Doping Of Transition Metal Dichalcogenides Research Articles

Two-dimensional Mo3-TiS2 monolayer hosting high moisture resistance and abundant surface-chemisorbed oxygen for effective detection of SF6 decomposition gases: Atomic-scale study

Monitoring the SF6 decomposition gas (DPA) by gas sensors can be used to predict the operating status of power equipment, and the sensitivity performance of gas sensors is deeply influenced by the humidity resistance and surface oxygen adsorption performance of their sensing materials. The present research aims to explore a high‐valence transition‐metal ion doped transition metal dichalcogenide (TMD) monolayer that hosts high moisture resistance and abundant surface-chemisorbed oxygen suitable for detecting DPA. Using first-principles methods, the adsorption behavior of three representative types of gases (HF, SO2, and CO2) in DPA, H2O, and O2 on Mo3-TiS2 monolayer are considered. Our results show that SO2 and CO2 molecules are adsorbed above the Mo3 cluster through chemical adsorption, while HF molecule is physically adsorbed. The desorption times for SO2 and CO2 on Mo3-TiS2 adsorbent are 9.89 s at 348 K and 34.96 s at 398 K, respectively. Introducing dopant Mo3 resulted in better anti-humidity performance of TiS2, as the force of Mo3-TiS2 monolayer towards H2O molecule is weakened compared to the intrinsic adsorption system. Additionally, Mo3-TiS2 monolayer exhibits an exceptionally high affinity for O2 molecule, leading to the rupture of chemical bond in O2, and the chemisorbed oxygen ions are acquired through the charge transfer between Mo3 cluster and O2 molecule. The computational findings in this article offer theoretical backing for advancing the development of gas sensors utilizing Mo3-TiS2 and its utilization in the realm of DPA detection.

Origin of hydrogen adsorption free energy variation in electrochemical hydrogen evolution in transition-metal dichalcogenides

Two-dimensional transition-metal (TM)-doped transition metal dichalcogenides (TMDs) are among the most promising materials for water-splitting catalysts. Among various methods applied to promote the hydrogen evolution reaction (HER) in TMDs, doping with TM heteroatoms has attracted attention because this strategy allows optimization of hydrogen adsorption and H2 generation reactions. Herein, we conduct in-depth, systematic analyses of the trends in the change in the hydrogen adsorption free energy (ΔGH∗)—the most well-known descriptor for evaluating HER performance—for doped TMDs. The total of 150 different doped TMDs are used to conduct an atomic-level analysis of the origin of ΔGH∗ changes upon TM heteroatom doping. Moreover, we suggest two key factors that govern hydrogen adsorption over doped TMDs: 1) changes in the charge of chalcogen atoms, where hydrogen atoms are adsorbed onto early-TM-doped structures and 2) structural deformation energies accompanying the introduced dopants of the late-TM-doped structures. Furthermore, we propose a new perspective on how vacancies in TM-doped TMDs can result in the enhanced ΔGH∗ for the HER. We suggest that introducing electrostatic and structural controls in early- and late-TM-doped systems, respectively, are effective strategies for achieving thermoneutral ΔGH∗ in TMDs and promote the design of new TMD catalysts with superior water-splitting capabilities.

Transition Metal Dichalcogenides: Making Atomic-Level Magnetism Tunable with Light at Room Temperature.

