Field emission resonance (FER) arises from the coupling of field-emitted electrons from the tip of scanning tunneling microscope (STM) with quantized states formed in the STM junction. The average lifetime of FER electrons, reflected in the linewidth according to the uncertainty principle, has been rarely explored. Here, we review our recent findings on using FER linewidth to probe field-induced effects within STM junction, including quantum trapping occurring on MoS2and Ag(100) surfaces, as well as the attractive deformation on graphite surfaces. We demonstrate that the FER linewidth on MoS2and Ag(100) surfaces can vary by up to tenfold, which is an outcome of the quantum trapping coupled with mechanisms such as correlated two-electron tunneling through exchange interaction, energy gap above the vacuum level, spin flip, light emission, and the Pauli exclusion principle. This substantial FER linewidth variation is absent on the Ag(111) surface due to the lack of an energy gap above the vacuum level. The finite lifetimes of resonant electrons signify that the FER wave function decays at a rate proportional to the FER linewidth. We find that this decay rate remains nearly unchanged with increasing FER electric field on Ag(111) surface, while it rises with field strength on graphite. This marked difference arises from the more pronounced attractive deformation of graphite, with the deformed top layer resembling monolayer graphene.

Our Previous study discovered that the linewidth of the field emission resonance (FER) observed on MoS2 surfaces varies significantly, resulting from the combined effects of correlated two-electron tunneling (CTET) and mechanisms involving spin flip, light emission, quantum trapping, and the Pauli exclusion principle. Due to CTET, the quantized state in FER can be simultaneously occupied by two electrons with opposite spins, in contrast to the conventional viewpoint that the quantized state is occupied by only one electron. Therefore, it is crucial to experimentally search for direct evidence of CTET in FER. In this study, we investigate whether the light emission associated with FER provides evidence of paired electrons on the Ag(100) surface, which possesses an energy gap above the vacuum level that may enhance the probability of CTET. This energy gap prevents electrons in the first-order FER from transmitting into the surface, thereby enabling light emission to reveal characteristics of triplet states and Auger-type excitation, both of which require two electrons occupying the quantized state, indicating that direct evidence of CTET has been obtained. Moreover, surface plasmons can be induced by electrons in the zeroth-order FER, and their radiative decay can also trigger Auger-type excitation.

We employ field emission resonance (FER) to observe the Smoluchowski effect on Ag(100) and Cu(100) surfaces, which is a charge transfer phenomenon, leading to electric dipole formation at surface steps. On Ag(100), pronounced charge transfer results in a discontinuity in FER energies at step sites. In contrast, this discontinuity is absent on Cu(100), indicating that the Smoluchowski effect is negligible. Density functional theory calculations confirm this significant difference in charge transfer at the step. By analyzing FER energies using the triangular potential model, we extract the spatial variation of the work function around the step on both surfaces. Our results for Cu(100) demonstrate that a reduction in the work function can occur even without a step electric dipole, contrary to the widely accepted explanation that the Smoluchowski effect reduces the work function. Furthermore, while it is generally accepted that as charge transfer occurs, local negative (positive) surface charge raises (lowers) the work function, our results for Ag(100) reveal the opposite trend. Additionally, the extracted work function enables spatially resolving the positive and negative charge densities within a step electric dipole, which has not yet been achieved using other local probe techniques.

Electric fields in the junction of a scanning tunneling microscope (STM) are generally considered to have a negligible impact on the vacuum level (VL) of materials. We employed field emission resonance (FER) in the STM, combined with the triangular potential model, to measure the VL of the Ag(100) surface under varying electric currents. Unexpectedly, our results reveal that the VL exhibits a linear positive energy shift with increasing electric field strength. We suggest that this Stark shift in the VL arises from local Fermi level alignment induced by the STM tip. Additionally, we examined the VL of Ag islands grown on Cu(111) and Au(111) surfaces under different currents. Despite the Ag island having a lower work function than the Cu(111) and Au(111) surfaces, the energy shift in the VL with respect to the electric field on the Ag island is almost identical to that on the substrate under the same tip structure. This suggests that the Stark shift of the VL is insensitive to the work function. These findings are crucial for utilizing FER to measure local work function variations on surfaces, as the measured value is not influenced by the STM tip structure or the tunneling current, both of which can alter the electric field.

