Delamination-Assisted Ultrafast Wrinkle Formation in a Freestanding Film.

Freestanding films provide a versatile platform for materials engineering thanks to additional structural motifs not found in films with a substrate. A ubiquitous example is wrinkles, yet little is known about how they can develop over as fast as a few picoseconds due to a lack of experimental probes to visualize their dynamics in real time on the nanoscopic scale. Here, we use time-resolved electron diffraction to directly observe light-activated wrinkling formation in freestanding La2/3Ca1/3MnO3 films. Via a "lock-in" analysis of oscillations in the diffraction peak position, intensity, and width, we quantitatively reconstructed how wrinkles develop on the time scale of lattice vibration. Contrary to the common assumption of fixed boundary conditions, we found that wrinkle development is associated with ultrafast delamination at the film boundaries. Our work provides a generic protocol to quantify wrinkling dynamics in freestanding films and highlights the importance of the film-substrate interaction in determining the properties of freestanding structures.

Disentangling Coexisting Structural Order Through Phase Lock-In Analysis of Atomic-Resolution STEM Data.

As a real-space technique, atomic-resolution STEM imaging contains both amplitude and geometric phase information about structural order in materials, with the latter encoding important information about local variations and heterogeneities present in crystalline lattices. Such phase information can be extracted using geometric phase analysis (GPA), a method which has generally focused on spatially mapping elastic strain. Here we demonstrate an alternative phase demodulation technique and its application to reveal complex structural phenomena in correlated quantum materials. As with other methods of image phase analysis, the phase lock-in approach can be implemented to extract detailed information about structural order and disorder, including dislocations and compound defects in crystals. Extending the application of this phase analysis to Fourier components that encode periodic modulations of the crystalline lattice, such as superlattice or secondary frequency peaks, we extract the behavior of multiple distinct order parameters within the same image, yielding insights into not only the crystalline heterogeneity but also subtle emergent order parameters such as antipolar displacements. When applied to atomic-resolution images spanning large (~0.5 × 0.5 µm2) fields of view, this approach enables vivid visualizations of the spatial interplay between various structural orders in novel materials.

Strain Gradient Elasticity in SrTiO3 Membranes: Bending versus Stretching.

Young's modulus determines the mechanical loads required to elastically stretch a material and also the loads required to bend it, given that bending stretches one surface while compressing the opposite one. Flexoelectric materials have the additional property of becoming electrically polarized when bent. The associated energy cost can additionally contribute to elasticity via strain gradients, particularly at small length scales where they are geometrically enhanced. Here, we present nanomechanical measurements of freely suspended SrTiO3 crystalline membrane drumheads. We observe an unexpected nonmonotonic thickness dependence of Young's modulus upon small deflections. Furthermore, the modulus inferred from a predominantly bending deformation is three times larger than that of a predominantly stretching deformation for membranes thinner than 20 nm. In this regime we extract a strain gradient elastic coupling of ∼2.2 µN, which could be used in new operational regimes of nanoelectro-mechanics.

Stabilization of Sr3Al2O6 Growth Templates for Ex Situ Synthesis of Freestanding Crystalline Oxide Membranes.

A new synthetic approach has recently been developed for the fabrication of freestanding crystalline perovskite oxide nanomembranes, which involves the epitaxial growth of a water-soluble sacrificial layer. By utilizing an ultrathin capping layer of SrTiO3, here we show that this sacrificial layer, as grown by pulsed laser deposition, can be stabilized in air and therefore be used as transferrable templates for ex situ epitaxial growth using other techniques. We find that the stability of these templates depends on the thickness of the capping layer. On these templates, freestanding superconducting SrTiO3 membranes were synthesized ex situ using molecular beam epitaxy, enabled by the lower growth temperature which preserves the sacrificial layer. This study paves the way for the synthesis of an expanded selection of freestanding oxide membranes and heterostructures with a wide variety of ex situ growth techniques.

Synthesis of freestanding single-crystal perovskite films and heterostructures by etching of sacrificial water-soluble layers.

The ability to create and manipulate materials in two-dimensional (2D) form has repeatedly had transformative impact on science and technology. In parallel with the exfoliation and stacking of intrinsically layered crystals, atomic-scale thin film growth of complex materials has enabled the creation of artificial 2D heterostructures with novel functionality and emergent phenomena, as seen in perovskite heterostructures. However, separation of these layers from the growth substrate has proved challenging, limiting the manipulation capabilities of these heterostructures with respect to exfoliated materials. Here we present a general method to create freestanding perovskite membranes. The key is the epitaxial growth of water-soluble Sr 3Al 2O 6 on perovskite substrates, followed by in situ growth of films and heterostructures. Millimetre-size single-crystalline membranes are produced by etching the Sr 3Al 2O 6 layer in water, providing the opportunity to transfer them to arbitrary substrates and integrate them with heterostructures of semiconductors and layered compounds.

