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We report high room-temperature mobility in single layer graphene grown by Chemical Vapor Deposition (CVD) after wet transfer on SiO$_2$ and hexagonal boron nitride (hBN) encapsulation. By removing contaminations trapped at the interfaces between single-crystal graphene and hBN, we achieve mobilities up to$\sim70000cm^2 V^{-1} s^{-1}$ at room temperature and$\sim120000cm^2 V^{-1} s^{-1}$ at 9K. These are over twice those of previous wet transferred graphene and comparable to samples prepared by dry transfer. We also investigate the combined approach of thermal annealing and encapsulation in polycrystalline graphene, achieving room temperature mobilities$\sim30000 cm^2 V^{-1} s^{-1}$. These results show that, with appropriate encapsulation and cleaning, room temperature mobilities well above $10000cm^2 V^{-1} s^{-1}$ can be obtained in samples grown by CVD and transferred using a conventional, easily scalable PMMA-based wet approach.

We investigate the photocatalytic performance of nanocomposites prepared in a one-step process by liquid-phase exfoliation of graphite in the presence of TiO$_2$ nanoparticles (NPs) at atmospheric pressure and in water, without heating or adding any surfactant, and starting from low-cost commercial reagents. The nanocomposites show enhanced photocatalytic activity, degrading up to 40$\%$ more pollutants with respect to the starting TiO$_2$-NPs. In order to understand the photo-physical mechanisms underlying this enhancement, we investigate the photo-generation of reactive species (trapped holes and electrons) by ultrafast transient absorption spectroscopy. We observe an electron transfer process from TiO$_2$ to the graphite flakes within the first picoseconds of the relaxation dynamics, which causes the decrease of the charge recombination rate, and increases the efficiency of the reactive species photo-production., Comment: 13 pages, 12 figures, 1 table

We report the photoresponse of niobium diselenide (NbSe$_2$), a transition metal dichalcogenide (TMD) which exhibits superconducting properties down to a single layer. Devices are built by using micro-mechanically cleaved 2 to 10 layers and tested under current bias using nano-optical mapping in the 350mK-5K range, where they are found to be superconducting. The superconducting state can be broken by absorption of light, resulting in a voltage signal when the devices are current biased. The response found to be energy dependent making the devices useful for applications requiring energy resolution, such as bolometry, spectroscopy and infrared imaging., Comment: 6 pages, 6 figures

We report Ultra-High-Frequency (UHF, 800MHz-1GHz) Radio Frequency Identification (RFID) transponders consisting of printed dipole antennas combined with RFID microchips. These are fabricated on Kapton via screen printing and on paper via spray coating, using inks obtained via microfluidization of graphite. We introduce a hybrid antenna structure, where an Al loop (small compared to the overall size of the antenna) is connected to a microchip with the double function of matching the impedances of antenna and microchip and avoiding bonding between exfoliated graphite and chip. The transponders have reading distance~11m at UHF RFID frequencies, larger than previously reported for graphene-based RFID and comparable with commercial transponders based on metallic antennas.

Hot electrons dominate the ultrafast ($\sim$fs-ps) optical and electronic properties of metals and semiconductors and they are exploited in a variety of applications including photovoltaics and photodetection. We perform power-dependent third harmonic generation measurements on gated single-layer graphene and detect a significant deviation from the cubic power-law expected for a third harmonic generation process. We assign this to the presence of hot electrons. Our results indicate that the performance of nonlinear photonics devices based on graphene, such as optical modulators and frequency converters, can be affected by changes in the electronic temperature, which might occur due to increase of absorbed optical power or Joule heating.

We report stimulated Raman spectroscopy of the G phonon in both single and multi-layer graphene, through Coherent anti-Stokes Raman Scattering (CARS). The signal generated by the third order nonlinearity is dominated by a vibrationally non-resonant background (NVRB), which obscures the Raman lineshape. We demonstrate that the vibrationally resonant CARS peak can be measured by reducing the temporal overlap of the laser excitation pulses, suppressing the NVRB. We model the observed spectra, taking into account the electronically resonant nature of both CARS and NVRB. We show that CARS can be used for graphene imaging with vibrational sensitivity., Comment: 10 pages, 5 figures

