Session X31: 2D Materials: Synthesis, Structure, and Properties

Observation of Image Potential State in Oxygen Intercalated Graphene on Iridium by Two-Photon-Photoemission Spectroscopy

In this talk, we report our experimental results on the first direct observation of image potential state (IPS) in oxygen-intercalated graphene on iridium by two-photo-photoemission spectroscopy. We demonstrate how oxygen intercalation influences the IPS in Gr/Ir and decouples the interlayer interaction. We present measurements of the electronic dispersion and work function in pristine Gr/Ir, oxygen-intercalated Gr/O/Ir, and deintercalated Gr/Ir. LEED patterns are measured during the pristine, oxygen-intercalated, and deintercalated phases of the Gr/Ir sample. Based on these measurements, relative to the pristine case, the work function and the energy location of n$=$1 IPS relative to the Fermi level increases by $\approx $0.39 eV and $\approx $0.3 eV, respectively, due to oxygen intercalation, whereas the effective mass of n$=$1 IPS is hardly influenced by the intercalation process. Moreover, we achieve the quenching and restoration of the resonance from Ir Rashba states to n$=$1 IPS in Gr/Ir by oxygen intercalation and deintercalation.   

Structural Corrugation of Graphene Oxides (C$_2$O Phase)

Although there are a lot of theoretical investigations on Graphene oxides (GO) configurations,\footnote{S. Mao, H. Pu, J. Chen,\textbf{ RSC Adv.}, 2012, 2, 2643.}$^{,}$\footnote{ H. J. Xiang, S. Wei, X. G. Gong,\textbf{ Phys. Rev. B: Condens. Matter Mater. Phys.}, 2010, 82, 035416.}$^{,}$\footnote{ D. K. Samarakoon, X. Q. Wang,\textbf{ Nanoscale}, 2011, 3, 192.} an in-depth explanation of the experimental observations in GO is still not there. The challenge is connected to inhomogeneous nature of the oxidation. We studied the structural, electronic, and vibrational properties of graphene oxide employing a particle swarm optimization search along with density functional theory calculations. Here we report a novel low-energy semiconducting configuration for the C$_2$O phase of graphene oxide that consists of a combination of 1,2 and 1,3-epoxides as well as carbonyl functional groups running along the armchair direction. A thorough examination of the corrugation and bonding reveals unique features of the new conformation which are in reasonable agreement with experimental observations. Our findings shed light on structural and electronic properties that are important for future improvement of graphene-based nanodevices.   

Graphene mediated electrically tunable emission enhancement of quantum dots

Graphene is known to strongly quench quantum dots and emitters in close proximity to its surface. While this aspect, enabled by a strong non-radiative energy transfer rate, is utilised in various applications, no pristine graphene based light emitting or display devices have been successfully demonstrated so far. On the other hand semiconductor quantum dots have been wodely used as a spectrally tunable, highly efficient, materials in light emitting devices and displays. Here we will discuss the first demonstration of electrically tunable enhanced photoluminescence in a hybrid device of CdSe quantum dots in close proximity to a single layer of graphene. The enhancement is maximum at the Dirac point and decreases symmetrically with positive or negative gate voltage. The photoluminescence enhancement is dependent on the emission wavelength of quantum dots in the visible regime and is also dependent on the surface density of quantum dots. FDTD simulations reveal that graphene mediated super-radiance of quantum dots is responsible for the observed emission enhancement of the quantum dots.   

Electrostatically-defined graphene nanoribbon

Electron confinement of Dirac fermions in graphene has remained a longstanding challenge.~ Owing to the gapless nature of the bandstructure, conventional depletion-gate schemes cannot be employed. Nano scale constrictions may be realized by etching, however this results in significant edge disorder, that tends to dominate the resulting device characteristics. ~ Here, we discuss a new approach to electrostatic confinement in graphene where we take advantage of either a Moiré induced energy gap (present when graphene is fabricated with zero-angle alignment to BN) as well as the $\nu =$ 0 quantum Hall state (a magnetic field induced energy gap without edge states). We use a dual-gated structure to set one region to the induced gap, while the other varies the Fermi energy in the confinement region.~ One dimensional nanoribbons are realized by utilizing carbon nanotubes as one of the electrostatic gates, demonstrated by the appearance of quantized step sin conductance.~   

