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Resistive random-access memory (RRAM) is a promising technology to develop nonvolatile memory and artificial synaptic devices for brain-inspired neuromorphic computing. Here, we have developed a STO:Ag/SiO2 bilayer based memristor that has exhibited a filamentary resistive switching with stable endurance and long-term data retention ability. The memristor also exhibits a tunable resistance modulation under positive and negative pulse trains, which could fully mimic the potentiation and depression behavior like a bio-synapse. Several synaptic plasticity functions, including long-term potentiation (LTP) and long-term depression (LTD), paired-pulsed facilitation (PPF), spike-rate-dependent-plasticity (SRDP), and post-tetanic potentiation (PTP), are faithfully implemented with the fabricated memristor. Moreover, to demonstrate the feasibility of our memristor synapse for neuromorphic applications, spike-time-dependent plasticity (STDP) is also investigated. Based on conductive atomic force microscopy observations and electrical transport model analyses, it can be concluded that it is the controlled formation and rupture of Ag filaments that are responsible for the resistive switching while exhibiting a switching ratio of ~103 along with a good endurance and stability suitable for nonvolatile memory applications. Before fully electroforming, the gradual conductance modulation of Ag/STO:Ag/SiO2/p++-Si memristor can be realized, and the working mechanism could be explained by the succeeding growth and contraction of Ag filaments promoted by a redox reaction. This newly fabricated memristor may enable the development of nonvolatile memory and realize controllable resistance/weight modulation when applied as an artificial synapse for neuromorphic computing.

Stable operations as resistive switching memory were demonstrated in amorphous TaOx (a-TaOx) thin films with very flat surfaces by conductive atomic force microscopy (c-AFM). The a-TaOx thin films fabricated on glass and Nb-doped SrTiO3 substrates by pulsed laser deposition showed high surface flatness with root-mean-square roughness of less than 0.2 nm. The c-AFM observations on the surfaces revealed that the resistive switching in a-TaOx causes almost no change in the topographic structures, and the significant structural deformation appears after the electrical breakdown by longer-range migration of the constituent ions. The possible mechanisms of the resistive switching phenomena were discussed based on the changes in the topographic structures and conduction states.

A combination of conductive atomic force microscopy (AFM) and confocal fluorescence microscopy was used to measure photocurrents passing through single trimeric photosytem I (PSI) complexes located in the vicinity of single gold nanorods (AuNRs). Simultaneous excitation of PSI and of the AuNR longitudinal plasmon mode and detection of photocurrents from individual PSI in relation to the position of single AuNRs enable insight into plasmon-induced phenomena that are otherwise inaccessible in ensemble experiments. We have observed photocurrent enhancement by the localized plasmons by a factor of 2.9 on average, with maximum enhancement values of up to 8. Selective excitation of the longitudinal plasmon modes by the polarization of the excitation laser enables controllable switch-on of the photocurrent enhancement. The dependence of the extent of enhancement on the distance between PSI and AuNRs indicates that, apart from the enhancement of absorption, there is an additional enhancement mechanism affecting directly the electron transport process. The present study provides deeper insight into the molecular mechanisms of plasmon-enhanced photocurrents, not only in PSI but also potentially in other systems as well.

Atomic layer deposition (ALD) derived ultrathin conformal Al2O3 coating has been identified as an effective strategy for enhancing the electrochemical performance of Ni-rich LiNixCoyMnzO2 (NCM; 0 ≤ x, y, z < 1) based cathode active materials (CAM) in Li-ion batteries. However, there is still a need to better understand the beneficial effect of ALD derived surface coatings on the performance of NCM based composite cathodes. In this work, we applied and optimized a low-temperature ALD derived Al2O3 coating on a series of Ni-rich NCM-based (NCM622, NCM71.51.5 and NCM811) ready-to-use composite cathodes and investigated the effect of coating on the surface conductivity of the electrode as well as its electrochemical performance. A highly uniform and conformal coating was successfully achieved on all three different cathode compositions under the same ALD deposition conditions. All the coated cathodes were found to exhibit an improved electrochemical performance during long-term cycling under moderate cycling conditions. The improvement in the electrochemical performance after Al2O3 coating is attributed to the suppression of parasitic side reactions between the electrode and the electrolyte during cycling. Furthermore, conductive atomic force microscopy (C-AFM) was performed on the electrode surface as a non-destructive technique to determine the difference in surface morphology and conductivity between uncoated and coated electrodes before and after cycling. C-AFM measurements on pristine cathodes before cycling allow clear separation between the conductive carbon additives and the embedded NCM secondary particles, which show an electrically insulating behavior. More importantly, the measurements reveal that the ALD-derived Al2O3 coating with an optimized thickness is thin enough to retain the original conduction properties of the coated electrodes, while thicker coating layers are insulating resulting in a worse cycling performance. After cycling, the surface conductivity of the coated electrodes is maintained, while in the case of uncoated electrodes the surface conductivity is completely suppressed confirming the formation of an insulating cathode electrolyte interface due to the parasitic side reactions. The results not only show the possibilities of C-AFM as a non-destructive evaluation of the surface properties, but also reveal that an optimized coating, which preserves the conductive properties of the electrode surface, is a crucial factor for stabilising the long-term battery performance.

The nanofriction properties of hexagonal boron nitride (h-BN) are vital for its application as a substrate for graphene devices and solid lubricants in micro- and nano-electromechanical devices. In this work, the nanofriction characteristics of h-BN on Si/SiO2 substrates with a bias voltage are explored using a conductive atomic force microscopy (AFM) tip sliding on the h-BN surface under different substrate bias voltages. The results show that the nanofriction on h-BN increases with an increase in the applied bias difference (Vt−s) between the conductive tip and the substrate. The nanofriction under negative Vt−s is larger than that under positive Vt−s. The variation in nanofriction is relevant to the electrostatic interaction caused by the charging effect. The electrostatic force between opposite charges localized on the conductive tip and at the SiO2/Si interface increases with an increase in Vt−s. Owing to the characteristics of p-type silicon, a positive Vt−s will first cause depletion of majority carriers, which results in a difference of nanofriction under positive and negative Vt−s. Our findings provide an approach for manipulating the nanofriction of 2D insulating material surfaces through an applied electric field, and are helpful for designing a substrate for graphene devices.

