Hybrid cavity-antenna architecture for strong and tunable sideband-selective molecular Raman scattering enhancement.

Plasmon resonances at the surface of metallic antennas allow for extreme enhancement of Raman scattering. Intrinsic to plasmonics, however, is that extreme field confinement lacks precise spectral control, which would hold great promise in shaping the optomechanical interaction between light and molecular vibrations. We demonstrate an experimental platform composed of a plasmonic nanocube-on-mirror antenna coupled to an open, tunable Fabry-Perot microcavity for selective addressing of individual vibrational lines of molecules with strong Raman scattering enhancement. Multiple narrow and intense optical resonances arising from the hybridization of the cavity modes and the plasmonic broad resonance are used to simultaneously enhance the laser pump and the local density of optical states, and are characterized using rigorous modal analysis. The versatile bottom-up fabrication approach permits quantitative comparison with the bare nanocube-on-mirror system, both theoretically and experimentally. This shows that the hybrid system allows for similar SERS enhancement ratios with narrow optical modes, paving the way for dynamical backaction effects in molecular optomechanics.

Sub-nanometer mapping of strain-induced band structure variations in planar nanowire core-shell heterostructures.

Strain relaxation mechanisms during epitaxial growth of core-shell nanostructures play a key role in determining their morphologies, crystal structure and properties. To unveil those mechanisms, we perform atomic-scale aberration-corrected scanning transmission electron microscopy studies on planar core-shell ZnSe@ZnTe nanowires on α-Al2O3 substrates. The core morphology affects the shell structure involving plane bending and the formation of low-angle polar boundaries. The origin of this phenomenon and its consequences on the electronic band structure are discussed. We further use monochromated valence electron energy-loss spectroscopy to obtain spatially resolved band-gap maps of the heterostructure with sub-nanometer spatial resolution. A decrease in band-gap energy at highly strained core-shell interfacial regions is found, along with a switch from direct to indirect band-gap. These findings represent an advance in the sub-nanometer-scale understanding of the interplay between structure and electronic properties associated with highly mismatched semiconductor heterostructures, especially with those related to the planar growth of heterostructured nanowire networks.

Holistic Determination of Optoelectronic Properties using High-Throughput Spectroscopy of Surface-Guided CsPbBr3 Nanowires.

Optoelectronic micro- and nanostructures have a vast parameter space to explore for modification and optimization of their functional performance. This paper reports on a data-led approach using high-throughput single nanostructure spectroscopy to probe >8000 structures, allowing for holistic analysis of multiple material and optoelectronic parameters with statistical confidence. The methodology is applied to surface-guided CsPbBr3 nanowires, which have complex and interrelated geometric, structural, and electronic properties. Photoluminescence-based measurements, studying both the surface and embedded interfaces, exploits the natural inter nanowire geometric variation to show that increasing the nanowire width reduces the optical bandgap, increases the recombination rate in the nanowire bulk, and reduces the rate at the surface interface. A model of carrier recombination and diffusion ascribes these trends to carrier density and strain effects at the interfaces and self-consistently retrieves values for carrier mobility, trap densities, bandgap, diffusion length, and internal quantum efficiency. The model predicts parameter trends, such as the variation of internal quantum efficiency with width, which is confirmed by experimental verification. As this approach requires minimal a priori information, it is widely applicable to nano- and microscale materials.

Energy-resolved plasmonic chemistry in individual nanoreactors.

Plasmonic resonances can concentrate light into exceptionally small volumes, which approach the molecular scale. The extreme light confinement provides an advantageous pathway to probe molecules at the surface of plasmonic nanostructures with highly sensitive spectroscopies, such as surface-enhanced Raman scattering. Unavoidable energy losses associated with metals, which are usually seen as a nuisance, carry invaluable information on energy transfer to the adsorbed molecules through the resonance linewidth. We measured a thousand single nanocavities with sharp gap plasmon resonances spanning the red to near-infrared spectral range and used changes in their linewidth, peak energy and surface-enhanced Raman scattering spectra to monitor energy transfer and plasmon-driven chemical reactions at their surface. Using methylene blue as a model system, we measured shifts in the absorption spectrum of molecules following surface adsorption and revealed a rich plasmon-driven reactivity landscape that consists of distinct reaction pathways that occur in separate resonance energy windows.

In Situ Imaging of Ferroelastic Domain Dynamics in CsPbBr3 Perovskite Nanowires by Nanofocused Scanning X-ray Diffraction.

The interest in metal halide perovskites has grown as impressive results have been shown in solar cells, light emitting devices, and scintillators, but this class of materials have a complex crystal structure that is only partially understood. In particular, the dynamics of the nanoscale ferroelastic domains in metal halide perovskites remains difficult to study. An ideal in situ imaging method for ferroelastic domains requires a challenging combination of high spatial resolution and long penetration depth. Here, we demonstrate in situ temperature-dependent imaging of ferroelastic domains in a single nanowire of metal halide perovskite, CsPbBr3. Scanning X-ray diffraction with a 60 nm beam was used to retrieve local structural properties for temperatures up to 140 °C. We observed a single Bragg peak at room temperature, but at 80 °C, four new Bragg peaks appeared, originating in different real-space domains. The domains were arranged in periodic stripes in the center and with a hatched pattern close to the edges. Reciprocal space mapping at 80 °C was used to quantify the local strain and lattice tilts, revealing the ferroelastic nature of the domains. The domains display a partial stability to further temperature changes. Our results show the dynamics of nanoscale ferroelastic domain formation within a single-crystal perovskite nanostructure, which is important both for the fundamental understanding of these materials and for the development of perovskite-based devices.

