A Thesis. Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of

Transcription

1 COLLOIDAL PBS AND PBS/CDS CORE/SHELL NANOSHEETS S K A Thesis Submitted to the Graduate College of Bowling Green State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE December 2015 Committee: Liangfeng Sun, Advisor Haowen Xi Mikhail Zamkov

2 2015 Simeen Khan All Rights Reserved

3 ABSTRACT iii Liangfeng Sun, Advisor PbS/CdS core/shell nanosheets were synthesized using the cation exchange method. A strong quantum confinement was observed in the PbS core due to a decrease in its thickness, indicated by a blue shift in photoluminescence. HRTEM revealed a flat atomic interface between PbS and CdS, and also the relationship between the energy-gap and the thickness in an extremely one- dimensionally confined nanostructure. The photoluminescence lifetime of the core/shell nanosheets was significantly longer than the core-only nanosheets, indicating better surface passivation.

4 ACKNOWLEDGEMENTS iv I would like to express my deepest gratitude to my supervisor Dr. Liangfeng Sun for giving me an opportunity to work under his supervision. I am extremely fortunate to work with him and learned under his guidance. I would also like to thank all the group members of Sun research group for their support and advice. I thank Department of Physics and Astronomy, BGSU for their support to complete my research and thesis. I also thank all the committee members for their advice and their time. I would also like to thank my parents and my husband to encourage me all the time.

5 v TABLE OF CONTENTS Page CHAPTER 1. INTRODUCTION GENERAL INTRODUCTION OF NANOMATERIALS QUANTUM CONFINEMENT OBJECTIVE OF THIS THESIS PROJECT KINETICS OF SHELL FORMATION 5 CHAPTER 2 EXPERIMENTAL METHODS EXPERIMENTAL SETUP SYNTHESIS OF PBS NANOSHEETS SYNTHESIS OF PBS/CDS CORE/SHELL NANOSHEETS PHOTOLUMINESCENCE SPECTROSCOPY TRANSMISSION ELECTRON MICROSCOPY (TEM) ABSORPTION MEASUREMENT PHOTOLUMINESCENCE LIFETIME MEASUREMENT ENERGY DISPERSIVE X-RAY SPECTROSCOPY THICKNESS MEASUREMENT SPECTROSCOPY 16 CHAPTER 3. RESULTS AND CONCLUSIONS TRANSMISSION ELECTRON MICROSCOPY IMAGES PHOTOLUMINESCENCE SPECTRA PHOTOLUMINESCENCE LIFETIME MEASUREMENT ENERGY DISPERSIVE X-RAY SPECTROSCOPY (EDS) CONCLUSION AND FUTURE WORK 29

6 vi REFERENCES APPENDIX -A... 35

7 vii LIST OF FIGURES Figure Page 1.1 Density of states of semiconductor nanoparticles and energy for, (a) 3-dimensions, (b) 2-dimesions, (c) 1-dimension, (d) 0-dimension Charge mobility in quantum dots and nanosheets Cation exchange for PbS/CdS core/shell structure showing total thickness remains same Diagram of Schlenk line system used for synthesis of PbS nanosheets and Core/shell Nanosheets An experimental setup using Schlenk line system for synthesis of PbS nanosheets An experimental setup for synthesis of core/shell PbS/CdS nanosheets Schematic setup for photoluminescence spectroscopy TEM image of nanosheets (a) Integrating sphere to diffuse the scattered light. (b) Absorption spectra using commercial device for quantum dot. (c) Absorption using commercial and home built setup a) TEM of PbS nanosheets synthesized using TCA, (b) SEM of PbS nanosheets synthesized using TCA, (c) TEM of PbS nanosheets synthesized using chloroform, (d) SEM of PbS nanosheets synthesized using chloroform Histogram showing the thickness of PbS nanosheets based on HRTEM using ImageJ (a) A TEM image of PbS nanosheets showing both standing-up and lying-down nanosheets. (b) The close-up HRTEM image of one standing-up nanosheets. (c)

