Doping nanocrystals

It is difficult to predict the effect of surface functionalization on the optical properties of nanoparticles in general. Surface ligands have only minor influence on the spectroscopic properties of nanoparticles, the properties of which are primarily dominated by the crystal field of the host lattice (e.g., rare-earth doped nanocrystals) or by plasmon resonance (e.g., gold nanoparticles). In the case of QDs, the fluorescence quantum yield and decay behavior respond to surface functionalization and bioconjugation, whereas the spectral position and shape of the absorption and emission are barely affected. 

Bhargava RN, Gallagher D, Hong X, Nurmikko A (1994) Optical-Properties of Manganese-Doped Nanocrystals of Zns. Phys Rev Lett 72 416-419... 

Figure 15 Relaxed structure of the S133BPH36 co-doped nanocrystal (diameter = 1.10 nm). Gray balls represent Si atoms, while the light gray balls are the H used to saturate the dangling bonds. B (dark gray) and P (black) impurities have been located at subsurface positions in substitutional sites on opposite sides of the nanocrystals. The relaxed impurity distance is DBP = 3.64 A.

The structural deformation occurring in the single and co-doped nanocrystals has a profound influence on the stability of the systems studied. 

Figure 16 Formation energy for single-doped and co-doped Si-NCs. In the co-doped nanocrystals, the impurities are placed as second neighbors in the first subsurface shell. Squares are related to S135H35, diamonds to S187H76, and circles to Si Hioo based nanocrystals. The lines are a guide for the eyes.

Now, what is important is that the electronic properties of B- and P- codoped Si-NCs are qualitatively and quantitatively different from those of either B- or P- single-doped Si-NCs. The presence of both a n and a p impurity leads to a HOMO level that contains two electrons and to a HOMO-LUMO gap that is strongly lowered with respect to that of the corresponding un-doped nanocrystals. 

Impurity ions in wurtzite lattices are described by the same expressions for P2, and P3c, with a numerically insignificant difference in P3o. These expressions are only quantitatively accurate in the dilute limit, but many of the doped nanocrystals discussed in this chapter fall in this limit. The reader is referred to Ref. 42 for a generalized treatment of the problem. Figure 2(b) plots the probabilities calculated from Eq. 4a-d as a function of impurity concentration. The fraction of dopants having at least one nearest-neighbor dopant is quite high even at moderate impurity concentrations (<5%). Needless to say, whereas purification to ensure size uniformity is possible (size-selective precipitation), no purification method has yet been developed for ensuring uniform dopant concentrations in an ensemble of nanocrystals. 

II. SYNTHESIS OF DOPED NANOCRYSTALS A. Synthetic Methods General Comments... 

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... 

An interesting approach recently applied to doped nanocrystals is the heterocrystalline core-shell method commonly applied to pure nanocrystals. In a series of papers (102, 103), Mn2+ CdS nanocrystals were synthesized in inverted micelles under conditions very similar to those described above and in Figs. 8, 9, and 13. The poor luminescent properties of the resulting Mn2+ CdS nanocrystals were attributed to nonradiative recombination at unpassivated CdS surface states. From the discussion in Section I and II.C, however, it is likely that a large fraction if not all of the Mn2+ ions resided on the surfaces of these as-prepared nanocrystals as observed for Co2+ (Fig. 9). This interpretation is supported by studies in other laboratories that showed large Mn2+ surface populations in Mn2+ CdS nanocrystals grown by the same inverted micelle approach (63). Nevertheless, growth of a ZnS shell around these Mn2+ CdS nanocrystals led to an approximately ninefold increase in Mn2+ 4T 1 > 6A i... 

Several examples have been reported recently of solution-processed multilayer electroluminescence devices incorporating semiconductor nanocrystals as the active recombination centers (16-18, 164). Recently, attention has also turned to hybrid electroluminescent devices involving transition metal-doped nanocrystals (104, 165-167). Although many challenges remain, including more specific exploitation of the dopants in many cases, the devices demonstrated to date represent a new direction in application of doped semiconductor nanocrystals made possible by the compatibility of these luminescent nanocrystals with solution processing methodologies. 

---