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Posted: Nov 28, 2012
Guided self-assembly of gold thin films
(Nanowerk News) 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 ("Nanoparticle assemblies in thin films of supramolecular nanocomposites").
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 morphologies.
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 threedimensional 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 welldefined 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 2-D nanoparticle assembly has been clearly achieved in multi-layers 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.
This simple yet versatile supramolecular approach allows for the control of the 3-D 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 3-D 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 2-D 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 nanoparticles 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 2-D hexagonally packed nanoparticle arrays. The inset of (a) shows the Fourier transform of a single grain of 2-D 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 2-D 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 2-D 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 2-D 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 2-D 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 inter-particle 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 inter-particle spacing in the 1-D chains and 2-D sheets, therefore allowing for their use in the investigation of unique plasmonic properties for next-generation electronic and photonic devices.