Light-Assisted Rechargeable Lithium Batteries: Organic Molecules for Simultaneous Energy Harvesting and Storage.

Lithium batteries that could be charged on exposure to sunlight will bring exciting new energy storage technologies. Here, we report a photorechargeable lithium battery employing nature-derived organic molecules as a photoactive and lithium storage electrode material. By absorbing sunlight of a desired frequency, lithiated tetrakislawsone electrodes generate electron-hole pairs. The holes oxidize the lithiated tetrakislawsone to tetrakislawsone while the generated electrons flow from the tetrakislawsone cathode to the Li metal anode. During electrochemical operation, the observed rise in charging current, specific capacity, and Coulombic efficiency under light irradiation in contrast to the absence of light indicates that the quinone-based organic electrode is acting as both photoactive and lithium storage material. Careful selection of electrode materials with optimal bandgap to absorb the intended frequency of sunlight and functional groups to accept Li-ions reversibly is a key to the progress of solar rechargeable batteries.

One-Dimensional Edge Contacts to a Monolayer Semiconductor.

Integration of electrical contacts into van der Waals (vdW) heterostructures is critical for realizing electronic and optoelectronic functionalities. However, to date no scalable methodology for gaining electrical access to buried monolayer two-dimensional (2D) semiconductors exists. Here we report viable edge contact formation to hexagonal boron nitride (hBN) encapsulated monolayer MoS2. By combining reactive ion etching, in situ Ar+ sputtering and annealing, we achieve a relatively low edge contact resistance, high mobility (up to ∼30 cm2 V-1 s-1) and high on-current density (>50 µA/µm at VDS = 3V), comparable to top contacts. Furthermore, the atomically smooth hBN environment also preserves the intrinsic MoS2 channel quality during fabrication, leading to a steep subthreshold swing of 116 mV/dec with a negligible hysteresis. Hence, edge contacts are highly promising for large-scale practical implementation of encapsulated heterostructure devices, especially those involving air sensitive materials, and can be arbitrarily narrow, which opens the door to further shrinkage of 2D device footprint.

Two-Dimensional Lateral Epitaxy of 2H (MoSe2)-1T' (ReSe2) Phases.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) heterostructures have been proposed as potential candidates for a variety of applications like quantum computing, neuromorphic computing, solar cells, and flexible field effective transistors. The 2D TMDC heterostructures at the present stage face difficulties being implemented in these applications because of lack of large and sharp heterostructure interfaces. Herein, we address this problem via a CVD technique to grow thermodynamically stable heterostructure of 2H/1T' MoSe2-ReSe2 using conventional transition metal phase diagrams as a reference. We demonstrate how the thermodynamics of mixing in the MoReSe2 system during CVD growth dictates the formation of atomically sharp interfaces between MoSe2 and ReSe2, which can be confirmed by high-resolution scanning transmission electron microscopy imaging, revealing zigzag selenium-terminated interface between the epitaxial 2H and 1T' lattices. Our work provides useful insights for understanding the stability of 2D heterostructures and interfaces between chemically, structurally, and electronically different phases.

Minimizing residues and strain in 2D materials transferred from PDMS.

Integrating layered two-dimensional (2D) materials into 3D heterostructures offers opportunities for novel material functionalities and applications in electronics and photonics. In order to build the highest quality heterostructures, it is crucial to preserve the cleanliness and morphology of 2D material surfaces that come in contact with polymers such as PDMS during transfer. Here we report that substantial residues and up to ∼0.22% compressive strain can be present in monolayer MoS2 transferred using PDMS. We show that a UV-ozone pre-cleaning of the PDMS surface before exfoliation significantly reduces organic residues on transferred MoS2 flakes. An additional 200 ◦C vacuum anneal after transfer efficiently removes interfacial bubbles and wrinkles as well as accumulated strain, thereby restoring the surface morphology of transferred flakes to their native state. Our recipe is important for building clean heterostructures of 2D materials and increasing the reproducibility and reliability of devices based on them.

Ultrasmall Plasmonic Single Nanoparticle Light Source Driven by a Graphene Tunnel Junction.

Metal nanoparticles that can couple light into tightly confined surface plasmons bridge the size mismatch between the wavelength of light and nanostructures are one of the smallest building blocks of nano-optics. However, plasmonic nanoparticles have been primarily studied to concentrate or scatter incident light as an ultrasmall antenna, while studies of their intrinsic plasmonic light emission properties have been limited. Although light emission from plasmonic structures can be achieved by inelastic electron tunneling, this strategy cannot easily be applied to isolated single nanoparticles due to the difficulty in making electrical connections without disrupting the particle plasmon mode. Here, we solve this problem by placing gold nanoparticles on a graphene tunnel junction. The monolayer graphene provides a transparent counter electrode for tunneling while preserving the ultrasmall footprint and plasmonic mode of nanoparticle. The tunneling electrons excite the plasmonic mode, followed by radiative decay of the plasmon. We also demonstrate that a dielectric overlayer atop the graphene tunnel junction can be used to tune the light emission. We show the simplicity and scalability of this approach by achieving electroluminescence from single nanoparticles without bulky contacts as well as millimeter-sized arrays of nanoparticles.

