MOCVD synthesis of compositionally tuned topological insulator nanowires

Device applications involving topological insulators (TIs) will require the development of scalable methods for fabricating TI samples with sub-micron dimensions, high quality surfaces, and controlled compositions. Here we use Bi-, Se-, and Te-bearing metalorganic precursors to synthesize TIs in the form of nanowires. Single crystal nanowires can be grown with compositions ranging from Bi2Se3 to Bi2Te3, including the ternary compound Bi2Te2Se. These high quality nanostructured TI compounds are suitable platforms for on-going searches for Majorana Fermions

Large anomalous Hall effect in ferromagnetic insulator-topological insulator heterostructures

We demonstrate the van der Waals epitaxy of the topological insulator compound Bi2Te3 on the ferromagnetic insulator Cr2Ge2Te6. The layers are oriented with (001) of Bi2Te3 parallel to (001) of Cr2Ge2Te6 and (110) of Bi2Te3 parallel to (100) of Cr2Ge2Te6. Cross-sectional transmission electron microscopy indicates the formation of a sharp interface. At low temperatures, bilayers consisting of Bi2Te3 on Cr2Ge2Te6 exhibit a large anomalous Hall effect (AHE). Tilted field studies of the AHE indicate that the easy axis lies along the c-axis of the heterostructure, consistent with magnetization measurements in bulk Cr2Ge2Te6. The 61 K Curie temperature of Cr2Ge2Te6 and the use of near-stoichiometric materials may lead to the development of spintronic devices based on the AHE.Comment: Related papers at http://pettagroup.princeton.ed

Controlled MOCVD growth of Bi2Se3 topological insulator nanoribbons

Topological insulators are a new class of materials that support topologically protected electronic surface states. Potential applications of the surface states in low dissipation electronic devices have motivated efforts to create nanoscale samples with large surface-to-volume ratios and highly controlled stoichiometry. Se vacancies in Bi2Se3 give rise to bulk conduction, which masks the transport properties of the surface states. We have therefore developed a new route for the synthesis of topological insulator nanostructures using metalorganic chemical vapour deposition (MOCVD). MOCVD allows for control of the Se/Bi flux ratio during growth. With the aim of rational growth, we vary the Se/Bi flux ratio, growth time, and substrate temperature, and observe morphological changes which indicate a growth regime in which nanoribbon formation is limited by the Bi precursor mass-flow. MOCVD growth of Bi2Se3 nanostructures occurs via a distinct growth mechanism that is nucleated by gold nanoparticles at the base of the nanowire. By tuning the reaction conditions, we obtain either single-crystalline ribbons up to 10 microns long or thin micron-sized platelets.Comment: Related papers at http://pettagroup.princeton.ed

Stem Cells to Regenerate Cardiac Tissue in Heart Failure

Myocardial regeneration is one of the most promising therapeutic strategies for heart failure patients. Many experimental studies have demonstrated that different types of stem cell can differentiate into myocardial cells and tissues necessary for regeneration of the damaged myocardium, while studies in experimental animals suggest that muscle (myoblast), bone marrow (mesenchymal, endothelial or hematopoietic progenitors) and even heart cells can help to improve heart contractility in vivo. These findings have led several groups to undertake studies in patients with myocardial infarction. However, the use of cellular therapy in clinical trials is not without controversy, mainly related with the need for better knowledge before these therapeutic strategies are used in clinical practice. Although significant enhancement of our knowledge of the processes involved is fundamental, we do not consider it unreasonable to initiate clinical trials in which specific questions are posed, whose answers will allow us to make further progress

Utilización de células madre para la regeneración miocárdica en la insuficiencia cardíaca

La terapia celular en la reparación miocárdica se vislumbra como una de las estrategias terapéuticas con mayor futuro en el tratamiento de la insuficiencia cardíaca. Numerosos estudios in vitro recientes apoyan la potencialidad de distintos tipos de células madre de diferenciarse hacia los tejidos necesarios para regenerar el tejido miocárdico dañado, mientras que estudios en animales de experimentación sugieren que células madre de músculo (mioblastos), médula ósea (progenitores mesenquimales, endoteliales o hematopoyéticos) e incluso del propio corazón pueden contribuir in vivo a mejorar la contractilidad cardíaca. Estos trabajos han conducido a que diversos grupos hayan iniciado estudios en pacientes con infarto de miocardio. Sin embargo, la utilización de la terapia celular en ensayos clínicos no está desprovista de controversia, fundamentalmente relacionada con la necesidad de aumentar nuestro conocimiento antes de pasar a la aplicación clínica de estas estrategias terapéuticas. Aunque es fundamental aumentar significativamente el conocimiento de los procesos, no consideramos irrazonable iniciar ensayos clínicos en los que se identifiquen preguntas concretas cuya respuesta nos permita avanzar en esta dirección