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In this work, Scanning Tunneling Microscopy (STM) images were simulated for the organic molecule 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) to study the effect of the number of tunneling states and the integrated Local Density of States (LDOS) isosurface value. Local Density Approximation of Density Functional Theory (DFT-LDA) calculations were performed to achieve the simulated images under the Tersoff and Hamann approximation. The number of tunneling states has a strong effect on the image appearance of the patterns. Intermediate isosurface values for the integrated LDOS produce good resolution and matching in relation to the experimental STM image. Both parameters seem to be of significant importance for STM image simulation.
This paper presents a new, general reconstruction algorithm that enables modeling of the experimental resonance spectrum of a prismatic microcantilever in a viscous fluid. A closed-form solution is obtained for the microcantilever frequency response from the equation of motion with fluid damping and internal friction terms, which allows direct calculation of the fluid damping and internal friction damping constants. In principle, the fluid damping constant has a simple relationship to viscosity thus potentially simplifying the process of obtaining viscosity from experimental data. Finally, the model is compared to experimental data.
Epitaxial calcium fluoride (CaF2) film surfaces grown on Si(111) were imaged with the atomic force microscopy operated in the noncontact mode in ultrahigh vacuum. Our experimental results reproducibly reveal two kind of topographic patterns with the atomic scale contrast. The line profiles obtained from the topographic image exhibit that the change of tip-polarity plays the important role for obtaining two atomic corrugation patterns by considering the interaction between the tip and the two topmost surface atoms. It is similar to the results from the literature obtained on the cleaved CaF2 surface with both positively and negatively terminated tip.
We propose molecular-scale photochemical-reaction control using triplet-triplet (T-T) energy transfer from a donor molecule attached on a probe to an acceptor on an insulator surface. In this work, we studied the feasibility of photochemical reactions on a substrate surface using a triplet sensitizer probe. We observed an efficient T-T energy transfer from Michler's ketone on the substrate to an acceptor molecule, cinnamoyl group, on the other substrate facing it. Approximately a quarter of the cinnamoyl groups were dimerized by triplet sensitization. We used a cone-shaped dendron molecule to avoid sensitizer self-quenching caused by the triplet energy migration within the probe surface. We then confirmed efficient site separation of the cone's focal point by measuring the absorption and fluorescent properties of the rhodamine B attached to the focal point. The generation-three dendrons provide enough distance between the functional sites on the probe to reduce singlet energy transfer and it should control triplet energy migration.
The effect of nitrogen composition on structural parameters of intrinsic quantum dots (QDs) has been studied in GaAs1-yNy and InxGa1-xAs1-yNy alloys (y∼0. 015–0.03) using low-temperature near-field scanning optical microscopy (NSOM) combined with magneto-photoluminescence spectroscopy. We used measurements of the diamagnetic shift (magnetic field strength 0–10T), temperature dependent spectra (temperature range 5–300K) and near-field monochromatic images for the estimation of the size, nitrogen excess and density of QDs. The obtained values (size ∼10–30 nm, nitrogen excess ∼0.005 and density ∼100 /μm-3) suggest spontaneous formation (phase separation) of QDs. Strong lateral inhomogeniety of the QD distribution on a micron length scale was observed.
Ballistic electron emission luminescence (BEEL) is a further development of ballistic electron emission microscopy (BEEM) combining three-terminal hot electron injection and interband radiative recombination in direct-gap semiconductor materials. By using a planar tunnel-junction emitter rather than a STM tip, a spectrographic analysis of the induced electroluminescence can be performed with the help of much higher current injection level. We demonstrate the operational principle of BEEL in a GaAs/AlxGa1−xAs heterostructure with a layer of InAs quantum dots (QDs) as the optical active layer. The wavelength-resolved BEEL spectra from planar tunnel-junction devices disclose the QD luminescence as a peak near 1.34 eV accompanied with a linear quantum-confined Stark shift. At higher collector voltage, luminescence from bulk states of GaAs peaked at 1.48 eV is observed. The spectrally integrated BEEL intensity as a function of collector voltage fits well with the results from STM tip injection, which is measured in a single-photon-counting mode. Measurement of ballistic electron current spectroscopy is made possible by freezing out the thermionic leakage current at low temperatures. Our results indicate that it is feasible to simultaneously acquire topographic, electronic and photonic information of buried light-emitting semiconductor heterostructures.
