Selective nanoparticle heating: Another form of nonequilibrium in dusty plasmas

Nanoparticles produced in low-temperature plasmas (LTPs) are often found with crystalline structure, which suggests rather high temperatures during synthesis. This even applies to particles of high-melting-point materials, which is surprising, because the gas temperature in LTPs is often close to room temperature and particles may reside in the plasma only for a few milliseconds. In this paper, we present a numerical study of nanoparticle heating in plasmas through energetic surface reactions. We find that, under realistic plasma conditions, particle temperatures can exceed the gas temperature by many hundreds of kelvins. However, as the particle temperature is highly unsteady, it is more reasonable to consider particle temperature distribution functions. The dependence of the particle temperature distribution on particle size and particle density in a dusty LTP is discussed.

Figure 1

Atomic hydrogen density and standard deviation of hydrogen densities obtained from different line ratios as a function of electron temperature.

Figure 2

Excess temperature with respect to the background gas for 2 (top), 3 (middle), and $10\mathrm{nm}$ (bottom) silicon nanoparticles.

Figure 3

Particle temperature distribution function as a function of particle size. The average temperature does not depend on the size, and it is equal to $+100\mathrm{K}$.

Figure 4

Temporal evolution of the hydrogen surface coverage and particle temperature for a small particle of $1.5\mathrm{nm}$ in diameter.

Figure 5

(a) Start phase of the simulation. Electron temperature is automatically adjusted so that ion density is constant and equal to the specified value. Charging of the particles induces a decrease in electron density. (b) Initial evolution of the charge of a test particle. At $t=0$ the particle charge is set to 1.

Figure 6

(a) Particle charge distribution as a function of particle density in the discharge. (b) Temperature distribution of a $5\mathrm{nm}$ particle as a function of particle density.