Armchair MoS2 Nanoribbons Research Articles

Polarization-induced giant thermoelectric effect in monolayer MoS2

The authors investigate the influence of local polarization on the thermoelectric properties of monolayer MoS2 using a tight-binding approach. It is found that strain-induced polarization in armchair monolayer MoS2 can be easily embedded to strongly enhance the thermoelectric efficiency ZT for quantum wells. The structure we proposed can achieve a figure of merit ZT of 0.8 at room temperature in an armchair MoS2 nanoribbon, and ZT achieves 1 at 500 K. The results facilitate insight and understanding of strain modulated thermoelectric properties of monolayer MoS2 nanodevices, provide theoretical guidance for experiments, and indicate their great potential in thermoelectric devices.

Tunable band gaps and optical absorption properties of bent MoS2 nanoribbons

The large tunability of band gaps and optical absorptions of armchair MoS2 nanoribbons of different widths under bending is studied using density functional theory and many-body perturbation GW and Bethe–Salpeter equation approaches. We find that there are three critical bending curvatures, and the non-edge and edge band gaps generally show a non-monotonic trend with bending. The non-degenerate edge gap splits show an oscillating feature with ribbon width n, with a period Delta n=3, due to quantum confinement effects. The complex strain patterns on the bent nanoribbons control the varying features of band structures and band gaps that result in varying exciton formations and optical properties. The binding energy and the spin singlet–triplet split of the exciton forming the lowest absorption peak generally decrease with bending curvatures. The large tunability of optical properties of bent MoS2 nanoribbons is promising and will find applications in tunable optoelectronic nanodevices.

Edge effect on flexoelectronic properties of Janus MoSSe nanoribbons: A first-principles study

The edge elasticity and its effect on flexoelectric response of the Janus MoSSe nanoribbons are systematically explored by means of density functional theory based first-principles calculations. We report edge stresses, edge elastic moduli, and structural deformations of the Janus MoSSe nanoribbons with various widths. It is shown that both armchair and zigzag terminated edges of the MoSSe nanoribbons are essentially subjected to tension, due to the existence of the edge stresses. The magnitude of average zigzag edge stresses is much larger than that of the average armchair ones. Furthermore, our results show that both misfit strain induced by asymmetric chalcogen atomic layers, and the edge stresses cause the spontaneous bending deformation of such Janus nanoribbons. More importantly, flexoelectronic properties of semiconducting armchair MoSSe nanoribbons are carefully evaluated and compared with those of armchair MoS2 and MoSTe nanoribbons. In particular, it is found that the out-of-plane flexoelectronic coefficients strongly depend on their widths. Additionally, the flexoelectric response resulting from spontaneous bending is weaker than that from the opposite one. The implicit mechanisms on deformations and flexoelectronic properties of such Janus nanoribbons have been carefully explored. The results in this work provide useful insights into their potential applications in nanoscale electromechanical systems.

Modulation of electronic and optical properties of line defected armchair MoS2 nanoribbon by vacancy passivation

In this study, the density functional theory (DFT) is used to investigate the effects of passivating line sulfur vacancies by non-metal species (C, N, O, F, OH and NH2) in armchair MoS2 nanoribbon (AMoS2NR) on its structural, electrical and optical properties. Calculated binding energies show that passivation of line vacancies by oxygen atoms leads to the most stable structures. Electronic calculations show that presence of single line vacancies decreases the bandgap of 0.68 eV in perfect AMoS2NR to 0.62 eV in single line vacant AMoS2NR (SV) and substituting the vacancies with carbon and oxygen (C-SV and O-SV) returns the bandgap to its initial value. It is also shown that passivation of SV with NH2 leads to a semiconductor with a small bandgap of 0.08 eV. However, substituting the vacancies of SV with N, F and OH results in metallic structures. In the case of double line vacant AMoS2NR (DV), the bandgap reduces significantly (0.38 eV) with respect to its perfect counterpart. Similar to what happened in SV, passivating DV with C and O again increases the bandgap to the bandgap of the perfect structure. Furthermore, N-, F-, OH- and NH2-DV show metallic behavior. In addition, we use total, projected and local density of states (TDOS, PDOS and LDOS) analysis to reveal the role of different atoms in different positions on the electronic properties of defective AMoS2NRs. In order to investigate the effect of passivation on the optical properties of defective AMoS2NRs we present the real and imaginary parts of dielectric function spectra. Our results suggest that passivation of line vacancies by different atoms can efficiently tune the absorption of AMoS2NR and open a new path to obtain MoS2-based optoelectronic devices.

