Hao Hong

Two-Dimensional (C4H9NH3)2PbBr4 Perovskite Crystals for High-Performance Photodetector

Two-dimensional (2D) layered hybrid perovskites of (RNH3)2PbX4 (R is an alkyl and X is a halide) have been recently synthesized and exhibited rich optical properties including fluorescence and exciton effects. However, few studies on transport and optoelectronic measurements of individual 2D perovskite crystals have been reported, presumably owing to the instability issue during electronic device fabrications. Here we report the first photodetector based on individual 2D (C4H9NH3)2PbBr4 perovskite crystals, built with the protection and top contact of graphene film. Both a high responsivity (∼2100 A/W) and extremely low dark current (∼10-10 A) are achieved with a design of interdigital graphene electrodes. Our study paves the way to build high-performance optoelectronic devices based on the emerging 2D single-crystal perovskite materials.

Strong Second-Harmonic Generation in Atomic Layered GaSe.

Nonlinear effects in two-dimensional (2D) atomic layered materials have recently attracted increasing interest. Phenomena such as nonlinear optical edge response, chiral electroluminescence, and valley and spin currents beyond linear orders have opened up a great opportunity to expand the functionalities and potential applications of 2D materials. Here we report the first observation of strong optical second-harmonic generation (SHG) in monolayer GaSe under nonresonant excitation and emission condition. Our experiments show that the nonresonant SHG intensity of GaSe is the strongest among all the 2D atomic crystals measured up to day. At the excitation wavelength of 1600 nm, the SHG signal from monolayer GaSe is around 1-2 orders of magnitude larger than that from monolayer MoS2 under the same excitation power. Such a strong nonlinear signal facilitates the use of polarization-dependent SHG intensity and SHG mapping to investigate the symmetry properties of this material: the monolayer GaSe shows 3-fold lattice symmetry with an intrinsic correspondence to its geometric triangular shape in our growth condition; whereas the bilayer GaSe exhibits two dominant stacking orders: AA and AB stacking. The correlation between the stacking orders and the interlayer twist angles in GaSe bilayer indicates that different triangular GaSe atomic layers have the same dominant edge configuration. Our results provide a route toward exploring the structural information and the possibility to observe other nonlinear effects in GaSe atomic layers.

Single-Crystal Atomic-Layered Molybdenum Disulfide Nanobelts with High Surface Activity

Nanostructured molybdenum disulfide (MoS2) has emerged as a promising catalytic alternative to the widely used Pt in the hydrogen evolution reaction from water because it is inexpensive and earth-abundant. The central prerequisite in realizing its potential is to enhance the surface activities by increasing the concentration of metallic edge sites. However, MoS2 thermodynamics favors the presence of a two-dimensional basal plane, and therefore, the one-dimensional edge sites surrounding the basal plane are very limited. Herein, we report the first synthesis of single-crystal MoS2 nanobelts with the top surface fully covered by edge sites. The nanobelt structure comprises parallel stacked atomic layers with the basal plane vertical to the substrate, and these layer edges form the top surface of the nanobelt. The surface is highly active: it optically quenches all of the indirect band gap excitons and chemically leads to a high electrocatalytic hydrogen evolution efficiency (a low onset overpotential of 170 mV for an electrocatalytic current density of 20 mA/cm(2) and a Tafel slope of 70 mV/decade).

Interlayer‐State‐Coupling Dependent Ultrafast Charge Transfer in MoS2/WS2 Bilayers

Light‐induced interlayer ultrafast charge transfer in 2D heterostructures provides a new platform for optoelectronic and photovoltaic applications. The charge separation process is generally hypothesized to be dependent on the interlayer stackings and interactions, however, the quantitative characteristic and detailed mechanism remain elusive. Here, a systematical study on the interlayer charge transfer in model MoS2/WS2 bilayer system with variable stacking configurations by time‐dependent density functional theory methods is demonstrated. The results show that the slight change of interlayer geometry can significantly modulate the charge transfer time from 100 fs to 1 ps scale. Detailed analysis further reveals that the transfer rate in MoS2/WS2 bilayers is governed by the electronic coupling between specific interlayer states, rather than the interlayer distances, and follows a universal dependence on the state‐coupling strength. The results establish the interlayer stacking as an effective freedom to control ultrafast charge transfer dynamics in 2D heterostructures and facilitate their future applications in optoelectronics and light harvesting.

