Bismuthene, a bismuth analogue of graphene, has a moderate band gap, has a high carrier mobility, has a topological nontriviality, has a high stability at room temperature, has an easy transferability, and is very attractive for electronics, optronics, and spintronics. The electrical contact plays a critical role in an actual device. The interfacial properties of monolayer (ML) bismuthene in contact with the metal electrodes spanning a wide work function range in a field-effect transistor configuration are systematically studied for the first time by using both first-principles electronic structure calculations and quantum transport simulations. The ML bismuthene always undergoes metallization upon contact with the six metal electrodes owing to a strong interaction. According to the quantum transport simulations, apparent metal-induced gap states (MIGSs) formed in the semiconductor–metal interface give rise to a strong Fermi-level pinning. As a result, the ML bismuthene forms an n-type Schottky contact with Ir/Ag/Ti electrodes with electron Schottky barrier heights (SBHs) of 0.17, 0.22, and 0.25 eV, respectively, and a p-type Schottky contact with Pt/Al/Au electrodes with hole SBHs of 0.09, 0.16, and 0.38 eV, respectively. The effective channel length of the ML bismuthene transistors is also significantly reduced by the MIGSs. However, the MIGSs are eliminated and the effective channel length is increased when ML graphene is used as an electrode, accompanied by a small hole SBH of 0.06 eV (quasi-Ohmic contact). Hence, an insight is provided into the interfacial properties of the ML bismuthene–metal composite systems and a guidance is provided for the choice of metal electrodes in ML bismuthene devices.Keywords: Bismuthene; density functional theory; interfacial properties; metal electrode; quantum transport simulation; Schottky barrier;

It is unreliable to evaluate the Schottky barrier height (SBH) in monolayer (ML) 2D material field effect transistors (FETs) with strongly interacted electrode from the work function approximation (WFA) because of existence of the Fermi-level pinning. Here, we report the first systematical study of bilayer (BL) phosphorene FETs in contact with a series of metals with a wide work function range (Al, Ag, Cu, Au, Cr, Ti, Ni, and Pd) by using both ab initio electronic band calculations and quantum transport simulation (QTS). Different from only one type of Schottky barrier (SB) identified in the ML phosphorene FETs, two types of SBs are identified in BL phosphorene FETs: the vertical SB between the metallized and the intact phosphorene layer, whose height is determined from the energy band analysis (EBA); the lateral SB between the metallized and the channel BL phosphorene, whose height is determined from the QTS. The vertical SBHs show a better consistency with the lateral SBHs of the ML phosphorene FETs from the QTS compared than that of the popular WFA. Therefore, we develop a better and more general method than the WFA to estimate the lateral SBHs of ML semiconductor transistors with strongly interacted electrodes based on the EBA for its BL counterpart. In terms of the QTS, n-type lateral Schottky contacts are formed between BL phosphorene and Cr, Al, and Cu electrodes with electron SBH of 0.27, 0.31, and 0.32 eV, respectively, while p-type lateral Schottky contacts are formed between BL phosphorene and Pd, Ti, Ni, Ag, and Au electrodes with hole SBH of 0.11, 0.18, 0.19, 0.20, and 0.21 eV, respectively. The theoretical polarity and SBHs are in good agreement with available experiments. Our study provides an insight into the BL phosphorene–metal interfaces that are crucial for designing the BL phosphorene device.Keywords: bilayer phosphorene transistor; energy band; interface; quantum transport simulation; Schottky barrier;

Contact engineering is a possible solution to decrease the pervasive Schottky barrier in a two dimensional (2D) material transistor with bulk metal electrodes. In this paper, two kinds of typical van der Waals (vdW)-type electrical contacts (a 2D metal contact and a 2D material/bulk metal hybrid contact) in monolayer (ML) black phosphorus (BP) transistors are investigated by ab initio energy band calculations and quantum transport simulations. Compared with the traditional bulk metal Ni contact, the gate electrostatic control is significantly improved by using both 2D graphene and borophene electrodes featuring a decrease of 30–50% in the subthreshold swing and an increase by a factor of 4–7 in the on-state current due to the depressed metal induced gap states and reduced screening of the 2D metal electrodes to the gate. In contrast, graphene insertion between the Ni electrode and ML BP shows only a slight improvement in the gate electrostatic control ability and BN insertion shows almost no improvement. The higher efficiency using the 2D metal contact than the 2D material/bulk metal hybrid contact in improving the ML BP FET device performance also provides helpful guidance in the selection of vdW-type electrical contacts of other 2D transistors.

The interfacial properties of β12 phase borophene contacts with other common two-dimensional materials (transition-metal dichalcogenides, group IV-enes and group V-enes) have been systematically studied using a density functional theory (DFT) method. The zero tunneling barrier is found for all of the investigated β12 phase borophene contacts except for the case of β12 borophene/graphene. The chemically reactive properties and high work function (4.9 eV) of the stable β12 borophene lead to the formation of Ohmic contacts with silicene, germanene, stanene, black phosphorene, arsenene and antimonene. The advantage of the zero tunnel barrier remains when changing the borophene from the β12 phase to the Δ phase. Therefore, a high carrier injection rate is expected in these borophene contacts. Our study provides guidance on borophene for future two dimensional materials based device designs.

