Electrostatic Force Microscopy Images Research Articles

Localized Charge on Surfactant-Wrapped Single-Walled Carbon Nanotubes

As-synthesized, semiconducting single-walled carbon nanotubes (SWCNTs) are nominally charge neutral. However, ionic surfactants that are commonly used to disperse SWCNTs in solution can lead to significantly charged aggregates adsorbed to the nanotube. Here, electrostatic force microscopy (EFM) was used to characterize the static-charge interactions between individual SWCNTs and the local environment. We report nonuniform spatial charge distributions with highly varying magnitudes ranging between ±15 e associated with surfactant coverage on long SWCNTs (>1.5 μm). EFM images acquired after resonant photoexcitation demonstrate charge carrier localization due to electrostatic interactions with charged surfactant aggregates. Charge densities as measured by EFM are used to estimate the depth of this electrostatically induced potential well, calculated to be on the order of hundreds of millielectronvolts, suggesting that surfactant charges heterogeneously covering SWCNTs provide traps for excitons potentially leading to their localization.

The detection of buried nanopillar based on electrostatic force microscopy simulation

Based on electrostatic force microscopy (EFM), the image of nano-objects buried below the surface was numerically simulated by using COMSOL Multiphysics® software. The shape and the approximate size of the buried pillar could be obtained from the simulated EFM images. It was demonstrated that the detection of the buried nanopillar based on EFM was feasible. When the image data measured by EFM were used as the input data for comparison with the simulated data, the three unknowns (relative dielectric constant εr,p, buried depth d, and side length l) of the buried pillar could be obtained. In this paper, the simulated EFM image data were used as the input data. The accurate values of εr,p, d, and l of the buried nanopillar could be obtained by comparing the input data with the data simulated with other known parameters. The results showed that EFM was a good candidate as a kind of three dimensional nanoscale tomographic technique. It could map the physical properties of buried nanomaterials, which are relevant to modern integrated circuits.

Mobility-Fluctuation-Controlled Linear Positive Magnetoresistance in 2D Semiconductor Bi2O2Se Nanoplates.

Linear magnetoresistance is generally observed in polycrystalline zero-gap semimetals and polycrystalline Dirac semimetals with ultrahigh carrier mobility. We report the observation of positive and linear magnetoresistance in a single-crystalline semiconductor Bi2O2Se grown by chemical vapor deposition. Both Se-poor and Se-rich Bi2O2Se single-crystalline nanoplates display a linear magnetoresistance at high fields. The Se-poor Bi2O2Se exhibits a typical 2D conduction feature with a small effective mass of 0.032m0. The average transport Hall mobility, which is lower than 5500 cm2 V-1 s-1, is significantly reduced, compared with the ultrahigh quantum mobility as high as 16260 cm2 V-1 s-1. More interestingly, the pronounced Shubnikov-de Hass oscillations can be clearly observed from the very large and nearly linear magnetoresistance (>500% at 14 T and 2 K) in Se-poor Bi2O2Se. A close analysis of the results reveals that the large and linear magnetoresistance observed can be ascribed to the spatial mobility fluctuation, which is strongly supported by Fermi energy inhomogeneity in the nanoplate samples detected using an electrostatic force microscopy images and multiple frequencies in a Shubnikov-de Hass oscillation. On the contrary, the Se-rich Bi2O2Se exhibits a transport mobility (<300 cm2 V-1 s-1) much smaller than that observed in Se-poor samples and shows a much smaller linear magnetoresistance ratio (less than 150% at 14 T and 2 K). More strikingly, no Shubnikov-de Hass oscillations can be observed. Therefore, the linear magnetoresistance in Se-rich Bi2O2Se is governed by the average mobility rather than the mobility fluctuation.

