Scanning probe microscopy

Summary

Scanning probe microscopy (SPM) is a branch of microscopy that forms images of surfaces using a physical probe that scans the specimen. SPM was founded in 1981, with the invention of the scanning tunneling microscope, an instrument for imaging surfaces at the atomic level. The first successful scanning tunneling microscope experiment was done by Gerd Binnig and Heinrich Rohrer. The key to their success was using a feedback loop to regulate gap distance between the sample and the probe. Many scanning probe microscopes can image several interactions simultaneously. The manner of using these interactions to obtain an image is generally called a mode. The resolution varies somewhat from technique to technique, but some probe techniques reach a rather impressive atomic resolution. This is due largely because piezoelectric actuators can execute motions with a precision and accuracy at the atomic level or better on electronic command. This family of techniques can be called "piezoelectric t

Official source

https://en.wikipedia.org/wiki/Scanning_probe_microscopy

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In situ Studies of Peptide and Protein Assemblies at the Solid-Liquid Interface

Bart Willem Stel

At the interface between science and engineering, there is the field of biomimicry: Innovations inspired by the observation of natural, evolutionary optimized biological structures and processes. To understand the challenges of biomimicry, the concept of âMolecular Tectonicsâ is particularly useful. It describes how different self-assembling processes are spatially and dynamically integrated to form successively more complex structures. This integration makes it difficult to isolate the underlying mechanisms, which are often on the scale of individual molecules. Molecular biomimicry therefore critically depends on the observation of natural systems at the molecular scale. Advances in scanning probe microscopy have enabled the study of molecular self-assembly in unprecedented detail. However, many of these advances critically depend on the use of Ultra High Vacuum (UHV) conditions. To study natural systems at the molecular scale, a transition from UHV towards solvated systems is required. In the first part of this thesis, small synthetic peptides are used as model system for molecular self-assembly. The self-assembly of peptides at interfaces has been studied by systematically varying the chemical structure and deposition method. By using ElectroSpray Ion Beam Deposition (ES-IBD) as well as in situ drop casting, the self-assembly at the solid-vacuum interface (UHV conditions) and at the solid-liquid interface was directly compared. This comprehensive study on the effect of solvent, structure and deposition method on the assembly of peptides at gas-solid and solid-liquid biointerfaces has shown that the same AcFA5K peptide can organize into various structures, from Î²-sheets to tubular and globular assemblies, depending on their local environment. The study of molecular self-assembly was extended to 2D protein crystals. By using high-speed and high-resolution and Atomic Force Microscopy (AFM), it was possible to resolve both spatially and dynamically the mechanisms involved in S-layer self-assembly at the solid-liquid interface, such as monomer adsorption, condensation into high-density clusters, crystallization and finally conformational changes of existing S-layer crystals. In the second part of this thesis, the integration of different self-assembling systems is explored and their functional properties are investigated. It combines the concepts of self-assembly and molecular tectonics. Self-assembled striped PS-b-PEO block copolymer (BCP) thin film substrates were used to confine and align S-layer self-assembly, without affecting the internal structure. Furthermore, the nucleation rate and S-layer growth was greatly enhanced on PS-b-PEO substrates, compared to the pure PS or PEO constituents. The hierarchical use of different self-assembling systems can be seen as a functional approach to the concept of molecular tectonics. The catalytic properties of S-layers regarding the formation of CaCO3 were studied. By studying the catalytic properties of S-layers ex vivo, their role can be investigated independently from the internal metabolism of their parent bacteria. Synchrotron based in situ X-ray Absorption Spectroscopy (XAS) and AFM are combined with continuous flow conditions, demonstrating that S-layers are able to stabilize amorphous forms of CaCO3 and catalyze the nucleation of calcite at extremely low supersaturations.

