Surface Recombination Limited Lifetimes of Photoexcited Carriers in Few-Layer Transition Metal Dichalcogenide MoS₂

Evaluation of Transport Parameters in MoS2/Graphene Junction Devices Fabricated by Chemical Vapor Deposition

We demonstrated imaging of the depletion layer in a MoS₂/graphene heterojunction fabricated by chemical vapor deposition and obtained their transport parameters such as diffusion length, lifetime, and mobility by using scanning photocurrent microscopy (SPCM). The device exhibited a n-type operation, which was determined by the MoS₂ layer with a lower mobility. The SPCM revealed the presence of the depletion layer at the heterojunction, whereas graphene provided an excellent electrical contact for the MoS₂ layer without resulting in a rectifying behavior, even if they were anchored within a very short range. The polarity of the photocurrent signal switched when we applied a drain-source bias voltage, from which we extracted the potential barrier at the junction. More importantly, a bias-dependent SPCM allowed us to simultaneously record the diffusion lengths of both majority and minority carriers for the respective MoS₂ and graphene layers. By combining the diffusion lengths with the lifetimes measured by femtosecond SPCM, we determined the electron and hole mobilities in each layer, from which we found that the electron mobility (160 cm² V^-1 s^-1) was higher than the hole mobility (80 cm² V^-1 s^-1) in MoS₂, whereas the hole mobility (15 000 cm² V^-1 s^-1) was relatively higher in graphene.

Carrier Trapping by Oxygen Impurities in Molybdenum Diselenide

Understanding defect effect on carrier dynamics is essential for both fundamental physics and potential applications of transition metal dichalcogenides (TMDs). Here, the phenomenon of oxygen impurities trapping photoexcited carriers has been studied with ultrafast pump-probe spectroscopy. Oxygen impurities are intentionally created in exfoliated multilayer MoSe₂ with Ar⁺ plasma irradiation and air exposure. After plasma treatment, the signal of transient absorption first increases and then decreases, which is a signature of defect-capturing carriers. With larger density of oxygen defects, the trapping effect becomes more prominent. The trapping defect densities are estimated from the transient absorption signal, and its increasing trend in the longer-irradiated sample agrees with the results from X-ray photoelectron spectroscopy. First-principle calculations with density functional theory reveal that oxygen atoms occupying Mo vacancies create mid-gap defect states, which are responsible for carrier trapping. Our findings shed light on the important role of oxygen defects as carrier trappers in TMDs, and facilitate defect engineering in relevant materials and device applications.

Weakly Trapped, Charged, and Free Excitons in Single-Layer MoS2 in the Presence of Defects, Strain, and Charged Impurities

Few- and single-layer MoS₂ host substantial densities of defects. They are thought to influence the doping level, the crystal structure, and the binding of electron-hole pairs. We disentangle the concomitant spectroscopic expression of all three effects and identify to what extent they are intrinsic to the material or extrinsic to it, i.e., related to its local environment. We do so by using different sources of MoS₂-a natural one and one prepared at high pressure and high temperature-and different substrates bringing varying amounts of charged impurities and by separating the contributions of internal strain and doping in Raman spectra. Photoluminescence unveils various optically active excitonic complexes. We discover a defect-bound state having a low binding energy of 20 meV that does not appear sensitive to strain and doping, unlike charged excitons. Conversely, the defect does not significantly dope or strain MoS₂. Scanning tunneling microscopy and density functional theory simulations point to substitutional atoms, presumably individual nitrogen atoms at the sulfur site. Our work shows the way to a systematic understanding of the effect of external and internal fields on the optical properties of two-dimensional materials.

Ultrafast non-radiative dynamics of atomically thin MoSe2

Photo-induced non-radiative energy dissipation is a potential pathway to induce structural-phase transitions in two-dimensional materials. For advancing this field, a quantitative understanding of real-time atomic motion and lattice temperature is required. However, this understanding has been incomplete due to a lack of suitable experimental techniques. Here, we use ultrafast electron diffraction to directly probe the subpicosecond conversion of photoenergy to lattice vibrations in a model bilayered semiconductor, molybdenum diselenide. We find that when creating a high charge carrier density, the energy is efficiently transferred to the lattice within one picosecond. First-principles nonadiabatic quantum molecular dynamics simulations reproduce the observed ultrafast increase in lattice temperature and the corresponding conversion of photoenergy to lattice vibrations. Nonadiabatic quantum simulations further suggest that a softening of vibrational modes in the excited state is involved in efficient and rapid energy transfer between the electronic system and the lattice.

