Insulated gold scanning tunneling microscopy probes for recognition tunneling in an aqueous environment

Chemically functionalized probes are required for tunneling measurements made via chemical contacts ("Recognition Tunneling"). Here, we describe the etching of gold STM probes suitable for chemical functionalization with moieties bearing thiol groups. Insulated with high density polyethylene, these probes may be used in aqueous electrolytes with sub pA leakage currents. The area of the exposed probe surface was characterized via the saturation current in an electroactive solution (0.1 M K(3)Fe(CN)(6)). Twenty five percent of the probes had an exposed region of 10 nm radius or less.     

Coated silica tips for use in high-pressure, high-temperature, scanning tunneling microscopy

A novel application of coated silica tips for use in high-pressure, high-temperature, scanning tunneling microscopy is introduced. Thermal drift is reduced in the Z-direction due to the low thermal expansion of silica. Virtually, any conducting material that can be evaporated or sputtered can be used as a tip material. Experimental results are shown for tips sputter coated with platinum, along with images obtained.     

Single-step electrochemical method for producing very sharp Au scanning tunneling microscopy tips

A single-step electrochemical method for making sharp gold scanning tunneling microscopy tips is described. 3.0M NaCl in 1% perchloric acid is compared to several previously reported etchants. The addition of perchloric acid to sodium chloride solutions drastically shortens etching times and is shown by transmission electron microscopy to produce very sharp tips with a mean radius of curvature of 15 nm.     

High-resolution scanning tunneling microscopy for molecules

Scanning tunneling microscopy (STM) can detect individual molecular configuration with its high spatial resolution ability, but some intrinsical and extrinsic factors result in the complexities of STM imaging of single molecules. By combining STM experimental work and theoretical simulation with the local density approximation based on Bardeen perturbation method, we have explored the atomic-scale configuration of the following molecular systems: C(60) molecules adsorbed on Si(111)-(7x7); alkanethiol self-assembly monolayers on Au(111); C(60) molecule imaged by STM tip adsorbed with another C(60) molecule; O(2) molecule adsorbed on Ag(110) and CO molecule adsorbed on Cu(111) imaged by CO chemically modified STM tip. Some related problems including: molecule-substrate interactions, STM imaging mechanism, chemically modified STM tip, etc., are discussed.     

Fabrication of silver tips for scanning tunneling microscope induced luminescence

We describe a reliable fabrication procedure of silver tips for scanning tunneling microscope (STM) induced luminescence experiments. The tip was first etched electrochemically to yield a sharp cone shape using selected electrolyte solutions and then sputter cleaned in ultrahigh vacuum to remove surface oxidation. The tip status, in particular the tip induced plasmon mode and its emission intensity, can be further tuned through field emission and voltage pulse. The quality of silver tips thus fabricated not only offers atomically resolved STM imaging, but more importantly, also allows us to perform challenging "color" photon mapping with emission spectra taken at each pixel simultaneously during the STM scan under relatively small tunnel currents and relatively short exposure time.     

Coating of tips for electrochemical scanning tunneling microscopy by means of silicon, magnesium, and tungsten oxides

Different combinations of metal tips and oxide coatings have been tested for possible operation in electrochemical scanning tunneling microscopy. Silicon and magnesium oxides have been thermally evaporated onto gold and platinum-iridium tips, respectively. Two different thickness values have been explored for both materials, namely, 40 and 120 nm for silicon oxide and 20 and 60 nm for magnesium oxide. Alternatively, tungsten oxide has been grown on tungsten tips via electrochemical anodization. In the latter case, to seek optimal results we have varied the pH of the anodizing electrolyte between one and four. The oxide coated tips have been first inspected by means of scanning electron microscopy equipped with microanalysis to determine the morphological results of the coating. Second, the coated tips have been electrically characterized ex situ for stability in time by means of cyclic voltammetry in 1 M aqueous KCl supporting electrolyte, both bare and supplemented with K(3)[Fe(CN)(6)] complex at 10 mM concentration in milliQ water as an analyte. Only the tungsten oxide coated tungsten tips have shown stable electrical behavior in the electrolyte. For these tips, the uncoated metal area has been estimated from the electrical current levels, and they have been successfully tested by imaging a gold grating in situ, which provided stable results for several hours. The successful tungsten oxide coating obtained at pH=4 has been assigned to the WO(3) form.     

