Low-temperature Scanning Microscopy Research Articles

The Tripartite Lichen Ricasolia virens: Involvement of Cyanobacteria and Bacteria in Its Morphogenesis.

Ricasolia virens is an epiphytic lichen-forming fungus mainly distributed in Western Europe and Macaronesia in well-structured forests with ecological continuity that lack eutrophication. It is considered to be threatened or extinct in many territories in Europe (IUCN). Despite its biological and ecological relevance, studies on this taxon are scarce. The thalli are tripartite, and the mycobiont has a simultaneous symbiotic relationship with cyanobacteria and green microalgae, which represent interesting models to analyse the strategies and adaptations resulting from the interactions of lichen symbionts. The present study was designed to contribute to a better understanding of this taxon, which has shown a clear decline over the last century. The symbionts were identified by molecular analysis. The phycobiont is Symbiochloris reticulata, and the cyanobionts (Nostoc) are embedded in internal cephalodia. Light, transmission electron and low-temperature scanning microscopy techniques were used to investigate the thallus anatomy, ultrastructure of microalgae and ontogeny of pycnidia and cephalodia. The thalli are very similar to its closest relative, Ricasolia quercizans. The cellular ultrastructure of S. reticulata by TEM is provided. Non-photosynthetic bacteria located outside the upper cortex are introduced through migratory channels into the subcortical zone by the splitting of fungal hyphae. Cephalodia were very abundant, but never as external photosymbiodemes.

Resolution enhancement for low-temperature scanning microscopy by cryo-immersion.

Here we report a simple way to enhance the resolution of a confocal scanning microscope under cryogenic conditions. Using a microscope objective (MO) with high numerical aperture (NA = 1.25) and 1-propanol as an immersion fluid with low freezing temperature we were able to reach an imaging resolution at 160 K comparable to ambient conditions. The MO and the sample were both placed inside the inner chamber of the cryostat to reduce distortions induced by temperature gradients. The image quality of our commercially available MO was further enhanced by scanning the sample (sample scanning) in contrast to beam scanning. The ease of the whole procedure marks an essential step towards the development of cryo high-resolution microscopy and correlative light and electron cryo microscopy (cryoCLEM).

Substrate-induced symmetry reduction of CuPc on Cu(1 1 1): An LT-STM study

The adsorption of copper phthalocyanine (CuPc) on Cu(1 1 1) was studied by means of low-temperature scanning microscopy. At very low coverage, individual molecules are randomly distributed over the surface. Increasing the coverage, the molecules align in chains before forming ordered domains with a rectangular unit cell. The molecules are centered on top of a copper atom aligning two opposite lobes with a principal axis of the substrate. The topographic images of the molecules show a reduction of the fourfold to a twofold symmetry. At negative sample bias, a switching between two states at a typical rate of 500 Hz is observed for isolated molecules, which are neither adsorbed at defects nor forming chains or domains.

Nonlinear microwave properties of YBa2Cu3O7−x block epitaxial films near the superconducting transition temperature

The structure and local properties of a nonlinear microwave response of YBa{sub 2}Cu{sub 3}O{sub 7-x} block films, as well as microbridges of these films, are studied using low-temperature scanning microscopy. Correlation between the halfwidth of the temperature dependence of the third harmonic power W{sub TH} and the curve of the electron-beam induced voltage W{sub EBIV} is observed for the medium size of the block. A theoretical model is proposed, which demonstrates that the nonlinear microwave response for large blocks is determined by intrablock pinning of Abrikosov vortices; for blocks of smaller size, the response acquires an additional contribution associated with pinning of hypervortices on the Josephson network of the boundaries between the blocks.

Application of EBIV technique for investigation of electrophysical parameters of devices based on HTSC

Local investigations with 4 μm spatial resolution using EBIV-signal registration of YBCO microstripes were made with low-temperature scanning microscopy. These investigations, coupled with microwave measurements of YBCO films (near-field microscopy) and structural investigations (X-ray diffractometry, SEM), gave the correlation between half-widths of the temperature dependence of the third harmonic power maximum WTH, EBIV-signal curve WEBIV verage microcrystallite size. In order to explain such a dependence, the diphasic medium model considering the non-linear voltage-current characteristic of the superconductor was proposed. Calculations allow us to conclude that a threefold increase of microcrystallite size results in hundredfold reduction of the non-linearity coefficient of YBCO films and microwave devices based on these films.

