The influence of conjugated alkynyl(aryl) surface groups on the optical properties of silicon nanocrystals: photoluminescence through in-gap states

"Turning the dials": controlling synthesis, structure, composition, and surface chemistry to tailor silicon nanoparticle properties

Silicon nanoparticles (SiNPs) can be challenging to prepare with defined size, crystallinity, composition, and surface chemistry. As is the case for any nanomaterial, controlling these parameters is essential if SiNPs are to realize their full potential in areas such as alternative energy generation and storage, sensors, and medical imaging. Numerous teams have explored and established innovative synthesis methods, as well as surface functionalization protocols to control these factors. Furthermore, substantial effort has been expended to understand how the abovementioned parameters influence material properties. In the present review we provide a commentary highlighting the benefits and limitations of available methods for preparing silicon nanoparticles as well as demonstrations of tailoring optical and electronic properties through definition of structure (i.e., crystalline vs. amorphous), composition and surface chemistry. Finally, we highlight potential opportunities for future SiNP studies.

Thermal Disproportionation for the Synthesis of Silicon Nanocrystals and Their Photoluminescent Properties

In the past decades, silicon nanocrystals have received vast attention and have been widely studied owing to not only their advantages including nontoxicity, high availability, and abundance but also their unique luminescent properties distinct from bulk silicon. Among the various synthetic methods of silicon nanocrystals, thermal disproportionation of silicon suboxides (often with H as another major composing element) bears the superiorities of unsophisticated equipment requirements, feasible processing conditions, and precise control of nanocrystals size and structure, which guarantee a bright industrial application prospect. In this paper, we summarize the recent progress of thermal disproportionation chemistry for the synthesis of silicon nanocrystals, with the focus on the effects of temperature, Si/O ratio, and the surface groups on the resulting silicon nanocrystals’ structure and their corresponding photoluminescent properties. Moreover, the paradigmatic application scenarios of the photoluminescent silicon nanocrystals synthesized via this method are showcased or envisioned.

Introductory lecture: origins and applications of efficient visible photoluminescence from silicon-based nanostructures

This review highlights many spectroscopy-based studies and selected phenomenological studies of silicon-based nanostructures that provide insight into their likely PL mechanisms, and also covers six application areas.

Space charge-limited current transport in thin films of alkyl-functionalized silicon nanocrystals

We describe the fabrication and electrical characterization of all-silicon electrode devices to study the electronic properties of thin films of silicon nanocrystals (SiNCs). Planar, highly doped Si electrodes with contact separation of 200 nm were fabricated from silicon-on-insulator substrates, by combination of electron beam lithography and reactive ion etching. The gaps between the electrodes of height 110 nm were filled with thin-films of hexyl functionalized SiNCs (diameter 3 nm) from colloidal dispersions, via a pressure-transducing PDMS (polydimethylsiloxane) membrane. This novel approach allowed the formation of homogeneous SiNC films with precise control of their thickness in the range of 15-90 nm, practically without any voids or cracks. The measured conductance of the highly resistive SiNC films at high bias voltages up to 60 V scaled approximately linearly with gap width (5-50 μm) and gap filling height, with little device-to-device variance. We attribute the observed, pronounced hysteretic current-voltage (I-V) characteristics to space-charge-limited current transport, which-after about twenty cycles-eventually blocks the current almost completely. We propose our all-silicon device scheme and gap filling methodology as a platform to investigate charge transport in novel hybrid materials at the nanoscale, in particular in the high resistivity regime.

Donor-Acceptor Pair Recombination in Size-Purified Silicon Quantum Dots

Shallow impurity doping is an efficient route to tailor optical and electronic features of semiconductor quantum dots (QDs). However, the effect of doping is often smeared by the size, shape, and composition inhomogeneities. In this paper, we study optical properties of almost monodispersed spherical silicon (Si) QDs that are heavily doped with boron (B) and phosphorus (P). The narrow size distribution achieved by a size-separation process enables us to extract doping-induced phenomena clearly. The degree of doping-induced shrinkage of the optical band gap is obtained in a wide size range. Comparison of the optical band gap with theoretical calculations allow us to estimate the number of active donor-acceptor pairs in a QD. Furthermore, we found that the size and detection energy dependence of the luminescence decay rate is significantly modified below a critical diameter, that is ∼5.5 nm. In the diameter range above 5.5 nm, the luminescence decay rate is distributed in a wide range depending on the detection energy even in size-purified Si QDs. The distribution may arise from that of donor-acceptor distances. On the other hand, in the diameter range below 5.5 nm the detection energy dependence of the decay rate almost disappears. In this size range, which is smaller than twice of the effective Bohr radius of B and P in bulk Si crystal, the donor-acceptor distance is not a crucial factor to determine the recombination rate.