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Halide mixing is one of the most powerful techniques to tune the optical bandgap of metal-halide perovskites. However, halide mixing has commonly been observed to result in phase segregation, which reduces excited-state transport and limits device performance. While the current emphasis lies on the development of strategies to prevent phase segregation, it remains unclear how halide mixing may affect excited-state transport even if phase purity is maintained. Here, we study exciton transport in phase pure mixed-halide 2D perovskites of (PEA) 2 Pb(I 1- x Br x ) 4 . Using transient photoluminescence microscopy, we show that, despite phase purity, halide mixing inhibits exciton transport. We find a significant reduction even for relatively low alloying concentrations. By performing Brownian dynamics simulations, we are able to reproduce our experimental results and attribute the decrease in diffusivity to the energetically disordered potential landscape that arises due to the intrinsic random distribution of alloying sites., Competing Interests: The authors declare no competing financial interest., (© 2021 The Authors. Published by American Chemical Society.)

Combining excellent optoelectronic properties with the benefits of solution processability, metal-halide perovskites are promising materials for photovoltaic and light-emitting technologies. To facilitate the integration of perovskite thin films into optoelectronic devices, a need exists for simple and efficient fabrication methods. Here, we present a template-stripping technique to produce ultraflat and flexible perovskite thin films. We apply a one-step spin-coating procedure to produce high-quality CH 3 NH 3 PbBr 3 perovskite thin films on top of ultraflat silicon templates. These films can be mechanically cleaved from the template using a polymer adhesive to reveal the ultraflat perovskite surfaces. We demonstrate how the flatness and flexibility of the template-stripped films enable new processing strategies based on dry interfacing, by interfacing them with plasmonic hole arrays. Moreover, we show that by using prepatterned silicon templates, structured perovskite surfaces can be produced with feature sizes down to a micrometer.

Exciton polaritons are hybrid light-matter quasiparticles that can serve as coherent light sources. Motivated by applications, room-temperature realization of polaritons requires narrow, excitonic transitions with large transition dipoles. Such transitions must then be strongly coupled to an electromagnetic mode confined in a small volume. While much work has explored polaritons in organic materials, semiconductor nanocrystals present an alternative excitonic system with enhanced photostability and spectral tunability. In particular, quasi-two-dimensional nanocrystals known as nanoplatelets (NPLs) exhibit intense, spectrally narrow excitonic transitions useful for polariton formation. Here, we place CdSe NPLs on silver hole arrays to demonstrate exciton-plasmon polaritons at room temperature. Angle-resolved reflection spectra reveal Rabi splittings up to 149 meV for the polariton states. We observe bright, polarized emission arising from the lowest polariton state. Furthermore, we assess the dependence of the Rabi splitting on the hole-array pitch and the number N of NPLs. While the pitch determines the in-plane momentum for which strong coupling is observed, it does not affect the size of the splitting. The Rabi splitting first increases with NPL film thickness before eventually saturating. Instead of the commonly used [Formula: see text] dependence, we develop an analytical expression that includes the transverse confinement of the plasmon modes to describe the measured Rabi splitting as a function of NPL film thickness.

To improve the photophysical performance of colloidal quantum dots for laser applications, sophisticated core/shell geometries have been developed. Typically, a wider bandgap semiconductor is added as a shell to enhance the gain from the quantum-dot core. This shell is designed to electronically isolate the core, funnel excitons to it, and reduce nonradiative Auger recombination. However, the shell could also potentially provide a secondary source of gain, leading to further versatility in these materials. Here we develop high-quality quantum-dot ring lasers that not only exhibit lasing from both the core and the shell but also the ability to switch between them. We fabricate ring resonators (with quality factors up to ∼2500) consisting only of CdSe/CdS/ZnS core/shell/shell quantum dots using a simple template-stripping process. We then examine lasing as a function of the optical excitation power and ring radius. In resonators with quality factors >1000, excitons in the CdSe cores lead to red lasing with thresholds at ∼25 μJ/cm 2 . With increasing power, green lasing from the CdS shell emerges (>100 μJ/cm 2 ) and then the red lasing begins to disappear (>250 μJ/cm 2 ). We present a rate-equation model that can explain this color switching as a competition between exciton localization into the core and stimulated emission from excitons in the shell. Moreover, by lowering the quality factor of the cavity we can engineer the device to exhibit only green lasing. The mechanism demonstrated here provides a potential route toward color-switchable quantum-dot lasers.

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

We report on a template-stripping method for the direct surface patterning of colloidal quantum-dot thin films to produce highly luminescent structures with feature sizes less than 100 nm. Through the careful design of high quality bull's-eye gratings we can produce strong directional beaming (10° divergence) with up to 6-fold out-coupling enhancement of spontaneous emission in the surface-normal direction. A transition to narrow single-mode lasing is observed in these same structures at thresholds as low as 120 μJ/cm 2 . In addition, we demonstrate that these structures can be fabricated on flexible substrates. Finally, making use of the size-tunable character of colloidal quantum dots, we demonstrate spectrally selective out-coupling of light from mixed quantum-dot films. Our results provide a straightforward route toward significantly improved optical properties of colloidal quantum-dot assemblies.

