Schematic of the experimental setup. The pendulum of about in length is the main design feature for providing mechanical vibrational insulation. At its end, the STM/AFM head is fixed in an ultrahigh vacuum environment while the upper end is suspended from a set of stainless steel bellows. The pendulum is placed inside of a helium gas filled exchange gas canister, which in turn is inserted into liquid helium. The helium gas is cooled by interacting with the walls of the gas canister in contact with the liquid cryogen (nitrogen or helium) and cools the STM/AFM down. The low temperature ac amplifier is placed near the STM/AFM head while the room temperature ac amplifier is placed outside of the Dewar.
Electronics block diagram. In the upper part the oscillation control loop for driving the double tuning fork assembly is shown. A low temperature ac amplifier is located near the STM/AFM head. It acts as a preamplifier with an amplification factor of about 10 at . Because of the high of about 30 000 of the tuning fork assembly only a small dither voltage amplitude in the order of is needed to excite the tuning forks. The sample-tip gap is controlled by the lower control loop. There is one path for STM operation and one path for AFM operation selectable by the two potentiometers.
Oscillation voltage amplifiers and calibration curve. (a) Low temperature amplifier. The amplifier has an input impedance of and is working at . Its power supply voltage is also used as bias voltage for the -channel MOSFET. filters (R1/C1 and R3/C2) are incorporated to suppress power line frequencies (50 or ). The amplified tuning fork signal is coupled by an audio transformer to the room temperature amplifier. In this circuit, gate-source and drain-source voltages have the same value. (b) Room temperature amplifier. It is a noninverting amplifier with an amplification factor of 1000. The input filter rejects low frequency noise. (c) Tip-sample displacement as a function of oscillation amplitude when the tunneling current is kept constant by the z-control loop. The slope of the linear part is and is used for calibration of the oscillation amplitude. For lower oscillation amplitude values the displacement does not change, because the mean value of the tunneling current is constant in this region.
Phase shifter/limiter. The phase of the tuning fork oscillation signal can be shifted by four bridges continuously by 480 while the amplitude is kept constant. The four bridges are separated by instrumentation amplifiers. Three outputs are available, output one for monitors the oscillation voltage, output two drives the dither tuning fork, and output three is connected with the phase detector input.
Block diagram of the FM detector. The input voltage is amplified and limited to a maximum value in the first stage for driving the bandpass filter with a constant amplitude. Two 90° phase shifted sinusoidal output signals from the bandpass filter are converted to rectangular voltages and compared in a logical AND gate. The now pulse width modulated AND gate output signal is converted to a precision analog voltage by a digital to analog converter. An adjustable low pass filter is necessary for a stable operation of the -control loop.
FM detector circuit. The input voltage drives the instrumentation amplifier A1 slightly into saturation, as shown in the scope image inset. The two filter voltages can be monitored by a dual beam oscilloscope connected to two OP05 amplifier buffered outputs. The tuning of the bandpass filter will be explained in the next chapter. The OP37 amplifier together with the AD790 comparator acts as a sinusoidal to rectangular converter. The OP37 is needed to overcome the offset error of the AD790. Two NAND gates act as an AND gate which drives the DAC. The temperature stability is determined by the zero offset drift of about of FSR/°C. The scope inset shows the filtered DAC output voltage. This voltage is filtered by two filters with an adjustable cutoff frequency between and . The output voltage is adjusted to zero with the potentiometer P1 for the undamped resonance frequency of the tuning fork. The slope of the output voltage as a function of frequency shift is set by S2.
Constant current STM images of a Cu(100) surface. (a) Overview shows atomic flat terraces, , , and . (b) Atomically resolved terrace, , , and .
Frequency shift of the tuning fork sensor. (a) Frequency shift as a function of sample bias voltage measured on a NiAl(110) surface. (b) Frequency shift as a function of tip-sample displacement measured on the thin alumina film. The hysteresis is caused by a tip change at high repulsive forces.
STM and ncAFM images of the thin alumina film on NiAl(110), , and . (a) Constant current STM image, , and . (b) Power spectrum of (a). (c) Autocorrelation function of (a) showing size and direction of the alumina unit cell. (d) Low pass filtered image. (e) ncAFM raw data image, , and repulsive region, . (f) Power spectrum of (e). (g) Autocorrelation function of (e) also showing size and direction of the alumina unit cell. (h) Low pass filtered image.
(a) AFM image of a thin alumina film grown on a NiAl(110) crystal, domain , , , , low pass filtered, and contrast inverted: bright, lower topographic sites, dark, higher topographic sites. Three unit cells and the atom positions in two unit cells have been marked. (b) Schematic of the alumina unit cells with respect to the NiAl(110) direction.
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