Carrier Delocalization in Two-Dimensional Coplanar p–n Junctions of Graphene and Metal Dichalcogenides

With the lateral coplanar heterojunctions of two-dimensional monolayer materials turning into reality, the quantitative understanding of their electronic, electrostatic, doping, and scaling properties becomes imperative. In contrast to traditional bulk 3D junctions where carrier equilibrium is reached through local charge redistribution, a highly nonlocalized charge transfer (trailing off as 1/x away from the interface) is present in lateral 2D junctions, increasing the junction size considerably. The depletion width scales as p–1, while the differential capacitance varies very little with the doping level p. The properties of lateral 2D junctions are further quantified through numerical analysis of realistic materials, with graphene, MoS2, and their hybrid serving as examples. Careful analysis of the built-in potential profile shows strong reduction of Fermi level pinning, suggesting better control of the barrier in 2D metal–semiconductor junctions

Mechanochemistry of One-Dimensional Boron: Structural and Electronic Transitions

Recent production of long carbyne chains, concurrent with advances in the synthesis of pure boron fullerenes and atom-thin layers, motivates an exploration of possible one-dimensional boron. By means of first-principles calculations, we find two isomers, two-atom wide ribbon and single-atom chain, linked by a tension-driven (negative-pressure) transformation. We explore the stability and unusual properties of both phases, demonstrating mechanical stiffness on par with the highest-performing known nanomaterials, and a phase transition between stable 1D metal and an antiferromagnetic semiconductor, with the phase boundary effectively forming a stretchable 1D Schottky junction. In addition, the two-phase system can serve as a constant-tension nanospring with a well-calibrated tension defined by enthalpic balance of the phases. Progress in the synthesis of boron nanostructures suggests that the predicted unusual behaviors of 1D boron may find powerful applications in nanoscale electronics and/or mechanical devices

Mechanochemistry of One-Dimensional Boron: Structural and Electronic Transitions

Recent production of long carbyne chains, concurrent with advances in the synthesis of pure boron fullerenes and atom-thin layers, motivates an exploration of possible one-dimensional boron. By means of first-principles calculations, we find two isomers, two-atom wide ribbon and single-atom chain, linked by a tension-driven (negative-pressure) transformation. We explore the stability and unusual properties of both phases, demonstrating mechanical stiffness on par with the highest-performing known nanomaterials, and a phase transition between stable 1D metal and an antiferromagnetic semiconductor, with the phase boundary effectively forming a stretchable 1D Schottky junction. In addition, the two-phase system can serve as a constant-tension nanospring with a well-calibrated tension defined by enthalpic balance of the phases. Progress in the synthesis of boron nanostructures suggests that the predicted unusual behaviors of 1D boron may find powerful applications in nanoscale electronics and/or mechanical devices

BN White Graphene with “Colorful” Edges: The Energies and Morphology

Interfaces play a key role in low dimensional materials like graphene or its boron nitrogen analog, white graphene. The edge energy of hexagonal boron nitride (h-BN) has not been determined as its lower symmetry makes it difficult to separate the opposite B-rich and N-rich zigzag sides. We report unambiguous energy values for arbitrary edges of BN, including the dependence on the elemental chemical potentials of B and N species. A useful manifestation of the additional Gibbs degree of freedom in the binary system, this dependence offers a way to control the morphology of pure BN or its carbon inclusions and to engineer their electronic and magnetic properties

Ballistic Thermal Conductance of Graphene Ribbons

An elastic-shell-based theory for calculating the thermal conductance of graphene ribbons of arbitrary width w is presented. The analysis of vibrational modes of a continuum thin plate leads to a general equation for ballistic conductance σ. At low temperature, it yields a power law σ ∼ Tβ, where the exponent β varies with the ribbon width w from β = 1 for a narrow ribbon (σ ∼ T, as a four-channel quantum wire) to β = 3/2 (σ ∼ wT3/2) in the limit of wider graphene sheets. The ballistic results can be augmented by the phenomenological value of a phonon mean free path to account for scattering and agree well with the reported experimental observations

Electron Optics and Valley Hall Effect of Undulated Graphene

Electron optics is the systematic use of electromagnetic (EM) fields to control electron motions. In graphene, strain induces pseudo-electromagnetic fields to guide electron motion. Here we demonstrate the use of substrate topography to impart desirable strain on graphene to induce static pseudo-EM fields. We derive the quasi-classical equation of motion for Dirac Fermions in a pseudo-EM field in graphene and establish the correspondence between the quasi-classical and quantum mechanical snake states. Based on the trajectory analysis, we design sculpted substrates to realize various “optical devices” such as a converging lens or a collimator, and further propose a setup to achieve valley Hall effect solely through substrate patterning, without any external fields, to be used in valleytronics applications. Finally, we discuss how the predicted strain/pseudo-EM field patterns can be experimentally sustained by typical substrates and generalized to other 2D materials

