Surface and Subsurface Physical and Chemical Characterization of Materials at the Nanoscale

Abstract The discontinuity in the atomic fabric of materials that defines the transition into a new medium gives rise to intriguing properties. Examples include the electronic tunneling behavior in scanning tunneling microscope or gigantic enhancement in the Raman emission from molecules near the surfaces of noble metals. In modern microscopy, spatial and spectral resolutions are of great importance in tackling questions related to material properties. The emergence of the atomic force microscopy (AFM), which surpasses what can be achieved optically due to the inherent diffraction limit, has opened numerous opportunities for investigating surfaces. However, a contemporary challenge in nanoscience is the non-destructive characterization of materials. The ability to non-invasively explore subsurface domains for presence of inhomogeneities is of tremendous importance. In addition, techniques providing both physical and chemical information are needed to reach a comprehensive understanding of the composition and behavior of complex systems. In order to tackle the subsurface and spectral imaging, here we propose to make use of the nonlinear interaction forces between the atoms of an AFM probe tip and those of a given sample surface. Such forces are known to contain a short range repulsive component and a long range van der Waals attractive contribution. This interfacial force can give rise to a multiple-order nanomechanical coupling between the probe and the sample, offering tremendous potential for obtaining a host of material characteristics. By applying a multi-harmonic mechanical forcing to the probe and another multi-harmonic forcing to the sample, we obtain, via frequency mixing a series of new operational modes. By varying the nature of the excitations, using elastic or photonic coupling, it is possible to obtain physical and chemical signature of a heterogeneous medium with nanoscale resolution. The technique, termed mode synthesizing atomic force microscopy (MSAFM) is therefore described as a generalized multifrequency AFM. We highlight the versatility of MSAFM and its potential to contribute to important problems in material sciences, toxicology and energy research, by presenting three specific studies: 1- imaging buried nanofabricated structures; 2- investigating the presence and distribution of embedded nanoparticles in a cell; and 3- characterizing the complex structures of plant cells

Photoluminescence quenching in gold - MoS2 hybrid nanoflakes

Achieving tunability of two dimensional (2D) transition metal dichalcogenides (TMDs) functions calls for the introduction of hybrid 2D materials by means of localized interactions with zero dimensional (0D) materials. A metal-semiconductor interface, as in gold (Au) - molybdenum disulfide (MoS2), is of great interest from the standpoint of fundamental science as it constitutes an outstanding platform to investigate plasmonic-exciton interactions and charge transfer. The applied aspects of such systems introduce new options for electronics, photovoltaics, detectors, gas sensing, catalysis, and biosensing. Here we consider pristine MoS2 and study its interaction with Au nanoislands, resulting in local variations of photoluminescence (PL) associated with various Au-MoS2 hybrid configurations. By controllably depositing monolayers of Au on MoS2 to form Au nanostructures of given size and thickness, we investigate the electronic structure of the resulting hybrid systems. We present strong evidence of PL quenching of MoS2 as a result of charge transfer from MoS2 to Au: p-doping of MoS2. The results suggest new avenues for 2D nanoelectronics, active control of transport or catalytic properties

Photoluminescence Quenching in Single-layer MoS2 via Oxygen Plasma Treatment

By creating defects via oxygen plasma treatment, we demonstrate optical properties variation of single-layer MoS2. We found that, with increasing plasma exposure time, the photoluminescence (PL) evolves from very high intensity to complete quenching, accompanied by gradual reduction and broadening of MoS2 Raman modes, indicative of distortion of the MoS2 lattice after oxygen bombardment. X-ray photoelectron spectroscopy study shows the appearance of Mo6+ peak, suggesting the creation of MoO3 disordered regions in the MoS2 flake. Finally, using band structure calculations, we demonstrate that the creation of MoO3 disordered domains upon exposure to oxygen plasma leads to a direct to indirect bandgap transition in single-layer MoS2, which explains the observed PL quenching.Comment: 12 pages, 7 figure

Non-destructively capturing CMAS degradation of EB-PVD thermal barrier coatings through 3D confocal Raman renderings

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Probing Chemical And Physical Properties Of Poplar Tension Wood Using Confocal Raman Microscopy And Pulsed Force Mode Atomic Force Microscopy

Lignocellulosic biofuels have been identified as a possible solution to contribute to the world\u27s demands in energy and environmental sustainability. However, the fundamental understanding of the physical and chemical traits hindering key reactions during biomass to biofuel conversion processes has been limited by the lack of suitable tools and by the large natural variability in such systems. Reaction wood constitutes a good model system to study variations of cellulose content, given the increase in cellulose content in the cell walls of the region under tension in the plant during growth. In this work, we use confocal Raman mapping and Pulsed Force Mode Atomic Force Microscopy (PFM) to explore the effect of variation in cellulose content on the structure and composition of the plant cell wall at the nanoscale. Using statistical analysis on Raman datasets, the characteristic peaks for cellulose and lignin are examined to reveal changes in peak positions across the different scanned regions of the cross section. PFM is used to study local mechanical properties of the different layers of the cell wall. Our approach facilitates the correlation of structure-composition traits of the plant cell wall for a more fundamental understanding of processes involved in biofuel research

