Photoluminescence size-dependent

Figure 17.1 (A) S ize-dependent photoluminescence colorofZnS-shelled CdSe quantum dots. (B) Schematic presentation of size in A, color, and photoluminescence spectral maxima of CdSe quantum dots. (C) Size-dependent absorption (solid lines) and photoluminescence (broken lines) spectra of CdSe quantum dots. Reprinted with permission from references [4] (A) and [5] (C) copyright [1997, 2001], American Chemical Society.

Gudiksen, M. S. Wang, J. Lieber, C. M. 2002. Size dependent photoluminescence from single indium phosphide nanowires. J. Phys. Chem B 106 4036 4039. 

Currently silicon is still one of the most important semiconductors as it is the basis of any computer chip. It exhibits an indirect band gap of 1.1 eV at room temperature in the microcrystalline phase. Similar to Ge, silicon nanoparticles show a size-dependent photoluminescence. It was reported by Katayama el al. that a thin Si layer can be electrodeposited in l-ethyl-3-methylimidazolium hexafluorosilicate at 90 °C [44], However, upon exposure to air the deposit reacted completely to SiC>2, which makes it difficult to decide whether the deposit was semiconducting or not. Recently, we showed for the first time that silicon can be well electrodeposited from SiCU in the air and water stable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([BMPJTfiN) [45, 46]. This ionic liquid can be... 

Semiconductor nanoparticles have been extensively studied in recent years owing to their strongly size-dependent optical properties. Among these nanomaterials, CdS and PbS are particularly attractive due to their nonlinear optical behavior and unusual fluorescence or photoluminescence properties [ 136,137]. A number of studies have been published recently regarding the preparation of CdS, PbS and ZnS nanoparticles in inverse microemulsion systems [138-143]. In these works, NP-5/NP-9 was the most commonly used surfactant and petroleum ether the most commonly used oil. The aqueous phase for each inverse microemulsion consisted of cadmium nitrate (0.1 M) and ammonia sulfide (0.1 M) respectively. CdS was recovered from the mixture of double microemulsions [141]. Electron microscopy revealed that the spherical particles were aroimd 10-20 nm in diameter, as seen in Fig. 14. 

Fig. 10.1 Size-dependent photoluminescence of CdTe NCs synthesized in water (2—5 nm size range, the smallest particles emit green, the largest red, quantum yield is up to 40% [3])-...

The dispersion and assembly behavior of silicon (Si) nanoclusters are controlled in organic and aqueous suspensions. Hydrogen-terminated Si nanoclusters are stably dispersed in non-polar solvents, but assemble on aqueous suspension surfaces. The nanoclusters spontaneously pack into lattice arrangements when a slow assembly rate is maintained. In addition, converting the nanocrystal surface to carboxylic acid termination stably disperses the nanocrystals in aqueous suspensions. They show size-dependent photoluminescence (PL) from yellow to green in the suspensions, while partly oxidizing the surface causes blue PL. 

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... 

The conductivity of Si and Ge can be modified by doping with group HI or V elements that lead to p- or n-type materials. Junctions of n- and p-doped Si are interesting for photovoltaic applications. Quantum confinement effects, such as the size-dependent photoluminescence of semiconductor nanoparticles, ate especially interesting for electronic and optoelectronic devices. 

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. 

Fig. 17.1 Scalable fabrication of p-Si modulates the color of the PL upper panel) showing the size dependency of emission wavelength (reprinted with permission from [22]). Experimental solid stars) and computed open circles) a first coordination numbers CN) of Si atoms b experimental photoluminescence PL) peak positions symbols)-, c experimental length of structural coherence symbols), d experimental Raman peak position symbols)-, e experimental quantum yield symbols), and f A(Si-Si-Si) bond angle distributions symbols) as a function of particle size. Solid litres in red) are nonhnear fits to the respective experimental data (reprint with permission from [26]. Copyright (2013) American Chemical Society)...

Figure 17.8 shows the BOLS reproduction of the measured size dependence of [79] (a) photoabsorption energy with inset showing an offset to lit hEpJ EpA = 0 (b) photoluminescence energy pl (c) bandgap Eq, and (d) Stokes shift... 

H. Shinojima, J. Yumoto, N. Uesugi, S. Omi, Y. Asahara, Microcrystallite size dependence of absorption and photoluminescence spectra in CdS Sei-x-doped glass Appl. Phys. Lett. 55 (1989) 1519-1521. 

The optical reflectance spectra were dependent on the nanocrystallite structure and dimensions, porosity and the layer thickness (Fig. 9.2). The maximal photosensitivity in the visible wavelength range of the spectra (30-35 mA/Lm) was typical of the sNPS layers with the nanocrystallite dimensions of 15 nm, and it decreased with increasing size of the nanocrystallites. The maximal sensitivity to the ultraviolet irradiation was obtained for sNPS layers with nanocrystalhte dimensions of 20-25 nm. sNPS layers obtained by electrochemical etching as well as by chemical etching showed the photoluminescence typical of this material a broad peak in the visible spectrum with the intensity sufficient for observation of the photoluminescence with a naked eye. sNPS samples obtained by chemical or electrochemical etching had intensive emission with the maximum at A, 640 nm and 700 nm. 

Time-resolved measurements of photogenerated (very intense illumination, up to 0.56 GW/cm ) electron/hole recombination on CD (selenosulphate/NTA bath) CdSe of different crystal sizes has shown that the trapping of electrons, probably in surface states, occurs in ca. 0.5 ps, and a combination of (intensity-dependent) Auger recombination and shallow-trapped recombination occurs in a time frame of ca. 50 ps. A much slower (not measured) decay due to deeply trapped charges also occurred [102]. A different time-resolved photoluminescence study on similar films attributed emission to recombination from localized states [103]. In particular, the large difference in luminescence efficiency and lifetime between samples annealed in air and in vacuum evidenced the surface nature of these states. 

Depending on the kind of synthesis, these quantum dots can be prepared or size separated into batches covering almost the entire visible spectral range from 400 to 750 nm with, in part, high photoluminescence quantum efficiencies (some stable in air [106], others not [107]). Weller et al. reported on a very efficient synthesis for hydrophilic, thiol-capped CdTe quantum dots [108,109], which can be transformed to lipophilic, alkanethiol-stabilized CdTe quantum dots using a place exchange reaction similar to that for metal nanoparticles described above [110]. A related strategy has also been successfully employed to produce hydrophobic or otherwise functionalized CdS [111] or CdSe quantum dots [112] (Fig. 1). 

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