Solid-state quantum emitters that couple coherent optical transitions to long-lived spin qubits are essential for quantum networks. Here we report on the spin and optical properties of individual tin-vacancy (SnV) centers in diamond nanostructures. Through cryogenic magneto-optical and spin spectroscopy, we verify the inversion-symmetric electronic structure of the SnV, identify spin-conserving and spin-flipping transitions, characterize transition linewidths, measure electron spin lifetimes and evaluate the spin dephasing time. We find that the optical transitions are consistent with the radiative lifetime limit even in nanofabricated structures. The spin lifetime is phononlimited with an exponential temperature scaling leading to $T_1$ $>$ 10 ms, and the coherence time, $T_2$ reaches the nuclear spin-bath limit upon cooling to 2.9 K. These spin properties exceed those of other inversion-symmetric color centers for which similar values require millikelvin temperatures. With a combination of coherent optical transitions and long spin coherence without dilution refrigeration, the SnV is a promising candidate for feasable and scalable quantum networking applications., Comment: 6 pages, 4 figures

We report soliton rains in a tuneable Tm-doped fiber laser mode locked by carbon nanotubes. We also detect their second- and third-harmonic. We achieve a tuneability of over 56nm, from 1877 to 1933nm, by introducing a polarization-maintaining isolator and two in-line polarization controllers. This makes our system promising as a tuneable filter for ultrafast spectroscopy.

We report a THz reflectarray metasurface which uses graphene as active element to achieve beam steering, shaping and broadband phase modulation. This is based on the creation of a voltage controlled reconfigurable phase hologram, which can impart different reflection angles and phases to an incident beam, replacing bulky and fragile rotating mirrors used for terahertz imaging. This can also find applications in other regions of the electromagnetic spectrum, paving the way to versatile optical devices including light radars, adaptive optics, electro-optical modulators and screens., Comment: 16 pages, 10 figures

In monolayer Transition Metal Dichalcogenides (TMDs) the valence and conduction bands are spin split because of the strong spin-orbit interaction. In tungsten-based TMDs the spin-ordering of the conduction band is such that the so-called dark exciton, consisting of an electron and a hole with opposite spin orientation, has lower energy than the A exciton. A possible mechanism leading to the transition from bright to dark excitons involves the scattering of the electrons from the upper to the lower conduction band state in K. Here we exploit the valley selective optical selection rules and use two-color helicity-resolved pump-probe spectroscopy to directly measure the intravalley spin-flip relaxation dynamics of electrons in the conduction band of single-layer WS$_2$. This process occurs on a sub-ps time scale and it is significantly dependent on the temperature, indicative of a phonon-assisted relaxation. These experimental results are supported by time-dependent ab-initio calculations which show that the intra-valley spin-flip scattering occurs on significantly longer time scales only exactly at the K point. In a realistic situation the occupation of states away from the minimum of the conduction band leads to a dramatic reduction of the scattering time.

Multi-exciton states such as biexcitons, albeit theoretically predicted, have remained challenging to identify in atomically thin transition metal dichalcogenides so far. Here, we use excitation-power, electric-field and magnetic-field dependence of photoluminescence to report direct experimental evidence of two biexciton complexes in monolayer tungsten diselenide: the neutral and the negatively charged biexciton. We demonstrate bias-controlled switching between these two states, we determine their internal structure and we resolve a fine-structure splitting of 2.5 meV for the neutral biexciton. Our results unveil multi-particle exciton complexes in transition metal dichalcogenides and offer direct routes to their deterministic control in many-body quantum phenomena.

Post-synthesis separation between metallic (m-SWNTs) and semiconducting (s-SWNTs) singlewall carbon nanotubes (SWNTs) is remaining a challenging process for the reliable fabrication of high performance electronic devices. Gel agarose chromatography is emerging as an efficient and large scale separation technique. However, the full (100%) separation of m-SWNTs and s-SWNTs has not been reached yet, mainly due to the lack of understanding of the separation mechanism. Here we study the temperature effect on the separation via gel agarose chromatography by varying the separation process temperatures between 6C and 50C, for four different SWNT sources. Exploiting the gel agarose micro-beads filtration technique we achieved up to 70% m-SWNTs and more than 90% s-SWNTs, independent of the source material. The process is temperature dependent, with improved yields up to 95% for s-SWNT (HiPco) at 6C. Temperature affects the sodium dodecyl sulfate SDS surfactant-micelle distribution along the SWNT sidewalls, thus helping to act as an aid in the sorting of SWNTs by electronic type. The sorted SWNTs are then used to fabricate transistors with very low OFF-currents (10^-13 A), high ON/OFF current ratio (>10^6) and improved charge carriers mobility.

We report an environmentally stable nanotube mode-locked fibre laser producing linearly-polarized, nanosecond pulses. A simple all-polarization-maintaining fibre ring cavity is used, including 300 m of highly nonlinear fibre to elongate the cavity and increase intracavity dispersion and nonlinearity. The laser generates scalar pulses with a duration of 1.23 ns at a centre wavelength of 1042 nm, with 1.3-nm bandwidth and at 641-kHz repetition rate. Despite the long cavity, the output characteristics show no significant variation when the cavity is perturbed, and the degree of polarization remains at 97%.

