Conductive Atomic Force Microscopy Studies Research Articles

The influence of silver and copper ions on the antibacterial activity and local electrical properties of single sepiolite fiber: A conductive atomic force microscopy (C-AFM) study

This is a study to analyze antibacterial and electrically-conductive sepiolite, and to evaluate its potential usage as biomaterials, medical products, sensors and electronics. Silver and copper ions were used to modify the properties of sepiolite through a multi-step procedure. During the modification of the sepiolite fibers, the most significant adsorption rates were obtained at metal uptake concentrations of 50mg/g for Ag+ and 80mg/g for Cu2+. This was found to fit the Langmuir isotherm model, thereby indicating the formation of a monolayer on the sepiolite surfaces by silver and copper ions. The effects of adsorbed silver and copper ions at the optimum metal uptake concentrations were then compared in terms of their antibacterial activity and electrical conductivity.Serial broth dilution and Kirby-Bauer disk diffusion susceptibility tests on Escherichia coli (ATCC 25922) and Staphylococcus aureus (ATCC 25923) were both utilized in this study. Metal ions that neutralized bacteria, and their minimum inhibitory concentrations (MIC), were measured at 50mg/L for Ag+ and 100mg/L for Cu2+. Additionally, the current distributions of metal ions over a single sepiolite fiber could also be characterized through the use of C-AFM. Interestingly, for the first time, local electric current variations and the potential gradients of Ag+ and Cu2+ sorped sepiolite fiber surfaces were detected. Their C-AFM generated current image and line scans showed that silver electrons transfer much more rapidly than those of copper; however, raw sepiolite demonstrates no antibacterial or current properties. Therefore, the fibrous clay mineral used in this study might be a good candidate for highly effective and innovative clay-based applications.

Communication—Reduced Off-Current of NbO2 by Thermal Oxidation of Polycrystalline Nb Wire

The origin of high leakage current in NbO2 is investigated on the basis of grain size and grain boundary distribution. We used thermally grown and sputtered NbO2 films on polycrystalline niobium microwires. The off-current of the thermally grown film was significantly decreased. This is attributed to the large size of grains in thermally grown film over sputtered one and better quality of oxide film could be grown in the thermal process than sputtering. Our assumptions are supported by Conductive Atomic Force Microscopy studies and simulations. In addition, by introducing 15 nm HfO2 dielectric layer further reduction of the off-current was achieved.

Plasmonic Periodic Nanodot Arrays via Laser Interference Lithography for Organic Photovoltaic Cells with >10% Efficiency.

In this study, we demonstrate a viable and promising optical engineering technique enabling the development of high-performance plasmonic organic photovoltaic devices. Laser interference lithography was explored to fabricate metal nanodot (MND) arrays with elaborately controlled dot size as well as periodicity, allowing spectral overlap between the absorption range of the active layers and the surface plasmon band of MND arrays. MND arrays with ∼91 nm dot size and ∼202 nm periodicity embedded in a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) hole transport layer remarkably enhanced the average power conversion efficiency (PCE) from 7.52% up to 10.11%, representing one of the highest PCE and degree of enhancement (∼34.4%) levels compared to the pristine device among plasmonic organic photovoltaics reported to date. The plasmonic enhancement mechanism was investigated by both optical and electrical analyses using finite difference time domain simulation and conductive atomic force microscopy studies.

Gold-decorated highly ordered self-organized grating-like nanostructures on Ge surface: Kelvin probe force microscopy and conductive atomic force microscopy studies

Nanoarchitecture by atomic manipulation is considered to be one of the emerging trends in advanced functional materials. It has a gamut of applications to offer in nanoelectronics, chemical sensing, and nanobiological science. In particular, highly ordered one-dimensional semiconductor nanostructures fabricated by self-organization methods are in high demand for their high aspect ratios and large number of applications. An efficient way of fabricating semiconductor nanostructures is by molecular beam epitaxy, where atoms are added to a crystalline surface at an elevated temperature during growth, yielding the desired structures in a self-assembled manner. In this article, we offer a room temperature process, in which atoms are sputtered away by ion impacts. Using gold ion implantation, the present study reports on the formation of highly ordered self-organized long grating-like nanostructures, with grooves between them, on a germanium surface. The ridges of the patterns are shown to have flower-like protruding nanostructures, which are mostly decorated by gold atoms. By employing local probe microscopic techniques like Kelvin probe force microscopy and conductive atomic force microscopy, we observe a spatial variation in the work function and different nanoscale electrical conductivity on the ridges of the patterns and the grooves between them, which can be attributed to gold atom decorated ridges. Thus, the architecture presented offers the advantage of using the patterned germanium substrates as periodic arrays of conducting ridges and poorly conducting grooves between them.

