The Fermi level dictates charge distribution and redox activity, yet its dynamic evolution under illumination remains poorly resolved. Using Kelvin probe force microscopy, we map time-dependent Fermi-level shifts in a model sample system, a single-crystal TiO2, under front- and back-side ultraviolet irradiation at two-photon energies. High-energy ultraviolet irradiation lowers the surface Fermi level through the formation of persistent photoinduced surface oxygen vacancies, while low-energy ultraviolet irradiation induces only short-lived electron-hole generation with rapid surface recovery. Moreover, front-side irradiation yields a transient response, followed by a uniform surface modification, whereas back-side excitation produces slower, bulk-mediated shifts. In addition, gold nanoparticle decoration of the surface further modifies the Fermi-level kinetics through the Schottky barrier at the interface. By decoupling photon energy from illumination geometry, this work reveals the paired dynamics of carrier transport and defect-mediated formation that shape oxide energetics. These findings establish a foundation for Fermi-level engineering in photocatalysis, sensing, and optoelectronic applications.

Thin film growth is a critical process enabling modern applications ranging from electronic devices to advanced coatings. Among the parameters that govern thin film growth, the Ehrlich-Schwoebel barrier stands out with its tight control over interlayer transfer and, consequently, kinetics-dominated film morphology. Despite its importance, the precise measurement of the Ehrlich-Schwoebel barrier remains complicated, presenting a critical impediment to rational thin film design. Here, we provide an insight into the Ehrlich-Schwoebel barrier over monoatomic step edges on Au (111) surfaces via three-dimensional atomic force microscopy (3D-AFM) with sub-nanometer spatial precision, minimizing the need for empirical model assumptions or theoretical calculations. Our measurements provide a quantitative, real-space view of the complex potential energy and force landscape near step edges, verifying the presence of energy barriers and wells at the top and bottom of step edges, respectively. The effect of the herringbone reconstruction on the potential energy landscape is also analyzed, revealing an enhancement of interactions near the elbows and a slight attenuation of the ridges.

The spatiotemporal behavior of charge carriers in metal oxides governs their performance in photocatalytic and electronic applications, yet remains poorly understood at the nanoscale. Here, we use time-resolved atomic force microscopy (TR-AFM) to map charge transport in TiO2 under controlled surface irradiation and thermal conditions. Our measurements reveal pronounced spatial variability in carrier migration times and activation energies, driven by local defect landscapes. Irradiation-induced surface defects are found to lower migration barriers, enhancing carrier relaxation. Notably, we observe electrostatic memory effects, with residual electric fields modulating migration dynamics across hundreds of nanometers. Temperature-dependent studies further reveal a tunable interaction between defect-mediated migration and thermal activation. These findings provide direct insight into nanoscale charge transport in TiO2 and highlight the role of defect engineering and thermal management in optimizing oxide-based devices for energy conversion and sensing.

Atomic force microscopy (AFM) enables high-resolution imaging and quantitative force measurement, which are critical for understanding nanoscale mechanical, chemical, and biological interactions. In dynamic AFM modes, however, interaction forces are not directly measured; they must be mathematically reconstructed from observables such as the amplitude, phase, or frequency shift. Many reconstruction techniques have been proposed over the last two decades, but they rely on different assumptions and have been applied inconsistently, limiting reproducibility and cross-study comparison. Here, we systematically evaluate major force reconstruction methods in both frequency- and amplitude-modulation AFM, detailing their theoretical foundations, performance regimes, and sources of error. To support benchmarking and reproducibility, we introduce an open-source software package that unifies all widely used methods, enabling side-by-side comparisons across different formulations. This work represents a critical step toward achieving consistent and interpretable AFM force spectroscopy, thereby supporting the more reliable application of AFM in fields ranging from materials science to biophysics.

