Scanning Probe Tip Research Articles

Nanoscale Adhesion and Material Transfer at 2D MoS2-MoS2 Interfaces Elucidated by In Situ Transmission Electron Microscopy and Atomistic Simulations.

Low-dimensional materials, such as MoS2, hold promise for use in a host of emerging applications, including flexible, wearable sensors due to their unique electrical, thermal, optical, mechanical, and tribological properties. The implementation of such devices requires an understanding of adhesive phenomena at the interfaces between these materials. Here, we describe combined nanoscale in situ transmission electron microscopy (TEM)/atomic force microscopy (AFM) experiments and simulations measuring the work of adhesion (Wadh) between self-mated contacts of ultrathin nominally amorphous and nanocrystalline MoS2 films deposited on Si scanning probe tips. A customized TEM/AFM nanoindenter permitted high-resolution imaging and force measurements in situ. The Wadh values for nanocrystalline and nominally amorphous MoS2 were 604 ± 323 mJ/m2 and 932 ± 647 mJ/m2, respectively, significantly higher than previously reported values for mechanically exfoliated MoS2 single crystals. Closely matched molecular dynamics (MD) simulations show that these high values can be explained by bonding between the opposing surfaces at defects such as grain boundaries. Simulations show that as grain size decreases, the number of bonds formed, the Wadh and its variability all increase, further supporting that interfacial covalent bond formation causes high adhesion. In some cases, sliding between delaminated MoS2 flakes during separation is observed, which further increases the Wadh and the range of adhesive interaction. These results indicate that for low adhesion, the MoS2 grains should be large relative to the contact area to limit the opportunity for bonding, whereas small grains may be beneficial, where high adhesion is needed to prevent device delamination in flexible systems.

Electric polarization near vortices in the extended Kitaev model

We formulate a Majorana mean-field theory for the extended JKΓ Kitaev model in a magnetic Zeeman field of arbitrary direction, and apply it for studying spatially inhomogeneous states harboring vortices. This mean-field theory is exact in the pure Kitaev limit and captures the essential physics throughout the Kitaev spin liquid phase. We determine the charge profile around vortices and the corresponding quadrupole tensor. The quadrupole-quadrupole interaction between distant vortices is shown to be either repulsive or attractive, depending on parameters. We predict that electrically biased scanning probe tips enable the creation of vortices at preselected positions. Our results paves the way for the electric manipulation of Ising anyons in Kitaev spin liquids.

Thermal nanoconversion of ferromagnetic nanoislands

In this work, we investigate the use of post-fabrication thermal nanoconversion (TNC), using a heated scanning probe tip, to modify the magnetic properties of Ni80Fe20 elliptical nanoislands with varying aspect ratio. Despite Ni80Fe20 being unoptimized for TNC, by comparing quasistatic and dynamic magneto-optical Kerr effect microscopy measurements, we demonstrate that TNC at a contact temperature of 250 °C increases the saturation magnetization of the treated nanoislands, reaching a value close to 800 kA/m. Micromagnetic simulations of the nanoislands indicate that the TNC technique can be used to alter the remanent state, from a single domain to a vortex. These results demonstrate the opportunities afforded by TNC to modify the properties of selected areas in a thin film or a patterned sample, particularly when designing magnonic crystals and other nanomagnetic devices.

Microfabricated double-tilt apparatus for transmission electron microscope imaging of atomic force microscope probe.

Atomic force microscopy (AFM) uses a scanning stylus to directly measure the surface characteristics of a sample. Since AFM relies on nanoscale interaction between the probe and the sample, the resolution of AFM-based measurement is critically dependent on the geometry of the scanning probe tip. This geometry, therefore, can limit the development of related applications. However, AFM itself cannot be effectively used to characterize AFM probe geometry, leading researchers to rely on indirect estimates based on force measurement results. Previous reports have described sample jigs that enable the observation of AFM probe tips using Transmission Electron Microscopy (TEM). However, such setups are too tall to allow sample tilting within more modern high-resolution TEM systems, which can only tilt samples less than a few millimeters in thickness. This makes it impossible to observe atomic-scale crystallographic lattice fringes by aligning the imaging angle perfectly or to view a flat probe tip profile exactly from the side. We have developed an apparatus that can hold an AFM tip for TEM observation while remaining thin enough for tilting, thereby enabling atomic-scale tip characterization. Using this technique, we demonstrated consistent observation of AFM tip crystal structures using tilting in TEM and found that the radii of curvature of nominally identical probes taken from a single box varied widely from 1.4nm for the sharpest to 50nm for the most blunt.