Silver Nanowires Binding with Sputtered ZnO to Fabricate Highly Conductive and Thermally Stable Transparent Electrode for Solar Cell Applications.

Silver nanowire (AgNW) film has been demonstrated as excellent and low cost transparent electrode in organic solar cells as an alternative to replace scarce and expensive indium tin oxide (ITO). However, the low contact area and weak adhesion with low-lying surface as well as junction resistance between nanowires have limited the applications of AgNW film to thin film solar cells. To resolve this problem, we fabricated AgNW film as transparent conductive electrode (TCE) by binding with a thin layer of sputtered ZnO (40 nm) which not only increased contact area with low-lying surface in thin film solar cell but also improved conductivity by connecting AgNWs at the junction. The TCE thus fabricated exhibited transparency and sheet resistance of 92% and 20Ω/□, respectively. Conductive atomic force microscopy (C-AFM) study revealed the enhancement of current collection vertically and laterally through AgNWs after coating with ZnO thin film. The CuInGaSe2 solar cell with TCE of our AgNW(ZnO) demonstrated the maximum power conversion efficiency of 13.5% with improved parameters in comparison to solar cell fabricated with conventional ITO as TCE.

Do defects get ordered in Cu2ZnSnS4?

Formation of ordered defect compounds and anomalous grain boundary physics are unique to Cu chalcogenides CuInX2 (S/Se) and its alloys. X-ray photoelectron spectroscopy (XPS) studies were carried on Cu2-xZn1.3SnS4 (x=0.0, 0.3, 0.5 and 0.7) to determine the position of valence band edge and explore the formation of ordered vacancy compounds along with absorption studies. Conductive atomic force microscopy (C-AFM) studies were carried out Cu2ZnSnS4 (CZTS) film deposited on Si and sodalime glass substrates to understand grain boundary physics.

Exfoliated graphite flakes as soft-electrodes for precisely contacting nanoobjects

We introduce a simple, clean and reliable method to transfer exfoliated graphite flakes as soft-electrodes for the electrical contacts of nano-objects. The microelectrodes thus produced exhibit extremely well-defined and thin edges and can be placed at any sample location with sub-micrometer precision. The procedure is carried out under ambient conditions and does not require chemical agents. We present electrical characterization of relevant examples including carbon nanotubes, metal–organic MMX nanoribbons, reduced graphene sheets and damaged circuit repair. The quality of the electrical contacts thus obtained is as good as those fabricated with conventional techniques. This technique is particularly relevant for conductive atomic force microscopy (AFM) studies.

Cross Kelvin force microscopy and conductive atomic force microscopy studies of organic bulk heterojunction blends for local morphology and electrical behavior analysis

Bulk Heterojunction (BHJ) organic photovoltaic devices performances depend on the relative organization and physical properties of the electron-donor and -acceptor materials. In this paper, BHJs of poly(3-hexyl-thiophene) (P3HT) associated with an electron acceptor material, 1-(3-methoxycarbonyl)-propyl-1-phenyl[6,6]C6 (PCBM) or [Ni(4dodpedt)2], are studied in terms of morphology, ordering, and electrical properties. First, comparison between the two BHJs performed by Atomic Force Microscopy (AFM) and Raman characterizations shows that P3HT structuration is improved by blending with [Ni(4dodpedt)2]. Then, the relationship between charges trapping, electrical properties, and film morphology is investigated using conductive AFM and Kelvin Force Microscopy. Measurements in dark condition and under solar cell simulator provide complementary information on electrical phenomena in these organic nanostructures. Finally, time dependent measurement highlights the influence of charges stacking on conduction. Specifically, we demonstrate that charge accumulation initiated by illumination remains valid after switching off the light, and induces the modification in current versus voltage characteristic of P3HT: PCBM blend. Finally, we observe a current increasing which can be attributed to the energy barrier decreasing due to charges trapping in PCBM.

Comparison of dimethyl sulfoxide treated highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) electrodes for use in indium tin oxide-free organic electronic photovoltaic devices

Indium tin oxide (ITO)-free organic photovoltaic (OPV) devices were fabricated using highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the transparent conductive electrode (TCE). The intrinsic conductivity of the PEDOT:PSS films was improved by two different dimethyl sulfoxide (DMSO) treatments – (i) DMSO was added directly to the PEDOT:PSS solution (PEDOT:PSSADD) and (ii) a pre-formed PEDOT:PSS film was immersed in DMSO (PEDOT:PSSIMM). X-ray photoelectron spectroscopy (XPS) and conductive atomic force microscopy (CAFM) studies showed a large amount of PSS was removed from the PEDOT:PSSIMM electrode surface. OPV devices based on a poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) bulk hetrojunction showed that the PEDOT:PSSIMM electrode out-performed the PEDOT:PSSADD electrode, primarily due to an increase in short circuit current density from 6.62mAcm−2 to 7.15mAcm−2. The results highlight the importance of optimising the treatment of PEDOT:PSS electrodes and demonstrate their potential as an alternative TCE for rapid processing and low-cost OPV and other organic electronic devices.

