Evaluation of lateral resolution of scanning surface microscopy by total internal reflection with thermal lens effect

We have developed a novel method for in situ and non-destructive surface analyses, or a total internal reflection with thermal lens spectroscopy (TIR-TLS), which has sufficient sensitivity to monitor phenomena in thin films, such as lipid bilayers. In this study, we applied TIR-TLS to microscopy for surface analyses, and we experimentally obtained its lateral resolution using the edge of a chromium film made by a photolithography technique. The obtained resolution was 20 microm, which was 60% of the diameter of an excitation beam at the interface. The estimated resolution with a simple model agreed with the experimental one, and from this model, TIR-TLS microscopy has the same resolution as that of ordinary optical microscopy. The microscopy by TIR-TLS was applied to a sample whose contrast was too weak to be visually seen, and an image of the sample was obtained without any loss of resolution.     

Spatial resolution and information transfer in scanning transmission electron microscopy

The relation between image resolution and information transfer is explored. It is shown that the existence of higher frequency transfer in the image is just a necessary but not sufficient condition for the achievement of higher resolution. Adopting a two-point resolution criterion, we suggest that a 10% contrast level between two features in an image should be used as a practical definition of resolution. In the context of scanning transmission electron microscopy, it is shown that the channeling effect does not have a direct connection with image resolution because sharp channeling peaks do not move with the scanning probe. Through a quantitative comparison between experimental image and simulation, a Fourier-space approach is proposed to estimate defocus and sample thickness. The effective atom size in Z-contrast imaging depends on the annular detector's inner angle. Therefore, an optimum angle exists for the highest resolution as a trade-off between reduced atom size and reduced signal with limited information transfer due to noise.     

Localised electrochemical impedance spectroscopy with high lateral resolution by means of alternating current scanning electrochemical microscopy

A new method for measuring local interfacial impedance properties with high lateral resolution was developed by combination of electrochemical impedance spectroscopy (EIS) with scanning electrochemical microscopy (SECM). Alternating current scanning electrochemical microscopy (AC-SECM) allowed to identify and visualise microscopic domains of different conductivity/electrochemical activities on solid/liquid interfaces immersed into an electrolyte. The performance of the method was illustrated by imaging an array of Pt-band microelectrodes in solutions of low conductivity in the absence of any redox mediator.     

Characterization of dental composites by thermal analysis, infrared spectroscopy and scanning electron microscopy

Commercial light-cured dental composites were used in this study. Two laboratorial composites, Resilab (Wilcos/Brazil), Epricord (Kuraray/Japan) were compared under cured and uncured conditions. Thermal analysis, infrared spectroscopy and scanning electron microscopy were used to evaluate the dental composites. The mass change and heat flow signals (TG–DSC) were recorded simultaneously by using STA 409 PC Luxx (NETZSCH), in the 25–800 °C temperature range at a heating rate of 10 °C/min under nitrogen atmosphere (70 mL/min). Employing thermo-microbalance TG 209 C F1 Iris (NETZSCH) coupled to the BRUKER Optics FTIR TENSOR, the samples were analyzed by combined thermogravimetric and spectroscopic methods (TG–FTIR). The initial sample mass was about ~12 mg, the data collection have been done in the 35–800 °C temperature range at a heating rate of 20 K/min in nitrogen atmosphere (flow rate: 40 mL/min). Finally, superficial topographic was analyzed by scanning electron microscopy (SEM). Dental composite evaluation suggests a high thermal stability and inorganic content in RES D sample. Degrees of conversion (DC) values were almost the same and there was no direct relationship between DC and amount of particles and size. Similar compositions were found in all samples.     

Electrochemical concept of scanning tunnel microscopy and scanning tunnel spectroscopy

The history of investigation, development and application of electrochemical scanning tunneling microscopy (ESTM) are reviewed.     

Conformational analysis of self-organized monolayers with scanning tunneling microscopy at near-atomic resolution

We describe the synthesis and a novel approach to the conformational analysis of 2,2'-bipyridines (bpy) bearing aromatic rich Frechet-type dendritic wedges of the first and second generation as substituents. The evaporation of solutions of these new ligands on graphite surfaces under ambient conditions results in the formation of self-organized monolayers. Scanning tunneling microscopy (STM) investigations of the monolayers under ambient conditions (air, 298 K) gave images at submolecular and near-atomic resolution. The analysis of the STM images includes the following processes: (i) identification and reproduction of potential homoconformational domains, (ii) exclusion of improper data using quality criteria for drift and feedback artifacts, (iii) compilation of running averages and checking for averaging artifacts, (iv) analysis of three-dimensional and contour plots, (v) calculation of the HOMO properties of the free molecules, and (vi) final conformational assignment based on all accessible information. Following this procedure, two different conformations could be assigned to domains observed in the monolayers of the first-generation (G1) and second-generation (G2) dendritic compounds. Homoconformational domains are observed side-by-side. The different conformations arise from syn or anti arrangements at the ether substituents. An additional conformational effect is found upon treating the G1 domains with HCl gas, when a partial rearrangement of the bpy from trans to cis occurs, concomitant with protonation.     

