Optoelectronic Switching of a Carbon Nanotube ... - ACS Publications

Lea Nienhaus,†,‡ Sarah Wieghold,§ Duc Nguyen,†,‡ Joseph W. Lyding,†,z Gregory E. Scott,# and Martin Gruebele*,†,‡,^

ARTICLE

Optoelectronic Switching of a Carbon Nanotube Chiral Junction Imaged with Nanometer Spatial Resolution †

Beckman Institute for Advanced Science and Technology, ‡Department of Chemistry, zDepartment of Electrical and Computer Engineering, and Department of Physics, University of Illinois, Urbana, Illinois 61801, United States, §Department of Chemistry, Technische Universität München, Lichtenbergstraße 4, 85748 Garching, Germany, and #Department of Chemistry and Biochemistry, California Polytechnic State University, San Luis Obispo, California 93407, United States ^

ABSTRACT Chiral junctions of carbon nanotubes have the potential of serving as

optically or electrically controllable switches. To investigate optoelectronic tuning of a chiral junction, we stamp carbon nanotubes onto a transparent gold surface and locate a tube with a semiconducting
metallic junction. We image topography, laser absorption at 532 nm, and measure I
V curves of the junction with nanometer spatial resolution. The bandgaps on both sides of the junction depend on the applied tip field (Stark effect), so the semiconducting
metallic nature of the junction can be tuned by varying the electric field from the STM tip. Although absolute field values can only be estimated because of the unknown tip geometry, the bandgap shifts are larger than expected from the tip field alone, so optical rectification of the laser and carrier generation by the laser must also affect the bandgap switching of the chiral junction. KEYWORDS: scanning tunneling microscopy . single molecule absorption . Stark effect . carbon nanotube

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olecular-scale devices typically switch 1 eV energy scales over 1 nm length scales, resulting in local electric fields on the V/nm scale. In addition, optical excitation can generate carriers that control the electrical properties of the device. Therefore, optoelectronic characterization of individual prototype devices is one of the two main thrusts in developing molecular electronics, alongside the even harder task of assembling devices at the molecular scale. As a particular example, chiral junctions of carbon nanotubes (CNTs) have the potential of acting as optically or electrically controlled switches, with metal
metal, metal
semiconductor, and semiconductor
semiconductor junctions possible. Such junctions have been imaged at the atomic level,1 and progress has been made in synthesizing and characterizing them.2 Raman spectroscopy3 and photoluminescence4 have been used to investigate the optical transitions of junctions with micrometer resolution, whereas the junctions themselves are about 3 orders of magnitude smaller. NIENHAUS ET AL.

The basis for understanding CNT chiral junctions at the nanometer resolution level in the presence of strong (V/nm) fields exists: The electronic structure of semiconducting
metallic (s
m)5 and m
s
m junctions6 has been modeled. The electrical character smoothly switches from metallic to semiconducting across the junction. Stark effect-tuning of CNT bandgaps in the presence of strong static and optical electric fields, but away from junctions, has been studied extensively.7
10 To a first approximation, tight binding models describe the band edges as van Hove singularities, although more sophisticated models11
13 and experiments14
16 highlight the electron
hole bound character of the optically brightest excited states, with exciton dimeters of 1.5
1.8 nm calculated and measured. The conclusion from various levels of theory is that static and optical fields on the order of several V/nm can cause semiconducting tubes to acquire metallic character (close the bandgap), and vice versa for metallic tubes.17
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* Address correspondence to [email protected]. Received for review April 30, 2015 and accepted September 8, 2015. Published online 10.1021/acsnano.5b04872 C XXXX American Chemical Society

