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Application fields of SAM-functionalized surfaces
Self-assembled monolayer appears to be the excellent tool for controlling and fine tuning of the surface properties. Since first experiments with thiols adsorbed on a gold surface, it has been clearly demonstrated possibility to control surface hydrophilic-hydrophobic character by simply changing the nature of the terminal group from CH3 to OH. Further development brought a large variety of new functional groups introduced into the SAM as terminal ones. Those works described main horizons for SAM applications such as surface coating and nanopatterning, sensors, tribology, molecular devices. Herein, only a few examples will be mentioned to give an idea of the available applications for SAMs.
Passivation, surface coating, nano-patterning
Dense monomolecular layers are useful in the prevention of corrosion. While noble metals usually do not suffer from it, copper protection has a practical interest. A work by Azzaroni et al.80 demonstrated that copper passivation by dodecanethiol could hinder copper oxide formation and copper dissolution in electrolyte solutions.
Superhydrophobic surfaces have attracted great attention due to their important applications in surface self-cleaning, stiction prevention and drag reduction.81 They have also been suggested to improve tribological performances of various devices such as microelectromechanical systems (MEMS)82 consisting of several moving parts. It has been proven that chemical modification of a smooth silicon surface can only lead to water contact angle value (WCA) of up to 120°83. However, the combination of a surface topography modification with a formation of SAM on it gives an access to the WCAs values above 150°. This has been demonstrated in the work of Y. Song where superhydrophobic surfaces were fabricated by applying an octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) on silicon micro- or nanotextured surfaces produced by the aluminum-induced crystallization of amorphous Si. This technique helps to archive WCA of 155° while OTS SAM on smooth Si showed usual value of 112°.
The construction of novel molecular devices asks for well-defined two-dimensional structure. Due to strong covalent connection of molecules to a surface and relatively fast kinetics, the great utility of SAMs for microcontact printing process has been shown. The group headed by Whitesides used poly(dimethylsiloxane) (PDMS) stamp inked by a solution of different thiol molecules to create nanopatterned surface. They achieved to “print” hydrophilic areas of different size and shape separated by hydrophobic regions of CH3-terminated alkanethiols.85 Then, the selective grows of calcite crystals have been demonstrated only in hydrophilic sites (Fig. 1.6).
Crooks et al. reported patterning with a high resolution employing photoreactive SAMs. A photomask was placed on top of a close-packed HOOC–(CH2)10–C≡C–C≡C– (CH2)10–SH monomolecular layer on Au substrate. Such sample was subsequently exposed to UV light provoking a polymerization of unmasked molecules. Nonpolymerized molecules were desorbed leaving patterned surface of good quality.
The finest resolution was demonstrated when the probe of the AFM was used like a pen for nanolithography. This approach has demonstrated a capability to fix the molecules only in the sites scanned by the probe87 or to catalyze a chemical reaction in a defined place.
Sensor Applications
A SAM with a functionalized terminal group can be constructed and used for selective interaction with some molecules or ions. In such way a sensor system can be developed. One of the typical examples was demonstrated in the work of Rubinstein and coworkers.89 The experiment has been carried out at the gold electrode modified with a mixed SAM of n-octadecyl mercaptan and 2,2′-thiobisethylacetoacetate (Fig. 1.7). This electrode has been immersed into an electrolyte solution containing Cu2+ and Fe3+ cations. While the electrochemical response of Fe3+ was completely suppressed, the response of Cu2+ was clearly observed due to selective chelation by functionalized SAM.
Self-organized monolayers of donor-acceptor molecules.
As was demonstrated in the previous part, the preparation of advanced surfaces often requires functionalization by complicated molecules. While alkanethiol-based SAMs are well studied and could be prepared easily, needed films may contain electron donor or electron acceptor groups, chromophores, moieties with unpaired spins, and so on. For this reason, there is a need to construct and study monolayer films with progressively more complicated polar and planar bulky π-systems. Nevertheless, such complex molecular structures may hinder compact organization of their SAM.
Depending on expected applications, different types of π-conjugated molecules were elaborated for SAM preparations. Fullerenes,98 diphenyldiacetylene99 and porphyrin100 derivatives have been described. Such systems usually provide different photoactive properties like photo-switching systems,98 light detectors,101 light-to-current energy convertors100 or can be interesting in molecular electronics studies.102 Between linearly conjugated molecules derivatives of oligothiophenes have attracted a lot of attention allowing to give SAM at same time molecular electronics103, as well as optical104,105 properties.
The main goal of this thesis work is to study SAMs of “donor-acceptor” molecules. Meanwhile, there are not many efforts made in this field.
Inspired by works in the domain of Langmuir-Blodgett films106, a hemicyanine type donor-acceptor molecules were synthesized and used for preparation of SAMs in series of works made by Ashwell and co-workers. They proposed that such polar structures, once connected covalently to the metal surface, should provide diode-like behavior. In the beginning, a study of SAM obtained from bis[1-{(3-propyl)-4-(2-(4-dimethylaminophenyl)vinyl)quinolinium}]disulfide diiodide (1.4.1) was described (Fig. 1.10).
