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Light matter interactions are greatly enhanced in two-dimensional (2D) semiconductors because of strong excitonic effects. Many optoelectronic applications would benefit from creating stacks of atomically thin 2D semiconductors separated by insulating barrier layers, forming multiquantum-well structures. However, most 2D transition metal chalcogenide systems require serial stacking to create van der Waals multilayers. Hybrid metal organic chalcogenolates (MOChas) are self-assembling hybrid materials that combine multiquantum-well properties with scalable chemical synthesis and air stability. In this work, we use spatially resolved linear and nonlinear optical spectroscopies over a range of temperatures to study the strongly excitonic optical properties of mithrene, that is, silver benzeneselenolate, and its synthetic isostructures. We experimentally probe s-type bright excitons and p-type excitonic dark states formed in the quantum confined 2D inorganic monolayers of silver selenide with exciton binding energy up to ∼0.4 eV, matching recent theoretical predictions of the material class. We further show that mithrene's highly efficient blue photoluminescence, ultrafast exciton radiative dynamics, as well as flexible tunability of molecular structure and optical properties demonstrate great potential of MOChas for constructing optoelectronic and quantum excitonic devices.

The equilibrium and non-equilibrium optical properties of single-layer transition metal dichalcogenides (TMDs) are determined by strongly bound excitons. Exciton relaxation dynamics in TMDs have been extensively studied by time-domain optical spectroscopies. However, the formation dynamics of excitons following non-resonant photoexcitation of free electron-hole pairs have been challenging to directly probe because of their inherently fast timescales. Here, we use extremely short optical pulses to non-resonantly excite an electron-hole plasma and show the formation of two-dimensional excitons in single-layer MoS2 on the timescale of 30 fs via the induced changes to photo-absorption. These formation dynamics are significantly faster than in conventional 2D quantum wells and are attributed to the intense Coulombic interactions present in 2D TMDs. A theoretical model of a coherent polarization that dephases and relaxes to an incoherent exciton population reproduces the experimental dynamics on the sub-100-fs timescale and sheds light into the underlying mechanism of how the lowest-energy excitons, which are the most important for optoelectronic applications, form from higher-energy excitations. Importantly, a phonon-mediated exciton cascade from higher energy states to the ground excitonic state is found to be the rate-limiting process. These results set an ultimate timescale of the exciton formation in TMDs and elucidate the exceptionally fast physical mechanism behind this process., The formation dynamics of excitons in 2D transition metal dichalcogenides are challenging to probe directly because of their inherently fast timescales. Here, the authors use extremely short optical pulses to excite an electron-hole plasma, and show the formation of 2D excitons in MoS2 on the timescale of 30 fs.

F\"orster Resonant Energy Transfer (FRET)-mediated exciton diffusion through artificial nanoscale building block assemblies could be used as a new optoelectronic design element to transport energy. However, so far nanocrystal (NC) systems supported only diffusion length of 30 nm, which are too small to be useful in devices. Here, we demonstrate a FRET-mediated exciton diffusion length of 200 nm with 0.5 cm2/s diffusivity through an ordered, two-dimensional assembly of cesium lead bromide perovskite nanocrystals (PNC). Exciton diffusion was directly measured via steady-state and time-resolved photoluminescence (PL) microscopy, with physical modeling providing deeper insight into the transport process. This exceptionally efficient exciton transport is facilitated by PNCs high PL quantum yield, large absorption cross-section, and high polarizability, together with minimal energetic and geometric disorder of the assembly. This FRET-mediated exciton diffusion length matches perovskites optical absorption depth, opening the possibility to design new optoelectronic device architectures with improved performances, and providing insight into the high conversion efficiencies of PNC-based optoelectronic devices.

A contactless charge-injection scheme that allows the local and quasi-permanent manipulation of atomically thin 2D materials, such as monolayer (1L-)MoS2, over spatial extents of several tens of micrometers, is reported. The possibility to precisely position and localize the charge-injection source to the micrometer scale post-fabrication allows the investigation of local unperturbed electronic structure of the 2D material. Thanks to this novel approach, the important impact of sample inhomogeneity on the charge-carrier percolation that occurs over the entire extent of the 2D flake and proliferates up to 40 μm away from the localized charge injection is elucidated. The apparent driving force for carrier relocation is the initial inhomogeneous electronic landscape of the 2D material. These studies demonstrate that local and contactless charge injection with submicrometer precision delivers an alternative route for charge injection and indicates that local 2D material electronic structure can serve as a key element for novel nanoscale device design.

