Oriented Graphene Nanoribbons Embedded in Hexagonal Boron Nitride Trenches

Graphene nanoribbons (GNRs) are ultra-narrow strips of graphene that have the potential to be used in high-performance graphene-based semiconductor electronics. However, controlled growth of GNRs on dielectric substrates remains a challenge. Here, we report the successful growth of GNRs directly on hexagonal boron nitride substrates with smooth edges and controllable widths using chemical vapour deposition. The approach is based on a type of template growth that allows for the in-plane epitaxy of mono-layered GNRs in nano-trenches on hexagonal boron nitride with edges following a zigzag direction. The embedded GNR channels show excellent electronic properties, even at room temperature. Such in-plane hetero-integration of GNRs, which is compatible with integrated circuit processing, creates a gapped channel with a width of a few benzene rings, enabling the development of digital integrated circuitry based on GNRs.Comment: 32 pages, 4 figures, Supplementary informatio

Precisely aligned graphene grown on hexagonal boron nitride by catalyst free chemical vapor deposition

To grow precisely aligned graphene on h-BN without metal catalyst is extremely important, which allows for intriguing physical properties and devices of graphene/h-BN hetero-structure to be studied in a controllable manner. In this report, such hetero-structures were fabricated and investigated by atomic resolution scanning probe microscopy. Moirre patterns are observed and the sensitivity of moirre interferometry proves that the graphene grains can align precisely with the underlying h-BN lattice within an error of less than 0.05 degree. The occurrence of moirre pattern clearly indicates that the graphene locks into h-BN via van der Waals epitaxy with its interfacial stress greatly released. It is worthy to note that the edges of the graphene grains are primarily oriented along the armchair direction. The field effect mobility in such graphene flakes exceeds 20,000 cm2/V.s at ambient condition. This work opens the door of atomic engineering of graphene on h-BN, and sheds light on fundamental research as well as electronic applications based on graphene/h-BN hetero-structure.Comment: 22 pages, 4 figures, the supporting information is also include

Behaviour of CFST columns with external confinement under uni-axial compression

This thesis presents both experimental and theoretical studies on the structural behaviour of externally confined concrete-filled-steel-tube (CFST) short columns under uni-axial compression. External confinement in the form of tie bars, steel rings, spirals and jackets is proposed. From the study, it can be concluded that the external confinement is highly effective in improving the uni-axial behaviour of CFST columns by improving confining stress and suppressing inelastic local buckling. The first part of the thesis covers the experimental behaviour of concentrically loaded confined CFST columns. The experimental programme involved testing a total of 208 specimens that consisted of 46 hollow-steel-tube columns with or without external confinement and CFST columns: 28 unconfined, 25 tie-confined, 77 ring-confined, 24 spiral-confined and 8 jacket-confined columns. The test results are summarized and discussed. From analysing the obtained experimental results, a better understanding of the structural behaviours, such as steel-concrete interface bonding, stress state of steel tube, behaviour of core concrete of CFST columns can be obtained. Besides, the test results obtained are useful for deriving and verifying theoretical models in this study. In the second part of the thesis, a theoretical stress-strain model is proposed based on the obtained test results. This model is obtained through a four-step procedure. As the first crucial step, a new and accurate hoop strain equation is proposed and verified based on experimental results. Having validated the assumption of stress path independence of the core concrete in CFST columns under monotonic uni-axial compression, an actively confined concrete constitutive model is adopted to predict the axial stress-strain response of the core concrete. Next, with the aid of Generalized Hooke’s Law and Prandtl-Reuss theory, the complicated stress-state of steel tube is accurately modelled. Lastly, by using the newly proposed hoop strain equation together with the interaction models among the core concrete, steel tube and external confinement, the uni-axial behaviours of CFST columns can be predicted. The validity of this model is subsequently verified with the author’s and published experimental results of CFST columns and FRP-confined concrete. Finally, a parametric study using the proposed model is conducted in this thesis, involving a wide range of parameters, i.e. steel tube yield strength, concrete compressive strength, steel ratio (steel tube outer diameter / thickness) and external confinement (types and arrangements). From the parametric study, two sets of critical steel ratios based on two levels of ductility at different steel yield and concrete compressive strengths are established. Moreover, the maximum column strengths of CFST columns obtained from the model and previous experimental studies are compared with the design strengths calculated using different design codes and the results indicate that the design codes cannot give reasonably accurate predictions. Therefore, new design equations are proposed in this thesis to predict the maximum strength of the CFST columns.published_or_final_versionCivil EngineeringDoctoralDoctor of Philosoph

Preparation and superconductivity of stoichiometric

The stoichiometric $\beta$-FeSe superconductor was prepared for the first time by mechanical alloying (MA) from elemental powders of iron and selenium (1 : 1) and subsequent annealing process. The formation of the stoichiometric $\beta$-FeSe is attributed to the MA resulting nanometric precursor containing high reaction activity for the following annealing process, and the MA mechanism here is a mechanic collision reaction (MCR). The crystalline phases in the MA precursor are tightly related to the milling time (t). Hexagonal γ-Fe7Se8, tetragonal $\beta$-FeSe, and residual Fe coexist when t $\leqslant$ 5 h. A single $\delta$-FeSe phase is observed when t = 10 h, and the crystalline size further decreases to ~ 10 nm. In the nanocrystalline state, hexagonal $\delta$-FeSe is the stable and preferred phase. With the increase of the annealing temperature, $\delta$-FeSe rapidly transforms to the well-crystallized $\beta$-FeSe. At 400 °C, the pure stoichiometric $\beta$-FeSe phase is successfully obtained with size of ~ 60 nm and the highest superconductor transition temperature $T_{c}^{onset}$ of about 8.9 K in the $\beta$-FeSe$_{1-x}$ series

Fillers to improve passing ability of concrete

Concrete possessing high-passing ability needs to be flowable and cohesive. Hence, passing ability cannot be improved by solely adding superplasticizer, which increases both flowability and segregation of concrete simultaneously. Decreasing the maximum size of aggregates so that concrete segregates at lower cohesiveness is a possible but undesirable way as it narrows the aggregates' grading and decrease dimensional stability of concrete. With the same maximum size of aggregates, passing ability can be improved by raising the concurrent flowability-segregation envelope of concrete. In this paper, fly ash and silica fume (cementitious fillers) and limestone (inert filler) were selected to replace cement partially and subsequently the passing ability of concrete was studied. From the results, it was evident that when either type of fillers were used, the passing ability and maximum limits of flowability and segregation achieved simultaneously increase. It is because these fillers are finer than cement that provides better filling effect to increase packing density and excess water leading to better flowability. Concurrently, the cohesiveness of concrete also increases as the content of fine particles increases. These allow concrete to hold the coarse aggregates more firmly when passing through narrow gaps, after which the concrete will keep flowing rapidly

An insight into voltage-biased superconducting quantum interference devices

We experimentally studied two important parameters of helium-cooled superconducting quantum interference devices (SQUIDs) in the voltage bias mode: the dynamic resistance Rd and the flux-to-current transfer coefficient ?i/?phi, with different junction shunt resistors RJ. We investigated a voltage-biased SQUID using the direct readout current-to-voltage converter scheme involving an operational amplifier. At higher RJ, the flux-to-voltage conversion coefficient ?V/?phi becomes sufficiently large to effectively suppress the room-temperature amplifier´s noise without any need for additional feedback circuits. The McCumber parameter limits the rise of ?V/?phi. We discuss the performance of voltage-biased SQUIDs at different effective McCumber parameters