Spatiotemporal Evolution of Coherent Elastic Strain Waves in a Single MoS2 Flake

We use bright-field imaging in an ultrafast electron microscope to spatiotemporally map the evolution of photoexcited coherent strain waves in a single, micrometer-size flake of MoS₂. Following in situ femtosecond photoexcitation, we observe individual wave trains emerge from discrete nanoscale morphological features and propagate in-plane along specific wave vectors at approximately the speed of sound (7 nm/ps). Over the span of several hundred picoseconds, the 50 GHz wave trains (20 ps periods) are observed to undergo phonon-phonon scattering and wave-train interference, resulting in a transition to larger-scale, incoherent structural dynamics. This incoherent motion further evolves into coherent nanomechanical oscillations over a few nanoseconds, ultimately leading to megahertz, whole-flake multimode resonances having microsecond lifetimes. These results provide insight into the low-frequency structural response of MoS₂ to relatively coherent optical photoexcitation by elucidating the origin and the evolution of high-velocity, gigahertz strain waves.

Centimeter-Scale Deposition of Mo0.5W0.5Se2 Alloy Film for High-Performance Photodetectors on Versatile Substrates

Because of their great potential for academic investigation and practical application in next-generation optoelectronic devices, ternary layered semiconductors have attracted considerable attention in recent years. Similar to the applications of traditional layered materials, practical applications of ternary layered semiconductor alloys require the synthesis of large-area samples. Here, we report the preparation of centimeter-scale and high-quality Mo_0.5W_0.5Se₂ alloy films on both a rigid SiO₂/Si substrate and a flexible polyimide (PI) substrate. Then, photodetectors based on these alloy films are fabricated, which are capable of conducting broad-band photodetection from ultraviolet to near-infrared region (370-808 nm) with high performance. The photodetector on SiO₂/Si substrates demonstrates a high responsivity (R) of 77.1 A/W, an outstanding detectivity (D*) of 1.1 × 10¹² Jones, and a fast response time of 8.3 ms. These figures-of-merit are much superior to those of the counterparts of binary material-based devices. Moreover, the photodetector on PI substrates also achieves high performance (R = 63.5 A/W, D* = 3.56 × 10¹² Jones). And no apparent degradation in the device properties is observed even after 100 bending cycles. These results make Mo_0.5W_0.5Se₂ alloy a highly qualified candidate for next-generation optoelectronic applications.

New Insights toward Efficient Charge-Separation Mechanism for High-Performance Photoelectrochemical Aptasensing: Enhanced Charge-Carrier Lifetime via Coupling Ultrathin MoS2 Nanoplates with Nitrogen-Doped Graphene Quantum Dots

Deeply understanding the internal mechanism of the photoelectrohemical (PEC) process is conducive to fabricate high-performance PEC biosensors. In this work, we proposed a new insight toward an efficient charge-separation mechanism in high-performance PEC biosensors. Specifically, we disclosed that the lifetimes of photogenerated charge carriers of ultrathin MoS₂ nanosheets could be prolonged by approximately millisecond time scales after a proper mole ratio of NGQDs were coupled, leading to the promoted charge separation and a giant photocurrent signal magnification. Benefiting from the dramatic signal amplification and the introduction of acetamiprid aptamer, subfemtomolar level detection of acetamiprid is achieved, which makes our strategy among the most sensitive electronic approaches for PEC-based monitoring of targets. This study was beneficial to further understand the charge-separation mechanism in PEC biosensing, which paved the way for the development of more efficient PEC biosensors.

Mutual Photoluminescence Quenching and Photovoltaic Effect in Large-Area Single-Layer MoS2-Polymer Heterojunctions

Two-dimensional transition metal dichalcogenides (TMDCs) have recently attracted attention due to their superlative optical and electronic properties. In particular, their extraordinary optical absorption and semiconducting band gap have enabled demonstrations of photovoltaic response from heterostructures composed of TMDCs and other organic or inorganic materials. However, these early studies were limited to devices at the micrometer scale and/or failed to exploit the unique optical absorption properties of single-layer TMDCs. Here we present an experimental realization of a large-area type-II photovoltaic heterojunction using single-layer molybdenum disulfide (MoS₂) as the primary absorber, by coupling it to the organic π-donor polymer PTB7. This TMDC-polymer heterojunction exhibits photoluminescence intensity that is tunable as a function of the thickness of the polymer layer, ultimately enabling complete quenching of the TMDC photoluminescence. The strong optical absorption in the TMDC-polymer heterojunction produces an internal quantum efficiency exceeding 40% for an overall cell thickness of less than 20 nm, resulting in exceptional current density per absorbing thickness in comparison to other organic and inorganic solar cells. Furthermore, this work provides insight into the recombination processes in type-II TMDC-polymer heterojunctions and thus provides quantitative guidance to ongoing efforts to realize efficient TMDC-based solar cells.