Mechanically Induced Metal–Insulator Transition in Carbyne

First-principles calculations for carbyne under strain predict that the Peierls transition from symmetric cumulene to broken-symmetry polyyne structure is enhanced as the material is stretched. Interpretation within a simple and instructive analytical model suggests that this behavior is valid for arbitrary 1D metals. Further, numerical calculations of the anharmonic quantum vibrational structure of carbyne show that zero-point atomic vibrations eliminate the Peierls distortion in the mechanically free chain, preserving the cumulene symmetry. The emergence and increase of Peierls dimerization under tension then implies a qualitative transition between the two forms, which our computations place around 3% strain. Thus, the competition between the zero-point vibrations and mechanical strain determines a switch in symmetry resulting in the transition from metallic state to a dielectric, with a small effective mass and a high carrier mobility. In any practical realization, it is important that the effect is also chemically modulated by the choice of terminating groups. These findings are promising for applications such as electromechanical switching and band gap tuning via strain, and besides carbyne itself, they directly extend to numerous other systems that show Peierls distortion

Dirac Cones and Nodal Line in Borophene

Two-dimensional single-layer boron (borophene) has emerged as a new material with several intriguing properties. Recently, the β₁₂ polymorph of borophene was grown on Ag(111), and observed to host Dirac fermions. Similar to graphene, β₁₂ borophene can be described as atom-vacancy pseudoalloy on a closed-packed triangular lattice; however, unlike graphene, the origin of its Dirac fermions  is yet unclear. Here, using first-principles calculations, we probe the origin of Dirac fermions in freestanding and Ag(111)-supported β₁₂ borophene. The freestanding β₁₂ sheet hosts two Dirac cones and a topologically nontrivial Dirac nodal line with interesting Dirac-like edge states. On Ag(111), the Dirac cones develop a gap, whereas the topologically protected nodal line remains intact, and its position in the Brillouin zone matches that of the Dirac-like electronic states seen in the experiment. The presence of nontrivial topological states near the Fermi level in borophene makes its electronic properties important for both fundamental and applied research

Mechanochemistry of One-Dimensional Boron: Structural and Electronic Transitions

Recent production of long carbyne chains, concurrent with advances in the synthesis of pure boron fullerenes and atom-thin layers, motivates an exploration of possible one-dimensional boron. By means of first-principles calculations, we find two isomers, two-atom wide ribbon and single-atom chain, linked by a tension-driven (negative-pressure) transformation. We explore the stability and unusual properties of both phases, demonstrating mechanical stiffness on par with the highest-performing known nanomaterials, and a phase transition between stable 1D metal and an antiferromagnetic semiconductor, with the phase boundary effectively forming a stretchable 1D Schottky junction. In addition, the two-phase system can serve as a constant-tension nanospring with a well-calibrated tension defined by enthalpic balance of the phases. Progress in the synthesis of boron nanostructures suggests that the predicted unusual behaviors of 1D boron may find powerful applications in nanoscale electronics and/or mechanical devices

Synthesis Landscapes for Ammonia Borane Chemical Vapor Deposition of <i>h</i>‑BN and BNNT: Unraveling Reactions and Intermediates from First-Principles

Planar hexagonal boron nitride (h-BN) and tubular BN nanotube (BNNT), known for their superior mechanical and thermal properties, as well as wide electronic band gap, hold great potential for nanoelectronic and optoelectronic devices. Chemical vapor deposition has demonstrated the best way to scalable synthesis of high-quality BN nanomaterials. Yet, the atomistic understanding of reactions from precursors to product-material remains elusive, posing challenges for experimental design. Here, performing first-principles calculations and ab initio molecular simulations, we explore pyrolytic decomposition pathways of the most used precursor ammonia borane (H3BNH3, AB) to BN, in gas-phase and on Ni(111) or amorphous boron (for BNNT growth) surfaces, for comparison. It reveals that in the gas phase, a pair of AB molecules cooperate to form intermediate NH3 and ammonia diborane, which further dissociates into H2BNH2, accompanied by critical BH4– and NH4+ ions. These ions act as H scavengers facilitating H2BNH2 dehydrogenation into HBNH. The consequent HBNH directly feeds BN flake growth by reacting with the crystal edge, while the addition of H2BNH2 to the edge is prohibited at 1500 K. In contrast, on Ni and boron surfaces, AB monomer dehydrogenates stepwise, deeper, yielding BNH and BN dimer as the primary building unit. Our study maps out three typical experimental conditions regarding the dissociation of AB-precursor, providing insights into the underlying reaction mechanisms of gas-phase precursors, to help as guidelines for the experimental growth of BN nanomaterials