arXiv subjects

Tony F. Heinz

Publications and source records attributed to Tony F. Heinz.

At least 73 records · Page 4Linked to original sources

arXiv2012

High-Contrast Electro-Optic Modulation of a Photonic Crystal Nanocavity by Electrical Gating of Graphene

We demonstrate a high-contrast electro-optic modulation of a photonic crystal nanocavity integrated with an electrically gated monolayer graphene. A high quality (Q) factor air-slot nanocavity design is employed for high overlap between the optical field and graphene sheet. Tuning of graphene's Fermi level up to 0.8 eV enables efficient control of its complex dielectric constant, which allows modulation of the cavity reflection in excess of 10 dB for a swing voltage of only 1.5 V. We also observe a controllable resonance wavelength shift close to 2 nm around a wavelength of 1570 nm and a Q factor modulation in excess of three. These observations allow cavity-enhanced measurements of the graphene complex dielectric constant under different chemical potentials, in agreement with a theoretical model of the graphene dielectric constant under gating. This graphene-based nanocavity modulation demonstrates the feasibility of high-contrast, low-power frequency-selective electro-optic nanocavity modulators in graphene-integrated silicon photonic chips.

physics.optics↗

arXiv2012

Observation of tightly bound trions in monolayer MoS2

Two-dimensional (2D) atomic crystals, such as graphene and transition-metal dichalcogenides, have emerged as a new class of materials with remarkable physical properties. In contrast to graphene, monolayer MoS2 is a non-centrosymmetric material with a direct energy gap. Strong photoluminescence, a current on-off ratio exceeding 10^8 in field-effect transistors, and efficient valley and spin control by optical helicity have recently been demonstrated in this material. Here we report the spectroscopic identification in doped monolayer MoS2 of tightly bound negative trions, a quasi-particle composed of two electrons and a hole. These quasi-particles, which can be created with valley and spin polarized holes, have no analogue in other semiconducting materials. They also possess a large binding energy (~ 20 meV), rendering them significant even at room temperature. Our results open up new avenues both for fundamental studies of many-body interactions and for opto-electronic and valleytronic applications in 2D atomic crystals.

cond-mat.mtrl-sci↗

arXiv2012

Layer breathing modes in few-layer graphene

Layer-layer coupling plays a critical role in defining the physical properties of few-layer graphene (FLG). With respect to vibrations, the interlayer coupling is predicted to create a set of N-1 finite frequency, out-of-plane interlayer modes in N-layer graphene. Unlike the widely studied in-plane vibrations, the properties of these layer-breathing modes (LBMs) are defined by the layer-layer interactions. Here we report direct observation of the distinct LBMs for graphene of thicknesses from 2 to 20 layers through electronically resonant overtone Raman bands observed in the range of 80 - 300 cm-1. The Raman bands exhibit multi-peak features that are unique for graphene samples of each layer thickness up to 20 layers. The frequencies of the set of layer breathing modes for all samples can be described within a simple model of nearest plane coupling.

cond-mat.mes-hall↗

arXiv2012

Control of valley polarization in monolayer MoS2 by optical helicity

Electronic and spintronic devices rely on the fact that free charge carriers in solids carry electric charge and spin, respectively. There are, however, other properties of charge carriers that might be exploited in new families of devices. In particular, if there are two or more conduction (or valence) band extrema in momentum space, then confining charge carriers in one of these valleys allows the possibility of valleytronic devices. Such valley polarization has been demonstrated by using strain and magnetic fields, but neither of these approaches allow for dynamic control. Recently, optical control of valley occupancy in graphene with broken inversion symmetry has been proposed but remains experimentally difficult to realize. Here we demonstrate that optical pumping with circularly-polarized light can achieve complete dynamic valley polarization in monolayer MoS2, a two dimensional (2D) non-centrosymmetric crystal with direct energy gaps at two valleys. Moreover, this polarization is retained for longer than 1 ns. Our results demonstrate the viability of optical valley control and valley-based electronic and optoelectronic applications in MoS2 monolayers.

cond-mat.mes-hall↗

arXiv2012

Observation of out-of-plane vibrations in few-layer graphene

We report the observation of layer breathing mode (LBM) vibrations in few-layer graphene (FLG) samples of thickness from 2 to 6 layers, exhibiting both Bernal (AB) and rhombohedral (ABC) stacking order. The LBM vibrations are identified using a Raman combination band lying around 1720 cm-1. From double resonance theory, we identify the feature as the LOZO' combination mode of the out-of-plane LBM (ZO') and the in-plane longitudinal optical mode (LO). The LOZO' Raman band is found to exhibit multiple peaks, with a unique line shape for each layer thickness and stacking order. These complex line shapes of the LOZO'-mode arise both from the material-dependent selection of different phonons in the double-resonance Raman process and from the detailed structure of the different branches of LBM in FLG.

