An intermolecular hydrogen bond plays a determining role in product selection of a surface confined Schiff-base reaction.

Herein, we illustrate how the cooperation of intermolecular hydrogen bonds and conformation flexibility leads to the formation of diverse complex covalent nanostructures on the surface, while the relative abundance of the final products can be further tuned by adjusting the molar ratio and concentration of monomers.

Boronic ester Sierpinski triangle fractals: from precursor design to on-surface synthesis and self-assembling superstructures.

Herein, we designed and synthesized a precursor with a three-fold node and successfully constructed covalent Sierpinski triangle (ST) fractals with boronic ester linkages both at the liquid/solid interface at room temperature and by thermal annealing in a water atmosphere under ambient conditions. Remarkably, large-scale ordered superstructures of covalent STs are constructed by thermal annealing, which paves the way for property investigation of STs.

Two-dimensional co-crystallization of two carboxylic acid derivatives having dissimilar symmetries at the liquid/solid interface.

By the co-assembly of two carboxylic acids with distinct symmetries and different numbers of carboxyl groups, we obtained two novel cocrystal structures at the n-octanoic acid/HOPG interface, one of which was sustained by unoptimized R22(8) hydrogen bonding. Benefiting from the bias-sensitivity of the BTB (1,3,5-tris(4-carboxyphenyl)benzene) molecule, a structure transition between the cocrystal network and a denser BTB lamella is achieved.

Imaging layer thickness of large-area graphene using reference-aided optical differential reflection technique.

Transparent layers are critical for enhancing optical contrast of graphene on a substrate. However, once the substrate is fully covered by large-area graphene, there are no accurate transparent layer and reference for optical contrast calculations. The thickness uncertainty of the transparent layer reduces the analytical accuracy of graphene. Thus, in this Letter, we propose a reference-aided differential reflection (DR) method with a dual-light path. The accurate thickness of the transparent layer is obtained by improving the DR spectrum sensitivity using a designable reference. Hence, the analytical accuracy of graphene thickness is guaranteed. To demonstrate this concept, a centimeter-scale chemical-vapor-deposition-synthesized graphene was measured on a SiO2/Si substrate. The thickness of underlying SiO2 was first identified with the 1 nm resolution by the DR spectrum. Then, the thickness distribution of graphene was directly deduced from a DR map with submonolayer resolution at a preferred wavelength. The results were also confirmed by ellipsometry and atomic force microscopy. As a result, this new method provides an extra degree of freedom for the DR method to accurately measure the thickness of large-area two-dimensional materials.

Ultrastiff, Strong, and Highly Thermally Conductive Crystalline Graphitic Films with Mixed Stacking Order.

A macroscopic film (2.5 cm × 2.5 cm) made by layer-by-layer assembly of 100 single-layer polycrystalline graphene films is reported. The graphene layers are transferred and stacked one by one using a wet process that leads to layer defects and interstitial contamination. Heat-treatment of the sample up to 2800 °C results in the removal of interstitial contaminants and the healing of graphene layer defects. The resulting stacked graphene sample is a freestanding film with near-perfect in-plane crystallinity but a mixed stacking order through the thickness, which separates it from all existing carbon materials. Macroscale tensile tests yields maximum values of 62 GPa for the Young's modulus and 0.70 GPa for the fracture strength, significantly higher than has been reported for any other macroscale carbon films; microscale tensile tests yield maximum values of 290 GPa for the Young's modulus and 5.8 GPa for the fracture strength. The measured in-plane thermal conductivity is exceptionally high, 2292 ± 159 W m-1 K-1 while in-plane electrical conductivity is 2.2 × 105 S m-1 . The high performance of these films is attributed to the combination of the high in-plane crystalline order and unique stacking configuration through the thickness.

Reversible Modification of Nitrogen-Doped Graphene Based on Se-N Dynamic Covalent Bonds for Field-Effect Transistors.

Temperature-dependent modification is an effective way to reversibly tailor graphene's electronic properties. We present the reversible modification of a uniform monolayer nitrogen-doped graphene (NG) film by the formation and cleavage of temperature-dependent Se-N dynamic covalent bonds. The increasing binding energy in X-ray photoelectron spectroscopy (XPS) indicates that phenylselenyl bromine (PhSeBr) bonds with pyridinic N and pyrrolic N rather than graphitic N by accepting the lone pair of electrons. The temperature dependence of Raman spectra (the increasing D band and the shifts of the 2D band) and XPS spectra (Se 3d and N 1s) indicates that the Se-N dynamic covalent bond is gradually cleaved by treatment at increasing temperatures and is also recovered by the reversible modification. Field-effect transistors (FETs) based on Se-NG exhibit a temperature-dependent change from n-type to p-type conduction and tunable electron and hole mobilities owing to the reversible formation or cleavage of Se-N dynamic covalent bonds. This result opens up opportunities for reversibly controlling electrical properties of FETs by optimizing dynamic covalent bonds.

