Hanyu Zhang

Modulating Optoelectronic Properties of Two-Dimensional Transition Metal Dichalcogenide Semiconductors by Photoinduced Charge Transfer

Atomically thin transition metal dichalcogenides (TMDCs) have attracted great interest as a new class of two-dimensional (2D) direct band gap semiconducting materials. The controllable modulation of optical and electrical properties of TMDCs is of fundamental importance to enable a wide range of future optoelectronic devices. Here we demonstrate a modulation of the optoelectronic properties of 2D TMDCs, including MoS2, MoSe2, and WSe2, by interfacing them with two metal-centered phthalocyanine (MPc) molecules: nickel Pc (NiPc) and magnesium Pc (MgPc). We show that the photoluminescence (PL) emission can be selectively and reversibly engineered through energetically favorable electron transfer from photoexcited TMDCs to MPcs. NiPc molecules, whose reduction potential is positioned below the conduction band minima (CBM) of monolayer MoSe2 and WSe2, but is higher than that of MoS2, quench the PL signatures of MoSe2 and WSe2, but not MoS2. Similarly, MgPc quenches only WSe2, as its reduction potential is situated below the CBM of WSe2, but above those of MoS2 and MoSe2. The quenched PL emission can be fully recovered when MPc molecules are removed from the TMDC surfaces, which may be refunctionalized and recycled multiple times. We also find that photocurrents from TMDCs, probed by photoconductive atomic force microscopy, increase over 2-fold only when the PL is quenched by MPcs, further supporting the photoinduced charge transfer mechanism. Our results should benefit design strategies for 2D inorganic-organic optoelectronic devices and systems with tunable properties and improved performances.

Understanding Solvent Effects on the Properties of Two-Dimensional Transition Metal Dichalcogenides

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have emerged as attractive direct bandgap semiconducting materials with remarkable properties. Recently, TMDC-based electronic and optoelectronic systems have been demonstrated with various chemical doping and functionalization approaches for modulating their physical properties and enhancing device performances. However, the dependence of intrinsic properties of TMDCs on diverse solvents, which are used necessarily in fabrication processes and chemical doping, remains largely unaddressed. Here we report a charge transfer mechanism in TMDCs by commonly used solvents such as chloroform, toluene, acetone, and 2-propanol, which significantly changes the physical properties of monolayer MoS2 and WSe2. We find that the relative difference in electronegativity between solvents and TMDCs drives the transfer of electrons from or to the TMDCs, which results in photoluminescence (PL) enhancement or quenching depending on the change of carrier density in TMDCs. The analysis of exciton and trion spectral weights in MoS2 as a function of solvent electronegativity provides evidence of charge transfer. Finally, conductive atomic force microscopy (C-AFM) on TMDCs before and after immersion in the solvents further supports the charge transfer mechanism and resulting changes in carrier density. Our results highlight the importance of selection of solvents for solution-processed 2D TMDC devices and systems.

DNA Oligonucleotide Templated Nanohybrids Using Electronic Type Sorted Carbon Nanotubes for Light Harvesting

Light harvesting nanostructure hybrids have been designed and demonstrated using single-wall carbon nanotubes (SWCNTs) and porphyrin chromophores. DNA oligonucleotides are used to conjugate SWCNTs with light-absorbing chromophores for transparent films which generate photocurrents. High-purity semiconducting SWCNTs demonstrate significant enhancement in the photocurrent compared to metallic or unsorted tubes.

Accessibility and external versus intercalative binding to DNA as assessed by oxygen-induced quenching of the palladium(II)-containing cationic porphyrins Pd(T4) and Pd(tD4).

