David Kreher, enseignant-chercheur
Self-Assembled Engineered Sub-Wavelength Emitters for STM-Excited Single Molecule Electroluminescence
INTRODUCTION and CONTEXT
So far, molecular self-assembly constitutes a promising route for the bottom-up manufacturing of nanostructures on atomically-flat surfaces. The mechanisms of molecular selforganization are currently under intense investigation aimed at the tailoring of mainly planar piconjugated organic building blocks, in order to obtain specific topologies. They involve a subtle interplay between molecule-substrate interactions and intermolecular interactions. The bottom-up approaches based on supramolecular chemistry have been found to be of particular interest for the formation of preprogrammed organic self-assembled monolayers.
The next objective is to exploit such techniques for the realization of self-assembled molecular architectures designed to exhibit original properties resulting from their nanometric structure, in view of future applications in molecular electronics, photonics or nanomechanical devices.[4,5]
Many of such applications are based on transient electronic excitations : luminescence processes, photo-isomerisation, photo-desorption, photoinduced charge transfers, fluorescence resonance energy transfers, and require a conducting substrate for electrical power supply, transmission or extraction of an electronic signal, application of an electric field.
However, a close proximity between the conjugated system and a conducting substrate results in the quenching of electronic excitations. At distances comparable to electron wave function decay lengths, i.e. 1-3 Å, ultra-fast electron-tunnelling takes place from molecule to substrate and reversely, which turn out into the so-called Dexter-type relaxations. For a conjugated system directly adsorbed on a metal substrate in face-on geometry, molecular excitations relax within a picosecond or less, thus preventing any other useful process to take place. This is why as concerns electronic or photonic applications, decoupling the molecule of interest from conducting substrate permits to preserve its electronic integrity, and to access both the understanding and the control of the electronic interaction between molecule and substrate. This is currently achieved either by covering the metallic substrate with an insulating layer or by maintaining the molecule above the substrate by adding spacer units (legs). All these results indicate that it is of prime importance to control the decoupling of the molecule from the metallic substrate.
However, if extensive efforts have been concentrated on the two-dimensional (2D) organization of planar conjugated molecules onto substrates, in contrast, only very few works were devoted to the organization out-of the plane of the substrate, that is to say the organization in the third dimension. Surprisingly, the combination of these two approaches has not been systematically addressed, whereas it appeared as an important challenge in nanotechnology.
We recently proposed a strategy for the realization of 2D patterns of well-defined 3D nanostructures on highly-oriented pyrolitic graphite (HOPG) at the liquid-solid interface at room temperature. Our strategy followed classical architectural paradigms based on the realization of a well-organized on-plane monolayer paving the HOPG, and the emergence, perpendicular to the substrate, of an array of standing organic nanopillars of tunable height in view of obtaining large atomically-precise alignments both in-plane and vertically controlled. To validate this concept, we chose the multilayered [2.2]paracyclophanyl (PCP) moiety as nanopillar of variable height. More precisely, we designed and synthesized a series of molecules bearing two functional 'clips' end-capping a central either benzene ring (molecule I), or the lower deck of a two- and three-layered PCP unit (molecules II and III, respectively). Due to this structural design, the 3D bifunctional building blocks I, II, and III can be considered as 1-, 2-, and 3-storied molecules. Here we demonstrate that all these multistory molecules are suitable to, self-assemble as linear chains and that consequently the new compounds II and III were well-designed for building up perpendicular to the HOPG substrate.
Figure 1. STM images at liquid solid interface of the planar base I and of multilayered PCP-based compounds II and III. (ref. 10)
OBJECTIVES and MEANS
At the present, our goals are the design and synthesis of sub-wavelength emitters on one hand, and the detection of single-molecule fluorescence induced by tunneling electrons from a STM tip (STM-excited single molecule electroluminescence) on the other one. This is why this project involves (i) chemists (ENSTA ParisTech & Molecular Foundry-Organic and Macromolecular Synthesis Facility), and (ii) physicists (CEA Paris).
The Figure 2 illustrates the possibility to produce a single-molecule electro-luminescence within a tunnel junction, by the injection of a hole in the LUMO level from the substrate side and the simultaneous injection of an electron from the top electrode side, in particular a STM tip. This Figure explains the important practical advantage of an insulating barrier between the conjugated molecular moiety and the conducting substrate is the possibility to dynamically adjust their relative energy levels. As a matter of fact, an electrical field can build up in the insulating gap as a result of a bias applied e.g. by an electrode, a metal tip (STM) or even an electrolyte in an electrochemical cell. This results in change in electrostatic potential at molecule location. Then, for instance, the HOMO level, normally located below the substrate's Fermi level, can be lifted so as to allow injection of a hole.
