Temperatures In Ultrahigh Vacuum Research Articles

Influence of recrystallization on stability of tungsten scanning tunneling microscope tips.

The structure and electron emission properties of scanning tunneling microscope tips electrochemically etched from polycrystalline and recrystallized tungsten wires were investigated using scanning electron microscopy and transmission electron microscopy. Tips etched using the recrystallized wire had single crystal domains larger than those seen in tips etched from the cold drawn wire. The stability of the tips under high electric fields was investigated using field emission. It was found that tips etched from the recrystallized wire tended to have improved stability compared to those etched from the polycrystalline wire and that annealing either type of tip to high temperature in ultra-high vacuum had the greater influence on tip stability.

Formation of well-ordered surfaces of Bi2-xSbxTe3-ySey topological insulators using wet chemical treatment

The surface preparation of topological insulators (TIs) is a critical task in order to realize their efficient applications. A chemical treatment in an anhydrous solution of hydrogen chloride in isopropanol (HCl-iPA) and a subsequent annealing at relatively low temperature in ultrahigh vacuum (UHV) was successfully used for the surface preparation of bulk 3D (0001) TIs Bi2Te3, Sb2Te3, Bi2Se3 and an MBE - grown Bi2-xSbxTe3-ySey (BSTS) thin films. The surface treatment showed a significant modification of the initial TI surfaces, which was free from the structural disorder, oxidation and chemical impurities determined by X-ray photoelectron spectroscopy and low-energy electron diffraction. The insulating nontrivial bulk gap and well resolved gapless surface states with a linear dispersion of a massless Dirac cone were observed by angle-resolved photoelectron spectroscopy (ARPES). In the BSTS film, the Fermi level is located within the bulk band gap. The negative magnetoconductance corresponding to weak antilocalization demonstrated the contribution of the surface states of BSTS, that promise to be protected from backscattering. The surface treatment method proposed in this work is highly efficient for both bulk and thin TI films that can be useful for the deposition of an insulators/metals to fabricate transistor and spin valve systems.

Polarized tip-enhanced Raman spectroscopy at liquid He temperature in ultrahigh vacuum using an off-axis parabolic mirror.

Tip-enhanced Raman spectroscopy (TERS) combines inelastic light scattering well below the diffraction limit down to the nanometer range and scanning probe microscopy and, possibly, spectroscopy. In this way, topographic and spectroscopic as well as single- and two-particle information may simultaneously be collected. While single molecules can now be studied successfully, bulk solids are still not meaningfully accessible. It is the purpose of the work presented here to outline approaches toward this objective. We describe a home-built, liquid helium cooled, ultrahigh vacuum TERS. The setup is based on a scanning tunneling microscope and, as an innovation, an off-axis parabolic mirror having a high numerical aperture of ∼0.85 and a large working distance. The system is equipped with a fast load-lock chamber, a chamber for the in situ preparation of tips, substrates, and samples, and a TERS chamber. Base pressure and temperature in the TERS chamber were ∼3 × 10-11 mbar and 15K, respectively. Polarization dependent tip-enhanced Raman spectra of the vibration modes of carbon nanotubes were successfully acquired at cryogenic temperature. The new features described here including very low pressure and temperature and the external access to the light polarizations, thus the selection rules, may pave the way toward the investigation of bulk and surface materials.

A study of Pt, Rh, Ni and Ir dispersion on anatase TiO2(101) and the role of water

Understanding how metal atoms are stabilized on metal oxide supports is important for predicting the stability of “single-atom” catalysts. In this study, we use scanning tunnelling microscopy (STM) and x-ray photoelectron spectroscopy (XPS) to investigate four catalytically active metals – platinum, rhodium, nickel and iridium – on the anatase TiO2(101) surface. The metals were vapor deposited at room temperature in ultrahigh vacuum (UHV) conditions, and also with a background water pressure of 2 × 10−8 mbar. Pt and Ni exist as a mixture of adatoms and nanoparticles in UHV at low coverage, with the adatoms immobilized at defect sites. Water has no discernible effect on the Pt dispersion, but significantly increases the amount of Ni single atoms. Ir is highly dispersed, but sinters to nanoparticles in the water vapor background leading to the formation of large clusters at step edges. Rh forms clusters on the terrace of anatase TiO2(101) irrespective of the environment. We conclude that introducing defect sites into metal oxide supports could be a strategy to aid the dispersion of single atoms on metal-oxide surfaces, and that the presence of water should be taken into account in the modelling of single-atom catalysts.

