Uncertainties in forces extracted from non-contact atomic force microscopy measurements by fitting of long-range background forces

In principle, non-contact atomic force microscopy (NC-AFM) now readily allows for the measurement of forces with sub-nanonewton precision on the atomic scale. In practice, however, the extraction of the often desired ‘short-range’ force from the experimental observable (frequency shift) is often far from trivial. In most cases there is a significant contribution to the total tip–sample force due to non-site-specific van der Waals and electrostatic forces. Typically, the contribution from these forces must be removed before the results of the experiment can be successfully interpreted, often by comparison to density functional theory calculations. In this paper we compare the ‘on-minus-off’ method for extracting site-specific forces to a commonly used extrapolation method modelling the long-range forces using a simple power law. By examining the behaviour of the fitting method in the case of two radically different interaction potentials we show that significant uncertainties in the final extracted forces may result from use of the extrapolation method

Forces at the nanoscale : interactions in atomic force microscopy and dielectrophoresis

Interactions at the nanoscale are governed almost exclusively by electromagnetic forces, but the interplay between different scaling laws produces a vast array of behaviours. We investigate radically different systems spanning almost three orders of magnitude of length scales, and use a variety of experimental techniques to determine the forces present in each regime, and the interplay between them. An important prototypical surface in SPM science has been the Si(100) surface, which due to it’s unstable buckling and complex electronic structure has fostered considerable debate in the surface science community. We have used small amplitude, high sensitivity combined qPlus STM/AFM to investigate tip -- sample interactions on the Si(100) surface at low-temperature in UHV, with a focus on the chemical, and electronic properties of the system and how these are modified by the probe. We present the first atomic resolution combined force/tunnel current results on the surface and show that great care must be taken in interpreting either pure AFM or pure STM data. We also examine tip -- sample interactions on arrays of thiol passivated spin-cast nanoparticles in both UHV and ambient conditions and show for the first time how minor modifications to the experimental parameters can radically alter the data collected, most likely due to the thiol -- surface -- tip interaction. We also present SKPM and voltage spectroscopy of the same samples and show the importance of electrostatic interactions in correct height determination of these network arrays, in parallel with the caution that must be maintained in interpreting CPD data. A key mechanism for the manipulation of meso-scale objects in solution is Dielelectrophoresis, which offers strong material and size specificity and a high degree of spatial control. In the final experimental chapter we investigate the effect of inhomogenous electric fields on nanoparticles in aqueous solution, and reveal how previously uninvestigated electrochemical effects can become important even at high frequencies, and may offer a new and exciting route for the control of self organised nanowires in solution

Forces at the nanoscale : interactions in atomic force microscopy and dielectrophoresis

Interactions at the nanoscale are governed almost exclusively by electromagnetic forces, but the interplay between different scaling laws produces a vast array of behaviours. We investigate radically different systems spanning almost three orders of magnitude of length scales, and use a variety of experimental techniques to determine the forces present in each regime, and the interplay between them. An important prototypical surface in SPM science has been the Si(100) surface, which due to it’s unstable buckling and complex electronic structure has fostered considerable debate in the surface science community. We have used small amplitude, high sensitivity combined qPlus STM/AFM to investigate tip -- sample interactions on the Si(100) surface at low-temperature in UHV, with a focus on the chemical, and electronic properties of the system and how these are modified by the probe. We present the first atomic resolution combined force/tunnel current results on the surface and show that great care must be taken in interpreting either pure AFM or pure STM data. We also examine tip -- sample interactions on arrays of thiol passivated spin-cast nanoparticles in both UHV and ambient conditions and show for the first time how minor modifications to the experimental parameters can radically alter the data collected, most likely due to the thiol -- surface -- tip interaction. We also present SKPM and voltage spectroscopy of the same samples and show the importance of electrostatic interactions in correct height determination of these network arrays, in parallel with the caution that must be maintained in interpreting CPD data. A key mechanism for the manipulation of meso-scale objects in solution is Dielelectrophoresis, which offers strong material and size specificity and a high degree of spatial control. In the final experimental chapter we investigate the effect of inhomogenous electric fields on nanoparticles in aqueous solution, and reveal how previously uninvestigated electrochemical effects can become important even at high frequencies, and may offer a new and exciting route for the control of self organised nanowires in solution

Measuring the reactivity of a silicon-terminated probe

It is generally accepted that the exposed surfaces of silicon crystals are highly reactive due to the dangling bonds which protrude into the vacuum. However, surface reconstruction can not only modify the reactivity of bulk silicon crystals, but plays a key role in determining the properties of silicon nanocrystals. In this study we probe the reactivity of silicon clusters at the end of a scanning probe tip by examining their interaction with closed shell fullerene molecules. Counter to intuitive expectations, many silicon clusters do not react strongly with the fullerene cage, and we find that only specific highly oriented clusters have sufficient reactivity to break open the existing carbon-carbon bonds

