Mihkel Rähn

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.

Atomic layer deposition and properties of ZrO2/Fe2O3 thin films

Thin solid films consisting of ZrO2 and Fe2O3 were grown by atomic layer deposition (ALD) at 400 °C. Metastable phases of ZrO2 were stabilized by Fe2O3 doping. The number of alternating ZrO2 and Fe2O3 deposition cycles were varied in order to achieve films with different cation ratios. The influence of annealing on the composition and structure of the thin films was investigated. Additionally, the influence of composition and structure on electrical and magnetic properties was studied. Several samples exhibited a measurable saturation magnetization and most of the samples exhibited a charge polarization. Both phenomena were observed in the sample with a Zr/Fe atomic ratio of 2.0.

Excitation of surface plasmons in Al-coated SNOM tips

The mesoscopic effect of spectral modulation occurring due to the interference of two photonic fiber modes filtered out by a metal-coated SNOM tip is used to observe the surface plasmon polariton (SPP) excitation in SNOM tips. In a spectrum of the broadband light transmitted by a SNOM tip a region of highly regular spectral modulation can be found, indicating the spectral interval in which only two photonic modes (apparently HE11 and TM01) are transmitted with significant and comparable amplitudes. The modulation period yields the value of optical path difference (OPD) for this pair of modes. Due to the multimode fiber’s inherent modal dispersion, this OPD value depends linearly on the fiber tail length l. An additional contribution to OPD can be generated in a metal-coated SNOM tip due to a mode-dependent photon-plasmon coupling strength resulting in generation of SPPs with different propagation velocities. For an Al-coated 200 nm SNOM tip spectra of transmitted light have been registered for ten different l values. An extrapolation of the linear OPD (l) dependence to l=0 yields a significant residual OPD value, indicating according to our theoretical considerations a mode-selective SPP excitation in the metal-coated tip. The modal dispersion is shown to switch its sign in the SNOM tip. First results of analogous experiments with an Al-coated 150 nm SNOM tip confirm our conclusions.

UVA-induced antimicrobial activity of ZnO/Ag nanocomposite covered surfaces

Application of efficient antimicrobial surfaces has been estimated to decrease both, the healthcare-associated infections and the spread of antibiotic-resistant bacteria. In this paper, we prepared ZnO and ZnO/Ag nanoparticle covered surfaces and evaluated their antimicrobial efficacy towards a Gram-negative bacterial model (Escherichia coli), a Gram-positive bacterial model (Staphylococcus aureus) and a fungal model (Candida albicans) in the dark and under UVA illumination. The surfaces were prepared by spin coating aliquots of ZnO and ZnO/Ag nanoparticle suspensions onto glass substrates. Surfaces contained 2 or 20 μg Zn/cm2 and 0-0.02 μg Ag/cm2. No significant antimicrobial activity of the surfaces, except of those with the highest Ag or Zn content was observed in the dark. On the other hand, UVA illuminated surfaces containing 20 μg Zn/cm2 and 2 μg Zn plus 0.02 μg Ag/cm2 caused >3 log decrease in the viable counts of E. coli and S. aureus in 30 min. As proven by brilliant blue FCF dye degradation and elemental analysis of the surfaces, this remarkable antimicrobial activity was a combined result of photocatalytic effect and release of Zn and Ag ions from surfaces. Surfaces retained significant antibacterial and photocatalytic properties after several usage cycles. Compared to bacteria, yeast C. albicans was significantly less sensitive to the prepared surfaces and only about 1 log reduction of viable count was observed after 60 min UVA illumination. In conclusion, the developed ZnO/Ag surfaces exhibit not only high antibacterial activity but also some antifungal activity.

Observation of Surface Plasmons in Metal-Coated Tapered Fiber Terminated by a Subwavelength Aperture

The effect of a tapered Al-coated optical fiber terminated by a subwavelength aperture (SWA) on the spectrum of the transmitted light is investigated experimentally. Under certain conditions a remarkable spectral modulation of the transmitted light can be observed [1]. This effect is of a mesoscopic origin, occurring only for a certain interval of SWA diameters. A noticeable modulation appears when the number of the transmitted fiber modes is small but exceeds unity, thus indicating the presence of a phase shift between different modes.

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].