Photoinduced Force Research Articles

Infrared photoinduced force near-field spectroscopy of silicon carbide

Silicon carbide (SiC) is a promising microelectronic semiconductor with an infrared response that depends on factors such as polytype, growth conditions, and structuring. Recent advances in infrared nano-imaging techniques provide new opportunities to characterize thin films at the nanoscale. We performed tip-enhanced near-field spectral characterization on various 4H-SiC sample surfaces using photoinduced force microscopy (PiFM). The near-field spectra feature a peak associated with a surface phonon polariton (SPhP) resonance, which proves to be highly sensitive to the material’s surface quality. Interestingly, we found that the power absorption at the tip-sample junction, enhanced by the effective tip polarizability, can account for these PiFM spectra, resulting in a blend of both dispersive and dissipative spectral line shapes. As a potential application, a junction-field effect transistor was imaged to evaluate the technique’s ability to map the surface of different p- and n-doped areas, demonstrating high sensitivity and spatial resolution compared to Raman analysis.

Interfacial Microenvironment-Regulated Coordinate Structures Dictate the Metal-Organic Framework Facet Orientation toward Efficient CO2 Cycloaddition.

The preparation of high-quality highly oriented metal-organic framework (MOF) thin films is desirable for developing advanced functional devices. However, the pathways for controlling the oriented growth of MOFs are largely unknown, and determining their microcosmic evolution at the complex solid-liquid interface remains a challenge. Herein, we investigate the critical early growth stage of typical HKUST-1 on the COOH-functionalized Au substrate utilizing a combination of in situ surface-enhanced infrared spectroscopy, X-ray photoelectron spectroscopy, and photoinduced force microscopy. Detailed molecular level information indicates that it is not only the COOH-terminated SAM itself but also the distinct interfacial structures of the first metal coordinate layer and second ligand coordinate layer, which can be regulated in aprotic and protic solvents, that dominate the initial growth behavior of MOF and thus lead to the [111], [100], and polycrystal facet-oriented growth of HKUST-1. Moreover, the prepared HKUST-1 films exhibit a crystal facet-dependent catalytic rate in the chemical fixation of CO2 into cyclic carbonate. Our observations provide a reasonable guide for designing MOF-based anisotropic functional equipment.

Near-Infrared Light-Driven CO2 Reduction on Cup-Stacked Carbon Nanotubes.

Carbon nanotubes feature one-dimensional nature of collective excitations, wherein strong confinement of surface plasmons severely hinders the liberation of hot electrons (HEs), posing grand challenges for their utilization in photochemistry. In this study, we prototypically achieved directed HEs flow and extraction in hybrid plasmonic CNN based on cup-stacked carbon nanotubes (CSCNTs), taking advantage of their privileged edge-plane sites. The localized pz electronic states and accessible intersubband plasmon excitations in the near-infrared (NIR) regime stands in striking contrast to the conventional concentric carbon nanotubes, as evidenced by combined photo-induced force microscopy (PiFM) and transient photocurrent response. The hybrid comprising intimately integrated CSCNTs-C3N4 effectively sustains interfacial electronic states and underlies the energy extraction out of plasmonic components. The CNN demonstrates almost near-unity NIR light-driven CO2 reduction to CO with a rate of 1.35 µmol g-1 h-1. This work sheds light on the exploitation of metal-free carbon-based plasmonic nanostructures for photocatalytic applications.

Phosphate-Driven Interfacial Self-Assembly of Silk Fibroin for Continuous Noncovalent Growth of Nanothin Defect-Free Coatings.

Silk fibroin is a fiber-forming protein derived from the thread of Bombyx mori silkworm cocoons. This biocompatible protein, under the kosmotropic influence of potassium phosphate, can undergo supramolecular self-assembly driven by a random coil to β-sheet secondary structure transition. By leveraging concurrent nonspecific adsorption and self-assembly of silk fibroin, we demonstrate an interfacial phenomenon that yields adherent, defect-free nanothin protein coatings that grow continuously in time, without observable saturation in mass deposition. This noncovalent growth of silk fibroin coatings is a departure from traditionally studied protein adsorption phenomena, which generally yield adsorbed layers that saturate in mass with time and often do not completely cover the surface. Here, we explore the fundamental mechanisms of coating growth by examining the effects of coating solution parameters that promote or inhibit silk fibroin self-assembly. Results show a strong dependence of coating kinetics and structure on solution pH, salt species, and salt concentration. Moreover, coating growth was observed to occur in two stages: an early stage driven by protein-surface interactions and a late stage driven by protein-protein interactions. To describe this phenomenon, we developed a kinetic adsorption model with Langmuir-like behavior at early times and a constant steady-state growth rate at later times. Structural analysis by FTIR and photoinduced force microscopy show that small β-sheet-rich structures serve as anchoring sites for absorbing protein nanoaggregates, which is critical for coating formation. Additionally, β-sheets are preferentially located at the interface between protein nanoaggregates in the coating, suggesting their role in forming stable, robust coatings.

