We report high magnetic field scanning tunneling microscopy and Landau level spectroscopy of twisted graphene layers grown by chemical vapor deposition. For twist angles exceeding ~3° the low energy carriers exhibit Landau level spectra characteristic of massless Dirac fermions. Above 20° the layers effectively decouple and the electronic properties are indistinguishable from those in single-layer graphene, while for smaller angles we observe a slowdown of the carrier velocity which is strongly angle dependent. At the smallest angles the spectra are dominated by twist-induced van Hove singularities and the Dirac fermions eventually become localized. An unexpected electron-hole asymmetry is observed which is substantially larger than the asymmetry in either single or untwisted bilayer graphene.

Single-Layer Behavior and Its Breakdown in Twisted Graphene Layers

Ultraflatbands in twisted bilayers of two-dimensional materials have the potential to host strong correlations, including the Mott-insulating phase at half-filling of the band. Using first-principles density functional theory calculations, we show the emergence of ultraflatbands at the valence band edge in twisted bilayer ${\mathrm{MoS}}_{2}$, a prototypical transition metal dichalcogenide. The computed band widths, 5 and 23 meV for 56.5\ifmmode^\circ\else\textdegree\fi{} and 3.5\ifmmode^\circ\else\textdegree\fi{} twist angles, respectively, are comparable to that of twisted bilayer graphene near ``magic'' angles. Large structural transformations in the moir\'e patterns lead to formation of shear solitons at stacking boundaries and strongly influence the electronic structure. We extend our analysis for twisted bilayer ${\mathrm{MoS}}_{2}$ to show that flatbands can occur at the valence band edge of twisted bilayer ${\mathrm{WS}}_{2}$, ${\mathrm{MoSe}}_{2}$, and ${\mathrm{WSe}}_{2}$ as well.

Ultraflatbands and Shear Solitons in Moiré Patterns of Twisted Bilayer Transition Metal Dichalcogenides.

Monolayer MoS2 is a direct-gap two-dimensional semiconductor that exhibits strong electron-hole interactions, leading to the formation of stable excitons and trions. Here we report the existence of efficient exciton-exciton annihilation, a four-body interaction, in this material. Exciton-exciton annihilation was identified experimentally in ultrafast transient absorption measurements through the emergence of a decay channel varying quadratically with exciton density. The rate of exciton-exciton annihilation was determined to be (4.3 ± 1.1) × 10(-2) cm(2)/s at room temperature.

http://absimage.aps.org/image/MAR15/MWS_MAR15-2014-006549.pdf

Observation of Rapid Exciton-Exciton Annihilation in Monolayer Molybdenum Disulfide

This article reviews the theoretical and experimental work related to the electronic properties of bilayer graphene systems. Three types of bilayer stackings are discussed: the AA, AB, and twisted bilayer graphene. This review covers single-electron properties, effects of static electric and magnetic fields, bilayer-based mesoscopic systems, spin-orbit coupling, dc transport and optical response, as well as spontaneous symmetry violation and other interaction effects. The selection of the material aims to introduce the reader to the most commonly studied topics of theoretical and experimental research in bilayer graphene.

/pdf/electronic-properties-of-graphene-based-bilayer-systems-rq753vqhkv.pdf

Electronic properties of graphene-based bilayer systems

Twisted bilayer graphene (tBLG) is a metallic material with two degenerate van Hove singularity transitions that can rehybridize to form interlayer exciton states. Here we report photoluminescence (PL) emission from tBLG after resonant 2-photon excitation, which tunes with the interlayer stacking angle, θ. We spatially image individual tBLG domains at room-temperature and show a five-fold resonant PL-enhancement over the background hot-electron emission. Prior theory predicts that interlayer orbitals mix to create 2-photon-accessible strongly-bound (~0.7 eV) exciton and continuum-edge states, which we observe as two spectral peaks in both PL excitation and excited-state absorption spectra. This peak splitting provides independent estimates of the exciton binding energy which scales from 0.5–0.7 eV with θ = 7.5° to 16.5°. A predicted vanishing exciton-continuum coupling strength helps explain both the weak resonant PL and the slower 1 ps−1 exciton relaxation rate observed. This hybrid metal-exciton behavior electron thermalization and PL emission are tunable with stacking angle for potential enhancements in optoelectronic and fast-photosensing graphene-based applications. Interlayer electronic states in twisted bilayer graphene are characterized by flat-band regions hosting many-body electronic effects. Here, the authors observe two-photon photoluminescence excitation and excited-state absorption spectra on graphene containing a variety of twist angles to access the dark exciton transitions and estimate the exciton binding energy.

