Three-Dimensional Optothermal Manipulation of Light-Absorbing Particles in Phase-Change Gel Media.

Rational manipulation and assembly of discrete colloidal particles into architected superstructures have enabled several applications in materials science and nanotechnology. Optical manipulation techniques, typically operated in fluid media, facilitate the precise arrangement of colloidal particles into superstructures by using focused laser beams. However, as the optical energy is turned off, the inherent Brownian motion of the particles in fluid media impedes the retention and reconfiguration of such superstructures. Overcoming this fundamental limitation, we present on-demand, three-dimensional (3D) optical manipulation of colloidal particles in a phase-change solid medium made of surfactant bilayers. Unlike liquid crystal media, the lack of fluid flow within the bilayer media enables the assembly and retention of colloids for diverse spatial configurations. By utilizing the optically controlled temperature-dependent interactions between the particles and their surrounding media, we experimentally exhibit the holonomic microscale control of diverse particles for repeatable, reconfigurable, and controlled colloidal arrangements in 3D. Finally, we demonstrate tunable light-matter interactions between the particles and 2D materials by successfully manipulating and retaining these particles at fixed distances from the 2D material layers. Our experimental results demonstrate that the particles can be retained for over 120 days without any change in their relative positions or degradation in the bilayers. With the capability of arranging particles in 3D configurations with long-term stability, our platform pushes the frontiers of optical manipulation for distinct applications such as metamaterial fabrication, information storage, and security.

Comparative Studies of Atomically Thin Proton Conductive Films to Reduce Crossover in Hydrogen Fuel Cells.

Hydrogen fuel cells based on proton exchange membrane fuel cell (PEMFC) technology are promising as a source of clean energy to power a decarbonized future. However, PEMFCs are limited by a number of major inefficiencies; one of the most significant is hydrogen crossover. In this work, we comprehensively study the effects of two-dimensional (2D) materials applied to the anode side of the membrane as H2 barrier coatings on Nafion to reduce crossover effects on hydrogen fuel cells, while studying adverse effects on conductivity and catalyst performance in the beginning of life testing. The barrier layers studied include graphene, hexagonal boron nitride (hBN), amorphous boron nitride (aBN), and varying thicknesses of molybdenum disulfide (MoS2), all chosen due to their expected stability in a fuel cell environment. Crossover mitigation in the materials studied ranges from 4.4% (1 nm MoS2) to 46.1% (graphene) as compared to Nafion 211. Effects on proton conductivity are also studied, suggesting high areal proton transport in materials previously thought to be effectively nonconductive, such as 2 nm MoS2 and amorphous boron nitride under the conditions studied. The results indicate that a number of 2D materials are able to improve crossover effects, with those coated with 8 nm MoS2 and 1 L graphene able to achieve greater crossover reduction while minimizing conductivity penalty.

Observation of Room-Temperature Exciton-Polariton Emission from Wide-Ranging 2D Semiconductors Coupled with a Broadband Mie Resonator.

Two-dimensional exciton-polaritons in monolayer transition metal dichalcogenides (TMDs) exhibit practical advantages in valley coherence, optical nonlinearities, and even bosonic condensation owing to their light-emission capability. To achieve robust exciton-polariton emission, strong photon-exciton couplings are required at the TMD monolayer, which is challenging due to its atomic thickness. High-quality (Q) factor optical cavities with narrowband resonances are an effective approach but typically limited to a specific excitonic state of a certain TMD material. Herein, we achieve on-demand exciton-polariton emission from a wide range of TMDs at room temperature by hybridizing excitons with broadband Mie resonances spanning the whole visible spectrum. By confining broadband light at the TMD monolayer, our one type of Mie resonator on different TMDs enables enhanced light-matter interactions with multiple excitonic states simultaneously. We demonstrate multi-Rabi splittings and robust polaritonic photoluminescence in monolayer WSe2, WS2, and MoS2. The hybrid system also shows the potential to approach the ultrastrong coupling regime.

