Discovered in 2013, 2D niobium carbide (Nb2C), a member of the MXene family, has been shown to have many extraordinary properties, such as high photothermal conversion efficiency, strong electron-phonon interactions, strong optical absorption in the near-infrared, and even saturable optical absorption. These unique properties of Nb2C render this MXene potentially useful for a variety of applications, including photonic and optoelectronic devices and even photothermal cancer therapy. Here, we employ both terahertz time-domain spectroscopy (TDS) and time-resolved terahertz spectroscopy (TRTS) to investigate intrinsic and photoinduced conductivity and dynamics of optically injected charge carriers with 1.55 eV excitations in order to understand the photoinduced processes taking place in Nb2C. We find that the photoinduced conductivity in this MXene shows an initial rapid decay over a picosecond time scale, followed by a much longer-lived component that lasts for nanoseconds. We also observe that the long-range conductivity is strongly limited by the nanoflake boundaries.
MXenes are a new class of intrinsically metallic 2D materials. Their wide range of optoelectronic properties they demonstrate as a function of their chemical composition suggest applications in electronic and photonic devices. In this work we present a comprehensive study of the optical properties of three members of the MXene family, Ti3C2Tz, Mo2Ti2C3Tz, and Nb2CTz, using ultrafast transient optical absorption and THz spectroscopy. We find that those properties result from a complicated interaction between free carriers, interband transitions and localized surface plasmon resonances. Elucidating the nature of photoexcitation and dynamics of carriers in these emergent materials will lay the foundation for their potential for optoelectronic applications.
We investigate mechanisms by which interaction of light and matter may be affected by electrons, and show how this can lead to optoelectronic devices with superior properties. In particular, confined cloud of electron gas allows sculpting a wave function that affects both emission and absorption of radiation, while its collective, plasmonic, excitation may be used for optical wave guiding, coupling and radiation. Such processes require much less energy and are much faster than classical kinetic energy-based charge transport in traditional electronics. Here we present thin-film photodetectors in which 2D electron and hole charges allow operation in hundreds of GHz, without applied bias, requiring a fraction of microwatt of optical power. The 2D channel can also be structured to provide the momentum change that is required for coupling to excitation at THz range. The confined charge is then used as a plate of (an unconventional) capacitor which changes states by a factor of >1000, in tens of fs, requiring atto-joules of energy which is also switchable by light. This opto-plasmonic capacitor finds application in threshold logic based neuromorphic systems. These thin-film devices are produced in bottom-up core-shell nanowire (CSNW) technology, resulting in resonant optical cavities whose properties are controlled by 2D and 1D charge plasma, with orders of magnitude increase in absorption and emission of light that leads to lasing at room temperature even without vertical structure. Since CSNWs can be grown on Si, they can be good candidate platforms for Photonic Integrated Circuits (PIC) and Silicon Photonics.
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