Automated Scanning Probe Tip State Classification without Machine Learning.

The manual identification and in situ correction of the state of the scanning probe tip is one of the most time-consuming and tedious processes in atomic-resolution scanning probe microscopy. This is due to the random nature of the probe tip on the atomic level, and the requirement for a human operator to compare the probe quality via manual inspection of the topographical images after any change in the probe. Previous attempts to automate the classification of the scanning probe state have focused on the use of machine learning techniques, but the training of these models relies on large, labeled data sets for each surface being studied. These data sets are extremely time-consuming to create and are not always available, especially when considering a new substrate or adsorbate system. In this paper, we show that the problem of tip classification from a topographical image can be solved by using only a single image of the surface along with a small amount of prior knowledge of the appearance of the system in question with a method utilizing template matching (TM). We find that by using these TM methods, comparable accuracy and precision can be achieved to values obtained with the use of machine learning. We demonstrate the efficacy of this technique by training a machine learning-based classifier and comparing the classifications with the TM classifier for two prototypical silicon-based surfaces. We also apply the TM classifier to a number of other systems where supervised machine learning-based training was not possible due to the nature of the training data sets. Finally, the applicability of the TM method to surfaces used in the literature, which have been classified using machine learning-based methods, is considered.

Identification of weak molecular absorption in single-wavelength s-SNOM images.

Scattering-type scanning near-field optical microscopy (s-SNOM) allows for nanoscale optical mapping of manifold material properties. It is based on interferometric recording of the light scattered at a scanning probe tip. For dielectric samples such as biological materials or polymers, the near-field amplitude and phase signals of the scattered field reveal the local reflectivity and absorption, respectively. Importantly, absorption in s-SNOM imaging corresponds to a positive phase contrast relative to a non-absorbing reference sample. Here, we describe that in certain conditions (weakly or non- absorbing material placed on a highly reflective substrate), a slight negative phase contrast may be observed, which can hinder the recognition of materials exhibiting a weak infrared absorption. We first document this effect and explore its origin using representative test samples. We then demonstrate straightforward simple correction methods that remove the negative phase contrast and that allow for the identification of weak absorption contrasts.

Toward augmenting tip-enhanced nanoscopy with optically resolved scanning probe tips

A thorough understanding of biological species and emerging nanomaterials requires, among other efforts, their in-depth characterization through optical techniques capable of nanoresolution. Nanoscopy techniques based on tip-enhanced optical effects have gained tremendous interest over the past years, given their potential to obtain optical information with resolutions limited only by the size of a sharp probe interacting with focused light, irrespective of the illumination wavelength. Although their popularity and number of applications is rising, tip-enhanced nanoscopy (TEN) techniques still largely rely on probes that are not specifically developed for such applications, but for atomic force microscopy. This limits their potential in many regards, e.g., in terms of signal-to-noise ratio, attainable image quality, or extent of applications. We take the first steps toward next-generation TEN by demonstrating the fabrication and modeling of specialized TEN probes with known optical properties. The proposed framework is highly flexible and can be easily adjusted to be used with diverse TEN techniques, building on various concepts and phenomena, significantly augmenting their function. Probes with known optical properties could potentially enable faster and more accurate imaging via different routes, such as direct signal enhancement or facile and ultrafast optical signal modulation. We consider that the reported development can pave the way for a vast number of novel TEN imaging protocols and applications, given the many advantages that it offers.

Intramolecular Force Mapping at Room Temperature.

