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Ultrasmall Plasmonic Single Nanoparticle Light Source Driven by a Graphene Tunnel Junction Seon Namgung, Daniel A Mohr, Daehan Yoo, Palash Bharadwaj, Steven J. Koester, and Sang-Hyun Oh ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b09163 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 2, 2018

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Ultrasmall Plasmonic Single Nanoparticle Light Source Driven by a Graphene Tunnel Junction Seon Namgung1, Daniel A. Mohr1, Daehan Yoo1, Palash Bharadwaj2, Steven J. Koester1, and Sang-Hyun Oh1*

1

Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota, 55455, USA. 2

Department of Electrical and Computer Engineering, Rice University, Houston, Texas, 77005, USA.

*Correspondence and requests for materials should be addressed to S.-H.O. (email: [email protected]).

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ABSTRACT Metal nanoparticles that can couple light into tightly confined surface plasmons bridge the size mismatch between the wavelength of light and nanostructures are one of the smallest building blocks of nano-optics. However, plasmonic nanoparticles have been primarily studied to concentrate or scatter incident light as an ultrasmall antenna, while studies of their intrinsic plasmonic light emission properties have been limited. Although light emission from plasmonic structures can be achieved by inelastic electron tunneling, this strategy cannot easily be applied to isolated single nanoparticles due to the difficulty in making electrical connections without disrupting the particle plasmon mode. Here, we solve this problem by placing gold nanoparticles on a graphene tunnel junction. The monolayer graphene provides a transparent counter electrode for tunneling while preserving the ultrasmall footprint and plasmonic mode of nanoparticle. The tunneling electrons excite the plasmonic mode, followed by radiative decay of the plasmon. We also demonstrate that a dielectric overlayer atop the graphene tunnel junction can be used to tune the light emission. We show the simplicity and scalability of this approach by achieving electroluminescence from single nanoparticles without bulky contacts as well as millimeter-sized arrays of nanoparticles.

KEYWORDS: electroluminescence, gap plasmon, inelastic tunneling, graphene, nanoparticle, plasmonics


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Surface plasmons,1–3 collective oscillations of conduction electrons, can localize optical fields within nanometer-scale structures. This tight sub-wavelength confinement of the optical field provides a promising solution to bridge the mismatch between photonic and electronic length scales, thus benefiting broad applications in optoelectronics.4–8 The sub-wavelength confinement of the electromagnetic field has been shown to enhance the performance in a diverse range of optoelectronic applications,9 including improved sensitivity in photodetectors,10 improved gain in plasmon-coupled optical modes,11,12 enhanced luminescence in quantum emitters,13–18 enhanced nonlinear effects,19–22 and molecular sensors with extraordinary sensitivity.23 While plasmonic effects have shown great promise in these and other applications, for the most part, the excitation of surface plasmons still requires bulky external light sources. To realize the true benefit of plasmonics for system-level miniaturization, it is highly desirable to realize electrically driven onchip nanoscale light sources. Toward this goal, plasmonic nanostructures can also be utilized to emit light through inelastic electron tunneling,24–29 where excess energy of the tunneling electrons can excite a plasmon and generate light by the radiative decay of the plasmon. Even though the efficiency of the light emission of the plasmonic devices is generally lower than for other types of light emitters, plasmonic emitters are extremely attractive due to their potential for tunable broadband light generation, ultrafast modulation, and ultrasmall size. For diverse future optoelectronic applications, plasmonic on-chip light sources are desirable especially in the single nanoparticle limit. Despite their tremendous potential, light-emitting single-nanoparticles structures based on inelastic tunneling have been difficult to achieve, since making electrical connections tends to disrupt the plasmonic mode.30 Although light emission from single plasmonic nanoparticles has been studied using scanning tunneling microscopes (STMs)31 and cathodoluminescence