Vertical heterostructure of two-dimensional MoS2 and WSe2 with vertically aligned layers.

Two-dimensional (2D) layered materials consist of covalently bonded 2D atomic layers stacked by van der Waals interactions. Such anisotropic bonding nature gives rise to the orientation-dependent functionalities of the 2D layered materials. Different from most studies of 2D materials with their atomic layers parallel to substrate, we have recently developed layer vertically aligned 2D material nanofilms. Built on these developments, here, we demonstrate the synthesis of vertical heterostructure of n-type MoS2 and p-type WSe2 with vertically aligned atomic layers. Thin film of MoS2/WSe2 vertical structure was successfully synthesized without significant alloy formation. The heterostructure synthesis is scalable to a large area over 1 cm(2). We demonstrated the pn junction diode behavior of the heterostructure device. This novel device geometry opens up exciting opportunities for a variety of electronic and optoelectronic devices, complementary to the recent interesting vertical heterostructures with horizontal atomic layers.

One-dimensional helical transport in topological insulator nanowire interferometers.

The discovery of three-dimensional (3D) topological insulators opens a gateway to generate unusual phases and particles made of the helical surface electrons, proposing new applications using unusual spin nature. Demonstration of the helical electron transport is a crucial step to both physics and device applications of topological insulators. Topological insulator nanowires, of which spin-textured surface electrons form 1D band manipulated by enclosed magnetic flux, offer a unique nanoscale platform to realize quantum transport of spin-momentum locking nature. Here, we report an observation of a topologically protected 1D mode of surface electrons in topological insulator nanowires existing at only two values of half magnetic quantum flux (±h/2e) due to a spin Berry's phase (π). The helical 1D mode is robust against disorder but fragile against a perpendicular magnetic field breaking-time-reversal symmetry. This result demonstrates a device with robust and easily accessible 1D helical electronic states from 3D topological insulators, a unique nanoscale electronic system to study topological phenomena.

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.

Weak antilocalization in Bi2(Se(x)Te(1-x))3 nanoribbons and nanoplates.

Studying the surface states of Bi(2)Se(3) and Bi(2)Te(3) topological insulators has proven challenging due to the high bulk carrier density that masks the surface states. Ternary compound Bi(2)(Se(x)Te(1-x))(3) may present a solution to the current materials challenge by lowering the bulk carrier mobility significantly. Here, we synthesized Bi(2)(Se(x)Te(1-x))(3) nanoribbons and nanoplates via vapor-liquid-solid and vapor-solid growth methods where the atomic ratio x was controlled by the molecular ratio of Bi(2)Se(3) to Bi(2)Te(3) in the source mixture and ranged between 0 and 1. For the whole range of x, the ternary nanostructures are single crystalline without phase segregation, and their carrier densities decrease with x. However, the lowest electron density is still high (~10(19) cm(-3)) and the mobility low, suggesting that the majority of these carriers may come from impurity states. Despite the high carrier density, weak antilocalization (WAL) is clearly observed. Angle-dependent magnetoconductance study shows that an appropriate magnetic field range is critical to capture a true, two-dimensional (2D) WAL effect, and a fit to the 2D localization theory gives α of -0.97, suggesting its origin may be the topological surface states. The power law dependence of the dephasing length on temperature is ~T(-0.49) within the appropriate field range (~0.3 T), again reflecting the 2D nature of the WAL. Careful analysis on WAL shows how the surface states and the bulk/impurity states may interact with each other.

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.

Unconventional Josephson effect in hybrid superconductor-topological insulator devices.

We report on transport properties of Josephson junctions in hybrid superconducting-topological insulator devices, which show two striking departures from the common Josephson junction behavior: a characteristic energy that scales inversely with the width of the junction, and a low characteristic magnetic field for suppressing supercurrent. To explain these effects, we propose a phenomenological model which expands on the existing theory for topological insulator Josephson junctions.

One nanometer resolution electrical probe via atomic metal filament formation.

