J. Guzman

Embedded Binary Eutectic Alloy Nanostructures: A New Class of Phase Change Materials

Phase change materials are essential to a number of technologies ranging from optical data storage to energy storage and transport applications. This widespread interest has given rise to a substantial effort to develop bulk phase change materials well suited for desired applications. Here, we suggest a novel and complementary approach, the use of binary eutectic alloy nanoparticles embedded within a matrix. Using GeSn nanoparticles embedded in silica as an example, we establish that the presence of a nanoparticle/matrix interface enables one to stabilize both nanobicrystal and homogeneous alloy morphologies. Further, the kinetics of switching between the two morphologies can be tuned simply by altering the composition.

Reversible phase changes in Ge–Au nanoparticles

We demonstrate a reversible phase transition in nanoparticles composed of a binary eutectic alloy, Ge–Au. The structure, 9 nm diameter nanoparticles embedded in silica, can be switched from bilobe to mixed using a 30 ns ultraviolet laser pulse. The structure can be switched back to bilobe by heating at 80 °C. The bilobe/mixed switching can be performed on the same sample at least ten times. Synchrotron X-ray diffraction studies reveal that the bilobe structure contains crystalline Ge and Au while the mixed structure consists of crystalline Ge and β Ge–Au.

Processing route for size distribution narrowing of ion beam synthesized nanoclusters

Ion beam synthesis of nanocrystals is explored using a recently developed kinetic Monte Carlo model for the process. The model suggests that temperature can be used to engineer nanocrystal size distributions. Specifically, by initiating implants at low temperature and then ramping the temperature upward, one can both tune the average size of the nanocrystals and restrict size distribution widths to less than 20% of the average size.

Photoluminescence enhancement of Er-doped silica containing Ge nanoclusters

The photoluminescence (PL) of Er-doped silica films containing Ge nanoclusters synthesized by ion implantation was investigated. The area of the 1540 nm Er3+ PL peak was enhanced by up to a factor of 200 by the addition of Ge nanoclusters. The PL enhancement was found to be proportional to the concentration of Ge atoms. Control experiments with argon ion implantation were used to show that the enhancement is due to the presence of Ge and not radiation damage. Furthermore, the Er3+ PL was found to be strongly influenced by the postgrowth annealing and the crystallinity of the Ge nanoclusters.

Modeling pulsed-laser melting of embedded semiconductor nanoparticles

Pulsed-laser melting (PLM) is commonly used to achieve a fast quench rate in both thin films and nanoparticles. A model for the size evolution during PLM of nanoparticles confined in a transparent matrix, such as those created by ion-beam synthesis, is presented. A self-consistent mean-field rate equations approach that has been used successfully to model ion beam synthesis of germanium nanoparticles in silica is extended to include the PLM process. The PLM model includes classical optical absorption, multiscale heat transport by both analytical and finite difference methods, and melting kinetics for confined nanoparticles. The treatment of nucleation and coarsening behavior developed for the ion beam synthesis model is modified to allow for a non-uniform temperature gradient and for interacting liquid and solid particles with different properties. The model allows prediction of the particle size distribution after PLM under various laser fluences, starting from any particle size distribution including as-implanted or annealed simulated samples. A route for narrowing the size distribution of embedded nanoparticles is suggested, with simulated distribution widths as low as 15% of the average size.

Self-consistent mean-field theory of size distribution narrowing during ramped temperature ion beam synthesis

A simple mathematical argument explains a recently identified route for the ion beam synthesis of nanoclusters with a narrowed size distribution. The key idea is that growth conditions for which the average nanocluster size is increasing rapidly can lead to narrowed size distributions. Modeling candidate processes using a self-consistent, mean-field theory shows that normalized nanocluster size distributions with full-width at half-maximum of 17% of the average can be attained.

Interfacial free energies determined from binary embedded alloy nanocluster geometry

The equilibrium geometries of embedded binary eutectic alloy nanostructures are used to determine the interfacial free energies between two phases of a strongly segregating alloy and the matrix. The solid Ge-SiO2 interfacial free energy at 600°C is determined to be 0.82–0.99 J/m2, in good agreement with estimates obtained from stress relaxation experiments.