In this study, we discovered that the energy gap above the vacuum level in the projected bulk band structure of Ag(100) prevents electrons in the first-order field emission resonance (FER) from inducing the surface plasmons. This mechanism allows light emission from FER to reveal characteristics of triplet states and Auger-type excitation resulting from two-electron tunneling in FER. According to optical spectra, surface plasmons can be induced by electrons in the zeroth-order FER. However, corresponding radiative decay can also trigger Auger-type excitation, whose energy state is influenced by the sharpness-dependent image potential acting on the scanning tunneling microscope tip.

The coupling of graphene with a ferromagnetic material opens opportunities for technological innovations in spintronics. To obtain this coupling it is necessary to control the elaboration of interfaces at the atomic scale. Here, we present results on cobalt intercalation between graphene and a buffer layer supported on a SiC(0001) substrate. As a result, we obtain cobalt islands covered by graphene whose local electronic properties are measured by scanning tunneling microscopy and spectroscopy. These islands reveal two very distinct shapes and properties. Small-islands with atomic height and very narrow size distribution and, more interestingly, flat cobalt nanodots lower than one nanometer high, that are encapsulated by graphene. Compared to a graphene monolayer on SiC, those nanodots exhibit very different spectroscopic signatures. Using dI/dV local differential conductance spectra together with an analysis of image potential surface states measured thanks to dz/dV spectra, we show that graphene on the nanodots is neutrally charged. Moreover, its 4.65 eV work function is surprisingly larger than the predicted value of 3.8 eV for graphene on Co. First principle calculations show that those Co nanodots can be seen as cobalt bilayer sandwiched between two carbon planes.

We demonstrated on Ag(111) and Ag(100) surfaces that the reciprocal of the field emission resonance (FER) linewidth, which is proportional to the mean lifetime of resonant electrons in FER, may vary with the electric field. The variation on Ag(111) was nearly smooth, whereas that on Ag(100) was sporadic and fluctuated remarkably. This drastic difference can be explained through their dissimilar projected bulk band structures and the ensemble interpretation of quantum mechanics, according to which all resonant electrons are governed by a single wave function. Ag(100) has an energy gap above its vacuum level, whereas Ag(111) does not. Consequently, the dissipation rate of the wave function, which is relevant to the FER linewidth, on Ag(111) was almost stable, whereas that on Ag(100) fluctuated. The fluctuation revealed that the quantum trapping effect and surface dipole layer on Ag(100) surface can be investigated through FER.

The apex structure of a scanning tunneling microscope (STM) tip consists of a base with radius of tens of nanometers and protrusion with atomic-scale sharpness. We characterized the tip base radius and sharpness on the basis of field emission resonance (FER) energies. We derived two quantities from the first- through sixth-order FER energies, which were related to tip sharpness and base radius. The base radius can remain unchanged while the sharpness varied, and the tips can have identical sharpness but different base radii. The base radius can significantly affect the peak intensities of FER, which corresponds to the mean lifetime of FER electrons, on a Ag(100) surface but not on those of FER on a Ag(111) surface. This difference results from the surface dipole layer and quantum trapping effect (QTE) on the Ag(100) surface which are greater than those on the Ag(111) surface.

We studied wave function dissipation (WFD) in field emission resonance (FER) by performing scanning tunneling microscopy on the highly oriented pyrolytic graphite (HOPG) and Ag(111) surfaces under two conditions: (1) the same current and FER number; (2) the same tip structure but different currents. Under the first condition, we observed that the decay rate corresponding to the WFD exhibited a larger variation on the HOPG surface than it did on the Ag(111) surface. Under the second condition, the decay rate was nearly independent of the FER electric field for the Ag(111) surface; by contrast, it was linearly proportional to the FER electric field for the HOPG surface. These remarkable differences can be attributed to the factors that the tip-induced attractive deformation caused by the electrostatic force was considerably more prominent on the HOPG surface than on the Ag(111) surface and that the deformed HOPG top layer had a unique electronic structure similar to that of single-layer graphene.

