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Amorphous metal oxides with analog resistive switching functions (i.e., continuous controllability of the electrical resistance) are gaining emerging interest due to their neuromorphic functionalities promising for energy efficient electronics. The mechanisms are currently attributed to field-driven migration of the constituent ions, but the applications are being hindered by the limited understanding of the physical mechanisms due to the difficulty in analyzing the causal ion migration, which occurs on a nanometer or even atomic scale. Here, the direct electrical transport measurement of analog resistive switching and angstro''m scale imaging of the causal ion migration is demonstrated in amorphous TaOx (a-TaOx) by conductive atomic force microscopy. Atomically flat thin films of a-TaOx, which is a practical material for commercial resistive random access memory, are fabricated in this study, and the mechanisms of the three known types of analog resistive switching phenomena (current-dependent set, voltage-dependent reset, and time-dependent switching) are directly visualized on the surfaces. The observations indicate that highly analog type of resistive switching can be induced in a-TaOx by inducing the continuous redox reactions for 2.0 < x < 2.5, which are characteristic of a-TaOx. The measurements also demonstrate drastic control of the switching stochasticity, which is attributable to controlled segregation of a metastable a-TaO2 phase. The findings provide direct clues for tuning the analog resistive switching characteristics of amorphous metal oxides and developing new functions for future neuromorphic computing.

Functional nanomaterials possess exceptional multi-physical (e.g., mechanical, electrical and optical) properties compared with their bulk counterparts. To facilitate both synthesis and device applications of these nanomaterials, it is highly desired to characterize their multi-physical properties with high accuracy and efficiency. The nanomanipulation techniques under scanning electron microscopy (SEM) has enabled the testing of mechanical and electrical properties of various nanomaterials. However, the seamless integration of mechanical, electrical, and optical testing techniques into an SEM for triple-field-coupled characterization of single nanostructures is still unexplored. In this work, we report the first SEM-based nanomanipulation system for high-resolution mechano-optoelectronic testing of single semiconductor InGaN/GaN nanowires (NWs). A custom-made optical measurement setup was integrated onto a four-probe nanomanipulator inside an SEM, with two optical microfibers actuated by the nanoma-nipulator for NW excitation and emission measurement. A conductive tungsten nanoprobe and a conductive atomic force microscopy (AFM) cantilever probe were integrated onto the nanomanipulator for electrical nanoprobing of single NWs for electroluminescence (EL) measurement. The AFM probe also served as a force sensor for quantifying the contact force applied to the NW during nanoprobing. Using this unique system, we examined, for the first time, the effect of mechanical compression applied to an InGaN/GaN NW on its optoelectronic properties.

Recently, there is a surge of research interest in configurable ferroelectric conductive domain walls which have been considered as possible fundamental building blocks for future electronic devices. In this work, by using piezoresponse force microscopy and conductive atomic force microscopy, we demonstrated the controlled manipulation of various conductive domain walls in epitaxial BiFeO3 thin films, e.g. neutral domain walls (NDW) and charged domain walls (CDWs). More interestingly, a specific type of nanoscale domains was also identified, which are surrounded by highly conductive circular CWDs. Similar nanoscale domains can also be controlled created and erasured by applying local field via conductive probe, which allow nondestructive current readout of different domain states with a large on/off resistance ratio up to 102. The results indicate the potential to design and develop high-density non-volatile ferroelectric memories by utilizing these programable conductive nanoscale domain walls.

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.

The conversion efficiency of energy-harvesting devices can be increased by utilizing hot-carriers (HCs). However, due to ultrafast carrier-carrier scattering and the lack of carrier injection dynamics, HC-based devices have low efficiencies. In the present work, we report the effective utilization of HCs at the nanoscale and their transfer dynamics from a non-noble metal to a metal oxide interface by means of real-space photocurrent mapping by using local probe techniques and conducting femtosecond transient absorption (TA) measurements. The photocurrent maps obtained under white light unambiguously show that the HCs are injected into the metal oxide layer from the TiN layer, as also confirmed by conductive atomic force microscopy. In addition, the increased photocurrent in the bilayer structure indicates the injection of HCs from both layers due to the broadband absorption efficiency of TiN layer, passivation of the surface states by the top TiN layer, and smaller barrier height of the interfaces. Furthermore, electrostatic force microscopy and Kelvin probe force microscopy provide direct evidence of charge injection from TiN to the MoOx film at the nanoscale. The TA absorption spectra show a strong photo-bleaching signal over wide spectral range and ultrafast decaying behavior at the picosecond time scale, which indicate efficient electron transfer from TiN to MoOx. Thus, our simple and effective approach can facilitate HC collection under white light, thereby achieving high conversion efficiency for optoelectronic devices.

