Chiroptically active quantum nanonails.

In recent years, extensive research efforts have been dedicated to the investigation of CdSe/CdS-based quantum-confined nanostructures, driven by their distinctive properties. The morphologies of these nanostructures have been shown to directly affect their properties, an area which has proven to be an important field of study. Herein, we report a new morphology of CdSe/CdS core-shell heterostructures in the form of a 'nanonail' - a modified nanorod-like morphology, in which a distinctive triangular head can be observed at one end of the structure. In-depth studies of this morphology reveal a material with tuneable rod length and width, as well as exceptional photoluminescent properties. Following this, we have demonstrated the ability to induce chiroptical activity via ligand exchange, revealing the important role of the specific morphology, shell thickness and chiral ligand concentration in the effect of ligand induced chirality. In addition, the cellular uptake and cytotoxicity of obtained chiral nanostructures were evaluated on human lung-derived A549 cancer cells, revealing a significant enantioselectivity in biological activity. Finally, analysis on monolayers of the material demonstrate the complete absence of FRET processes. Overall, this CdSe/CdS heterostructure is another tuneable morphology of a very important nanomaterial, one which shows great advantages and a range of potential applications.

Direct Laser Writing of Silica Nanoparticle Nanocomposites: Probing Mechanical Reinforcement and Understanding Structural Color from Design Parameters.

Nanocomposite materials have been thoroughly exploited in additive manufacturing, as a means to alter physical, chemical, and optical properties of resulting structures. Herein, nanocomposite materials suitable for direct laser writing (DLW) by two-photon polymerization are presented. These materials, comprising silica nanoparticles, bring significant added value to the technology through physical reinforcement and controllable photonic properties. Incorporation into acrylate photoresists, via a one-step fabrication process, enables the formation of complex structures with large overhangs. The inclusion of 150 nm silica nanoparticles in DLW photoresists at high concentrations, allows for the fabrication of composite microstructures that show reflected color, a product of the relative contributions from the quasi-ordering and random scattering. Using common DLW design parameters, such as slicing distance and structure dimension, a wide gamut of structural color, in solution, using a set concentration of nanoparticles is demonstrated. Numerical modeling is employed to predict the reflected wavelength of the pixel arrays, across the visible spectrum, and this information is used to encode reflected colors into different pixel arrays.

Spectral Tuning of a Nanoparticle-on-Mirror System by Graphene Doping and Gap Control with Nitric Acid.

Nanoparticle-on-mirror systems are a stable, robust, and reproducible method of squeezing light into sub-nanometer volumes. Graphene is a particularly interesting material to use as a spacer in such systems as it is the thinnest possible 2D material and can be doped both chemically and electrically to modulate the plasmonic modes. We investigate a simple nanoparticle-on-mirror system, consisting of a Au nanosphere on top of an Au mirror, separated by a monolayer of graphene. With this system, we demonstrate, with both experiments and numerical simulations, how the doping of the graphene and the control of the gap size can be controlled to tune the plasmonic response of the coupled nanosphere using nitric acid. The coupling of the Au nanosphere and Au thin film reveals multipolar modes which can be tuned by adjusting the gap size or doping an intermediate graphene monolayer. At high doping levels, the interaction between the charge-transfer plasmon and gap plasmon leads to splitting of the plasmon energies. The study provides evidence for the unification of theories proposed by previous works investigating similar systems.

High-Figure-of-Merit Biosensing and Enhanced Excitonic Absorption in an MoS2-Integrated Dielectric Metasurface.

