(263e) Shape- and Size-Controlled Fabrication of Lead Chalcogenide and Ferromagnetic Nanocrystals for Information Storage Devices

This report proposes the employment of self-assembled shape-controlled nanocrystals as building blocks of flash memory devices. Our approach consists on engineering the work function of the floating gate of memory devices with semiconductor nanocrystals of various compositions and morphology. Nanodot density, discreteness and size influence the charge capacity and the isolation of stored charges. To fully exploit the potential of nanodots as a floating gate, a homogeneous high-density nanodot array as the charge storage node is required. This work presents the synthesis of cubic-like PbSe and cubic and spherical PbTe nanocrystals (NCs) using a one-pot approach and the noncoordinating solvent, 1-octadecene (ODE) as capping agent, as well as the application of this material in floating gate memory device.

We employ an alternative method based on environmentally friendly solvent ODE. This one-pot technique does not need cooling of the precursors in a glovebox and is advantageously reproducible. We confirmed how lead chalcogenide NCs (PbSe, PbTe) undergo a shape evolution from spherical/polyhedron to cubic with increasing size. By controlling the average size of the colloids we could be able to manipulate the charge storage density. Recently fabricated PbSe NCs were compared with the cubic and spherical PbTe colloids. The next step in this research involves studying the relation among lead chalcogenide nanocrystals morphology and device performance. Further work would include the in-situ TEM observation in a heating stage to determine the thermal budget of lead chalcogenide NCs based devices.

A second approach involves using nanocrystals based magnetic random access memory (MRAM) devices. MRAM uses as the memory bit the magnetization orientation (parallel or antiparallel) of a free ferromagnetic layer with respect to a fixed layer separated by a thin non-magnet. Readout involves sensing the magnetoresistance of the stack, while writing and erasing ?i.e. switching of the free-layer magnetization ?is accomplished by the Oersted fields around the bit and word lines. This latter approach inherently lacks scalability and is improved upon by writing/erasing with current pulses using spin-torque switching (STS). We propose a STS based MRAM scheme with ferromagnetic nanocrystals (a single nanocrystal at the ultimate scaling limit) as the free ferromagnetic layer. The polarity of the current determines whether the nanocrystal magnetization ends up parallel or antiparallel. A much lower current is used to sense the magnetoresistance and thereby read out the state. This approach is scalable down to the superparamagnetic limit for the ferromagnetic nanocrystals ? which is the size limit below which ferromagnetism is not stable against thermal fluctuations. Our solution-phase synthesis of ferromagnetic colloids ? e.g. Co or PtFe ? is based on standard airless techniques on a Schlenk line. We have successfully demonstrated the organometallic-reduction of surfactant-protected dicobalt octacarbonyl in 1,2-dichlorobenzene, and platinum acetylacetonate, iron pentacarbonyl on octyl ether to prepare monodisperse ferromagnetic nanocrystals. In this case, we plan to operate at sizes somewhat above the superparamagnetic limit ? e.g. for spherical hcp Co nanoparticles, a diameter of about 10nm gives a retention of 10 years neglecting stray fields from neighboring cells. We will investigate the feasibility of controlling the nanocrystal shape to lower the superparamagnetic size limitation and thereby enable further scaling.