Electron Tunneling Spectroscopy Research Articles

Resonant electron tunneling spectroscopy of antibonding states in a Dirac semimetal

Recently, it was shown both theoretically and experimentally that certain three-dimensional (3D) materials have Dirac points in the Brillouin zone, thus being 3D analogs of graphene. Moreover, it was suggested that under specific conditions a pair of localized impurities placed inside a three-dimensional Dirac semimetal may lead to the formation of an unusual antibonding state. In the meantime, the effect of vibrational degrees of freedom which are present in any realistic system has avoided attention. In this work, we address the influence of phonons on characteristic features of (anti)bonding state, and discuss how these results can be tested experimentally via local probing, namely, inelastic electron tunneling spectroscopy curve obtained in STM measurements.

Tunneling anisotropic magnetoresistance in fully epitaxial magnetic tunnel junctions with different barriers

We report the tunneling anisotropic magnetoresistance (TAMR) in fully epitaxial Fe/Barrier/Fe (001) magnetic tunnel junctions (MTJs) where the Barrier is annealed MgO, MgAlOx, MgO-MgAlOx, or as-grown MgO/MgAlOx. The TAMR was measured as the magnetization of Fe electrodes rotated from in-plane to out-of-plane. The angular dependence of TAMR for all samples exhibited superposed behavior of twofold and fourfold symmetries. The proportion of fourfold symmetry is larger in MTJs with MgO and MgO-MgAlOx than that in MTJs with MgAlOx and MgO/MgAlOx barriers. By characterizing inelastic electron tunneling spectroscopy in the antiparallel state and parallel conductance of the MTJs, we revealed diverse minority interfacial resonant states (IRSs) and different contributions from Δ1 and Δ5 symmetry states to the conductance in the MTJs. Our results illustrate that the minority IRS dominated by Δ5 symmetry can mix with majority Δ1 states and give rise to the enhanced fourfold symmetric angular dependence in MTJs with MgO and MgO-MgAlOx barriers.

Investigating nuclear quantum effects of water

The mystery of water mainly arises from the intermolecular hydrogen-bonding (H-bonding) interaction. It is well known that H bonds have a strong classic component coming from electrostatics. However, quantum behaviors of hydrogen nuclei (protons), including tunneling and zero-point motion have significant effects on the macroscopic properties, structure, and dynamics of water even at room temperature or higher. Despite enormous theoretical efforts toward pursuing proper treatment of the nuclear motion at a quantum mechanical level, accurate and quantitative description of NQEs on the H-bonding interaction has proven to be experimentally challenging. This review highlights the recent advances in investigating nuclear quantum effects (NQEs) of water, with focus on the studies of surface water at atomic scale. We first introduce the conventional spectroscopic techniques and the emerging scanning tunneling microscopy/spectroscopy (STM/S) and non-contact atomic force microscopy (nc-AFM), which allow the access to the quantum degree of freedom of protons at atomic scale. Real-space imaging of interfacial water with submolecular resolution has been achieved, enabling discrimination of the orientation of the single water molecules and the H-bonding directionality of water clusters at surfaces. In addition, the limit of vibrational spectroscopy of water has been pushed down to the single-bond level using tip-enhanced inelastic electron tunneling spectroscopy (IETS). We then discuss how those techniques are used to investigate the NQEs of surface water, such as proton tunneling and the impact of zero point motion on the H-bonding interaction. Proton tunneling has been observed in water/ice under various conditions, such as surface water, high-pressure ice and confined water. Interestingly, proton tunneling in H-bonded networks tends to involve many H bonds simultaneously, leading to concerted many-body tunneling, which has been visualized in the experiment as well. NQEs, in terms of zero-point motion, could influence the H-bonding interactions and consequently the structure of H-bonded network. A general physical picture for the NQEs of H bonds has been unraveled, that is, the anharmonic quantum fluctuations of hydrogen nuclei weaken the weak H bonds and strengthen the strong ones due to the competing effect between O-H stretching and H-bond bending. For completeness, we also briefly review the NQEs of bulk water/ice and confined water using the macroscopic spectroscopic techniques. Those findings may completely renovate our understanding of water and provide answers to many weirdness and the macroscopic isotope effects of water from a quantum mechanical view. In the end, we present an outlook on the current challenges and further opportunities in this field. There is an urgent need to develop new techniques to study the NQEs of water at ambient condition with atomic precision. It would be also interesting to extend the NQEs studies of water to other H-rich materials and, more generally, to light-element materials, such as Li, He, C, N, B.

