Electron Microscopy and Analytical Spectroscopy of Silicon and Germanium Metalattices

Restricted (Penn State Only)
Author:
Yu, Shih Ying
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 25, 2017
Committee Members:
  • Suzanne E. Mohney, Dissertation Advisor
  • Suzanne E. Mohney, Committee Chair
  • John V. Badding, Committee Member
  • Chris Giebink, Committee Member
  • Nasim Alem, Outside Member
Keywords:
  • 3D artificial lattices
  • Inverse opal
  • Ordered nanostructures
  • Silicon
  • Germanium
  • Metalattice
  • Transmission electron microscopy
  • Electron energy loss spectroscopy
  • Electron tomography
  • Quantum confinement
Abstract:
A metalattice is an ordered nanostructure with a periodicity from 1-60 nm. The successful fabrication of metalattices opens up the possibility of designing new materials with previously inaccessible properties, instead of relying on the limited options offered by natural. Information about morphology and chemical bonding in the metalattices is critical for understanding their properties as feature sizes scale to mere nanometers and are reduced to the quantum regime. This thesis addresses a systematic characterization of Si and Ge metalattices fabricated using 60, 30, and 14 nm opal templates. We developed a protocol to release metalattices from their template using HF vapor. The metalattice, composed of amorphous or nanocrystalline silicon, shows long-range order and interconnectivity in two-dimensional (2D) microscopy images. High-resolution transmission electron microscopy (HRTEM) and electron dispersive spectroscopy (EDS) were used to investigate the nanostructures synthesized by high-pressure chemical vapor deposition (HPCVD), providing both crystallinity and chemical information of the infiltrated material. In addition, three-dimensional (3D) volume reconstruction using scanning transmission electron tomography was conducted to retrieve structural information in the third dimension. The face centered cubic (FCC) symmetry of the metalattice was confirmed by observations from many different angles using a tilting process and computer reconstruction. The metalattice closely adopts the shape of the inverse opal. Changes of lattice parameters and porosity of the metalattice due to contact between spheres in the template were observed. The relationship between the contact area and the resulting shape of the metalattice was compared to geometric calculations and found to be consistent with the measured results from our tomographic reconstruction. Finally, the cropping planes of the reconstructed octahedral and tetrahedral site meta-atoms confirmed the void-free infiltration by HPCVD and are consistent with conclusions about a lack of voids from positron annihilation lifetime spectroscopy (PALS). Electron energy loss spectroscopy (EELS) and scanning transmission electron microscopy (STEM) were applied to investigate the local electronic structure of the Si metalattices at the nanoscale. The Si L2,3 core-loss edge of the Si metalattice was blue shifted compared to the onset measured from bulk Si. In addition, a shape change was found for the Si L2,3 core-lose edge of the Si metalattice, suggesting modification of the band structure. HRTEM revealed that the Si metalattice is composed of Si nanocrystallites ranging from 2-10 nm, and the nanocrystal size decreases from the octahedral site meta-atoms to the tetrahedral site meta-atoms and is about 2-3 nm in the meta-bonds. The largest blueshift of 0.55 eV was found in the meta-bond, which is the smallest feature of the metalattice and has the smallest average nanocrystal size. Furthermore, an enhanced interband transition (E2) peak was found in the low-loss EELS of the small features in the Si metalattice, and this scattering event even surpassed the volume plasmon peak in the meta-bond region. In summary, we observed a systematic change in EEL spectra in both core-loss and low-loss regions, showing a gradual blueshift in energy which is smallest in the octahedral site meta-atom and largest in the meta-bonds. Both the special confinement applied by the nanocrystallite inside the metalattice and the periodic nature of the metalattice should contribute to the modification of the band structure of the Si metalattice. Finally, room temperature photoluminescence was observed from the Si metalattice. Surface passivation using atomic layer deposition (ALD) of Al2O3 successfully preserved the luminescence intensity that was diminished after the Si metalattice was released from the silica opal template. A broadband photoluminescence ranging from 600-900 nm was found, and the intensity was three times stronger after the passivation than that of the as-deposited Si metalattice. Mechanisms related to this emission were discussed. A dynamic PL study is suggested for further understanding of the origins of the photoluminescence from the Si metalattices.