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.     

Numerical study of the lateral resolution in electrostatic force microscopy for dielectric samples

We present a study of the lateral resolution in electrostatic force microscopy for dielectric samples in both force and gradient modes. Whereas previous studies have reported expressions for metallic surfaces having potential heterogeneities (Kelvin probe force microscopy), in this work we take into account the presence of a dielectric medium. We introduce a definition of the lateral resolution based on the force due to a test particle being either a point charge or a polarizable particle on the dielectric surface. The behaviour has been studied over a wide range of typical experimental parameters: tip-sample distance (1-20) nm, sample thickness (0-5) μm and dielectric constant (1-20), using the numerical simulation of the equivalent charge method. For potential heterogeneities on metallic surfaces expressions are in agreement with the bibliography. The lateral resolution of samples having a dielectric constant of more than 10 tends to metallic behaviour. We found a characteristic thickness of 100 nm, above which the lateral resolution measured on the dielectric surface is close to that of an infinite medium. As previously reported, the lateral resolution is better in the gradient mode than in the force mode. Finally, we showed that for the same experimental conditions, the lateral resolution is better for a polarizable particle than for a charge, i.e. dielectric heterogeneities should always look 'sharper' (better resolved) than inhomogeneous charge distributions. This fact should be taken into account when interpreting images of heterogeneous samples.     

Three-dimensional observation of nanoscale ferroelectric domains using scanning nonlinear dielectric microscopy with electric field correction by Kelvin probe force microscopy

An advanced technique for the measurement of three-dimensional ferroelectric domain structure is described. Scanning nonlinear dielectric microscopy is used to measure the polarization components both perpendicular and parallel to the specimen surface. A nanoscale electric field correction is devised and performed using Kelvin probe force microscopy to allow more precise measurement of the nanoscale polarization component parallel to the specimen surface. Using this electric field correction, three-dimensional imaging of the ferroelectric polarization orientation is demonstrated.     

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.     

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.     

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.     

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.     

Charge injection on insulators via scanning probe microscopy techniques: towards data storage devices

The idea of contact electrification aiming the development of nano-scale data storage devices has been explored through a careful investigation of charge injection on insulating films (SiO2 and PMMA) via scanning probe microscopy techniques. A complete route for data storage, showing simple and effective ways to write (inject the charge with an atomic force microscopy (AFM) tip), to read (detect the charge with electric force microscopy), to store (keep sample charge by changing ambient and surface conditions) and to erase the information (make the discharge process faster) is proposed and discussed. A detailed study of the influence of several parameters like AFM mode, bias voltage, relative humidity and surface hydrophobicity is also presented to optimize both charge injection and discharge processes. Results show that monitoring parameters such as ambient relative humidity and surface hydrophobic/hydrophilic character enable the control of pattern size, lateral dispersion, and storage time. The charge polarity is also dependent on the surface hydrophobicity and either positive or negative charges can become more appropriate for storage depending on the surface hydrophobic/hydrophilic character.     

Accuracy of the spring constant of atomic force microscopy cantilevers by finite element method

Atomic force microscopy (AFM) probe with different functions can be used to measure the bonding force between atoms or molecules. In order to have accurate results, AFM cantilevers must be calibrated precisely before use. The AFM cantilever's spring constant is usually provided by the manufacturer, and it is calculated from simple equations or some other calibration methods. The spring constant may have some uncertainty, which may cause large errors in force measurement. In this paper, finite element analysis was used to obtain the deformation behavior of the AFM cantilever and to calculate its spring constant. The influence of prestress, ignored by other methods, is discussed in this paper. The variations of Young's modulus, Poisson's ratio, cantilever geometries, tilt angle, and the influence of image tip mass were evaluated to find their effects on the cantilever's characteristics. The results were compared with those obtained from other methods.     

