Undoped MoS2 Research Articles

Electronic and optical properties of S vacancy and Br and I doped monolayer MoS2: A first-principle study

This study investigated the energy band structure (BS), electronic state density, and optical properties of monolayer molybdenum disulfide (ML-MoS2) with undoped, sulfur vacancy (VS), bromine (Br), and iodine (I) doped MoS2 systems using the first-principle approach based on density functional theory (DFT). Our results show that compared to undoped MoS2 system, Br, and I doped MoS2 systems induced lattice distortions. In addition, and the bandgap widths was reduced in VS, Br, and I doped systems, thereby effectively suppressing the compounding of electron–hole pairs. Moreover, the recombination rate of electron–hole pairs was reduced, which in turn increased the mobility of electrons transferred from the valence band (VB) to the conduction band (CB). Moreover, the VS and Br doped systems exhibited superior optical properties compared to undoped MoS2 system. Notably, the VS system exhibited the best optical properties, thereby demonstrating the strongest polarization capability. Our findings pave the way for realizing electronic devices and photocatalytic materials on MoS2 nanostructures.

Engineering surface state density of monolayer CVD grown 2D MoS2 for enhanced photodetector performance.

This study demonstrates the effect of nitrogen doping on the surface state densities (Nss) of monolayer MoS2 and its effect on the responsivity and the response time of the photodetector. Our experimental results shows that by doping monolayer MoS2 by nitrogen, the surface state (Nss) increases thereby increasing responsivity. The mathematical model included in the paper supports the relation of photocurrent gain and its dependency on trap level which states that the increasing the trap density increases the photocurrent gain and the same is observed experimentally. The experimental results at room temperature revealed that nitrogen doped MoS2 have a high NSS of 1.63 X 1013 states/m2/eV compared to undoped MoS2 of 4.2 x 1012 states/m2/eV. The increase in Nss in turn is the cause for rise in trap states which eventually increases the value of photo responsivity from 65.12 A/W (undoped MoS2) to 606.3 A/W (nitrogen doped MoS2). The response time calculated for undoped MoS2 was 0.85 sec and for doped MoS2 was 0.35 sec. Finally, to verify the dependence of surface states on the responsivity, the surface states were varied by varying temperature and it was observed that upon increment in temperature, the surface states decreases which causes the responsivity values also to decrease.

Investigating structural, optical, and electron-transport properties of lithium intercalated few-layer MoS2 films: Unraveling the influence of disorder

Molybdenum disulfide is a promising candidate for various applications in electronics, optoelectronics, or alkali-ion batteries. The natural presence of the van der Waals gap allows intercalating alkali ions, such as lithium, into MoS2 films. Intercalation can modify the electronic structure as well as the electrical and optical properties. Here, we present a structural, optical, and electrical characterization of Li-intercalated few-layer MoS2 films. The intercalation was carried out by annealing MoS2 film in the presence of Li2S powder, serving as a lithium source. The initial MoS2 layers were prepared by pulsed laser deposition (PLD) and by sulfurization of 1 nm thick Mo film (TAC). The presence of lithium was confirmed by synchrotron-based x-ray Photoelectron Spectroscopy. The Raman spectroscopy, x-ray diffraction, and optical absorption measurements confirmed semiconducting behavior for all samples. All samples exhibited the thermally activated dependence of the electrical resistance, R, typical for the Efros–Shklovskii variable range hopping in a disordered semiconductor, ln R(T) ∝ (TES/T)1/2, where kBTES is the hopping activation energy. The PLD-grown MoS2 samples exhibited a relatively mild initial disorder primarily caused by grain boundaries. Lithium intercalation led to an increase in disorder, evident in the increase in kBTES and a substantial rise in electrical resistance. The TAC-grown undoped MoS2 sample already exhibited significant resistance, and the impact of Li intercalation on resistance was minimal. This observation was attributed to the fact that the TAC-grown MoS2 samples exhibit a perturbed stoichiometry (the S:Mo ratio ∼ 2.20), causing strong disorder even before Li intercalation. The electron doping caused by lithium, if any, was completely obscured by the effect of disorder.

