Photodynamic therapy (PDT), which utilizes the combination of light and a photosensitizing drug to cause tissue damages, has emerged as a novel clinical approach for the treatment of numerous cancers, as well as some other non-malignant conditions. Although a few photosensitizers have been approved for clinical uses, the mechanism of drug action, especially the initial photochemical reactions that lead to the formation of the reactive oxygen species (ROS), is still not well understood. Moreover, the PDT efficiency of currently used drugs is limited due to the strong attenuation of light by tissues in the wavelength range of 630-690 nm, where these drugs are photo-activated. Photosensitizers which are sensitive to near infrared (NIR) light are believed to be able to overcome this limitation. In this thesis work, the molecular mechanism of action of indocyanine green (ICG), a potential NIR PDT drug, was investigated using our femtosecond time-resolved laser spectroscopy. Femtosecond time-resolved fluorescence decay profiles of ICG in water were obtained using the fluorescence up-conversion technique. The lifetime of ICG excited singlet state was determined to be about 150 ps, directly from the fluorescence decay kinetic traces. The excited triplet-state yield of ICG in water was found to be extremely low, according to the result of the ground-state bleaching recovery measurement. This observation is contrary to the conventional understanding that the ROS would be generated mainly from the excited triplet state of the photosensitizer and, therefore, suggests the existence of a new reaction pathway. Pump-probe transient absorption spectroscopy was applied to study the reaction between ICG and oxygen in more details. The results reveal that the formation of ICG and oxygen ground-state complexes ([ICG]_m:[O₂]_n) is a key step in the generation of the ROS. Electron transfer from the excited singlet state of ICG to oxygen has been proposed to be a possible pathway for the generation of ROS.

Photodynamic therapy (PDT), which utilizes the combination of light and a photosensitizing drug to cause tissue damages, has emerged as a novel clinical approach for the treatment of numerous cancers, as well as some other non-malignant conditions. Although a few photosensitizers have been approved for clinical uses, the mechanism of drug action, especially the initial photochemical reactions that lead to the formation of the reactive oxygen species (ROS), is still not well understood. Moreover, the PDT efficiency of currently used drugs is limited due to the strong attenuation of light by tissues in the wavelength range of 630-690 nm, where these drugs are photo-activated. Photosensitizers which are sensitive to near infrared (NIR) light are believed to be able to overcome this limitation. In this thesis work, the molecular mechanism of action of indocyanine green (ICG), a potential NIR PDT drug, was investigated using our femtosecond time-resolved laser spectroscopy. Femtosecond time-resolved fluorescence decay profiles of ICG in water were obtained using the fluorescence up-conversion technique. The lifetime of ICG excited singlet state was determined to be about 150 ps, directly from the fluorescence decay kinetic traces. The excited triplet-state yield of ICG in water was found to be extremely low, according to the result of the ground-state bleaching recovery measurement. This observation is contrary to the conventional understanding that the ROS would be generated mainly from the excited triplet state of the photosensitizer and, therefore, suggests the existence of a new reaction pathway. Pump-probe transient absorption spectroscopy was applied to study the reaction between ICG and oxygen in more details. The results reveal that the formation of ICG and oxygen ground-state complexes ([ICG]_m:[O₂]_n) is a key step in the generation of the ROS. Electron transfer from the excited singlet state of ICG to oxygen has been proposed to be a possible pathway for the generation of ROS.

Ultrafast time-resolved pump-probe (TRPP) apparatus has been applied to study the carrier dynamics of Si-doped InN thin films grown buffer by molecular beam expitaxy with and without a low-temperature growth GaN buffer layer. The peak of the PL has been found to increase from 0.7 to 0.8 eV with the back ground density. The total decay rates as a function of the delay time were obtained by the density-dependent TRPP peak intensity and the time-resolved TRPP signals. The total decay rates were interpreted as the sum of radiative and nonradiative recombination. The Shockley-Read-Hall decay rate derived from the TRPP signal at low photoexccitation density was found to increase with the doping density. At low concentration, the Auger recombination is not effective. The dominant recombination mechanism at room temperature is the Shockely-Read-Hall recombination.