8 viii Cartoon showing the arrangement of the ions projected in <110> direction. (d) In the thickness direction, the spacing between the neighbor ions a is nm, while in the orthogonal direction, the spacing between the neighbor ions b is 0.42 nm. The angle α is about 55 o (a) HRTEM image showing the clear interface between the PbS core and the CdS shell. (b) Zoomed-in HRTEM image showing the similar crystal structure of the CdS shell as the PbS core. The lines are to guide the eyes to the array of ions, making the angle α unique to {110} facet as mentioned earlier Histogram showing the thickness of PbS nanosheets based on HRTEM using ImageJ Elemental mapping of PbS/CdS core/shell nanosheets PL spectra for PbS NSs synthesized with different chloroalkanes. Chloroform (black), DCE (red), TCA (green) and DCB (blue), respectively PL spectra from the nanosheets before and after cation exchange showing the shift of the peak from 1760nm to 1520nm respectively a) The photoluminescence peak shifts from 0.75 ev to 0.83 ev and 0.85 ev after 15 minutes (dashed line) and 60 minutes (dotted line) cation exchanges, respectively. (b) The energy-gap dependence on the thickness follows the 1/L (L is the thickness of the PbS layer) model (solid line) 24 for both PbS nanosheets and PbS/CdS core/shell nanosheets. The increase of the energy gap of PbS quantum dots (dashed line) 29 is much faster than the nanosheets Photoluminescence lifetimes of the PbS (solid circles) and PbS/CdS (open circles) nanosheets

9 ix 3.11 Energy dispersive x-ray spectrum showing the presence of lead (Pb), sulfur (S), cadmium (Cd), and copper (Cu)

10 1 CHAPTER 1. INTRODUCTION 1.1 GENERAL INTRODUCTION OF NANOMATERIALS To understand the properties of semiconductor materials of size range 1-100nm made up of hundreds to thousands of atoms, and to utilize them in applications, various concepts of physics and chemistry are combined together, and that study of nanomaterials is known as nanoscience. Nanoparticles have been used from long time, but from past few decades, researchers are able to control their synthesis using either of the two approaches, that are top down or bottom up. In top down approaches, bulk semiconductors materials are miniaturize to produce nanoparticles, whereas in bottom up approaches, materials break down at atomic level to further combined together and construct the nanoparticles. These nanoparticles display many unique properties, which makes them suitable for variety of potential applications such as solar cells, light emitting diode, and in vitro and in vivo labeling 1-4. The most fundamental reasons for these unique properties are large surface to volume ratio and quantum confinement and both are related to the size and shape of nanoparticles 5. Synthesis of nanoparticles is very fascinating as their physical, chemical, and optical properties are considerably different from bulk material and can be changed by controlling their shape, size and structural morphology 6, 7. On the basis of structural dimensions, nanomaterials are classified into different groups: zero-dimension also referred as quantum dots, 1-dimension includes nanorods and nanowires, and 2-dimensional nanosheets 8. Unlike the bulk material, the properties of nanomaterials are function of size and can be controlled by Quantum Confinement Effect.

11 2 1.2 QUANTUM CONFINEMENT EFFECT As the bulk semiconductor reduces in size to nanomaterials, there is a change in the atomic structure as a result of change in the energy band structure of the material; these changes brings up the term Quantum confinement in nanoworld. This phenomenon results in when the energy bands do not overlap each other, unlike the bulk material, and the excitons squeeze to the size smaller or equal to exciton Bohr radius of semiconductor 9. The Bohr radius of a semiconductor depends on the dielectric constant of the material ( ε r ), mass of electron (m), rest mass of electron (µ) and the Bohr radius of hydrogen atom ( a b ); this can be formulated as 9 : a m = ε a µ * b r b When the semiconductor material absorbs a photon, electron gets excited and jumps to the conduction band, and creates a hole in the valence band. This pair of electron and photon known as exciton is bound together by Coulomb force and can be considered as a hydrogen atom whose electron orbits around the hole. As the size of nanoparticles become less than Bohr radius, the energy levels of electron become discrete, instead of forming continuous bands and this results in change in the properties of nanomaterials. Therefore, properties of nanomaterials like electric properties, magnetic, optical, and mechanical properties can all be characterized by change in the total energy and the morphology of the system. Different dimensional nanomaterials have different quantum confinement, which reduces from zero-dimensional quantum dots to 2- dimensional nanosheets. The density of states of different dimensional nanomaterials with respect to energy is shown in the Fig: (1.1) below 8 :

