Self-Trapped, Thermally Equilibrated Delayed Fluorescence Enables Low-Reabsorption Luminescent Solar Concentrators Based on Gold-Doped Silver Nanoclusters.

Reabsorption-free luminescent solar concentrators (LSCs) are crucial ingredients for photovoltaic windows. Atomically precise metal nanoclusters (NCs) with large Stokes-shifted photoluminescence (PL) hold great promise for applications in LSCs. However, a fundamental understanding of the PL mechanism, particularly on the excited-state interaction and exciton kinetics, is still lacking. Herein, we studied the exciton-phonon coupling and singlet/triplet exciton dynamics for gold-doped silver NCs in a solid matrix. Following photoexcitation, the excitons can be self-trapped via strong exciton-phonon coupling. Subsequently, rapid thermal equilibration between the singlet and triplet states occurs due to the coexistence of small energy splitting and spin-orbit coupling. Finally, broadband delayed fluorescence with a large Stokes shift can be generated, namely, self-trapped, thermally equilibrated delayed fluorescence (ST-TEDF). Benefiting from superior ST-TEDF, we demonstrated efficient LSCs with minimized reabsorption.

Quantum dots derived from two-dimensional transition metal dichalcogenides: synthesis, optical properties and optoelectronic applications.

Zero-dimensional transition metal dichalcogenides (TMD) quantum dots (QDs) have attracted a lot of attention due to their interesting fundamental properties and various applications. Compared to TMD monolayers, the QD counterpart exhibits larger values for direct transition energies, exciton binding energies, absorption coefficient, luminescence efficiency, and specific surface area. These characteristics make them useful in optoelectronic devices. In this review, recent exciting progress on synthesis, optical properties, and applications of TMD QDs is highlighted. The first part of this article begins with a brief description of the synthesis approaches, which focus on microwave-assistant heating and pulsed laser ablation methods. The second part introduces the fundamental optical properties of TMD QDs, including quantum confinement in optical absorption, excitation-wavelength-dependent photoluminescence, and many-body effects. These properties are highlighted. In the third part, we discuss lastest advancements in optoelectronic devices based on TMD QDs These devices include light-emitting diodes, solar cells, photodetectors, optical sensors, and light-controlled memory devices. Finally, a brief summary and outlook will be provided.

Enhanced Förster resonance energy transfer on layered metal-dielectric hyperbolic metamaterials: an excellent platform for low-threshold laser action.

Förster resonance energy transfer (FRET) is a well-known physical phenomenon, which has been widely used in a variety of fields, spanning from chemistry, and physics to optoelectronic devices. In this study, giant enhanced FRET for donor-acceptor CdSe/ZnS quantum dot (QD) pairs placed on top of Au/MoO3 multilayer hyperbolic metamaterials (HMMs) has been realized. An enhanced FRET transfer efficiency as high as 93% was achieved for the energy transfer from a blue-emitting QD to a red-emitting QD, greater than that of other QD-based FRET in previous studies. Experimental results show that the random laser action of the QD pairs is greatly increased on a hyperbolic metamaterial by the enhanced FRET effect. The lasing threshold with assistance of the FRET effect can be reduced by 33% for the mixed blue- and red-emitting as QDs compared to the pure red-emitting QDs. The underlying origins can be well understood based on the combination of several significant factors, including spectral overlap of donor emission and acceptor absorption, the formation of coherent closed loops due to multiple scatterings, an appropriate design of HMMs, and the enhanced FRET assisted by HMMs.

Dislocation characterization inc-plane GaN epitaxial layers on 6 inch Si wafer with a fast second-harmonic generation intensity mapping technique.

Second harmonic generation (SHG) intensity, Raman scattering stress, photoluminescence and reflected interference pattern are used to determine the distributions of threading dislocations (TDs) and horizontal dislocations (HDs) in thec-plane GaN epitaxial layers on 6 inch Si wafer which is a structure of high electron mobility transistor (HEMT). The Raman scattering spectra show that the TD and HD result in the tensile stress and compressive stress in the GaN epitaxial layers, respectively. Besides, the SHG intensity is confirmed that to be proportional to the stress value of GaN epitaxial layers, which explains the spatial distribution of SHG intensity for the first time. It is noted that the dislocation-mediated SHG intensity mapping image of the GaN epitaxial layers on 6 inch Si wafer can be obtained within 2 h, which can be used in the optimization of high-performance GaN based HEMTs.