Nanoscopic Observation of Chiro-Optical Effect by Photoinduced Force Microscopy

Nanoscopic Observation of Chiro-Optical Effect by Photoinduced Force Microscopy

Direct Detection of the Magnetic Force and Field Coupling of Electronic Spins Using Photoinduced Magnetic Force Microscopy.

The intrinsic spin of the electron and its associated magnetic moment can provide insights into spintronics. However, the interaction is extremely weak, as is the case with the coupling between an electron's spin and a magnetic field, and it poses significant experimental challenges. Here we demonstrate the direct measurement of polarized single NV- centers and their spin-spin coupling behaviors in diamond. By using photoinduced magnetic force microscopy, we obtain the extremely weak magnetic force coupling originating from the electron spin. The polarized spin state of NV- centers, transitioning from |0⟩ to |±1⟩, and their corresponding Zeeman effect can be characterized through their interaction with a magnetic tip. The result presents an advancement in achieving electron spin measurements by magnetic force, avoiding the need for manufacturing conductive substrates. This facilitates a better understanding and control of electron spin to novel electronic states for future quantum technologies.

Adsorption Dynamics of Disodium Octaborate Tetrahydrate on Clay Minerals: Implications for Construction Wood Protection.

The wood preservative disodium octaborate tetrahydrate (DOT) migration is studied in clay. Using boron analysis by inductively coupled plasma optical emission spectroscopy (ICP-OES), DOT spatial and temporal dynamics are surveyed to show how DOT permeates into the wood and the clay using concentration profiles as a function of depth, initial wood moisture, and direction of filling. Atomic force microscopy and chemical imaging using photoinduced force microscopy are used to show the morphology of the wood samples and the distribution of DOT on their surface. ICP-OES results show that the average DOT concentration in the wood samples is originally 0.8 and 1.5 wt% in the bulk and at the surface, respectively. Conditioning of the wood to a moisture content of 19% in a climatic chamber reduces DOT concentration by 8% for the fir and 17% for the spruce. After one week of contact with the clays, the results showed a rapid decrease of 25-40% in DOT concentration in wood. On longer periods (5 months), the spruce shows a tendency to reabsorb the DOT from the clay and the DOT migration stabilizes at 20%. These results contribute to defining the dosage of DOT when the wood is exposed to clay.

Taking Photoacoustic Force into Account in Liquid-Phase Peak Force Infrared Microscopy.

The microscopic structure of the material's solid-liquid interface significantly influences its physicochemical properties. Peak force infrared microscopy (PFIR) is a powerful technique for analyzing these interfaces at the nanoscale, revealing crucial structure-activity relationships. PFIR is recognized for its explicit photothermal signal generation mechanism but tends to overlook other photoinduced forces, which can disturb the obtained infrared spectra, thereby reducing spectral signal-to-noise ratio (SNR) and sensitivity. We have developed a multiphysics-coupled theoretical model to assess the magnitudes of various photoinduced forces in PFIR experiments and have found that the magnitude of the photoacoustic force is comparable to that of the photothermal expansion force in a liquid environment. Our calculations show that through simple modulation of the pulse waveform it is possible to effectively suppress the photoacoustic interference, thereby improving the SNR and sensitivity of PFIR. This work aims to alert researchers to the potential for strong photoacoustic interference in liquid-phase PFIR measurements and enhance the performance of PFIR by clarifying the photoinduced forces entangled in the signals.

Surface-phonon-polariton-enhanced photoinduced dipole force for nanoscale infrared imaging.

