Ming L. Tang

Nanoantenna-enhanced gas sensing in a single tailored nanofocus

We demonstrate antenna-enhanced hydrogen sensing at the single-particle level. We place a single palladium nanoparticle near the tip region of a gold nanoantenna and detect the changing optical properties of the system upon hydrogen exposure.

Ambipolar, high performance, acene-based organic thin film transistors.

We present a high performance, ambipolar organic field-effect transistor composed of a single material. Ambipolar molecules are rare, and they can enable low-power complementary-like circuits. This low band gap, asymmetric linear acene contains electron-withdrawing fluorine atoms, which lower the molecular orbital energies, allowing the injection of electrons. While hole and electron mobilities of up to 0.071 and 0.37 cm2/V.s, respectively, are reported on devices measured in nitrogen, hole mobilities of up to 0.12 cm2/V.s were found in ambient, with electron transport quenched. These devices were fabricated on octadecyltrimethoxysilane-treated surfaces at a substrate temperature of 60 degrees C.

Correlating Carrier Type with Frontier Molecular Orbital Energy Levels in Organic Thin Film Transistors of Functionalized Acene Derivatives

We investigate the relationship between the charge carrier type in organic thin film transistors (OTFTs) and molecular energy levels. We examine a series of functionalized acenes that collectively have their HOMOs range from -4.9 eV to -5.6 eV and LUMOs range from -2.8 eV to -3.7 eV, as measured by cyclic voltammetry. Placed together, these 20 molecules allow us to chart the transition from OTFTs that display only hole transport, to ambipolar, to solely electron transport. Specifically, we note that for octadecyltrimethoxysilane (OTS) treated substrates, with top contact gold electrodes, electron injection and transport occurs when the LUMO < -3.15 eV, while hole injection and transport ceases when the HOMO < -5.6 eV. Ambipolar transport prevails when molecules have HOMO/ LUMO levels within the aforementioned range. This is seen across channel lengths ranging from 50-150 microm and using only gold as electrodes. This empirical plot is the first time such a detailed study has been made on the onset of charge injection and transport for a class of organic semiconductors. It provides guidelines for future molecular design.

Structural and Electronic Study of an Amorphous MoS3 Hydrogen‐Generation Catalyst on a Quantum‐Controlled Photosensitizer

The design and synthesis of catalysts, especially for the production of solar fuels, is a major challenge in developing sources of renewable energy. Catalyst development requires an understanding of the mechanism(s) involved and the nature of the active site. While platinum group metals have unrivalled activity for both hydrogen and oxygen evolution, they are scarce and expensive. Photocatalytic systems relying on earth-abundant materials are therefore desirable for large scale energy production. Herein, we examine the structure and electronic properties of an amorphous molybdenum sulfide species and its possible use for photocatalytic hydrogen evolution. The catalyst was grown on a seeded quantum-rod sensitizer, a model system for investigating the photophysics of solar fuel generation. This catalyst s activity is shown experimentally to be associated with under-coordinated molybdenum centers, and we document that a reduced form of MoS3 is an active species for hydrogen generation. Molybdenum sulfides are prevalent in both biological enzymes and industrial catalysts. Mo metalloenzymes are involved in carbon, nitrogen, and sulfur metabolism, while synthetic molybdenum sulfides serve as industrial hydrotreating catalysts and are proven electrocatalysts for the hydrogen evolution reaction (HER). MoS2, [13,14] incomplete cubane [Mo3S4] 4+ clusters, molecular molybdenum catalysts, and amorphous MoS2 made by a reduction of MoS3 [19] have been shown to be active HER catalysts. Highly active HER catalysts, including Pt, have a Gibbs free energy of H adsorption (DGH) close to zero. [14] Density functional theory calculations show that the equatorial sulfur atoms in Fe–Mo cofactors in nitrogenase enzymes as well as the bridging S atom on the edge sites of MoS2 bind H atoms with DGH 0. These calculations, coupled with scanning tunneling microscopy (STM) studies, have indicated that molybdenum sulfide based hydrodesulfurization and HER catalysts derive their activities from under-coordinated atoms. Recent investigations of MoS2 nanoparticles using STM combined with electrochemical measurements have revealed that HER activity scales with the number of edge sites, rather than nanoparticle area, adding substantial evidence that undercoordination is critical to activity. There is also substantial current interest in molecularly thick and structurally disordered metal oxide and sulfide layers supported on electrodes, surfaces, and nanoparticles as potential catalysts for the HER and oxygen evolution reaction (OER). Such ultrathin films can support a variety of unusual and possibly favorable bonding geometries and may retain flexibility in healing and recovering. Despite their potential, such systems remain very difficult to characterize, impeding reproducibility and the communication of results between groups. Mechanisms are difficult to pin down when structural and electronic characterization is lacking. In this work, we use X-ray absorption techniques to obtain structural information on a catalytically active disordered molybdenum chalcogenide species that was grown on a wellcontrolled seeded quantum rod photosensitizer system with very high surface area. The high surface area of the colloidal system enables us to employ a variety of X-ray characterization techniques. Yet the system is also well-defined: Amorphous layers of MoS3 are deposited on quantumcontrolled photosensitizers. We take advantage of recent work showing that cadmium chalcogenide nanocrystals can be engineered to systematically control the separation of photogenerated holes and electrons, thus allowing us to modulate the photochemical yield of hydrogen. Nanorods of CdS grown on CdSe seeds with varying diameters and pure CdS nanorods of differing length were synthesized by a seeded-growth method previously reported. These particles have been of interest as a model system for investigating photochemical HER because their [*] Dr. M. L. Tang, D. C. Grauer, Dr. L. Amirav, Prof. J. R. Long, Prof. A. P. Alivisatos Department of Chemistry, University of California, Berkeley Material Sciences, Lawrence Berkeley National Laboratory (LBNL) Berkeley, CA 94720 (USA) E-mail: alivis@berkeley.edu jyano@lbl.gov

