Small Tip-sample Distances Research Articles

Switching from electromagnetic to chemical mechanism in quantum plasmonic tip-induced graphene oxide enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) is based on the ability of a surface substrate to increase Raman signals for sensing and imaging applications. The most widely used Au and Ag SERS substrates are primarily based on the electromagnetic mechanism (EM) with large enhancement factors (EFs), which are, however, limited by small gaps due to tunneling. Graphene has been explored as an alternative substrate for graphene-enhanced Raman scattering based on the chemical mechanism (CM). However, the limits of CM EFs in graphene-based substrates have not been well understood, especially as a function of tip-sample distance (TSD). Here we performed tip-enhanced Raman scattering of carbon nanotubes on Au and graphene-oxide (GO) hybrid substrates for different TSDs. We show evidence of quantum plasmonics with GO as a tunneling junction in an Au-GO-Au cavity with a 2-nm gap size. We demonstrate Raman signal enhancement by four orders of magnitude beyond the EM tunneling limit by switching to the CM regime for the resonant GO excitation at small TSD. Tip-induced GO-enhanced Raman scattering may be used to improve nanoimaging and biosensing on novel hybrid GO/Au substrates.

Atomic-scale visualization of surface-assisted orbital order.

Orbital-related physics attracts growing interest in condensed matter research, but direct real-space access of the orbital degree of freedom is challenging. We report a first, real-space, imaging of a surface-assisted orbital ordered structure on a cobalt-terminated surface of the well-studied heavy fermion compound CeCoIn5. Within small tip-sample distances, the cobalt atoms on a cleaved (001) surface take on dumbbell shapes alternatingly aligned in the [100] and [010] directions in scanning tunneling microscopy topographies. First-principles calculations reveal that this structure is a consequence of the staggered d xz -d yz orbital order triggered by enhanced on-site Coulomb interaction at the surface. This so far overlooked surface-assisted orbital ordering may prevail in transition metal oxides, heavy fermion superconductors, and other materials.

Feedforward neural network methodology to characterize thin films by Electrostatic Force Microscopy

The contribution of the present paper is in introducing a numerical method to improve the automatic characterization of thin films by increasing the effectiveness of numerical methods that take into account the macroscopic shape of the tip. To achieve this objective, we propose the combination of different feedforward neural networks architectures adapted to the specific requirements of the physical system under study. First, an Adaline architecture is redefined as a linear combination of Green functions obtained from the Laplace equation. The learning process is also redefined to accurately calculate the electrostatic charges inside the tip. We demonstrate that a complete training set for the characterization of thin films can be easily obtained by this methodology. The characterization of the sample is developed in a second stage where a multilayer perceptron is adapted to work efficiently in experimental conditions where some experimental data can be lost. We demonstrate that a very efficient strategy is to use evolutionary algorithms as training method. By the modulation of the fit function, we can improve the network performance in the characterization of thin films where some information is missing or altered by experimental noise due to the small tip-sample working distances. By doing so, we can discriminate the conductive properties of thin films from force curves that have been altered explicitly to simulate realistic experimental conditions.

Magnetic force microscopy using tip magnetization modulated by ferromagnetic resonance

In magnetic force microscopy (MFM), the tip–sample distance should be reduced to analyze the microscopic magnetic domain structure with high spatial resolution. However, achieving a small tip–sample distance has been difficult because of superimposition of interaction forces such as van der Waals and electrostatic forces induced by the sample surface. In this study, we propose a new method of MFM using ferromagnetic resonance (FMR) to extract only the magnetic field near the sample surface. In this method, the magnetization of a magnetic cantilever is modulated by FMR to separate the magnetic field and topographic structure. We demonstrate the modulation of the magnetization of the cantilever and the identification of the polarities of a perpendicular magnetic medium.

