Tip-sample Junction Research Articles

Infrared photoinduced force near-field spectroscopy of silicon carbide

Silicon carbide (SiC) is a promising microelectronic semiconductor with an infrared response that depends on factors such as polytype, growth conditions, and structuring. Recent advances in infrared nano-imaging techniques provide new opportunities to characterize thin films at the nanoscale. We performed tip-enhanced near-field spectral characterization on various 4H-SiC sample surfaces using photoinduced force microscopy (PiFM). The near-field spectra feature a peak associated with a surface phonon polariton (SPhP) resonance, which proves to be highly sensitive to the material’s surface quality. Interestingly, we found that the power absorption at the tip-sample junction, enhanced by the effective tip polarizability, can account for these PiFM spectra, resulting in a blend of both dispersive and dissipative spectral line shapes. As a potential application, a junction-field effect transistor was imaged to evaluate the technique’s ability to map the surface of different p- and n-doped areas, demonstrating high sensitivity and spatial resolution compared to Raman analysis.

Modification of transition pathways in polarized resonance Raman spectroscopy for carbon nanotubes by highly confined near-field light

We observed a modification of transition pathways in polarized resonance Raman spectroscopy during tip-enhanced Raman spectroscopy (TERS) analysis of metallic carbon nanotubes (CNTs). At a spatial resolution reaching up to the sub-nanometer regime, the signal intensity of the typical D-band is observed to be even higher than the intensity of the G-band all over the probed CNTs in TERS imaging. The measured D-band is attributed to the non-vertical transitions of electrons in k-space that are facilitated by highly confined near-field light at the tip–sample junction of our scanning tunneling microscope based TERS system. The D-band signal was observed even when the CNTs were excited by light polarized perpendicular to the tube axis that corresponds to electronic excitations between different cutting line numbers of a CNT. By combining the electron pathways brought about by both the near-field light and its polarization, we found a unique optical transition of electrons of CNTs in near-field Raman spectroscopy.

An ultracompact scanning tunneling microscope within a Φ 10 piezo tube in a 20T superconducting magnet.

Low-temperature scanning tunneling microscopy and spectroscopy (STM/S) help to better understand the fundamental physics of condensed matter. We present an ultracompact STM within a Φ 10 piezo tube in a 20T superconducting magnet. The carefully cut piezo tube contains the STM's coarse-positioning assembly. Loading an STM tip-sample mechanical loop into the piezo tube with special cut openings enables an ultracompact pencil-size dimension down to Φ 10mm, in which fine-machined nonmagnetic parts are assembled to enable slide-stick motion and xyz-scanning procedures. The small size leads to a higher resonant frequency, a typical feature of a rigid STM instrument, increasing its vibration immunity. Scanning by moving the sample while keeping the tip stationary improves the stability of the tip-sample junction compared to moving the tip. Taking advantage of its high-field compatibility and rigid design, our STM captures the atomically resolved topography of highly oriented pyrolytic graphite (HOPG) at 1.5K and in magnetic fields up to 17T. The topography of graphene lattice and graphite is simultaneously recorded on an atomic terrace of HOPG, unveiling a modified local charge density at a surface defect. The superconducting energy gaps of layered type-II superconductors NbSe2 and PdBi2 are well resolved through dI/dV tunneling spectra at sub-2K. Our unique STM is highly suitable for potential STM/S applications in world-class high-field facilities where the strong magnetic field can exceed 30T.

Efficient and Continuous Carrier-Envelope Phase Control for Terahertz Lightwave-Driven Scanning Probe Microscopy.

