Topological insulators (TI's) are a new class of quantum matter with extraordinary surface electronic states, which bear great potential for spintronics and error-tolerant quantum computing. In order to put a TI into any practical use, these materials need to be fabricated into devices whose basic units are often p-n junctions. Unique electronic properties of a 'topological' p-n junction were proposed theoretically such as the junction electronic state and the spin rectification. However, the fabrication of a lateral topological p-n junction has been challenging because of materials, process, and fundamental reasons. Here, we demonstrate an innovative approach to realize a p-n junction of topological surface states (TSS's) of a three-dimensional (3D) topological insulator (TI) with an atomically abrupt interface. When a ultrathin Sb film is grown on a 3D TI of Bi 2 Se 3 with a typical n-type TSS, the surface develops a strongly p-type TSS through the substantial hybridization between the 2D Sb film and the Bi 2 Se 3 surface. Thus, the Bi 2 Se 3 surface covered partially with Sb films bifurcates into areas of n-and p-type TSS's as separated by atomic step edges with a lateral electronic junction of as short as 2 nm. This approach opens a different avenue toward various electronic and spintronic devices based on well defined topological p-n junctions with the scalability down to atomic dimensions.Keywords topological insulator, topological p-n junction, angle-resolved photoemission spectroscopy, scanning tunneling microscopy/spectroscopy, ultrathin Sb film Surface states of topological insulators, 1,2 called topological surface states (TSS's), are robustly protected by the bulk topological nature and form necessarily a Dirac band with their spins locked helically with momentum. 1-6 These unique properties find obvious merits in spintronic applications and can yield a Majorana Fermion in proximity with supercon-2 ductivity. 7,8 However, there has been a huge barrier in making devices based on TSS's. The challenge is closely related to the notorious issue of controlling impurities or dopants in a TI crystal. While quite a few works tried to control the chemical potential of a TSS by impurity doping, 4,9-14 the deterioration of the surface channel and the inclusion of bulk channels were inevitable in many cases. Especially, the tunability of the chemical potential was often not enough to make a good p-type TSS. Fabricating a well defined topological p-n junction is even more challenging, 15-18 which represents one of the most important technological issues in staging applications of TI's. Nevertheless, a topological p-n junction, defined as an electronic junction of a p-and a n-type TSS, features unique properties, which are not shared by conventional p-n junctions of semiconductors but promise attractive new applications. 7,8 At a topological p-n junction, the electron scattering and transport are largely governed by the spin polarization of TSS's involved. This property provides the spin rectification eff...

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Here, we demonstrate a fully-flexible piezoelectric hybrid structure composed of sodium niobate nanocubes and polyvinylidene fluoride. Hydrothermally-grown sodium niobate nanocubes show an orthorhombic crystal structure with Pmc21 symmetry, and polyvinylidene fluoride is electrically poled for the β-phase, each of those represents piezoelectricity. Hybrid devices were electrically tested in two different modes by applying forces parallel to the in-plane and the out-of-plane directions of the piezoelectric layer. Our device shows an output voltage, current, and power density of 2.9 V, 68 nA, and 4.4 μWcm −3 , respectively, at a pushing force of 5 kgf and a frequency of 1 Hz.The output power generation also shows a linear relation with the applied force over a wide range of 1 − 5 kgf. This implies our flexible hybrid piezoelectric structure can be adapted to various mechanical environments. Our unique structure should open up various application areas of piezoelectric materials such as touch sensors, flexible energy harvesters and eco-friendly piezoelectric actuators.

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