Plasmon polaritons are hybrid excitations of light and mobile electrons that can confine the energy of long-wavelength radiation at the nanoscale. Plasmon polaritons may enable many enigmatic quantum effects, including lasing , topological protection and dipole-forbidden absorption . A necessary condition for realizing such phenomena is a long plasmonic lifetime, which is notoriously difficult to achieve for highly confined modes . Plasmon polaritons in graphene-hybrids of Dirac quasiparticles and infrared photons-provide a platform for exploring light-matter interaction at the nanoscale. However, plasmonic dissipation in graphene is substantial and its fundamental limits remain undetermined. Here we use nanometre-scale infrared imaging to investigate propagating plasmon polaritons in high-mobility encapsulated graphene at cryogenic temperatures. In this regime, the propagation of plasmon polaritons is primarily restricted by the dielectric losses of the encapsulated layers, with a minor contribution from electron-phonon interactions. At liquid-nitrogen temperatures, the intrinsic plasmonic propagation length can exceed 10 micrometres, or 50 plasmonic wavelengths, thus setting a record for highly confined and tunable polariton modes. Our nanoscale imaging results reveal the physics of plasmonic dissipation and will be instrumental in mitigating such losses in heterostructure engineering applications.

The success of metal-based plasmonics for manipulating light at the nanoscale has been empowered by imaginative designs and advanced nano-fabrication. However, the fundamental optical and electronic properties of elemental metals, the prevailing plasmonic media, are difficult to alter using external stimuli. This limitation is particularly restrictive in applications that require modification of the plasmonic response at subpicosecond timescales. This handicap has prompted the search for alternative plasmonic media 1-3 , with graphene emerging as one of the most capable candidates for infrared wavelengths. Here we visualize and elucidate the properties of non-equilibrium photo-induced plasmons in a high-mobility graphene monolayer 4 . We activate plasmons with femtosecond optical pulses in a specimen of graphene that otherwise lacks infrared plasmonic response at equilibrium. In combination with static nano-imaging results on plasmon propagation, our infrared pump-probe nano-spectroscopy investigation reveals new aspects of carrier relaxation in heterostructures based on high-purity graphene.Graphene plasmonics 5-7 has progressed rapidly, propelled by the electrical tunability, high field confinement 8,9 , potentially long lifetimes 10,11 of plasmons and the strong light-matter interactions 12-15 in graphene. An earlier spectroscopic study has reported photoinduced alteration of the plasmonic response of graphene on optical pumping 16 . In this work, we harnessed ultrafast optical pulses to generate mid-infrared (mid-IR) plasmons in a sample that lacks a plasmonic response at equilibrium. We examined the real-space aspects of non-equilibrium plasmon-polariton generation and propagation under femtosecond (fs) photo-excitation using a new ultrafast nano-infrared (IR) technique that fuses realspace plasmon imaging with spectroscopy. We applied this method to investigate high-quality graphene specimens encapsulated in hexagonal boron nitride: hBN/G/hBN 4 .We performed time-resolved broadband nano-IR experiments using antenna-based near-field nanoscopy (see Methods). This set-up (Fig. 1a,b) combines exceptional spatial, spectral and temporal resolution 16-18 , allowing an experimental probe of the dispersion of graphene plasmons under photo-excitation-a feat previously considered technologically infeasible. In our measurements, the metalized tip of an atomic force microscope (AFM) was illuminated by a focused IR probe beam, generating strong evanescent electric fields beneath the tip. These fields possess a wide range of in-plane momenta q and therefore facilitate efficient coupling to graphene plasmons 19 . Such evanescent fields extend ∼20 nm beneath the top surface of our structures, which is sufficient to launch and detect surface plasmons in a graphene microcrystal protected by a thin (10 nm) encapsulating layer of hBN 10 . The tip of the nanoscope acts as a launcher for surface plasmons of wavelength (λ p ) that propagate radially outwards from the tip. On reflection from the sample edge, these plasmons form sta...