Resistive-switching memories are alternative to Si-based ones, which face scaling and high power consumption issues. Tetrahedral amorphous carbon (ta-C) shows reversible, non-volatile resistive switching. Here we report polarity independent ta-C resistive memory devices with graphene-based electrodes. Our devices show ON/OFF resistance ratios$\sim$4x$10^5$, ten times higher than with metal electrodes, with no increase in switching power, and low power density$\sim$14$\mu$W/$\mu$m$^2$. We attribute this to a suppressed tunneling current due to the low density of states of graphene near the Dirac point, consistent with the current-voltage characteristics derived from a quantum point contact model. Our devices also have multiple resistive states. This allows storing more than one bit per cell. This can be exploited in a range of signal processing/computing-type operations, such as implementing logic, providing synaptic and neuron-like mimics, and performing analogue signal processing in non-von-Neumann architectures

Heterostructures formed by stacking layered materials require atomically clean interfaces. However, contaminants are usually trapped between the layers, aggregating into blisters. We report a process to remove such blisters, resulting in clean interfaces. We fabricate blister-free regions of graphene encapsulated in hexagonal boron nitride of$\sim$5000$\mu $m$^{2}$, limited only by the size of the exfoliated flakes. These have mobilities up to$\sim$180000cm$^2$V$^{-1}$s$^{-1}$ at room temperature, and$\sim$1.8$\times$10$^6$cm$^2$V$^{-1}$s$^{-1}$ at 9K. We further demonstrate the effectiveness of our approach by cleaning heterostructures assembled using graphene intentionally exposed to polymers and solvents. After cleaning, these samples reach similar high mobilities. We also showcase the general applicability of our approach to layered materials by cleaning blisters in other heterostructures based on MoS$_{2}$. This demonstrates that exposure of graphene to processing-related contaminants is compatible with the realization of high mobility samples, paving the way to the development of fab-based processes for the integration of layered materials in (opto)-electronic devices.

Layers of transition metal dichalcogenides (TMDs) combine the enhanced effects of correlations associated with the two-dimensional limit with electrostatic control over their phase transitions by means of an electric field. Several semiconducting TMDs, such as MoS$_2$, develop superconductivity (SC) at their surface when doped with an electrostatic field, but the mechanism is still debated. It is often assumed that Cooper pairs reside only in the two electron pockets at the K/K' points of the Brillouin Zone. However, experimental and theoretical results suggest that a multi-valley Fermi surface (FS) is associated with the SC state, involving 6 electron pockets at the Q/Q' points. Here, we perform low-temperature transport measurements in ion-gated MoS$_2$ flakes. We show that a fully multi-valley FS is associated with the SC onset. The Q/Q' valleys fill for doping$\gtrsim2\cdot10^{13}$cm$^{-2}$, and the SC transition does not appear until the Fermi level crosses both spin-orbit split sub-bands Q$_1$ and Q$_2$. The SC state is associated with the FS connectivity and promoted by a Lifshitz transition due to the simultaneous population of multiple electron pockets. This FS topology will serve as a guideline in the quest for new superconductors., Comment: 12 pages, 7 figures

We present quantum yield measurements of single layer $\textrm{WSe}_2$ (1L-$\textrm{WSe}_2$) integrated with high-Q ($Q>10^6$) optical microdisk cavities, using an efficient ($\eta>$90%) near-field coupling scheme based on a tapered optical fiber. Coupling of the excitonic emission is achieved by placing 1L-WSe$_2$ to the evanescent cavity field. This preserves the microresonator high intrinsic quality factor ($Q>10^6$) below the bandgap of 1L-WSe$_2$. The nonlinear excitation power dependence of the cavity quantum yield is in agreement with an exciton-exciton annihilation model. The cavity quantum yield is $\textrm{QY}_\textrm{c}\sim10^{-3}$, consistent with operation in the \textit{broad emitter} regime (i.e. the emission lifetime of 1L-WSe$_2$ is significantly shorter than the bare cavity decay time). This scheme can serve as a precise measurement tool for the excitonic emission of layered materials into cavity modes, for both in plane and out of plane excitation.

