Molecular attachment to a microscope tip: inelastic tunneling, Kondo screening, and thermopower

• Rouzhaji Tuerhong,
• Mauro Boero and
• Jean-Pierre Bucher

Beilstein J. Nanotechnol. 2019, 10, 1243–1250, doi:10.3762/bjnano.10.124

• microsope (STM) and a surface are investigated by combining the local manipulation capabilities of the STM with inelastic electron tunneling spectroscopy. By attachment of the molecule to the probe tip, the intrinsic physical properties similar to those exhibited by a free standing molecule become

• thermopower measured across the single-molecule junction. Keywords: inelastic electron tunneling; molecular quantum dot; Kondo physics; single molecule; thermopower; tunnel junction; Introduction Scanning tunneling microscopy (STM) has the capability to detect the electron transport through a molecule not

• only adsorbed on a surface, but also when it is attached to the probe tip itself [1][2][3][4]. The sharp tip of the STM is then used to lift a single molecule from the surface in order to efficiently minimize the influence of the substrate on the molecule. In this way, the intrinsic physical properties

Imaging the surface potential at the steps on the rutile TiO₂(110) surface by Kelvin probe force microscopy

• Masato Miyazaki,
• Huan Fei Wen,
• Quanzhen Zhang,
• Yuuki Adachi,
• Jan Brndiar,
• Ivan Štich,
• Yan Jun Li and
• Yasuhiro Sugawara

Beilstein J. Nanotechnol. 2019, 10, 1228–1236, doi:10.3762/bjnano.10.122

• observed with a lateral resolution of several nanometers by Kelvin probe force microscopy (KPFM) [29][30]. However, the dependence of surface potential on direction and structure of steps such as [001], and has not yet been clarified. In scanning tunneling microscopy (STM) [31] studies, three typical

• ]. Hence, we discuss a possible effect of orbit splitting. In STM studies, the local density of states (LDOS) of the orbit splitting has a large influence on the dipole moment of atomic species at the step edges. Although the orbit splitting has a big influence on the tunneling current, the influence on

Pure and mixed ordered monolayers of tetracyano-2,6-naphthoquinodimethane and hexathiapentacene on the Ag(100) surface

• Robert Harbers,
• Timo Heepenstrick,
• Dmitrii F. Perepichka and
• Moritz Sokolowski

Beilstein J. Nanotechnol. 2019, 10, 1188–1199, doi:10.3762/bjnano.10.118

• structures are formed on a surface by molecules that are otherwise typically used for the synthesis of bulk charge-transfer materials. The layers were obtained by vacuum deposition on the Ag(100) surface and analyzed by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). The

• :1 stoichiometry. Experimental The experiments were conducted in an ultra-high vacuum (UHV) chamber with a base pressure of 10−10 mbar. The chamber was equipped with a variable-temperature scanning tunneling microscope STM (type RHK UHV 300) from RHK Technologies and a multi-channel plate (MCP) low

• the screen. The LEED images displayed here were numerically enhanced in contrast. The simulated LEED patterns were calculated according to kinematic theory taking into account the presence of different symmetry-equivalent domains of the adsorbate. The STM images were recorded at RT using Pt/Ir tips in

In situ AFM visualization of Li–O₂ battery discharge products during redox cycling in an atmospherically controlled sample cell

• Kumar Virwani,
• Younes Ansari,
• Khanh Nguyen,
• Francisco José Alía Moreno-Ortiz,
• Jangwoo Kim,
• Maxwell J. Giammona,
• Ho-Cheol Kim and
• Young-Hye La

Beilstein J. Nanotechnol. 2019, 10, 930–940, doi:10.3762/bjnano.10.94

• these redox materials at the micrometer and nanometer scales. Gewirth et al. [2] reviewed the use of scanning tunneling microscopy (STM) and atomic force microscopy (AFM) investigations of phenomena such as reconstructions, restructuring and adsorption of ions. Phenomena such as under-potential

Capillary force-induced superlattice variation atop a nanometer-wide graphene flake and its moiré origin studied by STM

