Browsing by Subject "Nanocrystal"

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Assembling Inorganic Nanocrystal Gels
(2021-12-10) Green, Allison M.; Ofosu, Charles K.; Kang, Jiho; Anslyn, Eric V.; Truskett, Thomas M,; Milliron, Delia J.
Inorganic nanocrystal gels retain distinct properties of individual nanocrystals while offering tunable, network structure-dependent characteristics. We review different mechanisms for assembling gels from colloidal nanocrystals including (1) controlled destabilization, (2) direct bridging, (3) depletion, as well as linking mediated by (4) coordination bonding or (5) dynamic covalent bonding, and we highlight how each impacts gel properties. These approaches use nanocrystal surface chemistry or the addition of small molecules to mediate inter-nanocrystal attractions. Each method offers advantages in terms of gel stability, reversibility, or tunability and presents new opportunities for design of reconfigurable materials and fueled assemblies.
Assembly of colloidal nanocrystals into phospholipid structures and photothermal materials
(2012-08) Rasch, Michael; Korgel, Brian Allan, 1969-
There has been growing interest in developing colloidal metal and semiconductor nanocrystals as biomedical imaging contrast agents and therapeutics, since light excitation can cause the nanocrystals to fluoresce or heat up. Recent advances in synthetic chemistry produced fluorescent 2-4 nm diameter silicon and 1-2 nm diaemeter CuInSSe nanocrystals, as well as 16 nm diameter copper selenide (Cu₂₋[subscript x]Se) nanocrystals exhibiting strong absorbance of near infrared light suitable for biomedical applications. However, the syntheses yield nanocrystals that are stabilized by an adsorbed layer of hydrocarbons, making the nanocrystals hydrophobic and non-dispersible in aqueous solution. Encapsulating these nanocrystals in amphiphilic polymer micelles enables the nanocrystals to disperse in water. Subsequently, the Si nanocrystals were injected into tissue to demonstrate fluorescence imaging, the photothermal transduction efficiency of copper selenide nanocrystals was characterized in water, and the copper selenide nanocrystals were used enhance the photothermal destruction of cancer cells in vitro. The polymer-encapsulated copper selenide nanocrystals were found to have higher photothermal transduction efficiency than 140 nm diameter Au nanoshells, which have been widely investigated for photothermal therapy. Combining the optical properties of metal and semiconductor nanocrystals with the drug-carrying capability of lipid vesicles has received attention lately since it may create a nanomaterial capable of performing simultaneous drug delivery, optical contrast enhancement, and photo-induced therapy. Hydrophobic, dodecanethiol-coated Au nanocrystals were dispersed in water with phosphatidylcholine lipids and characterized using cryo transmission electron microscopy. 1.8 nm diameter Au nanocrystals completely load the bilayer of unsaturated lipid vesicles when the vesicles contain residual chloroform, and without chloroform the nanocrystals do not incorporate into the vesicle bilayer. 1.8 nm Au nanocrystals dispersed in water with saturated lipids to form lipid-coated nanocrystal agglomerates, which sometimes adhered to vesicles, and the shape of the agglomerates varied from linear nanocrystal chains, to flat sheets, to spherical clusters as the lipid fatty acid length was increased from 12 to 18 carbons. Including squalene formed lipid-stabilized emulsion droplets which were fully loaded with the Au nanocrystals. Results with 4.1 nm Au and 2-3 nm diameter Si nanocrystals were similar, but these nanocrystals could not completely load the bilayers of unsaturated lipids.
Colorimetric quantification of linking in thermoreversible nanocrystal gel assemblies
(American Association for the Advancement of Science, 2022-02-18) Kang, Jiho; Valenzuela, Stephanie A.; Lin, Emily Y.; Dominguez, Manuel N.; Sherman, Zachary M.; Truskett, Thomas M.; Anslyn, Eric V.; Milliron, Delia J.
