Nanocrystals electronic structure

This first example of the utility of quadrupolar nuclei as sources of information about nano-semiconductors showed that MAS-NMR could identify a poly type that is unstable in the bulk. A second example will show that information about changes in the electronic structure of nanocrystals as a function of synthesis and treatment conditions, as well as information about different polytypes, can be obtained via measurement of Knight shifts and chemical shifts. In this extensive study 69Ga... 

Charge transport through an array of semiconductor nanocrystals is strongly affected by the electronic structure of nanocrystal surfaces. It is possible to control the type of conductivity and doping level of quantum dot crystals by adsorbing/desorbing molecular species at the nanocrystal surface. As an... 

Electronic absorption spectroscopy has played a pivotal role in the development of methods for synthesizing pure semiconductor nanocrystals. Nanocrystal sizes, size distributions, growth kinetics, growth mechanisms, and electronic structures have all been studied in detail using electronic absorption spectroscopy. 

In this section, we discuss the application of several physical methods not as analytical techniques, but as probes of the electronic structures and functionally relevant properties of doped inorganic nanocrystals. 

Extensive investigations of metal nanocrystals of various sizes obtained, for example by the deposition of metals on amorphised graphite and other substrates, by X-ray photoelectron spectroscopy and related techniques 4-51 have yielded valuable information on their electronic structure. An important result from these experiments is that as the metal particle size decreases, the core-level binding energy of metals such as Au, Ag, Pd, Ni and Cu increases sharply. This is shown in the case of Pd in Figure 2, where the binding energy... 

Theoretical calculations of the electronic structure of metal and semiconductor nanocrystals throw light on the size-... 

We have hitherto discussed in the earlier sections, electronic structure and properties, chemical reactivity and self-assembly of nanocrystals, particularly those of metals. Hie discussion should suffice to illustrate how size if a crucial factor in deciding the chemistry in the nano regime. These size dependent properties form the basis of nanoscience, where the properties are exploited for possible applications. 

A consequence of absorption in the ultraviolet for many nanocrystals is the emission of the absorbed light as luminescence. In these cases, photoluminescence spectroscopy can yield both electronic structure information as well as details of excited electronic... 

Efros AL, Rosen M (2000) The electronic structure of semiconductor nanocrystals. Annual Rev. Mater. 

As already pointed out, the most direct consequence of a reduction in the nanocrystallite size on the electronic structure of semiconducting materials is a pronounced increase in the band gap due to the quantum confinement effect. While there are several ways to quantitatively understand this phenomenon from a theoretical standpoint, the experimental determination of the band gap variation as a function of size is most directly performed by ultraviolet-visible absorption spectroscopy, with the experimental absorption threshold corresponding to the direct band gap in the material. As the band gap shifts to higher energy, the blue-shift in the absorption edge signals the formation of progressively smaller sized nanocrystals. Therefore, UV-Vis absorption spectroscopy has played an immensely important role in the study of these systems and we discuss the essential aspects in Section 11.3. 

The UV-Vis absorption process generates an excited state of the system by generating an electron in the conduction band and a hole in the valence band this state most often de-excites via a radiative process, giving rise to fluorescence. The emission characteristics depend on various aspects of the electronic structure of the nanocrystallites. Thus, the tuning of the fluorescence wavelength in nanocrystals can be achieved by a variety of methods as discussed in Section 11.4. 

Photoemission experiments that probe the electronic structure of the nanocrystals are indispensable if one wishes to gain insight into the electronic structure-property relationship. Though very few studies have been carried out on semiconducting nanocrystalline systems to date, techniques such as photoemission and X-ray absorption spectroscopies are of immense value in probing the electronic structure and also in verifying various theories proposed for the nanocrystals. In Section 11.6 we discuss these spectroscopic studies. 

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