Real-Time Observation for MoS2 Growth Kinetics and Mechanism Promoted by the Na Droplet.

While the molten salt-catalyzed chemical vapor deposition (CVD) technique is recognized for its effectiveness in producing large-area transition metal chalcogenides, understanding their growth mechanisms involving alkali metals remains a challenge. Here, we investigate the kinetics and mechanism of sodium-catalyzed molybdenum disulfide (MoS2) growth and etching through image analysis conducted using an integrated CVD microscope. Sodium droplets, agglomerated via the thermal decomposition of the sodium cholate dispersant, catalyze the precipitation of supersaturated MoS2 laminates and induce growth despite fragmentation during this process. Triangular MoS2 crystals display a distinct self-exhausting exponential behavior and slow growth of thermodynamically favorable crystallographic faces, exhibiting a sulfur-dominant pressure. The growth and etching processes are facilitated by the scooting of sodium droplets along grain edges, displaying comparable rates. Leveraging these kinetics makes it possible to engineer atypical MoS2 shapes. This combined microscope not only enhances the understanding of growth mechanisms but also contributes to the facile development of next-generation nanomaterials.

Spatiotemporally Controllable Electrical Stimulator via Independent Photobending and Upconversion Photoluminescence Using Two Different Wavelengths of Near-Infrared/Visible Light as Dual Stimuli.

Multistimuli responsive materials are advantageous in that they can enhance the desired response or bypass unwanted reactions. Light is one of the most attractive stimuli since it allows remote spatiotemporal control and multiplexing of properties (e.g., wavelength, intensity, irradiation time, pulsed/continuous wave) for application on multiphotoresponsive materials. However, the operating wavelength for such photoresponsive systems often includes an ultraviolet (UV) range that limits its use in the biomedical field. Herein, we investigate near-infrared (NIR)/visible (Vis) light-responsive nanocomposite films composed of rare earth element (i.e., Yb, Er)-doped NaYF4 nanoparticles (NPs) embedded in azobenzene-incorporated poly(dimethylsiloxane) (AzoPDMS), silk fibroin, and silver nanowire (AgNW) layers. Photobending (PB) of the nanocomposite film is induced by a Vis light of 400-700 nm, while upconversion photoluminescence (UCPL) of embedded NPs is activated by an NIR light of 980 nm. The excitation wavelength of photoluminescence (PL) is shifted to the NIR (λ = 980 nm) range via photon upconversion in rare earth element-doped NPs. Independent operation of PB and UCPL enables both on-demand electrical switching and real-time location monitoring for spatiotemporally controlled electrical pulse stimulation. As a result, the dual-photoresponsive nanocomposite film can be utilized as a remotely controllable electrical stimulator and location indicator via different wavelengths of light.

The Effects of Lengths of Flavin Surfactant N-10-Alkyl Side Chains on Promoting Dispersion of a High-Purity and Diameter-Selective Single-Walled Nanotube.

Flavin with defined helical self-assembly helps to understand chemical designs for obtaining high-purity semiconducting (s)-single-walled carbon nanotubes (SWNT) in a diameter (dt)-selective manner for high-end applications. In this study, flavins containing 8, 12, 16, and 20 n-alkyl chains were synthesized, and their single/tandem effects on dt-selective s-SWNT dispersibility were investigated at isomolarity. Flavins with n-dodecyl and longer chain lengths (FC12, FC16, and FC20) act as good surfactants for stable SWNT dispersions whereas n-octyl flavin (FC8) exhibits poor dispersibility owing to the lack of SWNT buoyancy. When used with small-dt SWNT, FC8 displays chirality-selective SWNT dispersion. This behavior, along with various flavin helical motifs, prompts the development of criteria for 'side chain length (lS)' required for stable and dt-selective SWNT dispersion, which also explains lS-dependent dt-enrichment behavior. Moreover, SWNT dispersions with flavins with dodecyl and longer lS exhibit increased metallic (m)-SWNT, background absorption-contributing carbonaceous impurities (CIs) and preferential selectivity of s-SWNT with slightly larger dt. The increased CIs that affect the SWNT quantum yield were attributed to a solubility parameter. Furthermore, the effects of flavin lS, sonication bath temperature, centrifugal speed, and surfactant concentration on SWNT purity and s-/m-SWNT ratio were investigated. A tandem FC8/FC12 provides fine-tuning of dt-selective SWNT dispersion, wherein the FC8 ratio governs the tendency towards smaller dt. Kinetic and thermodynamic assemblies of tandem flavins result in different sorting behaviors in which wide dt-tunability was demonstrated using kinetic assembly. This study highlights the importance of appropriate side chain length and other extrinsic parameters to obtain dt-selective or high-purity s-SWNT.

