Tip-molecule Distance Research Articles

Gating Single-Molecule Fluorescence with Electrons.

Tip-enhanced photoluminescence (TEPL) measurements are performed with subnanometer spatial resolution on individual molecules decoupled from a metallic substrate by a thin NaCl layer. TEPL spectra reveal progressive fluorescence quenching with decreasing tip-molecule distance when electrons tunneling from the tip of a scanning tunneling microscope are injected at resonance with the molecular states. Rate equations based on a many-body model reveal that the luminescence quenching is due to a progressive population inversion between the ground neutral (S_{0}) and the ground charge (D_{0}^{-}) states of the molecule occurring when the current is raised. We demonstrate that the bias voltage and the lateral tip position can be used to gate the molecular emission. Our approach can be applied to any molecular system, providing unprecedented control over the fluorescence of a single molecule.

Chirality control of a single carbene molecule by tip-induced van der Waals interactions

Non-covalent interactions such as van der Waals interactions and hydrogen bonds are crucial for the chiral induction and control of molecules, but it remains difficult to study them at the single-molecule level. Here, we report a carbene molecule on a copper surface as a prototype of an anchored molecule with a facile chirality change. We examine the influence of the attractive van der Waals interactions on the chirality change by regulating the tip-molecule distance, resulting in an excess of a carbene enantiomer. Our model study provides insight into the change of molecular chirality controlled by van der Waals interactions, which is fundamental for understanding the mechanisms of chiral induction and amplification.

Orbital and Skeletal Structure of a Single Molecule on a Metal Surface Unveiled by Scanning Tunneling Microscopy.

Atomic-scale spatial characteristics of a phthalocyanine orbital and skeleton are obtained on a metal surface with a scanning tunneling microscope and a CO-functionalized tip. Intriguingly, the high spatial resolution of the intramolecular electronic patterns is achieved without resonant tunneling into the orbital and despite the hybridization of the molecule with the reactive Cu substrate. The resolution can be fine-tuned by the tip-molecule distance, which controls the p-wave and s-wave contribution of the molecular probe to the imaging process. The detailed structure is deployed to minutely track the translation of the molecule in a reversible interconversion of rotational variants and to quantify relaxations of the adsorption geometry. Entering into the Pauli repulsion imaging mode, the intramolecular contrast loses its orbital character and reflects the molecular skeleton instead. The assignment of pyrrolic-hydrogen sites becomes possible, which in the orbital patterns remains elusive.

Controlling the Spin States of FeTBrPP on Au(111).

Spin-flip excitations of iron porphyrin molecules on Au(111) are investigated with a low-temperature scanning tunneling microscope. The molecules adopt two distinct adsorption configurations on the surface that exhibit different magnetic anisotropy energies. Density functional theory calculations show that the different structures and excitation energies reflect unlike occupations of the Fe 3d levels. We demonstrate that the magnetic anisotropy energy can be controlled by changing the adsorption site, the orientation, or the tip-molecule distance.

Role of metal-nanostructure features on tip-enhanced photoluminescence of single molecules.

Tip-enhanced photoluminescence (TEPL) experiments have recently reached the ability to investigate single molecules exploiting resolution at the submolecular level. Localized surface plasmon resonances of metallic nanostructures have the capability of enhancing an impinging electromagnetic radiation in the proximity of their surface, with evident consequences both on absorption and emission of molecules placed in the same region. We propose a theoretical analysis of these phenomena in order to interpret TEPL experiments on single molecules, including a quantum mechanical description of the target molecule equilibrated with the presence of two nanostructures representative of the nanocavity usually employed in STMs. The approach has been applied to the zinc phthalocyanine molecule, previously considered in recent TEPL experiments [Yang et al., Nat. Photonics 14, 693-699 (2020)]. This work has the aim of providing a comprehensive theoretical understanding of the experimental results, particularly focusing on the investigation of the tip features that majorly influence the excitation and fluorescence processes of the molecule, such as the geometry, the dielectric function, and the tip-molecule distance.

