Costas P. Grigoropoulos

Fast Mass Transport Through Sub-2-Nanometer Carbon Nanotubes

We report gas and water flow measurements through microfabricated membranes in which aligned carbon nanotubes with diameters of less than 2 nanometers serve as pores. The measured gas flow exceeds predictions of the Knudsen diffusion model by more than an order of magnitude. The measured water flow exceeds values calculated from continuum hydrodynamics models by more than three orders of magnitude and is comparable to flow rates extrapolated from molecular dynamics simulations. The gas and water permeabilities of these nanotube-based membranes are several orders of magnitude higher than those of commercial polycarbonate membranes, despite having pore sizes an order of magnitude smaller. These membranes enable fundamental studies of mass transport in confined environments, as well as more energy-efficient nanoscale filtration.

All-inkjet-printed flexible electronics fabrication on a polymer substrate by low-temperature high-resolution selective laser sintering of metal nanoparticles

All-printed electronics is the key technology to ultra-low-cost, large-area electronics. As a critical step in this direction, we demonstrate that laser sintering of inkjet-printed metal nanoparticles enables low-temperature metal deposition as well as high-resolution patterning to overcome the resolution limitation of the current inkjet direct writing processes. To demonstrate this process combined with the implementation of air-stable carboxylate-functionalized polythiophenes, high-resolution organic transistors were fabricated in ambient pressure and room temperature without utilizing any photolithographic steps or requiring a vacuum deposition process. Local thermal control of the laser sintering process could minimize the heat-affected zone and the thermal damage to the substrate and further enhance the resolution of the process. This local nanoparticle deposition and energy coupling enable an environmentally friendly and cost-effective process as well as a low-temperature manufacturing sequence to realize large-area, flexible electronics on polymer substrates.

Ion Exclusion by Sub 2-nm Carbon Nanotube Pores

Biological pores regulate the cellular traffic of a large variety of solutes, often with high selectivity and fast flow rates. These pores share several common structural features: the inner surface of the pore is frequently lined with hydrophobic residues, and the selectivity filter regions often contain charged functional groups. Hydrophobic, narrow-diameter carbon nanotubes can provide a simplified model of membrane channels by reproducing these critical features in a simpler and more robust platform. Previous studies demonstrated that carbon nanotube pores can support a water flux comparable to natural aquaporin channels. Here, we investigate ion transport through these pores using a sub-2-nm, aligned carbon nanotube membrane nanofluidic platform. To mimic the charged groups at the selectivity region, we introduce negatively charged groups at the opening of the carbon nanotubes by plasma treatment. Pressure-driven filtration experiments, coupled with capillary electrophoresis analysis of the permeate and feed, are used to quantify ion exclusion in these membranes as a function of solution ionic strength, pH, and ion valence. We show that carbon nanotube membranes exhibit significant ion exclusion that can be as high as 98% under certain conditions. Our results strongly support a Donnan-type rejection mechanism, dominated by electrostatic interactions between fixed membrane charges and mobile ions, whereas steric and hydrodynamic effects appear to be less important.

Nanofluidics in carbon nanotubes

Extremely high aspect ratios, molecularly smooth hydrophobic graphitic walls, and nanoscale inner diameters of carbon nanotubes give rise to the unique phenomenon of ultra-efficient transport of water and gas through these ultra-narrow molecular pipes. Water and gas molecules move through nanotube pores orders of magnitude faster than through other pores of comparable size. The proposed water transport mechanism has a distinct similarity to the transport mechanisms of biological ion channels. Molecular dynamics simulations and experimental measurements of water transport underscore the importance of nanotube structure in enabling ultra-efficient transport through the pore.

Conductor microstructures by laser curing of printed gold nanoparticle ink

The laser-based curing of printed nanoparticle ink to create microlines (resistors) of electrical resistivity approaching that of bulk gold was investigated. The present work relies on laser absorption in both the nanoparticle ink and the sintered gold layer, as well as the transport of thermal energy in the substrate and the resulting solvent vaporization and nanoparticle deposition and sintering. The morphology and electrical properties of the gold line can be controlled by modulating the spatial distribution of the laser beam intensity. Based on the understanding of the underlying physics, a process that circumvents a serious drawback on the functionality of cured gold microlines is produced. Microconductors with resistivity approaching that of bulk gold are produced, while loss of gold nanoparticles and cross sectional nonuniformities are avoided.

