Mechanically stacked 1-nm-thick carbon nanosheets: ultrathin layered materials with tunable optical, chemical, and electrical properties

Carbon nanosheets are mechanically stable, free-standing two-dimensional materials with a thickness of ≈1 nm and well defined physical and chemical properties. They are made by radiation-induced cross-linking of aromatic self-assembled monolayers. Herein, a route is presented to the scalable fabrication of multilayer nanosheets with tunable electrical, optical, and chemical properties on insulating substrates. Stacks of up to five nanosheets with sizes of ≈1 cm(2) on oxidized silicon are studied. Their optical characteristics are investigated by visual inspection, optical microscopy, UV-vis reflection spectroscopy, and model calculations. Their chemical composition is studied by X-ray photoelectron spectroscopy. The multilayer samples are then annealed in an ultrahigh vacuum at various temperatures up to 1100 K. A subsequent investigation by Raman, X-ray photoelectron, and UV-vis reflection spectroscopy, as well as by electrical four-point probe measurements, demonstrates that the layered nanosheets transform into nanocrystalline graphene. This structural and chemical transformation is accompanied by changes in the optical properties and electrical conductivity and opens up a new path for the fabrication of ultrathin functional conductive coatings.     

Fabrication of Flexible and Transparent Conductive Nanosheets by the UV‐Irradiation of Gold Nanoparticle Monolayers

Conductive films that are highly transparent and flexible are extremely attractive for emerging optoelectronic applications. Currently, indium‐doped tin oxide films are the most widely used transparent conductive films and much research effort is devoted to developing alternative transparent conductive materials to overcome their drawbacks. In this work, a novel and facile approach for fabricating transparent conductive Au nanosheets from Au nanoparticles (AuNPs) is proposed. Irradiating an AuNP monolayer at the air–water interface with UV light results in a nanosheet with ≈3.5 nm thickness and ≈80% transparency in the UV–visible region. Further, the so‐fabricated nanosheets are highly flexible and can maintain their electrical conductivity even when they are bent to a radius of curvature of 0.6 mm. Fourier‐transform infrared and X‐ray photoelectron spectroscopy characterizations reveal that the transformation of the monolayer of AuNPs into the nanosheet is induced by the photodecomposition and/or photodetachment of the dodecanethiol ligands capping the AuNPs. Further, the UV‐irradiation of a hybrid monolayer consisting of AuNPs and silica particles affords the patterning of Au nanosheets with periodic hole arrays.     

Protein‐Directed Synthesis of NIR‐Emitting,Tunable HgS Quantum Dots and their Applications in Metal‐Ion Sensing

The development of luminescent mercury sulfide quantum dots (HgS QDs) through the bio‐mineralization process has remained unexplored. Herein, a simple, two‐step route for the synthesis of HgS quantum dots in bovine serum albumin (BSA) is reported. The QDs are characterized by UV–vis spectroscopy, Fourier transform infrared (FT‐IR) spectroscopy, luminescence, Raman spectroscopy, transmission electron microscopy (TEM), X‐ray photoelectron spectroscopy (XPS), circular dichroism (CD), energy dispersive X‐ray analysis (EDX), and picosecond‐resolved optical spectroscopy. Formation of various sizes of QDs is observed by modifying the conditions suitably. The QDs also show tunable luminescence over the 680–800 nm spectral regions, with a quantum yield of 4–5%. The as‐prepared QDs can serve as selective sensor materials for Hg(II) and Cu(II), based on selective luminescence quenching. The quenching mechanism is found to be based on Dexter energy transfer and photoinduced electron transfer for Hg(II) and Cu(II), respectively. The simple synthesis route of protein‐capped HgS QDs would provide additional impetus to explore applications for these materials.     

