Masato M. Maitani

Study of SERS Chemical Enhancement Factors Using Buffer Layer Assisted Growth of Metal Nanoparticles on Self-Assembled Monolayers

Highly controlled morphology Au nanoparticle films can be formed on the surfaces of self-assembled monolayers (SAMs) by vapor deposition at cryogenic temperatures (approximately 10 K) with intervening condensed Xe layers on the SAMs serving as a buffer to reduce the kinetic energy of the Au atoms impinging on the surface (buffer layer assisted growth or BLAG). Under these conditions pristine Au nanoparticles (AuNp) of a uniform shape and size were deposited onto two SAMs differing only by their terminal groups, 4-benzenedithiol (BDT) and 4-methylbenzenethiol (MBT), to form -S/Au and -CH(3)/Au interfaces with essentially identical AuNp overlayer morphologies. A surface enhanced Raman (SERS) enhancement factor ratio EF(BDT)/EF(MBT) of approximately 130 was observed uniformly across the surfaces (approximately <10% variation). Since equal electromagnetic contributions to the SERS enhancements are expected from the two identically structured Au overlayer films, the observed SERS intensity ratio accordingly reflects a pure chemical enhancement (CE) contribution arising from the -S/Au relative to the -CH(3)/Au interface and thereby provides the first quantitative experimental data for the magnitude of the SERS CE for well-defined Au-molecule contacts.

Crossed-Nanowire Molecular Junctions: A New Multispectroscopy Platform for Conduction-Structure Correlations

We report a crossed-nanowire molecular junction array platform that enables direct measurement of current-voltage-temperature characteristics simultaneously with inelastic electron tunneling and Raman vibrational spectra on the same junction. Measurements on dithiol-terminated oligo(phenylene-ethynylene) junctions show both spectroscopies interrogate the gap-confined molecules to reveal distinct molecular features. This versatile platform allows investigation of advanced phenomena such as molecular switching and cooperative effects with the flexible ability to scale both the junction geometries and array sizes.

Elucidating the Structure–Property Relationships of Donor–π-Acceptor Dyes for Dye-Sensitized Solar Cells (DSSCs) through Rapid Library Synthesis by a One-Pot Procedure

The creation of organic dyes with excellent high power conversion efficiency (PCE) is important for the further improvement of dye-sensitized solar cells. We wish to describe the rapid synthesis of a 112-membered donor-π-acceptor dye library by a one-pot procedure, evaluation of PCEs, and elucidation of structure-property relationships. No obvious correlations between ε, and the η were observed, whereas the HOMO and LUMO levels of the dyes were critical for η. The dyes with a more positive E(HOMO), and with an E(LUMO)<-0.80 V, exerted higher PCEs. The proper driving forces were crucial for a high J(sc), and it was the most important parameter for a high η. The above criteria of E(HOMO) and E(LUMO) should be useful for creating high PCE dyes; nevertheless, that was not sufficient for identifying the best combination of donor, π, and acceptor blocks. Combinatorial synthesis and evaluation was important for identifying the best dye.

Nascent metal atom condensation in self-assembled monolayer matrices: coverage-driven morphology transitions from buried adlayers to electrically active metal atom nanofilaments to overlayer clusters during aluminum atom deposition on alkanethiolate/gold monolayers.

Al atom deposition with controlled coverages has been carried out on self-assembled monolayers (SAMs), prepared by assembly of HS(CH(2))(15)X, with X = -CH(3) (M-SAM) and -CO(2)CH(3) (ME-SAM), on Au {111} substrates, and the resulting structures and electrical properties analyzed in situ by ultrahigh-vacuum, multiple mode atomic force microscopy (contact, noncontact, and conducting probe) and infrared reflection spectroscopy. The M-SAM data clearly reveal a distinct morphology transition at approximately 3 Al atoms per adsorbate molecule (3 EL) from formation of a buried approximately 1:1 Al-Au adlayer at low coverages to metal overlayer cluster nucleation and the appearance of isolated metal nanofilaments with varied behaviors including Ohmic conduction, resistive switching (memristor), and vestiges of quantum-like conductance steps. The ME-SAM data confirm our earlier report of a highly efficient, 1:1 chemical trapping of initial nascent Al atoms by the terminal ester group while also revealing formation of isolated, conducting filaments, mainly at SAM defects, and the presence of an insulating overlayer up to approximately 5 EL. For both SAMs, despite the large thermochemical driving forces to exhaustively form inorganic products, subtle kinetic pathways guide the evolution of metal nanostructures within and contiguous to the SAM. Overall the experiments demonstrate a highly controlled, quantitative strategy for exploring the chemistry of nascent metal atoms with organic moieties as well as providing opportunities to generate novel metal nanostructures with significant implications for molecular and organic device applications.