Splats are the most fundamental and paramount unit of any plasma sprayed coating and splats are having the potential to manipulate the overall property of the same. Current study shows the insulator-conductor transition of alumina (Al2O3) splat by the hybrid addition of carbonaceous nanofillers, namely carbon nanotubes (CNTs) and Graphene nanoplatelets (GNPs) into it. Reinforcement of CNTs and GNPs changed the splat shape from fragmented to nearly disk shaped as well as from porous splat to densified one upon the reinforcement. Further, Current mapping over the splats were carried out using conductive atomic force microscopy (cAFM). Current map for Al2O3 splat showed fully black region, evincing its insulating nature; whereas hybrid addition of CNTs and GNPs into the splat dramatically improved the electrical conductivity of Al2O3 splat, coining its non-conductive map to conductive one. This transition was attributed to electron tunneling mechanism, which is a function of electron hopping distance of these conductive nanofillers. The electron hopping distance of the matrix got minimized when CNTs and GNPs were simultaneously added to Al2O3 splat, due to their higher aspect ratio. This led to the formation of a 3-dimensional conductive channels for the electron transport in the matrix, converting the whole splat as conductive.

The current-carrying nanofriction characteristics play an important role in the performance, reliability, and lifetime of graphene-based micro/nanoelectromechanical systems and nanoelectronic devices. The atomic-scale friction characteristics of graphene were investigated using conductive atomic force microscopy by applying positive-bias and negative-bias voltages. The atomic-scale friction increased with applied voltages. Also, the friction under positive-bias voltages was lower than under negative-bias voltages, and the friction difference increased with the voltages. The different frictional behaviors resulted from the inherent work function difference and the water molecules between the tip and graphene. The applied voltages amplified the effect of the work function difference on the friction, and the water molecules played different roles under negative-bias and positive-bias voltages. The friction increased rapidly with the continuous increase of negative-bias voltages due to the electrochemical oxidation of graphene. Nevertheless, the friction under positive-bias voltages remained low and the structure of graphene was unchanged. These experimental observations were further explained by modeling the atomic-scale friction with a modified Prandtl-Tomlinson model. The model allowed the determination of the basic potential barrier and the voltage-induced potential barrier between the tip and graphene. The calculation based on the model indicated that the negative-bias voltages induced a larger potential barrier than the positive-bias voltages. The studies suggest that graphene can show a better lubricant performance by working as a lubricant coating for the cathodes of the sliding electrical contact interfaces.

In a bid to further reduce the cost of the front Ag contact metallization in Si solar cells, Cu is the potential alternative to replace the Ag in the Ag paste. However, this requires an understanding of the contact mechanism of screen-printable Ag/Cu paste in Si solar cell through rapid thermal process. The pastes with different weight percent of Cu (0 wt%, 25 wt% and 50 wt%) were used and the Voc of the cells was reduced with the increasing weight percent of Cu. This is because the presence of Cu in the paste changed the microstructure of the Ag/Cu/Si contact through Cu doping of the glass frits and hence increasing the Tg of the glass. The increased Tg of the glass impeded the uniform spreading of the molten glass and resulted in poor wetting and etching of the SiNx, which impacted the contact as evident in ideality factor of less than unity. This also led to the formation of agglomerated Ag crystallites with features of 700 nm in length and 200 nm in depth, which is close to the p-n junction, of which depth is ~300 nm. However, the interface glass layer acted as an effective diffusion barrier layer to prevent Cu atoms from diffusing into the Si emitter, which is quite remarkable for Cu not to diffuse into silicon at high temperature. Further investigation of the Ag/Cu contacts with the conductive AFM in conjunction with the SEM and STEM analyses revealed that the growth of Ag crystallites in the Si emitter is responsible for carrier conduction the gridlines as with the pure Ag paste.

The advent of two-dimensional materials has opened a plethora of opportunities in accessing ultrascaled device dimensions for future logic and memory applications. In this work, we demonstrate that a single layer of large-area chemical vapor deposition-grown molybdenum disulfide (MoS2) sandwiched between two metal electrodes can be tuned to show multilevel nonvolatile resistive memory states with resistance values separated by 5 orders of magnitude. The switching process is unipolar and thermochemically driven requiring significant Joule heating in the reset process. Temperature-dependent electrical measurements coupled with semiclassical charge transport models suggest that the transport in these devices varies significantly in the initial (pristine) state, high resistance state, and low resistance state. In the initial state, the transport is a one-step direct tunneling (at low voltage biases) and Fowler Nordeim tunneling (at higher bias) with an effective barrier height of 0.33 eV, which closely matches the Schottky barrier at the MoS2/Au interface. In the high resistive state, trap-assisted tunneling provides a reasonable fit to experimental data for a trap height of 0.82 eV. Density functional theory calculations suggest the possibility of single- and double-sulfur vacancies as the microscopic origins of these trap sites. The temperature-dependent behavior of the set and reset process are explained by invoking the probability of defect (sulfur vacancy) creation and mobility of sulfur ions. Finally, conductive atomic force microscopy measurements confirm that the multifilamentary resistive memory effects are inherent to a single-crystalline MoS2 triangle and not necessarily dependent on grain boundaries. The insights suggested in this work are envisioned to open up possibilities for ultrascaled, multistate, resistive memories for next-generation digital memory and neuromorphic applications.

In this work, the high temperature performance of a diamond Schottky PIN diode is reported in the range of 298-873 K. The diamond diode exhibited an explicit rectification up to 723 K with an excellent forward current density of >3000 A/cm2. The stability of the diode was investigated by exposing the sample to high temperature cycles (up to 873 K) for more than 10 times (totaling up to 120 hours), which exhibited no change between the I-V characteristics measured in each cycle. The dependence of ideality factor and Schottky barrier height on temperature along with an extracted Richardson's constant much smaller than the theoretical value (0.0461 A/cm2.K2), motivated us to study the possible reason for this anomaly. A modified thermionic emission model following Tung's analysis was used to explain the experimental observations. The model assumed the presence of inhomogeneous Schottky barrier heights leading to a reduced effective area and yielded a Richardson's constant closer to the theoretical value. Conductive atomic force microscopy studies were conducted, which concurred with the electrical data and confirmed the presence of inhomogeneous Schottky barrier heights.

