Intrazeolite Semiconductor Quantum Dots and Quantum Supralattices

Chapter 37

Intrazeolite Semiconductor Quantum Dots and Quantum Supralattices New Materials for Nonlinear Optical Applications Downloaded by NORTH CAROLINA STATE UNIV on November 27, 2012 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch037

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Geoffrey A. Ozin , Scott Kirkby , Michele Meszaros , Saim Özkar , Andreas Stein , and Galen D. Stucky 1

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Lash Miller Chemical Laboratories, University of Toronto, Toronto, Ontario M5S 1A1, Canada Department of Chemistry, University of California, Santa Barbara, CA 93106 2

Recent developments in host-guest inclusion chemistry have paved the way to the controlled and reproducible assembly of sodalite and faujasite quantum dots and supralattices, the latter being comprised of regular arrays of monodispersed semiconductor (eg. A g X , WO ) quantum dots confined in a dielectric material. This work has led to the synthesis of the first examples of mixed component semiconductor quantum supralattices represented by the new sodalite family of materials (8-2n)Na,2nAg,(2-p)X,pY-SOD. Collective electronic coupling between these encapsulated and stabilized nanostructures can be altered through judicious variations in the host structure and guest loading. When the carrier wave function is restricted to the region of the imbibed nanostructures, quantum size effects (QSE's) are observed which give rise to differences in the optical, vibrational and magnetic resonance properties of these materials with respect to those of the bulk semiconductor parent. In this regime of strong quantum confinement, one anticipates resonant and non-resonant excitonic optical nonlinearities associated with X to be enhanced with respect to those of the respective quantum wire, quantum well, and bulk semiconductor materials. 3

(3)

In the continuing quest for new materials with superior optical nonlinearities, fast response times, high photochemical and thermal stabilities for applications in optical switching and signal processing, chemists and physicists have recently turned their attention to semiconductor ultramicrostructures exhibiting reduced charge carrier mobility in one to three dimensions. Structures exhibiting quantum size effects (QSE's) caused by carrier confinement in three dimensions, that is, zero dimensional mobility, are commonly referred to as quantum dots (QD's) (2). 0097-6156/91/0455-0554$08.00/0 © 1991 American Chemical Society In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

Downloaded by NORTH CAROLINA STATE UNIV on November 27, 2012 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch037

37. OZIN ET AL.

Intrazeolite Semiconductor Quantum Dots

555

A t present the available literature invokes more questions than it solves, including: precisely what are the material requirements that make a Q D of value in the technology of nonlinear optics (NLO)? How do the properties of an idealized Q D differ from those of quantum wells (QW's, one dimensional confinement microstructures) and bulk materials? In which ways are the optical properties of QD's modified by local field effects (LFE's) arising from the embedding media? How do the optical properties of three dimensional arrays of electronically coupled QD's compare with those of isolated QD's? In addition, decisions concerning the selection of Q D atomic constituents, the degree of quantum and dielectric confinement, and the extent of electronic coupling between QD's all require a detailed assessment when attempting to fabricate new semiconductor materials with attractive N L O properties. A concern of particular importance, is the fabrication of monodispersed collections of QD's. In the experimental literature, great efforts have been expended to isolate the effects of a single particle size from those caused by even small size distributions (2). In our recent work, we have learned how to encapsulate, as clusters, the components of some well known semiconductors inside the accessible 0.66 nm and 1.3 nm void spaces of sodalite and faujasite host lattices (3, Ôzkar, S.; Ozin, G . A . / . Phys. Chem., in press.). This kind of guest-host inclusion chemistry provides a convenient route to strongly quantum confined nanostructures with densities ranging from isolated QD's through to perfectly organized, three dimensional periodic arrays of interacting QD's. While this synthetic route gives perfect monodispersed size distributions, the choice of particle size at present is limited by the range of currently available host cavity sizes (0.66 - 1.30 nm) and channel dimensions (0.50 - 1.05 nm). This situation could however, rapidly change with the synthesis of new generation large pore zeolites. One important question is not addressed by the current theory: whether monodispersed nanostructures this small (isolated or ordered, non-interacting or coupled), fabricated from the components of bulk semiconductors are likely to be interesting candidates as N L O materials. In order to advance current theories on linear and non-linear optical properties of semiconductor nanostructures, suitable materials must be made available. In the present article, we will survey the known synthetic procedures to intrazeolite semiconductor QD's and quantum supralattices (QS's; this name has been chosen to refer to these structures, since they do not alter the crystallographic unit cell of the host, as the more common name of superlattice would imply). This is followed by two examples from our recent work concerning the I-VII pure and mixed halide system A g , X - S O D (3) and the V I - V I system n(W03)-M56Y (Ôzkar, S.; Ozin, G . A . / . Phys. Chem., in press.). Some key properties of these materials will be briefly described that relate to QSE's, L F E ' s , electronic and vibrational coupling between QD's. The paper concludes with a very brief survey of some of the pertinent physics behind these early observations and how they might relate to the N L O properties of these materials.

