High oscillator strength interlayer excitons in two-dimensional heterostructures for mid-infrared photodetection

The development of infrared photodetectors is mainly limited by the choice of available materials and the intricate crystal growth process. Moreover, thermally activated carriers in traditional III-V and II-VI semiconductors enforce low operating temperatures in the infrared photodetectors. Here we demonstrate infrared photodetection enabled by interlayer excitons (ILEs) generated between tungsten and hafnium disulfide, WS₂/HfS₂. The photodetector operates at room temperature and shows an even higher performance at higher temperatures owing to the large exciton binding energy and phonon-assisted optical transition. The unique band alignment in the WS₂/HfS₂ heterostructure allows interlayer bandgap tuning from the mid- to long-wave infrared spectrum. We postulate that the sizeable charge delocalization and ILE accumulation at the interface result in a greatly enhanced oscillator strength of the ILEs and a high responsivity of the photodetector. The sensitivity of ILEs to the thickness of two-dimensional materials and the external field provides an excellent platform to realize robust tunable room temperature infrared photodetectors.

A molecular movie of ultrafast singlet fission

The complex dynamics of ultrafast photoinduced reactions are governed by their evolution along vibronically coupled potential energy surfaces. It is now often possible to identify such processes, but a detailed depiction of the crucial nuclear degrees of freedom involved typically remains elusive. Here, combining excited-state time-domain Raman spectroscopy and tree-tensor network state simulations, we construct the full 108-atom molecular movie of ultrafast singlet fission in a pentacene dimer, explicitly treating 252 vibrational modes on 5 electronic states. We assign the tuning and coupling modes, quantifying their relative intensities and contributions, and demonstrate how these modes coherently synchronise to drive the reaction. Our combined experimental and theoretical approach reveals the atomic-scale singlet fission mechanism and can be generalized to other ultrafast photoinduced reactions in complex systems. This will enable mechanistic insight on a detailed structural level, with the ultimate aim to rationally design molecules to maximise the efficiency of photoinduced reactions.

Ultrafast photo-induced processes in complex systems require theoretical models and their experimental validation which are still lacking. Here the authors investigate singlet fission in a pentacene dimer by a combined experimental and theoretical approach providing a real-time visualisation of the process.

The entangled triplet pair state in acene and heteroacene materials

Entanglement of states is one of the most surprising and counter-intuitive consequences of quantum mechanics, with potent applications in cryptography and computing. In organic materials, one particularly significant manifestation is the spin-entangled triplet-pair state, which mediates the spin-conserving fission of one spin-0 singlet exciton into two spin-1 triplet excitons. Despite long theoretical and experimental exploration, the nature of the triplet-pair state and inter-triplet interactions have proved elusive. Here we use a range of organic semiconductors that undergo singlet exciton fission to reveal the photophysical properties of entangled triplet-pair states. We find that the triplet pair is bound with respect to free triplets with an energy that is largely material independent (∼30 meV). During its lifetime, the component triplets behave cooperatively as a singlet and emit light through a Herzberg-Teller-type mechanism, resulting in vibronically structured photoluminescence. In photovoltaic blends, charge transfer can occur from the bound triplet pairs with >100% photon-to-charge conversion efficiency.

Hybrid sensor using gold nanoparticles and conjugated polyelectrolytes for studying sequence rule in protein-DNA interactions

Protein-DNA interactions play center roles in many biological processes. Studying sequence specific protein-DNA interactions and revealing sequence rules require sensitive and quantitative methodologies that are capable of capturing subtle affinity difference with high accuracy and in a high throughput manner. In this study, double stranded DNA-conjugated gold nanoparticles (dsDNA-AuNPs) and water-soluble conjugated polyelectrolytes (CPEs) are used as cooperative sensing elements to construct a suit of hybrid sensors for detecting protein-DNA interactions, exploiting the differential Förster resonance energy transfer (FRET) with and without protein binding. Through a proper selection of CPEs in terms of charge properties relative to the charge of dsDNA-AuNPs and emission wavelengths relative to the AuNP extinction peak, the hybrid sensors can be constructed into "light-on", "light-off", and "two-way" models. Protein binding can be detected by fluorescence recovery, fluorescence quenching, or both ways, respectively. The "two-way" sensor allows for detection of proteins of any charge properties or unknown charge properties. With estrogen receptor (ERα and ERβ), their consensus DNA (5'-GGTCAnnnTGACC-5') element, and all 15 possible singly mutated elements (i.e., 3 possible base substitutions at each of 1 to 5 positions from left to right of the 5' end half site, GGTCA), we have demonstrated the accuracy of the hybrids sensors for determination of binding affinity constant, binding stoichiometry, and site- and nucleotide-specific binding energy matrix. The in vitro binding energy determined by the hybrid sensors correlates very well with the energy matrix computed from in vivo genome-wide ERα binding data using Thermodynamic Modeling of ChIP-Seq (rank correlation coefficient 0.98). The high degree of correlation of the in vitro energy matrix versus the in vivo matrix renders the new method a highly reliable alternative for understanding in vivo protein binding in the whole genome.