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Semiconducting transition metal dichalcogenides (TMDs) have gained significant attention as a gain medium for nanolasers, owing to their unique ability to be easily placed and stacked on virtually any substrate. However, the atomically thin nature of the active material in existing TMD nanolasers presents a challenge, as their limited output power makes it difficult to distinguish between true laser operation and other "laser-like" phenomena. Here, we present comprehensive evidence of lasing from a CVD-grown tungsten disulphide (WS2) monolayer. The monolayer is placed on a dual-resonance dielectric metasurface with a rectangular lattice designed to enhance both absorption and emission; resulting in an ultralow threshold operation (threshold 2). We provide a thorough study of the laser performance at room temperature, paying special attention to directionality, output power, and spatial coherence. Notably, our lasers demonstrated a coherence length of over 30 μm, which is several times greater than what has been reported for 2D material lasers so far. Our realisation of a single-mode laser from a wafer-scale CVD-grown monolayer presents exciting opportunities for integration and the development of novel applications.

A promising strategy toward ultrathin, sensitive photodetectors is the combination of a photoactive semiconducting transition-metal dichalcogenide (TMDC) monolayer like MoS

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We present THz near-field transmission measurements for CVD-grown 2″ graphene mono-layer on sapphire before and after transfer to SiO 2 /Si substrate. The capability to detect changes in the graphene electronic properties after the transfer reveal the potential of THz-measurements as a non-destructive inspection tool for large scale samples with micron-scale resolution.

The development of deep blue OLED materials is a challenging problem – both for phosphorescent and TADF materials. We show that Ir(III) complexes with N-heterocyclic carbene ligands provide a route to deep blue OLED materials with CIE co-ordinates (0.15, 0.05) and external quantum efficiency above 12% at 100 Cd/m2.

In the past few decades Organic Light-Emitting Diodes (OLEDs) have matured to become a widespread display technology. Despite their commercial success in TVs, smartphones and tablet screens, several key issues still remain, especially if OLEDs are to become a viable technology for lighting. Metalorganic emitters based on Ir or Pt are widely used in OLED technology, due to their facile colour tunability, short phosphorescence lifetime and efficient intersystem crossing (ISC), which enables emission from both singlet and triplet states to be harnessed, leading to 100% internal quantum efficiency (IQE). However, these emitters are based on rare and toxic noble metals. In addition, efficient and stable blue and especially deep-blue phosphorescent emitters, which are essential for displays and lighting, are still to be demonstrated in order to replace currently used less efficient fluorescent blue OLEDs. Recently, purely organic emitters exhibiting thermally activated delayed fluorescence (TADF) mechanism were also shown to be capable of 100% IQE. Since efficient ISC in these compounds is based on the small singlet and triplet gap rather than heavy atom effect, TADF compounds represent a new class of potentially inexpensive emitters for next generation OLED displays and lighting. In this work, a series of four deep blue-to-green emitting TADF compounds have been synthesized and characterised. The compounds are based on the same scaffold as the dicarbazoyldicyanobenzene (2CzPN) reported by Adachi and co-workers, but with the replacement of the cyano groups by less electron-withdrawing oxadiazole moieties. The weaker acceptor strength of oxadiazole compared to cyano groups translates to more blue-shifted emission compared to 2CzPN. Additionally, higher spatial separation of HOMO and LUMO levels between acceptor and donor units, respectively, leads to a smaller singlet-triplet gap. This allows us to demonstrate efficient deep blue TADF OLEDs with CIE coordinates (0.16, 0.13), as compared to (0.16, 0.30) for 2CzPN devices. A thorough photophysical study of model compound 2CzPN and oxadiazole acceptor based derivatives will be presented both for solution and thin films. The blue oxadiazole compounds show up 75% quantum yield in solid state. The electrochemical and photophysical properties, as well as crystal structures are compared to the theoretical quantum chemical calculations of the studied compounds. The efficient emission properties coupled to the small singlet-triplet gap in these molecules allows us to demonstrate efficient electroluminescence (up to 10 % EQE) from the vacuum deposited OLEDs.

Room-temperature luminescence properties of dense electron–hole plasma were studied in GaN epilayers grown by various techniques (by LUMILOG grown on sapphire substrate, by AIXTRON via a MOCVD reactor grown on SiO and by UNIPRESS by the MOCVD technique grown on sapphire). By studying emission properties of plasma in lateral “thin stripe” excitation geometry the values of the threshold of the stimulated emission were estimated. A linear decrease of the stimulated emission threshold with a decrease in the density of the dislocations in GaN layers was observed. This dependence was shown to be mainly contributed by a decrease in carrier lifetime, while the impact of variation in carrier diffusion coefficient was found to be minor. Incorporation of iron into GaN layer significantly increased the stimulated emission threshold. The role of the light-scattering losses on the values of the stimulated emission threshold was discussed.

