Electrically Driven Highly Tunable Cavity Plasmons - ACS Photonics

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Electrically Driven Highly Tunable Cavity Plasmons Xiaobo He, Jibo Tang, Huatian Hu, Junjun Shi, Zhiqiang Guan, Shunping Zhang, and Hongxing Xu ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01620 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 15, 2019

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Electrically Driven Highly Tunable Cavity Plasmons Xiaobo He,† Jibo Tang,‡ Huatian Hu,‡ Junjun Shi,‡ Zhiqiang Guan,† Shunping Zhang,*, † and Hongxing Xu,*, †, ‡ †School

of Physics and Technology, Center for Nanoscience and Nanotechnology, and Key

Laboratory of Artificial Micro- and Nano-structures of Ministry of Education, Wuhan University, Wuhan 430072, China. ‡The

Institute for Advanced Studies, Wuhan University, Wuhan 430072, China.

*E-mail: [email protected], [email protected].

ABSTRACT: Electrically driven ultrafast plasmon sources with narrow linewidth and wide-range wavelength tunability are desirable choices for integrated nanophotonic circuits. These have a wide range of applications from the optical communication to the data processing. Here, we demonstrate a compact metal-insulator-metal tunneling junction as a plasmon source to meet these requirements simultaneously. It is consisted by a Ag nanowire (covered by a monolayer thiol molecular) crossplaced on a Au nanostripe. Cavity plasmons are excited by inelastic tunneling electrons as applying a bias voltage. The electroluminescence spectrum shows multiple peaks with linewidth as narrow as tens of nanometers, due to the excitation of third to fifth order cavity plasmon modes. The linearly tunable range of the third order cavity plasmon exceeds 200 nm by varying the diameter of the Ag nanowire ~70 nm. Our work can be further developed for the multi-channels and on-chip photonic light sources.

KEYWORDS: electrically driven, cavity plasmons, silver nanowires, tunneling junction, self-

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assembled monolayers, photonic devices

Ultracompact light emitting diodes are key components for high-speed integrated optoelectronic devices1-3. Comparing with conventional semiconductor light-emitting nanodiodes4-6, photon emission from a metal-insulator-metal (MIM) tunneling diode is an ultrafast process that can meet the ever-increasing requirement for high-speed optical communications. Quantum fluctuations of the tunneling current (quantum-shot noise) across the MIM junction excite the localized surface plasmons (LSPs)7-9 within the duration of electrons tunneling (~ 1 fs)10. The emission efficiency, spectrum shape and radiation pattern of the MIM tunneling sources are strongly optimized by using the virtue of the LSPs11-16. However, the emission spectrum of the common MIM tunneling sources covers a broad range of frequency11,15,17, originating from the excitation of bonding dipole plasmons that suffer from the strong radiative damping effect18. Light sources with narrow-band emission spectra are proper for on-chip optoelectronic circuits as they are conveniently tunable. Converting of broad spectra to narrow spectra in the integrated photonic circuits can be realized by integrating bandpass filters or using external optical transitions in the close vicinity19,20. But, these works are at an expense of increasing the device complexity or losing their ultrafast emission characteristic. Thus, further studies of the MIM tunneling sources with a wide-range wavelength tunability, narrow linewidth and no increasing the complexity are necessary. Cavity plasmons attract great interests due to their properties of high tunability21, narrow peak width22 and emission directionality23. Modes of different orders in the nanocavity can be simultaneously excited and resonance peaks are well separated in the far-field spectrum, and positions of emission peaks depend on the dimension of the nanocavity. Specifically, as the lateral


