Ultrahigh Vacuum Measurements Research Articles

Cold atom technology applied to ultra-high vacuum (UHV) measurements

Accurate measurements in ultrahigh vacuum (UHV) are increasingly critical for scientific research and industrial applications. Here, we report an experimental study of UHV measurements performed on a 7Li cold atom vacuum measurement setup. The setup enables cold atom vacuum measurements in the magneto-optical and magnetic traps (CAVM-MOT and CAVM-MT), respectively. The precision of the vacuum measurements in both traps was effectively improved after optimisation of the excited-state fraction data processing for CAVM-MOT and the initial loading control method for CAVM-MT. A comparison of the measurement methods was carried out in combination with an ionisation gauge, revealing the good linearity and accuracy of the CAVM. We believe that CAVM in the two potential traps have their own application situations and development directions, respectively.

Effect of the magnetic field on the operation of ionisation gauges

Hot cathode ionisation gauges are the only reliable pressure measurement devices suitable for both high and ultra-high vacuum measurement. These devices are characterized by low accuracy that is hardly better than 20%. The sources of loss of accuracy are documented in the literature although their quantification and details are not fully understood. In the present work, simulations of Bayard-Alpert and extractor gauges were performed under the influence of uniform magnetic fields up to 30 Gs. Results are in qualitative agreement with measurements taken previously with the Bayard-Alpert gauge. Both gauges are sensitive to low intensity magnetic fields which may be present in vacuum chambers. However, the extractor gauge proved to be more stable in the presence of a magnetic field due to its ring-shaped cathode.

Vacuum pressure measurement based on <sup>6</sup>Li cold atoms in a magneto-optical trap

Ultra-high vacuum measurement and extremely high vacuum (UHV/XHV) measurement play an important role in high-tech fields such as deep space exploration, particle accelerators, and nanoscience; with the continuous extension of the lower limit of measurement, especially when it reaches the order of 10<sup>–10</sup> Pa, higher requirements are placed on the accuracy of the measurement. At present, in the field of UHV/XHV measurement, ionization gauges based on the principle of neutral gas ionization are commonly applied to the vacuum measurement. However, traditional ionization vacuum gauges during use can create electronic excitation desorption effects, soft X-rays, and the effect of hot cathode outgassing, thereby affecting the accuracy of measurement and limiting the lower limit of measurement. Compared with the traditional measurement technology, this method uses the relationship between the loss rate and pressure caused by the collision of cold atoms trapped in the trap depth with the background gas to calculate the gas density and inversely calculate the vacuum pressure. Based on the intrinsic quantum mechanical properties of cold atom collisions, this method is expected to be developed into a new vacuum traceability standard. In this paper, based on the small-angle approximation and impulse approximation under the quantum scattering theory, the loss rate coefficient of the collision of <sup>6</sup>Li cold atoms with background gas molecules is calculated. According to the ideal gas equation, the pressure inversion formula is obtained. The collision loss rate is extracted by accurately fitting the loss curve of the cold atom. In order to improve the accuracy of vacuum inversion and reduce the influence of quantum diffractive collision on loss rate measurement, the trap depth under the conditions of a certain cooling laser intensity, detuning, and magnetic field gradient is determined by the photoassociation method. Finally, in a range of 1 × 10<sup>–8</sup>–5 × 10<sup>–6</sup> Pa, the inverted pressure value is compared with the measured value of the ionization meter, proving that this method has good accuracy and reliability in the inversion of vacuum pressure. At present, the main factor restricting the improvement of accuracy is the influence of the collision between the excited atoms in the magneto-optical trap and the background gas on the loss rate measurement. In the future, with the proportion of excited atoms and the excited state <i>C</i><sub>6</sub> coefficient to be precisely determined, the uncertainty of vacuum pressure measurement can be further reduced.

Comparative Study of Dimensionality and Symmetry Breaking on Nanoscale Friction in the Prandtl–Tomlinson Model with Varying Effective Stiffness

Friction Force Microscopy (FFM) measurements on NaCl immersed in ethanol display an increase of the effective contact stiffness with the applied load. This stiffness is estimated from the measured local contact interaction of the tip with the NaCl surface and the Prandtl–Tomlinson (PT) parameter, which reflects the relation between the corrugation stiffness and the effective contact stiffness. Different from FFM measurements in ultrahigh vacuum, for measurements in ethanol surroundings the PT parameters showed a maximum with the applied load. We incorporated this measured load-dependent effective stiffness together with the load-dependent amplitude of the corrugation energy into simulations based on the PT model, and studied its effect on the lateral friction for symmetric 1D and 2D potentials, as well as for an asymmetric 1D potentials. The simulations reproduced the experimentally observed non-monotonous behavior of the PT parameter, and enabled a glimpse on the relation of the characteristic observables (mean maximal slip forces and stiffness) with respect to their governing parameters (corrugation energy, effective stiffness). In all, apart from large deviations from symmetry in the interaction potential, the PT parameter provides a reliable estimate for nanoscale friction over periodic surfaces.

