Scanning Probe Microscopy Tip Research Articles

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.

Reversible Mechanical Switching of Ferroelastic Stripe Domains in Multiferroic Thin Films.

The application potential of ferroelectric thin films largely relies on the controllability of their domain structure. Among the various proposed strategies, mechanical switching is being considered as a potential alternative to replace electrical switching for control of the domain structure of ferroelectric thin films via, e.g., the flexoelectric effect. So far, studies on mechanical switching are confined to out-of-plane polarization switching in ferroelectric thin films, which are in pristine or prepoled single-domain states. In this work, we report reversible in-plane mechanical switching of the monoclinic phase (MC phase) stripe domains in BiFeO3 thin films can be realized by scanning tip force. Via controlling the fast scan direction of the scanning probe microscopy tip and the magnitude of the tip force, the effective trailing field induced by the local tip force can be rotated to consequently switch the net in-plane polarization of the two-variant stripe domain patterns by either 90° or 180°. Moreover, the monoclinic to rhombohedral (MC-R) phase transition occurs during mechanical switching with the distribution of R-phase domains dependent on the switching paths. These results extend our current understanding of the mechanical switching behavior in ferroelectric thin films and should be instructive for their future applications.

The odd-number cyclo[13]carbon and its dimer, cyclo[26]carbon

Molecular rings of N carbon atoms (cyclo[ N ]carbons, or C N ) are excellent benchmarking systems for testing quantum chemical theoretical methods and valuable precursors to other carbon-rich materials. Odd- N cyclocarbons, which have been elusive to date, are predicted to be even less stable than even- N cyclocarbons. We report the on-surface synthesis of cyclo[13]carbon, C 13 , by manipulation of decachlorofluorene with a scanning probe microscope tip. We elucidated the properties of C 13 by experiment and theoretical modeling. C 13 adopts an open-shell configuration with a triplet ground state and a kinked geometry, which shows different extents of distortion and carbene localization depending on the molecular environment. Moreover, we prepared and characterized the C 13 dimer, cyclo[26]carbon, demonstrating the potential of cyclocarbons and their precursors as building blocks for carbon allotropes.

A scanning probe microscope compatible with quantum sensing at ambient conditions.

We designed and built up a new type of ambient scanning probe microscope (SPM), which is fully compatible with state-of-the-art quantum sensing technology based on the nitrogen-vacancy (NV) centers in diamond. We chose a qPlus-type tuning fork (Q up to ∼4400) as the current/force sensor of SPM for its high stiffness and stability under various environments, which yields atomic resolution under scanning tunneling microscopy mode and 1.2-nm resolution under atomic force microscopy mode. The tip of SPM can be used to directly image the topography of nanoscale targets on diamond surfaces for quantum sensing and to manipulate the electrostatic environment of NV centers to enhance their sensitivity up to a single proton spin. In addition, we also demonstrated scanning magnetometry and electrometry with a spatial resolution of ∼20nm. Our new system not only paves the way for integrating atomic/molecular-scale color-center qubits onto SPM tips to produce quantum tips but also provides the possibility of fabricating color-center qubits with nanoscale or atomic precision.

Defining ferroelectric characteristics with reversible piezoresponse: PUND switching spectroscopy PFM characterization

Detecting ferroelectricity at micro- and nanoscales is crucial for advanced nanomaterials and materials with complicated topography. Switching spectroscopy piezoresponse force microscopy (SSPFM), which involves measuring piezoelectric hysteresis loops via a scanning probe microscopy tip, is a widely accepted approach to characterize polarization reversal at the local scale and confirm ferroelectricity. However, the local hysteresis loops acquired through this method often exhibit unpredictable shapes, a phenomenon often attributed to the influence of parasitic factors such as electrostatic forces and current flow. Our research has uncovered that the deviation in hysteresis loop shapes can be caused by spontaneous backswitching occurring after polarization reversal. Moreover, we’ve determined that the extent of this effect can be exacerbated when employing inappropriate SSPFM waveform parameters, including duration, frequency, and AC voltage amplitude. Notably, the conventional ‘pulse-mode’ SSPFM method has been found to intensify spontaneous backswitching. In response to these challenges, we have redesigned SSPFM approach by introducing the positive up-negative down (PUND) method within the ‘step-mode’ SSPFM. This modification allows for effective probing of local piezoelectric hysteresis loops in ferroelectrics with reversible piezoresponse while removing undesirable electrostatic contribution. This advancement extends the applicability of the technique to a diverse range of ferroelectrics, including semiconductor ferroelectrics and relaxors, promising a more reliable and accurate characterization of their properties.

