@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 {701, title = {Electronic, magnetic, and topological properties of ferromagnetic 2D perovskite-type oxides}, journal = {New J. Phys.}, volume = {26}, year = {2024}, pages = {123031}, abstract = {Two-dimensional (2D) materials within the hematene-type binary oxides and perovskites family have recently gathered huge research interest for nanoelectronic devices. However, the exploration of their fascinating topological properties remains limited. Herein, through first-principles calculations, we systematically examine the electronic, magnetic, and topological properties of substitutionally doped 2D ABO$_{3}$ (A = As, Sb, or Bi, and B = V, Nb, or Ta) perovskite structures at the B site of a B$_{2}$O$_{3}$ system. Interestingly, the atomic substitution makes the 2D ABO$_{3}$ structures dynamically stable. Our detailed calculations show the ferromagnetic (FM) and antiferromagnetic phases of these materials. The calculated Chern number (C) for the FM 2D ABO$_{3}$ (A = As, Sb, or Bi, B = Nb or Ta) suggests their topologically non-trivial phases. Furthermore, the computed nontrivial Berry curvature highlights the topological properties in AsNbO$_{3}$. These findings highlight opportunities in 2D-ABO$_{3}$ materials, for applications in spintronics.}, doi = {10.1088/1367-2630/ad9be0}, author = {Susaiammal Arokiasamy and Gennevieve M. Macam and Puthiya Covilakam Sreeparvathy and Rovi Angelo B. Villaos and Zhi-Quan Huang and Chia-Hsiu Hsu and Yoshinori Okada and Hsin Lin and Feng-Chuan Chuang} } @article {650, title = {Prediction of quantum spin Hall and Rashba effects in two-dimensional ilmenite oxides}, journal = {Chinese J. Phys.}, volume = {86}, year = {2023}, pages = {242}, abstract = {Using first-principles calculations, we investigate the structural, electronic, and topological properties of two-dimensional (2D) pristine ilmenite oxides ABO$_{3}$ (A~=~Au, Ag, or Cu; and B~=~Bi, Sb, or As) and their corresponding Janus structures. Phonon dispersions reveal the dynamic stability of these compounds. Interestingly, pristine CuBiO$_{3}$ and AuBiO$_{3}$, and Janus Cu$_{0}$.$_{5}$Ag$_{0}$.$_{5}$BiO$_{3}$, Au$_{0}$.$_{5}$Cu$_{0}$.$_{5}$BiO$_{3}$, Au$_{0}$.$_{5}$Ag$_{0}$.$_{5}$BiO$_{3}$, and CuBi$_{0}$.$_{5}$As$_{0}$.$_{5}$O$_{3}$ are topological insulators, while AuBi$_{0}$.$_{5}$As$_{0}$.$_{5}$O$_{3}$, CuBi$_{0}$.$_{5}$Sb$_{0}$.$_{5}$O$_{3}$, and AuBi$_{0}$.$_{5}$Sb$_{0}$.$_{5}$O$_{3}$ are topological semimetals, as confirmed by their Z2 invariance and conducting edge states under the hybrid functional approach. Moreover, we found van Hove singularities in Au$_{0}$.$_{5}$Ag$_{0}$.$_{5}$BiO$_{3}$ and Cu$_{0}$.$_{5}$Ag$_{0}$.$_{5}$BiO$_{3}$ near the Fermi level, suggesting the coexistence of superconductivity and nontrivial topology. Finally, isotropic Rashba spin-splitting is studied in detail for Au$_{0}$.$_{5}$Ag$_{0}$.$_{5}$BiO$_{3}$. Our findings demonstrate that 2D ilmenite oxides can be a new material playground for potential spintronic applications.}, doi = {10.1016/j.cjph.2023.09.022}, url = {https://www.sciencedirect.com/science/article/abs/pii/S0577907323001806}, author = {Susaiammal Arokiasamy and Gennevieve M. Macam and Rovi Angelo B. Villaos and Aniceto B. Maghirang and Zhi-Quan Huang and Chia-Hsiu Hsu and Guoqing Chang and Feng-Chuan Chuang} } @article {603, title = {Electronic and topological band evolution of VB-group transitionmetal monocarbides M$_{2}$C (M=V, Nb, or Ta) bulk and monolayer}, journal = {Mater. Today Commun.