Scanning probe microscopy has been widely used to investigate various interactions in microscopic nature. Particularly, conductive atomic force microscopy (C-AFM) can provide local electronic signals conveniently, but the probe resolution of C-AFM has been limited by the tip geometry. Here, we improve the probe resolution greatly by forming an atomic-size metallic filament on a commercial C-AFM tip. We demonstrate ∼1 nm lateral resolution in C-AFM using the metal filament tip. The filament tip is mechanically robust and electrically stable in repeated scans under ambient conditions since it is imbedded in a stable insulating matrix. The formation of the atomic filament is highly controllable and reproducible and can be easily integrated to existing AFM tip technologies to produce the next generation of high-resolution electrical and other scanning probes.

Hollow carbon nanofiber-encapsulated sulfur cathodes for high specific capacity rechargeable lithium batteries.

Sulfur has a high specific capacity of 1673 mAh/g as lithium battery cathodes, but its rapid capacity fading due to polysulfides dissolution presents a significant challenge for practical applications. Here we report a hollow carbon nanofiber-encapsulated sulfur cathode for effective trapping of polysulfides and demonstrate experimentally high specific capacity and excellent electrochemical cycling of the cells. The hollow carbon nanofiber arrays were fabricated using anodic aluminum oxide (AAO) templates, through thermal carbonization of polystyrene. The AAO template also facilitates sulfur infusion into the hollow fibers and prevents sulfur from coating onto the exterior carbon wall. The high aspect ratio of the carbon nanofibers provides an ideal structure for trapping polysulfides, and the thin carbon wall allows rapid transport of lithium ions. The small dimension of these nanofibers provides a large surface area per unit mass for Li(2)S deposition during cycling and reduces pulverization of electrode materials due to volumetric expansion. A high specific capacity of about 730 mAh/g was observed at C/5 rate after 150 cycles of charge/discharge. The introduction of LiNO(3) additive to the electrolyte was shown to improve the Coulombic efficiency to over 99% at C/5. The results show that the hollow carbon nanofiber-encapsulated sulfur structure could be a promising cathode design for rechargeable Li/S batteries with high specific energy.

New nanostructured Li2S/silicon rechargeable battery with high specific energy.

Rechargeable lithium ion batteries are important energy storage devices; however, the specific energy of existing lithium ion batteries is still insufficient for many applications due to the limited specific charge capacity of the electrode materials. The recent development of sulfur/mesoporous carbon nanocomposite cathodes represents a particularly exciting advance, but in full battery cells, sulfur-based cathodes have to be paired with metallic lithium anodes as the lithium source, which can result in serious safety issues. Here we report a novel lithium metal-free battery consisting of a Li(2)S/mesoporous carbon composite cathode and a silicon nanowire anode. This new battery yields a theoretical specific energy of 1550 Wh kg(-1), which is four times that of the theoretical specific energy of existing lithium-ion batteries based on LiCoO(2) cathodes and graphite anodes (approximately 410 Wh kg(-1)). The nanostructured design of both electrodes assists in overcoming the issues associated with using sulfur compounds and silicon in lithium-ion batteries, including poor electrical conductivity, significant structural changes, and volume expansion. We have experimentally realized an initial discharge specific energy of 630 Wh kg(-1) based on the mass of the active electrode materials.

Ultrathin topological insulator Bi2Se3 nanoribbons exfoliated by atomic force microscopy.

Ultrathin topological insulator nanostructures, in which coupling between top and bottom surface states takes place, are of great intellectual and practical importance. Due to the weak van der Waals interaction between adjacent quintuple layers (QLs), the layered bismuth selenide (Bi(2)Se(3)), a single Dirac-cone topological insulator with a large bulk gap, can be exfoliated down to a few QLs. In this paper, we report the first controlled mechanical exfoliation of Bi(2)Se(3) nanoribbons (>50 QLs) by an atomic force microscope (AFM) tip down to a single QL. Microwave impedance microscopy is employed to map out the local conductivity of such ultrathin nanoribbons, showing drastic difference in sheet resistance between 1-2 QLs and 4-5 QLs. Transport measurement carried out on an exfoliated (50 QLs) ribbons. These AFM-exfoliated thin nanoribbons afford interesting candidates for studying the transition from quantum spin Hall surface to edge states.

Extreme tensile strain states in La0.7Ca0.3MnO3 membranes.

A defining feature of emergent phenomena in complex oxides is the competition and cooperation between ground states. In manganites, the balance between metallic and insulating phases can be tuned by the lattice; extending the range of lattice control would enhance the ability to access other phases. We stabilized uniform extreme tensile strain in nanoscale La0.7Ca0.3MnO3 membranes, exceeding 8% uniaxially and 5% biaxially. Uniaxial and biaxial strain suppresses the ferromagnetic metal at distinctly different strain values, inducing an insulator that can be extinguished by a magnetic field. Electronic structure calculations indicate that the insulator consists of charge-ordered Mn4+ and Mn3+ with staggered strain-enhanced Jahn-Teller distortions within the plane. This highly tunable strained membrane approach provides a broad opportunity to design and manipulate correlated electron states.

