Chad R. Barry

Printing nanoparticle building blocks from the gas phase using nanoxerography

This letter reports on the electrostatic driven self-assembly of nanoparticles onto charged surface areas (receptors) 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 cm2 in size, were patterned with charge in 10 s with 100 nm scale resolution. Charge based receptors, 100 nm×100 nm in size, contained ∼100 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 ∼3 orders of magnitude greater than the resolution of existing xerographic printers.

Nanocontact Electrification through Forced Delamination of Dielectric Interfaces

This article reports patterned transfer of charge between conformal material interfaces through a concept referred to as nanocontact electrification. Nanocontacts of different size and shape are formed between surface-functionalized polydimethylsiloxane (PDMS) stamps and other dielectric materials (PMMA, SiO(2)). Forced delamination and cleavage of the interface yields a well-defined charge pattern with a minimal feature size of 100 nm. The process produces charged surfaces and associated fields that exceed the breakdown strength of air, leading to strong long-range adhesive forces and force-distance curves, which are recorded over macroscopic distances. The process is applied to fabricate charge-patterned surfaces for nanoxerography demonstrating 200 nm resolution nanoparticle prints and applied to thin film electronics where the patterned charges are used to shift the threshold voltages of underlying transistors.

Continuous nanoparticle generation and assembly by atmospheric pressure arc discharge

This letter describes a nanoparticle generation and deposition system which combines aspects of high temperature plasmas with room temperature aerosols. The process works at atmospheric pressure and produces nanoparticles of Au or ZnO through cathode erosion inside a dc arc discharge plasma. The particles are positively charged by the arc and form a room temperature aerosol. From the aerosol, nanoparticles assemble on conductive sample surfaces through openings in patterned resist with resolution enhanced by electrodynamic nanolenses. We report that continued operation of the system results in funneled deposition of nanoparticles into well positioned three dimensional nanostructures.

Nanocontact electrification: patterned surface charges affecting adhesion, transfer, and printing.

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.

Mimicking Electrodeposition in the Gas Phase: A Programmable Concept for Selected‐Area Fabrication of Multimaterial Nanostructures

An in situ gas-phase process that produces charged streams of Au, Si, TiO(2), ZnO, and Ge nanoparticles/clusters is reported together with a programmable concept for selected-area assembly/printing of more than one material type. The gas-phase process mimics solution electrodeposition whereby ions in the liquid phase are replaced with charged clusters in the gas phase. The pressure range in which the analogy applies is discussed and it is demonstrated that particles can be plated into pores vertically (minimum resolution 60 nm) or laterally to form low-resistivity (48 microOmega cm) interconnects. The process is applied to the formation of multimaterial nanoparticle films and sensors. The system works at atmospheric pressure and deposits material at room temperature onto electrically biased substrate regions. The combination of pumpless operation and parallel nozzle-free deposition provides a scalable tool for printable flexible electronics and the capability to mix and match materials.

Nanochemistry of Ceria Abrasive Particles

Surfaces of ceria (CeO2) particles have been studied by electron energy-loss spectroscopy in a field-emission gun scanning transmission electron microscope. All the ceria particles analyzed contained Ce3+ at the surface. Rare-earth impurities such as La were enriched at the surface and were observed for particles ranging from tens to hundreds of nanometers in size. The oxidation state of the cerium ion is measured from the Ce M5/M4white-line intensity ratio.

Gas Phase Nanoparticle Integration

We report on two gas phase nanoparticle integration processes to assemble nanomaterials onto desired areas on a substrate. We expect these processes to work with any material that can be charged. The processes offer self-aligned integration and could be applied to any nanomaterial device requiring site specific assembly. The Coulomb force process directs the assembly of nanoparticles onto charged surface areas with sub-100 nm resolution. The charging is accomplished using flexible nanostructured electrodes. Gas phase assembly systems are used to direct and monitor the assembly of nanoparticles onto the charge patterns with a lateral resolution of 50 nm. The second concept makes use of fringing fields. The fringing fields directed the assembly of nanoparticles into openings. The fringing fields can be confined to sub 50 nm sized areas and exceed 1 MV/m, acting as nanolenses. Gas phase assembly systems have been used to deposit silicon, germanium, metallic, and organic nanoparticles.

Dewetting on the Surface of Rutile

After annealing a continuous SiO2 film on the (001) surface of TiO2, the film dewets and then spreads to form a complex pattern. The final droplet morphology displays a densely branching morphology similar to those seen in computer-simulated models. It is proposed that Benard-Marangoni convection cells form within the film before dewetting occurs. The formation of Benard-Marangoni convection cells prior to dewetting results in the uniform size and spacing of the droplets on the surface. These convection cells form at temperature when the TiO2 substrate dissolves into the SiO2 thin film. The change in composition results in regions of differing surface tensions and therefore leads to the formation of the convection cells.