Modulating Electronic Properties of Monolayer MoS2 via Electron-Withdrawing Functional Groups of Graphene Oxide

Strong Photoluminescence Enhancement in Molybdenum Disulfide in Aqueous Media

The interface between conventional semiconductors and aqueous ionic solutions is an important target in chemistry and materials science. Recently, a wide variety of research has been done on transition-metal dichalcogenides (TMDCs) for use as 2D layered semiconductors, and their optoelectronic properties have been widely explored. One representative TMDC, monolayer (1L) MoS₂, is known to show a photoluminescence (PL) signal of a direct band gap nature, and the PL intensity is dependent on the carrier concentration. Various methods of 1L MoS₂ carrier modulation have been shown to enhance the PL intensity in dry environments. In contrast, enhancement in an aqueous environment is limited, and a strategy to design an interface with aqueous media has not yet been established. One proposed idea was an aqueous acid interface; however, the enhancement of the PL with this method was usually minimal, about 1 order of magnitude. In this study, we demonstrate a method to achieve strong PL enhancement in 1L MoS₂ in an aqueous media by incorporating bis(trifluoromethane)sulfonyl anion (TFSI^- ion) in an acidic environment. With the addition of the TFSI^- ion in an acidic environment, the enhancement factor of the PL in 1L MoS₂ is more than 100 times greater than its PL intensity in water. The molecular anion is the key factor, as the TFSI^- ion facilitates the oxidation of MoS₂. This anionic effect is the additional factor needed to modulate the optoelectronic properties of 2D semiconductors in aqueous media. The proposed idea could have potential applications for biochemical sensors in aqueous situations.

π-π stacking derived from graphene-like biochar/g-C3N4 with tunable band structure for photocatalytic antibiotics degradation via peroxymonosulfate activation

The severe pollution caused by antibiotics has raised serious concerns in recent decades. In this study, graphene-like Enteromorpha biochar modified g-C₃N₄ (BC/CN) was synthesized and applied to degrade tetracycline by activating PMS under visible light, obtaining around 90% removal rate within 1 h. The Enteromorpha biochar can provide electron-withdrawing groups to adjust the electronic structure of g-C₃N₄, and induces more π-π interaction to decline the recombination of photocarriers. The environmental adaptability of the BC/CN/PMS/vis system was confirmed by the TC degradation in different initial pH, coexisting ions, and natural organic materials. In most cases, the system maintained over 78% degradation rate. The kinetics and mechanism of the system indicating that ∙O₂^-, ¹O₂ contributed more to the TC photocatalytic degradation than ∙OH, SO₄∙^-, and h⁺. During the process, TC underwent serials hydroxylation, demethylation, and ring-opening processes, and produced more than 40 intermediates in three pathways. Moreover, the BC/CN/PMS/vis system was proved to have at least a 50% degradation rate for more tetracyclines and quinolone antibiotics with the same condition.

Large-Area MoS2 via Colloidal Nanosheet Ink for Integrated Memtransistor

2D transition metal dichalcogenides (TMDs) exhibit intriguing properties for applications in optoelectronics and electronics, among which memtransistors received extensive attention as multifunctional devices. For practical applications of 2D TMDs, large-area fabrication of the materials via reliable processes, which is in trade-off with their quality, has been a long-standing issue. Here, a simple and effective way is proposed to fabricate large-area and high-quality molybdenum disulfide thin films using MoS₂ colloidal ink through a spray coating, followed by a postsulfurization process. High-quality MoS₂ thin films exhibit excellent optical and electrical properties that can be utilized in field-effect transistors (FETs) and memtransistor arrays. The MoS₂ FETs show an average on/off ratio of 5 × 10⁶ and a high electron mobility of 10.34 cm²  V^-1  s^-1 , which can be understood by the healing of sulfur vacancies, recrystallization, and the removal of the carbon contamination of the MoS₂ . These MoS₂ -based memtransistors present stable operations with a high switching ratio tuned by back gate and light illumination, which is promising for multiple-levels memory and complex neuromorphic computing. This study demonstrates a new strategy to fabricate 2D TMDs with large-area and high quality for integrated optoelectronic and memory device applications.

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.

