Piezoelectric and ferroelectric properties in the two-dimensional (2D) limit are highly desired for nanoelectronic, electromechanical, and optoelectronic applications. Here we report the first experimental evidence of out-of-plane piezoelectricity and ferroelectricity in van der Waals layered α-InSe nanoflakes. The noncentrosymmetric R3m symmetry of the α-InSe samples is confirmed by scanning transmission electron microscopy, second-harmonic generation, and Raman spectroscopy measurements. Domains with opposite polarizations are visualized by piezo-response force microscopy. Single-point poling experiments suggest that the polarization is potentially switchable for α-InSe nanoflakes with thicknesses down to ∼10 nm. The piezotronic effect is demonstrated in two-terminal devices, where the Schottky barrier can be modulated by the strain-induced piezopotential. Our work on polar α-InSe, one of the model 2D piezoelectrics and ferroelectrics with simple crystal structures, shows its great potential in electronic and photonic applications.

Organic–inorganic perovskite solar cells have attracted tremendous attention because of their remarkably high power conversion efficiencies. To further improve device performance, it is imperative to obtain fundamental understandings on the photo-response and long-term stability down to the microscopic level. Here, we report the quantitative nanoscale photoconductivity imaging on two methylammonium lead triiodide thin films with different efficiencies by light-stimulated microwave impedance microscopy. The microwave signals are largely uniform across grains and grain boundaries, suggesting that microstructures do not lead to strong spatial variations of the intrinsic photo-response. In contrast, the measured photoconductivity and lifetime are strongly affected by bulk properties such as the sample crystallinity. As visualized by the spatial evolution of local photoconductivity, the degradation process begins with the disintegration of grains rather than nucleation and propagation from visible boundaries between grains. Our findings provide insights to improve the electro-optical properties of perovskite thin films towards large-scale commercialization.

It is well established that strain and geometry could affect the band structure of graphene monolayer dramatically. Here we study the evolution of local electronic properties of a twisted graphene bilayer induced by a strain and a high curvature, which are found to strongly affect the local band structures of the twisted graphene bilayer. The energy difference of the two low-energy van Hove singularities decreases with increasing lattice deformation and the states condensed into well-defined pseudo-Landau levels, which mimic the quantization of massive chiral fermions in a magnetic field of about 100 T, along a graphene wrinkle. The joint effect of strain and out-of-plane distortion in the graphene wrinkle also results in a valley polarization with a significant gap. These results suggest that strained graphene bilayer could be an ideal platform to realize the high-temperature zero-field quantum valley Hall effect.

The understanding of various types of disorders in atomically thin transition metal dichalcogenides (TMDs), including dangling bonds at the edges, chalcogen deficiencies in the bulk, and charges in the substrate, is of fundamental importance for TMD applications in electronics and photonics. Because of the imperfections, electrons moving on these 2D crystals experience a spatially nonuniform Coulomb environment, whose effect on the charge transport has not been microscopically studied. Here, we report the mesoscopic conductance mapping in monolayer and few-layer MoS 2 field-effect transistors by microwave impedance microscopy (MIM). The spatial evolution of the insulator-to-metal transition is clearly resolved. Interestingly, as the transistors are gradually turned on, electrical conduction emerges initially at the edges before appearing in the bulk of MoS 2 flakes, which can be explained by our firstprinciples calculations. The results unambiguously confirm that the contribution of edge states to the channel conductance is significant under the threshold voltage but negligible once the bulk of the TMD device becomes conductive. Strong conductance inhomogeneity, which is associated with the fluctuations of disorder potential in the 2D sheets, is also observed in the MIM images, providing a guideline for future improvement of the device performance.MoS 2 | microwave impedance microscopy | edge states | electrical inhomogeneity | metal-insulator transition E lectrostatic gating in the field-effect transistor (FET) configuration has played an essential role in the blooming field of semiconducting transition metal dichalcogenides (TMDs) such as MoS 2 and WSe 2 (1). The electrical control of carrier densities in these naturally formed 2D sheets is crucial for the realization of many intriguing phenomena, such as the metal−insulator transition (2-6), novel spin and valley physics (7-12), and superconducting phases (13-15). In addition, the carrier modulation provides an ideal tuning parameter to study the screening effect, which is particularly important for charge transport in 2D materials that are highly susceptible to local variations of the disorder potential (2-5, 16, 17). As a result, a complete understanding of the electronic properties of TMD FETs at all length scales, i.e., from local defects in the atomic scale, to electronic inhomogeneity in the mesoscale, to device performance in the macroscale, is imperative for both fundamental research on and practical applications of these fascinating materials.Transport and most optical measurements on TMD FETs are inherently macroscopic in nature, in which the sample response is averaged over large areas. TMD films in actual devices, however, are far from electronically uniform. Due to the relatively large amount of intrinsic defects and the inevitable charged states in the substrates, mesoscopic electrical inhomogeneity is not uncommon in TMDs, leading to hopping transport and percolation transition in the devices (6,(16)(17)(18)(19). Little is known, however, about...