Figure 2. Role of an insulator layer of height h between substrate and molecule on the shift of their relative energy levels induced by the bias of a tunnel junction.
To achieve these objectives, exploiting our previous approach, we try at the moment :
- to design a new type of molecule which combine the following functions (Scheme 1) :
- Self-assembly on the substrate
- Decoupling from the substrate
- Single molecule electroluminescence.
to record the STM-tip induced luminescence of this sub-wavelength single emitter.
Scheme 1 : Schematic representation of target architecture and its 2D self-organisation on HOPG surface.
More precisely (Scheme 2) we want to synthesize molecules bearing (i) two functional 'clips' end-capping the lower deck of a PCP unit, and (ii) an emitting unit attached on the upper deck of PCP typically a p-conjugated oligomer such as oligothiophene.
Scheme 2 : Schematic representation of target molecules (A).
The extension to the three-layered structure could be achieved as well using related molecules from our lab. Finally, the choice of the parameter n and of the height of the pillar may be guided by photoluminescence efficiency of adsorbed molecules outlined below (Task 4).
ACTUAL CHARACTERIZATIONS, STM and STL INVESTIGATIONS
First, we will use photophysics and electrochemistry to determine experimentally the optical (absorption/emission) and electronic properties (HOMO/LUMO) of :
- a) model compounds B (n = 0-3) to identify the best n value in adequation with the final application (see task 1).
- b) target compounds A (n = 0 or 1 or 2 or 3, tunable height)
Second, a pre-requisite is the self-organization of molecules A on HOPG or other atomically flat substrate. This will be probed by STM at the liquid/solid interface. At the molecular level this could be controlled by tuning alkyl-chain lengths and steric hindrance of bulky molecular moieties end-capping the upper level.
Thus, a first success indicator will be the deactivation of non-radiative relaxation pathways. This will be checked by two-photon excited photoluminescence microscopy (set-up available at CEA). The non-linear excitation permits to probe specifically the response of adsorbed molecules at the liquid/solid interface. Depending on these results we will thus tune the length (n parameter) and height of the pillars to observe the photoluminescence.
Finally the possibility to excite the luminescence locally by a tunnel current will be probed by STM. First the tunneling conditions will be analyzed by scanning tunneling microscopy. The tunnel barrier widths will be tuned at the molecular level to achieve simultaneous electron and hole injection. Finally, the STM-tip induced luminescence will be characterized. The luminescence map is expected to show localization over the thiophene moieties. n = 0, 1, 2 or 3 Electroluminescent experiments will be performed using tunnelling microscopes equipped with a photon detection line (photon collection optics, photon-counting based on avalanche photodiodes and CPM-PMTs, cooled-CCD spectrometer). Custom-built electronics permits simultaneous topographic and photonic imaging, in either collection or spectroscopic mode. One system is mounted on top of an inverted microscope equipped with time-correlated photon-pair detection.
IN THE FUTURE
In the next phase of this project, we propose the molecular engineering aimed at red shifting the oligomer emission to the nIR region. This could be achieved by using oligothiophene based on low bang gap (Donor-Acceptor-Donor) structures (Scheme 3).
Scheme 3 : p-conjugated D-A-D
Based on the know-how of different teams, we proposed the following structure Z (scheme 4) :
Scheme 4 :Schematic structure of target molecules Z with D-A-D upper deck.
It should be noted that such a red-shifted building block matches better with the requirements for STM-tip induced luminescence. Our models predict that as the band gap of the pi system is lowered, decoupling should be further diminished. later on, we would like to explore the possibly of orthogonal self assembly to furnish the multifunctional film. In our current strategy, the whole molecule is synthesized before adsorption on HOPG. Thus the potential wavelength emission is pre-determined. Here we propose to use a guest-host concept to reach a tunable emission. This coukd be achieved by using supramolecular chemistries (i.e. hydrogen bonding) to attach the chromophore. We aim to synthesize a building block X able to self-assemble on HOPG leading to a platform acting as a host matrix to catch emitting guest Y of tunable wavelengths (Scheme 5). Thus, we would obtain a versatile emitting platform.
Scheme 5 : Schematic structures of the host molecule X , emitting guest Y and their supramolecular self-assembly.
To finish, based on the recent publication. of a novel 'Clip' function designed for Gold (Au) substrate, we would like to develop synthetic routes to transpose this project initially dedicated to HOPG to reach the following self-assemblies described in scheme 6.
Scheme 6 : Chemical structures designed for Absorption/emission wavelengths tuning on Au.
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