One-dimensional van der Waals heterostructures: Growth mechanism and handedness correlation revealed by nondestructive TEM.

We recently synthesized one-dimensional (1D) van der Waals heterostructures in which different atomic layers (e.g., boron nitride or molybdenum disulfide) seamlessly wrap around a single-walled carbon nanotube (SWCNT) and form a coaxial, crystalized heteronanotube. The growth process of 1D heterostructure is unconventional-different crystals need to nucleate on a highly curved surface and extend nanotubes shell by shell-so understanding the formation mechanism is of fundamental research interest. In this work, we perform a follow-up and comprehensive study on the structural details and formation mechanism of chemical vapor deposition (CVD)-synthesized 1D heterostructures. Edge structures, nucleation sites, and crystal epitaxial relationships are clearly revealed using transmission electron microscopy (TEM). This is achieved by the direct synthesis of heteronanotubes on a CVD-compatible Si/SiO2 TEM grid, which enabled a transfer-free and nondestructive access to many intrinsic structural details. In particular, we have distinguished different-shaped boron nitride nanotube (BNNT) edges, which are confirmed by electron diffraction at the same location to be strictly associated with its own chiral angle and polarity. We also demonstrate the importance of surface cleanness and isolation for the formation of perfect 1D heterostructures. Furthermore, we elucidate the handedness correlation between the SWCNT template and BNNT crystals. This work not only provides an in-depth understanding of this 1D heterostructure material group but also, in a more general perspective, serves as an interesting investigation on crystal growth on highly curved (radius of a couple of nanometers) atomic substrates.

Synthesis of Siloxane Derivatives of Phenylboronic Acids of Different Structures.

The synthesis of a storage-stable organosilicon modifier with a dioxaborolane-protecting group is described. Its high reactivity and selective anti-Markovnikov addition in hydrosilylation reactions to afford siloxanes of various structures are shown. The possibility of deprotection of both the initial modifier and its siloxane derivatives under mild conditions using water in yields up to 96% is demonstrated. The existence of an equilibrium between the organosilicon derivatives of phenylboronic acids and their cyclic six-membered boroxines was confirmed by 1H NMR spectroscopy and X-ray diffraction analysis data. The use of siloxane derivatives of phenylboronic acids in Suzuki-Miyaura and Chan-Lam cross-coupling reactions was studied. All synthesized compounds were characterized by NMR (1H, 11B, 13C, and 29Si), IR spectroscopy, and high-resolution mass spectrometry.

Phenomenological model of thermal transport in carbon nanotube and hetero-nanotube films.

The thermal properties of individual single-walled carbon nanotubes (SWCNTs) have been well documented in the literature following decades of intensive study. However, when SWCNTs form a macroscale assembly, the thermal transport in these complex structures usually not only depends on the properties of the individual tubes, but also is affected and sometimes dominated by inner structural details, e.g. bundles and junctions. In this work, we first performed an experimental measurement of the thermal conductivities of individual SWCNT bundles of different sizes using a suspended micro-thermometer. The results, together with the data that we obtained from a previous work, give a complete experimental understanding of the effect of bundling on the thermal conductivity of SWCNTs. With these quantitative understandings, we propose a phenomenological model to describe the thermal transport in two-dimensional (2D) SWCNT films. The term 'line density' is defined to describe the effective thermal transport channels in this complex 2D network. Along with experimentally obtained geometric statistics and film transparency, the thermal conductance of SWCNTs is estimated, and the effects of bundle length, diameter, and contact conductance are systematically discussed. Finally, we extend this model to explain thermal transport in 2D networks of one-dimensional van der Waals heterostructures, which are coaxial hetero-nanotubes we recently synthesized using SWCNTs as the template. This extended model suggests that the contribution of boron nitride nanotubes (BNNTs) to the overall performance of a SWCNT-BNNT heterostructured film depends on the transparency of the original SWCNT film. The increase in the thermal conductance of a highly transparent film is estimated to be larger than that of a less transparent film, which shows a good agreement with our experimental observations and proves the validity of the proposed phenomenological model.

Ultrafast Optoelectronic Processes in 1D Radial van der Waals Heterostructures: Carbon, Boron Nitride, and MoS2 Nanotubes with Coexisting Excitons and Highly Mobile Charges.