Nanocrystal gels can be responsive, tunable materials, but designing their structure and properties is challenging. By using reversibly bonded molecular linkers, gelation can be realized under conditions predicted by thermody- namics. However, simulations have offered the only microscopic insights, with no experimental means to monitor linking leading to gelation. We introduce a metal coordination linkage with a distinct optical signature allowing us to quantify linking in situ and establish structural and thermodynamic bases for assembly. Because of coupling between linked indium tin oxide nanocrystals, their infrared absorption shifts abruptly at a chemically tunable gelation temperature. We quantify bonding spectroscopically and use molecular simulation to understand temperature-dependent bonding motifs, revealing that gel formation is governed by reaching a critical number of effective links that extend the nanocrystal network. Microscopic insights from our colorimetric linking chemistry enable switchable gels based on thermodynamic principles, opening the door to rational design of programmable nanocrystal networks.
Design and assembly of metal oxide nanocrystal gels via depletion attractions
(2020-04-14) Saez Cabezas, Camila A.; Milliron, Delia (Delia Jane); Truskett, Thomas Michael, 1973-; Lynd, Nathaniel A; Humphrey, Simon M
Achieving and implementing macroscopic materials capable of displaying the unique properties inherent to inorganic nanocrystals requires bridging the nanoscale and the length scales of larger orders of magnitude in a systematic, controllable, and scalable way. Over the last 15 years, nanocrystal gels have been developed and investigated as potential materials to tackle this need. However, available gelation methods rely on chemical reactions and interactions specific to the stabilizing molecules on the nanocrystal surface, and are therefore not readily adaptable across broad types of materials. Specifically, most studies have focused on gelation methods for metal chalcogenides and noble metals, whereas progress on metal oxide nanocrystal gels has lagged behind. This dissertation investigates and demonstrates an alternative gelation method based on entropic depletion attractions that are not dependent on specific surface chemistries and have not been explored in nanoscale colloidal gels. In the first study, a proof of concept system is developed, where depletion attractions induce the gelation of tin-doped indium oxide nanocrystals in the presence of a polymer depletant and achieve a macroscopic material with optical properties reflective of both the microstructure and the nanostructured building blocks. The mechanism of gelation is assessed by comparing the observed phase behavior to theoretical predictions and the microstructure is characterized by small-angle X-ray scattering (SAXS). Next, the universal applicability of depletion attractions is demonstrated by varying the composition and shape of the building blocks while fixing size and nanocrystal volume fraction. The gelation of spherical nanocrystals occurs at the same depletant concentration and this phase transition threshold does not depend on the specific composition of the metal oxide nanocrystal. Consistent with theoretical phase boundary calculations, cubic nanocrystals form gels at a lower depletant concentration than spherical nanocrystals due to the ability to pack face-to-face and therefore increase the overlap excluded volume during assembly. Finally, a method to polymer-wrap tin-doped indium oxide nanocrystals in a controllable way while maintaining colloidal stability is investigated in an effort to tune the physicochemical properties of the metal oxide building blocks available for gelation.
Designing the semiconductor nanocrystal to molecule interface for energy conversion
(2021-05-07) Raulerson, Emily Kate; Roberts, Sean T.; Neale, Nathan R; Rose, Michael J; Milliron, Delia J; Korgel, Brian A
Nanoscale confinement extends the desirable properties of inorganic semiconductors for electronics and energy applications by adding synthetic tunability and solution processability. Controlling for size via simple reaction parameters can change the energy and properties in a way that bulk semiconductors cannot reach, and produces inks of nanocrystals, making processing large solids facile via printing methods. However, these synthetic routes use an insulating layer of organic ligands at the interface of each of these inorganic nanocrystals, diminishing conductivity in nanocrystal solids—one of the largest barriers to adoption of these technologies. To circumvent this problem, the insulating ligands are often removed or exchanged for other ligands which restore conductivity or provide new capabilities. This process of ligand exchange is heterogeneous, complex, and system- dependent. This dissertation focuses on the interface of semiconductor nanocrystals with functional organic ligands via ultrafast spectroscopy and electrochemical methods to understand how ligand exchange alters the electronic properties of the combined system. System parameters can be tuned synthetically and investigated with transient absorption and photocurrent measurements in order to pin down the electronic dynamics in each of these systems. Transient absorption shows the change in the light absorption of a material over short time periods and thus gives information about migration of energy or charge in these materials, while photocurrent measurements correlate microscopic parameters with device-like performance. The overarching goal of this dissertation is to understand the effect of ligand functionalization on the single nanocrystal and extend that to the nanocrystal solid. Specifically of interest are infrared absorbing nanocrystals that can be used to improve the efficiency of solar energy harvesting by capturing more of the incident solar spectrum.
