Localization of lattice dynamics in low-angle twisted bilayer graphene.

Twisted bilayer graphene is created by slightly rotating the two crystal networks in bilayer graphene with respect to each other. For small twist angles, the material undergoes a self-organized lattice reconstruction, leading to the formation of a periodically repeated domain1-3. The resulting superlattice modulates the vibrational3,4 and electronic5,6 structures within the material, leading to changes in the behaviour of electron-phonon coupling7,8 and to the observation of strong correlations and superconductivity9. However, accessing these modulations and understanding the related effects are challenging, because the modulations are too small for experimental techniques to accurately resolve the relevant energy levels and too large for theoretical models to properly describe the localized effects. Here we report hyperspectral optical images, generated by a nano-Raman spectroscope10, of the crystal superlattice in reconstructed (low-angle) twisted bilayer graphene. Observations of the crystallographic structure with visible light are made possible by the nano-Raman technique, which reveals the localization of lattice dynamics, with the presence of strain solitons and topological points1 causing detectable spectral variations. The results are rationalized by an atomistic model that enables evaluation of the local density of the electronic and vibrational states of the superlattice. This evaluation highlights the relevance of solitons and topological points for the vibrational and electronic properties of the structures, particularly for small twist angles. Our results are an important step towards understanding phonon-related effects at atomic and nanometric scales, such as Jahn-Teller effects11 and electronic Cooper pairing12-14, and may help to improve device characterization15 in the context of the rapidly developing field of twistronics16.

Dielectric Screening inside Carbon Nanotubes.

Dielectric screening plays a vital role in determining physical properties at the nanoscale and affects our ability to detect and characterize nanomaterials using optical techniques. We study how dielectric screening changes electromagnetic fields and many-body effects in nanostructures encapsulated inside carbon nanotubes. First, we show that metallic outer walls reduce the scattering intensity of the inner tube by 2 orders of magnitude compared to that of air-suspended inner tubes, in line with our local field calculations. Second, we find that the dielectric shift of the optical transition energies in the inner walls is greater when the outer tube is metallic than when it is semiconducting. The magnitude of the shift suggests that the excitons in small-diameter inner metallic tubes are thermally dissociated at room temperature if the outer tube is also metallic, and in essence, we observe band-to-band transitions in thin metallic double-walled nanotubes.

Electro-optical properties of a graphene device on a tip-enhanced Raman spectroscopy system.

This paper investigates the impact of graphene on tip-enhanced Raman spectroscopy (TERS) by developing an electromagnetic characterization of the TERS-graphene device system. The study focuses on the interaction between the tip, the gate voltage, and the sample, specifically examining the electromagnetic effects in the system. Employing a finite element method (FEM)-based simulation model, we meticulously dissect the electric field distribution and the Raman amplification when graphene is introduced into the system. Our findings reveal that including graphene results in a marginal reduction in Raman amplification and a negligible variation in the induced charges within the system. To reinforce our simulations, we employ a simplified capacitor model, which corroborates our results, showcasing negligible induced charges and validating the obtained capacitance values. In this manuscript, we also explore the influence of the setup on the electro-optical properties of graphene, revealing a slight variation in conductivity despite strong changes in chemical potential. Overall, this work contributes to understanding TERS's electromagnetic aspects in the presence of graphene.

Detection of carbon nanotubes in bovine raw milk through Fourier transform Raman spectroscopy.

The potential use of carbon-based methodologies for drug delivery and reproductive biology in cows raises concerns about residues in milk and food safety. This study aimed to assess the potential of Fourier transform Raman spectroscopy and discriminant analysis using partial least squares (PLS-DA) to detect functionalized multiwalled carbon nanotubes (MWCNT) in bovine raw milk. Oxidized MWCNT were diluted in milk at different concentrations from 25.00 to 0.01 µg/mL. Raman spectroscopy measurements and PLS-DA were performed to identify low concentrations of MWCNT in milk samples. The PLS-DA model was characterized by the analysis of the variable importance in projection (VIP) scores. All the training samples were correctly classified by the model, resulting in no false-positive or false-negative classifications. For test samples, only one false-negative result was observed, for 0.01 µg/mL MWCNT dilution. The association between Raman spectroscopy and PLS-DA was able to identify MWCNT diluted in milk samples up to 0.1 µg/mL. The PLS-DA model was built and validated using a set of test samples and spectrally interpreted based on the highest VIP scores. This allowed the identification of the vibrational modes associated with the D and G bands of MWCNT, as well as the milk bands, which were the most important variables in this analysis.

