Pablo Jarillo-Herrero

Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

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Publications (101)792.93 Total impact

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    ABSTRACT: Photoexcitation of graphene leads to an interesting sequence of phenomena, some of which can be exploited in optoelectronic devices based on graphene. In particular, the efficient and ultrafast generation of an electron distribution with an elevated electron temperature and the concomitant generation of a photo-thermoelectric voltage at symmetry-breaking interfaces is of interest for photosensing and light harvesting. Here, we experimentally study the generated photocurrent at the graphene-metal interface, focusing on the time-resolved photocurrent, the effects of photon energy, Fermi energy and light polarization. We show that a single framework based on photo-thermoelectric photocurrent generation explains all experimental results.
    11/2014;
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    ABSTRACT: Controlling the energy flow processes and the associated energy relaxation rates of a light emitter is of high fundamental interest, and has many applications in the fields of quantum optics, photovoltaics, photodetection, biosensing and light emission. While advanced dielectric and metallic systems have been developed to tailor the interaction between an emitter and its environment, active control of the energy flow has remained challenging. Here, we demonstrate in-situ electrical control of the relaxation pathways of excited erbium ions, which emit light at the technologically relevant telecommunication wavelength of 1.5 $\mu$m. By placing the erbium at a few nanometres distance from graphene, we modify the relaxation rate by more than a factor of three, and control whether the emitter decays into either electron-hole pairs, emitted photons or graphene near-infrared plasmons, confined to $<$15 nm to the sheet. These capabilities to dictate optical energy transfer processes through electrical control of the local density of optical states constitute a new paradigm for active (quantum) photonics.
    10/2014;
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    ABSTRACT: We report on electronic transport measurements of dual-gated nano-devices of the low-carrier density topological insulator Bi1.5Sb0.5Te1.7Se1.3. In all devices the upper and lower surface states are independently tunable to the Dirac point by the top and bottom gate electrodes. In thin devices, electric fields are found to penetrate through the bulk, indicating finite capacitive coupling between the surface states. A charging model allows us to use the penetrating electric field as a measurement of the inter-surface capacitance $C_{TI}$ and the surface state energy-density relationship $\mu$(n), which is found to be consistent with independent ARPES measurements. At high magnetic fields, increased field penetration through the surface states is observed, strongly suggestive of the opening of a surface state band gap due to broken time-reversal symmetry.
    Physical review letters. 10/2014; 113(20).
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    ABSTRACT: A perpendicular electric field breaks the layer symmetry of Bernal-stacked bilayer graphene, resulting in the opening of a band gap and a modification of the effective mass of the charge carriers. Using scanning tunneling microscopy and spectroscopy, we examine standing waves in the local density of states of bilayer graphene formed by scattering from a bilayer/trilayer boundary. The quasiparticle interference properties are controlled by the bilayer graphene band structure, allowing a direct local probe of the evolution of the band structure of bilayer graphene as a function of electric field. We extract the Slonczewski-Weiss-McClure model tight binding parameters as $\gamma_0 = 3.1$ eV, $\gamma_1 = 0.39$ eV, and $\gamma_4 = 0.22$ eV.
    APL MATERIALS. 06/2014; 2(9).
  • Hugh O H Churchill, Pablo Jarillo-Herrero
    Nature Nanotechnology 05/2014; 9(5):330-1. · 31.17 Impact Factor
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    ABSTRACT: The crystal structure of a material plays an important role in determining its electronic properties. Changing from one crystal structure to another involves a phase transition that is usually controlled by a state variable such as temperature or pressure. In the case of trilayer graphene, there are two common stacking configurations (Bernal and rhombohedral) that exhibit very different electronic properties. In graphene flakes with both stacking configurations, the region between them consists of a localized strain soliton where the carbon atoms of one graphene layer shift by the carbon-carbon bond distance. Here we show the ability to move this strain soliton with a perpendicular electric field and hence control the stacking configuration of trilayer graphene with only an external voltage. Moreover, we find that the free-energy difference between the two stacking configurations scales quadratically with electric field, and thus rhombohedral stacking is favoured as the electric field increases. This ability to control the stacking order in graphene opens the way to new devices that combine structural and electrical properties.
