A multilayered composite structure formed by a random stacking of graphene oxide (GO) platelets is an attractive candidate for novel applications in nanoelectromechanical systems and paper-like composites. We employ molecular dynamics simulations with reactive force fields to elucidate the structural and mechanical properties of GO paper-like materials. We find that the large-scale properties of these composites are controlled by hydrogen bond networks that involve functional groups on individual GO platelets and water molecules within the interlayer cavities. Water content controls both the extent and collective strength of these interlayer hydrogen bond networks, thereby affecting the interlayer spacing and elastic moduli of the composite. Additionally, the chemical composition of the individual GO platelets also plays a critical role in establishing the mechanical properties of the composite--a higher density of functional groups leads to increased hydrogen bonding and a corresponding increase in stiffness. Our studies suggest the possibility of tuning the properties of GO composites by altering the density of functional groups on individual platelets, the water content, and possibly the functional groups participating in hydrogen bonding with interlayer water molecules.

Separation of molecules based on molecular size in zeolites with appropriate pore aperture dimensions has given rise to the definition of "molecular sieves" and has been the basis for a variety of separation applications. We show here that for a class of chabazite zeolites, what appears to be "molecular sieving" based on dimension is actually separation based on a difference in ability of a guest molecule to induce temporary and reversible cation deviation from the center of pore apertures, allowing for exclusive admission of certain molecules. This new mechanism of discrimination permits "size-inverse" separation: we illustrate the case of admission of a larger molecule (CO) in preference to a smaller molecule (N(2)). Through a combination of experimental and computational approaches, we have uncovered the underlying mechanism and show that it is similar to a "molecular trapdoor". Our materials show the highest selectivity of CO(2) over CH(4) reported to date with important application to natural gas purification.

We demonstrate that the energy bandgap of layered, high-dielectric α-MoO(3) can be reduced to values viable for the fabrication of 2D electronic devices. This is achieved through embedding Coulomb charges within the high dielectric media, advantageously limiting charge scattering. As a result, devices with α-MoO(3) of ∼11 nm thickness and carrier mobilities larger than 1100 cm(2) V(-1) s(-1) are obtained.

appreciable free carrier concentration. [5][6][7] The same strategy can potentially be implemented in selected 2D semiconductors. Another concern is the damping losses that should be kept low for applications such as optical communications, in which a long propagation of waves is necessary. [ 7 ] Reducing such damping losses requires that the product of the effective electron mass and the free charge mobility must be large in the 2D material. As a result, fi nding favorable materials that satisfy the aforementioned conditions are necessary for advancing the fi eld of 2D plasmonics.The creation of stable 2D semiconducting oxides of tungsten and molybdenum is possible, as we demonstrated previously. [ 8,9 ] In a recent topical feature article, Gregorieva and Geim have separated out these oxides as a unique group of 2D materials and predicted their signifi cant role in the future of planar structures. [ 10 ] The impact of these two metal oxides can be extended into the plasmonic realm, and, in fact, plasmon resonances in the one-dimensional (1D) morphologies of these two oxides have recently been demonstrated. Manthiram and Alivisatos reported plasmon resonances in 1D sub-stoichiometric semiconducting tungsten oxide, [ 6 ] while Huang et al. have shown the generation of plasmon resonances in 1D tubular reduced molybdenum oxide suspensions. [ 5 ] Advantageously both tungsten and molybdenum oxides can be ultra-doped and have also large dielectric constants, which both are important factors for obtaining plasmon resonances in the near IR and visible regions. [ 2 ] In 1D sub-stoichiometric tungsten and molybdenum oxides, the plasmon resonances are a function of two modest depolarization factors along the cross section of the 1D structure ( Figure 1 a -Supporting Information, Section S1 for the equations). However, the existence of one large depolarization factor reduces the wavelength of the plasmon resonances in 2D structures of similar stoichiometry.Accordingly, here, we explore tunable plasmonics in substoichiometric 2D molybdenum oxide nanofl akes in the visible range. The unique properties of 2D molybdenum oxide such as the possibility of high level ionic intercalation (hence ultradoping), large permittivity and the effect of the depolarization factor in 2D fl akes are used for demonstrating tunable plasmon resonance in this range. We investigate the effect of intercalating ions and changing the lateral dimensions of the fl akes on the plasmon resonance peaks of a reduced quasi-metallic form of molybdenum oxide.Molybdenum trioxide (MoO 3 ) is a stable n -type semiconductor under a wide range of conditions with a bandgap of ca. 3.2 eV, which is capable of adsorbing energy from a small portion of the visible light spectrum. [ 5,11 ] The most frequently 2D materials exhibit certain physical and chemical properties that are fundamentally different from their bulk counterparts. [ 1,2 ] The electronic and optical properties seen in the selected 2D materials may lead to signifi cantly altered plasmon dispersion relationsh...