Cesium Methylammonium Lead Iodide (Csx MA1-x PbI3 ) Nanocrystals with Wide Range Cation Composition Tuning and Enhanced Thermal Stability of the Perovskite Phase.

Cesium methylammonium lead iodide (Csx MA1-x PbI3 ) nanocrystals were obtained with a wide range of A-site Cs-MA compositions by post-synthetic, room temperature cation exchange between CsPbI3 nanocrystals and MAPbI3 nanocrystals. The alloyed Csx MA1-x PbI3 nanocrystals retain their photoactive perovskite phase with incorporated Cs content, x, as high as 0.74 and the expected composition-tunable photoluminescence (PL). Excess methylammonium oleate from the reaction mixture in the MAPbI3 nanocrystal dispersions was necessary to obtain fast Cs-MA cation exchange. The phase transformation and degradation kinetics of films of Csx MA1-x PbI3 nanocrystals were measured and modeled using an Avrami expression. The transformation kinetics were significantly slower than those of the parent CsPbI3 and MAPbI3 nanocrystals, with Avrami rate constants, k, at least an order of magnitude smaller. These results affirm that A-site cation alloying is a promising strategy for stabilizing iodide-based perovskites.

Synthetic Control of Intrinsic Defect Formation in Metal Oxide Nanocrystals Using Dissociated Spectator Metal Salts.

Crystallographic defects are essential to the functional properties of semiconductors, controlling everything from conductivity to optical properties and catalytic activity. In nanocrystals, too, defect engineering with extrinsic dopants has been fruitful. Although intrinsic defects like vacancies can be equally useful, synthetic strategies for controlling their generation are comparatively underdeveloped. Here, we show that intrinsic defect concentration can be tuned during the synthesis of colloidal metal oxide nanocrystals by the addition of metal salts. Although not incorporated in the nanocrystals, the metal salts dissociate at high temperatures, promoting the dissociation of carboxylate ligands from metal precursors, leading to the introduction of oxygen vacancies. For example, the concentration of oxygen vacancies can be controlled up to 9% in indium oxide nanocrystals. This method is broadly applicable as we demonstrate by generating intrinsic defects in metal oxide nanocrystals of various morphologies and compositions.

Two-Photon Excitation Spectroscopy of Silicon Quantum Dots and Ramifications for Bio-Imaging.

Two-photon excitation in the near-infrared (NIR) of colloidal nanocrystalline silicon quantum dots (nc-SiQDs) with photoluminescence also in the NIR has potential opportunities in the field of deep biological imaging. Spectra of the degenerate two-photon absorption (2PA) cross section of colloidal nc-SiQDs are measured using two-photon excitation over a spectral range 1.46 < ℏω < 1.91 eV (wavelength 850 > λ > 650 nm) above the two-photon band gap Eg(QD)/2, and at a representative photon energy ℏω = 0.99 eV (λ = 1250 nm) below this gap. Two-photon excited photoluminescence (2PE-PL) spectra of nc-SiQDs with diameters d = 1.8 ± 0.2 nm and d = 2.3 ± 0.3 nm, each passivated with 1-dodecene and dispersed in toluene, are calibrated in strength against 2PE-PL from a known concentration of Rhodamine B dye in methanol. The 2PA cross section is observed to be smaller for the smaller diameter nanocrystals, and the onset of 2PA is observed to be blue shifted from the two-photon indirect band gap of bulk Si, as expected for quantum confinement of excitons. The efficiencies of nc-SiQDs for bioimaging using 2PE-PL are simulated in various biological tissues and compared to efficiencies of other quantum dots and molecular fluorophores and found to be comparable or superior at greater depths.

Room-Temperature Observation of Near-Intrinsic Exciton Linewidth in Monolayer WS2.

