Molecular Emitters Research Articles

Inkjet printing of blue phosphorescent light-emitting layer based on bis(3,5-di(9H-carbazol-9-yl))diphenylsilane.

In this study, micropatterning of a blue light emitting, tetraphenylsilane-based phosphorescent material by inkjet printing was investigated. Bis(3,5-di(9H-carbazol-9-yl))diphenylsilane (SimCP2) doped with iridium bis(4,6-difluorophenypyridinato)picolate (FIrpic) was dissolved in a solvent mixture, and various conditions for the solvent composition and drying of films were examined. Homogeneous dot and line patterns with controllable thickness and smooth surface were obtained from a mixture of chlorobenzene and cyclohexanone at a moderate printing speed of 3 mm s−1 and a droplet ejection frequency of 70 Hz. An inkjet-printed device was designed and fabricated in [ITO/PEDOT:PSS /PVK/SimCP2:Flrpic/TSPO1/TPBi/LiF/Al] configuration, from which sky-blue light (0.14, 0.25) was obtained with a luminous efficiency of 10.73 cd A−1 and a power efficiency of 6.13 lm W−1. This amounted to 68% of the performance of an identical device where the emitting layer was spin coated. These results show the potential of inkjet printing as a low-cost patterning method for low molecular weight emitters in blue light emitting devices.

Single-molecule electroluminescence and its relevant latest progress

Research on the interaction and interconversion between electrons and photons on an individual molecular scale can provide scientific basis for the future developing of information and energy technology. Scanning tunneling microscope(STM) can offer abilities beyond atomic-resolution imaging and manipulation, and its highly localized tunneling electrons can also be used for exciting the molecules inside the tunnel junction, generating molecule-specific light emission, and thus enabling the investigation of molecular optoelectronic behavior in local nano-environment. In this paper, we present an overview of our recent research progress related to the single-molecule electroluminescence of zinc phthalocyanine (ZnPc) molecules. First, we demonstrate the realization of single-molecule electroluminescence from an isolated ZnPc by adopting a combined strategy of both efficient electronic decoupling and nanocavity plasmonic enhancement. By further combining the photon correlation measurements via the Hanbury-Brown-Twiss interferometry with STM induced luminescence technique, we demonstrate and confirm the single-photon emission nature of such an electrically driven single-molecule electroluminescence. Second, by developing the sub-nanometer resolved electroluminescence imaging technique, we demonstrate the real-space visualization of the coherent intermolecular dipole-dipole coupling of an artificially constructed non-bonded ZnPc dimer. By mapping the spatial distribution of the photon yield for the excitonic coupling in a well-defined molecular architecture, we can reveal the local optical response of the system and the dependence of the local optical response on the relative orientation and phase of the transition dipoles of the individual molecules in the dimer. Third, by using a single molecular emitter as a distinctive optical probe to coherently couple with the highly confined plasmonic nanocavity, we demonstrate the Fano resonance and photonic Lamb shift at a single-molecule level. The ability to spatially control the single-molecule Fano resonance with sub-nanometer precision can reveal the coherent and highly confined nature of the broadband nanocavity plasmon, as well as the coupling strength and the anisotropy of the field-matter interaction. These results not only shed light on the fabrication of electrically driven nano-emitters and single-photon sources, but also open up a new avenue to the study of intermolecular energy transfer, field-matter interaction, and molecular optoelectronics, all at the single-molecule level.

Electrically driven single-photon emission from an isolated single molecule

Electrically driven molecular light emitters are considered to be one of the promising candidates as single-photon sources. However, it is yet to be demonstrated that electrically driven single-photon emission can indeed be generated from an isolated single molecule notwithstanding fluorescence quenching and technical challenges. Here, we report such electrically driven single-photon emission from a well-defined single molecule located inside a precisely controlled nanocavity in a scanning tunneling microscope. The effective quenching suppression and nanocavity plasmonic enhancement allow us to achieve intense and stable single-molecule electroluminescence. Second-order photon correlation measurements reveal an evident photon antibunching dip with the single-photon purity down to g(2)(0) = 0.09, unambiguously confirming the single-photon emission nature of the single-molecule electroluminescence. Furthermore, we demonstrate an ultrahigh-density array of identical single-photon emitters.

