How to improve the structural stabilities of halide perovskite quantum dots: review of various strategies to enhance the structural stabilities of halide perovskite quantum dots.

Halide perovskites have emerged as promising materials for various optoelectronic devices because of their excellent optical and electrical properties. In particular, halide perovskite quantum dots (PQDs) have garnered considerable attention as emissive materials for light-emitting diodes (LEDs) because of their higher color purities and photoluminescence quantum yields compared to conventional inorganic quantum dots (CdSe, ZnSe, ZnS, etc.). However, PQDs exhibit poor structural stabilities in response to external stimuli (moisture, heat, etc.) owing to their inherent ionic nature. This review presents recent research trends and insights into improving the structural stabilities of PQDs. In addition, the origins of the poor structural stabilities of PQDs and various methods to overcome this drawback are discussed. The structural degradation of PQDs is mainly caused by two mechanisms: (1) defect formation on the surface of the PQDs by ligand dissociation (i.e., detachment of weakly bound ligands from the surface of PQDs), and (2) vacancy formation by halide migration in the lattices of the PQDs due to the low migration energy of halide ions. The structural stabilities of PQDs can be improved through four methods: (1) ligand modification, (2) core-shell structure, (3) crosslinking, and (4) metal doping, all of which are presented in detail herein. This review provides a comprehensive understanding of the structural stabilities and opto-electrical properties of PQDs and is expected to contribute to future research on improving the device performance of perovskite quantum dot LEDs (PeLEDs).

All-optical control of high-purity trions in nanoscale waveguide.

The generation of high-purity localized trions, dynamic exciton-trion interconversion, and their spatial modulation in two-dimensional (2D) semiconductors are building blocks for the realization of trion-based optoelectronic devices. Here, we present a method for the all-optical control of the exciton-to-trion conversion process and its spatial distributions in a MoS2 monolayer. We induce a nanoscale strain gradient in a 2D crystal transferred on a lateral metal-insulator-metal (MIM) waveguide and exploit propagating surface plasmon polaritons (SPPs) to localize hot electrons. These significantly increase the electrons and efficiently funnel excitons in the lateral MIM waveguide, facilitating complete exciton-to-trion conversion even at ambient conditions. Additionally, we modulate the SPP mode using adaptive wavefront shaping, enabling all-optical control of the exciton-to-trion conversion rate and trion distribution in a reversible manner. Our work provides a platform for harnessing excitonic quasiparticles efficiently in the form of trions at ambient conditions, enabling high-efficiency photoconversion.

Surface Active Salivary Metabolites Indicate Oxidative Stress and Inflammation in Obstructive Sleep Apnea.

PURPOSE: Obstructive sleep apnea (OSA), a highly prevalent and potentially serious sleep disorder, requires effective screening tools. Saliva is a useful biological fluid with various metabolites that might also influence upper airway patency by affecting surface tension in the upper airway. However, little is known about the composition and role of salivary metabolites in OSA. Therefore, we investigated the metabolomics signature in saliva from the OSA patients and evaluated the associations between identified metabolites and salivary surface tension. METHODS: We studied 68 subjects who visited sleep clinic due to the symptoms of OSA. All underwent full-night in-lab polysomnography. Patients with apnea-hypopnea index (AHI) < 10 were classified to the control, and those with AHI ≥ 10 were the OSA groups. Saliva samples were collected before and after sleep. The centrifuged saliva samples were analyzed by liquid chromatography with high-resolution mass spectrometry (ultra-performance liquid chromatography-tandem mass spectrometry; UPLC-MS/MS). Differentially expressed salivary metabolites were identified using open source software (XCMS) and Compound Discoverer 2.1. Metabolite set enrichment analysis (MSEA) was performed using MetaboAnalyst 5.0. The surface tension of the saliva samples was determined by the pendant drop method. RESULTS: Three human-derived metabolites (1-palmitoyl-2-[5-hydroxyl-8-oxo-6-octenoyl]-sn-glycerol-3-phosphatidylcholine [PHOOA-PC], 1-palmitoyl-2-[5-keto-8-oxo-6-octenoyl]-sn-glycerol-3-phosphatidylcholine [KPOO-PC], and 9-nitrooleate) were significantly upregulated in the after-sleep salivary samples from the OSA patients compared to the control group samples. Among the candidate metabolites, only PHOOA-PC was correlated with the AHI. In OSA samples, salivary surface tension decreased after sleep. The differences in surface tension were negatively correlated with PHOOA-PC and 9-nitrooleate concentrations. Furthermore, MSEA revealed that arachidonic acid-related metabolism pathways were upregulated in the after-sleep samples from the OSA group. CONCLUSIONS: This study revealed that salivary PHOOA-PC was correlated positively with the AHI and negatively with salivary surface tension in the OSA group. Salivary metabolomic analysis may improve our understanding of upper airway dynamics and provide new insights into novel biomarkers and therapeutic targets in OSA.