}, volume = {32}, year = {2022}, pages = {103875}, abstract = {Two-dimensional transition metal monocarbides (2D MXenes) have attracted intense interest due to their versatility, predicted topological phase, and immense applications. In this study, we investigated the electronic and topological band evolutions of both the bulk and monolayer MXenes, M$_{2}$C (M~= V, Nb, or Ta), using first-principles calculations. Our study shows that all the three bulk M$_{2}$C are semi-metallic and host topological phases. Interestingly, our study showed pristine monolayers V$_{2}$C and Nb$_{2}$C exhibit non-trivial topological phases, while Ta$_{2}$C exhibits trivial phase. Unlike other MXenes, monolayer V$_{2}$C and Nb$_{2}$C host the quantum spin Hall effect without functionalizations. Furthermore, thickness-dependent calculations intriguingly show Lifshitz electronic transition from semi-metallic to topological insulating phase in V$_{2}$C from bilayer to monolayer with a sufficiently large bandgap of 0.32~eV. Moreover, the topological phase transition between the TI state and the trivial state in V$_{2}$C is driven by quantum size effects as the Z$_{2}$ topological invariant notably oscillates between 1 and 0 with varying thickness. Finally, our study demonstrated that VB MXenes could be promising topological materials for spintronics applications.}, keywords = {Electronic structures, First-principles calculations, Mono-transition metal MXenes, Topological insulators, Topological phase transition, Transition metal monocarbides}, doi = {10.1016/j.mtcomm.2022.103875}, author = {Ali Sufyan and Aniceto B. Maghirang and Gennevieve M. Macam and Zhi-Quan Huang and Chia-Hsiu Hsu and Feng-Chuan Chuang} } @article {602, title = {Interplay between anisotropic spin texture and large gap topological insulating phases in functionalized MXenes}, journal = {Chinese J. Phys.}, volume = {77}, year = {2022}, pages = {2346}, abstract = {Massive attention has been given to two-dimensional (2D) MXenes due to their predicted topological phase and promising diverse applications. Using first-principles with hybrid functional calculations, we systematically conducted an extensive study on M$_{2}$C (M~=~Mo, or W) in 1T and 2H structures with various surface terminations T$_{2}$ (T~=~H, O, OH, F, Cl, Br, or I). Pristine Mo$_{2}$C and W$_{2}$C compounds are in the 2H phase. Upon functionalization of H, O, OH and F, they retain the 2H structure, whereas Cl, Br, and I functionalization, the structures transform to 1T phase. Notably, M$_{2}$CO$_{2}$ are found to be 2D topological insulators (TIs) with sizable nontrivial bandgaps as large as 666~meV. Interestingly, 2H M$_{2}$CO$_{2}$ exhibits not only large-gapped Zeeman-type spin splitting at K-point but also anisotropic Rashba spin splitting at Γ-point. The successful synthesis of pristine Mo$_{2}$C and our theoretical study pave a path for future applications of MXene-based 2D TIs.}, doi = {10.1016/j.cjph.2022.04.012}, author = {Aniceto B. Maghirang and Gennevieve M. Macam and Ali Sufyan and Zhi-Quan Huang and Chia-Hsiu Hsu and Feng-Chuan Chuang} } @article {601, title = {Robust tunable large-gap quantum spin Hall states in monolayer Cu$_{2}$S on insulating substrates}, journal = {ACS Omega}, volume = {7}, year = {2022}, pages = {15760}, abstract = {Quantum spin Hall (QSH) insulators with large band gaps and dissipationless edge states are of both technological and scientific interest. Although numerous two-dimensional (2D) systems have been predicted to host the QSH phase, very few of them harbor large band gaps and retain their nontrivial band topology when they are deposited on substrates. Here, based on a first-principles analysis with hybrid functional calculations, we investigated the electronic and topological properties of inversion-asymmetric monolayer copper sulfide (Cu$_{2}$S). Interestingly, we found that monolayer Cu$_{2}$S possesses an intrinsic QSH phase, Rashba spin splitting, and a large band gap of 220 meV that is suitable for room-temperature applications. Most importantly, we constructed heterostructures of a Cu$_{2}$S film on PtTe$_{2}$, h-BN, and Cu(111) substrates and found that the topological properties remain preserved upon an interface with these substrates. Our findings suggest Cu$_{2}$S as a possible platform to realize inversion-asymmetric QSH insulators with potential applications in low-dissipation electronic devices.