Phase selectivity triggered by nanoconfinement: the impact of corral dimensions

Article abstract of DOI:10.1039/C8CC08602H

By using a novel protocol to spatially confine molecules in well-defined small 2D areas, the so-called nanocorrals, we show using scanning tunneling microscopy (STM) how this kind of confinement affects self-assembled molecular network (SAMN) formation at a liquid-solid interface. The 2D lateral confinement, imposed by the size of the nanocorrals, has a clear impact on the phase selectivity of a molecule that can form both low-density and high-density SAMNs, the high-density phase being promoted by the confinement.

Full text of DOI:10.1039/C8CC08602H

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Phase selectivity triggered by nanoconfinement: the impact of
corral dimensions
Yi Hu,a,b Ana M. Bragança,a Lander Verstraete, a Oleksandr Ivasenko, a Brandon E. Hirsch, a Kazukuni
Received 00th January 20xx,
Accepted 00th January 20xx
Tahara, c,d Yoshito Tobe, c and Steven De Feyter*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
By using a novel protocol to spatially confine molecules in well- interface is an attractive environment to support the formation
defined small 2D areas, the so-called nanocorrals, we show using of such self-assembled supramolecular networks (SAMNs). If an
scanning tunneling microscopy (STM) how this kind of confinement atomically flat and conductive substrate is used, scanning
affects self-assembled molecular network (SAMN) formation at a tunneling microscopy (STM) is an ideal local probe microscopy
liquid-solid interface. The 2D lateral confinement, imposed by the technique to characterize the structure and dynamics in these
size of the nanocorrals, has a clear impact on the phase selectivity SAMNs, at the liquid-solid interface, at the nanoscale.
of a molecule that can form both low-density and high-density Numerous aspects, structural as well as functional, have been
SAMNs, the high-density phase being promoted by the investigated, for such SAMNs. A rather intriguing phenomenon
confinement.
is their concentration dependent phase behavior. The SAMN
structure might differ depending on the concentration of the
solute in the liquid-phase. In such cases, quite often low-density
SAMNs are favored at low concentrations while upon increasing
the solute concentration a transition towards one or more high-
density phases is observed.13–15 A similar effect can be obtained
upon changing the temperature.16,17 The gradual concentration
or temperature dependent transition of one SAMN structure
Many functional and sometimes structural properties of
materials change upon spatial confinement. Among the many
cases, the particle size dependent emissive properties of
inorganic nanoparticles such as CdSe is a textbook example.1
But also in organic materials such size dependent properties
arise.2,3 For instance, the deviation from bulk values of the glass
transition temperature of polymer thin films has been
attributed to the existence of confinement effects.4–6 Also,
properties of small molecules, such as their viscosity, can
change dramatically when confined in thin films.7,8
Nanoconfinement of materials in nanopores has led to a range
of interesting observations. For example, freezing and melting
points are often depressed in nanopores.9,10 Or the
polymorphism of some organic compounds can be significantly
affected as well.11,12
“Polymorphism” is a phenomenon that also occurs in
monolayer thin films of molecules self-assembled into
supramolecular networks on solid substrates. The liquid-solid
a. Department of Chemistry, Division of Molecular Imaging and Photonics, KU
Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium.
b. College of Materials Science and Engineering, South China University of
Technology, Guangzhou 510640, China.
c. Department of Applied Chemistry, School of Science and Technology, Meiji
University, 1-1-1 Higashimita, Tama-ku, Kawasaki, 214-8571, Japan.
d. Division of Frontier Materials Science, Graduate School of Engineering Science,
Osaka University, Toyonaka, Osaka 560-8531, Japan.
Fig. 1(a) Schematic illustration of the self-assembly of DBA derivatives on bare
HOPG and in confined spaces. (b) Chemical structures of DBA derivatives. (c)
Schematic models for porous and non-porous patterns. (d) Chemical structure
and schematic model of a covalently bound 3,4,5-trimethoxy phenyl group on a
graphene flake.
†
Electronic Supplementary Information (ESI) available: [Experimental details
regarding sample preparation, STM measurement, data analysis can be found in ESI.
Additional data on the self-assembly of DBA-OC13 and DBA-OC12 on pristine HOPG
or in corrals are also available]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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into another one can be explained based on thermodynamic done by measuring porous phase, and the 50% surface coverage
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grounds, albeit also kinetic effects may contribute. Motivated of porous phase was taken as the siDgnOI:o10f.1‘0n3o9/Cd8isCcCe0r8n6a0b2lHe
by the impact of nanopores on the polymorphism of certain selectivity towards either of the phases’.
organic systems,11,12,18,19 we reasoned that lateral confinement
For the nanoconfinement experiments, HOPG surface was
conditions could also reveal similar phenomena for SAMN fully grafted with a dense layer of 3,4,5-trimethoxyphenyl
formation at the liquid-solid interface.
