Three-dimensional field ion mass spectrometry

The three-dimensional atom probe (3 D-AP) is a new variant of the field ion microscope (FIM) combined with a time of flight mass spectrometer and single ion detection sensitivity (imaging atom probe). With the field ion microscope the topology of a surface, surface reactions and surface modifications can be studied in atomic detail. Using time of flight measurements, surface layers and interface layers can be chemically analyzed atom by atom and atomic layer by atomic layer. This three-dimensional atom probe permits the elemental reconstruction of a small volume of the specimen with near atomic resolution. This improvement is obtained by using the digitized video signal of the imaging atom probe detector and a separate time signal from the phosphor screen to achieve simultaneously the x and y position and the mass-to-charge ratio of individual ions striking the detector. Examples from a study on high speed steel are presented to demonstrate the usefulness of a recently built instrument.     

Quantitative investigations of atomic processes on metal surfaces at atomic resolution

Using the atomic resolution of the field-ion microscope (FIM) and the single-atom identification capability of the time-of-flight atom-probe field-ion microscope, fundamental properties of solid surfaces can be investigated at atomic level. Atomic structures of many surface planes are fully resolved. The chemical composition of a surface can be determined with a single atomic layer spatial resolution. Several interactions of single surface atoms, which are basic to the understanding of many surface phenomena, such as the atom to substrate surface interaction, the adatom-adatom interaction, the adatom-impurity atom interaction, the adatom-plane edge interaction, etc. can all be studied in atomic details. The charge distribution of a single atom, as manifest in its dipole moment and polarizability, can also be studied. Great advantages of FIM investigations include the availability of atomically perfect surface planes, which can be prepared by field evaporation process, and the capability of a direct observation of the atomic images. Although some electronegative atoms cannot be imaged, the surface conditions and the state of the atoms can be characterized by the atom-probe. Basic principles and results of these single-atom FIM studies are presented and discussed.     

Adsorption of phytosterol ethoxylates on silica in an aprotic room-temperature ionic liquid

The adsorption of a nonionic surfactant at a silica/room-temperature ionic liquid interface has been characterized on the basis of analytical data obtained through a combination of surface force measurements, in situ soft-contact atomic force microscope (AFM) images, and quartz crystal microbalance with dissipation monitoring (QCM-D) data. The surfactant employed in this study is a kind of phytosterol ethoxylate (BPS-20), and the ionic liquid selected here is aprotic 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide (EmimTFSI). This ionic liquid spontaneously forms solvation layers on silica, being composed of an Emim(+) cation layer and EmimTFSI ion pair layers. The addition of BPS-20 disrupts these solvation layers and suggests a surfactant layer adsorbed at the interface. This is the first report demonstrating the adsorption of nonionic surfactants at the solid/aprotic ionic liquid interface.     

Atomic, molecular and cluster dynamics on flat and stepped surfaces

Surface diffusion is responsible for transport of atoms at the surface. It is therefore essential for understanding many surface phenomena where transport of atoms and molecules are involved. Using the field ion microscope and scanning tunneling microscope, we have directly observed many elementary surface atomic and molecular processes at terraces and steps and have measured the activation barrier heights of these processes, and have also studied their atomic mechanisms in detail. For Si(111) surface, transport of atoms above 450°C appears to be achieved by magic clusters of about 10–15 atoms in size, and their diffusion behavior is very different from those of individual atoms and molecules. We discuss how atom dynamics affects the growth behavior, island shape transitions, and growth modes in the growth of crystals and epitaxial films.     

Temperature-programmed ESDIAD/TOF system as a new technique for characterization of adsorbed molecules and reaction intermediates

A temperature-programmed (TP-) electron stimulated desorption ion angular distribution (ESDIAD)/time-of-flight (TOF) system was developed to monitor changes in adsorbed layers in real time. This system can measure ESDIAD images and TOF spectra of desorbing ions stimulated by pulsed electron beam in temperature programmed surface processes. The coadsorption layers of CO and ammonia, the coadsorption layers of CO and methylamine, and the coadsorption layer of CO and acetylene on Ru(001), which were previously studied by our group using LEED, TPD and HREELS, have been observed by this system. The change in configuration of admolecules, the change in site occupation and the thermal evolution of adspecies can be monitored.     

