Tunable Electrical and Optical Characteristics in Monolayer Graphene and Few-Layer MoS2 Heterostructure Devices.

Lateral and vertical two-dimensional heterostructure devices, in particular graphene-MoS2, have attracted profound interest as they offer additional functionalities over normal two-dimensional devices. Here, we have carried out electrical and optical characterization of graphene-MoS2 heterostructure. The few-layer MoS2 devices with metal electrode at one end and monolayer graphene electrode at the other end show nonlinearity in drain current with drain voltage sweep due to asymmetrical Schottky barrier height at the contacts and can be modulated with an external gate field. The doping effect of MoS2 on graphene was observed as double Dirac points in the transfer characteristics of the graphene field-effect transistor (FET) with a few-layer MoS2 overlapping the middle part of the channel, whereas the underlapping of graphene have negligible effect on MoS2 FET characteristics, which showed typical n-type behavior. The heterostructure also exhibits a strongest optical response for 520 nm wavelength, which decreases with higher wavelengths. Another distinct feature observed in the heterostructure is the peak in the photocurrent around zero gate voltage. This peak is distinguished from conventional MoS2 FETs, which show a continuous increase in photocurrent with back-gate voltage. These results offer significant insight and further enhance the understanding of the graphene-MoS2 heterostructure.

Raman shift and electrical properties of MoS2 bilayer on boron nitride substrate.

We have fabricated a bilayer molybdenum disulphide (MoS2) transistor on boron nitride (BN) substrate and performed Raman spectroscopy and electrical measurements with this device. The characteristic Raman peaks show an upshift about 2.5 cm(-1) with the layer lying on BN, and a narrower line width in comparison with those on a SiO2 substrate. The device has a maximum drain current larger than 1 µA and a high current on/off ratio of greater than 10(8). In the temperature range of 100 K-293 K, the two terminal gate effect mobility and the carrier density do not change significantly with temperature. Results of the Raman and electrical measurements reveal that BN is a suitable substrate for atomic layer electrical devices.

Preparation and characterization of conducting mixed-valence 9,9'-dimethyl-3,3'-bicarbazyl rectangular nanowires.

The facile synthesis of an organic electric conducting nanowire is described. The simple oxidation of 9-methylcarbazole by iron(III) perchlorate in a methanol/acetonitrile mixture under atmospheric pressure and temperature produces abundant nanowires without using a template. The nanowire consists of 9,9'-dimethyl-3,3'-dicarbazyl and has a rectangular nanowire shape with an average diameter of 397 ± 50 nm and length of 17 ± 5 µm. The results of the elemental analysis, (1)H NMR, FT-IR, XPS, and ESR measurements revealed that the chemical composition of the nanowire is (dicarbazyl)(0.12)(dicarbazylium·ClO(4)(-))(0.88)·H(2)O. This result, combined with the UV-vis-NIR measurement, demonstrates that 9,9'-dimethyl-3,3'-dicarbazyl stacks in a mixed valence state. The nanowire is electroactive and has an electric conductivity of 3.0 × 10(-5) S cm(-1).

Evaluating the performance of quantum point contacts as nanoscale terahertz sensors.

Quantum point contacts (QPCs) are nanoscale constrictions that are realized in a high-mobility two-dimensional electron gas by applying negative bias to split Schottky gates on top of a semiconductor. Here, we explore the suitability of these nanodevices to THz detection, by making use of their ability to rectify THz signals via the strong nonlinearities that exist in their conductance. In addition to demonstrating the configuration of these devices that provides optimal THz sensitivity, we also determine their noise equivalent power and responsivity. Our studies suggest that, with further optimization, QPCs can provide a viable approach to broadband THz sensing in the range above 1 THz.

Shubnikov-de Haas measurements on a high mobility monolayer graphene flake sandwiched between boron nitride sheets.

A flake of monolayer graphene was sandwiched between boron nitride sheets. Temperature dependent Shubnikov-de Haas measurements were performed to access how this technique influences the electronic properties of the graphene sample. The maximum mobility found in this configuration was approximately 105 cm2 Vs -1. From the phase of the oscillations a Berry phase ß of 1/2 was obtained indicating the presence of Dirac fermions. We obtained Fermi velocities around [Formula: see text] m s-1 which imply hopping energies close to 2.5 eV. Furthermore, the carrier lifetime is typically higher than that found in graphene supported by SiO2, reaching values higher than 700 fs.

Nanoscale-Barrier Formation Induced by Low-Dose Electron-Beam Exposure in Ultrathin MoS2 Transistors.