Spatially Coherent Tip-Enhanced Raman Spectroscopy Measurements of Electron-Phonon Interaction in a Graphene Device.

Coherence length (Lc) of the Raman scattering process in graphene as a function of Fermi energy is obtained with spatially coherent tip-enhanced Raman spectroscopy. Lc decreases when the Fermi energy is moved into the neutrality point, consistent with the concept of the Kohn anomaly within a ballistic transport regime. Since the Raman scattering involves electrons and phonons, the observed results can be rationalized either as due to unusually large variation of the longitudinal optical phonon group velocity vg, reaching twice the value for the longitudinal acoustic phonon, or due to changes in the electron energy uncertainty, both properties being important for optical and transport phenomena that might not be observable by any other technique.

Electron-Phonon Coupling in a Magic-Angle Twisted-Bilayer Graphene Device from Gate-Dependent Raman Spectroscopy and Atomistic Modeling.

The importance of phonons in the strong correlation phenomena observed in twisted-bilayer graphene (TBG) at the so-called magic-angle is under debate. Here we apply gate-dependent micro-Raman spectroscopy to monitor the G band line width in TBG devices of twist angles θ = 0° (Bernal), ∼1.1° (magic-angle), and ∼7° (large-angle). The results show a broad and p-/n-asymmetric doping behavior at the magic angle, in clear contrast to the behavior observed in twist angles above and below this point. Atomistic modeling reproduces the experimental observations in close connection with the joint density of electronic states in the electron-phonon scattering process, revealing how the unique electronic structure of magic-angle TBGs influences the electron-phonon coupling and, consequently, the G band line width. Overall, the value of the G band line width in magic-angle TBG is larger when compared to that of the other samples, in qualitative agreement with our calculations.

Raman spectra-based structural classification analysis of quinoidal and derived molecular systems.

This work reports a classification analysis method based on the vibrational Raman spectra of 38 quinones and related structures, spectrally ordering and classifying the compounds. The molecular systems are relevant for chemical and biological processes, with applications in pharmacology, toxicology and medicine. The classification strategy uses a combination of principal component analysis with K-means clustering methods. Both theoretical simulations and experimental data are analysed, thus establishing their spectral characteristics, as related to their chemical structures and properties. The protocol introduced here should be broadly applicable in other molecular and solid state systems.

Nonlinear and vibrational microscopy for label-free characterization of amyloid-ß plaques in Alzheimer's disease model.

Given the long subclinical stage of Alzheimer's disease (AD), the study of biomarkers is relevant both for early diagnosis and the fundamental understanding of the pathophysiology of AD. Biomarkers provided by Amyloid-ß (Aß) plaques have led to an increasing interest in characterizing this hallmark of AD due to its promising potential. In this work, we characterize Aß plaques by label-free multimodal imaging: we combine two-photon excitation autofluorescence (TPEA), second harmonic generation (SHG), spontaneous Raman scattering (SpRS), coherent anti-Stokes Raman scattering (CARS), and stimulated Raman scattering (SRS) to describe and compare high-resolution images of Aß plaques in brain tissues of an AD mouse model. Comparing single-laser techniques images, we discuss the origin of the SHG, which can be used to locate the plaque core reliably. We study both the core and the halo with vibrational microscopy and compare SpRS and SRS microscopies for different frequencies. We also combine SpRS spectroscopy with SRS microscopy and present two core biomarkers unexplored with SRS microscopy: phenylalanine and amide B. We provide high-resolution SRS images with the spatial distribution of these biomarkers in the plaque and compared them with images of the amide I distribution. The obtained spatial correlation corroborates the feasibility of these biomarkers in the study of Aß plaques. Furthermore, since amide B enables rapid imaging, we discuss its potential as a novel fingerprint for diagnostic applications.

