Session Q1: Focus Session: Charge & Energy Transfer II

Optical Excitation and Energy Relaxation in Graphene: Layer by Layer

Intralayer and stacking-dependent interlayer optical excitations, and the cooling of hot electrons are the critical processes underlying the operation of exciting new graphene-based optoelectronic and plasmonic devices. In this talk, I will discuss the optical excitations and subsequent energy relaxation in single layer graphene and coupled graphene layers. We will first discuss the hot electron cooling rate near the Fermi level by measuring the electron temperature as it cools dynamically. We found that a disorder-enhanced supercollision cooling mechanism provides a complete and unified picture of energy loss near the Fermi level over the wide range of electronic (15 to $\sim$ 3000 K) and lattice (10 to 295 K) temperatures, supported by measurements done both electrically (using photocurrent thermometry) and optically. In bilayer graphene, the misorientation (or twist) angle between the two layers creates new pathways for interlayer optical excitations resulting in an interlayer optical resonance with a strongly angle-dependent energy. Our Raman, broadband absorption/reflection, and transient absorption studies on bilayer graphene samples with known twist angles confirm that the dispersion of the interlayer resonance peaks follows closely the band structure of single layer graphene; however, its energy relaxation is significantly slower. These studies may lead to new methods for customizing fundamental optical characteristics of multilayer 2D materials, including the optical conductivity, (circular) polarization dependence, and the cooling dynamics of hot carriers.   

Understanding the charge-transfer phenomena between prototypical electron-donors and acceptors: TTF-TCNQ as an example

It is widely accepted that the charge transfer between the conventional electron donor and acceptor molecules is independent of their relative configurations and electrons are always transferred from the molecule with the lower ionization potential, the electron-donor, to the high electron affinity molecule, the electron-acceptor. Conventional first-principles density functional theory (DFT) supports this conclusion. However, the computational results are dominated by a term in the DFT exchange-correlation functional, which often results in qualitatively and quantitatively wrong conclusion due to an artifact. In our study of prototypical electron donor-acceptor molecules, TTF-TCNQ, we show that the conventional electronic picture is not valid and the relative orientation between TTF and TCNQ is equally important as the electronic structure of the individual molecules. Our results show that the current understanding of the donor-acceptor interaction and charge transfer mechanism has to be modified. This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy.   

Fast electron transfer at molecule-substrate interfaces

The development of efficient organic electronic devices depends substantially on the electronic coupling of the molecules at interfaces and on their arrangement at the nanometer length-scale. As an example, $\pi$-conjugated electronic systems maximize their coupling to a contact when they adsorb flat. An effective molecule-substrate interaction is mandatory for solar cells where excited electrons should be collected before recombination. Core electron spectroscopies are possibly the most suitable experimental technique to access fast electron transfer times, but introduce significant perturbation on the valence orbitals by the presence of core holes and bound excitons, further calling for theoretical analysis. This talk will focus on the investigation of elastic electron transfer processes at the molecule-substrate interface based on first-principles Green's function methods. For the electronic coupling of chromophores at semiconductor interfaces we concentrate on the linewidth for electrons excited in molecular LUMO states, as occur in photovoltaic devices or in resonant core spectroscopies, and discuss the effect of core-level excited atoms on the system properties.   

Non-adiabatic vibrational-electronic resonance in a model dimer - Implications for Photosynthetic Energy Transfer

Recently Tiwari et al. [PNAS (2013)] have shown that for a model donor-acceptor system resonance between a vibrational quantum of energy of a weakly coupled Franck-Condon vibration and the excited state excitonic energy gap leads to an unavoidable nested energy funnel on the excited state of photosynthetic antennas. Anti-correlated nuclear motions on the two pigments are responsible for such non-adiabatic effects. Here we show that several vibrational modes lying close to an excitonic energy gap in the FMO antenna complex, and a finite width of vibrational-electronic resonance lead to an even stronger non-adiabatic vibrational-electronic mixing along a generalized energy tuning coordinate. Such a generalized tuning coordinate is similar to the ``tuning coordinate'' in a conical intersection. The 2D spectroscopic signatures of the resulting non-adiabatic effects are additive and lead to more than 2x enhancement of ground state anti-correlated vibrational wavepackets which are expected to dominate the long-lived 2D signatures. Thus, several near-resonant vibrations and a finite width of non-adiabatic coupling render the nested energy funnel in the FMO antenna as a robust and promising design principle for artificial energy and charge transport.   

Dynamics of electron transfer and exciton formation at interfaces

The combination of inorganic semiconductors with organic molecules to hybrid systems promises superior functionality of the interface compared to optoelectronic properties of the single materials. We have investigated the electron dynamics of the ZnO(10-10) surface and the influence of hydrogen and several organic molecules on the electronic structure using time-resolved two-photon-photoemission (2PPE) spectroscopy. Hydrogen termination leads to the formation a metallic ZnO surface, whereas e.g. by pyridine adsorption a substantial work function reduction up to 2.9 eV is achieved, which can be useful controlling the energy level alignment at inorganic/organic interfaces. Furthermore, we directly monitor the hot electron relaxation in the ZnO conduction band and the formation of an excitonic state at the surface within a few ps, which decays mediated a thermal activated process on a 100 ps timescale. In a second set of experiments we have studied the dynamics of photoinduced electron transfer and solvation processes at the water ice-metal interface and the effect of co-adsorbed alkali ions (Na, K, Cs). Time-resolved 2PPE provides direct access to elementary processes like electron injection and the subsequent solvation dynamics which competes with the electron transfer back to the Cu(111) substrate. In particular, we study the electronic structure changes and ultrafast dynamics for the bulid-up of a solvation shell (up to about 6 water molecules) around individual alkali atoms at the metal surface. For ice mulitlayers doped with alkali ions we observe the formation of longlived electron alkali-water complexes.   