Sub-nanometre control of the coherent interaction between a single molecule and a plasmonic nanocavity

The coherent interaction between quantum emitters and photonic modes in cavities underlies many of the current strategies aiming at generating and controlling photonic quantum states. A plasmonic nanocavity provides a powerful solution for reducing the effective mode volumes down to nanometre scale, but spatial control at the atomic scale of the coupling with a single molecular emitter is challenging. Here we demonstrate sub-nanometre spatial control over the coherent coupling between a single molecule and a plasmonic nanocavity in close proximity by monitoring the evolution of Fano lineshapes and photonic Lamb shifts in tunnelling electron-induced luminescence spectra. The evolution of the Fano dips allows the determination of the effective interaction distance of ∼1 nm, coupling strengths reaching ∼15 meV and a giant self-interaction induced photonic Lamb shift of up to ∼3 meV. These results open new pathways to control quantum interference and field–matter interaction at the nanoscale.

Method of images applied to driven solid-state emitters

Increasing the collection efficiency from solid-state emitters is an important step towards achieving robust single photon sources, as well as optically connecting different nodes of quantum hardware. A metallic substrate may be the most basic method of improving the collection of photons from quantum dots, with predicted collection efficiency increases of up to 50%. The established 'method-of-images' approach models the effects of a reflective surface for atomic and molecular emitters by replacing the metal surface with a second fictitious emitter which ensures appropriate electromagnetic boundary conditions. Here, we extend the approach to the case of driven solid-state emitters, where exciton-phonon interactions play a key role in determining the optical properties of the system. We derive an intuitive polaron master equation and demonstrate its agreement with the complementary half-sided cavity formulation of the same problem. Our extended image approach offers a straightforward route towards studying the dynamics of multiple solid-state emitters near a metallic surface.

The Infrared Spectra of Customized Magnesium/Teflon/Viton (MTV) Decoy Flares to Thermal Signature of Jet Engine

Decoy flares are customized materials, which are capable of yielding thermal signature to interfere with IR guided missile seekers. In this study, different decoy flares compositions based on Magnesium/Teflon/Viton (MTV) (with fuel percentage ranging from 40:70 wt %) were developed by granulation and subsequent pressing. The spectral performance of developed decoy flare formulations was measured to thermal signature of jet engine nozzle using (FT-MIR 2-6) spectrophotometer. The jet engine thermal signature was characterized with two characteristic peaks over α band (2-3μm) and β band (3-5μm); this signature was correlated to black body emission by the nozzle at 690 0C. Absorption of the IR emission by air could attenuate the signal over the band 3-4 μm due to CO2 and H2O IR absorption. The characteristic intensity ratio of jet engine nozzle (α/ β) = 0.26 . The developed decoy flares offered similar thermal signature but with higher intensity due to the formation carbon soot and MgF2 as black body and active IR emitter respectively. Quantification of these emitting species in the combustion flame was performed using the ICT thermodynamic code. MTV decoy flare with 65 wt % Mg offered an increase in the intensity of α band and β by 21 and 4 times respectively. This customized formulation offered the formation of MgF2 (molecular emitter) and C soot (grey emitter) at 42.4, and 8.8 wt % respectively. It is also offered 48 wt % Mg fuel which would be oxidized in the flame and add much more heat with the formation of MgO (grey body). The characteristic intensity ratio Ɵ was found to be 0.96. This is manuscript would open the route for the development of customized decoy flares with tailored spectral performance.

Synthetic Control of Exciton Behavior in Colloidal Quantum Dots.

Colloidal quantum dots are promising optical and optoelectronic materials for various applications, whose performance is dominated by their excited-state properties. This article illustrates synthetic control of their excited states. Description of the excited states of quantum-dot emitters can be centered around exciton. We shall discuss that, different from conventional molecular emitters, ground-state structures of quantum dots are not necessarily correlated with their excited states. Synthetic control of exciton behavior heavily relies on convenient and affordable monitoring tools. For synthetic development of ideal optical and optoelectronic emitters, the key process is decay of band-edge excitons, which renders transient photoluminescence as important monitoring tool. On the basis of extensive synthetic developments in the past 20-30 years, synthetic control of exciton behavior implies surface engineering of quantum dots, including surface cation/anion stoichiometry, organic ligands, inorganic epitaxial shells, etc. For phosphors based on quantum dots doped with transition metal ions, concentration and location of the dopant ions within a nanocrystal lattice are found to be as important as control of the surface states in order to obtain bright dopant emission with monoexponential yet tunable photoluminescence decay dynamics.

