Backbone Engineering of Monodisperse Conjugated Polymers via Integrated Iterative Binomial Synthesis.

Synthesis of sequence-defined monodisperse π-conjugated polymers with versatile backbones remains a substantial challenge. Here we report the development of an integrated iterative binomial synthesis (IIBS) strategy to enable backbone engineering of conjugated polymers with precisely controlled lengths and sequences as well as high molecular weights. The IIBS strategy capitalizes on the use of phenol as a surrogate for aryl bromide and represents the merge between protecting-group-aided iterative synthesis (PAIS) and iterative binomial synthesis (IBS). Long and monodisperse conjugated polymers with diverse irregular backbones, which are inaccessible by conventional polymerizations, can be efficiently prepared by IIBS. In addition, topology-dependent and chain-length-dependent properties have been investigated.

Low-Frequency Oscillations in Optical Measurements of Metal-Nanoparticle Vibrations.

Time-resolved optical measurements of vibrating metal nanoparticles have been used extensively to probe the ultrafast mechanical properties of the nanoparticles and of the surrounding liquid, but nearly all investigations so far have been limited to the linear regime. Here, we report the observation of a low-frequency oscillating signal in transient-absorption measurements of nanoparticles with octahedral gold cores and cubic silver shells; the signal appears at the difference of two mechanical vibrational frequencies in the particles, suggesting a nonlinear mixing process. We tentatively attribute this proposed mixing to a nonlinear coupling between a vibrational mode of the nanoparticle and its optical-frequency plasmon resonance. The optimization of this nonlinear transduction may enable high-efficiency opto-mechanical frequency mixing in the GHz-THz frequency regime.

Ingrained: An Automated Framework for Fusing Atomic-Scale Image Simulations into Experiments.

To fully leverage the power of image simulation to corroborate and explain patterns and structures in atomic resolution microscopy, an initial correspondence between the simulation and experimental image must be established at the outset of further high accuracy simulations or calculations. Furthermore, if simulation is to be used in context of highly automated processes or high-throughput optimization, the process of finding this correspondence itself must be automated. In this work, "ingrained," an open-source automation framework which solves for this correspondence and fuses atomic resolution image simulations into the experimental images to which they correspond, is introduced. Herein, the overall "ingrained" workflow, focusing on its application to interface structure approximations, and the development of an experimentally rationalized forward model for scanning tunneling microscopy simulation are described.

Three-dimensional optical trapping and orientation of microparticles for coherent X-ray diffraction imaging.

Optical trapping has been implemented in many areas of physics and biology as a noncontact sample manipulation technique to study the structure and dynamics of nano- and mesoscale objects. It provides a unique approach for manipulating microscopic objects without inducing undesired changes in structure. Combining optical trapping with hard X-ray microscopy techniques, such as coherent diffraction imaging and crystallography, provides a nonperturbing environment where electronic and structural dynamics of an individual particle in solution can be followed in situ. It was previously shown that optical trapping allows the manipulation of micrometer-sized objects for X-ray fluorescence imaging. However, questions remain over the ability of optical trapping to position objects for X-ray diffraction measurements, which have stringent requirements for angular stability. Our work demonstrates that dynamic holographic optical tweezers are capable of manipulating single micrometer-scale anisotropic particles in a microfluidic environment with the precision and stability required for X-ray Bragg diffraction experiments-thus functioning as an "optical goniometer." The methodology can be extended to a variety of X-ray experiments and the Bragg coherent diffractive imaging of individual particles in solution, as demonstrated here, will be markedly enhanced with the advent of brighter, coherent X-ray sources.

Stable aqueous dispersions of optically and electronically active phosphorene.

