Topography and Current Imaging
The STM normally operates in the Constant Current Mode: the topography is generated from
the Z scanner movement by keeping the tunnel current flow between tip and sample constant.
For relatively smooth samples it is possible to operate in the Constant Height Mode: variations
of the tunnel current across the surface are acquired during a scan whilst keeping the tip-sample
separation constant.
The AFM with a conductive cantilever can generate at the same time both topography and
current images by operating in the Constant Force Mode for topography and recording the
current flow between tip and sample for a given sample bias voltage during the scan.
I-V characteristics can be generated in both cases after repositioning the tip on the topographical
region (point) of interest of the just acquired image. The current flow for a specified range of
sample bias voltage is stored whilst the tip-sample separation (or force for the AFM) is held
constant with the feedback open.
Drifts in the tip-sample x-y position can result in a mismatch between topographical features
and I-V characteristics.
Scanning Tunneling Spectroscopy (STS)
Three measurement modes are available: I-V, I-S, and S-V.
The characteristics between probe and sample are measured by stopping the probe at the desired
point on the sample during a scan.
I-V: the tunnel current I is measured as function of the sample bias voltage V with the probesample distance S held constant.
I-S: the tunnel current I am measured as function of the probe-sample distance S for a constant
sample bias voltage V.
S-V: the probe-sample distance S is measured as function of sample bias voltage V whilst
applying feedback to maintain a constant tunnel current I.
From the I-V data the following information is available by differentiation in software:
I-V
I/V
(I/V)/(I/V)
2I/2V
Integrated value of the product of state densities of probe and sample
Product of state densities of probe and sample
True state density
Excited state of particle other than electron (phonon, magnon,)
Current Imaging Tunneling Spectroscopy
Scanning is carried in the Constant Current Mode (STM tip) or in the Constant Force Mode
(AFM with conductive cantilever). At each pixel of the topography image being generated, the
feedback is switched off and the probe-sample distance is kept constant. After an I-V
characteristic is generated, the next pixel of the image is acquired with the feedback applied
again. Each I-V characteristic is matched to the corresponding pixel in the topographical image.
Topographical
Image (128x128)
I
V
Up to 128
Current Images
At designated
bias voltages