Ambient operation poses a challenge to atomic force microscopy because in contrast to operation in vacuum or liquid environments, the cantilever dynamics change dramatically from oscillating in air to oscillating in a hydration layer when probing the sample. We demonstrate atomic resolution by imaging of the KBr(001) surface in ambient conditions by frequency-modulation atomic force microscopy with a cantilever based on a quartz tuning fork (qPlus sensor) and analyze both long-and short-range contributions to the damping. The thickness of the hydration layer increases with relative humidity; thus varying humidity enables us to study the influence of the hydration layer thickness on cantilever damping. Starting with measurements of damping versus amplitude, we analyzed the signal and the noise characteristics at the atomic scale. We then determined the optimal amplitude which enabled us to acquire high-quality atomically resolved images.

True atomic resolution was demonstrated for AFM more than 15 years ago, but only recently has it been possible to atomically engineer the tip apex (1-4), a technique pioneered in the STM community (5,6). Tip termination with a CO molecule has made studies of inter-and intramolecular bonding possible (7-11). The CO-terminated tip itself, however, has proven to be difficult to characterize, despite experimental and theoretical approaches (12, 13), in part because it is challenging to characterize lateral forces and stiffnesses with AFM. With any study of shortrange interactions, AFM requires that the long-range background interaction of the macroscopic tip and surface be subtracted (14). Although a full three-dimensional (3D) dataset can reconstruct the potential energy, lateral forces and stiffnesses are only accessible by taking a numerical derivative of experimentally determined energies (15), which results in substantial noise in the data.In LFM, the tip is oscillated parallel to the surface (16,17), and the recorded frequency shift (Δf) is a direct measure of the lateral stiffness (18). The subtraction of long-range contributions, necessary in normalforce AFM (where the tip oscillates normal to the surface) to identify the short-range force components, is obsolete in LFM, rendering LFM particularly appealing for measurements in which the short-range interaction is the signal of interest (e.g., Ref. (19)). We have used both normalforce AFM and LFM to characterize a CO terminated tip and quantify parameters of a model incorporating torsional springs to account for the response of the CO molecules to lateral forces.When the CO tip was first used to image pentacene, it was remarked that the image seemed to be distorted due to CO relaxation (7). Further experiments showed that the relaxation of the CO molecule on the tip was an important part of the tip-surface interaction (13). This relaxation is usually characterized as a torsional spring, with the CO bending around the metal atom to which it is bound (12).We used a CO-terminated tip to probe a single CO molecule on Cu(111). CO on Cu(111) has been well-studied and the system offers a high degree of symmetry. The asymmetry in a normal-force AFM image of a CO with a CO-terminated tip (Fig. 1A) is the result of the slight asymmetry of the tip with respect to the surface, amplified by the bending of both tip and surface CO molecules. Images collected at further distances [e.g., those shown in Fig. S1 (20)] do not show this degree of asymmetry.We acquired a 3D dataset by collecting constant-height normal-force AFM images at various distances from the surface (21). To isolate the shortrange interaction, we subtracted the raw Δf data from the Δf signal above the bare Cu surface (14). The data were then deconvoluted and integrated twice along the z-direction (where z denotes the distance to the surface) to yield the potential energy (21-23). The potential energy map of the short-range interaction at closest approach is shown in Fig. 1B.A plot of the energy as a functio...

Imaging at the atomic scale using atomic force microscopy in biocompatible environments is an ongoing challenge. We demonstrate atomic resolution of graphite and hydrogen-intercalated graphene on SiC in air. The main challenges arise from the overall surface cleanliness and the water layers which form on almost all surfaces. To further investigate the influence of the water layers, we compare data taken with a hydrophilic bulk-silicon tip to a hydrophobic bulk-sapphire tip. While atomic resolution can be achieved with both tip materials at moderate interaction forces, there are strong differences in force versus distance spectra which relate to the water layers on the tips and samples. Imaging at very low tip-sample interaction forces results in the observation of large terraces of a naturally occurring stripe structure on the hydrogen-intercalated graphene. This structure has been previously reported on graphitic surfaces that are not covered with disordered adsorbates in ambient conditions (i.e., on graphite and bilayer graphene on SiC, but not on monolayer graphene on SiC). Both these observations indicate that hydrogen-intercalated graphene is close to an ideal graphene sample in ambient environments.

