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The applications of two-dimensional (2D) materials in electronics, optoelectronics, and spintronics are limited by the high contact resistance at the metal/semiconductor interface owing to the strong Fermi-level pinning. In this study, an interlayer insertion strategy is proposed to solve this problem, and first principles calculations are done to study the influences of inserting a SiC layer on the Schottky barrier and electronic properties of MoS2/metals (Mg, Al, In, Cu, Ag, Au, Pd, Ti, and Sc). The average charge value substantially increased (≥0.060 e) at the interface between SiC and MoS2 layers, and then no tunneling barrier appeared except for the MoS2/Au contact by inserting the SiC layer. Moreover, ΦSB,N almost decreases for the MoS2/metal contacts by inserting the SiC layer. When Ti, Cu, Au, and Pd are used as electrodes, the n-type Schottky barrier is formed with the ΦSB,N values of 0.479 eV, −0.073 eV, 0.498 eV, and 0.225 eV, respectively. However, if Al, In, Mg, and Ag are used as electrodes, the systems are transformed into Ohmic contact. These findings provide a practical guideline for depinning the Fermi level at contact interfaces and designing the high performance TMD-based nanoelectronic devices.

The lithium–sulfur battery, despite having considerable advantages, is still far from its practical applications, which is primarily caused by the dissolution of polysulfide clusters of lithium (Li...

The applications of two-dimensional (2D) materials in electronics, optoelectronics, and spintronics are limited by the high contact resistance at the metal/semiconductor interface owing to the strong Fermi-level pinning. In this study, an interlayer insertion strategy is proposed to solve this problem, and first principles calculations are done to study the influences of inserting a SiC layer on the Schottky barrier and electronic properties of MoS2/metals (Mg, Al, In, Cu, Ag, Au, Pd, Ti, and Sc). The average charge value substantially increased (≥0.060 e) at the interface between SiC and MoS2 layers, and then no tunneling barrier appeared except for the MoS2/Au contact by inserting the SiC layer. Moreover, ΦSB,N almost decreases for the MoS2/metal contacts by inserting the SiC layer. When Ti, Cu, Au, and Pd are used as electrodes, the n-type Schottky barrier is formed with the ΦSB,N values of 0.479 eV, -0.073 eV, 0.498 eV, and 0.225 eV, respectively. However, if Al, In, Mg, and Ag are used as electrodes, the systems are transformed into Ohmic contact. These findings provide a practical guideline for depinning the Fermi level at contact interfaces and designing the high performance TMD-based nanoelectronic devices.

Structural defects are commonly undesirable in materials, however, atomic-level defect engineering is promising to improve the electronic, mechanical and chemical properties of graphene, if the density and types of defects could be well controlled. Herein, bismuth-mediated defect engineering method for epitaxial graphene (EG) grown on SiC(0001) is demonstrated. It is found that single defects and defect clusters could be facilitated by evaporating Bi atoms on SiC(0001) substrate before the standard EG preparation and, Bi atoms could be thoroughly cleaned away from the EG and the unwanted doping effects of Bi will be avoided by post-annealing at higher temperature. Scanning tunneling microscopy/spectroscopy characterization reveals the atomic structures, the electronic states and the Fermi level shift of flower-like, tube-like and point defects. This study sheds light on the metal-mediated formation of defects in graphene, and provides a practical defect engineering method.

Electrical contacts are crucial to the high performances of electronic devices, and they become more prominent for the popular two-dimensional (2D) semiconductors because they commonly have high contact resistances and are sensitive to the interfacial states. In this paper, taking monolayer MoS2 as an example, first-principles calculations are done to study and predict the influences of the contact mode on the interfacial electronic states of monolayer MoS2/metal (metal = Mg, Al, In, Cu, Ag, Au, Pd, Sc, and Ti). It is found that the interfacial properties are determined by the matching degree between the electronic states of the monolayer MoS2 and metal electrodes. The top contact configuration is preferred for the monolayer MoS2/Sc system as a result of an extremely low Schottky barrier (0.086 eV) as compared to that (0.439 eV) in the edge contact configuration, however, the edge contact configuration is preferred for Ag, Cu, Au, and Pd electrodes. Furthermore, metal electrodes in the top contact configuration might usually result in n-type doping of monolayer MoS2, but lead to p-type doping in the edge contact configuration. The pinning factor S (0.257, −0.009, −0.132, and −0.172) of monolayer MoS2 in both contact modes is close to zero, suggesting a strong electronic pinning effect. The findings provide theoretical guidance for the selection of electrodes for high-performance 2D material based devices.