Studies reveal that it is possible to design a palladium(II)-containing porphyrin to bind exclusively by intercalation to double-stranded DNA while simultaneously enhancing the ability to sensitize the formation of singlet oxygen. The comparisons revolve around the cations [5,10,15,20-tetra(N-methylpyridinium-4-yl)porphyrin]palladium(II), or Pd(T4), and [5,15-di(N-methylpyridinium-4-yl)porphyrin]palladium(II), or Pd(tD4), in conjunction with A═T and G≡C rich DNA binding sequences. Methods employed include X-ray crystallography of the ligands as well as absorbance, circular dichroism, and emission spectroscopies of the adducts and the emission from singlet oxygen in solution. In the case of the bulky Pd(T4) system, external binding is almost as effective as intercalation in slowing the rate of oxygen-induced quenching of the porphyrins triplet excited state. The fractional efficiency of quenching by oxygen nevertheless approaches 1 for intercalated forms of Pd(tD4), because of intrinsically long triplet lifetimes. The intensity of the sensitized, steady-state emission signal varies with the system and depends on many factors, but the Pd(tD4) system is impressive. Intercalated forms of Pd(tD4) produce higher sensitized emission yields than Pd(T4) is capable of in the absence of DNA.

Photodynamic inactivation of methicillin-resistant Staphylococcus aureus and Escherichia coli: A metalloporphyrin comparison

Increasing rates of antibiotic resistance coupled with the lack of novel antibiotics threatens proper clinical treatment and jeopardizes their use in prevention. A photodynamic approach appears to be an innovative treatment option, even for multi-drug resistant strains of bacteria. Three components are utilized in photodynamic inactivation: a photosensitizer, light source, and oxygen. Variations in photosensitizers strongly influence microbial binding and bactericidal activity. In this study, four different cationic metalloporphyrins (Cu2+, Fe2+, Pd2+, Zn2+) were compared to the free-base ligand 5,10,15,20-tetrakis(N-methylpyridinium-4-yl)porphyrin regarding their electronic properties and generation of reactive oxygen species upon subsequent 405nm violet-blue irradiation. Staphylococcus aureus and Escherichia coli were used as representatives of Gram-positive and -negative, respectively, to assess bactericidal effects by the photodynamic process. Bacterial cultures were pre-incubated with porphyrins and exposed to varying doses of 405nm irradiation (0-30J/cm2). Metalloporphyrins containing Cu2+ and Fe2+ demonstrated minimal effects on viability. Pronounced bactericidal activity was evident with free-base ligand, Zn2+, and Pd2+; though significantly stronger effects were apparent with Pd2+. Photodynamic killing was directly proportional to reactive oxygen species production post-illumination. These data provide new insight into the influence of metal chelation on photosensitizer activity on bactericidal singlet oxygen production. The strong anti-microbial photodynamic action through the use of a portable light-emitting diode over short time intervals (seconds) provides support for its potential use in self-treatment.

Dynamic and Progressive Control of DNA Origami Conformation by Modulating DNA Helicity with Chemical Adducts

DNA origami has received enormous attention for its ability to program complex nanostructures with a few nanometer precision. Dynamic origami structures that change conformation in response to environmental cues or external signals hold great promises in sensing and actuation at the nanoscale. The reconfiguration mechanism of existing dynamic origami structures is mostly limited to single-stranded hinges and relies almost exclusively on DNA hybridization or strand displacement. Here, we show an alternative approach by demonstrating on-demand conformation changes with DNA-binding molecules, which intercalate between base pairs and unwind DNA double helices. The unwinding effect modulates the helicity mismatch in DNA origami, which significantly influences the internal stress and the global conformation of the origami structure. We demonstrate the switching of a polymerized origami nanoribbon between different twisting states and a well-constrained torsional deformation in a monomeric origami shaft. The structural transformation is shown to be reversible, and binding isotherms confirm the reconfiguration mechanism. This approach provides a rapid and reversible means to change DNA origami conformation, which can be used for dynamic and progressive control at the nanoscale.