Experimental two-dimensional (2D) black phosphorus (BP) transistors typically appear in the form of Schottky barrier field effect transistors (SBFETs), but their performance limit remains open. We investigate the performance limit of monolayer BP SBFETs in the sub-10 nm scale by using ab initio quantum transport simulations. The devices with 2D graphene electrodes are apparently superior to those with bulk Ti electrodes due to their smaller and tunable Schottky barrier heights and the absence of metal induced gap states in the channels. With graphene electrodes, the performance limit of the sub-10 nm monolayer BP SBFETs outperforms the monolayer MoS2, carbon nanotube, and advanced silicon transistors and even can meet the requirements of both high performance and low power logic applications of the next decade in the latest International Technology Roadmap for Semiconductors. It appears that the ML BP SBFETs have the best intrinsic device performance among the reported sub-10 nm 2D material SBFETs.Keywords: 2D materials; ab initio calculations; electronic transport; ITRS; nanoelectronics;

Valleytronics is a promising paradigm to explore the emergent degree of freedom for charge carriers on the energy band edges. Using ab initio calculations, we reveal that the honeycomb boron nitride (h-BN) monolayer shows a pair of inequivalent valleys in the vicinities of the vertices of hexagonal Brillouin zone even without the protection of the C3 symmetry. The inequivalent valleys give rise to a 2-fold degree of freedom named the valley pseudospin. The valley pseudospin with a tunable bandgap from deep ultraviolet to far-infrared spectra can be obtained by doping h-BN monolayer with carbon atoms. For a low-concentration carbon periodically doped h-BN monolayer, the subbands with constant valley Hall conductance are predicted due to the interaction between the artificial superlattice and valleys. In addition, the valley pseudospin can be manipulated by visible light for high-concentration carbon doped h-BN monolayer. In agreement with our calculations, the circularly polarized photoluminescence spectra of the B0.92NC2.44 sample show a maximum valley-contrasting circular polarization of 40% and 70% at room temperature and 77 K, respectively. Our work demonstrates a class of valleytronic materials with a controllable bandgap.Keywords: ab initio calculation; BNC monolayer; photoluminescence spectra; pseudospin; valley polarization; Valleytronics;

Silicene, a silicon analogue of graphene, has attracted increasing attention during the past few years. As early as in 1994, the possibility of stage corrugation in the Si analogs of graphite had already been theoretically explored. But there were very few studies on silicene until 2009, when silicene with a low buckled structure was confirmed to be dynamically stable by ab initio calculations. In spite of the low buckled geometry, silicene shares most of the outstanding electronic properties of planar graphene (e.g., the “Dirac cone”, high Fermi velocity and carrier mobility). Compared with graphene, silicene has several prominent advantages: (1) a much stronger spin–orbit coupling, which may lead to a realization of quantum spin Hall effect in the experimentally accessible temperature, (2) a better tunability of the band gap, which is necessary for an effective field effect transistor (FET) operating at room temperature, (3) an easier valley polarization and more suitability for valleytronics study. From 2012, monolayer silicene sheets of different superstructures were successfully synthesized on various substrates, including Ag(1 1 1), Ir(1 1 1), ZrB2(0 0 0 1), ZrC(1 1 1) and MoS2 surfaces. Multilayer silicene sheets have also been grown on Ag(1 1 1) surface. The experimental successes have stimulated many efforts to explore the intrinsic properties as well as potential device applications of silicene, including quantum spin Hall effect, quantum anomalous Hall effect, quantum valley Hall effect, superconductivity, band engineering, magnetism, thermoelectric effect, gas sensor, tunneling FET, spin filter, and spin FET, etc. Recently, a silicene FET has been fabricated, which shows the expected ambipolar Dirac charge transport and paves the way towards silicene-based nanoelectronics. This comprehensive review covers all the important theoretical and experimental advances on silicene to date, from the basic theory of intrinsic properties, experimental synthesis and characterization, modulation of physical properties by modifications, and finally to device explorations.

Recently, phosphorene electronic and optoelectronic prototype devices have been fabricated with various metal electrodes. We systematically explore for the first time the contact properties of monolayer (ML) phosphorene with a series of commonly used metals in a transistor by using both ab initio electronic structure calculations and more reliable quantum transport simulations. ML phosphorene undergoes a metallization under the checked metals, and the metallized ML phosphorenes have an unnegligible coupling with channel ML phosphorene. ML phosphorene forms an n-type Schottky contact with Au, Cu, Cr, Al, and Ag electrodes and a p-type Schottky contact with Ti, Ni, and Pd electrodes upon inclusion of such a coupling. The calculated Schottky barrier heights are in good agreement with the available experimental data with Ni and Ti as electrodes. Our findings not only provide an insight into the ML phosphorene–metal interfaces but also help in ML phosphorene based device design.

Formation of low-resistance metal contacts is the biggest challenge that masks the intrinsic exceptional electronic properties of two dimensional WSe2 devices. We present the first comparative study of the interfacial properties between monolayer/bilayer (ML/BL) WSe2 and Sc, Al, Ag, Au, Pd, and Pt contacts by using ab initio energy band calculations with inclusion of the spin–orbital coupling (SOC) effects and quantum transport simulations. The interlayer coupling tends to reduce both the electron and hole Schottky barrier heights (SBHs) and alters the polarity for the WSe2–Au contact, while the SOC chiefly reduces the hole SBH. In the absence of the SOC, the Pd contact has the smallest hole SBH. Dramatically, the Pt contact surpasses the Pd contact and becomes the p-type ohmic or quasi-ohmic contact with inclusion of the SOC. Therefore, p-type ohmic or quasi-ohmic contact exists in WSe2–metal interfaces. Our study provides a theoretical foundation for the selection of favorable metal electrodes in ML/BL WSe2 devices.

