From Nanoparticle Chains to Nanorods: Control of Pb Doped ZnO Nanostructures by Laser Ablation Gordon Henry Waller1, and Xi-Wen Du2 1 Virginia Polytechnic Institute and State University, Department of Materials Science and Engineering, Blacksburg, VA 24060, USA. Email:
[email protected] 2 Tianjin University, Department of Materials Science and Engineering, Tianjin, 300072, PR China. E-mail:
[email protected]
ABSTRACT: Pb doped ZnO nanostructures were created using millisecond pulse width laser ablation. Nanoparticle chains (NPCs) formed during ablations in air or of a pure Zn target, and when argon (Ar) flow was introduced it could be used to mediate the nucleation and growth of nanorods along the NCA from a lead containing target. The nanorods are crystalline with lengths on the order of hundreds of nanometers and have diameters on the order of ten nanometers. Branching, kinking and a variety of growth directions were observed. Samples were analyzed using SEM, TEM and EDX. Liquid nanoparticles are the primary product of the low energy ablation technique which allows for the proposed nanorod growth mechanism through selective oxidation of the particle surfaces.
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INTRODUCTION Participation in the IREE China program was my first trip to China. I had traveled internationally previously, including a 1 month long exchange program to Cologne, Germany during high school however the purpose of for that trip was focused on culture and tourism rather than academics. I applied to the IREE program as a graduating senior at Virginia Tech in the department of Materials Science and Engineering. At Virginia Tech, students can apply early to the graduate school and begin their research and graduate coursework towards a masters degree – I had already elected to participate in this 5 year BS/MS program, and thus had already begun graduate level research before my participation in research activities in China. My research at Virginia Tech is and was under Dr. Jeremiah Abiade, focusing on oxide thin film deposition using pulsed laser deposition (PLD). Virginia Tech offers a number of opportunities for international experiences, and has recently developed a partnership with Tianjin University’s Materials Science and Engineering program. So, when I found out about the IREE program via a department wide email, I applied and discussed with the faculty in charge of this partnership the possibility of using the IREE award to travel to TJU. This proposition was accepted and encouraged, as the faculty at Virginia Tech felt it would help improve my research experience and help develop the fledging partnership between the two schools. Tianjin University is located in the center of Tianjin, a city of roughly 11 million people in northern China. Its’ development coincided with the development of Beijing due to their proximity; however TJU has a long history preceding the development of Tianjin. It was the first engineering school in China to adopt a westernized modern curriculum and remains as one of the best engineering schools in China today. When I was accepted for the IREE program, I had made contact with some of the faculty at TJU, however I did not know of my specific research advisor or assignment. Through the experience of students from Virginia Tech who had already traveled to TJU, I knew roughly what my accommodations would be. It was not until I arrived in Tianjin that I met Dr. Du and was introduced to his research group. Dr. Du also used laser ablation (a similar synthesis technique to PLD) for the fabrication of nanostructures. This common interest and base of research experience was the catalyst for my decision to join Dr. Du’s group for the summer. I arrived in China on the last weekend in May and worked with Dr. Du from Tuesday the following week to the second week in August – roughly three months. My motivation in applying to the IREE program was twofold. First, I wanted to expand my research experience in a competitive, competent environment to prepare me for a career as a research scientist. This criteria was fulfilled by Dr. Du. The second was to learn something of Chinese culture in regards to professional engineering and research, education and everyday life. As my career goal is to become a PhD holding researcher, I felt this experience would give me an advantage when it came to working as part of a research group domestically or internationally. Members of my own research group and many other graduate students at Virginia Tech are Chinese nationals, and before my trip to China I had trouble understanding their personal motivations for studying in America. Following my IREE experience, I feel I my experience satisfied or exceeded all of my original motivations for applying.