All-carbon stretchable and cavity-free white lasers: publisher's note.

This publisher's note contains corrections to [Opt. Express30, 20213 (2022)10.1364/OE.457921].

All-carbon stretchable and cavity-free white lasers.

Flexible, stretchable, and bendable electronics and optoelectronics have a great potential for wide applications in smart life. An environmentally friendly, cost effective and wide-angle emission laser is indispensable for the emerging technology. In this work, circumvent the challenge issue, cavity-free and stretchable white light lasers based on all carbon materials have been demonstrated by integration of fluorescent carbon quantum dots (CQDs) and crumpled graphene. The typical emission spectrum of the cavity-free laser based on all-carbon materials has a CIE chromaticity coordinate of (0.30, 0.38) exhibiting an intriguing broadband white-light emission. The unprecedented and non-toxic stretchable and white light cavity-free lasers based on all-carbon materials can serve as next-generation optoelectronic devices for a wide range application covering solid-state lighting and future wearable technologies.

Two-Dimensional Self-Assembly of Boric Acid-Functionalized Graphene Quantum Dots: Tunable and Superior Optical Properties for Efficient Eco-Friendly Luminescent Solar Concentrators.

Carbon-based nanomaterials hold promise for eco-friendly alternatives to heavy-metal-containing quantum dots (QDs) in optoelectronic applications. Here, boric acid-functionalized graphene quantum dots (B-GQDs) were prepared using bottom-up molecular fusion based on nitrated pyrenes and boric acid. Such B-GQDs with crystalline graphitic structures and hydrogen-bonding functionalities would be suitable model systems for unraveling the photoluminescence (PL) mechanism, while serving as versatile building blocks for supramolecular self-assembly. Unlike conventional GQDs with multiple emissive states, the B-GQDs exhibited excitation-wavelength-independent, vibronic-coupled excitonic emission. Interestingly, their PL spectra can be tuned without largely sacrificing the quantum yield (QY) due to two-dimensional self-assembly. In addition, such B-GQDs in a polystyrene matrix possessed an ultrahigh QY (∼90%) and large exciton binding energy (∼300 meV). Benefiting from broadband absorption, ultrahigh QY, and long-wavelength emission, efficient laminated luminescent solar concentrators (100 × 100 × 6.3 mm3) were fabricated, yielding a high power conversion efficiency (1.4%).

Mechanisms of negative differential resistance in glutamine-functionalized WS2quantum dots.

Understanding the mechanism of the negative differential resistance (NDR) in transition metal dichalcogenides is essential for fundamental science and the development of electronic devices. Here, the NDR of the current-voltage characteristics was observed based on the glutamine-functionalized WS2quantum dots (QDs). The NDR effect can be adjusted by varying the applied voltage range, air pressure, surrounding gases, and relative humidity. A peak-to-valley current ratio as high as 6.3 has been achieved at room temperature. Carrier trapping induced by water molecules was suggested to be responsible for the mechanism of the NDR in the glutamine-functionalized WS2QDs. Investigating the NDR of WS2QDs may promote the development of memory applications and emerging devices.

Aggregation-induced negative differential resistance in graphene oxide quantum dots.

Negative differential resistance (NDR) devices have attracted considerable interest due to their potential applications in switches, memory devices, and analog-to-digital converters. Modulation of the NDR is an essential issue for the development of NDR-based devices. In this study, we successfully synthesized graphene oxide quantum dots (GOQDs) using graphene oxide, cysteine, and H2O2. The current-voltage characteristics of the GOQDs exhibit a clear NDR in the ambient environment at room temperature. A peak-to-valley ratio as high as 4.7 has been achieved under an applied voltage sweep from -6 to 6 V. The behavior of the NDR and its corresponding peak-to-valley ratio can be controlled by adjusting the range of applied voltages, air pressure, and relative humidity. Also, the NDR is sensitive to the the concentration of H2O2 added in the synthesis. The charge carrier injection through the trapping states, induced by the GOQD aggregation, could be responsible for the NDR behavior in GOQDs.

