Freddie Withers

The University of Manchester, Manchester, England, United Kingdom

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Publications (25)142.02 Total impact

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    ABSTRACT: In this work we use Raman spectroscopy as a non-destructive and rapid technique for probing the Van der Waals (VdW) forces acting between two atomically thin crystals, where one is a transition metal dichalcogenide (TMDC). In this work, MoS2 is used as a Raman probe: we show that its two Raman active phonon modes can provide information on the interaction between the two crystals. In particular, the in-plane vibration (E2g) provides information on the in-plane strain, while the out-of-plane mode (A1g) gives evidence for the quality of the interfacial contact. We show that a VdW contact with MoS2 is characterized by a blue shift of +2 cm-1 of the A1g peak. In the case of a MoS2 /graphene heterostructure, the VdW contact is also characterized by a shift of +14 cm-1 of the 2D peak of graphene. Our approach offers a very simple, non-destructive and fast method to characterize the quality of the interface of heterostructures containing atomically thick TMDCs crystals.
    ACS Nano 09/2014; · 12.03 Impact Factor
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    ABSTRACT: Recent developments in the technology of van der Waals heterostructures made from two-dimensional atomic crystals have already led to the observation of new physical phenomena, such as the metal-insulator transition and Coulomb drag, and to the realization of functional devices, such as tunnel diodes, tunnel transistors and photovoltaic sensors. An unprecedented degree of control of the electronic properties is available not only by means of the selection of materials in the stack, but also through the additional fine-tuning achievable by adjusting the built-in strain and relative orientation of the component layers. Here we demonstrate how careful alignment of the crystallographic orientation of two graphene electrodes separated by a layer of hexagonal boron nitride in a transistor device can achieve resonant tunnelling with conservation of electron energy, momentum and, potentially, chirality. We show how the resonance peak and negative differential conductance in the device characteristics induce a tunable radiofrequency oscillatory current that has potential for future high-frequency technology.
    Nature Nanotechnology 09/2014; · 31.17 Impact Factor
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    ABSTRACT: Topological materials may exhibit Hall-like currents flowing transversely to the applied electric field even in the absence of a magnetic field. In graphene superlattices, which have broken inversion symmetry, topological currents originating from graphene's two valleys are predicted to flow in opposite directions and combine to produce long-range charge neutral flow. We observe this effect as a nonlocal voltage at zero magnetic field in a narrow energy range near Dirac points at distances as large as several microns away from the nominal current path. Locally, topological currents are comparable in strength to the applied current, indicating large valley-Hall angles. The long-range character of topological currents and their transistor-like control by gate voltage can be exploited for information processing based on the valley degrees of freedom.
    arXiv:1409.0113. 08/2014;
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    ABSTRACT: Quasi-two-dimensional (2D) films of layered metal-chalcogenides have attractive optoelectronic properties. However, photonic applications of thin films may be limited owing to weak light absorption and surface effects leading to reduced quantum yield. Integration of 2D films in optical microcavities will permit these limitations to be overcome owing to modified light coupling with the films. Here we present tunable microcavities with embedded monolayer MoS2 or few monolayer GaSe films. We observe significant modification of spectral and temporal properties of photoluminescence (PL): PL is emitted in spectrally narrow and wavelength-tunable cavity modes with quality factors up to 7400; PL life-time shortening by a factor of 10 is achieved, a consequence of Purcell enhancement of the spontaneous emission rate. This work has potential to pave the way to microcavity-enhanced light-emitting devices based on layered 2D materials and their heterostructures, and also opens possibilities for cavity QED in a new material system of van der Waals crystals.
    08/2014;
