MULTI PHOTON ABSORPTION INDUCED PHOTOLUMINESCENCE IN DOPED SEMICONDUCTOR QUANTUM DOTS AND HETERO NANOSTRUCTURES

MULTI-PHOTON ABSORPTION INDUCED PHOTOLUMINESCENCE IN DOPED SEMICONDUCTOR QUANTUM DOTS AND HETERO-NANOSTRUCTURES A THESIS submitted by Radhu S for the award of the degree of JOINT DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS INDIAN INSTITUTE OF TECHNOLOGY MADRAS AND NATIONAL UNIVERSITY OF SINGAPORE MAY 2015 DECLARATION I hereby declare that all the experiments embodied in this thesis entitled, “MULTI-PHOTON ABSORPTION INDUCED PHOTOLUMINESCENCE IN DOPED SEMICONDUCTOR QUANTUM DOTS AND HETERO-NANOSTRUCTURES”, submitted for the degree of Joint Doctor of Philosophy in Physical Sciences to Indian Institute of Technology Madras and National University of Singapore, under the Indo-NUS Joint PhD program, has been carried out by me at Department of Physics, IIT Madras, India and Department of Physics, National University of Singapore, Singapore under the supervision of Prof C Vijayan and Prof Wei Ji The contents of this thesis, in full or part, have not been submitted to any other Institute or University for the award of any degree or diploma Date: 19 May, 2015 Department of Physics Indian Institute of Technology Madras Chennai – 600036 RADHU S CERTIFICATE This is to certify that the work incorporated in the thesis entitled, “MULTI-PHOTON ABSORPTION INDUCED PHOTOLUMINESCENCE IN DOPED SEMICONDUCTOR QUANTUM DOTS AND HETERO-NANOSTRUCTURES” submitted by Ms RADHU S, has been carried out by her under our supervision at Department of Physics, Indian Institute of Technology Madras, Chennai, 600 036, India and Department of Physics, National University of Singapore, Singapore, 119077 The contents of this thesis, in full or part, have not been submitted to any other Institute or University for the award of any degree or diploma Prof C Vijayan Prof Wei Ji (Research Guide) (Research Guide) Photonics Laboratory Department of Physics Department of Physics Faculty of Science Indian Institute of Technology Madras National University of Singapore Chennai -600036 Science Drive 3, Singapore-117542 Tel: 91-44-2257 4877 Tel: (65)65166373 Email: cvijayan@iitm.ac.in Email: phyjiwei@nus.edu.sg Date: 19-05-2015 Place: Chennai Dedicated to… My Husband, Sasidevan V & My Parents, P.K Radhakrishna Panicker and S Subhakumari & My Brother, Sambhu R ACKNOWLEDGEMENT I would like to thank all the people and Institutions who helped and supported me for the completion of my PhD work as well as to finish this dissertation First and foremost, I would like to thank my supervisors, Prof C Vijayan (IITM) and Prof Wei Ji (NUS) for their constant encouragement, motivation and guidance I have faced many difficulties in my personal life during this PhD and I would like to express my sincere gratitude to Prof C Vijayan who cared me and supported me during these times, just like my father Also, his sincere efforts made it possible for me to work in National University of Singapore under the Joint PhD program It is a great privilege for me to work with Prof Ji Wei in NUS I would like to express my deepest gratitude to Prof Ji Wei for his inspiring and motivating attitude and willingness to come for discussions, even on holidays, which helped me a lot to improve myself I would also like to thank him for doing all the necessary arrangements to extend my candidature period in NUS and arranging financial support during this period, which helped me to complete this work I would like to thank IITM and NUS authorities, especially Prof Markandeylu (former HOD, Dept Of Physics, IITM), Prof Sunil Kumar (HOD, Dept of Physics, IITM) and Prof B.V.R Chowdhary (Dept of Physics, NUS) for giving me a wonderful opportunity to work in both the prestigious Institutes I would like to express my gratitude to my Doctoral Committee members, Prof M P Kothiyal, Prof Kasi Viswanathan, and Prof Nilesh J Vasa and Prof Edamana Prasad for their suggestions and comments in my work I would also like to thank Prof Lakshmi Bala and Prof Rajesh Narayanan for their encouragement and support The financial assistance from University Grants Commission, during my stay in IITM is also duly acknowledged I would