Potentials of thermal lens spectroscopy for polymethacrylate optical sensors

The sensitivity of the earlier proposed procedures for the determination of iron(II, III) with 1,10-phenanthroline, silver(I) with dithizone, mercury with copper(II) dithizonate, copper(II) with lead diethyldithiocarbamate, and ascorbic acid with 2,6-dichlorophenolindophenol using polymethacrylate optical sensitive elements for solid phase spectrophotometry is enhanced through the use of thermal lens spectrometry as the most sensitive method of molecular absorption spectroscopy. The limits of detection for all photometric reactions in the polymethacrylate matrix are reduced by an order of magnitude (to 10 nM) without substantial changes in the experimental conditions.     

4-Mercaptopyridine on Au(111): a scanning tunneling microscopy and spectroscopy study

The adsorption of 4-mercaptopyridine (4MPy) molecules on reconstructed Au(111) is investigated by Scanning Tunneling Microscopy (STM) and Spectroscopy (STS) at low temperature and under ultra-high vacuum (UHV) conditions. As made visible by STM, at low coverage (<10%) 4MPy adsorbs preferentially at elbow sites of the Herringbone reconstruction and at step edges of the Au(111). Increasing coverage (but still <30%) results in formation of molecular chains followed, at even higher coverage, by a 3-dimensional growth. Detailed analysis of z-V spectroscopy (ramping the tunneling bias V while keeping the tunneling current constant) provides information on the bias dependent apparent height of a single 4MPy/Au(111) as well as on the local density of states (LDOS) of single and chain 4MPy molecules in comparison to the bare Au(111) surface revealing a significant shift of the lowest unoccupied molecular orbital (LUMO) towards lower energy for molecules within chains. Additionally, the data provide no evidence that for these samples prepared in UHV the adsorption of 4MPy on Au(111) requires mediating Au adatoms. Also, clear indications are given that the adsorption does not induce a strong reduction of the Au DOS close to its Fermi energy. Finally, in context of the apparent STM height of 4MPy molecules, the behavior of the differential barrier height Φ(diff)(V) = (?(z)?(V)I/?(V)I)(2) on bare Au(111) and 4MPy/Au(111) is analyzed and the corresponding experimental values are applied to recover the LDOS of the molecule for unoccupied states according to a previously published numerical recipe [B. Koslowski, H. Pfeifer and P. Ziemann, Phys. Rev. B, 2009, 80, 165419 and M. Ziegler, N. Néel, A. Sperl, J. Kr?ger, and R. Berndt, Phys. Rev. B, 2009, 80, 125402]. In this way, one obtains a spectrum comprising a constant DOS of the Shockley-like surface state of Au(111) and a Lorentzian line attributed to the LUMO of 4MPy.     

Monitoring conformational diversity in self-organised monolayers with scanning tunnelling microscopy at near atomic resolution

The evaporation of solutions of dendron-functionalised 2,2'-bipyridines on a graphite surface gives highly ordered monolayers; near atomic resolution STM imaging has allowed a detailed conformational analysis to be made.     

In-lens cryo-high resolution scanning electron microscopy: methodologies for molecular imaging of self-assembled organic hydrogels.

The micro- and nanoarchitectures of water-swollen hydrogels were routinely analyzed in three dimensions at very high resolution by two cryopreparation methods that provide stable low-temperature specimens for in-lens high magnification recordings. Gemini surfactants (gS), poly-N-isopropylacrylamides (p-NIP Am), and elastin-mimetic di- (db-E) and triblock (tb-E) copolymer proteins that form hydrogels have been routinely analyzed to the sub-10-nm level in a single day. After they were quench or high pressure frozen, samples in bulk planchets were subsequently chromium coated and observed at low temperature in an in-lens field emission SEM. Pre-equilibrated planchets (4-40 degrees C) that hold 5-10 microl of hydrogel facilitate dynamic morphological studies above and below their transition temperatures. Rapidly frozen samples were fractured under liquid nitrogen, low-temperature metal coated, and observed in-lens to assess the dispersion characteristics of micelles and fragile colloidal assemblies within bulk frozen water. Utilizing the same planchet freezing system, the cryoetch-HRSEM technique removed bulk frozen water from the hydrogel matrix by low-temperature, high-vacuum sublimation. The remaining frozen solid-state sample faithfully represented the hydrogel matrix. Cryo- and cryoetch-HRSEM provided vast vistas of hydrogels at low and intermediate magnifications whereas high magnification recordings and anaglyphs (stereo images) provided a three-dimensional prospective and measurements on a molecular level.     

