Browsing by Subject "Ultrafast"
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Item Excited state dynamics of metalloporphyrins utilized in optoelectronic devices(2013-08) Hinke, Jonathan ArthurEnergy consumption in the world is currently dominated by fossil fuels (85%) which include coal, gas, and oil while photovoltaics constitute a small portion (0.1%). The hotovoltaic market is primarily comprised of silicon based photovoltaics which are currently unable to compete with fossil fuels in cost per kilowatt hour. However, emerging organic photovoltaics (OPVs) have shown potential to be surpass silicon based photovoltaics and be cost competitive with fossil fuels. One of the limitations in OPVs is the short diffusion length (10 nm) relative to the absorbing layer thickness (100-200 nm). Porphyrins, of which chlorophylls are derivatives, remain at the forefront of OPV investigation due to their success in natural photosynthesis and potential in photovoltaic devices. Furthermore, platinum octaethyl porphyrin (PtOEP) has been estimated to have a diusion length between 18-30 nm and long triplet lifetime (100 microsecondss). This long diffusion length indicates that platinum porphyrins are able to efficiently funnel excitons to the interface, showing promise as suitable donor materials. Other porphyrins, such as nickel, palladium, tin, and indium show similar properties including strong absorption, enhanced excited state lifetimes, and charge separated states. This thesis investigates the excited state properties of porphyrin materials. Ultrafast pump probe spectroscopy allows for investigation of excited state dynamics including intramolecular energy transfer observed in nickel porphyrins. Femtosecond dynamics of palladium and platinum porphyrins are explored as well as triplet fusion in PtOEP neat films, providing a unique way to study energy transfer and amorphous films. Finally, pump probe studies aim to explain photoluminescent quenching behavior in tin and indium porphyrins through observation of charge separated states. Investigation of these porphyrins is crucial to improving device efficiency through fundamental understanding of the excited state dynamics in films and neat films.Item The Investigation Of New Magnetic Materials And Their Phenomena Using Ultrafast Fresnel Transmission Electron Microscopy(2017-02) Schliep, KarlState-of-the-art technology drives scientific progress, pushing the boundaries of our current understanding of fundamental processes and mechanisms. Our continual scientific advancement is hindered only by what we can observe and experimentally verify; thus, it is reasonable to assert that instrument development and improvement is the cornerstone for technological and intellectual growth. For example, the invention of transmission electron microscopy (TEM) allowed us to observe nanoscale phenomena for the first time in the 1930s and even now it is invaluable in the development of smaller, faster electronics. As we uncover more about the fundamentals of nanoscale phenomena, we have realized that images alone reveal only a snapshot of the story; to continue progressing we need a way to observe the entire scene unfold (e.g. how defects affect the flow of current across a transistor or how thermal energy propagates in nanoscale systems like graphene). Recently, by combining the spatial resolution of a TEM with the temporal resolution of ultrafast lasers, ultrafast electron microscopy or microscope (UEM) has allowed us to simultaneously observe transient nanoscale phenomena at ultrafast timescales. Ultrafast characterization techniques allow for the investigation of a new realm of previously unseen phenomenon inherent to the transient electronic, magnetic, and structural properties of materials. However, despite the progress made in ultrafast techniques, capturing the nanoscale spatial sub-ns temporal mechanisms and phenomenon at play in magnetic materials (especially during the operation of magnetic devices) has only recently become possible using UEM. With only a handful of instruments available, magnetic characterization using UEM is far from commonplace and any advances made are sparsely reported, and further, specific to the individual instrument. In this dissertation, I outline the development of novel magnetic materials and the establishment of a UEM lab at the University of Minnesota and how I explored the application of it toward the investigation of magnetic materials. In my discussion of UEM, I have made a concerted effort to highlight the unique challenges faced when getting a UEM lab running so that new researchers may circumvent these challenges. Of note in my graduate studies, I assisted in the development of three different magnetic material systems, strained Fe nanoparticles