The homogeneous exciton linewidth, which captures the coherent quantum dynamics of an excitonic state, is a vital parameter in exploring light-matter interactions in 2D transition metal dichalcogenides (TMDs). An efficient control of the exciton linewidth is of great significance, and in particular of its intrinsic linewidth, which determines the minimum timescale for the coherent manipulation of excitons. However, such a control is rarely achieved in TMDs at room temperature (RT). While the intrinsic A exciton linewidth is down to 7 meV in monolayer WS2 , the reported RT linewidth is typically a few tens of meV due to inevitable homogeneous and inhomogeneous broadening effects. Here, it is shown that a 7.18 meV near-intrinsic linewidth can be observed at RT when monolayer WS2 is coupled with a moderate-refractive-index hydrogenated silicon nanosphere in water. By boosting the dynamic competition between exciton and trion decay channels in WS2 through the nanosphere-supported Mie resonances, the coherent linewidth can be tuned from 35 down to 7.18 meV. Such modulation of exciton linewidth and its associated mechanism are robust even in presence of defects, easing the sample quality requirement and providing new opportunities for TMD-based nanophotonics and optoelectronics.

Synthetic Ligand Selection Affects Stoichiometry, Carrier Dynamics, and Trapping in CuInSe2 Nanocrystals.

CuInSe2 nanocrystals exhibit tunable near-infrared bandgaps that bolster utility in photovoltaic applications as well as offer potential as substitutes for more toxic Cd- and Pb-based semiconductor compositions. However, they can present a variety of defect states and unusual photophysics. Here, we examine the effects of ligand composition (oleylamine, diphenylphosphine, and tributylphosphine) on carrier dynamics in these materials. Via spectroscopic measurements such as photoluminescence and transient absorption, we find that ligands present during the synthesis of CuInSe2 nanocrystals impart nonradiative electronic states which compete with radiative recombination and give rise to low photoluminescence quantum yields. We characterize the nature of these defect states (hole vs electron traps) and investigate whether they exist at the surface or interior of the nanocrystals. Carrier lifetimes are highly dependent on ligand identity where oleylamine-capped nanocrystals exhibit rapid trapping (<20 ps) followed by diphenylphosphine (<500 ps) and finally tributylphosphine (>2 ns). A majority of carrier population localizes at indium copper antisites (electrons), copper vacancies (holes), or surface traps (electrons and/or holes), all of which are nonemissive.

Photonic Lift-off Process to Fabricate Ultrathin Flexible Solar Cells.

A microsecond time-scale photonic lift-off (PLO) process was used to fabricate mechanically flexible photovoltaic devices (PVs) with a total thickness of less than 20 µm. PLO is a rapid, scalable photothermal technique for processing extremely thin, mechanically flexible electronic and optoelectronic devices. PLO is also compatible with large-area devices, roll-to-roll processing, and substrates with low temperature compatibility. As a proof of concept, PVs were fabricated using CuInSe2 nanocrystal ink deposited at room temperature under ambient conditions on thin, plastic substrates heated to 100 °C. It was necessary to prevent cracking of the brittle top contact layer of indium tin oxide (ITO) during lift-off, either by using a layer of silver nanowires (AgNW) as the top contact or by infusing the ITO layer with AgNW. This approach could generally be used to improve the mechanical versatility of current collectors in a variety of ultrathin electronic and optoelectronic devices requiring a transparent conductive contact layer.

State of the Art and Prospects for Halide Perovskite Nanocrystals.

Metal-halide perovskites have rapidly emerged as one of the most promising materials of the 21st century, with many exciting properties and great potential for a broad range of applications, from photovoltaics to optoelectronics and photocatalysis. The ease with which metal-halide perovskites can be synthesized in the form of brightly luminescent colloidal nanocrystals, as well as their tunable and intriguing optical and electronic properties, has attracted researchers from different disciplines of science and technology. In the last few years, there has been a significant progress in the shape-controlled synthesis of perovskite nanocrystals and understanding of their properties and applications. In this comprehensive review, researchers having expertise in different fields (chemistry, physics, and device engineering) of metal-halide perovskite nanocrystals have joined together to provide a state of the art overview and future prospects of metal-halide perovskite nanocrystal research.