High-energy ultraviolet (UVC) irradiation of metal oxides (MOs, e.g., TiO2) results in photoinduced surface oxygen vacancies (PI-SOVs), which can change the charge carrier (e.g., electrons and holes) migration dynamics. Although PI-SOVs alter the electronic and chemical properties of MOs, there is no consensus on the penetration depth of the UVC irradiation, which induces PI-SOVs and is an important variable for the design and operation of MO-based systems. Here, we performed optical transmission and time-resolved atomic force microscopy measurements on back-illuminated TiO2 samples. Our experiments show that the effect of UVC irradiation on MOs can be observed hundreds of micrometers across the bulk, i.e., orders of magnitude larger than previously postulated values. We believe that our findings would be important both for the fundamental understanding of UVC irradiation/penetration and for device design/fabrication processes.

The migration of holes in metal-oxide semiconductors such as TiO2 plays a vital role in (photo)catalytic applications. The dynamics of charge carriers under operation conditions can be influenced by both methanol addition and photoinduced surface oxygen vacancies (PI-SOVs). Nevertheless, the existing knowledge of the effect of methanol as a function of PI-SOVs solely concentrates on the chemical reduction process. For this reason, the fundamental understanding of the time-dependent charge carrier-vacancy interactions in the presence of methanol is impaired. Here, we conducted time-resolved atomic force microscopy measurements to quantitatively disclose the effect of methanol adsorption on the dynamics of hole migration in TiO2. Our results show that time constants associated with the migration of charge carriers significantly change due to methanol adsorption. Moreover, the energy landscape of the hole migration barrier was dominated and lowered by PI-SOVs. Our findings contribute to the physics of charge carrier dynamics by enabling the engineering of charge carrier-vacancy interactions.

Atomic force microscopy (AFM) is an analytical surface characterization tool that reveals the surface topography at a nanometer length scale while probing local chemical, mechanical, and even electronic sample properties. Both contact (performed with a constant deflection of the cantilever probe) and dynamic operation modes (enabled by demodulation of the oscillation signal under tip-sample interaction) can be employed to conduct AFM-based measurements. Although surface topography is accessible regardless of the operation mode, the resolution and the availability of the quantified surface properties depend on the mode of operation. However, advanced imaging techniques, such as frequency modulation, to achieve high resolution, quantitative surface properties are not implemented in many commercial systems. Here, we show the step-by-step customization of an atomic force microscope. The original system was capable of surface topography and basic force spectroscopy measurements while employing environmental control, such as temperature variation of the sample/tip, etc. We upgraded this original setup with additional hardware (e.g., a lock-in amplifier with phase-locked loop capacity, a high-voltage amplifier, and a new controller) and software integration while utilizing its environmental control features. We show the capabilities of the customized system with frequency modulation-based topography experiments and automated voltage and/or distance spectroscopy, time-resolved AFM, and two-dimensional force spectroscopy measurements under ambient conditions. We also illustrate the enhanced stability of the setup with active topography and frequency drift corrections. We believe that our methodology can be useful for the customization and automation of other scanning probe systems.

Inorganic-oxide-based sample systems are popular for applications in catalysis, sensing, renewable energy, and fuel cells, in which electronic properties play important roles. Environmental conditions, e.g., temperature, can greatly impact the electronic properties and thereby the performance. The lack of basic knowledge of the local variation of electronic properties as a function of temperature limits the fundamental understanding of systems and hampers their robustness. Here, we present temperature-dependent scanning probe microscopy experiments to reveal the variation of contact potential difference (CPD) across different length scales. Our measurements demonstrate that the CPD of inorganic perovskites (e.g., SrTiO3) and metal-oxide semiconductors (e.g., TiO2) evolves significantly with temperature. We show that CPD variation depends on the locality of the measurement and originates from a thermodynamically driven intrinsic doping state. These results will facilitate a fundamental understanding of the electronic properties of oxides and thus ease emerging technologies, rationalized by engineering temperature-dependent electronic properties.

