Tailoring Energy Transfer from Hot Electrons to Adsorbate Vibrations for Plasmon-Enhanced Catalysis

Chemical reactions can be enhanced on surfaces of bimetallic nanoparticles composed of a core plasmonic metal and a catalytically active shell when illuminated with light. However, the atomic-level details of the steps that govern such photochemical reactions are not yet understood. One critical process is the non-adiabatic energy transfer from hot electrons that transiently populate the unoccupied electronic orbitals of the adsorbate to the vibrational modes of the adsorbed reactants. This occurs via electron–vibration coupling and could potentially be tailored by changing the composition of the shell. Here, we apply an <i>ab initio</i> method based on density functional theory to investigate this coupling at various sp- and d-band metal–adsorbate interfaces. Our calculations demonstrate the importance of d-bands in enhancing and tuning this energy transfer at the interface. Further, they highlight specific choices of metals that could be utilized as shells for efficient photochemical reactions. From these calculations, we extract a simple descriptor (dependent on the coupling matrix element and equilibrium bond length) that can account for the coupling strength at a metal–adsorbate interface, thus representing a valuable tool for rational shell design for different reactions. We show the utility of this descriptor for photocatalysis with calculations for a specific photochemical reaction. The introduction of this descriptor should also impact other processes such as light-triggered drug release that exploit hot electrons, and surface-enhanced Raman spectroscopy, where electron–vibration coupling plays a key role

Observation of Thermal Beaming from Tungsten and Molybdenum Bull’s Eyes

Although losses in plasmonic films can be detrimental for optoelectronics, they can be exploited to create novel thermal emitters. Surface plasmon polaritons that are thermally excited on a heated metal surface can be converted to photons with useful properties. We demonstrate highly tailored thermal emission from tungsten and molybdenum films patterned with a series of circular concentric grooves (i.e., a bull’s eye). At 900 °C our structures emit an infrared beam normal to the film that is spectrally narrow (tens of nanometers) and highly directional (∼2° angular divergence). The peak wavelength (3.5 μm) can be tuned with groove periodicity. To enhance the thermal stability of the structures, we add a thin layer of HfO₂. Such devices, with their simple design and low thermal mass, provide interesting incandescent light sources for various applications

Template-Stripped Tunable Plasmonic Devices on Stretchable and Rollable Substrates

We use template stripping to integrate metallic nanostructures onto flexible, stretchable, and rollable substrates. Using this approach, high-quality patterned metals that are replicated from reusable silicon templates can be directly transferred to polydimethylsiloxane (PDMS) substrates. First we produce stretchable gold nanohole arrays and show that their optical transmission spectra can be modulated by mechanical stretching. Next we fabricate stretchable arrays of gold pyramids and demonstrate a modulation of the wavelength of light resonantly scattered from the tip of the pyramid by stretching the underlying PDMS film. The use of a flexible transfer layer also enables template stripping using a cylindrical roller as a substrate. As an example, we demonstrate roller template stripping of metallic nanoholes, nanodisks, wires, and pyramids onto the cylindrical surface of a glass rod lens. These nonplanar metallic structures produced <i>via</i> template stripping with flexible and stretchable films can facilitate many applications in sensing, display, plasmonics, metasurfaces, and roll-to-roll fabrication

Solid-Phase Flexibility in Ag₂Se Semiconductor Nanocrystals

Nanocrystals are known to alter the relative stability of bulk solid phases. Here we test the limits of this effect on Ag₂Se nanocrystals, a promising new electronic and infrared material. In the bulk, Ag₂Se exhibits a solid–solid phase transition to a superionic conducting phase at moderate temperatures. We map this phase transition as a function of size, temperature, and surface treatment in Ag₂Se core-only and core–shell nanocrystals. We show that the transition can be tuned not just below but also above the bulk phase-transition temperature. This phase flexibility has implications for applications in optoelectronic and phase-memory devices

Direct Patterning of Colloidal Quantum-Dot Thin Films for Enhanced and Spectrally Selective Out-Coupling of Emission

We report on a template-stripping method for the direct surface patterning of colloidal quantum-dot thin films to produce highly luminescent structures with feature sizes less than 100 nm. Through the careful design of high quality bull’s-eye gratings we can produce strong directional beaming (10° divergence) with up to 6-fold out-coupling enhancement of spontaneous emission in the surface-normal direction. A transition to narrow single-mode lasing is observed in these same structures at thresholds as low as 120 μJ/cm². In addition, we demonstrate that these structures can be fabricated on flexible substrates. Finally, making use of the size-tunable character of colloidal quantum dots, we demonstrate spectrally selective out-coupling of light from mixed quantum-dot films. Our results provide a straightforward route toward significantly improved optical properties of colloidal quantum-dot assemblies

