Browsing by Author "Zhang, Xu"

Now showing 1 - 6 of 6
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    Atomistic measurement and modeling of intrinsic fracture toughness of two-dimensional materials
    (PNAS, 2022) Zhang, Xu; Nguyen, Hoang; Zhang, Xiang; Ajayan, Pulickel M.; Wen, Jianguo; Espinosa, Horacio D.
    Quantifying the intrinsic mechanical properties of two-dimensional (2D) materials is essential to predict the long-term reliability of materials and systems in emerging applications ranging from energy to health to next-generation sensors and electronics. Currently, measurements of fracture toughness and identification of associated atomistic mechanisms remain challenging. Herein, we report an integrated experimental–computational framework in which in-situ high-resolution transmission electron microscopy (HRTEM) measurements of the intrinsic fracture energy of monolayer MoS 2 and MoSe 2 are in good agreement with atomistic model predictions based on an accurately parameterized interatomic potential. Changes in crystalline structures at the crack tip and crack edges, as observed in in-situ HRTEM crack extension tests, are properly predicted. Such a good agreement is the result of including large deformation pathways and phase transitions in the parameterization of the inter-atomic potential. The established framework emerges as a robust approach to determine the predictive capabilities of molecular dynamics models employed in the screening of 2D materials, in the spirit of the materials genome initiative. Moreover, it enables device-level predictions with superior accuracy (e.g., fatigue lifetime predictions of electro- and opto-electronic nanodevices).
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    New Security Threats in Multiple-Antenna Networks: Analysis and Experiments
    (2017-11-06) Zhang, Xu; Knightly, Edward W.
    Due to the multiple to massive number of antennas at the Access Point (AP), the performance of wireless network has substantially improved over the last decade. However, new security threats also arise, mainly because of the redesign of wireless protocols that adapt to these many antennas, as well as the critical dependence of the multi-fold increases on Channel State Information (CSI), a key parameter in multiple-antenna networks. In this thesis, I study two security threats of CSI that are closely related to its core properties. First, I analyze the confidentiality of CSI with a passive adversary. I discover that CSI is no longer confidential in a multi-user MIMO network, because there is a fundamental conflict between using CSI to optimize PHY design and hiding CSI from malicious nodes. I present CSIsnoop, a framework by which a passive adversary can infer any client's CSI, even when both channel sounding sequence from the AP and CSI measurement feedback from the clients are encrypted during downlink channel sounding, or when uplink channel sounding is employed. I implement CSIsnoop on a software defined radio and collect over 100,000 over-the-air transmissions in various indoor environments. CSIsnoop's high estimation accuracy urges reconsideration of the use of CSI as a tool to enhance physical layer security in multi-user MIMO networks. Second, I analyze the integrity of CSI with an active adversary. I present and model the Pilot Distortion Attack, a highly efficient yet devastating jamming strategy targeting the channel sounding process, in which the adversary distorts the AP's CSI measurement of even a single client leading to denial-of-service for all clients associated with the AP. As a countermeasure, I propose MACE, which exploits the AP's multiple antennas to detect Pilot Distortion Attack, as well as general jamming in wireless network, with zero startup cost, zero additional network overhead, and no coordination between the AP and the clients. I build a testbed with the Argos 72-antenna AP and collect over 3,000,000 over-the-air transmissions. My experiments demonstrate the devastating impacts of the Pilot Distortion Attack, as well as the superior detection performance of MACE.