The intrinsic electrical conductivity of graphene is one of the key factors affecting the electrical conductance of its assemblies, such as papers, films, powders, and composites. Here, the local electrical conductivity of the individual graphene flakes was investigated using conductive atomic force microscopy (C-AFM). An isolated graphene flake connected to a pre-fabricated electrode was scanned using an electrically biased tip, which generated a current map over the flake area. The current change as a function of the distance between the tip and the electrode was analyzed analytically to estimate the contact resistance as well as the local conductivity of the flake. This method was applied to characterize graphene materials obtained using two representative large-scale synthesis methods. Monolayer graphene flakes synthesized by chemical vapor deposition on copper exhibited an electrical conductivity of 1.46 ± 0.82 × 106 S/m. Reduced graphene oxide (rGO) flakes obtained by thermal annealing of graphene oxide at 300 and 600 °C exhibited electrical conductivities of 2.3 ± 1.0 and 14.6 ± 5.5 S/m, respectively, showing the effect of thermal reduction on the electrical conductivity of rGO flakes. This study demonstrates an alternative method to characterizing the intrinsic electrical conductivity of graphene-based materials, which affords a clear understanding of the local properties of individual graphene flakes.

LiCoO2, one of the most popular cathode materials for lithium-ion batteries, is well known for undergoing insulator–metal transitions depending on the amount of lithium ions. In this study, we succ...

Graphene nanoribbons (GNRs) are considered as major building blocks in future carbon-based electronics. The electronic performance of graphene nanostructures is essentially influenced and determined by their edge termination and their supporting substrate. In particular, semi-conducting, as well as metallic GNRs, can be fabricated by choosing the proper template which is favorable for device architecture designs. This study highlights the impact of microscopic details of the environment of the GNRs on the charge transport in GNRs. By means of lateral force, conductive atomic force and nanoprobe measurements, we explore the charge propagation in both zig-zag and armchair GNRs epitaxially grown on SiC templates. We directly image transport channels on the nanoscale and identify SiC substrate steps and nano-instabilities of SiC facets as dominant charge scattering centers. [Figure not available: see fulltext.].

The local transverse photoconductivity of ultrathin (~4 nm) ZrO2(Y) films with embedded single-layer arrays of Au nanoparticles (~2 nm in diameter) is studied by conductive atomic force microscopy. The ZrO2(Y) films with Au nanoparticles are formed on glass substrates with transparent conductive indium-tin-oxide sublayers using layer-by-layer magnetron deposition followed by annealing. The peaks observed in the optical absorption spectra of the samples at the wavelength λ ≈ 660 nm are attributed to collective plasmon resonance in dense arrays of Au nanoparticles. The photocurrent between the microscope probe and the indium-tin-oxide sublayer is measured during photoexcitation of the contact between the probe and the sample surface through the transparent substrate by the radiation of a semiconductor laser diode at the plasmon-resonance wavelength. The increase in current through the probe of the atomic force microscope under photoexcitation is attributed to the photon-assisted emission of electrons from the Au nanoparticle into the conduction band of ZrO2(Y) in a strong electric field applied between the probe and the indium-tin-oxide sublayer under plasmon resonance conditions.

As the cornerstone of the modern information technology, the development of memory devices with high speed, large capacity, long life time, low power consumption and ease of operation characteristics is beneficial for the fast revolution of the digital world. Using soluble 4-bromobenzene functionalized reduced graphene oxide (RGBr) as two dimensional template, in-situ growing of a novel donor-accept type conjugated polymer “poly[(1,4-diethynyl- benzene)-alt-(4,4′-(quinoxaline-2,3-diyl)bis(N,N-diphenylaniline))](PDQ)” from the RGO surface has been successfully realized via the palladium-catalyzed Sonogashira-Hagihara polymerization. The resultant material called as RGO-PDQ is highly soluble in some common organic solvents. The nonvolatile ternary rewritable memory performance has been observed in an electronic device with the RGO-PDQ film sandwiched between the Al and ITO electrodes. The switching threshold voltages on the ON-1 and ON-2 states are −0.74 and −1.44 V, respectively. The achieved current ratio of OFF: ON-1: ON-2 is about 1: 10: 104. The memory effect of the Al/RGO-PDQ/ITO device on the ON-1state can be assigned to the contribution of the PDQ moieties in the structure of RGO-PDQ. By using conductive atomic force microscopy (C-AFM) technique, the conductive nature of the RGO-PDQ film-based device was in-situ observed.

For the fabrication of high-performance top-gated MoS2 transistors, a uniform atomic layer deposition (ALD) of an ultrathin high-k gate dielectric film without abnormal leakage paths on a MoS2 channel is required. In this study, we fabricated a ~5.2 nm-thick monolithic HfO2 gate dielectric film by utilizing an e-beam-evaporated Hf seed layer (target thickness of 3 nm) prior to the ALD of a HfO2 film (~2 nm). The Hf seed layer was fully converted to HfO2 without metallic residues during the following ALD process, without damages to the Raman and photoluminescence characteristics of the underlying MoS2. The conformal and pinhole-free ALD of the subsequent HfO2 film was verified using conductive atomic force microscopy. In addition, operation of a top-gated MoS2 transistor was demonstrated by integrating the Hf-seeded HfO2 gate dielectric film.

Ag8In14Sb55Te23 (AIST) films were deposited on k9 glass substrates by DC magnetron sputtering. A laser direct writing system was used to fabricate the crystalline arrays on as-deposited amorphous films. Electrical properties of crystallized bits were investigated by using conductive atomic force microscopy (CAFM). Bipolar resistance switching and memory effects were observed on nanoscale area of crystalline bits. The crystalline arrays exhibit reproducible bistable resistive switching induced by bias amplitude and polarity. It is hoped that multistate memory can be achieved by using AgInSbTe phase change material.

