Conductive Atomic Force Microscopy Research Articles

Non-Volatile Resistive Switching in Nanoscaled Elemental Tellurium by Vapor Transport Deposition on Gold.

Two-dimensional (2D) materials are promising for resistive switching in neuromorphic and in-memory computing, as their atomic thickness substantially improve the energetic budget of the device and circuits. However, many 2D resistive switching materials struggle with complex growth methods or limited scalability. 2D tellurium exhibits striking characteristics such as simplicity in chemistry, structure, and synthesis making it suitable for various applications. This study reports the first memristor design based on nanoscaled tellurium synthesized by vapor transport deposition (VTD) at a temperature as low as 100°C fully compatible with back-end-of-line processing. The resistive switching behavior of tellurium nanosheets is studied by conductive atomic force microscopy, providing valuable insights into its memristive functionality, supported by microscale device measurements. Selecting gold as the substrate material enhances the memristive behavior of nanoscaled telluriumin terms of reduced values of set voltage and energy consumption. In addition, formation of conductive paths leading to resistive switching behavior on the gold substrate is driven by gold-tellurium interface reconfiguration during the VTD process as revealed by energy electron loss spectroscopy analysis. These findings reveal the potential of nanoscaled tellurium as a versatile and scalable material for neuromorphic computing and underscore the influential role of gold electrodes in enhancing its memristive performance.

Ultrathin natural biotite crystals as a dielectric layer for van der Waals heterostructure applications

Biotite, an iron-rich mineral belonging to the trioctahedral mica group, is a naturally abundant layered material (LM) exhibiting attractive electronic properties for application in nanodevices. Biotite stands out as a non-degradable LM under ambient conditions, featuring high-quality basal cleavage-a significant advantage for van der Waals heterostructure (vdWH) applications. In this work, we present the micro-mechanical exfoliation of biotite down to monolayers (1Ls), yielding ultrathin flakes with large areas and atomically flat surfaces. To identify and characterize the mineral, we conducted a multi-elemental analysis of biotite using energy-dispersive spectroscopy mapping. Additionally, synchrotron x-ray fluorescence and infrared nano-spectroscopy were employed to probe its iron content and vibrational signature in few-layer form, respectively, with sensitivity to the layer number. We have also observed good morphological and structural stability in time (up to 12 months) and no important changes in their physical properties after thermal annealing processes in ultrathin biotite flakes. Conductive atomic force microscopy evaluated its electrical capacity, revealing an electrical breakdown strength of approximately 1 V nm-1. Finally, we explore the use of biotite as a substrate and encapsulating LM in vdWH applications. We have performed optical and magneto-optical measurements at low temperatures. We find that ultrathin biotite flakes work as a good substrate for 1L-MoSe2, comparable to hexagonal boron nitride flakes, but it induces a small change of the 1L-MoSe2g-factor values, most likely due to natural impurities on its crystal structure. Furthermore, our results show that biotite flakes are useful systems to protect sensitive LMs such as black phosphorus from degradation for up to 60 days in ambient air. Our study introduces biotite as a promising, cost-effective LM for the advancement of future ultrathin nanotechnologies.

Engineering spin-dependent catalysts: chiral covalent organic frameworks with tunable electroactivity for electrochemical oxygen evolution

Abstract The chiral-induced spin selectivity (CISS) effect offers promising prospects for spintronics, yet designing chiral materials enabling efficient spin-polarized electron transport remains challenging. Here, we report the utility of covalent organic frameworks (COFs) in manipulating electron spin for spin-dependent catalysis via CISS. This facilitates us to design and synthesize three three-dimensional chiral COFs (CCOFs) with tunable electroactivity and spin-electron conductivity through imine condensations of enantiopure 1,1'-binaphthol-derived tetraaldehyde and tetraamines derived from 1,4-benzenediamine, pyrene, or tetrathiafulvalene skeletons. The CISS effect of CCOFs is verified by magnetic conductive atomic force microscopy. Compared with their achiral analogs, these CCOFs serve as efficient spin filters, reducing the overpotential of oxygen evolution and improving the Tafel slope. Particularly, the diarylamine-based CCOF showed a low overpotential of 430 mV (vs RHE) at 10 mA cm−2 with long-term stability comparable to the commercial RuO2. This enhanced spin-dependent OER activity stems from its excellent redox-activity, good electron conductivity and effective suppressed formation of H2O2 byproducts.

