Excited-state charge transfer enabling MoS2/Phthalocyanine photodetectors with extended spectral sensitivity
The combination of inorganic monolayer (ML) transition-metal dichalcogenides (TMDCs) with organic semiconductors holds the promise to further improve opto-electronic device properties with added functionality. The authors of this CRC 951 research highlight investigate a hybrid inorganic/organic system (HIOS) consisting of metal-free phthalocyanine (H2Pc) as thin organic absorber layer and ML MoS2 as TMDC. Via a combination of photoemission (PES), photoluminescence (PL), and photocurrent action spectroscopy they demonstrate, that excited-state charge transfer from the H2Pc layer enhances the photo response of ML MoS2 without loss in sensitivity extended to spectral regions where the TMDC is transparent. This observation is explained by the staggered type II energy-level alignment at the hybrid interface facilitating efficient exciton dissociation and excited-state charge transfer with the holes residing in the H2Pc HOMO and the electrons in the MoS2 conduction band. In hybrid photodetectors, these transferred charges increase the concentration of carriers in MoS2 and with that its photoconductivity. The present demonstration of a highly efficient carrier generation in TMDC/organic hybrid structures paves the way for future nanoscale photodetectors with very wide spectral sensitivity.
(a) Schematic design of the hybrid H2Pc/MoS2 photodetecting device. The H2Pc layer thickness is dH2Pc = 3.0 nm b Photoresponse of the hybrid (blue) and the reference MoS2-only (red) device. The spectra were normalized at the spectral position where H2Pc does not absorb, i.e., between 2.5 and 2.55 eV. The difference between the spectra of the hybrid (Rhyb) and reference (Rref) devices ΔR = Rhyb – Rref (green).
Excited-State Charge Transfer Enabling MoS2/Phthalocyanine Photodetectors with Extended Spectral Sensitivity
Insights into charge transfer at the atomically precise nanocluster/semiconductor interface for in-depth understanding the role of nanocluster in photocatalytic system
A TiO2/cluster composite of type II junction configuration for photocatalytic hydrogen evolution is built by deposition of atomically precise Ag44 nanocluster on TiO2. Besides photosensitizer, the cluster is found to serve as co-catalyst to improve the charge separation efficiency of the system, which is quite different from the well-known plasmonic nanoparticle (NP) enhanced systems. The hydrogen production rate by Ag44-TiO2 is ten times higher than that of the pure TiO2 and five times higher than that of the Ag NP-TiO2.
(a) Schematic illustration of the H2 production by Ag44-TiO2 under simulated sunlight; (b) Catalytic performance of TiO2 (black), Ag NP-TiO2 (yellow) and Ag44- TiO2 (red).
Insights into charge transfer at the atomically precise nanocluster/semiconductor interface for in‐depth understanding the role of nanocluster in photocatalytic system
Entangled Photons for Mid-Infrared Sensing - Quantum Futur Award 2019 for Aron Vanselow
Aron Vanselow receives the second prize of the Quantum Futur Award 2019, sponsored by Ministry of Science and Education for his master thesis at Humboldt University Berlin. It has long been anticipated that entangled photons hold the promise to drive a paradigm shift in imaging and sensing. Real-world implementations, however, have lagged behind their classical counterparts, because of low efficiency, loss and decoherence.
Aron Vanselow’s thesis, carried out iin the junior research group "Nonlinear Quantum Optics", led by Dr. Sven Ramelow, who is a member of IRIS Adlershof, presents the first experimental demonstration of mid-infrared frequency-domain optical coherence tomography (OCT) with entangled photons. OCT is an important depth-imaging method in biomedical diagnostics as well as non-destructive testing allowing for 3D microscopy. OCT in the mid-IR range enables looking inside strongly scattering media, where commercial systems which are all at shorter wavelengths don’t work.
The proof-of-principle setup developed by Aron Vanselow, Sven Ramelow and their colleagues is powered by quantum entanglement generated in a patented new crystal. Importantly, the reached performances are already comparable to the best conventional techniques while exposing the sample to 8 orders of magnitude less optical power. At the same time the technological overhead is drastically reduced compared with classical techniques using only compact and cost-effective components.
With the thesis demonstrating fast 2D and 3D imaging of highly scattering real-world samples (ceramics, paint layers) with 20 μm lateral and 10 μm depth resolution it has immediate relevance for applications in non-destructive testing such as quality control of coating thicknesses, cultural heritage conservation and microfluidics.