groups (ESI, section S5).26,27,28 The most relevant factor to
Herein we report the first observation of phase-selectivity in choose this aryl species was that even in low-resolution STM
molecular self-assembly under lateral nanoconfinement (Figure images 3,4,5-trimethoxyphenyls can be readily distinguished
1a). This was done for a well-investigated model system – from DBA-OC13. Via an in-situ STM based nanolithography
SAMNs based on alkoxylated dehydrobenzo[12]annulene (DBA) protocol removing locally grafted molecules at the interfaces of
derivatives.13,20 Controlled nanoconfinement was realized on the grafted graphite and a 1-phenyloctane solution containing
arylated highly oriented pyrolytic graphite (HOPG), the DBA-OC13, areas defined in size and shape are formed, the so-
substrate-of-choice for the custom design, in-situ modification called nanocorrals.22,23 The direct exposure of the newly-
and convenient STM characterization of graphite corrals.21–23 generated surface to DBA-OC13 solution prevented
Some rationales behind our system design choices are given contamination, otherwise, frequently observed when DBA-
below, while more technical aspects are detailed out in ESI.
The selected DBA-OC13 (Figure 1b) is a medium-size DBA
OC13 solution was introduced after ex-situ corral formation.
To probe the potential impact of the geometrical
building block that, just like other alkoxylated DBA nanoconfinement, square nanocorrals of different sizes were
derivates,13,24,25 when assembled from 1-phenyloctane (1-PO), created, ranging from 24 × 24 nm2 to 201 × 201 nm2. From here
can form two polymorphs: porous and linear (Figures 1c and 2). on, we denote the size of the nanocorral by referring to the
Control experiments (Figure S4, Table S1) on unmodified HOPG length of its side. The nanoshaving experiments were carried
have shown that ~ 50% : 50% porous : non-porous composition out at a DBA-OC13 concentration of 5.6 × 10–6 M, which on
is achieved in 5.6 × 10–6 M solution. That is why all pristine graphite surface ensures an equal surface coverage of
nanoconfinement experiments were done at this the porous and non-porous phases. The surface coverage of the
concentration. Furthermore, 1-PO leads to stable in-situ porous and non-porous phases for each of the nanocorrals is
nanoshaving conditions. It also turned out that the DBA pores summarized in Table S4 and presented in Fig. 3c. The trend is
are easier to identify than non-porous assemblies (especially, clear. In contrast to experiments carried out on pristine
when analyzing the content of the smallest nanocorrals (24-60 graphite, hardly any porous phase is observed in the smaller
nm) in which STM images often lacked sub-molecular corrals. Only locally, some hexagonal pores, indicated by green
resolution). Hence the surface coverage analysis was primarily dots in Fig. 3a, b, are observed. The larger the nanocorrals, the
larger the fraction of the porous phase, but it remains always
lower than the one observed on pristine graphite. Only, at a
corral size of about 200 nm, the surface coverage of the non-
porous phase equals the surface coverage of the porous phase,
reflecting the situation on pristine graphite. Note that we also
looked at the effect of DBA concentration, showing that
regardless of the nanocorral size, the porous : non-porous ratio
increases with decreasing DBA-concentration (Figure S7). The
larger the nanocorral, the more pronounced this trend is, as
expected based on the data for unmodified HOPG.
To rule out that odd-even effects are at play at the level of
the alkyl chains, similar experiments were carried out with DBA-
OC12 (ESI, Section S7-S8). Qualitatively and even quantitatively,
the results are very similar as those obtained with DBA-OC13.
We not only used square nanocorrals (in the main text), but
also circular and triangular corrals (Figure S12 and S13).
Irrespective of the shape of the nanocorrals, the results showed
similar tendency. So, the confinement effect is independent on
the corral shape.
Time-dependent effects were not investigated in detail, as
Fig. 2 High-resolution STM images and the corresponding structural models of
continued scanning leads to an increased risk of unintended
(a, b) porous and (c, d) non-porous self-assembled monolayers of DBA-OC13 at
nanoshaving and deterioration of the image quality. In-situ
cleaning of the tip via a voltage pulse is not an option as it resets
the self-assembly process. Nevertheless, some dynamics was
observed which however did not affect the non-porous to
porous ratio (Figure S14). In addition, for the smallest corral
dimensions, nanoshaving was performed at elevated
the 1-phenyloctane/HOPG interface. Concentration: (a) 2.0 × 10–6 M; (c) 6.7 ×
10–5 M. Scanning parameters: Vs = –0.20 V, It = 200 pA. Unit cells are overlaid on
the STM images. (a) Porous phase: unit cell parameters a = 4.7 ± 0.1 nm, b = 4.7
± 0.1 nm, γ = 60 ± 1°; area density per molecule is 9.57 nm2. (c) Non-porous
phase: unit cell parameters a = 2.5 ± 0.1 nm, b = 4.7 ± 0.1 nm, γ = 90 ± 3°; area
density per molecule is 5.88 nm2.
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not at play in defining the outcome of the self-assembly
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DOI: 10.1039/C8CC08602H
process. All of this suggests that the phenomena as observed
are independent of the nanoshaving process itself. The reason
can be found in the different geometries of the molecules.
