Quantum dot lattice

T. Takaghara, Quantum dot lattice and enhanced excitonic optical nonlinearity. Surf. Sci. 267, 310-314, 1992. 

Interesting results of the studies of the strong coupling regime of Wannier-Mott excitons in a quantum dot lattice embedded in organic medium and in dendrites and also unusual nonlinear properties of such structures can be found in the articles by Birman and coworkers (33)-(37). 

To improve control over nanoparticle formation while using wet impregnation, several authors used consecutive impregnations (four to five times) with drying the material in between to insure that mesopores are completely filled with metal precursor [31,32]. In this case, the amount of precursor is fixed and determined by the pore size and volume the recipe is well reproducible as the final particle size is controlled by the precursor amount and in some cases by the pore size. This avenue led to a 3D quantum dot lattice inside the mesoporous silica film due to regular CdS particle formation, when impregnation was followed by precipitation with H2S and the procedure was repeated until the film satura-... 

Besson S., Ricolleau C., Gacoin T., Jacquiod C., Boilot J.-P. 3D Quantum dot lattice inside meso-porous sdica films. Nano Lett. 2002 2 409... 

Fu, Y., Berglind, E., Thyldn, L., Agren, H. (2006b). Optical transmission and waveguiding by exci-tonic quantum dot lattices. Journal of the Optical Society of America B, 23, 2441-2447. 

Besson, S., Gacoin, T., Ricoleau, C., Jacquiod, C., and Boilot, J.P. (2002) 3D quantum dot lattice inside mesoporous silica films. Nano Lett., 2, 409-414. 

Fig. 56. TEM images of DNA-linked gold network (a) an assembly of 8 and 30 nm gold particles (b) higher resolution image of (a) (c) control experiment without DNA (d) HR-TEM image of a portion of a hybrid Au/quantum dot (QD) assembly. The lattice fringes of the QDs, which resemble fingerprints, appear near each Au nanoparticle, (e) A satellite structure formed using a 60-fold excess of the 8 nm particles. Reproduced with permission from Ref. (185). Copyright 2000, American Chemical Society.

Micic 01, Smith BB, Nozik AJ (2000) Core-shell quantum dots of lattice-matched ZnCdSe2 shells on InP cores Experiment and theory. J Phys Chem B 104 12149-12156... 

The ZnS nanotubes and nanorods were characterized by UV-visible absorption spectroscopy and PL spectroscopy. The inset in Fig. 2a shows the absorption spectrum of the ZnS nanotubes. The band appearing at 318nm is blue-shifted relative to that of the bulk ZnS (350 nm) [17]. Nanowires of ZnS of diameter 5 nm were reported to show an absorption maximum around 326 nm [18], An absorption band at 320 nm has been reported in the case of ZnS quantum dots [19], The PL spectrum of ZnS nanotubes given in the inset of Fig. 2b exhibits two bands, a weak blue emission at 485 nm and a strong green emission around 538 nm. The 485 nm band is attributed to zinc vacancies in the ZnS lattice. Emission bands at 470 nm [20] and 498nm [21] have been reported in ZnS nanobelts. The 538 emission band is similar to that reported for ZnS nanobelts [22] and is considered to result from vacancy or interstitial states [22,23]. 

As you may recall from Chapter 4, when an electron is promoted from the valence to conduction bands, an electron-hole pair known as an exciton is created in the bulk lattice. The physical separation between the electron and hole is referred to as the exciton Bohr radius (re) that varies depending on the semiconductor composition. In a bulk semiconductor crystal, re is significantly smaller than the overall size of the crystal hence, the exciton is free to migrate throughout the lattice. However, in a quantum dot, re is of the same order of magnitude as the diameter (D) of the nanocrystal, giving rise to quantum confinement of the exciton. Empirically, this translates to the strongest exciton confinement when D < 2r. ... 

The relative solubility of inorganic salts can be used to prepare more complex structures by such methods and examples indude CdS/ZnS [24], CdSe/AgS [25] HgS/CdS [26], PbS/CdS [27, 28], CdS/HgS [29], ZnS/CdSe [30] and ZnSe/CdSe [31] particles. The main constraints on the production of such structures involve the relative solubility of the solids and lattice mismatches between the phases. The preparation of quantum dot quantum well systems such as CdS/HgS/CdS [32, 33], has also been reported, in which a HgS quantum well of 1-3 monolayers is capped by 1-5 monolayers of CdS. The synthesis grows less soluble HgS on CdS (5.2 nm) by ion-replacement. The solubility products of CdS and HgS are 5 X 10 and 1.6 x 10 respectively. The authors reported fluorescence measurements in which the band edge emission for CdS/HgS/CdS is shifted to lower energy values with increasing thickness of the HgS well [33]. 

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