High solid-state photoluminescence quantum yield of carbon-dot-derived molecular fluorophores for light-emitting devices.

Several recent studies of carbon dots (CDs) synthesized by bottom-up methods under mild conditions have reported the presence of organic molecular fluorophores in CD dispersions. These fluorophores have a tendency to aggregate, and their properties strongly depend on whether they are present in the form of discrete molecules or aggregates. The aggregation becomes more prominent in the solid state, which motivates the study of the properties of the fluorophores associated with CDs in the solid state. Here, we report the solid-state characterization of N4,N11-dimethyldibenzo[a,h]phenazine-4,11-diamine (BPD) - a molecular fluorophore that forms CDs. Discrete BPD molecules show excitation-wavelength-independent photoluminescence (PL) emission in the green wavelength region at ∼520 nm. However, additional blue PL is also observed due to aggregation, making the PL emission significantly broad. For detailed studies, BPD is mixed in different solid matrices, and it is observed that the PL quantum yield (PLQY) of BPD films strongly depends on the concentration of BPD in the solid matrices. Increasing the concentration of BPD results in a considerable decrease in the PLQY. The PLQY of the films with an optimum concentration of BPD is 75.9% and 40.2% in polymethyl methacrylate and polystyrene, respectively. At higher concentrations, these PLQY values decrease to ∼11%. The significant decrease in the PLQY is ascribed to reabsorption and nonradiative exciton decay that is facilitated by BPD aggregation at higher concentrations. Finally, light-emitting devices (LEDs) were fabricated with almost pure white emission color, having CIE (International Commission on Illumination) coordinates of (0.35, 0.37) using BPD in the color-converting layer of blue-pumped LEDs. The device shows a luminous efficiency 3.8 lm W-1 and luminance of 43 331 cd m-2.

Photostability of blue phosphorescent films on plasmonic surfaces.

Organometallic phosphors are an important class of emissive materials used in high-efficiency organic light-emitting devices. However, problems of low photostability arise for blue-emitting phosphors due to chemical and environmental degradation and triplet quenching processes. Various approaches have been developed to improve the photostability of such phosphors, including the design of new organometallic molecules and control of host-dopant composition in thin films. Here, we demonstrate a different approach for improving the photostability of blue organometallic phosphors that uses localized surface plasmon resonances to increase the triplet recombination rate. The increased recombination rate improves the photostability of the phosphor due to the reduction in triplet quenching pathways. We show that the lifetime of phosphorescence is decreased significantly by nanoparticle-based plasmonic surfaces, which improves the photostability of the blue organometallic phosphor by up to a factor of 3.6. Other plasmonic surfaces are also tested and exhibit less significant photostability improvements due to a reduced spectral overlap of the plasmonic modes with the emitter and lower mode confinement. The use of plasmonic surfaces to improve phosphor photostability at blue wavelengths is distinct from other approaches because it involves modification to the local electromagnetic environment of the phosphor rather than modifications to the phosphor molecular structure or the emitting material composition.

Self-Assembled Monolayers for Improved Charge Injection of Silver Back Electrodes in Inverted Organic Electronic Devices.

Self-assembled monolayers (SAMs) formed from thiol compounds bound to Ag and Au electrodes have been used as an important strategy in improving the stability and efficiency of optoelectronic devices. Thiol compounds provide only one binding site with the metal electrode which limits their influence. Dithiolane/dithiol compounds can provide multiple binding sites and could be useful in enhancing the performance of the device. In this study, inverted organic semiconducting hole-only devices were fabricated by using Ag back electrodes in conjunction with SAMs formed from disulfide lipoic acid-based compounds and were compared to a long aliphatic chain thiol. The binding and the electronic properties as well as electrical characteristics of the SAMs on silver were studied to look at the influence of their structure on charge injection in the organic semiconductor devices. It was found that the SAMs formed with (±)-α-lipoic acid, isolipoic acid, and (±)-4-phenylbutyl 5-(1,2-dithiolan-3-yl) pentanoate significantly improved the charge injection by either changing the work function of the Ag or altering the physical interaction between the polymer and the metal surface. This study may lead to an understanding of how the nature of the functional groups of the SAM and the number of bonds formed between each SAM molecule and the metal electrode influence the contact resistance and the performance of organic semiconductor devices.

