Plenary lecture 2: nanostructured materials in solid state devices for energy

Designing solid-state devices is essentially limited by choosing available chemical elements found on the Periodic Table and forming various stable solid compounds made of these chemical elements. A key to developing novel solid-state devices is, therefore to find a route to combine a variety of such compounds that are often physically and/or chemically incompatible each other. In this talk, specific examples of "nanostructured materials" currently pursued at Nanostructured Energy Conversion Technology and Research of Advanced Studies Laboratories, University of California Santa Cruz and NASA Ames Research Center (http://asl.ucsc.edu/contact.php), will be presented with the view toward solid-state devices for energy harvesting and saving. The talk is divided into the following two topics. Nanostructured materials for energy harvesting (photovoltaics and thermoelectrics) Development of next-generation energy resources that are reliable and economically/environmentally acceptable is a key to harnessing and providing the resources essential for the life of mankind. Our research focuses on the development of novel nanostructured materials that would significantly benefit energy harvesting, in particular, from light and heat. In these critical applications, traditional semiconductor solid-state devices, such as photovoltaic (PV) and thermoelectric (TE) devices based on a stack of single-crystal semiconductor thin films or single-crystal bulk semiconductor have several drawbacks, for instance; scalability-limits when ultra-large-scale implementation is envisioned for PV devices and performance-limits for TE devices in which the interplay of both electronic and phonon systems is important. In our research, various types of nanostructured materials (e.g., nanowires and nanoparticles) coupled to or embedded within micrometer-scale semiconductor platforms are explored to build a variety of nonconventional PV and TE devices. Two core architectures are (1) single-crystal semiconductor nanowires electrically connected to amorphous semiconductor thin films and (2) semi-metallic nanoparticles embedded within a semiconductor thin film. These two architectures are studied within the context of their basic electronic, optical, and thermal properties, which will be further assessed and validated by comparison with theoretical approaches to draw comprehensive pictures of physicochemical properties of the nanostructured materials. Nanostructured materials for energy saving (low-power electronics) Fundamental building blocks of computers for the last half century have been based on three-terminal semiconductor electronic devices (i.e., transistors). Among various transistors, silicon metal oxide semiconductor field effect transistors (Si-MOSFETs) are the core devices for constructing prevailing CMOS logic families. Among many technical challenges in developing advanced Si-MOSFETs, reducing excessive heat generated by high performance CMOS chips ranks high. Lowering the operational voltage for a CMOS chip is one way to reduce the total electric power consumed by a CMOS chip while the size of Si-MOSFETs is scaled down, however, the lowering the operational voltage is negated by, for instance, the increase in off-state leakage current of smaller transistors. Replacing Si-MOSFETs with other types of devices that operate in ways fundamentally different from those of Si-MOSFETs will be the ultimate approach and could lead us to pave a entirely new way to construct computers in the future. We are currently developing unique two-terminal devices, resisters with memory functions "memristors", fabricated in sub-viral length scales to build memory and logic devices that can be operated at ultra-low power. A wide range of metal oxides that have been known to exhibit a variety of electrical properties including insulating, semiconducting, and metallic are used as core materials for memristors. Precise and reproducible control on thickness and chemical composition of metal oxide thin films is one of the critical factors in the fabrication of memristors. Among various fabrication techniques, we are currently employing atomic layer deposition (ALD) to develop variety of metal oxide thin films for memristors.