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. 2024 Dec 6;24(50):16099–16105. doi: 10.1021/acs.nanolett.4c04824

n- to p‑Type Conductivity Transition of Lu3N@C80 Due to Anisotropic Deformation of Fullerene and Pyramidalization of Endohedral Clusters

Cuiying Pei †,‡,§, Bertil Sundqvist ∥, Zhen Yao §,⊥, Zhipeng Yan §, Guangtao Liu §,⊥, Sudeshna Samanta §, Hong Xiao §, Yaofeng Wang #, Yanpeng Qi ‡,7,8, Ning Chen #, Lin Wang †,*, Yongjun Tian †
PMCID: PMC12349245  PMID: 39643935

Abstract

The endohedral fullerene Lu3N@C80 was examined using in situ high-pressure measurements, which included electrical transport, Fourier-transform infrared spectroscopy, and Raman spectroscopy, in combination with theoretical calculations. Lu3N@C80 was found to undergo a reversible n- to p-type conversion at ∼8.9 GPa. This p-type semiconductor remains stable up to 25 GPa. The fullerene cage collapses at ∼29 GPa, resulting in an irreversible p- to n-type conversion. Raman, infrared, and X-ray absorption spectroscopy reveal that an anisotropic distortion of the carbon cage and a pyramidalization of the planar Lu3N clusters occur during compression. Density functional theory simulations indicate that the p orbitals of C atoms in the fullerene cage primarily contribute to the density of states (DOS). Pressure-induced deformation of the fullerene cage dominates the DOS changes in the conduction and valence bands close to the Fermi level. The findings elucidate the relationship between the conductivity and structural changes in the endohedral clusters.

Keywords: endohedral fullerene, high pressure, structure evolution, conductivity conversion, electrical transport

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The transport properties of fullerenes and related materials have garnered a significant amount of attention since the initial discovery of alkali-doped superconductor K3C60. − C60, under ambient conditions, functions as a semiconductor with a band gap of ∼1.7 eV. In alkali-doped C60, each intercalated alkali atom contributes up to one valence electron, which occupies the t1u lowest unoccupied molecular orbital (LUMO) of C60. Endohedral metallofullerenes (EMFs), another form of doped fulleride, are particularly characteristic of larger carbon cages. They hold the potential to become superconductors with an even higher T c, provided that the internal clusters can supply sufficient charge carriers. However, a majority of them are semiconductors with a very low carrier concentration. A comprehensive understanding of their electronic structures and the charge transfer between the cage and the enclosed cluster will provide valuable insights into the conducting mechanism and further guidance to their potentially fascinating superconductivity.

EMFs demonstrate strong metal–cage hybridization and are characterized by incompletely delocalized metal valence electrons on the cage. − Dramatic changes in the molecular orbital configuration occur when the position of the metal atom changes. For instance, when the La atom moves from the center to an off-center stable position in La@C82, the energy of the La 5d-derived orbitals increases, which is accompanied by a formal charge state transition from 2+ to 3+. When more electropositive metals are encapsulated, the highest occupied molecular orbitals (HOMOs) are primarily localized on the carbon cage, while the LUMO shows a different contribution from the metal cluster based on the electron binding capability. High pressure serves as a potent fundamental parameter, capable of directly deforming fullerene cages and thereby manipulating the interaction and charge transfer between the cage and the endohedral clusters. On the contrary, high pressure could widen the partly occupied 3-fold degenerate t1u band, which transforms Cs3C60 in its A15 phase from an insulator to a superconductor. , Additionally, pressure has also been employed to manipulate the band gap of EMFs. Studies have revealed that the band gaps of Sm@C88 and Sm@C90 decrease significantly with an increase in pressure. Nevertheless, correlating the evolution of the geometric and electronic structures under high pressure remains a formidable challenge.

In this Letter, a high-pressure study on nitride cluster fullerenes (NCFs), Lu3N@C80, is reported. Hall effect and field effect transistor (FET) measurements reveal a reversible electronic transition from the n type to the p type at ∼8.9 GPa, with an n-type amorphous product resulting after an irreversible transition near 29 GPa. The evolution of the metal–cage interaction under high pressure is investigated by spectroscopy and calculations, demonstrating that the C80 cage undergoes anisotropic deformation and that the planar Lu3N cluster pyramidalizes under compression. Theoretical calculations are used to interpret the relationship between the structural evolution and charge carrier-type conversion under high pressure.