Applications of scanning electron microscopy and focused ion beam milling in dental research.

The abilities of scanning electron microscopy (SEM) and focused ion beam (FIB) milling for obtaining high-resolution images from top surfaces, cross-sectional surfaces, and even in three dimensions, are becoming increasingly important for imaging and analyzing tooth structures such as enamel and dentin. FIB was originally developed for material research in the semiconductor industry. However, use of SEM/FIB has been growing recently in dental research due to the versatility of dual platform instruments that can be used as a milling device to obtain low-artifact cross-sections of samples combined with high-resolution images. The advent of the SEM/FIB system and accessories may offer access to previously inaccessible length scales for characterizing tooth structures for dental research, opening exciting opportunities to address many central questions in dental research. New discoveries and fundamental breakthroughs in understanding are likely to follow. This review covers the applications, key findings, and future direction of SEM/FIB in dental research in morphology imaging, specimen preparation for transmission electron microscopy (TEM) analysis, and three-dimensional volume imaging using SEM/FIB tomography.

The integrity of synthetic magnesium silicate in charged compounds.

Magnesium silicate is an inorganic compound used as an ingredient in product formulations for many different purposes. Since its compatibility with other components is critical for product quality and stability, it is essential to characterize the integrity of magnesium silicate in different solutions used for formulations. In this paper, we have determined the magnitude of dissociation of synthetic magnesium silicate in solution with positively charged, neutral, and negatively charged compounds using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and Liquid Chromatography-High Resolution Mass Spectrometry (LC-HRMS). The EDS results were verified through Monte Carlo simulations of electron-sample interactions. The compounds chosen for this study were positively charged cetylpyridinium chloride (CPC), neutral lauryl glucoside, and negatively charged sodium cocoyl glutamate and sodium cocoyl glycinate since these are common compounds used in personal care and oral care formulations. Negatively charged compounds significantly impacted magnesium silicate dissociation, resulting in physio-chemical separation between magnesium and silicate ions. In contrast, the positively charged compound had a minor effect on dissociation due to ion competition, and the neutral compound did not have such an impact on magnesium silicate dissociation. Further, when the magnesium ions are dissociated from the synthetic magnesium silicate, the morphology is changed accordingly, and the structural integrity of the synthetic magnesium silicate is damaged. The results provide scientific confidence and guidance for product development using synthetic magnesium silicate.

Photon Recycling in Semiconductor Thin Films and Devices.

Photon recycling (PR) plays an important role in the study of semiconductor materials and impacts the properties of their optoelectronic applications. However, PR has not been investigated comprehensively and it has not been demonstrated experimentally in many different kinds of semiconductor materials and devices. In this review paper, first, the authors introduce the background of PR and describe how this phenomenon was originally identified in semiconductors. Then, the theory and modelling of PR is reviewed and some of the important parameters that are used to quantify PR are highlighted. Next, a variety of the methods used to achieve and characterize PR in materials and devices are discussed. Examples of how the performance parameters of different types of optoelectronic devices are affected by PR are described. Finally, a summary of the roles of PR in semiconductor materials and devices and an outlook on how PR can be used to solve existing problems and challenges in the field of optoelectronics are provided. From this review, it is apparent that PR can have a positive impact on optoelectronic device performance, and that further in-depth theoretical and experimental studies are needed to rigorously demonstrate the advantages and importance of PR.

Long-term effects of impurities on the particle size and optical emission of carbon dots.

Carbon dots (CDs) are fluorescent nanoparticles that exhibit strong photoluminescence (PL) emission throughout the visible range of the electromagnetic spectrum. Recent studies highlight the presence of fluorescent impurities in CD dispersions. Here, the long-term impact of these impurities on the stability of the physical and optical properties of CDs synthesized by the solvothermal method is studied. A significant increase in particle size is observed as a function of time after synthesis from transmission electron microscopy analysis of CDs. Furthermore, the quantum yield of blue PL emission, which is mostly caused by impurities that contain carboxyl groups, gradually decays from 30% to ∼3% over 13 weeks. The reduction in quantum yield is attributed to decomposition of impurities that, consequently, deposit on the particles and increase particle size. Finally, it is observed that the blue emission decreases considerably when CDs are properly purified and a solvent-dependent yellow emission arises. The yellow emission is almost negligible when CDs are dispersed in water; however, the intensity of yellow emission increases significantly when the concentration of ethanol is increased.