Micro-Raman spectroscopy of lipid halo and dense-core amyloid plaques: aging process characterization in the Alzheimer's disease APPswePS1ΔE9 mouse model.

The deposition of amyloid plaques is considered one of the main microscopic features of Alzheimer's disease (AD). Since plaque formation can precede extensive neurodegeneration and it is the main clinical manifestation of AD, it constitutes a relevant target for new treatment and diagnostic approaches. Micro-Raman spectroscopy, a label-free technique, is an accurate method for amyloid plaque identification and characterization. Here, we present a high spatial resolution micro-Raman hyperspectral study in transgenic APPswePS1ΔE9 mouse brains, showing details of AD tissue biochemical and histological changes without staining. First we used stimulated micro-Raman scattering to identify the lipid-rich halo surrounding the amyloid plaque, and then proceeded with spontaneous (conventional) micro-Raman spectral mapping, which shows a cholesterol and sphingomyelin lipid-rich halo structure around dense-core amyloid plaques. The detailed images of this lipid halo relate morphologically well with dystrophic neurites surrounding plaques. Principal Component Analysis (PCA) of the micro-Raman hyperspectral data indicates the feasibility of the optical biomarkers of AD progression with the potential for discriminating transgenic groups of young adult mice (6-month-old) from older ones (12-month-old). Frequency-specific PCA suggests that plaque-related neurodegeneration is the predominant change captured by Raman spectroscopy, and the main differences are highlighted by vibrational modes associated with cholesterol located majorly in the lipid halo.

Studying 2D materials with advanced Raman spectroscopy: CARS, SRS and TERS.

Raman spectroscopy has been established as a valuable tool to study and characterize two-dimensional (2D) systems, but it exhibits two drawbacks: a relatively weak signal response and a limited spatial resolution. Recently, advanced Raman spectroscopy techniques, such as coherent anti-Stokes spectroscopy (CARS), stimulated Raman scattering (SRS) and tip-enhanced Raman spectroscopy (TERS), have been shown to overcome these two limitations. In this article, we review how useful physical information can be retrieved from different 2D materials using these three advanced Raman spectroscopy and imaging techniques, discussing results on graphene, hexagonal boron-nitride, and transition metal di- and mono-chalcogenides, thus providing perspectives for future work in this early-stage field of research, including similar studies on unexplored 2D systems and open questions.

Few-Wall Carbon Nanotube Coils.

While various electronic components based on carbon nanotubes (CNTs) have already been demonstrated, the realization of miniature electromagnetic coils based on CNTs remains a challenge. Coils made of single-wall CNTs with accessible ends for contacting have been recently demonstrated but were found unsuitable to act as electromagnetic coils because of electrical shorting between their turns. Coils made of a few-wall CNT could in principle allow an insulated flow of current and thus be potential candidates for realizing CNT-based electromagnetic coils. However, no such CNT structure has been produced so far. Here, we demonstrate the formation of few-wall CNT coils and characterize their structural, optical, vibrational, and electrical properties using experimental and computational tools. The coils are made of CNTs with 2, 3, or 4 walls. They have accessible ends for electrical contacts and low defect densities. The coil diameters are on the order of one micron, like those of single-wall CNT coils, despite the higher rigidity of few-wall CNTs. Coils with as many as 163 turns were found, with their turns organized in a rippled raft configuration. These coils are promising candidates for a variety of miniature devices based on electromagnetic coils, such as electromagnets, inductors, transformers, and motors. Being chirally and enantiomerically pure few-wall CNT bundles, they are also ideal for fundamental studies of interwall coupling and superconductivity in CNTs.

Structural and elemental analysis of biochars in the search of a synthetic path to mimetize anthropic Amazon soils.