EPFL2018

Carbon nanotubes the long, cylindrical carbon molecules are the most exciting material discovered lately. They offer the possibility for a large number of applications. My task was to explore the way how we can use multi-walled carbon nanotubes (MWNT) in scanning probe applications: Atomic Force Microscopy (AFM) and Scanning Nearfield Optical Microscopy (SNOM). Carbon nanotubes (CNTs) are potentially ideal atomic force microscopy probes because they can have diameters as small as one nanometer, they have robust mechanical properties, and can be specifically functionalized with chemical and biological probes at the tip ends. CNTs will not wear, because they buckle elastically under applied load and thus will stay sharp. They also show exceptional field emission properties, and remarkably enough, the field emission at high currents is accompanied by light emission. This phenomenon offers the possibility to use CNTs as nanometer-size light sources for SNOM. In order to test CNTs for AFM and SNOM applications a mounting method has been elaborated. Two nanomanipulators have been designed and constructed for operations inside a Transmission and Scanning Electron Microscopes (TEM and SEM). This latter one with large work space and available electrical contacts allowed versatile use of nanomanipulator, including in situ field emission, and photon counting measurements using a photodiode. Hundreds of individual MWNT and double-wall nanotubes (DWNT) bundles, produced by arc-discharge (AD) or chemical vapour deposition (CVD) method, have been mounted, one-by-one, on either commercial AFM probes or sharpened metallic wires (W, Au, Pt). The tests show that in both case, AFM and SNOM probes, the weakest point is the attachment of the nanotube to the support. Amorphous carbon deposit is good for attaching nanotubes for limited time and limited applications, but it deforms under the forces that it has been submitted to during AFM scans. This weak bonding of nanotubes is the source of failure in AFM. In field/light emission the high contact resistance, which can change during the mechanical stress exerted by the applied electric field, will lead to a Joule heating of the nanotube/contact area and results a thermal breakdown of the tip. Attempts were made to elaborate a good mechanical and electrical attachment of the carbon nanotubes to the Si/metallic cantilever by using indium as bonding agent.

EPFL2006

Quantum size effects in ultrathin metallic islands

I-Po Hong

This thesis reports measurements concerning quantum size effects of single crystalline metallic islands by using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Different sample systems are presented in the following chapters. In chapter 2, several aspects of quantum well states (QWS) of Pb ultrathin islands grown on Si(111) substrate are reported. The differential conductance spectra of QWS can be understood by discrete energy levels with linewidth broadening because of finite quasiparticle lifetime. Using low temperature scanning tunneling spectroscopy, we studied the linewidth of unoccupied quantum-well states (QWS) in Pb islands, grown on Si(111) on two different Pb/Si interfaces, of thicknesses between 7 and 22 monolayers. A quantitative analysis of the differential conductance spectra allowed us to determine the QWS lifetime broadening as a function of energy, showing agreement with 3D Fermi-liquid theory, as well as the electron-phonon (e-ph) contribution between 5 and 50 K. Layer-dependent ab initio calculations of the e-ph linewidth contributions are in excellent agreement with the data. Importantly, the sum of the calculated e-e and e-ph lifetime broadening follows the experimentally observed quadratic energy dependence. In chapter 3, studies investigating reduction of the superconducting gap of ultrathin Pb islands are presented. The energy gap Δ of superconducting Pb islands grown on Si(111) was probed in situ between 5 and 60 monolayers by low-temperature scanning tunneling spectroscopy. Δ was found to decrease from its bulk value as a function of inverse island thickness. Corresponding Tc values, estimated using bulk gap-to-Tc ratio, are in quantitative agreement with ex situ magnetic susceptibility measurements, however, in strong contrast to previous scanning probe results. Layer-dependent ab initio density functional calculations for free-standing Pb films show that the electron-phonon coupling constant, determining Tc, decreases with diminishing film thickness. In chapter 4, we present preliminary results on single electron tunneling and Coulomb blockade phenomena of metallic islands decoupled from a Ag(111) substrate by dielectric NaCl layers. Using low temperature STM/STS, the geometry of the metallic island can be determined unambiguously and the single electron tunneling properties are characterized. Using orthodox theory of single electron tunneling, the tunneling spectra can be reproduced qualitatively. Despite minor quantitative disagreement between data and simulations, the parameters of the double barrier tunneling junction, including the capacitances and the resistances of both junctions, as well as the residual charge, can be determined.

EPFL2009