Plasmon resonance in individual nanogap electrodes studied using graphene nanoconstrictions as photodetectors
We achieve direct electrical read out of the wavelength and polarization dependence of the plasmon resonance in individual gold nanogap antennas by positioning a graphene nanoconstriction within the gap as a localized photo-detector . The polarization sensitivities can be as large as 99%, while the plasmon-induced photocurrent enhancement is 2-100. The plasmon peak frequency, polarization sensitivity, and photocurrent enhancement all vary between devices, indicating the degree to which the plasmon resonance is sensitive to nanometer-scale irregularities.
Sufei Shi, Xiaodong Xu, Paul McEuen and Dan Ralph
We grow single-layer graphene on copper foil using CVD in a furnace . After spin-coating a thick PMMA layer onto the graphene, we wet etch to dissolve the copper foil completely and transfer the graphene to a highly n-doped Si substrate with 300 nm thermal oxide on top. We confirm that the graphene is mostly single layer by optical imaging and Raman spectroscopy. We then define multiple Ti/Au contacts by photolithography. Electron beam lithography is used to define the critical feature, a gold nanowire ~100 nm wide. A highly n-doped silicon substrate is used as a back gate.
We apply two steps of an electromigration technique to make a graphene nanoconstriction coupled to a sub-10 nm gold break junction. In the first step, we use electromigration with electronic feedback  at room temperature in air to break the Au wire and leave a nanoscale gap that will correspond to the high-electric-field region of the plasmonic antenna. The graphene layer under the gold is unaffected by this step. We then narrow the graphene wire into a nanoconstriction without breaking it fully using a 2nd stage of electromigration in vacuum. Because graphene nanoribbons can sustain much higher current density than Au, this requires much larger voltages, 2-5 V, consistent with previous reports. Figure 1 shows an SEM image of the final device.
We perform scanning photocurrent (PC) measurements using a Ti-sapphire tunable continuous wave laser source focused to a 1.2 µm spot size with incident power ranging from 1 µW to 1 mW. We measure the PC and the reflected light simultaneously as we scan the position of the laser spot. Correlation between the reflection image and PC image (Figure 2) shows that a symmetric-in-position PC response arises from the narrowest region of the break junction device, to within the resolution of the laser spot size. A separate antisymmetric-in-position PC signal can also arise due to heating in the electrodes.
The amplitude of the symmetric PC signals for the narrow graphene nanoconstrictions vary strongly as a function of the wavelength and polarization of the incoming light. Figure 3 shows the wavelength response of the PC for a ~5 MΩ contact at room temperature. The PC is sharply peaked at 790 nm, typical for the plasmon resonance of an Au nanostructure, with a full width at half maximum of 40 nm. Figure 4 shows the dependence of the PC on the polarization of the incoming light for a different (R = 80 kΩ) device. The PC varies strongly with the polarization angle in a simple dipole pattern, with a factor of 11 variation from minimum to maximum response. The wavelength and polarization dependence studies show that the PC signal arises from the plasmon resonance of the gold nanogap electrode. The resonance properties vary from device to device, demonstrating the importance of geometrical variations in the performance of optical antennas at the nanometer scale.
Figure 1: SEM image of graphene nanoconstriction embedded in a gold break junction.
Figure 2: Spatially-resolved photocurrent at 800 nm continuous wave laser excitation. The dashed line is the outline of the gold electrode.
Figure 3: Wavelength dependence of the photocurrent from a device with room temperature resistance ~ 5 MW, showing a plasmon resonance at ~790 nm with a full width at half maximum of 40 nm.
Figure 4: Polarization dependence of the photocurrent for a device with room temperature resistance ~ 80 kW at 780 nm excitation, plotted relative to the long axis of the gold wire.
- X.Li et al., “Large-Area Synthesis of High-Quality and Uniform Graphene Films on Copper Foils,” Science 324, 1312 (2009).
- K.I.Bolotin et al., “From ballistic transport to tunneling in electromigrated ferromagnetic breakjunctions,” Nano Lett. 6, 123 (2006).
- S.-F. Shi et al., “Plasmon Resonance in Individual Nanogap Electrodes Studied Using Graphene Nanoconstrictions as Photodetectors.” Nano Lett. 11, 1814 (2011).
Last updated: 22-Jul-2011