Strong light-matter interactions and exciton-polaritons in carbon nanotubes

Exciton-polaritons are quasiparticles with hybrid light-matter character, offering a unique combination of photonic properties, such as a light mass, with those of excitons, for example strong nonlinearities and fast relaxation. Strong light-matter coupling enables a rich set of polaritonic quantum phenomena as well as applications. While originally observed in inorganic materials, organic semiconductors have recently attracted tremendous attention since their large oscillator strength facilitates particularly strong light-matter coupling and enabled polariton formation at room temperature. In particular, electrical excitation is pursued to apply these quantum-mechanical effects in practical polariton devices. However, a lack of organic materials with sufficiently high charge-carrier mobility and suitable device architectures impede their full utilization. Nanomaterials, in particular low-dimensional materials, present a novel material class that combines the excitonic properties of organic and electric characteristics of inorganic materials. In this thesis, single-walled carbon nanotubes (SWCNTs) were employed to demonstrate, for the first time, exciton-polariton formation in the near infrared (nIR) at room temperature. SWCNTs are identified as an ideal material facilitating strong light-matter coupling due to their high oscillator strength. Moreover, by implementing a strongly coupled microcavity into a light-emitting field effect transistor (LEFET), electrically pumped polariton emission at high current density was observed. These practical polariton devices emit in ranges relevant for telecommunication and support high currents due to the excellent optoelectronic properties of SWCNTs. Pumping polaritons at high rates presents a major step towards electrical lasing with carbon-based materials. For the realization of these experiments it was crucial to overcome current limitations in post-growth sorting of SWCNTs, which are intrinsically restricted to low-volume and damage the nanotubes. For this purpose, selective polymer wrapping by high-speed shear-force mixing, which can be easily scaled up, was developed. By using shear forces, the SWCNT-yield was drastically increased while, at the same time, the SWCNT-quality could be improved. In addition to strong light-matter coupling and polariton emission, the selected SWCNTs were employed in organic light-emitting diodes. These devices showed pure nIR emission with narrow linewidth at efficient electrical performance. This work paves the way for fundamental investigations as well as advanced applications of SWCNT-based optoelectronic devices

Near-infrared exciton-polaritons in strongly coupled single-walled carbon nanotube microcavities

This research was financially supported by the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. 306298 (EN-LUMINATE) and under the European Union’s Horizon 2020 Framework Programme (FP/2014-2020)/ERC Grant Agreement No. 640012 (ABLASE), by EPSRC through the CM-DTC (EP/L015110/1) and by the Scottish Funding Council through SUPA. J.Z. thanks the Alfried Krupp von Bohlen und Halbach-Stiftung via the “Alfried Krupp Förderpreis für junge Hochschullehrer” for general support.Exciton-polaritons form upon strong coupling between electronic excitations of a material and photonic states of a surrounding microcavity. In organic semiconductors the special nature of excited states leads to particularly strong coupling and facilitates condensation of exciton-polaritons at room temperature, which may lead to electrically pumped organic polariton lasers. However, charge carrier mobility and photo-stability in currently used materials is limited and exciton-polariton emission so far has been restricted to visible wavelengths. Here, we demonstrate strong light-matter coupling in the near infrared using single-walled carbon nanotubes (SWCNTs) in a polymer matrix in a planar metal-clad cavity. By exploiting the exceptional oscillator strength and sharp excitonic transition of (6,5) SWCNTs, we achieve large Rabi splitting (> 110 meV), efficient polariton relaxation and narrow band emission (< 15 meV). Given their high charge carrier mobility and excellent photostability, SWCNTs represent a promising new avenue towards practical exciton-polariton devices operating at telecommunication wavelengths. Publisher PDFPublisher PDFPeer reviewe

Preparation of WS2-PMMA composite films for optical applications

C. B. acknowledges the German research foundation DFG under Emmy-Noether grant BA4856/2-1. C. B., J. Z. and M. C. G. acknowledge the Volkswagen foundation under grant agreement no. 93404-93406. W. J. B. gratefully acknowledges support by a research grant from Science Foundation Ireland (SFI) under Grant Number 12/IA/1306.Thus far, research activities of 2D materials in optics, photonics and optoelectronics predominantly focus on micromechanically cleaved or grown nanosheets. Here, we show that high quality liquid-exfoliated nanosheets offer an alternative approach. Starting from well-defined, monolayer rich WS2 dispersions obtained after liquid exfoliation and size selection in aqueous surfactant, we present an optimised protocol facilitating transfer of the nanosheets to a polymer solution in organic media. From such dispersions, we fabricate WS2–polymer thin films by spin coating. The characteristic photoluminescence of WS2 monolayers is retained in the film at 2.04 eV without broadening (line width 40 meV) or significant changes in the line-shape. This confirms that nanosheet aggregation is efficiently prevented on transfer and deposition. The films are extremely smooth and uniform over large areas with a root mean square roughness <0.5 nm. To demonstrate the potential in optical applications, the nonlinear optical response was studied, revealing promise as optical limiter. In addition, we show that the photoluminescence can be manipulated by coupling the exciton response to cavity photons in a Ag microcavity.PostprintPeer reviewe

