Hyperbolic phonon polariton resonances in calcite nanopillars.

We report the first experimental observation of hyperbolic phonon polariton (HP) resonances in calcite nanopillars, demonstrate that the HP modes redshift with increasing aspect ratio (AR = 0.5 to 1.1), observe a new, possibly higher order mode as the pitch is reduced, and compare the results to both numerical simulations and an analytical model. This work shows that a wide variety of polar dielectric materials can support phonon polaritons by demonstrating HPs in a new material, which is an important first step towards creating a library of materials with the appropriate phonon properties to extend phonon polariton applications throughout the infrared.

Surfactant Modulated Phase Transitions of Liquid Crystals Confined in Electrospun Coaxial Fibers.

Confinement of liquid crystals (LCs) in polymeric fibers offers a promising strategy to control liquid crystal response to external stimuli. Here, the confinement of 4-cyano-4'-pentylbiphenyl (5CB), a nematic liquid crystal, within the core of coaxially electrospun fibers composed of poly(vinylpyrrolidone) (PVP) containing different surfactants is discussed. The effects of surfactant type, surfactant concentration, and core flow rate (confinement) on the LC behavior were demonstrated using polarized optical microscopy, scanning electron microscopy, differential scanning calorimetry, Raman, and dielectric spectroscopy. Introduction of surfactant dopants of varying hydrophilic and hydrophobic components into the sheath altered the interfacial interaction between the PVP sheath and the 5CB core of the fibers. Significant effects on the LC nematic to isotropic phase transition were attributed to changes in surface anchoring between the sheath and core. Confinement of nematic LCs in surfactant doped polymeric fibers demonstrates a facile method for tuning LC phase behavior.

Quantification of Efficient Plasmonic Hot-Electron Injection in Gold Nanoparticle-TiO2 Films.

Excitation of localized surface plasmons in metal nanostructures generates hot electrons that can be transferred to an adjacent semiconductor, greatly enhancing the potential light-harvesting capabilities of photovoltaic and photocatalytic devices. Typically, the external quantum efficiency of these hot-electron devices is too low for practical applications (<1%), and the physics underlying this low yield remains unclear. Here, we use transient absorption spectroscopy to quantify the efficiency of the initial electron transfer in model systems composed of gold nanoparticles (NPs) fully embedded in TiO2 or Al2O3 films. In independent experiments, we measure free carrier absorption and electron-phonon decay in the model systems and determine that the electron-injection efficiency from the Au NPs to the TiO2 ranges from about 25% to 45%. While much higher than some previous estimates, the measured injection efficiency is within an upper-bound estimate based on a simple approximation for the Au hot-electron energy distribution. These results have important implications for understanding the achievable injection efficiencies of hot-electron plasmonic devices and show that the injection efficiency can be high for Au NPs fully embedded within a semiconductor with dimensions less than the Au electron mean free path.

Pitch Control of Hexagonal Non-Close-Packed Nanosphere Arrays Using Isotropic Deformation of an Elastomer.

Self-assembly of colloidal nanospheres combined with various nanofabrication techniques produces an ever-increasing range of two-dimensional (2D) ordered nanostructures, although the pattern periodicity is typically bound to the original interparticle spacing. Deformable soft lithography using controlled deformation of elastomeric substrates and subsequent contact printing transfer offer a versatile method to systematically control the lattice spacing and arrangements of the 2D nanosphere array. However, the anisotropic nature of uniaxial and biaxial stretching as well as the strain limit of solvent swelling makes it difficult to create well-separated, ordered 2D nanosphere arrays with large-area hexagonal arrangements. In this paper, we report a simple, facile approach to fabricate such arrays of polystyrene nanospheres using a custom-made radial stretching apparatus. The maximum stretchability and spatial uniformity of the poly(dimethylsiloxane) (PDMS) elastomeric substrate is systematically investigated. A pitch increase as large as 213% is demonstrated using a single stretching-and-transfer process, which is at least 3 times larger than the maximum pitch increase achievable using a single swelling-and-transfer process. Unlike the colloidal arrays generated by the uniaxial and biaxial stretching, the isotropic expansion of radial stretching allows the hexagonal array to retain its original structure across the entire substrate. Upon radial strain applied to the PDMS sheet, the nanosphere array with modified pitch is transferred to a variety of target substrates, exhibiting different optical behaviors and serving as an etch mask or a template for molding.