The perfect transmission in a graphene monolayer and the perfect reflection in a Bernal graphene bilayer for electrons incident in the normal direction of a potential barrier are viewed as two incarnations of the Klein paradox. Here we show a new and unique incarnation of the Klein paradox. Owing to the different chiralities of the quasiparticles involved, the chiral fermions in a twisted graphene bilayer show an adjustable probability of chiral tunneling for normal incidence: they can be changed from perfect tunneling to partial or perfect reflection, or vice versa, by controlling either the height of the barrier or the incident energy. As well as addressing basic physics about how the chiral fermions with different chiralities tunnel through a barrier, our results provide a facile route to tune the electronic properties of the twisted graphene bilayer.

Theoretical research has predicted that ripple of graphene generates effective gauge field on its low energy electronic structure and could lead to Landau quantization. Here we demonstrate, using a combination of scanning tunneling microscopy and tight-binding approximation, that Landau levels will form when effective pseudomagnetic flux per ripple Φ ~ (h 2 /la)Φ 0 is larger than the flux quantum Φ 0 (here h is the height, l is the width of the ripple, a is the nearest C-C bond length). The strain induced gauge field in the ripple only results in one-dimensional (1D) Landau-level quantization along the ripple. Such 1D Landau quantization does not exist in two-dimensional systems in an external magnetic field. Its existence offers a unique opportunity to realize novel electronic properties in strained graphene.

Recent studies show that periodic potentials can generate superlattice Dirac points at energies ±ћν F |G|/2 in graphene (ν F is the Fermi velocity of graphene and G is the reciprocal superlattice vector). Here, we perform scanning tunneling microscopy and spectroscopy studies of a corrugated graphene monolayer on Rh foil. We show that the quasi-periodic ripples of nanometer wavelength in the corrugated graphene give rise to weak one-dimensional (1D) electronic potentials and thereby lead to the emergence of the superlattice Dirac points. The position of the superlattice Dirac point is space-dependent and shows a wide distribution of values. We demonstrated that the space-dependent superlattice Dirac points is closely related to the space-dependent Fermi velocity, which may arise from the effect of the local strain and the strong electron-electron interaction in the corrugated graphene.Since the laboratory realization of graphene in 2004 [1], this two-dimensional honeycomb lattice of carbon atoms has motivated intense theoretical and experimental investigations of its properties [2][3][4][5][6][7][8]. It was demonstrated that the electronic chirality (the spinorlike structure of the wavefunction) is of central importance to many of graphene's unique electronic properties [3,[9][10][11][12]. Recently, a number of theoretical studies predicted that the chiral nature of charge carriers results in highly anisotropic behaviours of massless Dirac fermions in graphene under periodic potentials and generates new Dirac points at energies E SD = ±ћν F |G|/2 in graphene superlattice (here ν F is the Fermi velocity of graphene and G is the reciprocal superlattice vector) [13][14][15][16]. Despite these suggestive findings [13][14][15][16] and many other interesting physics [17][18][19][20][21][22] in graphene superlattice, the experimental study of this system is scarce due to the difficulty in fabricating graphene under nano-scale periodic potentials [23]. Until recently, it was demonstrated that graphene superlattice (corrugated graphene or moiré pattern) induced between the top graphene layer and the substrate (or the underlayer graphene) acts as a weak periodic potential, which generates superlattice Dirac points at an energy determined by the period of the potential [24][25][26]. These seminal experiments provide a facile method to realize graphene superlattice and open opportunities for superlattice engineering of electronic properties in graphene.In this Letter, we address the electronic structures of a corrugated graphene monolayer on Rh foil. We show that the quasi-periodic ripples of nanometer wavelength give rise to a weak one-dimensional (1D) electronic potential in graphene. This 1D potential leads to the emergence of the superlattice Dirac points E SD , which are manifested by two dips in the density of states, symmetrically placed at energies flanking the pristine graphene Dirac point E D . The position of E SD is space-dependent and shows a wide distribution of values. Our experimental result demonstrates tha...

