Mn-intercalated MoSe2 under pressure: Electronic structure and vibrational characterization of a dilute magnetic semiconductor.

Intercalation offers a promising way to alter the physical properties of two-dimensional (2D) layered materials. Here, we investigate the electronic and vibrational properties of 2D layered MoSe2 intercalated with atomic manganese at ambient and high pressure up to 7 GPa by Raman scattering and electronic structure calculations. The behavior of optical phonons is studied experimentally with a diamond anvil cell and computationally through density functional theory calculations. Experiment and theory show excellent agreement in optical phonon behavior. The previously Raman inactive A2u mode is activated and enhanced with intercalation and pressure, and a new Raman mode appears upon decompression, indicating a possible onset of a localized structural transition, involving the bonding or trapping of the intercalant in 2D layered materials. Density functional theory calculations reveal a shift of the Fermi level into the conduction band and spin polarization in MnxMoSe2 that increases at low Mn concentrations and low pressure. Our results suggest that intercalation and pressurization of van der Waals materials may allow one to obtain dilute magnetic semiconductors with controllable properties, providing a viable route for the development of new materials for spintronic applications.

Chemically Tuning Quantized Acoustic Phonons in 2D Layered MoO3 Nanoribbons.

Molybdenum trioxide (α-MoO3) is a 2D layered metal oxide that can be altered in color from transparent white to dark blue with reversible intercalation of zerovalent metals, and whose mechanical properties can be controlled through intercalation. Here, we use Brillouin laser light spectroscopy to map the entire angular dispersion curves of multiple acoustic phonon branches of 2D layered MoO3, directly probing the effects of phonon quantum confinement when the phonon wavelength is comparable to the material thickness. Since acoustic phonons dictate elasticity, we thereby determine the full elastic stiffness tensor and the thickness of each nanoribbon to a statistical precision (derived from standard error propagation) corresponding to less than a monolayer. We show how intercalation of metallic Sn, Co, and Cu can chemically tune the quantized acoustic phonons and elasticity of MoO3 nanoribbons. This work provides the methodology to extract precise elastic constants from complex Brillouin scattering of 2D materials, taking advantage of phonon confinement to capture the complete elastic response with a single scattering geometry.

Mesoscale elastic properties of marine sponge spicules.

Marine sponge spicules are silicate fibers with an unusual combination of fracture toughness and optical light propagation properties due to their micro- and nano-scale hierarchical structure. We present optical measurements of the elastic properties of Tethya aurantia and Euplectella aspergillum marine sponge spicules using non-invasive Brillouin and Raman laser light scattering, thus probing the hierarchical structure on two very different scales. On the scale of single bonds, as probed by Raman scattering, the spicules resemble a combination of pure silica and mixed organic content. On the mesoscopic scale probed by Brillouin scattering, we show that while some properties (Young's moduli, shear moduli, one of the anisotropic Poisson ratios and refractive index) are nearly the same as those of artificial optical fiber, other properties (uniaxial moduli, bulk modulus and a distinctive anisotropic Poisson ratio) are significantly smaller. Thus this natural composite of largely isotropic materials yields anisotropic elastic properties on the mesoscale. We show that the spicules' optical waveguide properties lead to pronounced spontaneous Brillouin backscattering, a process related to the stimulated Brillouin backscattering process well known in artificial glass fibers. These measurements provide a clearer picture of the interplay of flexibility, strength, and material microstructure for future functional biomimicry.

A silicon-based two-dimensional chalcogenide: growth of Si2Te3 nanoribbons and nanoplates.

We report the synthesis of high-quality single-crystal two-dimensional, layered nanostructures of silicon telluride, Si2Te3, in multiple morphologies controlled by substrate temperature and Te seeding. Morphologies include nanoribbons formed by VLS growth from Te droplets, vertical hexagonal nanoplates through vapor-solid crystallographically oriented growth on amorphous oxide substrates, and flat hexagonal nanoplates formed through large-area VLS growth in liquid Te pools. We show the potential for doping through the choice of substrate and growth conditions. Vertical nanoplates grown on sapphire substrates, for example, can incorporate a uniform density of Al atoms from the substrate. We also show that the material may be modified after synthesis, including both mechanical exfoliation (reducing the thickness to as few as five layers) and intercalation of metal ions including Li(+) and Mg(2+), which suggests applications in energy storage materials. The material exhibits an intense red color corresponding to its strong and broad interband absorption extending from the red into the infrared. Si2Te3 enjoys chemical and processing compatibility with other silicon-based material including amorphous SiO2 but is very chemically sensitive to its environment, which suggests applications in silicon-based devices ranging from fully integrated thermoelectrics to optoelectronics to chemical sensors.

