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Langmuir 1997, 13, 365-368
Scanning Tunneling Microscopy under Polar Solvents with Uncoated Tips P. G. Van Patten,† J. D. Noll, M. L. McLester, Y.-G. Kim, and M. L. Myrick* Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208 Received August 5, 1996. In Final Form: September 27, 1996X
Introduction Scanning tunneling microscopy (STM) has seen rapid development and widespread use over the past decade.1 An extensive literature2,3 indicates that STM has great potential for high-resolution study of solid-liquid interfacial phenomena. One of the primary obstacles to STM under polar fluids is the interference of nontunneling currents (NTC’s) (both Faradaic and non-Faradaic) which swamp the much smaller tunneling current. Experiments in this lab indicate that NTC’s even in some pure solvents can be as large as 10 µA. Often, measures (such as use of highpurity solvents and limitation of solvent depth) can be taken to limit this current to ∼0.1 µA; however, even this value is too high to permit stable imaging with a reasonable (∼5 nA) set-point current. Currently, the best method for further reducing NTC's is to coat STM tips with an insulating material such as Apiezon wax,4 epoxy,5 or polyethylene glue.3 Tip-coating, though, suffers from several inherent drawbacks. Coatings must be highly resistant to any solvents used. Tipcoating introduces foreign matter into the reaction cell, which may affect experimental results. Moreover, coating reduces the reliability of tip production. Cutting or etching wire can produce tips suitable for atomic resolution with success rates of 50% or better. Coating complicates the preparation process and greatly reduces this success rate. Finally, the microscopic shape of the coating very near the tip is not, in general, controllable; bulky coatings can adversely affect tip-surface interactions. This can result in damage to the surface and/or tip and can affect imaging in unpredictable ways. An alternate method for imaging beneath polar solvents has been developed in this laboratory which should prove useful for future STM work. The method uses a modification to the STM's current measurement circuitry. The additional circuitry allows, with computer control, an “onthe-fly” voltage adjustment at one input of the current preamplifier to correct for the NT contributions to the current signal at the other input. Because the NTC is not strongly sensitive to small changes in tip-surface distance, it can be effectively subtracted out, allowing the instrument to function normally even in the presence of substantial NTC. * Author to whom all correspondence should be addressed. † Present address: MS C345, Los Alamos National Laboratory, Los Alamos, NM 87545. X Abstract published in Advance ACS Abstracts, December 1, 1996. (1) (a) Wiesdanger, R.; Guntherodt, H.-J. Scanning Tunneling Microscopy I; Springer-Verlag: Berlin, 1993. (b) Stroscio, J. A.; Kaiser, W. J. Scanning Tunneling Microscopy; Academic Press: Boston, 1993; Chapter 1. (2) (a) Bard, A. J. Faraday Discuss. 1992, 94, 1. (b) Wiesdanger, R. Scanning Probe Microscopy and Spectroscopy; Cambridge University Press: Cambridge, 1994; Chapter 5. (3) Noll, J. D.; Van Patten, P. G.; Nicholson, M. A.; Booksh, K.; Myrick, M. L. Rev. Sci. Instrum. 1995, 66, 4150-4156. (4) Heben, M. J.; Dovek, M. M.; Lewis, N. S.; Penner, R. M.; Quate, C. F. J. Microsc. 1988, 152, 651. (5) Uosaki, K.; Koinuma, M. Faraday Discuss. 1992, 94, 361.
