In our group, we are interested in learning how non-equilibrium (or kinetic) effects control the observable structures of solids. Research in this area currently centers around two main areas: the study of nanostructured composite materials, and the study of phase transitions in solids.

One particularly exciting class of nanostructured composites are ordered inorganic/organic composite materials. This broad class of materials carries the common bond that they are composed of a nanoscale mixture of organic and inorganic components. In most cases, the organic component controls the long range periodicity of the composite, while the inorganic component provides stability (thermal, chemical, and mechanical) and strength. Solids of this sort thus frequently have structures reminiscent of liquid crystals, but with a rigid inorganic framework. Further, the organic component can often be removed to produce a porous nanostructured inorganic material. Applications for these materials range from catalysis, to shape selective chemical sensors, to host/guest chemistry. To understand the factors that control structures in these materials we are studying the dynamics of formation and interconversion in silica/surfactant mesostructured composites under conditions of variable temperature, pressure, and chemical composition. Our primary tool for these experiments is real time X-ray diffraction. The goal of the experiments is to produce a fundamental understanding of the interplay between kinetic and thermodynamic factors in controlling structure in these complex materials.

Work is also underway to synthesize and characterize new composite materials with interesting electronic and optical properties. In particular, we are using host/guest chemistry and self organization to produce large quantities of material with supermolecular periodicities. We are currently involved in the synthesis of both inorganic/organic and inorganic/inorganic composites with potentially useful optical properties.

A final class of experiments focus on the general question of how solid- solid phase transitions occur. Many metastable (high-energy) phases can be useful because in solids, there is frequently no low energy pathway to get to a stable phase. By studying the dynamics and energetics of structural change in solids, we can learn what controls these changes and thus gain a predictive knowledge of what metastable phases can exist. Experiments such as X-ray diffraction under anisotropic pressure, AFM, and solid state NMR can all be used to study the kinetics of solid-solid phase transformations.

Representative Publications

35. B.J. Schwartz, T.-Q. Nguyen, J. Wu, and S.H. Tolbert, "Interchain and Intrachain Exciton Transport in Conjugated Polymers: Ultrafast Studies of Energy Migration in Aligned MEH-PPV/Mesoporous Silica Composites." Synthetic Metals, in press.

34. A.F. Gross, E.J. Ruiz, V. Le, and S.H. Tolbert, "The Role of Silica Chemistry in Controlling Phase Transitions in Silica/Surfactant Composite Materials." Microporous Mesoporous Mater., in press.

33. J. Wu, X. Liu, and S.H. Tolbert, "High Pressure Stability in Ordered Mesoporous Silicas - High Strength through Nanometer Scale Arches." Submitted.

32. T.-Q. Nguyen, J. Wu, S. H. Tolbert, B. J. Schwartz "Control of Energy Transport in Conjugated Polymers using an Ordered Mesoporous Silica Matrix." Adv. Mater., in press.

31. S.H. Tolbert, J. Wu, A.F. Gross, T.-Q. Nguyen, and B.J. Schwartz, "Directional Energy Migration in an Oriented Nanometer-Scale Host/Guest Composite: Semiconducting Polymers Threaded into Mesoporous Silica." Microporous Mesoporous Mater., in press.

30. S.H. Tolbert, C.C. Landry, G.D. Stucky, B.F. Chmelka, P. Norby, J. Hanson, and A. Monnier, "Phase Transitions in Mesostructured Silica/Surfactant Composites. Surfactant Packing and the Role of Charge Density Matching." Submitted to Chem. Mater.

29. C.C. Landry, S.H. Tolbert, K.W. Gallis, A. Monnier, G.D. Stucky, P. Norby, and J.C. Hanson, "Phase Transitions in Mesostructured Silica/Surfactant Composites. Mechanisms for Change and Applications to Materials Synthesis." Submitted to Chem. Mater.

28. A.F. Gross, V.H. Le, B.L. Kirsch, S.H. Tolbert, "Understanding Phase Stability in Silica/Surfactant Composites through the Study of a Curvature Driven Rectangular Intermediate." Submitted to Langmuir.

