Abstract
Keywords
1. Introduction
Single molecules are elementary building blocks for constructing integrated functional devices in the field of organic electronics and optoelectronics. Single-molecule spectroscopy conducted by a scanning tunneling microscope (STM), including STM-induced electroluminescence (STML)[
The fluorescence-quenching-suppression effect of an insulating decoupling layer might be attributed to at least two factors. Firstly, the insertion of an insulating decoupling layer will largely block the direct electron hybridization between the molecule and the substrate and thus suppress the ultrafast charge-transfer-induced quenching upon contact. Secondly, the decoupling layer will increase the distance between the molecular emitter and the metal substrate and thus suppress the Ohmic loss induced by the substrate. For practical reasons, one important question is whether and how the thickness and dielectric constant of the decoupling layer affect the molecular fluorescence. Previous STML experiments with single molecules adsorbed on ultrathin insulating NaCl films on Ag(100) surface show that as the thickness of NaCl films grows from two layers to five layers, the electroluminescence intensity grows monotonically, due to the better electronic and electromagnetic decoupling provided by the thicker decoupling layers[
In this Letter, we use classical electromagnetic simulations to systematically study the influence of a thin insulating decoupling layer on the local electric field enhancement and emission properties of a point dipole in the STM tunnel junction. For simplification, we only consider the electromagnetic response of the junction while the electronic structures of the metal junction and the decoupling layer are not considered. Our simulations show that the growth of a decoupling layer on the metal substrate surface will increase both the electric field intensity and lateral spatial confinement, compared to the situation with the same tip–substrate distance but without a decoupling layer. In addition, we find that there exists an optimal decoupling layer thickness to obtain the largest quantum efficiency of a dipole emitter, but this optimal thickness depends on the dielectric constant of the dielectric layer, the materials of the junction, as well as the dipole orientation. Furthermore, the simulations suggest that to obtain stronger molecular photoluminescence intensity and Raman intensity as well as higher spatial resolution, a decoupling layer with a larger dielectric constant is preferred.
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2. Numerical Methods
We performed electromagnetic simulations using frequency-domain finite-element method based on COMSOL Multiphysics to numerically study the local electric field enhancement and the radiative and non-radiative decay enhancement of an electric dipole emitter in model STM junctions[
Figure 1.(a) Schematic illustration of the model STM junction used for electromagnetic simulations. The tip is modeled as a truncated cone with a radius of 30 nm, a height of 300 nm, and a semi-cone angle of 15°. (b) Local electric field enhancement Mz = |Ez|/|E0| at position (0, 0, 1.2 nm) as a function of incident photon energy for different decoupling layer dielectric constants εr. (c) Spatial distribution of the local electric field enhancement Mz = |Ez|/|E0| in the xz plane (with y = 0) for different dielectric constants εr. (d) Instantaneous induced surface charge density at the surfaces of the tip and the substrate for εr = 1 (upper panel) and εr = 9 (lower panel). The density values are normalized to the maximum value of εr = 1. In (b)–(d), the thickness of the decoupling layer is set to td = 1 nm, and the distance between the tip apex and the top surface of the decoupling layer is set to dvac = 0.6 nm.
The quantum efficiency of a point dipole is calculated as follows. We first calculate the electromagnetic fields in space for a point dipole placed at a position . Then, the radiation power to the far field from the dipole is calculated through integrating the time-averaged Poynting vector () over the boundary of the simulation region, where and represent the electric and magnetic fields, respectively. The non-radiative power is calculated through integrating the power dissipation density () over the volumes of the tip and substrate, where represents their conductivity. Then, the quantum efficiency is evaluated as .
