Electronic structures,magnetic properties and band alignments of 3d transition metal atoms doped monolayer MoS2

Utilizing first-principles calculations, the electronic structures, magnetic properties and band alignments of monolayer MoS₂ doped by 3d transition metal atoms have been investigated. It is found that in V, Cr, Mn, Fe-doped monolayers, the nearest neighboring S atoms (S_NN) are antiferromagnetically polarized with the doped atoms. While in Co, Ni, Cu, Zn-doped systems, the S_NN are ferromagnetically coupled with the doped atoms. Moreover, the nearest neighboring Mo atoms also demonstrate spin polarization. Compared with pristine monolayer MoS₂, little change is found for the band edges' positions in the doped systems. The Fermi level is located in the spin-polarized impurity bands, implying a half-metallic state. These results provide fundamental insights for doped monolayer MoS₂ applying in spintronic, optoelectronic and electronic devices.     

First-principles study of B,C, N and F doped graphene-like MgO monolayer

Based on the first-principles calculations, we have investigated the stable geometries, electronic and magnetic properties of the graphene-like MgO monolayer with O atom substituted by B, C, N, and F atoms. The formation energy decreases in the order of B>C>N>F, which may be influenced by the different electronegativities. The band gaps of p-type doped MgO monolayers are tunable due to the emergence of impurity states within the band gap, while F-doped MgO monolayer realizes the transition from semiconductor to metal. The results show that p-type doped MgO monolayer exhibit magnetic behaviors due to polarizations of dopants and surrounding Mg or O atoms near the dopants, while no magnetism is observed in the case of F doped MgO monolayer. These results are potentially useful for spintronic applications and the development of magnetic nanostructures.     

Alkali-metal-embedded in monolayer MoS<Subscript>2</Subscript>: optical properties and work functions

Embedding alkali-metal in monolayer MoS₂ has been investigated by using first principles with density functional theory. The calculation of the electronic and optical properties indicates that alkali-metal was embedded in monolayer MoS₂ appearing almost metallic behavior, and the MoS₂ layer shows clear p-type doping behavior. The covalent bonding appears between the alkali-metal atoms and defective MoS₂. More importantly, embedding alkali-metal can increase the work function for monolayer MoS₂. Furthermore, the absorption spectrum of monolayer MoS₂ is red shifted because of alkali metal embedding. Accordingly, this study will provide the theoretical basis for producing the alkali-metal-doped monolayer MoS₂ radiation shielding and photoelectric devices.     

Impurity characteristics of group V and VII element-doped two-dimensional ZrSe2 monolayer

Through first-principles calculations, we investigate the electronic structures, formation energy and transition level of the selected group V and VII impurities in the two-dimensional ZrSe₂ monolayer. Our results indicate that group V and VII atoms substituting Se atom can be easier under Zr-rich experiment conditions. Moreover, group V element substituting doping can induce p-type carrier due to their negative formation energy and shallow transition level, while group VII doping can not be effective to induce the n-type impurities. In particular, N substituting Se exhibits the lowest formation energy and shallowest transition level, which indicates that N impurities can offer effective p-type carriers in the ZrSe₂ monolayer.     

Magnetic,electronic, optical,and photocatalytic properties of nonmetal- and halogen-doped anatase TiO2 nanotubes

The structure stability, magnetic, electronic, optical, and photocatalytic properties of nonmetal (B, C, N, P, and S), and halogen (F, Cl, Br, and I)-doped anatase TiO₂ nanotubes (TNTs) have been investigated using spin polarized density functional theory. The N- and F-doped TNTs are the most stable among other doped TNTs. It is found that the magnetic moment of doped TNT is the difference between the number of the valence electrons of the dopant and host anion. All dopants decrease the band gap of TNT. The decrease in the band gap of nonmetal (C, N, P, and S)-doped TNTs, in particular N and P, is larger than that of halogen-doped TNTs due to the created states of the nonmetal dopant in the band gap. There is a good agreement between the calculation results and the experimental observations. Even though C-, N-, and P-doped TNTs have the lowest band gap, they cannot be used as a photocatalysis for water splitting. The B-, S-, and I-doped TiO₂ nanotubes are of great potential as candidates for water splitting in the visible light range.     

