We have grown nearly freestanding single-layer 1T'-WTe2 on graphitized 6H-SiC(0001) by using molecular beam epitaxy (MBE), and characterized its electronic structure with scanning tunneling microscopy / spectroscopy (STM/STS). The existence of topological edge states at the periphery of single-layer WTe2 islands were confirmed. Surprisingly, a bulk band gap at the Fermi level and the insulating behaviors were also found in the single-layer WTe2 at low temperature, which is likely associated with an incommensurate charge order transition. The realization of two-dimensional topological insulators (2D TIs) in single-layer transition metal dichalcogenide (TMD) provides a promising platform for further exploration of the 2D TIs' physics and related applications.

single-layered materials have been predicted and realized, such as Si, [21][22][23] Ge, [24,25] Sn, [26] B, [27,28] Hf, [29] and Te, [30] but few of them share the same crystal structure as BP.It is believed that BP-structured monolayer (α-allotrope) can be formed in other group V elements, such as Bi (bismuthene), Sb (antimonene), or As (arsenene), and many theoretical efforts have been made to predict their structures and properties. [31][32][33][34][35][36][37] Comparing to their β-allotrope of hexagonal honeycomb structure that has been widely studied experimentally, [38][39][40][41][42][43][44] it still remains challenging to fabricate the large-scale and highquality monolayer α-allotrope of these group V monoelements, [36] even though small patches of the α-allotrope has been observed in some mixed structures. [45] In this study, we successfully synthesize the large-scale and high-quality α-antimonene with puckered BP structure on the T d -WTe 2 substrate, by using molecular beam epitaxy (MBE). In our experiment, the thickness of BP-structured antimonene can be well controlled in a layer-by-layer fashion. Owing to the high quality and large scale of the Sb monolayer, it becomes possible to map the electronic band structure via quasiparticle interference (QPI) with scanning tunneling microscopy (STM). The α-antimonene exhibits a hole-doped nature with a linearly dispersed band crossing the Fermi level and a high electrical Atomically thin 2D crystals have gained tremendous attention owing to their potential impact on future electronics technologies, as well as the exotic phenomena emerging in these materials. Monolayers of α-phase Sb (α-antimonene), which shares the same puckered structure as black phosphorous, are predicted to be stable with precious properties. However, the experimental realization still remains challenging. Here, high-quality monolayerα-antimonene is successfully grown, with the thickness finely controlled. The α-antimonene exhibits great stability upon exposure to air. Combining scanning tunneling microscopy, density functional theory calculations, and transport measurements, it is found that the electron band crossing the Fermi level exhibits a linear dispersion with a fairly small effective mass, and thus a good electrical conductivity. All of these properties make the α-antimonene promising for future electronic applications. AntimoneneSpurred by their prospect in electronic technologies, 2D crystals have been attracting increasing attentions. As the thickness is decreased down to the single-layer limit, 2D crystals usually exhibit different electronic properties from their bulk counterparts. [1] Exotic phenomena are also expected in single-layered materials, such as the quantum spin Hall effect, [2][3][4][5][6] 2D superconductivity, [7,8] charge density wave, [9][10][11][12] or magnetism. [13,14] Following the discovery of graphene, [15,16] black phosphorus (BP) has been revived as a potential candidate for optoelectronics and field-effect transistor (FET) applications, [17][18][19][...

The two-dimensional topological insulators host a full gap in the bulk band, induced by spin–orbit coupling (SOC) effect, together with the topologically protected gapless edge states. However, it is usually challenging to suppress the bulk conductance and thus to realize the quantum spin Hall (QSH) effect. In this study, we find a mechanism to effectively suppress the bulk conductance. By using the quasiparticle interference technique with scanning tunneling spectroscopy, we demonstrate that the QSH candidate single-layer 1T’-WTe2 has a semimetal bulk band structure with no full SOC-induced gap. Surprisingly, in this two-dimensional system, we find the electron–electron interactions open a Coulomb gap which is always pinned at the Fermi energy (EF). The opening of the Coulomb gap can efficiently diminish the bulk state at the EF and supports the observation of the quantized conduction of topological edge states.

Black phosphorus (BP) has recently attracted considerable attention due to its unique structure and fascinating optical and electronic properties as well as possible applications in photothermal agents. However, its main drawback is rapid degradation in ambient environments of HO and O, which has led to much research on the improvement of its stability. Unfortunately, this research has not shown great improvement in carrier mobilities. Here, we perform scanning tunneling microscopy observations of few-layer BP (FLBP) sheets exfoliated in ultrahigh vacuum and reveal, for the first time, the existence of lattice oxygen introduced during crystal growth. As a proof-of-concept application, hydrogenation is conducted to remove the lattice oxygen atoms followed by phosphorization, which repairs the phosphorous vacancies caused by mechanical exfoliation and hydrogenation. The resulting FLBP sheets show high ambipolar field-effect mobilities of 1374 cm V s for holes and 607 cm V s for electrons at 2 K. After storage in air for 3 days, the hole and electron mobilities only decrease to 1181 and 518 cm V s, respectively, and no structural degradation is observed. This work suggests an effective means to improve both the mobility and stability of BP sheets rendering practical application of FLBP sheets possible.

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By using scanning tunneling microscopy (STM) / spectroscopy (STS), we systematically characterize the electronic structure of lightly doped 1T-TiSe2, and demonstrate the existence of the electronic inhomogeneity and the pseudogap state. It is found that the intercalation induced lattice distortion impacts the local band structure and reduce the size of the charge density wave (CDW) gap with the persisted 2×2 spatial modulation. On the other hand, the delocalized doping electrons promote the formation of pseudogap. Domination by either of the two effects results in the separation of two characteristic regions in real space, exhibiting rather different electronic structures. Further doping electrons to the surface confirms that the pseudogap may be the precursor for the superconducting gap.This study suggests that the competition of local lattice distortion and the delocalized doping effect contribute to the complicated relationship between charge density wave and superconductivity for intercalated 1T-TiSe2.

Conformational arrangements in polymers on surfaces determine the overall shape as well as the potential properties. It is generally believed that conformational diversity leads to uncontrollable or disordered structures in on-surface synthesis. However, in this study, we obtain two well-ordered self-assembled covalently linked wavy chains with site-selective conformational switching via the Ullmann reaction of 1,2-bis(3-bromophenyl)ethane with multiple conformations on Ag(111). Two kinds of wavy chains exhibit distinct conformational arrangements, where chain I contains one repeating unit conformation of -cis-trans1-cis-trans1-cis-cis-trans1-, while the adjacent parallel parts in wavy chain II have two different conformational arrangements of -cis-cis-trans1- and -cis-cis-trans2-. Wavy chains coassemble with dissociated bromine atoms, suggesting that the Br···H–C interactions between Br atoms and molecular chains are crucial for the construction of ordered wavy chains. High-resolution scanning tunneling microscopy is employed to reveal the surface reaction process at the molecular scale. In depth growth mechanism analysis combined with density functional theory calculations unveils that the substrate also plays an important role in the fabrication of well-ordered wavy chains. The present work extends the surface reaction of conformational flexible precursors.

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