Direct measurement of quantum efficiency of single-photon emitters in hexagonal boron nitride
Two-dimensional materials like boron nitride (h-BN) have recently attracted the attention of the quantum optics and nano optics community. Individual single photon emitting (SPE) defects can be found even in single layers of h-BN. These emitters are bright and stable and have a narrow emission line, making them potentially suitable for use in quantum communication devices. As the field is still young, it is difficult to create SPEs with desired properties. One reason for this is the yet unknown atomic origin of the defect, which could help to identify processing steps that could lead to the desired outcome. In order to determine the atomic origin of an emitter, calculations are carried out under the assumption of different atomic configurations and compared with the observed spectra. Unfortunately, the h-BN SPEs spectra are distributed over a wide range, which makes the application of this method difficult. Another intrinsic property is the quantum efficiency (QE), i.e. the branching ratio between a radiative rate and the total (radiative and non-radiative) decay rate.
Schematics of the performed experiment and a distance dependent lifetime measurement.
(a) The AFM is equipped with a gold-coated hemispherical tip aligned with an SPE in h-BN and held at a variable distance. The objective lens on the bottom of the glass excites the SPE (green pulsed laser) and collects its emission. With this setup, distance-dependent lifetime measurements can be performed, one such measurement is shown in (b) (points). To determine the QE, an adjustment was performed (blue solid line). The same fit function with a QE of 1.0 is represented by the green solid line as a reference.
Researchers of Nanooptik AG of Humboldt-University of Berlin in cooperation with the Technical University of Sydney could now directly measure the absolute QE of single defects in h-BN. The underlying principle is based on the proportionality between a controlled change in the local density of the states into which the emitter can emit and the lifetime of the excited state. The researchers implemented this experimentally by controlling the distance between the SPE and a mirror with nanometer accuracy while measuring the lifetime of the excited state. In this way, not only the high QE of up to 87(7) % was determined, but also a correlation between fluorescence wavelength and QE was found. This paves the way for a better understanding of the origin of the emitters.
Direct measurement of quantum efficiency of single-photon emitters in hexagonal boron nitride N. Nikolay, N. Mendelson, E. Özelci, B. Sontheimer, F. Böhm, G. Kewes, M. Toth, I. Aharonovich, and O. Benson
Optica 6 (2019) 1084
Article of IRIS junior research group leader Michael J. Bojdys published in Nature Communications
The IRIS junior research group leader Michael J. Bojdys and his international team have achieved a great success: Their article “Real-time optical and electronic sensing with a β-amino enone linked, triazine-containing 2D covalent organic framework” has been selected to be published in the renowned journal Nature Communications. Bojdys article deals with aromatic two-dimensional covalent organic frameworks (2D COFs), which are a class of porous polymers that allow the precise incorporation of organic units into periodic structures.COFs can be chemically designed to incorporate particular surface functional groups which can be exploited to tune the optical and electronic properties. However, low stability towards chemical triggers has hampered their practical implementations.
Together with a team from the Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences (Prague, Czech Republic), IRIS junior research group leader Michael J. Bojdys and his team from Humboldt-Universität zu Berlin have explored a new design principle for COFs that makes use of strong, overall conjugation and incorporation of donor-acceptor domains. In this study a new, a highly stable chemoresistant β-amino enone linked, triazine-containing COF was used as a real-time, reversible optical and electronic sensor for volatile acids and bases. The team was further able to conclude that the sensing capabilities of the COF was achieved by preferential protonation of the electron accepotor – a triazine ring in the structure – , resulting in an optical response visible to the naked eye and an increase of bulk electrical conductivity by two orders of magnitude. These findings demonstrate a powerful approach to design more practical sensors and switches, and take genuine advantage of the chemoresistant make-up, porous structure, and overall conjugation of fully-aromatic systems. IRIS Adlershof would like to congratulate Michael J. Bojdys and his team on this successful study and its publication in Nature Communications!
Due to his great enthusiasm for the concept of IRIS Adlershof and the research carried out here, ERC-grant holder Bojdys joined the Humboldt-Universität zu Berlin and IRIS Adlershof in 2018 as leader of the junior research group “Functional Materials”. The group’s research aims at the development of metal-free, electronic components for transistors and sensors on the basis of functional materials made up of light, covalently-bonded atoms. At the heart of the project lies the challenge to transfer the control mechanisms and modularity known from molecular, organic chemistry to macroscopic structures.
Real-time optical and electronic sensing with a β-amino enone linked, triazine-containing 2D covalent organic framework
R. Kulkarni, Y. Noda, D.K Barange, Y.S. Kochergin, P. Lyu, B. Balcarova, P. Nachtigall, and M.J. Bojdys Nat. Commun 10 (2019) 3228
A novel semiconductor from the family of carbon nitrides
Research teams from the Humboldt-Universität and the Helmholtz Zentrum Berlin (HZB) have investigated a new material from the family of carbon nitrides. Triazine-based graphitic carbon nitride (TGCN) is a semiconductor that is useful in optoelectronic applications. Its structure is two-dimensional and layered, and it resembles that of graphene. Unlike graphene, its conductivity between the layers is 65-times higher than in-plane.
