Pulsed Force Kelvin Probe Force Microscopy-A New Type of Kelvin Probe Force Microscopy under Ambient Conditions.

Kelvin probe force microscopy (KPFM) is an increasingly popular scanning probe microscopy technique used for nanoscale imaging of surface potential for various materials, such as metals, semiconductors, biological samples, and photovoltaics, to reveal their surface work function and/or local accumulation of charges. This featured review outlines the operation principles and applications of KPFM, including several typical commercially available variants. We highlight the significance of surface potential measurements, present the details of the method operation, and discuss the causes of the limitation on spatial resolution. Then, we present the pulsed force Kelvin probe force microscopy (PF-KPFM) as an innovative improvement to KPFM, which provides an enhanced spatial resolution of <10 nm under ambient conditions. PF-KPFM is promising for the characterization of heterogeneous materials with spatial variations of electrical properties. It will be especially instrumental for investigating emerging perovskite photovoltaics, heterogeneous catalysts, 2D materials, and ferroelectric materials, among others.

Pulsed Force Kelvin Probe Force Microscopy through Integration of Lock-In Detection.

Kelvin probe force microscopy measures surface potential and delivers insights into nanoscale electronic properties, including work function, doping levels, and localized charges. Recently developed pulsed force Kelvin probe force microscopy (PF-KPFM) provides sub-10 nm spatial resolution under ambient conditions, but its original implementation is hampered by instrument complexity and limited operational speed. Here, we introduce a solution for overcoming these two limitations: a lock-in amplifier-based PF-KPFM. Our method involves phase-synchronized switching of a field effect transistor to mediate the Coulombic force between the probe and the sample. We validate its efficacy on two-dimensional material MXene and aged perovskite photovoltaic films. Lock-in-based PF-KPFM successfully identifies the contact potential difference (CPD) of stacked flakes and finds that the CPDs of monoflake MXene are different from those of their multiflake counterparts, which are otherwise similar in value. In perovskite films, we uncover electrical degradation that remains elusive with surface topography.

Visible to Mid-IR Spectromicroscopy with Top-Down Illumination and Nanoscale (≈10 nm) Resolution.

Photothermal induced resonance (PTIR), an atomic force microscopy (AFM) analogue of IR spectroscopy also known as AFM-IR, is capable of nanoscale lateral resolution and finds broad applications in biology and materials science. Here, the spectral range of a top-illumination PTIR setup operating in contact-mode is expanded for the first time to the visible and near-IR spectral ranges. The result is a tool that yields absorption spectra and maps of electronic and vibrational features with spatial resolution down to ≈10 nm. In addition to the improved resolution, the setup enables light-polarization-dependent PTIR experiments in the visible and near-IR ranges for the first time. While previous PTIR implementations in the visible used total internal reflection illumination requiring challenging sample preparations on an optically transparent prism, the top illumination used here greatly simplifies sample preparation and will foster a broad application of this method.

A guide to nanoscale IR spectroscopy: resonance enhanced transduction in contact and tapping mode AFM-IR.

Infrared (IR) spectroscopy is a broadly applicable, composition sensitive analytical technique. By leveraging the high spatial resolution of atomic force microscopy (AFM), the photothermal effect, and wavelength-tunable lasers, AFM-IR enables IR spectroscopy and imaging with nanoscale (< 10 nm) resolution. The transduction of a sample's photothermal expansion by an AFM probe tip ensures the proportionality between the AFM-IR signal and the sample absorption coefficient, producing images and spectra that are comparable to far-field IR databases and easily interpreted. This convergence of characteristics has spurred robust research efforts to extend AFM-IR capabilities and, in parallel, has enabled AFM-IR to impact numerous fields. In this tutorial review, we present the latest technical breakthroughs in AFM-IR spectroscopy and imaging and discuss its working principles, distinctive characteristics, and best practices for different AFM-IR measurement paradigms. Central to this review, appealing to both expert practitioners and novices alike, is the meticulous understanding of AFM-IR signal transduction, which is essential to take full advantage of AFM-IR capabilities. Here, we critically compile key information and discuss instructive experiments detailing AFM-IR signal transduction and provide guidelines linking experimental parameters to the measurement sensitivity, lateral resolution, and probed depth. Additionally, we provide in-depth tutorials on the most employed AFM-IR variants (resonance-enhanced and tapping mode AFM-IR), discussing technical details and representative applications. Finally, we briefly review recently developed AFM-IR modalities (peak force tapping IR and surface sensitivity mode) and provide insights on the next exciting opportunities and prospects for this fast-growing and evolving field.