The tunable nanofriction of graphene as a lubricant material is important for its application in micro/nano-electromechanical devices. A tunable nanofriction array of graphene on a silicon dioxide/silicon (SiO2/Si) substrate was achieved via functionalization using negative-bias conductive atomic force microscopy by introducing an electron releasing process to avoid electron saturation. Based on the electron releasing process, the functionalization degree of graphene and thus the nanofriction were controllably tuned by the tip voltage. Moreover, the nanofriction of the functionalized array was further enhanced by applying biased tip voltage. The nanofriction on the functionalized array increased with an increase in bias voltage due to the polarization-induced electrostatic force, and the voltage increased the meniscus force. The tuning of the nanofriction of functionalized graphene on an insulating substrate can provide insight into the electric-carrying friction behavior of graphene as a solid lubricant in micro/nano-electromechanical systems (MEMS/NEMS) and the fabrication of nanodevices with a patterned surface.

Two-dimensional graphene has remarkable properties that are revolutionary in many applications. Scrolling monolayer graphene with precise tunability would create further potential for niche applications but this has proved challenging. We have now established the ability to fabricate monolayer graphene scrolls in high yield directly from graphite flakes under non-equilibrium conditions at room temperature in dynamic thin films of liquid. Using conductive atomic force microscopy we demonstrate that the graphene scrolls form highly conducting electrical contacts to highly oriented pyrolytic graphite (HOPG). These highly conducting graphite–graphene contacts are attractive for the fabrication of interconnects in microcircuits and align with the increasing interest in building all sp2-carbon circuits. Above a temperature of 450 °C the scrolls unravel into buckled graphene sheets, and this process is understood on a theoretical basis. These findings augur well for new applications, in particular for incorporating the scrolls into miniaturized electronic devices.

The carrier transport through the silicon-oxide (SiOx) layer in tunnel oxide passivated contact (TOPCon) c-Si solar cells has been studied experimentally and by simulation. The current intensity versus voltage (J-V) characteristics of GaIn/n-c-Si/SiOx/n+-poly-Si/Al structures shows a linear Ohmic characteristic, while a non-Ohmic behavior is observed in the samples without the n+-poly-Si contact layer. Conductive Atomic Force Microscopy (c-AFM) images reveal some current spikes on the surface of the samples, which could be related to the transport through pinholes. The simulation results show that 1) a rectification characteristic is obtained when only the tunneling mechanism is included, 2) both the reverse saturation current and the forward current increase when a small amount of transport through pinholes is introduced, and 3) finally a linear Ohmic behavior is observed when the pinhole transport component reaches a certain level. Furthermore, the simulation for whole TOPCon solar cells provides some useful results. For very thin SiOx ( 1.2 nm) without the transport through pinholes, the TOPCon solar cell shows a poor fill factor (FF) with a high series resistance (Rs) because the tunneling does not provide a sufficient high transport channel for carrier transport, and the introduction of a small number of transports through pinholes improves the FF and reduces the Rs, hence improves the PCE. However, a high possibility for carrier going through pinholes reduces all of the performance parameters and degrades PCE for all the cases simulated. Therefore, an optimized pinhole density and size distribution is critical engineering for solar cell performance optimization. However, the establishment of an optimized method to precisely control the pinhole formation and characterization is still on the way.

We present an optical rectenna by engineering a rectifying diode at the interface between a metal probe of an atomic force microscope (AFM) and a single wall carbon nanotube (SWCNT) that acts as a nano-antenna. Individual SWCNT electrical and optical characteristics have been investigated using a conductive AFM nano-probe in contact with two device structures, one with a SWCNT placed on a CuO/Cu substrate and the other one with a SWCNT on a SiO2/Si substrate. The I–V measurements performed for both designs have exhibited an explicit rectification behavior and the sensitivity of carbon nanotube (CNT)-based rectenna to light. The measured output current at a set voltage value demonstrates the significant effect of the light irradiation on the current signal generated between the Au nano-probe and CNT interface. This effect is more prominent in the case of the CuO/Cu substrate. Detailed analysis of the system, including the energy band diagram, materials characterization and finite element simulation, is included to explain the experimental observations. This work will pave the way for more investigations and potential applications of CNTs as nano-rectennas in optical communication and energy harvesting systems.

Multiple studies have reported the observation of electro-synaptic response in different metal/insulator/metal devices; however, most of them analysed large (>1 µm2) devices that do not meet the integration density required by the industry (1010 devices/mm2). Some studies employed a scanning tunnelling microscope (STM) to explore nano-synaptic response in different materials, but in this setup there is a nanogap between the insulator and one of the metallic electrodes (i.e. the STM tip), which is not present in real devices. Here we show how to use a conductive atomic force microscope (CAFM) to explore the presence and quality of nano-synaptic response in confined areas

Efficient conversion of mechanical energy in our surrounding environment into electric power has become a promising strategy for meeting the ever-increasing energy consumption of small and distributed electronics. The contact-electrification-based triboelectric nanogenerators are one of the emerging devices to achieve such energy conversion. However, conventional contact electrifications between two insulators are limited by their low current density and alternating current output. Here we report a nanoscale contact electrification induced direct current output based on the flow of electrons from the defect states of the ZnO nanowires-packed film to the contact sliding conductive AFM tip. Combining experimental materials characterization and density functional theory (DFT) calculations, the direct current output is closely related to the concentration of oxygen vacancy defect states on the surface of ZnO nanowires: the higher the oxygen vacancy concentration, the higher the current output. Under optimized conditions, we obtain an ultrahigh current density of ~108 A m-2, which is several orders of magnitude higher than that of the conventional contact electrification and other effects. This work provides a new route of utilizing defect states contributed contact electrification for realizing nanoscale mechanical energy scavenging.