Self-organized formation of domain arrays in TGS crystal by moving SPM tip

In this paper, we studied the formation of self-organized domain arrays created by moving biased tip of scanning probe microscope (SPM) in deuterated TGS crystals. The shape of created domains depends significantly on scanning direction and applied voltage. For scanning along c axis, the domain shape drastically changed from arrays to stripe domains at 150[Formula: see text]V. Scanning perpendicular to [Formula: see text] axis led to formation of the array of the dashed domains. Increasing of the dashed domain length leads to change of domain shape from arrays of dashed domains to solid stripe domain. The obtained effect has been considered in terms of the kinetic approach as a result of formation of comb-like domain with charged domain walls in the bulk due to repetitive appearance of the domain spikes. The spontaneous backswitching under the action of the depolarization field leads to the fast growth of the tooth to the surface and results in the transformation of the domain shape. The computer simulation of the nonuniform motion of charged domain wall under the action of depolarization field has been done. The obtained results demonstrate the essential role of screening processes and pave the way for further improvement of domain engineering methods.

Concept for the Real-Time Monitoring of Molecular Configurations during Manipulation with a Scanning Probe Microscope.

A bold vision in nanofabrication is the assembly of functional molecular structures using a scanning probe microscope (SPM). This approach requires continuous monitoring of the molecular configuration during manipulation. Until now, this has been impossible because the SPM tip cannot simultaneously act as an actuator and an imaging probe. Here, we implement configuration monitoring using experimental data other than images collected during the manipulation process. We model the manipulation as a partially observable Markov decision process (POMDP) and approximate the actual configuration in real time using a particle filter. To achieve this, the models underlying the POMDP are precomputed and organized in the form of a finite-state automaton, allowing the use of complex atomistic simulations. We exemplify the configuration monitoring process and reveal structural motifs behind measured force gradients. The proposed methodology marks an important step toward the piece-by-piece creation of supramolecular structures in a robotic and possibly automated manner.

Controlled Vertical Transfer of Individual Au Atoms Using a Surface Supported Carbon Radical for Atomically Precise Manufacturing

To explore a proof-of-concept for atomically precise manufacturing (APM) using scanning probe microscopy (SPM), first principle theoretical calculations of atom-by-atom transfer from the apex of an SPM tip to an individual radical on a surface-bound organic molecule have been performed. Atom transfer is achieved by spatially controlled motion of a gold terminated tip to the radical. Two molecular tools for SPM-based APM have been designed and investigated, each comprising an adamantane core, a radical end group, and trithiol linkers to enable strong chemisorption on the Au(111) surface: ethynyl-adamantane-trithiol and adamantyl-trithiol. We demonstrate the details of controlled Au atom abstraction during tip approach toward and retraction from the radical species. Upon approach of the tip, the apical Au atom undergoes a transfer toward the carbon radical at a clearly defined threshold separation. This atomic displacement is accompanied by a net energy gain of the system in the range −0.5 to −1.5 eV, depending on the radical structure. In the case of a triangular pyramidal apex model, two tip configurations are possible after the tip atom displacement: (1) an Au atom is abstracted from the tip and bound to the C radical, not bound to the tip base anymore, and (2) apical tip atoms rearrange to form a continuous neck between the tip and radical. In the second case, subsequent tip retraction leads to the same final configuration as the first, with the abstracted Au atom bound to radical carbon atom of the molecular tool. For the less reactive adamantyl-trithiol radical molecular tool, Au atom transfer is less energetically favored, but this has the advantage of avoiding other apex gold atoms from rearrangement.

