22 research outputs found
Limits of Elemental Contrast by Low Energy Electron Point Source Holography
Motivated by the need for less destructive imaging of nanostructures, we
pursue point-source in-line holography (also known as point projection
microscopy, or PPM) with very low energy electrons (-100 eV). This technique
exploits the recent creation of ultrasharp and robust nanotips, which can field
emit electrons from a single atom at their apex, thus creating a path to an
extremely coherent source of electrons for holography. Our method has the
potential to achieve atom resolved images of nanostructures including
biological molecules. We demonstrate a further advantage of PPM emerging from
the fact that the very low energy electrons employed experience a large elastic
scattering cross section relative to many-keV electrons. Moreover, the
variation of scattering factors as a function of atom type allows for enhanced
elemental contrast. Low energy electrons arguably offer the further advantage
of causing minimum damage to most materials. Model results for small molecules
and adatoms on graphene substrates, where very small damage is expected,
indicate that a phase contrast is obtainable between elements with
significantly different Z-numbers. For example, for typical setup parameters,
atoms such as C and P are discernible, while C and N are not.Comment: 15 pages, 5 figure
Theory of Non-equilibrium Single Electron Dynamics in STM Imaging of Dangling Bonds on a Hydrogenated Silicon Surface
During fabrication and scanning-tunneling-microscope (STM) imaging of
dangling bonds (DBs) on a hydrogenated silicon surface, we consistently
observed halo-like features around isolated DBs for specific imaging
conditions. These surround individual or small groups of DBs, have abnormally
sharp edges, and cannot be explained by conventional STM theory. Here we
investigate the nature of these features by a comprehensive 3-dimensional model
of elastic and inelastic charge transfer in the vicinity of a DB. Our essential
finding is that non-equilibrium current through the localized electronic state
of a DB determines the charging state of the DB. This localized charge distorts
the electronic bands of the silicon sample, which in turn affects the STM
current in that vicinity causing the halo effect. The influence of various
imaging conditions and characteristics of the sample on STM images of DBs is
also investigated.Comment: 33 pages, 9 figure
Binary Atomic Silicon Logic
It has long been anticipated that the ultimate in miniature circuitry will be
crafted of single atoms. Despite many advances made in scanned probe microscopy
studies of molecules and atoms on surfaces, challenges with patterning and
limited thermal stability have remained. Here we make progress toward those
challenges and demonstrate rudimentary circuit elements through the patterning
of dangling bonds on a hydrogen terminated silicon surface. Dangling bonds
sequester electrons both spatially and energetically in the bulk band gap,
circumventing short circuiting by the substrate. We deploy paired dangling
bonds occupied by one movable electron to form a binary electronic building
block. Inspired by earlier quantum dot-based approaches, binary information is
encoded in the electron position allowing demonstration of a binary wire and an
OR gate
Characterizing the rate and coherence of single-electron tunneling between two dangling bonds on the surface of silicon
We devise a scheme to characterize tunneling of an excess electron shared by
a pair of tunnel-coupled dangling bonds on a silicon surface -- effectively a
two-level system. Theoretical estimates show that the tunneling should be
highly coherent but too fast to be measured by any conventional techniques. Our
approach is instead to measure the time-averaged charge distribution of our
dangling-bond pair by a capacitively coupled atomic-force-microscope tip in the
presence of both a surface-parallel electrostatic potential bias between the
two dangling bonds and a tunable midinfrared laser capable of inducing Rabi
oscillations in the system. With a nonresonant laser, the time-averaged charge
distribution in the dangling-bond pair is asymmetric as imposed by the bias.
However, as the laser becomes resonant with the coherent electron tunneling in
the biased pair the theory predicts that the time-averaged charge distribution
becomes symmetric. This resonant symmetry effect should not only reveal the
tunneling rate, but also the nature and rate of decoherence of single-electron
dynamics in our system
Single Electron Dynamics of an Atomic Silicon Quantum Dot on the H-Si(100) 2x1 Surface
Here we report the direct observation of single electron charging of a single
atomic Dangling Bond (DB) on the H-Si(100) 2x1 surface. The tip of a scanning
tunneling microscope is placed adjacent to the DB to serve as a single electron
sensitive charge-detector. Three distinct charge states of the dangling bond,
positive, neutral, and negative, are discerned. Charge state probabilities are
extracted from the data, and analysis of current traces reveals the
characteristic single electron charging dynamics. Filling rates are found to
decay exponentially with increasing tip-DB separation, but are not a function
of sample bias, while emptying rates show a very weak dependence on tip
position, but a strong dependence on sample bias, consistent with the notion of
an atomic quantum dot tunnel coupled to the tip on one side and the bulk
silicon on the other.Comment: 7 pages, 6 figure
Dangling-bond charge qubit on a silicon surface
Two closely spaced dangling bonds positioned on a silicon surface and sharing
an excess electron are revealed to be a strong candidate for a charge qubit.
Based on our study of the coherent dynamics of this qubit, its extremely high
tunneling rate ~ 10^14 1/s greatly exceeds the expected decoherence rates for a
silicon-based system, thereby overcoming a critical obstacle of charge qubit
quantum computing. We investigate possible configurations of dangling bond
qubits for quantum computing devices. A first-order analysis of coherent
dynamics of dangling bonds shows promise in this respect.Comment: 17 pages, 3 EPS figures, 1 tabl
Nanoscale structuring of tungsten tip yields most coherent electron point-source
This report demonstrates the most spatially-coherent electron source ever
reported. A coherence angle of 14.3 +/- 0.5 degrees was measured, indicating a
virtual source size of 1.7 +/-0.6 Angstrom using an extraction voltage of 89.5
V. The nanotips under study were crafted using a spatially-confined,
field-assisted nitrogen etch which removes material from the periphery of the
tip apex resulting in a sharp, tungsten-nitride stabilized, high-aspect ratio
source. The coherence properties are deduced from holographic measurements in a
low-energy electron point source microscope with a carbon nanotube bundle as
sample. Using the virtual source size and emission current the brightness
normalized to 100 kV is found to be 7.9x10^8 A/sr cm^2