95 research outputs found
Ionic Organic Small Molecules as Hosts for Light-Emitting Electrochemical Cells
No full textSupplementary material available at publisher's websiteFull text access from Treasures at UT Dallas is restricted to current UTD affiliates (use the provided Link to Article).Light-emitting electrochemical cells (LEECs) from ionic transition-metal complexes (iTMCs) offer the potential for high-efficiency electroluminescence in a simple, single-layer device. However, LEECs typically rely on the use of rare metal complexes. This has limited their cost effectiveness and put constraints on their applicability. With a view to leveraging the efficient emission of these complexes while mitigating costs, we describe here a host/guest LEEC strategy that relies on the use of carbazole (Cz)-based organic small-molecule hosts and iTMC guests. Three cationic host molecules were prepared via the coupling of 1-(4-bromophenyl)-2-phenylbenzimidazole (PBI-Br) with Cz. This has allowed a comparison between the hosts bearing methoxy (PBI-CzOMe) and tert-butyl (PBI-CztBu) substituents, as well as an unsubstituted analogue (PBI-CzH). Cyclic voltammetry and UV-visible absorption revealed that all three host materials have wide band gaps characterized by reversible oxidation and irreversible reduction events. On the basis of electronic structure calculations, the host highest occupied molecular orbital (HOMO) resides primarily on the Cz moiety, whereas the lowest unoccupied molecular orbital (LUMO) is located primarily on the phenyl-benzimidazolium unit. Photoluminescence analysis of thin-film blends of PBI-CzH with iTMC guests confirmed that the emission was blue-shifted relative to pristine iTMC films, which is consistent with what was seen in dilute dichloromethane solution. LEEC devices were prepared based on thin films of the pristine hosts, pristine guests, and 90%/10% (w/w) host/guest blends. Among these host/guest blends, LEECs based on PBI-CzH displayed the best performance, particularly when an iridium complex was used as the guest. The system in question yielded a luminance maximum of 624 cd/m2 at an external quantum efficiency of 3.80%. This result stands in contrast to what is seen with typical organic light-emitting diode host studies, where tert-butyl substitution of the host generally leads to a better performance. To rationalize the present observations, the host materials were subject to single-crystal X-ray diffraction analysis. The resulting structures revealed clear head-to-tail interactions in the case of both PBI-CzH and PBI-CzOMe. No such interactions were evident in the case of PBI-CztBu. Furthermore, PBI-CzH showed a relatively smaller spacing between the successive HOMO and successive LUMO levels relative to PBI-CzOMe and PBI-CztBu, a finding consistent with more favorable charge transport and energy transfer. The results presented here can help inform the design and preparation of host materials suitable for use in single-layer iTMC LEECs.School of Natural Sciences and Mathematic
Blue Light Emitting Electrochemical Cells Incorporating Triazole-based Luminophores
No full textWe report the electrochemical, photoluminescence, and electroluminescence properties of four fluorinated cationic iridium complexes bearing pyridyltriazole ancillary ligands. All the complexes display unstructured emission in the true blue region at 298 K with photoluminescent λem ranging from 452 to 487 nm in acetonitrile solution, in powder and in PMMA doped thin films. The nature of the emission is a mixed metal-to-ligand/ligand-to-ligand charge transfer state. Photoluminescence (PL) quantum efficiencies both in solution and in the solid state were low while excited state decay kinetics were found to be multiexponential. Each complex undergoes quasi-reversible oxidation and irreversible reduction with large HOMO-LUMO gaps. A detailed computational investigation corroborates the spectroscopic assignments. Additionally, light-emitting electrochemical cells (LEECs) were fabricated for each of the four complexes. The electroluminescence (EL) spectra of all complexes were red-shifted relative to the PL spectra. The LEEC containing 2a is the bluest emitter (λmax = 487 nm) of the family of complexes
Surface Characterization of Organic and Bioinspired Nanoscale Devices
No full textFor the past few decades, the research and industrial application of organic semiconducting materials has been very active. Compared to traditional semiconducting materials, facile chemical modification and processing are advantageous properties of organic electronics. This dissertation focuses on two classes of organic semiconducting devices: light-emitting electrochemical cells (LEECs) and bioinspired nanowires. Organic light-emitting diodes (OLEDs) have emerged in display applications, but not lighting due to high fabrication costs. To achieve high OLED performance at low cost, efforts have focused on light-emitting electrochemical cells (LEECs). LEECs, and particularly iridium LEECs, exhibit substantial efficiency, high luminance, and long lifetime in a simple, solution processable device architecture. Performance is facilitated by the redistribution of ions that assists charge injection. However, the physics of iridium LEECs has not been fully explored, particularly brightness enhancement with lithium additives. Scanning Kelvin Probe Microscopy (SKPM) was used to reveal the surface potential profile of iridium LEEC devices and clarify the effect of lithium addition. We found that ions do not pack densely at the cathode in pristine iridium LEECs devices. Li[PF6] addition produced a doubling of the peak electric field at the cathode from an increase of ionic space charge. This work was the first to clarify the nature of iridium device performance and enhancement from lithium salt additives: the additional mobile cations improves space charge accumulation for improved electron injection.
