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In recent years, correlated insulating states, unconventional superconductivity, and topologically non-trivial phases have all been observed in several moiré heterostructures. However, understanding of the physical mechanisms behind these phenomena is hampered by the lack of local electronic structure data. Here, we use scanning tunnelling microscopy and spectroscopy to demonstrate how the interplay between correlation, topology, and local atomic structure determines the behaviour of electron-doped twisted monolayer–bilayer graphene. Through gate- and magnetic field-dependent measurements, we observe local spectroscopic signatures indicating a quantum anomalous Hall insulating state with a total Chern number of ±2 at a doping level of three electrons per moiré unit cell. We show that the sign of the Chern number and associated magnetism can be electrostatically switched only over a limited range of twist angle and sample hetero-strain values. This results from a competition between the orbital magnetization of filled bulk bands and chiral edge states, which is sensitive to strain-induced distortions in the moiré superlattice. Twisted moiré heterostructures offer a highly tunable solid-state platform for exploring fundamental condensed matter physics. Here, the authors use scanning tunnelling microscopy to investigate the local electronic structure of the gate-controlled quantum anomalous Hall insulator state in twisted monolayer–bilayer graphene.

Transition metal dichalcogenides are attracting widespread attention for their appealing optoelectronic properties. Using a combination of numerical and experimental techniques, the exciton binding energy is now determined for MoSe 2 on graphene. Two-dimensional (2D) transition metal dichalcogenides (TMDs) are emerging as a new platform for exploring 2D semiconductor physics 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 . Reduced screening in two dimensions results in markedly enhanced electron–electron interactions, which have been predicted to generate giant bandgap renormalization and excitonic effects 10 , 11 , 12 , 13 . Here we present a rigorous experimental observation of extraordinarily large exciton binding energy in a 2D semiconducting TMD. We determine the single-particle electronic bandgap of single-layer MoSe 2 by means of scanning tunnelling spectroscopy (STS), as well as the two-particle exciton transition energy using photoluminescence (PL) spectroscopy. These yield an exciton binding energy of 0.55 eV for monolayer MoSe 2 on graphene—orders of magnitude larger than what is seen in conventional 3D semiconductors and significantly higher than what we see for MoSe 2 monolayers in more highly screening environments. This finding is corroborated by our ab initio GW and Bethe–Salpeter equation calculations 14 , 15 which include electron correlation effects. The renormalized bandgap and large exciton binding observed here will have a profound impact on electronic and optoelectronic device technologies based on single-layer semiconducting TMDs.

Layered transition metal dichalcogenides are ideal systems for exploring the effects of dimensionality on correlated electronic phases such as charge density wave (CDW) order and superconductivity. In bulk NbSe 2 a CDW sets in at T CDW = 33 K and superconductivity sets in at T c = 7.2 K. Below T c these electronic states coexist but their microscopic formation mechanisms remain controversial. Here we present an electronic characterization study of a single two-dimensional (2D) layer of NbSe 2 by means of low-temperature scanning tunnelling microscopy/spectroscopy (STM/STS), angle-resolved photoemission spectroscopy (ARPES), and electrical transport measurements. We demonstrate that 3 × 3 CDW order in NbSe 2 remains intact in two dimensions. Superconductivity also still remains in the 2D limit, but its onset temperature is depressed to 1.9 K. Our STS measurements at 5 K reveal a CDW gap of Δ = 4 meV at the Fermi energy, which is accessible by means of STS owing to the removal of bands crossing the Fermi level for a single layer. Our observations are consistent with the simplified (compared to bulk) electronic structure of single-layer NbSe 2 , thus providing insight into CDW formation and superconductivity in this model strongly correlated system. What happens to correlated electronic phases—superconductivity and charge density wave ordering—as a material is thinned? Experiments show that both can remain intact in just a single layer of niobium diselenide.