Hierarchical van der Waals Heterostructure Strategy to Form Stable Transition Metal Dichalcogenide Dispersions.

Although various methods have been developed to disperse transition metal dichalcogenides (TMDCs) in aqueous environments, the methodology to generate stable TMDC dispersions remains challenging. Here, we developed a hierarchical van der Waals (vdW) heterostructure-based strategy to disperse few-layered TMDCs (WS2, MoS2, WSe2, and MoSe2) using both hexagonal boron nitride (hBN) and sodium cholate (SC) as synergistic vdW surfactants. By showing long-term stability of up to 3 years, the extinction spectra of these TMDC/hBN/SC dispersions exhibit the most blue-shifted excitonic transitions, low background extinction, good colloidal stability, and dispersion stability upon ultracentrifugation compared to other dispersion methods. Hierarchical stacking having TMDCs and hBN/SC as core and shell parts is probed by electrostatic/atomic force microscopy and zeta potential, and its origin was attributed to surface energy matches. Along with the synergetic effect between TMDCs and hBN, the blue shift was ascribed to compressive strain on the TMDCs caused by hBN wrapping. The results of transmission electron microscopy show that the TMDCs in the dispersions have defective, few-layered structures with flake sizes that are less than a few hundred nm2. Raman spectroscopy is used to study not only the existence of compressive strain but also various interlayer coupling between TMDC and hBN. The hierarchical structures of TMDC/hBN/SC are discussed in terms of surface energies and topographies. This method is invaluable to provide a general methodology to disperse various surface-corrugated dimensional materials for various dispersion-based applications.

Quantification and removal of carbonaceous impurities in a surfactant-assisted carbon nanotube dispersion and its implication on electronic properties.

Carbonaceous impurities (CIs) affect the optoelectronic properties as well as the ability to use absorption spectroscopy to estimate the metallic content of a single-walled carbon nanotube (SWNT) dispersion. Therefore, a method for the accurate quantification and removal of CIs is required. We have devised methods to characterize and quantify CIs present in SWNT batches and to determine the effects of CIs on the optical and electrical properties of SWNT. Quantitative determination of CIs stems from the finding that chloroform selectively disperses CIs present in SWNT batches. CIs separated by dispersing the as-purchased SWNT batch in chloroform have the morphology of defective and agglomerated few-layered graphenes, whose sizes and locations depend on SWNT batches. Moreover, CIs exhibit a featureless UV-vis-mid-wavelength IR (MWIR) absorption curve and an extinction coefficient comparable to graphenes and show difference with carbon black, which is frequently used as the CI reference. The MWIR region that shows least absorptions caused by the transition of various SWNT types was utilized to assess the significant contribution made by CIs present in a surfactant-assisted SWNT dispersion, showing about 12-19 wt% of CIs in various SWNT dispersions. In addition, the extraction of CIs with chloroform results in a highly purified SWNT batch without any diameter distribution change originating from oxidative damage as compared to the commercially available purified SWNT batch. Finally, we found that increasing the weight of CIs present in a SWNT dispersion strongly lowers the thermal conductivity of a SWNT film when compared with the electrical conductivity. This study provides a way to understand the negative effects that CI has on the optoelectronic properties of SWNTs as well as the beneficial effects of excluding ubiquitous CIs in SWNTs batches.

Long-Term Exposure of MoS2 to Oxygen and Water Promoted Armchair-to-Zigzag-Directional Line Unzippings.