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spectroscopy,32 such setups are not practical for compact on-chip integration or large-scale production. Alternatively, on-chip metal-insulator-metal (MIM) structures have been used to study plasmonic emission.27 However, MIM structures cannot be used to generate light from an isolated nanoparticle, since electrical contact to nanoparticle cannot be made without distorting the plasmon. To overcome the limitations of prior work on plasmonic electroluminescence (EL), we place a single nanoparticle on top of a graphene tunnel junction, which allows electrical excitation of the nanoparticle plasmon mode without a direct electrical connection to each nanoparticle. Furthermore, our strategy can minimize disruption of the particle plasmon mode from the electrode present. In our strategy, a gap plasmon33,34 is formed between a base Au electrode and single Au nanoparticle separated by a thin (< 10 nm) dielectric gap. This gap mode is electrically excited by electrons tunneling from the base electrode, through a thin Al2O3 dielectric, and into a graphene layer located underneath the nanoparticle. The excess energy of the tunneling electrons is transferred to the gap plasmon mode, which then undergoes radiative decay, resulting in light emission with photon energy corresponding to the plasmon mode properties. While most light emission applications using two-dimensional (2D) materials rely on carrier recombination with photon energy corresponding to the semiconducting band gap,35 in our architecture, gapless graphene36 only acts as a transparent counter electrode. Critically, the monolayer nature of graphene minimizes unwanted dissipative pathways that may disrupt the formation of a gap plasmon. This approach allows the creation of plasmonic light sources over large area (~ mm2) using a scalable manufacturing process, where the emission wavelength can be controlled over a wide range by tuning the nanoparticle and gap sizes.


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RESULTS AND DISCUSSION A schematic image of the device structure and light emission mechanism are shown in Figure 1a. A graphene/Al2O3/Au tunnel junction is built on a standard silicon substrate with a 300 nm SiO2 layer, and single plasmonic nanoparticles are fabricated on top of the graphene sheet (left image in Figure 1a). Electrons tunnel through the thin Al2O3 layer with an applied voltage between the graphene and the Au electrode, and excess energy of the tunneling electrons is transferred to the gap plasmon mode (right image in Figure 1a). The excited gap plasmon undergoes radiative decay, resulting in far-field radiation whose energy corresponds to the gap plasmon. Our method to fabricate graphene tunnel junctions with nanoparticles is shown in Figure S1 in the Supporting Information. In brief, 10-100-µm wide Au electrodes for tunnel junctions are assembled first on a Si wafer substrate covered by a 300-nm SiO2 layer. Next, the Al2O3 tunnel barrier is formed by atomic layer deposition (ALD). Notably, the ALD process allows the thickness of the Al2O3 to be controlled with atomic-scale precision, and this ensures a uniform tunnel barrier over the entire Au metal electrode area. Source and drain electrodes are created next and are positioned adjacent to the base electrodes for tunneling, where these will be utilized as eventual contacts to the graphene. Next, a large-area sheet of graphene grown by chemical vapor deposition (CVD) is transferred over the electrodes via a PMMA-assisted transfer method. Finally, nanoparticles are formed on the graphene using either electron-beam lithography (EBL) or nano-imprint lithography and a standard evaporation and lift-off process. Figure 1b shows an optical micrograph of the device including the source/tunnel/drain electrode arrays. A scanning electron microscopy (SEM) image (Figure 1c) clearly shows the graphene and the source/tunnel/drain electrodes. An SEM image (Figure 2a) shows an array of symmetric nanoparticles (nanodisks) patterned with EBL. The height of the nanodisks is 30 nm while the thickness of the Al2O3 layer and the 5 ACS Paragon Plus Environment

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periodicity and the diameter of nanodisks were varied. We measured reflection spectra from the nano-disks assembled on tunnel junctions without graphene. Figure 2b shows the diameterdependence of the reflection spectra from the nanodisks assembled on tunnel junctions without graphene. We also characterized the dependence of the reflection spectra on the diameter of nanodisks, where the dips in reflection appear at larger wavelength as the diameter increases (solid lines in Figure 2b). Here, the periodicity of the nanodisks and the thickness of the Al2O3 are kept constant at 400 nm and 6 nm, respectively. We also measured the reflection spectra from an array of asymmetric (65 × 130 nm2) nanorods using polarized incident light on an Al2O3/Au structure (Figure 2c). The light absorption near 780 nm was maximized when incident light was polarized along the short axis of the nanorod (0° in the inset image) and decreased as the polarization axis of incident light rotates toward the long axis of the nanorod (90° in the inset image). The strong dependence of the polarization of incident light on the light absorption at the reflection dip (~ 780 nm) indicates that the observed reflection dips are associated with a plasmon mode along the nanoparticle structure. While larger polarizability along the long axis is expected, the fundamental plasmon mode along the long axis is in the near-infrared, which is out of range with our siliconbased charge coupled device (CCD) setup. Instead, a higher-order plasmon mode along the long axis of the nanorods was observed as the small dips appearing around 650 nm indicated by a green arrow. To reveal which plasmon mode of the nanoparticles determines the observed spectra, we measured reflection spectra of nanodisk arrays assembled on Al2O3 layers with different thickness (2, 3, and 6 nm), while keeping the periodicity (400 nm) and diameter (65 nm) of the disk arrays constant. As shown in Figure 2d, as the Al2O3 thickness increases, the spectral dips appear at shorter wavelengths. Considering all these results of reflection spectra, the observed spectra can


