Imaging of Carbon Nanotube Electronic States Polarized by the Field

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Imaging of Carbon Nanotube Electronic States Polarized by the Field of an Excited Quantum Dot Duc Nguyen, Alison Wallum, Huy A. Nguyen, Nhan T. Nguyen, Joseph W. Lyding, and Martin Gruebele ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b06806 • Publication Date (Web): 03 Jan 2019 Downloaded from http://pubs.acs.org on January 7, 2019

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Imaging of Carbon Nanotube Electronic States Polarized by the Field of an Excited Quantum Dot Duc Nguyen,†,‡,# Alison Wallum,†,‡,# Huy A. Nguyen,†,‡ Nhan T. Nguyen,⊥ Joseph W. Lyding‡,§ and Martin Gruebele†,‡,¶,* †Department

of Chemistry, ‡Beckman Institute for Advanced Science and Technology, §Department

of Electrical and Computer Engineering, ¶Department of Physics, University of Illinois at UrbanaChampaign, Urbana, Illinois 61801, United States. ⊥Faculty of Chemistry, VNU – University of Science, Hanoi 10000, Vietnam ABSTRACT Efficient heat dissipation and large gate capacitance have made carbon nanotube fieldeffect transistors (CNT FETs) devices of interest for over 20 years. The mechanism of CNT FETs involves localization of the electronic structure due to a transverse electric field, yet little is known about the localization effect, nor has the electronic polarization been visualized directly. Here, we co-deposit PbS quantum dots (QD) with CNTs and optically excite the QD so its excited state dipolar field biases the local environment of a CNT. Using single molecule absorption scanning tunneling microscopy, we show that the electronic states of the CNT become transversely localized. By nudging QDs to different distances from the CNT, the magnitude of the localization can be controlled. Different bias voltages probe the degree of localization in different CNT excited states. A simple tight binding model for the CNT in an electrostatic field provides a semi-quantitative model for the observed behavior. KEYWORDS: quantum dot, carbon nanotube, single molecule absorption scanning tunneling microscopy, tight binding calculation, dipole-induced polarization

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Localization of charge carriers is critical for applications in many fields spanning catalysis, spectroscopy, plasmonics and electronics.1–6 In catalysis, the localization of the electrons or holes at the catalytic active sites strongly influences the activity, selectivity and performance of the catalysts.1,2,4,7 The localization of electrons on the surface of metal nanoparticles upon optical excitation results in an enhanced electric field near the metal nanoparticle surface.3 That enhanced electric field and consequently hot charge carriers are the driving force for numerous phenomena in plasmon-driven chemistry8 and plasmonically enhanced spectroscopy.3 In electronics, the localization of charges obtained by doping or the field effect is essential for building electronic devices.9 A better understanding of charge localization at the sub-nm scale is needed to improve the design and performance of the active components in these applications. Efforts have been made to actively control and understand charge localization, particularly in nanomaterials at sub-nm length scales.4,10,11 For CNTs, a promising 1D material with exceptionally high electron mobility, mechanical stability and thermal conductivity, charges can be localized longitudinally or laterally. Longitudinal localization can be obtained by joining two CNTs of different diameter and chirality,12,13 or by introducing defects and dopants.14 Longitudinal localization in a single CNT has been studied extensively and visualized at sub-nm resolution.12,15,16 One of the most promising applications of CNTs is in CNT FETs, as demonstrated previously.5,17 In this application, a voltage (transverse electric field) is applied across the CNT to change its conductivity to turn on and off the current. Under the applied electric field, the CNT electronic states are laterally polarized. However, in contrast to longitudinal localization, much less is known about the lateral localization effect, nor has it been directly visualized. One of the main reasons is the small diameter of CNTs, on the order of ~1 nm, requiring ultrahigh spatial resolution characterization techniques. Single molecule absorption scanning tunneling microscopy (SMA-STM) is a technique capable of producing spectrally-resolved, sub-nm spatial resolution images of the change in electronic density of a chromophore or molecules near a chromophore as a result of optical absorption.15,16,18–21 Here we use SMA-STM to image the transverse localization of CNT electronic states induced by the electrostatic field of a nearby excited QD with sub-nm spatial resolution. The QD dipole acts as a switch that can turn on or off transverse polarization of the CNT. A strong localized SMA-STM signal is observed on the CNT when the QD is present and sufficiently close to the CNT. The polarization of the CNT electronic states, in particular the LUMO (lowest unoccupied molecular orbital) to LUMO+3 states, is strongly dependent on the state, separation and orientation of the CNT and the QD. Further, we show that the observed trend in polarization can be seen on non-metallic as well as metallic


