Rational design of Nd2O3 decorated functionalized carbon nanofiber composite for selective electrochemical detection of carbendazim fungicides in vegetables, water, and soil samples.

Abuse of carbendazim (CBZ) leaves excessive pesticide residues on agricultural products, which endangers human health because of the residues' high concentrations. Hence, a composite consisting of functionalized carbon nanofibers (f-CNF) with neodymium oxide (Nd2O3) was fabricated to monitor CBZ at trace levels. The Nd2O3/f-CNF composite-modified electrode displays higher electro-oxidation ability towards CBZ than Nd2O3 and f-CNF-modified electrodes. The combined unique properties of Nd2O3 and f-CNF result in a substantial specific surface area, superior structural stability, and excellent electrocatalytic activity of the composite yielding enhanced sensitivity to detecting CBZ with a detection limit of 4.3 nM. Also, the fabricated sensor electrode can detect CBZ in the linear concentration range of up to 243.0 µM with high selectivity, appropriate reproducibility, and stability. A demonstration of the sensing capability of CBZ in vegetables, water, and soil samples was reported paving the way for its use in practical applications.

Quantification of van der Waals forces in bimodal and trimodal AFM.

The multifrequency formalism is generalized and exploited to quantify attractive forces, i.e., van der Waals interactions, with small amplitudes or gentle forces in bimodal and trimodal atomic force microscopy (AFM). The multifrequency force spectroscopy formalism with higher modes, including trimodal AFM, can outperform bimodal AFM for material property quantification. Bimodal AFM with the second mode is valid when the drive amplitude of the first mode is approximately an order of magnitude larger than that of the second mode. The error increases in the second mode but decreases in the third mode with a decreasing drive amplitude ratio. Externally driving with higher modes provides a means to extract information from higher force derivatives while enhancing the range of parameter space where the multifrequency formalism holds. Thus, the present approach is compatible with robustly quantifying weak long range forces while extending the number of channels available for high resolution.

Random lasers from photonic crystal wings of butterfly and moth for speckle-free imaging.

Several biological membranes have been served as scattering materials of random lasers, but few of them include natural photonic crystals. Here, we propose and demonstrate a facile approach to fabricating high-performance biological photonic crystal random lasers, which is cost-effective and reproducible for mass production. As a benchmark, optical and lasing properties of dye-coated Lepidoptera wings, including Papilio ulysses butterfly and Chrysiridia rhipheus moth, are characterized and show a stable laser emission with a superior threshold of 0.016 mJ/cm2, as compared to previous studies. To deploy the proposed devices in practical implementation, we have applied the as-fabricated biological devices to bright speckle-free imaging applications, which is a more sustainable and more accessible imaging strategy.

A review of focused ion beam applications in optical fibers.

Focused ion beam (FIB) technology has become a promising technique in micro- and nano-prototyping due to several advantages over its counterparts such as direct (maskless) processing, sub-10 nm feature size, and high reproducibility. Moreover, FIB machining can be effectively implemented on both conventional planar substrates and unconventional curved surfaces such as optical fibers, which are popular as an effective medium for telecommunications. Optical fibers have also been widely used as intrinsically light-coupled substrates to create a wide variety of compact fiber-optic devices by FIB milling diverse micro- and nanostructures onto the fiber surface (endfacet or outer cladding). In this paper, the broad applications of the FIB technology in optical fibers are reviewed. After an introduction to the technology, incorporating the FIB system and its basic operating modes, a brief overview of the lab-on-fiber technology is presented. Furthermore, the typical and most recent applications of the FIB machining in optical fibers for various applications are summarized. Finally, the reviewed work is concluded by suggesting the possible future directions for improving the micro- and nanomachining capabilities of the FIB technology in optical fibers.

Hydration Dynamics and the Future of Small-Amplitude AFM Imaging in Air.

Here, we discuss the effects that the dynamics of the hydration layer and other variables, such as the tip radius, have on the availability of imaging regimes in dynamic AFM-including multifrequency AFM. Since small amplitudes are required for high-resolution imaging, we focus on these cases. It is possible to fully immerse a sharp tip under the hydration layer and image with amplitudes similar to or smaller than the height of the hydration layer, i.e., ~1 nm. When mica or HOPG surfaces are only cleaved, molecules adhere to their surfaces, and reaching a thermodynamically stable state for imaging might take hours. During these first hours, different possibilities for imaging emerge and change, implying that these conditions must be considered and reported when imaging.

Explaining doping in material research (Hf substitution in ZnO films) by directly quantifying the van der Waals force.