The design and development of a versatile time resolved fluorescence spectroscopy (TRFS) system capable of near real time data acquisition and processing for potential clinical diagnostic applications is reported. The TRFS apparatus is portable, versatile and compatible with the clinical environment. The main excitation source is a UV nitrogen laser with a nanosecond pulse width and the detection part consists of a dual grating spectrograph coupled with an MCP-PMT. The nitrogen laser also has a dye module attached to it, which enables broadband excitation of the sample. This setup allows rapid acquisition (250 ms for fluorescence decay at a wavelength) of time resolved fluorescence data with a high spectral (as low as 0.5 nm) and temporal (as low as 25 picoseconds) resolution. Alternatively, a state diode pumped pulsed laser can be used for excitation to improve data collection speed. The TRFS system is capable of measuring a broad range of fluorescence emission spectra (visible to near infra-red) and resolving a broad range of lifetimes (ranging from a few hundred picoseconds to several microseconds). The optical setup of the system is flexible permitting the connection of different light sources as well as optical fiber based probes for light delivery/collection depending on the need of the application. This permits the use of the TRFS apparatus in in vitro, ex vivo and in vivo applications. The system is fully automated for real-time data acquisition and processing, facilitating near-real time clinical diagnostic applications.

Time-resolved fluorescence spectroscopy (TRFS) is a powerful analytical tool for quantifying the biochemical composition of organic and inorganic materials. The potentials of TRFS as nondestructive clinical tool for tissue diagnosis have been recently demonstrated. To facilitate the translation of TRFS technology to the clinical arena, algorithms for online TRFS data analysis are of great need. A fast model-free TRFS deconvolution algorithm based on the Laguerre expansion method has been previously introduced, demonstrating faster performance than standard multiexponential methods, and the ability to estimate complex fluorescence decay without any a-priori assumption of its functional form. One limitation of this method, however, was the need to select, a priori, the Laguerre parameter a and the expansion order, which are crucial for accurate estimation of the fluorescence decay. In this thesis, a new implementation of the Laguerre deconvolution method is introduced, in which a nonlinear least-square optimization of the Laguerre parameter is performed, and the selection of optimal expansion order is attained based on a Minimum Description Length (MDL) criterion. In addition, estimation of the zero-time delay between the recorded instrument response and fluorescence decay is also performed based on a normalized means square error criterion. The method was fully validated on fluorescence lifetime, endogenous tissue fluorophores, and human tissue. The automated Laguerre deconvolution method is expected to facilitate online applications of TRFS, such as clinical real-time tissue diagnosis.

This thesis examines the luminescence properties of nanoscale silicon (Si) by using spectroscopic techniques. Since the development of new optical devices requires understanding light emission mechanism optical spectroscopy has become more important tool in the analysis of these structures. In this thesis, Si nanocrystals embedded in SiO2 matrix will be studied. Photoluminescence (PL) and Time-resolved photoluminescence spectroscopy (TRPL) have been used to detect the light emission in UV-Vis-NIR range. Experiments have been performed in the temperature range 10-300 K. PL is sensitive to impurities and defects that affect materials quality and device performance. In this context, the role of defects in limiting the luminescence of Si nanocrystals and the removal of these defects by hydrogen passivation has been investigated. v TRPL was employed to determine the time evolution of photoluminescence as function of temperature. The decay time of the PL spectra was determined by a stretched exponential function and perfectly fitted to an expression based on three excitonic levels. Carrier lifetimes associated with these three levels were determined and compared with literature. Additionally, temporal variation of PL from free-standing Si nanoparticles is studied under a strong laser illumination. The observed bleaching behavior (time dependent emission intensity), which is reversible, have discussed in terms of exciton trapping at the interface between nanocrystal and the surrounding oxide layer. The results of this thesis will provide new insight on the understanding of light emission mechanism of Si nanocrytals.

