Configurational control of photon emission from a molecular dimer

Abstract

Tin-phthalocyanine molecules adsorbed on a NaCl ultrathin film on Au(111) exhibit electrofluorescence excited by a current across a scanning tunneling microscope junction. Exploring the dependence of the molecular monomer photon yield on the injected current evidences the one-electron excitation process underlying the neutral-exciton luminescence. Photon spectra of the monomer exhibit vibrational progression and hot luminescence, while the dimer electrofluorescence spectroscopic fine structure results from the coupling of the adjacent optical transition dipoles. The photon yield of the dimer is significantly altered upon changing the configurational state of one of the two molecules. In one of the bistable configurations light emission is amplified compared to the monomer, and it is reduced in the other.

Figures (4)

• Figure 1: Electroluminescence spectroscopy of an Sn-Pc-$u$ monomer. (a) Sketch of the junction geometry including an Au-covered tip and the top surface layer (yellow) of Au(111) as well as a single Sn-Pc-$u$ molecule (H: white, C: black, N: blue, Sn: red) residing atop $4$ atomic NaCl (Na: green, Cl: violet) layers. (b) Top: top view of the relaxed vacuum structure of Sn-Pc. Bottom: constant-current STM image of a single Sn-Pc-$u$ molecule on NaCl (sample voltage: $1.9\,\text{V}$, current: $20\,\text{pA}$, the gray scale covers apparent heights ranging from $0\,\text{pm}$ (dark) to $695\,\text{pm}$ (bright)). The scale bar indicates $1\,\text{nm}$. (c) Wide range STML spectrum (dots) of Sn-Pc-$u$ acquired at $2.5\,\text{V}$, $80\,\text{pA}$ for $180\,\text{s}$ above the position indicated by the dot in (b). The solid line represents smoothed data.
• Figure 2: Photon emission characteristics of Sn-Pc-$u$ monomers and dimers. (a) Spectral photon yield of Sn-Pc-$u$ recorded at $1.9\,\text{V}$, $50\,\text{pA}$ for $180\,\text{s}$. The data are normalized by the spectrum of the tip-induced plasmon acquired above the bare NaCl island with the same tip. Inset: constant-height STM image of Sn-Pc-$u$ ($1.9\,\text{V}$, the gray scale encodes currents ranging from $10\,\text{pA}$ (dark) to $100\,\text{pA}$ (bright), scale bar: $1\,\text{nm}$). The dot indicates the tip position for luminescence spectroscopy. (b) Constant-height spectrum of $\text{d}I/\text{d}V$ (dots) recorded above the same position as the light spectrum (feedback loop parameters: $1.9\,\text{V}$, $20\,\text{pA}$). The solid line represents smoothed data. Squares depict the integrated spectral photon yield of (a) with the dashed line as a guide to the eye. Inset: illustration of alignment of tip and sample electron chemical potentials with molecular frontier orbitals for the resonant excitation of the molecule by tunneling electrons. Arrows indicate tunneling of electrons from the tip across the vacuum barrier to the LUMO and from the HOMO across the NaCl barrier to the sample. (c) Evolution of photon rate (squares) with the junction current at $1.9\,\text{V}$. The solid line is a power law fit to the data. (d)--(f) As (a)--(c) for an Sn-Pc-$u$ dimer. In (d), the acquisition time for the dimer spectrum is $240\,\text{s}$ and a monomer spectrum (red) recorded with the same tip as the dimer spectrum is added for comparison. Inset to (d): constant-height STM image of an artificially assembled Sn-Pc-$u$ dimer ($1.9\,\text{V}$, the gray scale encodes currents ranging from $10\,\text{pA}$ (dark) to $75\,\text{pA}$ (bright), scale bar: $1\,\text{nm}$).
• Figure 3: Spatially resolved electroluminescence spectra of the Sn-Pc-$u$ monomer and $uu$ dimer. (a) Site dependence of the normalized monomer spectral photon yield ($1.9\,\text{V}$, $50\,\text{pA}$, $120\,\text{s}$). The position of spectroscopy is marked by the dot in the constant-height STM images ($1.9\,\text{V}$, $2.3\,\text{nm}\times 2.3\,\text{nm}$, the gray scale encodes currents ranging from $2\,\text{pA}$ (dark) to $30\,\text{pA}$ (bright)). The vertical stripe has a width of $1\,\text{meV}$ and marks the $S_1\rightarrow S_0$ transition in all spectra. (b),(c) Site dependence of the normalized $uu$ dimer spectral photon yield ($1.9\,\text{V}$, $50\,\text{pA}$, $120 \,\text{s}$) atop the indicated (dots) positions. The position of spectroscopy is marked by the dot in the constant-height STM images ($1.9\,\text{V}$, $5\,\text{nm}\times 5\,\text{nm}$, the gray scale encodes currents ranging from $10\,\text{pA}$ (dark) to $75\,\text{pA}$ (bright)), where the individual monomer molecules of the dimer are labeled $A$ and $B$. The vertical lines $1$--$5$ indicate the dipole-coupled exciton states (see illustrations between (b) and (c)) underlying the fine structure. The spectra in (a)--(c) are normalized to their corresponding maximum and vertically offset.
• Figure 4: Electrofluorescence of $uu$ and $ud$ Sn-Pc dimers. (a),(b) Constant-height STM image of a $uu$ (a) and a $ud$ (b) dimer ($1.9\,\text{V}$, the gray scale encodes currents from (a) $10\,\text{pA}$, (b) $2\,\text{pA}$ (dark) to (a) $75\,\text{pA}$, (b) $25\,\text{pA}$ (bright), scale bar: $1\,\text{nm}$). (c) Normalized spectral photon yield of an Sn-Pc-$u$ monomer and Sn-Pc dimers in the $uu$ and $ud$ configuration ($1.9\,\text{V}$, $50\,\text{pA}$, $240\,\text{s}$). All spectra were recorded with the same tip above the positions indicated by the dots in (a),(b). They are normalized by the tip plasmon luminescence spectrum atop bare NaCl. (d) Same as (c) with the spectra normalized by their respective maximum and vertically offset.