Transferable Organic Semiconductor Nanosheets for Application in Electronic Devices

A method has been developed to stabilize and transfer nanofilms of functional organic semiconductors. The method is based on crosslinking of their topmost layers by low energy electron irradiation. The films can then be detached from their original substrates and subsequently deposited onto new solid or holey substrates retaining their structural integrity. Grazing incidence X‐ray diffraction, X‐ray specular reflectivity, and UV–Vis spectroscopy measurements reveal that the electron irradiation of ≈50 nm thick pentacene films results in crosslinking of their only topmost ≈5 nm (3–4 monolayers), whereas the deeper pentacene layers preserve their pristine crystallinity. The electronic performance of the transferred pentacene nanosheets in bottom contact field‐effect devices is studied and it is found that they are fully functional and demonstrate superior charge injection properties in comparison to the pentacene films directly grown on the contact structures by vapor deposition. The new approach paves the way to integration of the organic semiconductor nanofilms on substrates unfavorable for their direct growth as well as to their implementation in hybrid devices with unusual geometries, e.g., in devices incorporating free‐standing sheets.     

UV–Ozone Interfacial Modification in Organic Transistors for High‐Sensitivity NO2 Detection

A new type of nitrogen dioxide (NO₂) gas sensor based on copper phthalocyanine (CuPc) thin film transistors (TFTs) with a simple, low‐cost UV–ozone (UVO)‐treated polymeric gate dielectric is reported here. The NO₂ sensitivity of these TFTs with the dielectric surface UVO treatment is ≈400× greater for [NO₂] = 30 ppm than for those without UVO treatment. Importantly, the sensitivity is ≈50× greater for [NO₂] = 1 ppm with the UVO‐treated TFTs, and a limit of detection of ≈400 ppb is achieved with this sensing platform. The morphology, microstructure, and chemical composition of the gate dielectric and CuPc films are analyzed by atomic force microscopy, grazing incident X‐ray diffraction, X‐ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy, revealing that the enhanced sensing performance originates from UVO‐derived hydroxylated species on the dielectric surface and not from chemical reactions between NO₂ and the dielectric/semiconductor components. This work demonstrates that dielectric/semiconductor interface engineering is essential for readily manufacturable high‐performance TFT‐based gas sensors.     

Alumina‐Supported CoFe Alloy Catalysts Derived from Layered‐Double‐Hydroxide Nanosheets for Efficient Photothermal CO2 Hydrogenation to Hydrocarbons

A series of novel CoFe‐based catalysts are successfully fabricated by hydrogen reduction of CoFeAl layered‐double‐hydroxide (LDH) nanosheets at 300–700 °C. The chemical composition and morphology of the reaction products (denoted herein as CoFe‐x) are highly dependent on the reduction temperature (x). CO₂ hydrogenation experiments are conducted on the CoFe‐x catalysts under UV–vis excitation. With increasing LDH‐nanosheet reduction temperature, the CoFe‐x catalysts show a progressive selectivity shift from CO to CH₄, and eventually to high‐value hydrocarbons (C₂₊). CoFe‐650 shows remarkable selectivity toward hydrocarbons (60% CH₄, 35% C₂₊). X‐ray absorption fine structure, high‐resolution transmission electron microscopy, Mössbauer spectroscopy, and density functional theory calculations demonstrate that alumina‐supported CoFe‐alloy nanoparticles are responsible for the high selectivity of CoFe‐650 for C₂₊ hydrocarbons, also allowing exploitation of photothermal effects. This study demonstrates a vibrant new catalyst platform for harnessing clean, abundant solar‐energy to produce valuable chemicals and fuels from CO₂.     

Co Nanoislands Rooted on Co–N–C Nanosheets as Efficient Oxygen Electrocatalyst for Zn–Air Batteries

