Scanning tunneling microscopy of adsorbates on insulating films. From the imaging of individual molecular orbitals to the manipulation of the charge state

Ultrathin insulating films on metal substrates are unique systems for using a scanning tunneling microscope to study the electronic properties of single atoms and molecules that are electronically decoupled from the metallic substrate. Individual gold atoms on an ultrathin insulating sodium chloride film supported by a copper surface exhibit two different charge states, which are stabilized by the large ionic polarizability of the film. The charge state and associated physical and chemical properties such as diffusion can be controlled by adding or removing a single electron to or from the adatom with a scanning tunneling microscope tip. The simple physical mechanism behind the charge bistability in this case suggests that this is a common phenomenon for adsorbates on polar insulating films. In the case of molecules on ultrathin NaCl films, the electronic decoupling allows the direct imaging of the unperturbed molecular orbitals, as will be shown in the case of individual pentacene molecules. PACS 68.37.Ef; 73.61.Ng; 73.20.Hb     

Enhanced dielectric constant resolution of thin insulating films by electrostatic force microscopy

Electrostatic force microscopy has been shown to be a useful tool to determine the dielectric constant of insulating films of nanometer thicknesses that play a key role in many electrical, optical and biological phenomena. Previous approaches have made use of simple analytical formulas to analyze the experimental data for thin insulating films deposited directly on a metallic substrate. Here we show that the sensitivity of the EFM signal to changes in the dielectric constant of the thin film can be enhanced by using dielectric substrates with low dielectric constants. We present detailed numerical calculations of the tip-sample electrostatic interaction in the following setup: an insulating thin film, a dielectric substrate (or spacing layer) of known low dielectric constant and a metallic electrode. The EFM sensitivity to the dielectric constant increases with the thickness of the spacing layer and saturates for thicknesses above 100-300 nm, when it is close to that of an infinite medium.     

Signatures of irradiation-induced defects in scanning-tunneling microscopy images of carbon nanotubes

Using empirical-potential and tight-binding models, we study the structure and stability of irradiation-induced atomic-scale defects in the walls of carbon nanotubes. We model the temporal evolution of such defects and calculate their lifetimes at various temperatures. We also simulate scanning-tunneling microscopy (STM) images of irradiated nanotubes with such defects. Our simulations indicate that, at low temperatures, the defects live long enough to be detected by STM and that different defects manifest themselves in STM images in different ways, which allows one to distinguish the defects experimentally.     

Transmission electron microscopy imaging of individual functional groups of fullerene derivatives

Mobility and reactivity of the functionalized fullerenes with pyrrolidine (C60-C3NH7) incorporated in single-wall carbon nanotubes were examined by high-resolution transmission electron microscopy. An individual functional group attached to each fullerene cage is unambiguously visualized. This provides a direct evidence for the functionalized structure on a single-molecular basis. A rotational motion of the incorporated molecules tends to occur during the observation and, consequently, each fullerene molecule is likely to stand facing its functionalized group towards the nanotube wall. A fine structure analysis of electron energy-loss spectra for the nitrogen K(1s) edge shows a considerable change in the nitrogen chemical state and suggests a strong tube-fullerene interaction.     

Conformation effects on the molecular orbitals of serine

This paper calculates the five most stable conformers of serine with Hartree--Fock theory, density functional theory (B3LYP), Moller--Plesset perturbation theory (MP4(SDQ)) and electron propagation theory with the 6-311++G(2d,2p) basis set. The calculated vertical ionization energies for the valence molecular orbitals of each conformer are in agreement with the experimental data, indicating that a range of molecular conformations would coexist in an equilibrium sample. Information of the five outer valence molecular orbitals for each conformer is explored in coordinate and momentum spaces using dual space analysis to investigate the conformational processes, which are generated from the global minimum conformer Ser1 by rotation of C₂--C₃ (Ser4), C₁--C₂ (Ser5) and C₁--O₂ (Ser2 and Ser3). Orbitals 28a, 27a and 26a are identified as the fingerprint orbitals for all the conformational processes.     

Construction of molecular orbitals localized on polyatomic fragments

A simple method of localizing molecular orbitals on polyatomic molecular fragments is proposed; the method allows one to separate orbitals in the structural units of extended molecules. The method is illustrated by semiempirical calculations of the binuclear bridged complexes [(NH₃)₅Ru-py-(C₂H₂)_n- py-Ru(NH₃)₅]⁵⁺ (n = 0,1,2,3). One of possible application is construction of orbital bases for calculations by the configuration interaction method with limited sets of active MOs. Translated from ZhurnalStruktumoi Khimii, Vol. 39, No. 4, pp. 571–578, July–August, 1998.     

Ultrahigh-spatial-resolution field-emission projection microscopy of insulating samples

The method of ultrahigh-spatial-resolution field-emission projection imaging of nonconducting tips is implemented experimentally. An image of a glass microcapillary tip was obtained for the first time by the nonscanning method with spatial resolution no worse than 20 nm.     