A review on ultra‐small undoped MoS2 as advanced catalysts for renewable fuel production

AbstractMolybdenum disulfide (MoS2) has garnered significant attention in the field of catalysis due to the high density of active sites in its unique two‐dimensional (2D) structure, which could be developed into numerous high‐performance catalysts. The synthesis of ultra‐small MoS2 particles (<10 nm) is highly desired in various experimental studies. The ultra‐small structure could often lead to a distinct S–Mo coordination state and nonstoichiometric composition in MoSx, minimizing in‐plane active sites of the 2D structure and making it probable to regulate the atomic and electronic structure of its intrinsic active sites on a large extent, especially in MoSx clusters. This article summarizes the recent progress of catalysis over ultra‐small undoped MoS2 particles for renewable fuel production. Through a systematic review of their synthesis, structural, and spectral characteristics, as well as the relationship between their catalytic performance and inherent defects, we aim to provide insights into catalysis over this matrix that may potentially enable advancement in the development of high‐performance MoS2‐based catalysts for sustainable energy generation in the future.

Transition metal (Mn, Fe, Co, Ni) and nitrogen co-doping for improving the photocatalytic activity of monolayer MoS2

Exploring efficient photocatalyst is important for environmental protection. In this work, we systematically investigate the doping configuration, formation energy, band structures, density of state, recombination rate, catalytic active site, work function, redox potential, and optical absorption properties of transition metal and nitrogen co-doped monolayer MoS2. In transition doped MoS2, transition metal atoms replace Mo. Transition metal and nitrogen atoms substitute Mo and S atoms in transition metal and nitrogen co-doped MoS2, respectively. Compared with undoped MoS2, transition metal and nitrogen co-doped MoS2 have low photogenerated carrier recombination rate, abundant catalytic active sites, small work function, suitable redox potentials, and strong light absorption, especially Mn-N and Ni-N co-doped MoS2.

Impurity tuning for enhanced electrocatalytic reduction of N2 to NH3 over Fe/Co Co-doped MoS2 nanosheets

The electrochemical N2 reduction reaction (NRR) has attracted much attention for its use in the production of fertilizers and other chemicals. Herein, we demonstrate that the NRR can be enhanced when using Fe/Co co-doped MoS2 (FeCo–MoS2) nanosheets as the electrocatalyst. At the optimal doping concentration, the FeCo–MoS2 nanosheets provided a NH3 yield rate of 3.27 μg/h/cm2 at −0.55 V [vs reversible hydrogen electrode (RHE)], a value superior to those of undoped MoS2, Fe-doped MoS2, and Co-doped MoS2 samples by 191, 176, and 116%, respectively. Moreover, the Faradaic efficiency of 6.28% at −0.35 V (vs RHE) exceeded those of the undoped MoS2, Fe-doped MoS2, and Co-doped MoS2 samples by 217, 186, and 132%, respectively. Density functional theory calculations revealed that the preferred reaction mediated by the FeCo–MoS2 electrocatalyst followed a distal pathway, with its potential-determining step having an energy barrier (0.54 eV) lower than that of undoped MoS2 (1.13 eV), suggesting that the FeCo–MoS2 nanosheets possessed superior electrocatalytic activity. We conclude that Fe and Co are efficient dopants for boosting the NRR activity of MoS2 catalysts.

Multiple approaches of band engineering and mass fluctuation of solution-processed n-type Re-doped MoS2 nanosheets for enhanced thermoelectric power factor

The thermoelectric (TE) performance of Molybdenum disulpide (MoS2) can be improved by the incorporation of nanomaterials. MoS2 has been reported as promising thermoelectric materials due to their large bandgap and low thermal conductivity. In the present work, n-type MoS2 was successfully synthesized by facile hydrothermal route with an excellent thermoelectric performance by introducing rhenium (Re) dopant. The structural and morphological analyses confirmed the incorporation of Re into Mo (Molybdenum) lattice. The thermoelectric results showed that both the electrical conductivity (σ) and Seebeck coefficient (S) has been increased with the increase in Re content (2.5, 5, 7.5 and 10 at%) and temperature (303 K to 700 K), while the thermal conductivity (κ) was low. Doping with Re on MoS2 enhances the electrical conductivity through band engineering, improving carrier concentration and shifting the Fermi level to the conduction band. Introducing a heavy atomic element can reduce the total thermal conductivity by facilitating mass fluctuation. The maximum Seebeck coefficient was obtained as −100 μVK−1 at 500 K for Re 5 at% sample, which is 3.7 times greater than undoped MoS2. In addition, introducing electrons through Re doping induced bipolar conduction. These enhancements have increased the power factor of 8 μWm-1K−2 at 650 K.