Developing non‐precious‐metal bifunctional oxygen reduction and evolution reaction (ORR/OER) catalysts is a major task for promoting the reaction efficiency of Zn–air batteries. Co‐based catalysts have been regarded as promising ORR and OER catalysts owing to the multivalence characteristic of cobalt element. Herein, the synthesis of Co nanoislands rooted on Co–N–C nanosheets supported by carbon felts (Co/Co–N–C) is reported. Co nanosheets rooted on the carbon felt derived from electrodeposition are applied as the self‐template and cobalt source. The synergistic effect of metal Co islands with OER activity and Co–N–C nanosheets with superior ORR performance leads to good bifuctional catalytic performances. Wavelet transform extended X‐ray absorption fine spectroscopy and X‐ray photoelectron spectroscopy certify the formation of Co (mainly Co⁰) and the Co–N–C (mainly Co²⁺ and Co³⁺) structure. As the air‐cathode, the assembled aqueous Zn–air battery exhibits a small charge–discharge voltage gap (0.82 V@10 mA cm^?2) and high power density of 132 mW cm^?2, outperforming the commercial Pt/C catalyst. Additionally, the cable flexible rechargeable Zn–air battery exhibits excellent bendable and durability. Density functional theory calculation is combined with operando X‐ray absorption spectroscopy to further elucidate the active sites of oxygen reactions at the Co/Co–N–C cathode in Zn–air battery.     

One‐Step Formation of a Single Atomic‐Layer Transistor by the Selective Fluorination of a Graphene Film

In this study, the scalable and one‐step fabrication of single atomic‐layer transistors is demonstrated by the selective fluorination of graphene using a low‐damage CF₄ plasma treatment, where the generated F‐radicals preferentially fluorinated the graphene at low temperature (<200 °C) while defect formation was suppressed by screening out the effect of ion damage. The chemical structure of the C–F bonds is well correlated with their optical and electrical properties in fluorinated graphene, as determined by X‐ray photoelectron spectroscopy, Raman spectroscopy, and optical and electrical characterizations. The electrical conductivity of the resultant fluorinated graphene (F‐graphene) was demonstrated to be in the range between 1.6 kΩ/sq and 1 MΩ/sq by adjusting the stoichiometric ratio of C/F in the range between 27.4 and 5.6, respectively. Moreover, a unique heterojunction structure of semi‐metal/semiconductor/insulator can be directly formed in a single layer of graphene using a one‐step fluorination process by introducing a Au thin‐film as a buffer layer. With this heterojunction structure, it would be possible to fabricate transistors in a single graphene film via a one‐step fluorination process, in which pristine graphene, partial F‐graphene, and highly F‐graphene serve as the source/drain contacts, the channel, and the channel isolation in a transistor, respectively. The demonstrated graphene transistor exhibits an on‐off ratio above 10, which is 3‐fold higher than that of devices made from pristine graphene. This efficient transistor fabrication method produces electrical heterojunctions of graphene over a large area and with selective patterning, providing the potential for the integration of electronics down to the single atomic‐layer scale.     

Co‐Based Catalysts Derived from Layered‐Double‐Hydroxide Nanosheets for the Photothermal Production of Light Olefins

Solar‐driven Fischer–Tropsch synthesis represents an alternative and potentially low‐cost route for the direct production of light olefins from syngas (CO and H₂). Herein, a series of novel Co‐based photothermal catalysts with different chemical compositions are successfully fabricated by H₂ reduction of ZnCoAl‐layered double‐hydroxide nanosheets at 300–700 °C. Under UV–vis irradiation, the photothermal catalyst prepared at 450 °C demonstrates remarkable CO hydrogenation performance, affording an olefin (C_2–4⁼) selectivity of 36.0% and an olefin/paraffin ratio of 6.1 at a CO conversion of 15.4%. Characterization studies using X‐ray absorption fine structure and high‐resolution transmission electron microscopy reveal that the active catalyst comprises Co and Co₃O₄ nanoparticles on a ZnO–Al₂O₃ mixed metal oxide support. Density functional theory calculations further demonstrate that the oxide‐decorated metallic Co nanoparticle heterostructure weakens the further hydrogenation ability of the corresponding Co, leading to the high selectivity to light olefins. This study demonstrates a novel solar‐driven catalyst platform for the production of light olefins via CO hydrogenation.     

