Ultrahigh Vacuum Conditions Research Articles

Probing borophene oxidation at the atomic scale

Two-dimensional boron (i.e. borophene) holds promise for a variety of emerging nanoelectronic and quantum technologies. Since borophene is synthesized under ultrahigh vacuum (UHV) conditions, it is critical that the chemical stability and structural integrity of borophene in oxidizing environments are understood for practical borophene-based applications. In this work, we assess the mechanism of borophene oxidation upon controlled exposure to air and molecular oxygen in UHV via scanning tunneling microscopy andspectroscopy, x-ray photoelectron spectroscopy, and density functional theory calculations. While borophene catastrophically degrades almost instantaneously upon exposure to air, borophene undergoes considerably more controlled oxidation when exposed to molecular oxygen in UHV. In particular, UHV molecular oxygen dosing results in single-atom covalent modification of the borophene basal plane in addition to disordered borophene edge oxidation that shows altered electronic characteristics. By comparing these experimental observations with density functional theory calculations, further atomistic insight is gained including pathways for molecular oxygen dissociation, surface diffusion, and chemisorption to borophene. Overall, this study provides an atomic-scale perspective of borophene oxidation that will inform ongoing efforts to passivate and utilize borophene in ambient conditions.

Local Chiral Inversion of Thymine Dimers by Manipulating Single Water Molecules.

Water, as one of the most important and indispensable small molecules in vivo, plays a crucial role in driving biological self-assembly processes. Real-space detection and identification of water-induced organic structures and further capture of dynamic dehydration processes are important yet challenging, which would help to reveal the cooperation and competition mechanisms among water-involved noncovalent interactions. Herein, introduction of water molecules onto the self-assembled thymine (T) structures under ultrahigh vacuum (UHV) conditions results in the hydration of hydrogen-bonded T dimers forming a well-ordered water-involved T structure. Reversibly, a local dehydration process is achieved by in situ scanning tunneling microscopy (STM) manipulation on single water molecules, where the adjacent T dimers connected with water molecules undergo a local chiral inversion process with the hydrogen-bonding configuration preserved. Such a strategy enables real-space identification and detection of the interactions between water and organic molecules, which may also shed light on the understanding of biologically relevant self-assembly processes driven by water.

On‐surface synthesis and atomic scale characterization of unprotected indenofluorene polymers

AbstractPolycyclic hydrocarbons with nonzero radical character have attracted enormous interest as potential active media for organic electronics and spintronics. In this context, indenofluorenes are an intriguing class of formally antiaromatic, biradical materials with a radical character that depends on the connectivity of their six‐ and five‐membered rings. Synthesis of indenofluorene polymers and related compounds, first achieved in the early ‘90s with the production of ladder‐type chains, represents a major step toward incorporation of these systems into devices. However, solution‐based synthetic protocols require bulky protecting groups to stabilize the most reactive sites and, at the same time, to improve solubility and processability of such compounds. The preparation of various pristine – that is, unprotected—indenofluorene polymers has recently become possible via the on‐surface synthesis approach, where the resulting nanostructures are supported and efficiently stabilized by the underlying substrate in ultrahigh vacuum conditions. Here, an overview of these recent works is given, with a focus on synthetic challenges, structural details and electronic properties.

Design and characterization of a cryogenic linear Paul ion trap for ion-neutral reaction studies.

Ultra-high vacuum conditions are ideal for the study of trapped ions. They offer an almost perturbation-free environment, where ions confined in traps can be studied for extended periods of time-facilitating precision measurements and allowing infrequent events to be observed. However, if one wishes to study processes involving molecular ions, it is important to consider the effect of blackbody radiation (BBR). The vast majority of molecular ions interact with BBR. At 300K, state selection in trapped molecular ions can be rapidly lost (in a matter of seconds). To address this issue, and to maintain state selectivity in trapped molecular ions, a cryogenic ion trap chamber has been constructed and characterized. At the center of the apparatus is a linear Paul ion trap, where Coulomb crystals can be formed for ion-neutral reaction studies. Optical access is provided, for lasers and for imaging of the crystals, alongside ion optics and a flight tube for recording time-of-flight mass spectra. The ion trap region, encased within two nested temperature stages, reaches temperatures below 9K. To avoid vibrations from the cryocooler impeding laser cooling or imaging of the ions, vibration-damping elements are explicitly included. These components successfully inhibit the coupling of vibrations from the cold head to the ion trap-confirmed by accelerometer measurements and by the resolution of images recorded at the trap center (at 9 and 295K). These results confirm that the cryogenic ion trap apparatus meets all requirements for studying ion-neutral reactions under cold, controlled conditions.

