, n is the exciton effective mass parallel to the c axis and Lz is the (average) thickness of the WS2 nested structure (Lz=nx 0.6.2 nm, where n is the number of WS2 layers) in the nanoparticle. In a previous study of ultrathin films of 2H-WSe2, AEg of the A exciton was found to obey Eq. (1) over a limited thickness range. The parameter AEg exhibited a linear dependence on 1/Z for Lz in the range of 4-7 nm and became asymptotically constant for Lz> 8 nm (91). A similar trend is observed for IF-WS2 and MoS2, as shown in Fig. 23 (90). Therefore, the quantum size effect... [Pg.301]

As the radius of a semiconductor crystallite approaches the exciton-Bohr-radius its electronic properties begin to change, whereupon quantum size effects can be expected. The Bohr radius ub of an exciton is given by... [Pg.233]

Colloidal CdS particles 2-7 nm in diameter exhibit a blue shift in their absorption and luminescence characteristics due to quantum confinement effects [45,46]. It is known that particle size has a pronounced effect on semiconductor spectral properties when their size becomes comparable with that of an exciton. This so called quantum size effect occurs when R < as (R = particle radius, ub = Bohr radius see Chapter 4, coinciding with a gradual change in the energy bands of a semiconductor into a set of discrete electronic levels. The observation of a discrete excitonic transition in the absorption and luminescence spectra of such particles, so called Q-particles, requires samples of very narrow size distribution and well-defined crystal structure [47,48]. Semiconductor nanocrystals, or... [Pg.432]

Cadmium sulfide suspensions are characterized by an absorption spectrum in the visible range. In the case of small particles, a quantum size effect (28-37) is observed due to the perturbation of the electronic structure of the semiconductor with the change in the particle size. For the CdS semiconductor, as the diameter of the particles approaches the excitonic diameter, its electronic properties start to change (28,33,34). This gives a widening of the forbidden band and therefore a blue shift in the absorption threshold as the size decreases. This phenomenon occurs as the cristallite size is comparable or below the excitonic diameter of 50-60 A (34). In a first approximation, a simple electron hole in a box model can quantify this blue shift with the size variation (28,34,37). Thus the absorption threshold is directly related to the average size of the particles in solution. [Pg.219]

Lanthanides doped into nanocrystalline semiconductors have been the subject of numerous investigations in the past decades. If the size of a semiconductor particle is smaller than the Bohr radius of the excitons, the so-called quantum confinement occurs. As a result, the band gap of the semiconductor increases and discrete energy levels occur at the edges of the valence and conduction bands (Bol et al., 2002 Bras, 1986). These quantum size effects have stimulated extensive interest in both basic and applied research. [Pg.134]

Generally, quantum size effects are not expected in lanthanide-doped nanoinsulators such as oxides since the Bohr radius of the exciton in insulating oxides, like Y2O3 and Gd2C>3, is very small. By contrast, the exciton Bohr radius of semiconductors is larger (e.g., 2.5 nm for CdS) resulting in pronounced quantum confinement effects for nanoparticles of about 2.5 nm or smaller (Bol et al., 2002). Therefore, a possible influence of quantum size effects on the luminescence properties of lanthanide ions is expected in semiconductor nanocrystals. [Pg.134]

Size quantization effects in metals or semiconductors have attracted considerable attention in the past decade [104—107]. Semiconductor nanoparticles may experience a transition in terms of electronic properties from those typical for a solid to that of a molecule, in which a complete electron delocalization has not yet occurred. These quantum-size effects arise when the Bohr radius of the first exciton (an interacting electron-hole pair) and the semiconductor becomes comparable with or larger than that of the particle the Bohr radius [94]... [Pg.7]

Schmidt H. M. and Weller H. (1986), Photochemistry of colloidal semiconductors. 15. Quantum size effects in semiconductor crystallites—calculation of the energy spectrum for the confined exciton , Chem. Phys. Lett. 129, 615-618. [Pg.205]

Quantum Size Effects. The exciton radiative recombination can make a substantially larger contribution to the emission of AgBr and possibly AgCl... [Pg.166]

The energy spectra predicted by the hydrogenic model is a series of sharp and discrete levels right up to the continuum i.e. the conduction band. Real exciton spectra, however, have a finite, tenperature dependent width (and line shape) and the peak positions are a function of size of the quantum dot . This is the consequence of the finiteness of the nanocrystal (a macroscopic crystal is infinite) whose boundaries present a potential barrier for the motion of the carrier and whose size could be of the order of og. These quantum size effects as well as the confinement of carriers (either together or separately) are the basic phenomena, the consequences of which are to be understood and exploited in excitonics [3]. [Pg.321]

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,... [Pg.322]

Since bulk CdS shows free-exciton emission at low temperatures [34], it is interesting to compare these results on CdS with the discussion in Sect. 3.3.9b on the transition from semiconductors to insulators. There it was shown that narrow-line free-exciton emission transforms into broad-band localized emission, if the amount of delocalization of the excited state decreases. Since the valence band to conduction band transition of CdS is in principle a - Cd " " charge-transfer transition, this would bring the discussion on CdS in line with results from a different origin (see Sect. 3.3.9b). By all means the case of Cd32S 4(SC(,Hs)35.DMF4 is a nice example of luminescence research on a well-defined cluster showing the quantum-size effect. [Pg.217]


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See also in sourсe #XX -- [ Pg.183 , Pg.188 ]




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