UNIVERSITA’ DEGLI STUDI DI PADOVA
Laurea specialistica in Scienza e Ingegneria dei Materiali
Curriculum Scienza dei Materiali
Chimica Fisica dei Materiali Avanzati
Part 10 – Self-assembly at surfaces
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Supramolecular systems on surfaces
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Simple deposition on a solid surface
Langmuir-Blodgett (LB) films
Self-assembled monolayers (SAM)
Incorporation in membranes
Derivatized metal and metal oxide nanoparticles
Applications
 Sensoring
 New materials (e.g., molecule-based electronic circuits, display
systems, smart windows, ...)
 Heterogeneous catalysis
 Construction of nanostructures
 Molecular-level devices (”micro-nano” problem)
 Molecular-level electronic components
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The Langmuir-Blodgett technique
 An amphiphilic species, dissolved in a volatile organic solvent, is spread onto an
aqueous surface.
 After evaporation of the organic solvent, the amphiphilic molecules start to
reorganize, forming a film in which the nonpolar tails of the amphiphile rise
above the aqueous subphase, while the polar head groups remain immersed.
 At this point, the molecules are dispersed and the film is not continuous.
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The Langmuir-Blodgett technique - 2
 The surface is compressed by means of a moving barrier (which is part of the
trough). A feedback system, called Wilhelmy plate, controls the position of the
barrier, and thus the surface pressure.
Free-floating films on
the trough surface are
called Langmuir films.
Wilhelmy
plate
Moving
barrier
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The Langmuir-Blodgett technique - 3
 Langmuir films can be physically transferred onto solid supports by dipping the
support into the film. Deposition of a Langmuir film on the surface of a substrate
results in Langmuir-Blodgett films.
 Suitable substrates for the
preparation of LB films are
 Hydrophilic substrates like
glass and quartz (for
spectroscopic studies)
 Metal hydrophilic plates like
chromium, aluminium, tin, all in
the form of oxides
 Conductive substraates like
silver and gold (for
electrochemical studies:
modified electrodes)
 Semiconductor plates (silicon,
gallium arsenide, ...)
 Atomically flat and highly
hydrophobic substrates like
mica
 Monolayer LB films can be deposited on the surface by vertical deposition (VD)
in either of two possible orientations, which can be described simply as tails in
(X-type, transfer on the downstroke)
or heads in (Z-type, transfer on the
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upstroke).
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The Langmuir-Blodgett technique - 4
 To obtain information about the stability of the film, the reorientation of
the molecules, the phase transitions and conformational
transformations, pressure-area isotherms must be studied.
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The Langmuir-Blodgett technique - 5
 Note that the hydrophilicity of the substrate may influence the ease of preparation
of monolayers in a particular orientation. For example, the Z-type deposition is
likely to be easier on a hydrophilic substrate.
 Subsequent layers may be added by repeated vertical dipping in either mode.
 Films deposited in this fashion, while initially organized, are subject to relaxation of
the packed film. The organization of the film is degraded after transfer: without the
mechanical pressure provided by the floating barrier, the tight packing of the film
may be quickly lost.
 Important parameters for the preparation of the LB film:
 Surface pressure maintained during the transfer;
 Transfer (rising or dipping) speed;
 Time elapsed between successive monolayer deposition.
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The Langmuir-Blodgett technique - 6
 To overcome these problems, one can alternatively employ the horizontal
touching (HT) technique. In this method, the substrate is gently lowered onto the
surface of the trough and the amphiphilic molecules adhere to the substrate in
an X (tail in) orientation.
 Because the substrate remains in contact with the trough solution, which is
typically maintained at a constant pressure, the film deposited on the surface is
less subject to relaxation.
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LB technique for the fabrication of moleculear devices
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Developing molecule-based electronics
 Bi-functional amphiphilic rotaxanes
are self-organized in a LangmuirBlodgett film deposited on a SiO2
substrate patterned with preformed
poly-Si strips.
 An 8x8 cross-bar device is formed
between poly-Si electrodes (100 nm
wide) at the bottom and Ti/Al
electrodes (70 nm wide) on top.
Each molecular tunnel junction with
an area of 7000 nm2 contains about
5000 molecules.
 The device works as a 16-bit
memory unit by controlling the
voltage at each arm of the cross-bar.
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Self-assembled monolayers (SAM)
 The term self-assembled monolayers (SAMs) is used to described organized
molecular assemblies (one-molecule thick) whose spontaneous formation and
stability depend on favorable, if weak, intermolecular forces, as well as on forces
between each of the individual component molecules and the solid substrate.
 Molecules used to prepare SAMs and LB films differ in that the former must
interact chemically with the surface, that is, form chemical bonds, and are
soluble in the fluid from which they are prepared.
Surfactant
Alkanethiols
Dialkyl sulfides
Alcohols, Amines
Carboxylic and Phosphonic acids
Organosilicon compounds
Surface
Au, Ag, Cu
Au
Pt
TiO2, SnO2, Al2O3
Hydroxilated surfaces (SiO2, Al2O3)
 The most popular type of SAM is that formed by the chemisorption of long-chain
alkanethiol molecules on clean gold surfaces. The main driving forces for this selfassembly process are: (i) the formation of thiolate-gold bonds (estimated to release
40-50 kcal/mol), and (ii) the favorable lateral interactions among the aliphatic chains
of neighboring alkanethiol molecules.
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SAMs of thiol compounds on Au surfaces
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These SAMs are prepared simply by exposing a clean gold surface to
a solution of the alkanethiol.
For alkanethiols with chains longer than 10 carbon atoms, it is
generally true that:
1.
2.
3.
4.
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A stable, organized and densely
packed monolayer is readily
formed on the Au surface.
The thickness of the monolayer
is directly proportional to the
length of the alkanethiol chain.
The main axes of individual
molecules are tilted by an angle
of ca. 30° from the normal to the
surface.
Every chemisorbed molecule
loses the hydrogen atom from
the thiol group. The surfaceattached molecules are thus
better formulated as thiolates.
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Functional SAMs
 The figure below illustrates another important feature of SAMs.
Synthetic modification of the thiol permits the attachment of functional
groups (R) to the monolayer.
Among other applications, this
feature can be used to control the
wetting properties of the surface,
to attach redox-active or
photoactive groups at defined
distances to the gold surface, and
to incorporate binding sites which
can lead ultimately to molecular
recognition of suitable targets.
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Examples of SAMs for sensor applications
Adv. Mater. 2000, 12, 1315
Binding of b-cyclodextrin to a dansylfunctionalized monolayer on glass
SAMs of b-cyclodextrin and their
complexation with ferrocene
Binding of crown fullerene to a cystamine
Molecular recognition by a fluorescent receptor
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monolayer on gold
adsorbate on glass