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    ABSTRACT: One of the challenges associated with the development of next-generation electronics is to find alternatives to silicon oxide caused by the size-reduction constraints of the devices. The dielectric properties of two-dimensional (2D) crystals, added to their excellent chemical stability, mechanical and thermal properties, make them promising dielectrics. Here we show that liquid-phase exfoliation (LPE) in water by using low-cost commercial organic dyes as dispersant agents can efficiently produce defect-free 2D nanosheets, including mono-layers, in suspensions. We further show that these suspensions can be easily incorporated into current practical graphene-based devices. In particular, it is found that boron nitride thin films made by LPE are excellent dielectrics that are highly compatible with graphene-based electronics.
    2D Materials. 06/2014; 1(1):011012.
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    ABSTRACT: In graphene placed on hexagonal boron nitride, replicas of the original Dirac spectrum appear near edges of superlattice minibands. More such replicas develop in high magnetic fields, and their quantization gives rise to a fractal pattern of Landau levels, referred to as the Hofstadter butterfly. Some evidence for the butterfly has recently been reported by using transport measurements. Here we employ capacitance spectroscopy to probe directly the density of states and energy gaps in graphene superlattices. Without magnetic field, replica spectra are seen as pronounced minima in the density of states surrounded by van Hove singularities. The Hofstadter butterfly shows up in magnetocapacitance clearer than in transport measurements and, near one flux quantum per superlattice unit cell, we observe Landau fan diagrams related to quantization of Dirac replicas in a reduced magnetic field. Electron-electron interaction strongly modifies the superlattice spectrum. In particular, we find that graphene's quantum Hall ferromagnetism, due to lifted spin and valley degeneracies, exhibits a reverse Stoner transition at commensurable fluxes and that Landau levels of Dirac replicas support their own ferromagnetic states.
    Nature Physics 06/2014; · 19.35 Impact Factor
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    ABSTRACT: The new paradigm of heterostructures based on two-dimensional (2D) atomic crystals has already led to the observation of exciting physical phenomena and creation of novel devices. The possibility of combining layers of different 2D materials in one stack allows unprecedented control over the electronic and optical properties of the resulting material. Still, the current method of mechanical transfer of individual 2D crystals, though allowing exceptional control over the quality of such structures and interfaces, is not scalable. Here we show that such heterostructures can be assembled from chemically exfoliated 2D crystals, allowing for low-cost and scalable methods to be used in the device fabrication.
    Nano letters. 05/2014;
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    ABSTRACT: Hexagonal boron nitride is the only substrate that has so far allowed graphene devices exhibiting micron-scale ballistic transport. Can other atomically flat crystals be used as substrates for making quality graphene heterostructures? Here we report on our search for alternative substrates. The devices fabricated by encapsulating graphene with molybdenum or tungsten disulphides and hBN are found to exhibit consistently high carrier mobilities of about 60,000 cm2V-1s-1. In contrast, encapsulation with atomically flat layered oxides such as mica, bismuth strontium calcium copper oxide and vanadium pentoxide results in exceptionally low quality of graphene devices with mobilities of ~1,000 cm2V-1s-1. We attribute the difference mainly to self-cleansing that takes place at interfaces between graphene, hBN and transition metal dichalcogenides. Surface contamination assembles into large pockets allowing the rest of the interface to become atomically clean. The cleansing process does not occur for graphene on atomically flat oxide substrates.
    Nano Letters 05/2014; 14(6):3270–3276. · 13.03 Impact Factor
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    ABSTRACT: We present the first study of the intrinsic electrical properties of WS2 transistors fabricated with two different dielectric environments WS2 on SiO2 and WS2 on h-BN/SiO2, respectively. A comparative analysis of the electrical characteristics of multiple transistors fabricated from natural and synthetic WS2 with various thicknesses from single- up to four-layers and over a wide temperature range from 300 K down to 4.2 K shows that disorder intrinsic to WS2 is currently the limiting factor of the electrical properties of this material. These results shed light on the role played by extrinsic factors such as charge traps in the oxide dielectric thought to be the cause for the commonly observed small values of charge carrier mobility in transition metal dichalcogenides.
