«Studying Physical Properties at the Nano-Scale: Thin Films, Nano-Particles and Molecules by Alon Eisenstein A thesis submitted in conformity with the ...»
Studying Physical Properties at the Nano-Scale: Thin
Films, Nano-Particles and Molecules
A thesis submitted in conformity with the requirements
for the degree of Doctor of Philosophy
Department of Chemistry
University of Toronto
© Copyright by Alon Eisenstein 2014
Studying Physical Properties at the Nano-Scale: Thin Films,
Nano-Particles and Molecules
Doctor of Philosophy
Department of Chemistry
University of Toronto
Nanomaterials have been shown to be useful for many applications. The characterization of nanomaterials is a crucial step in understanding how to control their performance to tailor their properties for desired applications. In this thesis, several nanomaterials were studied using various methods, in an effort to characterize their properties. In the first chapter, the initial growth steps of nanometer thick polyelectrolyte film, grown layer-by-layer, were studied using Kelvin Probe Force Microscopy. The initially small domains grew with each added layer.
Surface potential contrast enabled the visualization of these domains far beyond the point where no topographical variation was visible. In the second and third chapters, the potential of using collapsed-polymer nanoparticles as a carrier platform for active chemicals was studied using dye molecules as probes. Two methods were implemented, spectroscopy and isothermal titration calorimetry. Following the measurements, a binding model was proposed, which also provided thermodynamic quantification of the binding process. In the fourth chapter, an atomic force microscope probe holder was custom designed and built to enable characterization of the probes using scanning electron microscopy in an effort to facilitate specific identification of composite collapsed-polymer nanoparticles using tip-enhanced Raman Spectroscopy. In the fifth chapter, an ultra high vacuum gas dosing attachment was custom designed and built to enable a study of ii self-assembly of organic molecules on silicon surface. Pulse dosing was found to affect the selfassembled pattern on the surface. In the final chapter, the surface halogenation of copper surfaces was studied using a scanning tunneling microscope. The reaction was induced by an electron pulse. The scattered halogens, dissociated from the initial molecule, provided information regarding the reaction dynamics of the process.
iii Acknowledgments I have learned many things throughout my research, but most important were the things I have learned about myself. Discovering my own interests and passions was the most rewarding part of my journey. Throughout my research, I have had the pleasure and honor of collaborating as well as receiving help and support from many people. I have worked with lots of wonderful people who have contributed to my work, for which I am grateful. I am grateful for all those with whom I have shared my thoughts and ideas. I am thankful for all who have provided me with their genuine interest and affection. I thank you all.
Liz Drolle contributed her knowledge and skills for the work using the FMKPFM to study the surface potential of the layer by layer polyelectrolyte films. Shashwat Sharma provided the first experiments showing the effect the collapsed polymer nanoparticles had on the methylene blue’s absorption spectrum and Tina Han made the deconvolution possible by writing the code for the computational process. Isaac Herrera was kind enough to teach me how to use the isothermal titration calorimeter. Iliya Gourevich was my SEM mentor. Harikumar Rajama was a pleasure to work with on the pulse gas dosing machine, which was planned and fabricated with the amazing skills of John Ford and his team of craftsmen. Lydie Leung has mentored me in my work using the low temperature scanning tunneling microscope. The entire Polanyi group and the Goh group have been a great pleasure to work with, especially having Calvin Cheng, Nari Kim and Ning Su as close friends.
Beyond those mentioned above, a few people were key factors in my success. First of all, I am grateful to Prof. John Polanyi for providing me with the opportunity to come to Canada and to the University of Toronto. I found John to be an admirable scientist and I value all that I have learned while working with him. I am also in debt to Iain McNabb for his charming character, his wit and breadth of knowledge. My gratitude to Prof. Cynthia Goh is ever growing. Cynthia has left a tremendously positive impression on my thinking and professional choices. Her relentless strive to bring scientific and technological innovation to the hands of all the people in the world in the form of real things which will improve their lives is truly inspirational to me. Her drive for making science accessible to everyone has led to my involvement with a non-profit doing just that, an experience which I cherish. Her ongoing support, both professionally and personally iv were invaluable to me during my work. I cannot find enough words to express the extent to which I am thankful to her.
Lastly, and most importantly, I am forever in debt to my dearest family, my partner and daughters. It is for them I work to achieve accomplishments, it is their love and support which fills me with happiness doing anything I do. Without them, everything would seem meaningless to me. You will always have my endless love for you.
IntroductionIn 1959, Richard Feynman gave a talk titled “There’s Plenty of Room at the Bottom” (1). The idea of creating machines (or structures) with atomic precision was brought forth. It was more than 20 years later that “Nano-technology” was coined, with the publication of Eric Drexler’s Engines of Creation, The Coming Era of Nanotechnology in 1981 (2). It was actually an article published by a magazine called OMNI titled “Nanotechnology, molecular machines that mimic life” in 1986 discussing the book which brought the term nanotechnology into public awareness (3). It is for this reason that it is commonly accepted that Drexler is credited as being one of the founding founders of nanotechnology. In its infancy, nanotechnology was perceived as the promise to fabricate new structures and materials with atomic precision. While Feynman initial talk considered a top-down approach, one in which material is chipped away until atomically precise machines are created; Drexler suggested using a bottom-up approach where atoms and molecules are used one by one to create larger useful machines. From a chemical point of view, the preparation of nanomaterials, materials which have length scale in the nanometer range, are fundamental to deliver the potential of nanotechnology.