Transmission electron microscopy and scanning force microscopy of poly r(A-U) and poly r(A-U)-ethidium bromide

Transmission electron microscopy and scanning force microscopy of negative-stained, carbon-coated replica and mica-adsorbed preparations of 200 μM poly r(A-U) and 50 μM ethidium bromide/200 μM poly r(A-U) have been employed to evaluate ethidium-induced changes in poly r(A-U) topology. Poly r(A-U) alone exhibits elongated conformations 85–115 nm in length that possess a number of hairpin loops as well as single-stranded domains. While the double-stranded domains are found predominately at the base of the hairpin loops (diameter = 5–30 nm), other rod-like (presumably double-stranded) regions ranging from 25–80 nm in length are present in other portions of the poly r(A-U). In contrast with the poly r(A-U) alone, the EB/poly r(A-U) combination appears as a heterogeneous population of condensed structures whose lengths and widths vary from 12–88 nm and 15–45 nm, respectively. These conformational changes are due to a number of factors, including the displacement of ordered water surrounding the poly r(A-U) and charge shielding of the phosphate groups of the poly r(A-U) upon the binding of the ethidium.     

High-yield synthesis of conductive carbon nanotube tips for multiprobe scanning tunneling microscope

We have established a fabrication process for conductive carbon nanotube (CNT) tips for multiprobe scanning tunneling microscope (STM) with high yield. This was achieved, first, by attaching a CNT at the apex of a supporting W tip by a dielectrophoresis method, second, by reinforcing the adhesion between the CNT and the W tip by electron beam deposition of hydrocarbon and subsequent heating, and finally by wholly coating it with a thin metal layer by pulsed laser deposition. More than 90% of the CNT tips survived after long-distance transportation in air, indicating the practical durability of the CNT tips. The shape of the CNT tip did not change even after making contact with another metal tip more than 100 times repeatedly, which evidenced its mechanical robustness. We exploited the CNT tips for the electronic transport measurement by a four-terminal method in a multiprobe STM, in which the PtIr-coated CNT portion of the tip exhibited diffusive transport with a low resistivity of 1.8 kOmega/microm. The contact resistance at the junction between the CNT and the supporting W tip was estimated to be less than 0.7 kOmega. We confirmed that the PtIr thin layer remained at the CNT-W junction portion after excess current passed through, although the PtIr layer was peeled off on the CNT to aggregate into particles, which was likely due to electromigration or a thermally activated diffusion process. These results indicate that the CNT tips fabricated by our recipe possess high reliability and reproducibility sufficient for multiprobe STM measurements.     

Recent advances in atomic-scale spin-polarized scanning tunneling microscopy

The Mn3N2 (010) surface has been studied using spin-polarized scanning tunneling microscopy at the atomic scale. The principle objective of this work is to elucidate the properties and potential of this technique to measure atomic-scale magnetic structures. The experimental approach involves the use of a combined molecular beam epitaxy/scanning tunneling microscopy system that allows the study of atomically clean magnetic surfaces. Several key findings have been obtained. First, both magnetic and non-magnetic atomic-scale information has been obtained in a single spin-polarized image. Magnetic modulation of the height profile having an antiferromagnetic super-period of c = 12.14 A (6 atomic rows) together with a non-magnetic superstructure having a period of c/2 = 6.07 A (3 atomic rows) was observed. Methods of separation of magnetic and non-magnetic profiles are presented. Second, bias voltage-dependent spin-polarized images show a reversal of the magnetic modulation at a particular voltage. This reversal is clearly due to a change in the sign of the magnetic term in the tunnel current. Since this term depends on both the tip's as well as the sample's magnetic local density of states, the reversal can be caused by either the sample or the tip. Third, the shape of the line profile was found to vary with the bias voltage, which is related to the energy-dependent spin contribution from the 2 chemically inequivalent Mn sites on the surface. Overall, the results shown here expand the application of the method of spin-polarized scanning tunneling microscopy to measure atomic-scale magnetic structures.     

