[Show abstract][Hide abstract] ABSTRACT: We report on the ground- and excited-state properties of Fe3+ centers in hydrothermally and chemical-vapor-transport grown single ZnO crystals studied by continuous-wave electron-paramagnetic resonance (EPR) under
dark and laser-illuminated conditions, pulsed-EPR and magneto-photoluminescence. By use of EPR experiments,
the fine-structure parameters of the Fe3+ spin Hamiltonian are determined. Three types of charge-compensated
Fe3+ centers are identified and the charge conversion from Fe2+ to Fe3+ is highlighted. The magneto-optical studies of the Zeeman components of the spin-forbidden electric-dipole transitions from excited 4^T_1(G) to
ground 6^A_1(6^S) states of the Fe3+ center indicate the trigonal symmetry of the fine structure of the lowest \Gamma_8(4^T_1) excited state. The energy positions of the Zeeman components are measured in the external magnetic field of 8 T rotated in (1210) and (0001) crystal planes. The angular variation of the Zeeman lines exhibits two magnetically nonequivalent Fe3+ centers. These features result from the contribution of high-rank Zeeman terms of dimension BJ^3 in the spin Hamiltonian. For the electron spin S = 5/2 system of the trigonal Fe3+ ion, we further demonstrate the tuning of one-photon Rabi oscillations by means of electron spin-echo measurements.
Phys Rev B. Solid State 11/2015; 92(19):195202. DOI:10.1103/PhysRevB.92.195202
[Show abstract][Hide abstract] ABSTRACT: Zinc oxide nanopowders doped with 1–15 mol % cobalt were produced by the microwave solvothermal synthesis (MSS) technique. The obtained nanoparticles were annealed at 800 °C in nitrogen (99.999%) and in synthetic air. The material nanostructure was investigated by means of the following techniques: X-ray diffraction (XRD), helium pycnometry density, specific surface area (SSA), inductively coupled plasma optical emission spectrometry (ICP-OES), extended X-ray absorption fine structure (EXAFS) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and with magnetometry using superconducting quantum interference device (SQUID). Irrespective of the Co content, nanoparticles in their initial state present a similar morphology. They are composed of loosely agglomerated spherical particles with wurtzite-type crystal structure with crystallites of a mean size of 30 nm. Annealing to temperatures of up to 800 °C induced the growth of crystallites up to a maximum of 2 μm in diameter. For samples annealed in high purity nitrogen, the precipitation of metallic α-Co was detected for a Co content of 5 mol % or more. For samples annealed in synthetic air, no change of phase structure was detected, except for precipitation of Co3O4 for a Co content of 15 mol %. The results of the magentometry investigation indicated that all as-synthesized samples displayed paramagnetic properties with a contribution of anti-ferromagnetic coupling of Co–Co pairs. After annealing in synthetic air, the samples remained paramagnetic and samples annealed under nitrogen flow showed a magnetic response under the influences of a magnetic field, likely related to the precipitation of metallic Co in nanoparticles.
[Show abstract][Hide abstract] ABSTRACT: In this work, the analysis of the temperature-dependent electrical conductivity of highly crystalline zinc oxide (ZnO) thin films obtained by the Atomic Layer Deposition (ALD) method is performed. It is deduced that the most important scattering mechanisms are: scattering by ionized defects (at low temperatures) as well as by phonons (mainly optical ones) at higher temperatures. Nevertheless, the role of grain boundaries in the carrier mobility limitation ought to be included as well. These conclusions are based on theoretical analysis and temperature-dependent Hall mobility measurements. The presented results prove that existing models can explain the mobility behavior in the ALD-ZnO films, being helpful for understanding their transport properties, which are strongly related both to the crystalline quality of deposited ZnO material and defects in its lattice.
[Show abstract][Hide abstract] ABSTRACT: Abstract In this paper, cheap and efficient photovoltaic cells based on ZnO/Si heterostructure are discussed. These cells contain zinc oxide nanorods (ZnONR) grown by a low temperature hydrothermal method on a p-type silicon surface. The hydrothermal method applied in the present work uses cheap precursors and allows reproducible and controllable growth of 3D systems. As-grown ZnONR on Si surface are uniformly covered by a zinc oxide (ZnO) layer followed by an aluminum doped zinc oxide (AZO) layer. The latter is deposited on top of the cell as transparent conductive oxide (TCO). Both zinc oxide and aluminum doped zinc oxide layers are grown by a low temperature atomic layer deposition (LT ALD) method. Thickness of ZnO layers is optimized to increase significantly the light-trapping effect and thus the photovoltaic (PV) response. We evaluate impact of ZnO thickness on the PV devices operation. It is found that PV efficiency increases when thickness of the ZnO layer changes from 50 nm to 500 nm. The best response of solar cells is achieved for a sample containing ZnO layer with a thickness equal to 500 nm. The overall photovoltaic response is 10.9% and can be further improved by contact and Si layer optimization.