Obviously, the question rises what is the mechanism behind
the nanoconfinement effect. Both kinetics and thermodynamics
are at play in the formation of self-assembled molecular
networks. A priory, it is not possible to state which will
dominate the network formation.29 Previously we have shown
that self-assembly of these alkoxylated DBA derivatives
represents a thermodynamic composition of linear and porous
structure at the interface between 1,2,4-trichlorobenzene (TCB)
and graphite. A similar, gradual decrease in the surface
coverage of the porous phase (and increase in the linear phase)
as a function of DBA-OC13 concentration, but now in 1-
phenyloctane, suggests that this is also the case in this system.
Note though that kinetic blockades cannot be ruled out. So, the
equal surface coverage of the linear and porous phases at 5.6 ×
10–6 M, does not necessarily represent an equilibrated surface
composition at the thermodynamic minimum. We have
identified two effects to explain the favored formation of the
high-density phase in smaller nanocorrals.
From a thermodynamic point of view, phase selectivity can
be explained as the result of additional interactions between
DBA-OC13 or DBA-OC12 and the walls of the corral. Such
interactions are simple in nature and their strength is additive.27
As such, the larger the interaction between the molecular
network and the wall, the larger the expected energy gain.
Higher flexibility in packing of linear phase allows easier
accommodation to the irregularities of grafts at the wall of the
corral. This effect becomes the more pronounced the larger the
perimeter to area ratio of the nanocorrals, i.e. for the smaller
nanocorrals. Hence, observed phase selectivity could be
interpreted as preferential stabilization of the linear phase by
the walls of the corral. However, we don’t find a bias for linear
phase formation at the corral walls.
From a kinetics point of view, the nanocorral size could
affect the relative rates of nucleation of the porous and the
linear phase.30 This could be due to size differences of the
critical nuclei of linear and porous phases: being more dense,
the critical nucleus of the linear phase can fit in a smaller
confinement than the honeycomb phase, and hence can have
higher nucleation probability. At present it is not possible to
distinguish between the two interpretations.
In conclusion, we have systematically investigated
geometrical confinement effects on the self-assembly of
alkoxylated DBAs that form two concentration dependent
phases at the interface between 1-phenyloctane and HOPG. The
effect of nanoconfinement is expressed by a strongly increased
tendency to form the non-porous phase. This nanoconfinement
is indeed size dependent as the effect disappears when the
corrals reach a size of about 200 × 200 nm2. This behavior can
be explained based both on thermodynamics as well as kinetics
Fig.
3 (a, b) STM images of the self-assembly of DBA-OC13 at the 1-
phenyloctane/graphite interface, in corrals of different size (approximately 24 ×
24 nm2, 39 × 39 nm2 and 60 × 60 nm2). These are the average dimensions of 15
nanocorrals of each type. Scanning parameters: Vs = –0.80 V, It = 80 pA. The
nanocorrals are formed via a nanoshaving process (Scanning parameters: Vs = –
0.001 V, It = 200 pA) at the interface between the 1-phenyloctane solution of
DBA-OC13 and graphite. DBA-OC13 concentration is 5.6 × 10–6 M. Green dots
represent hexagonal pores. (c) Dependence of the surface coverage of porous
and non-porous structures as a function of the size of the square corrals. Each
data point results from the average values of 15 corrals (24, 39 and 60 nm) or 40
zoom-in (50 × 50 nm2) images (90, 114, 145, 177 and 201 nm). The standard
deviation is indicated. For nanocorrals reaching a diameter of about 201 nm,
there is no nanoconfinement effect anymore.
temperature to assess the kinetic stability of the assemblies.
Compared to room temperature experiments, the porous : non-
porous ratio at 50 °C remained unaltered (Table S7). Thus, the
assembly composition in nanocorrals is kinetically stable up to
at least 50 °C.
In all these experiments, the relative orientation of the
square nanocorrals with respect to the symmetry axes of the
graphite surface is not known, and not controlled. To rule out
that there is an effect of substrate symmetry on the geometrical
nanoconfinement effects, we ran some tests on DBA-OC13
where we varied the orientation of the square nanocorrals with
respect to the HOPG lattice. In addition to the fact that these
experiments confirm that the alkyl chains run parallel to one of
the symmetry axes of graphite (Figure S15), the more important
outcome of this set of experiments is that regardless the
relative orientation of the square nanocorrals with respect to
the graphite lattice, the results are not different from those
discussed before (Figure S16, Table S8). Also the absence of
preferential molecular alignment with respect to the slow
nanoshaving direction, a phenomenon that was observed for a
chainlike molecule, 22 suggests that the nanoshaving process is
considerations, or
a combination of both. Temperature
dependent studies on polymorph selection might shine some
light on the relative importance of both mechanisms. Also chain
length dependent studies are of potential interest. Overall, this
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new approach to study confinement effects is an interesting 16
platform to reveal aspects of 2D supramolecular chemistry.
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This work is supported by the Fund of Scientific Research – 17
Flanders (FWO), Internal Funds KU Leuven. This work was in
part supported by FWO under EOS 30489208. The research
leading to these results has also received funding from the 18
European Research Council under the European Union’s
Seventh Framework Programme (FP7/2007–2013)/ERC Grant 19
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Conflicts of interest
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There are no conflicts to declare.
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Phase behavior of self-assembled molecular networks is affected by spatial confinement in corrals