Electrostatic Force Microscopy Technique Research Articles

Characterization of CVD-synthesized graphene films transferred on different substrates using the scanning probe microscopy electrical techniques

The use of local electrical characteristics of graphene films prepared by chemical vapor deposition (CVD) method on Cu and transferred on different substrates such as glass, SiO 2 /Si and Al 2 O 3 substrates was discussed. The topography, Kelvin probe force microscopy (KPFM), scanning capacitance microscopy (SCM) and electrostatic force microscopy (EFM) were compared for graphene on the three different dielectric substrates. The surface potentials of graphene films were investigated using KPFM. It was found that graphene films exhibited nonuniform surface potential distribution depending on the sort of the underlying substrate used, the number of graphene layers and the presence of graphene film defects such as edges, wrinkles etc. The EFM results demonstrated that the local electrostatic properties of graphene film on Al 2 O 3 substrate differ significantly compared to that on glass and SiO 2 /Si substrates. The cross-correlation coefficients between topographical image and SCM- and EFM-images were estimated in order to compare these techniques. It was shown that the techniques can bring complementary information for the characterization of graphene films on different substrates relevant for applications. • The local electrical properties of graphene films were investigated by KPFM, SCM and EFM techniques. • The quantitative values of local dielectric properties are sensitive to the underlying substrate. • Kelvin probe force microscopy. • Scanning capacitance microscopy. • Electrostatic force microscopy.

High-resolution electrical characterization of RuO2-borosilicate glass composites

RuO2-borosilicate glass composites representing nuclear glasses show a sudden increase in electrical conductivity when the RuO2 content reached a concentration of around 1% in volume. This percolation threshold is astonishingly low compared to a geometrical percolation threshold, i.e. 15%, considering the random distribution of phases. Herein, electrostatic force microscopy (EFM) analyses were used to investigate this anomalous behavior by performing local dielectric characterization of the RuO2-borosilicate glass composites that comprised clusters of precipitated RuO2 particles and "apparently” RuO2-free borosilicate regions. As expected, differences between the relative permittivity of the two zones, borosilicate matrix, and RuO2 particles, were found. Most interesting, the sensitivity of the EFM technique combined with numerical modeling based on a finite element method revealed that the relative permittivity of the borosilicate matrix was enhanced with the overall RuO2 content. Such an increase can be explained by a local enrichment of the borosilicate matrix in RuO2, which increases the overall relative permittivity due to its much higher electronic polarizability. The enrichment of the matrix could play a role in the conduction mechanism, changing the effective volume of the conductive medium and, therefore, decreasing the minimal volume of the RuO2 phase necessary for the electrical percolation.

Self-assembling crystalline peptide microrod for neuromorphic function implementation

Self-assembling crystalline peptide microrod for neuromorphic function implementation

Review of time-resolved non-contact electrostatic force microscopy techniques with applications to ionic transport measurements.

Recently, there have been a number of variations of electrostatic force microscopy (EFM) that allow for the measurement of time-varying forces arising from phenomena such as ion transport in battery materials or charge separation in photovoltaic systems. These forces reveal information about dynamic processes happening over nanometer length scales due to the nanometer-sized probe tips used in atomic force microscopy. Here, we review in detail several time-resolved EFM techniques based on non-contact atomic force microscopy, elaborating on their specific limitations and challenges. We also introduce a new experimental technique that can resolve time-varying signals well below the oscillation period of the cantilever and compare and contrast it with those previously established.

Electrostatic force microscopy for the accurate characterization of interphases in nanocomposites.

The unusual properties of nanocomposites are commonly explained by the structure of their interphase. Therefore, these nanoscale interphase regions need to be precisely characterized; however, the existing high resolution experimental methods have not been reliably adapted to this purpose. Electrostatic force microscopy (EFM) represents a promising technique to fulfill this objective, although no complete and accurate interphase study has been published to date and EFM signal interpretation is not straightforward. The aim of this work was to establish accurate EFM signal analysis methods to investigate interphases in nanodielectrics using three experimental protocols. Samples with well-known, controllable properties were designed and synthesized to electrostatically model nanodielectrics with the aim of “calibrating” the EFM technique for future interphase studies. EFM was demonstrated to be able to discriminate between alumina and silicon dioxide interphase layers of 50 and 100 nm thickness deposited over polystyrene spheres and different types of matrix materials. Consistent permittivity values were also deduced by comparison of experimental data and numerical simulations, as well as the interface state of silicone dioxide layers.

