Conductive Scanning Force Microscopy Research Articles

Depth resolution in piezoresponse force microscopy

Piezoresponse force microscopy (PFM) is one of the most widespread methods for investigating and visualizing ferroelectric domain structures down to the nanometer length scale. PFM makes use of the direct coupling of the piezoelectric response to the crystal lattice, and hence, it is most often applied to spatially map the three-dimensional (3D) near-surface domain distribution of any polar or ferroic sample. Nonetheless, since most samples investigated by PFM are at least semiconducting or fully insulating, the electric ac field emerging from the conductive scanning force microscopy (SFM) tip penetrates the sample and, hence, may also couple to polar features that are deeply buried into the bulk of the sample under investigation. Thus, in the work presented here, we experimentally and theoretically explore the contrast and depth resolution capabilities of PFM, by analyzing the dependence of several key parameters. These key parameters include the depth of the buried feature, i.e., here a domain wall (DW), as well as PFM-relevant technical parameters such as the tip radius, the PFM drive voltage and frequency, and the signal-to-noise ratio. The theoretical predictions are experimentally verified using x-cut periodically poled lithium niobate single crystals that are specially prepared into wedge-shaped samples, in order to allow the buried feature, here the DW, to be “positioned” at any depth into the bulk. This inspection essentially contributes to the fundamental understanding in PFM contrast analysis and to the reconstruction of 3D domain structures down to a 1 μm-penetration depth into the sample.

Enhanced Vertical Charge Transport of Homo- and Blended Semiconducting Polymers by Nanoconfinement.

The morphology of conjugated polymers has critical influences on electronic and optical properties of optoelectronic devices. Even though lots of techniques and methods are suggested to control the morphology of polymers, very few studies have been performed inducing high charge transport along out-of-plane direction. In this study, the self-assembly of homo- and blended conjugated polymers which are confined in nanostructures is utilized. The resulting structures lead to high charge mobility along vertical direction for both homo- and blended conjugated polymers. Both semicrystalline and amorphous polymers show highly increased population of face-on crystallite despite intrinsic crystallinity of polymers. They result in more than two orders of magnitude enhanced charge mobility along vertical direction revealed by nanoscale conductive scanning force microscopy and macroscale IV characteristic measurements. Moreover, blends of semicrystalline and amorphous polymers, which are known to show inferior optical and electrical properties due to their structural incompatibility, are formed into harmonious states by this approach. Assembly of blends of semicrystalline and amorphous polymers under nanoconfinement shows charge mobility in out-of-plane direction of 0.73 cm2 V-1 s-1 with wide range of absorption wavelength from 300 to 750 nm demonstrating the synergistic effects of two different polymers.

Removal of Surface Oxygen Vacancies Increases Conductance Through TiO2 Thin Films for Perovskite Solar Cells.

We report that UV–ozone treatment of TiO2 anatase thin films is an efficient method to increase the conductance through the film by more than 2 orders of magnitude. The increase in conductance is quantified via conductive scanning force microscopy on freshly annealed and UV–ozone-treated TiO2 anatase thin films on fluorine-doped tin oxide substrates. The increased conductance of TiO2 anatase thin films results in a 2% increase of the average power conversion efficiency (PCE) of methylammonium lead iodide-based perovskite solar cells. PCE values up to 19.5% for mesoporous solar cells are realized. The additional UV–ozone treatment results in a reduced number of oxygen vacancies at the surface, inferred from X-ray photoelectron spectroscopy. These oxygen vacancies at the surface act as charge carrier traps and hinder charge extraction from the adjacent material. Terahertz measurements indicate only minor changes of the bulk conductance, which underlines the importance of UV–ozone treatment to control surface-based defects.

Controlled Mutual Diffusion between Fullerene and Conjugated Polymer Nanopillars in Ordered Heterojunction Solar Cells

A new approach is presented to control the nanomorphology of organic solar cells in a predictable, controllable, and easily‐scalable way. The nanoimprint lithography (NIL) is combined with a subsequent molecular diffusion step controlled by thermal annealing. The new approach is realized by using nanointerdigitated donor–acceptor structure, consisting of poly(3‐hexylthiophene‐2,5‐diyl) nanopillar arrays surrounded by phenyl‐C61‐butyric acid methyl ester. Subsequent thermal annealing leads to vertically aligned ordered quasi‐bulk heterojunctions with hierarchical nanostructure. The changes are studied in nanostructural and electrical properties of the pillar samples using scanning probe microscopy. In addition, grazing‐incidence small and wide angle X‐ray scattering yield detailed quantitative information on the molecular‐ to domain‐scale nanostructures. The changes in crystal size, chain orientation, and domain composition as a function of thermal anneal temperature and time are obtained. In addition, the conductive scanning force microscopy in quantitative imaging mode, applied to the pillar‐based samples for the first time, allows us to establish a clear relationship between nanomorphology, nanoelectrical property, and macroscale device performance. It is believed that the NIL combined with controlled molecular diffusion is a powerful method, which could be easily extended to other materials and processes to realize a whole variety of other hierarchical nanomorphologies.