PREPARATION FOR IREE EXPERIENCE Preceding the IREE experience, I had no formal training in Mandarin Chinese. I had however engaged in a few months of self study which I began before applying to the IREE program. My original reason for studying Chinese was in preparation for a trip to China with a colleague, originally planned for the same summer of the IREE program. Other than working with Chinese students, I also no familiarity with Chinese culture beyond general a general history of the country. I participated in the Purdue orientation program. Here I was happy to interact with the IREE organizers at Purdue and my fellow IREE awardees. I was pleased with the accommodations and scheduled events for the orientation as well. While the language training was beneficial, it was too brief to provide any significant improvement to my language ability. It did however provide me with resources and helpful phrases which I later practiced in China. My favorite part of the orientation was the Chinese culture sections. This I felt was both interesting in terms of the broad history covered, but also relevant given the circumstances of our trip. Our discussions regarding the emergence of modern Chinese engineering education, aspects of Chinese professionalism, and most importantly the concept of “guanxi” proved to be remarkably accurate and prevalent once I arrived in China. Our introduction to Chinese history was especially useful when interpreting aspects of modern Chinese society. In discussions with my Chinese colleagues, I would refer to modern and historical events that I had learned about during my orientation and found that my colleagues were often pleased with the accuracy and breadth of my knowledge in this area (all of which was from the orientation sessions). For this reason I felt the selected topics were especially well chosen. I also found that the Chinese in general have a greater familiarity and pride in their national history. While many Americans are certainly well versed in our revolutionary origins, the time frame of our entire existence is short in comparison to the Chinese, which was a point that was often harped upon in good nature. At first I found the identification with historical events that happened before the fall of the Roman Empire as strange – certainly China wasn’t even really China so long ago, at the very least it wasn’t the current configuration, but many Chinese still discussed historical events as evidence for supporting modern affairs. My experience with the orientation program was also especially worthwhile because I left from Purdue directly for Tianjin. As I had begun orientation even before my own college graduation ceremony, it was worthwhile for me to have two weeks to prepare myself mentally for the following three months in China. The only aspect of the orientation program I considered to be a drawback is the underutilization of the panel discussions with Chinese students and scholars. I felt these were often superficially focused on discussing hobbies and food rather than a legitimate discussion regarding differences in education and professional mindsets. These discussions occasionally edged into these areas but perhaps made for uncomfortable subject matter. It is my opinion that frank, open and honest discourse between international parties is the key to progress in international collaboration; however this is often superseded by an emphasis on learning cultural idiosyncrasies.
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RESEARCH ACTIVITIES AND ACCOMPLISHMENTS OF THE INTERNATIONAL COOPERATION I became the equivalent of a full time PhD student while working with Dr. Du’s research group. I did this not because of an expectation from Dr. Du or the other students, but out of a desire to set a precedent for myself and American students in general. I was the first American to ever work with Dr. Du’s group, excluding collaborations with American universities on publications (i.e. through email or mailing samples for analysis). I took this as a great personal responsibility, not only because I was an American but because I was representing the NSF and Virginia Tech. Dr. Du is the leader of a very successful research group consisting of three PhD students, about 10 masters students and a number of other post doctoral researchers and assistant professors under him. He had a well supplied lab (even by American standards) with numerous pieces of synthesis and analytical equipment. He also had specially reserved time to use the TEM and SEM owned by the Materials Science Department collectively – a luxury that was not shared by any other research group. There were three main areas of research in