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1. Development of a scanning tunneling microscope for the carrier profiling of semiconductors by scanning frequency comb microscopy

   https://doi.org/10.1109/WMED.2017.7916926  

   Spencer, Greg; Hagmann, Mark J. (April 2017, Development of a scanning tunneling microscope for the carrier profiling of semiconductors by scanning frequency comb microscopy)

   Summary form only given. We are developing a scanning tunneling microscope that is portable and optimized for scanning frequency comb microscopy (SFCM) as one part of our effort to complete a prototype for the carrier profiling of semiconductors by SFCM. Conventional integral or integral plus proportion feedback control of the tunneling current in a scanning tunneling microscope (STM) is satisfactory once tunneling has been established but may cause tip-crash by integral windup during coarse approach. In tip-sample contact images with atomic-resolution may be obtained but the microwave frequency comb ceases because there is no optical rectification and scanning tunneling spectroscopy also fails. We are studying a new control algorithm based on approximating the tunneling current as a polynomial in the bias voltage where the coefficients in this polynomial are not required. It is noted that hanges in the apparatus, as well as the algorithms used for feedback control in the STM, are required to optimize this instrument for measuring the microwave frequency comb. 

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2. Adaptive Scalpel Scanning Probe Microscopy for Enhanced Volumetric Sensing in Tomographic Analysis

   https://doi.org/10.1002/admi.202400187  

   Laskar, Md_Ashiqur_Rahman; Leonetti, Giuseppe; Milano, Gianluca; Novotný, Ondřej; Neuman, Jan; Tongay, Sefaattin; Celano, Umberto (June 2024, Advanced Materials Interfaces)

   Abstract Controlling nanoscale tip‐induced material removal is crucial for achieving atomic‐level precision in tomographic sensing with atomic force microscopy (AFM). While advances have enabled volumetric probing of conductive features with nanometer accuracy in solid‐state devices, materials, and photovoltaics, limitations in spatial resolution and volumetric sensitivity persist. This work identifies and addresses in‐plane and vertical tip‐sample junction leakage as sources of parasitic contrast in tomographic AFM, hindering real‐space 3D reconstructions. Novel strategies are proposed to overcome these limitations. First, the contrast mechanisms analyzing nanosized conductive features are explored when confining current collection purely to in‐plane transport, thus allowing reconstruction with a reduction in the overestimation of the lateral dimensions. Furthermore, an adaptive tip‐sample biasing scheme is demonstrated for the mitigation of a class of artefacts induced by the high electric field inside the thin oxide when volumetrically reduced. This significantly enhances vertical sensitivity by approaching the intrinsic limits set by quantum tunneling processes, allowing detailed depth analysis in thin dielectrics. The effectiveness of these methods is showcased in tomographic reconstructions of conductive filaments in valence change memory, highlighting the potential for application in nanoelectronics devices and bulk materials and unlocking new limits for tomographic AFM. 

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3. Tunneling Mode of Scanning Electrochemical Microscopy: Probing Electrochemical Processes at Single Nanoparticles

   https://doi.org/10.1002/anie.201801115  

   Sun, Tong; Wang, Dengchao; Mirkin, Michael V. (May 2018, Angewandte Chemie International Edition)

   Abstract Electrochemical experiments at individual nanoparticles (NPs) can provide new insights into their structure–activity relationships. By using small nanoelectrodes as tips in a scanning electrochemical microscope (SECM), we recently imaged individual surface‐bound 10–50 nm metal NPs. Herein, we introduce a new mode of SECM operation based on tunneling between the tip and a nanoparticle immobilized on the insulating surface. The obtained current vs. distance curves show the transition from the conventional feedback response to electron tunneling between the tip and the NP at separation distances of less than about 3 nm. In addition to high‐resolution imaging of the NP topography, the tunneling mode enables measurement of the heterogeneous kinetics at a single NP without making an ohmic contact with it. The developed method should be useful for studying the effects of nanoparticle size and geometry on electrocatalytic activity in real‐world applications. 

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4. Numerical simulations for ferromagnetic resonance of nano-size island structures probed by radio-frequency scanning tunneling microscopy

   https://doi.org/10.35848/1347-4065/ac4556  

   Sato, Yudai; Haze, Masahiro; Yang, Hung-Hsiang; Asakawa, Kanta; Takahashi, Susumu; Hasegawa, Yukio (January 2022, Japanese Journal of Applied Physics)

   Abstract We numerically calculated ferromagnetic resonance (FMR) spectra taken on a single-domain nano-size ferromagnetic island structure in the configuration of radio-frequency (RF) scanning tunneling microscopy, where RF electromagnetic waves are introduced into the tunneling gap through the probe tip. In this scheme, near-field in-plane azimuthal RF magnetic field induces FMR of an out-of-plane magnetized island situated below the tip under the external out-of-plane magnetic field. The amount of the magnetization of the island is effectively reduced by the resonance and the reduction can be detected from the spin-polarized tunneling conductance. From the calculated spectra we found that the FMR signal becomes larger with a smaller tip-sample distance and a sharper tip. It is also revealed that the azimuthal RF magnetic field exerted on the island and therefore the FMR signal are enhanced when a tip is located near the edge of the island. 

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5. Theory of Josephson scanning microscopy with $s$ -wave tip on unconventional superconducting surface: Application to ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$

   https://doi.org/10.1103/PhysRevB.110.184519  

   Choubey, Peayush; Hirschfeld, P J (November 2024, Physical Review B)

   Josephson scanning tunneling microscopy (JSTM) is a powerful probe of the local superconducting order parameter, but studies have been largely limited to cases where the superconducting sample and superconducting tip both have the same gap symmetry—either s-wave or d-wave. It has been generally assumed that, in an ideal s-to-d JSTM experiment, the critical current would vanish everywhere, as expected for ideal c-axis planar junctions. We show here that this is not the case. Employing first-principlesWannier functions for Bi2Sr2CaCu2O8+δ , we develop a scheme to compute the Josephson critical current (Ic) and quasiparticle tunneling current measured by JSTM with subangstrom resolution. We demonstrate that the critical current for tunneling between an s-wave tip and a superconducting cuprate sample has the largest magnitude above O sites and it vanishes above Cu sites. Ic changes sign under π/2 rotation and its average over a unit cell vanishes, as a direct consequence of the d-wave gap symmetry in cuprates. Further, we show that Ic is strongly suppressed in the close vicinity of a Zn-like impurity owing to suppression of the superconducting order parameter. More interestingly, Ic acquires nonvanishing values above the Cu sites near the impurity. The critical current modulations produced by the impurity occur at characteristic wave vectors distinct from the quasiparticle interference (QPI) analog. Furthermore, the quasiparticle tunneling spectra in the JSTM setup shows coherence peaks and impurity-induced resonances shifted by the s-wave tip gap. We discuss the similarities and differences in JSTM observables and conventional STM observables, making specific predictions that can be tested in future JSTM experiments. 

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