Transition metal dichalcogenide semiconductors are ultrathin thin semiconductors that host a rich suite of photophysical phenomena. Excitons govern light-matter interactions in these 2D semiconductors and are fundamental packets of energy that can be manipulated with external stimuli and nanofabricated environments for next-generation technologies. Using nano-optical spectroscopy techniques to access excitonic physics at extreme length scales, a striking diversity of excitonic phenomena has been identified in these 2D materials, including heterogeneity due to edge states and charge puddles. Our latest findings highlight how strain localizes excitons on length scales that are commensurate with their size due to wrinkling phenomena that are unique to 2D materials. These results demonstrate the potential of monolayer semiconductors to be utilized for nanoscale optoelectronic devices with unique functionalities derived from excitonic physics.

We report a new localized and electrodeless charge injection scheme that quasi-permanently modifies monolayer (1L-)MoS2 doping levels to extents competing with electrostatic gating. The key innovation is to use Sn-doped In2O3 (ITO) nanocrystals (NCs) as contactless light-driven charge injection sources triggered solely by light. Each nanocrystal can store and transfer multiple charges after ultraviolet illumination within the diffraction limited laser spot. This results in reductions in carrier density in the underlying 1L-MoS2 up to 1×1013 cm-2 and is observed throughout the extent of the 2D material flake. The long-distance charge separation proliferates up to 40 µm away from the localized charge injection and persists over months. The apparent driving force for carrier relocation is the initial inhomogeneous electronic landscape of the 2D material. These studies demonstrate a novel all-optically controlled tool to locally inject carriers with sub-micrometer precision. This new ability allows us to extract important aspects of inhomogeneity in 2D materials undisturbed by bulky electronic contacts and indicates that local 2D material manipulation can serve as a key element for novel nanoscale device design.

Evaluating and tuning the properties of two-dimensional (2D) materials is a major focus of advancing 2D science and technology. While many claim that the photonic properties of a 2D layer provide evidence that the material is “high quality”, this may not be true for electronic performance. In this work, we deconvolute the photonic and electronic response of synthetic monolayer molybdenum disulfide. We demonstrate that enhanced photoluminescence can be robustly engineered via the proper choice of substrate, where growth of MoS2 on r-plane sapphire can yield >100x enhancement in PL and carrier lifetime due to increased molybdenum-oxygen bonding compared to that of traditionally grown MoS2 on c-plane sapphire. These dramatic enhancements in optical properties are similar to those of super-acid treated MoS2, and suggest that the electronic properties of the MoS2 are also superior. However, a direct comparison of the charge transport properties indicates that the enhanced PL due to increased Mo-O bonding leads to p-type compensation doping, and is accompanied by a 2x degradation in transport properties compared to MoS2 grown on c-plane sapphire. This work provides a foundation for understanding the link between photonic and electronic performance of 2D semiconducting layers, and demonstrates that they are not always correlated.

Broadband optoelectronics such as artificial light harvesting technologies necessitate efficient and, ideally, tunable coupling of excited states over a wide range of energies. In monolayer MoS2, a prototypical two-dimensional layered semiconductor, the excited state manifold spans the visible electromagnetic spectrum and is comprised of an interconnected network of excitonic and free-carrier excitations. Here, photoluminescence excitation spectroscopy is used to reveal the energetic and spatial dependence of broadband excited state coupling to the ground-state luminescent excitons of monolayer MoS2. Photoexcitation of the direct band gap excitons is found to strengthen with increasing energy, demonstrating that interexcitonic coupling across the Brillouin zone is more efficient than previously reported, and thus bolstering the import and appeal of these materials for broadband optoelectronic applications. Narrow excitation resonances that are superimposed on the broadband photoexcitation spectrum are ide...

Here we exploit transient reflectivity with sub-20fs temporal resolution to extract the exciton formation timescale from the build up of A/B exciton dynamics of 1L-MoS2. The timescale for this process ranges from ~15fs to ~35fs. © 2020 The Author(s)

Two-dimensional materials with engineered composition and structure will provide designer materials beyond conventional semiconductors. However, the potentials of defect engineering remain largely untapped, because it hinges on a precise understanding of electronic structure and excitonic properties, which are not yet predictable by theory alone. Here, we utilize correlative, nanoscale photoemission spectroscopy to visualize how local introduction of defects modifies electronic and excitonic properties of two-dimensional materials at the nanoscale. As a model system, we study chemical vapor deposition grown monolayer WS2, a prototypical, direct gap, two-dimensional semiconductor. By cross-correlating nanoscale angle-resolved photoemission spectroscopy, core level spectroscopy, and photoluminescence, we unravel how local variations in defect density influence electronic structure, lateral band alignment, and excitonic phenomena in synthetic WS2 monolayers.