A simple electrochemical sensor based on rGO/MoS2/CS modified GCE for highly sensitive detection of Pb(ii) in tobacco leaves

High-performance electrode modification materials play a crucial role in improving the sensitivity of sensor detection in electrochemical determination of heavy metals. In this study, a rGO/MoS₂/CS nanocomposite modified glassy carbon electrode (GCE) was used to construct a sensitive sensor for detecting lead ions in tobacco leaves. The reduced graphene oxide (rGO) was used to increase the conductivity of the sensor, and the nano-flowered MoS₂ could provide a large reaction specific surface area and a certain active site for heavy metal reaction. Chitosan (CS) was used to improve the enrichment ability of heavy metals and increase the electrocatalytic activity of electrode. Thus, an electrochemical sensor with excellent performance in reproducibility, stability and anti-interference ability was established. The stripping behavior of Pb(ii) and the application conditions of the sensor were studied by square wave anodic stripping voltammetry (SWASV). The investigation indicated that the sensor exhibited high detection sensitivity in the range of 0.005-0.05-2.0 μM, and the limit of detection (LOD) was 0.0016 μM. This work can provide a fast and effective method for determination of Pb(ii) in samples with low content, such as tobacco leaves.

Leveraging Molecular Properties to Tailor Mixed-Dimensional Heterostructures beyond Energy Level Alignment

The surface sensitivity and lack of dielectric screening in two-dimensional (2D) materials provide numerous intriguing opportunities to tailor their properties using adsorbed π-electron organic molecules. These organic-2D mixed-dimensional heterojunctions are often considered solely in terms of their energy level alignment, i.e., the relative energies of the frontier molecular orbitals versus the 2D material conduction and valence band edges. While this simple model is frequently adequate to describe doping and photoinduced charge transfer, the tools of molecular chemistry enable additional manipulation of properties in organic-2D heterojunctions that are not accessible in other solid-state systems. Fully exploiting these possibilities requires consideration of the details of the organic adlayer beyond its energy level alignment, including hybridization and electrostatics, molecular orientation and thin-film morphology, nonfrontier orbitals and defects, excitonic states, spin, and chirality. This Perspective explores how these relatively overlooked molecular properties offer unique opportunities for tuning optical and electronic characteristics, thereby guiding the rational design of organic-2D mixed-dimensional heterojunctions with emergent properties.

Surface-Modified Ultrathin InSe Nanosheets with Enhanced Stability and Photoluminescence for High-Performance Optoelectronics

Indium selenide (InSe) has become a research hotspot because of its favorable carrier mobility and thickness-tunable band gap, showing great application potential in high-performance optoelectronic devices. The trend of miniaturization in optoelectronics has forced the feature sizes of the electronic components to shrink accordingly. Therefore, atomically thin InSe crystals may play an important role in future optoelectronics. Given the instability and ultralow photoluminescent (PL) emission of mechanically exfoliated ultrathin InSe, synthesis of highly stable mono- and few-layer InSe nanosheets with high PL efficiency has become crucial. Herein, ultrathin InSe nanosheets were prepared via thermal annealing of electrochemically intercalated products from bulk InSe. The size and yield of the as-prepared nanosheets were up to ∼160 μm and ∼70%, respectively, and ∼80% of the nanosheets were less than five layer. Impressively, the as-prepared nanosheets showed greatly enhanced stability and PL emission because of surface modification by carbon species. Efficient photoresponsivity of 2 A/W was achieved in the as-prepared nanosheet-based devices. These nanosheets were further assembled into large-area thin films with photoresponsivity of 16 A/W and an average Hall mobility of about 5 cm² V^-1 s^-1. Finally, one-dimensional (1D) InSe nanoscrolls with a length up to 90 μm were constructed by solvent-assisted self-assembly of the exfoliated nanosheets.