The characteristics of FAMAPbI3-based write-once-read-many (WORM) devices were controlled by a cation-exchange process as part of a technique to alter the FA composition in FAMAPbI3 films. Interest...

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.

Chiral Pb(Sn)-I hybrid organic-inorganic perovskites exhibit outstanding chiral-induced spin selectivity (CISS) performance, but the nontoxic lead-free hybrid materials with high stability are still greatly desired for spin filtering in spintronic applications. We synthesize chiral hybrid copper halides (R/S-MBA)2 CuX4 (MBA=methylbenzylammonium; X=Cl, Br) with characteristic 0D CuX4 tetrahedral structural motifs, combining the low toxicity of Cu2+ and air stability of halide ions (Cl- and Br- ). Despite similar structural and electronic features, (R/S-MBA)2 CuBr4 shows much smaller chiroptical activity than the chloride counterpart. Magnetically conductive atomic force microscopy measurements display a typical spin-polarized charge-transport property with high efficiency up to 90 % for both copper halides. Our work expands the CISS effect into eco-friendly and stable metal-organic halides, which is promising for applications in spintronics based on transition-metal hybrid systems.

Atomic force microscopy techniques (conductive-AFM, I-V spectroscopy and PFM) were used for characterisation of the local electrical properties of bilayer polyaniline-polystyrene/P(VDF-TrFE) polymer nanocomposite. Observed hysteresis of current-voltage characteristics confirms its memristive properties. It was caused by the influence of the ferroelectric polarization of P(VDF-TrFE) layer, the domain structure of which was visualised by piezoelectric force microscopy on the transport of charge carriers at the interface.

Composite solid electrolytes (CSEs) which formed a flexible interface with electrodes are considered as promising electrolytes candidates for all-solid-state batteries (ASSBs). However, the role of inorganic particles and migration of Li-ions still need to be investigated. Herein, LLZO-PEO composite electrolyte is prepared with different weight ratios (0, 50, 75 wt. %) of LLZO and in-situ conductive atomic force microscopy (c-AFM) is employed to observe the changes in electrolyte topography and mechanical properties at different temperatures. Furthermore, c-AFM provides novel insights into the inhomogeneous migration of ions and electrons within the electrolyte. At high temperatures, Li-ions migrate along the amorphous PEO interphase of 0 and 50 wt. % LLZO-PEO electrolytes. With the increase of LLZO content (75 wt. %), LLZO particles form a continuous ionic network and Li-ions migrate through the LLZO particles. At low temperatures, Li-ions can only migrate along the amorphous PEO. Also, the addition of LLZO inhibits the migration of electrons and improves the insulative properties of the composite electrolyte. The current work provides novel insights into the design and development of composite electrolytes for next-generation ASSBs.

Dislocations often exhibit unique physical properties distinct from those of the bulk material. However, functional applications of dislocations are challenging due to difficulties in the construction of high-performance devices of dislocations. Here we demonstrate unidirectional single-dislocation Schottky diode arrays in a Fe2O3 thin film on Nb-doped SrTiO3 substrates. Conductivity measurements using conductive atomic force microscopy indicate that a net current will flow through individual dislocation Schottky diodes under forward bias and disappear under reverse bias. Under cyclic bias voltages, the single-dislocation Schottky diodes exhibit a distinct resistive switching behavior containing low-resistance and high-resistance states with a high resistance ratio of ∼103. A combined study of transmission electron microscopy and first-principles calculations reveals that the Fe2O3 dislocations comprise mixed Fe2+ and Fe3+ ions due to O deficiency and exhibit a one-dimensional electrical conductivity. The single-dislocation Schottky diodes may find applications for developing ultrahigh-density electronic and memory devices.

