Photon emission with the scanning tunneling microscope

By placing a photon detector near the tip-sample region of a scanning tunneling microscope, we have measured isochromat photon-emission spectra of polycrystalline tantalum and Si(111)7×7 at photon energies of 9.5 eV. Such spectra contain electronic-structure information comparable to inverse photoemission spectroscopy, but with high lateral/spatial resolution. The implications of this new observation are discussed.     

Forces with submolecular resolution between the probing tip and Cu-TBPP molecules on Cu(100) observed with a combined AFM/STM

Combined STM/AFM experiments were performed on monolayer islands of Cu-TBPP molecules on Cu(100). Pulling of single molecules on the substrate was observed and the imaging parameters were compared to STM studies. Local frequency shift vs. distance curves reveal three types of curves, which can be related to the interaction of the tip above the Cu(100) substrate, the centre of the molecule and the leg of the molecule. The frequency vs. distance curves above the legs reveal two minima, which are associated with the bending of the legs above a certain threshold value. Quantitative analysis yields force values of about 0.4 nN.     

One-dimensional metal structures at decorated steps

It is shown how wire structures a few nanometers wide can be fabricated by decorating step edges at vicinal surfaces. Their growth modes and electronic states are studied using Scanning Tunneling Microscopy (STM) and inverse photoemission. The observed growth modes are two-dimensional analogs of Stranski-Krastanov growth and layer-by-layer growth in three dimensions, e.g., for Cu on stepped Mo(1 1 0) and W(1 1 0), respectively. Contrast between different metals is achieved in STM pictures by resonant tunneling via surface states and image states, with the latter providing a map of the work function. The limit of single atomic rows decorating step edges is studied by inverse photoemission, and an energy shift of 0.4 eV is found for electronic states of step atoms. We expect stripe structures to become useful for the study of two- vs one-dimensional magnetism, for magnetoresistive films, and in the design of anisotropic materials.     

An accurate and compact high-pressure thermogravimetric analyser

The design and operation of a bench-top high-pressure thermogravimetric analyser is described and illustrated. It has the following specifications: sample mass 1–100 mg; temperature range 25–700°; heating rate 0.1–10 deg/min; pressure range 0–50 bar of air, O₂, N₂, CO₂, CO or H₂. Mass and temperature are measured with a maximum error of 0.1 mg and 5 K for any conditions of pressure, temperature and heating rate. The instrument can be used to study substances under industrially realistic conditions of pressure and temperature and to perform high-pressure accelerated oxidation tests on lubricants and polymers. In these areas of application, the instrument offers a new standard of accuracy and ease of use which make it comparable to commercially available high-pressure DSC systems.

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Zusammenfassung Bau und Funktion eines Hochdruck-TG-Analysators mit folgenden Spezifikationen wurden beschrieben: Probenmasse 1–100 mg; Temperaturbereich 25–700°C; Aufheizgeschwindigkeit 0.1–10 K/min; Druckbereich 0–50 bar für Luft, Sauerstoff, Stickstoff, Kohlendioxid, Kohlenmonoxid oder Wasserstoff. Die Meßgenauigkeit für Massen- bzw. Temperaturangaben beträgt 0.1 mg bzw. 5 K für alle Drücke, Temperaturen und Aufheizgeschwindigkeiten. Mit diesem Gerät können an Schmierstoffen und Polymeren unter Industriebedingungen für Druck und Temperatur Untersuchungen und hochdruckbeschleunigte Oxidationstests durchgeführt werden. Auf diesem Anwendungsgebiet ist das Gerät vollkommen vergleichbar mit kommerziell erhältlichen Hochdruck-D SC-Systemen.

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Dedicated to Professor Dr. H. J. Seifert on the occasion of his 60^th birthday

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The author thanks British Petroleum for permission to publish this work.     

Atomic-scale characterization of graphene grown on copper (100) single crystals

Growth of graphene on copper (100) single crystals by chemical vapor deposition has been accomplished. The atomic structure of the graphene overlayer was studied using scanning tunneling microscopy. A detailed analysis of moire? superstructures present in the graphene topography reveals that growth occurs in a variety of orientations over the square atomic lattice of the copper surface. Transmission electron microscopy was used to elucidate the crystallinity of the grown graphene. Pristine, defect-free graphene was observed over copper steps, corners, and screw dislocations. Distinct protrusions, known as "flower" structures, were observed on flat terraces, which are attributed to carbon structures that depart from the characteristic honeycomb lattice. Continuous graphene growth also occurs over copper adatoms and atomic vacancies present at the single-crystal surface. The copper atom mobility within vacancy islands covered with suspended graphene sheets reveals a weak graphene-substrate interaction. The observed continuity and room-temperature vacancy motion indicates that copper mobility likely plays a significant role in the mechanism of sheet extension on copper substrates. Lastly, these results suggest that the quality of graphene grown on copper substrates is ultimately limited by nucleation at the surface of the metal catalyst.     

Bias-dependent STM images of oxygen-induced structures on Ti(0001) facets

Titanium films grown on Si(111) surfaces in ultrahigh vacuum were investigated using scanning tunneling microscopy (STM). STM images of adsorbate structures induced by oxygen adsorption on Ti(0001) facets are observed to change from protrusions to depressions depending on the applied bias voltage. Using electron spectroscopic data we interpret our results in terms of local, oxygen-induced modifications of the density of states.     

Quantitative nanostructural and single-molecule force spectroscopy biomolecular analysis of human-saliva-derived exosomes

Exosomes are naturally occurring nanoparticles with unique structure, surface biochemistry, and mechanical characteristics. These distinct nanometer-sized bioparticles are secreted from the surfaces of oral epithelial cells into saliva and are of interest as oral-cancer biomarkers. We use high- resolution AFM to show single-vesicle quantitative differences between exosomes derived from normal and oral cancer patient's saliva. Compared to normal exosomes (circular, 67.4 ± 2.9 nm), our findings indicate that cancer exosome populations are significantly increased in saliva and display irregular morphologies, increased vesicle size (98.3 ± 4.6 nm), and higher intervesicular aggregation. At the single-vesicle level, cancer exosomes exhibit significantly (P < 0.05) increased CD63 surface densities. To our knowledge, it represents the first report detecting single-exosome surface protein variations. Additionally, high-resolution AFM imaging of cancer saliva samples revealed discrete multivesicular bodies with intraluminal exosomes enclosed. We discuss the use of quantitative, nanoscale ultrastructural and surface biomolecular analysis of saliva exosomes at single-vesicle- and single-protein-level sensitivities as a potentially new oral cancer diagnostic.