Abstract A water-induced electromechanical response in suspended graphene atop a microfluidic channel is reported. The graphene membrane resistivity rapidly decreases to ≈25% upon water injection into the channel, defining a sensitive “channel wetting” device—a wetristor. The physical mechanism of the wetristor operation is investigated using two graphene membrane geometries, either uncovered or covered by an inert and rigid lid (hexagonal boron nitride multilayer or poly(methyl methacrylate) film). The wetristor effect, namely the water-induced resistivity collapse, occurs in uncovered devices only. Atomic force microscopy and Raman spectroscopy indicate substantial morphology changes of graphene membranes in such devices, while covered membranes suffer no changes, upon channel water filling. The results suggest an electromechanical nature for the wetristor effect, where the resistivity reduction is caused by unwrinkling of the graphene membrane through channel filling, with an eventual direct doping caused by water being of much smaller magnitude, if any. The wetristor device should find useful sensing applications in general micro- and nanofluidics.

We investigate theoretically, through of first-principles calculations, the effect of the application of large in-plane uniaxial stress on single-layer of MoS2, MoSe2, and MoSSe alloys. For stress applied along the zigzag (zz) direction, we predict an anomalous behavior near the point fracture. This behavior is characterized by the reorientation of the MoS2 structure along the applied stress from zz to armchair due to the formation of transient square-lattice regions in the crystal, with an apparent crystal rotation of 30 degrees. After reorientation, a large plastic deformation remains after the stress is removed. This behavior is also observed in MoSe2 and in MoSSe alloys. This phenomenon is observed both in stress-constrained geometry optimizations and in ab initio molecular dynamics simulations at finite temperature and applied stress.

The objective of this work was to evaluate the influence of different altitudes on the epiphytic microbiota of coffee beans and on sensorial and chemical quality of coffees grown at 800, 1000, 1200, and 1400 m in Serra do Caparaó, Espírito Santo, Brazil. For microbiological analysis, the population counts of mesophilic bacteria, lactic acid bacteria (LAB), and yeasts were performed from the surface plating. The isolates were grouped and identified from the Matrix Assisted Laser Desorption Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF MS) and sequencing of the ribosomal region was used. The chemical composition of the green grains was evaluated by Raman spectroscopy, and the sensory analysis of the roasted grains was performed using temporal dominance of sensations (TDS). During fermentation, there was a decrease in the LAB in pulped coffee from 800 and 1000 m altitude, while an increase was observed at 1200 and 1400 m. In natural coffee, there was an increase of LAB population at all altitudes. The highest diversity of mesophilic bacteria and yeast were identified in natural 1400 m and 1000 m, respectively. However pulped coffee treatments it was at 1200 m and 800 m. The chlorogenic acid and fatty acids in the green bean changed with altitude variation and processing. The floral attribute was detected only at altitude 1400 m. Caramel, chocolate and almond attributes were most frequently detected in coffees at different altitudes and processing. Therefore, pulped coffee processing was most suitable at low altitude while at high altitudes, both processes can be conducted to obtain a beverage with unusual sensory profile.

Carbon nitride materials are promising for applications in electronics, clean energy production, and heat dissipation. Two-dimensional (2D) diamond-like carbon nitrides α-C2N2, β-C2N2, and γ-C4N4 rise as beyond graphene semiconductors. Here, we apply first-principles calculations and group theory to study their structure, mechanical properties, and vibrational signature. The α-C2N2 is the strongest among them, with a 2D Young’s modulus E2D equal to 616(6) N/m, followed by the γ-C4N4 with an E2D equal to 632(6) N/m and 581(7) N/m along its zigzag and armchair directions, respectively, and the β-C2N2 with an E2D equal to 582(9) N/m. These materials are about 2 times stiffer than graphene, and are the stiffest among 2D networks of carbon and nitrogen atoms. The zigzag direction of 2D γ-C4N4 is approximately 8% stronger than its armchair direction, unusual for in-plane anisotropic 2D materials, where the armchair direction is considerably weaker than the zigzag direction. These findings from stress–strain analysis are consistent with the high sound speed and elastic constants values we found by using 2D density-functional perturbation theory framework, suggesting them for mechanical reinforcement. We report the phonon wavenumber, atomic vibrational pattern, and Raman and infrared spectra for all polytypes. The longitudinal and transverse optical modes of the in-plane isotropic polytypes display the breakdown of LO–TO splitting, characteristic of 2D polar crystals. We found that the difference between their phonon wavenumbers can be probed in their unpolarized Raman and infrared spectra. The simulated angular dependency of the Raman intensity under backscattering parallel and cross polarizations show how to assign the A1′ and E′ modes of the α-C2N2, the A1g and Eg modes of the β-C2N2, and of the Ag and B1g modes of the γ-C4N4, being key for polytype identification. These results provide comprehensive information on the emerging 2D diamond-like carbon nitrides, necessary for further developments on their synthesis, characterization, and future device fabrication.

