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1.
Nano Lett ; 20(1): 658-663, 2020 Jan 08.
Artigo em Inglês | MEDLINE | ID: mdl-31809057

RESUMO

We present a versatile scheme dedicated to exerting strong electric fields up to 0.5 MV/cm on color centers in hexagonal silicon carbide, employing transparent epitaxial graphene electrodes. In both the axial and basal direction equally strong electric fields can be selectively controlled. Investigating the silicon vacancy (VSi) in ensemble photoluminescence experiments, we report Stark splitting of the V1' line of 3 meV by a basal electrical field and a Stark shift of the V1 line of 1 meV in an axial electric field. The spectral fine-tuning of the VSi, being an important candidate for realizing quantum networks, paves the way for truly indistinguishable single-photon sources.

2.
Angew Chem Int Ed Engl ; 59(14): 5602-5606, 2020 Mar 27.
Artigo em Inglês | MEDLINE | ID: mdl-31833618

RESUMO

Patterned graphene-functionalization with a tunable degree of functionalization can tailor the properties of graphene. Here, we present a new reductive functionalization approach combined with lithography rendering patterned graphene-functionalization easily accessible. Two types of covalent patterning of graphene were prepared and their structures were unambiguously characterized by statistical Raman spectroscopy together with scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM-EDS). The reversible defunctionalization processes, as revealed by temperature-dependent Raman spectroscopy, enable the possibility to accurately modulate the degree of functionalization by annealing. This allows for the management of chemical information through complete write/store/erase cycles. Based on our strategy, controllable and efficient patterning graphene-functionalization is no longer a challenge and facilitates the development of graphene-based devices.

3.
Sci Rep ; 9(1): 11205, 2019 Aug 01.
Artigo em Inglês | MEDLINE | ID: mdl-31371741

RESUMO

We present an efficient Schottky-diode detection scheme for Terahertz (THz) radiation, implemented on the material system epitaxial graphene on silicon carbide (SiC). It employs SiC as semiconductor and graphene as metal, with an epitaxially defined interface. For first prototypes, we report on broadband operation up to 580 GHz, limited only by the RC circuitry, with a responsivity of 1.1 A/W. Remarkably, the voltage dependence of the THz responsivity displays no deviations from DC responsivity, which encourages using this transparent device for exploring the high frequency limits of Schottky rectification in the optical regime. The performance of the detector is demonstrated by resolving sharp spectroscopic features of ethanol and acetone in a THz transmission experiment.

4.
Phys Rev Lett ; 121(20): 207401, 2018 Nov 16.
Artigo em Inglês | MEDLINE | ID: mdl-30500256

RESUMO

We investigate coherent electron dynamics in graphene, interacting with the electric field waveform of two orthogonally polarized, few-cycle laser pulses. Recently, we demonstrated that linearly polarized driving pulses lead to sub-optical-cycle Landau-Zener quantum path interference by virtue of the combination of intraband motion and interband transition [Higuchi et al., Nature 550, 224 (2017)NATUAS0028-083610.1038/nature23900]. Here we introduce a pulsed control laser beam, orthogonally polarized to the driving pulses, and observe the ensuing electron dynamics. The relative delay between the two pulses is a tuning parameter to control the electron trajectory, now in a complex fashion exploring the full two-dimensional reciprocal space in graphene. Depending on the relative phase, the electron trajectory in the reciprocal space can, e.g., be deformed to suppress the quantum path interference resulting from the driving laser pulse. Intriguingly, this strong-field-based complex matter wave manipulation in a two-dimensional conductor is driven by a high repetition rate laser oscillator, rendering unnecessary complex and expensive amplified laser systems.