Engineering of MoS2 nanoribbons as high-performance materials for biosensing applications

Today, biosensors play a key role in industry and medicine as detectors for a variety of biomolecules. The most important factors in these devices are the accuracy and rate of detection by the source selected for biosensing. In this work, we first placed Cytosine, Methionine, Thymine and Glutamine biomolecules on armchair MoS2 nanoribbons passivated with H and O atoms (AMoS2NR-H-O) and formed AMoS2NR-H-O-X structures, where X is the placed biomolecule. A first-principles theoretical study was performed to investigate the effects of these placements on the band structure, electron density, density of states, dielectric function and absorption spectrum of AMoS2NR-H-O. Our results showed that by placing biomolecules on the AMoS2NR-H-O, the band gap value changes between 0.3 and 0.75 eV. We found that sulfur atom has the main contribution in the interactions between AMoS2NR-H-O and biomolecules. The type of functional groups that make up the biomolecules plays an important role in the interactions between the biomolecules and S atoms (X-S interactions). Also, the placement of the biomolecules on AMoS2NR-H-O causes a red shift in the absorption edge. The extensive changes in the optoelectronic properties of AMoS2NR-H-O suggest that this compound can be a good platform for biosensing applications.

Effects of sulfur line vacancy defects on the electronic and optical properties of armchair MoS2 nanoribbon

Abstract Based on the density functional theory (DFT) calculations, we study the electronic and optical properties of armchair MoS2 nanoribbons (AMoS2NR) with single and double sulfur line vacancy defects in two different directions, i.e. parallel to the length (longitudinal) and width (transversal) of the structure. The formation energy calculations show that the structures with longitudinal vacancies are easier to create than the transversal ones. Investigating the band structures, it is observed that all the line defects proposed in this study decrease the band gap of the AMoS2NR. In fact, the band gap of 0.68 eV in perfect nanoribbon reduces to the lowest value of 0.11 eV in transversal double vacancy structure (T-DV). Analysis of the total and projected density of states (TDOS and PDOS) of the perfect and proposed defective structures indicates that in the defective structures the contribution of central atoms becomes more dominant especially in the case of the T-DV structure which has the most number of non-terminated bonds. In order to investigate the optical properties of the perfect and defective AMoS2NR, we present the real and imaginary parts of dielectric function, absorption coefficient and reflectance spectra for two different polarizations of light, along and perpendicular to the nanoribbon. The results show that all the structures in this study are anisotropic with respect to the type of polarization in the energies below 12 eV and are isotropic above it. It is also depicted that the light polarized along the nanoribbon has higher absorption in the energies below 3 eV. In addition, we show that the proposed line defects cause redshifts in the first peaks of the optical spectra which provides a tunable optical characteristic that can extend the application of AMoS2NR in a wide frequency range especially IR and visible.

Structural, electronic, and transport properties of Co-, Cr-, and Fe-doped functionalized armchair MoS2 nanoribbons

Using density functional theory, the structural, electronic, and transport properties of N, O, and F edge functionalized armchair molybdenum disulfide (AMoS2) nanoribbons (NRs) substituted with Cr, Fe, and Co impurity atoms were investigated. The near edge position of functionalized AMoS2NRs is preferred to substitute the impurity atoms, and all the structures are energetically stable. The bandgap of the structures is dramatically changed with 1% of the impurity metal atoms. In addition, multiple negative differential region phenomena exist with the substitution of these three metal impurities, and the peak to valley ratio of substituted NRs is more than that of unsubstituted nanoribbons.

Analytical study of the electronic and optical properties of the armchair MoS2 nanoribbons

In this paper, a tight binding approach is employed to compute the electronic properties of the armchair MoS2 nanoribbon. Among the three distinct groups of pristine armchair nanoribbons, the family with index N = 3p+2, (p being an integer), renders the highest band gap energy. Furthermore, having the Hamiltonian matrix and using the imaginary part of the dielectric function, the optical properties of the armchair nanoribbons are calculated for different widths. The transitions between subbands of the armchair MoS2 nanoribbons lead to peaks in the dielectric function. Simulation results illustrate how ribbon's width, may be tuned to design nanoribbon's band gap for a specific optoelectronic or digital electronic application.