Optical harmonic generation occurs when high intensity light ($>10^{10}$W/m$^{2}$) interacts with a nonlinear material. Electrical control of the nonlinear optical response enables applications such as gate-tunable switches and frequency converters. Graphene displays exceptionally strong-light matter interaction and electrically and broadband tunable third order nonlinear susceptibility. Here we show that the third harmonic generation efficiency in graphene can be tuned by over two orders of magnitude by controlling the Fermi energy and the incident photon energy. This is due to logarithmic resonances in the imaginary part of the nonlinear conductivity arising from multi-photon transitions. Thanks to the linear dispersion of the massless Dirac fermions, ultrabroadband electrical tunability can be achieved, paving the way to electrically-tuneable broadband frequency converters for applications in optical communications and signal processing.

We report vertically-illuminated, resonant cavity enhanced, graphene-Si Schottky photodetectors (PDs) operating at 1550nm. These exploit internal photoemission at the graphene-Si interface. To obtain spectral selectivity and enhance responsivity, the PDs are integrated with an optical cavity, resulting in multiple reflections at resonance, and enhanced absorption in graphene. Our devices have wavelength-dependent photoresponse with external (internal) responsivity~20mA/W (0.25A/W). The spectral-selectivity may be further tuned by varying the cavity resonant wavelength. Our devices pave the way for developing high responsivity hybrid graphene-Si free-space illuminated PDs for free-space optical communications, coherence optical tomography and light-radars

Transition metal dichalcogenides (TMDs) are emerging as promising two-dimensional (2d) semiconductors for optoelectronic and flexible devices. However, a microscopic explanation of their photophysics -- of pivotal importance for the understanding and optimization of device operation -- is still lacking. Here we use femtosecond transient absorption spectroscopy, with pump pulse tunability and broadband probing, to monitor the relaxation dynamics of single-layer MoS2 over the entire visible range, upon photoexcitation of different excitonic transitions. We find that, irrespective of excitation photon energy, the transient absorption spectrum shows the simultaneous bleaching of all excitonic transitions and corresponding red-shifted photoinduced absorption bands. First-principle modeling of the ultrafast optical response reveals that a transient bandgap renormalization, caused by the presence of photo-excited carriers, is primarily responsible for the observed features. Our results demonstrate the strong impact of many-body effects in the transient optical response of TMDs even in the low-excitation-density regime., Comment: This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Nano after peer review and technical editing by the publisher. To access the final edited and published work see: http://pubs.acs.org/articlesonrequest/AOR-fcCuTYn5m6wiDTIg98Kx

The equilibrium optical phonons of graphene are well characterized in terms of anharmonicity and electron-phonon interactions, however their non-equilibrium properties in the presence of hot charge carriers are still not fully explored. Here we study the Raman spectrum of graphene under ultrafast laser excitation with 3ps pulses, which trade off between impulsive stimulation and spectral resolution. We localize energy into hot carriers, generating non-equilibrium temperatures in the ~1700-3100K range, far exceeding that of the phonon bath, while simultaneously detecting the Raman response. The linewidth of both G and 2D peaks show an increase as function of the electronic temperature. We explain this as a result of the Dirac cones' broadening and electron-phonon scattering in the highly excited transient regime, important for the emerging field of graphene-based photonics and optoelectronics., Comment: 10 pages, 7 figures

We demonstrate a 10Gb/s Graphene Phase Modulator (GPM) integrated in a Mach-Zehnder interferometer configuration. This is a compact device, with a phase-shifter length of only 300$\mu$m, and 35dB extinction ratio. The GPM has modulation efficiency of 0.28Vcm, one order of magnitude higher compared to state-of-the-art depletion p-n junction Si phase modulators. Our GPM operates with 2V peak-to-peak driving voltage in a push-pull configuration, and it has been tested in a binary transmission of a non-return-to-zero data stream over 50km single mode fibre. This device is the key building block for graphene-based integrated photonics, enabling compact and energy efficient hybrid Si-graphene modulators for telecom, datacom and other applications

Van der Waals heterostructures have emerged as promising building blocks that offer access to new physics, novel device functionalities, and superior electrical and optoelectronic properties. Applications such as thermal management, photodetection, light emission, data communication, high-speed electronics and light harvesting require a thorough understanding of (nanoscale) heat flow. Here, using time-resolved photocurrent measurements we identify an efficient out-of-plane energy transfer channel, where charge carriers in graphene couple to hyperbolic phonon polaritons in the encapsulating layered material. This hyperbolic cooling is particularly efficient, giving picosecond cooling times, for hexagonal BN, where the high-momentum hyperbolic phonon polaritons enable efficient near-field energy transfer. We study this heat transfer mechanism through distinct control knobs to vary carrier density and lattice temperature, and find excellent agreement with theory without any adjustable parameters. These insights may lead to the ability to control heat flow in van der Waals heterostructures.

