Influence of Co bilayers and trilayers on the plasmon-driven light emission from Cu(111) in a scanning tunneling microscope

Light emission from the gap cavity formed by the tip of a scanning tunneling microscope (STM) and a flat metallic sample allows us to probe the dielectric response of metals at the atomic scale and presents a way to distinguish between different materials. The excitation mechanism of the charge carrier oscillations, which ultimately decay into light, is linked to inelastic electron tunneling as opposed to the mostly semiclassical picture of the electromagnetic resonance of the gap cavity. Thus, the observed light emission does not only reflect the electromagnetic resonance of the cavity but also involves the electronic density of states. In this paper, we compare light emission from Cu(111) and Co nanoislands on Cu(111). We find a strong intensity contrast but almost no alteration of the resonance wavelength except close to step edges. Our results show that the light emission from the STM junction is highly sensitive to a few atomic layers of alien material mostly due to the dielectric properties of the layer.

Figure 1

(a) STM overview of the area where the photon map was recorded. (b) Map of the total photon counts. The crosses indicate where example spectra of the full wavelength range were extracted and displayed in (c).

Figure 2

Change of the cavity resonance and emitted light intensity as the STM tip is moved across vertical steps of different heights. (a) Map of the fitted main peak energy, with color scale ranging from 1.92 (dark) to $1.94\mathrm{eV}$ (bright). The arrows indicate where the profiles of vertical tip displacement, fitted peak energy, and fitted peak intensity in (c), (d), and (e) were extracted, respectively. These profiles were averaged over six adjacent lines of measurement points. The height of the steps in atomic layers is given in the key in (c). (b) Model of the gap cavity in the presence of a step edge. The dotted line represents how a step edge is typically imaged in STM due to the presence of an image state.

Figure 3

Standing-wave patterns of the electronic surface state as observed in the photon map. (a) Maps of photon counts within the indicated wavelength intervals recorded on the trilayer Co island (top) and a section of the Cu(111) substrate (bottom). The false color scale was adapted to highlight the Friedel oscillations of decreasing wavelength as the photon wavelength increases. (b) Model of a single IET channel responsible for the emission of photons of a certain energy $\hslash \omega$. (c), (d) 2D Fourier transforms of the isochromatic maps (650 to $660\mathrm{nm}$) including only the Cu(111) area and trilayer Co island, respectively. The diameter of the solid disk in (c) and the length of the line in (d) were identified as twice the inverse wavelength of the wave patterns visible in (a). (e), (f) Extracted dispersion relations for the surface state of the Cu(111) and the Co trilayer nano-island, respectively. Black squares denote data points of this work, the solid red lines represent the dispersion relations with effective mass of 0.40 [29] and 0.37 [30] for Cu and Co, respectively.

Figure 4

(a) Geometry of the numerical simulation of the tip in front of the sample. (b) Antenna efficiency as function of wave length for a Cu sample, $1\mathrm{nm}$ of vacuum and an Ag tip (blue) and the same with an additional layer of $1\mathrm{nm}$ Co in the junction (orange).