Muonium in nano-crystalline II–VI semiconductors
Introduction
Quantum confinement in colloidal semiconductor nanocrystals (ncs) results in a set of discrete valence hole and conduction electron energy levels, instead of bands. As a consequence, the optical and electrical properties of such systems depend strongly on the nanocrystal size, leading to extensive research [1], [2]. A compelling question that has received a lot of interest recently is whether small nanocrystals can be doped with donor (acceptor) atoms, permitting wide applications of nanocrystal materials in electronic functional devices [3]. The incorporation of hydrogen into ZnO nanocrystals has been hypothesized to be important for the transport properties of ZnO nanocrystal assemblies [4], [5].
is a powerful method to study the isolated hydrogen (muonium) states in semiconductors. In particular, the formation and ionisation of a muonium shallow-donor state in some II–VI semiconductors in the bulk has been extensively studied [6], [7], [8], [9].
In this work we present the first results of a survey of muonium states in nanocrystalline (nc) II–VI semiconductors, focusing on those where the shallow muonium is formed in the bulk. By using the transverse- and longitudinal-field techniques we searched for signs of the different states that might be forming inside, on the surface, or in the space between the nanocrystal cores. The temperature dependences of the signals are compared with that observed in bulk samples for the ionisation of the shallow muonium state.
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Experimental details
Colloidal CdSe and CdTe nanocrystals used for these experiments were prepared by high-temperature organometallic synthesis. CdSe nanocrystals were prepared following de Mello Donega et al. [10], while CdTe particles were synthesized following the recipe of Wuister et al. [11]. The syntheses and subsequent steps are carried out in the inert atmosphere of an argon-purged glove box. This procedure yields small in diameter) particles with a narrow size distribution ( standard deviation).
Results
The low-temperature TF spectra have the same general traits for all samples studied, with a broad component and a central line, not well separated in most cases, as seen in Fig. 1 for the case of nc-CdTe.
Fig. 2 shows plots of the temperature variation of the parameters characterising the line shape of the spectra obtained for nc-CdTe and nc-ZnO. A single component was used for the fits to the nc-ZnO data and two component fits were used for nc-CdTe. These simple models do not reproduce the line
Conclusions
We have observed strong evidence in nc-ZnO of formation of surface states with dipolar interaction with nearby protons passivating dangling bonds. The plausibility of formation of states inside the nanocrystals is very low in the studied samples.
The data obtained for nc-CdSe also show strong evidence of dipolar interaction, presumably with N atoms of the HDA capping at the surface of the nanocrystals. A deep muonium state is formed, as expected for a substitution of hydrogen in the capping
Acknowledgements
This work was partially supported by the European Commission under the 6th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures. Contract no. HII3-CT-2003-505925. J.M.G., J.S.L., J.L.G. and S.F.J.C. have benefited from the Treaty of Windsor Grants B-2/05 and B-55/07. The Coimbra team (CEMDRX) was also supported by FCT and FEDER funds under Portuguese Contract SFA/2-30.
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