DOI: 10.1007/s00339-007-3987-8 Appl. Phys. A 87, 683–689 (2007) Materials Science & Processing Applied Physics A n.s. john 1 n.r. selvi 1 g.u. kulkarni 1, ✉ s. heun 2, ✉ e. cavaliere 3 m. fanetti 3 i. kholmanov 3 l. gavioli 3, ✉ m. sancrotti 3 Transformation of femtoliter metal cups to oxide cups: chemical mapping by scanning Auger spectroscopy 1 Chemistry and Physics of Materials Unit and DST Unit on Nanoscience, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur P.O., Bangalore 560064, India 2 CNR-INFM Laboratorio Nazionale TASC, Basovizza, 34012 Trieste, Italy 3 INFM-CNR and Dipartimento di Matematica e Fisica, Università Cattolica, via dei Musei 41, 25121 Brescia, Italy Received: 9 January 2007/Accepted: 20 February 2007 Published online: 4 April 2007 • © Springer-Verlag 2007 ABSTRACT Cup-like structures of In, Sn and Nb on Si sub- strates with femtoliter capacity obtained by pulsed laser abla- tion, have been subjected to different oxidation treatments and examined employing spatially resolved scanning Auger spec- troscopy and microscopy (SR-AES and SAM). The as-prepared cups, when exposed to ambient conditions are found to have a native oxide layer at the surface that could be easily removed by Ar ion sputtering, resulting in clean metal cups, suitable for functionalization. In the case of In cups, the thin metal layer at the bottom of the cups could be easily removed by sputtering to form In rings. Cups subjected to an external oxidation treatment have a thicker oxide layer in comparison to in-situ dosing of oxygen. In the case of Nb cups, the high temperature treatment employed during oxidation resulted in the segregation of Si to the surface of the cups. There is also evidence for the formation of a metal-silicon alloy at the center of the cups, especially in the case of Sn and Nb, during the oxidation treatment at elevated temperatures. PACS 68.37.Xy; 68.47.De; 81.15.Fg; 81.65.Mq; 81.70.Jb 1 Introduction Containers with ultrasmall capacities of the order of femtoliters or picoliters are important for monitoring sin- gle cell reactions and enzyme kinetics [1–3]. Such small volumes have been realized in the form of etched wells in silicon substrates [4] and polymer films [5], nanobowls ob- tained by chopping of polystyrene balls followed by chemical processing [6], and oxide nanopores [7] by various litho- graphic techniques, and recently in the form of biomimetic systems consisting of liposomes [8]. Other techniques re- ported in literature for making nanocups include layer by layer assembly of polyelectrolytes [9, 10], electrospraying of poly- mers and polymer-inorganic nanocomposites [11], or emul- sion based synthesis of inorganic nanoparticles embedded in polymer microcapsules [12]. We have demonstrated a sim- ple method of obtaining femtoliter cups of virtually any metal ✉ Fax: +91-80-22082766, E-mail: kulkarni@jncasr.ac.in ✉ Fax: +39-40-226767, E-mail: heun@tasc-infm.it ✉ Fax: +39-30-2406742, E-mail: luca.gavioli@unicatt.it (Ag, Au, Sn, In, Al, Cu, Zn, Nb, Cd) on flat substrates like silicon, graphite, or glass by pulsed laser ablation [13]. Dur- ing laser ablation, metal droplets from the laser plume im- pinge on the substrate and undergo a hydraulic jump, which is a common phenomenon in fluid mechanics, wherein the droplets spread out, thinly initially, and then undergo a sud- den increase in height at a distance from the center. In this case, surface tension drives the jump [14] rather than grav- ity, which is the common driver found in other cases [15]. A moderate laser energy of 100 mJ/pulse and a substrate temperature slightly below the metal melting point are em- ployed to obtain well-defined cups. Preliminary experiments have shown that the cups can be utilized to hold biomark- ers as well as nanocrystal sols. In such studies, surface con- tamination of the as-deposited cups is always a matter of concern. Further, the metal cups can, in general, be chemi- cally transformed to ceramics by suitable external treatment. However, retaining the cup morphology can be quite challeng- ing. Thus, it is not only important to understand the chem- ical nature of the surface of the as-prepared cups but it is also worthwhile to study the conditions under which a de- sired composition is obtained during chemical transformation without affecting morphology. A surface-sensitive probe with good spatial resolution is required for monitoring such local changes. In this article, we report scanning Auger microscopy (SAM) and spatially resolved Auger electron spectroscopy (SR-AES) characterization of oxidized cups. Since the inci- dent electron beam can be focused to small spot sizes with high current density, it is possible to perform Auger spec- troscopic measurements on nanostructures with good spatial resolution [16]. In general, this high lateral resolution makes it possible to use the Auger electrons for imaging and has been extensively used in the field of metallurgy to study diffusion process at grain boundaries [17], in geology as a microanalytic tool [18] and lately, to characterize nano- structures [19–22]. The energy resolution and sensitivity of AES in accurately monitoring metal oxidation are well doc- umented [23]. We have employed SR-AES to obtain chemical maps of oxidized metallic cups of In, Sn and Nb deposited on Si substrates. We also investigated how different oxidation treatments influence the chemical composition of the vari- ous regions of a cup. Our study has given useful insight into chemical transformation of metal femtoliter-cups.