Evaporable Glass-State Molecule-Assisted Transfer of Clean and Intact Graphene onto Arbitrary Substrates.

Graphene and its clean transfer methods have gathered growing interest and concern in recent decades. Here, we develop a novel large-scale intact transferring technology of paraffin wax onto arbitrary substrates. The wax will then be removed by thermal evaporation, avoiding uncontrollable reactions and leaving no residues. For characterizations, we adopt Raman, FT-IR, XPS, and DRS to measure the optical reflection difference on various surfaces and the thickness of graphene accurately. All the results demonstrate transferred surfaces' cleanliness and our method's validity. This technique allows for an effective transfer of graphene and enables a wider range of applications in many fields.

Eosin Y sensitized BiPO4 nanorods for bi-functionally enhanced visible-light-driven photocatalysis.

In this work, we report a novel photocatalyst of Eosin Y dye sensitized BiPO4 nanorods via a low-temperature and impregnation adsorption method. It shows enhanced visible-light-driven photocatalytic activity for degrading methylene blue (MB) and 2,4-DCP compared to that of pristine BiPO4 nanorods. The mass ratio of Eosin Y/BiPO4 is varied from 5 wt% to 30 wt% and the optimum value is 15 wt%, showing 46.7 and 10.5 fold greater apparent reaction rates than pristine BiPO4. Moreover, all of the reduced MB was transformed into CO2 and H2O during the photocatalysis, showing the good mineralization ability (almost 100%) of the composite. Furthermore, the photocatalytic mechanism of the composite is also investigated here by the zeta potential, scavenger experiments, Electron Paramagnetic Resonance (EPR), Photoluminescence Spectroscopy (PL), and a series of electrochemical analyses. The results show that (i) e- is the main reactive species and (ii) Eosin Y coated and adsorbed on BiPO4, thus widening the response range to the visible light region and accelerating the charge separation/transfer, resulting in bi-functionally promoted activity.

Self-Assembled Chiral Nanoparticle Superstructures and Identification of Their Collective Optical Activity from Ligand Asymmetry.

The spontaneous self-assembly of chiral nanoparticles (NPs) into stationary fabrication has garnered great interest in technique investigation and science advancement due to its expected apparent properties via orderly collective behaviors. However, this kind of characterization of assembled nanoparticles superstructure (NPS) is rarely reported and is distinguished with monodispersed chiral NPs. In this work, we used l-cysteine (Cys) as the chiral molecule in the form of functional surfactant, which had capped CdS/CdTe NPs and was treated as a linkage bridge for constructing orderly assembled NPS. Among the circular dichrosim (CD) phenomenon, Cys ligands exhibit related changes in CD absorption, while whole-molecule solution was used for treatment in different pH-controlling procedures. Synthesized chiral NPs are organized into ordered rod-shaped NPS during the spontaneous self-assembly process, and the CD response of NPS is monitored in different cultivating times; it showed a persuasive response appears in sum frequency generation (SFG) spectroscopy. Both experimental works and theory calculation convey that the ordered stacking of chiral stabilizer and the chirality of NPS, which are identified from chiral molecular status and their collective optical activity, originated from ligand asymmetry.

Camphor-Enabled Transfer and Mechanical Testing of Centimeter-Scale Ultrathin Films.

Camphor is used to transfer centimeter-scale ultrathin films onto custom-designed substrates for mechanical (tensile) testing. Compared to traditional transfer methods using dissolving/peeling to remove the support-layers, camphor is sublimed away in air at low temperature, thereby avoiding additional stress on the as-transferred films. Large-area ultrathin films can be transferred onto hollow substrates without damage by this method. Tensile measurements are made on centimeter-scale 300 nm-thick graphene oxide film specimens, much thinner than the ≈2 µm minimum thickness of macroscale graphene-oxide films previously reported. Tensile tests were also done on two different types of large-area samples of adlayer free CVD-grown single-layer graphene supported by a ≈100 nm thick polycarbonate film; graphene stiffens this sample significantly, thus the intrinsic mechanical response of the graphene can be extracted. This is the first tensile measurement of centimeter-scale monolayer graphene films. The Young's modulus of polycrystalline graphene ranges from 637 to 793 GPa, while for near single-crystal graphene, it ranges from 728 to 908 GPa (folds parallel to the tensile loading direction) and from 683 to 775 GPa (folds orthogonal to the tensile loading direction), demonstrating the mechanical performance of large-area graphene in a size scale relevant to many applications.

