An ultra-cold cloud of atoms (yellow) is trapped in a magnetic trap and scanned across a nanostructured surface. In “contact mode” a loss of atoms from the cloud can be measured, which depends on the surface topography. In the “dynamical mode” the frequency and amplitude of the cloud’s center-of-mass oscillation changes depending on the surface structure. Both methods allow the surface topography to be imaged. |
Microscopes
make tiny objects visible, as their name suggests. However, modern
microscopes often do this in a round-about way, not by optically imaging
the object with light, but by probing the surface with a fine,
needle-like tip. Here, where optical imaging methods reach their limits,
scanning probe microscopes can show, by different methods, structures
as small as one millionth of a millimetre. With their help, phenomena in
the nanoworld become visible and targeted manipulation becomes
possible. The heart of a scanning probe microscope is a moveable,
suspended tip, which, like the needle on a record player, reacts to
small height variations on the surface, and turns these into signals
that can be displayed on a computer.
Researchers
at the University of Tübingen in Germany have now been able to create
this tip, not out of solid material, as in the case of the record
player, but out of an ultra-cold, dilute gas of atoms. To do this, they
cooled an especially pure gas of rubidium atoms to a temperature less
than a millionth of a degree above absolute zero temperature, and stored
the atoms in a magnetic trap. This “quantum tip” can be precisely
positioned and enables the probing of nanostructured surfaces. With this
method, more accurate measurements of the interactions between atoms
and surfaces are possible and further cooling of the probe tip gives
rise to a so-called Bose-Einstein condensate, which allows a significant
increase in the resolution of the microscope. The work was led by Prof.
Dr. József Fortágh, head of the Nano-Atom-Optics group, and his
co-worker Dr. Andreas Günther. PhD student Michael Gierling is first
author of the study, which appeared on May 29 as an advance online
publication in the scientific journal Nature Nanotechnology.
The
scientists demonstrated the use of their cold-atom scanning probe tip
by testing a surface with vertically grown carbon nanotubes. The tip was
scanned over the sample using a type of magnetic conveyor belt. The
first measurements in the so-called “contact mode” revealed how the tall
tubes stripped some atoms out of the atom cloud. These atom losses told
the researchers about the location and height of the nanotubes and
enabled the imaging of the surface topography.
When
the temperature of an atomic gas approaches absolute zero, a quantum
mechanical phenomenon occurs, turning the cloud into what’s known as a
Bose-Einstein condensate. In this state it is no longer possible to
distinguish between the atoms. They become, so to speak, a single, giant
“super-atom”. With such a Bose-Einstein condensate it was possible for
the Tübingen scientists to microscopically resolve individual
freestanding nanotubes. According to the researchers, future
improvements to the cold-atom scanning probe microscope could, in
theory, increase the current resolution of about eight micrometres by a
factor of a thousand.
The
microscope also functions in the so-called “dynamical mode”. The
researchers again created a Bose-Einstein condensate close to the
nanotubes. They then allowed the condensate to oscillate perpendicular
to the surface, and observed how the frequency and size of these
oscillations changed, depending on the topography of the nanostructured
sample. In this way they were able to obtain a well resolved image of
the surface. The researchers write that this method has an advantage
because no atoms are loss from the cloud. This could be helpful in cases
where atoms that are adsorbed on the sample might influence subsequent
measurements.
The
researchers conclude: “the extreme purity of the probe tip and quantum
control over the atomic states in a Bose-Einstein condensate open up new
possibilities of scanning probe microscopy with non-classical probe
tips”. Beyond this, the researchers hope to develop new applications
from the demonstrated coupling between ultra-cold quantum gases and
nanostructures.
The
study was done within the framework of the BMBF programme “NanoFutur”
and in collaboration with several groups from the Center for Collective
Quantum Phenomena (CQ) Tübingen, to which various research groups from
the Faculty of Mathematics and Natural Science belong.
