Narrow Nanoribbons Research Articles

Thermoelectric properties of armchair graphene nanoribbons with array characteristics.

The thermoelectric properties of armchair graphene nanoribbons (AGNRs) with array characteristics are investigated theoretically using the tight-binding model and Green's function technique. The AGNR structures with array characteristics are created by embedding a narrow boron nitride nanoribbon (BNNR) into a wider AGNR, resulting in two narrow AGNRs. This system is denoted as w-AGNR/n-BNNR, where 'w' and 'n' represent the widths of the wider AGNR and narrow BNNR, respectively. We elucidate the coupling effect between two narrow symmetrical AGNRs on the electronic structure of w-AGNR/i-BNNR. A notable discovery is that the power factor of the 15-AGNR/5-BNNR with the minimum width surpasses the quantum limitation of power factor for 1D ideal systems. The energy level degeneracy observed in the first subbands of w-AGNR/n-BNNR structures proves to be highly advantageous in enhancing the electrical power outputs of graphene nanoribbon devices.

Quantum Transport of Topological and Trivial Edge States in Sn‐Doped SbH Nanoribbons

Although the transport of topological edge states may be insensitive to impurities, impurities may induce the interedge scattering and backscattering in topological nanoribbons due to the finite size effect. It may be interesting to compare the transport of topological edge states with that of trivial edge states in nanoribbons. Here, the quantum transport of edge states in the Sn‐doped SbH nanoribbons using the density functional theory combined with nonequilibrium Green's function method is studied. Both topological and trivial edge states can be found in the zigzag SbH nanoribbons. The impurity generates a higher barrier near the doping position and leads to the scattering of the states. From calculations, the transmission of topological edge states can be smaller than that of trivial edge states due to the antiresonance behavior. Compared with topological edge states, the transport of trivial edge states is more sensitive to the impurity position. Besides, the edge impurity is more likely to induce scattering for both topological and trivial edge states than the central impurity in narrow SbH nanoribbons, since the transport channels are limited to the edges. This study may be helpful for understanding the transport of edge states in nanoribbons.

Optical nanoimaging of highly-confined phonon polaritons in atomically-thin nanoribbons of α-MoO3.

Phonon polaritons (PhPs), collective modes hybridizing photons with lattice vibrations in polar insulators, enable nanoscale control of light. In recent years, the exploration of in-plane anisotropic PhPs has yielded new levels of confinement and directional manipulation of nano-light. However, the investigation of in-plane anisotropic PhPs at the atomic layer limit is still elusive. Here, we report the optical nanoimaging of highly-confined phonon polaritons in atomically-thin nanoribbons of α-MoO3 (5 atomic layers). We show that narrow α-MoO3 nanoribbons as thin as a few atomic layers can support anisotropic PhPs modes with a high confinement ratio (∼133 times smaller wavelength than that of light). The anisotropic PhPs interference fringe patterns in atomic layers are tunable depending on the PhP wavelength via changing the illumination frequency. Moreover, spatial control over the PhPs interference patterns is also achieved by varying the nanostructures' shape or nanoribbon width of atomically-thin α-MoO3. Our work may serve as an empirical reference point for other anisotropic PhPs that approach the thickness limit and pave the way for applications such as atomically integrated nano-photonics and sensing.

Nanoscale patterning on layered MoS2 with stacking-dependent morphologies and optical tunning for phototransistor applications

Nanoscale patterning has attracted considerable interest as a novel engineering technology for two-dimensional (2D) nanomaterials for various applications such as electronics, sensors, and catalysts. However, research on nanopatterning technologies that reflect the unique structural and electrical characteristics of 2D nanomaterials remains insufficient. In this study, we present a nanoscale patterning of two different polygons on layered molybdenum disulfide (MoS2), where the nanostructures are controlled by the stacking structure of MoS2. We investigate the dependence of nanopore edge stability on the stacking structure of MoS2 using density functional theory calculations and find the different geometric effects of the two nanopatterns on the band-structure modulation of semiconducting MoS2. Consistent with the theoretical calculations, we fabricate nanoporous MoS2 with hexagonal nanopores for AA’ and triangular nanopores for AB stacked MoS2 films. The AA’ MoS2 has narrow nanoribbons between the two hexagonal nanopores, resulting in a lateral quantum confinement effect, whereas the edge exposure effect is more dominant for the bandgap increase in the triangular nanoporous structure. Nanopatterns contribute to generating in-gap state of MoS2, significantly enhancing the photoelectrical characteristics of MoS2 phototransistors. Especially, the photogating effect is higher for AA’ MoS2 phototransistors with hexagonal nanoholes than AB MoS2 with triangular nanoholes. This study provides new insights into nanoscale patterning and band structure engineering of layered 2D materials for various optoelectrical applications.

