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MXenes are an emerging family of highly-conductive 2D materials which have demonstrated state-of-the-art performance in electromagnetic interference shielding, chemical sensing, and energy storage. To further improve performance, there is a need to increase MXenes’ electronic conductivity. Tailoring the MXene surface chemistry could achieve this goal, as density functional theory predicts that surface terminations strongly influence MXenes' Fermi level density of states and thereby MXenes’ electronic conductivity. Here, we directly correlate MXene surface de-functionalization with increased electronic conductivity through in situ vacuum annealing, electrical biasing, and spectroscopic analysis within the transmission electron microscope. Furthermore, we show that intercalation can induce transitions between metallic and semiconductor-like transport (transitions from a positive to negative temperature-dependence of resistance) through inter-flake effects. These findings lay the groundwork for intercalation- and termination-engineered MXenes, which promise improved electronic conductivity and could lead to the realization of semiconducting, magnetic, and topologically insulating MXenes. Two-dimensional transition metal carbides and nitrides (MXenes) have emerged as highly conductive and stable materials, of promise for electronic applications. Here, the authors use in situ electric biasing and transmission electron microscopy to investigate the effect of surface termination and intercalation on electronic properties.

The architectural design and fabrication of low-cost and reliable organic X-ray imaging scintillators with high light yield, ultralow detection limits and excellent imaging resolution is becoming one of the most attractive research directions for chemists, materials scientists, physicists and engineers due to the devices’ promising scientific and applied technological implications. However, the optimal balance among X-ray absorption capability, exciton utilization efficiency and photoluminescence quantum yield of organic scintillation materials is extremely difficult to achieve because of several competitive non-radiative processes, including intersystem crossing and internal conversion. Here we introduced heavy atoms (Cl, Br and I) into thermally activated delayed fluorescence (TADF) chromophores to significantly increase their X-ray absorption cross-section and maintaining their unique TADF properties and high photoluminescence quantum yield. The X-ray imaging screens fabricated using TADF-Br chromophores exhibited highly improved X-ray sensitivity and imaging resolution compared with the TADF-H counterpart. More importantly, the high X-ray imaging resolution of >18.0 line pairs per millimetre achieved from the TADF-Br screen exceeds most reported organic and conventional inorganic scintillators. This study could help revive research on organic X-ray imaging scintillators and pave the way towards exciting applications for radiology and security screening.Heavy atoms like Cl, Br and I introduced into thermally activated delayed fluorescence chromophores can increase the X-ray absorption cross-section. Light yield of ~20,000 photons MeV–1, detection limit of 45.5 nGy s−1 and imaging resolution of >18.0 line pairs per millimetre is demonstrated.

The ultrafast photoinduced ring-opening of 1,3-cyclohexadiene constitutes a textbook example of electrocyclic reactions in organic chemistry and a model for photobiological reactions in vitamin D synthesis. Although the relaxation from the photoexcited electronic state during the ring-opening has been investigated in numerous studies, the accompanying changes in atomic distance have not been resolved. Here we present a direct and unambiguous observation of the ring-opening reaction path on the femtosecond timescale and subångström length scale using megaelectronvolt ultrafast electron diffraction. We followed the carbon–carbon bond dissociation and the structural opening of the 1,3-cyclohexadiene ring by the direct measurement of time-dependent changes in the distribution of interatomic distances. We observed a substantial acceleration of the ring-opening motion after internal conversion to the ground state due to a steepening of the electronic potential gradient towards the product minima. The ring-opening motion transforms into rotation of the terminal ethylene groups in the photoproduct 1,3,5-hexatriene on the subpicosecond timescale. The photochemical electrocyclic ring-opening of 1,3-cyclohexadiene is a textbook organic chemistry reaction. Now, using ultrafast electron diffraction its reaction pathway has been resolved on the level of atomic distances and on its natural femtosecond timescale. Furthermore, coherent isomerization dynamics of the photoproduct 1,3,5-hexatriene were observed.

