A stable room-temperature molecular assembly of zwitterionic organic dipoles guided by a Si(111)-7 x 7 template effect

DOI: 10.1002/anie.200702794

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Angewandte Chemie - International Edition p. 9287 - 9290 (2007)

Update date:2022-08-30

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Authors:

Makoudi, Younes

Arab, Madjid

Palmino, Frank

Duverger, Eric

Ramseyer, Christophe

Picaud, Fabien

Cherioux, Frederic

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Article abstract of DOI:10.1002/anie.200702794

(Figure Presented) Chiral assemblies of achiral molecules: High-resolution STM images of zwitterionic organic dipoles deposited on Si(111)-7 x 7 show a chiral molecular assembly on this surface (see picture). Density functional calculations demonstrate that a sulfonato group can act as an electrostatic shield that protects the π skeleton of organic molecules from the dangling bonds of semiconductor surfaces, which is a major advance in the deposition of π-conjugated molecules.

Full text of DOI:10.1002/anie.200702794

Angewandte

Chemie

DOI: 10.1002/anie.200702794

Silicon Surface Chemistry

A Stable Room-Temperature Molecular Assembly of Zwitterionic

Organic Dipoles Guided by a Si(111)-7 7 Template Effect**

Younes Makoudi, Madjid Arab, Frank Palmino, Eric Duverger, Christophe Ramseyer,

Fabien Picaud, and FrØdØric ChØrioux*

The adsorption of functional molecules on surfaces plays a

vital role in the emerging field of nanoelectronics.^[1]In this

context, molecular and supramolecular ordering, which are

key steps in the development of complexes architectures, are

controlled by a balance between intermolecular forces and

molecule–substrate interactions.^[2]This restructuring is often

driven by cooperative molecule–substrate interactions involv-

ing many molecules and is not directly related to the shape of

the individual molecules.

The deposition of organic molecules and preservation of

their entire skeleton, especially on semiconductor surfaces, is

still a challenge, although much progress has been made in the

development of metallic surface-based devices with organic

molecules at low temperature, where the molecule–surface

interactions are weak and diffusion remains low.^[3]However,

the use of metallic substrates is less attractive for potential

applications as semiconductors, especially in the field of

nanoelectronics.

The deposition of molecules on a semiconductor surface

at room temperature with complete control of the adsorption

site and without altering their aromatic behavior is still a

challenge in view of the development of complex architec-

tures.^[4]Polanyi and co-workers, for example, have shown that

the adsorption of chlorododecane on Si(111)-7 7 at room

temperature leads to the formation of a bistable dimeric

corral of self-assembled molecules and have demonstrated

on/off electronic switching of a single silicon adatom using

molecular field effects.^[5]These authors have also described a

new strategy for preventing the dissociation of haloalkanes on

Si(111)-7 7 up to 373 K. This stability is ensured thanks to

the formation of dimeric circular corrals that override the

molecule–substrate interactions.^[5]Herein, we propose a new

concept for the room-temperature deposition of p-conjugated

organic molecules, without any modification of their elec-

tronic structure, at specific adsorption sites on the surface of

the semiconductor Si(111)-7 7 by adsorbing p-conjugated

zwitterionic molecules on this semiconductor surface. The

presence of a negative charge on the target molecules offsets

the electrophilic character of the Si(111)-7 7 adatoms and

preserves the p skeleton of the organic molecules after

deposition. The half-cells of Si(111)-7 7 act as a template

that guides the molecular assembly of achiral molecules, as

shown by the induction of chirality in the modified areas.

Experimental STM images as well as theoretical calculations

are in perfect agreement with the proposed concept.

The electronic properties of molecules are often modified

during their adsorption on a Si(111)-7 7 surface. Indeed,

Si C s bonds are usually formed by reaction of the electron-

deficient silicon adatoms of Si(111)-7 7 with the electron-

rich carbon atoms of the organic molecules.^[6]We propose an

original concept to prevent these cycloadditions by using the

negative site of a zwitterionic molecule as a shield (by

electrostatic interactions) for the molecule on the electro-

philic adatoms of Si(111)-7 7. Zwitterionic molecules are

ideal candidates since they are neutral but carry formal

positive and negative charges on different atoms. In principle,

the surface should offset its electron deficit with the negative

charge of the molecules instead of its extended electron-rich

p-conjugated system. To prove our concept, we synthesized 4-

methoxy-4’-(3-sulfonatopropyl)stilbazolium (MSPS), which is

terminated by a negatively charged sulfonato (SO₃) group

(Figure 1a), as a model zwitterionic organic molecule. All

experiments were carried out in an ultra-high vacuum

chamber with a pressure lower than 2 10 ^À10mbar. The

molecules were sublimed from a Knudsen cell at 390 K onto

the Si(111)-7 7 surface at room temperature. STM images

were acquired in the usual constant-current mode at room

temperature.