Lu3N@C80 (97% pure) was commercially purchased. A Mao-Bell-type diamond anvil cell (DAC) with a diamond culet size of 300 μm was used to apply pressure. The high-pressure spectra and transport measurements were taken as shown in other reports. − A Re sheet was drilled and serves as the gasket. For Raman and X-ray diffraction (XRD) measurements, silicon oil was utilized as a pressure-transmitting medium (PTM). KBr was used as a PTM in Fourier-transform infrared spectroscopy (FTIR) measurements. For electrical transport and EXAFS measurements, we did not use any PTM in DAC. The pressure was calibrated by a ruby luminescence method. A Raman spectrometer (Renishaw inVia) with a laser excitation wavelength of 633 nm, which matches the lowest-energy electronic transitions of X3N@C80 (X is the lanthanide), was used to enhance the resonance Raman signal. In situ high-pressure FTIR (Hyperion 2000) was employed to investigate the structural evolution of the sample. The four-probe method with the van der Pauw configuration was used for electrical transport property measurements. Alternating current impedance spectroscopy measurements were taken with a two-electrode microcircuit configuration using a ZENNIUM electrochemical workstation (ZAHNER-elektrik). The electrical resistivity and Hall coefficient were measured in a temperature range of 2–300 K and a magnetic field range of −9 to 9 T by a physical properties measurement system (PPMS, Quantum Design) with a van der Pauw four-probe method using silver paint for the contacts. High-pressure powder XRD was conducted at synchrotron beamline 15BLU1, Shanghai Synchrotron Radiation Facility (SSRF), with a wavelength λ of 0.6199 Å, and patterns of intensity versus 2θ were recorded. The dioptas and DICVOL programs were used for data integration and structural indexing. The Lu LIII-edge extended X-ray absorption fine structure (EXAFS) spectrum of Lu3N@C80 was recorded in transmission mode at the biological macromolecule station (beamline 1W2B) of the Beijing Synchrotron Radiation Facility (BSRF). Details about the theoretical calculations are presented in the Supporting Information.

Electrochemical impedance spectroscopy (EIS) shows that the electrical resistance of the Lu3N@C80 sample is >1011 Ω below 3 GPa (Figure S1a). By using a field effect transistor (FET) configuration as shown in Figure S1b, Lu3N@C80 was characterized to be an n-type semiconductor under ambient conditions, like Sc3N@C80, in which electrons are donated to the fullerene cage. With an increase in pressure, the resistance of the sample decreases and becomes measurable via the four-probe van der Pauw configuration at ∼8 GPa. The electrical resistance of compressed Lu3N@C80 is shown in Figure a. As the pressure increases, the electrical resistance decreases by an order of magnitude per gigapascal below 10.7 GPa. A plateau between 15.4 and 25.3 GPa is followed by an increase in resistance with further compression. No metallic state is achieved, as evidenced by temperature-dependent resistance measurements (Figure S2). Charge transport within fullerenes fundamentally involves two processes: intramolecular carrier movement and intermolecular charge transfer. Within a fullerene molecule, π-conjugation facilitates the free movement of charge carriers. Because of the weak van der Waals intermolecular coupling, charge carriers in bulk samples are localized within individual fullerene molecules. As the pressure increases, distances between fullerenes decrease, enhancing intercage interaction. Consequently, the conduction and valence bands become broader, the band gap shrinks, and the resistance decreases under high pressure. Furthermore, the charge state of Lu3N@C80 changes significantly compared to that of the empty fullerene due to the transfer of six electrons from the endohedral cluster to the cage. This modified intercage van der Waals binding energy contributes to the decrease in resistance.

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(a) Pressure-dependent resistance of Lu3N@C80 during the compression (filled circles) and decompression (empty circles) processes at room temperature. The inset provides a magnified view of the range between 15.4 and 35.1 GPa, along with the four-probe van der Pauw configuration utilized in the experiment. (b) Comparison of Hall coefficient R H across various compression cycles. Evolution of the (c) carrier concentration and (d) mobility of Lu3N@C80 at room temperature in different compression cycles. Prior to decompression, Lu3N@C80 was subjected to compression up to 29 GPa in round I and 19.5 GPa in round II.