    Scientific Reports 01/2014; 4:4967. · 5.08 Impact Factor
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    ABSTRACT: Plasmonics has established itself as a branch of physics which promises to revolutionize data processing, improve photovoltaics, and increase sensitivity of bio-detection. A widespread use of plasmonic devices is notably hindered by high losses and the absence of stable and inexpensive metal films suitable for plasmonic applications. To this end, there has been a continuous search for alternative plasmonic materials that are also compatible with complementary metal oxide semiconductor technology. Here we show that copper and silver protected by graphene are viable candidates. Copper films covered with one to a few graphene layers show excellent plasmonic characteristics. They can be used to fabricate plasmonic devices and survive for at least a year, even in wet and corroding conditions. As a proof of concept, we use the graphene-protected copper to demonstrate dielectric loaded plasmonic waveguides and test sensitivity of surface plasmon resonances. Our results are likely to initiate wide use of graphene-protected plasmonics.
    Scientific reports. 01/2014; 4:5517.
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    ABSTRACT: The next-nearest-neighbor hopping term t′ determines a magnitude, and, hence, the importance of several phenomena in graphene that include self-doping due to broken bonds and the Klein tunneling, which in the presence of t′, is no longer perfect. Theoretical estimates for t′ vary widely, whereas a few existing measurements by using polarization-resolved magnetospectroscopy have found surprisingly large t′, close to or even exceeding the highest theoretical values. Here, we report dedicated measurements of the density of states in graphene by using high-quality capacitance devices. The density of states exhibits a pronounced electron-hole asymmetry that increases linearly with energy. This behavior yields t′ ≈ −0.3 eV±15%, in agreement with the high end of theory estimates. We discuss the role of electron-electron interactions in determining t′ and overview phenomena, which can be influenced by such a large value of t′.
    Physical Review B 10/2013; 88(16):165427. · 3.66 Impact Factor
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    ABSTRACT: We investigate the optoelectronic properties of novel graphene/FeCl3-intercalated few-layer graphene (FeCl3-FLG, dubbed graphexeter) heterostructures using photovoltage spectroscopy. We observe a prominent photovoltage signal generated at the graphene/FeCl3-FLG and graphene/Au interfaces, whereas the photovoltage at the FeCl3-FLG/Au interface is negligible. The sign of the photovoltage changes upon sweeping the chemical potential of the pristine graphene through the charge neutrality point, and we show that this is due to the photothermoelectric effect. Our results are a first step toward all-graphene-based photodetectors and photovoltaics.
    ACS Nano 04/2013; · 12.03 Impact Factor
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    ABSTRACT: We demonstrate a novel method to tune the energy gap 1 between the localized states and the mobility edge of the valence band in chemically functionalized graphene by changing the coverage of fluorine adatoms via electron-beam irradiation. From the temperature dependence of the electrical transport properties we show that 1 in partially fluorinated graphene CF0.28 decreases upon electron irradiation up to a dose of 0.08 C cm−2. For low irradiation doses (<0.1 C cm−2) partially fluorinated graphene behaves as a lightly doped semiconductor with impurity bands close to the conduction and valence band edges, whereas for high irradiation doses (>0.2 C cm−2) the electrical conduction takes place via Mott variable range hopping.
    New Journal of Physics 03/2013; 15(3):033024. · 4.06 Impact Factor
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    ABSTRACT: The development of future flexible and transparent electronics relies on novel materials, which are mechanically flexible, lightweight and low-cost, in addition to being electrically conductive and optically transparent. Currently, tin doped indium oxide (ITO) is the most wide spread transparent conductor in consumer electronics. The mechanical rigidity of this material limits its use for future flexible electronic applications. We report novel graphene-based transparent conductors obtained by intercalating few-layer graphene (FLG) with ferric chloride (FeCl3). Through a combined study of electrical transport and optical transmission measurements we demonstrate that FeCl3 enhances the electrical conductivity of FLG by two orders of magnitude while leaving these materials highly transparent [1]. We find that the optical transmittance in the visible range of FeCl3-FLG is typically between 88% and 84%, whereas the resistivity is as low as 8.8 φ. These parameters outperform the best values found in ITO (i.e. resistivity of 10 φ at an optical transmittance of 85%), making therefore FeCl3-FLG the best candidate for flexible and transparent electronics. [4pt] [1] I. Khrapach, F. Withers, T. H. Bointon, D. K. Pplyushkin, W. L. Barnes, S. Russo, M. F. Craciun, Adv. Mater. 24, 2844 (2012).