Development of solution processed, flexible, CuInSe₂ nanocrystal solar cells
(2018-01-23) Voggu, Vikas Reddy; Korgel, Brian Allan, 1969-; Ekerdt, John G; Milliron, Delia; Truskett, Thomas; Vanden Bout, David A
Clean sources of energy, especially photovoltaics (PVs), are urgently needed to cope with global energy shortage and environmental pollution. For PVs to play a significant role in energy production, the current prices must be brought down. Thin film PVs made using layered Mo or Au/CuInGaSe₂(CIGS)/CdS/ZnO/ITO have already shown high efficiencies. Traditionally, most layers in CIGS solar cells are deposited using high-cost techniques requiring high temperatures and ultra-low pressures. By replacing the traditionally processed CIGS with a nanocrystal layer that can be deposited at mild processing conditions, the fabrication cost can be reduced. In this study, a high yielding synthesis method for CuInSe₂ nanocrystals has been developed which gives the best efficiency (3.1%), so far, for low-temperature processed CuInSe₂ nanocrystal PVs. An important challenge that nanocrystal solar cells currently face is low device efficiency, resulting in higher operating cost. CuInSe₂ nanocrystals can remain suspended in solution because of the long chain organic ligands attached to the surface. However, these ligands hinder charge transfer between nanocrystals causing low device efficiency. These ligands have been successfully replaced with smaller sulfide ions thereby improving the best efficiency of low-temperature processed CuInSe₂ nanocrystal solar cells from 3.1 % to 3.5%. Another approach to reducing the cost of CuInSe₂ PVs is by replacing the glass support medium with cheaper alternatives like paper. Flexible CuInSe₂ nanocrystal solar cells are successfully fabricated on paper with efficiencies reaching up to 2.25%. This is the first time a nanocrystal solar cell has been fabricated on paper. There is no significant loss in PV device performance after more than 100 flexes to 5 mm radius, and the devices continue to perform when folded into a crease. Apart from the absorber layer, the replacement of other high-temperature and vacuum processed device layers with ambient solution-processed layers lowers the manufacturing cost. This has been achieved by spin coating suitable nanomaterials as device layers. Lastly, for commercialization of CuInSe₂ nanocrystal solar cells, multiple devices need to be connected to achieve the desired current and voltage. A fabrication process has been developed for building multiple nanocrystal PVs on a single substrate using 3D printed masks
In Situ Optical Quantification of Extracellular Electron Transfer Using Plasmonic Metal Oxide Nanocrystals**
(John Wiley & Sons, 2021-11-11) Graham, Austin J.; Gibbs, Stephen L.; Saez Cabezas, Camila A.; Wang, Yongdan; Green, Allison M.; Milliron, Delia J.; Keitz, Benjamin K.
