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Image states at the interface with a dipolar organic semiconductor
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Image of FIG. 1.
FIG. 1.

(a) Structure of VONc calculated with DFT at the PBEh level with a basis set for C, H, N, and O and an extended SDD (Stuttgart RSC 1997 ECP) basis set for V. (b) Illustration of single-color TPPE process, here in resonance with the HOPG image state. Vacuum-level and experimentally determined molecular states of a 1 ML film of VONc on HOPG are also shown.

Image of FIG. 2.
FIG. 2.

(a) TPPE spectrum of the high final state energy/low binding energy region of 1 ML VONc on HOPG excited at 4.04 eV. The spectrum is background-subtracted with the TPPE spectrum of clean HOPG for clarity. The binding energy is referenced to the secondary-electron cut-off or global vacuum level. (b) Same as (a), but at 1.3 ML and for photon energy of 4.13 eV. The spectral decomposition is the result of a global fit to all coverages and all wavelengths investigated with minimal correlation. , , , IS1, and IS2 can clearly be identified in both (a) and (b). Peak shapes for the least-squares fit were chosen according to the nature of each feature: , the HOMO, is represented by an asymmetric contour resulting from a vibronic progression as analyzed in detail in Ref. 35; all other peaks were fit using Voigt profiles where IS1 and IS2 have a dominant Lorentzian component and and have a dominant Gaussian component. (c) Final-state energy dependence on photon energy for peaks , , , IS1, and IS2 in the 1.0 ML film. Similar plots were obtained for other coverages as well. The inset shows the expected slope for photoemission from occupied and unoccupied states . (d) Polarization-dependence of the low binding energy region TPPE spectrum of a 0.5 ML film at 4.43 eV.

Image of FIG. 3.
FIG. 3.

(a) 0–1 ML thickness dependence of the complete, scaled TPPE spectrum for VONc on HOPG, acquired at 4.04 eV. The gray line at serves to guide the eye and demonstrates the coverage-dependent increase of the global vacuum level. The gray line near is a guide to assess the IS1 position. The high-energy side of the spectrum is multiplied by a factor of 35 with respect to the low-energy side. (b) Image-state region of the coverage-dependent TPPE spectra at 4.04 eV. The colored lines represent a least-squares spectral decomposition of the spectra identifying IS1, IS2, , , and , using the same peak profiles as in Fig. 2. An asymmetric peak shape for is used, representing the vibrational envelope of the HOMO (Ref. 35). All spectra are background-subtracted for clarity.

Image of FIG. 4.
FIG. 4.

(a) Global vacuum level obtained from a fit of Eq. (2) to the secondary-electron cut-off and local HOPG vacuum level, derived from the work function of clean HOPG. (b) Coverage-dependent IS1 binding energy vs global vacuum level. (c) Coverage-dependent IS1 binding energy vs the HOPG vacuum level. VL: vacuum level.

Image of FIG. 5.
FIG. 5.

Fit of intermediate-(left axis)/final-state (right axis) energy of IS1 vs coverage based on near-field depolarization of IS1, Eq. (3).

Image of FIG. 6.
FIG. 6.

High final state energy region of background-subtracted TPPE spectrum for 1 ML VONc on HOPG, acquired at 4.04 eV as a function of annealing time. The colored line represents a least-squares spectral fit of IS1. IS1 shifts to lower final-state energies. The molecular peaks increase in intensity and shift, characteristic of conversion to a film of all O up molecules (Refs. 35 and 36). 65 min of annealing is less than the full annealing time used for all other spectra. The intensity scale is adjusted between the different panels for clarity.

Image of FIG. 7.
FIG. 7.

IS2 DCM probability density (upper panel) and potential for 1 ML VONc on HOPG, assuming an electron affinity of 0.80 eV. IS2 appears largely located inside the molecular layer.

Image of FIG. 8.
FIG. 8.

(a) Coverage-dependent global vacuum level obtained from a fit of Eq. (2) to the secondary-electron cut-off, fixed vacuum level for 1 ML-like domains, and local VONc vacuum level calculated from the near-field potential of a VONc film. Beyond 1 ML, IS2 is assumed to correspond to the screened image-derived state above 1 ML domains only. (b) IS2 binding energy vs coverage-dependent global vacuum level. (c) IS2 binding energy vs fixed 1 ML global vacuum level. (d) IS2 binding energy vs local VONc vacuum level.


Generic image for table
Table I.

Image state size and estimated affinity-level energies for 1 ML VONc/HOPG, obtained from the DCM for two different vacuum-levels. VL: Vacuum-level.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Image states at the interface with a dipolar organic semiconductor