Daniel A. Fischer

Molecular Packing of High-Mobility Diketo Pyrrolo-Pyrrole Polymer Semiconductors with Branched Alkyl Side Chains

We describe a series of highly soluble diketo pyrrolo-pyrrole (DPP)-bithiophene copolymers exhibiting field effect hole mobilities up to 0.74 cm(2) V(-1) s(-1), with a common synthetic motif of bulky 2-octyldodecyl side groups on the conjugated backbone. Spectroscopy, diffraction, and microscopy measurements reveal a transition in molecular packing behavior from a preferentially edge-on orientation of the conjugated plane to a preferentially face-on orientation as the attachment density of the side chains increases. Thermal annealing generally reduces both the face-on population and the misoriented edge-on domains. The highest hole mobilities of this series were obtained from edge-on molecular packing and in-plane liquid-crystalline texture, but films with a bimodal orientation distribution and no discernible in-plane texture exhibited surprisingly comparable mobilities. The high hole mobility may therefore arise from the molecular packing feature common to the entire polymer series: backbones that are strictly oriented parallel to the substrate plane and coplanar with other backbones in the same layer.

Substrate-dependent interface composition and charge transport in films for organic photovoltaics

The buried interface composition of polymer-fullerene blends is found by near-edge x-ray absorption fine structure spectroscopy to depend on the surface energy of the substrate upon which they are cast. The interface composition determines the type of charge transport measured with thin film transistors. These results have implications for organic photovoltaics device design and the use of transistors to evaluate bulk mobility in blends.

Connecting dopant bond type with electronic structure in n-doped graphene

Robust methods to tune the unique electronic properties of graphene by chemical modification are in great demand due to the potential of the two dimensional material to impact a range of device applications. Here we show that carbon and nitrogen core-level resonant X-ray spectroscopy is a sensitive probe of chemical bonding and electronic structure of chemical dopants introduced in single-sheet graphene films. In conjunction with density functional theory based calculations, we are able to obtain a detailed picture of bond types and electronic structure in graphene doped with nitrogen at the sub-percent level. We show that different N-bond types, including graphitic, pyridinic, and nitrilic, can exist in a single, dilutely N-doped graphene sheet. We show that these various bond types have profoundly different effects on the carrier concentration, indicating that control over the dopant bond type is a crucial requirement in advancing graphene electronics.

Microscopy and spectroscopy of lithium nickel oxide-based particles used in high power lithium-ion cells

Structural and electronic investigations were conducted on lithium nickel oxide-based particles used in positive electrodes of 18650-type high-power Li-ion cells. K-edge X-ray absorption spectroscopy (XAS) revealed trivalent Ni and Co ions in the bulk LiNi{sub 0.8}Co{sub 0.2}O{sub 2} powder used to prepare the high power electrode laminates. Using oxygen K-edge XAS, high resolution electron microscopy, nanoprobe diffraction, and electron energy-loss spectroscopy, we identified a <5 nm thick modified layer on the surface of the oxide particles, which results from the loss of Ni and Li ordering in the layered R{bar 3}m structure. This structural change was accompanied by oxygen loss and a lowering of the Ni- and Co-oxidation states in the surface layer. Growth of this surface layer may contribute to the impedance rise observed during accelerated aging of these Li-ion cells.

Molecular Characterization of Organic Electronic Films

Organic electronics have emerged as a viable competitor to amorphous silicon for the active layer in low-cost electronics. The critical performance of organic electronic materials is closely related to their morphology and molecular packing. Unlike their inorganic counterparts, polymers combine complex repeat unit structure and crystalline disorder. This combination prevents any single technique from being able to uniquely solve the packing arrangement of the molecules. Here, a general methodology for combining multiple, complementary techniques that provide accurate unit cell dimensions and molecular orientation is described. The combination of measurements results in a nearly complete picture of the organic film morphology.

Depressed Phase Transition in Solution-Grown VO2 Nanostructures

The first-order metal-insulator phase transition in VO(2) is characterized by an ultrafast several-orders-of-magnitude change in electrical conductivity and optical transmittance, which makes this material an attractive candidate for the fabrication of optical limiting elements, thermochromic coatings, and Mott field-effect transistors. Here, we demonstrate that the phase-transition temperature and hysteresis can be tuned by scaling VO(2) to nanoscale dimensions. A simple hydrothermal protocol yields anisotropic free-standing single-crystalline VO(2) nanostructures with a phase-transition temperature depressed to as low as 32 degrees C from 67 degrees C in the bulk. The observations here point to the importance of carefully controlling the stoichiometry and dimensions of VO(2) nanostructures to tune the phase transition in this system.