Utilizing an innovative combination of scanning-probe and spectroscopic techniques, supported by first-principles calculations, we demonstrate how electron-beam exposure of field-effect transistors, implemented from ultrathin molybdenum disulfide (MoS2), may cause nanoscale structural modifications that in turn significantly modify the electrical operation of these devices. Quite surprisingly, these modifications are induced by even the relatively low electron doses used in conventional electron-beam lithography, which are found to induce compressive strain in the atomically thin MoS2. Likely arising from sulfur-vacancy formation in the exposed regions, the strain gives rise to a local widening of the MoS2 bandgap, an idea that is supported both by our experiment and by the results of first-principles calculations. A nanoscale potential barrier develops at the boundary between exposed and unexposed regions and may cause extrinsic variations in the resulting electrical characteristics exhibited by the transistor. The widespread use of electron-beam lithography in nanofabrication implies that the presence of such strain must be carefully considered when seeking to harness the potential of atomically thin transistors. At the same time, this work also promises the possibility of exploiting the strain as a means to achieve "bandstructure engineering" in such devices.

Current Scaling and Dirac Fermion Heating in Multi-Layer Graphene.

We have performed transport measurements on a multi-layer graphene device fabricated by conventional mechanical exfoliation. By using the zero-field resistance of our graphene device as a self-thermometer, we are able to determine the effective Dirac fermion temperature TDF at various driving currents I while keeping the lattice constant fixed. Interesting, it is found that TDF is proportional to Ia where a ~ 1. According to theoretical and experimental studies, the exponent a is given by 2/(2+p) where the charge-phonon scattering rate 1/τph is proportional to TP. Therefore our results yield p ~ 0, suggesting that there is little Dirac fermion-phonon scattering, a great advantage for applications in nanoelectronics.

Total Synthesis of (-)-Verrucarol(1).

We have achieved the total synthesis of (-)-verrucarol, a trichothecene sesquiterpenoid obtained as a hydrolysis product of the naturally occurring verrucarin A. Our total synthesis began with the previously reported enantiomerically pure bicyclic alpha-methylated gamma-lactone, which was prepared from D-glucose. The key steps for the total synthesis were (1) aldol-like carbon-carbon bond formation applied to the starting lactone using a four-carbon aldehyde as an electrophile for introduction of the quaternary stereogenic carbon sharing the B and C-rings of the trichothecene skeleton, (2) Dieckmann cyclization of the derived epsilon-ester lactone for construction of the C-ring equivalent, (3) Barton's decarboxylative oxygenation for conversion of a carboxylic acid functionality in the Dieckmann cyclization product into a hydroxyl group, (4) skeletal enlargement strategy for the crucial trichothecene skeleton construction, and (5) the final stereoselective formation of the exo-epoxy ring at the methylene carbon in the bridge constituting the B and C-rings.

GR-FET application for high-frequency detection device.

A small forbidden gap matched to low-energy photons (meV) and a quasi-Dirac electron system are both definitive characteristics of bilayer graphene (GR) that has gained it considerable interest in realizing a broadly tunable sensor for application in the microwave region around gigahertz (GHz) and terahertz (THz) regimes. In this work, a systematic study is presented which explores the GHz/THz detection limit of both bilayer and single-layer graphene field-effect transistor (GR-FET) devices. Several major improvements to the wiring setup, insulation architecture, graphite source, and bolometric heating of the GR-FET sensor were made in order to extend microwave photoresponse past previous reports of 40 GHz and to further improve THz detection.

Experimental evidence for direct insulator-quantum Hall transition in multi-layer graphene.

We have performed magnetotransport measurements on a multi-layer graphene flake. At the crossing magnetic field Bc, an approximately temperature-independent point in the measured longitudinal resistivity ρxx, which is ascribed to the direct insulator-quantum Hall (I-QH) transition, is observed. By analyzing the amplitudes of the magnetoresistivity oscillations, we are able to measure the quantum mobility µq of our device. It is found that at the direct I-QH transition, µqBc ≈ 0.37 which is considerably smaller than 1. In contrast, at Bc, ρxx is close to the Hall resistivity ρxy, i.e., the classical mobility µBc is ≈ 1. Therefore, our results suggest that different mobilities need to be introduced for the direct I-QH transition observed in multi-layered graphene. Combined with existing experimental results obtained in various material systems, our data obtained on graphene suggest that the direct I-QH transition is a universal effect in 2D.