In-situ STS studies and first principles calculations on bare and Sn adsorbed UHV exfoliated WS2 layers

Abstarct. Two dimensional (2D) derivatives of tin (Sn) have obtained special deliberations recently due to practical realization of planar, as well as, buckled hexagonal lattice of Sn called stanene. However, it has been observed that proper choice of substrate is very important for growth of stanene like films owing to large core size of Sn that prefers sp 3 hybridization over sp 2. Transition metal dichalcogenides (TMDs) like MoS2 or WS2 with honey comb lattice structure seem to be promising substrate candidates for 2D growth of Sn. In the present work, we report mechanical exfoliation of few layers of WS2 under ultra-high vacuum (UHV) conditions and investigations of growth and local electronic structure by in-situ scanning tunneling microscopy (STM) and spectroscopy (STS) studies. Flat WS2 surface with honeycomb lattice structure in the atomic scale with a lattice constant of 0.34 nm is evident in the STM investigations, whereas, STS measurements reveal local density of states (LDOS) of WS2 with a bandgap of approximately 1.34 eV. Density functional theory (DFT) calculations performed by considering bulk WS2 reveal conduction and valence band states comprised of S p and W d at both sides of the Fermi energy (EF) and an indirect bandgap of 1.38 eV. Experimental observations upon Sn adsorption, reveal commensurate growth of Sn atoms on the sulfur `S’ sites with a buckling height of 40 ±10 pm. STS measurements exhibit local electronic structure of the Sn adsorbed surface with clear evidence of in-gap states. DFT calculations quantify the experimental results demonstrating `S’ sites as the most stable sites for the atomic adsorption of Sn with a buckling height of around 80 pm and reveal signature of in-gap hybridized states comprised of Sn p and W d orbitals.

In situ NAP-XPS study of CO2 and H2O adsorption on cerium oxide thin films

Insights of CO2 and H2O activation on cerium oxide based catalysts are of great interest to many important catalytic processes, which are widely studied in model systems under ultrahigh vacuum conditions. Here, in situ near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) was used to study CO2 and H2O adsorption on fully oxidized CeO2 and partially reduced CeO2−x surfaces. It is found that CO2 only weakly interacts with the stoichiometric CeO2 but strongly adsorbs on partially reduced CeO2−x surfaces with surface carbonate formation. H2O dissociatively adsorbs reversely on the CeO2 surface with H2O exposure pressure above 1 × 10−3 mbar, while H2O strongly dissociates on the partially reduced CeO2−x surfaces. The surface dissociated OH and H species are stable on the CeO2−x surface up to 600 K. Both CO2 and H2O adsorption cause clear reoxidation of partially reduced CeO2−x. This study provides new insights into CO2 and H2O interaction with cerium oxide surfaces at near ambient pressure conditions.

Electrical Double Layer on the Pt(111) Electrode Modeled under Ultrahigh Vacuum Conditions

An adsorbed hydroxide (OHad) layer is formed on platinum electrodes at positive potentials, which affects many important electrochemical reactions, such as the oxygen reduction reaction. The stability of the OHad layer is strongly governed by the hydrophilicity of the cation interacting with OHad in the electrical double layer (EDL). Therefore, elucidating the detailed hydration structure and role of cations in the EDL is a key research goal. In this study, the structures of H2O, OHad, and Li were investigated using infrared spectroscopy and density functional theory calculations. By optimizing the coverage of OHad (θOH) and Li (θLi), the bending mode of the PtOH (δPtOH) band on the Pt(111) surface under ultrahigh vacuum (UHV) conditions matches with that on the Pt(111) electrode at 0.8–0.9 V versus reversible hydrogen electrode (RHE) in LiOH solution, indicating that a quasi-EDL composed of OHad species interacting with hydrated Li+ on Pt(111) was successfully modeled under UHV conditions. According to the interfacial structure modeled under UHV conditions, θOH = 0.3, and Li interacts directly with OHad at a stoichiometric ratio (θOH/θLi) of 2. Modeling of the EDL, including the outer Helmholtz plane, is beneficial for identifying the microscopic details of the EDL under electrochemical conditions.