3. Results and Discussions
Surface plasmons have been widely used to enhance weak optical signals, e.g., from molecules[
We first study the influence of the dielectric constant of the decoupling layer on the electric field enhancement in the model STM junction under plane wave illumination. As schematically illustrated in Fig. 1(a), a -polarized plane wave with an incident angle of 60° with respect to the surface normal is used as an excitation source. We define the local electric field enhancement factor , where is the vertical component of the local electric field at position , and is the electric field of the incident plane wave. When the dielectric constant of the decoupling layer is set to , the plasmonic nanocavity exhibits two clear resonance peaks at and , which can be attributed to the dipolar and quadrupolar modes, respectively [Fig. 1(b)][
The phenomenon that the electric field is enhanced in the vacuum but weakened inside the decoupling layer is more clearly illustrated in Fig. 2(a), where the local electric field enhancement along the axis (with and ) is shown. Here, the positions at , , and correspond to the substrate surface, the upper surface of the decoupling layer, and the tip apex, respectively. When , the electric field in the gap region is vertically continuous. In addition, the electric field is with almost the same intensity close to the tip surface () and the substrate surface () since the tip radius is much larger than the gap distance. This is further supported by the spatial distribution of the normalized induced surface charge densities for in the lower panel of Fig. 1(d), from which we can see that the maximum value of is almost identical at the tip apex and at the substrate surface (all charge density values are normalized to the maximum value of ). However, as becomes larger than one, the electric field becomes discontinuous at the interface between the vacuum and the dielectric layer (i.e., at the plane): the electric field is stronger in the vacuum region but weaker in the decoupling layer. As is further increased, the discontinuity becomes increasingly more significant. Indeed, the continuous condition requires that the normal component of the electric displacement be continuous at the interface and thus , where and are the components of the electric field in the vacuum and in the decoupling layer, respectively. Therefore, the electric field in the vacuum region will approximately be times as large as the electric field in the decoupling layer, as can be verified in Fig. 2(b). As shown in the lower panel of Fig. 1(d), the maximum value of at the tip terminal is increased by over 210%, while it is decreased by over 80% at the substrate surface when is increased from 1 to 9. In other words, one can say that as the dielectric constant of the decoupling layer increases, the electric field at the dielectric/substrate interface is more strongly screened, and thus the induced surface charge at this interface decreases. Meanwhile, becomes more localized in the junction. In addition, with the increment of , not only the intensity of the electric field becomes stronger but its lateral spatial distribution also becomes more confined both horizontally and vertically in the vacuum region. As shown in Fig. 2(d), as increases from 1 to 9, the full width at half-maximum (FWHM) of in the lateral direction decreases from 14.6 nm to 9.2 nm. This is because as is increased, the electric field is more confined in the vacuum region between the tip and the decoupling layer, which is evidently smaller than the region between the tip and substrate. This decreases the vertical and horizontal spatial extension of the electric field and increases the electric field energy density, generating electric fields with higher intensity and smaller FWHM.
Figure 2.(a) Electric field enhancement Mz = |Ez|/|E0| along the z axis (with x = 0 and y = 0) for different decoupling layer dielectric constants εr. (b) Non-normalized and (c) normalized electric field enhancement Mz = |Ez|/|E0| along the x axis (with y = 0 and z = 1.2 nm). The gap distance is dgap = 1.6 nm, and the decoupling layer thickness is td = 1 nm.
In tip-enhanced single-molecule spectroscopy experiments, a larger electric enhancement and a better spatial confinement of the local electric field are usually in demand, since the former is usually related to the intensity, while the latter is related to the spatial resolution of molecular spectroscopy measurements. To achieve these goals, one could usually employ a sharper tip or decrease the gap distance. However, the introduction of the decoupling layer usually prevents one from shrinking the gap distance to extremely small values. Our calculations suggest that increasing the dielectric constant of the decoupling layer will help to increase the field enhancement and spatial confinement, although the distance between the surfaces of the metal tip and the metal substrate is still kept at a relatively large value.
In addition to the dielectric constant , the thickness of the dielectric layer could also affect the intensity and the lateral spatial confinement of the local electric fields. In Fig. 3, the distance between the tip apex and the upper surface of the decoupling layer is set to be , so the gap distance is 0.6 nm larger than the thickness of the decoupling layer: . As shown in Fig. 3(a), when , as the thickness of the decoupling layer increases (this is equivalent to increasing the gap distance), the field enhancement quickly decreases, and the field becomes laterally less confined. Specifically, increasing the gap distance from 0.6 nm to 3.6 nm will dramatically decrease the field enhancement from to [Figs. 3(a) and 3(b)], while significantly increasing the FWHM of from to . This is because the increment of the gap distance increases the effective volume of the plasmonic nanocavity, which will not only make the electromagnetic fields less confined both vertically and horizontally, but also reduce the local electromagnetic density and thus decrease the electric field intensity. On the other hand, if we choose a dielectric layer with a large dielectric constant of , as is increased from 0 to 3 nm, the field enhancement is reduced from to , while the FWHM of is only increased from to [Figs. 3(c) and 3(d)]. As illustrated, the reductions of the field intensity and lateral spatial confinement are much less significant with the increment of the gap distance if is large.
Figure 3.Electric field enhancement with and without a dielectric layer with different thicknesses td and dielectric constants εr. Electric field enhancement |Ez|/|E0| distribution in the xz plane (y = 0) for (a) εr = 1 and (c) εr = 9 with different decoupling layer thicknesses. (b) Enhancement and (d) FWHM of the electric field enhancement |Ez|/|E0| along the x axis (y = 0 and z = td + 0.2 nm) as functions of decoupling layer thickness for four different dielectric constants.