Oxygen vacancies (Ov) as the active sites have significant influences on the gas sensing performance of metal oxides, and self-doping of Ce3+ in CeO2 might promote the formation of oxygen vacancies. In this work, hydrothermal process is adopted to fabricate the composites of graphene and CeO2 nanoparticles, and the influences of oxygen vacancies as well as Ce3+ ions on the sensing response to NO2 are studied. It is found that the sensitivity of the composites to NO2 increases gradually, as the proportion of Ce3+ relative to all of the cerium ions is increased from 14.6% to 50.7% but decreases after that value. First-principles calculations illustrate that CeO2 becomes metallic at the Ce3+ proportion of

It is difficult to generate p–n junctions in atomically thin transition metal dichalcogenides (TMDs) because of the great challenge of selective doping. First-principles calculations demonstrate that the electronic states in monolayer MoS2 could be substantially tuned through contact with hydrogenated SiC sheets, as a result of interface-induced electronic doping. Specifically, monolayer MoS2 exhibits metallic characteristics when put in contact with the Si termination of SiC–H (MoS2/SiC–H), but exhibits ambipolar type polarization when in contact with the C termination of CSi–H (MoS2/CSi–H). Furthermore, monolayer MoS2 can be switched from p-type on H–Si terminations (MoS2/H–SiC and MoS2/H–SiC–H) to n-type on H–C terminations (MoS2/H–CSi and MoS2/H–CSi–H). Accordingly, p–n junctions can be generated in bilayer MoS2 if a fully hydrogenated monolayer SiC is inserted between the layers. In addition, the staggered band alignment of the top and bottom monolayers of MoS2 leads to considerable rectification of current. The results are helpful for the design of TMD based nanoelectronic devices.

Ferromagnetic (FM) two-dimensional (2D) transition metal dichalcogenides (TMDs) have potential applications in modern electronics and spintronics and doping of TMDs with transition metals can enhance the magnetic characteristics. In this work, the structural stability, electronic states, and magnetic properties of Mn-doped monolayer/bilayer MoS2 are studied systematically by first-principles calculations. Substitutional Mn dopants at the Mo sites are energetically favorable in both monolayer and bilayer MoS2 under the S-rich condition which is common in the synthesis of MoS2 nanosheets. Two Mn dopants participate in the FM interaction in monolayer MoS2 and magnetic coupling of two Mn dopants via the double-exchange mechanism can be mediated by the nearest neighboring S. Magnetic coupling can be ascribed to the competition between the double-exchange, direct-exchange, and super-exchange interactions, which take place between two Mn dopants in bilayer MoS2 with the MniMnMo, MniMnS and MnMo–MnMo configurations. Our results reveal the geometrical dependence of magnetic-exchange coupling suggesting that Mn-doped monolayer/bilayer MoS2 has large potential in spintronic devices.

The effects of vanadium (V) dopant on the electronic and magnetic properties of monolayer MoS2 are investigated by first-principles calculation. The substitutionally doped V produces antiferromagnetic (AFM) or ferromagnetic (FM) states depending on the separation between V dopants. When the separation between V dopants is smaller than 6.38 A and the maximum dopant concentration is 25%, the superexchange interaction between V atoms is stronger than the double exchange interaction between the localized V 3d orbitals and Mo 4d orbitals, resulting in the AFM state in monolayer MoS2. However, the double exchange interaction between the V and Mo atoms becomes stronger than the superexchange interaction between V atoms if the separation between V dopants is larger than 9.57 A when the maximum dopant concentration is 11.11%. Consequently, the FM state is observed from the monolayer MoS2 and 100% spin polarization takes place if the separation between V atoms is further increased to 12.76 A at a dopant concentration of 6.25%. The results suggest potential applications of monolayer MoS2 as diluted magnetic semiconductors (DMS) in spintronics.

The effects of vacancies and doping defects on the electronic and magnetic states of two–dimensional (2D) AlN/MoS2 heterostructure are investigated by first–principle calculation. Because of charge transfer from the AlN layer to the interstitial region between layers, the energy band structure of the AlN/MoS2 heterostructure is not a simple superposition of those of AlN and MoS2. The energy band alignment can be further tuned by introducing vacancies and doping at the interface. When the AlN layer is decorated by N vacancies or n–type doped, noticeable charge transfer to the conduction band of the MoS2 layer is observed and the band alignment maintains type–I. However, in the case of p–type doping, for instance, C substituting for N (CN) in the AlN sublayer, the band alignment changes to type–II. Moreover, Al vacancies and BeAl/CN doping produce asymmetrical spin–up and spin–down states, which leads to magnetization of the AlN/MoS2 heterostructure. The results demonstrate the significant effects of interf...

