Jerome K. Hyun

Three-dimensional nanonetworks for giant stretchability in dielectrics and conductors

The realization of levels of stretchability that extend beyond intrinsic limits of bulk materials is of great importance to stretchable electronics. Here we report large-area, three-dimensional nano-architectures that achieve this outcome in materials that offer both insulating and conductive properties. For the elastomer poly(dimethylsiloxane), such geometries enhance the stretchability and fracture strain by ~62% and ~225% over the bulk, unstructured case. The underlying physics involves local rotations of narrow structural elements in the three-dimensional network, as identified by mechanical modelling. To demonstrate the applications of three-dimensional poly(dimethylsiloxane), we create a stretchable conductor obtained by filling the interstitial regions with liquid metal. This stretchable composite shows extremely high electrical conductivity (~24,100 S cm(-1)) even at strains >200%, with good cyclic properties and with current-carrying capacities that are sufficient for interconnects in light-emitting diode systems. Collectively, these concepts provide new design opportunities for stretchable electronics.

Broadband plasmonic microlenses based on patches of nanoholes

This paper reports a new type of diffractive microlens based on finite-areas of two-dimensional arrays of circular nanoholes (patches). The plasmonic microlenses can focus single wavelengths of light across the entire visible spectrum as well as broadband white light with little divergence. The focal length is determined primarily by the overall size of the patch and is tolerant to significant changes in patch substructure, including lattice geometry and local order of the circular nanoholes. The optical throughput, however, depends sensitively on the patch substructure and is determined by the wavelengths of surface plasmon resonances. This simple diffractive lens design enables millions of broadband plasmonic microlenses to be fabricated in parallel using soft nanolithographic techniques.

Spatially Resolved Plasmonically Enhanced Photocurrent from Au Nanoparticles on a Si Nanowire

Semiconducting nanowires have been demonstrated as promising light-harvesting units with enhanced absorption compared to bulk films of equivalent volume. However, for small diameter nanowires, the ultrahigh aspect ratio constrains the absorption to be polarization selective by responding primarily to the transverse magnetic (TM) light. While this effect is useful for polarization-sensitive optoelectronic devices, practical light-harvesting applications demand efficient light absorption in both TM and transverse electric (TE) light. In this study, we engineer the polarization sensitivity and the charge carrier generation in a 50 nm Si nanowire by decorating the surface with plasmonic Au nanoparticles. Using scanning photocurrent microscopy (SPCM) with a tunable wavelength laser, we spatially and spectrally resolve the local enhancement in the TE photocurrent resulting from the plasmonic near-field response of individual nanoparticles and the broad-band enhancement due to surface-enhanced absorption. These results provide guidance to the development and the optimization of nanowire-nanoparticle light-harvesting systems.

Diameter and Polarization-Dependent Raman Scattering Intensities of Semiconductor Nanowires

Diameter-dependent Raman scattering in single tapered silicon nanowires is measured and quantitatively reproduced by modeling with finite-difference time-domain simulations. Single crystal tapered silicon nanowires were produced by homoepitaxial radial growth concurrent with vapor-liquid-solid axial growth. Multiple electromagnetic resonances along the nanowire induce broad band light absorption and scattering. Observed Raman scattering intensities for multiple polarization configurations are reproduced by a model that accounts for the internal electromagnetic mode structure of both the exciting and scattered light. Consequences for the application of Stokes to anti-Stokes intensity ratio for the estimation of lattice temperature are discussed.

Direct detection of hole gas in Ge-Si core-shell nanowires by enhanced Raman scattering.

We report the direct detection of hole accumulation in the core of Ge-Si core-shell nanowire heterostructures by a Fano resonance between free holes and the F2g mode in Raman spectra. Raman enhancements of 10-10 000 with respect to bulk were observed and explained using finite difference time domain simulations of the electric fields concentrated in the nanowire. Numerical modeling of the radial carrier concentration revealed that the asymmetric line-shape is strongly influenced by inhomogeneous broadening.

Barrier Height Measurement of Metal Contacts to Si Nanowires Using Internal Photoemission of Hot Carriers

Barrier heights between metal contacts and silicon nanowires were measured using spectrally resolved scanning photocurrent microscopy (SPCM). Illumination of the metal-semiconductor junction with sub-bandgap photons generates a photocurrent dominated by internal photoemission of hot electrons. Analysis of the dependence of photocurrent yield on photon energy enables quantitative extraction of the barrier height. Enhanced doping near the nanowire surface, mapped quantitatively with atom probe tomography, results in a lowering of the effective barrier height. Occupied interface states produce an additional lowering that depends strongly on diameter. The doping and diameter dependencies are explained quantitatively with finite element modeling. The combined tomography, electrical characterization, and numerical modeling approach represents a significant advance in the quantitative analysis of transport mechanisms at nanoscale interfaces that can be extended to other nanoscale devices and heterostructures.

Soft Elastomeric Nanopillar Stamps for Enhancing Absorption in Organic Thin‐Film Solar Cells

An elastomeric poly(dimethylsiloxane) (PDMS) block engraved with periodically arrayed nanopillars serves as a transferable light-trapping stamp for encapsulated organic thin-film solar cells. Diffracted light rays from the stamp interfere with one another and self-focus onto the active layer of the solar cell, generating enhanced absorption, as indicated in the current density-voltage measurements.

Raman concentrators in Ge nanowires with dielectric coatings

Raman spectroscopy is a powerful tool for investigating many fundamental properties of nanostructures, but extrinsic effects including background scattering and laser-induced heating can limit the analysis of intrinsic properties. A thin SiO2 dielectric coating is found to enhance the Raman signal from a single Ge nanowire by a factor of two as a result of wave interference. Consequently, the coated nanowire experiences less heating than a bare nanowire at equivalent signal intensities. The results demonstrate a simple and effective method to extend the limits of Raman analysis on single nanostructures and facilitate their characterization.

Mitigation of surface doping in VLS-grown Si nanowires

Semiconducting nanowires grown by the VLS method can develop non-uniform doping profiles along the growth direction due to unintentional surface doping during synthesis. For CVD growth using hydride precursors, surface doping can be suppressed by high H2 partial pressures, thereby improving the uniformity of the dopant distribution. Quantitative calculations of the electrostatic field and carrier concentration derived from scanning photocurrent microscopy measurements confirm suppression of surface doping by phosphine for Si nanowires grown in H2 compared with those grown in He. Nanowires grown in He show 100-fold increases in carrier concentration through surface doping, whereas nanowires grown in a large H2 partial pressure show only two-fold increases for similar growth times. As a result, the carrier concentration gradients are greatly reduced for nanowires grown in H2. These results demonstrate a general approach to in situ control of the surface doping in CVD of nanowires.