Electrostatic Force Spectroscopy Research Articles

High-low Kelvin probe force spectroscopy for measuring the interface state density.

The recently proposed high-low Kelvin probe force microscopy (KPFM) enables evaluation of the effects of semiconductor interface states with high spatial resolution using high and low AC bias frequencies compared with the cutoff frequency of the carrier transfer between the interface and bulk states. Information on the energy spectrum of the interface state density is important for actual semiconductor device evaluation, and there is a need to develop a method for obtaining such physical quantities. Here, we propose high-low Kelvin probe force spectroscopy (high-low KPFS), an electrostatic force spectroscopy method using high- and low-frequency AC bias voltages to measure the interface state density inside semiconductors. We derive an analytical expression for the electrostatic forces between a tip and a semiconductor sample in the accumulation, depletion, and inversion regions, taking into account the charge transfer between the bulk and interface states in semiconductors. We show that the analysis of electrostatic forces in the depletion region at high- and low-frequency AC bias voltages provides information about the interface state density in the semiconductor bandgap. As a preliminary experiment, high-low KPFS measurements were performed on ion-implanted silicon surfaces to confirm the dependence of the electrostatic force on the frequency of the AC bias voltage and obtain the interface state density.

Electrostatic force spectroscopy revealing the degree of reduction of individual graphene oxide sheets.

Electrostatic force spectroscopy (EFS) is a method for monitoring the electrostatic force microscopy (EFM) phase with high resolution as a function of the electrical direct current bias applied either to the probe or sample. Based on the dielectric constant difference of graphene oxide (GO) sheets (reduced using various methods), EFS can be used to characterize the degree of reduction of uniformly reduced one-atom-thick GO sheets at the nanoscale. In this paper, using thermally or chemically reduced individual GO sheets on mica substrates as examples, we characterize their degree of reduction at the nanoscale using EFS. For the reduced graphene oxide (rGO) sheets with a given degree of reduction (sample n), the EFS curve is very close to a parabola within a restricted area. We found that the change in parabola opening direction (or sign the parabola opening value) indicates the onset of reduction on GO sheets. Moreover, the parabola opening value, the peak bias value (tip bias leads to the peak or valley EFM phases) and the EFM phase contrast at a certain tip bias less than the peak value can all indicate the degree of reduction of rGO samples, which is positively correlated with the dielectric constant. In addition, we gave the ranking of degree for reduction on thermally or chemically reduced GO sheets and evaluated the effects of the reducing conditions. The identification of the degree of reduction of GO sheets using EFS is important for reduction strategy optimization and mass application of GO, which is highly desired owing to its mechanical, thermal, optical and electronic applications. Furthermore, as a general and quantitative technique for evaluating the small differences in the dielectric properties of nanomaterials, the EFS technique will extend and facilitate its nanoscale electronic devices applications in the future.

Charge-state dynamics in electrostatic force spectroscopy

We present a numerical model that allows us to study the response of an oscillating probe in electrostatic force spectroscopy to charge switching in quantum dots at various time scales. The model provides more insight into the behavior of frequency shift and dissipated energy under different scanning conditions when measuring a temporarily charged quantum dot on a surface. Namely, we analyze the dependence of the frequency shift, the dissipated energy, and their fluctuations on the resonance frequency of the tip and on the electron tunneling rates across the tip–quantum dot and quantum dot–sample junctions. We discuss two complementary approaches to simulating the charge dynamics, a stochastic and a deterministic one. In addition, we derive analytic formulas valid for small amplitudes, describing relations between the frequency shift, dissipated energy, and the characteristic rates driving the charging and discharging processes.

Nanoscale surface charge detection in epoxy resin materials using electrostatic force spectroscopy

Electrostatic force spectroscopy (EFS) operated in a conventional force gradient detection method allows determining local surface charges in epoxy samples. This is made possible through a detailed analysis of gradient versus DC voltage curves. The parabolic dependence of these curves is closely related to the charge density. Both maximum and origin-ordinate are key data from which it is possible to extract quantitative information on the detected charge. The study is based on the combined use of numerical and analytical simulations of the probe sample interaction. Excellent sensitivities to very low surface charge densities are reported.