When scaling the lateral size of a ferroelectric random access memory (FeRAM) device down to the nanometer range, the polarization switching-induced displacement current becomes small and challenging to detect, which greatly limits the storage density of FeRAM. Here, we report the observation of significantly enhanced injection currents, much larger than typical switching currents, induced by polarization switching in BiFeO3 thin films via conductive atomic force microscopy. Interestingly, this injected current can be effectively modulated by applying mechanical force. As the loading force increases from ∼50 to ∼750 nN, the magnitude of the injected current increases and the critical voltage to trigger the current injection decreases. Notably, changing the loading force by an order of magnitude increases the peak current by 2-3 orders of magnitude. The mechanically boosted injected current could be useful for the development of high-density FeRAM devices. The mechanical modulation of the injected current may be attributed to the mechanical force-induced changes in the barrier height and interfacial layer width.

Moire superlattices (MSLs) formed in van der Waals materials have become a promising platform to realize novel two-dimensional electronic states. Angle-aligned trilayer structures can form two sets of MSLs which could potentially interfere. In this work, we directly image the moire patterns in both monolayer and twisted bilayer graphene aligned on hexagonal boron nitride (hBN), using combined scanning microwave impedance microscopy and conductive atomic force microscopy. Correlation of the two techniques reveals the contrast mechanism for the achieved ultrahigh spatial resolution (

Electron transport through iron porphyrin (FeP) molecules self-assembled on a gold (Au) substrate was investigated using conductive atomic force microscopy (AFM) to measure current–voltage (I–V) ch...

Protein-based electronic biomaterials represent an attractive alternative to traditional metallic and semiconductor materials due to their environmentally benign production and purification. However, major challenges hindering further development of these materials include (1) limitations associated with processing proteins in organic solvents and (2) difficulties in forming higher-order structures or scaffolds with multilength scale control. This paper addresses both challenges, resulting in the formation of one-dimensional bundles composed of electrically conductive protein nanowires harvested from the microbes Geobacter sulfurreducens and Escherichia coli. Processing these bionanowires from common organic solvents, such as hexane, cyclohexane, and DMF, enabled the production of multilength scale structures composed of distinctly visible pili. Transmission electron microscopy revealed striking images of bundled protein nanowires up to 10 μm in length and with widths ranging from 50-500 nm (representing assembly of tens to hundreds of nanowires). Conductive atomic force microscopy confirmed the presence of an appreciable nanowire conductivity in their bundled state. These results greatly expand the possibilities for fabricating a diverse array of protein nanowire-based electronic device architectures.

We investigate hole-selective passivating contacts that consist of an interfacial layer of silicon oxide (SiOx) and a layer of boron-doped SiCx(p). The fabrication process of these contacts involves an annealing step at temperatures above 750 degrees C which crystallizes the initially amorphous layer and diffuses dopants across the interfacial oxide into the wafer to facilitate charge transport, but it can also disrupt the SiOx layer necessary for wafer-surface passivation. To investigate the transport mechanism of the charge carriers through the selective contact and its changes during the annealing process, we utilize various characterization methods, such as transmission electron microscopy, micro Raman spectroscopy, and conductive atomic force microscopy. Combining the latter with a sequential removal of material, we assemble a tomographic reconstruction of the crystallized layer that reveals the presence of preferential vertical transport channels.

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.

Resistive switching (RS) device behavior is highly dependent on both insulator and electrode material properties. In particular, the bottom electrode (BE) surface morphology can strongly affect RS characteristics. In this work, Ru films with different thicknesses grown on a TiN layer by radical-enhanced atomic layer deposition (REALD) are used as an inert BE in TaOx-based RS structures. The REALD Ru surface roughness is found to increase by more than 1 order of magnitude with the increase in the reaction cycle number. Simultaneously, a wide range of RS parameters, such as switching voltage, resistance both in low and high resistance states, endurance, and so forth, monotonically change. A simplified model is proposed to explain the linkage between RS properties and roughness of the Ru surface. The field distribution was simulated based on the observed surface morphologies, and the resulting conducting filament formation was anticipated based on the local field enhancement. Conductive atomic force microscopy confirmed the theoretical expectations.

