|Samorí , Paolo : Self-assembly of conjugated (macro)molecules: nanostructures for molecular electronics |
Making use of intramolecular, intermolecular and interfacial forces it is possible to design highly ordered 2D and 3D polymolecular architectures. A precise control of the molecular self-assembly both from solutions and from vacuum depositions can be obtained on such substrates that posses well defined chemical functionalities and physical properties [Ulm91]. A first requirement for a highly reproducible modification of a surface is a good flatness of the support. Because of this reasons it is worthwhile to put effort on the selection and careful preparation of the substrate. Then, atoms and molecules can attach to surfaces in two different ways: by means of physisorption (physical adsorption) or chemisorption (chemical adsorption).
The physisorption is the adsorption of molecules at surfaces which is characterized by the absence of a formation of a chemical bond both between molecule-molecule and molecule-substrate. It arises from the interplay of weak attraction forces (predominantly van der Waals and electrostatic which exist over long ranges) and hard core repulsions. Physisorption is an exothermic process; its enthalpy can be measured by monitoring the rise in temperature of a sample of known heat capacity during the adsorption or by investigating the thermal programmed desorption of the adsorbate. A typical enthalpy of physisorption is about 20 kJ/mol [Atk94]. This small enthalpy change is insufficient to lead to bond breaking, so a physisorbed molecule retains its identity, although it could be distorted by the surface.
Molecular organization in the case of physisorption is governed by intra-molecular, intermolecular and interfacial interactions, which may be described by the following potential:
Formulae 4.1: Physisorption of a molecule at surface [Hen93].
The first three terms in formula 4.1 describe the intramolecular forces, while the next two are intermolecular contributions to the overall interactions, and the last one describes the interfacial forces. The terms are respectively related to bond stretching, valence angle vibrations, torsional vibration, Lennard-Jones, Coulombic interactions and external static potential due to the surface acting on the adsorbate.
Besides, several experimental studies carried out in the recent past on the molecular physisorption at surfaces revealed distinctly different adsorbate arrangements on the molecular level when using conductive and insulating substrates.
Conjugated molecules tend to self-assemble into layers lying flat on the basal plane of conductive substrates, such as HOPG, dichalcogenides, Au or Ag surfaces. This is behavior, which is true mainly for molecular films with a nominal thickness of maximum a few monolayers, is due to a rehybridization of orbitals of the adsorbate with those of the substrate that exhibit metallic or semiconducting properties [Bis00]. Examples include several adsorbates at the interface between a liquid and the basal plane of on HOPG, namely oligomeric liquid crystals [Fos88, Smi89, Spo89], alkanes, alcohols [Rab91a], oligothiophenes [Bäu95], hexakis-dodecyl-hexabenzocoronene [Sta95b], diacetylenes [Rab93], isophthalic acids [Eic96], anthrone derivatives [Sta95c]; this tendency have been also detected on dried films prepared both by solution casting (dendronized poly(para-phenylene)s [Sto98a] and poly(styrene)s [Sto98b]) and by UHV sublimation (perylene derivatives [Lud94]). A similar arrangement have been detected also on dichalcogenides (alkanes [Cin93] and perylene derivatives [Lud94]), on Au (oligothiophenes [Buo96] and porphyrins [Jun97]), on Ag (oligothiophenes [Sou96] and porphyrins [Jun97]), on Cu
43(phthalocyanines [Lip89] and porphyrins [Jun97]) and on Pt (naphtalenes [Hal91] ).
Chain molecules usually do not lie flat on the insulating substrates. The main chain tends to stand on the basal plane of the non conductive support being sometimes oriented perpendicular to it. This has been observed for mica substrates (oligothiophenes [Bis95]) and for SiO2 (polymeric liquid crystals [Vix98] and oligothiophenes [Ser93]). This molecular arrangement is therefore characterized by a weak coupling with the substrate.
The chemisorption is the adsorption of molecules at surfaces which is characterized by the formation of a chemical bond between molecule and surface; in this type of adsorption the molecules tend to find sites that maximize their coordination number with the substrate. The enthalpy of chemisorption is bigger than that for physisorption, and amounts typically to ca. 200 kJ/mol [Atk94]. Chemisorption is commonly used to generate a well defined, tightly packed molecular structure that coats a surface uniformly. The film resulting is a so called Self-Assembled Monolayer (SAM). It can be formed spontaneously by the immersion of an appropriate substrate into a dilute solution of an active surfactant in an organic solvent or by sublimation of the molecular adlayer in vacuum. There are several types of SAMs that have been created. These include organosilicon on oxidized surfaces (SiO2, Al2O3), alkanethiols on Au, Ag, Cu, Pt; dialkyl sulfides and disulfides on Au; alcohols and amines on Pt; carboxylic acids on Al2O3 and Ag [Ulm91].
