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Tracking Optical and Electronic Behaviour of Quantum Contacts in Sub-Nanometre Plasmonic Cavities

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ABSTRACT

Plasmonic interactions between two metallic tips are dynamically studied in a supercontinuum dark-field microscope and the transition between coupled and charge-transfer plasmons is directly observed in the sub-nm regime. Simultaneous measurement of the dc current, applied force, and optical scattering as the tips come together is used to determine the effects of conductive pathways within the plasmonic nano-gap. Critical conductances are experimentally identified for the first time, determining the points at which quantum tunnelling and conductive charge transport begin to influence plasmon coupling. These results advance our understanding of the relationship between conduction and plasmonics, and the fundamental quantum mechanical behaviours of plasmonic coupling.

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A scan showing repeated approach and retraction using electrochemically-deposited AuNP-on-Pt tips, demonstrating the reproducibility.The applied force trace in (b) represents separation changes between tips, with tips approached, retracted and then finally approached into contact. Peak positions in the fitted model are denoted by dashed lines superimposed on the spectra (a). Reduced rates of redshift are found in the G ≈ 1 G0 regions with a discontinuous blueshift seen after the G > 2 G0 transition. Linear rates of amplitude variation are revealed from peak fits after removing the width contribution from the peak intensity. As in Fig. 4, blue shaded regions represent quantum tunneling and yellow shaded regions show conductive contact. (c–e) Extracted mode shifts, amplitudes, and linewidths for the longer wavelength (blue) and shorter wavelength (red) plasmon modes.
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f5: A scan showing repeated approach and retraction using electrochemically-deposited AuNP-on-Pt tips, demonstrating the reproducibility.The applied force trace in (b) represents separation changes between tips, with tips approached, retracted and then finally approached into contact. Peak positions in the fitted model are denoted by dashed lines superimposed on the spectra (a). Reduced rates of redshift are found in the G ≈ 1 G0 regions with a discontinuous blueshift seen after the G > 2 G0 transition. Linear rates of amplitude variation are revealed from peak fits after removing the width contribution from the peak intensity. As in Fig. 4, blue shaded regions represent quantum tunneling and yellow shaded regions show conductive contact. (c–e) Extracted mode shifts, amplitudes, and linewidths for the longer wavelength (blue) and shorter wavelength (red) plasmon modes.

Mentions: We emphasise that the behaviour shown so far can be quite different when the nano-geometry at the tip end is modified. As a contrasting example we show a scan using highly asymmetric AuNP-on-Pt tips, smoothed using piranha solution (Fig. 5a). The extracted mode spectral shifts, mode amplitudes, and mode linewidths are shown in Fig. 5(b–e). Both cantilevers have 40 Nm−1 spring constants, hence the force resolution during approach is limited. In this case, prior to tunnelling (G < 10−7 G0) a higher order mode begins to emerge. The transition into contact is so quick that few intermediate points can be captured, and initially a stable 0.8–1.5 G0 contact is formed (blue shaded background in Fig. 5). As this conductance increases up to the 1 G0 level, it triggers a 4% jump in the mode wavelength (marked as J on Fig. 5(c)) and changes the rate of spectral redshifts caused by screening. The initial BDP resonance quickly weakens with the rise in conductance and the higher order resonance gains intensity. As in Fig. 4, the decreasing amplitude of the BDP mode is accompanied by a rise in the BQP mode, suggesting they are coupled depending on the gap conductivity. The screening shown here is enough to enter into the tunnelling regime but without sufficient current to enter into the conductive regime and form CTPs. Since the conductivity remains below 2 G0, no blueshift from the screening is observed.


Tracking Optical and Electronic Behaviour of Quantum Contacts in Sub-Nanometre Plasmonic Cavities
A scan showing repeated approach and retraction using electrochemically-deposited AuNP-on-Pt tips, demonstrating the reproducibility.The applied force trace in (b) represents separation changes between tips, with tips approached, retracted and then finally approached into contact. Peak positions in the fitted model are denoted by dashed lines superimposed on the spectra (a). Reduced rates of redshift are found in the G ≈ 1 G0 regions with a discontinuous blueshift seen after the G > 2 G0 transition. Linear rates of amplitude variation are revealed from peak fits after removing the width contribution from the peak intensity. As in Fig. 4, blue shaded regions represent quantum tunneling and yellow shaded regions show conductive contact. (c–e) Extracted mode shifts, amplitudes, and linewidths for the longer wavelength (blue) and shorter wavelength (red) plasmon modes.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5016779&req=5

f5: A scan showing repeated approach and retraction using electrochemically-deposited AuNP-on-Pt tips, demonstrating the reproducibility.The applied force trace in (b) represents separation changes between tips, with tips approached, retracted and then finally approached into contact. Peak positions in the fitted model are denoted by dashed lines superimposed on the spectra (a). Reduced rates of redshift are found in the G ≈ 1 G0 regions with a discontinuous blueshift seen after the G > 2 G0 transition. Linear rates of amplitude variation are revealed from peak fits after removing the width contribution from the peak intensity. As in Fig. 4, blue shaded regions represent quantum tunneling and yellow shaded regions show conductive contact. (c–e) Extracted mode shifts, amplitudes, and linewidths for the longer wavelength (blue) and shorter wavelength (red) plasmon modes.
Mentions: We emphasise that the behaviour shown so far can be quite different when the nano-geometry at the tip end is modified. As a contrasting example we show a scan using highly asymmetric AuNP-on-Pt tips, smoothed using piranha solution (Fig. 5a). The extracted mode spectral shifts, mode amplitudes, and mode linewidths are shown in Fig. 5(b–e). Both cantilevers have 40 Nm−1 spring constants, hence the force resolution during approach is limited. In this case, prior to tunnelling (G < 10−7 G0) a higher order mode begins to emerge. The transition into contact is so quick that few intermediate points can be captured, and initially a stable 0.8–1.5 G0 contact is formed (blue shaded background in Fig. 5). As this conductance increases up to the 1 G0 level, it triggers a 4% jump in the mode wavelength (marked as J on Fig. 5(c)) and changes the rate of spectral redshifts caused by screening. The initial BDP resonance quickly weakens with the rise in conductance and the higher order resonance gains intensity. As in Fig. 4, the decreasing amplitude of the BDP mode is accompanied by a rise in the BQP mode, suggesting they are coupled depending on the gap conductivity. The screening shown here is enough to enter into the tunnelling regime but without sufficient current to enter into the conductive regime and form CTPs. Since the conductivity remains below 2 G0, no blueshift from the screening is observed.

View Article: PubMed Central - PubMed

ABSTRACT

Plasmonic interactions between two metallic tips are dynamically studied in a supercontinuum dark-field microscope and the transition between coupled and charge-transfer plasmons is directly observed in the sub-nm regime. Simultaneous measurement of the dc current, applied force, and optical scattering as the tips come together is used to determine the effects of conductive pathways within the plasmonic nano-gap. Critical conductances are experimentally identified for the first time, determining the points at which quantum tunnelling and conductive charge transport begin to influence plasmon coupling. These results advance our understanding of the relationship between conduction and plasmonics, and the fundamental quantum mechanical behaviours of plasmonic coupling.

No MeSH data available.


Related in: MedlinePlus