In situ observation of divergent phase transformations in individual sulfide nanocrystals.

Inorganic nanocrystals have attracted widespread attention both for their size-dependent properties and for their potential use as building blocks in an array of applications. A complete understanding of chemical transformations in nanocrystals is important for controlling structure, composition, and electronic properties. Here, we utilize in situ high-resolution transmission electron microscopy to study structural and morphological transformations in individual sulfide nanocrystals (copper sulfide, iron sulfide, and cobalt sulfide) as they react with lithium. The experiments reveal the influence of structure and composition on the transformation pathway (conversion versus displacement reactions), and they provide a high-resolution view of the unique displacement reaction mechanism in copper sulfide in which copper metal is extruded from the crystal. The structural similarity between the initial and final phases, as well as the mobility of ions within the crystal, are seen to exert a controlling influence on the reaction pathway.

Dual Element Intercalation into 2D Layered Bi2Se3 Nanoribbons.

We demonstrate the intercalation of multiple zero-valent atomic species into two-dimensional (2D) layered Bi2Se3 nanoribbons. Intercalation is performed chemically through a stepwise combination of disproportionation redox reactions, hydrazine reduction, or carbonyl decomposition. Traditional intercalation is electrochemical thus limiting intercalant guests to a single atomic species. We show that multiple zero-valent atoms can be intercalated through this chemical route into the host lattice of a 2D crystal. Intermetallic species exhibit unique structural ordering demonstrated in a variety of superlattice diffraction patterns. We believe this method is general and can be used to achieve a wide variety of new 2D materials previously inaccessible.

Non-invasive determination of the complete elastic moduli of spider silks.

Spider silks possess nature's most exceptional mechanical properties, with unrivalled extensibility and high tensile strength. Unfortunately, our understanding of silks is limited because the complete elastic response has never been measured-leaving a stark lack of essential fundamental information. Using non-invasive, non-destructive Brillouin light scattering, we obtain the entire stiffness tensors (revealing negative Poisson's ratios), refractive indices, and longitudinal and transverse sound velocities for major and minor ampullate spider silks: Argiope aurantia, Latrodectus hesperus, Nephila clavipes, Peucetia viridans. These results completely quantify the linear elastic response for all possible deformation modes, information unobtainable with traditional stress-strain tests. For completeness, we apply the principles of Brillouin imaging to spatially map the elastic stiffnesses on a spider web without deforming or disrupting the web in a non-invasive, non-contact measurement, finding variation among discrete fibres, junctions and glue spots. Finally, we provide the stiffness changes that occur with supercontraction.

Synthesis of MoS2 and MoSe2 films with vertically aligned layers.

Layered materials consist of molecular layers stacked together by weak interlayer interactions. They often crystallize to form atomically smooth thin films, nanotubes, and platelet or fullerene-like nanoparticles due to the anisotropic bonding. Structures that predominately expose edges of the layers exhibit high surface energy and are often considered unstable. In this communication, we present a synthesis process to grow MoS2 and MoSe2 thin films with vertically aligned layers, thereby maximally exposing the edges on the film surface. Such edge-terminated films are metastable structures of MoS2 and MoSe2, which may find applications in diverse catalytic reactions. We have confirmed their catalytic activity in a hydrogen evolution reaction (HER), in which the exchange current density correlates directly with the density of the exposed edge sites.

Ambipolar field effect in Sb-doped Bi2Se3 nanoplates by solvothermal synthesis.

A topological insulator is a new phase of quantum matter with a bulk band gap and spin-polarized surface states, which might find use in applications ranging from electronics to energy conversion. Despite much exciting progress in the field, high-yield solution synthesis has not been widely used for the study of topological insulator behavior. Here, we demonstrate that solvothermally synthesized Bi(2)Se(3) nanoplates are attractive for topological insulator studies. The carrier concentration of these Bi(2)Se(3) nanoplates is controlled by compensational Sb doping during the synthesis. In low-carrier-density, Sb-doped Bi(2)Se(3) nanoplates, we observe pronounced ambipolar field effect that demonstrates the flexible manipulation of carrier type and concentration for these nanostructures. Solvothermal synthesis offers an affordable, facile approach to produce high-quality nanomaterials to explore the properties of topological insulators.

Two-dimensional chalcogenide nanoplates as tunable metamaterials via chemical intercalation.

