M. Pärs

Raman scattering in nanosized nickel oxide NiO

Magnetic ordering in nanosized (100 and 1500 nm) nickel oxide NiO powders, prepared by the plasma synthesis method, was studied using Raman scattering spectroscopy in a wide range of temperatures from 10 to 300 K. It was observed that the intensity of two- magnon band decreases rapidly for smaller crystallites size. This effect is attributed to a decrease of antiferromagnetic spin correlations and leads to the antiferromagnetic-to- paramagnetic phase transition .

Structural study of TiO2 thin films by micro-Raman spectroscopy

The Raman spectroscopy method was used for structural characterization of TiO2 thin films prepared by atomic layer deposition (ALD) and pulsed laser deposition (PLD) on fused silica and single-crystal silicon and sapphire substrates. Using ALD, anatase thin films were grown on silica and silicon substrates at temperatures 125–425 °C. At higher deposition temperatures, mixed anatase and rutile phases grew on these substrates. Post-growth annealing resulted in anatase-to-rutile phase transitions at 750 °C in the case of pure anatase films. The films that contained chlorine residues and were amorphous in their as-grown stage transformed into anatase phase at 400 °C and retained this phase even after annealing at 900 °C. On single crystal sapphire substrates, phase-pure rutile films were obtained by ALD at 425 °C and higher temperatures without additional annealing. Thin films that predominantly contained brookite phase were grown by PLD on silica substrates using rutile as a starting material.

Polarisation dependent Raman study of single-crystal nickel oxide

The magnetic domain structure and Raman scattering have been studied in NiO single-crystals with three different (100), (110) and (111) orientations. Twin-domain structure was observed in NiO(100) and NiO(110) single-crystals using cross-polarized optical microscopy. We found that the ratio of the two-magnon (at 1500 cm−1) to the two-phonon (2LO, at 1100 cm−1) Raman bands intensity is sensitive in a particular way to the type of the twin-domain pattern.

Atomic layer deposition of HfO2 on graphene from HfCl4 and H2O

Atomic layer deposition of HfO2 on unmodified graphene from HfCl4 and H2O was investigated. Surface RMS roughness down to 0.5 nm was obtained for amorphous, 30 nm thick hafnia film grown at 180°C. HfO2 was also deposited in a two-step temperature process where the initial growth of about 1 nm at 170°C was continued up to 10–30 nm at 300°C. This process yielded uniform, monoclinic HfO2 films with RMS roughness of 1.7 nm for 10–12 nm thick films and 2.5 nm for 30 nm thick films. Raman spectroscopy studies revealed that the deposition process caused compressive biaxial strain in graphene, whereas no extra defects were generated. An 11 nm thick HfO2 film deposited onto bilayer graphene reduced the electron mobility by less than 10% at the Dirac point and by 30–40% far away from it.

Terrylene-doped biphenyl monocrystals for optical single-molecule spectroscopy

Biphenyl forms at normal pressure and temperatures below 40 K incommensurate crystalline structures with physical properties varying in space on a scale of nanometers. As biphenyl crystals are optically transparent, there is a possibility to study these natural nanostructures optically, by doping the host crystal with nanoscopic probes with optical properties depending on their local environment. It has already been demonstrated that terrylene impurity molecules in polycrystalline biphenyl sample can successfully play the role of such kind of sensitive nanoprobes when studied by the methods of high-resolution laser spectroscopy. We report growing of thin biphenyl monocrystals doped with terrylene molecules at very low concentrations. These sublimation-grown flakes can be studied at liquid helium temperatures using the technique of single-molecule spectroscopy. Compared to polycrystalline biphenyl samples, much higher signals and better signal-to-noise ratio can be achieved in single-molecule spectra. This allows to perform much faster spectral scans to find intensive single-molecule lines even in very dilute samples in spite of a very broad inhomogeneous absorption band of terrylene in biphenyl. Fast scanning also allows observation of single-molecule lines with much better temporal resolution, revealing processes of spectral diffusion occurring at different time scales. This can be helpful in our attempts to learn about the role of the matrix incommensurability in spectral features observed. Extremely high variability of temporal and spectral behaviour of terrylene single-molecule lines is reported, which is unusual for crystalline hosts.

Optical study of terrylene molecules in crystalline biphenyl: effects of pressure and temperature on the luminescence spectra

The photoluminescence spectra of terrylene guest molecules in polycrystalline biphenyl host were measured at various temperatures between 4.7 and 295 K under ambient pressure (1 atm) as well as under various pressures up to 4.4 kbar at 4.7 K and up to 4.9 kbar at 295 K. With increasing pressure, all four spectral peaks studied shift to the red. By elevating the temperature from 4.7 to 295 K at 1 atm, the most intense peak at 17,220 cm−1 first shifts slightly to the red, but at ∼40 K begins to shift to the blue. Comparison of the temperature and pressure shifts for this peak reveals that its temperature shift is mainly determined by the thermal expansion of the host crystal rather than by the change in the electron–phonon coupling with temperature.

Single-molecule probing of incommensurate biphenyl

Our data on the distribution of purely electronic linewidths of single molecules of terrylene in incommensurate biphenyl crystals are compared with the data obtained by other groups for different low-temperature organic solid hosts and with results of numerical simulations. The first two moments of the distributions measured within a narrow temperature interval have been used to calculate a single dimensionless parameter characterizing each of the respective hosts—the variation coefficient. It appears that different amorphous hosts have similar values of this coefficient, but the value obtained for the incommensurate crystal of biphenyl is significantly different. One can conclude that the remarkable single-molecule line broadening in biphenyl at 1.8K cannot be solely explained by the interaction with two-level systems, which is considered to cause the broadening in amorphous hosts.

Mesoscopic Spectral Modulation of Light Transmitted by a Subwavelength Aperture

We are currently studying the transmission of light through a tapered metal-coated optical fiber with a subwavelength aperture (SWA). The problem under investigation is the effect of SWA on the spectrum of the transmitted light. According to our experimental findings, one can observe, under certain conditions, a remarkable modulation of the spectrum of the transmitted light. The effect has a mesoscopic origin: the modulation takes place if the number of transmitted light modes is small but exceeds unity, which indicates the phase shifts between different modes. One possible source of such phase shift could be the different propagation speed for different modes in the fiber, but this effect should be small. In our opinion, the origin of the phase shifts is in the (different for different modes) slowdown of the light near the tip with SWA due to the interaction of propagating modes with surface plasmons of the metal coating of the fiber. One can expect that the interaction strength depend on the actual shape of the light field in the mode, which results in different modes getting different delays before passing through the tip. In case of sufficiently small SWA diameter only few modes can pass through the tapered fiber region [1], and their delay differences can cause an observable modulation of the transmitted light spectrum. In case of larger diameters many light modes can pass out, and no significant spectral modulation can be observed due to the effect of averaging. An observable modulation also disappears for SWA diameters as small as 100 nm, because in this case only one (the fundamental) light mode passes out [1].