The photoinduced dipole force (PiDF) is an attractive force arising from the Coulombic interaction between the light-induced dipoles on the illuminated tip and the sample. It shows extreme sample-tip distance and refractive index dependence, which is promising for nanoscale infrared (IR) imaging of ultrathin samples. However, the existence of PiDF in the mid-IR region has not been experimentally demonstrated due to the coexistence of photoinduced thermal force (PiTF), typically one to two orders of magnitude higher than PiDF. In this study, we demonstrate that, with the assistance of surface phonon polaritons, the PiDF of c-quartz can be enhanced to surpass its PiTF, enabling a clear observation of PiDF spectra reflecting the properties of the real part of permittivity. Leveraging the detection of the PiDF of phonon polaritonic substrate, we propose a strategy to enhance the sensitivity and contrast of photoinduced force responses in transmission images, facilitating the precise differentiation of the heterogeneous distribution of ultrathin samples.

Mapping electric field components of superchiral field with photo-induced force.

Circular dichroism (CD) of materials, difference in absorbance of left- and right-circularly polarized light, is a standard measure of chirality. Detection of the chirality for individual molecules is a frontier in analytical chemistry and optical science. The usage of a superchiral electromagnetic field near metallic structure is one promising way because it boosts the molecular far-field CD signal. However, it is still elusive as to how such a field actually interacts with the molecules. The cause is that the distribution of the electric field vector is unclear in the vicinity of the metal surface. In particular, it is difficult to directly measure the localized field, e.g., using aperture-type scanning near-field optical microscope. Here, we calculate the three-dimensional (3D) electric field vector, including the longitudinal field, and reveal the whole figure of the near-field CD on a two-dimensional (2D) plane just above the metal surface. Moreover, we propose a method to measure the near-field CD of the whole superchiral field by photo-induced force microscopy (PiFM), where the optical force distribution is mapped in a scanning 2D plane. We numerically demonstrate that, although the presence of the metallic probe tip affects the 3D electric field distribution, the PiFM is sufficiently capable to evaluate the superchiral field. Unveiling the whole figure of near-field is significantly beneficial in obtaining rich information of single molecules with multiple orientations and in analyzing the boosted far-field CD signals.

Optical Imaging of a Single Molecule with Subnanometer Resolution by Photoinduced Force Microscopy.

Visualizing the optical response of individual molecules is a long-standing goal in catalysis, molecular nanotechnology, and biotechnology. The molecular response is dominated not only by the electronic states in their isolated environment but also by neighboring molecules and the substrate. Information about the transfer of energy and charge in real environments is essential for the design of the desired molecular functions. However, visualizing these factors with spatial resolution beyond the molecular scale has been challenging. Here, by combining photoinduced force microscopy and Kelvin probe force microscopy, we have mapped the photoinduced force in a pentacene bilayer with a spatial resolution of 0.6 nm and observed its "multipole excitation". We identified the excitation as the result of energy and charge transfer between the molecules and to the Ag substrate. These findings can be achieved only by combining microscopy techniques to simultaneously visualize the optical response of the molecules and the charge transfer between the neighboring environments. Our approach and findings provide insights into designing molecular functions by considering the optical response at each step of layering molecules.

Sub-10 nm Nanoparticle Detection Using Multi-Technique-Based Micro-Raman Spectroscopy.

Microplastic pollution is a growing public concern as these particles are ubiquitous in various environments and can fragment into smaller nanoplastics. Another environmental concern arises from widely used engineered nanoparticles. Despite the increasing abundance of these nano-sized pollutants and the possibility of interactions with organisms at the sub cellular level, with many risks still being unknown, there are only a few publications on this topic due to the lack of reliable techniques for nanoparticle characterization. We propose a multi-technique approach for the characterization of nanoparticles down to the 10 nm level using standard micro-Raman spectroscopy combined with standard atomic force microscopy. We successfully obtained single-particle spectra from 25 nm sized polystyrene and 9 nm sized TiO2 nanoparticles with corresponding mass limits of detection of 8.6 ag (attogram) and 1.6 ag, respectively, thus demonstrating the possibility of achieving an unambiguous Raman signal from a single, small nanoparticle with a resolution comparable to more complex and time-consuming technologies such as Tip-Enhanced Raman Spectroscopy and Photo-Induced Force Microscopy.

Remote-Controllable Interfacial Electron Tunneling at Heterogeneous Molecular Junctions via Tip-Induced Optoelectrical Engineering.

