# Controlled high-current-induced scanning tunnelling microscope modification of C60

Ewan Scougall, Bastien Anézo, Yuri Tanuma, Henry Chandler, Chris Ewels, Renald Schaub, Eleanor E.B. Campbell

► To cite this version:

Ewan Scougall, Bastien Anézo, Yuri Tanuma, Henry Chandler, Chris Ewels, et al. Controlled high-current-induced scanning tunnelling microscope modification of C60. Carbon, 2025, 238, pp.120251. 10.1016/j.carbon.2025.120251. hal-05082631

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Submitted on 23 May 2025

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<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
Controlled High-Current-Induced Scanning Tunnelling Microscope
Modification of C<sub>60</sub>

Ewan S. Scougall¹, Bastien Anézo²,³, Yuri Tanuma³, Henry J. Chandler¹, Chris Ewels²,
Renald Schaub*¹, Eleanor E.B. Campbell*⁴,⁵

¹ EaStCHEM and School of Chemistry, University of St. Andrews, North Haugh, St Andrews
KY16 9ST, UK

² Institut des Matériaux de Nantes Jean Rouxel (IMN), UMR 6502 CNRS, Nantes Université,
44322 Nantes, France

³ Jožef Stefan Institute, Jamova c. 39, 1000 Ljubljana, Slovenia

⁴ EaStCHEM and School of Chemistry, University of Edinburgh, David Brewster Road,
Edinburgh EH9 3FJ, UK

⁵Dept. of Physics, Ewha Womans University, Seoul 03760, Republic of Korea

* Corresponding authors:

Renald Schaub: renald.schaub@st-andrews.ac.uk

Eleanor Campbell: eleanor.campbell@ed.ac.uk

Abstract

We demonstrate controlled STM-induced modification/destruction of Ih-C60 supported on a Cu(111) surface, showing that the molecule is more resilient to high currents for bias voltages greater than ca. 3.5 V. This is due to the enhanced charge transport through the diffuse SAMO orbitals of the molecule with lower probability for electron-vibration coupling than found for resonant low bias transport through π-molecular orbitals. Experimental and theoretical DFT results demonstrate the destruction mechanism comes from C2 emission from the fullerene cage and the formation of smaller fullerenes via sequential emission of C2.

Keywords: STM, fullerene destruction, SAMO, electron transport, sequential fragmentation, Stone-Wales
# 1. Introduction

Buckminsterfullerene, C<sub>60</sub>, is a fascinating molecule that has been used for many years as a model system that can serve to "bridge the gap" between typical molecular behaviour and that of nanoparticles or condensed matter. With its high symmetry and stability, it has been a popular target of Scanning Tunnelling Microscopy (STM) studies where it has been possible to obtain exquisitely detailed images of the electron density and the influence of the substrate on the orientation and charge state of the molecule.[1-9] STM studies also provided the first evidence for diffuse Rydberg-like "superatom molecular orbitals" (SAMO) [10] where, unlike conventional molecular Rydberg states, the electron density is centred at the centre of the hollow cage. Gas-phase femtosecond photoelectron spectroscopy studies later showed the existence of SAMO states in gas phase C<sub>60</sub>.[11, 12] These diffuse states have been shown to produce delocalised nearly-free electron bands in a C<sub>60</sub> monolayer [10, 13] and, although the C<sub>60</sub> bands are too high in energy to be accessible in conventional electronic applications, the energies can be lowered by endohedral doping.[14-16] The possibility of selectively removing C<sub>60</sub> from a monolayer in order to produce a patterned substrate has been demonstrated by injecting a high tunnelling current above an individual molecule which can then be selectively desorbed from the substrate without influencing the neighbouring molecules, as shown for C<sub>60</sub> on Si(111).[17] Low temperature studies of C<sub>60</sub> on different metal substrates showed the influence of the underlying metal with approximately one order of magnitude higher power needed for C<sub>60</sub> to undergo an irreversible degradation on Cu(110) compared to Pb(111) and Au(111).[18] This was attributed to the stronger coupling between C<sub>60</sub> and the Cu(110) surface that effectively quenched the molecular vibrational excitation. It was also shown that the molecular resonance structure was reflected in the power required to degrade the molecule with a lower power needed whenever a new molecular resonance was reached.

