Michael A. Filler

Contactless Determination of Electrical Conductivity of One-Dimensional Nanomaterials by Solution-Based Electro-orientation Spectroscopy.

Nanowires of the same composition, and even fabricated within the same batch, often exhibit electrical conductivities that can vary by orders of magnitude. Unfortunately, existing electrical characterization methods are time-consuming, making the statistical survey of highly variable samples essentially impractical. Here, we demonstrate a contactless, solution-based method to efficiently measure the electrical conductivity of 1D nanomaterials based on their transient alignment behavior in ac electric fields of different frequencies. Comparison with direct transport measurements by probe-based scanning tunneling microscopy shows that electro-orientation spectroscopy can quantitatively measure nanowire conductivity over a 5-order-of-magnitude range, 10(-5)-1 Ω(-1) m(-1) (corresponding to resistivities in the range 10(2)-10(7) Ω·cm). With this method, we statistically characterize the conductivity of a variety of nanowires and find significant variability in silicon nanowires grown by metal-assisted chemical etching from the same wafer. We also find that the active carrier concentration of n-type silicon nanowires is greatly reduced by surface traps and that surface passivation increases the effective conductivity by an order of magnitude. This simple method makes electrical characterization of insulating and semiconducting 1D nanomaterials far more efficient and accessible to more researchers than current approaches. Electro-orientation spectroscopy also has the potential to be integrated with other solution-based methods for the high-throughput sorting and manipulation of 1D nanomaterials for postgrowth device assembly.

Near-field coupling between plasmonic resonators in Si nanowires

Nanoscale semiconductors are emerging as promising plasmonic materials for applications in the infrared. Herein, we study the near-field coupling between adjacent plasmonic resonators embedded in Si nanowires with in-situ infrared spectroscopy and discrete dipole approximation calculations. Si nanowires containing multiple phosphorus-doped segments, each with a user-programmable aspect ratio and carrier density, are synthesized via the vapor-liquid- solid technique and support localized surface plasmon resonances (LSPRs) between 5 and 10 μm. Discrete dipole approximation calculations confirm that the observed spectral response results from resonant absorption and free carrier concentrations are on the order of 1020 cm-3. Near-field coupling occurs between neighboring doped segments and the observed trends agree with plasmon hybridization theory. Our results highlight the utility of vapor-liquid-solid (VLS) synthesis for investigating the basic physics of surface plasmons in nanoscale semiconductors and suggest new opportunities for engineering light absorption in Si.

Enhanced photodesorption from near- and mid-infrared plasmonic nanocrystal thin films

The authors show that the desorption rate of two model molecules, indole and benzoic acid, from thin films of indium tin oxide nanocrystals supporting near- and mid-infrared (0.33–0.48u2009eV) localized surface plasmon resonances (LSPRs) is enhanced by as much as 60% upon illumination with broadband infrared light. The desorption rate increases linearly with light intensity. No increase in the desorption rate is detected for undoped In2O3 nanocrystal thin films or when photons resonant with the LSPR are blocked. The authors study the desorption rate enhancement as a function of illumination intensity, LSPR energy, and isotopic substitution. Importantly, the authors demonstrate the accelerated desorption via in-coupling of light to LSPRs with energies well within the mid-infrared. Their work opens the door to using these low energy photons as choreographers of chemical processes and sets the stage for future mechanistic studies.