also like to express my gratitude to Prof K.R Somanatha Pillai, D B Pampa College, Parumala for his constant care, encouragement and discussions I would like to thank Prof Taeghwan Hyeon (Seoul National University, South Korea) and Prof Wee Shong Chin (Dept of Chemistry, NUS) for providing quantum dot and nanorod samples My lab members in IITM and NUS have been a big support for me I would like to express my heart-felt gratitude to Dr Venkatram Nalla for his invaluable help in Femtosecond laboratory, NUS, as well as for all the discussions during the different stages of my work which helped me to finish this dissertation I am glad to thank my labmates in IITM and NUS, Dr i Manas, Dr Jyotsana, Dr Anitha, Mr Jayachandra, Ms Radhika, Mr Shiva, Mr Xu Zhe, Mr Venkatesh Mamidala and Mr Chen Weiquiang for their support and encouragement I would like to remember with thanks the time I spent with Dr Aparna Devi, Dr Christie Thomas Cherian, Dr Sinu Thomas and their families I have immense pleasure in thanking my friends in IITM and NUS, Ms Saritha, Dr Safina Devi, Dr Shani Jose, Dr Pramitha V, Ms Lizbeth Zeta, Dr Anjana C.P, Dr Suruchi, Ms Kavitha K.G., Dr Lekha P.K, Ms Rusha, Dr Robin John, Ms Kavitha, Ms Deepthy, ms Divya and Ms Ganga who made my stay pleasant and memorable I am expressing my deepest gratitude to Ms Lincy for her love, care and support, which helped me a lot during thesis submission I would never have been able to finish this work without the invaluable support from my family My father, Radhakrishna Panicker (Late), have been my inspiration to pursue research in Science Words can’t express my gratitude to him for the love and care he gave me I remember with love at this moment, his decision to allow me to continue my studies even when he was struggling for life My mother, Subhakumari, has been my biggest support all these times It is her love, courage, support and prayers which helped me to move forward in life and reach this stage in both academic and personal life I am lucky to have a wonderful brother, Sambhu, who was there for me every time I needed a help, being it personal or academic My husband, Sasidevan came to my life just after the first year of my PhD From that time onwards, he has been with me as my best friend and soulmate It is just because of his invaluable support that I was able to go to Singapore and the decision was taken at a time, when he needed my help the most I recall at this moment, all the love and encouragement he has given me to finish this work I express my deepest gratitude to my grandparents, Kunjunnithan, Devakiamma and Saradaamma, and my sister-in-law, Nandinikutty, for their support to continue my studies Finally, I would like to thank all other family members especially, my uncles and aunts, my cousins, my mother-in-law Leela.P, Jayakrishnan, Harikrishnan and Unnikrishnan for their love and care Last, but not the least, I would like to thank God almighty for all his blessings to complete this work ii ABSTRACT Materials with high multi-photon absorption cross-section are of recent interest in optoelectronic and biological applications such as lasing, optical limiting, three-dimensional data storage, multi-photon microscopy etc Owing to the properties such as remarkable photo stability, brightness and size dependent absorption and emission, semiconductor nanocrystals (NCs) are preferred over other conventional fluorophores as bio-imaging probes in multi-photon microscopy The major impediment to use multi-photon absorption in NCs for practical applications is the requirement of high excitation intensity because of the low absorption crosssection of nanocrystals in near-infrared (NIR) wavelengths This dissertation aims at enhancing the multi-photon absorption cross-section of nanocrystals in NIR wavelengths by doping and by forming hetero-nanostructures Specifically, this thesis presents the nonlinear optical investigations of the multi-photon absorption induced photoluminescence in Mn2+-doped ZnS nanocrystals and CdS-CdSe-CdS segmented nanorods In addition to the nonlinear absorption studies, the charge transfer dynamics in CdS-CdSe-CdS segmented nanorods is also presented in this thesis