A Universal Wet-Chemistry Route to Metal Filling of Boron Nitride Nanotubes

We present a facile wet-chemistry method for efficient metal filling of the hollow inner cores of boron nitride nanotubes (BNNTs). The fillers conform to the cross-section of the tube cavity and extend in length from a few nm to hundreds of nm. The methodology is robust and is demonstrated for noble metals (Au, Pt, Pd, and Ag), transition metals (Co), and post-transition elements (In). Transmission electron microscopy and related electron spectroscopy confirm the composition and morphology of the filler nanoparticles. Up to 60% of BNNTs of a given preparation batch have some degree of metal encapsulation, and individual tubes can have up to 10% of their core volume filled during initial loading. The growth, movement, and fusing of metal nanoparticles within the BNNTs are also examined

Self-Assembled PCBM Nanosheets: A Facile Route to Electronic Layer-on-Layer Heterostructures

We report on the self-assembly of semicrystalline [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) nanosheets at the interface between a hydrophobic solvent and water, and utilize this opportunity for the realization of electronically active organic/organic molecular heterostructures. The self-assembled PCBM nanosheets can feature a lateral size of >1 cm² and be transferred from the water surface to both hydrophobic and hydrophilic surfaces using facile transfer techniques. We employ a transferred single PCBM nanosheet as the active material in a field-effect transistor (FET) and verify semiconductor function by a measured electron mobility of 1.2 × 10^–2 cm² V^–1 s^–1 and an on–off ratio of ∼1 × 10⁴. We further fabricate a planar organic/organic heterostructure with the p-type organic semiconductor poly­(3-hexylthiophene-2,5-diyl) as the bottom layer and the n-type PCBM nanosheet as the top layer and demonstrate ambipolar FET operation with an electron mobility of 8.7 × 10^–4 cm² V^–1 s^–1 and a hole mobility of 3.1 × 10^–4 cm² V^–1 s^–1

Real-Time Observation of Water-Soluble Mineral Precipitation in Aqueous Solution by <i>In Situ</i> High-Resolution Electron Microscopy

The precipitation and dissolution of water-soluble minerals in aqueous systems is a familiar process occurring commonly in nature. Understanding mineral nucleation and growth during its precipitation is highly desirable, but past <i>in situ</i> techniques have suffered from limited spatial and temporal resolution. Here, by using <i>in situ</i> graphene liquid cell electron microscopy, mineral nucleation and growth processes are demonstrated in high spatial and temporal resolution. We precipitate the mineral thenardite (Na₂SO₄) from aqueous solution with electron-beam-induced radiolysis of water. We demonstrate that minerals nucleate with a two-dimensional island structure on the graphene surfaces. We further reveal that mineral grains grow by grain boundary migration and grain rotation. Our findings provide a direct observation of the dynamics of crystal growth from ionic solutions

Synthesis of Highly Crystalline sp²‑Bonded Boron Nitride Aerogels

sp²-Bonded boron nitride aerogels are synthesized from graphene aerogels <i>via</i> carbothermal reduction of boron oxide and simultaneous nitridation. The color and chemical composition of the original gel change dramatically, while structural features down to the nanometer scale are maintained, suggesting a direct conversion of the carbon lattice to boron nitride. Scanning and transmission electron microscopies reveal a foliated architecture of wrinkled sheets, a unique morphology among low-density, porous BN materials. The converted gels display a high degree of chemical purity (>95%) and crystalline order and exhibit unique cross-linking structures

Real-Time Observation of Water-Soluble Mineral Precipitation in Aqueous Solution by <i>In Situ</i> High-Resolution Electron Microscopy

The precipitation and dissolution of water-soluble minerals in aqueous systems is a familiar process occurring commonly in nature. Understanding mineral nucleation and growth during its precipitation is highly desirable, but past <i>in situ</i> techniques have suffered from limited spatial and temporal resolution. Here, by using <i>in situ</i> graphene liquid cell electron microscopy, mineral nucleation and growth processes are demonstrated in high spatial and temporal resolution. We precipitate the mineral thenardite (Na₂SO₄) from aqueous solution with electron-beam-induced radiolysis of water. We demonstrate that minerals nucleate with a two-dimensional island structure on the graphene surfaces. We further reveal that mineral grains grow by grain boundary migration and grain rotation. Our findings provide a direct observation of the dynamics of crystal growth from ionic solutions

Graphene as a Long-Term Metal Oxidation Barrier: Worse Than Nothing

Anticorrosion and antioxidation surface treatments such as paint or anodization are a foundational component in nearly all industries. Graphene, a single-atom-thick sheet of carbon with impressive impermeability to gases, seems to hold promise as an effective anticorrosion barrier, and recent work supports this hope. We perform a complete study of the short- and long-term performance of graphene coatings for Cu and Si substrates. Our work reveals that although graphene indeed offers effective short-term oxidation protection, over long time scales it promotes more extensive wet corrosion than that seen for an initially bare, unprotected Cu surface. This surprising result has important implications for future scientific studies and industrial applications. In addition to informing any future work on graphene as a protective coating, the results presented here have implications for graphene’s performance in a wide range of applications

