Toward Gigahertz Photodetection with Transition Metal Dichalcogenides.

ConspectusTransition metal dichalcogenides (TMDCs) exhibit favorable properties for optical communication in the gigahertz (GHz) regime, such as large mobilities, high extinction coefficients, cheap fabrication, and silicon compatibility. While impressive improvements in their sensitivity have been realized over the past decade, the bandwidths of these devices have been mostly limited to a few megahertz. We argue that this shortcoming originates in the relatively large RC constants of TMDC-based photodetectors, which suffer from high surface defect densities, inefficient charge carrier injection at the electrode/TMDC interface, and long charging times. However, we show in a series of papers that rather simple adjustments in the device architecture afford TMDC-based photodetectors with bandwidths of several hundreds of megahertz. We rationalize the success of these adjustments in terms of the specific physical-chemical properties of TMDCs, namely their anisotropic in-plane/out-of-plane carrier behavior, large optical absorption, and chalcogenide-dependent surface chemistry. Just one surprisingly simple yet effective pathway to fast TMDC photodetection is the reduction of the photoresistance by using light-focusing optics, which enables bandwidths of 0.23 GHz with an energy consumption of only 27 fJ/bit.By reflecting on the ultrafast intrinsic photoresponse times of a few picoseconds in TMDC heterostructures, we motivate the application of more demanding chemical strategies to exploit such ultrafast intrinsic properties for true GHz operation in real devices. A key aspect in this regard is the management of surface defects, which we discuss in terms of its dependence on the layer thickness, its tunability by molecular adlayers, and the prospects of replacing thermally evaporated metal contacts by laser-printed electrodes fabricated with inks of metalloid clusters. We highlight the benefits of combining TMDCs with graphene to heterostructures that exhibit the ultrafast photoresponse and large spectral range of Dirac materials with the low dark currents and high responsivities of semiconductors. We introduce the bulk photovoltaic effect in TMDC-based materials with broken inversion symmetry as well as a combination of TMDCs with plasmonic nanostructures as means for increasing the bandwidth and responsivity simultaneously. Finally, we describe the prospects of embedding TMDC photodetectors into optical cavities with the objective of tuning the lifetime of the photoexcited state and increasing the carrier mobility in the photoactive layer.The findings and concepts detailed in this Account demonstrate that GHz photodetection with TMDCs is feasible, and we hope that these bright prospects for their application as next-generation optoelectronic materials motivate more chemists and material scientists to actively pursue the development of the more complicated material combinations outlined here.

Ultimate Limit in Optoelectronic Performances of Monolayer WSe2 Sloping-Channel Transistors.

Atomically thin monolayer two-dimensional (2D) semiconductors with natural immunity to short channel effects are promising candidates for sub-10 nm very large-scale integration technologies. Herein, the ultimate limit in optoelectronic performances of monolayer WSe2 field-effect transistors (FETs) is examined by constructing a sloping channel down to 6 nm. Using a simple scaling method compatible with current micro/nanofabrication technologies, we achieve a record high saturation current up to 1.3 mA/µm at room temperature, surpassing any reported monolayer 2D semiconductor transistors. Meanwhile, quasi-ballistic transport in WSe2 FETs is first demonstrated; the extracted high saturation velocity of 4.2 × 106 cm/s makes it suitable for extremely sensitive photodetectors. Furthermore, the photoresponse speed can be improved by reducing channel length due to an electric field-assisted detrapping process of photogenerated carriers in localized states. As a result, the sloping-channel device exhibits a faster response, higher detectivity, and additional polarization resolution ability compared to planar micrometer-scale devices.

Highly Anisotropic Second-Order Nonlinear Optical Effects in the Chiral Lead-Free Perovskite Spiral Microplates.

Chiral metal halide perovskites with intrinsic asymmetric structures have drawn increased research interest for the application of second-order nonlinear optics (NLO). However, designing chiral perovskites with the features of a large NLO coefficient, high laser-induced damage thresholds (LDT), and environmental friendliness remains a major challenge. Herein, we have synthesized two chiral hybrid bismuth halides: (R/S-MBA)4Bi2Br10 spiral structure microplates, templated by chiral (R/S)-methylbenzylamine (R/S-MBA). The as-grown chiral lead-free perovskite spiral microplates exhibit a recorded second harmonic generation (SHG) effect with a large effective second-order NLO coefficient (deff) of 11.9 pm V-1 and a high LDT of up to 59.2 mJ cm-2. More importantly, the twisted screw structures show competitive circular polarization sensitivity at 1200 nm with an anisotropy factor (gSHG-CD) of 0.58, which is about 3 times higher than that of reported Pb-based chiral perovskites. These findings provide a new platform to design multifunctional lead-free chiral perovskites for nonlinear photonic applications.