Investigation on Junction Contacts of Semiconducting Carbon Nanotube Networks Using Conductive Atomic Force Microscopy.

Semiconductor single-walled carbon nanotube (s-SWNT) networks have gained prominence in electronic devices due to their cost-effectiveness, relatively production-naturality, and satisfactory performance. Configuration, density, and resistance of SWNT-SWNT junctions are considered crucial factors influencing the overall conductivity of s-SWNT networks. In this study, we present a method for inferring the lower bounds of the SWNT-SWNT junction resistance in s-SWNT networks based on conductive atomic force microscopy TUNA images. This method further enables the proposal of a classification for SWNT-SWNT junctions based on the current behavior relative to their surroundings. The three types of SWNT-SWNT junctions are denoted as (i) true contact (T), (ii) poor contact (P), and (iii) false contact (F). Of them, the true and poor contacts, respectively, represent good and poor electrical contact for the subject SWNT-SWNT junctions whose electrical conductivity hardly improves under external tip pressure, while that of the false contact can be further improved by external pressure. Statistical analysis demonstrates that while T-type junctions make a significant contribution to network conductivity, their proportion accounts for only approximately 40%. The P-type and F-type junctions, which constitute over 60% of the total, may be a contributing factor that constrains the overall conductivity of the s-SWNT networks. The height ratio of the junction to the sum of two SWNTs was also observed to exhibit variations among the three types. Finally, we propose a three-dimensional model to elucidate the formation mechanism underlying each type of junction. The present study provides insights into the performance of spontaneous contacts between s-SWNTs in the networks, and the systematic image acquisition and junction classification processes may provide support for future advancements in these networks.

The Effect of Relative Humidity in Conductive Atomic Force Microscopy.

Conductive atomic force microscopy (CAFM) analyzes electronic phenomena in materials and devices with nanoscale lateral resolution, and it is widely used by companies, research institutions, and universities. Most data published in the field of CAFM is collected in air at a relative humidity (RH) of 30-60%. However, the effect of RH in CAFM remains unclear becauseprevious studies often made contradictory claims, plus the number of samples and locations tested is scarce. Moreover, previous studies on this topic didnot apply current limitations, which can degrade the CAFM tips and generate false data. This article systematically analyzes the effect of RH in CAFM by applying ramped voltage stresses at over 17,000 locations on ten different samples (insulating, semiconducting, and conducting) under seven different RH. An ultra-reliable setup with a 110-pA current limitation during electrical stresses is employed, andexcellent CAFM tip integrity after thousands of tests is demonstrated. It is found that higher RH results in increased currents due to the presence of a conductivewater meniscus at the tip/sample junction, which increases the effective area forelectron flow. This trend is observed in insulators and ultra-thin semiconductors; however, in thicker semiconductors the electron mean free path is high enough to mask this effect. Metallic samples show no dependence on RH. This study clarifies the effect of relative humidity in CAFM, enhances understanding of the technique, and teaches researchers how to improve the reliability of their studies in this field.

Scalable Layer-Controlled Oxidation of Bi2O2Se for Self-Rectifying Memristor Arrays With sub-pA Sneak Currents.