Justification of dipole mechanism over chemical charge transfer mechanism for dipole-based SERS platform with excellent chemical sensing performance

Molecular-dipole-induced surface-enhanced Raman scattering (SERS) has attracted a lot of attention due to its high sensitivity without the need for noble metals. However, its main mechanisms are still unclear. In this work, functional-group-rich graphene oxide (GO)/ZnO (GZ) films are fabricated as SERS platforms. A maximum enhancement factor of 70,000 is achieved by manipulating the chain reactions between the functional groups and ZnO under various annealing conditions. The electronic structures of the GZ composite films, characterized using ultraviolet photoelectron spectroscopy, indicate that the high sensitivity arises from molecular dipole interactions rather than photoinduced charge transfer. Electrostatic force microscopy images show that the amount and spatial uniformity of molecular dipoles on GZ are both significantly improved when appropriate heat treatment is employed, leading to a tremendous 43-fold sensitivity improvement compared to that of pristine GO. Stability tests demonstrate great retention of the dipole-induced enhancement, up to 87%, after more than six months of aging in the dark. The excellent sensitivity, uniformity, and stability make the GZ composite film an excellent SERS platform for practical applications in molecular identification and chemical sensing.

InVivo Bioengineering of Fluorescent Conductive Protein-Dye Microfibers.

SummaryEngineering protein-based biomaterials is extremely challenging in bioelectronics, medicine, and materials science, as mechanical, electrical, and optical properties need to be merged to biocompatibility and resistance to biodegradation. An effective strategy is the engineering of physiological processes in situ, by addition of new properties to endogenous components. Here we show that a green fluorescent semiconducting thiophene dye, DTTO, promotes, in vivo, the biogenesis of fluorescent conductive protein microfibers via metabolic pathways. By challenging the simple freshwater polyp Hydra vulgaris with DTTO, we demonstrate the stable incorporation of the dye into supramolecular protein-dye co-assembled microfibers without signs of toxicity. An integrated multilevel analysis including morphological, optical, spectroscopical, and electrical characterization shows electrical conductivity of biofibers, opening the door to new opportunities for augmenting electronic functionalities within living tissue, which may be exploited for the regulation of cell and animal physiology, or in pathological contexts to enhance bioelectrical signaling.

Effects of Thermal Annealing and Selective Chemical Etching on Structural and Optical Properties of GaAsBi Epilayer with Droplet Systems.

A GaAs1-xBix layer was grown by molecular beam epitaxy (MBE) with a low Bi content (2.3%) on GaAs maintaining the substrate at a non-rotating state and was then annealed at 750 °C, 800 °C and 850 °C. Each sample that was covered with droplets was investigated by using the Atomic Force Microscopy (AFM), Electrostatic Force Microscopy (EFM) and Photoluminescence (PL) techniques. The surface properties of the GaAs1-xBix layer were investigated by AFM and observed to have a droplet system, which was composed of a donut and a tail. The optical quality of the samples was enhanced after thermal annealing up to 800 °C, and the maximum PL intensity was obtained at 750 °C. AFM images revealed that the shape of the droplet and tail changed with increasing annealing temperature. EFM images revealed a phase separation on the surface droplet system. To explore the nature of the droplets, previously claimed to be made of Ga and/or Bi, and their effect on PL spectrum, a chemical etch procedure was carried out by using diluted solutions of H₂SO₄ and/or HCl. We showed that droplets may be efficiently removed from the surface, and PL intensity could be improved by using a proper sequence of chemical etching procedures. Furthermore, the presence of two different phases for the droplet-system observed by EFM was also confirmed by the selective etching procedure.

Epitaxial Synthesis of Monolayer PtSe2 Single Crystal on MoSe2 with Strong Interlayer Coupling.

PtSe2, a layered two-dimensional transition-metal dichalcogenide (TMD), has drawn intensive attention owing to its layer-dependent band structure, high air stability, and spin-layer locking effect which can be used in various applications for next-generation optoelectronic and electronic devices or catalysis applications. However, synthesis of PtSe2 is highly challenging due to the low chemical reactivity of Pt sources. Here, we report the chemical vapor deposition of monolayer PtSe2 single crystals on MoSe2. The periodic Moiré patterns from the vertically stacked heterostructure (PtSe2/MoSe2) are clearly identified via annular dark-field scanning transmission electron microscopy. First-principles calculations show a type II band alignment and reveal interface states originating from the strong-weak interlayer coupling (SWIC) between PtSe2 and MoSe2 monolayers, which is supported by the electrostatic force microscopy imaging. Ultrafast hole transfer between PtSe2 and MoSe2 monolayers is observed in the PtSe2/MoSe2 heterostructure, matching well with the theoretical results. Our study will shed light on the synthesis of Pt-based TMD heterostructures and boost the realization of SWIC-based optoelectronic devices.