The second class of devices concerns nanowires, specifically, the DNA-inspired self-assembly of nanoscale electronic devices. There is a need to fabricate nanoscale electronics with high yield and high purity. We created devices based on 20 nm long DNA nanowires incorporating a perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) derivative, an organic semiconductor with dimensions similar to two DNA bases. We synthesized these nanowires by automated DNA phosphoramidite chemistry and purified these wires by high performance liquid chromatography, thus achieving high control and purity of a nanoscale electronic element. We patterned gold nanogap electrodes and assembled the nanowires by gold-thiol self-assembly. Current voltage characterization revealed that the current of perylene nanowires was enhanced 4.4 fold over conventional DNA nanowires. Temperature dependence revealed that the current increased from room temperature up to 35 °C for each type of wire, and then lowered rapidly, consistent with DNA melting. We performed atomic force microscopy imaging studies to observe an instance of a single nanowire spanning a nanogap. This research provides a new approach to fabricate nanoscale devices with lower cost and high yield. Chapter 1 will serve as an introduction to organic semiconductor fundamentals and applications. Chapter 2 will discuss the state-of-the-art lithographic fabrication methods and the progress of molecular electronics, including research with DNA nanowires. Chapter 3 describes our research in performing surface characterization of iridium light-emitting electrochemical cells and the effect of lithium additives. Finally, the construction of bioinspired nanowire devices with the organic semiconductor perylene and associated electronic, thermal, and surface characterization is reported in Chapter 4
Ionic Additives in Light Emitting Electrochemical Cells from Ionic Transition Metal Complexes
No full textA light emitting electrochemical cell is an alternative technology for display and solid state lighting. It can either be based on conjugated polymers or ionic transition metal complexes. The ease in their fabrication process and simplicity of their architecture make them appealing in industrial processing. However, for this technology to be put into practical use, more research efforts are still needed to reduce their long turn on times, and to increase the duration of high luminance. Here we focused our attention in the effect of ionic additives and couterions on the performance of devices made from ionic transition metal complexes. With proper complex engineering and using appropriate additives, we were able to fabricate devices that meet the US DOE benchmark for luminance level with appreciable lifetimes. We applied spectroscopic techniques coupled with simulation studies to probe the origin of the enhancement. By employing these methods, we were able to prove our previous hypothesis that due to the immobility of the positively charged complex emitter in salt-free devices, the packing of accumulated positive ions in the negative electrode is less dense than the accumulation of negative ions in the positive electrode. These accumulations of uncompensated charges near the electrodes are called electric double layers (EDLs). The discrepancy in EDL formation leads to an imbalance of electron and hole injection because the EDLs aid in reducing the width of injection barriers. Addition of ionic additives of appropriate size and concentration remedies the offset and promote balanced densities of charge carriers thereby optimizing the device performance. Adding a higher fraction of ionic additive above the threshold concentration disrupts the balance and adversely impact the performance of the device, presumably due to side reactions such as incomplete dissociation or reassociation of ions. We also found out that the width of the EDLs for optimal device performance should not be too thick for facile injection and transport of charges through the bulk of the device. Large EDL widths induce a very strong Coulomb force that could hinder injected charges from reaching the middle of the device. Our finding also shows that the dielectric constant (ε) has an effect on the device performance and has a connection with the thickness of EDL formed. For materials considered, active layers with lower dielectric constants are favorable in the recombination of hole and electron as lower-ε materials exhibit strong exciton binding energy and greater bimolecular recombination strength; however, they also result in lower film conductivity thereby affecting the turn-on time of the devices. Consequently, as far as the materials considered in the study, a “sweet spot” of the dielectric constant is necessary for optimal device performance. Some simple methods in tuning the value of the dielectric constant as illustrated in our works are through addition of salts or choosing the right negative counterion to complement the complex emitter. As a recommendation, more studies by employing different complexes and additives should be conducted to verify the results mentioned above. In conclusion, careful implementation of the considerations mentioned above must be done to achieve optimal device performance