for permanent magnetic applications, FePd for applications in spintronic devices, and a rare-earth transition-metal (RE-TM) alloy that exhibits new magneto-optic phenomena. In studying the morphological and magnetic effects of lasers on these RE-TM alloys using the in situ laser irradiation capabilities of UEM along with standard TEM techniques and computational modeling, I uncovered a possible limitation in their utility for memory applications. Furthermore, with the aid of particle tracing software, I was able to optimize our UEM system for magnetic imaging and demonstrate the resolution of ultrafast demagnetization using UEM.Item Strongly-Bound Excitons In Transition Metal Dichalcogenides And Organic Semiconductors(2020-05) Schulzetenberg, AaronAtomically-thin, semiconducting transition metal dichalcogenides (TMDs) and organic semiconductors such as rubrene hold exceptional promise for unique and niche electronic applications which cannot be solved with conventional semiconducting crystalline materials. In particular, the process by which excitons relax in thin TMDs controls device engineering considerations including charge carrier mobility and exciton diffusion length. The decay mechanism and time scales can critically depend on interfaces, method of sample preparation and temperature. Here, I present ultrafast transient reflectivity studies of several chemical vapor deposition (CVD) grown TMD structures, including few-layer 2H MoTe2 on SiO2, MoTe2 1T’-2H homojunctions and monolayer MoS2-WS2 lateral heterojunctions on sapphire. The transient reflectivity of CVD-grown, few-layer (5-10 layers) 2H MoTe2 carried out a both room temperature and cryogenic temperatures demonstrates a temperature and fluence dependence consistent with defect-mediated exciton decay. The optical properties of MoTe2 were additionally found to be stable over the course of 8 months air exposure. The biexponential decay dynamics of monolayer MoS2 and WS2 were shown to be consistent with previous investigations. Both studies of interfaces, including the 2H-1T’ MoTe2 homojunctions and the MoS2-WS2 heterojunction were unable to observe signatures of interfacial charge transfer due to lack of sufficient spatial resolution near the interface crossover. In addition to studies on TMDs, the low-wavenumber Raman modes of both isotopically substituted 13C Rubrene and those of a structural analog to rubrene, fm-rubrene, were measured and compared to native rubrene. The 13C rubrene demonstrated a uniform shift to lower energy intermolecular mode vibrations. The modes of fm-rubrene were characterized for the first time and compared to a predicted computational Raman spectrum showing large (~4%) deviations with theory at low vibrational energies (<200cm-1), suggesting intermolecular coupling becomes influential at this threshold.Item Surface-Enhanced Raman Spectroscopy as a Probe to Understand Plasmon-Mediated Photochemistry(2019-09) Brooks, JamesThe development of plasmonic nanostructures as light-activated photocatalysts has proven to be a promising research avenue due to their ability to access and drive unfavorable chemical reactions. Theses chemical reactions are fueled by the presence of surface plasmons, which are the collective oscillation of the free electron density on the material’s surface. Once a surface plasmon is photoexcited, their initial energy rapidly decays into multiple different pathways, such as enhanced electromagnetic fields, an abundance of hot carriers, and dramatically elevated local thermal environments. To better understand the various chemistries that are enabled by plasmonic materials and the associated mechanisms driving these processes, we have employed surface-enhanced Raman spectroscopy to interrogate a plethora of plasmon-molecule coupled systems. Our initial studies investigated the relationship between the plasmonic local fields and a well-established plasmon-driven photochemical reaction. We found that there were no observable correlations between the two in our studies and identified a competing degradation pathway for the studied analytes. In addition to exploring well-studied plasmon-induced chemical photoreactions, we have highlighted two new reactions that were accessed on the gold film-over-nanosphere substrates. First, we were able to induce and subsequently monitor a selective intramolecular methyl migration on N-methylpyridinium using surface-enhanced Raman spectroscopy. This work emphasizes the growing potential of initiating highly-selective chemistries with plasmonic materials for synthetic or redox purposes. The second previously unreported plasmon-driven reaction involves the double cleavage of the C-N bond on a pair of viologen derivatives. While these viologens have traditionally been employed as robust