Tunneling explains efficient electron transport via protein junctions.

Metalloproteins, proteins containing a transition metal ion cofactor, are electron transfer agents that perform key functions in cells. Inspired by this fact, electron transport across these proteins has been widely studied in solid-state settings, triggering the interest in examining potential use of proteins as building blocks in bioelectronic devices. Here, we report results of low-temperature (10 K) electron transport measurements via monolayer junctions based on the blue copper protein azurin (Az), which strongly suggest quantum tunneling of electrons as the dominant charge transport mechanism. Specifically, we show that, weakening the protein-electrode coupling by introducing a spacer, one can switch the electron transport from off-resonant to resonant tunneling. This is a consequence of reducing the electrode's perturbation of the Cu(II)-localized electronic state, a pattern that has not been observed before in protein-based junctions. Moreover, we identify vibronic features of the Cu(II) coordination sphere in transport characteristics that show directly the active role of the metal ion in resonance tunneling. Our results illustrate how quantum mechanical effects may dominate electron transport via protein-based junctions.

Tip-induced local strain on MoS2/graphite detected by inelastic electron tunneling spectroscopy

We report the detection of tip-induced local strain applied to the monolayer $\mathrm{Mo}{\mathrm{S}}_{2}$ grown on a graphite substrate by scanning tunneling microscope. Monolayer $\mathrm{Mo}{\mathrm{S}}_{2}$ behaves as both mechanical and tunneling barriers that prevent the tip from contacting the graphite while maintaining the tunneling current. Inelastic tunneling electron spectroscopy (IETS) is utilized to probe the phonon modes in graphite. As the tip pushes the sample, IETS reveals a continuous phonon softening in graphite, corroborated by a downward shift of the phonon energy as calculated by density-functional theory. Our results demonstrate a way to apply local mechanical strain and simultaneously detect the induced change in phonon modes by unitizing IETS with two-dimensional materials as a tunneling barrier.

A Study of Vertical Transport through Graphene toward Control of Quantum Tunneling.

Vertical integration of van der Waals (vdW) materials with atomic precision is an intriguing possibility brought forward by these two-dimensional (2D) materials. Essential to the design and analysis of these structures is a fundamental understanding of the vertical transport of charge carriers into and across vdW materials, yet little has been done in this area. In this report, we explore the important roles of single layer graphene in the vertical tunneling process as a tunneling barrier. Although a semimetal in the lateral lattice plane, graphene together with the vdW gap act as a tunneling barrier that is nearly transparent to the vertically tunneling electrons due to its atomic thickness and the transverse momenta mismatch between the injected electrons and the graphene band structure. This is accentuated using electron tunneling spectroscopy (ETS) showing a lack of features corresponding to the Dirac cone band structure. Meanwhile, the graphene acts as a lateral conductor through which the potential and charge distribution across the tunneling barrier can be tuned. These unique properties make graphene an excellent 2D atomic grid, transparent to charge carriers, and yet can control the carrier flux via the electrical potential. A new model on the quantum capacitance's effect on vertical tunneling is developed to further elucidate the role of graphene in modulating the tunneling process. This work may serve as a general guideline for the design and analysis of vdW vertical tunneling devices and heterostructures, as well as the study of electron/spin injection through and into vdW materials.

A Quantum Biomimetic Electronic Nose Sensor

We propose a technologically feasible one-dimensional double barrier resonant tunneling diode (RTD) as electronic nose, inspired by the vibration theory of biological olfaction. The working principle is phonon-assisted inelastic electron tunneling spectroscopy (IETS), modeled here using the Non-Equilibrium Green Function formalism for quantum transport. While standard IETS requires low-temperature operation to obviate the thermal broadening of spectroscopic peaks, we show that quantum confinement in the well of the RTD provides electron energy filtering in this case and could thereby allow room-temperature operation. We also find that the IETS peaks - corresponding to adsorbed foreign molecules - shift monotonically along the bias voltage coordinate with their vibrational energy, promising a selective sensor.

