Local Electrostatic Forces Research Articles

Rewritable printing of ionic liquid nanofilm utilizing focused ion beam induced film wetting

Manipulating liquid flow over open solid substrate at nanoscale is important for printing, sensing, and energy devices. The predominant methods of liquid maneuvering usually involve complicated surface fabrications, while recent attempts employing external stimuli face difficulties in attaining nanoscale flow control. Here we report a largely unexplored ion beam induced film wetting (IBFW) technology for open surface nanofluidics. Local electrostatic forces, which are generated by the unique charging effect of Helium focused ion beam (HFIB), induce precursor film of ionic liquid and the disjoining pressure propels and stabilizes the nanofilm with desired patterns. The IBFW technique eliminates the complicated surface fabrication procedures to achieve nanoscale flow in a controllable and rewritable manner. By combining with electrochemical deposition, various solid materials with desired patterns can be produced.

Accurate Electrostatic Force Measurements by Atomic Force Microscopy Using Proper Distance Control

Electrostatic force microscopy (EFM) enables us to examine the local electrostatic force between an AFM tip and sample surface, from which an electrical capacitance between them and surface potential can be evaluated, by applying direct current (DC) and alternating current (AC) voltages. From the dependences of the electrostatic force on the DC voltage and on the frequency of AC voltage in EFM, carrier density, carrier type, and deep level states in a semiconductor can also be investigated. Since EFM is basically operated as an atomic force microscopy (AFM), special care should be taken in an effect of the electrostatic force on a distance control between the tip and sample, and robust distance control is necessary even under the strong electrostatic force in order to realize proper measurements of both topography and electrostatic force. In this paper, we have examined the effectiveness of the usage of the oscillation amplitude of a cantilever as a feedback target for the distance control, which is referred to as Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> -mode operation, while the cantilever oscillation frequency is always kept at its primary resonance, like the conventional frequency modulation AFM (FM-AFM) which uses a resonant frequency shift as the feedback target. First, we have verified that a distance change due to the strong electrostatic force was significantly reduced compared with FM-AFM. Secondly, we have confirmed that the Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</i> -mode operation in EFM realized proper measurements of the dependance of a tip-sample capacitance including a surface depletion capacitance on the DC voltage and on the frequency of AC voltage, indicating good robustness of the method against the strong electrostatic force.

Three-Dimensional Droplet Manipulation with Electrostatic Levitation.

An active and precise method for three-dimensional (3D) droplet manipulation is introduced. By modulating the local electrostatic force acting on droplets in carrier oil between needle plate electrodes, the vertical motion of droplets can be controlled, including the droplet levitation at the interface between the carrier oil and the air. Levitated droplets can be translated horizontally with high efficiency by the motion of the needle electrode. With controllable local deformation on the flexible plate electrode, selective manipulation can be realized for multiple droplets. Applying the manipulation method proposed, a platform is built and various droplet handling, such as transport, merging, and mixing, is performed effectively. Complex droplet transport trajectories are achieved by moving the needle electrode. The droplet transport velocity can reach up to 37 mm/s. The introduced method has fundamental advantages of avoiding cross-contamination between droplets, enhancing the flexibility, eliminating the transport track constraint, and lowering costs with straightforward and precise droplet manipulation.

Nanoscale Mapping of the Conductivity and Interfacial Capacitance of an Electrolyte‐Gated Organic Field‐Effect Transistor under Operation

AbstractProbing nanoscale electrical properties of organic semiconducting materials at the interface with an electrolyte solution under externally applied voltages is key in the field of organic bioelectronics. It is demonstrated that the conductivity and interfacial capacitance of the active channel of an electrolyte‐gated organic field‐effect transistor (EGOFET) under operation can be probed at the nanoscale using scanning dielectric microscopy in force detection mode in liquid environment. Local electrostatic force versus gate voltage transfer characteristics are obtained on the device and correlated with the global current–voltage transfer characteristics of the EGOFET. Nanoscale maps of the conductivity of the semiconducting channel show the dependence of the channel conductivity on the gate voltage and its variation along the channel due to the space charge limited conduction. The maps reveal very small electrical heterogeneities, which correspond to local interfacial capacitance variations due to an ultrathin non‐uniform insulating layer resulting from a phase separation in the organic semiconducting blend. Present results offer insights into the transduction mechanism at the organic semiconductor/electrolyte interfaces at scales down to ≈100 nm, which can bring substantial optimization of organic electronic devices for bioelectronic applications such as electrical recording on excitable cells or label‐free biosensing.

Energetic, Topological and Electric Field Analyses of Cation-Cation Nucleic Acid Interactions in Watson-Crick Disposition.

