BARIS ET AL.
’ NO. 8
June 30, 2012
C2012 American Chemical Society
Noncovalent Bicomponent Self-
Assemblies on a Silicon Surface
Bulent Baris,†Judicae ¨l Jeannoutot,†Vincent Luzet,†Frank Palmino,†Alain Rochefort,‡and
Fre ´de ´ric Che ´rioux†,*
†Institut FEMTO-ST, Université de Franche-Comté, CNRS, ENSMM, 32, Avenue de l0Observatoire, F-25044 Besanc -on Cedex, France and‡Département de génie
physique andRegroupementquébécois sur les matériauxdepointe(RQMP),?EcolePolytechnique deMontréal, CP6079, Succ.Centre-ville,Montréal, CanadaH3C 3A7
temperature, would constitute a break-
through for molecular nanodevice applica-
tions.1?3Moreover, in order to preserve the
pristine unique properties of the targeted
molecules in the nanostructures, the cohe-
sion of networks requires selective and
noncovalent interactions. To achieve func-
tional nanostructures, multicomponent su-
pramolecular assemblies on noble metal
molecular noncovalent architecture has
never been observed on a silicon adatom-
based surface. Indeed, for heteromolecular
networks, the interactions involved in the
due to combination of molecule?molecule
and molecule?substrate interactions. In con-
trast to noble metal or HOPG surfaces, this
semiconductive surfaces where molecule?
substrate interactions cannot be neglected
with respect to molecule?molecule interac-
tions. These strong molecule?substrate inter-
covalent grafting, but their presence can also
lecular edifice.21,22Successful attempts that
avoid strong molecule?substrate interac-
tions on semiconductors have been rarely
observed.23?29Nevertheless, the use of inex-
titutes a prerogative in the development of
many devices, such as for molecular electro-
nics, energy conversion, etc.
In the present paper, we describe the
creation of the first large-scale engineered
work on a silicon adatom surface in the
absence of covalent bonds between mole-
cules and substrate. This was achieved by
he fabrication of ordered nanostruc-
ture arrays over macroscopic areas,
with thermal stability at least at room
surface and specifically designed molecular
building blocks. The nanostructures were in-
vestigated by ultrahigh vacuum scanning
tunneling microscopy (UHV-STM), DFT cal-
culations, and STM image simulations. These
networks were further used as the template
for the growth of a periodic noncompact ful-
RESULTS AND DISCUSSION
The Si(111)-B√3 ?√3R30? surface, ob-
tained by specific ultrahigh vacuum (UHV)
thermal treatment of commercially avail-
able wafers, possesses the unique charac-
teristic of showing depopulated dangling
bonds due to the presence of boron atoms
Therefore, molecule?substrate interactions
are weak enough for molecules to diffuse
on the surface and strong enough for con-
trolling the growth of the supramolecular
network through a surface template effect.
*Address correspondence to
Received for review April 26, 2012
and accepted June 30, 2012.
ABSTRACT Two-dimensional supramole-
cular multicomponent networks on surfaces
are of major interest for the building of highly
ordered functional materials with nanometer-
sized features especially designed for applica-
tions in nanoelectronics, energy storage, sen-
sors, etc. If such molecular edifices have been
previously built on noble metals or HOPG
surfaces, we have successfully realized a 2D
fullerenes by site-specificity inclusion into a bicomponent supramolecular network.
KEYWORDS: supramolecular self-assembly.scanning probe microscopy.
semiconductors.surface chemistry.bicomponent network
BARIS ET AL.
’ NO. 8
is 0.66 nm. This value is also close to the distance
triphenylbenzene core (see Figure 1). This distance
matching has been recently used for the formation of
a commensurable large-scaled 2D open supramolecu-
lar network of 1,3,5-tri(40-bromophenyl)benzene (TBB,
Figure 1a) on the Si(111)-B surface.29For the present
work, we have chosen the 1,3,5-tri(400-bromo-4,40-
biphenyl)benzene molecule (BPB, Figure 1b) that is
built around a 1,3,5-triphenyl core. BPB possesses an
additional phenyl ring on each arm that allows us to
investigate the possibility of extending the size of the
nanopores and their periodicity in the network.