Enhanced photoluminescence of InGaAs/AlGaAs quantum well with tungsten disulfide quantum dots

The pristine and diethylenetriamine (DETA)-doped tungsten disulfide quantum dots (WS₂ QDs) with an average lateral size of about 5 nm have been synthesized using pulsed laser ablation (PLA). Introduction of the synthesized WS₂ QDs on the InGaAs/AlGaAs quantum wells (QWs) can improve the photoluminescence (PL) of the InGaAs/AlGaAs QW as high as 6 fold. On the basis of the time-resolved PL and Kelvin probe measurements, the PL enhancement is attributed to the carrier transfer from the pristine or DETA-doped WS₂ QDs to the InGaAs/AlGaAs QW. A heterostructure band diagram is proposed for explaining the carrier transfer, which increases the hole densities in the QW and enhances its PL intensity. This study is expected to be beneficial for the development of the InGaAs-based optoelectronic devices.

Surface group-modified MXene nano-flake doping of monolayer tungsten disulfides

Exciton/trion-involved optoelectronic properties have attracted exponential amount of attention for various applications ranging from optoelectronics, valleytronics to electronics. Herein, we report a new chemical (MXene) doping strategy to modulate the negative trion and neutral exciton for achieving high photoluminescence yield of atomically thin transition metal dichalcogenides, enabled by the regulation of carrier densities to promote electron-bound trion-to-exciton transition via charge transfer from TMDCs to MXene. As a proof of concept, the MXene nano-flake-doped tungsten disulfide is demonstrated to obtain an enhanced PL efficiency of up to ∼five folds, which obviously exceeds the reported efficiency upon electrical and/or plasma doping strategies. The PL enhancement degree can also be modulated by tuning the corresponding surface functional groups of MXene nano-flakes, reflecting that the electron-withdrawing functional groups play a vital role in this charge transfer process. These findings offer promising clues to control the optoelectronic properties of TMDCs and expand the scope of the application of MXene nano-flakes, suggesting a possibility to construct a new heterostructure junction based on MXenes and TMDCs.

Fabrication of Stacked MoS2 Bilayer with Weak Interlayer Coupling by Reduced Graphene Oxide Spacer

We fabricated the stacked bilayer molybdenum disulfide (MoS₂) by using reduced graphene oxide (rGO) as a spacer for increasing the optoelectronic properties of MoS₂. The rGO can decrease the interlayer coupling between the stacked bilayer MoS₂ and retain the direct band gap property of MoS₂. We observed a twofold enhancement of the photoluminescence intensity of the stacked MoS₂ bilayer. In the Raman scattering, we observed that the E¹_2g and A_1g modes of the stacked bilayer MoS₂ with rGO were further shifted compared to monolayer MoS₂, which is due to the van der Waals (vdW) interaction and the strain effect between the MoS₂ and rGO layers. The findings of this study will expand the applicability of monolayer MoS₂ for high-performance optoelectronic devices by enhancing the optical properties using a vdW spacer.

Systematic Study of Photoluminescence Enhancement in Monolayer Molybdenum Disulfide by Acid Treatment

Monolayer molybdenum disulfide (MoS₂) is an atomically thin semiconducting material with a direct band gap. This physical property is attributable to atomically thin optical devices such as sensors, light-emitting devices, and photovoltaic cells. Recently, a near-unity photoluminescence (PL) quantum yield of a monolayer MoS₂ was demonstrated via a treatment with a molecular acid, bis(trifluoromethane)sulfonimide (TFSI); however, the mechanism still remains a mystery. Here, we work on PL enhancement of monolayer MoS₂ by treatment of Brønsted acids (TFSI and sulfuric acid (H₂SO₄)) to identify the importance of the protonated environment. In TFSI as an acid, different solvents-1,2-dichloroethane (DCE), acetonitrile, and water-were studied, as they show quite different acidity in solution. All of the solvents showed PL enhancement, and the highest was observed in DCE. This behavior in DCE would be due to the higher acidity than others have. Acids from different anions can also be studied in water as a common solvent. Both TFSI and H₂SO₄ showed similar PL enhancement (∼4-8 enhancement) at the same proton concentration, indicating that the proton is a key factor to enhance the PL intensity. Finally, we considered another cation, Li⁺ from Li₂SO₄, instead of H₂SO₄, in water. Although Li and H atoms showed similar binding energy on MoS₂ from theoretical calculations, Li₂SO₄ treatment showed little PL enhancement; only coexisting H₂SO₄ reproduced the enhancement. This study demonstrated the importance of a protonated environment to increase the PL intensity of monolayer MoS₂. The study will lead to a solution to achieve high optical quality and to implementation for atomically thin optical devices.