Smart memristors with innovative properties are crucial for the advancement of next-generation information storage and bioinspired neuromorphic computing. However, the presence of significant sneak currents in large-scale memristor arrays results in operational errors and heat accumulation, hindering their practical utility. This study successfully synthesizes a quasi-free-standing Bi2O2Se single-crystalline film and achieves layer-controlled oxidation by developing large-scale UV-assisted intercalative oxidation, resulting β-Bi2SeO5/Bi2O2Se heterostructures. The resulting β-Bi2SeO5/Bi2O2Se memristor demonstrates remarkable self-rectifying resistive switching performance (over 105 for ON/OFF and rectification ratios, as well as nonlinearity) in both nanoscale (through conductive atomic force microscopy) and microscale (through memristor array) regimes. Furthermore, the potential for scalable production of self-rectifying β-Bi2SeO5/Bi2O2Se memristor, achieving sub-pA sneak currents to minimize cross-talk effects in high-density memristor arrays is demonstrated. The memristors also exhibit ultrafast resistive switching (sub-100ns) and low power consumption (1.2 pJ) as characterized by pulse-mode testing. The findings suggest a synergetic effect of interfacial Schottky barriers and oxygen vacancy migration as the self-rectifying switching mechanism, elucidated through controllable β-Bi2SeO5 thickness modulation and theoretical ab initio calculations.

Study on the electrical performance degradation mechanism of β -Ga2O3 p-n diode under heavy ion radiation

The impact of heavy ion irradiation on the β-Ga2O3 p-n diode and its physical mechanism have been studied in this Letter. After the irradiation fluence of 1 × 108 cm−2, it is observed that the electrical performance of the device is significantly degraded. The forward current density (JF) is reduced by 49.4%, the reverse current density (JR) is increased by more than two orders of magnitude, and the breakdown voltage (VBR) is decreased by 30%. Based on the results of the deep-level transient spectroscopy measurement, it is concluded that acceptor-like traps generated with an energy level of EC-0.75 eV in the β-Ga2O3 drift layer dominate the JF degradation of the device, which are most likely related to Ga vacancies. These acceptor-like traps result in the reduction of change carrier concentration, which in turn leads to a decrease in JF. In addition, according to the conductive atomic force microscope measurements and theoretical calculation, it is clearly observed that the latent tracks induced by heavy ion irradiation can act as leakage paths, leading to a significant degradation of JR and VBR.

Unveiling the Nanoscale Dielectric Gap and Its Influence on Ferroelectric Polarization Switching in Scanning Probe Microscopy

AbstractThe dielectric gap between the scanning probe microscopy (SPM) tip and the surface of a ferroelectric using conductive atomic force microscopy and piezoresponse force microscopy (PFM) is investigated. While the gap functions as a dielectric layer, it also allows tunneling current to inject charges into the ferroelectric when a critical loading force between 10–20 µN is applied to a tip with a radius of 25 nm under a bias voltage of 0.5 V. It is observed that the permittivity of the dielectric gap determines the coercive voltage measured by the piezoresponse hysteresis loop. While such studies done in air often produce coercive voltages much larger than those studied for the same materials in capacitor‐based studies, the use of high permittivity media such as water (ɛr = 79) or silicone oil (ɛr = 2.1‐2.8) produces coercive fields that more closely match those measured in conventional capacitor‐based polarization hysteresis loop measurements. Furthermore, using water as a dielectric medium in PFM imaging enhances the accuracy in extracting the amplitude and phase data from periodically poled lithium niobate crystals. These findings provide insight into the nanoscale phenomena of polarization switching instigated by the SPM tip and provide a pathway to improved quantitative studies.

