-
[show abstract]
[hide abstract]
ABSTRACT: Atomically thin two-dimensional materials have emerged as promising candidates for flexible and transparent electronic applications. Here we show non-volatile memory devices, based on field-effect transistors with large hysteresis, consisting entirely of stacked two-dimensional materials. Graphene and molybdenum disulphide were employed as both channel and charge-trapping layers, whereas hexagonal boron nitride was used as a tunnel barrier. In these ultrathin heterostructured memory devices, the atomically thin molybdenum disulphide or graphene-trapping layer stores charge tunnelled through hexagonal boron nitride, serving as a floating gate to control the charge transport in the graphene or molybdenum disulphide channel. By varying the thicknesses of two-dimensional materials and modifying the stacking order, the hysteresis and conductance polarity of the field-effect transistor can be controlled. These devices show high mobility, high on/off current ratio, large memory window and stable retention, providing a promising route towards flexible and transparent memory devices utilizing atomically thin two-dimensional materials.
Nature Communications 03/2013; 4:1624. · 7.40 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The most celebrated property of the quantum spin Hall effect is the presence
of spin-polarized counter-propagating edge states. This novel edge state
configuration has also been predicted to occur in graphene when spin-split
electron- and hole-like Landau levels are forced to cross at the edge of the
sample. In particular, a quantum spin Hall analogue has been predicted at
{\nu}=0 in bilayer graphene if the ground state is a spin ferromagnet. Previous
studies have demonstrated that the bilayer {\nu}=0 state is an insulator in a
perpendicular magnetic field, though the exact nature of this state has not
been identified. Here we present measurements of the {\nu}=0 state in a
dual-gated bilayer graphene device in tilted magnetic field. The application of
an in-plane magnetic field and perpendicular electric field allows us to map
out a full phase diagram of the {\nu}=0 state as a function of experimentally
tunable parameters. At large in-plane magnetic field we observe a quantum phase
transition to a metallic state with conductance of order 4e^2/h, consistent
with predictions for the ferromagnet.
12/2012;
-
Theanne Schiros,
Dennis Nordlund,
Lucia Pálová,
Deborah Prezzi,
Liuyan Zhao,
Keun Soo Kim,
Ulrich Wurstbauer,
Christopher Gutiérrez,
Dean Delongchamp,
Cherno Jaye,
Daniel Fischer,
Hirohito Ogasawara,
Lars G M Pettersson,
David R Reichman, Philip Kim,
Mark S Hybertsen,
Abhay N Pasupathy
[show abstract]
[hide abstract]
ABSTRACT: Robust methods to tune the unique electronic properties of graphene by chemical modification are in great demand due to the potential of the two dimensional material to impact a range of device applications. Here we show that carbon and nitrogen core-level resonant X-ray spectroscopy is a sensitive probe of chemical bonding and electronic structure of chemical dopants introduced in single-sheet graphene films. In conjunction with density functional theory based calculations, we are able to obtain a detailed picture of bond types and electronic structure in graphene doped with nitrogen at the sub-percent level. We show that different N-bond types, including graphitic, pyridinic, and nitrilic, can exist in a single, dilutely N-doped graphene sheet. We show that these various bond types have profoundly different effects on the carrier concentration, indicating that control over the dopant bond type is a crucial requirement in advancing graphene electronics.
Nano Letters 06/2012; 12(8):4025-31. · 13.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Graphene single layers grown by chemical vapor deposition on single crystal Cu substrates are subject to nonuniform physisorption strains that depend on the orientation of the Cu surface. The strains are revealed in Raman spectra and quantitatively interpreted by molecular dynamics (MD) simulations. An average compressive strain on the order of 0.5% is determined in graphene on Cu(111). In graphene on Cu (100), MD simulations interpret the observed highly nonuniform strains.
Nano Letters 04/2012; 12(5):2408-13. · 13.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We characterize the terahertz detection mechanism in antenna-coupled metallic
single-walled carbon nanotubes. At low temperature, 4.2 K, a peak in the
low-frequency differential resistance is observed at zero bias current due to
non-Ohmic contacts. This electrical contact nonlinearity gives rise to the
measured terahertz response. By modeling each nanotube contact as a nonlinear
resistor in parallel with a capacitor, we determine an upper bound for the
value of the contact capacitance that is smaller than previous experimental
estimates. The small magnitude of this contact capacitance has favorable
implications for the use of carbon nanotubes in high-frequency device
applications.