In this paper, we extend the study of Schulze *et al.* [18] to higher bias voltages where it is possible to resonantly access the SAMO states. We see a significant difference in behaviour as soon as the SAMO states are excited, with *higher* powers required for decomposition whenever a new SAMO resonance is reached. We attribute this to the efficient charge transport through SAMO states to the metal substrate that has been theoretically investigated for different curved aromatic molecular systems [19, 20] but, to our knowledge, has not yet been demonstrated experimentally.
Further, by combining STM topography imaging with Scanning Tunnelling Spectroscopy (STS) and Density Functional Theory (DFT) calculations, we show that the first product of $C_{60}$ degradation is two distinct isomers of $C_{58}$ with further degradation experiments at the same position leading to the formation of successively smaller fullerenes. We provide evidence for the fragmentation mechanisms initially postulated by Murry *et al.* [21]. The most energetically favourable products are predicted to be a heptagon-containing $C_{58}$ ($C_s$) isomer formed through direct $C_2$ loss, that can convert to a higher symmetry $C_{3v}$ isomer after a Stone-Wales bond rotation. Both isomers are identified on the Cu surface, with the $C_s$ heptagon isomer slightly more thermodynamically stable, inversely to their gas phase ordering. Although the Stone-Wales rearrangement leading to $C_2$ emission is the generally acknowledged mechanism for fragmentation of the cage at relatively low excitation energies, this is to our knowledge the first direct experimental evidence for the mechanism.

## 2. Methods

### 2.1 STM Experiments

All experiments were carried out in an UHV surface analysis system with a base pressure of below $1×10^{-10}$ mbar. The system consisted of a preparation chamber allowing for standard sample preparation and molecular deposition, and a microscope chamber housing a CreaTec low-temperature STM. STM acquisition was performed at liquid helium temperature, ~5 K, with electrochemically etched W tips. The bias voltage was applied to the sample with the tip remaining grounded. Constant-current mode was used for imaging, with both constant-current and constant-height modes used for spectroscopy ($dI/dV$), measured by a lock-in amplifier with a superimposed modulation amplitude of 25 mV and frequency of 1470 Hz. Clean Cu(111) single crystal surfaces were prepared by repeated cycles of sputtering with 1.5 keV $Ar^+$ followed by annealing to 843 K. Monolayers of $C_{60}$ were deposited using thermal sublimation inside a UHV sample preparation chamber onto the clean Cu(111) surface at 250 K. The sample was then annealed at 270 K to form molecular islands. This relatively low temperature prevents the formation of “nanopits” under the fullerenes, as has been previously reported.[6, 29, 30]

Fullerene decomposition was carried out by placing the STM tip above the molecule to be decomposed, setting a positive sample bias, disengaging the feedback loop, and approaching the tip towards the surface by applying a linear $z(t)$ ramp. This caused an exponential increase in the tunnelling current until a sudden drop was observed. The drop in current was caused by
the partial decomposition of the fullerene cage at which point the approach was stopped and
the decomposition current (maximum current observed) recorded. This was repeated about 20
times for each sample bias voltage. All the data reported here was obtained with a positive
sample bias i.e. injecting electrons into empty states of the fullerene/metal system.

## 2.2 *Calculations*

DFT calculations were performed using the AIMPRO code.[37-39] For all calculations, the generalised gradient approximation (PBE) was used for exchange correlation energy coupled with Grimme long-range dispersion correction (D2).[40] Wavefunctions were handled by a localised basis set consisting of 38 and 40 independent Gaussian type orbital functions for C and Cu atoms, respectively. The plane-wave energy cut-off for charge density was set to 175 Ha. Pseudopotentials given by Hartwigsen, Goedecker and Hutter [41] were used to handle core electrons, with a finite electron Fermi temperature of k_B T = 0.025 eV. Geometry optimisations were performed until the positions converged within 10⁻⁵ a₀ and energy within 10⁻⁶ Ha.

