Controlled manipulation of atoms in insulating surfaces with the virtual atomic force microscope

A Universal Solution of Controlling the Distribution of Multimaterials during Macroscopic Manipulation via a Microtopography-Guided Substrate

Any object can be considered as a spatial distribution of atoms and molecules; in this sense, we can manufacture any object as long as the precise distribution of atoms and molecules is achieved. However, the current point-by-point methods to precisely manipulate single atoms and single molecules, such as the scanning tunneling microscope (STM), have difficulty in manipulating a large quantity of materials within an acceptable time. The macroscopic manipulation techniques, such as magnetron sputtering, molecular beam epitaxy, and evaporation, could not precisely control the distribution of materials. Herein, we take a step back and present a universal method of controlling the distribution of multimaterails during macroscopic manipulation via microtopography-guided substrates. For any given target distribution of multimaterials in a plane, the complicated lateral distribution of multimaterials was firstly transformed into a simple spatial lamellar body. Then, a deposition mathematical model was first established based on a mathematical transformation. Meanwhile, the microtopographic substrate can be fabricated according to target distribution based on the deposition mathematical model. Following this, the deposition was implemented on the substrate according to the designed sequence and thickness of each material, resulting in the formation of the deposition body on the substrate. Finally, the actual distribution was obtained on a certain section in the deposition body by removing the upside materials. The actual distribution can mimic the target one with a controllable accuracy. Furthermore, two experiments were performed to validate our method. As a result, we provide a feasible and scalable solution for controlling the distribution of multimaterials, and point out the direction of improving the position accuracy of each material. We may achieve real molecular manufacturing and nano-manufacturing if the position accuracy of distribution approaches the atomic level.

Lateral manipulation of atomic vacancies in ultrathin insulating films

During the last 20 years, using scanning tunneling microscopy (STM) and atomic force microscopy, scientists have successfully achieved vertical and lateral repositioning of individual atoms on and in different types of surfaces. Such atom manipulation allows the bottom-up assembly of novel nanostructures that can otherwise not be fabricated. It is therefore surprising that controlled repositioning of virtual atoms, i.e., atomic vacancies, across atomic lattices has not yet been achieved experimentally. Here we use STM at liquid helium temperature (4.5 K) to create individual Cl vacancies and subsequently to laterally manipulate them across the surface of ultrathin sodium chloride films. This allows monitoring the interactions between two neighboring vacancies with different separations. Our findings are corroborated by density functional theory calculations and STM image simulations. The lateral manipulation of atomic vacancies opens up a new playground for the investigation of fundamental physical properties of vacancy nanostructures of any size and shape and their coupling with the supporting substrate, and of the interaction of various deposits with charged vacancies.

Friction through reversible jumps of surface atoms

We propose a microscopic model that incorporates the effect of thermally activated motion of surface atoms on nanoscopic friction. Our calculations demonstrate that the stick-slip motion of the tip is governed by two competing processes: (i) jumps of the surface atoms to the tip which tend to inhibit sliding, and (ii) jumps back to the sample which give rise to sliding. The energy dissipated during the reversible jumps of the surface atoms between the sample and tip contributes significantly to the friction force, and leads to a nonmonotonic dependence of friction on temperature, which has been observed in recent friction force microscopy experiments for different material classes. The proposed model elucidates the physical origin of microscopic instabilities introduced in phenomenological models for the interpretation of the experimental results.

Defect mediated manipulation of nanoclusters on an insulator

With modern scanning probe microscopes, it is possible to manipulate surface structures even at the atomic level. However, manipulation of nanoscale objects such as clusters is often more relevant and also more challenging due to the complicated interactions between the surface, cluster and apparatus. We demonstrate the manipulation of nanometer scale gold clusters on the NaCl(001) surface with a non-contact atomic force microscope, and show that the movement of clusters is in certain cases constrained to specific crystallographic directions. First principles calculations explain this kinetic anisotropy as the result of the cluster attaching to surface defects: cation vacancies allow the clusters to bond in such a way that they only move in one direction. Constraining the movement of clusters could be exploited in the construction of nanostructures or nanomechanical devices, and the manipulation signatures may also be used for identifying cluster-defect complexes.

