Coupled Charge Transfer Dynamics and Photoluminescence Quenching in Monolayer MoS2 Decorated with WS2 Quantum Dots

Mixed-Dimensional van der Waals Heterostructure (2D ReS2/0D MoS2 Quantum Dots)-Based Broad Spectral Range with Ultrahigh-Responsive Photodetector

The remarkable properties of two-dimensional (2D) materials have led to significant advancements in photodetection and optoelectronics research. Currently, there are many successful methods that are employed to improve the responsivity of photodetectors, but the limited spectral range of the device remains a limitation. This work demonstrates the development of a mixed-dimensional (2D/0D) hybrid photodetector device fabricated using chemical vapor deposition (CVD)-grown monolayer ReS2 and solution-processed MoS2 quantum dots (QDs). The mixed dimensionality of 2D (ReS2) and zero-dimensional (0D) MoS2 QDs assist in improving the spectral range of the device [ultraviolet (360 nm) to near-infrared (780 nm)]. Further, due to the work function difference between ReS2 and MoS2 QDs, the built-in electric field across the mixed-dimensional interface promotes effective charge separation and migration, resulting in improved responsivities of the device. The calculated responsivities of the fabricated photodetector are 5.4 × 102, 3.3 × 102, and 2.6 × 102 A/W when subjected to visible, UV, and NIR light illumination, which is remarkable when compared to the existing reports on broadband photodetection. The mixed-dimensionality heterostructure coupled with contact engineering paves the way for highly responsive broadband photodetectors for potential applications in security, healthcare, etc.

Synthesis of two-dimensional MoO2 nanoplatelets and its multistep sulfurization into MoS2

To control the growth of layered two-dimensional structures, such as transition metal dichalcogenide materials or heterostructures, understanding the growth mechanism is crucial. Here, we report the synthesis of ultra-thin MoO2 nanoplatelets through the sublimation of MoO3. Rhombus MoO2 nanoplatelets with the P21/c space group were characterized using various microscopic and spectroscopic techniques. Introducing sulfur sources into the chemical vapor deposition system also leads to the formation of monoclinic MoO2 nanoflakes due to the incomplete sulfurization of MoO3. With a gradual increase in the vapor concentration of sulfur, MoO3 undergoes stepwise reduction into MoS2/MoO2 and eventually into MoS2. Additionally, utilizing MoO2 as a precursor for Mo sources enables the formation of monolayer MoS2 single crystals. This work provides an effective approach for growing MoO2 nanoplatelets and elucidates the mechanism behind the stepwise sulfurization of MoO3.

On-water surface synthesis of electronically coupled 2D polyimide-MoS2 van der Waals heterostructure

The water surface provides a highly effective platform for the synthesis of two-dimensional polymers (2DP). In this study, we present an efficient on-water surface synthesis of crystalline monolayer 2D polyimide (2DPI) through the imidization reaction between tetra (4-aminophenyl) porphyrin (M1) and perylenetracarboxylic dianhydride (M2), resulting in excellent stability and coverage over a large area (tens of cm2). We further fabricate innovative organic-inorganic hybrid van der Waals heterostructures (vdWHs) by combining with exfoliated few-layer molybdenum sulfide (MoS2). High-resolution transmission electron microscopy (HRTEM) reveals face-to-face stacking between MoS2 and 2DPI within the vdWH. This stacking configuration facilitates remarkable charge transfer and noticeable n-type doping effects from monolayer 2DPI to MoS2, as corroborated by Raman spectroscopy, photoluminescence measurements, and field-effect transistor (FET) characterizations. Notably, the 2DPI-MoS2 vdWH exhibits an impressive electron mobility of 50 cm2/V·s, signifying a substantial improvement over pristine MoS2 (8 cm2/V·s). This study unveils the immense potential of integrating 2D polymers to enhance semiconductor device functionality through tailored vdWHs, thereby opening up exciting new avenues for exploring unique interfacial physical phenomena.

