Engineering electrode interfaces for telecom-band photodetection in MoS2/Au heterostructures via sub-band light absorption.

Transition metal dichalcogenide (TMD) layered semiconductors possess immense potential in the design of photonic, electronic, optoelectronic, and sensor devices. However, the sub-bandgap light absorption of TMD in the range from near-infrared (NIR) to short-wavelength infrared (SWIR) is insufficient for applications beyond the bandgap limit. Herein, we report that the sub-bandgap photoresponse of MoS2/Au heterostructures can be robustly modulated by the electrode fabrication method employed. We observed up to 60% sub-bandgap absorption in the MoS2/Au heterostructure, which includes the hybridized interface, where the Au layer was applied via sputter deposition. The greatly enhanced absorption of sub-bandgap light is due to the planar cavity formed by MoS2 and Au; as such, the absorption spectrum can be tuned by altering the thickness of the MoS2 layer. Photocurrent in the SWIR wavelength range increases due to increased absorption, which means that broad wavelength detection from visible toward SWIR is possible. We also achieved rapid photoresponse (~150 µs) and high responsivity (17 mA W-1) at an excitation wavelength of 1550 nm. Our findings demonstrate a facile method for optical property modulation using metal electrode engineering and for realizing SWIR photodetection in wide-bandgap 2D materials.

Highly Efficient Van Der Waals Heterojunction on Graphdiyne toward the High-Performance Photodetector.

Graphdiyne (GDY), a new 2D material, has recently proven excellent performance in photodetector applications due to its direct bandgap and high mobility. Different from the zero-gap of graphene, these preeminent properties made GDY emerge as a rising star for solving the bottleneck of graphene-based inefficient heterojunction. Herein, a highly effective graphdiyne/molybdenum (GDY/MoS2 ) type-II heterojunction in a charge separation is reported toward a high-performance photodetector. Characterized by robust electron repulsion of alkyne-rich skeleton, the GDY based junction facilitates the effective electron-hole pairs separation and transfer. This results in significant suppression of Auger recombination up to six times at the GDY/MoS2 interface compared with the pristine materials owing to an ultrafast hot hole transfer from MoS2 to GDY. GDY/MoS2 device demonstrates notable photovoltaic behavior with a short-circuit current of -1.3 × 10-5 A and a large open-circuit voltage of 0.23 V under visible irradiation. As a positive-charge-attracting magnet, under illumination, alkyne-rich framework induces positive photogating effect on the neighboring MoS2 , further enhancing photocurrent. Consequently, the device exhibits broadband detection (453-1064 nm) with a maximum responsivity of 78.5 A W-1 and a high speed of 50 µs. Results open up a new promising strategy using GDY toward effective junction for future optoelectronic applications.

Exciton Transfer at Heterointerfaces of MoS2 Monolayers and Fluorescent Molecular Aggregates.

Integration of distinct materials to form heterostructures enables the proposal of new functional devices based on emergent physical phenomena beyond the properties of the constituent materials. The optical responses and electrical transport characteristics of heterostructures depend on the charge and exciton transfer (CT and ET) at the interfaces, determined by the interfacial energy level alignment. In this work, heterostructures consisting of aggregates of fluorescent molecules (DY1) and 2D semiconductor MoS2 monolayers are fabricated. Photoluminescence spectra of DY1/MoS2 show quenching of the DY1 emission and enhancement of the MoS2 emission, indicating a strong electronic interaction between these two materials. Nanoscopic mappings of the light-induced contact potential difference changes rule out the CT process at the interface. Using femtosecond transient absorption spectroscopy, the rapid interfacial ET process from DY1 aggregates to MoS2 and a fourfold extension of the exciton lifetime in MoS2 are elucidated. These results suggest that the integration of 2D inorganic semiconductors with fluorescent molecules can provide versatile approaches to engineer the physical characteristics of materials for both fundamental studies and novel optoelectronic device applications.

