A. Reszka

Influence of substrate nitridation temperature on epitaxial alignment of GaN nanowires to Si(111) substrate

An arrangement of self-assembled GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy on a Si(111) substrate is studied as a function of the temperature at which the substrate is nitridized before GaN growth. We show that the NWs grow with the c-axis perpendicular to the substrate surface independently of nitridation temperature with only a slight improvement in tilt coherency for high nitridation temperatures. A much larger influence of the substrate nitridation process on the in-plane arrangement of NWs is found. For high (850 °C) and medium (450 °C) nitridation temperatures angular twist distributions are relatively narrow and NWs are epitaxially aligned to the substrate in the same way as commonly observed in GaN on Si(111) planar layers with an AlN buffer. However, if the substrate is nitridized at low temperature (~150 °C) the epitaxial relationship with the substrate is lost and an almost random in-plane orientation of GaN NWs is observed. These results are correlated with a microstructure of silicon nitride film created on the substrate as the result of the nitridation procedure.

Magnetic interactions in Ge1−xCrxTe semimagnetic semiconductors

We present the studies of magnetic properties of Ge/1-x/Cr/x/Te diluted magnetic semiconductor with changeable chemical composition 0.016 \leq x \leq 0.061. A spin-glass state (at T \leq 35 K) for x = 0.016 and 0.025 and a ferromagnetic phase (at T < 60 K) for x \geq 0.030 are observed. The long range carrier-mediated magnetic interactions are found to be responsible for the observed magnetic ordering for x < 0.045, while for x \geq 0.045 the spinodal decomposition of Cr ions leads to a maximum and decrease of the Curie temperature, TC, with increasing x. The calculations based on spin waves model are able to reproduce the observed magnetic properties at a homogeneous limit of Cr alloying, e.g. x < 0.04, and prove that carrier mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction is responsible for the observed magnetic states. The value of the Cr-hole exchange integral, Jpd, estimated via fitting of the experimental results with the theoretical model, is in the limits 0.77...0.88 eV.

Synthesis of ZnAl2O4:(Er3+,Yb3+) spinel-type nanocrystalline upconverting luminescent marker in HeLa carcinoma cells, using a combustion aerosol method route

Efficiently upconverting, spherical ZnAl2O4 nanoparticles (NPs), doped with erbium and ytterbium, were synthesized by a combustion aerosol method (CAM) and transported to cytosol of carcinoma cell line (HeLa) for the first time. Spherical, 82–140 nm spinels were obtained at various concentrations of substrates. The nanoparticles were optimized to emit in the red luminescence range (Er3+, 661 nm, 4F9/2 → 4I15/2) when excited with near infrared light. Lower absorption and scattering by aqueous biological samples, compared to the green emission (Er3+, 550 nm, 2H11/2 → 4I15/2, 2S3/2 → 4I15/2), was responsible for the preferred upconversion. In addition, the application of the near infrared light significantly reduced the cellular autofluorescence and light scattering. X-ray diffraction, transmission electron microscopy, scanning electron microscopy, and photoluminescence spectroscopy were employed to characterize the synthesized samples. Energy dispersive X-ray microanalysis was used to confirm the composition and distribution of the nanoparticles through the spectrum and elemental mapping. The hydrophilic, spherical NPs, coated with PVP (polyvinylpyrrolidone) in the presence of a liposomal transfection factor, lipofectamine, were endocytosed into living HeLa cells and followed as luminescent markers by confocal laser scanning microscopy. We present the optimized protocols for the NPs synthesis and delivery of the spinels to cancer cells for bioimaging.

Growth by molecular beam epitaxy and properties of inclined GaN nanowires on Si(001) substrate

The growth mode and structural and optical properties of novel type of inclined GaN nanowires (NWs) grown by plasma-assisted MBE on Si(001) substrate were investigated. We show that due to a specific nucleation mechanism the NWs grow epitaxially on the Si substrate without any Si(x)N(y) interlayer, first in the form of zinc-blende islands and then as double wurtzite GaN nanorods with Ga-polarity. X-ray measurements show that orientation of these nanowires is epitaxially linked to the symmetry of the substrate so that [0001] axis of w-GaN nanowire is directed along the [111]Si axis. This is different from commonly observed behavior of self-induced GaN NWs that are N-polar and grow perpendicularly to the surface of nitridized silicon substrate independently on its orientation. The inclined NWs exhibit bright luminescence of bulk donor-bound excitons (D(0)X) at 3.472 eV and exciton-related peak at 3.46 eV having a long lifetime (0.7 ns at 4 K) and observable up to 50 K.