Thermal conductivity of quasi-bilayer graphene without and with perfluorophenylazide molecules

The obstacles in further improving the heat-transport performance of the graphene-based films root in the random stacking configuration of the graphene flakes and their agglomeration in solvents. In order to overcome these obstacles, we design one kind of quasi-bilayer graphene device composed of one upper narrow nanoribbon and one lower wide nanoribbon with absorbed perfluorophenylazide (PFPA) molecules. By investigating the thermal conductivity (TC) of the quasi-bilayer graphene without absorbed PFPA molecules, we find the TC will increase as the length and width increase, while the TC will decrease with the increase of the layer number. This indicates that a quasi-bilayer graphene flake of a large area and a small layer number will be beneficial to increasing its TC. Furthermore, in the presence of the PFPA molecules, the TC of the quasi-bilayer graphene, albeit suppressed by the PFPA molecules, is found to decrease slowly with the increase of the PFPA molecule number. This demonstrates a feasible way to fabricate the high-quality graphene-based films based on the technology of absorbing PFPA molecules. This research should be an important reference for designing the graphene-based thermal functional devices.

Remnants of the nonrelativistic Casimir effect on the lattice

The Casimir effect is a fundamental quantum phenomenon induced by the zero-point energy for a quantum field. It is well-known for relativistic fields with a linear dispersion relation, while its existence or absence for nonrelativistic fields with a quadratic dispersion is an unsettled question. Here, we investigate the Casimir effects for various dispersion relations on the lattice. We find that Casimir effects for dispersions proportional to an even power of momentum are absent in a long distance but a remnant of the Casimir effect survives in a short distance. Such a remnant Casimir effect will be experimentally observed in materials with quantum fields on the lattice, such as thin films, narrow nanoribbons, and short nanowires. In terms of this effect, we also give a reinterpretation of the Casimir effect for massive fields.

First-Principles Calculations of Monolayer h-BN Nanosheets and Nanoribbons with Ultrahigh Phonon-Limited Hole Mobility for Wide Band Gap P-Channel Transistors

Hexagonal boron nitride (h-BN) has emerged as one of the most promising candidates for two-dimensional (2D) materials due to its exciting optoelectrical properties and a broad range of applications. In this work, we explore the potential applications of h-BN nanosheets and nanoribbons as wide band gap semiconductors in terms of carrier mobility. Based on the first-principles calculations and deformation potential (DP) theory, the phonon-limited carrier mobility of monolayer h-BN and nanoribbons at room temperature is predicted. We find that the hole mobility of armchair-edge h-BN nanoribbons (ABNNRs) oscillates regularly with the ribbon width Nac in 1–3 nm. The ABNNRs in the Nac = 3p + 1 family have larger hole mobility with the highest value of 1.9 × 104 cm2 V–1 s–1 in the narrow nanoribbons. Molecular orbital analyses reveal that the large hole mobility originates from the delocalization of the occupied orbitals of valence electrons in the transport direction. By studying the effect of ribbon width on mobility, we identify the role of quantum confinement in tuning the transport properties of h-BN nanoribbons. The potential technological application of h-BN nanostructures as a P-channel material in wide band gap 2D field effect transistors (FETs) is discussed.

Narrow Directed Black Phosphorus Nanoribbons Produced by A Reformative Mechanical Exfoliation Approach (Small 17/2023)

Narrow Directed Black-Phosphorus Nanoribbons The preparation of high-quality narrow black phosphorus nanoribbons (PNRs) aligned along the same direction is very challenging. In article number 2207538, Changxin Chen and co-workers develop a reformative mechanical exfoliation approach combining tape and polydimethylsiloxane (PDMS) exfoliations to fabricate high-quality, narrow and directed PNRs with smooth edges. The prepared PNRs have widths from a dozen to hundreds of nanometers and can align along a same direction with their length directions along the zigzag direction.