Optical Biosensors for Virus Detection: Prospects for SARS-CoV-2/COVID-19.

The recent pandemic of the novel coronavirus disease 2019 (COVID-19) has caused huge worldwide disruption due to the lack of available testing locations and equipment. The use of optical techniques for viral detection has flourished in the past 15 years, providing more reliable, inexpensive, and accurate detection methods. In the current minireview, optical phenomena including fluorescence, surface plasmons, surface-enhanced Raman scattering (SERS), and colorimetry are discussed in the context of detecting virus pathogens. The sensitivity of a viral detection method can be dramatically improved by using materials that exhibit surface plasmons or SERS, but often this requires advanced instrumentation for detection. Although fluorescence and colorimetry lack high sensitivity, they show promise as point-of-care diagnostics because of their relatively less complicated instrumentation, ease of use, lower costs, and the fact that they do not require nucleic acid amplification. The advantages and disadvantages of each optical detection method are presented, and prospects for applying optical biosensors in COVID-19 detection are discussed.

Strong Plasmon-Exciton Coupling in Ag Nanoparticle-Conjugated Polymer Core-Shell Hybrid Nanostructures.

Strong plasmon-exciton coupling between tightly-bound excitons in organic molecular semiconductors and surface plasmons in metal nanostructures has been studied extensively for a number of technical applications, including low-threshold lasing and room-temperature Bose-Einstein condensates. Typically, excitons with narrow resonances, such as J-aggregates, are employed to achieve strong plasmon-exciton coupling. However, J-aggregates have limited applications for optoelectronic devices compared with organic conjugated polymers. Here, using numerical and analytical calculations, we demonstrate that strong plasmon-exciton coupling can be achieved for Ag-conjugated polymer core-shell nanostructures, despite the broad spectral linewidth of conjugated polymers. We show that strong plasmon-exciton coupling can be achieved through the use of thick shells, large oscillator strengths, and multiple vibronic resonances characteristic of typical conjugated polymers, and that Rabi splitting energies of over 1000 meV can be obtained using realistic material dispersive relative permittivity parameters. The results presented herein give insight into the mechanisms of plasmon-exciton coupling when broadband excitonic materials featuring strong vibrational-electronic coupling are employed and are relevant to organic optoelectronic devices and hybrid metal-organic photonic nanostructures.

Optical and Electrical Properties of Organic Semiconductor Thin Films on Aperiodic Plasmonic Metasurfaces.

Metal electrodes are playing an increasingly important role in controlling photon absorption and in promoting optimal light management in thin-film semiconductor devices. For organic optoelectronic devices, the conventional fabrication approach is to build the device on top of a transparent electrode, with metal electrode deposition as the last step. This makes it challenging to control the surface of the metal electrode to promote good light management properties. An inverted fabrication approach that builds the device on top of a metal electrode makes it possible to control the morphology of the metal surface independently of the organic semiconductor active layer to achieve a variety of photonic and plasmonic behaviors useful for devices. However, there are few reports of inverted fabrication of organic optoelectronic devices and its impacts on device properties. Silver (Ag) is the most suitable metal for fabrication of nanostructured electrodes with plasmonic behavior (i.e., plasmonic electrodes) because of its low parasitic absorption loss and high reflectivity. In this project, we describe the facile fabrication of silver nanoparticle (AgNP) aperiodic plasmonic metasurfaces and study their physical and optical characteristics. Then, we investigate the photonic and electrical behaviors of the aperiodic plasmonic metasurfaces when interfaced with poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) organic semiconducting polymer thin films. The luminescence quantum yield of F8BT thin films increases from 29% on planar Ag up to 66% on AgNP metasurfaces due to the Purcell effect and the improved extraction of emission coupled to surface plasmon polariton modes. In particular, we show that plasmonic enhancement can overcome ohmic losses associated with metals and metal-induced exciton quenching. According to the current-voltage characteristics of hole-only devices with and without aperiodic plasmonic metasurfaces, we conclude that AgNP aperiodic plasmonic metasurfaces have comparable electrical behavior to planar metal electrodes while having superior light management capability.

