Morphological imperfections of epitaxial graphene: from a hindrance to the generation of new photo-responses in the visible domain

Functionalization of three-dimensional epitaxial graphene with metal nanoparticles

We demonstrate the first successful functionalization of epitaxial three-dimensional graphene with metal nanoparticles. The functionalization is obtained by immersing three-dimensional graphene in a nanoparticle colloidal solution. This method is versatile and demonstrated here for gold and palladium, but can be extended to other types of nanoparticles. We have measured the nanoparticle density on the top surface and in the porous layer volume by scanning electron microscopy and scanning transmission electron microscopy. The samples exhibit a wide coverage of nanoparticles with minimal clustering. We demonstrate that high-quality graphene promotes the functionalization, leading to higher nanoparticle density both on the surface and in the pores. X-ray photoelectron spectroscopy shows the absence of contamination after the functionalization process. Moreover, it confirms the thermal stability of the Au- and Pd-functionalized three-dimensional graphene up to 530 °C. Our approach opens new avenues for utilizing three-dimensional graphene as a versatile platform for catalytic applications, sensors, and energy storage and conversion.

Photon Drag Currents and Terahertz Generation in α-Sn/Ge Quantum Wells

We have fabricated α-Sn/Ge quantum well heterostructures by sandwiching nano-films of α-Sn between Ge nanolayers. The samples were grown via e-beam deposition and characterized by Raman spectroscopy, atomic force microscopy, temperature dependence of electrical resistivity and THz time-resolved spectroscopy. We have established the presence of α-Sn phase in the polycrystalline layers together with a high electron mobility μ = 2500 ± 100 cm² V^-1 s^-1. Here, the temperature behavior of the resistivity in a magnetic field is distinct from the semiconducting films and three-dimensional Dirac semimetals, which is consistent with the presence of linear two-dimensional electronic dispersion arising from the mutually inverted band structure at the α-Sn/Ge interface. As a result, the α-Sn/Ge interfaces of the quantum wells have topologically non-trivial electronic states. From THz time-resolved spectroscopy, we have discovered unusual photocurrent and THz radiation generation. The mechanisms for this process are significantly different from ambipolar diffusion currents that are responsible for THz generation in semiconducting thin films, e.g., Ge. Moreover, the THz generation in α-Sn/Ge quantum wells is almost an order of magnitude greater than that found in Ge. The substantial strength of the THz radiation emission and its polarization dependence may be explained by the photon drag current. The large amplitude of this current is a clear signature of the formation of conducting channels with high electron mobility, which are topologically protected.

Perfect Impedance Matching with Meta-Surfaces Made of Ultra-Thin Metal Films: A Phenomenological Approach to the Ideal THz Sensors

The terahertz (THz) frequency range is incredibly important as it covers electromagnetic emissions typical for biological and molecular processes. All molecules emit THz waves in a unique fingerprint pattern, although the intensity of such signals is usually too weak to be detected. To address the efficiency gap in existing THz devices it is extremely important to create surfaces with perfect anti-reflection properties. Although metals are absolutely reflective, here we show both theoretically and experimentally that by constructing meta-surfaces made of a superposition of ultra-thin metallic nano-films (a couple of nanometres thick) and oxide layers a unique property of perfect transmission and impedance matching may be realised. The perfect transmission rates can be as high as 100% and it may be achieved in both optical and THz regimes. The predicted effect has been observed for numerous meta-surfaces of different compositions. The effect found here is expected to impact the renewable energies sectors, optoelectronic and telecommunication industries, accelerating the arrival of the sensors for the new 6G-technology. The phenomenon is highly relevant to all scientific fields where minimising electromagnetic losses through reflection is important.

Kusmartsev F, Kusmartseva A, H. Alkallas F, AlFaify S, Shkir M. Raman Spectroscopy Imaging of Exceptional Electronic Properties in Epitaxial Graphene Grown on SiC

Graphene distinctive electronic and optical properties have sparked intense interest throughout the scientific community bringing innovation and progress to many sectors of academia and industry. Graphene manufacturing has rapidly evolved since its discovery in 2004. The diverse growth methods of graphene have many comparative advantages in terms of size, shape, quality and cost. Specifically, epitaxial graphene is thermally grown on a silicon carbide (SiC) substrate. This type of graphene is unique due to its coexistence with the SiC underneath which makes the process of transferring graphene layers for devices manufacturing simple and robust. Raman analysis is a sensitive technique extensively used to explore nanocarbon material properties. Indeed, this method has been widely used in graphene studies in fundamental research and application fields. We review the principal Raman scattering processes in SiC substrate and demonstrate epitaxial graphene growth. We have identified the Raman bands signature of graphene for different layers number. The method could be readily adopted to characterize structural and exceptional electrical properties for various epitaxial graphene systems. Particularly, the variation of the charge carrier concentration in epitaxial graphene of different shapes and layers number have been precisely imaged. By comparing the intensity ratio of 2D line and G line-"I2D/IG"-the density of charge across the graphene layers could be monitored. The obtained results were compared to previous electrical measurements. The substrate longitudinal optical phonon coupling "LOOPC" modes have also been examined for several epitaxial graphene layers. The LOOPC of the SiC substrate shows a precise map of the density of charge in epitaxial graphene systems for different graphene layers number. Correlations between the density of charge and particular graphene layer shape such as bubbles have been determined. All experimental probes show a high degree of consistency and efficiency. Our combined studies have revealed novel capacitor effect in diverse epitaxial graphene system. The SiC substrate self-compensates the graphene layer charge without any external doping. We have observed a new density of charge at the graphene-substrate interface. The located capacitor effects at epitaxial graphene-substrate interfaces give rise to an unexpected mini gap in graphene band structure.

Optical Transistor for Amplification of Radiation in a Broadband Terahertz Domain

We propose a new type of optical transistor for a broadband amplification of terahertz radiation. It is made of a graphene-superconductor hybrid, where electrons and Cooper pairs couple by Coulomb forces. The transistor operates via the propagation of surface plasmons in both layers, and the origin of amplification is the quantum capacitance of graphene. It leads to terahertz waves amplification, the negative power absorption, and as a result, the system yields positive gain, and the hybrid acts like an optical transistor, operating with the terahertz light. It can, in principle, amplify even a whole spectrum of chaotic signals (or noise), which is required for numerous biological applications.