How to Quantify Electrons in Plasmonic Colloidal Metal Oxide Nanocrystals

Abstract

Distinct from noble metal nanoparticles, doped metal oxide nanocrystals (NCs) exhibit localized surface plasmon resonance (LSPR) in the infrared region that can be tuned by changing the free electron concentration through both synthetic and postsynthetic doping. Redox reagents have commonly been used to postsynthetically modulate the LSPR, but to understand the relationship between the electron transfer processes and the resulting optical changes, it is imperative to quantify electrons in the NCs. Titration and LSPR peak fitting analysis are the most common methods used for quantifying electrons; however, a comparison between these methods has previously revealed discrepancies up to an order of magnitude without a clear explanation. Here, we apply these electron quantification techniques concurrently to Sn-doped In2O3 NCs with varying size, doping concentration, and extent of postsynthetic reduction. We find that oxidative titration consistently overestimates the number of electrons per NC, owing to the failure of the assumed stoichiometric equivalents between moles of oxidant added and moles of free electrons extracted from the NCs. The NC characteristics we examine strongly influence the driving force for the oxidation process, affecting the relative agreement between oxidative titration and LSPR fitting; the two methods more closely agree when the electron transfer driving force is larger. Overall, these analyses inform best practices for quantifying electrons in plasmonic semiconductor NCs and reveal how the accuracy is affected by NC characteristics.

Plasmon-Induced Hot-Carrier Excited-State Dynamics in Plasmonic Semiconductor Nanocrystals

“…The chemical titration yields a value of 1.80 × 10 22 cm –3 , which is in good agreement with the values obtained from SXDA and HEDA modeling, suggesting carrier compensation is minimal in WO 3– x . The observed agreement in carrier density suggests that NOBF 4 is an appropriate titrant for WO 3– x , which has been observed to not be the case for Sn:In 2 O 3 . Such an observation may reflect the enhanced stability due to TOPO passivation, removing interfacial compensation centers, as previously suggested by the Milliron and Strouse , groups as reasons for the lack of agreement between modeling and chemical titrations.…”

“…The observed agreement in carrier density suggests that NOBF 4 is an appropriate titrant for WO 3−x , which has been observed to not be the case for Sn:In 2 O 3 . 29 Such an observation may reflect the enhanced stability due to TOPO passivation, removing interfacial compensation centers, as previously suggested by the Milliron 29 and Strouse 19,30 groups as reasons for the lack of agreement between modeling and chemical titrations.…”

Size and Emission Control of Wurtzite InP Nanocrystals Synthesized from Cu3–xP by Cation Exchange

“…Within the size range discussed here, larger NCs manifest a more intense plasmonic absorption peak, while the frequency is only weakly dependent on the size. The lower plasmon absorption intensity in smaller sizes was attributed in other plasmonic NCs to faster relaxation in the NC surface or changes in the vacancy concentration . Moving on to the next reaction step of the cation exchange, Figure b shows the absorption of the resulting w-InP NCs after the exchange of Cu + to In 3+ .…”

Controlling Electron Delocalization in Vanadium‐Based Hybrid Bronzes through Molecular Templation

“…Redox titration measurements on H x V 2 O 5 (Figure S7; see later discussion and Supporting Information for details) also indicate an x value of at least 1.5, though the instability of partially re‐oxidized phases to air during the titration measurement likely makes this value an underestimate. Redox titrations of this type have also recently been shown to be less accurate due to inefficient oxidation [54] . We therefore denote the inorganic hydrogen bronze as H 2 V 2 O 5 moving forward.…”