Strain-induced room-temperature ferroelectricity in SrTiO3 membranes.

Advances in complex oxide heteroepitaxy have highlighted the enormous potential of utilizing strain engineering via lattice mismatch to control ferroelectricity in thin-film heterostructures. This approach, however, lacks the ability to produce large and continuously variable strain states, thus limiting the potential for designing and tuning the desired properties of ferroelectric films. Here, we observe and explore dynamic strain-induced ferroelectricity in SrTiO3 by laminating freestanding oxide films onto a stretchable polymer substrate. Using a combination of scanning probe microscopy, optical second harmonic generation measurements, and atomistic modeling, we demonstrate robust room-temperature ferroelectricity in SrTiO3 with 2.0% uniaxial tensile strain, corroborated by the notable features of 180° ferroelectric domains and an extrapolated transition temperature of 400 K. Our work reveals the enormous potential of employing oxide membranes to create and enhance ferroelectricity in environmentally benign lead-free oxides, which hold great promise for applications ranging from non-volatile memories and microwave electronics.

High-frequency micromechanical resonators from aluminium-carbon nanotube nanolaminates.

At micro- and nanoscales, materials with high Young's moduli and low densities are of great interest for high-frequency micromechanical resonator devices. Incorporating carbon nanotubes (CNTs), with their unmatched properties, has added functionality to many man-made composites. We report on the fabrication of < or = 100-nm-thick laminates by sputter-deposition of aluminium onto a two-dimensional single-walled CNT network. These nanolaminates--composed of Al, its native oxide Al(2)O(3) and CNTs--are fashioned, in a scalable manner, into suspended doubly clamped micromechanical beams. Dynamic flexural measurements show marked increases in resonant frequencies for nanolaminates with Al-CNT laminae. Such increases, further supported by quasi-static flexural measurements, are partly attributable to enhancements in elastic properties arising from the addition of CNTs. As a consequence, these nanolaminate micromechanical resonators show significant suppression of mechanical nonlinearity and enhanced strength, both of which are advantageous for practical applications and analogous to biological nanocomposites, similarly composed of high-aspect-ratio, mechanically superior mineral platelets in a soft protein matrix.

Two-dimensional limit of crystalline order in perovskite membrane films.

Long-range order and phase transitions in two-dimensional (2D) systems-such as magnetism, superconductivity, and crystallinity-have been important research topics for decades. The issue of 2D crystalline order has reemerged recently, with the development of exfoliated atomic crystals. Understanding the dimensional limit of crystalline phases, with different types of bonding and synthetic techniques, is at the foundation of low-dimensional materials design. We study ultrathin membranes of SrTiO3, an archetypal perovskite oxide with isotropic (3D) bonding. Atomically controlled membranes are released after synthesis by dissolving an underlying epitaxial layer. Although all unreleased films are initially single-crystalline, the SrTiO3 membrane lattice collapses below a critical thickness (5 unit cells). This crossover from algebraic to exponential decay of the crystalline coherence length is analogous to the 2D topological Berezinskii-Kosterlitz-Thouless (BKT) transition. The transition is likely driven by chemical bond breaking at the 2D layer-3D bulk interface, defining an effective dimensional phase boundary for coherent crystalline lattices.

Lateral and Vertical Two-Dimensional Layered Topological Insulator Heterostructures.

The heterostructured configuration between two-dimensional (2D) semiconductor materials has enabled the engineering of the band gap and the design of novel devices. So far, the synthesis of single-component topological insulator (TI) 2D materials such as Bi2Se3, Bi2Te3, and Sb2Te3 has been achieved through vapor phase growth and molecular beam epitaxy; however, the spatial controlled fabrication of 2D lateral heterostructures in these systems has not been demonstrated yet. Here, we report an in situ two-step synthesis process to form TI lateral heterostructures. Scanning transmission electron microscopy and energy-dispersive X-ray mapping results show the successful spatial control of chemical composition in these as-prepared heterostructures. The edge-induced growth mechanism is revealed by the ex situ atomic force microscope measurements. Electrical transport studies demonstrate the existence of p-n junctions in Bi2Te3/Sb2Te3 heterostructures.