Selfassembly And Catalysis

10.1. SELF-ASSEMBLY 10.1.1. Process of Self-Assembly

Proteins are large molecules, with molecular weights in the tens of thousands, found in virtually all the cells and tissues of the body, and they are essential for life. They are formed by the successive addition of hundreds of amino acids, each brought to its site of attachment by a transfer RNA molecule, in an order prescribed by a messenger RNA molecule. Bach amino acid readily bonds to the previous one when it arrives in place. Thus sequences of amino acids assemble into a polypeptide chain, which continually increases in length to eventually become a protein. This type of self-assembly process, which occurs naturally in all living systems, has its analog in nanoscience. Here we also encounter the spontaneous organization of small molecules into larger well-defined, stable, ordered molecular complexes or aggregates, and we encounter die spontaneous adsorption of atoms or molecules onto a substrate in a systematic, ordered manner. This process involves the use of weak, reversible interactions between parts of molecules without any central control, and the result is a configuration that is in equilibrium. The procedure is automatically

Introduction to Nanotechnology, by Charies P. Poole Jr. and Frank J. Owens. ISBN 0-471-07935-9. Copyright © 2003 John Wiley Sc. Sons, Inc.

error-checking, so faulty or improperly attached subunits can be replaced during the growth.

The traditional organic synthesis of very large molecules called macromolecules comprises a number of tune-consuming steps that involve breaking and remaking strong covalent bonds, and these steps are earned out under kinetic control. The yields are small, and errors are not readily recognized or corrected to contrast to this, the self-assembly variety of synthesis Brake» use of weak, ncocovaknt bonding interactions such as those involving hydrogen bonds and van der Waals forces, which .permit the ica&ioiis to proceed under thermodynamic control, with the continual correction of errors. The initial individual molecules or subunits are usually small in size and number and easy to synthesize, and the final product is produced in a thermodynamic equilibrium State.

10.1.2. Semiconductor Islands

One type of self-assembly involves the preparation of semiconductor islands, and it can be carried out by a technique called heteroepitaxy, which involves the placement or deposition of the material that forms the island on a supporting substance called a substrate made of a different material with a closely matched interface between them. Heteroepitaxy has been widely used for research, as well as for die fabrication of many semiconductor devices, so it is a well-developed technique. It involves bringing atoms or molecules to the surface of die substrate where they do one of three things. They either are adsorbed and diffuse about on the surface until they join or nucleate with another adatom to form an island, attach themselves to or aggregate into an existing island, or desorb and thereby leave fee surface. Small islands can continue to grow, migrate to other positions, or evaporate. There is a critical size at which they become stable, and no longer experience much evaporation. Thus there is an initial nucleation stage when the number of islands increases with fee coverage. This is followed by an aggregation stage when fee number of islands levels off and the existing ones grow in size. Finally there is fee coalescence stage when the main events that take place involve the merger of existing islands wife each other to form larger clusters.

The various stages can be described analytically or mathematically in terms of the rates of change dnjdt of the concentrations of individual adatoms nx, pairs of adatoms n2, clusters of size three n3, and so on, and an example of a kinetic equation feat is applicable at the initial or nucleation stage is fee following expression for isolated atoms (Weinberg et al. 2000).

where Ra<ls is the rate of adsorption, is the rate of detachment of atoms from clusters larger than pairs, and Ry is the rate of breakup of adatom pairs. The negative terms correspond to the rate of evaporation i?evap, the rate of capture of individual adatoms by clusters R^, and the rate of formation of pairs of adatoms 2R\. The factor of 2 associated with /?, and R\ accounts for the participation of two atoms in each pair process. Analogous expressions can be written for the rate of change of the number of pairs dn2/dt, for the rate of change of the number of triplet clusters dn3/dt, and so forth. Some of the terms for the various rates Rt depend on the extent of the coverage of the surface, and the equation itself is applicable mainly during the nucleation stage.

At the second or aggregation stage the percentage of isolated adatoms becomes negligible, and a free-energy approach can provide some insight into the island formation process. Consider the Gibbs fiee-energy density gsbetween the bare surface and the vacuum outside, the free-energy density g^-^y between the surface and the layers of adatoms, and the free-energy density gtay_va!. between these layers and die vacuum. These are related to the overall Gibbs free-energy density g through the expression g = gsur-racO ~ s) + igsur-iay + Sky-vac)« (10-2)

where 6 is the fraction of the surface covered. As the islands form and grow the relative contributions arising from these terms gradually change, and the growth process evolves to maintain the lowest thermodynamic free energy. These free energies can be used to define a spreading pressure Ps = g^— ^ + giay-v»c) that involves the difference between the bare surface free energy g^-^ and that of the layers C?sur-lay + £tay-v»cX and it is associated with the spreading of adatoms over the surfece. For the condition (g^-iay + Sky-vac) < gsur-vac> 1116 addition of adatoms increases s, and thereby causes the free energy to decrease. Thus the adatoms that adsorb will tend to remain directly on the bare surface leading to a horizontal growth of islands, and the eventual formation of a monolayer. The spreading pressure Ps is positive and contributes to the dispersal of the adatoms. This is referred to as the Franck-van der Merwe growth mode.