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be attributed to a gap plasmon mode with a highly confined electromagnetic field between the nanodisks and the metal substrate.34,37–39 The observed reflection dips of lower energy with larger diameter is attributed to the retardation effect in small nanoparticles (i.e. diameter < l), described by following equation40 D

2π n= mπ − ϕ λ

(1)

where D, l, n, and f is the diameter of nanodisk, wavelength, effective refractive index, and phase shift at the end of nanodisk, respectively. In addition, the red shift of the peaks with smaller gap thickness is attributed to the increased interaction between the nanoparticles and antiparallel image charge distribution in the base electrode. The dependence of the reflection spectra on the thickness of the Al2O3 strongly supports that the presence of a gap plasmon between the nanodisk and the base Au electrode. To further confirm that the observed reflection spectra originates from the gap plasmon of individual nanodisks, and not from the periodicity of the array, reflection spectra were taken for nanodisk arrays with different periodicity, while maintaining constant Al2O3 thickness and nanodisk diameter. As shown in Figure 2e, the position of spectral dips is independent of the periodicity of the nanodisk array. This result clearly confirms that the gap plasmon dominates the reflection spectra. Since the gap plasmon is tightly formed in the gap beneath individual nanodisks, the effect of periodicity of the nanoparticles should be negligible on the observed spectra. We also performed numerical simulations on the nanodisk structure and compared with experiments, and the results are shown in Figure 2b. The simulation result is in good agreement with the experimental result, where the slight difference can be ascribed to the tolerance in the measurement of the diameter using an SEM and the refractive index value of the Al2O3 layer used in the simulation. Figure 2f shows a side view of a spatial map of the electric field surrounding an


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80 nm nanodisk at the resonance condition. The highly squeezed electric field in the gap is clearly observed, supporting the presence of the gap plasmon mode. Notably, although we used nanoparticle arrays to obtain a strong reflection signal, the simulated field map and the wavelengthindependence from periodicity strongly support that gap plasmons of a single nanoparticle dominate the optical properties of our system. We also studied the spectral properties of the gap plasmon with a graphene sheet inserted beneath the nanoparticles. Here, we transferred a CVD graphene sheet on the Al2O3/base Au electrode structure, patterned a nanodisk array on the graphene and compared the reflection spectra of the nanodisk arrays with (blue lines) and without graphene (red lines), as shown in Figure 2g. When the nanodisks were assembled on graphene, the reflection dips appeared at larger wavelength and the linewidth of the dips increased. The red-shift of the dips and the increased linewidth are attributed to the large refractive index of graphene and the additional dissipative loss due to the graphene. This dissipative effect of graphene on the gap plasmon will be discussed in more detail later. Before studying the electroluminescence of the nanoparticles on graphene, we studied the electrical properties of the graphene/Al2O3/Au tunnel junctions. The tunneling current between graphene and the Au electrodes was measured in order to determine the charge neutrality point (CNP) of the graphene sheet, which helps to reveal the mechanism of the electroluminescence of the structure in next section. A schematic diagram of the experimental setup to measure the tunnel current and CNP of the graphene sheet is shown in Figure 3a. A tunneling voltage is applied between the base electrode and the graphene sheet connected to a source electrode, resulting in tunneling of electrons through a thin Al2O3 layer (marked as a red arrow in Figure 3a). This tunneling voltage also works as a gate voltage to control a source-drain current in a graphene based 8 ACS Paragon Plus Environment