Understanding the interfacial properties between two-dimensional (2D) semiconductors and metal electrodes is vital for designing and realization of electronic devices. In this paper, first-principle calculations are done to study the influences of the defects on the interfacial electronic states of mMoS2/metal (metal= Mg, Al, In, Cu, Ag, Au, and Pd). It is found that the electron states at the interface are substantially increased if defects are induced in mMoS2 contacted with metals. As a result, the tunneling barrier is reduced. The Schottky barrier height (SBH) is increased in mMoS2 contacted with Mg, Al, In, and Au, but is reduced in defective mMoS2 contacted with Cu, Ag, and Pd. Furthermore, the pinning factor S are 0.227, 0.238, and 0.238 for the VS, VMo, and SMo in mMoS2, but the S value is only 0.030 for substitution of S by Mo (MoS) in mMoS2. Our findings are in good agreement with the experimental results. The results give insight into the contact physics of 2D semiconductors with metals and it is suggested that defects play a key role on the future generation electronic applications.

Doping is an effective way to modulate the properties of two-dimensional (2D) materials to cater to new applications. In this work, the codoping effects of transition metals (TM) and nonmetals (NM) on the electronic states and magnetic polarization in monolayered MoS2 are investigated by first-principles calculation. The NM-doped MoS2 possesses semiconducting characteristics, but the B-, N-, and F-doped ones are magnetic showing a magnetic moment of 1.0 μ B per dopant. The metal dopants can induce magnetization in the monolayered MoS2, and the magnetic moment is related to the total number of outer electrons in the TM. In the case of codoping, the S and Mo atoms are substituted by the NM and TM atoms, respectively, and hence, the semiconducting characteristics are maintained despite the reduced band gap. Spin polarization resulting from codoping depends on the number of outer electrons in the TM and NM dopants. Spin polarization occurs if the total number of the outer electrons is odd, but does not if it is even. The magnetic moment of the codoped monolayered MoS2 is always 1 μ B and magnetism is enhanced considerably by (V + B) and (Mn + F) codoping.

Oxygen vacancies (O

Graphene as the thinnest material has an extremely large specific surface area, and thus the physical properties of graphene based devices should be sensitively dependent on the contacted metals. Moreover, the interfacial interaction between graphene and metals is complicated and it is difficult to probe. In this paper, epitaxial graphene is prepared by thermal decomposition of 6H-SiC(0001), and then Ag is deposited on it. It is found that the morphology and distribution of Ag particles on graphene domains are independent of the graphene thickness. The Ag particles induce the surface enhanced Raman scattering (SERS) effect and the doping effect in epitaxial graphene. The enhancement factor of SERS as well as the splitting of the G band and the shift of the 2D band decreases with increasing graphene thickness, which can be ascribed to the weakened interaction between Ag and EG. This is confirmed by the charge transfer between the Ag atom and epitaxial graphene on 6H-SiC predicted by first-principles calculations. The results are helpful to the design and development of graphene-based composites and devices.

Ferromagnetic (FM) two-dimensional (2D) transition metal dichalcogenides (TMDs) have potential applications in modern electronics and spintronics and doping of TMDs with transition metals can enhance the magnetic characteristics. In this work, the structural stability, electronic states, and magnetic properties of Mn-doped monolayer/bilayer MoS

Doping in epitaxial graphene (EG) is challenging because of the high-temperature process and the ultra-thin nature of graphene. In this work, a facile one-step method is demonstrated to generate doping in EG with bismuth (Bi) during thermal decomposition of SiC, in which Bi atom flux acts as the doping source. Raman spectroscopy and scanning tunneling microscopy/spectroscopy are employed to characterize the quality, morphology and electronic properties of Bi doped EG. Both the intercalated and incorporated Bi atoms can be considered as dopants. It was found that the Dirac point shifts away from the Fermi level as a result of electron transfer from Bi to EG, and thus enhances the n-doping behavior of EG significantly. First principles calculations were done to address the enhanced n-doping of EG by Bi. This in-situ doping procedure can be extended to other metals, showing great potential applications in tailoring the performance of EG and significance to electronics in the future.