Dielectric capacitors with high energy storage density (Wrec) and efficiency (η) are in great demand for high/pulsed power electronic systems, but the state-of-the-art lead-free dielectric materials are facing the challenge of increasing one parameter at the cost of the other. Herein, we report that high Wrec of 6.3 J cm-3 with η of 90% can be simultaneously achieved by constructing a room temperature M2–M3 phase boundary in (1-x)AgNbO3-xAgTaO3 solid solution system. The designed material exhibits high energy storage stability over a wide temperature range of 20–150 °C and excellent cycling reliability up to 106 cycles. All these merits achieved in the studied solid solution are attributed to the unique relaxor antiferroelectric features relevant to the local structure heterogeneity and antiferroelectric ordering, being confirmed by scanning transmission electron microscopy and synchrotron X-ray diffraction. This work provides a good paradigm for developing new lead-free dielectrics for high-power energy storage applications.

We report significant differences in the temperature-dependent and time-resolved photoluminescence ͑PL͒ from low and high surface density In x Ga 1Ϫx As/GaAs quantum dots ͑QD's͒. QD's in high densities are found to exhibit an Arrhenius dependence of the PL intensity, while low-density ͑isolated͒ QD's display more complex temperature-dependent behavior. The PL temperature dependence of high density QD samples is attributed to carrier thermal emission and recapture into neighboring QD's. Conversely, in low density QD samples, thermal transfer of carriers between neighboring QD's plays no significant role in the PL temperature dependence. The efficiency of carrier transfer into isolated dots is found to be limited by the rate of carrier transport in the In x Ga 1Ϫx As wetting layer. These interpretations are consistent with time-resolved PL measurements of carrier transfer times in low and high density QD's. ͓S0163-1829͑99͒04748-7͔

The composition distribution of Ge(Si)/Si (001) islands grown at 700 °C by molecular beam epitaxy is investigated using high-spatial resolution x-ray energy dispersive spectrometry in a scanning transmission electron microscope. Island shapes are investigated using cross-section transmission electron microscopy. Results show nonuniformity of the composition distribution in the islands, which affects the evolution of the aspect ratios of height-to-base diameter of dislocated islands.

Ge͑Si͒/Si͑001͒ quantum dots produced by gas-source molecular beam epitaxy at 575°C were investigated using energy-filtering transmission electron microscopy and x-ray energy dispersive spectrometry. Results show a nonuniform composition distribution in the quantum dots with the highest Ge content at the dot center. The average Ge content in the quantum dots is much higher than in the wetting layer. The quantum dot/ substrate interface has been moved to the substrate side. A growth mechanism of the quantum dots is discussed based on the composition distribution and interfacial structures.