Quantum dots derived from two-dimensional transition metal dichalcogenides: synthesis, optical properties and optoelectronic applications

Zero-dimensional transition metal dichalcogenides (TMD) quantum dots (QDs) have attracted a lot of attention due to their interesting fundamental properties and various applications. Compared to TMD monolayers, the QD counterpart exhibits larger values for direct transition energies, exciton binding energies, absorption coefficient, luminescence efficiency, and specific surface area. These characteristics make them useful in optoelectronic devices. In this review, recent exciting progress on synthesis, optical properties, and applications of TMD QDs is highlighted. The first part of this article begins with a brief description of the synthesis approaches, which focus on microwave-assistant heating and pulsed laser ablation methods. The second part introduces the fundamental optical properties of TMD QDs, including quantum confinement in optical absorption, excitation-wavelength-dependent photoluminescence, and many-body effects. These properties are highlighted. In the third part, we discuss lastest advancements in optoelectronic devices based on TMD QDs These devices include light-emitting diodes, solar cells, photodetectors, optical sensors, and light-controlled memory devices. Finally, a brief summary and outlook will be provided.

Two-dimensional bismuth oxyselenide quantum dots as nanosensors for selective metal ion detection over a wide dynamic range: sensing mechanism and selectivity

Bismuth oxyselenide (Bi2O2Se) nanosheets, a new 2D non-van der Waals nanomaterial having unique semiconducting properties, could be favorable for various sensing applications. In the present report, a top-down chemical approach was adopted to synthesize ultrathin Bi2O2Se quantum dots (QDs) in an appropriate solution. The as-prepared 2D Bi2O2Se QDs with an average size of ∼3 nm, exhibiting strong visible fluorescence, were utilized for heavy-metal ion detection with high selectivity. The QDs show a high optical band gap and a reasonably high fluorescence quantum yield (∼4%) in the green region without any functionalization. A series of heavy metal ions were detected using these QDs. The as-prepared QDs exhibit selective detection of Fe3+ over a wide dynamic range with a high quenching ratio and a low detection limit (<0.5 μM). The mechanism of visible fluorescence and Fe3+ ion-induced quenching was investigated in detail based on a model involving adsorption and charge transfer. Density functional theory (DFT) first principles calculations show that fluorescence quenching occurred selectively due to the efficient trapping of electrons in the bandgap states created by the Fe atoms. This work presents a sustainable and scalable method to synthesize 2D Bi2O2Se QDs for heavy metal ion sensing over a wide dynamic range and these 2D QDs could find potential uses in gas sensors, biosensors and optoelectronics.

Surface-dependent quenching of Qdot emission can be a new tool for high resolution measurements

Single quantum dots (Qdots) are often used in the field of single-molecule imaging. Qdots are sensitive to changes in the physical interactions between the Qdots and the surrounding materials. However, the spectral changes in a single Qdot emission have not been studied in detail. Low-temperature plasma treatment of glass surfaces reduced the intensity of the 655 nm emission peak of Qdot655 on glass surfaces, but did not significantly change the intensity of the 580 nm emission. Silanization of the glass surface increases the thickness of the silane layer, and the 655 nm emission peak increased. When single Qdots on the untreated glass were imaged, plasma treatment decreased the intensity of red emission and increased yellow emission. When Qdots were brought close to the glass surface in the range of 28-0 nm, the red emission intensity decreased and the yellow emission intensity increased slightly. When single actin filaments were labeled with Qdots, fluctuations of the yellow and red emission of the Qdot were detected, which reflected the very small distance changes. Our results indicate that the local interaction of Qdots with the glass surface improves the spatial and temporal resolution of optical measurements of biomolecules labeled with Qdots.

