This article reports on a new charging process and Coulomb-force-directed assembly of nanoparticles onto charged surface areas with sub-100-nm resolution. The charging is accomplished using a flexible nanostructured thin silicon electrode. Electrical nanocontacts have been created as small as 50 nm by placing the nanostructured electrode onto an electret surface. The nanocontacts have been used to inject charge into 50 nm sized areas. Nanoparticles were assembled onto the charge patterns, and a lateral resolution of 60 nm has been observed for the first time. A comparison of the nanoparticle patterns with the surface potential distribution recorded by Kelvin probe force microscopy (KFM) revealed a mismatch in the lateral resolution. One possible explanation is that nanoparticles may visualize charge patterns at a sub-60-nm length scale that is not well resolved using KFM.Inorganic, metallic, and semiconducting nanomaterials in the form of nanoparticles, nanowires, nanobelts, or nanodisks are considered key building blocks in the design of novel high-performance nanotechnological devices. The process to fabricate such devices, however, will require new additive concepts to integrate, orient, and assemble such building blocks at desired locations on a substrate. Current approaches that address the integration of nanomaterials at desired locations on a substrate include serial scanning probe based concepts to print 1,2 or manipulate 3 nanomaterials at the sub-100-nm length scale, semiparallel inkjet-based concepts 4,5 to print materials from suspensions with 10 µm scale resolution, parallel nanotransfer methods 6,7 to transfer nanomaterials from one substrate to another retaining a copy of the order, and a vast variety of programmable or "receptor based" assembly concepts 8-18 that use unordered nanomaterials as an input. Scanning probe and inkjet-based methods enable rapid reconfiguration of the patterns and the formation of heterogeneous assemblies at an early stage but remain too slow to print materials over large areas at the sub-10-µm length scale. Nanotransfer concepts are most suitable to transfer nanomaterials from one substrate to another over large areas. Nanotransfer maintains the arrangement of the nanomaterials on a donor substrate; i.e., it does not order or rearrange the materials as part of the process."Receptor"-based concepts focus on the directed assembly of randomly oriented nanomaterials. The materials are suspended in solution or gas phase and are assembled at desired locations (receptors) on a substrate using specific interactions. Most actively investigated areas, currently, use protein recognition, 19,20 DNA hybridization,9,21,22 hydrophobicity/hydrophilicity, surface tension and self-assembled monolayers, 10 topography-directed concepts, 23-25 magnetic 11 and dielectrophoretic assembly and transport, 22,[26][27][28] and electrostatic forces. [12][13][14][15][16][17][18]25,27 In recent years there has been an increased focus on the use of long range electrostatic forces to direct th...

This letter reports on the electrostatic driven self-assembly of nanoparticles onto charged surface areas ͑receptors͒ from the gas phase for nanoparticle based device fabrication. The charged areas were generated by a parallel technique that uses a flexible, conductive electrode to pattern electrons and holes in a thin film electret. Samples, 1 cm 2 in size, were patterned with charge in 10 s with 100 nm scale resolution. Charge based receptors, 100 nmϫ100 nm in size, contained ϳ100 elementary charges. A transparent particle assembly module was designed to direct and monitor the assembly of metallic nanoparticles at a resolution of 100 nm, which is ϳ3 orders of magnitude greater than the resolution of existing xerographic printers.

This paper reports on the directed self-assembly of nanoparticles onto charged surface areas with a resolution of 200 nm from the liquid phase and 100 nm from the gas phase. The charged areas required for this type of nanoxerographic printing were fabricated using a parallel method that employs a flexible, electrically conductive, electrode to charge a thin-film electret. As electrodes, we used metal-coated polymeric stamps and 10 µm thick doped silicon wafers carrying a pattern in topography. Each electrode was brought in contact with a thin-film electret on an n-doped silicon substrate. The charge pattern was transferred into the thin-film electret by applying a voltage pulse between the conductive electrode and the silicon substrate. Areas as large as 1 cm 2 were patterned with charge with 100 nm scale resolution in 10 s. These charge patterns attract nanoparticles. A liquid-phase assembly process where electrostatic forces compete with disordering forces due to ultrasonication has been developed to assemble nanoparticles onto charged based receptors in 10 s from a liquid suspension. A gas-phase assembly process was developed that uses a transparent particle assembly module to direct particles towards the charged surface while monitoring the total charge of assembled particles. Nanoparticles were generated using a tube furnace by evaporation and condensation at the outlet. The electrostatically directed assembly of 10-100 nm sized metal (gold, silver) and 30 nm sized carbon particles was accomplished with a resolution 500-1000 times greater than the resolution of existing xerographic printers.

This letter reports on a new gas-phase printing approach to deposit nanomaterials into addressable areas on a surface with 50 nm lateral accuracy. Localized fringing fields that form around conventional resist patterns (PMMA and SiO2) with openings to a silicon substrate are used to direct the assembly of nanomaterials into the openings. Directed assembly was observed due to a naturally occurring inbuilt charge differential at the material interface that was further enhanced by corona charging to yield a field strength exceeding 1 MV/m in Kelvin probe force microscopy (KFM) measurements. The assembly process is independent of the nanomaterial source and type: an evaporative, plasma, and electrospray source have been tested to deposit silicon and metallic nanoparticles. The results suggest a potential route to form nanolenses on the basis of charged resist structures; a 3-fold size reduction has been observed between the structures and the assembled particles. Applications range from the integration of functional nanomaterial building blocks to the elimination of lift-off steps in semiconductor processing.

Contact electrification creates an invisible mark, overlooked and often undetected by conventional surface spectroscopic measurements. It impacts our daily lives macroscopically during electrostatic discharge and is equally relevant on the nanoscale in areas such as soft lithography, transfer, and printing. This report describes a new conceptual approach to studying and utilizing contact electrification beyond prior surface force apparatus and point-contact implementations. Instead of a single point contact, our process studies nanocontact electrification that occurs between multiple nanocontacts of different sizes and shapes that can be formed using flexible materials, in particular, surface-functionalized poly(dimethylsiloxane) (PDMS) stamps and other common dielectrics (PMMA, SU-8, PS, PAA, and SiO(2)). Upon the formation of conformal contacts and forced delamination, contacted regions become charged, which is directly observed using Kelvin probe force microscopy revealing images of charge with sub-100-nm lateral resolution. The experiments reveal chemically driven interfacial proton exchange as the dominant charging mechanism for the materials that have been investigated so far. The recorded levels of uncompensated charges approach the theoretical limit that is set by the dielectric breakdown strength of the air gap that forms as the surfaces are delaminated. The macroscopic presence of the charges is recorded using force-distance curve measurements involving a balance and a micromanipulator to control the distance between the delaminated objects. Coulomb attraction between the delaminated surfaces reaches 150 N/m(2). At such a magnitude, the force finds many applications. We demonstrate the utility of printed charges in the fields of (i) nanoxerography and (ii) nanotransfer printing whereby the smallest objects are ∼10 nm in diameter and the largest objects are in the millimeter to centimeter range. The printed charges are also shown to affect the electronic properties of contacted surfaces. For example, in the case of a silicon-on-insulator field effect transistors are in contact with PDMS and subsequent delamination leads to threshold voltage shifts that exceed 500 mV.