Narrow Results

We summarize recent results on the impact of defects on the electronic properties of single-wall carbon nanotubes. We probe the influence of defects on electron transport in CNFETs by combined scanning gate microscopy (SGM) and scanning impedance microscopy (SIM). Depletion surface potential of individual defects is quantified from the SGM-imaged defect radius as a function of tip bias voltage. This provides a measure of the Fermi level at the defect with zero tip voltage. In the "off" state, transport is first dominated by barriers at depleted defects. It becomes diffusive as the CNFET is turned on, and finally is quasi-ballistic in the regime of "degenerate electrostatic doping". Metallic nanotubes with good contacts show a metal-to-insulator crossover as the gate voltage is varied. In the metallic state we see quantitative agreement with the "twiston" scattering picture.

The capability of graphitic networks to reorganize their structures under irradiation of energetic particles provides the tool for nano-engineering of carbon nanotubes (CNTs). We have studied the effect of 30 keV N⁺ ion irradiation with three different fluences 10¹², 10¹³, and 10¹⁴ ions/cm² on the structural and spectroscopic properties of single-walled carbon nanotubes (SWNTs) and multiwalled carbon nanotubes (MWNTs). Irradiation-induced structural defects and coalescence of the nanotubes are studied using high-resolution transmission electron microscopy (HRTEM). Upon irradiation, some of the radial breathing modes in Raman spectra disappear due to conversion from single-walled to multiwalled structure. We observed a systematic change in intensity of the intermediate frequency mode (IFM) with increasing dose of ion-irradiation and these IFM modes are attributed to structural defects of SWNTs. Dramatic improvement in the intensity ratio G-band to D-band (at ~1335 cm^-1) for ion fluence of 10¹³ ions/cm² indicates improved graphitic structure as a result of reconstruction. Similarly, X-ray Photoelectron spectroscopy studies show improvement in the amount of sp² carbon upon 10¹³ ions/cm²N⁺ ion irradiation dose. At a higher dose (10¹⁴N⁺ions/cm²), vacancy and bent structures having Stone–Wales defects were observed in HRTEM, whereas MWNTs show formation of surface hillock like protrusions leading to formation of fullerene-like structures.

The broad visible photoluminescence (PL) observed in ZnO nanocrystals (NCs) is widely attributed to multiple low lying surface-defects. We have performed steady state and time-resolved PL measurements on size-selected ZnO NCs in the strong confinement regimes. Our results show that radiative relaxation rates and coupling between excitons and surface defect states vary dramatically for sizes between 2 nm and 3 nm. Energy dependent PL lifetimes reveal that relaxation dynamics of these defect states in the blue- and red-edge of the emission are very different from each other.

The fatigue strength of components is affected by loading and environmental conditions, their geometry, the material and the manufacturing process adopted for their production. As a consequence of local stress concentrations, fatigue cracks can propagate to failure from the defects created by the manufacturing processes or introduced during the service life, as occurs in impact-damage. Ti6Al4V titanium alloy is commonly used in the aeronautical sector and other engineering applications due to its high specific strength. The study of the fatigue behavior of Ti6Al4V is therefore critical for designing reliable and durable structures. The work reports a synthesis of the results present in the literature for the fatigue behavior of Ti6Al4V in inert and aggressive environments, in the presence of a deposited coating, in the case of impact-damage, when components are produced by additive manufacturing and in the absence of solution treatment and over-aging.

In this paper we report on the preparation of nanobrushes of ZnO on quartz substrate by a direct atmosphere evaporation method using Zn metal flakes. Activated charcoal was used as a catalyst that facilitated the formation of nanobrushes in which the brush stem was about 15–20 μm in length and the bristles (100–200 nm thick) were made up of nanofibrous ZnO whose tips were 10–15 nm in width and were angled in some cases. These aligned nanobrushes can find potential applications as nanopower generators and high aspect ratio AFM probes by virtue of the piezoelectric property of ZnO. This technique is simple for realizing aligned ZnO nanobrushes with metallic Zn as the source material.

