Session L26: Focus Session: At the Interface of Molecules and Materials III

Exciton Dynamics in Quantum Dot Films and Interfaces

Colloidal quantum dots (QD) are a promising material platform for solution-processable optoelectronic devices such as solar cells, light-emitting diodes, thermoelectric modules, and flexible electronics. Central to the operation of these devices is the formation, transport, and conversion of free charges and excitons. In the first part of the talk, I will present a comprehensive study of exciton diffusion in inhomogeneously broadened QD assemblies, including spectrally-resolved transient photoluminescence spectroscopy, transient photoluminescence quenching, time-resolved optical imaging, and kinetic Monte Carlo simulations. In the second part of the talk, I will show how nanoscale dielectric screening phenomena in atomically thin semiconductors such as MoS$_{2}$ can lead to counterintuitive energy transfer behavior from QD donors.   

Ligand Chemistry and the Low-Frequency Vibrations of Semiconductor Nanocrystals

A variety of phonon-mediated processes centrally contribute to heat dissipation in colloidal quantum dot (QD) solids, and a method to tailor the QD vibrational spectrum may allow engineering of more efficient QD devices. Organic ligands, molecules attached to the surface of the inorganic core, are known to affect QD electronic transitions through the energy level alignment and degree of passivation; however, we demonstrate for the first time that ligands also affect the QD vibrational spectrum. We use low-frequency non-resonant Raman spectroscopy to non-destructively probe the acoustic phonon vibrational structure of CdSe QD cores with a variety of different attached ligands. The frequencies of the confined acoustic modes shift depending on the size and structure of the ligand, with more massive ligands resulting in red-shifted phonon energies. We develop a mathematical model based on vibrations of an elastic sphere to understand ligand-dependent shifts in the QD Raman spectrum upon ligand exchange. These data further our understanding of the factors affecting phonon energies and heat transport in QD solids.   

Evolution of ``waterproof'' photoluminescent complexes of rare earth ions in crowded environment

Understanding behavior of rare-earth ions (REI) in crowded environments is crucial for several nano- and bio-technological applications. Evolution of REI photoluminescence in small compartments inside a silica hydrogel, mimic to a soft matter bio-environment, has been studied [doi: 10.1039/C4CP04342A] and explained within a solvation model. The model uncovered the origin of high rare earth photoluminescence efficiency to be the formation of REI complexes, surrounded by sodium deoxycholate molecules. Comparative study of these REI-deoxycholate complexes in bulk water solution and those enclosed inside the hydrogel revealed a strong correlation between an up to 5 times longer photoluminescence lifetime of REIs and appearance of the deoxycholate ordered phase, further confirmed by dynamics of REI solvation shells, REI diffusion experiments and morphological characterization of microstructure of the hydrogel.   

Nanointerfaces in InAs-Sn$_2$S$_6$ nanocrystal-ligand networks: atomistic and electronic structure from \textit{first principles}

Semiconducting nanocomposites -- consisting of nanocrystals (NCs) embedded in a host matrix -- offer exciting prospects for solar energy conversion, light emission and electronic applications. Recent advances in wet chemical techniques allow for the synthesis of NCs, their assembly into superlattices and embedding into a host matrix using only inexpensive solution processing. However, the atomistic details of such composites are poorly understood, due to the complexity of the synthesis conditions and the unavailability of robust experimental techniques to probe nanointerfaces at the microscopic level. Here we present a density functional theory investigation of the interaction of Sn$_2$S$_6$ ligands with InAs NCs. Employing a grand canonical approach, we considered a multitude of structures possibly realized at the NC-ligand interface, such as surface termination, reconstructions, passivation, substitution of subsurface atoms, ligand dissociation, NC core-shell formation and the adsorption of the ligands on NCs with different structures. This study provides guidance about the experimental conditions which lead to specific structural motifs and highlights the impact of structural details on the composite's electronic properties.   

Defect Chemistry of Nanocarbon

Defects can rule the properties of a crystal. This effect is particularly intriguing in atom-thick materials such as single-walled carbon nanotubes and graphene, where electrons, excitons, phonons, and spin may strongly couple at the defect sites due to reduced dimensionality. In this talk, we will discuss our recent progress in fundamental understanding and molecular control of sp3 defects in sp2 carbon lattices, and their applications. An sp3 defect (tetrahedral bonding, diamond-like) is created by covalently attaching a functional group to the sp2 carbon lattice (trigonal planar, honeycomb-like) of a carbon nanotube or graphene. The beauty of this type of defect is its well-defined structure and chemical tunability at the molecular level. Our experimental results have unraveled a series of intriguing and surprising roles of defects. Specific examples will be given to illustrate how defects may be used to drive reaction propagation on sp2 carbon lattices, brighten carbon nanotube photoluminescence, and create selective chemical sensors.   

Localized Excitons in Carbon Nanotubes.

It has been historically known that unintentional defects in carbon nanotubes (CNTs) may fully quench the fluorescence. However, some dopants may enhance the fluorescence by one order of magnitude thus turning the CNTs, which are excellent light absorbers, in good emitters. We have correlated the experimentally observed photoluminescence spectra to the electronic structure simulations. Our experiment reveals multiple sharp asymmetric emission peaks at energies 50-300 meV red-shifted from that of the lowest bright exciton peak. Our simulations suggest an association of these peaks with deep trap states tied to different specific chemical adducts. While the wave functions of excitons in undoped CNTs are delocalized, those of the deep-trap states are strongly localized and pinned to the dopants. These findings are consistent with the experimental observation of asymmetric broadening of the deep trap emission peaks, which can result from scattering of acoustic phonons on localized excitons. Our work lays the foundation to utilize doping as a generalized route for wave function engineering and direct control of carrier dynamics in SWCNTs toward enhanced light emission properties for photonic applications.   

