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Interface-Engineered 3D porous MoS 2-ReS 2 in-plane heterojunction as efficient hydrogen evolution reaction electrocatalysts. J Colloid Interface Sci 2024; 661:957-965. [PMID: 38330667 DOI: 10.1016/j.jcis.2024.02.056] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/18/2023] [Revised: 02/03/2024] [Accepted: 02/05/2024] [Indexed: 02/10/2024]
Abstract
Constructing in-plane heterojunctions with high interfacial density using two-dimensional materials represents a promising yet challenging avenue for enhancing the hydrogen evolution reaction (HER) in water electrolysis. In this work, we report that three-dimensional porous MoS2-ReS2 in-plane heterojunctions, fabricated via chemical vapor deposition, exhibit robust electrocatalytic activity for the water splitting reaction. The optimized MoS2-ReS2 in-plane heterojunction achieves superior HER performance across a wide pH range, requiring an overpotential of only 200 mV to reach a current density of 10 mA cm-2 in alkaline seawater. Thus, it outperforms standalone MoS2 and ReS2. Furthermore, the catalyst exhibits remarkable stability, enduring up to 200 h in alkaline seawater. Experimental results coupled with density functional theory calculations confirm that electron redistribution at the MoS2-ReS2 heterointerface is likely driven by disparities in in-plane work functions between the two phases. This leads to charge accumulation at the interface, thereby enhancing the adsorptive activity of S atoms toward H* intermediates and facilitating the dissociation of water molecules at the interface. This discovery offers valuable insights into the electrocatalytic mechanisms at the interface and provides a roadmap for designing high-performance, earth-abundant HER electrocatalysts suitable for practical applications.
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Hydrothermal integration of MoO 2-MoS 2@rGO nanoframe networks: A promising approach for efficient bacterial disinfection in wastewater. CHEMOSPHERE 2023; 343:140273. [PMID: 37758069 DOI: 10.1016/j.chemosphere.2023.140273] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/01/2023] [Revised: 09/10/2023] [Accepted: 09/23/2023] [Indexed: 09/30/2023]
Abstract
The efficient disinfection of bacterial contaminants in wastewater is a critical challenge in the field of environmental remediation. Herein, we present a novel approach for efficient bacterial disinfection using hydrothermally integrated MoO2-MoS2@rGO nanoframe networks. The developed nanoframe networks exhibit a unique architecture comprising of molybdenum dioxide (MoO2) and molybdenum disulfide (MoS2) impregnated on algae biomass reduced graphene oxide (rGO). The as-synthesized nanoframe networks demonstrate exceptional antibacterial activity against Escherichia coli bacteria. The disinfection efficiency was evaluated by measuring the bacterial viability and observing the morphological changes using scanning electron microscopy. The MoO2-MoS2@rGO nanoframe networks exhibited a remarkable antibacterial effect, achieving a high disinfection rate of 95.8% within a short contact time of 10 min. The efficient bacterial disinfection capability of the nanoframe networks can be attributed to the synergistic effects of MoO2, MoS2, and rGO components. The MoO2 nanoparticles generate reactive oxygen species (ROS), persuading oxidative stress and leading to bacterial inactivation. The MoS2 nanoparticles possess inherent antibacterial properties through the release of Mo and S ions. The rGO nanosheets provide a conductive and stable platform, facilitating the charge transfer during the antibacterial process. Furthermore, the hydrothermal integration method enables easy scalability and cost-effectiveness of the MoO2-MoS2@rGO nanoframe networks. The nanoframe networks can be easily recovered and reused, reducing waste generation and promoting sustainability. Overall, this study presents a promising approach for efficient bacterial disinfection in wastewater using hydrothermally integrated MoO2-MoS2@rGO nanoframe networks. The remarkable antibacterial performance, along with the advantages of scalability and reusability, makes these nanoframe networks a potential candidate for practical applications in environmental remediation and water treatment processes.
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Spin-Stabilization by Coulomb Blockade in a Vanadium Dimer in WSe 2. ACS NANO 2023. [PMID: 37976219 DOI: 10.1021/acsnano.3c04841] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/19/2023]
Abstract
Charged dopants in 2D transition metal dichalcogenides (TMDs) have been associated with the formation of hydrogenic bound states, defect-bound trions, and gate-controlled magnetism. Charge-transfer at the TMD-substrate interface and the proximity to other charged defects can be used to regulate the occupation of the dopant's energy levels. In this study, we examine vanadium-doped WSe2 monolayers on quasi-freestanding epitaxial graphene, by high-resolution scanning probe microscopy and ab initio calculations. Vanadium atoms substitute W atoms and adopt a negative charge state through charge donation from the graphene substrate. VW-1 dopants exhibit a series of occupied p-type defect states, accompanied by an intriguing electronic fine-structure that we attribute to hydrogenic states bound to the charged impurity. We systematically studied the hybridization in V dimers with different separations. For large dimer separations, the 2e- charge state prevails, and the magnetic moment is quenched. However, the Coulomb blockade in the nearest-neighbor dimer configuration stabilizes a 1e- charge state. The nearest-neighbor V-dimer exhibits an open-shell character for the frontier defect orbital, giving rise to a paramagnetic ground state. Our findings provide microscopic insights into the charge stabilization and many-body effects of single dopants and dopant pairs in a TMD host material.
