[Show abstract][Hide abstract] ABSTRACT: We studied the magnetoresistance (MR) of twisted bilayer graphene (tBLG) on electron transparent substrate. Samples of tBLG were assembled on free-standing silicon nitride (SiNx) membranes (<100 nm thick) by transferring chemical vapor deposition (CVD)-grown single layer graphene (SLG) twice; this allowed the measurement of the angle of rotation between the two layers, the twist angle, by electron diffraction using a transmission electron microscope (TEM). To compare with the previous reports on tBLG, we performed Raman spectroscopy on our samples. We measured the MR of tBLG for two different twist angles: 2° (small) and 18° (large). The MR showed superposition of two Shubnikov de Haas (SdH) oscillations for both angles. An analysis of the oscillation peaks by Landau fan diagrams showed difference as twist angle. While the large twist angle (18°) sample had two anomalous π Berry’s phases, the small twist angle (2°) sample had conventional 2π and anomalous π Berry’s phase depending on carrier density.
[Show abstract][Hide abstract] ABSTRACT: The nucleation and growth of carbon on catalytically active metal surfaces is one of the most important techniques to produce nanomaterials such as graphene or nanotubes. Here it is shown by in-situ electron microscopy that fullerene-like spherical clusters with diameters down to 0.4 nm and thus much smaller than C60 grow in a polymerized state on Co, Fe, or Ru surfaces. The cages appear on the surface of metallic islands in contact with graphene under heating to at least 650°C and successively cooling to less than 500°C. The formation of the small cages is explained by the segregation of carbon on a supersaturated metal, driven by kinetics. First principles energy calculations show that the clusters polymerize and can be attached to defects in graphene. Under compression, the polymerized cages appear in a crystalline structure.
[Show abstract][Hide abstract] ABSTRACT: Nanopores are now being used not only as an ionic current sensor but also as a means to localize molecules near alternative sensors with higher sensitivity and/or selectivity. One example is a solid-state nanopore embedded in a graphene nanoribbon (GNR) transistor. Such a device possesses the high conductivity needed for higher bandwidth measurements and, because of its single-atomic-layer thickness, can improve the spatial resolution of the measurement. Here measurements of ionic current through the nanopore are shown during double-stranded DNA (dsDNA) translocation, along with the simultaneous response of the neighboring GNR due to changes in the surrounding electric potential. Cross-talk originating from capacitive coupling between the two measurement channels is observed, resulting in a transient response in the GNR during DNA translocation; however, a modulation in device conductivity is not observed via an electric-field-effect response during DNA translocation. A field-effect response would scale with GNR source-drain voltage (Vds ), whereas the capacitive coupling does not scale with Vds . In order to take advantage of the high bandwidth potential of such sensors, the field-effect response must be enhanced. Potential field calculations are presented to outline a phase diagram for detection within the device parameter space, charting a roadmap for future optimization of such devices.
[Show abstract][Hide abstract] ABSTRACT: Solid-state nanopores are single-molecule sensors that detect changes in ionic conductance (ΔG) when individual molecules pass through them. Producing high signal-to-noise ratio for the measurement of molecular structure in applications such as DNA sequencing requires low noise and large ΔG. The latter is achieved by reducing the nanopore diameter and membrane thickness. While the minimum diameter is limited by the molecule size, the membrane thickness is constrained by material properties. We use molecular dynamics simulations to determine the theoretical thickness limit of amorphous Si membranes to be ~ 1 nm, and we designed an electron-irradiation-based thinning method to reach that limit and drill nanopores in the thinned regions. Double-stranded DNA translocations through these nanopores (down to 1.4 nm in thickness and 2.5 nm in diameter) provide the intrinsic ionic conductance detection limit in Si-based nanopores. In this regime, where the access resistance is comparable to the nanopore resistance, we observe the appearance of two conductance levels during molecule translocation. Considering the overall performance of Si-based nanopores, our work highlights their potential as a leading material for sequencing applications.