Different graphene layers to enhance or prevent corrosion of polycrystalline copper.

Graphene was used as an anticorrosive coating for metals as it can effectively isolate the corrosion factors such as oxygen. However, we found that the anticorrosive and corrosive effects on metal surface were related to graphene layers and metal crystal faces. In this paper, we found that different layers of graphene had significantly different effects on the corrosion of polycrystalline copper during long-term storage under atmospheric conditions. Optical images and Raman spectra showed that single layer graphene (SLG)-coated copper had a higher degree of corrosion than bare copper. However, when covered with CVD in situ-grown bilayer graphene (BLG), the copper foil was effectively prevented from being etched as it exhibited a bright yellow color despite the differences in crystal faces. The surface potential differences measured by an electric force microscope (EFM) showed that a contact potential difference (V CPD) between 30 and 40 mV existed between Cu/SLG and bare copper. The SLG-coated areas had a higher surface potential (SP), which meant that the (SLG)-coated copper was more prone to lose electrons to exhibit galvanic corrosion. The BLG coating made SP of underlying copper lower making it harder to lose electrons; thus, BLG successfully protected the copper from being corroded. These findings have a foreseeable significance for graphene as a metal anti-corrosion coating.

Using multiple hydrogen bonding cross-linkers to access reversibly responsive three dimensional graphene oxide architecture.

Three-dimensional (3D) graphene materials have attracted a lot of attention for efficiently utilizing inherent properties of graphene sheets. However, 3D graphene materials reported in the previous literature are constructed through covalent or weak non-covalent interactions, causing permanent structure/property changes. In this paper, a novel 3D graphene material of dynamic interactions between lamellas with 2-ureido-4[1H]-pyrimidinone as a supra-molecular motif has been synthesized. This 3D graphene material shows enhanced sheet interactions while the cross-linking takes place. With proper solvent stimulation, the integrated 3D graphene material can disassemble as isolated sheets. The driving force for the 3D structure assembly or disassembly is considered to be the forming or breaking of the multiple hydrogen bonding pairs. Furthermore, the 3D material is used as an intelligent dye adsorber to adsorb methylene blue and release it. The controllable and reversible characteristic of this 3D graphene material may open an avenue to the synthesis and application of novel intelligent materials.

Facile synthesis of analogous graphene quantum dots with sp(2) hybridized carbon atom dominant structures and their photovoltaic application.

Graphene quantum dot (GQD) is an emerging class of zero-dimensional nanocarbon material with many novel applications. It is of scientific importance to prepare GQDs with more perfect structures, that is, GQDs containing negligible oxygenous defects, for both optimizing their optical properties and helping in their photovoltaic applications. Herein, a new strategy for the facile preparation of "pristine" GQDs is reported. The method we presented is a combination of a bottom-up synthetic and a solvent-induced interface separation process, during which the target products with highly crystalline structure were selected by the organic solvent. The obtained organic soluble GQDs (O-GQDs) showed a significant difference in structure and composition compared with ordinary aqueous soluble GQDs, thus leading to a series of novel properties. Furthermore, O-GQDs were applied as electron-acceptors in a poly(3-hexylthiophene) (P3HT)-based organic photovoltaic device. The performance highlights that O-GQD has potential to be a novel electron-acceptor material due to the sp(2) hybridized carbon atom dominant structure and good solubility in organic solvents.

Two-dimensional supramolecular spring: coordination driven reversible extension and contraction of bridged half rings.

A tetraethylene glycol ether bridged derivative 9 has been designed and synthesized, and its two-dimensional (2D) self-assembled behavior has been investigated at the single-molecule level. Our results revealed that 9 generally adopted the fully extended state but changed to the contracted state when triggered by K2CO3, and recovered the original fully extended conformation after subsequent addition of 18-crown-6. Such a coordination-controlled reversible assembly reveals supramolecular springs in response to chemical stimuli, which is of great interest in bionics and materials science.

Triangular-shaped molecular random tiling and molecular rotation in two-dimensional glassy networks.

