Conducting Probe Atomic Force Microscopy Research Articles

Electrochemical Synthesis of Compositionally Modulated In<SUB><I>x</I></SUB>Sb<SUB>1–<I>x</I></SUB>Nanowire Homojunctions and Their Tunneling AFM Characterization

Compositionally modulated InSb nanowires (200 nm in diameter) were electrodeposited from acidic chloride baths using anodized alumina membrane as templates. The compositionally modulated In x Sb 1-x (x ∼ 0.3, x ∼ 0.5) nanowires were synthesized by applying potential pulses between -0.6 V and -1.8 V. The segment thickness was controlled down to tens of nm by adjusting the deposition time. In this paper, we report the characteristics of the junctions by using tunneling AFM, which utilizes a conductive AFM probe to detect current passing through the sample and the probe while simultaneously acquiring a topographic image. Preliminary tunneling AFM measurements taken at room temperature of a 200 nm homojunction InSb Nanowire shows that the current-voltage characteristics of the two different segments exhibit a device resistance dependence that is proportional to the Sb content of the device.

“Water-cycle” mechanism for writing and erasing nanostructures at the LaAlO3/SrTiO3 interface

Nanoscale control of the metal-insulator transition in LaAlO3/ SrTiO3 heterostructures can be achieved using local voltages applied by a conductive atomic-force microscope probe. One proposed mechanism for the writing and erasing process involves an adsorbed H2O layer at the top LaAlO3 surface. In this picture, water molecules dissociates into OH- and H+ which are then selectively removed by a biased AFM probe. To test this mechanism, writing and erasing experiments are performed in a vacuum AFM using various gas mixtures. Writing ability is suppressed in those environments where H2O is not present. The stability of written nanostructures is found to be strongly associated with the ambient environment. The self-erasure process in air can be strongly suppressed by creating a modest vacuum or replacing the humid air with dry inert gas. These experiments provide strong constraints for theories of both the writing process as well as the origin of interfacial conductance.

Electrical transport along bacterial nanowires from Shewanella oneidensis MR-1

Bacterial nanowires are extracellular appendages that have been suggested as pathways for electron transport in phylogenetically diverse microorganisms, including dissimilatory metal-reducing bacteria and photosynthetic cyanobacteria. However, there has been no evidence presented to demonstrate electron transport along the length of bacterial nanowires. Here we report electron transport measurements along individually addressed bacterial nanowires derived from electron-acceptor-limited cultures of the dissimilatory metal-reducing bacterium Shewanella oneidensis MR-1. Transport along the bacterial nanowires was independently evaluated by two techniques: (i) nanofabricated electrodes patterned on top of individual nanowires, and (ii) conducting probe atomic force microscopy at various points along a single nanowire bridging a metallic electrode and the conductive atomic force microscopy tip. The S. oneidensis MR-1 nanowires were found to be electrically conductive along micrometer-length scales with electron transport rates up to 10(9)/s at 100 mV of applied bias and a measured resistivity on the order of 1 Ω·cm. Mutants deficient in genes for c-type decaheme cytochromes MtrC and OmcA produce appendages that are morphologically consistent with bacterial nanowires, but were found to be nonconductive. The measurements reported here allow for bacterial nanowires to serve as a viable microbial strategy for extracellular electron transport.

Enhanced Electronic Conductance across Bacteriorhodopsin, Induced by Coupling to Pt Nanoparticles

Studying solid-state electronic conductance of biological molecules requires interfacing the biomolecules with electronic conductors without altering the molecules. To this end, we developed and present here a simple, solution-based approach of conjugating Bacteriorhodopsin (bR)-containing membranes with metallic clusters. Our approach is based on selective electroless deposition of Pt nanoparticles on suspended membrane fragments through chemical interaction of the Pt precursor with the protein’s residues. Optical absorption measurements show that the membranes retain their photoactivity after this procedure. The result of the Pt deposition is best shown by conductive probe atomic force microscopy mapping of electronic current transport across such soft biological layers, which allows reproducible microscopic electrical characterization of the electronic conductance of the resulting junctions. The maps show that chemical contact between the protein and the deposited electrode yields better electronic cou...