Spin Lifetimes Exceeding 12 ns in Graphene Nonlocal Spin Valve Devices.

We show spin lifetimes of 12.6 ns and spin diffusion lengths as long as 30.5 μm in single layer graphene nonlocal spin transport devices at room temperature. This is accomplished by the fabrication of Co/MgO-electrodes on a Si/SiO2 substrate and the subsequent dry transfer of a graphene-hBN-stack on top of this electrode structure where a large hBN flake is needed in order to diminish the ingress of solvents along the hBN-to-substrate interface. Interestingly, long spin lifetimes are observed despite the fact that both conductive scanning force microscopy and contact resistance measurements reveal the existence of conducting pinholes throughout the MgO spin injection/detection barriers. Compared to previous devices, we observe an enhancement of the spin lifetime in single layer graphene by a factor of 6. We demonstrate that the spin lifetime does not depend on the contact resistance area products when comparing all bottom-up devices indicating that spin absorption at the contacts is not the predominant source for spin dephasing.

Electrical tip-sample contact in scanning conductive torsion mode

We investigated the nature of the mechanical and the electrical tip-sample contact in scanning conductive torsion mode microscopy (SCTMM). Experiments on the soft conducting polymer blend of poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) demonstrated that the tip-sample force and thus the danger of tip-induced sample damage can be minimized. Using current-voltage spectroscopy, we found a space-charge limited conduction behavior with no indication of a tunneling barrier. Spectroscopy and imaging experiments showed that SCTMM allows for a gentler tip-sample contact compared to conventional conductive scanning force microscopy. A gentle and well-defined contact is a prerequisite for reproducible scanning probe based conductivity measurements, in particular on soft organic materials.

Photoinduced Degradation Studies of Organic Solar Cell Materials Using Kelvin Probe Force and Conductive Scanning Force Microscopy

We investigated the photoinduced changes in the surface potential and conductivity for locally degraded active layers of organic solar cell materials using electrical modes of scanning force microscopy. Samples were degraded under different partial pressures of oxygen and humidity in the presence of light. Degraded and nondegraded areas were investigated by Kelvin Probe Force Microscopy (KPFM) and conductive scanning force microscopy (cSFM). The analysis allowed us to quantify the extent of degradation and compensate the contribution of the probe tip. Two typical blends used for organic solar cell, i.e., P3HT:PCBM and PCPDTBT:PCBM, were investigated. We observed that P3HT:PCBM photodegraded significantly more than PCPDTBT:PCBM for an environment containing oxygen. For short photodegradation times (1 h), we verified that changes in the surface potential and conductivity of P3HT:PCBM films were fully reversible after annealing. For individual layers of P3HT and PCBM, we found that only P3HT degrades. Howeve...

Redox active polymers with phenothiazine moieties for nanoscale patterning via conductive scanning force microscopy

Redox active polymers with phenothiazine moieties have been synthesized by Atomic Transfer Radical Polymerization (ATRP). These novel polymers reveal bistable behaviour upon application of a bias potential above the oxidation threshold value. Using conductive Scanning Force Microscopy, two distinguishable conductivity levels were induced on a nanoscale level. These levels were related to a high conducting “On” and a low conducting “Off” state. The “On” state is generated by the oxidation of the phenothiazine side chains to form stable phenothiazine radical cations. The formation and stability of the radical sites was examined by cyclic voltammetry, electron spin resonance and optical spectroscopy. Polymers with phenothiazine moieties show the ability to retain their redox state for several hours and can therefore be used for nonvolatile organic memory devices. Furthermore, thin films made from the phenothiazine containing polymers show high mechanical nanowear stability.

Surface polymerization of (3,4-ethylenedioxythiophene) probed by in situ scanning tunneling microscopy on Au(111) in ionic liquids

The electropolymerization of 3,4-ethylenedioxythiophene (EDOT) to poly(3,4-ethylenedioxythiophene) (PEDOT) was investigated in the air and water-stable ionic liquids 1-hexyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate [HMIm]FAP and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) amide [EMIm]TFSA. In situ scanning tunnelling microscopy (STM) results show that the electropolymerization of EDOT in the ionic liquid can be probed on the nanoscale. In contrast to present understanding, it was observed that the EDOT can be oxidised in ionic liquids well below its oxidation potential and the under potential growth of polymer was visualized by in situ STM. These results serve as the first study to confirm the under potential growth of conducting polymers in ionic liquids. Furthermore, ex situ microscopy measurements were performed. Quite a high current of 670 nA was observed on the nanoscale by conductive scanning force microscopy (CSFM).

Investigating morphology and electronic properties of self-assembled hybrid systems for solar cells

We investigate self-assembled network morphologies of hybrid materials and their application in solar cells. We show the use of a semiconducting functional block copolymer as a structure directing agent. The application of a functional block copolymer circumvents an additional filling step of the templated inorganic part with a hole conductor. Within a one-pot synthesis various morphologies adequate for active materials in photovoltaic devices are prepared. The solar cell performance was found to depend on the morphological design of the hybrid material since the formation of percolating networks is of general importance. Using conductive scanning force microscopy on a cross-section of a functional solar cell device, we proved that the titania in our system forms a percolating network. Measurements showed that percolation is not a limiting factor for the device performance. In combination with a functionalized titania-precursor, we were able to double the power conversion efficiency of our hybrid bulk heterojunction cells in comparison to our previous results.