}, doi = {10.1021/acsomega.2c00781}, author = {Ali Sufyan and Gennevieve M. Macam and Zhi-Quan Huang and Chia-Hsiu Hsu and Feng-Chuan Chuang} } @article {600, title = {Spin-lattice-charge coupling in quasi-one-dimensional spin-chain NiTe$_{2}$O$_{5}$}, journal = {Phys. Rev. Mater.}, volume = {6}, year = {2022}, pages = {044409}, abstract = {A high-quality NiTe$_{2}$O$_{5}$ single crystal was grown via the flux method and characterized using synchrotron x-ray diffraction (XRD) and electron probe microscopy techniques. The dc magnetization (M) confirms the antiferromagnetic long-range ordering temperature (TN) at 28.5 K. An apparent domelike dielectric anomaly near TN, with scaling of magnetodielectric (MD) coupling with magnetization (MD\% ∝ M{\texttwosuperior}), signifies higher-order magnetoelectric (ME) coupling. The critical finding is that magnetoelastic coupling plays a pivotal role in bridging the electrical and magnetic dipoles, which was further confirmed by temperature-dependent XRD. In addition, the theoretical charge density difference maps indicate that the emergence of electrical dipoles between the Ni and O atoms below TN originates through p-d hybridization. Thus, the p-d hybridization-induced magnetoelastic coupling is considered a possible mechanism for the higher-order ME effect in this quasi-onedimensional spin-chain NiTe$_{2}$O$_{5}$.}, doi = {10.1103/PhysRevMaterials.6.044409}, author = {Ajay Tiwari and D. Chandrasekhar Kakarla and Gennevieve M. Macam and Chia-Hsiu Hsu and Feng-Chuan Chuang and H. C. Wu and T. W. Kuo and Arkadeb Pal and H. Chou and D. P. Gulo and H. L. Liu and Y. C. Chuang and Y. C. Lai and C. A. Lee and Mitch M. C. Chou and H. D. Yang} } @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} } @article {605, title = {Theoretical prediction of topological insulators in two-dimensional ternary transition metal chalcogenides (MM{\textquoteright}X$_{4}$, M~=~Ta, Nb, or V; M{\textquoteright}= Ir, Rh, or Co; X~=~Se or Te)}, journal = {Chinese J. Phys.}, volume = {73}, year = {2021}, pages = {95}, abstract = {Ternary transition metal chalcogenides (TTMCs) have attracted interest due to the discovery of their Weyl semimetallic property and the recent synthesis of layered TTMCs which are regarded as potential candidates for two-dimensional (2D) topological insulators. Here, employing first-principles calculations, we predicted the emergence of non-trivial band topologies in the monolayer MM{\textquoteright}X$_{4}$ family (M= V, Nb, or Ta; M{\textquoteright}~=~Co, Rh, or Ir; and X~=~Se or Te) within hybrid functional calculations. Five of eighteen 2D materials were found to be topological insulators, while four of them are magnetic thin films. The nontrivial topologies were verified via the calculated Z$_{2}$ topological invariant and topologically protected edge states. Further calculations showed a strain-induced phase transition in VCoTe$_{4}$ from a magnetic phase to a nonmagnetic topological insulating phase. Our comprehensive study revealed a diverse family of monolayer ternary transition metal chalcogenides adding new members to the current catalog of 2D topological insulators and 2D magnetic materials.