We demonstrate the influence of surface terminated V-pits in tuning dark current and spectral responsivity of Al0.25Ga0.75N-based UV-B photodetectors with metal–semiconductor–metal geometry on Si (111) substrate. We show that the V-pit morphological defects contribute to a large internal gain in these photodetectors, thereby leading to a substantial enhancement in external quantum efficiency (EQE) at relatively low applied biases. For photodetectors fabricated on metal organic chemical vapor deposition grown Al0.25Ga0.75N epilayers with a surface pit density of $2 \times 10^{8}$ cm−2, an EQE of 100% was measured at a meager bias of 1.7 V, which increased significantly with bias. The EQE, photo-to-dark current ratio, and UV-to-visible rejection ratio measured $5 \times 10^{4}$ %, $1.2 \times 10^{4}$ , and $2 \times 10^{3}$ , respectively, at 5 V. The evidence of localized enhancement of photoresponse at the surface terminations of V-pits is exemplified by UV-assisted conductive atomic force microscopy. Temperature-dependent carrier transport analysis under dark and UV illumination revealed cumulative contributions of pit-induced thermionic field emission and hole-trapping-induced gain to the observed large EQE. This work presents the highest value of responsivity for III-nitride UV-B detectors at a given bias.

In this work, the local current-voltage characteristics of nanocomposite phase boundaries in the form of a poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) (70/30) matrix filled with conductive polyaniline (PANI) particles are studied by conductive atomic force microscopy. At negative external bias voltages, these dependencies are shown to have an interval with a negative differential resistance. This fact is caused by the switching of ferroelectric polarization of individual nanocrystallites PVDF-TrFE located near the phase boundaries in the amorphous matrix volume. The obtained results confirm assumptions about the memristive properties of the investigated nanocomposites.

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.

Using conductive atomic force microscopy, current images and I – V characteristics of tin oxide thin films deposited at various substrate temperatures were obtained. When analyzing current images and I – V characteristics, it was found that tin oxide films formed at a higher substrate temperature consist of the SnO2 phase, however, they include local areas with increased resistance that contain the SnO phase.

Defect states in a semiconductor are known to affect its optoelectronic properties and control its chemical composition. In this paper, based on conductive atomic force microscopy (cAFM) and electrostatic force microscopy (EFM) measurements, we demonstrate charge trapping at grain boundaries of ZnO:SnO2 (ZTO: zinc tin oxide) films, indicating the existence of local band-bending. Electrostatic force microscopy measurements reveal that charge trapping is more at grain boundaries in comparison to grains. On the other hand, cAFM and Kelvin probe force microscopy (KPFM) measurements provide an experimental existence of local band-bending at grain boundaries of ZTO films. In addition, compositional analysis of the films confirms the presence of oxygen vacancies, resulting in the formation of defect states within the band gap which further control the charge transport. The present observations are explained in the basis of defect-dependent potential barrier formation due to oxygen vacancies in the film. The present findings help to understand nanoscale charge transport in ZTO thin films using a combination of local probe techniques which should be useful in fabricating ZTO-based transparent optoelectronic devices.

Halide perovskite (HP) materials are actively researched for resistive switching (RS) memory devices due to their current-voltage hysteresis along with low-temperature processability, superior charge mobility, and simple fabrication. In this study, all-inorganic RbPbI3 perovskite has been doped with Cl in the halide site and incorporated as a switching media in the Ag/RbPbI3-xClx/ITO structure, since pure RbPbI3 is nonswitchable. Five compositions of the RbPbI3-xClx (x = 0, 0.3, 0.6, 0.9, and 1.2) films are fabricated, and the conductivity was found to be increasing upon increase in Cl concentration, as revealed by dielectric and I-V measurements. The device with a 20% chloride-substituted film exhibits a higher on/off ratio, extended endurance, long retention, and high-density storage ability. Finally, a plausible explanation of the switching mechanism from iodine vacancy-mediated growth of conducting filaments (CFs) is provided using conductive atomic force microscopy (c-AFM). The c-AFM measurements reveal that pure RbPbI3 is insulating in nature, whereas Cl-doped films demonstrate resistive switching behavior.

