Thermal Activation of Anti-Stokes Photoluminescence in CsPbBr3 Perovskite Nanocrystals: The Role of Surface Polaron States.

Optically driven cooling of a material, or optical refrigeration, is possible when optical up-conversion via anti-Stokes photoluminescence (ASPL) is achieved with near-unity quantum yield. The recent demonstration of optical cooling of CsPbBr3 perovskite nanocrystals (NCs) has provided a path forward in the development of semiconductor-based optical refrigeration strategies. However, the mechanism of ASPL in CsPbBr3 NCs is not yet settled, and the prospects for cooling technologies strongly depend on details of the mechanism. By analyzing the Arrhenius behavior of ASPL in CsPbBr3 NCs, we investigated the relationship between the average energy gained per photon during up conversion, ΔE, and the thermal activation energy, Ea. We find that Ea is systematically larger than ΔE, and that Ea increases for larger ΔE. We suggest that the additional energetic cost is due to a rearrangement of the crystal lattice as charge carriers pass from surface localized, structurally distinct sub-gap polaron states to the free exciton state during up-conversion. Our interpretation is further corroborated by quantifying the impact of ligand coverage on the NC surface. These findings help inform the development of CsPbBr3 NCs for applications in optical refrigeration.

Chemical and Structural Stability of CsPbX3 Nanorods during Postsynthetic Anion-Exchange: Implications for Optoelectronic Functionality.

We examine halide anion-exchange reactions on CsPbX3 nanorods (NRs), and we identify reaction conditions that provide complete anion exchange while retaining both the highly quantum-confined 1-D morphology and metastable crystal lattice configurations that span a range between tetragonal structures and thermodynamically preferred orthorhombic structures. We find that the chemical stability of CsPbBr3 NRs is degraded by the presence of alkyl amines that etch CsPbBr3 and result in the formation of Cs4PbBr6 and 2-D bromoplumbates. Our study outlines strategies for maintaining metastable states of the soft lattices of perovskite nanocrystals undergoing exchange reactions, despite the thermodynamic driving force toward more stable lattice configurations during this disruptive chemical transformation. These strategies can be used to fine-tune the band gap of LHP-based nanostructures while preserving structure-property relationships that are contingent on metastable shapes and crystal configurations, aiding optoelectronic applications of these materials.

Promoting solution-phase superlattices of CsPbBr3 nanocrystals.

We present a size-selective method for purifying and isolating perovskite CsPbBr3 nanocrystals (NCs) that preserves their as-synthesized surface chemistry and extremely high photoluminescence quantum yields (PLQYs). The isolation procedure is based on the stepwise evaporation of nonpolar co-solvents with high vapor pressure to promote precipitation of a size-selected product. As the sample fractions become more uniform in size, we observe that the NCs self-assemble into colloidally stable, solution-phase superlattices (SLs). Small angle X-ray scattering (SAXS) and dynamic light scattering (DLS) studies show that the solution-phase SLs contain 1000s of NCs per supercrystal in a simple cubic, face-to-face packing arrangement. The SLs also display systematically faster radiative decay dynamics and improved PLQYs, as well as unique spectral absorption features likely resulting from inter-particle electronic coupling effects. This study is the first demonstration of solution-phase CsPbBr3 SLs and highlights their potential for achieving collective optoelectronic phenomena previously observed from solid-state assemblies.

Mechanisms of Photothermalization in Plasmonic Nanostructures: Insights into the Steady State.

Localized surface plasmon resonances (LSPRs) in metallic nanostructures result in subwavelength optical confinement that enhances light-matter interactions, for example, aiding the sensitivity of surface spectroscopies. The dissipation of surface plasmons as electronic and vibrational excitations sets the limit for field confinement but also provides opportunities for photochemistry, photocatalysis, and photothermal heating. Optimization for either goal requires a deeper understanding of this photothermalization process. In this review, we focus on recent insights into the physics and dynamics governing photothermalization of LSPRs in metallic nanostructures, emphasizing comparisons between the steady-state behavior and ultrafast time-resolved studies. The differences between these regimes inform how to best optimize plasmonic systems for applications under relatively low-intensity, continuous illumination (e.g., sunlight).

