published_or_final_version / Physics / Master / Master of Philosophy

Thesis (M. Phil.)--University of Hong Kong, 1995. / "9th September 1994." Includes bibliographical references.

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Traps are ubiquitously present in semiconductors. Their presence results in ineffective charge transport and thus limited the device performance. For organic semiconductors, traps can present intrinsically via structural disorder or extrinsically during synthesis or device fabrication. A thorough understanding of traps is important to optimize the device performance and material design. This thesis employs two trap measurement techniques, photothermal deflection spectroscopy (PDS) and thermal admittance spectroscopy (TAS), to investigate the trap density in the materials. The subgap optical absorptions of several high performance bulk-heterojunction (BHJ) systems for organic solar cells have been studied by PDS. The charge transfer (CT) states are, in particular, looked into detail. CT states are intermediate bound electron-hole pairs at the donor/acceptor (D/A) interface of an organic solar cell. The dynamics and energetics of CT states are crucial to free charge generation and recombination processes. With the help of PDS and external quantum efficiency (EQE) measurements, the CT states the delocalized CT states (hot) from the localized CT states (cold) are observed and differentiated directly. It is discovered that the localized CT states are more pronounced when the acceptor concentration reaches its percolation limit. As the acceptor concentration reaches its optimized composition, the intensity of these CT states is significantly reduced due to the reduced recombination. Using the CT energies measured from PDS, the open-circuit voltage losses from the BHJs are determined. Besides PDS, thermal admittance spectroscopy (TAS) is employed as an alternative method to measure the trap densities. TAS measures the frequency dependent capacitance response of a semiconductor under a small ac signal excitation. This technique is useful to measure the trap depth and trap density of a semiconductor. The defect profiles in two classes of materials are investigated, they are perovskite compounds and an organic hole transporter with an intentional dopant. The trap density are determined by TAS is compared with that obtained by PDS.

Orientador: Edson Correa da Silva / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin / Made available in DSpace on 2018-09-26T17:36:33Z (GMT). No. of bitstreams: 1 Guimaraes_AndreOliveira_M.pdf: 2255240 bytes, checksum: 01c58731bfeea104f82fd18b44d26d19 (MD5) Previous issue date: 2004 / Resumo: O estudo de propriedades térmicas e espectroscópicas de materiais tem despertado o interesse não só no meio acadêmico, mas também entre diversos setores industriais. Muitas são as técnicas que possibilitam este tipo de pesquisa, porém as técnicas fototérmicas têm demonstrado extrema eficiência para uma diversidade enorme de estudos e materiais. O foco do trabalho é justamente apresentar potencialidades de algumas destas técnicas. Foi feita a descrição e caracterização de um aparato experimental que possibilita estudos de espectroscopia a partir de duas formas de detecção. Espectros de transmissão e absorção foram obtidos a partir da detecção fotopiroelétrica e fotoacústica, respectivamente, evidenciando a sensibilidade deste espectrômetro fototérmico. Um estudo sobre margarinas mostrou a sensibilidade da técnica fotopiroelétrica ao investigar o processo da quebra de emulsão para experimentos com variação¸ de temperatura. A técnica de Radiometria Infravermelho indicou que a margarina apresenta uma estrutura de camadas, tanto para a temperatura ambiente quanto para uma temperatura acima da transição de fase, e que neste caso, a concentração de gorduras é relevante. Através da configuração fotopiroelétrica de duplo sensor foi possível caracterizar termicamente um conjunto de amostras de polipropileno, carregadas com diferentes porcentagens de microesferas ocas de vidro. Obteve-se a esperada redução na condutividade do material, em função da inserção destas esferas, o que não foi possível por nenhuma outra técnica / Abstract: The study of thermal and spectroscopic properties of several sorts of materials has been increasing in interest both in the academy and in the industrial sector. There are several techniques able to develop these studies. In this thesis photothermal techniques were applied and its potentiality demonstrated. Among the photothermal techniques the photoacoustic and the photopyroelectric configurations were used as main techniques for spectroscopic purposes, allowing the obtainment of the optical spectra, from absorption and from transmission as well. The description and the characterization of a suitable apparatus for this purpose were presented. Margarine was studied showing the ability of the photopyroelectric methodology in the measurement of the thermal properties (diffusivity and effusivity) and in the detection of temperature dependent structural modifications, as melting and emulsion break as well. From frequency dependent photothermal infrared radiometry data it was possible to show that margarine presents a layered structure both for room temperature and above the transition point. Furthermore it was observed different effects for different fat content samples. Finally, we have applied for the first time the DSPPE configuration in the simultaneous determination of the thermal properties of a solid sample. Specifically, samples of a polymer composite based in the polypropylene material with hollow glass microspheres, for different proportions of microspheres were studied. It was possible to obtain the thermal conductivity of the samples from simultaneous measurement of the thermal diffusivity and effusivity. We have found a diminution in the conductivity of the samples increasing the hollow glass microspheres content, showing an increase of the thermal insulating properties of polypropylene, as expected by the composite producers / Mestrado / Física da Matéria Condensada / Mestre em Física

