Monolayer Structures on solid supports
The research is primarily focused on the preparation and characterization of ultrathin organic films on metal and semiconductor surfaces. Self-assembled monolayers (SAMs) of w-substituted alkanthiols on gold form the basis of the activities, but we are also investigating SAMs of organophosphine and organosilicon compounds as well as Langmuir-Blodgett (LB) films.
A large part of the work is of fundamental character, where a broad range of surface science apparatuses are employed to gain a deeper insight in the structure and phase behaviour of these 2-dimentional assemblies. We are also interested in developing strategies that can assist in the preparation of more complex monolayers with controlled lateral separation between functional entities, so-called mixed SAMs. A method for the preparation of so-called molecular gradients also has been developed. It is based on cross-diffusion of different thiols in a polysaccharide matrix. A novel type of nanoporous assemblies consisting of thiocholesterol attached to gold also has been exploited. The assembly of electrically conducting oligomers molecular wires attached to alkanthiols and disulphides have been prepared for applications in our molecular electronics and sensor related research.
Self-assembled monolayers (SAMs) can be prepared using different types of molecules and different substrates. Widespread examples are alkylsiloxane monolayers, fatty acids on oxidic materials and alkanethiolate monolayers. All these systems have been reviewed in great detail, and the interested reader is directed to, e.g., the book Ultrathin Organic Films by A. Ulman. Here, we will concentrate exclusively on SAMs of functionalized alkanethiols on gold surfaces. This type of SAMs holds great promise for applications in several different areas. Some examples of suggested and implemented applications are molecular recognition, SAMs as model substrates and biomembrane mimetics in studies of biomolecules at surfaces, selective binding of enzymes to surfaces, chemical force microscopy, metallization of organic materials, corrosion protection, molecular crystal growth, alignment of liquid crystals, pH-sensing devices, patterned surfaces on the µm scale, electrically conducting molecular wires and photoresists.
Research in this area began in 1983 and has seen an increasing number of published papers every year since then (see Figure1). The principle is simple: A molecule which is essentially an alkane chain, typically with 10-20 methylene units, is given a head group with a strong preferential adsorption to the substrate used. Thiol (S-H) head groups and Au(111) substrates have been shown to work excellently. The thiol molecules adsorb readily from solution onto the gold, creating a dense monolayer with the tail group pointing outwards from the surface. By using thiol molecules with different tail groups, the resulting chemical surface functionality can be varied within wide limits. Alternatively, it is also possible to chemically functionalize the tail groups by performing reactions after assembly of the SAM.
crystal face for alkanethiolate SAM preparation on gold substrates is the
(111) direction, which can be obtained either by using single crystal substrates
or by evaporation of thin Au films on flat supports, typically glass or
silicon. A schematic outline of the SAM preparation procedure on such gold
substrates is given in Figure 2, together with a schematic of a mixed SAM
(see below). Several different solvents are usable at the low thiol concentrations
(typically 1-2 mM) that are used in preparation of SAMs, but care must be
taken when using mixed thiol solutions, since the final composition of the
monolayer depends upon the relative solubilities of the different thiols.
The most commonly used solvent is ethanol. It is advisable to minimize the
water content in the solvent if the SAMs are to be used in UHV; this will
limit incorporation of water into the SAM structure which reduces outgassing
and increases repeatability in the UHV experiments. Even though a self-assembled
monolayer forms very rapidly on the substrate, it is necessary to use adsorption
times of 15 h or more to obtain well-ordered, defect-free SAMs. Multilayers
do not form, and adsorption times of two to three days are optimal in forming
highest-quality monolayers. In preparing SAMs for UHV use, meticulous rinsing
and drying are of course highly important.
A s mentioned above, the tail group that provides the functionality of the SAM can be widely varied. CH3-terminated SAMs are commercially available; other functional groups can be synthesized by any well-equipped chemical laboratory, providing almost infinite possibilities of variation. In addition, chemical modification of the tail group is entirely possible after formation of the SAM, expanding the available range of functionalities even further. Examples of functionalities used at our laboratory are:
By mixing two
differently terminated thiols in the preparation solution, we can prepare
mixed SAMs. The relative proportion of the two functionalities in the assembled
SAM will then depend upon several parameters, like the mixing ratio in solution,
the alkane chain lengths, the solubilities of the thiols in the solvent
used, and the properties of the chain-terminating groups. In general, the
composition will not be the same in the SAM as in the preparation solution.
Measurements with a surface- sensitive probe like, e.g., X-ray photoelectron
spectroscopy are necessary to calibrate the mixing ratio. In cases where
the two thiol molecules are of equal alkyl chain length and no special circumstances
(like bulky tail groups) are at hand, the SAM composition will be almost
identical to the composition of the solution, though. This is the case for
mixtures of HS(CH2)15CH3 and HS(CH2)16OH.
