Raman spectroscopy is a technique widely used to detect defects in semiconductors because it provides information of structural or chemical defects produced in its structure. In the case of graphene monolayer, the Raman spectrum presents two bands centered at 1582 cm⁻¹ (G band) and 2700 cm⁻¹ (2D band). However, when the periodic lattice of graphene is broken by different types of defects, new bands appear. This is the situation for the Raman spectrum of graphene oxide. It is well established that the existence of these bands, the position and the intensity or width of peaks can provide information about the origin of defects. However, in the case of the graphene oxide spectrum, we can find in the literature several discrepant results, probably due to differences in chemical composition and the type of defects of the graphene oxide used in these studies. Besides, theoretical calculations proved that the shape of bands, intensity and width, and the position of graphene oxide Raman spectrum depend on the atomic configuration. In the current work, we will summarize our current understanding of the effect of the chemical composition on the Raman spectrum of graphene oxide. Finally, we apply all this information to analyze the evolution of the structure of graphene oxide during the thermal annealing of the heterostructures formed by graphene oxide sandwiches in a hexagonal boron nitride.

Exciton physics in two-dimensional semiconductors are typically studied by photoluminescence spectroscopy. However, this technique does not allow for direct observation of non-radiating excitonic transitions. Here, we use low-temperature photocurrent spectroscopy as an alternative technique to investigate excitonic transitions in a high-quality monolayer MoS₂ phototransistor. The resulting spectra presents excitonic peaks with linewidths as low as 8 meV. We identify spectral features corresponding to the ground states of neutral excitons ( $${\mathrm{X}}_{1{\mathrm{s}}}^{\mathrm{A}}$$X 1 s A and $${\mathrm{X}}_{1{\mathrm{s}}}^{\mathrm{B}}$$X 1 s B ) and charged trions (T^A and T^B) as well as up to eight additional spectral lines at energies above the $${\mathrm{X}}_{1{\mathrm{s}}}^{\mathrm{B}}$$X 1 s B transition, which we attribute to the Rydberg series of excited states of X^A and X^B. The intensities of the spectral features can be tuned by the gate and drain-source voltages. Using an effective-mass theory for excitons in two-dimensional systems we are able to accurately fit the measured spectral lines and unambiguously associate them with their corresponding Rydberg states.

Terahertz (THz) waves have revealed a great potential for use in various fields and for a wide range of challenging applications. High‐performance detectors are, however, vital for exploitation of THz technology. Graphene plasmonic THz detectors have proven to be promising optoelectronic devices, but improving their performance is still necessary. In this work, an asymmetric‐dual‐grating‐gate graphene‐terahertz‐field‐effect‐transistor with a graphite back‐gate was fabricated and characterized under illumination of 0.3 THz radiation in the temperature range from 4.5 K up to the room temperature. The device was fabricated as a sub‐THz detector using a heterostructure of h‐BN/Graphene/h‐BN/Graphite to make a transistor with a double asymmetric‐grating‐top‐gate and a continuous graphite back‐gate. By biasing the metallic top‐gates and the graphite back‐gate, abrupt n ⁺ n (or p ⁺ p ) or np (or pn ) junctions with different potential barriers are formed along the graphene layer leading to enhancement of the THz rectified signal by about an order of magnitude. The plasmonic rectification for graphene containing np junctions is interpreted as due to the plasmonic electron‐hole ratchet mechanism, whereas, for graphene with n ⁺ n junctions, rectification is attributed to the differential plasmonic drag effect. This work shows a new way of responsivity enhancement and paves the way towards new record performances of graphene THz nano‐photodetectors.

A graphene-based field-effect-transistor with asymmetric dual-grating gates was fabricated and characterized under excitation of terahertz radiation at two frequencies: 0.15 THz and 0.3 THz. The graphene sheet was encapsulated between two flakes of h-BN and placed on a highly doped SiO₂/Si substrate. An asymmetric dual-grating gate was implemented on the h-BN top flake. Even though no antenna was used to couple the incoming radiation, a clear gate-bias-dependent photocurrent was measured under excitation at 0.3 THz up to room temperature. We subsequently demonstrated that the device can be used for terahertz sensing and inspection of hidden metallic objects at room temperature.

We identify a rapidly-responding mechanism for photogating in monolayer MoS₂ phototransistors that becomes the dominant contribution to photoresponse under high-frequency light modulation.

The quantum Hall (QH) effect in two-dimensional electron systems (2DESs) is conventionally observed at liquid-helium temperatures, where lattice vibrations are strongly suppressed and bulk carrier scattering is dominated by disorder. However, due to large Landau level (LL) separation (~2000 K atB = 30 T), graphene can support the QH effect up to room temperature (RT), concomitant with a non-negligible population of acoustic phonons with a wave-vector commensurate to the inverse electronic magnetic length. Here, we demonstrate that graphene encapsulated in hexagonal boron nitride (hBN) realizes a novel transport regime, where dissipation in the QH phase is governed predominantly by electron-phonon scattering. Investigating thermally-activated transport at filling factor 2 up to RT in an ensemble of back-gated devices, we show that the highB-field behaviour correlates with their zeroB-field transport mobility. By this means, we extend the well-accepted notion of phonon-limited resistivity in ultra-clean graphene to a hitherto unexplored high-field realm.