Efficiency improvement of P3CT-Na based MAPbI3solar cells with a simple wetting process.

The averaged power conversion efficiency of polyelectrolytes (P3CT-Na) based MAPbI3solar cells can be increased from 14.94% to 17.46% with a wetting method before the spin-coating process of MAPbI3precursor solutions. The effects of the wetting process on the surface, structural, optical and excitonic properties of MAPbI3thin films are investigated by using the atomic-force microscopic images, x-ray diffraction patterns, transmittance spectra, photoluminescence spectra and Raman scattering spectra. The experimental results show that the wetting process of MAPbI3precursor solution on top of the P3CT-Na/ITO/glass substrate can be used to manipulate the molecular packing structure of the P3CT-Na thin film, which determines the formation of MAPbI3thin films.

19% Efficient P3CT-Na Based MAPbI3 Solar Cells with a Simple Double-Filtering Process.

A high-efficiency inverted-type CH3NH3PbI3 (MAPbI3) solar cell was fabricated by using a ultrathin poly[3-(4-carboxybutyl)thiophene-2,5-diyl]-Na (P3CT-Na) film as the hole transport layer. The averaged power conversion efficiency (PCE) can be largely increased from 11.72 to 18.92% with a double-filtering process of the P3CT-Na solution mainly due to the increase in short-circuit current density (JSC) from 19.43 to 23.88 mA/cm2, which means that the molecular packing structure of P3CT-Na thin film can influence the formation of the MAPbI3 thin film and the contact quality at the MAPbI3/P3CT-Na interface. Zeta potentials, atomic-force microscopic images, absorbance spectra, photoluminescence spectra, X-ray diffraction patterns, and Raman scattering spectra are used to understand the improvement in the JSC. Besides, the light intensity-dependent and wavelength-dependent photovoltaic performance of the MAPbI3 solar cells shows that the P3CT-Na thin film is not only used as the hole transport layer but also plays an important role during the formation of a high-quality MAPbI3 thin film. It is noted that the PCE values of the best P3CT-Na based MAPbI3 solar cell are higher than 30% in the yellow-to-near infrared wavelength range under low light intensities. On the other hand, it is predicted that the double-filtering method can be readily used to increase the PCE of polymer based solar cells.

Time dependence of negative and positive photoconductivity for Si δ-doped AlGaAs/InGaAs/AlGaAs quantum well under various temperatures and various incident photon energies and intensities.

Si δ-doped AlGaAs/InGaAs/AlGaAs quantum well (QW) structure is commonly adopted as one of the core elements in modern electric and optoelectronic devices. Here, the time dependent photoconductivity spectra along the active InGaAs QW channel in a dual and symmetric Si δ-doped AlGaAs/InGaAs/AlGaAs QW structure are systematically studied under various temperatures (T = 80-300 K) and various incident photon energies (E in = 1.10-1.88 eV) and intensities. In addition to positive photoconductivity, negative photoconductivity (NPC) was observed and attributed to two origins. For T = 180-240 K with E in = 1.51-1.61 eV, the trapping of the photo-excited electrons by the interface states located inside the conduction band of InGaAs QW layer is one of the origins for NPC curves. For T = 80-120 K with E in = 1.10-1.63 eV, the photoexcitation of the excess 'supersaturated' electrons within the active InGaAs QW caused by the short cooling process is another origin.

PAA/ZnO Raspberry-Shaped Composite Microspheres Decorated with Ag Nanoparticles as Cleanable SERS Substrates.

A PAA/ZnO/Ag heterostructure composite material was prepared by a reduction method. The properties of composite particles are analyzed by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), UV-visible spectroscopy, and Raman spectroscopy. Ag nanoparticles play an important role in PAA/ZnO/Ag composite microspheres, conferring new SERS properties and functions to PAA/ZnO/Ag. The intensity of the SERS signal of PAA/ZnO/Ag irradiated with UV light decreased from 10 000 to below 500. The SERS detection limit of R6G obtained was reduced to 1 × 10-6 M. The PAA/ZnO/Ag composite particles show a very good degradation effect on R6G under UV light irradiation. This study has developed a new synthesis method to prepare SERS signal enhancement materials with self-cleaning effects. According to the experimental results, the PAA/ZnO/Ag composite material has higher sensitivity than the PAA/ZnO composite material.