Electroluminescence Generation in PbS Quantum Dot Light-Emitting Field-Effect Transistors with Solid-State Gating

The application of light-emitting field-effect transistors (LEFET) is an elegant way of combining electrical switching and light emission in a single device architecture instead of two. This allows for a higher degree of miniaturization and integration in future optoelectronic applications. Here, we report on a LEFET based on lead sulfide quantum dots processed from solution. Our device shows state-of-the-art electronic behavior and emits near infrared photons with a quantum yield exceeding 1% when cooled. We furthermore show how LEFETs can be used to simultaneously characterize the optical and electrical material properties on the same device and use this benefit dot film. to investigate the charge transport through the quantum dot film

Orientation distributions of vacuum-deposited organic emitters revealed by single-molecule microscopy

This work was supported by the Volkswagen Foundation (No. 93404) and the DFG-funded Research Training Group “Template-Designed Organic Electronics (TIDE)”, RTG2591. M.C.G. acknowledges support from the Alexander von Humboldt Stiftung through the Humboldt-Professorship. A.M. acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement No. 101023743 (PolDev).The orientation of luminescent molecules in organic light-emitting diodes strongly influences device performance. However, our understanding of the factors controlling emitter orientation is limited as current measurements only provide ensemble-averaged orientation values. Here, we use single-molecule imaging to measure the transition dipole orientation of individual emitter molecules in a state-of-the-art thermally evaporated host and thereby obtain complete orientation distributions of the hyperfluorescence-terminal emitter C545T. We achieve this by realizing ultra-low doping concentrations (10−6 wt%) of C545T and minimising background levels to reliably measure its photoluminescence. This approach yields the orientation distributions of >1000 individual emitter molecules in a system relevant to vacuum-processed devices. Analysis of solution- and vacuum-processed systems reveals that the orientation distributions strongly depend on the nanoscale environment of the emitter. This work opens the door to attaining unprecedented information on the factors that determine emitter orientation in current and future material systems for organic light-emitting devices.Publisher PDFPeer reviewe

Multispectral electroluminescence enhancement of single-walled carbon nanotubes coupled to periodic nanodisk arrays

The integration of periodic nanodisk arrays into the channel of a light-emitting field-effect transistor leads to enhanced and directional electroluminescence from thin films of purified semiconducting single-walled carbon nanotubes. The maximum enhancement wavelength is tunable across the near-infrared and is directly linked to the periodicity of the arrays. Numerical calculations confirm the role of increased local electric fields in the observed emission modification. Large current densities are easily achieved due to the high charge carrier mobilities of carbon nanotubes and will facilitate new electrically driven plasmonic devices

Infrared organic light-emitting diodes with carbon nanotube emitters

This research was financially supported by the Volkswagen Foundation (93404), the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. 306298 (EN-LUMINATE) and by EPSRC (EP/R010595/1). C.M. acknowledges funding by the European Commission through a Marie Skłodowska Curie Individual Fellowship (703387). J.Z. thanks the Alfried Krupp von Bohlen und Halbach-Stiftung via the “Alfried Krupp Förderpreis für junge Hochschullehrer” for general support.While organic light-emitting diodes (OLEDs) covering all colors of the visible spectrum have been demonstrated, suitable organic emitter materials in the near-infrared (nIR) beyond 800 nm are still lacking. Here, we demonstrate the first OLED based on single-walled carbon nanotubes (SWCNTs) as the emitter. By using a multi-layer stacked architecture with matching charge blocking and charge transport layers, we achieve narrow band electroluminescence at wavelengths between 1000 nm and 1200 nm, with spectral features characteristic of excitonic and trionic emission of the (6,5) SWCNTs used. We investigate the OLED performance in detail and find that local conduction hot-spots lead to pronounced trion emission. Analysis of the emissive dipole orientation shows a strong horizontal alignment of the SWCNTs with an average inclination angle of 12.9° with respect to the plane, leading to an exceptionally high outcoupling efficiency of 49 %. Our SWCNT-based OLEDs represent a highly attractive platform for emission across the entire nIR.PostprintPeer reviewe