The Anisotropic Complex Dielectric Function of CsPbBr3 Perovskite Nanorods Obtained via an Iterative Matrix Inversion Method.

Colloidal lead halide perovskite nanorods have recently emerged as promising optoelectronic materials. However, more information about how shape anisotropy impacts their complex dielectric function is required to aid the development of applications that take advantage of the strongly polarized absorption and emission. Here, we have determined the anisotropy of the complex dielectric function of CsPbBr3 nanorods by analyzing the ensemble absorption spectra in conjunction with the ensemble spectral fluorescence anisotropy. This strategy allows us to distinguish the absorption of light parallel and perpendicular to the main axis so that the real and imaginary components of the dielectric function along each direction can be determined by the use of an iterative matrix inversion (IMI) methodology. We find that quantum confinement gives rise to unique axis-dependent electronic features in the dielectric function that increase the overall fluorescence anisotropy in addition to the optical anisotropy that results from particle shape, even in the absence of quantum confinement. Further, the procedure outlined here provides a strategy for obtaining anisotropic complex dielectric functions of colloidal materials of varying composition and aspect ratios using ensemble solution-phase spectroscopy.

Active Tuning of Plasmon Damping via Light Induced Magnetism.

Circularly polarized optical excitation of plasmonic nanostructures causes coherent circulating motion of their electrons, which in turn gives rise to strong optically induced magnetization, a phenomenon known as the inverse Faraday effect (IFE). In this study we report how the IFE also significantly decreases plasmon damping. By modulating the optical polarization state incident on achiral plasmonic nanostructures from linear to circular, we observe reversible increases of reflectance by up to 8% and simultaneous increases of optical field concentration by 35.7% under 109 W/m2 continuous wave (CW) optical excitation. These signatures of decreased plasmon damping were also monitored in the presence of an external magnetic field (0.2 T). We rationalize the observed decreases in plasmon damping in terms of the Lorentz forces acting on the circulating electron trajectories. Our results outline strategies for actively modulating intrinsic losses in the metal via optomagnetic effects encoded in the polarization state of incident light.

Kinetic Control for Continuously Tunable Lattice Parameters, Size, and Composition during CsPbX3 (X = Cl, Br, I) Nanorod Synthesis.

The fast kinetics of all-inorganic CsPbX3 (X = Cl, Br, or I) nanocrystal growth entail that many synthetic strategies for structural control established in other semiconductor systems do not apply. Rather, products are often determined by thermodynamic factors, limiting the range of synthetic outcomes and functionality. In this study, we show how reaction kinetics are significantly slowed if nanocrystals are prepared using a dual injection strategy that moderates the crucial interaction between cesium and halide during nucleation and growth. The result is highly uniform nanorod or cuboid nanocrystals with a controllable size and aspect ratio across the quantum confinement regime, obtainable for both pure and mixed halide compositions. Further, the crystal lattice is continuously tunable between the tetragonal (I4/mcm) and orthorhombic (Pbnm) phases, independent of the overall nanorod morphology, enabling significantly more sophisticated structure-property relationships that can be tailored during this kinetically controlled synthesis.

Quantifying Order during Field-Driven Alignment of Colloidal Semiconductor Nanorods.

Aligning large populations of colloidal nanorods (NRs) into ordered assemblies provides a strategy for engineering macroscopic functional materials with strong optical anisotropy. The bulk optical properties of such systems depend not only on the individual NR building blocks but also on their meso- and macroscale ordering, in addition to more complex interparticle coupling effects. Here, we investigate the dynamic alignment of colloidal CdSe/CdS NRs in the presence of AC electric fields by measuring concurrent changes in optical transmission. Our work identifies two distinct scales of interaction that give rise to the field-driven optical response: (1) the spontaneous mesoscale self-assembly of colloidal NRs into structures with increased optical anisotropy and (2) the macroscopic ordering of NR assemblies along the direction of the applied AC field. By modeling the alignment of NR ensembles using directional statistics, we experimentally quantify the maximum degree of order in terms of the average deviation angle relative to the field axis. Results show a consistent improvement in alignment as a function of NR concentration─with a minimum average deviation of 36.2°â”€indicating that mesoscale assembly helps facilitate field-driven alignment of colloidal NRs.