The fundamental understanding of quantum dynamics in advanced materials requires precise characterization at the limit of spatiotemporal resolution. Ultrafast scanning tunneling microscopy is a powerful tool combining the benefits of picosecond time resolution provided by single-cycle terahertz (THz) pulses and atomic spatial resolution of a scanning tunneling microscope (STM). For the selective excitation of localized electronic states, the transient field profile must be tailored to the energetic structure of the system. Here, we present an advanced THz-STM setup combining multi-MHz repetition rates, strong THz near fields, and continuous carrier-envelope phase (CEP) control of the transient waveform. In particular, we employ frustrated total internal reflection as an efficient and cost-effective method for precise CEP control of single-cycle THz pulses with >60% field transmissivity, high pointing stability, and continuous phase shifting of up to 0.75 π in the far and near field. Efficient THz generation and dispersion management enable peak THz voltages at the tip-sample junction exceeding 20 V at 1 MHz and 1 V at 41 MHz. The system comprises two distinct THz generation arms, which facilitate individual pulse shaping and amplitude modulation. This unique feature enables the flexible implementation of various THz pump-probe schemes, thereby facilitating the study of electronic and excitonic excited-state propagation in nanostructures and low-dimensional materials systems. Scalability of the repetition rate up to 41 MHz, combined with a state-of-the-art low-temperature STM, paves the way toward the investigation of dynamical processes in atomic quantum systems at their native length and time scales.

A Novel Atomically Resolved Scanning Tunneling Microscope Capable of Working in Cryogen-Free Superconducting Magnet.

We present a novel homebuilt scanning tunneling microscope (STM) with atomic resolution integrated into a cryogen-free superconducting magnet system with a variable temperature insert. The STM head is designed as a nested structure of double piezoelectric tubes (PTs), which are connected coaxially through a sapphire frame whose top has a sample stage. A single shaft made of tantalum, with the STM tip on top, is held firmly by a spring strip inside the internal PT. The external PT drives the shaft to the tip-sample junction based on the SpiderDrive principle, and the internal PT completes the subsequent scanning and imaging work. The STM head is simple, compact, and easy to assemble. The excellent performance of the device was demonstrated by obtaining atomic-resolution images of graphite and low drift rates of 30.2 pm/min and 41.4 pm/min in the X-Y plane and Z direction, respectively, at 300K. In addition, we cooled the sample to 1.6 K and took atomic-resolution images of graphite and NbSe2. Finally, we performed a magnetic field sweep test from 0 T to 9 T at 70 K, obtaining distinct graphite images with atomic resolution under varying magnetic fields. These experiments show our newly developed STM's high stability, vibration resistance, and immunity to high magnetic fields.

Development of low-temperature and ultrahigh-vacuum photoinduced force microscopy.

In this paper, we develop optical and electronic systems for photoinduced force microscopy (PiFM) that can measure photoinduced forces under low temperature and ultrahigh vacuum (LT-UHV) without artifacts. For our LT-UHV PiFM, light is irradiated from the side on the tip-sample junction, which can be adjusted through the combination of an objective lens inside the vacuum chamber and a 90° mirror outside the vacuum chamber. We measured photoinduced forces due to the electric field enhancement between the tip and the Ag surface, and confirmed that photoinduced force mapping and measurement of photoinduced force curves were possible using the PiFM that we developed. The Ag surface was used to measure the photoinduced force with high sensitivity, and it is effective in enhancing the electric field using the plasmon gap mode between the metal tip and the metal surface. Additionally, we confirmed the necessity of Kelvin feedback during the measurement of photoinduced forces, to avoid artifacts due to electrostatic forces, by measuring photoinduced forces on organic thin films. The PiFM, operating under low temperature and ultrahigh vacuum developed here, is a promising tool to investigate the optical properties of various materials with very high spatial resolution.

Externally-triggerable optical pump-probe scanning tunneling microscopy with a time resolution of tens-picosecond

Photoinduced carrier dynamics of nanostructures play a crucial role in developing novel functionalities in advanced materials. Optical pump-probe scanning tunneling microscopy (OPP-STM) represents distinctive capabilities of real-space imaging of such carrier dynamics with nanoscale spatial resolution. However, combining the advanced technology of ultrafast pulsed lasers with STM for stable time-resolved measurements has remained challenging. The recent OPP-STM system, whose laser-pulse timing is electrically controlled by external triggers, has significantly simplified this combination but limited its application due to nanosecond temporal resolution. Here we report an externally-triggerable OPP-STM system with a temporal resolution in the tens-picosecond range. We also realize the stable laser illumination of the tip-sample junction by placing a position-movable aspheric lens driven by piezo actuators directly on the STM stage and by employing an optical beam stabilization system. We demonstrate the OPP-STM measurements on GaAs(110) surfaces, observing carrier dynamics with a decay time of sim 170 ps and revealing local carrier dynamics at features including a step edge and a nanoscale defect. The stable OPP-STM measurements with the tens-picosecond resolution by the electrical control of laser pulses highlight the potential capabilities of this system for investigating nanoscale carrier dynamics of a wide range of functional materials.