Blending Ionic and Coordinate Bonds in Hybrid Semiconductor Materials: A General Approach toward Robust and Solution-Processable Covalent/Coordinate Network Structures.

Inorganic semiconductor materials are best known for their superior physical properties, as well as their structural rigidity and stability. However, the poor solubility and solution-processability of these covalently bonded network structures has long been a serious drawback that limits their use in many important applications. Here, we present a unique and general approach to synthesize robust, solution-processable, and highly luminescent hybrid materials built on periodic and infinite inorganic modules. Structure analysis confirms that all compounds are composed of one-dimensional anionic chains of copper iodide (CumIm+22-) coordinated to cationic organic ligands via Cu-N bonds. The choice of ligands plays an important role in the coordination mode (µ1-MC or µ2-DC) and Cu-N bond strength. Greatly suppressed nonradiative decay is achieved for the µ2-DC structures. Record high quantum yields of 85% (λex = 360 nm) and 76% (λex = 450 nm) are obtained for an orange-emitting 1D-Cu4I6(L6). Temperature dependent PL measurements suggest that both phosphorescence and thermally activated delayed fluorescence contribute to the emission of these 1D-AIO compounds, and that the extent of nonradiative decay of the µ2-DC structures is much less than that of the µ1-DC structures. More significantly, all compounds are remarkably soluble in polar aprotic solvents, distinctly different from previously reported CuI based hybrid materials made of charge-neutral CumXm (X = Cl, Br, I), which are totally insoluble in all common solvents. The greatly enhanced solubility is a result of incorporation of ionic bonds into extended covalent/coordinate network structures, making it possible to fabricate large scale thin films by solution processes.

Rational design of a high-efficiency, multivariate metal-organic framework phosphor for white LED bulbs.

Developing rare-earth element (REE) free yellow phosphors that can be excited by 455 nm blue light will help to decrease the environmental impact of manufacturing energy efficient white light-emitting diodes (WLEDs), decrease their cost of production, and accelerate their adoption across the globe. Luminescent metal-organic frameworks (LMOFs) demonstrate strong potential for use as phosphor materials and have been investigated intensively in recent years. However, the majority are not suitable for the current WLED technology due to their lack of blue excitability. Therefore, designing highly efficient blue-excitable, yellow-emitting, REE free LMOFs is much needed. With an internal quantum yield of 76% at 455 nm excitation, LMOF-231 is the most efficient blue-excitable yellow-emitting LMOF phosphor reported to date. Spectroscopic studies suggest that this quantum yield could be further improved by narrowing the material's bandgap. Based on this information and guided by DFT calculations, we apply a ligand substitution strategy to produce a semi-fluorinated analogue of LMOF-231, LMOF-305. With an internal quantum yield of 88% (λ em = 550 nm) under 455 nm excitation, this LMOF sets a new record for luminescent efficiency in yellow-emitting, blue-excitable, REE free LMOF phosphors. Temperature-dependent and polarized photoluminescence (PL) studies have provided insight on the mechanism of emission and origin of the significant PL enhancement.

Identification of the local electrical properties of crystalline and amorphous domains in electrochemically doped conjugated polymer thin films.

Doped polymer thin films have several applications in electronic, optoelectronic and thermoelectric devices. Often the electrical properties of doped conjugated polymer thin films are affected by their local physical and mechanical characteristics. However, investigations into the effects of doping on local domain properties have not been carried out. Here, we study the physical, mechanical and optical properties of electrochemically doped P3HT thin films at the nanoscale and establish a relation between doping level and the physical properties of P3HT thin films. Bulk crystallinity of both pristine and doped P3HT thin films, characterized using grazing incidence X-ray diffraction, shows a clear loss in crystallinity upon doping. Nanoscale crystalline and amorphous domains in the films are visualized by multimode atomic force microscopy (AFM). It is apparent that the crystalline domains are most affected by doping and have a higher degree of doping compared to amorphous domains. This results in crystalline domains exhibiting superior electrical conductivity at a local level. These results are further supported by Raman mapping and elemental analysis of doped films. A direct relation is established between the physical, mechanical and electrical properties of doped P3HT thin films based on the AFM data. The findings demonstrate that higher dopant concentrations are found in crystalline domains compared to amorphous domains, which has not been shown before to the best of our knowledge. This study can be used to optimize the electronic properties of doped P3HT thin films for use in electronic and optoelectronic device applications.