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Here we study the optoelectronic properties of a chiral junction in a single CNT deposited on a transparent gold surface by scanning tunneling spectroscopy (STS) and by single molecule absorption scanning tunneling microscopy (SMA-STM).20,21 We tune the electric field at the chiral junction by adjusting the tip
sample spacing and the tip
sample voltage. STS and SMA-STM both produce signals even when photoluminescence is quenched by rapid relaxation, and both offer nanometer resolution well beyond the optical diffraction limit. By measuring I
V curves and optical absorption simultaneously at high spatial resolution, a more detailed picture of the junction emerges due to the complementary STS and SMA selection rules in our experiment: electron tunneling near the bandgap in one case, and photon absorption into an excited state well above the bandgap in the other. We start by briefly demonstrating that STS can detect bandgap changes induced by static and laser electric fields, and that we can use STS and SMA-STM to distinguish metallic and semiconducting nanotubes, similar to previous experiments.15 Next we examine a single CNT with an intramolecular semiconducting
metallic junction. After characterizing the junction by scanning tunneling spectroscopy (I
V curves), we show that the semiconducting and metallic regions absorb 532 nm light differentially, as expected from simple theoretical models. We also show that we can Stark-tune the optical transitions in the two regions to equalize their absorbance. STM does not allow absolute control of the field due to the unknown tip shape, but the results show that the character of intramolecular CNT junctions can be switched by an applied field and that the switching can be detected optically.

Figure 1. Energy levels of a (7,2) CNT as a function of electric field at the tight binding level of theory. S11 and S22 van Hove resonances of this semiconducting CNT are shown. Stark coefficients of 1.5 eV/(V/nm)2 for the S11 and 0.5 eV/(V/nm)2 for the S22 transitions are used. On the right, the corresponding Lorentzian absorption line shapes with a 40 nm width estimated for the S22 transition are shown. The S22 transition is first slowly tuned into resonance with the 532 nm excitation source (green line) as the electric field is increased, with the overlap peaking at an electric field of ca. 0.55 V/nm, and then tuned out of resonance as the electric field further increases.

RESULTS AND DISCUSSION Characterization of Carbon Nanotubes by STS and SMA-STM. All our measurements were done with CNTs stamped onto a gold surface transparent to visible light22,23 (Materials and Methods). The individual CNTs used in our sample (Materials and Methods) are 0.6
1 nm in diameter, with bandgaps of the semiconducting tubes ranging from ca. 1.3
0.8 eV. As shown in Figure 1, the STS of semiconducting tubes probes the S11 state (bandgap), while SMA-STM with 532 nm excitation probes the S22 state when it is Stark-tuned into resonance. To calibrate our electrical and optical measurements of a chiral junction, we first measured the Stark effect of individual tubes without laser by STS, and compared STS and SMA-STM of two adjacent semiconducting and metallic tubes. At a tip
sample distance of ca. 2 nm (dependent on set point), and sample bias voltages ranging from +0.5 to +2 V, the CNTs are subject to electric fields ranging from ca. 0.3
1 V/nm (Supporting Information, Figure S1). Our STS measurement shows that the bandgap of a semiconducting nanotube in our sample is lowered NIENHAUS ET AL.

Figure 2. (a) STM image of a semiconducting and a metallic nanotube on a transparent Pt
Au substrate; (b) in-phase, absorption image. The in-phase image shows a strong absorption signal (black) for the semiconducting nanotube. The metallic tube has a very small in-phase signal caused either by heating or an unoptimized phase. Apparent broadening of the CNTs is due to tip bluntness. Scanning conditions: 5 pA, 1 V. Scale bars are 20 nm. The out of phase signal and absorption at a different electric field are shown in Figure S8.

by ca. 30% without laser excitation when the STS set point changes from 1.5 to 1 V (Figure S2). A model (7,2) CNT described by a tight binding Hamiltonian10 with the addition of a quadratic Stark effect perpendicular VOL. XXX

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ARTICLE Figure 3. Top panels: topographs of a carbon nanotube junction at increasing electric field (left to right) under 532 nm illumination at 1250 mW/mm2 power density. The white arrows trace out where the bandgap was measured by STS in the bottom panels. The green arrows point at the location of the semiconducting (bottom half of the CNT) to metallic (top half of the CNT) junction. Bottom panels: the corresponding STS traces taken along the white arrows are analyzed to reveal a Stark effect on the observed bandgap (field strength increases as set point bias and tip
sample distance decrease). STS set point conditions: 5 pA at 1.5/1.2/1.0 V set point bias before scanning STS from
2 to 2 V with a 0.2 nm z-axis shift of the tip.