Using Hot Electrons and Hot Holes for Simultaneous Cocatalyst Deposition on Plasmonic Nanostructures.

Hot electrons generated in metal nanoparticles can drive chemical reactions and selectively deposit cocatalyst materials on the plasmonic hotspots, the areas where the decay of plasmons takes place and the hot electrons are created. While hot electrons have been extensively used for nanomaterial formation, the utilization of hot holes for simultaneous cocatalyst deposition has not yet been explored. Herein, we demonstrate that hot holes can drive an oxidation reaction for the deposition of the manganese oxide (MnOx) cocatalyst on different plasmonic gold (Au) nanostructures on a thin titanium dioxide (TiO2) layer, excited at their surface plasmon resonance. An 80% correlation between the hot-hole deposition sites and the simulated plasmonic hotspot location is showed when considering the typical hot-hole diffusion length. Simultaneous deposition of more than one cocatalyst is also achieved on one of the investigated plasmonic systems (Au plasmonic nanoislands) through the hot-hole oxidation of a manganese salt and the hot-electron reduction of a platinum precursor in the same solution. These results add more flexibility to the use of hot carriers and open up the way for the design of complex photocatalytic nanostructures.

Controlling Optically Driven Atomic Migration Using Crystal-Facet Control in Plasmonic Nanocavities.

Plasmonic nanoconstructs are widely exploited to confine light for applications ranging from quantum emitters to medical imaging and biosensing. However, accessing extreme near-field confinement using the surfaces of metallic nanoparticles often induces permanent structural changes from light, even at low intensities. Here, we report a robust and simple technique to exploit crystal facets and their atomic boundaries to prevent the hopping of atoms along and between facet planes. Avoiding X-ray or electron microscopy techniques that perturb these atomic restructurings, we use elastic and inelastic light scattering to resolve the influence of crystal habit. A clear increase in stability is found for {100} facets with steep inter-facet angles, compared to multiple atomic steps and shallow facet curvature on spherical nanoparticles. Avoiding atomic hopping allows Raman scattering on molecules with low Raman cross-section while circumventing effects of charging and adatom binding, even over long measurement times. These nanoconstructs allow the optical probing of dynamic reconstruction in nanoscale surface science, photocatalysis, and molecular electronics.

Large lattice distortions and size-dependent bandgap modulation in epitaxial halide perovskite nanowires.

Metal-halide perovskites have been shown to be remarkable and promising optoelectronic materials. However, despite ongoing research from multiple perspectives, some fundamental questions regarding their optoelectronic properties remain controversial. One reason is the high-variance of data collected from, often unstable, polycrystalline thin films. Here we use ordered arrays of stable, single-crystal cesium lead bromide (CsPbBr3) nanowires grown by surface-guided chemical vapor deposition to study fundamental properties of these semiconductors in a one-dimensional model system. Specifically, we uncover the origin of an unusually large size-dependent luminescence emission spectral blue-shift. Using multiple spatially resolved spectroscopy techniques, we establish that bandgap modulation causes the emission shift, and by correlation with state-of-the-art electron microscopy methods, we reveal its origin in substantial and uniform lattice rotations due to heteroepitaxial strain and lattice relaxation. Understanding strain and its effect on the optoelectronic properties of these dynamic materials, from the atomic scale up, is essential to evaluate their performance limits and fundamentals of charge carrier dynamics.

Surface-Guided CsPbBr3 Perovskite Nanowires on Flat and Faceted Sapphire with Size-Dependent Photoluminescence and Fast Photoconductive Response.

All-inorganic lead halide perovskite nanowires have been the focus of increasing interest since they exhibit improved stability compared to their hybrid organic-inorganic counterparts, while retaining their interesting optical and optoelectronic properties. Arrays of surface-guided nanowires with controlled orientations and morphology are promising as building blocks for various applications and for systematic research. We report the horizontal and aligned growth of CsPbBr3 nanowires with a uniform crystallographic orientation on flat and faceted sapphire surfaces to form arrays with 6-fold and 2-fold symmetries, respectively, along specific directions of the sapphire substrate. We observed waveguiding behavior and diameter-dependent photoluminescence emission well beyond the quantum confinement regime. The arrays were easily integrated into multiple devices, displaying p-type behavior and photoconductivity. Photodetectors based on those nanowires exhibit the fastest rise and decay times for any CsPbBr3-based photodetectors reported so far. One-dimensional arrays of halide perovskite nanowires are a promising platform for investigating the intriguing properties and potential applications of these unique materials.

Bottom-Up Tri-gate Transistors and Submicrosecond Photodetectors from Guided CdS Nanowalls.