Interfacial Superconductivity and Zero Bias Peak in Quasi‐One‐Dimensional Bi2Te3/Fe1+yTe Heterostructure Nanostructures

AbstractBismuth telluride/iron telluride (Bi2Te3/Fe1+yTe) heterostructures are known to exhibit interfacial superconductivity between two non‐superconducting materials: Fe1+yTe as the parent compound of Fe‐based superconducting materials and the topological insulator Bi2Te3. Here, a top‐down approach is presented starting from 2D heterostructures to fabricate 1D Bi2Te3/Fe1+yTe nanowires or narrow nanoribbons. It is demonstrated that the Bi2Te3/Fe1+yTe heterostructure remains intact in nanostructures of widths on the order of 100 nm and the interfacial superconductivity is preserved, as evidenced by electrical transport and Andreev reflection point contact spectroscopy experiments measured at the end of the nanowire. The differential conductance shows a similar superconducting twin‐gap structure as in 2D heterostructures, but with enhanced fluctuation effects due to the lower dimensionality. A zero‐bias conductance peak indicates the presence of an Andreev bound state and, given the involvement of the topological Bi2Te3 surface state, a possible topological nature of superconductivity is discussed with strong interplay with an emerging ferromagnetism due to the interstitial excess iron (Fe) in the Fe1+yTe layer, developing in parallel with superconductivity at low temperatures.

Electronic structures, transport properties, and optical absorption of bilayer blue phosphorene nanoribbons.

Based on first-principles density functional theory and nonequilibrium Green's function, we study the electronic band structures, the electronic transport properties, and the optical absorption of bilayer blue phosphorene nanoribbons (BPNRs). Both bilayer armchair BPNRs (a-BPNRs) and zigzag BPNRs (z-BPNRs) behave as semiconductors in the narrow nanoribbon case and metals in the wide nanoribbon case, sharply different from their monolayer counterparts where the monolayer a-BPNRs (z-BPNRs) are always semiconducting (metallic). This indicates that interlayer couplings or the increasing layer number may induce the switching of the conductivity of the monolayer BPNRs, which is absent in graphene and phosphorene nanoribbons. Furthermore, we explore the edge states of the energy bands near Fermi energy, and find that there are almost no pure edge-state band branches in the bilayer BPNRs, which can be attributed to the interlayer couplings between the edge-states in one layer and the bulk-states in the other. Consequently, the resulting complex band structures cannot be directly analyzed any more in the framework of the two-body coupling picture just according to the simple band structures of the monolayer BPNRs. Finally, we present the current-voltage characteristics and the optical absorption of the bilayer a-BPNRs and z-BPNRs. The influences of the nanoribbon width and the interlayer couplings on the current and the anisotropic optical absorption can be understood based on the complex energy band structures. This research should be an important reference of extending the field of BPNRs from the monolayer to the bilayer case, and deepen the understanding of the difference between the monolayer and bilayer nanoribbons in different materials.

The effect of circular antidots on electrical and magnetic properties of zigzag edge phosphorene nanoribbons

Electrical and magnetic properties of narrow phosphorene nanoribbons are theoretically studied. Within the Hubbard-type Hamiltonian combined with the self-consistent calculation method, the following topics are considered: spin- dependent electrical conductance, electronic thermal conductance, Seebeck coefficient and thermoelectric power factor. The emphasis is put on the modification of these physical quantities due to the appearance of antidots (nanopores). It is shown that the crucial role is played by the magnetic moments occurring both on external ribbon-edge atoms as well as on internal nanopore-edge atoms arranged in a zigzag way. Noteworthy, in case of structures with parallel magnetic configurations on the external edges, the systems show half-metallic behavior promising for expected spintronic applications.

Edge modes and their conductance in narrow nanoribbons of 2D materials in a topological phase

Employing novel 2D materials with topologically protected current-carrying edge states is promising to boost the on-current in electronic devices. Using nanoribbons is essential to reduce the contribution of the 2D bulk states to the current. Making the nanoribbon widths narrower allows one to put more current-carrying edge states under the gate of a fixed width thus increasing the current. However, the edge states from opposite edges may start to interact in narrow nanoribbons. Based on an effective k∙p model, we analyze the topologically protected edge states and their conductance for several 2D materials as a function of the normal electric field. We compare the 2D materials MoS2, MoSe2, WS2, and WSe2 in the topological 1T′ phase and find the largest electric field-induced conductance modulation in MoS2 nanoribbons.

Machine Learning for Shape Memory Graphene Nanoribbons and Applications in Biomedical Engineering.

