Nanoscale Electrical Measurements Research Articles

Progress in Traceable Nanoscale Capacitance Measurements Using Scanning Microwave Microscopy.

Reference samples are commonly used for the calibration and quantification of nanoscale electrical measurements of capacitances and dielectric constants in scanning microwave microscopy (SMM) and similar techniques. However, the traceability of these calibration samples is not established. In this work, we present a detailed investigation of most possible error sources that affect the uncertainty of capacitance measurements on the reference calibration samples. We establish a comprehensive uncertainty budget leading to a combined uncertainty of 3% in relative value (uncertainty given at one standard deviation) for capacitances ranging from 0.2 fF to 10 fF. This uncertainty level can be achieved even with the use of unshielded probes. We show that the weights of uncertainty sources vary with the values and dimensions of measured capacitances. Our work offers improvements on the classical calibration methods known in SMM and suggests possible new designs of reference standards for capacitance and dielectric traceable measurements. Experimental measurements are supported by numerical calculations of capacitances to reveal further paths for even higher improvements.

Surface defects state analysis of laser induced graphene from 4H-SiC

We demonstrate the surface defect type, density and related electrical modification of graphene sheet generated by 193-nm ArF excimer laser irradiation on n-type 4H-SiC. The charge density difference and partial density of states (PDOS) carried out by density functional theory (DFT) calculations predict the graphene with lattice imperfection can induce virtual bound states at the vacancy sites and Fermi level shift compared to perfect graphene. The surface defects density of laser generated graphene is characterized by nanoscale electrical measurements using the conductive atomic force microscope (c-AFM). The corresponding current–voltage (I–V) curves show that the electronic charge transfers, thereby electrical conductivities of laser generated graphene, are influenced by the lattice defects. The defect densities are tunable by change laser fluence and pulse number. This research provides an understanding of the interfacial transport properties between laser generated graphene and 4H-SiC through a detailed analysis of surface defects.

Phase Separation in Organic Semiconducting/Ferroelectric Blend Films with Air-Stable Resistive Properties

Organic semiconducting/ferroelectric blend films integrate both resistive and rectifying properties and thus well solve the cross-talk problem usually suffered in resistive memories. Till now several kinds of semiconducting/ferroelectric blend systems have been developed with good retention and ON/OFF properties. However, seldom work concerns air stability of their electrical properties. Here we report one kind of blend resistive films with air-stable Poly[(9,9-dioctylfluorenyl-2,7-diyl) -alt-co-(bithiophene)] (F8T2) as semiconducting material and the copolymer of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)) as ferroelectric material. We introduce AFM based nanoscale electrical measurements into the observation of phase separation in F8T2/P(VDF-TrFE) blend films. Conductive AFM measurements present discrete high-conductivity circular domains embedded in continuous low-conductivity matrix. Surface Potential Microscopy results further indicate that these discrete circular domains show lower surface potential than those from their surrounding matrix, implying the difference in chemical structures. It is reasonable to attribute those continuous surrounding matrix to ferroelectric P(VDF-TrFE) phase and those discrete circular domains to semiconducting F8T2 phase who penetrates the whole film resulting in large electrical conductivity. F8T2/P(VDF-TrFE) resistive blend films present good restive performance with high ON/OFF ratio up to 3.5×103. Such blend films show excellent electrical stability in air even after aging of 200 days.

Handling and Sensing of Single Enzyme Molecules: From Fluorescence Detection towards Nanoscale Electrical Measurements

Classical methods to study single enzyme molecules have provided valuable information about the distribution of conformational heterogeneities, reaction mechanisms, and transients in enzymatic reactions when individual molecules instead of an averaging ensemble are studied. Here, we highlight major advances in all-electrical single enzyme studies with a focus on recent micro- and nanofluidic tools, which offer new ways of handling and studying small numbers of molecules or even single enzyme molecules. We particularly emphasize nanofluidic devices, which enable the integration of electrochemical transduction and detection.

Atomic Scale Imaging and Energy Loss Spectroscopy of Epitaxial Graphene

ABSTRACTAtomic-resolution structural and spectroscopic characterization techniques (scanning transmission electron microscopy and electron energy loss spectroscopy) are combined with nanoscale electrical measurements (conductive atomic force microscopy) to study at the atomic scale the properties of graphene grown epitaxially through the controlled graphitisation of Si-face and C-face hexagonal SiC(0001) substrates by high temperature annealing. A scanning transmission electron microscopy analysis, carried out at 60KeV of beam energy, below the knock-on threshold for carbon to ensure no damage is imparted to the film by the electron beam, demonstrates that the buffer layer present on the planar SiC(0001) Si-face delaminates from it on the (11-2n) facets of SiC surface steps, In addition, electron energy loss spectroscopy reveals that the delaminated layer has a similar electronic configuration to purely sp2-hybridized graphene. A thin amorphous film is found on the C-face, instead, which strongly suppresses epitaxy with the SiC substrate. Structurally, the amorphous area is inhomgeneous, as its Si-concentration gradually decreases while approaching the first graphene layer, which is purely sp2-hybridized. Based on these features, we discuss differences and similarities between the C-only buffer layer that forms on the Si-face of SiC with respect to the thicker C/Si amorphous film of the C-face.

