Tip-sample System Research Articles

(Invited) Multiphysics Model of the sSNOM Spectroscopy: Can Near-Field Probes Report the Spectrum of Nanomaterials?

Last decade has been celebrated with a substantial increase of publications on imaging of nanomaterials with near-field probes. The latter include Tip-Enhanced Raman and/or Photoluminescence Spectroscopy (TERS/TEPL), various near-infrared (NIR) Atomic Force Microscopy (AFM) based methods, and several scattering Scanning Near-field optical methods (sSNOM). Using various coherent sources, often tunable in terms of excitation, these methods produce high spatial resolution images – near-field scanning maps, in terms of a pixel size claimed to be limited only by the size of the scanning tip.By design, TERS/TEPL detects a spectral response at each point of a map, which is, naively speaking, gives us a local probe of optical material properties within a single pixel (at the fixed excitation wavelength). NIR-AFM and sSNOM (leaving aside the FTIR mode) allow to obtain quasi-spectral information by tweaking the excitation. In this case, the collected (elastic) signal is restricted to the wavelength of the excitation.The essence of the near-field method is in breaking the laws of a plane-wave reflection from the sample, which requires a sub-wavelength separation between the tip and the sample and a strong enough near-field coupling between those. Still, it is routinely assumed that the tip is an “ideal probe” and its role is to “channel” the information about the local optical response to the detector. Up to date, little is known about the spectral response of the tip-sample system from the perspective of quantum physics.In this work, several aspects of the problem: mechanical motion of the scanning tip, electrodynamics of the excitation/scattered light, and quantum solid state description of the sample and the tip – are combined to obtain, within a simple model, a full picture of the near-field detection. The results, though not general and limited by the simplicity of model used for the sake of clarity of derivation, shed the light on the spectral response of a typical near-field microscope and describe effects, earlier called “artifacts”, as natural response for coupled tip-sample elementary excitations. Lastly, an application of the model for an analysis of spectra of nBN heteronanotubes (Y. Feng, et.al, ACS Nano 15, 5600, 2021) will be presented. Acknowledgement: This work has been partially supported by NSF MRSEC (DMR-2011839).Fig. 1. (left) Amplitude and phase of the near-field optical signal vs. the optical frequency: S(ω). (right) Same optical amplitude and phase vs. ω and the phase of mechanical motion of an AFM tip. Figure 1

Electrostatic force evolution during the tip-induced ferroelectric domain switching

Different from the signals of amplitude and phase lag in piezoresponse force microscopy (PFM) and piezoresponse force spectroscopy (PFS), the signals of the contact resonance frequency (f0) have not been clearly interpreted. Due to the complexity of the tip–sample system at the nanoscale, the relevant models and mechanisms were continuously improved in the past decades. The variation of f0 has been reported in a few previous studies, but the physical meaning of the unique variation in ferroelectric materials still needs to be investigated. Due to the imperfect tip–sample contact, the existence of a dielectric gap and screening charges causes the discontinuity in the conduction band. The evolution of the electrostatic force during the PFS measurements is significantly affected by the tip-induced ferroelectric domain evolution, resulting in the variation of the effective contact stiffness (k*) and f0. By involving the screening effect and tunneling effect in this model, the anomalies of the on-field f0 and off-field f0 can be well explained, respectively.

Interpreting Kelvin probe force microscopy on semiconductors by Fourier analysis

Kelvin probe force microscopy (KPFM) has become a popular surface scanning tool for functional materials and devices, and it has been widely interpreted by the contact potential difference (CPD) theory as the precedent Kelvin probe method. In this article, we developed a Fourier analysis framework for KPFM on the basis that the probe in KPFM is excited by a sinusoidal ac voltage, which is different from the classical Kelvin method. As a result, it was found that the KPFM signal will deviate from the CPD value if the sample charge quantity is not an odd function of the external bias, i.e., the CPD interpretation is invalid on those samples such as doped semiconductors. In order to further estimate the signal deviation from the CPD in the KPFM measurement on semiconductors, the tip–sample system was simulated as a one-dimensional metal–insulator–semiconductor capacitor using Fermi–Dirac statistics. The simulation results showed that the KPFM signals on doped semiconductors behave like those on an intrinsic one when the ac voltage is large, and therefore, the KPFM signal contrast on a pn junction could be flattened even if the sample surface has a clear CPD contrast without any Fermi level pinning due to surface states. Finally, possible ways for tuning KPFM operation parameters to suppress the signal deviation effect were also discussed.

