Ultrafast Imaging of the Jahn-Teller Topography in Carbon Tetrachloride.

We report the direct time-domain observation of ultrafast dynamics driven by the Jahn-Teller effect. Using time-resolved photoelectron spectroscopy with a vacuum-ultraviolet femtosecond source to prepare high-lying Rydberg states of carbon tetrachloride, our measurements reveal the local topography of a Jahn-Teller conical intersection. The pump pulse prepares a configurationally mixed superposition of the degenerate 1T2 4p-Rydberg states, which then distorts through spontaneous symmetry breaking that we identify to follow the t2 bending motion. Photoionization of these states to three cationic states 2T1, 2T2, and 2E reveals a shift in the center-of-mass of the photoelectron peaks associated with the 2Tn states which reveals the local topography of the Jahn-Teller conical intersection region prepared by the pump pulse. Time-dependent density functional theory calculations confirm that the dominant nuclear motion observed in the spectrum is the CCl4 t2 bending mode. The large density of states in the VUV spectral region at 9.33 eV of carbon tetrachloride and strong vibronic coupling result in ultrafast decay of the excited-state signal with a time constant of 75(4) fs.

Direct Time-Domain View of Auger Recombination in a Semiconductor.

The radiationless recombination of electron-hole pairs in semiconductors is detrimental to optoelectronic technologies. A prominent mechanism is Auger recombination, in which nonradiative recombination occurs efficiently by transferring the released energy-momentum to a third charge carrier. Here we use femtosecond photoemission to directly detect Auger electrons as they scatter into energy and momentum spaces from Auger recombination in a model semiconductor, GaSb. The Auger rate is modulated by a coherent phonon mode at 2 THz, confirming phonon participation in momentum conservation. The commonly assumed Auger rate constant is found not to be a constant, but rather decreases by 4 orders of magnitude as hot electrons cool down by ∼90 meV. These findings provide quantitative guidance in understanding Auger recombination and in designing materials for efficient optoelectronics.

Atomic-Scale Spectroscopy of Gated Monolayer MoS2.

The electronic properties of semiconducting monolayer transition-metal dichalcogenides can be tuned by electrostatic gate potentials. Here we report gate-tunable imaging and spectroscopy of monolayer MoS2 by atomic-resolution scanning tunneling microscopy/spectroscopy (STM/STS). Our measurements are performed on large-area samples grown by metal-organic chemical vapor deposition (MOCVD) techniques on a silicon oxide substrate. Topographic measurements of defect density indicate a sample quality comparable to single-crystal MoS2. From gate voltage dependent spectroscopic measurements, we determine that in-gap states exist in or near the MoS2 film at a density of 1.3 × 10(12) eV(-1) cm(-2). By combining the single-particle band gap measured by STS with optical measurements, we estimate an exciton binding energy of 230 meV on this substrate, in qualitative agreement with numerical simulation. Grain boundaries are observed in these polycrystalline samples, which are seen to not have strong electronic signatures in STM imaging.

Charge Transfer Excitons at van der Waals Interfaces.

The van der Waals interfaces of molecular donor/acceptor or graphene-like two-dimensional (2D) semiconductors are central to concepts and emerging technologies of light-electricity interconversion. Examples include, among others, solar cells, photodetectors, and light emitting diodes. A salient feature in both types of van der Waals interfaces is the poorly screened Coulomb potential that can give rise to bound electron-hole pairs across the interface, i.e., charge transfer (CT) or interlayer excitons. Here we address common features of CT excitons at both types of interfaces. We emphasize the competition between localization and delocalization in ensuring efficient charge separation. At the molecular donor/acceptor interface, electronic delocalization in real space can dictate charge carrier separation. In contrast, at the 2D semiconductor heterojunction, delocalization in momentum space due to strong exciton binding may assist in parallel momentum conservation in CT exciton formation.

Direct Observation of Entropy-Driven Electron-Hole Pair Separation at an Organic Semiconductor Interface.