Germanene, a germanium analogue of graphene and silicene, has been synthesized on metal substrates. It is predicted that the intrinsic germanene has a Dirac cone in its band structure, just like graphene and silicene. Using first-principles calculations, we investigate the geometrical structures and electronic properties of germanene on the Ag, Au, Cu, Al, Pt and Ir substrates. The Dirac cone of germanene is destroyed on the Al, Pt and Ir substrates but preserved on the Ag and Au substrates with a slight band hybridization. The upper part of the Dirac cone is destroyed for germanene on the Cu substrate while the lower part remains preserved. By contrast, the Dirac cone is always destroyed for silicene on these metal substrates because of a strong band hybridization. Our study suggests that it is possible to extract the intrinsic properties of germanene on the Ag and Au substrates although it appears impossible for silicene on these two substrates.

Semiconducting single-layer (SL) and few-layer MoS2 have a flat surface, free of dangling bonds. Using density functional theory coupled with non-equilibrium Green's function method, we investigate the spin-polarized transport properties of Co/2D MoS2/Co and Ni/2D MoS2/Ni junctions with MoS2 layer numbers of N = 1, 3, and 5. Well-defined interfaces are formed between MoS2 and metal electrodes. The junctions with a SL MoS2 spacer are almost metallic owing to the strong coupling between MoS2 and the ferromagnets, while those are tunneling with a few layer MoS2 spacer. Both large magnetoresistance and tunneling magnetoresistance are found when fcc or hcp Co is used as an electrode. Therefore, flat single- and few-layer MoS2 can serve as an effective nonmagnetic spacer in a magnetoresistance or tunneling magnetoresistance device with a well-defined interface.

Monolayer (ML) transition-metal dichalcogenides are considered as promising channel materials in next-generation transistors. Using ab initio energy band calculations and more reliable ab initio quantum transport simulations, we study the interfacial properties of ML MoSe2–metal interfaces (metals = Al, Ag, Pt, Cr, Ni, and Ti). Weak or medium adsorption is found between ML MoSe2 and the Al, Ag, and Pt surfaces with the band structure of ML MoSe2 preserved, while strong adsorption is found between ML MoSe2 and the Ni, Ti, and Cr surfaces with the band structure of ML MoSe2 destroyed. The two methods give similar polarity and height of Schottky barriers for ML MoSe2 with Al, Ag, Pt, and Ti electrodes. ML MoSe2 forms an n-type Schottky contact with Ag, Ti, and Al electrodes with electron Schottky barrier heights (SBH) of 0.25, 0.29, and 0.56 eV, respectively, and a p-type Schottky contact with Pt electrode with hole SBH of 0.78 eV according to ab initio quantum transport simulations. Our study offers a guidance for the choices of suitable metal electrodes in ML MoSe2 devices.

Graphdiyne was prepared on a metal surface, and the preparation of devices using it inevitably involves its contact with metals. Using density functional theory with dispersion correction, we systematically studied, for the first time, the interfacial properties of graphdiyne that is in contact with a series of metals (Al, Ag, Cu, Au, Ir, Pt, Ni, and Pd). Graphdiyne forms an n-type Ohmic or quasi-Ohmic contact with Al, Ag, and Cu, while it forms a Schottky contact with Pd, Au, Pt, Ni, and Ir (at the source/drain-channel interface), with high Schottky barrier heights of 0.21, 0.46 (n-type), 0.30, 0.41, and 0.46 (p-type) eV, respectively. A graphdiyne field effect transistor (FET) with Al electrodes was simulated using quantum transport calculations. This device exhibits an on–off ratio up to 104 and a very large on-state current of 1.3 × 104 mA mm−1 in a 10 nm channel length. Thus, a new prospect has opened up for graphdiyne in high performance nanoscale devices.

A large bulk band gap is critical for the application of quantum spin Hall (QSH) insulators or two-dimensional (2D) topological insulators (TIs) in spintronic devices operating at room temperature (RT). On the basis of first-principles calculations, we predicted a group of 2D TI BiX/SbX (X=H, F, Cl and Br) monolayers with extraordinarily large bulk gaps from 0.32 eV to a record value of 1.08 eV. These giant-gaps are entirely due to the result of the strong spin-orbit interaction related to the px and py orbitals of the Bi/Sb atoms around the two valleys K and K′ of the honeycomb lattice, which is significantly different from that consisting of the pz orbital as in graphene/silicene. The topological characteristic of BiX/SbX monolayers is confirmed by the calculated nontrivial Z2 index and an explicit construction of the low-energy effective Hamiltonian in these systems. We demonstrate that the honeycomb structures of BiX monolayers remain stable even at 600 K. Owing to these features, the giant-gap TIs BiX/SbX monolayers are an ideal platform to realize many exotic phenomena and fabricate new quantum devices operating at RT. Furthermore, biased BiX/SbX monolayers become a quantum valley Hall insulator, exhibiting valley-selective circular dichroism.

By using first-principles calculations, we predict that a sizable band gap can be opened at the Dirac point of silicene without degrading silicene's electronic properties with n-type doping by Cu, Ag, and Au adsorption, p-type doping by Ir adsorption, and neutral doping by Pt adsorption. A silicene p–i–n tunneling field effect transistor (TFET) model is designed by the adsorption of different transition metal atoms on different regions of silicene. Quantum transport simulations demonstrate that silicene TFETs have an on–off ratio of 103, a small sub-threshold swing of 77 mV dec−1, and a large on-state current of over 1 mA μm−1 under a supply voltage of about 1.7 V.