Hybrid plasmonic Au-TiN vertically aligned nanocomposites: a nanoscale platform towards tunable optical sensing.

Tunable plasmonic structure at the nanometer scale presents enormous opportunities for various photonic devices. In this work, we present a hybrid plasmonic thin film platform: i.e., a vertically aligned Au nanopillar array grown inside a TiN matrix with controllable Au pillar density. Compared to single phase plasmonic materials, the presented tunable hybrid nanostructures attain optical flexibility including gradual tuning and anisotropic behavior of the complex dielectric function, resonant peak shifting and change of surface plasmon resonances (SPRs) in the UV-visible range, all confirmed by numerical simulations. The tailorable hybrid platform also demonstrates enhanced surface plasmon Raman response for Fourier-transform infrared spectroscopy (FTIR) and photoluminescence (PL) measurements, and presents great potentials as designable hybrid platforms for tunable optical-based chemical sensing applications.

Plasmonic sphere-on-plane systems with semiconducting polymer spacer layers.

The optical properties of metal-film-coupled nanoparticles (NPs) are highly sensitive to physical and optical interactions between the NPs and the spacer medium in the gap between the NP and metal film. Here, we investigate the physical and optical interactions between gold NPs (AuNPs) and semiconducting conjugated polymer thin-film spacers in a "sphere-on-plane" type metal-film-coupled NP system, and their influence on the plasmonic scattering of individual AuNPs. We choose two different conjugated polymers: one with an absorption spectrum that is resonant with the plasmonic modes of the AuNPs and another that is non-resonant. By correlating dark-field back-scattering optical images with topographic atomic force microscope images, we find that partial embedding of the AuNPs occurs in both conjugated polymers to different extents. This can lead to partial quenching of certain plasmonic scattering modes, which results in a change of the back-scattering colors from the AuNPs. Pronounced, red-shifted scattering is observed due to deep embedding of the AuNPs, particularly for thicker conjugated polymer spacers that have resonant absorption with the plasmonic modes of the AuNPs. Polarization-controlled defocused dark-field imaging is employed to visualize the emergence of horizontally-polarized scattering modes upon embedding of AuNPs into the conjugated polymer spacer. These results demonstrate the importance of nanoparticle-spacer physical interactions to the control of the color and polarization of coupled plasmonic modes in nanoparticle-film systems relevant.

All-in-One: Achieving Robust, Strongly Luminescent and Highly Dispersible Hybrid Materials by Combining Ionic and Coordinate Bonds in Molecular Crystals.

Extensive research has been pursued to develop low-cost and high-performance functional inorganic-organic hybrid materials for clean/renewable energy related applications. While great progress has been made in the recent years, some key challenges remain to be tackled. One major issue is the generally poor stability of these materials, which originates from relatively fragile/weak bonds between inorganic and organic constituents. Herein, we report a unique "all-in-one" (AIO) approach in constructing robust structures with desired properties. Such approach allows formation of both ionic and coordinate bonds within a molecular cluster, which greatly enhances structural stability while maintaining the molecular identity of the cluster and its high luminescence. The novel AIO structures are composed of various anionic (CumIm+n)n- clusters and cationic N-ligands. They exhibit high luminescence efficiency, significantly improved chemical, thermal and moisture stability, and excellent solution processability. Both temperature dependent photoluminescence experiments and DFT calculations are performed to investigate the luminescence origin and emission mechanism of these materials, and their suitability as energy-saving LED lighting phosphors is assessed. This study offers a new material designing strategy that may be generalized to many other material classes.

Ultrafast Charge Transfer and Enhanced Absorption in MoS2-Organic van der Waals Heterojunctions Using Plasmonic Metasurfaces.