Understanding the effect of surface defects on sp2 carbon and HOPG

Basal planes of graphite are traditionally believed to be inert and electrochemical activity of graphitic materials was thought to occur at high energy defect sites, i.e., step edges. However, recent studies have shown the basal surface of graphite to be highly active and these results have significant implication on design of graphitic electrodes along with numerous other graphite, graphene, and carbon nanotube-based products. The mildly hydrophilic surface of fresh graphite subsequently adsorbs airborne contaminants causing the surface to transition towards hydrophobic behavior. A missing link between electrochemical activity and wettability requires elucidation of basal plane behavior and answering whether defect density on a graphite surface affects wettability. We have quantified defect density on various grades of highly ordered pyrolytic graphite (HOPG) through AFM imaging and contrasted wettability results to describe the effect of defect sites on wettability and surface contamination.   

Calculating exciton downconversion rates in Coulombically coupled chromophores

Exciton downconversion is a second order energy transfer process that splits a high energy exciton in a donor chromophore into multiple lower energy excitons in acceptor chromophores. Downconversion has been seen in a variety of materials including rare-earth doped glassy matrices, organic crystals, and semiconductor nanocrystals, and has the potential to efficiently convert a single high energy photon into a broad distribution of lower energy excitons. A comprehensive mechanistic understanding of the energy conversion process will allow for the rational engineering of materials that can control the flow of energy in a guided fashion. To this end we formulate and implement a method for calculating multi-chromophore F\"{o}rster-like exciton transfer rates using transition charges obtained from time-dependent density functional theory.   

Raman spectroscopy of electric-field-tuned molecule-semiconductor interface

In the search for methods of studying chemical properties of surfaces and atomic-scale heterogeneous interfaces, Raman scattering promises significant potential for measuring physical and chemical properties that vary on the scale of a few chemical bonds, reporting not only about a particular chemical species, but also about the immediate chemical environment. In this work we use first-principles (DFT) computations to investigate the chemical modification of Raman spectra of organic species after being chemically absorbed on semiconductor surfaces. We examine the binding of a trans-1,2-two(4-pyridyl) ethylene molecule to the PbSe semiconductor surface and show that we can tune the degree of the interfacial chemical coupling by means of an external electric field, and at the same time, observe the induced changes in Raman spectra. In the process of applying electric bias, we observed a crossover between two regimes of the interfacial electron-vibron coupling: with vibration-induced charge transfer; and without it.   

Electronic structures at the interface between Au and CH$_{3}$NH$_{3}$PbI$_{3}$

Organometal trihalide perovskite (CH$_{3}$NH$_{3}$PbI$_{3})$-based solar cells have been developed rapidly in decades. The electronic properties of interfaces formed between Au and CH$_{3}$NH$_{3}$PbI$_{3}$ are investigated with ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS) and inverse photoemission spectroscopy (IPES). The two-step method prepared CH$_{3}$NH$_{3}$PbI$_{3}$ film, coated onto the poly (3,4-ethylenedioxythiophene) poly (styrenesulfonate) (PEDOT:PSS)/indium tin oxide (ITO) substrate, presents n-type semiconductor behavior with a band gap of 1.7 eV and a valence band (VB) edge of 1.0 eV below the Fermi energy (E$_{\mathrm{F}})$. There is an interface dipole of 0.1 eV at CH$_{3}$NH$_{3}$PbI$_{3}$/Au interface. The energy level of CH$_{3}$NH$_{3}$PbI$_{3}$ is lifted ca.0.4 eV with Au coverage of 64 {\AA} upon it, resulting in band bending and a built-in field in CH$_{3}$NH$_{3}$PbI$_{3}$ that encourages hole transport to the interface. Hole accumulation near the interface facilitates the hole transfer from CH$_{3}$NH$_{3}$PbI$_{3}$ to Au. Furthermore, the decreasing offset between the VB maximum of CH$_{3}$NH$_{3}$PbI$_{3}$ and the E$_{\mathrm{F}}$ indicates a decrease of energy loss as extracting holes from CH$_{3}$NH$_{3}$PbI$_{3}$ to Au coverage.   

One-dimensional atomic chains: rich physics in simple structures

Carbyne---1D polymorph of carbon---has been hypothesized since 1960's, however reproducible techniques of fabricating long carbon chains and measuring their properties have arrived only in the last decade. More recently, 1D chains of other compositions have been fabricated, such as CsI and BN. Here we report our first-principles studies of the properties of carbyne, which shows extreme mechanical performance\footnote{M. Liu \emph{et al.}, \emph{ACS Nano} 7, 10075 (2013)} as well as strongly strain-sensitive electronic properties.\footnote{V. I. Artyukhov, M. Liu, and B. I. Yakobson, \emph{Nano Lett.} 14, 4224 (2014)} We also study 1D nanostructures of boron which shows a reversible tension-driven transition from metallic ribbons to insulating atomic chains with an unusual antiferromagnetic ground state.\footnote{V. I. Artyukhov, M. Liu, and B. I. Yakobson, in preparation}