Controlling Silicon Nanowire Growth Direction via Surface Chemistry

We report on the first <i>in situ</i> chemical investigation of vapor–liquid–solid semiconductor nanowire growth and reveal the important, and previously unrecognized, role of transient surface chemistry near the triple-phase line. Real-time infrared spectroscopy measurements coupled with postgrowth electron microscopy demonstrate that covalently bonded hydrogen atoms are responsible for the ⟨111⟩ to ⟨112⟩ growth orientation transition commonly observed during Si nanowire growth. Our findings provide insight into the root cause of this well-known nanowire growth phenomenon and open a new route to rationally engineer the crystal structure of these nanoscale semiconductors

Chemical Control of Semiconductor Nanowire Kinking and Superstructure

We show that methylgermane (GeH₃CH₃) can induce a transition from ⟨111⟩ to ⟨110⟩ oriented growth during the vapor–liquid–solid synthesis of Ge nanowires. This hydride-based chemistry is subsequently leveraged to rationally fabricate kinking superstructures based on combinations of ⟨111⟩ and ⟨110⟩ segments. The addition of GeH₃CH₃ also eliminates sidewall tapering and enables Ge nanowire growth at temperatures exceeding 475 °C, which greatly expands the process window and opens new avenues to create Si/Ge heterostructures

Solid–Liquid–Vapor Etching of Semiconductor Nanowires

The vapor–liquid–solid (VLS) mechanism enables the bottom-up, or additive, growth of semiconductor nanowires. Here, we demonstrate a reverse process, whereby catalyst atoms are selectively removed from the eutectic catalyst droplet. This process, which is driven by the dicarbonyl precursor 2,3-butanedione, results in axial nanowire etching. Experiments as a function of substrate temperature, etchant flow rate, and nanowire diameter support a solid–liquid–vapor (SLV) mechanism. An etch model with reaction at the liquid–vapor interface as the rate-limiting step is consistent with our experiments. These results identify a new mechanism to in situ tune the concentration of semiconductor atoms in the catalyst droplet

Interplay between Defect Propagation and Surface Hydrogen in Silicon Nanowire Kinking Superstructures

Semiconductor nanowire kinking superstructures, particularly those with long-range structural coherence, remain difficult to fabricate. Here, we combine high-resolution electron microscopy with <i>operando</i> infrared spectroscopy to show why this is the case for Si nanowires and, in doing so, reveal the interplay between defect propagation and surface chemistry during ⟨211⟩ → ⟨111⟩ and ⟨211⟩ → ⟨211⟩ kinking. Our experiments show that adsorbed hydrogen atoms are responsible for selecting ⟨211⟩-oriented growth and indicate that a twin boundary imparts structural coherence. The twin boundary, only continuous at ⟨211⟩ → ⟨211⟩ kinks, reduces the symmetry of the trijunction and limits the number of degenerate directions available to the nanowire. These findings constitute a general approach for rationally engineering kinking superstructures and also provide important insight into the role of surface chemical bonding during vapor–liquid–solid synthesis

Sidewall Morphology-Dependent Formation of Multiple Twins in Si Nanowires

Precise placement of twin boundaries and stacking faults promises new opportunities to fundamentally manipulate the optical, electrical, and thermal properties of semiconductor nanowires. Here we report on the appearance of consecutive twin boundaries in Si nanowires and show that sidewall morphology governs their spacing. Detailed electron microscopy analysis reveals that thin {111} sidewall facets, which elongate following the first twin boundary (TB₁), are responsible for deforming the triple-phase line and favoring the formation of the second twin boundary (TB₂). While multiple, geometrically correlated defect planes are known in group III–V nanowires, our findings show that this behavior is also possible in group IV materials

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

Rational Defect Introduction in Silicon Nanowires

The controlled introduction of planar defects, particularly twin boundaries and stacking faults, in group IV nanowires remains challenging despite the prevalence of these structural features in other nanowire systems (e.g., II–VI and III–V). Here we demonstrate how user-programmable changes to precursor pressure and growth temperature can rationally generate both transverse twin boundaries and angled stacking faults during the growth of ⟨111⟩ oriented Si nanowires. We leverage this new capability to demonstrate prototype defect superstructures. These findings yield important insight into the mechanism of defect generation in semiconductor nanowires and suggest new routes to engineer the properties of this ubiquitous semiconductor

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

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