Non-Injection and Low-Temperature Approach to Colloidal Photoluminescent PbS Nanocrystals with Narrow Bandwidth

Abstract

Colloidal photoluminescent (PL) PbS nanocrystals have attracted a lot of attention in various applications such as bioimaging and optical telecommunications due to their tunable bandgap in the near-infrared region of the electromagnetic spectrum. Hot-injection processes seem to be the best to engineer high-quality PbS nanocrystals. However, there is a limited body of literature documented on the syntheses, with little information on synthetic parameters affecting the optical properties of the product. Moreover, small PbS nanocrystals with large bandgap greater than 1.38 eV (ca. 900 nm) and narrow bandwidth are rarely reported, due to the fact that high-temperature growth in hot-injection processes leads to large nanocrystals rapidly. This manuscript deals with our noninjection and low temperature approach to small PbS nanocrystal ensembles with bandgap in wavelength shorter than 900 nm and with narrow bandwidth; the growth temperature can be as low as room temperature. For our noninjection approach, systematic study was performed on synthetic parameters affecting the growth, with the growth temperature in the range of 30-120°C and octadecene (ODE) as a reaction medium. Different acids including oleic aicd (OA) were explored as surface ligands, while two lead source compounds, which are lead oxide (PbO) and lead acetate, and three S source compounds, which are bis(trimethylsilyl)sulfide ((TMS) 2 S), thioacetamide (TAA), and elemental sulfur (S), were investigated. Generally, a solution of a lead precursor in ODE was first prepared via a reaction of a Pb-source compound and an acid; afterward, this solution was mixed with a S-source solution in ODE at room temperature. The use of (TMS) 2 S and OA bestows high-quality PbS nanocrystals, regarding narrow bandwidth of bandgap absorption and photoemission, without storage in dark for digestive and Ostwald ripening leading to selffocusing; in addition to the various acids and Pb and S source compounds explored, feed molar ratios of acid-to-Pb and Pb-to-S, as well as reactant concentrations were thoroughly investigated. Low acid-to-Pb and high Pb-to-S feed molar ratios together with high-reactant concentrations favor the formation of small PbS nanocrystals; meanwhile, from one synthetic batch, the growth of PbS nanocrystals in size is tunable mainly via temperature in addition to growth periods. The PbS nanocrystals exhibit bandwidth (full width at halfmaximum) as narrow as ca. 100 nm with growth temperature of 70°C. Thus, our noninjection approach features easy handling with high reproducibility and high-quality PbS nanocrystals with large bandgap but narrow bandwidth. Finally, bandgap engineering of our as-synthesized PbS nanocrystals was performed straightforwardly at room temperature via the mixing of a solution of Cd oleate in ODE; significant blueshift of bandgap absorption and photoemission with enhanced PL efficiency was accomplished.

From Hot‐Injection Synthesis to Heating‐Up Synthesis of Cobalt Nanoparticles: Observation of Kinetically Controllable Nucleation

“…The contribution of the injection to particle nucleation seems minor, and it is shown that injection can be replaced entirely by an accurately controlled heating up of the solution containing all reagents (including the cobalt precursor) from room temperature to the nucleation temperature. This synthetic method, which is often termed either "non-injection synthesis" [15,16] or "heating-up synthesis", [3,11] results in nanoparticles that are nearly identical to those made by the hot-injection method.We synthesized cobalt nanoparticles by injecting dicobalt octacarbonyl, [Co 2 (CO) 8 ], dissolved in a small amount of ortho-dichlorobenzene (o-DCB, b.p. 181 8C) into a solution of oleic acid and trioctylphosphine oxide (TOPO) in o-DCB at reflux.…”

Two‐Phase Approach to High‐Quality, Oil‐Soluble, Near‐Infrared‐Emitting PbS Quantum Dots by Using Various Water‐Soluble Anion Precursors

“…[19,[24][25][26] Very recently, the group of Yu developed a new non-injection approach to synthesize NIR-emitting PbS QDs in ODE. [27] These results represented the best optical quality of the NIR fluorescent PbS QDs in organic media. However, unfortunately, these methods are still complicated and involve the dangerous chemical bis(trimethylsilyl) sulfide [(TMS) 2 S] or poorly reactive elemental S.…”

“…[19,[21][22][23][24][25][26][27] As expected, if we can combine the respective advantages of both approaches properly, for instance, to synthesize oil-soluble QDs by using water-soluble anion precursors, the synthesis steps of the traditional organic-phase methods will be simplified greatly. However, this has proved to be a difficulty.…”

“…To the best of our knowledge, the use of the two-phase approach to synthesize NIR-emitting PbS QDs has not yet been reported, and the growth rate of PbS QDs is significantly different from those of CdS and CdSe QDs: the growth of PbS QDs is much faster, and could even occur at room temperature. [26,27] Thus, it is of great interest to investigate and better understand what role the activity of anion precursors plays in the nucleation and growth processes of PbS QDs in a two-phase system and how to adjust the properties of the products.…”

Low-Temperature Noninjection Approach to Homogeneously-Alloyed PbSexS1−x Colloidal Nanocrystals for Photovoltaic Applications

“…8,9 Furthermore, the role of DPP in eq 1 approach was not discussed and the DPP amount was not explored.…”