Chemical gradients on graphene via direct mechanochemical cleavage of atoms from chemically functionalized graphene surfaces.

Manipulating the surface chemistry of graphene is critical to many applications that are achievable by chemical functionalization. Specifically, tailoring the spatial distribution of functional groups offers more opportunities to explore functionality using continuous changes in surface energy. To this end, careful consideration is required to demonstrate the chemical gradient on graphene surfaces, and it is necessary to develop a technique to pattern the spatial distribution of functional groups. Here, we demonstrate the tailoring of a chemical gradient through direct mechanochemical cleavage of atoms from chemically functionalized graphene surfaces via an atomic force microscope. Additionally, we define the surface characteristics of the fabricated sample by using lateral force microscopy revealing the materials' intrinsic properties at the nanoscale. Furthermore, we perform the cleaning process of the obtained lateral force images by using a machine learning method of truncated singular value decomposition. This work provides a useful technique for many applications utilizing continuous changes in the surface energy of graphene.

Direct 3D-printed CdSe quantum dots via scanning micropipette.

The micropipette, pencil-shaped with an aperture diameter of a few micrometers, is a potentially promising tool for the three-dimensional (3D) printing of individual microstructures based on its capability to deliver low volumes of nanomaterial solution on a desired spot resulting in micro/nanoscale patterning. Here, we demonstrate a direct 3D printing technique in which a micropipette with a cadmium selenide (CdSe) quantum dot (QD) solution is guided by an atomic force microscope with no electric field and no piezo-pumping schemes. We define the printed CdSe QD wires, which are a composite material with a QD-liquid coexistence phase, by using photoluminescence and Raman spectroscopy to analyze their intrinsic properties and additionally demonstrate a means of directional falling.

Wear and Corrosion Resistance Technology of Thin Film Materials: A New Open Special Issue in Materials.

"Wear and Corrosion Resistance Technology of Thin Film Materials" is a new and open Special Issue published in Materials, presenting research and review papers that focus on the wear and corrosion resistance of various materials, new scientific issues and their useful applications in wear and corrosion research and industrial sectors [...].

Sorting Gold and Sand (Silica) Using Atomic Force Microscope-Based Dielectrophoresis.

Additive manufacturing-also known as 3D printing-has attracted much attention in recent years as a powerful method for the simple and versatile fabrication of complicated three-dimensional structures. However, the current technology still exhibits a limitation in realizing the selective deposition and sorting of various materials contained in the same reservoir, which can contribute significantly to additive printing or manufacturing by enabling simultaneous sorting and deposition of different substances through a single nozzle. Here, we propose a dielectrophoresis (DEP)-based material-selective deposition and sorting technique using a pipette-based quartz tuning fork (QTF)-atomic force microscope (AFM) platform DEPQA and demonstrate multi-material sorting through a single nozzle in ambient conditions. We used Au and silica nanoparticles for sorting and obtained 95% accuracy for spatial separation, which confirmed the surface-enhanced Raman spectroscopy (SERS). To validate the scheme, we also performed a simulation for the system and found qualitative agreement with the experimental results. The method that combines DEP, pipette-based AFM, and SERS may widely expand the unique capabilities of 3D printing and nano-micro patterning for multi-material patterning, materials sorting, and diverse advanced applications.

Quantitative Visualization of the Nanomechanical Young's Modulus of Soft Materials by Atomic Force Microscopy.

