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Title:Functional organic thin films under extreme confinement
Author(s):Kafle, Prapti
Director of Research:Diao, Ying
Doctoral Committee Chair(s):Diao, Ying
Doctoral Committee Member(s):Harley, Brendan A.C.; Leal, Cecilia; Schroeder, Charles M.
Department / Program:Chemical & Biomolecular Engr
Discipline:Chemical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Thin Films
Organic Electronics
Organic Semiconductors
Abstract:Confining materials can produce dramatic changes in their fundamental properties. As evident from graphene and other 2D materials, reducing film thickness of materials to a few layers of even monolayers can alter their optical, electronic and magnetic properties compared to bulk counterparts. One of the tenets of current materials research is to harvest these unique properties of thin film materials at "extreme confinement", i.e., at condition when the film thickness reduces below a critical value where one or more structural aspects such as molecular packing, orientation, conformation, as well as properties begin to deviate from bulk. So far, significant advancements have been made in extremely confined materials where individual entities are held together by strong covalent bonds, such as most inorganic materials. However, such studies on organic molecules are lacking. This is partly because of fabrication challenges that arise as the bonds that hold entities in the supramolecular structure of organic molecules are significantly weaker. My thesis aims to crystallize extremely confined thin films of two functional materials, active pharmaceutical ingredients (APIs) and conjugated polymers, and then to explore and elucidate the resulting variations in structure and properties compared to thicker films. Low aqueous solubility of APIs is one of the most significant impediments in pharmaceutical industry. Nanosizing has been recognized as an effective to address this challenge. My approach to nanosize poorly soluble APIs is to solidify them in to nanothin films by utilizing meniscus-guided solution-coating (MGC) technique. In my first study (Chapter 2), we demonstrated a control in film thickness of APIs in nanometer scale range (1.5 μm to 30 nm) as well as their morphology and molecular packing by varying simple processing parameters. Moreover, we also performed sequential deposition of multiple components in a layer-by-layer manner by utilizing the same approach. Interestingly, we also observed trapping and stabilization of metastable polymorphs of API when coated under highly non-equilibrium conditions. In my following study (Chapter 3), I aimed to elucidate the role of nanoconfinement in the observed phenomenon as well as to investigate its role on dissolution properties of APIs. Here, I observed abrupt order-to-disorder transition and stabilization of semi-crystalline and amorphous forms of three different APIs when the film thickness was decreased below a critical value of ~25-30 nm in all three APIs. I also developed a dissolution model that predicted an enhancement in solubility and intrinsic dissolution rate (IDR) upon decrease in film thickness. The experimental results closely matched this model and we demonstrated a drastic increase in IDR upon decreasing the thickness and increasing disorder in the films. Next, I explored into two-dimensionalization of conjugated polymers- a novel class of semiconducting materials that have enabled fabrication of inexpensive, transparent, flexible electronics. Monolayer films of conjugated polymers are gaining an increasing attention because of their potential in tuning optoelectronic properties compared to bulk films, application in fabrication of ultrasensitive sensors, and as a near-ideal platform for studying fundamental charge transport mechanisms. However, fabrication of 2D films with high coverage and high degree of order remains challenging, and fundamental questions on how multiscale morphology and electronic properties of these films change when confined to monolayer films remain unanswered. In my work (Chapter 4), I first utilized dynamic-template-directed MGC technique to fabricate highly ordered and aligned 2D monolayer films of conjugated polymers of ~3.5 nm thickness with 100% coverage, which contrasted with disordered monolayers produced on solid substrates. Furthermore, I observed that the polymer chains in monolayers on dynamic template were highly twisted and their backbone planarity increased monotonically until the film thickness was increased to a critical value of ~20-30 nm. This led to a decrease in interfacial trap state density and an increase in hole mobility of the polymer films by almost 3 orders of magnitude when film thickness was increased from monolayer to the critical thickness. In addition, I fabricated OFET-based sensors incorporating ultrathin films of polymers which showed sensitivity of ~83% to ammonia vapors of ~1ppb concentration. Next, I explored into graphene surfaces as a template for monolayer films of polymers (Chapter 5) and observed significant enhancement in multiscale morphology with higher fiber density, coverage, degree of crystallinity, alignment and polymer backbone planarity compared to those on control silicon substrates. This accompanied with p-doping of the polymer by the graphene substrates. OFET devices incorporating graphene nanoflakes in ultrathin films of polymer blends showed enhancement in hole mobility by ~2 orders of magnitude compared to pristine polymer films. We attributed this to a decrease in interfacial trap state density and change in morphology, resulting due to the interaction between graphene and the polymer. Overall, my thesis has demonstrated strategies to overcome the challenges associated with fabricating extremely confined thin films of organic materials and reported deviations in structure and properties of such films with respect to the bulk. The insights gained from these studies will be helpful in harvesting enhanced properties of extremely confined thin films of pharmaceuticals and organic electronics, and will hopefully inspire exploration into confinement effects of other organic materials.
Issue Date:2021-06-25
Rights Information:Copyright 2021 Prapti Kafle
Date Available in IDEALS:2022-01-12
Date Deposited:2021-08

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