Realization of semiconductor materials at the two-dimensional (2D) limit can elicit exceptional and diversified performance exercising transformative influence on modern technology. We report experimental evidence for the formation of conceptually new 2D indium oxide (InO) and its material characteristics. The formation of 2D InO was harvested through targeted intercalation of indium (In) atoms and deposition kinetics at graphene/SiC interface using a robust metal organic chemical vapor deposition (MOCVD) process. A distinct structural configuration of two sub-layers of In atoms in "atop" positions was imaged by scanning transmission electron microscopy (STEM). The bonding of oxygen atoms to indium atoms was indicated using electron energy loss spectroscopy (EELS). A wide bandgap energy measuring a value of 4.1 eV was estimated by conductive atomic force microscopy measurements (C-AFM) for the 2D InO. Funding Agencies|FLAG-ERA 2015 JTC project GRIFONE through Swedish Research Council [VR 2015-06816]; National Research Development and Innovation Office, Hungary [NN 118914]; Italian Ministry of Education and Research (MIUR) under the project EleGaNTeMinistry of Education, Universities and Research (MIUR) [PON ARS01_01007]; European Structural and Investment Funds [VEKOP-2.3.3-15-2016-00002]; Swedish Foundation for Strategic Research (SSF)Swedish Foundation for Strategic Research [RIF 14-0074]; Knut and Alice Wallenbergs FoundationKnut & Alice Wallenberg Foundation; [VR 2017-04071]

A conductive atomic force microscope has been used to electrically stress and to investigate the effects of degradation in the conduction properties of ultrathin (

The aim of this study was to clarify the influence of inter-metallic compounds (IMCs) on the electrical conductivity of Cu/Al joint. The longitudinal resistance and the lateral current distribution at flash welded and diffusion brazed Cu/Al joint interfaces were investigated using four-point method and conductive atomic force microscopy, respectively. A 2μm Cu9Al4/CuAl2 layer was formed in both joints. The IMCs layer was homogenous and the current distribution interface was planar at diffusion brazed joint. However, the IMCs layer was discontinuous and the current distribution interface was non-planar at flash welded joint. After heat treatment at 350°C for 500h, the thickness of interfacial layer was increased to 50μm. CuAl and a short crack were newly formed in the diffusion brazed joint. CuAl, (Cu,Al)xOy and a long crack were newly formed in the flash welded joint. A multilayer current distribution was found at both heat treated joints. The resistivity of all Cu/Al joints was higher than that of copper and lower than that of aluminum. The resistivity of diffusion brazed joint was the lowest, which was lower than the theoretical value. The resistivity of the heat treated flash welded joint was the highest among all the joints.

The open circuit voltage (VOC) is a critical and common indicator of solar cell performance as well as degradation, for panel down to lab-scale photovoltaics. Detecting VOC at the nanoscale is much more challenging, however, due to experimental limitations on spatial resolution, voltage resolution, and/or measurement times. Accordingly, an approach based on Conductive Atomic Force Microscopy is implemented to directly detect the local VOC, notably for monocrystalline Passivated Emitter Rear Contact (PERC) cells which are the most common industrial-scale solar panel technology in production worldwide. This is demonstrated with cross-sectioned monocrystalline PERC cells around the entire circumference of a poly-aluminum-silicide via through the rear emitter. The VOC maps reveal a local back surface field extending ~ 2 μm into the underlying p-type Si absorber due to Al in-diffusion as designed. Such high spatial resolution methods for photovoltaic performance mapping are especially promising for directly visualizing the effects of processing parameters, as well as identifying signatures of degradation for silicon and other solar cell technologies.

Piezoelectric and ferroelectric materials have garnered significant interest owing to their excellent physical properties and multiple potential applications. Accordingly, the need for evaluating piezoelectric and ferroelectric properties has also increased. The piezoelectric and ferroelectric properties are evaluated macroscopically using laser interferometers and polarization–electric field loop measurements. However, as the research focus is shifted from bulk to nanosized materials, scanning probe microscopy (SPM) techniques have been suggested as an alternative approach for evaluating piezoelectric and ferroelectric properties. In this Progress Report, the recent progress on the nanoscale evaluation of piezoelectric and ferroelectric properties of diverse materials using SPM‐based methods is summarized. Among the SPM techniques, the focus is on recent studies that are related to piezoresponse force microscopy and conductive atomic force microscopy; further, the utilization of these two modes to understand piezoelectric and ferroelectric properties at the nanoscale level is discussed. This work can provide guidelines for evaluating the piezoelectric and ferroelectric properties of materials based on SPM techniques., Piezoelectric and ferroelectric materials have been of significant interest owing to their remarkable physical properties. This Progress Report summarizes the recent advances in the nanoscale evaluation of piezoelectric and ferroelectric properties of diverse materials using approaches based on scanning probe microscopy.

The bottom-up DNA-templated nanoelectronics exploits the unparalleled self-assembly properties of DNA molecules and their amenability with various types of nanomaterials. In principle, nanoelectronic devices can be bottom-up assembled with near-atomic precision, which compares favorably with well-established top-down fabrication process with nanometer precision. Over the past decade, intensive effort has been made to develop DNA-based nanoassemblies including DNA-metal, DNA-polymer, and DNA-carbon nanotube complexes. This review introduces the history of DNA-based fabrication for nanoelectronics briefly and summarizes the state-of-art advances of DNA-based nanoelectronics. In particular, the most widely applied characterization techniques to explore their unique electronic properties at the nanoscale are described and discussed, including scanning tunneling microscopy, conductive atomic force microscopy, and Kelvin probe force microscopy. We also provide a perspective on potential applications of DNA-based nanoelectronics.

One common challenge highlighted in almost every review article on organic resistive memory is the lack of areal switching uniformity. This, in fact, is a puzzle because a molecular switching mechanism should ideally be isotropic and produce homogeneous current switching free from electroforming. Such a demonstration, however, remains elusive to date. The reports attempting to characterize a nanoscopic picture of switching in molecular films show random current spikes, just opposite to the expectation. Here, this longstanding conundrum is resolved by demonstrating 100% spatially homogeneous current switching (driven by molecular redox) in memristors based on Ru-complexes of azo-aromatic ligands. Through a concurrent nanoscopic spatial mapping using conductive atomic force microscopy and in operando tip-enhanced Raman spectroscopy (both with resolution

PC61BM is commonly used in perovskite solar cells (PSC) as the electron transport material (ETM). However, PC61BM film has various disadvantages, such as its low coverage or the many pinholes that appear due to its aggregation behavior. These faults may lead to undesirable direct contact between the metal cathode and perovskite film, which could result in charge recombination at the perovskite/metal interface. In order to overcome this problem, three alternative non-fullerene electron materials were applied to inverted PSCs, they were evaluated on suitability as electron transport layers. The roles and effects of these non-fullerene ETMs on device performance were studied using photoluminescence (PL) measurements, field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), internal resistance in PSC measurements, and conductive atomic force microscopy (C-AFM). It was found that one of the tested materials, IT-4f, showed excellent electron extraction ability and was associated with reduced recombination. The PSC with IT-4f as the ETM produced better cell-performance, it had an average PCE of 11.21%, which makes it better than the ITIC and COi8DFIC-based devices. Finally, IT-4f was compared with PC61BM, it was found that the two materials have quite comparable efficiency and stability levels.