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field effect transistor (marked as a blue arrow in Figure 3a). Thus, we can measure the transfer characteristics in the field-effect transistor (FET) with a tunneling voltage and a relatively low drain voltage (~ 0.1V). This transfer characteristic is used to determine the CNP of the graphene sheet, and concomitantly the Fermi energy of the graphene at the applied tunnel voltage. Figure 3b shows measured currents as a function of applied tunnel voltages between graphene sheets and Au electrodes, separated by thin Al2O3 barriers with thicknesses ranging from 2 to 4 nm. The measured current exponentially increases with increasing applied voltage and is significantly increased with reduced Al2O3 thickness, indicating the measured currents are dominated by the tunneling mechanism. To confirm the tunneling mechanism, a Fowler–Nordheim plot (log(I/V2) vs. 1/V) is shown in Figure 3c. The plot clearly shows the transition from Fowler–Nordheim tunneling (linear) indicated by the gray-shaded region to direct tunneling (logarithmic). The transition between these two mechanisms confirms the measured current between the graphene sheet and the base Au electrode relies upon the tunneling across the Al2O3 barrier.41 As noted, in the FET configuration shown in Figure 3a, we could simultaneously measure a source-drain current through a graphene sheet along with a tunneling current (Figure 3d). Here, the applied tunneling voltage works as a gate voltage in the FET configuration. The transfer characteristics in Figure 3d exhibits the conventional ambipolar behavior of source-drain current in a graphene FET, with a CNP voltage of +0.3 V. This CNP value is reliable since the tunneling current is several orders of magnitude smaller than the FET current for low values of the gate voltage. We then studied the light emission from nanoparticles on graphene tunnel junctions. A typical device used in these experiments is shown by the bright field micrograph in Figure 4a. A graphene sheet covers the entire image, and the regions patterned with nanoparticles are shown as darker gray. When we applied a tunnel voltage on the devices, stronger light emission is observed in the


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nanoparticle region compared to the bare graphene region. We analyzed the spectral distribution of the light emission from nanodisks with different diameters. Here, a 2-nm thick Al2O3 layer was utilized to ensure sufficient tunneling current at an applied tunnel voltage of −2V. As shown in Figure 4b, the spectral peak in the emitted light (marked as dots) appeared at longer wavelength as the diameter of the nanodisk increased. Interestingly, the emission spectra show good agreement with the gap plasmon-based reflection spectra of the nanodisk structures (marked as lines). Smaller nanodisks, exhibiting less light absorption in the reflection spectra, generated less light than larger nanodisks. The consistency between these spectra lead us to attribute the light emission to the excitation of gap plasmons via inelastic electron tunneling. To clarify the mechanism of the observed light emission, we measured light emission spectra from bare graphene tunnel junctions without nanoparticles. Some degree of light emission is observed in the region without nanoparticles. However, as shown in Figure 4a and Figure S2 in the Supporting Information, the light intensity is ~2 orders of magnitude lower than in the regions with nanoparticles. Regarding the source of this remnant light emission, thermal radiation from graphene can be excluded, since control devices without tunnel junctions (which had much larger injection current) did not show electroluminescence.42,43 Light can also be generated based on band-to-band recombination in bare graphene, though the quantum yield is low.44 However, the light emission observed in our experiments cannot be explained entirely by carrier recombination. The expected peak position (roughly twice the Fermi energy) based upon band-to-band recombination lies at much higher energy than the observed spectra (Fig. S3a in the Supporting Information). In addition, the light emission is roughly the same for positive and negative bias voltages (Fig.S3b in the Supporting Information), despite the fact that the charge-neutrality point (CNP) is shifted toward positive voltages. If the light emission were primarily due to band-to-band


