[Zurück]

G. Ramer, A. Balbekova, A. Schwaighofer, B. Lendl:
"Resonance Tracking in Resonance Enhanced Infrared Nanoscopy";
Vortrag: SCIX 2015, Providence, RI, USA; 27.09.2015 - 02.10.2015.

Resonance enhanced atomic force microscopy infrared nanoscopy (RE
-
AFMIR) is an analytical technique that allows non
-
destructive chemical
analysi
s of solid
-
state samples at better than 50 nm spatial resolution. This technique has been applied to a range of problems spanning from
biology, medicine and pharmaceutics to polymer
-
and material science, synthetic chemistry and plasmonics. In each of the
se fields the technique
has been shown to provide important information that is not easily accessible with other methods. Our own research is focused
on the extension
and improvement of the RE
-
AFMIR method itself. In our self built setup consisting of a Da
ylight Solutions external cavity
-
quantum cascade
laser (EC
-
QCL) and a Keysight 5400 AFM we have recently demonstrated a method for time
-
resolved analysis using RE
-
AFMIR, whereby a
repeating or non
-
repeating spectral change
-
such as the switch between tw
o secondary structures of a polypeptide film
-
can be followed at the
spatial resolution provided by RE
-
AFMIR and a speed of one spectrum every 1.5 seconds.
One of the main challenges in RE
-
AFMIR is keeping the repetition rate of the excitation laser at th
e contact resonance frequency of the
cantilever. Since this frequency depends on parameters of the cantilever (force constant, vibration mode, setpoint) as well a
s of the sample
(Young's modulus, surface properties) imaging across different materials leads
to shifts in the contact resonance. To the operator these shifts are
only visible as changes in the signal amplitude. For tracking resonance frequencies in scanning probe microscopy several tech
niques such as
phase
-
locked loops (PLL), dual
-
frequency excit
ation and probing of the amplitude around the resonance have been proposed.
While these published methods can serve as guide for the selection of a resonance tracking technique they were usually develo
ped for sine
-
wave
excitation which can not always be e
asily translated to the pulse
-
train excitation provided by lasers used in RE
-
AFMIR. For direct comparison of
different methods regarding their capability to accurately track the contact resonance in a RE
-
AFMIR system a flexible controller has been
designed
that allows to implement different tracking methods on the same sample position. We are thus able to experimentally select th
e most
suitable resonance tracking method

http://publik.tuwien.ac.at/files/PubDat_247503.pdf