This research demonstrates the capability of controlled, focused ion beam (FIB)–assisted tailoring of morphologies in both multiwall carbon nanotubes (CNTs) and Y junction nonlinear CNT systems through defect engineering. We have shown that a 30 keV FIB Ga⁺ ion beam at low ion milling currents of 1 pA can be used to partially reduce the CNT diameter, to provide electrical conduction bottleneck morphologies for linear CNTs, and to introduce both additive and subractive defects at Y junction locations of Y-CNT samples. Our aim is for this work to provide motivation for additional research to determine the effects of ion-beam-induced changes in modulating the physical and chemical properties of nanotubes.

Nanocrystalline ZnO particles substituted with different concentrations (0–30%) of Mn were synthesized by using a modified ceramic route and characterized by X-ray diffraction, transmission electron microscopy, selected area electron diffraction and energy dispersive X-ray analysis methods. Positron lifetime and coincidence Doppler broadening measurements were used as probes to identify the vacancy-type defects present in them and monitor the changes while doping. The predominant positron trapping center in the undoped ZnO is identified as the trivacancy-type cluster V_Zn+O+Zn, which is negatively charged, and it transformed to the neutral divacancy V_Zn+O on doping with Mn²⁺ ions. The intensity of the defect-specific positron lifetime component got reduced initially indicating partial occupancy of the vacancies by the doped cations but then recovered on further doping due to the additional Zn vacancies created as a result of the increasing strain introduced by the Mn ions of larger radius. The creation of a new phase ZnMn₂O₄ thereafter changed the course of variation of the annihilation parameters, as the positrons got increasingly trapped in the vacancies at the tetrahedral and octahedral sites of the spinel nanomanganite.

We report room temperature ferromagnetism (RTFM) in nanocrystalline Zn_1-xCu_xO(0.03 ≤ x ≤ 0.07) materials synthesized by autocombustion technique. The average particle sizes are in the range of 60 nm. The saturation magnetization and coercivity of 7% Cu-doped ZnO is enhanced significantly in comparison to 3% and 5% Cu-doped ZnO. There is not much variation in the optical band gap due to Cu doping, thus suggesting the uniform distribution of Cu in the ZnO matrix. Micro-Raman and photoluminescence analysis predict the presence of clusters of oxygen vacancies in Cu-doped system which improves with the increase in Cu concentration. This study provides further evidence that oxygen vacancies play an important role in the enhancement of room temperature ferromagnetic property in Cu-doped ZnO.

Spherical Bi₂S₃ nanoparticles (NPs) were prepared by a facile in situ thermal sulfuration method. Different Bi₂S₃ samples were obtained by controlling the sulfuration time. The products were characterized by X-ray diffractometer (XRD), scanning electron microscopy (SEM), Raman and Fourier-transform infrared (FT-IR) methods. The optical properties were examined by UV-visible-near-infrared (UV-Vis–NIR) and photoluminescence (PL) techniques. The results show that the phase of the products after sulfuration is pure and the spherical shape of Bi NPs has been successfully transmitted to Bi₂S₃ samples. The light absorption edges exhibit red shift to 1060 nm while the light emission displays blue shift to 868 nm, compared with the energy bandgap of bulk Bi₂S₃. The reason for the special optical properties of Bi₂S₃ NPs by this in situ sulfuration route is considered to associate with the defects and quantum size effect of NPs.

A molecular structural mechanics method has been implemented to investigate the vibrational characteristics of single-layer graphene (SLG) with defects. By adopting the lumped mass unit to replace carbon atoms, and the beam element with circular cross-section to mimic C–C covalent bonds, SLG is modeled as a space framework. The simulation results show that the chirality almost has no effect on the natural frequency and the vibration mode of SLG, while boundary conditions have great influences. The influences of defects with different number and location on the natural frequencies are also studied. It is concluded that vibration mode is insensitive to the vacancy defect, small hole and short flaw, but large holes and long flaws can affect the vibration characteristics. So the graphene sheet even with small defect effects might be selected as the nanosensor material as well as pristine graphene. The conclusions in this paper may provide some references for the design of nanosensor.