A weak microwave detection method based on resonator noise suppression technology

We put forward a simple method to test extremely weak microwaves by a self-made near field microwave detection system, which is made up of a λ/4 tunable resonance cavity and tungsten tip. By increasing the number of cycles of coupling loops, the technical problem of impedance matching is solved, and the background noise is suppressed. Hence, we can obtain a good sensitivity and spatial resolution of microwave power testing. In this way, the gigahertz electromagnetic wave is loaded to the tip-sample system efficiently, and the thermal parameter measurement of a chip with picowatt level can be obtained by non-contact scanning mode.

Investigation of the interaction between carbon nanotube tip and silicon sample through molecular dynamic simulation

Atomic Force Microscopy (AFM) has become a standard tool in experimental surface metrology, revealing atomic scale features of the surfaces. Carbon nanotube represents ideal structure for AFM tips because of their tiny diameter, high aspect ratio and high strength. This article employs molecular dynamics (MD) simulation approach to investigate the atomic-scale interaction between the AFM tips represented by carbon nanotube and sample. The interaction force between two different types of single walled carbon nanotube (SWCNT) tips including open ended carbon nanotube and capped carbon nanotube while the deformation mechanism during the measurement was discussed in detail. Firstly, the interaction force between SWCNT tips and sample in contact mode was simulated by using hybrid potential. The stiffness of the two different types of single walled carbon nanotubes were verified. The simulation results indicate that capped SWCNT tip is more prone to tilt than open ended SWCNT tip, but the ability to resist deformation is stronger. Then the dynamic model of tip-sample system in tapping mode was established. The relationship between the tip of load and the distance in the process of approaching to the sample was obtained by curve fitting. The numerical solution was achieved by solving the dynamic equation and frequency spectrum was obtained with the method of Fourier transforms.

In situ characterization of nanoscale contaminations adsorbed in air using atomic force microscopy.

Under ambient conditions, surfaces are rapidly modified and contaminated by absorbance of molecules and a variety of nanoparticles that drastically change their chemical and physical properties. The atomic force microscope tip–sample system can be considered a model system for investigating a variety of nanoscale phenomena. In the present work we use atomic force microscopy to directly image nanoscale contamination on surfaces, and to characterize this contamination by using multidimensional spectroscopy techniques. By acquisition of spectroscopy data as a function of tip–sample voltage and tip–sample distance, we are able to determine the contact potential, the Hamaker constant and the effective thickness of the dielectric layer within the tip–sample system. All these properties depend strongly on the contamination within the tip–sample system. We propose to access the state of contamination of real surfaces under ambient conditions using advanced atomic force microscopy techniques.

Water desorption in Kelvin-probe force microscopy: a generic model

Nanoparticles or similar, nanoscale objects such as proteins or biological fibrils usually have to be deposited from aqueous suspension onto a solid support surface for further characterization by atomic force microscopy (AFM) and related methods such as Kelvin-probe force microscopy (KFM). Here we show, on the examples of functionalized nanoparticles and collagen fibrils, that water desorption after sample preparation affects their electrostatic potential determined by KFM in a predictable manner. We explain this effect with a simple, analytical model based on the capacitance of the partially dielectric-filled tip-sample system. We also propose practical measures to avoid false interpretation of electrical AFM-based experiments. As the phenomenon is very generic it may have significant implications in the application of AFM to nanoparticles and other nanostructures including biological ones.

Unscrambling Structured Chirality with Structured Light at the Nanoscale Using Photoinduced Force

We introduce a microscopy technique that facilitates the prediction of spatial features of chirality of nanoscale samples by exploiting photo-induced optical force exerted on an achiral tip in the vicinity of the test specimen. The tip-sample interactive system is illuminated by structured light to probe both the transverse and longitudinal (with respect to the beam propagation direction) components of the sample magnetoelectric polarizability as the manifestation of its sense of handedness, i.e., chirality. We specifically prove that although circularly polarized waves are adequate to detect the transverse polarizability components of the sample, they are unable to probe the longitudinal component. To overcome this inadequacy, we propose a judiciously engineered combination of radially and azimuthally polarized beams, as optical vortices possessing pure longitudinal electric and magnetic field components along their vortex axis, respectively, hence probing longitudinal chirality. The proposed technique may benefit branches of science like stereochemistry, biomedicine, physical and material science, and pharmaceutics

Exploring load, velocity, and surface disorder dependence of friction with one-dimensional and two-dimensional models