Doping-free white organic light-emitting diodes without blue molecular emitter: An unexplored approach to achieve high performance via exciplex emission

Doping-free white organic light-emitting diodes (DF-WOLEDs) are promising for the low-cost commercialization because of their simplified device structures. However, DF-WOLEDs reported thus far in the literature are based on the use of blue single molecular emitters, whose processing can represent a crucial point in device manufacture. Herein, DF-WOLEDs without the blue single molecular emitter have been demonstrated by managing a blue exciplex system. For the single-molecular-emitter (orange or yellow emitter) DF-WOLEDs, (i) a color rendering index (CRI) of 81 at 1000 cd/m2 can be obtained, which is one of the highest for the single-molecular-emitter WOLEDs, or (ii) a high efficiency of 35.4 lm/W can be yielded. For the dual-molecular-emitter (yellow/red emitters) DF-WOLED, a high CRI of 85 and low correlated color temperature of 2376 K at 1000 cd/m2 have been simultaneously achieved, which has not been reported by previous DF-WOLEDs. Such presented findings may unlock an alternative avenue to the simplified but high-performance WOLEDs.

Deep-blue thermally activated delayed fluorescence dendrimers with reduced singlet-triplet energy gap for low roll-off non-doped solution-processed organic light-emitting diodes

Two TADF dendrimers composed of diphenylsulfone core and oligo-carbazole dendrons were developed. Compared with the reported small molecular TADF emitter bis[4-(3,6-di-tert-butylcarbazole)phenyl]sulfone (G1), dendrimers G2 and G3 inherited the blue light emission but exhibited a reduced singlet-triplet energy gap (ΔEST) with increasing the dendron generation. The efficient triplet-to-singlet (T1-to-S1) transition processes, excellent thermal and amorphous stabilities, good solution processability, and appropriate HOMO/LUMO energy levels enabled these dendrimers to serve as non-doped emitting layers for solution-processed OLEDs. The G2 device achieved deep-blue light with emission peak at 428 nm, a CIE coordinate at (0.15, 0.12) and relatively low efficiency roll-off at high current density.

Increasing the horizontal orientation of transition dipole moments in solution processed small molecular emitters.

The efficiency of organic light emitting diodes (OLEDs) can be improved by controlling the orientation of the transition dipole moment of the emitters. Currently, no effective methods exist for orienting the transition dipole moments in solution processed active layers for OLEDs. We investigate the orientation of the transition dipole moment of small molecular emitters in a host matrix of poly(9,9-dioctylfluorene) (PFO) by means of angle dependent luminescence intensity measurements. The polymer chains of the host orient predominantly in the plane of the film. Fluorescent p-phenylenevinylene oligomers with 6 and 7 repeat units (OPV6, OPV7) are found to also orient preferentially horizontally. The orientation of the emitters can be improved by thermal annealing with up to 90% of transition dipole moments oriented in the plane of the film. The phosphorescent emitter Ir(MDQ)2(acac) shows a degree of horizontal orientation in the polymeric host matrix lower than that which is observed for oligomers, but as high as is observed for the same emitter in evaporated layers. A carbazole derivative capable of thermally activated delayed fluorescence shows a small preference for vertical orientation within the polymer host. The strong orientation of OPV6 and OPV7 in the oriented polymer host is rationalized in terms of their high aspect ratios. The use of PFO as host material in host/guest systems allows achieving horizontal orientation of transition dipole moments in solution processed oligomers and small molecular emitters.

Submillimeter H2O and H2O+emission in lensed ultra- and hyper-luminous infrared galaxies atz ~ 2–4