Robust Stacking-Independent Ultrafast Charge Transfer in MoS2/WS2 Bilayers

Van der Waals-coupled two-dimensional (2D) heterostructures have attracted great attention recently due to their high potential in the next-generation photodetectors and solar cells. The understanding of charge-transfer process between adjacent atomic layers is the key to design optimal devices as it directly determines the fundamental response speed and photon-electron conversion efficiency. However, general belief and theoretical studies have shown that the charge transfer behavior depends sensitively on interlayer configurations, which is difficult to control accurately, bringing great uncertainties in device designing. Here we investigate the ultrafast dynamics of interlayer charge transfer in a prototype heterostructure, the MoS2/WS2 bilayer with various stacking configurations, by optical two-color ultrafast pump-probe spectroscopy. Surprisingly, we found that the charge transfer is robust against varying interlayer twist angles and interlayer coupling strength, in time scale of ∼90 fs. Our observation, together with atomic-resolved transmission electron characterization and time-dependent density functional theory simulations, reveals that the robust ultrafast charge transfer is attributed to the heterogeneous interlayer stretching/sliding, which provides additional channels for efficient charge transfer previously unknown. Our results elucidate the origin of transfer rate robustness against interlayer stacking configurations in optical devices based on 2D heterostructures, facilitating their applications in ultrafast and high-efficient optoelectronic and photovoltaic devices in the near future.

Monitoring Local Strain Vector in Atomic-Layered MoSe2 by Second-Harmonic Generation

Strain serves as a powerful freedom to effectively, reversibly, and continuously engineer the physical and chemical properties of two-dimensional (2D) materials, such as bandgap, phase diagram, and reaction activity. Although there is a high demand for full characterization of the strain vector at local points, it is still very challenging to measure the local strain amplitude and its direction. Here, we report a novel approach to monitor the local strain vector in 2D molybdenum diselenide (MoSe2) by polarization-dependent optical second-harmonic generation (SHG). The strain amplitude can be evaluated from the SHG intensity in a sensitive way (-49% relative change per 1% strain); while the strain direction can be directly indicated by the evolution of polarization-dependent SHG pattern. In addition, we employ this technique to investigate the interlayer locking effect in 2H MoSe2 bilayers when the bottom layer is under stretching but the top layer is free. Our observation, combined with ab initio calculations, demonstrates that the noncovalent interlayer interaction in 2H MoSe2 bilayers is strong enough to transfer the strain of at least 1.4% between the bottom and top layers to prevent interlayer sliding. Our results establish that SHG is an effective approach for in situ, sensitive, and noninvasive measurement of local strain vector in noncentrosymmetric 2D materials.

Ultrafast and highly sensitive infrared photodetectors based on two-dimensional oxyselenide crystals

Infrared light detection and sensing is deeply embedded in modern technology and human society and its development has always been benefitting from the discovery of various photoelectric materials. The rise of two-dimensional materials, thanks to their distinct electronic structures, extreme dimensional confinement and strong light–matter interactions, provides a material platform for next-generation infrared photodetection. Ideal infrared detectors should have fast respond, high sensitivity and air-stability, which are rare to meet at the same time in one two-dimensional material. Herein we demonstrate an infrared photodetector based on two-dimensional Bi2O2Se crystal, whose main characteristics are outstanding in the whole two-dimensional family: high sensitivity of 65 AW−1 at 1200 nm and ultrafast photoresponse of ~1 ps at room temperature, implying an intrinsic material-limited bandwidth up to 500 GHz. Such great performance is attributed to the suitable electronic bandgap and high carrier mobility of two-dimensional oxyselenide.Two-dimensional (2D) bismuth oxyselenide crystals with suitable electronic band-gap and ultrahigh carrier mobility enable near-infrared photodetection. Here, the authors report an infrared photodetector based on 2D-bismuth oxyselenide with high responsivity, ultrafast photoresponse of ~ 1 ps at room temperature and a detectable frequency limit of up to 500 GHz.

