Tip-sample Junction Research Articles

Tip-Enhanced Thermal Expansion Force for Nanoscale Chemical Imaging and Spectroscopy in Photoinduced Force Microscopy

We investigate the tip-enhanced thermal expansion force for nanoscale chemical imaging and spectroscopy in the tip-sample junction. It is found, both theoretically and experimentally, that the tip-enhanced absorption of the near-field at the tip followed by sample expansion shows characteristic behaviors with respect to the sample thickness and the incident laser pulse width. The van der Waals interaction plays a major role in exerting a force on the tip from the thermally expanded sample. The force behavior of the photoinduced force microscope (PiFM) is compared with that of the existing photothermal-induced resonance technique (PTIR) to unravel the ambiguous thermal expansion force mechanism. The present study opens up new opportunities for enhancing the performance of optical nanoscopy and spectroscopy such as chemical imaging of nanobiomaterials and the local field mapping of photonic devices, including surface polaritons on van der Waals materials with the assistance of the thermal expansion of a functionalized tip.

STM-induced light emission enhanced by weakly coupled organic ad-layers

We analyze the light emission induced by the tunneling current flowing in a scanning tunneling microscopy experiment. In particular, we study the influence of organic ad-layers on the light emission on the initial monolayer of bismuth (Bi) on Cu(111) in comparison to the well-known case of organic ad-layers on Ag(111). On the Bi/Cu(111)-surface, we find that the scanning tunneling microscopy-induced light emission is considerably enhanced if an organic layer, e.g., the fullerene C60 or the perylene derivate perylene-tetracarboxylic-dianhydride, is introduced into the tip-sample junction. The enhancement can be correlated with a peculiarly weak interaction between the adsorbed molecules and the underlying Bi/Cu(111) substrate as compared to the Ag(111) substrate. This allows us to efficiently enhance and tune the coupling of the tunneling current to localized excitations of the tip-sample junction, which in turn couple to radiative decay channels.

A proximal retarding field analyzer for scanning probe energy loss spectroscopy

A compact proximal retarding field analyzer for scanning probe energy loss spectroscopy measurements is described. Using the scanning tunneling microscope (STM) tip as a field emission (FE) electron source in conjunction with this analyzer, which is placed at a glancing angle to the surface plane, FE sample current and electron reflectivity imaging may be performed simultaneously. This is demonstrated in measurements of Ag nanostructures prepared on graphite by electron-beam lithography, where a material contrast of 13% is observed, with a lateral resolution of 25 nm, between the silver and graphite in electron reflectivity images. Topological contrast mechanisms such as edge enhancement and shadowing are also observed, giving rise to additional features in the electron reflectivity images. The same instrument configuration has been used to measure electron energy loss spectra on bare graphite, where the zero loss peak, π band plasmon loss peak and secondary electron peaks are observed. Using this simple and compact analyzer an STM, with sufficient open access to the tip-sample junction, may easily be augmented to provide simultaneous elemental and topographic mapping, supplementing STM image measurements with FE sample current and electron reflectivity images, as well as electron energy loss spectroscopy measurements, in the same instrument.

Revealing the Presence of Mobile Molecules on the Surface

Mobile molecules crossing freely underneath the scanning tip of a scanning tunneling microscope create a uniform diffusive noise, making the identification of single molecules on the surface a challenge. We demonstrate the possibility of detecting mobile molecules on a surface by scanning tunneling microscopy and reveal how the diffusive noise is created. Additionally, we show that a molecule caught in the tip–sample junction allows us to explore the potential energy surface of the system. Finally, voltage pulses disturb the mobile molecules, causing the loss of that ability. They also allow the creation of islands on the surface. Most of the investigations were done for Co- and Cu-phthalocyanine (Pc) on Ag(100). However, the concept is limited to neither Pc molecules nor Ag(100), as shown for a different organic molecule, astraphloxin, on Cu(111).

