Large-Scale Hot Spot Engineering for Quantitative SERS at the Single-Molecule Scale

Quantitative surface enhanced Raman spectroscopy (SERS) requires precise control of Raman enhancement factor and detection uniformity across the SERS substrate. Here, we show that alkanethiolate ligand-regulated silver (Ag) nanoparticle films can be used to achieve quantitative SERS measurements down to the single-molecule level. The two-dimensional hexagonal close-packed superlattices of Ag nanoparticles formed in these films allow for SERS detection over a large area with excellent uniformity and high Raman enhancement factor. In particular, the SERS signal from the thiolate ligands on Ag nanoparticle surfaces can be utilized as a stable internal calibration standard for reproducible quantitative measurements. We demonstrate the capability of quantitative SERS by measuring the areal densities of crystal violet molecules embedded in an ultrathin spin-on-glass detection “hot zone”, which is a planar and uniformly enhanced region several nanometers above the Ag nanoparticles. The Raman measurement results exhibit a linear response over a wide dynamic range of analyte concentration

Dynamic Visualization of Axial p–n Junctions in Single Gallium Nitride Nanorods under Electrical Bias

We demonstrate a direct visualization method based on secondary electron (SE) imaging in scanning electron microscopy for mapping electrostatic potentials across axial semiconductor nanorod p–n junctions. It is found that the SE doping contrast can be directly related to the spatial distribution of electrostatic potential across the axial nanorod p–n junction. In contrast to the conventional SE doping contrast achieved for planar p–n junctions, the quasi-one-dimensional geometry of nanorods allows for high-resolution, versatile SE imaging under high accelerating voltage, long working distance conditions. Furthermore, we are able to delineate the electric field profiles across the axial nanorod p–n junction as well as depletion widths at different reverse biases. By using standard p<i>–</i>n junction theory and secondary ion mass spectroscopy, the carrier concentrations of p- and n-regions can be further extracted from the depletion widths under reverse biasing conditions. This direct imaging method enables determination of electrostatic potential variation of p–n junctions in semiconductor nanorod and nanowire devices with a spatial resolution better than 10 nm

Dual-Band Planar Plasmonic Unidirectional Launching in a Semiannular Apertures Array

Multiple-band, frequency-adjustable unidirectional launching of planar surface plasmons is of great concern in plasmonic devices and circuits. We have designed and demonstrated a novel dual-band planar unidirectional surface plasmon polaritons (SPPs) launcher with narrow bandwidth (∼5 nm) and large band gap (∼50 nm) using a semiannular apertures array milled in a gold film. Symmetry breaking of the semiannular aperture brings significant advantages for the unidirectional launching, based on the excited asymmetrically distributed cylindrical surface plasmon resonance modes. During the unidirectional launching, the individual semiannular apertures function as unidirectional quasi-point SPP sources, and the grating coherently stacking amplitude of unidirectional SPPs functions as an amplifier. By controlling the semiannular aperture size, we achieved large range modulations of wavelengths beyond 60 nm for both bands. This efficient unidirectional launching is experimentally demonstrated for 632 nm, showing good agreement with numerical results

All-Color Plasmonic Nanolasers with Ultralow Thresholds: Autotuning Mechanism for Single-Mode Lasing

We report on the first demonstration of broadband tunable, single-mode plasmonic nanolasers (spasers) emitting in the full visible spectrum. These nanolasers are based on a single metal–oxide–semiconductor nanostructure platform comprising of InGaN/GaN semiconductor nanorods supported on an Al₂O₃-capped epitaxial Ag film. In particular, all-color lasing in subdiffraction plasmonic resonators is achieved via a novel mechanism based on a property of weak size dependence inherent in spasers. Moreover, we have successfully reduced the continuous-wave (CW) lasing thresholds to ultrasmall values for all three primary colors and have clearly demonstrated the possibility of “thresholdless” lasing for the blue plasmonic nanolaser