Chirality is a recurrent theme in the study of biological systems, in which active processes are driven by the internal conversion of chemical energy into work. Bacterial flagella, actomyosin filaments, and microtubule bundles are active systems that are also intrinsically chiral. Despite some exploratory attempt to capture the relations between chirality and motility, many features of intrinsically chiral systems still need to be explored and explained. To address this gap in knowledge, here we study the effects of internal active forces and torques on a 3-dimensional (3D) droplet of cholesteric liquid crystal (CLC) embedded in an isotropic liquid. We consider tangential anchoring of the liquid crystal director at the droplet surface. Contrary to what happens in nematics, where moderate extensile activity leads to droplet rotation, cholesteric active droplets exhibit more complex and variegated behaviors. We find that extensile force dipole activity stabilizes complex defect configurations, in which orbiting dynamics couples to thermodynamic chirality to propel screw-like droplet motion. Instead, dipolar torque activity may either tighten or unwind the cholesteric helix and if tuned, can power rotations with an oscillatory angular velocity of 0 mean.

Owing to its low excitation energy and long radiative lifetime, the first excited isomeric state of thorium-229, 229m Th, can be optically controlled by a laser 1 , 2 and is an ideal candidate for the creation of a nuclear optical clock 3 , which is expected to complement and outperform current electronic-shell-based atomic clocks 4 . A nuclear clock will have various applications—such as in relativistic geodesy 5 , dark matter research 6 and the observation of potential temporal variations of fundamental constants 7 —but its development has so far been impeded by the imprecise knowledge of the energy of 229m Th. Here we report a direct measurement of the transition energy of this isomeric state to the ground state with an uncertainty of 0.17 electronvolts (one standard deviation) using spectroscopy of the internal conversion electrons emitted in flight during the decay of neutral 229m Th atoms. The energy of the transition between the ground state and the first excited state corresponds to a wavelength of 149.7 ± 3.1 nanometres, which is accessible by laser spectroscopy through high-harmonic generation. Our method combines nuclear and atomic physics measurements to advance precision metrology, and our findings are expected to facilitate the application of high-resolution laser spectroscopy on nuclei and to enable the development of a nuclear optical clock of unprecedented accuracy. The transition energy of the first excited state of 229 Th to the ground state is determined through the measurement of internal conversion electrons to correspond to a wavelength of 149.7 ± 3.1 nanometres.

Singlet exciton fission is the process in organic semiconductors through which a spin-singlet exciton converts into a pair of spin-triplet excitons residing on different chromophores, entangled in an overall spin-zero state. For some systems, singlet fission has been shown to occur on the 100 fs timescale and with a 200% quantum yield, but the mechanism of this process remains uncertain. Here we study a model singlet fission system, TIPS-pentacene, using ultrafast vibronic spectroscopy. We observe that vibrational coherence in the initially photogenerated singlet state is transferred to the triplet state and show that this behaviour is effectively identical to ultrafast internal conversion for polyenes in solution. This similarity in vibronic dynamics suggests that both multi-molecular singlet fission and single-molecular internal conversion are mediated by the same underlying relaxation processes, based on strong coupling between nuclear and electronic degrees of freedom. In its most efficient form this leads to a conical intersection between the coupled electronic states. A vibrational wavepacket generated in a spin singlet is shown to be transferable to spin triplets during singlet fission in organic semiconductors, providing a link between multi-molecular singlet fission and single-molecular internal conversion.

Although there are some proposed explanations for aggregation-induced emission, a phenomenon with applications that range from biosensors to organic light-emitting diodes, current understanding of the quantum-mechanical origin of this photophysical behaviour is limited. To address this issue, we assessed the emission properties of a series of BF 2 –hydrazone-based dyes as a function of solvent viscosity. These molecules turned out to be highly efficient fluorescent molecular rotors. This property, in addition to them being aggregation-induced emission luminogens, enabled us to probe deeper into their emission mechanism. Time-dependent density functional theory calculations and experimental results showed that the emission is not from the S 1 state, as predicted from Kasha's rule, but from a higher energy (>S 1 ) state. Furthermore, we found that suppression of internal conversion to the dark S 1 state by restricting the rotor rotation enhances fluorescence, which leads to the proposal that suppression of Kasha's rule is the photophysical mechanism responsible for emission in both viscous solution and the solid state. A family of fluorescent molecular rotors has been developed and their mechanism for emission understood. It has been observed that, although most fluorescent molecules emit from their lowest energy excited state, S 1 (in accordance with Kasha's rule), BODIHY dyes do not. Furthermore, their fluorescence is enhanced through restricted rotor rotation, which suppresses internal conversion to the dark S 1 state.

