Search

Initial water saturation has a significant influence on spontaneous imbibition, and it impacts the imbibition rate and imbibed liquid dynamic distribution among pore networks. At present, the effect of initial water saturation on spontaneous imbibition is still unclear, especially the water film effect caused by initial water saturation varies during imbibition. To address these issues, we prepared the samples with varied initial water saturation through vapor adsorption and then conducted spontaneous imbibitions on these samples, which were monitored by nuclear magnetic resonance (NMR). Samples were selected from tight sandstone reservoirs in the eastern Ordos Basin. From the NMR spectrum corresponding to different imbibition duration, the left wing of the left peak at the initial water saturation is usually higher than the ones after imbibition, mainly caused by the water film effect. When the water film on the surface of large pores is too thin, it is prone to be interpreted as water in small pores because the water film has a quick relaxation time than the water in the center part of pores. The water film effect disappears when the whole pores are filled with water, and the inversed small pores decrease. The relaxation time of the water film is shorter than 0.51 ms in the tested samples. This study is conducive to understanding the effect of initial water saturation on liquid dynamic distribution during imbibition, especially the water film effect variation during imbibition and the T2 value of the water film. [ABSTRACT FROM AUTHOR]

Diarylethene (DAE) is one of the most widely used functional units for electrochromic or photochromic materials. To better understand the molecular modification effects on the electrochromic and photochromic properties of DAE, two modification strategies, substitution with functional groups or heteroatoms, were investigated theoretically by density functional theory calculations. It is found that red-shifted absorption spectra caused by a decreased highest occupied molecular orbital–lowest unoccupied molecular orbital energy gap and S0 → S1 transition energy during the ring-closing reaction become more significant by adding different functional substituents. In addition, for two isomers, the energy gap and S0 → S1 transition energy decreased by heteroatom substitution of S atoms with O or NH, while they increased by replacing two S atoms with CH2. For intramolecular isomerization, one-electron excitation is the most effective way to trigger the closed-ring (O → C) reaction, while the open-ring (C → O) reaction occurs most readily in the presence of one-electron reduction. Moreover, it is confirmed that substitution with strong electron donating groups (–OCH3/–NH2) or with one O/two CH2 heteroatoms leads to a more favorable closed-ring (O → C) reaction. Functionalized with strong electron-withdrawing groups (–NO2 and –COOH) or one/two NH heteroatom substitutions, the open-ring (C → O) reaction is easier. Our results confirmed that the photochromic and electrochromic properties of DAE can be tuned effectively by molecular modifications, which provides theoretical guidance for the design of new DAE-based photochromic/electrochromic materials. [ABSTRACT FROM AUTHOR]

Enhancing the conductivity in Al-rich n-AlGaN is a key issue for realizing AlGaN-based ultraviolet light-emitting diodes (UV-LEDs) with low operating voltage and high wall-plug efficiency, especially in a planar geometry of flip–chip configuration. An approach of modulation doping is herein proposed, where an alternating-layer structure consisting of Si-doped and unintentionally doped AlGaN is assembled to achieve the spatial separation of electron activation and transport. As massive electrons diffuse from the AlGaN:Si layer into the neighboring i-AlGaN ones and then drift, the ionized-donor scattering is effectively weakened, leading to a significant enhancement of mobility as well as conductivity. An impressive electrical property of n-Al0.6Ga0.4N with a lateral conductivity of 201.7 S/cm is realized as a consequence, being 2.1 times of that in the continuously doped one. Furthermore, the operating voltage of 280 nm UV-LEDs is correspondingly reduced by 0.1–0.2 V at 100 mA by adopting modulation-doped n-AlGaN in the n-cladding layer. [ABSTRACT FROM AUTHOR]

The formation of spiral hillocks during AlGaN growth is investigated by modulating the residual strain in the underlying AlN templates. It is demonstrated that the high-density hillocks are directly related to the compressive stress, in particular for AlGaN on high-temperature annealed (HTA) AlN. AlN/AlGaN stress modulation multilayer is then introduced and optimized before AlGaN growth, which is revealed to be helpful to release the compressive stress in AlGaN as well as to suppress the hillocks. Eventually, the hillocks in AlGaN/HTA-AlN are eliminated, leading to a great improvement of yield for deep-ultraviolet light-emitting diode (DUV-LED) wafers. This work will definitely promote further industrial development and application of DUV-LEDs. [ABSTRACT FROM AUTHOR]