Among the transitional metal dichalcogenides (TMDCs), molybdenum disulfide (MoS2) is considered an outstanding candidate for biosensing applications due to its high absorptivity and amenability to ionic current measurements. Dielectric metasurfaces have also emerged as a powerful platform for novel optical biosensing due to their low optical losses and strong near-field enhancements. Once functionalized with TMDCs, dielectric metasurfaces can also provide strong photon-exciton interactions. Here, we theoretically integrated a single layer of MoS2 into a CMOS-compatible asymmetric dielectric metasurface composed of TiO2 meta-atoms with a broken in-plane inversion symmetry on an SiO2 substrate. We numerically show that the designed MoS2-integrated metasurface can function as a high-figure-of-merit (FoM=137.5 RIU-1) van der Waals-based biosensor due to the support of quasi-bound states in the continuum. Moreover, owing to the critical coupling of the magnetic dipole resonances of the metasurface and the A exciton of the single layer of MoS2, one can achieve a 55% enhanced excitonic absorption by this two-port system. Therefore, the proposed design can function as an effective biosensor and is also practical for enhanced excitonic absorption and emission applications.

Electrically Driven Reprogrammable Vanadium Dioxide Metasurface Using Binary Control for Broadband Beam Steering.

Resonant optical phased arrays are a promising way to reach fully reconfigurable metasurfaces in the optical and near-infrared (NIR) regimes with low energy consumption, low footprint, and high reliability. Continuously tunable resonant structures suffer from inherent drawbacks such as low phase range, amplitude-phase correlation, or extreme sensitivity that makes precise control at the individual element level very challenging. We computationally investigate 1-bit (binary) control as a mechanism to bypass these issues. We consider a metasurface for beam steering using a nanoresonator antenna and explore the theoretical capabilities of such phased arrays. A thermally realistic structure based on vanadium dioxide sandwiched in a metal-insulator-metal structure is proposed and optimized using inverse design to enhance its performance at 1550 nm. Continuous beam steering over 90° range is successfully achieved using binary control, with excellent agreement with predictions based on the theoretical first-principles description of phased arrays. Furthermore, a broadband response from 1500 to 1700 nm is achieved. The robustness to the design manufacturing imperfections is also demonstrated. This simplified approach can be implemented to optimize tunable nanophotonic phased array metasurfaces based on other materials or phased shifting mechanisms for various functionalities.

Direct laser writing of vapour-responsive photonic arrays.

Using direct laser writing, arrays of optically responsive ionogel structures were fabricated. To demonstrate their responsive nature, visible colour changes in the presence of different solvent vapours were investigated. This represents a new departure for photonic structural colouration, in which the fabricating structure shows a programmable, controllable, and dynamic stimuli response.

Quasi-Guided Modes in Titanium Dioxide Arrays Fabricated via Soft Nanoimprint Lithography.

Reversible quasi-guided modes (QGMs) are observed in titanium dioxide (TiO2) metasurface arrays fabricated via soft nanoimprint lithography. A TiO2 layer between the nanopillar array and the substrate can facilitate the propagation of QGMs. This layer is porous, allowing for the tuning of the layer properties by incorporating another material. The presence of the QGMs is strongly dependent on the refractive index of the TiO2 layer. QGMs are not supported if the refractive index of the porous TiO2 is too low. It is demonstrated that after depositing R6G on the array QGMs can be observed as very strong and narrow reflectance peaks and transmittance dips. Furthermore, as the second material can penetrate through the pores into the layer it can experience the regions of high field enhancement associated with the QGMs. These results are of interest for a wide range of applications including but not limited to sensing, nonlinear optics, and emission control.

Plasmonic nanodiscs on vanadium dioxide thin films for tunable luminescence enhancement.