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    Solution-Deposited and Patternable Conductive Polymer Thin-Film Electrodes for Microbial Bioelectronics
    (Wiley, 2022) Tseng, Chia-Ping; Liu, Fangxin; Zhang, Xu; Huang, Po-Chun; Campbell, Ian; Li, Yilin; Atkinson, Joshua T.; Terlier, Tanguy; Ajo-Franklin, Caroline M.; Silberg, Jonathan J.; Verduzco, Rafael
    Microbial bioelectronic devices integrate naturally occurring or synthetically engineered electroactive microbes with microelectronics. These devices have a broad range of potential applications, but engineering the biotic–abiotic interface for biocompatibility, adhesion, electron transfer, and maximum surface area remains a challenge. Prior approaches to interface modification lack simple processability, the ability to pattern the materials, and/or a significant enhancement in currents. Here, a novel conductive polymer coating that significantly enhances current densities relative to unmodified electrodes in microbial bioelectronics is reported. The coating is based on a blend of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) crosslinked with poly(2-hydroxyethylacrylate) (PHEA) along with a thin polydopamine (PDA) layer for adhesion to an underlying indium tin oxide (ITO) electrode. When used as an interface layer with the current-producing bacterium Shewanella oneidensis MR-1, this material produces a 178-fold increase in the current density compared to unmodified electrodes, a current gain that is higher than previously reported thin-film 2D coatings and 3D conductive polymer coatings. The chemistry, morphology, and electronic properties of the coatings are characterized and the implementation of these coated electrodes for use in microbial fuel cells, multiplexed bioelectronic devices, and organic electrochemical transistor based microbial sensors are demonstrated. It is envisioned that this simple coating will advance the development of microbial bioelectronic devices.
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    System and method to enable secondary transmission in the presence of an active primary transmission
    (2019-01-22) Zhang, Xu; Knightly, Edward W.; Rice University; United States Patent and Trademark Office
    A method includes generating, by a wireless device, a sounding packet. The method includes sending, by the wireless device, copies of the sounding packet using a beam former and an antenna array to a second wireless device. Each copy of the copies of the sounding packet is sent using different beam weights. The method includes, in response to sending the copies of the sounding packet, obtaining, by the wireless device, a first correction beam weight and a second correction beam weight from the second wireless device and sending, by the wireless device, data to the second wireless device using the first correction beam weight and the second correction beam weight.
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    WATCH: Enabling WiFi in Active TV Channels
    (2014-10-08) Zhang, Xu; Knightly, Edward W.; Zhong, Lin; Aazhang, Behnaam
    "White space” model allows TV channels that are not being used regionally by a TV broadcaster to be re-purposed for secondary access. Unfortunately, populated areas have few these unused channels. Nonetheless, Nielsen data show severe under-utilization with vast regions in range of TV transmitters having no active TV receivers even at peak TV viewing time. In this thesis, I present the design, implementation, and evaluation of WATCH (WiFi in Active TV CHannels), the first system to (i) enable secondary WiFi transmission in the presence of kilowatt-scale TV transmitters by employing WATCH-IC (Interference Cancellation) and CAT (Constructive Addition Transmission); (ii) protect active TV receivers by employing their spatial-spectral requirements. With FCC permission to test WATCH implementation, I show that WATCH can provide at least 6.0 times the total achievable rate to 4 watt secondary devices compared to white space model, while increasing the TV channel switching time by less than 5%.
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    WATCH: WiFi in Active TV Channels
    (Association for Computing Machinery, 2015) Zhang, Xu; Knightly, Edward W.
    Today's "white space" model of spectrum sharing applied in the UHF TV band allows channels that are not being used regionally by a TV broadcaster to be re-purposed for unlicensed-style secondary access in 24 hour increments. Unfortunately, populated areas have few unused channels for white space usage. Nonetheless, from the UHF TV viewer's perspective, Nielsen data show severe under-utilization of this spectrum, with vast regions that are in range of TV transmitters having no active TV receivers on multiple channels even at peak TV viewing times. In this paper, we present the design, implementation, and experimental evaluation of WATCH (WiFi in Active TV CHannels), the first system to enable secondary WiFi transmission even in the presence of kilowatt-scale TV transmitters, while simultaneously protecting TV receivers when they are active. To protect active TV receivers, WATCH includes a smartphone-based TV remote or an Internet-connected TV to inform the WATCH controller of TV receivers' spatial-spectral requirements. To enable WiFi transmission in UHF bands, we design WATCH-IC (Interference Cancellation) and CAT (Constructive Addition Transmission) to (i) exploit the unique environment of asynchronous WiFi transmission in the presence of a strong streaming interferer, and (ii) require no coordination with legacy TV transmitters. With FCC permission to test our implementation, we show that WATCH can provide at least 6 times the total achievable rate to 4 watt secondary devices compared to current TV white space systems, while limiting the increase in TV channel switching time to less than 5%.