Influence of nonlocal dielectric response on the Au tip-enhanced fluorescence effect

Tip-enhanced fluorescence (TEF) with ultra-high detection sensitivity and spatial resolution has been a powerful characterization technique in the study of surface science and life science. Herein, a systematically theoretical investigation in the visible range had been performed to study TEF properties of a single molecule located inside a nanogap formed by Au tip and substrate. In the strong localized surface plasmon coupling effect, the contribution of nonlocal dielectric response to the fluorescence quantum yield as well as radiative and energy dissipated decay rates were calculated. It is found that the nonlocal dielectric effects become comparable to the radiative and energy dissipated decay rates with the increasing of the tip-molecule distance, as a result, the nonlocal dielectric effect significantly suppresses the fluorescence process. The huge excitation enhancement at the shorter tip-molecule distance can efficiently compensate the low quantum yield, leading to the great fluorescence enhancement. The results show that the maximum enhancement obtained from the calculations can reach as high as four orders of magnitude by optimizing the tip-molecule distance. These results are not only helpful to our understanding of the TEF mechanism but also valuable for its further applications.

Numerical investigations on the electromagnetic enhancement effect to tip-enhanced Raman scattering and fluorescence processes

In the present work, we theoretically study the electromagnetic (EM) enhancement of the Raman and fluorescence signals for a molecule placed in a nanocavity formed by a metallic tip and substrate that mimics a tip-enhanced Raman scattering (TERS) setup using three-dimensional finite element method calculations. The influence of tip size and tip-molecule distance on the EM enhancements of the incident field as well as the radiative and non-radiative decay rates of the molecule are systematically investigated. Simulation results show that the maximum EM enhancement to the incident light as provided by the localized surface plasmon resonance in the nanocavity can reach ∼285 for the configuration considered in the present work. Meanwhile, it was found that, at the classical limit, decreasing the apex radius or the tip-molecule distance can both reduce the spatial distribution (as characterized by the full width at half maximum) of the Raman enhancement in a linear fashion. Moreover, simulation results show that the nonlocal dielectric response of the tip and the substrate plays a key role to the fluorescence quantum yield of the molecule. However, it was found that the strong EM excitation enhancement is the dominating factor for the tip enhanced fluorescence (TEF) effect and stronger fluorescence enhancement has been found when increasing the apex radius or reducing the tip-molecule distance with an incident wavelength of 532 nm. The best TERS and TEF enhancements were found to be ∼ and ∼, respectively, with the tip-molecule distance around 1 nm.

Tuning the Coupling of an Individual Magnetic Impurity to a Superconductor: Quantum Phase Transition and Transport.

The exchange scattering at magnetic adsorbates on superconductors gives rise to Yu-Shiba-Rusinov (YSR) bound states. Depending on the strength of the exchange coupling, the magnetic moment perturbs the Cooper pair condensate only weakly, resulting in a free-spin ground state, or binds a quasiparticle in its vicinity, leading to a (partially) screened spin state. Here, we use the flexibility of Fe-porphin (FeP) molecules adsorbed on a Pb(111) surface to reversibly and continuously tune between these distinct ground states. We find that the FeP moment is screened in the pristine adsorption state. Approaching the tip of a scanning tunneling microscope, we exert a sufficiently strong attractive force to tune the molecule through the quantum phase transition into the free-spin state. We ascertain and characterize the transition by investigating the transport processes as function of tip-molecule distance, exciting the YSR states by single-electron tunneling as well as (multiple) Andreev reflections.

Modification of the Potential Landscape of Molecular Rotors on Au(111) by the Presence of an STM Tip.

Molecular rotors on solid surfaces are fundamental components of molecular machines. No matter whether the rotation is activated by heat, electric field or light, it is determined by the intrinsic rotational potential landscape. Therefore, tuning the potential landscape is of great importance for future applications of controlled molecular rotors. Here, using scanning tunneling microscopy (STM), we demonstrate that both tip-molecule distance and sample bias can modify the rotational potential of molecular rotors. We achieve the potential energy difference variations of ∼0.3 meV/pm and ∼18 meV/V between two configurations of a molecular rotor, a tetra- tert-butyl nickel phthalocyanine molecule on Au(111) substrate. Further analysis indicates that the mechanism of modifying the rotational potential is a combination of the van der Waals interaction and the interaction between the molecular dipole and an electric field. This work provides insight into the methods used to modify the effective rotational potential energy of molecular rotors.

Dynamics of copper-phthalocyanine molecules on Au/Ge(001).

