The zigzag-edged triangular graphene molecules (ZTGMs) have been predicted to host ferromagnetically coupled edge states with the net spin scaling with the molecular size, which affords large spin tunability crucial for next-generation molecular spintronics. However, the scalable synthesis of large ZTGMs and the direct observation of their edge states have been long-standing challenges because of the molecules’ high chemical instability. Here, we report the bottom-up synthesis of π-extended [5]triangulene with atomic precision via surface-assisted cyclodehydrogenation of a rationally designed molecular precursor on metallic surfaces. Atomic force microscopy measurements unambiguously resolve its ZTGM-like skeleton consisting of 15 fused benzene rings, while scanning tunneling spectroscopy measurements reveal edge-localized electronic states. Bolstered by density functional theory calculations, our results show that [5]triangulenes synthesized on Au(111) retain the open-shell π-conjugated character with magnetic ground states.

We use first-principles electronic structure calculations to predict a new class of two-dimensional (2D) topological insulators (TIs) in binary compositions of group III elements (B, Al, Ga, In, and Tl) and bismuth (Bi) in a buckled honeycomb structure. We identify band inversions in pristine GaBi, InBi, and TlBi bilayers, with gaps as large as 560 meV, making these materials suitable for room-temperature applications. Furthermore, we demonstrate the possibility of strain engineering in that the topological phase transition in BBi and AlBi could be driven at ∼6.6% strain. The buckled structure allows the formation of two different topological edge states in the zigzag and armchair edges. More importantly, isolated Dirac-cone edge states are predicted for armchair edges with the Dirac point lying in the middle of the 2D bulk gap. A room-temperature bulk band gap and an isolated Dirac cone allow these states to reach the long-sought topological spin-transport regime. Our findings suggest that the buckled honeycomb structure is a versatile platform for hosting nontrivial topological states and spin-polarized Dirac fermions with the flexibility of chemical and mechanical tunability.

Platinum-based transition metal dichalcogenides have been gaining renewed interest because of the development of a new method to synthesize thin film structures. Here, using first-principles calculation, we explore the electronic properties of PtX 2 (X = S, Se, and Te) with respect to film thickness. For bulk and layered structures (1 to 10 layers), octahedral 1T is the most stable. Surprisingly, we also find that the 3R structure has comparable stability relative to the 1T, implying possible synthesis of 3R. For a bulk 1T structure, PtS 2 is semiconducting with an indirect band gap of 0.25 eV, while PtSe 2 and PtTe 2 are both semi-metallic. Still, all their corresponding monolayers exhibit an indirect semiconducting phase with band gaps of 1.68, 1.18, and 0.40 eV for PtS 2 , PtSe 2 , and PtTe 2 , respectively. For the band properties, we observe that all these materials manifest decreasing/closing of indirect band gap with increasing thickness, a consequence of quantum confinement and interlayer interaction. Moreover, we discover that controlling the thickness and applying strain can manipulate van Hove singularity resulting to high density of states at the maximum valence band. Our results exhibit the sensitivity and tunability of electronic properties of PtX 2 , paving a new path for future potential applications.

We discuss two-dimensional (2D) topological insulators (TIs) based on planar Bi/Sb honeycombs on a SiC(0001) substrate using first-principles computations. The Bi/Sb planar honeycombs on SiC (0001) are shown to support a nontrivial band gap as large as 0.56 eV, which harbors a Dirac cone lying within the band gap. Effects of hydrogen atoms placed on either just one side or on both sides of the planar honeycombs are examined. The hydrogenated honeycombs are found to exhibit topologically protected edge states for zigzag as well as armchair edges, with a wide band gap of 1.03 and 0.41 eV in bismuth and antimony films, respectively. Our findings pave the way for using planar bismuth and antimony honeycombs as potential new 2D-TI platforms for room-temperature applications.Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence.Any further distribution of this work must maintain attribution to the author (s) and the title of the work, journal citation and DOI. 4 After this work was completed, we recently became aware of an independent study of BiX and SbX films by Song et al [23] who consider passivation with X atoms on both sides of the film. In contrast, we consider one-sided H-passivation, which is the key for breaking the inversion symmetry and inducing large spin-splittings. The effect of the one-sided H-passivation is similar to that of the SiC substrate.

Electronic structures and band topology of a single Sb(111) bilayer in the buckled honeycomb configuration are investigated using first-principles calculations. A nontrivial topological insulating phase can be induced by tensile strain, indicating the possibility of realizing the quantum spin Hall state for Sb thin films on suitable substrates. The presence of buckling provides an advantage in controlling the band gap through an out-of-plane external electric field, making a topological phase transition with six spin-polarized Dirac cones at the critical point. With a tunable gap and reversible spin polarization, Sb thin films are promising candidates for spintronic applications.