27. A.F. Gross, V.H. Le, B.L. Kirsch, A.E. Riley, S.H. Tolbert, "Correlations Between Silica Chemistry and Structural Changes in Hydrothermally Treated Hexagonal Silica/Surfactant Composites Examined by In-Situ X-ray Diffraction." Submitted to Chem. Mater.

26. J. Wu, M. Abu-Omar, S.H. Tolbert, "Fluorescent Probes of the Molecular Environment within Mesostructured Silica/Surfactant Composites under High Pressure." Nano Lett., 1, 27-31 (2001).

25. B.J. Schwartz, T.-Q. Nguyen, J. Wu, and S.H. Tolbert, "Interchain and Intrachain Exciton Transport in Conjugated Polymers: Ultrafast Studies of Energy Migration in Aligned MEH-PPV/Mesoporous Silica Composites." Synthetic Metals, 116, 35-40 (2001).

24. J. Wu, X. Liu, and S.H. Tolbert, "High Pressure Stability in Ordered Mesoporous Silicas – Rigidity and Elasticity through Nanometer Scale Arches." J. Phys. Chem. B., 104, 11837-11841 (2000).

23. T.-Q. Nguyen, J. Wu, V. Doan, B.J. Schwartz, and S.H. Tolbert, "Control of Energy Transfer in Oriented Conjugated Polymer-Mesoporous Silica Composites." Science, 288, 652-656 (2000).

22. A.F. Gross, E.J. Ruiz, and S.H. Tolbert, "Effect of Framework Polymerization on the Phase Stability of Periodic Silica/Surfactant Nanostructured Composites." J. Phys. Chem. B, June 15, 2000 issue.

21. T.-Q. Nguyen, J. Wu, V. Doan, B.J. Schwartz, and S.H. Tolbert, "Control of Energy Transfer in Oriented Conjugated Polymer-Mesoporous Silica Composites." Science, 288, 652-656 (2000). (See Semiconducting Polymers in Mesoporous Silica).

20. J. Wu, A.F. Gross, and S.H. Tolbert, "Host Guest Chemistry Using an Oriented Mesoporous Host: Alignment and Isolation of a Semiconducting Polymer in the Nano-Pores of an Ordered Silica Matrix." J. Phys. Chem. B., 103, 2374-2384 (1999).

19. A.B. Herhold, C.C. Chen, C.S. Johnson, S.H. Tolbert, and A.P. Alivisatos, "Structural Transformations and Metastability in Semiconductor Nanocrystals." Phase Transitions, 68, 1-25 (1999).

18. S.H. Tolbert, A. Firouzi, G.D. Stucky, and B.F. Chmelka, "Magnetic Field Alignment of Ordered Silicate-Surfactant Composites and Mesoporous Silica." Science, 278, 264-268 (1997).

17. Firouzi, D.J. Schaefer, S.H. Tolbert, G.D. Stucky, and B.F. Chmelka, "Magnetic-Field-Induced Orientational Ordering of Alkaline Lyotropic Silicate-Surfactant Liquid Crystals." J. Am. Chem. Soc., 119, 9466-9477 (1997).

16. S.H. Tolbert, P. Sieger, G.D. Stucky, S.M.J. Aubin, C.-C. Wu, and D.N. Hendrickson, "Control of Inorganic Layer Thickness in Self-Assembled Iron Oxide/Surfactant Composites." J. Am. Chem. Soc., 119, 8652-8661 (1997).

15. S.H. Tolbert, T.E. Schäffer, J. Feng, P.K. Hansma, and G.D. Stucky, "A New Phase of Oriented Mesoporous Silicate Thin Films." Chem. Mater., 9, 1962-1967 (1997).

14. P.F. Feng, X. Bu, S.H. Tolbert, and G.D. Stucky, "Synthesis and Characterizations of Chiral Tetrahedral Cobalt Phosphates with Zeolite ABW and Related Frameworks." J. Am. Chem. Soc., 119, 2497-2504 (1997).

13. A.V. Kadavanich, A. Mews, S.H. Tolbert, X. Peng, M.C. Schlamp, J.C. Lee, and A.P. Alivisatos, "Structural Investigations Of Colloidal Semiconductor Nanocrystal Heterostructures: Faceting And Epitaxy" in Advances in Microcrystalline and Nanocrystalline Semiconductors. R.W. Collins, P.M. Fauchet, I. Shimizu, and J.-C. Vial, eds. Mater. Res. Soc. Symp. Proc., 452, 353-358 (1997).

