NANO LETTERS
Laser Absorption Scanning Tunneling Microscopy of Carbon Nanotubes
2006 Vol. 6, No. 1 45-49
Joshua B. Ballard,† Erin S. Carmichael,†,‡ Dongxia Shi,†,§ Joseph W. Lyding,†,| and Martin Gruebele*,†,‡,⊥ Beckman Institute of AdVanced Science and Technology, Department of Chemistry, Department of Physics, Center for Biophysics and Computational Biology, and Department of Electrical and Computer Engineering, UniVersity of Illinois, Urbana, Illinois 61801 Received September 28, 2005; Revised Manuscript Received November 1, 2005
ABSTRACT We report single molecule laser absorption by carbon nanotubes stamped under ultrahigh vacuum onto Si(100)2×1:H surfaces. Absorption is detected by scanning tunneling microscopy. Images are obtained with and without modulated laser excitation using lock-in amplification and a rear-illumination geometry to reduce thermal effects. Noise appears at topographic edges and is analyzed by a quantitative model in terms of scan speed, mechanical instabilities, and feedback current fluctuations at the edge of the nanotubes. Noise due to mechanical instabilities is shown to persist even in the limit of slow scan speed.
The atomic resolution of scanning tunneling microscopy (STM) and the chemical selectivity of optical spectroscopy can be combined into a powerful tool for understanding molecular and submolecular structure on surfaces.1,2 Optical excitation of surface adsorbates, probed by STM, enhances molecular analysis, surface manipulation, and studies of molecular energy transfer and reactivity at the atomic level.3,4 Fluorescence techniques are not universally applicable in the often strongly quenching environment on surfaces, and apertureless near field scanning microscopy is limited to a spatial resolution of approximately 10 nm.5 Photocurrent detection during laser-assisted STM is not similarly limited because optically induced changes in junction current are detected via the spatial overlap of atomic wave functions on the tip and substrate/surface.6 The STM can spatially resolve molecular changes induced by interaction of the adsorbate with a laser at the tip-moleculesurface junction, effectively imaging laser absorption by a single molecule. Any form of atomic-resolution laser-assisted STM has been an elusive goal, with only a few cases of imaging of surface adsorbates with sub-nanometer resolution reported to date.7-9 The reason for this scarcity is simple: many problems have been reported at laser-illuminated tip-sample * Corresponding author. † Beckman Institute for Advanced Science and Technology. ‡ Department of Chemistry. § Current address: Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China. | Department of Electrical and Computer Engineering. ⊥ Department of Physics and Center for Biophysics and Computational Biology. 10.1021/nl0519231 CCC: $33.50 Published on Web 12/06/2005
© 2006 American Chemical Society
junctions, with laser heating being one major problem.6,10 Even with a total internal reflection (TIR) geometry, where the STM tip-sample junction is illuminated from behind, producing a standing evanescent wave, heating and laserinduced vibrations can dominate the signal.11 As long as a system (i.e., tip and sample) absorbs light and heats up, atomic registration is easily lost when comparing laser-driven signals to nonilluminated signals. To work around these difficulties, we have combined several approaches, insufficient by themselves, into a method that allows detection of single molecule absorption under laser illumination by STM imaging. Rear illumination of the junction through the substrate reduces tip heating, a sufficiently large semiconductor substrate band gap reduces substrate absorption, and rapid optical modulation combined with lock-in amplifier (LIA) detection circumvents the registration problem by providing a laser on/off subtracted scan at the same time as the topographic and current images. Most importantly, mechanical instabilities and current modulations that may be independent of the laser can be studied in detail to distinguish laser absorption from background effects. This detailed characterization of our technique will be the primary focus of this Letter. To demonstrate the critical differences between signal and noise in LIA-detected laser-assisted STM, we scan singlewalled carbon nanotubes (CNTs) on hydrogen passivated Si(100) surfaces using several different tips and CNTs with varying degrees of mechanical instability. The main reason for choosing this system is that CNTs are very regularly shaped, facilitating identification and providing a means for
Figure 1. Experimental setup. The STM control electronics (Ctrl) apply the bias to the sample and translate the STM tip in the x-y plane while using a feedback circuit to adjust the height of the tip above the surface to maintain constant current. The diode laser, modulated by an optical chopper (Mod) illuminates the STM junction from the rear of the silicon which has an polished wedge machined into it with θ ) 15°. The optically modulated current is monitored by a lock-in amplifier (LIA) whose signal, along with topographic and feedback current measurements from the control electronics, is monitored as a function of x and y by the computer (CPU).
