Metal Nanoparticles and Self-Assembly into Electronic Nanostructures Part 2(Nanotechnology)

3-D Arrays

Starting with metal MPNs that have a narrow size distribution, it is relatively easy to produce ordered 3-D arrays or superlattices, in which the MPNs take the place of atoms in conventional crystals. Ordered 3-D arrays are formed by supporting a drop of the colloidal nanoparticle suspension on a flat surface and slowly evaporating the solvent. Encapsulated metal nanoparticles typically self-assemble into an FCC or HCP superlattice.[38,50] Faceted, single-crystal Au MPNs often adopt preferred orientations in both 2-D and 3-D arrays in which the (111) atomic planes of each Au particle align parallel to the (111) atomic planes of all the other Au particles and to the surface of the supporting substrate.[38,39]

2-D Arrays

Uniform, close-packed monolayers of MPNs are an essential component of many proposed nanoelectronic devices. There are several techniques for fabricating nanoparticle monolayers by self-assembly. These fall into three categories: 1) field-enhanced or molecular interaction-induced deposition from a colloidal solution of MPNs onto a solid substrate; 2) drop casting or spin coating of a suspension of MPNs onto a solid substrate and allowing of the solvent to evaporate; and 3) spreading of a suspension of hydrophobic MPNs in an organic solvent on a water surface, allowing of the solvent to evaporate, and transferring of the floating MPN film to a solid substrate.

Giersig and Mulvaney[34] employed electrophoretic deposition at field strengths of 1 V/cm to deposit a monolayer of Au MPNs on a carbon film-coated TEM grid. The monolayer was polycrystalline with small hexagonal close-packed domains. When the nanoparticles were deposited without an electric field, no ordering was observed. The adsorption of MPNs from solution onto functionalized surfaces has also been used to form nanoparticle monolayers. In this case, the substrate is coated with a ”tether” molecule having a strong affinity for the particles. The substrate is immersed in a colloidal solution of MPNs, removed after a period of time, and rinsed to remove unbound particles. The MPN monolayer formed in this manner is amorphous and is seldom dense (Fig. 5a).[51,52] This approach can also be used to form multilayers if, after the deposition of each monolayer, the monofunctional molecule encapsulating the particles is replaced by a difunctional molecule such as a dithiol. The major drawback of these methods for electronic applications is the lack of long-range order in the particle film.

The simplest method of forming a monolayer of MPNs is to spread a thin film of a colloidal suspension containing the particles either by drop casting or spin coating on a substrate that is wet by the solvent, and to allow the solvent to evaporate.[17] As the solvent evaporates, small monolayer islands form on the substrate. These islands result from the breakup of the liquid film because of dewetting as the film thickness approaches molecular dimensions. The monolayer islands are dense and consist of small hexagonal close-packed domains (Fig. 5c). An important parameter for optimizing the size and degree of order in these monolayer domains is the rate of evaporation of the solvent. However, the key to forming large ordered regions are the uniformity and smoothness of the substrate.[53] Surface nonuniformities arrest the lateral mobility of the nanoparticles and result in microscopic voids and grain boundaries in the particle film.

Fig. 5 Examples of nanoparticle monolayers self-assembled by three different methods. (a) Scanning electron microscopy (SEM) micrograph of a monolayer formed by adsorption of the particles onto a substrate coated with a bifunctional tether molecule. (From Ref. [51]. ©ACS, 2000.) (b) SEM micrograph of a monolayer produced by compression of nanoparticle rafts on a Langmuir trough. (From Ref. [56]. ©AIP, 2001.) (c) TEM micrograph of a monolayer formed by drop casting a colloidal suspension of nanoparticles on a carbon film-coated TEM grid. 