cond-mat.mes-hall↗

arXiv2012

Water-Gated Charge Doping of Graphene Induced by Mica Substrates

We report on the existence of water-gated charge doping of graphene deposited on atomically flat mica substrates. Molecular films of water in units of ~0.4 nm-thick bilayers were found to be present in regions of the interface of graphene/mica hetero-stacks prepared by micromechanical exfoliation of kish graphite. The spectral variation of the G and 2D bands, as visualized by Raman mapping, shows that mica substrates induce strong p-type doping in graphene, with hole densities of $(9 \pm 2) \times 1012 cm${-2}$. The ultrathin water films, however, effectively block interfacial charge transfer, rendering graphene significantly less hole-doped. Scanning Kelvin probe microscopy independently confirmed a water-gated modulation of the Fermi level by 0.35 eV, in agreement with the optically determined hole density. The manipulation of the electronic properties of graphene demonstrated in this study should serve as a useful tool in realizing future graphene applications.

cond-mat.mtrl-sci↗

arXiv2011

High-resolution spatial mapping of the temperature distribution of a Joule self-heated graphene nanoribbon

We investigate the temperature distributions of Joule self-heated graphene nanoribbons (GNRs) with a spatial resolution finer than 100 nm by scanning thermal microscopy (SThM). The SThM probe is calibrated using the Raman G mode Stokes/anti-Stokes intensity ratio as a function of electric power applied to the GNR devices. From a spatial map of the temperature distribution, heat dissipation and transport pathways are investigated. By combining SThM and scanning gate microscopy data from a defected GNR, we observe hot spot formation at well-defined, localized sites.

cond-mat.mes-hall↗

arXiv2011

Observation of Electronic Raman Scattering in Metallic Carbon Nanotubes

We present experimental measurements of the electronic contribution to the Raman spectra of individual metallic single-walled carbon nanotubes (MSWNTs). Photoexcited carriers are inelastically scattered by a continuum of low-energy electron-hole pairs created across the graphenelike linear electronic subbands of the MSWNTs. The optical resonances in MSWNTs give rise to well-defined electronic Raman peaks. This resonant electronic Raman scattering is a unique feature of the electronic structure of these one-dimensional quasimetals.

cond-mat.mes-hall↗

arXiv2011

Structurally Dependent Fano Resonances in the Infrared Spectra of Phonons in Few-Layer Graphene

The in-plane optical phonons around 200 meV in few-layer graphene are investigated utilizing infrared absorption spectroscopy. The phonon spectra exhibit unusual asymmetric features characteristic of Fano resonances, which depend critically on the layer thickness and stacking order of the sample. The phonon intensities in samples with rhombohedral (ABC) stacking are significantly higher than those with Bernal (AB) stacking. These observations reflect the strong coupling between phonons and interband electronic transitions in these systems and the distinctive variation in the joint density of electronic states in samples of differing thickness and stacking order.

cond-mat.mes-hall↗

arXiv2011

Seeing many-body effects in single- and few-layer graphene: Observation of two-dimensional saddle-point excitons

Significant excitonic effects were observed in graphene by measuring its optical conductivity in a broad spectral range including the two-dimensional π-band saddle-point singularities in the electronic structure. The strong electron-hole interactions manifest themselves in an asymmetric resonance peaked at 4.62 eV, which is red-shifted by nearly 600 meV from the value predicted by ab-initio GW calculations for the band-to-band transitions. The observed excitonic resonance is explained within a phenomenological model as a Fano interference of a strongly coupled excitonic state and a band continuum. Our experiment also showed a weak dependence of the excitonic resonance in few-layer graphene on layer thickness. This result reflects the effective cancellation of the increasingly screened repulsive electron-electron (e-e) and attractive electron-hole (e-h) interactions.

cond-mat.mtrl-sci↗

arXiv2011

Visualizing Individual Nitrogen Dopants in Monolayer Graphene

In monolayer graphene, substitutional doping during growth can be used to alter its electronic properties. We used scanning tunneling microscopy (STM), Raman spectroscopy, x-ray spectroscopy, and first principles calculations to characterize individual nitrogen dopants in monolayer graphene grown on a copper substrate. Individual nitrogen atoms were incorporated as graphitic dopants, and a fraction of the extra electron on each nitrogen atom was delocalized into the graphene lattice. The electronic structure of nitrogen-doped graphene was strongly modified only within a few lattice spacings of the site of the nitrogen dopant. These findings show that chemical doping is a promising route to achieving high-quality graphene films with a large carrier concentration.

cond-mat.mtrl-sci↗

arXiv2011

Temperature dependence of the anharmonic decay of optical phonons in carbon nanotubes and graphite