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size of this MIM nanocavity increase (fixed gap distance), cavity plasmons of different order will linearly shift towards the lower frequency region24. This provides a reliable method to control the position of the emission spectrum of the MIM cavity. Remarkably, as the resonance wavelength changes, the linewidth of the cavity plasmons remains around tens of nanometers25, due to their subradiative features. In addition, the high directional emission properties of the cavity plasmons also improves the collection efficiency26. All these advantages make cavity plasmons suitable for the linearly tunable and narrowband integrated light source. Recently, some works27,28 have paid attention to the electrically driven cavity plasmon devices. Here, we demonstrate a tunneling device consisting of a thiol molecule covered Ag nanowire (NW) cross-placed on a Au nanostripe. The device can be utilized as an electrically driven and highly tunable multiband cavity plasmon source. The intersection region of the nanostructure is served as a MIM tunneling junction and an optical nanocavity simultaneously, and the cavity plasmons are excited by inelastic tunneling electrons. The thiol molecules (1-undecanethiol, 1-UT) self-assembled over the surface of Ag NWs, form a dense monolayer coating that builds the stable tunnel barrier. Multiple peaks corresponding to cavity plasmons of different orders were observed in the spectra. These peaks are relating to the diameters of the Ag NWs. Besides, the linewidth of peaks is as narrow as tens of nanometers. Third order cavity plasmon is linearly blue shifted over 200 nm in the emission spectra when the diameters of Ag NWs are varied from 152 nm to 81 nm, revealing the highly sensitive and tunable properties of the cavity plasmons.

RESULTS AND DISCUSSION The geometry of our device is schematically shown in Figure 1a. A cross between a Au nanostripe


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and a Ag NW forms a subwavelength MIM nanocavity. The rest parts of the nanostripe and the Ag NW serve as electrodes29. The Ag NW has a pentagonal cross section according to its five-fold twin structures. It provides an atomic flat bottom surface30 that is crucial to realize a well-defined tunneling barrier and ensures a sharp edge for the MIM nanocavity. The red solid line around the Ag NW indicates the monolayer thiol molecule. Figure 1b shows the energy diagram of the MIM junction with a bias voltage of Vb. Photons start to emit from the MIM junction as the electrons tunnel through the barrier at a sufficiently high applied bias. The Ag NW serves as an optical antenna that shapes the electroluminescence (EL) spectrum and redirects the emission. In order to obtain a stable MIM tunneling junction, we use the 1-UT molecules to substitute the PVP molecules around Ag NWs. PVP molecules around Ag NWs always have a uneven distribution and the thickness can be easily influenced by turning their local environment31. According to Ref 32, the n-alkyl thiols molecules can be adsorbed onto the surface of Ag NWs via the thiol group and form a dense monolayer32 and the number of carbon chain determines the height of the tunnel barrier33. The transmission electron microscope images of the Ag NWs with PVP and 1-UT molecules are shown in Supporting Information Fig. S2b. The densely packed monolayer 1-UT molecules between the Ag NWs and Au nanostripe provide a fixed thickness (~ 1.7 nm) and electrically stable tunnel junction. The fabrication process is shown in the Supporting Information Section 1.


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Figure 1. (a) Schematic illustration of the nanocavity structure. The bottom Au nanostripe and the Ag NW forms a finite width MIM junction. Inelastic electrons tunneling from the Au electrode to the Ag NW excite the cavity plasmons in the MIM junction. Inset: the cross sectional view of the Ag NWs on the Au electrode (the position of the dotted box), and D is the diameter of the NW. The red line is the surfactant 1-UT molecules defining the thickness g of the isolator layer. (b) Energy diagram of the tunnel junction with the applied voltage Vb. The red arrow indicates the generation of a cavity plasmon with energy ℏω ≤ e𝑉𝑏 by inelastic electron tunneling (black arrow). The polyline between Au and Ag indicates the surfactant molecules that defines the barrier height.

To investigate the properties of photon emission excited by inelastic tunneling electrons at the MIM junction, we mount the sample on the stage of an optical microscope. A variable voltage was applied to the junction by two tungsten probes connecting to a sourcemeter. Light emission from the junction was collected by a 50× long working distance objective (Olympus, NA = 0.5) and then recorded by a CCD camera or by a spectrometer (Andor, 550i). All experiments are performed at the room temperature and ambient conditions. The emission spot is superimposed on the scanning electron microscopy (SEM) image of a typical nanowire-on-mirror plasmonic structure is shown in Figure 2a. An enlarged SEM image in Figure 2b shows more details about this cross junction, the diameter of the Ag NWs is 152 nm and the width of Au nanostripe is 300 nm. Photons are emitted


