Effects of Surface Chemistry on the Photophysics of Colloidal InP Nanocrystals

Abstract

Indium phosphide (InP) semiconductor nanocrystals (NCs) provide a promising alternative to traditional heavy-metal-based luminescent materials for lighting and display technologies, and implementation of InP NCs in consumer products is rapidly increasing. As-synthesized InP NCs typically have very low photoluminescence quantum yields (PLQY), however. Although empirical methods have led to NCs with near-unity PLQYs, a fundamental understanding of how specific synthetic and post-synthetic protocols can alter the electronic landscape of InP NCs is still lacking. Here, we have studied a series of homologous InP NCs prepared from InP clusters using a combination of room-temperature and low-temperature time-resolved spectroscopies to elucidate how specific charge-carrier trapping processes are affected when various surface modifications are performed. The data allow identification of large PLQY increases that occur specifically through elimination of surface electron traps and provide a rationale for understanding the microscopic origins of this trap suppression in terms of elimination of undercoordinated surface In3+ ions. Despite essentially complete elimination of surface electron trapping when surface In3+ is addressed, hole trapping still exists. This hole trapping is shown to be partially suppressed by even very thin shell growth, attributable to elimination of undercoordinated surface phosphides. We also observe signatures of bright-dark excitonic splitting in InP NCs with only submonolayer surface coverage of select additives (divalent Lewis acids or fluoride anions)−signatures that have only been previously observed in thick-shelled InP NCs. Together, these synthetic and spectroscopic results improve our understanding of relationships between specific InP NC surface chemistries and the resulting NC photophysics.

Surface passivation extends single and biexciton lifetimes of InP quantum dots

“…With the above spectral assignment, we now focus on understanding the exciton decay dynamics of InP QDs. Due to the existence of trap states, 20 , 22 , 23 , 44 the XB decay in InP QDs should reflect the decay of photogenerated electrons through both the radiative channel by the electron–hole recombination process ( k e/h,rad ) and the nonradiative channel by the electron trapping process ( k e,non ) ( Scheme 1a ). Meanwhile, the PL decay of the band-edge exciton ( k PL ) reflects the radiative and nonradiative decay from both electrons and holes.…”

“…Meanwhile, the PL decay of the band-edge exciton ( k PL ) reflects the radiative and nonradiative decay from both electrons and holes. Therefore, by directly comparing the PL decay of the band-edge exciton with the XB decay from the TA measurement 23 , 45 ( Fig. 2d ), one can calculate the nonradiative decay component of holes ( k h,non , Scheme 1a ) through eqn (1).…”

“…22 On the other hand, our result is not consistent with other reports that suggest that HF treatment removes electron traps on InP QDs by the passivation of surface indium dangling bonds. 22,23 Exciton dynamics in InP@ZnS core@shell QDs: passivation of both electron and hole traps…”

“…[20][21][22][23][24][25][26] The rst passivation method employed a post-synthetic HF treatment of InP QDs under illumination. [20][21][22][23]25 The HF treatment has been suggested to remove the surface P dangling bonds through a photochemical reaction of trapped holes with the P atom, which is then attacked by the F À ions and eventually detach from the InP surface. 25 Meanwhile, a recent study suggested an alternative/complementary mechanism where the F À treatment may act through removing the electron traps caused by the surface indium dangling bonds.…”

Tuning the Optical Band Gap of Semiconductor Nanocomposites—A Case Study with ZnS/Carbon

“…In internally structured quantum dots control of band gap and alignment of band edges has made huge progress in the last years, allowing for instance the use of defects for broad band luminescence by internal diffusion processes [17] or electrical pumping of quantum dots by using graded so-called "giant" shells around CdSe cores [18]. Thick graded shells with proper alignment are also key to replace the near-perfect but toxic CdSe cores by environmentally more benign materials like InP [19], but it was also pointed out, that the control of surface defects as traps for electrons as well as holes is key to high efficiency [20]. The most general case in this context are ternary or quaternary semiconductors, which often allow tuning band edges simply by adjustment of the stoichiometry [21], but still need protective shells like the ZnS that is used in this study as proxy for other materials.…”