Selective growth and piezoelectric properties of highly ordered arrays of vertical ZnO nanowires on ultrathin alumina membranes

A well controlled and cost effective method of fabricating highly ordered arrays of vertical zinc oxide (ZnO) nanowires or nanopores is demonstrated where an ultrathin alumina membrane (UTAM) itself is utilized as a substrate for the selective growth of the ordered arrays. A thin film of gold was thermally evaporated on the UTAM followed by the growth of highly regular ZnO nanowires using chemical vapor deposition (CVD). Alternatively, highly ordered ZnO nanopores arrays were also grown by CVD on the bare UTAM. Additionally, piezoelectric currents were generated from the ZnO nanowires during the conductive atomic force microscopy probe tip scan across the array.

Barrier height and ideality factor dependency on identically produced small Au/p-Si Schottky barrier diodes

Small high-quality Au/P-Si Schottky barrier diodes (SBDs) with an extremely low reverse leakage current using wet lithography were produced. Their effective barrier heights (BHs) and ideality factors from current-voltage (I–V) characteristics were measured by a conducting probe atomic force microscope (C-AFM). In spite of the identical preparation of the diodes there was a diode-to-diode variation in ideality factor and barrier height parameters. By extrapolating the plots the built in potential of the Au/p-Si contact was obtained as Vbi = 0.5425 V and the barrier height value ΦB(C-V) was calculated to be ΦB(C-V) = 0.7145 V for Au/p-Si. It is found that for the diodes with diameters smaller than 100 μm, the diode barrier height and ideality factor dependency to their diameters and correlation between the diode barrier height and its ideality factor are nonlinear, where similar to the earlier reported different metal semiconductor diodes in the literature, these parameters for the here manufactured diodes with diameters more than 100 μm are also linear. Based on the very obvious sub-nanometer C-AFM produced pictures the scientific evidence behind this controversy is also explained.

Band-Gap Determination of the Native Oxide Capping Quantum Dots by Use of Different Kinds of Conductive AFM Probes: Example of InAs/GaAs Quantum Dots

In most of quantum-dot (QD) systems, the electrical behavior of a single QD is directly linked to the native oxide grown on their surface. Obtaining quantitative electrical measurements requires identifying this oxide well, which is not a trivial task. Due to the use of two conductive atomic force microscopy (C-AFM) probes of different behaviors, C-AFM experiments and local electrical measurements allow one to determine the barrier heights at the interface between InAs QD and the native oxide and then to deduce the oxide band gap. In the case of InAs/GaAs QDs, based on our work and on literature results, it may be assumed that the capping oxide is an InAs oxide enriched in indium.

An Analysis of Conductive-Probe Atomic Force Microscopy Applied to the Study of Electron Transport Mediating Properties of Self-Assembled Monolayers

We use conductive-probe atomic force microscopy (CP-AFM) and an AFM based nanolithographic technique, called nanografting, to investigate the electron transport mediating properties of self-assembled monolayers (SAMs) of thiols on gold surfaces. We investigated many factors that can influence the measurements, including the contact nature, the roughness of the substrate, the magnitude of the applied normal load, the diversity of different probes, and the tip wearing during the measurements. All these factors influence the measured current value but to a very variable extent. Molecules and impurities adsorbed at the tip/substrate interface are found to be the major cause of the spread of the experimental results. Our work results in the clarification of a series of phenomena to which more attention should be paid in future experimentation.

Dependency of barrier height and ideality factor on identically produced small Au/p-Si Schottky barrier diodes

Small high-quality Au/p-Si Schottky barrier diodes (SBDs) with extremely low reverse leakage current using wet lithography were produced. Their effective barrier heights (BHs) and ideality factors from current–voltage ( I– V) characteristics were measured by a conducting probe atomic force microscope (C-AFM). In spite of identical preparation of the diodes there was a diode-to-diode variation in ideality factor and barrier height parameters. By extrapolating the plots the built-in potential of the Au/p-Si contact was obtained as V bi =0.5425 V and the barrier height value ( Φ B( C− V) ) was calculated to be Φ B( C− V) =0.7145 V for Au/p-Si for a typical 100 μm diode diameters. In the present work the nonlinear dependency of BH and ideality factor on the diode diameter studied fundamentally and an experimental relation for this new behavior is derived. It is found that for the diodes with diameters smaller than 100 μm the diode barrier height and ideality factor dependency to their diameters and correlation between the diode barrier height and its ideality factor are nonlinear, which is similar to the earlier reported different metal semiconductor diodes in the literature, these parameters for the here manufactured diodes with diameters more than 100 μm are also linear. Based on the very obvious sub-nanometer C-AFM produced pictures, the scientific evidence behind this controversy is also explained.

