Formation of Ge alloy nanocrystals embedded in silica Eugene E. Haller, University of California-Berkeley, DMR Ge-Au particles Above: Transmission electron microscopy images of a) Ge-Au bi-lobed nanocrystals after anneal b) Ge-Au mixed phase nanoparticles after laser melting c) Ge-Ag bi-lobed nanoparticles formed from the melt. We have formed two new materials containing bi- lobed particles of Ge binary alloys: GeAu and GeAg. Two elements which are not miscible, such as Ge and Au, phase segregate under equilibrium conditions. Given the appropriate interface energies, bi-lobed particles will form from the melt in a constrained matrix. Subsequent melting and quenching can form non-equilibrium phase states in the particle. Indeed, reversible phase changes in the GeAu system were observed by cycling between pulsed laser melting and rapid thermal annealing.* * J. Guzman, et al., “Reversible phase changes in Ge-Au nanoparticles,” Appl. Phys. Lett. 98, (2011) Thin films of GeAu and GeAg are fabricated using rf sputtering and ion implantation. Transmission electron microscopy (images on right) is used to image the nanostructure. In the GeAu system, synchrotron X-ray diffraction established that laser melting induces a mixed phase of metastable β Ge- Au and Ge. Rapid thermal annealing restores the phase segregation. Raman spectroscopy and X-ray diffraction confirmed the two different states. 50 nm a) b) 5 nm Phase Change Ge-Ag particles c)

Formation of Ge alloy nanocrystals embedded in silica Eugene E. Haller, University of California- Berkeley, DMR Education / Broadening Participation of Underrepresented groups: Julian Guzman, who was born and raised in Columbia, has been the major contributor to this project. Julian completed his PhD in May 2011 and is employed in the semiconductor industry. Karen Bustillo, a returning graduate student, is fabricating the Ge-Ag films and learning transmission electron microscopy. Technological Outputs: Phase change materials are ubiquitous in memory applications. The ability to quickly and reversibly change the state of a discrete component establishes the storage of a 1 or 0. We have designed nanoparticles with diameters of nm, using materials and fabrication techniques commonly used in the semiconductor industry. In the equilibrium state, Ge-Au nanoparticles form a bi-lobed structure. Upon laser melting (30 nanosecond pulse), the structure becomes a mixed phase. Rapid thermal annealing at 80C restores the equilibrium bi-lobed particle. This phase change has been cycled 10 times demonstrating robust reversibility of this new phase change material. Above: Radial integration from an image plate detector of synchrotron X-ray diffraction. Crystalline peaks are observed after the anneal (blue), but disappear after laser melting (gold). The peaks reappear after rapid thermal annealing (green). The top two curves (grey and magenta) show results after 10 cycles of melting and annealing. The synchrotron data were collected with the cooperation of five graduate students and a post doc, providing them with hands-on experience at a beam-line.