Two-dimensional (2D) transition metal dichalcogenide semiconductors offer unique electronic and optical properties, which are significantly different from their bulk counterparts. It is known that the electronic structure of 2D MoS2, which is the most popular member of the family, depends on the number of layers. Its electronic structure alters dramatically at near atomically thin morphologies, producing strong photoluminescence (PL). Developing processes for controlling the 2D MoS2 PL is essential to efficiently harness many of its optical capabilities. So far, it has been shown that this PL can be electrically or mechanically gated. Here, we introduce an electrochemical approach to actively control the PL of liquid-phase-exfoliated 2D MoS2 nanoflakes by manipulating the amount of intercalated ions including Li(+), Na(+), and K(+) into and out of the 2D crystal structure. These ions are selected as they are crucial components in many bioprocesses. We show that this controlled intercalation allows for large PL modulations. The introduced electrochemically controlled PL will find significant applications in future chemical and bio-optical sensors as well as optical modulators/switches.

No abstract

No abstract

PN heterojunctions comprising layered van der Waals (vdW) semiconductors have been used to demonstrate current rectifiers, photodetectors, and photovoltaic devices. However, a direct or neardirect bandgap at the heterointerface that can significantly enhance optical generation, for high light absorbing few/multi-layer vdW materials, has not yet been shown. In this work, for the first time, few-layer group-6 transition metal dichalcogenide (TMD) WSe2 is shown to form a sizeable (0.7 eV) near-direct bandgap with type-II band alignment at its interface with the group-7 TMD ReS2 through density functional theory calculations. Further, the type-II alignment and photogeneration across the interlayer bandgap have been experimentally confirmed through micro-photoluminescence and IR 2 photodetection measurements, respectively. High optical absorption in few-layer flakes, large conduction and valence band offsets for efficient electron-hole separation and stacking of light facing, direct bandgap ReS2 on top of gate tunable WSe2 are shown to result in excellent and tunable photodetection as well as photovoltaic performance through flake thickness dependent optoelectronic measurements. Few-layer flakes demonstrate ultrafast response time (5 µs) at high responsivity (3 A/W) and large photocurrent generation and responsivity enhancement at the heterostructure overlap region (10-100×) for 532 nm laser illumination. Large open circuit voltage of 0.64 V and short circuit current of 2.6 µA enables high output electrical power. Finally, long term air-stability and a facile single contact metal fabrication process makes the multi-functional few-layer WSe2/ReS2 heterostructure diode technologically promising for next-generation optoelectronic applications.The semiconducting group-6 TMD WSe2, generally found in trigonal prismatic phase, 17 is an indirect bandgap material in its bulk form. 18,19 However, group-7 TMD e.g. 1T phase of ReS2 is distorted octahedral in structure 17,20 and exhibits direct (or, near-direct) bandgap at (or, close to) the Γ point of the Brillouin zone (BZ). Interestingly, unlike the group-6 TMDs, ReS2 exhibits a unique property owing to its distorted structure and weak interlayer coupling-the direct or near-direct nature of its bandgap remains unchanged from monolayer to bulk. [20][21][22] Closer examination of the bandstructure of group-7 TMDs reveals that the conduction band minimum of ReS2 remains at the Γ point, irrespective of the number of layers. 20,23 But for group-6 TMDs (such as WSe2) the valence band maximum relocates from K to Γ point of the BZ with increasing number of layers. 18,24 It is important to note that the valence band maximum at the Γ point differs in energy only slightly from that at the K point. 18 This gives rise to an increased probability of direct as well as indirect transitions from the Γ and the K valence maxima of WSe2 to the Γ conduction minimum of ReS2 respectively, for a predicted type-II band alignment. The possibility of a direct bandgap transition is not observed in a heter...