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recombination, the emission would be asymmetric around the zero point, and the spectral response would be strongly bias dependent. Alternatively, these results support inelastic tunneling as the main source of light emission, an explanation that is consistent with the observed symmetric emission response. In particular, we observed that the spectral response is independent of applied voltage within the allowed tunneling voltage range for a given dielectric thickness (Figure S4 in the Supporting Information), and the light emission from asymmetric nano-rods is polarized along the short axis of the structure (Figure 4c). From this behavior we can conclude that the light emission in the nanoparticle regions in Figure 4b is dominated by the out-coupling of excited gap plasmons. In tunneling device structures, including MIM and STM structures, the corresponding plasmonic modes can be electrically excited by inelastic tunneling electrons. In inelastic tunneling, the excess energy of the tunneling electrons is transferred to excite the plasmon of the tunneling structures. The excited plasmon decays to generate light, whose energy and directionality is determined by the gap plasmon.26 Generally, the quantum efficiency of light emission based on inelastic tunneling is limited to ~10-7-10-4 due to the low percentage of electrons that participate in inelastic tunneling compared to elastic tunneling and the poor out-coupling efficiency of gap plasmons in typical metal-insulator-metal geometries. However, as opposed to an electron-hole recombination process, a tunneling-based device can be used for ultrafast light generation, and therefore can have applications in ultrafast optical communication.45 In our case, graphene is used as an ultrathin counter electrode, allowing the tunneling of electrons from the base Au electrode which excite gap plasmons of individual nanoparticles to generate light. The calculated external quantum efficiency of the light emission in our devices is measured as 10-6~10-7 depending on the sample.


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Notably, with our measurement system (a Si-based CCD), the range of the Al2O3 thickness allowing sufficiently detectable light emission is limited to a range of only 2~3 nm. An Al2O3 layer less than 2 nm is too thin to support the bias voltages needed for visible or near-infrared emission, while an Al2O3 layer thicker than 3 nm exhibits tunneling current too low to generate a detectable amount of light. Due to the dependence of the gap plasmon mode on Al2O3 thickness (Figure 2d), the allowed emission spectra with 2~3 nm Al2O3 are limited to the near-infrared (Figure 4b). To tune the gap plasmon toward higher energy, we deposited an additional Al layer on top of the graphene tunnel junction, which was subsequently oxidized to AlOx (6 nm), “sandwiching” the graphene between two dielectric layers. Then, nanoparticles were created on top of the upper dielectric layer. In this strategy, the tunneling current magnitude remains sufficiently large to generate detectable light emission, as it is only determined by the initial thin Al2O3 layer (2 nm), while the gap plasmon mode is shifted to shorter wavelengths due to the increased distance between the nanoparticles and the base Au electrode (blue lines in Figure 4d). It should be also noted that, with the added AlOx, the difference between the spectra with (faint blue lines in Figure 4d) and without graphene (dark blue lines) is negligible, except for a small blue shift presumably due to a slight increase of the gap thickness from residual PMMA in the graphene transfer process. Without the added AlOx layer, however, the red-shift and the linewidth increase of reflection dips with graphene (faint red lines) compared to without graphene (dark red lines) are quite noticeable. To confirm this effect is not dependent on the dielectric thickness, we also measured the reflection spectra with the same AlOx/Al2O3 structures (Figure 4d, blue), except with the graphene located on top of the second dielectric layer, i.e. contacting the nanoparticles directly (Figure S5 in the Supporting Information). In this case, we also observe the redshift of the gap plasmon dip and large increase in linewidth. Numerical simulation results show that the


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electric field distribution of the sandwiched graphene case is very similar to the one without graphene, while the electric field of the structure with graphene contacting the nanodisk directly is noticeably weaker (Figure 4e), indicating graphene to have a dissipative effect. In a previous study, the dissipative effect of graphene on a nanoparticle plasmon becomes negligible with a distance larger than 5 nm.46 Thus, by sandwiching monolayer graphene between two dielectric layers, the gap plasmon of nanoparticles can be maintained without disruption from graphene in our tunnel junction devices. For further analysis, the linewidths of the reflection dips with and without graphene were determined by Lorentzian fit and compared. When graphene is sandwiched between two dielectric layers, the linewidth increased by 18.9±8.4 meV (n=3) compared to the linewidth without graphene, but when graphene is in contact with nanoparticles, the linewidth increased by 65.0±20.8 meV (n=3). The total linewidth, which represents the dissipative pathway of the plasmon in our structures, can be described by the following equation, Γtotal = Γnon-rad + Γrad + ΓET