A liquid/liquid interface, because of its inherent uniformity in the lateral direction and nonuniformity in the vertical direction, provides an ideal surface for self-assembling 2-D arrays of nanoparticles. Usually the self-assembly process is carried out by casting an organic suspension containing nanoparticles, which are encapsulated by a hydrophobic molecule, on a water surface and by allowing the solvent to evaporate. The number of nanoparticles spread on the water surface is taken to be smaller than the number needed to form a dense monolayer. When the organic solvent evaporates, discrete monolayer rafts of nanoparticles form on the water surface. These monolayer domains are often well-ordered but cover only a fraction of the surface area. A dense monolayer is obtained by decreasing the area available to the particle rafts using a Langmuir trough.[54-56] As the area of the trough is decreased, the monolayer rafts collide with each other and coalesce. Without an organic solvent present, the monolayer domains typically exhibit solidlike behavior and resist deformation. Collier et al.[57] were able to make use of the rigid behavior of a nanoparticle film supported on a water surface to carefully compress a film of Ag MPNs and to measure a reversible insulator-to-metal transition as the film was compressed and the tunneling distance between adjacent particles was decreased. Unfortunately, this rigid behavior often results in microscopic voids (Fig. 5b) and multilayer domains in monolayers assembled using a Langmuir trough.[56]

Schmid and Beyer[58] have proposed an intriguing variation on the classical technique. They introduce an amphiphilic molecule that has a strong affinity for the nanoparticles to assist in the formation of an ordered particle monolayer. They first self-assemble a layer of these molecules at an organic / water interface. When nanoparticles are introduced to the system, they adsorb on the amphiphilic layer and assemble into monolayer sheets or ribbons, depending on the experimental conditions.

Santhanam et al.[59] have also proposed a modification of the classical technique. They are able to self-assemble uniform, ordered 2-D arrays of Au MPNs on a water surface by controlling the nucleation and growth of the particle monolayer. This is accomplished by designing a cell that establishes a ”concave” lens of colloidal solution on the water surface (Fig. 6). As the solvent evaporates, the organic layer thins fastest at the center of the cell and, at some point, a monolayer array of MPNs nucleates at this spot. The periphery of this 2-D array is defined by a circular contact line. As more solvent evaporates, additional MPNs deposit at the edge of the particle monolayer, and the contact line moves steadily outward. The advantage of this technique is that new particles are added to the growing monolayer in the presence of solvent molecules. This ensures enough particle mobility to largely eliminate microscopic holes and grain boundaries, and promotes the formation of a close-packed crystalline monolayer. Fig. 7 shows a photograph of a film of 5-nm-diameter Au MPNs that was self-assembled on a water surface using this technique. The different hue seen near the edge of the cell arises from the increased curvature of the water interface at the cell wall, which leads to contact line instabilities. This results in alternating bands of multilayer regions surrounding the uniform monolayer region in the center of the cell. Fig. 7 also shows TEM micrographs of samples of this Au MPN film taken near the center and the edge of the cell. These images clearly illustrate the long-range translational ordering of the monolayer that forms in the center of the cell and the nature of the multilayer bands that form at the edge of the cell. A bilayer film can be produced on the water surface simply by increasing the concentration of the particles in the initial colloidal solution.

Fig. 6 Schematic illustration of the process proposed by Santhanam et al. for self-assembly of a uniform close-packed monolayer of metal MPNs by controlling the nucleation and growth of the monolayer film on a water surface.

Fig. 7 Photograph of a film of 5-nm-diameter Au MPNs that were self-assembled on a water surface using the method of Santhanam et al. The two inserts are TEM micrographs of samples of this nanoparticle film that were transferred to carbon film-coated TEM grids. The upper insert is taken from the central portion of the film, which is a uniform close-packed monolayer with a diameter of approximately 1 cm. The lower insert is taken from the outer edge of the film and shows one of the multilayer bands that form near the wall of the cell.

