Paper No. 5
Presentation Time: 10:00 AM


YANG, Fan, LIN, Yu and MAO, Wendy, Geological and Environmental Sciences, Stanford University, 450 Serra Mall Braun Hall, Building 320, Stanford, CA 94305,

Diamondoids are cage-like, ultra-stable, saturated hydrocarbons, with carbon-carbon frameworks superimposed on the diamond lattice and originally found in petroleum. The exceptional electron photoemission from thin film of diamondoids has excited interests in using these unique materials as functional elements to regulate energy flow at the nanoscale. Selective chemical functionalization further changes the electronic properties of the system and brings many more possible applications. Pressure (P), could lead to formation of novel phases of pure diamondoids and provide alternative routes for potentially synthesizing higher diamondoids. In addition, the electronic properties of functionalized diamondoids may also be enhanced with P.

We have compressed pure diamondoids: the [121] tetramantane up to 20 GPa and observed a phase transition at high pressure from powder x-ray diffraction (XRD). Meanwhile, we observed only continuous changes in the CCC bending and CC stretching regions from Raman spectroscopy, suggesting that the carbon framework of the diamondoid molecules remains intact with pressure. The phase transition observed in XRD is probably due to the molecular packing rearrangement.

We also discovered interesting pressure-induced electronic properties changes in newly synthesized functionalized diamondoids, particularly the Ag-adamantane-thiol, from Raman spectroscopy. First, there is an abrupt emergence of photoluminescence when the crystals are compressed to 2-3 GPa, suggesting an indirect to direct band gap transition. Second, the phonon modes begin to diminish, when the crystal is further compressed to 4-5 GPa. Pressure-induced disappearance of the Raman modes has been observed in inorganic strongly-correlated materials, and is associated with insulator-to-metal transitions. In our case this phenomenon happens at much lower pressures, presumably due to the higher compressibility of the organic-inorganic hybrid structure.

This project will be advanced by combining single crystal XRD to unambiguously determine the high pressure structure of [121] tetramantane and functionalized diamondoid molecules. We will also collaborate with theoreticians to develop a further understanding of this novel material for future energy application.