Shape memory materials have been playing an important role in a wide range of bioengineering applications. At the same time, recent developments of graphene-based nanostructures, such as nanoribbons, have demonstrated that, due to the unique properties of graphene, they can manifest superior electronic, thermal, mechanical, and optical characteristics ideally suited for their potential usage for the next generation of diagnostic devices, drug delivery systems, and other biomedical applications. One of the most intriguing parts of these new developments lies in the fact that certain types of such graphene nanoribbons can exhibit shape memory effects. In this paper, we apply machine learning tools to build an interatomic potential from DFT calculations for highly ordered graphene oxide nanoribbons, a material that had demonstrated shape memory effects with a recovery strain up to 14.5% for 2D layers. The graphene oxide layer can shrink to a metastable phase with lower constant lattice through the application of an electric field, and returns to the initial phase through an external mechanical force. The deformation leads to an electronic rearrangement and induces magnetization around the oxygen atoms. DFT calculations show no magnetization for sufficiently narrow nanoribbons, while the machine learning model can predict the suppression of the metastable phase for the same narrower nanoribbons. We can improve the prediction accuracy by analyzing only the evolution of the metastable phase, where no magnetization is found according to DFT calculations. The model developed here allows also us to study the evolution of the phases for wider nanoribbons, that would be computationally inaccessible through a pure DFT approach. Moreover, we extend our analysis to realistic systems that include vacancies and boron or nitrogen impurities at the oxygen atomic positions. Finally, we provide a brief overview of the current and potential applications of the materials exhibiting shape memory effects in bioengineering and biomedical fields, focusing on data-driven approaches with machine learning interatomic potentials.

Half-metallicity induced by out-of-plane electric field on phosphorene nanoribbons

Exploring the half-metallic nanostructures with large band gap and high carrier mobility is a crucial solution for developing high-performance spintronic devices. The electric and magnetic properties of monolayer zigzag black-phosphorene nanoribbons (ZBPNRs) with various widths are analyzed by means of the first-principles calculations. Our results show that the magnetic ground state is dependent on the width of the nanoribbons. The ground state of narrow nanoribbons smaller than 8ZBPNRs prefers ferromagnetic order in the same edge but antiferromagnetic order between two opposite edges. In addition, we also calculate the electronic band dispersion, density of states and charge density difference of 8ZBPNRs under the action of out-of-plane electric field. More interesting, the addition of out-of-plane field can modulate antiferromagnetic semiconductor to the half metal by splitting the antiferromagnetic degeneracy. Our results propose a new approach to realize half-metal in phosphorene, which overcomes the drawbacks of graphene/silicene with negligible band gap as well as the transitional metal sulfide (TMS) with low carrier mobility.

Fundamentals of Curvilinear Ferromagnetism: Statics and Dynamics of Geometrically Curved Wires and Narrow Ribbons.

Low-dimensional magnetic architectures including wires and thin films are key enablers of prospective ultrafast and energy efficient memory, logic, and sensor devices relying on spin-orbitronic and magnonic concepts. Curvilinear magnetism emerged as a novel approach in material science, which allows tailoring of the fundamental anisotropic and chiral responses relying on the geometrical curvature of magnetic architectures. Much attention is dedicated to magnetic wires of Möbius, helical, or DNA-like double helical shapes, which act as prototypical objects for the exploration of the fundamentals of curvilinear magnetism. Although there is a bulk number of original publications covering fabrication, characterization, and theory of magnetic wires, there is no comprehensive review of the theoretical framework of how to describe these architectures. Here, theoretical activities on the topic of curvilinear magnetic wires and narrow nanoribbons are summarized, providing a systematic review of the emergent interactions and novel physical effects caused by the curvature. Prospective research directions of curvilinear spintronics and spin-orbitronics are discussed, the fundamental framework for curvilinear magnonics are outlined, and mechanically flexible curvilinear architectures for soft robotics are introduced.

C6N3: A novel 2D carbon nitride with sp-N as support for efficient hydrogen production

Two-dimensional (2D) carbon nitrides (CxNy) have great potential in advanced fields such as energy harvest and storage. We report a novel 2D CxNy, C6N3, comprising sp-N and sp2-C atoms. Density functional theory calculations indicate that electronically, thermally, and dynamically stable C6N3 is metallic even when cut into very narrow nanoribbons. C6N3 adsorbed single atoms of 3d, 4d, and 5d transition metals to fabricate single-atom catalysts (SACs) for an electrocatalytic hydrogen evolution reaction (HER). Among all SACs studied, Ti-, Cr-, Zr-, and Hf-embedded C6N3 had excellent overall performance including promising stability, moderate binding strength of H and H2, and good electrical conductivity. Our results suggest a novel C6N3 with an sp-N skeleton that has great potential for HER. More investigations are needed to further understand the properties and applications of C6N3 in other fields.