For the fullerene-Cu slab systems, 4x4 supercell six layer Cu(111) [1] was optimised in an hexagonal supercell with a *k*-point mesh of 2×2×1 with a lattice parameter of *a*₀=10.12Å. The *c*-axis spacing of the supercell was fixed at >32 Å to avoid interactions between neighbouring systems. The slabs were thereafter held fixed. A single fullerene (C₆₀) was positioned in different orientations [2] and adsorption sites to determine the most stable configuration. Either a hex-hex bond, a pentagon centre or a hexagon centre of the fullerene was aligned above a Cu atom of either the first (on-top) or second layer (hcp-hollow), following Larsson *et al.* [3] (see S.I.). In each case the fullerene was fully geometrically optimised. The structure for the most stable C₆₀ and C₅₈ isomers is taken from the library of Tanuma *et al.*[42], which are C₆₀ in *I*<sub>h</sub> and C<sub>2v</sub> symmetry, C₅₈ in C<sub>3v</sub> and C<sub>s</sub> symmetry. For those presenting paired pentagons we refer the side where they are located as a defect and optimise the structure with the defect pointing away from the slab (up) or towards the slab (down) on the previously established most stable adsorption site. This gives a total of seven different configurations. All thermodynamics and charge transfer information are given in S.I. (Table S.3). Under the assumption that STM images are representations of the system localised density of states (LDOS), the simulated STM images are generated as an isosurface 2D map, integrated over a specific energy window of the computed LDOS.
**3. Results and Discussion**

**3.1 Current and Voltage Dependence of STM-Induced C<sub>60</sub> Destruction**

Constant-current STM images of a C<sub>60</sub> monolayer on Cu(111) are shown in Fig. 1. The fullerenes are chemisorbed on the Cu surface [6, 22] in a close-packed arrangement and remain immobile under the low temperature imaging conditions. The distance between neighbouring fullerenes is 10 Å, nearly corresponding to a 4×4 epitaxial relationship with the underlying substrate.[23] Fig. 1(a) was obtained with an imaging bias, V<sub>bias</sub>, of 0.4 V, corresponding to the LUMO of C<sub>60</sub>, while Fig. 1(b), obtained with V<sub>bias</sub> = 1.5 V, shows the LUMO+1.

Two fullerene adsorption conditions can be seen in the figure: A C₆ face prone to the surface (hex-C₆₀), appearing as a symmetric trefoil image, due to the enhanced electron density on the pentagons (Fig. 1(d)) and a hexagon-hexagon bond tilted towards the surface (hex:hex-C₆₀). The latter appears as a tilted, asymmetric trefoil with one lobe less visible, seen with lower intensity in images at a bias voltage of 1.5 V. All results presented in this paper will be for the hex-C₆₀ adsorption condition with the orientation circled in blue in Fig. 1(b) and shown schematically in Fig. 1(c), unless otherwise noted. These observations are in good accordance with our DFT calculations (Table S.2 of the S.I.), that show the most stable surface absorption arrangement for I<sub>h</sub>-C₆₀ is with a hexagon centre towards the Cu, sitting above a triangular interstitial site (with slight preference for a site with Cu directly beneath it in the next layer). Other orientations with either pentagon centre or hex:hex bond facing the Cu are around 0.5eV less stable. This is in overall agreement with previously studies by Larsson *et al.* [3] Fig. 1(d) shows the 2D LDOS map for the most stable arrangement, interpreted here as a simulated constant-current STM image, showing the threefold symmetry coming from the electron rich top three pentagon π-orbitals.
<figure><img src="image_2.png" /></figure>

Fig. 1. Molecular island of C₆₀ exhibiting a loose hexagonal order of C₆ prone C₆₀ molecules at (a) 0.4 V and (b) 1.5 V sample bias. (c) Schematic picture of C₆₀ adsorption orientation for the molecules circled in blue, used for all results in this paper unless otherwise indicated. (d) Simulated constant-current STM image for a C₆ prone C₆₀ on Cu(111) (hcp-hollow site), obtained by integrating the first electronic levels above the Fermi energy (see Fig. S.6).
<figure><img src="image_3.png" /><figcaption>Fig. 2. High resolution STM images of a C₆₀ monolayer island on Cu(111) before (a) V Bias = 0.4 V, (b) V Bias = 3.7 V, and after (c) V Bias = 0.4 V, (d) V Bias = 3.7 V STM-induced decomposition of the molecule marked with a blue dot in (a). The bias voltage and current used to decompose the cage were 1.5 V and 4.8 µA, respectively. e) Tunnel current versus tip distance for the same decomposition reaction. Inset shows the expected linear tunnel current behaviour on a semi-logarithmic plot. The resulting decomposed fullerene is identified as product A, see later in the main text.</figcaption></figure>