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supporting surfaces. Localization of different CNT UMOs is also imaged. Our experimental data agrees with a semi-quantitative tight binding model where the CNT electronic states are polarized in the electrostatic field of a point dipole. RESULTS AND DISCUSSION In our experiments, a semiconducting single-walled CNT with ~0.8 nm diameter that is not resonantly excited by 532 nm laser light was first located (Figure S1). An adjacent PbS quantum dot was then excited at 532 nm. SMA-STM can be used to image the QD excited states,18,20 and also to verify the lack of optical absorption signal on the CNT when the QD is far away. The large excitedstate dipole of a nearby QD (~20 D on non-plasmonic surfaces, ~40 D with plasmonic assistance)22 polarizes the CNT electronic structure in the transverse direction. The polarization can be imaged by SMA-STM as a tunneling signal localized on one side of the CNT and modulated at the same frequency and phase as the QD optical absorption signal. The localized SMA-STM signal can be a purely electrostatic effect due to the QD electric field polarizing the CNT, or it can also involve energy transfer from QD to CNT. By translating and rotating the quantum dot on the surface, the CNT-QD distance d as well as the orientation of the QD excited state dipole (angles 𝜃𝑎 and 𝜃𝑝) can be controlled, and their effect on polarizing the CNT can be measured. The azimuthal in-plane angle 𝜃𝑎 and the polar out-of-plane angle 𝜃𝑝 of the QD dipole moment are defined in the Methods and SI.

Figure 1. Dipole-induced polarization of a CNT by an excited QD. (a) STM topographic image showing a PbS QD (bottom right) and a CNT (top left). (b) At a separation of ~12 nm, no SMA-STM signal is observed on the CNT, while the QD shows a strong localized absorption signal. (c) Using the STM tip, the QD is moved toward the CNT. (d) At a separation of ~6 nm, a localized SMA-STM signal is observed (black stripe on the back side of the CNT), while the absorption signal on the QD changes. Scanning conditions 20 pA, 1.3 V, laser power density 1100 mW/mm2, scale bars 5 nm.

Figure 1a shows a CNT (top left) and a PbS QD (bottom right) on a PtAu surface, initially separated by d ≈ 12 nm in the orange topography image. The corresponding SMA-STM image is shown in Figure 1b in gray scale. A light signal means that tunneling is enhanced by laser excitation and a dark signal means that tunneling is blocked, while a gray signal indicates that tunneling is unaffected by laser excitation relative to the substrate. A strong SMA-STM signal is observed on the QD. The signal is non-uniform because the QD electronically excited state is localized due to a defect, as previously


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reported experimentally and simulated by DFT calculations.18,20 In contrast, no detectable SMA-STM signal is observed on the CNT, which does not absorb the light directly. In Figure 1c the QD has been nudged toward the CNT within d ≈ 6 nm. At this smaller separation, strongly reduced tunneling is observed by SMA-STM on the far side of the CNT from the quantum dot. This observation indicates polarization of the CNT by the electric field of the excited quantum dot such that an unoccupied orbital of the CNT, through which tunneling occurs, has reduced tunneling probability on the far side of the tube. The direct imaging of a localized SMA-STM signal along the back edge of a defect-free segment of CNT is different from resonant excitation of an isolated CNT, where the SMA-STM signal is uniform across the entire CNT, or from single point scanning tunneling spectroscopy (STS I-V curve).15,16 Based on the oblong shape of the QD in the topography image in Fig. 1a, it has been rotated by ~120° counterclockwise in the plane of the surface in Figure 1c in addition to being translated. This effect has been described previously and can be used to obtain multiple 2-D projections of excited states of a QD with sub-nm spatial resolution.20 However, here the SMA-STM image of the QD excited state changes even if the rotation is taken into account. Thus, the QD excited state imaged at d ≈ 6 nm separation is different from the one imaged at d ≈ 12 nm. The data in Figure 1 shows that there is a distance-dependent interaction between the QD and the CNT. The localized SMA-STM signal on the far side of the CNT is only observed when the QD is close (~6 nm separation), and the QD excited state changes when the QD is moved closer to the CNT. Given the long-range energy transfer between the QD and CNT with Förster radius R0 of >12 nm,23 it is unlikely that the observed asymmetry is due to energy transfer between the QD and CNT alone. Otherwise, a signal on the CNT would also be seen at ~12 nm (Figure 1b), and the CNT signal would be symmetric. We therefore hypothesize that polarization of the electronic structure of the CNT by the strongly distance-dependent dipole field of the QD is responsible for the observed asymmetry. The polarization of the nearby CNT may in turn modify the electronic structure of the QD, as evidenced by the different localized excited state shape of the QD at ~6 nm vs. ~12 nm (Figure 1b and d). To interpret such experimental data semi-quantitatively, we performed tight binding calculations on a 40-atom unit cell of a (10,0) CNT in the dipole electrostatic field of a QD. Such a field can have a transverse component across the CNT, and can consequently polarize the CNT electronic structure in the STM imaging plane. In Figure 2a, we can see a uniform probability density along the circumferential axis of the nanotube in the absence of a transverse electric field. In contrast, Figure