Electronically Coupled Gold Nanoclusters Render Deep-Red Emission with High Quantum Yields.

Electronic coupling can be used to tailor electronic states and optical properties of the luminophores. Therefore, electronically coupled systems would provide unique properties, which cannot be achieved by individual constituents. Here, electronically coupled gold nanoclusters (AuNCs) were prepared on the basis of organosilane grafting and a sol-gel-derived porous silica template. After prolonged drying, the formed AuNCs@silica composites exhibited red-shifted, line-width-narrowed, deep-red emission with high quantum yields (QYs) of ∼66% due to electronic-coupling-enhanced radiative rates and covalent-bonding-suppressed nonradiative relaxation. Meanwhile, the absorption maximum was slightly blue-shifted, leading to a large Stokes shift. All experimental findings revealed the formation of electronically coupled AuNC aggregates confined inside the nanopores and bonded to silica matrix. The mechanism is distinctly different from conventional aggregation-enhanced emission. Our work would provide great potential to engineer photophysical properties by controlling the packing modes.

Substrate-induced strain in 2D layered GaSe materials grown by molecular beam epitaxy.

Two-dimensional (2D) layered GaSe films were grown on GaAs (001), GaN/Sapphire, and Mica substrates by molecular beam epitaxy (MBE). The in situ reflective high-energy electron diffraction monitoring reveals randomly in-plane orientations of nucleated GaSe layers grown on hexagonal GaN/Sapphire and Mica substrates, whereas single-orientation GaSe domain is predominant in the GaSe/GaAs (001) sample. Strong red-shifts in the frequency of in-plane [Formula: see text] vibration modes and bound exciton emissions observed from Raman scattering and photoluminescence spectra in all samples are attributed to the unintentionally biaxial in-plane tensile strains, induced by the dissimilarity of symmetrical surface structure between the 2D-GaSe layers and the substrates during the epitaxial growth. The results in this study provide an important understanding of the MBE-growth process of 2D-GaSe on 2D/3D hybrid-heterostructures and pave the way in strain engineering and optical manipulation of 2D layered GaSe materials for novel optoelectronic integrated technologies.

Design of Core-Shell Quantum Dots-3D WS2 Nanowall Hybrid Nanostructures with High-Performance Bifunctional Sensing Applications.

Transition metal dichalcogenides (TMDCs) have recently attracted a tremendous amount of attention owing to their superior optical and electrical properties as well as the interesting and various nanostructures that are created by different synthesis processes. However, the atomic thickness of TMDCs limits the light absorption and results in the weak performance of optoelectronic devices, such as photodetectors. Here, we demonstrate the approach to increase the surface area of TMDCs by a one-step synthesis process of TMDC nanowalls from WOx into three-dimensional (3D) WS2 nanowalls. By utilizing a rapid heating and rapid cooling process, the formation of 3D nanowalls with a height of approximately 150 nm standing perpendicularly on top of the substrate can be achieved. The combination of core-shell colloidal quantum dots (QDs) with three different emission wavelengths and 3D WS2 nanowalls further improves the performance of WS2-based photodetector devices, including a photocurrent enhancement of 320-470% and shorter response time. The significant results of the core-shell QD-WS2 hybrid devices can be contributed by the high nonradiative energy transfer efficiency between core-shell QDs and the nanostructured material, which is caused by the spectral overlap between the emission of core-shell QDs and the absorption of WS2. Besides, outstanding NO2 gas-sensing performance of core-shell QDs/WS2 devices can be achieved with an extremely low detection limit of 50 ppb and a fast response time of 26.8 s because of local p-n junctions generated by p-type 3D WS2 nanowalls and n-type core-shell CdSe-ZnS QDs. Our work successfully reveals the energy transfer phenomenon in core-shell QD-WS2 hybrid devices and shows great potential in commercial multifunctional sensing applications.

Temperature-dependent charge-carrier transport between Si-δ-doped layers and AlGaAs/InGaAs/AlGaAs quantum well with various space layer thicknesses measured by Hall-effect analysis.

