Tip-sample Gap Research Articles

STM Light Emission Spectroscopy of Self-Assembled Monolayer of Alkanethiol on Au Film

In this paper, scanning tunneling microscope (STM) light emission (STM-LE) from alkanethiol self-assembled monolayer (SAM)-covered Au film has been probed in the Kretschmann configuration. The films were deposited on the smooth plane of a hemispherical glass prism. STM-LEs from the tip–sample gap into the vacuum (tip-side emission) and into the prism (prism-side emission) were investigated. Our experimental results showed that the tip-side emission was scarcely found, and the prism-side emissions were successfully detected due to the enhancement of Kretschmann configuration. It was also found from the experimental study that the peak intensity of STM-LE spectra become smaller, accompanying the redshift of the peak position with the rise in thickness of the alkanethiol SAM film. The main focus of this paper is to explore the behavior of electron tunneling into the SAM-covered Au film. In order to explain these phenomena, we have designed different models regarding the tip–sample gap structure. Among these models, the antenna factor successfully explains the cutoff energy shift of STM-LE spectra. According to this model, the cutoff energy shows redshift when the electronic transition occurs at the Au–S interface layer and the amount of the shift depends on the strength of the transitions.

Pump–probe STM light emission spectroscopy for detection of photo-induced semiconductor–metal phase transition of VO2

We attempted to observe pump–probe scanning tunneling microscopy (STM)-light emission (LE) from a VO2 thin film grown on a rutile TiO2(0 0 1) substrate, with an Ag tip fixed over a semiconducting domain. Laser pulses from a Ti:sapphire laser (wavelength 920 nm; pulse width less than 1.5 ps) irradiated the tip–sample gap as pump and probe light sources. With a photon energy of 2.7 eV, suggesting phase transition from semiconducting monoclinic (M) to metallic rutile (R) phases in relation to the electronic band structure, faint LE was observed roughly 30 ps after the irradiation of the pump pulse, followed by retention for roughly 20 ps. The incident energy fluence of the pump pulse at the gap was five orders of magnitude lower than the threshold value for reported photo-induced M–R phase transition. The mechanism that makes it possible to reduce the threshold fluence is discussed.

Two-photon luminescence contrast by tip-sample coupling in femtosecond near-field optical microscopy

We investigate the role of tip-sample interaction in nonlinear optical scanning near-field microscopy. The experiment was performed by tightly focusing femtosecond laser pulses onto a sharp gold tip that was positioned in close proximity to the surface of a sample with gold nanostructures on a Si-substrate by shear force feedback. The nonlinear optical signal consists of two-photon photoluminescence and second harmonic signal from the gold tip and the gold nanostructures. These signals can be used to characterize different coupling parameters such as geometry, material and width of the tip-sample gap and enable to reveal the mechanism responsible for the image contrast. Under the excitation with 776-nm and 110-fs laser pulses nonlinear imaging is almost background free and yields super resolution showing features with dimensions significantly below the diffraction limit with a signal intensity following quadratic excitation power law.

Visualizing and Calculating Tip-Substrate Distance in Nanoscale Scanning Electrochemical Microscopy Using 3-Dimensional Super-Resolution Optical Imaging.

We report a strategy for the optical determination of tip-substrate distance in nanoscale scanning electrochemical microscopy (SECM) using three-dimensional super-resolution fluorescence imaging. A phase mask is placed in the emission path of our dual SECM/optical microscope, generating a double helix point spread function at the image plane, which allows us to measure the height of emitting objects relative to the focus of the microscope. By exciting both a fluorogenic reaction at the nanoscale electrode tip as well as fluorescent nanoparticles at the substrate, we are able to calculate the tip-substrate distance as the tip approaches the surface with precision better than 25 nm. Attachment of a fluorescent particle to the insulating sheath of the SECM tip extends this technique to nonfluorogenic electrochemical reactions. Correlated electrochemical and optical determination of tip-substrate distance yielded excellent agreement between the two techniques. Not only does super-resolution imaging offer a secondary feedback mechanism for measuring the tip-sample gap during SECM experiments, it also enables facile tip alignment and a strategy for accounting for electrode tilt relative to the substrate.

Amplitude dependence of image quality in atomically-resolved bimodal atomic force microscopy

In bimodal frequency modulation atomic force microscopy (FM-AFM), two flexural modes are excited simultaneously. We show atomically resolved images of KBr(100) in ambient conditions in both modes that display a strong correlation between the image quality and amplitude. We define the sum amplitude as the sum of the amplitudes of both modes. When the sum amplitude becomes larger than about 100 pm, the signal-to-noise ratio (SNR) drastically decreases. We propose that this is caused by the temporary presence of one or more water layers in the tip-sample gap. These water layers screen the short range interaction and must be displaced with each oscillation cycle. Decreasing the amplitude of either mode, however, increases the noise. Therefore, the highest SNR in ambient conditions is achieved when twice the sum amplitude is slightly less than the thickness of the primary hydration layer.