Directional Modulation of Exciton Emission Using Single Dielectric Nanospheres.

Coupling emitters with nanoresonators is an effective strategy to control light emission at the subwavelength scale with high efficiency. Low-loss dielectric nanoantennas hold particular promise for this purpose, owing to their strong Mie resonances. Herein, a highly miniaturized platform is explored for the control of emission based on individual subwavelength Si nanospheres (SiNSs) to modulate the directional excitation and exciton emission of 2D transition metal dichalcogenides (2D TMDs). A modified Mie theory for dipole-sphere hybrid systems is derived to instruct the optimal design for desirable modulation performance. Controllable forward-to-backward intensity ratios are experimentally validated in 532 nm laser excitation and 635 nm exciton emission from a monolayer WS2 . Versatile light emission control is achieved for different emitters and excitation wavelengths, benefiting from the facile size control and isotropic shape of SiNSs. Simultaneous modulation of excitation and emission via a single SiNS at visible wavelengths significantly improves the efficiency and directionality of TMD exciton emission and leads to the potential of multifunctional integrated photonics. Overall, the work opens promising opportunities for nanophotonics and polaritonic systems, enabling efficient manipulation, enhancement, and reconfigurability of light-matter interactions.

Broadband Forward Light Scattering by Architectural Design of Core-Shell Silicon Particles.

A goal in the field of nanoscale optics is the fabrication of nanostructures with strong directional light scattering at visible frequencies. Here, the synthesis of Mie-resonant core-shell particles with overlapping electric and magnetic dipole resonances in the visible spectrum is demonstrated. The core consists of silicon surrounded by a lower index silicon oxynitride (SiOxNy) shell of an adjustable thickness. Optical spectroscopies coupled to Mie theory calculations give the first experimental evidence that the relative position and intensity of the magnetic and electric dipole resonances are tuned by changing the core-shell architecture. Specifically, coating a high-index particle with a low-index shell coalesces the dipoles, while maintaining a high scattering efficiency, thus generating broadband forward scattering. This synthetic strategy opens a route toward metamaterial fabrication with unprecedented control over visible light manipulation.

Tunable Chiral Optics in All-Solid-Phase Reconfigurable Dielectric Nanostructures.

Subwavelength nanostructures with tunable compositions and geometries show favorable optical functionalities for the implementation of nanophotonic systems. Precise and versatile control of structural configurations on solid substrates is essential for their applications in on-chip devices. Here, we report all-solid-phase reconfigurable chiral nanostructures with silicon nanoparticles and nanowires as the building blocks in which the configuration and chiroptical response can be tailored on-demand by dynamic manipulation of the silicon nanoparticle. We reveal that the optical chirality originates from the handedness-dependent coupling between optical resonances of the silicon nanoparticle and the silicon nanowire via numerical simulations and coupled-mode theory analysis. Furthermore, the coexisting electric and magnetic resonances support strong enhancement of optical near-field chirality, which enables label-free enantiodiscrimination of biomolecules in single nanostructures. Our results not only provide insight into the design of functional high-index materials but also bring new strategies to develop adaptive devices for photonic and electronic applications.

Suppressing material loss in the visible and near-infrared range for functional nanophotonics using bandgap engineering.

All-dielectric nanostructures have recently opened exciting opportunities for functional nanophotonics, owing to their strong optical resonances along with low material loss in the near-infrared range. Pushing these concepts to the visible range is hindered by their larger absorption coefficient, thus encouraging the search for alternative dielectrics for nanophotonics. Here, we employ bandgap engineering to synthesize hydrogenated amorphous Si nanoparticles (a-Si:H NPs) offering ideal features for functional nanophotonics. We observe significant material loss suppression in a-Si:H NPs in the visible range caused by hydrogenation-induced bandgap renormalization, producing strong higher-order resonant modes in single NPs with Q factors up to ~100 in the visible and near-IR range. We also realize highly tunable all-dielectric meta-atoms by coupling a-Si:H NPs to photochromic spiropyran molecules. ~70% reversible all-optical tuning of light scattering at the higher-order resonant mode under a low incident light intensity is demonstrated. Our results promote the development of high-efficiency visible nanophotonic devices.