Extracellular electron transfer (EET) is a critical form of microbial metabolism that enables respiration on a variety of inorganic substrates, including metal oxides. However, quantifying current generated by electroactive bacteria has been predominately limited to biofilms formed on electrodes. To address this, we developed a platform for quantifying EET flux from cell suspensions using aqueous dispersions of infrared plasmonic tin-doped indium oxide nanocrystals. Tracking the change in optical extinction during electron transfer enabled quantification of current generated by planktonic Shewanella oneidensis cultures. Using this method, we differentiated between starved and actively respiring cells, cells of varying genotype, and cells engineered to differentially express a key EET gene using an inducible genetic circuit. Overall, our results validate the utility of colloidally stable plasmonic metal oxide nanocrystals as quantitative biosensors in aqueous environments and contribute to a fundamental understanding of planktonic S. oneidensis electrophysiology using simple in situ spectroscopy.
Interaction between structural and electronic phase changes of metal oxide semiconductor nanocrystals
(2017-08-09) Dahlman, Clayton John; Milliron, Delia (Delia Jane); Korgel, Brian A; Mullins, Charles B; Willson, Carlton G; Henkelman, Graeme
Semiconducting metal oxides have emerged as a core class of materials in functional electronic devices because of their versatile compositions and tunable electronic and optical properties. Applying a charge to metal oxides can modulate carrier properties and induce structural changes from charge-compensating defects. However, charge-mediated transformations are contingent upon efficient transport of carriers, compensating species, or field biases into the bulk. Nanostructured materials, including colloidal metal oxide nanocrystals, can accommodate efficient charge transport across the semiconductor interface, and exhibit sensitive optical and electronic properties that arise from their nanoscale geometry. This dissertation studies the relationship between charge-mediated electronic and structural phase changes in metal oxide nanocrystals, and correlates these transformations with their nanoscale geometry and interfacial environment. The first investigation studies anatase TiO₂ nanocrystals during electrochemical charging. TiO₂ nanocrystal films can undergo two independent charging processes within a Li-ion electrolyte: surface capacitance, which raises the Fermi level upon reduction and induces Drude-like infrared localized surface plasmon resonance without affecting structure, and intercalative charging caused by the insertion of Li⁺ into the nanocrystal lattice. These two charging processes create independent dual-spectrum visible (Li-ion intercalation) and infrared (plasmon resonance) optical responses to applied bias, with applications for versatile electrochromic smart windows. The optical and electrochemical properties of both charging mechanisms are isolated and studied independently to examine the role of structure and interfacial environments on these transformations. The second part of this dissertation explores charge-mediated transformations in nanocrystalline VO₂, which has a highly non-ideal, charge-correlated electronic structure. A charge-mediated electrochemical insulator to metal transformation in VO₂ is found to be highly sensitive to nanoscale grain size, leading to a secondary metal-insulator transformation for sufficiently confined particles. The results of these studies establish general principles to control the interplay between defect-mediated structural transformations, ideal semiconductor gating behavior and interfacial environments in metal oxide nanocrystals.
Intrinsic Optical and Electronic Properties from Quantitative Analysis of Plasmonic Semiconductor Nanocrystal Ensemble Optical Extinction
(2020) Gibbs, Stephen L.; Staller, Corey M.; Agrawal, Ankit; Johns, Robert W.; Saez Cabezas, Camila A.; Milliron Delia J.
The optical extinction spectra arising from localized surface plasmon resonance in doped semiconductor nanocrystals (NCs) have intensities and lineshapes determined by free charge carrier concentrations and the various mechanisms for damping the oscillation of those free carriers. However, these intrinsic properties are convoluted by heterogeneous broadening when measuring spectra of ensembles. We reveal that the traditional Drude approximation is not equipped to fit spectra from a heterogeneous ensemble of doped semiconductor NCs and produces fit results that violate Mie scattering theory. The heterogeneous ensemble Drude approximation (HEDA) model rectifies this issue by accounting for ensemble heterogeneity and near-surface depletion. The HEDA model is applied to tin-doped indium oxide NCs for a range of sizes and doping levels but we expect it can be employed for any isotropic plasmonic particles in the quasistatic regime. It captures individual NC optical properties and their contributions to the ensemble spectra thereby enabling the analysis of intrinsic NC properties from an ensemble measurement. Quality factors for the average NC in each ensemble are quantified and found to be notably higher than those of the ensemble. Carrier mobility and conductivity derived from HEDA fits matches that measured in the bulk thin film literature.
