Schottkey barrier measurement of nanocrystalline Lu3N@ C80/Au contac

Electrical and structural properties of nanocrystalline solids of C 80 fullerene encapsulated with a Lu 3 N cluster, Lu 3 N@C 80 , have been studied by measuring x-ray photoemission spectra, x-ray diffraction, and current-voltage characteristics of the Lu 3 N@C 80 /Au Schottky contact in the temperature range of 300 - 500 K. The nanocrystalline solid sample of Lu 3 N@C 80 fullerene consists of grains characterized with an fcc structure and those grains become larger in size after pressing the powder sample at 1.25 GPa. The current-voltage characteristics measured at various temperatures showed that there are no significant dependences on both the Schottky barrier and the carrier mobility on electric field. The Schottky barrier of the Lu 3 N@C 80 /Au contact is determined to be 0.71 ± 0.04 eV.


Introduction
Higher fullerenes of C 80 are promising agent for organic photovoltaic devices particularly when they encapsulate a M 3 N cluster (M: Sc, Lu, Er, Dy, Y, Ho, La, Tm, Gd, etc.) inside the C 80 cage. The recent isolation technique has succeeded in isolating those M 3 N endohedral C 80 fullerenes, M 3 N@C 80 , with a production yield more than ten times higher than other endohedral metallofullerenes [1][2][3][4]. Electron transfer from a M 3 N cluster to the C 80 cage as well as the specific charge states of both the cluster and cage is of special interest in photovoltaic solar cells. Bare C 80 fullerenes that satisfy the isolated pentagon rule (IPR) [5] have seven isomers [6] with symmetries of I h , D 5d , D 5h , D 3 , D 2 , C 2v and C 2v , among which only the D 2 and D 5d isomers are able to be isolated because of their structural stability. It is known that excessive electrons can stabilize the molecular structure of the C 80 cage, so the M 3 N@C 80 molecules with I h and D 5h symmetries can also be obtained [7]. Also, the molecules with the excessive electrons from the endohedral cluster can interact with metal electrodes so that contact resistance at the metal/M 3 N@C 80 interface is reduced [8].
Furthermore, it is reported that the endohedral M 3 N cluster influences the carrier transport since the current through the Al/M 3 N@C 80 /Al system is obtained to be two times larger than that for the Al/C 80 /Al system [8].
With all the expectations for the promising abilities of M 3 N@C 80 fullerenes for electronic, magnetic and optical devices, their solid state properties have not been well understood, yet. So far, all the theoretical and experimental studies have focused mainly on the chemical and physical properties of an isolated single M 3 N@C 80 molecule, treating structure of the isomers [6,9], HOMO-LUMO energy gap [1,6], bond energy [6], electronic structure of neutral as well as ionized molecules [10], molecular dynamics [11,12], superatom [13], size effects of the endohedral clusters [14], magnetic moment [15,16], and optical and electrochemical properties [11,17].
In this study, we have prepared nanocrystalline Lu 3 N @C 80 solids, and revealed their properties about crystallographic structure, oxygen-induced chemical shifts of the core levels of constituting atoms, and effect of applied electric field on the Schottky barrier and the carrier mobility. Schottky barrier of the nanocrystalline Lu 3 N@ C 80 /Au contact is evaluated from the temperature-dependent current passing through the sample.

