19932 J. Phys. Chem., Vol. 100, No. 51, 1996
Mu et al.
Figure 7. Electronic spectra of bulk and the confined PbI2 in four
different pore-sized silica glasses.
Figure 8. Raman spectra of the bulk and the confined PbI2 in four
different pore-sized silica glasses.
STM measurements of PbI2 on graphite surfaces were also
conducted. However, no reliable experimental images were
obtained. This is expected to be due to the fact that the band
gap of the bulk PbI2 is about ∼2.4 eV. It is also expected that
the quantum confinement of the exciton in the small clusters
will result in a blue shift of the band gap. Based on the results
obtained by Sandroff et al. and others,1-3,11 the band gap of
the cluster may be even higher than 4 eV, making STM
measurements very difficult. In addition, the STM tip may
remove or destroy the PbI2 clusters on the graphite surface when
the bias voltage is not high enough, which has been observed
in a few cases in imaging SAMs on a Au surface.12 However,
more experiments are presently underway to elucidate the lateral
dimension or atomic resolution of these clusters.
Optical Characterization of PbI2 Impregnated in Porous
Glass. Figure 7 shows the electronic absorption spectra of bulk
PbI2 and the PbI2 physically confined in 2.5, 5, 10, and 20 nm
pores of silica substrates. As expected, the PbI2 confined in
large pores showed a little blue shift in the band gap. However,
a noticeable blue shift was observed at ∼505 nm when the PbI2
was confined in 2.5 nm pore, which is consistent with the
quantum confinement theory proposed by Brus.9 By comparing
the optical spectrum of PbI2 impregnated in 2.5 nm pores in
Figure 7 with the spectrum of PbI2 clusters synthesized in a
methanol solution at a 5 mM concentration in Figure 3, the same
absorption band edge was observed. This observation provided
a strong indication that the PbI2 clusters synthesized in the
methanol solution were less than 2.5 nm in size. As the
concentration decreases, the cluster size gets smaller so that the
absorption band of the PbI2 clusters is further blue-shifted.
Figure 8 illustrates the Raman spectra of PbI2 bulk and the
confined in four different pore-sized silica hosts. There were
three bands observed in the 60-150 cm-1 region. They are at
75, 96, and 116 cm-1, which have been assigned to the TO2,
LO2, and LO1 optical phonon modes of PbI2 crystals, respec-
tively. As the pore size decreases, the LO modes are broadened,
and the center frequencies seem to be red-shifted. The red shift
of the band at 116 cm-1 is more pronounced. We suggest that
the red shifts in LO modes may be due to the surface phonon
modes resulting from the local electric field at guest-host
interface for small particles. However, the experimental results
are far from being conclusive to make a claim that surface
phonons are observed in this system. More experiments are
currently underway to study surface phonons in layered
semiconductor quantum dots.
confirmed the PbI2 cluster formation. A spectral comparison
of the electronic transitions between the PbI2 clusters synthesized
in solution and physically confined in porous glasses suggests
that the cluster size in solution is less than 2.5 nm in lateral
dimension. This observation is consistent with the concept that
strong quantum confinement sets in when the particle size is
comparable or smaller than the exciton radius (aB| ) 1.9 nm).
AFM measurements of PbI2 clusters on mica, graphite, and
CH3 surfaces suggest that the clusters formed from solution
synthesis are disklike. A thickness of 1.0 ( 0.1 nm observed
in 0.5 mM solutions confirms, for the first time, that these
clusters are single-layered. The 40% expansion of the interlayer
distance can be attributed to the finite size effect of the clusters.
At small sizes, a strong intralayer chemical bonding can result
in the lateral contraction with respect to the bulk value and can
lead to the expansion in layer thickness.
The LO and TO modes of bulk and the confined PbI2 were
also characterized via Raman scattering measurements. The
observed red shifts and spectral band broadening of the LO
modes (LO1 and LO2) may be attributed to surface phonons of
PbI2 nanophase in the porous host. However, the experimental
results are not yet conclusive.
Acknowledgment. This work was supported in part by
NASA under Grant NAG8-1066 and in part by NASA funded
Center for Photonic materials and Devices.
References and Notes
(1) (a) Marino, M. M.; Sawamura, M.; Ermler, W. C.; Sandroff, C. J.
Phys. ReV. B 1990, 41, 1270. (b) Sawamura, M.; Ermler, W. C. J. Phys.
Chem. 1990, 94, 7805. (c) Sandroff, C. J.; Hwang, D. M.; Chung, W. M.
Phys. ReV. B 1986, 33, 5953. (d) Sandroff, C. J.; Kelty, S. P.; Hwang, D.
M. J. Chem. Phys. 1986, 85, 5337. (e) Sarid, D.; Henson, T.; Bell, L. S.;
Sandroff, C. J. J. Vac. Sci. Technol. A 1988, 6, 424.
(2) Roy, A.; Sarma, D. D.; Sood, A. K. Spectrochim. Acta 1992, 48A,
1779.
(3) Wang, Y.; Herron, N. J. Phys. Chem. 1987, 91, 5005.
(4) Grasso, V.; Mondio, G. Electronic Structure and Electronic
Transitions in Layered Materials; Grasso, V., Ed.; D. Reidel: Dordrecht,
1986; p 191.
(5) For example: (a) Feigelson, R. S. J. Cryst. Growth 1988, 90 1. (b)
Rosenberger, F. J. Cryst. Growth 1993, 76, 618.
(6) (a) Tang, Z. K.; Nozue, Y.; Goto, T. J. Phys. Soc. Jpn. 1992, 61,
2943. (b) Saito, S.; Goto, T. J. Lumin. 1994, 58, 127. (c) Goto, T.; Saito,
S. Solid State Commun. 1991, 80, 331.
(7) Lifshitz, E.; Yassen, M.; Bykov, L.; Dag, I. J. Phys. Chem. 1994,
98, 1459.
(8) Henderson, D. O.; Mu, R.; Ueda, A.; Burger, A.; Chen, K. T.;
Frazier, D. O. Mater. Res. Soc. Proc. 1995, 366, 283.
(9) Brus, L. J. Chem. Phys. 1984, 80, 4403.
(10) For example: Xu, S.; Arnsdorf, M. F. J. Microsc. 1994, 173, 199.
(11) Micic, O. I.; Li, Z.; Mills, G.; Sullivan, J. C.; Meisel, D. J. Chem.
Phys. 1987, 91, 6221.
(12) Schonenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.;
Fokkink, L. G. J. Langmuir 1994, 10, 611.
Conclusion
Optical characterization of PbI2 clusters synthesized in
methanol solution with an integrating sphere has unambiguously