Electrical detection of β-amyloid (1-40) using scanning tunneling microscopy

Numerous studies have shown that the presence of β-amyloid (1-40) in cerebrospinal fluid can be used as a potential biomarker for Alzheimer's disease. Identifying biomarkers for Alzheimer's disease is highly important because these biomarkers could be used to establish the diagnosis before the disease reaches clinical severity. In this study, a vertically configured electrical detection system associated with scanning tunneling microscopy (STM) was used to characterize antigen–antibody binding interactions. The proposed technique can be easily utilized to construct a multiple measurement system in a protein chip. The immunocomplexes used in the model protein comprise β-amyloid (1-40), corresponding antibody fragments, and gold nanoparticle–antibody conjugates. The electrical tunneling current between the STM tip and these complexes exhibited a peak-like pulse, where the frequency of these pulses was dependent on the surface density of bound complexes. Hence, a quantitative measurement of β-amyloid concentration from a periodogram analysis of peak frequency was successfully achieved at concentrations as low as 1 fg/mL.     

Characterization of nanowires and molecular switches with scanning tunneling microscopy

We consider the problem of finding the energy spectra, transmission and charge distribution on nanowires with scanning tunneling spectroscopy. Within this context, we first test the applicability of the method by comparing the output of our theory with available experimental results on In-based quantum wells patterned on nanowires. Then we study the correlation between density of states and electron transmission curves. Third, we investigate the induction of ultrafast electronic transitions due to the presence of a molecular switch or a scanning probe microscope tip.     

Compact device for cleaning scanner-mounted scanning tunneling microscope tips using electron bombardment

Most scanning probe techniques rely on the assumption that both sample and tip are free from adsorbates, residues, and oxide not deposited intentionally. Getting a clean sample surface can be readily accomplished by applying ion sputtering and subsequent annealing, whereas finding an adequate treatment for tips is much more complicated. The method of choice would effectively desorb undesired compounds without reducing the sharpness or the general geometry of the tip. Several devices which employ accelerated electrons to achieve this are described in the literature. To minimize both the effort to implement this technique in a UHV chamber and the overall duration of the cleaning procedure, we constructed a compact electron source fitted into a sample holder, which can be operated in a standard Omicron variable-temperature (VT)-STM while the tip stays in place. This way a maximum of compatibility with existing systems is achieved and short turnaround times are possible for tip cleaning.     

Preparation of TiO2(110)-(1x1) surface via UHV cleavage: an scanning tunneling microscopy study

TiO2(110) surface was successfully prepared in situ by UHV cleaving of a commercial TiO2 crystal. Scanning tunneling microscopy (STM) imaging revealed atomically flat more than 1 mum wide terraces with (110) orientation separated by steps running in [001] direction, with very low kink density. Atomically resolved STM images show periodicity in the [001] and [110] directions with the unit cell parameters measured to approximately 3 and 6.5 A, respectively, which are closed to the expected values of bulk terminated (1 x 1) surface.     