A espectroscopia foto-térmica tem sido largamente usada na investigação de propriedades térmicas e ópticas de materiais sólidos. Neste trabalho, desenvolvemos novas câmaras foto-térmicas as quais foram adaptadas para os estudos de materiais poliméricos. Estendemos o modelo desenvolvido por Mandelis para a espectroscopia fotopiroelétrica (PPES), incluindo nele um termo devido à reflexão de luz na interface amostra-detetor. A aplicação da técnica PPES em filmes de Poli(3-Butiltiofeno) não dopado nos permitiu obter sua condutividade e difusividade térmicas, como também seu gap de energia. Também aplicamos a técnica fotoacústica, com um flash de laser He-Ne, ao estudo de filmes de polímeros transparentes. Finalmente, desenvolvemos um método semi-empírico o qual significa a análise do sinal foto-térmico, quando ele é, principalmente, devido à difusão térmica. / Photothermal spectroscopies have been largely used in the investigation of thermal and optical properties of solids materials. In this work we developed new photothermal cells which were adapted for the study of polymerics materials. We also extended the model developed by Mandelis for the Photopyroeletric Spectroscopy (PPES), including in it a term due to the reflected light in the sample-detector interface. The application of the PPES technique in films of undopedpoly(3-butylthiophene) allowed us to obtain its thermal conductivity and diffusivity, as well as its gap energy. We also applied the photoacoustic technique, with a flash of He-Ne laser to study of transparent films of polymers. Finally we developed a semi-empiric method which simplifies the analysis of the photothermal signal, when it is mainly due to the thermal diffusion.

The mid-infrared is a promising region for detection different materials. Many vibrational modes, including bending and stretching, are located in this regime. Photothermal spectroscopy and imaging in the mid-infrared region is an emerging new method for non-contact detection of molecular groups. Our approach to photothermal spectroscopy and imaging utilizes a near-infrared erbium doped fiber laser (EDFL) to detect the photothermal induced changes in the refractive index. These changes are excited by a mid-infrared quantum cascade laser (QCL) pump beam. The probe beam is detected by a commercially available near-infrared photodetector. This method has advantages of high sensitivity, label-free detection, high spatial resolution and high signal-to-noise ratio (SNR). Hydrogels such as pHEMA are polymers that are of interest for contact lens, drug delivery and soft tissue replacement. The pHEMA hydrogel can retain water content, causing the material to swell. Additionally, pHEMA has a critical temperature at which the hydrogel undergoes a glass transition. Photothermal spectroscopy of pHEMA is demonstrated in this thesis where the presence of this glass transition temperature can be revealed. Additionally, photothermal imaging of a PMMA USAF target sample is shown and ideal parameters for high-resolution photothermal imaging are determined. In this thesis, we report a spatial resolution much smaller than the diffraction limited spot size of the mid-infrared beam. / 2018-07-09T00:00:00Z

Gold nanoparticles strongly absorb and scatter visible and near infrared light because of the strongly enhanced electric fields at the surface. This provides the potential of designing novel optically active reagents for simultaneous molecular imaging and photothermal cancer therapy. In this thesis, gold nanospheres and nanorods conjugated with anti-epidermal growth factor receptor (anti-EGFR) antibodies that specifically target EGFR on the cell surface are shown to be used for dual diagnostics and therapy. Using micro-absorption spectroscopy and light scattering imaging, cancerous (HOC 313 and HSC 3) and noncancerous cells (HaCat) can be differentiated due to the overexpression of EGFR on the surface of cancer cells. By irradiating the cells with a CW laser, selective photothermal cancer therapy is realized in visible region by using gold nanospheres and in near infrared region by using gold nanorods. The use of nanorods allow for in vivo therapy due to the fact that their absorption is in the near infrared region at which the laser light meets less interference from the tissue absorption. In addition, the catalytic effect of gold nanoparticles on the oxidization of NADH to NAD+ is investigated. The addition of gold nanoparticles is found to quench the NADH fluorescence intensities but has no effect on the fluorescence lifetime. This suggests that the fluorescence quenching is not due to coupling with the excited state, but due to changing the ground state of NADH. The intensity of the 340 nm absorption band of NADH is found to decrease while that of the 260 nm band of NAD+ is found to increase as the concentration of gold nanoparticles increase. This conversion reaction is further supported by nuclear magnetic resonance and mass spectroscopy. The linear relationship between the initial reaction rate of NADH and the concentration of gold nanoparticles strongly supports that NADH is surface catalyzed by the gold nanoparticles. The catalytic property of this important reaction might have important future applications in biological and medical fields.

Photothermal cancer therapy is a potential alternative to surgery and involves selective tissue destruction using thermal energy. Targeted photoabsorbers, used in conjunction with matching a continuous wave laser, make photothermal therapy both noninvasive and tumor-specific. However, to become clinically relevant, there is a need to develop an imaging technique to identify tissue composition and to detect the presence of photoabsorbers in the tumor volume before therapy; to monitor the temperature rise during therapy; and to assess the tumor damage after therapy. In this study, a combined ultrasound and photoacoustic imaging system was designed to assist photothermal therapy. The imaging system was tested on tissue mimicking phantoms, ex-vivo porcine tissue samples, ex-vivo mice and in-vivo mice. First, ultrasound imaging was utilized to differentiate between water-based and lipidbearing tissue. A combined ultrasound and photoacoustic imaging system was then assembled to identify the presence and spatial location of gold nanoparticles. Multiwavelength photoacoustic imaging was used to further confirm the presence of nanoparticles. Temperature monitoring algorithms, using both temperature-dependent time shifts in ultrasound signals and amplitude changes in photoacoustic signals, were developed. Finally, photothermal therapy was carried out on tumor-bearing nude mice using in-vivo ultrasound and photoacoustic imaging to identify the tumor boundary, detect the nanoparticles and monitor the temperature elevation. The results of the studies show that ultrasound and photoacoustic imaging provide complementary and clinically relevant information. Overall, there is potential of using the ultrasound and photoacoustic imaging system to plan, guide and monitor photothermal therapy. / text