Understanding Scanning Tunneling Microscopy Contrast Mechanisms on Metal Oxides: A Case Study

A comprehensive analysis of contrast formation mechanisms in scanning tunneling microscopy (STM) experiments on a metal oxide surface is presented with the oxygen-induced (2√2×√2)R45° missing row reconstruction of the Cu(100) surface as a model system. Density functional theory and electronic transport calculations were combined to simulate the STM imaging behavior of pure and oxygen-contaminated metal tips with structurally and chemically different apexes while systematically varying bias voltage and tip-sample distance. The resulting multiparameter database of computed images was used to conduct an extensive comparison with experimental data. Excellent agreement was attained for a large number of cases, suggesting that the assumed model tips reproduce most of the commonly encountered contrast-determining effects. Specifically, we find that depending on the bias voltage polarity, copper-terminated tips allow selective imaging of two structurally distinct surface Cu sites, while oxygen-terminated tips show complex contrasts with pronounced asymmetry and tip-sample distance dependence. Considering the structural and chemical stability of the tips reveals that the copper-terminated apexes tend to react with surface oxygen at small tip-sample distances. In contrast, oxygen-terminated tips are considerably more stable, allowing exclusive surface oxygen imaging at small tip-sample distances. Our results provide a conclusive understanding of fundamental STM imaging mechanisms, thereby providing guidelines for experimentalists to achieve chemically selective imaging by properly selecting imaging parameters.

Dynamic electrostatic force microscopy technique for the study of electrical properties with improved spatial resolution

The need to resolve the electrical properties of confined structures (CNTs, quantum dots, nanorods, etc) is becoming increasingly important in the field of electronic and optoelectronic devices. Here we propose an approach based on amplitude modulated electrostatic force microscopy to obtain measurements at small tip–sample distances, where highly nonlinear forces are present. We discuss how this improves the lateral resolution of the technique and allows probing of the electrical and surface properties. The complete force field at different tip biases is employed to derive the local work function difference. Then, by appropriately biasing the tip–sample system, short-range forces are reconstructed. The short-range component is then separated from the generic tip–sample force in order to recover the pure electrostatic contribution. This data can be employed to derive the tip–sample capacitance curve and the sample dielectric constant. After presenting a theoretical model that justifies the need for probing the electrical properties of the sample in the vicinity of the surface, the methodology is presented in detail and verified experimentally.

Exploring atomic-scale lateral forces in the attractive regime: a case study on graphite (0001)

A non-contact atomic force microscopy-based method has been used to map the static lateral forces exerted on an atomically sharp Pt/Ir probe tip by a graphite surface. With measurements carried out at low temperatures and in the attractive regime, where the atomic sharpness of the tip can be maintained over extended time periods, the method allows the quantification and directional analysis of lateral forces with piconewton and picometer resolution as a function of both the in-plane tip position and the vertical tip–sample distance, without limitations due to a finite contact area or to stick-slip-related sudden jumps of tip apex atoms. After reviewing the measurement principle, the data obtained in this case study are utilized to illustrate the unique insight that the method offers. In particular, the local lateral forces that are expected to determine frictional resistance in the attractive regime are found to depend linearly on the normal force for small tip–sample distances.

Interplay of the tip–sample junction stability and image contrast reversal on a Cu(111) surface revealed by the 3D force field

Non-contact atomic force microscopy is used to measure the 3D force field on a dense-packed Cu(111) surface. An unexpected image contrast reversal is observed as the tip is moved towards the surface, with atoms appearing first as bright spots, whereas hollow and bridge sites turn bright at smaller tip–sample distances. Computer modeling is used to elucidate the nature of the image contrast. We find that the contrast reversal is essentially a geometrical effect, which, unlike in gold, is observable in Cu due to an unusually large stability of the tip–sample junction over large distances.