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solely from this cross region. The current versus voltage (I-V) is plotted in Figure 2c. As the voltage increases, the current increases linearly for small voltage and then becomes superlinearly, the solid line shows a fit using Simmons model34 (with image force, barrier height: 2.54 eV, gap size: 1.65 nm). Furthermore, the tunneling current decreases significantly at negative bias voltage, since the work function of silver and gold is different. The I-V curve of our devices is asymmetric, which has been shown in Supporting Information Fig. S3a. EL spectra is much weaker at the negative bias voltage so that it is hardly collected by our optical setup. The spectra of the EL with applied voltage ranging from 1.4 to 1.8 V is shown in Figure 2d. For small bias voltage, the EL spectrum shows one broad peak at 1.41 eV (882 nm), denoted as peak I. As the voltage increases, the intensity of peak I increases simultaneously, while its position remains unchanged. The intensity of peak I as a function of the tunneling current is plot in the inset to Figure 2d, showing approximately a linear dependence. This indicates that the electron-to-photon conversion efficiency is almost independent of the bias voltage. As the applied voltage increases, the energy of inelastic electrons is getting larger so that higher energy cavity plasmons can be excited. The spectra of emitted light meets the high-frequency cutoff condition, ℏω𝑚𝑎𝑥 = |e𝑉𝑏|. The consequence is that high energy peaks start to show up in the EL spectra with increasing voltage. For Vb  1.7 V , two new peaks appear blue-shifted from the peak I, denoted as peak II and peak III. Lorentz fitting to the emission spectrum for Vb = 1.8 V yields three peaks at 882 nm, 818 nm and 748 nm, and their full-width at half-maximum of 68 nm, 27 nm and 52 nm, respectively. The narrow peak widths represent one of the key differences between the cavity plasmons and the conventional bonding dipole plasmons. The latter exhibits a broad peak covering a large spectral window so that the cutoff condition has a more apparent effect on the shape of the EL spectra. This


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is why the emission peak from a dipole antenna plasmon shows a blue shift as the bias voltage increases17, a negative effect that should be avoided for the application of wavelength-division signal encoding. On the contrary, the cavity plasmons driven here have a narrower peak width because of their lower radiative damping. Since the wavelength-dependent local density of state (LDOS) inside the junction region is modulated strongly by these LSPs, the EL spectra from this cross junction show multiple peaks accordingly. The external electron-to-photon conversion efficiency 𝜂𝑄𝐸 = 𝑁𝑃ℎ𝑜𝑡𝑜𝑛𝑠 𝑁𝐸𝑙𝑒𝑐𝑡𝑟𝑜𝑛𝑠 for the Ag NW tunneling source is ~1.85 × 10 ―5 at Vb = 1.8 V. Here, the total number of emission photons is estimated by integrating the corrected EL spectra (for details, see Supporting Information Section 4) and the number of electrons is calculated by the tunneling current at the same time.

Figure 2. (a) SEM image of the devices with EL spot superimposed on top of it. The applied voltage Vb was 1.8 V and the exposure time was 3 s for the EL collection. (b) Enlarged SEM image of the junction, corresponding to the boxed region in (a). The diameter of Ag NWs is 152 nm and the width of Au nanostripe is 300 nm. The red arrow indicates the direction of the tunneling electron across the junction. (c) I-V plot of the device. The black line is fitting according to the Simmons model. (d) EL spectra from the junction for different bias voltage, Vb = 1.4 V (blue), 1.5 V (dark green), 1.6


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V (light green), 1.7 V (khaki) and 1.8 V (orange). Solid black line is the Lorentz fit to the spectrum for Vb = 1.8 V. I, II and III denote the peak positions at 882 nm, 818 nm and 748 nm, respectively. Inset: the relationship of the intensity of Peak I and the tunneling current.