Due to the widespread applications of biosensors, such as in magnetic resonance imaging, cancer detection and drug delivery, the use of superparamagnetic materials for preparing biosensors has increased greatly. We report herein on a strategy toward fabrication of a nanoscale biosensor composed of superparamagnetic films. On increasing the film thickness of magnetic layers, a phase transition typically occurs from either a low-Curie-temperature state or a superparamagnetic state to a ferromagnetic state. A new finding is demonstrated wherein a phase transition of such a superparamagnetic phase can be induced by controlling the thickness of ultrathin ferromagnetic layers with perpendicular magnetic anisotropy. Both the M-H curve with zero coercive force at 300 K and deviations of the normalized hysteresis loop at 2 K confirm the superparamagnetic state of Co/Ir(111) at room temperature. An overstrained film transforming into clusters (OFTC) model based on the new finding and our experimental evidence is proposed for modeling this phenomenon. From the energetic point of view of the OFTC model, we propose a limited distortion mechanism that can be useful in determining the critical thickness for the phase transition. This mechanism considers the balance between interfacial strain energy and surface free energy. A method for producing superparamagnetic films by taking advantage of the accumulation of strain and relaxation is reported.

We demonstrate that the linewidth of the field emission resonance (FER) observed on the surface of MoS2 using scanning tunneling microscopy can vary by up to one order of magnitude with an increasing electric field. This phenomenon originates from quantum trapping, in which the electron relaxed from a resonant electron in the FER is momentarily trapped in a potential well on the MoS2 surface due to its wave nature. Because the relaxed electron and the resonant electron have the same spin, through the action of the Pauli exclusion principle, the lifetimes of the resonant electrons can be substantially prolonged when the relaxed electrons engage in resonance trapping. The linewidth of the FER is thus considerably reduced to as narrow as 12 meV. The coexistence of the resonant electron and the relaxed electron requires the emission of two electrons, which can occur through the exchange interaction.

By using localization techniques, we demonstrated that the morphology of a 2D material in three dimensions can be optically obtained with nanometer precision in z-axis. This technique provides a convenient method to study the correlation between the optical properties and the morphology of 2D materials for the same area. We utilized optical localization microscopy to directly study the correlation between the ultrafast photoluminescence and the morphology of graphene. We observed enhancement of the ultrafast photoluminescence from the deformed graphene. In comparison to the planar graphene, the enhancement factor of ultrafast photoluminescence could be up to several times at the highly curved region. We found that the intensity of photoluminescence from the uniaxially rippled graphene depends on the polarization of excitation light. Furthermore, Raman spectroscopy was used to measure the strain distribution. Pump-probe measurements were conducted to reveal the carrier dynamics. From the experimental results, two mechanisms were confirmed to mainly account for the enhancement of ultrafast photoluminescence from the deformed graphene. One is the deformation-induced strain increases the absorption of graphene. The other is the prolonged carrier relaxation time in the curved graphene.

Compound formation at a metal/semiconductor interface plays crucial roles in the properties of many material systems. Applications of Ni silicides span numerous areas and have the potential to be used as new functionalities. However, the magnetic properties of ultrathin Ni layers on silicon surfaces and related chemical compositions at the interface are not fully understood and the influence of Ag additives on the reactivity of Ni/Si(111) remain unclear. We report herein on the fact that the dominant species produced at the interface is NiSi, which is produced by the spontaneous formation of strong bonds between Ni and Si atoms. Assuming that a Ni layer is formed over a NiSi layer with the total coverage as a constraint, we established a chemical shift-related concentration model that, in effect, represents a practical method for determining the amount of ultrathin Ni silicides that are produced at the buried interface. The formation of Ag-Si particles provide a viable strategy for enhancing silicide formation via a specific interaction transfer mechanism, even at room temperature. The mechanism is related to differences in the enthalpies of formation ΔH Ag-Si , ΔH Ni-Ag , and ΔH Ni-Si , for these phases and provides insights into strategies for producing ultrathin silicides at a buried interface.