Tri-gate transistors offer better performance than planar transistors by exerting additional gate control over a channel from two lateral sides of semiconductor nanowalls (or "fins"). Here we report the bottom-up assembly of aligned CdS nanowalls by a simultaneous combination of horizontal catalytic vapor-liquid-solid growth and vertical facet-selective noncatalytic vapor-solid growth and their parallel integration into tri-gate transistors and photodetectors at wafer scale (cm2) without postgrowth transfer or alignment steps. These tri-gate transistors act as enhancement-mode transistors with an on/off current ratio on the order of 108, 4 orders of magnitude higher than the best results ever reported for planar enhancement-mode CdS transistors. The response time of the photodetector is reduced to the submicrosecond level, 1 order of magnitude shorter than the best results ever reported for photodetectors made of bottom-up semiconductor nanostructures. Guided semiconductor nanowalls open new opportunities for high-performance 3D nanodevices assembled from the bottom up.

Surface-Guided Core-Shell ZnSe@ZnTe Nanowires as Radial p-n Heterojunctions with Photovoltaic Behavior.

The organization of nanowires on surfaces remains a major obstacle toward their large-scale integration into functional devices. Surface-material interactions have been used, with different materials and substrates, to guide horizontal nanowires during their growth into well-organized assemblies, but the only guided nanowire heterostructures reported so far are axial and not radial. Here, we demonstrate the guided growth of horizontal core-shell nanowires, specifically of ZnSe@ZnTe, with control over their crystal phase and crystallographic orientations. We exploit the directional control of the guided growth for the parallel production of multiple radial p-n heterojunctions and probe their optoelectronic properties. The devices exhibit a rectifying behavior with photovoltaic characteristics upon illumination. Guided nanowire heterostructures enable the bottom-up assembly of complex semiconductor structures with controlled electronic and optoelectronic properties.

Guided CdSe Nanowires Parallelly Integrated into Fast Visible-Range Photodetectors.

One-dimensional semiconductor nanostructures, such as nanowires (NWs), have attracted tremendous attention due to their unique properties and potential applications in nanoelectronics, nano-optoelectronics, and sensors. One of the challenges toward their integration into practical devices is their large-scale controlled assembly. Here, we report the guided growth of horizontal CdSe nanowires on five different planes of sapphire. The growth direction and crystallographic orientation are controlled by the epitaxial relationship with the substrate as well as by a graphoepitaxial effect of surface nanosteps and grooves. CdSe is a promising direct-bandgap II-VI semiconductor active in the visible range, with potential applications in optoelectronics. The guided CdSe nanowires were found to have a wurtzite single-crystal structure. Field-effect transistors and photodetectors were fabricated to examine the nanowire electronic and optoelectronic properties, respectively. The latter exhibited the fastest rise and fall times ever reported for CdSe nanostructures as well as a relatively high gain, both features being essential for optoelectronic applications.

Guided Growth of Horizontal p-Type ZnTe Nanowires.

A major challenge toward large-scale integration of nanowires is the control over their alignment and position. A possible solution to this challenge is the guided growth process, which enables the synthesis of well-aligned horizontal nanowires that grow according to specific epitaxial or graphoepitaxial relations with the substrate. However, the guided growth of horizontal nanowires was demonstrated for a limited number of materials, most of which exhibit unintentional n-type behavior. Here we demonstrate the vapor-liquid-solid growth of guided horizontal ZnTe nanowires and nanowalls displaying p-type behavior on four different planes of sapphire. The growth directions of the nanowires are determined by epitaxial relations between the nanowires and the substrate or by a graphoepitaxial effect that guides their growth along nanogrooves or nanosteps along the surface. We characterized the crystallographic orientations and elemental composition of the nanowires using transmission electron microscopy and photoluminescence. The optoelectronic and electronic properties of the nanowires were studied by fabricating photodetectors and top-gate thin film transistors. These measurements showed that the guided ZnTe nanowires are p-type semiconductors and are photoconductive in the visible range. The guided growth of horizontal p-type nanowires opens up the possibility of parallel nanowire integration into functional systems with a variety of potential applications not available by other means.

Self-Assembled Organic Nanocrystals with Strong Nonlinear Optical Response.

Facile molecular self-assembly affords a new family of organic nanocrystals that, unintuitively, exhibit a significant nonlinear optical response (second harmonic generation, SHG) despite the relatively small molecular dipole moment of the constituent molecules. The nanocrystals are self-assembled in aqueous media from simple monosubstituted perylenediimide (PDI) molecular building blocks. Control over the crystal dimensions can be achieved via modification of the assembly conditions. The combination of a simple fabrication process with the ability to generate soluble SHG nanocrystals with tunable sizes may open new avenues in the area of organic SHG materials.

Guided Growth of Horizontal ZnSe Nanowires and their Integration into High-Performance Blue-UV Photodetectors.

Perfectly aligned horizontal ZnSe nano-wires are obtained by guided growth, and easily integrated into high-performance blue-UV photodetectors. Their crystal phase and crystallographic orientation are controlled by the epitaxial relations with six different sapphire planes. Guided growth paves the way for the large-scale integration of nanowires into optoelectronic devices.