Transient Lattice Response upon Photoexcitation in CuInSe2 Nanocrystals with Organic or Inorganic Surface Passivation.

CuInSe2 nanocrystals offer promise for optoelectronics including thin-film photovoltaics and printed electronics. Additive manufacturing methods such as photonic curing controllably sinter particles into quasi-continuous films and offer improved device performance. To gain understanding of nanocrystal response under such processing conditions, we investigate impacts of photoexcitation on colloidal nanocrystal lattices via time-resolved X-ray diffraction. We probe three sizes of particles and two capping ligands (oleylamine and inorganic S2-) to evaluate resultant crystal lattice temperature, phase stability, and thermal dissipation. Elevated fluences produce heating and loss of crystallinity, the onset of which exhibits particle size dependence. We find size-dependent recrystallization and cooling lifetimes ranging from 90 to 200 ps with additional slower cooling on the nanosecond time scale. Sulfide-capped nanocrystals show faster recrystallization and cooling compared to oleylamine-capped nanocrystals. Using these lifetimes, we find interfacial thermal conductivities from 3 to 28 MW/(m2 K), demonstrating that ligand identity strongly influences thermal dissipation.

Opto-thermoelectric pulling of light-absorbing particles.

Optomechanics arises from the photon momentum and its exchange with low-dimensional objects. It is well known that optical radiation exerts pressure on objects, pushing them along the light path. However, optical pulling of an object against the light path is still a counter-intuitive phenomenon. Herein, we present a general concept of optical pulling-opto-thermoelectric pulling (OTEP)-where the optical heating of a light-absorbing particle using a simple plane wave can pull the particle itself against the light path. This irradiation orientation-directed pulling force imparts self-restoring behaviour to the particles, and three-dimensional (3D) trapping of single particles is achieved at an extremely low optical intensity of 10-2 mW µm-2. Moreover, the OTEP force can overcome the short trapping range of conventional optical tweezers and optically drive the particle flow up to a macroscopic distance. The concept of self-induced opto-thermomechanical coupling is paving the way towards freeform optofluidic technology and lab-on-a-chip devices.

Enhanced Coloration Efficiency of Electrochromic Tungsten Oxide Nanorods by Site Selective Occupation of Sodium Ions.

Coloration efficiency is an important figure of merit in electrochromic windows. Though it is thought to be an intrinsic material property, we tune optical modulation by effective utilization of ion intercalation sites. Specifically, we enhance the coloration efficiency of m-WO2.72 nanocrystal films by selectively intercalating sodium ions into optically active hexagonal sites. To accurately measure coloration efficiencies, significant degradation during cycling is mitigated by introducing atomic-layer-deposited Al2O3 layers. Galvanostatic spectroscopic measurement shows that the site-selective intercalation of sodium ions in hexagonal tunnels enhances the coloration efficiency compared to a nonselective lithium ion-based electrolyte. Electrochemical rate analysis shows insertion of sodium ions to be capacitive-like, another indication of occupying hexagonal sites. Our results emphasize the importance of different site occupation on spectroelectrochemical properties, which can be used for designing materials and selecting electrolytes for enhanced electrochromic performance. In this context, we suggest sodium ion-based electrolytes hold unrealized potential for tungsten oxide electrochromic applications.

Spectrally tunable infrared plasmonic F,Sn:In2O3 nanocrystal cubes.