Magnetic noise spectroscopy as a probe of hydrodynamic transport in graphene

We develop the theoretical framework for calculating magnetic noise from conducting two dimensional (2D) materials. We describe how local measurements of this noise can directly probe the wave-vector dependent transport properties of the material over a broad range of length scales, thus providing new insight into a range of correlated phenomena in 2D electronic systems. As an example, we demonstrate how transport in the hydrodynamic regime in an electronic system exhibits a unique signature in the magnetic noise profile that distinguishes it from diffusive and ballistic transport and how it can be used to measure the viscosity of the electronic fluid. We employ a Boltzmann approach in a two-time relaxation-time approximation to compute the conductivity of graphene and quantitatively illustrate these transport regimes and the experimental feasibility of observing them. We also discuss signatures of isolated impurities lodged inside the conducting 2D material. The noise near an impurity is found to be suppressed compared to the background by an amount that is directly proportional to the cross-section of electrons/holes scattering off of the impurity. We use these results to outline an experimental proposal to measure the temperature dependent scattering properties of the impurity   

Hall viscosity and electromagnetic response of electrons in graphene

The Hall viscosity is a dissipationless component of the viscosity tensor of an electron liquid with broken time- reversal symmetry, such as a two-dimensional electron gas (2DEG) in the quantum Hall state. Similar to the Hall conductivity, the Hall viscosity is an anomalous transport coefficient; however, while the former is connected with the current response, the latter stems from the stress response to a geometric deformation. For a Galilean-invariant system such as 2DEG, the current density is indeed the generator of the geometric deformation: therefore a connection between the Hall connectivity and viscosity is expected and by now well established [1]. In the case of graphene, a non-Galilean-invariant system, the existence of such a connection is far from obvious, as the current operator is essentially different from the momentum operator. In this talk, I will first present our results of the geometric Hall viscosity of electrons in single-layer graphene [2]. Then, from the expansion of the nonlocal Hall conductivity for small wave vectors, I demonstrate that, in spite of the lack of Galilean invariance, an effective mass can be defined such that the relationship between the Hall conductivity and the viscosity retains the form it has in Galilean-invariant systems, not only for a large number of occupied Landau levels, but also, with very high accuracy, for the undoped system. [1] Hoyos and Son, PRL 108, 066805 (2012). [2] Sherafati, Principi, and Vignale, PRB 94, 125427 (2016).   

Modification of Electronic Band Structure in mL+nL Free-Stacking Graphene

We studied stacked mL+nL (m=1, 2; n=1-5) graphene layers using Raman spectroscopy. Our results indicate that the 2D band from stacked graphene can be considered as a superposition of those from the constituent nL and mL graphene layers, and a blueshift in the 2D band is observed when n or m = 1. The blueshift increases with the number of stacked layers. This is ascribed to the reduction of Fermi velocity in the single layer graphene. As the number of stacked layers changes from 1 to 5, the Fermi velocity in the single layer graphene reduces to about 85\% of its initial value. This study provides a convenient way to realize the modification of Fermi velocity in graphene and is of significance to the applications of graphene-based heterostructures.   

Quantum Tunneling of Thermal Protons Through Pristine Graphene

Atomically thin two-dimensional materials such as graphene and hexagonal boron nitride have recently been found to exhibit appreciable permeability to thermal protons, making these materials emerging candidates for separation technologies [S. Hu \textit{et al.}, Nature 516, 227 (2014); M. Lozada-Hidalgo \textit{et al.}, Science 351, 68 (2016).]. These remarkable findings remain unexplained by density-functional electronic structure calculations, which instead yield Arrhenius activation energies that exceed by $\sim$1.0~eV those found in experiments. Here we demonstrate that the thermal proton transfer through pristine graphene is driven by nuclear quantum effects, which substantially reduce the value of Arrhenius activation energy by up to 1.0~eV compared to the results of classical molecular dynamics. We show that an account for pre-exponential factors in the Arrhenius equation (entropic effects) is crucial in order to reproduce the observed isotope effect. Due to different delocalization of protons and deuterons these factors differ by more than seven orders of magnitude. Our findings offer new insights for controlling the underlying quantum ion transport mechanisms in nanostructured separation membranes.   

Coherent destruction of tunnelling in laser-graphene interactions

Coherent destruction of tunnelling (CDT) is defined as a critical slow-down of the dynamics of a quantum system that occurs when its adiabatic eigenstates exhibit a close avoided crossing. CDT has been observed in several quantum systems such as semiconductor superlattices, superconducting qubits and molecules in laser fields. In this work, CDT in low-dimensional Dirac materials is described using the viewpoint of Floquet theory. More specifically, the case of photo-excited graphene is considered. Conduction band populations are computed for various combinations of incident laser pulse shapes and polarizations. It is shown that these laser parameters provide control knobs over the phenomenon of CDT in graphene. Specifically, multiphoton peaks in momentum space can be selectively suppressed or enhanced. The potential of experimental techniques such as ARPES for the future observation of CDT in graphene is also discussed.   