Observations of Shape-Dependent Hydrogen Uptake Trajectories from Single Nanocrystals

In this work, H(2) absorption and desorption in faceted, crystalline Au/Pd core/shell nanocrystals and their interaction with a SiO(x)/Si support were studied at the single-particle level. Dark-field microscopy was used to monitor the changing optical properties of these Au/Pd nanoparticles (NPs) upon exposure to H(2) as reversible H(2) uptake from the Pd shell proceeded. Analysis of the heterogeneous ensemble of NPs revealed the H(2) uptake trajectory of each nanocrystal to be shape-dependent. Differences in particle uptake trajectories were observed for individual particles with different shapes, faceting, and Pd shell thickness. In addition to palladium hydride formation, the single-particle trajectories were able to decipher specific instances where palladium silicide formation and Au/Pd interdiffusion occurred and helped us determine that this was more frequently seen in those particles within an ensemble having thicker Pd shells. This noninvasive, plasmonic-based direct sensing technique shows the importance of single-particle experiments in catalytically active systems and provides a foundation for studying more complex catalytic processes in inhomogeneous NP systems.

Pentaceno[2,3-b]thiophene, a hexacene analogue for organic thin film transistors.

Hexacene and larger fused rings remain elusive targets for chemists. Here, we report a hexacene-like molecule containing six linearly fused rings, specifically a pentacene molecule fused with a terminal thiophene ring, pentaceno[2,3-b]thiophene. It can be purified and isolated as a purple-black powder at ambient conditions. This molecule has a low HOMO-LUMO gap of 1.75 eV in o-DCB and an optical band gap of 1.58 eV in thin film. Top contact organic thin film transistors (OTFTs) were made, and atomic force microscopy (AFM) reveals a dendritic thin film growth characteristic of pentacene. An OTFT mobility of 0.574 cm(2)/V s was measured for pentaceno[2,3-b]thiophene under nitrogen.

Thin film structure of tetraceno[2,3-b]thiophene characterized by grazing incidence X-ray scattering and near-edge X-ray absorption fine structure analysis.

Understanding the structure-property relationship for organic semiconductors is crucial in rational molecular design and organic thin film process control. Charge carrier transport in organic field-effect transistors predominantly occurs in a few semiconductor layers close to the interface in contact with the dielectric layer, and the transport properties depend sensitively on the precise molecular packing. Therefore, a better understanding of the impact of molecular packing and thin film morphology in the first few monolayers above the dielectric layer on charge transport is needed to improve the transistor performance. In this Article, we show that the detailed molecular packing in thin organic semiconductor films can be solved through a combination of grazing incidence X-ray diffraction (GIXD), near-edge X-ray absorption spectra fine structure (NEXAFS) spectroscopy, energy minimization packing calculations, and structure refinement of the diffraction data. We solve the thin film structure for 2 and 20 nm thick films of tetraceno[2,3-b]thiophene and detect only a single phase for these thicknesses. The GIXD yields accurate unit cell dimensions, while the precise molecular arrangement in the unit cell was found from the energy minimization and structure refinement; the NEXAFS yields a consistent molecular tilt. For the 20 nm film, the unit cell is triclinic with a = 5.96 A, b = 7.71 A, c = 15.16 A, alpha = 97.30 degrees, beta = 95.63 degrees, gamma = 90 degrees; there are two molecules per unit cell with herringbone packing (49-59 degree angle) and tilted about 7 degrees from the substrate normal. The thin film structure is significantly different from the bulk single-crystal structure, indicating the importance of characterizing thin film to correlate with thin film device performance. The results are compared to the corresponding data for the chemically similar and widely used pentacene. Possible effects of the observed thin film structure and morphology on charge carrier mobility are discussed.