Single-crystalline Ag 2 Te nanorods prepared by room temperature sputtering of GeTe

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Single‑crystalline ­Ag2Te nanorods prepared by room temperature sputtering of GeTe Kazuki Nakaya1 · Toshihiro Nakaoka1  Received: 23 March 2020 / Accepted: 21 August 2020 © Springer Nature Switzerland AG 2020

Abstract We report a facile, single-crystalline ­Ag2Te nanorod formation based on electrochemical diffusion of Ag. The nanorods were grown non-epitaxially by sputtering deposition of GeTe on ­Ag2Te nanoparticles at room temperature. For the nanorod growth, the source of the Ag supply is not deposition but the diffusion of Ag from the nanoparticles. The growth of the single-crystalline ­Ag2Te nanorods required GeTe deposition onto ­Ag2Te nanoparticles, in contrast to the growth of amorphous ­Ag2Te nanorods caused by Te deposition onto Ag nanoparticles and the growth of other nanostructures caused by GeTe deposition on Ag nanoparticles. The GeTe deposition onto the A ­ g2Te nanoparticles of an amount equivalent to a 100-nm-thick film produced nanorods of a length ranging from 3 to 5 μm and a diameter ranging from 100 to 400 nm. We propose a model for the nanorod growth based on solid-state electrochemical reaction between GeTe and movable Ag ions, inducing nanoscale phase separation and precipitation of amorphous Ge. We suggest that the nanorod structure with a crystalline ­Ag2Te core and an amorphous Ge shell is useful for thermoelectric applications. Keywords  GeTe · Ag2Te · Nanorod · Nanowire · Nanoparticle · Electrochemical reaction · RF magnetron sputtering

1 Introduction The electrochemical reaction of Ag in amorphous chalcogenides of solid electrolytes has been investigated intensively because of the fundamental interests in anomalous diffusion [1], modified phase change characteristics [2, 3], and optical properties [4–6] and also because of the potential device applications [7] such as RAM memories, sensors, and batteries. For example, the operation of conductive bridge RAM (CBRAM) devices [8, 9] is based on the formation of conductive filaments via the electrochemical reaction of Ag ions. We have directly observed the formation of conductive filaments in GeTe [10], which is a typical phase change material used in phase change RAM [11, 12], radio frequency switches [13, 14], CBRAM [15, 16], and thermoelectric devices [17–19]. Such an electrochemical reaction has also been used for crystal growth in silver chalcogenides [20, 21]. Bulk

single crystals of ­Ag2S and ­Ag2Se were grown based on the electrochemical diffusion of Ag that determined the growth rate [22, 23]. Amorphous [24] or polycrystalline [25] ­Ag2Te films were obtained by electrochemical interdiffusion between deposited Te and an Ag film at room temperature. Not only bulk crystals, but also nanostructures can be produced by the electrochemical reaction [26–28]. ­Ag2S whiskers or nanowires were grown electrochemically from an Ag plate, placed in a sulfur vapor environment, and kept at a fixed temperature for more than an hour. In this paper, we report on the electrochemical formation of single-crystalline ­Ag2Te nanorods by magnetron sputte

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