5.
Proc Natl Acad Sci U S A ; 115(38): 9509-9514, 2018 09 18.
Artigo em Inglês | MEDLINE | ID: mdl-30181293

RESUMO

Lead halide perovskites are used in thin-film solar cells, which owe their high efficiency to the long lifetimes of photocarriers. Various calculations find that a dynamical Rashba effect could significantly contribute to these long lifetimes. This effect is predicted to cause a spin splitting of the electronic bands of inversion-symmetric crystalline materials at finite temperatures, resulting in a slightly indirect band gap. Direct experimental evidence of the existence or the strength of the spin splitting is lacking. Here, we resonantly excite photocurrents in single crystalline ([Formula: see text])[Formula: see text] with circularly polarized light to clarify the existence of spin splittings in the band structure. We observe a circular photogalvanic effect, i.e., the photocurrent depends on the light helicity, in both orthorhombic and tetragonal ([Formula: see text])[Formula: see text] At room temperature, the effect peaks for excitation photon energies [Formula: see text] meV below the direct optical band gap. Temperature-dependent measurements reveal a sign change of the effect at the orthorhombic-tetragonal phase transition, indicating different microscopic origins in the two phases. Within the tetragonal phase, both [Formula: see text] and the amplitude of the circular photogalvanic effect increase with temperature. Our findings support a dynamical Rashba effect in this phase, i.e., a spin splitting caused by thermally induced structural fluctuations which break inversion symmetry.

6.
Nature ; 550(7675): 224-228, 2017 10 12.
Artigo em Inglês | MEDLINE | ID: mdl-28953882

RESUMO

The ability to steer electrons using the strong electromagnetic field of light has opened up the possibility of controlling electron dynamics on the sub-femtosecond (less than 10-15 seconds) timescale. In dielectrics and semiconductors, various light-field-driven effects have been explored, including high-harmonic generation, sub-optical-cycle interband population transfer and the non-perturbative change of the transient polarizability. In contrast, much less is known about light-field-driven electron dynamics in narrow-bandgap systems or in conductors, in which screening due to free carriers or light absorption hinders the application of strong optical fields. Graphene is a promising platform with which to achieve light-field-driven control of electrons in a conducting material, because of its broadband and ultrafast optical response, weak screening and high damage threshold. Here we show that a current induced in monolayer graphene by two-cycle laser pulses is sensitive to the electric-field waveform, that is, to the exact shape of the optical carrier field of the pulse, which is controlled by the carrier-envelope phase, with a precision on the attosecond (10-18 seconds) timescale. Such a current, dependent on the carrier-envelope phase, shows a striking reversal of the direction of the current as a function of the driving field amplitude at about two volts per nanometre. This reversal indicates a transition of light-matter interaction from the weak-field (photon-driven) regime to the strong-field (light-field-driven) regime, where the intraband dynamics influence interband transitions. We show that in this strong-field regime the electron dynamics are governed by sub-optical-cycle Landau-Zener-Stückelberg interference, composed of coherent repeated Landau-Zener transitions on the femtosecond timescale. Furthermore, the influence of this sub-optical-cycle interference can be controlled with the laser polarization state. These coherent electron dynamics in graphene take place on a hitherto unexplored timescale, faster than electron-electron scattering (tens of femtoseconds) and electron-phonon scattering (hundreds of femtoseconds). We expect these results to have direct ramifications for band-structure tomography and light-field-driven petahertz electronics.

7.
Nat Commun ; 8(1): 342, 2017 08 24.
Artigo em Inglês | MEDLINE | ID: mdl-28839136

RESUMO

Charge transport at the Dirac point in bilayer graphene exhibits two dramatically different transport states, insulating and metallic, that occur in apparently otherwise indistinguishable experimental samples. We demonstrate that the existence of these two transport states has its origin in an interplay between evanescent modes, that dominate charge transport near the Dirac point, and disordered configurations of extended defects in the form of partial dislocations. In a large ensemble of bilayer systems with randomly positioned partial dislocations, the distribution of conductivities is found to be strongly peaked at both the insulating and metallic limits. We argue that this distribution form, that occurs only at the Dirac point, lies at the heart of the observation of both metallic and insulating states in bilayer graphene.In seemingly indistinguishable bilayer graphene samples, two distinct transport regimes, insulating and metallic, have been identified experimentally. Here, the authors demonstrate that these two states originate from the interplay between extended defects and evanescent modes at the Dirac point.

