Showing papers on "Scanning tunneling spectroscopy published in 2006"

Persistent superconductivity in ultrathin Pb films: a scanning tunneling spectroscopy study.

Abstract: By using a low temperature scanning tunneling microscope we have probed the superconducting energy gap of epitaxially grown Pb films as a function of the layer thickness in an ultrathin regime (5-18 ML). The layer-dependent energy gap and transition temperature (Tc) show persistent quantum oscillations down to the lowest thickness without any sign of suppression. Moreover, by comparison with the quantum-well states measured above Tc and the theoretical calculations, we found that the Tc oscillation correlates directly with the density of states oscillation at E(F) . The oscillation is manifested by the phase matching of the Fermi wavelength and the layer thickness, resulting in a bilayer periodicity modulated by a longer wavelength quantum beat.

Manipulating kondo temperature via single molecule switching

Abstract: Two conformations of isolated single TBrPP-Co molecules on a Cu(111) surface are switched by applying +2.2 V voltage pulses from a scanning tunneling microscope tip at 4.6 K. The TBrPP-Co has a spin-active cobalt atom caged at its center, and the interaction between the spin of this cobalt atom and free electrons from the Cu(111) substrate can cause a Kondo resonance. Tunneling spectroscopy data reveal that switching from the saddle to a planar molecular conformation enhances spin-electron coupling, which increases the associated Kondo temperature from 130 to 170 K. This result demonstrates that the Kondo temperature can be manipulated just by changing molecular conformation without altering chemical composition of the molecule.

Electrochemistry at single-walled carbon nanotubes: the role of band structure and quantum capacitance.

Abstract: We present a theoretical description of the kinetics of electrochemical charge transfer at single-walled carbon nanotube (SWNT) electrodes, explicitly taking into account the SWNT electronic band structure. SWNTs have a distinct and low density of electronic states (DOS), as expressed by a small value of the quantum capacitance. We show that this greatly affects the alignment and occupation of electronic states in voltammetric experiments and thus the electrode kinetics. We model electrochemistry at metallic and semiconducting SWNTs as well as at graphene by applying the Gerischer−Marcus model of electron transfer kinetics. We predict that the semiconducting or metallic SWNT band structure and its distinct van Hove singularities can be resolved in voltammetry, in a manner analogous to scanning tunneling spectroscopy. Consequently, SWNTs of different atomic structure yield different rate constants due to structure-dependent variations in the DOS. Interestingly, the rate of charge transfer does not necessar...

Identification of color centers on MgO(001) thin films with scanning tunneling microscopy

Abstract: Localized electronic defects on the surface of a 4 monolayer (ML) thin MgO(001) film deposited on Ag(001) have been investigated by low-temperature scanning tunneling microscopy and spectroscopy. Depending on the location of the defect, we observe for the first time different defect energy levels in the band gap of MgO. The charge state of defects can be manipulated by interactions with the scanning tunneling microscope tip. Comparison with ground state energy levels of color centers on the MgO surface obtained from embedded cluster calculations corroborates the assignment of the defects to singly and doubly charged color centers.