Heterostructures built from 2D, atomically thin crystals are bound by the van der Waals force and exhibit unique optoelectronic properties. Here, we report the structure, composition and optoelectronic properties of 1D van der Waals heterostructures comprising carbon nanotubes wrapped by atomically thin nanotubes of boron nitride and molybdenum disulfide (MoS2). The high quality of the composite was directly made evident on the atomic scale by transmission electron microscopy, and on the macroscopic scale by a study of the heterostructure's equilibrium and ultrafast optoelectronics. Ultrafast pump-probe spectroscopy across the visible and terahertz frequency ranges identified that, in the MoS2 nanotubes, excitons coexisted with a prominent population of free charges. The electron mobility was comparable to that found in high-quality atomically thin crystals. The high mobility of the MoS2 nanotubes highlights the potential of 1D van der Waals heterostructures for nanoscale optoelectronic devices.

Ultrafast saturable absorption of large-diameter single-walled carbon nanotubes for passive mode locking in the mid-infrared.

We study the saturable absorption properties of single-walled carbon nanotubes (SWCNTs) with a large diameter of 2.2 nm and the corresponding exciton resonance at a wavelength of 2.4 µm. At resonant excitation, a large modulation depth of approximately 30 % and a small saturation fluence of a few tens of µJ/cm2 are evaluated. The temporal response is characterized by an instantaneous rise and a subpicosecond recovery. We also utilize the SWCNTs to realize sub-50 fs, self-start mode locking in a Cr:ZnS laser, revealing that the film thickness is an important parameter that affects the possible pulse energy and duration. The results prove that semiconductor SWCNTs with tailored diameters exceeding 2 nm are useful for passive mode locking in the mid-infrared range.

Organoboron Derivatives of Stereoregular Phenylcyclosilsesquioxanes.

This study presents the synthesis of organoboron derivatives of stereoregular 4-, 6-, and 12-unit phenylcyclosilsesquioxanes. All compounds obtained were isolated in good yields (70-80 %) and were fully characterized by 1 H, 13 C, 29 Si, 11 B NMR, IR spectroscopy, HRMS ESI, and elemental microanalysis. The structure of the key modifier, obtained for the first time, 4-(tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl) dimethylvinylsilane, was also confirmed by single-crystal XRD.

Vapor-Assisted Ex-Situ Doping of Carbon Nanotube toward Efficient and Stable Perovskite Solar Cells.

Single-walled carbon nanotubes (CNTs) has been considered as a promising material for a top electrode of perovskite solar cells owing to its hydrophobic nature, earth-abundance, and mechanical robustness. However, its poor conductivity, a shallow work function, and nonreflective nature have limited further enhancement in power conversion efficiency (PCE) of top CNT electrode-based perovskite solar cells. Here, we introduced a simple and scalable method to address these issues by utilizing an ex-situ vapor-assisted doping method. Trifluoromethanesulfonic acid (TFMS) vapor doping of the free-standing CNT sheet enabled tuning of conductivity and work function of the CNT electrode without damaging underneath layers. The sheet resistance of the CNT sheet was decreased by 21.3% with an increase in work function from 4.75 to 4.96 eV upon doping of TFMS. In addition, recently developed 2D perovskite-protected Cs-containing formamidium lead iodide (FACsPbI3) technology was employed to maximize the absorption. Because of the lowered resistance, better energy alignment, and improved absorption, the CNT electrode-based PSCs produced a PCE of 17.6% with a JSC of 24.21 mA/cm2, VOC of 1.005 V, and FF of 0.72. Furthermore, the resulting TFMS-doped CNT-PSCs demonstrated higher thermal and operational stability than bare CNT and metal electrode-based devices.

Controlled Redox of Lithium-Ion Endohedral Fullerene for Efficient and Stable Metal Electrode-Free Perovskite Solar Cells.

