Optical Properties of Cuprous Oxide Nanocrystals
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En =E
4 2 2g2e 2t
E=E h2 e 2n2
383 Mat. Res. Soc. Symp. Proc. Vol. 452 01997 Materials Research Society
(1) (1
p
where Eg is the band gap (2.17 eV for the lowest energy exciton series) [3], gt is the reduced exciton mass (0.365 mo) [16], e is the electron charge, h is Planck's constant, Eis the static dielectric constant (7.11) [16], and n is the principal quantum number. It has been shown that for n > 2, this model accurately predicts the energies of the Rydberg series of the yellow exciton when the static dielectric constant is used [17, 18], but fails for the n = 1 transition unless the high-frequency dielectric constant of 6.46 is used. The failure of Eq. 1 for the n = 1 transition has been attributed to the small exciton radius, which suggests a more localized exciton where the use of the static dielectric constant would be inaccurate [3, 14]. This work will focus on quantum confinement effects in the lowest energy forbidden n = 1 excitonic transition.
-8
-. 0.45 eV
cb
IF+
2.17 eV
0.13 eV
8
vb
EXPERIMENTAL Bulk Cu 2 0 powder was obtained from J. T. Baker and annealed in N 2 under different combinations of temperature and time with the most highly
structured photoluminescence spectra resulting from an anneal schedule of 500 'C for 17 hrs. The method used to grow
k FIG. 1 Cu20 band structure near the F point of the Brillouin zone. Valence and conduction bands are labeled as
vb and cb respectively. From [4].
Cu 2 0 microcrystals followed that of reference 19 with the modification of using ascorbic acid as a
reducing agent instead of glucose. In order to make nanocrystalline samples, the above preparation was further modified in a number of ways. One approach, following work done with AgBr [20], involved the creation of reverse micelles by introducing the aqueous reactants into an organic solution (heptane) containing a small amount of surfactant, dioctlsulfosuccinate sodium (AOT). In another approach nanocrystals dispersed in toluene were produced by modifying a preparation by Zou [21]. This dispersion was created by vigorously mixing toluene with aqueous prepared Cu 2 0 and a small amount of surfactant in a separation funnel. A third approach involved the addition of a small amount of polyvinyl alcohol (PVA) in water immediately after the addition of ascorbic acid. The polymer acted as a restraining agent, and some size control was achieved by varying the time at which the polymer was added. Transmission electron micrographs indicated that samples grown using this method were polydisperse, with a mean diameter of 11 nm and a standard deviation of 6 nm. This size metric can be deceiving as micrographs indicate that there is a noticeable population of crystallites as large as 50 nm. Quantifying size restriction effects would be difficult since each sized crystallite will undergo a transition at a slightly different energy which results in significant inhomogeneous broadening in both absorption and luminescence. Excitation spectra were recorded using an Ar-ion pumped tunable dye laser (Rhodamin
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