Synthesis and Characterization of Cu x S(x =1-2) Nanocrystals Formed by the Langmuir-Blodgett Technique Nikolay A. Yeryukov,* ,†,‡ Alexander G. Milekhin, †,‡ Larisa L. Sveshnikova, † Tatyana A. Duda, † Lev D. Pokrovsky, † Anton K. Gutakovskii, †,‡ Stepan A. Batsanov, † Ekaterina E. Rodyakina, †,‡ Alexander V. Latyshev, †,‡ and Dietrich R.T. Zahn § † A.V. Rzhanov Institute of Semiconductor Physics, pr. Lavrentjeva, 13, 630090 Novosibirsk, Russia ‡ Novosibirsk State University, Pirogov str. 2, 630090 Novosibirsk, Russia § Semiconductor Physics, Technische Universitä t Chemnitz, D-09107 Chemnitz, Germany ABSTRACT: Here, we present the results on the investigation of structural and vibrational properties of Cu x S(x =1-2) nanocrystals formed using the Langmuir-Blodgett technique. The synthesis requires deposition of high quality Langmuir- Blodgett films of copper behenates on a solid substrate (Si, Au, and Pt). The Langmuir-Blodgett film is then sulfidized, what results in the formation of the copper sulfide nanocrystals embedded in behenic acid matrix. Finally, free-standing Cu x S nanocrystals are obtained after temperature annealing at 120-400 °C in an Ar atmosphere. Morphology (size, shape, and areal density) and the crystal structure of nanocrystals were determined by direct structural methods including scanning and transmission electron microscopies and high-energy electron diffraction. Surface-enhanced Raman scattering (SERS) by optical phonons in Cu x S nanocrystals in the vicinity of metal nanoclusters provided a significant enhancement factor (about 25) and allowed the fine structure of their phonon spectrum to be observed. SERS spectra of Cu x S nanocrystals under annealing reveal the high frequency shift of optical phonon modes from 475 to 492 cm -1 , which is explained by the existence of minor copper- deficient crystal phases. The combination of surface-enhanced Raman scattering spectroscopy, electron diffraction, and electron transmission microscopy allowed us to establish at least three stable phases: CuS, Cu 1.8 S, and Cu 2 S. 1. INTRODUCTION Copper sulfides (Cu x S) having unique physical properties (superionic conductivity, 1 superconductivity, 2 or p-type con- ductivity 3 ) are considered by many researchers as promising materials for solar cells, 4-6 gas sensors, 7 switches, 8 and plasmonic applications. 9,10 It is well-known that Cu x S possesses at least seven stable crystal modifications (with x = 2, 1.96, 1.8, 1.75, 1.32, 1.12, and 1) at room temperature. 11 This allows the properties of Cu x S to be tuned in a broad range. In particular, the band gap of Cu x S has values from 1.2 eV for the “copper- rich” phase Cu 2 S to 2 eV for “copper deficient” CuS. 11,12 More interesting physical properties of Cu x S should appear if it is presented as nanoscale structures, in particular, nanocryst- als (NCs) which reveal quantum confinement effects. Nowa- days, there is a number of different methods to form Cu x S NCs including wet 13 and colloid 14,15 chemistry, sonochemical 16 and hydrothermal 17 approaches yielding spherical, platelet-shaped, rod-, flake-, and flower-like NCs. Among others, the Langmuir-Blodgett (LB) technique 18 proved itself as a relatively fast, flexible, and low-cost method which enables semiconductor NCs of various composition, including Cu x S NCs, and metal nanoclusters 19 to be synthesized. The structural parameters of the variety of the NCs were established by a combination of direct structural methods such as high resolution and scanning electron microscopies, atomic-force microscopy, and reflection high energy electron diffraction. 20,21 One of the most efficient optical methods suitable for the determination of phase composition of different materials including nanoscale Cu x S is Raman spectroscopy. However, only copper-deficient Cu x S phases (with x < 1.4) have covalent S-S bonds, the stretching vibrations of which can be detected by Raman spectroscopy. Indeed, the group theoretical analysis predicts eights Raman active phonon modes for CuS: 2A 1g , 2E 1g , and 4E 2g . 22 Three intense phonon modes at about 20, 60, and 475 cm -1 and three weaker bands at 115, 140, and 265 cm -1 are typically observed in the Raman spectra of CuS. The modes near 20 and 475 cm -1 are assigned to vibrational modes from the covalent S-S bonds. The latter is the most intensive and the frequently observed mode in Raman experiments. The other modes are assigned to bending and lattice vibrational Received: July 23, 2014 Revised: September 4, 2014 Published: September 29, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 23409 dx.doi.org/10.1021/jp507355t | J. Phys. Chem. C 2014, 118, 23409-23414