Title: Heterodyne transient vibrational SFG to reveal molecular responses to interfacial charge transfer
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. more »« less
Wang, Chenglai; Li, Yingmin; Xiong, Wei
(, Journal of Materials Chemistry C)
null
(Ed.)
We introduce a new data analysis method, which can be applied to transient vibrational sum-frequency generation spectroscopy to reveal hidden molecular dynamics of charge transfer at molecular heterojunction interfaces. After validating the method, we used it to extract molecular dynamics at organic semiconductor/metal interfaces, which was otherwise dominated by electronic dynamics. Such an ability can advance the understanding of the roles of molecules in interfacial charge transfer dynamics.
Garzón-Ramírez, Antonio J; Franco, Ignacio
(, The Journal of Chemical Physics)
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.
Ostovar, Behnaz; Lee, Stephen A; Mehmood, Arshad; Farrell, Kieran; Searles, Emily K; Bourgeois, Briley; Chiang, Wei-Yi; Misiura, Anastasiia; Gross, Niklas; Al-Zubeidi, Alexander; et al
(, Science Advances)
The lack of a detailed mechanistic understanding for plasmon-mediated charge transfer at metal-semiconductor interfaces severely limits the design of efficient photovoltaic and photocatalytic devices. A major remaining question is the relative contribution from indirect transfer of hot electrons generated by plasmon decay in the metal to the semiconductor compared to direct metal-to-semiconductor interfacial charge transfer. Here, we demonstrate an overall electron transfer efficiency of 44 ± 3% from gold nanorods to titanium oxide shells when excited on resonance. We prove that half of it originates from direct interfacial charge transfer mediated specifically by exciting the plasmon. We are able to distinguish between direct and indirect pathways through multimodal frequency-resolved approach measuring the homogeneous plasmon linewidth by single-particle scattering spectroscopy and time-resolved transient absorption spectroscopy with variable pump wavelengths. Our results signify that the direct plasmon-induced charge transfer pathway is a promising way to improve hot carrier extraction efficiency by circumventing metal intrinsic decay that results mainly in nonspecific heating.
Zhang, Tong; Brown, Jesse B; Fisher, Haley; Liebes, Mallory; Huang-Fu, Zhi-Chao; Qian, Yuqin; Rao, Yi
(, Chinese Journal of Chemical Physics)
The surface states of photoelectrodes as catalysts heavily influence their performance in photocatalysis and photoelectrocatalysis applications. These catalysts are necessary for developing robust solutions to the climate and global energy crises by promoting CO2 reduction, N2 reduction, contaminant degradation, and water splitting. The semiconductors that can fill this role are beholden as photoelectrodes to the processes of charge generation, separation, and utilization, which are in turn products of surface states, surface electric fields, and surface carrier dynamics. Methods which are typically used for studying these processes to improve semiconductors are indirect, invasive, not surface specific, not practical under ambient conditions, or a combination thereof. Recently, nonlinear optical processes such as electronic sum-frequency generation (ESFG) and second-harmonic generation (ESHG) have gained popularity in investigations of semiconductor catalysts systems. Such techniques possess many advantages of in-situ analysis, interfacial specificity, non-invasiveness, as well as the ability to be used under any conditions. In this review, we detail the importance of surface states and their intimate relationship with catalytic performance, outline methods to investigate semiconductor surface states, electric fields, and carrier dynamics and highlight recent contributions to the field through interface-specific spectroscopy. We will also discuss how the recent development of heterodyne-detected ESHG (HD-ESHG) was used to extract charged surface states through phase information, time-resolved ESFG (TR-ESFG) to obtain in-situ dynamic process monitoring, and two-dimensional ESFG (2D-ESFG) to explore surface state couplings, and how further advancements in spectroscopic technology can fill in knowledge gaps to accelerate photoelectrocatalyst utilization. We believe that this work will provide a valuable summary of the importance of semiconductor surface states and interfacial electronic properties, inform a broad audience of the capabilities of nonlinear optical techniques, and inspire future original approaches to improving photocatalytic and photoelectrocatalytic devices.
Schwinn, Madison C.; Rafiq, Shahnawaz; Lee, Changmin; Bland, Matthew P.; Song, Thomas W.; Sangwan, Vinod K.; Hersam, Mark C.; Chen, Lin X.
(, The Journal of Chemical Physics)
Mixed-dimensional van der Waals heterojunctions involve interfacing materials with different dimensionalities, such as a 2D transition metal dichalcogenide and a 0D organic semiconductor. These heterojunctions have shown unique interfacial properties not found in either individual component. Here, we use femtosecond transient absorption to reveal photoinduced charge transfer and interlayer exciton formation in a mixed-dimensional type-II heterojunction between monolayer MoS2 and vanadyl phthalocyanine (VOPc). Selective excitation of the MoS2 exciton leads to hole transfer from the MoS2 valence band to VOPc highest occupied molecular orbit in ∼710 fs. On the contrary, selective photoexcitation of the VOPc layer leads to instantaneous electron transfer from its excited state to the conduction band of MoS2 in less than 100 fs. This light-initiated ultrafast separation of electrons and holes across the heterojunction interface leads to the formation of an interlayer exciton. These interlayer excitons formed across the interface lead to longer-lived charge-separated states of up to 2.5 ns, longer than in each individual layer of this heterojunction. Thus, the longer charge-separated state along with ultrafast charge transfer times provide promising results for photovoltaic and optoelectronic device applications.
Li, Yingmin, Xiang, Bo, and Xiong, Wei. Heterodyne transient vibrational SFG to reveal molecular responses to interfacial charge transfer. Retrieved from https://par.nsf.gov/biblio/10101512. Journal of chemical physics 150. Web. doi:1.5066237.
Li, Yingmin, Xiang, Bo, and Xiong, Wei.
"Heterodyne transient vibrational SFG to reveal molecular responses to interfacial charge transfer". Journal of chemical physics 150 (). Country unknown/Code not available. https://doi.org/1.5066237.https://par.nsf.gov/biblio/10101512.
@article{osti_10101512,
place = {Country unknown/Code not available},
title = {Heterodyne transient vibrational SFG to reveal molecular responses to interfacial charge transfer},
url = {https://par.nsf.gov/biblio/10101512},
DOI = {1.5066237},
abstractNote = {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.},
journal = {Journal of chemical physics},
volume = {150},
author = {Li, Yingmin and Xiang, Bo and Xiong, Wei},
}
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