•Silicene on Ag(1 1 1) substrate is studied by using first-principles calculations.•The band structures of silicene on Ag(1 1 1) substrate are provided.•The Dirac cone of silicene on Ag(1 1 1) substrate is severely destructed.By using first-principles calculations, we systematically investigated several observed phases of silicene on Ag(1 1 1) substrates ((2×2)silicene/(7×7)Ag(1 1 1), (7×7)silicene/(23×23)Ag(1 1 1), (7×7)silicene/(13×13)Ag(1 1 1)) and their electronic structures. We find that the original Dirac cone of silicene is about 1.5–1.7 eV deeply below the Fermi level and severely destroyed by the band hybridization between silicene and Ag in all the examined phases. Thus, silicene synthesized on Ag(1 1 1) substrates could not preserve its excellent electronic property and new method is needed to develop in synthesizing silicene with its Dirac cone surviving.

•A band gap is opened at the Dirac point in alkali metal atoms-covered germanene.•The band gap is induced by the sublattice/bond symmetry breaking of germanene.•The largest global band gap we obtain is 0.26 eV.•The effective masses of charge carriers near the Dirac point are very small.•Germanene can be a candidate of effective transistors channel upon adsorption.Opening a sizable band gap in the zero-gap germanene without heavy loss of carrier mobility is a key issue for its application in nanoelectronic devices such as high-performance field effect transistors (FETs) operating at room temperature. Using the first-principles calculations, we find a band gap is opened at the Dirac point in germanene by single-side adsorption of alkali metal (AM) atoms. This band gap is tunable by varying the coverage and the species of AM atoms, ranging from 0.02 to 0.31 eV, and the maximum global band gap is 0.26 eV. Since the effective masses of electrons and holes in germanene near the Dirac point after surface adsorption (ranging from 0.005 to 0.106me) are small, the carrier mobility is expected not to degrade much. Therefore germanene is a potential candidate of effective FET channel operating at room temperature upon surface adsorption.

•Semihydrogenation can induce half-metallicity in silicene.•The transport properties of semihydrogenated silicene were studied by ab initio quantum transport theory.•A high on/off current ratio of 106 was obtained in the single-gated semihydrogenated silicene device.•A spin-polarized current was observed in the studied device, and the spin-filter efficiency can reach 100% at a voltage of 1.9 V.The first-principles calculations indicate that the semihydrogenated silicene (H@Silicene) is a ferromagnetic semiconductor. By the ab initio quantum transport theory, we study for the first time the transport properties of H@Silicene with pristine silicene as electrodes. A high on/off current ratio of 106 is obtained in the single-gated H@Silicene device. More importantly, a spin-polarized current can be generated. The spin-filter efficiency increases with the gate voltage and reaches 100% at a voltage of 1.9 V. Our results suggest that a gate voltage can induce half-metallicity in H@Silicene. Therefore, a new avenue is opened for H@Silicene in application of spintronics.

We investigated the dependence of the transport properties of heavily doped intratube single-walled carbon nanotube (SWCNT) p–i–n junctions on the length of the intrinsic region by using empirical self-consistent quantum transport simulations. When the length of the intrinsic region is scaled from a few angstroms to over 10 nanometers, the SWCNT p–i–n junction evolves from a tunneling diode with a large negative rectification and large negative differential resistance to one with a large positive rectification (like a conventional positive rectifying diode). The critical length of the intrinsic length is about 8.0 nm. Therefore, one can obtain nanoscale diodes of different performance types by changing the intrinsic region length.

It is well known that there is spin-filter efficiency (SFE) of a linear carbon atomic chain. In this article, we examine the quantum transport calculations of a linear carbon atomic chain connected to two half-planar graphene electrodes by using the first-principle method and reveal for the first time that sign of the SFE of such carbon atomic chain is changeable with the bias. This makes the carbon atomic chains attractive to potential application of spintronics.Highlights► We examine for the first time the quantum transport properties of a linear carbon atomic chain connected to two half-planar graphene electrodes at finite bias by using the first-principle method. ► We reveal that sign of the SFE of such carbon atomic chain is changeable with the bias. ► This property makes carbon atomic chains attractive to potential application of spintronic logic circuit.

Graphdiyne is a newly discovered 2D carbon allotrope with many special features. Using density functional theory plus van der Waals (vdW) density functional, we investigate the structural, electronic, and optical properties of several possible graphdiyne bulk structures. We find that bulk graphdiyne can be either a semiconductor or a metal, depending on its stacking configuration. The interlayer vdW force red shifts the optical absorption peaks of bulk graphdiyne relative to those of the monolayer, and spectra of different stackings display notable differences in the energy range below 1 eV. Finally, combining with previous electrical and optical experiments, we identify the structure of the recently synthesized graphdiyne film.

By using ab initio calculations, we predict that a vertical electric field is able to open a band gap in semimetallic single-layer buckled silicene and germanene. The sizes of the band gap in both silicene and germanene increase linearly with the electric field strength. Ab initio quantum transport simulation of a dual-gated silicene field effect transistor confirms that the vertical electric field opens a transport gap, and a significant switching effect by an applied gate voltage is also observed. Therefore, biased single-layer silicene and germanene can work effectively at room temperature as field effect transistors.

Opening a tunable and sizable band gap in single-layer graphene (SLG) without degrading its structural integrity and carrier mobility is a significant challenge. Using density functional theory calculations, we show that the band gap of SLG can be opened to 0.16 eV (without an electric field) and 0.34 eV (with a strong electric field) when properly sandwiched between two hexagonal boron nitride single layers. The zero-field band gaps are increased by more than 50% when the many-body effects are included. The ab initio quantum transport simulation of a dual-gated field effect transistor (FET) made of such a sandwich structure reveals an electric-field-enhanced transport gap, and the on/off current ratio is increased by a factor of 8.0 compared with that of a pure SLG FET. The tunable and sizeable band gap and structural integrity render this sandwich structure a promising candidate for high-performance SLG FETs.

