Composite stamps composed of two layerssa stiff layer supported by a flexible layersextend the capabilities of soft lithography to the generation of 50-100-nm features. The preparation of these stamps was adapted from a procedure originally developed by Schmid et al. (Macromolecules 2000, 33, 3042) for microcontact printing. This paper demonstrates how pattern transfer using other soft lithographic techniquessmicromolding in capillaries, microtransfer molding, and phase-shifting lithographyscan be improved using two-layer stamps relative to stamps made of Sylgard 184 poly(dimethylsiloxane).

The adsorption of n-alkanethiols onto polycrystalline thin films of palladium containing a strong (111) texture produces well-organized, self-assembled monolayers. The organization of the alkane chains in the monolayer and the nature of the bonding between the palladium and the thiol were studied by contact angle measurements, optical ellipsometry, reflection absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). The XPS data reveals that a compound palladium-sulfide interphase is present at the surface of the palladium film. The RAIR spectra, ellipsometry data, and wetting properties show that the palladium-sulfide phase is terminated with an organized, methyl-terminated monolayer of alkanethiolates. The local molecular environment of the alkane chains transitions from a conformationally disordered, liquidlike state to a mostly all-trans, crystalline-like structure with increasing chain length (n = 8-26). The intensities and dichroism of the methylene and methyl stretching modes support a model for the average orientation of an ensemble of all-trans-conformer chains with a tilt angle of approximately 14-18 degrees with respect to the surface normal and a twist angle of the CCC plane relative to the tilt plane of approximately 45 degrees. The SAMs are stable in air, although the sulfur present at the surface oxidizes in air over a period of 2-5 days at room temperature. The differences in chain organization between SAMs formed by microcontact printing and by solution deposition are also examined by RAIRS and XPS.

The deposition of a thin, metal film onto an array of spherical silica colloids, followed by dissolution of the colloidal template, produces metallic half-shells with nanometer-scale dimensions. Half-shells of gold, platinum, and palladium were fabricated, with diameters of the particles ranging from 100 to 500 nm, and shell thicknesses of 8−15 nm. The half-shells have three useful properties because of their geometries: (i) a high ratio of surface area to volume, (ii) a large length of edge relative to size, and (iii) an entropic resistance to assembling into close-packed structures. The surface properties of these half-shells can be modified with self-assembled monolayers (SAMs), formed by adsorption of alkanethiols. The surfaces composed of aggregated gold half-shells are superhydrophobic; the measured contact angle of water on a surface of unmodified gold half-shells was ∼151°and on a surface of gold half-shells functionalized with a hexadecanethiolate SAM was ∼163°. Aggregates of half-shells were patterned using template-assisted self-assembly. This paper describes a versatile and experimentally simple technique for fabricating hollow metallic hemispheres (which we call "half-shells") with diameters of 100-500 nm and thicknesses of 8-15 nm. The method uses monolayers or multilayers of spherical silica colloids on glass substrates as templates on which the half-shells are formed. The silica exposed at the surface of the array is coated with a thin film of metal by physical vapor deposition. Subsequent dissolution of the colloidal template releases the half-shells into a suspension. This work provides a route to another class of nanostructures useful for bottom-up assembly. 1 The semiconductor industry has developed a number of techniques, including molecular beam epitaxy (MBE), physical vapor deposition (PVD), and chemical vapor deposition (CVD), for depositing thin solid films. 2 These techniques provide control over the composition and thickness of a deposited film with precision on the order of Å. Natelson et al. used MBE and selective chemical etching to fabricate narrow trenches with critical dimensions of 3 nm on the cleaved edge of an alternating stack of AlGaAs/GaAs thin films. 3 Electron-beam and thermal evaporation sources can deposit a wide array of materials, including metals, semiconductors, alloys, and refractory compounds. 2 The combination of these methods for the deposition of thin films with the use of sacrificial structures as templates to guide the formation of particles provides a method of synthesizing nanoparticles of materials that are difficult to synthesize using other procedures.The use of templates in the formation of nanofeatured structures is common: examples include mesoporous silica and zeolites, 4 nanotubules and rods, 5 colloidal crystals and inverse opals, 6 core/shell particles, 7 and others. 8 Evaporation of metal into the spaces between crystallized arrays of spheres has been used to form tetrahedral nanoparticles with useful optical properties. 9 Evaporation of ...