Low cost processing of CuInSe2 nanocrystals for photovoltaic devices
(2015-05) Stolle, Carl Jackson; Korgel, Brian Allan, 1969-; Mullins, Charles B; Manthiram, Arumugam; Vanden Bout, David A; Markert, John T
Semiconductor nanocrystal-based photovoltaics are an interesting new technology with the potential to achieve high efficiencies at low cost. CuInSe2 nanocrystals have been synthesized in solution using arrested precipitation and dispersed in solvent to form a “solar ink”. The inks have been deposited under ambient conditions to fabricate photovoltaic devices with efficiency up to 3%. Despite the low cost spray coating deposition technique, device efficiencies remain too low for commercialization. Higher efficiencies up to 7% have been achieved using a high temperature selenization process, but this process is too expensive. New nanocrystal film treatment processes are necessary which can improve the device efficiency at low cost. To this end, CuInSe2 nanocrystals were synthesized using a diphenyl phosphine:Se precursor which allows for precise control over the nanocrystal size. The size is controlled by changing the temperature of the reaction. The smallest size nanocrystals demonstrated extremely high device open circuit voltage. Ligand exchange procedures were used to replace the insulating oleylamine capping ligand used during synthesis with more conductive halide ions or inorganic chalcogenidometallate cluster (ChaM) ligands. These ligands led to improved charge transport in the nanocrystal films. A high-intensity pulsed light processing technique known as photonic curing was used which allows for high temperature sintering of nanocrystal films on temperature-sensitive substrates. High energy pulses cause the nanocrystals to sinter into large grains, primarily through melting and resolidification. The choice of metal back contact has a dramatic effect on the final film morphology, with Au and MoSe2 back contacts providing much better adhesion with the CuInSe2 than Mo back contacts. Nanocrystal sintering without melting can be achieved by replacing the oleylamine ligands with ChaM ligands prior to photonic curing. Low energy photonic curing pulses vaporize the oleylamine ligands without inducing sintering or grain growth. This greatly improved nanocrystal coupling and interparticle charge transport. Multiexcitons were successfully extracted from these nanocrystal films and external quantum efficiencies over 100% were observed. Transient absorption spectroscopy was used to study the multiexciton generation process in CuInSe2 nanocrystal films and colloidal suspensions. The multiexciton generation efficiency, threshold, and Auger lifetimes for CuInSe2 compare well with other nanocrystal materials.
Photoluminescence and stability of perovskite-phase CsPbI₃ nanocrystals and development of nickel metal-organic decomposition inks
(2022-12-02) Abney, Michael Keith; Korgel, Brian Allan, 1969-; Ekerdt, John G; Milliron, Delia J; Vanden Bout, David A
Lead halide perovskite nanocrystals (LHP NCs) exhibit interesting and exceptional optical properties such as near-unity photoluminescence (PL) quantum yield and narrow PL emission line widths. As a result, LHP NCs have demonstrated promising potential in an array of optoelectronic devices such as photovoltaics (PVs), light-emitting diodes (LEDs), optical detectors, and lasers. For CsPbI₃ NCs in particular, one of the major obstacles to their commercial success is structural instability of the perovskite phase, which must be managed. Here, a closer look is taken at the PL emission of CsPbI₃ NC films and how it is affected by light and environmental conditions. The thermal phase stability of CsPbI₃ and the impact of surface area and composition are also investigated. Light-induced changes in photophysical and electronic properties in LHPs can affect their performance in device applications. It is revealed that light excitation induces a slow, reversible enhancement in PL lifetime and intensity in films of perovskite-phase CsPbI₃ NCs. Placing the films under vacuum or nitrogen for several minutes was also found to increase the PL lifetime and intensity. A model of slow, humidity and light-sensitive surface states in CsPbI₃ NCs is proposed. Perovskite-phase CsPbI₃ nanocrystals convert to the optically inactive δ-phase at elevated temperature. Alloying with Br was found to improve the phase stability when the films were relatively thin. Films of mixed-halide CsPbI₂.₅Br₀.₅ nanocrystals less than 30 nm thick showed no conversion to the δ-phase even after 1 hour of heating at 250°C, while thicker films still reverted to the δ-phase after heating. These results show that compositional changes and film thickness can have a significant, cumulative effect on the stability of perovskite nanocrystal films. The role of surface area/energy in CsPbI₃ nanocrystal stability is discussed. CsPbI₃ NC-based solar cells were successfully fabricated. This inspired collaborative work on the development of printable, conductive metal-organic decomposition (MOD) inks, which could play a role in the contacts of perovskite PV for low-cost, flexible, and low-temperature applications. Early development and characterization of a screen-printable, air-curable Ni-based MOD ink is detailed. Current performance of this ink is limited by residual carbon contamination, which is a focus of further development.