Solar Energy Materials and Solar Cells 07/2015; 143:99-104. DOI:10.1016/j.solmat.2015.06.042 · 5.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Thermally evaporated fullerene C60 porous films served as templates for a hybrid (molecular-inorganic) disordered blend formation. C60 films were covered with zinc oxide (ZnO) grown by atomic layer deposition. ZnO filled every pore in the C60 layer which led to the formation of C60–ZnO films with separate and distinguishable phases of C60 and ZnO constituents. Morphological, structural, optical, and electrical properties of the so-obtained films were investigated. Deposition of ZnO polycrystalline films on C60 porous layers resulted in the formation of ZnO with additional structural defects, compared to the films grown on planar substrates, which affected the electrical transport in the ZnO–C60 layers.
[Show abstract][Hide abstract] ABSTRACT: The test structures for photovoltaic (PV) applications based on zinc oxide nanorods (NRs) that were grown using a low-temperature hydrothermal method on p-type silicon substrates (100) covered with Ag
nanoparticles (NPs) were studied. The NPs of three different diameters, i.e., 5–10 nm, 20-30 nm, and 50–60 nm, were deposited using a sputtering method. The morphology and crystallinity of the structures were confirmed by scanning electron microscopy and Raman spectroscopy. It was found that the nanorods have a hexagonal wurtzite structure. An analysis of the Raman and photoluminescence spectra permitted the identification of the surface modes at 476 cm−1 and 561 cm−1. The presence of these modes is evidence of nanorods oriented along the wurtzite c-axis. The NRs with Ag NPs were covered with a ZnO:Al (AZO) layer that was grown using the low-temperature atomic layer deposition technique. The AZO layer served as a transparent ohmic contact to the ZnO
nanorods. The applicability of the AZO layer for this purpose and the influence of the Ag
nanoparticles on the effectiveness of light acquisition by such prepared PV
cells were checked by reflectance and transmittance measurements of the AZO/glass and AZO/NPs/glass reference structures. Based on these studies, the high-energy transmittance edge was assigned to the ZnO energy gap, although it is blueshifted with respect to the bulk ZnO energy gap because of Al doping. It was also shown that the most optimal PV performance is obtained from a structure containing Ag
nanoparticles with a diameter of 20–30 nm. This result is confirmed by the current-voltage measurements performed with 1-sun illumination. The structures show a plasmonic effect within the short wavelength range: the PV response for the structure with Ag
nanoparticles is twice that of the structure without the nanoparticles. However, the influence of the Ag
nanoparticle diameters on the plasmonic effect is ambiguous.
[Show abstract][Hide abstract] ABSTRACT: Electron Spin Resonance investigations allow to attribute a broad absorption band below the band-to-band transition of ZnO:Co to photoionization transitions of Co-ions. This absorption, with the onset at about 2.4–2.6 eV, is due to cobalt recharging, as concluded from the photo-ESR and pulsed ESR experiments. In these experiments light sensitive ESR signals of Co2+ and shallow donor are observed. Their response to light favors cobalt 2+ to 3+ photoionization.
[Show abstract][Hide abstract] ABSTRACT: We present results of cathodoluminescence (CL) investigations of high-quality zinc oxide (ZnO) nanorods obtained by an extremely fast hydrothermal method on a silicon substrate. A scanning electron microscopy (SEM) system equipped with CL allows direct comparison of SEM images and CL maps, taken from exactly the same areas of samples. Investigations are performed at a temperature of 5 K. An interlink between sample microstructure and emission properties is investigated. CL confirms a very high quality of ZnO nanorods produced by our method. In addition, the presence of super radiation effects in ZnO nanorod arrays is suggested.