We investigate insulating Cu 2 N islands grown on Cu(100) by means of combined scanning tunneling microscopy and atomic force microscopy with two vastly different tips: a bare metal tip and a CO-terminated tip. We use scanning tunneling microscopy data as proposed by Choi, Ruggiero, and Gupta to unambiguously identify atomic positions. Atomic force microscopy images taken with the two different tips show an inverted contrast over Cu 2 N. The observed force contrast can be explained with an electrostatic model, where the two tips have dipole moments of opposite directions. This highlights the importance of short-range electrostatic forces in the formation of atomic contrast on polar surfaces in noncontact atomic force microscopy. DOI: 10.1103/PhysRevLett.112.166102 PACS numbers: 68.37.Ps, 61.46.−w, 68.37.Ef The combination of scanning tunneling microscopy (STM) with noncontact atomic force microscopy (NC-AFM) in a single probe enables a wide range of atomic-scale studies on surfaces. Whereas contrast mechanisms in STM for different tip-sample systems are widely understood, the interpretation of NC-AFM data remains challenging. In NC-AFM the sum over all tip-sample interactions is measured, and the source of atomic resolution is often hard to identify. On semiconductors [1]-as well as on metals [2]-imaged with reactive tips (e.g., Si) atomic contrast is dominated by the formation of covalent bonds that often reach magnitudes of nanonewtons. For nonreactive CO-functionalized tips, Pauli repulsion was attributed to the observed intramolecular resolution [3,4]. Lantz et al. [5] showed that the dangling bonds of Si(111)-(7 × 7) can induce a dipole moment in (nonreactive) oxidized Si tips resulting in a short-range electrostatic interaction, which contributes to atomic resolution. Electrostatic interaction and an induced tip dipole moment was also used to explain atomic contrast on ionic crystals [6]. A similar model describes the interaction with charged adatoms on thin insulating layers [7,8]. Moreover, it was found that clean metallic tips carry an intrinsic dipole moment [9,10], which is caused by the Smoluchowski effect [11]. All of these examples underline the importance of atomic-scale electrostatic interactions in NC-AFM.Electrostatic forces become even more meaningful as polar thin insulating layers (e.g., NaCl, MgO, Cu 2 N) are used to decouple adsorbates in STM and AFM experiments [3,7,[12][13][14][15][16]. In this study we explore the influence of electrostatic forces in NC-AFM on Cu 2 N islands on Cu(100). N and Cu atoms on Cu 2 N form a periodic charge arrangement, as calculated by density functional theory (DFT) [17] [Figs. 1(c)-1(e)]. Compared to alkali halides, the Cu 2 N's cð2 × 2Þ unit cell structure has a lower symmetry; thus, its atomic positions are easier to designate. STM experiments led to two criteria to locate N atoms within the islands [18]: first, N adsorbs on the hollow sites of Cu(100) [19][20][21] and should therefore appear fourfold symmetric; second, island boundaries and sharp edg...

Simultaneous measurements of tunneling currents and atomic forces on surfaces and adsorbates provide new insights into the electronic and structural properties of matter on the atomic scale. We report on experimental observations and calculations of a strong impact the tunneling current can have on the measured force, which arises when the resistivity of the sample cannot be neglected. We present a study on Si(111)-7×7 with various doping levels, but this effect is expected to occur on other low-conductance samples like adsorbed molecules, and is likely to strongly affect Kelvin probe measurements on the atomic scale.Scanning tunneling microscopy (STM) sparked enthusiasm in scanning probe microscopy with images of the adatoms of Si (111) Frequency-modulation AFM (FM-AFM) is a technique in which the interaction between tip and sample is measured by the frequency shift, ∆f , of an oscillating tip from its eigenfrequency, f 0 [12]. ∆f can be formulated as a measure of the force gradient, k ts = − dF dz , where z is the distance from the surface. ∆f is also a function of the spring constant of the oscillator, k, the amplitude of oscillation, A, and z, and can be approximated at small amplitudes by ∆f ≈ (f 0 /2 k) k ts [13]. In short, a decrease in ∆f indicates that the force between the tip and sample is becoming more attractive.In contrast to the tunneling current, I, measured with STM, ∆f is not monotonic as a function of z. The local tip-sample interaction is usually well-represented by a Morse potential from which the force and the force gradient can be derived, as shown in Fig. 1(a). In region I, k ts decreases as z decreases. Long-range forces (e.g. van der Waals) cause attractive interaction between tip and sample. It is in this region STM is usually conducted on Si(111)-7×7, at setpoints under 10 nA at 1 V, corresponding to tip-sample distances greater than 5Å [14]. In region II, k ts increases as z decreases. The waveform overlap between tip and sample causes measurable energy increase due to Pauli repulsion, which states electrons may not occupy the same quantum state [15].In this Letter, we report upon the effect of bias voltage on FM-AFM of Si(111)-7×7. At tip-sample distances corresponding to normal STM setpoints, one expects a decrease in frequency shift as the tip moves laterally without feedback over an adatom, due to the increase in attractive force [16]. However, with the application of a moderate bias voltage (>1.0 V), one is able to observe a frequency shift increase as the tip moves over an adatom.Moreover, FM-AFM images taken with this applied bias voltage can show atomic contrast at tip-sample distances 300 pm further from the surface than is required to image with no applied bias. We propose a model incorporating sample resistance where the observed frequency shift is caused by a decrease in the electrostatic attraction between tip and sample.Experiments were performed with a qPlus sensor with k = 1800 N m −1 . Data were collected in constant height mode with both a home-built microscope oper...