Excimer laser-induced temperature field in melting and resolidification of silicon thin films

The liquid/solid interface motion and temperature history during excimer laser annealing of 50-nm-thick Si films on fused quartz substrates are investigated by in situ nanosecond time-resolved electrical conductance, optical reflectance, and transmittance at visible and near-IR wavelengths, combined with thermal emission measurements. The temperature response, melt propagation and evolution of the recrystallization process are fundamentally different in the partial-melting and the complete-melting regimes. Because it is necessary to balance the latent heat across the propagating phase-change interface, the maximum induced temperature in the partial-melting regime remains close to the melting point of amorphous Si. The peak temperature rises in the complete-melting regime, but the nonparticipating nature of the liquid Si/fused quartz interface allows substantial supercooling (>200 K), followed by spontaneous nucleation into fine-grained material. These phase transformations are consistent with the recrysta...

Air stable high resolution organic transistors by selective laser sintering of ink-jet printed metal nanoparticles

A high resolution organic field effect transistor (OFET) fabrication process has been developed based on the selective laser sintering of ink-jet printed nanoparticle inks and the recent development of an air stable carboxylate-functionalized polythiophene semiconducting polymer. The entire fabrication and device characterization are performed at room temperature, ambient pressure, and air environment without using complex lithographic methods. This low temperature OFET fabrication process based on nanoparticle laser sintering has great potential for realizing inexpensive, large area flexible electronics on heat sensitive polymer substrates.

Microstructuring by printing and laser curing of nanoparticle solutions

In this letter, the process of printing and laser curing of nanoparticle solutions is presented. A liquid solvent is employed as the carrier of gold nanoparticles (the material of interest in this study) possessing a low melting temperature compared to that of bulk gold. Using a specifically designed printing system, the gold nanoparticle solution is deposited on a substrate and cured with laser radiation. In this manner, the potential of writing gold structures on temperature sensitive substrates is demonstrated. The interaction between the laser radiation and nanoparticles drives the solvent evaporation and controls the quality of the microstructures printing process. The latter is also affected by thermocapillary flow at the free surface, developing during the curing process. An optical method for estimating the curing times is also developed and discussed.

COMPUTATIONAL STUDY OF HEAT TRANSFER AND GAS DYNAMICS IN THE PULSED LASER EVAPORATION OF METALS

Pulsed laser irradiation of nanosecond duration is used in a variety of applications, including laser deposition of thin films and micromachining. Of fundamental interest is the prediction of the evaporative material removal rates, as well as the velocity, density, and temperature distributions of the ejected particles as functions of the laser‐beam pulse energy, temporal distribution, and irradiance density on the target material surface. In order to address these issues, the present work establishes a new computational approach for the thorough treatment of the heat transfer and fluid flow phenomena in pulsed laser processing of metals. The heat conduction in the solid substrate and the liquid melt is solved by a one‐dimensional transient heat transfer model. The ejected high‐pressure vapor generates shock waves against the ambient background pressure. The compressible gas dynamics is computed numerically by solving the system of Euler equations for mass, momentum, and energy, supplemented by an isentro...

Plasma and ablation dynamics in ultrafast laser processing of crystalline silicon

Mechanisms of ultrafast (femtosecond) laser-induced ablation on crystalline silicon are investigated by time-resolved pump-and-probe microscopy in normal imaging and shadowgraph arrangements. A one-dimensional model of the energy transport is utilized to predict the carrier temperature and lattice temperature as well as the electron and vapor flux emitted from the surface. The temporal delay between the pump and probe pulses is set by a precision translation stage up to about 500 ps and then extended to the nanosecond regime by an optical fiber assembly. The ejection of material is observed at several picoseconds to tens of nanoseconds after the main (pump) pulse by high-resolution, ultrafast shadowgraphs.