Tip-enhanced Raman spectroscopy (TERS) has been rapidly improved over the past decade and opened up opportunities to study phonon properties of materials at the nanometer scale. In this Letter, we ...report on TERS of an ultrathin MoS2 flake on a nanostructured Au on silicon surface forming a two-dimensional (2D) crystal/plasmonic heterostructure. Au nanostructures (shaped in triangles) are prepared by nanosphere lithography, and then MoS2 is mechanically exfoliated on top of them. The TERS spectra acquired under resonance conditions at 638 nm excitation wavelength evidence strain changes spatially localized to regions as small as 25 nm in TERS imaging. We observe the highest Raman intensity enhancement for MoS2 on top of Au nanotriangles due to the strong electromagnetic confinement between the tip and a single triangle. Our results enable us to determine the local strain in MoS2 induced during heterostructure formation. The maximum frequency shift of E2g mode is determined to be (4.2 ± 0.8) cm–1, corresponding to 1.4% of biaxial strain induced in the MoS2 layer. We find that the regions of maximum local strain correspond to the regions of maximum topographic curvature as extracted from atomic force microscopy measurements. This tip-enhanced Raman spectroscopy study allows us to determine the built-in strain that arises when 2D materials interact with other nanostructures.
We provide a critical review of the current state of the synthesis and applications of nano- and micro-tubes of layered graphitic carbon nitride. This emerging material has a huge potential for ...light-harvesting applications, including light sensing, artificial photosynthesis, selective photocatalysis, hydrogen storage, light-induced motion, membrane technologies, and can become a major competitor for such established materials as carbon and titania dioxide nanotubes. Graphitic carbon nitride tubes (GCNTs) combine visible-light sensitivity, high charge carrier mobility, and exceptional chemical/photochemical stability, imparting this material with unrivaled photocatalytic activities in photosynthetic processes, such as water splitting and carbon dioxide reduction. The unique geometric GCNT structure and versatility of possible chemical modifications allow new photocatalytic applications of GCNTs to be envisaged including selective photocatalysts of multi-electron processes as well as light-induced and light-directed motion of GCNT-based microswimmers. Closely-packed arrays of aligned GCNTs show great promise as multifunctional membrane materials for the light energy conversion and storage, light-driven pumping of liquids, selective adsorption, and electrochemical applications. These emerging applications require synthetic routes to GCNTs with highly controlled morphological parameters and composition to be available. We recognize three major strategies for the GCNT synthesis including templating, supramolecular assembling of precursors, and scrolling of nano-/microsheets, and outline promising routes for further progress of these approaches in the light of the most important emerging applications of GCNTs.
The current state of the synthesis and applications of nano- and micro-tubes of graphitic carbon nitride is critically reviewed.
It is believed that plasmon-excited electrons from Ag and Au nanostructures can induce photochemical reactions. However, the influence of heat generated by nanoparticle (NP) hotspots during light ...irradiation was not systematically studied yet. To evaluate the role of plasmonic heating, we performed a surface-enhanced Raman spectroscopy study of the photocatalytic conversion of 4-nitrobenzenthiol to 4-aminobenzenthiol by metal NPs with different compositions and shapes having localized surface plasmon resonances (LSPRs) spanning the whole visible range. Our collective results based on temperature-dependent studies and multiwavelength analyses show that contrary to the previous reports, the photocatalytic reaction is not only determined by the excitation of LSPRs or by the NP material (Ag or Au) but also drastically dependent on the plasmon-induced heating. This work has strong implications for the development and engineering of novel plasmonic and photonic applications where the role of localized temperature must be considered.
