Inelastic light scattering spectroscopy has, since its first discovery, been an indispensable tool in physical science for probing elementary excitations, such as phonons, magnons and plasmons in both bulk and nanoscale materials. In the quantum mechanical picture of inelastic light scattering, incident photons first excite a set of intermediate electronic states, which then generate crystal elementary excitations and radiate energy-shifted photons. The intermediate electronic excitations therefore have a crucial role as quantum pathways in inelastic light scattering, and this is exemplified by resonant Raman scattering and Raman interference. The ability to control these excitation pathways can open up new opportunities to probe, manipulate and utilize inelastic light scattering. Here we achieve excitation pathway control in graphene with electrostatic doping. Our study reveals quantum interference between different Raman pathways in graphene: when some of the pathways are blocked, the one-phonon Raman intensity does not diminish, as commonly expected, but increases dramatically. This discovery sheds new light on the understanding of resonance Raman scattering in graphene. In addition, we demonstrate hot-electron luminescence in graphene as the Fermi energy approaches half the laser excitation energy. This hot luminescence, which is another form of inelastic light scattering, results from excited-state relaxation channels that become available only in heavily doped graphene.

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We report the synthesis and characterization of a new series of rod−coil block copolymers, regioregular poly(3-alkylthiophene)-b-polylactide (P3AT-PLA), where the alkyl chain in the polythiophene moiety is either 6 or 12 carbons in length. After utilizing a controlled polymerization technique to synthesize end-functionalized P3AT, these polymers were used as macroinitiators for the controlled ring-opening polymerization (ROP) of d,l-lactide. The block copolymers were characterized by 1H NMR spectroscopy, size exclusion chromatography (SEC), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), wide-angle X-ray scattering (WAXS), ultraviolet−visible (UV−vis) spectroscopy, and atomic force microscopy (AFM). In thin films of these materials (ca. 35 nm thickness), microphase separated domains are formed while the crystallinity of the P3AT majority phase is maintained. Upon chemical etching of the PLA block, we observed a nanopitted film where the crystallinity of the P3AT phase remains; characteristic pits are on the order of 35 nm in diameter with depths of up to 10 nm. The increase in the exposed surface area of the semiconducting polymer (∼150% that of the planar film) could be useful in a variety of organic electronic applications.

Exciton dynamics are investigated for size selected poly(3-hexylthiophene) samples in dilute chloroform solutions using time-resolved fluorescence. The sizes range from an average of 39 monomers (M w = 6490 Da) to an average of 168 monomers (M w = 27860 Da). Both isotropic emission transients, which monitor downhill energy migration, and time-resolved emission depolarization, used to measure orientational migration of the excitons, are reported as a function of emission energy. Downhill energy migration accelerates significantly as the chains become longer. While amplitude of the initial (sub-100 fs) depolarization increases with chain length, the subsequent rate of exciton reorientation is relatively insensitive to chain length for times less than 30 ps and then slows as the chains become longer. The chain length dependence provides additional insight into the connection between spectral diffusion and exciton spatial migration. The results are considered in terms of the distribution of accessible exciton states and how this distribution changes with chain length.

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Macromolecules with aliphatic backbones that bear pendant stable radical groups (i.e., radical polymers) have attracted much attention in applications where a supporting electrolyte is capable of aiding charge transport in solution; however, the utilization of these materials in solid state applications has been limited. Here, we synthesize a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), through a controlled reversible addition-fragmentation chain transfer (RAFT) mediated polymerization mechanism to generate well-defined and easily-tunable functional polymers. These completely amorphous, electronically-active polymers demonstrate relatively high glass transition temperatures (Tg ∼170 °C) and, because of the aliphatic nature of the backbone of the radical polymers, are almost completely transparent in the visible region of the electromagnetic spectrum. Additionally, we quantify the conductivity of PTMA (∼1×10(-6) S cm(-1)) and find it to be on par with pristine π-conjugated polymers such as poly(phenylene vinylenes) (PPVs) and poly(3-alkylthiophenes) (P3ATs). Furthermore, we demonstrate that the addition of small molecules bearing stable radical groups provides for more solid state charge hopping sites without altering the chemical nature of radical polymers; this, in turn, allows for an increase in the conductivity of PTMA relative to neat PTMA thin films while still retaining the same high degree of optical transparency and device stability. Because of the synthetic flexibility and easily-controlled doping mechanisms (that do not alter the PTMA chemistry), radical polymers present themselves as promising and tunable materials for transparent solid-state plastic electronic applications.

Conjugated rod-coil diblock copolymers self-assemble due to a balance of liquid crystalline (rod-rod) and enthalpic (rod-coil) interactions. Previous work has shown that while classical block copolymers self-assemble into a wide variety of nanostructures, when rod-rod interactions dominate self-assembly in rod-coil block copolymers, lamellar structures are preferred. Here, it is demonstrated that other, potentially more useful, nanostructures can be formed when these two interactions are more closely balanced. In particular, hexagonally packed polylactide (PLA) cylinders embedded in a semiconducting poly(3-alkylthiophene) (P3AT) matrix can be formed. This microstructure has been long-sought as it provides an opportunity to incorporate additional functionalities into a majority phase nanostructured conjugated polymer, for example in organic photovoltaic applications. Previous efforts to generate this phase in polythiophene-based block copolymers have failed due to the high driving force for P3AT crystallization. Here, we demonstrate that careful design of the P3AT moiety allows for a balance between crystallization and microphase separation due to chemical dissimilarity between copolymer blocks. In addition to hexagonally packed cylinders, P3AT-PLA block copolymers form nanostructures with long-range order at all block copolymer compositions. Importantly, the conjugated moiety of the P3AT-PLA block copolymers retains the crystalline packing structure and characteristic high time-of-flight charge transport of the homopolymer polythiophene (μ(h) ~10(-4) cm(2) V(-1) s(-1)) in the confined geometry of the block copolymer domains.