Electrical pumping and tuning of exciton-polaritons in carbon nanotube microcavities

This research was financially supported by the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. 306298 (EN-LUMINATE) and under the European Union’s Horizon 2020 Framework Programme (FP/2014-2020)/ERC Grant Agreement No. 640012 (ABLASE) and by the Scottish Funding Council (through SUPA). L.T. thanks the EPSRC for support through the CM-DTC (EP/L015110/1). J.Z. thanks the Alfried Krupp von Bohlen und Halbach-Stiftung via the “Alfried Krupp Förderpreis für junge Hochschullehrer” for general support.Exciton-polaritons are hybrid light–matter particles that form upon strong coupling of an excitonic transition to a cavity mode. As bosons, polaritons can form condensates with coherent laser-like emission. For organic materials, optically pumped condensation was achieved at room temperature but electrically pumped condensation remains elusive due to insufficient polariton densities. Here we combine the outstanding optical and electronic properties of purified, solution-processed semiconducting (6,5) single-walled carbon nanotubes (SWCNTs) in a microcavity-integrated light-emitting field-effect transistor to realize efficient electrical pumping of exciton-polaritons at room temperature with high current densities (>10 kA cm−2) and tunability in the near-infrared (1,060 nm to 1,530 nm). We demonstrate thermalization of SWCNT polaritons, exciton-polariton pumping rates ~104 times higher than in current organic polariton devices, direct control over the coupling strength (Rabi splitting) via the applied gate voltage, and a tenfold enhancement of polaritonic over excitonic emission. This powerful material–device combination paves the way to carbon-based polariton emitters and possibly lasers.PostprintPostprintPeer reviewe

Ultrastrong coupling of electrically pumped near-infrared exciton-polaritons in high mobility polymers

This research was financially supported by the European Research Council under the European Union's Seventh Framework Programme (FP/2007- 2013)/ERC Grant Agreement No. 306298 (EN -LUMINATE) and under the European Union’s Horizon 2020 Framework Programme (FP/2014- 2020)/ERC Grant Agreement No. 640012 (ABLASE) and by the EPSRC Programme Grant EP/P030017/1. L.T. thanks EPSRC for support through the CM -DTC (EP/L015110/1).Exciton-polaritons are quasiparticles with hybrid light–matter properties that may be used in new optoelectronic devices. Here, electrically pumped ultrastrongly coupled exciton-polaritons in a high-mobility donor–acceptor copolymer are demonstrated by integrating a light-emitting field-effect transistor into a metal-clad microcavity. Near-infrared electroluminescence is emitted exclusively from the lower polariton branch, which indicates efficient relaxation. A coupling strength of 24% of the exciton transition energy implies the system is in the ultrastrong coupling regime with a narrow and almost angle-independent emission. The lower polariton energy, which can be adjusted by the cavity detuning, strongly influences the external quantum efficiency of the device. Driving the transistors at ambipolar current densities of up to 4000 A cm−2 does not decrease the coupling strength or polariton emission efficiency. Cavity-integrated light-emitting field-effect transistors thus represent a versatile platform for polariton emission and polaritonic devices.Publisher PDFPeer reviewe

Correlating the transition dipole moment orientation of phosphorescent emitter molecules in OLEDs with basic material properties

This work received funding from the ESF/EU project OrganoMechanics and the European Community's Seventh Framework Programme under Grant Agreement no. FP7 267995 (NUDEV)The orientation of the emissive dipole moment of seven iridium-based phosphorescent emitter molecules commonly used in organic light-emitting diodes (OLEDs) is investigated. The orientation of Ir(ppy)3, Ir(ppy)2(acac), Ir(chpy)3, Ir(dhfpy)2(acac), Ir(BT)2(acac), Ir(MDQ)2(acac), and Ir(piq)3 is determined by measuring the angle dependent spectral radiant intensity of the transverse magnetic polarized emission from p–i–n OLEDs comprising these emitters. The experimental data are compared to the intensity calculated by a multilayer simulation method that includes the anisotropy factor describing the average dipole orientation. Surprisingly, among these molecules, Ir(ppy)3 is the only emitter showing an isotropically distributed transition dipole moment. In order to correlate our results with basic molecular properties, the permanent dipole moment and the size of the molecules are calculated by density functional theory (DFT). The dipole–dipole potential obtained for Ir(ppy)3 is more than 2.5 times larger than those for all other emitter molecules investigated here, indicating that this parameter is correlated with the transition dipole moment orientation.PostprintPeer reviewe