For the opposite condition -l-gi^-v«,) > ^ growth of the fractional surface coverage g increases the free energy, so it is thennodynamically unfavorable for the adsorbed layer to be thin said flat. The newly added adatoms tend to keep the free energy low by aggregating on the top of existing islands, leading to a vertical rather than a horizontal growth of islands. This is called the Volmer-Weber mode of growth.

We mentioned above that heteroepitaxy involves islands or a film with a nearly matched interface with the substrate. The fraction / of mismatch between the islands and fiie surface is given by file expression (see p. 18)

where Of is the lattice constant of the island or film and a, is the lattice constant of the substrate. For small mismatches, less than 2%, very little strain develops at the growth of a film consisting of many successive layers on top of each other. If the mismatch exceeds 3%, then the first layer is appreciably strained, and the extent of the strain builds up as several additional layers are added. Eventually, beyond a transition region the strain subsides, and thick films are only strained in the transition region near the substrate. This mismatch complicates (he free-energy discussion following Eq. (10.2), and favors the growth of three-dimensional islands to compensate for the strain and (hereby minimize the free energy. This is called the Stranski-Krastanov growth mode. A common occurrence in this mode is the initial formation of a monolayer that accommodates the strain, and acquires a critical thickness. The next stage is the aggregation of three-dimensional islands on the two-dimensional monolayer. Another eventuality is the coverage of die surface with monolayer islands of a preferred size to better accommodate the lattice mismatch strain. This can be followed by adding further layers to these islands. A typical size of such a monolayer island is perhaps 5 nm, and it might contain 12 unit cells.

In addition to the three free energies included in Eq. (10.2), the formation and growth of monolayer islands involves the free energies of the strain due to the lattice mismatch, the free energy associated with the edges of a monolayer island, and the free energy for the growth of a three-dimensional isknd on a monolayer. A great deal of experimental work has been done studying the adsorption of atoms on substrates for gradually increasing coverage from zero to several monolayers thick. The scanning tunneling microscope images presented in Fig. 10.1 show the successive growth of islands of InAs on a GaAs (001) substrate for several fractional monolayer coverages. We see from the data in Table B.l that the lattice constant a =i 0.606 nm for InAs and a = 0.565 run for GaAs, corresponding to a lattice mismatch/ = 7.0% from Eq. (10.3), which is quite large.

10.1.3. Monolayers

A model system that well illustrates the principles aid advantages of the self-assembly process is a self-assembled monolayer (Wilber and Whitesides 1999). The Langmuir-Blodgett technique, which historically preceded the self-assembled approach, had been widely used in the past for the preparation and study of optical coatings, biosensors, ligand-stabilized Au55 clusters, antibodies, and enzymes. It involves starting with clusters, forming them into a monolayer at an air-water interface, and then transferring the monolayer to a substrate in die form of what is called a Langmuir-Blodgett film. These films are difficult to prepare, however, and are not sufficiently rugged for most purposes. Self-assembled monolayers, on the other hand are stronger, are easier to make, and make use of a wider variety of available starting materials.

Self-assembled monolayers and multilayers have been prepared on various metallic and inorganic substrates such as Ag, Au, Cu, Ge, Pt, Si, GaAs, Si02, and other materials. This has been done with the aid of bonding molecules or ligands such as alkanethiols RSH, sulfides RSR', disulfides RSSR', acids RCOOH, and siloxanes RSiOR3, where the symbols R and R' designate organic molecule groups that bond to, for example, a thiol radical —SH or an acid radical —COOH. The binding to the surface for the thiols, sulfides, and disulfides is via the sulfur atom;

Figure 10.1. Transmission electron microscopy images of the successive growth of islands of InAs on a GaAs (001) substrate for fractional monolayer coverages of (a) 0.1, (b) 0.3, (c) 0.6, and (d) 1.0. The image sizes are 50 nm x 50 nm for (a) and 40 nm x 40 nm for (b) to (d). The inset for (a) is an enlarged view of the GaAs substrate. [From J. G. Belk, J. L Sudijono, M. M. Holmes, C. F. McConville, T. S. Jones, and B. A. Joyce, Surf. Sci. 365, 735 (1996).]

that is, the entity RS—Au is formed on a gold substrate, and the binding for the acid is RC02—(MO)„, where MO denotes a metal oxide substrate ion, and the hydrogen atom H of the acid is released at the formation of the bond. The alkanethiols RSH are the most widely used ligands because of their greater solubility, their compatibility with many organic functional groups, and their speed of reaction. They spontaneously adsorb on the surface; hence the term self-assemble is applicable. In this section we consider the self-assembly of the thiol ligand X(CH2)„ SH, where the terminal group X is methyl (CH3) and a typical value is n = 9 for decanethiol, corresponding to CioH2, for R.

