Localization to delocalization probed by magnetotransport of hBN/graphene/hBN stacks in the ultra-clean regime.

We report on magnetotransport in a high-quality graphene device, which is based on monolayer graphene (Gr) encapsulated by hexagonal boron nitride (hBN) layers, i.e., hBN/Gr/hBN stacks. In the vicinity of the Dirac point, a negative magnetoconductance is observed for high temperatures > ~ 40 K, whereas it becomes positive for low temperatures ≤ ~ 40 K, which implies an interplay of quantum interferences in Dirac materials. The elastic scattering mechanism in hBN/Gr/hBN stacks contrasts with that of conventional graphene on SiO2, and our ultra-clean graphene device shows nonzero magnetoconductance for high temperatures of up to 300 K.

Manipulation of phase slips in carbon-nanotube-templated niobium-nitride superconducting nanowires under microwave radiation.

We study the manipulation of thermal/quantum phase slips (tPSs/qPSs) in ultra-thin niobium-nitride superconducting nanowires (scNW) grown on carbon-nanotube templates. These NWs exhibit resistive steps in current-voltage (I-V) characteristics, and the number of phase slip centers (PSCs) in an NW can be tuned by the NW length. Under microwave (MW) radiation, emergence of each single PSC can be precisely controlled by varying the MW power. For thin and short scNW, a dip structure between the qPS-dominated low-temperature region and the tPS-dominated high-temperature region were observed owing to anti-proximity effect by electrodes.

Single-Carrier Transport in Graphene/hBN Superlattices.

Graphene/hexagonal boron nitride (hBN) moiré superlattices have attracted interest for use in the study of many-body effects and fractal physics in Dirac fermion systems. Many exotic transport properties have been intensively examined in such superlattices, but previous studies have not focused on single-carrier transport. The investigation of the single-carrier behavior in these superlattices would lead to an understanding of the transition of single-particle/correlated phenomena. Here, we show the single-carrier transport in a high-quality bilayer graphene/hBN superlattice-based quantum dot device. We demonstrate remarkable device controllability in the energy range near the charge neutrality point (CNP) and the hole-side satellite point. Under a perpendicular magnetic field, Coulomb oscillations disappear near the CNP, which could be a signature of the crossover between Coulomb blockade and quantum Hall regimes. Our results pave the way for exploring the relationship of single-electron transport and fractal quantum Hall effects with correlated phenomena in two-dimensional quantum materials.

Bubble-Free Transfer Technique for High-Quality Graphene/Hexagonal Boron Nitride van der Waals Heterostructures.

Bubbles at the interface of two-dimensional layered materials in van der Waals heterostructures cause deterioration in the quality of materials, thereby limiting the size and design of devices. In this paper, we report a simple all-dry transfer technique, with which the bubble formation can be avoided. As a key factor in the technique, a contact angle between a picked-up flake on a viscoelastic polymer stamp and another flake on a substrate was introduced by protrusion at the stamp surface. Using this technique, we demonstrated the fabrication of high-quality devices on the basis of graphene/hexagonal boron nitride heterostructures with a large bubble-free region. Additionally, the technique can be used to remove unnecessary flakes on a substrate under an optical microscopic scale. Most importantly, it improves the yield and throughput for the fabrication process of high-quality van der Waals heterostructure-based devices.

Fabry-Pérot resonances and a crossover to the quantum Hall regime in ballistic graphene quantum point contacts.

We report on the observation of quantum transport and interference in a graphene device that is attached with a pair of split gates to form an electrostatically-defined quantum point contact (QPC). In the low magnetic field regime, the resistance exhibited Fabry-Pérot (FP) resonances due to np'n(pn'p) cavities formed by the top gate. In the quantum Hall (QH) regime with a high magnetic field, the edge states governed the phenomena, presenting a unique condition where the edge channels of electrons and holes along a p-n junction acted as a solid-state analogue of a monochromatic light beam. We observed a crossover from the FP to QH regimes in ballistic graphene QPC under a magnetic field with varying temperatures. In particular, the collapse of the QH effect was elucidated as the magnetic field was decreased. Our high-mobility graphene device enabled observation of such quantum coherence effects up to several tens of kelvins. The presented device could serve as one of the key elements in future electronic quantum optic devices.

High-temperature operation of a silicon qubit.

This study alleviates the low operating temperature constraint of Si qubits. A qubit is a key element for quantum sensors, memories, and computers. Electron spin in Si is a promising qubit, as it allows both long coherence times and potential compatibility with current silicon technology. Si qubits have been implemented using gate-defined quantum dots or shallow impurities. However, operation of Si qubits has been restricted to milli-Kelvin temperatures, thus limiting the application of the quantum technology. In this study, we addressed a single deep impurity, having strong electron confinement of up to 0.3 eV, using single-electron tunnelling transport. We also achieved qubit operation at 5-10 K through a spin-blockade effect based on the tunnelling transport via two impurities. The deep impurity was implemented by tunnel field-effect transistors (TFETs) instead of conventional FETs. With further improvement in fabrication and controllability, this work presents the possibility of operating silicon spin qubits at elevated temperatures.