Using Patterned Self-Assembled Monolayers to Tune Graphene-Substrate Interactions.

Graphene has unique mechanical, electronic, and optical properties that make it of interest for an array of applications. These properties can be modulated by controlling the architecture of graphene and its interactions with surfaces. Self-assembled monolayers (SAMs) can tailor graphene-surface interactions; however, spatially controlling these interactions remains a challenge. Here, we blend colloidal lithography with varying SAM chemistries to create patterned architectures that modify the properties of graphene based on its chemical interactions with the substrate and to study how these interactions are spatially arrayed. The patterned systems and their resulting structural, nanomechanical, and optical properties have been characterized using atomic force microscopy, Raman and infrared spectroscopies, scattering-type scanning near-field optical microscopy, and X-ray photoelectron spectroscopy.

Markov chains for modeling complex luminescence, absorption, and scattering in nanophotonic systems.

We developed a method to model fluorescence, absorption, and scattering in nanophotonic systems using ergodic Markov chains. Past works have used absorbing Markov chains to find the long-run angle-dependent distribution of emitted photons. In contrast, we use ergodic Markov chains to focus on the steady state distribution of photons within various media, giving additional insight into the macroscopic optical response during illumination. We show that the method reproduces Beer-Lambert's Law and Kirchhoff's Law, and can quantify deviations from these laws when their assumptions are violated. We also use the method to model luminescent solar concentrators (LSCs) based on semiconductor nanocrystals.

Angle-independent plasmonic substrates for multi-mode vibrational strong coupling with molecular thin films.

Vibrational strong coupling of molecules to optical cavities based on plasmonic resonances has been explored recently because plasmonic near-fields can provide strong coupling in sub-diffraction limited volumes. Such field localization maximizes coupling strength, which is crucial for modifying the vibrational response of molecules and, thereby, manipulating chemical reactions. Here, we demonstrate an angle-independent plasmonic nanodisk substrate that overcomes limitations of traditional Fabry-Pérot optical cavities because the design can strongly couple with all molecules on the surface of the substrate regardless of molecular orientation. We demonstrate that the plasmonic substrate provides strong coupling with the C=O vibrational stretch of deposited films of PMMA. We also show that the large linewidths of the plasmon resonance allow for simultaneous strong coupling to two, orthogonal water symmetric and asymmetric vibrational modes in a thin film of copper sulfate monohydrate deposited on the substrate surface. A three-coupled-oscillator model is developed to analyze the coupling strength of the plasmon resonance with these two water modes. With precise control over the nanodisk diameter, the plasmon resonance is tuned systematically through the modes, with the Rabi splitting from both modes varying as a function of the plasmon frequency and with strong coupling to both modes achieved simultaneously for a range of diameters. This work may aid further studies into manipulation of the ground-state chemical landscape of molecules by perturbing multiple vibrational modes simultaneously and increasing the coupling strength in sub-diffraction limited volumes.

Optically Cooling Cesium Lead Tribromide Nanocrystals.

One photon up-conversion photoluminescence is an optical phenomenon whereby the thermal energy of a fluorescent material increases the energy of an emitted photon compared with the energy of the photon that was absorbed. When this occurs with near unity efficiency, the emitting material undergoes a net decrease in temperature, so-called optical cooling. Because the up-conversion mechanism is thermally activated, the yield of up-converted photoluminescence is also a reporter of the temperature of the emitter. Taking advantage of this optical signature, cesium lead trihalide nanocrystals are shown to cool during the up-conversion of 532 nm CW laser excitation. Raman thermometric analysis of a substrate on which the nanocrystals were deposited further verifies the decrease in the local environmental temperature by as much as 25 °C during optical pumping. This is the first demonstration of optical cooling driven by colloidal semiconductor nanocrystal up-conversion.

Size- and temperature-dependent photoluminescence spectra of strongly confined CsPbBr3 quantum dots.