understanding the statistics of the response of the LIA during imaging. Additionally, CNTs can be strong chromophores with optical resonances within the band gap of our silicon substrate12,13 that are accessible by our near-infrared laser system. The apparatus used to perform these experiments centers on a home-built ultrahigh vacuum (UHV) scanning tunneling microscope, similar to ones previously reported.14 The sample consists of a Si(100) wafer that has a 15° polished wedge machined into its rear face to allow a total internal reflection (TIR) excitation geometry, as shown in Figure 1. Upon introduction into the UHV preparation chamber, this sample is hydrogen passivated as previously reported, yielding a clean (2×1)-H surface reconstruction. Tips are made from electrochemically etched tungsten. HiPco carbon nanotubes are applied in situ using the dry contact transfer (DCT) method with a fiberglass brush, permitting investigation of the nanotubes on a well-characterized surface.14 A home-built diode laser is used for photoexcitation of the tip-sample junction. The laser consists of a commercial temperature-stabilized laser diode controller driving a custom InGaAs laser diode. The laser output is unpolarized, at an output power of up to 14 mW. The laser wavelength is set by the diode controller temperature with the primary longitudinal mode centered at 1250 ( 0.2 nm. A 30 cm focal length plano-convex lens focuses the laser onto a 100 µm diameter spot at the rear of the sample, at an angle of 30° relative to the polished wedge face. p-Polarized light is preferentially transmitted at the rear face of the sample and upon reaching the front surface of the sample undergoes TIR. The laser light is modulated by a mechanical optical chopper. Care is taken in choosing the modulation frequency to avoid driving any intrinsic tip/sample acoustic modes which are different for each tip and easily resolved on a spectrum analyzer. Within this constraint, all modulation is confined to the region of 0.8-1.3 kHz. For reference measurements to analyze noise sources, the laser is turned off with all other 46
Figure 2. Carbon nanotube images. Parts A-C are acquired simultaneously with the laser modulation on, and parts D-F are acquired simultaneously with the laser turned off. All images are taken with a sample bias of -2 V, current of 0.1 nA, and a horizontal tip velocity of ∼500 Å/s. Parts A and D are topographic images, B and E are feedback current images, and C and F are the LIA images. Notice the horizontal streakiness in parts C and F. The white scale bar in part A is 5 nm long.
equipment, including the mechanical chopper, operating as during laser excitation. The instrument constructs images by raster scanning in constant current mode, simultaneously saving topography, feedback current, and LIA output for each scan (see Figure 2). Scanning is typically performed at -2.5 V sample bias with a setpoint current of 100 pA unless otherwise noted. The LIA time constant is set to 10-30 ms, with the tip speed ranging from 0.06 to 1 Å/ms to maintain acceptable resolution in the scan direction. Figure 2 demonstrates the distinction between optically induced signal and noise. In Figure 2A, a carbon nanotube with a diameter of approximately 1.2 nm and length of 24 nm is imaged topographically. This image was scanned left to right with high tip velocity (≈0.8 Å/ms). During collection of this image, the surface was illuminated by the laser with 14 mW output at 1251.1 nm and 1.2 kHz modulation. Similar laser-exposed topographic imaging of the Si(100) substrate resolves dimer rows with a quality comparable to normal STM scans (data not shown). The modulated signal was simultaneously measured by the LIA with a 10 ms time constant. The LIA output of Figure 2C shows an optical signal (vertical black stripe) along the backbone of the CNT, not present with the laser off (Figure 2F). In addition, large amplitude noise (horizontal black/white stripes) occurs along the edges of the CNT whether the laser is on or off. The observed laser-on signal has two sources: electronic changes in the CNT from absorption, and driven mechanical vibrations at the junction. For the noise, we identify transient feedback current peaks at edges as the major source and provide a quantitative model. We discuss these phenomena in turn. Nano Lett., Vol. 6, No. 1, 2006
Figure 3. LIA buffer statistics. Multiple cross sections along the length of the CNT of Figure 2 are analyzed. In part A, filled symbols represent LIA data taken with the laser on, and open symbols represent data taken with the laser off. The solid black line is a rescaled topographic cross section of the (1.4 nm) diameter CNT. Squares (09) are the standard deviation, or noise, of the cross section statistics, and circles (Ob) are the background-subtracted mean signal. Although the noise is the same for laser on/laser off, the mean for laser on is nonzero where the noise is low. Part B is a comparison of the LIA cross section statistics for the upper versus the lower portions of the CNT of Figure 2 after co-adding six scans, half done left-to-right and half done right-to-left. The open symbols are for the bottom half of the tube, and the filled symbols are for the upper half. Blue triangles (23) are the standard deviation of the signal, and red circles (Ob) are the background subtracted mean. Part B (inset): expected E11 and E22 absorbances of this 1.2 nm diameter CNT, adapted from ref 21. The vertical dashed line is the photon energy used here.
