Directional Scattering and Sensing with Bimetallic Fanocubes: A Complex Fano-Resonant Plasmonic Nanostructure

Concentric nanostructures provide a unique architecture to manipulate light by modification of their internal geometry with minimal changes to their overall size. In this work, we have theoretically examined, using finite difference time domain simulations, the plasmonic properties of a concentric cubic nanostructure consisting of a silver (Ag) core, silica (SiO₂) interlayer, and gold (Au) shell. These “bimetallic fanocubes” display two separate geometry dependent Fano resonances in the visible and in the near-infrared. We employed a plasmon hybridization model to understand the origin of the spectral features and observe distinct hybridized modes contributed by the edges and corners, which is unique to the cubic geometry. Specifically, we note that the “nonbonding” mode that is essentially dark and not observable in spherical concentric nanostructures is enhanced in the bimetallic fanocubes. We show the far-field properties, and Fano resonances of the fanocubes can be tuned by altering the thickness of the silica layer, the thickness of the Au shell, and by breaking symmetry. Further, we have examined the refractive index sensing and directional scattering abilities of the fanocubes to ultimately enable their use in a range of applications, harnessing their absorption and scattering properties

Size-Dependent Phononic Properties of PdO Nanocrystals Probed by Nanoscale Optical Thermometry

With the advent of novel nanoscale devices, fast and reliable thermal mapping with high spatiotemporal resolution is imperative for probing the characteristics of phonons and evaluating the local temperature at the nanoscale. In this work, Raman spectroscopy is employed as a rapid and noncontact optical thermometry technique to investigate phononic properties of macroscopic assemblies of monodisperse palladium oxide (PdO) nanocrystals. PdO has been extensively employed in high temperature catalytic devices; however, the phonon behavior which determines the thermal stability of PdO remains unexplored thus far. Our study focuses on homogeneous, large-scale assemblies of monodisperse 4 and 10 nm nanocrystals synthesized using colloidal chemistry to understand size-dependent effects on the measured thermal properties. By monitoring the Raman peak shifts, peak broadening, and alterations in peak intensities as a function of laser power and particle concentration, a size-dependent trend is observed attributable to confinement of optical phonons within nanocrystal grain boundaries and laser-induced heating, both influenced by nanocrystal size. This study correlates size-dependent single-particle heating effects with size-dependent interparticle heat transfer under laser irradiation and is enabled by controlled nanocrystal synthesis

Solvent-Assisted Self-Assembly of CsPbBr₃ Perovskite Nanocrystals into One-Dimensional Superlattice

The self-assembly of colloidal nanocrystals into ordered architectures has attracted significant interest enabling innovative methods of manipulating physicochemical properties for targeted applications. This study reports the self-assembly of CsPbBr₃ perovskite nanocrystals (NCs) in one-dimensional (1D) superlattice chains mediated by ligand–solvent interactions. CsPbBr₃ NCs synthesized at ≥170 °C and purified in a nonpolar solvent, hexane, self-assembled into 1D chains, whereas those purified in polar solvents, including toluene and ethyl acetate, were disordered or formed short-range two-dimensional (2D) assemblies. The NCs assembled into 1D chains showed red shifts in both the absorbance and photoluminescence spectra relative to those of disordered NCs purified in a 50/50 hexane/ethyl acetate mixture. Microscopy and X-ray diffraction results confirmed the formation of polymeric nanostrands in hexane followed by organization of the NCs into 1D chains along the nanostrands. Our results suggest that excess aliphatic ligands remaining after purification of the NCs complex with ionic Cs⁺ and Br^– species through a hydrophobic effect; further, the alkyl chains of these ligands interlace with each other through van der Waals forces. Collectively, these interactions give rise to the nanostrands and subsequent self-assembly of CsPbBr₃ into 1D chains. In polar solvents, the minimization of repulsive forces between the solvent and the ligands drives proximal CsPbBr₃ NCs together into short-range 2D assemblies or disordered clusters. Our solvent-assisted self-assembly approach provides a general strategy for designing 1D superlattice chains of nanocrystals of any geometry, dimension, and composition by simply tuning the ligand–solvent interactions

Ultrafast Excited-State Dynamics in Shape- and Composition-Controlled Gold–Silver Bimetallic Nanostructures

