December 9, 2020.
Graphene is known to the scientists as the one-atom-thick material, the thinnest exotic material ever found! It composes of a hexagonal lattice of carbon atoms expanding in a two-dimensional plane. Each carbon atom forms sigma bond ( -bond) of the sp2 hybridization among the other three, thus promoting one free electron in the conduction band. This electron behaves like a massless relativistic particle with linear energy-momentum relation, which results in high electron mobility . Its linear band-energy profile and Fermi level tunability make graphene ideal for high-frequency electronic and photonic applications. Graphene is not a metal, but a zero-bandgap semiconducting material whose Fermi energy level can be adjusted by electrostatic doping from an external electric field . It can also absorb photons at various energies from ultraviolet (UV) to terahertz (THz) radiation and exhibits both electron and hole conductions, which is similar to that of an ambipolar transistor . In the past, there were continuous efforts on deploying this material to test and develop new theory and hypotheses of the quantum electrodynamics in condensed matter system. The discovery of graphene and the study of its electrical properties at low temperatures led to the Nobel Prize in Physics in 2010, awarded to Professor Andre Geim and Dr Konstantin Novoselov of the University of Manchester, the United Kingdom, for their groundbreaking experiments of the pioneering work that could pave the next generation of advanced electronic devices for the future .
After their achievement in 2010, a research team of Massachusetts Institute of Technology (MIT) proposed using a graphene sheet as an element to produce short-wavelength radiation. As shown in the diagram of Fig.1, high-energy electron beam (referred to Incoming electron) impacting on a graphene sheet generates the photons (X-rays) with the outgoing electron in the direction out of the x-y plane. The idea behind this mechanism requires (a) an accelerated-electron beam and (b) graphene plasmons to get their interaction on a semiconducting substrate .
Fig.1. Schematic drawing diagram of electron-plasmon interaction on the graphene surface that could lead to the X-ray emission. The incoming electron produced by an electron gun interacts with the graphene plasmons created by laser excitation. The interaction between these two components depends strongly on the momentum of the graphene plasmons and energy of the incoming electron. Adapted from .
In physics, a plasmon is a quantum state of plasma oscillations, like a photon - a quantum state of electromagnetic waves. We can say that Surface Plasmons (SPs) are electron oscillations at the interface between a thin-metallic conductor and an insulating material. In principle, when two materials possessing different permittivity (i.e. a metallic film exposing in the air or contacting with an oxide layer) are in contact, the SPs induced by the photoexcitation propagate in parallel to the conducting surface with the transverse magnetic (TM) mode. However, the amplitude of the SPs is attenuated inside the conducting layer due to energy absorption by impurity scattering. Surface plasmons in graphene are so called graphene plasmons (GPs). Here, in the context of the X-ray emission, graphene sheet will serve as a metamaterial that can retain plasmon wave with a long lifetime. The momentum of plasmons in graphene is also much higher than that of the photon in free space . When high-energy electron interacts with the GPs, electron oscillations take place in the transverse direction under the influence of GPs field. The result could lead to the production of the photons and the outgoing electron in the X-ray frequency. This mechanism is relatively different from the Thomson/Compton effect that considers scattering between electron and photon, while in graphene we consider the interaction between electron and plasmon of a much higher momentum at a given energy.
There are several uses of graphene plasmons in a wide variety of research fields. Examples are demonstrated in biosensing [7,8], optics [9-11], and photonics devices . To create graphene plasmons, a number of previous works have employed nanostructure objects lithographically patterned above the graphene sheet, such as the use of nanogratings , nanoribbons , metallic nanoparticles and nanorods . These nanostructures can be implemented by top-down and bottom-up nanofabrication approaches.
Why graphene plays a crucial role in the creation of plasmon waves for the THz emission? The answers lay between its electronic and optical properties. Besides the thinnest semiconducting nanomaterial, it shows unique linear band-energy profile up to room temperature, yielding photon absorption and emission via inter-band and intra-band energy transitions in the visible light and THz spectrum, respectively. These properties fascinate researchers and technologists to extend the limitations of conventional light-emitting and sensing devices in photonic applications.
In this project, we study the photoconductivity and plasmonic properties of thin graphene sheet for terahertz emission and detection that could lead to the development of an advanced photonic component. For instance, in the THz Time-Domain Spectroscopy (THz-TDS) the THz emitter and receiver are made of III-V semiconducting materials (i.e. InGaAs, GaP and GaAs), which requires sophisticated machining and high-cost production. Here, we aim at demonstrating some possibilities for THz sensing applications after those studies. We began with a design of the device prototype made of a thin graphene sheet prepared by chemical vapour deposition (CVD) technique. In the first-year experiment, we have made progress in device fabrication, installation of electronic equipment for monitoring, measuring, and collecting electrical signals excited by the laser light.
Fig.2 (a) Optical image of the graphene device created in this work. Using plasma etching, six narrow strips (dark blue) are defined for electrical paths, labelled as S1-6-D1-6. (b) Metallic nanorods are implemented using electron-beam lithography technique on the graphene sheet. Prior to depositing the nanorods, a thin layer of Al2O3 is coated on top of the graphene.
As shown in Fig. 2, we have created a graphene plasmonic device on CVD-grown graphene that has already been transferred onto SiO2/Si substrate with a thick oxide layer of 285 nm. After that the CVD graphene (10 x 10 mm2) was then shaped into narrow strips forming six conductive channels/paths whose length of each one is 900 μm long by the width of 20-100 μm, as shown in Fig.2(a). Standard cleanroom processing techniques were used to achieve such micropatterns. Next, an aluminium oxide (Al2O3) of a thickness 2-10 nm was deposited on top of the entire chip by thermal deposition and etched again by oxygen plasma. Then, small gold (Au) nanorods were implemented onto the Al2O3 layer by electron-beam lithography, as shown in Fig.2(b) indicating as the yellow squares. These nanorods of 100 - 300 nm long and 25 nm thick covers on different areas of the graphene around 20x20 µm2 to 80x80 µm2 on each strip.
With our device design and successful fabrication, these graphene samples will offer possibilities for the study of the photoconduction and surface plasmon confinements of the graphene via nanorods coupling and the laser excitation in the next following years.
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Dr. Yodchay Jompol, Department of Physics, Mahidol University