3D-Atom-probe study of oxygen-adsorption on stepped platinum surfaces

Oxygen adsorbed on Pt (011), (012) and (113) has been studied by means of field desorption in a three-dimensional atom-probe. The formal procedure of these experiments is quite similar to thermal desorption. Comprehensive investigations with the thermal desorption technique report on a highly stable atomic oxygen phase on Pt, which has been attributed to a subsurface oxygen layer or platinum oxide. In atom-probe depth profiling experiments oxygen was measured only in the topmost layer. Oxygen and platinum have been detected almost as single atomic ions, Pt-oxygen complexes have not been observed in the time-of-flight mass spectra. The 2D plots of the impact positions of the desorbed ions as well as field ion micrographs show most of the oxygen adsorbed in highly co-ordinated sites on the (012) and (113) planes.     

Helium atom scattering from oxide surfaces.

Many experimental methods of surface science employ electrons or photons of considerable incident energy as probe particles. However, insulating surfaces or delicate physisorbed layers may be damaged by these particles and should, therefore, be analyzed with a gentler probe: He atom scattering allows to determine the symmetry of the unit cell and the detection of phase transitions from diffraction measurements as well as the determination of surface and adsorbate vibrations by time-of-flight resolved detection. Herein, the application of He atom scattering to oxide surfaces is demonstrated on the basis of the examples of MgO and ZnO. MgO(001) is a very inert and stable surface, whereas hydrogen atoms are chemisorbed on the mixed-terminated ZnO(1010) and on both polar faces: ZnO(0001) and ZnO(0001). He atom scattering is very sensitive to the presence of hydrogen on surfaces. In addition ZnO reacts with molecules such as water, CO and CO(2). It is demonstrated that in combination with photoelectron spectroscopy and thermal desorption spectroscopy He atom scattering can also contribute to studies of surface chemistry.     

Sputter‐deposited Ni/Ti double‐bilayer thin film and the effect of intermetallics during annealing

In the present study, a double bilayer of a Ni/Ti thin film was investigated. A nanoscale NiTi thin film is deposited in a Ni–Ti–Ni–Ti manner to form a double‐bilayer structure on a Si(100) substrate. Ni and Ti depositions were carried out by using d.c. and r.f. power, respectively, in a magnetron sputtering chamber. Four types of bilayers are formed by varying the deposition time of each layer (i.e. 15, 20, 25, and 30 min). The as‐deposited amorphous thin films were annealed at 300, 400, 500, and 600 °C for 1 h to achieve the diffusion in between the layers. Microstructures were analyzed using field‐emission scanning electron microscope and high‐resolution transmission electron microscope. It was found that, with the increase in annealing temperature from 300 to 600 °C, the diffusion at the interface and atomic migration on the surface increase. Cross‐sectional micrographs exhibited the interdiffusion between the two‐layer constituents, especially at higher temperatures, which resulted in diffusion patches along the interface. Phase analyses, performed by grazing incidence X‐ray diffraction, showed the formation of intermetallic compounds with some silicide phases that enhance the mechanical properties. Nanoindentation and atomic force microscopy were carried out to know the mechanical properties and surface profiles of the films. The surface finish is better at higher annealing temperatures. It was found that for annealing temperatures varying from 300 to 600 °C, the increase in annealing temperature resulted in a gradual increase in atomic‐cluster coarsening with improved adatom mobility. Copyright © 2016 John Wiley & Sons, Ltd.     