Structural and chemical characterization of responsive nanocrystalline materials
(2021-01-21) Reimnitz, Lauren Christine; Milliron, Delia, (Delia Jane); Goodenough, John B.; Henkelman, Graeme A.; Hwang, Gyeong S.; Korgel, Brian A.
Nanocrystalline materials have interesting applications in many technological arenas, including catalysis, smart windows, and sensing. These crystals, with characteristic dimensions on the order of hundreds of nanometers or less, offer some key advantages over other materials for approaching these technologies. For heterogeneous catalysis, this small characteristic dimension translates to a very high surface-area-to-volume ratio. This extremely high specific surface area offers more chemically active sites than the same mass of material in a bulk form. Nanocrystals (NCs) can also exhibit unique chemical and physical properties in their very small form. For example, NCs of metallic materials can exhibit localized surface plasmon resonance (LSPR), a physical phenomenon not seen for bulk metals, that can make the material more useful for chemical sensing and electronic applications. Because they can be synthesized and processed using colloidal methods, NCs have also been investigated and employed as smart window materials, where solution-based processing keeps the cost of manufacturing window coatings much less expensive than the alternative low-pressure, high-temperature methods. During my PhD, I have had the privilege of working on many classes of nanocrystalline materials, with a broad variety of potential applications. In chapter 1 of this dissertation I describe my work synthesizing vanadium sesquioxide (V2O3) NCs doped with transition metal ions. These NCs reversibly absorb oxygen, with a remarkably low oxygen uptake onset temperature, which we propose for use in solving the cold start problem in automotive catalysis. Dopants added to the NC increase the initial oxygen storage capacity of the material, and have a substantial effect on the degradation of the material over ten oxidation and reduction cycles. In chapter 2, I discuss my collaboration with researchers from Brian Korgel's group. In one such collaboration, the reversible thermochromic phase transition of nickel iodide specific to thin films is explored. This thermochromism is found to be deliquescent, meaning it depends on the availability of humid air. These two properties give the material potential applications in both smart windows, where the phase transition causes a controllable color change on a window surface, and in humidity sensing, where the color change would indicate the presence of air with a relative humidity surpassing the critical point of the material. Using the same in situ heating techniques, the irreversible phase transition of perovskite cesium lead iodide (CsPbI3) NCs from the metastable black CsPbI3 phase to the yellow delta-orthorhombic phase is found to occur upon heating the NC films. This transition is destructive to the material, since the metastable gamma-phase has excellent electronic structure for photovoltaic applications, while the thermodynamic delta-orthorhomic phase is inactive. We found that this irreversible transition depended weakly on the humidity, but also proceeded when heated in dry nitrogen environments. In chapter 3, the design of a transparent conductive composite material is described. The goal material will be designed and engineered to be applied to the outside surface of aircraft windshields, where the conductive overlayer can shuttle charge built up by friction with ice and dust within the atmosphere to the aircraft body. This will prevent the build up of static charge on the windshield, which can interfere with electronics onboard the aircraft and even cause dielectric fracture of the windshield in severe cases. By incorporating cerium-doped indium oxide NCs and the conducting polymer PEDOT:PSS into an insulating, mechanically robust polymer matrix, we intend to create a thick material with high enough conductivity to be suitable for this anti-static application. By incorporating cerium-doped indium oxide NCs, the visible transparency of the resulting coating can be kept high despite the presence of visibly dark