For nanocrystals smaller than the Bohr exciton radius (~ a few nanometers), energy levels are quantized, with values directly related to the Quantum dot size. These phenomenon are called quantum confinement, hence the name “Quantum dots” (QDs). Semiconductor QDs are single crystals a few nanometers in diameter whose size and shape can be precisely controlled by the duration, temperature, and capping ligand molecules used in the synthesis process [1-2]. Otherwise, QDs, also as nanocrystals, are a special class of materials known as semiconductors, which are composed of periodic groups of II-VI, III-V, and IV-VI materials. The various compositions of QDs have shown the emitting from the UV to the infrared range, which depend on the band gap of QDs. Figure 1.1 represent QDs core materials scaled as a function of their emission wavelength superimposed over the spectrum [3].
Figure 1.1 Represent QDs core materials scaled as a function of their emission wavelength superimposed over the spectrum [3].
These QDs have composition and size dependent absorption and emission (as shown in Figure 1.2) [3-10]. When absorption a photon with energy above the semiconductor band gap energy would result in the creation of an electron-hole pair (or exciton).
Figure 1.2 Quantum dots can be synthesized from various types of semiconductor materials (II-VI: CdS, CdSe, CdTeI; III-V: InP, InAsI;
IV-VI: PbSeI) characterized by different bulk band gap energies. The curves represent experimental data from the literature on the dependence of peak emission wavelength on QDs diameter. The range of emission wavelength is 400 to 1350 nm, with size varying from 2 to 9.5 nm (organic passivation/solubilization layer not included). All spectra are typically around 30 to 50 nm (full width at half maximum).
Inset: Representative emission spectra for some materials. Data are from [3-10]. Data for CdHgTe/ZnS have been extrapolated to the maximum emission wavelength obtained in Michalet’s group.
According to Figure 1.3 [3], the broading absorption spectrum supported a wide excitation source chose. The radiative recombination of exciton leads to the emission of a photon in a narrow, symmetric energy band (shown in Figure 1.3). Bulk semiconductors only could display a rather uniform absorption spectrum; on the contrary, QDs would appear as a series of overlapping peaks in the absorption range.
Owing once more to the discrete nature of electron energy levels in QDs, each peak corresponds to an energy transition between electron-hole (exciton) energy levels.
Figure 1.3 Absorption (upper) and emission (lower) spectra of four CdSe/ZnS qdot samples. The blue vertical line indicates the 488-nm line of an argon-ion laser, which can be used to efficiently excite all four types of QDs simultaneously [3].
Furthermore, the QDs would not absorb light that has a wavelength longer than that of the first exciton peak. Therefore, the wavelength of the first exciton peak is a function of the composition and size of the quantum dot. The emission peak is bell-shaped (Gaussian) and occurs at a slightly longer wavelength than the lowest energy exciton peak (absorption peak). Consequently, the fluorescence or absorption could be tunable by synthesized process, such as temperature, growth time et al.
Unfortunately, surface defects in the crystal structure act as temporary traps for the electron or hole, hindering their radiative recombination. If the alternation of trapping and untrapping events results in intermittent fluorescence visible at the single molecule level [11] and reduces the overall quantum yield, which is the ratio of emitted to absorbed photons. Overcoming these shortcomings, and protecting surface atoms from oxidation and other chemical reactions, is to grow a shell of few atomic layers of a material with a larger band gap on top of the nanocrystal core. The properties of core/shell QDs are highly dependant on the inorganic surface passivation and obey the principles of quantum well. Using the organic surfactant (e.g. TOPO or TOP) passivated and stabilized on the CdSe QDs (as shown in Figure 1.4) could not improve the drawback in the fluorescence efficiency [12]. As a result of imperfect surface passivation and rearrangement of the surface atoms, incomplete quantum confinement effect on the surface may takes place. Nevertheless, overcoating nanocrystal core with inorganic materials of higher band offsets has been to improve the photoluminescence quantum yield (QY). It could improve and eliminate
the nonradiative recombination sites on the surface.
Figure 1.4 Nanocrystal surrounded by TOPO chains anchored to its surface [12].
As illustrated in Figure 1.5 (a-c) [13-15], ZnS or CdS shell capped CdSe QDs would provide higher quantum efficiency and photo stability due to the confinement of the carrier on the edge of the energy band offsets, and consequently reducing the loss of the carrier by trapping or tunneling [13-15]. This shell could be designed carefully to obtain quantum yields close to 90% [16]; this step also enhances QDs photostability by several orders of magnitude relative to conventional dyes [17]. As briefly noted above, QDs are made from inorganic semiconductors and have novel optical properties that can be used to optimize the signal-to-background ratio. Additionally, QDs have very large molar extinction coefficients in the order of 0.5-5×106 M cm-1[18], which makes them bringhter than organic dye. In photostability, QDs are several thousand times more stable against photobleaching than
organic dyes (shown in Figure 1.6) and are thus well suited for continuous tracking studies over a long period of time [19].
Figure 1.5 Bared CdSe QDs and inorganic surface passivation core/shell QDs (a) organic matrix (b) CdS shell (c) ZnS passivation on CdSe surface [13-15].
c
a b
Figure 1.6 Photobleaching curves showing that QDs are several thousand times more photostable than organic dyes (e.g. Texas red) under the same excitation conditions [19].