19,763 research outputs found

    Triply bonded pancake ŌÄ-dimers stabilized by tetravalent actinides

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    Aromatic ŌÄ-stacking is a weakly attractive, noncovalent interaction often found in biological macromolecules and synthetic supramolecular chemistry. The weak nondirectional nature of ŌÄ-stacking can present challenges in the design of materials owing to their weak, nondirectional nature. However, when aromatic ŌÄ-systems contain an unpaired electron, stronger attraction involving face-to-face ŌÄ-orbital overlap is possible, resulting in covalent so-called ‚Äúpancake‚ÄĚ bonds. Two-electron, multicenter single pancake bonds are well known, whereas four-electron double pancake bonds are rare. Higher-order pancake bonds have been predicted, but experimental systems are unknown. Here, we show that six-electron triple pancake bonds can be synthesized by a 3-fold reduction of hexaazatrinaphthylene (HAN) and subsequent stacking of the [HAN]3‚Äď triradicals. Our analysis reveals a multicenter covalent triple pancake bond consisting of a ŌÉ-orbital and two equivalent ŌÄ-orbitals. An electrostatic stabilizing role is established for the tetravalent thorium and uranium ions in these systems. We also show that the electronic absorption spectrum of the triple pancake bonds closely matches computational predictions, providing experimental verification of these unique interactions. The discovery of conductivity in thin films of triply bonded ŌÄ-dimers presents new opportunities for the discovery of single-component molecular conductors and other spin-based molecular materials

    Exciton Dynamics, Interaction, and Transport in Monolayers of Transition Metal Dichalcogenides

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    Monolayers Transition metal dichalcogenides (TMDs) have attracted much attention in recent years due to their promising optical and electronic properties for applications in optoelectronic devices. The rich multivalley band structure and sizable spin-orbit coupling in monolayer TMDs result in several optically bright and dark excitonic states with different spin and valley configurations. In the proposed works, we have developed experimental techniques and theoretical models to study the dynamics, interactions, and transport of both dark and bright excitons. In W-based monolayers of TMDs, the momentum dark exciton cannot typically recombine optically, but they represent the lowest excitonic state of the system and can severely affect the overall optical performances. We performed theoretical and experimental studies that show that compressive strain allows us to visualize intervalley momentum dark exciton in the PL spectrum and that these excitons can find application in strain sensing. We show that in monolayer WS2 the formation of momentum dark exciton is greatly enhanced even with small compressive strain due to intervalley electron-phonon coupling, and their spectral properties strongly correlated with the strain magnitude. Furthermore, we show a similar mechanism for WSe2, however, with tensile due to its qualitatively different band structure than WS2. We exploited this correlation for strain sensing in two-dimensional semiconductors, revealing an optical gauge factor exceeding 104. We then focused on spin dark excitons that possess an out-of-plane optical transition dipole, strong binding energy, and long lifetime. Therefore, spin forbidden excitons are promising candidates for interaction-driven long-range transportation. Moreover, these excitons are characterized by lower energy and exhibit a significantly higher density as supported by our theoretical model. By employing a high-resolution spatially resolved PL setup in an encapsulated monolayer of WS2, we demonstrated that the strong repulsive interaction arising from their high density and longer lifetime enables these dark excitons to diffuse up to several micrometers. Furthermore, we conduct experiments in the energy landscape and show that the repulsive interaction can provide energy to dark excitons for transportation even in an uphill energy landscape. This repulsion-driven long-range transport of dark states provides a new route for excitonic devices that could be used for both classical and quantum information processing. Last, we investigated the optical properties of monolayers of TMDs in different structural phases. Monolayers of TMDs occur in the semiconducting 1H phase, whose optical properties are dominated by excitons, and the metallic 1T phase, however less stable than the 1H phase. We developed a method to engineer stable the 1H/1T mixed phase starting with a pristine 1H phase monolayer WS2 by plasma irradiation process. We can control the size of 1T patches by tuning plasma irradiation time. We observe a novel resonance in mixed-phase WS2 monolayers characterized by a lower excitonic energy compared to the bright exciton and exhibits enhanced absorption, extended lifetime, and circular polarization. We attribute the emergence of these unique excitonic states to the interface that forms between two distinct phases. This interpretation gains additional support from our calculations of the dielectric function carried out on the mixed-phase supercell containing both 1H and 1T phases, revealing a novel optical response at lower energies