redox species, the unique and highly-powerful plasmonic local fields allowed the viologens to access an entirely new reaction pathway to transform into 4,4’-bipyridine. Lastly, we discuss our experimental approaches towards transiently studying the mechanism behind plasmon-mediated hot electron transfer. Using ultrafast surface-enhanced Raman spectroscopy, we interrogated the transient dynamics that occurred between surface plasmons and a bevy of electron accepting chemical adsorbates. Ultimately, the primary goal of this work is to provide a quantitative description of the transient interactions, which will assist in increasing the reported efficiencies and yields of plasmon-mediated chemical reaction and inspire the rational design of plasmonically-powered devices.Item Synthesis and characterization of ensembles containing zinc oxide nanocrystals and organic or transition metal dyes to probe the early events in a dye-sensitized solar cell.(2011-08) Saunders, Julia ErinThe synthesis of 3',4'-dibutyl-2-phenyl-2,2':5',2"-terthiophene-5"-carboxylic acid, and its behavior with monodispersed ZnO nanocrystals (NCs) having diameters from 2.7 to 3.2 nm are reported. The excited state of the dye (E0* = -1.61 V vs NHE) was quenched upon binding to ZnO Ncs. Adsorption isotherms were measured for the terthiophene dye in ethanol and fit with a Langmuir model, which gave a size-independent Kads of 2.3 ± 1.0 x 105 M-1. The maximum number of attached dyes per nanocrystal depended on the diameter and was consistent with each dye occupying 0.5 ± 0.1 nm2 at maximum coverage. Deviation from the Langmuir model observed at low dye concentrations was attributed to a small amount of free zinc ion present in solution that bind the carboxylate ions more strongly than do ZnO NCs. Incorporation of the equilibrium expression between zinc ion and free carboxylate into the model provided a satisfactory fit for both the adsorption isotherm experiments and the complex shape of the Stern-Volmer graphs. Treatment of the terthiophene dye-nanocrystal dyads with increasing concentrations of sodium acetate in ethanol resulted in gradual displacement of the dye. Time-resolved fluorescence and time- and frequency-resolved pump-probe spectroscopy confirm and characterize electron injection from the dye to the semiconductor nanocrystals in room temperature ethanol dispersions at a series of dye:ZnO NC concentration ratios. The spectrum of the oxidized dye was determined by spectroelectrochemistry. The singlet excited state of the dye (190 ps lifetime in ethanol) is quenched almost exclusively by electron transfer to the ZnO NC, and the electron transfer dynamics exhibit a single time scale of 3.5 ( 0.5 ps at all concentration ratios. In the measured transient responses at different dye:ZnO NC ratios, gain in the amplitude of the electron injection component is anticorrelated with loss of amplitude from unperturbed excited state dye molecules. The dependence of this amplitude on dye:ZnO NC ratio deviates significantly from the prediction of a standard Stern-Volmer model. This observation is in agreement with the static quenching studies. By identifying electron transfer as the quenching mechanism at all ratios, the work presented here helps to exclude concentration quenching as the basis for the complicated quenching results, and supports the model that incorporates competitive binding between ZnO NC s and free Zn2+ cations in solution.Item Time Resolved Vibrational Spectroscopies as a Tool for Exploring Dynamics of Confined Systems(2022-01) Pyles, CynthiaThis thesis examines a variety of vibrational probe-containing molecules such as triphenyl hydrides, CO2, and metal carbonyls with the goal of better understanding the dynamics for each system. Particular emphasis is placed on understanding how the behavior of a restricted probe, such as one dissolved in a rigid polymer or confined to a nanopore, may differ from the same probe placed in bulk solvent or a more rubbery polymer. The first study described herein scrutinized the vibrational heavy atom effect and its impact on ultrafast vibrational dynamics. A series of three triphenyl hydride compounds was investigated in a range of solvents by Fourier transform infrared (FTIR), infrared (IR) pump-probe, and two-dimensional infrared (2D-IR) spectroscopies. The mass of the central atom in the three compounds was varied systematically down the group 14 elements of silicon, germanium, and tin while keeping the rest of the molecule unaltered. Interestingly, frequency-frequency correlation functions obtained from 2D-IR spectra indicated that an increasingly large central atom produces small, but measurable changes in the dynamics of the solvation shell surrounding each compound. Next, CO2 (g) was examined via 2D-IR