Infrared spectroscopy of bis(4-nitrophenyl) disulfide grown on a Pb layer

Article abstract of DOI:10.1246/cl.2002.1138

Crystal whiskers of bis(4-nitrophenyl) disulfide have been grown on a 100-nm thick Pb film from a supersaturated solution of p-nitrothiophenol. The whiskers preferentially grow on Pb but not on Si, In, or Ag. The crystals were characterized by electron microscopy and infrared spectroscopy. The optimized structure of the molecule was determined by ab initio calculations.

Full text of DOI:10.1246/cl.2002.1138

1138
Chemistry Letters 2002
Infrared Spectroscopy of Bis(4-nitrophenyl) Disulfide Grown on a Pb Layer
M. D. McCluskey, D. I. Grover, and K. K. Zhuravlev
Department of Physics, Washington State University, Pullman, Washington 99164-2814, U.S.A.
(Received August 16, 2002; CL-020697)
Crystal whiskers of bis(4-nitrophenyl) disulfide have been
grown ona 100-nm thick Pb film from a supersaturated solution of
p-nitrothiophenol. The whiskers preferentially grow on Pb but not
on Si, In, or Ag. The crystals were characterized by electron
microscopy and infrared spectroscopy. The optimized structure
of the molecule was determined by ab initio calculations.
Surface-enhanced infrared absorption (SEIRA) is a phenom-
enon where the infrared (IR) absorption of molecules is enhanced
by the presence of a metal surface.1 P-nitrobenzoic acid ( p-NBA)
and p-nitrothiophenol ( p-NTP) are common examples of
molecules that exhibit SEIRA. Although Ag and Au are generally
used as metals in these studies, recently it was reported that p-
NBA on Pb showed evidence of SEIRA that depended strongly on
the thickness of the Pb film.2
In this letter, we present the results of the growth and
characterization of crystals grown from a p-NTP solution on Pb
films. Unlike Ag or Au layers, self-assembled monolayers were
not observed on thin (10 nm) Pb layers. On 100-nm thick Pb
layers, however, we obtained evidence for the growth of bis(4-
nitrophenyl) disulfide (NPD) crystals. Scanning electron micro-
scopy (SEM) revealed that the crystals were present in the form of
crystal whiskers a few microns in diameter and tens or hundreds
of microns long. The IR transmission spectra of these whiskers are
in good agreement with the published spectrum for NPD. In our
experiments, no direct evidence for surface enhancement was
found, perhaps due to oxidation of the Pb surface. Instead, the
larger IR absorption on the 100-nm thick Pb layer as compared to
the 10-nm layer was due to the fact that large crystals simply did
not grow on the thin layer. The thin layer likely consisted of Pb
islands, surrounded by Si which inhibited the growth of large
NPD crystals.
Figure 1. SEM micrograph of NPD grown on a Pb/Si substrate.
The dark region, where no crystals grew, is bare Si.
taken near the edge of the Pb film. The dark region, where no
crystals grew, is bare Si. This fact was verified by energy
dispersive X-ray (EDX) analysis, which showed no Pb signal in
the dark region. It is therefore apparent that the crystal whiskers
grow preferentially on Pb but not on Si. Other experiments were
performed with 100-nm layers of Ag and In, but crystals did not
grow on those metals.
An IR absorbance spectrum of the NPD sample is shown in
Figure 2. A bare Si substrate, with neither Pb nor NPD, was used
as the reference. The absence of a S–H mode at 2549 cmÀ1
indicates that the crystals are not p-NTP. Merklin et al.3 found a
Pb films were deposited on (100) Si substrates in a Perkin-
Elmer ion-pump vacuum deposition system at a pressure of
10À7 Torr. No steps were taken to remove the oxide layer from the
Si. A quartz gauge was used to monitor the Pb thickness during the
deposition. NPD crystals were grown, at room temperature, from
a supersaturated solution of p-NTP in methanol. It was found that
NPD crystals did not grow on bare Si. When Si coated with a 100-
nm layer of Pb was placed in the solution, however, the crystals
grew within a few minutes. The crystals were yellow in
appearance, and only weakly adhered to the Pb layer. To analyze
the crystal morphology, we used a JEOL 6400 SEM with a 20 keV
electron beam. The IR spectrum was obtained at room
temperature with a Bomem DA8 Fourier transform infrared
(FTIR) spectrometer with a KBr beamsplitter and mercury-
cadmium-telluride (MCT) detector. The spectral resolution was
0.7
0.6
C-H bend
0.5
0.4
0.3
0.2
0.1
0.0
C-H stretch
NO2
500
1000
1500
2000
2500
3000
3500
Wave numbers (cm-1)
2cm À1
.
Figure 2. IR spectrum of NPD grown on a Pb/Si substrate. Peaks
indicated by asterisks (Ã) are due to the instrumental background.
C–H and NO2 modes arising from the NPD molecules are
indicated by arrows.
An SEM micrograph of the NPD crystals is shown in Figure
1. It can be seen that the crystal whiskers are approximately 3
microns wide and 30–100 microns long. The micrograph was
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
1139
similar result for NPD films that were cast from a solution of p-
NTP. As in that work, our IR spectrum is in good agreement with
the published spectrum for bis(4-nitrophenyl) disulfide.4 The
overall increase in absorption versus wave numbers may be a
result of absorption by the Pb layer.
Ab initio calculations were performed in order to determine
the structure of NPD molecules, using density functional theory
(DFT).5 We used the basis sets 6-311G(d) and 6-311++G(d).
The larger basis set, 6-311++G(d), resulted in a nearly identical
structure as that obtained by the 6-311G(d) basis set. The
optimized structure for the NPD molecules is shown in Figure 3.
This skewed structure is similar to that found for numerous
disulfides.6 The two phenyl rings are connected by the S–S bond,
with a C–S–S–C dihedral angle of 81ꢀ. The NO2 groups are
rotated out of the plane of the phenyl rings by 33ꢀ.
our measurements. Strong IR peaks arising from C–H bending
modes were calculated at 679, 710, 745, and 860 cmÀ1, in good
agreement with the experimentally observed peaks at 637, 682,
739, and 845 cmÀ1. Calculated C–H stretch modes were centered
around 3200 cmÀ1, in reasonable agreement with the observed
peak at 3098 cmÀ1. However, the calculated NO2 IR peaks were
869 and 1154 cmÀ1, significantly lower than the observed
frequencies of 1326 and 1507 cmÀ1
.
In summary, NPD crystals have been characterized by SEM
and IR spectroscopy. The growth of NPD crystals has been found
to be strongly dependent on the substrate. This highly selective
growth process may be useful for the fabrication of devices based
on molecular crystals on Si.
We are pleased to acknowledge C. Delgado for assistance
with the SEM and P.R. Griffiths forhelpful discussions. This work
was supported by the U.S. National Science Foundation under
Grant No. DMR-0203832. Acknowledgment is made to the
donors of the Petroleum Research Fund, administered by the
ACS, for partial support of this research.
O
N
References
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53, 1448 (1999).
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Aldrich Co. (1997), Vol. 2, p 2340.
M. J. Frisch et al., ‘‘Gaussian 98, Revision A.9,’’ Gaussian,
Inc., Pittsburgh, Pennsylvania (1998).
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3
4
5
6
S
Figure 3. Calculated structure of the NPD molecule. Hydrogen
atoms are not shown.
IR-active vibrational frequencies were calculated in the
harmonic approximation. Modes involving significant motion of
the S atoms were below 500 cmÀ1, outside the spectral range of