DOI: 10.1007/s00339-004-3081-4 Appl. Phys. A 80, 753–758 (2005) Materials Science & Processing Applied Physics A j.-p. sylvestre a.v. kabashin e. sacher m. meunier ✉ Femtosecond laser ablation of gold in water: influence of the laser-produced plasma on the nanoparticle size distribution Laser Processing Laboratory, Department of Engineering Physics, Ecole Polytechnique de Montr´ eal, Case Postale 6079, succ. Centre-ville, Montr´ eal, Qu´ ebec, Canada, H3C 3A7 Received: 15 July 2004/Accepted: 29 September 2004 Published online: 15 December 2004 • © Springer-Verlag 2004 ABSTRACT Femtosecond laser radiation has been used to ablate a gold target in pure deionized water to produce gold colloids. The dimensional distribution of nanoparticles is characterized by the simultaneous presence of two distinct particle popula- tions: one with low dispersion, having a mean particle size of 5–20 nm, and one with high dispersion, having a much larger particle size. By changing the target position with respect to the radiation focus, we study the influence of the plasma formed after the laser pulse in front of the target, during nanofabrica- tion process. We show that the most intense plasma is produced by positioning the target slightly before the geometric focal point. Here, the plasma intensity was found to correlate with the amount of ablated material as well as with the mean size of nanoparticles associated with the second, highly dispersed, distribution of nanoparticles; this suggests the involvement of plasma-related processes in the ablation of material, and the for- mation of relatively large particles. The thermal heating of the target by the plasma, and its mechanical erosion by the col- lapse of a plasma-induced cavitation bubble are discussed as possible ablation mechanisms. The gold nanoparticles produced in ultrapure water are of importance for biosensing applications. PACS 81.07.-b; 81.16.-c 1 Introduction Nanoparticles of noble metals are predicted to be, or are already, successfully employed in a wide range of appli- cations, including catalysis, nanoelectronics and, particularly, biosensing. Gold nanoparticles (< 30 nm) are particularly in- teresting for these tasks since they are chemically stable and strongly absorb light around 520 nm, due to the presence of a resonant surface plasmon excitation [1]. Over the last decade, the major effort has been on the production of stable solutions of small nanoparticles with narrow size distribu- tions and controlled surface chemistry. Although 5– 100 nm nanoparticles can be produced by a relatively simple chem- ical reduction method [2], the surface of these nanoparticles is likely to be contaminated with reaction by-products such as anions and reducing agents, which can interfere with subse- quent stabilization and functionalization steps. ✉ Fax: +1-514-340-3218, E-mail: michel.meunier@polymtl.ca The laser ablation of a noble metal target immersed in a liquid was introduced as an alternative physical method for colloidal nanoparticle fabrication [3–19]. In contrast to the chemical reduction method, laser ablation offers the possibil- ity of nanoparticle growth in a controllable, contamination- free environment, a key requirement for the subsequent suc- cessful functionalization of the nanoparticle surface. To re- duce the size and size dispersion of nanoparticles produced by laser ablation, their growth has been controlled by both chem- ical and physical methods. The chemical approach consists of the addition of specific molecules, capable of interacting physically or chemically with the surface of the forming par- ticles, to the liquid fabrication environment to limit their sub- sequent coalescence. In particular, an efficient size reduction was observed by the use of ionic surfactants [6–10], although they are not always suited for biosensing applications because of biocompatibility problems. Much more biocompatible cy- clodextrins (torus-like macrocycles built up of glucose pyra- nose units [20]) appear more promising for the size reduction tasks providing nanoparticles with a mean size of 2 –2.5 nm and a size dispersion of 1 –1.5 nm [11, 12]. In contrast, the physical size control approach employs variations of physi- cal parameters to control the nanoparticle growth. Although limited size reductions can be achieved with nanosecond laser pulses [8, 16], femtosecond radiation gave much more effi- cient size control, permitting mean size particle variations between 4 to 150 nm [18]. Basically, nanofabrication with femtosecond radiation was characterized by the presence of two populations of nanoparticles. The first contained particles with a relatively small mean size (4– 15 nm) and a narrow dis- persion (8 –10 nm FWHM), whereas the second had particles with a much larger mean size (15–130 nm) and a broader size dispersion (20–90 nm FWHM) [18]. Although the data indi- cate the involvement of radiation- and plasma-related mech- anisms of material ablation, many aspects of the phenomenon remain unclear. This paper also focuses on the femtosecond laser ablation of gold in water. We vary the position of the target with re- spect to the focal plane of a focusing objective, permitting not only the variation of the radiation fluence, but also the con- trol of the position of the laser-produced plasma with respect to the target. In order to clarify the role of the plasma in the nanofabrication process, we examine the target mass loss, the sound and visible emission from the plasma, and the concen-