Hybrid organic-inorganic heterostructures are attracting tremendous attention for optoelectronic applications due to their low-cost processing and high performance in devices. In particular, van der Waals p-n heterojunctions formed between inorganic two-dimensional (2D) materials and organic semiconductors are of interest due to the quantum confinement effects of 2D materials and the synthetic control of the physical properties of organic semiconductors, enabling a high degree of tunable optoelectronic properties for the heterostructure. However, for photovoltaic applications, hybrid 2D-organic heterojunctions have demonstrated low power conversion efficiencies due to the limited absorption from constraints on the physical thickness of each layer. Here, we investigate the ultrafast charge transfer dynamics between an organic polymer:fullerene blend and 2D n-type MoS2 using transient pump-probe reflectometry. We employ plasmonic metasurfaces to enhance the absorption and charge photogeneration within the physically thin hybrid MoS2-organic heterojunction. For the hybrid MoS2-organic heterojunction in the presence of the plasmonic metasurface, the charge generation within the polymer is enhanced 6-fold, and the total active layer absorption bandwidth is increased by 90 nm relative to the polymer:fullerene blend alone. We demonstrate that MoS2-organic heterojunctions can serve as hybrid solar cells, and their efficiencies can be improved using plasmonic metasurfaces.

Absorption-induced scattering and surface plasmon out-coupling from absorber-coated plasmonic metasurfaces.

Interactions between absorbers and plasmonic metasurfaces can give rise to unique optical properties not present for either of the individual materials and can influence the performance of a host of optical sensing and thin-film optoelectronic applications. Here we identify three distinct mode types of absorber-coated plasmonic metasurfaces: localized and propagating surface plasmons and a previously unidentified optical mode type called absorption-induced scattering. The extinction of the latter mode type can be tuned by controlling the morphology of the absorber coating and the spectral overlap of the absorber with the plasmonic modes. Furthermore, we show that surface plasmons are backscattered when the crystallinity of the absorber is low but are absorbed for more crystalline absorber coatings. This work furthers our understanding of light-matter interactions between absorbers and surface plasmons to enable practical optoelectronic applications of metasurfaces.

Mode-specific study of nanoparticle-mediated optical interactions in an absorber/metal thin film system.

We present an experimental and theoretical study of the electromagnetic interaction between a single gold nanoparticle and a thin gold substrate separated by a sub-50 nm-thick optically absorptive polythiophene spacer layer. Single-particle dark-field scattering spectra show distinct resonance features assigned to four different modes: a horizontal image dipole coupling mode, a vertical image dipole coupling mode and horizontal and vertical coupling modes between localized surface plasmon resonances (LSPRs) and surface plasmon polaritons (SPPs). Relatively broadband spectral tuning of the modes can be achieved by modification of the thickness of either the absorptive spacer or the underlying metal film. Dark-field images also reveal the existence of particles for which the signal of the horizontal image dipole coupling mode is suppressed. This is attributed to partial-embedding of gold nanoparticles into the polythiophene spacer and leads to higher scattered light intensities at longer wavelengths. Full-field electromagnetic simulations show good agreement with the experimental results for the various sample conditions. Strong local electric field confinement at longer wavelengths in the polythiophene spacer, due to the vertical image dipole coupling mode and a LSPR-SPP coupling mode, is also observed in simulations and contributes to absorption enhancement in the spacer. Furthermore, we find absorption enhancement in the semiconducting polythiophene spacer increases with decreasing thickness, indicating the increased light trapping ability of the gold nanoparticles for ultra-thin semiconductor layers. The need for ever-thinner semiconductor layers in optoelectronic devices requires effective light trapping at deeply-subwavelength scales. This work demonstrates that light trapping in sub-50 nm-thick semiconductor layers is possible using a "sphere-on-plane" system and offers insight into how coupling modes can be manipulated in this system.

Enhancing surface plasmon leakage at the metal/semiconductor interface: towards increased light outcoupling efficiency in organic optoelectronics.

The light outcoupling efficiency of organic light-emitting optoelectronic devices is severely limited by excitation of tightly bound surface plasmon polaritons at the metal electrodes. We present a theoretical study of an organic semiconductor-silver-SiO(2) waveguide and demonstrate that by simple tuning of metal film thickness and the emission regime of the organic semiconductor, a significant fraction of surface plasmon polariton mode amplitude is leaked into the active semiconductor layer, thereby decreasing the amount of optical energy trapped by the metal. At visible wavelengths, mode leakage increases by factors of up to 3.8 and 88 by tuning metal film thickness and by addition of gain, respectively.