Fabrication of metallic nanodisc hexagonal arrays using nanosphere lithography and two-step lift-off.

Nanosphere lithography (NSL) has been widely used as an inexpensive method to create periodic arrays of metallic nanoparticles or nanodiscs on substrates. However, most nanodisc arrays derived from a NSL template are restricted to hexagonally-ordered triangular arrays because the metal layer is deposited onto the interstices between the nanospheres. Metallic nanodisc arrays with the same arrangement as the original nanosphere array have been rarely reported. Here, we demonstrate a facile, low-cost method to fabricate large-area hexagonal arrays of metallic nanodiscs using an NSL template combined with a two-step lift-off process. We employ a bi-layer of two dissimilar metals to create a re-entrant sidewall profile to undercut the sacrificial layer and facilitate the final lift-off of the metallic nanodiscs. The quality of the nanodisc pattern and the array periodicity is determined using statistical image analysis and compared to the original nanosphere array in terms of size distribution, surface smoothness, and array pitch. This nanodisc array is used as an etch mask to create a vertically-aligned Si nanowire array. This combined approach is a scalable and inexpensive fabrication method for creating relatively large-area, ordered arrays of various nanostructures.

Manipulating coupling between a single semiconductor quantum dot and single gold nanoparticle.

Using atomic force microscopy nanomanipulation, we position a single Au nanoparticle near a CdSe/ZnS quantum dot to construct a hybrid nanostructure with variable geometry. The coupling between the two particles is varied in a systematic and reversible manner. The photoluminescence lifetime and blinking of the same quantum dot are measured before and after assembly of the structure. In some hybrid structures, the total lifetime is reduced from about 30 ns to well below 1 ns. This dramatic change in lifetime is accompanied by the disappearance of blinking as the nonradiative energy transfer from the CdSe/ZnS quantum dot to the Au nanoparticle becomes the dominant decay channel. Both total lifetime and photoluminescence intensity changes are well described by simple analytical calculations.

Surface- and Structural-Dependent Reactivity of Titanium Oxide Nanostructures with 2-Chloroethyl Ethyl Sulfide under Ambient Conditions.

Robust materials capable of heterogeneous reactivity are valuable for addressing toxic chemical clean up. Synthetic manipulations for generating titanium oxide nanomaterials have been utilized to alter both photochemical (1000 nm > λ > 400 nm) and chemical heterogeneous reactivity with 2-chloroethyl ethyl sulfide (2-CEES). Synthesizing TiO2 nanomaterials in the presence of long-chain alkylphosphonic acids enhanced the visible light-driven oxidation of the thioether sulfur of 2-CEES. Photooxidation reaction rates of 99 and 168 µmol/g/h (quantum yields of 5.07 × 10-4 and 8.58 × 10-4 molecules/photon, respectively) were observed for samples made with two different alkylphosphonic acids (C14H29PO3H2 and C9H19PO3H2, respectively). These observations are correlated with (i) generation of new surface defects/states (i.e., oxygen vacancies) as a result of TiO2 grafting by alkylphosphonic acid that may serve as reaction active sites, (ii) better light absorption by assemblies of nanorods and nanowires in comparison to individual nanorods, (iii) surface area differences, and (iv) the exclusion of OH groups due to the surface functionalization with alkylphosphonic acids via Ti-O-P bonds on the TiO2. Alternatively, nanowire-form H2Ti2O5·H2O was produced and found to be capable of highly efficient hydrolysis of the carbon-chlorine (C-Cl) bond of 2-CEES in the dark with a reaction rate of 279.2 µmol/g/h due to the high surface area and chemical nature of the titanate structure.