Hall measurements of Lu3N@C80 were taken at 300 K to investigate the type of charge carrier. Two rounds of measurements were performed, extending up to 19.5 and 29.0 GPa. As shown in Figure b, it was found that the Hall coefficient (R H) of Lu3N@C80 at 8.2 GPa and 300 K was negative, which is characteristic for an n-type semiconductor, consistent with observations at ambient pressure. Interestingly, R H turned from negative to positive as the pressure reached 8.9 GPa, implying a carrier-type transition from the n to p type. As the pressure increased from 8.9 to 29.0 GPa, the Hall coefficient decreased by 4 orders of magnitude. Intriguingly, at 29 GPa, R H became negative once more, implying a p- to n-type transition. The n type persisted during decompression down to ambient pressure. The results from round II align with those from round I. However, a striking observation in round II was that upon decompression from 19.5 GPa, Lu3N@C80 maintained its p-type semiconductor nature down to 7.3 GPa, only transitioning to the n type at 1.2 GPa. Furthermore, the absolute value of R H in the sample that recovered to ∼1 GPa in round II was larger than that in round I. Thus, two distinct mechanisms were evident in two decompression cycles from different maximum pressures, even though both recovered products demonstrated characteristics of n-type semiconductors.

To investigate the different transport mechanisms of the samples decompressed from different pressures, the Raman spectra of the samples that recovered from 21.2 to 38.8 GPa were measured at room temperature. The spectra are shown in Figure a and Figure S3. Our findings indicate that Lu3N@C80 is maintained up to 27.3 GPa before collapsing at higher pressures. The transport results indicate that a reversible n- to p-type transition takes place at ∼8.9 GPa, significantly before the cage collapses. Upon further compression beyond 27.3 GPa, the Lu3N@C80 cage collapses, leading to an irreversible transformation into an amorphous phase. The pressure-induced amorphization occurs at pressures similar to those at which the sample transforms from the p to n type in the transport measurements. This suggests that the amorphous carbon phase with collapsed clusters generated at 29 GPa is an n-type semiconductor.

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(a) Raman spectra of Lu3N@C80 recovered from different pressures. (b) FTIR spectra of Lu3N@C80 at 1 atm and 0.8, 2.9, 7.1, and 13.2 GPa. (c) Lu–N bond lengths and (d) sums of all Lu–N–Lu bond angles as a function of pressure.

The pressure-induced changes in the charge carrier concentration (n) of Lu3N@C80 as depicted in Figure c confirm the carrier sign inversion. Under high pressure, the charge carrier concentration increases with pressure, peaking at a maximum pressure of 29.0 GPa, before receding during decompression. Notably, this concentration is still 1 order of magnitude larger at ∼10 GPa compared to the concentration at the corresponding pressure during the compression phase. In the second experimental cycle with a maximum pressure of 19.5 GPa, the carrier concentration follows the same trend as in round I on compression and remains almost unchanged when the pressure is released to <10 GPa upon decompression. The carrier concentration in the amorphous sample decompressed from 29 GPa is ∼1 order of magnitude higher than that in the recovered endohedral fullerene.

The carrier mobility of the sample was also studied at high pressure, as shown in Figure d. We can see that the mobility decreased from 8.2 to 8.9 GPa and then increased to 19.5 GPa, in agreement with the existence of the n- to p-type transition in the lower pressure range. As the pressure increases to ∼27 GPa, the carrier mobility starts to decrease. This is consistent with a p- to n-type conversion due to the pressure-induced collapse of fullerene. When the pressure is decreased from 29.0 to 11.0 GPa, the carrier mobility is found to be as high as 5.22 × 10–4 m2 V–1 s–1. This value is larger than the corresponding value upon compression, which supports a higher electron mobility in the n-type semiconductor than for the holes in the p-type material. In round II, the comparison of data for samples decompressed from different pressures suggests that the mobility of carriers is much higher in the recovered endohedral fullerene than in the collapsed clusters.