Dual Dirac points and odd–even oscillated energy gap in zigzag chlorinated stanene nanoribbon

Stanene has been predicted to be a two-dimensional topological insulator, providing an ideal platform for the realization of quantum spin Hall effect even at room temperature. Based on first-principles calculations, we studied the topological edge states in zigzag chlorinated stanene nanoribbon. From our calculations, dual Dirac points can be found near Fermi level. One Dirac point is localized at the edges and emerges in a narrow nanoribbon, while another is widespread and can only be found in a wide nanoribbon due to the coupling of two opposite edges. At the localized Dirac point, there is an interesting odd–even oscillated energy gap with the change of the width of nanoribbon. The energy gaps at both Dirac points and the coupling of two opposite edges can be modified by edge adsorption. Asymmetric adsorption of two edges was also discussed. Our calculations may be helpful for the potential applications of tin-based topological nanoribbons in nanodevices.

Subbands in a nanoribbon of topologically insulating MoS2 in the 1T′ phase

Continuous miniaturization has brought the feature size of silicon technology down into the nanometer scale, where performance enhancement cannot easily be achieved by further size reduction. The use of new materials with advanced properties has become mandatory to meet the needs for higher performance at reduced power. Topological insulators possess highly conductive topologically protected edge states which are insensitive to scattering and thus suitable for energy efficient high-speed devices. Here, we evaluate the subband structure in a narrow nanoribbon of 1T′ molybdenum disulfide by employing an effective k·p Hamiltonian. Highly conductive topologically protected edge modes whose energies lie within the bulk band gap are investigated on equal footing with traditional electron and hole subbands. Due to the interaction between the edge modes at opposite sides, a small gap in their linear spectrum opens up in a narrow nanoribbon. This gap is shown to sharply increase with the perpendicular out-of-plane electric field, in contrast to the behavior in a wide nanoribbon with negligible edge modes interaction. The gap between the traditional electron and hole subbands also increases with the perpendicular electric field. The increase of both gaps leads to a rapid decrease of the ballistic nanoribbon conductance and current with the electric field, which can be used for designing molybdenum disulfide nanoribbon-based current switches.

Li interaction-induced phase transition from black to blue phosphorene

A comprehensive first-principles calculation has been carried out revealing that at sufficiently high Li concentration and certain well-defined configurations a phase transition from black to blue phosphorene can take place. Blue phosphorene, a newly predicted allotrope of phosphorus, possesses unique crystalline and electronic structure and is a promising candidate, not only for fundamental research but also for electronic and optoelectronic applications. Methods of growing high-quality blue phosphorene layers are highly desirable but challenging. Here, a kinetic pathway to grow blue phosphorene layers from black phosphorene layers via Li intercalation is proposed based on first-principles study. This study pointed out that Li atoms intercalated in black phosphorene could act as a ``catalyst'' in the ``reactive region'' of the lone pair of P atoms, leading to a P-P bond breaking and, subsequently, a local structural transformation from an orthorhombic lattice to an assembly of parallel narrow nanoribbons with rhombohedra-like symmetry. During Li deintercalation, these nanoribbons are self-mended and form blue phosphorene layers. The interlayer distance was found to be 4.60 \AA{} for a double layer with AA stacking and 4.13 \AA{} for a multilayer with ABC stacking, indicating monolayer blue phosphorene can be mechanically exfoliated. This study also points out the possibility of new phases in other systems, where intercalation can lead to an unexpected structural phase transition and even a discovery of novel materials.

Disorder effects of vacancies on the electronic transport properties of realistic topological insulator nanoribbons: The case of bismuthene

The robustness of topological materials against disorder and defects is presumed but has not been demonstrated explicitly in realistic systems. In this work, we use state-of-the-art density functional theory and recursive nonequilibrium Green's functions methods to study the effect of disorder on the electronic transport of long nanoribbons, up to $157\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$, as a function of vacancy concentration. In narrow nanoribbons, a finite-size effect gives rise to hybridization between the edge states erasing topological protection. Hence, even small vacancy concentrations enable backscattering events. We show that the topological protection is more robust for wide nanoribbons, but surprisingly it breaks down at moderate structural disorder. Our study helps to establish some bounds on defective bismuthene nanoribbons as promising candidates for spintronic applications.