Probing cell membrane mechanics by magnetic particle actuation and 3D rotational particle tracking

The mechanical properties of the cell membrane and the actin cortex determine a variety of cellular processes. An accurate description of their mechanics and dynamics necessitates a measurement technique that can capture the inherent anisotropy of the system. We combine magnetic particle actuation with rotational and translational particle tracking to simultaneously measure the mechanical stiffness of the membrane and the actin cortex in living cells in three rotational and two translational directions. We demonstrate the technique by targeting various types of membrane receptors. When using particles that bind via integrins, we measured an isotropic stiffness and a characteristic power-law dependence of the shear modulus on the applied frequency. When using particles functionalized with immunoglobulin G, we measured an anisotropic stiffness with a strongly reduced value in one dimension. We suggest that the observed reduced stiffness is caused by a local detachment of the membrane from the subjacent cytoskeletal cortex. Furthermore, we use functionalized particles as phagocytic targets for macrophages. Although phagocytosis is an inherently mechanical process, little is known about the forces and energies that a cell requires for internalization. We use our technique to measure the stiffnesses of the phagocytic cup as a function of time. The measured values and their time-dependence can be interpreted with a model of a pre-stressed membrane connected to an elastically deformable actin cortex. A comparison of model and data allows a determination of the speed at which the membrane advances around the particle. This approach is a novel way of measuring the progression of phagocytic cups and their mechanical properties in real-time. We expect that our technique will enable new insights into the mechanical properties of cells and will help to better understand numerous cellular processes

Vanadium centers in ZnTe crystals. II. Electron paramagnetic resonance

Four V-related electron-paramagnetic-resonance (EPR) spectra are observed in Bridgman-grown ZnTe doped with vanadium. Two of them are attributed to the charge states VZn3+(A+) and VZn2+(A0) of the isolated V impurity. For the ionized donor, VZn3+(A+), the spectrum reveals the typical behavior of the expected 3A2(F) ground state in tetrahedral symmetry. The incorporation on a cation lattice site could be proved by the resolved superhyperfine interaction with four Te ions. The second spectrum showing triclinic symmetry and S=3/2 is interpreted as the neutral donor state VZn2+(A0). The origin of the triclinic distortion of the cubic (Td) crystal field could be a static Jahn-Teller effect. The two additionally observed EPR spectra are attributed to nearest-neighbor V-related defect pairs. The spectrum of the first one, V2+Zn-YTe, shows trigonal symmetry and can be explained by the S=3/2 manifold of an orbital singlet ground state. An associated defect "YTe" is responsible for the trigonal distortion of the tetrahedral crystal field of V2+Zn. The spectrum of the second pair defect also shows trigonal symmetry and can be described by S=1/2. The ground-state manifold implies a VZn3+−XTe pair as the most probable origin of this spectrum. The S=1/2 ground state is produced by a dominating isotropic exchange interaction coupling the S=1 ground-state manifold of V3+Zn to an assumed S=1/2 ground state of "XTe" in antiferromagnetic orientation. The nature of the associated defects "YTe" and "XTe" remains unknown for both pairs since no hyperfine structure has been observed, but most probably acceptorlike defects are involved

Vanadium centers in ZnTe crystals. I. Optical properties

In ZnTe:V bulk crystals with nominal vanadium concentrations between 1000 and 7000 ppm three vanadium-ion states V+, V2+, and V3+ were found in low-temperature optical measurements. No-phonon lines of the internal emissions were detected for the 5E(D)→5T2(D) transition of V+(d4) at 3401 cm−1 (0.422 eV), for 4T2(F)→4T1(F) of V2+(d3) at 4056 cm−1 (0.503 eV), and for 3T2(F)→3A2(F) of V3+(d2) at 4726 cm−1 (0.586 eV). The energies of the internal transitions are reduced with respect to the corresponding transitions in ZnS:V and ZnSe:V. The respective excitation spectra display, in addition to broad charge-transfer bands, higher excited levels of the individual charge states. Crystal-field calculations of the detected transition energies based on the Tanabe-Sugano scheme are presented. With the help of sensitization experiments, a one-electron model is designed, in which the donor level (V2+/V3+) is situated 12 500 cm−1 (1.55 eV) below the conduction-band edge and the acceptor level (V2+/V+) 9400 cm−1 (1.17 eV) above the valence-band edge. The dynamical behavior of the three infrared lurainescence bands was measured. Decay time constants of 43 μs (V+), 120 μs (V2+), and 420 μs (V3+) were found. Electron-paramagnetic-resonance (EPR) results measured on the same samples are presented in an accompanying paper and confirm the optical detection of isolated substitutional V2+(d3) and V3+(d2) ions. Relations between the EPR and optical results are discussed