Band-gap bowing and p-type doping of (Zn,Mg, Be)O wide-gap semiconductor alloys: a first-principles study

Using a first-principles band-structure method and a special quasirandom structure (SQS) approach, we systematically calculate the band gap bowing parameters and p-type doping properties of (Zn, Mg, Be)O related random ternary and quaternary alloys. We show that the bowing parameters for ZnBeO and MgBeO alloys are large and dependent on composition. This is due to the size difference and chemical mismatch between Be and Zn(Mg) atoms. We also demonstrate that adding a small amount of Be into MgO reduces the band gap indicating that the bowing parameter is larger than the band-gap difference. We select an ideal N atom with lower p atomic energy level as dopant to perform p-type doping of ZnBeO and ZnMgBeO alloys. For N doped in ZnBeO alloy, we show that the acceptor transition energies become shallower as the number of the nearest neighbor Be atoms increases. This is thought to be because of the reduction of p-d repulsion. The N_O acceptor transition energies are deep in the ZnMgBeO quaternary alloy lattice-matched to GaN substrate due to the lower valence band maximum. These decrease slightly as there are more nearest neighbor Mg atoms surrounding the N dopant. The important natural valence band alignment between ZnO, MgO, BeO, ZnBeO, and ZnMgBeO quaternary alloy is also investigated.     

Cr和W掺杂的单层MoS2电子结构的第一性原理研究

采用密度泛函理论框架下的第一性原理平面波赝势方法, 研究了Cr和W掺杂对单层二硫化钼(MoS₂)晶体的电子结构性质的影响. 计算结果表明: 当掺杂浓度较高时, W对MoS₂的能带结构几乎没有影响, 而Cr的掺杂则影响很大, 表现为能带由直接带隙变为间接带隙, 且禁带宽度减小. 通过进一步分析, 得出应力的产生是导致Cr掺杂的MoS₂电子结构变化的最直接的原因.     

Electronic properties of anatase TiO2 doped by lanthanides: A DFT+U study

The effects of mono-doping of 4f lanthanides with and without oxygen vacancy defect on the electronic structures of anatase TiO₂ have been studied by first-principles calculations with DFT+U (DFT with Hubbard U correction) to treat the strong correlation of Ti 3d electrons and lanthanides 4f electrons. Our results revealed that dopant Ce is easy to incorporate into the TiO₂ host by substituting Ti due to its lower substitutional energy (∼−2.0 eV), but the band gap of the system almost keeps intact after doping. The Ce 4f states are located at the bottom of conduction band, which mainly originates from Ti 3d states. The magnetic moment of doped Ce disappears due to electron transfer from Ce to the nearest O atoms. For Pr and Gd doping, their substitutional energies are similar and close to zero, indicating that both of them may also incorporate into the TiO₂ host. For Pr doping, some 4f spin-down states are located next to the bottom of the conduction band and narrow the band gap of the doping system. However, for Gd doping, the 4f states are located in deep valence band and there is no intermediate band in the band gap. The magnetic moment of dopant Gd is close to the value of isolated Gd atom (∼7 μB), indicating no overlapping between Gd 4f with other orbitals. For Eu, it is hard to incorporate into the TiO₂ host due to its very higher substitutional energy. The results also indicated that oxygen vacancy defect may enhance the adsorption of the visible light in Ln-doped TiO₂ system.     

Thermoelectric performance of monolayer Bi2Te2Se of ultra low lattice thermal conductivity

The electronic structure and thermoelectric properties of monolayer Bi₂Te₂Se were studied by density functional theory and semi-classical Boltzmann transport equation. The band gap with TB-mBJ can be improved for monolayer Bi₂Te₂Se. Monolayer Bi₂Te₂Se have ultra-low thermal conductivity comparing with other well-known two-dimensional materials. The monolayer Bi₂Te₂Se can improve electrical conductivities. ZT increases with increasing temperature for monolayer Bi₂Te₂Se. Comparing to GGA, TB-mBJ has larger ZT value in p-type doping. Monolayer Bi₂Te₂Se have larger ZT comparing with other well-known two-dimensional materials. Our calculated results show that our calculation greatly underestimates ZT value, therefore, monolayer Bi₂Te₂Se should have a higher ZT value.     