Chemically imaging nanostructures formed by the covalent assembly of molecular building blocks on a surface with ultrahigh vacuum tip-enhanced Raman spectroscopy

Surface-bound reactions have become a viable method to develop nanoarchitectures through bottom-up assembly with near atomic precision. However, the bottom-up fabrication of nanostructures on surfaces requires careful consideration of the intrinsic properties of the precursors and substrate as well as the complex interplay of any interactions that arise in the heterogeneous two-dimensional (2D) system. Therefore, it becomes necessary to consider these systems with characterization methods sensitive to such properties with suitable spatial resolution. Here, low temperature ultrahigh vacuum scanning tunneling microscopy (STM) and tip-enhanced Raman spectroscopy (TERS) were used to investigate the formation of 2D covalent networks via coupling reactions of tetra(4-bromophenyl)porphyrin (Br4TPP) molecules on a Ag(100) substrate. Through the combination of STM topographic imaging and TERS vibrational fingerprints, the conformation of molecular precursors on the substrate was understood. Following the thermally activated coupling reaction, STM and TERS imaging confirm the covalent nature of the 2D networks and suggest that the apparent disorder arises from molecular flexibility.

Research advancements and challenges in the design and fabrication of glass to metal seals for solar receiver tube applications

Glass to metal joints has a vast field of application due to their distinct properties in terms of electric conductivity, thermal conductivity, transparency, emissivity, reflectivity, coefficient of thermal expansion (CTE), density, wettability, and crystalline structure. This wide diversification in their properties makes this combination very useful in the special application like concentrated solar receiver tubes (CSRT), etc. This challenge becomes bigger when such seals are subjected to a very high temperature range, ultra-high pressure or ultra-high vacuum, and also gone through repeated number of thermal cycling over a period of time, as may be in the case of CSRT. Hence how to prepare a good quality seal for such constraints, study and control of many factors are essential covered such as seal design, oxidation layer thickness, manufacturing method, temperature and time of making seal, surface roughness, and residual stresses in glass, etc., that has been vigorously studied and reported in this paper.

Research advancements and challenges in the design and fabrication of glass to metal seals for solar receiver tube applications

Glass to metal joints has a vast field of application due to their distinct properties in terms of electric conductivity, thermal conductivity, transparency, emissivity, reflectivity, coefficient of thermal expansion (CTE), density, wettability, and crystalline structure. This wide diversification in their properties makes this combination very useful in the special application like concentrated solar receiver tubes (CSRT), etc. This challenge becomes bigger when such seals are subjected to a very high temperature range, ultra-high pressure or ultra-high vacuum, and also gone through repeated number of thermal cycling over a period of time, as may be in the case of CSRT. Hence how to prepare a good quality seal for such constraints, study and control of many factors are essential covered such as seal design, oxidation layer thickness, manufacturing method, temperature and time of making seal, surface roughness, and residual stresses in glass, etc., that has been vigorously studied and reported in this paper.

Activation characterization of the Ti-Zr-V getter films deposited by magnetron sputtering

Non-evaporable getter (NEG) thin films provide distributed pumping properties, allowing to achieve ultrahigh vacuum in narrow and conductance limited chambers. It is an ideal solution to deal with the small aperture of vacuum chambers. In this paper, ternary Ti-Zr-V getter film with approximate equal atomic ratio was deposited on oxygen free copper (OFC) substrate by using a developed DC magnetron sputtering system with an alloy target. Microstructural characterization by scanning electron microscopy, atomic force microscopy and energy dispersive spectroscopy (EDX) indicates that the porous getter film exhibit a columnar growth pattern and the grain sizes are very distributed ranging from several up to hundreds of nanometres. The compositions and the corresponding chemical bonding states are analyzed by using in-situ synchrotron radiation X-ray photoelectron spectroscopy after activation for 1 h at different temperatures in ultra-high vacuum. The results indicate that Ti, Zr and V are initially in their high oxidized states, which are gradually reduced in steps to oxides of lower valence states and finally to their metallic states. The reduction of vanadium oxides is more preferable than that of titanium- and zirconium-oxides. Such activation commences at a temperature as low as 150 °C, and can be enhanced by increasing temperature.

