Bawendi Group

Semiconductor nanocrystallites (quantum dots, QDs) whose radii are smaller than the bulk exciton Bohr radius constitute a class of materials intermediate between molecular and bulk forms of matter. Quantum confinement of both the electron and hole in all three dimensions leads to an increase in the effective band gap of the material with decreasing crystallite size. Consequently, both the optical absorption and emission of quantum dots shift to the blue (higher energies) as the size of the dots gets smaller. Although nanocrystallites have not yet completed their evolution into bulk solids, structural studies indicate that they retain the bulk crystal structure and lattice parameter. Recent advances in the synthesis of highly monodisperse nanocrystallites have paved the way for numerous spectroscopic studies assigning the quantum dot electronic states and mapping out their evolution as a function of size.

(*) Synthesis and Characterization -- In 1993 the Bawendi group developed a synthetic method for producing monodisperse cadmium chalcogenide nanocrystals. This synthetic method was based upon the classic work of La Mer and Dinegar in 1950 on colloid formation. The injection of precursors above a critical temperature creates a nucleation event and this is followed by rapid cooling to a growth temperature where no further nucleation is favorable.

Since 1993, the Bawendi group has pursued the synthesis of novel semiconductor nanocrystals. The unique c axis of the wurtzite CdSe structure allows for the growth of nanoscale heterostructures upon the terminal faces of the c axis. This has been exploited in the synthesis of CdSe/CdTe nanobarbells to spatially separate excitons for the potential purpose of creating more efficient solar cell materials.

(*) Microfluidic Synthesis of QDs -- In this lab, we design and fabricate microfluidic reactors as a synthesis tool and a mean to systematically study the mechanism of QD formation. Microfluidic reactors enable a number of advantages over conventional chemical processes including enhanced control of heat and mass transfer, lower reagent consumption during optimization, and sensor integration for in-situ reaction monitoring.

We make great efforts in improving the reactor design, such as a change from single-phase to segmented flow in our design work. In the single-phase laminar flow, diffusion is the only means of mixing. As a result of the parabolic fluid-velocity profile, particles near the wall spend more time in the reactor than those in the center, resulting in broad Residence Time Distributions (RTDs). In the two-phase case, recirculation within each liquid slug brings material from the wall to the center of the channel. This facilitates mixing, which narrows the RTD, and results in narrower size distributions.

Precursor solutions are delivered separately into the heated section and an Ar gas stream introduced further downstream results in a segmented gas-liquid flow. Recirculation within the liquid slugs rapidly mixes the heated precursors, thereby initiating the reaction. The fluids initially pass through a meandering section to induce good mixing across the channel before reaching a longer straight-channel section where the majority of the particle growth occurs. The segment lengths are very uniform during conditions of a typical QD synthesis and the resulting monodisperse samples exhibit excellent optical properties. The reaction is stopped when the fluids enter the cooled outlet region of the device.

(*) Magnetic Nanoparticles -- The development of monodisperse magnetic nanocrystals (NC) has been intensively pursued due to their fundamental and technological interest. Magnetic NC often exhibit very interesting physical and chemical properties, which are significantly different from those of their bulk counterparts. Recently, it has been reported that when sample containing an interface between a ferromagnet and an antiferromagnet is cooled in a magnetic field, it may exhibit an additional unidirectional anisotropy due to magnetic coupling at the interface.

We investigated systematically the magnetic properties of colloidal Co NPs after three extents of oxidation. The native sample has a thin (1.0 nm) CoO shell and exhibits no exchange biasing. The partially oxidized sample has a thicker CoO shell (3.2 nm), and is exchange biased. The sample fully oxidized to CoO loses exchange biasing. We observe three distinct magnetic properties that result from the finite-thickness antiferromagnet shell exchange coupled to a finite-size ferromagnet core, and from crystal and stoichiometric defects: (1) an enhancement of the thermal stability of the orientation of the magnetic moment due to exchange biasing in the partially oxidized sample, (2) a low-temperature paramagnetic response in the partially and fully oxidized samples due to defects in the CoO shell, and (3) an asymmetry in the field-dependent magnetization for the partially oxidized sample at low temperature due to small clusters of Co in a diffusion layer around the Co core.

(*) Ab-initio Studies of CdSe Growth -- Despite great efforts directed toward the development of adaptable chemistries for synthesis of nanocrystals, the exact reaction mechanisms leading to crystal growth in these systems are still poorly understood. Furthermore, since experimental methods to study reactive processes occurring on crystal surfaces are limited by the extreme difficulty of isolating elementary reaction steps, we decided to exploit first principles methods. In particular, we have employed periodic density functional theory (DFT) calculations to investigate the mechanistic details leading to nanocrystalline growth of CdSe.