Experimental
Solid samples were prepared using Lu 3 N@C 80 powder (purity > 97 wt%) purchased from LUNA Innovations. The Lu 3 N@C 80 powder was pressed into a pellet with two gold electrodes at room temperature at a pressure of 1.25 GPa for 50 min. The so formed pellet was 5 mm in diameter and 0.525 mm thick. Prior to the measurements the powder and pellet samples were characterized by field emission scanning electron microscope (FE-SEM, JEOL JSM-7000), x-ray photoemission spectroscopy (XPS; AXIS-NOVA, SHIMATSU/KRATOS) and x-ray diffraction (XRD; JEOL JDX-3500K). In the XPS analysis the beam diameter of Al K line was 55 μm, and the binding energy resolution was 0.15 eV.
In the electrical measurements, the current passing through the pellet sample were measured using a digital electrometer (ADVANTEST R8252) with a current resolution of 1 f A at various d.c. bias voltages, bi V , changing from 10 to 200 V. The Au/Lu 3 N@C 80 /Au structure consists of two Schottky contacts, connected backto-back. When a d.c. bias is applied, one of the Schottky diodes is forward and the other is reverse biased. At sufficiently high biases, the Au/Lu 3 N@C 80 /Au structure can be considered as a reverse-biased diode. Its currentvoltage characteristics can be written as where 0 J is the reverse saturation-current density, * A the effective Richardson constant, B q the Schottky barrier and B k the Boltzmann constant. Hence, from measurements of the temperature dependent current passing through the sample, I AJ  where A is the area of the electrodes on the sample, the Schottky barrier was obtained.
The pellet sample was set in a vacuum chamber of a cryostat during the electrical measurements. The base pressure of the vacuum chamber was less than 10 −5 Pa. The current measurements were carried out in the course of heating up or cooling down process between the temperatures from 300 K to 500 K. The rate of heating or cooling was 0.14 Kmin −1 with a stepwise increment of 1.0 K.