Molybdenum disulfide (MoS2) are emerging as an attractive material for next-generation electronics. Therefore, the growth of metals on MoS2 is of importance for future technologies. We used scanning tunneling microscopy to study the growth of Ag on MoS2. It was found that flat Ag islands with (111) surfaces and steep edges can be formed on MoS2 without a wetting layer. In particular, islands prefer to have thickness of six atomic layers at a low coverage. The manifestation of preferred thickness implies that the formation of Ag islands follows the electronic growth driven by the quantum size effect. Moreover, we annealed Ag islands to observe their thermal stability. We found that most of the Ag islands have thickness of 11 atomic layers after annealing, revealing another preferred thickness. This result implies that the applied thermal energy is insufficient for overriding the quantum size effect, although it can induce the reformation of Ag islands.

The form of the external potential (FEP) for generating field emission resonance (FER) in a scanning tunneling microscopy (STM) junction is usually assumed to be triangular. We demonstrate that this assumption can be examined using a plot that can characterize FEP. The plot is FER energies versus the corresponding distances between the tip and sample. Through this energy–distance relationship, we discover that the FEP is nearly triangular for a blunt STM tip. However, the assumption of a triangular potential form is invalid for a sharp tip. The disparity becomes more severe as the tip is sharper. We demonstrate that the energy–distance plot can be exploited to determine the barrier width in field emission and estimate the effective sharpness of an STM tip. Because FERs were observed on Pb islands grown on the Cu(111) surface in this study, determination of the tip sharpness enabled the derivation of the subtle expansion deformation of Pb islands due to electrostatic force in the STM junction.

We elucidate that the tip sharpness in scanning tunneling microscopy (STM) can be characterized through the number of field-emission (FE) resonances. A higher number of FE resonances indicates higher sharpness. We observe empty quantum well (QW) states in Pb islands on Cu(111) under different tip sharpness levels. We found that QW states observed by sharper tips always had lower energies, revealing negative energy shifts. This sharpness-induced energy shift originates from an inhomogeneous electric field in the STM gap. An increase in sharpness increases the electric field inhomogeneity, that is, enhances the electric field near the tip apex, but weakens the electric field near the sample. As a result, higher sharpness can increase the electronic phase in vacuum, causing the lowering of QW state energies. Moreover, the behaviors of negative energy shift as a function of state energy are entirely different for Pb islands with a thickness of two and nine atomic layers. This thickness-dependent behavior results from the electrostatic force in the STM gap decreasing with increasing tip sharpness. The variation of the phase contributed from the expansion deformation induced by the electrostatic force in a nine-layer Pb island is significantly greater, sufficient to effectively negate the increase of electronic phase in vacuum.

We employ ultra-high vacuum electron microscopy to investigate the interaction of metal nanoparticles through graphene. The nanoparticles attract those on the other side of graphene in the systems of Ag/graphene/Ag and Cu/graphene/Cu. In contrast, the system of Au/graphene/Au manifests the repelling interaction. Our density functional theory calculations demonstrate that for lower electron-affinity metals such as Cu and Ag, the clusters on opposite sides of graphene prefer the same site to share the electron-loss and reduce the energy. While for higher electron-affinity metals such as Au, they prefer to stay away from the clusters on the other side of graphene.

Field emission (FE) resonance (or Gundlach oscillation) in scanning tunneling microscopy (STM) is a phenomenon in which the FE electrons emitted from the microscope tip couple into the quantized standing-wave states within the STM tunneling gap. Although the occurrence of FE resonance peaks can be semi-quantitatively described using the triangular potential well model, it cannot explain the experimental observation that the number of resonance peaks may change under the same emission current. This study demonstrates that the aforementioned variation can be adequately explained by introducing a field enhancement factor that is related to the local electric field at the tip apex. The peak number of FE resonances increases with the field enhancement factor. The peak intensity of the FE resonance on the reconstructed Au(111) surface varies in the face-center cubic, hexagonal-close-packed, and ridge regions, thus providing the contrast in the mapping through FE resonances. The mapping contrast is demonstrated to be nearly independent of the tip–sample distance, implying that the FE electron beam is not divergent because of a self-focus function intrinsically involved in the STM configuration.

An ideal freestanding graphene in air is centrosymmetric in three dimensions, and its optical second harmonic generation is inhibited. However, we found strong second harmonic generation from suspended grapheme sheets, and attributed this observation to curved sheet in the long range.