    Enhancement of the Electroluminescence and Strain Properties of Dielectric Elastomeric Actuators Using Liquid Metal Reflectors

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    We report on the enhancement of the light-emitting and mechanical performance of multifunctional dielectric elastomeric actuators by combining liquid eutectic gallium indium metal with a stretchable and transparent hybrid electrode composed of silver nanowires (AgNWs) and carbon nanotubes (CNTs). The device shows improved optical properties, electrical conductivity, and stability for electroluminescent dielectric elastomer actuators compared with previous works. Combining single-walled CNTs (SWCNTs) with AgNWs impeded the chemical reaction between the liquid metal and AgNWs, resulting in a more stable operation of the device. The maximum luminance and maximum strain of the electroluminescent dielectric elastomer actuator increased by 50% (from 300 to 450 cd m‚Äď2) and 44% (from 85 to 122%), respectively

    Frontier Molecular Orbital Engineering of Aromatic Donor Fusion: Modularly Constructing Highly Efficient Narrowband Yellow Electroluminescence

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    The development of high-performance multiple resonance thermally activated delayed fluorescence (MR-TADF) materials with narrowband yellow emission is highly critical for various applications in industries, such as the automotive, aerospace, and microelectronic industries. However, the modular construction approaches to expeditiously access narrowband yellow-emitting materials is relatively rare. Here, a unique molecular design concept based on frontier molecular orbital engineering (FMOE) of aromatic donor fusion is proposed to strategically address this issue. Donor fusion is a modular approach with a ‚Äúleveraging effect‚ÄĚ; through direct polycyclization of donor attached to the MR parent core, it is facile to achieve red-shifted emission by a large margin. As a result, two representative model molecules, namely BN-Cz and BN-Cb, have been constructed successfully. The BN-Cz- and BN-Cb-based sensitized organic light-emitting diodes (OLEDs) exhibit bright yellow emission with peaks of 560 and 556 nm, full-width at half-maxima (fwhm‚Äôs) of 49 and 45 nm, Commission Internationale de L‚ÄôEclairage coordinates of (0.44, 0.55) and (0.43, 0.56), and maximum external quantum efficiencies (EQEs) of 32.9% and 29.7%, respectively. The excellent optoelectronic performances render BN-Cz and BN-Cb one of the most outstanding yellow-emitting MR-TADF materials

    A fluorene-bridged double carbonyl/amine multiresonant thermally activated delayed fluorescence emitter for efficient green OLEDs

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    S. W. thanks the China Scholarship Council (201906250199) for support. D.S. acknowledges support from the Royal Academy of Engineering Enterprise Fellowship (EF2122-13106). E. Z.-C. thanks the Engineering and Physical Sciences Research Council (EP/W015137/1, EP/W007517) for support. X.-H. Z. acknowledges support from the National Natural Science Foundation of China (Grant No. 52130304, 51821002) and the Collaborative Innovation Center of Suzhou Nano Science & Technology.Herein, we report a fluorene-bridged double carbonyl/amine-based MR TADF emitter DDiKTa-F, formed by locking the conformation of the previously reported compound DDiKTa. Using this strategy, DDiKTa-F exhibited narrower, brighter, and red-shifted emission. The OLEDs with DDiKTa-F emitted at 493 nm and showed an EQEmax of 15.3% with an efficiency roll-off of 35% at 100 cd m‚ąí2.Publisher PDFPeer reviewe

    Detection of Single-Diode Model Characteristic Values from Measured Current-Voltage Curves for Online Condition Monitoring Purposes of Photovoltaic Power Systems