[Show abstract][Hide abstract] ABSTRACT: Graphene nanoribbons (GNRs) are promising candidates for next generation integrated circuit (IC) components; this fact motivates exploration of the relationship between crystallographic structure and transport of graphene patterned at IC-relevant length scales (< 10 nm). We report on the controlled fabrication of pristine, freestanding GNRs with widths as small as 0.7 nm, paired with simultaneous lattice-resolution imaging and electrical transport characterization, all conducted within an aberration-corrected transmission electron microscope. Few-layer GNRs very frequently formed bonded-bilayers and were remarkably robust, sustaining currents in excess of 1.5 μA per carbon bond across a 5 atom-wide ribbon. We found that the intrinsic conductance of a sub-10 nm bonded bilayer GNR scaled with width as GBL(w) ≈ ¾(e2/h)w, where w is the width in nm, while a monolayer GNR was roughly five times less conductive. Nanosculpted, crystalline monolayer GNRs exhibited armchair-terminated edges after current annealing, presenting a pathway for the controlled fabrication of semiconducting GNRs with known edge geometry. Finally, we report on simulations of quantum transport in GNRs that are in qualitative agreement with the observations.
[Show abstract][Hide abstract] ABSTRACT: Graphene-based nanopore devices are promising candidates for next-generation DNA sequencing. Here we fabricated graphene nanoribbon-nanopore (GNR-NP) sensors for DNA detection. Nanopores with diameters in the range of 2-10 nm were formed at the edge or in the center of graphene nanoribbons (GNRs), with widths between 20 and 250 nm and a length of 600 nm, on 40 nm thick silicon nitride (SiNx) membranes. GNR conductance was monitored in situ during electron irradiation-induced nanopore formation inside a transmission electron microscope (TEM) operating at 200 kV. We show that GNR resistance increases linearly with electron dose and that GNR conductance and mobility decrease by a factor of ten or more when GNRs are imaged at relatively high magnification with a broad beam prior to making a nanopore. By operating the TEM in scanning TEM (STEM) mode, in which the position of the converged electron beam can be controlled with high spatial precision via automated feedback, we were able to prevent electron beam-induced damage and make nanopores in highly conducting GNR sensors. This method minimizes the exposure of the GNRs to the beam before and during nanopore formation. The resulting GNRs with unchanged resistances after nanopore formation can sustain microampere currents at low voltages (∼ 50 mV) in buffered electrolyte solution and exhibit high sensitivity, with a large relative change of resistance upon changes of gate voltage, similar to pristine GNRs without nanopores.
[Show abstract][Hide abstract] ABSTRACT: Graphene-boron nitride monolayer heterostructures contain adjacent electrically active and insulating regions in a continuous, single-atom thick layer. To date structures were grown at low pressure, resulting in irregular shapes and edge direction, so studies of the graphene-boron nitride interface were restricted to microscopy of nano-domains. Here we report templated growth of single crystalline hexagonal boron nitride directly from the oriented edge of hexagonal graphene flakes by atmospheric pressure chemical vapor deposition, and physical property measurements that inform the design of in-plane hybrid electronics. Ribbons of boron nitride monolayer were grown from the edge of a graphene template and inherited its crystallographic orientation. The relative sharpness of the interface was tuned through control of growth conditions. Frequent tearing at the graphene-boron nitride interface was observed, so density functional theory was used to determine that the nitrogen-terminated interface was prone to instability during cool down. The electronic functionality of monolayer heterostructures was demonstrated through fabrication of field effect transistors with boron nitride as an in-plane gate dielectric.
[Show abstract][Hide abstract] ABSTRACT: Crystalline hexagonally ordered silicon oxide layers with a thickness of less than a nanometer are grown on transition metal surfaces in an in-situ electron microscopy experiment. The nucleation and growth of silica bilayers and monolayers, which represent the thinnest possible ordered structures of silicon oxide, are monitored in real time. The emerging layers show structural defects reminiscent of those in graphene, and can also be vitreous. First-principles calculations provide atomistic insight into the energetics of the growth process. The interplay between the gain in silica-metal interaction energy due to their epitaxial match and energy loss associated with the mechanical strain of silica network is addressed. The results of calculations indicate that both ordered and vitreous mono/bilayer structures are possible so that the actual morphology of the layer is defined by the kinetics of the growth process.