A modern and comprehensive investigation of inelastic electron tunneling spectroscopy (IETS) and scanning tunneling spectroscopy on malignant and benign human cancer cells, tissues and tumors through optimizing synchrotron microbeam radiotherapy for human cancer treatments and diagnostics: An experimental biospectroscopic comparative study

A modern and comprehensive investigation of inelastic electron tunneling spectroscopy (IETS) and scanning tunneling spectroscopy on malignant and benign human cancer cells, tissues and tumors through optimizing synchrotron microbeam radiotherapy for human cancer treatments and diagnostics: An experimental biospectroscopic comparative study

Inelastic electron tunneling spectroscopy of difurylethene-based photochromic single-molecule junctions.

Diarylethene-derived molecules alter their electronic structure upon transformation between the open and closed forms of the diarylethene core, when exposed to ultraviolet (UV) or visible light. This transformation results in a significant variation of electrical conductance and vibrational properties of corresponding molecular junctions. We report here a combined experimental and theoretical analysis of charge transport through diarylethene-derived single-molecule devices, which are created using the mechanically controlled break-junction technique. Inelastic electron tunneling (IET) spectroscopy measurements performed at 4.2 K are compared with first-principles calculations in the two distinct forms of diarylethenes connected to gold electrodes. The combined approach clearly demonstrates that the IET spectra of single-molecule junctions show specific vibrational features that can be used to identify different isomeric molecular states by transport experiments.

Line shapes in inelastic electron tunneling spectroscopy of single-molecule junctions

Spectroscopic line-shape analyses for single-${\mathrm{C}}_{60}$ vibrational modes are presented for two kinds of scanning tunneling microscope experiments. Inelastic electron tunneling spectroscopy is performed for ${\mathrm{C}}_{60}$ molecules with different adsorption geometries on Pb(111). Depending on the ${\mathrm{C}}_{60}$ adsorption site and rotational orientation, the lowest unoccupied molecular orbital exhibits varying degrees of overlap with ${\mathrm{C}}_{60}$ vibrational energies. Concomitantly, the line shapes of vibrational modes are affected according to expectations for on-resonance inelastic electron tunneling. Inelastic electron transport is further studied for decreasing tip-${\mathrm{C}}_{60}$ distances covering tunneling and contact ranges. Line-shape changes signaling a conductance increase rather than the expected decrease upon exciting vibrational quanta are observed. A phenomenological approach is suggested to understand the dissimilar behavior of the junction conductance in the different electron transport ranges.

An Iron-Porphyrin Complex with Large Easy-Axis Magnetic Anisotropy on Metal Substrate.

Easy-axis magnetic anisotropy separates two magnetic states with opposite magnetic moments, and single magnetic atoms and molecules with large easy-axis magnetic anisotropy are highly desired for future applications in high-density data storage and quantum computation. By tuning the metalation reaction between tetra-pyridyl-porphyrin molecules and Fe atoms, we have stabilized the so-called initial complex, an intermediate state of the reaction, on Au(111) substrate, and investigated the magnetic property of this complex at a single-molecule level by low-temperature scanning tunneling microscopy and spectroscopy. As revealed by inelastic electron tunneling spectroscopy in magnetic field, this Fe-porphyrin complex has magnetic anisotropy energy of more than 15 meV with its easy-axis perpendicular to the molecular plane. Two magnetic states with opposite spin directions are discriminated by the dependence of spin-flip excitation energy on magnetic field and are found to have long spin lifetimes. Our density functional theory calculations reveal that the Fe atom in this complex, decoupled from Au substrate by a weak ligand field with elongated Fe-N bonds, has a high-spin state S = 2 and a large orbital angular momentum L = 2, which give rise to easy-axis anisotropy perpendicular to the molecular plane and large magnetic anisotropy energy by spin-orbit coupling. Since the Fe atom is protected by the molecular ligand, the complex can be processed at room or even higher temperatures. The reported system may have potential applications in nonvolatile data storage, and our work demonstrates on-surface metalation reactions can be utilized to synthesize organometallic complexes with large magnetic anisotropy.

Submolecular Resolution by Variation of the Inelastic Electron Tunneling Spectroscopy Amplitude and its Relation to the AFM/STM Signal.

