Transient Superhydrophilic Surface Modification of Polyimide by Metal Ion Beam Irradiation.

Polyimide is commonly used as a substrate for flexible electronic devices because of its excellent thermal, physical, and electrical properties. To enhance the adhesion between substrates and electrodes, it is necessary to improve the hydrophilic properties of the polyimide. Various surface treatments, such as plasma treatment, laser ablation, and ultraviolet treatments, have been applied for this purpose. In this study, we demonstrated that Cu and Ti ion beam irradiation can temporarily create a superhydrophilic surface on polyimide after irradiation. When Cu or Ti ions bombarded the polyimide, the contact angle changed systematically with the beam current density and over time. We present atomic force microscopy (AFM) data for polyimide irradiated with Cu and Ti ions at different beam current densities and discuss the possible mechanisms behind the changes in the contact angle.

Synthesis mechanism from graphene quantum dots to carbon nanotubes by ion-sputtering assisted chemical vapor deposition.

We present the first work of the synthesis mechanism from graphene quantum dots (GQDs) to carbon nanotubes (CNTs) by an ion-sputtering assisted chemical vapor deposition. During the annealing process, a Pt thin film deposited by the ion-sputtering was dewetted and agglomerated to form many nanometer-sized particles, leading to Pt nanoparticles (PtNPs) that can act as catalysts for creating carbon allotropes. The shape of the allotropes can be effectively tailored from GQDs to CNTs by controlling three key parameters such as the dose of catalytic ions (D), amounts of carbon source (S), and thermal energy (T). In our work, it was clearly proved that the growth control from GQDs to CNTs has a comparably proportional relationship with D and S, but has a reverse proportional relationship with T. Furthermore, high-purity GQDs without any other by-products and the CNTs with the cap of PtNPs were generated. Their shapes were appropriately controlled, respectively, based on the established synthesis mechanism.

Decoupling the metal insulator transition and crystal field effects of VO2.

VO2 is a highly correlated electron system which has a metal-to-insulator transition (MIT) with a dramatic change of conductivity accompanied by a first-order structural phase transition (SPT) near room temperature. The origin of the MIT is still controversial and there is ongoing debate over whether an SPT induces the MIT and whether the Tc can be engineered using artificial parameters. We examined the electrical and local structural properties of Cr- and Co-ion implanted VO2 (Cr-VO2 and Co-VO2) films using temperature-dependent resistance and X-ray absorption fine structure (XAFS) measurements at the V K edge. The temperature-dependent electrical resistance measurements of both Cr-VO2 and Co-VO2 films showed sharp MIT features. The Tc values of the Cr-VO2 and Co-VO2 films first decreased and then increased relative to that of pristine VO2 as the ion flux was increased. The pre-edge peak of the V K edge from the Cr-VO2 films with a Cr ion flux ≥ 1013 ions/cm2 showed no temperature-dependent behavior, implying no changes in the local density of states of V 3d t2g and eg orbitals during MIT. Extended XAFS (EXAFS) revealed that implanted Cr and Co ions and their tracks caused a substantial amount of structural disorder and distortion at both vanadium and oxygen sites. The resistance and XAFS measurements revealed that VO2 experiences a sharp MIT when the distance of V-V pairs undergoes an SPT without any transitions in either the VO6 octahedrons or the V 3d t2g and eg states. This indicates that the MIT of VO2 occurs with no changes of the crystal fields.

A mechanism of surface hardness enhancement for H+ irradiated polycarbonate.

H+ irradiation increases the surface hardness of polycarbonate. Nano indentation measurement shows that the hardness increases up to 3.7 GPa at the dose of 5 × 1016 # cm-2 and at the irradiation energy of 150 keV. In addition, the hardness increases with the dose and the energy of H+ irradiation. In accordance with the nano indentation measurement, the Fourier-transform infrared spectroscopy (FTIR) depends on the dose and energy of H+ irradiation. The peak at ∼1500 cm-1 for the aromatic ring and the peak at ∼1770 cm-1 for the C[double bond, length as m-dash]O stretch decrease with increasing dose and energy, while the increase of the dose and energy develops a new C[double bond, length as m-dash]O stretch vibration at ∼1700 cm-1 and forms aromatic hydrocarbons at ∼1600 cm-1. X-ray diffraction experiments are also consistent with the nano indentation measurement and FTIR spectra. Based on the experiments, we discuss a possible mechanism of the surface hardness enhancements by ion beam irradiation.

Coherence in defect evolution data for the ion beam irradiated graphene.

The defect evolution in graphene produced by ion beam bombardment is investigated by changing the ion species, irradiation energy and dose. Raman spectroscopy is performed to examine the defect yield produced under various ion beam bombardment conditions. The defect yields of the vacancy-type defect are well described by the linear energy transfer (L) and dose (d). By increasing Ld, the defect yields exhibit similar behaviours for all ion species. As a consequence, all the defect yields can be collapsed into a single curve by multiplying them by a single parameter, suggesting that the defect evolution under various ion beam bombardment conditions can be described in a simple formula.

Survival of Verwey transition in gadolinium-doped ultrasmall magnetite nanoparticles.

We have demonstrated that the Verwey transition, which is highly sensitive to impurities, survives in anisotropic Gd-doped magnetite nanoparticles. Transmission electron microscopy analysis shows that the nanoparticles are uniformly distributed. X-ray photoelectron spectroscopy and EDS mapping analysis confirm Gd-doping on the nanoparticles. The Verwey transition of the Gd-doped magnetite nanoparticles is robust and the temperature dependence of the magnetic moment (zero field cooling and field cooling) shows the same behaviour as that of the Verwey transition in bulk magnetite, at a lower transition temperature (∼110 K). In addition, irregularly shaped nanoparticles do not show the Verwey transition whereas square-shaped nanoparticles show the transition. Mössbauer spectral analysis shows that the slope of the magnetic hyperfine field and the electric quadrupole splitting change at the same temperature, meaning that the Verwey transition occurs at ∼110 K. These results would provide new insights into understanding the Verwey transition in nano-sized materials.