STM Control System:

Software:

The STM-software provides all the standard elements needed for flexible STM image acquisition, including a basic set of image analysis commands and powerful 3D image rendering. Special features are constant current, constant height and voltage dependant imaging or the automatically controlled acquisition of image series. Powerful I(V) and I(z) spectroscopy including user definable voltage and tip height ramps as well as dl/dV spectroscopy with an internal or external Lockin-Amplifier are also included.

The software offers a wide range of atomic/molecular manipulation techniques. Lateral manipulation and the formation of atomic scale structures can be performed either in constant current or constant height mode. With a scan speed of about 100nm/s you can acquire an image of (30nm)² size with (256 pixel)² resolution in 3min (typical @ 6K). The feedback loop is updated with 50kHz. Furthermore in combination with the lateral manipulation techniques the application of different tunneling voltage pulses together with simultaneous control over the tip surface distance can be used for controlled chemical bond breaking and bond formation. So far the dissociation, the synthesis, the desorption and the controlled conformational change of molecules have been demonstrated.

The program is written in Borland Delphi 6.0. A special unit in the source code has been reserved for user modifications. This opens up a straightforward way to extend the program in case the user wants to modify the software, while compatibility with future software upgrades is guaranteed.

Electronics:

The STM-electronics are based on a digital design. A DSP board controls the scanning parallel (X,Y) to the surface as well as the tip- sample separation. All scanning/feedback parameters are fully controlled through the STM program. An additional high voltage amplifier and a current preamplifier complement the electronics.

DSP-Board:

The heart of the STM-electronics is a DSP (TSM320C32-66) by Texas Instruments. It is integrated into an ISA-board which runs on a Windows 9X or NT4 operated PC. The board provides 4 D/A outputs (16bit, ± 10Volt, 200kHz), 4 A/D inputs (16bit, ± 10Volt, 100kHz) and 32 digital I/O ports (optional up to 16 D/A amd 16 A/D channels). The 4 output channels are used to control the piezo-voltages (X,Y,Z) and the sample bias voltage. One input channel records the tunneling current, the others may be used to read other signals (e.g. dl/dV from an external lock-in-amplifier).

HV-Amplifier:

Three channels (X,Y,Z) are amplified by a selectable factor of 1,3,10 or 30 to the dual voltage bipolar output. An offset voltage or a modulation voltage may be added to any channel by a potentiometer and via separate inputs. The maximum output voltage is ± 200V (Bandwidth: 80kHz, RMS: 1mV). An interface to the DSP-board allows to read the gain settings into the STM-program to set the appropriate scaling in the images.

Pre-Amplifier (I/V-Converter):

Any I/V-converter with the desired specifications can be used with the STM-program, but it should be externally gain switchable to exploit the manipulation techniques implemented in the software. We deliver the complete system with the Femto preamplifier (DLPCA-200) allowing you to set the required gain with the digital I/O of the DSP board (10³ ... 10¹¹ V/A, bandwidth: 500kHz ... 1.2kHz).

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