Understanding and exploiting the remarkable optical and electronic properties of phosphorene require mass production methods that avoid chemical degradation. Although solution-based strategies have been developed for scalable exfoliation of black phosphorus, these techniques have thus far used anhydrous organic solvents in an effort to minimize exposure to known oxidants, but at the cost of limited exfoliation yield and flake size distribution. Here, we present an alternative phosphorene production method based on surfactant-assisted exfoliation and postprocessing of black phosphorus in deoxygenated water. From comprehensive microscopic and spectroscopic analysis, this approach is shown to yield phosphorene dispersions that are stable, highly concentrated, and comparable to micromechanically exfoliated phosphorene in structure and chemistry. Due to the high exfoliation efficiency of this process, the resulting phosphorene flakes are thinner than anhydrous organic solvent dispersions, thus allowing the observation of layer-dependent photoluminescence down to the monolayer limit. Furthermore, to demonstrate preservation of electronic properties following solution processing, the aqueous-exfoliated phosphorene flakes are used in field-effect transistors with high drive currents and current modulation ratios. Overall, this method enables the isolation and mass production of few-layer phosphorene, which will accelerate ongoing efforts to realize a diverse range of phosphorene-based applications.

Silicon-Phosphorene Nanocavity-Enhanced Optical Emission at Telecommunications Wavelengths.

Generating and amplifying light in silicon (Si) continues to attract significant attention due to the possibility of integrating optical and electronic components in a single material platform. Unfortunately, silicon is an indirect band gap material and therefore an inefficient emitter of light. With the rise of integrated photonics, the search for silicon-based light sources has evolved from a scientific quest to a major technological bottleneck for scalable, CMOS-compatible, light sources. Recently, emerging two-dimensional materials have opened the prospect of tailoring material properties based on atomic layers. Few-layer phosphorene, which is isolated through exfoliation from black phosphorus (BP), is a great candidate to partner with silicon due to its layer-tunable direct band gap in the near-infrared where silicon is transparent. Here we demonstrate a hybrid silicon optical emitter composed of few-layer phosphorene nanomaterial flakes coupled to silicon photonic crystal resonators. We show single-mode emission in the telecommunications band of 1.55 µm ( Eg = 0.8 eV) under continuous wave optical excitation at room temperature. The solution-processed few-layer BP flakes enable tunable emission across a broad range of wavelengths and the simultaneous creation of multiple devices. Our work highlights the versatility of the Si-BP material platform for creating optically active devices in integrated silicon chips.

Atomistic determination of the surface structure of Cu2O(111): experiment and theory.

Cuprous oxide (Cu2O) is a promising catalyst for several important reactions. However, the atomic structures of defective Cu2O surfaces, which critically affect the catalytic properties both thermodynamically and kinetically, are not unambiguously characterized. High-resolution scanning tunneling microscopy (STM), combined with density functional theory (DFT) calculations and STM simulations, has been used to determine the atomic structure of the (111) surface of a Cu2O bulk crystal. The single crystal surface, processed by ultrahigh vacuum cleaning and oxygen annealing, shows a (1 × 1) periodicity in the low-energy electron diffraction pattern. The pristine (defect-free) Cu2O(111) surface exhibits a lattice of protrusions with hexagonal symmetry under STM, which is attributed to the dangling bonds of the coordinatively unsaturated copper (CuU) atoms on the surface. Two types of surface atomic defects are also identified, including the CuU vacancy and the oxygen-vacancy-induced local surface restructuring. The electronic structure of this surface measured by dI/dV spectroscopy shows an energy band gap of ∼1.6-2.1 eV. Consistent with dI/dV measurements, DFT calculations identified surface states within the electronic band gap arising from the Cu ions on the surface. Our results provide a clear picture of the pristine and defective Cu2O(111) surface structure in addition to the formation mechanism of the reconstructed surface, paving the way toward studying the site-dependent reactivity of this surface.

Room-temperature ferroelectricity in supramolecular networks of charge-transfer complexes.

Materials exhibiting a spontaneous electrical polarization that can be switched easily between antiparallel orientations are of potential value for sensors, photonics and energy-efficient memories. In this context, organic ferroelectrics are of particular interest because they promise to be lightweight, inexpensive and easily processed into devices. A recently identified family of organic ferroelectric structures is based on intermolecular charge transfer, where donor and acceptor molecules co-crystallize in an alternating fashion known as a mixed stack: in the crystalline lattice, a collective transfer of electrons from donor to acceptor molecules results in the formation of dipoles that can be realigned by an external field as molecules switch partners in the mixed stack. Although mixed stacks have been investigated extensively, only three systems are known to show ferroelectric switching, all below 71 kelvin. Here we describe supramolecular charge-transfer networks that undergo ferroelectric polarization switching with a ferroelectric Curie temperature above room temperature. These polar and switchable systems utilize a structural synergy between a hydrogen-bonded network and charge-transfer complexation of donor and acceptor molecules in a mixed stack. This supramolecular motif could help guide the development of other functional organic systems that can switch polarization under the influence of electric fields at ambient temperatures.