Handbook of Applied Solid State Spectroscopy

Abstract: Nuclear Magnetic Resonance Spectroscopy. -Introduction. -Properties Of Nuclear Spins. -Nuclear Spin Interactions in Solids. -Quantum Mechanical Calculations. -High Resolution Solid-State NMR Methods. -Principles of 2-Dimensional Spectroscopy. -Molecular Dynamics and Local Molecular Conformation in Solid Materials. -Nuclear Quadrupole Resonance Spectroscopy. -Introduction. -Basic Theory. -Instrumentation. -Interpretation of Coupling Constants. -Summary. -Electron Paramagnetic Resonance Spectroscopy. -Introduction. -Theoretical Background. -Experimental. -Applications of EPR Spectroscopy. ENDOR Spectroscopy. -Introduction. -Experimental Conditions for ENDOR. -ENDOR in The Solid State. -Pulsed ENDOR. -Applications. -Mossbauer Spectroscopy. -Introduction. -Methodology. -Applications. -Concluding Remarks. -Crystal-Field Spectroscopy. -Introduction. -The Crystal-Field Interaction. -Experimental Techniques. -Determination of Crystal-Field Parameters From Experimental Data. -Interactions of Crystal-Field Split Ions. -Crystal-Field Effects Related to High-Temperature Superconductivity. -Concluding Remarks. -Scanning Tunneling Spectroscopy. -Introduction. -The Scanning Tunneling Microscope (STM). -Scanning Tunneling Spectroscopy of Semiconductors & Metals. -Electron Tunneling Spectroscopy of Adsorbed Molecules. -Practical Considerations Relating to STM-IETS and STM-OMTS. -Some Concluding Points. -Resonance Acoustic Spectroscopy. -Introduction. -Scattering Of Waves. -Mathematical Models. -Method of Isolation and Identification of Resonances (MIIR). -Experimental and Numerical Results. -Fourier Transform Infrared Spectroscopy. -Introduction. -Historical Background. -FT-IR Spectroscopy. -Applications. -Auger Electron Spectroscopy. -Introduction. -Historical Perspective. -Basic Principlesof AES. -Instrumentation. -Experimental Procedures Including Sample Preparation. -Auger Spectra: Direct and Derivative Forms. -Applications of Auger Spectroscopy. -Recent Advances. -Conclusions. -X-Ray Photoelectron Spectroscopy. -Introduction And Basic Theory. -Historical Perspective. -Instrumentation. -Sample Selection and Preparation. -Spectral Analysis. -XPS Imaging. -Angle-Resolved XPS. -Recent Advances and Applications. -Conclusions. -Luminescence Spectroscopy. -Introduction. -Spontaneous Emission, Absorption And Induced Emission. -Measurements and Techniques. -Localized Systems. -Processes in Localized System Service. -Delocalized Systems. -Processes in Delocalized Systems. -Direction of Future Efforts. -Laser-Induced Fluorescence Spectroscopy. -Introduction. -Experimental Setup. -Fluorescence Spectroscopy of Minerals. -Fluorescence Spectroscopy of Surface Species and in Solid Phases. -Fluorescence Spectroscopy of Frozen Samples. -Fluorescence Spectroscopy of Non- Actinide Solid Matrices. -Outlook. -Soft X-Ray Emission and Resonant Inelastic Scattering Spectroscopy. -Introduction. -Properties of X-Ray Spectra. -Resonant Inelastic X-Ray Scattering. -Experimental Techniques. -Applications. -Summary. -Laser Raman Spectroscopy. -Introduction. -Spontaneous Raman Scattering. -Experimental Approaches. -Applications. -Conclusions and Outlook. -Polarization Spectroscopy of Ordered Samples. -Introduction. -Occurrence, Production and Optical Properties of Aligned, Solid Samples. -One-Photon Spectroscopy: Linear Dichroism. -Two-Photon Spectroscopy. -Conclusions.

Manipulation of the kondo effect via two-dimensional molecular assembly

Abstract: We report the manipulation of a Kondo resonance originating from the spin-electron interactions between a two-dimensional molecular assembly of TBrPP-Co molecules and a Cu(111) surface at 4.6 K. By manipulating nearest-neighbor molecules with a scanning tunneling microscope tip we are able to tune the spin-electron coupling of the center molecule inside a hexagonal molecular assembly in a controlled step-by-step manner. The Kondo temperature increases from 105 to 170 K with decreasing the number of nearest neighbor molecules from six to zero. The scattering of surface electrons by the molecules located at edges of the molecular layer reduces the spin-electron coupling strength for the molecules inside the layer. Investigations of different molecular arrangements indicate that the observed Kondo resonance is independent on the molecular lattice.