A synthetic challenge in faceted metal oxide nanocrystals (NCs) is realizing tunable localized surface plasmon resonance (LSPR) near-field response in the infrared (IR). Cube-shaped nanoparticles of noble metals exhibit LSPR spectral tunability limited to visible spectral range. Here, we describe the colloidal synthesis of fluorine, tin codoped indium oxide (F,Sn:In2O3) NC cubes with tunable IR range LSPR for around 10 nm particle sizes. Free carrier concentration is tuned through controlled Sn dopant incorporation, where Sn is an aliovalent n-type dopant in the In2O3 lattice. F shapes the NC morphology into cubes by functioning as a surfactant on the {100} crystallographic facets. Cube shaped F,Sn:In2O3 NCs exhibit narrow, shape-dependent multimodal LSPR due to corner, edge, and face centered modes. Monolayer NC arrays are fabricated through a liquid-air interface assembly, further demonstrating tunable LSPR response as NC film nanocavities that can heighten near-field enhancement (NFE). The tunable F,Sn:In2O3 NC near-field is coupled with PbS quantum dots, via the Purcell effect. The detuning frequency between the nanocavity and exciton is varied, resulting in IR near-field dependent enhanced exciton lifetime decay. LSPR near-field tunability is directly visualized through IR range scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS). STEM-EELS mapping of the spatially confined near-field in the F,Sn:In2O3 NC array interparticle gap demonstrates elevated NFE tunability in the arrays.

Scalable colloidal synthesis of Bi2Te2.7Se0.3 plate-like particles give access to a high-performing n-type thermoelectric material for low temperature application.

Colloidal synthesis is harnessed for the gram-scale preparation of hexagonal-shaped plate-like Bi2Te2.7Se0.3 particles, yielding nearly 5 g of the product in one experiment. The resultant textured particles are highly crystalline, phase-pure, chemically uniform, and can serve as a starting material for the preparation of bulk thermoelectrics for room temperature applications. The consolidation occurs via spark plasma sintering, which affords nanostructured n-type Bi2Te2.7Se0.3 material exhibiting a high figure of merit ZT ≈ 1 at 373 K with an average ZT ≈ 0.93 (300-473 K). Our experimental and theoretical studies indicate that the high thermoelectric performance is attributed to a favorable combination of the resultant transport properties. Specifically, bottom-up formation of the plate-like particles results in the substantial reduction of thermal conductivity by nanostructuring as observed experimentally and can be ascribed to phonon scattering at grain boundaries and suppressed bipolar conduction. When coupled with high electrical conductivity, which is preserved at the bulk scale as confirmed by ab initio calculations, these factors boost the thermoelectric performance of the as-synthesized n-type Bi2Te2.7Se0.3 bulk nanostructured alloy to the state-of-the-art level. The combination of a newly developed scalable colloidal synthesis with optimized spark plasma sintering constitutes a convenient route to nanostructured bulk thermoelectrics, which is an interesting pathway for the preparation of simple and complex thermoelectric chalcogenides.

Optical nanomanipulation on solid substrates via optothermally-gated photon nudging.

Constructing colloidal particles into functional nanostructures, materials, and devices is a promising yet challenging direction. Many optical techniques have been developed to trap, manipulate, assemble, and print colloidal particles from aqueous solutions into desired configurations on solid substrates. However, these techniques operated in liquid environments generally suffer from pattern collapses, Brownian motion, and challenges that come with reconfigurable assembly. Here, we develop an all-optical technique, termed optothermally-gated photon nudging (OPN), for the versatile manipulation and dynamic patterning of a variety of colloidal particles on a solid substrate at nanoscale accuracy. OPN takes advantage of a thin surfactant layer to optothermally modulate the particle-substrate interaction, which enables the manipulation of colloidal particles on solid substrates with optical scattering force. Along with in situ optical spectroscopy, our non-invasive and contactless nanomanipulation technique will find various applications in nanofabrication, nanophotonics, nanoelectronics, and colloidal sciences.