Electrostatic force microscopy analysis of Bi0.7Dy0.3FeO3 thin films prepared by pulsed laser deposition integrated with ZnO films for microelectromechanical systems and memory applications

In this paper, we report the charge trapping phenomena in zinc oxide (n-ZnO) and Bi0.7Dy0.3FeO3 (BDFO)/ZnO thin films deposited on p-type &amp;lt;100&amp;gt; conducting Si substrate. The significant change in contrast above the protrusions of ZnO verifies the possibility of heavy accumulation of injected holes in there. The ZnO and BDFO/ZnO films were characterized by the electrostatic force microscopy (EFM) to understand the phase dependence phenomenon on the bias supporting electron tunnelling. The EFM has an important role in the analysis of electrical transport mechanism characterization and electric charge distribution of local surface in nanoscale devices. It was observed that in BDFO/ZnO, the contrast of EFM images remains constant with the bias switching and that primarily indicates availability of trap sites to host electrons. The change in contrast over the protrusions of ZnO suggests that mobility of the electrical charge carriers may be through the grain boundary. The formation of these hole-trapped sites may be assumed by bond breaking phenomenon.

Surface potential extraction from electrostatic and Kelvin-probe force microscopy images

A comprehensive comparison study of electrostatic force microscopy (EFM) and Kelvin probe force microscopy (KPFM) is conducted in this manuscript. First, it is theoretically demonstrated that for metallic or semiconductor samples, both the EFM and KPFM signals are a convolution of the sample surface potential with their respective transfer functions. Then, an equivalent point-mass model describing cantilever deflection under distributed loads is developed to reevaluate the cantilever influence on detection signals, and it is shown that the cantilever has no influence on the EFM signal, while it will affect the KPFM signal intensity but not change the resolution. Finally, EFM and KPFM experiments are carried out, and the surface potential is extracted from the EFM and KPFM images by deconvolution processing, respectively. The extracted potential intensity is well consistent with each other and the detection resolution also complies with the theoretical analysis. Our work is helpful to perform a quantitative analysis of EFM and KPFM signals, and the developed point-mass model can also be used for other cantilever beam deflection problems.

Reconstruction of electrostatic force microscopy image for nanoscale potential distributions with potential steps

Given the stray capacitance between the probe and sample surface, electrostatic force microscopy (EFM) suffers from the probe averaging effect of electrostatic signals for measuring nanoscale potential distributions. A method for reconstructing an EFM image is presented by using the step response function (SRF) as the system transfer function. The SRF is constructed numerically by conducting finite element method simulations and reconsidering both the probe shape and tip-sample distance. The deconvolution of the probe averaging effect for the electrostatic image is demonstrated using an elaborated sample of graphene ribbons that are used as nanoscale surface potential steps. The lateral resolution of the electrostatic image is improved via deconvolution. The results present a powerful tool for explaining the EFM image to reduce the probe averaging effect effectively, especially for the sample with nanoscale potential steps.

Electrostatic force microscopy evaluation of the conductivity of individual multiwalled carbon nanotubes

The electric conductivity of individual multiwalled carbon nanotubes (CNTs) doped with nitrogen has been studied in as-synthesized, heat-treated, and argon-ion-irradiated states by the method of electrostatic force microscopy (EFM). Modelling of transverse cross-section profiles of EFM images were used to determine the potential difference across the probe tip–CNT gap (Utip-CNT), which is a parameter related to the conductivity of CNTs. A strong correlation between the specific volume conductivity of a CNT layer and average Utip-CNT value has been found for all types of samples. It is established that a change in the conductivity of N-doped CNTs upon thermal annealing and argon-ion irradiation is caused by modification of the composition and/or concentration of defects in CNT walls.

Nanoscale dielectric microscopy of non-planar samples by lift-mode electrostatic force microscopy

Lift-mode electrostatic force microscopy (EFM) is one of the most convenient imaging modes to study the local dielectric properties of non-planar samples. Here we present the quantitative analysis of this imaging mode. We introduce a method to quantify and subtract the topographic crosstalk from the lift-mode EFM images, and a 3D numerical approach that allows for extracting the local dielectric constant with nanoscale spatial resolution free from topographic artifacts. We demonstrate this procedure by measuring the dielectric properties of micropatterned SiO2 pillars and of single bacteria cells, thus illustrating the wide applicability of our approach from materials science to biology.

