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Hello,
We recently bought a Nanoscope V multimode SPM and we operate a separate STM base and a scanner head with the new controller box (software: nanoscope 8.10). Previously we operated the STM with Nanoscope III controller (software: nanoscope 5.12r4). With the new system, there are few things that I need to verify and I sincerely hope someone could help me with this. The questions I have are mainly about the STS ramp.
To get STS i(V) curves, these were some of the settings I used before; Feedback type: pixel, Feedback counts: 1, Feedback value: 0 mV, Average counts: 20. These setting allowed me to get nice STS i(V) curves of HOPG and SWNTs.
However, with the new software, if I put Feedback type as 'pixel' it gives out weird curves. But if I select 'cycle' as shown in the manual, it gives me reasonable I-V curves. And also, in the manual it shows an example with Feedback count: 2000 and Feedback value: 100 mV. I don't know what values I should use. Could somebody give me an example for HOPG?
Also, the I-V curves I’m getting (when I select ‘continuous’) change drastically. Is this because of low image quality? Would lowering the ramp rate help?
Any feedback on this would be greatly appreciated !
Thank you,
Dinushi
Dear Dinushi,
Let me first explain the parameters so to establish a base as what settings we should be using for STS i(V).
Feedback Type
FEEDBACK TYPE, selected from a pull-down list, is used to control the tip-to-sample separation and helps avoid tip crashes that can be caused by drift. For each feedback type, feedback is on for the duration specified by FEEDBACK COUNTS. Feedback uses the INTEGRAL and PROPORTIONAL GAINS set during imaging to reach the CURRENT SETPOINT, defined below, at a bias value set by the FEEDBACK VALUE.
The following FEEDBACK TYPEs are available:
• NONE: Open loop (i.e., no feedback).
• RAMP: Turns feedback on after each ramp (1/2 cycle), both approach and retract.
• CYCLE: Turns feedback on once per cycle when the tip is nearest to the sample; recommended.
• PIXEL: Enables feedback after every sample point of the ramp. This is useful for slow ramps with few sample points.
Feedback Counts
FEEDBACK COUNTS specifies the amount of time (in DSP interrupt cycles, ~25 us) that feedback will be on. Allow sufficient time for the control system to settle. A FEEDBACK COUNTS value greater than 1000 is recommended.
Feedback value
FEEDBACK VALUE specifies the bias voltage at which feedback runs to achieve the CURRENT SETPOINT.
Current Setpoint
CURRENT SETPOINT, sets the value (in nanoAmps) of the current between the tip and the sample surface when feedback is on.
Now, let me explain the example settings:
Feedback Type: CycleFeedback count: 2000 Feedback value: 100 mV
In order to prevent tip from crashing into the sample due to drift, the Feedback is turned on before each cycle begins, for a duration of 2000 counts (equivalant of 50ms) for the feedback to adjsut z position to achieve the setpoint current at 100 mV bias (defined by Feedback Value.).
So, if you use Feedback Type: Pixel, then the feedback will be on at each pixel to adjsut z to achive the setpoint current, you will end up seeing an I-V curve with 2 current levels -Current Setpoin at negative bias and +Current Setpoint at positive bias. This may be the the so-called "weired" I-V curve you have observed. So Feedback Type: Pixel should not be used at all.
If you wonder why it worked in your 5.12r4 version, that is because you were using a Feedback Count of 1 (corresponding to a very short time duration~25 us) that is so low that the Feedback did not have enough time to bring the current to the setpoint value.
Selecting "continuous" should not mess up the I-V curve, please send me some curves for me to take a look at.
Hope this helps.
Chunzeng
Dear Chunzeng,
Thank you so much for the information. I appreciate all the explanations and I guess now I can carefully look through, and adjust them to get better data. I will change parameters and soon send you few STS i(V) curves.
I do have few more question... when I switch from STM imaging to STS Ramp, it doesn't show any curves at once. I need to select either single or continuous to get the I-V curve. Is this the way or should it automatically show the curves as soon as I switch?
Also, I am still not sure why my I-V curves change drastically. I have decreased the ramp rate and sometimes it gives me very stable curves but sometimes the curve shape changes quickly and go flat after a while...would noisy images cause this?
Also, when I open my STS ramp data in offline software, the curves are inverted and the sample bias show different numbers (from 1.1 to 2.9 V - shown in top figure). During data acquisition, I change my bias from -1 to +1 V. If I open the same thing in realtime, it gives me the normal I-V curve with bias from -1 to +1 V (shown in bottom figure). Any idea why the software does this?
Thank you in advance!
Oh, It didn't show the images...I will email the figures to you
Hi Dinushi,
The guide to post images can be found here: http://nanoscaleworld.bruker-axs.com/nanoscaleworld/forums/t/526.aspx
Stefan
Thank you Stephen for the information on posting images!!
So here is the realtime plot:
And here is the offline plot of the same data. I looked through the parameters and the 'plot invert' only inverts data but doesn't correct the X axis values...