Evaluating Plasmonic Transport in Current-carrying Silver Nanowires

Summary

Silver nanowires can simultaneously transport electrons and optical information in the form of surface plasmons. A procedure is described here to realize such a shared circuitry and the limitations at propagating both information carriers are evaluated.

Abstract

Plasmonics is an emerging technology capable of simultaneously transporting a plasmonic signal and an electronic signal on the same information support^1,2,3. In this context, metal nanowires are especially desirable for realizing dense routing networks⁴. A prerequisite to operate such shared nanowire-based platform relies on our ability to electrically contact individual metal nanowires and efficiently excite surface plasmon polaritons⁵ in this information support. In this article, we describe a protocol to bring electrical terminals to chemically-synthesized silver nanowires⁶ randomly distributed on a glass substrate⁷. The positions of the nanowire ends with respect to predefined landmarks are precisely located using standard optical transmission microscopy before encapsulation in an electron-sensitive resist. Trenches representing the electrode layout are subsequently designed by electron-beam lithography. Metal electrodes are then fabricated by thermally evaporating a Cr/Au layer followed by a chemical lift-off. The contacted silver nanowires are finally transferred to a leakage radiation microscope for surface plasmon excitation and characterization^8,9. Surface plasmons are launched in the nanowires by focusing a near infrared laser beam on a diffraction-limited spot overlapping one nanowire extremity^5,9. For sufficiently large nanowires, the surface plasmon mode leaks into the glass substrate^9,10. This leakage radiation is readily detected, imaged, and analyzed in the different conjugate planes in leakage radiation microscopy^9,11. The electrical terminals do not affect the plasmon propagation. However, a current-induced morphological deterioration of the nanowire drastically degrades the flow of surface plasmons. The combination of surface plasmon leakage radiation microscopy with a simultaneous analysis of the nanowire electrical transport characteristics reveals the intrinsic limitations of such plasmonic circuitry.

Introduction

Plasmonics aims at merging electronics and photonics in a shared physical support via the mediation of an electron density wave called a surface plasmon polariton^1,2,3. Surface plasmon can travel in various waveguide geometries and interfaces. Among them, metal nanowires are especially desirable. As quasi-one dimensional structures they drastically confine the plasmon field to deep subwavelength scale while acting as electrical interconnects capable of sustaining an electron flow as depicted by the artistic drawing of Figure 1.

Both surface plasmon propagation and electron transport are sensitive to structural inhomogeneities of the nanowire (e.g. kinks, crystalline defects, etc.). Because they can grow as a single crystal with few defects, chemically-synthesized metal nanowires⁶ typically provide improved transport performances over amorphous metal nanowires fabricated by top-down approaches (e.g. electron beam lithography)¹². The realization of a plasmonic network unit cell requires transferring the nanowires from a colloidal solution to a glass substrate. Without any specific complex prepatterning like surface functionalization¹³ or self-assembly techniques¹⁴, nanowires are generally randomly oriented on the substrate. This uncontrolled distribution of orientations drastically complicates the electrical connection of the nanowire to an outside power source.

In this article, randomly oriented chemically-synthesized silver nanowires are successfully contacted by source and drain electrical terminals. To this purpose optical microscopy is combined with electron-beam lithography to precisely locate the nanowire and create electrical contacts^15,16. A characterization procedure evaluating the electro-plasmonic performances of the circuitry is described. After electrode fabrication, the contacted nanowires are transferred to a surface plasmon leakage radiation microscope for analyzing the effect of an electron flow on the propagation of surface plasmons. The microscope uses an inverted base equipped with a high numerical aperture oil-immersion objective and two charge-coupled device (CCD) cameras placed at the conjugate object plane and conjugate Fourier plane, respectively. These two conjugate planes provide complementary information on surface plasmon properties. Details of the propagation are directly inferred from image plane analysis, while the momentum distribution is visualized by Fourier plane imaging⁹.

Surface plasmons are excited in an individual nanowire by focusing a near-infrared laser beam in a diffraction-limited spot at the glass/air interface. When a nanowire extremity is aligned inside the focal region, the scattered incident laser light creates a broad distribution of wave-vectors, some of them resonant with the excitation of a surface plasmon. The propagation of this surface wave is visualized either by collecting the leakage of the mode emitted in the substrate or by observing the plasmon scattered at the nanowire distal end. The propagation length and effective index of the leaky surface plasmon mode are measured by analyzing the intensity distributions in a dual-plane leakage radiation microscopy.

Once a surface plasmon develops in the nanowire, the drain and source terminals at each extremity of the nanowire are connected to a regulated voltage supply. The CCD cameras monitor in real time the surface plasmon properties as a function of current flowing through the nanowire. For each value of the electrical transfer characteristic, the effective index and the propagation length of the surface plasmon mode are determined. This procedure enables to estimate the limitation of a nanowire-based circuitry to simultaneously sustain the transport of electrons and plasmons⁷.