A theoretical study of the effect of the diprotonation on the nucleic acid bases (A : U, A : T and G : C) in Watson-Crick conformation has been carried out by means of DFT computational methods in vacuum. In addition, the corresponding neutral and monoprotonated binary complexes have been considered. Most of the diprotonated species studied are stable, even though the binding energy is positive due to the overall repulsive electrostatic term. Local electrostatic attractive forces in the regions of hydrogen bonds (HBs) are responsible for equilibrium geometries, as shown by the electric field lines connecting the electrophilic and nucleophilic sites involved in the HB interactions. Secondary electrostatic effects also affect the assembling of the nucleic acid complexes in either neutral or cationic form. In particular, the electric field lines flowing from electrophilic sites in one base to nucleophilic sites in the other reinforce the linking between them. Hence, when the nucleophilic site concerns the free lone pair of the heteroatom involved in the HB interaction as acceptor, the HB distance shortens. However, if the free lone pair of the HB acceptor interacts with an electrophilic site in the same molecule, the HB distance elongates, weakening the HB interaction. The topological analysis of the electron density distribution in HB regions indicates that neutral, monoprotonated and diprotonated complexes show no differences in the nature of their HB's.

Electro-chemical aspects of IPMCs within the framework of the theory of porous media

Ionomeric polymer–metal composites (IPMCs) consist of an ionomer with bound anionic groups and mobile counterions. They are plated with noble impermeable metal cover layers. By application of an electric voltage, a transport of the mobile ions towards the respective electrode occurs. Due to local electrostatic and ionic forces, a local deformation of the IPMC can be observed. Therefore IPMCs are promising candidates for electrochemical transducers. In the present research, the chemo-electro-mechanical behavior of IPMCs is described within the framework of the theory of porous media. First, the field equations are derived with respect to the second law of thermodynamics. Second, a reduced set of equations for the chemo-electric behavior is formulated and discretized by applying the finite element method. In the numerical investigations a parametric study of the time and space dependent behavior is carried out in order to quantify the influence of different material compositions. Based on this study, the characteristic response of IPMC to the application of an electric voltage can be predicted. Concluding, the obtained computational framework is an excellent tool for the design of electrochemical transducers.

Quantification of in-contact probe-sample electrostatic forces with dynamic atomic force microscopy

Atomic force microscopy (AFM) methods utilizing resonant mechanical vibrations of cantilevers in contact with a sample surface have shown sensitivities as high as few picometers for detecting surface displacements. Such a high sensitivity is harnessed in several AFM imaging modes. Here, we demonstrate a cantilever-resonance-based method to quantify electrostatic forces on a probe in the probe-sample junction in the presence of a surface potential or when a bias voltage is applied to the AFM probe. We find that the electrostatic forces acting on the probe tip apex can produce signals equivalent to a few pm of surface displacement. In combination with modeling, the measurements of the force were used to access the strength of the electrical field at the probe tip apex in contact with a sample. We find an evidence that the electric field strength in the junction can reach ca. 1 V nm−1 at a bias voltage of a few volts and is limited by non-ideality of the tip-sample contact. This field is sufficiently strong to significantly influence material states and kinetic processes through charge injection, Maxwell stress, shifts of phase equilibria, and reduction of energy barriers for activated processes. Besides, the results provide a baseline for accounting for the effects of local electrostatic forces in electromechanical AFM measurements as well as offer additional means to probe ionic mobility and field-induced phenomena in solids.

The effect of thermal annealing on the optical and electrical properties of ZnO epitaxial films grown on n-GaAs (001)

The local electrostatic force Fωvs. Vsample curves recorded at points marked on the KFM image depicted in the inset shown in the upper right corner, and the schematic band diagrams of the (a) n-type and (b) annealed p-type ZnO layer.

Characterization of local charge distribution of polyethylene terephthalate film and influence as a graphene substrate

We examine local electrostatic force distribution of polyethylene terephthalate (PET) film surface before and after transferring graphene in order to compare the influence of local electrostatic interaction between graphene and PET film in terms of the sheet resistance value of deposited graphene. By using the electrical modulation of amplitude in a tapping mode AFM tip, we can obtain the distinguished electrostatic force amplitude mapping of graphene. We determine that different electrostatic attractive interactions between Au-coated tip and bare PET are present and the observed difference can affect the graphene formation and electrical performance, which correlates with the sheet resistance difference.

Drive frequency dependent phase imaging in piezoresponse force microscopy

The drive frequency dependent piezoresponse (PR) phase signal in near-stoichiometric lithium niobate crystals is studied by piezoresponse force microscopy. It is clearly shown that the local and nonlocal electrostatic forces have a great contribution to the PR phase signal. The significant PR phase difference of the antiparallel domains are observed at the contact resonances, which is related to the electrostatic dominated electromechanical interactions of the cantilever and tip-sample system. Moreover, the modulation voltage induced frequency shift at higher eigenmodes could be attributed to the change of indention force depending on the modulation amplitude with a piezoelectric origin. The PR phase of the silicon wafer is also measured for comparison. It is certificated that the electrostatic interactions are universal in voltage modulated scanning probe microscopy and could be extended to other phase imaging techniques.