Nano-contact microscopy of supracrystals

Background: Highly ordered three-dimensional colloidal crystals (supracrystals) comprised of 7.4 nm diameter Au nanocrystals (with a 5% size dispersion) have been imaged and analysed using a combination of scanning tunnelling microscopy and dynamic force microscopy. Results: By exploring the evolution of both the force and tunnel current with respect to tip–sample separation, we arrive at the surprising finding that single nanocrystal resolution is readily obtained in tunnelling microscopy images acquired more than 1 nm into the repulsive (i.e., positive force) regime of the probe–nanocrystal interaction potential. Constant height force microscopy has been used to map tip–sample interactions in this regime, revealing inhomogeneities which arise from the convolution of the tip structure with the ligand distribution at the nanocrystal surface. Conclusion: Our combined STM–AFM measurements show that the contrast mechanism underpinning high resolution imaging of nanoparticle supracrystals involves a form of nanoscale contact imaging, rather than the through-vacuum tunnelling which underpins traditional tunnelling microscopy and spectroscopy

Simulated structure and imaging of NTCDI on Si(1 1 1)-7 × 7 : a combined STM, NC-AFM and DFT study

The adsorption of naphthalene tetracarboxylic diimide (NTCDI) on Si(1 1 1)-7 × 7 is investigated through a combination of scanning tunnelling microscopy (STM), noncontact atomic force microscopy (NC-AFM) and density functional theory (DFT) calculations. We show that NTCDI adopts multiple planar adsorption geometries on the Si(1 1 1)-7 × 7 surface which can be imaged with intramolecular bond resolution using NC-AFM. DFT calculations reveal adsorption is dominated by covalent bond formation between the molecular oxygen atoms and the surface silicon adatoms. The chemisorption of the molecule is found to induce subtle distortions to the molecular structure, which are observed in NC-AFM images

Visualizing the 'invisible'

The ability of scientists to image and manipulate matter at the (sub)atomic scale is a result of stunning advances in microscopy. Foremost amongst these was the invention of the scanning probe microscope, which, despite its classification as a microscope, does not rely on optics to generate images. Instead, images are produced via the interaction of an atomically sharp probe with a surface. Here the author considers to what extent those images represent an accurate picture of ‘reality’ at a size regime where quantum physics holds sway, and where the image data can be acquired and manipulated in a variety of ways

Intermolecular artifacts in probe microscope images of C60 assemblies

Claims that dynamic force microscopy has the capability to resolve intermolecular bonds in real space continue to be vigorously debated. To date, studies have been restricted to planar molecular assemblies with small separations between neighboring molecules. Here we report the observation of intermolecular artifacts over much larger distances in 2D assemblies of C60 molecules, with compelling evidence that in our case the tip apex is terminated by a C60 molecule (rather than the CO termination typically exploited in ultrahigh resolution force microscopy). The complete absence of directional interactions such as hydrogen or halogen bonding, the nonplanar structure of C60, and the fullerene termination of the tip apex in our case highlight that intermolecular artifacts are ubiquitous in dynamic force microscopy

Intramolecular bonds resolved on a semiconductor surface

Noncontact atomic force microscopy (NC-AFM) is now routinely capable of obtaining submolecular resolution, readily resolving the carbon backbone structure of planar organic molecules adsorbed on metal substrates. Here we show that the same resolution may also be obtained for molecules adsorbed on a reactive semiconducting substrate. Surprisingly, this resolution is routinely obtained without the need for deliberate tip functionalization. Intriguingly, we observe two chemically distinct apex types capable of submolecular imaging. We characterize our tip apices by “inverse imaging” of the silicon adatoms of the Si(111)−7×7 surface and support our findings with detailed density functional theory (DFT) calculations. We also show that intramolecular resolution on individual molecules may be readily obtained at 78 K, rather than solely at 5 K as previously demonstrated. Our results suggest a wide range of tips may be capable of producing intramolecular contrast for molecules adsorbed on semiconductor surfaces, leading to a much broader applicability for submolecular imaging protocols

Critical assessment of the evidence for striped nanoparticles

There is now a significant body of literature which reports that stripes form in the ligand shell of suitably functionalised Au nanoparticles. This stripe morphology has been proposed to strongly affect the physicochemical and biochemical properties of the particles. We critique the published evidence for striped nanoparticles in detail, with a particular focus on the interpretation of scanning tunnelling microscopy (STM) data (as this is the only technique which ostensibly provides direct evidence for the presence of stripes). Through a combination of an exhaustive re-analysis of the original data, in addition to new experimental measurements of a simple control sample comprising entirely unfunctionalised particles, we show that all of the STM evidence for striped nanoparticles published to date can instead be explained by a combination of well-known instrumental artefacts, or by issues with data acquisition/analysis protocols. We also critically re-examine the evidence for the presence of ligand stripes which has been claimed to have been found from transmission electron microscopy, nuclear magnetic resonance spectroscopy, small angle neutron scattering experiments, and computer simulations. Although these data can indeed be interpreted in terms of stripe formation, we show that the reported results can alternatively be explained as arising from a combination of instrumental artefacts and inadequate data analysis techniques