Evaluating Plasmonic Transport in Current-carrying Silver Nanowires

Figure 2(a) shows a scanning electron micrograph of a typical 10 mm long Ag nanowire synthesized using the protocol outlined above. The width of the nanowire is here 200 nm as shown in the close up view of the left extremity in Figure 2(b). The figure readily shows the five-fold symmetry of the crystal growth. There are no visible defects on the surface of the nanowire (e.g. kinks, particles, etc.). The synthesis leads to a large distribution of nanowire lengths and widths. Shorter and smaller nanowires are notably visible around the thickest one in Figure 2(a).

Figure 3 displays typical scanning electron micrographs of the alignment marks lithographed on the glass substrate. The four large alignment marks located at the corners of the writing field (solid arrow) are used to report the coordinate systems between the successive lithography steps. To improve position accuracy in the repositioning, a second alignment is performed with the help of the smaller marks located at the each corner of the grid array (dashed arrow). The labeled crosses at the center of the writing field serve to precisely locate the randomly deposited nanowires with the help of an optical microscope.

Figure 4 shows optical images of silver nanowires deposited on a glass substrate before and after electrode fabrication. A diluted solution of nanowires is transferred to the substrate to avoid a high concentration on the surface that may jeopardize the electrical integrity of the connections. In Figure 4(a), the nanowire location is located with the four alignment marks prepatterned on the substrate (see details of the procedure 2). Figure 4(b) shows an image of the same nanowire after electron-beam lithography of the electrodes, subsequent deposition of a 70 nm thick Au layer and a chemical lift-off of the resist. Figures 4(c) and 4(d) show another nanowire before and after fabricating the electrical terminals. The use of gold for forming the contacts over silver brings long-term stability of the fabricated structures; the quality of evaporated Ag layer degrades within a few days. Note that the silver nanowires used in this work have a longer lifetime; the surfactant covering the nanowires slows down the surface degradation by atmospheric contaminants. Other metals with high conductivity like Pd, Pt, or Ti can easily substitute the Au contacts.

A schematic of the leakage radiation microscope used in this work is depicted in Figure 5. An inverted microscope is equipped with a high numerical aperture (N.A.) oil immersion objective (N.A.=1.49). The objective produces on one hand the necessary field distribution to excite a surface plasmon developing in the nanowire and collects on the other hand the plasmon leakage and related optical information. A laser source emitting at 780 nm excites the surface plasmons. Metallic losses in this near-infrared wavelength are significantly reduced compared to visible wavelengths providing thus longer plasmon propagation length. The lateral position of the nanowire and the focal spot are adjusted with the help of three-dimensional piezoelectric-controlled sample stage. Alternatively, galvanometric mirrors can also be utilized to steer the excitation beam. Two CCD cameras record the critical plasmon properties. These cameras are placed in the two conjugate planes of the microscope. A first CCD is placed at a secondary conjugate object plane (II") and records the intensity distribution of the leaky propagating surface plasmon mode. A 4f imaging system with two 100 mm focal length lenses forms a conjugate plane II" of the object. A second 4f imaging system forms a magnified image of the conjugate Fourier plane Σ" of the microscope. The two lenses in this system have focal lengths of L1=100 mm and L2=150 mm. The CCD placed at the Fourier plane Σ" monitors the wave-vector distribution. With this dual-plane leakage radiation imaging, the surface plasmon characteristics such as its effective index and attenuation length are determined. Depending on the nanowire diameter, different surface plasmon modes are excited. The Fourier content of the leaky mode radiating in the substrate features a single line located at the mode effective index. The width of the line is inversely proportional to the propagation length of the surface plasmon¹⁷. If a bound surface plasmon mode is excited, the numerical aperture of the objective is no longer sufficient to collect the associated in-plane wave-vector. However, the excitation of a bound mode is easily confirmed by an out-coupling of the surface plasmon intensity at the distal nanowire extremity.

