UVSOR-II BL5U

Surface Chemistry of Butyl-Passivated Silicon Nanoparticles Studied by
Synchrotron-Radiation Photoelectron Spectroscopy
M. Imamura, N. Takashima, J. Nakamura, S. Fujimasa, and H. Yasuda

Department of Mechanical Engineering, Kobe University, Kobe 657-8501, Japan
Various Si-based nanostructures have a greatinterest, since it has been reported that they show astrong photoluminescence, and therefore there is apossibility of future integrating electronic devices
with optical sensing technique. Numerous works
focused on the optical properties of Si nanoparticles
prepared by various methods have been reported todate. However, data in the literatures are nouniversally accepted, and depend on various factors
in the individual samples. Especially, the physical andchemical properties of nanoparticles are greatly
influenced by surface chemical states due to the large
surface to volume ratio with decreasing the size to thenanometer scale. In this work, we have synthesized
n-butyl-passivated Si nanoparticles with well
crystalline nature and well surface-passivated surface
by the solution routes, and have directly characterized
their electronic structures in the vicinity of Fermi
level using synchrotron-radiation photo-electron
spectroscopy. Furthermore, we have compared thevalence-band photoemission spectra of fully
n-butyl-passivated Si nanoparticles and those withoxygen contaminants in order to directly investigate
the effect of surface chemical nature on theirelectronic structures.
Figure 1 shows the synchrotron-radiation
valence-band photoemission spectra of as-preparedn-butyl- passivated Si nanoparticles with dc = 1.1 nm
and those exposed to ambient air for 10 min, on theHOPG substrate at room temperature with photonenergy of 195 eV. The spectral features around 7 and5.5 eV binding energies in the photoemission
spectrum of as-prepared n-butyl-passivated Si
nanoparticles originate from the C 2p derived states.
The spectral feature around 7 eV binding energy
originates from the C–H bonds in the butyl
surface-passivants, and that around 5.5 eV binding
energy originates from the Si-C bonds between the Si
nanoparticle and butyl surface-passivants. This
spectral feature around 5.5 eV in binding energy
indicates that the present Si nanoparticles are wellsurface-passivated by butyl molecules. The spectralfeature around 13.5 eV binding energy originatesfrom the C 2s-derived states. The other spectralfeatures around 3.5, 9, and 11 eV in binding energies
originate from Si 3s- and 3p-derived electronic states.
On the other hand, the valence-band photoemission
spectrum of n-butyl-passivated Si nanoparticles withdc = 1.1 nm after exposure to ambient air for 10 min
is similar to that of as-prepared n-butyl-passivated Si
nanoparticle. However, the spectral intensity derived
from Si-C bonds around 5.5 eV binding energy

decreases with oxygen contaminants. This clearly
indicates the removal of the surface-passivants ofbutyl molecules and subsequent contamination ofoxygen. It should be noted that the valence-band
photoemission spectrum of n-butyl-passivated Si
nanoparticles with oxygen contaminants exhibits anadditional feature centered around 2.6 eV binding
energy (shown by the arrow). Puzder et al. havepreviously reported the Density of State (DOS) ofhydrogen-passivated Si nanoparticles with various
contaminants by means of local density
approximation (LDA) in order to see physical originof the effect of oxygen on the gap. [1] When a double
bonded contaminant such as oxygen and sulfur isadded to Si nanoparticles, the Si sp3 network is
considerably distorted and the HOMO and LUMO
change their nature signiﬁcantly. As a result, HOMOand LUMO states localized in the vicinity of the
Si=O double bond, and the modiﬁed DOS originatedfrom the HOMO and LUMO appears near the Fermi
level as the additional features. From the comparison
with this LDA theoretical DOS, it is concluded that
this additional feature is ascribed to oxygencontaminants-induced states, and this result provides
a direct evidence of the effect of oxygen
contaminants on the electronic structure that theprevious calculation has predicted.

Figure. 1 Valence-band photoemission spectrum ofas-synthesized n-butyl- passivated Si nanoparticles
with dc = 1.1 nm and those exposed to air for 10 min.
The arrow indicates the additional feature originatedfrom the oxygen-contaminant-induced states.
[1] A. Puzder, A. J. Williamson, J. C. Grossman, andG. Galli, Phys. Rev. Lett., 88, 097401 (2002).