Electronic structures, minimum energy configurations, and band topology of strained Bi(111) single bilayers placed on a variety of semiconducting and insulating substrates are investigated using first-principles calculations. A topological phase diagram of a free-standing Bi bilayer is presented to help guide the selection of suitable substrates. The insulating hexagonal-BN is identified as the best candidate substrate material for supporting nontrivial topological insulating phase of Bi bilayer thin films. A planar hexagonal Bi layer is predicted under tensile strain, which we show could be realized on a SiC substrate. The Bi bilayer becomes metallic under the compressive strain induced by Si and Ge substrates.The recent discovery of topological insulators (TIs) which possess a nontrivial Z 2 topological invariant is attracting worldwide attention, making this a fast developing area in materials sciences. 1-4 TIs host spin-polarized surface/edge states, which are not allowed to backscatter due to the constraint of time-reversal symmetry and are thus very desirable for spintronics and other applications. 5,6 The development of topological band theory combined with the predictive power of first-principles calculations, has led to the discovery of many new families of TIs such as Li 2 AgSb, 18 ternary tetradymite, 19 quaternary chalcogenides, and famatinites. 20 While a number of three-dimensional (3D) topological insulators have been realized experimentally, there are only very few materials realizations of the two-dimensional (2D) TIs, also referred to as quantum spin Hall (QSH) insulators. The key feature of a QSH insulator is the presence of protected gapless edge states which carry two spin-polarized currents propagating in opposite directions. Graphene was the first system which was proposed to support a QSH state through spin-orbit coupling effects, but the associated gap in graphene is too small to be accessible experimentally. 4 To date, the only experimental realizations of the QSH state are HgTe/CdTe (Refs. 21-23) and InAs/GaSb/AlSb (Refs. 24 and 25) quantum well systems. No stand-alone thin film or a thin film supported on a suitable substrate has been experimentally demonstrated to harbor a QSH state. The great need for finding new QSH insulator materials is for these reasons clear.Theoretical studies have shown the sensitivity of the Z 2 topological invariant to film thickness in Bi 2 Se 3 and Bi 2 Te 3 ultrathin films, 26,27 suggesting that the 2D QSH phase could be induced through the reduced dimensionality in thin films of 3D TIs. 28-30 Accordingly, the search for 2D QSH phases has focused on Bi and Sb films in view of their strong spin-orbit interaction, 31-35 and the fact that Bi/Sb alloys were the first 3D TIs to be discovered experimentally. 36 In particular, a single Bi(111) bilayer (BL) film has been predicted to be an elemental 2D QSH insulator, 31,32 and ultrathin Bi(111) films are predicted to be topologically nontrivial for a wide range of film thicknesses. 33 In contrast, Sb(111) films w...

A large gap two-dimensional (2D) topological insulator (TI), also known as a quantum spin Hall (QSH) insulator, is highly desirable for low-power-consuming electronic devices owing to its spin-polarized backscattering-free edge conducting channels. Although many freestanding films have been predicted to harbor the QSH phase, band topology of a film can be modified substantially when it is placed or grown on a substrate, making the materials realization of a 2D TI challenging. Here we report a first-principles study of possible QSH phases in 75 binary combinations of group III (B, Al, Ga, In, and Tl) and group V (N, P, As, Sb, and Bi) elements in the 2D buckled honeycomb structure, including hydrogenation on one or both sides of the films to simulate substrate effects. A total of six compounds (GaBi, InBi, TlBi, TlAs, TlSb, and TlN) are identified to be nontrivial in unhydrogenated case; whereas for hydrogenated case, only four (GaBi, InBi, TlBi, and TlSb) remains nontrivial. The band gap is found to be as large as 855 meV for the hydrogenated TlBi film, making this class of III-V materials suitable for room temperature applications. TlBi remains topologically nontrivial with a large band gap at various hydrogen coverages, indicating the robustness of its band topology against bonding effects of substrates.

In this article we show that the reconstructions of semiconductor surfaces can be determined using a genetic procedure. Coupled with highly optimized interatomic potentials, the present approach represents an efficient tool for finding and sorting good structural candidates for further electronic structure calculations and comparison with scanning tunnelling microscope (STM) images. We illustrate the method for the case of Si (105), and build a database of structures that includes the previously found low-energy models, as well as a number of novel configurations.