Stabilization mechanism of edge states in graphene

Abstract: It has been known that edge states of a graphite ribbon are zero-energy, localized eigen-states. We show that next nearest-neighbor hopping process decreases the energy of the edge states at zigzag edge with respect to the Fermi energy. The energy reduction of the edge states is calculated analytically by first-order perturbation theory and numerically. The resultant model is consistent with the peak of recent scanning tunneling spectroscopy measurements.

Oxygen-deficient line defects in an ultrathin aluminum oxide film.

Abstract: A model for the straight antiphase domain boundary of the ultrathin aluminum oxide film on the NiAl(110) substrate is derived from scanning tunneling microscopy measurements and density-functional theory calculations. Although the local bonding environment of the perfect film is maintained, the structure is oxygen deficient and possesses a favorable adsorption site. The domain boundary exhibits a downwards band bending and three characteristic unoccupied electronic states, in excellent agreement with scanning tunneling spectroscopy measurements.

Realization of a four-step molecular switch in scanning tunneling microscope manipulation of single chlorophyll-a molecules

Abstract: Single chlorophyll-a molecules, a vital resource for the sustenance of life on Earth, have been investigated by using scanning tunneling microscope manipulation and spectroscopy on a gold substrate at 4.6 K. Chlorophyll-a binds on Au(111) via its porphyrin unit while the phytyl-chain is elevated from the surface by the support of four CH3 groups. By injecting tunneling electrons from the scanning tunneling microscope tip, we are able to bend the phytyl-chain, which enables the switching of four molecular conformations in a controlled manner. Statistical analyses and structural calculations reveal that all reversible switching mechanisms are initiated by a single tunneling-electron energy-transfer process, which induces bond rotation within the phytyl-chain.

Direct observation of half-metallic energy gap in Co2MnSi by tunneling conductance spectroscopy

Abstract: Magnetic tunnel junctions with a Co2MnSi∕Al–O∕CoFe structure are prepared by magnetron sputtering and investigated with respect to the energy gap near the Fermi energy level. The plasma oxidation time for the Al–O barrier is found to affect the condition of the Co2MnSi∕Al–O interface. The optimized sample (50s oxidation time) exhibits a magnetoresistance ratio of 159% and tunneling spin polarization of 0.89 at 2K. The bias voltage dependence of tunneling conductance (dI∕dV−V) reveals a clear half-metallic energy gap at 350–400meV for Co2MnSi, with an energy separation of just 10meV between the Fermi energy and the bottom edge of conduction band.

Disorder, defects and bandgaps in ultra thin (001) MgO tunnel barrier layers

Abstract: We report scanning tunneling spectroscopy studies of the electronic structure of 1.5 to 3 nm (001) textured MgO layers grown on (001) Fe. Thick MgO layers exhibit a bulk-like band gap, approximately 5-7 eV, and sparse, localized defect states with characteristics attributable to oxygen and, in some cases, Mg vacancies. Thin MgO layers exhibit electronic structure indicative of interacting defect states forming band tails which in the thinnest case extend to approximately 0.5 V of the Fermi level. These vacancy defects are ascribed to compressive strain from the MgO/Fe lattice mismatch, accommodated as the MgO grows.

Resonant Tunneling in Truly Axial Symmetry Mn 12 Single-Molecule Magnets: Sharp Crossover between Thermally Assisted and Pure Quantum Tunneling

Abstract: Magnetization measurements of a truly axial symmetry ${\mathrm{Mn}}_{12}\mathrm{\text{\ensuremath{-}}}t\mathrm{BuAc}$ molecular nanomagnet with a spin ground state of $S=10$ show resonant tunneling. This compound has the same magnetic anisotropy as ${\mathrm{Mn}}_{12}\mathrm{\text{\ensuremath{-}}}\mathrm{Ac}$ but the molecules are better isolated and the crystals have less disorder and a higher symmetry. Hysteresis loop measurements at several temperatures reveal a well-resolved step fine structure which is due to level crossings of excited states. All step positions can be modeled by a simple spin Hamiltonian. The results establish a sharp crossover between thermally assisted and pure quantum tunneling, as had been previously predicted.

