Donor-Acceptor Pair Quantum Emitters in Hexagonal Boron Nitride.

Quantum emitters are needed for a myriad of applications ranging from quantum sensing to quantum computing. Hexagonal boron nitride (hBN) quantum emitters are one of the most promising solid-state platforms to date due to their high brightness and stability and the possibility of a spin-photon interface. However, the understanding of the physical origins of the single-photon emitters (SPEs) is still limited. Here we report dense SPEs in hBN across the entire visible spectrum and present evidence that most of these SPEs can be well explained by donor-acceptor pairs (DAPs). On the basis of the DAP transition generation mechanism, we calculated their wavelength fingerprint, matching well with the experimentally observed photoluminescence spectrum. Our work serves as a step forward for the physical understanding of SPEs in hBN and their applications in quantum technologies.

Single-Photon Emission from Point Defects in Aluminum Nitride Films.

Quantum technologies require robust and photostable single-photon emitters. Here, room temperature operated single-photon emissions from isolated defects in aluminum nitride (AlN) films are reported. AlN films were grown on nanopatterned sapphire substrates by metal organic chemical vapor deposition. The observed emission lines range from visible to near-infrared, with highly linear polarization characteristics. The temperature-dependent line width increase shows T3 or single-exponential behavior. First-principle calculations based on density functional theory show that point defect species, such as antisite nitrogen vacancy complex (NAlVN) and divacancy (VAlVN) complexes, are considered to be an important physical origin of observed emission lines ranging from approximately 550 to 1000 nm. The results provide a new platform for on-chip quantum sources.

Anomalous Pressure Characteristics of Defects in Hexagonal Boron Nitride Flakes.

Research on hexagonal boron nitride (hBN) has been intensified recently due to the application of hBN as a promising system of single-photon emitters. To date, the single photon origin remains under debate even though many experiments and theoretical calculations have been performed. We have measured the pressure-dependent photoluminescence (PL) spectra of hBN flakes at low temperatures by using a diamond anvil cell device. The absolute values of the pressure coefficients of discrete PL emission lines are all below 15 meV/GPa, which is much lower than the pressure-induced 36 meV/GPa redshift rate of the hBN bandgap. These PL emission lines originate from atom-like localized defect levels confined within the bandgap of the hBN flakes. Interestingly, the experimental results of the pressure-dependent PL emission lines present three different types of pressure responses corresponding to a redshift (negative pressure coefficient), a blueshift (positive pressure coefficient), or even a sign change from negative to positive. Density functional theory calculations indicate the existence of competition between the intralayer and interlayer interaction contributions, which leads to the different pressure-dependent behaviors of the PL peak shift.

Polymer as an Additive in the Emitting Layer for High-Performance Quantum Dot Light-Emitting Diodes.

A facile but effective method is proposed to improve the performance of quantum dot light-emitting diodes (QLEDs) by incorporating a polymer, poly(9-vinlycarbazole) (PVK), as an additive into the CdSe/CdS/ZnS quantum dot (QD) emitting layer (EML). It is found that the charge balance of the device with the PVK-added EML was greatly improved. In addition, the film morphology of the hole-transporting layer (HTL) which is adjacent to the EML, is substantially improved. The surface roughness of the HTL is reduced from 5.87 to 1.38 nm, which promises a good contact between the HTL and the EML, resulting in low leakage current. With the improved charge balance and morphology, a maximum external quantum efficiency (EQE) of 16.8% corresponding to the current efficiency of 19.0 cd/A is achievable in the red QLEDs. The EQE is 1.6 times as high as that (10.5%) of the reference QLED, comprising a pure QD EML. This work demonstrates that incorporating some polymer molecules into the QD EML as additives could be a facile route toward high-performance QLEDs.

Large-scale assembly of highly sensitive Si-based flexible strain sensors for human motion monitoring.

Silicon is the dominant semiconductor in modern society, but the rigid nature of most Si structures hinders its applications in flexible electronics. In this work, Si-based flexible strain sensors are fabricated with Si fabric consisting of long Si nanowires. The as-obtained sensors demonstrate a large strain range of 50% and a gauge factor of up to 350, which are sufficient to detect human motions with superior performance over traditional sensors. The results reveal that the assembling strategy may potentially be applied to large-scale fabrication of highly sensitive, flexible strain sensors for emerging applications such as healthcare and sports monitoring. Moreover, the Si fabric would also enable broad applications of Si materials in other flexible and wearable devices such as flexible optoelectronics and displays.

The diameter-dependent photoelectrochemical performance of silicon nanowires.

We demonstrate the first systematic study of the diameter-dependent photoelectrochemical performance of single silicon nanowires within a broad size range from 200 to 2000 nm. SiNWs with a diameter of 1415 nm exhibit the highest solar energy conversion efficiency, which can be mainly traced to their diameter-dependent light absorption properties.

Occurrence and toxicity of sediment-associated contaminants in Guangzhou College City and its adjacent areas: the relationship to urbanization.

Guangzhou College City in the southeast of Guangzhou, China, became a home to 10 universities in 2003 after a largely agricultural past. The city has since experienced rapid urbanization with limited development of adjacent areas. Twenty-one sediment samples were collected in the city and its adjacent areas to evaluate the influence of urbanization in different functional zones on sediment quality in local waterways. Sediment toxicity was assessed by 10-day toxicity tests using two benthic invertebrates, Chironomus dilutus and Hyalella azteca. In addition, various organic contaminants-including current-use pesticides (pyrethroids and organophosphate insecticides) and polycyclic aromatic hydrocarbons-were analyzed, and a toxic unit (TU) approach was applied to identify possible toxicity contributors. In general, 38 and 4.8% of the sediments exhibited acute toxicity to C. dilutus and H. azteca, respectively, with 9.5% of the samples resulting in 100% mortality to C. dilutus. Distribution analysis showed that the rural industrial area, which is south of the city, had the greatest contaminant concentrations and greatest toxicity to both organisms compared with the other areas. Pyrethroids, especially cypermethrin, appeared to contribute the most to the observed toxicity, yet the lack of relationship between the toxicity and TU of pyrethroids may reflect toxicity associated with other contaminants.

Self-assembled quantum dot structures in a hexagonal nanowire for quantum photonics.

Two types of quantum nanostructures based on self-assembled GaAs quantumdots embedded into GaAs/AlGaAs hexagonal nanowire systems are reported, opening a new avenue to the fabrication of highly efficient single-photon sources, as well as the design of novel quantum optics experiments and robust quantum optoelectronic devices operating at higher temperature, which are required for practical quantum photonics applications.

[Up-conversion luminescent properties of SrAl2O4: Eu2+, Dy3+ phosphors].

Strong green up-conversion emission was observed for the first time in europium and dysprosium co-doped strontium aluminate (SrAl2O4: Eu2+, Dy3+) phosphor under 980 nm infrared excitation at room temperature. The up-conversion emission spectrum of phosphors was quite different from that of down-conversion emission spectrum excited by ultraviolet or visible lights. The two kinds of emission spectra implied that there are different mechanisms of emission. The X-ray diffraction (XRD) results revealed that the prepared phosphor has the same crystal lattice and composition as that reported previously.