Surface Science Letters
One-dimensional organic nanostructures: A novel approach based on the
selective adsorption of organic molecules on silicon nanowires
Eric Salomon
*
, Antoine Kahn
Department of Electrical Engineering, Princeton University, E-quad, Olden Street, Princeton, NJ 08544, United States
article info
Article history:
Received 11 March 2008
Accepted for publication 16 April 2008
Available online 26 April 2008
Keywords:
Chemisorption
Nanopatterning
Scanning tunneling microscopy
Scanning tunneling spectroscopy
abstract
Scanning tunneling microscopy (STM) is used to investigate the formation of one-dimensional organic
nanostructures by chemisorption of specific molecules on silicon nanowires. STM data demonstrate that
depending on the molecular functional groups, the molecules adsorb either randomly on the substrate or
preferentially on the nanowires. In the latter case, chemisorption of suitable organic molecules on the
nanowires leads to a well-defined one-dimensional aggregation and changes the metallic character of
the nanowires to a semi-conducting one.
Ó 2008 Elsevier B.V. All rights reserved.
One-dimensional (1D) metal, semiconductor and insulator
structures have attracted a great deal of interest from the scientific
community because of their potential for nanotechnology and the
opportunity they provide to understand the fundamentals of the
physics of low-dimensional systems. Fabricating these structures
with controlled pattern and dimensions remains experimentally

Torr) equipped with sputtering and anneal-
ing facilities, a Si evaporator for SiNWs formation, organic mole-
cules sublimation stations and a room-temperature Omicron
STM. STM images were recorded in constant current topographic
mode and processed with the WSxM software [5]. We present
the STS results as both the current–voltage I(V) and the normalized
differential conductance (dI/dV)/(I/V) curves, the latter serving as a
good approximation of the local density of states (LDOSs) [6–8].
A single crystal Ag(110) purchased from Mateck was prepared
by several cycles of Ar-ion sputtering (500 eV) and annealing
(690 K). This procedure produced an atomically clean surface with
80 nm wide terraces exhibiting a rectangular (1 Â 1) Ag unit cell.
The NWs were obtained by deposition of Si atoms on the clean
Ag(110) surface at room-temperature at a typical rate of 0.7 Å/
min. This process led to the formation of a 1D-template surface
consisting of SiNWs oriented along the ½

110 direction of the
Ag(110) surface. The THAP molecules, synthesized and purified
by the group of Marder [9], were deposited on the SiNWs/
Ag(110) surface by thermal evaporation ($480 K) from a quartz
crucible at a rate of 0.5 Å/min. Deposition rate and thickness were
estimated using a calibrated quartz crystal microbalance. The
0039-6028/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2008.04.023
* Corresponding author. Tel.: +1 609 258 3582; fax: +1 609 258 6279.
E-mail address: (E. Salomon).
Surface Science 602 (2008) L79–L83
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density of occupied and unoccupied states of the sample, respec-
tively. The data exhibit two clear peaks located at À0.89 V and
+1.82 V. These features correspond to two electronic quantum states
presumably due to the confinement of electrons inside the SiNWs.
The peak at À0.89 V is in good agreement with the first discrete state
previously observed by PES at a binding energy of 0.92 eV with re-
spect to E
F
[1]. A comparison of the STS data with DFT calculations
shows that the observed LDOS is mainly due to the Si3p orbitals of
the silicon atoms [4]. In addition, a non-zero LDOS is observed in
the proximity (À2 eV to +2 eV) of E
F
. This non-zero LDOS, as well
as the monotonic increase in current with bias (inset of Fig. 2), sug-
gest that the SiNWs present a metallic character. This supports the
observation made by Leandri et al. based on a detailed analysis of
the Si2p core levels and valence band of the surface [1].
To address the issue of whether or not any type of molecule will
react exclusively with the SiNWs, we used two different types of
molecules. For the control experiment, a molecule (THAP) with
no particular functional group that could specifically react with
the SiNWs was chosen. Subsequently, a molecule (PQ) with a dicar-
bonyl group, for which a strong and preferential reaction with the
SiNWs is expected, was selected. The THAP molecule has been
thoroughly studied in our group by means of PES and STM. Its
adsorption on a clean Ag(110) substrate was characterized by
STM [10], allowing us to compare it to the adsorption on the SiNWs
modified Ag(110) surface.
Two filled states STM images of the surface following the evap-

Ag(110) surface to 1 L of PQ, the result of which is shown in
Fig. 4a. Two different types of structures are observed: bright pro-
trusions, which are attributed to aggregates of PQ molecules, and
faint lines, which correspond to the SiNWs. The faintness of the
SiNWs, as compared to Fig. 1, is due to the presence of the molec-
ular aggregates on top of the SiNWs which decreases the contrast
between the SiNWs and the Ag surface. The molecular aggregates
form discontinuous lines oriented along the ½