Results
The SEM image of the surface of a Lu 3 N@C 80 pellet sample is shown in Figure 1. The inset shows the structure formula of the Lu 3 N@C 80 molecule. Figure 2 shows three x-ray photoemission spectra of the pellet sample at room temperature. They were obtained from the surface of the Lu 3 N@C 80 sample before and after Ar + ion sputtering for 10 and 30 s, respectively. Six peaks at biding energies of 12, 205, 287, 535 and 990 eV were observed in the spectra. The 12 eV peak is attributed to a photoemission from valence electrons of Lu atoms. The double peaks at 200 and 210 eV are the photoemissions from Lu  4 d 5/2 and 4 d 3/2 core levels. The peak around 287 and 535 eV comes from C 1s and O 1s core level, respectively. The peaks around 990 eV correspond to O KLL and C KVV Auger emissions. Also, the peaks around 240 eV observed after the Ar + ion sputtering are from Ar 2 p 1/2 and 2 p 3/2 core levers. The measured XPS spectra showed up that the Ar + ion sputtering causes both decrease in the O-related peaks and increase in the C and Lu-related peaks. This result indicates that the oxygen atoms adsorb only on the sample surface. From the spectrum after the 30 s Ar + ion sputtering, atomic ratio of Lu/C is evaluated to be 3.24 at%, close to the stoichio-Open Access MSA Schottkey Barrier Measurement of Nanocrystalline Lu 3 N@C 80 /Au Contact 810 metric ratio of 3.61 at% for Lu 3 N@C 80 . One should note that the photoemission from the N 1s level cannot be detected due to its smaller relative sensitivity factor (RSF) of 0.505 compared to the Lu 4 d RSF of 2.645. Although the RSF of C 1s is also small, 0.318, the XPS intensity by the C atom is somewhat strong because of the abundant concentration of C atoms in the Lu 3 N@C 80 solid.
The photoemission spectra from the Lu 4 d 5/2 and 4 d 3/2 and C 1s core levels were enlarged in the energy scaling and they were respectively plotted in 198 eV for Lu 4 d 5/2 and 287 eV for C 1s core level. Namely, the binding energies of the core levels shift to the low-energy side after eliminating oxygen adatoms by the Ar + ion sputtering. The XPS peak shift is 3.0 eV for the Lu 4 d 3/2 , 3.0 eV for the Lu 4 d 5/2 , and 2.0 eV for the C 1s, respectively. The peak of the Lu valence electrons at 12 eV shifts by 2.0 eV towards the low energy side. The observed chemical shifts to the lower energy side after the oxygen removal suggest that the oxygen adatoms accept electrons from the valence orbitals of both the C 80 cage and the endohedral Lu 3 N cluster. Figure 4 shows the XRD patterns of the Lu 3 N@C 80 powder material as-received and the pellet sample prepared by pressing under the pressure of 1.25 GPa. Several diffraction peaks can be recognized for the two patterns, a strong peak at 2 9.50    Figures 6(a) and (b) for the measurements during heating up and cooling down process, respectively. The good linear relationship found in the Arrhenius plots in Figure 6 suggests that carriers are generated by thermal activation over the Schottky barrier  Density 0 J of the current passing through the Lu 3 N @C 80 /Au structure at temperature of 450 K was plotted in Figure 8 as a function of E. The reverse current 0 J does not saturate and increases slowly with increasing E from 0 to 0.4 MVm −1 . As shown in Figure 7 there is not significant electric field dependence on the Schottky barrier. Therefore, the increase of 0 J with increasing electric field strength may be due to the tunneling effect of the carrier through the Schottky barrier and the carrier recombination in depletion layer. The bulk resistance decrease of the Lu 3 N@C 80 solid phase can also result in the increase of 0 J . C 70 fullerenes, that of the C 80 fullerene is small, 0.55 eV for D 2 , 0,12 eV for I h , 0.25 eV for D 5d , 0.12 eV for D 5h , 0.11 eV for C 2v' , 0.20 eV for D 3 , and 0.11 eV for C 2v isomer. Except for the D 2 and D 5d isomers, the bare C 80 isomers are unstable and still not be synthesized [6]. However, it is known that the encapsulation of the M 3 N cluster makes them stable, and then the M 3 N@C 80 molecule with I h and D 5h symmetries can be stably extracted [7]. Electron transfer from the M 3 N cluster to the C 80 cage plays the most crucial role to the stability of the fullerenes with highly symmetrical geometries. Photoemission and x-ray absorption studies have indeed revealed that the C 80 cages with I h and D 5h symmetries are stabilized by the excessive electrons transferred from the M 3 N cluster [1]. HOMO-LUMO energy gaps of anions and cations of C 80 fullerene with the I h symmetry have been calculated using ab initio molecular-orbital method [6]. It is 0.12 eV for neutral isomer, 0.38 eV for +2 cation, 0 eV for −2 anion, and 2.19 eV for −6 anion of the C 80 isomer. On the other hand, theoretical calculations for the [Lu 3 N] +6 @ [C 80 ] −6 molecule showed that its HOMO-LUMO energy gap is 1.54 -2.55 eV [4,18]. Previous experimental studies also showed that the HOMO-LUMO gap of a [Lu 3 N] +6 @[C 80 ] −6 molecule is 1.31 -1.65 eV [19,20] for optical absorption and 2.04 eV [4] for electrochemical measurement, respectively. In general, Schottky barrier of the semiconductor/metal is smaller than energy band gap of the semiconductor. In this study, the obtained Schottky barrier of the nanocrystalline [Lu 3 N] +6 @[C 80 ] −6 /Au contact is 0.71 ± 0.04 eV.

Electric Field Dependence of the Carrier Mobility
In general, the carrier mobility of fullerene crystals is very low due to impurity and poor crystallinity. For example, the electron drift mobility of C 60 crystal is in the 1 cm 2 V −1 s −1 range and the hole mobility is smaller than that of electron in three orders of magnitude [21][22][23]. Sato et al. [24] have reported a study for measuring mobility of the Sc 3 N@C 80 film with large grain size of several micrometers. The electron mobility of Sc 3 N@C 80 film under room temperature and atmospheric pressure is 5.7 × 10 −3 cm 2 V −1 s −1 . The low mobility values indicate that the carrier propagation in the fullerene crystals is by hopping. The mobility in the order of 10 −3 cm 2 V −1 s −1 gives a mean free path smaller than the molecular size, indicating that the band conduction model is not correct. A more likely mechanism of carrier transport in the fullerene solid would be hopping between grains or neighboring molecules. It is also reported that the carrier mobility of C 60 crystal at temperatures above 300 K is independent of temperature [22,25].
We have discussed the free time dependence of the mobility above. From the electron mobility of