Figure 1b,c shows STM images taken with two opposite

polarities, with atomic resolution, of both the Si(111)-7 7

reconstruction and the adsorbed molecules. One half-cell

exhibits a novel threefold star at the center of the image for

both filled and empty states. Each arm of the star consists of

two matching protrusions, one of which is more intense than

the other. The distance between the centers of the two paired

protrusions (0.67 nm) is much shorter than the length of a free

MSPS molecule (1.3 nm). This difference can be explained by

a conformational change caused by MSPS–substrate inter-

actions (see below). Three protrusions are located exactly on

top of corner adatoms and the other three are found between

a rest-atom and an adjacent center adatom. In the filled states

(Figure 1b), the brighter protrusions are situated exactly on

top of corner adatoms, whereas in the empty states (Fig-

[*] Y. Makoudi, Dr. M. Arab, Dr. F. Palmino, Dr. E. Duverger,

Dr. F. ChØrioux

Institut FEMTO-ST/LPMO, UMR CNRS 6174

32, Avenue de l’Observatoire, 25044 Besancon cedex (France)

Fax: (+33)8185-3998

E-mail: frederic.cherioux@femto-st.fr

Homepage: http://www.femto-st.fr

Prof. Dr. C. Ramseyer, Dr. F. Picaud

Laboratoire de Physique MolØculaire, UMR CNRS 6624

16 Route de Gray, 25030 Besancon cedex (France)

[**] The authors thank Dr. C. Joachim (CEMES, Toulouse, France) for

fruitfuldiscussions. This work was supported financiayl by the

CommunautØ d’AgglomØration du Pays de MontbØliard.

Supporting information for this article is available on the WWW

under http://www.angewandte.org or from the author.

Angew. Chem. Int. Ed. 2007, 46, 9287 –9290

ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

9287

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Communications

as described above, which means that this assembly process

implies high molecular diffusion onto the surface at room

temperature.

The assembly of three molecules into this particular star

shape can be explained by the template effect of the surface,

which possesses threefold symmetry. This assembly process is

probably guided by the template effect of the substrate.

Moreover, the intensities of the protrusions at each polarity

are not particularly affected by a change of the bias voltage,

which means that they are not likely to be due to the well-

known formation of C Si s bonds between adsorbed mole-

cules and a substrate.^[6,8]

The nature of MSPS adsorption on a Si(111)-7 7 surface

has been investigated by ab initio calculations at the extended

density functional theory (DFT) level using the Vienna Ab

Initio Simulation Package (VASP).^[9]To simulate the entire

system (MSPS + Si(111)-7 7) we first optimized the geom-

etry of one free MSPS molecule in vacuo in the cis

conformation using Gaussian 03 at the Hartree–Fock 6-

31G(d) level of theory.^[10]Three of these optimized molecules

were then positioned in the cis conformation thanks to the

help of the high-resolution STM images discussed above. The

adsorption energy was then varied at room temperature for

different assemblies of molecules and surface distances (d),

which correspond to the separation between the Si adatoms

and sulfur atom of the sulfonato group. Finally, the local

densities of states (LDOS) were determined at each point for

a constant current and at bias voltages of À2and + 2V,

respectively, with the Tersoff–Hamann scheme.^[11]

The adsorption energy for the entire system is depicted in

Figure 2a as a function of the distance d. The equilibrium

distance, which corresponds to the energy minimum of

À2.66 eV (À0.887 eV per molecule) is close to 0.26 nm, in

agreement with a physisorbed state. The LDOS images

calculated at the equilibrium distance compare well with the

experimental STM images (Figures 2b and 2c). Figure 2d

shows the DOS of the whole stable system at d = 0.26 nm. The

two weak interactions observed correspond to a very small

electronic transfer from the sulfonato groups to the corner

adatoms and from methoxy moieties to Si atoms, respectively,

which means that there is no bond formation between any

single oxygen atom and a silicon adatom. An interaction

between the molecular assembly and the substrate never-

theless exists, but it depends only on the distance, in agree-

ment with a weak electrostatic interaction between the

oxygen atoms of the sulfonato moieties and the electrophilic

Si adatoms. These results confirm that we have developed a

new strategy for stabilizing zwitterionic p-conjugated mole-

cules on a Si(111)-7 7 surface by taking advantage of the

electrostatic interactions between silicon adatoms and the

anionic groups of these molecules. These interactions are

strong enough to stabilize the molecular assembly at room

temperature and produce a template effect whereby the

Si(111)-7 7 surface imposes a cis conformation on the MSPS

molecules in the Si(111)-7 7 half-cells.

Figure 1. a) Molecular structure of MSPS. b,c) High-resolution, room-

temperature STM images (77nm²) of MSPS deposited on Si(111)-

77 in the filled state (sample negative bias voltage: V_s=À1.6 V) and

in the empty state (sample positive bias voltage: V_s=+1.9 V),

respectively. d) Both the above STM images with superimposed ball

and structural models showing the assembly of three MSPS molecules.