Graphene has attracted worldwide interest since its experimental discovery, but the preparation of large-area, continuous graphene film on SiO2/Si wafers, free from growth-related morphological defects or transfer-induced cracks and folds, remains a formidable challenge. Growth of graphene by chemical vapour deposition on Cu foils has emerged as a powerful technique owing to its compatibility with industrial-scale roll-to-roll technology. However, the polycrystalline nature and microscopic roughness of Cu foils means that such roll-to-roll transferred films are not devoid of cracks and folds. High-fidelity transfer or direct growth of high-quality graphene films on arbitrary substrates is needed to enable wide-ranging applications in photonics or electronics, which include devices such as optoelectronic modulators, transistors, on-chip biosensors and tunnelling barriers. The direct growth of graphene film on an insulating substrate, such as a SiO2/Si wafer, would be useful for this purpose, but current research efforts remain grounded at the proof-of-concept stage, where only discontinuous, nanometre-sized islands can be obtained. Here we develop a face-to-face transfer method for wafer-scale graphene films that is so far the only known way to accomplish both the growth and transfer steps on one wafer. This spontaneous transfer method relies on nascent gas bubbles and capillary bridges between the graphene film and the underlying substrate during etching of the metal catalyst, which is analogous to the method used by tree frogs to remain attached to submerged leaves. In contrast to the previous wet or dry transfer results, the face-to-face transfer does not have to be done by hand and is compatible with any size and shape of substrate; this approach also enjoys the benefit of a much reduced density of transfer defects compared with the conventional transfer method. Most importantly, the direct growth and spontaneous attachment of graphene on the underlying substrate is amenable to batch processing in a semiconductor production line, and thus will speed up the technological application of graphene.

In this letter, we demonstrate a non-volatile memory device in a graphene FET structure using ferroelectric gating. The binary information, i.e. "1" and "0", is represented by the high and low resistance states of the graphene working channels and is switched by controlling the polarization of the ferroelectric thin film using gate voltage sweep. A non-volatile resistance change exceeding 200% is achieved in our graphene-ferroelectric hybrid devices. The experimental observations are explained by the electrostatic doping of graphene by electric dipoles at the ferroelectric/graphene interface. PACS numbers: Valid PACS appear hereThe discovery of graphene in 2004 [1, 2, 3] has triggered enormous experimental and theoretical efforts [4,5]. As a gapless semiconductor, charge carriers in graphene can be tuned continuously from electrons to holes crossing the charge neutral Dirac point using an external electric field. Unlike conventional semiconductors, the doping process does not influence the mobility of charge carriers in graphene, which can exceed 10 5 cm 2 V −1 s −1 at low temperature [6,7]. Such doping-independent mobility leads to the field-dependent conductance in graphene. Based on these two properties, many novel graphene-based device applications have been predicted or demonstrated [8,9,10,11,12,13,14,15,16], including the heavilyexplored graphene-based field-effect transistor (GFET) [17,18,19,20,21,22]. However, a paradigm shift in the microelectronics industry from Si to graphene also requires graphene-based memory applications. Despite graphene intrinsically having a high resistance state at the Dirac point and a low resistance state when heavily doped, reports on graphene for non-volatile information storage is rarely seen. This is due to the difficulty in maintaining the resistance states in graphene without an external electric field. One chemical modification approach to achieve non-volatile switching in graphene has been recently proposed by Echtermeyer et al [19]. Although this method can achieve very high on-off ratio, it alters the unique crystalline structure of graphene upon which many of the extraordinary electronic properties and hence most novel device concepts are based [4,5].In this letter, we show non-volatile switching in graphene by using ferroelectric gating without having to break the lattice symmetry. We demonstrate basic writing and reading processes of this novel grapheneferroelectric memory device structure combining the field-dependent conductance of graphene with the remnant electric field of ferroelectric thin films. A bistable * Electronic address: phyob@nus.edu.sg FIG. 1: (a) Sample geometry of a finished grapheneferroelectric memory device. (b) Optical image of a graphene sample showing the Hall-bar geometry of the bottom electrodes. (c) R vs VBG of the graphene sample before P(VDF-TrFE) coating, measured in two-terminal configuration. (d) AFM image of another graphene sample after P(VDF-TrFE) spin-coating. The contrast comes from the slightly different crystallization of P(...

Graphene is an atomically thin plasmonic medium that supports highly confined plasmon polaritons, or nano-light, with very low loss. Electronic properties of graphene can be drastically altered when it is laid upon another graphene layer, resulting in a moiré superlattice. The relative twist angle between the two layers is a key tuning parameter of the interlayer coupling in thus obtained twisted bilayer graphene (TBG). We studied propagation of plasmon polaritons in TBG by infrared nano-imaging. We discovered that the atomic reconstruction occurring at small twist angles transforms the TBG into a natural plasmon photonic crystal for propagating nano-light. This discovery points to a pathway towards controlling nano-light by exploiting quantum properties of graphene and other atomically layered van der Waals materials eliminating need for arduous top-down nanofabrication.One Sentence Summary: Atomically relaxed twisted bilayer graphene hosts periodic arrays of topological conducting channels that act as a photonic crystal for surface plasmons.