International audience; Understanding the surface properties of hybrid perovskite materials is a key aspect to improve not only the interface properties in photovoltaic cells but also the stability against moisture degradation. In this work, we study the local electronic properties of two series of CH 3 NH 3 PbI 3 perovskite films by atomic force microscopybased methods. We correlate nanoscale features such as the local surface potential (as measured by Kelvin probe force microscopy) to the current response (as measured by conductive atomic force microscopy). CH 3 NH 3 PbI 3 perovskites made using lead acetate as a precursor result in films with high purity and crystallinity and also result in heterogeneous local electrical properties, attributed to variations in the density of surface states. In contrast, when using lead iodide as a precursor, the perovskite surface exhibits a uniform distribution of surface states. This work also aims to understand the early stages of water-induced degradation at the surface of those films. Through high-precision exposure to small amounts of water vapor, we observe a higher stability for surfaces prepared with lead iodide precursors. More importantly, each precursor-based fabrication route is associated with either nor p-type behavior of the films. These characteristics are determined by the type of surface states, which also and eventually preside over stability. This work should help discriminate between perovskite synthesis routes and improve their stability in photovoltaic cell applications.

CZTS is a compound semiconductor made from elements which are plainly available and nonpoisonous having favorable optoelectronic properties for thin film solar cell (TFSC) applications. In this study, Cu-poor CZTS thin film was fabricated on soda lime glass (SLG)/Mo-deposited substrate using cosputtering followed by post sulfurization in H2S atmosphere. Local electrical transport study was carried out by using conductive atomic force microscopy (C-AFM) for small bias voltage (100 mV). Here we observed that most of the dark current (Idark) flow through grain boundaries (GBs) than grain interiors. The positive high current about 3.4 nA and sharp C-AFM signal at the GBs, dips to the zero (0) value at the grain interior. Local surface potential (Vsurface) study was carried out using kelvin probe force microscopy (KPFM), which showed that the positive Vsurface potential about 175 mV in the vicinity of GBs in a Cu-poor CZTS sample. On the basis of these results we inferred a potential landscape (VL) around the GBs. All result shows that due to variation in elemental composition which creates Cu-deficit or CuZn anti site defects at GBs, leads reduced effective band gap (Eeff) than the bulk towards grain inner to GBs.[-2pt]

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.

The correlation between threading dislocations and leakage current in a CdZnTe(001)/GaAs epilayer grown by close-spaced sublimation was studied using scanning electron microscopy, Kelvin probe force microscopy, conductive atomic force microscopy (C-AFM) and current–voltage (I–V) systems. The electrostatic potential differences between the charged dislocations and the CZT epitaxial film were found to be lower than those in bulk CZT crystal. C-AFM tests in the CZT epitaxial film under forward bias demonstrated that dislocations guided the high current leakage paths. The leakage current at dislocation sites was found to be two orders higher than that at dislocation-free areas with an applied bias of 5 V. The rapid increase in leakage current under applied bias in the range of 3.5–7.5 V was associated with the Poole–Frenkel effect, where high leakage current arises from the high density of trapped carriers escaping from the traps segregated around dislocations.

Van der Waals layered materials, such as transition metal dichalcogenides (TMDs), are an exciting class of materials with weak interlayer bonding, which enables one to create so-called van der Waals heterostructures (vdWH). One promising attribute of vdWH is the ability to rotate the layers at arbitrary azimuthal angles relative to one another. Recent work has shown that control of the twist angle between layers can have a dramatic effect on TMD vdWH properties, but the twist angle has been treated solely through the use of rigid-lattice moire patterns. No atomic reconstruction, that is, any rearrangement of atoms within the individual layers, has been reported experimentally to date. Here, we demonstrate that vdWH of MoSe2/WSe2 and MoS2/WS2 at twist angles ≤1° undergo significant atomic level reconstruction leading to discrete commensurate domains divided by narrow domain walls, rather than a smoothly varying rigid-lattice moire pattern as has been assumed in prior experimental work. Using conductive atomic force microscopy (CAFM), we show that TMD vdWH at small twist angles exhibit large domains of constant conductivity. The domains in samples with R-type stacking are triangular, whereas the domains in samples with H-type stacking are hexagonal. Transmission electron microscopy provides additional evidence of atomic reconstruction in MoSe2/WSe2 structures and demonstrates the transition between a rigid-lattice moire pattern for large angles and atomic reconstruction for small angles. We use density functional theory to calculate the band structures of the commensurate reconstructed domains and find that the modulation of the relative electronic band edges is consistent with the CAFM results and photoluminescence spectra. The presence of atomic reconstruction in TMD heterostructures and the observed impact on nanometer-scale electronic properties provide fundamental insight into the behavior of this important class of heterostructures.

