Heating In Ultrahigh Vacuum Research Articles

Enhanced reducibility of well-defined Cr oxide nanostructures on Au(111) through Zn doping

Mixed oxides play important roles in many heterogeneous catalytic processes, and understanding of interaction between different oxide components is crucial for synthesizing the catalysts with high performances. Here, we introduce trace of metallic Zn species into well-defined chromium oxide (Cr oxide) nanostructures grown on Au(111) including CrO2-bilayer nanoislands and Cr2O7dinuclear clusters and study the impact of the dopant on reducibility of the Cr oxides. Combining low-temperature scanning tunneling microscopy (LT-STM) and X-ray photoelectron spectroscopy (XPS), we demonstrate that incorporation of 2.5 at.% Zn can lower the initial reduction temperature of Cr oxides by 70–160 K upon heating in ultrahigh vacuum. The reduction degree of Zn-doped Cr oxide is much larger than the pure oxide at the same heating temperature. Density functional theory (DFT) calculations reveal that the enhanced reducibility of the pristine oxide via introduction of Zn dopant mainly comes from the promoted activity of oxygen atoms due to Zn substitution of surface Cr.

Growth and Decomposition of Pt Surface Oxides.

The formation and thermal stability of Pt surface oxides on a Pt thin film were studied in situ using ambient-pressure X-ray photoelectron spectroscopy. At an oxygen pressure of 73 Pa (550 mTorr), the surface Pt oxide was gradually formed, evidenced by the O 1s peak at 529.5 eV as the Pt film was heated. The Pt oxide peak reached a maximum between 217 and 317 °C and then decreased as the sample temperature was further increased. A similar response was seen on cooling from 480 to 23 °C; the intensity of the Pt oxide peak first increased and then decreased. The remaining Pt surface oxides partially decomposed during ultra-high-vacuum (UHV) pumping and completely decomposed during heating in UHV, which highlights the challenge of characterizing these surfaces with UHV instruments. These results have important implications for the understanding of the surface states of platinum films in different environments and the roles of different catalytic mechanisms.

Thermal stability of the passive film formed on 316L stainless steel surface studied by ToF-SIMS

The modifications of the passive film formed on 316 L stainless steel surface during stepwise heating in ultra-high vacuum up to 300 °C have been studied in situ by ToF-SIMS. The pre-formed passive film (in 0.05 M H2SO4) has a bilayer structure, comprising Fe-rich, Mo-rich (outer) and Cr-rich (inner) layers. Below 100 °C, the passive film is stable. At 100 °C–250 °C, dehydroxylation and dehydration is observed. Above 250 °C, the main modification in the film is formation of chromium oxide at the expense of oxidized iron. At higher temperature, thicker Cr-rich inner layer with sharper inner/outer oxide interface is formed.

Hydrogen incorporation and release from nonevaporable getter coatings based on oxygen-free Pd/Ti thin films

Oxygen-free Pd/Ti thin films have been developed as novel nonevaporable getter coatings for ultrahigh vacuum (UHV) chamber walls, which can maintain high pumping speed even after many cycles of air-vent and only require activation at a relatively low temperature of 133°C. Here, the authors clarify the hydrogen (H) incorporation and release properties of such oxygen-free Pd/Ti thin films using scanning transmission electron microscopy (STEM), energy dispersive x-ray spectroscopy (EDS), and H depth profiling by 1H(15N,αγ)12C nuclear reaction analysis (NRA). EDS mapping and STEM show that the layers of Pd (6 nm) and Ti (14 nm) prepared by sequential UHV deposition on a stainless steel 304L substrate are clearly separated and that the Ti is completely covered by Pd, which prevents oxidation of Ti. NRA reveals that H accumulates preferentially in the Ti layer, where the H-content increases almost linearly with the H2 dosage. Quantitative analysis indicates that TiH1.92 is produced in the Ti layer after an exposure of 4×104 Langmuir, where the H uptake is still not saturated. Upon heating in UHV, H starts to desorb above 170°C and the release rate increases above 200°C. Repeated cycles of H incorporation at room temperature (20°C) and release at about 240°C are possible. Although the H incorporation rate changes during the initial absorption/desorption, it stabilizes after a few (∼3) incorporation/release cycles.

