Category: Highlights

Single Molecule Quantum State Switching with AFM Tip

C. Chotsuwan and S. C. Blackstock*

This figure illustrates the quantum state switching of a single 5PD molecule in a polymer film.  The polyredox molecule 5PD has 11 assessable and chemically stable charge states (0-+10) as determined electrochemically in solution.  When diluted in a thin polymer film, individual 5PD molecules can be reversibly switched to various charge states by contact with a biased AFM tip.  The 5PD charge state is monitored by Kelvin probe microscopy (KPM) via its associated surface potential. This result constitutes controlled quantum charge state switching in a discrete 3 nm3 volume that is the 5PD molecule in a thin film.

New Half-Metals

Metals (like copper and aluminum) can carry an electrical current; insulators on the other hand, (like glass) do not. Can a material be simultaneously a metal and an insulator? Surprisingly – the answer is yes! Materials have two different types of electrons – “up” spin and “down” spin. The electrical properties of these two different types of electrons are often quite different in ferromagnets. The most striking difference is seen in “half-metals”. These are ferromagnets in which electrons of one spin type act as if they are in a metal and those of the other spin type act as if they are in insulator. Scientists working at the MRSEC in the MINT Center at the University of Alabama have predicted two new families of half-metals. One is a variation on a well known family of materials called “Heusler” alloys. MINT scientists have developed simple recipes for mixing many different elements to make an infinite number of new half-metallic systems based on the simple body centered cubic crystal structure. The other family of half-metals is based on the spinel crystal structure (AB2X4) where A=Cu or Cd, B=Cr, and X=S or Se. The half-metals can be obtained by using combinations of Cu and Cd on the A sites or by replacing some of the S or Se atoms with Cr or Br or vacancies. These new half-metals may be used to make new types of “spintronic” devices. Spintronics is a new science and technology that utilizes the electron’s spin as well as its charge.

UA MRSEC Initiates Collaborations with Local Science Museums

The MRSEC at the University of Alabama has begun partnering with the McWane Science Center in Birmingham and the Children’s Hands on Museum (CHOM) in Tuscaloosa. Graduate students took part in “Show and Tell Saturday” at CHOM in February, letting museum visitors do experiments to explore the effect of detergents on oil/water mixtures, and to build simple electrical circuits.

A team of MRSEC faculty gave a presentation to McWane Science Center staff on Nanoscience, and developed plans for deploying faculty/students teams in the Museum to assist with Nanodays activities in March.

Controlling Bimetallic Composition at Individual Nanoparticle Level

Problem statement: Self-assembled magnetic arrays of FePt nanoparticles are candidate structures for ultra-high areal storage density. Compositional variation between individual FePt nanoparticles can lead to wide distributions in the magnetic properties and performance of these nanoparticles
Solution: Through extracting nanoaprticles at various stages of chemical synthesis, we have learned how FePt nanoparticles nucleate and grow. Subsequently, we have discovered that simultaneous decomposition/reduction of Fe-based and Pt-based precursors results in wide compositional distributions. Alternatively, the initial formation of Pt-rich seeds followed by the subsequent incorporation of Fe significantly narrows the compositional distribution (see Srivastava et al. JAP 102 (2007) p. 104310). We have tailored the surfactant chemistry of a previous method that exhibited wide compositional variation into a 2-step nucleation and growth process. The corresponding compositional histograms show a significant narrowing of the composition, demonstrating that the nucleation sequence (not precursor choice) is critical in controlling the nanoscale composition in these bimetallic nanoparticles.

High School Interns Named Semifinalists in National Science Competitions

The Center for Materials for Information Technology at the University of Alabama provides a summer research experience for high school students through its Nanoscience and Engineering High School Internship Program. Mr. Christopher D. Romanczuk (from the Randolph School in Huntsville) and Mr. Mike W. Zhang (from the Alabama School of Math and Sciences in Mobile) worked in the laboratories of Christopher S. Brazel (Department of Chemical & Biological Engineering) and David E. Nikles (Department of Chemistry) on magnetic particles for magnetic fluid hyperthermia therapy, a potential new form of cancer therapy. Chris Romanczuk worked on FePt and CuNi alloy nanoparticles, while Mike Zhang worked on FeNiPt nanoparticles. Both sought to understand the effect of magnetic anisotropy on the efficiency of ac magnetic field heating of the particles in aqueous solution. Chris entered the Siemens Competition and was the only person from the State of Alabama to be named a semifinalist. Mike entered the Intel Science Talent Search and was just named a semifinalist, the only one from the State of Alabama.

The Nanoscience and Engineering High School Internship Program was been supported in part by the NSF
Materials Research Science and Engineering Center award DMR-0213985 and the Office of the Provost of the
University of Alabama.

Writing charges into single molecules embedded in thin polymer films by Atomic Force Microscopy

C. Chotsuwan and S. C. Blackstock

  • Single molecule charging (electrochemistry) is accomplished in diluted polymer matrixes: this results provides a way to control electron placement on the nanometer scale, an important capability for the development of molecular electronic technologies, including information storage in molecules, the application in this case.
  • An AFM tip charging radius of 1.9 nm has been determined from these experiments: the charged molecule can be used to measure the resolution of the AFM charge injection process, an important technological parameter for the use of AFM-based charge patterning in other media.