Microscopy and Microanalysis 04/2015; 21(03):1-6. DOI:10.1017/S1431927615000264 · 1.88 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Abstract: In this paper, we discuss an impact of thin titanium dioxide
(TiO2) coatings on refractive index (RI) sensitivity and biofunctionalization
of long-period gratings (LPGs). The TiO2 overlays on the LPG surfaces
have been obtained using atomic layer deposition (ALD) method. This
method allows for a deposition of conformal, thickness-controlled, with
well-defined optical properties, and high-RI thin films which are highly
desired for optical fiber sensors. It has been found that for LPGs working at
a dispersion turning point of higher order cladding modes only tens of
nanometers of TiO2 overlay thickness allow to obtain cladding mode
transition effect, and thus significant improvement of RI sensitivity. When
the TiO2 overlay thickness reaches 70 nm, it is possible to obtain RI
sensitivity exceeding 6200 nm/RIU in RI range where label-free sensors
operate. Moreover, LPGs with TiO2-enhanced RI sensitivity have shown
improved sensitivity to bacteria endotoxin (E. coli B lipopolysaccharide)
detection, when TiO2 surface is functionalized with endotoxin binding
protein (adhesin) of T4 bacteriophage.
[Show abstract][Hide abstract] ABSTRACT: Dielectric films, such as hafnium dioxide (HfO2), aluminum oxide (Al2O3), zirconium dioxide (ZrO2), titanium dioxide (TiO2), and their composite layers are deposited on polycrystalline and amorphous substrates by the atomic layer deposition (ALD) method. We demonstrate that the use of this technology guarantees an uniform and controlled surface coverage in the nanometer scale at low temperatures (in our case below 100 °C). Modification of the composition of oxide layers allows the deposition of materials with quite different absorption coefficients, refractive indexes and dielectric constants. In particular, we demonstrate structural, electrical and optical properties of dielectric layers and test metal-oxide-semiconductor structures with these oxide materials. Our good quality dielectric layers, obtained at low temperature ALD, are characterized by a high dielectric constant (above 10), very smooth surface, wide energy gap (above 3 eV), low leakage current (in the range of 10- 8 A/cm2 at 1 V), high dielectric strength (even 6 MV/cm) and high refractive indexes (above 1.5 in visible spectral range).
Thin Solid Films 01/2015; 577. DOI:10.1016/j.tsf.2015.01.059 · 1.76 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Further development and commercialization of the new generation of optoelectronic and photovoltaic is often limited by the necessity of fabrication of low-cost and efficient transparent conductive electrodes. The development so far is hindered by the conveniently used indium tin oxide (ITO), which suffers from high cost and not high enough availability to support mass production. The zinc oxide is emerging as a convenient replacement for ITO for solar cell and light-emitting diode applications. The optical functions of aluminum-doped zinc oxide thin films are determined using optical spectroscopy measurements from 300 to 1100 nm. The dopant range studied varies from intrinsic ZnO to 5% Al content. Below the direct band gap there is a residual enhancement of the optical absorption coefficient by Al dopants, which is not related to surface roughness. We determined the dielectric functions and absorption coefficient evolution as the dopant concentration increases, as well as the free-carrier concentration.
29th European Photovoltaic Solar Energy Conference and Exhibition, Amsterdam; 11/2014
[Show abstract][Hide abstract] ABSTRACT: t. We achieved high conductivity of zinc oxide layers doped with aluminum atoms using atomic layer deposition (ALD) method. Their growth
mode, electrical and optical properties have been investigated. We discuss how the growth temperature and doping affect resistivity and optical
properties of the films. The obtained resistivities of ZnO:Al thin films ( 1.2x10-3 Ωcm) and high transparency make them suitable for the TCO
applications in photovoltaics. Developing of effective methods of aluminum doping of ZnO layers in ALD process and optimization of these
[Show abstract][Hide abstract] ABSTRACT: This paper we discusses an effect of coating long-period gratings (LPGs) with a thin titanium dioxide (TiO 2) overlays on refractive index (RI) sensitivity of the LPG. The overlays have been obtained using atomic layer deposition (ALD) method. The method allows for deposition well-controlled in thickness, well defined in optical properties, high-RI and very conformal thin films as required for optical fiber sensors. For the investigated LPGs we obtained sensitivity of 3490 to 6471 nm per RI unit depending on range RI. Keywords-Atomic Layer Deposition (ALD); thin films; long-period gratings (LPGs); titanium dioxide (TiO 2); optical fiber sensors; refractive index sensing
[Show abstract][Hide abstract] ABSTRACT: The Symposium F: “Novel materials for electronic, optoelectronic, photovoltaic and energy saving applications” of the 2013 E-MRS Fall Meeting (held in Warsaw, Poland, on 16–20 September 2013) was organized to cover many important aspects of basic and applied research of novel materials for present and future industrial applications. Particular emphasis was given to new technological and application concepts concerning both bulk crystals and various quantum structures of reduced dimensionality, like quantum wells and quantum dots (nanoparticles). The presented results on application-oriented materials were mostly focused on light emitting structures, light detectors, solar cells and materials for spintronics. Special attention was given to the technological aspects and challenges: growth techniques, defect control and structure optimization, doping procedures, optical properties and degradation processes. Relevant experimental techniques and crystal growth procedures were also discussed. Experimental results were compared with theoretical calculations giving deeper insight into many problems of modern materials science. Over 100 researchers from all over the world participated in this symposium, giving in all 15 invited lectures, 38 contributed talks and 70 poster presentations.
