46 research outputs found
The origin of high-resolution IETS-STM images of organic molecules with functionalized tips
Recently, the family of high-resolution scanning probe imaging techniques
using decorated tips has been complimented by a method based on inelastic
electron tunneling spectroscopy (IETS). The new technique resolves the inner
structure of organic molecules by mapping the vibrational energy of a single
carbonmonoxide (CO) molecule positioned at the apex of a scanning tunnelling
microscope (STM) tip. Here, we explain high-resolution IETS imaging by
extending the model developed earlier for STM and atomic force microscopy (AFM)
imaging with decorated tips. In particular, we show that the tip decorated with
CO acts as a nanoscale sensor that changes the energy of the CO frustrated
translation in response to the change of the local curvature of the surface
potential. In addition, we show that high resolution AFM, STM and IETS-STM
images can deliver information about intramolecular charge transfer for
molecules deposited on a~surface. To demonstrate this, we extended our
numerical model by taking into the account the electrostatic force acting
between the decorated tip and surface Hartree potential.Comment: 5 pages, 4 figure
The mechanism of high-resolution STM/AFM imaging with functionalized tips
High resolution Atomic Force Microscopy (AFM) and Scanning Tunnelling
Microscopy (STM) imaging with functionalized tips is well established, but a
detailed understanding of the imaging mechanism is still missing. We present a
numerical STM/AFM model, which takes into account the relaxation of the probe
due to the tip-sample interaction. We demonstrate that the model is able to
reproduce very well not only the experimental intra- and intermolecular
contrasts, but also their evolution upon tip approach. At close distances, the
simulations unveil a significant probe particle relaxation towards local minima
of the interaction potential. This effect is responsible for the sharp
sub-molecular resolution observed in AFM/STM experiments. In addition, we
demonstrate that sharp apparent intermolecular bonds should not be interpreted
as true hydrogen bonds, in the sense of representing areas of increased
electron density. Instead they represent the ridge between two minima of the
potential energy landscape due to neighbouring atoms
Scanning Quantum Dot Microscopy
Interactions between atomic and molecular objects are to a large extent
defined by the nanoscale electrostatic potentials which these objects produce.
We introduce a scanning probe technique that enables three-dimensional imaging
of local electrostatic potential fields with sub-nanometer resolution.
Registering single electron charging events of a molecular quantum dot attached
to the tip of a (qPlus tuning fork) atomic force microscope operated at 5 K, we
quantitatively measure the quadrupole field of a single molecule and the dipole
field of a single metal adatom, both adsorbed on a clean metal surface. Because
of its high sensitivity, the technique can record electrostatic potentials at
large distances from their sources, which above all will help to image complex
samples with increased surface roughness.Comment: main text: 5 pages, 4 figures, supplementary information file: 4
pages, 2 figure
How cold is the junction of a millikelvin scanning tunnelling microscope?
We employ a scanning tunnelling microscope (STM) cooled to millikelvin
temperatures by an adiabatic demagnetization refrigerator (ADR) to perform
scanning tunnelling spectroscopy (STS) on an atomically clean surface of
Al(100) in a superconducting state using normal-metal and superconducting STM
tips. Varying the ADR temperatures between 30 mK and 1.2 K, we show that the
temperature of the STM junction is decoupled from the temperature of the
surrounding environment . Simulating the STS data with the
theory, we determine that K, while the
fitting of the superconducting gap spectrum yields the lowest mK.Comment: 12 pages, 10 figure
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A millikelvin scanning tunneling microscope in ultra-high vacuum with adiabatic demagnetization refrigeration
We present the design and performance of an ultra-high vacuum (UHV) scanning
tunneling microscope (STM) that uses adiabatic demagnetization of electron
magnetic moments for controlling its operating temperature in the range between
30 mK and 1 K with the accuracy of up to 7 K. The time available for STM
experiments at 50 mK is longer than 20 h, at 100 mK about 40 h. The single-shot
adiabatic demagnetization refrigerator (ADR) can be regenerated automatically
within 7 hours while keeping the STM temperature below 5 K. The whole setup is
located in a vibrationally isolated, electromagnetically shielded laboratory
with no mechanical pumping lines penetrating through its isolation walls. The
1K pot of the ADR cryostat can be operated silently for more than 20 days in a
single-shot mode using a custom-built high-capacity cryopump. A high degree of
vibrational decoupling together with the use of a specially-designed
minimalistic STM head provides an outstanding mechanical stability,
demonstrated by the tunneling current noise, STM imaging, and scanning
tunneling spectroscopy measurements all performed on atomically clean Al(100)
surface.Comment: 12 pages, 15 figure
Electrostatic potentials of atomic nanostructures at metal surfaces quantified by scanning quantum dot microscopy
The discrete and charge-separated nature of matter — electrons and nuclei — results in local electrostatic fields that are ubiquitous in nanoscale structures and relevant in catalysis, nanoelectronics and quantum nanoscience. Surface-averaging techniques provide only limited experimental access to these potentials, which are determined by the shape, material, and environment of the nanostructure. Here, we image the potential over adatoms, chains, and clusters of Ag and Au atoms assembled on Ag(111) and quantify their surface dipole moments. By focusing on the total charge density, these data establish a benchmark for theory. Our density functional theory calculations show a very good agreement with experiment and allow a deeper analysis of the dipole formation mechanisms, their dependence on fundamental atomic properties and on the shape of the nanostructures. We formulate an intuitive picture of the basic mechanisms behind dipole formation, allowing better design choices for future nanoscale systems such as single-atom catalysts
Quantum transport through STM-lifted single PTCDA molecules
Using a scanning tunneling microscope we have measured the quantum
conductance through a PTCDA molecule for different configurations of the
tip-molecule-surface junction. A peculiar conductance resonance arises at the
Fermi level for certain tip to surface distances. We have relaxed the molecular
junction coordinates and calculated transport by means of the Landauer/Keldysh
approach. The zero bias transmission calculated for fixed tip positions in
lateral dimensions but different tip substrate distances show a clear shift and
sharpening of the molecular chemisorption level on increasing the STM-surface
distance, in agreement with experiment.Comment: accepted for publication in Applied Physics