Contact scanning near-field optical microscopy

A new method of scanning in near-field optical microscopy, which makes it possible to operate in contact with the experimental sample, is proposed and implemented. This method permits the practical utilization of the idea of using the dipole-dipole resonance transfer of excitation energy from the active element of the microscope to the sample for achieving a fundamental improvement in the resolution of near-field optical microscopy. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 4, 245–250 (25 February 1998)     

Theory of scanning near-field magnetooptical microscopy

An analytical theory of scanning near-field magnetooptical microscopy is developed. The theory is based on the elastic scattering of light by small, resonantly polarizable particles, which are used to scan the plane surface of a nonuniformly magnetized medium. The effective polarizability of the particles is calculated with the effect of dynamic “image forces” taken into account in all orders of perturbation theory with respect to the interaction of the particle with a demagnetized ferromagnet, and the magnetooptical perturbation is calculated to first order in the magnetization. The major contributions to the magnetooptical light scattering for a ferromagnetic structure magnetized perpendicular to the surface are found, including a quasistatic approximation for the near-field particle-magnet interaction. The optical size resolution of a magnetic (dielectric) inhomogeneity is estimated. Zh. Tekh. Fiz. 68, 86–91 (July 1998)     

Characterization of superconducting YBaCuO-films by low temperature scanning electron microscopy

Low-temperature scanning electron microscopy has been performed for imaging the spatial distribution of the critical current densityj _c(x,y) and of the critical temperatureT _c(x,y) in polycrystalline superconducting YBaCuO films. Strongly inhomogeneous behavior has been observed, and the spatial resolution limit has been found to be 1–2 m. The local temperature increment in the specimen film caused by the electron beam scanning has been demonstrated experimentally as the underlying mechanism of the imaging principle, and the beam-induced thermal perturbation of the high-T _c film/substrate configuration is discussed in detail. The radiation hardness of the sample films against the electron beam irradiation in our imaging experiments has been evaluated. No radiation damage could be detected up to the maximum applied dose of well above 10²⁰ electrons/cm² for a typical beam energy of 26 keV.     

Annealing effects on the microstructure of sputtered gold layers on oxidized silicon investigated by scanning electron microscopy and scanning probe microscopy

The structure of Au layers deposited by sputtering on oxidized p-type Si(100) substrates is investigated by a combination of scanning electron microscopy and scanning probe microscopy. The effect of the temperature on the grain structure of the layers has been determined, revealing that an annealing temperature of 300° C results in a larger grain size and smoother surfaces but generates some cracks in the film surface. At an annealing temperature of 500° C, further grain growth is observed, but a high density of cracks and voids also results while there is little enhancement regarding the smoothness of the grain surfaces.     

Network topology and subgap resonances observed by Fourier transform scanning tunnelling microscopy of cuprate high-temperature superconductors

Fourier transform scanning tunnelling microscopy (STM) on Bi₂Sr₂CaCu₂O_{8+ δ} (BSCCO) subgap resonances has deciphered an octet of ‘quasiparticle’ states that are consistent with the Fermi surface and energy gap observed by angle-resolved photoemission spectroscopy (ARPES), but the origin of the high-intensity k-space octets and the sharply defined r-space chequerboard is unexplained. The filamentary ferroelastic nanodomain model that predicted the r-space chequerboard also explains the k-space octets and the origin of the apparent anisotropic surface d-wave gap by using strong electron–phonon interactions outside the CuO₂ planes. The topological model identifies the factors that stabilize high-intensity k-space octets in the presence of a very high level of irregular r-space chequerboard noise.     

Synchrotron scanning photoemission microscopy of homogeneous and heterogeneous metal sulfide minerals

Scanning photoemission microscopy (SPEM) has been applied to the investigation of homogeneous and heterogeneous metal sulfide mineral surfaces. Three mineral samples were investigated: homogeneous chalcopyrite, heterogeneous chalcopyrite with bornite, and heterogeneous chalcopyrite with pyrite. Sulfur, copper and iron SPEM images, i.e. surface‐selective elemental maps with high spatial resolution acquired using the signal from the S 2p and Cu and Fe 3p photoemission peaks, were obtained for the surfaces after exposure to different oxidation conditions (either exposed to air or oxidized in pH 9 solution), in addition to high‐resolution photoemission spectra from individual pixel areas of the images. Investigation of the homogeneous chalcopyrite sample allowed for the identification of step edges using the topography SPEM image, and high‐resolution S 2p spectra acquired from the different parts of the sample image revealed a similar rate of surface oxidation from solution exposure for both step edge and a nearby terrace site. SPEM was able to successfully distinguish between chalcopyrite and bornite on the heterogeneous sample containing both minerals, based upon sulfur imaging. The high‐resolution S 2p spectra acquired from the two regions highlighted the faster air oxidation of the bornite relative to the chalcopyrite. Differentiation between chalcopyrite and pyrite based upon contrast in SPEM images was not successful, owing to either the poor photoionization cross section of the Cu and Fe 3p electrons or issues with rough fracture of the composite surface. In spite of this, high‐resolution S 2p spectra from each mineral phase were successfully obtained using a step‐scan approach.     