The effect of surface disorder, load, and velocity on friction between a single asperity contact and a model surface is explored with one-dimensional and two-dimensional Prandtl–Tomlinson (PT) models. We show that there are fundamental physical differences between the predictions of one-dimensional and two-dimensional models. The one-dimensional model estimates a monotonic increase in friction and energy dissipation with load, velocity, and surface disorder. However, a two-dimensional PT model, which is expected to approximate a tip–sample system more realistically, reveals a non-monotonic trend, i.e. friction is inert to surface disorder and roughness in wearless friction regime. The two-dimensional model discloses that the surface disorder starts to dominate the friction and energy dissipation when the tip and the sample interact predominantly deep into the repulsive regime. Our numerical calculations address that tracking the minimum energy path and the slip-stick motion are two competing effects that determine the load, velocity, and surface disorder dependence of friction. In the two-dimensional model, the single asperity can follow the minimum energy path in wearless regime; however, with increasing load and sliding velocity, the slip-stick movement dominates the dynamic motion and results in an increase in friction by impeding tracing the minimum energy path. Contrary to the two-dimensional model, when the one-dimensional PT model is employed, the single asperity cannot escape to the minimum energy minimum due to constraint motion and reveals only a trivial dependence of friction on load, velocity, and surface disorder. Our computational analyses clarify the physical differences between the predictions of the one-dimensional and two-dimensional models and open new avenues for disordered surfaces for low energy dissipation applications in wearless friction regime.

Phase transformation in AFM silicon tips

We confirmed the occurrence of phase transformations in an atomic force microscopy silicon tip during loading and unloading experiments performed on a polycrystalline Ti sample. The influence of the phase transformations on the effective mechanical and electrical properties of the tip was observed with the help of load–unload curves measured simultaneously for the tip-sample contact stiffness k* and the effective electrical resistance of the system Reff. We used the atomic force acoustic microscopy (AFAM) method to determine the values of k*. To measure the changes in Reff, we combined a high voltage source/measure unit with the existing AFAM system. The data obtained showed that the phase transformation from Si-I to Si-II is preceded by other structural changes such as formation of distorted diamond structures and formation of Si-III. This conclusion was reached after observing a small hysteretic behavior in the load–unload stiffness curve accompanied by only very small changes in the resistance of the tip-sample system occurring on the unloading. The coinciding of a sudden increase in the values of the contact stiffness with a decrease in the resistance of the system indicated that the formation of metallic Si-II occurred in the subsequent measurements. The interpretation of our results found confirmation in the results of molecular dynamics and atomistic simulations performed for silicon under nanoindentation experiments.

(Invited) Electrical Atomic Force Microscopy for 2D Transition Metal Dichalcogenide Materials

As Si-based electronic devices are approaching their projected scaling limits, layered two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs) are extensively studied as potential new channel materials and fundamental building blocks of emerging sensors and devices. In this context, MoS2 and WSe2 to name a few, are now available for deposition using different top down approaches. However, their outstanding properties are often degraded during the fabrication processes required for the device integration. For example, 2D TMDs selective growth, patterning and tuning of the electronics properties still remain elusive. Here, electrical atomic force microscopy (AFM) and beam analysis techniques are used to develop a framework of analysis for 2D materials. The latter is applied to understand the local properties of MoS2 comparing pristine material and structures which are selectively grown and patterned. Different growth techniques are investigated. After modelling the tip-sample contact system, we assess the impact of the plasma-induced damages combining the electrical AFMs and Auger emission spectroscopy. We study the local electrical properties of grain boundaries and their transport in patterned MoS2 structures for field-effect-transistor devices by conductive atomic force microscopy (C-AFM).

(Invited) Electrical Atomic Force Microscopy for 2D Transition Metal Dichalcogenide Materials

As Si-based electronic devices are approaching their projected scaling limits, layered two-dimensional (2D) materials such as transition metal dichalcogenides (TMDs) are extensively studied as potential new channel materials and fundamental building blocks of emerging sensors and devices.[1-2] In this context, MoS2, WS2 and WSe2 to name a few, are now available for deposition trough different top down approaches. However, their outstanding properties are often degraded during the fabrication processes required for the device integration. For example, the selective growth of 2D TMDs their patterning and the electronics properties fine tuning still remain elusive. Here we report on electrical atomic force microcopy (AFM) and beam analysis techniques which are used to develop a framework of analysis for 2D materials. The latter is applied to understand the local properties of MoS2 comparing pristine material and structures which are selectively grown and patterned.[3] Different growth techniques are investigated. After modelling the tip-sample contact system, we assess the impact of the plasma-induced damages combining the electrical AFMs and Auger emission spectroscopy. We study the local electrical properties of grain boundaries and their transport respectively in pristine and patterned structures for FET devices by conductive atomic force microscopy (C-AFM). [1] G. Fiori, F. Bonaccorso, G. Iannaccone, T. Palacios, D. Neumaier, A. Seabaugh, S. K. Banerjee, and L. Colombo, “Electronics based on two-dimensional materials,” Nat. Nanotechnol., vol. 9, no. 10, pp. 768–779, 2014.[2] Desai, S. B., Madhvapathy, S. R., Sachid, A. B., Llinas, J. P., Wang, Q., Ahn, G. H., Javey, A. (2016). MoS 2 transistors with 1-nanometer gate lengths, 354(6308), 2–6.[3] Chiappe, D., Asselberghs, I., Sutar, S., Iacovo, S., Afanas’Ev, V., Stesmans, A., … Thean, A. (2016). Controlled Sulfurization Process for the Synthesis of Large Area MoS2 Films and MoS2/WS2 Heterostructures. Advanced Materials Interfaces, 3(4), 1–10.