Dilute Rhenium Doping and its Impact on Defects in MoS2.

Substitutionally doped 2D transition metal dichalcogenides are primed for next-generation device applications such as field effect transistors (FET), sensors, and optoelectronic circuits. In this work, we demonstrate substitutional rhenium (Re) doping of MoS2 monolayers with controllable concentrations down to 500 ppm by metal-organic chemical vapor deposition (MOCVD). Surprisingly, we discover that even trace amounts of Re lead to a reduction in sulfur site defect density by 5-10×. Ab initio models indicate the origin of the reduction is an increase in the free-energy of sulfur-vacancy formation at the MoS2 growth-front when Re is introduced. Defect photoluminescence (PL) commonly seen in undoped MOCVD MoS2 is suppressed by 6× at 0.05 atomic percent (at. %) Re and completely quenched with 1 at. % Re. Furthermore, we find that Re-MoS2 transistors exhibit a 2× increase in drain current and carrier mobility compared to undoped MoS2, indicating that sulfur vacancy reduction improves carrier transport in the Re-MoS2. This work provides important insights on how dopants affect 2D semiconductor growth dynamics, which can lead to improved crystal quality and device performance.

Effect of Introducing Defects and Doping on Different Properties of Monolayer MoS2

Herein, the comprehensive study of different properties of undoped MoS2, MoS2 lattice with sulfur (S) and, molybdenum (Mo) vacancy, and MoS2 with substitutional doping of niobium (Nb), vanadium (V), and zinc (Zn) atoms is done. The density functional theory (DFT) is used and the electronic properties like density of states, band structure, electron density, and optical properties like dielectric function, optical conductivity, and refractive index are studied. It is observed that undoped MoS2 monolayer shows direct bandgap semiconductor characteristics with a bandgap of around 1.79 eV. P‐type characteristics are observed for Nb‐, V‐, and Zn‐doped MoS2 lattices. The real part and imaginary parts of all optical parameters along x and z directions for different MoS2 supercells are found to be anisotropic in nature up to a photon energy of almost 11 eV and thereafter they show nearly isotropic nature. Finally, it is found that the obtained properties of MoS2 monolayer as per literature are suitable for next‐generation MOSFET application.

Tuning the content of S vacancies in MoS2 by Cu doping for enhancing catalytic hydrogenation of CO2 to methanol

Hydrogenation of CO2 to methanol is one of effective ways to promote "carbon neutrality". The activity of MoS2 as an efficient catalyst for the catalytic hydrogenation of CO2 to methanol is influenced by the content of in-plane S vacancies. Theoretically, the higher the density of in-plane S vacancies is, the better the performance of the catalyst is. In this work, a series of MoS2 nanosheets with different copper (Cu) dopant concentrations were prepared via hydrothermal method. It was found that the highest selectivity of MoS2-catalyzed CO2 hydrogenation to methanol along with the spatial-temporal yield was achieved at the Cu doping of 5%. Compared to the undoped MoS2 catalyst, the increase in methanol selectivity and spatial-temporal yield after doping was 13.37% and 2.27 times, respectively. This was due to the fact that Cu doping multiplied the number of S vacancies on MoS2, which altered the microstructure and chemical properties of the catalyst. However, a further increase in Cu dopant concentration reduced the S vacancy content on the MoS2 substrate, hindering the hydrogenation of the decomposed CO and thus decreasing the methanol yield. Therefore, a new technique to enhance MoS2-catalyzed CO2 hydrogenation to methanol was proposed.