Rapid Synthesis of Thiophene‐Based, Organic Dyes for Dye‐Sensitized Solar Cells (DSSCs) by a One‐Pot, Four‐Component Coupling Approach

This one-pot, four-component coupling approach (Suzuki-Miyaura coupling/C-H direct arylation/Knoevenagel condensation) was developed for the rapid synthesis of thiophene-based organic dyes for dye-sensitized solar cells (DSSCs). Seven thiophene-based, organic dyes of various donor structures with/without the use of a 3,4-ethylenedioxythiophene (EDOT) moiety were successfully synthesized in good yields based on a readily available thiophene boronic acid pinacol ester scaffold (one-pot, 3-step, 35-61%). Evaluation of the photovoltaic properties of the solar cells that were prepared using the synthesized dyes revealed that the introduction of an EDOT structure beside a cyanoacrylic acid moiety improved the short-circuit current (Jsc) while decreasing the fill factor (FF). The donor structure significantly influenced the open-circuit voltage (Voc), the FF, and the power conversion efficiency (PCE). The use of a n-hexyloxyphenyl amine donor, and our originally developed, rigid, and nonplanar donor, both promoted good cell performance (η=5.2-5.6%).

Alternate Layered Nanostructures of Metal Oxides by a Click Reaction

The nanoconjugation of two metal oxides is expected to enhance and innovate their functionalities and properties. One of the best candidates for nanoconjugation is a photocatalyst consisting of titanium and tungsten oxides. Conjugation between titanium oxide and tungsten oxide facilitates a charge separation, because excited electrons of titanium oxide will move to the conduction band of tungsten oxide. Nanoconjugations precisely arranged on a nanometer scale are crucial for improved function and the development of new properties. Nanostructural control using layered metal oxide is one of the most useful methods for arranging metal-oxide species on a nanometer scale. Layered metal oxide composed of various elemental species is expected to develop unique properties and applications. The interlayers of a layered metal oxide possess the cations for insertion of a bulky quaternary ammonium ion, thereby resulting in the formation of nanosheets by exfoliation of the layers. Nanosheets are utilized in the formation of alternate-layer structures using selforganization and the Langmuir–Blodgett method. However, these methods require step-by-step operations, which takes a lot of time and labor for multilayer structures. On the other hand, a multilayer structure can also be created in one step. Sasaki et al. reported that an alternating layered structure can be synthesized by simply mixing a liquid dispersion of a double-hydroxide nanosheet with a positively charged surface and that of a metal-oxide nanosheet with a negatively charged surface. However, in this case, cationic nanosheets must be combined, and there is no other choice than to choose layered double hydroxides as a cationic nanosheet. Therefore, the method of creating an alternating laminate structure using various metal-oxide nanosheets through a one-step process creates possibilities for varied chemical compositions. To realize this new methodology of alternating nanoconjugation, we focused on click chemistry. Click chemistry is a reaction used to easily form stable connections between specific functional groups. Sharpless suggested that the Huisgen reaction, which is a cycloaddition reaction of alkyne and azide groups, is the main reaction of click chemistry. Dinolfo et al. reported porphyrin multilayer films on inorganic substrates by the reaction of alkyne and azide groups, which indicated that click chemistry is suitable for the synthesis of multilayer structures. However, explosive sodium azide is commonly used for introducing an azido group, and the experimental operation is not easy. Therefore, the thiol–ene reaction of click chemistry was used. A thiol–ene reaction starts when a radical initiator reacts with a thiol group, and alkene and thiol groups react in good yields. The thiol–ene reaction was also applied to ferrocene multilayer films. Therefore, heterogeneous materials can be selectively connected. Herein, we propose a new methodology to realize alternating multilayer powders by using a thiol–ene reaction (Figure 1). Two kinds of metal-oxide species were each modified with alkene and alkyl thiol groups, and dispersed into organic solvent to exfoliate the layers. A radical initiator was added to the dispersion to induce a thiol–ene reaction, then the powder of hundreds of layers of alternate stacks was synthesized in one step. Titanium oxide–tungsten oxide alternating laminate was used as a photocatalyst. Recently, heterojunctions between titanium oxide and tungsten oxide nanoparticles obtained by click chemistry showed enhancement of the photocatalytic reaction. Alternate layers of metal oxides are a promising structure because of the large area of nanoconjugations.