• Loji K. Thomas and
• Michael Reichling

Beilstein J. Nanotechnol. 2019, 10, 804–810, doi:10.3762/bjnano.10.80

• superlattices on graphite by imaging a live transition from one superlattice to another with concurrent and direct measurement of the orientation angle before and after rotation using scanning tunneling microscopy (STM). This has been possible due to a fortuitous observation of a superlattice on a nanometer

• -sized graphene flake wherein we have induced a further rotation of the flake utilizing the capillary forces at play at a solid–liquid interface using STM tip motion. We propose a more “realistic” tip–surface meniscus relevant to STM at solid–liquid interfaces and show that the capillary force is

• sufficient to account for the total expenditure of energy involved in the process. Keywords: capillary force; graphene; graphite; HOPG; moiré; solid–liquid interface; STM; superlattice; Introduction Graphite is a layered material with graphene sheets arranged in ABAB stacking. HOPG is an ordered form of

Polymorphic self-assembly of pyrazine-based tectons at the solution–solid interface

• Achintya Jana,
• Puneet Mishra and
• Neeladri Das

Beilstein J. Nanotechnol. 2019, 10, 494–499, doi:10.3762/bjnano.10.50

• polymorphic self-assembly of these molecules on a HOPG substrate. Two different molecular packing structures with equal distribution are observed. Detailed analysis of the STM images emphasizes the crucial role of weak intermolecular hydrogen bonding, and molecule–substrate interactions in the formation of

• –HOPG interface using scanning tunneling microscopy (STM) technique under ambient conditions. The molecules belong to a new class of pyrazine/triazine-based molecules, containing two or more pyridine pendant units, and can act as a precursor to several two- and three-dimensional supramolecular

• architectures with tunable nanocavities [18][19]. STM images reveal that the molecular adlayer spreads over an area of several hundred square nanometers at the 1-phenyloctane–HOPG interface. Furthermore, these molecules exhibit polymorphic self-assembly on the HOPG substrate where two different molecular

Intuitive human interface to a scanning tunnelling microscope: observation of parity oscillations for a single atomic chain

• Sumit Tewari,
• Jacob Bakermans,
• Christian Wagner,
• Federica Galli and
• Jan M. van Ruitenbeek

Beilstein J. Nanotechnol. 2019, 10, 337–348, doi:10.3762/bjnano.10.33

• –machine augmented system in which the operator and the machine are connected by a real-time simulation. Here, a 3D motion control system is integrated with an ultra-high vacuum (UHV) low-temperature scanning tunnelling microscope (STM). Moreover, we coupled a real-time molecular dynamics (MD) simulation

• refer to as point contact pushing (PCP) technique. Keywords: adatom imaging; mechanical annealing; scanning tunnelling microscopy (STM); STM tip; tip apex; Introduction It is of fundamental interest both for chemists and physicists to study the electronic transport through single atoms and molecules

• image the structural changes that happen at the junction using the STM, because the very STM tip used for imaging is also used for manipulation. It is known that the electronic transport of these nanoscale devices depends strongly on their structural conformations and coupling to the leads [3][4

Nitrous oxide as an effective AFM tip functionalization: a comparative study

• Taras Chutora,
• Bruno de la Torre,
• Pingo Mutombo,
• Jack Hellerstedt,
• Jaromír Kopeček,
• Pavel Jelínek and
• Martin Švec

Beilstein J. Nanotechnol. 2019, 10, 315–321, doi:10.3762/bjnano.10.30

• formation, the metallic tip (pre-treated by a gentle indentation into the substrate) was functionalized by an impurity CO molecule, which significantly improved the resolution in both STM and AFM. We performed high-resolution AFM/STM measurements on various clusters (comparable to the inset of Figure 1a

• contribution. To benchmark the performance of the N2O-decorated tip experimentally, we used it to obtain high-resolution STM/AFM images of a single FePc molecule, which is suitable as a standard due to its planar shape and the flat adsorption geometry on Au(111) [31]. A submonolayer coverage of FePc molecules

• was deposited on Au(111) at room temperature, and the FePc/Au(111) surface was subsequently cooled down in the microscope and exposed to N2O. Figure 4 shows an overview STM image of the obtained sample, where the FePc molecules predominantly occupy the fcc-stacked Au regions and the kinks of the Au