Understanding the long-term stability of MoS2 is important for various optoelectronic applications. Herein, we show that the long-term exposure to an oxygen atmosphere for up to a few months results in zigzag (zz)-directional line unzipping of the MoS2 basal plane. In contrast to exposure to dry or humid N2 atmospheres, dry O2 treatment promotes the initial formation of line defects, mainly along the armchair (ac) direction, and humid O2 treatment further promotes ac line unzipping near edges. Further incubation of MoS2 for a few months in an O2 atmosphere results in massive zz-directional line unzipping. The photoluminescence and the strain-doping plot based on two prominent bands in the Raman spectrum show that, in contrast to dry-N2-treated MoS2, the O2-treated MoS2 primarily exhibits hole doping, whereas humid-O2-treated MoS2 mainly exists in a neutral charge state with tension. This study provides a guideline for MoS2 preservation and a further method for generating controlled defects.

Flavin Mononucleotide-Mediated Formation of Highly Electrically Conductive Hierarchical Monoclinic Multiwalled Carbon Nanotube-Polyamide 6 Nanocomposites.

Although the multiwalled carbon nanotube (MWNT) is a promising material for use in the production of high electrical conductivity (σ) polymer nanocomposites, its tendency to aggregate and distribute randomly in a polymer matrix is a problematic issue. In the current study, we developed a highly conductive and monoclinically aligned MWNT-polyamide 6 (PA) nanocomposite containing interfacing flavin moieties. In this system, the flavin mononucleotide (FMN) initially serves as a noncovalent aqueous surfactant for individualizing MWNTs in the form of FMN-wrapped MWNTs (FMN-MWNT), and then partially decomposed FMN (dFMN) induces crystallization of the PA on the MWNTs. The results of experiments performed using material subjected to partial dissolution of PA matrix show that the nanocomposite PA-dFMN-MWNT, formed by melt extrusion of PA and dFMN-MWNT, contains a three-dimensional monoclinic MWNT network embedded in an equally monoclinic PA matrix. An increase in monoclinic network promoted by an increase in the content of MWNT increases σ of the nanocomposite up to 100 S/m, the highest value reported for a polymer-MWNT nanocomposite. X-ray diffraction along with transmission electron microscopy reveal that the presence of dFMN induces the formation of monoclinic PA on dFMN-MWNT. The high σ of the PA-dFMN-MWNT nanocomposite is also a consequence of a minimization of defect formation of MWNT by noncovalent functionalization. Hierarchical structural ordering, yet individualization of MWNTs, provides a viable strategy to improve the physical property of nanocomposites.

Helical Assembly of Flavin Mononucleotides on Carbon Nanotubes as Multimodal Near-IR Hg(II)-Selective Probes.

The development of novel methods to detect mercury is of paramount importance owing to the impact of this metal on human health and the environment. We observed that flavin mononucleotide (FMN) and its helical assembly with a single-walled carbon nanotube (SWNT) selectively bind Hg2+ arising from HgCl2 and MeHgCl. Absorption spectroscopic studies show that FMN preferentially forms a 2:1 rather than a 1:1 complex with Hg2+ at high FMN concentrations. On the basis of the analogy to the thymine-Hg-thymine complex, it is proposed that the 2:1 complex between FMN and Hg2+ comprises a Hg-bridged pair of FMN groups, regardless of the presence of SWNT. Upon addition of as little as a few hundred nanomoles of Hg2+, both FMN and FMN-SWNT exhibit absorption and photoluminescence (PL) changes. Moreover, FMN-SWNT displays simultaneous multiple sigmoidal changes in PL of SWNT tubes having different chiral vectors. Assessment of binding affinities using the Hill equation suggests that 2:1 and 1:1 complexes form between Hg2+ and FMN groups on the FMN-SWNT. Theoretical calculations indicate that optical changes of the FMN-SWNT originate from Hg-mediated conformational changes occurring on the helical array of FMN on the SWNT. High-resolution transmission electron microscopy revealed that the presence of Hg2+ in complexes with the FMN-SWNT enables visualization of helical periodic undulation of FMN groups along SWNT without the need for staining. Circular dichroism (CD) study revealed that FMN-SWNT whose CD signal mainly originates from FMN decreases dichroic bands upon the addition of Hg2+ owing to the formation of a centrosymmetric FMN-Hg-FMN triad on SWNT. The binding mode specificity and multimodal changes observed in response to Hg2+ ions suggest that systems based on FMN-SWNT can serve as in vivo NIR beacons for the detection of various mercury derivatives.