8.
Nanoscale ; 9(21): 7217-7226, 2017 Jun 01.
Artigo em Inglês | MEDLINE | ID: mdl-28513712

RESUMO

We investigate charge transport in C60-based single-molecule junctions with graphene electrodes employing a combination of density functional theory (DFT) electronic structure calculations and Landauer transport theory. In particular, the dependence of the transport properties on the conformation of the molecular bridge and the type of termination of the graphene electrodes is investigated. Furthermore, electron pathways through the junctions are analyzed using the theory of local currents. The results reveal, in agreement with previous experiments, a pronounced dependence of the transport properties on the bias polarity, which is rationalized in terms of the electronic structure of the molecule. It is also shown that the edge states of zigzag-terminated graphene induce additional transport channels, which dominate transport at low voltages. The importance of the edge states for transport depends profoundly on the interface geometry of the junctions.

9.
Nat Commun ; 7: 12894, 2016 Sep 27.
Artigo em Inglês | MEDLINE | ID: mdl-27671003

RESUMO

Mie scattering is an optical phenomenon that appears when electromagnetic waves, in particular light, are elastically scattered at a spherical or cylindrical object. A transfer of this phenomenon onto electron states in ballistic graphene has been proposed theoretically, assuming a well-defined incident wave scattered by a perfectly cylindrical nanometer scaled potential, but experimental fingerprints are lacking. We present an experimental demonstration of an electrical analogue to Mie scattering by using graphene as a conductor, and circular potentials arranged in a square two-dimensional array. The tabletop experiment is carried out under seemingly unfavourable conditions of diffusive transport at room-temperature. Nonetheless, when a canted arrangement of the array with respect to the incident current is chosen, cascaded Mie scattering results robustly in a transverse voltage. Its response on electrostatic gating and variation of potentials convincingly underscores Mie scattering as underlying mechanism. The findings presented here encourage the design of functional electronic metamaterials.

10.
Nano Lett ; 15(5): 3512-8, 2015 May 13.
Artigo em Inglês | MEDLINE | ID: mdl-25923590

RESUMO

On the way to ultraflat single-molecule junctions with transparent electrodes, we present a fabrication scheme based on epitaxial graphene nanoelectrodes. As a suitable molecule, we identified a molecular wire with fullerene anchor groups. With these two components, stable electrical characteristics could be recorded. Electrical measurements show that single-molecule junctions with graphene and with gold electrodes display a striking agreement. This motivated a hypothesis that the differential conductance spectra are rather insensitive to the electrode material. It is further corroborated by the assignment of asymmetries and spectral features to internal molecular degrees of freedom. The demonstrated open-access graphene electrodes and the electrode-insensitive molecules provide a model system that will allow for a thorough investigation of an individual single-molecule contact with additional probes.

11.
Nature ; 505(7484): 533-7, 2014 Jan 23.
Artigo em Inglês | MEDLINE | ID: mdl-24352231

RESUMO

Dislocations represent one of the most fascinating and fundamental concepts in materials science. Most importantly, dislocations are the main carriers of plastic deformation in crystalline materials. Furthermore, they can strongly affect the local electronic and optical properties of semiconductors and ionic crystals. In materials with small dimensions, they experience extensive image forces, which attract them to the surface to release strain energy. However, in layered crystals such as graphite, dislocation movement is mainly restricted to the basal plane. Thus, the dislocations cannot escape, enabling their confinement in crystals as thin as only two monolayers. To explore the nature of dislocations under such extreme boundary conditions, the material of choice is bilayer graphene, the thinnest possible quasi-two-dimensional crystal in which such linear defects can be confined. Homogeneous and robust graphene membranes derived from high-quality epitaxial graphene on silicon carbide provide an ideal platform for their investigation. Here we report the direct observation of basal-plane dislocations in freestanding bilayer graphene using transmission electron microscopy and their detailed investigation by diffraction contrast analysis and atomistic simulations. Our investigation reveals two striking size effects. First, the absence of stacking-fault energy, a unique property of bilayer graphene, leads to a characteristic dislocation pattern that corresponds to an alternating AB B[Symbol: see text]AC change of the stacking order. Second, our experiments in combination with atomistic simulations reveal a pronounced buckling of the bilayer graphene membrane that results directly from accommodation of strain. In fact, the buckling changes the strain state of the bilayer graphene and is of key importance for its electronic properties. Our findings will contribute to the understanding of dislocations and of their role in the structural, mechanical and electronic properties of bilayer and few-layer graphene.