The capacity to manipulate magnetization in 2D dilute magnetic semiconductors (2D-DMSs) using light, specifically in magnetically doped transition metal dichalcogenide (TMD) monolayers (M-doped TX2 , where M= V, Fe, and Cr; T= W, Mo; X= S, Se, and Te), may lead to innovative applications in spintronics, spin-caloritronics, valleytronics, and quantum computation. This Perspective paper explores the mediation of magnetization by light under ambient conditions in 2D-TMD DMSs and heterostructures. By combining magneto-LC resonance (MLCR) experiments with density functional theory (DFT) calculations, weshow that the magnetization can be enhanced using light in V-doped TMD monolayers (e.g., V-WS2 , V-WSe2 ). This phenomenon is attributed to excess holes in the conduction and valence bands, and carriers trapped in magnetic doping states, mediating the magnetization of the semiconducting layer. In 2D-TMD heterostructures (VSe2 /WS2 , VSe2 /MoS2 ), the significance of proximity, charge-transfer, and confinement effects in amplifying light-mediated magnetism is demonstrated. Weattributed this to photon absorption at the TMD layer that generates electron-hole pairs mediating the magnetization of the heterostructure. These findings will encourage further research in the field of 2D magnetism and establish a novel design of 2D-TMDs and heterostructures with optically tunable magnetic functionalities, paving the way for next-generation magneto-optic nanodevices.

Carbon Dioxide Reduction on Transition Metal Dichalcogenides with Ni and Cu Edge Doping: A Density-Functional Theory Study.

Transition-metal dichalcogenides (TMDs) have promising properties for their use as catalysts of CO2 reduction to methane via the Sabatier reaction. In this article we use density-functional theory calculations to gain insight into the energetics of this reaction for Mo/W-based and S/Se-based TMDs with non-, Ni- and Cu-doping. We show that sulfur-based TMDs with Ni/Cu doping exhibit better indicators for catalytic performance of the CO2 reduction reaction than non-doped and doped TMDs without active sites. In addition, the role of the transition metal was found to a much smaller influence in the reaction than the role of the chalcogen and dopant atoms, which influence the bonding strength and type, respectively.

N- and p-type doping of transition-metal dichalcogenides by Ar plasma treatment and its application in CMOS

Doping of two-dimensional (2D) semiconductors is necessary to achieve high performance and low power consumption in optoelectronic and logic devices. Herein, we report controllable p- and n-type doping of the transition-metal dichalcogenides (TMDs), tungsten diselenide (WSe2), and molybdenum ditelluride (MoTe2) via argon (Ar) plasma treatment. The doping of TMDs was tuned by controlling Ar plasma treatment conditions. The desired n-type doping in WSe2 and MoTe2 was obtained by applying a short treatment time, resulting in increased electron current and an upshift in binding energy. Owing to prolonged plasma treatment, abnormal p-type doping was achieved in WSe2 and MoTe2. Increased hole current, downshift in binding energy, appearance of oxygen bonds (O–W, Mo, Se, and Te), and redshift in Raman peaks revealed that the abnormal p-type doping behavior for WSe2 and MoTe2 was owing to considerable vacancies and edge defects induced in the top layer of TMDs by Ar plasma treatment. These were immediately occupied by oxygen atoms or oxidized to introduce oxygen doping when TMDs were exposed to air. The formation of a homogeneous oxide film on the top layer of TMDs was further demonstrated by the smooth surface and higher thickness of TMDs with heavy p-type doping compared to those of the pristine sample. A complementary metal-oxide-semiconductor (CMOS) inverter based on p- and n-WSe2 was demonstrated herein. Our study proposes a controllable way to modulate the doping type of TMDs, which has potential applications in the performance modulation of devices, design of novel device structures, and realization of 2D material-based logic circuits.

Enhanced Thermoelectric Power Factor in Carrier-Type-Controlled Platinum Diselenide Nanosheets by Molecular Charge-Transfer Doping.

2D transition metal dichalcogenides (TMDCs) have revealed great promise for realizing electronics at the nanoscale. Despite significant interests that have emerged for their thermoelectric applications due to their predicted high thermoelectric figure of merit, suitable doping methods to improve and optimize the thermoelectric power factor of TMDCs have not been studied extensively. In this respect, molecular charge-transfer doping is utilized effectively in TMDC-based nanoelectronic devices due to its facile and controllable nature owing to a diverse range of molecular designs available for modulating the degree of charge transfer. In this study, the power of molecular charge-transfer doping is demonstrated in controlling the carrier-type (n- and p-type) and thermoelectric power factor in platinum diselenide (PtSe2 ) nanosheets. This, combined with the tunability in the band overlap by changing the thickness of the nanosheets, allows a significant increase in the thermoelectric power factor of the n- and p-doped PtSe2 nanosheets to values as high as 160 and 250 µW mK-2 , respectively. The methodology employed in this study provides a simple and effective route for the molecular doping of TMDCs that can be used for the design and development of highly efficient thermoelectric energy conversion systems.

