Electric Force Microscopy Measurements Research Articles

Unraveling the Role of Sn Segregation in the Electronic Transport of Polycrystalline Hematite: Raising the Electronic Conductivity by Lowering the Grain‐Boundary Blocking Effect

AbstractThis paper describes the role of SnO2 in the electronic transport of polycrystalline hematite (α‐Fe2O3). The proper sintering process allows for freezing of a state of electronic defects, in which the electrical properties of hematite are controlled by the grain boundary and Sn segregation. Impedance spectroscopy and dc conductivity measurements show that current flows through preferential pathways associated with Sn segregation that occurs at the grain boundary, leading to a decrease in grain‐boundary resistance. Atomic force microscopy and electric force microscopy measurements confirm the results of the impedance analysis. The identification of preferential grain boundaries for electrical conductivity may have a direct influence on the light‐induced water‐splitting performance of the hematite photoanode.

Insight into the interphase in polymer nanocomposites

Enhanced electrical properties of insulating materials containing nanoscale filler particles (nanocomposites) are attributed to an interfacial area that is formed between the polymer matrix of the base material and the filler particles. So far the interfacial area could not be made visible. The two materials investigated in this study are based on the same silicone matrix material and either contain filler particles of the type F1 or filler particles of the type F2, both of them having nearly the same primary particle diameter. Differences in the electrical properties of the two materials are interpreted by an interfacial area that is larger around filler particles of the type F1 than around filler particles of the type F2. To gain an insight into the interfacial area the two materials were investigated by Atomic Force Microscopy (AFM) and Electric Force Microscopy (EFM). In the EFM images the both types of filler particles appear larger in the composite material than they do when they are not embedded in the composite material. This difference is explained by an interfacial area that surrounds both types of filler particles if they are embedded in the base material. As the particles of the type F1 appear significantly larger than the particles of the type F2 in the EFM images it can be concluded that the interfacial area around the filler particles of the type F1 is larger than the interfacial area around the filler particles of the type F2. By the combination of electrical measurements, their interpretation with models and AFM and EFM measurements the existence of the interfacial area in the investigated nanocomposites could be proven. The measurement results also give a hint on the nature of the interfacial area which could either be charge carriers forming an electrical double layer around the nanoparticles or a change in morphology of the polymer surrounding the particles by polymer-filler interaction resulting in a change of permittivity of the surrounding polymer region. Most notably the interfacial area could be made visible by EFM measurements.

Charge carrier dynamics and interactions in electric force microscopy

In electric force microscopy, a charged atomic force microscope tip in vacuum senses a fluctuating electrical force generated by the sample. Such measurements can in principle probe electrical noise generated by moving charge carriers in an organic semiconductor. We present a theory of cantilever frequency fluctuations in electric force microscopy, driven by coupled charge carrier dynamics and dielectric fluctuations. The connection between observable frequency fluctuations in electric force microscopy and the Casimir-Lifshitz force is described. This classical electrodynamic calculation is based on Maxwell's equations coupled to diffusive carrier transport. The effects of carrier transport and inter-carrier interactions on the spectrum of cantilever frequency noise are elucidated. We find that a simplified model of freely diffusing carriers can overestimate cantilever frequency noise by several orders of magnitude because of the neglect of interactions. Electric force microscopy measurements on an organic field effect transistor are reported and qualitatively interpreted in terms of the suppression of electrical noise from charge carriers by Coulomb interactions.

The use of a Ga+ focused ion beam to modify graphene for device applications

In this work, we clarify the features of the lateral damage of line defects in single layer graphene. The line defects were produced through well-controlled etching of graphene using a Ga+ focused ion beam. The lateral damage length was obtained from both the integrated intensity of the disorder induced Raman D band and the minimum ion fluence. Also, the line defects were characterized by polarized Raman spectroscopy. It was found that graphene is resilient under the etching conditions since the intensity of the defect induced Raman D peak exhibits a dependence on the direction of the lines relative to the crystalline lattice and also on the direction of the laser polarization relative to the lines. In addition, electrical measurements of the modified graphene were performed. Different ion fluences were used in order to obtain a completely insulating defect line in graphene, which was determined experimentally by means of charge injection and electric force microscopy measurements. These studies demonstrate that a Ga+ ion column combined with Raman spectroscopy is a powerful technique to produce and understand well-defined periodic arrays of defects in graphene, opening possibilities for better control of nanocarbon devices.