Atomically resolved low-temperature scanning tunneling microscope operating in a 22 T water-cooled magnet

We present the design and construction of a nonmetallic tip-sample mechanical loop featured Scanning Tunneling Microscope (STM) that operates in a 22 T water-cooled magnet at a low temperature of l.8 K. The STM head mainly consists of a spider-drive motor, stand-alone scanner, moveable sapphire sample holder, and sapphire frame. All parts exist in the tip-sample mechanical loop are made of sapphire to reduce the interference from high magnetic fields. Except for the necessary movement of the tip and scanner, all STM parts are stationary. More importantly, the tip-sample mechanical loop is separate from the motor after detecting the tunneling current, which helps prevent the high voltage signal interference from entering the tip-sample junction, leading to a high stable imaging. A Janis liquid helium cryostat is used to obtain a variable temperature range from 1.8 K to 300 K, and the STM head is cooled down via helium exchange gas. The STM head hangs at the bottom of a probe with a two-stage spring suspension to prevent the huge vibration generated by the water-cooled magnet from entering the tip-sample junction. The performance is demonstrated by atomically resolved STM images of graphite surface at 0 T and 22.8 T under room temperature. Furthermore, the obtained atomic-resolution images of NbSe2 at 1.8 K and 22 T, as well as high-resolution dI/dV spectrums at temperatures from 1.8 K to 8.5 K and magnetic fields from 0 T to 22 T are displayed. This is the first STM capable of atomic-resolution imaging and dI/dV measurement at 1.8 K in a 22 T water-cooled magnet. The high immunity to the magnetic field makes the nonmetallic tip-sample mechanical loop widely useable for atomic-resolution STM imaging in ultra-high magnetic field conditions.

Tip-Induced and Electrical Control of the Photoluminescence Yield of Monolayer WS2.

The photoluminescence (PL) of monolayer tungsten disulfide (WS2) is locally and electrically controlled using the nonplasmonic tip and tunneling current of a scanning tunneling microscope (STM). The spatial and spectral distribution of the emitted light is determined using an optical microscope. When the STM tip is engaged, short-range PL quenching due to near-field electromagnetic effects is present, independent of the sign and value of the bias voltage applied to the tip-sample tunneling junction. In addition, a bias-voltage-dependent long-range PL quenching is measured when the sample is positively biased. We explain these observations by considering the native n-doping of monolayer WS2 and the charge carrier density gradients induced by electron tunneling in micrometer-scale areas around the tip position. The combination of wide-field PL microscopy and charge carrier injection using an STM opens up new ways to explore the interplay between excitons and charge carriers in two-dimensional semiconductors.

Disentangling the electronic structure of an adsorbed graphene nanoring by scanning tunneling microscopy

Graphene nanorings are promising model structures to realize persistent ring currents and Aharonov–Bohm effect at the single molecular level. To investigate such intriguing effects, precise molecular characterization is crucial. Here, we combine low-temperature scanning tunneling imaging and spectroscopy with CO functionalized tips and algorithmic data analysis to investigate the electronic structure of the molecular cycloarene C108 (graphene nanoring) adsorbed on a Au(111) surface. We demonstrate that CO functionalized tips enhance the visibility of molecular resonances, both in differential conductance spectra and in real-space topographic images. Comparing our experimental data with ab-initio density functional theory reveals a remarkably precise agreement of the molecular orbitals and enables us to disentangle close-lying molecular states only separated by 50 meV at an energy of 2 eV below the Fermi level. We propose this combination of techniques as a promising new route for a precise electronic characterization of complex molecules and other physical properties which have electronic resonances in the tip-sample junction.