12.
Nat Nanotechnol ; 8(8): 575-9, 2013 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-23851359

RESUMO

Single-molecule spintronics investigates electron transport through magnetic molecules that have an internal spin degree of freedom. To understand and control these individual molecules it is important to read their spin state. For unpaired spins, the Kondo effect has been observed as a low-temperature anomaly at small voltages. Here, we show that a coupled spin pair in a single magnetic molecule can be detected and that a bias voltage can be used to switch between two states of the molecule. In particular, we use the mechanically controlled break-junction technique to measure electronic transport through a single-molecule junction containing two coupled spin centres that are confined on two Co(2+) ions. Spin-orbit configuration interaction methods are used to calculate the combined spin system, where the ground state is found to be a pseudo-singlet and the first excitations behave as a pseudo-triplet. Experimentally, these states can be assigned to the absence and occurrence of a Kondo-like zero-bias anomaly in the low-temperature conductance data, respectively. By applying finite bias, we can repeatedly switch between the pseudo-singlet state and the pseudo-triplet state.

13.
ACS Nano ; 7(5): 4441-8, 2013 May 28.
Artigo em Inglês | MEDLINE | ID: mdl-23586703

RESUMO

We present a fabrication process for freely suspended membranes consisting of bi- and trilayer graphene grown on silicon carbide. The procedure, involving photoelectrochemical etching, enables the simultaneous fabrication of hundreds of arbitrarily shaped membranes with an area up to 500 µm(2) and a yield of around 90%. Micro-Raman and atomic force microscopy measurements confirm that the graphene layer withstands the electrochemical etching and show that the membranes are virtually unstrained. The process delivers membranes with a cleanliness suited for high-resolution transmission electron microscopy (HRTEM) at atomic scale. The membrane, and its frame, is very robust with respect to thermal cycling above 1000 °C as well as harsh acidic or alkaline treatment.

14.
Phys Rev Lett ; 109(5): 056801, 2012 Aug 03.
Artigo em Inglês | MEDLINE | ID: mdl-23006194

RESUMO

We analyze quantum interference and decoherence effects in single-molecule junctions both experimentally and theoretically by means of the mechanically controlled break junction technique and density-functional theory. We consider the case where interference is provided by overlapping quasidegenerate states. Decoherence mechanisms arising from electronic-vibrational coupling strongly affect the electrical current flowing through a single-molecule contact and can be controlled by temperature variation. Our findings underline the universal relevance of vibrations for understanding charge transport through molecular junctions.

15.
Phys Rev Lett ; 108(10): 106601, 2012 Mar 09.
Artigo em Inglês | MEDLINE | ID: mdl-22463434

RESUMO

We investigate the magnetotransport in large area graphene Hall bars epitaxially grown on silicon carbide. In the intermediate field regime between weak localization and Landau quantization, the observed temperature-dependent parabolic magnetoresistivity is a manifestation of the electron-electron interaction. We can consistently describe the data with a model for diffusive (magneto)transport that also includes magnetic-field-dependent effects originating from ballistic time scales. We find an excellent agreement between the experimentally observed temperature dependence of magnetoresistivity and the theory of electron-electron interaction in the diffusive regime. We can further assign a temperature-driven crossover to the reduction of the multiplet modes contributing to electron-electron interaction from 7 to 3 due to intervalley scattering. In addition, we find a temperature-independent ballistic contribution to the magnetoresistivity in classically strong magnetic fields.

16.
Nat Mater ; 10(5): 357-60, 2011 May.
Artigo em Inglês | MEDLINE | ID: mdl-21460820

RESUMO

High-quality epitaxial graphene on silicon carbide (SiC) is today available in wafer size. Similar to exfoliated graphene, its charge carriers are governed by the Dirac-Weyl Hamiltonian and it shows excellent mobilities. For many experiments with graphene, in particular for surface science, a bottom gate is desirable. Commonly, exfoliated graphene flakes are placed on an oxidized silicon wafer that readily provides a bottom gate. However, this cannot be applied to epitaxial graphene as the SiC provides the source material out of which graphene grows. Here, we present a reliable scheme for the fabrication of bottom-gated epitaxial graphene devices, which is based on nitrogen (N) implantation into a SiC wafer and subsequent graphene growth. We demonstrate working devices in a broad temperature range from 6 to 300 K. Two gating regimes can be addressed, which opens a wide engineering space for tailored devices by controlling the doping of the gate structure.