Variable Orbital Coupling in a Two-Dimensional Quantum-Dot Solid Probed on a Local Scale

Abstract: The optoelectronic properties of semiconductor quantum-dot (QD) solids depend on the electronic structure of the building blocks and their interactions. Disorder may affect the coupling on a local scale. We have measured the density of states of 2D arrays of PbSe QDs site by site using scanning tunneling spectroscopy. It markedly differs from that of isolated QDs due to electronic coupling in the array. We observe strong local variations in the coupling strength with two prototypical cases: delocalization of the conduction electrons only, and full coupling with both hole and electron delocalization over the QD sites in the array.

Interplay between structural, chemical, and spectroscopic properties of Ag ∕ Au ( 111 ) epitaxial ultrathin films: A way to tune the Rashba coupling

Abstract: An overview of structural, chemical, and electronic properties of epitaxial $\mathrm{Ag}∕\mathrm{Au}(111)$ ultrathin films investigated by scanning tunneling microscopy and scanning tunneling spectroscopy and angle-resolved photoelectron spectroscopy is presented. New insights are exhibited: (i) a short-range ordered surface reconstruction is clearly observed for deposition at $300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$; (ii) self-organized ordering of Ag islands is obtained at $T\ensuremath{\simeq}80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In this whole temperature range, the $\mathrm{Ag}∕\mathrm{Au}(111)$ interface is shown to be (nearly) abrupt. An annealing of the elaborated room temperature leads to a strong intermixing and favors the formation of a chemically disordered surface alloy. The Shockley state parameters have been fully characterized for both interfaces. Surprisingly, the Rashba parameter is shown to scale the binding energy of the surface state in both cases. A simple one-dimensional model, taking into account the exponential decrease of the surface state wave function, allows us a quantitative understanding of the evolution of the surface state parameters. Indeed, the strength of the spin-orbit splitting is shown to be proportional to the number of heavy atoms probed by the surface state wave function, revealing its atomic character. Therefore, the strength of the Rasba coupling is shown to be tuned by adjusting the number of Ag epitaxial layers or the $\mathrm{Ag}\text{\ensuremath{-}}\mathrm{Au}$ alloy composition.

Electronic transport through electron-doped metal phthalocyanine materials

Abstract: FePc, and MnPc—that were investigated as a function of potassium concentration and temperature. For all of these systems, insulating in the pristine state, we found that the electrical conductivity can be increased to a value in excess of r= 100 Scm –1 upon potassium intercalation. In this state, the conductivity of all compounds remains high at cryogenic temperatures, indicating the existence of metallic behavior. Increasing the potassium concentration further brings all MPcs back into an insulating state. We will focus on one of these molecular systems, CuPc, to address the electronic and structural properties of this compound in more detail. We discuss scanning tunneling spectroscopy (STS) experiments that show the presence of a finite density of states in the metallic state and of a gap in the insulating states, thus confirming, at a local level, the occurrence of an insulator–metal–insulator transition. We also present structural investigations demonstrating the formation of intercalated phases and Raman spectroscopy measurements, which provide an independent microscopic

Bond fluctuation of S/Se anchoring observed in single-molecule conductance measurements using the point contact method with scanning tunneling microscopy.

Abstract: Conductance was measured for the single molecules with S/Se anchoring on a Au surface using the point contact method with scanning tunneling microscopy that enables us to selectively perform a repeated analysis of a chosen target molecule. Apparent conductance changes observed in sequential measurements suggest the existence of bond fluctuation among the adsorption sites.

Scanning tunneling spectroscopy in an ionic liquid.