Highly Ordered N-Heterocyclic Carbene Monolayers on Cu(111).

The benzannulated N-heterocyclic carbene, 1,3-dibenzylbenzimidazolylidene (NHCDBZ) forms large, highly ordered domains when adsorbed on Cu(111) under ultrahigh vacuum conditions. A combination of scanning tunnelling microscopy (STM), high-resolution electron energy loss spectroscopy (HREELS), and density functional theory (DFT) calculations reveals that the overlayer consists of vertical benzannulated NHC moieties coordinating to Cu adatoms. Long-range order results from the placement of the two benzyl substituents on opposite sides of the benzimidazole moiety, with their aromatic rings approximately parallel to the surface. The organization of three surface-bound benzyl substituents from three different NHCs into a triangular array controls the formation of a highly ordered Kagome-like surface lattice. By comparison with earlier studies of NHCs on Cu(111), we show that the binding geometry and self-assembly of NHCDBZ are influenced by intermolecular and adsorbate–substrate interactions and facilitated by the flexibility of the methylene linkage between the N-heterocycle and the aromatic wingtip substituents.

Interfacial Polarization of Thin Alq3, Gaq3, and Erq3 Films on GaN(0001).

This report presents results of research on electronic structure of three interfaces composed of organic layers of Alq3, Gaq3, or Erq3 deposited on GaN semiconductor. The formation of the interfaces and their characterization have been performed in situ under ultrahigh vacuum conditions. Thin layers have been vapor-deposited onto p-type GaN(0001) surfaces. Ultraviolet photoelectron spectroscopy (UPS) assisted by X-ray photoelectron spectroscopy (XPS) has been employed to construct the band energy diagrams of the substrate and interfaces. The highest occupied molecular orbitals (HOMOs) are found to be at 1.2, 1.7, and 2.2 eV for Alq3, Gaq3, and Erq3 layers, respectively. Alq3 layer does not change the position of the vacuum level of the substrate, in contrast to the other layers, which lower it by 0.8 eV (Gaq3) and 1.3 eV (Erq3). Interface dipoles at the phase boundaries are found to be −0.2, −0.9, −1.2 eV, respectively, for Alq3, Gaq3, Erq3 layers on GaN(0001) surfaces.

On the formation of Si nanowires by molecular beam epitaxy

Abstract Silicon nanowires can be successfully grown by applying the vapor – liquid – solid process. In the case of the commonly used chemical vapor deposition technique, a Si containing gas/precursor is cracked at Au droplets acting as seeds. Si adatoms are subsequently dissolved in the liquid metal. Due to a supersaturation within this droplet, Si precipitates predominantly at the liquid – solid interface – a nanowire grows. A different situation occurs if nanowires are grown by molecular beam epitaxy via the vapor– liquid – solid mechanism. The difference consists, for example, of the role of the metal seed, the morphology of the nanowires and their aspect ratio. In particular, surface diffusion including the metal used as well as Si, strongly influences the growth process. This article describes molecular beam epitaxy growth experiments of Si nanowires under ultra-high vacuum conditions and compares the results with other growth techniques.

Microstructure and Modifications of Cu/Al2O3 Interfaces

Abstract A combined approach of high-resolution transmission electron microscopy and electron energy-loss near-edge structure studies was employed to determine the atomic structure and bonding mechanisms at Cu/Al2O3 interfaces in dependence on the α-Al2O3 substrate orientation. The investigated specimens were prepared by molecular beam epitaxy using ultrahigh vacuum conditions, which led to atomically abrupt interfaces. The results show that intermetallic Cu–Al bonds occur at the investigated Cu/ (112̅0)Al2O3 interface, while ionic-covalent bonding contributions are observed at the Cu/(0001)Al2O3 interface. The interfacial microstructure of diffusion-bonded Cu/ (0001)Al2O3 samples was changed by annealing treatments under various oxygen partial pressures. Annealing resulted in the formation of a CuAlO2 reaction phase at the interface between Cu and Al2O3.