We present results of atomic-force-microscopy-based friction measurements on Re-doped molybdenum disulfide (MoS2). In stark contrast to the widespread observation of decreasing friction with increasing number of layers on two-dimensional (2D) materials, friction on Re-doped MoS2 exhibits an anomalous, i.e., inverse dependence on the number of layers. Raman spectroscopy measurements combined with ab initio calculations reveal signatures of Re intercalation. Calculations suggest an increase in out-of-plane stiffness that inversely correlates with the number of layers as the physical mechanism behind this remarkable observation, revealing a distinctive regime of puckering for 2D materials.

Metal-oxide semiconductors (MOS) are widely utilized for catalytic and photocatalytic applications in which the dynamics of charged carriers (e.g., electrons, holes) play important roles. Under operation conditions, photoinduced surface oxygen vacancies (PI-SOV) can greatly impact the dynamics of charge carriers. However, current knowledge regarding the effect of PI-SOV on the dynamics of hole migration in MOS films, such as titanium dioxide, is solely based upon volume-averaged measurements and/or vacuum conditions. This limits the basic understanding of hole-vacancy interactions, as they are not capable of revealing time-resolved variations during operation. Here, we measured the effect of PI-SOV on the dynamics of hole migration using time-resolved atomic force microscopy. Our findings demonstrate that the time constant associated with hole migration is strongly affected by PI-SOV, in a reversible manner. These results will nucleate an insightful understanding of the physics of hole dynamics and thus enable emerging technologies, facilitated by engineering hole-vacancy interactions.

The onset of yielding and the related atomic-scale plastic flow behavior of bulk metallic glasses at room temperature have not been fully understood due to the difficulty in performing the atomic-scale plastic deformation experiments needed to gain direct insight into the underlying fundamental deformation mechanisms. Here we overcome these limitations by combining a unique sample preparation method with atomic force microscopy-based indentation, which allows study of the yield stress, onset of yielding, and atomic-scale plastic flow of a platinum-based bulk metallic glass in volumes containing as little as approximately 1000 atoms. Yield stresses markedly higher than in conventional nanoindentation testing were observed, surpassing predictions from current models that relate yield stress to tested volumes; subsequent flow was then established to be homogeneous without exhibiting collective shear localization or loading rate dependence. Overall, variations in glass properties due to fluctuations of free volume are found to be much smaller than previously suggested. Metallic glasses display a high yield strength and typically deform via heterogeneous shear bands beyond the yield point. Here, deformation of as little as 1000 atoms in a Pt-based metallic glass at room temperature leads to near-theoretical yield strength, beyond which homogeneous deformation occurs.

The quantitative interatomic force measurements open a new pathway to materials characterization, surface science, and chemistry by elucidating the force between 'two' interacting atoms as a function of their separation. Atomic force microscope is the ideal platform to gauge interatomic forces between the tip and the sample. For such quantitative measurements, either the oscillation frequency or the oscillation amplitude and the phase of a vibrating cantilever are recorded as a function of the tip-sample separation. These experimental measures are subsequently converted into the interatomic force laws. Recently, it has been shown that the most commonly applied mathematical conversion techniques may suffer a significant deviation from the actual force laws. To avoid assessment of unphysical interatomic forces, either the use of very small (i.e., a few picometers) or very large oscillation amplitudes (i.e., a few nanometers) has been proposed. However, the use of marginal oscillation amplitudes gives rise to another problem as it lacks the feasibility due to the adverse signal to noise ratios. Here we show a new mathematical conversion principle that confronts interatomic force measurements while preserving the oscillation amplitude within the experimentally achievable and favorable limits, i.e. tens of picometers. We anticipate that our findings will be the nucleus of reliable evaluation of material properties with a more accurate measurement of interatomic force laws.

Perovskites are widely utilized either as a primary component or as a substrate in which the dynamics of charged oxygen vacancy defects play an important role. Current knowledge regarding the dynamics of vacancy mobility in perovskites is solely based upon volume- and/or time-averaged measurements. This impedes our understanding of the basic physical principles governing defect migration in inorganic materials. Here, we measure the ergodic and nonergodic dynamics of vacancy migration at the relevant spatial and temporal scales using time-resolved atomic force microscopy techniques. Our findings demonstrate that the time constant associated with oxygen vacancy migration is a local property and can change drastically on short length and time scales, such that nonergodic states lead to a dramatic increase in the migration barrier. This correlated spatial and temporal variation in oxygen vacancy dynamics can extend hundreds of nanometers across the surface in inorganic perovskites.