To investigate the mechanism behind the n-type to p-type to n-type conductivity conversions, we examined the structural evolution using Raman (Figures S4 and S5), FTIR (Figures S6 and S7), XRD (Figures S8–S10), EXAFS (Figure S11), and calculations under high pressure. The Raman spectra of Lu3N@C80 are shown in Figure S4. The spectra can be roughly divided into four parts (detailed information is presented in the Supporting Information). At a low energy, a peak at 154 cm–1 corresponds to a Lu3N cluster deformation (νdef) mode coupled with frustrated in-plane cluster translation. With an increase in pressure to ∼1.7 GPa, the intensities of all of the modes increase (Figure S5a). In addition, peaks attributed to cages at ∼290 and ∼450 cm–1 disappear, while new cage-dependent and cluster-dependent peaks at ∼500, ∼715, ∼728, ∼743, ∼746, ∼752, ∼760, ∼802, and ∼815 cm–1 appear in the Raman spectra. Given the enhanced peak intensity and the emergence of several new peaks, it is evident that the movement and rotation of both endohedral clusters and cages significantly slow at 1.7 GPa. Subsequently, the intensities of most modes decrease, and the peaks become broad. As shown in Figure S5b–f, we observe vibrational modes of Lu3N@C80 originating from both the cage and the metal cluster. − With an increase in pressure, a majority of Raman vibration modes exhibit blue shifts; however, a few lines at intermediate frequencies show red shifts. Generally, the bond length decreases under high pressure, resulting in a blue shift of the vibration modes, while bond length extension and bond angle changes may cause a red shift of the corresponding vibration modes. In addition, it has been observed that different pressure-induced slopes may be caused by different degrees of interaction between the cluster and the cage. Thus, blue and red shifts, along with varied pressure coefficients of the vibration energies, all signify an anisotropic deformation of the cage.

High-pressure FTIR measurements were also taken to provide insight into the n- to p-type transition (Figures S6 and S7). The FTIR results agree well with the Raman spectra. Notably, the υas(Lu–N) mode (714 cm–1 under ambient conditions) is sensitive to the structure of the M3N cluster. , This 2-fold degenerate vibrational mode splits into two components at 2.9 GPa due to the symmetry reduction (Figure b). This indicates a distortion of the trigonal geometry, suggesting pressure-induced pyramidalization of the planar Lu3N cluster. , Furthermore, the decreasing slope of υas(Lu–N) versus pressure can be attributed to a decrease in the force constant, which agrees with the pyramidalization of the planar triangle Lu3N under high pressure.

The pressure-induced pyramidalization is further confirmed by the combination of density functional calculations (detailed information is presented in the Supporting Information) and in situ high-pressure X-ray diffraction (Figures S8–S10). The Lu–N bond length and the sum of Lu–N–Lu bond angles at high pressure are calculated, as shown in panels c and d, respectively, of Figure . Upon compression to ∼1.8 GPa, the Lu–N bonds contract; however, beyond 1.8 GPa, two of these bonds extend while one continues to shrink. The sum of Lu–N–Lu bond angles decreases smoothly below 1.8 GPa, indicating a slow process of pressure-induced pyramidalization. This finding reinforces the conclusion drawn from Raman results at 1.7 GPa with regard to frozen endohedral clusters and supports the hypothesis that slightly pyramidal Lu3N adjusts its position to alleviate stress. As the pressure increases, the sum of the bond angles decreases drastically, indicating strong pyramidalization due to stronger interaction between carbon cage and the cluster.

The band structures of the material under high pressure were calculated using the pyramidal cluster structure obtained. These are shown in Figure to demonstrate the type of the material at different compressions. At 1 atm, Lu3N@C80 is a semiconductor with a band gap of 1.21 eV, in agreement with a previous report. With an increase in pressure, the band gap decreases but does not close before the cage collapses. The valence band maximum (VBM) occurs at symmetry points S and X/S at 3.59 GPa, compared to Q and a point between Z/Y and T/U in the lower pressure range. Meanwhile, the conduction band minimum (CBM) shifts to high-symmetry points S and X at 3.6 GPa, as well. That means that large changes in the electronic structure have taken place at the symmetry points. Lu3N@C80 transforms from an indirect band gap semiconductor to a direct band gap one just before Lu3N pyramidalization. On the contrary, the Fermi level is slightly closer to the conduction band at low pressure, which shows it to be an n-type semiconductor. However, at 7.7 GPa, it definitively moves closer to the valence band, suggesting that compressed Lu3N@C80 becomes a p-type semiconductor, in perfect agreement with the Hall effect measurement.