An introduction to the optical properties of semiconductor NCs is given in the beginning of Chapter followed by a brief introduction to hetero-junction nanomaterials The succeeding section deals with the carrier dynamics in single component and hetero-junction semiconductor NCs In the subsequent section the basics of multi-photon absorption (MPA) and related optical nonlinearities, which lay the foundation for the work presented in this thesis, are outlined This is followed by a review on the MPA studies in single component and hetero-junction semiconductor NCs This is followed by a discussion on the significance of excitation in NIR-I and NIR-II window The objectives and scope of the work presented in this thesis are outlined in the concluding section of Chapter The operational principles of the different techniques used for the non-linear optical characterization and carrier dynamics study such as Z-scan technique, transient pump-probe spectroscopy, and multi-photon absorption induced photoluminescence (MPA-PL) measurements are discussed in Chapter Doping can modify the photoluminescence (PL) emission wavelength of semiconductor NCs by introducing additional levels in the energy gap ZnS is a less cytotoxic material with less PL quantum yield The emission wavelength in these NCs can be shifted to NIR wavelengths, by iii finding appropriate dopants In Chapter 3, we present our results on the red emission in ZnS NCs where we propose that the emission in ZnS nanocrystals can be shifted to NIR-I window by doping with O2- ions, in the presence of interstitial sulfur ions in these NCs Besides changing the excitation and emission, the quantum confinement effect in QDs as well as the strain in the lattice due to defects can change other properties as well, such as shift in the vibrational modes of the lattice Raman spectroscopic studies in these O2- doped ZnS NCs were also presented in chapter which gives a better understanding on the shift in the vibrational modes on reducing the size of semiconductor from bulk to nanometer range Transition metal doping can change the multi-photon absorption cross-section in semiconductor NCs besides changing the emission wavelength and PL quantum yield Chapter primarily deals with understanding the effect of Mn2+-doping in the MPA properties of ZnS QDs on excitation in NIR-I and NIR-II window In the first section of Chapter 4, we present the derivation of 3PA theory for wide band gap semiconductor QDs (direct band gap) such as ZnS, without considering the presence of any defect levels in the band gap The details of the synthesis procedure and linear optical characterization studies in Mn2+-doped ZnS QDs are presented in the subsequent sections This section is followed by our investigation on the multiphoton action cross-section of Mn2+-doped ZnS QDs on excitation in NIR-I window With the intention of understanding the effect of Mn2+ doping on determining the multi-photon action cross-section in doped ZnS QDs, the results are compared with undoped ZnS QDs of similar size as well as the theoretical prediction under four band model for undoped ZnS QDs of similar size, without defects The succeeding section of Chapter describes our investigation of multi-photon action cross-section on excitation in NIR-II window The chapter is concluded with the transient PL measurements in Mn2+-doped ZnS QDs on multi-photon excitation Apart from changing the size of the NC or doping, a different method to enhance twophoton absorption over a wide range of wavelength is to form composites where the constituent components already possess large two-photon absorption Here, local field can determine the effective two-photon absorption cross-section, in addition to the weighted averages of the component semiconductors and hence providing a way to engineer the optical nonlinearity in semiconductor NCs Chapter deals with this aspect, in particular, the chapter focuses on the investigation of multi-photon absorption properties in CdS-CdSe-CdS segmented nanorods The iv procedure for the synthesis of CdS-CdSe-CdS segmented nanorods is presented in the first section of this chapter This is followed by the analysis of the linear optical