This dissertation focuses on developing a predictive method for determining the dynamic densification behavior of thermite powder mixtures consisting of equivolumetric mixtures of Ta + Fe₂O₃ and Ta + Bi₂O₃. Of primary importance to these highly reactive powder mixtures is the ability to characterize the stress at which full compaction occurs, the crush strength, which can significantly influence the stress required to initiate reaction during dynamic or impact loading. Examined specifically are the quasi-static and dynamic compaction responses of these mixtures. Experimentally obtained compaction responses in the quasi-static regime are analyzed using available compaction models, and an analysis technique is developed that allows for a correct measurement of the apparent yield strength of the powder mixtures. The correctly determined apparent yield strength is combined with an equation of state to yield a prediction of the shock densification response, including the dynamic crush strength of the thermite powder mixtures. The validated approach is also extended to the Al + Fe₂O₃ thermite system. It is found that accurate predictions of the crush strength can be obtained through determination of the apparent yield strength of the powder mixture when incorporated into the equation of state. It is observed that the predictive ability in the incomplete compaction region is configurationally dependent for highly heterogeneous thermite powder systems, which is in turn influenced by particle morphology and differences in intrinsic properties of constituents (density, strength, etc.).

This thesis aimed to investigate the features and properties of the ground states, transient species and photoproducts involved in the photophysical and photochemical processes for four kinds of drug compounds: lomefloxacin (LF), norfloxacin (NF), tiaprofenic acid (TPA), and flurbiprofen (Fp). The investigation used femtosecond transient absorption (fs-TA), nanosecond transient absorption (ns-TA), UV/Vis absorption spectra (UV/Vis), nanosecond transient resonance Raman (ns-TR2) and nanosecond time-resolved resonance Raman spectroscopy (ns-TR3), as well as density functional theory (DFT) calculations. Although many previous investigations have indicated that photo-defluorination or photo-decarboxylation reactions may account for the phototoxicity for these compounds, detailed information on the mechanisms remains unclear. In this thesis, the photo-defluorination reaction of LF was explored in neutral water at pH 7.2. The fs-TA results revealed that the lowest lying excited singlet state species (S1) partially decayed into the ground state through fluorescence emission and partially underwent cleavage of the carbon-fluorine bond at position 8 to generate into a singlet aryl cation. Subsequently, intersystem crossing (ISC) allowing the transformation from singlet cation to triplet carbene was observed. Finally, a cyclization reaction with the N-ethyl chain took place for the triplet carbene to generate the final product. The mechanism underlying NF phototoxicity involves a photo-defluorination reaction in neutral water (pH=7.2). The fs-TA spectra indicated that the S1 underwent efficient ISC to swiftly transform into lowest excited triplet (T1) The ns-TA gained under nitrogen-saturated condition observed a new transient species produced from T1 that was proposed to be a transient species derived from the photo-defluorination reaction involving a SN2Ar* mechanism. The photo-defluorinated product ultimately experienced an ISC process to produce the final product. The photo-decarboxylation mechanism of TPA was studied in a neutral phosphate buffered solution (PBS). The fs-TA data revealed that S1 went through an efficient ISC to rapidly transform into T1 that then undergoes a photo-decarboxylation reaction to produce a triplet biradical species (denoted as TB3). The ns-TA and ns-TR3 results supplied evidence of the protonation process of TB3 that produces the neutral species (denoted as TBP3) that then decayed through ISC to give rise to the singlet TBP species, which underwent further reaction to make the final product (DTPA). The photo-decarboxylation reaction of Fp was explored in pure acetonitrile (MeCN). The second excited singlet (S2) went through internal conversion (IC) to decay to S1. Intriguingly, three different pathways for S1 decay co-exist. One pathway is fluorescence emission and the second is an ISC process. The third pathway is the homolysis of the carbon α bond reaction that proceeds to generate two radical species, one being a carboxyl species and the other being the residual, denoted as FpR that was liable to be oxidized under an oxygen-saturated condition to yield a new radical species with the addition of one oxygen molecule which is denoted as FOR that then experienced intramolecular hydrogen transfer (IHT) and dehydroxylation (DHO) to produce the final product. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