(Invited) In-Situ Study of Electric Double Layer and Ionic Transport at the Solid/Liquid Interface Using Scanning Probe Microscopy

The structure and dynamics of the solid/liquid interface are of fundamental interest for electrochemical capacitors technology. Theories describing the electrical double layer (EDL) have been proposed; yet a full molecular level experimental description is lacking to fully understand its interfacial structures, dynamics and reactivity for both capacitive and pseudocapacitive processes. Due to the characteristic sizes of EDLs, this requires characterization techniques capable of resolving nm length scales. Atomic Force Microscopy (AFM) has recently shown great interest for characterizing the electrochemical storage processes at the nanoscale, where local phenomena at the tip-sample junction can be measured and analyzed. Here, we demonstrate the use of AFM based techniques to understand electrochemical processes at the solid/liquid interfaces. Using force spectroscopy reinforced with molecular dynamics (MD) simulations, we were able to image the EDL in a room temperature ionic liquid (RTIL) on electrified surfaces (1). In the dense layered structure of about 4 nanometers thick at graphite surfaces, single step edges were found to result only in short range changes in the ionic liquid ordering. At the same time, formations of dislocation type topological defects were observed on planar surfaces in a 3D manner. The influence of applied potential on the ions structure showed a complex cation/anion transition together with ionic reorientation, in excellent agreement with MD simulations. State-of-the-art AFM techniques in liquid environment also enabled to spatially resolve pseudocapacitive intercalation pathways. For the first time, we demonstrated how the ionic transport at the solid/liquid interface can be tracked using changes in mechanical properties when cations are inserted from an aqueous solution into 2D layered transition metal carbides (MXenes) (2). For Ti3C2, the insertion of ions results in a strong elastic coupling between MXene layers which results in a significant and measurable change of electrode stiffness. The tracking of spatially resolved elastic properties at the nanoscale allowed identifying fast ionic channels into MXene particles, strongly depending on the intercalated cation radius and charge. As the development of AFM techniques in liquid environment improves, achieving time-resolved tracking of ion storage in energy materials becomes more accessible, and can lead to tuning surface interactions for improved supercapacitor performance. This work was supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, and Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Measurements were performed at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences. References 1) J. M. Black, B. Okatan, G. Feng, P. T. Cummings, S. V. Kalinin, N. Balke. Nano Energy 2015, 15, 737-745. 2) J. Come, Y. Xie, M. Naguib, S. V. Kalinin, Y. Gogotsi, P. R. C. Kent, N. Balke. Advanced Energy Materials 2016, 1502290.

Scanning tunneling microscope light emission: Effect of the strong dc field on junction plasmons

The observed energies of the localized surface plasmons (LSPs) excited at the tip-sample junction of a scanning tunneling microscope, as identified by spectral peaks in the light output, are very significantly redshifted with respect to calculations that use standard optical data for the tip and sample material, gold in this case. We argue that this anomaly depends on the extreme field in the sub-nm tunneling proximity of the tip and the sample, across which a dc bias (1--2 V) is applied. Finite element modeling analysis is presented of a gold nanosphere-plane (NS-P) combination in tunneling proximity and, crucially, in the presence of a high static electric field ($\ensuremath{\sim}{10}^{9}\phantom{\rule{0.16em}{0ex}}\mathrm{V}/\mathrm{m}$). It is argued that the strong dc field induces nonlinear corrections to the dielectric function of the gold via the effect of a large background polarizability through the nonlinear, ${\ensuremath{\chi}}^{(3)}$ susceptibility contribution. When fed into the model system the modified optical data alters the LSP cavity modes of the NS-P system to indeed reveal a large redshift in energy compared to those of the virgin gold NS-P system. The net outcome may be regarded as equivalent to lowering the bulk plasmon energy, the physical interpretation being that the intense field of the tunneling environment leads to surface charge screening, effectively reducing the density of free electrons available to participate in the plasmon oscillations.