Determination of the Absolute Enantiomeric Excess of the Carbon Nanotube Ensemble by Symmetry Breaking Using the Optical Titration Method.

Symmetry breaking of single-walled carbon nanotubes (SWNTs) has profound effects on their optoelectronic properties that are essential for fundamental study and applications. Here, we show that isomeric SWNTs that exhibit identical photoluminescence (PL) undergo symmetry breaking by flavin mononucleotide (FMN) and exhibit dual PLs and different binding affinities (Ka). Increasing the FMN concentration leads to systematic PL shifts of SWNTs according to structural modality and handedness due to symmetry breaking. Density gradient ultracentrifugation using a FMN-SWNT dispersion displays PL shifts and different densities according to SWNT handedness. Using the optical titration method to determine the PL-based Ka of SWNTs against an achiral surfactant as a titrant, left- and right-handed SWNTs display two-step PL inflection corresponding to respective Ka values with FMN, which leads to the determination of the enantiomeric excess (ee) of the SWNT ensemble that was confirmed by circular dichroism measurement. Decreasing the FMN concentration for the SWNT dispersion leads to enantiomeric selection of SWNTs. The titration-based ee determination of the widely used sodium cholate-based SWNT dispersion was also demonstrated by using FMN as a cosurfactant.

Affinity-mediated sorting order reversal of single-walled carbon nanotubes in density gradient ultracentrifugation.

Sorted single-walled carbon nanotubes (SWNTs) are of paramount importance for their utilization in high-end optoelectronic applications. Sodium cholate (SC)-based density gradient ultracentrifugation (DGU) has been instrumental in isolating small diameter (d t) SWNTs. Here, we show that SWNTs wrapped by flavin mononucleotide (FMN) as a dispersing agent are sorted in DGU, and show sorting order reversal behavior, departing from prototypical SC-SWNT trends. Larger d t SWNTs are sorted in lower density (ρ), and buoyant ρ distribution of FMN-SWNT ranges from 1.15-1.25 g cm(-3). Such a nanotube layering pattern originates from both the binding affinity between FMN and SWNT and the less-susceptible hydrated volume of remote phosphate sidechains of FMN according to nanotube d t change.

Selective Dispersion of Highly Pure Large-Diameter Semiconducting Carbon Nanotubes by a Flavin for Thin-Film Transistors.

Scalable and simple methods for selective extraction of pure, semiconducting (s) single-walled carbon nanotubes (SWNTs) is of profound importance for electronic and photovoltaic applications. We report a new, one-step procedure to obtain respective large-diameter s- and metallic (m)-SWNT enrichment purity in excess of 99% and 78%, respectively, via interaction between the aromatic dispersing agent and SWNTs. The approach utilizes N-dodecyl isoalloxazine (FC12) as a surfactant in conjunction with sonication and benchtop centrifugation methods. After centrifugation, the supernatant is enriched in s-SWNTs with less carbonaceous impurities, whereas precipitate is enhanced in m-SWNTs. In addition, the use of an increased centrifugal force enhances both the purity and population of larger diameter s-SWNTs. Photoinduced energy transfer from FC12 to SWNTs is facilitated by respective electronic level alignment. Owing to its peculiar photoreduction capability, FC12 can be employed to precipitate SWNTs upon UV irradiation and observe absorption of higher optical transitions of SWNTs. A thin-film transistor prepared from a dispersion of enriched s-SWNTs was fabricated to verify electrical performance of the sorted sample and was observed to display p-type conductance with an average on/off ratio over 10(6) and an average mobility over 10 cm(2)/V·s.

Trap density probing on top-gate MoS2 nanosheet field-effect transistors by photo-excited charge collection spectroscopy.