/pdf/stacking-angle-tunable-photoluminescence-from-interlayer-1a9br1161z.pdf

Stacking angle-tunable photoluminescence from interlayer exciton states in twisted bilayer graphene.

When two sheets of graphene stack in a twisted bilayer graphene (tBLG) configuration, the resulting constrained overlap between interplanar 2p orbitals produce angle-tunable electronic absorption resonances. By applying a novel combination of multiphoton transient absorption (TA) microscopy and TEM, we resolve the electronic structure and ensuing relaxation by probing resonant excitations of single tBLG domains. Strikingly, we find that the transient electronic population in resonantly excited tBLG domains is enhanced many fold, forming a major electronic relaxation bottleneck. Two-photon TA microscopy shows this bottleneck effect originates from a strongly bound, dark exciton state lying ∼0.37 eV below the 1-photon absorption resonance. This stable coexistence of strongly bound excitons alongside free-electron continuum states has not been previously observed in a metallic, 2D material.

/pdf/tunable-optical-excitations-in-twisted-bilayer-graphene-form-4yyf70urqx.pdf

Tunable Optical Excitations in Twisted Bilayer Graphene Form Strongly Bound Excitons

Using resonant 2-photon excitation of interlayer electrons in twisted bilayer graphene (tBLG), we resolve photoluminescence (PL) that tunes spectrally with stacking angle, {\theta}. This weak signal is 4- 5$\times$ larger than the non-resonant background and is emitted from the interlayer band anti-crossing regions traditionally associated with van Hove singularity resonances. However, our observation of resonant PL emission with delayed ~1 ps electronic thermalization suggests interlayer carriers may instead form bound-excitons. Using both the 2-photon PL and intraband transient absorption spectra, we observe bright and dark state peak-splitting associated with an interlayer exciton binding energy ranging from 0.5 to 0.7 eV for {\theta} = 8$^o$ to 17$^o$. These results support theoretical models showing interlayer excitons in tBLG are stabilized by a vanishing exciton-coupling strength to the metallic continuum states. This unexpected dual metal-exciton optical property of tBLG suggests possible {\theta}-tuneable control over carrier thermalization, extraction and emission in optical graphene-based devices.

/pdf/theta-tunable-photoluminescence-from-interlayer-excitons-in-5wn2ydlk7r.pdf

{\theta}-Tunable Photoluminescence from Interlayer Excitons in Twisted Bilayer Graphene

We study how double-stranded DNA translocates through graphene nanogaps. Nanogaps are fabricated with a novel capillary-force induced graphene nanogap formation technique. DNA translocation signatures for nanogaps are qualitatively different from those obtained with circular nanopores, owing to the distinct shape of the gaps discussed here. Translocation time and conductance values vary by ∼ 100%, which we suggest are caused by local gap width variations. We also observe exponentially relaxing current traces. We suggest that slow relaxation of the graphene membrane following DNA translocation may be responsible. We conclude that DNA-graphene interactions are important, and need to be considered for graphene-nanogap based devices. This work further opens up new avenues for direct read of single molecule activitities, and possibly sequencing.

/pdf/dna-graphene-interactions-during-translocation-through-2wbf5j7ege.pdf

Hiral Patel

Papers

Stacking angle-tunable photoluminescence from interlayer exciton states in twisted bilayer graphene.

Tunable Optical Excitations in Twisted Bilayer Graphene Form Strongly Bound Excitons

{\theta}-Tunable Photoluminescence from Interlayer Excitons in Twisted Bilayer Graphene

DNA-graphene interactions during translocation through nanogaps.

Rare earth niobate coordination polymers