Spatially resolved current-time scanning tunneling spectroscopy combined with current-distance spectroscopy has been used to characterize the dynamic behavior of copper-phthalocyanine (CuPc) molecules adsorbed on a Au-modified Ge(001) surface. The analyzed CuPc molecules are adsorbed in a "molecular bridge" configuration, where two benzopyrrole groups (lobes) are connected to a Au-induced nanowire, whereas the other two lobes are connected to the adjacent nanowire. Three types of lobe configurations are found: a bright lobe, a dim lobe, and a fuzzy lobe. The dim and fuzzy lobes exhibit a well-defined switching behavior between two discrete levels, while the bright lobe shows a broad oscillation band. The observed dynamic behavior is induced by electrons that are injected into the LUMO+1 orbital of the CuPc molecule. By precisely adjusting the tip-molecule distance, the switching frequency of the lobes can be tuned accurately.

Competing Forces during Contact Formation between a Tip and a Single Molecule.

Sn-phthalocyanine adsorbs on Ag(111) in a physisorbed or a chemisorbed configuration. Both structures are contacted with the tip of a combined scanning tunneling and atomic force microscope. The tunneling conductances of both configurations exhibit similar exponential variations with the tip-molecule distance. The short-range forces, however, display nontrivial distance dependencies. First-principles calculations reproduce the experimental results. Both attractive and repulsive interactions occur between the tip and different parts of the molecule due to a combination of bond formation and electrostatic interactions with the tip electric dipole. Consequently, deformations occur and the force varies in the resulting unexpected fashion.

Tuning the Magnetic Anisotropy of Single Molecules.

The magnetism of single atoms and molecules is governed by the atomic scale environment. In general, the reduced symmetry of the surrounding splits the d states and aligns the magnetic moment along certain favorable directions. Here, we show that we can reversibly modify the magnetocrystalline anisotropy by manipulating the environment of single iron(II) porphyrin molecules adsorbed on Pb(111) with the tip of a scanning tunneling microscope. When we decrease the tip-molecule distance, we first observe a small increase followed by an exponential decrease of the axial anisotropy on the molecules. This is in contrast to the monotonous increase observed earlier for the same molecule with an additional axial Cl ligand ( Nat. Phys. 2013 , 9 , 765 ). We ascribe the changes in the anisotropy of both species to a deformation of the molecules in the presence of the attractive force of the tip, which leads to a change in the d level alignment. These experiments demonstrate the feasibility of a precise tuning of the magnetic anisotropy of an individual molecule by mechanical control.

Low-temperature scanning tunneling microscopy study on the electronic properties of a double-decker DyPc2 molecule at the surface.

To fully achieve potential applications of the double-decker molecules containing rare earth elements as single-molecule magnets in molecular spintronics, it is crucial to understand the 4f states of the rare earth atoms sandwiched in the double-decker molecules by metal electrodes. In this study, low-temperature scanning tunneling microscopy and spectroscopy were employed to investigate the isolated double-decker DyPc2 molecule adsorbed on Au(111) via its differential conductance measurements. The experimental results revealed that the differential conductance maps acquired at a constant height mode simply depicted the authentic molecular orbitals; moreover, the differential conductance maps achieved at a constant current mode could not directly probe the 4f states of the sandwiched Dy atom. This was consistent with the spectra obtained over the molecule center around the Fermi level, indicative of no Kondo feature. Upon decreasing the tip-molecule distance, the CH-mode images presented high-resolution structure but no information of the 4f states. All results indicated that the Dy atom barely contributed to the tunneling current because of the absence of coupling with the microscope tip, echoing the inaccessibility of the Dy 4f states in the double-decker DyPc2 molecule.

High‐resolution scanning tunneling and atomic force microscopy of stereochemically resolved dibenzo[a,h]thianthrene molecules

Recently, we reported on the bistable configurational switching of dibenzo[a,h]thianthrene (DBTH) molecules adsorbed on NaCl using combined low‐temperature scanning tunneling and atomic force microscopy (STM/AFM). Here, we discuss the intra‐molecular contrast in AFM images of the molecules as a function of the tip–molecule distance. Our experiments show that ridges in the frequency shift do not necessarily correlate with chemical bonds in this case of a non‐planar molecule. To explain this finding we compare images acquired at different tip–molecule distances to the calculated electron density of the molecules obtained from density functional theory calculations (DFT). In addition, we analyze the probability of finding different configurations after adsorption onto the surface.DBTH molecules in two configurations probed by a CO‐functionalized tip. Insets show AFM (left) and STM (right) images of a U molecule.