Band Alignments, Electronic Structure, and Core-Level Spectra of Bulk Molybdenum Dichalcogenides (MoS2, MoSe2, and MoTe2)

A comprehensive study of bulk molybdenum dichalcogenides is presented with the use of soft and hard X-ray photoelectron (SXPS and HAXPES) spectroscopy combined with hybrid density functional theory (DFT). The main core levels of MoS₂, MoSe₂, and MoTe₂ are explored. Laboratory-based X-ray photoelectron spectroscopy (XPS) is used to determine the ionization potential (IP) values of the MoX₂ series as 5.86, 5.40, and 5.00 eV for MoSe₂, MoSe₂, and MoTe₂, respectively, enabling the band alignment of the series to be established. Finally, the valence band measurements are compared with the calculated density of states which shows the role of p-d hybridization in these materials. Down the group, an increase in the p-d hybridization from the sulfide to the telluride is observed, explained by the configuration energy of the chalcogen p orbitals becoming closer to that of the valence Mo 4d orbitals. This pushes the valence band maximum closer to the vacuum level, explaining the decreasing IP down the series. High-resolution SXPS and HAXPES core-level spectra address the shortcomings of the XPS analysis in the literature. Furthermore, the experimentally determined band alignment can be used to inform future device work.

Defect-mediated carrier dynamics and third-order nonlinear optical response of WS2 quantum dots

In this Letter, we report the third-order absorptive and refractive nonlinear optical response of highly luminescent WS₂ quantum dots (QDs) in the off-resonant femtosecond and nanosecond pulses, which is beneficial for optical limiting and quantum information processing. For 800 nm femtosecond excitation, QDs show two-photon absorption (β =  (107 ± 2)×10^-3cm/GW) with positive nonlinearity originating from bound carriers. This picture changes significantly for 532 nm nanosecond excitation, where it shows reverse saturable absorption with negative nonlinearity primarily originating from the sequential absorption of two single photons through the shallow defects, creating free carriers. Our results provide a promising route toward low-dimensional optoelectronic devices.

Probing the charge transfer and electron-hole asymmetry in graphene-graphene quantum dot heterostructure

In recent years, graphene-based van der Waals (vdW) heterostructures have come into prominence showcasing interesting charge transfer dynamics which is significant for optoelectronic applications. These novel structures are highly tunable depending on several factors such as the combination of the two-dimensional materials, the number of layers and band alignment exhibiting interfacial charge transfer dynamics. Here, we report on a novel graphene based 0D-2D vdW heterostructure between graphene and amine-functionalized graphene quantum dots (GQD) to investigate the interfacial charge transfer and doping possibilities. Using a combination ofab initiosimulations and Kelvin probe force microscopy (KPFM) measurements, we confirm that the incorporation of functional GQDs leads to a charge transfer induced p-type doping in graphene. A shift of the Dirac point by 0.05 eV with respect to the Fermi level (E_F) in the graphene from the heterostructure was deduced from the calculated density of states. KPFM measurements revealed an increment in the surface potential of the GQD in the 0D-2D heterostructure by 29 mV with respect to graphene. Furthermore, we conducted power dependent Raman spectroscopy for both graphene and the heterostructure samples. An optical doping-induced gating effect resulted in a stiffening of theGband for electrons and holes in both samples (graphene and the heterostructure), suggesting a breakdown of the adiabatic Born-Oppenheimer approximation. Moreover, charge imbalance and renormalization of the electron-hole dispersion under the additional influence of the doped functional GQDs is pointing to an asymmetry in conduction and carrier mobility.

