We report atomic-resolution imaging and site-specific quantitative force measurements on a single-walled carbon nanotube by dynamic force microscopy and three-dimensional force field spectroscopy at low temperatures. The topography imaged in the attractive force regime reflects the trigonal arrangement of the hollow sites as maxima. Individual force curves were unambiguously assigned to carbon atoms and hollow sites, respectively. Site-specific quantitative evaluation revealed that the short-range interatomic van der Waals forces are responsible for the atomic-scale contrast.
The hollow core inside a carbon nanotube can be used to confine single molecules and it is now possible to image the movement of such molecules inside nanotubes. To date, however, it has not been possible to control this motion, nor to detect the forces moving the molecules, despite experimental and theoretical evidence suggesting that almost friction-free motion might be possible inside the nanotubes. Here, we report on precise measurements of the mechanical responses of individual metallofullerene molecules (Dy@C82) confined inside single-walled carbon nanotubes to the atom at the tip of an atomic force microscope operated in dynamic mode. Using three-dimensional force mapping with atomic resolution, we addressed the molecules from the exterior of the nanotube and measured their elastic and inelastic behaviour by simultaneously detecting the attractive forces and energy losses with three-dimensional, atomic-scale resolution.
A carbon nanotube (CNT) was used as a tip for a noncontact-mode atomic force microscope (NC-AFM). A CNT tip was attached to an Au/Si tip by a well-controlled procedure in a scanning-electron-microscope (SEM) chamber. The NC-AFM with the CNT tip produced highly reproducible images of right-handed helical turns of linear deoxyribonucleic acid (DNA) with a spacing of 3.5 ±1.0 nm. The full-width at half-maximum (FWHM) of the cross section of DNA measured was 3.1 ±0.6 nm.
We propose to use the damping signal of an oscillating cantilever in dynamic atomic force microscopy as a noninvasive tool to study the vibrational structure of the substrate. We present atomically resolved maps of damping in carbon nanotube peapods, capable of identifying the location and packing of enclosed Dy@C82 molecules as well as local excitations of vibrational modes inside nanotubes of different diameter. We elucidate the physical origin of damping in a microscopic model and provide quantitative interpretation of the observations by calculating the vibrational spectrum and damping of Dy@C82 inside nanotubes with different diameters using ab initio total energy and molecular dynamics calculations. [2,3] can be performed on electrically insulating systems, but yield subsurface images with nanoscale resolution at best. Building on the high spatial resolution and sensitivity of dynamic non-contact AFM [4, 5], we introduce Damping Force Spectroscopy (DFS) as a non-invasive tool to study subsurface structure and vibrational modes in complex molecular systems at the atomic scale. We have chosen carbon nanotube peapods [6,7,8] consisting of linear chains of fullerenes enclosed in single-wall carbon nanotubes (SWNTs) [9] as a prominent example of supramolecular compounds. Of particular interest are (M@C n )@SWNT peapods containing M@C n metallofullerenes, hollow cages of n carbon atoms surrounding the metal atom M, known for their unusual electronic transport behavior [10].Here, we demonstrate that monitoring the damping of an oscillating AFM tip provides invaluable information not only about topography, but also the subsurface vibrational modes that have not been observed before with atomic-scale spatial resolution. Our DFS studies of (Dy@C 82 )@SWNT indicate that the observed damping of the tip oscillation depends sensitively on its position and host tube diameter of (Dy@C 82 )@SWNT, in agreement with extensive molecular dynamics (MD) studies reported here. Results of our predictive calculations trace back the observed damping to the excitation of local vibrational modes by transferring energy from the oscillating AFM tip. This truly mechanical oscillator couples to the enclosing nanotube first and subsequently to the enclosed molecules, revealing their packing structure.(Dy@C 82 )@SWNT peapods were prepared by encapsulating Dy@C 82 metallofullerenes in open-ended SWNTs at a filling rate of ≈60%, as confirmed by high-resolution transmission electron microscopy and DFS [11]. The peapods were deposited at low coverage onto an insulating flat SiO 2 surface of a Si substrate and observed by a home-built dynamic AFM [11], shown schematically in Fig. 1(a). The AFM, equipped with a commercial Si cantilever (spring constant of 34.3 N/m, eigenfrequency of 159 kHz) and Si tip (nominal tip radius of 20Å), was operated at constant oscillation amplitude of 21 − 23Å under ultra-high vacuum (p < 1×10 −8 Pa) at low temperature (T < 13 K).High-resolution AFM topography observations of SWNTs and peapods, illustrated in Fig. 1(...
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