Temperature (T = 40 ~ 300 K) dependence of Hall-effect analysis on the dual Si-δ-doped AlGaAs/InGaAs/AlGaAs quantum-well (QW) structures with various space layer thicknesses (tS = 5, 10 and 15 nm) was performed. An interesting hysteresis behavior of electron sheet concentration [n2D(T)] was observed for tS = 10 and 15 nm but not for tS = 5 nm. A model involving two different activation barriers encountered respectively by electrons in the active QW and by electrons in the δ-doped layers is proposed to account for the hysteresis behavior. However, for small enough tS (= 5 nm ≤ 2.5 s, where s = 2.0 nm is the standard deviation of the Gaussian fit to the Si-δ-doped profile), the distribution of Si dopants near active QW acted as a specific form of "modulation doping" and can not be regarded as an ideal δ-doping. These Si dopants nearby the active QW effectively increase the magnitude of n2D, and hence no hysteresis curve was observed. Finally, effects from tS on the T-dependence of electron mobility in active QW channel are also discussed.

Highly transparent and luminescent gel glass based on reabsorption-free gold nanoclusters.

Luminescent and transparent composites formed by embedding luminophores in a solid matrix are essential components for several photonic applications, such as luminescent solar concentrators (LSCs) and luminescent down-shifting/conversion layers. For these applications, the optical losses, including reabsorption and scattering need to be minimized, while the photoluminescence (PL) emission must be stable against outdoor environments. Here, highly transparent and luminescent aluminosilicate glass doped with surface-engineered gold nanoclusters (AuNCs) was prepared without involving toxic elements and hazardous solvents. Such an AuNC@glass composite with a high loading (∼14 wt%) exhibits a unique absorption profile; near-unity absorptance in the absorption range but near-zero reabsorption in the emission region, and thus generates bright PL emission with negligible reabsorption losses. Meanwhile, the PL quantum yield was enhanced (from ∼1% to ∼14%) without sacrificing the Stokes shift, while still maintaining high optical transparency. In addition, they have high stability due to the effective protection of rigid inorganic matrices, and thus would be eco-friendly candidates for further preparation of efficient and reabsorption-free LSCs.

Nanoprobing of MoS2 by Synchrotron Radiation When van der Waals Epitaxy Is Locally Invalid.

In this work, we demonstrated nano-scaled Laue diffractions by a focused polychromatic synchrotron radiation beam to discover what happens in MoS2 when van der Waals epitaxy is locally invalid. A stronger exciton recombination with a local charge depletion in the density of 1 × 1013 cm-2, extrapolated by Raman scattering and photoluminescence, occurs in grains, which exhibits a preferred orientation of 30° rotation with respect to the c-plane of a sapphire substrate. Else, the charge doping and trion recombination dominate instead. In addition to the breakthrough in extrapolating mesoscopic crystallographic characteristics, this work opens the feasibility to manipulate charge density by the selection of the substrate-induced disturbances without external treatment and doping. Practically, the 30° rotated orientation in bilayer MoS2 films is promoted on inclined facets in the patterned sapphire substrate, which exhibits a periodic array of charge depletion of about 1.65 × 1013 cm-2. The built-in manipulation of carrier concentrations could be a potential candidate to lateral and large-area electronics based on 2D materials.

Enhanced photoluminescence of InGaAs/AlGaAs quantum well with tungsten disulfide quantum dots.

The pristine and diethylenetriamine (DETA)-doped tungsten disulfide quantum dots (WS2 QDs) with an average lateral size of about 5 nm have been synthesized using pulsed laser ablation (PLA). Introduction of the synthesized WS2 QDs on the InGaAs/AlGaAs quantum wells (QWs) can improve the photoluminescence (PL) of the InGaAs/AlGaAs QW as high as 6 fold. On the basis of the time-resolved PL and Kelvin probe measurements, the PL enhancement is attributed to the carrier transfer from the pristine or DETA-doped WS2 QDs to the InGaAs/AlGaAs QW. A heterostructure band diagram is proposed for explaining the carrier transfer, which increases the hole densities in the QW and enhances its PL intensity. This study is expected to be beneficial for the development of the InGaAs-based optoelectronic devices.