Revealing energy level structure of individual quantum dots by tunneling rate measured by single-electron sensitive electrostatic force spectroscopy.

We present theoretical and experimental studies of the effect of the density of states of a quantum dot (QD) on the rate of single-electron tunneling that can be directly measured by electrostatic force microscopy (e-EFM) experiments. In e-EFM, the motion of a biased atomic force microscope cantilever tip modulates the charge state of a QD in the Coulomb blockade regime. The charge dynamics of the dot, which is detected through its back-action on the capacitavely coupled cantilever, depends on the tunneling rate of the QD to a back-electrode. The density of states of the QD can therefore be measured through its effect on the energy dependence of tunneling rate. We present experimental data on individual 5 nm colloidal gold nanoparticles that exhibit a near continuous density of state at 77 K. In contrast, our analysis of already published data on self-assembled InAs QDs at 4 K clearly reveals discrete degenerate energy levels.

Epitaxial Graphene and Graphene–Based Devices Studied by Electrical Scanning Probe Microscopy

We present local electrical characterization of epitaxial graphene grown on both Si- and C-faces of 4H-SiC using Electrostatic Force Microscopy and Kelvin Probe Force Microscopy in ambient conditions and at elevated temperatures. These techniques provide a straightforward identification of graphene domains with various thicknesses on the substrate where topographical determination is hindered by adsorbates and SiC terraces. We also use Electrostatic Force Spectroscopy which allows quantitative surface potential measurements with high spatial resolution. Using these techniques, we study evolution of a layer of atmospheric water as a function of temperature, which is accompanied by a significant change of the absolute surface potential difference. We show that the nanoscale wettability of the material is strongly dependent on the number of graphene layers, where hydrophobicity increases with graphene thickness. We also use micron-sized graphene Hall bars with gold electrodes to calibrate work function of the electrically conductive probe and precisely and quantitatively define the work functions for single- and double-layer graphene.

High potential sensitivity in heterodyne amplitude-modulation Kelvin probe force microscopy

A surface potential measurement method using amplitude-modulation and heterodyne techniques is proposed. The effect of the stray capacitance between a cantilever and a sample in Kelvin probe force microscopy and the electrostatic force spectroscopy measurements are almost completely removed, because the distance (z) dependence of the modulated electrostatic force increases from 1/z to1/z2. This method improves the sensitivity of short range forces and reduces the surface potential measurement crosstalk that is induced by topographic feedback. This method has the advantage of high potential sensitivity due to the high cantilever Q value under vacuum. Quantitative surface potential measurements are demonstrated.

Nanoscale charge distribution and energy band modification in defect-patterned graphene

Defects were introduced precisely to exfoliated graphene (G) sheets on a SiO(2)/n(+) Si substrate to modulate the local energy band structure and the electron pathway using solution-phase oxidation followed by thermal reduction. The resulting nanoscale charge distribution and band gap modification were investigated by electrostatic force microscopy and spectroscopy. A transition phase with coexisting submicron-sized metallic and insulating regions in the moderately oxidized monolayer graphene were visualized and measured directly. It was determined that the delocalization of electrons/holes in a graphene "island" is confined by the surrounding defective C-O matrix, which acts as an energy barrier for mobile charge carriers. In contrast to the irreversible structural variations caused by the oxidation process, the electrical properties of graphene can be restored by annealing. The defect-patterned graphene and graphene oxide heterojunctions were further characterized by electrical transport measurement.

