Low Temperature Scanning Probe Microscopy

Topological Defects Induced High-Spin Quartet State in Truxene-Based Molecular Graphenoids

Scanning tunneling spectroscopy and microscopy investigations have been carried out in air on thermally evaporated tungsten oxide thin films deposited onto silicon substrates. The films have been submitted to thermal annealing at different temperatures and for different times. The microscopy images show strong effects of recrystallization and different grain sizes depending on the thermal treatment. The spectroscopic data exhibit a rectifying characteristics and the comparison between the topographic and current images allows a better understanding of the sample morphology in relation to the electrical behavior. X-ray photoelectron spectroscopy valence band measurements have evidenced a structure near the Fermi level, which has been assigned to a structural oxygen deficiency of the deposited films, which is also observed in the conductance curve of some samples.

Scanning tunneling microscopy and spectroscopy investigation of the atomic and electronic structure of CoO islands on Ag( [formula omitted

We have used scanning tunneling microscopy (STM) to elucidate the nanoscale electronic structures of graphene oxide (GO). The unreduced GO layer was imaged using STM without reduction processes when deposited on a Au(111) surface covered with an octanethiolate self-assembled monolayer (C8S-SAM). The STM image of the GO sheet exhibits a grainy structure having a thickness of about 1 nm, which is in good agreement with the previous results obtained using atomic force microscopy (AFM). We found that the C8S-SAM suppresses the adsorption of water remaining on the substrate, which would be important to accomplish the nanoscale imaging of the unreduced GO by STM. Furthermore, we successfully detected the π and π* states localized in the GO sheet using scanning tunneling spectroscopy (STS). The π-π* gap energy and the gap center are not uniform within the GO sheet, indicating the existence of various sizes of the sp2 domain and evidence for the local electronic doping by the substituents.

It is well known that scanning probe techniques such as scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) routinely offer atomic scale information on the geometric and the electronic structure of solids. Recent developments in STM and especially in non-contact AFM have allowed imaging and spectroscopy of individual molecules on surfaces with unprecedented spatial resolution, which makes it possible to study chemistry and physics at the single molecule level. In this feature article, we first review the physical concepts underlying image contrast in STM and AFM. We then focus on the key experimental considerations and use selected examples to demonstrate the capabilities of modern day low-temperature scanning probe microscopy in providing chemical insight at the single molecule level.

Epitaxial iron silicides on Si(001): an investigation with scanning tunneling microscopy and spectroscopy

Five dual-quinquethiophene self-assemblies are prepared on a highly oriented pyrolytic graphite (HOPG) surface. The dual-quinquethiophenes are quinquethiophenes (5T)-di to hexamethylene (n, 2-6)-quinquethiophenes (5T), abbreviated as 5T-n-5T (n = 2-6). The effect of the bridge alkylene chains on the structure and property of the five assemblies are investigated by scanning tunneling microscopy (STM). It is found that all 5T-n-5T molecules form ordered adlayers on a HOPG surface with stripe feature. The alkylene bridge part in a molecule appears in a dark contrast in an STM image. Intriguingly, the thiophene backbones of individual molecules in the adlayer always keep an angle with the direction of molecular stripes. With alkylene bridge length increasing, different structures are found in 5T-5-5T and 5T-6-5T assemblies. To understand the effect of bridge chains on single molecular property, scanning tunneling spectroscopy is used to probe the electronic property of the different adlayers. The results will be important in surface engineering by self-assembly and molecular device fabrication with oligothiophenes.

Standing wave patterns are formed near potential barriers or steps due to the interference of incident and reflected waves. Graphene systems show standing wave patterns near atomic step edges, defects, and impurities in scanning tunneling microscopy; however, there are still unexplored examples. In this study, we present our experimental results for graphene directly grown on hexagonal-BN/Cu by chemical vapor deposition. Standing wave patterns were observed in our scanning tunneling microscopy and spectroscopy, revealing linear dispersion relations with a Fermi velocity of about 106 m/s. Our study shows that graphene grown on hexagonal-BN/Cu provides a useful platform to study the electronic characteristics of graphene systems.

ABSTRACTIon beam sputter-deposition has been used to produce high temperature superconducting (HTSC) thin films with controlled orientation. Room temperature scanning tunneling microscopy (STM) studies of ion beam sputter-deposited Y-Ba-Cu-O thin films indicate that the growth mode depends on whether the films are a- or c-axis oriented. The c-axis oriented films appear to grow by a screw dislocation mechanism, producing layered spirals similar to those observed in films grown by plasma sputtering and laser ablation-deposition. STM images of the a-axis oriented films show a growth mode which appears to produce layered structures perpendicular to the substrate with no spirals. Scanning tunneling spectroscopy (STS) studies of the a- and c-axis oriented films tend to reflect the anisotropy of the Y-Ba-Cu-O structure. Both the c-axis and the a-axis oriented films have semiconducting characteristics, possibly due to a native oxide, with a band gap estimated to be 1.4 eV. The c-axis oriented film, however, exhibits more fine structure in its density of states. This apparent anisotropie band structure reflects the anisotropie Y-Ba-Cu-O microstructure and superconducting characteristics. Investigations with x-ray photcelectron spectroscopy (XPS) establish a substantial chemical difference between the two surfaces inferring more substantial native oxides and air-induced by-products on the a-axis oriented film.