Effects of the Adsorbed Oxygen Atoms
The XPS spectra of the Lu 3 N@C 80 solid convey information of the thin surface layer within the depth of only 5 nm. Therefore, effects of the adsorbed oxygen atoms on the binding energies of C 1s and Lu 4 d are sensitively detected as seen in Figure 3. Due to the adsorption of oxygen atoms, the binding energy increases by 2 eV for the C 1s level and by 3 eV for the Lu 4 d 5/2 and d 3/2 levels, respectively. Compared to the Schottky barrier of the Lu 3 N@C 80 solid, the shifts of the C and Lu-related levels are large. However, in this study the effect of impurity oxygen atoms on the measurement of the Schottky barrier of the Lu 3 N@C 80 solid phase can be ignored because of a small concentration or a just physical adsorption of oxygen atoms.

Interaction between the Applied Electric Field and the Dipoles in Lu 3 N@C 80 Molecule
The M 3 N cluster can adopt ten equivalent orientations in the C 80 cage by aligning its C 3 axis to ten different C 3 axes of the C 80 cage. For each orientation, the cluster can occupy four equivalent positions with different azimuths corresponding to the rotation about its C 3 axis. Popov et al. [26] reported a calculation of molecular orientational dynamics of the Lu 3 N cluster in the C 80 cage. A thermal activation behavior of the cluster rotation appears, strongly depending on the cluster orientation with respect to the C 80 cage. The barriers to the cluster rotation with respect to the C 3 axes of the C 80 cage are below 155 meV, and have a minimum closing to zero at room temperature. The barriers to the cluster rotation inside the C 80 cage are also found to somewhat increase in the charged states [26]. It is well known that there are dipoles in the M 3 N cluster due to the electron transfer from metal atoms to nitrogen atoms [27]. However, no coupling between the applied electric field and the dipoles in the M 3 N cluster is observed in this study. This is due to a high symmetry of three dipoles in the Lu 3 N molecule as well as a screening effect of both the high-speed rotating C 80 cage and the excessive electrons on it.
High-speed rotation of the Lu 3 N@C 80 molecule at room temperature have a high relaxation rate due to the enlarged cage size, the excessive electrons as well as the additional dipole-dipole interaction between the endohedral cluster and the C 80 cage, compared with the empty fullerenes such as C 60 and C 70 [12,28]. Furthermore, polarizations of the M 3 N@C 80 molecule have been reported and its polarizability increases with increasing molecular diameter [27]. In order to estimate the influence of the M 3 N@C 80 molecular polarization, we can calculate the potential energy of a dipole with elementary charge on the C 80 cage by using U qE   where q is the elementary charge, E the strength of applied electric field and  the diameter of the C 80 cage. We have the potential energy of the Lu 3 N@C 80 molecule having the dipole with elementary charge, U qE   meV where diameter of the C 80 cage is 1 nm and the maximum electric field strength used in this study is 0.4 MVm −1 . It is clear that the potential energy is much smaller than the mean kinetic energy of a molecule, 3 2 39 B meV at room temperature. Therefore, even there is a dipole in the Lu 3 N@C 80 molecule, its polarization effect cannot be observed due to strong thermal rotation and diffusion motions of the molecule. k T  Similar to the results of polarization effects of the endohedral Lu 3 N cluster, we can also conclude that polarization effects of the Lu 3 N@C 80 molecule can be ignored under the condition of the electric field strength used in this study, thus the capacitance or the Schottky barrier of the Lu 3 N@C 80 /Au contact is independent of applied electric field.

Conclusion
We have studied the structural properties of the nanocrystalline Lu 3 N@C 80 fullerene solid with fcc structure by x-ray photoemission spectroscopy and x-ray diffraction measurement. The current-voltage characteristics of the Lu 3 N@C 80 /Au Schottky contact were measured at various electric fields and temperatures. The Schottky barrier of the Lu 3 N@C 80 /Au contact is estimated to be 0.71 ± 0.04 eV and the effect from the applied electric field can be ignored. The interaction between the applied electric field and the dipoles in the endohedral Lu 3 N cluster and the C 80 cage cannot be observed.