Figure 2a shows a typical large-scale STM image,
obtained in the 100?300 K temperature range, of the
BPB/Si(111)-B interface around submonolayer mole-
cule coverage. Resolution of STM images acquired at
100 K is similar to those obtained at room temperature
is stable up to 400 K (see Figure S2 in Supporting
Information). No isolated molecule was observed on a
B leads to the formation of a monolayer (Figure S3 in
2Dnanoporous network showingthree-foldsymmetry
wherein protrusions have a different brightness from
each other. The distance between two disjoined pro-
trusions is 0.65 nm. The nanoporous network is based
on hexagonal nanopores (side: 1.1 nm) surrounded by
Figure 1. CPK (Corey?Pauling?Koltum) model of (a) 1,3,5-
tri(40-bromophenyl)benzene (TBB) and (b) 1,3,5-tri(400-
Figure 2. (a) STM imageof BPB deposited on theSi(111)-Bsurface (20 ? 20 nm2, Vs= 2.0 V, It= 0.015 nA, 100 K), with a large-
scale STM image as an inset (90 ? 100 nm2, Vs= 2.0 V, It= 0.02 nA, 100 K). (b) Simulated STM image of an adsorbed
BPB molecule with a geometry similar to the given model (top) obtained in a constant current mode (Vs= 2.0 V, It= 0.01 nA).
(c) Superimposed model of BPB network on both the Si(111)-B surface and molecular network.
BARIS ET AL.
’ NO. 8
six triangular nanopores (side: 2.0 nm), oriented at 60?
from one to the other. Seven protrusions are observed
within a hexagonal nanopore, while for a triangular
nanopore, only three protrusions (white dots in
Figure 2) can be seen. The periodicity of the network
is 7 ? 7, and the network forms a commensurable
structure with the
Si(111)-B surface (Figure 2).
Given the dimensions of BPB molecules, six dis-
joined protrusions are attributed to one BPB molecule
and two protrusions correspond to a BPB arm. This
hypothesis is strongly supported by STM image simu-
lation. Figure 2b shows the simulated STM image of an
isolated BPB molecule (top of Figure 2b) with multiple
individual tilted phenyl groups attached to the central
31G*) for an isolated BPB molecule and a dimer, we do
not clearly distinguish the contrast associated with term-
Fermi energy. Then, the different contrasts associated
with bromine that are observed experimentally have to
33? rotation ofanyphenylgroupinisolatedBPBnecessi-
tates less than 2.5 kcal/mol, and some of these phenyl
groups are even nearly free to rotate. Since the interac-
tion energy between BPB molecules is more significant
than energy for phenyl rotation and the surface corruga-
tion should also influence the adsorbate structure, the
(see Figure S5 in Supporting Information) can be drasti-
cally different from the gas phase structure. As a result,
the geometry of BPB used in the STM simulation corres-
ponds to the gas phase geometry but where we have
considered a rigid rotation of terminal bromophenyl by
33? with respect to the more central benzene ring.
Although the evaluation of an exact rotation angle of
extensive calculations on a large unit cell, the rigid rota-
tion considered here allows us to reproduce the main
features of experimental STM results.
From the high-resolution STM image observed near
the step edge island, and supported by our simula-
ted STM images, the proposed molecular network
adsorbed on the Si(111)-B surface schematized in
Figure 2c is fully consistent with an ordered commen-
surate BPB adlayer. The adsorption of BPB can be de-
scribed by the position of one bromophenyl group
and one Br atom from one of the two remaining
arms, respectively, located between three Si adatoms
(see green circles in Figure 3) and above a Si ada-
tom (see yellow circles in Figure 3). Overall molecular
√3 reconstruction of the
arrangement and the BPB molecular dimension ex-
plain the formation of the two types of nanopores. The
darker protrusions located in each nanopore (dots in
Figure 2c) are attributed to Si adatoms of the uncov-
ered√3 ?√3 reconstruction. According to this pro-
posed network, one can clearly see that the highest
constrast is always associated with the terminal Br
atomssitting onasurfaceSiadatomwherethe density
of states (DOS) is higher than that for the two other sites
where the remaining Br atoms are located. Hence, the
related to its adsorption site; the highest are observed for
the Br atom in the vicinity of a surface Si adatom.
range, we may conclude that BPB molecules diffuse
easily on the Si(111)-B surface in order to form the self-
assembly. In this supramolecular network, the stabiliza-
tion of molecule?molecule interactions should arise
the stability of the complex only by 5%. Although the
Figure 3. Position of Br atoms above silicon adatoms (yellow
molecule on a Si(111)-B surface.
Figure 4. STM images of C60onto the BPB supramolecular
2.2 V, It= 0.15 nA, 110 K). C60molecules are adsorbed in
hexagonal nanopores, in triangular nanopores, or onto the
BARIS ET AL.