Tunneling and thermionic emission as charge transport mechanisms in W-based Schottky contacts on AlGaN/GaN heterostructures

This paper investigates the forward conduction mechanism of W-based Schottky diodes on AlGaN/GaN heterostructures across a temperature range of 25–150 °C. Current-Voltage measurements carried out at different temperatures (I-V-T), allow to identify two coexisting mechanisms for charge transport. At lower bias the conduction mechanism is ruled by tunneling (TU), with a characteristic energy of E00 = 75 meV extracted from the temperature dependence of the ideality factor. At higher bias the Thermionic Emission (TE) mechanism dominates, thus revealing the presence of an inhomogeneous barrier that increases from 0.77 to 0.94 eV with increasing the measurement temperature. An ideal barrier of 1.22 eV was extrapolated for a unitary ideality factor. Structural and electrical analyses performed at nanoscale level revealed the presence of a density of defects (dislocations) in the order of 4 × 109 cm−2. Conductive Atomic Force Microscopy (C-AFM) provided local electrical information, uncovering a significant correlation between the observed electrical characteristics and the nanoscale defect distribution. This detailed insight highlights the crucial role of the electrical characteristics of defects in influencing the tunneling current component at low bias, thereby providing valuable context for understanding the electrical behavior and performance of microscopic devices.

Laser engineered architectures for magnetic flux manipulation on superconducting Nb thin films

Custom shaped magnetic flux guiding channels have been fabricated on superconducting Nb thin films by laser nanopatterning of their surface. Preferential pathways are defined by suitable combination of imprinted anisotropic pinning domains through laser-induced periodic surface structures (LIPSS). Generated by the selective energy deposition of femtosecond UV laser pulses, quasi-parallel ripple structures are formed under optimized irradiation conditions. On average, each domain is formed by grooves with a lateral period of 260-270 nm and a depth about 80 nm. By combination of scanning and transmission electron microscopy, magneto-optical imaging, and conductive atomic force microscopy techniques, we conclude that the boundaries of the LIPSS-covered domains play a prominent role in the magnetic flux diversion process within the film. This is confirmed by dedicated modelling of the flux dynamics, combined with the inversion of the magneto-optical signal. The created metasurfaces enable control of the flux penetration process at the microscale.

Vertical Current Transport in Monolayer MoS<sub>2</sub> Heterojunctions with 4H-SiC Fabricated by Sulfurization of Ultra-Thin MoO<sub>x</sub> Films

In this paper, we report on the growth of highly uniform MoS2 films, mostly consisting of monolayers, on SiC surfaces with different doping levels (n- SiC epitaxy, ~1016 cm-3, and n+ SiC substrate, ~1019 cm-3) by sulfurization of a pre-deposited ultra-thin MoOx films. MoS2 layers are lowly strained (~0.12% tensile strain) and highly p-type doped (<Nh>≈4×1019 cm−3), due to MoO3 residues still present after the sulfurization process. Nanoscale resolution I-V analyses by conductive atomic force microscopy (C-AFM) show a strongly rectifying behavior for MoS2 junction with n- SiC, whereas the p+ MoS2/n+ SiC junction exhibits an enhanced reverse current and a negative differential behavior under forward bias. This latter observation, indicating the occurrence of band-to-band-tunneling from the occupied states of n+ SiC conduction band to the empty states of p+ MoS2 valence band, is a confirmation of the very sharp hetero-interface between the two materials. These results pave the way to the fabrication of ultra-fast switching Esaki diodes on 4H-SiC.

Overcoming Microstructural Defects at the Buried Interface of Formamidinium-Based Perovskite Solar Cells.