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Experimental Section The instrument used for this work was a Digital Instruments NanoScope II with a modified scan head. A schematic of the modifications (black lines) along with the original equipment (gray lines) is shown in Figure 1. The necessary modifications include hardware that allows retraction of the tunneling tip and circuitry designed to allow nullification of the preamplifier output. By providing the proper voltage to point A via an added digitalto-analog converter (DAC1), the output voltage of the current preamplifier can be nullified. When the voltage applied to point A is equal to the voltage drop across R1, the preamplifier output (Vout) is nullified. In this way, a constant current flowing between the surface and tip can effectively be subtracted at the head by applying a constant countervoltage at DAC1. To determine how much countervoltage should be applied, the feedback loop is interrupted and the tunneling tip is retracted several nanometers to effectively eliminate the tunneling current. This is done by applying a slightly negative voltage to DAC0 and then switching S1. (While the tip is retracted, a “dummy” current signal is fed to the servomechanism (via DAC2 and SW2) to prevent it from crashing the tip during this process.) The output of the current preamplifier is then measured with an analog-to-digital converter (ADC), and the DAC1 voltage is varied in such a way as to minimize preamplifier output. This can be done either iteratively or by using the appropriate equations (vide infra) to determine the correct voltage to apply at DAC1. While our iterative loop takes up to 1.3 ms to complete (up to 12 steps for 12-bit resolution in DAC1), it seemed to nullify Vout more effectively than using the equations; however, both methods were used with success in our experiments. After Vout is nullified, SW1 and SW2 are switched back to return control to the servomechanism. The entire process (including the iterative loop) can be performed in less than 2.5 ms with minimal disruption of the image. Imaging conditions are given in the figure captions. The images in Figure 3 were low-pass filtered; otherwise all images are unfiltered. Tips were fashioned from 80% Pt/20% Ir wire obtained from Sigmund Cohn and cut with wire cutters. The highly-ordered pyrolytic graphite (HOPG) substrate was purchased from the Advanced Ceramics Division of Union Carbide. A fluid cell which has been described elsewhere3 was used to hold the samples during imaging; however, in this work Delrin was used in place of aluminum as the major component of the cell to eliminate electrochemical currents arising from the sample holder.
Results and Discussion Shown in Figure 2 is a 352 × 352 nm2 region of HOPG imaged (A) in air and (B) after the addition of 100% ethanol (iNT ) 65 nA). In part b, the correction algorithm has been executed to allow imaging under these conditions. Without the correction made as described above, the tip would retract fully from the surface and imaging would be impossible. The highest set-point current allowed with the NanoScope II is 50 nA, so this method truly allows imaging under conditions which would otherwise be prohibitive. The most important feature of this method, however, is that it allows quick “on-the-fly” updating of the correction algorithm so that imaging may continue even under changing conditions. In fact, we have been able to resume atomic resolution imaging within a few seconds of performing such an update. This is demonstrated in Figure 3, which shows (A) an atomic resolution image of HOPG in air, (B) a similar image under deionized water with the same uncoated tip after correction for the NTC (∼7 nA, set-point current ) 1.5 nA) by the method described above, and (C) another image immediately after the addition of more water to the cell and correction for the new NTC value (>10 nA). Similar images have been obtained under ethanol under similar circumstances, although our experiments indicate that the NTC’s gener-
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Notes
Figure 1. (Top) Existing hardware (black lines) and modifications required for offsetting nontunneling current (gray or dotted lines). See text for full explanation. R1 ) 1 MΩ, R2 ) 10 kΩ, R3 ) 1 kΩ, R4 ) 100 kΩ, and Rg ) effective resistance of the tip-surface gap. (Bottom) Detail of the preamplifier circuit showing definitions of quantities discussed in the text.
ated under ethanol are somewhat lower than those generated under water. We have had success imaging under solutions of various organic materials as well but have not been able to image with uncoated tips under ionic solutions. The effect of the DAC1 voltage on the output of the preamplifier can be expressed as follows (using the notation and sign conventions given in Figure 1):
(
R4 )= -10 R2 Vbias
(
∂Vout ∂Vbias
VDAC1
)
(
)(
)
R2R3 + R3R4 + R2R4 R1 ) R2R3 Rg + R1
(
)
R1 (3) Rg + R1
(1)
To nullify the output of the preamp, the voltages at the two inputs must be the same. This requires that
VDAC1 )
( )
111 MΩ
)
∂Vout ∂VDAC1
Noise derives from other sources as well. Two direct sources are the bias voltage signal and the DAC1 voltage signal. The preamplifier output as a function of bias is
) ()
R2R3 + R3R4 + R2R4 R2 (i1R1) ) R2R3 R4 (11.1 MΩ)(i1) (2)
where i1 represents the current through the preamp input resistor (R1).6 Thus, given a sufficient DAC1 voltage, compensation can be made for any excess voltage at the preamp input. Two factors limit this compensation. The first is the maximum input rating of the preamplifier, which must not be exceeded. In our case, that value is (10 V, allowing for a maximum correction of 901 nA (minus the desired set-point current). The second factor is the noise component present in the interfering current. When this noise becomes significant compared to the magnitude of the set-point current, imaging quality diminishes; this noise cannot be eliminated by this method and will degrade results even after the subtraction of the nontunneling current. (6) When the feedback mechanism has been interrupted and the tip is in the retracted position, i1 equals the nontunneling current through the gap.