Recent experiments on ferroelectric gating have introduced a novel functionality, i.e., nonvolatility, in graphene field-effect transistors. A comprehensive understanding in the nonlinear, hysteretic ferroelectric gating and an effective way to control it are still absent. In this Letter, we quantitatively characterize the hysteretic ferroelectric gating using the reference of an independent background doping (n(BG)) provided by normal dielectric gating. More importantly, we prove that n(BG) can be used to control the ferroelectric gating by unidirectionally shifting the hysteretic ferroelectric doping in graphene. Utilizing this electrostatic effect, we demonstrate symmetrical bit writing in graphene-ferroelectric field-effect transistors with resistance change over 500% and reproducible no-volatile switching over 10⁵ cycles.

Graphene has exceptional optical, mechanical and electrical properties, making it an emerging material for novel optoelectronics, photonics and for flexible transparent electrode applications. However, the relatively high sheet resistance of graphene is a major constrain for many of these applications. Here we propose a new approach to achieve low sheet resistance in large-scale CVD monolayer graphene using non-volatile ferroelectric polymer gating. In this hybrid structure, large-scale graphene is heavily doped up to 3×10 13 cm -2 by non-volatile ferroelectric dipoles, yielding a low sheet resistance of 120 Ω/□ at ambient conditions. The graphene-ferroelectric transparent conductors (GFeTCs) exhibit more than 95 % transmittance from the visible to the near infrared range owing to the highly transparent nature of the ferroelectric polymer. Together with its excellent mechanical flexibility, chemical inertness and the simple fabrication process of ferroelectric polymers, the proposed GFeTCs represent a new route towards large-scale graphene based transparent electrodes and optoelectronics.KEYWORDS CVD graphene, ferroelectric polymer gating, sheet resistance, high transparency, mechanical flexibility, charged impurity scattering 2 Graphene keeps attracting much attention with enormous amount of experimental and theoretical activity, since its first micromechanical exfoliation in 2004. [1][2][3][4] As one atomic layer membrane, graphene is highly transparent (97.3 %) over a wide range of wavelengths from the visible to the near infrared (IR). 5Owing to its covalent carbon-carbon bonding, graphene is also one of the strongest materials with a remarkably high Young's modulus of ~ 1 TPa. 6 The combination of its high transparency, wideband tunability and excellent mechanical properties make graphene a very promising candidate for flexible electronics, optoelectronics and phonotics. 7-9The technical breakthrough of large-scale graphene synthesis has further accelerated the use of graphene films as transparent electrodes. 10,11To utilize graphene as transparent electrodes for applications such as solar cells 12 , organic light emitting diodes, 13 touch panels and displays 14, the key challenge is to reduce the sheet resistance to values comparable with indium tin oxide (ITO), which provides the best known combination of transparency (> 90 %) and sheet resistance (< 100 Ω/□). 8,15 Conventional methods to reduce the sheet resistance like electrostatic doping of graphene requires complex fabrication steps of dielectric deposition and gate electrode preparations, which are not practical for doping large-scale graphene and consume power to maintain the doping levels. 12,14 Chemical doping has been shown to effectively reduce the sheet resistance of graphene. [16][17][18][19] However, the doping mechanism of chemical dopants is not yet fully understood and the relationship between charge density and carrier mobility is still under debate. [20][21][22] Furthermore, the adsorption of moisture and other chemical molecul...

Moiré patterns are periodic superlattice structures that appear when two crystals with a minor lattice mismatch are superimposed. A prominent recent example is that of monolayer graphene placed on a crystal of hexagonal boron nitride. As a result of the moiré pattern superlattice created by this stacking, the electronic band structure of graphene is radically altered, acquiring satellite sub-Dirac cones at the superlattice zone boundaries. To probe the dynamical response of the moiré graphene, we use infrared (IR) nano-imaging to explore propagation of surface plasmons, collective oscillations of electrons coupled to IR light. We show that interband transitions associated with the superlattice mini-bands in concert with free electrons in the Dirac bands produce two additive contributions to composite IR plasmons in graphene moiré superstructures. This novel form of collective modes is likely to be generic to other forms of moiré-forming superlattices, including van der Waals heterostructures.