to the tube axis yields a bandgap reduction of 0.2 eV when the Stark effect is included (Figures S3 and S4). Some of this bandgap reduction is also due to the screening effect which results in bandgap renormalization.24
26 Two more examples of bandgap reduction are shown in Figures S5 and S6. Thus, bandgap reduction of semiconducting nanotubes without laser illumination is modest, but easily detectable. Figure 2 shows that we can differentiate a semiconducting and metallic CNT by SMA-STM, analogous to previous results.15 The conductivity of the two tubes in Figure 2 was measured by STS (Figures S7 and S8). The two tubes appear very similar by topography (Figure 2a), but only the semiconducting tube on the left absorbs the 532 nm laser light significantly (Figure 2b). This result is expected because the first transition in metallic nanotubes (M11) is at higher energy than the S22 transition in semiconducting nanotubes of similar diameter.27 When the scan bias is increased from 1 to 1.5 V, the S22 absorption of the left tube decreases by about a factor of 3 (see Figure S8e). Figure 1 illustrates why this happens based on our model (7,2) CNT calculation. At a bias of 1 V and a field of roughly 0.6 V/nm (Figure S1), the S22 transition is in resonance, but at 1.5 V bias and a field of roughly 0.8 V/nm, it moves out of resonance. Tuning a Single Chiral CNT Junction with an Electric Field. Next, we searched for a smooth chiral junction within a single carbon nanotube. Such junctions have been studied theoretically5 and experimentally,28 and although challenging, experimental characterization of collinear NIENHAUS ET AL.

or Y junctions has appeared in recent years.2,29 No experiments have been able to resolve the optical absorption change across such a junction or its tuning in an applied field with nanometer spatial resolution. We provide such data here. Figure 3 shows STS in an area with three carbon nanotubes. The top middle panel shows all three CNTs: the one on the right is just a short fragment, the one on the left terminates as seen in the middle frame, and the CNT in the middle is of main interest here. The middle tube has ca. 0.65 nm diameter (height profiles in Figure S9). It contains a chiral junction marked by a green arrow in the top panels. Above the junction, there is no bandgap under laser illumination, and the tube is metallic under those conditions. Below the junction, the bandgap tunes from as high as 0.86 eV to as low as 0.18 eV under laser illumination when the tip field is increased (SI Figure S1 relates sample bias and calculated electric field under STS and under scanning conditions). Therefore, the tube is semiconducting below the junction under laser illumination. We attribute these observations to a defect that joins a semiconducting tube segment below to a larger diameter metallic or semimetallic segment above. As discussed in the previous section, the tip field or bandgap renormalization account for about a 30% reduction in bandgap at the fields employed here. In contrast, the bandgap reduction of the semiconducting part of the junction in Figure 3 is from 0.86 to 0.18 eV when the laser is on. A realistic Stark coefficient of VOL. XXX

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ARTICLE Figure 4. Stark tuning the semiconducting
metal junction (arrow in middle panels) of the CNT from Figure 2. Top panels: topographs at different set point biases between 0.8 and 1.7 V. The calculated electric field strength is listed for each panel. Middle panels: in-phase absorption signals changing with the set point bias indicate a strong differential Stark shift at the junction marked by a green arrow. Bottom panels: 90 out-of-phase lock-in channel. Absorption (CNT signal
background) is highest at about 1.0 V, as shown in Figure 5. Scanning conditions: 5 pA, variable bias as shown in top panels. Scale bars are 10 nm.

1.5 eV/(V/nm)2 for the S11 state based on the theoretical literature11,16,17 cannot account for the full shift by only the tip electric field. Instead we invoke laser excitation as an additional source of bandgap shift. Under our STS conditions, the S22 transition of the CNT is nearly saturated by the 532 nm laser even at the high relaxation rate through S11 into phonons and into the surface.21 The resulting appreciable excited state population is necessary to allow imaging of the optical absorption by SMA-STM. Thus, we propose that a large contribution to the apparent bandgap decrease is caused by optically excited carriers and not by the tip field alone. Photoexcitation produces excitons or free carriers in the conduction band of a semiconductor, which have been shown to contribute to the observation of a more metallic-like small bandgap.30 As discussed below, alternative explanations exist, but we believe they are less likely. The switching of the semiconducting
metallic character of the chiral junction by the tip field can also be imaged by SMA-STM, which probes the S22 state instead of the bandgap. We imaged the same junction at constant current (5 pA) and variable bias (0.8 to 1.7 V) as shown in Figure 4. For additional images at different voltages see Figures S10
S12. The junction runs perpendicular to the axis of the CNT within experimental error. The discontinuous absorption cross section of the CNT at the chiral junction is readily seen at 0.8 V bias. Figure 5a shows how the absorption cross-section of the S22 transition below the junction and of the M11 transition above the junction varies with tip electric field (compare SI Figure S11 for signal histogram). The S22 transition below the junction goes through an NIENHAUS ET AL.

