We demonstrate an entirely new method of nanoparticle chemical synthesis based on liquid droplet irradiation with ultralow (<0.1 eV) energy electrons. While nanoparticle formation via high energy radiolysis or transmission electron microscopy-based electron bombardment is well-understood, we have developed a source of electrons with energies close to thermal which leads to a number of important and unique benefits. The charged species, including the growing nanoparticles, are held in an ultrathin surface reaction zone which enables extremely rapid precursor reduction. In a proof-of-principle demonstration, we obtain small-diameter Au nanoparticles (∼4 nm) with tight control of polydispersity, in under 150 μs. The precursor was almost completely reduced in this period, and the resultant nanoparticles were water-soluble and free of surfactant or additional ligand chemistry. Nanoparticle synthesis rates within the droplets were many orders of magnitude greater than equivalent rates reported for radiolysis, electron beam irradiation, or colloidal chemical synthesis where reaction times vary from seconds to hours. In our device, a stream of precursor loaded microdroplets, ∼15 μm in diameter, were transported rapidly through a cold atmospheric pressure plasma with a high charge concentration. A high electron flux, electron and nanoparticle confinement at the surface of the droplet, and the picoliter reactor volume are thought to be responsible for the remarkable enhancement in nanoparticle synthesis rates. While this approach exhibits considerable potential for scale-up of synthesis rates, it also offers the more immediate prospect of continuous on-demand delivery of high-quality nanomaterials directly to their point of use by avoiding the necessity of collection, recovery, and purification. A range of new applications can be envisaged, from theranostics and biomedical imaging in tissue to inline catalyst production for pollution remediation in automobiles.

We have established, through time correlated plasma emission and electrode and plasma potential measurements, that the near electrode emission observed in asymmetric capacitively coupled 13.56 MHz-driven hydrogen plasmas is caused by field reversal that leads to sheath collapse. Near-electrode emission has now been observed in Ar and He. The field reversal appears to be due to collision-induced electron drag.

We give preliminary results on the breakdown and low current limit of volt-ampere characteristics of simple parallel plate non-equilibrium dc discharges at standard (centimetre size) and micro-discharge conditions. Experiments with micro-discharges are reported attempting to establish the maintenance of E/N, pd and j/p 2 scalings at small dimensions down to 20 µm. It was found that it may not be possible to obtain properly the left-hand side of the Paschen curve. The possible causes are numerous but we believe that it is possible that long path prevention techniques do not work at high pressures. Nevertheless, the standard scaling laws seem to be maintained down to these dimensions which are consistent with simulations that predict violation of scaling below 10 µm. Volt-ampere characteristics are also presented and compared with those of the standard size discharges.

We report, for the first time, quadrupole mass spectrometry of neutral and positive ionic hydrocarbon species measured at the rf biased substrate electrode of an inductively coupled plasma for acetylene rich C 2 H 2 :Ar mixtures under various bias, frequency and pressure conditions. It has been observed that, irrespective of initial gas mixture, the resultant plasma is dominated by argon neutrals and ions. This is attributed to highly efficient conversion of acetylene to C 2 H due to the enhanced electron density compared to a standard capacitive plasma where the acetylene (neutral and ion) species remain dominant. This conversion may be crucial to film formation via inert rather than hydrocarbon ion bombardment. In addition, the transient formation of CH 4 from acetylene has been discovered using IR absorption spectroscopy with time constants similar to observed pressure variations. Rate coefficients and rates for many of the reaction mechanisms, calculated using measured EEDFs and species densities, are given. These results have important application in plasma models and growth studies for hydrogenated amorphous or diamondlike carbon film deposition. Film growth under similar plasma conditions is reported in an associated paper along with ion energy distributions for important growth species.

We report the controlled injection of near-isolated micron-sized liquid droplets into a low temperature He-Ne steady-state rf plasma at atmospheric pressure. The H 2 O droplet stream is constrained within a 2 mm diameter quartz tube. Imaging at the tube exit indicates a log-normal droplet size distribution with an initial count mean diameter of 15 m falling to 13 m with plasma exposure. The radial velocity profile is approximately parabolic indicating near laminar flow conditions with the majority of droplets travelling at > 75% of the local gas speed and having a plasma transit time of < 100 s. The maximum gas temperature, determined from nitrogen spectral lines, was below 400 K and the observed droplet size reduction implies additional factors beyond standard evaporation, including charge and surface chemistry effects. The successful demonstration of controlled microdroplet streams opens up possibilities for gas-phase microreactors and remote delivery of active species for plasma medicine.