thick C 60 acceptor layer, and an EBL consisting of either BCP (see Fig. 4, structure) or Ru(acac) 3 (see Fig. 5, structure). Finally, a 1000 Å thick Ag cathode was evaporated through a shadow mask with 1 mm diameter openings.The J-V characteristics were measured in the dark and under simulated AM 1.5G solar illumination (Oriel Instruments) using an 4155B semiconductor parameter analyzer (Hewlett-Packard). Illumination intensity was measured using a calibrated broadband optical power meter. Photocurrent spectra were recorded using a monochromatic beam of variable-wavelength light from an Oriel Instruments quartz tungsten-halogen lamp and chopped at 400 Hz. The monochromatic light was calibrated using a Si photodetector, and photocurrent was measured using a lock-in amplifier referenced to the chopper frequency. Absorption spectra were measured on quartz substrates using a Perkin-Elmer Lambda 800 UV/vis spectrometer referenced to clean quartz substrates to cancel out absorption losses in the quartz. The Ru(acac) 3 hole (electron) conductivity was measured for ohmic devices consisting of a 2000 Å thick Ru(acac) 3 layer in between Au (Ag) contacts. Organic materials studied by UPS were grown by ultrahigh-vacuum organic molecular beam deposition [15] on highly doped n-Si(100) substrates coated with 500 Å thick in-situ-deposited Ag layers. HeI emission (21.22 eV) from a VG UPS/2 lamp (Thermo VG Scientific) was used as a photon source, and the spectra were collected with a multichannel hemispherical VG CLAM4 electron-energy analyzer. The UPS measurement resolution [13] [1][2][3] in order to produce colloidal patterns that fulfill the requirements of order and generic design. One method makes use of topographical templates, made of silicon [4] or a polymer, [5] to confine non-functionalized particles within the pattern. Pattern confinement has been used, for instance, to obtain special nanoparticle arrangements, [6][7][8] to control lattice and superlattice symmetry, [9][10][11] and to pattern multilayers of particles on both large [12] and small scales.[5] Another method employs self-assembled monolayer (SAM) templates to fabricate chemically patterned substrates. SAMs introduce differences in wettability [13,14] or electrostatic charge [15,16] to direct the particles to the intended areas. For this purpose, microcontact printing (lCP) [16] and scanning probe lithography (SPL) [17] have been used to chemically modify substrates on both large and small scales. Hammond and co-workers [18,19] have shown an alternative to these methods that creates a combination of chemical and topographical patterns by printing polyelectrolyte multilayers which then direct the positioning of particles. Patterns obtained using any of these methods can potentially be used for photonic-bandgap devices, [20][21][22] ionic and biological sensors on surfaces, [23] molecular recognition, [24,25] single-electron transistors, [26] and high-density data-storage systems.[27]Nanoparticles have been attached to substrates using immersion in a s...
The implementation of high‐resolution polymer templates fabricated by capillary force lithography (CFL) is explored both in nanoimprint lithography (NIL) and in the wet‐etching of metals. Several different thermoplastic and UV‐curable polymers and types of substrates are incorporated into the general CFL procedure to meet the diverging requirements of these two applications. The mechanical stability of UV‐curable templates for imprinting in polymers, as examined by atomic force microscopy (AFM), and their anti‐adhesive properties are excellent for application in NIL. The conditions for curing the UV‐curable polymer are optimized in order to obtain high‐stability polymer templates. Gold patterns on silicon with a lateral resolution of 150 nm are fabricated by subsequent lift‐off in acetone. Similar patterns with a lateral resolution of 100 nm are fabricated using templates of thermoplastic polymers on gold layers on silicon as an etch mask. The transfer of stamp residues during CFL with these polymer templates is proven by X‐ray photoelectron spectroscopy (XPS) and AFM friction analysis. For poly(methylmethacrylate) (PMMA), the presence of large amounts of silicon‐containing residues is found to compromise the processability of the resulting template in subsequent O2 reactive‐ion etching (RIE) treatment. The extent of silicon contamination is up to six times less for polystyrene (PS). At this level, the etch performance of the PS etch mask is not affected, as was the case for PMMA. Accurate downscaling of the lateral dimensions of the resulting metal patterns by several factors with respect to the dimensions of the PS etch mask is achieved by over‐etching of the gold. Overall, the results in this paper demonstrate the potential of CFL templates as tools for high‐resolution soft lithography.
Nanoimprint lithography (NIL) is used as a tool to pattern self‐assembled monolayers (SAMs) on silicon substrates because of its ability to pattern in the micrometer and nanometer ranges. The polymer template behaves as a physical barrier preventing the formation of a SAM in the covered areas of the substrate. After polymer removal, SAM patterns are obtained. The versatility of the method is shown in various nanofabrication schemes. Substrates are functionalized with a second type of silane adsorbate. Pattern enhancement via selective electrostatic attachment of carboxylate‐functionalized particles is achieved. Further applications of the NIL‐patterned substrates include template‐directed adsorption of particles, as well as the fabrication of electrodes on top of a SAM.
Various patterning strategies have been developed to create hybrid nanostructures of dendrimers and gold nanoparticles on cyclodextrin self-assembled monolayers (CD SAMs) based on multiple supramolecular interactions using a layer-by-layer (LBL) approach. A lack of specificity of the adsorption of the dendrimer prevented the use of LBL assembly on chemically patterned SAMs, which were prepared by microcontact printing (μCP) or nanoimprint lithography (NIL). Nanotransfer printing (nTP) and nanoimprint lithography solved that problem and resulted in patterned LBL assemblies on the CD SAMs. nTP was achieved by LBL assembly on a PDMS stamp followed by transfer onto a full CD SAM. NIL-prepared PMMA patterns provided patterned CD SAMs and functioned as a physical mask for LBL assembly. For these methods, differences in thickness of the LBL assemblies were observed when compared to LBL assembly on full CD SAMs. These differences were shown not to originate from rinsing or lift-off procedures, but probably from differences in wetting.
An accurate and versatile process for the fabrication of high-resolution 3D nanostructures combining top-down and bottom-up nanofabrication schemes is described here. The method is based on layer-by-layer (LBL) assembly of functionalized nanoparticles (NPs) bound together by means of supramolecular interactions between a layer of adamantyl-functionalized dendrimers, the guest, and cyclodextrin (CD)-functionalized nanoparticles, the host. First, a self-assembled CD monolayer (CD SAM) was patterned using nanoimprint lithography (NIL) and later used to anchor supramolecular LBL assemblies onto it. The versatility of the process was demonstrated by using NPs of different size and nature. Two types of LBL assemblies were fabricated based on (i) 2.8 nm CD-functionalized Au NPs, which allow an accurate height control and (ii) 60 nm CD-functionalized SiO2 particles, which permit the fabrication of nanostructures. In one of the cases vertical deposition was used to obtain high particle ordering. Both types of NP were used to produce nanostructured LBL assemblies with lateral sizes below 100 nm. Physical confinement was observed when using 60 nm CD-functionalized SiO2 particles in the sub-300 nm scale on the first and second bilayers. Finally, periodic patterns of single nanoparticles were achieved.
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