Motional Sideband Asymmetry of a Nanoparticle Optically Levitated in Free Space.

The hallmark of quantum physics is Planck's constant h, whose finite value entails the quantization that gave the theory its name. The finite value of h gives rise to inevitable zero-point fluctuations even at vanishing temperature. The zero-point fluctuation of mechanical motion becomes smaller with growing mass of an object, making it challenging to observe at macroscopic scales. Here, we transition a dielectric particle with a diameter of 136nm from the classical realm to the regime where its zero-point motion emerges as a sizable contribution to its energy. To this end, we optically trap the particle at ambient temperature in ultrahigh vacuum and apply active feedback cooling to its center-of-mass motion. We measure an asymmetry between the Stokes and anti-Stokes sidebands of photons scattered by the levitated particle, which is a signature of the particle's quantum ground state of motion.

Ultrathin Chromia on a Hexagonally-Ordered d0 Ferromagnet: Evidence of Interfacial Exchange Bias at the Cr2O3/TiO2-x Interface.

Heterostructures consisting of 10 Å thick chromia films and 50 Å thick titania films display significant exchange bias at and above room temperature. Chromia films ∼10 Å thick were deposited by molecular beam epitaxy (MBE) of Cr at room temperature in ultrahigh vacuum on 50 Å thick TiO2-x(111) films (x < 0.3) also deposited epitaxially by MBE on Al2O3(0001). Cr deposition yields increased Ti(III) formation in the titania substrate and the formation of a Cr2O3 overlayer, without Cr/Ti interfacial mixing, as determined by in situ photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS). In situ low-energy electron diffraction (LEED) and XPS data indicate that the chromia overlayer is hexagonally ordered and ∼10 Å thick. Longitudinal and polar magneto-optic Kerr effect (MOKE) measurements at 285-315 K provide evidence of strong exchange bias between the boundary layer magnetization of chromia and the ferromagnetic substrate. These data demonstrate the robust room-temperature interaction of the boundary layer magnetization of a multiferroic antiferromagnet with a d0 ferromagnetic substrate.

SOI wafer fabricated with a diamond BOX layer using surface activated bonding at room temperature

We propose a fabrication process for a silicon on insulator (SOI) wafer with a diamond buried oxide (BOX) layer by combining nanodiamond-seeding deposition and a surface-activated bonding technique for high-frequency and power device applications. The diamond layer was deposited on a base wafer by the spin-coating of nanodiamonds and microwave-plasma-enhanced chemical vapor deposition. The thermal conductivity of this deposited diamond layer was three times that of a conventional SiO2 layer. A silicon wafer was then bonded to the diamond layer at room temperature in ultrahigh vacuum without forming any voids. Additionally, this SOI wafer was used to fabricate devices at 1000 °C. Therefore, we believe that this SOI wafer with a diamond BOX layer and its fabrication process are important for the realization of self-heating devices such as next-generation high-frequency and power devices.

A compact ultrahigh vacuum scanning tunneling microscope with dilution refrigeration.

We have designed and built a scanning tunneling microscope (STM) setup for operation at millikelvin temperatures in an ultrahigh vacuum. A compact cryostat with an integrated dilution refrigerator has been built that allows measurements at a base temperature of 25 mK in the magnetic field up to 7.5 T with low mechanical and electronic noise. The cryostat is not larger than conventional helium bath cryostats (23 and 13 l of nitrogen and helium, respectively) so that the setup does not require a large experimental hall and fits easily into a standard lab space. Mechanical vibrations with running dilution circulation were kept below 1 pm/ by mechanically decoupling the STM from the cryostat and the pumping system. All electronic input lines were low-pass filtered, reducing the electronic temperature to below 100 mK, as deduced from the quasiparticle peaks of superconducting aluminum. The microscope is optically accessible in the parked position, making sample and tip exchange fast and user-friendly. For measurement, the STM is lowered 60 mm down so that the sample ends in the middle of a wet superconducting magnetic coil.

