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Rattling motion of a single atom in a fullerene cage molecule sensed by terahertz spectroscopy

Upon the discovery of the superatom states in endohedral metallofullerenes (EMFs), the superatom properties have become attractive, because ultrafast motion of the trapped atom modifies the charge distribution in the fullerene cage. However, the observation of ultrafast atom motion in the fullerene cage is very challenging, since dynamical processes take place in the terahertz (THz) frequency range in a picometer region. Here, we report on the THz spectroscopy of single Ce@C82 EMF molecules by using the single molecule transistor geometry. Due to the vibron-assisted tunneling process, two broad photocurrent peaks are observed in the THz spectra and are ascribed to the bending and stretching motions of the encapsulated single Ce atom. This work demonstrates that THz spectroscopy using nanogap electrodes can detect a motion of a single atom, opening a door to ultrafast THz nanoscience on the picometer-scale.

photovoltaic effect. This work demonstrates that THz spectroscopy using nanogap electrodes can detect a motion of a single atom, opening a door to ultrafast THz nanoscience at the sub-nm scale.
Observation of an ultrafast motion of an encapsulated single atom is very challenging due to a huge difference in size between the single atom and the THz wavelength [13][14][15] . One of the difficulties arises from the so-called "diffraction limit". Another difficulty is that, since the THz absorption by a single atom is extremely small, conventional absorption measurements cannot be used. Here, we report on the THz spectroscopy of an ultrafast motion of a single atom by using a single molecule transistor (SMT) geometry. The SMT is a structure in which a single molecule is captured in a subnm gap created between the source and drain metal electrodes fabricated on a field plate. [16][17][18][19] Figure   1a shows a schematic sample configuration used in this work. We used a Ce@C82 endohedral metallofullerene (EMF) molecule, in which a single cerium (Ce) atom is encapsulated in a C82 cage. [4][5][6][7]20 We captured a single Ce@C82 molecule by the nanogap electrodes created by electromigration and fabricated Ce@C82 SMTs. Using the source and drain electrodes as a THz antenna, we tightly focused THz radiation onto a Ce@C82 molecule. 21,22 Furthermore, we have detected a very small THz absorption by a single atom by measuring THz-induced photocurrent. 22,23 Figure 1b shows the scanning electron microscope (SEM) image of a sample used in this study.
We deposited a 10-nm-thick NiCr layer on a high-resistivity Si substrate, which served as a semitransparent backgate electrode. Then, a 30-nm-thick Al2O3 gate-insulation film was grown by atomic layer deposition. We fabricated gold nanojunctions on the surface of the wafer, using the shadow evaporation technique, [24][25][26] and formed the source and drain electrodes by applying the electrical break junction (EBJ) method to the nanojunctions. Details of the feedback control of the EBJ process are described elsewhere. 25 A bowtie antenna shape was used for the source and drain electrodes to achieve a good coupling efficiency between the THz radiation and the single Ce@C82 molecule. As shown in Fig. 1c, the broad antenna resonance covers a frequency range from 0 to 5 THz (photon energy range ħω = 0 -20 meV). The electric-field distribution under the resonance condition (ħω ~ 10 meV) is plotted in the inset of Fig. 1c, showing that the THz field is concentrated 3 in the nanogap region. 27 It was shown that the local THz field in the nanogap is enhanced by a factor of ~10 5 by the plasmonic effect of the metal electrodes. 21,27,28 The sample was glued on a hemispherical Si lens. A dilute toluene solution of Ce@C82 molecules was deposited on the surface of the gold nanojunctions and dried off with nitrogen gas. The sample was then mounted in a vacuum space of a 4 He cryostat. All the measurements were performed at ~4.6 K in a vacuum. Figure 2a shows a color-coded differential conductance (IDS/VDS) map of a Ce@C82 SMT plotted as a function of the source-drain voltage, VDS, and the gate voltage, VG (Coulomb stability diagram). A crossing pattern in the diagram indicates that this device operates as a single electron transistor. Furthermore, vibrational excitation lines are observed at ~5 meV, which suggests that a single Ce@C82 molecule was trapped in the nanogap and served as a Coulomb island. 29 After the transport measurements, we illuminated the sample with a broadband THz radiation from a blackbody light source and searched for a THz-induced photocurrent. By sweeping VG while applying a very small VDS (= 0.1 mV), we measured the THz-induced photocurrent as a function of VG. The black curve in Fig. 2b is the current measured in the dark condition, while the red trace is the THz-induced photocurrent. The peak of the black curve corresponds to the charge degeneracy point of the Ce@C82 SMT. Due to thermal excitation, the Coulomb peak has a slightly asymmetric shape. The linear conductance at the Coulomb peak was 1.2 μS. Note that a small, but finite photocurrent of the order of 0.3 pA was observed near the charge degeneracy point.
