Negative Differential Resistance on the Atomic Scale: Implications for Atomic Scale Devices

The conductance-voltage spectrum of molecular nanostructures measured by scanning tunneling spectroscopy (STS) is generally assumed to reflect the local density of states of the molecule. This excludes the possibility of observing negative differential resistance (NDR). We report here the observation of NDR in the scanning tunneling microscope (STM) current-voltage (I-V) characteristics of self-assembled monolayer (SAM) of 4-p-Terphenylthiol molecules on gold substrate measured using a platinum probe. We argue that the NDR arises from narrow structures in the local density of states at the tip apex atom and show that depending on the electrostatic potential profile across the system, NDR could be observed in one or both bias directions.

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.

Redox properties of α2-K8P2W17(M·OH2)O61 (M = MnII, ZnII, FeII, CoII, and NiII) Wells–Dawson heteropolyacids probed by scanning tunneling microscopy and tunneling spectroscopy

The controlled manipulation of silicon at the nanometer scale will facilitate the fabrication of new types of electronic devices. The scanning tunneling microscope (STM) can be used to manipulate strongly bound silicon atoms or clusters at room temperature. Specifically, by using a combination of electrostatic and chemical forces, surface atoms can be removed and deposited on the STM tip. The tip can then move to a predetermined surface site, and the atom or cluster can be redeposited. The magnitude of such forces and the amount of material removed can be controlled by applying voltage pulses at different tip-surface separations.

The scanning tunnelling microscope is capable of the real-space imaging and spectroscopy of molecules on an atomic scale. Numerous attempts have been made to use the scanning tunnelling microscope to sequence single DNA molecules, but difficulties in preparing samples of long-chain DNA molecules on surfaces, and problems in reproducing results have limited these experiments. Here, we report single-molecule DNA sequencing with a scanning tunnelling microscope by using an oblique pulse-injection method to deposit the molecules onto a copper surface. First, we show that guanine bases have a distinct electronic state that allows them to be distinguished from the other nucleic acid bases. Then, by comparing data on M13mp18, a single-stranded phage DNA, with a known base sequence, the 'electronic fingerprint' of guanine bases in the DNA molecule is identified. These results show that it is possible to sequence individual guanine bases in real long-chain DNA molecules with high-resolution scanning tunnelling microscope imaging and spectroscopy.

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.

Scanning tunneling spectroscopy observations of the Coulomb staircase at room temperature from the dioctyldithiophosphate sodium salt molecules deposited on the (0001) graphite surface

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.

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.

Using scanning tunneling microscopy and transmission electron microscopy, we demonstrate the existence of antiphase boundaries between neighboring grains shifted by a fraction of a quintuple layer in epitaxial (0001) films of the three-dimensional topological insulator Bi(2)Se(3). Scanning tunneling spectroscopy and first-principles calculations reveal that these antiphase boundaries provide electrostatic fields on the order of 10(8) V/m that locally charge the Dirac states, modulating the carrier density, and shift the Dirac point by up to 120 meV. This intrinsic electric field effect, demonstrated here near interfaces between Bi(2)Se(3) grains, provides direct experimental evidence at the atomic scale that the Dirac states are indeed robust against extended structural defects and tunable by electric field. These results also shed light on the recent observation of coexistence of Dirac states and two-dimensional electron gas on Bi(2)Se(3) (0001) after adsorption of metal atoms and gas molecules.