The impact of colloidal silicon quantum dots (SiQDs) on next-generation light sources is promising. However, factors determining the efficiency of SiQDs, such as the photoluminescence (PL) wavelength, PL quantum yield (PLQY), and the SiQD LED performance based on the type of ligand, ligand coverage, stress, and dangling bonds, have not been quantified. Characterizing these variables would accelerate the design and implementation of SiQDs. Herein, colloidal SiQDs were synthesized by pyrolyzing hydrogen silsesquioxane and their surfaces were terminated with 1-decene by either thermal hydrosilylation (HT-SiQDs) or room-temperature hydrosilylation using PCl 5 (RT-SiQD). As a result, PL, PL-excitation, and ultraviolet−visible absorption spectra were similar, but their PLQYs were significantly different: 54% (RT-SiQDs) vs 19% (HT-SiQDs). To understand their similarities and differences, surface coverages (dangling bonds, Si−H (Si−H 1 , Si−H 2 , and −Si−H 3 ), Si−O−Si, Si−C, Si−Cl) were determined. A core stress analysis established that a single ligand terminated to a SiQD bond site stretched the Si−Si bond length by 0.3%. From the two well-defined SiQDs, the PLQY and SiQD LED efficiency were attributed to four factors: low coverage of insulator ligands, the Cl ligand effect on radiative and nonradiative rates, negligible dangling bonds, and a SiQD core with low tensile stress. The PLQY of the RT-SiQDs in toluene was 80%. In addition, the 20× electroluminescence intensity difference of the LEDs originated from a 10× difference in current density and a 2× difference in Auger recombination. The concepts demonstrated here can be applied to further improve the PLQY and LED efficiencies of SiQDs with other ligands.

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We report the electrical resistivity , Hall coefficient R H , thermoelectric power S, specific heat C, and thermal conductivity on single crystals of the type-VIII clathrate Ba 8 Ga 16 Sn 30 grown from Sn-flux. Negative S and R H over a wide temperature range indicate that electrons dominate electrical transport properties. Both ͑T͒ and S͑T͒ show typical behavior of a heavily doped semiconductor. The absolute value of S increases monotonically to 243 V / K with increasing temperature up to 550 K. The large S may originate from the low carrier concentration n = 3.7ϫ 10 19 cm −3 . Hall mobility H shows a maximum of 62 cm 2 / V s around 70 K. The analysis of temperature dependence of H suggests a crossover of a dominant scattering mechanism from ionized impurity to acoustic phonon scattering with increasing temperature. The existence of local vibration modes of Ba atoms in cages composed of Ga and Sn atoms is evidenced by analysis of experimental data of structural refinement and specific heat, which give an Einstein temperature of 50 K and a Debye temperature of 200 K. This local vibration of Ba atoms should be responsible for the low thermal conductivity ͑1.1 W / m K at 150 K͒. The potential of type-VIII clathrate compounds for thermoelectric application is discussed.

We have grown single crystals of the type-VIII intermetallic clathrate Ba 8 Ga 16 Sn 30 from both Sn and Ga flux, evaluated their compositions through electron microprobe analysis and studied their transport properties through measurements on temperature dependent resistivity, thermopower and Hall coefficient. Crystals grown in Sn flux show n-type carriers and those from Ga flux show p-type carriers, whereas all measured compositions remain very close to the stoichiometric 8:16:30 proportion of Ba:Ga:Sn, expected from charge-balance principles. Our results indicate a very high sensitivity of the charge carrier nature and density with respect to the growth conditions, leading to relevant differences in transport properties which point to the importance of tuning this material for optimal thermoelectric performance.

Brush printing is a unique method used to obtain uniaxially oriented films, whereby a polymer solution is brushed onto a substrate. However, there have been only a few reports on the brushprinting method. Here, we report the preparation of a uniaxially oriented film of a green light-emitting conductive polymer, poly(9,9dioctylfluorene-alt-benzothiadiazole) (F8BT). The fluorescence polarization ratio of the oriented F8BT films was as high as 11.3, and the average orientation factor reached 0.74 ± 0.06. The orientation factor and the torsion angle of F8BT were visualized by two mappings of fluorescence and Raman spectral measurements by confocal spectromicroscopy, respectively. These two x-y mapping data with many pixels (∼750 pixels) were evaluated by x-y-z mapping of the film thickness at a single position and were used to reveal the threedimensional (3D) orientation mechanism from a stochastic approach. Polarized green fluorescence originates from polymer chains uniaxially oriented along the brush direction. The high orientation for a film thickness < 100 nm is established by shear stress, faster capillary flow, and flow-induced chain extension for a thin solution film on a substrate. The high orientation factor was also demonstrated by a high brushing speed, whereas an optimized brushing speed existed. We found that this optimization is attributed to the property of a non-Newtonian fluid. By applying this brush-printing method to the fabrication of an optoelectrical device, polarized green electroluminescence was preliminarily demonstrated by the OLED assembled from an oriented F8BT film.