Cable bacteria are an emerging class of electroactive organisms that sustain unprecedented long-range electron transport across centimeter-scale distances. The local pathways of the electrical currents in these filamentous microorganisms remain unresolved. Here, the electrical circuitry in a single cable bacterium is visualized with nanoscopic resolution using conductive atomic force microscopy. Combined with perturbation experiments, it is demonstrated that electrical currents are conveyed through a parallel network of conductive fibers embedded in the cell envelope, which are electrically interconnected between adjacent cells. This structural organization provides a fail-safe electrical network for long-distance electron transport in these filamentous microorganisms. The observed electrical circuit architecture is unique in biology and can inspire future technological applications in bioelectronics. R.T.E. and R.B. contributed equally to this work. The authors thank the colleagues from X-LAB from Hasselt University and the Microbial Electricity team from University of Antwerp for discussions and feedback. Special thanks to H. T. S. Boschker, I. Cardinaletti, J. Drijkoningen, J. L. Hou, and S. Thijs for their insights and discussions. Thanks to K. Ceyssens and T. Custers for the graphics. This research was financially supported by the Research Foundation Flanders (FWO project grant G031416N to FJRM and JM, FWO aspirant grant 1180517N to RB) and Dutch Research Council (NWO Vici grant 016.VICI.170.072 to FJRM). All measurements and data analysis were performed by R.T.E. and R.B. in equal contribution. J.M. and B.C. coordinated the study. Conceptualization and discussion were done by J.M., R.V., B.C., R.C., R.T.E., and R.B. Cable bacteria enrichment and fiber sheath extraction was performed by S.H.-M., R.B., and F.J.R.M. Funding was acquired by J.M., J.V., and F.J.R.M. Writing was done by R.B., R.T.E., J.M., B.C., and F.J.R.M., with contributions from all authors.

In this chapter, we summarize the recent developments in scanning probe microscopy techniques to isolate individual defects and characterize them using random telegraph noise (RTN) signatures, considering HfO2 high-κ dielectric as the material of interest. The chapter also provides an overview on the challenges associated with the existing probe station-based device-level RTN spectroscopy and the potential for the conductive atomic force microscopy (CAFM) and scanning tunneling microscopy (STM) technique to effectively address some of these challenges. As a proof of concept, we have demonstrated the application of CAFM and STM for the measurement of spatially resolved RTN spectra from grain and grain boundaries in HfO2, which provides an opportunity to directly correlate the electrical defects at the microstructural sites to their specific noise contributions.

Chemical vapour deposition (CVD) is a promising method for producing large-scale graphene (Gr). Nevertheless, microscopic inhomogeneity of Gr grown on traditional metal substrates such as copper or nickel results in a spatial variation of Gr properties due to long wrinkles formed when the metal substrate shrinks during the cooling part of the production cycle. Recently, molybdenum (Mo) has emerged as an alternative substrate for CVD growth of Gr, mainly due to a better matching of the thermal expansion coefficient of the substrate and Gr. We investigate the quality of multilayer Gr grown on Mo and the relation between Gr morphology and nanoscale mechanical and electrical properties, and spatial homogeneity of these parameters. With atomic force microscopy (AFM) based scratching, Kelvin probe force microscopy, and conductive AFM, we measure friction and wear, surface potential, and local conductivity, respectively. We find that Gr grown on Mo is free of large wrinkles that are common with growth on other metals, although it contains a dense network of small wrinkles. We demonstrate that as a result of this unique and favorable morphology, the Gr studied here has low friction, high wear resistance, and excellent homogeneity of electrical surface potential and conductivity., Accepted for publication in Applied Surface Science, 31 pages, 10 figures

7 pags., 7 figs., 1 tab., Nanostructuring is one promising alternative to modify the functionalities of conducting polymers aiming atapplications in technologies like organic solar cells, organicfield-effect transistors and organic light-emittingdiodes. Nowadays, there is an increasing interest in alternative ways of nanostructuring more economic, quickerand reproducible, avoiding the necessity of stringent environmental conditions like clean rooms, high vacuum orcomplex mask fabrication. Here we prove that Laser Induced Periodic Surface Structures (LIPSS) can be formedinfilms of Poly(3,4-ethylenedioxythiophene) complexed with Poly(styrenesulfonate) (PEDOT:PSS) prepared byspin-coating on planar Indium Tin Oxide. PEDOT:PSS has been irradiated with the 4th harmonic of a Nd:YAGlaser resulting in LIPSS parallel to the laser polarization with a period close to the laser wavelength. LIPSSmorphology has been characterized by Atomic Force Microscopy (AFM) and synchrotron Grazing IncidenceSmall Angle X-ray Scattering while thefilm structure was studied by Grazing-Incidence Wide-Angle X-rayScattering and Raman spectroscopy. To analyze the impact of laser treatment on the electrical properties ofPEDOT:PSS, conductive AFM was employed showing that LIPSS partially preserve the original electrical con-ductivity. These results can be of interest for achieving functional polymer nanostructures by a quick and re-producible approach with no need for clean room conditions., This work has been supported by Ministry of Economy and Competitiveness (MINECO, Spain), Spanish State Research Agency(AEI) and European Regional Development (FEDER) under the projects MAT2014-59187-R, MAT2015-66443-C02-1-R-MINECO/FEDER, UEand CTQ2016-75880-P-AEI/FEDER, UE. E.R. also thanks for the tenure of a Ramón y Cajal contract (No. RYC-2011-08069). GISAXS and GIWAXS experiments were performed at NCD-SWEET beamline at ALBA Synchrotron with the collaboration of ALBA staff.