▪ Abstract  Nanostructures are fabricated using either conventional or unconventional tools—that is, by techniques that are highly developed and widely used or by techniques that are relatively new and still being developed. This chapter reviews techniques of unconventional nanofabrication, and focuses on experimentally simple and inexpensive approaches to pattern features with dimensions <100 nm. The techniques discussed include soft lithography, scanning probe lithography, and edge lithography. The chapter includes recent advances in fabricating nanostructures using each set of techniques, together with demonstrated advantages, limitations, and applications for each.

This report describes the manipulation of light in waveguides that comprise a liquid core and a liquid cladding (liq͞liq waveguide). These waveguides are dynamic: Their structure and function depend on a continuous, laminar flow of the core and cladding liquids. Because they are dynamic, they can be reconfigured and adapted continuously in ways that are not possible with solid-state waveguides. The liquids are introduced into the channels of a microfluidic network designed to sandwich the flowing core liquid between flowing slabs of the cladding fluid. At low and moderate Reynolds numbers, flow is laminar, and the liq͞liq interfaces are optically smooth. Small irregularities in the solid walls of the channels do not propagate into these interfaces, and liq͞liq waveguides therefore exhibit low optical loss because of scattering. Manipulating the rate of flow and the composition of the liquids tunes the characteristics of these optical systems.T his report describes the design and operation of a waveguide, an optical switch, and an evanescent coupler in which both the light-guiding and cladding structures are liquids flowing at low Reynolds number (Re Ϸ 5-500) (1) in microchannels fabricated in poly(dimethylsiloxane) (PDMS). The liquid͞liquid interfaces in these systems [liquid-core͞liquid-cladding (liq͞liq) waveguides] are optically smooth at low Re flow; modest roughness in the PDMS channel walls has little effect on the quality of this interface. We estimate that the optical loss because of scattering at the liq͞liq interface is Ͻ1 dB͞cm, ϭ 400-1,100 nm; this estimate is similar to losses observed in polymer and inorganic planar optical waveguides (2). Because the dimensions of the waveguiding region (i.e., the liquid core) in the flowing liquid streams are smaller than the dimensions of the channel and because the roughness of the edges of the channel does not degrade performance, liq͞liq waveguides can be fabricated easily and rapidly in organic polymers by using the convenient techniques of rapid prototyping (3-6). Here we demonstrate the operation of liq͞liq waveguides and show that their properties can be changed by manipulating the rate of flow and͞or the composition of the liquids they comprise. We believe that these systems provide the basis for a tunable class of optical devices.We (7) and others (8-15) have prepared and characterized liquid-core͞solid-cladding waveguides in solid [glass (8, 12, 13, 15) or polymeric (9 -11, 14)] microchannels. Liquid-core waveguiding is commonly used to increase the length of the optical path (and thus the sensitivity) of chip-based spectrophotometers (15, 16). Solid-core͞liquid-cladding waveguides have been prepared by pumping fluid through air holes within the cladding layer of optical fibers (17-19) and by integration of bare fiber cores with liquids contained in microfluidic channels (20,21). Although all of these configurations are useful, they allow reconfiguration of the optical properties of the system (e.g., the contrast in refractive index between core an...