12 3 Figure 1.1: Density of states of semiconductor nanoparticles and energy for, (a) 3- dimensions, (b) 2-dimesions, (c) 1-dimension, (d) 0-dimension. 1.3 OBJECTIVE OF THIS THESIS PROJECT Currently 2D nanosheets are drawing attention of the researchers in the field of nanotechnology because they have the advantage that they do not exhibit tunnel barriers in lateral dimension, which makes them promising for photovoltaic or optoelectronics 10. Its difficult to use quantum dots in various optoelectronics devices because of there surface ligands which acts like an obstacle for charge transport ; in such case, synthesis of 2-dimensional nanosheets not only reduces this obstacle but also maintain quantum confinement in one dimension 14 (figure1.2).

13 4 Figure 1.2: Charge mobility in quantum dot and nanosheets Due to one-dimension quantum confinement, nanosheets exhibits narrow emission spectra 15 and ultrafast radiative fluorescent lifetime 16. These unique properties of nanosheets can further be enhanced by growing a shell around the core nanosheets by the process of cation exchange 17,18. Cation exchange not only grows a shell around the core nanosheets but also improves the morphology of core nanosheets to an extent, which cannot be achieved by direct synthesis method 19, 20. Direct band gap column IV-VI semiconductor are very promising for 2- dimensional structure as they have large exciton Bohr radii which helps in achieving strong quantum confinement 21. In our project, we synthesized PbS 2-dimensional nanosheets as it shows strong multiple exciton generation and it also shows a wide range of tunable band gap energy 22. Furthermore,

14 5 growing an inorganic shell not only prevents the core nanosheets physically and chemically from degradation, but also improves the surface morphology. For shell formation over PbS nanosheets we used CdS material as they have very less lattice mismatch. The new PbS / CdS core shell nanosheet heterostructure exhibits the high efficiency of PbS nanosheets and chemical stability of CdS shell. Therefore, in our research, different properties of core/shell nanosheets were studied using different characterization methods like photoluminescence, TEM, PL lifetime measurements etc., and compared with the properties of core PbS nanosheets. 1.4 KINETICS OF SHELL FORMATION In this project we present surface modification of colloidal PbS nanosheets by growing a CdS shell on it. For 0-dimensional quantum dots and 1-dimensional nanowires, it has been already verified that core/sell structure improves the surface passivation 23,24. For 2-dimensioanl PbS nanosheets, growth of CdS shell was verified using different methods like Photoluminescence, TEM, EDS, and PL lifetime. The core/shell structure of 2-dimensional nanosheets was achieved using the process of cation exchange. This method allows shrinking of core nanosheet in size by replacing the outer layer of Pb ion by Cd ion while maintaining the total thickness of nanosheets Figure (1.3).

15 6 L PbS L L PbS CdS Figure 1.3: Cation exchange for PbS / CdS core/shell structure showing total thickness remains same The thickness of PbS nanosheets and the thickness of CdS shell was also are found by this method. Core/shell nanosheets exhibits a longer photoluminescence lifetime than the core only nanosheets, which shows that surface passivation has been improved.

16 7 CHAPTER 2. EXPERIMENTAL METHODS 2.1 EXPERIMENTAL SETUP All the syntheses were performed under the air free nitrogen environment using standard Schlenk line system. The function of Schlenk line system is to perform vacuum or supply nitrogen gas in strict air free conditions. This system is also known as vacuum gas manifold which is a glass apparatus consists of two manifolds with various ports; one manifold is connected to the supply of nitrogen or argon gas which is further connected to an oil bubbler to release the pressure due to excess N 2 supply; the other manifold is connected to a high pressure vacuum system to remove the organic vapors and gaseous reaction products. The vacuum system is prevented from contamination caused by these organics vapors through a liquid nitrogen cold trap. The ports of both the manifolds are connected by a stopcock, which delivers either N 2 gas or vacuum to the connected tube as desired by the user. When the stopcock is open towards the nitrogen gas, it delivers N 2 to the tube and finally to the connected flask, but when the stopcock is open towards the vacuum, it supplies vacuum to the system. The whole Schlenk line system is operated within a fumehood. The figure 2.1 below shows the schematic diagram of Schlenk line system 26.