By performing first-principle quantum transport calculations, we predict a giant magnetoresistance in zigzag silicene nanoribbons (ZSiNRs) connecting two semi-infinite silicene electrodes through switch of the edge spin direction of ZSiNRs. Spin-filter efficiency of both the antiferromagnetic and ferromagnetic ZSiNRs is sign-changeable with the bias voltage. Therefore, potential application of silicene in spintronics devices is suggested.

Interest in the two-dimensional MoS2 material is consistently increasing because of its many potential applications, in particular in the next-generation nanoelectronic devices. By means of density functional theory computations, we systematically examined the effect of vertical electric field on the electronic structure of MoS2 bilayer. The bandgaps of the bilayer MoS2 monotonically decrease with an increasing vertical electric field. The critical electric fields, at which the semiconductor-to-metal transition occurs, are predicted to be in the range of 1.0–1.5 V/Å depending on different stacked conformations. Ab initio quantum transport simulations of a dual-gated bilayer MoS2 channel clearly confirm that the vertical electric field continuously manipulates the transmission gap of bilayer MoS2.

We imposed screwing operation to a metallic ferromagnetic zigzag-edged graphene nanoribbon (ZGNR) with a narrow width and a finite length, and the polarized charge transport is investigated by using Nonequilibrium Green's function in combination with density functional theory. The current are nearly completely suppressed when the ZGNRs are overturned. Inspiringly, this metal-to-semiconductor transition tuned by screwing operation is reversible. Hence our investigation brings forward a novel electromechanical switch, and such a switch is equivalent to a spin valve without resort to an external magnetic field.Graphical abstractWe propose a novel electromechanical switch via twisting a metallic ferromagnetic symmetric zigzag-edged graphene nanoribbon. The switch realizes a spin valve function in a flexible nanoribbon by twisting in addition to change in the magnetic field direction.Highlights► Twisting a metallic ferromagnetic ZGNR is a possible way to make an electromechanical switch. ► Twisting would cause orbital symmetry mismatch of the two leads in a symmetric ferromagnetic ZGNR. ► Zero transmission gap is thus generated, and hence the current is suppressed. ► The switch series can be obtained via multiply overturning the nanoribbon.

Layered tungsten disulfide nanostructures are of both fundamental and technological interest. The widths of currently synthesized WS2 ribbons are in the microscale. By using single-walled carbon nanotubes and double-walled carbon nanotubes as templates, we fabricate WS2 nanoribbons with smooth zigzag edges and uniform widths down to 1–3 nm and layer numbers down to 1–3, dependent on the nanotube diameter. Although bulk WS2 is a nonmagnetic semiconductor, the ultra-narrow free-standing zigzag-edged WS2 nanoribbons turn out to be magnetic or nonmagnetic metals depending on the edge passivation way according to our first-principles calculations, whereas the ultra-narrow armchair-edged WS2 nanoribbons remain nonmagnetic semiconductors with a narrow gap.

Nanoribbons are suggested to be among the most promising candidates being considered as building blocks in future electronics. In this study, we use density functional calculations to examine the structures and electronic properties of BC2N nanoribbons with bare zigzag-shaped edges (zz-BC2NNRs). Four different types of atomistic edge configurations are considered, including ribbons terminated with two C edges, B and N edges, B an C edges, and C and N edges. We find the existence of half-metallicity in the ground state of the zz-BC2NNRs with two bare C edges and with bare C and N edges. The other two configurations of the zz-BC2NNRs can be either semiconducting or metallic, depending on the specific configuration. We also find that the stability of the zz-BC2NNRs are largely dependent on ribbon width. The zz-BC2NNRs become energetically more stable when the nanoribbon width exceeds 3.3 nm. It is interesting to find that half-metallic zz-BC2NNRs with a width of 0.7 nm are thermodynamically more stable than either metallic or semiconducting counterparts. Therefore, the possibility of synthesizing half-metallic zz-BC2NNRs exists.

Structural, electronic, and magnetic properties of the Fe-, Co-, Ni-, and V-intercalated graphene bilayer sandwich (denoted by C2|M|C2, M = Fe, Co, Ni, and V) and graphene on hexagonal boron nitride (h-BN) bilayer sandwich (denoted by C2|M|BN, M = Fe, Co, Ni, and V) are studied by using density functional theory method. We find that both the graphene bilayer and graphene-h-BN bilayer in all the C2|M|C2 and C2|M|BN sandwiches favor AB stacking over AA stacking mode. The Fe, Co, and Ni atoms prefer to be located over the center of C–C bonds whereas V atoms prefer to be located above the C atoms on graphene, and they all prefer to be located above the N atoms on h-BN sheet, regardless of the stacking mode. The C2|Fe|C2, C2|Co|C2, C2|Fe|BN, and C2|Co|BN sandwiches of AB stacking are all ferromagnetic metals with the spin polarization of 86%, 67%, 65%, and 46% at the Fermi level, respectively. By contrast, both C2|Ni|C2 and C2|Ni|BN sandwiches of AB stacking are nonmagnetic semiconductors with bandgaps of 0.64 and 0.23 eV, respectively, which provide a novel strategy of opening a bandgap of graphene. From the quantum transport calculation, we obtain a giant room-temperature magnetoresistance of ∼200% in the spin valve device based on AB stacking C2|Fe|C2 sandwich.

We study the nonresonant Raman scattering of armchair and zigzag graphene nanoribbons (GNRs) using density functional perturbation theory. We find that, in both GNR types, the Raman spectrum is extremely polarized along the ribbon axis direction with the scattering intensity being over 102 times greater than those of the other polarizations because of the geometrical confinement. Along the dominant polarization direction, the scattering intensity and frequency oscillate strongly with the ribbon width in the armchair GNRs, while the scattering intensity initially increases and then decreases with the ribbon width and the frequency monotonically changes with the ribbon width in the zigzag GNRs. Such a difference is closely associated with the different width dependences of band structure between the two types of GNRs.