Protocol

1. Synthesis of a Colloidal Solution of Silver Nanowires

Silver nanowires (Ag NWs) with smooth surface and homogeneous structure are synthesized using a modified polyols-assisted method⁶. In a typical synthesis of Ag NWs, the following procedures are carried out step by step:

1. Pour 12 ml ethylene glycol (EG) in a 100 ml precleaned, round-bottomed capped flask.
2. Heat the solution at 150 °C for 1 hr in the oil bath (dimethylsilicone) with magnetic stirring at 260 rpm.
3. Add 4 ml AgNO₃ (0.2 M in EG solution) by dropping. During this process, the color of the reaction solution changes from colorless to transparent bright yellow, which indicates the growth of silver crystal seeds.
4. Slowly inject 4 ml poly-(vinylpyrrolidone) (PVP) solution (the concentration is 22.2 mg/ml in EG solution) in the reaction solution with a pipette. In the reaction, EG serves as both the solvent and reducing agent, and the PVP enables the silver single crystal to grow along a certain direction.
5. Cover the reaction flask with a cap. After 150 min, the color of the solution changes from light yellow to sage green. Finally the colloid appears turbid silver-grey.
6. Stop the reaction by placing the flask in an ice-cold water bath to decrease the temperature of the solution.
7. Transfer 1 ml of the colloidal solution to a centrifuge tube filled with 3 ml of acetone.
8. Centrifuge the tube at 600 x g for 25 min.
9. Remove the supernatant and add 4 ml of DI water.
10. Repeat steps 1.8-1.9 at least 3x to remove the excess of EG and PVP.

2. Preparing Macroscopic Alignment Landmarks on a Glass Substrate by Electron-beam Lithography

1. Clean a microscope cover glass N°1 ½ calibrated for oil immersion microscopy in a diluted concentrated soap solution [1:3] using an ultrasonic bath.
2. Rinse first in a large volume of DI water, followed by a second wash with isopropanol. Blow-dry the cover slip with nitrogen.
3. Pipette 160 ml of poly(methyl methacrylate) (PMMA) electron beam resist (see table of reagents) and spin coat it the NW covered substrate with the following successive coating parameters:
   1. 10 sec at a speed of 4 x g and an acceleration of 4 x g/sec.
   2. 1 sec at a speed of 350 x g and an acceleration of 200 x g/sec.
   3. 60 sec at a speed of 700 x g and an acceleration of 200 x g/sec.
4. Bake the sample at 170 °C for 10 min on a hot plate.
5. Repeat steps 2.2-2.4 to produce a second layer of resist to facilitate liftoff at the end of the procedure. Use the same electron-beam resist for both layers.
6. Sputter a thin conductive gold layer (20 nm) on the top of the sample before proceeding to electron beam lithography.
7. Transfer the sample to an electron-beam microscope equipped for lithography. Using a Faraday cage, set the beam current to 150 pA.
8. Expose the sample following the design layout of the grid landmarks with a step size set at 48 nm, a dwell-time per pixel at 0.0243 msec for a dose of 150 µA/cm².
9. Develop the sample using MIBK (methyl isobutyl ketone): IPA (isopropanol) with a ratio [1:3] for 45 sec. Stop the process by dipping the sample in isopropanol for another 45 sec.
10. Transfer the substrate in a metal evaporator operating at a base pressure of 8 x 10^-8 mbar. Deposit at an evaporation rate of 0.1 nm/sec 2 nm of Cr (adhesion layer) followed by 70 nm of Au.
11. Proceed to a chemical lift-off of the sample using acetone warmed at 70 °C for about 1.5 hr.

3. Deposition of the Nanowires on the Substrate

1. Dilute the nanowire solution in ethanol to obtain a nanowire density on the substrate of about one nanowire every 10 µm².
2. Pipette 50 µl of the solution and drop-cast it on the prepatterned substrate. Blow dry with nitrogen.

4. Locating the Nanowires by Optical Microscope

1. Place the substrate decorated with nanowires on a standard calibrated optical microscope.
2. Precisely locate the extremities of a series of isolated high-quality nanowires with respect to the closest alignment marks.

