P30S

22BG044, 23BG033

BL-13

Charge Transfer in Gold Substrates and Nanoparticles Coatedwith Methyl Ester Substituted Aromatic Thiol Molecules
Shogo Tendo1, Akinobu Niozu1, Kakuto Yoshioka1, Jun-ichi Adachi2,Hirokazu Tanaka2, and Shin-ichi Wada1,3
1

Graduate School of Science and Engineering, Hiroshima University, Higashi-Hiroshima 739-8526, Japan2
Photon Factory, Institute of Materials Structure Science, KEK, Tsukuba 305-0801, Japan
3
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-0046, Japan
Keywords: Self-assembled monolayers (SAMs), Gold nanoparticles, Site-selective desorption, Core-hole-clock (CHC)

Charge transfer between molecules and metal surfaces
is a crucial process with implications for molecularelectronics, energy storage, and catalysis. Particularly in
nanoparticle films deposited on substrates, where organic
molecules coat gold nanoparticles (AuNPs), chargetransfer between π-conjugated molecules and metal playsa pivotal role in determining the photoelectric conversion
efficiency of dye-sensitized solar cells. In this study, we
focused on oligophenyl molecules, which have differentmolecular lengths, and directly measured the chargetransfer time using soft X-rays (Figure 1 (a)–(c)) [1].
Subsequently, we elucidated the electronic properties of
the nanoparticle films by comparing the charge transfer
dynamics from molecules to flat metal surfaces.
Monophenylandbiphenyl
self-assembled
monolayers (MP and MBP SAMs, Figure 1 (a), (b)) wereprepared on two different metal surfaces. The first SAMs
were formed on two-dimensional flat gold substrates (2DSAMs, Figure 1 (d)). The second SAMs were formed onthree-dimensional AuNPs, which were drop-cast onto thesubstrates (3D SAMs, Figure 1 (e)). AuNPs weresynthesized by pulsed laser ablation in liquid. The size of
the AuNPs was determined to be 7 nm from the surface
plasmon resonant peak wavelength, as shown in Figure
2, based on the analytical equation.
Near-edge X-ray absorption fine structure (NEXAFS)
spectra and X-ray photoelectron spectra (XPS) werecollected at the HiSOR BL-13. The XPS results for the2D-MP and 3D-MBP SAMs are presented in Figure 3.
An important observation from XPS is the similarity inchemical shifts, with no clear discrepancy in the energy
position between 2D and 3D SAMs. Any slight variations
were attributed to the inhomogeneity of the NP surfaces;
nevertheless, the electronic structures of the 2D and 3D
SAM remained identical. Figure 4 shows the C and O Kedge NEXAFS spectra. The dominant peaks included
π*(C=C) at ~285 eV for the C K-edge and π*(C=O) at
532.0 eV for the O K-edge. Polarization angular(a)(b)(d)(c)(e)

Au
Au

FIGURE 1. Molecular structures of (a) MP and (b)MBP. Images of (c) charge transfer process, and(d) 2D-MP, and (e) 3D-MP SAMs.

FIGURE 2. UV-VIS spectrum of AuNP colloids.

FIGURE 3. XPS of 2D-MP and 3D-MP SAMs.

dependence was observed in 2D-MP, and the subsequentfitting analysis suggested that the molecules adsorbed in
an upright orientation to the surfaces [2]. In contrast, thisphenomenon was not observed in 3D SAMs, likely due
to the averaging of the transition dipole moment ofmolecules on spherical particles. The energy level of theπ*(C=O) peaks exceeded the binding energy of the C=Opeaks at 531.7 eV in the O 1s XPS spectra, indicatingthat it was suitable for subsequent resonant Auger
electron spectroscopy experiments.
We conducted time-of-flight mass spectrometry(TOF-MS) at Photon Factory BL-2B and utilized thedata obtained to construct near-edge X-ray absorption
mass spectra (NEXAMS), as shown in Figure 5. The
total ion yield (TIY) and partial ion yield (PIY) spectrafor both 2D and 3D SAMs exhibited a primary peaks
corresponding to σ*(O–CH3) at 288.9 eV. Thisobservation indicates that 3D SAMs form monolayers
similar to those formed by 2D SAMs. The presence of asingle sharp peak at 288.9 eV for CHn+ (n = 0–3) ionssuggests site-selective desorption of CHn+ ions from themethoxy group of SAMs [3].
Resonant Auger electron spectra (RAES) weremeasured at HiSOR BL-13, and the charge transfer
times of the 2D and 3D SAMs were determined using
the core-hole-clock (CHC) approach. In Figure 6, RAES
at O 1s→π*(C=O) excitation is presented, with peaks
corresponding to spectator Auger decay processes
observed at ~486 eV and ~511 eV. The RAES atπ*(C=O) excitation was fitted with the RAES at postedge excitation and π*(C=O) excitation of 2D-aliphaticSAMs, which exhibit insulation properties. Using thefitting coefficient and core hole lifetime of O 1s, thecharge transfer time for each 2D and 3D SAMs weredetermined. The charge transfer time of 3D SAMs
decreased with increasing chain length, similar to that of2D SAMs. Furthermore, the charge transfer time
between the 2D and 3D SAMs remained unchanged.
Our results suggest that the spectroscopic andelectrical properties of 3D SAMs closely resemble thoseof 2D SAMs. This similarity can be attributed to the factthat the AuNPs synthesized in our study had a diameterof ~7 nm, predominantly comprising {111} terraces on
their surfaces [4]. Notably, unique absorption features atthe corners and edges of the NPs were not observed in
the spectra. In essence, this study serves as a crucial link,extending the applicability of insights into the chargetransfer times obtained from diverse molecules observedin 2D to the context of 3D SAMs.

FIGURE 4. (a) Polarization dependence spectra of CK-edge NEXAFS spectra of 2D and 3D-MP. (b) O Kedge NEXAFS spectra of 2D and 3D-MP.

FIGURE 5. C K-edge NEXAMS spectra of (a) 2DMP and (b) 3D-MP. Total ion yield spectra and partial
ion yield spectra of major products.

REFERENCES
1. M. Zharnikov, Acc. Chem. Res., 53, 2975–2984 (2020).
2. J. Stöhr, NEXAFS Spectroscopy, Surface Sciences, vol. 25,Berlin, Springer,1992.
3. S. Wada, H. Kizaki, et al. J. Phys.: Condens. Matter, 18,S1629-S1653 (2006).
4. A. C. Templeton, W. P. Wuelfing and R. W. Murray, Acc.
Chem. Res., 33, 27–36 (2000).

FIGURE 6. RAES of (a) MP and (b) MBP underπ*(C=O) excitation after Shirley backgroundsubtraction (dotted lines). The reproduced spectra(gray lines) were obtained by fitting with the normal
Auger spectra of each MP or MBP SAM (green lines)
and the spectator Auger spectra of 2D-aliphatic SAM(light blue lines).