Since the advent of formamidinium (FA)-based perovskite photovoltaics (PVs), significant performance enhancements have been achieved. However, a critical challenge persists: the propensity for void formation in the perovskite film at the buried perovskite-interlayer interface has a deleterious effect on device performance. With most emerging perovskite PVs adopting the p-i-n architecture, the specific challenge lies at the perovskite-hole transport layer (HTL) interface, with previous strategies to overcome this limitation being limited to specific perovskite-HTL combinations; thus, the lack of universal approaches represents a bottleneck. Here, we present a novel strategy that overcomes the formation of such voids (microstructural defects) through a film treatment with methylammonium chloride (MACl). Specifically, our work introduces MACl via a sequential deposition method, having a profound impact on the microstructural defect density at the critical buried interface. Our technique is independent of both the HTL and the perovskite film thickness, highlighting the universal nature of this approach. By employing device photoluminescence measurements and conductive atomic force microscopy, we reveal that when present, such voids impede charge extraction, thereby diminishing device short-circuit current. Through comprehensive steady-state and transient photoluminescence spectroscopy analysis, we demonstrate that by implementing our MACl treatment to remedy these voids, devices with reduced defect states, suppressed nonradiative recombination, and extended carrier lifetimes of up to 2.3 μs can be prepared. Furthermore, our novel treatment reduces the stringent constraints around antisolvent choice and dripping time, significantly extending the processing window for the perovskite absorber layer and offering significantly greater flexibility for device fabrication.

Multi-step switchable superdomain architecture with enhanced photoelectrical performance in epitaxial ferroelectrics

Ferroic domains and relevant topological defects, such as domain walls and vortices, have gained significant attention as functional units for potential advancements in nanoelectronics. Pb(ZrxTi1-x)O3 (PZT) is a tetragonal ferroelectric material at room-temperature, exhibiting remarkable piezoelectricity and intricate domain structures. In this work, we explore the ferroelectric properties, photoelectric reactions, and efficient manipulation pathways of the unconventional superstructures in epitaxial (101)-oriented PZT thin films. Employing piezoresponse force microscopy (PFM) and conductive atomic force microscopy (cAFM), we unveil the three-dimensional polarization configurations of the superdomain structures inherently featuring conductive charged domain walls. Our findings reveal an increase in photoactivity at the head-side charged domain walls, attributed to the band-bending mechanism. Additionally, we discover the enhanced photoelectrochemical (PEC) performance in the superdomain structures compared to the (101)-oriented PZT films with conventional c/a domains. Furthermore, time-dependent pulse voltages are utilized to dynamically assess local currents and realize direct conductivity modulation by manipulating distinct polarization states. The elucidation of the photoelectrical mechanism and delineation of diverse pathways for intermediate state control underscore the potential of ferroelectric superdomains in constructing functional photoelectronic nanodevices.

Scanning Probe Microscopy of Halide Perovskite Solar Cells.

Scanning probe microscopy (SPM) has enabled significant new insights into the nanoscale and microscale properties of solar cell materials and underlying working principles of photovoltaic and optoelectronic technology. Various SPM modes, including atomic force microscopy, Kelvin probe force microscopy, conductive atomic force microscopy, piezoresponse force microscopy, and scanning near-field optical microscopy, can be used for the investigation of electrical, optical and chemical properties of associated functional materials. A large body of work has improved the understanding of solar cell device processing and synthesis in close synergy with SPM investigations in recent years. This review provides an overview of SPM measurement capabilities and attainable insight with a focus on recently widely investigated halide perovskite materials.

Photovoltaic Performance, Optical Current Loss Simulation, and Conductive Atomic Force Microscopy Analysis of Organic Solar Cells Based on PM6:Y7

Photovoltaic Performance, Optical Current Loss Simulation, and Conductive Atomic Force Microscopy Analysis of Organic Solar Cells Based on PM6:Y7

Chirality-Induced Magnetic Polarization by Charge Localization in a Chiral Supramolecular Crystal.

The chirality-induced spin selectivity (CISS) effect is a fascinating phenomenon that correlates the molecular structure with electron spin-polarization (SP). Experimental procedures to quantify the spin-filtering magnitude have extensively used magnetic-field-dependent conductive AFM. In this work chiral crystals of imide-substituted coronene bisimide ((S)-CBI-GCH) are studied to explain the dynamics of the current-voltage I - V spectra and the origin of superimposed peaks are investigated. A dynamic voltage-sweep rate-dependent phenomenon can give rise to complex I - V curves. The redox group, capable of localization of charge, acts as a localized state that interferes with the continuum of the π - π stacking, giving rise to Fano resonances. A novel mechanism for dynamic transport is introduced, which provides insight into the origin of spin-polarized charge in crystallized CBI-GCH molecules after absorption on a metallic substrate, guided by transient charge polarization. Crucially, interference between charge localization and delocalization during transport may be important properties in understanding the magnetochiral phenomena observed by electrostatic force microscopy. Finally, it is observed that charge trapping sensitively modifies the injection barrier from direct tunneling to Fowler-Nordheim tunneling transport supporting nonlinearity in CISS for this class of molecules.