Another advantage of cavity plasmons is their wide-range wavelength tunability upon changing the structural parameters, such as the thickness of the insulation layer and the diameter of the Ag NW. However, the thickness of the insulation layer has already been fixed to the tunnel barrier. Here, we demonstrate tunability by showing that the EL peak can be achieved easily by changing the diameters of the Ag NWs. Figure 3 shows the SEM images of three different Ag NWs-Au nanostripe junctions and the corresponding EL spectrum. As the diameter of the Ag NW decreases from 142 nm, 98 nm to 83 nm, and the EL peak get blue-shift from 924 nm, 806 nm to 738 nm. What’s more, the number of cavity plasmon modes that can be excited is reduced as the diameter decreases. This is because all the cavity plasmons will shift to the higher energy region where they cannot be excited according to the cutoff condition. The tunability of both the emission peaks position and the number of peaks is important for further application of MIM plasmonic source. On the other hand, the characteristic of cavity plasmons is influenced by the size of cavity have been well studied in the previous work21,22,24-26. For our devices, the diameter of Ag NWs will affect the radiation efficiency and LDOS for the MIM nanocavity. The ratio of radiative damping rate to the total damping rate becomes larger and LDOS is also enhancement as the size of MIM cavity becomes smaller. Thus, using a thinner Ag NWs maybe benefit for improving the conversion efficiency.


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Figure 3. The SEM image (left) and EL spectra (right) from different Ag NWs-Au nanostripe junction, for Ag NWs with diameter of 142 nm (a), 98 nm (b) and 82 nm (c). The bias voltage at which the EL spectra were acquired are indicated in each figure.

Comparing with EL, photoluminescence (PL) measurement is much more stable over time to allow a complete study on the spatial, spectral and polarization dependence, etc. Both EL and PL process are proportional to the local photonic density of states which is strongly modulated due to the presence of the LSPs12,35-37. Here, we measured the PL of this MIM nanocavity to further confirm that the exact types of the cavity mode. A continuous laser (wavelength  = 532 nm) was used to excite the PL. The polarization of the laser was aligned perpendicular to the Ag NWs. By subtracting the PL background from the Au nanostripe, we obtained the PL emission from the junction region


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where the PL intensity is obviously enhanced. Figure 4a shows peak positions fitted in PL spectra by Lorentzian function and the black, green and blue markers represent the different resonance peaks. The width of the Au nanostripe is fixed to 300 nm while the diameter of the Ag NWs is changed from 152 nm to 81 nm. In consist with the EL observation, the PL peak shows a gradual blueshift when decreasing the diameter of Ag NW. High order cavity plasmons only appear when the diameter of Ag NWs is larger than 100 nm. The resonance wavelength of the lowest order peak shows a linear dependence on the diameter of Ag NWs. When the diameter is varied from 152 nm to 81 nm, the resonance peaks are blueshifted by nearly 200 nm, revealing the sensitive characteristic of the cavity plasmons. The resonance peak from the EL spectra in Figure 2 and 3 are also included in Figure 4a for comparison. The consistency in the trend of resonance wavelength vs. diameter verifies that the PL and EL are probing the same cavity plasmons. Since the MIM junction can in principle support cavity plasmons perpendicular either to the Ag NW or to the Au nanostripe, a convincing assignment of the observed peaks should be given. To do this, we added an analyzer in the collection path in the PL measurement. For a nearly orthogonal Ag NW-Au nanostripe junction, when the analyzer is perpendicular to the Ag NW, we observe the cavity plasmons mentioned above. On the contrary, when the analyzer is parallel to the Ag NWs, all the peaks disappear and the PL spectrum is similar to that collected from a flat Au film (Figure 4b). What’s more, the overall PL intensity for perpendicular collection is about 10 times compared with that for parallel collection, suggesting that the emitted photons have strong polarization perpendicular to the Ag NW. The polarization dependence of EL spectrum presents the same phenomenon (Supporting Information S8). Therefore, we conclude that the observed EL/PL peaks corresponds to the Fabry-Pérot cavity perpendicular to the Ag NW. This assignment is reasonable