Scaling of Time-Dependent Tunneling Current in Terahertz Scanning Tunneling Microscopes

Terahertz scanning tunneling microscopy (THz-STM) has enabled spatiotemporal imaging with femtosecond temporal resolution and angstrom spatial resolution. In THz-STMs, the junction bias is modulated by coupling THz pulses, which results in transient voltage bias and extremely high transient tunneling currents. In order to have efficient imaging of the sample surface, it is important to understand the nonlinear tunneling current response and its parametric dependence. In this work, we theoretically investigate the basic scaling of rectified electrons in a THz-STM junction. We use a self-consistent quantum model that includes both space-charge potential and exchange-correlation potential, which were ignored in previous studies. Since THz-STMs are operated at high transient voltage in field emission regime, the effects of exchange-correlation potential become crucial. We validate our calculation with recently reported experimental data and investigate the rectification property of the tip-sample junction for different parameters. We find that the time-dependent tunneling current and the electron transport can be manipulated by varying the dc bias voltage (polarity, amplitude), incident THz field (polarity, shape, peak amplitude), work functions of STM tip and sample---especially their difference $\mathrm{\ensuremath{\Delta}}W$, and the tip-sample separation. Our study provides an important framework that can be used in the future to characterize, control, and improve THz-induced currents and probing techniques at the nanometer scale over subpicosecond time periods.

Characterizations of two-dimensional materials with cryogenic ultrahigh vacuum near-field optical microscopy in the visible range

The development of new characterization methods has resulted in innovative studies of the properties of two-dimensional (2D) materials. Observations of nanoscale heterogeneity with scanning probe microscopy methods have led to efforts to further understand these systems and observe new local phenomena by coupling light-based measurement methods into the tip-sample junction. Bringing optical spectroscopy into the near-field in ultrahigh vacuum at cryogenic temperatures has led to highly unique studies of molecules and materials, yielding new insight into otherwise unobservable properties nearing the atomic scale. Here, we discuss studies of 2D materials at the subnanoscale where the measurement method relies on the detection of visible light scattered or emitted from the scanning tunneling microscope (STM). We focus on tip-enhanced Raman spectroscopy, a subset of scattering-type scanning near-field optical microscopy, where incident light is confined and enhanced by a plasmonic STM tip. We also mention scanning tunneling microscope induced luminescence, where the STM tip is used as a highly local light source. The measurement of light-matter interactions within the atomic STM cavity is expected to continue to provide a useful platform to study new materials.

Multimodal Tip-Enhanced Nonlinear Optical Nanoimaging of Plasmonic Silver Nanocubes.

Optical field localization at plasmonic tip-sample nanojunctions has enabled high-spatial-resolution chemical analysis through tip-enhanced linear optical spectroscopies, including Raman scattering and photoluminescence. Here, we illustrate that nonlinear optical processes, including parametric four-wave mixing (4WM), second-harmonic/sum-frequency generation (SHG and SFG), and two-photon photoluminescence (TPPL), can be enhanced at plasmonic junctions and spatiospectrally resolved simultaneously with few-nm spatial resolution under ambient conditions. Through a detailed analysis of our spectral nanoimages, we find that the efficiencies of the local nonlinear signals are determined by sharp tip-sample junction resonances that vary over the few-nanometer length scale. Namely, plasmon resonances centered at or around the different nonlinear signals are tracked through TPPL, and they are found to selectively enhance nonlinear signals with closely matched optical resonances.

Electrochemical Tip-Enhanced Raman Spectroscopy for the In Situ Characterization of Functional Materials