17 8 Figure 2.1: Experimental set up of Schlenk line system used for synthesis of PbS nanosheets and Core/shell Nanosheets SYNTHESIS OF PbS NANOSHEETS The method used for synthesis of PbS nanosheets basically follows the method given by Weller group 14 and developed later 25. In this process, the lead precursor was prepared by dissolving g of lead(ii) acetate trihydrate in 3.5 ml of oleic acid and 10 ml of diphenylether in a 3 neck flask. The whole setup was kept under nitrogen environment to remove air. The solution was then kept at 85 o C and degassed for 2 hours using a high vacuum to transform lead acetate to lead oleate and to remove acetic acid. After degassing, the Pb precursor was again put under the nitrogen environment, and a chloroalkane, 1,1,2-trichloroethane (TCA) was added at a temperature 4 o C less than its boiling point that is 110 o C and kept for 30 minutes. Chloroalkane plays a key role in the formation of nanosheets 25. In an another 3 neck flask, sulfur precursor was

18 9 prepared by adding g of thioacetamide (TAA) in 70 µl of N, N-dimethylformamide under nitrogen and then 930 µl of Tri-n-octylposphine (TOP) was added after replacing air from the flask by nitrogen to prevent the formation of TOPO, which is formed by the oxidation of TOP. Figure 2.2: An experimental setup using Schlenk line system for synthesis of PbS nanosheets 27. The solution was kept at room temperature for about an hour. After preparing both the precursors, the temperature of lead precursor was increased to 130 o C, which is 20 o C more than the boiling point of the chloroalkane used and then the sulfur precursor was added to the lead precursor and mixed for about 5 minutes to grow 2-dimensional PbS nanosheets. The final solution was allowed to cool down slowly to room temperature for several hours. The reaction mixture was then purified by using centrifugation for 5 minutes at 3000 rpm and then again washed and centrifuged for several times using toluene. Final precipitation was dispersed in toluene or tetracholorethylene.

19 SYNTHESIS OF PbS / CdS CORE/SHELL NANOSHEETS The growth of CdS shell on PbS nanosheets was achieved by cation exchange method. In this process, 1g of cadmium oxide (CdO) was dissolved in 6ml of oleic acid and 16ml of diphenyl ether and the mixture was heated until 255 o C under nitrogen until all the cadmium oxide was dissolved and a clean and colorless solution was obtained. This solution was then cooled down to 155 o C under nitrogen flow to remove water. In another flask, PbS nanosheets dispersed in toluene were kept for degassing by nitrogen flow for 30 minutes, and then the temperature was set to 100 o C. Immediately after setting the temperature at 100 o C (before it reaches to 100 o C), the Cd precursor was mixed with PbS nanosheets and allowed to react for 15 minutes. After 15 minutes, the reaction was quenched by adding the solution to cold hexane. The aliquots were removed by centrifugation at 2500 rpm for 5 minutes. The precipitate was dispersed in toluene and washed again by centrifugation for at least three times and finally dispersed in toluene or tetrachloroethylene. Figure 2.3: An experimental setup for synthesis of core/shell PbS/CdS nanosheets.

20 PHOTOLUMINESCENCE SPECTROSCOPY Photoluminescence spectroscopy is a technique to measure the luminescence emitted by the excited electron. When the sample absorbs a photon, the electrons get excited from valence band to conduction band resulting in formation of electron-hole pair known as exciton. After the process of photoexcitation, electron loses its energy and returns to the lower energy level. The energy from electron is re-radiated as a luminescent photon, which is known as photoluminescence. A home built spectrometer was used to measure the photoluminescence of the core and core/shell nanosheets. An argon laser was used as a light source to excite the specimens. Along with that, the setup also consists of a monochromator and an IR detector. The laser beam is aligned using two irises after being reflected from two mirrors. This laser beam was modulated before it falls on the sample using a chopper; the intensity of the laser beam was controlled by using a filter. After emission through the sample, two convex lenses were used to converge the light and another filter was used which blocks the laser light to pass through it and enters the monochromator. The IR detector detects the light near infrared region and the detected signals were sent to lock-in amplifier that is further connected to a computer to convert the signals into digital data. The figure 2.4 below shows the setup for the photoluminescence spectroscopy.

