Alexandru Korotcov

One-dimensional conductive IrO2 nanocrystals

We review the results of the synthesis of IrO2 nanocrystals (NCs) on different substrates via metal-organic chemical vapour deposition (MOCVD) using (MeCp)(COD)Ir as the source reagent. The surface morphology, structural and spectroscopic properties of the as-deposited NCs were characterized using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), selected-area electron diffractometry (SAD), x-ray diffractometry (XRD), x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. The roles of different substrates for the formation of various textures of nanocrystalline IrO2 are studied. Several one-dimensional (1D) nanostructures have evolved by decreasing the degree of interface instability. The morphological evolution occurs from triangular/wedged nanorods via incomplete/scrolled nanotubes to square nanotubes and square nanorods (NRs), with increasing morphological stability. The results show that the three-dimensional (3D) grains composing traditional film belong to the most stable form as compared to all the 1D NCs, and the sequential shape evolution has been found to be highly correlated to a morphological phase diagram based on the growth kinetics. In addition, area selective growth of IrO2 NRs has been demonstrated on sapphire(012) and sapphire(100) substrates which consist of patterned SiO2 as the nongrowth surface. The initial growth of IrO2 nuclei is studied. Selectivity, rod orientation, and other morphological features of the nanorod forest can find their origins in the nucleation behaviour during initial growth. XPS analyses show the coexistence of higher oxidation states of iridium in the as-grown IrO2 NCs. The usefulness of the experimental Raman scattering together with the modified spatial correlation (MSC) model analysis as a residual stress and structural characterization technique for 1D IrO2 NCs has been demonstrated. The field emission properties of the vertically aligned IrO2 NRs are studied and demonstrated as a high-performance and robust field emitter material owing to its low work function, low resistivity and excellent stability against oxygen.

Growth and characterization of well aligned densely packed IrO2 nanocrystals on sapphire via reactive sputtering

Well aligned densely packed IrO2 nanocrystals (NCs) have been grown on sapphire (SA) substrates with different orientations by reactive magnetron sputtering using an Ir metal target. The surface morphology, structural and spectroscopic properties of the as-deposited NCs were characterized using field-emission scanning electron microscopy (FESEM), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS) and micro-Raman spectroscopy. FESEM micrographs reveal that NCs with parallel in-plane alignment were gown on SA(001), vertically aligned NCs were grown on SA(100), while the NCs on SA(012) and SA(110) contained, respectively, single- and double-aligned directions with a tilt angle of ~35° from the normal to the substrates. The XRD results indicate that the NCs are (100), (001), and (101) oriented on SA(001), SA(001), and SA(012)/SA(110) substrates, respectively. A strong substrate effect on the alignment of the IrO2 NCs growth has been demonstrated and the probable mechanism for the formation of these NCs has been discussed. XPS analyses show the coexistence of higher oxidation states of iridium in the as-grown IrO2 NCs. The Raman spectra show the red-shift and asymmetric peak broadening with a low frequency tail of the IrO2 signatures with respect to that of the bulk counterpart, which are attributed to both the size and residual stress effects.

Effect of length, spacing and morphology of vertically aligned RuO2 nanostructures on field-emission properties

The length, spacing and morphology of vertical aligned nanostructural RuO2 one-dimensional (1D) arrays are varied independently to investigate their effect on the field-emission properties. It is shown that, in general, the lowest turn-on field, highest emission current density and apparent field enhancement factor can be achieved in nanocrystals (NCs) with the smallest emitter radius for the tips and the highest aspect ratio. Moreover, the lower-density NCs exhibit better field-emission characteristics, which are related to the screening effect. The results could be valuable for the application of field-emission-based devices using RuO2 NCs arrays as cathode materials.

Selective growth of IrO2 nanorods using metalorganic chemical vapor deposition

Area-selective growth of iridium dioxide (IrO2) nanorods has been demonstrated via metalorganic chemical vapor deposition (MOCVD) using precursor (methylcyclopentadienyl)(1,5-cyclooctadiene)Ir on a sapphire (012) or (100) substrate which consists of patterned SiO2 as the nongrowth surface. The optimal substrate temperature for selective growth is 450 ± 10 °C at a chamber pressure of 20 mbar. Orientation of nanorods is dictated by the in-plane epitaxial relation between the IrO2 crystal and sapphire, along with the IrO2 growth habit in the [001] direction. The photolithography method is shown to be a superior patterning method since it gives a better resolution in preserving rod orientation at the growth and nongrowth boundary zone. The initial growth of IrO2 nuclei is also studied. Selectivity, rod orientation, and other morphological features of the nanorod forest can find their origins in nucleation behavior during initial growth.

Raman scattering characterization of well-aligned RuO2 nanocrystals grown on sapphire substrates

Raman scattering (RS) spectroscopy is a popular measurement technique that uses inelastic scattering of monochromatic light to study vibrational characteristics of a material system. A typical application of RS is for material structure determination. This paper describes the application of RS for the characterization of the preferable growth direction of well-aligned nanocrystals (NCs) deposited on sapphire substrates. The results indicate that RS could become a powerful technique for the quick determination of the NCs orientation. The redshifts and asymmetric linewidth broadening of the Raman features of RuO2 NCs are analysed by a modified spatial correlation (MSC) model, which includes the factor of stress-induced shift. The usefulness of experimental RS together with the MSC model analysis as a nondestructive structural and residual stress characterization technique for NCs has been demonstrated.

Well-aligned IrO 2 nanocrystals

We review the results of synthesis of well-aligned IrO2 nanocrystals (NCs) on sapphire (SA), LiNbO3 (LNO), LiTaO3 (LTO) substrates via reactive magnetron sputtering and metal-organic chemical vapor deposition. The surface morphology and structural properties of the as-deposited NCs were characterized. Field emission scanning electron microscopy micrographs reveal that NCs grown on SA(100)/LNO(100) are vertically aligned, while the NCs on SA(012)/LTO(012) and SA(110) contain singly and doubly tilted alignments, respectively, with a tilt angle of ∼ 35° from the normal to the substrates. NCs grown on SA(001) show in-plane alignment with mosaic structure. The X-ray diffraction results indicate that the NCs are (001), (101), and (100) oriented on SA(100)/LNO(100), SA(012)/LTO(012)/SA(110), and SA(001) substrates, respectively. A strong substrate effect on the alignment of the IrO2 NCs deposition has been demonstrated. The roles of different substrates in the formation of various textures of nanocrystalline IrO2 are studied and the possible mechanisms have been discussed.

Growth and characterization of vertically aligned 1D IrO/sub 2/ nanocrystals via reactive sputtering

We report the preparation of 1D vertically aligned IrO/sub 2/ nanocrystals on LiNbO/sub 3/ (100) substrates by reactive magnetron sputtering with Ir metal target. The effects of sputtering conditions such as pressure, rf power, substrate temperature etc. have been presented and discussed. The surface morphology, structural and spectroscopic properties of the as-grown nanocrystals have been characterized using field-emission scanning electron microscopy (FESEM), X-ray diffractometry (XRD) and micro-Raman scattering. The red-shift and asymmetric broadening of Raman lineshape have been analyzed and attributed to both the size effect and residual stress.