High efficiency perovskite solar cells have underpinned the rapid growth of the field. However, their low device stability limits further advancement. Hygroscopic lithium bis(trifluoromethanesulfonyl)imide (Li+TFSI-) and metal electrode are the main causes of the device instability. In this work, the redox reaction between lithium-ion endohedral fullerenes and 2,2',7,7'-tetrakis(N,N-di-p-methoxyphenylamine)-9,9'-spirobi-fluorene (spiro-MeOTAD) was controlled to optimize the amount of oxidized spiro-MeOTAD and antioxidizing neutral endohedral fullerenes. Application of this mixture to metal-free carbon nanotube (CNT)-laminated perovskite solar cells resulted in 17.2% efficiency with a stability time of more than 1100 h under severe conditions (temperature = 60 °C, humidity = 70%). Such high performance is attributed to the uninhibited charge flow, no metal-ion migration, and the enhanced antioxidizing activity of the devices.

Aerobic Co-/ N-Hydroxysuccinimide-Catalyzed Oxidation of p-Tolylsiloxanes to p-Carboxyphenylsiloxanes: Synthesis of Functionalized Siloxanes as Promising Building Blocks for Siloxane-Based Materials.

Synthesis of organosilicon products with a "polar" functional group within organic substituents is one of the most fundamentally and practically important challenges in today's chemistry of silicones. In our study, we suggest a solution to this problem, viz., a high-efficiency preparative method based on aerobic Co-/ N-hydroxysuccinimide (NHSI) catalyzed oxidation of p-tolylsiloxanes to p-carboxyphenylsiloxanes. This approach is based on "green", commercially available, simple, and inexpensive reagents and employs mild reaction conditions: Co(OAc)2/NHSI catalytic system, O2 as the oxidant, process temperature from 40 to 60 °C, atmospheric pressure. This reaction is general and allows for synthesizing both mono- and di-, tri-, and poly( p-carboxyphenyl)siloxanes with p-carboxyphenyl groups at 1,1-, 1,3-, 1,5-, and 1,1,1-positions. All the products were obtained and isolated in gram amounts (up to 5 g) and in high yields (80-96%) and characterized by NMR, ESI-HRMS, GPC, IR, and X-ray data: p-carboxyphenylsiloxanes in crystalline state form HOF-like structures. Furthermore, it was shown that the suggested method is applicable for the oxidation of organic alkylarene derivatives (Ar-CH3, Ar-CH2-R) to the corresponding acids and ketones (Ar-C(O)OH and Ar-C(O)-R), as well as hydride silanes ([Si]-H) to silanols ([Si]-OH). The possibility of synthesizing monomeric (methyl) and polymeric (siloxane-containing PET analogue, Sila-PET) esters based on 1,3-bis( p-carboxyphenyl)disiloxane was studied. These processes occur with retention of the organosiloxane frame and allow to obtain the corresponding products in 90 and 99% yields.

Self-starting mode-locked Cr:ZnS laser using single-walled carbon nanotubes with resonant absorption at 2.4 µm.

We develop a mode-locked Cr:ZnS polycrystalline laser using single-walled carbon nanotubes (SWCNTs) that have resonant absorption at the wavelength of 2.4 µm. The laser generates ultrashort pulses of 49 fs duration, a 2.4 µm center wavelength, and a 9.2 THz (176 nm) spectral span at a repetition rate of 76 MHz. We also confirm self-starting of the mode-locked operation. SWCNTs, if appropriately controlled in terms of their diameters, prove to be useful as ultrafast saturable absorbers in the mid-infrared region.

Enhanced efficiency of hybrid amorphous silicon solar cells based on single-walled carbon nanotubes and polymer composite thin film.

We report a simple approach to fabricate hybrid solar cells (HSCs) based on a single-walled carbon nanotube (SWCNT) film and thin film hydrogenated amorphous silicon (a-Si:H). Randomly oriented high-quality SWCNTs with conductivity enhanced by means of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate are used as a window layer and a front electrode. A series of HSCs are fabricated in ambient conditions with varying SWCNT film thicknesses. The polymethylmethacrylate layer drop-casted on fabricated HSCs reduces the reflection fourfold and enhances the short-circuit J sc , open-circuit V oc , and efficiency by nearly 10%. A state-of-the-art J-V performance is shown for SWCNT/a-Si HSC with an open-circuit voltage of 900 mV and an efficiency of 3.4% under simulated one-sun AM 1.5 G direct illumination.

