The achievement of luminescent nanocrystalline solid films, with good optical quality, will be crucial to the development of opto-electronic devices based on such materials. Although (CdSe)ZnS "core-shell" nanocrystals are typically found to have solution photoluminescence (PL) efficiencies in excess of 60%, the values associated with solid films are found to be an order of magnitude lower. Care of surface chemistry and control of nanocrystal/matrix interactions are of paramount importance. Furthermore, the PL efficiency exhibits a dependence on nanocrystal concentration consistent with a semi-quantitative model describing the effects of Förster energy transfer between nanocrystals and the associated trapping at surface sites. In addition to the ability to control optical properties by variation of the nanocrystal dimensions, it is also possible to alter the optical environment in which the nanocrystals are situated. By placing films of nanocrystals into high-Q, planar microcavities, it is possible to produce significant alteration of photoluminescence into very narrow resonant modes of the cavity. This is an important technical step towards the realisation of a nanocrystal laser. The combination of robust semiconductor emitters with the convenience of solution processing also offers considerable advantages over conventional molecular beam epitaxy (MBE) techniques. Finally, the PL emission from close-packed core-shell nanocrystalline thin films under intense picosecond UV excitation is studied. Strong, stable line-narrowing features are observed as the excitation intensity is increased, both at 77K and at room temperature; these are attributed to waveguiding and amplified spontaneous emission (ASE) in the films. Such behaviour would usually be considered as the signature of optical gain. Lasing from microcavities based on these films has yet to be observed, however, and a semi-empirical model of line-narrowing threshold intensities and cavity-photon lifetimes suggests that higher gain, lower losses or greater cavity finesse may be required for this.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:599029 |
| Date | January 2002 |
| Creators | Finlayson, C. E. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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