Application of atomic force microscopy to microbial surfaces: from reconstituted cell surface layers to living cells.

The application of atomic force microscopy (AFM) to probe the ultrastructure and physical properties of microbial cell surfaces is reviewed. The unique capabilities of AFM can be summarized as follows: imaging surface topography with (sub)nanometer lateral resolution; examining biological specimens under physiological conditions; measuring local properties and interaction forces. AFM is being used increasingly for: (i) visualizing the surface ultrastructure of microbial cell surface layers, including bacterial S-layers, purple membranes, porin OmpF crystals and fungal rodlet layers; (ii) monitoring conformational changes of individual membrane proteins; (iii) examining the morphology of bacterial biofilms, (iv) revealing the nanoscale structure of living microbial cells, including fungi, yeasts and bacteria, (v) mapping interaction forces at microbial surfaces, such as van der Waals and electrostatic forces, solvation forces, and steric/bridging forces; and (vi) probing the local mechanical properties of cell surface layers and of single cells.     

Reflection of a slow cesium atomic beam from a naturally magnetized Nd-Fe-B surface

We have demonstrated the partly directed reflection of a slow cesium atomic beam by using the natural magnetic stray field above a Nd-Fe-B surface. From these experiments we determine the reflectivity and a minimum value for the magnetic stray field directly at the surface. Received: 5 July 1999 / Revised version: 6 October 1999 / Published online: 3 November 1999     

Lobular scattering of heavy molecules from low mass surface layers; Br2 from a graphite covered surface

The scattering of Br₂ molecules from thin graphite layers on platinum foils has been studied during UHV conditions with a room temperature, almost thermal Br₂ molecular beam. The angular variation of Br⁺₂ and Br⁺ signals at surface temperatures of 1300–1600 K has been measured with a mass spectrometer. The scattering pattern for Br⁺₂ shows a main lobe at angles 30–40° from the normal, when the specular direction is at 60–70°. The best but not satisfactory fit of a hard cube model is found with a surface mass equal to the mass of four to five C atoms. This result is not easily interpreted. Since multiple collisions with several C atoms in the surface are expected, “cube” models are really not applicable in their usual form, which is only valid for single collisions with the surface. To circumvent the problem, calculations have been made with a novel phonon transfer model. Good agreement with experiments is found, assuming transfer of one single perpendicular phonon in the graphite layer to translation of the Br₂ molecule in the normal direction. The maximum phonon energy in graphite corresponds to 1900 K. This is much larger than the Debye temperature in most metals (θ_D = 310 K for W), which makes it understandable that one-phonon processes can dominate in the scattering.     

Dispersion relations of surface phonons from atomic scattering

We show the existence of a spatial forbidden zone for one-phonon atomic scattering by surfaces. In case of scattering by surface modes, this region is bounded by sharp observable maxima of the scattering intensity, whose angular locations depend only on the dispersion relation of the modes involved. A method for obtaining the dispersion relation of surface modes needing no measurement of scattering energies then follows. The method is applied to the He-NaF system.     

Ion scattering from nanodimensional surface layers of emitter structures

The elemental composition of gallium arsenide-based outer emitter monolayers with oxygencesium films that have negative electron affinity is investigated via the scattering low-energy ions, along with the charge state of atoms during the deposition of cesium and oxygen. The physical mechanism for the formation of negative affinity is considered.     

Selective reflection of laser radiation from submicron layers of Rb and Cs atomic vapors: Applications in atomic spectroscopy

We studied selective reflection (SR) of laser radiation from a window of a nanocell with thickness L ~ λ_1,2/2 filled with Rb and Cs atoms, where λ₁ = 780 nm and λ₂ = 852 nm are the wavelengths resonant with the D₂ laser lines for Rb and Cs, respectively. It is demonstrated that the negative derivative of the SR signal profile for L > λ/2 changes to the positive one for L < λ/2. It is shown that the real-time formation of the SR signal profile derivative (SRD) with the spectral width 30–40 MHz and located at the atomic transition is, in particular, a convenient frequency marker of D₂ transitions in Rb and Cs. The amplitudes of SRD signals are proportional to the atomic transition probabilities. A comparison with the known saturated absorption (SA) method demonstrated a number of advantages, such as the absence of cross-over resonances in the SRD spectrum, the simplicity of realization, a low required power, etc. An SRD frequency marker also operates in the presence of the Ne buffer gas at a pressure of 6 Torr, which allowed us to determine the Ne–Rb collisional broadening, whereas the SA method is already inapplicable at buffer gas pressures above 0.1 Torr. The realization simplicity makes the SRD method a convenient tool for atomic spectroscopy. Our theoretical model well describes the SRD signal.     