Abstract: Molecular redox levels can be used to modulate tunneling currents through single or small numbers of molecules and induce molecular electronic device function While most of these devices require cryogenic conditions, room temperature operation has been demonstrated by using electrochemical gating in aqueous environments The latter have, however, serious shortcomings with a view on their relatively high volatility and narrow stability ranges in terms of potential Here we report the first-time use of an ionic liquid, 1-butyl-3-methylimidazoliumhexafluorophosphate (BMI), as an electrochemical gate in a Scanning Tunneling Microscope (STM) configuration Ionic liquids are known to have a very low vapor pressure, and accessible potential ranges are in principle large, up to 6 V In a proof-of-principle experiment, we show how a heteroleptic redox-active Os bisterpyridine complex (Ossac) can be brought to exhibit both transistor and diode function in this novel environment at room temperature This renders ionic liquids an attractive gating medium for configurations where back-gating is difficult to implement (eg, break-junction techniques) or experimental conditions prohibit the use of aqueous or organic electrolyte media (vacuum or high temperatures) From an applied perspective, they represent a step toward solid-state molecular electronics with electrochemical gating

Gap-inhomogeneity-induced electronic states in superconducting Bi2Sr2CaCu2O(8+delta).

Abstract: In this Letter, we analyze, using scanning tunneling spectroscopy, the density of electronic states in nearly optimally doped Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} in zero magnetic field. Focusing on the superconducting gap, we find patches of what appear to be two different phases in a background of some average gap, one with a relatively small gap and sharp large coherence peaks and one characterized by a large gap with broad weak coherence peaks. We compare these spectra with calculations of the local density of states for a simple phenomenological model in which a 2{zeta}{sub 0} x 2{zeta}{sub 0} patch with an enhanced or suppressed d-wave gap amplitude is embedded in a region with a uniform average d-wave gap.

Detailed studies of molecular conductance using atomic resolution scanning tunneling microscopy.

Abstract: Atomically resolved scanning tunneling microscopy and spectroscopy (STM/STS) have been used to carefully examine the relationship between molecular conductivity and the adsorption state of various organic molecules on silicon surfaces. We show that the particular configuration of styrene and cyclopentene molecules on silicon affects the conductivity of the molecules. Detailed correlation of STM images with point specific current-voltage spectroscopy reveal that observed negative peaks are due to random configuration changes driven by inelastically scattered electrons and not due to tuned alignment of molecule and electrode levels. These random processes, which include molecular rearrangement, desorption, and/or decomposition occur with increasing frequency at larger voltage and current settings.

Scanning probe microscopies beyond imaging : manipulation of molecules and nanostructures