The pure organic dye sensitized solar cells (DSSCs) were studied and a new organic dye of donor-π-2acceptors (D-π-2A configuration) was fabricated. This dye, denoted as B2, was investigated and applied in DSSCs. Density functional theory (DFT) was used to examine the electronic distribution of the frontier orbitals of the B2 dye. It was found that intramolecular charge transfer (ICT) between the donor moieties and acceptor moieties of the B2 dye may take place under photo irradiation. The LUMO, LUMO+1 and LUMO+2 of B2 are all distributed on the acceptor moieties and this is very helpful to enhance the intramolecular electron transfer from the donor moieties to the acceptor moieties, which will consequently promote the chance of electron injection into the semiconductor. DSSCs based on B2 demonstrated an power conversion efficiency of 3.62 %. This efficiency value is approximately half of the power conversion efficiency of DSSCs based on N719 (7.69 %) under the same conditions. Femtosecond transient absorption and nanosecond transient absorption (TA), and time-correlated single photon counting (TCSPC) technique were applied to examine the electron transfer processes occurring on the surface of B2/TiO2. B2 dye has life time of the excited states three orders in magnitude shorter than that of N719. The electron injection time from excited B2 to TiO2 is also three orders in magnitude shorter than that from excited N719 to TiO2. It was revealed that the delocalized electrons of π → π* transition for both the B2 dye and the N719 dye could be further guided into the semiconductor, while such injection processes may not happen for the localized electrons in π → π* transition of these dyes. The nanosecond transient absorption and transient emission spectroscopy of the ruthenium bipyridyl sensitizer N719 in different solvents were studied. Three kinds of ZnO nanoparticles were utilized to study the electron transfer process taking place on the interface of N719/ZnO with and without electrolyte by Time-Correlated Single Photon Counting (TCSPC) technique, TA and transient emission spectroscopy. Isopropanol was found to stabilize the singlet excited state of N719 and a related emission band centered at 460 nm was observed in nanosecond time scale. It was revealed that the electrolyte has a significant impact upon the electron transfer dynamics on the N719/ZnO interface. In the absence of electrolyte, the electron transfer process on the N719/ZnO interface is dependent upon the depth of defects in ZnO nanoparticles. Conversely, in the presence of electrolyte, the impact of ZnO defects upon the electron transfer process is eliminated and the effective electron injection happens from the excited states of N719 to ZnO, in spite of the ZnO particle sizes. The polymer based solar cells were studied and a polymer incorporated with a pyrenylcarbazole pendant was synthesized and applied in the functionalization of multi-wall carbon nanotubes (MWCNT) by noncovalent π-π interaction. The polymer/MWCNT hybrids were isolated and examined. The strong interaction between the polymer and MWCNT in a 1,1,2,2-tetrachloroethane (TCE) solution was investigated. The emission spectra demonstrated an effective quenching of emission from the polymer by the MWCNT. DFT calculations showed an electron delocalization phenomenon between the pyrene and carbazole moieties. The LUMO of the polymer is mainly located on the pyrene moiety while the LUMO+1 of the polymer is predominantly positioned on the carbazole moiety. The electronic transition of LUMO+1→LUMO results in intramolecular charge transfer (ICT) from the carbazole moieties to the pyrene moieties. Femtosecond TA determined the characteristic TA feature of the excited states, which are contributed from both the pyrene and carbazole moieties. The excited state lifetime of the polymer was calculated to be 659 ps and the photo excited electrons can inject into the MWCNT very fast on a time scale of 420 fs. / published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Silicon carbide (SiC) is currently under development for high power bipolar devices such as insulated gate bipolar transistors (IGBTs). A major issue for these devices is the charge carrier lifetime, which, in the absence of structural defects such as dislocations, is influenced by point defects and their associated deep levels. These defects provide energy levels within the bandgap and may act as either recombination or trapping centers, depending on whether they interact with both conduction and valence band or