New plasmonic materials with tunable properties are in great need for nanophotonics and metamaterials applications. Here we present two-dimensional layered, metal chalcogenides as tunable metamaterials that feature both dielectric photonic and plasmonic modes across a wide spectral range from the infrared to ultraviolet. The anisotropic layered structure allows intercalation of organic molecules and metal atoms at the van der Waals gap of the host chalcogenide, presenting a chemical route to create heterostructures with molecular and atomic precision for photonic and plasmonic applications. This marks a departure from a lithographic method to create metamaterials. Monochromated electron energy-loss spectroscopy in a scanning transmission electron microscope was used to first establish the presence of the dielectric photonic and plasmonic modes in M2E3 (M = Bi, Sb; E = Se, Te) nanoplates and to observe marked changes in these modes after chemical intercalation. We show that these modal properties can also be tuned effectively by more conventional methods such as thickness control and alloy composition of the nanoplates.

Tunable Chemochromism and Elastic Properties in Intercalated MoO3: Au-, Cr-, Fe-, Ge-, Mn-, and Ni-MoO3.

Chemical tunability of the elastic constants of α-MoO3, a two-dimensional layered oxide, is demonstrated with mutability on the order of tens of GPa, simply by choice of a metal intercalant including Au, Cr, Fe, Ge, Mn, and Ni. Using Brillouin laser light scattering from confined acoustic phonons in nanometer-thick materials, the in-plane angular dispersion of the quantized acoustic phonon branches of 2D layered, intercalated MoO3 is measured and used to determine the bulk modulus (K), Young's moduli (E11, E22, and E33), each of the nine independent elastic tensor elements (cij), and the thickness. Intercalation of metals generally reduces the anisotropy in MoO3 except in Ge-MoO3, for which the in-plane longitudinal elastic anisotropy is unaffected. Chemochromism from transparent white (MoO3 and Fe-MoO3) to near black (Ni-MoO3) to brilliant dark blue (Ge-MoO3) is demonstrated and is associated with a reduction in electronic band gap with intercalation and an increase in absorption >600 nm for some intercalants (Cr-, Ge-, and Mn-MoO3).

Tailoring Ultrafast Near-Band Gap Photoconductive Response in GeS by Zero-Valent Cu Intercalation.

Zero-valent intercalation of atomic metals into the van der Waals gap of layered materials can be used to tune their electronic, optical, thermal, and mechanical properties. Here, we report the impact of intercalating ∼3 atm percent of zero-valent copper into germanium sulfide (GeS). Advanced many-body calculations predict that copper introduces quasi-localized intermediate band states, and time-resolved THz spectroscopy studies demonstrate that those states have prominent effects on the photoconductivity of GeS. Cu-intercalated GeS exhibits a faster rise of transient photoconductivity and a shorter lifetime of optically injected carriers following near-gap excitation with 800 nm pulses. At the same time, Cu intercalation improves free carrier mobility from 1100 to 1300 cm2 V-1 s-1, which we attribute to the damping of acoustic phonons observed in Brillouin scattering and consequent reduction of phonon scattering.

Luminescent nanocrystal stress gauge.

Microscale mechanical forces can determine important outcomes ranging from the site of material fracture to stem cell fate. However, local stresses in a vast majority of systems cannot be measured due to the limitations of current techniques. In this work, we present the design and implementation of the CdSe-CdS core-shell tetrapod nanocrystal, a local stress sensor with bright luminescence readout. We calibrate the tetrapod luminescence response to stress and use the luminescence signal to report the spatial distribution of local stresses in single polyester fibers under uniaxial strain. The bright stress-dependent emission of the tetrapod, its nanoscale size, and its colloidal nature provide a unique tool that may be incorporated into a variety of micromechanical systems including materials and biological samples to quantify local stresses with high spatial resolution.

Effects of magnetic doping on weak antilocalization in narrow Bi2Se3 nanoribbons.

We report low-temperature, magnetotransport measurements of ferrocene-doped Bi(2)Se(3) nanoribbons grown by vapor-liquid-solid method. The Kondo effect, a saturating resistance upturn at low temperatures, is observed in these ribbons to indicate presence of localized impurity spins. Magnetoconductances of the ferrocene-doped ribbons display both weak localization and weak antilocalization, which is in contrast with those of undoped ribbons that show only weak antilocalization. We show that the observed magnetoconductances are governed by a one-dimensional localization theory that includes spin orbit coupling and magnetic impurity scattering, yielding various scattering and dephasing lengths for Bi(2)Se(3). The power law decay of the dephasing length on temperature also reflects one-dimensional localization regime in these narrow Bi(2)Se(3) nanoribbons. The emergence of weak localization in ferrocene-doped Bi(2)Se(3) nanoribbons presents ferrocene as an effective magnetic dopant source.