An original instrument intended to integrate the advantages of scanning probe, near and far field optical microscopy is presented. The device named Apertureless Head was specially designed to be incorporated to Ntegra scanning probe laboratory, the powerful analytical instrumentation in the nanotechnology field produced by NT-MDT Company, Russia. The device is supposed to be a powerful tool for high resolution analysis in different fields of investigations such as material sciences (optical and optoelectronic, magnetic, semi- and superconducting materials), polymers and biological sciences (structural biology, molecular and cell biology, microbiology, etc.). Probe tip plane light distributions are calculated and different light propagation schemes are discussed.
We report on the direct observation of optical near-field enhancement around metallic nanoparticles. We used an easy to set up approach which consists in irradiating a photosensitive azo-dye polymer film spin-coated on metallic nanostructures. Photoinduced topographical modifications of the polymer film surface are characterized after irradiation by atomic force microscopy (AFM). Comparisons between AFM images and numerical simulations show that these photo-induced topography agrees with the near-field intensity distribution around the nano-structures. The possibility of generating complex structures is demonstrated.
We use an improved setup for deducing quantitative surface potential values by means of frequency modulated Kelvin-probe force microscopy (FM-KPFM). This method is sensitive to the electrostatic force gradient rather than the absolute force probed in KPFM so far, and therefore provides both a higher lateral resolution and quantitative values. Furthermore, FM-KPFM allows using cantilevers with high spring constants which even favors both the stability and increased topographic resolution. Here, we apply FM-KPFM to deduce interfacial electrical properties of the sub-monolayer coverage of three adsorbates on metal substrates: lithium chloride films, Copper-porphyrines, and C60 molecules.
Electrochemical experiments with tetracyanoquinodimethane (TCNQ) modified electrodes in contact with aqueous copper containing electrolytes leads to the incorporation and expulsion of copper ions. This process occurs concomitantly with nucleation and growth processes and significant crystal fragmentation to produce particles with dimensions of the order of 10's of nanometres. During reduction of TCNQ and intercalation of copper ions, different phases of the semiconducting compound CuTCNQ are formed.[1,2] The preparation of both conducting and insulating substrates coated with electroactive TCNQ and CuTCNQ particles of variable size have been made by dip and spin coating procedures. Results suggest that the phase and hence electronic properties of CuTCNQ is dependent on the size of particles that decorate the electrode surface. Combining atomic force microscope (AFM) based methods that interrogate the morphological and electronic properties of nanometre sized particles with use of a scanning electrochemical microscope (SECM) is a new advance in materials characterisation that has proved highly valuable in understanding the highly complex behaviour of these semi-conducting particles.
The ability to probe chemical heterogeneity with nanometer scale resolution is essential for developing a molecular–level understanding of a variety of phenomena occurring at surfaces of materials. One area that could benefit greatly from nanoscale chemical measurement is an understanding of the degradation mechanisms of polymeric materials exposed to the environment. For example, the degradation (photo and hydrolytic) of polymers and polymeric materials has been observed to occur non-uniformly in which nanometer pits form locally, which deepen and enlarge with exposure (1, 2). The pitting has been postulated to initiate in the hydrophilic degradation-susceptible regions of the films (3). However, due to the lack of spatial resolution of the most current surface analytical techniques, the chemical nature of the degradation-initiated locations has not been identified. The use of a chemically-functionalized probe in an AFM (chemical force microscopy CFM) (4) has been shown to be capable of discriminating chemically-different domains of self-assembled monolayer (SAM) surfaces at the nanoscale spatial resolution. This study provides data to demonstrate that, by using proper RH at the tip-sample environment, the contrast between the hydrophilic and hydrophobic domains in SAM and polymer samples can be discerned, and presents results on the effects of RH on tipsample adhesion forces for different substrates.