A first-principles insight into Pd-doped MoSe2 monolayer: A toxic gas scavenger

Using first-principles theory, we investigate the Pd-doping effect on the geometric and electronic behaviors of MoSe₂ monolayer, and the adsorption behavior of Pd-MoSe₂ monolayer upon four toxic gases, namely NO, NO₂ SO₂ and H₂S. Desorption property of Pd-MoSe₂ monolayer upon four gases at diverse temperatures is analyzed as well to help explore its potential application. For Pd dopant adsorption onto MoSe₂ monolayer, somewhat n-type doping is determined, which accounts for the increased conductivity for intrinsic MoSe₂ monolayer. The strong adsorption ability and poor desorption performance of Pd-MoSe₂ monolayer upon four gases indicate its large potential as gas scavenger to remove these toxic gases from their surroundings. Moreover, it could be explored as a gas sensor for detection of NO, NO₂ and H₂S as well, given the obvious change in conductivity after gas adsorption based on band structure analysis. Our calculations would be beneficial to understand the TM doping effect on intrinsic MoSe₂ monolayer and to provide a first insight into the potential application as gas sensor or sweeper for Pd-MoSe₂ monolayer.     

Electronic and optical properties of single-layer MoS<Subscript>2</Subscript>

The electronic structures of a MoS₂ monolayer are investigated with the all-electron first principle calculations based on the density functional theory (DFT) and the spin-orbital couplings (SOCs). Our results show that the monolayer MoS₂ is a direct band gap semiconductor with a band gap of 1.8 eV. The SOCs and d-electrons in Mo play a very significant role in deciding its electronic and optical properties. Moreover, electronic elementary excitations are studied theoretically within the diagrammatic self-consistent field theory. Under random phase approximation, it shows that two branches of plasmon modes can be achieved via the conduction-band transitions due to the SOCs, which are different from the plasmons in a two-dimensional electron gas and graphene owing to the quasi-linear energy dispersion in single-layer MoS₂. Moreover, the strong optical absorption up to 10⁵ cm^-1 and two optical absorption edges I and II can be observed. This study is relevant to the applications of monolayer MoS₂ as an advanced photoelectronic device.     

Insulator-superconductor-metal transitions induced by spin fluctuations in the paramagnetic phase of doped insulators

AbstractThe band structure of cuprates as a doped 2D insulator is modeled assuming that the excess charge carriers are associated with the corresponding substitution atoms, and the phase diagram of the paramagnetic states as a function of the degree x of doping at zero temperature is studied. The Hamiltonian contains electronic correlations on impurity orbitals and hybridization between them and the initial band states of the insulator. It is shown that the change in the electronic structure of a doped compound includes the formation of impurity bands of distributed and localized electronic states in the initial insulator gap. It is established that in the case of one excess electron per substitution atom the spin fluctuations (1) give rise to an insulator state of the doped compound for x < x _thr, 1, (2) lead to a superconducting state for x _thr, 1 < x < x _thr, 2, and (3) decay as x > x _thr, 2 increases further, and the doped compound transforms into a paramagnetic state of a “poor” metal with a high density of localized electronic states at the Fermi level.     

Electronic and optical properties of black phosphorus doped with Au,Sn and I atoms

In this study, the electronic and the optical properties of monolayer black phosphorus (BP) doped with Gold (Au), Tin (Sn) and Iodine (I) atoms have been investigated by the density-functional theory (DFT) method. In the calculations, the electronic and the optical properties of monolayer BP have been substantially changed with doping. Monolayer BP has a narrow bandgap as 0.85 eV, BP doped with these atoms, results in a metallic behaviour and nearly spin gapless band gap behaviour. The dielectric constant of BP which shows anisotropic optical properties due to different edge states as zigzag and armchair has been changed with doping especially with Au.     

