No abstract

No abstract

The direct piezoelectric properties of BiFeO3 epitaxial thin films with different crystal orientation were investigated. Epitaxial films of (100) and (111) rhombohedral BiFeO3 fabricated using pulsed laser deposition showed rectangular hysteresis loops with remanent polarizations of 54 and 83 μC/cm2, respectively. Effective transverse piezoelectric coefficients (e31,f) of −3.5 and −1.3 C/m2 were obtained, for (100) and (111) films, respectively. Results suggest that the strong direct piezoelectric response of the (100) rhombohedral film results from the effects of the engineered-domain configuration.

To advance the development of atomically thin optoelectronics using two-dimensional (2D) materials, engineering strong luminescence with a physicochemical basis is crucial. Semiconducting monolayer transition-metal dichalcogenides (TMDCs) are candidates for this, but their quantum yield (QY) is known to be poor. Recently, a molecular superacid treatment of bis­(trifluoromethane)­sulfonimide (TFSI) generated unambiguously bright monolayer TMDCs and a high QY. However, this method is highly dependent on the processing conditions and therefore has not been generalized. Here, we shed light on environmental factors to activate the photoluminescence (PL) intensity of the TFSI-treated monolayer MoS2, with a factor of more than 2 orders of magnitude greater than the original by photoactivation. The method is useful for both mechanically exfoliated and chemically deposited samples. The existence of photoirradiation larger than the band gap demonstrates enhancement of the PL of MoS2; on the other hand, activation by thermal annealing, as demonstrated in the previous report, is less effective for enhancing the PL intensity. The photoactivated monolayer MoS2 shows a long lifetime of ∼1.35 ns, more than a 30-fold improvement over the original as exfoliated. The consistent realization of the bright monolayer MoS2 reveals that air exposure is an essential factor in the process. TFSI treatment in a N2 environment was not effective for achieving a strong PL, even after the photoactivation. These findings can serve as a basis for engineering the bright atomically thin materials for 2D optoelectronics.

Monolayer molybdenum disulfide (MoS) is an atomically thin semiconducting material with a direct band gap. This physical property is attributable to atomically thin optical devices such as sensors, light-emitting devices, and photovoltaic cells. Recently, a near-unity photoluminescence (PL) quantum yield of a monolayer MoS was demonstrated via a treatment with a molecular acid, bis(trifluoromethane)sulfonimide (TFSI); however, the mechanism still remains a mystery. Here, we work on PL enhancement of monolayer MoS by treatment of Brønsted acids (TFSI and sulfuric acid (HSO)) to identify the importance of the protonated environment. In TFSI as an acid, different solvents-1,2-dichloroethane (DCE), acetonitrile, and water-were studied, as they show quite different acidity in solution. All of the solvents showed PL enhancement, and the highest was observed in DCE. This behavior in DCE would be due to the higher acidity than others have. Acids from different anions can also be studied in water as a common solvent. Both TFSI and HSO showed similar PL enhancement (∼4-8 enhancement) at the same proton concentration, indicating that the proton is a key factor to enhance the PL intensity. Finally, we considered another cation, Li from LiSO, instead of HSO, in water. Although Li and H atoms showed similar binding energy on MoS from theoretical calculations, LiSO treatment showed little PL enhancement; only coexisting HSO reproduced the enhancement. This study demonstrated the importance of a protonated environment to increase the PL intensity of monolayer MoS. The study will lead to a solution to achieve high optical quality and to implementation for atomically thin optical devices.