Direct detection of density of gap states in ${\mathrm{C}}_{60}$ single crystals by photoemission spectroscopy

Direct detection of density of gap states in $C_{60}$ single crystals by photoemission spectroscopy

Fabio Bussolotti, Janpeng Yang, Masahiro Hiramoto, Toshihiko Kaji, Satoshi Kera, and Nobuo Ueno

Phys. Rev. B 92, 115102 – Published 1 September 2015

Abstract

We report on the direct and quantitative evaluation of density of gap states (DOGS) in large-size $C_{60}$ single crystals by using ultralow-background, high-sensitivity ultraviolet photoemission spectroscopy. The charging of the crystals during photoionization was overcome using photoconduction induced by simultaneous laser irradiation. By comparison with the spectra of as-deposited and gas exposed $C_{60}$ thin films the following results were found: (i) The DOGS near the highest occupied molecular orbital edge in the $C_{60}$ single crystals $(10^{19} - 10^{21} states e V^{- 1} c m^{- 3})$ mainly originates from the exposure to inert and ambient gas atmosphere during the sample preparation, storage, and transfer; (ii) the contribution of other sources of gap states such as structural imperfections at grain boundaries is negligible $(< 10^{18} states e V^{- 1} c m^{- 3})$ .

Received 27 February 2015
Publisher error corrected 23 September 2015

DOI:https://doi.org/10.1103/PhysRevB.92.115102

Physics Subject Headings (PhySH)

Density of states

Fullerenes Organic compounds

Ultraviolet photoelectron spectroscopy

Condensed Matter, Materials & Applied Physics

Corrections

23 September 2015

Erratum

Publisher's Note: Direct detection of density of gap states in $C_{60}$ single crystals by photoemission spectroscopy [Phys. Rev. B 92, 115102 (2015)]

Fabio Bussolotti, Janpeng Yang, Masahiro Hiramoto, Toshihiko Kaji, Satoshi Kera, and Nobuo Ueno

Phys. Rev. B 92, 119907 (2015)

Authors & Affiliations

Fabio Bussolotti^1,*, Janpeng Yang^1,2, Masahiro Hiramoto³, Toshihiko Kaji^3,†, Satoshi Kera^1,3, and Nobuo Ueno^1,‡

¹Department of Nanomaterial Science, Graduate School of Advanced Integration Science, Chiba University, Chiba 263-8522, Japan
²College of Physical Science and Technology, Yangzhou University, Jiangsu 225009, People's Republic of China
³Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan

^*Corresponding author: fabio@ims.ac.jp; Present address: Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan.
^†Present address: Department of Applied Physics, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei 184-8588, Japan.
^‡Corresponding author: uenon@faculty.chiba-u.jp

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Vol. 92, Iss. 11 — 15 September 2015