21 12 Figure 2.4: Schematic setup for photoluminescence spectroscopy. A software was programmed to operate and control all the measurements in LABVIEW. For measuring the photoluminescence of core and core/shell nanosheets, they are dispersed in tetrachloroethene and stored in quartz cuvette. 2.5 TRANSMISSION ELECTRON MICROSCOPY (TEM) To understand the morphology of core nanosheets and core/shell nanosheets, transmission electron microscopy was used. TEM technique confirms the 2-dimensional structure of nanosheets and helps in measuring the lateral dimensions of nanosheets. To obtain the images for nanosheets samples before and after cation exchange, a drop of nanosheets sample diluted in toluene was drop

22 13 casted on a copper TEM grid and allowed to dry for several minutes. For TEM measurement, a beam of electrons is used to obtain the image of nanosheets. 1µm Figure 2.5: TEM image of nanosheets 2.6 ABSORPTION MEASUREMENT For measuring the absorption spectra of nanosheets before and after cation exchanges, a home-built system was used. Due to strong scattering of light from the lateral dimension of nanosheets, the commercial instrument failed to measure the absorption spectra for nanosheets unlike quantum dots, therefore, an integrating sphere was used to diffuse the scattered light and allow it to go to the spectrometer (figure2.5). This method of measuring the absorption is based on method used by Mello et al to calculate the PL quantum yield of thin films 28.

23 14 Figure 2.6: (a) Integrating sphere to diffuse the scattered light. (b) Absorption spectra using commercial device for quantum dot. (c) Absorption using commercial and home built setup. For absorption measurement, PbS core nanosheets and PbS/CdS core/shell nanosheets were dispersed in tetrachloroethylene (TCE) in two different quartz cuvettes and one cuvette is filled with only TCE for baseline corrections. The electrons of semiconductor nanocrystals absorb the energy of photon bombarded on them and the amount of absorption varies with materials and synthesis conditions. To calculate the absorption of light, Beer-lambert law is used 29. log I 0 A = I where, A is the absorption at certain wavelength of light, I 0 is the intensity of light before it passes through the sample and I is the intensity of light after passing through the sample.

24 PHOTOLUMINESCENCE LIFETIME MEASUREMENT Photoluminescence lifetime spectroscopy is a technique use to study the decay of photoluminescence in time. when the electrons in excited states decays to ground state producing a photon. The single exponential decay from excited state to ground state can be calculated as: I(t) = I 0 exp (-t/τ) where, I 0 = Intensity upon excitation when t = 0, τ = lifetime, t = time after absorption, I(t) = intensity of excited electrons at time t. However, for nanosheets, the photoluminescence decay is not single exponential rather bi-exponential, which can be calculated as: f t = A! exp t τ! + A! exp t τ! where, τ 1 and τ 2 are the decay time and A 1 and A 2 are the pre-exponential factor to represent the fraction of fluorophores with respective lifetime From the given equation, lifetime is defined as when the time taken by the originally excited population to decrease by a factor of 1/e or 37%. The advantage of using PL lifetime measurement is that, even by changing the concentration of the sample, its lifetime does not vary, which means, it gives an absolute measurement irrespective of concentration. For measuring PL lifetime at t = 0, sample is excited by using short pulse of light source. When the pulse is generated, time digitizer starts the counting and stops as soon as the emitted photon hits the detector. This time interval between two signals is then plotted against the intensity of emitted photon to calculate the lifetime.