Multiplexed Optical Detection of Plasma Porphyrins Using DNA Aptamer-Functionalized Carbon Nanotubes

A novel optical platform based on DNA aptamer-functionalized SWCNTs (a-SWCNTs) is developed for multiplexed detection of plasma porphyrins. We have investigated the interactions of a-SWCNTs with heme (FePP), protoporphyrin (PP), coproporphyrin (CP), and uroporphyrin (UP). Two interaction mechanisms, specific binding, and nonspecific adsorption between porphyrins and a-SWCNTs are proposed based on observed optical signal modulations. The optical transduction signals are used to formulate a multiplexed detection strategy for the four porphyrin species without a laborious separation process. The detection scheme is sensitive, selective, and can readily be used for porphyrin detection in plasma samples when combined with a solvent extraction method. Our optical platform offers novel analytical tools for probing the surface chemistry at the porphyrin/a-SWCNTs interface, showing great promise for both research and clinical applications.

Engineering Chemically Exfoliated Large‐Area Two‐Dimensional MoS2 Nanolayers with Porphyrins for Improved Light Harvesting

Molybdenum disulfide (MoS2 ) is a promising candidate for electronic and optoelectronic applications. However, its application in light harvesting has been limited in part due to crystal defects, often related to small crystallite sizes, which diminish charge separation and transfer. Here we demonstrate a surface-engineering strategy for 2D MoS2 to improve its photoelectrochemical properties. Chemically exfoliated large-area MoS2 thin films were interfaced with eight molecules from three porphyrin families: zinc(II)-, gallium(III)-, iron(III)-centered, and metal-free protoporphyrin IX (ZnPP, GaPP, FePP, H2 PP); metal-free and zinc(II) tetra-(N-methyl-4-pyridyl)porphyrin (H2 T4, ZnT4); and metal-free and zinc(II) tetraphenylporphyrin (H2 TPP, ZnTPP). We found that the photocurrents from MoS2 films under visible-light illumination are strongly dependent on the interfacial molecules and that the photocurrent enhancement is closely correlated with the highest occupied molecular orbital (HOMO) levels of the porphyrins, which suppress the recombination of electron-hole pairs in the photoexcited MoS2 films. A maximum tenfold increase was observed for MoS2 functionalized with ZnPP compared with pristine MoS2 films, whereas ZnT4-functionalized MoS2 demonstrated small increases in photocurrent. The application of bias voltage on MoS2 films can further promote photocurrent enhancements and control current directions. Our results suggest a facile route to render 2D MoS2 films useful for potential high-performance light-harvesting applications.

Regeneration of Light-Harvesting Complexes via Dynamic Replacement of Photodegraded Chromophores

All-synthetic molecular donor-acceptor complexes are designed, which are capable of counteracting the effect of photoinduced degradation of donor chromophores. Anionic gallium protoporphyrin IX (GaPP) and semiconducting carbon nanotube (CNT) are used as a model donor-acceptor complex, which is assembled using DNA oligonucleotides. The GaPP-DNA-CNT complex produces an anodic photocurrent in a photoelectrochemical cell, which steadily decays due to photo-oxidation. By modulating the chemical environment, we showed that the photodegraded chromophores may be dissociated from the complex, whereas the DNA-coated carbon nanotube acceptors are kept intact. Reassociation with fresh porphyrins leads to the full recovery of GaPP absorption and photocurrents. This strategy could form a basis for improving the light-harvesting performance of molecular donor-acceptor complexes and extending their operation lifetime.

Carbon nanotube solar cells

Abstract: Carbon nanotubes have been explored in light-harvesting and photovoltaic devices because of their unique optoelectronic properties. This chapter provides a brief description of the optoelectronic properties of carbon nanotubes, particularly single-wall carbon nanotubes (SWCNTs), and their implication in various solar cell applications including donor–acceptor solar cells, polymer solar cells, and dye-sensitized solar cells, where carbon nanotubes are utilized as photoactive materials. Carbon-nanotube-based electrodes in photovoltaic devices are also introduced. Carbon-nanotube-based light-harvesting devices are reviewed in terms of fabrication and material processing as well as performance. Finally, advanced emerging methods and the future outlook for carbon-nanotube-based solar cells are discussed.