Let us discuss how the photocurrent is generated in the Ce@C82 SMT. When we have N electrons on the molecule and the system is in the Coulomb blockaded region, as shown in Fig. 2c, the lowest unoccupied molecular orbital (LUMO) is above the source/drain Fermi levels. Therefore, an electron cannot enter the molecule in the dark condition. However, when the Ce@C82 molecule is illuminated by THz radiation, an electron can hop onto the LUMO by absorbing a vibron (the vibron-assisted tunneling-in process). The electron excited to the LUMO level subsequently escapes the molecule to one of the electrodes, as shown in Fig. 2d. Similarly, when we have N+1 electrons on the molecule and the system is in the Coulomb blockaded region, the highest occupied 4 molecular orbital (HOMO) is below the Fermi levels (see Fig. 2e) and the electron on the HOMO cannot escape the molecule in the dark condition. However, when the molecule is illuminated by THz radiation, the electron on the molecule absorbs a vibron and can escape the molecule (the vibronassisted tunneling-out process). Subsequently, the emptied HOMO is refilled by an electron from one of the electrodes, as shown in Fig. 2f. This is consistent with the fact that the photocurrent measured by sweeping VG gradually decreases as the VG approaches the voltage where the extrapolated vibron excitation line intersects the VDS = 0 line (see the dashed curve in Fig. 2b).
Note here that the photocurrent does not vanish even at VDS approaches 0, as seen in Fig. 2b, which implies that the THz photocurrent is partly generated by the photovoltaic effect. To confirm this, we plot in Supplementary Information the maps for the dark current and the photocurrent measured as a function of VG and VDS on a sample which exhibited a higher conductance (~ 10 μS) (see Fig. S1). The black dotted lines in Figs. S1c and S1d denote the boundaries of the Coulomb diamonds. The shot noise in the photocurrent is large in the single electron tunneling regions. In  30 Here, the subscripts S and D denote the electrode from/to which an electron tunnels, and the subscripts i and o denote the tunnel-in and tunnel-out processes, respectively. Since there always exists unavoidable electrode-and energy-dependent asymmetry in tunneling probabilities, we have a finite current flow even at VDS = 0. 30 For spectroscopic measurements, we used a Ce@C82 SMT itself as a THz detector and performed Fourier transform infrared spectroscopy 23 , as shown in Fig. 3a. Figure 3b shows an interferogram of the THz-induced photocurrent measured on a Ce@C82 SMT when the gate voltage 5 was set at the peak position of the photocurrent (VG = -6.82 V; black arrow in Fig. 2b). A clear center peak and an interference pattern can be seen in the interferogram. By calculating its Fourier spectrum, we obtained a THz spectrum for a Ce@C82 SMT. As seen in Fig. 3c, two broad peaks with opposite polarities are observed at ~5 meV and ~15 meV. In our previous work, 22 we observed the center-of-mass oscillation of the Ce@C82 molecule by using THz bursts generated by femtosecond laser pulses (we replotted the previously obtained THz spectrum for the center-of-mass vibration by a dashed line in Fig. 3c). 22 Since its frequency is too low (~ 500 GHz = ~2 meV) for the measurements with a blackbody light source, the center-of-mass motion was not visible in the present measurements.