The Effect of Ambient Conditions on the Potential Screening at Ionic Liquid – Electrode Interfaces

In comparison to the clean conditions of ultrahigh vacuum (UHV), we investigate the influence of ambient conditions on the potential screening (PS) at the interfaces of Au and Pt electrodes with the ionic liquids [C8C1Im][Tf2N] and [C8C1Im]Cl. Our study is based on a proof-of-principle experiment that shows that PS measurements performed by XPS yield the same results as measurements using a specific 3-electrode sample holder setup, both in UHV. Based on this, we compare PS measurements under ultraclean conditions by XPS with PS measurements with a 3-electrode setup in N2 and in air. We demonstrate that there is indeed an influence of the ambient gas atmosphere, which depends on applied voltage and combination of IL and electrode material. For Pt electrodes, there is a pronounced influence of the ambient conditions on the PS for both [C8C1Im][Tf2N] and [C8C1Im]Cl, while for Au electrodes hardly any influence is seen. We attribute the observed effects mainly to water affecting the electrical double layers at the IL/electrode interfaces. In addition, we observe that reaction products formed under faradaic conditions can lead to a contamination of the 3rd electrode (reference electrode), yielding time-dependent deviations from the true behavior.

Laser-induced thermal source for cold atoms

We demonstrate a simple and compact approach to laser cool and trap atoms based on laser-induced thermal ablation (LITA) of a pure solid granule. A rapid thermalisation of the granule leads to a fast recovery of the ultra-high vacuum condition required for a long trapping lifetime of the cold gas. We give a proof-of-concept of the technique, performing a magneto-optical trap on the 461 nm ^1S_0rightarrow ,^1P_1 transition of strontium. We get up to 3.5 million of cold strontium-88 atoms with a trapping lifetime of more than 4 s. The lifetime is limited by the pressure of the strontium-free residual background vapour. We also implement an original configuration of permanent magnets to create the quadruple magnetic field of the magneto-optical trap. The LITA technique can be generalized to other atomic elements such as transition metals and lanthanide atoms, and shows a strong potential for applications in quantum technologies ranging from quantum computing to precision measurements such as outdoor inertial sensing.

Fineline circuits realization with liquid photoresist and DMD-based photolithographic technique for space electronics applications

Background: High-density interconnect (HDI) printed circuit board (PCB) packaging technology has gained greater significance in military and aerospace sectors. Manufacturing with fineline features and the impedance-controlled circuit pattern is a major technical challenge for which several methods have been reported. Aim: Herein, a novel process of HDI PCB fabrication has been presented using liquid photosensitive resist spray coating followed by digital micromirror device (DMD)-based micro-opto-electromechanical direct imaging photolithography to realize fineline circuit pattern. Approach: A fineline HDI circuit has been developed with minimum conductor trace width of <40 μm and spacing from the copper foil (18-μm thickness) cladded to dielectric substrate that comply the requirements of high reliability space electronics packaging. Such fineline conductor features have been exemplified with optical microscopy, SEM, and subjected to micro-sectioning. Results: The resulting undercut values and etched pattern have been found to be excellent. Leveraging the standard test method, the peel strength values of the realised fineline circuit patterns were measured to be more than 1.2 kg / cm2 required for high reliability space electronics packaging applications. Conclusions: The output paraments meet the space high-reliability requirements withstanding the stringent environmental conditions of ultra-high vacuum, vibration, and the extreme temperatures for longer durations.

Изменение электронной структуры поверхности GaN/Si(111) при адсорбции Li

The electronic structure of the epitaxial GaN/Si(111) layers and the Li/GaN/Si(111) interface with a monolayer Li coverages has been studied in situ under ultrahigh vacuum conditions. The experiments were carried out using photoelectron spectroscopy with synchrotron radiation in the photon energy range 75 eV – 850 eV. The photoemission spectra in the valence band and the core levels of Ga 3d, N 1s, and Li 1s are studied for the monolayer Li coating. It is found that Li adsorption causes a significant decrease in the intensity of the photoemission line of the intrinsic surface state and the appearance of an induced surface state due to charge transfer between the adsorbed Li layer and surface Ga atoms. It has been found that the GaN/Si(111) surface has predominantly Ga polarity. The Li/GaN/Si(111) interface has a semiconductor character.

Defects of thin CaO(001) on Mo(001): an EPR spectroscopic perspective.