The understanding of the mechanism of operation and failure of functional (nano)materials and surfaces is a prerequisite to the optimization of their performance and design. This can be address with nanoscale-sensitive techniques which can be implemented, in situ/operando under the operating conditions of such functional material. Tip-Enhanced Raman Spectroscopy (TERS) has already demonstrated its ability to characterize functional materials at the nanoscale, though its operation in liquid conditions and under polarization remains challenging.[1] After the first demonstration of liquid TERS in organic solvent in 2016 in our group [2], we could access electrochemical TERS (EC-TERS) mapping with 8-nm resolution in 2019.[3]Our most recent work focuses on the in situ investigation of the reactivity of electrochemically-active molecular architectures immobilized on electrode surfaces by EC-TERS. To this purpose we have developed a configurations enabling EC-TERS implementation in the Scanning Tunneling Microscopy mode (EC-STM-TERS). In this setup using a home-made bipotentiostat synchronized to the CCD detector, relatively high potential sweep rate[4] and broad potential ranges are accessible, enabling real-time assessments of electrochemical processes without compromising the plasmonic coupling at the tip-sample junction. A complex electrochemical mechanism, involving reaction intermediates and multiple reaction paths, could be resolved on electroactive architectures based on nitrobenzene derivatives. Further EC-TERS investigations on nitrobenzene derivatives assembled as mono- or multilayers on the electrode surface emphasized the influence of the structure of the molecular assemblies on their reactivity.In our latest developments, EC-TERS measurements implemented in the Atomic Force Microscopy mode (EC-AFM-TERS) are also accessible, therefore broadening the range of functional materials which can be studied in situ/operando.[1] T Touzalin, S Joiret, E Maisonhaute, IT Lucas “Capturing electrochemical transformations by tip-enhanced Raman spectroscopy”, Current Opinion in Electrochemistry 6 (2017) 46-52[2] T Touzalin, AL Dauphin, S Joiret, IT Lucas, E Maisonhaute “Tip enhanced Raman spectroscopy imaging of opaque samples in organic liquid”, Physical Chemistry Chemical Physics 18 (2016) 15510-15513[3] T Touzalin, S Joiret, IT Lucas, E Maisonhaute “Electrochemical tip-enhanced Raman spectroscopy imaging with 8 nm lateral resolution”, Electrochemistry Communications 108 (2019) 106557[4] M Mattei, G. Kang, G. Goubert, D.V. Chulhai, G.C. Schatz, L. Jensen and R.P. Van Duyne., “Tip-Enhanced Raman Voltammetry: Coverage Dependence and Quantitative Modeling” Nano Lett.(2017) 17, 590−596 Figure 1

Probing Magnetic Exchange Interactions with Helium.

Controlling and sensing spin polarization of electrons forms the basis of spintronics. Here, we report a study of the effect of helium on the spin polarization of the tunneling current and magnetic contrast in spin-polarized scanning tunneling microscopy (SP STM). We show that the magnetic contrast in SP STM images recorded in the presence of helium depends sensitively on the tunneling conditions. From tunneling spectra and their variation across the atomic lattice we establish that the helium can be reversibly ejected from the tunneling junction by the tunneling electrons. The energy of the tunneling electrons required to eject the helium depends on the relative spin polarization of the tip and sample, making the microscope sensitive to the magnetic exchange interactions. We show that the time-averaged spin polarization of the tunneling current is suppressed in the presence of helium and thereby demonstrate voltage control of the spin polarization of the tunneling current across the tip-sample junction.

Nanoscale phase change on Ge2Sb2Te5 thin films induced by optical near fields with photoassisted scanning tunneling microscope

A scanning probe microscope coupled with either femtosecond laser pulses or terahertz pulses holds great promise not only for observing ultrafast phenomena but also for fabricating desirable structures at the nanoscale. In this study, we demonstrate that a few-nanometer-scale phase change can be non-thermally stored on the Ge2Sb2Te5 surface by a laser-driven scanning tunneling microscope (STM). An atomically flat Ge2Sb2Te5 surface was irradiated with the optical near-field generated by introducing femtosecond laser pulses to the STM tip-sample junction. The STM topographic images showed that few-nanometer-scale mounds appeared after irradiation. In addition, tunneling conductance spectra showed that the bandgap increased by 0.2 eV in the area of 5 × 5 nm2. These indicate that the nanoscale crystal-to-amorphous phase change was induced by the STM-tip-induced near field. Our approach presented here offers an unprecedented increase in the recording density of optical storage devices and is, therefore, expected to facilitate the development of next-generation information technology.

A fiber scanning tunneling microscope for optical analysis at the nanoscale.