Electrochemical Measurements on Self-Assembled Monolayers of DNA to Follow Anti-Cancer Drug Activity and Helicase Interactions
No full textBiological study would benefit greatly from techniques that detect cellular processes typically constrained within cells. Electrochemistry using DNA-mediated charge transport enables studies of DNA interactions with proteins and drugs. We developed DNA-based multiplexed chips to mimic cellular environments or operate in cellular extracts. In this dissertation, electrochemical measurements were performed with chips bearing monolayers of DNA to follow anticancer drug activity and DNA-helicase interactions. Many cancer treatments involve DNA damage, and understanding these drugs involves controlling activation pathways and precisely following DNA damage repair. We designed an electrochemical chip of DNA modified electrodes to follow DNA damage, offering benefits over gel electrophoresis assays. We used chips to study the anticancer agent β-lapachone (ß-lap), which generates DNA damaging peroxide in the presence of overexpressed NAD(P)H: quinone oxidoreductase 1 (NQO1), a hallmark of many cancer cells. Initially, ß-lap was studied in a model system reproducing certain pathways of drug activation, DNA damage repair, and drug abrogation. We observed drug-specific changes from these chips and demonstrated a high correlation with the ß-lap-induced redox cycle.Our study revealed significant signal changes at clinically relevant levels and sub-lethal concentrations. Catalase, an enzyme decomposing peroxide, suppressed signal changes under conditions specific to cancer. Thus, this chip-based platform enabled unique tracking of ß-lap-induced DNA damage repair. Subsequently, we followed ß-lap activity in cellular lysates with these devices to correlate cell death activity with DNA damage. Cells were prepared to be proficient or deficient in NQO1 to mimic cancerous and healthy cells. Cells were lysed and added to chips, and β-lap activity was followed by signal changes arising from DNA damage. Devices showed an approximate fourfold difference in electrochemical response to NQO1+ over NQO1− cells, as well as great selectivity to controls deactivating the drug-induced DNA damage pathways. Saturation of DNA damage on the chip correlated with the onset of cell death from viability assays. These devices could be applied for screening of multiple anticancer drugs from small samples to guide cancer treatment.
Xeroderma pigmentosum group B (XPB) is an essential helicase involved in both DNA repair and transcription. Significant characteristics of XPB binding and activity remains to be established. We utilized DNA electrochemistry to sense the DNA-helicase interactions of three distinct XPB helicases. Changes in DNA duplex stability were quantified upon helicase binding. Binding dissociation constants were estimated in the range from 10-50 nM. and ATP-stimulated DNA unwinding activity was followed, revealing distinct modes of operation confirmed by crystal structures. These devices provided a sensitive measure of the structural thermodynamics and kinetics of DNA-helicase interactions. Chapter 1 introduces DNA, electrochemistry, and the specific field of DNA electrochemistry. Chapter 2 relates our research of β-lap in a model system incorporating the drug activation cycle, DNA base-excision repair by a glycosylase, and a drug abrogation pathway. Chapter 3 builds on this study to investigate β-lap activity in cellular lysates with differing concentrations of NQO1 that were proficient or deficient in DNA damaging activity. Chapter 4 describes the DNA binding and unwinding activity of XPB helicases with DNA devices
Enhancement of the Electrical Properties of DNA Molecular Wires through Incorporation of Perylenediimide DNA Base Surrogates
No full textCMMI-1246762; Air Force Office of Scientific Research. Grant Number: FA9550-13-1-0096; Office of Naval Research. Grant Number: N00014-16-1-2741.Due to copyright restrictions and/or publisher's policy full text access from Treasures at UT Dallas is not available. UTD affiliates may be able to acquire a copy by using the link below to contact university Interlibrary Loan.DNA has long been viewed as a promising material for nanoscale electronics, in part due to its well-ordered arrangement of stacked, pi-conjugated base pairs. Within this context, a number of studies have investigated how structural changes, backbone modifications, or artificial base substitutions affect the conductivity of DNA. Herein, we present a comparative study of the electrical properties of both well-matched and perylene-3,4,9,10-tetracarboxylic diimide (PTCDI)-containing DNA molecular wires that bridge nanoscale gold electrodes. By performing current-voltage measurements for such devices, we find that the incorporation of PTCDI DNA base surrogates within our macromolecular constructs leads to an approximately 6-fold enhancement in the observed current levels. Together, these findings suggest that PTCDI DNA base surrogates may enable the preparation of designer DNA-based nanoscale electronic components. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, WeinheimSchool of Natural Sciences and MathematicsErik Jonsson School of Engineering and Computer Scienc