Contact angles measurements
Measurements of the contact angles (CA) probe the surface wettability towards the chosen solvent. This technique also allows to determine the free surface energy by the use of different solvents. In practice, often the solvent of choice is water and after CA analysis the hydrophilic or hydrophobic character of the surface can be estimated helping to understand better the nature of the film covering the substrate surface. Once a water drop is deposited on the surface two situations are possible:
– a drop does not spread and keeps the stable shape forming at borders the angle with support calling “contact angle” between 0 and 180°; this is a situation of partial wetting; Above the angle of 90° the surface is hydrophobic, whereas hydrophilic below.
– a drop spontaneously spreads over the surface forming a liquid film; the wetting is complete, and contact angle is considered as 0°. The wetting phenomenon is directly connected to surface tensions in all interfaces present in the system, notably, the solid-vapor, solid-liquid and liquid-vapor.
The theoretical underground of the drop form lies in thermodynamic equilibrium between three phases. If the solid–vapor interfacial energy is denoted by γSV, the solid–liquid interfacial energy by γSL, and the liquid–vapor interfacial energy (i.e. the surface tension) by γLV, then the equilibrium contact angle θC is determined by Young’s equation (Eq. 2.2 & Fig. 2.2).
Scanning probe microscopy
Scanning probe microscope (SPM) uses a probe sensitive to some kind of physical force or property and a controlling system moving the probe relatively to the sample in X-Y plane for scanning the image, and in Z-direction. Modern scanning probe microscopes achieve the nanometric or even atomic scale resolutions thanks to very accurate piezoelectric elements of the position controlling system, sensitive probes and careful experimental conditions (low temperatures, vacuum setup). To form images microscope is moving the tip over the analyzing surface or the surface under the tip. At discrete points, some physical value is recorded (which depends on the type of SPM and the mode of operation). These recorded values are then displayed as a map producing images, usually using a color scale. Tyically, SPM operating modes are divided into two major kinds.
In the first one, the force of interaction is fixed by the driving software and the feedback system is used to move physically the probe closer to or further from the surface using the piezoelectrics in the Z direction to achieve this desired value. In this way, a topographic image is formed by the value of the Z-displacement of the piezoelectrics at each (X,Y) point of the surface. In the second one, constant height is fixed, and the probe keeps the same Z-position at any point of the image. In this way, the value of interaction is recorded. Constant height imaging is more delicate than constant interaction because its case there is a possibility that the probe can crash into the sample if the surface is too rough and/or tilted.
While the nature of the probe is determining the physical interaction with sample and, appropriately, the type of SPM, the piezo-electrics X-Y and Z- moving system together with the feedback and the data collect electronics can be used. In our laboratory, we have used a Nanoscope IIIa and Nanoscope V Multimode system from Bruker Corp. enabling to perform low-current (picoamper range) scanning tunneling microscopy (STM) as well as atomic-force microscopy (AFM). Images were recorded and treated by Nanoscope and WSxM 5.0114 software.
Scanning tunneling microscopy
After its development in 1981, scanning tunneling microscopy (STM) demonstrates resolution up to 0.1 nm laterally and 0.01 nm in the vertical direction. This permits to “see” individual molecules and even atoms. The STM microscope can be operated not only in the high vacuum but also in any gas (including air) or liquid conditions in a wide temperature range. However, STM is a challenging technique also, requiring clean and stable surfaces, sharp tips, vibration control, and sophisticated electronics. The working principle of STM microscope is based on quantum tunneling. The conducting tip is placed very close to the studied surface and under application of a voltage the tunneling current flows between the tip and the surface through vacuum or air. It usually depends on various parameters such as the tip position, the applied bias, as well as on the local density of states of the sample.
Table of contents :
Résumé
Abstract
Table des matières
Introduction
List of abbreviations
Chapter 1. “Push-pull” chromophores and molecular layers
1.1 General characteristics of donor – acceptor structures
1.2 D-A molecules in materials engineering
1.3 Ultrathin organic films
1.4 Self-organized monolayers of donor-acceptor molecules.
Chapter 2. Thin film characterization technics
2.1. Ellipsometry
2.2 Contact angles measurements
2.3. Scanning probe microscopy
2.3.1. Scanning tunneling microscopy
2.3.2. Atomic force microscopy
2.4. UV-Visible absorption spectroscopy
2.5. X-ray photoelectron spectroscopy
2.6. Cyclic voltammetry
2.7. Characterization of synthesized compounds by physic-chemical analysis
Chapter 3. Synthesis of “push-pull” chromophores
3.1 Synthetic methods for preparation of functionalized π-conjugated systems
3.2 Multi-step approach to surface-active D – A molecules
3.3 Physicochemical properties of obtained “push-pull” chromophores
3.4 Experimental part
Chapter 4. Self-assembled monolayers of “push-pull” chromophores
4.1 SAMs of a model aliphatic and aromatic thiols deposited on gold
4.2 SAMs of “push-pull” molecule on gold
4.3 SAMs on ITO and incorporation of gold nanoparticles
Conclusions and prospects
Bibliography