Integrated Tapping Mode Kelvin Probe Force Microscopy with Photoinduced Force Microscopy for Correlative Chemical and Surface Potential Mapping.

Kelvin probe force microscopy (KPFM) is a popular technique for mapping the surface potential at the nanoscale through measurement of the Coulombic force between an atomic force microscopy (AFM) tip and sample. The lateral resolution of conventional KPFM variants is limited to between ≈35 and 100 nm in ambient conditions due to the long-range nature of the Coulombic force. In this article, a novel way of generating the Coulombic force in tapping mode KPFM without the need for an external AC driving voltage is presented. A field-effect transistor (FET) is used to directly switch the electrical connectivity of the tip and sample on and off periodically. The resulting Coulomb force induced by Fermi level alignment of the tip and sample results in a detectable change of the cantilever oscillation at the FET-switching frequency. The resulting FET-switched KPFM delivers a spatial resolution of ≈25 nm and inherits the high operational speed of the AFM tapping mode. Moreover, the FET-switched KPFM is integrated with photoinduced force microscopy (PiFM), enabling simultaneous acquisitions of high spatial resolution chemical distributions and surface potential maps. The integrated FET-switched KPFM with PiFM is expected to facilitate characterizations of nanoscale electrical properties of photoactive materials, semiconductors, and ferroelectric materials.

Liquid medium annealing for fabricating durable perovskite solar cells with improved reproducibility.

Solution processing of semiconductors is highly promising for the high-throughput production of cost-effective electronics and optoelectronics. Although hybrid perovskites have potential in various device applications, challenges remain in the development of high-quality materials with simultaneously improved processing reproducibility and scalability. Here, we report a liquid medium annealing (LMA) technology that creates a robust chemical environment and constant heating field to modulate crystal growth over the entire film. Our method produces films with high crystallinity, fewer defects, desired stoichiometry, and overall film homogeneity. The resulting perovskite solar cells (PSCs) yield a stabilized power output of 24.04% (certified 23.7%, 0.08 cm2) and maintain 95% of their initial power conversion efficiency (PCE) after 2000 hours of operation. In addition, the 1-cm2 PSCs exhibit a stabilized power output of 23.15% (certified PCE 22.3%) and keep 90% of their initial PCE after 1120 hours of operation, which illustrates their feasibility for scalable fabrication. LMA is less climate dependent and produces devices in-house with negligible performance variance year round. This method thus opens a new and effective avenue to improving the quality of perovskite films and photovoltaic devices in a scalable and reproducible manner.

Nanoscale Chemical Features of the Natural Fibrous Material Wood.

Peak force infrared (PFIR) microscopy is a recently developed approach to acquire multiple chemical and physical material properties simultaneously and with nanometer resolution: topographical features, infrared (IR)-sensitive maps, adhesion, stiffness, and locally resolved IR spectra. This multifunctional mapping is enabled by the ability of an atomic force microscope tip in the peak force tapping mode to detect photothermal expansion of the sample. We report the use of the PFIR to characterize the chemical modification of bio-based native and intact wooden matrices, which has evolved into an increasingly active research field. The distribution of functional groups of wood cellulose aggregates, either in native or carboxylated states, was detected with a remarkable spatial resolution of 16 nm. Furthermore, mechanical and chemical maps of the distinct cell wall layers were obtained on polymerized wooden matrices to localize the exact position of the modified regions. These findings shall support the development and understanding of functionalized wood materials.

Peak Force Infrared-Kelvin Probe Force Microscopy.

