Browsing by Author "Liu, Xiaoyang"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Generation of Heptagon-Containing Fullerene Structures by Computational Methods
    Liu, Xiaoyang (Virginia Tech, 2016-12-14)
    Since the discovery three decades ago, fullerenes as well as metallofullerenes have been extensively investigated. However, almost all known fullerenes follow the classical definition, that is, classic fullerenes are comprised of only pentagons and hexagons. Nowadays, more and more evidence, from both theoretical and experimental studies, suggests that non-classical fullerenes, especially heptagon-containing fullerenes, are important as intermediates in fullerene formation mechanisms. To obtain fundamental understandings of fullerenes and their formation mechanisms, new systematic studies should be undertaken. Although necessary tools, such as isomer generating programs, have been developed for classical fullerenes, none of them are able to solve problems related to non-classical fullerenes. In this thesis, existing theories and algorithms of classical fullerenes are generalized to accommodate non-classical fullerenes. A new program based on these generalized principles is provided for generating non-classical isomers. Along with this program, other tools are also attached for accelerating future investigations of non-classical fullerenes. In addition, research to date is also reviewed.
  • Thumbnail Image
    LDA Team Project in CS5604, Spring 2015: Extracting Topics from Tweets and Webpages for IDEAL
    Pumma, Sarunya; Liu, Xiaoyang (2015-05-10)
    IDEAL or Integrated Digital Event Archiving and Library is a project of Virginia Tech to implement a state-of-the-art event-based information retrieval system. A practice project of CS 5604 Information Retrieval is a part of the IDEAL project. The main objective of this project is to build a robust search engine on top of Solr, a general purpose open-source search engine, and Hadoop, a big data processing platform. The search engine can provide documents, which are tweets and webpages, that are relevant to a query that a user provides. To enhance the performance of the search engine, the documents in the archive have been indexed by various approaches including LDA (Latent Dirichlet Allocation), NER (Name-Entity Recognition), Clustering, Classification, and Social Network Analysis. As CS 5604 is a problem-based learning class, teams are responsible for implementation and development of solutions for each technique. In this report, the implementation of the LDA component is presented. LDA aids extracting collections of topics from the documents. A topic in this context is a set of words that can be used to represent a document. Details of how LDA worked with both small and large collections are described. Once the implementation of the LDA component is integrated with other processing and Solr, we are confident that performance of the information retrieval system of the IDEAL project will be enhanced.
  • Thumbnail Image
    Machine Learning Models in Fullerene/Metallofullerene Chromatography Studies
    Liu, Xiaoyang (Virginia Tech, 2019-08-08)
    Machine learning methods are now extensively applied in various scientific research areas to make models. Unlike regular models, machine learning based models use a data-driven approach. Machine learning algorithms can learn knowledge that are hard to be recognized, from available data. The data-driven approaches enhance the role of algorithms and computers and then accelerate the computation using alternative views. In this thesis, we explore the possibility of applying machine learning models in the prediction of chromatographic retention behaviors. Chromatographic separation is a key technique for the discovery and analysis of fullerenes. In previous studies, differential equation models have achieved great success in predictions of chromatographic retentions. However, most of the differential equation models require experimental measurements or theoretical computations for many parameters, which are not easy to obtain. Fullerenes/metallofullerenes are rigid and spherical molecules with only carbon atoms, which makes the predictions of chromatographic retention behaviors as well as other properties much simpler than other flexible molecules that have more variations on conformations. In this thesis, I propose the polarizability of a fullerene molecule is able to be estimated directly from the structures. Structural motifs are used to simplify the model and the models with motifs provide satisfying predictions. The data set contains 31947 isomers and their polarizability data and is split into a training set with 90% data points and a complementary testing set. In addition, a second testing set of large fullerene isomers is also prepared and it is used to testing whether a model can be trained by small fullerenes and then gives ideal predictions on large fullerenes.
  • Thumbnail Image
    The Studies of Fullerenes and Metallofullerenes in Geometry, Electron Transfer, Chromatography and Characterization
    Liu, Xiaoyang (Virginia Tech, 2019-08-14)
    Since their discovery, fullerenes and metallofullerenes have been investigated regarding their structures, synthesis, isolations, and applications. The highly symmetric structures of fullerenes and metallofullerenes lead to extraordinary physical properties, such as electron transfers, and attract major attention from the science community. It has been well established that the stabilities of fullerenes and metallofullerenes can be estimated by recognizing structural patterns. Recently, we developed a generalized spiral program and additional codes and believe they are useful for fullerene/metallofullerene researchers. The higher fullerenes, those with more than 90 carbon atoms, also follow certain structural patterns. In our studies, we have shown that the higher fullerenes with tubular structures are stable in thermodynamics and can survive the aminopropanol reaction, but other spherical fullerenes cannot. For the past three decades, great efforts have been devoted to applying fullerenes and metallofullerenes as electronic materials. In our studies, we find the ground state electron transfer properties endow metallofullerenes as an ideal material for perovskite solar cells to enhance the stabilities. It has been shown in our investigations that common metallofullerenes, such as Sc3N@C80, are capable to be as the electron transfer layers in perovskite solar cells, and the test demonstrates that our novel perovskite solar cells may achieve high stability and high efficiency. The electron transfer abilities of metallofullerenes are studied with the M2@C79N since electron densities located in between the two metal atoms convert between a single electron bond and a double electron bond. The huge spherical electron delocalized structures of fullerenes and metallofullerenes lead to strong interactions with other delocalized systems, such as graphene. Previous studies have shown that graphene has a unique ability in molecular adsorptions. However, the graphene surface is not always flat and the rippled areas have effects on the packing styles. Therefore, we examined the behavior of fullerenes on the rippled graphene surface and then compared with another flat molecule, PTCDA. The results show that the effect of rippled areas varies due to molecular structures. This study gives instructions for electronic device manufacturing using graphene and fullerenes. In our studies, polarizability is a key factor of fullerenes and metallofullerenes. It has been shown that the chromatographic retention behavior has a strong relationship with the average polarizability of a molecule. Based on the experimental data, we built a model for the prediction of chromatographic retention times using computational polarizabilities. After that, we validated the model by two series of chromatographic data. The characterization of carbon-based materials has been long investigated. In the last chapter, we introduce a dynamic nuclear polarization-based method to characterize the structures of chars and studied the adsorption of oxygen on the activated radical sites. Overall, the dissertation reports my Ph. D. studies in the areas including theoretical studies of fullerene geometries, chromatographic models, applications and also experimental studies of the applications of fullerenes/metallofullerenes and characterization.