Quantitative scanning thermal microscopy of graphene devices on flexible polyimide substrates

A triple-scan scanning thermal microscopy (SThM) method and a zero-heat flux laser-heated SThM technique are investigated for quantitative thermal imaging of flexible graphene devices. A similar local tip-sample thermal resistance is observed on both the graphene and metal areas of the sample, and is attributed to the presence of a polymer residue layer on the sample surface and a liquid meniscus at the tip-sample junction. In addition, it is found that the tip-sample thermal resistance is insensitive to the temperature until it begins to increase as the temperature increases to 80 °C and exhibits an abrupt increase at 110 °C because of evaporation of the liquid meniscus at the tip-sample junction. Moreover, the variation in the tip-sample thermal resistance due to surface roughness is within the experimental tolerance except at areas with roughness height exceeding tens of nanometers. Because of the low thermal conductivity of the flexible polyimide substrate, the SThM measurements have found that the temperature rise in flexible graphene devices is more than one order of magnitude higher than those reported for graphene devices fabricated on a silicon substrate with comparable dimensions and power density. Unlike a graphene device on a silicon substrate where the majority of the electrical heating in the graphene device is conducted vertically through the thin silicon dioxide dielectric layer to the high-thermal conductivity silicon substrate, lateral heat spreading is important in the flexible graphene devices, as shown by the observed decrease in the average temperature rise normalized by the power density with decreasing graphene channel length from about 30 μm to 10 μm. However, it is shown by numerical heat transfer analysis that this trend is mainly caused by the size scaling of the thermal spreading resistance of the polymer substrate instead of lateral heat spreading by the graphene. In addition, thermoelectric effects are found to be negligible compared to Joule heating in the flexible graphene devices measured in this work.

Probing Redox Reactions at the Nanoscale with Electrochemical Tip-Enhanced Raman Spectroscopy.

A fundamental understanding of electrochemical processes at the nanoscale is crucial to solving problems in research areas as diverse as electrocatalysis, energy storage, biological electron transfer, and plasmon-driven chemistry. However, there is currently no technique capable of directly providing chemical information about molecules undergoing heterogeneous charge transfer at the nanoscale. Tip-enhanced Raman spectroscopy (TERS) uniquely offers subnanometer spatial resolution and single-molecule sensitivity, making it the ideal tool for studying nanoscale electrochemical processes with high chemical specificity. In this work, we demonstrate the first electrochemical TERS (EC-TERS) study of the nanoscale redox behavior of Nile Blue (NB), and compare these results with conventional cyclic voltammetry (CV). We successfully monitor the disappearance of the 591 cm(-1) band of NB upon reduction and its reversible reappearance upon oxidation during the CV. Interestingly, we observe a negative shift of more than 100 mV in the onset of the potential response of the TERS intensity of the 591 cm(-1) band, compared to the onset of faradaic current in the CV. We hypothesize that perturbation of the electrical double-layer by the TERS tip locally alters the effective potential experienced by NB molecules in the tip-sample junction. However, we demonstrate that the tip has no effect on the local charge transfer kinetics. Additionally, we observe step-like behavior in some TERS voltammograms corresponding to reduction and oxidation of single or few NB molecules. We also show that the coverage of NB is nonuniform across the ITO surface. We conclude with a discussion of methods to overcome the perturbation of the double-layer and general considerations for using TERS to study nanoscale electrochemical processes.

Plasmon-Mediated Electron Transport in Tip-Enhanced Raman Spectroscopic Junctions.

We combine experiment, theory, and first-principles-based calculations to study the light-induced plasmon-mediated electron transport characteristics of a molecular-scale junction. The experimental data show a nonlinear increase in electronic current perturbation when the focus of a chopped laser beam moves laterally toward the tip-sample junction. To understand this behavior and generalize it, we apply a combined theory of the electronic nonequilibrium formed upon decoherence of an optically triggered plasmon and first-principles transport calculations. Our model illustrates that the current via an adsorbed molecular monolayer increases nonlinearly as more energy is pumped into the junction due to the increasing availability of virtual molecular orbital channels for transport with higher injection energies. Our results thus illustrate light-triggered, plasmon-enhanced tunneling current in the presence of a molecular linker.