The creation of van der Waals heterostructures based on a graphene monolayer and other two-dimensional crystals has attracted great interest because atomic registry of the two-dimensional crystals can modify the electronic spectra and properties of graphene. Twisted graphene bilayer can be viewed as a special van der Waals structure composed of two mutual misoriented graphene layers, where the sublayer graphene not only plays the role of a substrate, but also acts as an equivalent role as the top graphene layer in the structure. Here we report the electronic spectra of slightly twisted graphene bilayers studied by scanning tunneling microscopy and spectroscopy. Our experiment demonstrates that twist-induced van Hove singularities are ubiquitously present for rotation angles θ less than about 3.5 o , corresponding to moiré-pattern periods D longer than 4 nm. However, they totally vanish for θ > 5.5 o (D < 2.5 nm). Such a behavior indicates that the continuum models, which capture moiré-pattern periodicity more accurately at small rotation angles, are no longer applicable at large rotation angles.Graphene's novel electronic properties are a consequence of its two-dimensional honeycomb lattice [1]. Its electronic spectra are relatively easy to be tuned because graphene is a single-atom-thick membrane of carbon [2][3][4]. Very recently, it was demonstrated that a layer of hexagonal boron nitride (hBN) in contact with graphene can generate a periodic potential felt by graphene and lead to profound changes in graphene's electronic spectrum [5][6][7][8][9][10]. This provides an effective route to control the electronic spectra and properties of graphene via the creation of van der Waals heterostructures [5][6][7][8][9][10]. Graphene placed on top of another graphene monolayer with stacked misorientation forms a unique two dimensional van der Waals structure, i.e., twisted graphene bilayer [11][12][13][14][15][16][17][18][19][20][21], in which the graphene-on-graphene moiré modifies the electronic spectra [16,17,19]. The period of the moiré pattern D is related to the rotation angle θ by D = a/[2sin(θ/2)] with a = 0.246 nm the lattice parameter of graphene. This unique layered structure exhibits many fascinating physical properties beyond that of graphene monolayer due to interlayer coupling [16][17][18][19][20]. For example, the quasiparticles in twisted graphene bilayer are expected to show tunable chirality and adjustable probability of chiral tunneling [20].At small rotation angles electronic spectra of twisted graphene bilayer have been experimentally demonstrated to follow the predictions of the continuum models [11] and show twist-induced van Hove singularities (VHSs) [13][14][15]22,23], which directly arise from the finite interlayer coupling. However, the VHSs were not always observed and several experiments indicate that the electronic properties of the twisted graphene bilayer resemble a single graphene sheet [13,14,[24][25][26][27]. Obviously, the