To understand the origin of the observed peaks, we have performed transport measurements 29 on a hollow-cage single C84 molecule as a reference (see Fig. S2). There are no vibrational modes below 20 meV. In addition, the calculated THz absorption spectrum 31 of an empty C82 molecule does not show any vibrational modes below ~25 meV. In contrast, new low-energy excitation modes appear when a metal atom is injected into the hollow C82 cage 31,32 . This clearly indicates that the two vibrational modes around 5 meV and 15 meV are associated with the Ce atom in the cage.
Indeed, it is predicted that the Ce atom in the C82 cage has two vibrational modes 33 , as schematically illustrated in the inset of Fig. 3c; the vibrational mode at ~5-6 meV is the bending motion of the Ce atom and the peak at around ~15-16 meV is due to the stretching mode. Previous THz measurements on an ensemble of Ce@C82 molecules observed only one broad peak 33 extending from 1 meV to 12 meV and could not resolve the two separate peaks, most likely due to inhomogeneous broadening. The observation of the two rattling modes has become possible only after inhomogeneous broadening has been removed by measuring a single molecule. Figure 4a plots the interferograms measured at various VDS. When VDS is varied even for a small range (-5 mV < VDS < 5 mV), the polarity and the shape of the interferogram change dramatically. Figure 4b shows the obtained photocurrent spectra measured at various VDS. We can clearly see two broad photocurrent peaks at ~5 meV and ~15 meV, which are attributed to the bending 6 and stretching modes, respectively. 33 Figure 4c plots the magnitudes of the two photocurrent peaks as a function of VDS. The magnitude and the polarity of the photocurrent sensitively depend on the bias voltage. The photocurrents for the two modes at VDS = 0.1 mV have opposite polarities. When a larger positive or negative bias voltage is applied, however, the two components start to have the same polarities. As a result, the photocurrent spectra exhibit a rather complicated behavior. For example, when VDS = 1 mV, the photocurrent peak for the bending mode almost vanishes, while the peak for the stretching mode is suppressed when VDS = -1 mV. Such a complicated behavior can be understood by considering the fact that the single electron photovoltaic effect arises from a subtle imbalance in the tunnel coupling probabilities of the HOMO and LUMO states with the source and drain electrodes. 30 Furthermore, note that the photocurrent peaks are very broad; the peak width for the bending mode is ~ 4 meV and that for the stretching mode is even broader (~8 meV). These broad peak widths are consistent with theoretical predictions that the confinement potential for the Ce atom in the C82 cage is very anharmonic 11,12 . In addition, we have found that the spectrum for the stretching mode is broader than that for the bending mode, which is in agreement with theory 34 ; it predicted that the curvature of the confinement potential of the Ce atom is more nonparabolic along the radius direction of the cage. 34 In summary, we have demonstrated that the THz spectroscopy that uses nanogap electrodes as THz antennas can detect ultrafast rattling motion of a single Ce atom in a C82 fullerene cage. The THz-induced photocurrent spectra exhibited two broad peaks due to the vibron-assisted tunneling process. The vibrational modes observed at ~5 meV and ~15 meV are respectively attributed to the bending and stretching motions of the Ce atom in the C82 cage. By mapping the THz-induced photocurrent as a function of VG and VDS, we have found that the THz-induced photocurrent originates not only from the photoconductive effect but also from the photovoltaic effect that arises from asymmetry in the tunnel coupling of the HOMO/LUMO with the electrodes. The present result have demonstrated that the THz spectroscopy have attained an atom-level spatial resolution by using nanogap electrodes. We believe that such an ultrahigh spatial resolution, together with 7 sub-ps time resolution inherent to THz spectroscopy, will provide a great impact to physics, chemistry, molecular biology, and pharmaceutical sciences.