Ville Pale

Influence of aluminium doping on thermoelectric performance of atomic layer deposited ZnO thin films

We study the effect of Al doping on thermoelectric power factor of ZnO films grown using atomic layer deposition method. The overall doping level is tuned by either varying the precursor pulsing sequence or by varying the number of precursor pulses while keeping the sequence unchanged. We observe that commonly utilized doping approach when periodic dopant layers are densely packed results in reduced power factor. At the same time, we find that thermoelectric performance can be improved by clustering the dopants. In addition, the clustering was found to tune the preferred crystal orientation of the polycrystalline film.

Biomimetic zinc chlorin–poly(4-vinylpyridine) assemblies: doping level dependent emission–absorption regimes

To develop biomimetic dye–polymers for photonics, two different types of Zn chlorin–poly(4-vinylpyridine) (P4VP) assemblies were prepared by varying Zn pyro-pheophorbide a methylester (ZnPPME) and Zn 31-OH-pyro-pheophorbide a methylester (Zn-31-OH-PPME) doping levels. 1H NMR spectroscopy and diffusion ordered NMR spectroscopy (DOSY) studies revealed that a coordinative interaction between Zn chlorin and P4VP was predominant in solution (d5-nitrobenzene). Small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) characterization of bulk samples of polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) doped with variable amounts of Zn chlorin showed that the pigment doping transformed the native cylindrical block copolymer nanostructures to lamellar morphologies. The result indicates that the pyridine moiety–Zn chlorin coordination is stronger than the aggregation tendency between the pigment molecules even in the solid state. UV-Vis absorption spectroscopy studies of a Zn chlorin–P4VP thin film showed characteristic monomeric chlorin spectra, while steady-state fluorescence spectroscopy displayed quenching of fluorescence and time-resolved studies indicated shortening of fluorescence lifetimes with an increasing chlorin doping level. Notably, time-resolved fluorescence spectroscopy revealed that the lifetime decay changed from monoexponential to biexponential above 0.5 wt% (ca. 0.001 equiv.) loadings. The Forster analysis implies that excitonic chlorin–chlorin interactions are observed in the thin films when the distance between the pigment molecules is approximately 50 A. The Zn chlorin–P4VP solid films emit strongly up to 1 wt% (ca. 0.002 equiv.) doping level above which the chlorin–chlorin interactions start to linearly dominate with an increase of doping level, while with 10 wt% (ca. 0.02 equiv.) loading less than 10% of fluorescence remains. Doping levels up to 300 wt% (0.5 equiv.) can be used in absorbing materials without the formation of chlorin aggregates. These defined optical response regions pave the way for photonic materials based on biopigment assemblies.

ALD-Assisted Multiorder Dispersion Engineering of Nanophotonic Strip Waveguides

We propose a new technique for the multiorder dispersion engineering of nanophotonic strip waveguides. Unlike other techniques, the method does not require wafers with customized parameters and is fully compatible with standard wafers used in nanophotonics. The dispersion management is based on the application of nanometer-thick TiO2 layer formed by atomic layer deposition. The method is simple and reliable and allows good control of dispersion up to the fourth-order terms. The additional advantages are the reduction of propagation losses and partial compensation of fabrication tolerances.

Fluorescence-enhancing plasmonic silver nanostructures using azopolymer lithography

The performance of fluorescence-based measurement techniques is fundamentally limited by the intrinsic quantum yield of a fluorophore. Radiative decay engineering using metallic nanostructures has significant potential to improve the fluorescence emission of a fluorophore. Especially, periodic arrays of metallic nanostructures have the advantage of exhibiting a strong optical response, which is beneficial for improving sensitivity in surface enhanced spectroscopic techniques. In this work, we present a cost-effective and large-scale fabrication scheme for creating periodic plasmonic nanoparticle arrays for fluorescence enhancement. The fabrication process uses an azopolymer mask that forms a two-dimensional surface relief grating when illuminated with two orthogonal exposures, which can directly be used as a soft etching mask to define the nanoparticle array. This approach allows the creation of periodic arrays of symmetrical metallic nanostructures that exhibit good long-range order. Furthermore, the dimensions of the array and the structures can be tuned by changing the exposure or process parameters. The plasmonic behaviour of the fabricated structures was studied both experimentally and by numerical simulations. The fluorescence enhancement performance for the blue and green wavelength regions was verified by using Rhodamine 6G and Cascade Blue as fluorophores. A significant 14-fold fluorescence intensity increase for Rhodamine 6G was observed, whereas the fluorescence intensity for Cascade Blue was roughly doubled. In addition, time-resolved measurements displayed a shortening of the fluorescence lifetime for both of the fluorophores when deposited on the nanoparticle grating. We expect that this approach could be advantageous for other application areas of plasmonics, such as SERS or sensing.

Excitation-dependent fluorescence from atomic/molecular layer deposited sodium-uracil thin films

Atomic/molecular layer deposition (ALD/MLD) offers unique possibilities in the fabrication of inorganic-organic thin films with novel functionalities. Especially, incorporating nucleobases in the thin-film structures could open new avenues in the development of bio-electronic and photonic devices. Here we report an intense blue and widely excitation-dependent fluorescence in the visible region for ALD/MLD fabricated sodium-uracil thin films, where the crystalline network is formed from hydrogen-bonded uracil molecules linked via Na atoms. The excitation-dependent fluorescence is caused by the red-edge excitation shift (REES) effect taking place in the red-edge of the absorption spectrum, where the spectral relaxation occurs in continuous manner as demonstrated by the time-resolved measurements.

Light-harvesting zinc chlorin-poly (4-vinylpyridine) complexes

To investigate the potential of light-harvesting chlorin - poly(4-vinylpyridine) (P4VP) complexes for photonic applications, we prepared solid state assemblies from chlorophyll a derived Zn pyro-pheophorbide a methylester (ZnPPME) and Purpurin 18 (P18) molecules in P4VP matrix. Here, we demonstrate the use of Förster resonant energy transfer (FRET) to enhance the optical density and Stokes shift of the system. ZnPPMe and P18 were employed as donor and acceptor molecules, respectively, and the optical response of the assembly was studied using absorption and steady-state fluorescence spectroscopy. The FRET efficiency was elucidated using time-resolved fluorescence measurements and the Förster distance was calculated using the measured specroscopic data yielding a value of 42.2 Å. These preliminary results will enable the development of new biomimetic photonic materials.

Fabrication of large-area plasmonic nanostructures for surface enhanced fluorescence

Two different types of large-area plasmonic nanostructures are fabricated on a silicon wafer and on a glass substrate for surface enhanced fluorescence. Ag coated silicon spikes are formed on a silicon substrate by cryogenic inductively coupled plasma reactive ion etching together with e-beam evaporation, and Ag nanoparticles are embedded in glass by two-step ion exchange. Both of them are demonstrated to enhance the light emission of fluorescence by using rhodamin 6G as the analyte. The presented fabrication methods are simple, low-cost and suitable for large-scale fabrication, providing a possibility to make substrates with improved fluorescence sensitivity for commercial applications.