[Show abstract][Hide abstract] ABSTRACT: In the last two decades, new techniques that monitor ionic current modulations as single molecules pass through a nanoscale pore have enabled numerous single-molecule studies. While biological nanopores have recently shown the ability to resolve single nucleotides within individual DNA molecules, similar developments with solid-state nanopores have lagged, due to challenges both in fabricating stable nanopores of similar dimensions as biological nanopores and in achieving sufficiently low-noise and high-bandwidth recordings. Here we show that small silicon nitride nanopores (0.8 to 2-nm-diameter in 5 to 8-nm-thick membranes) can resolve differences between ionic current signals produced by short (30 base) ssDNA homopolymers (poly(dA), poly(dC), poly(dT)), when combined with measurement electronics that allow a signal-to-noise ratio of better than 10 to be achieved at 1 MHz bandwidth. While identifying intramolecular DNA sequences with silicon nitride nanopores will require further improvements in nanopore sensitivity and noise levels, homopolymer differentiation represents an important milestone in the development of solid-state nanopores.
[Show abstract][Hide abstract] ABSTRACT: We report electronic measurements on high quality single layer
junction-confined graphene nanoribbons fabricated in a transmission
electron microscope (TEM). In this work, a process is demonstrated for
the fabrication and confirmation of pristine single layer graphene
nanoribbons using high vacuum current annealing and precision
nano-sculpting, both conducted within the vacuum chamber of a TEM.
Briefly, CVD-grown graphene is patterned into a freely-suspended
nanoribbon connected to large area contacts. The sample is then mounted
on a TEM holder with electrical feedthroughs to allow for simultaneous
imaging and in-situ electrical transport measurements within the TEM. A
focused electron beam is used to progressively narrow the ribbon,
providing a platform to controllably sculpt and define the device
geometry while characterizing its electrical properties. In-situ
electrical measurements and TEM imaging with sub-nm resolution revealed
the dependence of the nanoribbon resistance as a function of width in
the range 17 -- 280 nm. Monolayer graphene were found to sustain current
densities in excess of 5 x 10^9 A/cm^2, orders of magnitude higher than
copper while the conductance varied approximately as w^0.75, where w is
the ribbon width in nanometers. These results demonstrates graphene's
potential as a next generation, high performance interconnects material
with the ability to reach single-digit technology nodes at the level of
a single atomic layer.
[Show abstract][Hide abstract] ABSTRACT: The industry's march towards higher transistor density has called for an
ever-increasing number of interconnect levels in logic devices. The
historic transition from aluminum to copper was necessary in reducing
timing delays while future technology nodes presents an opportunity for
new materials and patterning techniques. One material for consideration
is graphene, a single atomic layer of carbon atoms. Graphene is known to
have excellent electrical properties , driving strong interest in its
integration into the wafer fabrication processes for future electronics
, and its ballistic transport properties give promise for use in
on-chip interconnects . This study demonstrates the feasibility of a
direct electron beam lithography technique to pattern sub-5nm metallic
graphene ribbons, without using a mask or photoresist, to act as next
generation interconnects. Sub-5nm monolayer and multilayer graphene
ribbons were patterned using a focused electron beam in a transmission
electron microscope (TEM) through direct knock-on ejection of carbon
atoms. These ribbons were measured during fabrication to quantify their
electrical performance. Multilayered graphene nanoribbons were found to
sustain current densities in excess of 109 A/cm2,
orders of magnitude higher than copper, while monolayer graphene
provides comparable performance to copper but at the level of a single
atomic layer. High volume manufacturing could utilize wafer-size
chemical vapor deposition (CVD) graphene  transferred directly onto
the substrate paired with a direct write multi-beam tool to knock off
carbon atoms for patterning of nanometer sized interconnects. The
patterning technique introduced here allows for the fabrication of small
foot-print high performance next generation graphene interconnects that
bypass the use of a mask and resist process.