physica status solidi (c) 09/2014; 11(9‐10). DOI:10.1002/pssc.201470060
[Show abstract][Hide abstract] ABSTRACT: Zinc oxide (ZnO) films were used in silicon-based photovoltaic (PV) structures both as an n-type partner to p-type Si and (on top) as a transparent electrode (as a so-called TCO film). Photovoltaic response of such PV structures depends on the electrical properties of n-ZnO films as well as p-Si (100) substrates. The ZnO-based TCO films were deposited by an Atomic Layer Deposition (ALD) method. The films show a high electron concentration (similar to 10(20) cm(-3)) and a relatively high electron mobility ( > 10 cm(2)/V s). Considering Si, we used fairly cheap commercial substrates with 360 mu m thickness, i.e., not optimized for PV cells applications. The best results were obtained for Si substrates with a moderate hole concentration (similar to 10(15) cm(-3)) and resistivity of 10 Omega cm. The photovoltaic efficiency achieved is 4.4%, with the possibility of further improvement.
[Show abstract][Hide abstract] ABSTRACT: Oxide nanoparticles doped with rare-earth ions are tested for applications as nanomarkers in biology and medicine, e.g. for early cancer diagnostics. In this work we describe properties of such markers obtained by a microwave hydrothermal technology. Nanomarkers based on oxide materials (ZnO) are grown and tested for medical and biological applications.
[Show abstract][Hide abstract] ABSTRACT: Aluminum-doped zinc oxide (AZO) films were grown on polyethylene terephthalate (PET) substrates by atomic layer deposition (ALD) at low deposition temperatures (110-140 degrees C). The films have low resistivities, similar to 10(-3) Omega cm, and high transparency (similar to 90%) in the visible range. Bending tests indicated a critical bending radius of approximate to 1.2cm, below which the resistivity changes became irreversible. The films deposited on PET with additional buffer layer are more stable upon bending and temperature changes.
Materials Science and Engineering B 08/2014; 186(1). DOI:10.1016/j.mseb.2014.03.002 · 2.17 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Luminescent nanocrystals and quantum dots show great potential for use as fluorescent markers in biology and medicine. Preliminary results have been encouraging and show the visualising properties of quantum dots in the diagnosis of tumours (1, 6, 10, 31). However, the first generations of nanocrystals were based on a heavy-metal core, which was un-stable and shed heavy-metal ions into biological me-dia (8). This, coupled with a lack of information on their biodistribution and pharmacokinetics, rendered them unusable for purposes outside research. Thus the search started for a non-heavy metal solution for highly fluorescent nanocrystals. Recently developed rare-earth-doped nanocrystals are a promising material
[Show abstract][Hide abstract] ABSTRACT: We report on the properties of photovoltaic (PV) structures based on thin films of n-type zinc oxide grown by atomic layer deposition method on a cheap silicon substrate. Thin films of ZnO are used as n-type partner to p-type Si (110) and, when doped with Al, as a transparent electrode. PV structures with different thicknesses of ZnO layers (from 600 nm to 1600 nm) were deposited to determine the optimal performance of PV structures. The best response we obtained for the structure with ZnO layer thickness of 800 nm. The so-obtained PV structures show 6% efficiency.
Thin Solid Films 07/2014; 563:28–31. DOI:10.1016/j.tsf.2013.10.110 · 1.76 Impact Factor