Electric conductivity-tunable transparent flexible nanowire-filled polymer composites: orientation control of nanowires in a magnetic field.

Cobalt compound nanowires were dispersed in a transparent nonconductive polymer film by merely stirring, and the film's transparency and electrical conductivity were examined. This composite film is a unique system in which the average length of the nanowires exceeds the film's thickness. Even in such a system, a percolation threshold existed for the electric conductivity in the direction of the film thickness, and the value was 0.18 vol%. The electric conductivity value changed from ∼1 × 10(-12) S/cm to ∼1 × 10(-3) S/cm when the volume fraction exceeded the threshold. The electric conductivity apparently followed the percolation model until the volume fraction of the nanowires was about 0.45 vol %. The visible light transmission and electric conductivity of the composite film of about 1 vol % nanowires were 92% and 5 × 10(-3) S/cm, respectively. Moreover, the electric conductivity in the direction parallel to the film surface did not depend on the amount of the dispersed nanowires, and its value was about 1 × 10(-14) S/cm. Even in a weak magnetic field of about 100 mT, the nanowires were aligned in a vertical and parallel direction to the film surface, and the electric conductivity of each aligned composite film was 2.0 × 10(-2) S/cm and 2.1 × 10(-12) S/cm. The relation between the average wire length and the electric conductivity was examined, and the effect of the magnetic alignment on that relation was also examined.

On the direct insulator-quantum Hall transition in two-dimensional electron systems in the vicinity of nanoscaled scatterers.

A direct insulator-quantum Hall (I-QH) transition corresponds to a crossover/transition from the insulating regime to a high Landau level filling factor ν > 2 QH state. Such a transition has been attracting a great deal of both experimental and theoretical interests. In this study, we present three different two-dimensional electron systems (2DESs) which are in the vicinity of nanoscaled scatterers. All these three devices exhibit a direct I-QH transition, and the transport properties under different nanaoscaled scatterers are discussed.

Depletion of CD8alpha+ lymphocytes attenuates CCL28 expression in villus epithelia in rats.

To examine the involvement of CD8alpha+ intraepithelial lymphocytes (IELs) in chemokine expression by villus epithelial cells, villus and crypt fractions were collected by mechanical isolation using a chelating buffer and specific antibodies in CD8alpha+ cell-depleted rats. A larger population of CD8alpha+ cells was observed by histochemical evaluation in villus epithelia than in crypt epithelia in rat small intestine, and CCL9 and CCL28 expression was higher in the crypt fraction than in the villus fraction. The mRNA expression of CCL28 in villus fractions isolated from rat small intestinal mucosa was significantly reduced compared to that of CCL9, and was accompanied by CD8alpha depletion. Using a combined histochemical and flow cytometric approach, CD8alphaalpha+ cells were detected in the intraepithelial region of the villus epithelium. Thus, CCL28 expression in villus epithelial cells is partially supported by CD8alphaalpha+ cells, and CD8alpha+ IELs are involved in CCL28 expression.

Effects of gas adsorption on the electrical conductivity of single-wall carbon nanohorns.

We present significant electrical conductivity responses of the pelletized as-prepared and oxidized (ox-) single-wall carbon nanohorns (SWNHs) on adsorption of CO(2) and O(2). The morphological and pore structures of both pelletized SWNHs were examined by transmission electron microscopy (TEM) and nitrogen adsorption isotherm, leading to explicit evidences of the formation of nanoscale windows on the wall by oxidation. The SWNH and ox-SWNH induced a semiconducting behavior, strongly responded to CO(2) and O(2) adsorptions, and each exhibited n-type- and p-type-like conductivities. The electrical conductivity increase and decrease for CO(2) and O(2) adsorption, respectively, were observed for SWNH, whereas ox-SWNH showed a marked electrical conductivity drop on CO(2) adsorption and almost no change on O(2) adsorption. The dramatically different electrical conductivity response of ox-SWNH is presumed to be ascribed to the annihilation of pentagons in the single graphene wall by oxidation.

4-(Pyrrolidinyl)methoxybenzoic acid derivatives as a potent, orally active VLA-4 antagonist.

A novel series of benzoic acid derivatives as VLA-4 antagonists were synthesized. Optimization, focusing on activity and lipophilicity needed for cell permeability, resulted in the identification of 15b and 15e with good activity (IC50 = 1.6 nM each) and moderate lipophilicity (Log D = 2.0, 1.8). Furthermore, 15e demonstrated efficacy in murine asthma model by an oral dose of 30 mg/kg.