Fabricating Quantum Dot Monolayers Sandwiched Between van der Waals Monolayer Electrodes

Using methods to form order monolayers of quantum dots on a liquid surface, the Herman group made sandwiches of a monolayer of CdSe quantum dots (or nanoparticles) between two graphene layers, as shown in the below figure. This is a new type of vertical heterostructure made of nanocomponents, which can provide new functionality.

Rapid and Conformal Coverage of Nanocrystal Films by Using EPD With Backside Electrodes

The Herman group continued its work on the use of electrophoretic deposition (EPD) to fabricate films of nanocrystals to show that use of electrodes on the backside (or buried electrodes) need not decrease the EPD rate, as shown in the top figure, because much or most of the the voltage drop can still be across the high-resistivity colloid (with nonpolar solvents).

SAXS Monitoring Growth of Thick Single and Binary Nanoparticle Superlattices

Using small angle x-ray scattering (SAXS) at the Brookhaven National Laboratory the Herman group has followed in real time the kinetics of the growth of ordered 3D NP superlattices (SLs, SNSLs) (of one size of iron oxide NPs) and binary NP SLs (BNSLs) (of two sizes of iron oxide NPs) in solution are understood by combining controlled solvent evaporation. We have learned that the larger the NP the farther apart are the NPs when the SNSLs begin to precipitate and the closer they are after ordering, as seen in the below figures, which we have explained by using a model of the energies of interactions betweent the NPs.

Precisely Placed 100-Layer Thick Nanocrystal Superlattices

The Herman group has developed a method to form 100-layer thick supercrystals of nanoparticles at lithographically-determined sites using a two-solvent microfluid flow method. These micrometer-dimension supercrystals form as a result of flow from a reservoir to a channel (due to capillary flow) and subsequent controlled drying. Previously, very few layered-superlattices could be formed on surfaces and supercrystals could be formed only after drying in beakers. These new materials open the prospect of cooperative optical, electrical, magnetic and magenetic properties. The left SEM shows the top of a CdSe nanocrystal superlattice and the right one shows the side of a cleaved Fe₂O₃ nanocrystal superlattice. The trace represents small angle x-ray scattering (SAXS) of the Fe₂O₃ nanocrystal superlattice.

Formation of Nanocrystal-Carbon Nanotube Hybrid Materials

The Herman group is using methods to form hybrids of nanocrystals and carbon nanotubes that may have novel optical and electrical properties. Both components have signature optical properties and coupling between them may be very interesting. The left SEM shows HIPCo carbon nanotubes with (a) CdSe nanparticles, (b) core shell particles and (c) CdSe nanorods, and the right SEM shows a hybrid of a CNT and CdSe nanoparticles on a surface, with an AFM inset.

Spatially Controlled AC Dielectrophoretic Placement of Carbon Nanotubes in Device Structures

The Herman group is developing and using methods to controllably place single walled carbon nanotubes (CNTs) in multi-electrode device geometries by using dielectrophoretic deposition (DEPD) and other methods. The top row shows SEMs of DEPD without (left) and with (right) control by series resistors. The bottom row shows control using floating posts (left) and controlled deposition of crossed CNTs (right).

Controlled Synthesis of CdSe Nanorods

The Herman group usually synthesizes nanocrystals using well-established procedures or obtains them from collaborators. Howevever in this study it developed controlled methods for synthesizing CdSe nanotubes using phosphonic acid ligands (left) of different alkyl chain lenghts and by using mixtures of these ligands. The shorter the ligand, the longer the rod and the more likely it is branched. The figure on the right demonstrates quantum confinement as measured by energy of the first exciton peak in these nanorods.

Electrophoretic Deposition of Films of Mixtures of CdSe and Fe₂O₃ Nanocrystals

In some cases, electrophoretic deposition using mixtures of nanocrystals produces films composed of mixtures of the nanocrystals. This is shown below schematically for mixtures of CdSe and Fe₂O₃ nanocrystals. Such film mixtures are potentially multifunctional nanomaterials. When Au nanocrystals are added to a hexane solvent containing either CdSe or Fe₂O₃ nanocrystals, there is deposition of a film containing either only CdSe or Fe₂O₃ nanocrystals, respectively, and this occurs on only one electrode. This is a collaboration with the O'Brien and Levicky groups.

Electrophoretic Deposition of Films of CdSe Nanocrystals

The Herman group is examining the use of electrophoretic deposition methods to form thick, smooth, robust films of CdSe nanocrystals. The figure on the top shows particles selectively depostied on metal regions in contact with the electrode (gray), with no deposition on oxide (dark) or metal regions isolated from the electrode (bright). Depositon about of a particle film (gray) around an oxide spacer (dark) is shown on the bottom. For very thick films, strain leads to film fracture, as is seen in the SEM (lower, right). Such cracking is not uncommon when there is some solvent evaporation during film drying.

Real-time Monitoring of Organic Surface Ligands and Solvent During the Self-Assembly of Nanocrystal Arrays

The densities of surface ligands and solvent molecules are being followed during the self-assembly of CdSe nanocrystals into arrays by using multiple-reflection attenuated total internal reflection (ATIR) spectroscopy. This is performed in a Fourier transform infrared (FTIR) spectrometer in which the arrays formed on a ZnSe prism. During the self-assembly of CdSe nanocrystals passivated by pyridine that are dissolved in pyridine, the 1436.1 cm^-1 peak of neat pyridine is followed along with that at 1445.2 cm^-1 due to pyridine bound to the CdSe surface (below).

The solvent evaporates in about 30 minutes during the self-assembly of ~200-monolayer thick arrays in an argon ambient (inset below). The pyridine bound to the surface slowly leaves the surface (inset, open symbols), but about 35% remains after drying for several days (main figure, below). Since pyridine only weakly binds to the surface, it had been commonly thought that no pyridine would remain after extensive drying. While this is not true, there are still significant changes to the surface during drying. This result is important for the MRSEC studies of the self-assembly of arrays and the properties of these arrays. This monitoring method is also being used to follow the surface of exchange of TOPO and pyridine ligands, and related processes.