Electrostructural and morphological features of etch pits in boron-doped HPHT-diamond single crystals and multisectoral plates

Morphological, structural, and nano-electrical features of the growth defects and sectoral boundaries revealed by selective etching in synthetic boron-doped single crystals of diamond (BDD) and multisectoral plates are characterized by a set of scanning probe microscopy (SPM) methods, confocal micro-Raman, and micro-FTIR spectroscopy. Diamond single crystals were grown under high pressure and high temperature conditions (HPHT) in the FeAlBC system. Morphology of dislocation etch pits and micro-defect related pits/protrusions compared both on the growth facets and multisectoral plates cut parallel to the {110} facet of BDD single crystals. Ex situ AFM observation of the etched surface revealed non-homogeneously distributed pits and protrusions over the growth facets correlated with irregular macroscopic growth defects. There is a center-to-periphery gradient of dislocation density in multisectoral plates. The protrusions are homogeneously distributed over plates; some often decorate dislocation pits. Micro-Raman mapping on dislocation pits (boron-rich regions) decorated by low-boron content regions revealed crystalline imperfections in three dimensions, with apparent compressive/tensile strain. The defects were mainly electrically neutral under surface potential mapping by Kelvin probe force microscopy, opposite to the inter-sectoral boundaries having pronounced surface potential steps. The scanning spreading resistance microscopy shows resistivity variations at etching-reviled surface defects and sectoral boundaries if the appropriate DC bias is applied to the SPM tip.

Spatially-Resolved Study of the Electronic Transport and Resistive Switching in Polycrystalline Bismuth Ferrite

Ferroelectric materials attract much attention for applications in resistive memory devices due to the large current difference between insulating and conductive states and the ability of carefully controlling electronic transport via the polarization set-up. Bismuth ferrite films are of special interest due to the combination of high spontaneous polarization and antiferromagnetism, implying the possibility to provide multiple physical mechanisms for data storage and operations. Macroscopic conductivity measurements are often hampered to unambiguously characterize the electric transport, because of the strong influence of the diverse material microstructure. Here, we studied the electronic transport and resistive switching phenomena in polycrystalline bismuth ferrite using advanced conductive atomic force microscopy (CAFM) at different temperatures and electric fields. The new approach to the CAFM spectroscopy and corresponding data analysis are proposed, which allow deep insight into the material band structure at high lateral resolution. Contrary to many studies via macroscopic methods, postulating electromigration of the oxygen vacancies, we demonstrate resistive switching in bismuth ferrite to be caused by the pure electronic processes of trapping/releasing electrons and injection of the electrons by the scanning probe microscopy tip. The electronic transport was shown to be comprehensively described by the combination of the space charge limited current model, while a Schottky barrier at the interface is less important due to the presence of the built-in subsurface charge.

Highly Anisotropic Tip-Induced Domain Growth in Polydomain Triglycine Sulfate

Ferroelectrics are widely used as modern functional materials for nonlinear optics, optoelectronics, nonvolatile data storage, and nanoelectronics. Most of these applications require creation of precise micron-scale domain patterns. We have studied the current limited wall motion regime driven by anisotropic bulk screening processes during local switching in polydomain triglycine sulfate single crystals. The field application using a conductive tip of a scanning probe microscope (SPM) in a dry atmosphere resulted in formation of a partially switched area, strongly elongated in the c crystallographic direction. At the same time, switching at high humidity led to a circular shape of the area. The calculation of the spatial distribution of the external field demonstrated the possibility of domain wall shift at distances up to hundreds of microns from the SPM tip. However, after external field switch-off the depolarization field in nonscreened areas led to reverse domain wall motion. Thus, the domain wall stabilization is possible only in the regions with sufficient screening of the depolarization field. We have shown that the growth of the area size with time obeys the same current-limited wall motion expressions that were derived previously for domain growth during local switching by a highly resistive electrode. The change in the area shape with humidity was attributed to the change in the governing screening mechanism from anisotropic bulk conductivity to isotropic surface conductivity through an adsorbed water layer. The obtained results demonstrate the essential role of the screening processes during periodical poling and pave the way for further improvement of the domain engineering methods.

Tip-induced domain growth on the non-polar cut of lithium niobate with various stoichiometry deviations

Appearance of wedge-like domains during local switching by a biased tip of a scanning probe microscope (SPM) was studied in non-polar cut plates of lithium niobate (LN) with the spatial distribution of the deviation of Li concentration from stoichiometry composition (ΔcLi) created by vapor transport equilibration. The voltage dependences of the domain sizes were measured in the areas of LN plates with various values of ΔcLi. It was shown that the domain length increased linearly with the voltage, while the domain base width demonstrated a square root voltage dependence. The obtained dependence of the base width was attributed to the spatial distribution of the polar component of the external field produced by a biased SPM tip. The obtained results were considered in terms of the kinetic approach to domain growth. The growth of wedge-like domains was attributed to step generation and kink motion. The velocities of the base growth and kink motion were defined by the excess of the local value of superposition of the polar components of an external field produced by a biased tip, depolarization field, and screening fields over the threshold values. The average length of the elementary steps at the domain walls revealed from the domain length to width ratio demonstrated the square root voltage dependence. It was revealed that the domain length and the width of the base inversely depended on ΔcLi. The obtained dependences of the domain growth parameters on the composition allow for improving the periodical poling technique used for the fabrication of the nonlinear optical devices.