Fig. 2 compares images obtained before and after a decomposition experiment on the molecule indicated with a blue dot in Fig. 2(a). The decomposition procedure reported by Schulze *et al*. was followed.[18] The STM tip was approached towards the selected C₆₀
molecule while holding the sample bias constant. For the image shown in Fig. 2(a), the applied imaging sample bias was set to 0.4 V, close to the C₆₀ LUMO orbital, clearly showing the trefoil geometry. The image taken at 3.7 V, Fig. 2(b), highlights the diffuse S-SAMO orbital of the same C₆₀ monolayer.[7, 10] During approach, the current flowing through the molecular junction was recorded. The *I* vs *z* data for the decomposition experiment is shown in Fig. 2(e). The semi-logarithmic plot clearly shows that the system is in the tunnelling regime where the current flowing through the junction has an exponential dependence on the distance of approach of the tip. For all experiments reported in this paper, the *I* vs *z* behaviour was found to be exponential over the entire range of approach distances investigated. At a certain distance of approach, a sudden jump in the current to a lower value is taken as being indicative of a change to the molecule. Figs. 2(c) and 2(d) show the STM images, obtained for the same imaging bias values as Figs. 2(a) and 2(b), after the decomposition experiment, showing that only the molecule under the STM tip has been altered. The intramolecular trefoil structure has disappeared (0.4 V image) as has the very bright diffuse image due to tunnelling through the C₆₀ S-SAMO (3.7 V image).

In contrast to Schulze *et al.* [18] who investigated the C₆₀ destruction up to a bias voltage of 3 V (encompassing only LUMO+X orbitals), in the present studies the voltage bias is scanned up to 6 V, covering the range where S-, P- and D-SAMO orbitals are accessed.[24] A plot of average tunnel current/power required to induce an irreversible change in C₆₀ is shown in Fig. 3, overlayed on a set of constant-height dI/dV scans of C₆₀ within a molecular island. Multiple constant-height dI/dV spectra were required due to the large range of sample biases covered. The preamplifier gain and current setpoints were adjusted between spectra to avoid saturating the preamplifier or damaging the tip due to very high tunnelling currents and electric fields. Fig. 3(b) shows a single constant-current dI/dV scan spanning the entire range of energies probed for comparison (see S.I. for a discussion of the differences in spectra observed in constant-height and constant-current modes; the peak position is more accurately determined in the constant-height mode scans). The calculated density of states for the C₆₀/Cu (111) system is shown in Fig. 3(c). The agreement with the experimental STS in the LUMO+X region is very good with the peak positions corresponding well with the constant-height dI/dV data (Fig. 3(a)). Although the relative peak heights are not well reproduced in the SAMO region (due to the use of localised basis sets), the peak positions are in good agreement with the experiments. The calculations show a clear downshift in electronic levels of C₆₀ caused by the proximity to Cu.
For low bias values (< 3 V) the experimental decomposition results are consistent with those of Schulze *et al.* on Cu(110).[18] The monotonically decreasing current transforms to an almost flat behaviour of the average power required to induce decomposition (ca. 10 µW) with small variations that are anti-correlated with the resonant LUMO, LUMO+1 and LUMO+2 peaks in the STS. This was interpreted as being due to the increased vibronic coupling that takes place on resonance, leading to an increase in the temperature of the molecule (vibrational heating) and hence slightly lower decomposition power.[18]

For a given set of current and bias values, the molecular temperature reached is, in general, considered to depend on the balance between the heat generated by the inelastic scattering of electrons with molecular vibrational modes and the heat dissipated into the cold substrate,[25] and thus expected to show substrate-dependent decomposition powers. By preparing the C<sub>60</sub>/Cu(111) sample differently so as to alter the molecule-surface interface, we also observed a change in the decomposition behaviour, as discussed in the S.I.

The behaviour shown in Fig. 3 changes quite dramatically as the bias voltage is increased beyond 3 V. Here we see a non-monotonic increase of the decomposition current and corresponding power, with the power required for decomposition at a bias of 5.5 V being more than twice the power required at < 3 V. The decomposition current/power increases strongly whenever a SAMO resonance is reached. This is very apparent at 3.8 V in correspondence with the S-SAMO, and at 4.8 V in correspondence with the P-SAMO. We relate this to the diffuse nature of these SAMO orbitals and the strong hybridisation with the underlying metal. This provides an efficient electron transport channel between the STM tip and metal surface particularly for the $M_L = 0$ (i.e. S and $P_z$) SAMOs with a decreased probability for electronic-vibronic coupling in the molecule compared to the resonant excitation of the LUMO states localised on the carbon cage at bias voltages < 3.6 V. The $P_{x,y}$ SAMOs can form a delocalised nearly-free-electron (NFE) band in the plane of the C<sub>60</sub> monolayer [13] which can also serve to disperse electronic energy throughout the plane, again reducing the coupling to vibrational modes of the decomposed fullerene and raising the tunnelling power required for decomposition.