PEDOT:PSS. Lastly, in chapter 4, I describe my supporting contributions to work in my research group involving indium oxide and tin-doped indium oxide NCs. Chapter 4 is broken into three parts. First, the independent synthetic control of size and shape of indium oxide NCs was achieved through the addition of spectating alkali ions Na+ and K+. Since size and shape are critical parameters for controlling the sensing, catalytic, and assembly properties of the resulting NCs, this is an enabling discovery for many studies in the future. Next, I describe my contributions to the demonstration of depletion effects in degenerately doped tin-doped indium oxide (Sn:In2O3) NCs. A well-known effect in other semiconducting materials, depletion causes noticeable changes in the LSPR that can be controlled using electrochemical charging, and depends strongly on the NCs size and dopant concentration. In the last part of chapter 4, I discuss my work using X-ray photoemission spectroscopy (XPS) to evaluate Sn:In2O3 NCs before and after use in electrochemical CO2 reduction. This reaction has received interest for its ability to convert the greenhouse gas CO2 into more industrially relevant chemicals. Sn:In2O3 NCs show high selectivity for formate and CO, and our analysis show the NCs are stable during the electrochemical reaction, with no change in particle size or dopant concentration. This dissertation describes work on a broad range of nanocrystalline materials, including metal oxides, inorganic perovskites, and nickel iodide thin films. In evaluating their characteristics for their varied applications, I gained experience with a broad range of analytical tools, including electron microscopy, optical microscopy, UV-visible spectroscopy, and X-ray diffraction, among others. With an eye toward technological applications, fundamental studies help de fine the properties and limitations of the materials at our disposal for solving a wide range of problems.
Studies of singlet exciton fission in perylenediimide films and triplet exciton transfer at organic:inorganic interfaces
(2021-01-27) Bender, Jon Alexander; Roberts, Sean T.; Rose, Michael J; Biaz, Carlos R; Milliron, Delia J; Vanden Bout, David A
Herein, I showcase three studies that have formed the backbone of my work at UT Austin. First, we studied singlet exciton fission (SF) in a common perylenediimide (PDI) derivative often used as a molecular organic semiconductor, C8-PDI, using pump-probe spectroscopy to model SF and singlet-singlet annihilation (SSA). Therein, we were surprised to report a SF rate orders of magnitude slower than predicted by computational studies of PDI dimers. In Chapter 3, we study a suite of six PDI derivatives that adopt different crystal structures to further assess the nature of SF in polycrystalline films grown on sapphire and fused-quartz. We developed a kinetic model across the PDI series based on the analysis of time-resolved photoluminescence, free from SSA contributions to kinetics that must otherwise be carefully modelled for TA data of these films. We confirm that Redfield theoretic approximations of the SF rate in these materials better captures the trend in kinetics implying the important role the charge-transfer character in these excitons play in mediating the process. The variation in intermolecular organization and associated changes in the Coulombic and exchange coupling between nearest-neighbor molecules correctly captures a qualitative trend in the observed SF rate, though the observed rates are an order of magnitude smaller than expected. We propose this discrepancy arises because the PDI dimer model we use for our predictive model for polycrystalline PDI thin films neglects changes in the excited state character/energetics that become important in strongly interacting molecular solids. The contents of Chapter 4 are then a study of colloidal suspensions of PbS nanocrystals (NC) decorated with a TIPS-pentacene ligand, 2-CP. We set out to search for evidence of the formation of spin-triplet excitons on the 2-CP ligands after photoexcitation of the PbS NC. Triplet exciton formation is observed with no clear observation of an intermediate charge separated species. However, an intermediary state is observed and carefully assigned to a surface associated state on the PbS NC. This hypothesis is further supported by the presence of a multitude of triplet excited states found in constrained DFT computations and fluence dependent pump-probe data providing evidence for the photopassivation the intermediate surface state. In totality, my studies have elucidated excited state dynamics in singlet fission capable polycrystalline films of perylenediimide molecules and contributed to our growing understanding of triplet exciton transfer between PbS NCs and molecular ligands.