We propose an alternative method to dynamically tune luminescence enhancement in the near infrared spectral range using noble metal nanostructures on top of phase change material vanadium dioxide (VO2) thin films. The VO2 phase change is used to tune the nanodisc plasmon resonance providing a luminescence modification mechanism. We employ a model to calculate the emission of quantum emitters, such as dye molecules, in hybrid systems comprising single silver (Ag) nanodiscs on top of a thin layer of VO2. The model considers different dipole orientations and positions with respect to the nanostructure-VO2 film and determines the degree of observable luminescence modification. In the NIR spectral region, the observable photoluminescence of Alexa Dyes in the hybrid systems at room temperature is enhanced by more than a factor of 2.5 as compared to the same system without plasmonic particles. An additional photoluminescence enhancement by more than a factor of 2 can be achieved with the Ag nanodisc-VO2 film systems after the phase transition of the VO2. These systems can be used for tunable luminescence modification and for compensation of thermally induced luminescence quenching. Through optimization of the Ag nanodisc-VO2 film system, luminescence enhancements of up to a factor of 4 can be seen in the metallic VO2 compared to the semiconducting phase and would therefore compensate for a thermal quenching of up to 70% between room temperature and 70° C, rendering the hybrid systems as promising candidates for improved photon management in optoelectronic devices where elevated temperatures minimize the efficiencies of such devices.

Node-Dependent Photoinduced Electron Transfer in Third-Generation 2D MOFs Containing Earth-Abundant Metal Ions.

Five isostructural 2D metal-organic frameworks (MOFs), based on a photoactive CuI metallolinker and mixed mono-/dinuclear secondary building units (SBUs), are reported. The MOFs 1(M) (M = Mn, Co, Cu, Zn, and Cd) exhibit broad absorption across the visible-light spectrum and emission centered at ca. 730 nm. Upon photoexcitation, the rigidity of the framework hinders the pseudo-Jahn-Teller distortion of the metallolinker's excited state, providing efficient intersystem crossing into the triplet state. Rapid luminescence quenching in 1(Cu) and 1(Co) suggests photoinduced electron transfer (PET) to the SBUs, whereas lifetimes of up to 22.2 ns are observed in 1(Zn). The quantum yields relative to the parent photosensitizer (PS) decrease for metal nodes containing transition metal ions with partially occupied d-orbitals but increase for the d10 systems CdII and ZnII by a factor of up to 6. Importantly, the excited state decay rates directly correlate with the occupancy of the [MII(OH2)]x moieties in the MOFs providing nonradiative decay pathways via O-H oscillators. Cyclovoltammetry reveals minor changes in CuI/II oxidation potential, with excited-state reduction potentials for 1(M) rivalling Ru analogues. These results establish bis(diimine)copper(I) photosensitizers as viable metallolinkers for MOFs and present a rare example of an isostructural series obtained from a photosensitive metallolinker.

Influence of Gold Nano-Bipyramid Dimensions on Strong Coupling with Excitons of Monolayer MoS2.

Rabi splitting between the longitudinal plasmon of a gold nano-bipyramid and the A exciton of monolayer MoS2 is observed at room temperature. The dependence of the Rabi splitting on the physical dimensions of the nano-bipyramid is reported. The impact of bipyramid length, aspect ratio, and tip radius on the coupling strength is investigated. The mode volume of the nanoresonator is significantly reduced because of the sharp tips of the bipyramid, and the Rabi splitting increases with tip sharpness. The results also reveal that greater Rabi splitting is observed for larger bipyramids, contrasting with results previously reported for different nanoresonator shapes. This shows, for the first time, how the magnitude of the splitting has a different response for particular nanoresonators when tuning the size, without increasing the number of excitons coupled into the system. The Rabi splitting, at zero energy detuning between plasmon and A exciton, increases from ∼55 meV with a 70 nm-long bipyramid to ∼80 meV with a 100 nm-long bipyramid. The increase in coupling strength with size arises because of increasing confinement of the field enhancement at the bipyramid tip.

Plasmonic Colour Printing by Light Trapping in Two-Metal Nanostructures.