Electronic and magnetic properties of 3d-metal trioxides superhalogen cluster-doped monolayer MoS2: A first-principles study

Utilizing first-principle calculations, the structural, electronic, and magnetic properties of monolayer MoS₂ doped with 3d transition-metal (TM) atoms and 3d-metal trioxides (TMO₃) superhalogen clusters are investigated. 3d-metal TMO₃ superhalogen cluster-doped monolayers MoS₂ almost have negative formation energies except CoO₃ and NiO₃ doped monolayer MoS₂, which are much lower than those of 3d TM-doped structures. 3d-metal TMO₃ superhalogen clusters are more easily embedded in monolayer MoS₂ than 3d-metal atoms. MnO₃, FeO₃, CoO₃, and NiO₃ incorporated into monolayer MoS₂ are magnetic, and the total magnetic moments are approximately 1.0, 2.0, 3.0, and 4.0 μB per supercell, respectively. MnO₃ and FeO₃ incorporated into monolayer MoS₂ become semiconductors, whereas CoO₃ and NiO₃ incorporated into monolayer MoS₂ become half-metallic. Our studies demonstrate that the half-metallic ferromagnetic nature of 3d-metal TMO₃ superhalogen clusters-doped monolayer MoS₂ has a great potential for MoS₂-based spintronic device applications.     

Theoretical and experimental investigation on structural and electronic properties of Al/O/Al,O-doped WS2

Effects of the doping atom (O, Al, and (Al, O)) on structural and electronic properties of the monolayer WS₂ have been studied by using first-principles calculations. Results show that the covalent character of W–S bonding has been enhanced after doping. Meanwhile, W–O, Al–S and W–S bonds of (Al, O) co-doped WS₂ monolayer have higher covalent character compared with O-doped and Al-doped WS₂ monolayer of this work. After doping with Al (or Al, O) atoms, Fermi level moves close to the valence band and the dopant atoms produce the defect energy levels, indicating that Al doped and (Al, O) co-doped WS₂ monolayer both have p-type conductivity. O-doped and (Al, O) co-doped WS₂ ultrathin films was prepared on Si substrates. Results of Raman spectra show the formation of the O-doped and (Al, O) co-doped WS₂ films. Moreover, compared with the pure WS₂, the approximate reduction of 0.43 eV and 0.46 eV for W 4f and S 2p in binding energy after (Al, O) co-doped shows that p-type doping of (Al, O) co-doped WS₂ has been verified.     

Electronic and optical properties of pure and Mo doped anatase TiO2 using GGA and GGA+U calculations

Comparative GGA and GGA+U calculations for pure and Mo doped anatase TiO₂ are performed based on first principle theory, whose results show that GGA+U calculation provide more reliable results as compared to the experimental findings. The direct band gap nature of the anatase TiO₂ is confirmed, both by using GGA and GGA+U calculations. Mo doping in anatase TiO₂ narrows the band gap of TiO₂ by introducing Mo 4d states below the conduction band minimum. Significant reduction of the band gap of anatase TiO₂ is found with increasing Mo doping concentration due to the introduction of widely distributed Mo 4d states below the conduction band minimum. The increase in the width of the conduction band with increasing doping concentration shows enhancement in the conductivity which may be helpful in increasing electron–hole pairs separation and consequently decreases the carrier recombination. The Mo doped anatase TiO₂ exhibits the n-type characteristic due to the shifting of Fermi level from the top of the valence band to the bottom of the conduction band. Furthermore, a shift in the absorption edge towards visible light region is apparent from the absorption spectrum which will enhance its photocatalytic activity. All the doped models have depicted visible light absorption and the absorption peaks shift towards higher energies in the visible region with increasing doping concentration. Our results describe the way to tailor the band gap of anatase TiO₂ by changing Mo doping concentration. The Mo doped anatase TiO₂ will be a very useful photocatalyst with enhanced visible light photocatalytic activity.     

铂掺杂扶手椅型石墨烯纳米带的电学特性研究

本文采用基于密度泛函理论(DFT)的第一性原理计算了铂原子填充扶手椅型石墨烯纳米带(AGNR)中双空位结构的电学性能.计算结果表明: 通过控制铂原子的掺杂位置, 可以实现纳米带循环经历小带隙半导体—金属—大带隙半导体的相变过程; 纳米带边缘位置是铂原子掺杂的最稳定位置, 边缘掺杂纳米带的带隙值随宽度的变化与本征AGNR一样可用三簇曲线表示, 但在较大宽度时简并成两条曲线, 一定程度上抑制了带隙值的振荡; 并且铂原子边缘掺杂导致宽度系数N_a = 3p和3p + 1(p是一个整数)的几个较窄纳米带的带隙中出现杂质能级, 有效地降低了其过大的带隙值. 此外, 铂掺杂AGNR的能带结构对掺杂浓度不是很敏感, 从而降低了对实验精度的挑战. 本文的计算有利于推动石墨烯纳米带在纳米电子学方面的应用.     