Light- and environment-sensitive electrospun ZnO nanofibers

One-dimensional (1D) ZnO nanostructures have been widely studied because of their electronic and optoelectronic applications. This report discusses the morphology, optical, electrical and sensory properties of polycrystalline ZnO nanofibers (NFs). We observed that the electrospun ceramic NFs interband emission increases with the nanocrystal size, consistent with decreasing of the surface-to-volume ratio. The observation is novel for the electrospun ceramic NFs. The chemical composition and structural characterization reveal that the NFs consist of ZnO wurzite nanocrystals, whose mean diameters increase from 7 to 22 nm with calcination temperature. Emission properties are studied by cathodo- and photoluminescence. The NFs are applied to construct light, gas and liquid sensors. We find an increase of the NFs conductivity by three orders of magnitude under UV illumination as a result of desorption of molecular oxygen from the nanocrystal surface. We study the influence of oxygen on NF conductivity by purging the NFs with air or nitrogen. We show that the flow of nitrogen removes the oxygen resulting in an important increase of the conductivity. Also, we study the dynamics of this process with and without UV illumination. We show sensitivity of the NFs to liquid environment by studying the conductivity of NFs immersed in water and ethanol and find an increased conductivity with respect to a dry air environment. These light- and environmental-sensitive ZnO NFs have useful optical and electronic properties for building high-performance sensors.

Ferromagnetic Transition in Ge_{1-x}Mn_{x}Te Layers

Ferromagnetic transition temperature in thin layers of diluted magnetic (semimagnetic) semiconductor Ge1−xMnxTe was studied experimentally by SQUID magnetometry method and analyzed theoretically for a model Ising-type diluted magnetic system with Ruderman–Kittel–Kasuya–Yosida indirect exchange interaction. The key features of the experimentally observed dependence of the Curie temperature on Mn content (x ≤ 0.12) and conducting hole concentration p = (1–10)× 10 cm−3 were reproduced theoretically for realistic valence band and crystal lattice parameters of p-Ge1−xMnxTe taking into account short carrier mean free path encountered in this material and Ruderman–Kittel–Kasuya–Yosida mechanism with both delta-like and diffused character of spatial dependence of the exchange coupling between magnetic ions and free carriers.

Characterization of the Nonpolar GaN Substrate Obtained by Multistep Regrowth by Hydride Vapor Phase Epitaxy

One of the most promising methods of obtaining nonpolar GaN substrates is regrowth of thick GaN crystals using hydride vapor phase epitaxy (HVPE). The multistep growth was performed along the c polar direction. After HVPE depositions, the crystal was sliced along (1120) nonpolar planes. On such samples, we performed structural high-resolution X-ray characterization. The full width at half maximum of the X-ray rocking curves for the 1120 reflection achieved 27 arcsec. The interfaces between each regrowth step were clearly visible in cathodoluminescence (CL), due to different concentrations of residual dopants before and after a regrowth step.

Pressure control of magnetic clusters in strongly inhomogeneous ferromagnetic chalcopyrites

Room-temperature ferromagnetism in Mn-doped chalcopyrites is a desire aspect when applying those materials to spin electronics. However, dominance of high Curie-temperatures due to cluster formation or inhomogeneities limited their consideration. Here we report how an external perturbation such as applied hydrostatic pressure in CdGeP2:Mn induces a two serial magnetic transitions from ferromagnet to non-magnet state at room temperature. This effect is related to the unconventional properties of created MnP magnetic clusters within the host material. Such behavior is also discussed in connection with ab initio density functional calculations, where the structural properties of MnP indicate magnetic transitions as function of pressure as observed experimentally. Our results point out new ways to obtain controlled response of embedded magnetic clusters.

Structural and optical characterization of GaN nanowires

Optical properties of GaN nanowires (NWs) grown by plasma-assisted molecular beam epitaxy on Si(111) substrates were studied with the use of micro-Raman spectroscopy and photoluminescence. Two types of NWs, hereafter labeled as A and B, grown with different values of Ga flux were studied. Morphology of the samples was probed by high resolution scanning electron microscopy. It was found that large Ga flux has led to a partial coalescence of nanowires in sample A. Reduction of Ga flux during growth of sample B resulted in an ensemble of separated nanowires. Micro-Raman and photoluminescence spectra were taken under illumination of 325 nm He-Cd laser line. Micro-Raman data reveal hexagonal phase of GaN NWs as well as a slight strain of Si substrate. Photoluminescence data yield that ensembles of separated NWs in sample B are defect free, whereas the spectra for coalesced wires in sample A exhibit both broad yellow luminescence band and defect-related band centered around 3.36 eV. Moreover, it was found that ...

Optical properties of ZnO microrods grown by a hydrothermal method – a cathodoluminescence study

Optical properties of individual zinc oxide microrods grown by a microwave-assisted hydrothermal method are investigated by spatially and spectrally resolved cathodoluminescence spectroscopy and imaging at liquid-helium temperature. For optimized growth conditions the strong localization of cathodoluminescence emission at the corners of the individual hexagonal ZnO microrods has been revealed. Locally distributed luminescence and fine structure of near-band-edge emission are discussed as a manifestation of whispering gallery modes of the hexagonal resonator occurring for the near band-gap luminescence in the investigated microrods.