Structural colour generation by nanoscale plasmonic structures is of major interest for non-bleaching colour printing, anti-counterfeit measures and decoration applications. We explore the physics of a two-metal plasmonic nanostructure consisting of metallic nanodiscs separated from a metallic back-reflector by a uniform thin polymer film and investigate the potential for vibrant structural colour in reflection. We demonstrate that light trapping within the nanostructures is the primary mechanism for colour generation. The use of planar back-reflector and polymer layers allows for less complex fabrication requirements and robust structures, but most significantly allows for the easy incorporation of two different metals for the back-reflector and the nanodiscs. The simplicity of the structure is also suitable for scalability. Combinations of gold, silver, aluminium and copper are considered, with wide colour gamuts observed as a function of the polymer layer thickness. The structural colours are also shown to be insensitive to the viewing angle. Structures of copper nanodiscs with an aluminium back-reflector produce the widest colour gamut.

Light-harvesting, 3rd generation RuII/CoII MOF with a large, tubular channel aperture.

A photoactive, hetero-metallic CoII/RuII-based metal-organic framework (MOF) with a large channel aperture, ca. 21 Å, is reported. The photophysical properties of the MOF are derived from the RuII nodes giving rise to emission centred at ca. 620 nm and relatively long triplet 3MLCT lifetimes. In addition to the optical attributes, the 1H-imidazo [4,5-f][1,10]-phenanthroline ligand imparts structural functionality to the MOF which is composed of alternating CoII- and RuII-based nodes of Δ and Λ helicity. The framework maintains its integrity upon activation and shows gas sorption behaviour that is characteristic of mesoporous materials promoting high CO2 sorption capacities and selectivities over N2.

Oxide-mediated recovery of field-effect mobility in plasma-treated MoS2.

Precise tunability of electronic properties of two-dimensional (2D) nanomaterials is a key goal of current research in this field of materials science. Chemical modification of layered transition metal dichalcogenides leads to the creation of heterostructures of low-dimensional variants of these materials. In particular, the effect of oxygen-containing plasma treatment on molybdenum disulfide (MoS2) has long been thought to be detrimental to the electrical performance of the material. We show that the mobility and conductivity of MoS2 can be precisely controlled and improved by systematic exposure to oxygen/argon plasma and characterize the material using advanced spectroscopy and microscopy. Through complementary theoretical modeling, which confirms conductivity enhancement, we infer the role of a transient 2D substoichiometric phase of molybdenum trioxide (2D-MoO x ) in modulating the electronic behavior of the material. Deduction of the beneficial role of MoO x will serve to open the field to new approaches with regard to the tunability of 2D semiconductors by their low-dimensional oxides in nano-modified heterostructures.

Nanoplasmonic Sensing at the Carbon-Bio Interface: Study of Protein Adsorption at Graphitic and Hydrogenated Carbon Surfaces.

Various forms of carbon are known to perform well as biomaterials in a variety of applications and an improved understanding of their interactions with biomolecules, cells, and tissues is of interest for improving and tailoring their performance. Nanoplasmonic sensing (NPS) has emerged as a powerful technique for studying the thermodynamics and kinetics of interfacial reactions. In this work, the in situ adsorption of two proteins, bovine serum albumin and fibrinogen, were studied at carbon surfaces with differing chemical and optical properties using nanoplasmonic sensors. The carbon material was deposited as a thin film onto NPS surfaces consisting of 100 nm Au nanodisks with a localized plasmon absorption peak in the visible region. Carbon films were fully characterized by X-ray photoelectron spectroscopy, atomic force microscopy, and spectroscopic ellipsometry. Two types of material were investigated: amorphous carbon (a-C), with high graphitic content and high optical absorptivity, and hydrogenated amorphous carbon (a-C:H), with low graphitic content and high optical transparency. The optical response of the Au/carbon NPS elements was modeled using the finite difference time domain (FDTD) method, yielding simulated analytical sensitivities that compare well with those observed experimentally at the two carbon surfaces. Protein adsorption was investigated on a-C and a-C:H, and the protein layer thicknesses were obtained from FDTD simulations of the expected response, yielding values in the 1.8-3.3 nm range. A comparison of the results at a-C and a-C:H indicates that in both cases fibrinogen layers are thicker than those formed by albumin by up to 80%.