Figure 6(a) illustrates the propagation of surface plasmons excited in a bare Ag nanowire without electrical connections. The image is recorded in the object plane II". The focused excitation appears as an intensity-saturated region overlapping with one extremity of the nanowire. The two luminous lines located at either side of the nanowire confirm the radiation leakage of the plasmon during its propagation. Figure 6(b) displays the corresponding momentum distribution (k_x/k_o,k_y/k_o) recorded in the Fourier plane Σ". The numerical aperture of the objective (here 1.49) limits the detection window. The inner ring corresponds to the critical angle at the glass/air interface. The line at a constant k_x/k_o in the red box is the signature of the surface plasmon leaky mode. The effective index of the mode N_eff is evaluated by fitting the experimental wave-vector profile with a Lorentzian curve. The peak of the curve corresponds to N_eff and is here determined at 1.02. The full-width at half-maximum of the Lorentzian curve is inversely proportional to the surface plasmon propagation length¹⁷. Using this dual plane imaging, the principal properties of the plasmon mode are completely determined.

Figure 7 shows a typical image illustrating the propagation of a leaky surface plasmon mode in an electrically connected Ag nanowire. The white lines indicate the position of the electrodes. The image generally resembles that of Figure 6(a) obtained for a plasmon propagating in a nanowire without any electrodes. This strongly suggests that the propagation of the mode is not affected by the presence of the additional metal layer at the electrodes. Note that the mode no longer in the region where the electrodes are in contact with the surface because of the different geometry. However, the plasmon propagation and its associated leakage radiation resume in the bare nanowire after the electrode. Figure 7(b) displays the wave-vector distribution of the detected signal; a bright line similar to that observed in Figure 6(b) is the signature of the leaky mode. Using the information contained in this dual-plane imaging scheme, the properties of the surface plasmons are then evaluated when a current flow is established in the nanowire.

Figure 8 shows representative results of the evolution of the surface plasmon properties with voltage bias applied at the electrodes. An electronic control of a scanning tunneling microscope delivers the bias voltage and acquires the data. The advantage of using this type of acquisition device is the possibility to generate handshaking synchronization signals with the CCD cameras at each value of the voltage to simultaneously evaluate the transport of electrical current and propagation of surface plasmons. Figure 8(a) displays the current-voltage characteristic of the nanowire. For small biases, a linear relationship indicates an ohmic transport. Above approximately 0.4 V, the trend is no longer linear; a temperature-dependent resistance, amorphization of the nanowire and electromigration-induced mass transport affect the electron flow¹⁸. Electrical failure of the nanowire occurs at 1.2 V. The plasmon effective index N_eff and the full-width at Half-Maximum (FWHM) deduced from Fourier plane analysis (see Figure 6) are reported for each bias voltage in Figure 8(b). These two plasmon characteristics are independent of voltage as long as the current scales linearly with the bias (<0.4 V). At the onset of electromigration (≈ 1.1 V) N_eff decreases significantly. The evolution of N_eff results from a modification of the nanowire diameter by the electromigration process⁷. The effective index is varying around 1.027 with voltage and the loss encoded in the FWHM of the Fourier signature follows the evolution of N_eff. The variation of these two parameters results from the dynamic morphological change of the nanowire with voltage. Close to the electrical rupture, the plasmon no longer reaches the distal end of the nanowire as shown by Figure 8(c). This is confirmed by the leakage radiation images displayed in Figures 8(d), 8(e) and 8(f). These images are acquired at different operating conditions reported in Figure 8(a) and correspond to a bias-free nanowire (i), a current value above the electromigration threshold (ii) and after electrical failure (iii). The current flow affects both surface plasmon leakage and light scattered at the extremity of the nanowire. After the nanowire structural breakdown at 1.2 V (Figure 8(f)), there is no evidence of a surface plasmon launched in this electrically and structurally damaged nanowire.

Figure 1. Artistic rendering of a basic plasmonic circuitry. An electrically contacted Ag nanowire simultaneously transports electrons as well as optical information encoded in the form of surface plasmon polaritons. Click here to view larger image.

Figure 2. (a) Scanning electron micrograph of a chemically-synthesized silver nanowire drop casted and dried on a substrate. The nanowire is 10 mm long with a section of 200 nm. (b) Close up view of the left termination of the nanowire emphasizing its pentagonal shape. Click here to view larger image.

Figure 3. Scanning electron micrographs of the grid landmark pattern at different magnifications. Click here to view larger image.