Optical absorbance of the CNT is nonzero as long as the excitation frequency of 7993 cm-1 exceeds the CNT band gap.13 For semiconducting CNTs, this happens for tube diameters greater than 1.0 nm, as is the case here (see Figure 3B, inset).12 As previously reported, optical absorption by semiconducting CNTs can result in an increase or decrease in bulk photoconductivity at a junction, depending on CNT/ substrate band alignment.15 We believe that the large negative contrast originates from optimal band alignment between the CNT and sample, as in discussed in ref 15. Other CNTs we scanned show reproducible increases in the LIA amplitude, consistent with this picture (data not shown). In either case, CNT absorption directly modulates the STM current, providing the LIA signal. Nano Lett., Vol. 6, No. 1, 2006
Optical absorption by the bulk silicon, tip, and CNT also heats the junction, driving tip-substrate vibration through temperature-dependent expansion.10,11 This driven vibration adds to the LIA signal. Additionally, slight changes in the STM junction resonant frequencies also cause phase changes of the LIA signal.16 Indeed, a slight contrast matching the topography is achieved with the laser off when simulating thermal modulation by electronic tip height modulation. The optical signal in Figure 3 has much higher contrast than that expected based upon differences in tip/sample and tip/surface vibration alone, supporting a direct optical absorption mechanism. The images were analyzed in detail to understand the source of the edge noise, which may have been mistaken for optical signal in the past. Multiple (∼200) LIA signal cross sections perpendicular to the CNT axis were collected. Figure 3A shows the cross sections’ mean and standard deviation, along with a rescaled topographic cross section. The mean deviates from zero most near the center of the CNT with the laser on, with a slight scan direction-dependent shift at high scan speeds attributed to the LIA time constant. The mean is close to zero with the laser off. The standard deviation is maximized near the edges, whether the laser is on or off. The height and width of the standard deviation peaks provide important clues to the origin of the noise. The height of the standard deviation peak is always greater on the leading edge of a scan over a CNT. The deviation of the feedback current from setpoint is also always larger along the leading edge of the CNT due to nonlinearity of the current with respect to tip-sample distance. For the data in Figures 2 and 3, the scan speed was inversely related to the feedback current, slowing the scan rate on the leading edge of the CNT, thus narrowing the leading edge standard deviation peak. The two observations point to current fluctuations near steps as the major source of noise. We now construct a quantitative model for the edge noise in terms of the feedback current response of our STM when the topography deviates from flat. Under the rigid surface approximation, this can be expressed as N(x) ∝
dz dz dx ) dt dx dt
(1)
where limitations in the temporal response of the tip-sample separation feedback (dz/dt) depend on the local contour of the surface in the fast scan direction (dz/dx) and the tip velocity in the fast scan direction (dx/dt). At nonzero surface roughness and scan velocity, there will be a transient peak in the feedback current before tip retraction (or extension) can occur. These transient peaks (or valleys) in the feedback current (as in Figure 2B) are read by the LIA as a “signal” with random phase from scan line to scan line. Thus to simulate the noise, a convolution can be applied to the feedback current image. First, each line of the feedback current image is interpolated using a cubic spline method. Next, the laser-off LIA signal is simulated according to 47
f ) FFT[i(x - 80/ωm:x)] S(x) )
1 N
∑n e-x /rRe[f(ωm)] n
(2a) (2b)
where S(x) is the signal at position x in the image, n sums over points in the current buffer collected before the point at x, r is the LIA time constant in spatial units as determined by the scan rate, ωm is the LIA modulation frequency in spatial units, f is the Fourier transform of the current buffer from x - 80/ωm to x (80 being approximately 5r in this case), and N is a scaling factor. This convolution is valid only when the current smoothly varies from point to point, as is the case for limitations in the feedback response. Figure 4 shows the agreement between edge noise standard deviations from a mechanically stable “laser off” LIA image, and the convoluted current image from eq 2. This agreement verifies that the dominant contribution to the LIA noise arises from smoothly varying modulations in the feedback current that originate from scanning faster than the feedback can respond. Noise can be reduced by image averaging or by reducing the rate of topographic changes. Noise was reduced approximately 2.5-fold (expected: x6) by averaging three pairs of interleaved right-left and left-right scans (Figure 3B). This noise reduction permits reliable quantification of features along the length of the CNT. Figure 3B shows a comparison of the cross section statistics separately for the upper and lower halves of the CNT. The mean, now centered on the topographic image because LIA time constant delays cancel, reveals significant signal differences between the top and bottom halves of the CNT. Much smaller features of less than 1 nm can be resolved with a signal-to-noise ratio of approximately 4 for a single cross section. Figure 5 demonstrates the noise-reducing effect of reducing the rate of topographic changes, either by scanning more slowly or by scanning along the long axis of a feature, along
Figure 4. Comparison of LIA noise and feedback current convolution noise. The squares (0) show the directly collected LIA data from a (0.9 nm) diameter CNT image using -2 V, 0.01 nA, and a constant tip velocity of 330 Å/s. The solid black line is the statistics of the image based on convoluting the feedback current using eq 2. 48
Figure 5. Dependence of noise on scan speed and direction. The cross section statistics for varying scan speeds with a fast scan direction perpendicular to the primary CNT axis are shown in part A. The solid black line is the topographic cross section. The scan speeds are 170 Å/s (blue ×), 238 Å/s (green O), 298 Å/s (yellow 4), 397 Å/s (orange 0), and 595 Å/s (red +). Part B compares cross section statistics for fast scan directions parallel (red b, 600 Å/s) and perpendicular (blue ×, 170 Å/s) to the primary CNT axis.