Hydrogen desorption from tantalum with segregated oxide surface layers

The combination of thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES) was used to examine the correlation between elementary processes on an oxidized tantalum surface and the acceleration of the dehydriding rates at increasing temperatures. We present the TDS of TaD_x foils (foil thickness ~ 25μm) for various initial concentrations (0.038 ⩽ x ⩽ 0.108; α-phase), various heating rates and for high and ultra high vacuum conditions. Activation energies for desorption were derived from the TDS spectra on the assumption that the rate determining process is the recombination of hydrogen atoms to hydrogen molecules at the surface. We find two different activation energies for the low and for the high temperature region. The AES measurements show that the corresponding change in the desorption process is correlated with the dissolution of the segregated oxide surface layer.     

Structure of Al2O3 thin layers synthesized on the silicon surface by atomic layer deposition

The distribution of the phase and chemical composition at an Al₂O₃/Si interface is studied by depth-resolved ultrasoft x-ray emission spectroscopy. The interface is formed by atomic layer deposition of Al₂O₃ films of various thicknesses (from several to several nanometers to several hundreds of nanometers) on the Si(100) surface (c-Si) or on a 50-nm-thick SiO₂ buffer layer on Si. L _2,3 bands of Al and Si are used for analysis. It is found that the properties of coatings and Al₂O₃/Si interfaces substantially depend on the thickness of the Al₂O₃ layer, which is explained by the complicated character of the process kinetics. At a small thickness of coatings (up to 10–30 nm), the Al₂O₃ layer contains inclusions of oxidized Si atoms, whose concentration increases as the interface is approached. As the thickness increases, a layer containing inclusions of metallic Al clusters forms. A thin interlayer of Si atoms occurring in an unconventional chemical state is found. When the SiO₂ buffer layer is used (Al₂O₃/SiO₂/Si), the structure of the interface and the coating becomes more perfect. The Al₂O₃ layer does not contain inclusions of metallic aluminum, does not vary with the sample thickness, and has a distinguished boundary with silicon.     

Electron-stimulated desorption of cesium atoms from cesium layers deposited on a germanium-coated tungsten surface

The yield and energy distribution of Cs atoms from cesium layers adsorbed on germanium-coated tungsten were measured, using the time-of-flight technique with a surface-ionization-based detector, as a function of the energy of bombarding electrons, germanium film thickness, the amount of adsorbed cesium, and substrate temperature. The threshold for the appearance of Cs atoms is ～30 eV, which correlates well with the germanium 3d-level ionization energy. As the electron energy increases, the Cs atom yield passes through a broad maximum at ～120 eV. For germanium film thicknesses from 0.5 to 2 monolayers, resonance Cs yield peaks were observed at electron energies of 50 and 80 eV, which can be related to the tungsten 5p and 5s core-level ionization energies. As the cesium coverage increases, the Cs atom yield passes through a flat maximum at monolayer coverage. The energy distribution of Cs atoms follows a bell-shaped curve. With increasing cesium coverage, this curve shifts to higher energies for thin germanium films and to lower energies for thick films. The Cs energy distribution measured at a substrate temperature T = 160 K exhibits two bell-shaped peaks, namely, a narrow peak with a maximum at ～0.35 eV, associated with tungsten core-level excitation, and a broad peak with a maximum at ～0.5 eV, deriving from the excitation of the germanium 3d core level. The results obtained can be described within a model of Auger-stimulated desorption.     

Perturbation of atomic energy levels by a metal surface

The energy level shifts of one-electron atomic particles H, He⁺, Li⁺⁺, etc. which interact with a metal surface have been investigated. In the approximation of image charges, an operator describing perturbations of atomic levels has been obtained. By numerically solving the Schro dinger equation, we have calculated energy levels of H(1s), H*(n=2), and C⁵⁺(n) as functions of the distance between an atom and surface. Asymptotic behavior of atomic levels at large distances from the surface has been studied. The linear Stark effect for excited states, which was earlier mentioned by A. V. Chaplik, has been found and investigated in detail. Zh. éksp. Teor. Fiz. 116, 236–256 (July 1999)     

Narrow-band N-resonance formed in thin rubidium atomic layers

The narrow-band N-resonance formed in a ?? system of D ₁-line rubidium atoms is studied in the presence of a buffer gas (neon) and the radiations of two continuous narrow-band diode lasers. Special-purpose cells are used to investigate the dependence of the process on vapor column thickness L in millimeter, micrometer, and nanometer ranges. A comparison of the dependences of the N-resonance and the electromagnetically induced transparency (EIT) resonance on L demonstrates that the minimum (record) thickness at which the N-resonance can be detected is L = 50 ??m and that a high-contrast EIT resonance can easily be formed even at L ?? 800 nm. The N-resonance in a magnetic field for ⁸⁵Rb atoms is shown to split into five or six components depending on the magnetic field and laser radiation directions. The results obtained indicate that levels F _g = 2, 3 are initial and final in the N-resonance formation. The dependence of the N-resonance on the angle between the laser beams is analyzed, and practical applications are noted.     