Full-text · Article · Mar 2013 · Proceedings of SPIE - The International Society for Optical Engineering
[Show abstract][Hide abstract] ABSTRACT: Carbon atoms are displaced in pre-selected locations of carbon nanotubes by using a focused electron beam in a scanning transmission electron microscope. Sub-nanometer-sized holes are created that change the morphology of double and triple-walled carbon nanotubes and connect the shells in a unique way. By combining in situ transmission electron microscopy experiments with atomistic simulations, we study the bonding between defective shells in the new structures which are reminiscent of the shape of a flute. We demonstrate that in double-walled nanotubes the shells locally merge by forming nanoarches while atoms with dangling bonds can be preserved in triple-walled carbon nanotubes. In the latter system, nanoarches are formed between the inner- and outermost shells, shielding small graphenic islands with open edges between the neighboring shells. Our results indicate that arrays of quantum dots may be produced in carbon nanotubes by spatially localized electron irradiation, generating atoms with dangling bonds that may give rise to localized magnetic moments.
[Show abstract][Hide abstract] ABSTRACT: Single-walled carbon nanotubes (SWCNTs) containing traces of iron oxide were functionalized by noncovalent lipid-PEG or covalent carboxylic acid function to supply new efficient MRI contrast agents for in vitro and in vivo applications. Longitudinal (r(1)) and transversal (r(2)) water proton relaxivities were measured at 300 MHz, showing a stronger T(2) feature as an MRI contrast agent (r(2)/r(1) = 190 for CO(2) H functionalisation). The r(2) relaxivity was demonstrated to be correlated to the presence of iron oxide in the SWNT-carboxylic function COOH, in comparison to iron-free ones. Biodistribution studies on mice after a systemic injection showed a negative MRI contrast in liver, suggesting the presence of the nanotubes in this organ until 48 h after i.v. injection. The presence of carbon nanotubes in liver was confirmed after ex vivo carbon extraction. Finally, cytotoxicity studies showed no apparent effect owing to the presence of the carbon nanotubes. The functionalized carbon nanotubes were well tolerated by the animals at the dose of 10 µg g(-1) body weight.
Full-text · Article · Mar 2012 · Contrast Media & Molecular Imaging
[Show abstract][Hide abstract] ABSTRACT: Nanocrystals of different metals with sizes of 2–6 nm are deposited on graphene, carbon nanotubes, or amorphous carbon films. Irradiation with a highly focused electron beam is used to split clusters of a few metal atoms (<1 nm in diameter) from the crystals. The metal clusters follow the electron beam spot on the graphitic surface when the beam is slowly deflected away from the clusters. This unusual behaviour of metals on graphitic sur-faces is explained in terms of electron beam-induced activation of the graphitic surfaces and covalent bonding between metal and carbon atoms. The technique might be applicable in (sub-)nanometre structuring of graphene with metal dots.
[Show abstract][Hide abstract] ABSTRACT: Thin Co and Ni lamellae grow under electron irradiation of metal crystals supported on multilayer graphene or amorphous carbon films. The lateral growth of a lamella from a source crystal is achieved by directing an electron beam to the periphery of the metal crystal and moving the beam over the surrounding carbon. Patterns of linear, branched, or ringlike metal lamellae can be created. The patterning is carried out in situ in a transmission electron microscope, allowing simultaneous structuring and imaging. The process is driven by the metal-carbon interaction at a beam-activated carbon surface.
No preview · Article · May 2011 · Applied Physics Letters
[Show abstract][Hide abstract] ABSTRACT: Single and few-layer graphene is grown by a solid-state transformation of amorphous carbon on a catalytically active metal. The process is carried out and monitored in situ in an electron microscope. It is observed that an amorphous carbon film is taken up by Fe, Co, or Ni crystals at temperatures above 600 °C. The nucleation and growth of graphene layers on the metal surfaces happen after the amorphous carbon film has been dissolved. It is shown that the transformation of the energetically less favorable amorphous carbon to the more favorable phase of graphene occurs by diffusion of carbon atoms through the catalytically active metal.
[Show abstract][Hide abstract] ABSTRACT: Reconstructed point defects in graphene are created by electron irradiation and annealing. By applying electron microscopy and density functional theory, it is shown that the strain field around these defects reaches far into the unperturbed hexagonal network and that metal atoms have a high affinity to the nonperfect and strained regions of graphene. Metal atoms are attracted by reconstructed defects and bonded with energies of about 2 eV. The increased reactivity of the distorted π-electron system in strained graphene allows us to attach metal atoms and to tailor the properties of graphene.
Full-text · Article · Nov 2010 · Physical Review Letters