}, doi = {10.1016/j.cjph.2021.06.014}, author = {Ali Sufyan and Gennevieve M. Macam and Chia-Hsiu Hsu and Zhi-Quan Huang and Shin-Ming Huang and Hsin Lin and Feng-Chuan Chuang} } @article {607, title = {Tuning topological phases and electronic properties of monolayer ternary transition metal chalcogenides (ABX$_{4}$, A/B = Zr, Hf, or Ti; X = S, Se, or Te)}, journal = {Appl. Phys. Lett.}, volume = {118}, year = {2021}, pages = {111901}, abstract = {Topological materials are very promising materials for technological applications ranging from spintronics to quantum computation. Here, based on first-principles calculations, we predict a family of two-dimensional (2D) topological materials in nine ternary transition metal chalcogenides (TTMCs) ABX$_{4}$, where A/B = Zr, Hf, or Ti and X = S, Se, or Te. A total of three compounds (ZrTiTe$_{4}$, HfZrTe$_{4}$, and HfTiTe$_{4}$) are identified to be nontrivial within a hybrid functional calculation. The nontrivial phase originated from the p-d band inversion at the Г point with spin-orbit coupling. The structural stability of these monolayers is confirmed by phonon spectrum analysis, showing no negative phonon frequencies. The diversity of TTMCs will open a wide possibility for tuning the bandgap and will provide a variety of opportunities for 2D and topological materials research.}, doi = {10.1063/5.0036838}, author = {Gennevieve M. Macam and Ali Sufyan and Zhi-Quan Huang and Chia-Hsiu Hsu and Shin-Ming Huang and Hsin Lin and Feng-Chuan Chuang} } @article {610, title = {Large-gap topological insulators in functionalized ordered double transition metal carbide MXenes}, journal = {Phys. Rev. B}, volume = {102}, year = {2020}, pages = {075306}, abstract = {Two-dimensional MXenes continue to receive much research attention owing to their versatility and predicted topological phase, which are yet to be fully explored. Here, we conduct a rigorous search on M$_{2}${\textquoteright}M{\textquoteright}{\textquoteright}C$_{2}$ (M{\textquoteright} = V , Nb , or Ta ; M{\textquoteright}{\textquoteright} = Ti , Zr , or Hf) with various surface terminations, X$_{2}$ (X = F , Cl , Br , I , O , H , or OH) , using first-principles calculations. The majority of the systems exhibit the topological phase with semimetallic band structures. Most importantly, fluorinated MXenes, M$_{2}${\textquoteright}M{\textquoteright}{\textquoteright}C$_{2}$F$_{2}$, are topological insulators. They possess sizable nontrivial band gaps from 34 to 318 meV using Heyd-Scuseria-Ernzerhof (HSE) hybrid functional calculations which are within the range capable of realizing quantum spin-Hall effects even at room temperature. Furthermore, the d orbitals of M{\textquoteright} and M{\textquoteright}{\textquoteright} atoms mostly contribute to the spin-orbit coupling-induced band gaps. Selecting V$_{2}$TiC$_{2}$F$_{2}$ as an exemplar, we demonstrate the presence of edge states, verifying the calculated Z$_{2}$ invariant, and reveal its robustness against tensile strain. Finally, we propose SiC(0001) as a candidate substrate for material realization as it can preserve the nontrivial band topology.}, doi = {10.1103/PhysRevB.102.075306}, author = {Zhi-Quan Huang and Mei-Ling Xu and Gennevieve M. Macam and Chia-Hsiu Hsu and Feng-Chuan Chuang} } @article {608, title = {Quantum anomalous Hall insulator phases in Fe-doped GaBi honeycomb}, journal = {Chinese J. Phys.}, volume = {67}, year = {2020}, pages = {246}, abstract = {We discuss electronic and magnetic properties of the Fe-doped GaBi honeycomb using first principles calculations. Our analysis shows that the pristine GaBi honeycomb transitions from being a two-dimensional quantum spin Hall (QSH) insulator to a quantum anomalous Hall (QAH) insulator when it is doped with one Fe atom in a 4 {\texttimes} 4 GaBi honeycomb. The QAH phase in Fe-doped GaBi is found to be robust in that it maintains its Chern number (C = 1) under fairly large strains (\~{} 4\%) and supports a gap as large as 112~meV at 2.21\% strain. The QAH phase is also retained when the Fe-doped GaBi is placed on a CdTe substrate, suggesting that Fe-doped GaBi films could be useful for spintronics applications.