Fine structures of organic photovoltaic thin films probed by frequency-shift electrostatic force microscopy

The localized charge and electrostatic properties of organic photovoltaic thin films are predominating factors for controlling energy conversion efficiency. The surface potential and electrostatic structures of organic photovoltaic thin films were investigated by frequency shift mode Kelvin force microscopy (KFM) and electrostatic force microscopy (EFM). The KFM images of a poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene]/phenyl-C61-butyric-acid-methyl ester (PCBM) blend thin film reveals that the PCBM domains precipitate as the topmost layer on the thin films. We find fine structures that were not observed in the topography and KFM images. The bias dependence of the EFM images suggests that the EFM contrast reflects the field-induced polarization, indicating the presence of charge trapping sites.

Polydiphenylamine/zeolite Y composite as a sensor material for chemical vapors

A composite of nanoscale polydiphenylamine and dealuminated zeolite Y was fabricated to be used as a sensor material for discriminating various kinds of chemical solvent vapors: non-polar solvents, low-polar solvents, and high-polar solvents. Discriminant analysis was used to confirm that the composite could distinguish different chemical solvents. The effects of surfactant type and concentration, dealuminated zeolite Y content on the electrical conductivity, relative response, and selectivity of the composite were investigated. The response and recovery time, and cyclic response of the composite were also investigated. The composite with sodium dodecyl sulfate as a surfactant with 15% (v/v) dealuminated zeolite Y showed a relatively high relative response toward dichloromethane (DCM) vapor. Discriminant analysis confirmed that the response patterns of the composite toward each chemical solvent could be distinguished among non-polar and low-polar solvents, but not high-polar solvents. Moreover, the interaction of the composite and DCM vapor was reversible as confirmed by cyclic response, Fourier transform infrared spectroscopy spectra, and electrostatic force microscopy images. When compared with conventional microscale polydiphenylamine composites, the nanoscale polydiphenylamine composited with dealuminated zeolite Y (DYH80) is a promising sensor material for detecting non-polar and low-polar solvents.

Carrier trapping study on a Ge nanocrystal by two-pass lift mode electrostatic force microscopy

Trapped charges inside an isolated germanium nanocrystal (Ge NC) have been studied by two-pass lift mode electrostatic force microscopy (EFM) measurements at room temperature. From visualized EFM images, electrons and holes were proven to be successfully injected and trapped in the Ge NC and distributed homogenously at the edge of its truncated spherical morphology. The Ge NC is found to have iso-potential surface and behave as a conductive material after being charged. It is also shown that the dominant charge decay mechanism during discharging of Ge NCs is related to the leakage of these trapped charges. A truncated capacitor model is used to approximate the real capacitance between the tip and Ge NC surface and to quantitatively study these trapped charges. These investigations demonstrate the potential for Ge nanocrystal memory applications.

Enhanced Electrostatic Force Microscopy Imaging Reveals Mechanism of TRF2 Mediated DNA Compaction

T-loop formation at telomeres is proposed to play an important role in telomere protection through the sequestration of the 3’ single-stranded overhang. Previous studies indicated that shelterin protein TRF2 modulates telomere structure by promoting DNA compaction and T-loop formation. To further understand the mechanism underlying TRF2-mediated DNA compaction, we applied Dual-Resonance-frequency-Enhanced Electrostatic force Microscopy (DREEM), a recently developed technique capable of high-resolution imaging of weak electrostatic potentials. DREEM images of nucleosomes clearly reveal DNA strands wrapped around histone proteins in nucleosomal arrays. In contrast, DREEM imaging shows DNA compacted inside TRF2 complexes through a 3-dimensional stacking of TRF2 dimers mediated by collective actions of multiple copies of TRF2 proteins. TRF2-mediated DNA compaction leads to electric potential gradients across the complexes. Surprisingly, while DNA wrapped around histones displays similar electrostatic potential signals compared to bare DNA, TRF2 DNA compaction leads to significant differences in the electrical potential signals inside multi-oligomeric TRF2 complexes compared to protein or DNA alone. These results clearly demonstrate the electrostatic changes in TRF2-DNA complexes upon oligomerizaton of proteins and DNA compaction, and underscore the importance of developing new electrostatic force microscopy imaging techniques for studying biological systems.

