Self-Assembled Monolayers

Transcription

1 CHE499 : A Nanotechnology Course in Chemical & Materials Engineering Spring 2006 Self-Assembled Monolayers By Drs. Lloyd Lee, Winny Dong 5GD6ER

2 Self-Assembled Monolayers (SAMs)

3 History Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481.

4 Introduction Self-assembled monolayers (SAMs) can be prepared using alkylsiloxane monolayers, fatty acids on oxidic materials and alkanethiolate monolayers. O O The principle is simple: A molecule which is essentially an alkane chain, typically with 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. Book: Ultrathin Organic Films by A. Ulman, H 5 C 2 O O H2N Si O Si OH HO OH Si H 2 N Si O Si O O H 2 N Si O Si O O H 2 N H 2 N H 2 N HO HO Si OH Si O Si O O OH Si Si O

5 Applications: 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.

6 Manufacture The preferred 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 the Figure, together with a schematic of a mixed SAM (see below). Several different solvents are usable at the low thiol concentrations (typically 1-2 mm). The most commonly used solvent is ethanol. Even though a selfassembled 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 highestquality monolayers. The substrate, Au on Si, is immersed in an ethanol solution of the desired thiol(s). Initial adsorption is fast (seconds); then an organization phase follows which should be allowed to continue for >15 h for best results. A schematic of a fully assembled SAM is shown to the right.

7 Tail Groups: Functionalization As 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.

8 SAM on AU (111) Surface A schematic model of the (sqrt(3) sqrt(3))r30 overlayer structure formed by alkanethiolate SAMs on Au(111).

9 Characterization: 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 ). The distance between pinning sites in this geometry is 5.0 Å, resulting in an available area for each molecule of 21.4 Å2. Since the van der Waals diameter of the alkane chain is somewhat too small (4.6 Å) 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.

10 The Au-thiolate 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 kcal/mol. The latter forces add up to significant strength for alkyl chains of methylenes and play an important role in aligning the alkyl chains parallel to each other in a nearly alltrans 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 highquality 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 the following Figure...

11 IRAS spectrum of a hexadecanethiolate SAM in the CH stretching region. The most prominent vibrations are indicated. d+ and d- are the symmetric and antisymmetric CH2 stretches; r+ and r- are the symmetric and antisymmetric CH3 stretches, respectively. At the measurement temperature (82 K), the ra- and rb-components of the r- peak are resolved.

12 Thickness 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 21±1 Å range, to compare with the 21.2 Å that result if a fully extended hexadecanethiol molecule of 24.5 Å 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.

13 Lithography

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15 U Bielefeld

16 In Si(100), the anisotropic etch characteristic of a KOH was exploited for the fabrication of 35 nm wide and 30 nm deep grooves. The grating pattern was written in Octadecylthrichlorosilane (OTS) adsorbed onto hydroxilized Si(100).

17 DNA-Self Self-assembled monolayers

18 DNA SAMs

19 Biotinylated SAMs (NIST) NIST

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21 Mercury Removal: SAMs in Mesopores (Pacific NN Lab)

22 Mesoporous Silica for Mercury Removal Xiangdong Feng, while a researcher for the Pacific Northwest National Laboratory, developed a process where molecules that can grab mercury out of the water are placed inside mesoporous silica. This spongelike rock has a surface area thousands of times larger than its size allowing it to grab the mercury quickly and efficiently. Eventually, Feng says, it should be possible to reduce the presence of mercury to a few parts per trillion, compared with a few parts per billion for current techniques, and do it more quickly as well. In addition, Feng says his invention can remove just about any pollutant or heavy metal from contaminated water. "People say I should quit my day job and start mining for gold."

23 Mercury Removal: SAM in Mesopores (Pacific NN Lab) Under this task, a proprietary new technology, Self-Assembled Monolayers on Mesoporous Supports (SAMMS), for RCRA metal ion removal from aqueous wastewater and mercury removal from organic wastes such as vacuum pump oils is being developed at Pacific Northwest National Laboratory (PNNL). The six key features of the SAMMS technology are 1) large surface area (>900 m2/g) of the mesoporous oxides (SiO2, ZrO2, TiO2) ensures high capacity for metal loading (more than 1 g Hg/g SAMMS); 2) molecular recognition of the interfacial functional groups ensures the high affinity and selectivity for heavy metals without interference from other abundant cations (such as calcium and iron) in wastewater; 3) suitability for removal of mercury from both aqueous wastes and organic wastes; 4) the Hg-laden SAMMS not only pass TCLP tests, but also have good long-term durability as a waste form because the covalent binding between mercury and SAMMS has good resistance to ion exchange, oxidation, and hydrolysis; 5) the uniform and small pore size (2 to 40 nm) of the mesoporous silica prevents bacteria (>2000 nm) from solubilizing the bound mercury; and 6) SAMMS can also be used for RCRA metal removal from gaseous mercury waste, sludge, sediment, and soil. Resource Conservation and Recovery Act (RCRA)

24 Hg-removal by SAM in Silica (SiO2, ZrO2, TiO2)

25 Molecular self-assembly is a unique phenomenon in which functional molecules aggregate on an active surface, resulting in an organized assembly having both order and orientation.9-11 In this approach, bifunctional molecules containing a hydrophilic head group and a hydrophobic tail group adsorb onto a substrate or an interface as closely packed monolayers. The driving forces for the self-assembly are the inter- and intra-molecular interactions between the functional molecules (such as van der Waals forces). The tail group and the head group can be chemically modified to contain certain functional groups to promote covalent bonding between the functional organic molecules and the substrate on one end, and molecular bonding between the organic molecules and the metals on the other.9 By populating the outer interface with specific functional groups, an effective means for scavenging heavy metals is made available. The metal-loading capability is determined by the available surface area of the underlying inorganic support. A high surface area support allows for high RCRA metal loading.

26 High-Surface Mesoporous Supports The unique mesoporous oxide supports provide high surface area (>900 m2/g), thereby enhancing the metal loading capacity. They also provide an extremely narrow pore size distribution, which can be specifically tailored from 15 Å to 400 Å, thereby minimizing biodegradation from microbes and bacteria. Mesoporous structures can be disposed of as stable waste forms. The porous supporting materials used in this research (SiO2, ZrO2, TiO2) are synthesized through a co-assembly process using oxide precursors and surfactant molecules The material synthesis is accomplished by mixing surfactants and oxide precursors in a solvent and reacting the solution under mild hydrothermal conditions. The surfactant molecules form ordered liquid crystalline structures, such as hexagonally ordered rod-like micelles, and the oxide materials precipitate on the micellar surfaces to replicate the organic templates formed by the rod-like micelles. Subsequent calcination to 500 C removes the surfactant templates and leave a high surface area oxide skeleton. The pore size of the mesoporous materials is then determined by the rod-like micelles, which are extremely uniform. Using different chain length surfactants produces mesoporous materials with different pore sizes.