Scalable Design of Two‐Dimensional Oxide Nanosheets for Construction of Ultrathin Multilayer Nanocapacitor

Large size of capacitors is the main hurdle in miniaturization of current electronic devices. Herein, a scalable solution‐based layer‐by‐layer engineering of metallic and high‐κ dielectric nanosheets into multilayer nanosheet capacitors (MNCs) with overall thickness of ≈20 nm is presented. The MNCs are built through neat tiling of 2D metallic Ru_0.95O₂^0.2? and high‐κ dielectric Ca₂NaNb₄O₁₃^? nanosheets via the Langmuir–Blodgett (LB) approach at room temperature which is verified by cross‐sectional high‐resolution transmission electron microscopy (HRTEM). The resultant MNCs demonstrate a high capacitance of 40–52 µF cm^?2 and low leakage currents down to 10^?5–10^?6 A cm^?2. Such MNCs also possess complimentary in situ robust dielectric properties under high‐temperature measurements up to 250 °C. Based on capacitance normalized by the thickness, the developed MNC outperforms state‐of‐the‐art multilayer ceramic capacitors (MLCC, ≈22 µF cm^?2/5 × 10⁴ nm) present in the market. The strategy is effective due to the advantages of facile, economical, and ambient temperature solution assembly.     

Analysis of Carbon Materials by X‐ray Photoelectron Spectroscopy and X‐ray Absorption Spectroscopy

Since hard coating such as ta:C films are of growing interest for protecting surfaces of industrial machines and high capacity tools against wear and friction or for biomedical applications, more information about the structure‐to‐property relation is required. Therefore, core level X‐ray photoelectron sprectroscopy and X‐ray absorption spectroscopy and spectromicroscopy are used to derive chemical information about selected carbon species.     

Ultrathin Molybdenum Dioxide Nanosheets as Uniform and Reusable Surface‐Enhanced Raman Spectroscopy Substrates with High Sensitivity

Metal oxides have advantages over the traditional noble metals to be used as substrate materials for surface‐enhanced Raman spectroscopy (SERS) with low cost, versatility, and biocompatibility, but their enhancement factors are generally quite low with a poor limit of detection. Here, ultrathin molybdenum dioxide (MoO₂) nanosheets synthesized by chemical vapor deposition demonstrated in large area are used as SERS substrates with superior signal uniformity in the whole area with a limit of detectable concentration down to 4 × 10^?8m and enhancement factor up to 2.1 × 10⁵, exceeding that of 2D materials and comparable to that of noble metal films. More practically important, the planar MoO₂ substrate is more robust than noble metals and shows excellent reusability and uniformity, which is usually prohibited for nanostructured or nanoparticle‐based metal oxide substrates. The enhancement is mainly attributed to the surface plasmon resonance effect as evidenced by the first principle calculations and UV–vis absorption spectroscopy characterization, which can be further increased by decreasing the thickness of the MoO₂ nanosheets. The overall superior performance makes the MoO₂ nanosheets an ideal substrate for practical SERS applications.     

Black Phosphorus/Platinum Heterostructure: A Highly Efficient Photocatalyst for Solar‐Driven Chemical Reactions

A 2D black phosphorus/platinum heterostructure (Pt/BP) is developed as a highly efficient photocatalyst for solar‐driven chemical reactions. The heterostructure, synthesized by depositing BP nanosheets with ultrasmall (≈1.1 nm) Pt nanoparticles, shows strong Pt–P interactions and excellent stability. The Pt/BP heterostructure exhibits obvious P‐type semiconducting characteristics and efficient absorption of solar energy extending into the infrared region. Furthermore, during light illumination, accelerated charge separation is observed from Pt/BP as manifested by the ultrafast electron migration (0.11 ps) detected by a femtosecond pump‐probe microscopic optical system as well as efficient electron accumulation on Pt revealed by in situ X‐ray photoelectron spectroscopy. These unique properties result in remarkable performance of Pt/BP in typical hydrogenation and oxidation reactions under simulated solar light illumination, and its efficiency is much higher than that of other common Pt catalysts and even much superior to that of conventional thermal catalysis. The 2D Pt/BP heterostructure has enormous potential in photochemical reactions involving solar light and the results of this study provide insights into the design of next‐generation high‐efficiency photocatalysts.     