    Nature Material 04/2014; · 35.75 Impact Factor
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    ABSTRACT: We report on temperature dependent photocurrent measurements of high-quality dual-gated monolayer graphene (MLG) p-n junction devices. A photothermoelectric (PTE) effect governs the photocurrent response in our devices, allowing us to track the hot electron temperature and probe hot electron cooling channels over a wide temperature range (4 K to 300 K). At high temperatures ($T > T^*$), we found that both the peak photocurrent and the hot spot size decreased with temperature, while at low temperatures ($T < T^*$), we found the opposite, namely that the peak photocurrent and the hot spot size increased with temperature. This non-monotonic temperature dependence can be understood as resulting from the competition between two hot electron cooling pathways: (a) (intrinsic) momentum-conserving normal collisions (NC) that dominates at low temperatures and (b) (extrinsic) disorder-assisted supercollisions (SC) that dominates at high temperatures. Gate control in our high quality samples allows us to resolve the two processes in the same device for the first time. The peak temperature $T^*$ depends on carrier density and disorder concentration, thus allowing for an unprecedented way of controlling graphene's photoresponse.
    Physical review letters. 03/2014; 112(24).
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    ABSTRACT: The p-n junction is the functional element of many electronic and optoelectronic devices, including diodes, bipolar transistors, photodetectors, light-emitting diodes and solar cells. In conventional p-n junctions, the adjacent p- and n-type regions of a semiconductor are formed by chemical doping. Ambipolar semiconductors, such as carbon nanotubes, nanowires and organic molecules, allow for p-n junctions to be configured and modified by electrostatic gating. This electrical control enables a single device to have multiple functionalities. Here, we report ambipolar monolayer WSe2 devices in which two local gates are used to define a p-n junction within the WSe2 sheet. With these electrically tunable p-n junctions, we demonstrate both p-n and n-p diodes with ideality factors better than 2. Under optical excitation, the diodes demonstrate a photodetection responsivity of 210 mA W(-1) and photovoltaic power generation with a peak external quantum efficiency of 0.2%, promising values for a nearly transparent monolayer material in a lateral device geometry. Finally, we demonstrate a light-emitting diode based on monolayer WSe2. These devices provide a building block for ultrathin, flexible and nearly transparent optoelectronic and electronic applications based on ambipolar dichalcogenide materials.
    Nature Nanotechnology 03/2014; · 31.17 Impact Factor
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    ABSTRACT: van der Waals heterostructures assembled from atomically thin crystalline layers of diverse two-dimensional solids are emerging as a new paradigm in the physics of materials. We used infrared nanoimaging to study the properties of surface phonon polaritons in a representative van der Waals crystal, hexagonal boron nitride. We launched, detected, and imaged the polaritonic waves in real space and altered their wavelength by varying the number of crystal layers in our specimens. The measured dispersion of polaritonic waves was shown to be governed by the crystal thickness according to a scaling law that persists down to a few atomic layers. Our results are likely to hold true in other polar van der Waals crystals and may lead to new functionalities.
    Science 03/2014; 343(6175):1125-9. · 31.20 Impact Factor
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    ABSTRACT: We explore the photoresponse of an ambipolar graphene infrared thermocouple at photon energies close to or below monolayer graphene's optical phonon energy and electrostatically accessible Fermi energy levels. The ambipolar graphene infrared thermocouple consists of monolayer graphene supported by an infrared absorbing material, controlled by two independent electrostatic gates embedded below the absorber. Using a scanning infrared laser microscope, we characterize these devices as a function of carrier type and carrier density difference controlled at the junction between the two electrostatic gates. Based on these measurements, conducted at both mid- and near-infrared wavelengths, the primary detection mechanism can be modeled as a thermoelectric response. By studying the effect of different infrared absorbers, we determine that the optical absorption and thermal conduction of the substrate play the dominant role in the measured photoresponse of our devices. These experiments indicate a path toward hybrid graphene thermal detectors for sensing applications such as thermography and chemical spectroscopy.
    Nano Letters 01/2014; · 13.03 Impact Factor
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    ABSTRACT: licenses/by-nc-sa/4.0/ The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters.
    arXiv:1401.7663. 01/2014;