The accurate measurement of nanoscale mechanical characteristics is crucial in the emerging field of soft condensed matter for industrial applications. An atomic force microscope (AFM) can be used to conduct nanoscale evaluation of the Young's modulus on the target surface based on site-specific force spectroscopy. However, there is still a lack of well-organized study about the nanomechanical interpretation model dependence along with cantilever stiffness and radius of the tip apex for the Young's modulus measurement on the soft materials. Here, we present the fast and accurate measurement of the Young's modulus of a sample's entire scan surface using the AFM in a newly developed PinPointTM nanomechanical mode. This approach enables simultaneous measurements of topographical data and force-distance data at each pixel within the scan area, from which quantitative visualization of the pixel-by-pixel topographical height and Young's modulus of the entire scan surface was realized. We examined several models of contact mechanics and showed that cantilevers with proper mechanical characteristics such as stiffness and tip radius can be used with the PinPointTM mode to accurately evaluate the Young's modulus depending on the sample type.

Probing the shear viscoelasticity of a nanoscale ionic liquid meniscus.

Ionic liquids (ILs) are emerging as novel solvents that exhibit peculiar mechanical properties in the form of thin films on metal surfaces under normal pressure. However, the mechanical properties of ILs in the form of nano-meniscus have not been analyzed yet. Here, we investigate the shear viscoelasticity of a single IL meniscus at the nanoscale. To characterize the shear rheological properties of ILs, we employ a quartz tuning fork-based atomic force microscope, conduct dynamic force spectroscopy, and analyse shear properties using the non-Newtonian-Maxwell model. The elastic response of the IL nanomeniscus is found to be about 25 times higher than that of the bulk IL bridge, whereas the viscous responses are similar. In addition, by conducting shear velocity-dependent measurements, we find that the IL meniscus shows nonlinear rheological behaviours. Interestingly, we observe that the relaxation time of the IL increases at a tip-substrate distance of about 60 nm.

Direct measurement of curvature-dependent surface tension of an alcohol nanomeniscus.

Surface tension is a key parameter for understanding nucleation in the very initial stage of phase transformation. Although surface tension has been predicted to vary with the curvature of the liquid-vapor interface, particularly at the large curvature of, e.g., the subnanometric critical nucleus, experimental study still remains challenging due to inaccessibility to such a small cluster. Here, by directly measuring the critical size of a single capillary-condensed nanomeniscus using atomic force microscopy, we address the curvature dependence of surface tension of alcohols and observe that the surface tension is doubled for ethanol and n-propanol with a radius-of-curvature of ∼-0.46 nm. We also find that the interface of larger negative (positive) curvature exhibits larger (smaller) surface tension, which evidently governs nucleation at the ∼1 nm scale and below, indicating more facilitated nucleation than normally expected. Such well characterized curvature effects contribute to better understanding and accurate analysis of nucleation occurring in various fields including materials science and atmospheric science.

Dynamic Responses of Electrically Driven Quartz Tuning Fork and qPlus Sensor: A Comprehensive Electromechanical Model for Quartz Tuning Fork.

A quartz tuning fork and its qPlus configuration show different characteristics in their dynamic features, including peak amplitude, resonance frequency, and quality factor. Here, we present an electromechanical model that comprehensively describes the dynamic responses of an electrically driven tuning fork and its qPlus configuration. Based on the model, we theoretically derive and experimentally validate how the peak amplitude, resonance frequency, quality factor, and normalized capacitance are changed when transforming a tuning fork to its qPlus configuration. Furthermore, we introduce two experimentally measurable parameters that are intrinsic for a given tuning fork and not changed by the qPlus configuration. The present model and analysis allow quantitative prediction of the dynamic characteristics in tuning fork and qPlus, and thus could be useful to optimize the sensors' performance.

Theoretical study of buckling-based nonlinear transition near a static bifurcation point.

We theoretically investigate the nonlinear behavior of a buckled tip near the bifurcation point under external stress. We present a mechanical model for the buckled tip and derive the governing equation that describes the "buckling-to-flipping" nonlinear transition of the tip motion. Our minimal mechanistic model fully captures the velocity-dependent flipping phenomena, in which the flip position of the tip varies with the speed of the surface motion, as consistently observed in previous experiments. The present study could be applicable for sensitive detection of directional surface motion such as seismic waves.