Abstract: Foreword. Preface. List of Authors. I Scanning Tunneling Microscopy-Based Approaches. Nanoscale Structural, Mechanical and Electrical Properties. 1 Chirality in 2D (Steven De Feyter and Frans C. De Schryver). 1.1 Introduction. 1.2 Chirality and STM: From 0D to 2D. 1.3 Conclusion. Acknowledgements. References. 2 Scanning Tunneling Spectroscopy of Complex Molecular Architectures at Solid/Liquid Interfaces: Toward Single-Molecule Electronic Devices (Frank Jackel and Jurgen P. Rabe) 2.1 Introduction. 2.2 STM/STS of Molecular Adsorbates. 2.3 An Early Example of STS at the Solid/Liquid Interface. 2.4 Ultrahigh Vacuum versus Solid/Liquid Interface. 2.5 Probing p-Coupling at the Single-Molecule Level by STS. 2.6 Molecular Diodes and Prototypical Transistors. 2.7 Conclusions. Acknowledgements. References. 3 Molecular Repositioning to Study Mechanical and Electronic Properties of Large Molecules (Francesca Moresco). 3.1 Introduction. 3.2 Specially Designed Molecules. 3.3 STM-Induced Manipulation. 3.4 Mechanical Properties: Controlled Manipulation of Complex Molecules. 3.5 Inducing Conformational Changes: A Route to Molecular Switching. 3.6 The Role of the Substrate. 3.7 Electronic Properties: Investigation of the Molecule-Metal Contact. 3.8 Perspectives. Acknowledgements. References. 4 Inelastic Electron Tunneling Microscopy and Spectroscopy of Single Molecules by STM (Jose Ignacio Pascual and Nicola-s Lorente). 4.1 Introduction. 4.2 Experimental Results. 4.3 Theory. 4.4 Conclusion. References. II Scanning Force Microscopy-Based Approaches. Patterning. 5 Patterning Organic Nanostructures by Scanning Probe Nanolithography (Cristiano Albonetti, Rajendra Kshirsagar, Massimiliano Cavallini, and Fabio Biscarini). 5.1 Importance of Patterning Organic Nanostructures. 5.2 Direct Patterning of Organic Thin Films. 5.3 Assembly of Organic Structures on Nanofabricated Patterns. 5.4 Outlook and Conclusions. Acknowledgements. References. 6 Dip-Pen Nanolithography (Seunghun Hong, Ray Eby, Sung Myung, Byung Yang Lee, Saleem G. Rao, and Joonkyung Jang). 6.1 Introduction. 6.2 Basics of Dip-Pen Nanolithography. 6.3 Lithographic Capability of Dip-Pen Nanolithography. 6.4 Other Applications. 6.5 Nanoscale Statistical Physics Inspired by DPN. 6.6 Conclusions. Acknowledgements. References. Mechanical Properties. 7 Scanning Probe Microscopy of Complex Polymer Systems: Beyond Imaging their Morphology (Philippe Leclere, Pascal Viville, Melanie Jeusette, Jean-Pierre Aime, and Roberto Lazzaroni). 7.1 Introduction. 7.2 Microscopic Morphologies of Multicomponent Polymer Systems. 7.3 Methodology. 7.4 Combined Analysis of Height and Phase Images. 7.5 Concluding Remarks. Acknowledgements. References. 8 Pulsed Force Mode SFM (Alexander Gigler and Othmar Marti). 8.1 Introduction. 8.2 Modes of SPM Operation. 8.3 Pulsed Force Mode. 8.4 Theoretical Description of Contact Mechanics. 8.5 AFM Measurements Using Pulsed Force Mode. 8.6 Applications of Pulsed Force Mode. 8.7 Conclusions. Acknowledgements. References. 9 Force Spectroscopy (Phil Williams). 9.1 Introduction. 9.2 Basic Experiments. 9.3 Theory. 9.4 The Ramp-of-Force Experiment. 9.5 Multiple Transition States. 9.6 Multiple Bonds. 9.7 Distributions. 9.8 Simulations. 