We report on the temperature dependence of the anharmonic decay rate of zone-center (G mode) optical phonons in both single-walled carbon nanotubes and graphite. The measurements are performed using a pump-probe Raman scattering scheme with femtosecond laser pulses. For nanotubes, measured over a temperature range of 6 K-700 K, we observe little temperature dependence of the decay rate below room temperature. Above 300 K, the decay rate increases from 0.8 to 1.7 ps-1. The decay rates observed for graphite range from 0.5 to 0.8 ps-1 for temperatures from 300 K-700 K. We compare the behavior observed in carbon nanotubes and graphite and discuss the implications of our results for the mechanism of the anharmonic decay of optical phonons in both systems.

cond-mat.mtrl-sci↗

arXiv2011

Observation of an electrically tunable band gap in trilayer graphene

A striking feature of bilayer graphene is the induction of a significant band gap in the electronic states by the application of a perpendicular electric field. Thicker graphene layers are also highly attractive materials. The ability to produce a band gap in these systems is of great fundamental and practical interest. Both experimental and theoretical investigations of graphene trilayers with the typical ABA layer stacking have, however, revealed the lack of any appreciable induced gap. Here we contrast this behavior with that exhibited by graphene trilayers with ABC crystallographic stacking. The symmetry of this structure is similar to that of AB stacked graphene bilayers and, as shown by infrared conductivity measurements, permits a large band gap to be formed by an applied electric field. Our results demonstrate the critical and hitherto neglected role of the crystallographic stacking sequence on the induction of a band gap in few-layer graphene.

cond-mat.mes-hall↗

arXiv2011

Observation of Intra- and Inter-band Transitions in the Optical Response of Graphene

The optical conductivity of freely suspended graphene was examined under non-equilibrium conditions using femtosecond pump-probe spectroscopy. We observed a conductivity transient that varied strongly with the electronic temperature, exhibiting a crossover from enhanced to decreased absorbance with increasing pump fluence. The response arises from a combination of bleaching of the inter-band transitions by Pauli blocking and induced absorption from the intra-band transitions of the carriers. The latter dominates at low electronic temperature, but, despite an increase in Drude scattering rate, is overwhelmed by the former at high electronic temperature. The time-evolution of the optical conductivity in all regimes can described in terms of a time-varying electronic temperature.

cond-mat.mes-hall↗

arXiv2010

Imaging Stacking Order in Few-Layer Graphene

Few-layer graphene (FLG) has been predicted to exist in various crystallographic stacking sequences, which can strongly influence the electronic properties of FLG. We demonstrate an accurate and efficient method to characterize stacking order in FLG using the distinctive features of the Raman 2D-mode. Raman imaging allows us to visualize directly the spatial distribution of Bernal (ABA) and rhombohedral (ABC) stacking in tri- and tetra-layer graphene. We find that 15% of exfoliated graphene tri- and tetra-layers is comprised of micron-sized domains of rhombohedral stacking, rather than of usual Bernal stacking. These domains are stable and remain unchanged for temperatures exceeding $800^{\circ}$C.

cond-mat.mtrl-sci↗

arXiv2010

Thermal conductance at the graphene-SiO2 interface measured by optical pump-probe spectroscopy

We have examined the interfacial thermal conductance σint of single and multi-layer graphene samples prepared on fused SiO2 substrates by mechanical exfoliation of graphite. By using an ultrafast optical pump pulse and monitoring the transient reflectivity on the picosecond time scale, we obtained an average σint of 5,000 W/cm2K for the graphene-SiO2 system. We observed significant variation in σint between individual samples, but found no systematic dependence on the thickness of the graphene layers.

cond-mat.mtrl-sci↗

arXiv2010

Ultrafast Photoluminescence from Graphene

Since graphene has no band gap, photoluminescence is not expected from relaxed charge carriers. We have, however, observed significant light emission from graphene under excitation by ultrashort (30-fs) laser pulses. Light emission was found to occur across the visible spectral range (1.7 - 3.5 eV), with emitted photon energies exceeding that of the excitation laser (1.5 eV). The emission exhibits a nonlinear dependence on the laser fluence. In two-pulse correlation measurements of the time-domain response, a dominant relaxation time of tens of femtoseconds is observed. A two-temperature model describing the electrons and their interaction with strongly coupled optical phonons can account for the experimental observations.

cond-mat.mes-hall↗

arXiv2010

Anomalous Lattice Vibrations of Single and Few-Layer MoS2

Molybdenum disulfide (MoS2) of single and few-layer thickness was exfoliated on SiO2/Si substrate and characterized by Raman spectroscopy. The number of S-Mo-S layers of the samples was independently determined by contact-mode atomic-force microscopy. Two Raman modes, E12g and A1g, exhibited sensitive thickness dependence, with the frequency of the former decreasing and that of the latter increasing with thickness. The results provide a convenient and reliable means for determining layer thickness with atomic-level precision. The opposite direction of the frequency shifts, which cannot be explained solely by van der Waals interlayer coupling, is attributed to Coulombic interactions and possible stacking-induced changes of the intralayer bonding. This work exemplifies the evolution of structural parameters in layered materials in changing from the 3-dimensional to the 2-dimensional regime.

cond-mat.mtrl-sci↗