Quantitative microwave impedance microscopy with effective medium approximations

Microwave impedance microscopy (MIM) is a scanning probe technique to measure local changes in tip-sample admittance. The imaginary part of the reported change is calibrated with finite element simulations and physical measurements of a standard capacitive sample, and thereafter the output ΔY is given a reference value in siemens. Simulations also provide a means of extracting sample conductivity and permittivity from admittance, a procedure verified by comparing the estimated permittivity of polytetrafluoroethlyene (PTFE) to the accepted value. Simulations published by others have investigated the tip-sample system for permittivity at a given conductivity, or conversely conductivity and a given permittivity; here we supply the full behavior for multiple values of both parameters. Finally, the well-known effective medium approximation of Bruggeman is considered as a means of estimating the volume fractions of the constituents in inhomogeneous two-phase systems. Specifically, we consider the estimation of porosity in carbide-derived carbon, a nanostructured material known for its use in energy storage devices.

First-Principles Atomic Force Microscopy Image Simulations with Density Embedding Theory.

We present an efficient first-principles method for simulating noncontact atomic force microscopy (nc-AFM) images using a "frozen density" embedding theory. Frozen density embedding theory enables one to efficiently compute the tip-sample interaction by considering a sample as a frozen external field. This method reduces the extensive computational load of first-principles AFM simulations by avoiding consideration of the entire tip-sample system and focusing on the tip alone. We demonstrate that our simulation with frozen density embedding theory accurately reproduces full density functional theory simulations of freestanding hydrocarbon molecules while the computational time is significantly reduced. Our method also captures the electronic effect of a Cu(111) substrate on the AFM image of pentacene and reproduces the experimental AFM image of Cu2N on a Cu(100) surface. This approach is applicable for theoretical imaging applications on large molecules, two-dimensional materials, and materials surfaces.

Structural, surface potential and optical studies of AlGaN based double heterostructures irradiated by 120 MeV Si9+ swift heavy ions

The Al0.33Ga0.77N/Al0.14Ga0.86N based double heterostructure was irradiated using Si9+ ion at room temperature (RT) and liquid nitrogen temperature (LNT) with four dissimilar ion fluence. The effect of Si9+ ion irradiation in dislocation densities and in-plane strain of GaN layer were discussed. The in-plane strain values of AlxGa1-xN layers were calculated from asymmetric reciprocal space mapping (RSM). The surface modification and the variation in phase shift on Al0.33Ga0.77N surfaces due to the irradiation were measured by Electrostatic Force Microscopy (EFM). The capacitance of the tip-sample system was determined from EFM. The band edge emissions of heterostructures were measured by the room temperature phototluminescence (PL) and the shift in the Al0.14Ga0.86N active layer emission peaks towards the low energy side at low fluence ion irradiation has been noted.

The weight function for charges—A rigorous theoretical concept for Kelvin probe force microscopy

A comprehensive discussion of the physical origins of Kelvin probe force microscopy (KPFM) signals for charged systems is given. We extend the existing descriptions by including the open-loop operation mode, which is relevant when performing KPFM in electrolyte solutions. We define the contribution of charges to the KPFM signal by a weight function, which depends on the electric potential and on the capacitance of the tip-sample system. We analyze the sign as well as the lateral decay of this weight function for different sample types, namely, conductive samples as well as dielectric samples with permittivities both larger and smaller than the permittivity of the surrounding medium. Depending on the surrounding medium the sign of the weight function can be positive or negative, which can lead to a contrast inversion for single charges. We furthermore demonstrate that the KPFM signal on thick dielectric samples can scale with the sample size—rendering quantitative statements regarding the charge density challenging. Thus, knowledge on the weight function for charges is crucial for qualitative as well as quantitative statements regarding charges beneath the tip.