Surface structure of 1,4-benzenedithiol on Au(111)

The structure of self-assembled monolayers of 1,4-benzene dithiol (1,4-BDT) oligomers on Au(111) has been investigated using scanning tunneling microscopy (STM) measurements in ultrahigh vacuum combined with density function theory (DFT) calculations. The dominant adsorption geometry of 1,4-BDT oligomer chains involves an Au adatom linker forming a complex with two trans anchoring groups, resulting in a zigzag structure. STM measurements show preferential growth from step edges where gold adatoms can be extracted to form the oligomers. Ladder-like structures involving two parallel oligomer chains are observed at medium coverages, suggesting a stabilizing interaction between neighboring chains and reduced mobility following oligomerization. At high-coverages, the gold surface is covered with close-packed domains of oligomers. DFT STM simulations were performed using the Tersoff-Hamann method in order to elucidate structures observed in STM measurements.

In situ study of oxygen and Mg effects on current-induced magnetization switching in Pt/Co bilayers in ultrahigh vacuum

We report on an in situ study of oxygen exposure and Mg dusting effects on current-induced magnetization switching in Pt/Co bilayers via controlled oxygen exposure, Co and Mg deposition, and electrical measurements in ultrahigh vacuum. We show that the oxygen exposure and Mg dusting induce changes in saturation magnetization, effective magnetic anisotropy, spin–orbit torque, and switching current density. From the correlation between these parameters and the switching current density for current-induced switching, we found that the reduction in switching current density by oxygen exposure is caused by the combined effect of enhancement in spin–orbit torque and lowering of the switching energy barrier required for incoherent switching with the latter playing a dominant role in large samples.

Two-dimensional conducting layer on the SrTiO3 surface induced by hydrogenation

We found that a surface state induced by hydrogenation on the surface of SrTiO$_{3}$(001) (STO) did not obey the rigid band model, which was confirmed by in situ electrical resistivity measurements in ultrahigh vacuum. With exposure of atomic hydrogen on the STO, a new surface state (H-induced donor state, HDS) appears within the bulk band gap (an in-gap state), which donates electrons thermally activated to the bulk conduction band, resulting in downward bending of the bulk bands beneath the surface. The doped electrons flow through the space-charge layer in two-dimensional manner parallel to the surface. The observed semiconducting behavior in the temperature dependence of electronic transport is explained by the thermal activation of carriers. The HDS and the bulk conduction band are non-rigid in energy position; they come closer with increasing the hydrogen adsorption. Eventually the HDS saturates its position around 88 meV below the bottom of the bulk conduction band. The sheet conductivity, accordingly, also saturates at 1.95$\pm$ 0.02 $\mu$ S/sq. with increasing hydrogen adsorption, corresponding to completion of the hydrogenation of the surface.

In-situ study of oxygen exposure effect on spin-orbit torque in Pt/Co bilayers in ultrahigh vacuum

Oxygen incorporation has been reported to increase the current-induced spin-orbit torque in ferromagnetic heterostructures, but the underlying mechanism is still under active debate. Here, we report on an in-situ study of the oxygen exposure effect on spin-orbit torque in Pt/Co bilayers via controlled oxygen exposure, Co and Mg deposition, and electrical measurements in ultrahigh vacuum. We show that the oxygen exposure on Pt/Co indeed leads to an increase of spin-orbit torque, but the enhancement is not as large as those reported previously. Similar enhancement of spin-orbit torque is also observed after the deposition of an MgO capping layer. The results of ab initio calculations on the Rashba splitting of Pt/Co and Pt/Co/O suggest that the enhancement is due to enhanced Rashba-Edelstein effect by surface-adsorbed oxygen. Our findings shed some light on the varying roles of oxygen in modifying the spin torque efficiency reported previously.

