Alfred J. Weymouth

Quantifying Molecular Stiffness and Interaction with Lateral Force Microscopy

A Soft Molecular Spring In noncontact atomic force microscopy, extremely high resolution has been achieved by attaching a terminal CO molecule to the metal scanning probe tip. This CO molecule can undergo torsional vibrations, and a full understanding of the imaging mechanism requires a measure of this spring constant. Weymouth et al. (p. 1120, published online 6 February; see the Perspective by Salmeron) used lateral force microscopy, in which the tip vibrates laterally across the surface, to determine this torsional constant. The stiffness of the isolated CO molecules on the tip was much less than that for CO molecules adsorbed on planar surfaces. Lateral force microscopy reveals the torsional spring constant of a carbon monoxide molecule at the end of an atomic force microscope tip. [Also see Perspective by Salmeron] The spatial resolution of atomic force microscopy (AFM) can be drastically increased by terminating the tip with a single carbon monoxide (CO) molecule. However, the CO molecule is not stiff, and lateral forces, such as those around the sides of molecules, distort images. This issue begs a larger question of how AFM can probe structures that are laterally weak. Lateral force microscopy (LFM) can probe lateral stiffnesses that are not accessible to normal-force AFM, resulting in higher spatial resolution. With LFM, we determined the torsional spring constant of a CO-terminated tip molecule to be 0.24 newtons per meter. This value is less than that of a surface molecule and an example of a system whose stiffness is a product not only of bonding partners but also local environment.

Phantom force induced by tunneling current: a characterization on Si(111).

Simultaneous measurements of tunneling current and atomic forces provide complementary atomic-scale data of the electronic and structural properties of surfaces and adsorbates. With these data, we characterize a strong impact of the tunneling current on the measured force on samples with limited conductivity. The effect is a lowering of the effective gap voltage through sample resistance which in turn lowers the electrostatic attraction, resulting in an apparently repulsive force. This effect is expected to occur on other low-conductance samples, such as adsorbed molecules, and to strongly affect Kelvin probe measurements when tunneling occurs.

Optimizing atomic resolution of force microscopy in ambient conditions

(Received 22 March 2013; published 12 June 2013)Ambient operation poses a challenge to atomic force microscopy because in contrast to operation in vacuumor liquid environments, the cantilever dynamics change dramatically from oscillating in air to oscillating in ahydrationlayerwhenprobingthesample.WedemonstrateatomicresolutionbyimagingoftheKBr(001)surfacein ambient conditions by frequency-modulation atomic force microscopy with a cantilever based on a quartztuningfork(qPlussensor)andanalyzebothlong-andshort-rangecontributionstothedamping.Thethicknessofthehydrationlayerincreaseswithrelativehumidity;thusvaryinghumidityenablesustostudytheinfluenceofthehydration layer thickness on cantilever damping. Starting with measurements of damping versus amplitude, weanalyzed the signal and the noise characteristics at the atomic scale. We then determined the optimal amplitudewhich enabled us to acquire high-quality atomically resolved images.DOI: 10.1103/PhysRevB.87.245415 PACS number(s): 07

Atomically resolved graphitic surfaces in air by atomic force microscopy.

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.

CO tip functionalization inverts atomic force microscopy contrast via short-range electrostatic forces.

We investigate insulating Cu2N 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 Cu2N. 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.

The influence of chemical bonding configuration on atomic identification by force spectroscopy.

The force between two atoms depends not only on their chemical species and distance, but also on the configuration of their chemical bonds to other atoms. This strongly affects atomic force spectroscopy, in which the force between the tip of an atomic force microscope and a sample is measured as a function of distance. We show that the short-range forces between tip and sample atoms depend strongly on the configuration of the tip, to the point of preventing atom identification with a poorly defined tip. Our solution is to control the tip apex before using it for spectroscopy. We demonstrate a method by which a CO molecule on Cu can be used to characterize the tip. In combination with gentle pokes, this can be used to engineer a specific tip apex. This CO Front atom Identification (COFI) method allows us to use a well-defined tip to conduct force spectroscopy.