Structural and electronic properties of endohedral doped SWCNTs: A DFT study

The structural and electronic properties of semiconductors (Si and Ge) and metal (Au and Tl) atoms doped armchair (n, n) and zigzag (n, 0); n=4–6, single wall carbon nanotubes (SWCNTs) have been studied using an ab-initio method. We have considered a linear chain of dopant atoms inside CNTs of different diameters but of same length. We have studied variation of B.E./atom, ionization potential, electron affinity and HOMO–LUMO gap of doped armchair and zigzag CNTs with diameter and dopant type. For armchair undoped CNTs, the B.E./atom increases with the increase in diameter of the tubes. For Si, Ge and Tl doped CNTs, B.E./atom is maximum for (6, 6) CNT whereas for Au doped CNTs, it is maximum for (5, 5) CNTs. For pure CNTs, IP decreases slightly with increasing diameter whereas EA increases with diameter. The study of HOMO–LUMO gap shows that on doping metallic character of the armchair CNTs increases whereas for zigzag CNTs semiconducting character increases. In case of zigzag tubes only Si doped (5, 0), (6, 0) and Ge doped (6, 0) CNTs are stable. The IP and EA for doped zigzag CNTs remain almost independent of tube diameter and dopant type whereas for doped armchair CNTs, maximum IP and EA are observed for (5, 5) tube for all dopants.     

Structural and magnetic properties of MoS2 monolayer zigzag nanoribbon doped by Ti,V, Cr,and Mn

In order to find new functions of monolayer MoS₂ in nanoelectronics or spin electronic devices, using spin-polarized density functional theory (DFT) calculations with on-site coulomb interaction (U), we investigated substitutional doping of Mo atoms of monolayer zigzag MoS₂ nanoribbon (ZZ-MoS₂ NR) by transition metals (TM) (where TM = Ti, V, Cr, Mn) at the Mo-edge, S-edge, and the middle of the NRs. The results of this study indicate the NR widened irrespective of the doped TM position and type, and the Mo-edge was found as the easiest substitutional position. For ZZ-MoS₂ NR doped by Mn, Cr or V atoms, the preferred magnetic coupling state is the edge atoms of S at the S-edge, exhibiting the same spin polarization with TM (named the FM1 state), attributing the NR with metallic magnetism. For Ti-doped monolayer ZZ-MoS₂ NR, in addition to the FM1 state, other preferred magnetic coupling state was observed in which the edge atoms of S at the S-edge exhibit the opposite spin polarization with that of Ti (named the FM2 state). Thus, the NR doped by Ti atom possesses metallic (FM1 state) or half-metallic (FM2 state) magnetism. The total magnetic moments of the ZZ-MoS₂ NR doped by TM follows a linear relationship as a function of the TM dopants (Mn, Cr, V, and Ti). Under $>4\text{\%}$ applied strain, the NR doped by Ti atom only presents the characteristics of half-metallic magnetism as the initial one in the FM2 state, and its total magnetic moment always remained 0 μB, i.e., it was not affected by the width of the NR. This study provides a rational route of tuning the magnetic properties of ZZ-MoS₂ NRs for their promising applications in nanoelectronics and spin electronic devices.     

Chirality effect of mechanical and electronic properties of monolayer MoS2 with vacancies

We present first principles theory calculations about the chirality and vacancy effects of the mechanical and electronic properties of monolayer MoS₂. In the uni-axial tensile tests, chirality effect of the mechanical properties is negligible at zero strain and becomes significant with the increasing strain, regardless of vacancies. The existence of vacancies decreases the Young's modulus and ultimate strength of the MoS₂ structure. During the uni-axial tensile tests, the band gap decreases with the increasing strain, regardless of chirality and vacancies. The band gap is reduced with the intermediate state brought by the existence of vacancies. No chirality effect can be observed on the band gap variations of perfect MoS₂. Chirality effect appears to the band gap variation of defected MoS₂ due to the local lattice relaxation near the vacancies.