How an electron-hole pair escapes the Coulomb potential at a donor-acceptor interface has been a key issue in organic photovoltaic research. Recent evidence suggests that long-distance charge separation can occur on ultrafast time scales, yet the underlying mechanism remains unclear. Here we use charge transfer excitons (CTEs) across an organic semiconductor-vacuum interface as a model and show that nascent hot CTEs can spontaneously climb up the Coulomb potential within 100 fs. This process is driven by entropic gain due to the rapid rise in density of states with increasing electron-hole separation. In contrast, the lowest CTE cannot delocalize, but undergoes self-trapping and recombination.

The quantum coherent mechanism for singlet fission: experiment and theory.

The absorption of one photon by a semiconductor material usually creates one electron-hole pair. However, this general rule breaks down in a few organic semiconductors, such as pentacene and tetracene, where one photon absorption may result in two electron-hole pairs. This process, where a singlet exciton transforms to two triplet excitons, can have quantum yields as high as 200%. Singlet fission may be useful to solar cell technologies to increase the power conversion efficiency beyond the so-called Shockley-Queisser limit. Through time-resolved two-photon photoemission (TR-2PPE) spectroscopy in crystalline pentacene and tetracene, our lab has recently provided the first spectroscopic signatures in singlet fission of a critical intermediate known as the multiexciton state (also called a correlated triplet pair). More importantly, we found that population of the multiexciton state rises at the same time as the singlet state on the ultrafast time scale upon photoexcitation. This observation does not fit with the traditional view of singlet fission involving the incoherent conversion of a singlet to a triplet pair. However, it provides an experimental foundation for a quantum coherent mechanism in which the electronic coupling creates a quantum superposition of the singlet and the multiexciton state immediately after optical excitation. In this Account, we review key experimental findings from TR-2PPE experiments and present a theoretical analysis of the quantum coherent mechanism based on electronic structural and density matrix calculations for crystalline tetracene lattices. Using multistate density functional theory, we find that the direct electronic coupling between singlet and multiexciton states is too weak to explain the experimental observation. Instead, indirect coupling via charge transfer intermediate states is two orders of magnitude stronger, and dominates the dynamics for ultrafast multiexciton formation. Density matrix calculation for the crystalline tetracene lattice satisfactorily accounts for the experimental observations. It also reveals the critical roles of the charge transfer states and the high dephasing rates in ensuring the ultrafast formation of multiexciton states. In addition, we address the origins of microscopic relaxation and dephasing rates, and adopt these rates in a quantum master equation description. We show the need to take the theoretical effort one step further in the near future by combining high-level electronic structure calculations with accurate quantum relaxation dynamics for large systems.

Dynamics of the triplet-pair state reveals the likely coexistence of coherent and incoherent singlet fission in crystalline hexacene.

The absorption of a photon usually creates a singlet exciton (S1) in molecular systems, but in some cases S1 may split into two triplets (2×T1) in a process called singlet fission. Singlet fission is believed to proceed through the correlated triplet-pair 1(TT) state. Here, we probe the 1(TT) state in crystalline hexacene using time-resolved photoemission and transient absorption spectroscopies. We find a distinctive 1(TT) state, which decays to 2×T1 with a time constant of 270 fs. However, the decay of S1 and the formation of 1(TT) occur on different timescales of 180 fs and <50 fs, respectively. Theoretical analysis suggests that, in addition to an incoherent S1→1(TT) rate process responsible for the 180 fs timescale, S1 may couple coherently to a vibronically excited 1(TT) on ultrafast timescales (<50 fs). The coexistence of coherent and incoherent singlet fission may also reconcile different experimental observations in other acenes.

Harvesting singlet fission for solar energy conversion via triplet energy transfer.

The efficiency of a conventional solar cell may be enhanced if one incorporates a molecular material capable of singlet fission, that is, the production of two triplet excitons from the absorption of a single photon. To implement this, we need to successfully harvest the two triplets from the singlet fission material. Here we show in the tetracene (Tc)/copper phthalocyanine (CuPc) model system that triplets produced from singlet fission in the former can transfer to the later on the timescale of 45±5 ps. However, the efficiency of triplet energy transfer is limited by a loss channel due to faster formation (400±100 fs) and recombination (2.6±0.5 ps) of charge transfer excitons at the interface. These findings suggest a design principle for efficient energy harvesting from singlet fission: one must reduce interfacial area between the two organic chromophores to minimize charge transfer/recombination while optimizing light absorption, singlet fission and triplet rather than singlet transfer.