Lead-halide perovskite nanocrystals (NCs) are receiving much attention as a potential high-quality source of photons due to their superior luminescence properties in comparison to other semiconductor NCs. To date, research has focused mostly on NCs with little or no quantum confinement. Here, we measured the size- and temperature-dependent photoluminescence (PL) from strongly confined CsPbBr3 quantum dots (QDs) with highly uniform size distributions, and examined the factors determining the evolution of the energy and linewidth of the PL with varying temperature and QD size. Compared to the extensively studied II-VI QDs, the spectral position of PL from CsPbBr3 QDs shows an opposite dependence on temperature, with weaker dependence overall. On the other hand, the PL linewidth is much more sensitive to the temperature and size of the QDs compared to II-VI QDs, indicating much stronger coupling of excitons to the vibrational degrees of freedom both in the lattice and at the surface of the QDs.

Comparing steady state photothermalization dynamics in copper and gold nanostructures.

Metal nanostructures have been the focus of several recent studies due to their ability to generate high energy, non-equilibrium "hot" electrons for use in photochemical and photocatalytic applications. In particular, there is growing interest to understand how differences in the electronic structure and optical response of different metals may impact the behavior and utility of their hot electrons in chemical reactions. Using a continuous wave anti-Stokes Raman spectroscopy technique recently developed in our laboratory, in this study, we measured the temperature and lifetime of hot electrons in gold and copper nanostructures in order to understand how the choice of metal impacts hot electron dynamics during steady state illumination. We found that hot electrons in copper are more abundant and more reactive than those in gold, suggesting that copper nanostructures may be a more promising platform for performing hot electron photochemistry.

The role of gold oxidation state in the synthesis of Au-CsPbX3 heterostructure or lead-free Cs2AuIAuIIIX6 perovskite nanoparticles.

Here, we report that the oxidation state of gold plays a dominant role in determining the reaction products when gold halide salts are mixed with all-inorganic lead halide perovskite nanocrystals. When CsPbX3 nanocrystals react with Au(i) halide salts, Au nanoparticles are deposited on the surface of the perovskites through the reduction of Au1+ ions by the surfactant ligand shell, to produce Au-CsPbX3 heterostructures. These heterostructures preserve comparably high photoluminescence quantum yield (PLQY) and show identical XRD diffractograms as the parent CsPbX3 nanocrystals. In contrast, the reaction of CsPbX3 nanocrystals with Au(iii) halide salts promotes complete cation exchange of Pb ions by Au ions in the nanocrystal perovskite lattice. The cation exchange products, Cs2AuIAuIIIBr6 or Cs2AuIAuIIICl6, show XRD patterns corresponding to a tetragonal mixed halide perovskite crystal structure and show no visible photoluminescence. This crucial dependence on the oxidation state of the Au ion informs synthetic strategies for producing and optimizing metal-perovskite heterostructures and lead-free perovskite nanoparticles.

Nanoscale Artificial Plasmonic Lattice in Self-Assembled Vertically Aligned Nitride-Metal Hybrid Metamaterials.

Nanoscale metamaterials exhibit extraordinary optical properties and are proposed for various technological applications. Here, a new class of novel nanoscale two-phase hybrid metamaterials is achieved by combining two major classes of traditional plasmonic materials, metals (e.g., Au) and transition metal nitrides (e.g., TaN, TiN, and ZrN) in an epitaxial thin film form via the vertically aligned nanocomposite platform. By properly controlling the nucleation of the two phases, the nanoscale artificial plasmonic lattices (APLs) consisting of highly ordered hexagonal close packed Au nanopillars in a TaN matrix are demonstrated. More specifically, uniform Au nanopillars with an average diameter of 3 nm are embedded in epitaxial TaN platform and thus form highly 3D ordered APL nanoscale metamaterials. Novel optical properties include highly anisotropic reflectance, obvious nonlinear optical properties indicating inversion symmetry breaking of the hybrid material, large permittivity tuning and negative permittivity response over a broad wavelength regime, and superior mechanical strength and ductility. The study demonstrates the novelty of the new hybrid plasmonic scheme with great potentials in versatile material selection, and, tunable APL spacing and pillar dimension, all important steps toward future designable hybrid plasmonic materials.

The role of mid-gap states in all-inorganic CsPbBr3 nanoparticle one photon up-conversion.