03/2012;
-
[show abstract]
[hide abstract]
ABSTRACT: We report on transport measurements of the insulating state that forms at the charge neutrality point of graphene in a magnetic field. Using both conventional two-terminal measurements, sensitive to bulk and edge conductance, and Corbino measurements, sensitive only to the bulk conductance, we observed a vanishing conductance with increasing magnetic fields. By examining the resistance changes of this insulating state with varying perpendicular and in-plane fields, we probe the spin-active components of the excitations in total fields of up to 45 T. Our results indicate that the ν=0 quantum Hall state in single layer graphene is not spin-polarized.
Physical Review Letters 03/2012; 108(10):106804. · 7.37 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: 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 heterostacks 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 ± 2) × 10(12) 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, which is 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.
Nano Letters 02/2012; 12(2):648-54. · 13.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: In a graphene Landau level (LL), strong Coulomb interactions and the fourfold
spin/valley degeneracy lead to an approximate SU(4) isospin symmetry. At
partial filling, exchange interactions can spontaneously break this symmetry,
manifesting as additional integer quantum Hall plateaus outside the normal
sequence. Here we report the observation of a large number of these quantum
Hall isospin ferromagnetic (QHIFM) states, which we classify according to their
real spin structure using temperature-dependent tilted field magnetotransport.
The large measured activation gaps confirm the Coulomb origin of the broken
symmetry states, but the order is strongly dependent on LL index. In the high
energy LLs, the Zeeman effect is the dominant aligning field, leading to real
spin ferromagnets with Skyrmionic excitations at half filling, whereas in the
`relativistic' zero energy LL, lattice scale anisotropies drive the system to a
spin unpolarized state, likely a charge- or spin-density wave.
01/2012;
-
Jaesung Park,
Sae Byeok Jo,
Young-Jun Yu,
Youngsoo Kim,
Jae Won Yang,
Wi Hyoung Lee,
Hyun Ho Kim,
Byung Hee Hong, Philip Kim,
Kilwon Cho,
Kwang S Kim
[show abstract]
[hide abstract]
ABSTRACT: Dual doping-driven perpendicular electric field with opposite directions remarkably increase the on/off current ratio of bilayer graphene field-effect transistors. This unambiguously proves that it is possible to open a bandgap with two molecular dopants (F4-TCNQ and NH(2) -functionalized self-assembled monolayers (SAMs)) even in a single-gate device structure.
Advanced Materials 12/2011; · 13.88 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: 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.
10/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: We have performed temperature-dependent electrical transport measurements on known structure single wall carbon nanotubes at low bias. The experiments show a superlinear increase in nanotube resistivity with temperature, which is in contradiction with the linear dependence expected from nanotube acoustic-phonon scattering. The measured electron mean free path is also much lower than expected, especially at medium to high temperatures (>100 K). A theoretical model that includes scattering due to surface polar phonon modes of the substrates reproduces the experiments very well. The role of surface phonons is further confirmed by resistivity measurements of nanotubes on aluminum nitride.
Physical Review Letters 09/2011; 107(14):146601. · 7.37 Impact Factor
-
Liuyan Zhao,
Rui He,
Kwang Taeg Rim,
Theanne Schiros,
Keun Soo Kim,
Hui Zhou,
Christopher Gutiérrez,
S P Chockalingam,
Carlos J Arguello,
Lucia Pálová,
Dennis Nordlund,
Mark S Hybertsen,
David R Reichman,
Tony F Heinz, Philip Kim,
Aron Pinczuk,
George W Flynn,
Abhay N Pasupathy
[show abstract]
[hide abstract]
ABSTRACT: In monolayer graphene, substitutional doping during growth can be used to alter its electronic properties. We used scanning tunneling microscopy, 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.
Science 08/2011; 333(6045):999-1003. · 31.20 Impact Factor
-
Advanced Materials 08/2011; 23(31):3531-5. · 13.88 Impact Factor
-
Nature 06/2011; · 36.28 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report a multiband transport study of bilayer graphene at high carrier
densities. Employing a poly(ethylene)oxide-CsClO$_4$ solid polymer electrolyte
gate we demonstrate the filling of the high energy subbands in bilayer graphene
samples at carrier densities $|n|\geq2.4\times 10^{13}$ cm$^{-2}$. We observe a
sudden increase of resistance and the onset of a second family of Shubnikov de
Haas (SdH) oscillations as these high energy subbands are populated. From
simultaneous Hall and magnetoresistance measurements together with SdH
oscillations in the multiband conduction regime, we deduce the carrier
densities and mobilities for the higher energy bands separately and find the
mobilities to be at least a factor of two higher than those in the low energy
bands.