absorption maximum at 1 V sample bias (blue squares), whereas the M11 transition above the junction goes through a maximum at a higher bias of 1.5 V (red circles). When the sample bias is increased to 1.7 V bias (field ca. 0.85 V/nm), the semiconducting and metallic segment signals are equalized, as seen in Figures 4 and 5. The combination of laser field and tip field Stark effect can tune the junction from strong semiconductor
metallic character to nearly equal optical properties for the two halves of the junction. A simple model explains the switching of the chiral junction quantitatively. We assumed a (7,2) semiconducting tube joined to a metallic (6,6) or (10,1) tube. The model S22 transition is at at ca. 2.7 eV, the model M11 transition at ca. 3 eV, and the model tube diameters are close to the measured height (Figure S9). We also adjusted the width of the S22 absorption peak to 40 nm, and the width of the M11 absorption peak to 60 nm, to best match our experimental data in Figure 5a. These spectral widths are close to spectral line widths of 45
100 meV reported in the literature.31,32 The model has no further adjustable parameters. In accordance with calculations by Perebeinos and Avouris,11 we used one-third of the Stark coefficient of the S11 transition in Figure 1 for the semiconducting S22 transition (0.5 eV/(V/nm)2), and estimated the metallic M11 transition to have the same Stark coefficient as the S22 transition. Figure 5b plots the calculated S22 transition center wavelength for the model (7,2) CNT as a function of the electric field, showing the red-shift of the absorption caused by the quadratic Stark effect (blue curve). Additionally the S22 line shape centered at 532 nm is VOL. XXX

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A(x) ¼ 1
2 x
σ A(x) ¼ e

Figure 5. (a) Experimental absorption signals as a function of bias (approximate field at the bottom) for the semiconducting segment below the chiral junction in Figures 3 and 4 (red circles), and for the metallic segment above the chiral junction (blue squares). (b) The calculated S22 transition wavelength as a function of the electric field, showing the red-shift of the absorption caused by the quadratic Stark effect (blue). The S22 line shape centered at 532 nm accounts for a 40 nm absorption width (green). From the line shape and Stark shift, the signal in panel a was calculated: S22 transition (black dotted curve), M11 transition (black dashed curve), and their sum (black solid curve).

shown (green curve). Shifting the center of the absorption line shape according to the quadratic Stark effect, and extracting its intensity at the 532 nm laser wavelength, yields the calculated signal for the semiconducting part of the junction (Figure 5a, dotted black curve). The calculation matches the experimentally observed peak very well. The analogous calculation for the metallic part of the junction matches equally well (dashed black curve). In Figure 5a, it appears that the sum of the S22 and M11 absorption profiles (solid black curve) actually fits the data below the junction (red circles) better than the S22 profile alone. This observation could be explained by energy transfer from the upper part to the lower part of the CNT across the junction. NIENHAUS ET AL.

ARTICLE

Thus, SMA-STM under variable bias provides a new way to measure line widths of CNT excited states. However, a quantitative calibration of bias vs field would be required. If such a calibration still yields the good agreement between experiment and model seen in Figure 5, then increased carrier density of the laser-excited CNT would suffice to explain the anomalously large bandgap reduction discussed earlier. There are other possible explanations: Additional fields from the rectified laser light at the tip
gold surface junction,33 from the dynamic Stark effect as investigated by Orellana and Pacheco,10 and from image dipoles formed in the gold surface are also candidates for reducing the bandgap more than expected from the static field of the tip alone. It will be interesting to see if these effects play a role when more rigorous models of the substrate
CNT
tip junction under laser illumination are developed. We measured the width of the transition zone from metallic to semiconducting character by averaging slices of the absorption image along the axis of the CNT (Figure 6). We fitted the data by least-squares to truncated Gaussian:15 (x < 0) (xg0)