Plasmonic metasurfaces have spawned the field of flat optics using nanostructured planar metallic or dielectric surfaces that can replace bulky optical elements and enhance the capabilities of traditional far-field optics. Furthermore, the potential of flat optics can go far beyond far-field modulation, and can be exploited for functionality in the near-field itself. Here, we design metasurfaces based on aperiodic arrays of plasmonic Au nanostructures for tailoring the optical near-field in the visible and near-infrared spectral range. The basic element of the arrays is a rhomboid that is modulated in size, orientation and position to achieve the desired functionality of the micron-size metasurface structure. Using two-photon-photoluminescence as a tool to probethe near-field profiles in the plane of the metasurfaces, we demonstrate the molding of light into different near-field intensity patterns and active pattern control via the far-field illumination. Finite element method simulations reveal that the near-field modulation occurs via a combination of the plasmonic resonances of the rhomboids and field enhancement in the nanoscale gaps in between the elements. This approach enables optical elements that can switch the near-field distribution across the metasurface via wavelength and polarization of the incident far-field light, and provides pathways for light matter interaction in integrated devices., Comment: 26 pages, 7 figures

Reduced-dimensionality materials for photonic and optoelectronic applications including energy conversion, solid-state lighting, sensing, and information technology are undergoing rapid development. The search for novel materials based on reduced-dimensionality is driven by new physics. Understanding and optimizing material properties requires characterization at the relevant length scale, which is often below the diffraction limit. Three important material systems are chosen for review here, all of which are under investigation at the Molecular Foundry, to illustrate the current state of the art in nanoscale optical characterization: 2D semiconducting transition metal dichalcogenides; 1D semiconducting nanowires; and energy-transfer in assemblies of 0D semiconducting nanocrystals. For each system, the key optical properties, the principal experimental techniques, and important recent results are discussed. Applications and new developments in near-field optical microscopy and spectroscopy, scanning probe microscopy, and cathodoluminescence in the electron microscope are given detailed attention. Work done at the Molecular Foundry is placed in context within the fields under review. A discussion of emerging opportunities and directions for the future closes the review.

Reducing the size of lasers to microscale dimensions enables new technologies1 that are specifically tailored for operation in confined spaces ranging from ultra-high-speed microprocessors2 to live brain tissue3. However, reduced cavity sizes increase optical losses and require greater input powers to reach lasing thresholds. Multiphoton-pumped lasers4-7 that have been miniaturized using nanomaterials such as lanthanide-doped upconverting nanoparticles (UCNPs)8 as lasing media require high pump intensities to achieve ultraviolet and visible emission and therefore operate under pulsed excitation schemes. Here, we make use of the recently described energy-looping excitation mechanism in Tm3+-doped UCNPs9 to achieve continuous-wave upconverted lasing action in stand-alone microcavities at excitation fluences as low as 14 kW cm-2. Continuous-wave lasing is uninterrupted, maximizing signal and enabling modulation of optical interactions10. By coupling energy-looping nanoparticles to whispering-gallery modes of polystyrene microspheres, we induce stable lasing for more than 5 h at blue and near-infrared wavelengths simultaneously. These microcavities are excited in the biologically transmissive second near-infrared (NIR-II) window and are small enough to be embedded in organisms, tissues or devices. The ability to produce continuous-wave lasing in microcavities immersed in blood serum highlights practical applications of these microscale lasers for sensing and illumination in complex biological environments.

Optoelectronic excitations in monolayer ${\mathrm{MoS}}_{2}$ manifest from a hierarchy of electrically tunable, Coulombic free-carrier and excitonic many-body phenomena. Investigating the fundamental interactions underpinning these phenomena---critical to both many-body physics exploration and device applications---presents challenges, however, due to a complex balance of competing optoelectronic effects and interdependent properties. Here, optical detection of bound- and free-carrier photoexcitations is used to directly quantify carrier-induced changes of the quasiparticle band gap and exciton binding energies. The results explicitly disentangle the competing effects and highlight longstanding theoretical predictions of large carrier-induced band gap and exciton renormalization in two-dimensional semiconductors.

