Nanocrystals surface

Wlrile size distribution is important, control over tire nanocrystal surface is equally important. The best nanocrystal syntlieses provide avenues for nanocrystals to be purified, collected as powders, and tlien redissolved in appropriate solvents. This requires control over tire surface chemistry, in order to control tire solubility of tire nanocrystals. 

The simplest approach to understanding the reduced melting point in nanocrystals relies on a simple thennodynamic model which considers the volume and surface as separate components. Wliether solid or melted, a nanocrystal surface contains atoms which are not bound to interior atoms. This raises the net free energy of the system because of the positive surface free energy, but the energetic cost of the surface is higher for a solid cluster than for a liquid cluster. Thus the free-energy difference between the two phases of a nanocrystal becomes smaller as the cluster size... 

Katari, J. E. B., Colvin, V. L. and Alivisatos, A. P. (1994) X-Ray photoelectron-spectroscopy of CdSe nanocrystals with applications to studies of the nanocrystal surface./. Phys. Chem., 98, 4109 117. 

Figure 9.59 One of the first methods of preparing water-soluble QDs was to use thioacetic acid modification of the nanocrystal surface. This resulted in a negative charge on the surface of each dot that provides like charge repulsion of particles suspended in aqueous solution. The carboxylate group also could be used for conjugation with amine-containing molecules.

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... 

Figure 10.12. Device characteristics of PbSe nanocrystal FETs activated with hydrazine. (a) Plots of 7d and 7 2 versus VG at constant VDS = 40 V for an n-channel FET assembled from 8.1-nm PbSe nanocrystals. L = 8 xm, W = 2300 tm. (b) ID versus VG plot at constant VDS = IV for an ambipolar FET assembled from 8.1-nm PbSe nanocrystals. L = 8 xm, W = 2300 xm. (c) Plots of 7D and 7 2 versus VG at constant Fps = -40V for a p-channel FET assembled from 8.2-nm PbSe nanocrystals. L = 10 tm, W = 3000 xm. (d) Plot of 7D versus FDS, as a function of VG for a p-channel FET assembled from 8.4-nm PbSe nanocrystals. L = 8 xm, W = 2300 xm. The changes in the transistor polarity were induced by controllable adsorption/desorption of hydrazine molecules from the nanocrystal surface. Reproduced from Ref.68, Copyright 2005, with permission from the AAAS.

Thus, the authors theorized that the photorelease was initiated by electron transfer from the CdSe nanocrystal surface to 78 to form the anion of 78, which undergoes cis-trans isomerization, followed by lactonization to release 79. The authors suggested that photorelease from CdSe nanocrystals may have potential use in the areas of drug delivery and imaging. 

This chapter deals mainly with quantum size effects in CD nanocrystalline films. However, another, quite separate property of such films is related to the large percentage of atoms located on the surface of the nanocrystals of these films, e.g. —50% for a crystal size of a few nm this is the effect of adsorption of molecular and ionic species on the nanocrystal surfaces. This aspect has been dealt with much less than has size quantization therefore, it constitutes only a very small part of this chapter, mainly Section 10.2.3, which discusses the effect of adsorbed water on CD CdSe films. Section 9.2.2.2 deals in somewhat more detail with this particular issue. 

The significance of these solvation intermediates lies in their relationship to intermediates along the growth pathway to internally doped nanocrystals, since these data reveal the thermodynamic stability of tetrahedral surface-bound Co2+ ions. Binding of impurity ions to nanocrystal surfaces is a necessary step in doping a growing nanocrystal. The absence of a detectable intermediate between... 

The solvation of transition metal ions bound to the surfaces of nanocrystals clearly relates to the thermodynamics of their interaction with the surface. It is interesting to note that Mn2+ solvation from CdSe nanocrystal surfaces appeared to be complete after a Py ligand-exchange procedure that took 24h (47), whereas Co2+ on the surfaces of CdS nanocrystals requires weeks to be solvated by Py (68), and Co2+ on the surfaces of ZnS nanocrystals was not solvated by Py to any measurable extent (91). The thermodynamic variations thus depend sensitively on the geometries of the surface-binding sites offered to the dopants. For example, the S S separations of CdS surfaces are apparently too large to stabilize Co2+ ions to the same extent as those of ZnS. As discussed in Section II.C, the capacity a surface has to stabilize bound dopants is intimately related to... 

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