Semiconductor nanowires with precisely controlled structure, and hence well-defined electronic and optical properties, can be grown by self-assembly using the vapour–liquid–solid process. The structure and chemical composition of the growing nanowire is typically determined by global parameters such as source gas pressure, gas composition and growth temperature. Here we describe a more local approach to the control of nanowire structure. We apply an electric field during growth to control nanowire diameter and growth direction. Growth experiments carried out while imaging within an in situ transmission electron microscope show that the electric field modifies growth by changing the shape, position and contact angle of the catalytic droplet. This droplet engineering can be used to modify nanowires into three dimensional structures, relevant to a range of applications, and also to measure the droplet surface tension, important for quantitative development of strategies to control nanowire growth.
Studied dynamic behavior of nanoscale liquids in graphene liquid cells using in situ TEM. Captured fluctuations of liquid-gas bubble interfaces. Imaged liquid nanodroplet formation resulting from dynamic motion of liquid-gas interfaces. Discovered that improving wettability of graphene liquid cells by ultraviolet-ozone treatment has drastic effects on the dynamic behavior of liquid during TEM imaging.
The formation of self-assembled contacts between vapor-liquid-solid grown silicon nanowires and flat silicon surfaces was imaged in situ using electron microscopy. By measuring the structural evolution of the contact formation process, we demonstrate how different contact geometries are created by adjusting the balance between silicon deposition and Au migration. We show that electromigration provides an efficient way of controlling the contact. The results point to novel device geometries achieved by direct nanowire growth on devices.
Avalanching
nanoparticles (ANPs) are a new class of lanthanide-based
upconverting material demonstrating steep optical nonlinearities with
the potential to advance applications ranging from subwavelength bioimaging
to neuromorphic computing, nanothermometry, and pressure transduction.
Here, we use single-nanocrystal imaging to uncover design-dependent
heterogeneity in ANP threshold intensity (I
th). Quantitative comparisons between distributions of I
th and ANP shell properties reveal correlations between
mean I
th values, histogram widths, and
nanocrystal shell thickness. Evaluating avalanching behaviors using
an established model of shell-dependent surface energy transfer shows
that variations in shell thickness–and the resultant energy
transfer through the shell to the surface and environment–are
likely the primary contributor to ANP-to-ANP I
th heterogeneity. Further, nanocrystals with an ∼6 nm
average shell thickness show I
th heterogeneity
beyond the extent expected from statistical measurements of shell
size and variability using transmission electron microscopy (TEM).
These results provide a principal guide for the design and application
of ANPs to environmental sensing.
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