In this work, chemical and structural properties of various biochars were analyzed and compared with those from a highly stable anthropic soil, Terra Preta de Índio (TPI). TPI is believed to be responsible for the fertility of Amazonian soils and their stability; therefore, the production of a synthetic TPI would be of great interest for agricultural applications. Biochar produced from different raw biomasses were comprehensively characterized and, based on the obtained results, a preliminary study was performed testing three different routes of chemical activation using nitric acid, phosphoric acid, and potassium hydroxide as activating agents. After chemical activations, metal contents in the biochars decreased, as expected, and high degrees of carbonization were observed. In the case of the activation performed with HNO3, intense signals related to carboxylic groups in TG-MS analysis and in potentiometric titrations point out to a highly oxygenated biochar. Structural analysis showed that activations generated point defects in sp2-carbon structures of biochar, with the material obtained after KOH activation showing a high surface area (569 m2 g-1), an important feature for the use as soil amendment.

Temperature-dependent phonon dynamics and anharmonicity of suspended and supported few-layer gallium sulfide.

Phonons play a fundamental role in the electronic and thermal transport of 2D materials which is crucial for device applications. In this work, we investigate the temperature-dependence of A[Formula: see text] and A[Formula: see text] Raman modes of suspended and supported mechanically exfoliated few-layer gallium sulfide (GaS), accessing their relevant thermodynamic Grüneisen parameters and anharmonicity. The Raman frequencies of these two phonons soften with increasing temperature with different [Formula: see text] temperature coefficients. The first-order temperature coefficients θ of A[Formula: see text] mode is ∼ -0.016 cm-1/K, independent of the number of layers and the support. In contrast, the θ of A[Formula: see text] mode is smaller for two-layer GaS and constant for thicker samples (∼ -0.006 2 cm-1 K-1). Furthermore, for two-layer GaS, the θ value is ∼ -0.004 4 cm-1 K-1 for the supported sample, while it is even smaller for the suspended one (∼ -0.002 9 cm-1 K-1). The higher θ value for supported and thicker samples was attributed to the increase in phonon anharmonicity induced by the substrate surface roughness and Umklapp phonon scattering. Our results shed new light on the influence of the substrate and number of layers on the thermal properties of few-layer GaS, which are fundamental for developing atomically-thin GaS electronic devices.

Nanofabrication of plasmon-tunable nanoantennas for tip-enhanced Raman spectroscopy.

Plasmon-tunable tip pyramids (PTTPs) are reproducible and efficient nanoantennas for tip-enhanced Raman spectroscopy (TERS). Their fabrication method is based on template stripping of a segmented gold pyramid with a size-adjustable nanopyramid end, which is capable of supporting monopole localized surface plasmon resonance (LSPR) modes leading to high spectral enhancement when its resonance energy is matched with the excitation laser energy. Here, we describe in detail the PTTP fabrication method and report a statistical analysis based on 530 PTTPs' and 185 ordinary gold micropyramids' templates. Our results indicate that the PTTP method generates probes with an apex diameter smaller than 30 nm on 92.4% of the batch, which is a parameter directly related to the achievable TERS spatial resolution. Moreover, the PTTPs' nanopyramid edge size L, a critical parameter for LSPR spectral tuning, shows variability typically smaller than 12.5%. The PTTP's performance was tested in TERS experiments performed on graphene, and the results show a spectral enhancement of up to 72-fold, which is at least one order of magnitude higher than that typically achieved with gold micropyramids. Imaging resolution is in the order of 20 nm.

Probing Spatial Phonon Correlation Length in Post-Transition Metal Monochalcogenide GaS Using Tip-Enhanced Raman Spectroscopy.

The knowledge of the phonon coherence length is of great importance for two-dimensional-based materials since phonons can limit the lifetime of charge carriers and heat dissipation. Here we use tip-enhanced Raman spectroscopy (TERS) to measure the spatial correlation length Lc of the A1g1 and A1g2 phonons of monolayer and few-layer gallium sulfide (GaS). The differences in Lc values are responsible for different enhancements of the A1g modes, with A1g1 always enhancing more than the A1g2, independently of the number of GaS layers. For five layers, the results show an Lc of 64 and 47 nm for A1g1 and A1g2, respectively, and the coherence lengths decrease when decreasing the number of layers, indicating that scattering with the surface roughness plays an important role.