Tribological manufacturing of ZDDP tribofilms functionalised by graphene nanoplatelets

Abstract 3D Tribo-Nanoprinting (3D TNP), which uses a highly controllable tribological contact to deposit tribofilms, has been proposed as a manufacturing method for nanoscale structures. Inspired by this, we show for the first time, as a proof of concept, the ability to electrically functionalise tribofilms for potential use in the manufacture of structures with nanoscale thickness. Zinc dialkyldithiophosphate (ZDDP) tribofilms have been generated to include varying concentrations of graphene nanoplatelets (GNPs) resulting in them becoming electrically conductive when tested using conductive atomic force microscopy. In its highest GNP concentration, approximately 55% of the surface of the tribofilm was able to sustain current up to a threshold of 245 pA. The higher graphene content led to a suppression in film formation and decreased substrate coverage. Transmission electron microscopy revealed a dual-layered tribofilm with a carbon-rich layer above a pure layer of ZDDP tribofilm. Within the carbon-rich layer, the GNPs formed into scrolls which created an internal network through which current could flow, being limited by the insulating pure ZDDP layer at the film-substrate interface, and the presence of surface graphene sheets. A modified lateral force microscopy procedure supported the presence of surface graphene sheets. Despite limited deposition precision in terms of homogeneity and distribution of the tribofilms, this work provides a step towards the use of 3D TNP for the manufacture of electronic structures on the nanoscale by proving that tribofilms can be functionalised by the addition of particle additives.

Benefits and Limitations of Encapsulating Platinum Electrocatalysts with Niobium Oxide

The development of energy infrastructure based on renewable sources is vital to reduce our reliance on fossil fuels. Technologies such as hydrogen fuel cells and water electrolyzers are able to bridge the gap between the intermittency issues associated with green energy sources such as wind and solar, and the demand for reliable electricity by being able to convert between electrical and chemical energy. Nanostructured and mesoporous platinum (Pt) prepared by electrodeposition or sputter coating have been shown to have increased performance in hydrogen fuel cells compared to the industry standard of Pt nanoparticles supported on carbon.1,2 Over the lifetime of a fuel cell Pt nanostructures and nanoparticles are, however, known to restructure and lose their surface area by mechanisms such as Ostwald ripening, dissolution, and coalescence. Previously, thin film Pt catalysts have been shown to be stabilized by a nanoscale film of niobium oxide.3 Herein, nanoscale thin films of niobium oxide are investigated as a stabilizing material through use by encapsulating nanostructured Pt catalysts.Nanostructured Pt catalysts were electrodeposited onto Pt-coated Si wafer substrates. Subsequently, niobium oxide films with thicknesses of 0.5 nm, 3 nm, and 4.5 nm were deposited onto the Pt catalyst using atomic layer deposition. X-ray fluorescence and conductive atomic force microscopy techniques were used to confirm the presence and coverage of the niobium oxide layer on the Pt catalyst. The performance of the encapsulated and un-encapsulated Pt catalysts as oxygen reduction reaction catalysts over the course of up to a thousand degradation cycles were evaluated using electrochemical techniques. The morphology of the catalyst before and after the degradation testing was also assessed using scanning electron microscopy techniques. Electrochemical impedance spectroscopy indicated that increasing the thickness of the niobium oxide layer increased the resistance of the catalyst towards the oxygen reduction reaction. The combined results of the analyses performed by electrochemical techniques and electron microscopy measurements revealed a range of film thicknesses for the niobium oxide that successfully stabilized the electrochemically active surface area and surface morphology of the Pt catalyst. Paul, M. T. Y.; Gates, B. D. Mesoporous Platinum Prepared by Electrodeposition for Ultralow Loading Proton Exchange Membrane Fuel Cells. Sci Rep 2019, 9, 4161.Debe, M. K. Tutorial on the Fundamental Characteristics and Practical Properties of Nanostructured Thin Film (NSTF) Catalysts. Electrochem. Soc. 2013, 160, F522.Eastcott, J.; Gates, B. D. Nanoscale Thin Films of Niobium Oxide on Platinum Surfaces: Creating a Platform for Optimizing Material Composition and Electrochemical Stability. Can. J. Chem. 2017, 96, 260–266.