Microscopic theory of exciton and trion polaritons in doped monolayers of transition metal dichalcogenides

We study a doped transition metal dichalcogenide (TMDC) monolayer in an optical microcavity. Using the microscopic theory, we simulate spectra of quasiparticles emerging due to the interaction of material excitations and a high-finesse optical mode, providing a comprehensive analysis of optical spectra as a function of Fermi energy and predicting several modes in the strong light-matter coupling regime. In addition to exciton-polaritons and trion-polaritons, we report polaritonic modes that become bright due to the interaction of excitons with free carriers. At large doping, we reveal strongly coupled modes corresponding to excited trions that hybridize with a cavity mode. We also demonstrate that the increase of carrier concentration can change the nature of the system’s ground state from the dark to the bright one. Our results offer a unified description of polaritonic modes in a wide range of free electron densities.

Unveiling Unintentional Fluorine Doping in TMDs During the Reactive Ion Etching: Root Cause Analysis, Physical Insights, and Solution

Layered semiconductors, such as transition metal dichalcogenides (TMDs), gained popularity due to their unique properties favoring different applications. For all practical applications, it is essential to have the ability to pattern these layered semiconductors as per the desired dimensions, which are derived from device design or circuit design. Due to its anisotropic nature, plasma or reactive ion etching (RIE) has been the most used technique for patterning bulk semiconductors or thin films. This article unveils the challenges associated with the fluorine-based (SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> and CHF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) plasma etching of TMDs. Subsequently, a detailed root cause analysis is presented with the solution to the problems thereof. We have discovered: 1) high affinity of resists leading to unwanted resist residues over the TMD surface and 2) unintentional doping induced by the fluorine plasma, due to fluorine occupation at the interstitial sites near sulfur (S) defects or its occupation of an S-defect site in the TMD. This was observed despite the surface being masked by the resist. Physical insights were developed using atomic force microscopy (AFM), Raman spectroscopy, electrical characterization, field-effect mobility and transfer length method (TLM)-based contact resistance comparison, and density functional theory (DFT)-based atomistic computations. Furthermore, these insights are used to demonstrate an improved plasma recipe, which mitigates these challenges and allows realization of transistors over a patterned channel while offering characteristic similar to an intrinsic/pristine layer. Finally, the improved plasma recipe, developed for MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , is also shown to be effective for MoSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , WS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and WSe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> layers.

Signature of Spin-Resolved Quantum Point Contact in p-Type Trilayer WSe2 van der Waals Heterostructure.

In this study, an electrostatically induced quantum confinement structure, so-called quantum point contact, has been realized in a p-type trilayer tungsten diselenide-based van der Waals heterostructure with modified van der Waals contact method with degenerately doped transition metal dichalcogenide crystals. Clear quantized conductance and pinch-off state through the one-dimensional confinement were observed by dual-gating of split gate electrodes and top gate. Conductance plateaus were observed at a step of e2/h in addition to quarter plateaus such as 0.25 × 2e2/h at a finite bias voltage condition indicating the signature of intrinsic spin-polarized quantum point contact.