Multi-scale electrical response of silicon nitride/multi-walled carbon nanotubes composites

Dense silicon nitride (Si 3N 4) composites with various amounts (0–8.6 vol%) of multi-walled carbon nanotubes (MWCNTs) are electrically characterised by combining macroscopic dc–ac and nanoscale conductive scanning force microscopy (C-SFM) measurements. In this way, a coherent picture of the dominant charge transport mechanisms in Si 3N 4/MWCNTs composites is presented. A raise of more than 10 orders of magnitude in the electrical dc conductivity compared to the blank specimen is measured for MWCNTs contents above 0.9 vol%. Semiconductor and metallic-like behaviours are observed depending on both the temperature and the MWCNTs content. Macroscopic measurements are further supported at the nanoscale by means of C-SFM. The metallic-type conduction is associated to charge transporting along the nanotube shells, whereas the semiconductor behaviour is linked to hopping conduction across nanotube–nanotube contacts and across intrinsic defect clusters within the nanotubes.

Electrical Scanning Probe Microscopy of an Integrated Blocking Layer

Scanning probe microscopy was performed on an integrated blocking layer system developed for hybrid organic solar cells. A nanocomposite consisting of titania and an amphiphilic triblock copolymer ((PEO)MA-PDMS-MA(PEO)) was prepared by sol-gel chemistry. After plasma treatment and annealing of a spin casted film of 30-100 nm thickness a granular structure with a typical titania grain diameter of 20 nm was found. Conductive scanning force microscopy revealed that on top of almost every grain on the surface there is an increased conductivity compared to the average value. The correlation of grains and conductivity indicated that titania particles formed interconnecting paths through the film. For the resistivity of these pathways we found that effects of tip-sample and sample-electrode resistivity dominate. Additionally, conductive scanning force microscopy revealed non-conducting structures attributed to the thermal treatment. Kelvin probe microscopy of pristine samples on one side and plasma treated plus annealed samples on the other side showed that there is a shift in work function (0.8 +/- 0.2 eV) as expected for the transition of amorphous to anatase titania.

Mapping of Local Conductivity Variations on Fragile Nanopillar Arrays by Scanning Conductive Torsion Mode Microscopy

A gentle method that combines torsion mode topography imaging with conductive scanning force microscopy is presented. By applying an electrical bias voltage between tip and sample surface, changes in the local sample conductivity can be mapped. The topography and local conductivity variations on fragile free-standing nanopillar arrays were investigated. These samples were fabricated by an anodized aluminum oxide template process using a thermally cross-linked triphenylamine-derivate semiconductor. The nanoscale characterization method is shown to be nondestructive. Individual nanopillars were clearly resolved in topography and current images that were recorded simultaneously. Local current-voltage characteristics suggest a space-charge limited conduction in the semiconducting nanopillars.

Load-Free Determination of Film Structure Dependent Tunneling Decay Factors in Molecular Junctions

Two supramolecular orders of alkanethiol molecules self-assembled on Au(111) differing in molecular tilt angles, molecular density, and in-plane order are investigated, for a variety of chain lengths, by means of conductive scanning force microscopy. A load-free approach is presented as an alternative to the common pressure-induced methods used for investigating electron transport mechanisms in these films. The methodology, which compares molecular junctions with different thickness but exactly the same number of through bond channels, allows analyzing the molecular tilt influence (e.g., through space probability) on the current response while avoiding pressure-induced defects or film restructuring under pressure. In addition, it constitutes a direct strategy that has permitted us to evaluate tunneling decay factors which are specific of the supramolecular order within the film.

Absence of self-heated bistable resistivity inLa0.7Sr0.3MnO3films up to high current densities

The $E(J)$ characteristics of well-characterized fully strained epitaxial ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_{3}$ films have been investigated up to high current densities $(J\ensuremath{\sim}{10}^{4}--{10}^{5}\text{ }\text{A}/{\text{cm}}^{2})$. Different electric contact resistances and different current path configurations have been used to sort out the role of Joule self-heating from contact heating in the transport properties. It is demonstrated through macroscopic transport and conductive scanning force microscopy measurements that contact heating may lead to nonlinear and irreversible $E(J)$ characteristics when high contact resistances are used. Low dissipative contact power measurements are crucial to define the upper $J$ limits to keep linear and reversible $E(J)$ characteristics. We demonstrate that Joule self-heating in ${\text{La}}_{0.7}{\text{Sr}}_{0.3}{\text{MnO}}_{3}$ thin films only induces moderate warming at high temperatures while achieving bistable resistivity at low temperatures would require the use of very high current densities $(J\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{5}\text{ }\text{A}/{\text{cm}}^{2})$.