High resolution scanning of DNA autoradiographs.

The sizes of DNA fragments (alleles) on autoradiographs are measured by means of a computerised video scanner developed in this laboratory. This report investigates the advantages of using higher resolution equipment for this purpose. High resolution is simulated by using existing equipment and moving the video camera closer to the autoradiograph. Test autoradiographs, used to validate the original scanner, have been rescanned to determine the degree of improvement that could be obtained using expensive high resolution equipment.     

Correlating surface microstructures with reactivity on commercially pure zirconium using scanning electrochemical microscopy and scanning electron microscopy

Scanning electrochemical microscopy and scanning electron microscopy were employed to correlate the surface microstructures with surface reactivity of commercially pure zirconium. It was found that heightened reactivity was associated with iron impurities lying beneath the oxide surface. This could result in failure of nuclear reactor components fabricated using zirconium alloys due to hydrogen ingress and corrosion. COMSOL multiphysics software was used to quantify the electrochemical kinetic constants associated with the differences in surface reactivity.     

Inelastic tunneling spectroscopy using scanning tunneling microscopy on trans-2-butene molecule: spectroscopy and mapping of vibrational feature

Inelastic tunneling spectroscopy (IETS) measurement using scanning tunneling microscopy (STM) with a commercially available STM set up is presented. The STM-IETS spectrum measured on an isolated trans-2-butene molecule on the Pd(110) shows a clear vibrational feature in d2I/dV2 at the bias voltage of 360 mV and -363 mV, which corresponds to the nu(C-H) mode (d2I/dV2 approximately 10 nA/V2). In addition, we have obtained an image by mapping the vibrational feature of nu(C-H) in d2I/dV2. The image is obtained by scanning the tip on the surface with the feedback loop activated while the modulation voltage is superimposed on the sample voltage. With the method that is readily performable with conventional software, we have clearly differentiated the molecules of trans-2-butene and butadiene through the mapping of the vibrational feature, demonstrating its capability of chemical identification in atomic scale.     

Spatial arrangement of alpha-cyclodextrins in a rotaxane. Insights from free-energy calculations

The rotaxane formed by alpha-cyclodextrins (alpha-CDs) threaded onto a poly(ethylene glycol) (PEG) chain was investigated in the gas phase and in an aqueous solution by means of molecular dynamics simulations. The free-energy difference between the three possible spatial arrangements of consecutive alpha-CD--viz.. head-to-head (HH), head-to-tail (HT), and tail-to-tail, was determined using free-energy perturbation calculations. These simulations reveal that the interaction of alpha-CD with the PEG chain is very similar in the two surroundings, whereas the mutual interaction of the macrocycles is stronger in the gas phase than in the aqueous solution. Moreover, difference in the overall interaction appears to stem primarily from changes in the electrostatic contribution. Analysis of intermolecular hydrogen bonds indicates that hydrogen bonds created between alpha-CD and water molecules weaken the hydrogen-bonding interaction of adjacent alpha-CDs. Comparison of the free-energy differences characteristic of the three possible spatial arrangements of contiguous alpha-CDs reveals that the HH motif of the rotaxane is the most stable in the gas phase due to the hydrogen-bond formation between the secondary hydroxyl groups of the two alpha-CDs, and the slight preference for the HT motif in aqueous solution, which can be related to the directionality of the dipole moment borne by the macrocycles in these two motifs.     

The probe profile and lateral resolution of scanning transmission electron microscopy of thick specimens

Lateral profiles of the electron probe of scanning transmission electron microscopy (STEM) were simulated at different vertical positions in a micrometers-thick carbon sample. The simulations were carried out using the Monte Carlo method in CASINO software. A model was developed to fit the probe profiles. The model consisted of the sum of a Gaussian function describing the central peak of the profile and two exponential decay functions describing the tail of the profile. Calculations were performed to investigate the fraction of unscattered electrons as a function of the vertical position of the probe in the sample. Line scans were also simulated over gold nanoparticles at the bottom of a carbon film to calculate the achievable resolution as a function of the sample thickness and the number of electrons. The resolution was shown to be noise limited for film thicknesses less than 1 μm. Probe broadening limited the resolution for thicker films. The validity of the simulation method was verified by comparing simulated data with experimental data. The simulation method can be used as quantitative method to predict STEM performance or to interpret STEM images of thick specimens.     