25 ENERGY DISPERSIVE X-RAY SPECTROSCOPY Energy dispersive x-ray spectroscopy is used to analyze the presence of elements on areas of TEM image. A beam of electron is incident on the sample image, which generates an x-ray spectrum, which characterizes the presence of different element on sample, and their atomic percent, as different elements has distinct atomic structure, which gives distinct peaks in x-ray spectrum. The electron beam excites the electron from ground state of the sample to the excited state, creating a pair of electron and hole. The electron of high energy level fills the hole, and an x-ray is released due to difference in the energy of two energy levels of electrons. In our PbS/CdS core/shell nanosheets projects, EDS confirms the presence of Cd after the cation exchange has been done. The energy-dispersive X-ray spectroscopy was measured in situ using an EDX detector mounted on Scanning Transmission Electron Microscope 2.9 THICKNESS MEASUREMENT To verify the calculated thickness of nanosheets, we measured the thickness of nanosheets using HRTEM also. The study of thickness, spacing and angle between the neighboring ions, enables us to understand the different facets of PbS crystal before and after cation exchanges. For measuring different parameters, a software ImageJ was used to process the data. In this software, measurement scale and unit were set, and then HRTEM of nanosheets were imported to do measurements. A line was drawn from the top of the nanosheets to the bottom of the nanosheets, and then software compares this thickness with set scale and unit. The process is repeated for lot of nanosheets and then the data was saved for further calculations of standard deviation and

26 17 mean. The values obtained by this measurement are very much close to the exact values. The distribution of measured values gives the Gaussian distribution.

27 18 CHAPTER 3. RESULTS AND CONCLUSIONS In this chapter, characterizations of core PbS nanosheets and core/shell PbS/CdS nanosheets have been discussed. Further, there is comparison between core and core/shell nanosheets characteristics using different techniques like optical spectroscopy, TEM, HRTEM, PL lifetime measurement. 3.1 TRANSMISSION ELECTRON MICROSCOPY IMAGES To study the shape and size of 2-dimensional nanosheets, transmission electron microscopy and scanning electron microscopy were used. TEM images confirm the 2-dimensional structure of PbS nanosheets and also the lateral size of nanosheets to be about few hundred nanometers. Furthermore, it is also observed from TEM images that nanosheets do not form a single layer, but they stack over one another to form multiple layers due to strong interactions. The figure 3.1 below shows the TEM and SEM for PbS nanosheets synthesized with TCA and chloroform respectively.

28 19 Figure 3.1: (a) TEM of PbS nanosheets synthesized using TCA, (b) SEM of PbS nanosheets synthesized using TCA, (c) TEM of PbS nanosheets synthesized using chloroform, (d) SEM of PbS nanosheets synthesized using chloroform. Furthermore, to study the cross section of nanosheets, high-resolution transmission electron microscopy (HRTEM) was used. For this, nanosheets with a width of about 20 nm was synthesized, which makes them easier to stand on edge on the TEM substrate. The HRTEM (figure 3.3b) clearly reveals that PbS nanosheets have a single crystal structure of 12 atomic layers in the direction of thickness. From the HRTEM image, thickness was measured using a software ImageJ. Figure 3.2 shows the histogram for the thickness of PbS nanosheets which is found to be around 3.3 nm ± 0.1 nm; this thickness is identical to the thickness calculated by

29 20 multiplying the lattice constant of PbS (0.594 nm) with multiple of lattice constant thickness span (5.5), resulting a thickness of about 3.27 nm. Figure 3.2: Histogram showing the thickness of PbS nanosheets based on HRTEM using ImageJ. The growth mechanism of 2-dimensional PbS nanosheets is already reported using 2D oriented attachment model 14, according to which <110> facets of quantum dots attached together to form nanosheets, leaving <110> facet at the edges. This result is also confirmed using the HRTEM image. The neighboring ions along the <001> facet are arranged at a distance of a = c/2 = nm and the neighboring ions along <110> direction are at a distance of b = c/ 2 = nm (Figure 3.3d). The angle α between the line of same ions and<110> direction is found to be 55 o using the following equation. α = tan!! c/b = tan!! c/(c/ 2) = 54.7

30 21 All these dimensions measured using HRTEM are consistent with the calculated values. Hence, this shows that the edges of nanosheets have <110> facet, whereas the upper and lower surfaces of nanosheets possess <001> facet. Figure 3.3: (a) A TEM image of PbS nanosheets showing both standing-up and lying-down nanosheets. (b) The close-up HRTEM image of one standing-up nanosheets shows a singlecrystalline structure with 12 layers of ions in the thickness direction. (c) Cartoon showing the arrangement of the ions projected in <110> direction. (d) In the thickness direction, the spacing between the neighbor ions a is nm, while in the orthogonal direction, the spacing between the neighbor ions b is 0.42 nm. The angle α is about 55 o.