Molecular electronics enables functional electronic behavior via single molecules or molecular self-assembled monolayers, providing versatile opportunities for hybrid molecular-scale electronic devices. Although various molecular junction structures are constructed to investigate charge transfer dynamics, significant challenges remain in terms of interfacial charging effects and far-field background signals, which dominantly block the optoelectrical observation of interfacial charge transfer dynamics. Here, tip-induced optoelectrical engineering is presented that synergistically correlates photo-induced force microscopy and Kelvin probe force microscopyto remotely control and probe the interfacial charge transfer dynamics with sub-10nm spatial resolution. Based on this approach, the optoelectrical origin of metal-molecule interfaces is clearly revealed by the nanoscale heterogeneity of the tip-sample interaction and optoelectrical reactivity, which theoretically aligned with density functional theory calculations. For a practical device-scale demonstration of tip-induced optoelectrical engineering, interfacial tunneling is remotely controlled at a 4-inch wafer-scale metal-insulator-metal capacitor, facilitating a 5.211-fold current amplification with the tip-induced electrical field. In conclusion, tip-induced optoelectrical engineering provides a novel strategy to comprehensively understand interfacial charge transfer dynamics and a non-destructive tunneling control platform that enables real-time and real-space investigation of ultrathin hybrid molecular systems.

Characterizing and controlling infrared phonon anomaly of bilayer graphene in optical-electrical force nanoscopy

We, for the first time, report the nanoscopic imaging study of anomalous infrared (IR) phonon enhancement of bilayer graphene, originated from the charge imbalance between the top and bottom layers, resulting in the enhancement of E1u mode of bilayer graphene near 0.2 eV. We modified the multifrequency atomic force microscope platform to combine photo-induced force microscope with electrostatic/Kelvin probe force microscope constituting a novel hybrid nanoscale optical-electrical force imaging system. This enables to observe a correlation between the IR response, doping level, and topographic information of the graphene layers. Through the nanoscale spectroscopic image measurements, we demonstrate that the charge imbalance at the graphene interface can be controlled by chemical (doping effect via Redox mechanism) and mechanical (triboelectric effect by the doped cantilever) approaches. Moreover, we can also diagnosis the subsurface cracks on the stacked few-layer graphene at nanoscale, by monitoring the strain-induced IR phonon shift. Our approach provides new insights into the development of graphene-based electronic and photonic devices and their potential applications.

Visualizing the Structure, Composition and Activity of Single Catalyst Particles for Olefin Polymerization and Polyolefin Decomposition.

The structural and morphological characterization of individual catalyst particles for olefin polymerization, as well as for the reverse process of polyolefin decomposition, can provide an improved understanding for how these catalyst materials operate under relevant reaction conditions. In this review, we discuss an emerging analytical toolbox of 2D and 3D chemical imaging techniques that is suitable for investigating the chemistry and reactivity of related catalyst systems. While synchrotron-based X-ray microscopy still provides unparalleled spatial resolutions in 2D and 3D, a number of laboratory-based techniques, most notably focused ion beam-scanning electron microscopy, confocal fluorescence microscopy, infrared photoinduced force microscopy and laboratory-based X-ray nano-computed tomography, have helped to significantly expand the arsenal of analytical tools available to scientists in heterogeneous catalysis and polymer science. In terms of future research, the review outlines the role and impact of in situ and operando (spectro-)microscopy experiments, involving sophisticated reactors as well as online reactant and product analysis, to obtain real-time information on the formation, decomposition, and mobility of polymer phases within single catalyst particles. Furthermore, the potential of fluorescence microscopy, X-ray microscopy and optical microscopy is highlighted for the high-throughput characterization of olefin polymerization and polyolefin decomposition catalysts. By combining these chemical imaging techniques with, for example, chemical staining methodologies, selective probe molecules as well as particle sorting approaches, representative structure-activity relationships can be derived at the level of single catalyst particles.

Nanoscale chemical characterization of secondary protein structure of F-Actin using mid-infrared photoinduced force microscopy (PiF-IR)

The recently developed photoinduced force microscopy for mid-infrared (PiF-IR) offers high spectral resolution in combination with surface sensitivity and a spatial resolution in the range of a few nanometers. Although PiF-IR has primarily been applied to polymer materials, this technology presents significant potential for the chemical characterization of cellular structures approaching single-molecule sensitivity. We applied PiF-IR to differently polymerized F-Actin samples finding general agreement with FTIR spectra from the same samples. Single PiF-IR spectra of F-Actin show variations in the amide I band spectral region, which is related to secondary protein structure. Local variations are also seen in PiF-IR hyperspectra in this region. Such high sensitivity is a necessary requirement for discriminating Actin organization into bundles and other networks in cells and tissue. We applied PiF-IR to mouse liver tissue ex vivo. Single-frequency PiF-IR scans at three different IR frequencies show significant variations in local contrast. However, the presence of other proteins and the unique spatial resolution of PiF-IR pose a challenge to interpreting and validating such data. Careful design of model systems and further theoretical understanding of PiF-IR data far from bulk averages are needed to fully unfold the potential of PiF-IR for high-resolution chemical investigation in the Life Sciences.

