This paper investigates a critical access control issue in the Internet of Things (IoT). In particular, we propose a smart contract-based framework, which consists of multiple access control contracts (ACCs), one judge contract (JC) and one register contract (RC), to achieve distributed and trustworthy access control for IoT systems. Each ACC provides one access control method for a subject-object pair, and implements both static access right validation based on predefined policies and dynamic access right validation by checking the behavior of the subject. The JC implements a misbehavior-judging method to facilitate the dynamic validation of the ACCs by receiving misbehavior reports from the ACCs, judging the misbehavior and returning the corresponding penalty. The RC registers the information of the access control and misbehavior-judging methods as well as their smart contracts, and also provides functions (e.g., register, update and delete) to manage these methods. To demonstrate the application of the framework, we provide a case study in an IoT system with one desktop computer, one laptop and two Raspberry Pi single-board computers, where the ACCs, JC and RC are implemented based on the Ethereum smart contract platform to achieve the access control.
Femtosecond pump-probe experiments with a time resolution of 100 fs have been performed for copper particles with a radius of 4 nm. Differential absorption spectra for a pump centered at 2.05 eV indicates the broadening of the absorption band due to the surface plasmon. The nonlinear response time derived from the recovery time of the nonlinear absorption is dependent on the pumping laser fluences, and is as short as 0.7 ps for 210 μJ/cm2. The relaxation dynamics of nonequilibrium electrons can be described by the usual electron-phonon coupling model.
A dye-sensitized photoelectrochemical cell consisting of a film of
SnO2 crystallites coated with ultrafine particles of Al2O3 generates
an exceptionally high open-circuit voltage as compared to a cell made only
from SnO2. Al2O3 coating on SnO2 improves the efficiency and the
fill factor while delivering reasonably high photocurrents. Photoexcited dye
molecules on Al2O3 injects electrons into the conduction band of SnO2
via tunnelling through the Al2O3 barrier. Suppression of recombinations
of electrons with the dye cations and the acceptors at the electrolytic
interface build up the quasi-Fermi level in SnO2 with an impressive
increase of the open-circuit voltage.
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