In addition to semiconductor heterostructures, highly luminescent colloidal quantum dots (QDs) or semiconductor nanocrystals (NCs) are attracting increasing interest in science, with potential for use in diverse fields, including light-emitting diodes [34], photodetectors [35], biosensing [36], and biolabeling [37]. Very recently, increasing research effort has been devoted to the new generation of solar cells which contain NCs as active materials [38-40].
Colloidal QDs are easily synthesized with low energy and material consumption [41]. They are perfect building blocks for controlled nano assemblies on a molecular scale, such as spatially ordered structures [42] or colloidal supercrystals [43]. The typical length scale of QDs is responsible for chemical and physical properties that differ markedly from those of both their bulk and molecular counterparts. The properties of QDs depend on their size, shape and surface effects. Over the last two decades, extensive studies that have applied optical techniques to CdSe QDs.
Energy transfer between semiconductor NCs has attracted substantial interest in recent years [44-51]. Förster resonance energy transfer (FRET) is a nonradiative process that is driven by dipole–dipole interactions [52-54]. The efficiency of FRET depends on the degree of spectral overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor, and on the sixth power of the separation between the donor and acceptor pair.
The rate of nonradiative energy transfer according to the Förster theory is given by the formula [54],
where τD represents the excited-state radiative lifetime of the donor in the absence of transfer and R0 is the Förster critical radius. R0 (in angstrom) is given by
( ) ( )
162 4 4
0 0.2108 D 0 D A
R = ⎡⎢⎣κ Φ n−
∫
∞I λ ε λ λ λd ⎤⎥⎦ , (2)where κ2 is the orientational factor and depends on the relative orientation of the donor and acceptor dipoles. The values of κ2 range from 0 (perpendicular) to 4 (collinear). ΦD denotes the fluorescence quantum yield of the donor in the absence of transfer; n is the average refractive index of the medium in the wavelength range where spectral overlap is significant;
ID(λ) is the fluorescence spectrum of the donor normalized such that
∫
0∞ID( )
λ λd =1; εA(λ) is the molar absorption coefficient of the acceptor, and λ is the wavelength in nanometers.The transfer efficiency is given by According to Eq. (3), the dipole-dipole coupling mechanism is most sensitive to the donor-acceptor distance when this distance is comparable to the Förster critical radius. The characteristics of FRET can be adopted as a “spectroscopic ruler” [55]. FRET has been widely employed in vivo and in vitro biological studies, such as the monitoring of DNA hybridization and sequencing, protein conformation studies, and diffusion dynamics [55].
In contrast, radiative transfer is a two-step process: an acceptor absorbs a photon that is emitted by a donor. This process occurs when the average distance between the donor and acceptor exceeds the wavelength. Such a transfer does not involve any interaction between particles. Radiative transfer results in a decrease of the donor fluorescence intensity in the region of spectral overlap. The fraction a of photons emitted by donors and absorbed by acceptors is given by where CA is the molar concentration of acceptors [54].
The FRET process in semiconductor nanocrystals has recently attracted great interest.
Bawendi’s group was the first to demonstrate the FRET process between close-packed CdSe
QDs [44,45].Crooker et al. elucidated FRET dynamics in monodisperse, mixed-size, and layered assemblies of CdSe/ZnS QDs using time-resolved and spectrally-resolved PL [46].
They all found an enhancement in luminescence and lifetime of the acceptor that was accompanied by a reduction of both of the donor. These phenomena are direct evidence of energy transfer from small to large QDs, eliminating the reabsorption effect, which would not increase the decay rate from small QDs. Very recently, Feldmann’s group also studied the cascaded FRET in a funnel-like structure and FRET in layer-by-layer assemblies of CdSe and water-soluble CdTe QDs [47-51]. The FRET process in an aqueous solution of QDs, which holds much promise for use in biological studies, is limited by the inter-dot distance and the spectral overlap between the donor emission and the acceptor absorption. Chapter 7 compares in detail the electronic energy transfer behavior of mixed CdTe QDs solution with that of the solid using time-resolved and spectrally-resolved PL.
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