Hafnium, Titanium, and Zirconium Intercalation in 2D Layered Nanomaterials.

Altering the physical and chemical properties of a layered material through intercalation has emerged as a unique strategy toward tunable applications. In this work, we demonstrate a wet chemical method to intercalate titanium, hafnium, and zirconium into 2D layered nanomaterials. The metals are intercalated using bis-tetrahydrofuran metal halide complexes. Metal intercalation is demonstrated in nanomaterials of Bi2Se3, Si2Te3, MoO3, and GeS. This strategy intercalates, on average, 3 atm % or less of Hf, Ti, and Zr that share charge with the host nanomaterial. This methodology is used to chemochromically alter MoO3 from transparent white to dark blue.

Chemical intercalation of zerovalent metals into 2D layered Bi2Se3 nanoribbons.

We have developed a chemical method to intercalate a variety of zerovalent metal atoms into two-dimensional (2D) layered Bi(2)Se(3) chalcogenide nanoribbons. We use a chemical reaction, such as a disproportionation redox reaction, to generate dilute zerovalent metal atoms in a refluxing solution, which intercalate into the layered Bi(2)Se(3) structure. The zerovalent nature of the intercalant allows superstoichiometric intercalation of metal atoms such as Ag, Au, Co, Cu, Fe, In, Ni, and Sn. We foresee the impact of this methodology in establishing novel fundamental physical behaviors and in possible energy applications.

High-density chemical intercalation of zero-valent copper into Bi2Se3 nanoribbons.

A major goal of intercalation chemistry is to intercalate high densities of guest species without disrupting the host lattice. Many intercalant concentrations, however, are limited by the charge of the guest species. Here we have developed a general solution-based chemical method for intercalating extraordinarily high densities of zero-valent copper metal into layered Bi(2)Se(3) nanoribbons. Up to 60 atom % copper (Cu(7.5)Bi(2)Se(3)) can be intercalated with no disruption to the host lattice using a solution disproportionation redox reaction.

Optoelectronics of Atomic Metal-Semiconductor Interfaces in Tin-Intercalated MoS2.

Metal-semiconductor interfaces are ubiquitous in modern electronics. These quantum-confined interfaces allow for the formation of atomically thin polarizable metals and feature rich optical and optoelectronic phenomena, including plasmon-induced hot-electron transfer from metal to semiconductors. Here, we report on the metal-semiconductor interface formed during the intercalation of zero-valent atomic layers of tin (Sn) between layers of MoS2, a van der Waals layered material. We demonstrate that Sn interaction leads to the emergence of gap states within the MoS2 band gap and to corresponding plasmonic features between 1 and 2 eV (0.6-1.2 µm). The observed stimulation of the photoconductivity, as well as the extension of the spectral response from the visible regime toward the mid-infrared suggests that hot-carrier generation and internal photoemission take place.

Quantitative Hole Mobility Simulation and Validation in Substituted Acenes.

Knowledge of the full phonon spectrum is essential to accurately calculate the dynamic disorder (σ) and hole mobility (µh) in organic semiconductors (OSCs). However, most vibrational spectroscopy techniques under-measure the phonons, thus limiting the phonon validation. Here, we measure and model the full phonon spectrum using multiple spectroscopic techniques and predict µh using σ from only the Γ-point and the full Brillouin zone (FBZ). We find that only inelastic neutron scattering (INS) provides validation of all phonon modes, and that σ in a set of small molecule semiconductors can be miscalculated by up to 28% when comparing Γ-point against FBZ calculations. A subsequent mode analysis shows that many modes contribute to σ and that no single mode dominates. Our results demonstrate the importance of a thoroughly validated phonon calculation, and a need to develop design rules considering the full spectrum of phonon modes.

Enhancing Light-Matter Interactions in MoS2 by Copper Intercalation.

The intercalation of layered compounds opens up a vast space of new host-guest hybrids, providing new routes for tuning the properties of materials. Here, it is shown that uniform and continuous layers of copper can be intercalated within the van der Waals gap of bulk MoS2 resulting in a unique Cu-MoS2 hybrid. The new Cu-MoS2 hybrid, which remains semiconducting, possesses a unique plasmon resonance at an energy of ≈1eV, giving rise to enhanced optoelectronic activity. Compared with high-performance MoS2 photodetectors, copper-enhanced devices are superior in their spectral response, which extends into the infrared, and also in their total responsivity, which exceeds 104 A W-1 . The Cu-MoS2 hybrids hold promise for supplanting current night-vision technology with compact, advanced multicolor night vision.