New Pathway for Hot Electron Relaxation in Two-Dimensional Heterostructures

Two-dimensional (2D) heterostructures composed of transition-metal dichalcogenide atomic layers are the new frontier for novel optoelectronic and photovoltaic device applications. Some key properties that make these materials appealing, yet are not well understood, are ultrafast hole/electron dynamics, interlayer energy transfer and the formation of interlayer hot excitons. Here, we study photoexcited electron/hole dynamics in a representative heterostructure, the MoS2/WSe2 interface, which exhibits type II band alignment. Employing time-dependent density functional theory in the time domain, we observe ultrafast charge dynamics with lifetimes of tens to hundreds of femtoseconds. Most importantly, we report the discovery of an interfacial pathway in 2D heterostructures for the relaxation of photoexcited hot electrons through interlayer hopping, which is significantly faster than intralayer relaxation. This finding is of particular importance for understanding many experimentally observed photoinduced processes, including charge and energy transfer at an ultrafast time scale (<1 ps).

Probing Phonon Dynamics in Individual Single-Walled Carbon Nanotubes

Interactions between elementary excitations, such as carriers, phonons, and plasmons, are critical for understanding the optical and electronic properties of materials. The significance of these interactions is more prominent in low-dimensional materials and can dominate their physical properties due to the enhanced interactions between these excitations. One-dimensional single-walled carbon nanotubes provide an ideal system for studying such interactions due to their perfect physical structures and rich electronic properties. Here we investigated G-mode phonon dynamics in individual suspended chirality-resolved single-walled carbon nanotubes by time-resolved anti-Stokes Raman spectroscopy. The improved technique allowed us to probe the intrinsic phonon information on a single-tube level and exclude the influences of tube-tube and tube-substrate interactions. We found that the G-mode phonon lifetime ranges from 0.75-2.25 ps and critically depends on whether the tube is metallic or semiconducting. In comparison with the phonon lifetimes in graphene and graphite, we revealed structure-dependent carrier-phonon and phonon-phonon interactions in nanotubes. Our results provide new information for optimizing the design of nanotube electronic/optoelectronic devices by better understanding and utilizing their phonon decay channels.

Ultrafast Broadband Charge Collection from Clean Graphene/CH3NH3PbI3 Interface

Photocarrier generation in a material, transportation to the material surface, and collection at the electrode interface are of paramount importance in any optoelectronic and photovoltaic device. In the last collection process, ideal performance comprises ultrafast charge collection to enhance current conversion efficiency and broadband collection to enhance energy conversion efficiency. Here, for the first time, we demonstrate ultrafast broadband charge collection achieved simultaneously at the clean graphene/organic-inorganic halide perovskite interface. The clean interface is realized by directly growing perovskite on graphene surface without polymer contamination. The tunable two-color pump-probe spectroscopy, time-resolved photoluminescence spectroscopy, and time-dependent density functional theory all reveal that the clean-interfacial graphene collects band-edge photocarriers of perovskite in an ultrashort time of ∼100 fs, with a current collection efficiency close to 99%. In addition, graphene can extract deep-band hot carriers of perovskite within only ∼50 fs, several orders faster than hot carrier relaxation and cooling in perovskite itself, due to the unique Dirac linear band structure of graphene, indicating a potential high energy conversion efficiency exceeding the Shockley-Queisser limit. Adding other graphene superiority of good transparency, high carrier mobility, and extreme flexibility, clean-interfacial graphene provides an ideal charge collection layer and electrode candidate for future optoelectronic and photovoltaic applications in two dimensions.