Combining electrochemistry and direct force measurements: from the control of surface properties towards applications

Direct force measurements contributed in the last years much to our understanding of the diffuse double layer of charged interfaces in electrolyte solutions. Such measurements have been performed with the atomic force microscope or the surface force apparatus. This review gives an overview over the recent studies based on force measurements with electrode surfaces. Not only bare metal electrodes but also electrodes modified by different organic layers, including electroactive films, have been studied by these techniques. Direct force measurements indicate that further effects besides classical Gouy–Chapman–Stern theory have to be taken into consideration in order to describe the force profiles. In addition to the long-range forces also the adhesion between surfaces can be tuned by potentiostatic control. New single-molecule techniques based on the atomic force microscope allow to probe the extension of polymer strands or their desorption from solid interfaces. In combination with electrochemistry, it became now possible to tune the desorption behavior of polymer strands or to measure the electromechanical coupling of motors from single strands of electroactive polymers.     

Time-of-flight secondary ion mass spectroscopy analysis of Na adatoms interacting with water-ice film

The origins of a slow reaction rate between the sodium adatoms and the water-ice film have been investigated by analyzing the surface composition using time-of-flight secondary ion mass spectroscopy in the temperature range of 13-230 K. An unhydrated NaOH layer is formed at the water-Na interface at 13 K which is followed by the growth of the metallic Na layer, whereas domains of both NaOH and unreacted Na are created only in the multilayer regime at 100 K. The NaOH layer plays a role as a separator between the water and Na layers, and its poor solubility in water is responsible for the small reaction rate of Na on glassy water. The solubility of NaOH in the deeply supercooled liquid water is low as well, but the mobile water molecules diffusing to the surface react with the Na adatoms, thereby quenching the growth of the metallic Na overlayer.     

Surface‐interface microstructures and binding strength of cathodic arc ion plated TiCN coatings on YT14 cutting tools

A titanium carbonitride (TiCN) coating was deposited on YT14 cutting tool by using a CAIP (cathodic arc ion plating). The surface‐interface morphologies, chemical compositions, and phases of TiCN coatings were observed by using a FESEM (field emission scanning electron microscopy), EDS (energy dispersive spectroscopy), XRD (X‐ray diffraction), respectively, and the bonding energy, surface roughness, and bonding strength were characterized with an XPS (X‐ray photoelectron spectroscopy), AFM (atomic force microscope), and scratch test, respectively. The results show that the phases of the TiCN coating are primarily composed of TiN, TiC, and amorphous C, of which the TiC and TiN increases the coating hardness, and the amorphous C atom improves friction and lubrication properties of the coating. The effect of CAIP on the topography of the TiCN coating is at nano‐scale, the Ti and N atoms are enriched in the coating at the bonding interface, and the part of chemical elements are diffused in the gradual transformation layer. The bonding form of the TiCN coating interface is primarily composed of mechanical combination, accompanying with slight metallurgical combination, and the bonding strength is characterized with 60.85 N by scratch test. Copyright © 2016 John Wiley & Sons, Ltd.     

Misfit-Dislocation Induced Surface Morphology of InGaAs/GaAs Heterostructures

The correlation between the surface cross-hatched morphology and the interfacial misfit dislocations in partially relaxed InGaAs/GaAs heterostructures was studied by means of atomic force microscopy and electron-beam induced current mode in a scanning electron microscope. A close correspondence between the misfit-dislocation network at the interface and the surface morphology shows that the cross-hatch development results primarily from the misfit-dislocation generation. Statistical analysis of the surface roughness reveals an anisotropy in strain relaxation of the epitaxial layers, which results from an asymmetry in the misfit-dislocation formation.     