DNA‐Mediated Morphological Control of Silver Nanoparticles

It is demonstrated that DNA can be used to control the synthesis of silver nanoplates with different morphologies using spherical silver seeds. UV–vis spectroscopy, transmission electron microscopy, scanning electron microscopy, X‐ray photoelectron spectroscopy, and Raman spectroscopy are used to characterize the synthesized nanoparticles. Silver nanoprisms are encoded by poly C and poly G, while silver flower bouquets and silver nanodiscs are synthesized using poly A and poly T, respectively. The length of DNA is found to have little effect on the morphology of silver nanoparticles. Moreover, the synthesized silver nanoplates are found to have high surface enhanced Raman scattering enhancement ability, good antibacterial activity, and good biocompatibility. These discoveries will broaden the application of DNA in nanoscience and will provide a new platform to investigate the interaction between DNA sequences and silver nanoparticles.     

Synergistic Impacts of Electrolyte Adsorption on the Thermoelectric Properties of Single‐Walled Carbon Nanotubes

Single‐walled carbon nanotubes are promising candidates for light‐weight and flexible energy materials. Recently, the thermoelectric properties of single‐walled carbon nanotubes have been dramatically improved by ionic liquid addition; however, controlling factors remain unsolved. Here the thermoelectric properties of single‐walled carbon nanotubes enhanced by electrolytes are investigated. Complementary characterization with absorption, Raman, and X‐ray photoelectron spectroscopy reveals that shallow hole doping plays a partial role in the enhanced electrical conductivity. The molecular factors controlling the thermoelectric properties of carbon nanotubes are systematically investigated in terms of the ionic functionalities of ionic liquids. It is revealed that appropriate ionic liquids show a synergistic enhancement in conductivity and the Seebeck coefficient. The discovery of significantly precise doping enables the generation of thermoelectric power factor exceeding 460 µW m^–1 K^–2.     

Synthesis and layer-by-layer self-assembly of silver nanoparticles capped by mercaptosulfonic acid

Spherical silver nanoparticles capped by mercaptosulfonic acid with a diameter of about 8 nm were prepared by a simple chemical reaction. The resulting silver nanoparticles were characterized by UV–vis spectroscopy (UV–vis) and transmission electron microscopy (TEM). Using layer-by-layer (LBL) self-assembly technique, the multilayer films containing silver nanoparticles and polycation poly(dialyldimethylammonium chloride) (PDDA) were successfully fabricated. The fabrication process was monitored by UV–vis spectra and the morphology of the multilayer films was investigated by atomic force microscopy (AFM). The cyclic voltammogram (CV) measurements further confirmed that the silver nanoparticles had assembled into the multilayer films successfully. Surface-enhanced Raman spectroscopy (SERS) measurements showed that the multilayer films containing silver nanoparticles could serve as SERS-active substrate.     

Multilayer Polypyrrole Nanosheets with Self‐Organized Surface Structures for Flexible and Efficient Solar–Thermal Energy Conversion

Converting solar energy into concentrated heat is very appealing for various applications. Polypyrrole (PPy) is known to possess excellent photothermal property with low thermal conductivity, and thus is an ideal candidate for solar–thermal energy conversion. However, solar–thermal materials based on PPy or other conducting polymers still exhibit limited energy conversion efficiency due to the lack of effective light‐trapping schemes. Here, it is demonstrated that multilayer PPy nanosheets with spontaneously formed surface structures such as wrinkles and ridges via sequential polymerization on paper substrates can dramatically enhance broadband and wide‐angle light absorption across the full solar spectrum, leading to an impressive solar–thermal conversion efficiency of 95.33%. The intriguing solar–thermal properties and structural features of multilayer PPy nanosheets can be used for solar heating and photoactuators. Meanwhile, when used for solar steam generation, the measured efficiency could achieve ≈92% under one sun irradiation. The hierarchically multilayer structure is mechanically flexible and robust, holding great potential for practical solar energy utilization. This study provides a simple and straightforward approach toward engineering light‐weight and thermally insulating polymers into efficient solar–thermal materials for emerging solar energy‐related applications.     