Visualization of electron orbitals in scanning tunneling microscopy

Scanning tunneling microscopy (STM) is one of the main techniques for direct visualization of the surface electronic structure and chemical analysis of multi-component surfaces at the atomic scale. This review is focused on the role of the tip orbital structure and tip-surface interaction in STM imaging with picometer spatial resolution. Fabrication of STM probes with well-defined structure and selective visualization of individual electron orbitals in the STM experiments with controlled tunneling gap and probe structure are demonstrated.     

Topographic study by scanning-tunneling microscopy of a two-dimensional graphite film on (10\bar 10)

Two-dimensional graphite films on $(10\bar 10)$ were produced in ultrahigh vacuum by adsorption of benzene vapor on the metal heated to T=1800 K. High-resolution Auger spectroscopy used for the film characterization showed the film indeed to have graphitic structure and monolayer thickness. The surface topography was studied in air by scanning-tunneling microscopy. The monolayer thickness was confirmed, and it was shown that a two-dimensional graphite film has a complex topography featuring numerous hillocks with linear dimensions of ～3000 Å and height differences of ～300 Å, while retaining graphitic structure on the atomic scale. The lack of planarity of such a film at room temperature is considered to be due to deformation occurring under cooling from the temperature of formation down to 300 K, which is caused by the difference in thermal expansion coefficients between the graphite sheet and rhenium.     

Three-dimensional molecular imaging using mass spectrometry and atomic force microscopy

We combine imaging ToF-SIMS depth profiling and wide area atomic force microscopy to analyze a test structure consisting of a 300 nm trehalose film deposited on a Si substrate and pre-structured by means of a focused 15-keV Ga⁺ ion beam. Depth profiling is performed using a 40-keV C₆₀⁺ cluster ion beam for erosion and mass spectral data acquisition. A generic protocol for depth axis calibration is described which takes into account both lateral and in-depth variations of the erosion rate. By extrapolation towards zero analyzed lateral area, an “intrinsic” depth resolution of about 8 nm is found which appears to be characteristic of the cluster-surface interaction process.     

Thermal diffusivity measurement of thermally conducting films on insulating substrates

We report thermal diffusivity measurements of thin Niobium films on glass substrates, using OptoThermal Transient Emission Radiometry (OTTER). The result is a thermal diffusivity value ofD=(2.79±0.36)×10^–5 m²/s, which is within 17% of the accepted value for multicrystaline bulk niobium. The technique is remote sensing, non-destructive, and the measurements depend only on the thickness of the film and the thermal properties of the substrate.     

Electrostatic force microscopy on oriented graphite surfaces: coexistence of insulating and conducting behaviors

We present measurements of the electric potential fluctuations on the surface of highly oriented pyrolytic graphite using electrostatic force and atomic force microscopy. Micrometric domainlike potential distributions are observed even when the sample is grounded. Such potential distributions are unexpected given the good metallic conductivity of graphite because the surface should be an equipotential. Our results indicate the coexistence of regions with "metalliclike" and "insulatinglike" behaviors showing large potential fluctuations of the order of 0.25 V. In lower quality graphite, this effect is not observed. Experiments are performed in Ar and air atmospheres.     

Plasmonics-based spatially activated light microscopy for super-resolution imaging of molecular fluorescence

In this Letter, we explore plasmonics-based spatially activated light microscopy (PSALM) for sub-diffraction-limited imaging of biomolecules. PSALM is based on the spatially switched activation of local amplified electromagnetic hot spots under multiple light incidence conditions. The hot spots are associated with surface plasmons that are excited and localized by surface nanostructures. The feasibility of the concept was demonstrated by imaging fluorescent nanobeads on a two-dimensional gold nanograting of a 100-nm-wide grating ridge, the size of which is the measure of the imaging resolution. The result confirms the performance of PSALM for imaging nanobeads at a resolution below the conventional diffraction limit.     

Electrical aging of insulating films in MIS structures

Insulating films of thickness 10²–10⁴ Å are used in microelectronic devices for insulation and charge storage and control. The basic electrical parameters of such films are the electrical strength and the charge. Real MIS and MIM systems made by the best methods are essentially inhomogeneous objects with numerous macroscopic and microscopic defects, which occur not only within the insulator (pores, cracks, inclusions, and so on) but also at the metal-insulator interface (microscopic ridges on the electrodes). Therefore, a major problem is to examine the behavior of insulating film in relation to time in strong electric fields, at high temperatures, with high humidity, and other such factors in the environment. This enables one to evaluate the degree of defectiveness in actual MIS systems and to identify defects during electrical aging.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 116–118, July, 1976.     

Nondestructive Raman and atomic force microscopy measurement of molecular structure for individual diphenylalanine nanotubes

Polarized Raman microspectroscopy and atomic force microscopy were used to measure molecular orientation in individual diphenylalanine nanotubes (diameters ranging from 100 nm to 1000 nm). Analysis of the amide I Raman bands (1686 cm(-1)) indicated that the C=O side chains have a parallel alignment with the nanotube axis. The amide III Raman band (1249 cm(-1)) associated with the peptide backbone C-N vibrations showed that these bonds are preferentially aligned perpendicular to the nanotube axis. However, the Raman band corresponding to the symmetric breathing mode of the aromatic rings (1002 cm(-1)) indicated a rather random orientation. These results support the theoretical molecular structure models proposed recently.