Laser alignment as a route to ultrafast control of electron transport through junctions

We consider the extension of ultrafast laser alignment schemes to surface-adsorbed molecules, where the laser field coerces the molecule to reorient itself relative to the surface. When probed by a scanning tunneling microscope tip, this reorientation modifies the tip-molecule distance, and thus the tunneling current, suggesting a route to an ultrafast, nanoscale current switch. In addition to exploring the controllability of adsorbed molecules by moderately intense laser fields and discussing the fundamental differences of alignment behavior between surface-adsorbed molecules and gas phase molecules, we computationally investigate the quality of orientation with respect to field intensity, field duration, and the location of the tip. Overall, the molecule moves directly to its oriented configuration, which is reasonably insensitive to the tip location. These results collectively suggest the efficacy of using laser alignment schemes to control electron transport through junctions.

Scanning Tunneling Spectroscopy of Metal Phthalocyanines on a Au(111) Surface with a Ni Tip

Scanning tunneling spectroscopy of metal phthalocyanines (MPc) adsorbed on a Au(111) surface with a Ni(111) scanning tunneling microscopy tip is simulated on the basis of first-principles calculations and a modified Bardeen approximation. Local d orbital symmetry matching between the molecule and the Ni tip brings obvious negative differential resistance (NDR) phenomena, of which, bias voltage and resonant orbitals can be tuned sensitively by the central ion of the molecule. Different dependences of the NDR peak on the tip-molecule distance at two bias polarities and rectifying phenomena are also interpreted in terms of specific structures of 3d orbitals of the adsorbed MPc and Ni tip.

Controlled Contact to aC60Molecule

The tip of a low-temperature scanning tunneling microscope is approached towards a C60 molecule adsorbed at a pentagon-hexagon bond on Cu(100) to form a tip-molecule contact. The conductance rapidly increases to approximately 0.25 conductance quanta in the transition region from tunneling to contact. Ab-initio calculations within density functional theory and nonequilibrium Green's function techniques explain the experimental data in terms of the conductance of an essentially undeformed C60. The conductance in the transition region is affected by structural fluctuations which modulate the tip-molecule distance.

Probing charge transport at the single-molecule level on silicon by using cryogenic ultra-high vacuum scanning tunneling microscopy

A cryogenic variable-temperature ultra-high vacuum scanning tunneling microscope is used for measuring the electrical properties of isolated cyclopentene molecules adsorbed to the degenerately p-type Si(100)-2x1 surface at a temperature of 80 K. Current-voltage curves taken under these conditions show negative differential resistance at positive sample bias, in agreement with previous observations at room temperature. Because of the enhanced stability of the scanning tunneling microscope at cryogenic temperatures, repeated measurements can be routinely taken over the same molecule. Taking advantage of this improved stability, we show that current-voltage curves on isolated cyclopentene molecules are reproducible and possess negligible hysteresis for a given tip-molecule distance. On the other hand, subsequent measurements with variable tip position show that the negative differential resistance voltage increases with increasing tip-molecule distance. By using a one-dimensional capacitive equivalent circuit and a resonant tunneling model, this behavior can be quantitatively explained, thus providing insight into the electrostatic potential distribution across a semiconductor-molecule-vacuum-metal tunnel junction. This model also provides a quantitative estimate for the alignment of the highest occupied molecular orbital of cyclopentene with respect to the Fermi level of the silicon substrate, thus suggesting that this experimental approach can be used for performing chemical spectroscopy at the single-molecule level on semiconductor surfaces. Overall, these results serve as the basis for a series of design rules that can be applied to silicon-based molecular electronic devices.

Current-Voltage Characteristics of Self-Assembled Monolayers by Scanning Tunneling Microscopy

This paper presents a comparison of the theoretical and experimental current-voltage (I-V) characteristics of a self-assembled monolayer of $\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{-}\mathrm{x}\mathrm{y}\mathrm{l}\mathrm{y}\mathrm{l}$ dithiol molecules on a gold substrate measured with a scanning tunneling microscope probe. Good quantitative agreement is obtained with the tip-molecule distance as the only ``fitting parameter.'' Several other thiol-coupled molecules that we have studied also show similar agreement. The conceptual picture presented in this paper could be useful for the interpretation of I-V measurements on molecular monolayers in general.