Effects of Stone–Wales Defect on the Electronic and Optical Properties of Armchair MoS2 Nanoribbon: First-Principles Calculations

The electronic and optical properties of armchair $$\hbox {MoS}_{{2}}$$ nanoribbons with Stone–Wales (SW) defect have been investigated using first-principles calculations. The structure of OH-passivated armchair $$\hbox {MoS}_{{2}}$$ nanoribbons with SW-2 at the edge is unstable, and can be relaxed to perfect nanoribbon. SW decreases the band gaps. The gaps of armchair $$\hbox {MoS}_{{2}}$$ nanoribbons with SW-1 at the center is greater than that of SW-1 at the edge, it is inverse for SW-2. The band gaps of armchair $$\hbox {MoS}_{{2}}$$ nanoribbons with OH-passivated are greater than that of armchair $$\hbox {MoS}_{{2}}$$ nanoribbons with H-passivated. The partial charge densities indicate that the defect levels are originating from the SW. The optical response, such as the dielectric function, the absorption, the reflectance, and the electron energy loss spectra, are also presented. The optical results show that $$\varepsilon _{1}^{\perp }$$ (0) and $$\varepsilon _{1}^{\Vert }$$ (0) of armchair $$\hbox {MoS}_{{2}}$$ nanoribbons with SW are greater than that of perfect nanoribbons. Besides, armchair $$\hbox {MoS}_{{2}}$$ nanoribbon with SW is a promising donor material for solar cells due to high power conversion efficiency.

Structural, electronic and transport properties of an edge terminated armchair MoS2 nanoribbon with N, O and F atoms

Structural, electronic and transport properties of an edge terminated armchair molybdenum disulfide nanoribbon with N, O and F atoms are investigated using ab initio calculations. This functionalization leads to more stable structures. Also the band gap of the MoS2 ribbon is essentially altered, and varying values of the direct and indirect band gaps are obtained because of such novel edge functionalization. An external electric fields were applied to these edge terminated MoS2 nanoribbons and electronic properties were studied. According to the findings, the enhancement of the field strength reduces the band gap of the edge terminated AMoS2NRs. Furthermore, the transport properties of the structures were studied with equilibrium Green’s function formalism. It is intriguing that multiple negative differential resistance was observed in these non-metal edge functionalized armchair MoS2.

Armchair MoS2 nanoribbons turned into half metals through deposition of transition-metal and Si atomic chains

MoS2 nanoribbons with armchair-terminated edges are semiconductors suitable for the tuning of electronic and magnetic properties. Our first-principles density function calculations reveal that a variety of transition-metal atomic chains deposited on some of the ribbons is able to transform the semiconductors into half metals, allowing transport of 100% spin-polarized currents. Furthermore, we found that a Si atomic chain is equally capable of achieving half metallicity when adsorbed on the same nanoribbon. These results should be useful for spintronic application.

Single-band negative differential resistance in metallic armchair MoS2 nanoribbons

Semiconductor armchair MoS2 nanoribbons can be converted into conductors by edge functionalization of H atoms or OH groups. Those metallic nanoribbons exhibit I–V characteristics of a single half-filled band with strong negative differential resistance (NDR) under a voltage bias less than 1 V. This originates from the spatial separation between electrons in the conduction and valence bands. The NDR becomes spin dependent if the H atoms or OH groups are not uniformly adsorbed on the edge. Furthermore, the spin polarization can be greatly enhanced in heterojunctions of H- and OH-passivated nanoribbons.

Transport properties and device-design of Z-shaped MoS2 nanoribbon planar junctions

Based on MoS2 nanoribbons, metal-semiconductor-metal planar junction devices were constructed. The electronic and transport properties of the devices were studied by using density function theory (DFT) and nonequilibrium Green's functions (NEGF). It is found that a band gap about 0.4eV occurs in the planar junction. The electron and hole transmissions of the devices are mainly contributed by the Mo atomic orbitals. The electron transport channel is located at the edge of armchair MoS2 nanoribbon, while the hole transport channel is delocalized in the channel region. The I-V curve of the two-probe device shows typical transport behavior of Schottky barrier, and the threshold voltage is of about 0.2V. The field effect transistors (FET) based on the planar junction turn out to be good bipolar transistors, the maximum current on/off ratio can reach up to 1 × 104, and the subthreshold swing is 243mV/dec. It is found that the off-state current is dependent on the length and width of the channel, while the on-state current is almost unaffected. The switching performance of the FET is improved with increasing the length of the channel, and shows oscillation behavior with the change of the channel width.