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since the diameter of the Ag NW (D = 145 nm) is smaller than the width of the Au nanostripe (300 nm). The excitation efficiency of single cavity plasmon by single tunneling electron is higher for a mode with stronger spatial confinement. Also, the radiative efficiency is larger for a Fabry-Pérot cavity with shorter length. Another evidence for the mode assignment is the resonance wavelength of the PL peak show minor change when the Au nanostripe is replaced by an Au film. In addition, PL from a 300 nm nanostripe exhibits a small polarization angle dependence (Supporting Information S7), meaning that the Au nanostripe is wide enough to be treated as an Au mirror in the experiments. Therefore, the emitted EL/PL from the junction is majorly determined by the Ag NWs. We calculated the scattering and absorption spectra using finite element method (FEM) simulations (for details, see Methods), to identify the order of the Fabry-Pérot type cavity plasmons in our experiments. The Au nanostripe was modeled by an infinitely large Au film for simplicity. The length of the Ag NWs (> 5 m) was assumed to have no effect on the cavity plasmon perpendicular to the NWs. In this way, the system can be simplified into a 2D problem. Figure 4a shows the relationship between the absorption peak positions and the diameters of the Ag NW, overlaid by the experimental data from the PL and EL peaks. The agreement between the calculation and experiment reveals that the multiple peaks observed in both PL and EL spectra correspond to Fabry-Pérot type cavity plasmons of different orders. In Figure 2, it’s shown that for a Ag NW with diameter of 152 nm, there are three peaks in the EL spectrum, labeled by I, II and III. From Figure 4a, it is now clear that these peaks fall into three distinct regions that correspond to the third (m = 3), fourth (m = 4) and fifth (m = 5) order. The near-field distribution of these LSPs show a pronounce field concentration inside the gap region (Figures 4c-e). In addition, as the optical density of the states harbored in the nanocavity strongly modify the rates of electron tunneling/transition, we


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calculated the LDOS of the system with structure following Figure 2(a) to directly analyze the EL/PL process (Supporting Information S10). The LDOS of the third order mode is the largest compared with the others, which will increase its excitation efficiency. Peak I (m = 3) have both the maximum field enhancement and LDOS, which explains why the intensity of emission light is strongest at the lowest-energy side of the spectrum. In short, the electromagnetic modeling confirms that the multi-peaks in the EL experiment stems from the Fabry-Pérot type cavity plasmons of different order in the MIM tunneling junction. To the best of our knowledge, our work is the first to realize multiple channels electrically driven ultrafast plasmon source. Such voltage dependent multicolor EL emission would provide an additional degree of freedom for optical data processing.

Figure 4. (a) The identification of the order of cavity plasmons in EL and PL experiments by comparison with FEM simulated absorption spectra. The solid markers represent the peak positions obtained in PL spectra and the open markers represent the peaks from the EL spectra. The dash lines


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divide the whole map into five regimes each characterized by the order of the cavity plasmons, m = 1 ~ 5. The experiment data fall into the m = 3 (black square), 4 (green diamond) and 5 (blue triangle) regimes. (b) PL spectra when the collection polarizer is perpendicular to the Ag NW (black) or parallel to the Ag NW (red). The diameter of the Ag NW D = 145 nm. (c)-(e) Calculated electric field distribution of the cavity plasmons for m = 3 (c), 4 (d) and 5 (e), when the diameter of Ag NW is 152 nm.

The electron-to-photon conversion efficiency is determined by the LDOS inside the junction and the radiation efficiency of the cavity plasmons38. For our system, using shorter carbon chain molecules (decreasing the tunneling gap) can in principle help to increase the conversion efficiency (Supplementary Section 12). Further optimization may include using single crystal metal as the bottom electrode to reduce the nonradiative loss, or constructing resonant tunnel junctions in the gap39. In addition, our previous work demonstrated that the cavity plasmons in the nanowire-onmirror configuration is very sensitive to the environment changing21, which may provide an unique method to tune the peak positions of the plasmon source, by, e.g. changing temperature or relative humidity. The stability of the MIM junction is influenced by the magnitude of tunneling current, the strength of applied electric field and the migration of silver atoms. The breakdown field of the insulator was determined to be about 12.9 MV/cm (see Supporting Information Fig. S3b). These factors limit the working time of our device. The stability could be improved by replacing Au NWs instead of Ag NWs, since the mobility rate of Au atom is much smaller. On the other hand, Ag NW networks have been developed as transparent electrodes for flexible electronics40. Extending the current work to a Ag NW network with multiple junctions may be one


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of the future direction for the flexible display with the pixel size down to tens of nanometers. More works on improving the fabrication procedure to control the position and orientation of Ag NWs are required41, and the stability of the junction could be further optimized by packaging the final devices. Furthermore, narrower resonance can be achieved through using the sensitivity nanopatch antenna structures, such as a silver nanocube dimer that the linewidth of scattering spectra can be smaller than 10 nm in theory22.