Apparent tunneling barrier height and local work function of atomic arrays

• Neda Noei,
• Alexander Weismann and
• Richard Berndt

Beilstein J. Nanotechnol. 2018, 9, 3048–3052, doi:10.3762/bjnano.9.283

• of a barrier height increase with decreased lateral size of the tunneling-current path is expected to be reliable. These results predict a kinetic energy contribution of the order of 1 eV for tunneling between two atomically sharp structures. Experiments were performed in an ultrahigh vacuum STM

• of the vertical tip excursion Δz the feedback loop of the STM was disabled at a sample voltage of V = 20 mV and a current of I = 200 pA. The tip was then brought closer to the structure under investigation at a rate of 1.7 nm/s while recording I. Figure 2 shows typical results from a clean (111

• local surface normal is at an angle α with the z direction of the STM [38]. However, this is expected to reduce Φapp by a factor cos2α and thus cannot explain our large experimental values. In any event, we measured Φapp of atoms and chains on maxima of the topographies, where α ≈ 0. We therefore

Investigation of CVD graphene as-grown on Cu foil using simultaneous scanning tunneling/atomic force microscopy

• Majid Fazeli Jadidi,
• Umut Kamber,
• Oğuzhan Gürlü and
• H. Özgür Özer

Beilstein J. Nanotechnol. 2018, 9, 2953–2959, doi:10.3762/bjnano.9.274

• Majid Fazeli Jadidi Umut Kamber Oguzhan Gurlu H. Ozgur Ozer Department of Physics Engineering, İstanbul Technical University, 34469, İstanbul, Turkey 10.3762/bjnano.9.274 Abstract Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) images of graphene reveal either a triangular

• an inequivalent electronic structure in HOPG or multilayer graphene due to the presence of a carbon atom or a hollow site underneath. In this work, we report small-amplitude, simultaneous STM/AFM imaging using a metallic (tungsten) tip, of the graphene surface as-grown by chemical vapor deposition

• (CVD) on Cu foils. Truly simultaneous operation is possible only with the use of small oscillation amplitudes. Under a typical STM imaging regime the force interaction is found to be repulsive. Force–distance spectroscopy revealed a maximum attractive force of about 7 nN between the tip and carbon

Charged particle single nanometre manufacturing

• Philip D. Prewett,
• Cornelis W. Hagen,
• Claudia Lenk,
• Steve Lenk,
• Marcus Kaestner,
• Tzvetan Ivanov,
• Ahmad Ahmad,
• Ivo W. Rangelow,
• Xiaoqing Shi,
• Stuart A. Boden,
• Alex P. G. Robinson,
• Dongxu Yang,
• Sangeetha Hari,
• Marijke Scotuzzi and
• Ejaz Huq

Beilstein J. Nanotechnol. 2018, 9, 2855–2882, doi:10.3762/bjnano.9.266

• wavelength is more than 10% of the target 1 nm resolution. This is of particular relevance to lithography using electrons generated in proximity probe (STM) systems and will be discussed further in Section 3. After the ultimate probe size is determined, the ability to perform lithography using charged

• offered by this technique that are not easily achievable by any other, as well as on the ultimate resolution achievable. We also give a few examples of ultrahigh-resolution work performed using STM-based EBID and pattern transfer. As described above, EBID can be carried out in an electron microscope by

• Electron beam induced deposition. We begin with a brief review of EBID, which addresses the fabrication of dots and lines in SEM, TEM and STM on bulk and thin film substrates, as well as sub-10 nm FEBIP for specialised applications. Since the darkening due to decomposition of surface contaminants was first

Low cost tips for tip-enhanced Raman spectroscopy fabricated by two-step electrochemical etching of 125 µm diameter gold wires

• Antonino Foti,
• Francesco Barreca,
• Enza Fazio,
• Cristiano D’Andrea,
• Paolo Matteini,
• Onofrio Maria Maragò and
• Pietro Giuseppe Gucciardi

Beilstein J. Nanotechnol. 2018, 9, 2718–2729, doi:10.3762/bjnano.9.254

• ]. The tips efficiently enhance and confine the electromagnetic field at the nanoscale [8][9] or even at sub-nanometer levels [10]. TERS has a sensitivity that can reach the single molecule level [11][12]. TERS setups based on atomic force microscopy (AFM) [1][13], scanning tunneling microscopy (STM) [14