Study of Gaseous Interactions on Co3O4 Thin Film Coatings by Ambient Pressure Soft X-ray Absorption Spectroscopy

The redox process of pretreated Co3O4 thin film coatings has been studied by ambient pressure soft X-ray absorption spectroscopy. The Co3O4 coatings were composed of nanoparticles of about 10 nm in size as prepared by pulsed laser deposition. The thin film coatings were pretreated in He or in H2 up to 150 °C prior to exposure to the reactive gases. The reactivity toward carbon monoxide and oxygen was monitored by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy during gas exposures. The results indicate that the samples pretreated in He show reactivity only at high temperature, while the samples pretreated in H2 are reactive also at room temperature. X-ray photoemission spectroscopy measurements in ultra-high vacuum and NEXAFS simulations with the CTM4XAS code further specify the results.

Adsorption and Interaction of CO and NO Molecules on Pure and Oxidized Surfaces of Al−Mo(110) Alloy

The coadsorption of carbon-monoxide- and nitric-oxide molecules and their interactions on surfaces of Al−Mo(110) alloy and the system resulting from its oxidation (Al−Mo−O) are investigated using an array of spectroscopic techniques (X-ray photoelectron, Auger electron, infrared, and thermal-desorption spectroscopies), low-energy electron diffraction, and work-function measurements in ultra-high vacuum. Al−Mo(110) alloy is fabricated by the thermal annealing (at 800 K) of an aluminum film several layers thick deposited onto the Mo(110) surface. Aluminum atoms diffuse into the substrate to yield a surface alloy of hexagonal structure characteristic of stoichiometric Al2Mo alloy. Unlike dissociative adsorption on both Mo(110) and Al(111) surfaces, CO and NO adsorb molecularly at the surface of Al−Mo(110) alloy. At 200 K, the adsorption of CO molecules on the Al−Mo(110) surface containing pre-adsorbed NO molecules has a dramatic effect on the latter by displacing them to higher-coordinated adsorption sites and at the same time causing their molecular axis to tilt toward the adsorbent surface plane. Heating this system to 320 K results in the reduction of nitric oxide by carbon monoxide to yield CO2 and surface nitrides. This can be a consequence of surface reconstruction that leads to the formation of additional adsorption/reaction sites at the Al/Mo interface and to changes in the substrate’s d-band filling as a result of alloying. The oxidation of CO by NO proceeds with a markedly higher efficiency at the surface of the Al−Mo−O system; this system results from oxidation of the Al−Mo(110) alloy by oxygen at 700 K and exposure up to 1500 L.

Development of an CNT field emission UHV ionization gauge with customized electronics

An ionization vacuum gauge with carbon nanotube (CNT) cathode was designed for measurement of ultra high vacuum (UHV). The sensor was constructed based on Bayard-Alpert type gauge. The key index of the control unit is determined by theoretical analysis and experimental results. A customized detection circuit was designed considering the characteristics of the output ion current of the sensor, the performance of which, such as linearity, noise, bandwidth and gain, were determined by a series of tests and calibrations. At last, the vacuum measurement range and uncertainty of this gauge were determined. The test results showed that the lower limit of the CNT-cathode ionization vacuum gauge enters 10-8Pa, The measurement uncertainty is about 4.0% in the pressure range of 10-4Pa-10-7Pa. This work can provide a new solution for UHV measurement.

Transport Spectroscopy of Sublattice-Resolved Resonant Scattering in Hydrogen-Doped Bilayer Graphene.

We report the experimental observation of sublattice-resolved resonant scattering in bilayer graphene by performing simultaneous cryogenic atomic hydrogen doping and electron transport measurements in an ultrahigh vacuum. This allows us to monitor the hydrogen adsorption on the different sublattices of bilayer graphene without atomic-scale microscopy. Specifically, we detect two distinct resonant scattering peaks in the gate-dependent resistance, which evolve as a function of the atomic hydrogen dosage. Theoretical calculations show that one of the peaks originates from resonant scattering by hydrogen adatoms on the α sublattice (dimer site) while the other originates from hydrogen adatoms on the β sublattice (nondimer site), thereby enabling a method for characterizing the relative sublattice occupancy via transport measurements. Utilizing this new capability, we investigate the adsorption and thermal desorption of hydrogen adatoms via controlled annealing and conclude that hydrogen adsorption on the β sublattice is energetically favored. Through site-selective desorption from the α sublattice, we realize hydrogen doping with adatoms primarily on a single sublattice, which is highly desired for generating ferromagnetism.

An ionization gauge for ultrahigh vacuum measurement based on a carbon nanotube cathode.