Study of resonant tunneling in Au nanoclusters on the surface of SiO2/Si thin films using the combined scanning tunneling microscopy and atomic-force microscopy technique

The morphology and electronic properties of Au nanoclusters on the surface of SiO₂ thin films on n ⁺-Si substrates are studied using the combined scanning tunneling microscopy (STM) and atomic-force microscopy (AFM) technique. The peaks associated with the resonant tunneling of electrons from the states of the valence band of the probe material to the states of the conduction band of the substrate material through Au nanoclusters are observed on the current-voltage characteristics for the contact of a p ⁺-Si AFM probe with Au nanoclusters. Experimental results are interpreted by calculating the tunnel transparency of the SiO₂/Au/SiO₂ double barrier structure in a strong electric field.     

Hollow-tip scanning photoelectron microscopy

A new type of microscopy based on scanning in vacuum by a beam of charged particles transmitted through a hollow probe has been implemented. This approach provides controllable motion of spatially localized ion, electron, molecular (atomic), and soft X-ray beams and investigation of the surface in the shear force mode. In the photoelectron mode, in which electrons are transmitted through a 2-μm quartz capillary, a surface profile of gadolinium irradiated by 400-nm femtosecond laser pulses has been visualized with a subwave spatial resolution. The new method of microscopy opens an opportunity of investigations in the field of nanometer local photodesorption of molecular ions (one of the last ideas of V.S. Letokhov).     

Investigation of the electronic properties of Au nanoclusters in SiO<Subscript>2</Subscript> by combined scanning tunneling/atomic force microscopy

The tunneling of electrons through Au nanoc lusters formed by pulsed laser deposition in a SiO₂ thin film on a Si substrate has been investigated by combined scanning/atomic force microscopy (STM/AFM). Conducting Pt-coated Si cantilevers were used. The feedback was maintained via the AFM channel, and the current-voltage (I-V) characteristics of the tunnel contact between the AFM probe and the n ⁺-Si substrate through a =4-nm-thick SiO₂ film with Au nanoclusters =2 nm in diameter were measured simultaneously. The current image of the structure contained areas of increased current (tunnel-current channels) 2–15 nm in size, related to tunneling of electrons through Au nanoclusters in SiO₂. The I-V characteristics recorded in the tunnel-current channels exhibit specific features related to the Coulomb blockade of electron tunneling through Au nanoclusters.     

Energy loss of 1000 keV electrons in thin silicon crystals as measured by high voltage electron microscopy

Abstract

Energy loss spectra of 1000 keV electrons transmitted by [111]-: riented thin silicon crystals were observed by an energy analyzer attached to the HVEM. The crystals were set to the systematic 220 Bragg reflection. Measurements were made for crystal thickness ranging from 1000 to 10,000 Å, which were determined by observations of pendellösung fringes.

Results were analyzed with Landau's transport equation, giving the : onclusion that the loss probability, which is the reciprocal of the mean free path, is 0.52 ± 0.02 × 10^?3 A^?1 for plasmon excitation and 1.50 ± 0.02 × 10^?3 A^?1 for L-electron excitation.     

Some aspects of scanning electron microscopy

The main components of the SEM are the signal generation and the signal detection and display systems. A finely focussed electron beam scans the specimen surface. Due to the interaction between electrons and solid, signals such as secondary emission, light, X-radiation and currents are generated. Depending on the selected detection system the displayed images provide a variety of information, such as topography, element distribution, surface voltage, luminescence, conductivity, etc. The useful application of the SEM ranges from surface study to bulk analysis to device fabrication.     

Electron multiplicity in slow collisions of Ar ions with C

Electron capture by Ar⁸⁺ in collisions with C₆₀ fullerene has been investigated using coincident measurements of the number n of ejected electrons, the mass and charge of multicharged Cr+ ₆₀ recoil ions and their fragments Ci+ m and the final charge state of outgoing projectiles Ar(8-s)+ (). The number of captured electrons r is the sum of the numbers of stabilized and emitted electrons: r = n + s. The ratio n / s decreases by a factor three with s increasing from 1 to 7 showing that the multiply excited states populated by capture of a large number of electrons are rather stable against auto-ionisation. Each kinetic energy spectrum of Ar⁺ and Ar²⁺ projectiles is composed of two peaks which we attribute to collisions “inside” and “outside” the C₆₀ cage. The measured energy shift of the projectile keV is consistent with the corresponding energy loss keV in a carbon foil with an equivalent thickness. Inside collisions are characterized by a strong dissociation of recoil ions into light monocharged fragments and by a high multiplicity of ejected electrons. Received: 25 March 1998 / Received in final form and Accepted: 9 June 1998     

High-resolution microscopy of plasmon field distributions by scanning tunneling luminescence and photoemission electron microscopies