Individual Template-Stripped Conductive Gold Pyramids for Tip-Enhanced Dielectrophoresis.

Gradient fields of optical, magnetic, or electrical origin are widely used for the manipulation of micro- and nanoscale objects. Among various device geometries to generate gradient forces, sharp metallic tips are one of the most effective. Surface roughness and asperities present on traditionally produced tips reduce trapping efficiencies and limit plasmonic applications. Template-stripped, noble metal surfaces and structures have sub-nm roughness and can overcome these limits. We have developed a process using a mix of conductive and dielectric epoxies to mount template-stripped gold pyramids on tungsten wires that can be integrated with a movable stage. When coupled with a transparent indium tin oxide (ITO) electrode, the conductive pyramidal tip functions as a movable three-dimensional dielectrophoretic trap which can be used to manipulate submicrometer-scale particles. We experimentally demonstrate the electrically conductive functionality of the pyramidal tip by dielectrophoretic manipulation of fluorescent beads and concentration of single-walled carbon nanotubes, detected with fluorescent microscopy and Raman spectroscopy.

Electroluminescence from graphene excited by electron tunneling.

We use low-energy electron tunneling to excite electroluminescence in single layer graphene. Electrons are injected locally using a scanning tunneling microscope and the luminescence is analyzed using a wide-angle optical imaging system. The luminescence can be switched on and off by inverting the tip-sample bias voltage. The observed luminescence is explained in terms of a hot luminescence mechanism.

The role of gap plasmons in light emission from tunnel junctions.

Light emission from the junction of a scanning tunneling microscope (STM) is examined in the presence of 20 nm topographical features in thin gold films. These features significantly modify the emission rates of the junction. Contributions to this modification are discriminated by examining emission rates on samples where the material is varied spatially. It is found that the variability in STM photoemission rates between a gold tip and a gold sample under ambient conditions is due to the modification of localized gap plasmon modes and not to the presence of an electroluminescent gold cluster on the STM probe apex.

Self-similar gold-nanoparticle antennas for a cascaded enhancement of the optical field.

We experimentally demonstrate cascaded field enhancement by means of gold nanoparticle dimer and trimer antennas. The local field enhancement is probed by single-molecule fluorescence using fluorophores with high intrinsic quantum efficiency (Q(0)>80%). Using a self-similar trimer antenna consisting of 80, 40, and 20 nm gold nanoparticles, we demonstrate a fluorescence enhancement of 40 and a spatial confinement of 15 nm. Compared with a single gold nanoparticle, the self-similar trimer antenna improves the enhancement-confinement ratio by more than an order of magnitude. Self-similar antennas hold promise for high-resolution imaging and spectroscopy, ultrasensitive detection, and efficient single-photon sources.

Electrical excitation of surface plasmons.

We exploit a plasmon mediated two-step momentum down-conversion scheme to convert low-energy tunneling electrons into propagating photons. Surface plasmon polaritons (SPPs) propagating along an extended gold nanowire are excited on one end by low-energy electron tunneling and are then converted to free-propagating photons at the other end. The separation of excitation and outcoupling proves that tunneling electrons excite gap plasmons that subsequently couple to propagating plasmons. Our work shows that electron tunneling provides a nonoptical, voltage-controlled, and low-energy pathway for launching SPPs in nanostructures, such as plasmonic waveguides.

Robustness of quantum dot power-law blinking.

Photon emission from quantum dots (QDs) and other quantum emitters is characterized by abrupt jumps between an "on" and an "off" state. In contrast to ions and atoms however, the durations of bright and dark periods in colloidal QDs curiously defy a characteristic time scale and are best described by a power-law probability distribution, i.e., ρ(τ) ∝ τ(-α). We controllably couple a single colloidal QD to a single gold nanoparticle and find that power-law blinking is preserved unaltered even as the gold nanoparticle drastically modifies the excitonic decay rate of the QD. This resilience of the power law to change provides evidence that blinking statistics are not swayed by environment-induced variations in kinetics and provides clues toward the mechanism responsible for universal fluorescence intermittency.

Spectral dependence of single molecule fluorescence enhancement.

The fluorescence from a single molecule can be strongly enhanced near a metal nanoparticle acting as an optical antenna. We demonstrate the spectral tunability of this antenna effect and show that maximum enhancement is achieved when the emission frequency is red-shifted from the surface plasmon resonance of the particle. Our experimental results, using individual gold and silver particles excited at different laser-frequencies, are in good agreement with an analytical theory which predicts a different spectral dependence of the radiative and non-radiative decay rates.

Enhancement and quenching of single-molecule fluorescence.

We present an experimental and theoretical study of the fluorescence rate of a single molecule as a function of its distance to a laser-irradiated gold nanoparticle. The local field enhancement leads to an increased excitation rate whereas nonradiative energy transfer to the particle leads to a decrease of the quantum yield (quenching). Because of these competing effects, previous experiments showed either fluorescence enhancement or fluorescence quenching. By varying the distance between molecule and particle we show the first experimental measurement demonstrating the continuous transition from fluorescence enhancement to fluorescence quenching. This transition cannot be explained by treating the particle as a polarizable sphere in the dipole approximation.