The maximum in current and power required to decompose the molecules in the SAMO region is seen at 5.5 V. We correlate this to the efficient dissipation of tunnelling electrons by the P-SAMOs, but is higher in energy than the P-SAMO $dI/dV$ peak of 4.8 V. This is due to the Stark effect, which shifts the energy of electronic states, particularly image potential
states, to higher energies in the presence of an electric field.[26] The constant-height dI/dV spectra were acquired with a peak current of around 1 nA, while the decompositions occurred at much higher currents. This means that, during the decomposition experiments, the tip was much closer to the sample than in the dI/dV spectra, causing an increased electric field, and thus Stark shift of the SAMOs to higher energies, with a greater shift for higher order SAMOs.[27] The Stark-shifted SAMOs during decomposition then cause the peak in decomposition current to be at a greater energy than would be expected from the dI/dV spectra. As D-SAMOs were near the edge of the energy range examined in the low current dI/dV spectra in Figs. 3(a) and 3(b), the Stark shift under decomposition conditions moves these outside of the experimental energy range, which is why no decomposition current peak corresponding to the D-SAMOs is recorded. This is further discussed in the S.I.

These results support the hypothesis of efficient electron transport through the diffuse SAMO orbitals as first hypothesised by Petek and co-workers [28] and investigated theoretically for a wide range of curved aromatic molecular systems [19], including C₆₀.[20]
<figure><img src="image_4.png" /><figcaption>Fig. 3. a) Current (left hand axis, blue circles) and power (right hand axis, red squares) at which the first irreversible change to C₆₀ occurs, averaged over 20 experiments for each bias value. The data is superimposed on a series of constant-height dI/dV spectra of a C₆₀ molecule within a C₆₀ monolayer island on Cu(111). Peaks have been normalised to the same maximum I value for each constant-height scan. FES indicates the high bias region where field emission occurs. b) constant-current dI/dV spectrum over the full range of voltages. c) Calculated unoccupied density of states for C₆₀ on copper. Purple curve indicates projected states from the fullerene in the C₆₀/Cu(111) system, top inset shows eigenvalues of the isolated C₆₀ unoccupied electronic states, and dashed lines show the shift of clearly identified C₆₀ states towards the Cu Fermi level.</figcaption></figure>
## 3.2 Decomposition Product

Although it may look as if no trace of $C_{60}$ remains in Fig. 2(d), this is not actually the case and is a consequence of the image settings and the high contrast of the intact $C_{60}$ S-SAMO. A faint image can be seen at the position of the decomposed molecule in Fig. 2(c) for an image bias voltage of 0.4 V. A series of images at different imaging bias voltages is shown in Fig. 4 before (upper row) and after (lower row) decomposition, carried out using a voltage bias of 1.5 V. The position of the targeted molecule is indicated by the dashed and full red circles, respectively. The constant-current $dI/dV$ spectrum of the decomposed $C_{60}$ covering the bias voltage range from 0.1 V to 6 V is shown in Fig. 5(a) (red trace), together with the spectrum produced from the same molecule before decomposition (dashed orange trace). There is a significant difference in the signal from the LUMO+X, $\pi$ molecular orbitals with the peaks broadening and shifting to higher energies. In contrast, the SAMO peaks look very similar and only show a very small shift to higher energies. The obvious presence of the SAMO peaks is a strong indication that the decomposition product remaining on the surface is itself a fullerene. Figs. 5(b) and 5(c) show constant-height spectra around the S-SAMO peak and in the HOMO-LUMO range, respectively. The constant-height spectra provide a more reliable peak position than the constant-current spectra although can only cover a small range of bias voltages as mentioned above and described in more detail in the S.I. Major changes to the density of states in the HOMO-LUMO region can be seen, explaining the change in the STM images of the targeted molecule in Figs. 2 and 4.