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2b reveals an antisymmetric LUMO density in the transverse direction in the presence of an electric potential modeled by a dipole moment of 40 D located ~3 nm from the center of the CNT. In the example of this semiconducting nanotube, the probability density of the LUMO increases on the side of the CNT furthest from the point dipole source. This would increase tunneling through the LUMO on the far side of the CNT. When we change the orientation of the dipole moment, CNT bandgap properties, or the excited state (here the LUMO) of the CNT used in our calculations, the probability density varies. An example of the LUMO probability density in the presence of an external electric field for a metallic nanotube and a (10,0) nanotube with an opposite dipole is included in Figure S3.

Figure 2. Tight-binding analysis of a (10,0) CNT polarized by an external dipolar electric field, showing a cross section view, top view, and unrolled views of the first half of three consecutive CNT unit cells. Carbon atoms are plotted along the azimuthal angle (𝜑) which corresponds to the angle along the circumferential axis, where 𝜑 = 0˚ is associated with the carbon atom furthest from the field source and 𝜑 = 180˚ is associated with the carbon atom closest to the field source. (a) When no electric field is present, no polarization of the carbon nanotube is observed. (b) In the presence of a QD modeled as a point dipole with a moment of 40 D at 3 nm away from the CNT, the CNT becomes polarized. (c) A 3D representation of the CNT modeled in (b) rolled up, with points along the black arrow corresponding to different azimuthal angles. All calculations were done on a single CNT unit cell with a dipole rotation of 𝜃𝑎 = 0˚ and 𝜃𝑝 = 37˚.

Figure 3 directly compares another set of experimental data with tight binding calculations. The nanotube LUMO shown in Figure 2 was calculated at varying distances and orientations of the neighboring point dipole of ~40 D. Figure 3a shows a STM topography image of a CNT with a kink defect (left) and a PbS QD (right) on a PtAu surface separated by ~10 nm. A strong absorption signal is observed on the QD, but only a weak SMA-STM signal is observed on the far side of the CNT (Figure


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3b). The tight binding calculation in Figure 3c shows that when the QD is modeled as a dipole 10 nm away from the CNT, little to no polarization of the nanotube is seen. The QD was then nudged toward the CNT and settled at a separation of ~5 nm, as shown in Figure 3d. Strongly enhanced tunneling is now observed by SMA-STM on the far side of the CNT, with reduced tunneling seen on the inner side (Figure 3e). The sign of the SMA-STM signal in Figure 1 is approximately equal and opposite to the data in Figure 3 (positive vs. negative). This is consistent with a reversal of the dipole moment associated with the two different QDs in Figure 1 and Figure 3 (see Figures S3c,d for a calculation). QD dipole orientation depends on the nature of defects in the QD. Figure 3f shows a tight binding calculation that models the experimental condition for Figure 3d, with the appropriate dipole orientation of the QD and a shorter separation of ~4 nm.