Sequential Coupling Approach to the Synthesis of Nickel(II) Complexes with N-aryl-2-amino Phenolates

A sequential multicomponent coupling approach is a powerful method for the construction of combinatorial libraries because structurally complex and diverse molecules can be synthesized from simple materials in short steps. In this paper, an efficient synthesis of nickel(II) complexes with N-aryl-2-amino phenols via a sequential three-step coupling approach is described, for potential use in nonlinear optical materials, bioinspired catalytic systems, and near-infrared absorbing filters. Seventeen N-aryl-2-amino phenolates were successfully synthesized in high yields based on the coupling of 3,5-di-tert-butylbenzene-1,2-diol with a pivotal aromatic scaffold, 4-bromo-2-iodo-aniline, followed by sequential Suzuki-Miyaura coupling with aryl boronates. A total of 16 analytically pure nickel(II) complexes with N-aryl-2-amino phenolates were obtained from 17 complexation trials. The procedure allowed us to assemble 4 components in high yields without protection, deprotection, oxidation or reduction steps. Various building blocks that included electron-donating, electron-withdrawing, and basic were used, and readily available, nontoxic and environmentally benign substrates and reagents were employed with no generation of toxic compounds. No strict anhydrous or degassed conditions were required. Absorption spectroscopic measurement of the synthesized nickel(II) complexes revealed that the ortho-substituent Ar(1) exerted more influence on the absorption wavelength of the complexes than the para-substituent Ar(2). On the other hand, both substituents Ar(1) and Ar(2) influenced the molar absorptivity values. These observations should be useful for the design of new and useful nickel(II) complexes as near-infrared chromophores.

Microwave-enhanced photocatalysis on CdS quantum dots--Evidence of acceleration of photoinduced electron transfer.

The rate of electron transfer is critical in determining the efficiency of photoenergy conversion systems and is controlled by changing the relative energy gap of components, their geometries, or surroundings. However, the rate of electron transfer has not been controlled by the remote input of an external field without changing the geometries or materials of the systems. We demonstrate here that an applied microwave field can enhance the photocatalytic reduction of bipyridinium ion using CdS quantum dots (QDs) by accelerating electron transfer. Analysis of the time-resolved emission decay profiles of CdS quantum dots immersed in aqueous solutions of bipyridinium exhibited the shortening of their emission lifetimes, because of the accelerated electron transfer from QDs to bipyridinium under microwave irradiation. This discovery leads us to a new methodology using microwaves as an external field to enhance photocatalytic reactions.

Issues and Challenges in Vapor-Deposited Top Metal Contacts for Molecule-Based Electronic Devices

Metal vapor deposition to form ohmic contacts is commonly used in the fabrication of organic electronic devices because of significant manufacturability advantages. In the case of single molecular layer devices, however, the extremely small thickness, typically ~1-2nm, presents serious challenges in achieving good contacts and device integrity. This review focuses on recent scientific aspects of metal vapor deposition on monolayer thickness molecular films, particularly self-assembled monolayers, ranging across mechanisms of metal nucleation, metal-molecular group interactions and chemical reactions, diffusion of metal atoms within and through organic films, and the correlations of these and other factors with device function. Results for both non-reactive and reactive metal deposition are reviewed. Finally, novel strategies are considered which show promise for providing highly reliable and durable metal/organic top contacts for use in metal-molecule-metal junctions for device applications.

Microwave Effects on Co–Pi Cocatalysts Deposited on α-Fe2O3 for Application to Photocatalytic Oxygen Evolution

We analyze the effects of microwave applied in the process of photoelectrochemical deposition of cobalt-based cocatalysts, Co-Pi, onto well-orientated flat α-Fe2O3 thin films, which were fabricated by pulsed laser deposition. As compared with conventional heating, microwave significantly affects the morphology, chemical composition, and photocatalytic activity of Co-Pi/α-Fe2O3 composite. A significant enhancement in photocurrent related to photocatalytic water oxidation is achieved by the Co-Pi catalyst prepared under microwave irradiation. This, along with its interfacial electron-transfer properties, is studied by means of electrochemical impedance spectroscopy.