66 research outputs found
First-principles study of the atomic and electronic structure of the Si(111)-(5x2-Au surface reconstruction
We present a systematic study of the atomic and electronic structure of the
Si(111)-(5x2)-Au reconstruction using first-principles electronic structure
calculations based on the density functional theory. We analyze the structural
models proposed by Marks and Plass [Phys. Rev. Lett.75, 2172 (1995)], those
proposed recently by Erwin [Phys. Rev. Lett.91, 206101 (2003)], and a
completely new structure that was found during our structural optimizations. We
study in detail the energetics and the structural and electronic properties of
the different models. For the two most stable models, we also calculate the
change in the surface energy as a function of the content of silicon adatoms
for a realistic range of concentrations. Our new model is the energetically
most favorable in the range of low adatom concentrations, while Erwin's "5x2"
model becomes favorable for larger adatom concentrations. The crossing between
the surface energies of both structures is found close to 1/2 adatoms per 5x2
unit cell, i.e. near the maximum adatom coverage observed in the experiments.
Both models, the new structure and Erwin's "5x2" model, seem to provide a good
description of many of the available experimental data, particularly of the
angle-resolved photoemission measurements
Atomic Scale Memory at a Silicon Surface
The limits of pushing storage density to the atomic scale are explored with a
memory that stores a bit by the presence or absence of one silicon atom. These
atoms are positioned at lattice sites along self-assembled tracks with a pitch
of 5 atom rows. The writing process involves removal of Si atoms with the tip
of a scanning tunneling microscope. The memory can be reformatted by controlled
deposition of silicon. The constraints on speed and reliability are compared
with data storage in magnetic hard disks and DNA.Comment: 13 pages, 5 figures, accepted by Nanotechnolog
Massively parallel computing on an organic molecular layer
Current computers operate at enormous speeds of ~10^13 bits/s, but their
principle of sequential logic operation has remained unchanged since the 1950s.
Though our brain is much slower on a per-neuron base (~10^3 firings/s), it is
capable of remarkable decision-making based on the collective operations of
millions of neurons at a time in ever-evolving neural circuitry. Here we use
molecular switches to build an assembly where each molecule communicates-like
neurons-with many neighbors simultaneously. The assembly's ability to
reconfigure itself spontaneously for a new problem allows us to realize
conventional computing constructs like logic gates and Voronoi decompositions,
as well as to reproduce two natural phenomena: heat diffusion and the mutation
of normal cells to cancer cells. This is a shift from the current static
computing paradigm of serial bit-processing to a regime in which a large number
of bits are processed in parallel in dynamically changing hardware.Comment: 25 pages, 6 figure
Gd disilicide nanowires attached to Si(111) steps
Self-assembled electronic devices, such as quantum dots or switchable
molecules, need self-assembled nanowires as connections. We explore the growth
of conducting Gd disilicide nanowires at step arrays on Si(111). Atomically
smooth wires with large aspect ratios are formed at low coverage and high
growth rate (length >1 micron, width 10nm, height 0.6nm). They grow parallel to
the steps in the [-1 1 0 ] direction, which is consistent with a lattice match
of 0.8% with the a-axis of the hexagonal silicide, together with a large
mismatch in all other directions. This mechanism is similar to that observed
previously on Si(100). In contrast to Si(100), the wires are always attached to
step edges on Si(111) and can thus be grown selectively on regular step arrays.Comment: 3 pages including 4 figure
Reversible Photomechanical Switching of Individual Engineered Molecules at a Surface
We have observed reversible light-induced mechanical switching for a single
organic molecule bound to a metal surface. Scanning tunneling microscopy (STM)
was used to image the features of an individual azobenzene molecule on Au(111)
before and after reversibly cycling its mechanical structure between trans and
cis states using light. Azobenzene molecules were engineered to increase their
surface photomechanical activity by attaching varying numbers of tert-butyl
(TB) ligands ("legs") to the azobenzene phenyl rings. STM images show that
increasing the number of TB legs "lifts" the azobenzene molecules from the
substrate, thereby increasing molecular photomechanical activity by decreasing
molecule-surface coupling.Comment: related theoretical paper: cond-mat/061220
X-ray-Induced Reversible Switching of an Azobenzene Derivative Adsorbed on Bi(111)
We report on the adsorption of a submonolayer of di-m-cyanoazobenzene (DMC) on
Bi(111) and on the reversible switching of these molecules induced by resonant
X-ray illumination. DMC adsorbs in at least two configurations, the flat trans
and the nonflat cis isomer. We find that in 0.8 monolayers at least 26% of the
molecules change their configuration at 110 K by excitation of the N1s → LUMO
transition at the azo group, and by a thermally induced back reaction at 120
K. Nonresonant excitation with X-ray light does not induce any reversible
changes
Selective adsorption of metallocenes on clean and chemically modified Si(111) surfaces
Metallocene adsorption on clean Si(111) and CaF2/CaF1/Si(111) substrates has been investigated with scanning tunneling microscopy. The surface chemical composition is found to strongly change the adsorption site selectivity, leading to an enhanced edge selectivity on modified substrates. Templates with well-defined local chemical reactivity have been created via self-assembly. The selective adsorption of metallocenes on such tailored substrates facilitates patterning ordered arrays of magnetic nanowires and stripes on the single digit nanometer scale. ©1999 American Institute of Physics
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