Electronic structure and polymerization of a self-assembled monolayer with multiple arene rings

We find evidence of intermolecular interactions for a self-assembled monolayer (SAM) formed from a large molecular adsorbate, [1,$1^{\prime }$;$4^{\prime }$,$1^{″}$-terphenyl]-4,$4^{″}$-dimethanethiol, from the dispersion of the molecular orbitals with changing wave vector $k$. With the formation self-assembled molecular (SAM) layer, the molecular orbitals hybridize to electronic bands, with indications of significant band dispersion of the unoccupied molecular orbitals. The electronic structure is also seen to be dependent upon temperature, and cross linking between the neighbor molecules, indicating that the electronic structure may be subtly altered by changes in molecular conformation and packing.

Figure 1

(Color) Combined UPS (left) and IPES (right) spectra (top) of TPDMT/Au (top curve), along with the calculated ground state molecular orbital energies for a single TPDMT molecule (bottom line). The UPS and IPES signals are related to the contributions from the occupied and unoccupied molecular orbitals, respectively. Selected calculated molecular orbitals of TPDMT are shown on the top. The photoemission spectrum was taken with unpolarized $\mathrm{He}\mathrm{I}$ $\left(21.2\mathrm{eV}\right)$, with the photoelectrons collected normal to the surface. The inverse photoemission spectrum was taken at normal incidence.

Figure 2

(Color) The geometry optimization calculated ground state molecular orbital energies of TPDMT on Au substrate are compared with experimental binding energies with respect to the Fermi level. This includes a few unoccupied molecular orbitals down to the lowest unoccupied molecular orbital, as well as the occupied molecular orbitals referenced to the vacuum level. Selected molecular orbitals are shown left. The calculated ground state free molecular HOMO-LUMO gap is $7.7\mathrm{eV}$.

Figure 3

(Color online) Intermolecular dispersion of a band originating from the sulfur weighted $\pi$ molecular orbitals of [1,$1^{\prime }$; $4^{\prime }$,$1^{″}$-terphenyl]-4,$4^{″}$-dimethanethiol (TPDMT) molecules assembled on $\mathrm{Au}\left(111\right)$ in the reduced zone scheme, determined from high resolution photoemission spectra taken with unpolarized $\mathrm{He}\mathrm{I}$ $\left(21.2\mathrm{eV}\right)$ light.

Figure 4

(Color) Indications of intermolecular band dispersion from the LUMO $\pi$ molecular orbitals of [1,$1^{\prime }$;$4^{\prime }$,$1^{″}$-terphenyl]-4,$4^{″}$-dimethanethiol. The angle-resolved inverse photoemission spectra are shown for zone center (black) and for $k_{\parallel }=0.8{\mathrm{\AA }}^{-1}$ (red). The corresponding LUMO is shown in the margin.

Figure 5

(Color) The relative (with respect to the Fermi level) position of LUMO in the TPDMT SAM, obtained from the IPES data, as a function of irradiation dose ($50\mathrm{eV}$ electrons). The insets show [1,$1^{\prime }$;$4^{\prime }$,$1^{″}$-terphenyl]-4,$4^{″}$-dimethanethiol (TPDMT) before (left) and after (right) electron-induced cross linking. The error bars indicate the relative error between the measurements and do not include the systemic errors arising from the instrumental function.

Figure 6

(Color online) Combined UPS (left) and IPES (right) spectra (solid line) of pristine (a) and irradiated (b) TPDMT/Au, along with the calculated density-of-states (dashed line) for a pair of the separate and cross-linked TPDMT molecules. The UPS and IPES signals are related to the contributions from the occupied and unoccupied molecular orbitals, respectively. The experimental HOMO-LUMO gaps are indicated. The irradiated films were exposed to $54\mathrm{mC}∕{\mathrm{cm}}^2$ of $50\mathrm{eV}$ electrons (IPES) and synchrotron white light exposure of about $600\mathrm{mA}\min$ (UPS). The photoemission spectra were taken with unpolarized $\mathrm{He}\mathrm{I}$ $\left(21.2\mathrm{eV}\right)$ light.

Figure 7

(Color) Comparison of the light polarization photoemission spectra of TPDMT taken with $p$-polarized light (black lines) and $s+p$-polarized light (red lines) as a function of temperature. In (a), the energy region with the temperature reversible polarized light effects are enhanced compared to the broader energy scale in (b). The proposed model for the molecular arrangement in the TPDMT film at $295\mathrm{K}$, in accordance with Ref. 9, is shown in the inset. The spectra were taken at photon energy of $80\mathrm{eV}$.

Figure 8

(Color) Comparison of the photoemission spectra taken with linearly polarized $p$-polarized light (black) and $s+p$-polarized light (red) for the pristine (a) and irradiated (b,c) TPDMT film after its exposure to $100\mathrm{mA}\min$ (b) and $500\mathrm{mA}\min$ (c) of synchrotron white light. The data for $295\mathrm{K}$ (left panel) and $135\mathrm{K}$ (right panel) are shown. The spectra were taken at photon energy of $80\mathrm{eV}$ and the photoelectrons collected at normal emission.