Structural and optical properties of nitrogen‐iron co‐doped titanium dioxide films prepared via sol‐gel dip‐coating: Effect of urea and iron nitrate concentration in the sol

In this study, nitrogen‐iron co‐doped titanium dioxide films were prepared via sol‐gel dip‐coating method using urea and iron nitrate as nitrogen and iron source, respectively. Nonmetal doping of TiO₂ have some disadvantages such as massive charge carrier recombination and losing the photo‐catalytic capability. Three different nitrogen‐iron co‐doped titanium dioxide sols with different urea and iron nitrate concentration were prepared. The resulting sols were homogeneous and transparent, and no precipitation was observed in any of them. It was observed that the film prepared with middle urea‐iron nitrate concentration sol got opaque in a short time after the dip‐coating process. All prepared films were characterized by X‐ray diffraction, X‐ray photoelectron spectroscopy, scanning electron microscopy, confocal microscopy and ultraviolet–visible (UV‐Vis) spectroscopy. It was found that the concentration of the urea and iron nitrate in the sol had an effect on the crystal structure, microstructure, surface morphology and optical properties of the resulting films. Samples with middle concentrations had amorphous structure and bigger particle size. It was seen that sample with higher iron amount has lower band‐gap. It is concluded that we can prepare transparent anatase, transparent amorphous and opaque amorphous titanium dioxide films by changing the urea and iron nitrate concentration in the sol.     

Ruthenium Nanoparticles on Cobalt‐Doped 1T′ Phase MoS2 Nanosheets for Overall Water Splitting

2D MoS₂ nanostructures have recently attracted considerable attention because of their outstanding electrocatalytic properties. The synthesis of unique Co–Ru–MoS₂ hybrid nanosheets with excellent catalytic activity toward overall water splitting in alkaline solution is reported. 1T′ phase MoS₂ nanosheets are doped homogeneously with Co atoms and decorated with Ru nanoparticles. The catalytic performance of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) is characterized by low overpotentials of 52 and 308 mV at 10 mA cm^?2 and Tafel slopes of 55 and 50 mV decade^?1 in 1.0 m KOH, respectively. Analysis of X‐ray photoelectron and absorption spectra of the catalysts show that the MoS₂ well retained its metallic 1T′ phase, which guarantees good electrical conductivity during the reaction. The Gibbs free energy calculation for the reaction pathway in alkaline electrolyte confirms that the Ru nanoparticles on the Co‐doped MoS₂ greatly enhance the HER activity. Water adsorption and dissociation take place favorably on the Ru, and the doped Co further catalyzes HER by making the reaction intermediates more favorable. The high OER performance is attributed to the catalytically active RuO₂ nanoparticles that are produced via oxidation of Ru nanoparticles.     

Nanosheets of Nonlayered Aluminum Metal–Organic Frameworks through a Surfactant‐Assisted Method

During the last decade, the synthesis and application of metal–organic framework (MOF) nanosheets has received growing interest, showing unique performances for different technological applications. Despite the potential of this type of nanolamellar materials, the synthetic routes developed so far are restricted to MOFs possessing layered structures, limiting further development in this field. Here, a bottom‐up surfactant‐assisted synthetic approach is presented for the fabrication of nanosheets of various nonlayered MOFs, broadening the scope of MOF nanosheets application. Surfactant‐assisted preorganization of the metallic precursor prior to MOF synthesis enables the manufacture of nonlayered Al‐containing MOF lamellae. These MOF nanosheets are shown to exhibit a superior performance over other crystal morphologies for both chemical sensing and gas separation. As revealed by electron microscopy and diffraction, this superior performance arises from the shorter diffusion pathway in the MOF nanosheets, whose 1D channels are oriented along the shortest particle dimension.