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Figure 1
(a) Structure of the $C_{60}$ molecule (left) and $C_{60}$ molecular packing in the face centered cubic (fcc) unit cell (right). The lattice parameter of the fcc unit cell is 14.156 Å [23]. (b) Photographic image of the $C_{60}$ cleaved single crystal on top of conductive carbon (C) tape. The upper face of the single crystal is parallel to the (111) plane of the fcc lattice. The hexagonal packing of the $C_{60}$ molecules in the (111) plane is schematically represented ( $C_{60}$ molecules = white circles). The primitive unit cell of the (111) surface is indicated by the gray shaded area. The high-symmetry directions [-11-1] and [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] in the (111) plane are also indicated. The long side of the $C_{60}$ single crystal is parallel to the [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] direction. (c) Schematic drawing of the experimental geometry.
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Figure 2
(a) XeI-UPS spectrum of the $C_{60}$ single crystal (red filled circles) and $C_{60} (15 nm) / Si O_{2}$ thin film (green continuous line). The UPS intensity was normalized to the highest-intensity point of the HOMO bands. Inset: Comparison of the $C_{60}$ single crystal and thin film in the cutoff energy range. The UPS data were normalized at the highest-intensity point of the cutoff region. The energy was aligned at the cutoff position of the $C_{60}$ thin film (vertical line) (b) Energy level alignment for $C_{60}$ single crystal and $C_{60} (15 nm) / Si O_{2}$ thin film as obtained from the UPS data in panel (a). The vacuum level (VL) and the HOMO edge positions of $C_{60}$ single crystal $(E_{HOMO}^{SC})$ and thin film $(E_{HOMO}^{TF})$ were evaluated by the linear extrapolation of the secondary cutoff and HOMO bands, as described in the main text. The LUMO positions ( $E_{LUMO}^{SC}$ and $E_{LUMO}^{TF}$ ) were determined from the value of the HOMO-LUMO energy gap of $C_{60}$ (2.4 eV [27]). (c) Comparison between the HOMO band of $C_{60}$ single crystal (red filled circles) and $C_{60} (15 nm) / Si O_{2}$ thin film (green continuous line), after removal of the inelastic background (see Fig. S3 of Supplemental Material [28] for further details). The UPS data were normalized to the highest-intensity point of the HOMO bands and aligned at the HOMO peak position of the $C_{60}$ single crystal. The HOMO band components of $C_{60}$ single crystal are indicated by vertical (black) arrows (see text for details).
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Figure 3
XeI-UPS spectrum (log scale intensity plot) of $C_{60}$ single crystal in the HOMO and gap binding energy region (red filled circles). The XeI-UPS spectrum of the C-tape (gray continuous curve) was rescaled to reproduce the $C_{60}$ data in the 0–1.7 -V binding energy range. The rescaled C-tape spectra (for different rescaling factor α) are indicated by dash-dotted lines. Continuous lines are the cumulative fitting curve of the $C_{60}$ -HOMO band resulting from convolution of Gaussian functions. For clarity, only the lower-binding-energy Gaussian component was shown (dark yellow short dotted curve). The DOS value at the HOMO edge $(E_{HOMO}^{SC})$ is $N_{0} = 1.42 \times 10^{21} states e V^{- 1} c m^{- 3}$ (gray horizontal dotted line). Above $E_{HOMO}^{SC}$ the DOGS decrease very rapidly (black short dashed line) from $\sim 10^{21}$ to $\sim 10^{19} eV$ . Below 1.7 eV no clear DOGS feature is detected. The DOS values (right scale) were obtained from UPS data according to the same procedure described in Refs. [6, 8]. The molecular packing density of the fcc phase of $C_{60}$ was used $(1.4 \times 10^{21} molecules c m^{- 3})$ [23].
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Figure 4
Comparison between the DOGS of $C_{60}$ single crystal (continuous curves, C signal removed; see Fig. 3) and $C_{60} (15 nm) / Si O_{2}$ thin film, as evaluated from the corresponding XeI-UPS spectra [36]. The data were aligned at the position of the HOMO edge of the $C_{60}$ single crystal ( $E_{HOMO}^{SC}$ , red dash-dotted line]. The DOS values (right scale) were obtained from UPS data according to the same procedure described in Refs. [6, 8]. The molecular packing density of the fcc phase of $C_{60}$ was used $(1.4 \times 10^{21} molecules c m^{- 3})$ [23].
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Physical Review B

covering condensed matter and materials physics

Direct detection of density of gap states in $C_{60}$ single crystals by photoemission spectroscopy

Fabio Bussolotti, Janpeng Yang, Masahiro Hiramoto, Toshihiko Kaji, Satoshi Kera, and Nobuo Ueno

Phys. Rev. B 92, 115102 – Published 1 September 2015

Abstract

Physics Subject Headings (PhySH)

Corrections

Erratum

Publisher's Note: Direct detection of density of gap states in $C_{60}$ single crystals by photoemission spectroscopy [Phys. Rev. B 92, 115102 (2015)]

Fabio Bussolotti, Janpeng Yang, Masahiro Hiramoto, Toshihiko Kaji, Satoshi Kera, and Nobuo Ueno

Phys. Rev. B 92, 119907 (2015)

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Physical Review B

covering condensed matter and materials physics

Direct detection of density of gap states in C60 single crystals by photoemission spectroscopy

Fabio Bussolotti, Janpeng Yang, Masahiro Hiramoto, Toshihiko Kaji, Satoshi Kera, and Nobuo Ueno

Phys. Rev. B 92, 115102 – Published 1 September 2015

Abstract

Physics Subject Headings (PhySH)

Corrections

Erratum

Publisher's Note: Direct detection of density of gap states in C60 single crystals by photoemission spectroscopy [Phys. Rev. B 92, 115102 (2015)]

Fabio Bussolotti, Janpeng Yang, Masahiro Hiramoto, Toshihiko Kaji, Satoshi Kera, and Nobuo Ueno

Phys. Rev. B 92, 119907 (2015)

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Direct detection of density of gap states in $C_{60}$ single crystals by photoemission spectroscopy

Publisher's Note: Direct detection of density of gap states in $C_{60}$ single crystals by photoemission spectroscopy [Phys. Rev. B 92, 115102 (2015)]