Some organic materials can be used in optoelectronics just like silicon-based semiconductors. Whether in solar cells, light-emitting diodes, or as transistors – the important property is the bandgap, i.e. the energy-difference of the electrons in the valence band and the conduction band. The basic principle underlying all electronic components is that electrons can be promoted by light or by voltage between the valence and the conduction band. Here, bandgaps between 1 and 2 eV are ideal.
A team led by the chemist Dr. Michael J. Bojdys from the chemistry department and IRIS Adlershof of the Humboldt-Universität zu Berlin, has recently synthesized an organic semiconductor from the family of carbon nitrides. This triazine-based graphitic carbon nitride (TGCN) consists exclusively from carbon and nitrogen atoms and can be grown as a brown film on quartz glass substrates. The C- and N-atoms connect in hexagonal, honeycomb patterns like carbon atoms in graphene. Just like in graphene, the crystal structure of TGCN is based on layered, two-dimensional sheets. In graphene, in-plane conductivity is excellent, however, it is much lower through the planes. In the case of TGCN, the opposite is observed: through-plane conductivity is 65-times higher than in-plane. With a bandgap of 1.7 eV TGCN is a good candidate for optoelectronic applications.
The HZB-physicist Dr. Christoph Merschjann has examined the charge carrier transport in samples of TGCN using time-resolved absorption measurements in the femto- to nanosecond regime at the laser lab JULiq – a joint lab between the HZB and the Freie Universität Berlin. Such laser experiments offer a unique way to correlate macroscopic conductivity and microscopic transport models. From his measurements, he was able to deduce how the charge carriers diffuse throughout the material. “Electrons do not exit the hexagonal honeycombs of triazine units horizontally, but they move at a slope to the nearest triazine-unit in the neighboring layer. The crystal structure of the material leads to a preferred movement of charge carriers along tube-like channels.” This mechanism could explain why the conductivity of TGCN is fundamentally higher through-plane than in-plane. “TGCN is the hitherto best candidate to replace silicon semiconductors and the critical, rare-earth dopants used in their manufacture”, says Michael Bojdys. “The production method for TGCN that we developed in my group at the Humboldt-Universität zu Berlin yields flat layers of semiconducting TGCN on insulating quartz glass. This enables relatively easy upscaling and device production.”
Directional charge transport in layered, two‐dimensional triazine‐based graphitic carbon nitride
Y. Noda, C. Merschjann, J. Tarábek, P. Amsalem, N. Koch, and M.J. Bojdys Angew. Chem. Int. Ed. 58 (2019) 9394
Researchers demonstrate very large electric tuning of a single quantum emitter at room temperature
Bright and tunable solid-state single-photon emitters (SPEs) are required for the realization of scalable quantum photonic technologies. Recently, optically active defects in a two-dimensional material, boron nitride (h-BN), have been extensively studied as bright single-photon emitters with a narrow linewidth and operating at room temperature. The layered nature of h-BN also offers potential advantages for integration in novel opto-electronic hybrid elements including photonic resonators, waveguides, modulator, and detectors. In order to exploit the functionality of such elements a tuning of the emitter’s fluorescence line is essential. Tuning via the Stark effect using a static electric field has been suggested for various solid-state emitters, such as quantum dots or color centers in diamond. Researcher from the Institute of Physics of Humboldt-University together with coworkers from the University of Technology in Sydney were now able to demonstrate controlled and reversible Stark tuning of individual emitters in hBN. They used a metallic tip of an atomic force microscope (AFM) to locally select a single emitter and tune it over a record range of up to 5.5 nanometers at room temperature.
a) Structure of a defect in hexagonal Boron Nitride. b) Schematic of the experiment, where a metallic AFM tip is placed above a single defect emitter and a bias voltage is applied. C) Measured Stark-shift of the narrow fluorescence line.
Based on their results the researchers suggest building a room-temperature single photon source, which can be tuned electrically in or out of a resonance of a plasmonic resonator. “Such a source would be highly desirable as a reliable non-classical light source for applications in quantum-enhanced sensing and metrology or in quantum key distribution.” says Prof. Oliver Benson, who is researcher in IRIS Adlershof and leads the Humboldt-team.
