| AVS 55th International Symposium & Exhibition | |
| Surface Science | Thursday Sessions |
| Session SS2-ThA |
| Session: | Novel Reactive Surfaces |
| Presenter: | A.B. De La Ree, University of California, Irvine |
| Authors: | A.B. De La Ree, University of California, Irvine V. Johánek, University of Virginia J.C. Hemminger, University of California, Irvine |
| Correspondent: | Click to Email |
New insight has been obtained on the conversion of ethylene to carbon clusters by thermal dehydrogenation on Pt(111). Previous experiments have shown that thermal dehydrogenation following saturation exposure of ethylene to Pt(111) results in the formation of well defined carbon clusters. This occurs through a process of aggregation during the dehydrogenation process in the temperature range from 400K to 700K. Previous experiments have shown that heating an ethylene covered Pt(111) surface to temperatures above 800K leads to the formation of large sheets of monolayer graphite (graphene). The aggregation to form carbon clusters leaves open Pt areas available for additional adsorption. It had not been previously determined whether the adsorption and dehydrogenation of additional ethylene would lead to the growth of the initial clusters or to the nucleation of more clusters of the same size. We have utilized UHV-Variable temperature scanning tunneling microscopy (VT-STM) to address this question. In our experiments, a saturation coverage of ethylene is adsorbed on the Pt(111) surface at room temperature forming a layer of the well known ethylidyne dehydrogenation intermediate. The surface was then heated to 700K leading to further dehydrogenation and formation of carbon clusters that are easily observed by VT-STM. Our STM images show that these carbon clusters are 15±2 Å in diameter and containing an average number of 34±9 carbon atoms per cluster. The surface was then exposed to additional ethylene at room temperature and again heated to 700K. This process was repeated in a number of cycles until the dehydrogenation activity of the Pt(111) surface was completely suppressed. Repeated dosing of ethylene onto the surface followed by annealing led to an increase in the number of carbon clusters without an increase in the size of previously produced clusters. The Pt(111) dehydrogenation activity was completely suppressed after 4 such cycles. The only growth in cluster size that was observed was due to agglomeration of clusters once the surface was covered with a high density of clusters. Continued cycles, such as were utilized in these experiments, leads to the formation of a graphite adlayer on the platinum (presumably via agglomeration of the clusters at high cluster density). This was observed at temperatures well below the onset of long range graphene formation in single ethylene dose experiments.