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Two-dimensional (2D) materials occupy a noteworthy place in nanophotonics providing for subwavelength light confinement and optical phenomena dissimilar to those of their bulk counterparts. In the mid-infrared, graphenebased heterostructures and van der Waals crystals of hexagonal boron nitride (hBN) overwhelmingly concentrate the attention by exhibiting real-space nano-optics features from plasmons, phonon−polaritons, and hybrid plasmon phonon− polaritons. Here we present a prime study on mid-infrared nanophotonics of talc, a natural atomically flat layered material, and graphene-talc (G-talc) heterostructures using broadband synchrotron infrared nanospectroscopy. Wavelength tuning of the talc vibrational resonances, assigned to in-and out-of-plane molecular vibrations, are achieved by changing the thickness of the crystals, which configures a tunable infrared signature for the 2D talc. In G-talc nanostructures, we unveil a coupling of the graphene plasmons polaritons with surface phonons polaritons of talc, originating hybrid surface plasmon−phonon polaritons modes. In analogy to G-hBN and G-SiO 2 heterostructures, the coupling in G-talc produces a large increase in the optovibrational activity for the out-of-plane mode as well as it induces a blue-shift for the in-plane mode. Moreover, the coupling can be modulated by an external gate voltage to the heterostructure when mounted in a transistor configuration. Therefore, our results introduce talc and G-talc heterostructures as appealing materials for nanophotonics, especially for applications involving long wavelengths and active electric tuning of opto-vibrational activity.

Steady doping, added to its remarkable electronic properties, would make graphene a valuable commodity in the solar cell market, as energy power conversion could be substantially increased. Here we report a graphene van der Waals heterostructure which is able to spontaneously dope graphene (ptype) up to n ~ 2.2 × 10 13 cm −2 while providing excellent charge mobility (µ ~ 25 000 cm 2 V −1 s −1 ). Such properties are achieved via deposition of graphene on atomically flat layered talc, a natural and abundant dielectric crystal. Raman investigation shows a preferential charge accumulation on graphene-talc van der Waals heterostructures, which are investigated through the electronic properties of talc/graphene/hBN heterostructure devices. These heterostructures preserve graphene's good electronic quality, verified by the observation of quantum Hall effect at low magnetic fields (B = 0.4 T) at T = 4.2 K. In order to investigate the physical mechanisms behind graphene-on-talc p-type doping, we performed first-principles calculations of their interface structural and electronic properties. In addition to potentially improving solar cell efficiency, graphene doping via van der Waals stacking is also a promising route towards controlling the band gap opening in bilayer graphene, promoting a steady n or p type doping in graphene and, eventually, providing a new path to access superconducting states in graphene, predicted to exist only at very high doping. LETTER RECEIVED

Combining experiment and theory, we investigate how a naturally created heterojunction (pn junction) at a graphene and metallic contact interface is modulated via interaction with molecular hydrogen (H2). Due to an electrostatic interaction, metallic electrodes induce pn junctions in graphene, leading to an asymmetrical resistance for electronic transport via electrons and holes. We report that an asymmetry in the resistance can be tuned in a reversible manner by exposing graphene devices to H2. The interaction between the H2 and graphene occurs solely at the graphene-contact pn junction and might be due to a modification on the electrostatic interaction between graphene and metallic contacts. We confront the experimental data with theory providing information concerning the length of the heterojunction, and how it changes as a function of H2 adsorption. Our results are valuable for understanding the nature of the metal-graphene interfaces and point out to a novel route towards selective hydrogen sensor application.Graphene is a zero-gap semiconductor which charge carrier density and conductance can be controlled electrostatically by preparing graphene devices as field effect transistors.1,2 In such architecture, the contact resistance considerably impairs device performance and is responsible for a conduction asymmetry for p-doped and n-doped graphene.3-8 This asymmetry stems from heterojunctions (pn junctions) formed at metal-graphene interfaces due to different work functions between graphene and metal -i.e. Fermi level pinning. 9-13 Effectively, the advent of the pn junction results in an additional charge scattering at the metal-graphene interface increasing device resistance. On the other hand, interesting effects can be observed as well. For instance: heterojunctions at metal-graphene interface have been used to design FabryPerot cavities 11 and to observe resonances in Josephson junctions in graphene devices. 14 In both systems, the metal-graphene interfaces have been considered a static problem, where the doping at the graphene underneath the contact is solely defined by the type of metal used. However, so far, a controllable method to probe and modulate the pn junction induced by contacts has not been reported. In addition, there are still open questions about how far a pn junction can extend from the contact into the graphene channel and if there are technological applications based on specialties of graphene contact resistance. 3,4,6,7,[14][15][16]

We report on gate hysteresis in resistance on high quality graphene/h-BN devices. We observe a thermal activated hysteretic behavior in resistance as a function of the applied gate voltage at temperatures above 375K. In order to investigate the origin of the hysteretic phenomenon, we design heterostructures involving graphene/h-BN devices with different underlying substrates such as: SiO 2 /Si and graphite; where heavily doped silicon and graphite are used as a back gate electrodes, respectively. The gate hysteretic behavior of the resistance shows to be present only in devices with an h-BN/SiO 2 interface and is dependent on the orientation of the applied gate electric field and sweep rate. Finally, we suggest a phenomenological model, which captures all of our findings based on charges trapped at the h-BN/SiO 2 . Certainly, such hysteretic behavior in graphene resistance represents a technological problem for the application of graphene devices at high temperatures, but conversely, it can open new routes for applications on digital electronics and graphene memory devices.Graphene is currently attracting attention as a potential building blocks for future nanoelectronics devices due to its physical and electrical properties.1-3 The ongoing development on graphene device technology is supported by high mobility and fast response of graphene field effect transistors (FET). 4 However, the intrinsic graphene characteristics can be significantly affected by the underlying substrates. For instance: hysteretic behaviors of graphene FET have been observed in graphene devices on different substrates, 5-13 opening several questions regarding the ideal platform for graphene transistors, limitations concerning temperature and influence of the environment on their properties. Out of a variety of dielectric materials, so far, hexagonal boron nitride (h-BN) stands up as the best platform for graphene devices. Up to now, graphene transistors on h-BN substrates show low charge inhomogeneity, and they free of hysteretic effects at temperatures ranging from room temperature (RT) down to low temperatures. 14-16 Also, a significant amount of attention regarding graphene h-BN heterostructures have been performed at low temperatures, demonstrating its ultrahigh mobility and showing new physical properties. 3,17 Moreover, as the demand for technological applications of graphene increases, various possibilities start to be considered, for instance, graphene devices based on high field and high current measurements. [18][19][20] Curiously, little attention has been given to graphene/h-BN devices working at operating temperatures of transistors, sensors and digital devices. Therefore, it is crucial