Improving resolution in quantum subnanometre-gap tip-enhanced Raman nanoimaging.

Two-dimensional (2D) materials beyond graphene such as transition metal dichalcogenides (TMDs) have unique mechanical, optical and electronic properties with promising applications in flexible devices, catalysis and sensing. Optical imaging of TMDs using photoluminescence and Raman spectroscopy can reveal the effects of structure, strain, doping, edge states, and surface functionalization from materials to bioscience. However, Raman signals are inherently weak and so far have been limited in spatial resolution in TMDs to a few hundred nanometres which is much larger than the intrinsic scale of these effects. Here we overcome the diffraction limit by using resonant tip-enhanced Raman scattering (TERS) of few-layer MoS2, and obtain nanoscale optical images with ~20 nm spatial resolution. This becomes possible due to electric field enhancement in an optimized subnanometre-gap resonant tip-substrate configuration. We investigate the limits of signal enhancement by varying the tip-sample gap with sub-Angstrom precision and observe a quantum quenching behavior, as well as a Schottky-Ohmic transition, for subnanometre gaps, which enable surface mapping based on this new contrast mechanism. This quantum regime of plasmonic gap-mode enhancement with a few nanometre thick MoS2 junction may be used for designing new quantum optoelectronic devices and sensors with a wide range of applications.

Capacitance Behavior of Alkanethiol Self-Assembled Monolayer Studied by Scanning Tunneling Microscope Light Emission Spectroscopy

We have studied the capacitance behavior of alkanethiol self-assembled monolayer (SAM)-covered Au film by scanning tunneling microscope light emission (STM-LE) spectroscopy in the Kretschmann geometry. Although, the STM-LE from tip-sample gap into the vacuum (tip-side emission) is fundamentally weak and very difficult to detect, we have succeeded in detecting the STM-LE radiated into the prism (prism-side emission) by virtue of the enhancement of prism-coupled geometry. Our experimental results shows that the cutoff energy of STM-LE spectra have been redshifted with increase in thickness (chain length) of the SAM film. In order to explain the cutoff energy shift, we have designed a two-layer tunnel junction model by considering the capacitance response of the SAM film depending on the molecular chain length. It has been found from the model analysis, that the capacitance of the SAM changes with changing of the molecular thickness. Hence, it is concluded that the shift of the cutoff energy has originated from the change of the capacitance of the SAM film depending on the molecular chain length.

STM-induced light emission from vacuum-evaporated gold film

A vacuum evaporation system has been used to evaporate gold film on glass substrate in order to probe the scanning tunneling microscope-light emission (STM-LE) from the evaporated film. The surface morphology of the evaporated Au film has been checked by atomic force microscope (AFM). In order to estimate the appropriate thickness of the Au film, which is essential for the enhancement of STM-LE in the prism-coupled geometry, a theoretical calculation has been performed. Our theoretical simulation revealed that the light emission from the prism-coupled STM junction is strongly enhanced when the Au film has a thickness of 40 nm. AFM observation also showed that the morphology of the gold films strongly depends on the cleanliness of glass substrates and the deposition temperature. Relatively smooth surface was observed when a 40-nm-thick Au film was evaporated at room temperature on the preannealed glass substrate. Finally, the evaporated films were deposited on the flat bottom of a hemispherical glass prism, and STM-LE from the tip–sample gap into the vacuum (tip-side emission) and into the prism (prism-side emission) were measured. It was found from the experimental results that the prism-side emission is much stronger than the tip-side emission by virtue of the enhancement of the prism-coupled geometry.

Electromagnetic properties of scanning tunneling microscope tip–sample gap in the terahertz frequency range

We have investigated electromagnetic properties of the tip–sample gap of a scanning tunneling microscope (STM) in the terahertz (THz) spectral range. Light in the STM is emitted from the current source excited by tunneling electrons through dipole radiation processes. The antenna factor that measures the efficiency of the dipole radiation contains a factor proportional to the square of the angular frequency ω of the STM light. Hence, it might occur that STM light emissions in the THz spectral range become undetectably weak. The antenna factors for samples of Au and TiO2, whose plasma frequencies are in the UV and THz spectral ranges, respectively, were investigated as a function of the radius a of curvarure of the W tip. We have found that the effect of ω2 in the antenna factor can be compensated for using a tip having an a of approximately 1500 nm.