Non-monotonic behavior has been observed in the optoelectronic properties of ZnO thin films as doped with Hf (HZO). Here we propose that two competing mechanisms are responsible for such behaviour. Specifically, we propose that provided two crystal orientations dominate film growth, only one of them might be responsible for direct Hf substitution. Nonmonotonic behaviour is expected at once by considering that preferential growth of the crystal that allows for direct Hf substitution is inhibited by Hf concentration in the manufacturing process. This inhibition would also be a thermodynamic consequence of Hf substitution. Maxima in Hf substitution is thus reached at a point where enough crystals exhibit the preferential orientation, and where enough Hf is present on the surface for substitution. Outside this optimum scenario, Hf substitution can only decrease. We interpret the surface phenomena by discussing surface energy and the van der Waals forces as measured experimentally by means of atomic force microscopy.

Hybrid graphene metasurface for near-infrared absorbers.

We experimentally demonstrated an amorphous graphene-based metasurface yielding near-infrared super absorber characteristic. The structure is obtained by alternatively combining magnetron-sputtering deposition and graphene transfer coating fabrication techniques. The thickness constraint of the physical vapor-deposited amorphous metallic layer is unlocked and as a result, the as-fabricated graphene-based metasurface absorber achieves near-perfect absorption in the near-infrared region with an ultra-broad spectral bandwidth of 3.0 µm. Our experimental characterization and theoretical analysis further point out that the strong light-matter interaction observed is caused by localized surface plasmon resonance of the metal film's particle-like surface morphology. In addition to the enhanced light absorption characteristics, such an amorphous metasurface can be used for surface-enhanced Raman scattering applications. Meanwhile, the proposed graphene-based metasurface relies solely on CMOS-compatible, low cost and large-area processing, which can be flexibly scaled up for mass production.

Direct Measurement of the Magnitude of the van der Waals Interaction of Single and Multilayer Graphene.

Vertical stacking of monolayers via van der Waals (vdW) assembly is an emerging field that opens promising routes toward engineering physical properties of two-dimensional materials. Industrial exploitation of these engineering heterostructures as robust functional materials still requires bounding their measured properties so as to enhance theoretical tractability and assist in experimental designs. Specifically, the short-range attractive vdW forces are responsible for the adhesion of chemically inert components and are recognized to play a dominant role in the functionality of these structures. Here, we reliably quantify the strength of ambient vdW forces in terms of an effective Hamaker coefficient for chemical vapor deposition-grown graphene and show how it scales by a factor of two or three from single to multiple layers on standard supporting surfaces such as copper or silicon oxide. Furthermore, direct measurements on freestanding graphene provide the means to discern the interplay between the vdW potential of graphene and its supporting substrate. Our results demonstrated that the underlying substrates could be controllably exploited to enhance or reduce the vdW force of graphene surfaces. We interpret the physical phenomena in terms of a Lifshitz theory-based analytical model.

Surface aging investigation by means of an AFM-based methodology and the evolution of conservative nanoscale interactions.

The divergence in physical properties of surfaces exposed to airborne contaminants in the atmosphere has been widely investigated in recent years, but agreement regarding the role that airborne hydrocarbons and water contamination have on surface property evolution remains elusive. Here we investigate the evolution of a freshly grown highly oriented pyrolytic graphite (HOPG) surface exposed to a controlled environment solely containing airborne water contamination, i.e. water vapor. Our approach combines standard electromagnetic spectrum-based spectrometry methods and atomic force microscopy based techniques to provide a holistic view of the surface properties. We provide evidence of the affect that water adsorption kinetics has on surface properties, interpreting time dependent force-distance profiles and force of adhesion maps directly obtained from the standard observables in a bimodal AFM. To demonstrate the generality of our approach we also apply it to aged calcite surfaces, proving its advantages in providing comprehensive transient characterization of surfaces with sufficient spatial resolution.

The evolution in graphitic surface wettability with first-principles quantum simulations: the counterintuitive role of water.

The surface wettability of graphite has gained a lot of interest in nanotechnology and fundamental studies alike, but the types of adsorptions that dominate its time resolved surface property variations in ambient environment are still elusive. Prediction of the intrinsic surface wettability of graphite from first-principles simulations offers an opportunity to clarify the overall evolution. In this study, by combining experimental temporal Fourier transform infrared spectroscopy, atomic force microscopy (AFM), and static contact angle measurements with density functional theory (DFT)-predicted contact angles and DFT AFM force simulations, we provide conclusive evidence to demonstrate the role played by water adsorption in the evolution of surface properties of aged graphite in ambient air. Moreover, this study has the merit of linking DFT-predicted adhesive energy at the solid/liquid interface and cohesive energy at the liquid/liquid interface with the DFT AFM-predicted force of adhesion through the Young-Dupre equation. This establishes the basis of the quantum surface wettability theory by combining two independent atomic-level quantum physics simulation methodologies.