(abridged) We report rest-frame submillimeter H2O emission line observations of 11 HyLIRGs/ULIRGs at z~2-4 selected among the brightest lensed galaxies discovered in the Herschel-ATLAS. Using the IRAM NOEMA, we have detected 14 new H2O emission lines. The apparent luminosities of the H2O emission lines are $\mu L_{\rm{H_2O}} \sim 6-21 \times 10^8 L_\odot$, with velocity-integrated line fluxes ranging from 4-15 Jy km s$^{-1}$. We have also observed CO emission lines using EMIR on the IRAM 30m telescope in seven sources. The velocity widths for CO and H2O lines are found to be similar. With almost comparable integrated flux densities to those of the high-J CO line, H2O is found to be among the strongest molecular emitters in high-z Hy/ULIRGs. We also confirm our previously found correlation between luminosity of H2O ($L_{\rm{H_2O}}$) and infrared ($L_{\rm{IR}}$) that $L_{\rm{H_2O}} \sim L_{\rm{IR}}^{1.1-1.2}$, with our new detections. This correlation could be explained by a dominant role of far-infrared (FIR) pumping in the H2O excitation. Modelling reveals the FIR radiation fields have warm dust temperature $T_\rm{warm}$~45-75 K, H2O column density per unit velocity interval $N_{\rm{H_2O}}/\Delta V \gtrsim 0.3 \times 10^{15}$ cm$^{-2}$ km$^{-1}$ s and 100 $\mu$m continuum opacity $\tau_{100} > 1$ (optically thick), indicating that H2O is likely to trace highly obscured warm dense gas. However, further observations of $J\geq4$ H2O lines are needed to better constrain the continuum optical depth and other physical conditions of the molecular gas and dust. We have also detected H2O+ emission in three sources. A tight correlation between $L_{\rm{H_2O}}$ and $L_{\rm{H_2O^+}}$ has been found in galaxies from low to high redshift. The velocity-integrated flux density ratio between H2O+ and H2O suggests that cosmic rays generated by strong star formation are possibly driving the H2O+ formation.

Theoretical study of plasmonic lasing in junctions with many molecules

We calculate the quantum state of the plasmon field excited by an ensemble of molecular emitters, which are driven by exchange of electrons with metallic nano-particle electrodes. Assuming identical emitters that are coupled collectively to the plasmon mode but are otherwise subject to independent relaxation channels, we show that symmetry constraints on the total system density matrix imply a drastic reduction in the numerical complexity. For $N_{\text{m}}$ three-level molecules we may thus represent the density matrix by a number of terms scaling as $(N_{\rm m}+8)!/(8!N_{\rm m}!)$ instead of $9^{N_{\text{m}}}$, and this allows exact simulations of up to $N_{\text{m}}=10$ molecules. Our simulations demonstrate that many emitters compensate strong plasmon damping and lead to the population of high plasmon number states and a narrowed linewidth of the plasmon field. For large $N_{\text{m}}$, our exact results are reproduced by an approximate approach based on the plasmon reduced density matrix. With this approach, we have extended the simulations to more than $50$ molecules and shown that the plasmon number state population follows a Poisson-like distribution. An alternative approach based on nonlinear rate equations for the molecular state populations and the mean plasmon number also reproduce the main lasing characteristics of the system.

Engineering and mapping nanocavity emission via precision placement of DNA origami.

Many hybrid devices integrate functional molecular or nanoparticle components with microstructures, as exemplified by the nanophotonic devices that couple emitters to optical resonators for potential use in single-molecule detection, precision magnetometry low threshold lasing and quantum information processing. These systems also illustrate a common difficulty for hybrid devices: although many proof-of-principle devices exist, practical applications face the challenge of how to incorporate large numbers of chemically diverse functional components into microfabricated resonators at precise locations. Here we show that the directed self-assembly of DNA origami onto lithographically patterned binding sites allows reliable and controllable coupling of molecular emitters to photonic crystal cavities (PCCs). The precision of this method is sufficient to enable us to visualize the local density of states within PCCs by simple wide-field microscopy and to resolve the antinodes of the cavity mode at a resolution of about one-tenth of a wavelength. By simply changing the number of binding sites, we program the delivery of up to seven DNA origami onto distinct antinodes within a single cavity and thereby digitally vary the intensity of the cavity emission. To demonstrate the scalability of our technique, we fabricate 65,536 independently programmed PCCs on a single chip. These features, in combination with the widely used modularity of DNA origami, suggest that our method is well suited for the rapid prototyping of a broad array of hybrid nanophotonic devices.

Modular Synthesis of Diarylalkynes and Their Efficient Conversion into Luminescent Tetraarylbutadienes

A series of electronically distinct symmetrical diarylalkynes were prepared via a general Suzuki–Miyaura cross-coupling protocol. These alkynes underwent zirconium-mediated coupling to yield zirconacycles that afford new tetraaryl-1,3-butadienes upon subsequent protonolysis; these butadienes display deep blue or green emission and represent promising building blocks for light-emitting devices. The presented synthetic pathway allows access to new libraries of molecular light emitters with tunable luminescence and considerable thermal- and photostabilities.

A highly efficient directional molecular white-light emitter driven by a continuous-wave laser diode.