In this work, the thermoelectric (TE) properties of poly(3,4-ethylenedioxylthiophene):poly(styrene sulfonate) (PEDOT:PSS) thin films at room temperature are studied. Different methods have been ...applied for tuning the TE properties: 1 super(st) addition of polar solvent, dimethyl sulfoxide (DMSO), into the PEDOT:PSS solution; 2 super(nd) post-treatment of thin films with a mixture of DMSO and ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF sub(4)). It is verified that DMSO post-treatment is more efficient than DMSO addition in improving the electrical conductivity with a trivial change in the Seebeck coefficient. The power factor is increased up to 30.1 mu W mK super(-2) for the film with DMSO post-treatment, while the optimized power factor by DMSO addition is 18.2 mu W mK super(-2). It is shown that both DMSO addition and post-treatment induce morphological changes: an interconnected network of elongated PEDOT grains is generated, leading to higher electrical conductivity. In contrast, for those films post-treated in the presence of EMIMBF sub(4), an interconnected network of short and circular PEDOT grains with increased polaron density is created, resulting in the improvement in the Seebeck coefficient and a concomitant compromise in the electrical conductivity. An optimized power factor of 38.46 mu W mK super(-2) is achieved at 50 vol% of EMIMBF sub(4), which is the highest reported so far for PEDOT:PSS thin films to our knowledge. Assuming a thermal conductivity of 0.17 W mK super(-1), the corresponding ZT is 0.068 at 300 K. These results demonstrate that post-treatment is a promising approach to enhance the TE properties of PEDOT:PSS thin films. Furthermore, ionic liquid, EMIMBF sub(4), shows the potential for tuning the TE properties of PEDOT:PSS thin films viaa more environmentally benign process.
Abstract
Stability is one of the most important challenges facing material research for organic solar cells (OSC) on their path to further commercialization. In the high-performance material system ...PM6:Y6 studied here, we investigate degradation mechanisms of inverted photovoltaic devices. We have identified two distinct degradation pathways: one requires the presence of both illumination and oxygen and features a short-circuit current reduction, the other one is induced thermally and marked by severe losses of open-circuit voltage and fill factor. We focus our investigation on the thermally accelerated degradation. Our findings show that bulk material properties and interfaces remain remarkably stable, however, aging-induced defect state formation in the active layer remains the primary cause of thermal degradation. The increased trap density leads to higher non-radiative recombination, which limits the open-circuit voltage and lowers the charge carrier mobility in the photoactive layer. Furthermore, we find the trap-induced transport resistance to be the major reason for the drop in fill factor. Our results suggest that device lifetimes could be significantly increased by marginally suppressing trap formation, leading to a bright future for OSC.
The engineering of acetylenic carbon-rich nanostructures has great potential in many applications, such as nanoelectronics, chemical sensors, energy storage, and conversion, etc. Here we show the ...synthesis of acetylenic carbon-rich nanofibers via copper-surface-mediated Glaser polycondensation of 1,3,5-triethynylbenzene on a variety of conducting (e.g., copper, graphite, fluorine-doped tin oxide, and titanium) and non-conducting (e.g., Kapton, glass, and silicon dioxide) substrates. The obtained nanofibers (with optical bandgap of 2.51 eV) exhibit photocatalytic activity in photoelectrochemical cells, yielding saturated cathodic photocurrent of ca. 10 µA cm
(0.3-0 V vs. reversible hydrogen electrode). By incorporating thieno3,2-bthiophene units into the nanofibers, a redshift (ca. 100 nm) of light absorption edge and twofold of the photocurrent are achieved, rivalling those of state-of-the-art metal-free photocathodes (e.g., graphitic carbon nitride of 0.1-1 µA cm
). This work highlights the promise of utilizing acetylenic carbon-rich materials as efficient and sustainable photocathodes for water reduction.
This Perspective provides a critical summary of the current state of the art in the synthesis and properties of polyheptazine single-layer carbon nitride (SLCN). The summary combines the authors' ...research and literature reports on SLCN concerning the synthesis of single-layer polyheptazine sheets, light absorption and emission by SLCN, photochemical and photocatalytic properties of SLCN as well as examples of applications of SLCN sheets as "building blocks" in heterostructures with nanocrystalline semiconductors and metals. The Perspective is concluded with an outlook discussing the most promising directions for further studies and applications of SLCN and related composites.
This Perspective provides a critical summary of the current state of the art in the synthesis and properties of polyheptazine single-layer carbon nitride (SLCN).