The comparison of images at 0.4 V and the lack of the diffuse S-SAMO image at 3.7 V as well as the comparison of the $dI/dV$ spectra in Fig. 5 (dashed green trace) rule out a simple rotation of the $C_{60}$ as an explanation for the observed changes.

Similarly, the formation of a Cu nanopit under the fullerene, due to the high tunnelling current, can be ruled out as an explanation based on a comparison of the STS spectra of $C_{60}$ molecules sitting in a Cu nanopit formed at a high annealing temperature,[29, 30] as shown in the S.I. (Fig. S.3).
<figure><img src="image_5.png" /><figcaption>Fig. 4. Series of STM images acquired at different bias voltages before (a) and after (b) decomposition of the molecule indicated by the dashed (upper) and full (lower) red circles. A decomposition bias voltage of 1.5 V was used. The decomposed species is identified as product A.</figcaption></figure>

In all the decomposition experiments that were carried out (a total of 440), 19 different products were detected (Fig. S.4 in the S.I.). The SAMO peaks are present in the STS for all 19 observed products. The dI/dV spectra of the decomposition products are thus consistent with all the observed decomposition products being fullerenes. Of these, the product shown in Fig. 4 was the most common (product A) and was observed 231 times. The second most common product (product B) appeared on 85 occasions and the topographical images and STS spectra for this product are shown in Fig. 6. The S-SAMO is shifted to slightly higher energy than for product A at 4.26 V compared to 4.23 V in the constant-height spectra, with the main differences between the two products visible in the LUMO range. The small shift upwards in energy of the S-SAMO peaks for Product A and B compared to the original C₆₀ is consistent with a smaller fullerene cage.[28] Product A appears predominantly for low decomposition voltages (< 4 V) while product B is more prevalent for high decomposition voltages (> 4 V). The prevalence of different products for different decomposition voltages is shown in Fig. S.5 in the S.I.
<figure><img src="image_6.png" /></figure>

Fig. 5. dI/dV spectra for the fullerene highlighted in Fig. 4, product A. Dashed orange: before decomposition, solid red: after decomposition. The spectra of an intact hex:hex-C₆₀ are also shown by the green dotted line. (a) constant-current dI/dV. The labels indicate the peak assignments for the intact C₆₀. (b) constant-height dI/dV around the S-SAMO energy. The position of the peak is seen to shift by ca. 0.2 V to higher voltage. (c) constant-height dI/dV for the region around the Fermi energy showing a large change in the density of states of the occupied and unoccupied states after decomposition.
<figure><img src="image_7.png" /><figcaption>Fig. 6. Decomposition product B (blue dashed line) compared to product A (full red line). (a) STM images for different bias voltages; (b) constant-current dI/dV; (c) constant-height dI/dV for HOMO-LUMO regions; (d) constant-height dI/dV for S-SAMO region.</figcaption></figure>

Previous studies of C₆₀ degradation suggest C₂ loss giving C₅₈ [31]. The two most stable resultant C₅₈ isomers in the literature are a C₈-C₅₈ cage containing a single heptagon, and a C₃ᵥ-C₅₈ isomer consisting of three paired pentagons surrounding a single shared carbon atom, with previous modelling suggesting the C₃ᵥ isomer to be slightly more stable in the gas phase [32]. A proposed formation route for the C₈-C₅₈ heptagon isomer occurs through direct C₂ removal from Iₕ-C₆₀, requiring a further Stone-Wales (SW) rotation of two carbon atoms around their bond centre to convert to the C₃ᵥ-C₅₈ isomer. An alternative formation route for the C₃ᵥ-C₅₈ isomer occurs by first SW-rotating a hex-hex bond in Iₕ-C₆₀ and then ejecting C₂ [33]. Our calculated energy schematic for these routes is given in Fig. 7a, including metastable intermediates where the C₂ dimer remains attached to the fullerene cage.
<figure><img src="image_8.png" /></figure>

