After the development and use of several scanning tunneling microscopes for different environments (air, UHV and low temperatures) we designed and built a simple STM. Only standard and affordable components are used, to make this microscope equally applicable to a student laboratory. The controller generates a very smooth scan with a resolution much greater than the digital-to-analogue converters in the control computer. In the last 2 years it has proved to be a reliable system with good resolution. It has been used, for example, to study dye molecules on silver-bromide surfaces.The scanning tunneling microscopy (STM) [1] has proven to be a powerful tool and the number of applications is still expanding. In our laboratory we have designed and built several STM systems for operation in different environments (air, liquid, UHV, and low-temperature high magnetic field). After building these microscopes with increasing complexity, we decided to design a simple, cheap and rigid system that can be used both in undergraduate-student [2] and our own research laboratories. The use of microscopes in a student laboratory implies low costs, simplicity, transparent mode of operation, and ruggidity; our own research requires low noise, operation with a small tunnel current, and no spurious drift due to the electronics.We have chosen to use a simple analogue feedback system and an analogue I-O board [3] in a personal computer.To avoid noise problems with the D-A converter we control the lateral tip velocity instead of the lateral tip position. By integrating a voltage [4] (tip velocity) over a certain time we generate the desired lateral tip positions. In this way we get a more accurate control of the tip position as well as more time to acquire and average the incoming data. Special care is needed to minimize the intrinsic drift of these integrators. To reduce this drift we designed special software that accomplishes the reduction without the need of additional hardware.To process the measured images we have a large library of noise-reduction and STM image manipulation routines, which is available via Ethernet.One of these microscopes has been used in our group for two years and has shown good resolution and reliability. A second STM has now been used by students for six months, also without problems. Figure 1 shows the three important parts of the STM. The vibration isolation system (essential for good images) is not shown. The STM head shown in Fig. 2 is made of simple stainless steel parts: a differential screw [5] for coarse approach and piezo tube [6] for scanning. The pitch of the differential screw (50 microns) is reduced by a factor of 7 at the tip position (lever) [7]. Usually the coarse approach is done manually but we also used a small piezo motor instead of the differential screw [8]. With our scan tube the sensitivity is about 30 nm/V for X and Y (when driven symmetrically) and 5 nm/V for Z.
DesignConversion of the tunnel current into a voltage for the feedback system is made by a 10 8 -Ω resistor and a OPA111 opa...
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