}, doi = {10.1016/j.cjph.2020.07.007}, url = {https://www.sciencedirect.com/science/article/pii/S0577907320301866}, author = {Zhi-Quan Huang and Chia-Hsiu Hsu and Christian P. Crisostomo and Gennevieve M. Macam and Jing-Rong Su and Hsin Lin and Arun Bansil and Feng-Chuan Chuang} } @article {613, title = {Growth of a predicted two-dimensional topological insulator based on InBi-Si(111)-√7{\texttimes}√7}, journal = {Phys. Rev. B}, volume = {98}, year = {2018}, pages = {121404}, abstract = {Using combined scanning tunneling microscopy (STM) measurements and first-principles electronic structure calculations, we extensively studied the atomic and electronic properties of a √7 -InBi overlayer on Si(111). We propose and demonstrate an effective experimental process to successfully form a large well-ordered √7 surface by depositing Bi atoms on the In-Si(111)4{\texttimes}1 substrate. The STM images exhibit a honeycomb pattern. After performing an exhaustive computational search, we identified the atomic structures of the surface at In and Bi coverages of 6/7 and 3/7 monolayers, respectively. We discovered a trimer model with a lower energy than the previously proposed model. The simulated STM images of trimer models confirm the presence of the honeycomb pattern in accord with our experimental STM images. Most importantly, we found that the surface is robust, preserving the topologically nontrivial phase. Our edge state calculations verify that the InBi overlayer on Si(111) is indeed a two-dimensional (2D) topological insulator (TI). Moreover, hybrid functional calculations result in band gaps up to 70 meV, which is high enough for room-temperature experiments. Our findings lay the foundation for the materials realization of 2D TIs by growing an InBi overlayer on a Si(111) substrate.}, doi = {10.1103/PhysRevB.98.121404}, author = {Chia-Hsiu Hsu and Zhi-Quan Huang and Cho-Ying Lin and Gennevieve M. Macam and Yu-Zhang Huang and Deng-Sung Lin and Tai Chang Chiang and Hsin Lin and Feng-Chuan Chuang and Li Huang} } @article {614, title = {Prediction of two-dimensional organic topological insulator in metal-DCB lattices}, journal = {Appl. Phys. Lett.}, volume = {113}, year = {2018}, pages = {233301}, abstract = {Based on first-principles calculations, we systematically investigated a two-dimensional (2D) organometallic framework consisting of metal atoms (elements from groups IIIA, IVA, VA, VIA, IB, and Pt) and dicyanobenzenes (DCBs). Our stability analysis showed that the system prefers the buckled structure in metals with p-orbital valence electrons, whereas in metals with d-orbitals, the planar phase is preferable. Topological invariants (Z$_{2}$) of these systems were calculated, and they are identified as 2D intrinsic organic topological insulators. Au- and Bi-DCB are exemplar materials with the largest bandgaps within IB and VA groups. Moreover, Au-DCB exhibits robustness of its topological phase against strain. Furthermore, the topologically protected edge states in Au-DCB are identified to further verify the Z$_{2}$ invariant. Interestingly, utilizing hole doping in Bi-DCB leads to a nearly flat Chern band and results in the quantum anomalous Hall phase. Our results suggest that these organometallic frameworks are promising for potential applications in quantum spintronics with the merits of low cost and ease of synthesis.}, doi = {10.1063/1.5064610}, author = {Chia-Hsiu Hsu and Zhi-Quan Huang and Gennevieve M. Macam and Feng-Chuan Chuang and Li Huang} }