We provide the first systematic ab initio investigation of the possibility to create a band gap in few-layer graphene (FLG) via a perpendicular electric field. Bernal (ABA) and arbitrarily stacked FLG remain semimetallic, but rhombohedral (ABC) stacked FLG demonstrates a variable band gap. The maximum band gap in ABC stacked FLG decreases with increasing layer number and can be fitted by the relationship Δmax = 1/(2.378 + 0.521N + 0.035N2) eV. The effective masses of carriers over a wide range around the maximum band gap point in ABC stacked FLG are comparable with that in AB bilayer graphene under zero field. It is therefore possible to fabricate an effective field effect transistor operating at room temperature with high carrier mobility out of ABC stacked FLG.

We present the first transport property investigation of a-few-nanometer-long armchair graphene nanoribbon (AGNR) p–n junctions by using first-principles method. Intriguingly, family dependent rectification is observed. To be specific, traditional rectification effect in the forward direction is observed in the AGNR p–n junctions with 3n and 3n + 2 widths, whereas reverse rectification effect is observed in the AGNR p–n junctions with 3n + 1 width. The analysis of the spatial distribution of molecular projected self-consistent Hamiltonian eigenstates and the projected density of states give an insight into the observed results.

It has been well established that the electrical resistance of metal is insensitive to gate voltage and unsuitable for making field effect transistors. However, we find that telescoping pristine double-walled metallic carbon nanotubes are extremely sensitive to gate voltage with an on/off ratio up to 104 based on the first principles quantum transport calculations. This remarkable feature is closely related to the antiresonances in the transmission spectra. Besides, robust negative differential resistance effects are also found in the same device.

Using first-principles calculations, we explore the possibility of functionalized graphene as a high-performance two-dimensional spintronics device. Graphene functionalized with O on one side and H on the other side in the chair conformation is found to be a ferromagnetic metal with a spin-filter efficiency up to 54% at finite bias. The ground state of graphene semifunctionalized with F in the chair conformation is an antiferromagnetic semiconductor, and we construct a spin-valve device from it by introducing a magnetic field to stabilize its metallic ferromagnetic state. The resulting room-temperature magnetoresistance is up to 2200%, which is 1 order of magnitude larger than the available experimental values.Keywords: first-principles calculations; functionalized graphene; magnetoresistance; spin-filter efficiency; spintronics;

Quasi-one-dimensional nanotubes and two-dimensional nanoribbons are two fundamental forms of nanostructures, and integrating them into a novel mixed low-dimensional nanomaterial is fascinating and challenging. We have synthesized a stable mixed low-dimensional nanomaterial consisting of MoS2 inorganic nanoribbons encapsulated in carbon nanotubes (which we call nanoburritos). This route can be extended to the synthesis of nanoburritos composed of other ultranarrow transition-metal chalcogenide nanoribbons and carbon nanotubes. The widths of previously synthesized MoS2 ribbons are greater than 50 nm, while the encapsulated MoS2 nanoribbons have uniform widths down to 1−4 nm and layer numbers down to 1−3, depending on the nanotube diameter. The edges of the MoS2 nanoribbons have been identified as zigzag-shaped using both high-resolution transmission electron microscopy and density functional theory calculations.

Using density functional theory and nonequilibrium Green’s function method, we construct organometallic nanowires that consist of Fe or V atoms sandwiched between composite molecules (Cp*FeCp*, where Cp* is C5(CH3)5). For the first time, we demonstrate that half-metallicity, negative differential resistance, and sign-reversible high spin-filter capability can coexist remarkably in one organometallic nanowire (FeCp* wire). This renders FeCp* wire promising in electronics and spintronics.

We propose a novel single-molecule organic field effect transistor (FET) fabricated via covalent functionalization of an individual metallic single-walled carbon nanotube (SWCNT). The transfer characteristic of this FET is calculated by using ab initio quantum transport calculations. Because of the significantly reduced screening effect of the quasi-one-dimensional electrode and seamless connection between the electrode and scattering region, the optimized device shows an excellent overall performance over the experimental single-molecule organic field effect transistors. This renders functionalized metallic SWCNTs a promising candidate for a high-performance single-molecule organic field effect transistor.

Structural, electronic, and transport properties of Gd/Eu atomic chains encapsulated in single-walled carbon nanotubes (SWCNTs) are studied by using first-principles density functional theory and the nonequilibrium Green’s function method. We find that the linear single-atom Gd and Eu chains occupy an off-centered position when encapsulated in the (8,0), (10,0), and (6,6) SWCNTs and considerable electrons are transferred from the Gd and Eu chains to the SWCNTs. The resulting composites are all ferromagnetic metals, with the conductivity significantly larger than those of the pristine SWCNTs and the free-standing Gd/Eu linear single-atom atomic chains. The spin polarization of the finite Gd linear single-atom chain at the Fermi level is 67% when encapsulated in the (8,0) SWCNT from the quantum transport calculation.

