Tadashi Ogitsu Quantum Simulations Group

1. Performance Measures x.x, x.x, and x.x Quantum Simulation of Nanomaterials Under Pressure Tier-2 allocation on ThunderFeb. 21st, 2008 Tadashi Ogitsu Quantum Simulations Group Physical Sciences

2. Adios! Andrew Williamson (PI) Tadashi Ogitsu (PI) Research Team External collaborators: • Jeff Grossman, UC Berkeley • Nicola Marzari, MIT Sebastien Hamel Heather Whitley Byeongchan Lee Robert Rudd Physical Sciences

3. Outline of the talk • Nanomaterials under pressure • Control of properties by size and pressure • Are there general rules? • Synthesis issues? • Growth mechanism of silicon nanowires on gold nanoparticles • Summary and future developments • Geometry is crucial • Extended ensemble methods to deal with the complexity of real world Si nano-dots embedded in amorphous SiN -Sebastian Hamel Physical Sciences

4. Size Motivation: Why “nano” & “pressure”? • Nano: • Properties of nanomaterials are known to change as a function of size • For ex. color of emitted light versus size of nano-dots • Pressure: • Pressure changes the properties of materials, sometimes dramatically (i.e. phase transitions) Small Large + liquid solid 2 Temperature solid 1 • Nano + pressure: • Additional degrees of freedom to achieve desired properties! Pressure Physical Sciences

5. Successful application of “pressure” in modern technologies: Blue-Ray • What is “pressure” to do with “nano”? • Ex: Pressure annealing on GaN greatly reduces defects • Blue-violet lasers for new storage device technologies (Blue-Ray) • Access to a different region in the phase diagram is the key • Complementary to CVD (non-equilibrium approach) Wasilewski 2004 Physical Sciences

6. From a fundamental viewpoint • There are systematics in each field • Ex1: Optical gaps decrease in larger dots • Ex2: Analogy in the phase diagrams of elements in the same group • Ex3: Electron counting rule in metallic cluster lead to magic numbers • The combined effects are not necessarily linear • Nanomaterials are not uniform materials (surface and bulk) Thiolates stabilize 102-atom gold clusters Science 318, 430 (2007): 58 is the magic number There is a real need to develop a basic understanding of the systematic pressure behavior of nanomaterials Physical Sciences

7. DJ Norris et al. University of Manchester Optical gap decreases as the size of nano-dot increases • Electronic excitation gaps tend to decrease as the size increases - quantum confinement effects • The gap of CdSe nano-dot decreases smoothly and monotonically Gap Size Physical Sciences

8. Boron Ga Al Analogy in the phase diagrams of the elements in the same group First-principles phase diagram of the III-group elements PRL 90, 065701 (2003) • Elements in the same group tend to have similar phase diagrams • Shift of boundaries are mostly due to the size of the atoms • However, there are notable exceptions! -B -Ga fcc Pressure Physical Sciences

9. T P PT~22GPa L~10nm dia -tin P L~1nm ??? Tolbert et al. PRL 76, 4384 (1996) Takeguchi et al. JJAP 38, 7140 (1999) Non-linear effects when combined! • Traditional phase diagram are two dimensional (P, T) • Thermodynamic limit • Bulk phase (infinite size limit) • Phase diagram of a finite size material is different from the bulk • Phase boundaries can shift • Solid/solid boundaries tend to rise, while melting temperature tend to decrease • Completely different phases can emerge Si phase diagram PT=11GPa L= T dia -tin P T Physical Sciences

10. How do we define “pressure” on a nano-material? • Pressure media, i.e., atoms and molecules hit the surface of a nanomaterial • Far from an image of static compression…? • Dynamical effects (individual collisions) can be ignored if the system is large (surface to volume ratio is small) • Is it true for a nano-material at finite temperature? • (perhaps, it depends…) Physical Sciences

11. How do we define “pressure” on a nano-material? A. Brute force: MD with explicit pressure media • Pros • All effects are included • Surface chemistry as well (minor effect in bulk) • Cons • Computationally expensive! • Classical pressure media particles can be used Physical Sciences

12. How do we define “pressure” on a nano-material? B. The enthalpy functional by Marzari et al. • Stability of the system is described by “enthalpy” • H=E+PV • Unit volume for a bulk system is well-defined. (unit-cell) • What is the volume for an isolated “nano-material”? • Exclusion volume of charge! • By comparing with the explicit media simulation, one can distinguish “collisions” from “thermal fluctuations” Physical Sciences

13. Small silicon nano-dot (~1nm) compressed at T=0K • The transition pressure from diamond to -tin structure increases for smaller dots • Below a critical size, the dots become amorphous • The critical size is unclear • Effect of foreign chemical species (such as oxygen) Physical Sciences

14. Hydro static Uni-axial Photoluminescence of CdSe nano-dots under pressure • CdSe nano-dots shows smooth change of optical gap • Transition pressure rises for small dot • Ideal material to study “pressure control” of gap • Saturation on the increase of the optical gap has been observed • Enhanced tunneling between dots has been suggested • Our theory/experiments clearly show that it is due to the strain Physical Sciences

15. Growth mechanism of silicon-nanowires on gold nano-particles • CNT growth from catalyst particles • SiNW growth from catalyst particles J. Westwater et al, J. Vac. Technol. B 15, 554 (1997) Physical Sciences

16. Growth mechanism: the conventional view Au Si atoms landed on Au surface Si atoms diffuse into Au droplet SiNW wire grows more Si atoms introduced more Si in Si/Au droplet Si/Au supersaturated and excessive Si atoms go to interface Physical Sciences

17. The growth mechanism observed in our simulation Au Si atoms landed on Au surface Si atoms diffuse underneath the surface SiNW wire grows more Si atoms introduced all subsurface spots are filled excessive Si atoms go to interface by surface Physical Sciences

18. Summary • Computer simulation tools to apply “pressure” on an isolated nano-material is in place • The “phase diagram” of silicon nano-dot is being studied systematically • Saturation on the increase of optical gap in CdSe is explained by strain • First-principles simulations of silicon nano-wire growth on gold nano-particles • Provide a novel atomistic insight on the growth mechanism Physical Sciences

19. “QMC Calculations” Surface structure and chemistry Future work: How do we study the geometrical uncertainty of nano-particles? Extended Ensemble Methods 5 Experiments 4 3 Optical Absorption Gap (eV) 2 1 0 0 2 4 6 8 Q-Dot Diameter (nm) Physical Sciences

20. In the nano-world, stability is sometimes dictated by different principles PRL 52, 2141 (1984) • “Magic numbers” are well known for some metallic nano-clusters • Analogy to the shell structure of an atom (pseudo atom picture) • Surface effect is large • Eg. surface decoration Thiolates stabilize 102-atom gold clusters Science 318, 430 (2007): 58 is the magic number Closed shell in a spherical 3D well Physical Sciences

21. Are ultimate goal is to enable first-principles based materials design Current scope: Understand the nature of nanomaterials under pressure and the grow mechanism Ultimate goal: Propose new materials based on first-principles Physical Sciences

22. “QMC Calculations” Surface structure and chemistry Smooth monotonic change of the gap is not always the case: silicon nano-dot [Williamson et al.] 5 Experiments 4 3 Optical Absorption Gap (eV) 2 1 0 0 2 4 6 8 Q-Dot Diameter (nm) Physical Sciences