Band-gap variation

Shaukat, A. 1990. Composition-dependent band gap variation of mixed chalcopy-rites. J. Phys. Chem. Solids 51 1413-1418. 

Fig. 16.4 (a) The crystal ceiis of anatase Ti02. (b) View of the octahedra-packing of anatase Ti02 the thick red lines indicate the axis in representative octahedra which, under stress, results in a change of band gap. (c) Band gap variation in anatase Ti02 under hydrostatic, epitaxial, and uniaxial stress. Adapted from Yin etal. [72] with kind permission from American Institute of Physics (2010). 

Figure 4. Band gap variations calculated by using equation 1 and experimental data for poly(dimethylsilane) (a) and a-Si H-a-Si N H superlattices (b). (Data are from refs. 20 and 21.)...

Figure 10. Band gap variation with alkyl-side-chain substitution. Abbreviations are defined as follows HO, highest occupied LU, lowest unoccupied HH, polysilane HMe, poly(monomethylsilane) HEt, poly(monoethylsilane) MeMe, poly(dimethylsilane) MeEt, poly(methylethylsilane) and MePr, poly-...

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. 

EMA gives a good description of the band gap variation vith size. However, it grossly overestimates the change, Eg, in the band gap for smaller nanocrystals. 

Ramakrishna and Friesner [57] calculated the band structure of CdS and GaP nanocrystals up to 30 A radius in the zinc-blende phase. The calculated variation agrees quite well with the experimental data points. The band gap variation curve in the case of GaP shows a red shifted band gap in the case of very small nanocrystals. This has not yet been experimentally verified. 

In view of the flexibility of the perovskite stracture, many modifications are possible. The band gap of these phases and thus the absorption spectrum of the perovskite and its light-harvesting efficiency can be tuned by doping the organic and the halogen sites. The perovskites CH3NH3Pb(Ij Br )3 have a band gap variation given by... 

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