• times of approximately 2 min. The tips can be easily manipulated and safely mounted, by gluing or clamping them into STM- or ShF-based TERS setups. The good performance of the tips is highlighted by TERS spectra of dyes, pigments and biomolecules. The enhancement factor in the range of 104–105 was found

• . The analysis of the tips’ TERS performance is carried out in gap-mode [14], using a commercial setup that couples a micro-Raman spectrometer (XploRA Plus, Horiba) with an AFM/STM (Smart SPM-1000, AIST-NT). The setup, shown in Supporting Information File 1, works in a side-illumination configuration

Directional light beams by design from electrically driven elliptical slit antennas

• Shuiyan Cao,
• Eric Le Moal,
• Quanbo Jiang,
• Aurélien Drezet,
• Serge Huant,
• Jean-Paul Hugonin,
• Gérald Dujardin and
• Elizabeth Boer-Duchemin

Beilstein J. Nanotechnol. 2018, 9, 2361–2371, doi:10.3762/bjnano.9.221

• spread of the emitted beam is inversely proportional to the length of the ellipse axes. Results and Discussion Figure 1 shows schematics of the experiment performed in this study. All experiments are carried out in air and at room temperature using a scanning tunneling microscope (STM) head mounted on

• top of an inverted optical microscope. The setup is described in detail in the Experimental section. Circular or elliptical slits are etched in an optically thick (200 nm) gold film deposited on a glass coverslip. The inelastic effects of the tunnel current between the STM tip and the surface of the

• numerical aperture (NA) microscope objective. The angular distribution of the emitted light is acquired from Fourier-space images [24][25]. The angular emission pattern results from the far-field interference of the light scattered from all along the slit. When the STM tip is positioned in the center of the

Metal-free catalysis based on nitrogen-doped carbon nanomaterials: a photoelectron spectroscopy point of view

• Mattia Scardamaglia and
• Carla Bittencourt

Beilstein J. Nanotechnol. 2018, 9, 2015–2031, doi:10.3762/bjnano.9.191

• the electronic states of graphene. In general, point defects generate localized states at the Fermi level, easily identifiable as protrusion by scanning tunneling microscopy (STM) [56], while carbon vacancies are responsible for an opening of the energy gap [57]. The increase in the density of states

• of the C atom due to the localized DOS near the Fermi level, as observed by STM [117]. Conclusion In all the experiments reported in this review, the picture of the nitrogen active sites in carbon nanomaterials for ORR activity is far from being complete, since many results in similarly designed

Recent highlights in nanoscale and mesoscale friction

• Andrea Vanossi,
• Dirk Dietzel,
• Andre Schirmeisen,
• Ernst Meyer,
• Rémy Pawlak,
• Thilo Glatzel,
• Marcin Kisiel,
• Shigeki Kawai and
• Nicola Manini

Beilstein J. Nanotechnol. 2018, 9, 1995–2014, doi:10.3762/bjnano.9.190

Interplay between pairing and correlations in spin-polarized bound states

• Szczepan Głodzik,
• Aksel Kobiałka,
• Anna Gorczyca-Goraj,
• Andrzej Ptok,
• Grzegorz Górski,
• Maciej M. Maśka and
• Tadeusz Domański

Beilstein J. Nanotechnol. 2018, 9, 1370–1380, doi:10.3762/bjnano.9.129

• the Kondo and the leaking Majorana quasiparticle can be confronted with each other. These magnetically polarized YSR and Majorana quasiparticles as well as the subgap Kondo effect can be experimentally verified using tunneling heterostructures with ferromagnetic lead (STM tip). Results and Discussion

• polarized bound states, focusing on the proximity-induced nontrivial superconducting phase. In practice, the quasiparticle spectrum can be probed within STM-type setups, by attaching a conducting [39][40], superconducting [41], or a magnetically polarized tip [42]. We assume the spin–orbit interaction

• have recently emphasized [43], that the amplitude of intersite pairing (between identical spin electrons) differs several times for ↑ and ↓ sectors. This leads to an obvious polarization of the YSR and Majorana quasiparticles (the latter appearing near the nanochain edges). Let us consider the STM-type

Heterostuctures of 4-(chloromethyl)phenyltrichlorosilane and 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine prepared on Si(111) using particle lithography: Nanoscale characterization of the main steps of nanopatterning