31 22 Further, cation exchange was done using these PbS nanosheets to form PbS/CdS core/shell nanosheets. After the synthesis of core/shell nanosheets, again the HRTEM was taken which clearly shows the interference between PbS and CdS (Figure: 3.4). Figure 3.4: (a) HRTEM image showing the clear interface between the PbS core and the CdS shell. (b) Zoomed-in HRTEM image showing the similar crystal structure of the CdS shell as the PbS core. The lines are to guide the eyes to the array of ions, making the angle α unique to {110} facet as mentioned earlier. Out of 12 atomic layers of PbS, 4 atomic layers undergo cation exchange and turned into CdS (2 layer from top surface and 2 layers from bottom surface), which reduces the size of core PbS nanosheets to 2.1 nm as per the HRTEM measurement. Figure 3.5 below shows the histogram of thickness of nanosheets based on HRTEM using ImageJ.

32 23 Figure 3.5: Histogram showing the thickness OF PbS/CdS based on HRTEM using ImageJ. As the atomic weight of Pb is higher than the atomic weight of Cd, we can clearly see the PbS/CdS heterostructure in the HRTEM images (Figure 3.4). After cation exchanges, the surface of PbS/CdS core/shell nanosheets seems uniform, which indicates flat and uniform interfaces between PbS and CdS. The images obtained for Pb, Cd, and S by elemental mapping was identical with the high angle annular dark field image, which confirms that the cation exchange occurred evenly over the surface of nanosheets (Figure 3.6). From the elemental mapping, the molar ratio of Pb to Cd is found to be 2.4:1 which is identical to the values obtained from measurement done from HRTEM image i.e. 8 layers of PbS and 4 layers of CdS.

33 24 Figure 3.6: Elemental mapping of PbS/CdS core/shell nanosheets 3.2 PHOTOLUMINESCENCE SPECTRA The photoluminescence spectra for PbS nanosheets synthesized with different chloroalkanes was observed in the infrared region, which is identical to the result obtained earlier 25 (figure3.7). The peak of emission spectra moves from higher wavelength to lower wavelength with decreasing boiling point of chloroalkanes. Higher the boiling point of chloroalkane, lower is the photoluminescence intensity. As the chloroform has lowest boiling point among other chloroalkanes used, its intensity is maximum. Moreover, peak positions at different wavelength indicate different thicknesses of nanosheets. Thus, by changing the chloroalkane used during the synthesis of nanosheets, its thickness can be controlled 25.

34 25 Figure 3.7: PL spectra for PbS NSs synthesized with different chloroalkanes. Chloroform (black), DCE (red), TCA (green) and DCB (blue), respectively 25. For core/shell PbS/CdS nanosheets also, photoluminescence was observed in infrared region, but there is a significant blue shift of photoluminescence with respect to the photoluminescence of core PbS nanosheets. As the thickness of core PbS nanosheets reduced after cation exchanges, there is a strong quantum confinement observed. The shift in the photoluminescence was observed within 15 minutes of cation exchange reaction (figure 3.8).

35 26 Figure 3.8: Photoluminescence spectra from the nanosheets before (open circles) and after (solid circles) cation exchange showing the shift of the peak from 1760 nm to 1520 nm. After 15 minutes, the cation exchange reaction was carried out for 60 minutes, but not a significant shift was observed. Figure 3.9a shows the shift in energy from original nanosheets to core/shell nanosheets for 15 minute and 60 minute of reaction respectively. The thickness measured from HRTEM and the energy gap obtained from photoluminescence was plotted together in Figure 3.9b. As the quantum dots has 3-dimensional confinement, whereas nanosheets has only 1-dimensional confinement 25, it has been observed that the confinement energy for quantum dots of diameter same as the thickness of nanosheets is much more than the PbS core nanosheets, as well as for core/shell nanosheets