Fabrication of Smooth Patterned Structures of Refractory Metals, Semiconductors, and Oxides via Template Stripping

The template-stripping method can yield smooth patterned films without surface contamination. However, the process is typically limited to coinage metals such as silver and gold because other materials cannot be readily stripped from silicon templates due to strong adhesion. Herein, we report a more general template-stripping method that is applicable to a larger variety of materials, including refractory metals, semiconductors, and oxides. To address the adhesion issue, we introduce a thin gold layer between the template and the deposited materials. After peeling off the combined film from the template, the gold layer can be selectively removed via wet etching to reveal a smooth patterned structure of the desired material. Further, we demonstrate template-stripped multilayer structures that have potential applications for photovoltaics and solar absorbers. An entire patterned device, which can include a transparent conductor, semiconductor absorber, and back contact, can be fabricated. Since our approach can also produce many copies of the patterned structure with high fidelity by reusing the template, a low-cost and high-throughput process in micro- and nanofabrication is provided that is useful for electronics, plasmonics, and nanophotonics

Near-Field Light Design with Colloidal Quantum Dots for Photonics and Plasmonics

Colloidal quantum-dots are bright, tunable emitters that are ideal for studying near-field quantum-optical interactions. However, their colloidal nature has hindered their facile and precise placement at desired near-field positions, particularly on the structured substrates prevalent in plasmonics. Here, we use high-resolution electro-hydrodynamic printing (<100 nm feature size) to deposit countable numbers of quantum dots on both flat and structured substrates with a few nanometer precision. We also demonstrate that the autofocusing capability of the printing method enables placement of quantum dots preferentially at plasmonic hot spots. We exploit this control and design diffraction-limited photonic and plasmonic sources with arbitrary wavelength, shape, and intensity. We show that simple far-field illumination can excite these near-field sources and generate fundamental plasmonic wave-patterns (plane and spherical waves). The ability to tailor subdiffraction sources of plasmons with quantum dots provides a complementary technique to traditional scattering approaches, offering new capabilities for nanophotonics

Optical Chirality Flux as a Useful Far-Field Probe of Chiral Near Fields

To optimize the interaction between chiral matter and highly twisted light, quantities that can help characterize chiral electromagnetic fields near nanostructures are needed. Here, by analogy with Poynting’s theorem, we formulate the time-averaged conservation law of optical chirality in lossy dispersive media and identify the optical chirality flux as an ideal far-field observable for characterizing chiral optical near fields. Bounded by the conservation law, we show that it provides precise information, unavailable from circular dichroism spectroscopy, on the magnitude and handedness of highly twisted fields near nanostructures

Polarization Multiplexing of Fluorescent Emission Using Multiresonant Plasmonic Antennas

Combining the ability to localize electromagnetic fields at the nanoscale with a directional response, plasmonic antennas offer an effective strategy to shape the far-field pattern of coupled emitters. Here, we introduce a family of directional multiresonant antennas that allows for polarization-resolved spectral identification of fluorescent emission. The geometry consists of a central aperture surrounded by concentric polygonal corrugations. By varying the periodicity of each axis of the polygon individually, this structure can support multiple resonances that provide independent control over emission directionality for multiple wavelengths. Moreover, since each resonant wavelength is directly mapped to a specific polarization orientation, spectral information can be encoded in the polarization state of the out-scattered beam. To demonstrate the potential of such structures in enabling simplified detection schemes and additional functionalities in sensing and imaging applications, we use the central subwavelength aperture as a built-in nanocuvette and manipulate the fluorescent response of colloidal-quantum-dot emitters coupled to the multiresonant antenna

Electronic Impurity Doping in CdSe Nanocrystals

We dope CdSe nanocrystals with Ag impurities and investigate their optical and electrical properties. Doping leads not only to dramatic changes but surprising complexity. The addition of just a few Ag atoms per nanocrystal causes a large enhancement in the fluorescence, reaching efficiencies comparable to core–shell nanocrystals. While Ag was expected to be a substitutional acceptor, nonmonotonic trends in the fluorescence and Fermi level suggest that Ag changes from an interstitial (n-type) to a substitutional (p-type) impurity with increased doping