Structural Characterization of Colloidal Ag 2 Se Nanocrystals V. Buschmann* and G. Van Tendeloo EMAT, University of Antwerp (RUCA), Groenenborgerlaan 171, B-2020 Antwerp, Belgium Ph. Monnoyer and J. B. Nagy Faculte ´ s Universitaires Notre-Dame de la Paix, Laboratoire de RMN, rue de Bruxelles 61, B-5000 Namur, Belgium Received December 2, 1997. In Final Form: February 2, 1998 Ag2Se nanocrystals of less than 15 nm have been investigated by means of high-resolution electron microscopy and nano-energy-dispersive X-ray spectroscopy. Two groups of clusters are distinguished, depending upon their preparation technique. Those obtained by double jet precipitation show a limited stability both in time and under the electron beam. Their structure is orthorhombic, although a cubic phase is encountered if an excess Ag is present. Those prepared by microemulsion mixing have an orthorhombic structure as well but show a greater stability. Introduction Its low activation energy for diffusion and conduction makes Ag 2 Se a typical superionic conductor, similar to the high-temperature phases of AgI and Ag 2 S. Ag 2 Se nanocrystals may be used as chemical sensitizers in the photographic industry, equivalent to the traditional Ag 2 S. 1 The high-temperature phase, above 406 K, of Ag 2 Se films was determined by Ralphs et al. 2 as body-centered cubic (bcc) with a lattice parameter of 0.498 nm. As to the low-temperature phase, the influence on the structure of both stoichiometry and preparation conditions has re- sulted in a huge amount of structural data. Nevertheless, most authors agree that in the case of a correct stoichi- ometry the structure of the investigated films is ortho- rhombic with a ) 0.433 nm, b ) 0.706 nm, and c ) 0.776 nm, belonging to the space group P2 1 2 1 2 1 (19). 3 When the compound is nonstoichiometric, as a result of processing conditions, a different structure may be encountered. Okabe et al. 4 used the flash evaporation technique for preparing Ag 2 Se thin films and sandwiched them between two carbon layers in order to maintain their initial composition throughout the processing and investigation. Using this procedure, they found for a correct stoichiom- etry the orthorhombic phase, as described above. Films containing an excess of silver were face-centered cubic (FCC) with a lattice parameter of 1.09 nm. On the other hand, in the presence of an excess of selenium, the structure was monoclinic with a ) 0.705 nm, b ) 0.817 nm, c ) 0.434 nm, and R) 101.0°. Also a metastable phase, only found for grains measuring less than 50 nm, was characterized exhibiting the tetragonal structure. 4 The influence of the thickness on the film structure has been studied by Gu ¨ nter et al. 5 and Sa ´ fra ´ n et al. 6 This was done by selenizing silver films on 〈100〉-cleaved NaCl substrates, meanwhile gradually increasing the selenium and silver deposition. For the thinnest films a cubic together with a tetragonal phase was formed. A meta- stable monoclinic, pseudotetragonal phase was encoun- tered when investigating films with a thickness of about 20-30 nm. Thicker Ag 2 Se films transformed into the stable orthorhombic phase. Many other structures are also described in the litera- ture, e.g., fcc with lattice parameter 0.565 nm, 7 a triclinic - pseudo-orthorhombic phase by De Ridder et al. 8 Mo- lecular dynamics simulation at constant pressure and temperature revealed an orthorhombic phase with lattice parameters a ) 0.429 nm, b ) 0.682 nm, and c ) 0.825 nm and space group Pmnb. 9,10 In the present study individual Ag 2 Se nanocrystals with diameter less than 15 nm are investigated. The clusters are prepared in solution, in regard to which two groups may be distinguished, depending on the preparation method. One group is made by double jet precipitation, the other by microemulsion mixing. In the first group a protective colloid, in the latter a surfactant, helps to maintain the correct stoichiometry during the examination of the clusters. The close relationship between stoichiometry and structure favors a high-resolution electron microscopy (HREM) study in combination with nano-energy-disper- sive X-ray spectroscopy (nano-EDX) analyses. This indeed enables us to obtain maximal information about individual nanocrystals. Experimental Section As mentioned above, the Ag2Se nanoclusters are divided into two groups depending on their origin: double jet precipitation and microemulsion mixing. Double jet precipitation is a well-established technique in the photographic industry for the manufacture of silver halides. 1 For the preparation of the Ag2Se nanocrystals 1,1-dimethyl-2- selenurea and AgNO3 are precipitated in a reaction vessel in the * Corresponding author, present address: TU Darmstadt, Fach- gebiet Materialwissenschaften-Fachbereich Strukturforschung, Petersenstrasse 23, D-64287 Darmstadt, Germany. (1) Buschmann, V.; Schryvers D.; Van Landuyt J.; Van Roost C.; De Keyzer R. J. Imaging Sci. Technol. 1996, 40, 189. (2) Ralphs, P. Z. Phys. Chem. 1936, 31B, 157. (3) Wiegers, G. A. Am. Mineral. 1971, 56, 1882. (4) Okabe, T.; Ura, K. J. Appl. Crystallogr. 1994, 27, 140. (5) Gu ¨ nter, J. R.; Keusch, P. Ultramicroscopy 1993, 49, 293. (6) Sa ´ fra ´n, G.; Keusch, P.; Gu ¨ nter, J. R.; Barna, P. B. Thin Solid Films 1992, 215, 147. (7) De Ridder, R.; Amelinckx, S. Phys. Status Solidi 1973, 18, 99. (8) De Ridder, R.; De Sitter, J.; Amelinckx, S. Phys. Status. Solidi A 1974, 23, 615. (9) Shimojo, F.; Okazaki, H. J. Phys. Soc. Jpn. 1991, 60, 3745. (10) Shimojo, F.; Okazaki, H. J. Phys. Soc. Jpn. 1993, 62, 179. 1528 Langmuir 1998, 14, 1528-1531 S0743-7463(97)01321-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 03/04/1998