Sungkyunkwan University

About: Sungkyunkwan University is a education organization based out in Seoul, South Korea. It is known for research contribution in the topics: Thin film & Graphene. The organization has 28229 authors who have published 56428 publications receiving 1352733 citations. The organization is also known as: 성균관대학교.

Joint Secure Beamforming Design at the Source and the Relay for an Amplify-and-Forward MIMO Untrusted Relay System

Abstract: An amplify-and-forward (AF) multiple-input multiple-output (MIMO) relay network composed of a source, a relay, and a destination is considered, where transmit beamforming is employed both at the source and at the relay. The relay is a user who is willing to help the communication from the source to the destination. In our paper, however, the relay is untrusted in the sense that it may make a passive security attack; that is, it may decode messages of the source. We consider two ways to transmit confidential information of the source to the destination: noncooperative secure beamforming and cooperative secure beamforming. In the noncooperative scheme, the relay is simply treated as an eavesdropper, and does not participate in communication. In the cooperative scheme, the relay is asked to relay signals from the source to the destination. In this paper, the source and relay beamforming is jointly designed to maximize the secrecy rate in the cooperative scheme. The conditions under which the cooperative scheme achieves a higher secrecy rate than the noncooperative scheme are derived in the low and high signal-to-noise ratio (SNR) regimes of the source-relay and relay-destination links. The performance of the secure beamforming schemes is compared through extensive numerical simulations.

Water Splitting Progress in Tandem Devices: Moving Photolysis beyond Electrolysis

Abstract: Water photolysis is a sustainable technology to convert natural solar energy and water into chemical fuels and is thus considered a thorough solution to the forthcoming energy crises. Unassisted water splitting that could directly harvest solar light and subsequently split water in a single device has become an important research theme. Three types of tandem devices including photoelectrochemical (PEC), photovoltaic (PV) cell/PEC and PV/electrolyser tandem cells are proposed to realize water photolysis at different levels of integration and component. Recent progress in tandem water splitting devices is summarized, and crucial issues on device optimization from the perspective of each photo-absorber functionalities in band edge potential, light absorptivity and transmittance are discussed. By increasing the performances of stand-alone PEC or PV devices, a 20% solar to hydrogen efficiency is predicted that is a significant value towards further application in practice. Accordingly, the challenges for materials development and configuration optimization are further outlined.

Correction: Surface group modification and carrier transport properties of layered transition metal carbides (Ti2CT(x), T: -OH, -F and -O).

Abstract: In spite of recent significant research into various two-dimensional (2D) materials after the emergence of graphene, the development of a new 2D material that provides both high mobility and an appropriate energy band gap (which are crucial for various device applications) remains elusive. In this report, we demonstrate that the carrier transport behaviour of 2D Ti2CTx, which belongs to the family of 2D transition metal carbides and nitrides, can be tuned by modifying the surface group Tx (-OH, -F, and -O). Our results show that 2D Ti2C(OH)xFy and Ti2COx films can be obtained via simple chemical treatment, thermal annealing, and mechanical exfoliation processes. For the first time, we study the carrier transport properties of 2D Ti2CTx field effect transistors (FETs), obtaining the high field effect carrier mobilities of 10(4) cm(2) V(-1) s(-1) at room temperature. The temperature dependent resistivity of the Ti2COx film exhibits semiconductor like Arrhenius behaviour at zero gate voltage, from which we estimate the energy gap of 80 meV. One interesting feature of the FETs based on transition metal carbides is that the field effect mobility at room temperature is less sensitive to the measured transport gaps, which may arise from the dominant charge transport of activated carriers over the narrow energy gaps of the transition metal carbides. Our results open up the possibility that new 2D materials with high mobilities and appropriate band gaps can be achieved, and broaden the range of electronic device applications of Ti2CTx films.

Boosting Power-Generating Performance of Triboelectric Nanogenerators via Artificial Control of Ferroelectric Polarization and Dielectric Properties

Abstract: Low output current represents a critical challenge that has interrupted the use of triboelectric nanogenerators (TNGs) in a wide range of applications as sustainable power sources. Many approaches (e.g., operation at high frequency, parallel stacks of individual devices, and hybridization with other energy harvesters) remain limited in solving the challenge of low output current from TNGs. Here, a nanocomposite material system having a superior surface charge density as a triboelectric active material is reported. The nanocomposite material consists of a high dielectric ceramic material, barium titanate, showing great charge-trapping capability, together with a ferroelectric copolymer matrix, Poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)), with electrically manipulated polarization with strong triboelectric charge transfer characteristics. Based on a contact potential difference study showing that poled P(VDF-TrFE) has 18 times higher charge attracting properties, a fraction between two components is optimized. Boosting power-generating performance is achieved for 1130 V of output voltage and 1.5 mA of output current with this ferroelectric composite-based TNG, under 6 kgf of pushing force at 5 Hz. An enormously faster charging property than traditional polymer film-based TNGs is demonstrated in this study. Finally, the charging of a self-powering smartwatch with a charging management circuit system with no external power sources is demonstrated successfully.