The ability to pattern planar and freestanding 3D metallic architectures at the microscale would enable myriad applications, including flexible electronics, displays, sensors, and electrically small antennas. A 3D printing method is introduced that combines direct ink writing with a focused laser that locally anneals printed metallic features "on-the-fly." To optimize the nozzle-to-laser separation distance, the heat transfer along the printed silver wire is modeled as a function of printing speed, laser intensity, and pulse duration. Laser-assisted direct ink writing is used to pattern highly conductive, ductile metallic interconnects, springs, and freestanding spiral architectures on flexible and rigid substrates. T he ability to create planar and freestanding 3D metal structures on demand at the microscale would enable myriad applications, including electronics (1-8), microelectromechanical systems (MEMS) (9, 10), metamaterials (11-13), and biomedical devices (14-16). For example, many electronic devices, such as inductors and antennas (4, 7), operate more efficiently in 3D form. However, that format is not well suited to standard photolithographic techniques. A onestep process for directly writing conductive, ductile metal wires and complex 3D architectures, such as freestanding spiral motifs, onto low-cost plastic and rigid substrates would enable highperformance, customizable electronic and other devices to be manufactured in a cost-effective and space-efficient manner. To date, several printing approaches have been developed to directly deposit conductive features, including roll-to-roll (17-19), inkjet printing (20, 21), meniscus printing (8), and direct ink writing (DIW) (1,22). Although DIW has demonstrated spanning linear traces or short arcs printed out-of-plane, only meniscus-based electrodeposition printing has been used to generate freestanding 3D solid metal structures in arbitrary geometries. However, its ultralow print speed (<1 μm/s) coupled with the need for a conductive substrate have limited its widespread adoption (8).Here, we introduce laser-assisted direct ink writing (laser-DIW), which combines printing of concentrated silver nanoparticle inks with focused infrared laser annealing to rapidly create high conductivity, ductile metallic wires and 3D architectures "on-the-fly" in a one-step, additive process. Laser-DIW offers three key advantages over other 3D printing techniques. First, by combining patterning and annealing in a single step, the printed metallic features exhibit the requisite mechanical properties needed to precisely fabricate arbitrary objects in midair, enabling complex curvilinear structures to be generated without the need for support material. Due to localized annealing, such features can be printed on low-cost plastic substrates, such as poly(ethylene terphthalate) (PET). Finally, the patterned features exhibit high electrical conductivity approaching that of bulk silver. Results and DiscussionDuring laser-DIW, an 808-nm IR laser is focused to a 100-μm spot adjacent ...

Single-molecule topological insulators are promising candidates as conducting wires over nanometer length scales. In past, most conjugated molecular wires exhibit low conductance that decays as the wire length increases. To overcome this limitation, we studied a family of oligophenylene-bridged bis(triarylamines) with tunable and stable (mono-/di-)radicaloid character. The wires can undergo one-and two-electron chemical oxidations to the corresponding monocation and dication, respectively. We found that the oxidized wires exhibit high reversed conductance decay with increasing length, consistent with the expectation for the Su-Schrieffer-Heeger-type one-dimensional (1D) topological insulators. The champion 2.6 nm long dication displays a significantly high conductance greater than 0.1 G0 (2e 2 /h, the conductance quantum), 5400-fold greater than the neutral form. The observed conductance-length relationship is similar between monocation and dication series. DFT calculations elucidate how the frontier orbitals and delocalization of radicals facilitate the observed nonclassical quasi-metallic behavior. These findings offer new insights into molecular design of highly conducting 1D topological insulators.

Electron transport across a molecular junction is characterized by an energy-dependent transmission function. The transmission function accounts for electrons tunneling through multiple molecular orbitals (MOs) with different phases, which gives rise to quantum interference (QI) effects. Because the transmission function comprises both interfering and noninterfering effects, individual interferences between MOs cannot be deduced from the transmission function directly. Herein, we demonstrate how the transmission function can be deconstructed into its constituent interfering and noninterfering contributions for any model molecular junction. These contributions are arranged in a matrix and displayed pictorially as a QI map, which allows one to easily identify individual QI effects. Importantly, we show that exponential conductance decay with increasing oligomer length is primarily due to an increase in destructive QI. With an ability to "see" QI effects using the QI map, we find that QI is vital to all molecular-scale electron transport.

Polymethine dyes are linear π-conjugated compounds with an odd number of carbons that display a much greater delocalization in comparison to polyenes that have an even number of carbon atoms in their main chain. Herein, we perform scanning tunneling microscope based break-junction measurements on a series of three cyanine dyes of increasing length. We demonstrate, at the single molecule level, that these short chain polymethine systems exhibit a substantially smaller decay in conductance with length (attenuation factor β = 0.04 Å) compared to traditional polyenes (β ≈ 0.2 Å). Furthermore, we show that by changing solvent we are able to shift the β value, demonstrating a remarkable negative β value, with conductance increasing with molecular length. First principle calculations provide support for the experimentally observed near-uniform length dependent conductance and further suggest that the variations in β with solvent are due to solvent-induced changes in the alignment of the frontier molecular orbitals relative to the Fermi energy of the leads. A simplified Hückel model suggests that the smaller decay in conductance correlates with the smaller degree of bond order alternation present in polymethine compounds compared to polyenes. These findings may enable the design of molecular wires without a length-dependent decay for efficient electron transport at the nanoscale.

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