The realization of high‐contrast modulation in optically transparent media is of great significance for emerging mechano‐responsive smart windows. However, no study has provided fundamental strategies for maximizing light scattering during mechanical deformations. Here, a new type of 3D nanocomposite film consisting of an ultrathin (≈60 nm) Al2O3 nanoshell inserted between the elastomers in a periodic 3D nanonetwork is proposed. Regardless of the stretching direction, numerous light‐scattering nanogaps (corresponding to the porosity of up to ≈37.4 vol%) form at the interfaces of Al2O3 and the elastomers under stretching. This results in the gradual modulation of transmission from ≈90% to 16% at visible wavelengths and does not degrade with repeated stretching/releasing over more than 10 000 cycles. The underlying physics is precisely predicted by finite element analysis of the unit cells. As a proof of concept, a mobile‐app‐enabled smart window device for Internet of Things applications is realized using the proposed 3D nanocomposite with successful expansion to the 3 × 3 in. scale.
Sensor networks are expected to be used at anywhere in the near future and recently their security problems have been rising. Due to the limited computing power of senor nodes, it is impossible to use asymmetric cryptography approaches for the session key establishment or to apply advanced encryption method such as AES and DES for the data encryption. In this paper, therefore, we propose a key establishment and a data encryption scheme for secure and energy-efficient data transmission in wireless sensor networks (WSNs). The proposed schemes just treat simple operations as a message authentication code (MAC), exclusive-OR (XOR) and time-spacing key derivation function (TSDF). Additionally, the security of the proposed protocols is analyzed.
Two-dimensional (2D) semiconductors
have emerged as an excellent
platform for studying various excitonic matter under strong quantum
and dielectric confinements. However, such effects can be seriously
overestimated for Coulomb binding of two excitons to form a biexciton
by a naive interpretation of the corresponding photoluminescence (PL)
spectrum. By using 2D halide perovskite single crystals of [CH3(CH2)3NH3]2Pb1–x
Mn
x
Br4 (x = 0–0.09) as a model system, we
investigated both population and relaxation kinetics of biexcitons
as a function of excitation density, temperature, polarization, and
Mn doping. We show that the biexciton is formed by binding of two
dark excitons, which are partially bright, but they radiatively recombine
to yield a bright exciton in the final state. This renders the spectral
distance between the exciton peak and the biexciton peak as very different
from the actual biexciton binding energy (ϕ) because of large
bright–dark splitting. We show that Mn doping introduces paramagnetism
to our 2D system and improves the biexciton stability as evidenced
by increase in ϕ from 18.8 ± 0.7 to 20.0 ± 0.7 meV
and the increase of the exciton–exciton capture coefficient C from 2.4 × 10–11 to 4.3 ×
10–11cm2/ns within our doping range.
The precisely determined ϕ values are significantly smaller
than the previously reported ones, but they are consistent with the
instability of the biexciton against thermal dissociation at room
temperature. Our results demonstrate that electron–hole exchange
interaction must be considered for precisely locating the biexciton
level; therefore, the ϕ values should be reassessed for other
2D halide perovskites that even do not exhibit any dark exciton PL.
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