Flexible self-powered piezo-supercapacitor system for wearable electronics.

The integration of energy harvesting and energy storage in a single device both enables the conversion of ambient energy into electricity and provides a sustainable power source for various electronic devices and systems. On the other hand, mechanical flexibility, coupled with optical transparency of the energy storage devices, is required for many applications, ranging from self-powered rolled-up displays to wearable optoelectronic devices. We integrate a piezoelectric poly(vinylidene-trifluoroethylene) (P(VDF-TrFE)) film into a flexible supercapacitor system to harvest and store the energy. The asymmetric output characteristics of the piezoelectric P(VDF-TrFE) film under mechanical impacts results in effective charging of the supercapacitors. The integrated piezo-supercapacitor exhibits a specific capacitance of 50 F g-1. The open-circuit voltage of the flexible and transparent supercapacitor reached 500 mV within 20 s during the mechanical action. Our hybridized energy harvesting and storage device can be further extended to provide a sustainable power source for various types of sensors integrated into wearable units.

Ultrafast all-fibre laser mode-locked by polymer-free carbon nanotube film.

This work for the first time reports the results on study of a polymer-free carbon nanotube (CNT) films used as a saturable absorber in an all-fibre laser. It is demonstrated that free-standing single-walled CNT films fabricated by an aerosol method are able to ensure generation of transform-limited pulses in an Er all-fibre ring laser with duration of several picoseconds and high quality of mode locking. The optimal average output power levels are identified, amounting to 0.4-0.5 mW depending on the linear transmission of the studied samples (60% or 80%). Application of polymer-free CNT films solves problems related to degradation of conventional polymer matrices of CNT-based saturable absorbers and paves the way to longer-lasting and more reliable saturable absorbers compatible with all-fibre laser configurations.

Carbon nanotube-amorphous silicon hybrid solar cell with improved conversion efficiency.

We report a hybrid solar cell based on single walled carbon nanotubes (SWNTs) interfaced with amorphous silicon (a-Si). The high quality carbon nanotube network was dry transferred onto intrinsic a-Si forming Schottky junction for metallic SWNT bundles and heterojunctions for semiconducting SWNT bundles. The nanotube chemical doping and a-Si surface treatment minimized the hysteresis effect in current-voltage characteristics allowing an increase in the conversion efficiency to 1.5% under an air mass 1.5 solar spectrum simulator. We demonstrated that the thin SWNT film is able to replace a simultaneously p-doped a-Si layer and transparent conductive electrode in conventional amorphous silicon thin film photovoltaics.

Direct and Dry Deposited Single-Walled Carbon Nanotube Films Doped with MoO(x) as Electron-Blocking Transparent Electrodes for Flexible Organic Solar Cells.

Organic solar cells have been regarded as a promising electrical energy source. Transparent and conductive carbon nanotube film offers an alternative to commonly used ITO in photovoltaics with superior flexibility. This communication reports carbon nanotube-based indium-free organic solar cells and their flexible application. Direct and dry deposited carbon nanotube film doped with MoO(x) functions as an electron-blocking transparent electrode, and its performance is enhanced further by overcoating with PEDOT: PSS. The single-walled carbon nanotube organic solar cell in this work shows a power conversion efficiency of 6.04%. This value is 83% of the leading ITO-based device performance (7.48%). Flexible application shows 3.91% efficiency and is capable of withstanding a severe cyclic flex test.

Pervaporation and Gas Separation Properties of High-Molecular Ladder-like Polyphenylsilsesquioxanes.

This paper presents the results of studies on the pervaporation properties (for benzene/hexane mixtures) and gas permeability (for He, H2, N2, O2, CO2, CH4, C2H6, and C4H10) of ladder-like polyphenylsesquioxanes (L-PPSQ) with improved physical and chemical properties. These polymers were obtained by condensation of cis-tetraphenylcyclotetrasiloxanetetraol in ammonia medium. The structure of L-PPSQ was fully confirmed by a combination of physicochemical analysis methods: 1H, 29Si NMR, IR spectroscopy, HPLC, powder XRD, and viscometry in solution. For the first time, a high molecular weight of the polymer (Mn = 238 kDa, Mw = 540 kDa) was achieved, which determines its improved mechanical properties and high potential for use in membrane separation. Using TGA and mechanical analysis methods, it was found that this polymer has high thermal (Td5% = 537 °C) and thermal-oxidative stability (Td5% = 587 °C) and good mechanical properties (Young's module (E) = 1700 MPa, ultimate tensile stress (σ) = 44 MPa, elongation at break (ε) = 6%), which is important for making membranes workable under various conditions. The polymer showed a high separation factor for a mixture of 10% wt. benzene in n-hexane (126) at a benzene flow of 33 g/(m2h).