To prepare a gold substrate as an extended site for the self-assembly, an electron beam or high-temperature heating element is used to evaporate a polycrystalline layer of gold that is 5-300 nm thick on to a polished support such as a glass slide, a

Figure 10.2. Hexagonal close-packed array of adsorbed n-alkanethiolate molecules (small solid circles) occupying one-sixth of the threefold sites on the lattice of close-packed gold atoms (large circles).

silicon wafer, or mica. The outermost gold layer, although polycrystalline, exposes local regions of a planar hexagonal close-packing atom arrangement, as indicated in Figs. 10.2 and 10.3. Various properties such as the conductivity, opacity, domain size, and surface roughness of the film depend on its thickness. The adsorption sites are in the hollow depressions between triplets of gold atoms on the surface. The number of depressions is the same as the number of gold atoms on the surface. Sometimes Cr or Ti is added to facilitate the adhesion. When molecules in the liquid (or vapor) phase come into contact with the substrate, they spontaneously adsorb on the clean gold surface in an ordered manner; that is, they self-assemble.

The mechanism or process whereby the adsorption takes place involves each particular alkanethiol molecule CH3(CH2)„SH losing the hydrogen of its sulfhydryl group HS—, acquiring a negative charge, and adhering to the surface as a thiolate compound by embedding its terminal S atom in a hollow between a triplet of Au

Figure 10.3. Illustration of the self-assembly of a monolayer of r>atkanethiolates on gold. The terminal sulphur resides in the hollow between three close-packed gold atoms, as shown in Fig. 10.2. The terminal groups labeled by X represent methyl. [From J: L. Wittier and G. M. Whitesides, in Nanotechnology, G. Timp, ed., Springer-Vertag, Berlin, 1999, Chapter 8, p. 336.)

Figure 10.3. Illustration of the self-assembly of a monolayer of r>atkanethiolates on gold. The terminal sulphur resides in the hollow between three close-packed gold atoms, as shown in Fig. 10.2. The terminal groups labeled by X represent methyl. [From J: L. Wittier and G. M. Whitesides, in Nanotechnology, G. Timp, ed., Springer-Vertag, Berlin, 1999, Chapter 8, p. 336.)

atoms, as noted above and indicated in Fig. 10.3. The reaction at the surface might be written

CH3(CH2)„SH 4- Aum => CH3(CH2)„S-(Au3+) • Aum_3 + ±H2 (10.4)

where Aum denotes the outer layer of the gold film, which contains m atoms. The quantity (AuJ) is the positively charged triplet of gold atoms that forms the hollow depression in the surface where die terminal sulfur ion (S~) forms a bond with the gold ion AuJ. Figure 10.3 gives a schematic view of this adsorption process. The sulfur-gold bond that holds the alkanethiol in place is a fairly strong one (~44kcal/mol) so the adhesion is stable. The bonding to the surface takes place at one-sixth of die sites on the (111) close-packed layer, and these sites are occupied in a regular manner so they form a hexagonal close-packed layer with the lattice constant equal to V3 Oo = 0.865 nm, where a0 = 0.4995 nm is the distance between Au atoms on the surface. Figures 10.2 and 10.3 indicate die location of the alkanethiol sites on the gold close-packed surface.

The alkanethiol molecules RS—, bound together sideways to each other by weak van der Waals forces with a strength of ~1.75kcal/mol, arrange themselves extending lengthwise up from the gold surface at an angle of ~30° with the normal, as indicated in the sketch of Fig. 10.4. The alkyl chains R extend out ~2.2nm to form the layer of hexadecanethiol CH3(CH2)10S~. A number of functional groups can be attached at the end of the alkyl chains in place of the methyl group such as acids, alcohols, amines, esters, fluorocaibons, and nitriles. Thin layers of alkanethiols with n < 6 tend to exhibit significant disorder, while thicker ones with n > 6 are more regular, but polycrystalline, with domains of differing alkane twist angle. Increasing die temperature can cause die domains to alter their size and shape.

For self-assembled monolayers to be useful in commercial microstructures, they can be arranged in structured regions or patterns on the surface. An alkanethiol "ink" can systematically form or write patterns on a gold surface with alkanethio-late. The monolayer-forming "ink" can be applied to the surface by a process called microcontact printing, which utilizes an elastomer, which is a material with rubber-like properties, as a "stamp" to transfer the pattern. The process can be employed to produce thin radiation-sensitive layers called resists for nanoscale lithography, as discussed in Section 9.2. The monolayers themselves can serve for a process called

Figure 10.4. Sketch of n-aikanethiotate molecules adsorbed on a gold surface at an angle of 30° with respect to the normal. [From Wilber and Whitesides (1999), p. 336.]

Figure 10.4. Sketch of n-aikanethiotate molecules adsorbed on a gold surface at an angle of 30° with respect to the normal. [From Wilber and Whitesides (1999), p. 336.]

passivation by protecting the underlying surface from corrosion. Alkanediols can assist in colloid preparation by controlling the size and properties of the colloids, and this application can be very helpful in improving the efficacy of catalysts.

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