The structuralmodelof Si(111)-77 (right) with a superimposed bal

model of the self-assembly of three MSPS molecules summarizes the

STM images in the two polarities.

ure 1c) the brighter protrusions appear between a rest-atom

and a center adatom. The sulfonato moiety in MSPS possesses

a negative charge, and the methoxy group is electron-poor

owing to conjugation with the pyridinium ring. Therefore, the

intense protrusions observed in the filled states on top of

corner adatoms can be attributed to the sulfonato groups

(Figure 1b) and the intense protrusions observed in the empty

states between a rest-atom and an adjacent center adatom to

the methoxy groups (Figure 1c).

The position of the MSPS molecules in this assembly can

be determined unequivocally thanks to the use of two

different polarities when recording the STM images. The

star shape in each 7 7 half-cell can be attributed to the

assembly of three MSPS molecules by molecule–substrate

interactions, which change the conformation of the three

molecules from trans to cis. The proposed structural model of

the 7 7 assembly is superimposed on the STM images in

Figure 1d.

At very low molecule coverage this adsorption and

formation of star shapes occurs preferentially on the faulted

half-cell, which is known to be more reactive (Figure 1b).^[7]

The occupied half-cells only contain three MSPS molecules,

Figure 3 shows two half-cells, each of which contains an

assembly of three MSPS molecules that is a mirror image of

the other reflected in a mirror plane. This means that the

threefold stars are chiral even though free MSPS is achiral.

9288 www.angewandte.org

ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Angew. Chem. Int. Ed. 2007, 46, 9287 –9290

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Angewandte

Chemie

Figure 3. A room-temperature STM image (V_s=À2.1 V, 79 nm²)

showing the two enantiomers of the chiral supramolecular self-

assembly of MSPS deposited on Si(111)-77 (left) and a schematic

modelof these two enantiomers on Si(111)-77 (right).

these previous studies on semiconductors is due to the

formation of at least one stereogenic center after the

chemisorption, for example by cycloaddition, of molecules

on silicon or germanium atoms. The results reported here

show, to the best of our knowledge, the first example of chiral

assembly of achiral molecules on Si(111)-7 7.

In conclusion, the results reported herein are remarkable

from several different points of view, especially the molecular

assembly of organic dipoles at room temperature, the chirality

of the assembly, and the stability and conservation of the p-

conjugated skeleton on the semiconductor surface at room

temperature. This zwitterionic strategy, where the sulfonato

group acts as an electrostatic shield that protects the

p skeleton of organic molecules against the dangling bonds

of the semiconductor surface, may become an important

method for the deposition of p-conjugated molecules.

Experimental Section

The Si(111) substrate was heated under ultra-high vacuum by direct

current. Clean Si(111)-7 7 surface reconstruction was obtained by

repeated cycles of heating at 12008C and slow cooling to room

temperature. Deposition of the MSPS molecules from an Mo crucible

onto the sample at room temperature was performed at 1208C and a

pressure lower than 10^À10mbar.

Electron–ion interactions were described using the projector-

augmented wave (PAW) method, which was expanded within a plane-

wave basis-set up to a cut-off energy of 400 eV.^[14]Electron exchange

and correlation effects were described by the Perdew–Burke–

Ernzerhof generalized gradient approximation (PBE GGA)

exchange-correlation functional.^[15]The Brillouin zone was sampled

using the gamma point only. Simulations were performed using a

volume box (21.67 nm³) containing one 7 7 unit cell (DAS model

with four silicon layers).^[16]The adsorption energy was calculated

from the difference between the total energy of the entire system

(substrate + molecules) when interacting and the sum of the total

energies of each free component.

Figure 2. a) Adsorption energy as a function of distance. b,c) Inte-

grated LDOS images of three MSPS molecules on Si(111)-77 (left)

and with a superimposed ball model (right) with a negative bias

voltage (À2 V, b) and a positive bias voltage (+2 V, c). d) A side view

of the density of states of MSPS adsorbed on Si(111)-77 showing

weak molecule–substrate interactions.

The chirality of these molecular assemblies can be explained

by the template effect of the Si(111)-7 7 surface. The

sulfonato group can swivel upon the corner adatom, whereas

the surface forces the methoxy moieties to be fixed between a

rest-atom and a center adatom (Figure 3). The two possible

enantiomers in this case, which are formed by a clockwise or

anticlockwise folding, are termed the (P)-enantiomer and the

(M)-enantiomer, respectively. Most previous studies regard-

ing chiral structures have been concerned with metal surfa-

ces,^[12]and relatively little work has been reported for

semiconductor surfaces.^[13]However, the chirality induced in

All synthetic procedures and calculations are fully described in

the Supporting Information.

Received: June 25, 2007

Revised: September 13, 2007

Published online: November 6, 2007

Keywords: chirality · scanning probe microscopy ·

ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

semiconductors · surface chemistry · zwitterions

Angew. Chem. Int. Ed. 2007, 46, 9287 –9290

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Communications

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