Plasmonic structures can provide deep-subwavelength electromagnetic fields that are useful for enhancing light-matter interactions. However, because these localized modes are also dissipative, structures that offer the best compromise between field confinement and loss have been sought. Metallic wedge waveguides were initially identified as an ideal candidate but have been largely abandoned because to date their experimental performance has been limited. We combine state-of-the-art metallic wedges with integrated reflectors and precisely placed colloidal quantum dots (down to the single-emitter level) and demonstrate quantum-plasmonic waveguides and resonators with performance approaching theoretical limits. By exploiting a nearly 10-fold improvement in wedge-plasmon propagation (19 μm at a vacuum wavelength, λvac, of 630 nm), efficient reflectors (93%), and effective coupling (estimated to be >70%) to highly emissive (~90%) quantum dots, we obtain Ag plasmonic resonators at visible wavelengths with quality factors approaching 200 (3.3 nm line widths). As our structures offer modal volumes down to ~0.004λvac(3) in an exposed single-mode waveguide-resonator geometry, they provide advantages over both traditional photonic microcavities and localized-plasmonic resonators for enhancing light-matter interactions. Our results confirm the promise of wedges for creating plasmonic devices and for studying coherent quantum-plasmonic effects such as long-distance plasmon-mediated entanglement and strong plasmon-matter coupling.

We report highly efficient nonradiative energy transfer from cadmium selenide (CdSe) quantum dots to monolayer and few-layer molybdenum disulfide (MoS2). The quenching of the donor quantum dot photoluminescence increases as the MoS2 flake thickness decreases with the highest efficiency (>95%) observed for monolayer MoS2. This counterintuitive result arises from reduced dielectric screening in thin layer semiconductors having unusually large permittivity and a strong in-plane transition dipole moment, as found in MoS2. Excitonic energy transfer between a zero-dimensional emitter and a two-dimensional absorber is fundamentally interesting and enables a wide range of applications including broadband optical down-conversion, optical detection, photovoltaic sensitization, and color shifting in light-emitting devices.

Despite their technological importance, lead sulfide (PbS) nanocrystals have lagged behind nanocrystals of cadmium selenide (CdSe) and lead selenide (PbSe) in terms of size and energy homogeneity. Here, we show that the ratio of lead to sulfur precursor available during nucleation is a critical parameter affecting subsequent growth and monodispersity of PbS nanocrystal ensembles. Applying this knowledge, we synthesize highly monodisperse (size dispersity <5%) PbS nanocrystals over a wide range of sizes (exciton energies from 0.70 to 1.25 eV, or 1000-1800 nm) without the use of size-selective precipitations. This degree of monodispersity results in absorption peak half width at half max (HWHM) values as small as 20 meV, indicating an ensemble that is close to the homogeneous limit. Photoluminescence emission is correspondingly narrow and exhibits small Stokes shifts and quantum efficiencies of 30-60%. The nanocrystals readily self-assemble into ordered superlattices and exhibit exceptional air stability over several months.

Colloidal quantum dots (QDs) are promising materials for use in solar cells, light-emitting diodes, lasers, and photodetectors, but the mechanism and length of exciton transport in QD materials is not well understood. We use time-resolved optical microscopy to spatially visualize exciton transport in CdSe/ZnCdS core/shell QD assemblies. We find that the exciton diffusion length, which exceeds 30 nm in some cases, can be tuned by adjusting the inorganic shell thickness and organic ligand length, offering a powerful strategy for controlling exciton movement. Moreover, we show experimentally and through kinetic Monte Carlo simulations that exciton diffusion in QD solids does not occur by a random-walk process; instead, energetic disorder within the inhomogeneously broadened ensemble causes the exciton diffusivity to decrease over time. These findings reveal new insights into exciton dynamics in disordered systems and demonstrate the flexibility of QD materials for photonic and optoelectronic applications.

Spatial uniformity is a key consideration in high-resolution displays and light-emitting structures fabricated from colloidal quantum dots (QDs). We report spatially and spectrally resolved transient photoluminescence measurements of laterally patterned QD heterostructures. We show, using a microcontact printing technique, that spatially uniform energy transfer can be achieved in a QD donor-acceptor bilayer system, highlighting the promising potential of colloidal QDs as flexible photonic components in next-generation optoelectronic technologies.

We report on a photodetector in which colloidal quantum dots directly bridge nanometer-spaced electrodes. Unlike in conventional quantum-dot thin film photodetectors, charge mobility no longer plays a role in our quantum-dot junctions as charge extraction requires only two individual tunnel events. We find an efficient photoconductive gain mechanism with external quantum efficiencies of 38 electrons-per-photon in combination with response times faster than 300 ns. This compact device-architecture may open up new routes for improved photodetector performance in which efficiency and bandwidth do not go at the cost of one another.

We report on a method to fabricate and measure gateable molecular junctions that are stable at room temperature. The devices are made by depositing molecules inside a few-layer graphene nanogap, formed by feedback controlled electroburning. The gaps have separations on the order of 1-2 nm as estimated from a Simmons model for tunneling. The molecular junctions display gateable I-V-characteristics at room temperature.

Many types of molecular motors have been proposed and synthesized in recent years, displaying different kinds of motion, and fueled by different driving forces such as light, heat, or chemical reactions. We propose a new type of molecular motor based on electric field actuation and electric current detection of the rotational motion of a molecular dipole embedded in a three-terminal single-molecule device. The key aspect of this all-electronic design is the conjugated backbone of the molecule, which simultaneously provides the potential landscape of the rotor orientation and a real-time measure of that orientation through the modulation of the conductivity. Using quantum chemistry calculations, we show that this approach provides full control over the speed and continuity of motion, thereby combining electrical and mechanical control at the molecular level over a wide range of temperatures. Moreover, chemistry can be used to change all key parameters of the device, enabling a variety of new experiments on molecular motors.