We present for the first time the linearly polarized vibrational infrared spectra of edge-hydrogenated armchair and zigzag graphene nanoribbons (GNRs) through first-principles calculations. We find that there exists a prominent and width-insensitive peak in both GNRs for out-of-plane polarization, which can be used as a benchmark in measurements. In the major absorption regions of all polarizations, we observe noticeable differences between the two GNR types. In armchair GNRs, the spectra for in-plane polarization oscillate with the ribbon width, while in zigzag GNRs the dispersion relationship bears close resemblance with that of graphene. We also discover a special peak that reflects the mirror symmetry between the two edges of zigzag GNRs. Finally, the quenching of magnetism and oxygen-passivation are revealed to remarkably influence the infrared spectra. Our work provides a new insight into GNR fundamental property and is expected to help confront the current problem of edge structure and magnetism identification of GNRs.

A magnetic ground state is revealed for the first time in zigzag-edged carbon nanoscrolls (ZCNSs) from spinunrestricted density functional theory calculations. Unlike their flat counterpart—zigzag-edged carbon nanoribbons, which are semiconductors with spin-degenerate electronic structure—ZCNSs show a variety of magnetic configurations, namely spin-selective semiconductors, metals, semimetals, quasi-half-metals, and half-metals. To the best of our knowledge, this is the first discovery of quasi-half-metals and half-metals in a pure hydrocarbon without resort to an external electric field. In addition, we calculated the spin-dependent transportation of the semiconducting ZCNSs with 12 and 20 zigzag chains, and found that they are 13% and 17% at the Fermi level, respectively, suggesting that ZCNS can be an effective spin filter.

We calculate the electronic structures of the fully bare and half-bare zigzag-edged boron nitride nanoribbons by using density functional theory. We find that the ground states of both the fully bare boron nitride nanoribbons and the boron nitride nanoribbons with a bare N edge and a H-terminated B edge are half-metallic. The alignment of the spin at the bare B edge is antiferromagnetic, while that at the bare N edge is ferromagnetic in the ground states of both the fully bare and half-bare zigzag-edged boron nitride nanoribbons. The H-terminated B or N edge of the half-bare zigzag-edged boron nitride nanoribbon exhibits no magnetism.

We calculated the optical absorption spectrum response of single-walled carbon nanotubes under charge doping by using density functional theory. We find that the spectrum responses can be generally divided into two categories: one is similar to those obtained from the graphene zone-folding and rigid-band model, while the other deviates from the expectation and shows several special features. Our analysis reveals that the doping type and curvature effects play the primary role. Finally, we argue that the present results will probably prevail in more elaborate methods and other similar nanotubes.

We investigated the geometric and electronic properties and doping efficiency of phosphorus-doped zinc oxide nanowires along the [0001] direction using the first-principle calculation. For isolated point defects, the substitutional and vacancy defects prefer the edge of the nanowire, the Zn interstitial defects favor the tetrahedral site, and the most stable P and O interstitial defects have a dumbbell-like structure. A complex defect of PZn−2VZn is formed by the combination of a substitutional P at a Zn site (PZn) and two Zn vacancies (VZn), and it prefers the edge site. We found that PZn defects could be effective donors while VZn and PZn−2VZn defects could be effective acceptors. The PZn defects have low formation energies and high concentrations under the Zn- and P-rich conditions, and they can lead to n-type ZnO nanowires. The VZn defects have low formation energies and high concentrations under the O- and P-rich conditions. The VZn defects can greatly suppress the PZn defects. VZn and PZn−2VZn defects can lead to p-type ZnO nanowires under the O- and P-rich conditions.

By using the density functional theory, we find that organometallic multidecker sandwich clusters V2n+1Cp2n+2, Vn(FeCp2)n+1 (Cp = cyclopentadienyl), and V2nAntn+1 (Ant = anthracene) may have linear structures, and their total magnetic moments generally increase with the cluster size. The one-dimensional (VCp)∞, (VBzVCp)∞ (Bz = benzene), and (V2Ant)∞ wires are predicted to be ferromagnetic half-metals, while the one-dimensional (VCpFeCp)∞ wire is a ferromagnetic semiconductor. The spin transportation calculations show that the finite V2n+1Cp2n+2 and Vn(FeCp2)n+1 sandwich clusters coupled to gold electrodes are nearly perfect spin-filters.

We present a simple approach of tuning the hole doping level of iodine-doped single-walled carbon nanotubes by adjusting temperature, utilizing the structural conversion of iodine species encapsulated in SWNTs.

The selective adsorption of NO2+ cation on single-walled carbon nanotubes (SWNTs) is systemically studied by using density functional theory calculations. It is found that the adsorption energy of cations on SWNTs depends on the concentration of cations and the diameter and the electronic structure of SWNTs. The binding strength of NO2+ on each SWNT increases monotonically as the concentration of NO2+ decreases, undergoing a change from endothermic to exothermic reaction. Generally speaking, the binding of NO2+ on SWNTs becomes weaker as the diameter increases. In the medium-diameter region (9 < d < 11 Å), NO2+ prefers to interact with metallic SWNTs (m-SWNTs) rather than semiconducting SWNTs (s-SWNTs) at the same concentration of NO2+. In the small-diameter region (d < 9 Å), the binding of NO2+ is nearly independent of metallicity, but it is stronger than that of on the medium-diameter s-SWNTs. In the large-diameter region (d > 11 Å), the dependence of adsorption on the electronic structure is complicated, but the binding of NO2+ is weaker than that on the medium-diameter s-SWNTs. Our results are in agreement with the experimental report that the small-diameter m- and s-SWNTs and the medium-diameter m-SWNTs are etched away by NO2+ while the medium-diameter s-SWNTs and the large-diameter m- and s-SWNTs are intact.

