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Guided Self-Assembly of Gold Thin Films Print
Wednesday, 21 November 2012 12:18

Nanoparticles—man-made atoms with unique optical, electrical, and mechanical properties—have become key components in many fields of science. If nanoparticles could be coaxed into routinely assembling themselves into predictable complex structures and hierarchical patterns, devices could be mass-produced that are one thousand times smaller than today’s microtechnologies. Berkeley Lab and UC Berkeley scientists have made progress toward this goal, successfully directing the self­-assembly of nanoparticles into device-ready thin films, which have potential applications in fields ranging from computer memory storage to energy harvesting and storage, from catalysis to light management, and into the emerging new field of plasmonics.

Nanoparticles on the Mesoscale

The collective properties of 3D nanoparticle assemblies have not been widely explored. For the past decade, Xu et al. have been working on ways to build and study such assemblies. Earlier this year, they induced rod-shaped semiconductor nanocrystals to self-assemble into one-, two-, and three-dimensional macroscopic structures. Now, they have coerced 2D nanoparticles to assemble into multilayer thin films using a scalable and easily reproducible technique.

Organizing nanoparticles into ordered arrays can illuminate hidden, interesting properties that arise from synergistic interparticle coupling. This is yet another example of researchers looking to expand their knowledge of a phenomenon on the mesoscale, which looks at functionality, complexity, interactions, and other properties on multiple length and time scales (see Not All Nanodisk Magnetic Vortices Are Created Equally for an example).

Development of a fabrication platform to generate 3D ordered arrays of nanoparticles will enable not only a better understanding of this new family of materials, but also the fabrication of a wide range of next-generation nanodevices including optical lenses and polarizers, memory storage, nanoelectronic circuits, photovoltaics, and batteries.

Through a relatively easy and inexpensive technique where solutions of block copolymer supramolecules direct the self-assembly of nanoparticles, researchers produced multiple layers of thin films from highly ordered one-­, two- and three-dimensional arrays of gold nanoparticles. Block copolymers are long sequences, or “blocks,” of one type of monomer bound to blocks of another type of monomer that have an innate ability to self-assemble into well-­defined arrays of nanosized structures over macroscopic distances. A supramolecule is a group of molecules that act as a single molecule able to perform a specific set of functions.

This research represents the first time that 2D nanoparticle assembly has been clearly achieved in multilayers in supramolecule-based nanocomposite thin films. Block copolymer supramolecules self­-assemble to form a wide range of morphologies that feature microdomains smaller than tens of nanometers in size. Since their size is comparable to that of nanoparticles, the microdomains of block copolymer supramolecules provide an ideal structural framework for nanoparticle co-self­-assembly.

 

A relatively simple and inexpensive technique allows researchers to direct the self-assembly of nanoparticles into device­-ready thin films with microdomains of lamellar (left) or cylindrical (right) morphologies.

This simple yet versatile supramolecular approach allows for the control of the 3D spatial organization of nanoparticles with single-particle precision over macroscopic distances in thin films. It does not require chemical modification to any of the components in the composite system and, in addition to providing a means of building nanoparticle-based devices, should also provide a powerful platform for studying nanoparticle structure–property correlations. The technique can easily produce much larger films, and it can be used on nanoparticles of many materials besides gold, making it well suited for scalable manufacturing.

Incorporating gold nanoparticles into solutions of block copolymer supramolecules forms films ranging in thickness between 100 to 200 nanometers. Upon incorporation of nanoparticles, the supramolecules experience conformational changes, resulting in entropy that determines the placement and distribution of the nanoparticles, as well as the overall morphology of the nanocomposite thin films. Results indicate that it should be possible to generate highly ordered lattices of nanoparticles within block co­polymer microdomains and obtain 3D hierarchical assemblies of nanoparticles with precise structural control.