Tailoring the Electrical Characteristics of MoS2 FETs through Controllable Surface Charge Transfer Doping Using Selective Inkjet Printing

Surface charge transfer doping (SCTD) has been regarded as an effective approach to tailor the electrical characteristics of atomically thin transition metal dichalcogenides (TMDs) in a nondestructive manner due to their two-dimensional nature. However, the difficulty of achieving rationally controlled SCTD on TMDs via conventional doping methods, such as solution immersion and dopant vaporization, has impeded the realization of practical optoelectronic and electronic devices. Here, we demonstrate controllable SCTD of molybdenum disulfide (MoS₂) field-effect transistors using inkjet-printed benzyl viologen (BV) as an n-type dopant. By adjusting the BV concentration and the areal coverage of inkjet-printed BV dopants, controllable SCTD results in BV-doped MoS₂ FETs with elaborately tailored electrical performance. Specifically, the suggested solvent system creates well-defined droplets of BV ink having a volume of ∼2 pL, which allows the high spatial selectivity of SCTD onto the MoS₂ channels by depositing the BV dopant on demand. Our inkjet-printed SCTD method provides a feasible solution for achieving controllable doping to modulate the electrical and optical performances of TMD-based devices.

Broadband, Ultra-High-Responsive Monolayer MoS2/SnS2 Quantum-Dot-Based Mixed-Dimensional Photodetector

Atomically thin two-dimensional (2D) materials have gained significant attention from the research community in the fabrication of high-performance optoelectronic devices. Even though there are various techniques to improve the responsivity of the photodetector, the key factor limiting the performance of the photodetectors is constrained photodetection spectral range in the electromagnetic spectrum. In this work, a mixed-dimensional 0D/2D SnS₂-QDs/monolayer MoS₂ hybrid is fabricated for high-performance and broadband (UV-visible-near-infrared (NIR)) photodetector. Monolayer MoS₂ is deposited on SiO₂/Si using chemical vapor deposition (CVD), and SnS₂-QDs are prepared using a low-cost solution-processing method. The high performance of the fabricated 0D/2D photodetector is ascribed to the band bending and built-in potential created at the junction of SnS₂-QDs and MoS₂, which enhances the injection and separation efficiency of the photoexcited charge carriers. The mixed-dimensional structure also suppresses the dark current of the photodetector. The decorated SnS₂-QDs on monolayer MoS₂ not only improve the performance of the device but also extends the spectral range to the UV region. Photoresponsivity of the device for UV, visible, and NIR region is found to be ∼278, ∼ 435, and ∼189 A/W, respectively. Fabricated devices showed maximum responsivity under the visible region attributed to the high absorbance of monolayer MoS₂. The response time of the fabricated device is measured as ∼100 ms. These results reveal that the development of a mixed-dimensional (0D/2D) SnS₂-QDs/MoS₂-based high-performance and broadband photodetector is technologically promising for next-generation optoelectronic applications.

Excitons and Trions in MoS2 Quantum Dots: The Influence of the Dispersing Medium

Single-layer MoS₂ has been reported to exhibit strong excitonic and trionic signatures in its photoluminescence (PL) spectra. Here, we report that the emission spectra of MoS₂ QDs strongly depend on the dielectric constant of the solvent and the relative difference in the electronegativity between the solvent and QDs. Due to the difference in electronegativity, electrons are either added to the QD or withdrawn from it. Consequently, depending upon the dielectric permittivity and the electronegativity of the surrounding medium, the signature peaks of excitons and trions exhibit a significant change in the PL spectra of MoS₂ QDs. Our findings are helpful to understand the effect of the surrounding environment on the optical properties of QDs and the importance of the selection of solvent since MoS₂ QDs are potential candidates for valleytronics applications.

Mechanisms of negative differential resistance in glutamine-functionalized WS2quantum dots

Understanding the mechanism of the negative differential resistance (NDR) in transition metal dichalcogenides is essential for fundamental science and the development of electronic devices. Here, the NDR of the current-voltage characteristics was observed based on the glutamine-functionalized WS₂quantum dots (QDs). The NDR effect can be adjusted by varying the applied voltage range, air pressure, surrounding gases, and relative humidity. A peak-to-valley current ratio as high as 6.3 has been achieved at room temperature. Carrier trapping induced by water molecules was suggested to be responsible for the mechanism of the NDR in the glutamine-functionalized WS₂QDs. Investigating the NDR of WS₂QDs may promote the development of memory applications and emerging devices.