’ NO. 8
exact nature of such π?π interactions is still not totally
clear, a mechanism involving a sort of dipole?dipole
the chemical nature of BPB, a stabilization of the assem-
realistic. Nevertheless, molecule?substrate interaction
appears strong enough in the self-assembly to direct
BPB molecules onto in asingle type of adsorption site (Si
forces the formation of an open network instead of a
compact self-assembly (see Figure 2c and Figure S7 in
Supporting Information). Therefore, thermal stability of
the BPB network above room temperature is justified by
molecule?molecule and molecule?substrate attractive
Trapping of fullerenes in molecular nanoporous net-
formation of a periodic C60array.12?19In our case, the
covalent diameter of C60(0.8 nm) are nicely matching.
Figure 4 shows an STM image, recorded at 110 K, of
C60 molecules adsorbed onto nanopores of a BPB
molecular network. The measured diameter of these
very bright protrusions (2 nm) is compatible with the
van der Waals diameter of C60observed with STM by
to a single C60molecule adsorbed above the nanopo-
rous BPB network. No clear fullerene organization is
observed onto the BPB network: 66% of C60molecules
are adsorbed onto BPB network as small clusters. For
isolated C60molecules, 6% are adsorbed in triangular
nanopores and 28% are adsorbed in hexagonal nano-
pores (Figure 4).
In order to form a high-level periodic array of fullerene
molecules onto the BPB network, we need to find a
strategy for compelling a more specific C60adsorption in
hexagonal nanopores of the BPB network. This could be
achieved by avoiding any adsorption in triangular nano-
pores of the BPB network. Triangular nanopores have
2.0 nm sides (Figure 2a), which is strongly consistent with
dimension of TBB molecules (see Figure 1b). This size
TBB molecules were deposited onto BPB network. STM
Images are described in Figure 5a?c.
Large-scale STM images of the TBB/BPB/Si(111)-B
at 300 K (Figure S8 in in Supporting Information). No
TBB molecule is observed on the free Si(111)-B surface.
Monolayers are constituted by a 2D nanoporous net-
work showing a three-fold symmetry. By comparison
with STM images of the BPB network (Figure 2a), only
hexagonal nanopores (side: 1.1 nm)are now observed,
Figure 5. (a) STM images of TBB onto the BPB supramolecular network on Si(111)-B surface (34 ? 34 nm2, Vs= 2.0 V, It=
in (a) with triangle filling highlighted in blue. (c) Superimposed model of the TBB/BPB network on the Si(111)-B surface.
BARIS ET AL.
’ NO. 8
and the entire amount of triangular nanopores has
disappeared. All triangular nanopores previously ob-
served in the BPB network are now filled by three
disjoined protrusions. These three protrusions arecon-
sistent with STM images recorded for the TBB network
are attributed to adsorbed TBB molecules in triangular
nanopores of the BPB network. The periodicity of the
network is still 7 ? 7 but is rotated by 21.7? and is
always commensurable with the√3 ?√3 reconstruc-
tion of the Si(111)-B surface (Figure 5a).
On the basis of STM images, a superimposed model
for the TBB/BPB/Si(111)-B interface is described in
into hexagonal nanopores. This result is consistent
with the adsorption site of TBB on Si(111)-B that is only
existing in triangular nanopores,29leading to a specific
that TBB?BPB interactions are related to π?π interac-
biphenyl arms of BPB molecules. The 21.7? rotation of
TBB/BPB/Si(111)-B relative to BPB/Si(111)-B accentuates
the TBB?BPB π?π interactions (see Figure S9 in Suppor-
ting Information). All of these attractive interactions en-
sure thermal stability of the supramolecular network.
As the TBB/BPB supramolecular network exhibits
only a single type of nanopores, with sides equal to
1.1 nm, C60molecules were deposited onto this inter-
face. A periodic array of bright protrusions onto the
TBB/BPB/Si(111)-B network is observed in STM images
(Figure 6). Less than 5% of C60molecules appear as a
are exactly located on hexagonal nanopores of the TBB/
BPB network. Therefore, these protrusions are attributed
to a single fullerene molecule sitting on a hexagonal
nanopore of the TBB/BPB network.
The periodicity of C60/TBB/BPB/Si(111)-B network is
still 7?7andis alwayscommensurable withthe√3?