The accuracy of an optically supported fast approach solution for scanning probe microscopy (SPM)-measuring devices

Atomic force microscopy is a diagnostic technique offering sub-micron resolution in both lateral and vertical scales due to observation of interactions between a sharp scanning tip and the surface. Before the sample is scanned, the tip must approach the surface and this process is one of the most critical procedures in terms of the risk of damaging the tip and the sample. Automatic approach methods based on only near-interaction detection cannot provide a fully safe procedure when the distance of hundreds of micrometers is reduced to about 10 nm within a reasonable time period. In this paper we present a method of determining the tip–sample distance using advanced processing of the optical picture. The obtained information can be used during the coarse approach procedure. Eventually, the final approach can be done using near-field interaction detection, more carefully than before, due to the small tip–sample distance. The results of the tests of this method proving its efficiency are presented as well.

Contrast inversion in electrostatic force microscopy imaging of trapped charges: tip–sampledistance and dielectric constant dependence

We present a numerical and analytical study of the behavior of both electrostatic force andforce gradient created by a charge trapped below the surface of a dielectric on an atomicforce microscope tip as a function of the dielectric constant and tip–sample distance. Asexpected, the force decreases monotonously when the dielectric constant increases.However, a maximum in the dielectric constant dependence of the force gradient is found.This maximum occurs in the typical experimental parameters’ range and depends on thetip–sample distance and the sample thickness. The analytical study permits us tounderstand the physical origin of this phenomenon and is in good agreement withthe numerical simulation for small tip–sample distances. We also report a studyexemplifying a possible contrast inversion in electrostatic force microscopy (EFM)signals while scanning, at different heights, two charges trapped in a sample havingheterogeneous dielectric domains. In addition to this particular contrast inversion effect,this study can be considered as a way to gain insight into the mechanisms ofEFM image formation as a function of the dielectric constant and tip–sample.

Method to calculate electric fields at very small tip-sample distances in atomic force microscopy

A method to calculate electric magnitudes at very small tip-sample distances in atomic force microscopy is presented. We show that the method accurately calculates the electrostatic potential and vertical force for electrostatic force microscopy geometries that cannot be correctly simulated by the standard techniques. This technique can accurately calculate tip-sample distances four orders of magnitude smaller than the tip radius. We also demonstrate that, at this range, traditional techniques underestimate the electrostatic force in almost 30%. Finally, we calculate the jump-to-contact distance for geometries obtained from experiments that combine atomic force microscopy and scanning tunneling microscopy.

Contrast and Resolution of Nanowires in Electrostatic Force Microscopy

A detailed analysis of the contrast and lateral resolution between a dc-biased tip and metallic nanowires over a dielectric sample is presented. The theoretical technique used to analyze the interaction intrinsically includes the mutual polarization between the tip, the sample, and the metallic objects. A good selection of the dielectric constant of the sample is found to be critical since it can increase the contrast by more than an order of magnitude. On the other hand, the dielectric constant does not have any influence on the lateral resolution. In this case, the tip-sample distance and the tip radius are the most relevant parameters. For small tip-sample distances, the length of the nanowires must also be taken into account since it can produce differences of up to 20% in the lateral resolution.

Effect of antiferromagnetic layers on the spin-dependent transport in magnetic tunnel junctions

Spin-dependent tunneling between a layer-wise antiferromagnet grown on a ferromagnetic substrate and a ferromagnetic tip electrode was studied using spin-polarized scanning tunneling microscopy (Sp-STM). The systems under investigation consist of thin Mn films deposited on an Fe(001) substrate, a vacuum barrier, and an Fe-coated STM electrode. The asymmetry of the tunnel currents, measured over a wide voltage range, is nonzero but small, only a few percent and exhibits a change of sign as a function of voltage. Surface states are found to contribute significantly to the tunnel current only for small tip-sample distances. The comparison with model calculations for ballistic transport supports that the current asymmetry can be attributed mainly to the symmetry breaking at the surface, that is to the surface layer of the Mn film.

Light-modulated scanning tunneling spectroscopy for nanoscale imaging of surface photovoltage

Light-modulated scanning tunneling spectroscopy (LM–STS) is proposed as a useful method for investigating spatially resolved surface photovoltage (SR–SPV). LM–STS provides the dependences of SR–SPV on bias voltage under constant tip-sample distance simultaneously with the entire dark/illuminated I–V curves. With this method, it is shown that SPV of a metallic Si(111) surface can be bias-dependent and SPV at zero bias voltage for Si(001) can be tip-sample-distance-dependent under conditions of small tip-sample distance and high illumination intensity. The importance of the experimental condition for interpreting experimentally obtained SR–SPV was suggested.