Previous STML experiments for single molecules on ultrathin NaCl films suggested that as the thickness of the NaCl decoupling layer grows from two layers to five layers, the fluorescence intensity grows monotonically[
Figure 4.Schematics showing the configurations of (a) a vertical dipole and (d) a horizontal dipole in the model STM junctions. In (a), the shape of the tip and substrate are the same as in Fig.
However, when the vertical dipole is placed above a more lossy W substrate, choosing a proper decoupling layer thickness becomes more important [Fig. 4(c)]. When the thickness is very small, the non-radiative channels induced by the lossy W substrate will largely decrease . As is increased, the distance between the dipole and the substrate is increased, and the quantum efficiency first increases and then decreases, showing an optimal value that is sensitive to . For example, the optimal thickness to reach the largest is for , further increasing will decrease rather than increase . The optimal thickness grows with increasing . For , further increasing will not decrease . This is because, when is large enough, the electric field is primarily confined within the small region between the tip terminal and the upper surface of the decoupling layer, and thus the electromagnetic energy density is not much decreased with increasing decoupling layer thickness, as we have already learned from Fig. 3. In other words, the plasmonic nanocavity formed by a metal tip and a dielectric substrate with large dielectric constant can induce large field intensity and sustain relatively efficient molecular emission.
Considering that many STML experiments were carried out for molecules with transition dipoles oriented parallel to the substrate, we also calculate the dependence of the quantum efficiency on the dielectric constant and thickness of the decoupling layer for a horizontal dipole [see Fig. 4(d) for schematic illustration]. To ensure comparatively large quantum efficiency, a spherical protrusion with a radius of 0.5 nm is superimposed at the apex of the tip shaft, and the horizontal dipole is laterally placed 0.5 nm away from the tip apex, following Ref. [9]. It is worthwhile to note that the protrusion at the tip apex is essentially important in preventing the emission from dipoles that are oriented parallel to the substrate from quenching. Moreover, to ensure an effective coupling between the horizontal dipole and the nanocavity plasmon, the dipole is placed 0.5 nm away laterally from the tip apex[
Finally, we briefly discuss the influence of the thickness and dielectric constant of the decoupling layer on the intensity of single-molecule TEPL and TERS. In the calculations of Fig. 5, the TEPL intensity is approximated as , and the TERS intensity is approximated as . As shown in Figs. 5(a) and 5(b), both the intensities of TEPL and TERS show a strong dependence on the thickness and dielectric constant of the decoupling layer. For , as the decoupling layer thickness is increased from 0.2 nm to 5 nm, the TEPL and TERS intensities, respectively, decrease by 3 and 4 orders of magnitude due to the much weaker electric field in larger gaps. However, as is increased, the decrease of TEPL and TERS intensities with increasing decoupling layer thickness becomes much slower. Moreover, at a fixed decoupling layer thickness, both the TEPL and TERS intensities become larger for larger , especially when is large. As shown in the simulated single-molecule TEPL and TERS photon images in Figs. 5(c) and 5(d), for a fixed value of , as is increased from 1 to 9, the TEPL intensity increases by times, and the TERS intensity increases by times. Moreover, the photon images become more spatially confined, suggesting that applying a decoupling layer with larger dielectric constant could further improve the spatial resolution of single-molecule TEPL and TERS imaging. Thus, it is believed that a dielectric layer with larger is very helpful to increase the intensities as well as the spatial confinement of STML, TEPL, and TERS of individual molecules.
Figure 5.Influence of the dielectric constant of the decoupling layer on the TEPL and TERS intensities of a single dipole emitter. (a) TEPL intensity and (b) TERS intensity as functions of the decoupling layer thickness for different dielectric constants. (c) and (d) show the TEPL and TERS images for εr = 1 and εr = 9. In (c) and (d), the molecule is approximated as a vertical dipole, the gap distance is 1.6 nm, the thickness of the decoupling layer is 1 nm, and the plane for simulation of the photon image is z = 1.2 nm.
4. Conclusion
In this work, we have used electromagnetic simulations to numerically study the influence of a thin dielectric decoupling layer on the local field enhancement and molecular spectroscopy intensity in the junction of an STM. Our simulations show that the growth of a decoupling layer on the metal substrate surface will increase both the electric field intensity and lateral spatial confinement, compared to the situation with the same tip–substrate distance but without a decoupling layer. In addition, we find that there exists an optimal decoupling layer thickness to obtain the largest quantum efficiency of a dipole emitter, but this optimal thickness depends on the dielectric constant of the dielectric layer, the materials of the junction, as well as the dipole orientation. To obtain higher molecular photoluminescence, Raman intensities, and spatial resolution, a decoupling layer with a larger dielectric constant is preferred. These results may be instructive for further studies in molecular optics and optoelectronics in plasmonic junctions.
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