Evidence for Negative Electron Affinity in Laser Irradiated ZnTe Thin Films

Local transport properties of laser irradiated ZnTe thin films are reported. By rastering the laser beam (of 532 nm wavelength) appropriately, ZnTe decomposes into n-type ZnTe and Te and a grating like struct ure with micro-stripesof ZnTe separated by grooves of Te is obtained. Conductive atomic force microscopy studies and local I -V measurements made on thestripesand grooves show that the film properties are mainly determined by chemical composition, ratherthan by the topography of the film. When the tip is positively biased, the current images closely match the topography images. In contrast, when the tip is negatively biased the current and topography images are very different and a large negative curren t was also observed in the grooves. This is attributed to the variation in charge separation (interface capacitance) caused by the rough surface. It is shown that Te forms ohmic contact with Au tip, but the junction exhibits Schottky diode behavior under l ow biasing voltages. The large current at both high positive and negative tip biasing may arise due to semi-metallic properties of Te. The I -V characteristics measurement reveals formation of Schottky barrier between ZnTe-Au junctions with a very low valu e (32.4 �78.3 meV) of barrier which indicates the presence of negative electron affinity . OriginalResearch Article

Morphology and conductivity of PEDOT layers produced by different electrochemical routes

The electrochemical polymerization of 3,4-ethylenedioxythiophene (EDOT) in a polyanion, sodium poly(styrene-4-sulfonate)(NaPSS), has been performed through three different oxidative electropolymerizations on flexible electrodes. These electrodes have been fully characterized both at a macroscopic and microscopic scale. The morphology, characterized by Scanning Electron Microscope (SEM) and Atomic Force Microscope (AFM) reveals a tendency of PEDOT:PSS to arrange in cauliflower-like shape. Homogeneity, grain dimension and electrical properties of the PEDOT layers can be tuned by choosing different polymerization routes and different parameters of electropolymerization. At the macroscopic level, we achieved a significant decrease of the impedance of the whole electrode after the electropolymerization thanks to an increase of both the intrinsic conductivity and the roughness of the electrode surface. Moreover Conductive Atomic Force Microscopy (C-AFM) study shows for the first time a clear relation between surface topography and local conductivity of the deposed film and help to clarify the conduction mechanism in electrodeposited PEDOT:PSS layers. The deposition was also shown to be very stable and resistant through thermal accelerated ageing tests that can mimic the behaviour of an implanted electrode after several months at human body temperature. The long term stability coupled with the low impedance make these electrodes suitable candidates for long-term neural probe applications.

Towards a unified description of the charge transport mechanisms in conductive atomic force microscopy studies of semiconducting polymers.

In this work, conductive atomic force microscopy (C-AFM) is used to study the local electrical properties in thin films of self-organized fibrillate poly(3-hexylthiophene) (P3HT), as a reference polymer semiconductor. Depending on the geometrical confinement in the transport channel, the C-AFM current is shown to be governed either by the charge transport in the film or by the carrier injection at the tip-sample contact, leading to either bulk or local electrical characterization of the semiconducting polymer, respectively. Local I-V profiles allow discrimination of the different dominating electrical mechanisms, i.e., resistive in the transport regime and space charge limited current (SCLC) in the local regime. A modified Mott-Gurney law is analytically derived for the contact regime, taking into account the point-probe geometry of the contact and the radial injection of carriers. Within the SCLC regime, the probed depth is shown to remain below 12 nm with a lateral electrical resolution below 5 nm. This confirms that high resolution is reached in those C-AFM measurements, which therefore allows for the analysis of single organic semiconducting nanostructures. The carrier density and mobility in the volume probed under the tip under steady-state conditions are also determined in the SCLC regime.

Highly conductive spray deposited poly(3, 4-ethylenedioxythiophene):poly (styrenesulfonate) electrodes for indium tin oxide-free small molecule organic photovoltaic devices

Spray deposition is currently attracting significant attention as a low cost method for organic photovoltaic (OPV) device fabrication. We report indium tin oxide (ITO)-free small moleule OPV devices comparing spin coated and spray deposited highly conductive poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as the transparent conducting electrode (TCE). Conductive atomic force microscopy studies indicate that despite an increase in surface roughness, spray deposited PEDOT:PSS TCEs display a similar distribution in the surface composition of PEDOT and PSS and provide comparable device performance to spin coated PEDOT:PSS TCEs. Therefore, the TCEs fabricated by spray deposition show potential as an inexpensive solution processable alternative to the conventionally used ITO TCE for OPV devices.