Modeling recombination processes and predicting energy conversion efficiency of dye sensitized solar cells from first principles

We present a set of algorithms based on solo first principles calculations, to accurately calculate key properties of a DSC device including sunlight harvest, electron injection, electron-hole recombination, and open circuit voltages. Two series of D-$\pi $-A dyes are adopted as sample dyes. The short circuit current can be predicted by calculating the dyes' photo absorption, and the electron injection and recombination lifetime using real-time time-dependent density functional theory (TDDFT) simulations. Open circuit voltage can be reproduced by calculating energy difference between the quasi-Fermi level of electrons in the semiconductor and the electrolyte redox potential, considering the influence of electron recombination. Based on timescales obtained from real time TDDFT dynamics for excited states, the estimated power conversion efficiency of DSC fits nicely with the experiment, with deviation below 1-2{\%}. Light harvesting efficiency, incident photon-to-electron conversion efficiency and the current-voltage characteristics can also be well reproduced. The predicted efficiency can serve as either an ideal limit for optimizing photovoltaic performance of a given dye, or a virtual device that closely mimicking the performance of a real device under different experimental settings.   

Tunable Band Edges of TiO2 via Functionalization with Phosphonic Acid Adsorbates

The deliberate design of semiconductor surfaces with band edge energies optimal for electro- or photoelectrochemical applications is a grand challenge. We examine the extent that the band edges of anatase TiO2(101) can be effectively tuned via molecular adsorption on its surface. Using density functional theory, we compute TiO2 band edge energies for a series of phosphonic acid molecules, whose intrinsic dipole moments are significantly different in both magnitude and direction. The results reveal that the molecule-substrate binding leads to a large induced dipole moment, and the induced dipole upon adsorption varies with the binding nature and configuration. Repulsive dipole-dipole interactions between molecules lead to a striking coverage-dependence of the effective dipole moments. Interestingly, computed band edge shifts in TiO2 are in excellent agreement with the experimentally measured work-function changes. Implications for the role of such adsorbates on photoelectrochemical devices will be discussed.   

A novel theoretical probe of the SrTiO$_3$ surface under water-splitting conditions

Understanding the reaction mechanisms required to generate hydrogen fuel by photoelectrolysis of water is essential to energy conversion research. These reaction pathways are strongly influenced by the geometry and electronic structure of the electrode surface under water-splitting conditions. Electrochemical microscopy has demonstrated that biasing a SrTiO$_3$ (001) surface can lead to an increase in water-splitting activity. {\it In operando} X-ray reflectivity measurements at the Cornell High Energy Synchrotron Source (CHESS) correlate this increase in activity to a significant reorganization in the surface structure but are unable to determine the exact nature of this change. Joint Density-Functional Theory (JDFT), a rigorous yet computationally efficient alternative to molecular dynamics, provides a quantum-mechanical description of an electrode surface in contact with an aqueous environment, and a microscopically detailed description of the interfacial liquid structure. Our JDFT calculations determine the structure of the activated SrTiO$_3$ surface and explore why it is correlated with higher activity for water splitting. With no empirical parameters whatsoever, we predict the X-ray crystal truncation rods for SrTiO$_3$, finding excellent agreement with experiment.   

Efficient Plasmon-Induced Hot Electron Transfer and Photochemistry in Semiconductor-Au Nanorod Heterostructures

In recent years, it has been shown that excitation of plasmons in metal nanostructures can lead to the injection of hot electrons into semiconductors and enhanced photochemistry. This novel plasmon-exciton interaction mechanism suggests that plasmonic nanostructures can potentially function as a new class of widely tunable and robust light harvesting materials for photo-detection or solar energy conversion. However, plasmon-induced hot electron injections from metal to semiconductor or molecules are still inefficient because of the competing ultrafast hot electron relaxation (via ultrafast electron-electron and electron-phonon scattering) processes within the metallic domain. In this paper we discuss a recent study on the plasmon-exciton interaction mechanisms in colloidal quantum-confined epitaxially-grown semiconductor-gold plexcitonic nanorod heterostructures. Using transient absorption spectroscopy, we show that optical excitation of plasmons in the Au tip leads to efficient hot electron injection into the semiconductor nanorod. In the presence of sacrificial electron donors, this plasmon induced hot electron transfer process can be utilized to drive photoreduction reactions under continuous illumination. Ongoing studies are examining how to further improve the plasmon induced hot electron injection efficiency through controlling the size and shape of the plasmonic and excitonic domains.