Modulation of Schottky Barrier Height by Nitrogen Doping and Its Influence on Responsivity of Monolayer MoS2 Photodetector

AbstractMonolayer MoS2 flakes are prepared by low‐pressure chemical vapor deposition on p‐type and n‐type silicon substrates and post‐treated under nitrogen (N2)‐rich conditions to incorporate nitrogen atoms in sulfur vacancies. Ultraviolet photoelectron spectroscopy (UPS) shows an increase in work function value by 0.47 eV and 0.53 eV compared to undoped MoS2 when grown on p and n‐type substrates, respectively. Photodetection experiments conducted for doped and undoped MoS2 grown on p‐type substrate reveal a decrease in the value of photo responsivity for N2 doped MoS2 (191 A W−1) compared with undoped MoS2 (572 A W−1). Also, MoS2 crystals grown and doped on an n‐type substrate display an important enhancement of the photoresponsivity from 63 A W−1 for undoped to 606 A W−1 for N2 doped MoS2. The modulation of Schottky barrier height for N2 doped MoS2 on p type substrate decreased whereas for n type substrate the high electric field created due to the difference in the Fermi level allows for greater separation of photogenerated charge carriers. This modulation in the photoresponsivity due to the selection of the type of substrate opens up new avenues of research and engineering of atomically thin optoelectronic devices.

Ultrathin Rare-Earth-Doped MoS2 Crystalline Films Prepared with Magnetron Sputtering and Ar + H2 Post-Annealing

In this work, we propose a method to prepare large-area, crystalline ultrathin rare-earth (RE, i.e., Eu, Yb, Er and Tb)-doped MoS2 thin films, using magnetron sputtering and subsequent Ar + H2 annealing. The film thickness of as-deposited samples varied from 60 to 100 nm, and decreases to be below 10 nm after annealing at 550 °C for 30 min. X-ray diffraction and Raman spectra analysis revealed that the sample films were crystallized after the annealing, which resulted in a MoS2 crystallite size of about 4–5 nm. X-ray photoelectron spectroscopy indicated that most of the RE ions existed in the films in trivalent states. The optical bandgap of the RE-doped MoS2 samples decreased from 1.6 eV (undoped) to 1.3 eV (Eu-doped) in the UV-vis absorption spectra. Electrical measurements showed that the electrical resistance decreased from 9.13 MΩ (undoped) to 0.34 MΩ (Yb-doped), the carrier density increased by one to two orders of magnitude and the carrier mobility decreased from 5.4 cm2/V·s (undoped) to 0.65 cm2/V·s (Yb-doped). The sign of the Hall coefficients indicated that the undoped MoS2 and the Yb-, Tb- and Er-doped MoS2 samples were n-type semiconductors, while the Eu-doped sample showed p-type characteristics. This study may be helpful to broaden the photoelectronic applications of these two-dimensional materials.

Enhanced Visible-Light Response of Ultrathin Er-Doped MoS2 Nanosheets through a Wet-Chemical Method

Doping is one of the important technologies for modifying the properties of two-dimensional (2D) transition metal dichalcogenides (TMDs). Herein, ultrathin Er-doped MoS2 nanosheets with an enhanced visible-light response have been synthesized by a wet-chemical method. The few-layer characteristic of the MoS2:Er nanosheets is beneficial from the H2S generated during the preparation process. Near-infrared emission at ∼1530 nm from MoS2:Er nanosheets has been observed under 980 nm laser excitation, which corresponds to the energy transition from 4I13/2 to 4I15/2 of Er3+. Meanwhile, the enhanced visible-light response of MoS2:Er has been demonstrated by the photocatalytic performance (removing rhodamine B) and photocurrent response compared with undoped MoS2. Based on the experiments and density functional theory calculation, the enhanced visible-light response is attributed to the improved separation capability of photogenerated electron–hole pairs of the ultrathin MoS2:Er nanosheets. Er-doping induces an increase of S vacancies and the energy transfer between Er3+ ions and the host. The work demonstrates a developed universal and recyclable synthesis strategy for various metal-doped MoS2 nanosheets and their applications in the fields of photocatalysis and photodetection.