The origin of the increased efficiency of perovskite solar cells by controlling environmental humidity was investigated using conductive atomic force microscopy (C-AFM) and friction force microscop...

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.

We report the synthesis of high-quality single monolayer MoS2 samples using a novel technique that utilizes direct liquid injection (DLI) for the delivery of precursors. The DLI system vaporizes a liquid consisting of a selected precursor dissolved in a solvent into small, micron-sized droplets in an expansion chamber maintained at a selected temperature and pressure, before delivery to the deposition chamber. We demonstrate the synthesis of monolayer MoS2 on SiO2/Si substrates using the DLI technique with film quality superior to exfoliated samples or those grown by traditional tube furnace chemical vapor deposition (CVD) methods. Photoluminescence measurements of DLI monolayers exhibit consistently brighter emission, narrower line width, and higher emission energy than their exfoliated and CVD counterparts. Conductive atomic force microscopy identifies a defect density of 8.3 × 1011/cm2 in DLI MoS2, lower than the measured density in CVD material and nearly an order of magnitude improvement over the exfoliated MoS2 investigated under the same conditions. The DLI method is directly applicable to many other van der Waals materials, which require the use of challenging low vapor pressure precursors, to the growth of alloys, and sequential growths of dissimilar materials leading to van der Waals heterostructures.

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.

Organosilane monolayers are part of many process flows in nanoelectronics and biotechnology because of their versatility. Monolayers that inhibit reactions on silicon/silicon oxide surfaces are nee...

Integration of yttrium iron garnet (YIG) with Si can bring new avenues for garnet-based complementary metal-oxide semiconductor compatible devices. Herein, investigations on the morphological, electrical, and magnetic properties of postgrowth annealed YIG thin films grown on Si substrate were systematically performed at nanoscale lateral resolution, employing the variants of atomic force microscopy (AFM) viz., conductive AFM (C-AFM), kelvin probe force microscopy, and magnetic force microscopy (MFM). Scanning electron microscopy analysis has revealed the formation of three different regions with varied crystallinity defined as center, dark, and surrounding matrix. Interestingly, AFM topography has not shown any surface variation of the obtained three different regions. Despite no variation in their surface topography, notable changes occur in their local conductivity, surface potential, and microstructure of their magnetic domains. C-AFM studies have shown the formation of conducting channels with three different resistivity regions. The tunneling current was enhanced nearly 50 times from the dark region (~1 pA) to the center region (~50 pA). MFM image analysis reveals the formation of two different magnetically active domains in the form of circles (-0.3°) distributed in a surrounding matrix (+0.3°) with a steep change in their magnetic phase degree. The formation of circular magnetic domains with highly distinguishable regions has suggested the potential of YIG/Si films for magnetic memory application. This work has shed light on the prospective of YIG/Si films for resistive and magnetic memory applications and fundamental aspects of growth of YIG on nongarnet substrates.

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.

Charged domain walls in ferroelectrics hold great promise for the design of novel electronic devices due to their enhanced local conductivity. In fact, charged domain walls show unique properties including the possibility of being created, moved and erased by an applied voltage. Here, we demonstrate that the charged domain walls are constituted by a core region where most of the screening charge is localized and such charge accumulation is responsible for their enhanced conductivity. In particular, the link between the local structural distortions and charge screening phenomena in 109° tail-to-tail domain walls of BiFeO3 is elucidated by a series of multiscale analysis performed by means of scanning probe techniques, including conductive atomic force microscopy (cAFM) and atomic resolution differential phase contrast scanning transmission electron microscopy (DPC-STEM). The results prove that an accumulation of oxygen vacancies occurs at the tail-to-tail domain walls as the leading charge screening process. This work constitutes a new insight in understanding the behavior of such complex systems and lays down the fundaments for their implementation into novel nanoelectronic devices.