Ion Jump Dynamics in Nanoscopic Subvolumes Analyzed by Electrostatic Force Spectroscopy

Abstract A nanoscopic technique based on electrostatic force spectroscopy in the time domain is introduced. This technique is used to characterize the ion dynamics in nanoscale subvolumes of solid electrolytes. Nanoscopic polarization spots can be created and directly visualized, and their time evolution can be studied. In the case of partially crystallized glass ceramics, the dynamic processes in different phases (glassy, crystalline and interface) can be distinguished and their activation energies and pre-exponential factors can be quantified. By applying grid-type spectroscopic measurements, maps of the local relaxation strength are obtained, giving information about the spatial distribution of the glassy and crystalline phase.

Electrostatic force spectroscopy on insulating surfaces: the effect of capacitiveinteraction

Scanning surface probes delivered from atomic force microscopy (AFM) are expected toinvestigate local electrostatic properties on insulating surfaces by forces. Electrostatic forcespectroscopy is especially suitable to clarify the capacitive interaction. In order to performit at a well-defined tip–surface separation, we developed a dynamic mode, in which thetip–surface separation is regulated by maintaining the cantilever oscillation amplitude withan active feedback, while the electrostatic force gradient is simultaneously detected with avariable resonant frequency shift. Using the method, it turns out that the quadraticdependence of the electrostatic force gradient on an applied bias observed on an insulatingAl2O3(0001) is comparable to those on a metallic Au(111). It results from the potential differencebetween the tip and the insulator surface being only one order smaller than that betweenthe tip and the metal surface despite the spacing between electrodes for the insulator being106 times larger than for the metal, because the capacitive interaction is modified primarilybetween the sharp AFM tip and the surface.

Characterizing ion dynamics in nanoscopic volumes: time-domain electrostatic force spectroscopy on solid electrolytes

We use time-domain electrostatic force spectroscopy (TD-EFS) for characterizing the ion dynamics in solid electrolytes on a nanoscopic level. For partially crystallized glass ceramics we are able to distinguish between dynamic processes in the glassy phase, in crystallites, and at interfaces. Nanoscopic polarization spots can be created and directly visualized, and their time evolution can be studied. By applying grid-type spectroscopic measurements, maps of the local relaxation strength can be obtained, giving information about the spatial distribution of the glassy and crystalline phases.

Time-domain electrostatic force spectroscopy on nanostructured lithium-ion conducting glass ceramics: analysis and interpretation of relaxation times

The nanoscopic electrical properties of LiAlSiO(4) glass ceramics with different degrees of crystallinity chi were studied by means of time-domain electrostatic force spectroscopy (TDEFS). Thereby, a faster relaxation process due to lithium ion movements in the glassy phase and a slower process due to lithium ion movements in the crystallites could be distinguished. Over a broad range of crystallinity values, the TDEFS relaxation times of both processes are Arrhenius activated, with an activation energy being essentially independent of chi and with a pre-exponential factor depending in a systematic fashion on chi. With increasing crystallinity, the pre-exponential factor of the faster process (glassy phase) increases, while that of the slower process (crystallites) decreases. In order to explain this observation, we consider simple equivalent circuit models for the capacitance relaxation of the system AFM tip/gap/sample.

Nanoscale charging hysteresis measurement by multifrequency electrostatic force spectroscopy

We report a scanning probe technique that can be used to measure charging of localized states on conducting or partially insulating substrates at room temperature under ambient conditions. Electrostatic interactions in the presence of a charged particle between the tip and the sample is monitored by the second order flexural mode, while the fundamental mode is used for stabilizing the tip-sample separation. Cycling the bias voltage between two limits, it is possible to observe hysteresis of the second order mode amplitude due to charging. Results are presented on silicon nitride films containing silicon nanocrystals.