Interlayer electron modulation in van der Waals heterostructures assembled by stacking monolayer MoS2 onto monolayer graphene with different electron transfer ability

Achieving tunable optoelectronic properties and clarifying interlayer interactions are key challenges in the development of 2D heterostructures. Herein, we report the feasible modulation of the optoelectronic properties of monolayer MoS₂ (1L-MoS₂) on three different graphene monolayers with varying ability in extracting electrons. Monolayer oxygen-functionalized graphene (1L-oxo-G, a high amount of oxygen of 60%) with a work function (WF) of 5.67 eV and its lowly oxidized reduction product, namely reduced-oxo-G (1L-r-oxo-G, a low amount of oxygen of 0.1%), with a WF of 5.85 eV serving as hole injection layers significantly enhance the photoluminescence (PL) intensity of MoS₂, whereas pristine monolayer graphene (1L-G) with a work function (WF) of 5.02 eV results in PL quenching of MoS₂. The enhancement in the PL intensity is due to increase of neutral exciton recombination. Furthermore, 1L-r-oxo-G/MoS₂ exhibited a higher increase (5-fold) in PL than 1L-oxo-G/MoS₂ (3-fold). Our research can help modulate the carrier concentration and electronic type of 1L-MoS₂ and has promising applications in optoelectronic devices.

Understanding the interfacial charge transfer in the CVD grown Bi2O2Se/CsPbBr3 nanocrystal heterostructure and its exploitation in superior photodetection: experiment vs. theory

Efficient charge transfer in a 2D semiconductor heterostructure plays a crucial role in high-performance photodetectors and energy harvesting devices. Non-van der Waals 2D Bi₂O₂Se has enormous potential for high-performance optoelectronics, though very little is known about the interfacial charge transport at the corresponding 2D heterojunction. Herein, we report a combined experimental and theoretical investigation of interfacial charge transfer in the Bi₂O₂Se/CsPbBr₃ heterostructure through various microscopic and spectroscopic tools corroborated with density functional theory calculations. The CVD-grown few-layer Bi₂O₂Se nanosheet possesses high crystallinity and a high absorption coefficient in the visible-near IR region. We integrated the few-layer Bi₂O₂Se nanosheet possessing superior electron mobility and CsPbBr₃ nanocrystals with high light-harvesting capability for efficient broadband photodetection. The band alignment reveals a type-I heterojunction, and the device under reverse bias reveals a fast response time of 12 μs/24 μs (rise time/fall time) and an improved responsivity in the 390 to 840 nm range due to the effective interfacial charge transfer and efficient interlayer coupling at the Bi₂O₂Se/CsPbBr₃ interface. Notably, a photodetector with a better light on/off ratio and a peak responsivity of ∼10³ A W^-1 was achieved in the Bi₂O₂Se/CsPbBr₃ heterostructure due to the synergistic effects in the heterostructure under ambient conditions. The DFT analysis of the density of states and charge density plots in the heterostructure revealed a net transfer of electrons/holes from perovskite nanocrystals to Bi₂O₂Se layers and additional density of states in Bi₂O₂Se. These results are significant for the development of non-van der Waals heterostructure based high-performance low-powered photodetectors.

Boron vacancy: a strategy to boost the oxygen reduction reaction of hexagonal boron nitride nanosheet in hBN-MoS2 heterostructure