Conductive AFM Study to Differentiate between the Surface Ionic Conductivity of Nafion and Electrospun Membranes

Multi-mode atomic force microscopy (AFM) was used to differentiate between Nafion® and electrospun composite nanofiber membranes. The results of this experiment will guide the design of new class of proton exchange membranes (PEM). Characterizing the electrospun composite nanofiber membranes will allow us to identify properties that are ideal for the next generation PEM. Engineering the proton conducting membrane for a PEM fuel cell that allows protons and inhibits other species to cross is vital to its long-term operation. When species other than protons are allowed to cross a PEM, issues arise in the form of electrode flooding, adverse chemical reactions, and reduced fuel cell efficiency.

Conductive atomic force microscopy study of single molecule electron transport through the Azurin-gold nanoparticle system

Transduction of biorecognition events into electrical signals through integration of single redox metalloproteins in bioelectronic nanodevices requires both a reliable electrical contact between the biomolecule and the metallic electrode and an efficient overall conduction mechanism. These conditions have been met in the hybrid system obtained by linking gold nanoparticles on top of Azurin proteins, in turn assembled on gold surfaces. Such an assembling strategy, combined with a conductive atomic force microscopy investigation, has allowed us to put into evidence an unprecedented matching between current and topography features and to attribute the intramolecular charge transport to a non-resonant tunnelling mechanism.

Conductive atomic force microscopy studies on dielectric breakdown behavior of ultrathin Al2O3 films

Ultrathin films of Al2O3 prepared by atomic layer deposition have been subjected to local electrical stress analysis using conducting atomic force microscopy. The loss of local dielectric integrity through current leakage in these extremely thin films is studied using scanning spreading resistance imaging. Our experimental results shows that repeated voltage stress progressively increases number of leakage spots. While the density of leakage spots increase with higher applied bias for thin oxide films, initial increase and reduction in leakage spots are observed for thick films.

Influence of composition and bottom electrode properties on the local conductivity of TiN/HfTiO2 and TiN/Ru/HfTiO2 stacks

HfTiO 2 layers of various stoichiometries where deposited by physical vapor depostion on TiN and TiN/Ru bottom electrodes (BE) in order to determine the influence of composition, conduction band offset, and BE morphology on the overall leakage current characteristics. Current-voltage spectroscopy, transmission electron microscopy, electron energy loss spectroscopy, and conductive atomic force microscopy studies show increased leakage current and charge trapping with increased Ti content. The interplay of conduction band offset and trap density were studied. The influence of Ru bottom electrode roughness on the leakage current is higher than the influence of Ti content and low conduction band offset.

Macroscopic and microscopic electrical characterizations of high-k ZrO2 and ZrO2/Al2O3/ZrO2 metal-insulator-metal structures

In order for sub-10 nm thin films of ZrO2 to have a dielectric constant larger than 30 they need to be crystalline. This is done by either depositing the layer at higher temperatures or by a postdeposition annealing step. Both methods induce high leakage currents in ZrO2 based dielectrics. In order to understand the leakage a thickness series of ultrathin ZrO2 and nanolaminate ZrO2/Al2O3/ZrO2 (ZAZ) films, deposited by atomic layer deposition, was investigated. After deposition these films were subjected to different rapid thermal annealing (RTA) processes. Grazing incidence x-ray diffraction and transmission electron microscopy yield that the crystallization of ZrO2 during deposition is dependent on film thickness and on the presence of an Al2O3 sublayer. Moreover, the incorporation of Al2O3 prevents crystallites from spanning across the entire film during RTA. C-V and I-V spectroscopies show that after a 650 °C RTA in N2 the capacitance equivalent oxide thickness of 10 nm ZAZ films is reduced to 1.0 nm while maintaining low leakage currents of 3.2×10−8 A/cm2 at 1 V. Conductive atomic force microscopy studies yield that currents are not associated with significant morphological features in amorphous layers. However, after crystallization, the currents at crystallite grain boundaries are increased in ZrO2 and ZAZ films.

Conductive Atomic Force Microscopy Study of Doped Tin Oxide Thin Film

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Conductive atomic force microscopy study of self-assembled silicon nanostructures

Understanding the electrical transport properties of nanostructures and metal-nanostructure contacts is important before these can be fabricated into electronic devices. Conductive atomic force microscopy investigations of self-assembled silicon nanostructures are reported here, where the nanostructures are fabricated using electron-beam rapid thermal annealing, a self-assembly process that has been shown to produce field-emission devices using CMOS-compatible technology. Unambiguous correlations between current flow and topography are found, and local current-voltage (IV) spectroscopy measurements are used to determine the ideality factors (1.83–3.20) and barrier heights (0.28–0.49eV) for the metal-nanostructure contacts.