Complex design of dissipation signals in non-contact atomic force microscopy

Complex interplay between topography and dissipation signals in Non-Contact Atomic Force Microscopy (NC-AFM) is studied by a combination of state-of-the-art theory and experiment applied to the Si(001) surface prone to instabilities. Considering a wide range of tip-sample separations down to the near-contact regime and several tip models, both stiff and more flexible, a sophisticated architecture of hysteresis loops in the simulated tip force-distance curves is revealed. At small tip-surface distances the dissipation was found to be comprised of two related contributions due to both the surface and tip. These are accompanied by the corresponding surface and tip distortion approach-retraction dynamics. Qualitative conclusions drawn from the theoretical simulations such as large dissipation signals (>1.0 eV) and a step-like dissipation dependent on the tip-surface distance are broadly supported by the experimental observations. In view of the obtained results we also discuss the reproducibility of NC-AFM imaging.

Single-molecule switching with non-contact atomic force microscopy

We report upon controlled switching of a single 3,4,9,10-perylene tetracarboxylic diimide derivative molecule on a rutile TiO(2)(110) surface using a non-contact atomic force microscope at room temperature. After submonolayer deposition, the molecules adsorb tilted on the bridging oxygen row. Individual molecules can be manipulated by the atomic force microscope tip in a well-controlled manner. The molecules are switched from one side of the row to the other using a simple approach, taking benefit of the sample tilt and the topography of the titania substrate. From density functional theory investigations we obtain the adsorption energies of different positions of the molecule. These adsorption energies are in very good agreement with our experimental observations.

Recent trends in surface characterization and chemistry with high-resolution scanning force methods

The current status and future prospects of non-contact atomic force microscopy (nc-AFM) and Kelvin probe force microscopy (KPFM) for studying insulating surfaces and thin insulating films in high resolution are discussed. The rapid development of these techniques and their use in combination with other scanning probe microscopy methods over the last few years has made them increasingly relevant for studying, controlling, and functionalizing the surfaces of many key materials. After introducing the instruments and the basic terminology associated with them, state-of-the-art experimental and theoretical studies of insulating surfaces and thin films are discussed, with specific focus on defects, atomic and molecular adsorbates, doping, and metallic nanoclusters. The latest achievements in atomic site-specific force spectroscopy and the identification of defects by crystal doping, work function, and surface charge imaging are reviewed and recent progress being made in high-resolution imaging in air and liquids is detailed. Finally, some of the key challenges for the future development of the considered fields are identified.

Modelling components of future molecular devices

We discuss challenges involved in modelling different components of molecular devices and give several examples that demonstrate how computer modelling evolved over the last few years to become a comprehensive tool for designing molecules, predicting their adsorption and diffusion at surfaces, simulating atomic force microscopy imaging and manipulation of atoms and molecules at insulating surfaces and studying electron conduction in prototype molecular devices. We describe some of the computational techniques used for modelling adsorption, diffusion, imaging and manipulation of organic molecules at surfaces and challenges pertaining to these studies, give several examples of applications and discuss further prospects for theoretical modelling of complex organic molecules at surfaces.

Atomic force microscopy as a tool for atom manipulation

During the past 20 years, the manipulation of atoms and molecules at surfaces has allowed the construction and characterization of model systems that could, potentially, act as building blocks for future nanoscale devices. The majority of these experiments were performed with scanning tunnelling microscopy at cryogenic temperatures. Recently, it has been shown that another scanning probe technique, the atomic force microscope, is capable of positioning single atoms even at room temperature. Here, we review progress in the manipulation of atoms and molecules with the atomic force microscope, and discuss the new opportunities presented by this technique.

