The use of boron nitride (BN) as a substrate for graphene nanodevices has attracted much interest since the recent report that BN greatly improves the mobility of charge carriers in graphene compared to standard SiO(2) substrates. We have explored the local microscopic properties of graphene on a BN substrate using scanning tunneling microscopy. We find that BN substrates result in extraordinarily flat graphene layers that display microscopic Moiré patterns arising from the relative orientation of the graphene and BN lattices. Gate-dependent dI/dV spectra of graphene on BN exhibit spectroscopic features that are sharper than those obtained for graphene on SiO(2). We observe a significant reduction in local microscopic charge inhomogeneity for graphene on BN compared to graphene on SiO(2).

Relativistic quantum mechanics predicts that when the charge of a superheavy atomic nucleus surpasses a certain threshold, the resulting strong Coulomb field causes an unusual atomic collapse state; this state exhibits an electron wave function component that falls toward the nucleus, as well as a positron component that escapes to infinity. In graphene, where charge carriers behave as massless relativistic particles, it has been predicted that highly charged impurities should exhibit resonances corresponding to these atomic collapse states. We have observed the formation of such resonances around artificial nuclei (clusters of charged calcium dimers) fabricated on gated graphene devices via atomic manipulation with a scanning tunneling microscope. The energy and spatial dependence of the atomic collapse state measured with scanning tunneling microscopy revealed unexpected behavior when occupied by electrons.

Generating electron coherence in quantum materials is essential in optimal control of many-body interactions and correlations. In a multidomain system this signifies nonlocal coherence and emergence of collective phenomena, particularly in layered 2D quantum materials possessing novel electronic structures and high carrier mobilities. Here we report nonlocal ac electron coherence induced in dispersed MoS 2 flake domains, using coherent spatial self-phase modulation (SSPM). The gap-dependent nonlinear dielectric susceptibility χ (3) measured is surprisingly large, where direct interband transition and two-photon SSPM are responsible for excitations above and below the bandgap, respectively. A windchime model is proposed to account for the emergence of the ac electron coherence. Furthermore, all-optical switching is achieved based on SSPM, especially with two-color intraband coherence, demonstrating that electron coherence generation is a ubiquitous property of layered quantum materials.electron coherence | transition metal dichalcogenide | self-phase modulation | optical switching | emergent phenomena R ecently 2D layered quantum materials have attracted tremendous interest since the discovery of graphene a decade ago (1). Various layered materials, ranging from boron nitride sheets to transition metal dichalcogenides and from topological insulators to high-temperature superconductors, have been intensively investigated (2-11). Strict 2D atomic crystals can now be produced at a macroscopic scale, using a variety of methods (11,12). Among them molybdenum disulfide (MoS 2 ) and related layered quantum materials are particularly interesting due to their novel optical properties and potential valleytronics applications (4,5,8,9) at a thickness of monolayer and few layers (2, 10, 13). Layered materials share common physical properties rooted in their ubiquitous 2D quantum nature, for which achieving pure coherence among electrons (lattices) is of particular interest (14-19). The presence of multiple domains is ubiquitous in many known 2D quantum materials, ranging from stripe-order cuprate superconductors to polycrystalline strongly correlated systems. For example, phase locking between different layers of stripe orders is crucial for enhancing the superconducting phase in layered hightemperature superconductors (20,21).In this work we demonstrate unambiguously that nonlocal and intraband ac electron coherence, of which the electronic wave function oscillates at an optical frequency of 10 14 Hz, can be generated in separate MoS 2 flakes, using spatial self-phase modulation (SSPM). The SSPM is a coherent third-order nonlinear optical process systematically investigated decades ago (22), where the nonlinear optical susceptibility χ (3) is uniquely determined by the laser-intensity-dependent refractive index n = n 0 + n 2 I, where n 0 and n 2 are linear and nonlinear refractive indexes, respectively. If this effect is strong enough in a material, the phenomenon of selffocusing can be directly observed. The SSPM is also f...

