My research focuses on the rationalization of the fundamental principles governing the structure, stability and thermodynamics of materials and the prediction of material behaviour. The objective is to provide the guiding principles for the rational design of novel functional materials. To achieve this goal, we have developed new theoretical models and implement new computational techniques. In many instances, we planned and performed synchrotron x-ray and neutron experiments to complement the theoretical studies. A few highlights of categorize my accomplishments in the last 10 years is listed as follows:
We have established ourselves to be one of the major computational material science groups in Canada. This recognition is reflected by many invitations to major international conferences, working visits by established researchers and their students such as from
- assembled a strong team in material modelling employing high performance computing which is unique in Canada;
contributed significantly to the understanding of novel phenomena under high pressure. Particularly on the electronic and structural stability and the transformations mechanisms in polymorphic solid phases.
- established a principle for the rationalization of chemical bonds of metals at high pressure
experimental and theoretical characterization of large complex systems such as the bonding and the optical properties in the technologically important luminescent silicon and semiconductor clusters encapsulated in zeolites,
- the application of electron band theory to investigate the principles of alloying and the prediction of the physical properties of metallic alloys such as the discharge voltage profile of Li batteries,
- theoretical and experimental characterization of the phonon scattering mechanism leading an explanation for the anomalous glass-like thermal conductivity in crystalline clathrates and established the principles for rational design of high efficiency thermoelectric materials,
My early scientific work was on the development of theoretical models for the elucidation of many electron phenomena accompanying the photoionization processes. Some notable achievements are the formulation of a theory for the “ligand field splittings” in main group compounds (Phil. Trans. Roy. Soc., 1980; Comment Inorg. Chem., 1986); the rationalization of the basic principles of valence level photoionization (a series of papers from 1980-1990; e.g. Inorg. Chem., 1979 and J. Chem. Phys., 1988). These studies have helped to open a new field in using tunable synchrotron radiation photoelectron spectroscopy in the characterization of the bonding in organometallic compounds (see a series of paper co-authored with G.M. Bancroft and R.J. Puddaphatt)
I have been involved in the research of gas hydrates and inclusion compounds since my undergraduate days in the early 1970s. One of my first publications was the x-ray structure determination of H2S enclathrated b-quinol (J. Chem. Soc., 1976). Upon obtaining the Ph.D degree in 1980, I started my research at in Dr. Davidson’s gas hydrate group at NRC. Several important contributions were made in early 1980s. The most significant contribution was the discovery of gas hydrates with small guest atoms prefers the type II structure (Nature 1984). This observation changed fundamentally the conception towards the structures of gas hydrates. We have prepared and identified the structure of CO hydrate (Nature, 1987a) and the discovery of a new hexagonal hydrate structure (Nature, 1987b). I am also responsible for the characterization of the glassy-like thermal conductivity of gas hydrates (J. Phys. Chem., 1988) in spite of the crystalline structure of the gas hydrates. To rationalize this anomalous thermal property, I was first in applying molecular dynamics (MD) calculations to the study of the structure, stability and physical properties of gas hydrates (J. Phys. Chem., 1983, J. Chem. Phys., 1983 and 1984). A model based on the resonant scattering of the lattice acoustic phonons by the localized guest vibrations was proposed as the mechanism for the amorphous-like thermal conductivity (J. Incl. Phenom. 1994). This is followed by a thorough theoretical investigation of the guest-host coupling using MD and lattice dynamics methods (J. Chem. Phys., 1997, Europhys. Lett. 2001). The proposal was fully substantiated through the study of the phonon spectra of selected hydrates ( J. Phys. Chem., 1997 and series of papers co-authored with W. Press) obtained from neutron incoherent-inelastic scattering experiments and more recently from x-ray inelastic scattering (Phys. Rev. B, 2003). The study on gas hydrate has been expanded into the high pressure regime as a model for probing hydrophobic interactions between water and small organic molecules. The first result has been very encouraging where we have determined several new structures of gas hydrate stable under substantial pressure (Nature, 2001 and Phys. Rev. Lett., 2001). Recently, we show that hydrogen ice clathrate at low temperature and moderate pressure satisfies the 6.5% weigh ratio and maybe used a storage medium (PNAS, 2003).
Research in the unusual thermal conductivity in ice clathrates has led to a new and practical project on the search for advanced thermoelectric materials The “resonant scatterings” model proposed by us forms the basis of a new paradigm - phonon glass electron crystal (PGEC), for the design of high efficiency thermoelectric materials (Phys. Rev. Lett., 2000). Based of this principle, our recent work focuses on metal-doped semi-conductor clathrates (Chem. Eur. J, 2002; Phys. Rev. Lett., 2002) and p-doped low bandgap polymers.