Ferromagnetic resonance and magnetic force microscopy evidence for above room temperature ferromagnetism in Mn doped Si made by a solid state sintering process

Silicon doped with manganese made by a solid state sintering process starting with nanoparticles of Si and MnO is shown to be ferromagnetic above room temperature. The evidence for ferromagnetism is obtained from ferromagnetic resonance and magnetic force microscopy measurements. The magnetic force microscopy measurements show that the ferromagnetic resonance occurs in nanosized regions in the sample as has been suggested by Dietl. Raman and electric force microscopy measurements also suggest that then material is doped inhomogenously.

Molecular organization and doping in poly(2-methoxyaniline)/Ni(dmit)2 films obtained with the Langmuir–Blodgett technique

The control of the properties of materials at the molecular level is pursued for many applications, especially those associated with nanostructures. In this paper, we show that the coordination compound [Ni(dmit)2], where (dmit) is the 1,3-dithiole-2-thione-4,5-dithiolate ligand, can induce doping of poly(2-methoxyaniline) (POMA) in molecularly ordered Langmuir and Langmuir–Blodgett (LB) films. Doping was associated with interactions between the components and the compression of the Langmuir film at the air–water interface, according to polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS) data. Taking these results together with in situ UV-Vis absorption measurements, we could identify the molecular groups involved in the interaction, including the way they were reoriented upon film compression. The Langmuir films were sufficiently stable to be transferred as Y-type LB films, while the hybrid POMA/[Ni(dmit)2] films remain doped in the solid state. As expected, the molecular charges affected the film morphology, as observed from combined atomic and electric force microscopy measurements. In summary, with adequate spectroscopy and microscopy tools we characterized molecular-level interactions, which may allow one to design molecular electronic devices with controlled electrical properties.

Room‐Temperature Compression‐Induced Diamondization of Few‐Layer Graphene

Experimental and theoretical evidence for compression-induced diamondization of few-layer graphene is reported. Ab initio calculations predict that hydroxyl groups stabilize the sp3 hybridization of the two topmost carbon layers, creating a single layer of hydroxylated diamond, or diamondol, when graphene layers are compressed in the presence of water. Electric force microscopy measurements reveal a compression-induced charging inhibition of bilayer and multilayer graphene.

Lateral piezoelectric response across ferroelectric domain walls in thin films

In purely c-axis oriented PbZr0.2Ti0.8O3 ferroelectric thin films, a lateral piezoresponse force microscopy signal is observed at the position of 180° domain walls, where the out-of-plane oriented polarization is reversed. Using electric force microscopy measurements we exclude electrostatic effects as the origin of this signal. Moreover, our mechanical simulations of the tip/cantilever system show that the small tilt of the surface at the domain wall below the tip does not satisfactorily explain the observed signal either. We thus attribute this lateral piezoresponse at domain walls to their sideways motion (shear) under the applied electric field. From simple elastic considerations and the conservation of volume of the unit cell, we would expect a similar lateral signal more generally in other ferroelectric materials, and for all types of domain walls in which the out-of-plane component of the polarization is reversed through the domain wall. We show that in BiFeO3 thin films, with 180°, 109°, and 71° domain walls, this is indeed the case.

Time-Resolved Electric Force Microscopy of Charge Trapping in Polycrystalline Pentacene

Here we introduce time-resolved electric force microscopy measurements to directly and locally probe the kinetics of charge trap formation in a polycrystalline pentacene thin-film transistor. We find that the trapping rate depends strongly on the initial concentration of free holes and that trapped charge is highly localized. The observed dependence of trapping rate on the hole chemical potential suggests that the trapping process should not be viewed as a filling of midgap energy levels, but instead as a process in which the very creation of trapped states requires the presence of free holes.

Investigation of surface properties of Si-doped GaN films by electric force microscopy and photoluminescence

Results of the electric force microscopy (EFM) and photoluminescence (PL) measurements in a set of Si-doped GaN films are presented. The EFM measurements indicate that substantial surface states exist in GaN thin films and the density of surface states is reduced with the Si doping concentration. It is found that this result strongly correlates with the optical properties of the epifilms. We show that the ratio of near-band edge luminescence of the front-side and back-side PL spectra increases with the doping concentration. This behavior manifests the effects of the surface states on the luminescent properties of GaN. Our investigation therefore not only shows the important role of surface states on the optical properties of GaN, but it also reveals that the combination of EFM and optical techniques is a very powerful way to understand the physical properties of a thin film.