A novel polyurea nanofiltration membrane constructed by PEI/TA-MoS2 for efficient removal of heavy metal ions

Heavy metal pollution poses a serious threat to human health. Positively charged nanofiltration membranes show excellent application prospects in the field of heavy metal removal due to the combined effect of electrostatic repulsion and size sieving. Here, molybdenum disulfide (MoS2) nanosheets were dispersed in aqueous solution polyethyleneimine (PEI) by virtue of the adhesion effect of tannic acid (TA). Positively charged polyurea NF membranes (TSPU) were prepared by interfacial polymerization of aqueous phase PEI/TA-MoS2 and organic phase 2,4-toluene diisocyanate (TDI). The surface of the optimized membrane TSPU-0.015 showed better hydrophilicity, the water contact angle was 74.53°, and the functional layer thickness of TSPU-0.015 was 114.8 nm. TSPU-0.015 interface was positively charged (IEPTSPU-0.015 = 5.62), so the rejection of magnesium salts is significantly higher than that of sodium salts. It shows a retention performance of more than 99% for high-valent heavy metal ions such as Fe3+ and Cu2+, and the retention for Cd2+ is as high as 92.53%. The permeation flux of TSPU-0.015 is 2.88 times higher than that of undoped MoS2 (TSPU-0), and the flux to MgSO4 solution reaches 34.5 L/m2·h (6 bar, R(MgSO4) = 93.18%). TSPU has long-term stable operation and excellent anti-pollution properties, showing wide application potential in the fields of inorganic salt separation and heavy metal pollution.

Growth of highly conducting MoS2-xNx thin films with enhanced 1T' phase by pulsed laser deposition and exploration of their nanogenerator application

SummaryHigh-quality growth of MoS2-xNx films is realized on single-crystal c-Al2O3 substrates by the pulsed laser deposition (PLD) in ammonia rendering highly stable and tunable 1Tʹ/2H biphasic constitution. Raman spectroscopy reveals systematic enhancement of 1Tʹ phase component due to the incorporation of covalently bonded N-doping in MoS2 lattice, inducing compressive strain. Interestingly, the film deposited at 300 mTorr NH3 shows ∼80% 1Tʹ phase. The transport measurements performed on MoS2-xNx films deposited at 300 mTorr NH3 display very low room temperature resistivity of 0.03 mΩ-cm which is 100 times enhanced over the undoped MoS2 grown under comparable conditions. A triboelectric nanogenerator (TENG) device containing biphasic MoS2-xNx film as an electron acceptor exhibits a clear enhancement in the output voltage as compared to the pristine MoS2. Device architecture, p-type N doping in MoS2 lattice, favorably increased work-function, multiphasic component of MoS2, and increased surface roughness synergistically contribute to superior TENG performance.

Complementary Two-Dimensional (2-D) FET Technology With MoS2/hBN/Graphene Stack

This paper demonstrates a complementary 2-D field-effect transistor (FET) technology with molybdenum disulfide (MoS2) as the active film, hexagonal boron nitride (hBN) as the gate dielectric, and graphene as the gate electrode. The active region of the n-channel (n-FET) and p-channel (p-FET) devices are formed by undoped and Nb-doped MoS2, respectively. The complementary FETs have the desirable threshold voltage polarity, similar current drive, and high on-off current ratios of more than 106. A complementary metal-oxidesemiconductor (CMOS) inverter has been fabricated and the measured gain is about 14 with 90% noise margin at a 2 V power supply.

Interfacial engineering effect and bipolar conduction of Ni- doped MoS2 nanostructures for thermoelectric application

Transition Metal Chalcogenides play an important role in the thermoelectric (TE) application due to their unique properties such as stability, easily tunable carrier transport and charge carrier mobility which enhance the power factor. Molybdenum disulphide (MoS2) is a hopeful TE material with a layered structure, but it possesses low electrical conductivity. Herein, our present work focuses to improve the electrical conductivity of MoS2 by Ni-doping and the effect of Ni-concentration was studied extensively. Undoped and Ni-doped MoS2 samples were successfully prepared by hydrothermal route. The hexagonal structured 2H-MoS2 and the formation of nickel sulphide (NiS) secondary phase at 10 at% of Ni-concentration was confirmed by XRD analysis. HRTEM confirmed the influence of Ni content in MoS2 layers which create the bonding between the layers and reduces the interlayer distance of MoS2. The reduced thermal conductivity was observed around 0.48 Wm−1K−1 for 7.5 at% of Ni-doped MoS2 which is two times lower than that of undoped MoS2. Meanwhile, the enhancement of electrical conductivity of Ni-doped samples from 0.012 S/cm to 0.049 S/cm observed for 7.5 at% of Ni-doped MoS2 sample. Because of NiS, the conduction behavior was changed from p-type to n-type Seebeck coefficient. The maximum zT was obtained as 0.28 × 10−3 for undoped sample. However, Ni-doping was experimentally proved to the simultaneous optimization of enhanced electrical conductivity and the suppressed thermal conductivity, the change in carrier type due to nickel sulphide (NiS) phase reduced the figure of merit.