Manipulation of single atoms by atomic force microscopy as a resonance effect

Extraction and deposition of single atoms using an atomic force microscope tip is a promising technique for building nanostructures. Previous theoretical models for this technique, based on adiabatic atom motion in either classical or quantum mechanics settings, encountered an apparent difficulty in explaining atom extraction and deposition for the same experimental conditions. We resolve that difficulty by showing that both extraction and deposition of atoms can be formulated in terms of pure classical mechanics as a resonance effect, arising from a combination of interatomic forces and vibrations of individual atoms.

Imaging of the hydrogen subsurface site in rutile TiO2

From an interplay between simultaneously recorded noncontact atomic force microscopy and scanning tunneling microscopy images and simulations based on density functional theory, we reveal the location of single hydrogen species in the surface and subsurface layers of rutile TiO2. Subsurface hydrogen atoms (H(sub)) are found to reside in a stable interstitial site as subsurface OH groups detectable in scanning tunneling microscopy as a characteristic electronic state but imperceptible to atomic force microscopy. The combined atomic force microscopy, scanning tunneling microscopy, and density functional theory study demonstrates a general scheme to reveal near surface defects and interstitials in poorly conducting materials.

Modelling the manipulation of C60 on the Si001 surface performed with NC-AFM

We present a theoretical model of manipulation of the C(60) molecule on the Si(001) surface with a non-contact atomic force microscope (NC-AFM). The model relies on the lowering of the energy barrier for the C(60) manipulation due to the interaction of the C(60) with an AFM tip and the subsequent thermal movement of the molecule over this barrier. We performed numerical simulations of these energy barriers for a series of tip positions relative to the molecule to show how the barriers change with the tip position. The values of these barriers are then used in kinetic Monte Carlo simulations to estimate the probability of the C(60) movement for different tip positions and temperatures. Virtual atomic force microscope simulations, which include the kinetic Monte Carlo treatment of the C(60) movement, are then performed to describe in real time the process of movement of the C(60) molecule during an NC-AFM scan. Our results demonstrate that manipulation of the C(60) molecule, which is covalently bound to the surface, is possible with NC-AFM, even though there is no continuous tip-molecule contact, which is known to be a necessary requirement for the C(60) manipulation with scanning tunnelling microscopy. We show that the manipulation event can be identified in real NC-AFM experiments as an abrupt change in the distance of the tip closest approach (topography), and as spikes in the frequency shift and dissipation signals.

VEDA: a web-based virtual environment for dynamic atomic force microscopy

We describe here the theory and applications of virtual environment dynamic atomic force microscopy (VEDA), a suite of state-of-the-art simulation tools deployed on nanoHUB (www.nanohub.org) for the accurate simulation of tip motion in dynamic atomic force microscopy (dAFM) over organic and inorganic samples. VEDA takes advantage of nanoHUB's cyberinfrastructure to run high-fidelity dAFM tip dynamics computations on local clusters and the teragrid. Consequently, these tools are freely accessible and the dAFM simulations are run using standard web-based browsers without requiring additional software. A wide range of issues in dAFM ranging from optimal probe choice, probe stability, and tip-sample interaction forces, power dissipation, to material property extraction and scanning dynamics over hetereogeneous samples can be addressed.

Mechanical manifestations of rare atomic jumps in dynamic force microscopy

The resonance frequency and the excitation amplitude of a silicon cantilever have been measured as a function of distance to a cleaved KBr(001) surface with a low-temperature scanning force microscope (SFM) in ultrahigh vacuum. We identify two regimes of tip-sample distances. Above a site-dependent critical tip-sample distance reproducible data with low noise and no interaction-induced energy dissipation are measured. In this regime reproducible SFM images can be recorded. At closer tip-sample distances, above two distinct atomic sites, the frequency values jump between two limiting curves on a timescale of tens of milliseconds. Furthermore, additional energy dissipation occurs wherever jumps are observed. We attribute both phenomena to rarely occurring changes in the tip apex configuration which are affected by short-range interactions with the sample. Their respective magnitudes are related to each other. A specific candidate two-level system is also proposed.