skip to main content

Title: Ultrathin-shell epitaxial Ag@Au core-shell nanowires for high-performance and chemically-stable electronic, optical, and mechanical devices
Abstract Silver nanowires (AgNWs) hold great promise for applications in wearable electronics, flexible solar cells, chemical and biological sensors, photonic/plasmonic circuits, and scanning probe microscopy (SPM) due to their unique plasmonic, mechanical, and electronic properties. However, the lifetime, reliability, and operating conditions of AgNW-based devices are significantly restricted by their poor chemical stability, limiting their commercial potentials. Therefore, it is crucial to create a reliable oxidation barrier on AgNWs that provides long-term chemical stability to various optical, electrical, and mechanical devices while maintaining their high performance. Here we report a room-temperature solution-phase approach to grow an ultra-thin, epitaxial gold coating on AgNWs to effectively shield the Ag surface from environmental oxidation. The Ag@Au core-shell nanowires (Ag@Au NWs) remain stable in air for over six months, under elevated temperature and humidity (80 °C and 100% humidity) for twelve weeks, in physiological buffer solutions for three weeks, and can survive overnight treatment of an oxidative solution (2% H 2 O 2 ). The Ag@Au core-shell NWs demonstrated comparable performance as pristine AgNWs in various electronic, optical, and mechanical devices, such as transparent mesh electrodes, surface-enhanced Raman spectroscopy (SERS) substrates, plasmonic waveguides, plasmonic nanofocusing probes, and high-aspect-ratio, high-resolution atomic force microscopy (AFM) probes. These Au@Ag core-shell NWs offer a universal solution towards chemically-stable AgNW-based devices without compromising material property or device performance.  more » « less
Award ID(s):
Author(s) / Creator(s):
; ; ; ; ; ; ; ; ; ; ; ; ; ;
Date Published:
Journal Name:
Nano Research
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. Germanium nanowires (NWs) have attractive properties for a variety of applications, including micro- and optoelectronics, memory devices, solar energy conversion, and energy storage, among others. For applications that involve exposure to air, the poor chemical stability and electronic surface passivation of native oxides have remained a long-standing concern. Termination by sulfur-rich surface layers has emerged as a promising strategy for passivation of planar Ge surfaces. Here we discuss experiments on solid-state sulfurization of Ge nanowires in sulfur vapor at near-ambient pressures and at different temperatures. Combined transmission electron microscopy imaging and chemical mapping establishes that Ge NWs remain intact during vapor-phase reaction with S at elevated temperatures, and show the formation of sulfur-rich shells with T-dependent morphology and thickness on the Ge NW surface. Photoluminescence of ensembles of such core–shell nanowires is dominated by strong emission at ∼1.85 eV, consistent with luminescence of GeS. Cathodoluminescence spectroscopy on individual NWs establishes that this luminescence originates in thin GeS shells formed by sulfurization of the NWs. Our work establishes direct sulfurization as a viable approach for forming stable, wide-bandgap surface terminations on Ge NWs. 
    more » « less
  2. Chemically-synthesized single-crystalline silver nanowire (AgNW) probes can combine the scanning tunneling microscopy (STM) technique with tip-enhanced Raman scattering spectroscopy (TERS) for complementary morphological and chemical information with nanoscale spatial resolution. However, its performance has been limited by the blunt nanowire tip geometry, the insulating surfactant layer coating AgNW surfaces, and the thermal-induced mechanical vibrations. Here, we report a reproducible fabrication method for the preparation of sharp-tip AgNW-based TERS probes. By removing the polyvinylpyrrolidone (PVP) surfactant molecules from the AgNW surfaces for stable electrical conductivity and controlling the protruding length with μm-level accuracy for improved mechanical stability, we demonstrate atomic-resolution STM imaging with the sharp-tip AgNW probe. Furthermore, the sharp-tip AgNW has an excellent TER enhancement (∼1.1 × 10 6 ), which is about 66 folds of that achieved by regular AgNWs. Our experiments demonstrate that AgNWs with clean interfaces and the proper tip geometry can provide reliable and reproducible STM and TER characterizations, which remove the hurdles preventing the implementation of AgNW in STM-based near-field optical applications for a broad community. 
    more » « less
  3. Abstract