Ag colloids and arrays for plasmonic non-radiative energy transfer from quantum dots to a quantum well.

Non-radiative energy transfer (NRET) can be an efficient process of benefit to many applications including photovoltaics, sensors, light emitting diodes and photodetectors. Combining the remarkable optical properties of quantum dots (QDs) with the electrical properties of quantum wells (QWs) allows for the formation of hybrid devices which can utilize NRET as a means of transferring absorbed optical energy from the QDs to the QW. Here we report on plasmon-enhanced NRET from semiconductor nanocrystal QDs to a QW. Ag nanoparticles in the form of colloids and ordered arrays are used to demonstrate plasmon-mediated NRET from QDs to QWs with varying top barrier thicknesses. Plasmon-mediated energy transfer (ET) efficiencies of up to ∼25% are observed with the Ag colloids. The distance dependence of the plasmon-mediated ET is found to follow the same d -4 dependence as the direct QD to QW ET. There is also evidence for an increase in the characteristic distance of the interaction, thus indicating that it follows a Förster-like model with the Ag nanoparticle-QD acting as an enhanced donor dipole. Ordered Ag nanoparticle arrays display plasmon-mediated ET efficiencies up to ∼21%. To explore the tunability of the array system, two arrays with different geometries are presented. It is demonstrated that changing the geometry of the array allows a transition from overall quenching of the acceptor QW emission to enhancement, as well as control of the competition between the QD donor quenching and ET rates.

Influence of plasmonic array geometry on energy transfer from a quantum well to a quantum dot layer.

A range of seven different Ag plasmonic arrays formed using nanostructures of varying shape, size and gap were fabricated using helium-ion lithography (HIL) on an InGaN/GaN quantum well (QW) substrate. The influence of the array geometry on plasmon-enhanced Förster resonance energy transfer (FRET) from a single InGaN QW to a ∼80 nm layer of CdSe/ZnS quantum dots (QDs) embedded in a poly(methyl methacrylate) (PMMA) matrix is investigated. It is shown that the energy transfer efficiency is strongly dependent on the array properties and an efficiency of ∼51% is observed for a nanoring array. There were no signatures of FRET in the absence of the arrays. The QD acceptor layer emission is highly sensitive to the array geometry. A model was developed to confirm that the increase in the QD emission on the QW substrate compared with a GaN substrate can be attributed solely to plasmon-enhanced FRET. The individual contributions of direct enhancement of the QD layer emission by the array and the plasmon-enhanced FRET are separated out, with the QD emission described by the product of an array emission factor and an energy transfer factor. It is shown that while the nanoring geometry results in an energy transfer factor of ∼1.7 the competing quenching by the array, with an array emission factor of ∼0.7, results in only an overall gain of ∼14% in the QD emission. The QD emission was enhanced by ∼71% for a nanobox array, resulting from the combination of a more modest energy transfer factor of 1.2 coupled with an array emission factor of ∼1.4.

A theoretical investigation of the influence of gold nanosphere size on the decay and energy transfer rates and efficiencies of quantum emitters.