Probing the shape of atoms in real space

The structure of single atoms in real space is investigated by scanning tunneling microscopy. Very high resolution is possible by a dramatic reduction of the tip-sample distance. The instabilities which are normally encountered when using small tip-sample distances are avoided by oscillating the tip of the scanning tunneling microscope vertically with respect to the sample. The surface atoms of Si(111)-(7 x 7) with their well-known electronic configuration are used to image individual samarium, cobalt, iron and silicon atoms. The resulting images resemble the charge density corresponding to 4f, 3d and 3p atomic orbitals.

Measuring electrical current during scanning probe oxidation

Electrical current is measured during scanning probe oxidation by performing force versus distance curves under the application of a positive sample voltage. It is shown how the time dependence of the current provides information about the kinetics of oxide growth under conditions in which the tip–surface distance is known unequivocally during current acquisition. Current measurements at finite tip–sample distance, in particular, unveil how the geometry of the meniscus influences its electrical conduction properties as well as the role of space charge at very small tip–sample distances.

Time-dependent oscillations of tunneling current on partially oxidized Si (111) surfaces

Clean and partially oxidized Si (111) surfaces were investigated by scanning tunneling microscopy and spectroscopy (STM/STS). In contrast to STS spectra measured on clean Si surfaces, time-dependent periodic oscillations of tunneling current were detected on partially oxidized Si surfaces. The STM current images of the clean Si surface with atomic resolution and the partially oxidized Si surfaces were analyzed by two-dimensional fast Fourier transformation (2D-FFT). The oscillation frequencies determined by STS and Fourier analysis are roughly multiples of a basic frequency of 30 Hz. The oscillations disappear at small tip–sample distance.

Probes for magnetic force microscopy imaging of soft magneticsamples

Magnetic force imaging of soft magnetic materials is often achallenge due to perturbing effects of the probe's stray field on themagnetic structure of the sample if the probe and the sample are too close.Especially with one commonly employed imaging mode, the lift mode, which isused in some commercial instruments, this problem is very severe, sincevery small tip-sample distances frequently occur during the imaging of thesample topography. In this study the effect is demonstrated by the imagingof arrays of micro-structured Permalloy films. It is shown thatperturbations can be diminished by depositing a mechanical spacer on top ofthe probe's magnetic layer, which ensures that a minimum distance betweenthe magnetic part of the probe and the sample is always kept.

Scanning tunneling microscopy of the atomic structure of the GaAs (110) surface at reduced tip sample separation

A scanning tunneling microscope was used to study the GaAs (110) surface at small tip-sample distances of about 2–3 Å. Atomically resolved images in this new mode referred to as near contact imaging mode revealed corrugation maxima both on the As and on the Ga surface sites at one sample voltage. These dual atom images differ significantly from the simultaneously measured conventional filled-state images at larger tunneling gap widths of 4–9 Å which show the As sublattice only. We interpret the dual atom images in terms of local tip induced modifications of the electronic surface states. These modifications are dragged along the surface with the scanning movement of the tip.

Chemical analysis of PtxNi1?x alloy single crystal surfaces by scanning tunnelling microscopy

Two STM investigations are presented, in which irregular tip conditions enable direct access to chemical and structural information of a surface on an atomic scale, otherwise invisible for the STM. They allow a study of surface ordering of a Pt25Ni75(111) crystal by chemical contrast between the alloy components, and a study of carbon superstructures on a Pt10Ni90(100) surface by simultaneous imaging of substrate lattice and carbon atoms. All these images were obtained at very low tunnelling resistances and thus at small tip-sample distances. A chemical interaction between the probably adsorbate covered tip and the sample is proposed to explain these images.