Correlative scanning probe microscopy of chemical identity, surface potential, and mechanical properties provide insight into the structure-function relationships of nanomaterials. However, simultaneous measurement with comparable and high resolution is a challenge. We seamlessly integrated nanoscale photothermal infrared imaging with Coulomb force detection to form peak force infrared-Kelvin probe force microscopy (PFIR-KPFM), which enables simultaneous nanomapping of infrared absorption, surface potential, and mechanical properties with approximately 10 nm spatial resolution in a single-pass scan. MAPbBr3 perovskite crystals of different degradation pathways were studied in situ. Nanoscale charge accumulations were observed in MAPbBr3 near the boundary to PbBr2 . PFIR-KPFM also revealed correlations between residual charges and secondary conformation in amyloid fibrils. PFIR-KPFM is applicable to other heterogeneous materials at the nanoscale for correlative multimodal characterizations.

Pulsed Force Kelvin Probe Force Microscopy.

Measurement of the contact potential difference (CPD) and work functions of materials are important in analyzing their electronic structures and surface residual charges. Kelvin probe force microscopy (KPFM), an imaging technique of atomic force microscopy, has been widely used for surface potential and work function mapping at the nanoscale. However, the conventional KPFM variants are often limited in their spatial resolution to 30-100 nm under ambient conditions. The continuingly decreasing size and increasing complexity of photoactive materials and semiconductor devices will present future challenges in uncovering their nanometer-scale electrical properties through KPFM. Here, we introduce a KPFM technique based on the pulsed force mode of atomic force microscopy. Our technique, named pulsed force Kelvin Probe Force Microscopy (PF-KPFM), is a single-pass technique that utilizes the intrinsic Fermi level alignment between the AFM tip and the conductive sample without the need for an external oscillating voltage. Induced cantilever oscillations generated by a spontaneous redistribution of electrons between tip and sample are extracted and used to obtain the cantilever oscillation amplitude and to derive the surface potential. The spatial resolution of PF-KPFM is shown to be <10 nm under ambient conditions. The high spatial resolution surface potential mapping enables in situ determination of ohmic and nonohmic contacts between metals and semiconductors, mapping boundaries of ferroelectric domains of BaTiO3, as well as characterization of protein aggregates. High spatial resolution measurements with PF-KPFM will facilitate further studies directed at uncovering electrical properties for emerging photoactive materials, biological samples, and semiconductor devices.

Differences in surface chemistry of regenerated lignocellulose fibers determined by chemically sensitive scanning probe microscopy.

Tuning the composition of regenerated lignocellulosic fibers in the production process enables targeting of specific material properties. In composite materials, such properties could be manipulated by controlled heterogeneous distribution of chemical components of regenerated fibers. This attribute requires a visualization method to show their inherent chemical characteristics. We compared complementary microscopic techniques to analyze the surface chemistry of four differently tuned regenerated lignocellulosic fibers. Adhesion properties were visualized with chemical force microscopy and showed contrasts towards hydrophilic and hydrophobic atomic force microscopy tips. Fibers containing xylan showed heterogeneous adhesion properties within the fiber structure towards hydrophilic tips. Additionally, peak force infrared microscopy mapped spectroscopic contrasts with nanometer resolution and provided point infrared spectra, which were consistent to classical infrared microscopy data. With this setup, infrared signals with a spatial resolution below 20 nm reveal chemical gradients in specific fiber types.

Generalized Heterodyne Configurations for Photoinduced Force Microscopy.

Infrared chemical microscopy through mechanical probing of light-matter interactions by atomic force microscopy (AFM) bypasses the diffraction limit. One increasingly popular technique is photoinduced force microscopy (PiFM), which utilizes the mechanical heterodyne signal detection between cantilever mechanical resonant oscillations and the photoinduced force from the light-matter interaction. So far, PiFM has been operated in only one heterodyne configuration. In this Article, we generalize heterodyne configurations of PiFM by introducing two new schemes: harmonic heterodyne detection and sequential heterodyne detection. In harmonic heterodyne detection, the laser repetition rate matches integer fractions of the difference between the two mechanical resonant modes of the AFM cantilever. The high harmonic of the beating from the photothermal expansion mixes with the AFM cantilever oscillation to provide the PiFM signal. In sequential heterodyne detection, the combination of the repetition rate of laser pulses and the polarization modulation frequency matches the difference between two AFM mechanical modes, leading to detectable PiFM signals. These two generalized heterodyne configurations for PiFM deliver new avenues for chemical imaging and broadband spectroscopy at ∼10 nm spatial resolution. They are suitable for a wide range of heterogeneous materials across various disciplines: from structured polymer film, to polaritonic boron nitride materials, to isolated bacterial peptidoglycan cell walls. The generalized heterodyne configurations introduce flexibility for the implementation of PiFM and the related tapping-mode AFM-IR and provide possibilities for an additional modulation channel in PiFM for targeted signal extraction with nanoscale spatial resolution.

