Magnetic materials are widely used for information storage while semiconducting materials are essential for high speed processing of information, thus these two materials form the most important components of the vast majority of electronic devices today. Developing ferromagnetism in conventional semiconductor materials is highly desired to make electronic devices faster, smaller, cheaper, and more energy efficient. This project investigates ways to develop stable above-room temperature ferromagnetism in oxide semiconductors using multiple approaches, most importantly using the unique and novel properties of oxide materials when prepared in nanoscale size range. By engaging graduate students in the research, this project provides significant support for the new doctoral programs at Boise State University in the areas of Materials Science and Engineering, and Biomolecular Sciences (starting in Fall 2012). Significant aspects of this research project are integrated into several existing as well as new interdisciplinary graduate courses in the Materials Science and Engineering, and Physics programs. Also, this project provides research opportunities for several undergraduate students, and students and science teachers from local high schools.
TECHNICAL DETAILS: In spite of the lack of success in producing stable, reliable and reproducible room temperature ferromagnetism in conventional semiconductors using dilute level doping of 3d cations(dilute magnetic semiconductors) during the past 10 years, research in this direction remains even more exciting due to its potential to make electronic devices faster, smaller, more energy efficient and less expensive. Most of the experimental data reported so far indicate the presence of ferromagnetism in transition-metal doped oxide semiconductor nanoparticles. However, several recent findings such as the lack of a systematic dependence of the magnetic moment with dopant concentration, observation of ferromagnetism even in undoped oxide semiconductors, and absence of properties expected from spin-orbit interaction do not convincingly support it as a true dilute magnetic semiconductor system. Thus, a new mechanism to understand the novel ferromagnetism is needed and this is one of the major goals of this project. A novel approach is being undertaken where the oxide semiconductor nanoparticles are capped with various organic molecules to investigate if charge transfer between nanoparticles and these linked molecules modifies the electronic structure and thereby produces ferromagnetism in these oxides. X-ray absorption near edge structure (XANES) studies in collaboration with Professor Steven Bernasek at Princeton University is being utilized to evaluate the charge transfer between the nanoparticles and the organic molecules. Other surface sensitive techniques including X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy are also available for additional studies. Finally, modification of the physicochemical properties, especially fluorescence emission, of the surface bound organic dyes due to their interaction with oxide semiconductor nanoparticles and charge transfer are also being investigated based on the PI's recent observation of a 90-fold increase in the fluorescence emission of fluorescein isothiocyanate dye when chemically bound to ZnO nanoscale tripod structures. Graduate, undergraduate and high school students are being trained on cutting-edge research techniques such as XANES, transmission electron microscopy, X-ray photoelectron spectroscopy, superconducting quantum interference device magnetometry and X-ray diffraction.
Magnetic materials are widely used for information storage while semiconducting materials are essential for high speed processing of information, thus these two materials form the most important components of the vast majority of electronic devices today. Developing ferromagnetism in conventional semiconductor materials is highly desired because it will lead to faster, smaller, more energy efficient and cheaper devices. Above-room temperature ferromagnetism in semiconductors doped with a small amount of transition metal (TM) ions, such as Fe, Co, Mn, Ni etc, known as dilute magnetic semiconductors, have been theoretically predicted, and a decade-long experimental studies on accomplishing it, mostly in oxide semiconductor systems, have been reported. Ferromagnetism in TM doped oxide semiconductors have been reported by numerous groups, including the PI. However, there is widespread agreement in the research community in this area that the observed ferromagnetism cannot be understood using the conventional superexchange or double exchange interactions, or by the dilute magnetic semiconductor model. The magnetic moment per dopant ion in TM doped ZnO, SnO2 and Ceria obtained by the PI shows that the magnetic moment per ion decreases with increasing dopant %. Another study by the PI showed that the ferromagnetism in pure undoped metal oxides strengthens when structural defects are intentionally introduced by changing the oxygen stoichiometry. The PI demonstrated that the magnetism of undoped ZnO nanoparticles at 300K increases with decreasing oxygen stoichiometry. More detailed studies by the PI have shows the actual role of TM in producing magnetism in oxide semiconductors. PI's studies of Co doped ZnO and Fe doped SnO2 have clearly shown that (1)magnetism occurs only if the dopants exist as multivalent ions (Co2+ and Co3+ in ZnO; Fe3+ and Fe4+ in SnO2); (2)Ferromagnetism is present in samples that have large number of oxygen vacancies to act as defect states and (3) Magnetism disappears when the dopant exists as monovalent ion. Finally, the role of surface linked organic molecules including dodecanethiol, tryoctylphosphine, dodecylamine, in producing ferromagnetism in oxides semiconductors via charge transfer between nanoparticles and these linked molecules was investigated by the PI . These studies have shown that a more probable role for multi-valent TM ions in oxide semiconductors is to act as charge reservoirs from which electrons can be transferred to local defect density of states (mostly oxygen vacancies), leading to charge transfer ferromagnetism. These results have been published in about a dozen peer reviewed journal articles. This project supported the research of a graduate student Ms. Catherine Anders in the interdisciplinary Biomolecular Sciences Ph.D. programs at Boise State University. The nanoparticle synthesis data is used in a new course PHYS 620 Nanobiotechnology and the characterization data of the samples are used to demonstrate the use of several characterization techniques such as x-ray diffraction, and x-ray photoelectron spectroscopy in PHYS 523 Physical Methods of Materials Characterization. This project provided research opportunities for as many as 5 undergraduate students. Two of these students, Mr. Jordan Chess and Mr. Josh Eixenberger completed BS in Physics, published multiple research papers and got admitted to PhD programs. The work of Ms. Kelsey Dodge resulted in two published manuscripts, and she and Ms. Katherine Rainey have submitted one manuscript each for publication recently.
