Molecular dynamics simulations and atomic force microscopy are used to investigate the atomistic mechanisms of adhesion, contact formation, nanoindentation, separation, and fracture that occur when a nickel tip interacts with a gold surface. The theoretically predicted and experimentally measured hysteresis in the force versus tip-to-sample distance relationship, found upon approach and subsequent separation of the tip from the sample, is related to inelastic deformation of the sample surface characterized by adhesion of gold atoms to the nickel tip and formation of a connective neck of atoms. At small tipsample distances, mechanical instability causes the tip and surface to jump-to-contact, which in turn leads to adhesion-induced wetting of the nickel tip by gold atoms. Subsequent indentation of the substrate results in the onset of plastic deformation of the gold surface. The atomic-scale mechanisms underlying the formation and elongation of a connective neck, which forms upon separation, consist of structural transformations involving elastic and yielding stages.

https://science.sciencemag.org/content/sci/248/4954/454.full.pdf

Atomistic Mechanisms and Dynamics of Adhesion, Nanoindentation, and Fracture

Recent initial experiments in magnetic resonance force microscopy (MRFM) have detected the magnetic force exerted by electrons and nuclei in microscopic samples. The experiments generate a force signal by modulating the sample magnetization with standard magnetic resonance techniques. Sample sizes of a few nanograms generate readily detected force signals of order ${10}^{\ensuremath{-}14}$ to ${10}^{\ensuremath{-}16}$ Newtons. This article describes the present status of MRFM technology, with particular attention to the feasibility of detecting single-electron magnetic moments, and the possible applications of MRFM in biological imaging.

Magnetic resonance force microscopy

Monolayer MoS2 is a direct band gap semiconductor which has been recently investigated for low-power field effect transistors. The initial studies have shown promising performance, including a high on/off current ratio and carrier mobility with a high-κ gate dielectric. However, the performance of these devices strongly depends on the crystalline quality and defect morphology of the monolayers. In order to obtain a detailed understanding of the MoS2 electronic device properties, we examine possible defect structures and their impact on the MoS2 monolayer electronic properties, using density functional theory in combination with scanning tunneling microscopy to identify the nature of the most likely defects. Quantitative understanding based on a detailed knowledge of the atomic and electronic structures will facilitate the search of suitable defect passivation techniques. Our results show that S adatoms are the most energetically favorable type of defect and that S vacancies are energetically more favorable than Mo vacancies. This approach may be extended to other transition-metal dichalcogenides (TMDs), thus providing useful insights to optimize TMD-based electronic devices.

https://iopscience.iop.org/article/10.1088/0957-4484/25/37/375703/pdf

Impact of Intrinsic Atomic Defects on the Electronic Structure of MoS2 Monolayers

A scanning tunnelling microscope can image surfaces down to the atomic scale. Its very high resolution is caused by its local probing mechanism: a tunnel current which flows through the outermost atom of a scanning tip to the sample. Its invention, about ten years ago, was based on an ingenious blend of quantum physics, mechanical design and electronic control. Since then, it has proved to be a very versatile instrument. Its applications range from the characterization of surface roughness to atomic reconstructions, and from doped semiconductors to cell membranes. It can operate in ultra high vacuum, in air, in reactive gases, in corrosive solutions or at cryogenic temperatures. This paper reviews the evolution of scanning tunnelling microscopy in the first decade after its invention. Attention is focused on the basis of STM: tunnelling theory, mechanical design and modes of operation. Representative examples of applications in various fields of research are discussed.

http://idwebhost-202-73.ethz.ch/praktika/analytisch/stm/Rep%20Prog%20phys%2055,%20p1165.pdf

Scanning tunnelling microscopy

The fundamental unit of synthetic melanin: a verification by tunneling microscopy of X-ray scattering results.

We have been successful in obtaining atomically resolved images of highly oriented pyrolytic graphite (HOPG) in air at point contact. Direct contact between tip and sample or contact through a contamination layer provides a conduction mechanism in addition to the exponential tunneling mechanism responsible for scanning tunneling microscopy (STM) imaging. Current–voltage (I–V) spectra were obtained while scanning in the current imaging mode with the feedback circuit interrupted in order to study the graphite imaging mechanism. Multiple tunneling tips are probably responsible for images without the expected hexagonal or trigonal symmetry. Our observations indicate that the use of HOPG for testing and calibration of STM instrumentation may be misleading.

/pdf/imaging-graphite-in-air-by-scanning-tunneling-microscopy-4vl1istwkj.pdf

Imaging graphite in air by scanning tunneling microscopy: Role of the tip

We have shown that it is possible to image DNA with atomic resolution using scanning tunneling microscopy (STM), [R. J. Driscoll, M. G. Youngquist, and J. D. Baldeschwieler, Nature 346, 294 (1990)]. Here we describe that data together with our general observations on STM of DNA in ultrahigh vacuum. We also suggest a possible contrast mechanism for DNA imaging by STM based on wave function orthogonality requirements between a molecule and its substrate. Topographic images are presented which resolve atomic features in addition to the double helical structure and nucleotide pairs of the DNA molecule. Comparisons of experimental STM profiles and modeled contours of the van der Waals surface of A-DNA show excellent correlation. Successive scans show that the imaging is nondestructive and reproducible. For this study, double-stranded DNA was deposited on highly oriented pyrolitic graphite without coating, shadowing, or chemical modification.

/pdf/scanning-tunneling-microscopy-of-dna-atom-resolved-imaging-1msqz9929h.pdf

Scanning tunneling microscopy of DNA: Atom‐resolved imaging, general observations and possible contrast mechanism

Images of DNA obtained in a scanning tunneling microscope operating under ultra‐high vacuum are described, and a contrast mechanism for STM imaging of weakly‐bound adsorbates is proposed. The capability of the STM for atom‐scale imaging of DNA is demonstrated through comparison of STM data with the accepted structure of A‐DNA derived from x‐ray crystallography; seven different structural elements of the molecule are identified and used to support the data interpretation. The contrast mechanism proposed invokes the Pauli‐principle based interference of the adsorbate with the substrate wavefunction vacuum tail. The adsorbate wavefunctions themselves lie far from the Fermi level and are unlikely to support electron conduction; their interaction with substrate wavefunction tail, however, enhances the tip‐substrate electron tunneling probability at the adsorption site. The suggested mechanism is supported with ab initio calculations. The paper summarizes a talk presented at the Engineering Foundation conference ‘‘Scanned Probe Microscopies: STM and Beyond.’’

M. G. Youngquist

Papers

Imaging graphite in air by scanning tunneling microscopy: Role of the tip

Scanning tunneling microscopy of DNA: Atom‐resolved imaging, general observations and possible contrast mechanism

STM of DNA in UHV