• Phillip C. Chambers and
• Jayne C. Garno

Beilstein J. Nanotechnol. 2018, 9, 1211–1219, doi:10.3762/bjnano.9.112

• studied with scanning tunneling microscopy (STM) to evaluate the surface conformation and molecular geometry [14]. Individual molecules of nonplanar freebase and copper-metallated tetraphenyl porphyrins adsorbed on Cu(111) were investigated using frequency modulated noncontact AFM to resolve subtle

• differences in structure and conformation [15]. The submolecular structure of cobalt and copper phthalocyanines on gold substrates were resolved with STM by Lu et al. [16]. The differences in central metals were resolved for a mixed sample. The molecular orientation and molecular switching properties of a

• triple-decker sandwich complex of phthalocyanine compounds prepared on graphite was studied using STM by Lei et al. [17]. A method of photocatalytic lithography was reported for making porphyrin surface structures that were applied for preparing protein arrays [18][19]. The assembly of porphyrins at

Combined pulsed laser deposition and non-contact atomic force microscopy system for studies of insulator metal oxide thin films

• Daiki Katsube,
• Hayato Yamashita,
• Satoshi Abo and
• Masayuki Abe

Beilstein J. Nanotechnol. 2018, 9, 686–692, doi:10.3762/bjnano.9.63

• transmission electron microscopy [5][8][9][10][11][12][13]. As atomic resolution methods, scanning probe microscopy including scanning tunneling microscopy (STM) [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] and non-contact atomic force microscopy (NC-AFM) [19][23][29][31][32][33][34

• possible to use it to observe any surface regardless of the conductivity of the material [19][23][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45][46][47]. STM on the other hand can only be used for observations of insulators with a thickness of few atomic layers on conductive substrates [20][21

• ][22]. For observations of surface atoms of metal oxides using NC-AFM and STM, it is critical to prepare atomically flat and clean surfaces. A standard method for obtaining clean surfaces of metal oxides is performing iterations of Ar+ sputtering and annealing at high temperatures [23][24][25][26][37

Revealing the interference effect of Majorana fermions in a topological Josephson junction

• Jie Liu,
• Tiantian Yu and
• Juntao Song

Beilstein J. Nanotechnol. 2018, 9, 520–529, doi:10.3762/bjnano.9.50

• DOS can be detected by using two STM leads or two normal leads. A single side lead can only detect the Andreev reflection tunneling process in the junction, which cannot reveal information about the interference effect in general. However, using two side leads, we can reveal information about the

• information. In general, the interference effects in the trivial Andreev bound states are unrelated, and their period is 2π. Thus, it may be a way to distinguish them using information contained in the DOS. We suggest that the interference effect can be detected using two STM leads or two normal leads. We

• parity related and shows very different oscillation behavior between the DOS of the electron part and the hole part. Thus, they can be well distinguished by considering the DOS. Detecting the 4π oscillation through two STM leads In the last section, we have shown that the main features of the DOS for the

Engineering of oriented carbon nanotubes in composite materials

• Razieh Beigmoradi,
• Abdolreza Samimi and
• Davod Mohebbi-Kalhori

Beilstein J. Nanotechnol. 2018, 9, 415–435, doi:10.3762/bjnano.9.41

• microscopes, respectively. In analyzing the alignment of CNTs, the most common microscopic methods are atomic force microscopy (AFM), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and scanning transmission microscopy (STM). Electron microscope: By changing the curvature and number

Review: Electrostatically actuated nanobeam-based nanoelectromechanical switches – materials solutions and operational conditions

• Liga Jasulaneca,
• Jelena Kosmaca,
• Raimonds Meija,
• Jana Andzane and
• Donats Erts

Beilstein J. Nanotechnol. 2018, 9, 271–300, doi:10.3762/bjnano.9.29

• microscope [8][10][11][12][13][14][15]. In situ studies of the dynamics of force interactions, conductance and adhesion in gold point contacts using combined transmission electron microscopy/scanning tunnelling microscopy (TEM-STM) [51][52] and atomic force microscopy/transmission electron microscopy (AFM

Anchoring of a dye precursor on NiO(001) studied by non-contact atomic force microscopy

