book of abstracts - IM2NP

A B S T R A C T S WEDNESDAY, JUNE 30 N A N O S E A 2 0 1 0
surface scale of molecules showing particular electronic properties (insulator, molecular wire, memory,
diode, …) [3]. Using silicon as a SAM substrate appears to be a promising challenge in molecular
electronics. Indeed, this hybrid approach benefits from both the well-developed silicon technology and the
specific properties of molecules. Therefore, preparation of self-assembled monolayers of aromatic
conjugated molecules on silicon is a key point in molecular electronics [4]. Moreover, regarding potential
applications, it is important to be able to prepare nano-islands of such active molecules on silicon.
Nevertheless few works addressed this subject [5]. To achieve these two points, strong interactions between
aromatic molecules are mandatory. Varying these interactions in order to identify the right interaction
strength suitable for preparing dense aromatic SAMs either at a large surface scale or within nano-islands is
the scope of this work. For this purpose we used two aromatic molecules bearing a phenyl ring on a small
alkyl chain, with (Fig. 1b) and without (Fig. 1a) fluorine substitution.
F
F
Cl
Si Cl
Cl
F
F
F
Cl
Cl
Si
Cl
Figure 1. Aromatic trichlorosilane molecules used in this study: (a) phenylbutyltrichlorosilane (PBTCl),
(b) pentafluoro-phenylpropyltrichlorosilane (FPPTCl).
2 – Abstract
In a first part of our work, in order to control the formation of conjugated molecular nano-domains on native
oxide covered silicon, we studied how various tri-functionalized silane molecules bearing a phenyl cycle,
modified or not, interact during their self-assembly [6].
Concerning phenyl rings without alkyl chain, SAM growth is shown to occur in a single step: chemisorption
on the surface. This step is thermally activated and does not depend on ring to ring interactions. We show
that adding a short alkyl chain (3-4 carbon atoms) to the phenyl ring gives the molecules enough flexibility
to generate an additional second growth step. The latter is independent from the deposition temperature and
corresponds to the arrangement between molecules. We found that this packing step is accelerated by
replacing phenyl by pentafluoro-phenyl rings, possibly due to quadrupolar interactions between fluorinated
cycles. Furthermore we demonstrate that mixing phenyl and pentafluoro-phenyl molecules leads to an even
faster packing step which is accounted for by hydrogen bonding CH FC in a face to face
phenyl/pentafluoro-phenyl arrangement [7,8,9]. We believe these results allow improving charge
delocalization over conjugated molecular domains.
In a second part, we studied the phase separation between phenyl-alkylsilane and octadecyltrichlorosilane
(OTS) molecules. Improving the phase separation was studied using two parameters: ring to ring interactions
afore-analyzed and reactive heads with different grafting kinetics. Using the same trichlorosilane grafting
moiety for phenyl molecules as for OTS, we show that phase separation is improved and OTS islands are
smaller with phenyl species that involve stronger ring to ring interactions. The best case is obtained with
mixing phenyl and pentafluoro-phenyl rings using hydrogen bonds for packing together the aromatic species
of the SAM. Small phenyl species islands (40-100 nm in diameter) could be obtained inside the OTS SAM
using a less reactive grafting head for the aromatic molecules. These two cases demonstrate an improved
control of SAM composition and morphology essential to further use the obtained islands for building
molecular devices.
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