Magnetic order and critical temperature of substitutionally doped transition metal dichalcogenide monolayers

Using first-principles calculations, we investigate the magnetic order in two-dimensional (2D) transition-metal-dichalcogenide (TMD) monolayers: MoS2, MoSe2, MoTe2, WSe2, and WS2 substitutionally doped with period four transition-metals (Ti, V, Cr, Mn, Fe, Co, Ni). We uncover five distinct magnetically ordered states among the 35 distinct TMD-dopant pairs: the non-magnetic (NM), the ferromagnetic with out-of-plane spin polarization (Z FM), the out-of-plane polarized clustered FMs (clustered Z FM), the in-plane polarized FMs (X–Y FM), and the anti-ferromagnetic (AFM) state. Ni and Ti dopants result in an NM state for all considered TMDs, while Cr dopants result in an anti-ferromagnetically ordered state for all the TMDs. Most remarkably, we find that Fe, Mn, Co, and V result in an FM ordered state for all the TMDs, except for MoTe2. Finally, we show that V-doped MoSe2 and WSe2, and Mn-doped MoS2, are the most suitable candidates for realizing a room-temperature FM at a 16–18% atomic substitution.

Tunable Doping of Rhenium and Vanadium into Transition Metal Dichalcogenides for Two-Dimensional Electronics.

Two‐dimensional (2D) transition metal dichalcogenides (TMDCs) with unique electrical properties are fascinating materials used for future electronics. However, the strong Fermi level pinning effect at the interface of TMDCs and metal electrodes always leads to high contact resistance, which seriously hinders their application in 2D electronics. One effective way to overcome this is to use metallic TMDCs or transferred metal electrodes as van der Waals (vdW) contacts. Alternatively, using highly conductive doped TMDCs will have a profound impact on the contact engineering of 2D electronics. Here, a novel chemical vapor deposition (CVD) using mixed molten salts is established for vapor–liquid–solid growth of high‐quality rhenium (Re) and vanadium (V) doped TMDC monolayers with high controllability and reproducibility. A tunable semiconductor to metal transition is observed in the Re‐ and V‐doped TMDCs. Electrical conductivity increases up to a factor of 108 in the degenerate V‐doped WS2 and WSe2. Using V‐doped WSe2 as vdW contact, the on‐state current and on/off ratio of WSe2‐based field‐effect transistors have been substantially improved (from ≈10–8 to 10–5 A; ≈104 to 108), compared to metal contacts. Future studies on lateral contacts and interconnects using doped TMDCs will pave the way for 2D integrated circuits and flexible electronics.

Combined healing and doping of transition metal dichalcogenides through molecular functionalization

Twofold effect of molecular functionalization of defective transition metal dichalcogenides (TMDCs).

Air-stable and efficient electron doping of monolayer MoS2 by salt-crown ether treatment.

To maximize the potential of transition-metal dichalcogenides (TMDCs) in device applications, the development of a sophisticated technique for stable and highly efficient carrier doping is critical. Here, we report the efficient n-type doping of monolayer MoS2 using KOH/benzo-18-crown-6, resulting in a doped TMDC that is air-stable. MoS2 field-effect transistors show an increase in on-current of three orders of magnitude and degenerate the n-type behaviour with high air-stability for ∼1 month as the dopant concentration increases. Transport measurements indicate a high electron density of 3.4 × 1013 cm-2 and metallic-type temperature dependence for highly doped MoS2. First-principles calculations support electron doping via surface charge transfer from the K/benzo-18-crown-6 complex to monolayer MoS2. Patterned doping is demonstrated to improve the contact resistance in MoS2-based devices.

Controlled Re doping in MoS2 by chemical vapor deposition

2D materials have been extensively studied due to their unique optical and electronic properties. Compositional doping in these materials could further enable useful utilization of these materials in electronics, optoelectronics and energy harvesting. However, scalable one pot synthesis of doped Transition Metal Dichalcogenides (TMDs) has been challenging. Here we develop a facile approach for controlled one step Rhenium (Re) doping in multilayer Molybdenum Disulphide (MoS2) by powder Chemical Vapor Deposition (CVD). Interestingly, we find the morphology of MoS2 is also dependent on the dopant concentrations with a flower like morphology observed at higher doping concentrations. Further we demonstrate a clear correlation between the shift in PL and Raman peaks as a function of Re doping concentrations. A red shift in PL is observed with increased concentration of Re atoms. The ability to controllably dope thin layers of MoS2 could open up unique applications of this material in areas hitherto unexplored.

