Tailorable, Visible Light Emission From Silicon Nanocrystals
J. P. Wilcoxon and G. A. Samara
Abstract
Crystalline, size-selected Si nanocrystals in the size range 1.8-10 nm grown in inverse
micellar cages exhibit highly structured optical absorption and photoluminescence (PL) across
the visible range of the spectrum.
The most intense PL for the smallest nanocrystals produced
(-2 nm) was in the blue (-365 nm) with a radiative lifetime of -1 ns and is attributed to direct
recombination at zone center.
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There is presently a large research effort aimed at exploring physical and chemical means
to induce a useful level of visible photoluminescence (PL) from silicon (Si). The approaches
includel porous silicon and quantum wires and dots.
These two approaches, which may be
mechanistically related via quantum confinement, have considerable potential but are poorly
understood. Visible PL has been observedl from Si nanocrystals, or quantum dots, produced by
a variety of techniques including aerosols,2 colloids,3 and ion implantation.4 However, all of
these techniques produce large distributions of cluster sizes resulting in broad, unstructured
optical absorption and PL features which limit usefulness and make definitive interpretation of
results difficult.
We have used inverse micelles as reaction vessels5 to produce useful quantities of Si
nanocrystals which have been remarkable in their size monodispersity, the sharpness and
richness of their spectral features and tailorability of their properties. In what follows we
describe the synthesis and some of the properties.
Details of the work will be published
elsewhere.6 An anhydrous ionic salt (e.g., SiX4, where X = Cl, Br, or I) is dissolved in the
hydrophilic interior of a solution of micelles in octane with complete absence of water. The
surfactants used, nonionic aliphatic pol yethers or quatemary ammonium cationic surfactants, are
dissolved in anhydrous tetrahydro furan (THF) and dried over Na metal. We next reduce Si(IV)
to Si(0) using an anhydrous metal hydride, (usually 1M LiAl~ in THF). The reduction is rapid
with vigorous bubbling of Hz gas. One can determine the progress of the reaction by following
the disappearance of the Si(IV) charge transfer absorption from the precursor solution.
Nanocrystals with diameters between 1.8 and -10 nm were produced. Spectroscopy and high
pressure liquid chromatography (HPLC) with on-line absorbance, conductivity and refractive
index detectors were used to demonstrate 100% reduction of the Si(IV) to the final Si(0)
nanocrystal form. The nanocrystals were characterized by high-resolution transmission electron
microscopy.
The optical absorption spectra of our nanocrystals are much richer in spectral features
than earlier nanocrystal spectra making it possible to assess the role of quantum confinement in
Si. An example is shown in Fig. 1 for a sample of 2 nm nanocrystals. The figure also shows the
spectrum of bulk Si where the spectral features reflect the details of the band structure shown in
the inset in Fig. 2. Specifically, the long absorption tail between 1.2 and -3 eV reflects the
indirect nature of the bandgap.
The sharp rise in absorption with increasing photon energy
starting around 3.2 eV (380 nm) is associated with the direct transition at the r point [r25 -+ r15]
whose energy is 3.4 eV (365 rim), and the second sharp rise starting around 4 eV (320 nm) is
associated with a second direct transition, most likely the r~ - r~ transition whose energy is 4.2
eV (295 nm) or possibly the direct transition at X.
The close resemblance in the shape of the two spectra in Fig. 1 is remarkable indicating
that the bulk-like character of the band structure of Si is preserved down to the d = 2 nm size (i.e.
-200 atoms or less). The results show clear evidence for quantum confinement; specifically,
both direct transitions of the nanocrystals are blue shifted by about 0.4 eV compared to the bulk.
This is larger than is predicted by model calculations .9 The indirect absorption tail is also blue-
shifted, and the gap appears to remain indirect-
Because our samples are very dilute (-10-4
molar) the signal-to-noise ratio for the sample in Fig. 1 is low for the low absorbance associated
with the indirect transition. For other samples, however, the signal-to-noise was considerably
higher and the spectrum less noisy in this region allowing meaningful analysis of the data.
Analysis of results on 1.8 nm nanocrystals yielded an indirect bandgap of 2.2 * 0.3 eV.6 This
result is in close agreement with the 2.06 eV obtained by Brus et a12 on SiOz-capped Si