Quantum dots spectra

Figure 8.11a shows steady-state absorption spectra of the CdTe quantum dots in water. Each spectrum in the figure exhibits a distinct peak at a different band corresponding to its size, indicating that all of these suspensions include mono-dispersed nanocrystals. This mono-dispersibility is also supported by their emission spectra with different peak bands corresponding to particle size, as in Figure 8.11b. 

Discrete energy levels are to be observed for position (a) as well as for position (b) at exactly the same values, in case (b) somewhat better expressed than in (a). The level spacing is 135 mV. This spectrum clearly identifies the Au55 cluster as a quantum dot in the classical sense, having discrete electronic energy levels, though broader than in an atom, but nevertheless existent. The description of such quantum dots as artificial, big atoms seems indeed to be justified. 

Fig. 17.10 Schafer et al.14 have observed single mode lasing from core shell CdSe/ZnS nano crystal quantum dots in a glycerine water mixture. The fluorescence spectrum (black line) showed clear peaks of WGM and single mode lasing (grey line) was observed for sufficiently small droplets ( 10 pm) and high pump laser intensities (53 mJ crrT2 in 10 ns pulses at 532 nm). Insert shows the droplet trapped between the electrodes. Reprinted from Ref. 14 with permission. 2008 American Chemical Society...

The energy spectrum of two electrons confined in a quasi-fwo-dimensional Gaussian potential has also been studied for the same set of the strengths of confinement as the corresponding quasi-one-dimensional cases, and are compared to them. The energy spectrum of the quasi-two-dimensional quantum dot is qualitatively different from that of the quasi-one-dimensional quantum dot in the small confinement regime. The origin of the differences is due to the difference in the structure of the internal space. 

Nano-scale and molecular-scale systems are naturally described by discrete-level models, for example eigenstates of quantum dots, molecular orbitals, or atomic orbitals. But the leads are very large (infinite) and have a continuous energy spectrum. To include the lead effects systematically, it is reasonable to start from the discrete-level representation for the whole system. It can be made by the tight-binding (TB) model, which was proposed to describe quantum systems in which the localized electronic states play an essential role, it is widely used as an alternative to the plane wave description of electrons in solids, and also as a method to calculate the electronic structure of molecules in quantum chemistry. 

Figure 5. Selected time-resolved a) UV-Vis spectra and b) Zn K-edge EXAFS k3%(k) spectra recorded during the formation of ZnO quantum dot nanoparticles by heating of a precursor solution at 70°C in presence of LiOH. The ageing time ranges from 1 to 40 minutes. At the top of b) is reported for comparison purpose the EXAFS spectrum of a crystalline ZnO reference.

Bottom The broad color spectrum of quantum dots that is now available. These nanoscale particles can be functionalized and attached to a variety of different chemical species for tagging purposes. The great advantages of quantum dots are that they can all be excited at the same wavelength and are very resistant to photobleaching. 

The size-dependent properties of nanoparticles differ greatly from the corresponding bulk materials. An example is the size quantization phenomenon commonly observed in II-VI and III-V inorganic semiconductor nanocrystals.6 During the intermediate transition towards that of the bulk metal (usually between 2 and 20 nm), localization of electrons and holes in a confined volume causes an increase in its effective optical band gap as the size of the nanoparticle decreases, observed as a blue shift in its optical spectrum. Bms predicted that there should also be a dependence on the redox potential for these same classes of quantum dots.7 Bard and coworkers showed this experimentally and have reported on the direct observation between the... 

The energy of the emitted photon depends also on the particle size (see equation 7.1). Lowering the particle size within the nanometre scale the maximum of emission spectrum of the quantum dot may shift within a full range of visible light (Figure 7.11). 

The ZnS nanotubes and nanorods were characterized by UV-visible absorption spectroscopy and PL spectroscopy. The inset in Fig. 2a shows the absorption spectrum of the ZnS nanotubes. The band appearing at 318nm is blue-shifted relative to that of the bulk ZnS (350 nm) [17]. Nanowires of ZnS of diameter 5 nm were reported to show an absorption maximum around 326 nm [18], An absorption band at 320 nm has been reported in the case of ZnS quantum dots [19], The PL spectrum of ZnS nanotubes given in the inset of Fig. 2b exhibits two bands, a weak blue emission at 485 nm and a strong green emission around 538 nm. The 485 nm band is attributed to zinc vacancies in the ZnS lattice. Emission bands at 470 nm [20] and 498nm [21] have been reported in ZnS nanobelts. The 538 emission band is similar to that reported for ZnS nanobelts [22] and is considered to result from vacancy or interstitial states [22,23]. 

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