Figure 3. Distance- and orientation-dependent polarization of a CNT by the dipole of an excited QD. (a, d, g) Topography and (b, e, h) SMA-STM experimental images of a PbS QD and a CNT on a PtAu film. Scanning conditions: 1.2V, 20 pA, laser power density 1630 mW/mm2, scale bars 5 nm. Figure S4 shows the bias dependence, which is further analyzed for a different CNT-QD system in Figure 4. (b) When the QD is ~10 nm separated from the CNT, weak polarization of the CNT is seen. (e) Upon moving the QD closer to the nanotube (~5 nm separation), a strong localized and positive SMA-STM signal is observed on the CNT. (h) When the CNT is moved into contact with the QD, the strong positive SMA-STM signal on both QD and CNT disappears. (c, f, i) Tight-binding analysis shows semi-quantitative agreement with experimental results. (c) At d = 10 nm, a dipole of 40 D, and 𝜃𝑝 = 37˚, no significant polarization is seen. (f) At d = 4 nm, the probability density decreases on the side of the nanotube closest to dipole source, and increases on the side furthest


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from dipole source. (i) If 𝜃𝑝 is then rotated to 90˚ at d = 3.5 nm, the polarization is in the line of sight of the STM tip and cannot be detected. All calculations were done with the dipole angle 𝜃𝑎 = 0˚.

Finally, the CNT was slid into contact (edge-to-edge separation of ~1.5 nm) with the QD’s oleic acid envelope (Figure 3g). A much smaller negative SMA-STM signal is observed on the CNT and QD (Figure 3h). Our hypothesis is that the QD has been perturbed by near-contact with the CNT in one of two ways: either the QD relaxes more rapidly to the ground state in vicinity of the CNT, or the laser is in resonance with a different QD excited state, which has a dipole nearly perpendicular to the surface due to a defect. In the first scenario, there is no excited state dipole moment to polarize the CNT. In the second scenario, the CNT is polarized perpendicular to the surface, and the increased signal from the top of the nanotube and decreased signal from the bottom of the nanotube will cancel, resulting in a small SMA-STM signal. The second scenario is plausible according to our tight binding calculation in Figure 3i. When the dipole distance is reduced to 3.5 nm (center-to-center) and the dipole is rotated to 𝜃𝑝=90˚ (pointing away from substrate plane), the polarization results in no transverse asymmetry of the CNT orbitals. The metallic PtAu substrate could have a significant effect on the electronic properties of the CNT because of the image charge and the strong interaction with the CNT; the metallic substrate also can have an effect on the QD due to the plasmonic enhancement of its absorption.24 Thus, we performed measurements on a non-metallic crystalline SiC surface. The SMA-STM signal is shown in Figure 4, and the topography image in Figure S5. Polarization of the CNT electronic states is still observed on the SiC surface (Figures 4b,c). The SMA-STM signal on the SiC surface is smaller than on PtAu surfaces due to a lack of surface plasmon enhancement and the reduced dipole moment of the QD. In Figure 4, scanning the tunneling voltage from 0.8 to 1.5 V allowed us to image different excited states of the CNT. Little signal is seen on the CNT at 0.8 V, just below the bandgap. A strong SMA-STM signal localized near the right middle of the CNT is observed from 1.0 V to 1.1 V. A weak signal that fluctuates across the CNT is seen at 1.3 and 1.5 V. In contrast, a previous study of isolated CNTs on a PtAu surface16 showed uniform signal changes across the CNTs, attributed to a Stark effect-induced spectral shift in the absorption spectrum while the CNT was probed at a fixed wavelength. The experimentally observed trend from little signal at the lowest bias, to stronger signal at intermediate biases, to little signal again at the highest biases, can be rationalized by our tight binding calculations, which used d = 8 nm and reduced dipole moment of 20 D to represent the excited QD on SiC. At lower biases the LUMO contributes most to the signal, and indeed it shows little polarization in our calculation in Figure 4f. At intermediate biases, higher lying excited states begin to contribute


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to the signal, and indeed the calculation shows a strong polarization to the right center for the LUMO +1, mirroring the experimental trend in polarization seen for 1.0-1.1 V. At higher biases, higher states including the LUMO+2 and LUMO+3 states can contribute to the signal, for which the tight binding calculations show reduced polarization in agreement with the smaller SMA-STM signal in Figure 4de.