spectroscopy as a precursory study to understanding its behavior inside polymers. Processes which lead to dephasing of the vibrational echo such as collisions were largely circumvented by using CO2 diluted in N2 under ambient pressure and temperature. Off diagonal features in the 2D-IR spectra were observed which correspond to population and coherence exchange between rovibrational transitions. Then, CO2 (g) was dissolved inside polymers such as poly(methyl methacrylate), poly (methyl acrylate), and poly(dimethylsiloxane). These polymers with differing properties were chosen to study the impact of the glass transition on the dynamics of the dissolved CO2 probe. Interactions between the polymeric backbone and probe also impacted the dynamics. The parameters obtained from 2D-IR studies directly correlated with the diffusivity of CO2 through the polymer matrices. Next, I inspected CO2 (g) adsorbed to microporous systems such as MIL-53(Al) and ZIF-8. Preliminary FTIR studies suggest that these samples could possess a wealth of dynamic information despite narrow FTIR peaks, much like CO2 dissolved in polymers. Experimental limitations regarding these novel systems are briefly discussed. Lastly, I compared the dynamics of three ruthenium-bound carbonyl complexes: Ru3CO12 in bulk THF, [HRu3(CO)11]- entrapped in an aluminum sol-gel, and [NEt4][HRu3(CO)11] in bulk THF. Ru3CO12 is catalytically inactive but becomes active upon incorporation into an alumina sol-gel matrix. Pump probe and 2D-IR studies indicated that the changed dynamics are primarily due to an altered solvent shell which most likely exhibits long-range ordering. Though it is uncertain whether the increased catalytic activity of [HRu3(CO)11]- is due to the presence of the hydride or this newly ordered solvent shell, the results nonetheless showcase 2D-IR’s efficacy in sensing dynamics of confined environments.Item Vibrational Spectroscopy on the Silicon Hydride Mode: Probing Ultrafast Dynamics in Small Molecules to Macromolecular Polymer Systems(2019-06) Olson, Courtney MarieThis thesis describes Fourier transform infrared (FTIR) and two-dimensional infrared (2D-IR) spectroscopy applied to small molecule silanes (trimethoxysilane and triphenylsilane) and polydimethylsiloxane (PDMS). 2D-IR spectroscopy gives information about the dynamics that the vibrational probe is sensitive to and the heterogeneous and homogeneous contributions to the linear FTIR lineshape. The vibrational probe used for all the studies in this thesis is the silicon hydride stretch due to being present in the small molecule silanes and in PDMS. The studies presented show how the silicon hydride mode was first characterized in small molecules to understand the probe more. Then, the probe was utilized in polymer systems to study more complex motions to make the connection between the ultrafast dynamics of polymers to the macroscopic properties. The first study involved studying the solvation dynamics of two small molecule silanes in three neat solvents using FTIR and 2D-IR spectroscopies along with molecular dynamics simulations. The two different molecules exhibited different degrees of vibrational solvatochromism, and the differences was found to be a result of higher mode polarization with more electron withdrawing ligands using density functional theory calculations. The solvent dynamics were found to be dominated by their interactions with neighboring solvent molecules rather than with the solute. Next, FTIR and 2D-IR spectroscopies were used to study PDMS cross-linked films and siloxane oligomers without solvent and swollen or dissolved in various solvents. There is an absence of vibrational solvatochromism in these systems, which was shown by 2D-IR spectroscopy to be due to the heterogeneity. The silicon hydride mode in the cross-linked, solvent-free PDMS film exhibited spectral diffusion, which must be due to the polymer structural motions. However, once the solvent penetrates the network, the dynamics become a convolution of the solvent and polymer motions due to the motions being of similar timescale. In the last study discussed, FTIR and 2D-IR spectroscopies were used to study the ultrafast structural dynamics of PDMS thin films with various physical and chemical changes done to the polymer, which included elevated curing temperature, increased cross-linker agent concentration, compression, and cooling near the glass transition temperature. The FTIR spectra were found to be relatively insensitive to all of these perturbations, which 2D-IR spectroscopy revealed was caused by the overwhelming heterogeneity. There is clearly a disconnect between the microscopic and macroscopic behavior in this polymer due to having only slight differences in the heterogeneous and homogeneous dynamics.