Topological phases in a multi-band 2D electron gas.

Recent experiments have established the phase diagrams of mono-, bilayer and ABC-stacked trilayer graphene at the charge neutrality, but that in more complex cases of graphene multiband structures such as ABA-stacked trilayer graphene (TLG) have not been thoroughly investigated. We report transport studies in ABA-TLG where the effect of Coulomb interactions plays a crucial role in formation of the different orbital/spin/valley symmetries. We explore theoretically and experimentally the effect of magnetic and displacement fields on these symmetries within the $\nu {\rm g}=$ 0 quantum Hall state. Our experimental results indicate at least 5 different phases arise from multiband nature of TLG, with conductance ranging from 0.1e$^{\mathrm{2}}$/h, 2e$^{\mathrm{2}}$/h to 4e$^{\mathrm{2}}$/h, reflecting the rich interplay between crystal symmetry and Coulomb interactions.   

Method for Transferring High-Mobility CVD-Grown Graphene with Perfluoropolymers

The transfer of graphene grown by chemical vapor deposition (CVD) using amorphous polymers represents a widely implemented method for graphene-based electronic device fabrication. However, the most commonly used polymer, poly(methyl methacrylate) (PMMA), leaves a residue on the graphene that limits the mobility. Here we report a method for graphene transfer and patterning that employs a perfluoropolymer --- Hyflon --- as a transfer handle and to protect the graphene against contamination from photoresists or other polymers. CVD-grown graphene transferred this way onto LaAlO$_3$/SrTiO$_3$ heterostructures is atomically clean, with high mobility (˜30,000 cm$^2$V$^{−1}$s$^{−1}$) near the Dirac point at 2 K and clear, quantized Hall and magnetoresistance. Local control of the LaAlO$_3$/SrTiO$_3$ interfacial metal-insulator transition --- through the graphene --- is preserved with this transfer method. The use of perfluoropolymers, such as Hyflon, with CVD-grown graphene and other 2D materials can readily be implemented with other polymers or photoresists.   

Detection of ionized gas molecules in air by graphene and carbon nanotube networks

The liquid phase ions sensing by graphene and carbon nanotube has been demonstrated in many publications due to the minimum gate voltage easily shift induced by ionic gating effect, but it is still unclear for vapor phase ions sensing. Here we want to report that the ionized gas molecules in air can be also very sensitively detected by graphene and carbon nanotube networks under very low applied voltage, which shows the very high charge to current amplification factor, the value can be up to 10$^{\mathrm{8}}$~A/C, and the direction of current-change can be used to differentiate the positive and negative ions. In further, the field effect of graphene device induced by vapor phase ions was discussed.   

Casimir Force Phase Transitions in the Graphene Family

The Casimir force is a universal interaction induced by electromagnetic quantum fluctuations between any types of objects. The expansion of the graphene family by adding silicene, germanene, and stanene is a promising platform to probe Dirac-like physics in honeycomb staggered systems in such a ubiquitous interaction. Here, we discuss the quantum mechanical regime of the Casimir interaction between layers of the graphene family. We discover Casimir force phase transitions between these staggered 2D materials induced by the complex interplay between Dirac physics, spin-orbit coupling, and externally applied fields. We find that the interaction energy experiences different power law distance decays, magnitudes, and dependences on characteristic physical constants. Furthermore, due to the topological properties of these materials, repulsive and quantized Casimir interactions are possible. Finally, thermal corrections to the Casimir interaction owing to finite temperature of the system are also addressed.   

Many-body dynamics and Floquet gap opening in interacting periodically driven systems.

Periodically driven quantum systems have recently attracted a lot of interest due to peculiar phases they can host. In particular, driving by circularly polarized light was predicted to open a gap with non-trivial topological properties in graphene-like systems. We study the dynamics in such a system at transient times after the switch-on of the driving, taking into account electron-electron interactions self-consistently up to second order. We analyze the emergence and time-dependent renormalization of the Floquet gap. We investigate the time scales for the emergence of the Floquet-Bloch bands in the non-equilibrium correlation functions, accessible in ARPES experiments.   

Topological Thouless pumping in graphene and 2D Dirac materials

We present a comprehensive analysis of strain-induced topological Thouless pumping of charge and valley currents in graphene and 2D Dirac materials. We analyze the role of strain deformations with all possible symmetries and classify the charge and valley currents that are adiabatically pumped in response. These manifest as transport without an applied bias. The production of valley currents implies that strained Dirac materials are a candidate platform for valleytronic applications.