Significant dependence of morphology and charge carrier mobility on substrate surface chemistry in high performance polythiophene semiconductor films

The authors report a significant dependence of the morphology and charge carrier mobility of poly(2,5-bis(3-dodecylthiophene-2-yl)thieno[3,2-b]thiophene) (pBTTT) films on the substrate surface chemistry upon heating into its liquid crystal phase. In contrast with films on bare silicon oxide surfaces, pBTTT films on oxide functionalized with octyltrichlorosilane exhibit substantial increases in the lateral dimensions of molecular terraces from nanometers to micrometers, increased orientational order, and higher charge carrier mobility. The large-scale crystallinity of this polymer plays an important role in the high carrier mobility observed in devices, but renders it more sensitive to substrate surface chemistry than other conjugated polymers.

Imaging local electronic corrugations and doped regions in graphene

Electronic structure heterogeneities are ubiquitous in two-dimensional graphene and profoundly impact the transport properties of this material. Here we show the mapping of discrete electronic domains within a single graphene sheet using scanning transmission X-ray microscopy in conjunction with ab initio density functional theory calculations. Scanning transmission X-ray microscopy imaging provides a wealth of detail regarding the extent to which the unoccupied levels of graphene are modified by corrugation, doping and adventitious impurities, as a result of synthesis and processing. Local electronic corrugations, visualized as distortions of the π*cloud, have been imaged alongside inhomogeneously doped regions characterized by distinctive spectral signatures of altered unoccupied density of states. The combination of density functional theory calculations, scanning transmission X-ray microscopy imaging, and in situ near-edge X-ray absorption fine structure spectroscopy experiments also provide resolution of a longstanding debate in the literature regarding the spectral assignments of pre-edge and interlayer states.

Combined NMR and XAS Study on Local Environments and Electronic Structures of Electrochemically Li-Ion Deintercalated Li1 − x Co1 / 3Ni1 / 3Mn1 / 3 O 2 Electrode System

Combined 6 Li magic-angle spinning (MAS) NMR, in situ metal K-edge (hard) X-ray absorption spectroscopy (XAS), and O K-edge (soft) XAS have been carried out during the first charging process for layered Li 1 - x Co 1 / 3 Ni 1 / 3 Mn 1 / 3 O 2 cathode material. The 6 Li MAS NMR results showed the presence of Li in the Ni 2 + /Mn 4 + layers, in addition to the expected sites for Li in the lithium layers. On charging, Li ions in both the transition metals and lithium layers are removed and no new resonances are observed. The metal K-edge XAS results suggest that the major charge compensation at the metal site during charge is achieved by oxidation of Ni 2 + ions, while manganese ions remain mostly unchanged in the Mn 4 + state. From observation of O K-edge XAS results, one can conclude that a large portion of the charge compensation during charge is achieved in the oxygen site. This work provides the possibility of larger capacity of the electrode material using Li in the transition metal layers and contribution of oxygen during charge.

Chemistry of NO2 on CeO2 and MgO: Experimental and theoretical studies on the formation of NO3

In environmental catalysis the destruction or removal of nitrogen oxides (DeNOx process) is receiving a lot of attention. Synchrotron-based x-ray absorption near-edge spectroscopy, high-resolution photoemission, and first-principles density-functional calculations (DFT-GGA) were used to study the interaction of nitrogen dioxide with CeO2 and MgO. The only product of the reaction of NO2 with pure CeO2 at 300 K is adsorbed nitrate. The NO3 is a thermally stable species which mostly decomposes at temperatures between 450 and 600 K. For the adsorption of NO2 on partially reduced ceria (CeO2−x), there is full decomposition of the adsorbate and a mixture of N, NO, and NO3 coexists on the surface of the oxide at room temperature. Ce3+ cations can assist in the transformation of NO and NO2 in DeNOx operations. Adsorbed NO3 (main product) and NO2 are detected after exposing MgO to NO2 gas. A partial NO2,ads→NO3,ads transformation is observed on MgO(100) from 150 to 300 K. DFT-GGA calculations show strong bonding i...