The nanocomposite films featured microdomains in one of two common morphologies­: lamellar or cylindrical. In the lamellar microdomains, the nanoparticles formed hexagonally packed 2D sheets stacked into multiple layers parallel to the surface. For the cylindrical microdomains, the nanoparticles formed 1­D chains (single-particle width) that were packed into distorted hexagonal lattices in parallel orientation with the surface.

AFM phase image and the cross-sectional TEM image of a ∼150-nm-thick PS(40K)-b-P4VP(5.6K)(PDP)1/6 vol% Au nanoparticle thin film are shown in (a) and (b), respectively. At 6 vol% nanoparticle loading, the Au nanoparticles covered the entire air−polymer interface and form 2D hexagonally packed nanoparticle arrays. The inset of (a) shows the Fourier transform of a single grain of 2D nanoparticle array at the film surface, confirming that the nanoparticles are hexagonally packed with an interparticle distance of ∼10 nm. The cross-sectional TEM image and the image analysis suggest that the nanoparticles formed a 2D nanoparticle sheet in the middle of P4VP(PDP)1 lamellae, and the concentration of nanoparticles in each nanoparticle sheet is uniform throughout the entire thickness of the nanocomposite thin film. The schematic drawing in (c) depicts the spatial distribution of nanoparticles in this particular sample. (d) GISAXS pattern of the nanocomposite thin film at an incident angle α of 0.1°, confirming the nanoparticles form hexagonally packed ordered arrays on the film surface. (e) Horizontal (qy) line cuts of the GISAXS patterns of the same film at six different incident angles. As the incident angle went from below to above the critical angle of the film, the scattering profiles along the qy direction remain similar, confirming that the hexagonally packed 2D nanoparticle lattice is present at the air−polymer interface as well as the interior of the film. The plots were shifted vertically for clarity.

To study these two morphologies, grazing-incidence small-angle x-ray scattering (GISAXS) was performed on ALS Beamline 7.3.3 to probe internal nanoparticle structures. The incident angle was selected to probe only the in-plane structure of nanoparticles at the film surface. In-plane diffraction peaks of up to the seventh order can be clearly seen, corresponding to highly ordered 2D lattice domains of hexagonally packed nanoparticles. To probe nanoparticle assemblies in the interior of the film, GISAXS patterns were collected at six incident angles below and above the critical angle of the film. The 2D in-plane nanoparticle assemblies remain similar at each incident angle, confirming that the nanoparticles formed hexagonally packed ordered arrays with similar grain sizes not only on the surface, but also in the interior of the film.

The interparticle distance between gold nanoparticles in the 1­D chains and the 2­D sheets was 8 to 10 nanometers, which raises intriguing possibilities with regards to plasmonics, the phenomenon by which a beam of light is confined to ultra­cramped spaces. Plasmonic technology holds great promise for advancing superfast computers and optical microscopy. One major challenge for developing plasmonics has been the difficulty of fabricating metamaterials with noble metal nanoparticles, like gold. The gold thin films in this study display strong plasmonic coupling along the interparticle spacing in the 1D chains and 2D sheets, therefore allowing for their use in the investigation of unique plasmonic properties for next-generation electronic and photonic devices.

 

From left: Peter Bai, Joseph Kao, and Ting Xu incorporated gold nanoparticles into solutions of block copolymer supramolecules to form multiple layers of self­-assembled thin films.

 


 

Research conducted by: J. Kao, P. Bai, and V.P. Chuang (University of California, Berkeley); Z. Jiang (Advanced Photon Source); P. Ercius (Berkeley Lab); and T. Xu (UC Berkeley and Berkeley Lab).

Research funding: Office of Naval Research Young Investigator Program. Operation of the ALS is supported by the U.S. Department of Energy, Office of Basic Energy Sciences.

Publication about this research: J. Kao, P. Bai, V.P. Chuang, Z. Jiang, P. Ercius, and T. Xu, “Nanoparticle assemblies in thin films of supramolecular nanocomposites,” Nano Lett. 12, 2610 (2012).

ALS Science Highlight #261

 

ALSNews Vol. 337