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

Carbon-assisted chemical vapor deposition of hexagonal boron nitride

We show that in a low-pressure chemical vapor deposition (CVD) system, the residual oxygen and/or air play a crucial role in the mechanism of the growth of hexagonal boron nitride (h-BN) films on Ni foil 'enclosures'. Hexagonal-BN films grow on the Ni foil surface via the formation of an intermediate boric-oxide (BOx) phase followed by a thermal reduction of the BOx by a carbon source (either amorphous carbon powder or methane), leading to the formation of single-and bi-layer h-N. Low energy electron microscopy (LEEM) and diffraction (LEED) were used to map the number of layers over large areas; Raman spectroscopy, time-of-flight secondary ion mass spectrometry (ToF-SIMS), x-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM) were used to characterize the structure and physical quality of the ultra-thin h-BN film. The growth procedure reported here leads to a better understanding and control of the synthesis of ultra-thin h-BN films

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

Critical community-engaged scholarship in an undergraduate food systems capstone: A case study from Public Health

In this perspective paper, we present a case study of food systems pedagogy and critical community-university engagement within a school of public health at a large and public research university. We start by providing a contextual foundation for the importance of intentionally centering equity-oriented curriculum and community partnerships in academic settings. After highlighting institutional mandates and curricular innovations from a food systems capstone course, we utilize key questions of critical community-engaged scholarship to analyze the case and critically reflect on gaps and opportunities for ongoing growth

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

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

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

Tungsten oxide mediated quasi-van der Waals Epitaxy of WS2 on Sapphire

Conventional epitaxy plays a crucial role in current state-of-the art semiconductor technology, as it provides a path for accurate control at the atomic scale of thin films and nanostructures, to be used as the building blocks in nanoelectronics, optoelectronics, sensors, etc. Four decades ago, the terms “van der Waals” (vdW) and “quasi-vdW (Q-vdW) epitaxy” were coined to explain the oriented growth of vdW layers on 2D and 3D substrates, respectively. The major difference with conventional epitaxy is the weaker interaction between the epi-layer and the epi-substrates. Indeed, research on Q-vdW epitaxial growth of transition metal dichalcogenides (TMDCs) has been intense, with oriented growth of atomically thin semiconductors on sapphire being one of the most studied systems. Nonetheless, there are some striking and not yet understood differences in the literature regarding the orientation registry between the epi-layers and epi-substrate and the interface chemistry. Here we study the growth of WS2 via a sequential exposure of the metal and the chalcogen precursors in a metal–organic chemical vapor deposition (MOCVD) system, introducing a metal-seeding step prior to the growth. The ability to control the delivery of the precursor made it possible to study the formation of a continuous and apparently ordered WO3 mono- or few-layer at the surface of a c-plane sapphire. Such an interfacial layer is shown to strongly influence the subsequent quasi-vdW epitaxial growth of the atomically thin semiconductor layers on sapphire. Hence, here we elucidate an epitaxial growth mechanism and demonstrate the robustness of the metal-seeding approach for the oriented formation of other TMDC layers. This work may enable the rational design of vdW and quasi-vdW epitaxial growth on different material systems