Subsurface imaging of metal lines embedded in a dielectric with a scanning microwave microscope**Official contribution of the National Institute of Standards and Technology; not subject to copyright in the United States.

We demonstrate the ability of the scanning microwave microscope (SMM) to detect subsurface metal lines embedded in a dielectric film with sub-micrometer resolution. The SMM was used to image 1.2 μm-wide Al–Si–Cu metal lines encapsulated with either 800 nm or 2300 nm of plasma deposited silicon dioxide. Both the reflected microwave (S11) amplitude and phase shifted near resonance frequency while the tip scanned across these buried lines. The shallower line edge could be resolved within 900 nm ± 70 nm, while the deeper line was resolved within 1200 nm ± 260 nm. The spatial resolution obtained in this work is substantially better that the 50 μm previously reported in the literature. Our observations agree very well with the calculated change in peak frequency and phase using a simple lumped element model for an SMM with a resonant transmission line. By conducting experiments at various eigenmodes, different contrast levels and signal-to-noise ratios have been compared. With detailed sensitivity studies, centered around 9.3 GHz, it has been revealed that the highest amplitude contrast is obtained when the probe microwave frequency matches the exact resonance frequency of the experimental setup. By RLC equivalent circuit modeling of the tip-sample system, two competing effects have been identified to account for the positive and negative S11 amplitude and phase contrasts, which can be leveraged to further improve the contrast and resolution.

Structural and electrical characteristics of GaN, n-GaN and A1xGa1−xN

GaN, n-GaN (n∼̴ 1 × 1018/cm3) and AlxGa1−xN (x = 0. 14, 0.26, 0.45) were epitaxially grown by metal organic chemical vapor deposition (MOCVD) on c-plane sapphire substrate. The composition and thickness of AlxGa1-xN layers were found by employing ω-2θ scan with simulation fit. The dislocation densities of epilayers were determined from symmetric and asymmetric planes of ω-scan rocking curve. The (105) plane reciprocal space mapping was employed to measure the in-plane strain (εxx) of the AlxGa1−xN layers grown on GaN. The in-plane strain of GaN and n-GaN were calculated from 2θ scan. The second derivative of the capacitance of the AFM tip-sample system was determined using electrostatic force microscopy (EFM). Photon decay time was measured using time resolved photoluminescence and the results correlated with structural property of the epilayers. The Schottky behavior of Ni/Au contacts on grown layers were studied using current–voltage (I–V) measurements. The Schottky barrier height of each device was compared with the edge dislocation density of the layers.

Insights into Kelvin probe force microscopy data of insulator-supported molecules

We present a detailed analysis and understanding of Kelvin probe force microscopy (KPFM) data for a system of point charges in a vacuum-dielectric tip-sample system. Explicit formulae describing the KPFM signal $\ensuremath{\Delta}V$ are derived for the two KPFM operation modes, namely amplitude modulation and frequency modulation (FM). The formulae allow for a physical interpretation of the resulting KPFM signal, reveal contributing parameters, and especially disclose an additive behavior. We numerically evaluate these equations and show exemplary KPFM slice data for a single point charge. The theoretical analysis is complemented by two-dimensional FM-KPFM maps obtained experimentally on 2,5-dihydrobenzoic acid on calcite $(1\phantom{\rule{0.59998pt}{0ex}}0\phantom{\rule{0.59998pt}{0ex}}.\phantom{\rule{0.59998pt}{0ex}}4)$. The molecules assemble in two coexisting phases understood as protonated and deprotonated molecules. The two-dimensional maps reveal a convex shape of the KPFM signal $\ensuremath{\Delta}V$ across the islands and a clear difference between the two phases is revealed. Furthermore, we apply the theoretically suggested difference strategy to extract the molecular component from the measured total KPFM signal.

Accurate formula for dissipative interaction in frequency modulation atomic force microscopy

Much interest has recently focused on the viscosity of nano-confined liquids. Frequency modulation atomic force microscopy (FM-AFM) is a powerful technique that can detect variations in the conservative and dissipative forces between a nanometer-scale tip and a sample surface. We now present an accurate formula to convert the dissipation power of the cantilever measured during the experiment to damping of the tip-sample system. We demonstrated the conversion of the dissipation power versus tip-sample separation curve measured using a colloidal probe cantilever on a mica surface in water to the damping curve, which showed a good agreement with the theoretical curve. Moreover, we obtained the damping curve from the dissipation power curve measured on the hydration layers on the mica surface using a nanometer-scale tip, demonstrating that the formula allows us to quantitatively measure the viscosity of a nano-confined liquid using FM-AFM.