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WS 2-intercalated Ti 3C 2T x MXene/TiO 2-stacked hybrid structure as an excellent sonophotocatalyst for tetracycline degradation and nitrogen fixation. ULTRASONICS SONOCHEMISTRY 2023; 100:106623. [PMID: 37832252 PMCID: PMC10585321 DOI: 10.1016/j.ultsonch.2023.106623] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/26/2023] [Revised: 09/21/2023] [Accepted: 09/24/2023] [Indexed: 10/15/2023]
Abstract
Designing a heterostructure nanoscale catalytic site to facilitate N2 adsorption and photogenerated electron transfer would maximize the potential for photocatalytic activity and N2 reduction reactions. Herein, we have explored the interfacial TiO2 nanograins between the Ti3C2TxMXene-WS2 heterostructure and addressed the beneficial active sites to expand the effective charge transfer rate and promote sonophotocatalytic N2 fixation. Benefiting from the interfacial contact and dual heterostructure interface maximizes the photogenerated carrier separation between WS2 and MXene/TiO2. The sonophotocatalytic activity of the MXene@TiO2/WS2 hybrid, which was assessed by examining the photoreduction of N2 with ultrasonic irradiation, was much higher than that of either sonocatalytic and photocatalytic activity because of the synergistic sonocatalytic effect under photoirradiation. The Schottky junction between the MXene and TiO2 on the hybrid MXene/TiO2-WS2 heterostructure resulted in the sonophotocatalytic performance through effective charge transfer, which is 1.47 and 1.24 times greater than MXene-WS2 for nitrogen fixation and pollutant degradation, respectively. Under the sonophotocatalytic process, the MXene/TiO2-WS2 heterostructure exhibits a decomposition efficiency of 98.9 % over tetracycline in 90 min, which is 5.46, 1.73, and 1.10 times greater than those of sonolysis, sonocatalysis, and photocatalysis, respectively. The production rate of NH3 on MXene/TiO2-WS2 reached 526 μmol g-1h-1, which is 3.17, 3.61, and 1.47 times higher than that of MXene, WS2, and MXene-WS2, respectively. The hybridized structure of MXene-WS2 with interfacial surface oxidized TiO2 nanograins minimizes the band potential and improves photocarrier use efficiency, contributing directly to the remarkable catalytic performance towards N2 photo fixation under visible irradiation under ultrasonic irradiation. This report provides the strategic outcome for the mass carrier transfer rate and reveals a high conversion efficiency in the hybridized heterostructure.
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Construction of an electrochemical sensor towards environmental hazardous 4-nitrophenol based on Nd(OH) 3-embedded VSe 2 nanocomposite. ENVIRONMENTAL SCIENCE AND POLLUTION RESEARCH INTERNATIONAL 2023:10.1007/s11356-023-25688-0. [PMID: 36781666 DOI: 10.1007/s11356-023-25688-0] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/23/2022] [Accepted: 01/30/2023] [Indexed: 06/18/2023]
Abstract
The toxicity of 4-nitrophenol (4-NP) is one of the most common threats to the environment; therefore, developing a simple and sensitive analytical method to detect 4-NP is crucial. In this study, we prepared the Nd(OH)3/VSe2 nanocomposite using the simple hydrothermally assisted ultrasonication method and it was used to detect the 4-NP. Different characterization techniques were used to investigate the morphological and chemical compositions of Nd(OH)3/VSe2 nanocomposite. All of these investigations revealed that Nd(OH)3 nanoparticles were finely dispersed on the surface of the VSe2 nanosheet. The electrical conductivity of our prepared samples was evaluated by the electrochemical impedance spectroscopic technique. The CV and DPV methods were used to explore the electrochemical activity of 4-NP at the Nd(OH)3/VSe2/GCE sensor which exhibited a wide linear range (0.001 to 640 µM), low limit of detection (0.008 µM), and good sensitivity (0.41 µA µM-1 cm-2), respectively. Additionally, Nd(OH)3/VSe2/GCE sensor was tested in water samples for the detection of 4-NP, which exhibited good recovery results. The Nd(OH)3/VSe2 electrode material is a novel one for the electrochemical sensor field, and the obtained overall results also proved that our proposed material is an active material for sensor applications.
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Scalable Moiré Lattice with Oriented TMD Monolayers. NANOSCALE RESEARCH LETTERS 2022; 17:34. [PMID: 35286495 PMCID: PMC8921411 DOI: 10.1186/s11671-022-03670-y] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 12/28/2021] [Accepted: 01/30/2022] [Indexed: 06/14/2023]
Abstract
Moiré lattice in artificially stacked monolayers of two-dimensional (2D) materials effectively modulates the electronic structures of materials, which is widely highlighted. Formation of the electronic Moiré superlattice promises the prospect of uniformity among different moiré cells across the lattice, enabling a new platform for novel properties, such as unconventional superconductivity, and scalable quantum emitters. Recently, epitaxial growth of the monolayer transition metal dichalcogenide (TMD) is achieved on the sapphire substrate by chemical vapor deposition (CVD) to realize scalable growth of highly-oriented monolayers. However, fabrication of the scalable Moiré lattice remains challenging due to the lack of essential manipulation of the well-aligned monolayers for clean interface quality and precise twisting angle control. Here, scalable and highly-oriented monolayers of TMD are realized on the sapphire substrates by using the customized CVD process. Controlled growth of the epitaxial monolayers is achieved by promoting the rotation of the nuclei-like domains in the initial growth stage, enabling aligned domains for further grain growth in the steady-state stage. A full coverage and distribution of the highly-oriented domains are verified by second-harmonic generation (SHG) microscopy. By developing the method for clean monolayer manipulation, hetero-stacked bilayer (epi-WS2/epi-MoS2) is fabricated with the specific angular alignment of the two major oriented monolayers at the edge direction of 0°/ ± 60°. On account of the optimization for scalable Moiré lattice with a high-quality interface, the observation of interlayer exciton at low temperature illustrates the feasibility of scalable Moiré superlattice based on the oriented monolayers.