Acquisition of dense, three-dimensional, force fields with intramolecular resolution via noncontact atomic force microscopy (NC-AFM) has yielded enormous progress in our ability to characterize molecular and two-dimensional materials at the atomic scale. To date, intramolecular force mapping has been performed exclusively at cryogenic temperatures, due to the stability afforded by low temperature operation, and as the carbon monoxide functionalization of the metallic scanning probe tip, normally required for submolecular resolution, is only stable at low temperature. In this paper we show that high-resolution, three-dimensional force mapping of a single organic molecule is possible even at room temperature. The physical limitations of room temperature operation are overcome using semiconducting materials to inhibit molecular diffusion and create robust tip apexes, while challenges due to thermal drift are overcome with atom tracking based feedforward correction. Three-dimensional force maps comparable in spatial and force resolution to those acquired at low temperature are demonstrated, permitting a quantitative analysis of the adsorption induced changes in the geometry of the molecule at the picometer level.

Atomically precise control of rotational dynamics in charged rare-earth complexes on a metal surface

Complexes containing rare-earth ions attract great attention for their technological applications ranging from spintronic devices to quantum information science. While charged rare-earth coordination complexes are ubiquitous in solution, they are challenging to form on materials surfaces that would allow investigations for potential solid-state applications. Here we report formation and atomically precise manipulation of rare-earth complexes on a gold surface. Although they are composed of multiple units held together by electrostatic interactions, the entire complex rotates as a single unit when electrical energy is supplied from a scanning tunneling microscope tip. Despite the hexagonal symmetry of the gold surface, a counterion at the side of the complex guides precise three-fold rotations and 100% control of their rotational directions is achieved using a negative electric field from the scanning probe tip. This work demonstrates that counterions can be used to control dynamics of rare-earth complexes on materials surfaces for quantum and nanomechanical applications.

Thermal scanning probe lithography using Parylene C as thermal resist

Thermal scanning probe lithography is a direct-write patterning method that uses a heated scanning probe tip to remove thermal resist. The most widely used thermal resist is polyphthalaldehyde. Another alternative thermal resist, Parylene C is introduced in this letter. It has been found that Parylene C has a wide process latitude as a thermal resist under our experimental conditions. A high resolution of ∼40 nm can be achieved in our optimised process. In addition, patterns in parylene layer can be transferred directly into the substrate with deep reactive ion etching without an additional hard mask, thus simplifying the fabrication process.

Automated tip functionalization via machine learning in scanning probe microscopy

Auto-CO-AFM is an open-source software package for scanning probe microscopes that enables the automatic functionalization of scanning probe tips with carbon monoxide molecules. This enables machine operators to specify the quality of the tip needed utilizing a pre-trained library with off-the-shelf software. From a single image, the software package can determine which molecules on a surface are carbon monoxide, perform the necessary tip functionalization procedures, interface with microscope software to control the tip position, and determines the centeredness of the tip after a successful functionalization. This is of particular interest for atomic force microscopy imaging of molecules on surfaces, where the tip functionalization is a necessary and time consuming step needed for sub-molecular resolution imaging. This package is freely available under the MIT License. Program summaryProgram Title: Auto-CO-AFMCPC Library link to program files:https://doi.org/10.17632/khfvjhx4vb.1Developer's repository link:https://github.com/SINGROUP/Auto-CO-AFMLicensing provisions: MITProgramming language: PythonSupplementary material: Supplementary Information as PDFNature of problem: Scanning probe microscopy experiments are limited by the lack of automated tools in tip preparation, particularly when functionalizing an adatom or molecule on the end of a tip. These tip functionalizations are commonly manually-driven processes that are time-consuming for the operator.Solution method: We have developed an automated solution for tip functionalization for carbon monoxide molecules, which combines machine learning descriptors with automated software control of the tip preparation process. The Python program interfaces with a major vendor's software to scan, functionalize, and verify the tip quality for the operator. The source code, documentation, and basic program are posted online.Additional comments including restrictions and unusual features: Current version of software in publication requires STMAFM software from CreaTec GmbH in physical connection with a CreaTec DSP.

High-fidelity nano-FTIR spectroscopy by on-pixel normalization of signal harmonics

Abstract Scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier transform infrared nanospectroscopy (nano-FTIR) are emerging tools for physical and chemical nanocharacterization of organic and inorganic composite materials. Being based on (i) diffraction-limited illumination of a scanning probe tip for nanofocusing of light and (ii) recording of the tip-scattered radiation, the efficient suppression of background scattering has been critical for their success. Here, we show that indirect tip illumination via far-field reflection and scattering at the sample can produce s-SNOM and nano-FTIR signals of materials that are not present at the tip position – despite full background suppression. Although these artefacts occur primarily on or near large sample structures, their understanding and recognition are of utmost importance to ensure correct interpretation of images and spectra. Detailed experimental and theoretical results show how such artefacts can be identified and eliminated by a simple signal normalization step, thus critically strengthening the analytical capabilities of s-SNOM and nano-FTIR spectroscopy.

