@article {702,
	title = {Effect of MA orientation and thickness on the bandgap in quasi-2D perovskites (PEA)$_{2}$(MA)ₙ$_{-}$$_{1}$PbₙI$_{3}$ₙ$_{+}$$_{1}$},
	journal = {J. Phys. Chem. C},
	volume = {128},
	year = {2024},
	pages = {17091},
	abstract = {Three-dimensional organic{\textendash}inorganic perovskite photovoltaic materials such as MAPbI$_{3}$ (MA is methylammonium) have rapidly advanced in their performance over the past decade, but their instability remains a problem. One of the methods to improve their stability is by reducing their dimensionality. This dimensional reduction is performed by adding a large organic cation as a spacer to separate the original bulk lattice, forming a new quasi-two-dimensional (quasi-2D) structure. In this study, we have investigated the electronic properties of the quasi-2D structures of (PEA)$_{2}$(MA)ₙ$_{-}$$_{1}$PbₙI$_{3}$ₙ$_{+}$$_{1}$ (PEA is phenylethylammonium) with various thicknesses (n) and MA orientations using first-principles calculations. Our results show that the bandgap decreased as the number of layers (n) increased for quasi-2D structures. The structures with in-plane MA orientations and mirror symmetry along the z-axis are found to be the most stable and exhibit a converging trend in the bulk bandgap when n > 6. Our calculations reveal how the orientations of the MA ions affect the electronic structure via symmetry breaking and electric dipole formation. These two factors change the electrostatic potential and band energies of the individual PbI$_{3}$ layers, which lead to enhanced band splitting and bandgap reduction. The results of this work would give some insights into the electronic structures and may provide directions for the crystal growth design of quasi-2D perovskite materials in photovoltaic applications.},
	doi = {10.1021/acs.jpcc.4c03540},
	author = {Yang-Hao Lee and Gennevieve M. Macam and Rovi Angelo B. Villaos and Liang-Ying Feng and Chia-Hsiu Hsu and Zhi-Quan Huang and Deng-Chi Wang and Hsin Lin and Feng-Chuan Chuang}
}
@article {604,
	title = {Band engineering and van Hove singularity on HfX$_{2}$ thin films (X = S, Se, or Te)},
	journal = {ACS Appl. Electron. Mater.},
	volume = {3},
	year = {2021},
	pages = {1071},
	abstract = {Two-dimensional transition metal dichalcogenides (TMDs) have become well-known due to their versatile and tunable physical properties for potential applications, specifically on low-power and optical devices. Here, we explored the structural stability and electronic properties of bulk and thin-film (from 1 up to 6 layers) structures of hafnium dichalcogenides (HfX$_{2}$, X = S, Se, or Te) using first-principles calculations. Our calculations reveal that the most stable phase is 1T for both thin films and bulk. The bulk and thin-film structures of HfTe$_{2}$ are semimetallic, while those of HfS$_{2}$ and HfSe$_{2}$ are insulating. Both HfS$_{2}$ and HfSe$_{2}$ thin films exhibit a decreasing band gap with increasing thickness, while HfTe$_{2}$ thin films remain semimetallic with increasing number of layers. Moreover, van Hove singularity (vHs), due to the contribution of the pz orbital from S atoms, is observed in 3L-HfS$_{2}$ at the valence band maximum, which can be further enhanced by applying an in-plane biaxial strain, suggesting possible superconductivity. Finally, the bulk and monolayer band structures of HfTe$_{2}$, under HSE06 and GGA + U with the effective Hubbard U parameter of 4.6 eV, are in good agreement with the experimental ARPES data. Our results indeed show that HfX$_{2}$ have sensitive and tunable electronic properties through film thickness control and strain for future potential applications.},
	doi = {10.1021/acsaelm.0c00907},
	author = {Harvey N. Cruzado and John Symon C. Dizon and Gennevieve M. Macam and Rovi Angelo B. Villaos and Thi My Duyen Huynh and Liang-Ying Feng and Zhi-Quan Huang and Chia-Hsiu Hsu and Shin-Ming Huang and Hsin Lin and Feng-Chuan Chuang}
}