Fingerprints of carbon defects in vibrational spectra of gallium nitride (GaN) consider-ing the isotope effect

This work examines the carbon defects associated with recently reported and novel peaks of infrared (IR) absorption and Raman scattering appearing in GaN crystals at carbon ($^{12}C$) doping in the range of concentrations from $3.2*10^{17}$ to $3.5*10^{19} cm^{-3}$. 14 unique vibrational modes of defects are observed in GaN samples grown by hydride vapor phase epitaxy (HVPE) and then compared with defect properties predicted from first-principles calculations. The vibrational frequency shift in two $^{13}C$ enriched samples related to the effect of the isotope mass indicates six distinct configurations of the carbon-containing point defects. The effect of the isotope replacement is well reproduced by the density functional theory (DFT) calculations. Specific attention is paid to the most pronounced defects, namely tri-carbon complexes($C_N=C=C_N$) and carbon substituting for nitrogen $C_N$. The position of the transition level (+/0) in the bandgap found for $C_N=C=C_N$ defects by DFT at 1.1 eV above the valence band maximum, suggest that $(C_N=C=C_N)^+$ provides compensation of ${C_N}^-$. $C_N=C=C_N$ defects are observed to be prominent, yet have high formation energies in DFT calculations. Regarding ${C_N}$ defects, it is shown that the host Ga and N atoms are involved in the defect's delocalized vibrations and significantly affect the isotopic frequency shift. Much more faint vibrational modes are found from di-atomic carbon-carbon and carbon-hydrogen (C-H) complexes. Also, we note changes of vibrational mode intensities of $C_N$, $C_N=C=C_N$, C-H, and $C_N-C_i$ defects in the IR absorption spectra upon irradiation in the defect-related UV/visible absorption range. Finally, it is demonstrated that the resonant enhancement of the Raman process in the range of defect absorption above 2.5 eV enables the detection of defects at carbon doping concentrations as low as $3.2*10^{17} cm^{-3}$

Experimental Hall electron mobility of bulk single crystals of transparent semiconducting oxides

We provide a comparative study of basic electrical properties of bulk single crystals of transparent semiconducting oxides (TSOs) obtained directly from the melt (9 compounds) and from the gas phase (1 compound), including binary (β-Ga2O3, In2O3, ZnO, SnO2), ternary (ZnSnO3, BaSnO3, MgGa2O4, ZnGa2O4), and quaternary (Zn1−xMgxGa2O4, InGaZnO4) systems. Experimental outcome, covering over 200 samples measured at room temperature, revealed n-type conductivity of all TSOs with free electron concentrations (ne) between 5 × 1015 and 5 × 1020 cm−3 and Hall electron mobilities (μH) up to 240 cm2 V−1 s−1. The widest range of ne values was achieved for β-Ga2O3 and In2O3. The most electrically conducting bulk crystals are InGaZnO4 and ZnSnO3 with ne > 1020 cm−3 and μH > 100 cm2 V−1 s−1. The highest μH values > 200 cm2 V−1 s−1 were measured for SnO2, followed by BaSnO3 and In2O3 single crystals. In2O3, ZnO, ZnSnO3, and InGaZnO4 crystals were always conducting, while others could be turned into electrical insulators

Experimental Hall electron mobility of bulk single crystals of transparent semiconducting oxides