Figure 4. Example of contacted Ag nanowires. (a) and (c) Isolated silver nanowires randomly deposited in the grid landmarks. (b) and (d) The same nanowires electrically connected to a set of Au electrodes. The distance between each mark is 10 mm. The electrodes are designed by electron-beam lithography and subsequent chromium (2 nm) and gold (70 nm) depositions. Click here to view larger image.

Figure 5. Schematic of a dual-plane leakage radiation microscope. A collimated laser beam is tightly focused at the glass/air interface by a high numerical aperture objective (N.A.=1.49). If a silver nanowire is laterally displaced to have one extremity overlapping the focal spot, a surface plasmon is excited. Depending on the nature of the excited mode, surface plasmon propagation may be seen as leakage radiation along the nanowire or as a light intensity scattered from the distal end. In both cases, the same objective collects the light and sends it to two different conjugate planes S" and P" for CCD recording. The CCD placed at the conjugate image plane P" provides a magnified image of the sample used to visualize the propagation of the surface plasmon. The CCD placed at the conjugate Fourier plane S" measures the wave-vector distribution contained in the plasmon leakage. A regulated power supply provides the bias applied to the nanowire and a home-made current-to-voltage converter monitors the current flowing. Click here to view larger image.

Figure 6. Leakage radiation images of a surface plasmon developing in a bare Ag nanowire. (a) Record of a plane conjugate to the object plane. The propagation of the mode is readily seen by detecting its leakage along the nanowire. The excitation spot is the intensity saturated region. (b) Intensity distribution recorded in the Fourier plane. This image represents the wave-vector content of the collected light signal. The bright line at a constant k_x/k_o in the red dashed box is the wave-vector distribution of the surface plasmon mode leaking in the substrate. The profile along the k_x/k_o axis (doted line in the Fourier plane) is shown with blue dot. The position of the effective index of the plasmon mode and its propagation length are extracted from a Lorentzian fit through the data (red curve). Here N_eff=1.02 and the propagation length is inversely proportional to the full-width at half-maximum (FWHM) of the fit. Click here to view larger image.

Figure 7. Leakage radiation image of a surface plasmon developing in a Ag nanowire contacted by two electrodes. (a) Record of the conjugate object plane. The dashed white lines outline the position of the electrodes. (b) Record of the corresponding conjugate Fourier plane. Click here to view larger image.

Figure 8. Effect of electron flow on the surface plasmon properties. (a) Current-voltage characteristic of a Ag nanowire showing its electromigration-induced breakdown. (b) Evolution of the plasmon effective index N_eff and the full-width at half-maximum (FWHM) as a function of applied bias. (c) Bias-dependence of the intensity signal scattered at the distal end of the nanowire. (d) to (f) Leakage radiation images taken at different points of the output characteristic labeled i, ii, and iii, respectively. The intensity level reported in (c) is extracted from the red boxes displayed in (d) to (f). Click here to view larger image.

Figure 9. High-resolution transmission electron micrographs showing the presence of surfactant on the surface of the synthesized Ag and Au nanowires. For silver synthesis (a), a 3.5 nm thick surfactant covers the entire surface of the nanowire (arrows). An inhomogeneous coverage of the surface is however typical from Au synthesis (b). For instance, the surface of the nanowire located between the two dashed lines is free of surfactant. The presence of the surfactant on the nanowire surface affects the electronic transport. Click here to view larger image.

Figure 10. (a) and (b) Current-voltage characteristics for two different connected nanowires during an initial bias sweep (semi-logarithmic scales). In both graphs, sudden current jumps punctuate the curves indicative of a partial destruction of the surfactant layer located between the Au electrodes and the nanowire surface. After burning off the surfactant layer, a subsequent bias sweep typically shows a monotonous characteristic similar to that of Figure 8(a). Click here to view larger image.

Figure 11. Position-dependent surface plasmon mode excitation. In (a), the two luminous light distributed on either side of the nanowire confirms the excitation of leaky surface plasmon mode. In (b), the nanowire extremity is slightly displaced in the focal spot to excite the bound surface plasmon. This mode has an effective index greater than the collection angle of the objective and remains undetected during its propagation. However, the presence of a strong signal at the distal end indicates that the bound mode is efficiently scattered out. Click here to view larger image.