Nanopipette/Nanorod-Combined Quartz Tuning Fork⁻Atomic Force Microscope.

We introduce a nanopipette/quartz tuning fork (QTF)-atomic force microscope (AFM) for nanolithography and a nanorod/QTF-AFM for nanoscratching with in situ detection of shear dynamics during performance. Capillary-condensed nanoscale water meniscus-mediated and electric field-assisted small-volume liquid ejection and nanolithography in ambient conditions are performed at a low bias voltage (~10 V) via a nanopipette/QTF-AFM. We produce and analyze Au nanoparticle-aggregated nanowire by using nanomeniscus-based particle stacking via a nanopipette/QTF-AFM. In addition, we perform a nanoscratching technique using in situ detection of the mechanical interactions of shear dynamics via a nanorod/QTF-AFM with force sensor capability and high sensitivity.

Buckling-Based Non-Linear Mechanical Sensor.

Mechanical sensors provide core keys for high-end research in quantitative understanding of fundamental phenomena and practical applications such as the force or pressure sensor, accelerometer and gyroscope. In particular, in situ sensitive and reliable detection is essential for measurements of the mechanical vibration and displacement forces in inertial sensors or seismometers. However, enhancing sensitivity, reducing response time and equipping sensors with a measurement capability of bidirectional mechanical perturbations remains challenging. Here, we demonstrate the buckling cantilever-based non-linear dynamic mechanical sensor which addresses intrinsic limitations associated with high sensitivity, reliability and durability. The cantilever is attached on to a high-Q tuning fork and initially buckled by being pressed against a solid surface while a flexural stress is applied. Then, buckling instability occurs near the bifurcation region due to lateral movement, which allows high-sensitive detection of the lateral and perpendicular surface acoustic waves with bandwidth-limited temporal response of less than 1 ms.

Adhesive force measurement of steady-state water nano-meniscus: Effective surface tension at nanoscale.

When the surface of water is curved at nanoscale as a bubble, droplet and meniscus, its surface tension is expected to be smaller than that of planar interface, which still awaits experimental studies. Here, we report static and dynamic force spectroscopy that measures the capillary force of a single nanoscale water meniscus at constant curvature condition. Based on the Young-Laplace equation, the results are used to obtain the effective surface tension (ST) of the meniscus, which decreases to less than 20% of the bulk value at the radius-of-curvature (ROC) below 25 nm, while indicating the bulk behaviour above ~130 nm ROC. Interestingly, such a possibility provides a qualitative resolution of the unsettled discrepancies between experiments and theories in the thermodynamic activation processes for the mentioned three types of nano-curvatured water. Our results may not only lead to development of microscopic theories of ST as well as further experimental investigations, but also help better understanding of the ST-induced nanoscale dynamics such as cluster growth or protein folding, and the ST-controlled design of nano-biomaterials using the nano-meniscus.

Bifurcation-enhanced ultrahigh sensitivity of a buckled cantilever.

Buckling, first introduced by Euler in 1744 [Euler L (1744) Opera Omnia I 24:231], a sudden mechanical sideways deflection of a structural member under compressive stress, represents a bifurcation in the solution to the equations of static equilibrium. Although it has been investigated in diverse research areas, such a common nonlinear phenomenon may be useful to devise a unique mechanical sensor that addresses the still-challenging features, such as the enhanced sensitivity and polarization-dependent detection capability. We demonstrate the bifurcation-enhanced sensitive measurement of mechanical vibrations using the nonlinear buckled cantilever tip in ambient conditions. The cantilever, initially buckled with its tip pinned, flips its buckling near the bifurcation point (BP), where the buckled tip becomes softened. The enhanced mechanical sensitivity results from the increasing fluctuations, unlike the typical linear sensors, which facilitate the noise-induced buckling-to-flipping transition of the softened cantilever. This allows the in situ continuous or repeated single-shot detection of the surface acoustic waves of different polarizations without any noticeable wear of the tip. We obtained the sensitivity above 106 V(m/s)-1, a 1,000-fold enhancement over the conventional seismometers. Our results lead to development of mechanical sensors of high sensitivity, reproducibility, and durability, which may be applied to detect, e.g., the directional surface waves on the laboratory as well as the geological scale.