Pd layers on a W(100) surface

Pd films from zero to several monolayers in thickness on a W(100) surface are studied by AES, LEED, work function change (A0) measurements and TDS. Similar to other metals on W, the adsorption and annealing behaviour differs drastically from that on the W(110) surface but resembles the behaviour of other metals on the W(100) and Mo(100) surface.     

Diffraction of fast atomic projectiles during grazing scattering from a LiF(001) surface

Light atoms and molecules with energies from 300 eV to 25 keV are scattered under a grazing angle of incidence from a LiF(001) surface. For impact of neutral projectiles along low index directions for strings of atoms in the surface plane we observe a defined pattern of intensity spots in the angular distribution of reflected particles which is consistently described using concepts of diffraction theory and specific features of grazing scattering of atoms from insulator surfaces. Experimental results for scattering of H, D, 3He, and 4He atoms as well as H2 and D2 molecules can be unequivocally referred to atom diffraction with de Broglie wavelengths as low as about 0.001 Angstroms.     

Role of the curvature of atomic layers in electron field emission from graphitic nanostructured carbon

Layers of oriented carbon nanotubes and nanometer-size plate-shaped graphite crystallites are obtained by chemical vapor deposition in a glow-discharge plasma. A structural-morphological investigation of a carbon material consisting of nanotubes and nanocrystallites is performed, and the field-emission properties of the material are also investigated. It is shown that electron field emission is observed in an electric field with average intensity equal to or greater than 1.5 V/μm. The low fields giving rise to electron emission can be explained by a decrease in the electronic work function as a result of the curvature of the atomic layers of graphitic carbon. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 5, 381–386 (10 March 1999)     

Adsorption compression in surface layers

A recently elucidated aspect of adsorption, compression in confined phases, is discussed. Grand Canonical Monte Carlo simulations were performed for the adsorption of Lennard–Jones molecules and new details of intermolecular interactions in adsorbed layers are analysed. It is shown that a strong attraction to a surface can cause adsorption compression not only in the first layer, but also in higher layers. Compression of the first layer creates a pattern of active sites; the second layer tends to be commensurate with this pattern and has density higher than that of a ‘free’ layer. This pattern propagates to higher layers. However, there is a wide range of chemical potentials where the first layer is compressed and the second layer is not yet formed. It was found that transition to adsorption compression results in oscillations of the isosteric heat of adsorption. These oscillations are determined by a combination of (a) changes in adsorbed layers’ structure and (b) exchange of molecules between layers. In particular, at high affinity to adsorbent, the adsorption isotherm for the first layer has a slight maximum because an increase of the chemical potential causes molecules to leave the compressed first layer and move to the second layer. For this reason, the isosteric heat of adsorption decreases and can become negative. Analysis of adsorption compression mechanisms in the context of theory and emerging experimental results indicates that the significance of this phenomenon is not limited to fundamental aspects of adsorption and capillarity. These mechanisms play a crucial role in various applications, such as heterogeneous catalysis, membrane separations, and self-assembly on surfaces. Results are discussed in a broader context of theory, experiments and previous simulations.     

Interfacial analysis of InP surface preparation using atomic hydrogen cleaning and Si interfacial control layers prior to MgO deposition

The objective of this study is to investigate how the surface characteristics of indium phosphide (InP) can be modified through the use of atomic hydrogen (H^*) cleaning and silicon interfacial control layers (Si ICL), prior to the deposition of MgO dielectric layers. X-ray photoelectron spectroscopy (XPS) analysis shows that the InP native oxide can be successfully removed using atomic hydrogen cleaning at a substrate temperature of 300 °C. However, atomic force microscopy (AFM) images display evidence for the growth of metallic In island features after H^* cleaning, and subsequent deposition of MgO thin films on the H^* cleaned surface resulted in high levels of interfacial indium oxide growth. It has also been shown that the deposition of thin (∼1 nm) Si layers on InP native oxide surfaces results in the transfer of oxygen from the InP substrate to the Si ICL and the formation of Si-InP bonds. XPS analysis indicates that MgO deposition and subsequent 500 °C annealing results in further oxidation of the Si layer. However, no evidence for the re-growth of interfacial In or P oxide species was observed, in contrast to observations on the H^* cleaned surface.     

Nonlinear dynamics of incommensurate surface layers

We describe analytically the nonlinear dynamics of the incommensurate surface layer ("self-modulated" system) with a spatially periodical structure. In the framework of the Frenkel-Kontorova model the nonlinear excitations of the periodic soliton lattice, such as moving additional kinks and gap solitons, are investigated.