Figure 4. Imaging different excited states of a CNT by varying sample bias. (a-e) Experimental SMA-STM images of a CNT ~8 nm center-to-center from a PbS QD on a crystalline SiC surface. At low sample bias voltage (0.8 V), no polarization of the carbon nanotube is observed. At intermediate bias voltages (1.0 and 1.1 V), the SMA-STM signal shows enhanced tunneling near the middle of the CNT and reduced tunneling around the edges. When the bias voltage is increased further (1.3 and 1.5 V), the SMA-STM signal shows multiple smaller fluctuations across the width of the CNT. Scanning conditions: 10 pA, laser power density 2620 mW/mm2, scale bars 4 nm. (fi) Probability densities for different UMOs by tight-binding analysis of a (10,0) carbon nanotube color-mapped onto a CNT half unit cell with two additional rows of neighboring atoms. Corresponding graphs for each unit cell in (f-i) reflect the smoothed probability density variation across one row of carbon atoms for the given unit cell. (f) When the QD is modeled as a point dipole with a moment of 20 D, no polarization of the LUMO state is seen at d = 8 nm. (g) The next highest energy UMO (LUMO +1) shows a significant increase in probability density around the center of the nanotube. (h, i) The next two higher non-degenerate UMOs (LUMO +2 and LUMO + 3) show drastically reduced variations in the probability density. All calculations were carried out with dipole angles of 𝜃𝑎 = 0˚ and 𝜃𝑝 = -37˚.

It should be noted that the polarizability of different CNT excited states changes nonmonotonically with energy, as seen in the LUMO to LUMO+3 calculations. in Figure 4. This is a


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consequence of differences in the distribution of electron density along the direction of the electric field among the different CNT orbitals. The SI shows calculations of the polarizabilities for the different electronic states of a (10, 0) nanotube included in Figure S8.

CONCLUSION Using SMA-STM we image the transverse polarization of individual nanotubes’ electronic states induced by the electrostatic field of an excited QD with sub-nm spatial resolution. SMA-STM images show strong dependence of the CNT electronic polarization on quantum dot distance and orientation. Localization of different CNT UMOs (assuming a one-electron approximation) is also imaged by tuning the tunneling voltage, revealing different localization patterns. Our experimental data semiquantitatively agrees with a simple tight-binding model of the CNT, in which the excited QD is treated as a point dipole inducing the polarization of the CNT electronic states. Our results highlight the possibility of optically gating CNTs using a nearby QD as a switch. MATERIALS AND METHODS Sample preparation and SMA-STM measurements Two different substrates were employed for CNTQD co-deposition: an ultrathin PtAu film on sapphire (10 nm Au on 5 nm Pt on sapphire),19,25 and crystalline silicon carbide (c-SiC, bulk bandgap ~3.2 eV and surface bandgap ~2.8 eV).26 PbS QDs (1.01 eV bandgap, ~4.2 nm diameter, capped with oleic acid, Evident Thermoelectrics) and CNTs (HiPCO produced single-wall CNTs, ~1.00 eV bandgap, Carbon Nanotechnologies, Inc.) were simultaneously deposited onto clean surfaces using matrix-assisted dry contact transfer.16 The bandgap of the CNTs was determined by optical absorption measurements and scanning tunneling spectroscopy of single CNTs in our previous studies.15,16,27 The experimental setup has been described previously16,19,20 using a home-built optical UHV-STM (base pressure ≤7x10-9 Pa).28 Both mechanically cut Pt/Ir (80/20) tips and electrochemically etched W tips were used to image adjacent CNTs and QDs. To perform SMA-STM, a 532 nm laser (diode pumped solid state laser, Thorlabs, 1100-2600 mW/mm2, modulated at 2.2 kHz) was used to excite QDs and CNTs from the back of the substrate by total internal reflection. Total internal reflection was obtained by either attaching a 45° right angle prism to the back of PtAu ultrathin samples or machining a 15° wedge to the back of the c-SiC samples. The SMA-STM signal was collected using the STM with a lock-in amplifier modulated at the same frequency (ca. 2 kHz) as the excitation laser. The ms timescale detection of SMASTM is much longer than electronic excitation and dipole-induced polarization timescales. Thus we imaged steady-state changes to the CNTs and QDs in our measurements.