Dr. Du’s group – these were the synthesis of nanostructures by laser ablation (my research area), the synthesis of nanostructures by solution routes, and the integration of these structures into devices for photovoltaics. Every student in Dr. Du’s lab was expected to work for a minimum of 40 hours a week (masters students only had to work for 10 while the semester was still in session), and submit a weekly report detailing their experimental progress. These reports were compiled and presented every Sunday evening by one student on a rotating basis. Once every three weeks, each research subgroup had a meeting with Dr. Du to discuss relevant literature each team member had read. Additionally to the weekly report, every student was expected to present once a month for approximately 45 minutes at the Sunday evening meeting following the weekly report synopsis. During my stay, I gave two of these presentations. Another informal meeting was held daily in the late afternoon as a kind of break. We drank green tea and ate snacks, and Dr. Du would ask students to discuss issues with their research they needed help on. The majority of the interaction between Dr. Du and me regarding my own research took place in this setting. On occasion I would answer questions during this time about America. Politics and the American social structure were the most common topics, which I treated with as much candor as possible. I was surprised that Dr. Du and a few of the students more comfortable with their English would respond in turn with similarly objective analysis of China. My normal daily routine would be to arrive at our office, which was a two room renovated section of a building that would have been condemned in the U.S., by 9 am. I would usually spend the morning reading research papers, reviewing collected SEM or TEM data from previous experiments, or organizing any of the two for the weekly or monthly presentations. After lunch, I went to our laboratory which was in another building and worked until dinner time. Saturdays was our reserved SEM day, so I would work for most of the day on Saturday between SEM and finishing my weekly report. I did travel on occasion, which was happily accepted by Dr. Du. He encouraged me to travel and enjoy China as much as I wanted, and placed no obligations on me to make up lost work time, though I usually did anyways. My interaction with Dr. Du and the other students was always extremely friendly and accommodating on their end. One of my
chief motivations for working hard was to make my presence in their lab as painless for them as possible, at which I feel I was successful. My research dealt with fabrication of novel Pb doped ZnO nanostructures using laser ablation. The basis for this work was some previous work done by Dr. Du’s group, which suggested a special growth scenario occurred during our laser ablation technique1,2. It was this work that I expanded upon. The work was very successful, and complete enough that I was able to draft and edit a manuscript for publication co-authored with Dr. Du. We are currently in the final stages of revisions. This work was related to my previous work at Virginia Tech as it used a similar fabrication technique. My master’s research, which I will also complete at Virginia Tech, also deals with one dimensional nanostructures. My work in China gave me an invaluable introduction to this field. Figure 1 shows a representative SEM image of the structures I fabricated in my research, and Figure 2 shows the growth mechanism established by Dr. Du and myself to explain their formation. The mechanism in Figure can be explained by an excerpt from our publication: “Nanodroplets (light grey) are formed during ablation, and form into NCAs via a random collision during expansion towards the substrate (Figure 2 top). When deposited in air (route A), the nanodroplets are oxidized to form a compact amorphous shell (marked by a black border in A1), which defines the shape of the particles and prevents further oxidization in any direction. Subsequently, the center crystallizes (A2) into a zinc particle and results in the NPC morphology (A3). When deposited in the flowing argon (oxygen deficient) environment (route B), oxidation is slowed down by the limited availability of the oxygen reactant. Oxide nuclei (dark grey) first form on the surface of nanodroplets (B1) and grow into tapered nanorods by depleting the liquid and reducing their size (B2). At the B1 stage, lead may increase surface tension of the nuclei and create local inert regions on the droplets, thus prevent the formation of a compact oxide shell and promote the continuous growth of nanorods until the liquid zinc is consumed (B3).”
