Optoelectronic and Magnetic Properties of 2D Layered Organic-Inorganic Hybrids and Selected Transition Metal Oxides

Abstract

Perovskites with the general chemical formula ABX3 can be categorized into oxides and halides depending on the nature of the X anion. 3D organic-inorganic halide perovskites are extensively studied in the context of solar cell and photo- and electro-luminescence applications due to their outstanding optoelectronic properties, while oxide perovskites have attracted a great deal of attention for their many interesting physical properties such as structural, electrical, magnetic, and magnetocaloric effects. More recently, 2D layered organic-inorganic hybrid (OIH) materials have emerged as a new class of materials with rich optoelectronic properties. They exhibit proven advantages over their 3D counterparts due to their large structural diversity and improved environmental stability against heat and moisture. 2D OIH materials have exhibited many interesting physical properties such as high exciton binding energies, intense photoluminescence, ferroelectricity, and chiro-optical properties. While the Lead-based hybrid perovskites have been very well studied for their spectacular optoelectronic properties, lately there have been attempts to design new materials such as Cd2+, Cu2+, Sn2+ -based hybrid halide materials which offer a new playground in the field of photovoltaic research. The work reported in this thesis explores the ferroelectric properties of Cd2+ - and Cu2+ -based halide materials and discusses the possible microscopic mechanisms for the origin of ferroelectricity in these materials. Further, using experimental and theoretical inputs, it is shown that the Cu2+ -based hybrid materials have outstanding chiro-optical properties. In addition, we also explore the interesting magnetic properties of a few transition metal compounds, exhibiting inverse magnetocaloric effect as well as Griffiths phase in some temperature ranges. Chapter 1 briefly introduces various concepts relevant to the investigations reported in subsequent chapters of this thesis. The present status of the research in the field of 2D organic- inorganic hybrid materials with an emphasis on various exciting properties such as ferroelectricity, bandgap modulation, and chiro-optical properties has been discussed. This chapter also presents discussions relevant to the family of oxide perovskites with reference to magnetic properties from fundamental and technological standpoints. Chapter 2 describes different experimental and theoretical methods that were employed to carry out the studies presented in this thesis. Chapter 3 presents a detailed study of the successive structural phase transitions of BA2CdCl4. These results establish that these structural phase transitions are associated with intrinsic ferroelectric transitions, from room temperature paraelectric to intermediate temperature ferroelectric, followed by another low-temperature ferroelectric phase. It was widely believed that ferroelectricity in these 2D organic-inorganic hybrid materials originated due to the order-disorder transition of molecular dipoles associated with the organic spacer cations. However, results in this thesis show that there are dipoles also associated with the inorganic components due to local structural distortions. The thesis presents a combination of experimental and theoretical results suggesting that the dipoles associated with the organic spacers and the dipoles originating from these local structural distortions within the inorganic units both play significant roles in the observed ferroelectricity of BA2CdCl4. Chapter 4 deals with the chiro-optical properties of quasi 2D (R-/S-MBA)2CuBr4 hybrid material. We discussed the role of chiral organic amine cations on the optical properties of these hybrid materials. Few Lead based compounds and one copper-based sample show giant chiro-optical properties but these are chirally active only for wavelengths < 490nm, limited by their large bandgaps. (R-/S-MBA)2CuBr4 shows a relatively large chiro-optical property in the orange-red part of the visible spectrum. Structural analyses of these compounds show that these are made of alternating layers of the chiral organic units and an inorganic layer of isolated CuBr4 units. Such isolated inorganic units distinguish this class of compounds from the more intensely investigated hybrid lead halide systems where the basic PbX6 (X = Cl, Br, or I) units are linked together by corner sharing of the halide ions, making them intrinsically 2D systems. The present Cu-based system would have qualified as a 0D system but for the Cu-Br….Br-Cu interactions that allow the separated CuBr4 units to interact, making the system a quasi-2D system. This subtle structural aspect plays an important role in giving this system remarkable chiro-optical properties. The semi-isolation of the CuBr4 units allows them to be rotated along the 21 screw-axis by the