/pdf/self-assembled-monolayers-of-thiolates-on-metals-as-a-form-1neb0hj3k4.pdf

Self-assembled monolayers of thiolates on metals as a form of nanotechnology.

▪ Abstract Solution phase syntheses and size-selective separation methods to prepare semiconductor and metal nanocrystals, tunable in size from ∼1 to 20 nm and monodisperse to ≤5%, are presented. Preparation of monodisperse samples enables systematic characterization of the structural, electronic, and optical properties of materials as they evolve from molecular to bulk in the nanometer size range. Sample uniformity makes it possible to manipulate nanocrystals into close-packed, glassy, and ordered nanocrystal assemblies (superlattices, colloidal crystals, supercrystals). Rigorous structural characterization is critical to understanding the electronic and optical properties of both nanocrystals and their assemblies. At inter-particle separations 5–100 A, dipole-dipole interactions lead to energy transfer between neighboring nanocrystals, and electronic tunneling between proximal nanocrystals gives rise to dark and photoconductivity. At separations <5 A, exchange interactions cause otherwise insulating ass...

Synthesis and Characterization of Monodisperse Nanocrystals and Close-Packed Nanocrystal Assemblies

The purpose of this article is to describe the basic physical processes involved in the nucleation and growth of thin films of materials on solid surfaces. In this introduction the three modes of crystal growth which are thought to occur on surfaces in the absence of interdiffusion are described, and the relationships between the thermodynamics of adsorption and the kinetics of crystal growth are explored in general terms. This is followed by a brief review of atomistic nucleation theory, explaining the relations of such theories to experimental observables. In the next three sections, recent experimental examples of these three growth modes are given, which are interpreted where possible in terms of nucleation and growth theory. The last section discusses observations on the shapes of growing crystallites and the relation of such observations to nucleation and surface diffusion processes.

https://iopscience.iop.org/article/10.1088/0034-4885/47/4/002/pdf

Nucleation and growth of thin films

The fabrication methods of the microelectronics industry have been refined to produce ever smaller devices, but will soon reach their fundamental limits. A promising alternative route to even smaller functional systems with nanometre dimensions is the autonomous ordering and assembly of atoms and molecules on atomically well-defined surfaces. This approach combines ease of fabrication with exquisite control over the shape, composition and mesoscale organization of the surface structures formed. Once the mechanisms controlling the self-ordering phenomena are fully understood, the self-assembly and growth processes can be steered to create a wide range of surface nanostructures from metallic, semiconducting and molecular materials.

/pdf/engineering-atomic-and-molecular-nanostructures-at-surfaces-2sqr7sjd15.pdf

Engineering atomic and molecular nanostructures at surfaces

“Spintronics,” in which both the spin and charge of electrons are used for logic and memory operations, promises an alternate route to traditional semiconductor electronics. A complete logic architecture can be constructed, which uses planar magnetic wires that are less than a micrometer in width. Logical NOT, logical AND, signal fan-out, and signal cross-over elements each have a simple geometric design, and they can be integrated together into one circuit. An additional element for data input allows information to be written to domain-wall logic circuits.

http://science.sciencemag.org/content/sci/309/5741/1688.full.pdf

Magnetic Domain-Wall Logic

The Nano-Devices Group at the University of Notre Dame proposed a new device that encodes information in the geometrical charge distribution of artificial (or natural) molecules. Functional units are composed by electrostatic coupling. In these units, processing takes place by reshaping the electron density of the molecules, and not by switching currents. Signal processing potential of next-neighbor-coupled cellular nonlinear networks (CNNs) has been recently explored with the conclusion that local-activity of the cells is necessary to exhibit complexity. It will be shown that Coulomb-coupled time-invariant artificial molecules behave like nonlinear locally passive devices, thus signal-power-gain or multiple equilibria cannot be achieved by integrating them. However, the signal input-output relation of strongly nonlinear molecules can be varied in time by adiabatic pumping, called clock control. It will be shown that strongly nonlinear time-varying molecules can transform the necessary amount of clock energy into the signal flow, thereby enabling the network of molecules to perform signal processing.