Coherent growth of oxide films on a cleaved layered metal oxide substrate

Understanding oxide interface-induced effects requires controlled epitaxial growth of films on well-defined substrate surfaces. While conventional film growth on ex situ prepared substrates has proven to be a successful route, the choices of appropriate substrates with atomically defined surfaces are limited. Here, by depositing $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ on $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ (001), we present an alternative method of growing oxide thin films on in situ cleaved surfaces of layered-structured substrates. Cleaving $\mathrm{S}{\mathrm{r}}_{2}\mathrm{Ru}{\mathrm{O}}_{4}$ at low temperature in ultrahigh vacuum exposes an atomically flat, solely SrO-terminated surface with up to micrometer-scale terraces. The deposition of $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_{3}$ spontaneously diminishes the surface $\mathrm{Ru}{\mathrm{O}}_{6}$ in-plane rotational distortion of the substrate and results in a cubic-like perovskite film structure with (La/Sr)-O layer termination. The interface is atomically sharp without obvious deviation of lattice spacing and chemical valence, except in the first unit cell where Ru-Mn intermixing is observed. These results demonstrate that film growth on a cleaved substrate can be an alternative route to obtain well-defined interfaces and in addition increase the availability of substrates for future oxide films.

Unravelling the GLY-PRO-GLU tripeptide induced reconstruction of the Au(110) surface at the molecular scale

The adsorption of GLY-PRO-GLU tripeptide on Au(110) is investigated within the frame of all atom classical mechanics simulations and Density Functional Theory, focusing on the surface reconstruction. It is shown that the tripeptide adsorption reorganizes and restructures the Au(110) surface. A mechanism for the surface restructuration is proposed for both the neutral and zwitterionic form of the peptide at room temperature in Ultra High Vacuum. Diverse residues may be involved in the Au atoms displacement, and in particular glutamic acid, triggering a double proton transfer and the formation of a zwitter ionic state, is found to be responsible for the triggering of the surface reconstruction.

Decomposition of La2−xSrxCuO4 into several La2O3 phases at elevated temperatures in ultrahigh vacuum inside a transmission electron microscope

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The structural and mechanical characterization of TiC and TiC/Ti thin films grown by DC magnetron sputtering

The formation of TiC and Ti phases and their influence on their mechanical properties was studied in this work. Thin layers were deposited by DC magnetron sputtering at room temperature in ultrahigh vacuum from Ti and C targets.Cubic TiC phase (c-TiC) was formed from 58 to 86 at.% Ti content. First formation of hexagonal Ti (h-Ti) occurred from 86 at.% Ti content. The c-TiC disappears from 90 at.% Ti content. Films with 86 at.% Ti content the c-TiC structure can transform to h-Ti by sequential stacking faults. Dominance of c-TiC(111) texture with increasing Ti content was observed.The hardness of thin films agree with structural observations. The highest hardness value (∼26 GPa) showed the c-TiC thin film with 67 at% Ti content. The nanohardness values showed decreasing character with increasing Ti content over 70 at.%. The lowest values of nanohardness (∼10 GPa) was observed for thin films with only h-Ti phase.

Spectroscopic Observation and Molecular Dynamics Simulation of Ga Surface Segregation in Liquid Pd-Ga Alloys.

Liquid binary Pd-Ga alloys with low Pd contents of 0.8, 1.8, and 4.7 at % of Pd were examined as a function of sample temperature in ultra-high vacuum by using angle-resolved XPS. Upon cooling from 750 to 400 K, a pronounced temperature-dependence of the Pd concentration in the liquid phase was observed, which was explained by the transition from the pure liquid phase to a two-phase system, consisting of a solid Ga5 Pd phase and a Pd-depleted liquid Pd-Ga alloy. In the liquid Pd-Ga alloy, Pd is always depleted from the topmost interface layer, as deduced from angle-resolved XPS at 0 and 80° emission, independent of temperature and Pd concentration. This observation is interpreted as an inhomogeneous depth distribution function of Pd, that is, the segregation of Ga to the surface of the liquid phase. The results of a DFT-based molecular dynamics simulation (MD) independently show interfacial stratification of Ga and an inhomogeneous Pd distribution along the surface normal. The evaluation of the experimental data with a rigid layer model based on the MD calculations leads to excellent agreement with the simulation.

