Evaluating Plasmonic Transport in Current-carrying Silver Nanowires

This nanowire based plasmonic circuitry simultaneously transports electrons and surface plasmons. In a shared information support first drop cast a colloidal solution of silver nano wires on a glass cover slip pre-PA with alignment marks. Then using a design software, select extremities of nanowires to precisely fabricate the contacting electrodes by electron beam lithography.

Next, investigate the property of the contacted silver nano wires under a surface plasma leakage radiation microscope. Results of the surface plasma effective index and propagation length can show the evolution of the surface plasma characteristics as a function of biased voltage applied. I first had the idea to develop this method when it became clear that crystalline seven nanowire can be used to transport an electrical command and carry an optical signal simultaneously.

Demonstrating the excitation of a surface plasma in a nanowire will be Ana cus an engineer from my laboratory Dilute the nanowire solution. In ethanol to obtain a nanowire density on the substrate of about one nanowire every 10 square microns. Then pipette 50 microliters of the solution and drop cast it on the pre-PA substrate, blow dry with nitrogen.

Place the substrate decorated with nanowires on a standard calibrated optical microscope, precisely locate the extremities of a series of isolated high quality nanowires with respect to the closest alignment marks. Use the measured positions of the nano wires to design an electrode layout. Incorporate large receiving pads to connect tungsten voltage probes to the electrode.

Reaching the nano wires pipette 160 microliters of polymethyl methacrylate electron beam. Resist then spin coat the nano wire covered substrate with these successive coating parameters. Bake the sample at 170 degrees Celsius for 10 minutes.

Repeat the process for a second layer of resist using the same electron beam. Resist for both layers. Now sputter a thin conductive gold layer on top of the sample.

Transfer the sample to an electron beam microscope equipped for lithography using a faraday cage. Measure the beam current. Then using the alignment marks, calibrate the coordinate system of the electron beam microscope.

Expose the sample following the design layout. Set a step size and determine the dwell time per pixel for a specific dose and beam current. Now develop the sample using a one to three ratio of methyl isobutyl ketone to isopropanol for 45 seconds.

Then to stop the process, dip the developed sample in isopropanol for another 45 seconds. Transfer the substrate in a metal evaporator, operating at a base pressure of eight times 10 to the negative eight millibar at an evaporation rate of 0.1 nanometers per second Deposit an adhesion layer of two nanometers of chromium followed by 70 nanometers of gold. Proceed to a chemical liftoff of the sample using acetone warmed at 70 degrees Celsius for about 1.5 hours.

Set up a surface plasma and leakage microscope, equipped with a high numerical aperture objective and a two axis pizo electric stage. To adjust sample position, prepare a coated Gaussian beam from a near infrared laser. A longer excitation wavelength provides a longer plasma propagation distance.

Align the beam in the microscope. Verify that the diameter of the collated beam over fills the entrance pupil of the microscope for formation of a diffraction limited focal spot at the glass air interface with the help of beam splitters and a series of relay lenses positioned at one exit port of the microscope formed two planes conjugate with the object plane and with the Fourier plane respectively placed two charge coupled device cameras at the location of these conjugate planes. Now using the Pizo electric stage, align the extremity of a selected contacted nanowire to overlap the focal spot.

Adjust the position to maximize light coupling into the plasma mode. 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 or a current meter in the circuitry.

Then monitor the applied bias, the current flowing through the nano wire and the intensity distributions recorded by the two CCD cameras. This scanning electron micrograph shows a typical 10 micrometer long silver nano wire. A closeup view of the left extremity indicates a 200 nanometer width of the nano wire with fivefold symmetry of the crystal growth.

These scanning electron indicate the alignment marks lithographed on the glass substrate. 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. These electrodes are designed by electron beam lithography in subsequent depositions of two nanometers chromium and 70 nanometers gold.

Here a leakage radiation image shows a surface plasma developing in a bare silver nanowire without electrical connections. Note that the focused excitation appears as an intensity saturated region overlapping with one extremity of the nano wire intensity distribution was then recorded in the Fourier plane. The position of the effective index of the plasma mode and its propagation length are extracted from a Lian fit through the data.

This image illustrates the propagation of a leaky surface plasma mode in an electrically connected silver nano wire. Using the information contained in this dual plane imaging scheme, the properties of the surface plasmas are then evaluated when a current flow is established in the nano wire. These representative results show the evolution of the surface plasma properties with voltage bias applied at the electrodes.

The current voltage characteristic of a silver nano wire shows its electro migration induced breakdown. While the plasma effective index and the full width at half maximum are deduced from four a plane analysis and are reported for each bias voltage close to the electrical rupture. The plasma no longer reaches the distal end of the nano wire.

This is confirmed by the leakage radiation images. These high resolution transmission electron micrographs show the presence of surfactants on the surface of the synthesized silver nanowires affecting the current flow. This layer creates a dielectric barrier preventing the current from flowing through the interface.

These graphs show current voltage characteristics for two different connected nanowires measured during an initial bias sweep. Sudden current jumps punctuate the curves indicative of a partial destruction of the surfactant layer located between the gold electrodes and the nano wire surface. An omic contact is established when a subsequent voltage sweep shows a linear characteristic.

After watching this video, you should have a good understanding on how to bring electrical contact to randomly deposited metal nano wires, and to measure the critical parameters governing surface plasma and propagation in this one dimensional wave guides.