Interlocking of Single-Walled Carbon Nanotubes with Metal-Tethered Tetragonal Nanobrackets to Enrich a Few Hundredths of a Nanometer Range in Their Diameters.

We have separated carbon nanotubes through host-guest complexation using host molecules named "nanotweezers" and "nanocalipers". In this work, a host molecule named tetragonal "M-nanobrackets", consisting of a pair of dipyrrin nanocalipers corresponding to two brackets "[" and "]" tethered by two metals (M), is designed, synthesized, and employed to separate single-walled carbon nanotubes (SWNTs). A facile three-step process including one-pot Suzuki coupling is developed to synthesize M-nanobrackets in a 37% total yield (M = Cu). Upon extraction of SWNTs with a square nanobracket and Cu(II), in situ formed tetragonal M-nanobrackets are found to interlock SWNTs to disperse them in 2-propanol. The interlocking is confirmed by absorption and Raman spectroscopy as well as transmission electron and atomic force microscopy. Especially, Raman spectroscopy is utilized to prove the interlocking of SWNTs; Cu-nanobrackets are found to show inherent resonance Raman signals and affect the SWNT signals, or a radial breathing vibration, due to the rigid rectangular structure of Cu-nanobrackets. The interlocking is facilely and thoroughly released through demetalation to recover the pristine SWNTs as well as the square nanobracket. Such chemically controlled locking and unlocking for SWNTs are one of the characteristics of our separation process. This enables a precise evaluation by Raman, photoluminescence, and absorption spectroscopy of the diameter selectivity to SWNTs, revealing the diameter enrichment of only three kinds of SWNTs, (7,6), (9,4), and (8,5), in the 0.02 nm diameter range from 0.90 to 0.92 nm among ∼20 kinds of SWNTs from 0.76 to 1.17 nm in their diameter range.

Visible-Light-Activated Response Originating from Carrier-Mobility Modulation of NO2 Gas Sensors Based on MoS2 Monolayers.

Some gas sensors exhibit significant increases in their sensitivity and response/recovery rates under light illumination. This photoactivation of the gas response is considered a promising alternative to conventional thermal activation, which requires high power consumption. Thin layers of molybdenum disulfide (MoS2) are known to exhibit an effective photoactivated gas response under visible light. However, the mechanism of the photoactivated response has not yet been studied in detail. In this study, we fabricated field-effect-transistor (FET) gas sensors based on MoS2 monolayers and investigated their photoactivated gas responses to NO2 gas under illumination at various irradiances of visible light. A photocurrent was generated mainly due to the photovoltaic effect, which decreased upon exposure to NO2. The conductance-based sensor response showed a dependence on NO2 concentration according to the Langmuir adsorption isotherm, thereby suggesting that the response is proportional to the surface coverage of NO2 molecules on the MoS2 layer. The response and recovery rates showed a linear increase with increasing irradiance. Analysis based on the Langmuir adsorption model revealed that both photostimulated adsorption and desorption are involved in the photoactivated response. In contrast, despite the strong dependence of the photocurrent on the irradiance, the magnitude of the sensor response was independent of the irradiance. Based on this result and the change in transfer characteristics of the FET during NO2 exposure, we concluded that the fast response/recovery of the photoactivated response is due to the carrier mobility modulation of MoS2, which is caused by the dipole scattering of adsorbed NO2 molecules.

Bias- and Gate-Tunable Gas Sensor Response Originating from Modulation in the Schottky Barrier Height of a Graphene/MoS2 van der Waals Heterojunction.

We report on the gas-sensing characteristics of a van der Waals heterojunction consisting of graphene and a MoS2 flake. To extract the response actually originating from the heterojunction area, the other gas-sensitive parts were passivated by gas barrier layers. The graphene/MoS2 heterojunction device demonstrated a significant change in resistance, by a factor of greater than 103, upon exposure to 1 ppm NO2 under a reverse-bias condition, which was revealed to be a direct reflection of the modulation of the Schottky barrier height at the graphene/MoS2 interface. The magnitude of the response demonstrated strong dependences on the bias and back-gate voltages. The response further increased with increasing reverse bias. Conversely, it dramatically decreased when measured at a large forward bias or a large positive back-gate voltage. These behaviors were analyzed using a metal-semiconductor-metal diode model consisting of graphene/MoS2 and counter Ti/MoS2 Schottky diodes.

Four-point probe resistance measurements using PtIr-coated carbon nanotube tips.

We performed four-terminal conductivity measurements on a CoSi2 nanowire (NW) at room temperature by using PtIr-coated carbon nanotube (CNT) tips in a four-tip scanning tunneling microscope. The physical stability and high aspect ratio of the CNT tips made it possible to reduce the probe spacing down to ca. 30 nm. The probe-spacing dependence of resistance showed diffusive transport even at 30 nm and no current leakage to the Si substrate.