Realizing and manipulating strong light–matter coupling in 2D monolayer semiconductors are of the utmost importance in the development of photonic devices. Hollow nanostructures of noble metals are particularly interesting because of their stronger local electromagnetic field compared with solid nanoparticles, which facilitate the strong coupling of single metal nanostructures. Here, the tunable single nanocavity plasmon–exciton coupling was demonstrated at room temperature in hybrid systems consisting of Ag@Au hollow nanocubes (HNCs) and monolayer WS2 underneath, where a large vacuum Rabi splitting of 131.3 meV was observed. Mode splitting can be clearly observed from the dark-field scattering spectrum of the single hybrid nanocavity, which is ascribed to the strong coupling between the nanocavity mode and the excitonic mode. Then, we used the finite difference time domain method to simulate these hybrid systems. By changing the thickness of the shell of the Ag@Au HNC, we can tune the surface plasmon resonance peak position of HNCs to match the exciton energy of the monolayer WS2. The strong couplings were realized via the calculated scattering spectra. The calculated results were consistent with the experimental results. Furthermore, the mode volume of different nanostructures was discussed, and the mode volume of HNCs is smaller than other solid ones at the same plasmonic resonance wavelength, which also indicates that its ability to restrict an electromagnetic field is stronger. This study provides an ideal platform for the strong coupling of a single nanocavity at room temperature and has broad application prospects in the field of single-photon devices. [ABSTRACT FROM AUTHOR]

Organic–inorganic hybrid perovskite solar cells are considered promising due to their strong light absorption capability, long carrier diffusion distance, and rapid increase in energy conversion efficiency in the past decade. However, there is a high concentration of defects between the perovskite layer and the electron transport layer, which can lead to limited carrier extraction and, thus, affect the device efficiency. Black phosphorus quantum dots (BPQDs) have the advantages of tunable bandgap size, high absorption coefficient, and high electron mobility. Therefore, we introduced black phosphorus quantum dots (BPQDs) in aqueous SnO2 colloidal solution by laser liquid-phase ablation to improve the carrier extraction rate and reduce the trap density. By characterizing the prepared complete devices, we found that the device efficiency reached 15.83% after the introduction of BPQDs, which is 12% higher compared to 14.06% for the blank sample. Our work provides a simple and fast method to enhance the efficiency of perovskite solar cells. [ABSTRACT FROM AUTHOR]

Organic-inorganic hybrid perovskite solar cells (PSCs) have witnessed a rapid rising in power conversion efficiency (PCE) over the past few years; however, they still suffer from recombination loss via interface defects in perovskite films. In this study, we implement an efficient phase purification strategy by incorporating isopropyl alcohol (IPA) post-treatment of perovskite films that reduces defect states and improves charge transport. It is found that the DMSO-PbI2-MAI complex in the perovskite film is eliminated after IPA post-treatment. A suit of opto-electric characterizations demonstrates that the nonradiative recombination is greatly diminished, and charge extraction is effectively boosted in the modified perovskite films. The perovskite solar cells with phase-pure MAPbI3 achieve an impressively larger PCE of 18.78% than that of 17.1% for the control devices. Our work presents a facile and efficient path to performance improvement of PSCs. [ABSTRACT FROM AUTHOR]

Phosphorene quantum dots (PQDs) with high photoluminescence (PL) quantum yield were fabricated through pulsed laser ablation of black phosphorus in liquids. The PL emission showed excitation wavelength-independent features due to the sufficient passivation by oxygen-containing substituents derived from the ablation-decomposed solvent molecules. There were four peaks in the PL profile showing a multifrequency PL effect, which provided a viable route to investigate the PL mechanism via the detailed structures of the PL profile. The two peaks at shorter wavelengths were attributed to the intrinsic PL transitions. The two peaks at longer wavelengths were assigned to the transitions from the lowest unoccupied molecular orbital to the two-surface states. The positions of the four peaks are redshifted with increasing solvent molecular chain length due to the passivation effect. The intense, excitation wavelength-independent, multifrequency and blue-violet PL emissions indicate the potential application in semiconductor light sources. [ABSTRACT FROM AUTHOR]