Observation of the quantum valley Hall state in ballistic graphene superlattices.

In graphene superlattices, bulk topological currents can lead to long-range charge-neutral flow and nonlocal resistance near Dirac points. A ballistic version of these phenomena has never been explored. We report transport properties of ballistic graphene superlattices. This allows us to study and exploit giant nonlocal resistances with a large valley Hall angle without a magnetic field. In a low-temperature regime, a crossover occurs toward a new state of matter, referred to as a quantum valley Hall state (qVHS), which is an analog of the quantum Hall state without a magnetic field. Furthermore, a nonlocal resistance plateau, implying rigidity of the qVHS, emerges as a function of magnetic field, and this plateau collapses, which is considered a manifestation of valley/pseudospin magnetism.

One-Dimensional Anhydrous Proton Conducting Channel Formation at High Temperature in a Pt(II)-Based Metallo-Supramolecular Polymer and Imidazole System.

One dimensional (1D) Pt(II)-based metallo-supramolecular polymer with carboxylic acids (polyPtC) was synthesized using a new asymmetrical ditopic ligand with a pyridine moiety bearing two carboxylic acids. The carboxylic acids in the polymer successfully served as apohosts for imidazole loaded in the polymer interlayer scaffold to generate highly ordered 1D imidazole channels through the metallo-supramolecular polymer chains. The 1D structure of imidazole loaded polymer (polyPtC-Im) was analyzed in detail by thermogravimetric analysis, powder X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and ultraviolet-visible and photoluminescence spectroscopic measurements. PolyPtC-Im exhibited proton conductivity of 1.5 × 10-5 S cm-1 at 120 °C under completely anhydrous conditions, which is 6 orders of magnitude higher than that of the pristine metallo-supramolecular polymer.

Proton Conductive Nanosheets Formed by Alignment of Metallo-Supramolecular Polymers.

Linear Fe(II)-based metallo-supramolecular polymer chains were precisely aligned by the simple replacement of the counteranion with an N,N'-bis(4-benzosulfonic acid)perylene-3,4,9,10-tetracarboxylbisimide (PSA) dianion, which linked the polymer chains strongly. A parallel alignment of the polymer chains promoted by the PSA dianions yielded nanosheets formation. The nanosheets' structure was analyzed with FESEM, HRTEM, UV-vis, and XRD in detail. The nanosheets showed more than 5 times higher proton conductivity than the original polymer due to the smooth ionic conduction through the aligned polymer chains. The complex impedance plot with two semicircles also suggested the presence of grain boundaries in the polymer nanosheets.

Platinum(II)-Based Metallo-Supramolecular Polymer with Controlled Unidirectional Dipoles for Tunable Rectification.

A platinum(II)-based, luminescent, metallo-supramolecular polymer (PolyPtL1) having an inherent dipole moment was synthesized via complexation of Pt(II) ions with an asymmetric ligand L1, containing terpyridyl and pyridyl moieties. The synthesized ligand and polymer were well characterized by various NMR techniques, optical spectroscopy, and cyclic voltammetry studies. The morphological study by atomic force microscopy revealed the individual and assembled polymer chains of 1-4 nm height. The polymer was specifically attached on Au-electrodes to produce two types of film (films 1 and 2) in which the polymer chains were aligned with their dipoles in opposite directions. The Au-surface bounded films were characterized by UV-vis, Raman spectroscopy, cyclic voltammetry, and atomic force microscopy study. The quantum mechanical calculation determined the average dipole moment for each monomer unit in PolyPtL1 to be about 5.8 D. The precise surface derivatization permitted effective tuning of the direction dipole moment, as well as the direction of rectification of the resulting polymer-attached molecular diodes. Film 1 was more conductive in positive bias region with an average rectification ratio (RR = I(+4 V)/I(-4 V)) ≈ 20, whereas film 2 was more conducting in negative bias with an average rectification ratio (RR = I(-4 V)/I(+4 V)) ≈ 18.

Black-to-Transmissive Electrochromism with Visible-to-Near-Infrared Switching of a Co(II)-Based Metallo-Supramolecular Polymer for Smart Window and Digital Signage Applications.

