Temperature Dependence of Photoluminescence Spectra In
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TEMPERATURE DEPENDENCE OF PHOTOLUMINESCENCE SPECTRA IN POLYSTYRENE A Thesis Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Master of Science Kelvin Xorla Tsagli June, 2021 TEMPERATURE DEPENDENCE OF PHOTOLUMINESCENCE SPECTRA IN POLYSTYRENE Kelvin Xorla Tsagli Thesis Approved: Accepted: Advisor Dean of the College Dr. Sasa V. Dordevic Dr Mitchell McKinney Faculty Reader Dean of the Graduate School Dr. Robert R. Mallik Dr Marnie Saunders Faculty Reader Date Dr. Ben Yu-Kuang Hu Department Chair Dr, Christopher Ziegler ii ABSTRACT A previous study of low temperature photoluminescence (PL) in several common polymers revealed that photoluminescence intensity was, to a different degree, tem- perature dependent in all of them. Even though polystyrene showed only moderate temperature dependence, it was the only studied polymer in which the wavelength, and therefore the energy of the photoluminescence peak changed with temperature. In this work I explored that effect in more details. A careful photoluminescence study was done of four polystyrene samples from different manufacturers. Samples were la- beled PS1 PS2, PS3, and PS4, with measurements done at 77 K, 100 K, 200 K and 292 K. All samples showed that fluorescence quantum yield increased at cryogenic temperatures, causing the peak intensity to increase at temperatures below room temperature. However, only PS4 sample showed a peak shift, i.e. the energy of the peak changed with temperature. In PS1, PS2 and PS3 samples the peak position did not change with temperature. Therefore, I concluded that the effect was not an intrinsic property of polystyrene. Moreover, the data analysis revealed that the shift of the peak in PS4 was caused by a second photoluminescence peak, which displays a very strong temperature dependence. This second peak was not observed in PS1, iii PS2 and P3 and therefore is not intrinsic to polystyrene. I speculate that it is due to impurities. iv ACKNOWLEDGEMENTS My most gratitude goes to God Almighty, whom I believe is the author and originator of all understanding. I sincerely appreciate the effort of my supervisor Dr. Sasa V. Dordevic for his time, advice and encouragement all through my study at The University of Akron. It has been a long time coming but finally, the moment is here . I want to express my sincere gratitude and honor to my lovely and distin- guished parents, Torgbi and Mrs Tsagli II for their immeasurable care, love and support. Thank you for being my pillar of strength. To my most precious, gifted, loving and wonderful siblings, I say thank you for always believing in me and more importantly, standing by me with all that you have. I would like to thank Dr. Jutta Luettmer-Strathmann for her wealth of knowledge and understanding spirit. For your sake, my transition from Ghana has been a memorable one. In particular, special thanks to Ebenezer Awadzie and Eric Osei Boateng. You guys are amazing. Your support is immeasurable . I dedicate this work to you. v TABLE OF CONTENTS Page LIST OF FIGURES . viii CHAPTER I. INTRODUCTION . 1 1.1 Overview . 1 1.2 Thesis Outline . 4 II. PHOTOLUMINESCENCE . 5 2.1 Photoluminescence Spectroscopy . 7 III. POLYSTYRENE . 16 3.1 Structure and Polymerization of Polystyrene . 18 3.2 Properties of Polystyrene . 21 IV. EXPERIMENTAL METHOD . 24 4.1 Cary Eclipse Fluorescent Spectrophotometer . 24 4.2 The Software . 28 4.3 Low Temperature Measurements and Experimental Setup . 30 4.4 Experimental Procedure . 33 4.5 Samples Used In The Thesis . 36 V. RESULTS AND EXPERIMENTAL DATA . 38 vi 5.1 Previous Photoluminescence Spectra of Polystyrene . 40 5.2 2-D Contour Plots of Photoluminescence at Different Temperatures . 44 5.3 Temperature Dependence of Photoluminiscence Spectra . 49 5.4 Mathematical Functions Used For Fitting . 52 5.5 Fitted Spectra . 53 5.6 Temperature Dependence of Fitting Parameters . 60 VI. CONCLUSIONS . 70 vii LIST OF FIGURES Figure Page 2.1 Schematic Diagram Of A Photoluminescence Phenomenon[9] . 6 2.2 A diagram showing ways in which molecules can deexcite from the excited state[9] . 7 2.3 Energy Level Diagram that shows the difference between absorption and emission spectra during photoluminescence[13] . 8 2.4 A diagram illustrating the band gap between the valence band and conduction band of a photoluminescence system[2] . 9 2.5 A Jablonski diagram that illustrates the processes involved in the photoluminiscence phenomena . The diagram shows fluorescence from an excites singlet state and an excited triplet state.[17] . 11 2.6 Illustration of Beer Lambert Attenuation[18] . 13 3.1 Different Types of Polystyrene . 17 3.2 Chemical Structure of Polystyrene[30] . 