Complementing with the gas hydrate research, I collaborated with Dr. D. Klug in the study of the structure and dynamics of ices. I performed the first MD calculations of the phonon spectra of ice Ih, ice VIII and ice XI (J. Chem. Phys. 1984). We have resolved the controversy on the origin of the high frequency vibrations in the lattice translation region of ice Ih (J. Chem. Phys., 1986 and J. Chem. Phys., 1991). More importantly, I proposed a mechanism based on the mechanical stability of crystal to explain the pressure-induced amorphization of ice Ih at low temperature (Phys. Rev. Lett., 1987 and J. Chem. Phys. 1992). This theoretical model was extended to the rationalization of similar amorphization phenomena in a-quartz (Phys. Rev. Lett., 1991) and a-berlinite (Science, 1992). This theory is confirmed by an experiment (Nature, 1994) and more recently, from a complete theoretical characterization of the pressure-amorphization phenomenon in ice Ih (Nature, 1999). During the course of the gas hydrate and ice research, I have familiarized myself with a variety of experimental and theoretical techniques including diffraction using synchrotron x-ray and neutron, the measurement of vibrational density of states using neutron incoherent-inelastic scattering and theoretical molecular dynamics and lattice dynamics methods. More recently, we focus our attention in resolving the controversy in the proposal of two-liquid structure model for water. Our investigation leads to a deeper understanding of the structure and thermodynamics of pressure induced amorphiztion transformations (Phys. Rev. Lett., 2001)
Apart from research relating to gas hydrates and ices, a new research direction employing ab initio molecular dynamics techniques in the study of the structure and dynamics of molecular clusters, surfaces and condensed matter was initiated in 1993. Subsequently we have installed and implemented other new computational techniques to perform large-scale electronic structure calculations on metallic alloys, nano-metallic and semiconductor clusters encapsulated in framework compounds. In less than three years, with no prior experience in electronic band structure theory, we have established ourselves in the applications of quantum molecular dynamics methods in the study of structural phase transitions. Most notably scientific contributions in this period are:
We have developed computational techniques for the prediction of the properties of complex systems from their electronic band structures. To characterize the guest-host interactions, we have studied the electronic band structures and the optical spectra of photoluminescence semi-conducting Si and Cd-chalcognides clusters encapsulated in the zeolites. These calculations have led to a fundamental understanding on the interactions between the framework and the cluster and identified the role of the cluster in modifying the electronic property of the combined system. This information is essential for the future design of tunable optical devices.
- the understanding of the phase transitions and possible structures for high pressure solid hydrogen (Nature, (1995))
- the theoretical characterization of the structure and dynamics of carbo-cation in the gas phase (Phys. Rev. Lett., (1995); Science, (1995); J. Phys. Chem (1998))
- proposed a theoretical model based on the crystal mechanical stability to rationalize the pressure-driven amorphization in quartz (Phys. Rev. Lett, (1991) and the related reversible amorphization transformations in aluminium phosphate (Science, (1992)).
- conceived and performed an experimental study to verify this theoretical model (Nature (1994))
- Elucidated the mechanism for pressure-induced amorphization in ice Ih proposed earlier (Phys. Rev. Lett. (1987), J. Chem. Phys. (1992)) through a combined theoretical lattice dynamics and experimental neutron scattering studies (Nature, (1999)).
- applied First Principles calculations to characterize the thermodynamics of iso-structural transformation in ice VIII at low pressure (Phys. Rev. Lett., (1998)).
- developed a theoretical model to rationalize the structure-property relationship of binary alloys (Phys. Rev. Lett., (1999)).
- Theoretical characterization of phase transition anomaly of simple metals under pressure e.g Zn (Phys. Rev. Lett., 2000), Li (Nature, 200 and Cs (Natl. Acad. Sci., 2001, submitted)
Since 1995, I led an experimental research program focussed on the synthesis and characterization of nanoclusters and advanced materials with novel optical properties for the purpose of band-gap engineering.
- The multistage voltage discharge profile of a complex Li/Sn battery system has been accurately predicted from ab initio calculations (Phys. Rev. B (1998)). The implication of this work is the possibility on the evaluation of potential useful battery materials using theoretical calculations.
- Developed and implemented computer codes for the calculation of the thermolectric power and Hall coefficients (Phys, Rev. B submitted) and First Principle molecular dynamics using the FLAPW method (Mol. Simul. 2000).
More recently, we observed resonant vibrational modes of Xe enclathrated in ice hydrate from a neutron incoherent inelastic scattering experiment. Green’s function method was employed to compute the effects of the coupling modes to the lifetime of the acoustic phonons (Europhys. Lett., 2001). A related work has led to the understand of the origin of boson peak in amorphous solids (Phys. Rev. Lett., 2000) The results supported that the resonant scattering mechanism proposed in an earlier study (J. Phys. Chem. (1988)) is indeed responsible for the amorphous-like thermal conductivity behaviour of crystalline gas hydrates. The scattering of thermal phonons by localized excitation of enclathrated molecules or atoms is a general phenomenon. This is confirmed by our recent measurement on the thermal conductivity of a Na doped Si clathrate. This result has a significant consequence in the search for high efficiency thermoelectric materials (Phys. Rev. Lett., 2000).
- the discovery of a crystalline “graphitic” layered polysilane (Phys. Rev.B (1993), J. Appl. Phys. (1994), J. Mat. Chem (1998)) which exhibit quantum confinement effects analogous to porous silicon.
- the synthesis of new inclusion compounds of pure silicon and boron or phosphorous doped nanoclusters with strong optical photoluminescence (Chem. Comm., 1997)
- the successful determination of the average size of the encapsulated cluster in HY zeolite using an innovative application of synchrotron radiation photoabsorption and x-ray photoelectron spectroscopies and the 2H spin counting technique (Appl. Phys. Lett., (1997) and (1999); J. Amer. Chem. Soc. (1998)).