Minggang Xia

Extremely stretchable all-carbon-nanotube transistor on flexible and transparent substrates

The response of carbon-nanotube (CNT) transistors to large tensile strains has not been studied because of lack of stretchable devices. In this letter, we fabricate extremely stretchable single-wall CNT (SWCNT) conductive coatings on flexible and transparent elastomer substrates. We then measure the mechanical and electrical properties of the coatings and found excellent stretchability (Poisson ratiou2009≈u20090.31). The sheet resistances of the coatings remain largely unchanged under a large tensile strain. We then construct an active transistor on SWCNT thin films, which serve as active channel and electrodes, with polydimethylsiloxane thin film as the gate dielectric layer. The transistor exhibits excellent mechanical stability, showing no noticeable change (less than 5%) in electrical performance up to a large strain of 22.5%. The stretchable SWCNT thin-film transistor exhibits a current on–off ratio of ∼50 and field-effect mobility of ∼24u2009cm2 V−1 s−1, with 75% transmissivity in visible wavelength. We also fo...

Raman spectra of bilayer graphene covered with Poly(methyl methacrylate) thin film

The Raman spectra of bilayer graphene covered with poly(methyl methacrylate) (PMMA) were investigated. Both the G and 2D peaks of PMMA-coated graphene were stiff and broad compared with those of uncovered graphene. This could be attributed to the residual strain induced by high-temperature baking during fabrication of the nanodevice. Furthermore, the two 2D peaks stiffened and broadened with increasing laser power, which is just the reverse to uncovered graphene. The stiffness is likely caused by graphene compression induced by the circular bubble of the thin PMMA film generated by laser irradiation. Our findings may contribute to the application of PMMA in the strain engineering of graphene nanodevices.

Width-dependent structural stability and magnetic properties of monolayer zigzag MoS2 nanoribbons

First-principles study based on density functional theory has been employed to investigate width-dependent structural stability and magnetic properties of monolayer zigzag MoS2 nanoribbons (ZZ-MoS2 NRs). The width N = 4–6 (the numbers of zigzag Mo–S chains along the ribbon length) are considered. The results show that all studied ZZ-MoS2 NRs are less stable than two-dimensional MoS2 monolayer, exhibiting that a broader width ribbon behaves better structural stability and an inversely proportional relationship between the structural stability (or the ribbon with) and boundary S–Mo interaction. Electronic states imply that all ZZ-MoS2 NRs exhibit magnetic properties, regardless of their widths. Total magnetic moment increases with the increasing width N, which is mainly ascribed to the decreasing S–Mo interaction of the two zigzag edges. In order to confirm this reason, a uniaxial tension strain is applied to ZZ-MoS2 NRs. It has been found that, with the increasing tension strain, the bond length of boundary S–Mo increases, at the same time, the magnetic moment increases also. Our results suggest the rational applications of ZZ-MoS2 NRs in nanoelectronics and spintronics.

Anomalous junctions characterized by Raman spectroscopy in SixGe1−x nanowires with axially degraded components

The characterization of junctions in nanowires by high-resolution transmission electron microscopy with spherical aberration correction is tricky and tedious. Many disadvantages also exist, including rigorous sample preparation and structural damage inflicted by high-energy electrons. In this work, we present a simple, low-cost, and non-destructive Raman spectroscopy method of characterizing anomalous junctions in nanowires with axially degraded components. The Raman spectra of SixGe1−x nanowires with axially degraded components are studied in detail using a confocal micro-Raman spectrometer. Three Raman peaks (νSi–Siu2009=u2009490u2009cm−1, νSi–Geu2009=u2009400u2009cm−1, and νGe–Geu2009=u2009284u2009cm−1) up-shift with increased Si content. This up-shift originates in the bond compression induced by a confined effect on the radial direction of nanowire. The anomalous junctions in SixGe1−x nanowires with axially degraded components are then observed by Raman spectroscopy and verified by transmission electron microscopy energy-dispersive X-r...