Adding Ag to FePt Nanoparticles Lowers the Temperature for L10 Ordering

Chemically synthesized FePt nanoparticles have attached considerable attention as future, high density magnetic recording media. A fundamental problem has been that the chemical synthesis gives the disordered A1 phase and the particles must be heated to temperatures above 550ºC to obtain the chemically ordered L10 phase with high magnetocrystalline anisotropy. However the heating also caused undesirable grains growth and this has led to a search for means of obtaining the L10 phase without grain growth. The Nanoparticles Team in our MRSEC was the first to develop the chemistry to prepare ternary alloys, FePtM, where M is Ag, Au, Co, Cu, Mn, Ni or Pd. The particles had a chemically disordered fcc structure with Fe, Pt and M randomly placed on the fcc lattice. Adding Ag or Ag lowers the temperature for chemical ordering; however for self-assembled films the addition also lowered the temperature for the onset of grain growth. A recently developed salt annealing procedure (J. P. Liu at U. T. Arlington) allowed us to heat the particles without sintering. We found that adding Ag intrinsically lowers the temperature required fro chemical ordering, while adding Mn had no effect. Further studies will seek to understand the mechanism of the temperature lowering and the fate of the Ag additive.

High School Intern Prepares Micro-Nanostructured Metals

Catherine Cook, a high school senior at Tuscaloosa Academy carried out research in the summer of 2007 as a Nanoscience and Engineering Intern working with graduate student Jason Manning and middle school teachers Brenda O’Neil and Leigh McKenzie. Catherine formed both microstructures and nanostructures using surfactant/oil/water mixtures as template. The combination of micro and nanostructure produces very high surface areas which are of interest for applications such as supercapacitors and separations. This work was recently accepted for publication in the Proceedings of the Materials Research Society, and was presented by Catherine at the Southeast Regional American Chemical Society in Greenville SC in October 2007.

Size-controlled Synthesis of Magnetic Chalcospinel Nanocrystals

The controlled synthesis of inorganic nanocrystals (NCs) has generated much interest in recent years, motivated in large part by the unusual electronic, optical and magnetic properties that are exhibited in this size regime. The solution-based synthesis of a wide range of magnetic NCs of different morphologies has been reported, including those of transition metals (Fe, Co, Ni), alloys (FePt, CoPt, etc), and oxide materials (ferrites, perovskites, etc). Their properties have been studied quite extensively, both from the viewpoint of fundamental understanding and applications, ranging from magnetic resonance imaging, drug delivery, biosensing, and nanoelectronic devices. These studies notwithstanding, the synthesis and properties of a number of other nanocrystalline magnetic materials, such as those based on the rare earths and chalcogenides, remain largely unexplored. Of particular interest are the chromium-based spinel chalcogenides, ACr2X4 (A= Cu, Cd, Hg, Fe, Co; X = S, Se, Te), which are ferro/ferrimagnetic insulators, semiconductors, or even metals that display unique properties in the bulk. As in the case of the standard magnetic systems, the utility of the chalcospinels can be augmented if they can be synthesized as colloidal nanocrystals with highly controlled dimensions.

Room temperature ferromagnetism with a Curie temperature of 430 K makes CuCr2Se4 an interesting system of study amongst the chalcospinels. Solution-based synthesis of CuCr2Se4 and other chalcospinels has, however, been limited by the complexity involved in the synthesis; particularly since they cannot be precipitated from aqueous solution. Furthermore, it is difficult to synthesize the stoichiometric compound and/or the desired spinel phase because of the volatility of Se and the availability of a number of oxidation states of chromium. Recently, there have been reports of solution-based solvothermal and microwave synthesis of CuCr2Se4 crystals. While the phase-pure material has been stabilized, these reactions result in the formation of relatively large crystallites or agglomerates with a broad size and shape distribution. We have developed a facile synthesis process for size-controlled CuCr2Se4 nanocrystals (∼15-30 nm, see Figure) that are nearly monodisperse and readily form a colloidal suspension. The process involves the thermal decomposition and reaction of the metal-acetylacetonate precursors with selenium in a high boiling organic solvent mixture of oleylamine (OLA) and 1-octadecene (ODE). The coordinating solvents play the dual role of forming an organo-Se intermediate and in selectively adsorbing on the surface of the nucleated product to passivate and control their size.

Y-H. A. Wang, N. Bao, L. Shen, P. Padhan, and A. Gupta, J. Am. Chem. Soc., in print.

Very low defect density in electroplated epitaxial Ni films

Electrodeposition technique was used to grow epitaxial Ni(111) epitaxial films on GaAs(110) and resistivity measurements were performed as a function of film thickness. The phenomenological Fuch’s model was used to analyze the data which describes the transport properties in terms of a surface and a bulk contribution to electron scattering. As shown in the figure, the slope of a straight line fit to the data corresponds to the bulk-resistivity which is the resistivity of the film assuming the surfaces would not scatter the electrons. Thus this value is a measure for electron scattering in the interior of the film and thus the value for a very thick film.

The bulk resistivity value at room temperature (red data) is close to the literature value for Ni whereas the low temperature (blue data) bulk resistivity value is 0.1 μΩcm or less. This indicates very low intra-layer electron scattering and thus a very low defect density due to the high quality of the epitaxial growth. Such a low bulk resistivity value normally is found only for films prepared under the cleanest vacuum conditions using Molecular Beam Epitaxy.