ABSTRACT
Traditional semiconductors are known to exhibit excellent electrical properties but oversized lattice thermal conductivities, thus limiting their thermoelectric performance. Herein, we have discovered a low-energy allotrope of those traditional semiconductors. Compared with the wurtzite structure, the lattice thermal conductivity is reduced by more than five times in the haeckelite structure. This is attributed to the softening of acoustic phonon modes and concurrently enhanced anharmonicity in the haeckelite structure. Benefiting from the suppressed lattice thermal conductivity while retaining the excellent electrical properties of wurtzite structure, haeckelite compounds have been proven to be a novel category of high-performance thermoelectric materials. As an excellent representative, haeckelite CdTe exhibits a peak figure of merit approaching 1.3 at n-type doping and high temperature, which experiences a 3-fold improvement compared with its wurtzite counterpart. This work provides an alternative pathway of engineering the lattice thermal conductivities of traditional semiconductors toward superior thermoelectric properties.
ABSTRACT
Traditional semiconductor quantum dots of groups II-VI are key ingredients of next-generation display technology. Yet, the majority of them contain toxic heavy-metal elements, thus calling for alternative light-emitting materials. Herein, we have explored three novel categories of multicomponent compounds, namely, tetragonal II-III2-VI4 porous ternary compounds, cubic I2-II3-VI4 ternary compounds, and cubic I-II-III3-V4 quaternary compounds. This is achieved by judicious introduction of a "super atom" perspective and concurrently varying the solid-state lattice packing of involved super atoms or the population of surrounding counter cations. Based on first-principles calculations of 392 candidate materials with designed crystal structures, 53 highly stable materials have been screened. Strikingly, 34 of them are direct-bandgap semiconductors with emitting wavelengths covering the near-infrared and visible-light regions. This work provides a comprehensive database of highly efficient light-emitting materials, which may be of interest for a broad field of optoelectronic applications.
ABSTRACT
Evidence suggests that neurometabolite alterations may be involved in the pathophysiology of autism spectrum disorders (ASDs). We performed a meta-analysis of proton magnetic resonance spectroscopy (1H-MRS) studies to examine the neurometabolite levels in the brains of patients with ASD. A systematic search of PubMed and Web of Science identified 54 studies for the meta-analysis. A random-effects meta-analysis demonstrated that compared with the healthy controls, patients with ASD had lower N-acetyl-aspartate-containing compound (NAA) and choline-containing compound (Cho) levels and NAA/(creatine-containing compound) Cr ratios in the gray matter and lower NAA and glutamate + glutamine (Glx) levels in the white matter. Furthermore, NAA and gamma-aminobutyric acid (GABA) levels, NAA/Cr ratios, and GABA/Cr ratios were significantly decreased in the frontal cortex of patients with ASD, whereas glutamate (Glu) levels were increased in the prefrontal cortex. Additionally, low NAA levels and GABA/Cr ratios in the temporal cortex, low NAA levels and NAA/Cr ratios in the parietal and dorsolateral prefrontal cortices, and low NAA levels in the cerebellum and occipital cortex were observed in patients with ASD. Meta-regression analysis revealed that age was positively associated with effect size in studies analyzing the levels of gray matter NAA and white matter Glx. Taken together, these results provide strong clinical evidence that neurometabolite alterations in specific brain regions are associated with ASD and age is a confounding factor for certain neurometabolite levels in patients with ASD.
