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Register a new userPhonon-assisted Auger enables ultrafast charge transfer in CdSe Quantum Dot/Organic Molecule
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Charge transfer between photoexcited quantum dots and molecular acceptors is one of the key limiting processes in most applications of colloidal nanostructures, most prominently in photovoltaics. An atomistic detailed description of this process would open new ways to optimize existing and create new structures with targeted properties. We achieve a one-to-one comparison between ab-initio non-adiabatic molecular dynamics calculations and transient absorption spectroscopy experiments, which allows us to draw a comprehensive atomistic picture of the charge transfer process, following the time evolution of the charge carrier across the electronic landscape and identifying the thereby induced vibrations. For two quantum dot sizes we find two qualitatively different processes. For the larger structure we find a relatively slow (tau = 516 fs) transfer process that we explain by the existence of a large energy detuning and weak vibronic coupling. For the smaller structure the process is ultrafast (tau = 20 fs) due to an efficient, phonon-assisted Auger process triggered by a strong electron-hole coupling.
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Organic ferroelectric materials are in demand in the growing field of environmentally friendly, lightweight electronics. Donor-Acceptor charge transfer crystals have been recently proposed as a new class of organic ferroelectrics, which may possess a new kind of ferroelectricity, the so-called electronic ferroelectricity, larger and with faster polarity switching in comparison with conventional, inorganic or organic, ferroelectrics. The current research aimed at achieving ambient conditions electronic ferroelectricity in organic charge transfer crystals is shortly reviewed, in such a way to evidence the emerging criteria that have to be fulfilled to reach this challenging goal.
We report the site-specific probing of charge-transfer dynamics in a prototype system for organic photovoltaics (OPV) by picosecond time-resolved X-ray photoelectron spectroscopy. A layered system consisting of approximately two monolayers of C$_{60}$ deposited on top of a thin film of Copper-Phthalocyanine (CuPC) is excited by an optical pump pulse and the induced electronic dynamics are probed with 590 eV X-ray pulses. Charge transfer from the electron donor (CuPC) to the acceptor (C$_{60}$) and subsequent charge carrier dynamics are monitored by recording the time-dependent C 1$s$ core level photoemission spectrum of the system. The arrival of electrons in the C$_{60}$ layer is readily observed as a completely reversible, transient shift of the C$_{60}$ associated C 1$s$ core level, while the C 1$s$ level of the CuPC remains unchanged. The capability to probe charge transfer and recombination dynamics in OPV assemblies directly in the time domain and from the perspective of well-defined domains is expected to open additional pathways to better understand and optimize the performance of this emerging technology.
Atomically thin layered van der Waals heterostructures feature exotic and emergent optoelectronic properties. With growing interest in these novel quantum materials, the microscopic understanding of fundamental interfacial coupling mechanisms is of capital importance. Here, using multidimensional photoemission spectroscopy, we provide a layer- and momentum-resolved view on ultrafast interlayer electron and energy transfer in a monolayer-WSe$_2$/graphene heterostructure. Depending on the nature of the optically prepared state, we find the different dominating transfer mechanisms: while electron injection from graphene to WSe$_2$ is observed after photoexcitation of quasi-free hot carriers in the graphene layer, we establish an interfacial Meitner-Auger energy transfer process following the excitation of excitons in WSe$_2$. By analysing the time-energy-momentum distributions of excited-state carriers with a rate-equation model, we distinguish these two types of interfacial dynamics and identify the ultrafast conversion of excitons in WSe$_2$ to valence band transitions in graphene. Microscopic calculations find interfacial dipole-monopole coupling underlying the Meitner-Auger energy transfer to dominate over conventional Forster- and Dexter-type interactions, in agreement with the experimental observations. The energy transfer mechanism revealed here might enable new hot-carrier-based device concepts with van der Waals heterostructures.
Motivated by recent experimental suggestions of charge-order-driven ferroelectricity in organic charge-transfer salts, such as $kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl, we investigate magnetic and charge-ordered phases that emerge in an extended two-orbital Hubbard model on the anisotropic triangular lattice at $3/4$ filling. This model takes into account the presence of two organic BEDT-TTF molecules, which form a dimer on each site of the lattice, and includes short-range intramolecular and intermolecular interactions and hoppings. By using variational wave functions and quantum Monte Carlo techniques, we find two polar states with charge disproportionation inside the dimer, hinting to ferroelectricity. These charge-ordered insulating phases are stabilized in the strongly correlated limit and their actual charge pattern is determined by the relative strength of intradimer to interdimer couplings. Our results suggest that ferroelectricity is not driven by magnetism, since these polar phases can be stabilized also without antiferromagnetic order and provide a possible microscopic explanation of the experimental observations. In addition, a conventional dimer-Mott state (with uniform density and antiferromagnetic order) and a nonpolar charge-ordered state (with charge-rich and charge-poor dimers forming a checkerboard pattern) can be stabilized in the strong-coupling regime. Finally, when electron-electron interactions are weak, metallic states appear, with either uniform charge distribution or a peculiar $12$-site periodicity that generates honeycomb-like charge order.
We have observed the characteristic temperature dependence of the intermolecular phonon spectrum in the organic dimer Mott insulator kappa-(ET)2Cu2(CN)3 exhibiting a dielectric anomaly at 30 K. The anomalous spectral narrowing of the 55 cm-1 phonon peak at 30 K was analyzed in terms of motional narrowing within the framework of a stationary Gaussian process, i. e., the phonon frequency is modulated by the ultrafast charge fluctuation. The spectral narrowing occurs because the time constant of the correlation time tau_c and the amplitude of the frequency modulation delta satisfy the relation tau_c<delta at 30 K. At temperatures below 30 K, the motional narrowing is disturbed by the increasing of tau_c, near the charge-glass or the short-range order at 6 K. On the other hand, for temperatures above 30 K, the motional narrowing is disturbed by the increase of delta with increasing temperature.
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