Figure 4.1: Chemisorption reaction of a surfactant on a substrate.
A self-assembling surfactant molecule is typically composed of 3 parts (Fig. 4.1):
441) head group: it provides the chemisorption to the substrate (exothermic process). The very strong molecule-substrate interactions result in a pinning of the head group to a specific site on the surface through a chemical bond. This can be:
As a result of the exothermic head group - substrate interaction, molecules try to occupy every available binding site on the surface, and in this process molecules that have been already adsorbed tend to pack one beside the other. Therefore, often a crystalline assembly is formed upon annealing.
2) alkyl chain: it plays an important role in the packing at surfaces because when the distance between adjacent molecules is so small that short-range, dispersive, London-type, van der Waals forces become effective molecules tend to reorganize due to these interactions between the alkyl chains. When the alkyl chain is substituted with a polar bulky function there are also long-range interactions that sometimes are energetically predominant.
3) ??functionalization: the function can be such that it opens the possibility of successive adsorptions (anchoring) or chemical reactions (like the photopolymerization of diacetylenes [Kim96]).
The most studied case, as previously mentioned, is the one of thiol functionalized molecules self-assembled on a Au(111) surface [Del96, Poi97]. The rate of the chemisorption from solution is such that it can occur on a 20 hours time scale. A faster adsorption can be obtained on an Ag substrate, which, however, has the disadvantage of getting oxidized easily in air environment. Nevertheless reproducible SAMs on Ag(111) have been developed and compared to Au(111): they exhibit the advantage that the aliphatic chains pack perpendicular to the substrate whereas for Au(111) they prefer to assemble at 30° from the normal [Hei95]. This causes the packing of alkanethiols to be more tight for silver than for gold.
The electronic properties or the conductivity of single molecules or of a small ensemble of molecules can be elucidated by making use of 2 different types of contacts (Fig 4.2).
Figure 4.2: Junctions for probing electronic properties of a single molecule or molecular object.
In the first case (A) the STM set-up is used to measure the conductivity of a molecule or a molecular aggregate. The measurement can be performed by imaging the current at constant height and voltage if the adsorbate exhibits a high degree of order, like in the case of a SAM [Hei95]. Alternatively one my perform a spectroscopical study on single molecules (STS) [Sta95b]. In the latter case (B), one requires a particular test sample: one molecule or a well defined array of molecules assembled between two Au nanoelectrodes.
One method that can be used to produce gold nanoelectrodes is Electron Beam Lithography (EBL). It utilizes the fact that certain chemicals change their properties when irradiated with electrons just as a photographic film does when irradiated with light. The electron beam is generated in a Scanning Electron Microscope which normally is set up to provide an image of an object by rastering with a well focused beam of electrons over it.
Collecting electrons that are scattered or emanated from that object for each raster point provides an image. With computer control of the position of the electron beam it is possible to write arbitrary structures onto a surface. The steps to produce a structure by EBL are shown below: the sample is covered with a thin layer of poly(methylmetacrylate) (PMMA), then the desired structure is exposed with a certain dose of electrons. The exposed PMMA changes its solubility towards certain chemicals. This can be used to produce a trench in the thin layer. If one wants to produce a metallic structure, a metallic film is evaporated onto the sample and after dissolving the unexposed PMMA with its cover (lift-off) the desired metallic nanostructure remains on the substrate. This method, shown in Fig. 4.3, allows gaps to be
46engineered down to the 10 nanometer regime [Bez97a, Bez97b, Her98]. This gap might be bridged by the molecular aggregate. This would enable to relate the electronic properties of the object to its order on a molecular scale. The resulting 10-20 nm gap can be visualized with SFM.
Figure. 4.3: Scheme of the e-beam lithography procedure used for producing Au nanogaps.
Figure 4.4: Method for developing Mechanically controllable break junctions.
Another procedure for developing metallic nano-contacts, the so called Mechanically controllable break junctions [Ree97] (see also chapter 3) provides metallic nanoprobes with a gap of 1-2 nm. The production is based on a fracture of a Au wire that takes place upon
47applying pressure on it with a piezoelectric crystal (Fig. 4.4). Unfortunately, in my opinion, the reproducibility of these type of gaps is rather poor. The procedure includes molecular self-assembly that is carried out bathing the filament during the breakage with a solution of benzene-1,4 dithiol. A successive relaxation of the pressure induced by the piezo cause the two probes to approach each other again. The authors claim that the nanoprobes are separated by an organic monolayer of self-assembled molecules and current vs. voltage characterization of the system gives insight into the conductivity of the monolayer, thus of the single molecules.
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