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MoS2 nanoribbons have attracted increased interest due to their properties, which can be tailored by tuning their dimensions. Herein, the growth of MoS2 nanoribbons and triangular crystals formed by the reaction between films of MoOx (2
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This work demonstrates the enhancement in plasmonic sensing efficacy resulting from spatially-localized functionalization on nanostructured surfaces, whereby probe molecules are concentrated in areas of high field concentration. Comparison between SERS measurements on nanostructured surfaces (arrays of nanodisks 110 and 220 nm in diameter) with homogeneous and spatially-localized functionalization with thiophenol demonstrates that the Raman signal originates mainly from areas with high field concentration. TERS measurements with 10 nm spatial resolution confirm the field distribution profiles predicted by the numerical modeling. Though this enhancement in plasmonic sensing efficacy is demonstrated with SERS, results apply equally well to any type of optical/plasmonic sensing on functionalized surfaces with nanostructuring.
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Graphene nanoribbons (GNRs) fabricated using electron beam lithography are investigated using tip-enhanced Raman spectroscopy (TERS) with a spatial resolution of 5 nm under ambient conditions. High-resolution TERS imaging reveals a structurally modified 5-10 nm strip of disordered graphene at the edge of the GNRs. Furthermore, hyperspectral TERS imaging discovers the presence of nanoscale organic contaminants on the GNRs. These results pave the way for nanoscale chemical and structural characterisation of graphene-based devices using TERS.
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One common challenge highlighted in almost every review article on organic resistive memory is the lack of areal switching uniformity. This, in fact, is a puzzle because a molecular switching mechanism should ideally be isotropic and produce homogeneous current switching free from electroforming. Such a demonstration, however, remains elusive to date. The reports attempting to characterize a nanoscopic picture of switching in molecular films show random current spikes, just opposite to the expectation. Here, this longstanding conundrum is resolved by demonstrating 100% spatially homogeneous current switching (driven by molecular redox) in memristors based on Ru-complexes of azo-aromatic ligands. Through a concurrent nanoscopic spatial mapping using conductive atomic force microscopy and in operando tip-enhanced Raman spectroscopy (both with resolution <7 nm), it is shown that molecular switching in the films is uniform from hundreds of micrometers down to the nanoscale and that conductance value exactly correlates with spectroscopically determined molecular redox states. This provides a deterministic molecular route to obtain spatially homogeneous, forming-free switching that can conceivably overcome the chronic problems of robustness, consistency, reproducibility, and scalability in organic memristors.
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Two-dimensional transition metal dichalcogenides are gaining increasing interest due to their promising optical properties. In particular, molybdenum disulfide (MoS2) which displays a band-gap change from indirect at 1.29 eV for bulk materials to direct at 1.8 eV for the material monolayer. This particular effect can lead to a strong light interaction which can pave the way for a new approach to the next generation of visible light emitting devices. In this work we show the nanoscale variation of light emission properties by tip-enhanced photoluminescence microscopy and spectroscopy in the MoS2 monolayer, grown by chemical vapour deposition. The variations of the light emission properties are due to different effects depending on the shape of the MoS2 single layer, for instance, a different concentration of point defect in an irregularly shaped flake and the presence of a nanoscale terrace in a triangular monolayer. Simultaneously, atomic force microscopy reveals indeed the presence of a nanometric terrace, composed of an additional layer of MoS2, and tip-enhanced PL intensity imaging shows a localized intensity decrease.
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Visualising the distribution of structural defects and functional groups present on the surface of two-dimensional (2D) materials such as graphene oxide challenges the sensitivity and spatial resolution of the most advanced analytical techniques. Here we demonstrate mapping of functional groups on a carboxyl-modified graphene oxide (GO-COOH) surface with a spatial resolution of ≈10 nm using tip-enhanced Raman spectroscopy (TERS). Furthermore, we extend the capability of TERS by measuring local electronic properties in situ, in addition to the surface topography and chemical composition. Our results reveal that the Fermi level at the GO-COOH surface decreases as the ID/IG ratio increases, correlating the local defect density with the Fermi level at nanometre length-scales. The in situ multi-parameter microscopy demonstrated in this work significantly improves the accuracy of nanoscale surface characterisation, eliminates measurement artefacts, and opens up the possibilities for characterising optoelectronic devices based on 2D materials under operational conditions.