17.
Phys Rev Lett ; 106(13): 136807, 2011 Apr 01.
Artigo em Inglês | MEDLINE | ID: mdl-21517410

RESUMO

We carry out experiments on single-molecule junctions at low temperatures, using the mechanically controlled break junction technique. Analyzing the results obtained with various molecules, the nature of the first peak in the differential conductance spectra is elucidated. We observe an electronic transition with a vibronic fine structure, if the first peak occurs at small voltages. This regime can accurately be described by the resonant tunneling model. At higher voltages, additional smearing is observed and no fine structure can be resolved. A detailed analysis of the noise signal indicates that the onset of current is associated with strong fluctuations as a precursor of current flow. The data indicate that a complex fluctuation-driven transport mechanism takes over in this regime.

18.
Chemphyschem ; 11(10): 2256-60, 2010 Jul 12.
Artigo em Inglês | MEDLINE | ID: mdl-20521299

RESUMO

We investigate the effect of vibrations on the electronic transport through single-molecule junctions, using the mechanically controlled break junction technique. The molecules under investigation are oligoyne chains with appropriate end groups, which represent both an ideally linear electrical wire and an ideal molecular vibrating string. Vibronic features can be detected as satellites to the electronic transitions, which are assigned to longitudinal modes of the string by comparison with density functional theory data.

19.
Nat Mater ; 8(3): 203-7, 2009 Mar.
Artigo em Inglês | MEDLINE | ID: mdl-19202545

RESUMO

Graphene, a single monolayer of graphite, has recently attracted considerable interest owing to its novel magneto-transport properties, high carrier mobility and ballistic transport up to room temperature. It has the potential for technological applications as a successor of silicon in the post Moore's law era, as a single-molecule gas sensor, in spintronics, in quantum computing or as a terahertz oscillator. For such applications, uniform ordered growth of graphene on an insulating substrate is necessary. The growth of graphene on insulating silicon carbide (SiC) surfaces by high-temperature annealing in vacuum was previously proposed to open a route for large-scale production of graphene-based devices. However, vacuum decomposition of SiC yields graphene layers with small grains (30-200 nm; refs 14-16). Here, we show that the ex situ graphitization of Si-terminated SiC(0001) in an argon atmosphere of about 1 bar produces monolayer graphene films with much larger domain sizes than previously attainable. Raman spectroscopy and Hall measurements confirm the improved quality of the films thus obtained. High electronic mobilities were found, which reach mu=2,000 cm (2) V(-1) s(-1) at T=27 K. The new growth process introduced here establishes a method for the synthesis of graphene films on a technologically viable basis.

20.
Small ; 4(12): 2229-35, 2008 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-19016500

RESUMO

The charge transport through a single ruthenium atom clamped by two terpyridine hinges is investigated, both experimentally and theoretically. The metal-bis(terpyridyl) core is equipped with rigid, conjugated linkers of para-acetyl-mercapto phenylacetylene to establish electrical contact in a two-terminal configuration using Au electrodes. The structure of the [Ru(II)(L)(2)](PF(6))(2) molecule is determined using single-crystal X-ray crystallography, which yields good agreement with calculations based on density functional theory (DFT). By means of the mechanically controllable break-junction technique, current-voltage (I-V), characteristics of [Ru(II)(L)(2)](PF(6))(2) are acquired on a single-molecule level under ultra-high vacuum (UHV) conditions at various temperatures. These results are compared to ab initio transport calculations based on DFT. The simulations show that the cardan-joint structural element of the molecule controls the magnitude of the current. Moreover, the fluctuations in the cardan angle leave the positions of steps in the I-V curve largely invariant. As a consequence, the experimental I-V characteristics exhibit lowest-unoccupied-molecular-orbit-based conductance peaks at particular voltages, which are also found to be temperature independent.


Assuntos
Rutênio/química , Cristalização , Cristalografia por Raios X , Modelos Moleculares , Estrutura Molecular
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