Nucleosome-like, Single-stranded DNA (ssDNA)-Histone Octamer Complexes and the Implication for DNA Double Strand Break Repair

Repair of DNA double strand breaks (DSBs) is key for maintenance of genome integrity. When DSBs are repaired by homologous recombination, DNA ends can undergo extensive processing, producing long stretches of single-stranded DNA (ssDNA). In vivo, DSB processing occurs in the context of chromatin, and studies indicate that histones may remain associated with processed DSBs. Here we demonstrate that histones are not evicted from ssDNA after in vitro chromatin resection. In addition, we reconstitute histone-ssDNA complexes (termed ssNucs) with ssDNA and recombinant histones and analyze these particles by a combination of native gel electrophoresis, sedimentation velocity, electron microscopy, and a recently developed electrostatic force microscopy technique, DREEM (dual-resonance frequency-enhanced electrostatic force microscopy). The reconstituted ssNucs are homogenous and relatively stable, and DREEM reveals ssDNA wrapping around histones. We also find that histone octamers are easily transferred in trans from ssNucs to either double-stranded DNA or ssDNA. Furthermore, the Fun30 remodeling enzyme, which has been implicated in DNA repair, binds ssNucs preferentially over nucleosomes, and ssNucs are effective at activating Fun30 ATPase activity. Our results indicate that ssNucs may be a hallmark of processes that generate ssDNA, and that posttranslational modification of ssNucs may generate novel signaling platforms involved in genome stability.

High resolution dynamic electrostatic force microscopy technique: quantifying electrical properties at the nanoscale.

AbstractIn electrostatic force microscopy (EFM), a conductive atomic force microscopy (AFM) tip is electrically biased against a grounded sample and electrostatic forces are investigated. This methodology has been broadly used in the scientific community to characterize dielectric properties of samples at the nanoscale. Two are the main operating conditions associated with this technique. The oscillation amplitude is usually kept to very small values to allow a linearized approach to the force reconstruction and the tip-sample distance is maintained elevated. However, this latter condition negatively affects the lateral resolution of the technique. Thus, electrostatic interaction should be probed in the vicinity of the sample. Theoretically, in this region the force can be linearized using oscillation amplitudes in the order of Å. This might cause the trapping of the tip on the surface (snap-in). Furthermore, at small distances, short-range forces (i.e. Van der Waals’) might reach values comparable to electrostatic forces.Here we present a framework that combines EFM and dynamic amplitude modulation AFM to achieve decoupled reconstruction of forces. It permits reconstructing the real shape of the electrostatic force and the capacitance of the tip-sample system even in the vicinity of the surface. This is done using a technique proposed in literature by Sader and Katan to reconstruct the force without the linearization approximation. The steps needed to decouple short-range and electrostatic forces are explained in detail. This data can be employed to derive the electrical properties of thin films with enhanced lateral resolution with respect to the commonly used EFM techniques.

Dynamic electrostatic force microscopy technique for the study of electrical properties with improved spatial resolution

The need to resolve the electrical properties of confined structures (CNTs, quantum dots, nanorods, etc) is becoming increasingly important in the field of electronic and optoelectronic devices. Here we propose an approach based on amplitude modulated electrostatic force microscopy to obtain measurements at small tip–sample distances, where highly nonlinear forces are present. We discuss how this improves the lateral resolution of the technique and allows probing of the electrical and surface properties. The complete force field at different tip biases is employed to derive the local work function difference. Then, by appropriately biasing the tip–sample system, short-range forces are reconstructed. The short-range component is then separated from the generic tip–sample force in order to recover the pure electrostatic contribution. This data can be employed to derive the tip–sample capacitance curve and the sample dielectric constant. After presenting a theoretical model that justifies the need for probing the electrical properties of the sample in the vicinity of the surface, the methodology is presented in detail and verified experimentally.

Compatibility studies of polystyrene and poly(vinyl acetate) blends using electrostatic force microscopy

AbstractThe effect of thermal treatment on the phase separation process of the components of a polymer blend was investigated using electrostatic force microscopy (EFM). EFM technique is an advance on conventional atomic force microscopy, which enables us to measure locally the dielectric properties of the samples under investigation providing compositional information. In this work, we studied the phase separation process of the polymer blend thin films made of polystyrene and poly(vinyl acetate) (PS/PVAc) (75/25 weight fraction). The samples were subjected to different thermal treatments. It was found that at low annealing temperature, PVAc forms many small islands within PS matrix. As the annealing temperature increases, the number of PVAc islands decreases with an increase in the size of the islands. These islands take spherical‐like shape when annealed at a temperature well above the glass transition temperatures of both the component polymers. Despite these morphological/topographical changes, EFM images evidence that there is no interdiffusion which was further confirmed by quantitatively measuring the value of the dielectric permittivity across the interphase. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1332–1338, 2011

