Probing the Gas-Phase Dynamics of Graphene Chemical Vapour Deposition using <i>in-situ</i> UV Absorption Spectroscopy

AbstractThe processes governing multilayer nucleation in the chemical vapour deposition (CVD) of graphene are important for obtaining high-quality monolayer sheets, but remain poorly understood. Here we show that higher-order carbon species in the gas-phase play a major role in multilayer nucleation, through the use of in-situ ultraviolet (UV) absorption spectroscopy. These species are the volatilized products of reactions between hydrogen and carbon contaminants that have backstreamed into the reaction chamber from downstream system components. Consequently, we observe a dramatic suppression of multilayer nucleation when backstreaming is suppressed. These results point to an important and previously undescribed mechanism for multilayer nucleation, wherein higher-order gas-phase carbon species play an integral role. Our work highlights the importance of gas-phase dynamics in understanding the overall mechanism of graphene growth.</jats:p

Copper Oxidation through Nucleation Sites of Chemical Vapor Deposited Graphene

We investigate the nucleation defect-triggered oxidation of Cu covered by CVD graphene during postannealing in air. The results reveal that different growth conditions may induce imperfect nucleation of graphene, and cause creation of defects near the nucleation point such as pin holes and amorphous carbon. These defects would serve as a pathway for the diffusion of O₂ during thermal annealing, allowing oxidation of Cu to progress gradually from the nucleation center toward the growth edge. The oxidation process follows the graphene morphology closely; the shape of the oxidized area of Cu has a striking resemblance to that of the graphene flakes. Our work demonstrates that inferior graphene nucleation in CVD processes can compromise the oxidation resistance of a graphene-coated Cu substrate, and indirectly reveal the structure and integrity of graphene, which is of fundamental importance for the quality monitoring and control of graphene growth, for understanding the mechanisms of graphene nucleation and growth, and has implications for graphene’s use in electronic and passivation applications

Supramolecular Assembly and Stimuli-Responsive Behavior of Multielement Hybrid Copolymers

Toward the organic polymer, hybrid elements can be defined as those beyond C, H, O, and N. Polymers comprising hybrid elements, such as Si, P, B, or metal ions have attracted great attention in the design of high performance or smart materials. Introduction of hybrid elements into a polymeric network may also lead to the formation of new intermolecular interactions, thus promote the self-organization of polymer chains to form controllable structures and morphologies. In this chapter, we introduce some of the recent important development in the design and self-assembly of hybrid amphiphilic copolymers. Specific attention was paid on the hybrid amphiphilic copolymers containing POSS, boronic acid, or boronate functional moieties. We introduce the design, synthesis, self-assembly behavior, and properties of these hybrid amphiphilic copolymers in detail. Also, the advantages and drawbacks of these polymers and their corresponding nanoassemblies are discussed

Quantitative optical mapping of two-dimensional materials

The pace of two-dimensional materials (2DM) research has been greatly accelerated by the ability to identify exfoliated thicknesses down to a monolayer from their optical contrast. Since this process requires time-consuming and error-prone manual assignment to avoid false-positives from image features with similar contrast, efforts towards fast and reliable automated assignments schemes is essential. We show that by modelling the expected 2DM contrast in digitally captured images, we can automatically identify candidate regions of 2DM. More importantly, we show a computationally-light machine vision strategy for eliminating false-positives from this set of 2DM candidates through the combined use of binary thresholding, opening and closing filters, and shape-analysis from edge detection. Calculation of data pyramids for arbitrarily high-resolution optical coverage maps of two-dimensional materials produced in this way allows the real-time presentation and processing of this image data in a zoomable interface, enabling large datasets to be explored and analysed with ease. The result is that a standard optical microscope with CCD camera can be used as an analysis tool able to accurately determine the coverage, residue/contamination concentration, and layer number for a wide range of presented 2DMs

Graphene-Si CMOS oscillators

Graphene field-effect transistors (GFETs) offer a possibility of exploiting unique physical properties of graphene in realizing novel electronic circuits. However, graphene circuits often lack the voltage swing and switchability of Si complementary metal-oxide-semiconductor (CMOS) circuits, which are the main building block of modern electronics. Here we introduce graphene in Si CMOS circuits to exploit favorable electronic properties of both technologies and realize a new class of simple oscillators using only a GFET, Si CMOS D latch, and timing RC circuit. The operation of the two types of realized oscillators is based on the ambipolarity of graphene, i.e., the symmetry of the transfer curve of GFETs around the Dirac point. The ambipolarity of graphene also allowed to turn the oscillators into pulse-width modulators (with a duty cycle ratio ∼1 : 4) and voltage-controlled oscillators (with a frequency ratio ∼1 : 8) without any circuit modifications. The oscillation frequency was in the range from 4 kHz to 4 MHz and limited only by the external circuit connections, rather than components themselves. The demonstrated graphene-Si CMOS hybrid circuits pave the way to the more widespread adoption of graphene in electronics

An airflow-controlled solvent evaporation route to hollow microspheres and colloidosomes

National Natural Science Foundation of China [51173153, U1205113, 51103122]; Scientific and Technical Project of Fujian Province of China [2009J1009]A facile and large-scale method combining airflow-controlled solvent evaporation and amphiphilic copolymer self-assembly has been developed for the generation of hollow polymer microspheres, colloidosomes or even organic-inorganic hybrid colloidosomes. By replacing traditional agitation with the controllable airflow, this surfactant free route showed promising prospect in the fabrication of microcapsules with closed pore morphology. While the hollow polymer microspheres had an adjustable pore structure, the polymer colloidosomes and the hybrid colloidosomes possessed seamless surfaces, making them suitable for the stable encapsulation of small molecules. The hybrid colloidosomes constructed from polymer and Fe3O4 nanoparticles, and the ternary hybrid colloidosomes derived from polymer, polymer nanospheres and Fe3O4 nanoparticles displayed superparamagnetic properties and were excellent contrast agents for magnetic resonance imaging. More importantly, both hybrid colloidosomes and ternary hybrid colloidosomes exhibited a significant evolution of pore morphology from a closed pore structure to an open pore structure in response to the temperature variation, which induced a controllable release of guest molecules

<i>NuSTAR </i>reveals the extreme properties of the super-Eddington accreting supermassive black hole in PG 1247+267

PG1247+267 is one of the most luminous known quasars at z similar to 2 and is a strongly super-Eddington accreting supermassive black hole (SMBH) candidate. We obtained NuSTAR data of this intriguing source in December 2014 with the aim of studying its high-energy emission, leveraging the broad band covered by the new NuSTAR and the archival XMM-Newton data. Several measurements are in agreement with the super-Eddington scenario for PG1247+267: the soft power law (Gamma = 2.3 +/- 0.1); the weak ionized Fe emission line; and a hint of the presence of outflowing ionized gas surrounding the SMBH. The presence of an extreme reflection component is instead at odds with the high accretion rate proposed for this quasar. This can be explained with three different scenarios; all of them are in good agreement with the existing data, but imply very different conclusions: i) a variable primary power law observed in a low state, superimposed on a reflection component echoing a past, higher flux state; ii) a power law continuum obscured by an ionized, Compton thick, partial covering absorber; and iii) a relativistic disk reflector in a lamp-post geometry, with low coronal height and high BH spin. The first model is able to explain the high reflection component in terms of variability. The second does not require any reflection to reproduce the hard emission, while a rather low high-energy cutoff of similar to 100 keV is detected for the first time in such a high redshift source. The third model require a face-on geometry, which may affect the SMBH mass and Eddington ratio measurements. Deeper X-ray broad-band data are required in order to distinguish between these possibilities