Large Spatially Resolved Rectification in a Donor-Acceptor Molecular Heterojunction.

We demonstrate that rectification ratios (RR) of ≳250 (≳1000) at biases of 0.5 V (1.2 V) are achievable at the two-molecule limit for donor-acceptor bilayers of pentacene on C60 on Cu using scanning tunneling spectroscopy and microscopy. Using first-principles calculations, we show that the system behaves as a molecular Schottky diode with a tunneling transport mechanism from semiconducting pentacene to Cu-hybridized metallic C60. Low-bias RRs vary by two orders-of-magnitude at the edge of these molecular heterojunctions due to increased Stark shifts and confinement effects.

Self-Sustained Micromechanical Oscillator with Linear Feedback.

Autonomous oscillators, such as clocks and lasers, produce periodic signals without any external frequency reference. In order to sustain stable periodic motion, there needs to be an external energy supply as well as nonlinearity built into the oscillator to regulate the amplitude. Usually, nonlinearity is provided by the sustaining feedback mechanism, which also supplies energy, whereas the constituent resonator that determines the output frequency stays linear. Here, we propose a new self-sustaining scheme that relies on the nonlinearity originating from the resonator itself to limit the oscillation amplitude, while the feedback remains linear. We introduce a model for describing the working principle of the self-sustained oscillations and validate it with experiments performed on a nonlinear microelectromechanical oscillator.

Nonlinearity-induced synchronization enhancement in micromechanical oscillators.

An autonomous oscillator synchronizes to an external harmonic force only when the forcing frequency lies within a certain interval-known as the synchronization range-around the oscillator's natural frequency. Under ordinary conditions, the width of the synchronization range decreases when the oscillation amplitude grows, which constrains synchronized motion of micro- and nanomechanical resonators to narrow frequency and amplitude bounds. Here, we show that nonlinearity in the oscillator can be exploited to manifest a regime where the synchronization range increases with increasing oscillation amplitude. Experimental data are provided for self-sustained micromechanical oscillators operating in this regime, and analytical results show that nonlinearities are the key determinants of this effect. Our results provide a new strategy to enhance the synchronization of micromechanical oscillators by capitalizing on their intrinsic nonlinear dynamics.

Self-assembled nanoparticle drumhead resonators.

The self-assembly of nanoscale structures from functional nanoparticles has provided a powerful path to developing devices with emergent properties from the bottom-up. Here we demonstrate that freestanding sheets self-assembled from various nanoparticles form versatile nanomechanical resonators in the megahertz frequency range. Using spatially resolved laser-interferometry to measure thermal vibrational spectra and image vibration modes, we show that their dynamic behavior is in excellent agreement with linear elastic response for prestressed drumheads of negligible bending stiffness. Fabricated in a simple one-step drying-mediated process, these resonators are highly robust and their inorganic-organic hybrid nature offers an extremely low mass, low stiffness, and the potential to couple the intrinsic functionality of the nanoparticle building blocks to nanomechanical motion.

Imaging Valley Excitons in a 2D Semiconductor with Scanning Tunneling Microscope-Induced Luminescence.

Valley excitons dominate the optoelectronic response of transition-metal dichalcogenides and are drastically affected by structural and environmental inhomogeneities localized in these materials. Critical to understanding and controlling these nanoscale excitonic changes is the ability to correlate the imaging of excitonic states with crystalline structures on the atomic scale. Here, we apply scanning tunneling microscope-induced luminescence microscopy to image valley excitons in a semiconducting transition-metal dichalcogenide monolayer decoupled by a 10 nanometer-thick hexagonal-boron-nitride flake incorporated in a lateral homojunction on an Au electrode surface. This design enables the observation of chiral excitonic emission arising from neutral and charged valley excitons of the monolayer semiconductor at ambipolar voltages with a quantum efficiency up to ∼10-5 photon/electron. The measured light helicity demonstrates considerable circular polarization dependent on the sample voltage, reaching as much as 40%. The real-space luminescence imaging maps─at subnanometer resolution─of the valley excitons reveal striking spatial variations associated with localized inhomogeneities, including surface impurities and possibly nanoscale dielectric and/or potential disorders in the monolayer. Our study introduces a promising format for 2D materials to explore and tailor their optoelectronic processes at the atomic scale.

