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.

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.

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.

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.

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.

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.

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.

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.