where Rg is the effective gap resistance of the tunneling gap. As the interfering current increases and the effective gap resistance decreases, fluctuations in the bias voltage have a more pronounced effect upon the measured current. Given the system outlined above and a gap resistance of 1 MΩ (a reasonable value for an experiment under water), a bias modulation of only 4 mV (smaller than the precision of a 12-bit bias DAC which outputs (10 V) would modulate current by about 2 nA. This additional noise component arising from the bias signal is muffled by coated tips, which artificially raise the effective gap resistance. Noise due to this source is shown in Figure 2b, apparent as a very faint pattern of lines across the image (contrast in this image has been exaggerated to accentuate the effect). It is obvious, of course, that some noise derives naturally from the electrochemical processes occurring in the liquid cell. No attempt has yet been made to gather quantitative data regarding the amplitude and dependencies of this noise, and discussion of such data would be beyond the scope of this paper. Nevertheless, it is quite apparent from the images obtained that this noise does not present a serious obstacle to STM imaging in the situations discussed herein. Because the subtraction can be performed in only a few milliseconds without completely disengaging the tip, this technique could be very useful for flow-injection experiments which involve multiple solvents added in series to
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Figure 2. Two 352 × 352 nm2 areas of HOPG imaged (A) in air and (B) under 100% ethanol with the same uncoated tip and after execution of the corrective algorithm. NTC ≈ 65 nA. It ) 2 nA. Vb ) 100 mV.
the sample cell.3 As the Faradaic and non-Faradaic currents change with variations in solvent depth or with the arrival of a new solution in the flow cell, the algorithm can be performed as necessary to keep the DAC1 voltage properly updated. If updating is not executed with sufficient frequency, either loss of tip-surface registry (with increasing NTC) or a tip crash (with decreasing NTC) may result. In these experiments, two effects have been observed which seriously affect image quality. The first problem is brief glitches or data dropout that accompanies execution of the algorithm. This problem results from the fact that the scanning tunneling microscope continues to image during execution of the algorithm. This is especially problematic when frequent updating of the DAC1 voltage is required (i.e. when NTC's are expected to be changing quickly). The second problem is occasional tip crashes associated with returning feedback control to the servomechanism. This usually occurs when the servomecha-
Figure 3. Three atomic resolution STM images of HOPG: It ) 1.5 nA; Vb ) 35 mV; (A) taken in air with an uncoated tip; (B) taken under DI water with the same uncoated tip after the corrective algorithm has been run to subtract nontunneling current; NTC ≈ 7 nA; (C) taken under DI water immediately after the fluid depth (and thus nontunneling current) has been increased and the NTC subtraction has been updated; NTC ≈ 10 nA.
nism gain values are high (i.e. the feedback response is quick and extreme). When the feedback control is returned to the servomechanism, the tip is far from the surface and a very low tunneling current is measured; the response of the servomechanism is to extend the tip very quickly. If the gains are high, the tip is crashed into the surface before tip extension is halted.
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In this work, these problems have been eliminated by using low gain values and by executing the algorithm during the return sweep of the X piezoelectric tube (i.e. between consecutive scan lines). Some images cannot be obtained with very low gains, and it is difficult to make a computer recognize the return sweep of the X piezo with high efficiency. For these reasons our solutions are imperfect and unsatisfactory; however, both of the problems at hand can be resolved entirely by temporary suspension of imaging during the execution of the algorithm. Unfortunately, we have not found a way to accomplish this feat without information which is proprietary to the microscope manufacturer. We believe that the modifications described in this work could be integrated into the instrument by the manufacturers in such a way as to totally eliminate the adverse effects encoun-
Notes
tered in our work. The price would be slower image acquisition while imaging in this mode, but the gain would be glitch-free, crash-free imaging. This method presently shows great promise in correcting for nontunneling currents in STM experiments under a fluid medium which has significant conductivity. Though this approach is unlikely to work in ionic solutions with very large Faradaic currents, it has the potential to be very successful for nominally nonconducting solutions where the NTC's are moderate. Future work with a wide variety of solvent/substrate systems and perhaps with an improved instrument will determine the complete potential of this new technique. LA960768Q
