HAADF images provide unambiguous information on the position of the atomic columns in a solid with sub-Ångstrom resolution. Since the HAADF contrast is strongly affected by the average atomic number of the atomic column, the relevant images contain direct information on the specimen chemistry [1]. It has been recently demonstrated how, by choosing the proper experimental condition in HAADF experiments, the concentration profile of a chemical species in a host matrix can be probed even without image simulation [2]. Nevertheless, a quantitative chemical analysis requires image simulation and a more accurate determination of experimental parameters like defocus and sample thickness. There are two main approaches to the measure of the parameters necessary to quantify the information contained in a HAADF experiment. One class of approaches is based on adaptive parameter refinement techniques aimed to maximize the likelihood of matching experimental and simulated images [3, 4]. These approaches require an extensive use of computer simulation often in the order of several hundreds of hours of calculation. Another class of approaches, that could be called "parametric", consists in the a priori calculation of the parametric dependencies of the features of the images. Parameters are then extracted from experiment exploiting in the back direction the calculated relations. This kind of approach has been used, for example, to measure the focus value of an image acquired from an amorphous area [5]. Through focus technique is widely used in electron microscopy, so far mainly in phase-contrast high resolution TEM, to measure experimental parameters and to extract accurate information on the specimen at atomic resolution [6]. In this paper the aim is to accurately measure the defocus value of the atomic resolution HAADF image as acquired from the region of interest. The defocus value is evaluated by studying the atomic fringes in a through focus HAADF image series. The p
A novel method for the focus assessment in atomic resolution STEM HAADF experiments
2004
Abstract
HAADF images provide unambiguous information on the position of the atomic columns in a solid with sub-Ångstrom resolution. Since the HAADF contrast is strongly affected by the average atomic number of the atomic column, the relevant images contain direct information on the specimen chemistry [1]. It has been recently demonstrated how, by choosing the proper experimental condition in HAADF experiments, the concentration profile of a chemical species in a host matrix can be probed even without image simulation [2]. Nevertheless, a quantitative chemical analysis requires image simulation and a more accurate determination of experimental parameters like defocus and sample thickness. There are two main approaches to the measure of the parameters necessary to quantify the information contained in a HAADF experiment. One class of approaches is based on adaptive parameter refinement techniques aimed to maximize the likelihood of matching experimental and simulated images [3, 4]. These approaches require an extensive use of computer simulation often in the order of several hundreds of hours of calculation. Another class of approaches, that could be called "parametric", consists in the a priori calculation of the parametric dependencies of the features of the images. Parameters are then extracted from experiment exploiting in the back direction the calculated relations. This kind of approach has been used, for example, to measure the focus value of an image acquired from an amorphous area [5]. Through focus technique is widely used in electron microscopy, so far mainly in phase-contrast high resolution TEM, to measure experimental parameters and to extract accurate information on the specimen at atomic resolution [6]. In this paper the aim is to accurately measure the defocus value of the atomic resolution HAADF image as acquired from the region of interest. The defocus value is evaluated by studying the atomic fringes in a through focus HAADF image series. The pI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