CVD growth of ultrapure diamond, generation of NV centers by ion implantation, and their spectroscopic characterization for quantum technological applications

Abstract Applications of nitrogen-vacancy (NV) centers in diamond in quantum technology have attracted considerable attention in recent years. Deterministic generation of ensembles of NV centers can advance the research on quantum sensing, many-body quantum systems, multipartite entanglement and so on. Here we report the complete process of controlled generation of NV centers in diamond as well as their characterisation: growing diamond films through chemical vapor deposition (CVD), ion implantation and spectroscopic characterization of the defect centers using a confocal microscope. A microwave-assisted CVD set-up is presented which we constructed for the preparation of single-crystalline homoepitaxial diamond films. The films were prepared with minimized nitrogen concentration, which is confirmed through photoluminescence measurements. We demonstrate an in situ ultra high vacuum (UHV) implantation and heating process for creation of NV centers using a novel experimental set-up. For the first time hot implantation has been shown which prevents surface charging effects. We do not observe graphitization due to UHV heating. By optimizing the implantation parameters it has been possible to implant NV centers in a precise way. We present large area mapping of the samples to determine the distribution of the centers and describe the characterization of the centers by spectroscopic techniques. Reducing the decoherence caused by environmental noise is of primary importance for many applications in quantum technology. We demonstrate improvement on coherence time T_{2} of the NV spins by suppression of their interaction with the surrounding spin-bath using robust dynamical decoupling sequences.

Improved pumping speeds of oxygen-free palladium/titanium nonevaporable getter coatings and suppression of outgassing by baking under oxygen

Oxygen-free palladium/titanium (Pd/Ti) is a new nonevaporable getter material with an activation temperature as low as 133 °C. Because pumping speeds of oxygen-free Pd/Ti for H2 and CO have been reported to be improved by baking under an O2 atmosphere, the authors investigated oxygen-free Pd/Ti samples heated in an ultrahigh vacuum (UHV) or under an O2 pressure of 1.3 × 10−4 Pa by x-ray photoelectron spectroscopy. The authors found that carbon contamination decreased to an extent on heating in UHV, but decreased considerably on heating in O2. Pressure-curve measurements in an oxygen-free Pd/Ti-coated chamber show that pumping speeds for H2 and CO were considerably improved after baking the chamber under an O2 pressure of 1.3 × 10−4 Pa (O2 baking) in comparison with baking in UHV. Furthermore, partial-pressure measurements suggested that O2 baking removes adsorbed carbon and hydrogen, and consequently suppresses the formation of H2, CO, H2O, and CH4. Catalytic chemical reactions on the Pd surface appear to be responsible for the removal of adsorbed carbon and hydrogen. The pumping speeds of the oxygen-free Pd/Ti-coated chamber for H2 and CO were measured by using the orifice method and were found to be improved after O2 baking. O2 baking is therefore useful for improving pumping speeds for H2 and CO and for reducing the partial pressures of H2, CO, H2O, and CH4.

Stoichiometric and Oxygen-Deficient VO2 as Versatile Hole Injection Electrode for Organic Semiconductors.

Using photoemission spectroscopy, we show that the surface electronic structure of VO2 is determined by the temperature-dependent metal-insulator phase transition and the density of oxygen vacancies, which depends on the temperature and ultrahigh vacuum (UHV) conditions. The atomically clean and stoichiometric VO2 surface is insulating at room temperature and features an ultrahigh work function of up to 6.7 eV. Heating in UHV just above the phase transition temperature induces the expected metallic phase, which goes in hand with the formation of oxygen defects (up to 6% in this study), but a high work function >6 eV is maintained. To demonstrate the suitability of VO2 as hole injection contact for organic semiconductors, we investigated the energy-level alignment with the prototypical organic hole transport material N, N'-di(1-naphthyl)- N, N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (NPB). Evidence for strong Fermi-level pinning and the associated energy-level bending in NPB is found, rendering an Ohmic contact for holes.

SYNTHESIS, ELECTRONIC STRUCTURE AND STUDY OF YTTERBIUM TETRAPHENYLPORPHYRIN COMPLEXES STABILITY BY PHOTOELECTRON SPECTROSCOPY AND THERMOGRAVIMETRY