Tunable spin polarization of MoS2 nanoribbons without time-reversal breaking

In this work, we present a detailed investigation of the influences of intrinsic spin-orbit coupling (ISOC) as well as Rashba spin-orbit coupling (RSOC) on the spin-resolved transport properties and net spin polarization of molybdenum disulfide (MoS2) nanoribbons including different configurations. For this reason, we generalize the tight-binding model by including the effects of the ISOC on all the atoms and a RSOC induced by a vertical electric field. The results predict a noticeable spin polarization in along to the field for the armchair MoS2 nanoribbon close to the Fermi level, which is originated mostly from edge states. Where as, the induced spin polarization in the zigzag MoS2 ribbons is much smaller than that of the considered armchair ones. Contrary to the RSOC, which it can not introduce significant spin polarization in the ZMoS2 the ribbon with width about 1–2 nm, the combination effect of an exchange field and Rashba spin-orbit interaction induces a spin-selective current especially close to the Fermi level. The numerical results may be useful to engineer and design magnetic-field-free spintronic devices based on the MoS2 nanoribbons.

The indirect–direct band gap tuning in armchair MoS2 nanoribbon by edge passivation

We performed density functional theory calculations to investigate the electronic structure of armchair MoS2 nanoribbons (AMoS2NRs) with and without passivation. The AMoS2NRs are nonmagnetic semiconductors and there exists a transition from indirect to direct band gap with the increasing of ribbon widths. Their electronic structures are determined by edge states. Edge passivation is an effective way to engineer the band structure of AMoS2NRs because it can vary the edge charge distribution. By choosing passivated groups and controlling passivated patterns, we can also tune the band gap of AMoS2NRs from indirect to direct. As a result, full hydrogen (H) or amidogen (NH2) passivation and partial hydroxyl (OH) passivation can not only tune AMoS2NRs from the indirect to direct band gap semiconductors, but also enhance the stability of the systems. Our interesting findings on indirect–direct band gap tuning may be helpful for engineering the band structure of AMoS2NRs and open the possibility of their application in optoelectronics.

Transition-metal doping induces the transition of electronic and magnetic properties in armchair MoS2 nanoribbons.

The electronic structure, magnetic properties and stability of transition-metal (TM) doped armchair MoS2 nanoribbons (AMoS2NRs) with full hydrogen passivation have been investigated using density functional theory. The hydrogen passivated AMoS2NRs are non-magnetic semiconductors, but TM doping can make the AMoS2NRs display diverse characteristics (such as non-magnetic metal, magnetic semiconductor, non-magnetic semiconductor and semi-metal properties), in which a transition of the electronic and magnetic properties is observed. Electronic structure analysis shows that the magnetism of the TM-doped AMoS2NRs is concentrated on the TM dopant and the edge Mo atoms, which mainly comes from the competition between the exchange splitting and crystal-field splitting. More importantly, Mn-doped AMoS2NRs may be good candidates for spintronic devices due to their good ferromagnetism with long-range FM magnetic coupling, reliable Curie temperature and high stability. These interesting findings on AMoS2NRs may open the possibility of their application in nanodevices and spintronic devices based on low-dimensional nanostructures.

Monolayer MoS2 Nanoribbons as a Promising Material for Both n-type and p-type Legs in Thermoelectric Generators

First-principles calculations have been performed to study the thermoelectric properties of monolayer MoS2 armchair nanoribbons (ACNRs). The electronic behavior of nanoribbons is dominated by the presence of edge states that are dependent on the number of zigzag chains across the nanoribbon. In addition, it is found that the phonon thermal conductance of monolayer MoS2 ACNRs is smaller than monolayer films due to phonon edge scattering. This effect is more pronounced in narrower nanoribbons, which leads to a higher ZT value compared to a monolayer MoS2 sheet. The effects of sulfur vacancy and edge roughness on the thermoelectric properties of MoS2 ACNRs have also been studied. We found that edge roughness decreased ZT values compared to those of perfect nanoribbons, as its impact on electrical conductance is more severe than on phonon thermal conductance. Sulfur vacancy, however, improved ZT in some subbands. It is shown that ZT values as high as 4 for electron-doped and 3 for hole-doped nanoribbons can be achieved at T = 500 K. The ability to achieve high ZT values for both p-type and n-type nanoribbons makes monolayer MoS2 ACNR a promising candidate for future solid-state thermoelectric generators.