Structural and electronic decoupling of C60 from epitaxial graphene on SiC.

We have investigated the initial stages of growth and the electronic structure of C(60) molecules on graphene grown epitaxially on SiC(0001) at the single-molecule level using cryogenic ultrahigh vacuum scanning tunneling microscopy and spectroscopy. We observe that the first layer of C(60) molecules self-assembles into a well-ordered, close-packed arrangement on graphene upon molecular deposition at room temperature while exhibiting a subtle C(60) superlattice. We measure a highest occupied molecular orbital-lowest unoccupied molecular orbital gap of ∼3.5 eV for the C(60) molecules on graphene in submonolayer regime, indicating a significantly smaller amount of charge transfer from the graphene to C(60) and substrate-induced screening as compared to C(60) adsorbed on metallic substrates. Our results have important implications for the use of graphene for future device applications that require electronic decoupling between functional molecular adsorbates and substrates.

Emergence of excited-state plasmon modes in linear hydrogen chains from time-dependent quantum mechanical methods.

Explicitly time-dependent configuration-interaction theory is used to predict a new type of plasmonic behavior in linear hydrogen chains. After an intense ultrashort laser pulse brings the system into a broad superposition of excited states, the electronic dipole of the entire chain oscillates coherently, and the system is predicted to emit radiation at energies significantly lower than the first absorption band. A simple classical model accurately predicts the energy of this plasmon resonance for different hydrogen chain lengths and electron densities, demonstrating that collective, free-electron-like behavior can arise in chains of as few as 20 hydrogen atoms. The excitation mechanism for this plasmonic resonance is a highly nonlinear, multiphoton process, different from the linear excitation of ordinary surface plasmons.

Epitaxial graphene on Cu(111).

The growth of graphene on single crystal Cu(111) has been achieved by thermal decomposition of ethylene in an ultrahigh vacuum chamber for the first time. The structural and electronic properties of graphene on Cu(111) have been investigated by scanning tunneling microscopy and spectroscopy. The nucleation of monolayer islands and two predominant domain orientations have been observed, which lead to the formation of numerous domain boundaries with increasing coverage. These results reveal that reducing the density of domain boundaries is one challenge of growing high-quality graphene on copper.

Probing the surface of transition-metal nanocrystals by chemiluminescence.

We propose a simple chemiluminescence (CL) method for investigation of the surface of Co-based nanocrystals (NCs). Using a combination of CL and spin-trap electron paramagnetic resonance techniques, we systematically studied the generation of reactive oxygen species (ROS) at the surface of differently sized CoPt(3) spherical NCs and CoPt(3)/Au nanodumbbells. We have shown that differently sized CoPt(3) NCs can promote the formation of ROS and as a result can lead to the oxidation of luminol accompanied by the emission of the light. CL allows monitoring the stability of transition-metal-based NCs against oxidation and dissolution. We found by CL that cobalt ions slowly leach from the surface of CoPt(3) NCs even under very mild conditions; however, the amount of the leached cobalt ions does not exceed the maximal concentration of cobalt at the NC surface indicating that only surface atoms can go into solution.

Patterning graphene at the nanometer scale via hydrogen desorption.

We have demonstrated the reversible and local modification of the electronic properties of graphene by hydrogen passivation and subsequent electron-stimulated hydrogen desorption with an scanning tunneling microscope tip. In addition to changing the morphology, we show that the hydrogen passivation is stable at room temperature and modifies the electronic properties of graphene, opening a gap in the local density of states. This insulating state is reversed by local desorption of the hydrogen, and the unaltered electronic properties of graphene are recovered. Using this mechanism, we have "written" graphene patterns on nanometer length scales. For patterned regions that are roughly 20 nm or greater, the inherent electronic properties of graphene are completely recovered. Below 20 nm we observe dramatic variations in the electronic properties of the graphene as a function of pattern size. This reversible and local mechanism for modifying the electronic properties of graphene has far-reaching implications for nanoscale circuitry fabricated from this revolutionary material.