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Band structure and electronic DOS of Lu3N@C80 at 1 atm and 3.6, 7.7, and 12.0 GPa.

Upon comparing the projected density of states (PDOS) of carbon atoms in Lu3N@C80, we find that distinct p orbitals of C atoms make a predominate contribution to the lower conduction and upper valence bands (Figure ). Notably, the carbon atoms of fullerene cages close to Lu (C″) contribute to the lowest conduction band, while those distant from the metal (C′) contribute to the highest valence band. Under high pressure, the fullerene cage undergoes varying degrees of deformation due to different interactions between the endohedral cluster and the fullerene cage. As a result, pressure-induced carbon cage distortion together with the pyramidalization of the endohedral cluster leads to the conversion from the n to p type at ∼8 GPa. This evidence highlights the fact that anisotropic deformation of the cage and a strong metal–cage interaction, shown by the Lu3N pyramidalization, are intrinsic behaviors above the n- to p-type conversion at ∼8.9 GPa.

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Band structure and electronic DOS projected to s, p, and d orbitals of C, N, and Lu, respectively, for Lu3N@C80 at 1 atm. The DOS is projected to p orbitals of C″ atoms close to the metal (orange) and C′ atoms distant from the metal (blue) in a C80 cage of Lu3N@C80.

The periodically ordered structure of Lu3N@C80, with the freely rotating endohedral fullerene serving as a structural building block, is analogous to a nanoparticle superlattice. In the case of a rhombohedral superlattice constructed from Pt nanocubes, synchrotron-based in situ pressure small- and wide-angle X-ray scattering studies have demonstrated a pressure-driven anisotropic strain distribution. This observation is further supported by high-pressure resistance measurements and electron microscopy analyses, which indicate an anisotropic interplay among the components. Similarly, Lu3N@C80 exhibits deformation that accompanies the transformation of Lu3N into a pyramidal shape under high pressure. This structural evolution results in several changes in the physical properties, including carrier-type inversion, carrier concentration abnormal, and spectroscopic alternations. Our findings elucidate the anisotropic interactions among building blocks in nanobased architectures, highlighting their relationship to the manifestation of these properties.

To summarize, we have identified and described the pressure-induced structural evolution of Lu3N@C80. At the initial stage of the compression, the translation/vibration and rotation of the cages slow at ∼1.7 GPa. Lattice parameter a decreases much faster than do b and c. A further increase in pressure makes Lu3N@C80 transform from a monoclinic structure to an orthorhombic structure at ∼3.6 GPa. Simultaneously, Lu3N@C80 converts from an indirect band gap semiconductor to a direct band gap one. The increased density improves the conductivity but to a limited degree. The next stage is characterized by an n- to p-type conversion accompanied by a sharp decrease in resistance. An enhanced metal–cage interaction forces C80 to deform anisotropically. Carbon atoms close to and distant from the metal in the fullerene cage contribute to the DOS of the conduction band and valence band near the Fermi level, respectively. As a result, the anisotropic fullerene cage deformation leads to a band gap reduction, and the Fermi level moves closer to the valence band. In the final stage, an obvious pyramidalization of the Lu3N cluster causes the resistance to change gently before a final irreversible collapse of the fullerene cages results in an amorphous product characterized as an n-type semiconductor. The high-pressure treatment scheme used here will lead to new pathways for modifying and enhancing the properties of fullerenes at the atomic level. Furthermore, this discovery should have significant implications for energy and electronic device applications based on fullerenes and their derivatives.

Supplementary Material

nl4c04824_si_001.pdf (1.8MB, pdf)

Acknowledgments

This work was mainly supported by the National Natural Science Foundation of China (Grants 52288102, 52090020, and 11874076), National Science Associated Funding (NSAF, Grant U1530402), the Science Challenging Program (Grant TZ2016001), and the Postdoctoral Science Foundation (2015M572499). The authors thank Ye Yuan, Xin Li, Dr. Bo Gao, Dr. Jinbo Zhang, and Dr. Feng Ke for their help with the XRD and PPMS measurements.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.nanolett.4c04824.

  • DFT simulation method, electrical measurements under ambient conditions, electrical resistance, and Raman, EXAFS, XRD, and FTIR measurements of Lu3N@C80 under high pressure (PDF)

The authors declare no competing financial interest.

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