Controlled AFM manipulation of small nanoparticles and assembly of hybrid nanostructures.

We demonstrate controlled manipulation of semiconductor and metallic nanoparticles (NPs) with 5-15 nm diameters and assemble these NPs into hybrid structures. The manipulation is accomplished under ambient environment using a commercial atomic force microscope (AFM). There are particular difficulties associated with manipulating NPs this small. In addition to spatial drift, the shape of an asymmetric AFM tip has to be taken into account in order to understand the intended and actual manipulation results. Furthermore, small NPs often attach to the tip via electrostatic interaction and modify the effective tip shape. We suggest a method for detaching the NPs by performing a pseudo-manipulation step. Finally, we show by example the ability to assemble these small NPs into prototypical hybrid nanostructures with well-defined composition and geometry.

Propagating surface plasmon induced photon emission from quantum dots.

We investigate the interaction between propagating surface plasmons in silver nanowires and excitons generated in quantum dots. We show propagating surface plasmons can excite excitons, which results in quantum dot emission. In this process, the energy is directly transferred from the propagating surface plasmons to the excitons without converting to photons. Furthermore, we demonstrate the reverse process where the decay of excitons generates surface plasmons.

Si Nanocrystals/ZnO Nanowires Hybrid Structures as Immobilized Photocatalysts for Photodegradation.

Numerous semiconductor-based hybrid nanostructures have been studied for improved photodegradation performance resulting from their broadband optical response and enhanced charge separation/transport characteristics. However, these hybrid structures often involve elements that are rare or toxic. Here, we present the synthesis and material characterization of hybrid nanostructures consisting of zinc oxide (ZnO) nanowires (NWs) and silicon nanocrystals (Si-NCs), both abundant and environmentally benign, and evaluate them for photodegradation performance under various illumination conditions. When incorporating Si-NCs into the vertically-aligned ZnO NWs immobilized on substrates, the resulting photocatalysts exhibited a narrowed band gap, i.e., more responsive to visible light, and enhanced charge separation at the interface, i.e., more reactive species produced for degradation. Consequently, the hybrid Si-NCs/ZnO-NWs displayed a superior photodegradability for methylene blue under UV and white light in comparison to the pristine ZnO NWs. Based on the optical measurements, we hypothesize the band structures of Si-NCs/ZnO-NWs and the potential mechanism for the improved photodegradability.

Enabling remote quantum emission in 2D semiconductors via porous metallic networks.

Here we report how two-dimensional crystal (2DC) overlayers influence the recrystallization of relatively thick metal films and the subsequent synergetic benefits this provides for coupling surface plasmon-polaritons (SPPs) to photon emission in 2D semiconductors. We show that annealing 2DC/Au films on SiO2 results in a reverse epitaxial process where initially nanocrystalline Au films gain texture, crystallographically orient with the 2D crystal overlayer, and form an oriented porous metallic network (OPEN) structure in which the 2DC can suspend above or coat the inside of the metal pores. Both laser excitation and exciton recombination in the 2DC semiconductor launch propagating SPPs in the OPEN film. Energy in-/out- coupling occurs at metal pore sites, alleviating the need for dielectric spacers between the metal and 2DC layer. At low temperatures, single-photon emitters (SPEs) are present across an OPEN-WSe2 film, and we demonstrate remote SPP-mediated excitation of SPEs at a distance of 17 µm.

Plasmon-Induced Charge Transfer: Challenges and Outlook.

