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Towards plasmonic tunnel gaps for nanoscale photoemission currents by on-chip laser ablation Philipp Zimmermann, Alexander Hötger, Noelia Fernandez, Anna Nolinder, Kai Müller, Jonathan Finley, and Alexander W. Holleitner Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b04612 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 8, 2019
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Nano Letters
Towards plasmonic tunnel gaps for nanoscale photoemission currents by on-chip laser ablation Philipp Zimmermann1,2, Alexander Hötger1, Noelia Fernandez1, Anna Nolinder1, Kai Müller1, Jonathan J. Finley1,2 and Alexander W. Holleitner1,2
1) Walter
Schottky Institute and Physics Department, Technical University of Munich, Am Coulombwall 4a, 85748 Garching, Germany. 2)
Nanosystems Initiative Munich (NIM), Schellingstr. 4, 80799 Munich, Germany.
ABSTRACT: We demonstrate that prestructured metal nanogaps can be shaped on-chip to below 10 nm
by femtosecond laser ablation. We explore the plasmonic properties and the non-linear photocurrent characteristics of the formed tunnel junctions. The photocurrent can be tuned from multi-photon absorption towards the laser-induced strong-field tunneling regime in the nanogaps. We demonstrate that a unipolar ballistic electron current is achieved by designing the plasmonic junctions to be asymmetric, which allows ultrafast electronics on the nanometer scale. KEYWORDS: Nanoscale gaps, nanoelectronics, ultrafast photoemission, laser ablation, photodetector
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In the last years, a remarkable trend in the fabrication of nanometer gaps has evolved with the goal to electronically contact single molecules and atoms
1–4
, and to reach the strong coupling regime in
plasmonic systems with few to single molecules in the gaps 5,6. Within this scope, various techniques to form nanogaps down to the atom scale have been established, such as shadow mask evaporation, electromigration, and the utilization of mechanical break junctions help to avoid thermal runaway
11,15
emission induced electromigration
7–17
. Special feedback mechanisms
and to reduce the width of lithographically defined gaps by field12–14
. In the final nanogap devices, electrons tunnel onto single
molecules and atoms allowing to characterize the involved energy levels as well as the electron-spin and electron-phonon interactions
1–4
. Another well-established tool to structure metal surfaces is the
ablation process induced by nano- to femtosecond laser pulses, as has been reported for plasmonic bowtie antennas 18,19 and nanoparticles 20–22. In particular, laser ablation has been used to increase the surface enhanced Raman signal on nanoparticles
23
. Laser-ablation processes are typically
24
distinguished as thermal and non-thermal
. The first one dominates after the thermalization of the
lattice has taken place, while the second one occurs only during the femtosecond pulse duration, which is much shorter than the lattice heating in metals 24–26. Combining the optical ablation with lithographic approaches seems very desirable particularly for plasmonic junctions, such that plasmonic fields enhance in-situ the laser ablation processes exactly at the positions within the metal structures where nanoelectronic gaps are desired. As we demonstrate in this work, laser ablation processes allow forming sub-10 nm gaps in plasmonic junctions. Before the laser ablation, the metal junctions are pre-structured by electron beam lithography. In the final nanogaps, tunneling processes become equally important to describe the nanoelectronic characteristics (without laser illumination) as well as the photoemission processes of electrons during a femtosecond optical excitation. We utilize scanning electron microscopy (SEM), nanoelectronic tunneling, and photoemission currents to characterize the sub-10 nm gaps. Particularly, the photoemission processes are very sensitive to the specific form and gap distance within the plasmonic nanojunctions moderate laser fields, the multiphoton photoemission process dominates
27–37
. At
33,34,38–40
called strong-field regime, carrier envelope phase effects are reported to appear
, whereas in the so-
27–33,35
. If the electron
quiver amplitude is smaller than the near-field optical decay length, the electrons are accelerated during multiple optical cycles, while at longer quiver amplitudes, the electrons can escape from the metal even in one sub-cycle of the optical pulse
27,30
. To drive a dc unipolar photocurrent as is necessary for
nanoelectronic applications, symmetry breaking of the spatio-temporal dynamics is required. So far, this has been realized by asymmetric few cycle laser pulses
32
, by applying strong dc fields at the emitter
electrodes 33, or by asymmetric plasmonic nanostructures 34,37 and rectifying plasmonic antennas 41,42.