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    Aurinkosähkövoimalan aurinkokennot ovat järjestelmän elinkaaren myötä taipuvaisia ikääntymään ja rappeutumaan. Ikääntymisilmiöt aiheuttavat merkittäviä tehohäviöitä, mikä ilmenee taloudellisina tappioina. Näin ollen tarvitaan luotettava kunnonvalvontamenetelmä niin kennojen ikääntymisen toteamiseksi kuin ikääntymisen asteen määrittämiseksi. Tällaisia menetelmiä löytyy, mutta niiden käyttö on tyypillisesti työlästä ja kallista. Tämän väitöstutkimuksen tarkoitus on löytää ratkaisu kyseiseen ongelmaan. Käyttökelpoinen lähestymistapa aurinkosähkövoimalan kunnonvalvontaan on virta-jännitekäyrien mittaaminen aurinkopaneelin tai -paneelikokonaisuuden liittimistä ja aurinkokennon toimintaa kuvaavan matemaattisen mallin sovittaminen mitattuihin käyriin. Malliksi soveltuu laajasti käytetty yksidiodimalli. Mallin sovitteen parametrit tarjoavat diagnostisesti arvokasta tietoa aurinkokennojen kunnosta. Tässä on kuitenkin eräitä käytännön haasteita. Ensinnäkin yksidiodimallin parametreihin vaikuttavat myös toimintaolosuhteet, joiden mittauksia on harvoin käytännön aurinkosähkövoimaloissa. Täten on tarpeen tunnistaa toimintaolosuhteet laskennallisesti yhdessä yksidiodimallin parametrien kanssa. Toiseksi kokonaisten virta-jännitekäyrien käyttö sovitukseen vaatii voimalan alasajon mittausjakson ajaksi, kun taas osittaisten virta-jännitekäyrien käyttö heikentää sovitteen laatua. Näin ollen järjestelmällinen tutkimus mittausalueen rajoittamisen vaikutuksesta sovitteen antamiin parametreihin on tarpeen. Kolmanneksi virta-jännitemittausdatassa esiintyvät poikkeamat tekevät mittausalueen rajoittamisesta entistä hankalampaa. Täten tarvitaan sopiva mittausdatan esikäsittelymenetelmä. Näitä kysymyksiä käsitellään tässä väitöskirjassa kokeellisesta näkökulmasta. Ensiksi kehitetään uusi esikäsittelymenetelmä mitatuille virta-jännitekäyrille. Menetelmää voidaan käyttää parantamaan mittausdatan laatua, mikä tekee datan soveltuvammaksi yksidiodimallin sovittamiseen. Seuraavaksi kehitetään uusi yksidiodimallin sovitusmenetelmä, joka tunnistaa laskennallisesti varsinaisten yksidiodimallin parametrien ohella toimintalämpötilan ja -säteilyvoimakkuuden. Menetelmää voidaan käyttää täysin ilman ulkoisia säteilyvoimakkuus- ja lämpötilamittauksia. Lopuksi tarkastellaan järjestelmällisesti virta-jännitekäyrien mittausalueen rajoittamista maksimitehopisteen ympäristöön keskittyen lähinnä ikääntymisen tunnistamiseen ja näytetään, miten virta-jännitekäyrien mittausaluetta voidaan rajoittaa niin, että ikääntyminen saadaan yhä tunnistettua luotettavasti. Merkittävä tulos on, että kehitetty yksidiodimallin sovitusmenetelmä mahdollistaa sopivasti mitattujen osittaisten virtajännitekäyrien käytön parametrien tunnistamisessa.Photovoltaic (PV) power systems are prone to ageing and degradation occurring in the PV cells during their lifespan. Such phenomena cause significant output power degradation which manifests as economic losses. Hence, a reliable condition monitoring procedure to detect and quantify ageing is a necessity. Such procedures do exist, but they are typically laborious and costly. The present study aims at finding a solution for this problem. A feasible approach for monitoring the condition of a PV system is to measure the current-voltage curves from the terminals of a PV unit and fit a mathematical model describing the operation of a PV module or a larger PV unit to the curves. The widely used single-diode model is a suitable choice. The fitted model parameters provide valuable diagnostic information on the condition of the PV cells. However, there are some practical challenges. Firstly, the model parameters are affected by the operating conditions, the measurements of which seldom exist at practical PV sites. This makes it necessary to identify the operating conditions jointly with the model parameters. Secondly, using entire measured current-voltage curves in fitting requires the rundown of the PV system for the measurement period, while using partial current-voltage curves reduces the fit quality. Hence, a systematic study of the effect of the limitation of the measurement range on the fitted parameters is needed. Thirdly, the discrepancies in the current-voltage measurement data make such limitation even more involved. Hence a suitable pre-processing procedure for the measurement data is needed. These issues are addressed in this thesis from an empirical viewpoint. First, a new pre-processing procedure for the measured current-voltage curves is developed. It can be used to improve the quality of such measurement data, making it more suitable for fitting. Thereafter, a novel single-diode model fitting procedure identifying the operating irradiance and temperature jointly with the actual model parameters is developed. It can be utilized fully without external irradiance or temperature measurements. Finally, the effect of limiting the measurement range of the current-voltage curves to the vicinity of the maximum power point is systematically investigated, particularly focusing on ageing detection. It is shown how the measurement range of the current-voltage curves can be limited while maintaining the reliable detection of ageing. As a significant result, the developed single-diode model fitting procedure allows for the usage of suitably formed partial current-voltage curves in the parameter identification