In this thesis, ultra-thin dielectric layers on metal substrates are studied mainly by scanning tunneling microscopy and scanning tunneling spectroscopy. In chapter 2, field emission resonances, i.e. bound states in the potential well between sample and tip with an applied voltage, are used to determine the local work function on a sample with patches of different structural composition. To do so, a simple theoretical model is developed and used to simulate the peak positions of the measured dI/dV spectra. In the present case the sample is a Ag(100) surface covered by up to three monolayers of NaCl. The presented method is also applicable to other systems suitable for STM measurements. Additionally, the surface potential is locally probed in the dipole layer region between domains of different work function. In chapter 3, new molecular structures on silver surfaces are presented, that are formed by a mixture of gases. This mixture contains the small molecules H2, H2O, CO, CO2, and N2. No final conclusion can be drawn on the chemical nature of these structures due to the impossibility of the scanning tunneling microscope to determine chemical species. But already the presence of these structures indicates that there is a multitude of unknown interactions, that are possible between a transition metal surface and even smallest molecules. In chapter 4, the moiré pattern is described that a BC3 film creates on a NbB2(0001) surface. As the growth of the BC3 sheet is incommensurate, a moiré pattern is formed that modulates the apparent height of the atoms on the STM images, i.e. the local density of states. The BC3 layer exhibits a completely different behavior than the closely related graphite layer, because it is transparent for the tunnel current. In the present case, the BC3 layer is visible on the STM images only by its moiré effect.

We show that a scanning capacitance microscope (SCM) can image buried delta-doped donor nanostructures fabricated in Si via a recently developed atomic-precision scanning tunneling microscopy (STM) lithography technique. A critical challenge in completing atomic-precision nanoelectronic devices is to accurately align mesoscopic metal contacts to the STM defined nanostructures. Utilizing the SCMs ability to image buried dopant nanostructures, we have developed a technique by which we are able to position metal electrodes on the surface to form contacts to underlying STM fabricated donor nanostructures with a measured accuracy of 300 nm. Low temperature (T = 4 K) transport measurements confirm successful placement of the contacts to the donor nanostructures.

Confocal scanning laser — scanning probe hybrid microscope for biological applications

In this chapter, we discuss recent developments in the characterization of polarization doped wide bandgap semiconductor (WBS) heterojunctions with an emphasis on characterization by scanning probe microscopy (SPM). Wide bandgap semiconductors, such as SiC and GaN, have a wide variety of applications in electronic and optoelectronic devices [1–6]. The operation of devices made from these materials is dominated by the polarization properties of III-N and SiC semiconductor crystals. For example, polarization doped 2 dimensional electron gas (2DEG) channels in undoped GaN/AlGaN and SiC polytypic heterojunctions have been fabricated with current densites in excess of 1A/mm. Other effects, such as surface charge instability stemming from polarization, offer interesting potentials and challenges.While some of these effects may be studied using more conventional techniques such as currentvoltage (IV) and capacitance-voltage (CV) techniques (described in sections 2 and 3), others require probes capable of investigation on the nanometer scale. Transmission electron microscopy (TEM) can be used to investigate such polarizationinduced phenomena with atomic resolution, but involves destructive and laborious sample preparation. SPM techniques offer powerful, non-destructive tools to effectively and conveniently probe electronic materials and devices. Since the discovery of scanning tunneling microscopy (STM) in 1981 by Binnig and Rohrer [7], the field of SPM has advanced to encompass a large number of variations including, (i) STM, (ii) Atomic force microscopy (AFM), (iii) Electrostatic force microscopy (EFM), (iv) Scanning capacitance microscopy (SCM), (v) Scanning conduction microscopy (SCM), (vi) Magnetic force microscopy (MFM), (vii) Piezoelectric force microscopy (PFM), and (vii) Near field scanning optical microscopy (NSOM). Of the above modes, the first five modes have been extensively used for semiconductor characterization [8–15]. For WBS characterization, especially III-N, AFM, EFM and SCM modes are the most important due to the strong polarization of III-N and SiC. In the latter half of this chapter, we focus on the characterization of III-N by EFM operated in

The fascinating many-body physics involved in the interaction of a single magnetic impurity with the conduction electrons of its nonmagnetic metallic host is reflected in unconventional phenomena in magnetism, transport properties and the specific heat. Characteristic low-energy excitations, termed the Kondo resonance, are generally believed to be responsible for this striking behaviour. However, in spite of an intense research for over more than 30 years, a direct spectroscopic observation of the Kondo resonance on individual magnetic adatoms withstood experimental efforts hitherto. The development of low-temperature scanning tunneling microscopes (STM) operating under ultrahigh vacuum (UHV) conditions has provided new opportunities for investigating locally the electronic structure at surfaces. At low temperatures, due to the reduced broadening of the Fermi level of the STM tip and the sample, rather high energy resolution (≤ 1 meV) in scanning tunneling spectroscopy (STS) is achievable. Moreover, the absence of diffusion together with the spatial resolution of the STM enables detailed studies of electronic states on and near single adsorbed atoms and other nanoscale structures. Recently, for the first time, two such STS/STM experiments spatially resolved the electronic properties of individual magnetic adatoms displaying the Kondo effect. In particular, the observed Fano lineshape of the Kondo resonance reveals unambiguously the details of the quantum mechanical interference between the localized orbital and the conduction electrons on an atomic length scale [1,2]. This achievement of the detection of individual magnetic atoms with atomic resolution opens new perspectives for probing magnetic nanostructures.

This article presents theoretical simulations of scanning tunnelling microscope (STM) images and spectra of nanostructures. It begins with an overview of the theories of STM and scanning tunnelling spectroscopy (STS), focusing on four main approaches: the perturbation or Bardeen approach, the Tersoff–Hamann approach and its extension, the scattering theory or Landauer–Bütticker approach, and the non-equilibrium Green's function or Keldysh approach. It then considers conventional STM and STS experimental investigations of various systems including clean surfaces, ad-atoms, single molecules, self-assembled monolayers, and nanostructures. It also discusses STM activities that go beyond conventional STM images and STS, such as functionalized STM tip, inelastic spectroscopy identification, manipulation, molecular electronics and molecular machines.