Effect of the tip state during qPlus noncontact atomic force microscopy of Si(100) at 5 K: Probing the probe

Background: Noncontact atomic force microscopy (NC-AFM) now regularly produces atomic-resolution images on a wide range of surfaces, and has demonstrated the capability for atomic manipulation solely using chemical forces. Nonetheless, the role of the tip apex in both imaging and manipulation remains poorly understood and is an active area of research both experimentally and theoretically. Recent work employing specially functionalised tips has provided additional impetus to elucidating the role of the tip apex in the observed contrast. Results: We present an analysis of the influence of the tip apex during imaging of the Si(100) substrate in ultra-high vacuum (UHV) at 5 K using a qPlus sensor for noncontact atomic force microscopy (NC-AFM). Data demonstrating stable imaging with a range of tip apexes, each with a characteristic imaging signature, have been acquired. By imaging at close to zero applied bias we eliminate the influence of tunnel current on the force between tip and surface, and also the tunnel-current-induced excitation of silicon dimers, which is a key issue in scanning probe studies of Si(100). Conclusion: A wide range of novel imaging mechanisms are demonstrated on the Si(100) surface, which can only be explained by variations in the precise structural configuration at the apex of the tip. Such images provide a valuable resource for theoreticians working on the development of realistic tip structures for NC-AFM simulations. Force spectroscopy measurements show that the tip termination critically affects both the short-range force and dissipated energy.

current was detectedin Figure S2 line profiles from two scans are presented showing that there is significant variation in the topography but no detected tunnel current in either instance. We also checked for AC displacement currents induced by the CPD between tip and sample (although with a different tip apex to those used to acquire the results presented in the paper). The displacement current was measured using a lock-in amplifier whilst the tip was oscillating (using a method similar to that described in A. D. Müller et al. Appl. Phys. Lett. 74, 2963Lett. 74, (1999) at a range of biases (including 0 V). We were unable to detect any displacement current within the noise level of the low-gain preamplifier (~1 pA).
Consequently our data (both imaging and force spectroscopy) cannot be assigned to any form of electronic crosstalk and represent the characteristic imaging related to different physical tip apices, and their interaction with the surface.

Calibration of dissipation
The dissipation recorded was determined by measurement of the DC gain of the amplitude regulation (i.e., the damping signal) as described in the Habilitationsschrift of Dr. Franz Giessibl. (Universität Augsburg 2000). The ratio of the free damping to the damping at closest approach is the same as the ratio of the free driving signal to the driving signal at closest approach, and may therefore be used to calculate the dissipated energy by

Atom assignment in inverted imaging
Because the Si(100) surface is composed of atoms in two distinct configurations at different absolute heights, particular care must be taken in the interpretation of "inverted" images as depressions may also correspond to the location of the "down"' atoms . In Figure S3 we show that true contrast inversion is observed by considering the imaging of B-Si dopantrelated defect structure [1,2], These ad-dimer structures protrude above the native dimer rows and thus provide a structure that is unambiguously topographically higher than the surrounding atoms. In addition we observe split-off dimers at a step edge, which remain symmetric (i.e., unbuckled) and are thus at (approximately) the same height as "up" atoms of normal buckled dimers. Comparison of conventional high-bias STM, "conventional" imaging, and "inverted" imaging of the same regions ( Figure S3) highlights that the protrusions in STM and conventional imaging correspond to the depressions in the inverted image. Although there is weak evidence for a "conventional" double tip over the B-Si ad-dimer during inverted

S3
imaging (as noted in the main paper) we stress that the registry of these features does not match with the position of the noninverted features between rows, and thus the contrast over the clean surface cannot be assigned to the imaging of the down-atom positions. It is possible that the tip structure that produces the inverted contrast may always produce an offset "conventional" image at close approach, and this may in turn provide a valuable clue as the apex structure that produces this contrast. We also note that, in contrast to the data presented in the main paper, a very small tunnel-current signal was detected in some regions of the surface for these images. These are presented alongside the raw topography to highlight that the two signals are well decoupled, and that the variations in imaging are not due to the detected tunnel current.   (e,f ) and (g,h) raw (planesubtracted) topography and tunnelcurrent images of conventional and inverted contrast, respectively. Levels have been adjusted to maximise contrast on the upper terrace. Range of tunnel current in displayed images ~10 pA, applied bias ~1 mV