Another useful SAM preparation method is the formation of two-component molecular gradients, as first described by Liedberg and Tengvall (Langmuir 11 (1995), 3821). By cross-diffusion of two differently terminated thiols through an ethanol-soaked polysaccharide gel (Sephadex LH-20, a chromatography material) that is covering the gold substrate, a continuous gradient of 10-20 mm length may be formed. The principle of preparation is outlined in Figure 3. Ethanol solutions of each of the two thiols are simultaneously injected into two glass filters at opposite ends of the gold substrate. The presence of the polysaccharide gel makes the diffusion and the thiol attachment to the surface slow enough for a gradient of macroscopic dimension (several mm) to form.
SAMs have been thoroughly characterized using a large number of surface analytical tools. Among the most frequently used techniques are infrared spectroscopy, ellipsometry, studies of wetting by different liquids, x-ray photoelectron spectroscopy, electrochemistry, and scanning probe measurements. It has been clearly shown that SAMs with an alkane chain length of 12 or more methylene units form well-ordered and dense monolayers on Au(111) surfaces. The thiols are believed to attach primarily to the threefold hollow sites of the gold surface, losing the proton in the process and forming a (sqrt(3)×sqrt(3))R30° overlayer structure (shown in Figure 4). The distance between pinning sites in this geometry is 5.0 Ċ, resulting in an available area for each molecule of 21.4 Ċ2. Since the van der Waals diameter of the alkane chain is somewhat too small (4.6 Ċ) for the chain to completely occupy that area, the chains will tilt, forming an angle of approximately 30° with the surface normal. Depending on chain length and chain-terminating group, various superlattice structures are superimposed on the (sqrt(3)×sqrt(3))R30° overlayer structure. The most commonly seen superlattice is the c(4×2) reconstruction, where the four alkanethiolate molecules of a unit cell display slightly different orientations when compared with each other.
bond is strong - homolytic bond strength 44 kcal/mol - and contributes to
the stability of the SAMs together with the van der Waals forces between
adjacent methylene groups, which amount to 1.4-1.8 kcal/mol. The latter
forces add up to significant strength for alkyl chains of 10-20 methylenes
and play an important role in aligning the alkyl chains parallel to each
other in a nearly all-trans configuration. At low temperatures, typically
100 K, the order is nearly perfect, but even at room temperature there are
only few gauche defects, concentrated to the outermost alkyl units.
One convenient method of checking a SAM for well-ordered and dense structure is infrared reflection-absorption spectroscopy (IRAS). The CH stretching vibrations of the alkyl chain are very sensitive to packing density and to the presence of gauche defects, which makes them ideally suited as probes to determine SAM quality. In particular, the antisymmetric CH2 stretching vibration (d-) at ~2918 cm-1 is a useful indicator; its position varies from 2916 or 2917 cm-1 for SAMs of exceptional quality or cooled below room temperature, via 2918 cm-1 which is the normal value for a high-quality SAM, to ~2926 cm-1 which is indicative of a heavily disordered, "spaghetti-like" SAM. A typical IRAS spectrum of the CH stretching region of a hexadecanethiolate (HS(CH2)15CH3 ) SAM is shown in Figure 5.
T hickness measurements
using ellipsometry yield SAM thicknesses that are in good agreement with
the 30° chain tilt mentioned above. For example, reported ellipsometric
thicknesses of hexadecanethiolate SAMs lie in the 21ħ1 Ċ range,
to compare with the 21.2 Ċ that result if a fully extended hexadecanethiol
molecule of 24.5 Ċ length is tilted 30°. Contact
angle measurements further confirm that alkanethiolate SAMs are very dense
and that the contacting liquid only interacts with the topmost chemical
groups. Reported advancing contact angles with water range from 111°
to 115° for hexadecanethiolate SAMs. At the other end of the wettability
scale, there are hydrophilic monolayers, e.g., SAMs of 16-mercaptohexadecanol
(HS(CH2)16OH), that display water contact angles of <10°. These two
extremes are only possible to achieve if the SAM surfaces are uniform and
expose only the chain-terminating group at the interface. Mixed SAMs of
CH3- and OH-terminated thiols can be tailor-made with any wettability (in
terms of contact angle) between these limiting values. The
characteristics of mixed two-component SAMs depend strongly upon the precise
chemical identity of the components and upon their proportion in the preparation
solution, as already stated above. Apart from the composition of the SAMs,
the issue of island formation is very important for mixed monolayers. In
mixed CH3/CO2CH3 SAMs, scanning tunnelling microscopy (P. Weiss et.al.)
has revealed island formation on the 20-50 Ċ scale. For mixed SAMs
of hexadecanethiol and 16-mercaptohexadecanol, which is a commonly used
model system at our lab, IRAS, wetting, laser desorption spectroscopy and
TOF-SIMS data (both by us and other investigators) support a picture of
randomly pinned, well-mixed monolayers, although mixing at a true molecular
level has neither been contradicted nor confirmed at the present stage.
Undoubtedly though, macroscopic phase segregation into single component
domains does not occur.
Responsible for this page: Erik Martinsson
Last updated: 05/16/06