Point contact tunneling spectroscopy apparatus for large scale mapping of surface superconducting properties

We describe the design and testing of a point contact tunneling spectroscopy device that can measure material surface superconducting properties (i.e., the superconducting gap Δ and the critical temperature T(C)) and density of states over large surface areas with size up to mm(2). The tip lateral (X,Y) motion, mounted on a (X,Y,Z) piezo-stage, was calibrated on a patterned substrate consisting of Nb lines sputtered on a gold film using both normal (Al) and superconducting (PbSn) tips at 1.5 K. The tip vertical (Z) motion control enables some adjustment of the tip-sample junction resistance that can be measured over 7 orders of magnitudes from a quasi-ohmic regime (few hundred Ω) to the tunnel regime (from tens of kΩ up to few GΩ). The low noise electronic and LabVIEW program interface are also presented. The point contact regime and the large-scale motion capabilities are of particular interest for mapping and testing the superconducting properties of macroscopic scale superconductor-based devices.

Resolving Intra- and Inter-Molecular Structure with Non-Contact Atomic Force Microscopy.

A major challenge in molecular investigations at surfaces has been to image individual molecules, and the assemblies they form, with single-bond resolution. Scanning probe microscopy, with its exceptionally high resolution, is ideally suited to this goal. With the introduction of methods exploiting molecularly-terminated tips, where the apex of the probe is, for example, terminated with a single CO, Xe or H2 molecule, scanning probe methods can now achieve higher resolution than ever before. In this review, some of the landmark results related to attaining intramolecular resolution with non-contact atomic force microscopy (NC-AFM) are summarised before focussing on recent reports probing molecular assemblies where apparent intermolecular features have been observed. Several groups have now highlighted the critical role that flexure in the tip-sample junction plays in producing the exceptionally sharp images of both intra- and apparent inter-molecular structure. In the latter case, the features have been identified as imaging artefacts, rather than real intermolecular bonds. This review discusses the potential for NC-AFM to provide exceptional resolution of supramolecular assemblies stabilised via a variety of intermolecular forces and highlights the potential challenges and pitfalls involved in interpreting bonding interactions.

Atomic Force Microscopy Applications for Energy Storage: Detecting the Structure and Dynamics of Fluid/Electrode Interfaces

The structure of ions at the electrode-electrolyte interface and their transport properties within electrode materials control the properties of energy storage devices, such as the energy density and rate capability. Tremendous efforts have been undergone to achieve fundamental understanding of interfacial structures, dynamics and reactivity in order to improve the energy efficiency of electric devices. However characterization tools able to probe interaction lengths as small as few nanometers are needed. Atomic Force Microscopy (AFM) techniques are well suited to investigate the nanoscale properties of energy storage materials and to characterize the electrode-electrolyte interface due to the high lateral and vertical resolution and sensitivity to weak forces. In this presentation we will discuss several AFM techniques used to investigate ionic transport across and near solid-liquid interfaces. The sensitivity of the AFM probe to very weak forces allows determining the structure of an ionic liquid within the electrical double layer with molecular-level resolution [1]. The influence of applied potential on the ions structure can be detected, and the excellent agreement with molecular dynamics simulations provides insight into the ion layering reactivity. Moreover, the unmatched lateral resolution of scanning probe techniques also opens the pathway to mapping the structure of the electrical double layer in a 3D manner. We will also show that AFM is capable of characterizing the changes induced during the electrochemical storage at the nanoscale for Li-ion electrode materials, where local phenomena at the tip-sample junction can be measured, analyzed and interpreted in terms of local ionic transport [2]. Furthermore, Electrochemical Strain Microscopy (ESM) technique has been used to study the ionic transport from the electrolyte into a variety of materials in-operando. For example, an ESM study of activated carbons of different pore sizes revealed faster cation adsorption kinetics vs. anion in an ionic liquid electrolyte [ 3]. More recently, AFM was used to evidence the anomalously large volume contraction of a 2D titanium carbide (MXene) electrode upon cation intercalation [4]. As the development of AFM techniques in electrolyte environment improve, achieving time-resolved tracking of ion storage in energy materials becomes more accessible and can lead to tuning surface interactions to improve the energy density of supercapacitors. This work was supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, and Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Measurements were performed at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences. [1] J. M. Black, D. Walters, A. Labuda, G. Feng, P. C. Hillesheim, S. Dai, P. T. Cummings, S. V. Kalinin, R. Proksch, N. Balke, Nano Lett. 13 (2013) 5954-5960. [2] N. Balke, S. Jesse, A. N. Morozovska, E. Eliseev, D. W. Chung, Y. Kim, L. Adamczyk, R. E. García, N. Dudney, S.V. Kalinin, Nat. Nanotechnol. 5 (2010) 749-754. [3] J. M. Black, G. Feng, P. F. Fulvio, P. C. Hillesheim, S. Dai, Y. Gogotsi, P. T. Cummings, S. V. Kalinin, N. Balke, Adv. Energy Mater. 4 (2014). [4] J. Come, J. M. Black, M. Naguib, M. R. Lukatskaya, M. Beidaghi, A. J. Rondinone, D. J. Wesolowski, S. V. Kalinin, Y. Gogotsi, N. Balke, Nano Energy, accepted.