√3 reconstruction of the Si(111)-B surface (Figure 6b
and Figure S10 in Supporting Information). This net-
Information). The periodic self-assembly is based on
specific adsorption of TBB molecules on the BPB net-
work and specific adsorption of C60onto the TBB/BPB
network. Therefore, intermolecular distance between
C60is 4.65 nm. In such bicomponent self-assembly, the
geometrical design of C60patterns is of primary inter-
est for heterojunction solar cells, in order to ensure the
commensurability of exciton dissociation interfacial
area and the exciton diffusion length (5?10 nm).34
We report the formation of a bicomponent supra-
molecular network on a silicon surface with a thermal
stability up to 300 K. This framework has been achieved
by exploiting the interactions between molecules and
substrate. By tuning the size of molecules built from a
1,3,5-triphenyl core, well-defined supramolecular net-
works have been obtained with a controlled long dis-
the route toward a new class of robust and commensur-
able organic networks on a silicon substrate.
Molecule and Substrate Preparation. 1,3,5-Tri(40-bromophenyl)-
benzene was purchased from Aldrich and then purified by
column chromatography on silica gel and then sublimated. C60
1,3,5-Tri(400-bromo-4,40-biphenyl)benzene was synthetized by cy-
clotrimerization of 40-bromophenyl-4-acetophenone.35?37Mol-
ecules were then purified by column chromatography on silica
gel and sublimated.
by annealing of the (111) surface of a highly B-doped Si wafer
(0.001 W3cm resistivity). Si(111) surface is carefully outgassed and
cleaned in situ by a series of rapid heating up to 1200 ?C under
a pressure lower than 5 ? 10?10mbar. A thermal process (1 h at
800 ?C) activates the boron segregation at the surface, and a
maximum boron atom concentration of 1/3 monolayer (ML) can
density with 7.8 ? 1014atoms3cm?2). In these conditions, the
surface exhibits a perfect3 ?
STM Experiments. STM experiments were performed in an
ultrahigh vacuum chamber with a base pressure lower than
2?10?10mbarequipped with avariable temperature Omicron
scanning tunneling microscope (STM). STM images were
Figure 6. (a) STM images of C60adsorbed on the TBB/BPB network on the Si(111)-B surface (36 ? 50 nm2, Vs= 2.2 V, It=
0.01 nA, room temperature). (b) STM image showing electronic substructure of C60and TBB onto the BPB network on the
Si(111)-B surface (20 ? 17 nm2, Vs= 2.4 V, It= 0.01 nA, 100 K).
BARIS ET AL.
’ NO. 8
acquired in a constant-current mode at room temperature or
100 K. 1,3,5-Tri(40-bromophenyl)benzene (TBB) and C60mol-
ecules were deposited from aquartz crucible at 160and350 ?C,
respectively. TheSi(111)-Bsubstrate waskeptatroomtempera-
using WSXM software.38
STM Image Simulation. STM simulations were carried out with
the SPAGS-STM software,39in which we used an electron
scattering approach included in our parallel Landauer?Buttiker
solver in conjunction with a tight-binding Hamiltonian that
reproduces DFT results. DFT calculations on isolated species
were performed with the NwChem software40where we used
the B3LYP functional (6-31G*) and the semiempirical approach
of Grimme to evaluate the contribution of dispersion and van
der Waals interaction to the DFT energy.41
Prior to any STM simulations, DFT calculations were per-
formed on an isolated BPB molecule to obtain the optimized
geometry. Since the size of a single unit cell representing the
multicomponent system is computationally prohibitive, in ad-
dition to the fact that the adsorbed molecules are weakly
interacting with the substrate, we have limited our STM simula-
tion to a single BPB molecule physisorbed on a Cu(111) surface.
In this manner, we exclude a contribution from the substrate to
the STM contrast. In the STM simulations, we have studied the
influence of the rotation of the terminal bromophenyl groups
on the resulting STM images. We found that a rotation of þ33?
of terminal groups (the middle phenyl groups are rotated by
?33?) that were originally flat with respect to the central
benzene ring gives a good agreement with experimental STM
images. On the basis of our DFT calculations, a single phenyl
rotation of 33? would need less than 2.5 kcal/mol to occur for a
fully relaxed structure.
Conflict of Interest: The authors declare no competing
Acknowledgment. This work is supported by the Pays de
the French Agency ANR (ANR-09-NANO-038). A.R. is grateful to
Calcul Québec and Calcul Canada for providing computational
Supporting Information Available: Additional figures. This
material is available free of charge via the Internet at http://
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