We present in this contribution a comprehensive investigation of the effect of the size of gold nanospheres on the decay and energy transfer rates of quantum systems placed close to these nanospheres. These phenomena have been investigated before, theoretically and experimentally, but no comprehensive study of the influence of the nanoparticle size on important dependences of the decay and energy transfer rates, such as the dependence on the donor-acceptor spectral overlap and the relative positions of the donor, acceptor, and nanoparticle, exists. As such, different accounts of the energy transfer mechanism have been presented in the literature. We perform an investigation of the energy transfer mechanisms between emitters and gold nanospheres and between donor-acceptor pairs in the presence of the gold nanospheres using a Green's tensor formalism, experimentally verified in our lab. We find that the energy transfer rate to small nanospheres is greatly enhanced, leading to a strong quenching of the emission of the emitter. When the nanosphere size is increased, it acts as an antenna, increasing the emission of the emitter. We also investigate the emission wavelength and intrinsic quantum yield dependence of the energy transfer to the nanosphere. As evidenced from the literature, the energy transfer process between the quantum system and the nanosphere can have a complicated distance dependence, with a r(-6) regime, characteristic of the Förster energy transfer mechanism, but also exhibiting other distance dependences. In the case of a donor-acceptor pair of quantum systems in the presence of a gold nanosphere, when the donor couples strongly to the nanosphere, acting as an enhanced dipole; the donor-acceptor energy transfer rate then follows a Förster trend, with an increased Förster radius. The coupling of the acceptor to the nanosphere has a different distance dependence. The angular dependence of the energy transfer efficiency between donor and acceptor exhibits a strong focusing effect and the same enhanced donor-dipole character in different angular arrangements. The spectral overlap of the donor emission and acceptor absorption spectra shows that the energy transfer follows the near-field scattering efficiency, with a red-shift from the localized surface plasmon peak for small sphere sizes.

Experimental and theoretical investigation of the distance dependence of localized surface plasmon coupled Förster resonance energy transfer.

The distance dependence of localized surface plasmon (LSP) coupled Förster resonance energy transfer (FRET) is experimentally and theoretically investigated using a trilayer structure composed of separated monolayers of donor and acceptor quantum dots with an intermediate Au nanoparticle layer. The dependence of the energy transfer efficiency, rate, and characteristic distance, as well as the enhancement of the acceptor emission, on the separations between the three constituent layers is examined. A d(-4) dependence of the energy transfer rate is observed for LSP-coupled FRET between the donor and acceptor planes with the increased energy transfer range described by an enhanced Förster radius. The conventional FRET rate also follows a d(-4) dependence in this geometry. The conditions under which this distance dependence is valid for LSP-coupled FRET are theoretically investigated. The influence of the placement of the intermediate Au NP is investigated, and it is shown that donor-plasmon coupling has a greater influence on the characteristic energy transfer range in this LSP-coupled FRET system. The LSP-enhanced Förster radius is dependent on the Au nanoparticle concentration. The potential to tune the characteristic energy transfer distance has implications for applications in nanophotonic devices or sensors.

Wavelength, concentration, and distance dependence of nonradiative energy transfer to a plane of gold nanoparticles.

Nonradiative energy transfer to metal nanoparticles is a technique used for optical-based distance measurements which is often implemented in sensing. Both Förster resonant energy transfer (FRET) and nanometal surface energy transfer (NSET) mechanisms have been proposed for emission quenching in proximity to metal nanoparticles. Here quenching of emission of colloidal quantum dots in proximity to a monolayer of gold nanoparticles is investigated. Five differently sized CdTe quantum dots are used to probe the wavelength dependence of the quenching mechanism as their emission peak moves from on resonance to off resonance with respect to the localized surface plasmon peak of the gold nanoparticle layer. The gold nanoparticle concentration and distance dependences of energy transfer are discussed. Photoluminescence quenching and lifetime data are analyzed using both FRET and NSET models and the extracted characteristic distances are compared with theory. Good agreement with FRET theory has been found for quantum dots with emission close to the localized surface plasmon resonance, though larger than expected Förster radii are observed for quantum dots with emission red-shifted with respect to the localized surface plasmon peak. Closer agreement between experimental and theoretical characteristic distances can be found across the full wavelength range within a NSET approach.

Impact of bias current distribution on the noise figure and power saturation of a multicontact semiconductor optical amplifier.

We present an experimental investigation of a multicontact semiconductor optical amplifier. This first-generation device allows for direct control of the carrier density profile along the length of the waveguide. This is used to control the device noise figure, with a minimum value of 5 dB observed at a gain of 15 dB for an optimum carrier density profile. The opposite carrier density profile results in an increase of the power saturation by 3 dB.