The decay of a surface plasmon, a collective electron oscillation at the surface of a metal, can generate hot charge carriers that may be transferred to an adjacent semiconductor. This plasmon-induced charge transfer process can be used to enhance photocatalysis, to create photodetectors, or to drive selective photochemistry. However, the charge transfer efficiency in many fabricated devices remains too low for practical applications, typically <1%. In this Perspective, I discuss critical aspects of designing plasmonic systems for improved performance and highlight important findings for maximizing the transfer efficiency. In particular, I draw attention to the article by Ma and Gao in this issue of ACS Nano that describes using real-time time-dependent density functional theory to give a detailed and informative look at the charge transfer dynamics at a TiO2-Ag nanocluster interface.

Silicon nanowire arrays for the preconcentration and separation of trace explosives vapors.

Silicon nanowire (SiNW) arrays are demonstrated as a suitable platform for the preconcentration of trace nitroaromatic compounds and subsequent desorption via Joule heating of the array. Arrays are fabricated from Si wafers containing an epitaxially grown layer of low conductivity intrinsic Si sandwiched between layers of high conductivity p-type Si. Passage of current through the nanowires results in nanowire temperatures in excess of 200 °C during heating of the arrays as verified by using the temperature-dependent shift of the Si Raman band at ˜520 cm-1. Analyte vapor preconcentration and partial separation is achieved on the array at analyte concentrations nearly two orders-of-magnitude below saturated vapor concentrations at room temperature. The effects of desorption carrier gas flow rate and temperature on the ability to preconcentrate and resolve the analytes of interest are determined. 2,6-dinitrotoluene (2,6-DNT) and 2,4-dinitrotoluene (2,4-DNT) were detected at nominal vapor concentrations of 800 pptv with a 1 min sample time (1.1 ng nominal mass load) and trinitrotoluene (TNT) was detected at a nominal vapor concentration of 65 pptv with a 10 min sample time (1.1 ng nominal mass load).

Controlling the Infrared Dielectric Function through Atomic-Scale Heterostructures.

Surface phonon polaritons (SPhPs), the surface-bound electromagnetic modes of a polar material resulting from the coupling of light with optic phonons, offer immense technological opportunities for nanophotonics in the infrared (IR) spectral region. However, once a particular material is chosen, the SPhP characteristics are fixed by the spectral positions of the optic phonon frequencies. Here, we provide a demonstration of how the frequency of these optic phonons can be altered by employing atomic-scale superlattices (SLs) of polar semiconductors using AlN/GaN SLs as an example. Using second harmonic generation (SHG) spectroscopy, we show that the optic phonon frequencies of the SLs exhibit a strong dependence on the layer thicknesses of the constituent materials. Furthermore, new vibrational modes emerge that are confined to the layers, while others are centered at the AlN/GaN interfaces. As the IR dielectric function is governed by the optic phonon behavior in polar materials, controlling the optic phonons provides a means to induce and potentially design a dielectric function distinct from the constituent materials and from the effective-medium approximation of the SL. We show that atomic-scale AlN/GaN SLs instead have multiple Reststrahlen bands featuring spectral regions that exhibit either normal or extreme hyperbolic dispersion with both positive and negative permittivities dispersing rapidly with frequency. Apart from the ability to engineer the SPhP properties, SL structures may also lead to multifunctional devices that combine the mechanical, electrical, thermal, or optoelectronic functionality of the constituent layers. We propose that this effort is another step toward realizing user-defined, actively tunable IR optics and sources.

Surface plasmon polariton-induced hot carrier generation for photocatalysis.

