Fabricating planar nanoparticle assemblies with number density gradients. (General Developments).

Nanoparticle-based structures are envisioned to play an important role in futuristic devices such as single-electron tunneling-based computer chips, high-density information storage devices, and other commercially-important applications such as sensors and high-efficiency solar cells. In order to position nanocomponents onto functional devices, it is vital to develop methods for placing them into chemically and structurally well-defined environments. To this end, scientists at NIST and their collaborators at North Carolina State University have prepared assemblies of 15 nm gold nanoparticles with continuous gradients in number density on flat silica-covered substrates. A description of the material appears as the cover story in the July 23 issue of Langmuir [Fabricating planar nanoparticle assemblies with number density gradients, Rajendra R. Bhat, Daniel A. Fischer, and Jan Genzer, Langmuir 18, 5640-5643 (2002)].

For surface-property optimization, it is useful to prepare samples with a continuously-varying number density of immobilized particles along the surface. Such a nanoparticle gradient surface provides a combinatorial platform for surface adsorption selectivity. The gold nanoparticle surface was created by forming a one-dimensional molecular gradient of amino groups on the substrate, and then attaching the gold nanoparticles to the amino groups by immersing the substrate in a colloidal gold solution. Atomic force microscopy demonstrated that the number density of the nanoparticles varied continuously as a function of position on the substrate from 500 particles/[mu][m.sup.2] to zero particles/[mu][m.sup.2] over a distance of 45 mm. Near-edge x-ray absorption fine-structure studies confirmed that the nanoparticle number density gradient was closely correlated with the concentration gradient of amino groups anchored to the substrate. As a result of this work, the number density of nanoparticles within the g radient and the length of the gradient can now be tuned by controlling the initial amino group gradient.

CONTACT: Daniel A. Fischer, [phone]; [e-mail] or Jan Genzer, [phone]; [e-mail].