In this work, we have examined the ultrafast dynamics of shape- and composition-controlled bimetallic Au/Ag core/shell nanostructures with transient absorption spectroscopy (TAS) as a function of Ag layer thickness (0–15 nm) and pump excitation fluence (50–500 nJ/pulse). Our synthesis approach generated both bimetallic nanocubes and nanopyramids with distinct dipolar plasmon resonances and plasmon dephasing behavior at the resonance. Lifetimes obtained from TAS at low powers (50 nJ/pulse) demonstrated minimal dependence on the Ag layer thickness, whereas at high power (500 nJ/pulse) a rise in electron–phonon coupling lifetime (τ₁) was observed with increasing Ag shell thickness for both nanocubes and nanopyramids. This is attributable to the stronger absorption of the 400 nm pump pulse with higher Ag content, which induced higher electron temperatures. The phonon–phonon scattering lifetime (τ₂) also rises with increasing Ag layer, contributed both by the increasing size of the Au/Ag nanostructures as well as by surface chemistry effects. Further, we observed that even the thinnest, 2 nm, Ag shell strongly impacts both τ₁ and τ₂ at high power despite minimal change in overall size, indicating that the nanostructure composition also strongly impacts the thermalization temperature following absorption of 400 nm light. We also observed a shape-dependent trend at high power, where τ₂ increased for the nanopyramids with increasing Ag shell thickness and nanostructure size, but bimetallic nanocubes demonstrated an unexpected decrease in τ₂ for the thickest, 15 nm, Ag shell. This was attributed to the larger number of corners and edges in the nanocubes relative to the nanopyramids

Engineered Porous Silicon Counter Electrodes for High Efficiency Dye-Sensitized Solar Cells

In this work, we demonstrate for the first time, the use of porous silicon (P-Si) as counter electrodes in dye-sensitized solar cells (DSSCs) with efficiencies (5.38%) comparable to that achieved with platinum counter electrodes (5.80%). To activate the P-Si for triiodide reduction, few layer carbon passivation is utilized to enable electrochemical stability of the silicon surface. Our results suggest porous silicon as a promising sustainable and manufacturable alternative to rare metals for electrochemical solar cells, following appropriate surface modification

Enhanced Efficiency in Dye-Sensitized Solar Cells with Shape-Controlled Plasmonic Nanostructures

In this work, we demonstrate enhanced light harvesting in dye-sensitized solar cells (DSSCs) with gold nanocubes of controlled shape. Silica-coated nanocubes (Au@SiO₂ nanocubes) embedded in the photoanodes of DSSCs had a power conversion efficiency of 7.8% relative to 5.8% of reference (TiO₂ only) devices, resulting in a 34% improvement in DSSC performance. Photocurrent behavior and incident photon to current efficiency spectra revealed that device performance is controlled by the particle density of Au@SiO₂ nanocubes and monotonically decreases at very high nanocube concentration. Finite difference time domain simulations suggest that, at the 45 nm size regime, the nanocubes predominantly absorb incident light, giving rise to the lightning rod effect, which results in intense electromagnetic fields at the edges and corners. These intense fields increase the plasmonic molecular coupling, amplifying the carrier generation and DSSC efficiency

Electrochemical and Corrosion Stability of Nanostructured Silicon by Graphene Coatings: Toward High Power Porous Silicon Supercapacitors

We demonstrate the electrochemical stability of nanostructured silicon in corrosive aqueous, organic, and ionic liquid media enabled by conformal few-layered graphene heterogeneous interfaces. We demonstrate direct gas-phase few-layered graphene passivation (<i>d</i> = 0.35 nm) at temperatures that preserve the structural integrity of the nanostructured silicon. This passivation technique is transferrable both to silicon nanoparticles (Si-NPs) as well as to electrochemically etched porous silicon (P-Si) materials. For Si-NPs, we find the graphene-passivated silicon to withstand physical corrosion in NaOH aqueous conditions where unpassivated Si-NPs spontaneously dissolve. For P-Si, we demonstrate electrochemical stability with widely different electrolytes, including NaOH, enabling these materials for electrochemical supercapacitors. This leads us to develop high-power on-chip porous silicon supercapacitors capable of up to 10 Wh/kg and 65 kW/kg energy and power densities, respectively, and 5 Wh/kg energy density at 35 kW/kgcomparable to many of the best high-power carbon-based supercapacitors. As surface reactivity wholly dictates the utilization of nanoscale silicon in diverse applications across electronics, energy storage, biological systems, energy conversion, and sensing, we emphasize direct formation of few-layered graphene on nanostructured silicon as a means to form heterogeneous on-chip interfaces that can maintain stability in even the most reactive of environments