Multibit MoS2 Photoelectronic Memory with Ultrahigh Sensitivity

A novel multibit MoS₂ photoelectronic nonvolatile memory device is developed by synergistically combining rational device designs and the efficient transfer of large-area MoS₂ flakes. The MoS₂ photoelectronic memory exhibits excellent memory characteristics, including a large programming/erasing current ratio that exceeds 10⁷ , multilevel data storage of 3 bits (corresponding to eight levels), performance stability over 200 cycles, and stable data retention over 10⁴ s.

Bipolar Junction Transistors in Two-Dimensional WSe2 with Large Current and Photocurrent Gains

In the development of semiconductor devices, the bipolar junction transistor (BJT) features prominently as being the first solid state transistor that helped to usher in the digital revolution. For any new semiconductor, therefore, the fabrication and characterization of the BJT are important for both technological importance and historical significance. Here, we demonstrate a BJT device in exfoliated TMD semiconductor WSe2. We use buried gates to electrostatically create doped regions with back-to-back p-n junctions. We demonstrate two central characteristics of a bipolar device: current gain when operated as a BJT and a photocurrent gain when operated as a phototransistor. We demonstrate a current gain of 1000 and photocurrent gain of 40 and describe features that enhance these properties due to the doping technique that we employ.

Exciton Dynamics in Monolayer Transition Metal Dichalcogenides

Since the discovery of semiconducting monolayer transition metal dichalcogenides, a variety of experimental and theoretical studies have been carried out seeking to understand the intrinsic exciton population recombination and valley relaxation dynamics. Reports of the exciton decay time range from hundreds of femtoseconds to ten nanoseconds, while the valley depolarization time can exceed one nanosecond. At present, however, a consensus on the microscopic mechanisms governing exciton radiative and non-radiative recombination is lacking. The strong exciton oscillator strength resulting in up to ~ 20% absorption for a single monolayer points to ultrafast radiative recombination. However, the low quantum yield and large variance in the reported lifetimes suggest that non-radiative Auger-type processes obscure the intrinsic exciton radiative lifetime. In either case, the electron-hole exchange interaction plays an important role in the exciton spin and valley dynamics. In this article, we review the experiments and theory that have led to these conclusions and comment on future experiments that could complement our current understanding.

Promoting the Performance of Layered-Material Photodetectors by Alloy Engineering

The successful peeling of graphene heralded the era of van der Waals material (vdWM) electronics. However, photodetectors based on semiconducting transition metal dichalcogenides (TMDs), formulated as MX2 (M = Mo, W; X = S, Se), often suffer either poor responsivity or long response time because of their high density of deep-level defect states (DLDSs). Alloy engineering, which can shift the DLDSs to shallow-level defect states, is proposed to be an efficient strategy to solve this problem. However, proof-of-concept is still lacking, which is probably because of the absence of a facile technology to grow high-quality alloyed TMDs. Here, we report the growth of large-scale and high-quality Mo0.5W0.5S2 alloy films via pulsed laser deposition (PLD). We demonstrate that the resulting Mo0.5W0.5S2 photodetector possesses a stable photoresponse from 370 to 1064 nm. The photocurrent exhibits positive dependence on both the source-drain voltage and incident power density, providing good tunability for multifunctional photoelectrical applications. We also establish that, because of the suppression of DLDSs with alloy engineering, the Mo0.5W0.5S2 photodetector achieves a good responsivity of 5.8 A/W and a response time shorter than 150 ms. The working mechanism for the suppression of DLDSs in Mo0.5W0.5S2 is unveiled by qualitatively analyzing the alloying-dressed band structure. In conclusion, the excellent performance of the PLD-grown Mo0.5W0.5S2 photodetector may pave the way for next-generation photodetection. The approach shown here represents a fundamental and universal scenario for the development of alloyed TMDs.