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A billion-fold increase in the Raman signal over conventional tip-enhanced Raman spectroscopy/microscopy (TERS) is reported. It is achieved by introducing a stimulating beam confocal with the pump beam into a conventional TERS setup. A stimulated TERS spectrum, closely corresponding to its spontaneous TERS counterpart, is obtained by plotting the signal intensity of the strongest Raman peak of an azobenzene thiol self-assembled monolayer versus the stimulating laser frequency. The stimulated TERS image of azobenzene thiol molecules grafted onto Au ⟨111⟩ clearly shows the surface distribution of the molecules, whereas, when compared to the simultaneously recorded surface topography, it presents an image contrast of different nature. The experimentally obtained stimulated gain is estimated at 1.0 × 10(9), which is in reasonable agreement with the theoretically predicted value. In addition to the signal increase, the signal-to-noise ratio was 3 orders of magnitude higher than in conventional spontaneous TERS. The proposed stimulated TERS technique offers the possibility for a substantially faster imaging of the surface with respect to normal TERS.
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Mixed thiol self-assembled monolayers (SAMs) presenting methyl and azobenzene head groups were prepared by chemical substitution from the original single-component n-decanethiol or [4-(phenylazo)phenoxy]hexane-1-thiol SAMs on polycrystalline gold substrates. Static contact-angle measurements were carried out to confirm a change in the hydrophobicity of the functionalized surfaces following the exchange reaction. The mixed SAMs presented contact-angle values between those of the more hydrophobic n-decanethiol and the more hydrophilic [4-(phenylazo)phenoxy]hexane-1-thiol single-component SAMs. By means of tip-enhanced Raman spectroscopy (TERS) mapping experiments, it was possible to highlight that molecular replacement takes place easily and first at grain boundaries: for two different mixed SAM compositions, TERS point-by-point maps with <50 nm step sizes showed different spectral signatures in correspondence to the grain boundaries. An example of the substitution extending beyond grain boundaries and affecting flat areas of the gold surface is also shown.
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Post-growth transfer and high growth temperature are two major hurdles that research has to overcome to get graphene out of research laboratories. Here, using a plasma-enhanced chemical vapour deposition process, we demonstrate the large-area formation of continuous transparent graphene layers at temperatures as low as 450 °C. Our few-layer graphene grows at the interface between a pre-deposited 200 nm Ni catalytic film and an insulating glass substrate. After nickel etching, we are able to measure the optical transmittance of the layers without any transfer. We also measure their sheet resistance directly and after inkjet printing of electrical contacts: sheet resistance is locally as low as 500 Ω sq⻹. Finally the samples equipped with printed contacts appear to be efficient humidity sensors.
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6-[4-(phenylazo)phenoxy]hexane-1-thiol self-assembled monolayers deposited on a gold surface form domain-like structures possessing a high degree of order with virtually all the molecules being identically oriented with respect to the surface plane. We show that, by using polarized near-field Raman spectroscopy, it is possible to derive the Raman scattering tensor of the ordered layer and consequently, the in-plane molecular orientation at the individual domain level. More generally, this study extends the application domain of the near-field Raman scattering selection rules from crystals to ordered organic structures.
Assuntos
Compostos Azo/química , Compostos de Sulfidrila/química , Compostos Azo/síntese química , Análise Espectral Raman , Compostos de Sulfidrila/síntese químicaRESUMO
The present study focuses on the interaction of hydrogen microwave CVD plasma with nanodiamonds (NDs). Hydrogen treated NDs (H-NDs) were characterized using electron spectroscopies (XPS, AES) without air exposure. A surface temperature higher than 700 °C is needed to remove the oxygen present on raw NDs. The kinetics of oxygen removal were investigated. Moreover, UHV annealings of H-NDs after ageing in ambient air clearly underline that 75% of the oxygen is related to physisorbed species. Finally, H-NDs were efficiently grafted using photochemical reaction with alkenes and a spontaneous coupling of aryldiazonium salts. These results confirm similar electronic surface properties between bulk and nano diamond materials.
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Nanodiamond (ND) seeding is a well-established route toward the CVD (chemical vapor deposition) synthesis of diamond ultrathin films. This method is based on the deposition onto a substrate of diamond nanoparticles which act as pre-existing sp(3) seeds. Here, we report on a straightforward method to disperse diamond nanoparticles on a substrate by taking advantage of the electrostatic interactions between the nanodiamonds and the substrate surface coated with a cationic polymer. This layer-by-layer deposition technique leads to reproducible and homogeneous large-scale nanoparticle deposits independent of the substrate's nature and shape. No specific functionalization of the nanoparticles is required, and low concentrated solutions can be used. The density of NDs on the substrate can be controlled, as shown by in situ ATR-FTIR (attenuated total reflection Fourier transform infrared) analysis and QCM (quartz crystal microbalance) measurements. Highly dense and compact ND deposits can be obtained, allowing CVD growth of nanocrystalline diamond ultrathin films (70 nm) on various substrates. The synthesis of 3D structured and patterned diamond thin films has also been demonstrated with this method.