This results in a fitted width of σ = 1.2 ( 0.3 nm. (The fitted error is smaller; we base our error estimate on STM resolution). For the shortest junction from a (10,0) semiconducting to a (5,5) metallic tube, modeled by Rochefort and Avouris5 by a single girdle of 7- and 5-membered rings, the transition of electronic structure occurs over ∼4 girdles or ca. 1.5 nm. Thus, our width detected by optical absorption across the junction is consistent with the most abrupt geometrical change possible. Assuming that exciton penetration in the semiconducting to metallic direction across the junction is on the order of exciton size, our result is also consistent with measured exciton sizes (ca. 1.5 nm15,34). The calculations by Rochefort and Avouris indicate that electronic structure is perturbed more deeply into the semiconducting part of the junction than into the metallic part. Unfortunately we were not able to measure the topographic transition relative to the SMASTM transition with enough accuracy to check whether this is also the case experimentally. However, Ruppalt and Lyding detected enhanced conductance in the semiconducting part of such a junction, attributed to metal-induced gap states in the semiconducting part of the tube.28 As a final note, the short CNT piece (not unambiguously identifiable as such) on the right in Figure 4 shows a 180 out-of-phase (white instead of black) signal, so its local density of states changes in the opposite direction of the CNT when the laser is on. VOL. XXX

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Figure 6. Transition zone from semiconducting to metallic character is 1.2 ( 0.3 nm in width, about as short as a connector between two different characteristic nanotube segments can be made. Inset (transition at 0.8 V) shows the cross section used for the fit. The inset scale bar is 5 nm.

Perhaps this is due to plasmonic excitation or energy transfer into the metal surface from a metal catalyst particle. Such change of density of states would be an interesting subject for future investigation, using metal nanoparticles on metal surfaces.

MATERIALS AND METHODS Substrate Preparation for SMA-STM. For visible excitation, platinum
gold ultrathin films of 15 nm thickness on c-plane sapphire, fabricated by electron beam deposition at elevated temperatures as previously reported22 are used as flat, transparent, and conductive substrates in the visible spectrum. To ensure better conductivity between the delicate thin film and the sample holder, thick silver contacts are applied on two sides of the sample, using colloidal silver paint (TedPella Inc.). A 3 mm fused silica right angle prism (Thorlabs) is glued to the backside of the sample using an ultrahigh vacuum compatible epoxy (302-3M, Epotek) to couple light into the back of the sample and allow total internal reflection at the front. The sample undergoes a 120 C degas for 12 h prior to STM imaging. Dry Contact Transfer of CNTs. HiPCO-synthesized CNTs of ∼1 nm diameter are transferred in situ onto clean surfaces using a modified version of dry contact transfer (DCT).35 The DCT applicators are constructed of a piece of frayed fiberglass, which is loaded with CNTs by lightly rubbing it in CNT powder. The applicators are degassed at elevated temperatures overnight, while keeping the pressure below 1.5 μPa ( 0 means the edge is shifted toward the Fermi energy, giving a smaller bandgap. As the sample bias voltage is decreased, the tip moves slightly closer to the surface at the reference current (typically 5 pA here), and the effective electric field during the STS sweep actually increases. The bandgap accordingly decreases by a small amount. Conflict of Interest: The authors declare no competing financial interest. Acknowledgment. L.N. would like to thank J. C. Koepke for support and helpful discussions. Financial support was provided by the National Science Foundation Grant NSF CHE 13-07002. (L.N., D.N., J.L., M.G.). S.W. would like to thank Nanosystems Initiative Munich (NIM) for financial support. Supporting Information Available: The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b04872. Calculated electric field under scanning and STS conditions (Figure S1); STS without illumination (Figures S2, S5, S6); calculated LDOS for a (7,2) CNT (Figure S3) and the resulting theoretical I
V curves (Figure S4); CNT height profiles (Figures S7 and S9) and SMA-STM scans of the CNTs at different voltages (Figures S8, S10, S11, S12); histograms showing the absorption signal (Figure S11); experimental setup TIR geometry (Figure S13) (PDF)

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STM junction capacitance on the order of 0.5 pF,38 which is much larger than the expected tip
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