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Electrochemical determination of quercetin using glassy carbon electrode modified with WS 2/GdCoO 3 nanocomposite. Mikrochim Acta 2022; 189:118. [PMID: 35195788 DOI: 10.1007/s00604-022-05219-3] [Citation(s) in RCA: 5] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/13/2021] [Accepted: 02/03/2022] [Indexed: 11/28/2022]
Abstract
A WS2/GdCoO3 nanocomposite was successfully prepared using hydrothermal-assisted synthesis. Our prepared WS2/GdCoO3 nanocomposite was fabricated on a glassy carbon electrode (GCE) for the detection of quercetin (QCT). The WS2/GdCoO3 nanomaterial was characterized by powder XRD, micro-Raman, FT-IR, XPS, FE-SEM, and HR-TEM, which proved that WS2 nanoplates were finely dispersed on the surface of the GdCoO3 nanoflakes. The electrocatalytic performance of WS2/GdCoO3 was investigated by the EIS technique, and it exhibited a small semi-circle, which confirms that it has a large active surface area and high electrical conductivity. The electrochemical behavior of QCT at the WS2/GdCoO3 sensor was explored by using the CV and DPV methods. The proposed electrochemical sensor exhibited excellent electrochemical response toward QCT with a wide linear range of 0.001 to 329 µM, low limit of detection (LOD) of 0.003 µM, and limit of quantification (LOQ) of 0.0101 µM. The sensor also displayed excellent selectivity, sensitivity, reproducibility, and stability. Additionally, the WS2/GdCoO3 sensor was utilized for the detection of QCT in apple juice and grape juice samples, and it exhibited good recovery results.
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Computer vision AC-STEM automated image analysis for 2D nanopore applications. Ultramicroscopy 2021; 231:113249. [PMID: 33902953 DOI: 10.1016/j.ultramic.2021.113249] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/21/2020] [Revised: 01/15/2021] [Accepted: 02/27/2021] [Indexed: 01/17/2023]
Abstract
Transmission electron microscopy (TEM) has led to important discoveries in atomic imaging and as an atom-by-atom fabrication tool. Using electron beams, atomic structures can be patterned, annealed and crystallized, and nanopores can be drilled in thin membranes. We review current progress in TEM analysis and implement a computer vision nanopore-detection algorithm that achieves a 96% pixelwise precision in TEM images of nanopores in 2D membranes (WS2), and discuss parameter optimization including a variation on the traditional grid search and gradient ascent. Such nanopores have applications in ion detection, water filtration, and DNA sequencing, where ionic conductance through the pore should be concordant with its TEM-measured size. Standard computer vision methods have their advantages as they are intuitive and do not require extensive training data. For completeness, we briefly comment on related machine learning for 2D materials analysis and discuss relevant progress in these fields. Image analysis alongside TEM allows correlated fabrication and analysis done simultaneously in situ to engineer devices at the atomic scale.
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Photoelectrochemical immunosensor for methylated RNA detection based on WS 2 and poly(U) polymerase-triggered signal amplification. Mikrochim Acta 2020; 187:596. [PMID: 33033870 DOI: 10.1007/s00604-020-04572-5] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/18/2020] [Accepted: 09/25/2020] [Indexed: 10/23/2022]
Abstract
A novel photoelectrochemical immunosensor has been constructed for the determination of methylated RNA. MoS2 nanosheets with large specific area were employed as photoactive material, gold nanoparticles were used as signal amplification unit and immobilization matrix of 4-mercaptophenylboronic acid, anti-m6A antibody was adopted as methylated RNA recognition reagent, and poly(U) polymerase-mediated RNA chain extension and Ru(NH3)63+ were used as assisted signal amplification unit. With the sensitization effect of Ru(NH3)63+, the photoactivity of WS2 nanosheets was improved greatly, which also improved the sensitivity. Using visible-light excitation and ascorbic acid as electron donor, the sensitive determination of methylated RNA was achieved by monitoring the photocurrent change with different concentrations of methylated RNA. This photoelectrochemical immunosensor has a wide linear relationship with methylated RNA concentration from 0.05 to 35 nM under optimal experimental conditions. The low detection limit of 14.5 pM was realized based on 3σ criterion. In addition to the good selectivity, this sensor also presents high reproducibility with a relative standard deviation of 1.4% for the photocurrent of seven electrodes. The applicability of the developed method was also investigated by detecting the level of methylated RNA in corn seedling leaves with and without sulfadiazine treatment. Graphical abstract A novel photoelectrochemical immunosensor was developed for methylated RNA detection using the photoactive material of MoS2 and poly(U) polymerase-mediated RNA chain extension.
