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Using scanning transmission electron microscopy we image ~ 4 nm platinum nanoparticles deposited on an insulating membrane, where the membrane is one of two electron-transparent windows separating an aqueous environment from the microscope’s high vacuum. Upon receiving a relatively moderate dose of ~ 104 e/nm2, initially immobile nanoparticles begin to move along trajectories that are directed radially outward from the center of the field of view. With larger dose rates the particle motion becomes increasingly dramatic. These observations demonstrate that even under mild imaging conditions, the in situ electron microscopy of aqueous environments can produce electrophoretic charging effects that dominate the dynamics of nanoparticles under observation.

The relative orientation of successive sheets, i.e. the stacking sequence, in layered two-dimensional materials is central to the electronic, thermal, and mechanical properties of the material. Often different stacking sequences have comparable cohesive energy, leading to alternative stable crystal structures. Here we theoretically and experimentally explore different stacking sequences in the van der Waals bonded material hexagonal boron nitride (h-BN). We examine the total energy, electronic bandgap, and dielectric response tensor for five distinct high symmetry stacking sequences for both bulk and bilayer forms of h-BN. Two sequences, the generally assumed AA' sequence and the relatively unknown (for h-BN) AB (Bernal) sequence, are predicted to have comparably low energy. We present a scalable modified chemical vapor deposition method that produces large flakes of virtually pure AB stacked h-BN; this new material complements the generally available AA' stacked h-BN.In recent years there has been a dramatic resurgence in interest in van der Waals bonded layered materials, including graphite, boron-nitride, and transition metal dichalcogenides. 1,2 These materials display strong intraplane (typically covalent) bonding and weak van der Waals interplane bonding, which facilitates exfoliation into mono-layer or few-layer forms, and further allows custom stack-ups or laminations of sheets with different chemical composition or crystallographic orientation. 1-8 Even for a material composed of identical sheets, the stacking order of the successive sheets or layers, which may be translationally and/or rotationally shifted, can profoundly influence the overall physical properties. 9-12 For example, for naturally occurring graphite the usual stacking sequence is Bernal (AB) stacking, but rhombohedral (ABC) stacking is also possible, which has a completely different electronic band structure. 13,14 Hexagonal boron nitride (h-BN) is structurally very similar to graphite, with successively stacked (and van der Waals bonded) sheets of hexagonally arranged sp 2bonded boron and nitrogen. 5 However, unlike graphite, h-BN is purely synthetic, with a wide electronic band gap (hence the nickname "white graphite"). [15][16][17] Virtually all synthesis methods for h-BN lead to an AA' stacking sequence, where atoms in one layer all lie directly above atoms in the next layer. 4,[18][19][20] Successive layers are rotated such that all nitrogens lie above borons, and all borons lie above nitrogens. 20 Potential alternative stacking sequences for h-BN are of great theoretical and experimental interest.Here we explore these alternative h-BN stacking sequences. We employ Density Functional Theory (DFT) to determine the total energy, electronic band structure, and dielectric tensor elements for five different high-symmetry h-BN stackings. We find that Bernal (AB) stacked h-BN has a total energy comparable to, and indeed a bit lower than,

Two-dimensional (2D) excitons arise from electron-hole confinement along one spatial dimension. Such excitations are often described in terms of Frenkel or Wannier limits according to the degree of exciton spatial...

We demonstrate the fabrication of individual nanopores in hexagonal boron nitride (h-BN) with atomically precise control of the pore shape and size. Previous methods of pore production in other 2D materials typically create pores with irregular geometry and imprecise diameters. In contrast, other studies have shown that with careful control of electron irradiation, defects in h-BN grow with pristine zig-zag edges at quantized triangular sizes, but they have failed to demonstrate production and control of isolated defects. In this work, we combine these techniques to yield a method in which we can create individual size-quantized triangular nanopores through an h-BN sheet. The pores are created using the electron beam of a conventional transmission electron microscope; which can strip away multiple layers of h-BN exposing single-layer regions, introduce single vacancies, and preferentially grow vacancies only in the single-layer region. We further demonstrate how the geometry of these pores can be altered beyond triangular by changing beam conditions. Precisely size- and geometry-tuned nanopores could find application in molecular sensing, DNA sequencing, water desalination, and molecular separation.

The important role of water in growth of monolayer transition metal dichalcogenides Interest in transition metal dichalcogenides (TMDs) has been renewed by the discovery of emergent properties when reduced to single, two-dimensional (2D) layers. The transition to direct band gap [1,2], emerging charge density waves [3,4], high mobility [5][6][7], and valley polarization [8][9][10] are some of the many exciting properties that have been reported in the TMD literature recently. A major bottleneck to this research is the lack of reproducible and large scale synthetic methods for high quality, consistent monolayer TMD samples. The dominant growth method is the vaporization and subsequent chalcogenization of solid metal oxides in the presence of gaseous chalcogen precursors. This process is commonly referred to as chemical vapor deposition (CVD) or powder vaporization [11][12][13][14]. Due to its simplicity, CVD is extensively used by the TMD community to produce high quality, micron-sized single crystals [11][12][13][15][16][17][18][19]. Understanding the vaporization chemistry of solid transition metal precursors and vapor transport of volatilized precursors, particularly with respect to the influence of water vapor, is critical. Humidity, i.e. water content of the reaction environment, is an important parameter in the gas phase synthesis of inorganic materials, and while it is typically thought of as a contaminant, water is also an effective transport agent [20][21][22][23]. In this communication, we describe the synthesis of luminescent monolayer TMD islands by introducing water vapor as a simple means of controlling the volatilization and transport of the metal oxide precursor. Our experiments demonstrate a direct correlation between gas phase water content and the morphology of the resulting films. In particular, explicit control of the in situ water vapor concentration allows us to switch between two modes of growth: one in an effectively dry environment, in which the transition metal oxide source is converted directly to TMD material through a solid state reaction with the chalcogen source, and another in which the transition metal oxide undergoes vapor transport followed by reaction with the chalcogen source. We show that a small amount of water enhances the volatilization, and hence vapor transport, of the oxides of tungsten and molybdenum at the elevated temperatures (500-800 °C) used in the conversion or growth of their TMD counterparts. We attribute this effect to the enhanced vaporization of WO 3 and MoO 3 in the presence of water, first demonstrated in the 1930s and Our results show a direct correlation between gas phase water content and the morphology of TMD films. In particular, we show that the presence of water enhances volatilization, and therefore the vapor transport of tungsten and molybdenum oxide. Surprisingly, we find that water not only plays an important role in volatilization but is also compatible with TMD growth. In fact, carefully controlled humidity can consistently produce high qual...

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Graphene's structure bears on both the material's electronic properties and fundamental questions about long range order in two-dimensional crystals. We present an analytic calculation of selected area electron diffraction from multi-layer graphene and compare it with data from samples prepared by chemical vapor deposition and mechanical exfoliation. A single layer scatters only 0.5% of the incident electrons, so this kinematical calculation can be considered reliable for five or fewer layers. Dark-field transmission electron micrographs of multi-layer graphene illustrate how knowledge of the diffraction peak intensities can be applied for rapid mapping of thickness, stacking, and grain boundaries. The diffraction peak intensities also depend on the mean-square displacement of atoms from their ideal lattice locations, which is parameterized by a Debye-Waller factor. We measure the Debye-Waller factor of a suspended monolayer of exfoliated graphene and find a result consistent with an estimate based on the Debye model. For laboratory-scale graphene samples, finite size effects are sufficient to stabilize the graphene lattice against melting, indicating that ripples in the third dimension are not necessary.