A comparative study of Ag doping effects on the electronic, optical, carrier conversion, photocatalytic and electrical properties of MoS2

MoS2 is an interesting material due to its intriguing properties and potential applications. Theoretical and experimental studies demonstrated p-type of Ag doped MoS2 system. Band structures, density of states and I-V characteristics confirmed that Ag is an efficient acceptor and indirect band gap is reduced with Ag doping. Formation energy is negative which indicates the substitution reactions could be possible. MoS2 is a strong photocatalyst for O2 production and the oxidation ability is enhanced with Ag doping. Mo100-xAgxS2 (x = 0.0 to 10) thin films seems to be stacked of many layers and XRD patterns justify hexagonal crystal structure. EDX and XPS measurements ensured the films are composed of Mo, S and Ag. Rectification is observed for Au/Mo100-xAgxS2 and the forward current decreases with Ag. The built-in potential is just opposite side to undoped MoS2 insights p-MoS2. Thus Ag doped MoS2 would enable for fabrication of efficient electronic, photocatalytic and optoelectronic devices.

Tuning Transport and Chemical Sensitivity via Niobium Doping of Synthetic MoS2

AbstractBeyond the intrinsic properties of 2D materials, another advantage is the tunability that follows from their low dimensionality. Here, large‐area Nb‐doped MoS2 monolayer films deposited by metal organic chemical vapor deposition that can function as electrical contacts or chemical sensors are demonstrated. Compared to pristine MoS2, Nb‐doped MoS2 exhibits a relatively faster growth rate and quenched PL due to formation of mid‐gap energy bands. When the Nb concentration reaches 5 at%, doped MoS2 shows clear p‐type characteristics, evident by a 1.7 eV shift of the Fermi level toward the valence band maximum. Doping also impacts transport at the metal/MoS2 interface, demonstrated by Pt–Ir metallization that is Schottky‐limited when in contact with undoped MoS2 but Ohmic on Nb‐MoS2. Moreover, a 50 × improved signal‐to‐noise ratio is demonstrated in sensing triethylamine compared to undoped MoS2, with <15 parts‐per‐billion detection limit.

Structural, morphological, optical, and photocatalytic properties of Ag-doped MoS2 nanoparticles

Nanoparticles of Ag doped molybdenum disulfide (Ag–MoS2) were synthesized by hydrothermal process. The structural, morphological, and optical characterisics of these nanoparticles were characterized by scanning electron microscopy (SEM), Raman spectroscopy, and photoluminescence spectroscopy (PL), respectively. Raman spectra confirmed the formation of MoS2 and Ag-doped MoS2 nanoparticles. SEM images indicated that sheet-like structure is prepared in undoped MoS2, while Ag–MoS2 has a particle-like structure. PL peaks are observed at 600 nm (2.06 eV), 610 nm (2.03 eV), 700 nm (1.77 eV), 725 nm (1.71 eV), and 730 nm (1.69 eV) for pure, 1% and 2% Ag– MoS2. When the doping amount of Ag is increased further (i.e., to 3% Ag-doped MoS2), new peaks at 636 nm (1.9 eV) and 663 nm (1.87 eV) are observed due to an increase in structural defects. The photocatalytic activity (PCA) of undoped and Ag–MoS2 is estimated by monitoring the degradation of rhodamine blue (RhB) and methylene blue (MB) dyes. The PCA of Ag–MoS2 samples are greater than that of undoped MoS2 when irradiated with visible light, which is attributed to increased electron–hole pair separation.