Metalloorganic complexes of rare earth elements with porphyrins have attracted attention over the past years in view of potential applications as templates for biology, nanotechnology and medicine. For deeper understanding and studying of their properties the knowledge of electronic structure and bonding in these compounds is required and should be investigated essentially by a photoelectron spectroscopy. In present work the electronic structure and stability of tetrakis-porphyrins and its ytterbium complexes were studied experimentally by X-ray (XPS) photoemission spectroscopy. The X-ray photoemission data show the different atomic constituents in accordance with its states in molecules which can be related to the peaks of N1s, C1s, O1s, Yb4d appearing in the electronic spectra. In the tetraphenylporphyrin free base spectrum of the N1s state core levels there are two peaks of N1s which were assigned to sp3 and sp2 nitrogen respectively (pyrrol- and azastates). In Yb metalloporphyrin charge distribution is more uniform for N1s spectra and thus wide single peak of N1s states reflects small difference between pyrol- and aza-nitrogen. The analysis of Yb4d electronic states shows that spectra do not consist of the spin-orbit split doublet, but instead is composed of asymmetric peak with multiplet splitting. Divalent Yb has a filled 4f shell, i.e. a 4f14 configuration, and the 4d spectra shows the doublet with 3:2 ratio, while for trivalent Yb, 4f13, the 4d peaks consist of a multiplet. The spectrum analysis demonstrated the multiplet splitting of Yb 4d and trivalent state in Yb(acac)-5,10,15,20-tetrakis-porphyrins. The research of thermal stability of tetraphenylporphyrin, octabrominetetraphenylporphyrin, ytterbium acetylacetonate octabrominetetraphenylporphyrin by X-ray photoelectron spectroscopy and thermogravimetry measurements in the range of temperatures of 30-450°C (when heating in ultrahigh vacuum) has shown destruction of the octabrominetetraphenylporphyrin and ytterbium acetylacetonate octabrominetetraphenylporphyrin after warming up higher than 150ºС while free base porphyrines (tetraphenylporphyrines) have shown thermal stability under vacuum conditions.

Well-Ordered In Adatoms at the In2O3(111) Surface Created by Fe Deposition

Metal deposition on oxide surfaces usually results in adatoms, clusters, or islands of the deposited material, where defects in the surface often act as nucleation centers. Here an alternate configuration is reported. After the vapor deposition of Fe on the In_{2}O_{3}(111) surface at room temperature, ordered adatoms are observed with scanning tunneling microscopy. These are identical to the In adatoms that form when the sample is reduced by heating in ultrahigh vacuum. Density functional theory calculations confirm that Fe interchanges with In in the topmost layer, pushing the excess In atoms to the surface where they arrange as a well-ordered adatom array.

Structural and magnetic properties of Cu-alloyed FePd films

Abstract Multilayer films [Cu( d A)/Fe(9 A)/Pd(11 A)] 5 were deposited at room temperature on Si(001)/SiO 2 (400 nm) substrates. In order to induce chemical L1 0 ordering, the as-deposited samples were post-annealed by rapid thermal annealing (RTA) at 600 °C for 90 s followed additionally by heating in ultra-high vacuum (UHV) at 700 °C up to several hours. In this study the impact of post-annealing on the structural and magnetic properties of FePdCu alloy films in dependence on the Cu content was investigated. It was found that the addition of Cu to the FePd alloy has a strong influence on the chemical ordering process and the (001) texture formation. After the RTA treatment only an isotropic distribution of the easy axis of magnetization with coercive fields in the range of a few hundred mT was observed. In contrast, samples which were additionally heated for 1 h at 700 °C revealed an out-of-plane easy axis of magnetization with an effective magnetic anisotropy of about 2×10 5 J/m 3 for the sample containing 10 at% of Cu.

Patterning Quasi-Periodic Co 2D-Clusters underneath Graphene on SiC(0001)

The behavior of Co nanoparticles (NPs) grown on graphene/SiC(0001) after oxygen exposure and heating in ultrahigh vacuum is investigated. The results of photoelectron spectroscopy (XPS) show that, as grown, the metal is on the surface of the graphene/SiC and suffers oxidation forming a single phase CoO when exposed to O2, even at low doses. After heating in ultrahigh vacuum (UHV), there is a deoxidation of cobalt and intercalation between the graphene (G) and the buffer layer (BL), as indicated by scanning tunneling microscopy (STM) and XPS. Cobalt forms almost regular small 2D clusters between G and BL. Moreover, graphene acts as a barrier to oxidation, preserving the metallic and the magnetic character of the material even when exposed to O2. This paper shows a method for patterning chemically protected Co NPs on graphene/SiC(0001) which could be used in nanomagnetism based devices.