Conserved Atomic Bonding Sequences and Strain Organization of Graphene Grain Boundaries

The bulk properties of polycrystalline materials are directly influenced by the atomic structure at the grain boundaries that join neighboring crystallites. In this work, we show that graphene grain boundaries are comprised of structural building blocks of conserved atomic bonding sequences using aberration corrected high-resolution transmission electron microscopy. These sequences appear as stretches of identically arranged periodic or aperiodic regions of dislocations. Atomic scale strain and lattice rotation of these interfaces is derived by mapping the exact positions of every carbon atom at the boundary with ultrahigh precision. Strain fields are organized into local tensile and compressive dipoles in both periodic and aperiodic dislocation regions. Using molecular dynamics tension simulations, we find that experimental grain boundary structures maintain strengths that are comparable to idealized periodic boundaries despite the presence of local aperiodic dislocation sequences

The Use of Graphene and Its Derivatives for Liquid-Phase Transmission Electron Microscopy of Radiation-Sensitive Specimens

One of the key challenges facing liquid-phase transmission electron microscopy (TEM) of biological specimens has been the damaging effects of electron beam irradiation. The strongly ionizing electron beam is known to induce radiolysis of surrounding water molecules, leading to the formation of reactive radical species. In this study, we employ DNA-assembled Au nanoparticle superlattices (DNA-AuNP superlattices) as a model system to demonstrate that graphene and its derivatives can be used to mitigate electron beam-induced damage. We can image DNA-AuNP superlattices in their native saline environment when the liquid cell window material is graphene, but not when it is silicon nitride. In the latter case, initial dissociation of assembled AuNPs was followed by their random aggregation and etching. Using graphene-coated silicon nitride windows, we were able to replicate the observation of stable DNA-AuNP superlattices achieved with graphene liquid cells. We then carried out a correlative Raman spectroscopy and TEM study to compare the effect of electron beam irradiation on graphene with and without the presence of water and found that graphene reacts with the products of water radiolysis. We attribute the protective effect of graphene to its ability to efficiently scavenge reactive radical species, especially the hydroxyl radicals which are known to cause DNA strand breaks. We confirmed this by showing that stable DNA-AuNP assemblies can be imaged in silicon nitride liquid cells when graphene oxide and graphene quantum dots, which have also recently been reported as efficient radical scavengers, are added directly to the solution. We anticipate that our study will open up more opportunities for studying biological specimens using liquid-phase TEM with the use of graphene and its derivatives as biocompatible radical scavengers to alleviate the effects of radiation damage

In Situ Localized Growth of Ordered Metal Oxide Hollow Sphere Array on Microheater Platform for Sensitive, Ultra-Fast Gas Sensing

A simple and versatile strategy is presented for the localized on-chip synthesis of an ordered metal oxide hollow sphere array directly on a low power microheater platform to form a closely integrated miniaturized gas sensor. Selective microheater surface modification through fluorinated monolayer self-assembly and its subsequent microheater-induced thermal decomposition enables the position-controlled deposition of an ordered two-dimensional colloidal sphere array, which serves as a sacrificial template for metal oxide growth via homogeneous chemical precipitation; this strategy ensures control in both the morphology and placement of the sensing material on only the active heated area of the microheater platform, providing a major advantage over other methods of presynthesized nanomaterial integration via suspension coating or printing. A fabricated tin oxide hollow sphere-based sensor shows high sensitivity (6.5 ppb detection limit) and selectivity toward formaldehyde, and extremely fast response (1.8 s) and recovery (5.4 s) times. This flexible and scalable method can be used to fabricate high performance miniaturized gas sensors with a variety of hollow nanostructured metal oxides for a range of applications, including combining multiple metal oxides for superior sensitivity and tunable selectivity

Simultaneous Sheet Cross-Linking and Deoxygenation in the Graphene Oxide Sol–Gel Transition

The precursor material to graphene aerogels is a hydrogel formed from an aqueous solution of graphene oxide. We investigate the time evolution of the physical and chemical properties of a graphene oxide suspension as it transitions to a hydrogel. Fully formed hydrogels undergo densification during reaction, forming mechanically stable monoliths. We demonstrate that the gelation process removes oxygen functional groups, partially re-forms the sp² network, and creates bonds between graphene oxide sheets. Furthermore, these changes to the physical and chemical properties occur on exactly the same time scale, suggesting that they have a common origin. This discovery lends greater understanding to the formation of graphene oxide-based hydrogels, which could allow more flexibility and tunability in synthetic methods for graphene-like materials