Relaxation of Plasma Carriers in Graphene: An Approach by Frequency‐Dependent Optical Conductivity Measurement

Abstract

resulting in ultrahigh carrier mobilities, long mean free paths, and anomalous quantum Hall effects. [3][4][5][6][7] The most important advantage of graphene is the tunability of the carrier densities, which can be easily controlled by a gate bias or doping. [8][9][10][11][12] Consequently, graphene has been applied as convenient tunable terahertz (THz) metamaterials. [9,10,13,14] In photonics, the optical properties of graphene have been discussed with the surface plasmon model, corresponding to the Drude model with the free carrier concept. In general, in the plasmon model, the restoring force by the displacement from equilibrium is neglected to simplify the solution of the Maxwell equation. It has successfully been applied to the analysis of optical conductivity in metals including graphene over the THz regime [15][16][17][18] as the frequency of electromagnetic waves is significantly higher, compared to the plasma frequency. Recently, the applications of graphene have been extended to microwave absorption in the gigahertz (GHz) regime. [19,20] However, in most of these studies, the optical conductivity of graphene, related to the optical absorption, is discussed with the Debye model [21] or free carrier model. [22] The effective mass of electrons in graphene is extremely small, resulting in a high plasma frequency, comparable to the GHz regime. Hence, the analysis of the optical conductivity of graphene in the GHz regime should be modified from the free carrier model to the plasma carrier model considering the restoring force.In contrast, the field-induced time-resolved microwave conductivity (FI-TRMC) technique has recently been applied to complex permittivity analysis for organic semiconductors, leading to the assessment of the trap depth for charge carriers. [23] FI-TRMC is a microwave-based alternating current technique to evaluate the dielectric loss of materials with a resonant cavity. [23][24][25][26][27] In contrast to direct current (DC) methods represented by field-effect transistor (FET) and Hall effect measurements, FI-TRMC is advantageous in that it avoids the effect of extrinsic factors because of the small displacement distance of charge carriers. [28] For complex permittivity analysis with FI-TRMC, the real change in permittivity is estimated simultaneously with the dielectric loss, providing a detailed range of charge transport characteristics in the materials. FI-TRMC is suitable for investigation of the electronic properties The relaxation time and mobility of electrons on graphene are deduced quantitatively by complex permittivity analysis with microwave dielectric loss spectroscopy. Hexagonal graphene sheets with uniform domain sizes from 6 to 30 µm, which target the mean free path of electrons on graphene based on the significantly longer turnover time of microwave probing at gigahertz (GHz) frequencies as compared to the relaxation time of electrons, are employed to determine the factors for carrier relaxation at the intradomain and grain boundaries. The changes in the complex permittiv...

Terahertz Spectroscopy of Nanomaterials: a Close Look at Charge‐Carrier Transport

“…Nanoribbons with variable width were prepared by optical lithography and oxygen plasma etching: the frequency of the observed localized plasmons was shown to depend on the ribbon dimensions and on the doping. Choi et al [209] used microwave dielectric loss Adv. They showed that substrate terraces and wrinkles on the graphene surface due to strain relaxation during annealing serve as confining potential for carriers and can be also used to excite THz localized plasmons.…”

Rapid Charge Separation Boosts Solar Hydrogen Generation at the Graphene–MoS2 Junction: Time-Domain Ab Initio Analysis

“…The intraband relaxation of the injected electron exhibits multiple pathways on ultrafast time scales, such as down to the MoS 2 conduction band minimum (CBM) at the K and M points, because all of these processes are ultrafast, ranging from 150 fs to (sub-) 1 ps. , Then, it transfers back and recombines with the hole residing on the graphene. The K–K transition should be slow because it occurs across a wide energy gap, evidenced by the calculated MoS 2 CBM population decay taking place over 230 ps (Figure S1), which is 2–3 orders of magnitude longer than the experimentally reported time scales. − Alternatively, the M–K transition becomes dominant due to the presence of multiple intermediate states between the M and K points (Figure a).…”

Impact of oxygen plasma treatment on carrier transport and molecular adsorption in graphene

“…On the other hand, the increased carrier density can be attributed to an increase in oxygen adsorption on the graphene surface following plasma treatment, as reported in earlier studies. [12,17,18] Keeping in mind the very low power and plasma exposure time, XPS studies were carried out to confirm if indeed such an increase in oxygen bonding on the graphene surface occurred. In addition, since exposure to H2 plasma is expected to act in an opposite way to O2 plasma exposure, we also performed electrical and XPS characterization on H2 plasma treated graphene samples, and the XPS spectra and carrier transport characteristics were compared between initially untreated, O2 plasma treated, and H2 plasma treated graphene.…”