A graphene flake under external electric fields reconstructed from field-perturbed kernels

The energy, dipole moment, and polarizability of a finite hydrogen terminated zigzag graphene flake (C46H20, in 2×7 rings) are calculated in the absence of and in the presence of external electric fields reaching 5×109Vm−1 [=0.01atomic units (a.u.)]. The field intensities studied are typically found in nanoelectronics and in the tip-sample gap of a scanning tunneling microscope (STM). The change in the total energy ΔE(eV) can be closely fitted to a quadratic function of the field {1.549×104 [E (a.u.)]2} while the dipole moment μ(debye) to a linear function [2.7×E (a.u.)]. The results obtained with three different chemical models [MP2, density functional theory (B3LYP), and Hartree–Fock calculations, all with a 6-311G(2d,2p) basis set] are consistent in both trends and in absolute values. These results obtained from direct calculations are reproduced with a remarkable accuracy from a linear scaling fragmentation scheme called the kernel energy method (KEM). The KEM reproduces all studied field-free and response properties of this graphene flake model, in relative and absolute terms, independently of the underlying chemical model. An observation consistent with the known stiffness of graphene is that geometry optimization under a field as strong as 0.01a.u. insignificantly alters the total energy and the geometry of a (smaller) zigzag C28H14 flake, the difference in the field-induced stabilization energy (ΔΔE) being only 0.006eV (less than 1% of ΔE) and the average field-induced displacement of nuclear positions ∼0.0046Å [B3LYP/6-31G(d,p)].

Electrostatic Force Curves in Finite-Size-Ion Electrolytes

We obtain analytical expressions for electrostatic forces between an atomic force microscope tip and a sample immersed in an electrolyte. These simple expressions relate force to tip-sample separation explicitly incorporating tip size, solvent ion size, and solvent ion concentration. If the ions are much smaller than the tip-sample gap, the force decays monotonically, a consequence of the corresponding monotonic decays of the correlation function in the Debye-Hückel context. If the ions are of size comparable to the tip-sample gap, then oscillations appear superimposed on the overall decay, a consequence of the geometric mismatch between ion cluster size and the gap size.

Measurement of phonon energy of Sb2Te3 by scanning tunneling microscope light-emission spectroscopy

We have measured the energy of the A2u phonon mode of Sb2Te3 by scanning tunneling microscope light-emission (STM-LE) spectroscopy combined with ps-pulsed laser irradiation of the STM tip-sample gap. It is known that the STM-LE from Sb2Te3 is excited by electronic transitions at a photon energy of around 1.6eV, i.e, a single peak with a narrow spectral width appears at the photon energy in the STM-LE spectrum. For STM-LE occurring with the STM tip-sample gap irradiated by ps laser pulses, several supplementary peaks appear in the STM-LE spectra (in addition to the single original peak). The difference in energy between adjacent peaks gives the phonon energy of the A2u mode of Sb2Te3.

Fabrication of the Nanoscale Flat-Bottomed and Lamellar Structures on HOPG Surface by STM-Based Electric Lithography

The requirement for fabrication of the nanometer-scale structures has grown up recently due to the advance in the development of the nanoscale electronic-devices or bio-devices. Scanning tunneling microscope (STM)-based electric lithography is one of the potential fabrication approaches to produce nanoscale structures on a variety of materials. This study of the STM-based electric lithography intends to fabricate flat-bottomed and lamellar structures on the graphite surface, which differs from the conventionally fabricated tapered structures. The formation and the comparison of both the lamellar and tapered structures are obtained by applying distinct STM tip geometries in the STM-based electric lithography. On the basis of the experimental results, it is found that the formation of lamellar structures should be attributed to the local electrochemical reaction, while the generation of tapered structures is resulted from the dielectric breakdown in the tip-sample gap.

Measurements of Electrostatic Potential Across p–n Junctions on Oxidized Si Surfaces by Scanning Multimode Tunneling Spectroscopy

We investigated the variation in contact potential difference (CPD) voltage across p–n junctions on oxygen-passivated Si(110) surfaces by scanning multimode tunneling spectroscopy, which detects probe–sample interaction force simultaneously with tunneling current. The enhancement of sensitivity to electrostatic force was achieved with a small amplitude of probe vibration (0.3 nm) when the tip–sample gap was adjusted to reduce short-range interactions by maintaining the tunneling current at a specified bias voltage. At the optimal tip–sample gap, the CPD voltage, derived from force gradient spectra, agrees with the expected built-in potential across the p–n junction. The CPD voltage showed a standard deviation of ∼30 mV on atomically flat terraces. Larger fluctuations were ascribed to structural and charge variations on the oxidized surfaces.

