Droplet Supersaturation Research Articles

Polytype formation in GaAs/GaP axial nanowire heterostructures

Transmission electron microscopy (TEM) studies are presented for Au-assisted vapor–liquid–solid (VLS) nanowire growth of gallium arsenide/gallium phosphide (GaAs/GaP) axial heterostructures. The supersaturation of the liquid Au-III–V alloy droplet during MBE growth was found to have a profound impact on both the crystal phase and growth rate of the nanowire heterostructures. Among these effects was the appearance of a previously unreported 4H GaP crystal phase. 4H GaP occurred at low Au-III–V droplet supersaturation following the transition from GaAs to GaP growth. Both crystal phase and growth rate were different for GaAs and GaP, underlining the importance of group V elements in Au-III–V droplet supersaturation and thus VLS nanowire growth.

Kinetic Effects in InP Nanowire Growth and Stacking Fault Formation: The Role of Interface Roughening

InP nanowire polytypic growth was thoroughly studied using electron microscopy techniques as a function of the In precursor flow. The dominant InP crystal structure is wurtzite, and growth parameters determine the density of stacking faults (SF) and zinc blende segments along the nanowires (NWs). Our results show that SF formation in InP NWs cannot be univocally attributed to the droplet supersaturation, if we assume this variable to be proportional to the ex situ In atomic concentration at the catalyst particle. An imbalance between this concentration and the axial growth rate was detected for growth conditions associated with larger SF densities along the NWs, suggesting a different route of precursor incorporation at the triple phase line in that case. The formation of SFs can be further enhanced by varying the In supply during growth and is suppressed for small diameter NWs grown under the same conditions. We attribute the observed behaviors to kinetically driven roughening of the semiconductor/metal interface. The consequent deformation of the triple phase line increases the probability of a phase change at the growth interface in an effort to reach local minima of system interface and surface energy.

Low pressure evaporative cooling of micron-sized droplets of solutions and its novel applications

For free molecular regime the mathematical model of low pressure evaporative cooling of binary droplets in gas flow is developed. The model includes five ordinary differential equations and takes into account effects such as the release of the latent heat of condensation of both components and the release of the latent heat of dissolution. Simulations were made for weak aqueous solutions of ammonia. It was discovered that compositions of gas flow and the aqueous solution affect the rate of evaporative cooling of droplets. The ratio of mass flow of solution and gas flow is also an important parameter. The cooling rate of such binary droplets can reach the value of about 2 × 10 5 K/s. As first applications we consider the air cooler based on evaporative cooling of droplets. For pressure of 20–80 Torr in aerosol reactor, it is shown that in the cooler with length of about 1 m temperature of air flow may drop to about 10–15 °C. The second application is the formation of nanoparticle in evaporating multicomponent droplet with two volatile components. Simulation was made for aqueous solution of ammonia which is widely used by experimentalists and engineers now. Effects of the number of precursors in droplet and supersaturation in droplet on the final size of nanoparticles were investigated.

Droplet mass transfer, intradroplet nucleation and submicron particle production in two-phase flow of solvent-supercritical antisolvent emulsion

Droplet formation, droplet interaction and coagulation together with droplet mass transfer are major sub-processes in the developing technology of nanoparticle production by means of solute nucleation inside the emulsion droplet. The solvent (ethanol) droplets containing the solute form during the solvent jet dispergation in the pressurized flow of solvent CO 2. In the formed two phases flow of solvent–antisolvent emulsion, the solvent diffuses from droplets into antisolvent, while antisolvent dissolves inside solvent droplets. The solvent replacement by the antisolvent causes droplet supersaturation by solute when it occurs near the critical point of solvent–antisolvent emulsion (∼80 bar and 31 °C) and the intradroplet nucleation of solute. To provide the same droplet lifetime and the uniform droplet supersaturation, the hydrodynamic relaxation time for droplets and for two phases flow have to be shorter than their relaxation time of the mass transfer. Above a critical volume fraction of solvent, droplets dissolve partially. Afterwards, i.e. downstream, antisolvent is saturated with solvent, i.e. phase equilibrium establishes within two-phase flow with uniform solute supersaturation inside droplets. Under these conditions, an additional mechanism of the supersaturation is identified, which is droplet specific (the supersaturation caused by increasing solute concentration), and is favorable for small particle production. As long as droplets move along the last quasi-equilibrium section of uniform two-phase flow with length L, the nucleation and a solute precipitation proceed within droplets, i.e. the dimension of formed particles is controlled by droplet residence time τ res which is proportional to L and inversely proportional to stream velocity. The maximal τ res and the precipitation time which affect nanoparticle dimension is restricted by the rate of turbulent droplet coagulation.