Figure 1 Pb Doped ZnO Nanorod Chains
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Figure 2 Nanorod Chain Growth Mechanism
BROADER IMPACTS OF THE INTERNATIONAL TRAVEL Because of the standing relationship between the Virginia Tech and Tianjin University’s Materials Science and Engineering departments, I was asked on a few occasions to participate in discussions with faculty representatives from both Virginia Tech and Tianjin University. The goal of these discussions was to help the program at TJU develop into a stronger program to benefit their students. Unfortunately, many of the aspects of the program at Virginia Tech that myself and the other students from Virginia Tech (two additional students came from Virginia Tech at the end of my stay for the purpose of these discussions and tourism) would be difficult to employ at Tianjin University due to the size of their program, which is roughly ten times larger than ours. However, these activities directly contributed to the future collaboration between our universities. While in China, I engaged in everyday activities with my coworkers and other students at the university. From eating meals, to shopping and visiting local tourist attractions, I was able to communicate and bond with them outside of the workplace. This allowed me to practice the Chinese language and more importantly to develop a genuine friendship with my coworkers. It was this friendship that opened the door to some of the most revealing conversations regarding Chinese culture I had while abroad. Originally reserved students eventually humored my inquiries, which ranged from their experiences in education, the social lives of Chinese students, to their thoughts on America and China from a global standpoint. From a personal broader impacts perspective, the most significant effect of my trip to China was on my identity as an American and a professional engineer. I was struck by the strong sense of national identity that was held and encouraged by the Chinese. It was clear to me that the Chinese were trying very hard at the governmental and educational level to improve China economically. Also the personal motivations of individual students in China were vastly different from that of the engineering students in America. Specifically in regards to why they wanted to become engineers. In America, many students shy away from a field such as engineering unless they are dedicated to the pursuit of an engineering degree. I would go so far as to say that engineering is pursued by some simply because it is a challenge. In China the focus is exclusively on finding a job. While this is true in general for American college students, engineers do not typically find themselves in this mindset. My work in China has given me insight into what I believe will become the future relationship between China and the United States. China is striving to become independent in many realms, and education is one of them. Perhaps in the coming years the desire for Chinese students to study abroad will subside, as domestic options become available to them. This will coincide with a downturn in attendance and graduation rates in United States graduate programs from Chinese nationals. It will be the responsibility of those American engineers who have already been trained to participate in the encouragement and tutoring of the future generation of American engineers. China has convinced me of the importance of fostering my own national identity, as it will be my generation of engineers who interact with this new China.
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For the future, I would certainly consider continuing my career abroad. Specifically, I hope to secure a post-doctoral position in another country. I believe that international experience cannot be confined to a single event – no matter the duration.
DISCUSSION AND SUMMARY My most significant research accomplishment was the vast amount of new knowledge I acquired through literature reviews and exposure to a new experimental technique. I also feel that I developed a strong sense for the scientific method. This was reinforced by the occasional language barrier – in other words I had to rely on myself to solve problems that I could not easily explain. Hopefully my research manuscript will be published and serve as a record of my work in China. For future IREE participants, I recommend you bring a strong work ethic and an open mind to China or wherever you travel abroad. Despite whatever goals you set for yourself regarding research, remember that social interaction is also a key part of the collaborative experience. This will allow you and your host to benefit from your research assignment fully. Serve as a representative for yourself and your home institution and country.
REFERENCES 1.
K.Y. Niu,; J Yang; S.A. Kulinich; J Sun; H Li; X.W. Du Journal of the American Chemical Society 2010, 132, 9814
2.
K.Y. Niu; Yang J; J Sun; H Li; X.W. Du Nanotechnology 2010, 21, 295604
ACKNOWLEDGEMENTS I would like to thank the IREE program and the NSF, grant number 0965733 for their roles in organization and financial support. I would also like to thank Dr. Xi Wen Du and Dean Zhen Duo Cui of the Materials Science and Engineering program and Tianjin University, and Dr. GQ Lu of the Materials Science and Engineering program at Virginia Tech. Gordon Henry Waller received his bachelors of Science degree in Materials Science and Engineering at Virginia Tech in 2010. Currently he is pursuing his masters degree at Virginia Tech, hoping to complete the studies for his MS in 2011. Following his masters study, he will apply for PhD programs in Materials Science and Engineering. His research interests include thin films and other nanostructures, electronic materials and energy harvesting technologies. Dr. Xi Wen Du received his PhD from Tsinghua University in 2001. Following this he served as chief engineer at Motorola in Tianjin, China from 2001 to 2002, served as a postdoctoral researcher at the National Materials Research
center in Japan, and jointed Tianjin Universities Materials Science and Engineering program in 2003. He is currently a full professor in the College of Materials Science and Engineering, with research focused on quantum dots and functional nanomaterials.