Spin-Selective Hole-Exciton Coupling in a V-Doped WSe2 Ferromagnetic Semiconductor at Room Temperature.

While valley polarization with strong Zeeman splitting is the most prominent characteristic of two-dimensional (2D) transition metal dichalcogenide (TMD) semiconductors under magnetic fields, enhancement of the Zeeman splitting has been demonstrated by incorporating magnetic dopants into the host materials. Unlike Fe, Mn, and Co, V is a distinctive dopant for ferromagnetic semiconducting properties at room temperature with large Zeeman shifting of band edges. Nevertheless, little known is the excitons interacting with spin-polarized carriers in V-doped TMDs. Here, we report anomalous circularly polarized photoluminescence (CPL) in a V-doped WSe2 monolayer at room temperature. Excitons couple to V-induced spin-polarized holes to generate spin-selective positive trions, leading to differences in the populations of neutral excitons and trions between left and right CPL. Using transient absorption spectroscopy, we elucidate the origin of excitons and trions that are inherently distinct for defect-mediated and impurity-mediated trions. Ferromagnetic characteristics are further confirmed by the significant Zeeman splitting of nanodiamonds deposited on the V-doped WSe2 monolayer.

Inclined Ultrathin Bi2O2Se Films: A Building Block for Functional van der Waals Heterostructures.

As an emerging ultrathin semiconductor material, Bi2O2Se exhibits prominent performances in electronics, optoelectronics, ultrafast optics, etc. However, until now, the in-plane growth of Bi2O2Se thin films is mostly fulfilled on atomically flat mica substrates with interfacial electrostatic forces setting obstacles for Bi2O2Se transfer to fabricate functional van der Waals heterostructures. In this work, controlled growth of inclined Bi2O2Se ultrathin films is realized with apparently reduced interfacial contact areas upon mica flakes. Consequently, the transfer of Bi2O2Se could be facile by overcoming weaker electrostatic interactions. From cross-sectional characterizations at the Bi2O2Se/mica interfaces, it is found that there are no oxide buffer layers in existence for both in-plane and inclined growths, while the un-neutralized charge density is apparently decreased for inclined films. By mechanical pressing, inclined Bi2O2Se could be transferred onto SiO2/Si substrates, and back-gated Bi2O2Se field effect transistors are fabricated, outperforming previously reported in-plane Bi2O2Se devices transferred with the assistance of corrosive acids and adhesive polymers. Furthermore, Bi2O2Se/graphene heterostructures are fulfilled by a probe tip to fabricate hybrid phototransistors with pristine interfaces, exhibiting highly efficient photoresponses. The results in this work demonstrate the potential of inclined Bi2O2Se to act as a building block for prospective van der Waals heterostructures.

Highly efficient broadband photodetectors based on lithography-free Au/Bi2O2Se/Au heterostructures.

As one of the bismuth-based oxychalcogenide materials, Bi2O2Se ultrathin films have received intense research interest due to their high carrier mobility, narrow bandgaps, ultrafast intrinsic photoresponse and long-term ambient stability; they exhibit great potential in electronic and optoelectronic applications. However, the device performance of photodetectors based on metal/Bi2O2Se/metal structures has degraded due to the undesirable defects or contaminants from the electrode deposition or the sample transfer processes. In this work, highly efficient photodetectors based on Au/Bi2O2Se junctions were achieved with Au electrodes transferred under the assistance of a probe tip to avoid contaminants from traditional lighography methods. Furthermore, to improve the charge transfer efficiency, specifically by increasing the intensity of the electrical field at the Au/Bi2O2Se interface and along the Bi2O2Se channels, the device annealing temperature was optimized to narrow the van der Waals gap at the Au/Bi2O2Se interface and the device channel length was shortened to improve the overall device performance. Among all the devices, the maximum device photoresponsivity was 9.1 A W-1, and the device response time could approach 36 µs; moreover, the photodetectors featured broadband spectral responses from 360 nm to 1090 nm.