Composites fabricated by metal nanoparticles supported on graphene sheets are promising for many technological applications ranging from sensing, energy production and storage, catalysis to electronics and optoelectronics. This is due to the benefits offered by the properties of both components. In particular, charge transfer effects at graphene-nanoparticles interface are the basis for the operation of several devices. Here we present an approach to fabricate hybrid composites consisting of graphene-noble metal nanoparticles. Mono- and bimetallic platinum and palladium nanoparticles are produced by laser ablation in a liquid environment and their morphology is characterized by transmission electron microscopy. In addition, in order to fabricate low-dimensional hybrid composites, the nanoparticles are loaded on graphene layer by spin coating of colloidal solutions and the density of the nanoparticles on graphene was evaluated by scanning electron microscopy. The nanoparticles-graphene charge transfer was studied by employing Raman spectroscopy and conductive atomic force microscopy. In particular, the degree of the G and 2D Raman peak shifts (evidenced by the Raman Spectroscopy measurements) and the characteristics of the current-voltage curves (evidenced by conductive atomic force microscopy) allowed inferring an n-type doping effect of the metal nanoparticles on the graphene properties, i. e. electrons transfer from the nanoparticles to the graphene layers. A combined analysis of these results allow us to demonstrate that the chemical composition of the metal nanoparticles is a key parameter in determining the charge transfer phenomenon in low-dimensional systems.

The integration of graphene (Gr) with nitride semiconductors is highly interesting for applications in high-power/high-frequency electronics and optoelectronics. In this work, we demonstrated the direct growth of Gr on Al0.5Ga0.5N/sapphire templates by propane (C3H8) chemical vapor deposition (CVD) at temperature of 1350{\deg}C. After optimization of the C3H8 flow rate, a uniform and conformal Gr coverage was achieved, which proved beneficial to prevent degradation of AlGaN morphology. X-ray photoemission spectroscopy (XPS) revealed Ga loss and partial oxidation of Al in the near-surface AlGaN region. Such chemical modification of a 2 nm thick AlGaN surface region was confirmed by cross-sectional scanning transmission electron microscopy (STEM) combined with electron energy loss spectroscopy (EELS), which also showed the presence of a bilayer of Gr with partial sp2/sp3 hybridization. Raman spectra indicated that the deposited Gr is nanocrystalline (with domain size 7 nm) and compressively strained. A Gr sheet resistance of 15.8 kOhm/sq was evaluated by four-point-probe measurements, consistently with the nanocrystalline nature of these films. Furthermore, nanoscale resolution current mapping by conductive atomic force microscopy (C-AFM) indicated local variations of the Gr carrier density at a mesoscopic scale, which can be ascribed to changes in the charge transfer from the substrate due to local oxidation of AlGaN or to the presence of Gr wrinkles., Comment: 20 pages, 8 figures

The use of graphene for applications such as micro- and nano-scale electronic devices often involves incorporating the two-dimensional material onto various substrates. However, the effects of the substrate's mechanical properties on electrical contact conductance are not fully understood. Here, we explore these effects by measuring the conductance between a nanoscale probe and a single layer of graphene with three different levels of substrate support: no substrate, i.e. free-standing graphene, an elastic substrate, and a rigid substrate. These three systems are studied using conductive atomic force microscopy experiments complemented by molecular dynamics simulations using the electrochemical dynamics with implicit degrees of freedom method. In both experiments and simulations, at a given normal force, current increases as: rigid substrate

Hybrid solar cells based on semiconducting polymers and metal oxides offer the possibility of combining the best attributes of both constituents to achieve efficient and low-cost solar energy harvesting. Active layers consisting of ZnO nanorods and semiconducting polymer poly(3-hexylthiophene), P3HT, have been studied extensively over the past decade. This type of solar cell, however, still exhibits a relatively low performance and poor device-to-device reproducibility. For insight into the performance bottlenecks in P3HT:ZnO nanorod solar cells, we employ a point-by-point current–voltage mapping method using conductive atomic force microscopy to probe the local photovoltaic properties of hybrid P3HT:ZnO nanorod active layers with nanoscale spatial resolution. We observe that the short-circuit current density, open-circuit voltage, fill factor, and power conversion efficiency are highly heterogeneous and sensitive to the local thickness of the P3HT hole transport layer that sits atop the active layer, wit...

Cable Bacteria are an emerging class of electroactive organisms that sustain unprecedented long-range electron transport across centimeter-scale distances. The pathways of the electrical currents in these filamentous microorganisms remain unresolved. Here, the electrical circuitry in a single cable bacterium is visualized with nanoscopic resolution using Conductive Atomic Force Microscopy. Combined with perturbation experiments, it is demonstrated that electrical currents are conveyed through a parallel network of conductive fibers embedded in the cell envelope which are electrically interconnected between adjacent cells. This redundant structural organization of electrical pathways likely forms a crucial adaptive trait for organisms capable of long-distance electron transport. The observed electrical circuit architecture is unique in biology and could inspire future technological applications in bioelectronics.

Electrospray (ES) has emerged as an attractive approach to fabricate thin-film organic photovoltaic devices. However, the morphological properties and their implication to electrical characteristics of polymer:fullerene blends deposited by this method have not been clearly understood yet. In order to uncover the interplay of nanoscale morphology and electrical properties of ES-deposited films, both the residues of single droplets and thin films from the overlapping of multiple droplets were investigated by various characterization techniques. The surface morphology was demonstrated in details by the optical microscope (OM) and atom force microscope (AFM). The ordering structures and aggregation of the polymer were identified by the Raman and UV–vis spectroscopic investigations. The spatially distribution map of the P3HT aggregation states was analyzed by fitting the C˭C mode of P3HT with Lorentzian functions. The conductive atomic force microscopy (C-AFM) was used to quantify local currents and reveal the correlation between the nanostructure and charge-transport mobility. Both surface morphology and aggregation of the P3HT were found to strongly depend on the overlapping boundaries formed by the dry residues of individual droplets. More P3HT aggregation present at the boundaries. However, boundaries also show significantly higher charge-transport resistance thus lower current. Thin films deposited with less droplet evaporation exhibit more homogenous morphology, more uniform phase segregation, and consequently higher charge mobility.

Numerous studies have reported the use of halide perovskites as highly functional light-harvesting materials. The development of optimized compositions and deposition approaches has led to impressive improvements; however, no noticeable breakthrough in performance has been observed for these materials recently. Here, a breakthrough that enables the fabrication of vertically grown halide perovskite (VGHP) nanopillar photodetectors via a nanoimprinting crystallization technique is demonstrated. We used engraved nanopatterned polymer stamps to form VGHP nanopillars during the pressurized crystallization of the softly baked gel state of a methylammonium lead iodide (CH3NH3PbI3, denoted MAPI) film. The VGHP films exhibit much lower defect density and higher conductivity, as supported by current–voltage characteristic measurements and conductive atomic force microscopy measurements. Ultimately, two-terminal lateral photodetectors based on the VGHP nanopillar films show a greatly enhanced photoresponse compared ...