Electromechanical Properties of Self-Assembled Monolayers of Tetrathiafulvalene Derivatives Studied by Conducting Probe Atomic Force Microscopy

The electrical conduction of metal−molecule−metal junctions formed between Au-supported self-assembled monolayers (SAMs) of symmetric tetrathiafulvalene (TTF) derivatives and a Pt-coated atomic force microscope (AFM) tip has been measured under different compression forces using conducting probe AFM. The TTFs are linked to the metal through double metal−thiol linkers on both sides of the molecule. The TTF junctions exhibit remarkably high electrical conduction with estimated single molecule resistance of 14.7 ± 3.4 GΩ, corresponding to the molecular resistivity of 390 Ω·cm. A single molecule resistance of a comparable length 1-decanethiol molecule is estimated to be several orders of magnitude higher with the value of 950 ± 22 GΩ. The TTF junction resistance displayed a unique nonlinear dependence with the compression force. The dependence is attributed to the change in the intermolecular electronic coupling between the TTF molecules.

Transition from Tunneling to Hopping Transport in Long, Conjugated Oligo-imine Wires Connected to Metals

We report the electrical transport characteristics of conjugated oligonaphthalenefluoreneimine (ONI) wires having systematically varied lengths up to 10 nm. Using aryl imine addition chemistry, ONI wires were built from gold substrates by extending the conjugation length through imine linkages between highly conjugated building blocks of alternating naphthalenes and fluorenes. The resistance and current-voltage characteristics of ONI wires were measured as a function of molecular length, temperature, and electric field using conducting probe atomic force microscopy (CP-AFM). We have observed a transition in direct current (DC) transport from tunneling to hopping near 4 nm as previously established for oligophenyleneimine (OPI) wires. Furthermore, we have found that long ONI wires are less resistive than OPI wires. The single-wire conductivity of ONI wires is approximately 1.8 +/- 0.1 x 10(-4) S/cm, a factor of approximately 2 greater than that of OPI wires, and consistent with the lower transport activation energy ( approximately 0.58 eV versus 0.65 eV or 13 versus 15 kcal/mol). Quantum chemical calculations reveal that charge is preferentially localized on the fluorene subunits and that the molecules are substantially twisted. Overall, this work confirms that imine addition chemistry can be used to build molecular wires long enough to probe the hopping transport regime. The versatility of this chemistry, in combination with CP-AFM, opens up substantial opportunities to probe the physical organic chemistry of hopping conduction in long conjugated molecules.

Polythiophenes containing in-chain cobaltabisdicarbollide centers.

New cobalt(III) bis(dicarbollide) complexes covalently linked to two 2-oligothienyl units have been synthesized and electropolymerized in acetonitrile electrolyte in order to produce the corresponding polythiophene films containing in-chain metallic centers. The polymer films electrogenerated from the bithienyl (4b) and terthienyl (4c) derivatives display redox processes attributed to the Co(III)/Co(II) couple at ca. -1.1 V vs SCE and to the p-doping/undoping of the expected quaterthienyl and sexithienyl segments at ca. 0.8 V vs SCE. In contrast, the anodic oxidation of the thienyl (4a) derivative leads to passivation of the electrode surface. As the length of the oligothiophene substituents increases, the metallic and dicarbollide cage carbon atoms contributions in the HOMO decrease dramatically so that the highest occupied frontier orbitals of 4b and 4c can be considered as almost purely oligothiophene-based. From further UV-vis spectroscopy analysis, it is demonstrated that the polymer incorporating the sexithienyl segments is more conjugated than that with the quaterthienyl segments as the absorption maximum for the interband pi-pi* transition was observed at 410 and 448 nm for poly(4b) and poly(4c) respectively. Furthermore, these polymers display a more extended degree of conjugation than the parent oligothiophenes. Such features indicate a significant electronic delocalization through the cobaltabisdicarbollide moiety. Their conducting probe atomic force microscopy characterization indicates that poly(4b) and poly(4c) behave like heavily doped semiconductors rather than pure semiconductors. Mean conductivity values extracted from the current-voltage profiles are 1.4 x 10(-4) and 7.5 x 10(-4) S cm(-1) for poly(4b) and poly(4c), respectively. Such materials are found to be efficient for the electrocatalytic reduction of protons to dihydrogen, as exemplified for poly(4b). The overpotential for hydrogen evolution is significantly decreased by ca. 230 mV with respect to that obtained with the bare electrode (measured for a current density of 1.4 mA cm(-2) in the presence of 20 mM HBF(4)).