Prism-Coupled Scanning Tunneling Microscope Light Emission Spectroscopy of Au Film Covered with Self-Assembled Alkanethiol Monolayer

We have investigated scanning tunneling microscope (STM) light emission from bare and alkanethiol self-assembled monolayer (SAM)-covered Au films in the Kretschmann geometry. The films were deposited on the flat bottom of a hemispherical prism, and the STM light emissions from the tip–sample gap into the vacuum (tip-side emission) and into the prism (prism-side emission) were measured. The prism-side emission was much stronger than the tip-side emission for the bare Au film. Theoretical analysis revealed that this enhancement of emission intensity is caused by the fact that surface plasmon polaritons (SPPs) localized at the Au surface become radiative on the prism side. This geometry was applied to the investigations of STM light emission from the Au film covered with an alkanethiol SAM. The prism-side emission was successfully detected by virtue of the enhancement of STM light emission.

Temperature dependence of solvation forces as measured in atomic force microscopy

An atomic force microscope (AFM) has been used to study the effect of temperature on solvation forces in the liquids octamethylcyclotetrasiloxane, n-hexadecane, and n-dodecanol confined between the AFM tip and a graphite surface. Discrete solvation layers can be observed for all three liquids at all the temperatures measured (298-348K). However, with increasing temperature there is a significant decrease in the magnitude of the measured solvation forces and a reduction in the number of solvation oscillations which can be observed. Solvation forces per se are only weakly temperature dependent and the most plausible explanation is that we are measuring how the layers are squeezed from the tip-sample gap. The squeeze out process is a thermally activated phenomenon and gives rise to the large change in the magnitude of the force oscillations with temperature. A simple analysis is provided based on transition rate theory. The large change in solvation force with temperature has implications for the measurement of localized forces using AFM, particularly in interpreting biological interactions and single asperity friction.

Plasmon enhanced luminescence from fullerene molecules excited by local electron tunneling

Tunneling electrons from a scanning tunneling microscope (STM) induce luminescence from C(60) and C(70) molecules forming fullerene nanocrystals grown on ultrathin NaCl films on Au(111). Intramolecular fluorescence and phosphorescence associated with the transitions between the lowest electronic excited state and ground state of C(70) molecules are identified, leading to unambiguous chemical recognition on the nanoscale. Moreover we demonstrate that the molecular luminescence is selectively enhanced by localized surface plasmons in the STM tip-sample gap.

All low voltage lateral junction scanning tunneling microscope with very high precision and stability

We describe the first lateral junction and fully low voltage scanning tunneling microscope, featuring very high precision, stability, compactness, and image quality (highly oriented pyrolytic graphite atomic resolution images). In its core, the tip and sample each sit on one of two parallel-mounted piezoelectric tube scanners so that the tip-sample gap is regulated along the scanners' pairing direction. The scanner's large lateral deflection provides a large gap regulation range even under low voltages, allowing exclusively using only low voltage (less than +/-15 V) operational amplifiers to precisely implement the coarse (inertial slider) and fine approach, feedback control, and hence the entire electronics. Because the scanners are identical and adjacent, thermal drifts are minimal.

Photon emission spectroscopy of thin MgO films with the STM: from a tip-mediated to an intrinsic emission characteristic

Electron injection from the tip of a scanning tunnelling microscope (STM) has been used to stimulate local photon emission from the surface of a thin MgO film grown on Mo(001). Depending on the excitation energy, several emission regimes have been identified on the basis of the energy and the spatial distribution of the emitted photons. At low excitation bias, tip-induced plasmons are excited in the tip–sample gap, carrying little information on the oxide film. With increasing tip bias, radiative electron transitions between field-emission resonances dominate the photon response from the MgO surface. Intrinsic optical modes of the oxide material, such as radiative decays of electron–hole pairs, are only observed when operating the STM in the field emission regime, where the direct correlation between the photon signal and the local surface morphology gets partially lost due to the increased tip–sample distance.

Vibration-induced structures in scanning tunneling microscope light emission spectra of Ni(1 1 0)-streaky (1 × 2)-H

We have measured the scanning tunneling microscope (STM) light emission spectra of Ni(1 1 0)-streaky (1 × 2) surfaces. When the tip was fixed over atomic hydrogen adsorbed on the surfaces, two types of vibration-induced structure were observed in the STM light emission spectra. One is the periodic fine structures that were already reported in our previous paper [Y. Uehara, S. Ushioda, Phys. Rev. Lett. 92 (2004) 066102] and the other newly found in the present experiments is a stepwise structure that is located at the vibrational energy of hydrogen below the cutoff energy of the STM light emission. They are ascribed to different excitation mechanisms of the vibration in the STM light emission process; the periodic fine structures appear when the vibrating motion is directly excited by the electrons injected from the tip. Conversely, the stepwise structure is observed when it is excited by the electromagnetic fields confined in the tip-sample gap, i.e., by localized surface plasmons.