The stretch of Aβ peptide spanning residues 14 through 23 is the segment of the Aβ-42 peptide (designated as Aβ(14-23) from hereon) that adopts β-sheet structure when Aβ-42 assembles into fibrils, and Aβ(14-23) itself forms fibrils in solution2324. The on-surface aggregation experiments were performed under the same conditions as in the above described experiments (100 nM Aβ(14-23), 10 mM sodium phosphate buffer (pH 7.4) at room temperature), and the results are shown in Fig. 2. A few aggregates appear as globular features after 24 h (Fig. 2A-plate i) and their sizes have increased at the 72 h incubation time point (Fig. 2A, plate iii). Fibrillar features were also observed and are indicated with arrows. Zoomed images of the fibrils are shown as insets (Fig. 2Ai–iii), and a 3D view is shown in Fig. 2A(iii). Statistical analysis for the globular aggregates sizes was performed, and the data are shown in Fig. 2B. There is a considerable growth of the aggregates between 48 h (Fig. 2B(ii)) and 72 h (Fig. 2B(iii)), with a bimodal distribution reflecting the presence of large aggregates evident in the histogram corresponding to the later time point (Fig. 2B(iii)). A graph in which these values as well as the number of aggregates were plotted against incubation time (Fig. 2C) illustrates a monotonic growth of aggregates in size and number. As a control, aggregation in the bulk solution was monitored in parallel. Typical AFM images for these samples are shown in Supplementary Fig. 3. Only a few aggregates were formed after 72 h, and no fibrillar features were found.