Time Strengthening of Crystal Nanocontacts.

We demonstrate how an exponentially saturating increase of the contact area between a nanoasperity and a crystal surface, occurring on time scales governed by the Arrhenius equation, is consistent with measurements of the static friction and lateral contact stiffness on a model alkali-halide surface at different temperatures in ultrahigh vacuum. The "contact ageing" effect is attributed to atomic attrition and is eventually broken by thermally activated slip of the nanoasperity on the surface. The combination of the two effects also leads to regions of strengthening and weakening in the velocity dependence of the friction, which are well-reproduced by an extended version of the Prandtl-Tomlinson model.

A versatile UHV transport and measurement chamber for neutron reflectometry under UHV conditions.

We report on a versatile mini ultra-high vacuum (UHV) chamber which is designed to be used on the MAgnetic Reflectometer with high Incident Angle of the Jülich Centre for Neutron Science at Heinz Maier-Leibnitz Zentrum in Garching, Germany. Samples are prepared in the adjacent thin film laboratory by molecular beam epitaxy and moved into the compact chamber for transfer without exposure to ambient air. The chamber is based on DN 40 CF flanges and equipped with sapphire view ports, a small getter pump, and a wobble stick, which serves also as sample holder. Here, we present polarized neutron reflectivity measurements which have been performed on Co thin films at room temperature in UHV and in ambient air in a magnetic field of 200 mT and in the Q-range of 0.18 Å-1. The results confirm that the Co film is not contaminated during the polarized neutron reflectivity measurement. Herewith it is demonstrated that the mini UHV transport chamber also works as a measurement chamber which opens new possibilities for polarized neutron measurements under UHV conditions.

Study on evaporation from alloys used in microwave vacuum electron devices

The development of modern satellite communication technologies is imposing higher demands on the lifetime and reliability of the microwave vacuum electronic devices, which directly depend on the evaporation properties of the extensively used Monel and stainless steel. Therefore, it is of vital importance to study the evaporation properties of these two types of metallic materials. For the first time, as far as we know, this paper proposes to study the evaporation properties of metallic materials using time-of-flight mass spectrometer (TOFMS). The components and the contents of the vacuum background, the evaporants from the Monel and from the stainless steel have been measured using the TOFMS, respectively. After the pressure of the measurement chamber is below 4.010-8 Pa, the TOFMS is used for the metallic materials working at different temperatures. They are respectively acquired when the Monel and stainless steel are at room temperature on operate between 750 to 900 ℃ under a pressure of 1.010-6 Pa. The measurements are carried out rapidly and in high sensitivity. As disclosed by the measurements, Mn and Cu began to evaporate when the Monel and the stainless steel are heated to 800 ℃, which is still far below the melting points of the two alloys (1243 ℃ and 1080 ℃). When the Monel and the stainless steel are further heated to 900 ℃, the evaporation of Mn, Cu, and Cr becomes quite considerable. Once the evaporated Mn, Cu, or Cr deposit on the ceramics for the insulation in an electron gun, its insulation will be deteriorated. Hence, the Monel and the stainless steel are not suitable to be use as the components in cathode electron guns, especially those used in the devices that are to work a long lifetime in high vacuum. Moreover, the Monel and the stainless steel are not suitable for used as the components that are often under the electron bombardment, e.g., anodes and collectors, either. The SEM images and XRD of the heat treated surface structures of the Monel and the stainless steel in ultrahigh vacuum (1.010-6 Pa) have also been studied. On heating at 900 ℃ for 30 and 120 min the surface structure and composition change remarkably and a significant reduction in Mn and Cr is visible, and also a large number of holes and crystal boundaries emerge on the surfaces of the two metallic alloys. With increasing heating time, the boundaries will grow larger and larger. As a result, the strength of the two metallic materials becomes weaker and gas permeation and leakage even occur. Therefore, it can be concluded that the components made from Monel and stainless steel, especially those with thin walls, should not be heated to high temperatures in ultrahigh vacuum for a long time. The above phenomena are analyzed in detail theoretically and the proper and feasible application methods of the metallic materials are explored in device design and technological process control. These works are expected to contribute to the prolonging of the lifetimes of the satellites, and will lead to tremendous economic benefits.