9.9 The Future. References. Appendix. Bond Strength and Tracking Chemical Reactions. 10 Chemical Force Microscopy: Nanometer-Scale Surface Analysis with Chemical Sensitivity (Holger Scho nherr and G. Julius Vancso). 10.1 Introduction: Mapping of Surface Composition by AFM Approaches. 10.2 Chemical Force Microscopy: Basics. 10.3 Applications of CFM. 10.4 Outlook. Acknowledgements. References. 11 Atomic Force Microscopy-Based Single-Molecule Force Spectroscopy of Synthetic Supramolecular Dimers and Polymers (Shan Zou, Holger Schonherr, and G. Julius Vancso). 11.1 Introduction. 11.2 Supramolecular Interactions. 11.3 AFM-Based Single-Molecule Force Spectroscopy (SMFS). 11.4 SMFS of Synthetic Supramolecular Dimers and Polymers. 11.5 Conclusions and Outlook. Acknowledgments. References. Electrical Properties of Nanoscale Objects. 12 Electrical Measurements with SFM-Based Techniques (Pedro. J. de Pablo and Julio Gomez-Herrero). 12.1 Introduction. 12.2 SFM Tips. 12.3 Setups for Short Molecules. 12.4 Experiments with Molecular Wires (MWs). 12.5 Noncontact Experiments. References. 13 Electronic Characterization of Organic Thin Films by Kelvin Probe Force Microscopy (Vincenzo Palermo, Matteo Palma, and Paolo Samori) 13.1 Introduction. 13.2 Kelvin Probe Scanning Force Microscopy. 13.3 Interpretation of the Signal in KPFM Measurements. 13.4 Electronic Characterization of Organic Semiconductors. 13.5 KPFM of Conventional Inorganic Materials. 13.6 KPFM on Organic Monolayers, Supramolecular Systems, and Biological Molecules. 13.7 KPFM on Organic Electronic Devices. 13.8 Conclusions and Future Challenges. Acknowledgements. References. Appendix: Practical Aspects of KPFM. III Other SPM Methodologies. 14 Scanning Electrochemical Microscopy Beyond Imaging (FrancCois O. Laforge and Michael V. Mirkin). 14.1 Introduction. 14.2 SECM Principle of Operation. 14.3 Instrumentation. 14.4 Theory. 14.5 Applications. Acknowledgements. References. IV Theoretical Approaches. 15 Theory of Elastic and Inelastic Transport from Tunneling to Contact (Nicolas Lorente and Mads Brandbyge). 15.1 Introduction. 15.2 Theory of Tunneling Conductance. 15.3 Theory of Inelastic Processes in Electron Transport. 15.4 Elastic High-Transmission Regime. 15.5 Inelastic High-Transmission Regime. 15.6 Conclusions and Outlook. Acknowledgements. References. Appendix A. Appendix B. 16 Mechanical Properties of Single Molecules: A Theoretical Approach (Pasquale De Santis, Raffaella Paparcone, Maria Savino, and Anita Scipioni). 16.1 Introduction. 16.2 DNA Curvature. 16.3 DNA Flexibility. 16.4 The Worm-Like Chain Model. 16.5 DNA Persistence Length in Two Dimensions. 16.6 A Model for Predicting the DNA Intrinsic Curvature and Flexibility. 16.7 Mapping Sequence-Dependent DNA Curvature and Flexibility from Microscopy Images. 16.8 The Ensemble Curvature and the Corresponding Standard Deviation for a Segmented DNA. 16.9 The Symmetry of Palindromic DNA Images. 16.10 Experimental Evidence of DNA Sequence Recognition by Mica Surfaces. 16.11 Comparison between Theoretical and Experimental DNA Curvature and Flexibility. 16.12 Sequence-Dependent DNA Dynamics from SFM Time-Resolved DNA Images. 16.13 Conclusions. Acknowledgements. References. Index.