Fast Interfacial Ionic Conduction in Nanostructured Glass Ceramics

The hopping movements of mobile ions in a nanostructured LiAlSiO4 glass ceramic are characterized by time-domain electrostatic force spectroscopy (TDEFS). While the macroscopic conductivity spectra are governed by a single activation energy, the nanoscopic TDEFS measurements reveal three different dynamic processes with distinct activation energies. Apart from the ion transport processes in the glassy and crystalline phases, we identify a third process with a very low activation energy, which is assigned to ionic movements at the interfaces between the crystallites and glassy phase. Such interfacial processes are believed to play a key role for obtaining high ionic conductivities in nanostructured solid electrolytes.

Light induced electrostatic force spectroscopy: Application to local electronic transitions in InN epifilms

A technique based on electrostatic force microscopy in which light is used to change the charge states of the local region in a solid is introduced and demonstrated. This technique provides a unique feature that it can be used to probe local electronic transitions of a solid in a submicron scale. As an illustration, it has been applied to study local electronic structure in InN epifilms. Combining with atomic force microscopy, it is found that surface state density in the dale region is larger than that in the pinnacle region and an electron accumulation layer does exist on the surface. In addition, the magnitude of the surface band bending obtained for the regions with different surface states is consistent with the result measured by other techniques. We point out that light induced scanning electrostatic force spectroscopy is a very useful tool to probe the local electronic transitions of a solid in a submicron scale with high sensitivity.

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.

Electrostatic force spectroscopy of near surface localized states

Electrostatic force microscopy at cryogenic temperatures is used to probe the electrostaticinteraction of a conductive atomic force microscopy tip and electronic charges trapped inlocalized states in an insulating layer on a semiconductor. Measurement of the frequencyshift of the cantilever as a function of tip–sample bias voltage shows discrete peaks atcertain voltages when the tip is located near trap centres. These discrete changes infrequency are attributed to one by one filling of individual electronic states when thequantized energies traverse the substrate conduction band Fermi energy as the tip–samplevoltage is increased. Theoretical analysis of the experiment suggests that such ameasurement of the cantilever frequency shift as a function of bias voltage can beinterpreted as an AC force measurement, from which spectroscopic informationabout the location and energy of localized states can be deduced. Experimentalresults from the study of a sample with InAs quantum dots as trap centres arepresented.

Nanoscopic study of the ion dynamics in a LiAlSiO4 glass ceramic by means of electrostatic force spectroscopy

We use time-domain electrostatic force spectroscopy (TD-EFS) for characterising the dynamics of mobile ions in a partially crystallised LiAlSiO4 glass ceramic, and we compare the results of the TD-EFS measurements to macroscopic electrical conductivity measurements. While the macroscopic conductivity spectra are determined by a single dynamic process with an activation energy of 0.72 eV, the TD-EFS measurements provide information about two distinct relaxation processes with different activation energies. Our results indicate that the faster process is due to ionic movements in the glassy phase and at the glass-crystal interfaces, while the slower process is caused by ionic movements in the crystallites. The spatially varying electrical relaxation strengths of the fast and of the slow process provide information about the nano- and mesostructure of the glass ceramic.

Probing ion transport at the nanoscale: Time-domain electrostatic force spectroscopy on glassy electrolytes

We have carried out time-domain electrostatic force spectroscopy on two different ion-conducting glasses using an atomic force microscope. We compare the electrostatic force spectroscopic data obtained at different temperatures with macroscopic electrical data of the glasses. The overall consistency of the data shows that electrostatic force spectroscopy is capable of probing the ion dynamics and transport in nanoscopic subvolumes of the samples.

Electrostatic Force Spectroscopy and Imaging of Bi Wires: Spatially Resolved Quantum Confinement

We demonstrate that scanning probe electrostatic force microscopy applied to a nanoscale electronic structure can be used for studying the spatially resolved carrier quantized states as determined by the screening properties of local surface regions of the structure. The results presented for the cross-sectional surface of Bi quantum wires elucidate the microscopic nature of quasi-one-dimensional confined states excited by an applied bias voltage, where the single-particle-in-a-box energy quantization competes with the wire boundary enhanced intercarrier Coulomb repulsion.