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

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

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

High intrinsic noise and absence of hysteresis in superconducting quantum interference devices with large Steward-McCumber parameter

We investigated niobium thin film superconducting quantum interference devices (SQUIDs) with large Steward-McCumber parameter (βc > 1). No hysteresis was observed in the current-voltage (I-V) characteristics of the SQUIDs, even for βc ≈ 17. We attribute the absence of hysteresis to an excess voltage noise of the junctions which increases the SQUID intrinsic noise δΦs. It can be represented by an effective noise temperature T* of the SQUID which is higher than the bath temperature T. We simulated SQUID I-V characteristics using the measured device parameters and confirmed the absence of hysteresis