Spin quantum tunneling in single molecular magnets: fingerprints in transport spectroscopy of current and noise.

Abstract: We demonstrate that transport spectroscopy of single molecular magnets shows signatures of quantum tunneling at low temperatures. We find current and noise oscillations as a function of bias voltage due to a weak violation of spin-selection rules by quantum tunneling processes. The interplay with Boltzmann suppression factors leads to fake resonances with temperature-dependent position which do not correspond to any charge excitation energy. Furthermore, we find that quantum tunneling can completely suppress transport if the transverse anisotropy has a high symmetry.

Size-dependent single-particle energy levels and interparticle Coulomb interactions in CdSe quantum dots measured by scanning tunneling spectroscopy

Abstract: We report on tunneling spectroscopy measurements on colloidal CdSe quantum dots of different sizes. The size-dependent energy level structure and electron-hole Coulomb attraction in CdSe quantum dots are obtained by a combination of shell-tunneling spectroscopy and optical spectroscopy. The results are in good agreement with tight-binding calculations. The electron-electron interactions are investigated by shell-filling spectroscopy. The tunneling spectra in this regime are analyzed by solving the master equation for the electron and hole occupancy of the quantum dot.

Spin-resolved electronic structure of nanoscale cobalt islands on Cu(111).

Abstract: Using spin-polarized scanning tunneling spectroscopy, we reveal how the standing wave patterns of confined surface state electrons on top of nanometer-scale ferromagnetic Co islands on Cu(111) are affected by the spin character of the responsible state, thus experimentally confirming a very recent theoretical result. Furthermore, at the rim of the islands a spin-polarized state is found giving rise to enhanced zero bias conductance. Its polarization is opposite to that of the islands. The experimental findings are in accordance with ab initio spin-density calculations.

Tunneling spectroscopy and vortex imaging in boron-doped diamond.

Abstract: We present the first scanning tunneling spectroscopy study of single-crystalline boron-doped diamond. The measurements were performed below 100 mK with a low temperature scanning tunneling microscope. The tunneling density of states displays a clear superconducting gap. The temperature evolution of the order parameter follows the weak-coupling BCS law with Delta(0)/kBTc approximately 1.74. Vortex imaging at low magnetic field also reveals localized states inside the vortex core that are unexpected for such a dirty superconductor.

Effect of tip geometry on contrast and spatial resolution of the near-field microwave microscope

Abstract: The near-field scanning microwave microscope (NSMM) can quantitatively image materials properties at length scales far shorter than the free space wavelength (λ). Here we report a study of the effect of tip geometry on the NSMM signals. This particular NSMM utilizes scanning tunneling microscopy (STM) for distance-following control. We systematically examined many commercially available STM tips and found them to have a conical structure on the macroscopic scale, with an embedded sphere (of radius rsphere) at the apex of the tip. The rsphere values used in the study ranged from 0.1to12.6μm. Tips with larger rsphere show good signal contrast [as measured by the frequency shift (Δf) signal between tunneling height and 2μm away from the sample] with NSMM. For example, the tips with rsphere=8μm give signal contrast of 1000kHz compared to 85kHz with a tip of rsphere=0.55μm. However, large rsphere tips distort the topographic features acquired through STM. A theoretical model is used to understand the tip-to-sa...

Monolayer to multilayer nanostructural growth transition in N-type oligothiophenes on Au(111) and implications for organic field-effect transistor performance.

Abstract: The evolution in growth morphology and molecular orientation of n-type semiconducting α,ω-diperfluorohexyl-quaterthiophene (DFH-4T) on Au(111) is investigated by scanning tunneling microscopy and scanning tunneling spectroscopy as the film thickness is increased from one monolayer to multilayers. Monolayer-thick DFH-4T films are amorphous and morphologically featureless with a large pit density, whereas multilayer films exhibit drastically different terraced structures consisting of overlapping platelets. Large changes in DFH-4T molecular orientation are observed on transitioning from two to four monolayers. Parallel electrical characterization of top-versus-bottom contact configuration DFH-4T FETs with Au source/drain electrodes reveals greatly different mobilities (μTOP = 1.1 ± 0.2 10-2 cm2V-1s-1 versus μBOTTOM = 2.3 ± 0.5 10-5 cm2V-1s-1) and contact resistances (RC-TOP = 4−12 MΩcm vs RC-BOTTOM > 1 GΩcm). This study provides important information on the organic semiconductor-source\drain electrode inter...

Probing the electronic properties of self-organized poly(3-dodecylthiophene) monolayers by two-dimensional scanning tunneling spectroscopy imaging at the single chain scale.

Abstract: Regioregular poly(3-dodecylthiophene) films self-organized on highly oriented pyrolytic graphite have been investigated by scanning tunneling microscopy and two-dimensional scanning tunneling spectroscopy (STS). Simulated spectra in very good agreement with the experimental data have been obtained by a method combining ab initio and semiempirical approaches, which allows a careful discussion of the polymer electronic states. From the experimental data, with the support of modeling, it is shown that the STS spectra give a direct access to the polymer semiconducting band gap without noticeable charge-transfer effects from the substrate. Spectroscopic images are achieved at the single chain scale, which allows scrutinizing the electronic consequences of chain folds and π-stacking effects through spectroscopic contrasts. While chain folds do not locally increase the polymer band gap more than a few tens of millielectonvolt, a striking widening of the STS conductance gap is observed in the case of electronic t...