Synthesis and characterization of germanium-based nanocrystals
(2021-04-06) Kim, Hyun Gyung; Korgel, Brian Allan, 1969-; Mullins, Charles B.; Delia, Milliron; Roberts, Sean T.
Approaches to colloidal synthesis have rapidly developed to control the size, shape, and composition of various semiconductors, offering cost reductions, controllability, and scalability. Of semiconductor materials, germanium nanomaterials are known to be the most difficult to synthesize in solution-based methods because of their high crystallization temperature. Zero-dimensional germanium nanocrystals were synthesized by the heat-up method, without any strong reducing agent. Subsequently, finely controlled size-selective precipitation narrowed size distributions, and size-selected nanocrystals successfully created a monolayer germanium nanocrystals superlattice. One-dimensional germanium nanorods were synthesized by the solution–liquid–solid method using tin nanoparticles as seeds. By forming a liquid alloy with the tin seed at the eutectic temperature, which is much lower than the crystallization temperature, germanium nanorods were grown from the tin seed. A monophenylsilane enhanced the yield of germanium nanorods by promoting the phenyl redistribution of diphenylgermane, a germanium precursor. Using a mixture of HCl and HF, tin seeds were completely removed from the tips of the germanium nanorods, leaving germanium crystalline nanorods. Nonvolatile memories, a key component in various electronics and portable systems, include phase-change memory, a leading technology that has seen exponential growth in demand over the last decade. One important class of phase change materials are compounds on the GeTe–Sb2Te3 tie line. Despite interesting properties of the nanomaterials, colloidal synthesis of phase change material nanocrystals has only been rarely reported. In the present study, three representative phase change material nanocrystals, GeTe, Sb2Te3, and Ge2Sb2Te5, were successfully synthesized using the hot-injection method. A poly(vinylpyrrolidinone)–hexadecane (PVP–HDE) polymer was essential for the nanocrystal dispersion and making ternary Ge2Sb2Te5 nanocrystals. Two solvents, oleylamine and trioctylphosphine, were studied for synthesizing all three nanocrystals and reveal the conversion chemistry of phase change material precursors.
The assembly of inorganic nanocrystals using dynamic covalent chemistry
(2022-05) Dominguez, Manuel Nicholas; Milliron, Delia (Delia Jane); Truskett, Thomas M; Anslyn, Eric V; Rose, Michael
Here I show the work accomplished in my graduate career on metal oxide nanocrystals and their assembly into unique gels. Nanocrystals were synthesized using a well-established method that was adapted to improve subsequent functionalization. Collaborations between the Anslyn group led to ligands that were designed for tunable orthogonal covalent linkages between particles. Establishing a well solid foundation for controllable assembly through this collaboration was the goal, along with a better understanding of the system through collaboration with the Truskett lab and the work they do on simulations and theory. Applying what was learned and theorized into studying differences in similar systems for changes to the optical response also became a goal of the work presented. All put together an understanding of the building blocks of assembly, the assembly method, and finally the properties of the assembly are discussed within.