The exploitation of plasmon resonances to promote the interaction between conjugated molecules and optical fields motivates intensive research. The objectives are to understand the mechanisms of plasmon-mediated interactions, and to realize molecularly- or atomically-precise metal nanostructures, combining field enhancements and optical antenna effects. In this review paper, we present examples of plasmonic-field mappings based on scanning tunneling microscope (STM)-induced light emission or multiphoton photoemission (PEEM), two techniques among those which offer todayʼs best spatial resolutions for plasmon microscopy. An unfamiliar property of the junction of an STM is its ability to behave as a highly localized source of light. It can be exploited to probe optoelectronic properties, in particular plasmonic fields, with ultimate subnanometer spatial resolution, an advantage balanced by a sometimes delicate deconvolution of local-probe influence. Alternatively, local-probe disadvantages can be overcome by imaging the photoemitted electrons, using well-established electron optics. This allows obtaining two-dimensional intensity maps reflecting the unperturbed distribution of the optical near field. This approach provides full field spectroscopic images with a routine spatial resolution of the order of 20 nm (down to 5 nm with recent aberration corrected instruments).     

Effect of backscattered electrons on the analysis area in scanning Auger microscopy

A simple analytical model has been used to determine the effects of backscattered electrons on the analysis area in scanning Auger microscopy. For normally incident electrons, the radius r_a of the analysis area is calculated corresponding to detection of 80, 90, and 95% of the total Auger-electron signal as a function of two sample parameters, the backscattering factor R and the Gaussian parameter σ_b describing the radial distribution of the backscattered electrons. For a reasonable range of these parameters, r_a depends linearly on σ_b and to a lesser extent on R. Values of r_a can also be appreciably larger, by more than a factor of 100, than the widths of the incident beam in modern instruments, and need to be considered in quantitative analyses of particles and inclusions. Monte-Carlo calculations are needed for more realistic evaluations of the analysis area and to determine this area for non-normal incidence of the electron beam.     

Theory of light rectification in scanning tunneling microscopy in the presence of an adsorbed molecule

We consider the problem of the rectified current induced by laser radiation in the STM junction when the tip is placed above a small molecule like CO or NO. This is calculated assuming a simple tight-binding model for the tunneling junction including the adsorbate and using nonequilibrium Green's functions techniques. The coupling between tunneling electrons and the molecule vibrational modes is taken into account by a local electron-phonon interaction term. In a second step we estimate the excitation rate of the molecule vibrations for a given laser power. This value is then used to obtain the relative change in the rectified current when the laser is in resonance with a molecule vibration. For a moderate laser power of 2 kW/cm² a relative change of 1 to 3% is predicted.     

The effects of nonequilibrium charge distribution in scanning tunneling spectroscopy of semiconductors

Results are presented from a low-temperature scanning tunneling microscopy (STM) investigation of III-V semiconductor surfaces cleaved in situ along a (110) plane. The STM topographic images reveal the presence of surface charge structures. The possibility of their observation depends on the charge state of the apex of the STM tip. Peaks are also observed in the local tunneling conductivity spectra. The energy position of these peaks and the energy position of the edges of the band gap change with distance from the defect. A theoretical model is proposed which demonstrates that the experimental scanning tunneling spectroscopy (STS) data can be explained by effects due to a nonequilibrium electron distribution in the contact area, which gives rise to localized charges. In this model the on-site Coulomb repulsion of localized charges and their interaction with semiconductor electrons are treated self-consistently. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 4, 299–304 (25 August 1998)     

Electric quadrupole interactions and the γ–α phase transition in Ce: the role of conduction electrons

We present a theoretical model of the “isostructural" - phase transition in Ce which is based on quadrupolar interactions due to coupled charge density fluctuations of 4f electrons and of conduction electrons. The latter are treated in tight-binding approximation. The - transition is described as an orientational ordering of quadrupolar electronic densities in a structure. The quadrupolar order of the conduction electron densities is complementary to the quadrupolar order of 4f electron densities. The inclusion of conduction electrons leads to an increase of the lattice contraction at the - transition in comparison to the sole effect of 4f electrons. We calculate the Bragg scattering law and suggest synchrotron radiation experiments in order to check the structure. Received 21 September 1999 and Received in final form 2 May 2000     

Convoy electrons produced at glancing angle scattering of MeV HeH+ ions from a crystal surface

Abstract

Convoy electrons produced at glancing angle scattering of MeV HeH⁺ ions from an atomically clean (001) surface of SnTe crystal are observed. Energy spectrum of the convoy electrons shows a peak broader than that at scattering of atomic projectiles and the most probable energy of convoy electrons at HeH⁺ scattering is larger than those at scattering of isotachic He ions. This acceleration of convoy electrons is qualitatively explained by the force due to surface wake induced by Coulomb exploding fragment He²⁺ and H⁺.     

Drag effects in a two-layer system of spatially separated electrons and excitons

We discuss drag effects in a two-layer system of spatially separated electrons and excitons: the entrainment of excitons by moving electrons, and the entrainment of electrons by moving excitons. For the case of excitons entrained by electrons we find the drag velocity υ _drag, and for electrons entrained by excitons we compute the induced electric field E ₂. These drag effects can be sensitive indicators of the phase state of the excitons and of phase transitions in the exciton system (to a liquid phase, superfluid state, etc.) Zh. éksp. Teor. Fiz. 111, 1107–1119 (March 1997)