Non-radiative plasmon decay in noble metals generates highly energetic carriers under visible light irradiation, which opens new prospects in the fields of photocatalysis, photovoltaics, and photodetection. While localized surface plasmon-induced hot carrier generation occurs in diverse metal nanostructures, inhomogeneities typical of many metal-semiconductor plasmonic nanostructures hinder predictable control of photocarrier generation and therefore reproducible carrier-mediated photochemistry. Here, we generate traveling surface plasmon polaritons (SPPs) at the interface between a noble metal/titanium dioxide (TiO2) heterostructure film and aqueous solution, enabling simultaneous optical and electrochemical interrogation of plasmon-mediated chemistry in a system whose resonance may be continuously tuned via the incident optical excitation angle. To the best of our knowledge, this is the first experimental demonstration of SPP-induced hot carrier generation for photocatalysis. We found electrochemical photovoltage and photocurrent responses as SPP-induced hot carriers drive both solution-based oxidation of methanol and the anodic half-reaction of photoelectrochemical water-splitting in sodium hydroxide solution. A strong excitation angle dependence and linear power dependence in the electrochemical photocurrent confirm that the photoelectrochemical reactions are SPP-driven. SPP-generated hot carrier chemistry was recorded on gold and silver and with two different excitation wavelengths, demonstrating potential for mapping resonant charge transfer processes with this technique. These results will provide the design criteria for a metal-semiconductor hybrid system with enhanced hot carrier generation and transport, which is important for the understanding and application of plasmon-induced photocatalysis.

Influence of inhomogeneous porosity on silicon nanowire Raman enhancement and leaky mode modulated photoluminescence.

Metal-assisted chemical etching (MACE) offers an inexpensive, massively parallel fabrication process for producing silicon nanowires (SiNWs). These nanowires can possess a degree of porosity depending on etch conditions. Because the porosity is often spatially inhomogeneous, there is a need to better understand its nature if applications exploiting the porosity are to be pursued. Here, the resolution afforded by micro-Raman and micro-photoluminescence (PL) is used to elucidate the effects of porosity heterogeneity on the optical properties of individual SiNWs produced in large arrays with MACE, while also determining the spatial character of the heterogeneity. For highly porous SiNWs, there is a dramatic reduction in Raman signal and an increase in PL near the SiNW tips. PL spectra collected along the SiNW length exhibit peaks due to leaky mode resonances. Analysis of the PL resonance peaks, Raman spectrum line shape, SEM images, and EDS spectra indicate that the SiNWs possess both radial and axial heterogeneity wherein, from base to SiNW tip, the SiNWs comprise a shell of increasingly thick porous Si surrounding a tapering core of bulk Si. This work describes how structural porosity variation shapes SiNW optical properties, which will influence the design of new SiNW-based photonic devices and chemical/biological sensors.

Modular assembly of optical nanocircuits.

A key element enabling the microelectronic technology advances of the past decades has been the conceptualization of complex circuits with versatile functionalities as being composed of the proper combination of basic 'lumped' circuit elements (for example, inductors and capacitors). In contrast, modern nanophotonic systems are still far from a similar level of sophistication, partially because of the lack of modularization of their response in terms of basic building blocks. Here we demonstrate the design, assembly and characterization of relatively complex photonic nanocircuits by accurately positioning a number of metallic and dielectric nanoparticles acting as modular lumped elements. The nanoparticle clusters produce the desired spectral response described by simple circuit rules and are shown to be dynamically reconfigurable by modifying the direction or polarization of impinging signals. Our work represents an important step towards extending the powerful modular design tools of electronic circuits into nanophotonic systems.

Atomic force microscope nanomanipulation with simultaneous visual guidance.

Atomic force microscopy (AFM) has been used to assemble prototype nanostructures consisting of colloidal nanoparticles. In the standard manipulation protocol, the AFM is used either as a manipulation tool or an imaging tool, but not both at the same time. We developed a new nanomanipulation protocol in which simultaneous visual guidance is obtained during manipulation. As an example, Au nanoparticles were manipulated on a substrate in two steps. First, a nanoparticle is kicked with the z feedback off. This kicking event reduces the static friction. Second, the nanoparticle is dribbled to a target position in tapping mode, and visual guidance is provided by a ghost trace of the nanoparticle. The new manipulation protocol greatly improves efficiency of manipulating small nanoparticles (15 nm in diameter or smaller). Our work highlights the importance and challenges of understanding friction at the nanoscale.