Small Semiconductor Particles - ACS Symposium Series (ACS

Chapter 38

Small Semiconductor Particles Preparation and Characterization

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on August 26, 2015 | http://pubs.acs.org Publication Date: March 11, 1991 | doi: 10.1021/bk-1991-0455.ch038

Norman Herron Central Research and Development Department, Ε. I. du Pont de Nemours and Company, Wilmington, DE 19880-0328

The construction of discreet particles of semiconductors, either inside porous hosts or as free-standing, surface-capped clusters, has generated a new class of materials where confinement effects on the semiconductor optical properties are pronounced. In porous zeolite hosts, in addition to the size-quantization effects, novel intercluster phenomena become manifest as the individual semiconductor clusters reach a volume density above the percolation limit and begin to interact three-dimensionally. This interaction is modulated by the zeolite framework topology and hence leads to an ordered array of clusters in what we have termed cluster crystals. Novel absorption, emission and excitation behaviors of these materials, dominated by defect sites, result. Detailed characterization of the semiconductor species responsible (by x-ray powder diffraction and EXAFS) reveal a cubane like (CdS) unit as the basic building block of the structure. The random porosity but good optical properties of sol gel glasses allow the generation of optical materials containing related quantum-dot semiconductor clusters (prepared by organometallic means) where now, nonlinear 4

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optical ( χ ) properties have been estimated. Finally, non-resonant non-linearity has been observed in free-standing surface-capped 0097-6156/91/0455-0582$06.00A) © 1991 American Chemical Society In Materials for Nonlinear Optics; Marder, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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semiconductor clusters of CdS, CdSe and CdTe whose surfaces have been terminated and passivated using thiophenolate ligands. These latter materials are highly soluble in polar organics and therefore processable into thin films or polymer composites. The control of cluster size available via synthesis conditions in these materials makes possible a unique and systematic study of optical properties as a function of cluster size and thus of quantum confinement. Small metal and semiconductor clusters, having hybrid molecular and bulk properties, represent a new class of materials and are under intensive investigationl. The basic problem facing researchers in this area is the control of surface reactions of such particles so as to arrest their growth at the small cluster stage. Many approaches have been explored for the preparation of these small clusters including the use of micelles2, colloids3, polymers4 and glasses5 to control the aggregation problem. In all cases, however, the cluster sizes and crystallinities are poorly defined and one would like to find an approach to this class of materials which produces a mono-dispersion of cluster sizes in a well defined and characterizable array. These criteria would seem well met by an inclusion type approach using a porous host lattice as the template within which the clusters could be constructed and confined. Alternately, one may use synthetic chemistry to control the cluster surface such that it is terminated by capping groups which both passivate the cluster electronically and prevent its further aggregation and growth. This paper describes our efforts in both directions and includes: 1) the synthesis and characterization of CdS clusters in zeolites Y, X and A; 2) the preparation of a variety of semiconductors in the "poor-man's zeolite" - porous glass and 3) use of thiophenol capping chemistry to generate free-standing passivated soluble clusters of CdS and CdSe. The resulting effects of sizeconfinement on the semiconductor optical and nonlinear optical properties will be described. Why small semiconducting particles are interesting It is important to understand why there is the current interest in very small particles of semiconductors 1 and what limitations this interest places on the nature of the 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

The concept of an all-optical or opto-electronic computer technology has attracted attention because of its potential for extreme speeds and parallel processing capabilities in such areas as image recognition. Such a technology requires several basic optical materials for the construction of devices which m i m i c their electronic counterparts. One such fundamental computing element is the optical transistor or bistable device which acts as a light switch or valve/amplifier. Basic requirements placed on materials for such a device are that they have a very rapid switching speed (ideally picosecond) and extreme photostability in order to perform trillions of switching operations/sec for years at a time. One realization of such a material could involve the use of third-order nonlinear optical properties, X3, to effect a transient refractive index change. Illumination of such a material with intense laser light w i l l cause a change of its refractive index leading to a switch from an opaque to transmissive state in an interferometertype bistable device. W h i l e semiconductor materials themselves w i l l perform this kind of switching at their bandedge wavelengths, the speed of the effect is slow - usually as a consequence of a long free-carrier lifetime. This speed can be increased by providing more sites for efficient removal of these free-carriers - i n other words more defect sites. One can view surface sites on a semiconductor particle as such defect sites and one simple way to increase their concentration is to go to very small particles. The commercial color filters of Schott and Corning based on CdS/Se nanoparticulates in a silica matrix have verified the utility of this kind of material for nonlinear- optical devices 5. W e would like to explore a wide range of other semiconductors and matrices for these purposes and the zeolite host provides an almost ideal starting point. C d S i n zeolite Y 6 The zeolite Y occurs naturally as the mineral faujasite and consists of a porous network of aluminate and silicate tetrahedra linked through bridging oxygen atoms (Figure 1). The structure consists of truncated octahedra called sodalite units arranged in a diamond net and linked through double six-rings7. This gives rise to two types of cavity within the structure - the sodalite cavity of - 6 . 6 Å free diameter with access through - 2 . 3 Å windows and the supercage of ~ 1 3 Å diameter with access through ~ 7 . 5 Å windows. Whenever an aluminum atom occurs in the framework this introduces one