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QDs and CNTs were manipulated on the substrate following the procedure described in ref. 29. After scanning a predefined line over the QDs or CNTs, the STM tip was then moved down by ~0.5-0.7 nm and the same line was re-scanned. This procedure has been shown to effectively move nanostructures on the surfaces without damaging them. We positioned CNTs and QDs at different distances, and with different orientations of the QD localized excited state20 using this approach. Tight binding calculations To further support our experimental findings, π-orbital nearest neighbor tight-binding calculations were performed for a CNT in the presence of an external electric field to model semi-quantitatively the change in CNT electron density due to an adjacent QD. The open-source Python package Pybinding was used to write code for these calculations.30 Based on the measured STM heights (~0.8 nm) and bulk absorption measurement (~1.0 eV bandgap) of the CNT used in our experiments (Figure S1), a (10, 0) semiconducting carbon nanotube was chosen as a representative geometry in our calculations. Variations in structure between different semiconducting nanotubes are predicted to have little effect on the transverse polarizability of the nanotube.31 Nevertheless, we have included two calculations for semiconducting nanotubes with similar diameters in Figure S2 to show we predict the same trend in polarization seen in the (10,0) nanotube. The model CNT was constructed using the zone-folding approximation, which simplifies tight-binding calculations by modeling the band structure from an unrolled carbon nanotube unit cell with the appropriate periodic boundary conditions around the circumference of the nanotube.32 This unit cell can be wrapped up into a circumferential cross-section of the desired nanotube. The size and geometry of these unit cells can be uniformly determined by a set of well-known equations described by Dresselhaus et al.33 A band structure determination for an unperturbed (10,0) CNT modeled with our code can be found in Figure S6 and matches those found in the literature.34 Probability density calculations were performed at the edges of the Brillouin zone of the CNT for the LUMO (Lowest Unoccupied Molecular Orbital) and higher energy UMOs just above the bandgap. More precisely, the probability density corresponds to the square of the wavefunction for a given position on the CNT and set of degenerate states. This defines the probability of finding the electron in a particular orbital at a particular location. In the zone-folding approximation, the edges of the Brillouin zone lie at 𝑘 = 𝜋 𝑇

and 𝑘 =

―𝜋 𝑇 ,

where T is the magnitude of the translational vector that defines the longitudinal symmetry

of the carbon nanotube.33 This particular point in the Brillouin zone was chosen because it reflects the circumferential (transverse) position dependence of the CNT electronic states, of main interest in our experiments imaging lateral asymmetry of CNT electronic structure. To capture the effect of a QD neighboring the CNT, the onsite energy of each carbon atom in the tightbinding unit cell was modified by the electric potential of the QD at each carbon position.35 The excited


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QD was approximated as a point dipole of magnitude 𝜇. The distance and orientation dependence of the QD potential in our system can be expressed as

𝑉=

μ cos 𝜃1cos 𝜃2 4𝜋𝜀 ⊥ 𝑞2

.

[1]

𝜃1=𝜃𝑎 is the azimuthal rotation of the dipole in the plane defined by CNT and QD such that 𝜃𝑎 = 0° has

the dipole pointing straight at the CNT. 𝜃2 is defined by 𝜃2 = cos ―1

𝑑 + 𝑟cos 𝜑 (𝑑 + 𝑟cos 𝜑)2 + (𝑟sin 𝜑)2

± 𝜃𝑝

[2]

where +𝜃𝑝 is applied to the top half of the carbon nanotube and ―𝜃𝑝 is applied to the bottom half of

the nanotube. Here 𝜑 is the azimuthal angle of each carbon atom in the CNT (described in Figure 2). 𝜃𝑝 is the polar angle of the QD dipole relative to the QD–CNT central axis such that -90° points straight

down towards the surface. Figure S7 illustrates the angles with pictorial examples. d is the center-tocenter separation between CNT and QD, r is the radius of the CNT, q is the distance of the dipole from each carbon atom, and 𝜀 ⊥ is the transverse permittivity at the surface of the CNT, which we approximate by 2𝜀0 (see ref. 31 and SI).31 An excited state QD dipole value of 20 Debye was used for calculations on nonplasmonic surfaces, as this magnitude is in the range expected for a PbS QD with a diameter of approximately 4 nm.22 To account for surface plasmon effects seen on PtAu films, which will enhance the transverse dipole moment of the quantum dot, we employed a larger dipole of 40 D when modeling the PbS QD on the PtAu surface.36,37

ASSOCIATED CONTENT The authors declare no competing financial interest. Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.xxxxxxx. It consists of additional methods and data figures (PDF) AUTHOR INFORMATION #D.