One of the major challenges to the widespread adoption of plasmonic and nano-optical devices in real-life applications is the difficulty to mass-fabricate nano-optical antennas in parallel and reproducible fashion, and the capability to precisely place nanoantennas into devices with nanometer-scale precision. In this study, we present a solution to this challenge using the state-of-the-art ultraviolet nanoimprint lithography (UV-NIL) to fabricate functional optical transformers onto the core of an optical fiber in a single step, mimicking the ‘campanile’ near-field probes. Imprinted probes were fabricated using a custom-built imprinter tool with co-axial alignment capability with sub

The reabsorption of photoluminescence within a medium, an effect known as the inner filter effect (IFE), has been well studied in solutions, but has garnered less attention in regards to solid-state nanocomposites. Photoluminescence from a quantum dot (QD) can selectively excite larger QDs around it resulting in a net red-shift in the reemitted photon. In CdSe/CdS core/shell QD-polymer nanocomposites, we observe a large spectral red-shift of over a third of the line width of the photoluminescence of the nanocomposites over a distance of 100 μm resulting from the IFE. Unlike fluorescent dyes, which do not show a large IFE red-shift, QDs have a component of inhomogeneous broadening that originates from their size distribution and quantum confinement. By controlling the photoluminescence broadening as well as the sample dispersion and concentration, we show that the magnitude of the IFE within the nanocomposite can be tuned. We further demonstrate that this shift can be exploited in order to spectroscopically monitor the vertical displacement of a nanocomposite in a fluorescence microscope. Large energetic shifts in the measured emission with displacement can be maximized, resulting in a displacement sensor with submicrometer resolution. We further show that the composite can be easily attached to biological samples and is able to measure deformations with high temporal and spatial precision.

Spatially resolved 3D carrier-lifetime maps were measured using 2P tomography to uncover carrier dynamics at buried structures in CdTe thin-films photovoltaics. When thin-film CdTe solar cells are initially deposited, we observe increased nonradiative carrier recombination at GBs throughout the 3D volume of the film, which is most pronounced in the critical junction region where small nucleation grains form at the buried CdTe/CdS interface. The data on CdTe films treated by exposure to CdCl2vapor reveal that a critical function of the treatment is to reduce surface, interface and GB recombination throughout the entirety of these polycrystalline films. The CdCl2treatment may also compensate CdTe films, making it difficult to exceed a hole density of 1015m-3and achieve higher current density and efficiency.

The optical properties of fractal silver films grown using the Tollens silver mirror reaction enable highly reproducible single molecule surface-enhanced Raman scattering (SERS). These characteristics are a result of the nanostructure morphology which supports strongly localized surface plasmon polariton excitations that dramatically enhance the incident electromagnetic field in nanoscale regions called hot spots. Besides SERS, the local field amplification enhances an array of other linear and nonlinear optical effects such as silver luminescence and second-harmonic and continuum generation. By varying film growth conditions, we establish that the intrinsic linear and nonlinear responses qualitatively correlate with film morphology in the same manner as reported for SERS. We probe the polarization anisotropy, polarization memory, power dependence, emission spectra, excitation wavelength and temperature dependence, blinking, spectral diffusion, and emission decay dynamics of both hot spot species. Strikin...

Two-dimensional monolayer transition metal dichalcogenide semiconductors are ideal building blocks for atomically thin, flexible optoelectronic and catalytic devices. Although challenging for two-dimensional systems, sub-diffraction optical microscopy provides a nanoscale material understanding that is vital for optimizing their optoelectronic properties. Here we use the ‘Campanile' nano-optical probe to spectroscopically image exciton recombination within monolayer MoS2 with sub-wavelength resolution (60 nm), at the length scale relevant to many critical optoelectronic processes. Synthetic monolayer MoS2 is found to be composed of two distinct optoelectronic regions: an interior, locally ordered but mesoscopically heterogeneous two-dimensional quantum well and an unexpected ∼300-nm wide, energetically disordered edge region. Further, grain boundaries are imaged with sufficient resolution to quantify local exciton-quenching phenomena, and complimentary nano-Auger microscopy reveals that the optically defective grain boundary and edge regions are sulfur deficient. The nanoscale structure–property relationships established here are critical for the interpretation of edge- and boundary-related phenomena and the development of next-generation two-dimensional optoelectronic devices., Understanding the dynamics of light-induced carriers is vital for employing two-dimensional materials in optoelectronic applications. Here, the authors use a sub diffraction-limit optical technique to reveal the excitonic properties of monolayer molybdenum disulfide at the nanoscale.