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negative charge onto the zeolite skeleton which is compensated by loosely attached cations which give rise to the w e l l known ion-exchange properties of zeolites. Cadmium ion-exchange of the zeolite is carried out by slurrying 10g of zeolite L Z Y - 5 2 (sodium zeolite Y from Linde) in 1L of distilled water and the p H is adjusted to 5 with nitric acid. A calculated amount of cadmium nitrate designed to give a specific exchange level is stirred into the slurry and the mixture is stirred at room temperature overnight. Collection of the exchanged zeolite by filtration and washing with distilled water is followed by drying and calcination. The powder is heated to 4 0 0 ° C at 3°/min in flowing dry oxygen (lOOmL/min) then cooled in vacuo to 100°C. The zeolite is then exposed to flowing hydrogen sulfide (40mL/min) at 100°C for 30min. Finally the still white zeolite is evacuated at 100°C for 30mins then sealed and transferred to an inert atmosphere dry box for handling and storage. The zeolite turns pale yellow/cream during the final evacuation step. A l l zeolites prepared in this manner are moisture sensitive becoming deep yellow (zeolite Y or X ) or pale yellow (zeolite A ) on prolonged exposure to atmospheric h u m i d i t y . Chemical analysis confirms C d and S are present in from 0 to 25wt% depending on exchange conditions. X P S shows that there is no detectable C d on the exterior surface of the zeolite crystallites. IR spectra show no S H groups but there are the expected O H groups attached to the zeolite frameworko. The exact nature of the C d S cluster units is revealed by a combined application of optical spectroscopies and x-ray techniques. S t r u c t u r e of C d S i n Y a n d its O p t i c a l Consequences. Detailed analysis of the powder x-ray diffraction data on a series of CdS loaded zeolite Y samples reveals the fundamental C d S cluster present consists of interlocking tetrahedra of C d and S atoms (although some of the S atoms are occasionally replaced by Ο atomso) forming a distorted cube (Cd-S = 2.47Â) (Figure 2). This structure, which is heavily dictated by the zeolite symmetry, is confirmed by E X A F S data at the C d edge w h i c h reveals the local symmetry and coordination environment of the C d 6 . T o our initial surprise, these C d 4 S 4 cubes were not located in the supercages of the Y structure but were instead sited within the smaller sodalite cages. In retrospect this location is entirely reasonable since these cages are the preferred sites for the original C d ions upon exchange8



MATERIALS FOR NONLINEAR OPTICS: CHEMICAL PERSPECTIVES

Representative zeolite structures where the open framework is represented by sticks j o i n i n g the S i or A l atoms. Oxygen bridge atoms lie roughly at the mid-point of these atoms and are omitted for clarity, a) zeolite Y b) zeolite A

Structure of the ( C d S ) 4 unit located within the zeolite sodalite units, (hatched circles = C d ; open circles = S).



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and all that needs to occur upon exposure to the H 2 S is that the cube zips together. The C d ions of the ideal cluster are i n octahedral coordination to 3 sulfur atoms of the cube and 3 oxygen atoms of the zeolite framework six-ring window (Figure 3). The sodalite cage seems to have been made for this C d S cluster!, (for structural details of this material, the reader is referred to ref. 6) The evolution of the optical spectra as a function of C d S loading density is particularly revealing. A t loading densities of

Small Semiconductor Particles - ACS Symposium Series (ACS

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