only one of the two bands. Of all deep levels know in 4H-SiC, the intrinsic carbon vacancy related Z1/2 is the most problematic since it is a very effective recombination center which is unavoidably formed during growth. Its concentration in the epilayer can be decreased for the production of high voltage devices by injecting interstitial carbon, for example by oxidation, which, however, results in the formation of other new deep levels. Apart from intrinsic crystal flaws, extrinsic defects such as transition metals may also produce deep levels within the bandgap, which in literature have so far only been shown to produce trapping effects. The focus of the thesis is the transient electrical and optical characterization of deep levels in SiC and their influence on the carrier lifetime. For this purpose, deep level transient spectroscopy (DLTS) and minority carrier transient spectroscopy (MCTS) variations were used in combination with time-resolved photoluminescence (TRPL). Paper 1 deals with a lifetime limiting deep level related to Fe-incorporation in n-type 4H-SiC during growth and papers 2 and 3 focus on identifying the main intrinsic recombination center in p-type 4H-SiC. In paper 4, the details of the charge carrier capture behavior of the deeper donor levels of the carbon vacancy, EH6/7, are investigated. Paper 5 deals with trapping effects created by unwanted incorporation of high amounts of boron during growth of n-type 4H-SiC which hinders the measurement of the carrier lifetime by room temperature TRPL. Finally, paper 6 is concerned with the characterization of oxidation-induced deep levels created in n- and p-type 4H- and 6H-SiC as a side-product of lifetime improvement by oxidation. In paper 1, the appearance of a new recombination center in n-type 4H-SiC, the RB1 level is discussed and the material is analyzed using room temperature TRPL, DLTS and pnjunction DLTS. The level appears to originate from a reactor contamination with Fe, a transition metal that generally leads to the formation of several trapping centers in the bandgap. Here it is found that under specific circumstances beneficial to the growth of high-quality material with a low Z1/2 concentration, the Fe incorporation also creates an additional recombination center capable of limiting the carrier lifetime. In paper 2, all deep levels found in p-type 4H-SiC grown at Linköping University which are accessible by DLTS and MCTS are investigated with regard to their efficiency as recombination centers. We find that none of the detectable levels is able to reduce carrier lifetime in p-type significantly, which points to the lifetime killer being located in the top half of the bandgap and having a large hole to electron capture cross section ratio (such as Z1/2, which is found in n-type material), making it undetectable by DLTS and MCTS. Paper 3 compares carrier lifetimes measured by temperature-dependent TRPL measurements in n- and p-type 4H-SiC and it is shown that the lifetime development over a large temperature range (77 - 1000 K) is similar in both types. This is interpreted as a further indication that the carbon vacancy related Z1/2 level is the main lifetime killer in p-type. In paper 4, the hole and electron capture cross sections of the near midgap deep levels EH6/7 are characterized. Both levels are capable of rapid electron capture but have only small hole capture rates, making them insignificant as recombination centers, despite their advantageous position near midgap. Minority carrier trapping by boron, which is both a p-type dopant and an unavoidable contaminant in 4H-SiC grown by CVD, is investigated in paper 5. Since even the shallow boron acceptor levels are relatively deep in the bandgap, minority trap and-release effects are detectable in room-temperature TRPL measurements. In case a high density of boron exists in n-type 4H-SiC, for example leached out from damaged graphite reactor parts during growth, we demonstrate that these trapping effects may be misinterpreted in room temperature TRPL measurements as a long free carrier lifetime. Paper 6 uses MCTS, DLTS, and room temperature TRPL to characterize the oxidation induced deep levels ON1 and ON2 in n- and p-type 4H- and their counterparts OS1-OS3 in 6H-SiC. The levels are found to all be positive-U, coupled two-levels defects which trap electrons efficiently but exhibit very inefficient hole capture once the defect is fully occupied by electrons. It is shown that these levels are incapable of significantly influencing carrier lifetime in epilayers which underwent high temperature lifetime enhancement oxidations. Due to their high density after oxidation and their high thermal stability they may, however, act to compensate n-type doping in low-doped material.