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We demonstrate heterodyne detected transient vibrational sum frequency generation (VSFG) spectroscopy and use it to probe transient electric fields caused by interfacial charge transfer at organic semiconductor and metal interfaces. The static and transient VSFG spectra are composed of both non-resonant and molecular resonant responses. To further disentangle both contributions, we apply phase rotation to make the imaginary part of the spectra be purely molecular responses and the real part of the spectra be dominated by non-resonant signals. By separating non-resonant and molecular signals, we can track their responses to the transient electric-fields at interfaces independently. This technique combined with the phase sensitivity gained by heterodyne detection allows us to successfully identify three types of photoinduced dynamics at organic semiconductor/metal interfaces: coherent artifacts, optical excitations that do not lead to charge transfer, and direct charge transfers. The ability to separately follow the influence of built-in electric fields to interfacial molecules, regardless of strong nonresonant signals, will enable tracking of ultrafast charge dynamics with molecular specificities on molecular optoelectronics, photovoltaics, and solar materials.
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Stark control of electrons across the molecule–semiconductor interface
Controlling matter at the level of electrons using ultrafast laser sources represents an important challenge for science and technology. Recently, we introduced a general laser control scheme (the Stark control of electrons at interfaces or SCELI) based on the Stark effect that uses the subcycle structure of light to manipulate electron dynamics at semiconductor interfaces [A. Garzón-Ramírez and I. Franco, Phys. Rev. B 98, 121305 (2018)]. Here, we demonstrate that SCELI is also of general applicability in molecule–semiconductor interfaces. We do so by following the quantum dynamics induced by non-resonant few-cycle laser pulses of intermediate intensity (non-perturbative but non-ionizing) across model molecule–semiconductor interfaces of varying level alignments. We show that SCELI induces interfacial charge transfer regardless of the energy level alignment of the interface and even in situations where charge exchange is forbidden via resonant photoexcitation. We further show that the SCELI rate of charge transfer is faster than those offered by resonant photoexcitation routes as it is controlled by the subcycle structure of light. The results underscore the general applicability of SCELI to manipulate electron dynamics at interfaces on ultrafast timescales.
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- Award ID(s):
- 2102386
- PAR ID:
- 10549791
- Publisher / Repository:
- Journal of Chemical Physics
- Date Published:
- Journal Name:
- The Journal of Chemical Physics
- Volume:
- 159
- Issue:
- 4
- ISSN:
- 0021-9606
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0154862