110 direction of the
Ag that corresponds to the direction of the SiNWs. Furthermore,
from this image one can notice that the molecular aggregates are
exclusively adsorbed on top of the SiNWs. This observation is even
clearer on the bottom right inset image. For this exposure, no
molecular aggregate can be observed in between two SiNWs,
clearly demonstrating a highly selective reaction between the PQ
molecules and the SiNWs. As the exposure to PQ increases up to
4L(Fig. 4b), the 1D alignment and the selective adsorption of the
organic structure are preserved, confirming that the first step in
the adsorption process of the PQ on the SiNWs/Ag(110) surface
is the specific interaction with the SiNWs.
We propose that the selective adsorption of PQ molecules on
the SiNWs occurs via chemical reaction. In order to describe the
adsorption mechanism, we refer to the structural model of an indi-
vidual SiNW proposed by Sahaf et al. [2]. According to this model,
the top layer of the NWs consists of Si dimers (Fig. 5a). Following
the experimental work done by Fang et al. and Hacker et al.
[13,14] as well as the first principle calculations performed by Her-
mann et al. [15] on the adsorption of PQ on Si(001), we propose
that an interaction occurs here between the Si dimers and the

ond, as inferred from both the I(V) (inset of Fig. 2) and (dI/dV)/(I/V)
curves, the ONWs do not exhibit a metallic character. The plateau
observed in the I(V) curve is clear evidence of the semi-conducting
nature of the ONWs. The 0.3 eV gap observed in the (dI/dV)/(I/V)
curve is consistent with the semi-conducting character of the
ONWs, even though the gap appears smaller than expected for
the PQ molecule ($2 eV for an isolated molecule [15]). Since the
thickness of the ONWs is of the order of several ångström, the
observed LDOS is a convolution of the LDOS of the SiNWs and that
of the adsorbed PQ aggregates. Therefore, the contribution of the
LDOS of the SiNWs at the proximity of the Fermi level is presumably
Fig. 2. STM filled states images of the SiNWs adsorbed on Ag(110) (I
t
= 0.35 nA, V
s
= À50 mV) for (a) 50 Â 50 nm and (b) 20 Â 20 nm.
Fig. 3. Average STS spectra representing the normalized differential conductivity of
both the SiNWs (dotted red line) and the ONWs (full blue line) obtained upon the
adsorption of 4 L of PQ. The inset shows the corresponding I(V) curves.
Fig. 4. STM filled states images of the adsorption of 4 Å of THAP on the SiNWs/Ag(11 0) surface (I
t
= 0.35 nA, V
s
= À1.29 V) for (a) 110 Â 110 nm and (b) 22 Â 22 nm.
E. Salomon, A. Kahn / Surface Science 602 (2008) L79–L83
L81
SURFACE SCIENCE
LETTERS
responsible for the shrinkage of the gap. For the same reason, the
onset of the feature appearing at À0.5 V is presumably due to the

= À1.10 V). The inset corresponds to a zoom-in of
25 Â 25 nm. (b) About 90 Â 90 nm STM filled state images of the SiNWs/Ag(11 0) surface exposed to 4 L of PQ (I
t
= 0.2 nA, V
s
= À1.50 V).
Fig. 6. (a) Top view of the template SiNWs/Ag(1 10) surface corresponding to the structure proposed by Sahaf et al. [2] and (b) side view of the PQ/SiNWs adsorption
mechanism.
L82 E. Salomon, A. Kahn / Surface Science 602 (2008) L79–L83
SURFACE SCIENCE
LETTERS
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.susc.2008.04.023.
References
[1] C. Leandri, G. Le Lay, B. Aufray, C. Girardeaux, J. Avila, M.E. Da
´
vila, M.C. Asensio,
C. Ottaviani, A. Cricenti, Surf. Sci. 574 (2005) L9.
[2] H. Sahaf, L. Masson, C. Leandri, B. Auffray, G. Le Lay, F. Ronci, Appl. Phys. Lett.
90 (2007) 263110.
[3] M.A. Valbuena, J. Avila, M.E. Davila, C. Leandri, B. Aufray, G. Le Lay, M.C.
Asensio, Appl. Surf. Sci. 254 (2007) 50.
[4] G M. He, Phys. Rev. B 73 (2006) 035311.
[5] I. Horcas, R. Fernandez, J.M. Gomez-Rodriguez, J. Colchero, J. Gomez-Herrero,
A.M. Baro, Rev. Sci. Instrum. 78 (2007) 013705.
[6] J.A. Kubby, J.E. Griffith, R.S. Becker, J.S. Vickers, Phys. Rev. B 36 (1987) 6079.
[7] R.M. Feenstra, Phys. Rev. Lett. 63 (1989) 1412.
[8] G. Garreau, S. Hajjar, G. Gewinner, C. Pirri, Phys. Rev. B 71 (2005)
193308.