Introduction One of the most requisite semiconducting sulfides is the well known silver sulfide Ag 2 S. As early as in 1833, Faraday found that lead fluoride and silver sulfide possessed high ion conductivity comparable to the conductivity of metals in a wide temperature range [1] . He wrote: "I formerly described a substance, sulfuret of silver, whose conducting power was increased by heat... When a piece of that substance, which had been fused and cooled, was introduced into the circuit of a

more »

... oltaic battery, it stopped the current. Being heated, it acquired conducting powers..." [2] . Unique chemical, structural, optical and conductive properties make silver sulfide an excellent substance for preparation of hetero-structures. Among composite hetero-structures of silver sulfide, the semiconductor/metal hetero-nanostructure Ag 2 S/Ag attracts special attention. It can be used in resistive switches and nonvolatile memory devices [3] [4] [5] [6] . The action of the switch is based on the phase transformation between nonconducting α-Ag 2 S acanthite and superionic β-Ag 2 S argentite. According to the phase diagram of the system Ag -S [7] , silver sulfide Ag 2 S has three basic polymorphic modifications]. Low-temperature semiconducting phase α-Ag 2 S (acanthite) with monoclinic crystal structure exists at temperatures below ~450 K. Monoclinic acanthite has a direct band gap of 0.9-1.05 eV. Under equilibrium conditions, cubic phase β-Ag 2 S (argentite) exists in the temperature range 452-859 K and has a superionic conductivity. High-temperature Face Centered Cubic (FCC) phase γ-Ag 2 S stable from ~860 K up to melting temperature. Known methods for the preparation of a hetero-nanostructures of Ag 2 S or Ag mainly deal with the synthesis of nanoparticles of one species with the subsequent growth of other species nanoparticles [8] [9] [10] [11] [12] . These methods are rather expensive and time-consuming. Abstract Ag 2 S/Ag hetero-nanostructure has been produced by a simple one-stage chemical deposition from aqueous solutions of silver nitrate, sodium sulfide, and sodium citrate with the use of monochromatic light irradiation. For simultaneous synthesis of Ag 2 S and Ag nanoparticles, deposition has been performed from reaction mixtures with reduced sodium sulfide concentration. The formation of Ag 2 S/Ag nanocomposite structures is confirmed by X-ray analysis, high-resolution electron microscopy, energy dispersion analysis and dynamic light scattering methods. It is established that in the contact layer between silver sulfide and silver, non conducting α-Ag 2 S acanthite transforms into superionic β-Ag 2 S argentite under the action of external electric field. The scheme of the operation of a resistive switch based on an Ag 2 S/Ag hetero-nanostructure is proposed.