Electrically Conductive Peptide Fibers as Direct Electron Transfer Scaffolds for Enzymatic Electrocatalysis

Biology provides many sources of inspiration for synthetic and multi-functional nanomaterials. Naturally evolved proteins possess a range of unique functions and self-assembly behavior. Some of these have electron transport functions as part of metabolic processes. Their flexible design and potential for stimuli responsiveness make proteins ideal building blocks of bioelectronic interfaces. In biosensor and electrocatalysis applications, bioelectronic materials are designed to interconvert biological signals or processes into electronic signals and vice versa. In this work, we designed a peptide fiber structure that exhibits stimuli-responsive self-assembly and the capacity to transport electrical current over micrometer long distances. Hierarchically ordered nanofibers of α-helical coiled-coil peptides were assembled using a pH responsive structure ordering motif comprising of two neighboring lysine residues. Cryo-EM structures of these assemblies reveal a novel organization of α-helical peptides in a cross coiled coil arrangement that exhibits electrical conductivity. Both solid-state and solution-based electrochemical characterization show that these fibers are conductive. Single-fiber conductive AFM determined that the solid-state electrical conductivity is on a similar order of magnitude with conductive cytochrome filaments from bacteria. Solution deposited fiber films approximately doubled the electroactive surface area of the electrode, confirming their conductivity in aqueous environments. Furthermore, these fibers can be used as scaffolds for redox enzyme immobilization for electrochemical biosensing. Progress towards immobilized enzymatic devices will be discussed as well. This work further expands the field of stimuli-responsive and multifunctional nanomaterials by using peptide assemblies as structural supports for future implementation in bioelectronic and bioelectrochemical systems.

Revealing the Properties of Electrically Driven Optical Antennas via Conductive Atomic Force Microscope.

Light emission from ultracompact electrically driven optical antennas (EDOAs) has garnered significant attention due to its terahertz modulation bandwidth. Typically, the EDOAs are fixed and nonadjustable once fabricated, thus hindering the attempts to investigate the influence of structural geometry on light emission properties. Here, we propose and demonstrate that the EDOAs can be constructed by carefully manipulating the gold-coated tips of atomic force microscopy operated in conductive mode into contact with the optical antennas covered by insulating film, where the position of the tunnel junction on the antenna surface can be controlled with high accuracy and flexibility. Taking gold nanorod antennas covered by HfO2 film as an example, we found that the highest light generation efficiency is obtained when the tunnel junction is located at the shoulder edge of the nanorod antenna, where the bonding dipolar surface plasmon mode in the junction is spectrally and spatially coupled with the longitudinal radiation mode of the EDOAs. Besides, position variation of the tunnel junction on the nanorod surface also strongly influences the far-field radiation angular distribution and emission spectrum. Numerical simulations are in good agreement with the experimental results. Our findings offer fundamental insights into the influence of structural parameters on the light emission performance of EDOAs, thus leading to better design of EDOAs with high light generation efficiency and powerful functionality.