Very large and Reversible Stark-Shift Tuning of Single Emitters in Layered Hexagonal Boron Nitride N. Nikolay, N. Mendelson, N. Sadzak, F. Böhm, T. T. Tran, B. Sontheimer, I. Aharonovich, and O. Benson Phys. Rev. Applied 11 (2019) 041001
Enlightening full-color displays
Researchers from the University of Strasbourg & CNRS (France), in collaboration with University College London (United Kingdom), and Humboldt University Berlin (Germany), have shown that a subtle combination of light-emitting semiconducting polymers and small photoswitchable molecules can be used to fabricate light-emitting organic transistors operating under optical remote control, paving the way to the next generation of multifunctional optoelectronic devices. These achievements have now been published in Nature Nanotechnology. Organic light-emitting transistors are widely recognized as key components in numerous optoelectronic applications. However, the integration of multiple functionalities into a single electronic device remains a grand challenge in this technological sector. Moreover, the next generation of displays requires to encode high-density visual information into single and ultra-small pixels. Now a team of researchers from Strasbourg, London, and Berlin has taken a big step forward by creating the first organic light-emitting transistor that can be remote-controlled by light itself. They have been blending a custom-designed molecule as a miniaturized optical switch with a light-emitting semiconducting polymer. Upon illumination with ultraviolet and visible light, the molecular switch reversibly changes its electronic properties. As a consequence, the electrical and optical response of the device can be modulated simultaneously by light, which serves as an optical remote control. However, having a device capable of producing only one color is not sufficient for daily-life applications, such as full-color displays. By choosing appropriate photoswitchable molecules and blending them with suitable light-emitting polymers, the researchers have demonstrated that this new type of organic light-emitting transistors can shine in the range of the three primary colors (red, green, and blue), thereby covering the entire visible spectrum. The disruptive potential of such approach was demonstrated by writing and erasing spatially defined emitting patterns (a letter for example) within a single device with a beam of laser light, allowing a non-invasive and mask-free process, with a response time on the microsecond scale and a spatial resolution of a few micrometers, thus outperforming the best “retina” displays. Clearly, these findings represent a major breakthrough that offers multiple perspectives for smart displays, active optical memories, and light-controlled logic circuits.
Optically switchable organic light-emitting transistors
L. Hou, X. Zhang, G.F. Cotella, G. Carnicella, M. Herder, B.M. Schmidt, M. Pätzel, S. Hecht, F. Cacialli, and P. Samorì Nature Nanotechnology 14 (2019) 347
Hybrid Organic-Inorganic Perovskites: Promising Substrates for Single-Atom Catalysts
Mononuclear metal species are widespread in enzymes and homogeneous catalysts. When such isolated single metal atoms are placed on a solid surface, they can also play an important role in heterogeneous catalysis. In the past few years, great attention has been paid to single-atom catalysts, not only because they can exhibit superior catalytic performance, but also, because they offer a novel way of maximizing the efficiency of utilizing atoms, which is especially desirable in the use of scarce metal elements like platinum. However, single atoms cannot work in isolation but need to be dispersed on suitable substrates.
Qiang Fu and Claudia Draxl have recently demonstrated that hybrid organic-inorganic perovskites ˗ the emerging candidates in solar-cell applications ˗ are highly promising substrates for Pt single atom catalysts. Through systematic first-principles calculations, they found that single Pt atoms are stabilized on such substrates through a synergistic cooperation between covalent bond formation and charge transfer. The generated Pt sites possess excellent catalytic properties in CO oxidation and may be able to play a role in CO2 reduction. This work not only has promising consequences in single-atom catalysis but also sheds light on potential applications of hybrid perovskites as photocatalysts.
Ab initio modeling of novel photocathode materials for high brightness electron beams
The development of laser-driven photocathode radio-frequency electron injectors has become a significant enabling technology for free electron lasers and for the fourth generation of light sources. Such remarkable progress come with quest for novel materials that are able to operate in the visible region with optimized quantum efficiency and minimized intrinsic emittance. Multi-alkali antimonides have recently emerged as ideal materials for photocathode applications in spite of the little fundamental knowledge regarding their electronic and optical properties. A team composed of scientists from the HU Berlin and HZB carried out a systematic investigation of the electronic structure and excitations of CsK2Sb, an exemplary and promising multi-alkali antimonide, by means of first-principles many-body methods. The results of their study confirm that this material is an excellent candidate for photocathode applications and pioneers a new research line bridging solid-state theory, material science, and accelerator physics in view of an improved modelling and design of materials for the next-generation electron sources.
This work was published on The Journal of Physics: Condensed Matter (http://iopscience.iop.org/article/10.1088/1361-648X/aaedee) as an invited contribution to Prof. Caterina Cocchi, a member of IRIS Adlershof since 2017, to the special issue “Emerging leaders 2018” (http://iopscience.iop.org/journal/0953-8984/page/Emerging-leaders-2018).
First-principles many-body study of the electronic and optical properties of CsK2Sb, a semiconducting material for ultra-bright electron sources C. Cocchi, S. Mistry, M. Schmeißer, J. Kühn, and T. Kamps J. Phys.: Condens. Matter 31 (2019) 014002