The authors kindly acknowledge the financial support by the Free State of Saxony, the Sächsische Aufbaubank (SAB, Project Nr. 100087862), and the European Social Fund (ESF). Photolithography offers a scalable solution for large area pixel patterning of organic light-emitting diode (OLED) displays. It has been limited due to the incompatibility of traditional processing steps with the majority of organic materials. In this work, photolithographic structuring of state-of-the-art phosphorescent red p-i-n OLEDs is demonstrated using fluorinated polymers in combination with hydrofluoroether solvents. This is the first report on photolithographically patterned OLEDs with state-of-the-art efficiency and good long-term stability. Postprint

New pyrenyl mono- and disubstituted fluorene and carbazole derivatives were synthesized and investigated as blue-emitting materials. Most of the synthesized compounds were capable of forming glasses with glass transition temperatures up to 105 °C. The mono- and disubstituted compounds exhibited efficient emission in a diluted form with the peak wavelengths of 416 and 422 nm and quantum yields of 0.72 and 0.82, respectively. Photophysical properties of the compounds in dilute solutions and solid state were investigated experimentally and rationalized by density functional theory calculations. The calculations revealed a low energy barrier for intramolecular twisting in the ground and excited states. The electronic spectra of the compounds were dominated by an allowed “pyrene-like” S0 → S2 transition prevailing in nearly perpendicularly twisted molecular structures and competitive S0 → S1 transition becoming increasingly allowed for more planar conformers. The disubstituted compounds demonstrated a 3-fold e...

Light-induced transient grating and photoluminescence measurements were employed for carrier recombination studies in variously Mg doped GaN layers. Carrier lifetime and ambipolar diffusion coefficient were found to decrease with doping from 210 to 20 ps and from 2.0 to 0.9 cm2/s,respectively, which proved the degradation of electrical quality of the layers. A threshold of stimulated emission was found to depend non-monotonously on doping and had the lowest value of 0.19 mJ/cm2 in the most doped layer. This dependence was explained in terms of degeneracy of the hole system.

Optically induced carrier dynamics was investigated in highly excited m -plane GaN films grown by metalorganic vapor phase epitaxy (MOVPE) on LiAlO 2 substrates and compared to a c -plane GaN reference. Photoluminescence (PL) spectra showing spontaneous and stimulated emission (SE) were investigated at room temperature in both backscattering and lateral geometries, the latter using two different orientations of the excitation stripe with respect to the m -plane GaN c -axis. A lower SE threshold was observed for the m -plane GaN film, which may be attributed to a different defect structure and in-plane strain. The threshold in the m -plane GaN layer is the lowest when using stripe excitation along [0 0 0 1], which is caused by a structural anisotropy in the film. The maximum degree of polarization for SE and spontaneous emission in the m -plane GaN film was found to be 0.47 and 0.23, respectively. The light-induced transient grating technique (LITG) provides carrier lifetimes of 40 and 380 ps for the m -plane and c -plane orientation, respectively, and reveals SE threshold values comparable to the ones obtained by PL.

White organic light-emitting diodes (OLEDs) are promising candidates for future solid-state lighting applications and backplane illumination in large-area displays. One very specific feature of OLEDs, which is currently gaining momentum, is that they can enable tunable white light emission. This feature is conventionally realized either through the vertical stacking of independent OLEDs emitting different colors or in lateral arrangement of OLEDs. The vertical design is optically difficult to optimize and often results in efficiency compromises between the units. In contrast, the lateral concept introduces severe area losses to dark regions between the subunits, which requires a significantly larger overall device area to achieve equal brightness. Here we demonstrate a color-tunable, two-color OLED device realized by side-by-side alignment of yellow and blue p-i-n OLEDs structured down to 20 μm by a simple and up-scalable orthogonal photolithography technique. This layout eliminates the problems of conventional lateral approaches by utilizing all area for light emission. The corresponding emission of the photo-patterned two-unit OLED can be tuned over a wide range from yellow to white to blue colors. The independent control of the different units allows the desired overall spectrum to be set at any given brightness level. Operated as a white light source, the microstructured OLED reaches a luminous efficacy of 13 lm W−1 at 1000 cd m−2 without an additional light outcoupling enhancement and reaches a color rendering index of 68 when operated near the color point E. Finally, we demonstrate an improved device lifetime by means of size variation of the subunits.

Recently, bilayer resist processing combined with development in hydrofluoroether (HFE) solvents has been shown to enable single color structuring of vacuum-deposited state-of-the-art organic light-emitting diodes (OLED). In this work, we focus on further steps required to achieve multicolor structuring of p-i-n OLEDs using a bilayer resist approach. We show that the green phosphorescent OLED stack is undamaged after lift-off in HFEs, which is a necessary step in order to achieve RGB pixel array structured by means of photolithography. Furthermore, we investigate the influence of both, double resist processing on red OLEDs and exposure of the devices to ambient conditions, on the basis of the electrical, optical and lifetime parameters of the devices. Additionally, water vapor transmission rates of single and bilayer system are evaluated with thin Ca film conductance test. We conclude that diffusion of propylene glycol methyl ether acetate (PGMEA) through the fluoropolymer film is the main mechanism behind OLED degradation observed after bilayer processing.

Novel donor-acceptor compounds consisting of singly bonded fluorene (Fl), benzothiadiazole (BT), and carbazole (Cz) functional units in the same molecule were investigated. Analysis of the optical spectra and fluorescence transients of the compounds revealed the domination of intramolecular charge transfer (ICT) states with high fluorescence quantum yield (72%-85%). A similar Cz-Fl-Cz compound exhibiting 100% fluorescence quantum yield and no ICT character was also studied as a reference to reveal the impact of electron-accepting BT groups. Thorough examination of the optical properties of the compounds in different media, i.e., dilute solution and polymer matrix, indicated their twisted conformations due to steric hindrance in the ground state and flattened geometry in the excited state for both reference and ICT compounds. Remarkable fluorescence efficiency losses (amounting to 70%) observed upon casting the molecular solutions into neat films were determined to originate from the low-fluorescent twisted conformers and migration-facilitated exciton quenching. The majority of emission efficiency losses (over 70%) were caused by the twisted conformers, whereas only less than 30% by exciton-migration-induced nonradiative deactivation.