Spectro-Mechanical Characterizations of Kerogen Heterogeneity and Mechanical Properties of Source Rocks at 6 nm Spatial Resolution.

In situmeasurements of the chemical compositions and mechanical properties of kerogen help understand the formation, transformation, and utilization of organic matter in oil shale source rocks. However, the optical diffraction limit prevents attainment of nanoscale resolution using conventional spectroscopy and microscopy. Here, we developed peak force infrared (PFIR) microscopy for multimodal characterization of kerogen in organic shales. PFIR microscopy provides correlative infrared imaging, mechanical mapping, and broadband infrared spectroscopy capabilities with 6 nm spatial resolution within the frequency region of 2400-4000 cm-1. We have observed nanoscale heterogeneity in the chemical composition, aromaticity, and the level of maturity of the kerogens from source rocks obtained from the Eagle Ford shale play in Texas. The level of aromaticity of the kerogen positively correlates with the local mechanical moduli of the surrounding inorganic matrix, offering insights into the effect of kerogen heterogeneity on the nanoscale mechanical properties of the source rock. Our method and investigation advances the understanding toward the origin and transformation of kerogen in geological settings.

Tomographic and multimodal scattering-type scanning near-field optical microscopy with peak force tapping mode.

Scattering-type scanning near-field optical microscopy (s-SNOM) enables nanoscale spectroscopic imaging and has been instrumental for many nano-photonic discoveries and in situ studies. However, conventional s-SNOM techniques with atomic force microscopy tapping mode operation and lock-in detections do not provide direct tomographic information with explicit tip-sample distance. Here, we present a non-traditional s-SNOM technique, named peak force scattering-type scanning near-field optical microscopy (PF-SNOM), by combination of peak force tapping mode and time-gated light detection. PF-SNOM enables direct sectioning of vertical near-field signals from a sample surface for both three-dimensional near-field imaging and spectroscopic analysis. Tip-induced relaxation of surface phonon polaritons are revealed and modeled by considering tip damping. PF-SNOM also delivers a spatial resolution of 5 nm and can simultaneously measure mechanical and electrical properties together with optical near-field signals. PF-SNOM is expected to facilitate three-dimensional nanoscale near-field characterizations and correlative in situ investigations on light-induced mechanical and electrical effects.

Nanoscale simultaneous chemical and mechanical imaging via peak force infrared microscopy.

Nondestructive chemical and mechanical measurements of materials with ~10-nm spatial resolution together with topography provide rich information on the compositions and organizations of heterogeneous materials and nanoscale objects. However, multimodal nanoscale correlations are difficult to achieve because of the limitation on spatial resolution of optical microscopy and constraints from instrumental complexities. We report a novel noninvasive spectroscopic scanning probe microscopy method-peak force infrared (PFIR) microscopy-that allows chemical imaging, collection of broadband infrared spectra, and mechanical mapping at a spatial resolution of 10 nm. In our technique, chemical absorption information is directly encoded in the withdraw curve of the peak force tapping cycle after illumination with synchronized infrared laser pulses in a simple apparatus. Nanoscale phase separation in block copolymers and inhomogeneity in CH3NH3PbBr3 perovskite crystals are studied with correlative infrared/mechanical nanoimaging. Furthermore, we show that the PFIR method is sensitive to the presence of surface phonon polaritons in boron nitride nanotubes. PFIR microscopy will provide a powerful analytical tool for explorations at the nanoscale across wide disciplines.