Nanophotonic Atomic Force Microscope Transducers Enable Chemical Composition and Thermal Conductivity Measurements at the Nanoscale.

The atomic force microscope (AFM) offers a rich observation window on the nanoscale, yet many dynamic phenomena are too fast and too weak for direct AFM detection. Integrated cavity-optomechanics is revolutionizing micromechanical sensing; however, it has not yet impacted AFM. Here, we make a groundbreaking advance by fabricating picogram-scale probes integrated with photonic resonators to realize functional AFM detection that achieve high temporal resolution (<10 ns) and picometer vertical displacement uncertainty simultaneously. The ability to capture fast events with high precision is leveraged to measure the thermal conductivity (η), for the first time, concurrently with chemical composition at the nanoscale in photothermal induced resonance experiments. The intrinsic η of metal-organic-framework individual microcrystals, not measurable by macroscale techniques, is obtained with a small measurement uncertainty (8%). The improved sensitivity (50×) increases the measurement throughput 2500-fold and enables chemical composition measurement of molecular monolayer-thin samples. Our paradigm-shifting photonic readout for small probes breaks the common trade-off between AFM measurement precision and ability to capture transient events, thus transforming the ability to observe nanoscale dynamics in materials.

Nanoscale imaging and spectroscopy of band gap and defects in polycrystalline photovoltaic devices.

Improving the power conversion efficiency of photovoltaic (PV) devices is challenging because the generation, separation and collection of electron-hole pairs are strongly dependent on details of the nanoscale chemical composition and defects which are often poorly known. In this work, two novel scanning probe nano-spectroscopy techniques, direct-transmission near-field scanning optical microscopy (dt-NSOM) and photothermal induced resonance (PTIR), are implemented to probe the distribution of defects and the bandgap variation in thin lamellae extracted from polycrystalline CdTe PV devices. dt-NSOM provides high-contrast spatially-resolved maps of light transmitted through the sample at selected wavelengths. PTIR provides absorption maps and spectra over a broad spectral range, from visible to mid-infrared. Results show variation of the bandgap through the CdTe thickness and from grain to grain that is spatially uncorrelated with the distributions of shallow and deep defects.

Fabrication of Scanning Electrochemical Microscopy-Atomic Force Microscopy Probes to Image Surface Topography and Reactivity at the Nanoscale.

Concurrent mapping of chemical reactivity and morphology of heterogeneous electrocatalysts at the nanoscale allows identification of active areas (protrusions, flat film surface, or cracks) responsible for productive chemistry in these materials. Scanning electrochemical microscopy (SECM) can map surface characteristics, record catalyst activity, and identify chemical products at solid-liquid electrochemical interfaces. It lacks, however, the ability to distinguish topographic features where surface reactivity occurs. Here, we report the design and fabrication of scanning probe tips that combine SECM with atomic force microscopy (AFM) to perform measurements at the nanoscale. Our probes are fabricated by integrating nanoelectrodes with quartz tuning forks (QTFs). Using a calibration standard fabricated in our lab to test our probes, we obtain simultaneous topographic and electrochemical reactivity maps with a lateral resolution of 150 nm.

Viscometry of single nanoliter-volume droplets using dynamic force spectroscopy.

The viscometry of minute amounts of liquid has been in high demand as a novel tool for medical diagnosis and biological assays. Various microrheological techniques have shown the capability to handle small volumes. However, as the liquid volume decreases down to nanoliter scale, increasingly dominant surface effects complicate the measurement and analysis, which remain a challenge in microrheology. Here, we demonstrate an atomic force microscope-based platform that determines the viscosity of single sessile drops of 1 nanoliter Newtonian fluids. We circumvent interfacial effects by measuring the negative-valued shear elasticity, originating from the retarded fluidic response inside the drop. Our measurement is independent of the liquid-boundary effects, and thus is valid without a priori knowledge of surface tension or contact angle, and consistently holds at a 1 milliliter-scale volume. Importantly, while previous methods typically need a much larger 'unrecoverable' volume above 1 microliter, our simple platform uses only ∼1 nanoliter. Our results offer a quantitative and unambiguous methodology for viscosity measurements of extremely minute volumes of Newtonian liquids on the nanoliter scale.