Selecting and controlling nanoparticles in flight

Researchers at IIT Delhi have developed a gas phase synthesis setup with the ability of accurately selecting and controlling nanoparticle size and shell thickness. For realizing the size dependent properties of nanoparticles, synthesis of nanoparticle having controllable size and narrow size distribution is one of the important prerequisites.

The setup has been designed and fabricated by integrating thin film technology and particle size measurement technology for growing, size selecting and sintering while nanoparticles are in flight and monosized and crystalline nanoparticles with a controllable size are deposited onto the devices in the last process step. The nanoparticle can be dispersed onto substrates or incorporated into growing thin film or deposited onto organic and inorganic layers.

The gas phase synthesis setup has been used to synthesize size selected metal, alloy and core-shell nanoparticles, which have applications in sensing, plasmonics, solar cell, photoelectrochemical devices. In particular, the setup has been used to grow Pd, Cu, Ag, Pd-Cu, Pd-Ag nanoparticles. A novel modification in the synthesis set up has been carried out for growing core-shell nanoparticles of metal-graphene, Pd-C, SnO2-C, Si-Sn etc. It has been shown that the core size and the shell thickness can be independently controlled and thus the effect of size and shell thickness has been used to optimize the properties for specific applications.

Nanoparticle synthesis setup consists of a spark generator (SG), an ultraviolet (UV) charger, a differential mobility analyzer (DMA) and a sintering furnace (SF). The primary agglomerates of the material to be synthesized are produced in the spark generator which is composed of two electrodes (rods) of the suitable material. Due to the short duration of the pulsed discharge, evaporated material cools down rapidly, resulting in supersaturation and hence primary particle formation by nucleation and condensation. A higher number concentrationof primary particles results in Brownian coagulation andformation of aerosol containing agglomerates of different sizes in the carrier gas. The agglomerates of desired size are selected bypassing the aerosol through a differential mobility analyzer (DMA). For this process, aerosol first enters the UV charger wherethe majority of the agglomerates (>95%) acquire a unitcharge. The charged agglomerates subsequently enter the DMA, which selects the agglomerates on the basis oftheir electrical mobility, which depends on the size,charge level, and shape of the agglomerates. After the size-selection step,agglomerates pass through a sintering furnace, whichenables in-flight sintering for obtaining spherical andwell-crystallized monodisperse nanoparticles. Online particle-size distribution andnumber concentration measurements were carried outby a scanning mobility particle sizer (SMPS). The size-selected and subsequentlysintered nanoparticles are deposited on thedifferent substrates by using an electrostatic precipitator atroom temperature and atmospheric pressure with almost 100% efficiency.

The work is being carried out as a part of major research projects sponsored by Nanomission Programme of Department of Science and Technology, Department of Information Technology and European commission with Prof B.R. Mehta, Department of Physics as the principal investigator.