A fingerprint of amyloid plaques in a bitransgenic animal model of Alzheimer's disease obtained by statistical unmixing analysis of hyperspectral Raman data.

The global prevalence of Alzheimer's disease (AD) points to endemic levels, especially considering the increase of average life expectancy worldwide. AD diagnosis based on early biomarkers and better knowledge of related pathophysiology are both crucial in the search for medical interventions that are able to modify AD progression. In this study we used unsupervised spectral unmixing statistical techniques to identify the vibrational spectral signature of amyloid ß aggregation in neural tissues, as early biomarkers of AD in an animal model. We analyzed spectral images composed of a total of 55 051 Raman spectra obtained from the frontal cortex and hippocampus of five bitransgenic APPswePS1ΔE9 mice, and colocalized amyloid ß plaques by other fluorescence techniques. The Raman signatures provided a multifrequency fingerprint consistent with the results of synthesized amyloid ß fibrils. The fingerprint obtained from unmixed analysis in neural tissues is shown to provide a detailed image of amyloid plaques in the brain, with the potential to be used as biomarkers for non-invasive early diagnosis and pathophysiology studies in AD on the retina.

Anomalous Nonlinear Optical Response of Graphene Near Phonon Resonances.

In this work we probe the third-order nonlinear optical property of graphene and hexagonal boron nitride and their heterostructure by the use of coherent anti-Stokes Raman spectroscopy. When the energy difference of the two input fields matches the phonon energy, the anti-Stokes emission intensity is enhanced in h-BN, as usually expected, while for graphene an anomalous decrease is observed. This behavior can be understood in terms of a coupling between the electronic continuum and a discrete phonon state. We have also measured a graphene/h-BN heterostructure and demonstrate that the anomalous effect in graphene dominates the heterostructure nonlinear optical response.

Photonic Counterparts of Cooper Pairs.

The microscopic theory of superconductivity raised the disruptive idea that electrons couple through the elusive exchange of virtual phonons, overcoming the strong Coulomb repulsion to form Cooper pairs. Light is also known to interact with atomic vibrations, as, for example, in the Raman effect. We show that photon pairs exchange virtual vibrations in transparent media, leading to an effective photon-photon interaction identical to that for electrons in the BCS theory of superconductivity, in spite of the fact that photons are bosons. In this scenario, photons may exchange energy without matching a quantum of vibration of the medium. As a result, pair correlations for photons scattered away from the Raman resonances are expected to be enhanced. An experimental demonstration of this effect is provided here by time-correlated Raman measurements in different media. The experimental data confirm our theoretical interpretation of a photonic Cooper pairing, without the need for any fitting parameters.

Multi-walled carbon nanotubes functionalized with recombinant Dengue virus 3 envelope proteins induce significant and specific immune responses in mice.

BACKGROUND: Dengue is the most prevalent arthropod-borne viral disease in the world. In this article we present results on the development, characterization and immunogenic evaluation of an alternative vaccine candidate against Dengue. METHODS: The MWNT-DENV3E nanoconjugate was developed by covalent functionalization of carboxylated multi-walled carbon nanotubes (MWNT) with recombinant dengue envelope (DENV3E) proteins. The recombinant antigens were bound to the MWNT using a diimide-activated amidation process and the immunogen was characterized by TEM, AFM and Raman Spectroscopy. Furthermore, the immunogenicity of this vaccine candidate was evaluated in a murine model. RESULTS: Immunization with MWNT-DENV3E induced comparable IgG responses in relation to the immunization with non-conjugated proteins; however, the inoculation of the nanoconjugate into mice generated higher titers of neutralizing antibodies. Cell-mediated responses were also evaluated, and higher dengue-specific splenocyte proliferation was observed in cell cultures derived from mice immunized with MWNT-DENV3E when compared to animals immunized with the non-conjugated DENV3E. CONCLUSIONS: Despite the recent licensure of the CYD-TDV dengue vaccine in some countries, results from the vaccine's phase III trial have cast doubts about its overall efficacy and global applicability. While questions about the effectiveness of the CYD-TDV vaccine still lingers, it is wise to keep at hand an array of vaccine candidates, including alternative non-classical approaches like the one presented here.