Block diagram of the electronics for scanning noise microscopy. The color coded letters (a) to (f) mark the different signals corresponding to the respective plots in Fig. 2 . T = pulse duration, tc = time constant.
Analogue and digital signals at the different steps of the analysis. (a) The input signal (current as a function of time) shows random telegraph noise (RTN). (b) The derivative of (a) with respect to time. Thresholds are set to discriminate the background noise against peaks corresponding to the jumps of the RTN. Pulses of defined width and height are generated for every peak that exceeds the upper (c) or the lower (d) threshold. (e) The sum of these pulses yields the total rate of events. (f) A RS flip-flop triggered by the pulses (c) and (d) reproduces the RTN signal in an idealized way.
Scanning noise spectroscopy for CuPc on Cu(111). All data were measured with a home-built low-temperature STM operated at 7 K: (a) Ball and stick model of the planar copper phthalocyanine molecule with a copper atom at its center (red), surrounded by nitrogen (blue), carbon (green), and hydrogen atoms (white). (b) STM image of the molecule showing fluctuations on two opposite lobes. V sample = −1.5 V, I set = 100 pA. (c) Rate of switching events as a function of the average tunneling current obtained by varying the tip-surface distance. The rate depends linear on the current, revealing a one electron process triggering the molecular switch. The slope indicates an electron yield in the order of 10−7 events per electron. (d) Duty cycle as a function of sample voltage. Extracting electrons close to the Fermi level, the molecular switch is preferably found in the on-state, with a duty cycle close to one. Increasing the negative sample voltage, the duty cycle is reduced until it saturates at ∼0.2–0.4. (e) Scanning noise spectroscopy. (Upper part) The rate as a function of the tunneling voltage is plotted by the dotted line. To compensate for the changes of the tip-surface distance induced by the constant current operation, the rate is multiplied by the bias providing a “pseudo constant-distance signal” (red). (Lower part) In analogy to standard scanning tunneling spectroscopy, the derivative of the resulting spectrum with respect to the bias (blue) reveals the molecular states responsible for the RTN. Data in (d) and (e) were averaged and smoothed. For more details see Ref. 1 .
Scanning noise microscopy data for several individually adsorbed CuPc molecules on a Cu(111) surface. I set = 100 pA, V sample = −2 V, T = 7 K. (a) STM topography; (b) map of the rate; (c) map of the amplitude; and (d) map of the duty cycle. Areas with very low rate (∼0 Hz) are masked in green since the duty cycle is not well defined.
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