CsPbBr3 nanoparticles that have been treated with NH4SCN to produce essentially trap-free surfaces show an increase of one photon up-conversion quantum yield with little change to the up-conversion energy of activation, as estimated using an Arrhenius analysis. This suggests that mid-gap trap states constitute a loss pathway, and may not be integral to the one-photon excitation mechanism.

Using Particle Lithography to Tailor the Architecture of Au Nanoparticle Plasmonic Nanoring Arrays.

The facile assembly of metal nanostructured arrays is a fundamental step in the design of plasmon enhanced chemical sensing and solar cell architectures. Here we have investigated methods of creating controlled formations of two-dimensional periodic arrays comprised of 20 nm Au nanoparticles (NPs) on a hydrophilic polymer surface using particle lithography. To direct the assembly process, capillary force and NP concentration both play critical roles on the resulting nanostructured arrays. As such, tuning these experimental parameters can directly be used to modify the nature of the nanostructures formed. To explore this, two different concentrations of Au NP solutions (∼7 × 1011 or 4 × 1012 NPs/mL) were used in conjunction with a fixed concentration of polystyrene microspheres (PS MS, ∼6 × 109 PS MS/mL). Assembly at a relative humidity (RH) of 45% with the higher concentration resulted in the formation of well-defined Au nanorings of ca. 23 nm in height and 881 nm in diameter with a pitch of 2.5 µm. Assembly at 65% RH with the lower concentration of NPs resulted in Au nanodonut arrays comprised of isolated single Au NPs. To explore the extent of coupling in the well-defined structures, dark field scattering spectra were collected and showed a broad localized surface plasmon resonance (LSPR) peak with a shoulder, which full-wave electrodynamics modeling (finite-difference time domain (FDTD) method) attributed to be a result of pronounced particle-particle coupling along the circumference of the nanoring array.

Optoelectronic phenomena in gold metal nanostructures due to the inverse Faraday effect.

The inverse Faraday effect (IFE) is an opto-magnetic phenomenon that produces static magnetic fields in a wide range of materials during illumination with circularly polarized light. This study analyzes non-magnetic gold (Au) metal nanostructures, providing insight into plasmonic enhancement of the magnetic and optoelectronic phenomena associated with the IFE. We report a simple numerical approach in combination with full-wave optical simulations (finite-difference time-domain method) for tracking the optically-induced motion of electrons inside plasmonic nanostructures that gives rise to the IFE. In addition to static magnetic fields, a circulating drift current is observed, where the direction of current is the same as the chirality of the circularly polarized light. Our results indicate a significant enhancement of this drift current by ~100 times in Au nanoparticles due to larger optical field gradients in comparison with bulk Au films. We also report on the size, geometry, and spectral dependence of the induced drift currents and static magnetic fields, which we predict can exceed 1×10-3 T under 1015 W m-2 optical intensity for spherical Au nanoparticles. Our results inform the development of new classes of magneto-optic and optoelectronic behavior that can be obtained via direct manipulation of electron dynamics by the optical fields inside metals.

Au Exchange or Au Deposition: Dual Reaction Pathways in Au-CsPbBr3 Heterostructure Nanoparticles.

We have designed a facile synthetic strategy for the selective deposition of Au metal on all-inorganic CsPbBr3 perovskite nanocrystals that includes the addition of PbBr2 salt along with AuBr3 salt. PbBr2 is necessary because the addition of Au3+ to solutions of CsPbBr3 nanocrystals otherwise results in the exchange of Au3+ ions from solution with Pb2+ cations within the nanocrystal lattice to produce Cs2AuIAuIIIBr6 nanocrystals with a tetragonal crystal structure and a band gap of about 1.6 eV, in addition to Au metal deposition. Including excess Pb2+ ions in solution prevents the exchange reaction. Au metal deposits on the surface of the nanocrystals to produce the Au-CsPbBr3 heterostructure nanoparticles with an Au particle diameter determined by the Au3+ ion concentration. Fluorescence quenching caused by Au deposition monotonically increases with deposition size, but the fluorescence quantum yield (QY) is significantly greater than if any cation exchange has occurred. An optimized synthesis can produce Au-CsPbBr3 nanoparticles with 70% QY and no evidence of cation exchange.