    Transparent microelectrodes have received much attention from the biomedical community due to their unique advantages in concurrent crosstalk‐free electrical and optical interrogation of cell/tissue activity. Despite recent progress in constructing transparent microelectrodes, a major challenge is to simultaneously achieve desirable mechanical stretchability, optical transparency, electrochemical performance, and chemical stability for high‐fidelity, conformal, and stable interfacing with soft tissue/organ systems. To address this challenge, we have designed microelectrode arrays (MEAs) with gold‐coated silver nanowires (Au–Ag NWs) by combining technical advances in materials, fabrication, and mechanics. The Au coating improves both the chemical stability and electrochemical impedance of the Au–Ag NW microelectrodes with only slight changes in optical properties. The MEAs exhibit a high optical transparency >80% at 550 nm, a low normalized 1 kHz electrochemical impedance of 1.2–7.5 Ω cm2, stable chemical and electromechanical performance after exposure to oxygen plasma for 5 min, and cyclic stretching for 600 cycles at 20% strain, superior to other transparent microelectrode alternatives. The MEAs easily conform to curvilinear heart surfaces for colocalized electrophysiological and optical mapping of cardiac function. This work demonstrates that stretchable transparent metal nanowire MEAs are promising candidates for diverse biomedical science and engineering applications, particularly under mechanically dynamic conditions.

    more » « less
  4. Dye-doped nanoparticles have been investigated as bright, fluorescent probes for localization-based super-resolution microscopy. Nanoparticle size is important in super-resolution microscopy to get an accurate size of the object of interest from image analysis. Due to their self-blinking behavior and metal-enhanced fluorescence (MEF), Ag@SiO2and Au@Ag@SiO2nanoparticles have shown promise as probes for localization-based super-resolution microscopy. Here, several noble metal-based dye-doped core-shell nanoparticles have been investigated as self-blinking nanomaterial probes. It was observed that both the gold- and silver-plated nanoparticle cores exhibit weak luminescence under certain conditions due to the surface plasmon resonance bands produced by each metal, and the gold cores exhibit blinking behavior which enhances the blinking and fluorescence of the dye-doped nanoparticle. However, the silver-plated nanoparticle cores, while weakly luminescent, did not exhibit any blinking; the dye-doped nanoparticle exhibited the same behavior as the core fluorescent, but did not blink. Because of the blinking behavior, stochastic optical reconstruction microscopy (STORM) super-resolution analysis was able to be performed with performed on the gold core nanoparticles. A preliminary study on the use of these nanoparticles for localization-based super-resolution showed that these nanoparticles are suitable for use in STORM super resolution. Resolution enhancement was two times better than the diffraction limited images, with core sizes reduced to 15 nm using the hybrid Au–Ag cores.

    more » « less
  5. Abstract

    Zinc oxide (ZnO) nanowires are widely studied for use in ultraviolet optoelectronic devices, such as nanolasers and sensors. Nanowires (NWs) with an MgO shell exhibit enhanced band‐edge photoluminescence (PL), a result previously attributed to passivation of ZnO defects. However, we find that processing the ZnO NWs under low oxygen partial pressure leads to an MgO‐thickness‐dependent PL enhancement owing to the formation of optical cavity modes. Conversely, processing under higher oxygen partial pressure leads to NWs that support neither mode formation nor band‐edge PL enhancement. High‐resolution electron microscopy and density‐functional calculations implicate the ZnOm‐plane surface morphology as the key determinant of core‐shell structure and cavity‐mode optics. A ZnO surface with atomic steps along them‐plane in thec‐axis direction stimulates the growth of a smooth MgO shell that supports guided‐wave optical modes and enhanced UV PL. On the other hand, a smoother ZnO surface leads to nucleation of a rough cladding layer which supports neither enhanced UV PL nor optical cavity modes. Finite‐element analysis shows a clear correlation between allowed Fabry‐Perot and whispering gallery modes and enhanced UV‐PL. These results point the way to fabricating ZnO/MgO core‐shell nanowires for more efficient UV nanolasers, scintillators, and sensors.

    more » « less