The incorporation of vacancies in a system is considered a proficient method of defect engineering in general catalytic modulation. Among two-dimensional materials, the deficiency of surface active sites and a high band gap restrict the catalytic activity of hexagonal boron nitride (hBN) material towards the oxygen reduction reaction (ORR), which hinders its applicability in fuel cells. A bane to boon strategy has been introduced here by coupling two sluggish ORR materials (hBN & MoS2) by a probe-sonication method to form a heterostructure (termed HBPS) which fosters four electron pathways to assist the reduction of oxygen. Theoretical and experimental studies suggest the kinetically and thermodynamically favorable formation of boron vacancies (B-vacancies) in the presence of MoS2, which act as active sites for oxygen adsorption in HBPS. B-vacancy induced uneven charge distribution together with band gap depression promote rapid electron transfer from the valance band to the conduction band which prevails over the kinetic limitation of pure hBN nanosheets towards ORR kinetics. The formed B-vacancy induced HBPS further exhibits a low Tafel slope (66 mV dec-1), and a high onset potential (0.80 V vs. RHE) with an unaltered electrochemically active surface area (ESCA) after long-term cycling. Thus, vacancy engineering in hBN has proved to be an efficient approach to unlock the potential of catalytic performance enhancement.

Enhancing and quantifying spatial homogeneity in monolayer WS2

Controlling the radiative properties of monolayer transition metal dichalcogenides is key to the development of atomically thin optoelectronic devices applicable to a wide range of industries. A common problem for exfoliated materials is the inherent disorder causing spatially varying nonradiative losses and therefore inhomogeneity. Here we demonstrate a five-fold reduction in the spatial inhomogeneity in monolayer WS2, resulting in enhanced overall photoluminescence emission and quality of WS2 flakes, by using an ambient-compatible laser illumination process. We propose a method to quantify spatial uniformity using statistics of spectral photoluminescence mapping. Analysis of the dynamic spectral changes shows that the enhancement is due to a spatially sensitive reduction of the charged exciton spectral weighting. The methods presented here are based on widely adopted instrumentation. They can be easily automated, making them ideal candidates for quality assessment of transition metal dichalcogenide materials, both in the laboratory and industrial environments.

Emerging of Heterostructure Materials in Energy Storage: A Review

With the ever-increasing adaption of large-scale energy storage systems and electric devices, the energy storage capability of batteries and supercapacitors has faced increased demand and challenges. The electrodes of these devices have experienced radical change with the introduction of nano-scale materials. As new generation materials, heterostructure materials have attracted increasing attention due to their unique interfaces, robust architectures, and synergistic effects, and thus, the ability to enhance the energy/power outputs as well as the lifespan of batteries. In this review, the recent progress in heterostructure from energy storage fields is summarized. Specifically, the fundamental natures of heterostructures, including charge redistribution, built-in electric field, and associated energy storage mechanisms, are summarized and discussed in detail. Furthermore, various synthesis routes for heterostructures in energy storage fields are roundly reviewed, and their advantages and drawbacks are analyzed. The superiorities and current achievements of heterostructure materials in lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), lithium-sulfur batteries (Li-S batteries), supercapacitors, and other energy storage devices are discussed. Finally, the authors conclude with the current challenges and perspectives of the heterostructure materials for the fields of energy storage.

WS2 /MoS2 Heterostructures through Thermal Treatment of MoS2 Layers Electrostatically Functionalized with W3 S4 Molecular Clusters

The preparation of 2D stacked layers combining flakes of different nature gives rise to countless numbers of heterostructures where new band alignments, defined at the interfaces, control the electronic properties of the system. Among the large family of 2D/2D heterostructures, the one formed by the combination of the most common semiconducting transition metal dichalcogenides, WS₂ /MoS₂ , has awakened great interest owing to its photovoltaic and photoelectrochemical properties. Solution as well as dry physical methods have been developed to optimize the synthesis of these heterostructures. Here, a suspension of negatively charged MoS₂ flakes is mixed with a methanolic solution of a cationic W₃ S₄ -core cluster, giving rise to a homogeneous distribution of the clusters over the layers. In a second step, a calcination of this molecular/2D heterostructure under N₂ leads to the formation of clean WS₂ /MoS₂ heterostructures, where the photoluminescence of both counterparts is quenched, proving an efficient interlayer coupling. Thus, this chemical method combines the advantages of a solution approach (simple, scalable, and low-cost) with the good quality interfaces reached by using more complicated traditional physical methods.