(2)

where Γnon-rad, Γrad, and ΓET corresponds to linewidth contributions from non-radiative decay, radiative decay, and electron transfer from nanoparticle to graphene, respectively.47 Since we expect the secondary dielectric layer (6 nm) atop the graphene to block direct electron transfer, the difference of the linewidth increase between the two cases corresponds to ΓET, measured as 46.1 meV. This barrier to direct electron transfer allows us to preserve the gap plasmon. In addition, the insertion of monolayer graphene changes the gap between the nanoparticle and base electrode by only 4% (i.e. 0.34 nm of 8.34 nm), which also has negligible effect on the gap plasmon. We also measured EL from the nanoparticles with sandwiched graphene (Figure 4f). In this case, we again observed the EL spectra to be in good agreement with the corresponding reflection 13 ACS Paragon Plus Environment

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spectra. Notably, the EL spectrum is blue-shifted compared to the one before adding the AlOx layer (Figure 4b). Thus, we could engineer the EL spectra simply by adding an additional dielectric layer on top of graphene tunnel junctions without the dissipative effect of graphene on gap plasmons. To demonstrate the utility of our device architecture, we produced single nanoparticle light sources as well as larger-area (millimeter-scale) light emission devices. As shown in Figure 5a, we assembled individual nanodisks separated by 5 µm on a graphene tunnel junction and measured light emission. We observed emission at the location of individual nanoparticles, which demonstrates our system can be used as a single ultrasmall light source. In this study, we largely used nanoparticles created by electron beam lithography. However, placing virtually any kind of plasmonic nanoparticle on our graphene tunnel junction platform can also lead to light emission. For example, we placed chemically synthesized colloidal nanoparticles on a graphene tunnel junction and observed light emission consistent with the plasmonic mode of the nanoparticles (Figure 5b). We also assembled nanoparticles on a graphene tunnel junction over large areas (~cm2) using nanoimprint and by merely annealing a thin gold film, with light emission observed in both cases (Figure 5c and d). These results indicate graphene tunnel junctions can be used as a scalable platform for light emission from isolated nanoparticles with our simple method.

CONCLUSION In summary, we developed a graphene-based light emission device architecture that allows a single nanoparticle atop graphene to be used as an ultrasmall light source driven by inelastic tunneling. We used monolayer graphene as an ultrathin and transparent electrode to electrically excite gap plasmons of a single film-coupled nanoparticle. Light emission from the gap plasmon is tuned from the near-infrared to the visible range by adding an additional dielectric layer on top


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of the graphene tunnel junction. We further showed that sandwiched graphene can be used to minimize damping of the gap plasmon mode, while still acting as an effective counter electrode for inelastic tunneling. An important feature of our scheme is its simplicity: an ultra-small, electrically driven light source can be achieved simply by placing an isolated plasmonic nanoparticle on a graphene tunnel junction. The scalability of our approach is shown by fabricating millimeter-scale light emission tunnel junctions with a conventional microfabrication method. These electrically driven single nanoparticle light sources can prove useful for broad applications in optoelectronics, single-molecule studies,48 nanoscale sensors,49,50 optical rectification,51 quantum plasmonics,37,52 and 2D plasmonics.53


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METHODS Sample fabrication. First, Au/Ti (50/5 nm) electrodes for the tunnel junctions were created on a SiO2 (300 nm)/Si wafer by photolithography and metal evaporation via an electron beam evaporator. Second, a uniform Al2O3 layer was deposited over the entire wafer via ALD. Next, additional Au/Ti (50/5 nm) electrodes for the source/drain electrodes were created adjacent to the tunnel junction electrodes by photolithography/metal evaporation. Then, a single layer graphene sheet (Graphene Supermarket) grown by chemical vapor deposition (CVD) was transferred on the electrodes using a PMMA-assisted wet transfer method, and the PMMA was removed by acetone. Finally, nanoparticles were created on the tunnel junctions using several different techniques: (1) performing electron beam lithography, Au evaporation (30 nm), and liftoff; (2) dispersing colloidal Au particles (100 nm diameter stabilized in citrate buffer, Sigma-Aldrich) (3) using a nanoimprinter (NX-B200, Nanonex) over large areas (~cm2); and (4) annealing a 5 nm thick Au film at 400 °C in Ar. The colloidal Au particles were used after removing citrate buffer by centrifugation and resuspending in deionized water. In general, nanoparticles were directly assembled on the tunnel junctions. In the case of Figure 4a-c, however, nanoparticles were first assembled on a bare SiO2/Si wafer, and then transferred on the tunnel junctions via a PMMAassisted wet transfer method after etching the SiO2 layer. This process enabled better quality particles/devices as well as particle distribution, in some cases. Electrical and optical characterization. Source measure units (Keithley 2450 SourceMeter, Tektronics) were used to apply tunneling/source-drain voltages and to measure corresponding currents. Optical characterization was performed using an inverted microscope (ECLIPSE Ti, Nikon) with an EM-CCD camera (iXon3, Andor) to observe light emission. Light emission and reflection spectra were measured by an imaging spectrometer (Acton SpectraPro SP-2300,


