The effects of electronic coupling between capping molecules and quantum dots on the light absorption and emission of octyl, styryl, and 4-ethynylstyryl terminated silicon quantum dots

Abstract

Optical properties of silicon quantum dots (Si QDs) are greatly influenced by their size and surface chemistry. We report the micro-emulsion synthesis of hydrogen terminated Si QDs, with the modification of quenching the remaining reducing agent LiAlH4 with CuSO4. Subsequent functionalization was carried out with different capping molecules, including 1-octene, phenylacetylene, and 1,4-diethynylbenzene, to give octyl, styryl, and 4-ethynylstyryl terminated silicon quantum dots, respectively. The optical properties of the three kinds of Si QD synthesized, with the extended conjugation length, were examined. The effects of surface chemistry on the optical properties of the Si QD, obtained using ultraviolet-visible absorption spectroscopy and photoluminescence spectroscopy, were compared to the extension of electron and hole wavefunctions into the capping molecules, which were estimated from modified particles in a box calculation. The observed quantum yield increased from 2% to 2.5% and 9.0% and the average lifetime decreased with increasing conjugation length of the octyl Si QD, the styryl Si QD, and the 4-ethynylstyryl Si QD, which were ascribed to the effect of electronic coupling between the capping molecules and the Si QD. A tentative model in which the strong electronic interaction through covalent bonding induced a more direct band gap character for light emission was proposed by tuning the ground state wavefunctions of the electron and hole in wave vector space.

A comparative study of electron transport in benzene molecule covalently bonded to gold and silicon electrodes for pioneering the electron transport properties of silicon quantum dot-molecule hybrid polymers

“…1. It is well known that the carbon readily binds to Si usually by hydrosilylation reaction between unsaturated hydrocarbon and SieH in the assistance of thermal energy [44] or Pt catalyst [3]. Moreover, in recent, Chen et al reported a direct AueC bond formation by using trimethyltin end group and such a direct coupling between the molecule and Au electrode is expected to yield highly conducting s-bonded systems [45,46].…”

“…Surface functionalization of the Si QD has been diversely investigated not only to protect the surface especially from oxidation, but also to tune the optoelectronic properties [2]. Recently, we have found that the surface modification of the Si QD with conjugated organic molecules led to strong electronic coupling between the Si QD and conjugated molecules, resulting in significant alteration of the electronic structure of the Si QD directing to more direct band gap character [3]. Also we have demonstrated new material concept, Si QD polymers, in which properly surface-functionalized Si QD by introducing a,u-bifunctional molecule can be allowed for further chemical reactions [4].…”

Tailoring the Charge Transport Properties of Printable Si‐NCs By Conjugated Organic Ligands

“…Between these, styryl moieties and derivatives have been described as outstanding molecular wires because of their capability of intramolecular and intermolecular charge transport. Additionally, these ligands – obtained by hydrosylilation of phenylacetylene and derivatives (also named ethynilbenzene) – showed strong electronic coupling with the silicon surfaces …”

“…When turning on the Si‐NCs, the post synthesis doping has instead received very little attention although the tunability of their work function (W f ) by grafting organic molecules on silicon has been already predicted by periodic density functional theory . Styryl moieties were previously grafted on Si‐NCs by thermal or photochemical activation as well as by catalysis with chloroplatinic acid (H 2 PtCl 6 ) …”

Organo‐Functionalization of Silicon Nanocrystals Synthesized by Inductively Coupled Plasma Chemical Vapor Deposition

“…Organo‐functionalization can provide not only oxygen protection but also solution processability, and the solution process provides a superior adaptability to conventional applications, easy to synthesize, and the ability for mass production. Recently, our group reported on the surface functionalization of Si NCs or Si QDs by using a hydrosilylation reaction . Although this method provides unoxidized Si NCs with a narrow size distribution, it is challenging to mass produce high‐quality crystals.…”