Strong Second- and Third-Harmonic Generation in 1D Chiral Hybrid Bismuth Halides.

Breaking the symmetry of a crystal structure can enable even-order nonlinear activities, including second-harmonic generation (SHG). The emerging chiral hybrid organic-inorganic metal halides feature unique optical and electronic properties and flexible crystal structures, making them a class of promising nonlinear optical materials. However, their nonlinear response performances are currently inferior to traditional nonlinear crystals, because of the lack of research on resonant enhancement and third-harmonic generation (THG). Herein, we designed chiral hybrid bismuth halides with naturally nonsymmetrical structure to enable SHG. Simultaneously, these chiral compounds preserve 1D crystal structures to create strong free exciton, broad self-trapped exciton (STE), and discrete band energy levels, which facilitate the resonant enhancement of SHG and THG susceptibilities. These new chiral films showcase superior effective SHG susceptibility (χ(2) ∼ 130.5 pm V-1 at an interesting wavelength of 1550 nm), exceeding that of the reference, a commercial LiNbO3 (χ(2) ∼ 83.4 pm V-1) single-crystal film. Furthermore, their THG intensities are even higher than their SHG intensities, with effective THG susceptibility (χ(3)) being ∼9.0 × 106 pm2 V-2 at 1550 nm (37 times that of the reference monolayer WS2). Their high SHG and THG performances indicate the promising future of these 1D chiral hybrid bismuth halides toward nonlinear optical applications.

Second-harmonic generation in single CdSe nanowires by focused cylindrical vector beams.

Cylindrical vector beams with radial or azimuthal polarization have created great interest due to their unique focusing characteristics and focal components. In this Letter, we investigate second-harmonic general (SHG) of single CdSe nanowires (NWs) excited by tightly focused cylindrical vector beams of 150 fs pulses at 800 nm. With the specific polarizations in the focal region, we demonstrate a three-dimensional interaction between the focal electric field components and the NWs. The excitation anisotropy of the SHG can directly be derived from the imaging patterns with the cylindrical vector beams. The highest SHG excitation efficiency is observed when the polarization is parallel to the long axis of the NW, which is confirmed by the conventional linear polarization approach. Our work with cylindrical vector beams provides a new approach to study the nonlinear phenomenon of single semiconductor NWs in three dimensions and it could be applied to many other nanoscale systems.

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.

A universal growth method for high-quality phase-engineered germanium chalcogenide nanosheets.

Low-dimensional group IV-VI metal chalcogenide-based semiconductors hold great promise for opto-electronic device applications owing to their diverse crystalline phases and intriguing properties related to thermoelectric and ferroelectric effects. Herein, we demonstrate a universal chemical vapor deposition (CVD) growth method to synthesize stable germanium chalcogenide-based (GeS, GeS2, GeSe, GeSe2) nanosheets, which increases the library of the p-type semiconductor. The phase transition between different crystalline polytypes can be deterministically controlled by hydrogen concentration in the reaction chamber. Structural characterization and synthesis experiments identify the behavior, where the higher hydrogen concentration promotes the transiton from germanium dichalcogenides to germanium monochalcogenides. The angle-polarized and temperature-dependent Raman spectra demonstrate the strong interlayer coupling and lattice orientation. Based on the optimized growth scheme and systematic comparison of electrical properties, GeSe nanosheet photodetectors were demonstrated, which exhibit superior device performance on SiO2/Si and HfO2/Si substrate with a high photoresponsivity up to 104 A W-1, fast response time less than 15 ms, and high mobility of 3.2 cm2 V-1 s-1, which is comparable to the mechanically exfoliated crystals. Our results manifest the hydrogen-mediated deposition strategy as a facile control knob to engineer crystalline phases of germanium chalcogenides for high performance optoelectronic 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.

Giant nonlinear optical activity in two-dimensional palladium diselenide.

Nonlinear optical effects in layered two-dimensional transition metal chalcogenides have been extensively explored recently because of the promising prospect of the nonlinear optical effects for various optoelectronic applications. However, these materials possess sizable bandgaps ranging from visible to ultraviolet region, so the investigation of narrow-bandgap materials remains deficient. Here, we report our comprehensive study on the nonlinear optical processes in palladium diselenide (PdSe2) that has a near-infrared bandgap. Interestingly, this material exhibits a unique thickness-dependent second harmonic generation feature, which is in contrast to other transition metal chalcogenides. Furthermore, the two-photon absorption coefficients of 1-3 layer PdSe2 (ß ~ 4.16 × 105, 2.58 × 105, and 1.51 × 105 cm GW-1) are larger by two and three orders of magnitude than that of the conventional two-dimensional materials, and giant modulation depths (αs ~ 32%, 27%, and 24%) were obtained in 1-3 layer PdSe2. Such unique nonlinear optical characteristics make PdSe2 a potential candidate for technological innovations in nonlinear optoelectronic devices.

