Covalent network glasses are usually composed of the group IV and/or V selenides, and exist in three basic types of elastic phases- flexible, intermediate and stressed-rigid. Flexible phases are made up of weakly cross-linked structures and possess "floppy" modes, while stressed-rigid phases consist of heavily cross-linked structures that have stress creating "redundant" bonds. Intermediate phases (IPs) are optimally cross-linked structures that are marginally rigid but stress-free features that lead to unforeseen functionalities, such as thermal reversibility of the glass transition, absence of aging, liquid-like configurational entropies, and network adaptability to expel stress and lower their free energies. This individual investigator award supports a project to extend investigations of IPs from selenides to sulfides and oxides. Modulated DSC, Raman, IR reflectance and Molar volume measurements will be undertaken as a function of glass composition to establish IPs. These measurements will permit elucidating the role of chemical bonding and structure on IPs as fractional ionicity of bonding increases. Possible applications include optimization of thin-film gate dielectrics in 3-terminal devices, flat panel displays, programmable cell metallization memory devices, protein folding and drug design. Research on amorphous semiconductors and insulators has proven to be an outstanding training ground for young physicists and electrical engineers seeking jobs in industry and academia.
Advances in synthesis, purification of starting materials and detailed compositional studies have led to the recognition that physical properties of amorphous semiconductors including glasses can change abruptly with chemical composition. These developments have permitted probing the rigidity transition (between flexible and intermediate phase glasses) and the stress transition (between intermediate and stressed-rigid phases) in network glasses. This individual investigator award supports a project to investigate Intermediate phases (IPs) in select sulfides and oxides, extending present results of IPs from selenides to systems that are more ionic. Thermal, optical, mechanical and electrical measurements will be undertaken to establish IPs and their aging behavior. The goal of the project is to establish IPs of these technologically important materials and to understand them in terms of glass sub-structure. These findings will also serve as bounds for numerical simulations of these phases using first principles methods that are currently ongoing in the US and Europe. Students will be trained in state-of the art techniques and semiconducting materials growth, which will open jobs in Science and Engineering infrastructure of the US.
Applications of vitreous or glassy solids in modern day living are pervasive. Optical fibers made of fused quartz (SiO2) as the backbone of communications on this planet have nearly completely displaced traditional Copper wires. Rewritable DVDs, memories based on "Phase Change Materials" are widely used in consumer electronics. "Flat panel displays" made from Silicate glasses suitably modified, are widely used in cell phones, I-pads, home TVs, and in large scale Jumbotrons in football fields. Scientists and engineers view glasses as "atomically disordered solids", and in contrast to crystals like quartz and diamond that are "atomically ordered solids". The former relax with time, i.e., age, while the latter do not. This means that their physical properties, which are manifestations of underlying network structures also change. So for increased reliability in precision applications, the glasses used must be of a very special variety. Intellectual Merit. Glasses used in many commercial applications including window glass belong to a "new phase of matter", which show drastically reduced aging due to highly optimized molecular network structures. These networks are elastically rigid but mechanically unstressed, also known as the "Intermediate Phase"(IP). The IP was discovered by the PI and his students at University of Cincinnati in 1997. In the present research proposal we have identified the existence of this phase in "heavy metal oxide" glasses and in "alkali-borate" glasses for the first time. Such new results permit one to better understand the intimate connections between "glass molecular structure" and the "stress-free" nature of networks formed in the IP. Further, the discovery of the IP in a new class of materials holds the key to applications of these materials in the real world. The second significant outcome of this grant support is that melts of covalent material systems, such as those containing Sulfur, Selenium and Tellurium alloyed with Germanium and/or Arsenic, do not readily homogenize upon heating, thereby yielding heterogeneous glasses when supercooled. The very special properties attributed to IP glass compositions are then not easily observed in these heterogeneous glasses. We introduced a new method to unambiguously establish heterogeneity of melts in-situe using "Raman scattering". Here one examines light scattered from various part of the encased melt in a tube and establishes variations of glass structure optically. Furthermore, we have developed a basic understanding of the reasons why melt mixing is slow, and found a way to homogenize them, and establish their actual physical behavior revealing the IP. Broader impacts. ?5 graduate students, ? 2 UC undergraduates and ? 2 high school juniors have benefited directly by participating in the basic materials research on the grant support. Three graduate students have completed their MS and Ph.D. Thesis. Two of the graduate students (Siddhesh Bhosle, Kapila Gunasekera) were hired as full time employee by IM Flash based in Salt Lake city, Utah, and the third one Vignarooban was hired as a Post-Doctoral Fellow at Arizona State University, Phoenix. The two undergraduates ( Aaron Diebold, Zachary Tucker) worked closely with Sriram Ravindren ( Ph.D. student) measuring mechanical and calorimetric properties on bulk AsxSe100-x glasses. Such direct involvement in specific measurements in the laboratory permits students to get a direct feel for the discovery process. The two high school students interacted directly with the PI, and two graduate students, and have assisted ongoing projects at their level. One of these high school students (Rajat Bhageria) contributed towards volumetric measurements of glasses of controlled heterogeneity, and found that as glasses homogenize their molar volumes steadily increase, and then saturate when the glasses are fully homogeneous. The finding along with other results has significant consequences on understanding how melts/glasses homogenize. A manuscript based on this work has been submitted to a refereed journal of International standing ( Physica Status Solidi B). Project Outcomes ? Observed the Intermediate Phase in a new class of oxide glass system- Heavy Metal oxide- Telluro-Vanadates ? Observed the Intermediate phase in Li- and Na-borate glasses highlighting the role of the alkali-ion field in determining the intermediate range order ?Slow homogenization of covalent melts established, decoded and overcome, permitting observation of Intermediate Phases in this class of materials ? A Master and 2 Ph.D. students have gained full time employment in US industry and academia. ? A High school senior from Cincinnati area co-authors a research publication in an internationally recognized refereed publication. He is currently applying for a Siemens and separately Intel Math-Science-Technology competition. All terms used in inverted commas (" ") can be accessed on Wikipedia.
