[en] Synchrotron radiation photoelectron spectroscopy is used to measure the valence-band offsets for Si/ZnS(111) and (100) heterojunctions. The valence band discontinuities obtained from the measurements are both (1.9 +- 0.1) eV for Si/ZnS(111) and Si/ZnS(100) interfaces, which is in excellent agreement with the theoretical predictions but considerably different from the experimental result of ZnS/Si(111) reported by Maierhofer et al. This suggests that the commutativity rule of band offset may not be valid for Si/ZnS polar interface and the reason is discussed

[en] We present detailed studies of the high photocurrent gain behavior in multilayer organic photodiodes containing tailored carrier blocking layers we reported earlier in a Letter [W. T. Hammond and J. Xue, Appl. Phys. Lett. 97, 073302 (2010)], in which a high photocurrent gain of up to 500 was attributed to the accumulation of photogenerated holes at the anode/organic active layer interface and the subsequent drastic increase in secondary electron injection from the anode. Here, we show that both the hole-blocking layer structure and layer thickness strongly influence the magnitude of the photocurrent gain. Temporal studies revealed that the frequency response of such devices is limited by three different processes with lifetimes of 10 μs, 202 μs, and 2.72 ms for the removal of confined holes, which limit the 3 dB bandwidth of these devices to 1.4 kHz. Furthermore, the composition in the mixed organic donor-acceptor photoactive layer affects both gain and bandwidth, which is attributed to the varying charge transport characteristics, and the optimal gain-bandwidth product is achieved with approximately 30% donor content. Finally, these devices show a high dynamic range of more than seven orders of magnitude, although the photocurrent shows a sublinear dependence on the incident optical power

[en] Upconversion phosphors Sr₂LaF₇ co-doped with Yb³⁺ and Er³⁺ are synthesized via hydrothermal method. The phase compositions of Sr₂LaF₇ phosphors with different amounts of Yb³⁺ and Er³⁺ dopants, pH values, and complexing agents are characterized by X-ray diffraction (XRD). The crystal structure and morphology of Sr₂LaF₇ phosphor doped with 15% Yb³⁺/ 1% Er³⁺ is refined by the Rietveld method and transmission electron microscopy (TEM). Under 980 nm laser excitation, Yb³⁺ and Er³⁺ co-doped Sr₂LaF₇ phosphors exhibit red emission around 670 nm generated by the ⁴F_9/2 → ⁴I_15/2 transition and green emissions centering around 550 nm and 523 nm generated by the ⁴S_3/2 → ⁴I_15/2 and ²H_11/2 → ⁴I_15/2 transitions of Er³⁺, respectively. The optimum synthetic conditions for upconversion luminescence for Sr₂LaF₇ co-doped with Yb³⁺ and Er³⁺ are discussed and determined to be 15% Yb³⁺/ 1% Er³⁺ with EDTA-2Na as the complexing agent under pH value = 8, and the corresponding Commission International de L′Eclairage (CIE) coordinates are calculated to be x = 0.4293 and y = 0.5589 with reddish yellow color visible to naked eyes under the 980 nm laser excitation. The related upconversion mechanisms of Yb³⁺ and Er³⁺ co-doped Sr₂LaF₇ depending on the pump power are also studied in detail.