Computational insights into the molecular mechanisms for chromium passivation of stainless-steel surfaces

First principles DFT calculations are used to gain insights into the molecular mechanism of Cr passivation of FeCr alloy surfaces. The systems studied represent early stages of oxidation of FeCr alloys when the oxide layers extend just a few atomic layers into the bulk. A Monte-Carlo atom-swapping algorithm was developed to efficiently explore possible atomic positions and identify the most promising structures that yield overall energy lowering. Analysis of the resulting low energy structures show that the surface oxide layer is rich in chromium while there is a reduction in chromium in the metallic phase near the alloy-oxide interface. Furthermore, there is an increased concentration of Fe near the oxide-air surface. Analysis of the molecular structure of the oxide layers found that oxidized Cr was predominantly in the Cr₂O₃ phase, while oxidized Fe was present as both FeO and Fe₂O₃. We propose that the oxidative variability of Fe facilitates O diffusion in the iron-rich phases because of the range of geometries available for accommodating the O atom. In contrast, O diffusion is less facile in Cr, which has little variability in oxidation state.     

Probing liquid surfaces under vacuum using SEM and ToF-SIMS

We report a newly developed self-contained interface for high-vapor pressure liquid surfaces to vacuum-based analytical instruments. It requires no wires or tubing connections to the outside of the instrument and uses a microfluidic channel with a 3 μm diameter window into the flowing fluid beneath it. This window supports the liquid against the vacuum by the liquid's surface tension and limits the high-density vapor region traversed by the probe beams to only a few microns. We demonstrate this microfluidic interface for in situ liquid surfaces in a time-of-flight secondary ion mass spectrometer (ToF-SIMS) and a scanning electron microscope (SEM) with chemical analysis.     

Large- and small-angle ion-scattering spectroscopy for studying cold, clean and gas-covered metal surfaces

New possibilities of the large-angle and small-angle ion scattering spectroscopy are demonstrated. If the target material consists of two or more isotope components it was found that the ratio of isotope parts of the quasi-single-scattering peak reaches a maximum at some incidence angle. The maximum is attributed to the fact that the ions doubly scattered from the atoms with smaller masses can have the same energy and exit angle as the ions singly scattered from the atoms with larger masses. Based on the atomic pair model (shadowing atom plus scattering atom) the mean distance between two atoms of the topmost atomic layer can be found. In the range of small scattering angles the substrate quasi-single-scattering peak as well as the recoil peak show a fine structure if the cold metal surface is covered by a thin film of condensed gases. It is postulated that such effect is connected with a multiple scattering of projectiles and recoils inside the condensed film. Multiple scattering direct the ions to the detector after single scattering from the substrate atom over the angles larger or smaller that the detection angle. This effect can be used for determination of a cleanliness of the target surface or parameters of the condensed layer (thickness or density).     

An in situ STM/AFM and impedance spectroscopy study of the extremely pure 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate/Au(111) interface: potential dependent solvation layers and the herringbone reconstruction

The structure and dynamics of the interfacial layers between the extremely pure air- and water-stable ionic liquid 1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl)trifluorophosphate and Au(111) has been investigated using in situ scanning tunneling microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and atomic force microscopy measurements. The in situ scanning tunnelling microscopy measurements reveal that the Au(111) surface undergoes a reconstruction, and at -1.2 V versus Pt quasi-reference the famous (22 × √3) herringbone superstructure is probed. Atomic force microscopy measurements show that multiple ion pair layers are present at the ionic liquid/Au interface which are dependent on the electrode potential. Upon applying cathodic electrode potentials, stronger ionic liquid near surface structure is detected: both the number of near surface layers and the force required to rupture these layers increases. The electrochemical impedance spectroscopy results reveal that three distinct processes take place at the interface. The fastest process is capacitive in its low-frequency limit and is identified with electrochemical double layer formation. The differential electrochemical double layer capacitance exhibits a local maximum at -0.2 V versus Pt quasi-reference, which is most likely caused by changes in the orientation of cations in the innermost layer. In the potential range between -0.84 V and -1.04 V, a second capacitive process is observed which is slower than electrochemical double layer formation. This process seems to be related to the herringbone reconstruction. In the frequency range below 1 Hz, the onset of an ultraslow faradaic process is found. This process becomes faster when the electrode potential is shifted to more negative potentials.     