Fig. 7. (a) Energy schematic of DFT converged metastable states for Stone-Wales transformation of (i) $I_h-C_{60}$ fullerene towards the most stable $C_{58}$ products (iii) $C_{3v}-C_{58}$ and (iv) $C_s-C_{58}$. Energies are calculated for isolated gas phase species (the values are slightly shifted in the presence of Cu). Left: red steps going through $C_2$ emission from (ii) $C_{2v}-C_{60}$ leading to Product **A** and isolated $C_2$. Right: blue steps directly displaying $I_h-C_{60}$ bond breaking and $C_2$ emission resulting in Product **B** and isolated $C_2$. All configurations are within a range of 11eV. $\Delta_1=0.2(2)$ eV, $\Delta_2=0.2(4)$ eV. (b) STM and DFT image comparison for the best matches. Identification of the most stable isomer pointing down as the Product **A** and **B** of the tunnelling current induced decomposition. DFT images are 2D localised density of states (LDOS) map (see Fig. S.6 in the S.I.) of the same computed configuration presenting the most similarities with the experimental STM images (see Fig. S.7 in the S.I.). (top) monolayer of $I_h-C_{60}$ 2D LDOS map matching the STM images of the same monolayer. In (center) the $C_{3v}-C_{58}$ fused-pentagon-down configuration bears a close resemblance to Product **A**, obtained mostly at low decomposition bias voltages. In (bottom) the $C_s-C_{58}$ heptagon-down configuration matches Product **B**, the second most common product, mainly seen for higher decomposition bias voltages.

Our calculations show the $C_s-C_{58}$ heptagon isomer is only slightly less stable than the $C_{3v}-C_{58}$ in the gas phase, in agreement with Murry and Scuseria [21], see also Table S.2 in S.I.
However when bound to the Cu(111) surface this relative stability is inverted (Table S.3 in S.I.), finding the $C_s-C_{58}$ heptagon configuration to be slightly more stable than the fused-
pentagon C₃ᵥ-C₅₈. This is similar to observations made by Bajales *et al.* for STM-imaging of deposited C₅₈⁺ ions on Au(111).[34] The calculations show that for both isomers it is significantly more energetically stable to orient the damaged section of fullerene containing paired pentagons/heptagons towards rather than away from the Cu(111) surface (by 1.4-2.1 eV, see Table S.3 in S.I.). The most stable configuration is with the heptagon (Cₛ) or a hexagon (C₃ᵥ) directly above a Cu interstitial site between three Cu neighbours. The C₅₈ bind much more strongly to the Cu surface than Iₕ-C₆₀ does (by 1.7-1.9 eV), consistent also with higher calculated charge transfer from the Cu to the fullerene cage (1.5-1.6e compared to 1.3e for Iₕ-C₆₀).

It is likely that if the C₆₀ in our experiments is fragmenting to leave a C₅₈ fullerene on the surface, these two lowest energy isomers may be found. To identify our products, we generate 2D LDOS maps from the most stable configurations, as shown in Fig. 7b. The trefoil images observed for Product **A** when imaging at 1.5 V (Fig. 4(b)) are rotated 30° with respect to the initial pre-decomposition image which is consistent with the image expected for the C₃ᵥ-C₅₈ paired-pentagon-down isomer shown in Fig. 7(b)(centre) where we see a slightly tilted symmetry that could explain both the 0.4V and 1.5V images. In Fig. 7b(bottom) (see also Fig. S.7 in the S.I.) the heptagon-down configuration has only one pentagon pointing up without any specific symmetry, in good agreement with the images obtained from Product **B**.

The fragmentation energetics illustrated in Fig. 7a are consistent with the prevalence of Product **A** at low decomposition bias voltages and the prevalence of Product **B** at high bias voltages (Fig. S.5 in the S.I.). We observe smaller energy steps leading to product **A** where there first must be a bond rotation to C₂ᵥ-C₆₀ before the C-C bond breaking, leading to C₂ emission. This is consistent with the lower bias voltage (and decomposition power, Fig. 3(a)) producing predominantly the C₃ᵥ-C₅₈ product (**A**). Conversely, direct Iₕ C-C bond breaking to eject C₂ giving the Cₛ-C₅₈ product (**B**) requires a larger energy difference consistent with the prevailing product **B** population observed for higher decomposition bias voltage (power).

Finally, we consider the result of successive decomposition at the same position. Fig. 8 shows a series of images where multiple successive decomposition experiments were carried out with the images and STS spectra being measured between the decomposition stages. All decomposition stages were done at a bias voltage of 1.5 V. The topographical image changes after each decomposition process. The constant-height dI/dV spectra in the region of the S-SAMO are plotted together in Fig. 8. The S-SAMO shifts to higher energy after each
decomposition by 0.04 V - 0.06 V after each step. This is consistent with the molecule continuing to fragment by emitting C₂ units and reducing in size.