19 3.3 Formation and Polymerization of Polystyrene[21] . 20 4.1 The Varian Cary Eclipse Fluorescence Spectrophotometer . 25 4.2 Internal component of Cary Eclipse Fluorescent Spectrophotome- ter. The colored beam represents the path of light. The blue light beam is the excitation light emerging out from a Xenon flash lamp, which is finally transitioned into an orange-colored beam called the emission light[28] . 26 4.3 Schematic Diagram of a Fluorescence Spectrometer.[1] . 27 4.4 Control Interface of Cary Eclipse Fluorescence Spectrophotometer[28] . 29 viii 4.5 The Full Experimental Setup . 30 4.6 Cryostat Used to hold a sample for measurement at low temperatures . 32 4.7 Interface of temperature regulator for roughing pump . 32 4.8 Interface of the turbopump controller used to monitor vacuum and the pressure sensor . 33 4.9 Adding liquid nitrogen to the cyrostat for cryogenic measurements . 35 4.10 Samples Used . 36 4.11 Scale Measurements of Samples . 37 5.1 A sample 2-D figure obtained with MATLAB showing a plot of exci- tation against emission wavelength. The Photoluminescence emis- sion peak of the sample is found around an excitation wavelength of 255 and an emission wavelength of 270 nm . 40 5.2 The figure shows the PL intensity variation of six different polymers as temperature changes . In exception of PS, all samples showed growth in PL peak as temperature decreased but stayed at the same wavelength however in PS, the peak shifted to a shorter wavelength with temperature decrease [29] . 42 5.3 Previous [29] photoluminescence emission spectra of sample PS4 at 77 K, 100 K, 150 K, 200 K, 250 K and 292 K. The peak intensity measurements are in arbitrary units. The photoluminescence peak intensity increases as temperature decreases; however, the peak in- tensity shift is only noticed at 77 K and 100 K. 43 5.4 A 2D contour plot of PS1 at 77 K, 100 K, 200 K and 292 K. The maximum excitation wavelength was found to be 270 nm. The approximate Photoluminescence peak is indicated with an arrow. There is minimal or no shift in intensity peak for this sample as temperature changes; however, it shows a slight increase in peak intensity. 45 5.5 A 2D contour plot of PS2 at 77 K, 100 K, 200 K and 292 K. The maximum excitation wavelength was found to be 260 nm. The approximate Photoluminescence peaks are indicated with an arrow. The Photoluminescence peak does not shift with a decrease in temperature. 46 ix 5.6 A 2D contour plot of PS3 at 77 K, 100 K, 200 K and 292 K. The maximum excitation wavelength was found to be 265 nm. The approximate Photoluminescence peaks are indicated with an arrow. 47 5.7 A 2D contour plot of PS4 at 77 K, 100 K, 200 K and 29 2K. The maximum excitation wavelength was found to be 260 nm. The approximate Photoluminescence peaks are indicated with an arrow. This sample showed a peak shift at 77 K and 100 K. On the graph, the peak shift can be seen to vary from 325 nm at 29 2K to 290 nm at 77 K. The PL emission was measured from 240 nm to 380 nm. 48 5.8 Photoluminescence emission spectra of all samples at 77 K, 100 K, 200 K and 292 K. PL luminescence intensity is in arbitrary units. Excitation wavelength for each sample is kept fixed at a value that corresponds to the maximum of the peak for that sample (See Table 5.3). 51 5.9 Photoluminescence emission spectra of PS1 fitted with a Gauss-mod peak function. Good fits were obtained at all four temperatures. 54 5.10 Photoluminescence emission spectra PS2 fitted with a Gauss-mod peak function. 56 5.11 Photoluminescence emission spectra PS3 fitted with a Gauss-mod peak analyzer . 57 5.12 Figure 5.12 shows Lorentz fits of photoluminescence intensity (in ar- bitrary units) at 77 (blue line), 100 (green line), 150(magenta line), and 200K (red line). Thick black lines represent the total fits. The yellow and orange modes represent the two Lorentz oscillators. The yellow mode does not change significantly as the temperature de- creases. However, the orange mode grows dramatically, its intensity increasing by a factor of five between 150 K and 77 K. This creates the appearance that the PL peak shifts to lower wavelengths. The yellow mode corresponds to the peak observed in the other three samples, whereas the orange mode is most likely due to impurities [30]. 59 5.13 Plot of Area, Center, and Width Vrs Temperature of fitted Gaus- mod curve of PS1 polymer at various temperatures . 60 5.14 Plot of Area, Center, and Width Vrs Temperature of fitted Gaus- mod curve of PS2 polymer at various temperatures . 62 5.15 Plot of Area, Center, and Width Vrs Temperature of fitted Gaus- mod curve of PS3 polymer at various temperatures. 64 x 5.16 Plot of Area, Center, and Width Vrs Temperature of fitted Gaus- mod curve of PS4 polymer at various temperatures.