Unprecedented covalently attached ATRP initiator onto OH-functionalized mica surfaces

Mica substrates were activated by a plasma method leading to OH-functionalized surfaces to which an atom transfer radical polymerization (ATRP) radical initiator was covalently bound using standard siloxane protocols. The unprecedented covalently immobilized initiator underwent radical polymerization with tert-butyl acrylate, yielding for the first time end-grafted polymer brushes that are covalently linked to mica. The initiator grafting on the mica substrate was confirmed by time-of-flight secondary ion mass spectrometry (TOF-SIMS), while the change in the water contact angle of the OH-activated mica surface was used to follow the change in surface coverage of the initiator on the surface. The polymer brush and initiator film thicknesses relative to the virgin mica were confirmed by atomic force microscopy (AFM). This was done by comparing the atomic step-height difference between a protected area of freshly cleaved mica and a zone exposed to plasma activation, initiator immobilization, and then ATRP.     

Dynamic forces between bubbles and surfaces and hydrodynamic boundary conditions

A bubble attached to the end of an atomic force microscope cantilever and driven toward or away from a flat mica surface across an aqueous film is used to characterize the dynamic force that arises from hydrodynamic drainage and electrical double layer interactions across the nanometer thick intervening aqueous film. The hydrodynamic response of the air/water interface can range from a classical fully immobile, no-slip surface in the presence of added surfactants to a partially mobile interface in an electrolyte solution without added surfactants. A model that includes the convection and diffusion of trace surface contaminants can account for the observed behavior presented. This model predicts quantitatively different interfacial dynamics to the Navier slip model that can also be used to fit dynamic force data with a post hoc choice of a slip length.     

First‐principle investigation of the relaxed structure and electronic properties of the Cu(110) vicinal surface

We performed first‐principle calculations, by employing the ultrasoft pseudopotential plane‐wave (UPPW) method, for the multilayer relaxation and the local density of states (LDOS) of Cu(430) and its corresponding flat Cu(110) surfaces. The shift directions of the atoms in the terrace (layers 1–4), sub‐terrace (layers 5–7) and third‐terrace (layers 8–11) of the Cu(430) surface are similar to the layers 1, 2 and 3 of the Cu(110) surface, respectively. And the stepped surface changes to a flatter version with a thin compacted layer, including the topmost seven atomic layers and separating slightly from the underneath atomic layers. Because of a reduction in the coordination numbers (CNs),there is a sharp rise in the higher energy region and a slight reduction in the lower energy region of the LDOSfor the topmost three layer atoms of the Cu(430) surface and the first layer atom of the Cu(110) surface. The shape of the LDOS of the corresponding layer with the same CN is similar for both the surfaces. Copyright © 2009 John Wiley & Sons, Ltd.     

Using (18)O/(16)O exchange to probe an equilibrium space-charge layer at the surface of a crystalline oxide: method and application

The use of an (18)O/(16)O exchange experiment as a means for probing surface space-charge layers in oxides is examined theoretically and experimentally. On the basis of a theoretical treatment, isotope penetration profiles are calculated for (18)O/(16)O exchange across a gas-solid interface and subsequent diffusion of the labelled isotope through an equilibrium space-charge layer depleted of mobile oxygen vacancies and into a homogeneous bulk phase. Profiles calculated for a range of conditions all have a characteristic shape: a sharp drop in isotope fraction close to the surface followed by a normal bulk diffusion profile. Experimental (18)O profiles in an exchanged (001) oriented single crystal of Fe-doped SrTiO(3) were measured by time-of-flight secondary ion mass spectrometry (ToF-SIMS). By extracting the space-charge potential from such profiles, we demonstrate that this method allows the spatially resolved characterization of space-charge layers at the surfaces of crystalline oxides under thermodynamically well-defined conditions.