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    ABSTRACT: Low-dimensional electronic systems have traditionally been obtained by electrostatically confining electrons, either in heterostructures or in intrinsically nanoscale materials such as single molecules, nanowires and graphene. Recently, a new method has emerged with the recognition that symmetry-protected topological (SPT) phases, which occur in systems with an energy gap to quasiparticle excitations (such as insulators or superconductors), can host robust surface states that remain gapless as long as the relevant global symmetry remains unbroken. The nature of the charge carriers in SPT surface states is intimately tied to the symmetry of the bulk, resulting in one- and two-dimensional electronic systems with novel properties. For example, time reversal symmetry endows the massless charge carriers on the surface of a three-dimensional topological insulator with helicity, fixing the orientation of their spin relative to their momentum. Weakly breaking this symmetry generates a gap on the surface, resulting in charge carriers with finite effective mass and exotic spin textures. Analogous manipulations have yet to be demonstrated in two-dimensional topological insulators, where the primary example of a SPT phase is the quantum spin Hall state. Here we demonstrate experimentally that charge-neutral monolayer graphene has a quantum spin Hall state when it is subjected to a very large magnetic field angled with respect to the graphene plane. In contrast to time-reversal-symmetric systems, this state is protected by a symmetry of planar spin rotations that emerges as electron spins in a half-filled Landau level are polarized by the large magnetic field. The properties of the resulting helical edge states can be modulated by balancing the applied field against an intrinsic antiferromagnetic instability, which tends to spontaneously break the spin-rotation symmetry. In the resulting canted antiferromagnetic state, we observe transport signatures of gapped edge states, which constitute a new kind of one-dimensional electronic system with a tunable bandgap and an associated spin texture.
    Nature 12/2013; · 38.60 Impact Factor
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    Y H Wang, H Steinberg, P Jarillo-Herrero, N Gedik
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    ABSTRACT: The unique electronic properties of the surface electrons in a topological insulator are protected by time-reversal symmetry. Circularly polarized light naturally breaks time-reversal symmetry, which may lead to an exotic surface quantum Hall state. Using time- and angle-resolved photoemission spectroscopy, we show that an intense ultrashort midinfrared pulse with energy below the bulk band gap hybridizes with the surface Dirac fermions of a topological insulator to form Floquet-Bloch bands. These photon-dressed surface bands exhibit polarization-dependent band gaps at avoided crossings. Circularly polarized photons induce an additional gap at the Dirac point, which is a signature of broken time-reversal symmetry on the surface. These observations establish the Floquet-Bloch bands in solids and pave the way for optical manipulation of topological quantum states of matter.
    Science 10/2013; 342(6157):453-7. · 31.20 Impact Factor
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    ABSTRACT: We report electronic transport measurements of devices based on monolayers and bilayers of the transition-metal dichalcogenide MoS2. Through a combination of in situ vacuum annealing and electrostatic gating we obtained ohmic contact to the MoS2 down to 4 K at high carrier densities. At lower carrier densities, low temperature four probe transport measurements show a metal-insulator transition in both monolayer and bilayer samples. In the metallic regime, the high temperature behavior of the mobility showed strong temperature dependence consistent with phonon dominated transport. At low temperature, intrinsic field-effect mobilities greater than 1000 cm2/Vs were observed for both monolayer and bilayer devices. Mobilities extracted from Hall effect measurements were several times lower and showed a strong dependence on density, likely caused by screening of charged impurity scattering at higher densities.
    Nano Letters 08/2013; · 13.03 Impact Factor
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    ABSTRACT: Van der Waals heterostructures comprise a new class of artificial materials formed by stacking atomically-thin planar crystals. Here, we demonstrate band structure engineering of a van der Waals heterostructure composed of a monolayer graphene flake coupled to a rotationally-aligned hexagonal boron nitride substrate. The spatially-varying interlayer atomic registry results both in a local breaking of the carbon sublattice symmetry and a long-range moir\'e superlattice potential in the graphene. This interplay between short- and long-wavelength effects results in a band structure described by isolated superlattice minibands and an unexpectedly large band gap at charge neutrality, both of which can be tuned by varying the interlayer alignment. Magnetocapacitance measurements reveal previously unobserved fractional quantum Hall states reflecting the massive Dirac dispersion that results from broken sublattice symmetry. At ultra-high fields, integer conductance plateaus are observed at non-integer filling factors due to the emergence of the Hofstadter butterfly in a symmetry-broken Landau level.