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Class-selective voltammetric determination of hydroxycinnamic acids structural analogs using a WS 2/catechin-capped AuNPs/carbon black-based nanocomposite sensor. Mikrochim Acta 2020; 187:296. [PMID: 32347378 DOI: 10.1007/s00604-020-04281-z] [Citation(s) in RCA: 17] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/01/2020] [Accepted: 04/13/2020] [Indexed: 12/13/2022]
Abstract
A high-performance screen-printed electrode (SPE) based nanocomposite sensor integrating tungsten disulfide (WS2) flakes decorated with catechin-capped gold nanoparticles (AuNP-CT) and carbon black (CB) has been developed. The excellent antifouling properties of WS2 decorated with AuNP-CT into a high conductivity network of CB results in high selectivity, sensitivity, and reproducibility for the simultaneous determination of hydroxycinnamic acid (hCN) structural analogs: caffeic (CF), sinapic (SP), and p-coumaric acids (CM). Using differential pulse voltammetry (DPV), the target hCNs resulted in three well-resolved oxidation peaks at SPE-CB-WS2/AuNP-CT sensor. Excellent antifouling performance (RSD ip,a ≤ 3%, n = 15 for three analytes' simultaneous measure) and low detection limits (CF 0.10 μmol L-1; SP, 0.40 μmol L-1; CM, 0.40 μmol L-1) are obtained despite the analyzed compounds having a high passivation tendency towards carbon-based sensors. The SPE-CB-WS2/AuNP-CT sensor was successfully applied to determine CF, SP, and CM in food samples with good precision (RSD ≤ 4%, n = 3) and recoveries (86-109%; RSD ≤ 5%, n = 3). The proposed sensor is the first example exploiting the simultaneous determination of these compounds in food samples. Given its excellent electrochemical performance, low cost, disposability, and ease of use, this SPE-CB-WS2/AuNP-CT nanocomposite sensor represents a powerful candidate for the realization of electrochemical devices for the determination of (bio)compounds with high passivation tendency. Graphical abstract.
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Analyzing the microstructure and related properties of 2D materials by transmission electron microscopy. Appl Microsc 2019; 49:10. [PMID: 33580317 PMCID: PMC7809582 DOI: 10.1186/s42649-019-0013-5] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/23/2019] [Accepted: 10/10/2019] [Indexed: 11/25/2022] Open
Abstract
Two-dimensional materials such as transition metal dichalcogenide and graphene are of great interest due to their intriguing electronic and optical properties such as metal-insulator transition based on structural variation. Accordingly, detailed analyses of structural tunability with transmission electron microscopy have become increasingly important for understanding atomic configurations. This review presents a few analyses that can be applied to two-dimensional materials using transmission electron microscopy.
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Quantum Enhancement of Charge Density Wave in NbS 2 in the Two-Dimensional Limit. NANO LETTERS 2019; 19:3098-3103. [PMID: 30932501 DOI: 10.1021/acs.nanolett.9b00504] [Citation(s) in RCA: 23] [Impact Index Per Article: 4.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
At ambient pressure, bulk 2H-NbS2 displays no charge density wave instability, which is at odds with the isostructural and isoelectronic compounds 2H-NbSe2, 2H-TaS2, and 2H-TaSe2, and in disagreement with harmonic calculations. Contradictory experimental results have been reported in supported single layers, as 1H-NbS2 on Au(111) does not display a charge density wave, whereas 1H-NbS2 on 6H-SiC(0001) endures a 3 × 3 reconstruction. Here, by carrying out quantum anharmonic calculations from first-principles, we evaluate the temperature dependence of phonon spectra in NbS2 bulk and single layer as a function of pressure/strain. For bulk 2H-NbS2, we find excellent agreement with inelastic X-ray spectra and demonstrate the removal of charge ordering due to anharmonicity. In the two-dimensional limit, we find an enhanced tendency toward charge density wave order. Freestanding 1H-NbS2 undergoes a 3 × 3 reconstruction, in agreement with data on 6H-SiC(0001) supported samples. Moreover, as strains smaller than 0.5% in the lattice parameter are enough to completely remove the 3 × 3 superstructure, deposition of 1H-NbS2 on flexible substrates or a small charge transfer via field-effect could lead to devices with dynamical switching on/off of charge order.
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Determining Interaction Enhanced Valley Susceptibility in Spin-Valley-Locked MoS 2. NANO LETTERS 2019; 19:1736-1742. [PMID: 30720286 DOI: 10.1021/acs.nanolett.8b04731] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/09/2023]
Abstract
Two-dimensional transition metal dichalcogenides (TMDCs) are recently emerged electronic systems with various novel properties, such as spin-valley locking, circular dichroism, valley Hall effect, and superconductivity. The reduced dimensionality and large effective masses further produce unconventional many-body interaction effects. Here we reveal strong interaction effects in the conduction band of MoS2 by transport experiment. We study the massive Dirac electron Landau levels (LL) in high-quality MoS2 samples with field-effect mobilities of 24 000 cm2/(V·s) at 1.2 K. We identify the valley-resolved LLs and low-lying polarized LLs using the Lifshitz-Kosevitch formula. By further tracing the LL crossings in the Landau fan diagram, we unambiguously determine the density-dependent valley susceptibility and the interaction enhanced g-factor from 12.7 to 23.6. Near integer ratios of Zeeman-to-cyclotron energies, we discover LL anticrossings due to the formation of quantum Hall Ising ferromagnets, the valley polarizations of which appear to be reversible by tuning the density or an in-plane magnetic field. Our results provide evidence for many-body interaction effects in the conduction band of MoS2 and establish a fertile ground for exploring strongly correlated phenomena of massive Dirac electrons.