CONCLUSION In conclusions, we present an electrically driven plasmon source consisting of a Ag NW laying on a gold nanostripe where the cross junction behaves as a MIM tunneling junction. This simple cross geometry provides a convenient way of modulating the emission spectrum by changing the diameter of the Ag NW. We demonstrate a multipeak EL from the cavity plasmon of different order. The lower-order mode linearly shifts over 200 nm in spectra when the diameter of Ag NW varies ~70 nm. The linewidth of the peaks is as narrow as tens of nanometers. The narrow peak width of the cavity plasmons is beneficial for further application in electron-to-photon signal encoding, compared with previous works using broad bonding dipole plasmons. The tunability of the cavity plasmons and the compatibility with other nanophotonic devices will bring new functionality for data encoding of on-chip ultrafast devices.

METHODS Self-assembled monolayers. Ag NWs were synthesized following by the polyol reduction


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method30. The Ag NWs were cleaned by centrifugation and re-immerse for three times before surface modification. Then, the Ag NWs were incubated in 1-UT ethanol solution (10 mM) for 16 hours, at room temperature. After that, the NWs were stored in the 1 mM 1-UT ethanol solution. Devices fabrication. A two-step lithography process was applied to fabricate the devices. Firstly, we fabricated the bottom electrode, by subsequently, electron beam lithography (EBL), reaction-ion etching (using CHF3 gas flow) and thermal evaporation of Au film (60 nm). After lift-off, we got an embedded Au nanostripe as bottom electrode. Then, the modified Ag NWs were deposited onto the electrodes by drop-casting and dry, and a top electrode (1.5 m width and 80 nm height) was fabricated onto the Ag NWs by EBL, Au evaporation, and lift-off. The details fabrication process is shown in the Supplementary Information S1. Electrical and optical characterization. For electrical measurements, two tungsten probes with 10 m diameter were used to contact the Au electrodes with the sourcemeter (Keithley 2634B). The parameters were 5 NPLC (number of power line cycles) for the analog-to-digital converter, integration times of 1 s and the current using the automatic range switch. EL image were recorded by an optical microscope (Olympus BX51) equipped with a 50× long working distance objective (NA = 0.5) and a CCD camera (DVC 710M-00-MW). The emission spectrum were collected by a spectrometer (Andor 550i, 150 lines/mm blazing at 800 nm) and detected by the EMCCD (Andor Newton). The PL spectrum is collected by another microscope using a 100× objective (NA = 0.8) and a Raman spectrometer (Renishaw inVia). The diameter of excitation laser spot was about 1.0 m and collected region was about 2.0 × 2.0 m2. FEM simulations. Full wave FEM simulations were carried using commercial software (COMSOL Multiphysics, V5.2a). Two dimensional model was established considering a Ag NW positioned on


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the top of a Au film. The permittivity of Ag and Au were taken from the Johnson and Christy42. The insulator gaps within the structures has a thickness of 1.3 nm and a refractive index of n = 1.55. The pentagonal cross sections of Ag NWs were taken into account, with a rounding in the sharp edges by a D/15 curvature, where D is the diameter of the NW. The absorption spectra were obtained by the surface integral of Ohmic heat over the whole Ag NW cross section. The scattering spectra were obtained by integrating the Poynting vector on the upper half space of the system. All the spectra were normalized into arbitrary unit for comparison.