The effect of grain boundaries (GBs) especially twin boundaries (TBs) on the performance of polycrystalline CdTe thin film (1–2 μm in thickness) solar cells remains largely controversial. Understanding the GB’s microstructure and physical property and their impact on the photovoltaic efficiency is critically important to the optimization of the solar cell performance. In this work, we present a systematic study of GBs in CdTe thin film solar cells of different efficiencies in the range of 1.97–9.68%, aiming to elucidate a quantitative correlation of GBs especially TBs with the solar cell charge transport by integrating several advanced approaches including electron backscattering diffraction, transmission electron microscopy, conductive atomic force microscopy, and scanning Kelvin probe force microscopy. Importantly, we have confirmed {111} Σ3 TBs form in the PLD CdTe solar cells and are beneficial by providing an efficient channel for charge transport. The percentage of {111} Σ3 TBs was found to be corre...

Transparent conductive networks are important for flexible electronics and solar cells. Often interwire (junction) conductivity is the limiting factor for network conductivity and can be improved by various treatments. The conductivity of individual junctions in a single walled carbon nanotube network was measured by conductive atomic force microscopy before and after exposure to nitric acid. The measurements show that this exposure improves the conductivity of each one of the junctions within the network. Our results suggest that the acid improves the conductivity by p-type charge transfer doping and by surfactant degradation.

Sub-10 nm high-K gate dielectrics are of critical importance in two-dimensional transition metal dichalcogenides (TMDs) transistors. However, the chemical inertness of TMDs gives rise to a lot of pinholes in gate dielectrics, resulting in large gate leakage current. In this study, sub-10 nm, uniform and pinhole-free Al2O3 high-K gate dielectrics on MoS2 were achieved by atomic layer deposition without surface functionalization, in which an ultrathin Al2O3 layer prepared with a short purge time at a low temperature of 80 °C offers the nucleation cites for the deposition of the overlaying oxide at a higher temperature. Conductive atomic force microscopy reveals the significant suppression of gate leakage current in the sub-10 nm Al2O3 gate dielectrics with the low-temperature nucleation layer. Raman and X-ray photoelectron spectroscopies indicate that no oxidation occurred during the deposition of the low-temperature Al2O3 nucleation layer on MoS2. With the high-quality sub-10 nm Al2O3 high-K gate dielectrics, low hysteresis and subthreshold swing were demonstrated on the normally-off top-gated MoS2 transistors.

Non-volatile memories (NVMs) using nanocrystals (NCs) as active materials can be applied to soft electronic devices requiring a low-temperature process because NCs do not require a heat treatment process for crystallization. In addition, memory devices can be implemented simply by using a patterning technique using a solution process. In this study, a flexible NiO ReRAM device was fabricated using a simple NC patterning method that controls the capillary force and dewetting of a NiO NC solution at low temperature. The switching behavior of a NiO NC based memory was clearly observed by conductive atomic force microscopy (c-AFM).

Obstacles to the commercialization of carbon nanotubes (CNTs) in alternative energy and clean water applications include high production costs, aggregation problems, and poor water solubility. We report the use of Laponite nanoparticles to improve the aqueous phase fabrication of single-walled CNT (SWCNT) percolating networks. SWCNT/Laponite films were cast at varied Laponite and CNT ink concentrations to optimize polydispersity. Structural and morphological characterization identified isolated aggregates, coalesced aggregates, and interconnected networks. Conductive atomic force microscopy studies confirmed that the nanocomposite films maintained their electrical transport properties after heteromixing. Incorporating Laponite improved CNT aqueous dispersibility, prevented aggregation, and promoted continuous, homogenous film formation.

This paper proposes a simple non-toxic simultaneous selenization/sulfurization process to improve cell efficiency in Cu2ZnSn(S,Se)4 (CZTSSe) solar cells with the aim of modulating the S/(S+Se) ratio. Experiment results show that the S content in the thin film increases with the S/(S+Se) ratio, and the S content on the surface is higher than inside the device when S/(S+Se) ratio ≧ 0.55. Higher S content on the surface results in a V-shaped energy gap profile. The simultaneous selenization/sulfurization process was shown to inhibit the formation of an excessively thick interfacial MoSe2 /Mo(S,Se)x layer, while enhancing the uniformity of Cu, Zn and Sn distribution in the absorber layer. The Zn/Sn ratio of the CZTSSe absorber layer was shown to decrease with an increase in the S/(S+Se) ratio, thereby increasing the likelihood that secondary phases of SnSe2, CuSn(S,Se), ZnS, SnS and Cu2SnS3 will form in the absorber layer. Conductive atomic force microscopy (C-AFM) results revealed that secondary phases of SnSe2 and Cu2Sn(S,Se)3 on the surface of the film increased the number of leakage current pathways along the grains of secondary phases. The proposed simultaneous selenization/sulfurization method increases device efficiency from 5.2% (conventional non-toxic selenization) to 6.3% when S/(S+Se) = 0.55.

As the most emergent target for the construction of neuromorphic networks, the emulation of synaptic responses with a memristor on the device level has attracted much attention in both microelectronic industries and academic institutes. For this reason, a newly synthesized soluble push–pull type random conjugated polymer, that is, poly[{4,4′-(((9-H-fluorene-9,9-diyl)bis(4,1-phenylene))bis(oxy))diphthalonitrile}-(9,9-di-octylfluorene)-(macrocyclic triphenylamine)] (hereafter called PFFMT), was used to fabricate a memristor with a configuration of Al/PFFMT/ITO. The device shows interesting history-dependent memristive switching performance. The synaptic potentiation and depression, the transition from short-term plasticity to long-term plasticity, the relaxation time constant of the memory decay process, and the biological stimuli of the “learning–forgetting–relearning” processes associated to the human's learning/memory functions have been demonstrated. Upon being subjected to different voltages, the conductive nature of the as-prepared memristor has also been successfully revealed via the in situ conductive atomic force microscopy technique.