Early experiments with transiting circular Rydberg atoms in a superconducting resonator laid the foundations of modern cavity and circuit quantum electrodynamics 1 , and helped explore the defining features of quantum mechanics such as entanglement. Whereas ultracold atoms and superconducting circuits have since taken rather independent paths in the exploration of new physics, taking advantage of their complementary strengths in an integrated system enables access to fundamentally new parameter regimes and device capabilities 2 , 3 . Here we report on such a system, coupling an ensemble of cold 85 Rb atoms simultaneously to an, as far as we are aware, first-of-its-kind optically accessible, three-dimensional superconducting resonator 4 and a vibration-suppressed optical cavity in a cryogenic (5 K) environment. To demonstrate the capabilities of this platform, and with an eye towards quantum networking 5 , we leverage the strong coupling between Rydberg atoms and the superconducting resonator to implement a quantum-enabled millimetre wave (mmwave) photon to optical photon transducer 6 . We measured an internal conversion efficiency of 58(11)%, a conversion bandwidth of 360(20) kHz and added thermal noise of 0.6 photons, in agreement with a parameter-free theory. Extensions of this technique will allow near-unity efficiency transduction in both the mmwave and microwave regimes. More broadly, our results open a new field of hybrid mmwave/optical quantum science, with prospects for operation deep in the strong coupling regime for efficient generation of metrologically or computationally useful entangled states 7 and quantum simulation/computation with strong non-local interactions 8 . We report an ensemble of cold 85 Rb atoms strongly coupled to a superconducting resonator and optical cavity, resulting in the demonstration of quantum-enabled transduction of millimetre wave photons to optical photons.

Thorium-229 ( 229 Th) possesses an optical nuclear transition between the ground state ( 229g Th) and low-lying isomer ( 229m Th). A nuclear clock based on this nuclear-transition frequency is expected to surpass existing atomic clocks owing to its insusceptibility to surrounding fields 1 – 5 . In contrast to other charge states, triply charged 229 Th ( 229 Th 3+ ) is the most suitable for highly accurate nuclear clocks because it has closed electronic transitions that enable laser cooling, laser-induced fluorescence detection and state preparation of ions 1 , 6 – 8 . Although laser spectroscopic studies of 229 Th 3+ in the nuclear ground state have been performed 8 , properties of 229m Th 3+ , including its nuclear decay lifetime that is essential to specify the intrinsic linewidth of the nuclear-clock transition, remain unknown. Here we report the trapping of 229m Th 3+ continuously supplied by a 233 U source and the determination of nuclear decay half-life of the isolated 229m Th 3+ to be 1,400 − 300 + 600 s through nuclear-state-selective laser spectroscopy. Furthermore, by determining the hyperfine constants of 229m Th 3+ , we reduced the uncertainty of the sensitivity of the 229 Th nuclear clock to variations in the fine-structure constant by a factor of four. These results offer key parameters for the 229 Th 3+ nuclear clock and its applications in the search for new physics. The trapping of triply charged 229m Th 3+ is described and its nuclear decay half-life determined, showing useful properties for the development of a nuclear clock and applications in the search for new physics.

Optical frequency combs have revolutionized precision measurement, time-keeping and molecular spectroscopy 1 – 7 . A substantial effort has developed around ‘microcombs’: integrating comb-generating technologies into compact photonic platforms 5 , 7 – 9 . Current approaches for generating these microcombs involve either the electro-optic 10 or Kerr mechanisms 11 . Despite rapid progress, maintaining high efficiency and wide bandwidth remains challenging. Here we introduce a previously unknown class of microcomb—an integrated device that combines electro-optics and parametric amplification to yield a frequency-modulated optical parametric oscillator (FM-OPO). In contrast to the other solutions, it does not form pulses but maintains operational simplicity and highly efficient pump power use with an output resembling a frequency-modulated laser 12 . We outline the working principles of our device and demonstrate it by fabricating the complete optical system in thin-film lithium niobate. We measure pump-to-comb internal conversion efficiency exceeding 93% (34% out-coupled) over a nearly flat-top spectral distribution spanning about 200 modes (over 1 THz). Compared with an electro-optic comb, the cavity dispersion rather than loss determines the FM-OPO bandwidth, enabling broadband combs with a smaller radio-frequency modulation power. The FM-OPO microcomb offers robust operational dynamics, high efficiency and broad bandwidth, promising compact precision tools for metrology, spectroscopy, telecommunications, sensing and computing. An integrated device that combines optical parametric oscillation and electro-optic modulation in lithium niobate creates a flat-top frequency-comb-like output with low power requirements.