The CREST established collaborative research programs and has put forth a strong effort to foster scientific advances by bringing together researchers and educators and creating intellectual synergy among scientists and engineers. The collaboration includes NCCU, NASA, NREL, CSM (nanostructures and photovoltaics project 1), CCMR (nanostructure characterization, development of biochemical detector and IR sensor), Jefferson National Laboratory (JLab) (on material formation, few body calculations, hypernuclear physics, and development of the world's most intensive positron beam), the National Renewable Energy Laboratory (NREL) (photovoltaics theory and material characterizations), local centers at Duke University (material formation and meta-materials), UNC, NCSU, and Shaw University (photovoltaics); local community colleges Wake Technical Community College and Durham Technical Community College (education). The goal of the CREST is to develop a program that will include cutting edge facilities for fundamental and applied multidisciplinary material research and an outstanding scientific and educational program built through strong collaborations between members.
The CREST experimental program is based on the unique nanostructure deposition system available at NCCU that includes ultrafast Pulsed Laser Deposition (PLD) and pulsed electron deposition (PED) that is enhanced with Pulsed Free Electron Laser Deposition (PFELD) at nearby Duke FEL facility and (PFELD) at Jefferson National Laboratory that are available to our group. Our system is unique in the sense that it allows for picosecond to femtosecond deposition with two types of pulsed beams, electrons and photons, which have the same overall intensity and pulse width. The Duke FEL has the broadest tuning range of any FEL, with output tunable from 15 micrometers to 150 nanometers, and the JLab FEL is the world's most powerful FEL (up to 10 kW). The pulsed width of both FELs is also in the picosecond region that allows for unique systematic studies of the material deposition. We started the first systematic study in which the beam wavelength and energy will be systematically changed in such broad range, while holding all other deposition parameters constant. Through collaboration with Fisk University, which has nanosecond pulsed width PLD, we will also study the impact of the pulse duration from the femtosecond to nanosecond range. This study will be feasible due to fact that the ultra high vacuum deposition chamber at NCCU has the same parameters as that in Fisk, since it is built through collaborative efforts.
During this year, one of the main goals was to increase the depth of existing materials research at NCCU focused on the development of novel nanoscale materials for advanced devices. The major component of this program leverages the previous development of experimental and theoretical tools at NCCU for rapid modeling and fabrication of nanoscale materials outside of boundaries imposed by conventional fabrication methods. A unique ultrafast pulsed laser deposition (PLD) / pulsed electron deposition (PED) facility at NCCU, which produces plumes of ablated materials with stoichiometries identical to those of the target, have been used to systematically study complex ternary / quaternary III-V and chalcopyrite semiconductor quantum dots (QDs) and nanowires (NWs) that are anticipated to play an important role in the development of advanced photovoltaic cells and sensors. The complexity of these little studied semiconductors provides additional degrees of freedom to tune bandgaps, lattice constants and carrier effective masses, enabling production of materials with properties tailored for specific applications.
Collaboration with Cornell and the Colorado School of Mines that we established during this year provided us with an additional access to expertise in nanostructure characterization, fabrication of prototype device heterostructures featuring nanomaterials, device processing and device characterization that are not available at NCCU. Previous work at NCCU, which featured only limited characterization of QD monolayers, has been thus extended to studies of QD size/composition – optoelectronic property correlations and the properties of QDs and NWs in realistic device heterostructures.
Theoretical modeling of the electronic structure and carrier dynamics of QDs and NWs complements the experimental research. Synergy between computational simulations and experiments – theoretical efforts provide guidance for material selection and aid in the understanding of experimental data, while experimental results allow fine tuning of theoretical models – has been exploited to provide new insights into the dependence of material properties on composition and size. This program also supported the seed projects to foster other related innovative materials research at NCCU that can benefit significantly from the infrastructure developed through this CREST project. Initial seed projects are based on: free electron laser modification and study of amorphous and nanocrystalline thin films and integration of these films with nanostructures; fabrication and characterization of photonic metal nanostructures for control and guidance of light; and investigation into the mechanisms of millimeter wave absorption by wide bandgap semiconductors.
Collaboration with NASA and Jefferson National Laboratory includes development of two new linearly polarized photons polarimeter prototypes that will have an analyzing power and efficiency of 10% for E up to 250 MeV; one based on pair production on the nucleus and another on triplet production on electrons.