Self-assembled monolayers (SAMs) formed from alkanethiols on palladium resist corrosion by solution-phase chemical etchants, regardless of the chain length and wettability of the SAM. This insensitivity to chain length contrasts with SAMs on gold, for which a hydrophobic, well-ordered, crystalline alkane film is required to prevent access of water-soluble etchants to the underlying metal. 1,2 This paper demonstrates that dense SAMs form on contact of hexadecanethiol (C 16 SH) and palladium, and that these SAMs are notably more useful resists against wet-chemical etchants than are those on gold. 3 This system, however, differs from SAMs on gold in that the interfacial layer between the palladium and the organic film contributes the major part of the etch resistance of the system: a number of thiols differing substantially in hydrophobicity and chain length (C 16 SH, C 12 SH, HO(CH 2 ) 2 SH, C 6 H 5 SH, HO 2 -CCH 2 SH) provide comparable levels of contrast as etch resists.SAMs of alkanethiolates on the coinage metals (Au, Ag, Cu) have been used for a range of purposes, 4 including etch resists for microcontact printing (µCP). 5 The use of µCP of SAMs is generally not practical for patterning sub-500-nm features on these metals for two reasons: (i) it leaves defects and surface pits during etching (for gold) and (ii) spontaneous formation of an oxide layer in air complicates the system (for copper and silver). In addition, the coinage metals are incompatible with complementary metal-oxide semiconductor (CMOS) fabrication, and require barrier layers to prevent their contamination of the silicon with deep-level traps for minority carriers. 6 Palladium and its alloys are commonly used in microelectronic elements such as resistors and capacitors. 7 Palladium has several physical properties that suggest it would be a useful substrate for patterning SAMs of alkanethiolates: (i) it is very reactive toward sulfur-containing compounds, (ii) it has a slightly smaller lattice spacing in the (111) plane (2.75 Å) than gold and silver, 8 (iii) it resists oxidation in air below 400°C, and (iv) it forms smaller grains (15-25 nm) by e-beam evaporation than either gold (50-100 nm) or silver (30-50 nm). Here we report that alkanethiols form well-ordered SAMs on palladium, and resist etching with greater selectivity than SAMs on gold. 9 The details of the formation and characterization of the hexadecanethiolate SAM on palladium are described in the Supporting Information. Infrared data indicate that a hexadecanethiolate monolayer forms rapidly on palladium with crystalline molecular packing, and that the order of the SAM is comparable to that of C 16 S SAMs on gold and silver. A high advancing contact angle of water (θ a ∼ 120°) and high hysteresis (θ a -θ r ∼ 20°) suggest that this SAM may be more heterogeneous or disordered at the molecular scale than C 16 Sderived SAMs on either gold or silver. 10 Figure 1 shows structures that we can generate by the combination of µCP and etching on palladium and on gold. 11 Both films were pat...

This paper demonstrates the application of projection photolithography, using a standard commercial microscope, for the generation of masters for soft lithography. The procedure is rapid and convenient and produces features smaller in size (as small as 0.6 μm) than those available from other methods of rapid prototyping, albeit over a limited area (∼4 × 104 μm2 per exposure). A transparency photomask (prepared using high-resolution printing) is inserted into the light path of the microscope and projected through the microscope objective onto a photoresist-coated substrate. Features on the order of 1 μm can be produced routinely over the area of sharp focus (a circle of radius r ≅ 100 μm with a 100× objective) by this method without modification or precise calibration of the microscope. The microscope platform also provides two other useful functions, both characteristic of commercial steppers:  step-and-repeat exposures and pattern alignment. The developed photoresist is used as a master for the fabrication of stamps and replica molds for soft lithography. These elements are used in demonstrations of fabrication of microstructures with feature sizes in the range from 1 to 10 μm. Although the technique is limited in the area it can produce in a single exposure, it can fabricate many kinds of structures useful in chemistry, biology, and materials science.

This letter demonstrates the patterning of thin films of metallic palladium by microcontact printing ͑CP͒ of octadecanethiol, and the use of the patterned films in the fabrication of a functional sensor. This technique was also used to prepare templates of palladium for the electroless deposition of copper. The resistivity of the palladium and copper microstructures was 13.8 and 2.8 ⍀ cm, respectively; these values are approximately 40% larger than the values for the pure bulk metals. Palladium patterned into serpentine wires using CP functioned as a hydrogen sensor with sensitivity of 0.03 vol % H 2 in N 2 , and a response time of ϳ10 s ͑at room temperature͒.