Surface state control of III–V semiconductors using molecular modification

Abstract An attempt to control surface electronics of III–V semiconductor using wet chemical processes has been performed. Here, we report results on the use of self-assembled monolayers (SAMs) of organic molecules on (0 0 1) GaAs surface. Octadecanethiol (ODT) and benzenethiol (BT) have been the choice in the present study. GaAs wafers were modified by thiol molecules on the flat surface after the native oxide layers are removed by chemical etching under optimized conditions. The change in the electronic properties was measured in terms of transport properties via the SAM layer by conductive probe atomic force microscopy. The current–voltage characteristics thus obtained show that ODT functions as a tunnel barrier while BT is conductive due to the presence of π -electrons. As a result, we can control the electronic states of GaAs–molecule interface for realizing novel device structures by the selection of functional molecules.

Dependence of Pentacene Crystal Growth on Dielectric Roughness for Fabrication of Flexible Field-Effect Transistors

The dependence of pentacene nanostructures on gate dielectric surfaces were investigated for flexible organic field-effect transistor (OFET) applications. Two bilayer types of polymer/aluminum oxide (Al(2)O(3)) gate dielectrics were fabricated on commercial Al foils laminated onto a polymer back plate. Some Al foils were directly used as gate electrodes, and others were smoothly polished by an electrolytic etching. These Al surfaces were then anodized and coated with poly(alpha-methyl styrene) (PAMS). For PAMS/Al(2)O(3) dielectrics onto etched Al foils, surface roughness up to approximately 1 nm could be reached, although isolated dimples with a lateral diameter of several micrometers were still present. On PAMS/Al(2)O(3) dielectrics (surface roughness >40 nm) containing mechanical grooves of Al foil, average hole mobility (mu(FET)) of 50 nm thick pentacene-FETs under the low operating voltages (|V| < 6 V) was approximately 0.15 cm(2) V(-1) s(-1). In contrast, pentacene-FETs employing the etched Al gates exhibited mu(FET) of approximately 0.39 cm(2) V(-1) s(-1), which was comparable to that of reference samples with PAMS/Al(2)O(3) dielectrics onto flat sputtered Al gates. Conducting-probe atomic force microscopy and two-dimensional X-ray diffraction of pentacene films with various thicknesses revealed different out-of-plane and in-plane crystal orderings of pentacene, depending on the surface roughness of the gate dielectrics.

From Tunneling to Hopping: A Comprehensive Investigation of Charge Transport Mechanism in Molecular Junctions Based on Oligo(p-phenylene ethynylene)s

The charge transport mechanism of oligo(p-phenylene ethynylene)s with lengths ranging from 0.98 to 5.11 nm was investigated using modified scanning tunneling microscopy break junction and conducting probe atomic force microscopy methods. The methods were based on observing the length dependence of molecular resistance at single molecule level and the current-voltage characteristics in a wide length distribution. An intrinsic transition from tunneling to hopping charge transport mechanism was observed near 2.75 nm. A new transitional zone was observed in the long length molecular wires compared to short ones. This was not a simple transition between direct tunneling and field emission, which may provide new insights into transport mechanism investigations. Theoretical calculations provided an essential explanation for these phenomena in terms of molecular electronic structures.

Addressable photocharging of single quantum dots assisted with atomic force microscopy probe

Here we report on addressable photocharging of individual cadmium selenide quantum dots (QDs). The charging is a result of synergetic action of light and an atomic force microscope (AFM) probe. The probe squeezes the coating layer of QDs helping the photoelectron to tunnel to either conductive AFM probe or substrate. The charge can be induced on individual QDs by locating the QDs with AFM. The charges are stable in ambient conditions. These results may be of interest for QDs based sensors, memory, and solar cell applications. For instance, this method could provide recording information at a density of 1 Tb/cm2.