chiral organic units via strong hydrogen bonding, thereby imparting the giant chirality to the entire hybrid system. Simultaneously, the connectivity of the CuBr4 units via Br…Br interactions imparts a quasi-2D character helping to achieve a broadband absorption, thereby extending the chiro-optical properties to longer wavelengths. Chapter 5 discusses tuning of the bandgap while retaining ferroelectricity through halide substitution in Cu2+ -based chiral 2D hybrid materials to obtain small bandgap ferroelectric materials. Search for such small bandgap ferroelectric materials has been popular in the literature, not only because most ferroelectrics tend to have large bandgaps but also because of their obvious applications in solar photovoltaics. The ferroelectric Lead-based hybrid perovskites have been very well studied for their rich optoelectronic properties that are relevant in photovoltaic applications, but all are having a relatively higher bandgap. We have lowered the ligand to metal charge transfer bandgap from ~2.53 to 2.09 eV while retaining ferroelectricity in a copper chloride based low-dimensional hybrid material through partial substitution of chlorine with bromine. Our results show that a complete substitution of Cl- by Br- leads to a bandgap of ~1.62 eV with a loss of the ferroelectric state in the pure bromide material. Chapter 6 discusses the low temperature magnetic state along with the main ferromagnetic ordering at ~200 K and the magnetocaloric effect of double perovskite, Nd2NiMnO6. An earlier study on this compound established that for any applied magnetic field lower than 3 T, these samples show a downturn in M(T), and any field higher than 3 T, shows an upturn in M(T) for the temperature range below 100 K. This has been interpreted as Nd moments experience an effective ~3 T internal magnetic field due to the presence of the ordered Ni-Mn ferromagnetic sublattices. This indicates that the low temperature magnetic state of this compound is easily influenced by an externally applied magnetic field in the tune of 3 T, suggesting possible interesting magnetocaloric effects in this material. This chapter presents a detailed study of the magnetocaloric properties of Nd2NiMnO6. Interestingly, it shows a significant inverse magnetocaloric effect (IMCE) at low temperatures (T < 50 K) together with a significant conventional magnetocaloric effect (CMCE) at the ferromagnetic ordering temperature (Tc ~200 K). IMCE and CMCE correspond to the antiferromagnetic arrangement of Nd and Ni–Mn sublattices and ferromagnetic ordering of Ni–Mn sublattices, respectively. Nd2NiMnO6 with its second order phase transition follows the universal behavior of magnetic entropy change, ΔSM(T); it also shows a power law dependency on the magnetic field as ΔSM ∝ 𝐻𝜂. Chapter 7 deals with Griffiths phase-like magnetic anomalies in disordered La0.85Sr0.15CoO3 induced by chemical doping. In earlier studies on doped LaCoO3 with doping concentration higher than the percolation threshold (18%) shows non-Griffiths phase-like behavior. But no reports are available for the composition just below the percolation limit in this context. So, we chose the 15% doping concentration and explored its magnetic properties carefully. Our results establish that this composition shows typical Griffiths phase-like behavior in the intermediate temperature range and followed by spin glass behavior below 60 K. The existence of nanoscale ferromagnetic clusters below 240 K contributes to the total magnetization of the system for low applied magnetic fields resulting in a downturn of the χ-1 vs. T plot. The extent of this downturn is strongly suppressed by increasing the dc applied magnetic field, a typical signature of Griffiths phase. In the appendix, we present results of investigating Cs- and Na-doped WO3 exhibiting strong absorption in the near-infrared (NIR) and transmittivity in the visible range. Despite several publications, there is a lack of agreement in the community on the origin of this strong optical absorption with competing claims of polaronic and plasmonic origins. We address this controversy by first investigating bulk samples that are relatively free of complications arising from any shape anisotropy; we show by combining experimental and theoretical results that all spectral features in both bulk and nanoparticle samples are consistent with plasmonic excitations, without any need to invoke a polaronic mechanism. Doped WO3 exhibits strong optical absorption primarily due to surface plasmon resonances in colloidal nanoparticles, while their bulk counterparts are dominated by bulk plasmonic features. Investigating systems with different crystal structures and charge doping levels, we established that the complex spectral features of these plasmonic absorption bands for both bulk and nano samples are dominated by the underlying structure-dependent anisotropic electronic properties that determine the plasmonic features