A resolution study for electrostatic force microscopy on bimetallic samples using the boundary element method

Electrostatic force microscopy (EFM) is a special design of non-contact atomic force microscopy used for detecting electrostatic interactions between the probe tip and the sample. Its resolution is limited by the finite probe size and the long-range characteristics of electrostatic forces. Therefore, quantitative analysis is crucial to understanding the relationship between the actual local surface potential distribution and the quantities obtained from EFM measurements. To study EFM measurements on bimetallic samples with surface potential inhomogeneities as a special case, we have simulated such measurements using the boundary element method and calculated the force component and force gradient component that would be measured by amplitude modulation (AM) EFM and frequency modulation (FM) EFM, respectively. Such analyses have been performed for inhomogeneities of various shapes and sizes, for different tip-sample separations and tip geometries, for different applied voltages, and for different media (e.g., vacuum or water) in which the experiment is performed. For a sample with a surface potential discontinuity, the FM-EFM resolution expression agrees with the literature; however, the simulation for AM-EFM suggests the existence of an optimal tip radius of curvature in terms of resolution. On the other hand, for samples with strip-?and disk-shaped surface potential inhomogeneities, we have obtained quantitative expressions for the detectability size requirements as a function of experimental conditions for both AM-?and FM-EFMs, which suggest that a larger tip radius of curvature is moderately favored for detecting the presence of such inhomogeneities.     

Contrast inversion in electrostatic force microscopy imaging of trapped charges: tip-sample distance and dielectric constant dependence

We present a numerical and analytical study of the behavior of both electrostatic force and force gradient created by a charge trapped below the surface of a dielectric on an atomic force microscope tip as a function of the dielectric constant and tip-sample distance. As expected, the force decreases monotonously when the dielectric constant increases. However, a maximum in the dielectric constant dependence of the force gradient is found. This maximum occurs in the typical experimental parameters' range and depends on the tip-sample distance and the sample thickness. The analytical study permits us to understand the physical origin of this phenomenon and is in good agreement with the numerical simulation for small tip-sample distances. We also report a study exemplifying a possible contrast inversion in electrostatic force microscopy (EFM) signals while scanning, at different heights, two charges trapped in a sample having heterogeneous dielectric domains. In addition to this particular contrast inversion effect, this study can be considered as a way to gain insight into the mechanisms of EFM image formation as a function of the dielectric constant and tip-sample.     

Time-resolved electrostatic force microscopy of polymer solar cells

Blends of conjugated polymers with fullerenes, polymers, or nanocrystals make promising materials for low-cost photovoltaic applications. Different processing conditions affect the efficiencies of these solar cells by creating a variety of nanostructured morphologies, however, the relationship between film structure and device efficiency is not fully understood. We introduce time-resolved electrostatic force microscopy (EFM) as a means to measure photoexcited charge in polymer films with a resolution of 100 nm and 100 micros. These EFM measurements correlate well with the external quantum efficiencies measured for a series of polymer photodiodes, providing a direct link between local morphology, local optoelectronic properties and device performance. The data show that the domain centres account for the majority of the photoinduced charge collected in polyfluorene blend devices. These results underscore the importance of controlling not only the length scale of phase separation, but also the composition of the domains when optimizing nanostructured solar cells.     

Electric force microscopy study of the surface electrostatic property of rubbed polyimide alignment layers

Two polyimides were synthesized for use as alignment layers. The pretilt angles of the liquid crystals, 4-cyano-4′-n-pentylbiphenyl, on the two polyimides were measured by the crystal rotation method. The relative surface atomic concentrations of F/C (%) were measured by X-ray photoelectron spectroscopy. Electric force microscopy was utilized to investigate the surface electrostatic property of the two thin polyimide alignment layers before and after rubbing. All results demonstrate that rubbing causes trifluoromethyl moieties to migrate towards the surface, absorb negative charges and orient along the rubbing direction. Thus, it is proposed that distributions of functional groups on the surface of the polyimide after rubbing are anisotropic and the van der Waals forces between the polar groups and liquid crystal molecules play an important role in the uniform orientation of the liquid crystal molecules.     

Quantifying the dielectric constant of thick insulators by electrostatic force microscopy: effects of the microscopic parts of the probe

We present a systematic analysis of the effects that the microscopic parts of electrostatic force microscopy probes (the cone and cantilever) have on the electrostatic interaction between the tip apex and thick insulating substrates (thickness > 100 μm). We discuss how these effects can influence the measurement and quantification of the local dielectric constant of the substrates. We propose and experimentally validate a general methodology that takes into account the influence of the cone and the cantilever, thus enabling us to obtain very accurate values of the dielectric constants of thick insulators.     