Computational Study on Compound I Redox-Active Species in Horseradish Peroxydase Enzyme: Conformational Fluctuations and Solvation Effects

Molecular dynamics simulations for the compound I species in horseradish peroxidase were carried out over a nanoseconds time-scale. Results indicate that the supramolecular assembly composed of compound I in interaction with highly conserved distal residues (His42 and Arg38) exists in two well-defined conformations basically differing in the local position of the distal histidine (i.e., His42). Furthermore, we observe the presence of a biological channel in the distal side of the heme cavity, between Arg38 and Pro139 residues, that represents a direct link connecting the compound I (CpdI) species to bulk molecules. Our investigation supports the idea that when CpdI is formed, the biological machinery relaxes the local electrostatic forces, opening a structural channel through which an exchange of water molecules with the bulk solvent takes place without any significant kinetic barrier. Interestingly, we also show that the combined effect of enzyme and solvent, modulated by thermal fluctuations, affects the order and the energy difference between the lowest doublet and quartet magnetic states of the CpdI-His42-Arg38 complex. Consequently, when passing from the gas phase to the biological environment, the doublet spin state becomes slightly more stable than the higher spin multiplicity state. The distribution of the perturbed low-lying states energy variation, induced by surrounding fluctuations, yields results rather close to that obtained by other state of the art quantum-mechanics/molecular mechanics calculations, and still in line with previous polarizable continuum calculations.

Influence of feedback parameters on ferroelectric domain imaging with piezoresponse force microscopy

The piezoresponse (PR) amplitude and phase signals for antiparallel ferroelectric domains were investigated with different feedback parameters. It is found that the drive frequency and setpoint can strongly alternate the PR image contrast, while the drive phase can be considered as a constant adding to the PR phase signal. The PR amplitude is proportional to the drive amplitude while the PR phase is drive amplitude independent. The larger piezoelectric vibration amplitude and fitting piezoelectric constants (PCs) obtained by vectorial analysis compared with the known values are originated from the sample resonance, and the local electrostatic force can lead to a nonlinear shift in the measured PCs from the theoretical expectations.

Determination of local electrostatic forces for EUVL mask chucks

Determination of local electrostatic forces across the surface of an ultra-planar, pin-structured mask chuck for EUVL is addressed. By measuring the flatness of a thin Si-wafer across the supporting pins at high chucking voltages, local bending values are obtained. Comparing these values with FE calculations based on the wafer parameter and chuck design data, the local electrostatic forces can be derived. Our measurements reveal considerable stochastic bending variation across the surface, but are (on the whole) in good agreement with calculated electrostatic forces from the chuck design and demonstrate the potential of this method for chuck characterization.

Electric charges and forces in atomic force microscopy and nano-xerography

Electrostatic forces generated by contact potential differences, localized charges, or externally applied voltages play a crucial role in atomic force microscopy. Electrostatic forces mediate the non-contact measurement of local potentials by the Kelvin probe technique, enabling compositional mapping of surfaces. However, if not compensated properly, electrostatic forces lead to height errors in topography images acquired in tapping mode. We present a single scan Kelvin probe force microscopy technique that compensates local electrostatic forces and allows simultaneous height and potential measurements in tapping mode. Electrostatic forces also direct the localized assembly of structures in nano-xerography. Here we describe how positive charges, written into a thin film of poly(methyl)methacrylate with the conductive tip of an atomic force microscope, guide the deposition of carboxyl-functionalized multiwalled carbon nanotubes suspended in isopropyl-acohol.

Myoglobin adsorption onto poly(glycidyl methacrylate) microbeads with surface functionalized iminodiacetic acid

In this study, poly(glycidyl methacrylate) [PGMA] microbeads with surface modified iminodiacetic acid (IDA) were used for myoglobin (Mb) adsorption from buffer solutions at different pHs and ionic strengths in a packed-bed column. Attenuated Total Reflectance Fourier Transformed Infrared (ATR-FTIR) spectroscopy and scanning electron microscopy (SEM) measurements before and after the adsorption process confirmed the structural stability of adsorbed Mb. The effects of initial concentration, flow-rate, pH and ionic strength on the adsorption were investigated. The results showed that the maximum amount of protein was adsorbed at a pH 7.0, which is the protein isoelectric point. The adsorption is rationalized in terms of local electrostatic forces acting between the protein and the IDA modified PGMA surface as well as hydrophobic interactions close to the protein isoelectric point, whereas at low pH the global changes give rise to protein–protein repulsion and at high pH protein-support material repulsion.

