Multiscale self-assembly is ubiquitous in nature but its deliberate use to synthesize multifunctional three-dimensional materials remains rare, partly due to the notoriously difficult problem of controlling topology from atomic to macroscopic scales to obtain intended material properties. Here, we propose a simple, modular, noncolloidal methodology that is based on exploiting universality in stochastic growth dynamics and driving the growth process under far-from-equilibrium conditions toward a preplanned structure. As proof of principle, we demonstrate a confined-but-connected solid structure, comprising an anisotropic random network of silicon quantum-dots that hierarchically self-assembles from the atomic to the microscopic scales. First, quantum-dots form to subsequently interconnect without inflating their diameters to form a random network, and this network then grows in a preferential direction to form undulated and branching nanowire-like structures. This specific topology simultaneously achieves two scale-dependent features, which were previously thought to be mutually exclusive: good electrical conduction on the microscale and a bandgap tunable over a range of energies on the nanoscale.
Understanding
the dynamics of the laser crystallization (LC) process
of Ge thin films by nanosecond (ns) pulsed infrared (IR) lasers is
important for producing homogeneous, crack-free crystalline device-grade
films for use in thin-film transistors, photo-detectors, particle
detectors, and photovoltaic applications. Our motivation is to describe
a ns IR laser-based crystallization process of Ge by implementing
suitable parameters to fabricate thin-film devices. Our LC technique
was applied to crystallize thin amorphous Ge (a-Ge) films with thicknesses
suitable for device applications. The LC process was applied to a
300 nm-thick a-Ge thin film utilizing a 200 ns pulsed IR laser with
a wavelength of 1064 nm. Electron-beam-evaporation-deposited a-Ge
on glass substrates were subject to successive ns laser pulses with
a line focus. The crystallinity of the polycrystalline Ge (pc-Ge)
films was evaluated by Raman spectroscopy, optical microscopy, and
electron backscatter diffraction (EBSD). LC-Ge exhibited a Raman peak
of around 300 cm–1, confirming successful crystallization
of a-Ge. pc-Ge domain sizes exceeding several tens of micrometers
were observed in EBSD scans. LC of a-Ge minimizes the thermal energy
budget of processing and provides flexibility to locally crystallize
the film. Our work is the first demonstration of the LC of a-Ge thin
films, resulting in domain sizes exceeding tens of micrometers via
a ns pulsed IR laser.
Gold-induced (Au-) crystallization of amorphous germanium (α-Ge) thin films was investigated by depositing Ge on aluminum-doped zinc oxide and glass substrates through electron beam evaporation at room temperature. The influence of the postannealing temperatures on the structural properties of the Ge thin films was investigated by employing Raman spectra, X-ray diffraction, and scanning electron microscopy. The Raman and X-ray diffraction results indicated that the Au-induced crystallization of the Ge films yielded crystallization at temperature as low as 300°Cfor 1 hour. The amount of crystallization fraction and the film quality were improved with increasing the postannealing temperatures. The scanning electron microscopy images show that Au clusters are found on the front surface of the Ge films after the films were annealed at 500°C for 1 hour. This suggests that Au atoms move toward the surface of Ge film during annealing. The effects of annealing temperatures on the electrical conductivity of Ge films were investigated through current-voltage measurements. The room temperature conductivity was estimated as 0.54 and 0.73 Scm −1 for annealed samples grown on aluminum-doped zinc oxide and glass substrates, respectively. These findings could be very useful to realize inexpensive Ge-based electronic and photovoltaic applications.
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