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We demonstrate how to exploit the impact of femtosecond lasers to fabricate sub-10 nm gaps in asymmetric gold nanojunctions on sapphire. The laser-induced nanogaps show current-voltage characteristics consistent with electron tunneling processes. Moreover, the photoemission processes are characterized by intensity dependent photocurrent measurements, indicating that the non-linear photoemission in the nanogaps stems from a multiphoton-absorption process in combination with laserinduced strong-field tunneling processes. We verify the impact of the structural changes to form sub-10 nm gaps on the photoemission processes. The observations are consistent with a non-thermal laser ablation of metal atoms in the nanogaps. We discuss the plasmonic characteristics of sub-10 nm gaps with different nanoscopic morphologies with respect to the photon energy and the overall spectrum of the utilized ultrafast laser. Our results open the way to produce sub-10 nm gaps in plasmonic structures for nanoelectronic application in an on-chip manner by optical means. Figure 1a sketches the photoemission process of an asymmetric metal nanogap excited by a short laser pulse. The laser field is plasmonically enhanced by the diamond shape of the emitter electrode, such that in average, electrons are emitted from this electrode into the vacuum and then to the collector electrode. We fabricate such asymmetric nanostructures from Ti / Au (2 nm / 20 nm) by electron beam lithography on sapphire in an interdigital gate geometry with 42 nanogaps per circuit (Figure 1b and Supporting Information). In total, we characterize 19 circuits with such asymmetric nanogaps. The data shown are measured at a bath temperature of 80 K and a vacuum of about 10-6 mbar. All emitters (‘E’) are connected to one metal contact and a bias voltage, whereas all collector electrodes (‘C’) are connected to a second metal contact (Figure 1b). We measure both the tunnel current Idc without laser illumination and the photoemission current Iemission during laser excitation. The lateral distance between the diamond-shaped emitters (‘E’) and the flat collector electrodes is dgap ~ 50 nm before the laser excitation. Under illumination with near-infrared (NIR) optical broadband pulses, a unipolar photocurrent Iemission is observed for zero bias voltage (Vbias = 0) at the position of the nanojunctions (Figure 1c). The figure shows a corresponding photocurrent map of Iemission overlaid by a scanning electron microscope (SEM) image of the electrodes. The ultra-broadband laser has a photon energy of Ephoton = (0.9 – 1.3) eV, a Gaussian pulse length of ~14 fs, and a spot size of ~ 7.5 µm (FWHM) on the chip. The polarity of Iemission reveals that electrons flow from the emitter to the collector. The symmetry breaking reflects the different geometric forms of the emitters and collectors, as will be discussed below in detail. The dc current Idc without laser illumination between the two contacts shows no measurable signal directly after the fabrication of the nanojunctions (Figure 1d). However, a non-linear Idc-Vbias-characteristic evolves after typically 90 measurement cycles with laser illumination (in total ~200 min). The characteristic can be fitted by an asymmetric Fowler-Nordheim equation (Figure 1d and Supporting Information)43. This
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change in the current-voltage characteristic already points towards a tunneling current in the structurally changed nanoscale gaps after laser excitation. In order to highlight the structural changes of the nanogaps, Figure 2 shows a comparison of SEM images before and after laser illumination. Before the first measurement, a high magnification SEM image of the samples verifies that no material is left in the gap between the emitter and collector (Figure 2a). The length of the initial diamond-shaped emitters is l ~ (255 ± 10) nm. Connector lines with a width of ~90 nm contact them, and they exhibit a gap distance of dgap ~ 50 nm. After laser illumination, the SEM analysis reveals a structural change predominantly of the diamond-shaped emitters (Figure 2b and Supporting Information). Especially at the tips, a brighter region evolves in the SEM images suggesting that this area consists of Au or a mixture of Ti and Au. The overall emitter length increases to l ~ (340 ± 13) nm, and dgap decreases to below 10 nm. This structural change explains the conversion of an insulating current-voltage characteristic to a tunneling one as in Figure 1d. Generally, two processes can explain the structural change: laser ablation caused by a photoemission current or a thermal melting of the tips
24,25,44
. Both effects are maximum in the vicinity of the tips where the photon
intensities are plasmonically enhanced tips
46
35,38,45
, which explains the structural change towards rounded
with decreased gap distances.
For the laser-ablated nanogaps, we observe a superlinear dependence of Iemission vs the peak electric 2 laser field Flaser at zero bias (Vbias = 0) (with the laser pulse energy 𝐸pulse ∝ 𝐹laser ). Figure 3a shows 2 Iemission vs Flaser in a double-logarithmic scale fitted by a power law (𝐹laser )δ resulting in a power law
coefficient of δ = 3.55 with a general noise floor of ~ 100 fA. All measured asymmetric nanogaps show a superlinear behavior with power law coefficients for Iemission in the range between 2.1 ≤ δ ≤ 3.6. The non-linear photoemission processes can be described by the Keldysh theory 47 𝐼emission ∝ exp (−
2𝛷𝑏𝑎𝑟𝑟𝑖𝑒𝑟 1 √1+γ2 + ) arcsinh(γ) − [(1 ]) 2 ħ𝜔 2γ 2γ
,
with barrier the barrier height (in eV), ħω the photon energy, and γ the Keldysh parameter (blue line in Figure 3b). The Keldysh parameter γ is defined as 𝛾=
𝜔√2𝑚𝛷𝑏𝑎𝑟𝑟𝑖𝑒𝑟 𝑒𝐹𝑙𝑎𝑠𝑒𝑟
where m is the electron mass, and e the electron charge
, 47
. In the limit of low laser fields (Keldysh
parameter >> 1), the multiphoton absorption process dominates the emission current (dashed red line in Figure 3b) 47, and the power law coefficient resembles the number of absorbed photons 47. For a work function of gold (barrier ~ 5.1 eV)
48
and a laser energy of Ephoton = (0.9 – 1.3) eV, 6 - 4 photons are
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involved according to this theory. For