Photo-induced bending of chalcogenide glass fiber

Optical fibers of a typical chalcogenide glass, As2S3, can be sharply bent only by sideward irradiation of polarized light. The bending direction depends on the polarization; the fiber bends in the forward/backward directions of light propagation for bandgap light that is linearly-polarized orthogonal/parallel to the fiber axis. A reciprocity law between the intensity and the exposure time holds, which suggests that thermal effects are irrelevant. Alternatively, the bending appears to arise from photo-induced optomechanical forces and fluidity. This phenomenon could be developed to a photo-manipulation method of chalcogenide-glass fibers, promising for all-optical switching/modulation devices working at infrared wavelengths.

Bionanocomposites with Enhanced Physical Properties from Curli Amyloid Assemblies and Cellulose Nanofibrils.

Proteinaceous amyloid fibrils are one of the stiffest biopolymers due to their extensive cross-β-sheet quaternary structure, whereas cellulose nanofibrils (CNFs) exhibit interesting properties associated with their nanoscale size, morphology, large surface area, and biodegradability. Herein, CNFs were supplemented with amyloid fibrils assembled from the Curli-specific gene A (CsgA) protein, the main component of bacterial biofilms. The resulting composites showed superior mechanical properties, up to a 7-fold increase compared to unmodified CNF films. Wettability and thermogravimetric analyses demonstrated high surface hydrophobicity and robust thermal tolerance. Bulk spectroscopic characterization of CNF-CsgA films revealed key insights into the molecular organization within the bionanocomposites. Atomic force microscopy and photoinduced force microscopy revealed the high-resolution location of curli assemblies into the CNF films. This novel sustainable and cost-effective CNF-based bionanocomposites supplemented with intertwined bacterial amyloid fibrils opens novel directions for environmentally friendly applications demanding high mechanical, water-repelling properties, and thermal resistance.

Three-dimensional photoinduced force microscopy reveals artifacts from photothermal tip vibrations

Photoinduced force microscopy (PiFM) is a technique for nanoscale characterization of optically active materials. For correct data analysis, it is important to understand and recognize potential artifacts. In this study, we provide a Fourier-based framework for the different origins of the photoinduced force, including the photothermal motion of the atomic force microscopy (AFM) tip in PiFM measurements. We use individual gold nanoparticles as a model system to explore such tip-related phenomena and to demonstrate the benefits of three-dimensional analysis of the photoinduced force. Comparison of the in-phase heterodyne PiFM signal and the derivative of the tip–sample interaction, ∂⟨kts⟩/∂zc, directly reveals a photothermal origin for the PiFM signal in our experiments. Furthermore, we observe a lateral photothermal vibration of the tip apex induced by tip asymmetry. This lateral vibration emphasizes the importance of considering the motion of the AFM tip during data analysis. The improved lateral resolution conferred by ∂⟨kts⟩/∂zc can lead to misinterpretation. Our findings highlight that meticulous data analysis is required to ensure the correct interpretation of PiFM data.

Spatial Tuning of Light-Matter Interaction via Strain-Gradient-Induced Polarization in Freestanding Wrinkled 2D Materials.

To date, controlled deformation of two-dimensional (2D) materials has been extensively demonstrated with substrate-supported structures. However, interfacial effects arising from these supporting materials may suppress or alter the unique behavior of the deformed 2D materials. To address interfacial effects, we report, for the first time, the formation of a micrometer-scale freestanding wrinkled structure of 2D material without any encapsulation layers where we observed the enhanced light-matter interactions with a spatial modulation. Freestanding wrinkled monolayer WSe2 exhibited about a 330% enhancement relative to supported wrinkled WSe2 quantified through photoinduced force microscopy. Spatial modulation and enhancement of light interaction in the freestanding wrinkled structures are attributed to the enhanced strain-gradient effect (i.e., out-of-plane polarization) enabled by removing the constraining support and proximate dielectrics. Our findings offer an additional degree of freedom to modulate the out-of-plane polarization and enhance the out-of-plane light-matter interaction in 2D materials.