Pulsed Current for Diameter-Controlled Carbon Nanotubes and Hybrid Carbon Nanostructures in Electrolysis of Captured Carbon Dioxide

Here, we demonstrate how temporally controlled pulses of current can control the physical properties of multiwalled carbon nanotubes (MWCNTs) synthesized from the electrolysis of air-captured carbon dioxide. Our findings demonstrate that a transient 1 min 7.5-fold rate increase of current or carbonate reduction during nucleation of MWCNTs leads to 2.5 times smaller average MWCNT diameters, a higher degree of graphitization in the walls, and an overall 10% lower energy consumption over the full growth duration. Conversely, when identical transient current pulses are applied after MWCNT nucleation in the middle of CNT growth, our findings indicate the deposition of noncatalytic onionlike carbons on the surface of the MWCNTs to form hybrid nanostructured materials, but no changes are observed to MWCNT diameters, energy consumption, or wall graphitization of the MWCNTs. A detailed study of this system by three-electrode cyclic voltammetry, imaging, X-ray diffraction (XRD), and Raman spectroscopy supports the mechanistic role of current pulses in nucleation to facilitate rapid catalyst reduction and minimize coarsening to sustain catalysts with high activity. This work demonstrates how temporally controlled electrochemical current density, and hence carbon flux, in molten carbonate electrolysis is a powerful tool to engineer the production of carbon nanostructures with tailored physical properties and a total energy consumption footprint

Morphology-Directed Catalysis with Branched Gold Nanoantennas

We synthesized multibranched gold nanoantennas (MGNs) of two morphologies by varying the core-to-branch ratio. We compared their efficacy in catalytic reduction of <i>p</i>-nitrophenol (PNP) to <i>p</i>-aminiphenol (PAP). We observed that MGNs with shorter protrusions had a faster induction time and higher apparent rate constant, <i>k</i>_app, for PNP catalysis relative to the MGNs with longer protrusions. By examining the reaction as a function of temperature, we observed significantly lower activation energy for the MGNs with shorter protrusions (80 J/g) compared to MGNs with longer protrusions (200 J/g). The Langmuir–Hinshelwood model was used to fit the change in <i>k</i>_app as a function of increasing [PNP], which demonstrated more efficient PNP adsorption on the surfaces of MGNs with shorter protrusions. For the MGNs with longer protrusions, PNP adsorption is affected by the heterogeneity of the surface sites resulting in a lower adsorption coefficient. We attributed the improved efficiency of the MGNs with shorter protrusions to the presence of {100} and {110} crystal planes, which have a high density of atomic steps and kinks that promote higher catalytic activity for PNP degradation. MGNs with long protrusions are bound by low index {111} facets; the highly coordinated atoms of {111} reduce the adsorption efficiency of PNP

Solution Assembled Single-Walled Carbon Nanotube Foams: Superior Performance in Supercapacitors, Lithium-Ion, and Lithium–Air Batteries

We demonstrate a surfactant-free, solution processing route to form three-dimensional freestanding foams of pristine single-walled carbon nanotubes (SWCNTs) and explore the diverse electrochemical energy storage applications of these materials. This route utilizes SWCNT dispersions in organic <i>n</i>-methylpyrrolidone solvents and subsequent electrophoretic assembly onto a metal foam sacrificial template which can be dissolved to yield surfactant-free, binder-free freestanding SWCNT foams. We further provide a comparison between surfactant-free foams and conventional surfactant-based solvent processing routes and assess performance of these foams in supercapacitors, lithium-ion batteries, and lithium–air batteries. For pristine SWCNT foams, we measure up to 83 F/g specific capacitance in supercapacitors, specific capacity up to 2210 mAh/g for lithium-ion batteries with up to 50% energy efficiency, and specific discharge capacity up to 8275 mAh/g in lithium–air batteries. For lithium–air batteries, this corresponds to a total energy density of 21.2 and 3.3 kWh/kg for the active mass and total battery device, respectively, approaching the 12.7 kWh/kg target energy density of gasoline. In comparison, SWCNT foams prepared with surfactant exhibit poorer gravimetric behavior in all devices and compromised Faradaic storage that leads to depreciated amounts of usable, stored energy. This work demonstrates the broad promise of SWCNTs as lightweight and highly efficient energy storage materials but also emphasizes the importance of clean nanomanufacturing routes which are critical to achieve optimized performance with nanostructures