Photoelectric resistive random access memory (RRAM) is a promising optoelectronic technology that is being developed to overcome the Von Neumann bottleneck and improve the computing and access performance of current computer systems. In this work, an ultra-stable broadband photoelectric tunable RRAM device based on PbS quantum dots (QDs) and poly(methyl methacrylate) (PMMA) hybrid materials was demonstrated. The device exhibits a long retention capability (>104 s), a high ON/OFF current ratio (104), fast response time (170 ns), cycle-to-cycle consistency, impressive environmental stability (>90 days) and flexibility. Multilevel data storage can be achieved through the appropriate setting of a series of compliance currents. In addition, the SET voltage of the device shows a broadband tunability from the ultraviolet (405 nm) to the near infrared (1177 nm). Furthermore, conductive atomic force microscopy (CAFM) measurements confirm that the formation and rupture of Ag conducting filaments are responsible for the resistive switching behavior. This study paves the way toward the development of next-generation high-density data storage technology and broadband photoelectric memory and computer technologies.

We report the effects of the composition on the stretchability and conductivity of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) polymer blends with soft polymers including poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA), and poly(methacrylic acid) (PMAA) and their application in pressure sensors. The composition–stretchability relationship was investigated with the stress–strain curves of the bulk films. Among these polymer blends, PAA-based blends outperformed in stretchability, attributed to the low glass transition temperature and ductile nature of PAA. And the composition–conductivity relationship was studied with conductive atomic force microscopy, Gaussian simulation and infrared spectroscopy. The PAA-based blends presented well-dispersed morphology for PEDOT and the strongest hydrogen bonding network between PAA and PEDOT:PSS was created through the blending, which gave rise to stable and high conductivity. Furthermore, the conductivity of the PEDOT:PSS/PAA blend at 20/80 wt ratio was largely increased (13 to 125 S cm−1) and stretchability (elongation at break) increased from 20 to 40% through methanol treatment. Finally, PAA-based blends treated with methanol were applied to the pressure sensor device, achieving a very high sensitivity of 39.90 kPa−1 at 20% tensile strain, a quick response time of around 49 ms, and a low detection limit of about 27.4 Pa. This study suggested a novel and facile approach to manipulate the structure and stretchability of the PEDOT:PSS polymer blending system for stretchable electronics.

Currents play critical roles in neurons. Direct observation of current flows in cells at nanometre dimensions and picoampere current resolution is still a daunting task. In this study, we investigated the current flows in hippocampal neurons, PC12 cells and astrocytes in response to voltages applied to the cell membranes using conductive atomic force microscopy (CAFM). The spines in the hippocampal neurons play crucial roles in nerve signal transfer. When the applied voltage was greater than 7.2 V, PC12 cells even show metallic nanowire-like characteristics. Both the cell body and glial filaments of astrocytes yielded CAFM test results that reflect different electrical conductance. To our best knowledge, the electrical characteristics and current transport through components of cells (especially neurons) in response to an applied external voltage have been revealed for the first time at nanometre dimensions and picoampere current levels. We believe that such studies will pave new ways to study and model the electrical characteristics and physiological behaviours in cells and other biological samples.

This paper presents multiple-modes Scanning Probe Microscopy (SPM) studies on characterize the correlation of resistance switching (RS) and polarization rotation (PR) in copper doped ZnO (ZnO:Cu) thin films. Firstly, the bipolar RS behavior is confirmed by conductive Atomic Force Microscopy (c-AFM). The PR with almost 180° phase angle is confirmed by using the Piezoresponse Force Microscopy (PFM) on the same location. In addition, it elucidates that obvious PR behavior can be observed in the sample with increasing Cu concentration by combining Kelvin Probe Force Microscopy (KPFM). Furthermore, it is found that the region with downward polarization has low resistance state (LRS), whereas the region with upward polarization has high resistance state (HRS). Moreover, the Piezoresponse Force Spectroscopy (PFS) and Switching Spectroscopy PFM (SS-PFM) measurements further confirm that the existence of the built-in voltage, Vbuilt-in is largest in the ZnO:Cu (8 at.%) film deposited at the oxygen partial pressure of 2 × 10−4 Torr. The schematic diagrams of energy band diagram with varied built-in field, Ebuilt-in, polarization directions and redistributed charges are presented to explain the correlation between RS and PR behavior. Keywords: Resistive switching, Polarization rotation, Built-in voltage, ZnO:Cu thin film, Next generation memory