Doping transition of doped ZnO nanorods measured by Kelvin probe force microscopy

We have investigated the doping transition of one-dimensional (1-D) doped-ZnO nanorods with Kelvin probe force microscopy (KPFM). Vertically aligned (undoped, As-doped, and undoped/As-doped homo-junction) ZnO nanorods were grown on Si (111) substrates without any catalyst by vapor phase transport. Individual ZnO nanorods are removed from the substrates and transferred onto thin Au films grown on Si substrates. The morphology and surface potentials of the nanorods were measured simultaneously by the KPFM. For the homo-junction nanorods with ~ 250 nm in diameter, the KPFM image shows localization of the doping transition along the nanorods. The measured Kelvin signal (surface potential) across the junction induces the work function difference between the undoped and the As-doped region of ~ 85 meV. Also, the work function of As-doped nanorods is ~ 95 meV higher than that of intrinsically undoped nanorods grown in similar conditions. These consistent results indicate that the KPFM is reliable to determine the localization of the doping transition in 1-D structures.     

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.     

Nanoscale mapping and affinity constant measurement of signal-transducing proteins by atomic force microscopy

Atomic force microscope (AFM) was used to measure the interaction force between two signal-transducing proteins, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and Ras homologue enriched in brain (Rheb), and to analyze the binding of glyceraldehyde-3-phosphate (Gly-3-P) to GAPDH. To enhance the recognition efficiency and avoid undesirable multiple interactions, the AFM probe and the substrate were each modified with a dendron, glutathione S-transferase (GST)-fused proteins were employed, and reduced glutathione (GSH) was conjugated at the apex of each immobilized dendron. The resulting median specific force between GAPDH and Rheb was 38 ± 1 pN at a loading rate of 3.7 × 10(3) pN/s. The measurements showed that the GAPDH-Rheb interaction was inhibited by binding of Gly-3-P. An adhesion force map showed individual GADPHs on the surface and that the number density of GAPDH decreased with the concentration of Gly-3-P. Maps obtained in the presence of various Gly-3-P concentrations provided information on the binding behavior, yielding a thermodynamic association constant of 2.7 × 10(5) M(-1).     

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.     

Electrical characterization of Ge microcrystallites by atomic force microscopy using a conducting probe

We have studied the nucleation and growth of Ge microcrystallities on Si(100) or evaporated Cr substrates from an rf glow discharge decomposition of GeH₄ highly-diluted with H₂, where the crystallinity, the surface microroughness and the local electric transport of the films have been measured as a function of the film thickness. For the film growth thicker than ∼65 nm, Raman scattering spectra show that the evolution of the microcrystalline phase tends to be saturated. In the thickness range of 7-65 nm, the nucleation and/or microcrystalline grain formation with progressive film growth and corresponding significant difference in the electrical conductivity in the direction of the film thickness between the grains and their boundaries have been demonstrated from topographic and current images taken simultaneously by an atomic force microscope with a conducting probe.     

The work function of doped polyaniline nanoparticles observed by Kelvin probe force microscopy

The work function of polyaniline nanoparticles in the emeraldine base state was determined by Kelvin probe force microscopy to be ～270 meV higher than that of similar nanoparticles in the emeraldine salt state. Normal tapping mode atomic force microscopy could not be used to distinguish between the particles due to their similar morphologies and sizes. Moreover, other potential measurement systems, such as using zeta potentials, were not suitable for the measurement of surface charges of doped nanoparticles due to their encapsulation by interfering chemical groups. Kelvin probe force microscopy can be used to overcome these limitations and unambiguously distinguish between the bare and doped polyaniline nanoparticles.     

Reduction of tip-sample contact using dielectrophoretic force scanning probe microscopy

Dielectrophoretic force microscopy is shown to allow for facile noncontact imaging of systems in aqueous media. Electrokinetic tip-sample forces were predicted from topography measurements of an interface and compared with experimental images. Correlation function and power spectral density analyses indicated that image feedback was maintained without mechanical contact using moderate potentials (e.g., approximately 18 nm off the surface for a 7-Vpp, 100-kHz waveform). The applied dielectrophoretic force and the corresponding increase in effective tip radius were predictably adjusted by changing the peak potential.     

Studying electric field profiles in GaAs-based detector structures by Kelvin probe force microscopy

The method of scanning Kelvin probe force microscopy has been used to study the electric field distribution in GaAs-based p ⁺-π-n-n ⁺ detector structures. In the active layer volume, two maxima in the field strength profiles have been found, which are localized in the regions of p ⁺-π and π-n junctions. A volt-age drop on the π-n junction expands the region of collection of nonequilibrium holes, thus increasing the charge collection efficiency for the absorption of γ photons with an energy of 59.5 keV.     

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.     

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.