Dual polarization-enabled ultrafast bulk photovoltaic response in van der Waals heterostructures.

The bulk photovoltaic effect (BPVE) originating from spontaneous charge polarizations can reach high conversion efficiency exceeding the Shockley-Queisser limit. Emerging van der Waals (vdW) heterostructures provide the ideal platform for BPVE due to interfacial interactions naturally breaking the crystal symmetries of the individual constituents and thus inducing charge polarizations. Here, we show an approach to obtain ultrafast BPVE by taking advantage of dual interfacial polarizations in vdW heterostructures. While the in-plane polarization gives rise to the BPVE in the overlayer, the charge carrier transfer assisted by the out-of-plane polarization further accelerates the interlayer electronic transport and enhances the BPVE. We illustrate the concept in MoS2/black phosphorus heterostructures, where the experimentally observed intrinsic BPVE response time achieves 26 ps, orders of magnitude faster than that of conventional non-centrosymmetric materials. Moreover, the heterostructure device possesses an extrinsic response time of approximately 2.2 ns and a bulk photovoltaic coefficient of 0.6 V-1, which is among the highest values for vdW BPV devices reported so far. Our study thus points to an effective way of designing ultrafast BPVE for high-speed photodetection.

Observation and Modulation of High-Temperature Moiré-Locale Excitons in van der Waals Heterobilayers.

Transition metal dichalcogenide heterobilayers feature strong moiré potentials with multiple local minima, which can spatially trap interlayer excitons at different locations within one moiré unit cell (dubbed moiré locales). However, current studies mainly focus on moiré excitons trapped at a single moiré locale. Exploring interlayer excitons trapped at different moiré locales is highly desirable for building polarized light-emitter arrays and studying multiorbital correlated and topological physics. Here, via enhancing the interlayer coupling and engineering the heterointerface, we report the observation and modulation of high-temperature interlayer excitons trapped at separate moiré locales in WS2/WSe2 heterobilayers. These moiré-locale excitons are identified by two emission peaks with an energy separation of ∼60 meV, exhibiting opposite circular polarizations due to their distinct local stacking registries. With the increase of temperature, two momentum-indirect moiré-locale excitons are observed, which show a distinct strain dependence with the momentum-direct one. The emission of these moiré-locale excitons can be controlled via engineering the heterointerface with different phonon scattering, while their emission energy can be further modulated via strain engineering. Our reported highly tunable interlayer excitons provide important information on understanding moiré excitonic physics, with possible applications in building high-temperature excitonic devices.

Picosecond electrical response in graphene/MoTe2 heterojunction with high responsivity in the near infrared region.

Understanding the fundamental charge carrier dynamics is of great significance for photodetectors with both high speed and high responsivity. Devices based on two-dimensional (2D) transition metal dichalcogenides can exhibit picosecond photoresponse speed. However, 2D materials naturally have low absorption, and when increasing thickness to gain higher responsivity, the response time usually slows to nanoseconds, limiting their photodetection performance. Here, by taking time-resolved photocurrent measurements, we demonstrated that graphene/MoTe2 van der Waals heterojunctions realize a fast 10 ps photoresponse time owing to the reduced average photocurrent drift time in the heterojunction, which is fundamentally distinct from traditional Dirac semimetal photodetectors such as graphene or Cd3As2 and implies a photodetection bandwidth as wide as 100 GHz. Furthermore, we found that an additional charge carrier transport channel provided by graphene can effectively decrease the photocurrent recombination loss to the entire device, preserving a high responsivity in the near-infrared region. Our study provides a deeper understanding of the ultrafast electrical response in van der Waals heterojunctions and offers a promising approach for the realization of photodetectors with both high responsivity and ultrafast electrical response.

Ultra-long distance carrier transportation in bandgap-graded CdSxSe1-x nanowire waveguides.

Carrier transportation in semiconductor nanowires is essential for their application in integrated opto-electronic devices. Therefore, it is of importance to manipulate and enhance the transportation performance of nanowires through micro-nano scale engineering. In this work, the carrier dynamics of the waveguides in the bandgap-graded CdSxSe1-x nanowires is systematically investigated. By developing a spatially separated time-resolved photoluminescence spectroscopy system, the dependence between the propagation distance/direction and the dynamics of the bandgap gradient driven long-range carrier transportation of the nanowires is characterized. In the meantime, the dynamics of carrier concentration driven spontaneous diffusion is also characterized to be compared to. It is found that the continuous carrier transportation which is driven by the bandgap gradient is the dominant process in the active waveguide, where the maximum transportation distance of 100 µm is detected. Such a transportation distance is up to ∼8-fold larger than the spontaneous carrier diffusion distance in the bandgap-graded CdSxSe1-x nanowires. The ultra-long carrier transportation capability in the bandgap gradient nanowires makes them ideal structures for applications in long-distance photo-energy delivery and micro-nanoscale opto-electronics.

Trion-Induced Distinct Transient Behavior and Stokes Shift in WS2 Monolayers.

Understanding the excitonic behavior in two-dimensional transition-metal dichalcogenides (2D TMDs) is of both fundamental interest and critical importance for optoelectronic applications. Here, we investigate the transient excitonic behavior and Stokes shift in WS2 monolayers on both sapphire and glass substrates. Trion formation was confirmed as the origin of the distinct photoluminescence (PL) emission and Stokes shift in WS2 monolayers. Moreover, the transient studies demonstrate faster recombination of both the exciton and the short-lived trion on the glass substrate as compared to that on the sapphire substrate, owing to the heavier n-doping and greater number of defects introduced by the glass substrate. In addition, a long-lived trion species attributed to the intervalley triplet trion was observed on the glass substrate, with a lifetime on the nanosecond time scale. These findings offer a comprehensive understanding of the excitonic behavior and Stokes shift in WS2 monolayers and will lay the foundation for further fundamental investigations in the field.