Coupling Enhancement And Giant Rabi-Splitting In Large Arrays Of Tunable Plexcitonic Substrates

Advances in active manipulation of light at the nanoscale are rapidly emerging with the concept of plexcitonic coupling at the interface between plasmonics nanostructures and excitonic molecules. In this work, we devise a simple fabrication scheme to produce and optimize large area tunable plasmonic substrates for strong plasmon-exciton interactions. By tuning the diameter of the nanoholes using a simple plasma etching process, we demonstrate the potential of our approach to deliver tunable plasmonic substrates. Thus, large enhancements of fluorescence and Raman scattering could be measured. Moreover, hybridized states appearing in the presence of excitonic molecules (RG6) give rise to anticrossing behaviors in extinction spectroscopy, a phenomenon also known as Rabi-splitting. The results demonstrate the great potential of our large nanofabricated arrays as plexcitonic substrates for numerous applications, including sensors, light harvesters, and all-optical switches

Supercapacitor Electrode Materials: Nanostructures From 0 To 3 Dimensions

Supercapacitors have drawn considerable attention in recent years due to their high specific power, long cycle life, and ability to bridge the power/energy gap between conventional capacitors and batteries/fuel cells. Nanostructured electrode materials have demonstrated superior electrochemical properties in producing high-performance supercapacitors. In this review article, we describe the recent progress and advances in designing nanostructured supercapacitor electrode materials based on various dimensions ranging from zero to three. We highlight the effect of nanostructures on the properties of supercapacitors including specific capacitance, rate capability and cycle stability, which may serve as a guideline for the next generation of supercapacitor electrode design

Translocation Of N-Acetyl Cysteine Capped Fluorescent Quantum Dots In Plant Tissue: Confocal Imaging Studies

Semiconductor fluorescent quantum dots (Qdots) are popularly used as bioimaging taggants in live cell imaging and spectroscopy. In recent years, Qdots taggants are emerging in agricultural applications. Studies are primarily focused on nanotoxicity of ultra-small size watersoluble Qdots in plant systems. Nanotoxicity is correlated with Qdot core composition and surface coating. However, Qdots with certain chemical composition and surface coating may boost plant growth. In this study, we report that N-acetyl cysteine (NAC) capped ∼3.5 nm size ZnS: Mn/ZnS Qdots (NAC-Qdot) are efficiently uptaken by the snow pea (Pisum sativum L., a model plant) vascular system, enhancing the root growth at a dose level of 80 μg/mL. Fluorescence microscopy studies confirmed localization of NAC-Qdots in the intercellular regions. Germination and growth of the snow pea seeds were found to be strongly dependent on Qdot dosage and incubation time with Qdots. Seed germination reached 100% within 48 hours of NAC-Qdot exposure. Based on our preliminary findings, it is suggested that NAC-Qdot can be used as systemic plant nutrient material for boosting the seed germination and plant growth

Hydrothermally Derived Water-Dispersible Mixed Valence Copper-Chitosan Nanocomposite As Exceptionally Potent Antimicrobial Agent

We report, for the first time, a one-step hydrothermal (HT) process to design and synthesize water-dispersible chitosan nanoparticles loaded with mixed valence copper. Interestingly, this HT copper-chitosan biocompatible composite exhibits exceptionally high antimicrobial properties. A comprehensive characterization of the composite indicates that the hydrothermal process results in the formation of monodispersed nanoparticles with average size of 40 ± 10 nm. FT-IR and Raman spectroscopic studies unveiled that the hydrolysis of the glycoside bonds as the origin of the depolymerization of chitosan. Furthermore, X-Ray Photoelectron Spectroscopy measurements confirmed the presence of mixed valence copper states in the composite, while UV–Vis and FT-IR studies revealed the chemical interaction of copper with the chitosan matrix. Hence, the extensive spectroscopic data provide strong evidence that the chitosan structure was rearranged to capture copper oxide nanoparticles. Finally, HT copper-chitosan composite showed a complete killing effect when tested against both Gram negative (E. coli) and Gram positive (S. aureus) bacteria at metallic copper concentration of 100 μg/ml (1.57 mM). At the same concentration, neither pure chitosan nor copper elicited such antimicrobial efficacy. Thus, we show that HT process significantly enhances the synergistic antimicrobial effect of chitosan and copper in addition to increasing the water dispersibility