Macrocycle-1 molecules can self-assemble into glassy state networks via van der Waals force and form many triangular nanopores in networks. The nanopores can be expressed by triangular tilings, which lead to a particularly rich range of arrangements. Moreover an interesting molecular rotation phenomenon was observed in the glassy networks.

A layer-nanostructured assembly of PbS quantum dot/multiwalled carbon nanotube for a high-performance photoswitch.

A layered nanostructure of a lead sulfide (PbS) quantum dot (QD)/multi-walled carbon nanotube (MWNT) hybrid was prepared by the electrostatic assembly after the phase transfer of PbS QDs from an organic to an aqueous phase. Well-crystallized PbS QDs with a narrow diameter (5.5 nm) was mono-dispersed on the sidewalls of MWNT by the electrostatic adsorption. Near-infrared absorption of PbS/MWNT nanostructures was improved and controlled by the packing density of PbS QDs. Efficient charge transfer between PbS and MWNT at the interface resulted in a remarkable quenching of photoluminescence up to 28.6% and a blue-shift of emission band by 300 nm. This feature was facilitated by band energy levels based on the intimate contact through the electrostatic interaction. Two-terminal devices using PbS/MWNT nanostructures showed an excellent on/off switching photocurrent and good stability during 20 cycles under light illumination due to electron transfer from PbS to MWNT. The photoswitch exhibited a high photo sensitivity up to 31.3% with the photocurrent of 18.3 µA under the light of 3.85 mW/cm(2), which outperformed many QD/carbon-based nanocomposites. Results indicate that the electrostatic layered assembly of QD/MWNT nanostructure is an excellent platform for the fabrication of high-performance optoelectronic devices.

Topological structural transformations of nanoparticle self-assemblies mediated by phase transfer and their application as organic-inorganic hybrid photodetectors.

Nanoparticle (NP) self-assemblies have attracted an increasing amount of attention in recent years because of their potential application in the construction of novel nanodevices. The controllable transformation of NP self-assemblies (NPS) between a polar and nonpolar environment is required for many specific applications because of their different properties in different environments. In this article, water-soluble luminescent CdS/CdTe NPS were synthesized using thioglycolic acid as a capping agent. The stiff and straight NPS bundles became loose after phase transfer from an aqueous to an organic phase. Subsequently, the NPS transferred to the aqueous phase. The loose structure transformed into many twisted nanoribbons. Additionally, hybrid photodetectors made using the organic-soluble NPS and P3HT polymers were fabricated, and we found that the NPS/P3HT blend may be perfect for light detection. The organic-soluble NPS are potentially useful for the fabrication of semiconductor nanojunctions.

Solvent dependent supramolecular self-assembly and surface reversal of a modified porphyrin.

In this paper, a novel core-modified porphyrin with meso-aryl substituents and phenanthrene-fused pyrrole rings (N2S2-OR) is synthesized. Scanning tunneling microscopy (STM) has been used to probe its self-assembly behavior on a highly-oriented pyrolytic graphite (HOPG) surface. Our STM results have shown that there is an obvious solvent-dependent self-assembly for the surface-confined target molecules. In n-tetradecane, N2S2-OR assembles into a perfect alternating structure. At the 1-phenyloctane-graphite interface, disordered structures are formed and nonperiodic alternation is observed, whereas the target molecule in 1-heptanoic acid is assumed to form homogeneous close-packed monolayers with no alternating. Interestingly, such solvent-dependent supramolecular assembled behavior also involves the structural transformation of the backbone of the core-modified porphyrin derivative from saddle to reversed-saddle in these three solvents with different polarities.

Site-selective effects on guest-molecular adsorption and fabrication of four-component architecture by higher order networks.

2D porous networks have attracted great attention as they can be used to immobilize functional units as guest molecules in a spatially ordered arrangement. In this work, a novel molecular hybrid network with two kinds of cavities was fabricated. Several kinds of guest molecules, such as coronene, copper(II) phthalocyanine (CuPc), triphenylene, heptanoic acid and fullerene molecules, can be immobilized into this template. Site- and size-selective effects can be observed. Furthermore, we have also fabricated interesting 2D crystal architecture with complex four-component structure at the liquid-solid interface, following investigation by scanning tunnelling microscopy (STM). The current findings provide a convenient approach towards the formation of more complex and functionalized surface nanopatterns, which can benefit the study of host-guest assembly behaviour within a monolayer composed by several components at interfaces.