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Tungsten disulfide (WS 2) nanosheet-based photoelectrochemical aptasensing of chloramphenicol. Mikrochim Acta 2018; 185:453. [PMID: 30209622 DOI: 10.1007/s00604-018-2970-8] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/20/2018] [Accepted: 08/18/2018] [Indexed: 01/23/2023]
Abstract
A method is described for photoelectrochemical determination of chloramphenicol (CLOA). It is based on the use of (a) aptamers protected with photoactive WS2 nanosheets, and (b) DNase I-assisted target recycling. The DNA aptamer without label was employed for recognition of CLOA. In the absence of CLOA, the aptamer is adsorbed on the surface of WS2. This leads to a decrease of photocurrent due to the steric-hindrance effect of aptamer DNA. The adsorption of WS2 also protects the aptamer from digestion by DNase. In the presence of CLOA, the aptamer will be desorbed from the WS2 surface due to formation of an aptamer/CLOA conjugate. This results in an increased photocurrent due to a decreased amount of aptamer DNA on the electrode surface. The increase of photocurrent can be further improved by applying DNase triggered catalytic recycling of CLOA. Under optimal experimental conditions, the response is linear 10 pM - 10 nM CLOA concentration range, with a 3.6 pM lower detection limit (at 3σ). This method is acceptably selective, accurate and stable. It was applied to the determination of CLOA in spiked milk samples and gave satisfactory results. Graphical abstract A simple and sensitive photoelectrochemical apta-biosensor was fabricated for chloramphenicol detection. In this work, WS2 nanosheets were employed as photoactive material, and DNase I catalytic chloramphenicol recycling strategy was adopted to amplify the detection signal.
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Atomic-Scale Probing of Reversible Li Migration in 1T-V 1+ xSe 2 and the Interactions between Interstitial V and Li. NANO LETTERS 2018; 18:6094-6099. [PMID: 30142274 DOI: 10.1021/acs.nanolett.8b03154] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Ionic doping and migration in solids underpins a wide range of applications including lithium ion batteries, fuel cells, resistive memories, and catalysis. Here, by in situ transmission electron microscopy technique we directly track the structural evolution during Li ions insertion and extraction in transition metal dichalcogenide 1T-V1+ xSe2 nanostructures which feature spontaneous localized superstructures due to the periodical interstitial V atoms within the van der Waals interlayers. We find that lithium ion migration destroys the cationic orderings and leads to a phase transition from superstructure to nonsuperstructure. This phase transition is reversible, that is, the superstructure returns back after extraction of lithium ion from Li yV1+ xSe2. These findings provide valuable insights into understanding and controlling the structure and properties of 2D materials by general ionic and electric doping.
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Augmented Quantum Yield of a 2D Monolayer Photodetector by Surface Plasmon Coupling. NANO LETTERS 2018; 18:2316-2323. [PMID: 29561626 DOI: 10.1021/acs.nanolett.7b05060] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Monolayer (1L) transition metal dichalcogenides (TMDCs) are promising materials for nanoscale optoelectronic devices because of their direct band gap and wide absorption range (ultraviolet to infrared). However, 1L-TMDCs cannot be easily utilized for practical optoelectronic device applications (e.g., photodetectors, solar cells, and light-emitting diodes) because of their extremely low optical quantum yields (QYs). In this investigation, a high-gain 1L-MoS2 photodetector was successfully realized, based on the surface plasmon (SP) of the Ag nanowire (NW) network. Through systematic optical characterization of the hybrid structure consisting of a 1L-MoS2 and the Ag NW network, it was determined that a strong SP and strain relaxation effect influenced a greatly enhanced optical QY. The photoluminescence (PL) emission was drastically increased by a factor of 560, and the main peak was shifted to the neutral exciton of 1L-MoS2. Consequently, the overall photocurrent of the hybrid 1L-MoS2 photodetector was observed to be 250 times better than that of the pristine 1L-MoS2 photodetector. In addition, the photoresponsivity and photodetectivity of the hybrid photodetector were effectively improved by a factor of ∼1000. This study provides a new approach for realizing highly efficient optoelectronic devices based on TMDCs.
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Abstract
Semiconducting transition metal dichalcogenide (TMDC) monolayers have exceptional physical properties. They show bright photoluminescence due to their unique band structure and absorb more than 10% of the light at their excitonic resonances despite their atomic thickness. At room temperature, the width of the exciton transitions is governed by the exciton-phonon interaction leading to strongly asymmetric line shapes. TMDC monolayers are also extremely flexible, sustaining mechanical strain of about 10% without breaking. The excitonic properties strongly depend on strain. For example, exciton energies of TMDC monolayers significantly redshift under uniaxial tensile strain. Here, we demonstrate that the width and the asymmetric line shape of excitonic resonances in TMDC monolayers can be controlled with applied strain. We measure photoluminescence and absorption spectra of the A exciton in monolayer MoSe2, WSe2, WS2, and MoS2 under uniaxial tensile strain. We find that the A exciton substantially narrows and becomes more symmetric for the selenium-based monolayer materials, while no change is observed for atomically thin WS2. For MoS2 monolayers, the line width increases. These effects are due to a modified exciton-phonon coupling at increasing strain levels because of changes in the electronic band structure of the respective monolayer materials. This interpretation based on steady-state experiments is corroborated by time-resolved photoluminescence measurements. Our results demonstrate that moderate strain values on the order of only 1% are already sufficient to globally tune the exciton-phonon interaction in TMDC monolayers and hold the promise for controlling the coupling on the nanoscale.