Amplitude dependence of image quality in atomically-resolved bimodal atomic force microscopy

In bimodal frequency modulation atomic force microscopy (FM-AFM), two flexural modes are excited simultaneously. We show atomically resolved images of KBr(100) in ambient conditions in both modes that display a strong correlation between the image quality and amplitude. We define the sum amplitude as the sum of the amplitudes of both modes. When the sum amplitude becomes larger than about 100 pm, the signal-to-noise ratio (SNR) drastically decreases. We propose that this is caused by the temporary presence of one or more water layers in the tip-sample gap. These water layers screen the short range interaction and must be displaced with each oscillation cycle. Decreasing the amplitude of either mode, however, increases the noise. Therefore, the highest SNR in ambient conditions is achieved when twice the sum amplitude is slightly less than the thickness of the primary hydration layer.

Imaging in Biologically-Relevant Environments with AFM Using Stiff qPlus Sensors

High-resolution imaging of soft biological samples with atomic force microscopy (AFM) is challenging because they must be imaged with small forces to prevent deformation. Typically, AFM of those samples is performed with soft silicon cantilevers (k ≈ 0.1–10 N/m) and optical detection in a liquid environment. We set up a new microscope that uses a stiff qPlus sensor (k ≥ 1 kN/m). Several complex biologically-relevant solutions are non-transparent, and even change their optical properties over time, such as the cell culture medium we used. While this would be problematic for AFM setups with optical detection, it is no problem for our qPlus setup which uses electrical detection. The high stiffness of the qPlus sensor allows us to use small amplitudes in frequency-modulation mode and obtain high Q factors even in liquid. The samples are immersed in solution in a liquid cell and long tips are used, with only the tip apex submerged. We discuss the noise terms and compare the minimal detectable signal to that of soft cantilevers. Atomic resolution of muscovite mica was achieved in various liquids: H2O, Tris buffer and a cell culture medium. We show images of lipid membranes in which the individual head groups are resolved.

Evaluating the potential energy landscape over single molecules at room temperature with lateral force microscopy

One of the challenges of AFM, in contrast to STM, is that the measured signal includes both long-range and short-range components. The most accurate method for removing long-range components is to measure both on and off an adsorbate and to subtract the difference. This on-off method is challenging at room temperature due to thermal drift. By moving to a non-contact scheme in which the lateral component of the force interaction is probed, the measurement is dominated by short-range interactions. We use frequency-modulation lateral force microscopy to measure individual PTCDA molecules adsorbed on Ag/Si(111)-( 3×3). By fitting the data to a model potential, we can extract the depth and width of the potential. When the tip is closer to the sample, a repulsive feature can be observed in the data.One of the challenges of AFM, in contrast to STM, is that the measured signal includes both long-range and short-range components. The most accurate method for removing long-range components is to measure both on and off an adsorbate and to subtract the difference. This on-off method is challenging at room temperature due to thermal drift. By moving to a non-contact scheme in which the lateral component of the force interaction is probed, the measurement is dominated by short-range interactions. We use frequency-modulation lateral force microscopy to measure individual PTCDA molecules adsorbed on Ag/Si(111)-( 3×3). By fitting the data to a model potential, we can extract the depth and width of the potential. When the tip is closer to the sample, a repulsive feature can be observed in the data.

The Phantom Force

While atomic resolution in an AFM image is usually assumed to originate from the formation of a chemical bond or Pauli repulsion, it can also be caused by a phenomenon we called the phantom force. When there is an electric potential difference between tip and sample, they will be attracted to one another. It is quite common in AFM experiments to apply a voltage between the tip and the sample. At distances required for atomic resolution, this can result in a tunneling current. If there is a tunneling current, then there will also be a potential difference within the sample (as charge carriers do not accumulate after they have tunnelled). The magnitude of this potential difference within the sample is related to the resistivity of the sample. The total potential difference between the sample bulk and the tip is fixed by the applied voltage, so any potential difference within the sample reduces the potential drop in the junction between the tip and sample. This phantom force is an apparently repulsive force caused by a decrease in the electrostatic attraction between tip and sample. If the total resistance within the tip or sample is high enough, then the phantom force can be the dominant contrast mechanism in AFM images. It can also dominate features in bias and distance spectroscopy. This chapter includes a comprehensive description of our theory of the phantom force and data which demonstrate this effect.