We present two new approaches that significantly enhance the analytic power of Scanning Conductance Microscopy (SCM) and Scanning Gate Microscopy (SGM). First, we present a quantitative model that explains the phase shifts observed in SCM, by considering the change in the total capacitance of the tip-sample-substrate system. We show excellent agreement with data on samples of (conducting) single wall nanotubes and insulating polyethylene oxide (PEO) nanofibers. This model is also used to determine the dielectric constant of PEO nanofibers, a general approach that can be extended to other dielectric nanowires. Second, we extend the SGM to frequencies up to 15MHz, and use it to image changes in the impedance of carbon nanotube field effect transistor (CNFET) circuits induced by the SGM-tip gate. We show that these measurements are consistent with a simple RC parallel circuit model of the CNFET, with a time constant of 0.3 μs.
Single crystalline ZnSe nanowires were fabricated on GaAs substrates by molecular beam epitaxy technique via Au-catalyzed vapor-liquid-solid reaction. The nucleation and initial growth of the nanowires were investigated by high-resolution transmission electron microscopy. It was revealed that Au catalysts initially reacted with the substrate forming binary AuGa2 alloy droplets. The sizes of the catalysts determined the growth direction of ZnSe nanowires. A model based on the surface energies of the nanowire nuclei was proposed to explain the size dependence of growth direction for ZnSe nanowires.
Ge nanocrystals embedded in SiO2 via a low temperature thermal oxidation of a Si/Ge/Si stack structure grown by low pressure chemical vapour deposition or by molecular beam epitaxy in localized focalized ion beam nanopatterns are characterized by scanning capacitance microscopy. Local electrical spectroscopy on the Ge structure shows hole or electron charging by the Ge nanocrystal, thanks to the complete electrical isolation induced by the oxidation process. The scanning capacitance microscope allows measuring the discharging kinetics of the electron, giving an order of the retention time value of several hours.
Mesoamerican copper metallurgy emerged in West Mexico sometime between A.D. 600 and 800. Over a period of approximately 900 years a wide variety of artifacts, typically decorations and other valuable non-utilitarian goods, were produced. By A.D. 1450, the Tarascan kingdom in the state of Michoacan had become the most important center of pre-Hispanic metalworking. Metallurgy played a significant role in the structure of political and economic power in the Tarascan Empire. Metal adornments used as insignia of social status and public ritual became even more associated with political power. While metal was used for an array of goods, virtually nothing is known about the manufacture and the organization of production of this material. Archaeological research at the site of Itziparátzico, near the modern Tarascan community of Santa Clara del Cobre, has recently located potential production areas where concentrations of smelting slag were recorded.
The smelting of ores is almost invariably related to the formation of slags, which form from the various impurities introduced into the smelting process, such as gangue minerals, furnace wall material, and fuel ash. Slag analysis thus has the potential for revealing important information about metallurgical technology. Copper smelting slag recovered from the excavations at Itziparátzico has been analyzed for microstructure and compositional properties using light microscopy, x-ray fluorescence spectrometry (XRF), and scanning electron microscopy with energy-dispersive x-ray spectrometry (SEM/EDS). Preliminary results indicate a smelting technology that used sulfidic ores and highly efficient furnaces. While further archaeological investigations are required to precisely date these activities, this technological information is important for establishing the context and scale of production of copper at the site.
The Si/SiO2 interface roughness has received tremendous interest due to its relation to channel mobility degradation and dielectric reliability. We have used ultra-high vacuum scanning tunneling microscopy to directly examine the Si/SiO2 interface and study the roughening effect caused by chemical etching. The rms-roughness extracted quantitatively from the STM topography was found to be doubled from 0.111nm to 0.232nm by the normal NH4OH/H2O2 treatment, and further increased to 0.285nm for additional etching steps. It was also found that there were no regular single steps on the SiO2/Si(100) interface.