Three-dimensional woodpile photonic crystals for the visible light regime were fabricated using a layer-by-layer electron beam lithography process. The materials consisted of silicon nitride for the high index material and the planarization was accomplished using a spin-coated nano-porous silica material which was left in place as the interstitial material. This fabrication procedure presents a simplified route for constructing multilayer structures where planarization is necessary, including three-dimensional photonic crystals (3DPCs) or three-dimensional metamaterials. To demonstrate the robustness of the approach, multiple 3DPCs were fabricated and optically characterized. Multiplane waveguide defects were inserted in the crystals and characterized. The vertical waveguides demonstrate enhanced transmission of light, and work continues to realize waveguiding through the full three-dimensional network of waveguides.

The plasmonic resonances of nanostructured silver films produce exceptional surface enhancement, enabling reproducible single-molecule Raman scattering measurements. Supporting a broad range of plasmonic resonances, these disordered systems are difficult to investigate with conventional far-field spectroscopy. Here, we use nonlinear excitation spectroscopy and polarization anisotropy of single optical hot spots of supercontinuum generation to track the transformation of these plasmon modes as the mesoscopic structure is tuned from a film of discrete nanoparticles to a semicontinuous layer of aggregated particles. We demonstrate how hot spot formation from diffractively-coupled nanoparticles with broad spectral resonances transitions to that from spatially delocalized surface plasmon excitations, exhibiting multiple excitation resonances as narrow as 13 meV. Photon-localization microscopy reveals that the delocalized plasmons are capable of focusing multiple narrow radiation bands over a broadband range to the same spatial region within 6 nm, underscoring the existence of novel plasmonic nanoresonators embedded in highly disordered systems.

CdSe/CdS nanocrystal tetrapods are interesting building blocks for excitonic circuits, where the flow of excitation energy is gated by an external stimulus. The physical morphology of the nanoparticle, along with the electronic structure, which favors electron delocalization between the two semiconductors, suggests that all orientations of a particle relative to an external electric field will allow for excitons to be dissociated, stored, and released at a later time. While this approach, in principle, works, and fluorescence quenching of over 95$%$ can be achieved electrically, we find that discrete trap states within the CdS are required to dissociate and store the exciton. These states are rapidly filled up with increasing excitation density, leading to a dramatic reduction in quenching efficiency. Charge separation is not instantaneous on the CdS excitonic antennae in which light absorption occurs, but arises from the relaxed exciton following hole localization in the core. Consequently, whereas strong electromodulation of the core exciton is observed, the core multiexciton and the CdS arm exciton are not affected by an external electric field.

Single-molecule spectroscopy can help to uncover the underlying heterogeneity of conjugated polymers used in organic electronics, revealing the most effective molecules in an ensemble in terms of the transport of charge and excitation energy. We demonstrate that β-phase polyfluorene chains can form a near-perfect π-electron system, whereas conventional polymers exhibit chromophoric localization due to perturbation of the conjugation. Broad-band excitation spectroscopy demonstrates that only one absorbing and emitting unit is present on the polymer chain with an average length of ∼500 repeat units, illustrating that the material effectively behaves as a molecular quantum wire with strong electronic coupling throughout the entire system.

An Upside of Asymmetry Advances in synthetic techniques have enabled the preparation of nanometer-scale semiconductors in a wide range of precise shapes and sizes, including core-shell morphologies that layer several different materials in the same particle. Such two-in-one motifs are promising for light-harvesting applications because they allow optically induced charge separation across the internal interface. Borys et al. (p. 1371 ) studied a series of rod-shaped cadmium sulfide–cadmium selenide hybrid particles using single-particle–resolved optical spectroscopy and found that smooth versus bulbous geometries produced distinct emission spectra. Further analysis of more complex, tetrapodal particles (with four arms aligned tetrahedrally) suggested that nonuniform geometries facilitate interfacial charge transfer by reducing the likelihood of electronic band misalignment.

We demonstrate high resolution transmission microscopy in a conventional two-photon wide-field fluorescence microscope by exploiting nonlinear white light generation from clusters of silver nanoparticles placed beneath the specimen. Surface-enhanced two-photon luminescence occurs at nanoparticle hot spots in the form of spectrally broad, spatially confined light which can be exploited to determine the transmission properties of a sample placed on the silver nanoparticles. We demonstrate the versatility of the technique by revealing individual crystalline domains formed in the diffuse biological photonic crystals of the scales of a beetle. We can identify submicron changes between photonic crystal facets as well as the occurrence of stacked domains invisible to surface-sensitive methods. Control over wavelength, polarization, and pulse shape promises selective addressing of hot spots in nanoparticle assemblies for motionless spatial scanning of the transmission properties with subdiffraction resolution.