Paramagnetic defects of thin CaO(001) films grown on Mo(001) are characterized using electron paramagnetic resonance (EPR) spectroscopy under ultrahigh vacuum conditions. A variety of paramagnetic centers located in the volume of the films are identified whose speciation as well as relative abundance was found to depend on the growth rate of the films. Pristine films prepared at a lower growth rate exhibited a larger number and a different speciation of paramagnetic defects than films grown at a higher rate. Annealing of the films to 1030 K, which improves their long-range order, results in quenching of most of the paramagnetic species observed for the pristine film; however, films prepared at a lower growth rate exhibit new paramagnetic signals upon annealing, which are absent in films prepared at a higher growth rate. The signals can be assigned to paramagnetic Mo ions previously shown to diffuse into these films. These results indicate that the amount and speciation of the transition metal ions depend on the preparation conditions which in turn can also affect the surface chemistry of these systems.

Электронная структура наноинтерфейса K/ALN

The electronic structure of the AlN surface and the ultrathin K/AlN interface was studied using in situ photoelectron spectroscopy under ultra-high vacuum conditions. Core level spectra from the N 1s, Al 2p and K 3p and from the valence band were studied for the clean AlN surface and for the K/AlN interface in the regime of K submonolayer coatings. During K adsorption, significant changes in all the spectra were found. Surface states in the valence band region below the EVBM were found. It was determined that the K/AlN interface has the semiconductor-like character.

Changes in the electronic properties of the GaN/Si(111) surface under Li adsorption

The electronic structure of the epitaxial GaN/Si(111) layers and the Li/GaN/Si(111) interface with a monolayer Li coverage has been studied in situ under ultrahigh vacuum conditions. The experiments were carried out using photoelectron spectroscopy with synchrotron radiation in the photon energy range 75-850 eV. The photoemission spectra in the valence band and the core levels of Ga 3d, N 1s, and Li 1s are studied for the monolayer Li coating. It is found that Li adsorption causes a significant decrease in the intensity of the photoemission line of the intrinsic surface state and the appearance of an induced surface state due to charge transfer between the adsorbed Li layer and surface Ga atoms. It has been found that the GaN/Si(111) surface has predominantly Ga polarity. The Li/GaN/Si(111) interface has a semiconductor character. Keywords: III-nitrides, electronic structure, metal-GaN interface, photoelectron spectroscopy.

Electrodeposition of Indium from an Ionic Liquid Investigated by In Situ Electrochemical XPS

The electrochemical behavior and electrodeposition of indium in an electrolyte composed of 0.1 mol/L InCl3 in 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py1,4]TFSI) on a gold electrode were investigated. The cyclic voltammogram revealed several reduction and oxidation peaks, indicating a complex electrochemical behavior. In the cathodic regime, with the formation of an In-Au alloy, the reduction of In(III) to In(I) and of In(I) to In(0) takes place. In situ electrochemical X-ray photoelectron spectroscopy (XPS) was employed to investigate the reduction process by monitoring the oxidation states of the components during the cathodic polarization of 0.1 mol/L InCl3/[Py1,4]TFSI on a gold working electrode under ultra-high vacuum (UHV) conditions. The core electron binding energies of the IL components (C 1s, O 1s, F 1s, N 1s, and S 2p) shift almost linearly to more negative values as a function of the applied cell voltage. At −2.0 V versus Pt-quasi reference, In(I) was identified as the intermediate species during the reduction process. In the anodic regime, a strong increase in the pressure in the XPS chamber was recorded at a cell voltage of more than −0.5 V versus Pt quasi reference, which indicated, in addition to the oxidation reactions of In species, that the oxidation of Cl− occurs. Ex situ XPS and XRD results revealed the formation of metallic In and of an In-Au alloy.

Abiotic Formation of an Amide Bond via Surface-Supported Direct Carboxyl-Amine Coupling.

Amide bond formation is one of the most important reactions in biochemistry, notably being of crucial importance for the origin of life. Herein, we combine scanning tunneling microscopy and X-ray photoelectron spectroscopy studies to provide evidence for thermally activated abiotic formation of amide bonds between adsorbed precursors through direct carboxyl-amine coupling under ultrahigh-vacuum conditions by means of on-surface synthesis. Complementary insights from temperature-programmed desorption measurements and density functional theory calculations reveal the competition between cross-coupling amide formation and decarboxylation reactions on the Au(111) surface. Furthermore, we demonstrate the critical influence of the employed metal support: whereas on Au(111) the coupling readily occurs, different reaction scenarios prevail on Ag(111) and Cu(111). The systematic experiments signal that archetypical bio-related molecules can be abiotically synthesized in clean environments without water or oxygen.