12. A.M. Belcher, C. Zaremba, Q. Huo, C.C. Landry, S.H. Tolbert, A. Firouzi, M. Janicke, P.K. Hansma, D. E. Morse, B.F. Chmelka, S. Schacht, I.G. Voigt-Martin, F. Schüth, and G.D. Stucky, "The Organic/Inorganic Interface and Materials Synthesis in the Nano- to Macroscale Continuum" in Chemistry on the Nanometer Scale, the Robert A. Welch Foundation 40th Conference on Chemical Research, (Robert A. Welch Foundation, Houston TX, 1996) pp. 101-112.

11. S.H. Tolbert, A.B. Herhold, L.E. Brus, and A.P. Alivisatos, "Pressure Induced Structural Transformations in Silicon Nanocrystals: Surface and Shape Effects." Phys. Rev. Lett., 76, 4384-4387 (1996).

10. A.B. Herhold, S.H. Tolbert, A.A. Guzelian, and A.P. Alivisatos, "A Comparison of Pressure-Induced Structural Transformations in CdSe, InP, and Si Nanocrystals." in Fine Particles Science and Technology From Micro to Nanoparticles, E. Pilizzetti, Ed., NATO ASI Proceedings (Kluwer Academic Publishers, Dordrecht, 1996) pp. 331-342.

9. S.H. Tolbert and A.P. Alivisatos, "High Pressure Phase Transitions in Semiconductor Nanocrystals." Ann. Rev. Phys. Chem., 46, 595-625 (1995).

8. S.H. Tolbert, A.P. Alivisatos, "The Wurtzite to Rock Salt Structural Transformation in CdSe Nanocrystals under High Pressure." J. Chem. Phys., 102, 4642-4656 (1995).

7. S.H. Tolbert, A.B. Herhold, C.S. Johnson, and A.P Alivisatos, "A Comparison of Quantum Confinement Effects on the Electronic Absorption Spectra of Direct and Indirect Gap Semiconductor Nanocrystals." Phys. Rev. Lett., 73, 3266-3269 (1994).

6. S.H. Tolbert and A.P Alivisatos, "Size Dependence of a First Order Solid-Solid Phase Transition: The Wurtzite to Rock Salt Transformation in CdSe Nanocrystals." Science, 265, 373-376 (1994).

5. S.H. Tolbert and A.P Alivisatos, "Se EXAFS Study of the Elevated Wurtzite to Rock Salt Structural Phase Transition in CdSe Nanocrystals" in Nanophase Materials: Synthesis Properties-Applications, G.C. Hadipanayis and R.W. Siegel, eds., NATO ASI Proceedings (Kluwer Academic Publishers, Dordrecht, 1993) pp. 471-482.

4. S.H. Tolbert and A.P Alivisatos, "Size Dependence of the Solid-Solid Phase Transition in CdSe Nanocrystals." Zeit. Phys. D - Atoms, Molecules and Clusters, 26, 56-58 (1993).

3. S.H. Tolbert, A.P. Alivisatos, H.E. Lorenzana, M.B. Kruger, and R. Jeanloz, "Raman Studies on C60 at High Pressures and Low Temperatures." Chem. Phys. Lett., 188, 163-167 (1992).

2. J.R Heath, J.M. Hawkins, A.P. Alivisatos, R.J. Saykally, T.A. Lewis, S.D. Loren, A. Meyer, Y. Shibato, S.H. Tolbert, and J.J. Shiang, "C60 Research at the University of California at Berkeley." Proc. of the 1990 Materials Research Conference, Boston, MA, Nov. 25-29 (1990).

1. R.B. Markowitz, S. Tolbert, and W.S. Dynan, "Promoter Evolution in BK Virus - Functional Elements Are Created at Sequence Junctions." J. Virology, 64, 2411-2415 (1990).

Contact Info

Department of Chemistry & Biochemistry
UCLA
Box 951569 (post)
607 Charles E. Young Drive East (courier)
Los Angeles, CA 90095-1569