    Science 05/2013; 340:1427. · 31.20 Impact Factor
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    ABSTRACT: An exchange gap in the Dirac surface states of a topological insulator (TI) is necessary for observing the predicted unique features such as the topological magnetoelectric effect as well as to confine Majorana fermions. We experimentally demonstrate proximity-induced ferromagnetism in a TI, combining a ferromagnetic insulator EuS layer with Bi_{2}Se_{3}, without introducing defects. By magnetic and magnetotransport studies, including anomalous Hall effect and magnetoresistance measurements, we show the emergence of a ferromagnetic phase in TI, a step forward in unveiling their exotic properties.
    Physical Review Letters 05/2013; 110(18):186807. · 7.73 Impact Factor
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    ABSTRACT: Graphene is a two-dimensional (2D) material that has attracted great interest for electronic devices since its discovery in 2004. Due to its zero band gap band structure, it has a broad-band optical absorption ranging from the far-infrared all the way to the visible making it potentially useful for infrared photodetectors. Electrostatically gated p-n junctions have demonstrated photocurrents in the near-IR (λ= 850nm), primarily due to hot carrier mechanisms. In order to study these mechanisms at longer wavelengths (λ= 10 μm), high quality chemically vapor grown (CVD) graphene is necessary to fabricate electrostatically controlled p-n junctions due to the longer optical length scales. Moreover, at these low energies (˜ 125 meV), optical phonon scattering is suppressed and is predicted to lead to increased carrier lifetimes and enhanced photo-response. Using electrostatic gating, we are able to study the absorption mechanisms in graphene by selecting between conventional photovoltaic effects and photo-thermoelectric effects. Experiments suggest that the photocurrent signal is enhanced by electrostatic gating near the Dirac peak and reduced disorder in the graphene sample.
    03/2013;
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    ABSTRACT: In order to investigate the predicted exotic behavior of topological insulators (TIs) epitaxial films with near ideal electronic properties are essential. Obtaining high quality TI films requires careful control of not only growth parameters but also a good understanding of the dynamics of film formation. We have developed methods to obtain consistently high mobility and low carrier density by carefully controlling the nucleation and growth process of Bi2Se3 epitaxial films. Such MBE grown epitaxial films have been well characterized by different diffraction based techniques and electrical transport to obtain a correlation between structural and electrical properties. This has allowed us to see their systematic dependence. For example, in thin films, carrier density in low 10^12/cm^2 range with bulk mobilities higher than 3000 cm^2/V-s are routinely seen which nicely compares very well with structural data. Acknowledgements: NSF grant DMR 1207469 and NSF DMR 08-19762 (CMSE -- Initiative 2).
    03/2013;
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    ABSTRACT: We report progress on the fabrication and measurement of multi-terminal devices based on few-layer MoS2. By using different contact metal recipes, we describe efforts to significantly decrease contact resistance and gain access to the intrinsic transport properties of MoS2. We measured four-terminal resistance of monolayer, bilayer, and trilayer MoS2 with Ohmic contacts to obtain the intrinsic field-effect mobility of these materials on SiO2 substrates at temperatures down to 4 K. We also probed Hall transport of MoS2 and extracted the temperature dependence of its Hall mobility.
    03/2013;
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    ABSTRACT: We report electronic transport measurements on double-gated topological insulator Bi2Se3 devices. To obtain both top- and bottom-gating, we exfoliate the Bi2Se3 on standard SiO2-capped Si and coat it with an ultrathin layer of hexagonal Boron Nitride (h-BN), which serves as a dielectric for a top gate. Using both top and bottom gates, we are able to identify the individual contributions of both surfaces and the bulk channel, and show that all three channels have mobilities exceeding 1000 cm2/Vs. Our results suggest that the h-BN transfer technique holds potential for providing a future path for high quality TI density-tunable devices.
    03/2013;

Publication Stats

2k Citations
792.93 Total Impact Points

Institutions

  • 2009–2014
    • Massachusetts Institute of Technology
      • Department of Physics
      Cambridge, Massachusetts, United States
  • 2011–2012
    • The University of Arizona
      • Department of Physics
      Tucson, AZ, United States
  • 2010–2012
    • Harvard University
      • Department of Physics
      Cambridge, Massachusetts, United States
  • 2007
    • Columbia University
      • Department of Physics
      New York City, NY, United States
  • 2004–2005
    • Delft University Of Technology
      • Applied Geophysics and Petrophysics
      Delft, South Holland, Netherlands