First-principles study on structural, thermal, mechanical and dynamic stability of T’-MoS2

Using first-principles density functional theory calculations, we investigate the structure, stability, optical modes and electronic band gap of a distorted tetragonal MoS2 monolayer (T-MoS2). Our simulated scanning tunnel microscopy (STM) images of T-MoS2 are dramatically similar to those STM images which were identified as K x (H2O) y MoS2 from a previous experimental study. This similarity suggests that T-MoS2 might have already been experimentally observed, but due to being unexpected was misidentified. Furthermore, we verify the stability of T-MoS2 from the thermal, mechanical and dynamic aspects, by ab initio molecular dynamics simulation, elastic constants evaluation and phonon band structure calculation based on density functional perturbation theory, respectively. In addition, we calculate the eigenfrequencies and eigenvectors of the optical modes of T-MoS2 at [Formula: see text] point and distinguish their Raman and infrared activity by pointing out their irreducible representations using group theory. At the same time, we compare the Raman modes of T-MoS2 with those of H-MoS2 and T-MoS2. Our results provide useful guidance for further experimental identification and characterization of T-MoS2.

Gallium ion implantation greatly reduces thermal conductivity and enhances electronic one of ZnO nanowires

The electrical and thermal conductivities are measured for individual zinc oxide (ZnO) nanowires with and without gallium ion (Ga+) implantation at room temperature. Our results show that Ga+ implantation enhances electrical conductivity by one order of magnitude from 1.01 × 103 Ω−1m−1 to 1.46 × 104 Ω−1m−1 and reduces its thermal conductivity by one order of magnitude from 12.7 Wm−1K−1 to 1.22 Wm−1K−1 for ZnO nanowires of 100 nm in diameter. The measured thermal conductivities are in good agreement with those in theoretical simulation. The increase of electrical conductivity origins in electron donor doping by Ga+ implantation and the decrease of thermal conductivity is due to the longitudinal and transverse acoustic phonons scattering by Ga+ point scattering. For pristine ZnO nanowires, the thermal conductivity decreases only two times when its diameter reduces from 100 nm to 46 nm. Therefore, Ga+-implantation may be a more effective method than diameter reduction in improving thermoelectric performance.

THE TRANSPORT PROPERTIES OF NANOTUBES AND ITS CURVATURE EFFECTS

The transport properties across contacts between quantum tubes and metals are studied. The curvature effects on the transmission are also considered. We find that the transmission probability of an electron transporting from the tube into the metal through the contact can be close to 100%. The curvature has a larger effect on the transmission for smaller radii and the effects decrease as the radii increase.

Spontaneous formation of non-uniform double helices for elastic rods under torsion

Abstract The spontaneous formation of double helices for filaments under torsion is common and significant. For example, the research on the supercoiling of DNA is helpful for understanding the replication and transcription of DNA. Similar double helices can appear in carbon nanotube yarns, cables, telephone wires and so forth. We noticed that non-uniform double helices can be produced due to the surface friction induced by the self-contact. Therefore an ideal model was presented to investigate the formation of double helices for elastic rods under torque. A general equilibrium condition which is valid for both the smooth surface and the rough surface situations is derived by using the variational method. By adding further constraints, the smooth and rough surface situations are investigated in detail respectively. Additionally, the model showed that the specific process of how to twist and slack the rod can determine the surface friction and hence influence the configuration of the double helix formed by rods with rough surfaces. Based on this principle, a method of manufacturing double helices with designed configurations was proposed and demonstrated. Finally, experiments were performed to verify the model and the results agreed well with the theory.

Structure–property relationships of cell clusters in biotissues: 2D analysis

Gaining insight into the relationships between the self-organized cell structures and the properties of biotissues is helpful for revealing the function of biomaterials and its designing principle. However, the traditionally used random foam model neglects several important details of the frameworks of cell clusters resulting in incomplete conclusions. Herein, we use a more complete model, the cell adhesion model, to investigate the mechanical and morphological properties of the two-dimensional (2D) dry cell foams. Since these 2D structures are formed by cell adhesion, the system can reach equilibrium through minimizing free energy. Under the equilibrium conditions without volume constraint, shape equations for highly symmetrical structures are derived, and the analytical results of the corresponding mechanical parameters, such as the Youngs modulus, bulk modulus and failure strength, are obtained. Moreover, with volume constraint, numerical simulation method is applied to study the complex shapes and obtain several stable multicellular structures. Symmetry breaking caused by the volume change is also observed. Moreover, typical periodic shapes and the corresponding phase transformations are also explored. Our study provides a new potential method to bridge the microstructure and macro-mechanical parameters of biotissues. The results are also useful for understanding the formation mechanism of biotissue structures.