Liquid-Metal-Assisted Growth of Vertical GaSe/MoS2 p-n Heterojunctions for Sensitive Self-Driven Photodetectors.

van der Waals (vdW) vertical p-n junctions based on two-dimensional (2D) materials have shown great potential in flexible, self-driven, high-efficiency electronic and optoelectronic applications. However, due to the complex nucleation dynamics, the controllable synthesis of vertical heterostructures remains a daunting challenge. Here, we report the controlled growth of vertical GaSe/MoS2 p-n heterojunctions via a liquid gallium (Ga)-assisted chemical vapor deposition method. The growth mechanism can be interpreted by theoretical calculations based on the Burton-Cabrera-Frank theory. By analyzing the diffusion barriers and the Ehrlich-Schwoebel barriers of adatoms, we found that the growth modes between vertical and lateral can be precisely switched by means of adjusting the amount of Ga. Based on the achieved high-quality vertical GaSe/MoS2 p-n heterojunctions, photosensing devices are further designed and systematically investigated. Upon light illumination, prominent photovoltaic effects with large open-circuit voltage (0.61 V) and broadband detection capability from 375 to 633 nm are observed, which can further be employed for self-powered photodetection with high responsivity (900 mA/W) and fast response speed (5 ms). The developed liquid-metal-assisted strategy provides an effective method for controllable synthesis of vdW heterostructures and will give impetus to their applications in high-performance optoelectronic device.

Epitaxial synthesis of ultrathin ß-In2Se3/MoS2 heterostructures with high visible/near-infrared photoresponse.

van der Waals (vdWs) heterostructures, combining different two-dimensional (2D) layered materials with diverse properties, have been demonstrated to be a very promising platform to explore a new physical phenomenon and realize various potential applications in atomically thin electronic and optoelectronic devices. Here, we report the controlled growth of vertically stacked ß-In2Se3/MoS2 vdWs heterostructures (despite the existence of large lattice mismatching ∼29%) through a typical two-step chemical vapor deposition (CVD) method. The crystal structure of the achieved heterostructures is characterized by transmission electron microscopy, where evident Moiré patterns are observed, indicating well-aligned lattice orientation. Strong photoluminescence quenching is obeserved in the heterostructure, revealing effective interlayer charge transfer at the interface. Electrical devices are further constructed based on the achieved heterostructures, which have a high on/off ratio and a typical rectifying behavior. Upon laser irradiation, the devices show excellent photosensing properties. A high responsivity of 4.47 A W-1 and a detectivity of 1.07 × 109 Jones are obtained under 450 nm laser illumination with a bias voltage of 1 V, which are much better than those of heterostructures grown via CVD. Most significantly, the detection range can be extended to near-infrared due to the relatively small bandgap nature of ß-In2Se3. With 830 nm laser illumination, the devices also show distinct photoresponses with fast response speed even when operating at room temperature. The high-quality ß-In2Se3/MoS2 heterostructures broaden the family of the 2D layered heterostructure system and should have significant potential applications in high-performance broadband photodetectors.

Near-Unity Polarization of Valley-Dependent Second-Harmonic Generation in Stacked TMDC Layers and Heterostructures at Room Temperature.

With unique valley-dependent optical and optoelectronic properties, 2D transition metal dichalcogenides (2D TMDCs) are promising materials for valleytronics. Second-harmonic generation (SHG) in 2D TMDCs monolayers has shown valley-dependent optical selection rules. However, SHG in monolayer TMDCs is generally weak; it is important to obtain materials with both strong SHG signals and a large degree of polarization. In the work, a variety of inversion-symmetry-breaking (3R-like phase) TMDCs (WSe2 , WS2 , MoS2 ) atomic layers, spiral structures, and heterostructures are prepared, and their SHG polarization is studied. Through circular-polarization-resolved SHG experiments, it is demonstrated that the SHG intensity is enhanced in thicker samples by breaking inversion symmetry while maintaining the degree of polarization close to unity at room temperature. By studying TMDCs with different twist angles and the spiral structures, it is found that there is no significant effect of multilayer interlayer interaction on valley-dependent SHG. The realization of strong SHG with high degree of polarization may pave the way toward a new platform for nonlinear optical valleytronics devices based on 2D semiconductors.