Nanomaterials have been found useful for an amazing variety of applications, with new materials emerging and being put to new applications all the time (4). From a research point of view, any preparation of nanomaterials also merits a fundamental study of its properties. Having the ability to characterize nanomaterials is an integral process in the development of the materials. One must first gain basic understating of the nanomaterial’s properties and how they are affected by various conditions in order to obtain the ability to produce the material with a desired property.
This is true for both conventional materials as it is for nanomaterials. However, a crucial difference lies in the potential applicability of nanomaterials. For conventional materials, the amount of material used for any specific application is a large one, considering the number of individual molecules/atoms constituting the material. Measuring bulk properties, as is traditionally done, is therefore adequate for bulk materials, since incidental variation within the material are not likely to affect the final performance of the material. For nanomaterials, on the other hand, with applications using small numbers of molecules, or even single atoms at times, a bulk measurement may be inadequate. A machine which is only 100 atoms high, as Richard Feynman suggested, will likely affected by similar sized defects on the surface on which it rests, vi defects which may seem insignificant for traditional applications. For this reason much advancement has been made in the last few decades developing instruments with atomic resolution. The invention of the Scanning Tunneling Microscope in 1982 (5) has delivered the much sought after atomic resolution. From this pivotal moment, individual atoms and molecules could be probed, and later manipulated to provide physicists and chemists the capability to study how matter behaves at the atomic/molecular level. It was now possible for to study both bulk properties as well as individual properties of nanomaterials, depending on need and relevance.
In our lab, two classes of polymer based nanomaterials have been developed and explored for their potential applications. The layer-by-layer growth of polyelectrolyte thin film has been studied with respect to its relaxation mechanism (6), salt effect on morphology (7) and its kinetics (8). There are many variations of layer by layer films and many methods have been used to study them (9). Methods of characterization typically fall into one of two categories. Optical methods (e.g. UV-Vis, fluorescence, FTIR, ellipsometry and X-ray reflectivity) can provide a myriad of information, with the limitation of lateral resolution and are therefore mainly bulk averaging measurements. Surface morphology methods (e.g. SEM, TEM and AFM) are useful in providing nanometer resolution of the surface’s features, although limited additional information is available. It is therefore of interest to extend the applicability of localized measurement, typically studied with nanometer resolution using atomic force microscopy (10). In this work surface potential is probed with nanometer resolution, and it is this method which is the focus of the first chapter. By examining the initial growth steps of the thin polyelectrolyte films, new insight is gained about the extent of contribution lower layers have on subsequent additions of layers.
The second type of polymer based nanomaterial explored in our lab is collapsed-polymer nanoparticles (11). These nanoparticles are prepared through the collapsing of a polyelectrolyte in an aqueous solution, followed by a cross-linking step to stabilize the collapsed form, and finally chemically treating the nanoparticle to obtain a composite material. The uniqueness of the method is in its versatility, enabling the preparation of an endless variety of different nanoparticle using a single method simply by varying the initial chemicals. Although these nanoparticles have already been found useful for several commercial applications, their full potential has yet to be attained. One such potential use entails the loading of active chemicals into the ‘empty’ collapsed-polymer nano particles. Depending on the choice of active chemical, vii multiple applications can be explored (e.g. targeted drug delivery). Studying the loading behavior and mechanism of these particles has yet to be undertaken, and in chapters 2 and 3 two approaches are discussed, with a proposed mechanism. As noted before, the method of preparation of the nanoparticle is universal, thereby enabling the preparation of numerous variations of the collapsed nanoparticles. The applicability of the methods of study described in chapters 2 and 3 provides a benchmark for further studying the properties of the variety of collapsed nanoparticles which can be potentially prepared.
Perhaps the most challenging of experiments with respect to studying the nanoparticles is the possibility of chemical identification of individual nanoparticles. While scanning probe microscopy can routinely provide nanometer and even atomic lateral resolution, an inherent limitation is the lack of specific identification of the probed objects. Two different nanoparticles with different chemical composition will be indistinguishable using conventional probe microscopy methods. A potential hope to augment this issue is seen with Tip Enhanced Raman Spectroscopy (12); a localized probe method providing nanometer resolution based on chemical signature utilizing Raman spectroscopy integrated with probe microscopy. Still relatively a novel measurement, specialized probes are required to successfully measure the Raman scattering from a nanometer scale location. The special probes needed for the method are still not reliably commercially available and require in house fabrication. For this purpose, a custom build holder was designed and is described in chapter 4. The described holder is used in the characterization process following the fabrication of the specialized probes, an important step in the process of establishing a reproducible fabrication methodology.
Following Feynman (1) and Drexler’s (3) prediction for nanotechnology, it is obvious that any nanometer scale material with atomic precision must be able to couple to at least microscopic size components in order for it to be included in real life size structures. In order to produce large scale materials using nanotechnology self assembly is commonly used because it has atomic precision in the preparation process. It was noted in the field of self assembly that “we want to understand how nature self-assembles structures, we want to understand her principles and techniques, and, we want to learn how to use self-assembly to build engineered systems” (13). In my work, an ordered surface is used as a template to guide the added molecules into ordered structures through self-assembly (14). The control of such self assembly processes can provide means of tailoring the properties and usages of the materials, requiring the understanding of the viii factors which affect the self assembly process. In the fifth chapter, an extension to an ultra-highvacuum scanning tunneling microscope was fabricated. With this addition to the system, ultrafast gas dosing was enabled, providing the opportunity to study the effect the dosing time has on the surface binding process of single molecules onto the surface of a silicon substrate.