/pdf/signal-processing-with-near-neighbor-coupled-time-varying-4nfbmej72h.pdf

Signal processing with near-neighbor-coupled time-varying quantum-dot arrays

The preparation of 7-Fc(+) -8-Fc-7,8-nido-[C2 B9 H10 ](-) (Fc(+) FcC2 B9 (-) ) demonstrates the successful incorporation of a carborane cage as an internal counteranion bridging between ferrocene and ferrocenium units. This neutral mixed-valence Fe(II) /Fe(III) complex overcomes the proximal electronic bias imposed by external counterions, a practical limitation in the use of molecular switches. A combination of UV/Vis-NIR spectroscopic and TD-DFT computational studies indicate that electron transfer within Fc(+) FcC2 B9 (-) is achieved through a bridge-mediated mechanism. This electronic framework therefore provides the possibility of an all-neutral null state, a key requirement for the implementation of quantum-dot cellular automata (QCA) molecular computing. The adhesion, ordering, and characterization of Fc(+) FcC2 B9 (-) on Au(111) has been observed by scanning tunneling microscopy.

Synthesis of a Neutral Mixed‐Valence Diferrocenyl Carborane for Molecular Quantum‐Dot Cellular Automata Applications

The quantum-dot cellular automata (QCA) paradigm is a revolutionary approach to molecular-scale computing which represents binary information using the charge configuration of nanostructures in lieu of current switching devices. The basic building-block of QCA devices is the QCA cell. Electrostatic interaction between neighboring cells allows the design of QCA wires, logic devices and even simple microprocessors. The geometry of molecular six-dot QCA cells enables the clocking of QCA devices via an electric field generated by a layout of clocking wires. Thus, precise control over the timing and direction of data flow in QCA circuits is possible. The design of QCA circuits now lies not only in the logic structure of the cells, but also in the layout of clocking wires. We discuss the clocking of QCA devices and connect layout to architecture.

/pdf/quantum-dot-cellular-automata-an-architecture-for-molecular-ko87wclfap.pdf

Quantum-dot cellular automata: an architecture for molecular computing

The behavior of rings of four small‐capacitance tunnel junctions that are charged with two extra electrons is examined. Single electron charging effects result in quantization of charge on the metal electrode islands. To minimize the total electrostatic energy, the electrons localize on opposite electrodes, leading to a charge alignment in one of two configurations. We consider such rings as cells that may be capacitively coupled to each other in a cellular automaton architecture. The interaction between cells results in strong bistable saturation in the cell’s charge alignment which may be used to encode binary information. Lines of such cells can be viewed as binary wires.

Bistable saturation due to single electron charging in rings of tunnel junctions

Measurable disorder exists on GaAs(001) surfaces during MBE even though these surfaces subsequently incorporate into exceedingly well‐ordered bulk GaAs lattices. We have used reflection high energy electron diffraction (RHEED) to characterize this disorder. On the As stabilized surface obtained at 850 K, some of the fractional order beams are broadened indicating that there is antiphase disorder present on the surface. Models of this disorder are presented. In addition, if the growth is stopped by shuttering the Ga flux both integral and quarter order beams sharpen, indicating that a second type of disorder exists. From the lengths of the RHEED streaks we determine that during growth the long range order in the twofold direction is about 500 A. About 10 min after growth is stopped, the extent of order in this direction increases to 3000 A. At a given As flux the ordering process is slightly temperature dependent over the narrow range that this disordered 2×4 exists. At lower temperatures, the disordered s...

Craig S. Lent

Papers

Signal processing with near-neighbor-coupled time-varying quantum-dot arrays

Synthesis of a Neutral Mixed‐Valence Diferrocenyl Carborane for Molecular Quantum‐Dot Cellular Automata Applications

Quantum-dot cellular automata: an architecture for molecular computing

Bistable saturation due to single electron charging in rings of tunnel junctions

Disorder on GaAs(001) surfaces prepared by molecular beam epitaxy