Wei Bao

Visualizing nanoscale excitonic relaxation properties of disordered edges and grain boundaries in monolayer molybdenum disulfide

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 (60u2009nm), 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.

A polarizing situation: Taking an in-plane perspective for next-generation near-field studies

By enabling the probing of light–matter interactions at the functionally relevant length scales of most materials, near-field optical imaging and spectroscopy accesses information that is unobtainable with other methods. The advent of apertureless techniques, which exploit the ultralocalized and enhanced near-fields created by sharp metallic tips or plasmonic nanoparticles, has resulted in rapid adoption of near-field approaches for studying novel materials and phenomena, with spatial resolution approaching sub-molecular levels. However, these approaches are generally limited by the dominant out-of-plane polarization response of apertureless tips, restricting the exploration and discovery of many material properties. This has led to recent design and fabrication breakthroughs in near-field tips engineered specifically for enhancing in-plane interactions with near-field light components. This mini-review provides a perspective on recent progress and emerging directions aimed at utilizing and controlling in-plane optical polarization, highlighting key application spaces where in-plane near-field tip responses have enabled recent advancements in the understanding and development of new nanostructured materials and devices.

Anomalous Above-Gap Photoexcitations and Optical Signatures of Localized Charge Puddles in Monolayer Molybdenum Disulfide

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 identified and coincide with the energetic positions of the higher-energy excitons and the electronic band gap as predicted by first-principles calculations. Identification of such features outlines a facile route to measure the optical and electronic band gaps and thus the exciton binding energy in the more sophisticated device architectures that are necessary for untangling the rich many-body phenomena and complex photophysics of these layered semiconductors. In as-grown materials, the excited states exhibit microscopic spatial variations that are characteristic of local carrier density fluctuations, similar to charge puddling phenomena in graphene. Such variations likely arise from substrate inhomogeneity and demonstrate the possibility to use substrate patterning to tune local carrier density and dynamically control excited states for designer optoelectronics.

Elucidating heterogeneity in nanoplasmonic structures using nonlinear photon localization microscopy

Using nonperturbative photon localization microscopy and electromagnetic simulation, it is observed that localized modes in plasmonic devices are significantly impacted by small, and frequently time-dependent, structural variations on the nanometer scale. This is important because many such devices rely on the concentration of electromagnetic energy at the ?10 nm length scale and below for applications ranging from ultrasensitive molecular spectroscopy and detection, to chemical nano-imaging and plasmo-catalysis. In all devices, but particularly those based on noble metals, structural heterogeneity at these length scales is unavoidable, emphasizing the need for characterizing and understanding its effects. By exploiting the two-photon photoluminescence signal, one addresses the specific challenge of probing local electromagnetic fields inside the metal, which directly determine hot carrier generation and photoemission. It is found that heterogeneous nanoscale asperities serve as energy localization centers, and that functional impact is influenced primarily by two factors: position relative to a plasmonic mode volume, and how the asperity affects the smallest critical dimension, such as the size of a nanogap, in the structure.

Experimental Demonstration of Hyperbolic Metamaterial Assisted Illumination Nanoscopy

An optical metamaterial is capable of manipulating light in nanometer scale that goes beyond what is possible with conventional materials. Taking advantage of this special property, metamaterial-assisted illumination nanoscopy (MAIN) possesses tremendous potential to extend the resolution far beyond conventional structured illumination microscopy. Among the available MAIN designs, hyperstructured illumination that utilizes strong dispersion of a hyperbolic metamaterial (HMM) is one of the most promising and practical approaches, but it is only theoretically studied. In this paper, we experimentally demonstrate the concept of hyperstructured illumination. A ∼80 nm resolution has been achieved in a well-known Ag/SiO2 multilayer HMM system by using a low numerical aperture objective (NA = 0.5), representing a 6-fold resolution enhancement of the diffraction limit. The resolution can be significantly improved by further material optimization.

Near‐Field Imaging: Revealing Optical Properties of Reduced‐Dimensionality Materials at Relevant Length Scales (Adv. Mater. 38/2015)

Optimizing the optical properties of reduced-dimensionality materials requires characterization at the relevant length scale, often below the diffraction limit. On page 5693, D. F. Ogletree and co-workers review the current state of the art for 0D, 1D, and 2D nanomaterials, including novel techniques like the Molecular Foundrys Campanile probe.