Here we show scanning tunneling microscopy (STM), noncontact atomic force microscopy (AFM), and inelastic electron tunneling spectroscopy (IETS) measurements on an organic molecule with a CO-terminated tip at 5K. The high-resolution contrast observed simultaneously in all channels unambiguously demonstrates the common imaging mechanism in STM/AFM/IETS, related to the lateral bending of the CO-functionalized tip. The IETS spectroscopy reveals that the submolecular contrast at 5K consists of both renormalization of vibrational frequency and variation of the amplitude of the IETS signal. This finding is also corroborated by first principles simulations. We extend accordingly the probe-particle AFM/STM/IETS model to include these two main ingredients necessary to reproduce the high-resolution IETS contrast. We also employ the first principles simulations to get more insight into a different response of frustrated translation and rotational modes of the CO tip during imaging.

Atomic-scale characterization of the interfacial phonon in graphene/SiC

Epitaxial graphene on SiC that provides wafer-scale and high-quality graphene sheets on an insulating substrate is a promising material to realize graphene-based nanodevices. The presence of the insulating substrate changes the physical properties of free-standing graphene through the interfacial phonon, e.g., limiting the mobility. Despite such known impacts on the material properties, a complete and microscopic picture is missing. Here, we report on atomically resolved inelastic electron tunneling spectroscopy (IETS) with a scanning tunneling microscope for epitaxial graphene grown on $4H$-SiC(0001). Our data reveal a strong spatial dependence in the IETS spectrum, which cannot be explained by intrinsic graphene properties. We show that this variation in the IETS spectrum originates from a localized low-energy vibration of the interfacial Si atom with a dangling bond via ab initio electronic and phononic state calculations. This insight may help advancing graphene device performance through interfacial control.

Thermometry of Plasmonic Heating by Inelastic Electron Tunneling Spectroscopy (IETS)

The electronic and lattice heating accompanying plasmonic structures under illumination is suggested to be utilized in a broad range of thermoplasmonic applications. Specifically, in molecular electronics precise determination of the temperature of illuminated junctions is crucial, because the temperature-dependent energy distribution of charge carriers in the leads affects the possibility to steer various light-controlled conductance processes. Existing optical methods to characterize the local temperature in all these applications lack the spatial resolution to probe the few nanometers in size hot spots and therefore typically report average values over a diffraction limited length scale. Here we demonstrate that inelastic electron tunneling spectroscopy of molecular junctions based on thiol-alkyl chains can be used to precisely measure the temperature of metal nanoscale gaps under illumination. The nature of this measurement guarantees that the reported temperature indeed characterizes the confined volume in which heat is produced by the relaxation of hot carriers. Using a simple model, we suggest that the accuracy of the method enables also one to semiquantify the energy distribution of the hot carriers.

Inelastic electron tunneling into graphene nanostructures on a metal surface

Wrinkles and blisters of graphene on Ir(111) give rise to remarkably high signals in inelastic electron tunneling spectroscopy due to graphene phonons. Spatially resolved spectra unravel the gradual increase of the graphene phonon signatures with progressive delamination of graphene from the metal surface. Spectroscopy of the Ir(111) surface resonance evidences that the graphene-metal hybridization is efficiently reduced in wrinkles and blisters. A high phonon density of states favors the observation of phonons with wave vectors near the $\overline{M}$ point of the surface Brillouin zone.

π-Electron-Assisted Relaxation of Spin Excited States in Cobalt Phthalocyanine Molecules on Au(111) Surface

We present our investigation on the spin relaxation of cobalt phthalocyanine (CoPc) films on Au(111) (CoPc/Au(111)) surface using scanning tunneling microscopy and spectroscopy. The spin relaxation time derived from the linewidth of spin-flip inelastic electron tunneling spectroscopy is quantitatively analyzed according to the Korringa-like formula. We find that although this regime of the spin relaxation time calculation by just considering the exchange interaction between itinerant conduction electrons and localized d-shells (s-d exchange interaction) can successfully reproduce the experimental value of the adsorbed magnetic atom, it fails in our case of CoPc/Au(111). Instead, we can obtain the relaxation time that is in good agreement with the experimental result by considering the fact that the π electrons in CoPc molecules are spin polarized, where the spin polarized π electrons extended at the Pc macrocycle may also scatter the conduction electrons in addition to the localized d spins. Our analyses indicate that the scattering by the π electrons provides an efficient spin relaxation channel in addition to the s-d interaction and thus leads to much short relaxation time in such a kind of molecular system on a metal substrate.