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Princeton Instruments) equipped with a CCD camera (Pixis 400, Princeton Instruments). A broadband white light source (LDLS, Energetiq) was used for the reflection measurements. Numerical simulation. COMSOL Multiphysics was used to optically model the nanodisk arrays under plane-wave illumination. Only one fourth of a unit cell was simulated due to the symmetry available in the structure using the appropriate PEC and PMC boundary conditions. Perfectly matched layers were used with scattering boundary conditions normal to the light propagation direction. A refractive index of n=1.6 was used for both Al2O3 and AlOx, while the dielectric function of graphene54 was assumed to be isotropic.


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ACKNOWLEDGMENT This work was supported by grants from the National Science Foundation (NSF Grant No. ECCS 1610333 to S.N., D.Y., S.-H.O.), Seagate Technology (D.A.M., S.-H.O.). This work was also supported in part by the Air Force Office of Scientific Research under Award No. FA9550-14-1-0277 (S.N., S.J.K.) and a startup grant from Rice University (P.B.). Device fabrication was performed at the Minnesota Nanofabrication Center at the University of Minnesota, which receives partial support from the NSF through the National Nanotechnology Coordinated Infrastructure (NNCI). Portions of this work were also carried out in the University of Minnesota Characterization Facility, which received capital equipment funding from the NSF MRSEC.

SUPPORTING INFORMATION The Supporting Information is available free of charge on the ACS Publications website. Fabrication process of graphene-metal tunnel junction hybridized with nanoparticles, optical characterization of gap plasmons of nanoparticles, light emission spectrum in a graphene tunnel device without nanoparticles, Electroluminescence spectra of the nanodisk array as a function of applied voltage, comparison of light emission spectrum before and after nano-disk array assembly, and reflection spectrum of the structures with graphene right beneath nanoparticles.

CONFLICT OF INTEREST The authors declare no competing financial interest.


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FIGURES

Figure 1. Plasmonic nanoparticles on a graphene tunnel junction for electroluminescence. (a) Schematic image of light emission from a nanoparticle array using a graphene tunnel junction (left) and underlying mechanism (right). A single plasmonic nanoparticle generates light when a voltage is applied across the tunnel barrier. The tunneling electrons lose their energy by exciting gap plasmons of the single nanoparticle, and the excited gap plasmons decay into far-field radiation. Graphene acts as an ultrathin transparent counter electrode for tunneling, while minimizing disruption of the plasmon mode. (b) Optical image of a tunnel junction device array. (c) Scanning electron microscopy (SEM) of a graphene tunnel junction (inset, scale bar: 50 µm).