Reducing the Molecule-Substrate Coupling inC60-Based Nanostructures by Molecular Interactions

Codeposition of C60 and the three-dimensional molecular hydrocarbon 1,3,5,7-tetraphenyladamantane (TPA) on Au(111) leads to the spontaneous formation of molecular nanostructures in which each fullerene is locked into a specific orientation by three surrounding TPA. Scanning tunneling spectroscopy shows that the electronic coupling of C60 with the surface is significantly reduced in these nanostructures, enhancing the free-molecule properties. As evidenced by density functional theory simulations, the nanostructures are stabilized by 18 local electrostatic forces between C60 and TPA, resulting in a lifting of the C60 cage from the surface.

Influence of pH and ionic strength on the adsorption, leaching and activity of myoglobin immobilized onto ordered mesoporous silicates

Myoglobin has been immobilized onto different ordered mesoporous silicates. The effect of the pH on the adsorption, leaching and activity was studied. The results showed that the maximum amount of protein was adsorbed at a pH 6.5, just below the protein isoelectric point (7–7.3). There was no effect of increasing ionic strength on the adsorption profile at different pH values. The adsorption is rationalized in terms of local electrostatic forces acting between the enzyme and the silica surface as well as hydrophobic interactions close to the protein isoelectric point, whereas at low pH the global charges give rise to protein–protein repulsion and at high pH enzyme–silica repulsion. Higher amounts of immobilized myoglobin were leached at a pH 4, while lower amounts were leached at pH 6.5. The catalytic activity of myoglobin immobilized onto SBA-15 showed optimal activity at a pH 6.5 in comparison to a pH of 5 for the free form.

Anharmonicity of water stretching vibrations in series of isomorphous crystalline hydrates: Copper and manganese saccharinates hexahydrates

On the basis of the experimentally obtained frequencies of isotopically isolated OH and OD species, the anharmonicity constants, as well as the harmonic eigenvalues of the OH and OD stretching vibrations for two members of the isomorphous series of metal(II) saccharinates hexahydrates (those of Mn and Cu) were calculated using several theoretical models [B. Berglund, J. Lindgren, J. Tegenfeldt, J. Mol. Struct. 43 (1978) 169, M.G. Sceats, S.A. Rice, J. Chem. Phys. 71 (1979) 973, H. Engstrom, J.B. Bates, L.A. Boatner, J. Chem. Phys. 73 (1980) 1073]. The anharmonicity constants and the ν(OH)/ ν(OD) isotopic ratios correlate well with the stretching frequencies of the isotopically isolated OH and OD oscillators. Both anharmonicity constants and isotopic ratios as criteria for the anharmonicity of the OH (OD) vibrations show that, with a very few exceptions, it increases with the increase in the hydrogen bond strength. The exceptions from the trend are explained in terms of local electrostatic field differences and force constant changes due to the coordination to the metal ion. The obtained regression equations were used to predict the anharmonicity constants of the ν(OD) modes in other members of the series. Within the three-particle model of the hydrogen bonded complex, the parameters characterising the coupling of the ν(OH)/ ν(OD)/modes with the low-frequency ν(O⋯O) ones were estimated. The positions of the overtones of OH and OD stretching vibrations in the compounds of copper and manganese were estimated using the calculated anharmonicity parameters. The predicted values for the OD oscillators were compared with the experimentally obtained data. Model calculations of the described type are shown to be valuable in the assignment of the second-order transitions, especially in complex systems.

Dynamic behaviour in piezoresponse force microscopy

Frequency-dependent dynamic behaviour in piezoresponse force microscopy (PFM)implemented on a beam-deflection atomic force microscope (AFM) is analysed using acombination of modelling and experimental measurements. The PFM signal is comprised ofcontributions from local electrostatic forces acting on the tip, distributed forces acting onthe cantilever, and three components of the electromechanical response vector. Theseinteractions result in the flexural and torsional oscillations of the cantilever, detected asvertical and lateral PFM signals. The relative magnitudes of these contributions depend ongeometric parameters of the system, on the stiffnesses and frictional forces of thetip–surface junction, and on the frequency of operation. The dynamic signal formationmechanism in PFM is analysed and conditions for optimal PFM imaging are formulated.An experimental approach for probing cantilever dynamics using frequency–biasspectroscopy and deconvolution of electromechanical and electrostatic contrast isimplemented.

Quantitative electrostatic force microscopy on heterogeneous nanoscale samples

Locally resolved electrostatic force spectroscopy is combined with Kelvin force microscopy to compare the results obtained using either the force or the frequency as signal source for tip-sample interaction. A two-component locally heterogeneous sample—islands of octadecanethiol molecules self-assembled on Au(111)—is used as a nanometer scale model system. On this kind of sample, electrostatic force spectroscopy as well as Kelvin force microscopy clearly demonstrate that local and quantitative electrostatic force microscopy has to be implemented with the frequency as the signal source.