Trion Formation Resolves Observed Peak Shifts in the Optical Spectra of Transition-Metal Dichalcogenides.

Monolayer transition-metal dichalcogenides (ML-TMDs) have the potential to unlock novel photonic and chemical technologies if their optoelectronic properties can be understood and controlled. Yet, recent work has offered contradictory explanations for how TMD absorption spectra change with carrier concentration, fluence, and time. Here, we test our hypothesis that the large broadening and shifting of the strong band-edge features observed in optical spectra arise from the formation of negative trions. We do this by fitting an ab initio based, many-body model to our experimental electrochemical data. Our approach provides an excellent, global description of the potential-dependent linear absorption data. We further leverage our model to demonstrate that trion formation explains the nonmonotonic potential dependence of the transient absorption spectra, including through photoinduced derivative line shapes for the trion peak. Our results motivate the continued development of theoretical methods to describe cutting-edge experiments in a physically transparent way.

Hot carrier extraction from 2D semiconductor photoelectrodes.

Hot carrier-based energy conversion systems could double the efficiency of conventional solar energy technology or drive photochemical reactions that would not be possible using fully thermalized, "cool" carriers, but current strategies require expensive multijunction architectures. Using an unprecedented combination of photoelectrochemical and in situ transient absorption spectroscopy measurements, we demonstrate ultrafast (<50 fs) hot exciton and free carrier extraction under applied bias in a proof-of-concept photoelectrochemical solar cell made from earth-abundant and potentially inexpensive monolayer (ML) MoS2. Our approach facilitates ultrathin 7 Å charge transport distances over 1 cm2 areas by intimately coupling ML-MoS2 to an electron-selective solid contact and a hole-selective electrolyte contact. Our theoretical investigations of the spatial distribution of exciton states suggest greater electronic coupling between hot exciton states located on peripheral S atoms and neighboring contacts likely facilitates ultrafast charge transfer. Our work delineates future two-dimensional (2D) semiconductor design strategies for practical implementation in ultrathin photovoltaic and solar fuel applications.

The N3/TiO2 interfacial structure is dependent on the pH conditions during sensitization.

The electronic structure of the N3/TiO2 interface can directly influence the performance of a dye sensitized solar cell (DSSC). Therefore, it is crucial to understand the parameters that control the dye's orientation on the semiconductor's surface. A typical step in DSSC fabrication is to submerge the nanoparticulate semiconductor film in a solution containing the dye, the sensitizing solution. The pH of the N3 sensitizing solution determines the distribution of the N3 protonation states that exist in solution. Altering the pH of the sensitizing solution changes the N3 protonation states that exist in solution and, subsequently, the N3 protonation states that anchor to the TiO2 substrate. We utilize the surface specific technique of heterodyne detected vibrational sum frequency generation spectroscopy to determine the binding geometry of N3 on a TiO2 surface as a function of the sensitizing solution pH conditions. It is determined that significant reorientation of the dye occurs in pH ≤2.0 conditions due to the lack of N3-dye carboxylate anchoring groups participating in adsorption to the TiO2 substrate. Consequently, the change in molecular geometry is met with a change in the interfacial electronic structure that can hinder electron transfer in DSSC architectures.

The pH-dependent orientation of N3 dye on a gold substrate is revealed using heterodyne-detected vibrational sum frequency generation spectroscopy.

We report on systematic changes to the adsorption geometry of the dye N3 {[cis-bis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato ruthenium(II)]} on a gold substrate as the pH of the deposition environment is altered. The protonation states of the four -COOH groups of the N3 dye change according to the modified pH conditions, thus affecting the number of -COOH and -NCS functional groups that participate in the adsorption to gold. Here, we use heterodyne detected vibrational sum frequency generation (HD-VSFG) spectroscopy to obtain surface specific vibrational information on both -COOH and -NCS groups as a function of pH of the deposition conditions. Polarization-dependent HD-VSFG yields sets of complex χ(2) spectra, enabling us to perform a simultaneous fitting procedure to the polarization-dependent real and imaginary components and thus extract detailed structural information of the N3/gold interface. Our results show that N3 preferentially adsorbs to gold either with two -COOH groups and one -NCS group in more acidic conditions or with one -COOH group and two -NCS groups in more basic conditions.