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Figure 2. Optical characterization of gap plasmons. (a) SEM image of Au nanodisk array. (b) Reflection spectra (solid) of nanodisks with different diameters (50, 65, and 80 nm) and corresponding simulation results (dotted). Periodicity of the nanodisk arrays and Al2O3 thickness are fixed at 400 nm and 6 nm, respectively. The reflection dips are determined by gap plasmon modes of a single nanoparticle. (c) Reflection spectra of asymmetric nanorods (65 nm × 130 nm) dependent on the polarization of incident light. Inset image shows an SEM image of an asymmetric nanorod. When the polarization of incident light is aligned along the short axis of the rods, the light absorption in the reflection measurement increased. The green arrow indicates the higher mode absorption along the long axis (90°). (d) Reflection spectra of nanodisks assembled on different Al2O3 layer thicknesses. The periodicity and the diameter of the nanodisks are fixed at 400 nm and 65 nm, respectively. (e) Reflection spectra of nanodisks with 300, 400, and 500 nm periodicity. The Al2O3 thickness and the diameter of the nanodisks are fixed at 6 and 60 nm, respectively. The reflection spectra are maintained regardless of the periodicity, supporting the gap plasmon of individual nanodisk governs the reflection spectra. (f) Side view of a spatial map of electric field surrounding a nanodisk with an 80 nm diameter at a resonance condition. The electric field is highly squeezed inside the gap between a nanodisk and an underlying electrode, indicating the observed reflection spectra is attributed to gap plasmon. (g) Reflection spectra of nanodisks (50,65, and 80 nm diameter) with (blue) and without (red) graphene electrode. Reflection dips are red-shifted and the linewidth of the dips increase when graphene is added.


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Figure 3. Electrical characterization of graphene tunnel junction. (a) Schematic diagram showing the electrical measurement of tunneling current and source-drain current. (b) Tunneling current density of the graphene tunnel junction devices dependent on the thickness of the Al O layer deposited by ALD. (c) Fowler–Nordheim plot of tunneling devices in (b). Fowler–Nordheim tunneling dominates in the gray-shaded region, while direct tunneling dominates in the other. (d) Tunneling current (orange dots) and source-drain current (blue line) of a graphene tunnel junction as a function of tunneling voltage. 2

3


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Figure 4. Electroluminescence (EL) from plasmonic nanoparticles on graphene tunnel junctions. (a) Optical image of nanodisk arrays on a graphene tunnel junction (left) and corresponding EL image (right). Stronger light emission is observed from the regions where nanodisks are assembled. (b) Spectra of EL (dots) and reflection (solid lines) of nanodisk arrays (diameters 50, 65, and 80 nm) of a graphene tunnel junction. Inset image shows a schematic side view of the structure. The light emission spectra are in good agreement with the corresponding reflection spectra, indicating the gap plasmon of a single nanoparticle governs the light emission. (c) Polarization-dependent EL spectra of asymmetric nanorod (65 × 130 nm) array. The polarization direction of emitted light along the short axis strongly supports our gap plasmon based mechanism. Inset: SEM of an asymmetric nanorod. (d) Reflection spectra of nanodisks without (red) and with (blue) an additional AlO layer on top. Spectra with graphene are shown in lighter color. Inset images show schematic structures of corresponding cases. Note, after adding AlO , there is a negligible difference between the spectra with (faint blue) and without graphene (dark blue). (e) Cross-sectional electric field maps (on resonance) of a 80 nm diameter-nanodisk with different graphene positions and their schematic structures (right inset images). Note the sandwiched graphene has a negligible effect on the electric field distribution, while the graphene in direct contact with the nanoparticle has a noticeable dissipative effect, due to electron transfer. (f) Spectra of EL (dots) and reflection (solid lines) of nanodisk arrays on graphene tunnel junctions with added AlO on top (red arrow in inset). Inset contains a cross-sectional schematic of the structure. The light emission spectra can be tuned by an additional dielectric layer on top of the graphene tunnel junctions, without disrupting the plasmon mode. x

x

x


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Figure 5. Light emission from a single nanoparticle to millimeter-scale tunnel junction arrays hybridized with nanoparticles. Diverse applications of light emitters based on plasmonic nanoparticles. (a) EL image of single nanodisks with 80 nm diameter. Inset image shows a corresponding dark-field image. (b) EL (dots) and reflection (solid line) spectra of colloidal nanoparticles with 100 nm diameter randomly spread on a graphene tunnel junction. Inset image shows a SEM of the colloidal nanoparticles. (c) SEM of a dense array of nanodisks over large area (~cm2) created by nanoimprint (top) and corresponding light emission (bottom). (d) SEM of nanoparticles created by annealing a thin Au film on a graphene tunneling junction (top) and corresponding EL image (bottom). Large area light emission is achieved in (c) and (d).


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