Strong electron correlation can induce Mott insulating behaviour and produce intriguing states of matter such as unconventional superconductivity and quantum spin liquids. Recent advances in van der Waals material synthesis enable the exploration of Mott systems in the two-dimensional limit. Here we report characterization of the local electronic properties of single- and few-layer 1T-TaSe 2 via spatial- and momentum-resolved spectroscopy involving scanning tunnelling microscopy and angle-resolved photoemission. Our results indicate that electron correlation induces a robust Mott insulator state in single-layer 1T-TaSe 2 that is accompanied by unusual orbital texture. Interlayer coupling weakens the insulating phase, as shown by reduction of the energy gap and quenching of the correlation-driven orbital texture in bilayer and trilayer 1T-TaSe 2 . This establishes single-layer 1T-TaSe 2 as a useful platform for investigating strong correlation physics in two dimensions. The electrons that contribute to the Mott insulator state in single-layer 1T-TaSe2 are shown to also have a rich variation in their orbital occupation. As more layers are added, both the insulating state and orbital texture weaken.

Width-modulated heterostructures are created in graphene nanoribbons using a bottom-up approach, thus achieving molecular-scale bandgap engineering. Bandgap engineering is used to create semiconductor heterostructure devices that perform processes such as resonant tunnelling 1 , 2 and solar energy conversion 3 , 4 . However, the performance of such devices degrades as their size is reduced 5 , 6 . Graphene-based molecular electronics has emerged as a candidate to enable high performance down to the single-molecule scale 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 . Graphene nanoribbons, for example, can have widths of less than 2 nm and bandgaps that are tunable via their width and symmetry 6 , 18 , 19 . It has been predicted that bandgap engineering within a single graphene nanoribbon may be achieved by varying the width of covalently bonded segments within the nanoribbon 20 , 21 , 22 . Here, we demonstrate the bottom-up synthesis of such width-modulated armchair graphene nanoribbon heterostructures, obtained by fusing segments made from two different molecular building blocks. We study these heterojunctions at subnanometre length scales with scanning tunnelling microscopy and spectroscopy, and identify their spatially modulated electronic structure, demonstrating molecular-scale bandgap engineering, including type I heterojunction behaviour. First-principles calculations support these findings and provide insight into the microscopic electronic structure of bandgap-engineered graphene nanoribbon heterojunctions.

Chalcogen vacancies are generally considered to be the most common point defects in transition metal dichalcogenide (TMD) semiconductors because of their low formation energy in vacuum and their frequent observation in transmission electron microscopy studies. Consequently, unexpected optical, transport, and catalytic properties in 2D-TMDs have been attributed to in-gap states associated with chalcogen vacancies, even in the absence of direct experimental evidence. Here, we combine low-temperature non-contact atomic force microscopy, scanning tunneling microscopy and spectroscopy, and state-of-the-art ab initio density functional theory and GW calculations to determine both the atomic structure and electronic properties of an abundant chalcogen-site point defect common to MoSe 2 and WS 2 monolayers grown by molecular beam epitaxy and chemical vapor deposition, respectively. Surprisingly, we observe no in-gap states. Our results strongly suggest that the common chalcogen defects in the described 2D-TMD semiconductors, measured in vacuum environment after gentle annealing, are oxygen substitutional defects, rather than vacancies. The nature of defects in transition metal dichalcogenide semiconductors is still under debate. Here, the authors determine the atomic structure and electronic properties of chalcogen-site point defects common to monolayer MoSe 2 and WS 2 , and find that these are substitutional defects, where a chalcogen atom is substituted by an oxygen atom, rather than vacancies.

