(a) Schematic of the measuring setup. (b) Optical microscope view in the AFM. The Veeco CPII AFM has a built in optical microscope with high resolution. Individual nanowires scatter light very efficiently and makes it possible to locate them. Nanowires are identified as small white rods scattering the light as indicated by the arrow. Alignment of the cantilever using the optical microscope is, as the image indicates, insufficient and is therefore done using the A-B signal thus testing a possible contact. (c) To verify the nanowire bending behavior, in situ experiments in SEM were performed using a Smaract nanomanipulator. These experiments, however, do not provide any force measurements.
Illustration of the nanowire-cantilever measurement method and the FD curves as function of y. (a) The nanowire is regarded as a circular beam, fixed at one end and free at the other. The bending of the nanowire can be found from the nanowire stage position and cantilever deflection measured from the A-B diode FD curves. (b) As the cantilever approaches the nanowire substrate, the nanowire spring constant increases causing a stronger cantilever bending, thus the slope of the FD curve will increase. (c) A representative plot of the slopes of the FD curves, α(y), as a function of y. These plots are fitted to Eq. (11) to obtain the mechanical properties of the nanorods.
Plots of α(y) for two nanorods having dimensions comparable with the measured samples presented later on, showing the importance of the tapered model. (a) Wire with D r = 100 nm, D t = 70 nm, and L = 3 μm. (b) Wire with D r = 350 nm, D t = 90 nm, and L = 5 μm. The three untapered curves are calculated using the mean diameter (dashed), the tip diameter (dotted), and the root diameter (dash-dotted) in order to compare with the linear tapered wire having a y dependent diameter (full). All curves in both graphs are calculated for E = 85.9 GPa, Young's modulus of bulk GaAs.
Example of FD curve measurements on a CNF with D r = 190 nm, D t = 120 nm, and L = 3 μm. The different colors of the curves in (a) indicates different measurements on the CNF at various y positions plotted in the same graph, with the stage position on the horizontal axis and the cantilever deflection on the vertical axis. The entire plot of the FD curves for the CNF is shown and the various regions are indicated (and explained in Fig. 5). From region “A” the noise can be found and in “B” the constant slopes are seen. In region “C”, a representative window is chosen for the slope calculations. (b) α(y) from region “C” is plotted and fitted to Eq. (12). (c) Example of a complex multi-nanowire bundle giving a more complex signal. Three different region “D”s can be identified (indicated by the dashed circles), revealing three different nanowires.
SEM images and illustration of the mechanism behind the FD behavior. The in situ SEM experiments were performed on a CNF array using a Smaract nanomanipulator, where the CNF sample is moved upwards toward the cantilever. “A” The fiber and cantilever are brought into contact. The cantilever will always make first contact to the apex of the fiber, due to the difference in angle. “B” As the fiber moves downwards, it becomes parallel with the cantilever. “C” The interaction between fiber and cantilever is now y-dependent and the slope of the curve in this region, α(y), is used for the calculations. “D” and “E” When the fiber is moved further down, it slips in small steps along the cantilever before it finally slides off the cantilever.
SEM images of the measured GaAs nanowire array. The wires are slightly tapered with little dimension variation. Root diameter: D r = 99 ± 1 nm, tip diameter: D t = 68 ± 1 nm, and length: L = 3 ± 0.2 μm.
Measured α(y) for GaAs nanowires (a) and CNFs (b). The small α(y) values for the GaAs nanowires indicates a small bending stiffness of the wires compared to the fibers. The small bending stiffness of the wires can be ascribed to their small diameter and a low Young's modulus. The black (a) and green (b) lines are examples of fitted curves from (12).
SEM image of the measured CNF array, with a strong tapering structure and a large dimension variation.
GaAs nanowires mechanical measurements.
CNFs mechanical measurements.
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