Characterization of Nanoclusters in MgO Created by Means of Ion Implantation.
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R8.10.1
Characterization of Nanoclusters in MgO Created by Means of Ion Implantation. M.A. van Huis1, A. van Veen1, H. Schut1, B.J. Kooi2, J.Th.M. De Hosson2 1 Interfaculty Reactor Institute, Delft University of Technology, Mekelweg 15, NL-2629JB, Delft, The Netherlands. 2 Materials Science Center, University of Groningen, Nijenborgh 4, NL-4797 AG Groningen, The Netherlands.
ABSTRACT Metal nanoclusters (NCs) of lithium, zinc, silver and gold embedded in MgO were created by means of ion implantation of Li, Zn, Ag and Au ions into single crystals of MgO(100) and subsequent thermal annealing. Nanoclusters of the compound semiconductor CdSe were obtained by implantation of both Cd and Se ions. Solid noble gas clusters were formed by Kr ion implantation. Optical and structural properties of the NCs were investigated using optical absorption spectroscopy (OAS), high-resolution X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (XTEM). The mean nanocluster size is estimated from the broadening of the Mie plasmon optical absorption bands using the Doyle formula. These results are compared with the NC size as obtained from XRD (using the Scherrer formula) and from direct XTEM observations. The three methods are found to be in reasonable agreement with a mean size of 4.0 and 10 nm found for the Au and Ag clusters, respectively. Using TEM observations, the relative interface energies of MgO//Au and MgO//Ag interfaces are also determined. In the case of MgO//Au, they are found not to be in agreement with theoretical predictions in the literature. CdSe nanoclusters were found to adopt different crystal structures dependent on the size. Small ones ( 40 nm: bcc Li a=3.51 Å 2.0 % MgO(111)//Li(110) 2.0 % MgO[0 1 1]//Li[11 1 ]
MgO:Zn MgO:Ag MgO:Au MgO;CdSe Rock salt MgO:Kr MgO
? MgO(001)//Zn(0001) MgO(001)//Ag(001) MgO[010]//Ag[010] MgO(001)//Au(001) MgO[010]//Au[010] MgO(001)//CdSe(001) MgO[010]//CdSe[010] MgO(001)//Kr(001) MgO[010]//Kr[010] -
a= 2.665 Å c= 4.947 Å a= 4.085Å
[14]
a= 4.078 Å
[14]
a= 5.61 Å
[14]
[14]
a= 5.3-5.8 Å a= 4.213 Å
? 17 % 3.0 % 3.0 % 3.2 % 3.2 % 33%
1550 K > 1500 K > 1500 K
30-37% [14]
0%
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Comparing the NC cluster sizes as obtained from optical absorption, XRD and XTEM, it can be concluded that the cluster sizes are in reasonable agreement, although the size of the Ag clusters as deduced using the Doyle formula underestimates the size (as observed by TEM). Possibly the Ag clusters are already too large for application of Doyle-Mie theory. For small CdSe clusters ( 5 nm) the rock salt structure is observed. The observed lattice parameter corresponds with a very high pressure of 9 GPa imposed onto the cluster by the surrounding matrix. This explains why this high density structure exists in the MgO sample. Similarly, the solid Krypton clusters are under high pressure. From the TEM pictures the lattice parameter of individual clusters could be determined. The relation between lattice parameter (aKr) and cluster size (bcluster) is given by aKr = 4.6 + 0.39 bc
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