Spin-dependent vibronic response of a carbon radical ion in two-dimensional WS2

Atomic spin centers in 2D materials are a highly anticipated building block for quantum technologies. Here, we demonstrate the creation of an effective spin-1/2 system via the atomically controlled generation of magnetic carbon radical ions (CRIs) in synthetic two-dimensional transition metal dichalcogenides. Hydrogenated carbon impurities located at chalcogen sites introduced by chemical doping are activated with atomic precision by hydrogen depassivation using a scanning probe tip. In its anionic state, the carbon impurity is computed to have a magnetic moment of 1 μB resulting from an unpaired electron populating a spin-polarized in-gap orbital. We show that the CRI defect states couple to a small number of local vibrational modes. The vibronic coupling strength critically depends on the spin state and differs for monolayer and bilayer WS2. The carbon radical ion is a surface-bound atomic defect that can be selectively introduced, features a well-understood vibronic spectrum, and is charge state controlled.

Probing long-range current-carrying edge modes by two quantum point contacts

The origin of anomalous current-carrying edge states in quasi-two-dimensional quantum samples with an insulating interior is currently mysterious. We propose to address this issue using a hybrid setup, an interferometric phase-sensitive configuration of two independent scanning probe tips, normal and superconducting, able to realize the quantum interference effect of quasiparticle currents moving in different directions along the metallic-like one-dimensional near-boundary channels. To simulate the dissipationless edge currents, we consider a quantum material with a simple Corbino disk geometry and analyze how the differential conductance spectrum depends on the distance between the two tips, the applied voltage bias, and the presence of a magnetic field. An essential difference between classical and quantum expectations should clarify the enigmatic origin of the long-range conducting modes observed in different materials at low temperatures. Strong dependence on the applied magnetic field can be useful for practical implementation of the quantum effects associated with the phase difference of electron wave functions in the ring geometry.

Lightwave-driven scanning tunnelling spectroscopy of atomically precise graphene nanoribbons

Atomically precise electronics operating at optical frequencies require tools that can characterize them on their intrinsic length and time scales to guide device design. Lightwave-driven scanning tunnelling microscopy is a promising technique towards this purpose. It achieves simultaneous sub-ångström and sub-picosecond spatio-temporal resolution through ultrafast coherent control by single-cycle field transients that are coupled to the scanning probe tip from free space. Here, we utilize lightwave-driven terahertz scanning tunnelling microscopy and spectroscopy to investigate atomically precise seven-atom-wide armchair graphene nanoribbons on a gold surface at ultralow tip heights, unveiling highly localized wavefunctions that are inaccessible by conventional scanning tunnelling microscopy. Tomographic imaging of their electron densities reveals vertical decays that depend sensitively on wavefunction and lateral position. Lightwave-driven scanning tunnelling spectroscopy on the ångström scale paves the way for ultrafast measurements of wavefunction dynamics in atomically precise nanostructures and future optoelectronic devices based on locally tailored electronic properties.

Scanning probe-induced thermoelectrics in a quantum point contact

We study three-terminal thermoelectric transport in a two-dimensional Quantum Point Contact (QPC) connected to left and right electronic reservoirs, as well as a third one represented by a scanning probe tip. The latter acts as a voltage probe exchanging heat with the system but no charges on average. The thermoelectric coefficients are calculated numerically within the Landauer–Büttiker formalism in the low-temperature and linear response regimes. We find tip-induced oscillations of the local and non-local thermopowers and study their dependence on the QPC opening. If the latter is tuned on a conductance plateau, the system behaves as a perfect thermoelectric diode: for some tip positions, the charge current through the QPC, driven by a local Seebeck effect, can flow in one direction only.