Evaluation of Sr2MMoO6 (M = Mg, Mn) as anode materials in solid-oxide fuel cells: A neutron diffraction study

Sr2MMoO6 (M = Mg, Mn) double perovskites have recently been proposed as anode materials in solid-oxide fuel cells (SOFC). The evolution of their crystal structures has been followed by “in situ” temperature-dependent neutron powder diffraction from 25 °C room temperature (RT) to 930 °C by heating in ultrahigh vacuum (PO2 ≈ 10−6 Torr) in order to simulate the reducing atmosphere corresponding to the working conditions of an anode in a SOFC. At RT, the samples are described as tetragonal (I4/m space group) and monoclinic (P21/n) for M = Mg, Mn, respectively. Sr2MgMoO6 undergoes a structural phase transition from tetragonal to cubic (Fm-3m) below 300 °C; Sr2MnMoO6 experiences two consecutive phase transitions to tetragonal (I4/m) and finally cubic (Fm-3m) at 600 °C and above. In the cubic phases, the absence of octahedral tilting accounts for a good overlap between the oxygen and transition-metal orbitals, resulting in a good electronic conductivity; a high mobility of the oxygen atoms is derived from the elevated displacement parameters, for instance 3.0 Å2 and 4.6 Å2 at 930 °C for M = Mg, Mn, respectively. Both factors contribute to the excellent performance described for these mixed ionic and electronic conductor oxides as anodes in single fuel cells. From dilatometric measurements, the thermal expansion coefficients (TEC) in the cubic region are 12.7 × 10−6 K−1 and 13.0 × 10−6 K−1 for M = Mg and Mn, respectively. These figures are comparable to those obtained from the mentioned structural analysis; moreover, the TECs for the cubic phases perfectly match those of the usual electrolytes in a SOFC.

Precise measurements of nanostructure parameters

The precision of measurements performed by atomic-force microscopy (AFM) and high-resolution electron microscopy (HREM) for solving problems of metrology and diagnostics of solid nanostructures is discussed. The HREM-measured height of a monatomic step on a Si(111) surface covered by a thin natural oxide film is demonstrated to be 0.314 ± 0.001 nm. The same accuracy is ensured by AFM measurements through controlling the Si surface relief with heating in ultra-high vacuum on specially created test objects with the distance between the steps being approximately 2 µm. It is shown that the geometric phase method can be used to quantify the strains in the crystal lattice of strained heterostructures on the basis of HREM images with accuracy to 10−4%, and in situ irradiation by electrons in HREM measurements can be used to visualize ordered clusterization of vacancies and self-interstitial atoms in {113} planes in Si samples.

Nanostructure formation in the initial roughening of a thin silicon sheet

Silicon-on-insulator (SOI) presents a unique model system for exploring the stability of crystalline nanomaterials in metastable configurations. We show that the initial destabilization of ultrathin SOI is related to mechanical stress, in contrast to phenomena at later times driven by the energy of the ${\text{SiO}}_{2}/\text{Si}$ interface. Stepped rectangular truncated pyramids, with lateral dimensions of tens of nanometers, are formed on the outer Si layer of ultrathin (001)-oriented SOI during heating in ultrahigh vacuum. Pyramid edges are bounded by doubled atomic steps, with corners consisting of a complex series of single-layer steps. The shape of these nanopyramids represents a balance between stress-induced roughening and the elastic interaction between steps. SOI allows the precisely known energetics of silicon surfaces to be readily adapted to materials with nanoscale dimensions.

Electro-mechanics of resonating nanotubes and nanowires in the field emission environment

In this paper, we review our recent experimental investigations of phenomena associated with the mechanical resonances of nanotubes and nanowires (NN) during field emission (FE). The goal is to show how this configuration can be used for measuring the physical parameters of an NN and how it opens interesting perspectives in the emerging field of Nano Electro-Mechanical Systems (NEMS). In the basic experiment the resonances are excited capacitively in ultra high vacuum (UHV) and are detected by either a variation in the FE current or an increase in the size of the FE pattern. The high electric field also induces a giant tuning of the NN resonance frequency making our NNs tunable oscillators. An image analysis technique has been developed to exploit the FE pattern changes for measuring low vibration amplitudes. High Q-factors of SiC nanowires have been determined as a function of in UHV heating cycles. The effect of the tuning on the resonance frequency has been modelled allowing the extraction of the natural frequency and thus the Young's modulus. Finally, an example of a new NEMS phenomena in the FE configuration is amechanical 'self oscillations' observed without any AC drive: it is a nanometric DC/AC conversion device.

Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy

Several nanometer-thick graphene oxide films deposited on silicon nitride-on silicon substrates were exposed to nine different heat treatments (three in Argon, three in Argon and Hydrogen, and three in ultra-high vacuum), and also a film was held at 70 °C while being exposed to a vapor from hydrazine monohydrate. The films were characterized with atomic force microscopy to obtain local thickness and variation in thickness over extended regions. X-ray photoelectron spectroscopy was used to measure significant reduction of the oxygen content of the films; heating in ultra-high vacuum was particularly effective. The overtone region of the Raman spectrum was used, for the first time, to provide a “fingerprint” of changing oxygen content.

Intact sublimation of silicon nanocrystals evidenced via HREM imaging and EELS in a dedicated STEM

Silicon nano crystals (NCs) have attracted considerable interest for possible uses in optoelectronics1 As the particle size decreases the properties of NCs become increasingly sensitive to the surface termination.2, 3 Monolayer chemistries4-10 have been exploited to control the physicochemical properties. NCs are often prepared by vapour-phase deposition techniques; using these they can be conveniently analysed via gas phase analysis techniques, such as mass spectrometry. This cannot be employed, however, if NCs are not synthesized in the gas phase. Here we present a STEM study of undecyl-capped SiNCs, evaporated intact upon heating in ultrahigh vacuum at 200 °C and collected on a variety of solid substrates, including carbon-coated TEM grids. The BF- and HAADF lattice images confirm that the particles have a crystalline core with Si-lattice spacings. The presence of Si in the core is also confirmed by Si-L edge EELS, which reveals furthermore the presence of a surface oxide.

Fe–Ni Surface Alloy Formation on Ni(111) Investigated by Scanning Tunneling Microscopy

The structure of Fe films grown on Ni(111) with an Fe coverage of 3.2 monolayers and its conversions during heating in ultrahigh vacuum have been investigated by scanning tunneling microscopy (STM). The Fe films consist of elongated domains stacked with four to six monatomic layers of Fe, forming ridgelike structures. A phase transition from fcc(111) to bcc(110) with the Nishiyama–Wassermann orientation relationship occurs from at least the third monatomic layer. A triangular network structure and large accumulated bcc-Fe islands are formed on the surface after heating the Fe films at 550 K. The network is considered to be misfit dislocation loops caused by the lattice misfit between the fcc-Fe films and the Ni substrate. A surface structure consisting of two distinct randomly distributed spots and a well-ordered (5 ×5) superstructure are observed after further heating at 620 and 700 K, respectively, and they are caused by the formation of the Fe–Ni surface alloy on the Ni(111) surface.

Evaporation and deposition of alkyl-capped silicon nanocrystals in ultrahigh vacuum

Nanocrystals are under active investigation because of their interesting size-dependent properties and potential applications. Silicon nanocrystals have been studied for possible uses in optoelectronics, and may be relevant to the understanding of natural processes such as lightning strikes. Gas-phase methods can be used to prepare nanocrystals, and mass spectrometric techniques have been used to analyse Au and CdSe clusters. However, it is difficult to study nanocrystals by such methods unless they are synthesized in the gas phase. In particular, pre-prepared nanocrystals are generally difficult to sublime without decomposition. Here we report the observation that films of alkyl-capped silicon nanocrystals evaporate upon heating in ultrahigh vacuum at 200 degrees C, and the vapour of intact nanocrystals can be collected on a variety of solid substrates. This effect may be useful for the controlled preparation of new quantum-confined silicon structures and could facilitate their mass spectroscopic study and size-selection.

XPS investigation of Mn valence in lanthanum manganite thin films under variation of oxygen content

The question whether $\mathrm{La}\mathrm{Mn}{\mathrm{O}}_{3}$ accepts doping with tetravalent cations such as cerium and thus allows the preparation of electron-doped mixed-valent lanthanum manganites has been discussed controversially so far. Against the background of this problem, we present a comparative x-ray photoemission (XPS) study of epitaxial ${\mathrm{La}}_{0.7}{\mathrm{Ce}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCeMO) and ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LCMO) thin films. We focus on the exchange splitting of the Mn $3s$ core level peak, which is a direct indicator of the Mn valence and allows us to quantify the Mn valence in the outermost $3\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ of the films. We demonstrate that, depending on the oxygen content, the Mn valence can be tuned between a mixed ${\mathrm{Mn}}^{3+∕4+}$ state and a mixed ${\mathrm{Mn}}^{2+∕3+}$ state in both the LCeMO and the LCMO film. The oxygen content was varied by heating in ultrahigh vacuum for deoxygenation and in an oxygen atmosphere for reoxidation. In the LCeMO film, the deoxygenation not only changes the Mn valence, but also the Ce valence is driven from the $4+$ towards the $3+$ state.