Quantum modelling of organic materials for electronics, thermoelectric, and photoluminescent applications / Mohamad Syafie Mahmood

Organic semiconductors are dominating a niche segment of applications in electronic devices, as an alternative to inorganic semiconductors. They are becoming mainstream in electronic devices such as organic light emitting diode (OLED) and nanowires due to their promising advantages such as low cost, ease of synthesis and high throughput fabrication methods. In this thesis, 3 families of organic materials; discotic liquid crystal (DLC), protic ionic liquid (PIL), and chalcone are chosen for their potential applications in electronics, thermoelectric, and fluorescence respectively. Therefore, this work is separated into 3 corresponding major sections as follows; (1) correlation between molecule structure and electronic properties of the 2,3,6,7,10,11-hexahexyloxytriphenylene (HAT6) DLC molecule using first principle Density Functional Theory (DFT), (2) quantum thermodynamic calculations of entropy for amine based PILs, and (3) photoluminescent efficiency evaluation of 4-dimethylamino-2ʹ-hydroxychalcone (DHC) based on molecular conformation. In (1), the columnar stacking of HAT6 molecules allows π-π orbital overlap at the molecular core which allows electronic charge transfer along the column. The electronic transfer is affected by conformation of DLC molecules in columnar phases, namely core-core facial separation (D), angular twist (θ), and lateral slide (L). The correlation between molecular structure and electronic properties are evaluated in terms of its formation energy, band gap (BG), and density of state (DOS). The preservation of π-π interaction by maintaining the integrity of its columnar structural characteristics is key to maintaining the DLC’s charge transfer efficacy. In (2), quantum thermodynamic simulation of a series of amine based PILs (ethylammonium triflate (EaTf), diethylammonium triflate (DieaTf), triethylammonium triflate (TrieaTf), and 2-methylpyridinium triflate (2mpTf)) was carried out to calculate the entropy of the system. Analysis in terms of vibrational and conformational contribution to entropy was then extended to evaluate the role of PIL in a thermoelectric solution. In this section, the focus is on the vibration of proton attachment, which has the biggest influence on their thermodynamic properties. Then, a hypothetical thermodynamic cycle is built around the states where the thermodynamic-conformation mechanism was discussed. Thus, the relationship between molecule properties to the thermoelectric properties such as Seebeck coefficient was elucidated. In (3), the photoluminescence capability of the DHC from the chalcone family was evaluated using first principle DFT calculation. The conformation of the DHC in the ground and excited states were evaluated in crystalline state and in solutions. The conservation of planarity in the ground to excited state are expected to provide large photoluminescence yield. In solution, it is shown that the minimum energy in the excited state are achieved by a twist in the conformation, this provided a non-radiative pathway which quenches photoluminescence. In the crystalline state, the calculations show that the planarity is conserved going from the ground to excited state which encourage a high quantum yield of photoluminescence. These simulations are strongly supported by femtosecond spectroscopy results and provide a promising pathway for the use of chalcone in optoelectronic/bioimaging applications. Thus, this thesis provides a pathway for molecular design using first principle DFT calculations, to provide an optimisation strategy of better organic electronic materials for applications in electronics, thermoelectric, and optoelectronic

A DFT study of the chemical and optical properties of 7-atom Ag-X [X = Li, Na] nanoalloys for potential applications in opto-electronics and catalysis

In this paper, Ag atoms are substituted by X (Li, Na) atoms to form AgmX(7 - m) clusters to explore their electronic, chemical and optical properties in the framework of density functional theory (DFT). The clusters are geometrically optimized without imposing symmetry and later, vibrational analysis is carried out to test the stability of the optimized structures. The calculation of ionization potential and electron affinity asserted that the Li and Na doped bimetallic clusters (especially, Ag4Li3 and Ag3Li4) are very stable in the neutral state, but their anions are expected to be very reactive. The calculated absorption spectra of the AgmX7-m clusters have revealed that the doping of Li and Na has made the absorption band wider with regards to undoped Ag-7 clusters. Therefore, this work suggests that Li and Na doping (especially, Ag4X3, Ag3X4 and Ag2X5 clusters) will result in improvement of the absorption band in the 1-5 eV range, which is the prime absorption band for opto-electronic devices such as solar cells

New Insight into the Origin of the Red/Near-Infrared Intense Fluorescence of a Crystalline 2‑Hydroxychalcone Derivative: A Comprehensive Picture from the Excited-State Femtosecond Dynamics

Fluorescence upconversion and transient absorption techniques are used to explain the source of the intense red/near-infrared emission of crystalline 4-dimethylamino-2′-hydroxychalcone. We found that the initially excited enol form undergoes tautomerization in 3 ps to form the keto tautomer. The latter is stable in the ground state as a consequence of J-type aggregation in the crystal packing and is manifested in an absorption peak at 550 nm that spectrally overlaps with the short-lived enol emission, leading to self-reabsorption and adding a factor to the complete depletion of the enol emission. Relaxation of the keto tautomer takes place in the form of intense fluorescence (600–750 nm) with 1.7 ns lifetime. The different spectroscopy in solution is due to vibrational cooling (300 fs), followed by solvation dynamics (5 ps in methanol) and twisting of the hydroxyphenyl ring (16 ps), before relaxation of the enol tautomer in the form of weak green fluorescence with 350 ps lifetime

DFT studies of structural–electronic correlation for the HAT6 discotic liquid crystal columnar stacking

Discotic liquid crystals (DLC) have shown much promise for 1-D charge transport and relatively high charge mobility due to π-π conjugation of their cores. However, development of DLCs in molecular electronics are hindered by a lack of systematic study of DLCs' molecular conformation and its effect on the electronic properties. This study takes a model DLC molecule, 2, 3, 6, 7, 10, 11-hexahexyloxytriphenylene (HAT6) and calculates the electronic properties as a function of the columnar structural parameters (co-facial distance, twist and lateral slide) from (density functional theory) DFT calculations. The relationship between the structural parameter to band gap, electronic density of state (DOS) and partial DOS, charge population analysis, and electronic density mapping is discussed

In-Filled La0.5Co4Sb12 Skutterudite System with High Thermoelectric Figure of Merit

The contribution of In addition to the La0.5Co4Sb12 skutterudite structure to improve its thermoelectric properties has been demonstrated. InxLa0.5Co4Sb12 (0 ≤ x ≤ 0.3) samples were prepared through mechanical alloying followed by spark plasma sintering. Characterization of the phase structure and morphology of the sintered InxLa0.5Co4Sb12 bulk samples was carried out using x-ray diffraction (XRD) analysis, scanning electron microscopy, and energy-dispersive x-ray spectroscopy. Rietveld analysis of the XRD spectra indicated that double filling at the 2a (000) interstitial site with La and In was successfully achieved, significantly improving the thermoelectric performance of the La0.5Co4Sb12 compound through simultaneous increase in the electrical conductivity and Seebeck coefficient. A maximum power factor of 3.39 × 10−3 W/m-K2 was recorded at 644 K for the In0.3La0.5Co4Sb12 sample, more than 96% of that of the La0.5Co4Sb12 sample. Double filling also effectively reduced the lattice thermal conductivity by about 46%, thus demonstrating that the overall improvement in ZT was primarily due to the reduced thermal conductivity. A maximum ZT value of 1.15 was achieved at 692 K for In0.3La0.5Co4Sb12