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Absence of a Band Gap at the Interface of a Metal and Highly Doped Monolayer MoS 2. NANO LETTERS 2017; 17:5962-5968. [PMID: 28920701 DOI: 10.1021/acs.nanolett.7b01986] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
High quality electrical contact to semiconducting transition metal dichalcogenides (TMDCs) such as MoS2 is key to unlocking their unique electronic and optoelectronic properties for fundamental research and device applications. Despite extensive experimental and theoretical efforts reliable ohmic contact to doped TMDCs remains elusive and would benefit from a better understanding of the underlying physics of the metal-TMDC interface. Here we present measurements of the atomic-scale energy band diagram of junctions between various metals and heavily doped monolayer MoS2 using ultrahigh vacuum scanning tunneling microscopy (UHV-STM). Our measurements reveal that the electronic properties of these junctions are dominated by two-dimensional metal-induced gap states (MIGS). These MIGS are characterized by a spatially growing measured gap in the local density of states (L-DOS) of the MoS2 within 2 nm of the metal-semiconductor interface. Their decay lengths extend from a minimum of ∼0.55 nm near midgap to as long as 2 nm near the band edges and are nearly identical for Au, Pd, and graphite contacts, indicating that it is a universal property of the monolayer semiconductor. Our findings indicate that even in heavily doped semiconductors, the presence of MIGS sets the ultimate limit for electrical contact.
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Abstract
Understanding edge effects and quantifying their impact on the carrier properties of two-dimensional (2D) semiconductors is an essential step toward utilizing this material for high performance electronic and optoelectronic devices. WS2 monolayers patterned into disks of varying diameters are used to experimentally explore the influence of edges on the material's optical properties. Carrier lifetime measurements show a decrease in the effective lifetime, τeffective, as a function of decreasing diameter, suggesting that the edges are active sites for carrier recombination. Accordingly, we introduce a metric called edge recombination velocity (ERV) to characterize the impact of 2D material edges on nonradiative carrier recombination. The unpassivated WS2 monolayer disks yield an ERV ∼ 4 × 104 cm/s. This work quantifies the nonradiative recombination edge effects in monolayer semiconductors, while simultaneously establishing a practical characterization approach that can be used to experimentally explore edge passivation methods for 2D materials.
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Colloidal 2D nanosheets of MoS 2 and other transition metal dichalcogenides through liquid-phase exfoliation. Adv Colloid Interface Sci 2017; 245:40-61. [PMID: 28477866 DOI: 10.1016/j.cis.2017.04.014] [Citation(s) in RCA: 118] [Impact Index Per Article: 16.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/06/2017] [Revised: 04/21/2017] [Accepted: 04/24/2017] [Indexed: 12/19/2022]
Abstract
This review focuses on the exfoliation of transition metal dichalcogenides MQ2 (TMD, M=Mo, W, etc., Q=S, Se, Te) in liquid media, leading to the formation of 2D nanosheets dispersed in colloids. Nowadays, colloidal dispersions of MoS2, MoSe2, WS2 and other related materials are considered for a wide range of applications, including electronic and optoelectronic devices, energy storage and conversion, sensors for gases, catalysts and catalyst supports, biomedicine, etc. We address various methods developed so far for transferring these materials from bulk to nanoscale thickness, and discuss their stabilization and factors influencing it. Long-time known exfoliation through Li intercalation has received renewed attention in recent years, and is recognized as a method yielding highest dispersed concentrations of single-layer MoS2 and related materials. Latest trends in the intercalation/exfoliation approach include electrochemical lithium intercalation, experimenting with various intercalating agents, multi-step intercalation, etc. On the other hand, direct sonication in solvents is a much simpler technique that allows one to avoid dangerous reagents, long reaction times and purifying steps. The influence of the solvent characteristics on the colloid formation was closely investigated in numerous recent studies. Moreover, it is being recognized that, besides solvent properties, sonication parameters and solvent transformations may affect the process in a crucial way. The latest data on the interaction of MoS2 with solvents evidence that not only solution thermodynamics should be employed to understand the formation and stabilization of such colloids, but also general and organic chemistry. It appears that due to the sonolysis of the solvents and cutting of the MoS2 layers in various directions, the reactive edges of the colloidal nanosheets may bear various functionalities, which participate in their stabilization in the colloidal state. In most cases, direct exfoliation of MQ2 into colloidal nanosheets is conducted in organic solvents, while a small amount of works report low-concentrated colloids in pure water. To improve the dispersion abilities of transition metal dichalcogenides in water, various stabilizers are often introduced into the reaction media, and their interactions with nanosheets play an important role in the stabilization of the dispersions. Surfactants, polymers and biomolecules usually interact with transition metal dichalcogenide nanosheets through non-covalent mechanisms, similarly to the cases of graphene and carbon nanotubes. Finally, we survey covalent chemical modification of colloidal MQ2 nanosheets, a special and different approach, consisting in the functionalization of MQ2 surfaces with help of thiol chemistry, interaction with electrophiles, or formation of inorganic coordination complexes. The intentional design of surface chemistry of the nanosheets is a very promising way to control their solubility, compatibility with other moieties and incorporation into hybrid structures. Although the scope of the present review is limited to transition metal dichalcogenides, the dispersion in colloids of other chalcogenides (such as NbS3, VS4, Mo2S3, etc.) in many ways follows similar trends. We conclude the review by discussing current challenges in the area of exfoliation of MoS2 and its related materials.