Author(s): Acikgoz, Ogulcan; Yanilmaz, Alper; Dagdeviren, Omur E; Celebi, Cem; Baykara, Mehmet Z | Abstract: We present the results of atomic-force-microscopy-based friction measurements on Re-doped molybdenum disulfide (MoS2). In stark contrast to the seemingly universal observation of decreasing friction with increasing number of layers on two-dimensional (2D) materials, friction on Re-doped MoS2 exhibits an anomalous, i.e. inverse dependency on the number of layers. Raman spectroscopy measurements revealed signatures of Re intercalation, leading to a decoupling between neighboring MoS2 layers and enhanced electron-phonon interactions, thus resulting in increasing friction with increasing number of layers: a new paradigm in the mechanics of 2D materials.

Theory predicts that two-dimensional (2D) materials may only exist in the presence of out-of-plane deformations on atomic length scales, frequently referred to as ripples. While such ripples can be detected via electron microscopy, their direct observation via surface-based techniques and characterization in terms of interaction forces and energies remain limited, preventing an unambiguous study of their effect on mechanical characteristics, including but not limited to friction anisotropy. Here, we employ high-resolution atomic force microscopy to demonstrate the presence of atomic-scale ripples on supported samples of few-layer molybdenum disulfide (MoS 2 ). Three-dimensional force/energy spectroscopy is utilized to study the effect of ripples on the interaction landscape. Friction force microscopy reveals multiple symmetries for friction anisotropy, explained by studying rippled sample areas as a function of scan size. Our experiments contribute to the continuing development of a rigorous understanding of the nanoscale mechanics of 2D materials.

Ever since their dawn in 2009 1, hybrid organic–inorganic perovskite materials have been experiencing an ever increase in research investment to finally emerge as a strong alternative for high power conversion efficiency (PCE) photovoltaic devices at low costs. One of the main challenges that is currently impeding the upscaling of these solar cells is the phenomenon of hysteresis on their j–V response that has been correlated with slow ion migration which, in turn, affects their photovoltaic properties. Although hysteresis has been described to some extent of detail, the precise role of the external contacts remains under debate. Furthermore, there has been a great deal of focus on materials and interface design while less attention has been given to optical aspects of these cells. Our work highlights the role of electron transport layer (ETL) of different compositions and synthesized through various routes on the hysteresis response of these systems. We show that there’s a clear correlation between thickness, morphology and composition of the ETL on the photovoltaic response these systems. To better elucidate such a response, we have utilized photoluminescence, electroluminescence, absorption and kelvin probe force microscopy to get a substantial grasp of the optoelectronic phenomena occurring at the interfaces in perovskite solar cells. Acknowledgments: This research was supported by a Hydro-Quebec/NSERC CRD grant and the McGill Sustainability Systems Initiative (MSSI). 1- Kojima, A., Teshima, K., Shirai, Y. and Miyasaka, T., 2009. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. Journal of the American Chemical Society, 131(17), pp.6050-6051.

The evolution of the surface morphology of Pt57.5Cu14.7Ni5.3P22.5 bulk metallic glasses thermoplastically formed by replication of SrTiO3 single crystals during heat treatments is investigated by atomic force microscopy. Due to the unique imprinting process, the surface of the bulk metallic glass features a terraced structure with sub-angstrom surface roughness. Upon heating over the material's glass transition temperature, the surface rearranges and relaxes towards a more stable, denser packed glass by smoothening out steps and increasing on-terrace surface roughness. With further increasing annealing temperatures, substantial surface alterations are observed because of increased arrangement kinetics and the onset of crystallization.