We provide a comparative study of basic electrical properties of bulk single crystals of transparent semiconducting oxides (TSOs) obtained directly from the melt (9 compounds) and from the gas phase (1 compound), including binary (β-Ga2O3, In2O3, ZnO, SnO2), ternary (ZnSnO3, BaSnO3, MgGa2O4, ZnGa2O4), and quaternary (Zn1−xMgxGa2O4, InGaZnO4) systems. Experimental outcome, covering over 200 samples measured at room temperature, revealed n-type conductivity of all TSOs with free electron concentrations (ne) between 5 × 1015 and 5 × 1020 cm−3 and Hall electron mobilities (μH) up to 240 cm2 V−1 s−1. The widest range of ne values was achieved for β-Ga2O3 and In2O3. The most electrically conducting bulk crystals are InGaZnO4 and ZnSnO3 with ne > 1020 cm−3 and μH > 100 cm2 V−1 s−1. The highest μH values > 200 cm2 V−1 s−1 were measured for SnO2, followed by BaSnO3 and In2O3 single crystals. In2O3, ZnO, ZnSnO3, and InGaZnO4 crystals were always conducting, while others could be turned into electrical insulators.Leibniz-Gemeinschaft http://dx.doi.org/10.13039/501100001664Leibniz-Institut für Kristallzüchtung (IKZ) im Forschungsverbund Berlin e.V. (3477)Peer Reviewe

Evolution of planar defects during homoepitaxial growth of β-Ga2O3 layers on (100) substrates—A quantitative model

We study the homoepitaxial growth of β-Ga2O3 (100) grown by metal-organic vapour phase as dependent on miscut-angle vs. the c direction. Atomic force microscopy of layers grown on substrates with miscut-angles smaller than 2° reveals the growth proceeding through nucleation and growth of two-dimensional islands. With increasing miscut-angle, step meandering and finally step flow growth take place. While step-flow growth results in layers with high crystalline perfection, independent nucleation of two-dimensional islands causes double positioning on the (100) plane, resulting in twin lamellae and stacking mismatch boundaries. Applying nucleation theory in the mean field approach for vicinal surfaces, we can fit experimentally found values for the density of twin lamellae in epitaxial layers as dependent on the miscut-angle. The model yields a diffusion coefficient for Ga adatoms of D = 7 × 10−9 cm2 s−1 at a growth temperature of 850 °C, two orders of magnitude lower than the values published for GaAs

Characterization of Silicon Crystals Grown from Melt in a Granulate Crucible

The growth of silicon crystals from a melt contained in a granulate crucible significantly differs from the classical growth techniques because of the granulate feedstock and the continuous growth process. We performed a systematic study of impurities and structural defects in several such crystals with diameters up to 60 mm. The possible origin of various defects is discussed and attributed to feedstock (concentration of transition metals), growth setup (carbon concentration), or growth process (dislocation density), showing the potential for further optimization. A distinct correlation between crystal defects and bulk carrier lifetime is observed. A bulk carrier lifetime with values up to 600 μs on passivated surfaces of dislocation-free parts of the crystal is currently achieved

Distinction between the Poole-Frenkel and tunneling models of electric field-stimulated carrier emission from deep levels in semiconductors

The enhancement of the emission rate of charge carriers from deep-level defects in electric field is routinely used to determine the charge state of the defects. However, only a limited number of defects can be satisfactorily described by the Poole-Frenkel theory. An electric field dependence different from that expected from the Poole-Frenkel theory has been repeatedly reported in the literature, and no unambiguous identification of the charge state of the defect could be made. In this article, the electric field dependencies of emission of carriers from DX centers in AlxGa1-xAs:Te, Cu pairs in silicon, and Ge:Hg have been studied applying static and terahertz electric fields, and analyzed by using the models of Poole-Frenkel and phonon assisted tunneling. It is shown that phonon assisted tunneling and Poole-Frenkel emission are two competitive mechanisms of enhancement of emission of carriers, and their relative contribution is determined by the charge state of the defect and by the electric-field strength. At high-electric field strengths carrier emission is dominated by tunneling independently of the charge state of the impurity. For neutral impurities, where Poole-Frenkel lowering of the emission barrier does not occur, the phonon assisted tunneling model describes well the experimental data also in the low-field region. For charged impurities the transition from phonon assisted tunneling at high fields to Poole-Frenkel effect at low fields can be traced back. It is suggested that the Poole-Frenkel and tunneling models can be distinguished by plotting logarithm of the emission rate against the square root or against the square of the electric field, respectively. This analysis enables one to unambiguously determine the charge state of a deep-level defect