/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

Quantum-dot cellular automata (QCA) is an approach to computing which eliminates the need for transistors by representing binary digits as charge configurations rather than current levels. Coulomb interactions provide device-device coupling without current flow. Clocked control of the device allows power gain, control of power dissipation, and pipelined computation. Molecular QCA uses redox sites of molecules as quantum dots. We present an ab initio analysis of a simple molecular system which acts as a clocked molecular QCA cell.

Molecular quantum-dot cellular automata

We present an experimental demonstration of power gain in quantum-dot cellular automata (QCA) devices. Power gain is necessary in all practical electronic circuits where power dissipation leads to decay of logic levels. In QCA devices, charge configurations in quantum dots are used to encode and process binary information. The energy required to restore logic levels in QCA devices is drawn from the clock signal. We measure the energy flow through a clocked QCA latch and show that power gain is achieved.

https://www3.nd.edu/~lent/pdf/nd/Power_gain_in_a_quantum-dot_cellular_automata_latch.pdf

Power gain in a quantum-dot cellular automata latch

Quantum cellular automata

Molecular electronics is commonly conceived as reproducing diode or transistor action at the molecular level. The quantum-dot cellular automata (QCA) approach offers an attractive alternative in which binary information is encoded in the configuration of charge among redox-active molecular sites. The Coulomb interaction between neighboring molecules provides device-device coupling. No current flow between molecules is required. We present an ab initio analysis of a simple molecular system which acts as a molecular QCA cell. The intrinsic bistability of the charge configuration results in dipole or quadrupole fields which couple strongly to the state of neighboring molecules. We show how logic gates can be implemented. We examine the role of the relaxation of nuclear coordinates in the molecular charge reconfiguration.

{Cp*(dppe)Fe(C≡C−)}2(1,3-C6H4) is studied both as a neutral molecule, Fe(II)−Fe(II), and as a mixed-valence complex, Fe(II)−Fe(III). Scanning tunneling microscopy (STM) is used to image these species at 77 K under ultrahigh-vacuum conditions. The neutral molecule Fe(II)−Fe(II) has a symmetric, “dumbbell” appearance in STM images, while the mixed-valence complex Fe(II)−Fe(III) demonstrates an asymmetric, bright-dim double-dot structure. This asymmetry results from localization of the electron to one of the iron-ligand centers, a result which is confirmed through comparison to theoretical STM images calculated using constrained density-functional theory (CDFT). The observation of charge localization in mixed-valence complexes outside of the solution environment opens up new avenues for the control and patterning of charge on surfaces, with potential applications in smart materials and molecular electronic devices.

/pdf/charge-localization-in-isolated-mixed-valence-complexes-an-47q2r38ce0.pdf

Craig S. Lent

Papers

Molecular quantum-dot cellular automata

Power gain in a quantum-dot cellular automata latch

Quantum cellular automata

Molecular quantum-dot cellular automata

Charge localization in isolated mixed-valence complexes: an STM and theoretical study.