Analysis of flexochemical effect and its application in scanning probe microscopy

Electrochemical processes in solids are affected by the properties of various interfaces, where the flexoelectric effect manifests itself considerably due to the inevitable strong gradient fields. Thus, it is crucial to study the coupling between the electrochemical process and the flexoelectric effect. Based on the continuum theory, we conducted the finite element implementation for the flexochemical effect, being the coupling between flexoelectricity, Vegard effect and chemical reactions. Then, the developed method is employed to investigate the flexochemical effect arising in scanning probe microscopy (SPM), including evaluating the contributions from the flexoelectric effect and Vegard effect to the electromechanical response on material SrTiO3 (STO) in piezoresponse force microscopy (PFM) as well as to mechanical redistribution of oxygen vacancy in STO. It is found that at room temperature the nanoscale electromechanical response of the undoped STO in PFM imaging is mainly induced by the converse flexoelectricity while the contribution of direct Vegard effect is negligible. Furthermore, the contact force exerted by SPM tip in manipulating the redistribution of oxygen vacancies is multifunctional, including diminishing vacancies underneath the contact area and enriching the regions around the tip-surface contact edge and inside the sample below the tip, resulting from the synergy of the converse Vegard effect and the direct flexoelectricity. These analyses explain some experimental observations well. This paper provides a continuum framework for the analysis of electrochemomechanical systems with the flexoelectric effect.

Discrete switching in the ion sliced lithium niobate thin films with thick dielectric layer

Domain growth and shape evolution during local switching under the action of biased conductive tip of scanning probe microscope were studied in Z + cut lithium niobate on insulator (LNOI) with 1 µm-thick SiO2 layer between LN film and grounded electrode. Piezoresponse force microscopy and confocal Raman microscopy were used for domain imaging. The domain shape evolution during switching was qualitatively reconstructed. The crucial role of discrete switching on the kinetics of domain growth was revealed. The stability of the switched domains was confirmed. The obtained knowledge will be used for creation of the periodical domain structures by local switching.

Tip-Enhanced Photoluminescence of Freestanding Lateral Heterobubbles.

Two-dimensional (2D) semiconducting materials have promising applications in flexible optoelectronics, nanophotonics, and sensing based on the broad tunability of their optical and electronic properties. 2D nanobubbles form exciton funnels due to localized strain that can be used as local emitters for information processing. Their nanoscale optical characterization requires the use of near-field scanning probe microscopy (SPM). However, previous near-field studies of 2D materials were performed on SiO2/Si and metallic substrates using the plasmonic gap mode to increase the signal-to-noise ratio. Another challenge is the deterministic control of bubble size and location. We addressed these challenges by investigating the photoluminescence (PL) signals of freestanding monolayer lateral WSe2-MoSe2 heterostructures under the influence of strain exerted by a plasmonic SPM tip. For first time, we performed tip-enhanced PL imaging of freestanding 2D materials and studied the competition between the PL enhancement mechanisms by nanoindentation as a function of the tip-sample distance. We observed the tunability of PL as a function of bubble size, which opens new possibilities to design optoelectronic nanodevices.

Response and Implication of NASICON Solid-State Electrolytes to Local Electrical Stimulation: From Surface Engineering to Interfacial Manipulation.

The surface feature of solid electrolytes fundamentally governs their own physical properties and significantly affects the interaction with the electrode materials. The evaluation of interfacial contact between the electrolyte and the metallic anode is largely relied on the macroscopic contact angle measurement, which is influenced by the intrinsic wettability and the microstructure of the electrolyte. In this work, the surface chemistry of the solid electrolyte is first regulated via facile thermal treatments. Then, scanning probe microscopy (SPM)-based techniques are comprehensively adopted to study the interaction between the electrolyte and metallic anode at the nanoscale. By manipulating the overpotential applied on the SPM tip, the mobile sodium ions at the subsurface of the solid electrolyte can be extracted toward the surface, and the eventual topography of the products is deliberately correlated with the sodium wettability. In this context, the impact of surface treatment on the sodium wettability of the surface layer is systematically evaluated based on the topographic evolution at the nanoscale. Furthermore, the local electrochemical reaction dynamics is revealed by correlating the surface ionic activity and current-voltage (I-V) curves. This work presents a new methodology to effectively evaluate the sodium wettability of the solid electrolyte, and these findings can provide meaningful implications to the surface engineering of ceramic electrolytes for high-performance solid-state batteries.