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Photoresponse of an Organic Semiconductor/Two-Dimensional Transition Metal Dichalcogenide Heterojunction. NANO LETTERS 2017; 17:3176-3181. [PMID: 28388064 DOI: 10.1021/acs.nanolett.7b00695] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/07/2023]
Abstract
We study the optoelectronic properties of a type-II heterojunction (HJ) comprising a monolayer of the transition metal dichalcogenide (TMDC), WS2, and a thin film of the organic semiconductor, 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA). Both theoretical and experimental investigations of the HJ indicate that Frenkel states in the organic layer and two-dimensional Wannier-Mott states in the TMDC dissociate to form hybrid charge transfer excitons at the interface that subsequently dissociate into free charges that are collected at opposing electrodes. A photodiode employing the HJ achieves a peak external quantum efficiency of 1.8 ± 0.2% at a wavelength of 430 ± 10 nm, corresponding to an internal quantum efficiency (IQE) as high as 11 ± 1% in these ultrathin devices. The photoluminescence spectra of PTCDA and PTCDA/WS2 thin films show that excitons in the WS2 have a quenching rate that is approximately seven times higher than in PTCDA. This difference leads to strong wavelength dependence in IQE.
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Lattice Dynamics of the Rhenium and Technetium Dichalcogenides. NANOSCALE RESEARCH LETTERS 2016; 11:250. [PMID: 27178055 PMCID: PMC4870478 DOI: 10.1186/s11671-016-1459-9] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 03/24/2016] [Accepted: 05/02/2016] [Indexed: 06/05/2023]
Abstract
The rhenium and technetium dichalcogenides are layered van der Waals semiconductors which show a large number of Raman-active zone-centre phonon modes as a result of their unusually large unit cells and deviation from hexagonal symmetry. They thus offer the possibility of introducing in-plane anisotropy into composite heterostructures based on van der Waals materials, and Raman spectroscopy is generally used to determine their in-plane orientation. We show that first-principles calculations give a good description of the lattice dynamics of this family of materials and thus predict the zone-centre phonon frequencies and Raman activities of TcS2. We consider the distribution of the phonon modes in frequency and their atomic displacements and give a unified understanding of the phonon frequencies and Raman spectra of ReS2, TcS2 and ReSe2 in terms of the scaling of Raman frequency with the chalcogen mass.
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Quantitative measurement of mean inner potential and specimen thickness from high-resolution off-axis electron holograms of ultra-thin layered WSe 2. Ultramicroscopy 2016; 178:38-47. [PMID: 27554459 DOI: 10.1016/j.ultramic.2016.07.016] [Citation(s) in RCA: 20] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/15/2016] [Revised: 07/27/2016] [Accepted: 07/29/2016] [Indexed: 11/29/2022]
Abstract
The phase and amplitude of the electron wavefunction that has passed through ultra-thin flakes of WSe2 is measured from high-resolution off-axis electron holograms. Both the experimental measurements and corresponding computer simulations are used to show that, as a result of dynamical diffraction, the spatially averaged phase does not increase linearly with specimen thickness close to an [001] zone axis orientation even when the specimen has a thickness of only a few layers. It is then not possible to infer the local specimen thickness of the WSe2 from either the phase or the amplitude alone. Instead, we show that the combined analysis of phase and amplitude from experimental measurements and simulations allows an accurate determination of the local specimen thickness. The relationship between phase and projected potential is shown to be approximately linear for extremely thin specimens that are tilted by several degrees in certain directions from the [001] zone axis. A knowledge of the specimen thickness then allows the electrostatic potential to be determined from the measured phase. By using this combined approach, we determine a value for the mean inner potential of WSe2 of 18.9±0.8V, which is 12% lower than the value calculated from neutral atom scattering factors.
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Monolayer Single-Crystal 1T'-MoTe2 Grown by Chemical Vapor Deposition Exhibits Weak Antilocalization Effect. NANO LETTERS 2016; 16:4297-304. [PMID: 27223343 PMCID: PMC5893939 DOI: 10.1021/acs.nanolett.6b01342] [Citation(s) in RCA: 45] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
Growth of transition metal dichalcogenide (TMD) monolayers is of interest due to their unique electrical and optical properties. Films in the 2H and 1T phases have been widely studied but monolayers of some 1T'-TMDs are predicted to be large-gap quantum spin Hall insulators, suitable for innovative transistor structures that can be switched via a topological phase transition rather than conventional carrier depletion [ Qian et al. Science 2014 , 346 , 1344 - 1347 ]. Here we detail a reproducible method for chemical vapor deposition of monolayer, single-crystal flakes of 1T'-MoTe2. Atomic force microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy confirm the composition and structure of MoTe2 flakes. Variable temperature magnetotransport shows weak antilocalization at low temperatures, an effect seen in topological insulators and evidence of strong spin-orbit coupling. Our approach provides a pathway to systematic investigation of monolayer, single-crystal 1T'-MoTe2 and implementation in next-generation nanoelectronic devices.
