J. Sankar and D. Kumar Center for Advanced Materials and Smart Structures

1. Pulsed Laser Deposition Assisted Fabrication and Characterization of the Nanostructured Quantum Wells J. Sankar and D. Kumar Center for Advanced Materials and Smart Structures Department of Mechanical Engineering North Carolina A&T State University

3. Most Common Problems in the Synthesis of Nano Composites • Nanoparticles, large surface-area to volume ratio, tend to reduce their surface energy (>100 dyn/cm) • Concurrent coarsening of grains • Control of composition and structure • Reproducibility and scale-up ability

4. Success in Synthesis and Fabrication of Metal-Ceramic Thin Film Nanocomposites New and improved control of the size and manipulation of nanoscale building blocks New and improved characterization of materials at the nanoscale New and improved understanding of the relationship between nanostructure and properties

5. Self-Assembly of Nanoparticels Successful self-assembly of nanoparticles array depends on ability to: -To prepare monodisperse particles -To balance the interparticle forces This results into the formation of ordered structures spontaneously Previous methods of self-assembly of magnetic nanoparticles: -Lithography: Perfect but extending to nanometer scale is difficult -STM: Very small structures possible but not good for large quantities -Electrodeposition of metal in cylindrical pores of anodized Al Self-assembly can produce nanostructures and is scaleable at low cost

6. OBJECTIVE • When the width of the layer separating semiconductor quantum well is sufficiently thin (typically less than about 5 nm, electron can tunnel between the wells. • This ability of electrons to tunnel between quantum wells is the basis of many technologically important applications.

7. Proposed work • Selection of materials • Optimization of pulsed laser deposition parameters • Investigation of Electrical and Optical Properties • Microstructure Property Correlations • Education and training of minority students • Participation of minority students in MS and Ph.D. programs

8. Approach and Materials of Interest • The two-dimensional quantum wells which we propose to fabricate and test will consist of alternate layers of different semiconductor materials. • We will use PLD to fabricate the test structures. • The typical layer thicknesses will be of 10's to 100's of Ao. One layer- type serves as the active conductor and the other as a “barrier.” • The materials of interest are: Si/SiGe, Si/SiC, Ag, Au, etc.

9. Pulsed Laser Deposition System • Laser energy of 642 mJ • Pre-deposition vacuum of 8.3X10-6 Torr • Target-to-Substrate of 8 cm • Pulse Repetition Rate of 10 Hz

10. Optimization of PLD Parameters • PLD technique is one of the most popular and effective techniques used in the present days for the deposition of thin films. In this technique, a pulsed laser is directed on a solid target. The nanosecond laser pulse is focused to give an energy density sufficient to vaporize a few hundred angstroms of surface material in the form of neutral or ionic atoms and molecules with kinetic energies of a few eV, which then get deposited onto the substrate. • The plasma temperature is high (~ 103 K) and the evaporants become more energetic when they pass through the plume. This affects the film deposition in a positive manner due to increase in the adatom surface mobility. • Use of short pulses helps to maintain high laser power density in a small area of the target and produces congruent evaporation. • Our initial work will be focused on the optimization of deposition parameters such as substrate temperature, laser fluence, pulse repetition rate, and target substrate distance to realize the best quality heterostructures

11. Investigation of Electrical Properties • In a layered semiconductor heterostructures with layer thickness smaller than the electron mean free path of bulk three-dimensional semiconductors the motion of electrons and their interaction with photons are significantly modified. Such modification will result in novel electronic behavior that could be exploited to produce new electronic and photonic devices. • In this respect we plan to study superlattice effects on the electronic transport properties of semiconductor superlattices. • The principal experimental observation regarding normal-state transport in semiconductor superlattices will be contribution to the resistivity owing to electron scattering at the interfaces. This will be understood in terms of the classical Drude model for electrical conductivity, where the resistivity will be predicted to have a contribution from a term inversely proportional to the average layer thickness. • The resistivity as a function of the individual layer thickness in equal layer thickness multilayer will be measured at different temperatures. The measurements will be done at low temperatures (<50K) in order to greatly reduce the phonon scattering contribution to the resistivity.

12. Investigation of Optical Properties • Optical absorption measurements will be done to estimate exciton absorption energy. The exciton energy will be tuned by a precise control of the width of the quantum wells. • The absorption spectrum will be used to characterize the quality of the layering since the more uniform the layer, the smaller the width of the emission peak.

13. Direct Atomic Scale Imaging of Nanoparticle-Host Matrix Interface • The structure and chemistry of interfaces are known to affect the electrical and magnetic properties. • The ability to retrieve atomic structures directly from experiments is a great advantage for first principle simulations. • Scanning transmission electron microscopy with atomic number (Z) contrast (STEM-Z) imaging is an incoherent imaging process. So, the phase ambiguity inherent to HRTEM images is removed. • Since the high angles electrons are used for imaging formation, there is chemical sensitivity in the images. • STEM-Z allows Electron Energy Loss Spectroscopy (EELS) to be performed simultaneously allowing compositional analysis and local band structures to be determined at atomic resolution.

14. Z-contrast imaging in STEM-Z

15. TEM Images Cross sectional STEM-Z micrograph Of Ni nano particles embedded in Al 2O3 matrix

16. High Resolution STEM-Z micrograph of a single Ni nano particle in alumina matrix

17. TUNABLE MAGENETIC PROPERTIES

18. Education and Outreach A team of A & T undergraduate students participating in a “Day of Science” meeting at Oak Ridge national Laboratory, Oak Ridge.

19. External Collaborations Atomic structure characterization will be conducted in collaboration with • STEVE PENNYCOOK: OAK RIDGE NATIONAL LABORATORY Devices such as thermoelectric generators and coolers using quantum-well structures will be fabricated in collaboration with • CATERPILLAR • HI-Z INC.

20. ACKNOWLEDGEMENTS • HBCU/MI Environmental Technology Consortium • US Department of Energy