Tuning the activity of the inert MoS2 surface via graphene oxide support doping towards chemical functionalization and hydrogen evolution: a density functional study

The basal plane of MoS₂ provides a promising platform for chemical functionalization and the hydrogen evolution reaction (HER); however, its practical utilization remains challenging due to the lack of active sites and its low conductivity. Herein, using first principles simulations, we first proposed a novel and effective strategy for significantly enhancing the activity of the inert MoS₂ surface using a graphene oxide (GO) support (MoS₂/GOs). The chemical bonding of the functional groups (CH₃ and NH₂) on the MoS₂-GO hybrid is stronger than that in freestanding MoS₂ or MoS₂-graphene. Upon increasing the oxygen group concentration or introducing N heteroatoms into the GO support, the stability of the chemically functionalized MoS₂ is improved. Furthermore, use of GOs to support pristine and defective MoS₂ with a S vacancy (S-MoS₂) can greatly promote the HER activity of the basal plane. The catalytic activity of S-MoS₂ is further enhanced by doping N into GOs; this results in a hydrogen adsorption free energy of almost zero (ΔG_H = ∼-0.014 eV). The coupling interaction with the GO substrate reduces the p-type Schottky barrier heights (SBH) of S-MoS₂ and modifies its electronic properties, which facilitate charge transfer between them. Our calculated results are consistent with the experimental observations. Thus, the present results open new avenues for the chemical functionalization of MoS₂-based nanosheets and HER catalysts.

Controlling electronic properties of MoS2/graphene oxide heterojunctions for enhancing photocatalytic performance: the role of oxygen

The manipulation of the constituents of novel hetero-photocatalysts is an effective method for improving photocatalytic efficiency, but a theoretical understanding of the relationship between interlayer interaction and photocatalytic activity is still lacking.

Highly Efficient Thin-Film Transistor via Cross-Linking of 1T Edge Functional 2H Molybdenum Disulfides

Thin-film transistors (TFTs) have received great attention for their use in lightweight, large area, and wearable devices. However, low crystalline materials and inhomogeneous film formation limit the realization of high-quality electrical properties for channels in commercial TFTs, especially for flexible electronics. Here, we report a field-effect TFT fabricated via cross-linking of edge-1T basal-2H MoS₂ sheets that are prepared by edge functional exfoliation of bulk MoS₂ with soft organic exfoliation reagents. For edge functional exfoliation, the electrophilic 4-carboxy-benzenediazonium used as the soft organic reagent attacks the nucleophilic thiolates exposed at the edge of the bulk MoS₂ with the help of an amine catalyst, resulting in 1T edge-functional HOOC-benzene-2H basal MoS₂ nanosheets (e-MoS₂). The cross-linking via hydrogen bonding of the negatively charged HOOC of the e-MoS₂ sheets with the help of a cationic polymer, polydiallyldimethylammonium chloride, results in a good film formation for a channel of the solution processing TFT. The TFT exhibits an extremely high mobility of 170 cm²/(V s) at 1 V (on/off ratio of 10⁶) on SiO₂/Si substrate and also a high mobility of 36.34 cm²/(V s) (on/off ratio of 10³) on PDMS/PET substrate.

Phosphorus Enhanced Intermolecular Interactions of SnO2 and Graphene as an Ultrastable Lithium Battery Anode

SnO₂ suffers from fast capacity fading in lithium-ion batteries due to large volume expansion as well as unstable solid electrolyte interphase. Herein, the design and synthesis of phosphorus bridging SnO₂ and graphene through covalent bonding are demonstrated to achieve a robust structure. In this unique structure, the phosphorus is able to covalently "bridge" graphene and tin oxide nanocrystal through PC and SnOP bonding, respectively, and act as a buffer layer to keep the structure stable during charging-discharging. As a result, when applied as a lithium battery anode, SnO₂ @P@GO shows very stable performance and retains 95% of 2nd capacity onward after 700 cycles. Such unique structural design opens up new avenues for the rational design of other high-capacity materials for lithium battery, and as a proof-of-concept, creates new opportunities in the synthesis of advanced functional materials for high-performance energy storage devices.