characterization in these nanorods The next section deals with the Z-scan measurements as well as the multi-photon absorption induced PL measurements in CdS-CdSe-CdS nanorods The effect of the composite structure on the optical non-linear properties of CdS-CdSe-CdS segmented nanorods is also examined in this chapter by considering the effect of local field, as suggested by MaxwellGarnett theory The superior photoluminescence of CdS-CdSe hetero-nanostructures is a result of the efficient charge transfer from CdS to CdSe and subsequent recombination in CdSe In the final chapter (Chapter 6), we investigate the dynamical properties of photo excited carriers in CdSCdSe-CdS segmented nanorods using femtosecond transient pump-probe spectroscopy The carriers generated in CdS can have different relaxation channels with charge transfer to CdSe being a dominant mechanism Different relaxation mechanisms of the photo generated carriers in CdS segments have been investigated in this chapter Excitation at higher intensities is required for applications such as multi-photon microscopy As the intensity increases, other relaxation mechanisms such as Auger recombination can become significant in semiconductor NCs and these mechanisms can considerably affect the charge transfer also, resulting in reduced PL quantum yield at high intensities In Chapter 6, we also present our investigations on the charge transfer dynamics in CdS-CdSe-CdS segmented nanorods at high excitation intensities (when the average number of electron-hole pair per nanorod greater than unity) In particular, we examined the effect of Auger recombination on charge transfer in CdS-CdSe-CdS segmented nanorods The present work is expected to lead to a better understanding of the non-linear optical properties of nanomaterials and provide a platform for engineering their non-linear absorption, thereby making it suitable for applications in bio-imaging and photonics v TABLE OF CONTENTS ACKNOWLEDGEMENT……………………………………………………………………….i ABSTRACT…………………………………………………………………………………… iii LIST OF FIGURES…………………………………………………………………………….ix LIST OF TABLES…………………………………………………………………………… xiii LIST OF PUBLICATIONS………………………………………………………………… xiv ABBREVIATIONS…………………………………………………………………………… xv Chapter Introduction…………………………………………………………………………1 1.1 Background…………………………………………………………………… …1 1.2 Previous Research on Quantum Dots, Nanorods and Hetero-junction nanocomposites……………………………………………………………… … 1.2.1 Quantum Dots and Nanorods………………………………………… 1.2.2 Hetero-junction nanomaterials……………………………………… .10 1.2.3 Carrier dynamics in nanomaterials…………………………………… 12 1.2.4 Multi-photon absorption and related optical nonlinearities in semiconductor NCs………………………………………………… ….14 1.2.4.1 Nonlinear absorption………………………………………… 15 1.2.4.2 Nonlinear refraction………………………………………… .19 1.2.4.3 Nonlinear scattering…………………………………………… 19 1.2.4.4 Previous research on MPA in semiconductor NCs………… 20 1.2.5 Significance of NIR-I and NIR-II windows…………………………… 23 1.3 Objectives and Scope of the thesis………………………………………… … 26 Chapter Experimental methods and techniques………………………………….……… 27 2.1 Z-scan technique……………………………………………………… …………27 vi Ellis-Davies G C R (2011) Two-Photon Microscopy for Chemical Neuroscience ACS Chem Neurosci., 2, 185-197 El-Sayed M A (2004) Small Is Different:  Shape-, Size-, and Composition-Dependent Properties of Some Colloidal Semiconductor Nanocrystals Acc Chem Res., 37, 326-333 Fang, Bando Y., Ye, Shen, 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undoped ZnS nanoparticles embedded in SiO2 matrices J Appl Phys., 92, 6828-6832 154 ... experiments embodied in this thesis entitled, ? ?MULTI- PHOTON ABSORPTION INDUCED PHOTOLUMINESCENCE IN DOPED SEMICONDUCTOR QUANTUM DOTS AND HETERO- NANOSTRUCTURES? ??, submitted for the degree of Joint Doctor... is to certify that the work incorporated in the thesis entitled, ? ?MULTI- PHOTON ABSORPTION INDUCED PHOTOLUMINESCENCE IN DOPED SEMICONDUCTOR QUANTUM DOTS AND HETERO- NANOSTRUCTURES? ?? submitted by... by doping and by forming hetero- nanostructures Specifically, this thesis presents the nonlinear optical investigations of the multi- photon absorption induced photoluminescence in Mn2+ -doped ZnS