Lateral resolution and potential sensitivity in Kelvin probe force microscopy:
Towards understanding of the sub-nanometer resolution
F. Krok,* K. Sajewicz, J. Konior, M. Goryl, P. Piatkowski, and M. Szymonski
Research Centre for Nanometer-Scale Science and Advanced Materials (NANOSAM), Faculty of Physics,
Astronomy and Applied Computer Science, Jagiellonian University, Reymonta 4, 30-059 Krakow, Poland
Received 20 December 2007; revised manuscript received 5 May 2008; published 18 June 2008
We report on high-resolution potential imaging of heterogeneous surfaces by means of Kelvin probe force
microscopy, working in frequency modulation mode FM-KPFM, performed in ultrahigh vacuum. To study
the limits of potential and lateral resolutions in FM-KPFM, we have investigated clean surface of compound
semiconductor InSb001 and the same surface with some submonolayer coverages of KBr and Au. It was
found that long- and short-range bias-dependent interactions, acting between the tip and the surface, could be
detected and that both interactions contribute to the measured contact potential difference CPD signal. On the
one hand, when only the long-range electrostatic interactions between the tip and the surface are active, the
CPD map provides the distribution of the local surface potential on the imaged sample with the lateral
resolution and the correctness of the measured values depending on the measurement conditions. For this case,
the experimental findings were compared with the predictions of theoretical calculations based on a realistic
model for the cantilever-sample geometry. On the other hand, when the short-range and bias-dependent inter-
actions are detected, FM-KPFM provides even the sub-nanometer contrast in the CPD signal. In this situation,
however, the measured CPD signal is not related to the sample surface potential but reflects the properties of
the front tip atom-surface atom interactions.
DOI: 10.1103/PhysRevB.77.235427 PACS numbers: 68.37.Ps, 81.05.Ea, 68.55.-a, 73.40.Cg
I. INTRODUCTION
Design and production of nanoscale structures is one of
the primary tasks in current research related to nanotechnol-
ogy. Therefore, there is a requirement for developing a non-
destructive, diagnostic tool that could probe a variety of sur-
face related properties down to a nanometer scale range. The
Kelvin probe force microscopy KPFM technique,
1,2
based
on dynamic force microscopy DFM principles, gives both
topography and potential distribution of the sample together
with a high spatial resolution. In DFM, the vibrating tip
moves over the surface at distances where the tip-surface
forces are attractive. One of the interactions acting between
the tip and the surface is the electrostatic one due to the
difference between the tip and the sample work functions
. In general, if the electrons are allowed to flow between
two different conducting materials in this case, the tip and
the surface are connected by an effective wire, there will be
a contact potential between them as the electrons must pay
some energy to travel from the material with the smaller
work function to the material with the higher one. The dif-
ference of the work functions defines the contact potential
difference CPD between the tip and the sample, V
CPD
=
tip
-
sample
/ e. By applying an external bias voltage and,
thus, compensating V
CPD
, KPFM technique effectively en-
ables to remove the electrostatic interaction from the total
tip-sample interactions, and that allows for both topography
and contact potential images of the sample to be acquired
simultaneously. It is assumed that the presence of the tip in
close proximity to the sample surface does not change its
electronic properties as long as the chemical composition of
the tip apex remains unchanged. As a consequence, the val-
ues of sample work function provided with KPFM are the
same as those measured with other spectroscopic techniques,
for example, with ultraviolet photoemission spectroscopy
UPS.
3
Among many applications, KPFM technique has been ap-
plied for work function mapping,
4
evaluation of doping con-
centration in semiconductors,
5
and for determining a local
work function distribution in quantum dots.
6
For imaging of
heterogeneous surfaces, the KPFM has already shown its ad-
vantage over the standard DFM microscopy as due to the
compensation of the electrostatic forces, the height profiles
obtained in the topography images are more reliable.
7
More-
over, for some heterogeneous surfaces, by using KPFM, not
only true topography is recorded but also the chemical con-
trast can be obtained.
8
From many crucial aspects concerning KPFM technique,
the limits of potential sensitivity and lateral resolution are
particularly important. Recently, lateral resolution on the
atomic scale in the CPD signal has been reported
9,10
but its
origin is still poorly understood, mainly due to the long-
range nature of electrostatic forces. A big effort is under-
taken, both on the experimental as well as on the theoretical
side, to explain the observed features.
In this work, we present the experimental and theoretical
results concerning the lateral resolution of surface potential,
measured in ultrahigh vacuum UHV FM-KPFM. The mea-
sured CPD contrast is then compared with the predictions of
a theoretical model that takes into account long-range tip-
sample electrostatic interactions. We have also confirmed ex-
perimentally the existence of some short-range and bias-
dependent tip-surface interactions. A detection of these short-
range interactions with KPFM could provide, finally, the
CPD contrast with lateral resolution in the range of atomic
distances.
The paper is organized as follows. In Sec. II, we describe
the experimental system used in the present study. In Secs.
PHYSICAL REVIEW B 77, 235427 2008
1098-0121/2008/7723/2354279 ©2008 The American Physical Society 235427-1