Observation of Electronic States on Si(111)-(7×7)through Short-Range Attractive Force with Noncontact Atomic Force Spectroscopy

Abstract

We experimentally reveal that the short-range attractive force between a Si tip and a Si(111)-7 7 surface is enhanced at specified bias voltages; we conduct force spectroscopy based on noncontact atomic force microscopy with changing bias voltage at a fixed separation. The spectra exhibit prominent peaks and a broad peak, which are attributed to quantum mechanical resonance as the energy levels of sample surface states are tuned to those of the tip states by shifting the Fermi level through changing bias voltage, and to the resonating states over a lowered tunneling barrier, respectively. DOI: 10.1103/PhysRevLett.93.256101 PACS numbers: 68.37.Ps, 73.20.-r, 81.05.Cy The adhesive force between two solid bodies at close separations is one of the most fundamental interactions in condensed matter, which involve chemical bond, metallic bond, van der Waals force, electrostatic force, and so on. For a simple case of chemical covalent bonding between two isolated atoms, Pauling successfully introduced the concept of quantum mechanical resonance between their electronic states at close energy levels [1]. By extending this concept of the resonance to condensed matter, the force interaction between two bodies at small separations can be regarded as the resonating overlap of their electron wave functions.Since the advent of scanning tunneling microscopy (STM) [2] and atomic force microscopy (AFM) [3], the adhesive force and the tunneling conductance between two bodies have simultaneously been examined by bringing a tip closer to a sample, i.e., by taking force-distance curves [4]. For the correlation between the force and the tunneling current, Chen theoretically evaluated it on the basis of the quantum mechanical resonance between the surface electronic states of both tip and sample through the tunneling barrier [5]. The force and the tunneling conductance can be derived from the overlapping of their wave functions at close energy levels through a tunneling barrier: the Bardeen's matrix element, which is used to evaluate the tunneling current, is fundamentally equal to the Heisenberg's resonance energy E, corresponding to the covalent bonding. The correlation between the force and the tunneling current has been shown experimentally for metals [6 -8], but it is still under debate: the decay length of force is larger than that of typical chemical interactions, and the patched charges around a tip may change the interaction.It is noted that the energy level of the surface electronic state in one body of condensed matter can be shifted relatively with respect to the other by changing bias voltage, corresponding to the Fermi level shift. Since the resonance occurs between the electronic states having closer energy levels, the quantum mechanical resonance is expected to be enhanced by tuning the energy levels of surface electronic states in the two bodies through the Fermi level shift. Thus, the interaction between two bodies should be analyzed by force spectroscopy with a capability of changing bias voltage.The noncontact (n...

Multiscale approach for simulations of Kelvin probe force microscopy with atomic resolution

“…63 In particular, above a Si adatom, they found a narrow peak growing with decreasing d superposed on the usual parabolic dependence around the plotted minimum of −∆f 1 (V b ) in their Fig. 1, i.e.…”

Atomic resolution force microscopy imaging on a strongly ionic surface with differently functionalized tips

“…The apparent atomic contrast is about 1.5 Hz and close to the detection limit. A clean tip is known to be appropriate for imaging silicon surfaces because of the strong chemical interaction between the Si atoms at the tip apex and silicon atoms in the surface 8,42 and has been proposed to yield a good chemical contrast also on CaF 2 (111). 34 It has, however, also been pointed out that a silicon based tip generally exhibits a considerably smaller interaction with the ionic surface than a polar tip.…”

“…Noncontact atomic force microscopy (NC-AFM) has greatly succeeded in imaging and manipulating a variety of surfaces of metals, 1, 2 semi-metals, [3][4][5] semiconductors, [6][7][8][9][10] and insulators. [11][12][13][14] In particular, atom-resolved imaging on terraces 15 and at step edges [16][17][18] on CaF 2 (111) that is the surface of interest for this study, is well established and understood by extensive theoretical modelling.…”

Local Spectroscopy and Atomic Imaging of Tunneling Current, Forces, and Dissipation on Graphite

“…4), it is evidently more symmetric than in [17]. Arai and Tomitori [25] have recently shown that force interactions are also a function of bias. Figure 4d)-f) shows constant-height images at a different tip bias of -60 mV.…”