Ultrafast Carrier Dynamics in Two-Dimensional Electron Gas-like K-Doped MoS2

Electronic interactions associated with atomic adsorbates on transition metal dichalcogenides such as MoS2 can induce massive electronic reconstruction that results in the formation of a new type of heavily doped transition metal dichalcogenide that exhibits characteristics of a two-dimensional electron gas. The impact of quantum confinement and reduced dimensionality on the carrier dynamics in such two-dimensional systems is at present not known, but is of paramount importance if they are to find application in optoelectronic devices. Here we show by a combination of angle-resolved photoemission and advanced x-ray spectroscopies that many-body interactions in reduced dimension drastically shorten carrier lifetimes to below 0.5 fs, and reveal how potassium intercalation in MoS2 forces orbital rehybridization to create a two-dimensional electron gas.

Conversion of Intercalated MoO3 to Multi-Heteroatoms-Doped MoS2 with High Hydrogen Evolution Activity.

Lack of effective strategies to regulate the internal activity of MoS2 limits its practical application for hydrogen evolution reactions (HERs). Doping of heteroatoms without forming aggregation or an edge enrichment is still challenging, and its effect on the HER needs to be further explored. Herein, a two-step method is developed to obtain multi-metal-doped H-MoS2 , which includes intercalation of the layered MoO3 precursor with a following sulfurization. Benefiting from the capability of the intercalation method to uniformly and simultaneously introduce different elements into the van der Waals gap, this method is universal to obtain multi-heteroatoms co-doped MoS2 without forming clusters, phase separation, and an edge enrichment. It is demonstrated that the doping of adjacent cobalt and palladium monomers on MoS2 greatly enhances the HER catalytic activity. The overpotential at 10 mA cm-2 and Tafel slope of Co and Pd co-doped MoS2 is found to be 49.3 mV and 43.2 mV dec-1 , respectively, representing a superior acidic HER catalytic activity. This intercalation-assisted method also provides a new and general strategy to synthesize uniformly doped transition metal dichalcogenides for various applications.

Substitutional Fluorine Doping of Large-Area Molybdenum Disulfide Monolayer Films for Flexible Inverter Device Arrays.

Reliable and controllable doping of transition metal dichalcogenides (TMDCs) is a mandatory requirement for practical large-scale electronic applications. However, most of the literature on the doping methodologies of TMDCs has focused on n-type doping and multilayer TMDC rather than a monolayer one enabling large-scale growth. Herein, we report substitutional fluorine doping of a chemical vapor deposition (CVD)-grown molybdenum disulfide (MoS2) monolayer film using a delicate SF6 plasma treatment. Our SF6-treated MoS2 monolayer shows a p-type doping effect with fluorine substitution. The doping concentration is controlled by the plasma treatment time (2-4.9 atom %) while maintaining the structural integrity of the MoS2 monolayer. Such reliable and tunable substitutional doping is attributed to preventing direct ion bombardment to the MoS2 monolayer by our gentle plasma treatment system. Finally, we fabricated MoS2 homojunction flexible inverter device arrays based on the pristine and SF6-treated MoS2 monolayer. A clear switching behavior is observed, and the voltage gain is approximately 8 at an applied VDD of 2 V, which is comparable to that of CVD-grown MoS2-based inverter devices reported previously. Obtained voltage gain is also stable even after 500 bending cycles at an applied strain of 0.5%.