36 27 Figure 3.9: (a) The photoluminescence peak shifts from 0.75 ev to 0.83 ev and 0.85 ev after 15 minutes (dashed line) and 60 minutes (dotted line) cation exchanges, respectively. (b) The energy-gap dependence on the thickness follows the 1/L (L is the thickness of the PbS layer) model (solid line) 25 for both PbS nanosheets and PbS/CdS core/shell nanosheets. The increase of the energy gap of PbS quantum dots (dashed line) 30 is much faster than the nanosheets. 3.3 PHOTOLUMINESCENCE LIFETIME MEASUREMENT After doing cation exchange to obtain core/shell PbS/CdS nanosheets, increase in the PL lifetime was observed. For PbS nanosheets and PbS/CdS core/shell nanosheets, the 1/e photoluminescence intensity occurs at 4 ns and 7ns respectively (figure 3.10). The increase in decay time for core/shell nanosheets confirms that after cation exchanges, the surface passivation of PbS core nanosheets improves. By using double exponential function, PL decay yields to a fast decay of 2.9 ns and 4.9 ns for PbS core and PbS/CdS core/shell respectively; where as a slow decay 30 ns and 51 ns for core and core/shell respectively.

37 28 Figure 3.10: Photoluminescence lifetimes of the PbS (solid circles) and PbS/CdS (open circles) nanosheets. Each lifetime is measured at the peak intensity of the photoluminescence, 0.75 ev for PbS nanosheets and 0.83 ev for PbS/CdS nanosheets. The solid lines passing through the data points are the fitting curves. The system response function (SRF) is also shown. 3.4 ENERGY DISPERSIVE X-RAY SPECTROSCOPY (EDX) An EDS detector was used to determine the atomic percentage of lead (Pb), sulfur (S) and cadmium (Cd). Presence of different elements lead (Pb), sulfur (S), cadmium (Cd), and copper (Cu) was observed in EDS spectrum. The copper element is from the substrate for TEM.

38 29 Figure 3.11: Energy dispersive x-ray spectrum showing the presence of lead (Pb), sulfur (S), cadmium (Cd), and copper (Cu). 3.5 CONCLUSIONS AND FUTURE WORK Colloidal PbS nanosheets are very promising for optoelectronics devices, due to their high charge mobility along with 1D confinement. However, the surface trap of PbS nanosheets makes them inferior for optical devices. In this project, the surface of PbS nanosheets was modified using a very evident method to develop a CdS shell around the core nanosheets. The surface of PbS nanosheets was improved by exposing to Cd to form PbS/CdS core/shell heterostructure, which passivates the dangling bonds on the surface of nanosheets resulting an improved quantum yield and also protects the nanosheets physically and chemically. Different characterizations of core/shell nanosheets were done like PL, TEM, SEM, HRTEM, etc., and it has been confirmed through our study that, cation exchange method increases the energy gap of PbS nanosheets which was confirmed by photoluminescence lifetime measurement. An atomic flat interface between PbS and CdS was revealed by the HRTEM. This method can further be developed to create thin PbS nanosheets with shell of CdS; change in thickness will change the

39 30 structure and optical properties of nanosheets. In 2D materials, exciton dynamics can be studied with the help of enhanced optical properties. We expect that this research will help in further study of core/shell nanosheets and will enhance the feasibility and stability of nanosheets for the applications in many optoelectronics devices.

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44 35 APPENDIX-A A) Secondary-electron microscopy images of PbS nanosheets and PbS/CdS core/shell nanosheets were taken using the Hitachi HD-2300 Scanning Transmission Electron Microscope located in University of Toledo. B) The energy-dispersive X-ray spectroscopy was measured in situ using an EDX detector mounted on the Hitachi HD-2300 Scanning Transmission Electron Microscope located in University of Toledo. C) HRTEM images were measured in Air Force Research Laboratory, Dayton. D) The scanning/transmission electron microscope (STEM) characterizations were performed on 3 mm copper grids dispersed with the nanoparticles by using a FEI Talos F200 S/TEM, which was equipped with a field emission gun and was operated at an accelerating voltage of 200kV.