Noncontact Atomic Force Microscopy - Yale School of Engineering ...

P.II-10
Transverse conductance image of DNA probed by current-feedback
noncontact AFM
T. Matsumoto 1, Y. Maeda 2, and T Kawai 1
1 Institute of Scientific and Industrial Research (ISIR), Osaka Universit,Osaka, Japan
2 National Institute of Advanced Industrial Science and Technology (AIST), Osaka Japan
Scanning probe microscopy (SPM), which provides means to access to individual
molecule, has a special significance for investigations of molecular-scale electronics. In
particular, scanning tunneling microscopy (STM) has been used to evaluate molecular
conductivity. However, STM measurement is effective only for height-foreknown system
such as the molecules embedded in self-assembled monolayer. For the molecules having
conformational variations, STM measurement is unable to separate the information of
transverse conduction through the molecules from the unknown height variation.
We demonstrated that simultaneous measurement of tunnel current and frequency
shift realized to separate electrical properties from topographic variations. Figure 1(a)
shows simultaneously obtained STM topography (constant-current mode) and frequency
shift image of DNA on Cu (111) surface. These images are totally similar to each other
but the contrast distribution of these images shows differences. For the quantitative study
of these variances, the correlation between STM height and frequency shift is analyzed by
plotting each pixel as shown in Figure 2(a). The negative frequency shift overall increase
as increasing STM height. This indicates that the tip close to the molecule at the high
region of STM topography. Assuming the transverse electron tunneling through DNA
molecules, frequency shift and STM height show linear relationship as a function of
transconductance G0 at molecule/surface interface and attenuation decay constant β. The
line represented in Figure 2(a) means the data sets of STM height and frequency shift
satisfying a parameter set of G0=0.27 and β= 0.26 . The pixel data picked from the plots
around this line is used for reconstructing an image which shows a slice indicating the
conduction with G0=0.27 and β= 0.26. As shown in Figure 2(b), the conductance image
differ from either STM topography and frequency shift image in that it reflects the
conductance variations affected by molecular conformation and/or molecules/substrate
interface.
STM topography Frequency shift image
Fig. 1. Simultaneously obtained STM
topography and frequency shift image
of DNA on Cu (111).
(a) Correlation analysis (b) Conductance image
Frequency shift (Hz)
4
2
0
-2
-4
-6
(G 0 ,β ) = (0.26, 0.27)
-8
-6 -4 -2 0 2 4 6 8
STM height (nm)
Fig. 2. (a) Correlation analysis between STM height
and frequency. (b) Conductance image reconstructed
from the data around the line of G0=0.27, β= 0.26.
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