Quantum dots experimental

The birth of the field of carbon nanotubes is marked by the publication by lijima of the observation of multi-walled nanotubes with outer diameters as small as 55 A, and inner diameters as small as 23 A, and a nanotube consisting of only two coaxial cylinders [2]. This paper was important in making the connection between carbon fullerenes, which are quantum dots, with carbon nanotubes, which are quantum wires. FurtheiTnore this seminal paper [2] has stimulated extensive theoretical and experimental research for the past five years and has led to the creation of a rapidly developing research field. 

In conclusion, wc have shown the interesting information which one can get from electrical resistivity measurements on SWCNT and MWCNT and the exciting applications which can be derived. MWCNTs behave as an ultimate carbon fibre revealing specific 2D quantum transport features at low temperatures weak localisation and universal conductance fluctuations. SWCNTs behave as pure quantum wires which, if limited in length, reduce to quantum dots. Thus, each type of CNT has its own features which are strongly dependent on the dimensionality of the electronic gas. We have also briefly discussed the very recent experimental results obtained on the thermopower of SWCNT bundles and the effect of intercalation on the electrical resistivity of these systems. 

The above theoretical derivation of a quantum dot does not spare to experimentally find and characterize... 

Theoretical investigations of AEV and AEC have indicated that the ratio AEV/AEC is greater than 1 [Zh2, Vo2, Ne2] and may be as high as 3 for the case of hydrogenated silicon clusters [Re3], In a similar calculation for thin silicon films, even-odd oscillations of A v according to the number of Si monolayers have been found [Zh2], For the latter case the ratio AEy/AEc showed values between 1 and 2. The results of a study [Wa5] including the decrease in the static dielectric constant with size are close to the experimentally [Bu2] observed values of about 2 for the ratio AEy/AEc, as shown in Fig. 7.16. In this work [Wa5], it is concluded that the electron-hole pair is confined by the physical dimension of the quantum dot, not by Coulomb attraction. 

In a study that addressed the effect of doping on quantum dots, the donor and acceptor levels were found to be practically independent of particle size [De3]. In other words, shallow impurities become deep ones if the dot size is reduced. Experimental observations show that the luminescence is not affected by doping if a thermal diffusion process, for example using a POCl3 source, is used [Ell]. Implantation, in contrast, is observed to effectively quench the PL [Tal4]. If the pores are filled with a medium of a large low-frequency dielectric constant, such as water or any other polar solvent, it is found that deep impurity states still exist,... 

There are two very broad, general conclusions resulting from the above review. The first is that MoS2-type nanoparticles are very different than other types of semiconductor nanoparticles. Nanoparticles of several different types of semiconductors, such as CdSe, CdS, and InP, have been extensively studied. Experimental and theoretical studies have elucidated much of their spectroscopy, photophysics, and dynamics. The results reviewed above are, in many places, in sharp contrast with those obtained on other types of quantum dots. This does not come as a surprise. The properties of the bulk semiconductor are reflected in those of the nanoparticle, and properties of layered semiconductors are vastly different from those of semiconductors having three-dimensional crystal structures. Although the electronic and spectroscopic properties of nanoparticles are strongly influenced by quantum confinement effects, the differences in the semiconductors cause there to be few generalizations about semiconductor quantum dots that can be made. 

In recent years much effort has been spent on the development of experimental techniques to grow well defined nanoscale materials, due to their possible applications in nanometric electronic devices. Indeed the creation of nanowire field effect transistors [128-132], nano-sensors [133,134], atomic scale light emitting diodes and lasers [135,136], has been made possible by the development of new techniques, which allow one to control the growth processes of nanotubes, nanowires and quantum dots. Of particular importance, among the different atomic scale systems experimentally studied, are... 

A detailed discussion of experimental in vitro and in vivo testing methodologies and results is not the purpose of this chapter. There are some comprehensive reviews covering this topic. For instance, the contribution by Oberdorster et al. [45] who have summarized recent data and highlighted gaps in this field two works [60, 61] review data on environmental and human effects of carbon nanotubes in relation to their properties a paper [62] that discusses toxicological endpoints of combustion-derived nanoparticles a review [63] of quantum dots toxicity an excellent review devoted to toxicity of particular nanomaterials classes by Borm et al. [26] and many others. 

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... 

One can say that the obtained by us experimental results upon 2D exciton localization (taking place due to the growth of the crystal dielectric permeability anisotropy parameter) with o are very close to [27] where the behaviour of polaron excitons in parabolic quantum dots were considered and shown that the dot size decrease results in increasing the exciton binding energy. 

Before discussing the experimental evidence, it is worthwhile to consider lasing-related properties of quantum dots from a fundamental point of view. The theoretical description of the optical gain in bulk and quantum well semiconductors is discussed in Datareview C5.3. 

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