Nonequilibrium Synthesis of TiO2 Nanoparticle “Building Blocks” for Crystal Growth by Sequential Attachment in Pulsed Laser Deposition

Masoud Mahjouri-Samani, Mengkun Tian, Alexander A. Puretzky, Miaofang Chi, Kai Wang, Gerd Duscher, Christopher M. Rouleau, Gyula Eres, Mina Yoon, John Lasseter, Kai Xiao, David B. Geohegan

Non-equilibrium growth pathways for crystalline nanostructures with metastable phases are demonstrated through the gas-phase formation, attachment, and crystallization of ultrasmall amorphous nanoparticles as building blocks in pulsed laser deposition (PLD). Temporally- and spatially-resolved gated-ICCD imaging and ion probe measurements are employed as in situ diagnostics to understand and control the plume expansion conditions for the synthesis of nearly pure fluxes of ultrasmall (~3 nm) amorphous TiO2 nanoparticles in background gases and their selective delivery to substrates. These amorphous nanoparticles assemble into loose, mesoporous assemblies on substrates at room temperature, but dynamically crystallize by sequential particle attachment at higher substrate temperatures to grow nanostructures with different phases and morphologies. Molecular dynamics calculations are used to simulate and understand the crystallization dynamics. This work demonstrates that non-equilibrium crystallization by particle attachment of metastable ultrasmall nanoscale "building blocks" provides a versatile approach for exploring and controlling the growth of nanoarchitectures with desirable crystalline phases and morphologies.