5. Fabrication of the Electrodes by Electron Beam Lithography, Thermal Evaporation, and Chemical Lift-off

1. Use the measured positions of the nanowires to design an electrode layout. Incorporate large receiving pads (50 µm x 50 µm) to connect tungsten voltage probes to the electrode reaching the nanowires.
2. Pipette 160 ml of poly(methyl methacrylate) (PMMA) electron beam resist (see table of reagents) and spin coat the nanowire-covered substrate with the following successive coating parameters:
   1. 10 sec at a speed of 4 x g and an acceleration of 4 x g/sec.
   2. 1 sec at a speed of 350 x g and an acceleration of 200 x g/sec.
   3. 60 sec at a speed of 700 x g and an acceleration of 200 x g/sec.
3. Bake the sample at 170 °C for 10 min on a hot plate.
4. Repeat steps 5.2-5.3 to produce a second layer of resist to facilitate liftoff at the end of the procedure. Use the same electron-beam resist for both layers.
5. Sputter a thin conductive gold layer (20 nm) on the top of the sample before proceeding to electron beam lithography.
6. Transfer the sample to an electron-beam microscope equipped for lithography. Using a Faraday cage, set the beam current to 150 pA.
7. Using the alignment marks, calibrate the coordinate system of the electron-beam microscope.
8. Expose the sample following the design layout with a step size set at 48 nm, a dwell-time per pixel at 0.0243 msec for a dose of 150 µA/cm².
9. Develop the sample using MIBK (methyl isobutyl ketone): IPA (isopropanol) with a ratio [1:3] for 45 sec. Stop the process by dipping the developed sample in isopropanol for another 45 sec.
10. Transfer the substrate in a metal evaporator operating at a base pressure of 8 x 10^-8 mbar. Deposit at an evaporation rate of 0.1 nm/sec, 2 nm of Cr (adhesion layer) followed by 70 nm of Au.
11. Proceed to a chemical lift-off of the sample using acetone warmed at 70 °C for about 1.5 hr.

6. Transfer to a Surface Plasmon Leakage Radiation Microscope

1. Set up a surface plasmon leakage microscope equipped with a high numerical aperture objective (N.A.>1) and a two-axis piezoelectric stage to adjust sample position.
2. Prepare a collimated Gaussian beam from a near-infrared laser. Longer excitation wavelength provides longer plasmon propagation distance. Align the beam in the microscope. The diameter of the collimated beam should overfill the entrance pupil of the microscope. This insures the formation of a diffraction-limited focal spot at the glass/air interface.
3. With the help of beam splitters and a series of relay lenses positioned at one exit port of the microscope, form two planes conjugate with the object plane and with the Fourier plane (objective back focal plane), respectively. Place two charge-coupled device (CCD) cameras at the location of these conjugate planes.
4. Using the piezoelectric stage, align the extremity of a selected contacted nanowire to overlap the focal spot. Adjust the position to maximize light coupling into the plasmon mode.
5. Connect a regulated power supply to the receiving pads of the electrical terminals with tungsten tips mounted on three-dimensional mechanical probers. Insert a low gain current-to-voltage converter (gain 10mA/V) or a current meter in the circuitry.
6. Monitor the applied bias, the current flowing through the nanowire and the intensity distributions recorded by the two CCDs cameras.

Results

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 lengt...

Discussion

The synthesis uses an excess of chemical surfactant remaining bound to the surface of the nanowire as illustrated in the transmission electron micrograph of Figure 9(a). This layer creates a dielectric barrier preventing the current to flow through the electrode-nanowire interface. Figures 10(a) and (b) show typical examples of the current-voltage characteristics. Sharp current steps at certain biases punctuate the curves. These steps occur when the current den...

Materials

Name	Company	Catalog Number	Comments
DMEM	Invitrogen	ABCD1234
Ethylene glycol (EG)	Sinopharm Chemical Reagent Co., Ltd	T20111130
PMMA	Allresist	AR-P 679
Acetone Analar Normapur	VWR Prolabo	20066.296
Isopropanol (IPA) Analar Normapur	VWR Prolabo	20842.298
AgNO3	Sinopharm Chemical Reagent Co., Ltd	20080826
Poly-(vinylpyrrolidone) (PVP)	Aladdin Chemistry Co., Ltd	1041671-31744
Dimethysilicone	Sinopharm Group Company Limited	H201-500
Propylene Glycol Methyl Ether Acetate	Microchemicals Gmbh	AZ EBR
Inverted optical microscope	Nikon	TE 2000
Microscope objective	Nikon	1.49/100X TIRF Plan-Apo
CCD Cameras (2x)	Andor	Luca-S
Regulated power supply	RHK	SPM 1000
Acquisition data	RHK	SPM 1000
Current to voltage converter	homemade	Gain 10 mA/V
Electron beam microscope	JEOL	FEG 6500
Lithography addon	RAITH	Elphy
Spincoater	PRIMUS	STT15
Thermal evaporator	PLASSYS	MEB 400
Micrometer probing stage (2x)	SÜSS MicroTec	PH110
Piezoelectric stage	Mad City Labs	Nano LP100