By using the density functional theory method, we calculate the 13C NMR isotropic chemical shifts of the semiconducting and semimetallic infinite single-walled carbon nanotubes (SWNTs). We find that the 13C chemical shifts of SWNTs with the diameter smaller than 1.4 Å can be classified into two distinct groups according to their electronic structures: the semiconducting group and the semimetallic group. The chemical shifts of the semiconducting group decrease monotonously with the increasing nanotube diameter, and are 0−12 ppm strikingly larger than those of their semimetallic counterparts in the typical diameter range (1.05 ± 0.2 nm) of SWNTs produced by the common high-pressure CO decomposition method (HiPCO). The chemical shifts of the two groups overlap around the diameter of 1.4 Å. Then the chemical shift of the semimetallic group becomes larger than that of the similar-sized semiconducting group as the diameter is larger than 1.4 Å. The chemical shifts of the four examined helical SWNTs are very close to those of the zigzag SWNTs with similar diameters and electronic structures.

Using first principle calculations, we find that hole doping remarkably enhances adsorption strength of naphthalene on single-wall carbon nanotubes (SWNTs). The enhancement in adsorption strength is more significant for the larger-sized SWNTs, and the adsorption of naphthalene on the larger-sized (10,0) SWNTs becomes stronger than that on the smaller-sized (5,5) SWNTs at a higher doping level. However, the enhancement in adsorption strength appears insensitive to the electronic structure of SWNTs. For similar-sized SWNTs, hole doping does not reverse the selective adsorption of naphthalene towards metallic SWNTs versus their semiconducting counterparts. This is in sharp contrast to NH2CH3 adsorption, where hole doping can reverse the selectivity of NH2CH3 towards metallic SWNTs versus similar-sized semiconducting SWNTs.

Water soluble Gd-based metallofullerenes have potential application as magnetic resonance imaging contrast agent due to its higher proton relaxivities and lower toxicity. In this Letter, we have investigated the structural and electronic properties of Gd@C60, which is the most abundantly produced Gd-based metallofullerene in carbon arc process, by using all-electron relativistic density functional theory. It is found that the Gd3+ ion is bonded to a hexagonal ring of C60 by electrostatic interaction. The total spin multiplicity of Gd@C60 is S = 7. The Gd atomic orbitals are hybridized with the C60 molecular orbitals.We have investigated the structural and electronic properties of Gd@C60 by using all-electron relativistic density functional theory. It is found that the Gd3+ ion is bonded to a hexagonal ring of C60 by electrostatic interaction. The total spin multiplicity of Gd@C60 is S = 7.

Possibility of encapsulations of metallofullerenes inside single-walled boron nitride nanotubes (BNNTs) is studied by using first-principles calculations. We find that both La@C82 and La2@C80 can be exothermically encapsulated inside the (17, 0) and (14, 7) BNNTs. The minimum diameters of exothermically encapsulating both La@C82 and La2@C80 inside BNNTs are predicated to be about 13.4 Å.

By using density functional theory calculations, it is found that the [2+1] cycloaddition derivatives of armchair (m,m) single-wall carbon nanotube (SWNT) evolve from open structure to closed three-membered ring structure as m>11 for NH addition, m>10 for O and CH2 additions, and m>5 for SiH2 addition. The diameter upper limit of the opening of the sidewall of SWNTs upon [2+1] cycloaddition is predicted to be about 15 Å.

We present a simple approach of tuning the hole doping level of iodine-doped single-walled carbon nanotubes by adjusting temperature, utilizing the structural conversion of iodine species encapsulated in SWNTs.

Semiconducting single-layer (SL) and few-layer MoS2 have a flat surface, free of dangling bonds. Using density functional theory coupled with non-equilibrium Green's function method, we investigate the spin-polarized transport properties of Co/2D MoS2/Co and Ni/2D MoS2/Ni junctions with MoS2 layer numbers of N = 1, 3, and 5. Well-defined interfaces are formed between MoS2 and metal electrodes. The junctions with a SL MoS2 spacer are almost metallic owing to the strong coupling between MoS2 and the ferromagnets, while those are tunneling with a few layer MoS2 spacer. Both large magnetoresistance and tunneling magnetoresistance are found when fcc or hcp Co is used as an electrode. Therefore, flat single- and few-layer MoS2 can serve as an effective nonmagnetic spacer in a magnetoresistance or tunneling magnetoresistance device with a well-defined interface.

Germanene, a germanium analogue of graphene and silicene, has been synthesized on metal substrates. It is predicted that the intrinsic germanene has a Dirac cone in its band structure, just like graphene and silicene. Using first-principles calculations, we investigate the geometrical structures and electronic properties of germanene on the Ag, Au, Cu, Al, Pt and Ir substrates. The Dirac cone of germanene is destroyed on the Al, Pt and Ir substrates but preserved on the Ag and Au substrates with a slight band hybridization. The upper part of the Dirac cone is destroyed for germanene on the Cu substrate while the lower part remains preserved. By contrast, the Dirac cone is always destroyed for silicene on these metal substrates because of a strong band hybridization. Our study suggests that it is possible to extract the intrinsic properties of germanene on the Ag and Au substrates although it appears impossible for silicene on these two substrates.

Layered tungsten disulfide nanostructures are of both fundamental and technological interest. The widths of currently synthesized WS2 ribbons are in the microscale. By using single-walled carbon nanotubes and double-walled carbon nanotubes as templates, we fabricate WS2 nanoribbons with smooth zigzag edges and uniform widths down to 1–3 nm and layer numbers down to 1–3, dependent on the nanotube diameter. Although bulk WS2 is a nonmagnetic semiconductor, the ultra-narrow free-standing zigzag-edged WS2 nanoribbons turn out to be magnetic or nonmagnetic metals depending on the edge passivation way according to our first-principles calculations, whereas the ultra-narrow armchair-edged WS2 nanoribbons remain nonmagnetic semiconductors with a narrow gap.