Abstract Jacutingaite (Pt2HgSe3) is a recently discovered layered platinum-group mineral. Recent experimental studies have shown that it displays the properties of a quantum spin Hall insulator (QSHI), and theoretical studies indicate that its two-dimensional monolayer is a QSHI with a robust topological gap of ∼0.5 eV. Jacutingaite is thus promising for potential applications to nanoelectronics and spintronics. The Raman spectrum of three-dimensional bulk jacutingaite and the symmetries of its vibrational modes, fundamental for understanding structural modifications of this material, are still unexplored. Here, we address the zone-center Raman optical phonons of bulk jacutingaite by experiments, symmetry, and first-principles calculations. The improved synthesis used here provided crystals of higher purity and of micrometer size, allowing the study of single crystals. Polarized Raman spectroscopy was used to assign the symmetries of nine out of the 11 Raman-active modes expected by group theory and their respective selection rules. The calculated wavenumbers of the Raman-active modes, in addition to their atomic displacements, are in very good agreement with experiments. In addition, we discuss the use of different exchange correlation functionals within density functional theory, as local functionals and nonlocal functionals that best describe van der Waals interactions. The influence of the inclusion of spin–orbit coupling on calculated vibrational phonon wavenumbers and lattice parameters is commented, and it was found that the local density approximation provides a good description. Our results are of paramount importance to further exploitation of the effects of jacutingaite's structural modifications to tune its properties, as well as for its structural, optical, electronic, mechanical, and thermal applications.

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 and A 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 temperature coefficients. The first-order temperature coefficients θ of A mode is ∼ −0.016 cm−1/K, independent of the number of layers and the support. In contrast, the θ of A 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.

Raman spectroscopy is one of the most important optical techniques for the study of two-dimensional systems, providing fundamental information for the development of applications using these materials in optoelectronics and valleytronics. The emerging area of two-dimensional layered materials demands the characterization and understanding of the basic physical properties of the material under study and is indispensable to pave the way for the engineering of devices. In this review we cover the recent development of resonance Raman spectroscopy on transition metal dichalcogenides, discussing the exciton-phonon coupling and intervalley double-resonance Raman scattering process. A brief discussion of the effect of defects and disorder on the Raman spectra of these materials is also presented. The results of Raman spectroscopy in TMDs are compared to those observed in graphene, showing that this technique also provides physical information about TMDs that were previously reported in graphene systems. We also discuss the possible future perspectives and directions that the field may go to.

Abstract The Raman spectra of graphene-based matter exhibit a set of defect/disorder-induced bands. The D band, which exhibits a strong dispersion up to  50 cm−1/eV, comes from transverse optical phonons around K or K′ in the first Brillouin zone and involves an intervalley double resonance (DR) Raman process. In the present work, resonant Raman scattering (lines ranging from 1.58 to 3.81 eV) is used to study the unusual behavior of the one-phonon Raman band of a carbonaceous material (anthracene-based carbon which is one of the graphitizable carbons) upon its secondary carbonization stage (450°C–1000°C). While the G band appears to be nondispersive, the D band exhibits a change in both position and intensity. Its dispersion progressively rises from  6 cm−1/eV to values close to what is usually observed in defected graphene-based systems when anthracene-based carbon becomes almost pure. This evolution appears to be correlated with a release of hydrogen (fixed on the edges of polyaromatic layers) questioning their role in changing the D band resonance conditions.

The double-resonance (DR) Raman process is a signature of all sp2 carbon material and provide fundamental information of the electronic structure and phonon dispersion in graphene, carbon nanotubes and different graphite-type materials. We have performed in this work the study of different DR Raman bands of rhombohedral graphite using eight different excitation laser energies and obtained the dispersion of the different DR features by changing the laser energy. Results are compared with those of Bernal graphite and shows that rhombohedral graphite exhibit a richer DR Raman spectrum. For example, the 2D band of rhombohedral graphite is broader and composed by several maxima that exhibit different dispersive behavior. The occurrence of more DR conditions in rhombohedral graphite is ascribed to the fact that the volume of its Brillouin zone (BZ) is twice the volume of the Bernal BZ, allowing thus more channels for the resonance condition. The spectra of the intervalley TO-LA band of rhombohedral graphite, around 2450 cm−1, is also broader and richer in features compared to that of Bernal graphite. Results and analysis of the spectral region 1700-1850 cm−1, where different intravalley processes involving acoustic and optical phonons occurs, are also presented.