N and A. W. contributed equally to the work

D. N. current address: Department of Chemistry, Northwestern University, Evanston, IL Corresponding Author *[email protected]

ORCID D. Nguyen: 0000-0002-6591-6429


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H. A. Nguyen: 0000-0001-6729-9498 J. W. Lyding: 0000-0001-7285-4310 M. Gruebele: 0000-0001-9291-8123 ACKNOWLEDGMENTS This work was supported by The James R. Eiszner Family. D.N. thanks the Beckman Institute for a Beckman Graduate Fellowship while this work was carried out. REFERENCES (1) (2)

(3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14)

(15) (16)

Chen, X.; Shen, S.; Guo, L.; Mao, S. S. Semiconductor-Based Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503–6570. Wang, H.; Zhang, L.; Chen, Z.; Hu, J.; Li, S.; Wang, Z.; Liu, J.; Wang, X. Semiconductor Heterojunction Photocatalysts: Design, Construction, and Photocatalytic Performances. Chem. Soc. Rev. 2014, 43, 5234. Willets, K. A.; Van Duyne, R. P. Localized Surface Plasmon Resonance Spectroscopy and Sensing. Annu. Rev. Phys. Chem. 2007, 58, 267–297. Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science 2009, 323, 760–764. Avouris, P.; Freitag, M.; Perebeinos, V. Carbon-Nanotube Photonics and Optoelectronics. Nat. Photon. 2008, 2, 341–350. Dekker, C.; Tans, S. J.; Verschueren, A. R. M. Room-Temperature Transistor Based on a Single Carbon Nanotube. Nature 1998, 393, 49–52. Thomas, J. M.; Raja, R.; Lewis, D. W. Single-Site Heterogeneous Catalysts. Angew. Chemie Int. Ed. 2005, 44, 6456–6482. Brongersma, M. L.; Halas, N. J.; Nordlander, P. Plasmon-Induced Hot Carrier Science and Technology. Nat. Nanotechnol. 2015, 10, 25–34. Tsividis, Y.; McAndrew, C. Operation and Modeling of the MOS Transistor; Oxford University Press, 2011. Erwin, S. C.; Zu, L.; Haftel, M. I.; Efros, A. L.; Kennedy, T. A.; Norris, D. J. Doping Semiconductor Nanocrystals. Nature 2005, 436, 91–94. Luther, J. M.; Jain, P. K.; Ewers, T.; Alivisatos, A. P. Localized Surface Plasmon Resonances Arising from Free Carriers in Doped Quantum Dots. Nat. Mater. 2011, 10, 361–366. Ouyang, M.; Huang, J. L.; Cheung, C. L.; Lieber, C. M. Atomically Resolved Single-Walled Carbon Nanotube Intramolecular Junctions. Science 2001, 291, 97–100. Yao, Z.; Postma, H. W. C.; Balents, L.; Dekker, C. Carbon Nanotube Intramolecular Junctions. Nature 1999, 402, 273–276. Czerw, R.; Terrones, M.; Charlier, J.-C.; Blase, X.; Foley, B.; Kamalakaran, R.; Grobert, N.; Terrones, H.; Tekleab, D.; Ajayan, P. M.; Blau, W.; Rühle, M.; Carroll, D. L. Identification of Electron Donor States in N-Doped Carbon Nanotubes. Nano Lett. 2001, 1, 457–460. Scott, G.; Ashtekar, S.; Lyding, J.; Gruebele, M. Direct Imaging of Room Temperature Optical Absorption with Subnanometer Spatial Resolution. Nano Lett. 2010, 10, 4897–4900. Nienhaus, L.; Wieghold, S.; Nguyen, D.; Lyding, J. W.; Scott, G. E.; Gruebele, M. Optoelectronic


Page 12 of 15

Page 13 of 15 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

(17) (18)

(19) (20) (21)

(22) (23) (24) (25)

(26) (27)

(28) (29)

(30) (31) (32) (33) (34) (35)