Kelvin Probe Microscopy has been used to characterize the magnitude and spatial distribution of reproducible characteristic residual potential in electron beam irradiated silicon on insulator specimens (SIMOX). Focussed electron beam irradiation produces trapped charge within the insulating buried oxide layer which produces highly localized electric fields. The charging processes are dynamic, localized, and dependent on pre-existing and irradiation induced defect concentrations. The characteristic experimental surface potential distributions are compared with calculated model surface potential distributions. This work demonstrates that proximal probe methods which are usually considered to be surface analysis techniques, can be used to investigate subsurface properties and give insight into subsurface charging processes.
Atomically resolved images of single-wall carbon nanotubes (CNT) grown in a chemical vapor deposition (CVD) chamber were obtained with the scanning tunneling microscope (STM) under ambient conditions. We found that the average diameters d of the CVD-grown CNTs appear to fall into a bimodal distribution of 1.0 and 0.6 nm, and the chiral angle Ø was observed to be close to zero degree. The summation of the lattice indices (n+m) was determined to be 14 and 9 for d= 1.0nm and d= 0.6nm, respectively. The most possible lattice index pairs (n, m) with a chiral angle close to zero degree are (7, 7) and (5, 4), which indicates that the larger nanotubes are metallic and the smaller nanotubes are semi-conductive.
The analytical technique micro-thermal analysis (μ-TA) will be introduced and evaluated. It combines the visualisation power of atomic force microscopy (AFM) and it's ability to image topography, phase shifts, friction, stiffness, and adhesion with the characterisation capabilities of thermal analysis (thermal conductivity, micro-differential thermal analysis and micro-thermo mechanical analysis) resulting in a characterisation of surfaces with respect to their thermal and thermo-mechanical properties. The scanning mode may be used for the inspection of surfaces with respect to their thermal properties using a scanner with an active thermal sensor (heater, height and thermal sensor) as probe (tip), simultaneously acquiring topography, thermal conductivity and thermal diffusivity images. A second mode gives the option to perform a ocal thermo-mechanical analysis of discrete areas of a few square microns (L-TMA and L-CA), and detects at the same time during heating in contact mode changes of the sensor position and the heat flow to the sample in a very short time. Any thermodynamic phase transition, connected with a change in mechanical properties (softening, expansion, melting) and thermal properties (heat of fusion, change of heat capacity) will thus be detected by both methods. Applications for the analysis of especialy soft materials are presented and critically discussed. Moreover, the use of the instrument for controlled surface treatment is demonstrated resulting in the creation of super smooth polymeric surfaces. Furthermore, attempts to determine thermal conductivity's on surfaces on a quantitative matter wil be presented and discussed.
Scanning capacitance microscopy (SCM) has been commonly used to image dopant gradients in silicon and other semiconductors. As a mobile, high-resolution (to 10 nm) metal-oxide-semiconductor (MOS) probe, SCM also is a non-destructive, contactless tool with which to examine local variations in dielectric thin film quality and local variations in semiconductor substrate properties. Virtually any measurement that can be made with fabricated metal electrodes can also be made with SCM. Two particular applications being pursued are characterization of high-κ dielectric films on silicon for next generation integrated circuits and characterization of native and deposited insulators on wide bandgap semiconductors.
Local differential capacitance (ΔC) versus tip bias (Vdc) measurements can be made with SCM using an ac voltage to generate the differential capacitance signal. These measurements differ from conventional C-V measurements due to the 3-D nature of the scanning capacitance microscope tip and the method used to generate the differential capacitance signal. Theoretical predictions and experimental measurements are made of SCM differential capacitance versus dc bias voltage (ΔC-V) curves for MOS capacitors with various levels of fixed and interface traps. The goal of this work is to determine if quantitative interface trap distributions can be made using SCM and if variations in interface density can be observed near defects or device structures. The response of the SCM MOS capacitance measurement to a local electric field stress and optical pumping from the atomic force microscope (AFM) laser will also be discussed.