05/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: We characterize radio frequency detection in a high-quality metallic
single-walled carbon nanotube. At a bath temperature of 77 K, only bolometric
(thermal) detection is seen. At a bath temperature of 4.2 K and low bias
current, the response is due instead to the electrical nonlinearity of the
non-ohmic contacts. At higher bias currents, the contacts recover ohmic
behavior and the observed response agrees well with the calculated bolometric
responsivity. The bolometric response is expected to operate at terahertz
frequencies, and we discuss some of the practical issues associated with
developing high frequency detectors based on carbon nanotubes.
05/2011;
-
[show abstract]
[hide abstract]
ABSTRACT: Nanometer-scale graphene objects are attracting much research interest because of newly emerging properties originating from quantum confinement effects. We present Raman spectroscopy studies of graphene nanoribbons (GNRs), which are known to have nonzero electronic bandgap. GNRs of width ranging from 15 to 100 nm have been prepared by e-beam lithographic patterning of mechanically exfoliated graphene followed by oxygen plasma etching. Raman spectra of narrow GNRs can be characterized by an upshifted G band and a prominent disorder-related D band originating from scattering at the ribbon edges. The D-to-G band intensity ratio generally increases with decreasing ribbon width. However, its decrease in width of <25 nm, partly attributed to amorphization at the edges, provides a valuable experimental estimate on D mode relaxation length of <5 nm. The upshift in the G band of the narrowest GNRs can be attributed to confinement effect or chemical doping by functional groups on the GNR edges. Notably, GNRs are much more susceptible to photothermal effects resulting in reversible hole doping caused by atmospheric oxygen than bulk graphene sheets. Finally we show that the 2D band is still a reliable marker in determining the number of layers of GNRs despite its significant broadening for very narrow GNRs.
ACS Nano 03/2011; 5(5):4123-30. · 10.77 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The elementary excitations of monolayer graphene, which behave as massless Dirac particles, make it a fascinating venue in which to study relativistic quantum phenomena. One notable example is Klein tunneling, a phenomena in which electrons convert to holes to tunnel through a potential barrier. However, the omnipresence of charged impurities in substrate-supported samples keep the overall charge distribution nonuniform, obscuring much of this “Dirac” point physics in large samples. Using local gates, one can create tunable heterojunctions in graphene, isolating the contribution of small regions of the samples to transport. In this review, we give an overview of quantum transport theory and experiment on locally gated graphene heterostructures, with an emphasis on bipolar junctions.
02/2011; 2:101-120.
-
[show abstract]
[hide abstract]
ABSTRACT: Single-molecule measurements of biomolecules can provide information about the molecular interactions and kinetics that are hidden in ensemble measurements. However, there is a requirement for techniques with improved sensitivity and time resolution for use in exploring biomolecular systems with fast dynamics. Here, we report the detection of DNA hybridization at the single-molecule level using a carbon nanotube field-effect transistor. By covalently attaching a single-stranded probe DNA sequence to a point defect in a carbon nanotube, we are able to measure two-level fluctuations in the conductance of the nanotube in the presence of a complementary DNA target. The kinetics of the system are studied as a function of temperature, allowing the measurement of rate constants, melting curves and activation energies for different sequences and target concentrations. The kinetics demonstrate non-Arrhenius behaviour, in agreement with DNA hybridization experiments using fluorescence correlation spectroscopy. This technique is label-free and could be used to probe single-molecule dynamics at microsecond timescales.
Nature Nanotechnology 02/2011; 6(2):126-32. · 27.27 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report synthesis and transport properties of the minimal graphite intercalation compound, a ferric chloride (FeCl(3))(n) island monolayer inside bilayer graphene. Chemical doping by the intercalant is simultaneously probed by micro-Raman spectroscopy and Hall measurements. Quantum oscillations of conductivity originate from microscopic domains of intercalated and unintercalated regions. A slight upturn in resistance related to magnetic transition is observed. Two-dimensional intercalation in bilayer graphene opens new possibilities to engineer two-dimensional properties of intercalates.
Nano Letters 02/2011; 11(2):860-5. · 13.20 Impact Factor