    Achieving Long‚ÄźWavelength Electroluminescence Using Two‚ÄźCoordinate Gold(I) Complexes: Overcoming the Energy Gap Law

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    Abstract Two‚Äźcoordinate coinage metal complexes have emerged as promising emitters for highly efficient organic light‚Äźemitting devices (OLEDs). However, achieving efficient long‚Äźwavelength electroluminescence emission from these complexes remains as a daunting challenge. To address this challenge, molecular design strategies aimed at bolstering the photoluminescence quantum yield (ő¶) of Au(I) complex emitters in low‚Äźenergy emission regions are investigated. By varying amido ligands, a series of two‚Äźcoordinate Au(I) complexes is developed that exhibit photoluminescence peak wavelengths over a broad range of 533‚ąí750¬†nm. These complexes, in particular, maintain ő¶ values up to 10% even in the near‚Äźinfrared emission region, overcoming the constraints imposed by an energy gap. Quantum chemical calculations and photophysical analyses reveal the action of radiative control, which serves to overcome the energy gap law, becomes more pronounced as the overlap between hole and electron distributions (Sr(r)) in the excited state increases. It is further elucidated that Sr(r) increases with the distance between the hole‚Äźdistribution centroid and the nitrogen atom in an amido ligand. Finally, multilayer OLEDs involving the Au(I) complex emitters exhibit performances beyond the borderline of the electroluminescence wavelength‚ąíexternal quantum efficiency space set by previous devices of coinage metal complexes

    Constraints on directionality effect of nuclear recoils in a liquid argon time projection chamber

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    The direct search for dark matter in the form of weakly interacting massive particles (WIMP) is performed by detecting nuclear recoils (NR) produced in a target material from the WIMP elastic scattering. A promising experimental strategy for direct dark matter search employs argon dual-phase time projection chambers (TPC). One of the advantages of the TPC is the capability to detect both the scintillation and charge signals produced by NRs. Furthermore, the existence of a drift electric field in the TPC breaks the rotational symmetry: the angle between the drift field and the momentum of the recoiling nucleus can potentially affect the charge recombination probability in liquid argon and then the relative balance between the two signal channels. This fact could make the detector sensitive to the directionality of the WIMP-induced signal, enabling unmistakable annual and daily modulation signatures for future searches aiming for discovery. The Recoil Directionality (ReD) experiment was designed to probe for such directional sensitivity. The TPC of ReD was irradiated with neutrons at the INFN Laboratori Nazionali del Sud, and data were taken with 72 keV NRs of known recoil directions. The direction-dependent liquid argon charge recombination model by Cataudella et al. was adopted and a likelihood statistical analysis was performed, which gave no indications of significant dependence of the detector response to the recoil direction. The aspect ratio R of the initial ionization cloud is estimated to be 1.037 +/- 0.027 and the upper limit is R < 1.072 with 90% confidence leve
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