    03/2013;
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    ABSTRACT: Transparent conductors based on few-layer graphene (FLG) intercalated with ferric chloride (FeCl(3)) have an outstandingly low sheet resistance and high optical transparency. FeCl(3)-FLGs outperform the current limit of transparent conductors such as indium tin oxide, carbon-nanotube films, and doped graphene materials. This makes FeCl(3)-FLG materials the best transparent conductor for optoelectronic devices.
    Advanced Materials 04/2012; 24(21):2844-9. · 14.83 Impact Factor
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    ABSTRACT: We present a fabrication process for high quality suspended and double gated trilayer graphene devices. The electrical transport measurements in these transistors reveal a high charge carrier mobility (higher than 20000 cm^2/Vs) and ballistic electric transport on a scale larger than 200nm. We report a particularly large on/off ratio of the current in ABC-stacked trilayers, up to 250 for an average electric displacement of -0.08 V/nm, compatible with an electric field induced energy gap. The high quality of these devices is also demonstrated by the appearance of quantum Hall plateaus at magnetic fields as low as 500mT.
    Applied Physics Letters 12/2011; 100(1). · 3.79 Impact Factor
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    ABSTRACT: ABSTRACT: We demonstrate the possibility to tune the electronic transport properties of graphene mono-layers and multi-layers by functionalisation with fluorine. For mono-layer samples, with increasing the fluorine content, we observe a transition from electronic transport through Mott variable range hopping (VRH) in two dimensions to Efros-Shklovskii VRH. Multi-layer fluorinated graphene with high concentration of fluorine show two-dimensional Mott VRH transport, whereas CF0.28 multi-layer flakes exhibit thermally activated transport through near neighbour hopping. Our experimental findings demonstrate that the ability to control the degree of functionalisation of graphene is instrumental to engineer different electronic properties in graphene materials.
    Nanoscale Research Letters 09/2011; 6:526. · 2.52 Impact Factor
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    ABSTRACT: We demonstrate the possibility to selectively reduce insulating fluorinated graphene to conducting and semiconducting graphene by electron beam irradiation. Electron-irradiated fluorinated graphene microstructures show 7 orders of magnitude decrease in resistivity (from 1 TΩ to 100 kΩ), whereas nanostructures show a transport gap in the source-drain bias voltage. In this transport gap, electrons are localized, and charge transport is dominated by variable range hopping. Our findings demonstrate a step forward to all-graphene transparent and flexible electronics.
    Nano Letters 08/2011; 11(9):3912-6. · 13.03 Impact Factor
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    ABSTRACT: Engineering the electronic properties of graphene has triggered great interest for potential applications in electronics and opto-electronics. Here we demonstrate the possibility to tune the electronic transport properties of graphene monolayers and multilayers by functionalisation with fluorine. We show that by adjusting the fluorine content different electronic transport regimes can be accessed. For monolayer samples, with increasing the fluorine content, we observe a transition from electronic transport through Mott variable range hopping in two dimensions to Efros - Shklovskii variable range hopping. Multilayer fluorinated graphene with high concentration of fluorine show two-dimensional Mott variable range hopping transport, whereas CF0.28 multilayer flakes have a band gap of 0.25eV and exhibit thermally activated transport. Our experimental findings demonstrate that the ability to control the degree of functionalisation of graphene is instrumental to engineer different electronic properties in graphene materials.
    05/2011;

Publication Stats

144 Citations
142.02 Total Impact Points

Institutions

  • 2013–2014
    • The University of Manchester
      • School of Physics and Astronomy
      Manchester, England, United Kingdom
  • 2011–2013
    • University of Exeter
      • • College of Engineering, Mathematics and Physical Sciences
      • • Department of Engineering
      Exeter, ENG, United Kingdom