Rotons in optical excitation spectra of monolayer semiconductors

Optically generated excitons dictate the absorption and emission spectrum of doped semiconductor transition metal dichalcogenide monolayers. We show that upon increasing the electron density, the elementary optical excitations develop a roton-like dispersion, evidenced by a shift of the lowest energy state to a finite momentum on the order of the Fermi momentum. This effect emerges due to Pauli exclusion between excitons and the electron Fermi sea, but the robustness of the roton minimum in these systems is a direct consequence of the long-range nature of the Coulomb interaction and the nonlocal dielectric screening characteristic of monolayers. Finally, we show that the emergence of rotons could be related to hitherto unexplained aspects of photoluminescence spectra in doped transition metal dichalcogenide monolayers.

Chemical doping of transition metal dichalcogenides (TMDCs) based field effect transistors: A review

Abstract Transition metal dichalcogenides (TMDCs), the two dimensional layered structures with significant band gap have been considered as noteworthy candidates for advanced electronic applications. Here we review various chemical dopants in TMDC materials that modulate charge carrier concentrations and hence influence other electrical attributes. It is reported that different features of TMDCs based devices such as mobility, channel and contact resistances and on/off current ratio have been significantly upgraded. Chemicals such as LiF, Benzyl viologen, P-toluene sulfonic acid, Chloride molecules, Potassium (K) and Hydrazine are reported to induce n-type doping in TMDC materials while AuCl3, chemical sulfur treatment (ST) utilizing ammonium sulfide [(NH4)2S] and Octadecyltrichlorosilane are reported as p-type dopants. The oxide Al2O3 acts as p-type and Amorphous Titanium Suboxide (ATO) acts n-type dopant. The literature reviewed here, reports that p-type doping improves the device stability thereby minimizing the hysteresis effect, while n-type doping improves the mobility of the samples. It also reviewed the temperature-dependent in situ calculations performed via DFT, these results showed that conduction because of n-type carriers is intrinsic property that is indorsed to defects (tellurium vacancies) in TMDCs. This review presents the comprehensive summary of the role of dopants to modulate the numerous electronic transport, electronic and optical properties of TMDCs based field-effect transistors and provide future insights towards state of the art nanoelectronic devices.

How Substitutional Point Defects in Two-Dimensional WS2 Induce Charge Localization, Spin–Orbit Splitting, and Strain

Control of impurity concentrations in semiconducting materials is essential to device technology. Because of their intrinsic confinement, the properties of two-dimensional semiconductors such as transition metal dichalcogenides (TMDs) are more sensitive to defects than traditional bulk materials. The technological adoption of TMDs is dependent on the mitigation of deleterious defects and guided incorporation of functional foreign atoms. The first step toward impurity control is the identification of defects and assessment of their electronic properties. Here, we present a comprehensive study of point defects in monolayer tungsten disulfide (WS2) grown by chemical vapor deposition using scanning tunneling microscopy/spectroscopy, CO-tip noncontact atomic force microscopy, Kelvin probe force spectroscopy, density functional theory, and tight-binding calculations. We observe four different substitutional defects: chromium (CrW) and molybdenum (MoW) at a tungsten site, oxygen at sulfur sites in both top and bottom layers (OS top/bottom), and two negatively charged defects (CD type I and CD type II). Their electronic fingerprints unambiguously corroborate the defect assignment and reveal the presence or absence of in-gap defect states. CrW forms three deep unoccupied defect states, two of which arise from spin-orbit splitting. The formation of such localized trap states for CrW differs from the MoW case and can be explained by their different d shell energetics and local strain, which we directly measured. Utilizing a tight-binding model the electronic spectra of the isolectronic substitutions OS and CrW are mimicked in the limit of a zero hopping term and infinite on-site energy at a S and W site, respectively. The abundant CDs are negatively charged, which leads to a significant band bending around the defect and a local increase of the contact potential difference. In addition, CD-rich domains larger than 100 nm are observed, causing a work function increase of 1.1 V. While most defects are electronically isolated, we also observed hybrid states formed between CrW dimers. The important role of charge localization, spin-orbit coupling, and strain for the formation of deep defect states observed at substitutional defects in WS2 as reported here will guide future efforts of targeted defect engineering and doping of TMDs.