• Sara Freund,
• Antoine Hinaut,
• Nathalie Marinakis,
• Edwin C. Constable,
• Ernst Meyer,
• Catherine E. Housecroft and
• Thilo Glatzel

Beilstein J. Nanotechnol. 2018, 9, 242–249, doi:10.3762/bjnano.9.26

• lie between 3.5 and 4.3 eV [25][26][27][28][29], and given that scanning tunnelling microscopy (STM) can only be performed on thin NiO films grown on metals [30][31], non-contact atomic force microscopy (nc-AFM) in ultra-high vacuum is the technique of choice. Due to its hardness and high reactivity

Transition from silicene monolayer to thin Si films on Ag(111): comparison between experimental data and Monte Carlo simulation

• Alberto Curcella,
• Romain Bernard,
• Yves Borensztein,
• Silvia Pandolfi and
• Geoffroy Prévot

Beilstein J. Nanotechnol. 2018, 9, 48–56, doi:10.3762/bjnano.9.7

• Alberto Curcella Romain Bernard Yves Borensztein Silvia Pandolfi Geoffroy Prevot Sorbonne Universités, UPMC Univ Paris 06, CNRS-UMR 7588, Institut des NanoSciences de Paris, F-75005, Paris, France 10.3762/bjnano.9.7 Abstract Scanning tunneling microscopy (STM), Auger electron spectroscopy (AES

• regime where multilayer silicene has been claimed to form (470–500 K), a good agreement is found with AES intensity variations and STM measurements within a Ag surfactant mediated growth, whereas a model with multilayer silicene growth fails to reproduce the AES measurements. Keywords: Auger electron

• ], ARPES [31] and grazing incidence X-ray diffraction [32]. The diamond-like structure of the film has been confirmed by scanning tunneling microscopy (STM) [33] and optical measurements [34]. The Ag termination of the surface has been also demonstrated by Auger electron spectroscopy (AES) [34], metastable

Adsorption of iron tetraphenylporphyrin on (111) surfaces of coinage metals: a density functional theory study

• Hao Tang,
• Nathalie Tarrat,
• Véronique Langlais and
• Yongfeng Wang

Beilstein J. Nanotechnol. 2017, 8, 2484–2491, doi:10.3762/bjnano.8.248

• vacuum disappears upon adsorption on the metal surfaces. The high-spin state of physisorbed FeTPP is stable on all adsorption sites. This result reveals that an external permanent element such as a STM tip or an additional molecule is needed to use FeTPP or similar molecules as model system for molecular

• molecule (e.g., NO, CO) or atom (Cl) in order to modify the coordination number [8][9][10][11]. Only few reversible manipulations of spin were achieved without additional ligand, as the one shown by N. Lin et al. on a single FeTPP molecule junction in a scanning tunnelling microscope (STM) [12]. In this

• S = 1 associated with a conformational change by passing from a saddle shape to a planar shape in the presence of the STM tip. To the best of our knowledge, the stability as well as the activation barrier between HS and IS FeTPP have not yet been investigated. In this paper, a brief analysis of the

Robust procedure for creating and characterizing the atomic structure of scanning tunneling microscope tips

• Sumit Tewari,
• Koen M. Bastiaans,
• Milan P. Allan and
• Jan M. van Ruitenbeek

Beilstein J. Nanotechnol. 2017, 8, 2389–2395, doi:10.3762/bjnano.8.238

• Sumit Tewari Koen M. Bastiaans Milan P. Allan Jan M. van Ruitenbeek Huygens–Kamerlingh Onnes Laboratory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, Netherlands 10.3762/bjnano.8.238 Abstract Scanning tunneling microscopes (STM) are used extensively for studying and manipulating matter at

• the atomic scale. In spite of the critical role of the STM tip, procedures for controlling the atomic-scale shape of STM tips have not been rigorously justified. Here, we present a method for preparing tips in situ while ensuring the crystalline structure and a reproducibly prepared tip structure up

• to the second atomic layer. We demonstrate a controlled evolution of such tips starting from undefined tip shapes. Keywords: adatom imaging; mechanical annealing; scanning tunneling microscopy (STM); STM tip; tip apex; Introduction After the advent of the scanning tunneling microscope (STM) in 1981