Switching of a Carbon Nanotube Chiral Junction Imaged with Nanometer Spatial Resolution. ACS Nano 2015, 9, 10563–10570. Javey, A.; Guo, J.; Wang, Q.; Lundstrom, M.; Dai, H. Ballistic Carbon Nanotube Field-Effect Transistors. Nature 2003, 424, 654–657. Nienhaus, L.; Goings, J. J.; Nguyen, D.; Wieghold, S.; Lyding, J. W.; Li, X.; Gruebele, M. Imaging Excited Orbitals of Quantum Dots: Experiment and Electronic Structure Theory. J. Am. Chem. Soc. 2015, 137, 14743–14750. Nguyen, D.; Nguyen, H. A.; Lyding, J. W.; Gruebele, M. Imaging and Manipulating Energy Transfer among Quantum Dots at Individual Dot Resolution. ACS Nano 2017, 11, 6328–6335. Nguyen, D.; Goings, J. J.; Nguyen, H. A.; Lyding, J.; Li, X.; Gruebele, M. Orientation-Dependent Imaging of Electronically Excited Quantum Dots. J. Chem. Phys. 2018, 148, 64701. Nguyen, H. A.; Banerjee, P.; Nguyen, D.; Lyding, J. W.; Gruebele, M.; Jain, P. K. STM Imaging of Localized Surface Plasmons on Individual Gold Nanoislands. J. Phys. Chem. Lett. 2018, 9, 1970– 1976. Liu, X.; Lü, Y.; Dai, Z.; Tang, X.; Yan, L. Electroabsorption Spectra of Lead Sulfide (PbS) Quantum Dots in a Polymer Film. J. Phys. Chem. C 2013, 117, 21483–21489. Shafran, E.; Mangum, B. D.; Gerton, J. M. Energy Transfer from an Individual Quantum Dot to a Carbon Nanotube. Nano Lett. 2010, 10, 4049–4054. Shin, H.-J.; Clair, S.; Kim, Y.; Kawai, M. Substrate-Induced Array of Quantum Dots in a SingleWalled Carbon Nanotube. Nat. Nanotechnol. 2009, 4, 567–570. Nienhaus, L.; Scott, G. E.; Haasch, R. T.; Wieghold, S.; Lyding, J. W.; Gruebele, M. Transparent Metal Films for Detection of Single-Molecule Optical Absorption by Scanning Tunneling Microscopy. J. Phys. Chem. C 2014, 118, 13196–13202. Nguyen, D.; Nienhaus, L.; Haasch, R. T.; Lyding, J.; Gruebele, M. Sub-Nanometer Glass Surface Dynamics Induced by Illumination. J. Chem. Phys. 2015, 142, 234505. Albrecht, P. M.; Lyding, J. W. Ultrahigh-Vacuum Scanning Tunneling Microscopy and Spectroscopy of Single-Walled Carbon Nanotubes on Hydrogen-Passivated Si(100) Surfaces. Appl. Phys. Lett. 2003, 83, 5029–5031. Lyding, J. W.; Skala, S.; Hubacek, J. S.; Brockenbrough, R.; Gammie, G. Variable-Temperature Scanning Tunneling Microscope. Rev. Sci. Instrum. 1988, 59, 1897–1902. Albrecht, P. M.; Lyding, J. W. Lateral Manipulation of Single-Walted Carbon Nanotubes on HPassivated Si(100) Surfaces with an Ultrahigh-Vacuum Scanning Tunneling Microscope. Small 2007, 3, 146–152. Moldovan, D.; Anđelković, M.; Peeters, F. Pybinding v0.9.4: A Python Package for Tight-Binding Calculations. 2017. Kozinsky, B.; Marzari, N. Static Dielectric Properties of Carbon Nanotubes from First Principles. Phys. Rev. Lett. 2006, 96, 166801. Saito, R.; Fujita, M.; Dresselhaus, G.; Dresselhaus, M. S. Electronic Structure of Graphene Tubules Based on Cqo. Phys. Rev. B 1991, 46, 1804–1811. Dresselhaus, M. S.; Dresselhaus, G.; Saito, R. Physics of Carbon Nanotubes. Carbon N. Y. 1995, 33, 883–891. Reich, S.; Thomsen, C.; Ordejó, P. Electronic Band Structure of Isolated and Bundled Carbon Nanotubes. Phys. Rev. B 2002, 65, 155411. Pacheco, M.; Barticevic, Z.; Latgé, A.; Rocha, C. G. Optical Properties of Carbon Nanotubes under External Electric Fields. Brazilian J. Phys. 2006, 36, 440–442.


ACS Nano 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(36) Wieghold, S.; Nienhaus, L.; Knoller, F. L.; Schweinberger, F. F.; Shepherd, J. J.; Lyding, J. W.; Heiz, U.; Gruebele, M.; Esch, F. Plasmonic Support-Mediated Activation of 1 Nm Platinum Clusters for Catalysis. Phys. Chem. Chem. Phys. 2017, 19, 30570–30577. (37) Amendola, V.; Pilot, R.; Frasconi, M.; Maragò, O. M.; Iatì, M. A. Surface Plasmon Resonance in Gold Nanoparticles: A Review. J. Phys. Condens. Matter 2017, 29, 203002.


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TOC Figure:


Imaging of Carbon Nanotube Electronic States Polarized by the Field

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