Nanowire probes for high resolution combined scanning electrochemical microscopy - atomic force microscopy

We describe a method for the production of nanoelectrodes at the apex of atomic force microscopy (AFM) probes. The nanoelectrodes are formed from single-walled carbon nanotube AFM tips which act as the template for the formation of nanowire tips through sputter coating with metal. Subsequent deposition of a conformal insulating coating, and cutting of the probe end, yields a disk-shaped nanoelectrode at the AFM tip apex whose diameter is defined by the amount of metal deposited. We demonstrate that these probes are capable of high-resolution combined electrochemical and topographical imaging. The flexibility of this approach will allow the fabrication of nanoelectrodes of controllable size and composition, enabling the study of electrochemical activity at the nanoscale.     

Submicrosecond time resolution atomic force microscopy for probing nanoscale dynamics

We propose, simulate, and experimentally validate a new mechanical detection method to analyze atomic force microscopy (AFM) cantilever motion that enables noncontact discrimination of transient events with ~100 ns temporal resolution without the need for custom AFM probes, specialized instrumentation, or expensive add-on hardware. As an example application, we use the method to screen thermally annealed poly(3-hexylthiophene):phenyl-C(61)-butyric acid methyl ester photovoltaic devices under realistic testing conditions over a technologically relevant performance window. We show that variations in device efficiency and nanoscale transient charging behavior are correlated, thereby linking local dynamics with device behavior. We anticipate that this method will find application in scanning probe experiments of dynamic local mechanical, electronic, magnetic, and biophysical phenomena.     

Charge trapping in carbon nanotube loops demonstrated by electrostatic force microscopy

Electronic devices made from carbon nanotubes (CNTs) can be greatly affected by substrate charges, which, for instance, induce strong hysteresis in CNT field effect transistors. In this work, electrostatic force microscopy (EFM) is employed to investigate single-walled nanotubes grown by chemical vapor deposition on SiO2 substrates. We demonstrate the use of this technique to gain quantitative information on the substrate charges. It is found that charge pools with densities around 10(-8) C/cm2 can be trapped inside nanotube loops for extended periods of time, showing that nanotubes can act as confining barriers for substrate charges. The trapped charges can be removed by scanning probe manipulation.     

Mapping of local electrical properties in epitaxial graphene using electrostatic force microscopy

Local electrical characterization of epitaxial graphene grown on 4H-SiC(0001) using electrostatic force microscopy (EFM) in ambient conditions and at elevated temperatures is presented. EFM provides a straightforward identification of graphene with different numbers of layers on the substrate where topographical determination is hindered by adsorbates. Novel EFM spectroscopy has been developed measuring the EFM phase as a function of the electrical DC bias, establishing a rigorous way to distinguish graphene domains and facilitating optimization of EFM imaging.     

Local electronic structure of single-walled carbon nanotubes from electrostatic force microscopy

An atomic force microscope was used to locally perturb and detect the charge density in carbon nanotubes. Changing the tip voltage varied the Fermi level in the nanotube. The local charge density increased abruptly whenever the Fermi level was swept through a van Hove singularity in the density of states, thereby coupling the cantilever's mechanical oscillations to the nanotube's local electronic properties. By using our technique to measure the local band gap of an intratube quantum-well structure, created by a nonuniform uniaxial strain, we have estimated the nanotube chiral angle. Our technique does not require attached electrodes or a specialized substrate, yielding a unique high-resolution spectroscopic tool that facilitates the comparison between local electronic structure of nanomaterials and further transport, optical, or sensing experiments.     

Polarizability of G4-DNA observed by electrostatic force microscopy measurements

G4-DNA, a quadruple helical motif of stacked guanine tetrads, is stiffer and more resistant to surface forces than double-stranded DNA (dsDNA), yet it enables self-assembly. Therefore, it is more likely to enable charge transport upon deposition on hard supports. We report clear evidence of polarizability of long G4-DNA molecules measured by electrostatic force microscopy, while coadsorbed dsDNA molecules on mica are electrically silent. This is another sign that G4-DNA is potentially better than dsDNA as a conducting molecular wire.     

Tapping-mode scanning force microscopy: Metallic tips and samples

The operation of a tapping-mode scanning force microscope using a metallic tip and metallic sample, with a bias voltage applied between the two, is modelled as a driven nonlinear oscillator, where metal-metal adhesion and electric forces are taken into account. The model, which applies to the case where the sample indentation by the tip is minimal, shows that one can obtain a good estimate of the tip-sample contact time from the tip-sample current.     