with a second type of noise caused by mechanical instability. The CNT in Figure 5A contains two parallel topographic features (a double tip artifact) with different noise characteristics. In Figure 5A, with the fast scan direction perpendicular to the long CNT axis, the standard deviations are plotted for scan speeds ranging from 160 to 600 Å/s. In the left hand feature, the LIA standard deviation decreases at slower scan speeds, verifying primarily dz/dt based noise discussed above. On the right-hand feature, the peak heights are insensitive to scan speed. The reason for this is a highfrequency (>800 Hz) mechanical instability of that CNT along its right edge.17,18 As this instability depends on tip position and sample bias, it remains a factor at any scan speed. At the fastest scan speed, a shoulder appears, resulting from the large dz/dx along the right-hand side of the CNT. Figure 5B shows that noise is also reduced by scanning the long axis of a tube (reducing dz/dx). An important implication for doing absorbance spectroscopy on CNTs is that the width of the low-noise region along the CNT axis is greater in the parallel scan case than the perpendicular scan case. This suggests that even with edge-dependent noise or mechanical instability, low-noise LIA signals differentiating different regions of the CNT backbone may be acquired.18 Figure 6 demonstrates the present resolution capabilities of the technique using our STM under laser illumination. The topography in Figure 6A shows clear dimer row resolution in the Si-H substrate, in line with the finest lateral resolution we obtain without laser illumination. Atomic resolution of CNTs has been reported previously using a very similar STM design but has not yet been achieved under optical excitation.14 Although we discussed that noise can be reduced at slow scan speeds, slower, lower-noise scans will be more sensitive to residual thermal drift. As an example, the vertical dimer rows in Figure 6A are slightly Nano Lett., Vol. 6, No. 1, 2006
spatial resolution simply by increasing the amplitude of voltage modulation, albeit at the cost of decreased energy resolution.19,20 In summary, UHV rear-illuminated STM capable of lockin detecting single molecule optical absorption on surfaces is presented. It is shown that topographic edges induce laserindependent noise in the LIA signal, which can be modeled quantitatively based on the feedback current image. These artifacts are compared to the true laser-induced signal. A second source of noise was also investigated: mechanical instability of carbon nanotubes at the surface results in a scan-speed-independent noise. References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12)
Figure 6. High-resolution topography and LIA signal. A lateral scan speed of 60 Å/s was used for these scans. Part A shows the topography of a 7.4 nm long CNT spanning a terrace step edge on H passivated silicon, with dimer rows clearly visible. The inset shows the LIA signal using ∼15 mW laser power. Notice the lack of signal on the circled topographic feature to the right of the CNT. Part B shows the cross sectional mean (b) and standard deviation (0) for the CNT based upon the single LIA signal of part A.
tilted due to this thermal drift. Increasing the localized laser power will increase the signal but perturb the imaging by increasing thermal drift or by inducing thermal tip vibration due to laser modulation. It is estimated that compared to the 15 mW laser power of Figure 6 modulated at 1.2 kHz, a 10-fold increase in laser power is possible before tip vibrations begin to limit spatial resolution. This is in contrast with other LIA detection techniques, such as dI/dV spectroscopy, where the signal can be increased without limiting
Nano Lett., Vol. 6, No. 1, 2006
(13) (14) (15) (16)
(17) (18) (19) (20) (21)
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