Two-dimensional (2D) molybdenum disulfide (MoS2) field-effect transistors (FETs) have been extensively studied, but most of the FETs with gate insulators have displayed negative threshold voltage values, which indicates the presence of interfacial traps both shallow and deep in energy level. Despite such interface trap issues, reports on trap densities in MoS2 are quite limited. Here, we probed top-gate MoS2 FETs with two- (2L), three- (3L), and four-layer (4L) MoS2/dielectric interfaces to quantify deep-level interface trap densities by photo-excited charge collection spectroscopy (PECCS), and reported the result that deep-level trap densities over 10(12) cm(-2) may exist in the interface and bulk MoS2 near the interface. Transfer curve hysteresis and PECCS measurements show that shallow traps and deep traps are not that different in density order from each other. We conclude that our PECCS analysis distinguishably provides valuable information on deep level interface/bulk trap densities in 2D-based FETs.

Few-Layer MoS2-Organic Thin-Film Hybrid Complementary Inverter Pixel Fabricated on a Glass Substrate.

Few-layer MoS2-organic thin-film hybrid complementary inverters demonstrate a great deal of device performance with a decent voltage gain of ≈12, a few hundred pW power consumption, and 480 Hz switching speed. As fabricated on glass, this hybrid CMOS inverter operates as a light-detecting pixel as well, using a thin MoS2 channel.

Multifunctional Schottky-diode circuit comprising palladium/molybdenum disulfide nanosheet.

Many electron devices using two-dimensional dichalcogenide MoS2 have been reported beyond graphene, but those were mostly field-effect transistors except few while P-N or Schottky diode form devices should be also important. In the present study, we have fabricated a Pd-driven MoS2 Schottky diode and its related circuits for multifunctional applications: dynamic electrical rectifier, visible light sensor, and hydrogen gas sensor.

The effects of dendrimer size and central metal ions on photosensitizing properties of dendrimer porphyrins.

A series of dendrimer porphyrins (G(n)DP(M); n = generation of dendrimer, n = 1-3; M = coordination metal, M = freebase, Zn, Pt) were prepared and their photosensitizing properties were compared. All G(n)DP(M) exhibited sharp absorption in organic solvents. However, the Soret absorptions of G(n)DP(M)(CO(2)H) in 10 mM phosphate buffer solution (pH = 7.4) are broader than those of G(n)DP(M) in organic solvents, indicating inhomogeneous microenvironments of the focal porphyrin derivatives. All G(3)DP(M)(CO(2)H) successfully formed globular polyion complex micelles that were uniform in size. Under dark conditions, all G(n)DP(M)(CO(2)H) showed negligible cytotoxicity. However, all samples exhibited concentration-dependent photocytotoxicity under light irradiation. In vitro photocytotoxicity as well as singlet oxygen generation revealed that G(3)DP(Zn)(CO(2)H) is the best dendritic PS of the three different dendrimer porphyrin species.

Effect of tight flavin mononucleotide wrapping and its binding affinity on carbon nanotube covalent reactivities.

The controlled functionalization of single-walled carbon nanotubes (SWNTs) is a key to using them in high-end applications. We show that nanotube reactivity after covalent diazonium modification is governed by a chirality-specific surfactant binding affinity to SWNTs. Both metallic and semiconducting SWNTs tightly organized by a helical flavin mononucleotide (FMN) assembly exhibit two hundred times slower reactivity toward 4-methoxy benzenediazonium (4-MBD) than those wrapped by sodium dodecyl sulfate and this reactivity enables chirality- and metallicity-specific behaviours to be probed, as confirmed by absorption, Raman, and photoluminescence (PL) spectroscopy. Each reaction kinetic of the two-step SWNT PL decays originating from electron transfer and the covalent reaction of 4-MBD, respectively, is inversely proportional to the binding affinity (Ka) between FMN and the SWNTs. The observed marginally higher reaction rate of the metallic nanotube compared to the semiconducting one results from the weaker Ka value of the metallic nanotubes with FMN. An enrichment demonstration of a few nanotube chiralities using selective and slow covalent diazonium chemistry demonstrates the importance of the binding affinity between the surfactant and the SWNTs. The study provides a handle on chirality-specific covalent chemistry via surfactant-SWNT binding affinity and impacts on future-sensing schemes.