Effect of the microstructure of epitaxial YBa2Cu3O7 − x films on their electrophysical and nonlinear microwave properties

The local nonlinear microwave response of YBa2Cu3O7 − x films was measured by near-zone field microscopy with a spatial resolution of 50 μm, and YBa2Cu3O7 − x film microbridges were locally studied by low-temperature scanning microscopy with a spatial resolution of 4 μm. The microstructure of epitaxial YBa2Cu3O7 − x films was examined using x-ray diffraction and electron microscopy. A correlation is detected between the average crystallite size and the half-widths of the temperature dependences of the third-harmonic power (W TH) and the electron-beam-induced voltage (W EBIV). The experimental results are described in terms of a model of a two-phase medium taking into account the nonlinear I-V characteristic of the superconductor. For large crystallites, the nonlinear microwave response is shown to be caused by intracrystallite vortex pinning. As the average crystallite size decreases, an additional contribution to the nonlinear response appears due to the pinning of a magnetic flux by the Josephson network of crystallite boundaries. Calculations show that a three-fold increase in the crystallite size decreases the nonlinearity coefficient of YBa2Cu3O7 − x films by two orders of magnitude.

Effects of locust bean, xanthan and guar gums on the ice crystals of a sucrose solution frozen at high pressure

Hydrocolloids are added in ice-creams and frozen desserts to produce smooth texture and protect the product during its storage. The high-pressure freezing techniques also aimed to enhance product qualities. In this work, both the hydrocolloid and the high-pressure processing actions are combined. A 0.3% (w/w) of guar gum (viscous solution) or of locust bean gum plus xanthan gum (gel) was mixed in sucrose solutions (16% w/w) to analyze whether water mobility affected ice crystal formation. Sucrose solutions with or without hydrocolloids were frozen by high-pressure assisted freezing (HPAF) at 100 MPa and by high-pressure shift freezing (HPSF) from 210 MPa to 0.1 and 100 MPa, to compare their effects on the ice crystal characteristics. Ice crystal sizes were determined from images obtained by low temperature scanning microscopy (LT-SEM). Ice crystals were smaller after HPSF than after HPAF, due to greater supercooling following expansion and to shorter phase transition times. As regards the hydrocolloids, ice crystals were smaller when the mixture of locust bean and xanthan gums were added irrespective of the freezing method. The formation of a gel-like structure at ambient temperature strengthened by a cryo-gelation effect in the frozen state may limit water molecule diffusion and ice crystal growth.

Exogenous gibberellins inhibit coffee (Coffea arabica cv. Rubi) seed germination and cause cell death in the embryo

The mechanism of inhibition of coffee (Coffea arabica cv. Rubi) seed germination by exogenous gibberellins (GAs) and the requirement of germination for endogenous GA were studied. Exogenous GA(4+7) inhibited coffee seed germination. The response to GA(4+7) showed two sensitivity thresholds: a lower one between 0 and 1 microM and a higher one between 10 and 100 microM. However, radicle protrusion in coffee seed depended on the de novo synthesis of GAs. Endogenous GAs were required for embryo cell elongation and endosperm cap weakening. Incubation of coffee seed in exogenous GA(4+7) led to loss of embryo viability and dead cells were observed by low temperature scanning microscopy only when the endosperm was surrounding the embryo. The results described here indicate that the inhibition of germination by exogenous GAs is caused by factors that are released from the endosperm during or after its weakening, causing cell death in the embryo and leading to inhibition of radicle protrusion.

Spatial variation of the current in grain boundary Josephson junctions

The spatial variation of the current across the boundary in several YBa2Cu3O7−x grain boundary Josephson junctions was determined using direct methods. A phase retrieval algorithm was used to calculate the positional critical current density J(x) from critical current versus applied magnetic field, Ic(B), measurements. The current distributions were highly nonuniform along the length of the junctions. These measurements are consistent with existing filamentary grain boundary models, low temperature scanning microscopy studies, and laser scanning microscopy studies of high Tc grain boundaries. The very large scatter in the critical currents reported in the literature for grain boundaries of the same macroscopic geometry appear to be due to the underlying variations in local critical currents.

Bandwidth enhancement of a shear-force-controlled distance regulation in near-field microscopy

The distance between sample and probe in a scanning near-field optical microscope is regulated via tracing the shear-force on the tip which is glued to a tuning fork piezo. A lock-in technique is used. We demonstrate that the bandwidth of the control loop is increased if not only amplitude or phase, but a favorable combination of both is used as feedback signal. The enhancement of bandwidth is connected with a reduction of signal-to-noise ratio. The optimum combination of both, bandwidth and signal-to-noise ratio, can be adjusted purely electronically to the specific needs of an experiment. A theoretical model is developed that discloses the relation between the mechanical and electrical properties of the combination of tuning fork and fiber tip. The frequency response of the shear-force detection system is calculated with a numerical simulation based on this model. Experimental frequency response curves are well fitted by these simulations. Our results are especially important for low-temperature scanning microscopy, where the bandwidth enhancement is essential for obtaining a reasonable scanning speed.