Discerning the Contribution of Morphology and Chemistry in Wettability Studies.

The wetting behavior of homogeneous systems is now well understood at the macroscopic scale. However, this understanding offers little predictive power regarding wettability when mesoscopic chemical and morphological heterogeneities come into play. The fundamental interest in the effect of heterogeneity on wettability is derived from its high technological relevance in several industries, including the petroleum industry where wettability is recognized as a key determinant of the overall efficiency of the water-flooding-based enhanced oil recovery process. Here, we demonstrate the use of the atomic force microscopy force curve measurements to distinguish the roles of chemistry and morphology in the wetting properties of rock formations, thus providing a clear interpretation and deeper insight into the wetting behavior of heterogeneous formations. Density functional theory calculations further prove the versatility of our approach by establishing benchmarks on ideal surfaces that differ in chemistry and morphology in a predefined manner.

Dependence of surface aging on DNA topography investigated in attractive bimodal atomic force microscopy.

Here we employ bimodal atomic force microscopy (AFM) to investigate the relevance of the aging of the surface and accumulation of adsorbates on the resolved topography of biomolecules. We produce raw bimodal images and a set of contrast channels derived from these to show that the imaging of DNA molecules on hydrophilic model substrates such as mica should be performed immediately after the sample is prepared. Days after preparation, i.e. 48 hours, the adsorbates shield the forces arising from the true substrate and molecule and the molecule might become "invisible" in the images. We employ dsDNA molecules on mica as a model system since the nominal height of dsDNA is comparable to the height of the adsorbed films. With this set up, the molecules can fully disappear under attractive imaging due to the shielding effects of the adsorbates. We further transform the images obtained immediately after cleaving the mica surface and show that the data are then suitable to be transformed into more physically meaningful maps such as Hamaker maps.

Surface alteration of calcite: interpreting macroscopic observations by means of AFM.

Wettability has been recognized to play a fundamental role in the efficacy of water flooding processes of carbonate oil and gas reservoirs. However, the theoretical mechanism governing this process is still not entirely understood. This can be partly attributed to the absence of ad hoc tools and standardized sample-preparation methodologies for comprehensive transient characterization of the mineral surface. Here, we use atomic force microscopy (AFM) to investigate the effect of different calcite sample-preparation methodologies in estimating the macroscopic water static contact angle (SCA). Single crystal calcite surfaces are aged in deionized (DI) water baths, for different exposure times, and dried by different techniques, to reveal SCA discrepancies. Trends and observations are explained with the use of time-dependent adhesion maps of the surface obtained by bimodal AFM. In this context, the AFM interpretation of macroscopic observations provides a means to single out the different factors influencing wettability, thus allowing for a more standardized description of the processes responsible for the modification of the affinity between the mineral rock and injected water.

Efficiency enhancement in two-cell CIGS photovoltaic system with low-cost optical spectral splitter.

Spectrum splitting represents a valid alternative to multi-junction solar cells for broadband light-to-electricity conversion. While this concept has existed for decades, its adoption at the industrial scale is still stifled by high manufacturing costs and inability to scale to large areas. Here we report the experimental validation of a novel design that could allow the widespread adoption of spectrum splitting as a low-cost approach to high efficiency photovoltaic conversion. Our system consists of a prismatic lens that can be manufactured using the same methods employed for conventional CPV optic production, and two inexpensive CuInGaSe(2) (CIGS) solar cells having different composition and, thus, band gaps. We demonstrate a large improvement in cell efficiency under the splitter and show how this can lead to substantial increases in system output at competitive cost using existing technologies.

Integrated Nano- and Macroscale Investigation of Photoinduced Hydrophilicity in TiO2 Thin Films.

The hydrophilicity of titanium dioxide has been investigated for films, deposited on glass by e-beam evaporation, being exposed to UV radiation and subjected to thermal annealing. The wettability alteration has been showed to depend upon both treatments, and insights into how to introduce more stable hydrophilicity into these films have been presented for the sake of boosting their commercial value. Observations from multiple length scales to assess the wetting behavior of as-deposited and high-temperature annealed samples were assessed through macroscopic measurements, i.e., water contact angle measurements, showing that the annealed crystalline samples, treated at 500 °C, are much more hydrophilic (SCA ≈ 20°) than as-deposited TiO2 films (SCA ≈ 90°), and the nanoscopic experiments performed by amplitude modulation (AM) atomic force microscopy (AFM) indicated that this increased hydrophilicity is related to an enhanced adhesion force and surface energy, resulting in the partial crystallization of TiO2 and the consequent formation of crystals on its surface rather than being related to morphologic differences. XRD and Raman measurements have highlighted that the crystallinity of the TiO2 film is crucial in determining its hydrophilicity, in good agreement with the AFM study. The results also indicated that, after irradiation, the samples treated at 500 °C preserve their hydrophilicity for a significant time compared to previous studies.