Black-to-transmissive electrochromism has been obtained with a Co(II)-based metallo-supramolecular polymer (polyCo). Thin films of polyCo, based on bisterpyridine ligand assembled with Co(II) metal ion, were constructed by spray casting the polymer onto ITO glass. With such simple fabricating means to form good-quality films, polyCo films show stable switching at the central metal ion of the Co(II)/Co(I) redox reaction when immersed in aqueous solution. With an increase in the pH of the aqueous electrolyte solution from neutral, the film exhibits a color response due to the interaction between the d-orbital electron and hydroxide ions affecting the d-d* transition. As a result, a nearly transparent-to-black electrochromic performance can be achieved with a transmittance difference at 550 nm of 74.3% (81.9-7.6%) in pH 13 solution. The light absorption of the film can be tuned over light regions from visible to near-infrared with a large attenuation.

Proton conduction in Mo(VI)-based metallo-supramolecular polymers.

High proton conduction (8.5 × 10(-2) mS cm(-1)) was observed in a Mo(vi)-based metallo-supramolecular polymer with carboxylic acids at 95%RH. The integration of OH groups into the polymer was analysed using FTIR spectroscopy and found to be crucial for the proton transport in the polymer.

Three-dimensional Fe(II)-based metallo-supramolecular polymers with electrochromic properties of quick switching, large contrast, and high coloration efficiency.

A series of Fe(II)-based metallo-supramolecular polymers with three-dimensional (3-D) structures were synthesized by the stepwise complexation of an Fe(II) salt with different ratios of a linear bis(terpyridine) ligand and a branched tris(terpyridine) ligand. Atomic force microscopy images of the polymer films showed a drastic change in the surface morphology upon varying the amount of the branched ligand. The surface of a designed 3-D construction film showed a highly porous structure (pore size: approximately 30-50 nm in diameter), probably due to the formation of a hyperbranched polymer structure. All the 3-D polymers had a blue color based on the metal-to-ligand charge-transfer (MLCT) absorption and exhibited excellent electrochromic properties. The most highly porous 3-D-structured film showed the best electrochromic performance; as compared with a 1-D linear polymer, the switching times were improved 38.7% for the coloring (0.31 → 0.19 s) and 37.9% for the bleaching (0.58 → 0.36 s). The transmittance change (ΔT) increased 21.8% (41.6 → 50.7%). Also, the coloration efficiency (η) was enhanced 45.3% (263.8 → 383.4 cm(2) C(-1)). The redox in the 3-D film was diffusion-controlled, as supported by the linear relationship between the current and square root of the scan rate. It is considered that the porous structure of the 3-D polymer films contributed to smooth ionic transfer during the redox and to the improved electrochromic properties.

Selective edge modification in graphene and graphite by chemical oxidation.

The effect of edge structures in graphene sheets has been well investigated theoretically but most experimental demonstrations of the functionalization have been for the bulk structures because of only a few reports on chemical methods to modify the edges selectively. We herein report a chemical method using the Lemieux-von Rudloff reagent that selectively oxidizes only the edges of graphene sheets. The selective oxidation at the edges of the graphene sheet was confirmed by thermogravimetric analysis (TGA), Raman mapping measurements, and X-ray photoelectron spectroscopy (XPS). The TGA result of the oxidized graphite with different particle sizes showed a slight weight loss at approximately 350 degrees C (2.29% for the middle particles (35 microm)), which indicates thermal decomposition of the oxidized edge part. The Raman mapping measurement in the inner part of graphene sheets didn't detect any defects or translational symmetry breaking after the oxidation. The XPS data clearly showed that the total carbon atom content present as C--O, C==O, and O--C==C increased from 4.65 to 6.18% by the oxidation. Using the obtained edge-oxidized graphene as a starting material, various functionalizations of the edge structure are expected in the future.

Fabrication of quantum-dot devices in graphene.

We describe our recent experimental results on the fabrication of quantum-dot devices in a graphene-based two-dimensional system. Graphene samples were prepared by micromechanical cleavage of graphite crystals on a SiO2/Si substrate. We performed micro-Raman spectroscopy measurements to determine the number of layers of graphene flakes during the device fabrication process. By applying a nanofabrication process to the identified graphene flakes, we prepared a double-quantum-dot device structure comprising two lateral quantum dots coupled in series. Measurements of low-temperature electrical transport show the device to be a series-coupled double-dot system with varied interdot tunnel coupling, the strength of which changes continuously and non-monotonically as a function of gate voltage.

Coupled quantum dots in a graphene-based two-dimensional semimetal.

We present an experimental demonstration of a graphene-based double quantum dot system, which exhibits single-electron transport of two lateral quantum dots coupled in series. Low-temperature transport measurements revealed honeycomb charge stability diagrams with a varied (from weak to strong) interdot tunnel-coupling regime, and we have extracted the relevant parameters associated with the double quantum dot system. These results are important for the realization of integrated quantum circuits in graphene-based electronics.