Optomechanical Mapping of Ferroelectric Domains and the Piezo‐Photovoltaic Effect in Ba‐ and Ni‐Doped (K0.5Na0.5)NbO3

The piezo‐photovoltaic effect has been recently proposed as an analogy of the flexo‐photovoltaic effect in noncentrosymmetric ferroelectrics that are also piezoelectric in nature. It has been demonstrated to boost the photovoltaic performance of ferroelectrics under applied uniaxial mechanical loads. The impact of the piezo‐photovoltaic effect on ferroelectric domains, however, has not yet been studied. In this context, a nanoscale insight into mechanical as well as optomechanical control of domains in a novel bandgap‐engineered ferroelectric, namely—KNBNNO ((K0.5Na0.5)NbO3–2 mol% Ba(Ni0.5Nb0.5)O3−δ), is provided. It is found that the applied mechanical force of 1 μN (pressure: 0.25 GPa) using a scanning probe tip evinces a 67% amplification in piezoresponse in the material, and on removing mechanical load, the material retains 33% higher piezoresponse than the pristine state. Mechanically induced property changes in the material are found to be strongly influenced by light illumination. Applied mechanical stress due to an atomic force microscopy (AFM) tip is highly nonuniform and can induce additional polarization via the flexoelectric effect which further enhances the photovoltaic charge screening.

Stabilization and manipulation of in-plane polarization in a ferroelectric|dielectric superlattice

We investigate in-plane ferroelectricity in an epitaxial ferroelectric|dielectric PbTiO3|SrTiO3 (PTO|STO) superlattice under tensile strain. Using a combination of x-ray diffraction and piezoresponse force microscopy, we identify a strain-induced periodic in-plane nanoscale domain architecture with polarization along the base diagonals of the PTO unit cell. We scrutinize its switching behavior using local in-plane trailing fields from the scanning-probe tip. For an electric field pointing in the polarization direction of a specific domain state, we create stable single-domain areas of in-plane polarization at the scale of tens of micrometers. Our findings on controlling robust in-plane ferroelectricity in ultrathin ferroelectric layers can help one to develop device concepts unaffected by the detrimental effects of the depolarizing field.

Mechanics of electrochemical strain microscopy: Computational simulations and experimental validations

In the last decade, electrochemical strain microscopy (ESM) has emerged as a powerful tool to study electrochemical processes at the nanoscale, yet its quantitative analysis is quite challenging, involving complex electrochemo-mechanical coupling under highly concentrated electromechanical fields. In this work, we develop a theoretical framework of thermodynamics and kinetics for mobile ions in electrochemically-active solids, wherein full electrochemo-mechanical coupling is considered via concentration-dependent Vegard strain and stress-dependent diffusivity. The theory is applied to model electrochemical processes underneath a charged scanning probe tip, and implemented numerically to solve for the highly inhomogeneous electrochemo-mechanical field via combined fast Fourier transform and finite difference analysis. The simulations reveal that the ESM amplitude correlates linearly with both ionic concentration and diffusivity, while relaxation time constant depends only on diffusivity, making it possible to decouple these two material parameters. In order to validate our model analysis, ESM mapping and point-wise relaxation studies are carried out on solid state electrolyte ceria, and the experimental data agree with model predictions well. The analysis thus provides a powerful technique to analyze ESM experiments, and sheds deep insight into the nanoscale electrochemical process under a tip.

Variable Repetition Rate THz Source for Ultrafast Scanning Tunneling Microscopy.

Broadband THz pulses enable ultrafast electronic transport experiments on the nanoscale by coupling THz electric fields into the devices with antennas, asperities, or scanning probe tips. Here, we design a versatile THz source optimized for driving the highly resistive tunnel junction of a scanning tunneling microscope. The source uses optical rectification in lithium niobate to generate arbitrary THz pulse trains with freely adjustable repetition rates between 0.5 and 41 MHz. These induce subpicosecond voltage transients in the tunnel junction with peak amplitudes between 0.1 and 12 V, achieving a conversion efficiency of 0.4 V/(kV/cm) from far-field THz peak electric field strength to peak junction voltage in the STM. Tunnel currents in the quantum limit of less than one electron per THz pulse are readily detected at multi-MHz repetition rates. The ability to tune between high pulse energy and high signal fidelity makes this THz source design effective for exploration of ultrafast and atomic-scale electron dynamics.