Bulk Assembly of Organic Metal Halide Nanoribbons

Organic metal halide hybrids with low-dimensional structures at the molecular level have received great attention recently for their exceptional structural tunability and unique photophysical properties. Here we report for the first time the synthesis and characterization of a one-dimensional (1D) organic metal halide hybrid material, which contains metal halide nanoribbons with a width of three octahedral units. It is found that this material with a chemical formula C$_8$H$_{28}$N$_5$Pb$_3$Cl$_{11}$ shows a dual emission with a photoluminescence quantum efficiency (PLQE) of around 25% under ultraviolet (UV) light irradiation. Photophysical studies and density functional theory (DFT) calculations suggest the coexisting of delocalized free excitons and localized self-trapped excitons in metal halide nanoribbons leading to the dual emission. This work shows once again the exceptional tunability of organic metal halide hybrids that bridge between molecular systems with localized states and crystalline ones with electronic bands.Comment: 6 pages, 4 figures, plus supporting informatio

Surface Effects on Anisotropic Photoluminescence in One-Dimensional Organic Metal Halide Hybrids

One-dimensional (1D) organic metal halide hybrids exhibit strongly anisotropic optical properties, highly efficient light emission, and large Stokes shift, holding promises for novel photodetection and lighting applications. However, the fundamental mechanisms governing their unique optical properties and in particular the impacts of surface effects are not understood. Here, we investigate 1D C4N2H14PbBr4 by polarization-dependent time-averaged and time-resolved photoluminescence (TRPL) spectroscopy, as a function of photoexcitation energy. Surprisingly, we find that the emission under photoexcitation polarized parallel to the 1D metal halide chains can be either stronger or weaker than that under perpendicular polarization, depending on the excitation energy. We attribute the excitation-energy-dependent anisotropic emission to fast surface recombination, supported by first-principles calculations of optical absorption in this material. The fast surface recombination is directly confirmed by TRPL measurements, when the excitation is polarized parallel to the chains. Our comprehensive studies provide a more complete picture for a deeper understanding of the optical anisotropy in 1D organic metal halide hybrids

Understanding the Structure, Electronic, Phononic, Optical, and Mechanical Properties of Low-Dimensional Materials

Low-dimensional materials have attracted significant attention in the field of materials science and technology. These materials, characterized by their limited dimensions in one, two or three directions, exhibit exceptional properties different from those of their bulk counterparts. This deviation results from the interaction of quantum effects, surface interactions, and unique electronic structures, creating a wide range of innovative applications in various fields such as electronics, photonics, energy storage and more. This study not only reveals the scientific complexities of their behavior but also paves the way for disruptive innovations that could transform industries and transform our interactions with the world around us. In this work, I have studied different classes of low-dimensional materials such as bulk and monolayer transition-metal dichalcogenides, one-dimensional organic-metal halide hybrids, organic metal halide nanoribbons, Bi$_2$Te$_3$ and HfTe$_5$.In the opening chapter, I lay the groundwork by discussing fundamental theories crucial for our subsequent calculations. These theories encompass density functional theory, density functional perturbation theory, GW and Bethe-Salpeter equations and other fundamental concepts on Raman, and calculation methods for elastic tensors. Chapter 2 delves into a comprehensive analysis of the impact of introducing Ni into the bulk phases of MoS$_2$, with a particular focus on the mechanism supporting low wear for lubrication applications. This chapter also presents systematic doping studies conducted through first-principles calculations. Chapter 3 narrows the focus to examine the consequences of Ni doping on the monolayer phases of MoS$_2$, revealing the induction of various known and novel distorted phases. These induced phases bring forth unique properties, including those of ferroelectric materials, multiferroic semimetals, ferromagnetic polar metals, and the presence of multiple gaps in conduction bands. These findings have promising implications for applications in spintronics, intermediate band solar cells, and non-linear optics. In Chapter 4, the attention shifts to a low-energy nine-layer phase within transition-metal dichalcogenides, where we explore methods for characterizing this phase through Raman spectroscopy and powder diffraction. Additionally, we investigate its potential applications in piezoelectricity and valleytronics. Next Chapter 5 delves into understanding the anisotropic properties of one-dimensional organic-metal halide hybrids and the mechanism of broadband emission. This chapter introduces an innovative approach for determining exciton-phonon coupling and the self-trapped exciton structure using excited state forces within a GW/Bethe-Salpeter framework. Chapter 6 offers a concise overview of my contributions to other collaborative works on low-dimensional materials, encompassing organic-metal halide hybrids, organic metal halide nanoribbons, Bi$_2$Te$_3$ and HfTe$_5$. Finally, I conclude with reflections on the potential of low-dimensional materials, their current progress, and future prospects

Panoply of Ni-Doping-Induced Reconstructions, Electronic Phases, and Ferroelectricity in 1T-MoS₂

The distorted phases of monolayer 1T-MoS2 have distinct electronic properties, with potential applications in optoelectronics, catalysis, and batteries. We theoretically investigate the use of Ni-doping to generate distorted 1T phases and find not only the ones usually reported but also two further phases (3 × 3 and 4 × 4), depending on the concentration and the substitutional or adatom doping site. Corresponding pristine phases are stable after removal of dopants, which might offer a potential route to experimental synthesis. We find large ferroelectric polarizations, most notably in 3 × 3 whichcompared to the recently measured 1T″has 100 times greater ferroelectric polarization, a lower energy, and a larger band gap. Doped phases include exotic multiferroic semimetals, ferromagnetic polar metals, and improper ferroelectrics with only in-plane polarization switchable. The pristine phases have unusual multiple gaps in the conduction bands with possible applications for intermediate band solar cells, transparent conductors, and nonlinear optics