Local Conductivity Properties Research Articles

Polarization-dependent local conductivity and activation energy in KTiOPO4

We study the local conductivity properties of KTiOPO4 using conductive atomic force microscopy in ultrahigh vacuum (UHV). We show that domains with opposite orientations have different conductivity values. We investigate the temperature dependence of the local conductivity and report a difference in the activation energy of 25% between different domains. Furthermore, we show that the local conductivity increases with the number of biased-scans. Finally, it is found that the domain wall conductivity previously observed at ambient conditions vanishes in UHV. Our results are discussed in terms of the screening effects and surface conditions.

Mapping Free-Carriers in Multijunction Silicon Nanowires Using Infrared Near-Field Optical Microscopy.

We report the use of infrared (IR) scattering-type scanning near-field optical microscopy (s-SNOM) as a nondestructive method to map free-carriers in axially modulation-doped silicon nanowires (SiNWs) with nanoscale spatial resolution. Using this technique, we can detect local changes in the electrically active doping concentration based on the infrared free-carrier response in SiNWs grown using the vapor-liquid-solid (VLS) method. We demonstrate that IR s-SNOM is sensitive to both p-type and n-type free-carriers for carrier densities above ∼1 × 1019 cm-3. We also resolve subtle changes in local conductivity properties, which can be correlated with growth conditions and surface effects. The use of s-SNOM is especially valuable in low mobility materials such as boron-doped p-type SiNWs, where optimization of growth has been difficult to achieve due to the lack of information on dopant distribution and junction properties. s-SNOM can be widely employed for the nondestructive characterization of nanostructured material synthesis and local electronic properties without the need for contacts or inert atmosphere.

Structure-Property Relations of Methylamine Vapor Treated Hybrid Perovskite CH3NH3PbI3 Films and Solar Cells.

The power conversion efficiency of halide perovskite solar cells is heavily dependent on the perovskite layer being sufficiently smooth and pinhole-free. It has been shown that these features can be obtained even when starting out from rough and discontinuous perovskite film by briefly exposing the film to methylamine (MA) vapor. The exact underlying physical mechanisms of this phenomenon are, however, still unclear. By investigating smooth, MA treated films based on very rough and discontinuous reference films of methylammonium triiode (MAPbI3) and considering their morphology, crystalline features, local conductive properties, and charge carrier lifetime, we unraveled the relation between their characteristic physical qualities and their performance in corresponding solar cells. We discovered that the extensive improvement in photovoltaic performance upon MA treatment is a consequence of the induced morphological enhancement of the perovskite layer together with improved electron injection into TiO2, which in fact compensates for an otherwise compromised bulk electronic quality simultaneously caused by the MA treatment.

Self-assembled Carbon Nanotube-DNA Hybrids at the Nanoscale: Morphological and Conductive Properties Probed by Atomic Force Microscopy

ABSTRACTOur research is focused on the engineering of novel, highly sensitive and miniaturized hybrid materials from carbon nanotubes (CNTs) and DNA molecules for applications in biosensors and medical devices. These hybrid sensors allow for a high degree of miniaturization, a key factor in the design of lightweight components while maintaining the advantages of in-situ and real-time analysis capabilities. In the first phase of the sensor design process, we investigated the structural and electrical properties of the supramolecular complexes made of amide-functionalized CNTs and double-stranded DNA. The solubilization properties of the hybrid nanotubes in aqueous solutions with different concentrations of DNA were studied, and an optimal ratio of nanotubes and biomolecules to achieve a good level of dispersion was found. Complexes formed in aqueous solution from CNTs and DNA are highly stable and maintain their properties up to one month from preparation. The morphology of the CNT-DNA composites was investigated at the nanoscale level using atomic force microscopy (AFM) and electron microscopy (SEM). Results from these experiments show the strong affinity between the surface of the amide-functionalized CNTs and the DNA strands. Further, the CNT-DNA films were investigated by atomic force microscopy in the PeakForce TUNA mode to assess the suitability of this technique in determining the local conductive properties of the hybrid films.

Nanoscale resistance switching in manganite thin films: Sharp voltage threshold and pulse-width dependence

We report the local-conductivity properties of a ${\text{La}}_{0.8}{\text{Ca}}_{0.2}{\text{MnO}}_{3}$ thin film, studied by conductive atomic force microscopy. Nonvolatile and bipolar reversible switching of nanometer-sized regions was observed. A threshold voltage, ${U}_{c}\ensuremath{\approx}3\text{ }\text{V}$, and a logarithmic pulse-width dependence compatible with domain-wall creep were revealed. The results are difficult to explain in terms of an ionic drift scenario but rather indicate a switching mechanism based on orbital and accompanying structural changes. A phenomenological model of an electric field-induced structural transition is proposed.