Quantitative non-contact voltage profiling of quasi one-dimensional nanoelectronic devices

Local electrical characterization tools, such as Electrostatic force microscopy (EFM), can provide local electrical information of nanoelectronic devices, albeit mostly qualitative. For example, EFM images are convolution of local surface potential, capacitance, and contact potential variations in the device. In this study, we demonstrate a calibration procedure to obtain quantitative local voltage distributions of quasi one-dimensional nanoelectronic devices based on carbon nanotubes and ZnO nanowires. By comparing the results with IV measurements of the same devices, we can obtain local electrical properties of devices such as contact resistance, intrinsic resistivity of the nanomaterial, and resistance of a defect.

Investigation of effects of ionizing radiation exposure on material properties of organic semiconducting oligomer – Pentacene

Organic semiconducting materials are known for their chemical affinity and sensitivity to environmental conditions. In this manuscript we investigate the changes in various material properties of a very well-known organic semiconducting oligomer – pentacene, upon exposure to ionizing radiation of γ-rays using Cobalt-60 radiation source. The changes in the optical properties were observed by studying UV–Visible Absorbance spectra of the pentacene thin-film. The effects on the valance electronic structure of the pentacene were studied by ultraviolet photoelectron spectroscopy of pristine pentacene, and after exposure of the same to different total-doses of ionizing radiation. Moreover, X-ray photoelectron spectra of the material were recorded to study the core-level changes in the pentacene when exposed to ionizing radiation. These photoelectron spectroscopy techniques suggested p-type doping into the material and presence of newly generated oxidized energy-states in the energy band gap of the material upon exposure to ionizing radiation. Electrostatic force microscopy imaging was done to observe the changes in the electrical properties of pentacene thin-film which suggested an increase in the conductivity upon exposure to ionizing radiation. Moreover, the effect of newly generated oxidized energy-states, in the band gap of the material, on the hopping transport in the pentacene thin-film was studied by measuring changes in the resistance as a function of temperature before and after different total-doses of ionizing radiation, and observing the changes in the slope. Thus, exposure to ionizing radiation is proposed here as a controlled method of doping the organic semiconducting materials and changing their conductivity. Moreover, the changes in the electrical properties of the thin-film upon exposure to ionizing radiation enable its use in sensors for determining ionizing radiation using organic electronic devices as sensors.

Identification of epitaxial graphene domains and adsorbed species in ambient conditions using quantified topography measurements

We discuss general limitations of topographical studies of epitaxial graphene in ambient conditions, in particular, when an accurate determination of the layers thickness is required. We demonstrate that the histogram method is the most accurate for measurements of small vertical distances (&amp;lt;0.5 nm) and generally should be applied to epitaxial graphene and similar types of samples in order to get the correct and reproducible values. Experimental determination of the step height between different domains of epitaxial graphene shows excellent agreement with the predicted values once the adsorption of a 2D monolayer is taken into account on top of the one layer graphene. In contrast to general limitations of AFM topography, electrostatic force microscopy imaging allows a straightforward identification of domains of epitaxial graphene of different thickness.

Analysis of Operation Mechanism of Field Effect Transistor Composed of Network of High-Quality Single Wall Carbon Nanotubes by Scanning Gate Microscopy

Field effect transistors (FETs) whose channel is composed of a network of high-quality single wall carbon nanotubes (SWNTs) have been studied to investigate the mechanism of the device operation via scanning gate microscopy (SGM) at room temperature. SWNTs synthesized by CoMoCAT® process was used for the formation of the network. Clear SGM responses were observed only at some points but not uniformly in a whole of the channel. The observed responses correspond to positions where two SWNTs are crossing. Back gate voltage dependence of the SGM images and an electrostatic force microscopy image were also studied. One of the possible mechanisms of the SGM response is considered as a modulation of Schottky barrier formed at junctions between metallic and semiconducting SWNTs. Such junctions suggestively play an important role in the FET operation.