Ladder Polyphenylsilsesquioxanes and Their Niobium-Siloxane Composite as Coating Materials: Spectroscopy and Atomic Oxygen Resistance Study.

In order to expand the range of materials that can be used in outer space and in development of small spacecraft, ladder polyphenylsilsesquioxanes with different molar weights and the Nb-siloxane composites based on them were studied. The properties of the polymer films were studied, including tests in an oxygen plasma flow. Both initial and filled ladder polymers feature extremely low erosion coefficients in the region of 10-26 cm3/atom O at a high fluence of atomic oxygen of 1.0 × 1021 atom O/cm2. Ladder polyphenylsilsesquioxane films irradiated with atomic oxygen (AO) retain their integrity, do not crack, and exhibit good optical properties, in particular, a high transmittance. The latter slightly decreases during AO exposure. The Nb-siloxane filling retains the AO resistance and slight decrease in optical transmission due to diffuse scattering on the formed Nb-[(SiO)x] nanoparticles. Ladder polyphenylsilsesquioxanes demonstrate their suitability for creating protective, optically transparent coatings for small spacecraft that are resistant to the erosive effects of incoming oxygen plasma.

Tetrachromophoric Derivatives of Dibenzoylmethanatoboron Difluoride Based on Stereoregular Cyclotetrasiloxanes: Synthesis and Properties.

A series of new tetrachromophoric systems based on stereoregular tetracyclosiloxanes and dibenzoylmethanatoboron difluoride derivatives have been synthesized and characterized by a complex of physicochemical methods. The photophysical properties of the synthesized compounds are studied by electronic absorption, steady-state, and time-resolved fluorescence spectroscopy. In the synthesized compounds, four dibenzoylmethanatoboron difluoride (DBMBF2)-based fluorophores are in an all-cis arrangement with respect to a cyclotetrasiloxane scaffold. DFT calculations predict that they can form H-type dimers, trimers, or tetramers with an antiparallel orientation of their ground-state dipole moments. Under UV excitation, solutions of these compounds in polar and nonpolar solvents exhibit complex fluorescence consisting of monomer- and excimer-like emissions with different lifetimes. Global fitting analysis reveals the presence of at least four kinetically distinguishable species in the excited state. The studied compounds in solutions have CIE chromaticity coordinates very close to the white color point and are promising objects for the development of next-generation single-emission materials for white illumination.

The Use of the Thiol-Ene Addition Click Reaction in the Chemistry of Organosilicon Compounds: An Alternative or a Supplement to the Classical Hydrosilylation?

This review presents the main achievements in the use of the thiol-ene reaction in the chemistry of silicones. Works are considered, starting from monomers and ending with materials.The main advantages and disadvantages of this reaction are demonstrated using various examples. A critical analysis of the use of this reaction is made in comparison with the hydrosilylation reaction.

Star-Shaped Polydimethylsiloxanes with Organocyclotetrasilsesquioxane Branching-Out Centers: Synthesis and Properties.

New non-crystallizable low-dispersity star-shaped polydimethylsiloxanes (PDMS) containing stereoregular cis-tetra(organo)(dimethylsiloxy)cyclotetrasiloxanes containing methyl-, tolyl- and phenyl-substituents at silicon atoms and the mixture of four stereoisomers of tetra[phenyl(dimethylsiloxy)]cyclotetrasiloxane as the cores were synthesized. Their thermal and viscous properties were studied. All synthesized compounds were characterized by a complex of physicochemical analysis methods: nuclear magnetic resonance (NMR), FT-IR spectroscopy, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), viscometry in solution, rheometry, and Langmuir trough study.