Field-tunable bandgap in bilayer graphene The electronic bandgap of a material refers to an energy region where electrons are not 'allowed' to reside because of quantum mechanical considerations related to the symmetries and atomic constituents of the underlying crystal structure. It is a fundamental property of semiconductors and insulators and determines their electrical and optical response, which is why it is a crucial consideration in modern device physics and technologies. Ideally, the bandgap would be tunable by electric fields, which would allow great flexibility in device design and functionality. Until now electrical tunability has proved elusive, but now Zhang et al . demonstrate such a tunable bandgap in a bilayer-graphene-based device, spanning a spectral range from zero to mid-infrared. The ability to electrically control the bandgap, a fundamental property of semiconductors and insulators that determines electrical and optical response, is highly desirable for device design and functionality. Experiments now demonstrate versatile control of the bandgap in bilayer graphene-based devices by use of electric fields. The electronic bandgap is an intrinsic property of semiconductors and insulators that largely determines their transport and optical properties. As such, it has a central role in modern device physics and technology and governs the operation of semiconductor devices such as p–n junctions, transistors, photodiodes and lasers 1 . A tunable bandgap would be highly desirable because it would allow great flexibility in design and optimization of such devices, in particular if it could be tuned by applying a variable external electric field. However, in conventional materials, the bandgap is fixed by their crystalline structure, preventing such bandgap control. Here we demonstrate the realization of a widely tunable electronic bandgap in electrically gated bilayer graphene. Using a dual-gate bilayer graphene field-effect transistor (FET) 2 and infrared microspectroscopy 3 , 4 , 5 , we demonstrate a gate-controlled, continuously tunable bandgap of up to 250 meV. Our technique avoids uncontrolled chemical doping 6 , 7 , 8 and provides direct evidence of a widely tunable bandgap—spanning a spectral range from zero to mid-infrared—that has eluded previous attempts 2 , 9 . Combined with the remarkable electrical transport properties of such systems, this electrostatic bandgap control suggests novel nanoelectronic and nanophotonic device applications based on graphene.

We introduce a new type of liquid cell for in situ transmission electron microscopy (TEM) based on entrapment of a liquid film between layers of graphene. The graphene liquid cell facilitates atomic-level resolution imaging while sustaining the most realistic liquid conditions achievable under electron-beam radiation. We employ this cell to explore the mechanism of colloidal platinum nanocrystal growth. Direct atomic-resolution imaging allows us to visualize critical steps in the process, including site-selective coalescence, structural reshaping after coalescence, and surface faceting.

Transition metal dichalcogenide materials are unique in the wide variety of structural and electronic phases they exhibit in the two-dimensional limit. Here we show how such polymorphic flexibility can be used to achieve topological states at highly ordered phase boundaries in a new quantum spin Hall insulator (QSHI), 1 T ′-WSe 2 . We observe edge states at the crystallographically aligned interface between a quantum spin Hall insulating domain of 1 T ′-WSe 2 and a semiconducting domain of 1 H -WSe 2 in contiguous single layers. The QSHI nature of single-layer 1 T ′-WSe 2 is verified using angle-resolved photoemission spectroscopy to determine band inversion around a 120 meV energy gap, as well as scanning tunneling spectroscopy to directly image edge-state formation. Using this edge-state geometry we confirm the predicted penetration depth of one-dimensional interface states into the two-dimensional bulk of a QSHI for a well-specified crystallographic direction. These interfaces create opportunities for testing predictions of the microscopic behavior of topologically protected boundary states. Transition metal dichalcogenides may host exotic topological phases in the two-dimensional limit, but detailed atomic properties have rarely been explored. Here, Ugeda et al. observe edge-states at the interface between a single layer quantum spin Hall insulator 1 T ′-WSe 2 and a semiconductor 1 H -WSe 2 .

One of the many unusual characteristics of graphene is that it shows ‘puddles’ of positive and negative charge throughout. A systematic scanning tunnelling microscope study shows that these puddles are not a consequence of ripples in graphene’s structure as originally thought, but are due to charged impurities below its surface. In an ideal graphene sheet, charge carriers behave as two-dimensional Dirac fermions 1 . This has been confirmed by the discovery of a half-integer quantum Hall effect in graphene flakes placed on a SiO 2 substrate. The Dirac fermions in graphene, however, are subject to microscopic perturbations that include topographic corrugations and electron-density inhomogeneities (that is, charge puddles). Such perturbations profoundly alter Dirac-fermion behaviour, with implications for their fundamental physics as well as for future graphene device applications. Here we report a new technique of Dirac-point mapping that we have used to determine the origin of charge inhomogeneities in graphene. We find that fluctuations in graphene charge density are caused not by topographical corrugations, but rather by charge-donating impurities below the graphene. These impurities induce surprising standing wave patterns due to unexpected backscattering of Dirac fermions. Such wave patterns can be continuously modulated by electric gating. Our observations provide new insight into impurity scattering of Dirac fermions and the microscopic mechanisms limiting electronic mobility in graphene.