*These authors contributed equally to this workThe response of Dirac fermions to a Coulomb potential is predicted to differ significantly from the behavior of non-relativistic electrons seen in traditional atomic and impurity systems [1][2][3] . Surprisingly, many key theoretical predictions for this ultra-relativistic regime have yet to be tested in a laboratory [4][5][6][7][8][9][10][11][12] . This small value of g ε indicates that microscopic electron-electron interactions can contribute significantly to graphene properties.Our experiment was performed using a scanning tunneling microscope (STM) in UHV at T = 4.8K to probe back-gated devices consisting of CVD-grown graphene 17 placed on top of boron nitride (BN) flakes 18 on a SiO 2 /Si surface (see Supplementary Materials for methods).Utilization of BN substrates significantly reduces the charge inhomogeneity of graphene 19,20 , thus allowing us to probe the intrinsic graphene electronic response to individual charged impurities. The charged impurities probed in this work were cobalt trimers constructed on graphene by atomically manipulating cobalt monomers with the tip of an STM 21 (cobalt atoms were deposited via e-beam evaporation onto low temperature graphene samples).Figs.1 a-f show the process of manipulating three cobalt monomers to create a single Co trimer on graphene (the detailed interior structure of the Co trimer cannot be resolved due to its instability under high current measurement).Co trimers were used in this study because they form a robust, reproducible impurity whose charge can be toggled on and off through the use of a back-gate electrode. Co monomers (which can also be charge-toggled) were unsuitable for this study due to the fact that their charge state switches in the proximity of an STM tip 22 (thus leading to spatially inhomogeneous ionization features that mask the intrinsic graphene response to a Coulomb impurity of fixed charge). Co trimers are ideal because they can be prepared in different charge states (through back-gating) that are charge-stable in the proximity of an STM tip. This is shown in the spectroscopic data of Figs. 1g and 2. Fig. 1g shows STM spectra 3 acquired with the STM tip held directly over a single Co trimer for two different back-gate voltages (V g ). Following the analysis of Ref. 22, the Co trimer impurity state marked R is seen to lie below the Fermi level (E F ) for V g = +14V, in which case it is filled by an electron, while it lies above E F for V g = -2V, in which case the electron has been removed from the trimer. The R state arises from the local cobalt-graphene hybridization and is undetectable at distances greater than r 0 = 1.5nm from the trimer center, demonstrating that short-range cobalt-graphene interactions end at r 0 . The spectroscopic feature marked S arises due to tip-induced ionization of the trimer and thus confirms the charging nature of the impurity state R 22-25 .The gate-dependent charge states of the Co trimer were determined by performing dI/dV mapping of the surrounding grap...

AbstractAs an emerging material, nanomaterials have attracted extensive attention due to their small size, surface effect and quantum tunneling effect, as well as potential applications in traditional materials, medical devices, electronic devices, coatings and other industries. Herein, the influence of nanoparticle selection, production process, grain size, and grain boundary structures on the mechanical properties of nanomaterials is introduced. The current research progress and application range of nano-materials are presented. The unique properties of nano-materials make them superior over traditional materials. Therefore, nanomaterials will have a broader application prospect in the future. Research on nanomaterials is significant for the development and application of materials science.

Hierarchical TiO(2) nanostructures would be desirable for preparing dye-sensitized solar cells because of their large amount of dye adsorption and superior light harvesting efficiency, as well as efficient charge separation and transport properties. In this study, rutile TiO(2) nano-branched arrays grown directly on transparent conductive glass (FTO) were prepared by a facile two-step wet chemical synthesis process, using a simple aqueous chemical growth method involving immersing the TiO(2) nanorod arrays in an aqueous TiCl(4) solution as seeds, which were prepared by a hydrothermal method. The dye-sensitized solar cells based on the TiO(2) nano-branched arrays which were only about 3 μm in length show a short-circuit current intensity of 10.05 mA cm(-2) and a light-to-electricity conversion efficiency of 3.75%, which is nearly three times as high as that of bare nanorod arrays, due to the preferable nanostructure, which not only retains the efficient charge separation and transport properties of the nanorod arrays, but also can improve the amount of dye adsorption due to the increased specific surface area from the nanobranches.

The idea flat surface, superb thermal conductivity and excellent optical transmittance of single-layer graphene promise tremendous potential for graphene as a material for transparent defoggers. However, the resistance of defoggers made from conventional transferred graphene increases sharply once both sides of the film are covered by water molecules which, in turn, leads to a temperature drop that is inefficient for fog removal. Here, the direct growth of large-area and continuous graphene films on quartz is reported, and the first practical single-layer graphene defogger is fabricated. The advantages of this single-layer graphene defogger lie in its ultrafast defogging time for relatively low input voltages and excellent defogging robustness. It can completely remove fog within 6 s when supplied a safe voltage of 32 V. No visible changes in the full defogging time after 50 defogging cycles are observed. This outstanding performance is attributed to the strong interaction forces between the graphene films and the substrates, which prevents the permeation of water molecules. These directly grown transparent graphene defoggers are expected to have excellent prospects in various applications such as anti-fog glasses, auto window and mirror defogging.

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