Person:

Whitesides, George

This paper examines the factors that influence the quality of nanostructures fabricated by sectioning thin films with an ultramicrotome (“nanoskiving”). It surveys different materials (metals, ceramics, semiconductors, and conjugated polymers), deposition techniques (evaporation, sputter deposition, electroless deposition, chemical-vapor deposition, solution-phase synthesis, and spin-coating), and geometries (nanowires or two-dimensional arrays of rings and crescents). It then correlates the extent of fragmentation of the nanostructures with the composition of the thin films, the methods used to deposit them, and the parameters used for sectioning. There are four major conclusions. (i) Films of soft and compliant metals (those that have bulk values of hardness less than or equal to those of palladium, or ≤500 MPa) tend to remain intact upon sectioning, whereas hard and stiff metals (those that have values of hardness greater than or equal to those of platinum, or ≥500 MPa) tend to fragment. (ii) All conjugated polymers tested form intact nanostructures. (iii) The extent of fragmentation is lowest when the direction of cutting is perpendicular to the exposed edge of the embedded film. (iv) The speed of cutting−from 0.1 to 8 mm/s−has no effect on the frequency of defects. Defects generated during sectioning include scoring from defects in the knife, delamination of the film from the matrix, and compression of the matrix. The materials tested were: aluminum, titanium, nickel, copper, palladium, silver, platinum, gold, lead, bismuth, germanium, silicon dioxide ((\textrm{SiO}_2)), alumina ((\textrm{Al}_2\textrm{O}_3)), tin-doped indium oxide (ITO), lead sulfide nanocrystals, the semiconducting polymers poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV), poly(3-hexylthiophene) (P3HT), and poly(benzimidazobenzophenanthroline ladder) (BBL), and the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS).

This paper describes the fabrication of arrays of nanostructures (rings, crescents, counterfacing split rings, cylinders, coaxial cylinders, and other structures) by a four-step process: (i) molding an array of epoxy posts by soft lithography, (ii) depositing thin films on the posts, (iii) embedding the posts in epoxy, and (iv) sectioning in a plane parallel to the plane defined by the array of posts, into slabs, with an ultramicrotome (“nanoskiving”). This work demonstrates the combination of four capabilities: (i) formation of structures that are submicrometer in all dimensions; (ii) fabrication of 3D structures, and arrays of structures, with gradients of height; (iii) patterning of arrays containing two or more materials, including metals, semiconductors, oxides, and polymers; and (iv) generation of as many as 60 consecutive slabs bearing contiguous arrays of nanostructures. These arrays can be transferred to different substrates, and arrays of gold rings exhibit plasmonic resonances in the range of wavelengths spanning 2−5 μm.

This Review discusses nanoskiving—a simple and inexpensive method of nanofabrication, which minimizes requirements for access to cleanrooms and associated facilities, and which makes it possible to fabricate nanostructures from materials, and of geometries, to which more familiar methods of nanofabrication are not applicable. Nanoskiving requires three steps: 1) deposition of a metallic, semiconducting, ceramic, or polymeric thin film onto an epoxy substrate; 2) embedding this film in epoxy, to form an epoxy block, with the film as an inclusion; and 3) sectioning the epoxy block into slabs with an ultramicrotome. These slabs, which can be 30 nm–10 μm thick, contain nanostructures whose lateral dimensions are equal to the thicknesses of the embedded thin films. Electronic applications of structures produced by this method include nanoelectrodes for electrochemistry, chemoresistive nanowires, and heterostructures of organic semiconductors. Optical applications include surface plasmon resonators, plasmonic waveguides, and frequency-selective surfaces.

Optical metasurfaces—patterned arrays of plasmonic nanoantennas that enable the precise manipulation of light–matter interactions—are emerging as critical components in many nanophotonic materials, including planar metamaterials, chemical and biological sensors, and photovoltaics. The development of these materials has been slowed by the difficulty of efficiently fabricating patterns with the required combinations of intricate nanoscale structure, high areal density, and/or heterogeneous composition. One convenient strategy that enables parallel fabrication of periodic nanopatterns uses self-assembled colloidal monolayers as shadow masks; this method has, however, not been extended beyond a small set of simple patterns and, thus, has remained incompatible with the broad design requirements of metasurfaces. This paper demonstrates a technique—shadow-sphere lithography (SSL)—that uses sequential deposition from multiple angles through plasma-etched microspheres to expand the variety and complexity of structures accessible by colloidal masks. SSL harnesses the entire, relatively unexplored, space of shadow-derived shapes and—with custom software to guide multiangled deposition—contains sufficient degrees of freedom to (i) design and fabricate a wide variety of metasurfaces that incorporate complex structures with small feature sizes and multiple materials and (ii) generate, in parallel, thousands of variations of structures for high-throughput screening of new patterns that may yield unexpected optical spectra. This generalized approach to engineering shadows of spheres provides a new strategy for efficient prototyping and discovery of periodic metasurfaces.

Convenient and inexpensive methods to pattern the facets of optical fibers with metallic nanostructures would enable many applications. This communication reports a method to generate and transfer arrays of metallic nanostructures to the cleaved facets of optical fibers. The process relies on nanoskiving, in which an ultramicrotome, equipped with a diamond knife, sections epoxy nanostructures coated with thin metallic films and embedded in a block of epoxy. Sectioning produces arrays of nanostructures embedded in thin epoxy slabs, which can be transferred manually to the tips of optical fibers at a rate of approximately 2 min−1, with 88% yield. Etching the epoxy matrices leaves arrays of nanostructures supported directly by the facets of the optical fibers. Examples of structures transferred include gold crescents, rings, high-aspect-ratio concentric cylinders, and gratings of parallel nanowires.

This paper describes the use of a nanoindenter, equipped with a diamond tip, to form patterns of indentations on planar substrates (epoxy, silicon, and (SiO_2)). The process is called “Indentation Lithography” (IndL). The indentations have the form of pits and furrows, whose cross-sectional profiles are determined by the shapes of the diamond indenters, and whose dimensions are determined by the applied load and hardness of the substrate. IndL makes it possible to indent hard materials, to produce patterns with multiple levels of relief by changing the loading force, and to control the profiles of the indentations by using indenters with different shapes. This paper also demonstrates the transfer of indented patterns to elastomeric PDMS stamps for soft lithography, and to thin films of evaporated gold or silver. Stripping an evaporated film from an indented template produces patterns of gold or silver pyramids, whose tips concentrate electric fields. Patterns produced by IndL can thus be used as substrates for surface-enhanced Raman scattering (SERS) and for other plasmonic applications.

Conventional lithographic methods (e.g., electron-beam lithography, photolithography) are capable of producing high-resolution structures over large areas but are generally limited to large (>1 cm2) planar substrates. Incorporation of these features on unconventional substrates (i.e., small (<1 mm2) and/or non-planar substrates) would open possibilities for many applications, including remote fiber-based sensing, nanoscale optical lithography, three-dimensional fabrication, and integration of compact optical elements on fiber and semiconductor lasers. Here we introduce a simple method in which a thin thiol-ene film strips arbitrary nanoscale metallic features from one substrate and is then transferred, along with the attached features, to a substrate that would be difficult or impossible to pattern with conventional lithographic techniques. An oxygen plasma removes the sacrificial film, leaving behind the metallic features. The transfer of dense and sparse patterns of isolated and connected gold features ranging from 30 nm to 1 μm, to both an optical fiber facet and a silica microsphere, demonstrates the versatility of the method. A distinguishing feature of this technique is the use of a thin, sacrificial film to strip and transfer metallic nanopatterns and its ability to directly transfer metallic structures produced by conventional lithography.