Once a high-quality monolayer of MPNs is self-assembled on the water surface, if it is to be used for constructing an electronic device, it must be transferred to a solid substrate. This transfer has been accomplished by either dipping the substrate through the water surface and slowly withdrawing it [Langmuir-Blodgett (LB) method], or by holding the substrate parallel to the water surface and touching it to the nanoparticle film [Langmuir-Schaefer (LS) method]. Although Santhanam et al.[59] found the LS method preferable, it proved unsatisfactory when the substrate was large and/or hydrophilic. As a result, they developed a two-step transfer process. First, using the LS method, the nanoparticle film is transferred from the water surface to a polydimethylsiloxane (PDMS) pad. After carefully wicking off any water drops that adhere to the transferred nanoparticle film, the PDMS pad is then pressed conformally onto the desired substrate. This technique is analogous to conventional microcontact printing,[60] with the nanoparticle monolayer taking the place of the molecular ink. This two-step printing technique facilitates the formation of MPN films with a controlled number of layers. This can be achieved either by repeating the process for a desired number of cycles and printing the monolayers one on top of the other, or by picking up the monolayers successively onto the same elastomeric pad and then using a single printing step. The use of a PDMS stamp also allows the formation of patterned monolayer and multilayer arrays of MPNs by using an appropriately structured elastomeric pad.[24,61] Fig. 8 shows TEM images at three different magnifications of a bilayer film of 5-nm-diameter Au MPNs that has been patterned into a series of micron-scale lines and printed on a Si3N4 substrate.[24,61]

Fig. 8 TEM micrographs of a bilayer film of 5-nm-diameter Au MPNs that were deposited as a pattern of parallel lines on a silicon nitride substrate using the two-step process proposed by Santhanam et al. The low-magnification micrograph on the left shows the lines printed on the substrate. The higher-magnification micrograph in the center shows the relatively sharp edge of one of the lines. The highest-magnification micrograph on the right shows the dense close-packed nanostructure of the film at the edge of one of the lines.

1-D Arrays

The ability to form 1-D arrays or thin ribbons of metal MPNs is important in the context of using linked metal MPNs as interconnects. However, because metal MPNs are nearly spherical in shape, this is a difficult pattern to produce by self-assembly and can be accomplished only with the help of appropriate templates to direct the self-assembly process. For example, carbon nanotubes, long-chain polymers, and DNA strands can be decorated with MPNs to produce quasi-1-D chains, and MPNs can be assembled inside a nanotube or adsorbed on a thin line fabricated on a substrate. Hornyak et al.[62] have synthesized nanoporous alumina membranes with controlled pore size and narrow pore size distribution, and used the pores as a template to form quasi-1-D chains of Au MPNs. A colloidal solution of the particles was drawn by means of a vacuum into the pores. On evaporation of the solvent, some of the pores were found to be filled with MPN chains. Gleich et al.[63] made use of the wetting instability of a monolayer transferred onto a solid substrate to produce channels on the order of 200 nm in width and were able to form quasi-1-D chains of MPNs by drop casting a colloidal solution onto this template. Quasi-1-D arrays or ribbons of MPNs have also been prepared by patterning a 2-D monolayer of MPNs using e-beam lithography.[64] In principle, the PDMS stamping technique described in ”2-D Arrays” could be extended to produce nanometer-scale ribbons of metal MPNs using an appropriate master to mold the PDMS stamp. The technique of Melosh et al.,[15] which makes use of selective etching of a GaAs/AlGaAs superlattice to generate thin parallel trenches, could possibly be used as the master, or a master could be generated using e-beam lithography and PMMA resist.

CONCLUSION

Au MPNs are attractive building blocks for fabricating nanoelectronic devices by self-assembly. The synthesis of Au MPNs with a mean diameter in the 2- to 20-nm range, with a narrow size distribution, and with a monolayer coating of alkanethiol molecules is, by now, a standard procedure. The assembly of these particles to form chemiresistive films and low-resistivity printable conductors is well established. Recent results describing the self-assembly of high-quality monolayer films of Au MPNs on a water surface and the discovery that these films can be transferred as patterned close-packed arrays onto any reasonably flat substrate have opened up a wide range of potential nanoelectronic applications. What remains is the need to establish methods to reproducibly link these ordered arrays of alkanethiol-encapsulated gold particles with conjugated organic molecules to form nanometer-scale interconnects and molecular electronic circuits.