Tailored light sources have greatly advanced technological and scientific progress by optimizing the emission spectrum or color and the emission characteristics. We demonstrate an efficient spectrally broadband and highly directional warm-white-light emitter based on a nonlinear process driven by a cheap, low-power continuous-wave infrared laser diode. The nonlinear medium is a specially designed amorphous material composed of symmetry-free, diamondoid-like cluster molecules that are readily obtained from ubiquitous resources. The visible part of the spectrum resembles the color of a tungsten-halogen lamp at 2900 kelvin while retaining the superior beam divergence of the driving laser. This approach of functionalizing energy-efficient state-of-the-art semiconductor lasers enables a technology complementary to light-emitting diodes for replacing incandescent white-light emitters in high-brilliance applications.

Cavity-enhanced light emission from electrically driven carbon nanotubes

An important advancement towards optical communication on a chip would be the development of integratable, nanoscale photonic emitters with tailored optical properties. Here we demonstrate the use of carbon nanotubes as electrically driven high-speed emitters in combination with a nanophotonic cavity that allows for exceptionally narrow linewidths. The one-dimensional photonic crystal cavities are shown to spectrally select desired emission wavelengths, enhance intensity and efficiently couple light into the underlying photonic network with high reproducibility. Under pulsed voltage excitation, we realize on-chip modulation rates in the GHz range, compatible with active photonic networks. Because the linewidth of the molecular emitter is determined by the quality factor of the photonic crystal, our approach effectively eliminates linewidth broadening due to temperature, surface interaction and hot-carrier injection. Carbon nanotubes in a nanocavity offer a route to narrow-linewidth on-chip light emitters.

(Invited) Cavity Enhanced Light Emission from Electrically Driven Carbon Nanotubes

An important advancement towards optical communication on a chip would be the development of integrable, nanoscale photonic emitters with tailored optical properties. Here we demonstrate the use of carbon nanotubes as electrically driven high-speed emitters in combination with a nanophotonic cavity which allows for exceptionally narrow linewidths [1]. The one-dimensional photonic crystal cavities are shown to spectrally select desired emission wavelengths, enhance intensity and efficiently couple light into the underlying photonic network with high reproducibility. Under pulsed voltage excitation we realize on-chip modulation rates in the GHz range, compatible with active photonic networks. Because the linewidth of the molecular emitter is determined by the quality factor of the photonic crystal, our approach effectively eliminates linewidth broadening due to temperature, surface interaction and hot-carrier injection. This work was supported by the Volkswagen Foundation, by the Helmholtz Society through the program STN and the KNMF. [1] Felix Pyatkov, Valentin Fütterling, Svetlana Khasminskaya, Benjamin Flavel, Frank Hennrich, Manfred M. Kappes, Ralph Krupke, and Wolfram H.P. Pernice, under review

Molecular MIMO: From Theory to Prototype

In diffusion-based molecular communication, information transport is governed by diffusion through a fluid medium. The achievable data rates for these channels are very low compared to the radio-based communication system, since diffusion can be a slow process. To improve the data rate, a novel multiple-input multiple-output (MIMO) design for molecular communication is proposed that utilizes multiple molecular emitters at the transmitter and multiple molecular detectors at the receiver (in RF communication these all correspond to antennas). Using particle-based simulators, the channel's impulse response is obtained and mathematically modeled. These models are then used to determine interlink interference (ILI) and intersymbol interference (ISI). It is assumed that when the receiver has incomplete information regarding the system and the channel state, low complexity symbol detection methods are preferred since the receiver is small and simple. Thus, four detection algorithms are proposed-adaptive thresholding, practical zero forcing with channel models excluding/including the ILI and ISI, and Genie-aided zero forcing. The proposed algorithms are evaluated extensively using numerical and analytical evaluations.

Anthracene-based molecular emitters for non-doped deep-blue organic light emitting transistors

Twisted anthracene derivatives, including the newly synthesized DIPAXA compound, are here investigated as blue emitters in non-doped organic light emitting transistors (OLETs).

Stapled helical o-OPE foldamers as new circularly polarized luminescence emitters based on carbophilic interactions with Ag(i)-sensitivity.

ortho-Oligo(phenylene)ethynylenes (o-OPEs) stapled with enantiopure 2,3-dihydroxybutane diethers have highly intense circular dichroism (CD) spectra and excellent circular polarized luminescence (CPL) responses (glum values up to 1.1 × 10-2), which are consistent with homochiral helically folded structures. In the presence of Ag(i), a change in the CPL emission is observed, representing the first example of CPL active small organic molecular emitters, which can be modulated by carbophilic interactions in a reversible manner.