Electrostatic force microscopy of self‐assembled peptide structures

In this report electrostatic force microscopy (EFM) is used to study different peptide self-assembled structures such as tubes and particles. It is shown that not only geometrical information can be obtained using EFM, but also information about the composition of different structures. In particular we use EFM to investigate the structures of diphenylalanine peptide tubes, particles, and CSGAITIG peptide particles placed on pre-fabricated SiO(2) surfaces with a backgate. We show that the cavity in the peptide tubes could be due to the presence of water residues. Additionally we show that self-assembled amyloid peptides form spherical solid structures containing the same self-assembled peptide in its interior. In both cases transmission electron microscopy is used to verify these structures. Further, the limitations of the EFM technique are discussed, especially when the observed structures become small compared with the radius of the AFM tip used. Finally, an agreement between the detected signal and the structure of the hollow peptide tubes is demonstrated.

Mapping of carbon nanotubes in the polystyrene domains of a polystyrene-b-polyisoprene-b-polystyrene block copolymer matrix using electrostatic force microscopy

Electrostatic force microscopy (EFM) was used to localize and characterize conducting nanofillers inside the domains of a block copolymer matrix. The confinement of carbon nanotubes into polystyrene-b-polyisoprene-b-polystyrene block copolymer (SIS) matrix was investigated. The EFM technique provided information with unprecedented imaging of octadecylamine functionalized single-walled carbon nanotubes, sequestered in the self-assembled lamellar polystyrene domains of the SIS block copolymer matrix.

Characterisation of trapped electric charge carriers behaviour at nanometer scale by electrostatic force microscopy

Atomic force microscopy (AFM) and related electrical probe techniques such as electrostatic force microscopy (EFM) can be used to perform injection and detection of electric charge carriers in nanostructures or oxide layer at the nanometer scale. In this paper the control and the deposition of both positive and negative local charges are described and discussed. A basic introduction to both the theoretical and experimental techniques of EFM is also presented. In addition a review of the analytical calculation of tip–surface capacitance interaction is described and then utilised to estimate charge storage in oxide layers and nanostructures from EFM images or spectroscopy curves. The charge resolution of EFM at room temperature and a controlled atmosphere is estimated to be about 20 charge carriers. The EFM technique is also used to inject charges and to study their behaviour in confined silicon nanostructures covered by a thin layer of oxide and separated from the Si substrate by a SiO 2 layer. The total injected charge is found to depend on the thickness of the oxide layer. The electric field has emerged to be a key parameter in the injection mechanism. The dynamics and propagation of the deposited charge carriers have been studied and a homogenous distribution of the charge in the nanostructure has been observed. Thanks to these studies and observation, the localisation of the trapped charges has been determined: it occurs mainly in the silicon pattern rather than on the thin covering oxide layer. This charge localisation together with charge energy calculation leads to a better understanding of the origin of the charge dissipation.

Macroscopic and microscopic charging effects of Si nanocrystals embedded in a SiO 2 layer

Capacitance–voltage ( C– V) and electrostatic force microscopy (EFM) measurements on Si nanocrystals (Si-NCs) formed by using the sonochemical method were carried out to investigate the charging effects of the Si-NCs. Transmission electron microscopy images and atomic force microscopy images showed that the Si-NCs were created inside the SiO 2 layer. The C– V curve and the EFM image showed that the Si-NCs embedded in the SiO 2 layer experienced charging effects. The macroscopic surface charge density determined from the C– V curve was in reasonable agreement with the microscopic local value obtained from the EFM image. The present results indicate that the EFM technique might provide a promising method for investigating charging effects in various kinds of nanocrystals embedded in the insulating layer.

Electrostatic force microscopy as a tool to estimate the number of active potential barriers in dense non-Ohmic polycrystalline SnO2 devices

In the present work, electroactive grain boundaries of highly dense metal oxide SnO2-based polycrystalline varistors were determined by electrostatic force microscopy (EFM). The EFM technique was applied to identify electroactive grain boundaries and thus estimate the amount of active grain boundary, which, in the metal oxide SnO2-based varistor, was calculated at around 85%, i.e., much higher than that found in traditional metal oxide ZnO-based varistors. The mean potential barrier height value obtained from the EFM analysis was in complete agreement with the values calculated from the C-V measurements, together with a complex capacitance plane analysis that validates the methodology proposed here.