In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES


Synthesis Intrazeolite semiconductors (IZS's) of several types, II-VI, IV-VI, I-VII, III-V, V I and VI-VI, exemplified by CdSe, PbS, AgBr, GaP, Se and W O ^ respectively, have been fabricated. These composites are normally prepared by aqueous (Ozin, G.A.; Stein, Α.; Stucky, G.D.; Godber, J.P. /. Inch Phenom., in press; 4,5) and melt (3) ion-exchange, metal-organic chemical vapour deposition (6), vapour phase impregnation (7) and phototopotactic (Ôzkar, S.; Ozin G . A . /. Phys Chem, in press.) methods. A major synthetic challenge with these materials concerns control of the nuclearity, population, location, distribution, dimensionality, purity, defect and doping concentration of the encapsulated semiconductor guest in the zeolite host lattice. In what follows we will focus attention on some of the interesting aspects of the use of melt ion-exchange techniques and simple binary metal carbonyl phototopotaxy for the growth and stabilization of IZS quantum nanostructures of the type exemplified by A g , X - S O D and n(W03)-M56Y respectively. Sodalite Quantum Supralattices: Preamble Sodalite, 8M,2X-SOD (where S0D^SÎ6A1 024, M^cation and X ^ a n i o n reflect the framework, cation and anion content of the sodalite unit cell), is unique as a host material, as it consists of bcc packed cubo-octahedral cavities (called /?-cages) having a free diameter of about 0.66 nm (Figure 1) (8). A network of S1O4 and A I O 4 tetrahedra form densely packed cubo-octahedral cavities (non-rigid, originating from Si-O-Al angular flexibility) with eight six-ring and six four-ring openings. The negative charge on the framework is balanced by exchangeable cations at tetrahedral sites near the six-rings of the /?-cage. A n additional six-ring cation and an anion at the centre of each β-cage are often present as well. Thus sodalite can be viewed as the archetype QS boasting perfectly periodic arrays of all-space fillings-cages (a Federov solid) containing atomically precise, organized populations of M 4 X clusters. 6

Class A Quantum Supralattices. As an example, silver halide exhibiting molecular behaviour has been produced inside sodalite by silver ion exchange of sodium halo-sodalites using a AgN03/NaN03 melt containing substoichiometric amounts of silver (5). Rietveld refinement of high resolution X-ray powder data for a sample containing 0.3 A g per unit cell, or an average of one A g X molecule in every eighth cage, indicates that the A g X molecules can be considered isolated. A t increased A g concentrations up to complete silver exchange, the product is best described as a sodalite lattice containing expanded silver halide, that is, a three dimensional array composed of monodispersed zero dimensional silver halide dots. Rietveld refinement showed that the A g 4 X units in 8Ag,2X-SOD are perfectly ordered. Control over the silver halide environment is possible by varying the anion and cation compositions as illustrated below and in Figure 1: +

+


O Z I N E T AL.


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37.

Figure 1. (a) The sodalite β-cage exhibiting the imbibed tetrahedral M X cluster, (b) The bcc packing arrangement of £-cages in the sodalite unit cell, (c) Quantum supralattices. 4


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MATERIALS F O R NONLINEAR OPTICS: C H E M I C A L PERSPECTIVES

8Na,2X-SOD

. (8-2n)Na, 2nAg, 2 X - S O D + 2 n N a

+

(1)

where η = 0-4. Results obtained from far and mid-IR spectroscopy and powder X R D indicate that in mixed sodium-silver sodalite the cations are distributed statistically (a 3-D commensurate compositionally disordered solid-solution of N a 4 - A g X , η = 0-4) rather than in aggregates (domains of Na4X and Ag4X) or in an ordered fashion (Na4_ Ag X, η fixed). Both far-IR and X R D data of (8-2n)Na,2nAg,2X-SOD show breaks at a silver loading of n ~ 1 corresponding to more than one A g per sodalite cage possibly indicating a percolative threshold reflecting collective interactions between cavity contents. As the Na4_ Ag X cavity nuclearity is limited to five, no significant band shift in the electronic spectrum occurs on increasing the loadings of A g from n = 1-4. This is to be contrasted with most other supported and unsupported quantum size particles (e.g. glasses, micelles, vesicles, clays, L B films, and surface-capped particles respectively), where an increase in the loading of the semiconductor components results in a red-shift as the particle size increases (9,10) The band edges of the 8Ag,2X-SOD quantum supralattices (described in terms of the tight binding and miniband approximations (11,12)) all lie at higher energy than in the bulk semiconducting A g X (band-gap, VB[X'(np), A g (4d)] -> C B [ A g (5s)] excitation). As in the bulk A g X , the band positions are affected by the type of halide. A red-shift is observed for the bigger anions in larger unit cells in the case of the isolated molecular and fee bulk forms of A g X (3). In contrast, the estimated energies (computer fit using a = K(Eg-E) ^) of the absorption edge of the silver halide quantum supralattices (Figure 2) follow the order C l < B r > I. This is indicative of an interplay of decreasing band-gap with decreasing bandwidth down the halide series, implying that the extent of inter-/?-cage electronic coupling follows the order of the observed distances between the centres of the β-cages, that is, CI < Br < I (Figure 2). The absorption edges show no temperature dependence down to 10K, therefore indicating a direct band-gap for the 8Ag,2X-SOD QS's in accordance with the estimation of E from a . n

n

n

n


+

n

n

+

+

d a

+

1

d a

g

This investigation shows that an organized assembly ranging from isolated molecules to expanded structures stabilized inside a sodalite host matrix (Figure 1) can be readily fabricated out of a material that is normally a I-VII semiconductor. The (8-2n)Na,2nAg,2X-SOD sodalites might find applications as electronically tunable nonlinear optical materials (see later).

Class Β Quantum Supralattices. A n interesting series of sodalite materials that for the first time offer the opportunity to manipulate the degree of collective electronic and vibrational interactions between monodispersed semiconductor components in neighboring β-cavities (Figure 1) can be synthesized as follows:


37.

OZIN ET AL.


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Molecular Absorptions/Absorption Edges (eV) AgCl

AgBr

Agi

Order

Molecule

5.12,5.85

4.00,5.04

3.56,4.72

Cl>Br>I

Expanded SC

3.83

3.85

3.76

Cl^Br>I

BulkSC

3-25

2.68

233

Cl>Br>I

SOD Unit Cell Size (A)

8.8708

8.9109

8.9523

CKBr

Intrazeolite Semiconductor Quantum Dots and Quantum Supralattices

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