Optically Abrupt Localized Surface Plasmon Resonances in Si Nanowires by Mitigation of Carrier Density Gradients

Spatial control of carrier density is critical for engineering and exploring the interactions of localized surface plasmon resonances (LSPRs) in nanoscale semiconductors. Here, we couple <i>in situ</i> infrared spectral response measurements and discrete dipole approximation (DDA) calculations to show the impact of axially graded carrier density profiles on the optical properties of mid-infrared LSPRs supported by Si nanowires synthesized by the vapor–liquid–solid technique. The region immediately adjacent to each intentionally encoded resonator (<i>i.e.</i>, doped segment) can exhibit residual carrier densities as high as 10²⁰ cm^–3, which strongly modifies both near- and far-field behavior. Lowering substrate temperature during the spacer segment growth reduces this residual carrier density and results in a spectral response that is indistinguishable from nanowires with ideal, atomically abrupt carrier density profiles. Our experiments have important implications for the control of near-field plasmonic phenomena in semiconductor nanowires, and demonstrate methods for determining and controlling axial dopant profile in these systems

Strong Near-Field Coupling of Plasmonic Resonators Embedded in Si Nanowires

The strength of localized surface plasmon resonance (LSPR) near-field interactions scales in a well-known, nearly universal manner. Here, we show that embedding resonators in an anisotropic dielectric with a large permittivity can substantially increase coupling strength. We experimentally demonstrate this effect with Si nanowires containing two phosphorus-doped segments. The near-field decay length scaling factor is extracted from <i>in situ</i> infrared spectral response measurements using the “plasmon ruler” equation and found to be ca. 4−5 times larger than for the same resonators in isotropic vacuum or Si. Discrete dipole approximation calculations support the observed coupling behavior for nanowires and show how it is affected by the resonator geometry, carrier density, and embedding material (Si, Ge, GaAs, etc.). Our findings demonstrate that equivalent near-field interactions are achievable with a smaller total volume and/or at increased resonator spacing, offering new opportunities to engineer plasmon-based chemical sensors, catalysts, and waveguides

Tunable Mid-Infrared Localized Surface Plasmon Resonances in Silicon Nanowires

We observe and systematically tune an intense mid-infrared absorption mode that results from phosphorus doping in silicon nanowires synthesized via the vapor–liquid–solid technique. The angle- and shape-dependence of this spectral feature, as determined via <i>in-situ</i> transmission infrared spectroscopy, supports its assignment as a longitudinal localized surface plasmon resonance (LSPR). Modulation of resonant frequency (740–1620 cm^–1) is accomplished by varying nanowire length (135–1160 nm). The observed frequency shift is consistent with Mie–Gans theory, which indicates electrically active dopant concentrations between 10¹⁹ and 10²⁰ cm^–3. Our findings suggest new opportunities to confine light in this ubiquitous semiconductor and engineer the optical properties of nontraditional plasmonic materials

Direct Observation of Transient Surface Species during Ge Nanowire Growth and Their Influence on Growth Stability

Surface adsorbates are well-established choreographers of material synthesis, but the presence and impact of these short-lived species on semiconductor nanowire growth are largely unknown. Here, we use infrared spectroscopy to directly observe surface adsorbates, hydrogen atoms and methyl groups, chemisorbed to the nanowire sidewall and show they are essential for the stable growth of Ge nanowires via the vapor–liquid–solid mechanism. We quantitatively determine the surface coverage of hydrogen atoms during nanowire growth by comparing ν­(Ge–H) absorption bands from <i>operando</i> measurements (i.e., during growth) to those after saturating the nanowire sidewall with hydrogen atoms. This method provides sub-monolayer chemical information at relevant reaction conditions while accounting for the heterogeneity of sidewall surface sites and their evolution during elongation. Our findings demonstrate that changes to surface bonding are critical to understand Ge nanowire synthesis and provide new guidelines for rationally selecting catalysts, forming heterostructures, and controlling dopant profiles

Influence of Dielectric Anisotropy on the Absorption Properties of Localized Surface Plasmon Resonances Embedded in Si Nanowires

We utilize discrete dipole approximation simulations to provide a detailed picture of the scattering behavior of mid-infrared localized surface plasmon resonances (LSPRs) in selectively doped (i.e., <i>i</i>–<i>n</i>^<i>++</i>–<i>i</i>) Si nanowires. Our simulations, and their quantitative comparison to recent experimental results, show that the large refractive indices (<i>n</i> ≈ 3–4) of undoped semiconductors in the infrared and the anisotropic dielectric environment inherent in the nanowire geometry strongly enhance/depress absorption by the longitudinal/transverse LSPR. An examination of “cladding” materials other than Si (e.g., GaAs, Ge, etc.) reveals that this behavior scales with refractive index and that absorption enhancements of at least 35× are possible relative to an isotropic vacuum. We also show how scattering and absorption contribute to the overall extinction and extract a value for the carrier density of Si-based resonators synthesized via the vapor–liquid–solid (VLS) mechanism. Our findings establish a framework for rationally engineering LSPR spectral response in semiconductor nanowires and highlight the promise of the VLS technique for this purpose

Blood pressure and pulse rate changes at rest, Foley insertion and during the process of bladder infusion.

<p>Blood pressure and pulse rate changes at rest, Foley insertion and during the process of bladder infusion.</p

Power spectrum analysis (LF/HF ratio) of HRV at rest, Foley insertion and during the process of bladder infusion.

<p>Power spectrum analysis (LF/HF ratio) of HRV at rest, Foley insertion and during the process of bladder infusion.</p

The changes of HRV parameters, LF/HF ratio during UDS of the three groups.

<p>These seven time points were: after lying quietly for 15 min (p1), Foley catheter insertion (p2), start of infusion (p3), and infused volume at 1/4 (p4), 2/4 (p5), 3/4 (p6) and 4/4 (p7) of maximum capacity.</p

Basic demographic data of our subjects.

<p>Basic demographic data of our subjects.</p

Comparison of Autonomic Reactions during Urodynamic Examination in Patients with Spinal Cord Injuries and Able-Bodied Subjects - Fig 1

<p><b>The changes of SBP (1A) and PR (1B) throughout the examination of these three groups.</b> These seven time points were: after lying quietly for 15 min (p1), Foley catheter insertion (p2), start of infusion (p3), and infused volume at 1/4 (p4), 2/4 (p5), 3/4 (p6) and 4/4 (p7) of maximum capacity.</p