Local charge transport in two-dimensional PbSe nanocrystal arrays studied by electrostatic force microscopy

Two-dimensional PbSe nanocrystal arrays on silicon nitride membranes were investigated using electrostatic force microscopy (EFM) and transmission electron microscopy (TEM). Changes in lattice and transport properties upon annealing in a vacuum were revealed. Local charge transport behavior was directly imaged by EFM and correlated to nanopatterns observed with TEM. Charge transport through nanochannels in complex two-dimensional nanocrystal networks was identified. Our results demonstrate the importance of measurements of local transport details complementary to the conventional current-voltage (I-V) measurements.     

Two-dimensional dopant profiling by electrostatic force microscopy using carbon nanotube modified cantilevers

A two-dimensional (2D) dopant profiling technique is demonstrated in this work. We apply a unique cantilever probe in electrostatic force microscopy (EFM) modified by the attachment of a multiwalled carbon nanotube (MWNT). Furthermore, the tip apex of the MWNT was trimmed to the sharpness of a single-walled carbon nanotube (SWNT). This ultra-sharp MWNT tip helps us to resolve dopant features to within 10?nm in air, which approaches the resolution achieved by ultra-high vacuum scanning tunnelling microscopy (UHV STM). In this study, the CNT-probed EFM is used to profile 2D buried dopant distribution under a nano-scale device structure and shows the feasibility of device characterization for sub-45?nm complementary metal-oxide-semiconductor (CMOS) field-effect transistors.     

Low-noise magnetic force microscopy with high resolution by tip cooling

Low-noise magnetic force microscopy (MFM) was realized by using a conventional high-vacuum MFM with homemade tip-cooling equipment. The noise level of the MFM at a tip temperature of 130 K was estimated at /spl mu/N/m order. High spatial resolution of 10 nm was obtained for observing high-density recording media with recording density of 1000 kfci. The improvement of resolution by tip cooling was a result of the reduction of thermodynamic noise of a cantilever and the effective reduction of tip-sample distance due to the magnetic hardening of a tip.     

High spatial resolution Kelvin probe force microscopy with coaxial probes

Kelvin probe force microscopy (KPFM) is a widely used technique to measure the local contact potential difference (CPD) between an AFM probe and the sample surface via the electrostatic force. The spatial resolution of KPFM is intrinsically limited by the long range of the electrostatic interaction, which includes contributions from the macroscopic cantilever and the conical tip. Here, we present coaxial AFM probes in which the cantilever and cone are shielded by a conducting shell, confining the tip-sample electrostatic interaction to a small region near the end of the tip. We have developed a technique to measure the true CPD despite the presence of the shell electrode. We find that the behavior of these probes agrees with an electrostatic model of the force, and we observe a factor of five improvement in spatial resolution relative to unshielded probes. Our discussion centers on KPFM, but the field confinement offered by these probes may improve any variant of electrostatic force microscopy.     

Chemical imaging of patterned inorganic thin-film structures by lateral force microscopy

Factors influencing the chemical image formation by lateral force microscopy (LFM, or friction force microscopy, FFM) under normal ambient conditions were studied by applying LFM to patterned specimens of inorganic thin films deposited predominantly by atomic layer epitaxy. The patterned steps on SnO(2)/Si, CaS/Si, CeO(2)/Si, and Pt/Al(2)O(3) samples were formed by chemical etching or lift-off processing. The results of semiquantitative AFM and LFM studies were compared to the static contact angle studies using capillary force evaluation. The chemical contrast in LFM images of the patterned specimens was the highest in cases where silicon was present. This is in accordance with contact angle data, which showed much higher hydrophilicity on Si than on the other materials studied. Further experiments with a patterned SnO(2)/Si specimen indicated that chemical contrast can be significantly affected (i) by whether the surface was pretreated with ethanol, (ii) by the loading force (2-50 nN or 1-10 μN) applied, and (iii) by using SnO(2)-coated AFM probes instead of the conventional Si probes.     

High resolution electron microscopy study of GeAsSe

Layer type GeAsSe crystals have been investigated by High Resolution Electron Microscopy (HREM). Structure images observed along the [001] direction allow a straightforward interpretation. The image contrast was calculated using the multi-slice approximation.     

Atomic force microscopy reveals drebrin induced remodeling of f-actin with subnanometer resolution

We show by high-resolution atomic force microscopy analysis that drebrin A (a major neuronal actin binding protein) induced F-actin structural and mechanical remodeling involves significant changes in helical twist and filament stiffness (+55% persistence length). These results provide evidence of a unique mechanical role of drebrin in the dendrites, contribute to current molecular-level understanding of the properties of the neuronal cytoskeleton, and reflect the role of biomechanics at the nanoscale, to modulate nanofilament-structure assemblies such as F-actin.