Isotropic charge transport in highly ordered regioregular poly(3-hexylthiophene) monolayer

Charge transport anisotropy in π-stacked poly(3-hexylthiophene-2,5-diyl) (P3HT) monolayers was investigated. The monolayers were prepared using a Langmuir–Blodgett technique and were uniaxial but homogeneous two-dimensional sheets. Nanoscale electrical measurements were carried out using metal electrodes with a submicrometre gap between them in order to exclude breaches that occasionally occur along the chains. A remarkable degree of isotropy in both the conductivity and mobility was found. The conductivity isotropy implies that charge transport is dominated by fatal defects in polymers arising at structural and electrical discontinuities, even in the absence of large morphological defects. It was found that the mobility along the π-stacking direction can exceed that along the polymer chain in monolayers with good crystallinity. This high mobility along the π-stacking direction is thought to be an inherent charge transport characteristic that has so far been obscured in solid state conjugated polymers with complex microstructures.

Delaminated Graphene at Silicon Carbide Facets: Atomic Scale Imaging and Spectroscopy

Atomic-resolution structural and spectroscopic characterization techniques (scanning transmission electron microscopy and electron energy loss spectroscopy) are combined with nanoscale electrical measurements (conductive atomic force microscopy) to study at the atomic scale the properties of graphene grown epitaxially through the controlled graphitization of a hexagonal SiC(0001) substrate by high temperature annealing. This growth technique is known to result in a pronounced electron-doping (∼10(13) cm(-2)) of graphene, which is thought to originate from an interface carbon buffer layer strongly bound to the substrate. The scanning transmission electron microscopy analysis, carried out at an energy below the knock-on threshold for carbon to ensure no damage is imparted to the film by the electron beam, demonstrates that the buffer layer present on the planar SiC(0001) face delaminates from it on the (112n) facets of SiC surface steps. In addition, electron energy loss spectroscopy reveals that the delaminated layer has a similar electronic configuration to purely sp2-hybridized graphene. These observations are used to explain the local increase of the graphene sheet resistance measured around the surface steps by conductive atomic force microscopy, which we suggest is due to significantly lower substrate-induced doping and a resonant scattering mechanism at the step regions. A first-principles-calibrated theoretical model is proposed to explain the structural instability of the buffer layer on the SiC facets and the resulting delamination.

Nanogap based graphene coated AFM tips with high spatial resolution, conductivity and durability

After one decade of analyzing the intrinsic properties of graphene, interest into the development of graphene-based devices and micro electromechanical systems is increasing. Here, we fabricate graphene-coated atomic force microscope tips by growing the graphene on copper foil and transferring it onto the apex of a commercially available AFM tip. The resulting tip exhibits surprising enhanced resolution in nanoscale electrical measurements. By means of topographic AFM maps and statistical analyses we determine that this superior performance may be related to the presence of a nanogap between the graphene and the tip apex, which reduces the tip radius and tip-sample contact area. In addition, the graphene-coated tips show a low tip-sample interaction, high conductivity and long life times. The novel fabrication-friendly tip could improve the quality and reliability of AFM experiments, while reducing the cost of AFM-based research.

Multiple-scanning-probe tunneling microscope with nanoscale positional recognition function

Over the past decade, multiple-scanning-probe microscope systems with independently controlled probes have been developed for nanoscale electrical measurements. We developed a quadruple-scanning-probe tunneling microscope (QSPTM) that can determine and control the probe position through scanning-probe imaging. The difficulty of operating multiple probes with submicrometer precision drastically increases with the number of probes. To solve problems such as determining the relative positions of the probes and avoiding of contact between the probes, we adopted sample-scanning methods to obtain four images simultaneously and developed an original control system for QSPTM operation with a function of automatic positional recognition. These improvements make the QSPTM a more practical and useful instrument since four images can now be reliably produced, and consequently the positioning of the four probes becomes easier owing to the reduced chance of accidental contact between the probes.

Biomimetic Metallic Electrodes for Intracellular Electrical Measurements

Electrical interfaces represent a powerful approach for interrogating or perturbing biological systems, with applications in neural prostheses, the regulation of artificial neuronal networks and arrayed patch clamp diagnostics. A key step towards achieving this interface is the development of inorganic nanostructures that can specifically and non-destructively incorporate into biological membranes. Here, we report the development of nanoscale metallic posts that mimic transmembrane proteins, allowing their spontaneous insertion into lipid membranes. These electrode posts were formed by metal evaporation with a nanoscale gold band that can be functionalized with alkanethiols to be hydrophobic. We describe nanoscale electrical measurements with these post-electrodes on cells and demonstrate Giga-ohm seal formation at the electrode-membrane interface. Moreover, we use coarse-grained molecular dynamics simulations to elucidate the mechanism and structures of electrode-membrane interface formation.

Conductive atomic force microscopy (C-AFM) analysis of photoactive layers in inert atmosphere

Conductive atomic force microscopy (C-AFM) measurements have been performed on the MDMO-PPV/PCBM system which is potentially interesting for applications as active layer in polymer solar cells (PSCs). It is demonstrated that C-AFM analysis performed in air for ambient conditions provides only inadequate information about the local electrical properties. The main reason is that the samples chemically degrade when in contact with air. Moreover, we speculate that also the adsorbed water layer interferes with reliable nanoscale electrical measurements. In contrast, when performed in inert atmosphere C-AFM analysis offers consistent results of e.g. the I–V characteristics with lateral resolution better than 50nm, and is able to detect local heterogeneities of these I–V characteristics at the sample surface.