Practical Realization of Apertureless Scanning Near-field Optical Microscopy Using Hybrid Mode Atomic Force Microscopy

ABSTRACTThe local detection of optical response at the sub-wavelength scale on a materials’ surface is an invaluable characterization capability of apertureless scanning near-field optical microscopy (ASNOM). The technique is traditionally realized in amplitude modulation (AM) AFM mode. We have expanded this method by employing an alternative scheme for the detection of the near-field and far-field responses with the use of Hybrid (HD) AFM mode. In HD mode the sample is brought to the intermittent contact with the tip in a periodic oscillation at a frequency (1-2 kHz) much smaller than the probe resonance. In every oscillation cycle the probe deflection to a set-point value is used for surface profiling. For optical measurements the metal coated AFM tip was top-illuminated by visible laser. Simultaneously with surface profiling the light scattered from tip-sample junction was collected by a sensitive photomultiplier (PMT). The homodyne optical signal detection scheme was applied to discriminate near- and far-field optical components. Our method was verified by the studies of various materials (semiconductors, metals, polymers, etc.). The presented results show that the contrast of ASNOM images can be used for compositional mapping of heterogeneous systems.

Unique Determination of “Subatomic” Contrast by Imaging Covalent Backbonding

The origin of so-called "subatomic" resolution in dynamic force microscopy has remained controversial since its first observation in 2000. A number of detailed experimental and theoretical studies have identified different possible physicochemical mechanisms potentially giving rise to subatomic contrast. In this study, for the first time we are able to assign the origin of a specific instance of subatomic contrast as being due to the back bonding of a surface atom in the tip-sample junction.

Seebeck effect at the atomic scale.

The atomic variations of electronic wave functions at the surface and electron scattering near a defect have been detected unprecedentedly by tracing thermoelectric voltages given a temperature bias [Cho et al., Nat. Mater. 12, 913 (2013)]. Because thermoelectricity, or the Seebeck effect, is associated with heat-induced electron diffusion, how the thermoelectric signal is related to the atomic-scale wave functions and what the role of the temperature is at such a length scale remain very unclear. Here we show that coherent electron and heat transport through a pointlike contact produces an atomic Seebeck effect, which is described by the mesoscopic Seebeck coefficient multiplied by an effective temperature drop at the interface. The mesoscopic Seebeck coefficient is approximately proportional to the logarithmic energy derivative of local density of states at the Fermi energy. We deduced that the effective temperature drop at the tip-sample junction could vary at a subangstrom scale depending on atom-to-atom interaction at the interface. A computer-based simulation method of thermoelectric images is proposed, and a point defect in graphene was identified by comparing experiment and the simulation of thermoelectric imaging.

