No abstract

Three blue‐light emitting dipyrenylbenzene derivatives, 1‐(4‐(1‐pyrenyl)phenyl)pyrene (PPP), 1‐(2,5‐dimethoxy‐4‐(1‐pyrenyl)phenyl)pyrene (DOPPP), and 1‐(2,5‐dimethyl‐4‐(1‐pyrenyl)phenyl)pyrene (DMPPP), have been prepared by the Suzuki coupling reaction of aryl dibromides with pyreneboronic acid in high yields. These compounds exhibit high glass‐transition temperatures of 97–137 °C and good film‐forming ability. As revealed from single‐crystal X‐ray analysis, these dipyrenylbenzenes adopt a twisted conformation with inter‐ring torsion angles of 44.5°–63.2° in the solid state. The twisted structure is responsible for the low degree of aggregation in the thin films that leads to fluorescence emission of the neat films at 446–463 nm, which is shorter than that of the typical pyrene excimer emission. The low degree of aggregation is also conducive for the observed high fluorescence quantum yields of 63–75%. In organic light‐emitting diode (OLED) applications, these dipyrenylbenzenes can be used as either the charge transporter or host emitter. The non‐doped blue OLEDs that employ these compounds as the emissive layer can achieve a very high external quantum efficiency (ηext) of 4.3–5.2%. In particular, the most efficient DMPPP‐based device can reach a maximum ηext of 5.2% and a very high luminescence of 40 400 cd m–2 in the deep‐blue region with Commission Internationale d'Énclairage (CIE) coordinates of (0.15, 0.11).

Single-crystalline zinc oxide (ZnO) nanowires (NWs) with diameters of 90-200 nm were synthesized by the thermal evaporation method. Four-probe Ti/Au electrodes were made by the standard electron-beam lithography technique, and the intrinsic resistivities, rho(T), of individual NWs were measured over a wide range of temperature from 300 down to 0.25 K. The temperature behavior of rho(T) between 300 and 5 K reveals that the intrinsic electrical-transport mechanisms through individual ZnO NWs are due to a combination of the thermal activation conduction and the nearest-neighbor hopping conduction processes. Three distinct activation and hopping contributions with discrete characteristic activation energies are observed. Above about 100 K, the charge transport mechanism is dominated by the thermal activation of electrons from the Fermi level, mu, to the conduction band. Between approximately 20 and 100 K, the charge transport mechanism is due to the activation of electrons from mu to the upper impurity (D-) band. Between approximately 5 and 20 K, the charge transport mechanism arises from the nearest-neighbor hopping conduction within the lower impurity (D) band. Such unique electrical conduction behaviors can be explained in terms of the intricate material properties (in particular, the presence of moderately high concentrations of n-type defects accompanied with a slight self-compensation) in natively doped ZnO NWs. In one heavily doped NW, a surface-related conduction process manifesting the two-dimensional attributes of quantum-interference transport phenomena is observed. The carrier concentrations in our NWs have been estimated, and they were found to lie close to the critical concentration for the Mott metal-insulator transition.

Metal contacts to n-type Ge have poor performance due to the Fermi level pinning near the Ge valence band at metal/Ge interfaces. The electron barrier height can be reduced by inserting ultrathin dielectrics at the metal-semiconductor interface. However, this technique introduces tunneling resistance from the large conduction band offset (CBO) between the insulator and Ge. In this work, the CBO between TiO2 and Ge is estimated to range from −0.06 to −0.26 eV so tunneling resistance can be reduced. By inserting 7.1 nm TiO2 between Al and n-Ge, current densities increased by about 900× at 0.1 V and 1200× at −0.1 V compared to contacts without TiO2.

There is an abundance of low-frequency and irregular human motion energy that can be harvested. In this work, a non-resonant rotational electromagnetic energy harvester (REH) for scavenging low-frequency (<10 Hz) and irregular human motion is presented. The energy harvester simply introduces a cylindrical stator and a disk-shaped rotor forming a movement of a higher pair. Without any complicated transmission mechanism, the rotor can easily rotate around the stator by magnetic attractive force. Driven by a broadband frequency vibration, the magnetic rotor is coupled with surrounding wound coils to operate electromagnetic energy harvesting. Theoretical and experimental investigations of the REH are studied, and numerical simulations show good agreement with the experimental results. The treadmill tests at various motion speeds are performed to demonstrate the advantage of the REH in harvesting energy from irregular human motion. At a driving frequency of 8 Hz, the electromagnetic coils can provide the maximum power of 10.4 mW at a load resistance of 100 Ω. The REH exhibits outstanding output performance and has potential applications for powering intelligent wearable or portable electronic devices.