Functional characterization and evolutionary implication of the internal 157-amino-acid sequence of an exoinulinase from Penicillium sp. strain TN-88.

An extracellular exoinulinase from the filamentous fungus Penicillium sp. strain TN-88 has a 14-fold higher specific activity of 743 U/mg toward inulin than its equivalent from the Aspergillus niger strain 12 and possesses an internal 157-amino-acid sequence whose corresponding region is absent in the A. niger enzyme. On the basis of sequence alignment, the internal region D' encoding the 157-amino-acid sequence in the Penicillium exoinulinase gene inuD cDNA was inserted into the site between the nucleotides 897 and 898 of the A. niger exoinulinase gene inuE cDNA. The resultant inuE::D' fusion was expressed in the methylotrophic yeast Pichia pastoris. The K(m) value of the secreted hybrid enzyme InuE::D' for inulin hydrolysis was about 1/15 that of the A. niger InuE, whereas its k(cat) value did not differ greatly from that of the InuE. These observations indicate that the Penicillium exoinulinase has evolved by the horizontal transfer and integration of a relevant DNA segment and that the internal sequence D' functions as an additional noncatalytic inulin-affinity region.

Quantitative expression analysis of inulinase gene cluster of Penicillium sp. strain TN-88.

The filamentous fungus Penicillium sp. strain TN-88 carries the endoinulinase gene inuC and the exoinulinase gene inuD that are linked head-to-head on the genome and divergently transcribed from an 859-bp intergenic region [Moriyama et al., Biosci. Biotechnol. Biochem., 66, 1887-1896 (2002)]. Quantitative real-time PCR amplification revealed that the transcription levels of the inuC and inuD genes increased 42- and 3260-fold in inulin-grown mycelia, respectively. Sucrose as well as fructose did not induce the expression of the inuC or inuD gene at all. The levels of inuC and inuD transcripts in mycelia grown on the glucose/inulin mixture were both below their basal levels in glucose-grown mycelia. Thus, glucose exerts a strong carbon catabolite repression on the expression of the two genes.

Acidophilic xylanase from Aureobasidium pullulans: efficient expression and secretion in Pichia pastoris and mutational analysis.

A yeast-like fungus Aureobasidium pullulans var. melanigenum strain ATCC 20524 produces an extracellular acidophilic endo-1,4-beta-xylanase with an optimum pH of 2.0 [Ohta et al., J. Biosci. Bioeng., 92, 262-270 (2001)]. The xynI cDNA encoding the precursor protein (XynI) was expressed in the methylotrophic yeast Pichia pastoris under the control of the alcohol oxidase I gene promoter. The 34 amino acid prepro-signal peptide of the A. pullulans XynI directed the efficient secretion of 178 mg of active xylanase per liter of the culture medium. The secretion level of the xylanase with its own signal peptide was comparable to that of the mature protein fused to the prepro leader from Saccharomyces cerevisiae alpha-mating factor and twofold higher than that of the mature protein fused to the pre-type signal peptide from P. pastoris acid phosphatase. The N-terminal amino acid sequence and the apparent M(r) of 24 kDa of the secreted recombinant protein indicated the native-like processing of the A. pullulans XynI signal sequence in P. pastoris. The three-dimensional model and mutational analysis of the xynI gene product showed that Asp-73 and Glu-157 residues located at the upper and lower edges of the active site cleft, respectively, play a significant role in its low pH optimum.

Molecular cloning and characterization of an exoinulinase gene from Aspergillus niger strain 12 and its expression in Pichia pastoris.

A genomic DNA segment and cDNAs encoding an extracellular exoinulinase from Aspergillus niger strain 12 were cloned and sequenced. Southern blot analysis indicated that the exoinulinase gene (inuE) was present as a single copy in the genome. An open reading frame of 1611 by was interrupted by a single intron of 60 bp, and encoded a 19-amino acid signal peptide and a 518-amino acid mature protein. The mature protein contained a single Cys residue and nine potential N-linked glycosylation sites. Three distinct transcription start points were observed at positions -41 (A), -35 (A), and -31 (A) from the start codon. The 5'-noncoding region had a putative TATA at position -75 (TATAAA). Transcription of the inuE gene was induced by inulin or sucrose and repressed by fructose or glucose. The inuE cDNA was functionally expressed under the control of the alcohol oxidase gene promoter in the methylotrophic yeast Pichia pastoris. The deduced amino acid sequence of the inuE gene product was 91% identical to that of an exoinulinase from Aspergillus awamori. A neighbor-joining tree showed that exo- and endoinulinases found in Aspergillus and Penicillium spp. have independently evolved the respective hydrolytic activities toward terminal and internal beta-2,1-fructofuranosidic linkages in inulin.