The effect of sample resistivity on Kelvin probe force microscopy

Kelvin probe force microscopy (KPFM) is a powerful technique to probe the local electronic structure of materials with atomic force microscopy. One assumption often made is that the applied bias drops fully in the tip-sample junction. We have recently identified an effect, the Phantom force, which can be explained by an ohmic voltage drop near the tip-sample junction causing a reduction of the electrostatic attraction when a tunneling current is present. Here, we demonstrate the strong effect of the Phantom force upon KPFM that can even produce Kelvin parabolae of opposite curvature.

18/20 T high magnetic field scanning tunneling microscope with fully low voltage operability, high current resolution, and large scale searching ability

We present a home-built 18/20 T high magnetic field scanning tunneling microscope (STM) featuring fully low voltage (lower than ±15 V) operability in low temperatures, large scale searching ability, and 20 fA high current resolution (measured by using a 100 GOhm dummy resistor to replace the tip-sample junction) with a bandwidth of 3.03 kHz. To accomplish low voltage operation which is important in achieving high precision, low noise, and low interference with the strong magnetic field, the coarse approach is implemented with an inertial slider driven by the lateral bending of a piezoelectric scanner tube (PST) whose inner electrode is axially split into two for enhanced bending per volt. The PST can also drive the same sliding piece to inertial slide in the other bending direction (along the sample surface) of the PST, which realizes the large area searching ability. The STM head is housed in a three segment tubular chamber, which is detachable near the STM head for the convenience of sample and tip changes. Atomic resolution images of a graphite sample taken under 17.6 T and 18.0001 T are presented to show its performance.

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.

Observation of Multiple Vibrational Modes in Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy Combined with Molecular-Resolution Scanning Tunneling Microscopy

Multiple vibrational modes have been observed for copper phthalocyanine (CuPc) adlayers on Ag(111) using ultrahigh vacuum (UHV) tip-enhanced Raman spectroscopy (TERS). Several important new experimental features are introduced in this work that significantly advance the state-of-the-art in UHV-TERS. These include (1) concurrent sub-nm molecular resolution STM imaging using Ag tips with laser illumination of the tip-sample junction, (2) laser focusing and Raman collection optics that are external to the UHV-STM that has two cryoshrouds for future low temperature experiments, and (3) all sample preparation steps are carried out in UHV to minimize contamination and maximize spatial resolution. Using this apparatus we have been able to demonstrate a TERS enhancement factor of 7.1 × 10(5). Further, density-functional theory calculations have been carried out that allow quantitative identification of eight different vibrational modes in the TER spectra. The combination of molecular-resolution UHV-STM imaging with the detailed chemical information content of UHV-TERS allows the interactions between large polyatomic molecular adsorbates and specific binding sites on solid surfaces to be probed with unprecedented spatial and spectroscopic resolution.

Antenna-based ultrahigh vacuum microwave frequency scanning tunneling microscopy system

The instrumental synthesis of high resolution scanning tunneling microscopy (STM) with the ability to measure differential capacitance with atomic scale resolution is highly desirable for fundamental metrology and for the study of novel physical characteristics. Microwave frequency radiation directed at the tip-sample junction in an STM system allows for such high-resolution differential capacitance information. This ability is particularly critical in ultrahigh vacuum environments, where the additional parameter space afforded by including a capacitance measurement would prove powerful. Here we describe the modifications made to a commercial scanning tunneling microscope to allow for broad microwave frequency alternating current scanning tunneling microscopy (ACSTM) in ultrahigh vacuum conditions using a relatively simple loop antenna and microwave difference frequency detection. The advantages of our system are twofold. First, the use of a removable antenna on a commercial STM prevents interference with other UHV processes while providing a simple method to retrofit any commercial UHV-STM with UHV-ACSTM capability. Second, mounting the microwave antenna on a translator allows for specific tuning of the system to replicate experimental conditions between samples, which is particularly critical in sensitive systems like organic thin films or single molecules where small changes in incident power can affect the results. Our innovation therefore provides a valuable approach to give nearly any commercial STM, be it an ambient or UHV system, the capability to measure atomic-scale microwave studies such as differential capacitance or even single molecule microwave response, and it ensures that experimental ACSTM conditions can be held constant between different samples.