<figure><img src="image_9.png" /></figure>

Fig. 8. Effect of successive decompositions on the S-SAMO. The initial, intact fullerene has an S-SAMO peak just above 4.0 eV. This shifts to higher energies after each successive decomposition, as shown by the colour coded constant-height dI/dV spectra. The corresponding topography images measured for V<sub>bias</sub> = 0.4 V also appear different.

**4. Conclusion**

The selective destruction of C₆₀ on Cu(111) has been studied by inducing a high STM tunnelling current under low temperature, UHV conditions. As was shown previously [18], when the voltage bias is resonant with low-lying unoccupied orbitals (LUMO+X, X = 0 to 3) there is a small but significant decrease in the power required to induce a change in the C₆₀ cage. This is attributed to an enhanced probability for electronic excitation in addition to an efficient coupling of the excitation energy to vibrational modes leading to strong heating and consequent fragmentation of the cage. Here, we show that at higher bias voltages, in the region of the diffuse SAMO states the opposite effect is observed. When the bias voltage is resonant with the S- or P-SAMOs a much higher tunnelling current/power is needed to induce
a change in the fullerene cage. This is due to the efficient current transport through the SAMOs and into the Cu(111) surface, bypassing the carbon cage and thus avoiding vibrational excitation of the molecule.

In addition, the product of the tunnelling current-induced cage destruction was studied by a combination of STM topography imaging, STS and DFT calculations. We find evidence for the formation of the two most stable isomers(s) of C₅₈. STM images and calculations are consistent with fragmentation occurring via C₂ loss from Iₕ-C₆₀ giving the C₅-C₅₈ heptagon isomer, combined with Stones-Wales bond rotations for the C₃ᵥ-C₅₈ triple paired pentagon isomer, as originally predicted by Murry *et al.* [21] Although long regarded as the accepted mechanism for C₆₀ fragmentation, to our knowledge this is the first direct experimental evidence of the proposed mechanism for fragmentation of C₆₀. Evidence is also shown for further successive loss of C₂ to produce smaller fullerenes. The results could be relevant for understanding the origin of a wide range of fullerene species found recently in the AhS meteorite [35] and provide a potentially important pathway for formation of C₂ in the interstellar medium. [36]

**5 Data Availability**

The research data supporting this publication can be accessed at:

https://doi.org/10.17630/cc18c6a4-f269-43e6-96c8-c4b5940cd8ab

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## Acknowledgements

This article is based upon work from COST Action CA 21126 Carbon molecular nanostructures in space (NanoSpace), supported by COST (European Cooperation in Science and Technology). RS and ESS acknowledge support from EPSRC through grant EP/T518062/1. BA and CE acknowledge the EUR LUMOMAT project the Investments for the Future ANR-18-EURE-0012. DFT calculations were performed at the CCIPL (Centre de Calcul Intensif des Pays de la Loire).

## Author Information

### *Authors and Affiliations*

**EaStCHEM and School of Chemistry, University of St. Andrews, North Haugh, St Andrews KY16 9ST, UK**

Ewan S. Scougall, Henry J. Chandler and Renald Schaub

**Institut des Matériaux de Nantes Jean Rouxel (IMN), UMR 6502 CNRS, Nantes Université, 44322 Nantes, France**

Bastien Anézo and Chris Ewels

**Jožef Stefan Institute, Jamova c. 39, 1000 Ljubljana, Slovenia**

Yuri Tanuma and Bastien Anézo.

**EaStCHEM and School of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh EH9 3FJ, UK**

Eleanor E. B. Campbell

**Dept. of Physics, Ewha Womans University, Seoul 03760, Republic of Korea**

Eleanor E. B. Campbell

### *Contributions*

EEBC and RS conceived and supervised the project. ES and HJC performed the STM measurements and analysis. CE supervised the theoretical work. DFT calculations were
performed by BA and YT. ES, EEBC, RS, BA and CE wrote the manuscript. All authors
contributed to the discussion of the results and the editing of the manuscript.

### *Corresponding Authors*

Correspondence to [Eleanor E. B. Campbell](mailto:Eleanor.E.B.Campbell@eebc.ox.ac.uk) or [Renald Schaub](mailto:Renald.Schaub@eebc.ox.ac.uk)

### Ethics Declarations

#### *Competing Interests*

The authors declare no competing interests.