40 research outputs found
Ultra-thin film NbN depositions for HEB heterodyne mixer on Si-substrates
The key of improving hot-electron bolometer (HEB) mixer performance lies inevitably in the quality of ultra-thin NbN films itself. This work presents a thorough investigation of crucial process parameters of NbN films deposited by means of reactive DC-sputtering on Si-substrates at elevated temperatures up to 750°C.
The polycrystalline NbN films with thickness of 4 to 10nm were characterized by DC resistivity measurements, ellipsometry and high resolution transmission electron microscopy (HRTEM) in order to confirm thickness and film structure. Since the macroscopic properties such as critical temperature, thickness as well as the transition width to the superconducting state are directly linked to HEB mixer noise temperature and IF bandwidth, a set of experiments were conducted to enhance aforementioned properties. We considered deposition temperature, RF biasing, nitrogen and argon partial and total pressure during deposition as major process variable parameters. Careful optimization of the deposition conditions allowed setting up a process resulting in high-quality NbN ultra-thin films with thickness of 5.5nm exhibiting Tc of 10.5K. Moreover, the transition width could be kept as low as 1.4K. The produced films were stored at ambient conditions and re-characterized over a period of 4 month without measurable degradation
Strong and ductile platelet-reinforced polymer films inspired by nature: Microstructure and mechanical properties
The unique structure and mechanical properties of platelet-reinforced biological materials such as bone and seashells have motivated the development of artificial composites exhibiting new, unusual mechanical behavior. On the basis of designing principles found in these biological structures, we combined high-performance artificial building blocks to fabricate platelet-reinforced polymer matrix composites that exhibit simultaneously high tensile strength and ductility. The mechanical properties are correlated with the underlying microstructure of the composites before and after mechanical loading using transmission electron microscopy. The critical role of the strength of the platelet-polymer interface and its dependence on the platelet surface chemistry and the type of matrix polymer are studied. Thin multilayered films with highly oriented platelets were produced through the bottom-up layer-by-layer assembly of submicrometer-thin alumina platelets and either polyimide or chitosan as polymer matrix. The tensile strength and strain at rupture of the prepared composites exceeded that of nacre, whereas the elastic modulus reached values similar to that of lamellar bones. In contrast to the brittle failure of clay-reinforced composites of similar or higher strength and stiffness, our composites exhibit plastic deformation in the range of 2-90% before failure. In addition to the high reinforcing efficiency and ductility achieved, several toughening mechanisms were identified in fractured composites, namely friction, debonding, and formation of microcracks at the platelet-polymer interface, as well as plastic deformation and void formation within the continuous polymeric phase. The combination of high strength, ductility, and toughness was achieved by selecting platelets that exhibit an aspect ratio high enough to carry significant load but small enough to allow for fracture under the platelet pull-out mode. At high concentrations of platelets, the ductility gets lost because of out-of-plane misalignment of the platelets and incorporation of voids in the microstructure during processing. The designing principles applied in this study can potentially be extended to other types of platelets and polymers to obtain new, hybrid materials with tunable mechanical propertie
Superconductivity in Weyl Semimetal Candidate MoTe2
In recent years, layered transition-metal dichalcogenides (TMDs) have
attracted considerable attention because of their rich physics; for example,
these materials exhibit superconductivity, charge density waves, and the valley
Hall effect. As a result, TMDs have promising potential applications in
electronics, catalysis, and spintronics. Despite the fact that the majority of
related research focuses on semiconducting TMDs (e.g., MoS2), the
characteristics of WTe2 are provoking strong interest in semimetallic TMDs with
extremely large magnetoresistance, pressure-driven superconductivity, and the
predicted Weyl semimetal (WSM) state. In this work, we investigate the sister
compound of WTe2, MoTe2, which is also predicted to be a WSM and a quantum spin
Hall insulator in bulk and monolayer form, respectively. We find that MoTe2
exhibits superconductivity with a resistive transition temperature Tc of 0.1 K.
The application of a small pressure (such as 0.4 GPa) is shown to dramatically
enhance the Tc, with a maximum value of 8.2 K being obtained at 11.7 GPa (a
more than 80-fold increase in Tc). This yields a dome-shaped superconducting
phase diagram. Further explorations into the nature of the superconductivity in
this system may provide insights into the interplay between strong correlations
and topological physics.Comment: 20 pages, 5 figure
Phase evolution of a barium tin 1,2-ethanediolato complex to barium stannate during thermal decomposition
International audienceThe thermal behaviour of [Ba(C2H6O2)4][Sn(C2H4O2)3] used as a BaSnO3 precursor, and its phase evolution during thermal decomposition are described. The initially formed transient barium-tin-oxycarbonate phase disintegrates into BaCO 3 and SnO 2 , reacting subsequently to BaSnO3. The existence of the intermediate oxycarbonate phase is evidenced by FT-IR, XRD, and EELS (ELNES) investigations
Microstructure of highly strained BiFeO3 thin films : transmission electron microscopy and electron-energy loss spectroscopy studies
Microstructure and electronic structure of highly strained bismuth ferrite (BiFeO3) thin films grown on lanthanum aluminate substrates are studied using high-resolution transmission and scanning transmission electron microscopies and electron energy loss spectroscopy (EELS). Monoclinic and tetragonal phases were observed in films grown at different temperatures, and a mix of both phases was detected in a film grown at intermediate temperature. In this film, a smooth transition of the microstructure was found between the monoclinic and the tetragonal phases. A considerable increase in the c-axis parameters was observed in both phases compared with the rhombohedral bulk phase. The off-center displacement of iron (Fe) ions was increased in the monoclinic phase as compared with the tetragonal phase. EEL spectra show different electronic structures in the monoclinic and the tetragonal phases. These experimental observations are well consistent with the results of theoretical first-principle calculations performed
Metal Seed Loss Throughout the Nanowire Growth: Bulk Trapping and Surface Mass Transport
The
physical and chemical properties of metal-catalyzed semiconductor
nanowires are very sensitive to their composition and morphology,
which are very sensitive to the behavior of the catalyst nanodroplet
during growth. Herein, we identify and investigate the main atomic
pathways and processes governing the metal mass transport and the
associated variation in the nanodroplet size throughout the growth
of metal-catalyzed silicon nanowires. This includes surface diffusion
and catalyst trapping in addition to the shift in phase boundaries
of the eutectic nanodroplet to count for surface effects, capillarity,
and related nanoscale stresses. On the basis of thermodynamic and
kinetic considerations, a theoretical framework is presented to elucidate
these catalyst nanodroplet instabilities. Moreover, we also address
the influence of these phenomena on the shape and impurity concentration
in silicon nanowires. Modeling results along with experimental data
demonstrate that the combined effects of the kinetically driven catalyst
trapping and surface diffusion play the key role in tailoring the
nanowire morphology and composition. The proposed model can be extended
straightforwardly to describe the evolution of the morphology during
the growth of any other system of metal-catalyzed semiconductor nanowires