We show how an oxidative pretreatment of Fe, Co, or Ni growth catalyst on SiO2 support can be used to switch the growth mode of carbon nanotubes from tip growth to root growth, thus favoring the growth of dense, vertically aligned nanotube forests. The oxidative treatment creates a strong catalyst–support interaction at the catalyst–silica interface, which limits the surface diffusion and sintering of the catalyst nanoparticles and binds the catalyst to the SiO2 surface. This shows that the alignment and growth mode of nanotubes can be controlled, increasing the range of support materials giving dense nanotube forests.

X-ray absorption near-edge spectroscopy, photoluminescence, cathodoluminescence, and Raman spectroscopy have been used to investigate the chemical states of nitrogen dopants in ZnO nanowires. It is found that nitrogen exists in multiple states: N O , N Zn , and loosely bound N 2 molecule. The results establish a direct link between a donor-acceptor pair emission at 3.232 eV and the concentration of loosely bound N 2 . This work confirms that N 2 at Zn site is a potential candidate for producing a shallow acceptor state in N-doped ZnO as theoretically predicted by Lambrecht and Boonchun [Phys. Rev. B 87, 195207 (2013)]. Additionally, shallow acceptor states arising from N O complexes have been ruled out in this paper.

We study the high pressure response, up to 8 GPa, of silicon nanowires (SiNWs) with ∼ 15 nm diameter, by Raman spectroscopy. The first order Raman peak shows a superlinear trend, more pronounced compared to bulk Si. Combining transmission electron microscopy and Raman measurements we estimate the SiNWs' bulk modulus and the Grüneisen parameters. We detect an increase of Raman linewidth at ∼ 4 GPa, and assign it to pressure induced activation of a decay process into LO and TA phonons. This pressure is smaller compared to the ∼ 7 GPa reported for bulk Si. We do not observe evidence of phase transitions, such as discontinuities or change in the pressure slopes, in the investigated pressure range.

We report the Raman spectra of silicon nanowires (SiNWs) in a wide temperature range, between 2 K and 850 K. At room temperature we find a strong influence on the spectrum from applied laser excitation powers. These effects can be attributed a laser heated sample, leading to an inhomogeneous temperature distribution within the laser‐spot. If the laser excitation power is small (below 100 μW) such effects are negligible, and we find a temperature dependence governed by threephonon decay processes. The results from temperature dependent measurements indicate a change of sample morphology due to heating. Raman measurements on SiNWs immersed in superfluid helium at ≈ 2 K show very strong red‐shifts, even though they still have the perfect thermal contact via the superfluid helium. Considering anharmonic effects we find massively increased Si core temperatures. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Raman spectra of silicon nanowires (SiNWs) are studied as a function of laser excitation power and temperature. With increase in temperature and power a red-shift of the SiNWs first order optical mode is observed. At ambient conditions, the Raman shift shows a saturated behavior beyond a critical power. Similar measurements, under vacuum, rule out effects due to thermal convection as a reason for the observed saturation. We assign this to a change in sample morphology. A simulation of the SiNW peak position reveals that its temperature behavior can be explained by a four-phonon anharmonic process.SEM image of a typical SiNWs sample. The SiNW are $15 nm in diameter and up to a few microns long. The sample is a large bundle of SiNWs (up to 50 mm thick) with strong porosity.