Abstract LT-STM

The LT-STM is specially designed for working at atomic scale at low temperature (i.e. to perform atomic / molecular manipulation or local spectroscopy). The STM has a modified Besocke beetle design which ensures its high stability (dz<1pm) and only a small drift.

The STM is thermally isolated. Its temperature can be varied between 6 ... 300 K. The low temperature of the STM freezes the motion of most adsorbates that can be evaporated onto the sample surface either inside or outside the STM.

The surface / adsorbates can be investigated for weeks. No degredation of the sample will be observed as long as the 4K radiation shield that surrounds the STM remains cold.

Due to th design of the cryostat a very low helium consumption is achieved which requires to refill only 4 liters of liquid helium every 48 hours when the STM is at its lowest temperature.

The whole scan unit is digitally controlled. Therefore a precise tip-postioning on any spot of the sample with any tunneling parameter can be made. This can be used to peform local spectroscopic measurements (i.e. I(V), I(Z)) to reveal electronic or vibrational properties of the sample or the interaction between tip and sample as well a different kinds of manipulation techniques.

Any information (I(t), U(t), XYZ(t), dI/dU ...) can also be recorded during the maniulation.

The standard scan range of 1.5µm x 1.5µm x 0.3µm can be adopted to the desired range. The minimum tunneling current is 2pA.

The system provides optical access onto the sample to control the exact position of the tip with the help of a telescope. The sample transfer can also be done at low temperature. A helium cooled manipulator allows you to prepare, analyze and transfer the samples at temperatures down to 20K.

Electronics

The STM-electronics are based on a digital design. The DSP controlling the 'digital' feedback loop (maximum speed 200.000pixels/s) is electronically decoupled from the analog I/O board, which provides 6 D/A outputs (20bit, >200kHz, +/- 10V), 4 A/D inputs (18bit, >200kHz, +/-10V) and 32 digital I/O ports (16 input, 16 output). The DSP is able to provide a lock in amplifier for the first and second harmonics of the tunneling current by software. All features are controlled by a Windows PC using a high speed USB2.0 link. The HV-amplifier has 3 channels with dual voltage bipolar output (±200V, 60kHz, RMS: 1mV). Each channel offers modulation and offset voltage input and manual gain settings (with digital readouts) of 1x, 3x, 10x, 30x. The preamplifier (DLPCA-200) allows to set the required gain with the digital I/O of the DSP-board (10³ … 10¹¹ V/A, bandwidth: 500kHz … 1.2kHz). With a typical scan speed of about 100nm/s an image of (30nm)² with (256 pixel)² is acquired in less than 3min (typical @6K).

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Software

The STM software provides all the standard elements needed for flexible STM image acquisition, including a basic set of image analysis commands. Special features are constant current, constant height and voltage dependant imaging and 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 dI/dV spectroscopy with an external Lock-In amplifier are also included.

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 to future software upgrades is guaranteed.

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Atomic / Molecular Manipulation

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. 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 the 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 (see references).

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UHV-system

In order to minimize internal vibrations we reduced the size of the UHV-system. Instead of having separate chambers for preparation and analysis we choose to have only one chamber with an integrated gate valve. This also minimizes the length of the LHe-coolable manipulator, which can be used to prepare, analyze and transfer the samples and tips (T > 10K). The preparation chamber can be adapted to the customers needs.