Elastic, Adhesive, and Charge Transport Properties of a Metal−Molecule−Metal Junction: The Role of Molecular Orientation, Order, and Coverage

The elastic, adhesive, and charge transport properties of a metal-molecule-metal junction were studied via conducting-probe atomic force microscopy (AFM) and correlated with molecular structure by near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The junctions consisted of Co-Cr-coated AFM tips in contact with methyl-terminated alkanethiols (CH(3)(CH(2))(n-1)SH, denoted by C(n), where n is the number of carbons in the molecular chain) on Au substrates. AFM contact data were analyzed with the Derjaguin-Muller-Toporov contact model, modified by a first-order elastic perturbation method to account for substrate effects, and a parabolic tunneling model, appropriate for a metal-insulator-metal junction in which the thickness of the insulator is comparable to the Fermi wavelength of the conducting electrons. NEXAFS carbon K-edge spectra were used to compute the dichroic ratio R(I) for each film, which provided a quantitative measure of the molecular structure as a function of n. As n decreased from 18 to 5, there was a change in the molecular phase from crystalline to amorphous (R(I) --> 0) and loss of surface coverage, and as a result, the work of adhesion w increased from 82.8 mJ m(-2) to 168.3 mJ m(-2), the Young's modulus of the film E(film) decreased from 1.0 to 0.15 GPa, and the tunneling barrier height phi(0) - E(F) decreased from 2.4 to 2.1 eV. For all n, the barrier thickness t decreased for small applied loads F and remained constant at approximately 2.2 nm for large F. The change in behavior was explained by the presence of two insulating layers: an oxide layer on the Co-Cr tip, and the alkanethiol monolayer on the Au surface. X-ray photoelectron spectroscopy confirmed the presence of an oxide layer on the Co-Cr tip, and by performing high-resolution region scans through the film, the thickness of the oxide layer t(oxide) was found to be between 1.9 and 3.9 nm. Finally, it was shown that phi(0) - E(F) is strain-dependent, and the strain at which the film is completely displaced from under the tip is -0.17 for all values of n.

Spin transfer torque switching of magnetic tunnel junctions using a conductive atomic force microscope

We show that a nonmagnetic conductive atomic force microscopy probe can be used to read and write magnetic bits using current passed between the tip and bit. The bits were patterned using electron beam lithography from a magnetic tunnel junction (MTJ) film with in-plane shape anisotropy using an MgO tunnel barrier. Probes were made having a thick Pt coating and could deliver up to several milliamps, so that MTJ structures were easily switched repeatedly using the spin transfer torque effect.

Theoretical and Experimental Studies of Electronic Transport of Dithienothiophene

Electronic transport through metal-dithieno[3,2-b:2′,3′-d]thiophene (DTT) junctions with two interface geometries were studied experimentally by conducting probe atomic force microscopy on a self-assembled monolayer of DTT sandwiched between pairs of Au contacts and studied theoretically by using the fully self-consistent nonequilibrium Green’s function combined with the density functional theory to calculate the transport current in the DTT nanostructures. The experimental and simulation results reveal a similar electrical transport nature of a steplike current−voltage (I−V) curve and negative differential resistance (NDR). The characteristics of the I−V curves were elucidated with evolution of the transmission spectra, and the dependence of transmission peaks on the spatial distribution of renormalized molecular orbitals were further analyzed. In two designed test molecular schemes, the highest occupied molecular orbital (HOMO) is mainly responsible for the transmission resonance at a lower external bias voltage, while the interface states originating from coupling between the electrode surface and DTT molecule are suggested to produce the NDR peak in an asymmetrical metal−DTT junction. The study of electronic transport of monomers at the molecular scale in this work demonstrates a new mechanistic approach to engineer and optimize organic material-based electronic devices.

Electric Conduction Properties of Self-assembled Monolayer Films of Ru Complexes with Disulfide/Phosphonate Anchors in a Au–(Molecular Ensemble)–(Au Nanoparticle) Junction

Abstract Electric conduction properties of SAM films of Ru complexes with disulfide or phosphonate anchors have been measured by Au-nanoparticle-assisted conducting probe atomic force microscopy. The I–V curves of these films were nonlinear, and the resistivities depend on the headgroups and the layered structure in ways that can be explained by the contact resistance and the insulating effect of the Zn–phosphonate interlayer in metal–molecule–metal junctions. Asymmetry in these films causes a rectification effect on the I–V response.