A hybrid scanning tunneling/optical near-field microscope is presented, in which an optical fiber tip coated with 100 nm thick Ag/Cr films scans the surface. The tip metallization enables operating the instrument via a current-based distance control and guarantees sub-nanometer spatial resolution in the topographic channel. The fiber tip simultaneously serves as nanoscale light source, given the optical transparency of the metal coating. The emission response of the tip-sample junction is collected with two parabolic mirrors and probed with a far-field detector. To test the capabilities of the new setup, the evolution of the optical signal is monitored when the tip approaches a gold surface. The intensity rise and frequency shift of the emission provide evidence for the development of coupled plasmon modes in the tip-sample cavity. Photon mapping is employed to probe the optical inhomogeneity of Ru(0001) and TiO2(110) surfaces covered with silver deposits. While the 2D Ag flakes on Ru give rise to a near-field enhancement, the 3D particles on titania locally damp the gap plasmons and lower the emitted intensity. The lateral resolution in the optical channel has been estimated to be ∼1 nm, and optical and topographic signals are well correlated. Our fiber microscope thus appears to be suitable for probing optical surface properties at the nanoscale.

A hybrid magnet based scanning tunneling microscope.

In this paper, a scanning tunneling microscope (STM) is presented that operates in a 27.5 T magnetic field within a hybrid magnet. The coarse approach of the STM is realized by using an inertial piezoelectric motor, and the scanning is realized by using a miniature scanner, which stands alone on a sapphire base. A combined vibration isolation system consisting of a brick-rubber-brick stack and two springs is used to isolate the vibration generated from the magnet. An enclosed copper shield is used to prevent sound from entering the tip-sample junction. The sound and vibration isolation measures highly improve the stability of the STM imaging. All the materials selected to construct the STM head are nonmagnetic. The drift rates of the STM in the X-Y plane and Z direction are as low as 26.2 pm/min and 34.6 pm/min, respectively, under ambient conditions. The high performance of the homebuilt STM was demonstrated by graphite hexagonal lattice images obtained in magnet fields ranging from 0 T to 27.5 T even without the protection of a vacuum and low temperatures. As far as known, this is the first STM that operates in a hybrid magnet. It is also the first STM that can obtain graphite hexagonal lattice images in magnetic fields up to 27.5 T. Our results greatly contribute to the further STM studies under ambient conditions and ultrahigh magnetic fields.

Nanoscale spectroscopic origins of photoinduced tip–sample force in the midinfrared

When light illuminates the junction formed between a sharp metal tip and a sample, different mechanisms can contribute to the measured photoinduced force simultaneously. Of particular interest are the instantaneous force between the induced dipoles in the tip and in the sample, and the force related to thermal heating of the junction. A key difference between these 2 force mechanisms is their spectral behavior. The magnitude of the thermal response follows a dissipative (absorptive) Lorentzian line shape, which measures the heat exchange between light and matter, while the induced dipole response exhibits a dispersive spectrum and relates to the real part of the material polarizability. Because the 2 interactions are sometimes comparable in magnitude, the origin of the chemical selectivity in nanoscale spectroscopic imaging through force detection is often unclear. Here, we demonstrate theoretically and experimentally how the light illumination gives rise to the 2 kinds of photoinduced forces at the tip-sample junction in the midinfrared. We comprehensively address the origin of the spectroscopic forces by discussing cases where the 2 spectrally dependent forces are entwined. The analysis presented here provides a clear and quantitative interpretation of nanoscale chemical measurements of heterogeneous materials and sheds light on the nature of light-matter coupling in optomechanical force-based spectronanoscopy.

A variable-temperature scanning tunneling microscope operated in a continuous flow cryostat.

We present a continuous flow cryostat scanning tunneling microscope (STM) which can be operated at temperatures between 4.9 K and 300 K. A variable temperature insert with 20 mm aperture is installed in the cryostat. A base temperature of 4.9 K at the STM head was achieved with helium exchange gas cooling. By using a novel design of zirconia structure, the size of the STM head was minimized to 9.2 mm in outer diameter. The high compactness and rigidity of the STM head make it less sensitive to vibrations, contributing to the high stability of the tip-sample junction. The drifting rates of the STM at 4.9 K in the X-Y plane and Z direction are 1.96 and 3.05 pm/min, respectively. The performance of the STM was demonstrated by atomically resolved imaging of graphite and NbSe2 without using external vibrational isolation. Furthermore, the dI/dV spectra of NbSe2 were resolved near its critical temperature, illustrating the formation process of the superconducting gap as a function of temperature. This STM is ultrahigh vacuum and strong magnetic field compatible, making it promising for direct application in the extreme conditions involving water-cooled magnets and hybrid magnets.