This work reports on the complete design and the properties of an ionization gauge based on a carbon nanotube cathode, which can measure ultrahigh vacuum without thermal effects. The gauge is composed of a pressure sensor and an electronic controller. This pressure sensor is constructed based on a hot-cathode ionization gauge, where the traditional hot filament is replaced by an electron source prepared with multi-wall nanotubes. Besides, an electronic controller was developed for bias voltage supply, low current detection, and pressure indication. The gauge was calibrated in the pressure range of 10-8 to 10-4 Pa in a XHV/UHV calibration apparatus. The gauge shows good linear characteristics in different gases. The calibrated sensitivity is 0.035 Pa-1 in N2, and the standard deviation of the sensitivity is about 1.1%. In addition, the stability of the sensitivity was learned in a long period. The standard deviation of the sensitivity factor "S" during one year is 2.0% for Ar and 1.6% for N2.

Effect of ultrathin Fe dusting layer on electrical transport properties of few-layer MoS2 field-effect transistors

Transition-metal-doped MoS2 has been predicted to be a potential candidate for a two dimensional dilute magnetic semiconductor, while the effect of transition metal dopants on the electrical properties of MoS2-based devices has received relatively less attention so far. Here, the authors report on a systematic electrical transport study of Fe-dusted few-layer molybdenum disulfide (MoS2) field-effect transistors via repeated in situ Fe deposition (total nominal thickness ≤ 2 nm) and electrical measurements in ultrahigh vacuum. It is found that an ultrathin Fe adsorption layer (≪0.5 nm) n-dopes MoS2 without noticeably affecting the electrical characteristics of the device. In contrast, a thicker Fe layer (0.5–2 nm) results in the loss of the carrier tunability and a nonlinear current-voltage characteristic with the differential conductance approximately linear to drain voltage. The authors show through result analysis and qualitative modeling that Fermi level pinning in MoS2 directly underneath Fe nanoclusters plays an important role in causing these degradations. The pinning effect can be partially removed by in situ oxidizing the Fe nanoclusters for a short duration of 16 min. The mechanism of Fermi level pinning is discussed.

Superconductivity in thallium double atomic layer and transition into an insulating phase intermediated by a quantum metal state

We report on the first observation of superconductivity in a double atomic layer of Tl on Si(1 1 1) using in situ electrical resistivity measurements in ultrahigh vacuum. The structure of the Tl bilayer was characterized by a set of techniques, including scanning tunneling microscopy, electron diffraction and photoemission spectroscopy, which confirmed the metastability and metallic nature of the Tl bilayer. The epitaxial growth of atomically thin ‘soft’ metallic film over the entire surface of substrate enabled us to find a macroscopic superconducting transition at 0.96 K, accompanied by thermal and quantum fluctuations of order parameter. The system also demonstrates a perpendicular-magnetic-field-induced superconductor-insulator transition, together with an intermediate metallic state. We have found that the magnetoresitivity at the lowest temperature is consistent with the Bose metal picture, which is a consequence of strong quantum fluctuations.

Metal Evaporation-Induced Degradation of Fullerene Acceptors in Polymer/Fullerene Solar Cells.

Surface-sensitive NEXAFS spectroscopy is used to probe the interaction between low work function metal electrodes and fullerene derivatives in organic solar cells. Evaporation of either Ca or Al electrodes onto films of the fullerene derivatives (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) and indene-C60 bisadduct (ICBA) leads to a dramatic change in the observed NEXAFS spectrum. The observed changes cannot be explained only in terms of interfacial electronic doping or charge transfer, but rather point to the formation of new chemical bonds that destroy the extensive electron delocalization on the C60 cage. A combination of ex situ and in situ ultrahigh vacuum measurements indicates that metal evaporation results in a change in the electronic structure of PCBM that then facilitates chemical degradation and oxidation in the presence of oxygen. To investigate the effect of this chemical interaction on device performance, a unique transfer method to laminate the Al electrode to the top of polymer blend is used, in which case, the chemical degradation of the fullerene is not observed. Device performance of P3HT/PCBM blend solar cells in which the top metal electrode has either been thermally evaporated or transferred is then compared. These results highlight that chemical, as well as electronic, interactions between metals and organic semiconductors must be considered.

