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The incorporation of a certain amount of Cl ions into methylammonium lead iodide (MAPbI3) perovskite films and how these incorporated Cl ions affect the structural and electronic properties of these films have been an intensively studied topic. In this study, we comprehensively investigated Cl incorporation in MAPbI3 at the atomic scale by a combined study of scanning tunneling microscopy, X-ray photoelectron spectroscopy, ultraviolet and inverse photoemission spectroscopy, density functional theory and molecular dynamics calculations. At a Cl concentration of 14.8 ± 0.6%, scanning tunneling microscopy images confirm the incorporation of Cl ions on the MAPbI3 surface, which also corresponds to the highest surface stability of MAPbI3 found from the viewpoint of both thermodynamics and kinetics by density functional theory and molecular dynamics calculations. Our results show that the Cl concentration is crucial to the surface bandgap and stability of MAPbI3.

All-inorganic halide perovskites are promising materials for optoelectronic applications. The surface or interface structure of the perovskites plays a crucial role in determining the optoelectronic conversion efficiency, as well as the material stability. A thorough understanding of surface atomic structures of the inorganic perovskites and their contributions to their optoelectronic properties and stability is lacking. Here we show a scanning tunneling microscopy investigation on the atomic and electronic structure of CsPbBr3 perovskite. Two different surface structures with a stripe and an armchair domain are identified, which originates from a complex interplay between Cs cations and Br anions. Our findings are further supported and correlated with density functional theory calculations and photoemission spectroscopy measurements. The stability evaluation of photovoltaic devices indicates a higher stability for CsPbBr3 in comparison with MAPbBr3, which is closely related to the low volatility of Cs from the perovskite surface.

Silver (Ag) is one of the most sensitive noble metals for nanoplasmonic applications. However, Ag is prone to oxidation in wet environments (e.g., aqueous and organic solvents), which usually leads...

We present an active droplet merging device, which can merge various sizes of micro droplets in different numbers by using pneumatically controlled horizontal PDMS microvalves. The merging part consists of a main and side channels separated by a pillar array. The pillar array structure is contained within a microfuidic channel. The function of the pillar array provides a bypass path to the continuous flow (oil) inside the merging chamber. Droplets are successfully generated within the channel and achieve merging by controlling the selective different numbers and diameters of droplets through varying the flow resistance of main and side channel. In the merging chamber, a droplet will enter and slow down its movement. It will wait and then merge with the sequential droplets. These experiments demonstrate that such a merging device can controllably select and adjust the distance between the different adjacent micro droplets without any generation of sister droplets in the side channel. The device has no desynchronization problems. Thus, it can be applied for efficiently mixing the droplets in various diameters and numbers without changing the structure of the merging chamber. Hence, this device can be a more effective choice when applying microfluidics to chemical and biological applications.

This paper describes a microdroplet merging device that can actively control the merging of various droplets under a wide range of flow conditions, using a simple structure. The microdroplets were trapped and merged in a wide chamber divided by pillars, and their behavior was controlled by two horizontal pneumatic microactuators. Hydrodynamic flow control by the actuation was evaluated numerically, and the trapping and merging of droplets were achieved experimentally and controlled via pressure applied to the microactuators. Furthermore, two independently generated droplets were merged under four different modes, ranging from no merging to four-droplet merging, with different ratios and volumes. The pneumatic actuators allowed not only the control of the number of merged droplets, but also a wide range of applied droplet volumes. The device was fabricated simply using a single-layer PDMS (polydimethylsiloxane) structure, and the continuous merging performance operated using only hydrodynamic flow control without any surfactant. Finally, chemical synthesis of a metal complex was performed by the droplet merging method. Crystallization of the complex was visualized in real time, and the synthesis was verified by ultraviolet-visible spectroscopy.

Multiple core-sheath flow was realized using matrix arrangement of 3D sheath flows. The sheath flow was hydrodynamically formed in a flow shift area which has symmetrical microgrooves on channel walls. Vertical and horizontal alignments of the sample streams are a key element of matrix configuration. The flow shift areas were connected in parallel to achieve horizontal alignment of the sheath flow. The cascade connection of the flow shift areas is used for vertical alignment of the sheath flow. In order to achieve matrix arrangement of core-sheath flow, combination of the parallel and cascade connections was utilized. In this work, the horizontal and vertical configurations of the 2-sample sheath flow were demonstrated. Two streams of the vertically aligned 2-sample sheath flow were joined horizontally, and, as a result, 4-sample core-sheath flow of matrix configuration was obtained successfully.

This thesis discusses the understanding of the structural and electronic properties of metal halide perovskite materials (e.g., CH3NH3PbI3) as well as the interaction with additive materials at the atomic scale. Metal halide perovskite materials are currently at the core of attention because of their high-efficiency attainable when utilized in solar cell applications as well as lost fabrication cost. However, the stability of these perovskite materials and perovskite-based solar cells is still a major challenge. The proposed atomic-scale research helps address the thermal instability and ambient air induced degradation issues commonly observed in perovskites. Incorporation of additive materials into the perovskite was reported to be an efficient strategy to enhance the stability. Cl and KI have been found to be eligible candidates but studies on the fundamental processes at the atomic scale of these additives are scarce. Therefore, we comprehensively investigated Cl incorporation in MAPbI3 at the atomic scale by combining scanning tunneling microscopy (STM), X-ray photoelectron, ultraviolet, and inverse photoemission spectroscopy. For the Cl concentration of 14.8 ± 0.6%, STM images confirm the presence of Cl ions on the MAPbI3 (MAPbI2.59Cl0.21) surface leading to the highest surface stability found from the viewpoint of both thermodynamics and kinetics by density functional theory and molecular dynamics calculations. This study evidence that Cl can substitute I ions of the surface structure and/or fill the surface I-vacancies and further enhance the structural stability of MAPbI3. Upon the PbCl2deposition on MAPbI3, only Cl ions were observed in MAPbI3 surface crystal structure while Pb ions were not observed by LT-STM. Before investigating the PbCl2 interaction with MAPbI3, the atomic structures of metallic-Pb and PbCl2 on Au (111) substrate need be studied by LT-STM. For the PbCl2/Au (111) system, a myriad of structures such as the intact form of PbCl2 as well as dissociated species in the forms of Cl ions, Cl-Cl dimer, Pb, and Pb-Pb dimers were observed. Furthermore, the KI additive was investigated by LT-STM, which reveals the I-I square atomic structure. After deposition protocol optimization, KI was deposited on MAPbI3 and characterized by XPS confirming K incorporation in MAPbI3. As a future plan, LT-STM studies can be conducted to find the optimal concentration of KI for enhancing the MAPbI3 surface stability. Also, it is important to point out that STM can be used to characterize mainly the surface atomic structures of perovskite materials.