Binding affinities and thermodynamics of noncovalent functionalization of carbon nanotubes with surfactants.

Binding affinity and thermodynamic understanding between a surfactant and carbon nanotube is essential to develop various carbon nanotube applications. Flavin mononucleotide-wrapped carbon nanotubes showing a large redshift in optical signature were utilized to determine the binding affinity and related thermodynamic parameters of 12 different nanotube chiralities upon exchange with other surfactants. Determined from the midpoint of sigmoidal transition, the equilibrium constant (K), which is inversely proportional to the binding affinity of the initial surfactant-carbon nanotube, provided quantitative binding strengths of surfactants as SDBS > SC ≈ FMN > SDS, irrespective of electronic types of SWNTs. Binding affinity of metallic tubes is weaker than that of semiconducting tubes. The complex K patterns from semiconducting tubes show preference to certain SWNT chiralities and surfactant-specific cooperativity according to nanotube chirality. Controlling temperature was effective to modulate K values by 30% and enables us to probe thermodynamic parameters. Equally signed enthalpy and entropy changes produce Gibbs energy changes with a magnitude of a few kJ/mol. A greater negative Gibbs energy upon exchange of surfactant produces an enhanced nanotube photoluminescence, implying the importance of understanding thermodynamics for designing nanotube separation and supramolecular assembly of surfactant.

Handedness enantioselection of carbon nanotubes using helical assemblies of flavin mononucleotide.

In order to truly unlock advanced applications of single-walled carbon nanotubes (SWNTs), one needs to separate them according to both chirality and handedness. Here we show that the chiral D-ribityl phosphate chain of flavin mononucleotide (FMN) induces a right-handed helix that enriches the left-handed SWNTs for all suspended (n,m) species. Such enantioselectivity stems from the sp(3) hybridization of the N atom anchoring the sugar moiety to the flavin ring. This produces two FMN conformations (syn and anti) analogous to DNA. Electrostatic interactions between the neighboring uracil moiety and the 2'-OH group of the side chain provide greater stability to the anti-FMN conformation that leads to a right-handed FMN helix. The right-handed twist that the FMN helix imposes to the underlying nanotube, similar to "Indian burn", causes diameter dilation of only the left-handed SWNTs, whose improved intermolecular interactions with the overlaying FMN helix, impart enantioselection.

High-throughput graphene imaging on arbitrary substrates with widefield Raman spectroscopy.

Raman spectroscopy has been used extensively to study graphene and other sp(2)-bonded carbon materials, but the imaging capability of conventional micro-Raman spectroscopy is limited by the technique's low throughput. In this work, we apply an existing alternative imaging mode, widefield Raman imaging (WRI), to image and characterize graphene films on arbitrary substrates with high throughput. We show that WRI can be used to image graphene orders of magnitude faster than micro-Raman imaging allows, while still obtaining detailed spectral information about the sample. The advantages of WRI allow characterization of graphene under conditions that would be impossible or prohibitively time-consuming with other techniques, such as micro-Raman imaging or reflected optical microscopy. To demonstrate these advantages, we show that WRI enables graphene imaging on a large variety of substrates (copper, unoxidized silicon, suspended), large-scale studies of defect distribution in CVD graphene samples, and real-time imaging of dynamic processes.

Oxidation resistance of graphene-coated Cu and Cu/Ni alloy.

The ability to protect refined metals from reactive environments is vital to many industrial and academic applications. Current solutions, however, typically introduce several negative effects, including increased thickness and changes in the metal physical properties. In this paper, we demonstrate for the first time the ability of graphene films grown by chemical vapor deposition to protect the surface of the metallic growth substrates of Cu and Cu/Ni alloy from air oxidation. In particular, graphene prevents the formation of any oxide on the protected metal surfaces, thus allowing pure metal surfaces only one atom away from reactive environments. SEM, Raman spectroscopy, and XPS studies show that the metal surface is well protected from oxidation even after heating at 200 °C in air for up to 4 h. Our work further shows that graphene provides effective resistance against hydrogen peroxide. This protection method offers significant advantages and can be used on any metal that catalyzes graphene growth.