Spatial Dependence of the Dipolar Interaction between Quantum Dots and Organic Molecules Probed by Two-Color Sum-Frequency Generation Spectroscopy

Abstract

Given the tunability of their optical properties over the UV–Visible–Near IR spectral range, ligand-capped quantum dots (QDs) are employed for the design of optical biosensors with low detection threshold. Thanks to non-linear optical spectroscopies, the absorption properties of QDs are indeed used to selectively enhance the local vibrational response of molecules located in their vicinity. Previous studies led to assume the existence of a vibroelectronic QD–molecule coupling based on dipolar interaction. However, no systematic study on the strength of this coupling has been performed to date. In order to address this issue, we use non-linear optical Two-Color Sum-Frequency Generation (2C-SFG) spectroscopy to probe thick QD layers deposited on calcium fluoride (CaF2) prisms previously functionalized by a self-assembled monolayer of phenyltriethoxysilane (PhTES) molecules. Here, 2C-SFG is performed in Attenuated Total Reflection (ATR) configuration. By comparing the molecular vibrational enhancement measured for QD–ligand coupling and QD–PhTES coupling, we show that the spatial dependence of the QD–molecule interactions (∼1/r3, with r the QD–molecule distance) is in agreement with the hypothesis of a dipole–dipole interaction.

Two-Colour Sum-Frequency Generation Spectroscopy Coupled to Plasmonics with the CLIO Free Electron Laser

“…Nonlinear optical Two-colour Sum-Frequency Generation (2C-SFG) spectroscopy of metal [1][2][3] and semiconducting [4][5][6] nanoparticles constitutes a reliable probe of the surface molecular chemistry of nanostructured samples in catalysis and (bio)chemical sensing [7]. For instance, in such systems, the evolution of various physico-chemical properties impacted by surface plasmons and excitons can be analysed [8,9]. In a general way, whatever the probed scale, 2C-SFG spectroscopy gives access to intramolecular vibronic couplings [10][11][12], molecule/substrate interactions [13][14][15][16] and molecule/nanostructure interactions [17][18][19][20][21][22][23][24][25][26].…”

Modification of Vibrational Parameters of a D∞h-Symmetric Triatomic Molecule in a Laser Plasma

“…The role of the laser-modified nuclear motion in a molecule is also important for the material science. In particular, recent paper [8] presents the experimental study of a vibroelectronic quantum dots-molecule coupling based on dipolar interaction. Namely, the amplitude of the vibrational response of the phenyltriethoxysilane ligands (over the infrared range) was modulated by the linear optical susceptibility of the Cd Te 0.25 S 0.75 quantum dots (over the visible range).…”

Special Issue of Symmetry: “Recent Advances in Linear and Nonlinear Optics”

“…Subsequently, at the nanoscale, it is possible to perform a quantitative analysis of composite or hybrid systems made of molecules/nanoparticles interfaces designed for optical (bio or chemical) sensing with an improved detection threshold at a low cost. By taking profit of quantum properties of small CdTe Quantum dots (QDs~3.4 nm diameter), researchers use advanced nonlinear optical Two-Colour Sum-Frequency Generation (2C-SFG) spectroscopy to check the hypothesis of the existence of a dipolar coupling from QD excitons to vibrations in their molecular environment to explain enhanced molecular sensitivity [5]. They demonstrate this physical process by comparing the dipolar coupling strength between the close chemical ligands of QDs with a farther molecular monolayer by highlighting 1/r 3 spatial dependence compatible with dipole-dipole interactions.…”