The “controlled” synthesis of metastable γ-Bi2O3 by solution based approaches was reported several times recently, but the formation of Bi12SiO20 in the presence of trace amounts of silicates renders ...the results to be questionable. Here, the preparation of the Sillenite γ-Bi2O3 and the Sillenite-type Bi12SiO20 starting from the polynuclear bismuth oxido cluster Bi38O45(O2CC3H5)24(DMSO)9 is reported. γ-Bi2O3 crystallizes after calcination at 800 °C of the silicate-free hydrolysis product “Bi38O45(OH)24” on a silver sheet. Corrosion of the substrate causes contamination with silver, which is not incorporated into the Bi–O lattice, and was removed by treatment with an aqueous KCN-solution. Bi12SiO20 was obtained after hydrothermal treatment of the bismuth oxido cluster in the presence of NaOH in glass vessels or Na2SiO3 in a Teflon-lined reactor vessel followed by calcination at 600 °C. PXRD studies, scanning electron microscopy, nitrogen adsorption measurements, IR- and Raman spectroscopy, diffuse UV–vis spectroscopy, and DSC were used for characterization. The phase transition of γ-Bi2O3 to give α-Bi2O3 occurred slowly in the temperature range of 348–510 °C (ΔH γ→α = 6.57 kJ·mol–1). The silver-containing γ-Bi2O3 exhibits slightly increased Raman modes compared to the silver-free sample due to the SERS effect. In the diffuse UV–vis spectrum γ-Bi2O3 exhibits an absorption edge at λ = 485 nm (E g = 2.76 eV), and the contamination with silver results in an additional absorption edge at λ = 572 nm. Silver-free γ-Bi2O3 exhibits an absorption edge at λ = 460 nm (E g = 2.83 eV) and Bi12SiO20 at λ = 422 nm (E g = 3.16 eV). The photocatalytic activity of the compounds was investigated in the decomposition of aqueous rhodamine B under visible light irradiation, showing silver-containing γ-Bi2O3 to be slightly more effective compared to Bi12SiO20 and significantly more effective than the silver-free γ-Bi2O3.
In this work, we developed a method to study in situ the optical properties of Cu2–x Se and CuS nanocrystals upon electrochemical reduction and oxidation. Both these materials possess a strong ...localized surface plasmon resonance (LSPR) in the near-infrared region. First, the nanoparticles were embedded into a transparent film made of a perfluorinated sulfonic-acid copolymer Nafion deposited onto an ITO-coated glass. This substrate was employed as a working electrode for chronoamperometry and cyclic voltammetry measurements directly in a transparent cell allowing for simultaneous acquisition of absorption spectra of the system upon its charging/discharging. We observed that LSPR of the Cu2–x Se NCs can be well-controlled and tuned in a wide range simply by potentiostatic potential switching. Starting with an intensive plasmon of the initial as-synthesized Cu2–x Se NCs we were able to completely damp it via reduction (electron injection). Moreover, this electrochemical tuning was demonstrated to be reversible by subsequent oxidation (extracting electrons from the system). At the same time, CuS NCs did not exhibit such prominent LSPR modulation upon the same experimental conditions due to their more metallic-like electronic structure. Hence, our findings demonstrate for the first time a reversible tuning of the LSPR of copper chalcogenide NCs without any chemical or structural modification. Such a wide LSPR tunability is of paramount importance, for example in applications of these materials in photovoltaics to amplify light absorption, in systems involving plasmon–exciton interactions to controllably quench/enhance light emission, and in electrochromic devices to control their transmittance.
Conjugated polymers featuring tunable band gaps/positions and tailored active centers, are attractive photoelectrode materials for water splitting. However, their exploration falls far behind their ...inorganic counterparts. Herein, we demonstrate a molecular engineering strategy for the tailoring aromatic units of conjugated acetylenic polymers from benzene‐ to thiophene‐based. The polarized thiophene‐based monomers of conjugated acetylenic polymers can largely extend the light absorption and promote charge separation/transport. The C≡C bonds are activated for catalyzing water reduction. Using on‐surface Glaser polycondensation, as‐fabricated poly(2,5‐diethynylthieno3,2‐bthiophene) on commercial Cu foam exhibits a record H2‐evolution photocurrent density of 370 μA cm−2 at 0.3 V vs. reversible hydrogen electrode among current cocatalyst‐free organic photocathodes (1–100 μA cm−2). This approach to modulate the optical, charge transfer, and catalytic properties of conjugated polymers paves a critical way toward high‐activity organic photoelectrodes.
Photoelectrode materials for water splitting: As‐fabricated poly(2,5‐diethynylthieno3,2‐bthiophene) on Cu foam exhibits a record H2‐evolution photocurrent density of 370 μA cm−2 at 0.3 V vs. reversible hydrogen electrode, in comparison to current cocatalyst‐free organic photocathodes (1–100 μA cm−2), during water splitting. This paves a critical way towards the development of high‐activity organic photoelectrodes.
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