When two periodic two-dimensional structures are superposed, any mismatch rotation angle between the layers generates a Moiré pattern superlattice, whose size depends on the twisting angle θ. If the layers are composed by different materials, this effect is also dependent on the lattice parameters of each layer. Moiré superlattices are commonly observed in bilayer graphene, where the mismatch angle between layers can be produced by growing twisted bilayer graphene (TBG) samples by CVD or folding the monolayer back upon itself. In TBG, it was shown that the coupling between the Dirac cones of the two layers gives rise to van Hove singularities (vHs) in the density of electronic states, whose energies vary with θ. The understanding of the behavior of electrons and their interactions with phonons in atomically thin heterostructures is crucial for the engineering of novel 2D devices. Raman spectroscopy has been often used to characterize twisted bilayer graphene and graphene heterostructures. Here, we review the main important effects in the Raman spectra of TBG discussing firstly the appearance of new peaks in the spectra associated with phonons with wavevectors within the interior of the Brillouin zone of graphene corresponding to the reciprocal unit vectors of the Moiré superlattice, and that are folded to the center of the reduced Brillouin Zone (BZ) becoming Raman active. Another important effect is the giant enhancement of G band intensity of TBG that occurs only in a narrow range of laser excitation energies and for a given twisting angle. Results show that the vHs in the density of states is not only related to the folding of the commensurate BZ, but mainly associated with the Moiré pattern that does not necessarily have a translational symmetry. Finally, we show that there are two different resonance mechanisms that activate the appearance of the extra peaks: the intralayer and interlayer electron–phonon processes, involving electrons of the same layer or from different layers, respectively. Both effects are observed for twisted bilayer graphene, but Raman spectroscopy can also be used to probe the intralayer process in any kind of graphene-based heterostructure, like in the graphene/h-BN junctions.

Raman spectroscopy has been extensively used to probe disorder in graphene and other carbon-related materials, and disorder-induced (DI) Raman bands are prominent even for low defect densities. The DI bands in MoS2 have been studied in the last years, but a multiple excitation study using laser excitation energies near the excitonic energies was still lacking. In this work, we investigate the low-frequency defect-induced Raman bands in MoS2 coming from the acoustic phonon branches near the Brillouin zone edge using samples produced by mechanical exfoliation and chemical vapor deposition, recorded with different laser excitation energies close to the resonance with the excitonic transitions, and measured at different temperatures, from 100 K to 400 K. Our results show that the defect-induced Raman processes are affected by both excitation energy and temperature. We find that the temperature of measurement affects the linear dependence between the intensities of the DI peaks and the defect concentration. In particular, we observed that the ratio of intensities of the DI longitudinal acoustic (LA) and transversal acoustic (TA) modes with respect to the first-order E′ mode is about the same for the two different samples when results are corrected by the defect density. We show in this work that the largest intensity of the DI peaks occurs for laser energies in the resonance with the excitonic transitions. Finally, we introduce a general expression that provides the parameters for the quantification of defects in MoS2 samples based on the intensity of the DI Raman bands, measured at different laser energies across the excitonic transitions.

Abstract In this work, we present measurements of angle-resolved polarized Raman spectra of single-layer (1L) and bulk ReSe2 recorded with excitation energies of 1.92 eV (647.1 nm) and 2.34 eV (530.8 nm). The Raman tensors for all modes were obtained by fitting simultaneously the angular dependence of the parallel (I‖) and crossed (I⊥) polarized intensities. We observed that the tensor elements are, in general, complex numbers, and their magnitudes and phases depend on both the dimensionality of the sample (1L or bulk) and the excitation energy. Results are discussed by considering the intrinsic contribution of a single layer to the tensor elements and the macroscopic contribution coming from the stacking of several layers. We show that the different behaviour of angle-resolved polarized Raman spectra for different excitation energies is due to the resonant Raman effect, which affects both the real and imaginary parts of the Raman tensor elements. Our work highlights the importance of understanding the fundamental physics of low symmetry 2D materials that can be used to fabricate devices sensitive to the direction of light polarization or electrical current.

Low symmetry 2D materials offer an alternative for the fabrication of optoelectronic devices which are sensitive to light polarization. The investigation of electron–phonon interactions in these materials is essential since they affect the electrical conductivity. Raman scattering probes light–matter and electron–phonon interactions, and their anisotropies are described by the Raman tensor. The tensor elements can have complex values, but the origin of this behavior in 2D materials is not yet well established. In this work, we studied a single-layer triclinic ReSe2 by angle-dependent polarized Raman spectroscopy. The obtained values of the Raman tensor elements for each mode can be understood by considering a new coordinate system, which determines the physical origin of the complex nature of the Raman tensor elements. Our results are explained in terms of anisotropy of the electron–phonon coupling relevant to the engineering of new optoelectronic devices based on low-symmetry 2D materials.

Inducing electrostatic doping in 2D materials by laser exposure (photodoping effect) is an exciting route to tune optoelectronic phenomena. However, there is a lack of investigation concerning in what respect the action of photodoping in optoelectronic devices is local. Here, we employ scanning photocurrent microscopy (SPCM) techniques to investigate how a permanent photodoping modulates the photocurrent generation in MoS2 transistors locally. We claim that the photodoping fills the electronic states in MoS2 conduction band, preventing the photon-absorption and the photocurrent generation by the MoS2 sheet. Moreover, by comparing the persistent photocurrent (PPC) generation of MoS2 on top of different substrates, we elucidate that the interface between the material used for the gate and the insulator (gate-insulator interface) is essential for the photodoping generation. Our work gives a step forward to the understanding of the photodoping effect in MoS2 transistors and the implementation of such an effect in integrated devices.