扫描隧道探针的制备研究

本文概述针尖对扫描隧道显微镜的重要性,总结制备探针的各种技术,并着重介绍电化学腐蚀制备钨和铂铱合金探针的原理及电路设计。     

Safe fabrication of sharp gold tips for light emission in scanning tunnelling microscopy

Gold is the optimal tip metal for light emission in scanning tunnelling microscopy (LESTM) under ambient conditions. Sharp Au-tips of 10 nm radius were produced reliably using a safe, two-step etching method in 20% (w/w) CaCl₂ solution. Previous CaCl₂-based methods have tended to produce blunter tips, while other etching techniques that do produce sharp Au-tips, do so with the use of toxic or hazardous electrolytes. The tips are characterised using scanning electron microscopy and their efficacy in LESTM is evidenced by high-resolution, simultaneous topographic and photon mapping of Au(1 1 1)- and polycrystalline Au-surfaces. Spectra of the optical emission exhibit only one or two peaks with etched tips in contrast to the more complex spectra typical of cut tips; this feature, together with the highly symmetric geometry of the tips, facilitates a definitive analysis of the light emission process.     

Adlayer structure of octa-alkoxy-substituted copper(II) phthalocyanine on Au(111) by electrochemical scanning tunneling microscopy

Electrochemical scanning tunneling microscopy (ECSTM) has been used to examine the adlayer of octa-alkoxy-substituted copper(II) phthalocyanines (CuPc(OC(8)H(17))(8)) on Au(111) in 0.1 M HClO(4), where the molecular adlayer was prepared by spontaneous adsorption from a benzene solution containing this molecule. Topography STM scans revealed long-range ordered, interweaved arrays of CuPc(OC(8)H(17))(8) with coexistent rectangular and hexagonal symmetries. High-quality STM molecular resolution yielded the internal molecular structure and the orientation of CuPc(OC(8)H(17))(8) admolecules. These STM results could shed insight into the method of generating ordered molecular assemblies of phthalocyanine molecules with long-chained substitutes on metal surface.     

Imaging of metal-coated biological samples by scanning tunneling microscopy

A method for imaging biological samples by scanning tunneling microscopy (STM) is presented. There are two main difficulties in imaging biological samples by STM: (1) the low conductivity of biological material and (2) finding a method of reliably depositing the sample on a flat conducting surface. The first of these difficulties was solved by coating the samples with a thin film of platinum-carbon. The deposition problem was solved by a method similar to a procedure used to deposit biological molecules onto field ion microscope (FIM) tips. STM images of bacteriophage T7 and filamentous phage fd are shown. The substrate on which the samples were absorbed was atomically flat gold. The images do not show molecular detail due to the metal coating, but the gross dimensions and morphology are correct for each type of virus. Also, the surface density of virus particles increases and decreases in the way expected when the conditions of deposition are changed. These methods allow reliable and reproducible STM imaging of biological samples.     

Two-step controllable electrochemical etching of tungsten scanning probe microscopy tips

Dynamic electrochemical etching technique is optimized to produce tungsten tips with controllable shape and radius of curvature of less than 10 nm. Nascent features such as "dynamic electrochemical etching" and reverse biasing after "drop-off" are utilized, and "two-step dynamic electrochemical etching" is introduced to produce extremely sharp tips with controllable aspect ratio. Electronic current shut-off time for conventional dc "drop-off" technique is reduced to ～36 ns using high speed analog electronics. Undesirable variability in tip shape, which is innate to static dc electrochemical etching, is mitigated with novel "dynamic electrochemical etching." Overall, we present a facile and robust approach, whereby using a novel etchant level adjustment mechanism, 30° variability in cone angle and 1.5 mm controllability in cone length were achieved, while routinely producing ultra-sharp probes.     

A data acquisition and image processing system for scanning tunneling microscopy

We have built a dedicated data acquisition and image processing system for scanning tunneling microscopy (STM). Data acquisition is accomplished by a fast 8-bit add-on frame grabber which may be synchronized either to standard TV frequencies or to asynchronous slow scan data sources such as STM or SEM. A 32-bit minicomputer is used for image processing by means of a comprehensive interactive-language which also allows the data acquisition process to be controlled. To speed up time-consuming computations, a floating-point array processor was linked to the system.