Reconstruction of height of sub-nanometer steps with bimodal atomic force microscopy.

Obtaining topographic images of surfaces presenting terraces with heights in the nanometer and sub-nanometer range has become routine since the advent of atomic force microscopy (AFM). There remain however several open questions regarding the validity of direct topographic measurements. Here we turn to recent advances in AFM to correct the height of nanometric terraces by exploiting the four observables of bimodal AFM operated in the non-invasive attractive regime. We first derive expressions based on the van der Waals theory and then image model terraces in air in standard bimodal AFM while simultaneously correcting and decoupling the sources of loss/gain of height.

Divergent surface properties of multidimensional sp (2) carbon allotropes: the effect of aging phenomena.

Despite the current interest in the scientific community in exploiting divergent surface properties of graphitic carbon allotropes, conclusive differentiation remains elusive even when dealing with parameters as fundamental as adhesion. Here, we set out to provide conclusive experimental evidence on the time evolution of the surface properties of highly oriented pyrolytic graphite (HOPG), graphene monolayer (GML) and multiwalled carbon nanotubes (MWCNTs) as we expose these materials to airborne contaminants, by providing (1) statistically significant results based on large datasets consisting of thousands of force measurements, and (2) errors sufficiently self-consistent to treat the comparison between datasets in atomic force microscopy (AFM) measurements. We first consider HOPG as a model system and then employ our results to draw conclusions from the GML and MWCNT samples. We find that the surface properties of aged HOPG are indistinguishable from those of aged GML and MWCNT, while being distinct from those of cleaved HOPG. Herein, we provide a sufficient body of evidence to disregard any divergence in surface properties for multidimensional sp (2) carbon allotropes that undergo similar aging processes.

Surface and Bulk Effects in Photochemical Reactions and Photomechanical Effects in Dynamic Molecular Crystals.

The increasing number of reports on photomechanical effects in molecular crystals necessitates systematic studies to understand the intrinsic and external effectors that determine and have predictive power of their type and magnitude. Differential light absorption and product gradient between the surface and the bulk of the crystal are often invoked to qualitatively explain the mechanical response of crystals to light; however, the details on how this difference in photochemical response accounts for macroscopic effects such as surface modification, deformation, or disintegration of crystals are yet to be established. Using both bulk- and surface-sensitive analytical techniques, a rare instance of benzylidenefuranone crystals is studied here, and it is capable of several distinct types of photomechanical response including surface striation and delamination, photosalient effect (ballistic disintegration and motion), and photoinduced bending by dimerization. The results provide a holistic view on these effects and set the stage for the development of overarching theoretical models to describe the photomechanics in the ordered solid state.

Sun-tracking optical element realized using thermally activated transparency-switching material.

We present a proof of concept demonstration of a novel optical element: a light-responsive aperture that can track a moving light beam. The element is created using a thermally-activated transparency-switching material composed of paraffin wax and polydimethylsiloxane (PDMS). Illumination of the material with a focused beam causes the formation of a localized transparency at the focal spot location, due to local heating caused by absorption of a portion of the incident light. An application is proposed in a new design for a self-tracking solar collector.

Time dependent wettability of graphite upon ambient exposure: the role of water adsorption.

We report the temporal evolution of the wettability of highly ordered pyrolytic graphite (HOPG) exposed to environmental conditions. Macroscopic wettability is investigated by static and dynamic contact angles (SCA and DCA) obtaining values comparable to the ones presented in the literature. SCA increases from ∼68° to ∼90° during the first hour of exposure after cleaving, whereas DCA is characterized by longer-scale (24 h) time evolution. We interpret these results in light of Fourier transform infrared spectroscopy, which indicates that the evolution of the HOPG wettability is due to adsorption of molecules from the surrounding atmosphere. This hypothesis is further confirmed by nanoscopic observations obtained by atomic force microscope (AFM)-based force spectroscopy, which monitor the evolution of surface properties with a spatial resolution superior to macroscopic experiments. Moreover, we observe that the results of macro- and nanoscale measurements evolve in similar fashion with time and we propose a quantitative correlation between SCA and AFM measurements. Our results suggest that the cause of the transition in the wettability of HOPG is due to the adsorption of hydrocarbon contaminations and water molecules from the environment. This is corroborated by annealing the HOPG is vacuum conditions at 150°, allowing the desorption of molecules on the surface, and thus re-establishing the initial macro and nano surface properties. Our findings can be used in the interpretation of the wettability of more complicated systems derived from HOPG (i.e., graphene).