Abstract

Concerns over possible toxicities of conventional metal-containing quantum dots have inspired growing research interests in colloidal silicon nanocrystals (SiNCs), or silicon quantum dots (SiQDs). This is related to their potential applications in a number of fields such as solar cells, optoelectronic devices and fluorescent bio-labelling agents. The past decade has seen significant progress in the understanding of fundamental physics and surface properties of silicon nanocrystals. Such understanding is based on the advances in the preparation and characterization of surface passivated colloidal silicon nanocrystals. In this critical review, we summarize recent advances in the methods of preparing high quality silicon nanocrystals and strategies for forming self-assembled monolayers (SAMs), with a focus on their bio-applications. We highlight some of the major challenges that remain, as well as lessons learnt when working with silicon nanocrystals (239 references).

Highlights

  • Semiconductor nanocrystals, or quantum dots (QDs), are attractive nano-materials because of their unique optoelectronic properties

  • silicon quantum dots (SiQDs) possess many of the desired physical properties for bio-applications, in order for them to be effective in practical contexts, the effect of preparation method and surface functionalization must be considered with respect to both photophysical features and biological interactions

  • The wet-chemical approach of surface modification to colloidal silicon nanocrystals has shown notable advantages: its compatibility with conventional bench-top chemistry allowed relatively simple experimental set-ups; most procedures are performed in solution, which is fundamentally important for applications such as ink printing and fluorescent labelling agents

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Summary

Introduction

Semiconductor nanocrystals, or quantum dots (QDs), are attractive nano-materials because of their unique optoelectronic properties. This requires the particles to be made relatively with controlled size and optical properties For this aim size tunable within 1–5 nm,[37] emission wavelength spanning from blue to near-IR (NIR) and QY above 10–15% are essential.[38,39] Another challenge is how to effectively modify the surface of silicon quantum dots, as freshly prepared silicon surface is prone to oxidation.[40,41] The impact of surface states is significant for particles at this dimension,[42] due to the small exciton Bohr radius for silicon of merely 4.2 nm.[43] Both challenges, combined with difficulties in characterization, make SiNCs considerably more difficult to work with compared with conventional metal based quantum dots.

Physical properties
Comparison between quantum dots and organic dyes
Methods of preparation
The top-down approach
The bottom-up approach
Precursor decomposition and re-assembly
Surface modifications
Wet-chemical based modification strategies
Surface passivation by plasma-surface interactions
Multi-step surface modifications
Characterizing the surface
Impacts of surface chemistry on properties and theoretical studies
Fluorescent imaging
SiQDs for magnetic resonance imaging
Silicon quantum dots as a less toxic alternative to conventional quantum dots
Findings
Summary and future perspectives

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