Cationic iridium(III) complexes bearing ancillary 2,5-dipyridyl(pyrazine) (2,5-dpp) and 2,2 ':5 ',2 ''-terpyridine (2,5-tpy) ligands : synthesis, optoelectronic characterization and light-emitting electrochemical cells
No full textFour cationic iridium(III) complexes of the form [Ir(C^N)2(N^N)]+ bearing either a 2,5-dipyridylpyrazine (2,5-dpp) or a 2,2′:5′,2′′-terpyridine (2,5-tpy) ancillary ligand and either 2-phenylpyridine (ppy) or a 2-(2,4-difluorophenyl)-5-methylpyridine (dFMeppy) cyclometalating ligands were synthesized. The optoelectronic properties of all complexes have been fully characterized by UV-visible absorption, cyclic voltammetry and emission spectroscopy. The conclusions drawn from these studies have been corroborated by DFT and TDDFT calculations. The four complexes were assessed as emitters in light-emitting electrochemical cells. Complex 1a, [Ir(ppy)2(2,5-dpp)]PF6, was found to be a deep red emitter (666 nm) both in acetonitrile solution and in the electroluminescent device. Complex 2a, [Ir(ppy)2(2,5-tpy)]PF6 was found to be an orange emitter (604 nm) both in solution and in the LEEC. LEECs incorporating both of these complexes were stable over the course of around 4–6 hours. Complex 1b, [Ir(dFMeppy)2(2,5-dpp)]PF6, was also determined to emit in the orange (605 nm) but with a photoluminescent quantum yield (ΦPL) double that of 2a. Complex 2b, [Ir(dFMeppy)2(2,5-tpy)]PF6 is an extremely bright green emitter (544 nm, 93%). All four complexes exhibited quasireversible electrochemistry and all four complexes phosphoresce from a mixed charge-transfer excited state.Peer reviewe
POD
No full textThis thesis explores the themes which motivate the creation of abstract sculpture, searching for common forms and universal shapes throughout the pieces examined in the thesis. The examples presented were created by other artists and by the author, which include ceramic sculpture and earthenware oxides and glazes
Light Amplification Using Colloidal Quantum Shells Nanoparticles
No full textColloidal semiconductor nanocrystals (NCs), also known as quantum dots (QDs), have been
widely researched for their optical properties, such as size-dependent bandgaps, narrow band
emission, high luminescence, and nonlinearities. However, when electron-hole pairs (excitons)
are confined within the small volume of the dot upon absorption, luminescence is significantly
compromised due to nonradiative Auger recombination. In this study, we introduce novel
colloidal quantum shells (QSs) with an “inverted” QD geometry, which effectively suppress
Auger decay. Transient Absorption (TA) analysis demonstrates the successful suppression
of biexciton (BX) and multiexciton (MX) Auger recombination, resulting in extended gain
lifetimes. By integrating the QSs with a photonic crystal (PhC) cavity with variable array
periods, we achieved tunable lasing with a near-record low lasing threshold. Our results
closely agree with theoretical model predictions, marking a significant advancement in the
development of colloidal nanocrystal lasers
Ultrafast Charge Recombination Mechanism in Single 0D All-inorganic Perovskite Nanocrystals
No full textNanoscale semiconductors possess many attractive properties as compared to bulk counterparts. Main advantages are tunable energy levels, size-controlled carrier-carrier interactions
and, often times, facile synthesis methods offering broad possibilities in optoelectronic applications. A rigorous understanding of the elemental physical properties in these nanos-
tructures would provide deep insights for designing and improving the quality of materials.
Among many spectroscopic detection methods, single particle spectroscopy is a powerful
and sensitive tool for understanding interactions of multiple charge-carrier species in a wide
range of materials, from bulk crystals down to individual nanoparticles . In this work, we
study various types of so-called “0D” cesium-based perovskite nanocrystals, whose photo
and environmental stability has been reported to exceed the conventional 3D perovskites
nanocrystals. Due to specific, nearly complete isolation of halide octahedra in 0D structures,
their optical properties bear strong resemblance to molecular-type defects, necessitating use
of single particle detection methods. The following work concerns with perovskites’ ultrafast
charge-carrier dynamics, stability and change of photoluminescence quantum yield (PLQY)
inferred from single particle blinking signatures and modified by application of nanoscale
alumina layers via the atomic layer deposition (ALD) encapsulation
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