135 research outputs found
Mechanism of Near-Field Raman Enhancement in One-Dimensional Systems
We develop a theory of near-field Raman enhancement in one-dimensional systems, and report supporting experimental results for carbon nanotubes. The enhancement is established by a laser-irradiated nanoplasmonic structure acting as an optical antenna. The near-field Raman intensity is inversely proportional to the 10th power of the separation between the enhancing structure and the one-dimensional system. Experimental data obtained from single-wall carbon nanotubes indicate that the Raman enhancement process is not significantly influenced by the specific phonon eigenvector, and is mainly defined by the properties of the nanoplasmonic structure
Laser-induced etching of few-layer graphene synthesized by Rapid-Chemical Vapour Deposition on Cu thin films
The outstanding electrical and mechanical properties of graphene make it very
attractive for several applications, Nanoelectronics above all. However a
reproducible and non destructive way to produce high quality, large-scale area,
single layer graphene sheets is still lacking. Chemical Vapour Deposition of
graphene on Cu catalytic thin films represents a promising method to reach this
goal, because of the low temperatures (T < 900 Celsius degrees) involved during
the process and of the theoretically expected monolayer self-limiting growth.
On the contrary such self-limiting growth is not commonly observed in
experiments, thus making the development of techniques allowing for a better
control of graphene growth highly desirable. Here we report about the local
ablation effect, arising in Raman analysis, due to the heat transfer induced by
the laser incident beam onto the graphene sample.Comment: v1:9 pages, 8 figures, submitted to SpringerPlus; v2: 11 pages,
PDFLaTeX, 9 figures, revised peer-reviewed version resubmitted to
SpringerPlus; 1 figure added, figure 1 and 4 replaced,typos corrected,
"Results and discussion" section significantly extended to better explain
etching mechanism and features of Raman spectra, references adde
Revealing the planar chemistry of two-dimensional heterostructures at the atomic level
Two-dimensional (2D) atomic crystals and their heterostructures are an intense area of study owing to their unique properties that result from structural planar confinement. Intrinsically, the performance of a planar vertical device is linked to the quality of its 2D components and their interfaces, therefore requiring characterization tools that can reveal both its planar chemistry and morphology. Here, we propose a characterization methodology combining (micro-) Raman spectroscopy, atomic force microscopy and time-of-flight secondary ion mass spectrometry to provide structural information, morphology and planar chemical composition at virtually the atomic level, aimed specifically at studying 2D vertical heterostructures. As an example system, a graphene-on-h-BN heterostructure is analysed to reveal, with an unprecedented level of detail, the subtle chemistry and interactions within its layer structure that can be assigned to specific fabrication steps. Such detailed chemical information is of crucial importance for the complete integration of 2D heterostructures into functional devicesopen2
Fermi velocity engineering in graphene by substrate modification
The Fermi velocity is one of the key concepts in the study of a material, as
it bears information on a variety of fundamental properties. Upon increasing
demand on the device applications, graphene is viewed as a prototypical system
for engineering Fermi velocity. Indeed, several efforts have succeeded in
modifying Fermi velocity by varying charge carrier concentration. Here we
present a powerful but simple new way to engineer Fermi velocity while holding
the charge carrier concentration constant. We find that when the environment
embedding graphene is modified, the Fermi velocity of graphene is (i) inversely
proportional to its dielectric constant, reaching ~2.5 m/s, the
highest value for graphene on any substrate studied so far and (ii) clearly
distinguished from an ordinary Fermi liquid. The method demonstrated here
provides a new route toward Fermi velocity engineering in a variety of
two-dimensional electron systems including topological insulators.Comment: accepted in Scientific Report
Hexagonal boron nitride tunnel barriers grown on graphite by high temperature molecular beam epitaxy
We demonstrate direct epitaxial growth of high-quality hexagonal boron nitride (hBN) layers on graphite using high-temperature plasma-assisted molecular beam epitaxy. Atomic force microscopy reveals mono- and few-layer island growth, while conducting atomic force microscopy shows that the grown hBN has a resistance which increases exponentially with the number of layers, and has electrical properties comparable to exfoliated hBN. X-ray photoelectron spectroscopy, Raman microscopy and spectroscopic ellipsometry measurements on hBN confirm the formation of sp2-bonded hBN and a band gap of 5.9 ± 0.1 eV with no chemical intermixing with graphite. We also observe hexagonal moiré patterns with a period of 15 nm, consistent with the alignment of the hBN lattice and the graphite substrate
Transfer-free electrical insulation of epitaxial graphene from its metal substrate
High-quality, large-area epitaxial graphene can be grown on metal surfaces
but its transport properties cannot be exploited because the electrical
conduction is dominated by the substrate. Here we insulate epitaxial graphene
on Ru(0001) by a step-wise intercalation of silicon and oxygen, and the
eventual formation of a SiO layer between the graphene and the metal. We
follow the reaction steps by x-ray photoemission spectroscopy and demonstrate
the electrical insulation using a nano-scale multipoint probe technique.Comment: Accepted for publication in Nano Letter
In Situ Observations during Chemical Vapor Deposition of Hexagonal Boron Nitride on Polycrystalline Copper.
Using a combination of complementary in situ X-ray photoelectron spectroscopy and X-ray diffraction, we study the fundamental mechanisms underlying the chemical vapor deposition (CVD) of hexagonal boron nitride (h-BN) on polycrystalline Cu. The nucleation and growth of h-BN layers is found to occur isothermally, i.e., at constant elevated temperature, on the Cu surface during exposure to borazine. A Cu lattice expansion during borazine exposure and B precipitation from Cu upon cooling highlight that B is incorporated into the Cu bulk, i.e., that growth is not just surface-mediated. On this basis we suggest that B is taken up in the Cu catalyst while N is not (by relative amounts), indicating element-specific feeding mechanisms including the bulk of the catalyst. We further show that oxygen intercalation readily occurs under as-grown h-BN during ambient air exposure, as is common in further processing, and that this negatively affects the stability of h-BN on the catalyst. For extended air exposure Cu oxidation is observed, and upon re-heating in vacuum an oxygen-mediated disintegration of the h-BN film via volatile boron oxides occurs. Importantly, this disintegration is catalyst mediated, i.e., occurs at the catalyst/h-BN interface and depends on the level of oxygen fed to this interface. In turn, however, deliberate feeding of oxygen during h-BN deposition can positively affect control over film morphology. We discuss the implications of these observations in the context of corrosion protection and relate them to challenges in process integration and heterostructure CVD.P.R.K. acknowledges funding from the Cambridge Commonwealth Trust and the Lindemann
Trust Fellowship. R.S.W. acknowledges a research fellowship from St. John’s College,
Cambridge. S.H. acknowledges funding from ERC grant InsituNANO (no. 279342), EPSRC
under grant GRAPHTED (project reference EP/K016636/1), Grant EP/H047565/1 and EU FP7
Work Programme under grant GRAFOL (project reference 285275). The European Synchrotron
Radiation Facility (ESRF) is acknowledged for provision of synchrotron radiation and assistance
in using beamline BM20/ROBL. We acknowledge Helmholtz-Zentrum-Berlin Electron storage
ring BESSY II for synchrotron radiation at the ISISS beamline and continuous support of our
experiments.This is the final version. It was first published by ACS at http://pubs.acs.org/doi/abs/10.1021/cm502603
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