Rubbing-induced charge domains observed by electrostatic force microscopy: effect on liquid crystal alignment

The rubbing of polymer-coated substrates is one of the most frequently used techniques for liquid crystal alignment. However, the aligning mechanisms are not completely understood. The influence of friction charges induced by the rubbing process has been taken into account in theoretical publications. In this work we investigate the rubbing-induced charge domains of three polymers (PMMA, PI and PVA) with the electrostatic force microscopy technique, which allows the simultaneous determination of the surface topography and electrostatic potential. We observed a large intensity of the potential for the PMMA substrate, whereas no charge domains were observed for PVA. In addition, we followed the time evolution of the surface charge domains, and surprisingly after five days the charges were still present with a small reduction of intesity. Using polarizing optical microscopy we studied the influence of the charge domains on the LC aligning properties.

Humidity-dependent open-circuit photovoltage from a bacteriorhodopsin/indiumtin oxide bioelectronic heterostructure

Non-contact electrostatic force microscopy techniques were used tostudy the open-circuit photovoltage of purple membrane multilayerselectrodeposited on indium tin oxide. A humidity-dependent photovoltageresponse under illumination by a 635 nm photodiode was observed. Peakphotovoltages in excess of 2 V at ∼36 W m−2 were obtainedfor ∼15%relative humidity.

Equivalent-time sampling force microscopy using pulse position modulation method

This paper presents a homodyne pulse-sampling electrostatic force microscopy technique using a pulse position modulation method for the noncontact measurement of internal voltage waveforms in integrated circuits. The measurement system operates by monitoring the mechanical deflection of a micromachined probe as it responds to electric forces induced by the circuit voltage waveform. Although the mechanical response of typical probes used in such systems is limited to few kHz, measurement of high frequency repetitive waveforms is enabled by applying a high speed sampling pulse signal to the probe with the pulse position modulated at a rate below the probe mechanical resonance. This results in down conversion of the circuit induced electric force harmonics to within the probe mechanical response, thus allowing the measurement of high frequency signals. The proposed technique is modeled using Fourier analysis of the measurement system response, and is demonstrated by the measurement of a 0.8 Mbit/s digital pattern on a CMOS test pad. The performance of the measurement system is analyzed based on obtained measurements and simulated system response. An analysis of the method capabilities shows a measurement sensitivity of 13 mVrms/√Hz.

Polar diffraction gratings made by spatially periodic photopoling Langmuir–Blodgett films

Polar diffraction gratings have been prepared by spatially periodic photopoling Langmuir–Blodgett films consisting of azobenzene compounds. The gratings were made by film irradiation with two left circularly polarized Ar-ion laser beams under strong electric field from a corona discharge. Therefore, simultaneously with the spatial modulation of the linear refractive index, the local polar order of the film becomes periodically modulated (through symmetry, the latter is related to the modulation of second-order optical nonlinearity χ(2)). The local polarity is shown by an electrostatic force microscopy technique: an image of the polar grating is recorded without any trace of periodicity in the film topography.

Epitaxial layer sheet resistance outside and under ohmic contacts measurements using electrostatic force microscopy

The electrostatic force microscopy (EFM) is used to measure the absolute surface potential on a transmission line model (TLM) structure.The classic electrical measurement taken on the TLM structure has been used to determine the sheet resistance outside the ohmic contact and the contact resistance. Modeling the potential profile along TLM structures has been made using a distributed array of resistances. The sheet resistance of the epitaxial layer under the ohmic contact, the interfacial resistance and the gold layer resistance are taken into account.The measured potential profile using the EFM technique allows to determine the sheet resistance of the epitaxial layer outside and under the ohmic contact which gives the best fit to the model. These values can be determined with an accuracy better than 5%.

Cross-sectional electrostatic force microscopy of thin-film solar cells

In a recent work, we showed that atomic force microscopy (AFM) is a powerful technique to image cross sections of polycrystalline thin films. In this work, we apply a modification of AFM, namely, electrostatic force microscopy (EFM), to investigate the electronic properties of cleaved II–VI and multijunction thin-film solar cells. We cleave the devices in such a way that they are still working with their nominal photovoltaic efficiencies and can be polarized for the measurements. This allows us to differentiate between surface effects (work function and surface band bending) and bulk device properties. In the case of polycrystalline CdTe/CdS/SnO2/glass solar cells, we find a drop of the EFM signal in the area of the CdTe/CdS interface (±50 nm). This drop varies in amplitude and sign according to the applied external bias and is compatible with an n-CdS/p-CdTe heterojunction model, thereby invalidating the possibility of a deeply buried n-p CdTe homojunction. In the case of a triple-junction GaInP/GaAs/Ge device, we observe a variation of the EFM signal linked to both the material work-function differences and to the voltage bias applied to the cell. We attempt a qualitative explanation of the results and discuss the implications and difficulties of the EFM technique for the study of such thin-film devices.