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Abstract
This study describes a new and simple approach to dope two-dimensional transition metal dichalcogenides (TMDCs) using the superatom Co6Se8(PEt3)6 as the electron dopant. Semiconducting TMDCs are wired into field-effect transistor devices and then immersed into a solution of these superatoms. The degree of doping is determined by the concentration of the superatoms in solution and by the length of time the films are immersed in the dopant solution. Using this chemical approach, we are able to turn mono- and few-layer MoS2 samples from moderately to heavily electron-doped states. The same approach applied on WSe2 films changes their characteristics from hole transporting to electron transporting. Moreover, we show that the superatom doping can be patterned on specific areas of TMDC films. To illustrate the power of this technique, we demonstrate the fabrication of a lateral p-n junction by selectively doping only a portion of the channel in a WSe2 device. Finally, encapsulation of the doped films with crystalline hydrocarbon layers stabilizes their properties in an ambient environment.
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Recombination Kinetics and Effects of Superacid Treatment in Sulfur- and Selenium-Based Transition Metal Dichalcogenides. NANO LETTERS 2016; 16:2786-2791. [PMID: 26978038 DOI: 10.1021/acs.nanolett.6b00536] [Citation(s) in RCA: 117] [Impact Index Per Article: 14.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/05/2023]
Abstract
Optoelectronic devices based on two-dimensional (2D) materials have shown tremendous promise over the past few years; however, there are still numerous challenges that need to be overcome to enable their application in devices. These include improving their poor photoluminescence (PL) quantum yield (QY) as well as better understanding of exciton-based recombination kinetics. Recently, we developed a chemical treatment technique using an organic superacid, bis(trifluoromethane)sulfonimide (TFSI), which was shown to improve the quantum yield in MoS2 from less than 1% to over 95%. Here, we perform detailed steady-state and transient optical characterization on some of the most heavily studied direct bandgap 2D materials, specifically WS2, MoS2, WSe2, and MoSe2, over a large pump dynamic range to study the recombination mechanisms present in these materials. We then explore the effects of TFSI treatment on the PL QY and recombination kinetics for each case. Our results suggest that sulfur-based 2D materials are amenable to repair/passivation by TFSI, while the mechanism is thus far ineffective on selenium based systems. We also show that biexcitonic recombination is the dominant nonradiative pathway in these materials and that the kinetics for TFSI treated MoS2 and WS2 can be described using a simple two parameter model.
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Abstract
Heterostructures of compositionally and electronically variant two-dimensional (2D) atomic layers are viable building blocks for ultrathin optoelectronic devices. We show that the composition of interfacial transition region between semiconducting WSe2 atomic layer channels and metallic NbSe2 contact layers can be engineered through interfacial doping with Nb atoms. WxNb1-xSe2 interfacial regions considerably lower the potential barrier height of the junction, significantly improving the performance of the corresponding WSe2-based field-effect transistor devices. The creation of such alloyed 2D junctions between dissimilar atomic layer domains could be the most important factor in controlling the electronic properties of 2D junctions and the design and fabrication of 2D atomic layer devices.
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Transport Properties of a MoS2/WSe2 Heterojunction Transistor and Its Potential for Application. NANO LETTERS 2016; 16:1359-66. [PMID: 26784325 DOI: 10.1021/acs.nanolett.5b04791] [Citation(s) in RCA: 194] [Impact Index Per Article: 24.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/28/2023]
Abstract
This paper studies band-to-band tunneling in the transverse and lateral directions of van der Waals MoS2/WSe2 heterojunctions. We observe room-temperature negative differential resistance (NDR) in a heterojunction diode comprised of few-layer WSe2 stacked on multilayer MoS2. The presence of NDR is attributed to the lateral band-to-band tunneling at the edge of the MoS2/WSe2 heterojunction. The backward tunneling diode shows an average conductance slope of 75 mV/dec with a high curvature coefficient of 62 V(-1). Associated with the tunnel-diode characteristics, a positive-to-negative transconductance in the MoS2/WSe2 heterojunction transistors is observed. The transition is induced by strong interlayer coupling between the films, which results in charge density and energy-band modulation. The sign change in transconductance is particularly useful for multivalued logic (MVL) circuits, and we therefore propose and demonstrate for the first time an MVL-inverter that shows three levels of logic using one pair of p-type transistors.
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Abstract
Substitutional doping of transition metal dichalcogenides (TMDs) may provide routes to achieving tunable p-n junctions, bandgaps, chemical sensitivity, and magnetism in these materials. In this study, we demonstrate in situ doping of monolayer molybdenum disulfide (MoS2) with manganese (Mn) via vapor phase deposition techniques. Successful incorporation of Mn in MoS2 leads to modifications of the band structure as evidenced by photoluminescence and X-ray photoelectron spectroscopy, but this is heavily dependent on the choice of substrate. We show that inert substrates (i.e., graphene) permit the incorporation of several percent Mn in MoS2, while substrates with reactive surface terminations (i.e., SiO2 and sapphire) preclude Mn incorporation and merely lead to defective MoS2. The results presented here demonstrate that tailoring the substrate surface could be the most significant factor in substitutional doping of TMDs with non-TMD elements.
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Abstract
Optimizing new generations of two-dimensional devices based on van der Waals materials will require techniques capable of measuring variations in electronic properties in situ and with nanometer spatial resolution. We perform scanning microwave microscopy (SMM) imaging of single layers of MoS2 and n- and p-doped WSe2. By controlling the sample charge carrier concentration through the applied tip bias, we are able to reversibly control and optimize the SMM contrast to image variations in electronic structure and the localized effects of surface contaminants. By further performing tip bias-dependent point spectroscopy together with finite element simulations, we distinguish the effects of the quantum capacitance and determine the local dominant charge carrier species and dopant concentration. These results underscore the capability of SMM for the study of 2D materials to image, identify, and study electronic defects.
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