Characterization and theory of electrocatalysts based on scanning electrochemical microscopy screening methods

A strategy for finding new electrocatalysts for the oxygen reduction reaction (ORR) in acidic solutions is outlined and illustrated with results for Pd-Co catalysts. This is based on establishing guidelines for selecting test systems, rapid preparation of arrays, and rapid screening by scanning electrochemical microscopy. Promising candidates are further tested as supported electrocatalysts by larger scale electrochemical methods and in fuel cells, with optimization of the composition and structure. Those that emerge are characterized by a variety of methods, including X-ray diffraction, scanning electron microscopy, and X-ray photoemission spectroscopy. Finally, density functional theory is used for detailed calculations of oxygen adsorption and dissociation on the material and provides better guidelines for further testing.     

Simultaneous detection of uric acid and glucose on a dual-enzyme chip using scanning electrochemical microscopy/scanning chemiluminescence microscopy

Scanning electrochemical microscopy (SECM) and scanning chemiluminescence microscopy (SCLM) were used for imaging an enzyme chip with spatially-addressed spots for glucose oxidase (GOD) and uricase microspots. For the SECM imaging, hydrogen peroxide generated from the GOD and/or uricase spots was directly oxidized at the tip microelectrode in a solution containing glucose and/or uric acid (electrochemical (EC) detection). For the SCLM imaging, a tapered glass capillary (i.d. of 1∼2 μm) filled with luminol and horseradish peroxidase (HRP) was used as the scanning probe for generating the chemiluminescence (CL). The inner solution was injected from the capillary tip at 78 pl s⁻¹ while scanning above the enzyme-immobilized chip. The CL generated when the capillary tip was scanned above the enzyme spots was detected using a photon-counter (CL detection). Two-dimensional mapping of the oxidation current and photon-counting intensity against the tip position affords images of which their contrast reflects the activity of the immobilized GOD and uricase. For both the EC and CL detections, the signal responses were plotted as a function of the glucose and uric acid concentrations in solution. The sensitivities for the EC and CL detection were found to be comparable.     

Writing and reading methodology for biochips with sub-100-nm chemical patterns based on near-field scanning optical microscopy.

This paper demonstrates a writing and reading methodology, which allows both to create and to detect sub-100-nm carboxyl-terminated patterns on light-transmissive quartz substrates by the same instrumental system. Such a technique, capable of creating carboxyl-terminated nanopatterns, offers several benefits for the miniaturization of biochips, since the carboxyl-terminated nanopatterns allow the easy immobilization of biomolecules by amide bond formation. As a consequence, increasingly miniaturized biochips require suitable analytical methods for the detection of nanopatterns. In our approach, carboxyl-terminated nanopatterns of down to 80 nm width were created using a photolabile silane coupling agent and a UV laser coupled to a near-field scanning optical microscope (NSOM). The same NSOM system was then used in a next step to detect the fabricated carboxyl-terminated nanopatterns after modification with a fluorescent label. Furthermore, as a first step towards biochip applications, the successful immobilization of several biomolecules, such as streptavidin, IgG and DNA on carboxyl-terminated nanopatterns was demonstrated. We have shown that our approach has the potential to lead to a new bioanalytical method, which enables one to write and to read biochips on a sub-100-nm scale by the same system.     

Isolated supramolecules on surfaces studied with scanning tunneling microscopy

To date, supramolecular chemistry is an ever growing research field owing to its crucial role in molecular catalysis, recognition, medicine, data storage and processing as well as artificial photosynthetic devices.Different isolated supramolecules were prepared by molecular self-assembly on surfaces. This review mainly focuses on supramolecular aggregations on noble metal surfaces studied by scanning tunneling microscopy, including dimers, trimers, tetramers, pentamers, wire-like assemblies and Sierpin′ ski triangular fractals. The variety of self-assembled structures reflects the subtle balance between intermolecular and molecule–substrate interactions, which to some extent may be controlled by molecules, substrates and the molecular coverage. The comparative study of different architectures helps identifying the operative mechanisms that lead to the structural motifs. The application of these mechanisms may lead to novel assemblies with tailored physicochemical properties.     

Structure and topography of molecular assemblies on solid substrates by infrared spectroscopy and scanning tunneling microscopy

Recently we have combined infrared spectroscopy and atomic resolution scanning tunneling microscopy (STM) to probe the local structure and intermolecular arrangement of molecules within thin films. IR spectroscopy provides spatially averaged information about orientation of the molecules with respect to the surface and about intermolecular arrangement within the crystallographic unit cell. STM data yields a local picture of molecular packing within the film. The requirements of an atomically flat (over distances of hundreds of angstroms) conducting substrate for the STM are fulfilled by an epitaxially grown film of gold on a cleaved mica substrate which also provides a good infrared reflective surface, enabling IR and STM measurements on identical samples. Systems investigated include Langmuir-Blodgett films of cadmium arachidate and self-assembled films of octadecyltrichlorosilane.