Phonon thermal conductivity of monolayer MoS2

We use nonequilibrium molecular dynamics modeling using Stillinger–Weber interatomic potential to investigate the thermal properties of monolayer molybdenum disulfide (MoS2) nanoribbons. We study the impact of factors such as length, edge chirality, monovacancies, and uniaxial stretching on the thermal conductivity of MoS2 nanoribbons. Our results show that longer ribbons have a higher thermal conductivity, and the thermal conductivity of infinitely long zigzag and armchair MoS2 nanoribbons is, respectively, 54 W/mK and 33 W/mK. This is significantly lower than the thermal conductivity of some other graphene-like two-dimensional materials such as graphene and boron nitride. While the presence of molybdenum or sulfur vacancies reduces the thermal conductivity of ribbons, molybdenum vacancies have a more deteriorating effect on thermal conductivities. We also have studied the impact of uniaxial stretching on the thermal conductivity of MoS2 nanoribbons. The results show that in contrast to three dimensional materials, thermal conductivity of MoS2 is fairly insensitive to stretching. We have used the phonon dispersion curves and group velocities to investigate the mechanism of this unexpected behavior. Our results show that tensile strain does not alter the phonon dispersion curves and hence the thermal conductivity does not change.

Hydroxyl induced edge magnetism and metallicity in armchair MoS2 nanoribbons

Based on ab initio density functional theory, we demonstrate systematically how nonmagnetic semiconductor armchair MoS2 nanoribbons (AMoS2NRs) become magnetic or/and metallic when being edge-passivated by OH groups. Both the Mo and S edge atoms of an AMoS2NR can adsorb OH groups but an S atom can catch one OH group only when each of its neighbor Mo atoms has already been passivated by two. The AMoS2NR becomes edge magnetic in low passivation density and edge conductive in high density. In the case of uniform edge passivation, one or both of the edges usually become metallic and nonmagnetic if the number i of OH groups per primitive cell satisfies . In case a non-passivated edge Mo atom may be spin polarized if its neighbor Mo atom has adsorbed one OH group and the nanoribbon becomes magnetic semiconductor. For the nanoribbon become nonmagnetic semiconductor again.

Molecular dynamics study on relaxation properties of monolayer MoS2 nanoribbons

In order to study the essential structural characteristics of monolayer MoS2 nanoribbons in natural state, and also the effects of the aspect ratio and the ambient temperature on the relaxation properties of the nanoribbons, the relaxation properties of monolayer MoS2 nanoribbons with different aspect ratios are simulated by molecular dynamics (MD) method based on REBO potential functions at different thermodynamic temperatures from 0.01 K to 1500 K. The energy curves and surface morphologies of monolayer MoS2 nanoribbon are obtained, and the dynamic equilibrium process of the monolayer MoS2 nanoribbons is also discussed in all the simulation process. The simulation results show that the monolayer MoS2 nanoribbons do not generate a fluctuation at the ideal temperature (0.01 K) for the reason that the kinetic energy of the monolayer MoS2 nanoribbons is almost zero and the vibration amplitude is small. However, a certain degree of fluctuations occurs at the edges and inside of the monolayer MoS2 nanoribbons at the room temperature or high temperature. The fluctuation height and the fluctuation degree also increase with increasing the ambient temperature and the aspect ratio of the MoS2 nanoribbon, even a high aspect ratio monolayer MoS2 nanoribbon exhibits a surface curved fluctuation, which is perpendicular to the surface of the MoS2 nanoribbons under high temperature condition. Finally, the influences of chirality on relaxation property under different temperature conditions are studied in this paper further, the results show that unlike the armchair structure, the zigzag monolayer MoS2 nanoribbons not only present a surface fluctuation, but also exhibit an obvious bending phenomenon along the width direction simultaneously. Like the armchair nanoribbons, the surface fluctuation height and the surface fluctuation degree of the zigzag nanoribbons also increase with increasing both the ambient temperature and the aspect ratio of the MoS2 nanoribbons. It is also observed that the armchair and zigzag monolayer MoS2 nanoribbons with a similar aspect ratio have a similar surface fluctuation degree at the same ambient temperature. Unlike the armchair nanoribbons, the bending phenomenon along the width direction of the zigzag nanoribbons is more significant, and the bending width and the bending degree increase with increasing the ambient temperature and the aspect ratio of the MoS2 nanoribbons. Although the bending degree of the zigzag nanoribbons becomes larger with the increase of temperature, the increasing rate of the bending degree will become smaller and smaller until the bending degree reaches a maximum value.