SUPPORTING INFORMATION Fabrication process; The thickness of PVP and 1-UT molecule around Ag NWs; I-V characterization for the tunneling devices; Calibration of the electroluminescence spectrum; Dark field spectra of the Au nanostripe and Ag NW cross junctions; Photoluminescence of the Au nanostripe and Ag NW cross junctions; Polarization characteristics of the light emission from the Au nanostripe; Polarization characteristics for the electroluminescence spectrum; Simulated scattering and absorption spectra; Simulation of the LDOS for the Au nanostripe and Ag NW cross junction; Simulation of the radiation efficiency for the nanocavity; Improvement of the conversion efficiency by changing the size of tunnel gap;

ACKNOWLEDGEMENTS This work was supported by the National Key Basic Research Program (Grant No. 2015CB932400), the National Natural Science Foundation of China (Grants Nos. 91850207, 11674255 and 11674256), the National Key R&D Program of China (Grant No. 2017YFA0303504, 2017YFA0205800) and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB30000000).


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CONFLICT OF INTERESTS The authors declare no competing financial interest.

CONTRIBUTIONS S.P.Z. conceived the idea. X.B.H. and J.B.T. prepared the devices and performed the experiments. H.T.H. performed the electromagnetic calculations. J.J.S. help with the PL measurements. S.P.Z, X.B.H. and J.B.T. analyzed the data. S.P.Z, X.B.H., and H.T.H. wrote the manuscript. All authors discussed and commented on the manuscript.

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TOC Graphic


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Figure 1. (a) Schematic illustration of the nanocavity structure. The bottom Au nanostripe and a Ag NW forms a finite width MIM junction. Inelastic tunneling electrons from the Au electrode to the Ag NW excite the nanocavity plasmons in the MIM junction. Inset: the cross sectional view of the Ag NWs on the Au electrode (the position of the dotted box), and D is the diameter of NW. The red line is the surfactant 1-UT molecule defining the thickness g of the isolator layer. (b) Energy diagram of the tunnel junction with the applied voltage Vb. The red arrow indicates the generation of a nanocavity plasmon with energy by inelastic electron tunneling (black arrow). The polyline between Au and Ag indicates the surfactant molecules that defines the barrier height.


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Figure 2. (a) SEM image of the devices with EL spot superimposed on top of it. The applied voltage Vb was 1.8 V and the exposure time was 3 s for the EL collection. (b) Enlarged SEM image of the junction, corresponding to the boxed region in (a). The diameter of Ag NWs is 152 nm and the width of Au nanostripe is 300 nm. The red arrow indicates the direction of the tunneling electron across the junction. (c) I-V plot of the device. The black line is fitting according to the Simmons model. (d) EL spectra from the junction for different bias voltage, Vb = 1.4 V (blue), 1.5 V (dark green), 1.6 V (light green), 1.7 V (khaki) and 1.8 V (orange). Solid black line is the Lorentz fit to the spectrum for Vb = 1.8 V. I, II and III denote the peak positions at 882 nm, 818 nm and 748 nm, respectively. Inset: the relationship of the intensity of Peak I and the tunneling current.


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Figure 3. The SEM image (left) and EL spectra (right) from different Ag NWs-Au nanostripe junction, for Ag NWs with diameter of 142 nm (a), 98 nm (b) and 82 nm (c). The bias voltage at which the EL spectra were acquired are indicated in each figure.


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Figure 4. (a) The identification of the order of nanocavity plasmons in EL and PL experiments by comparison with FEM simulated absorption spectra. The solid markers represent the peak positions obtained in PL spectra and the open markers represent the peaks from the EL spectra. The dash lines divide the whole map into five regimes each characterized by its order of the nanocavity plasmons, m = 1 ~ 5. The experiment data fall into the m = 3 (black square), 4 (green diamond) and 5 (blue triangle) regimes. (b) PL spectra when the collection polarizer is perpendicular to the Ag NW (black) or parallel to the Ag NW (red). The diameter of the Ag NW D = 145 nm. (c)-(e) Calculated electric field distribution of the nanocavity plasmons for m = 3 (c), 4 (d) and 5 (e), when the diameter of Ag NW is 152 nm.


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Electrically Driven Highly Tunable Cavity Plasmons - ACS Photonics

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