In this work, the influences of thermal annealing and chemical passivation on the optical and electrical properties of ultraviolet light-emitting-diode (UV-LED) were investigated. The electroluminescence (EL) intensities of the LEDs under KOH treatment and thermal annealing increased by 48% and 81%, respectively compared to as-fabricated LED under current level of 10 mA. Cathodoluminescence (CL) mapping of UV-LEDs confirmed no variation of the density of the non-radiative recombination centers after surface treatments, and no obvious change in surface morphology was identified due to lacking of energy for surface atom migration. However, Raman spectroscopy indicates a relaxation of compressive strains inside the thin film after both thermal and chemical treatments, and conductive atomic force microscopy (c-AFM) also illustrated reduced leakage current after KOH passivation, which are responsible for the improved luminescence properties of UV-LEDs.

3D nanoprinting via focused electron beam induced deposition (FEBID) is applied for fabrication of all-metal nanoprobes for atomic force microscopy (AFM)-based electrical operation modes. The 3D tip concept is based on a hollow-cone (HC) design, with all-metal material properties and apex radii in the sub-10 nm regime to allow for high-resolution imaging during morphological imaging, conductive AFM (CAFM) and electrostatic force microscopy (EFM). The study starts with design aspects to motivate the proposed HC architecture, followed by detailed fabrication characterization to identify and optimize FEBID process parameters. To arrive at desired material properties, e-beam assisted purification in low-pressure water atmospheres was applied at room temperature, which enabled the removal of carbon impurities from as-deposited structures. The microstructure of final HCs was analyzed via scanning transmission electron microscopy—high-angle annular dark field (STEM-HAADF), whereas electrical and mechanical properties were investigated in situ using micromanipulators. Finally, AFM/EFM/CAFM measurements were performed in comparison to non-functional, high-resolution tips and commercially available electric probes. In essence, we demonstrate that the proposed all-metal HCs provide the resolution capabilities of the former, with the electric conductivity of the latter onboard, combining both assets in one design.

The origin of nitrogen-incorporated boron-doped nanocrystalline diamond (NB-NCD) nanowires as a function of substrate temperature (Ts) in H2/CH4/B2H6/N2 reactant gases is systematically addressed. Because of Ts, there is a drastic modification in the dimensional structure and microstructure and hence in the several properties of the NB-NCD films. The NB-NCD films grown at low Ts (400 °C) contain faceted diamond grains. The morphology changes to nanosized diamond grains for NB-NCD films grown at 550 °C (or 700 °C). Interestingly, the NB-NCD films grown at 850 °C possess one-dimensional nanowire-like morphological grains. These nanowire-like NB-NCD films possess the co-existence of the sp3-diamond phase and the sp2-graphitic phase, where diamond nanowires are surrounded by sp2-graphitic phases at grain boundaries. The optical emission spectroscopy studies stated that the CN, BH, and C2 species in the plasma are the main factors for the origin of nanowire-like conducting diamond grains and the materialization of graphitic phases at the grain boundaries. Moreover, conductive atomic force microscopy studies reveal that the NB-NCD films grown at 850 °C show a large number of emission sites from the grains and the grain boundaries. While boron doping improved the electrical conductivity of the NCD grains, the nitrogen incorporation eased the generation of graphitic phases at the grain boundaries that afford conducting channels for the electrons, thus achieving a high electrical conductivity for the NB-NCD films grown at 850 °C. The microplasma devices using these nanowire-like NB-NCD films as cathodes display superior plasma illumination properties with a threshold field of 3300 V/μm and plasma current density of 1.04 mA/cm2 with a supplied voltage of 520 V and a lifetime stability of 520 min. The outstanding plasma illumination characteristics of these conducting nanowire-like NB-NCD films make them appropriate as cathodes and pave the way for the utilization of these materials in various microplasma device applications.