Electron pairing in the vast majority of superconductors follows the Bardeen-Cooper-Schrieffer theory of superconductivity, which describes the condensation of electrons into pairs with antiparallel spins in a singlet state with an s-wave symmetry. Unconventional superconductivity is predicted in single layer graphene where the electrons pair with a p-wave or chiral d-wave symmetry, depending on the position of the Fermi energy with respect to the Dirac point. By placing single layer graphene on an electron-doped (non-chiral) d-wave superconductor and performing local scanning tunnelling microscopy and spectroscopy, here we show evidence for a p-wave triggered superconducting density of states in single layer graphene. The realization of unconventional superconductivity in single layer graphene offers an exciting new route for the development of p-wave superconductivity using two-dimensional materials with transition temperatures above 4.2 K.

We report the exfoliation of graphite in aqueous solutions under high shear rate [$\sim10^8s^{-1}$] turbulent flow conditions, with a 100\% exfoliation yield. The material is stabilized without centrifugation at concentrations up to 100 g/L using carboxymethylcellulose sodium salt to formulate conductive printable inks. The sheet resistance of blade coated films is below$\sim2\Omega/\square$. This is a simple and scalable production route for graphene-based conductive inks for large area printing in flexible electronics.

We induce surface carrier densities up to $\sim7\cdot 10^{14}$cm$^{-2}$ in few-layer graphene devices by electric double layer gating with a polymeric electrolyte. In 3-, 4- and 5-layer graphene below 20-30K we observe a logarithmic upturn of resistance that we attribute to weak localization in the diffusive regime. By studying this effect as a function of carrier density and with ab-initio calculations we derive the dependence of transport, intervalley and phase coherence scattering lifetimes on total carrier density. We find that electron-electron scattering in the Nyquist regime is the main source of dephasing at temperatures lower than 30K in the $\sim10^{13}$cm$^{-2}$ to $\sim7 \cdot 10^{14}$cm$^{-2}$ range of carrier densities. With the increase of gate voltage, transport elastic scattering is dominated by the competing effects due to the increase in both carrier density and charged scattering centers at the surface. We also tune our devices into a crossover regime between weak and strong localization, indicating that simultaneous tunability of both carrier and defect density at the surface of electric double layer gated materials is possible., Comment: 13 pages, 7 color figures

The flourishing field of two-dimensional (2D) nanophotonics has generated much excitement in the quantum technologies community after the identification of quantum emitters (QEs) in layered materials (LMs). LMs offer many advantages as platforms for quantum circuits, such as integration within hybrid technologies, valley degree of freedom and strong spin-orbit coupling. QEs in LMs, however, suffer from uncontrolled occurrences, added to the uncertainty over their origin, which has been linked to defects and strain gradients. Here, we report a scalable method to create arrays of single-photon emitting QEs in tungsten diselenide (WSe2) and tungsten disulphide (WS2) using a nanopatterned silica substrate. We obtain devices with QE numbers in the range of hundreds, limited only by the flake size, and a QE yield approaching unity. The overall quality of these deterministic QEs surpasses that of their randomly appearing counterparts, with spectral wanderings of around 0.1 meV, an order of magnitude lower than previous reports. Our technique solves the scalability challenge for LM-based quantum photonic devices.

We report ultra-strong high-order nonlinear multiphoton processes in monolayer MoS$_2$ (1L-MoS$_2$): the third harmonic is thirty times stronger than the second harmonic, and the fourth harmonic is comparable to the second harmonic. We find that second and third harmonic processes are strongly dependent on elliptical polarization, which can be used to selectively tune harmonic generation with different orders. We explain this by calculating the nonlinear response functions of 1L-MoS$_2$ with a continuum-model Hamiltonian and quantum-mechanical diagrammatic perturbation theory, highlighting the crucial role of trigonal warping. A similar effect is expected for all other transition-metal dichalcogenides. Our results pave the way for efficient and tunable harmonic generation based on layered materials for various applications, including microscopy and imaging

Graphene is ideally suited for photonic and optoelectronic applications, with a variety of photodetectors (PDs) in the visible, near-infrared (NIR), and THz reported to date, as well as thermal detectors in the mid-infrared (MIR). Here, we present a room temperature-MIR-PD where the pyroelectric response of a LiNbO3 crystal is transduced with high gain (up to 200) into resistivity modulation for graphene, leading to a temperature coefficient of resistance up to 900%/K, two orders of magnitude higher than the state of the art, for a device area of 300x300um2. This is achieved by fabricating a floating metallic structure that concentrates the charge generated by the pyroelectric substrate on the top-gate capacitor of the graphene channel. This allows us to resolve temperature variations down to 15umK at 1 Hz, paving the way for a new generation of detectors for MIR imaging and spectroscopy