"May the Force Be with You!" Force-Volume Mapping with Atomic Force Microscopy.

Information of the chemical, mechanical, and electrical properties of materials can be obtained using force volume mapping (FVM), a measurement mode of scanning probe microscopy (SPM). Protocols have been developed with FVM for a broad range of materials, including polymers, organic films, inorganic materials, and biological samples. Multiple force measurements are acquired with the FVM mode within a defined 3D volume of the sample to map interactions (i.e., chemical, electrical, or physical) between the probe and the sample. Forces of adhesion, elasticity, stiffness, deformation, chemical binding interactions, viscoelasticity, and electrical properties have all been mapped at the nanoscale with FVM. Subsequently, force maps can be correlated with features of topographic images for identifying certain chemical groups presented at a sample interface. The SPM tip can be coated to investigate-specific reactions; for example, biological interactions can be probed when the tip is coated with biomolecules such as for recognition of ligand–receptor pairs or antigen–antibody interactions. This review highlights the versatility and diverse measurement protocols that have emerged for studies applying FVM for the analysis of material properties at the nanoscale.

Ab inito exploration of nanocars as potential corrosion inhibitors

With the intent to search for new unexplored potential inhibitors – a series of newly synthesized NanoCars possessing a permanent dipole moment that makes them able for a controlled surface movement (by electric field gradient from a scanning probe microscopy tip) is ab initio explored as corrosion inhibitors. The adsorption of the NanoCars onto Fe (110) surface was assessed using Density Functional Theory (DFT), Monte Carlo simulation (MC), Molecular Dynamics simulation (MD). The acquired results offered molecular level details about the adsorption ability, adsorption centers, geometry, and adsorption energetics of NanoCars onto the Fe(110) interface.

Automated Tip Conditioning for Scanning Tunneling Spectroscopy

Scanning tunneling spectroscopy (STS), a technique that records the change in the tunneling current as a function of the bias (dI/dV) across the gap between a tip and the sample, is a powerful tool to characterize the electronic structure of single molecules and nanomaterials. While performing STS, the structure and condition of the scanning probe microscopy (SPM) tips are critical for reliably obtaining high quality point spectra. Here, we present an automated program based on machine learning models that can identify the Au(111) Shockley surface state in dI/dV point spectra and perform tip conditioning on clean or sparsely covered gold surfaces with minimal user intervention. We employed a straightforward height-based segmentation algorithm to analyze STM topographic images to identify tip conditioning positions and used 1789 archived dI/dV spectra to train machine learning models that can ascertain the condition of the tip by evaluating the quality of the spectroscopic data. Decision tree based ensemble and boosting models and deep neural networks (DNNs) have been shown to reliably identify tips in suitable conditions for STS. We expect the automated program to reduce operational costs and time, increase reproducibility in surface science studies, and accelerate the discovery and characterization of novel nanomaterials by STM. The strategies presented in this paper can readily be adapted to STM tip conditioning on a wide variety of other common substrates.

Tip-induced domain protrusion in ferroelectric films with in-plane polarization

Charge manipulation and fabrication of stable domain patterns in ferroelectric materials by scanning probe microscopy open up broad avenues for the development of tunable electronics. Harnessing the polarization energy and electrostatic forces with specific geometry of the system enables producing the nanoscale domains by-design. Along with that, domain engineering requires mastery of underlying physical mechanisms that govern the domain formation. Here, we present a theoretical description of the domain formation by a scanning probe microscopy tip in a ferroelectric film with strong in-plane anisotropy of polarization. We demonstrate that local charge injection produces wedge-shaped domains that propagate along the anisotropy axis, whereas the tip-written lines of charge generate a comb-like domain structure. The results of our calculations agree with earlier experimental observations and allow for the optimization of the targeted domain structures.