In situ resistivity of endotaxial FeSi2 nanowires on Si(110)

We present in situ ultra-high vacuum measurements of the resistivity ρ of self-assembled endotaxial FeSi2 nanowires (NWs) on Si(110) using a variable-spacing two-point method with a moveable scanning tunneling microscope tip and fixed contact pad. The resistivity at room temperature was found to be nearly constant down to NW width W = 4 nm, but rose sharply to nearly double the bulk value at W = 3 nm. These data are not well-fit by a simple Fuch-Sondheimer model for boundary scattering, suggesting that other factors, possibly quantum effects, may be significant at the smallest dimensions. For a NW width of 4 nm, partial oxidation increased ρ by approximately 50%, while cooling from 300 K to 150 K decreased ρ by approximately 10%. The relative insensitivity of ρ to NW size or oxidation or cooling is attributed to a high concentration of vacancies in the FeSi2 structure, with a correspondingly short length for inelastic electron scattering, which obscures boundary scattering except in the smallest NWs. It is remarkable that the vacancy concentration persists in very small structures.

Electronic Structure Calculations and Synergistic Experimental Work in the Nanostructures for Electrical Energy Storage (NEES) Energy Frontier Research Center

In this presentation, we will review computational effort and synergistic experimental activities in the Nanostructures for Electrical Energy Storage (NEES) Energy Frontier Research Center, and discuss future research directions. Interfaces and interphases constitute a main theme of our research. Nanoscale structures exhibit enhanced surface areas, and fundamental understanding and smart design of interfaces can help exploit their kinetic advantages while mitigating undesirable parasitic reactions. Thus electrochemical reactions responsible for liquid electrolyte degradation are examined using both large scale ab initio molecular dynamics (AIMD) simulations and complementary ultrahigh vacuum (UHV) measurements. Electrolyte decomposition on lithium peroxide surfaces, a key obstacle in Li-air battery implementation, is examined using analytical/spectroscopic techniques and DFT calculations. Artificial "solid electrolyte interphase" are deposited on amorphous carbon electrodes via atomic layer deposition (ALD) processes, and its electrode-passivating property is characterized by microgravimetric measurements and modelled using DFT calculations coupled with Marcus Theory constructions. This ALD study highlights a second key theme in our research: that necessary fundamental science, computational- and nano-electrochemistry advances emerge naturally in the ostensibly "applied" field of energy storage. We will discuss other examples of method development, such as rigorous voltage calibration in AIMD/DFT calculations, and measurement/control of minute currents associated with single nanowires. Going forward, our new computational approaches will be applied to model solid-solid interfaces pertinent to solid electrolytes, artificially-coated, nanostructured interfaces, and electrode degradation studies.This work was supported by Nanostructures for Electrical Energy Storage (NEES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under award no. DESC0001160. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corpo ration, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.

Decisive influence of substitution positions in molecular self-assembly.

Molecular self-assembly provides a versatile tool for creating functional molecular structures at surfaces. A rational design of molecular structure formation requires not only an in-depth understanding of the subtle balance between intermolecular and molecule-surface interactions, but might also involve considering chemical changes of the molecules, such as deprotonation. Here, we present a systematic investigation of a comparatively simple class of molecules, namely dihydroxybenzoic acid, which, nevertheless, enables creating a rich variety of structures when deposited onto calcite (10.4) held at room temperature. Based on non-contact atomic force microscopy measurements in ultra-high vacuum, our study demonstrates the decisive impact of the positions of the hydroxyl groups on the structure formation. Six isomers of dihydroxybenzoic acid exist which form six different molecular structures on the calcite surface. Surprisingly, only two isomers arrange into stable, ordered structures at sub-monolayer coverage: 2,5-dihydroxybenzoic acid forms a commensurate (1 × 5) structure, composed of deprotonated molecules. A double-row structure consisting of protonated molecules is observed for 3,5-dihydroxybenzoic acid. The positions of the functional groups steer the molecular self-assembly of dihydroxybenzoic acids in three distinct ways, namely by (a) affecting the deprotonation tendency of the acid group, (b) influencing the intermolecular interaction as already indicated by greatly different bulk structures and (c) altering the molecule-substrate matching. Our results, thus, shed light on the impact of rather small changes in the molecular structure on the structural variety in molecular self-assembly on surfaces.

Note: A simple approach to fabricate a microscopic four-point probe for conductivity measurements in ultrahigh vacuum

We present a simple method to fabricate microscopic four-point probe (M4PP) with spacing of 70-100 μm for conductivity measurements in ultrahigh vacuum. The probe includes four gold wires with 30 μm diameter and a 0.5 mm thickness sapphire slice as cantilever. One of the dual scanning tunneling microscope (DSTM) is replaced by M4PP. As a result, in situ transport measurement could be performed by M4PP and investigation of surface morphology by STM. Finally, we measure conductivity of 14 monolayer Bi(111) epitaxial film on n type Si which is 1.6 × 10(-3) Ω(-1)∕[larger open square].