Inelastic electron tunneling spectroscopy in molecular junctions showing quantum interference

Destructive quantum interference effect is implemented in large area molecular junctions to improve signatures of electron-phonon interaction. Vertical molecular junctions based on a cross-conjugated anthraquinone layer were fabricated and low-noise transport measurements were performed by acquiring simultaneously the current-voltage characteristics, its second derivative, and the differential conductance. Signatures of vibrational modes excited by inelastic events are present in the whole measured voltage range and superpose to the conductance suppression induced by destructive quantum interference. As a consequence vibrational modes have improved visibility in the low energy window $(<80\phantom{\rule{0.28em}{0ex}}\mathrm{meV})$. Inelastic electron transport spectroscopy data are compared to infrared attenuated total reflection spectroscopy on Au/anthraquinone thin films. Common vibrational modes can be clearly identified, but inelastic electron tunneling spectroscopy reveals the existence of vibrational modes in a wider energy range $(0--400\phantom{\rule{0.28em}{0ex}}\mathrm{meV})$ where infrared spectroscopy is lacking.

Transport mirages in single-molecule devices

Molecular systems can exhibit a complex, chemically tailorable inner structure which allows for targeting of specific mechanical, electronic, and optical properties. At the single-molecule level, two major complementary ways to explore these properties are molecular quantum-dot structures and scanning probes. This article outlines comprehensive principles of electron-transport spectroscopy relevant to both these approaches and presents a new, high-resolution experiment on a high-spin single-molecule junction exemplifying these principles. Such spectroscopy plays a key role in further advancing our understanding of molecular and atomic systems, in particular, the relaxation of their spin. In this joint experimental and theoretical analysis, particular focus is put on the crossover between the resonant regime [single-electron tunneling] and the off-resonant regime [inelastic electron (co)tunneling spectroscopy (IETS)]. We show that the interplay of these two processes leads to unexpected mirages of resonances not captured by either of the two pictures alone. Although this turns out to be important in a large fraction of the possible regimes of level positions and bias voltages, it has been given little attention in molecular transport studies. Combined with nonequilibrium IETS—four-electron pump-probe excitations—these mirages provide crucial information on the relaxation of spin excitations. Our encompassing physical picture is supported by a master-equation approach that goes beyond weak coupling. The present work encourages the development of a broader connection between the fields of molecular quantum-dot and scanning probe spectroscopy.

Efficient Spin-Flip Excitation of a Nickelocene Molecule.

Inelastic electron tunneling spectroscopy (IETS) within the junction of a scanning tunneling microscope (STM) uses current-driven spin-flip excitations for an all-electrical characterization of the spin state of a single object. Usually decoupling layers between the single object, atom or molecule, and the supporting surface are needed to observe these excitations. Here we study the surface magnetism of a sandwich nickelocene molecule (Nc) adsorbed directly on Cu(100) by means of X-ray magnetic circular dichroism (XMCD) and density functional theory (DFT) calculations and show with IETS that it exhibits an exceptionally efficient spin-flip excitation. The molecule preserves its magnetic moment and magnetic anisotropy not only on Cu(100), but also in different metallic environments including the tip apex. By taking advantage of this robusteness, we are able to functionalize the microscope tip with a Nc, which can be employed as a portable source of inelastic excitations as exemplified by a double spin-flip excitation process.

Electrical measurement of the linewidth of a quantum well bound state

We investigate electron tunneling spectroscopy in the presence of a bound state within a double quantum barrier, single quantum well structure. We demonstrate a new technique to directly measure the intrinsic linewidth of the bound state within the quantum well from the current-voltage signature of the resonant tunneling phenomena and contrast our results with the standing approach in the literature. We then examine the signal behavior for the influence of device temperature and find support for electron-electron interactions within the well. The measured intrinsic bound-state width, ΓE, in the negative differential conductance regime is 1.11± 0.01 meV.