Examining the Effect of Dopant Ionic Radius on Plasmonic M:ZnO Nanocrystals (M = Al³⁺, Ga³⁺, In³⁺)

Abstract

Understanding the role of dopant deactivation on plasmon frequency and extinction is important for the rational design of plasmonic semiconductor nanocrystals (PSNCs). Aliovalent dopants do not always contribute a free carrier to a localized surface plasmon resonance (LSPR) for many reasons, including the existence of a depletion region, the pinning of carriers at neutral defect sites, or even the formation of a secondary insulating microphase (inclusions) not observable in the powder X-ray diffraction (pXRD). Here, we investigate such possibilities and their role in determining the LSPR frequency of Al-, Ga-, and In-doped ZnO NCs. Elemental analysis, pXRD, and absorption measurements are utilized to examine the impact of dopant incorporation on the resulting properties. Both simple and advanced effective mass Drude models are used to fit the mid-infrared plasmons, while one-electron oxidant chemical titrations are used as an independent measure of the free electron concentrations. The results of these analyses indicate that dopant/host lattice mismatch leads to inefficient carrier generation for aliovalent substitution, potentially due to local spinel-like inclusions. Smaller dopant ions are more likely to incorporate interstitially and form spinel phases, which results in an increased number of pinned carriers. Improved size matching from Al3+ to In3+ results in increased substitution efficiency and subsequently higher free carrier concentrations and LSPR frequencies. Drude model correction factors are calculated for each sample and compared to the literature value for n-ZnO determined via full band structure calculations. Each dopant is shown to have a unique correction factor, further illustrating the effect of differing ionic radii on the resulting LSPR.

Surface-Ligand-Controlled Enhancement of Carrier Density in Plasmonic Tungsten Oxide Nanocrystals: Spectroscopic Observation of Trap-State Passivation via Multidentate Metal Phosphonate Bonding

“…Furthermore, the N e value of TDPA-treated WO 3– x NPLs has a nearly 57% higher in comparison to a Drude approximation of 5.56 × 10 22 cm –3 . This is not surprising because in a recent report on cation-doped zinc oxide NCs, the authors showed nearly 80% difference in N e values between redox titration and Drude model-based calculations . Nevertheless, we predict that this large difference between the experimental and empirical formula-based calculations in N e values of WO 3– x NPLs is due to improper approximation of several parameters: (1) the original Drude model was developed for spherical, NCs where shape factor (κ) is 2.0 (eq ).…”

“…Next, we sought to determine how quantitative is the previously determined N e values, which were calculated using the empirical Drude approximation. To experimentally determine the N e value, we applied a single electron redox titration using nitrosonium tetrafluoroborate (NOBF 4 ) as reported by Strouse and coworkers and others on plasmonic metal oxide and nitride NCs. , A known concentration of NOBF 4 was added to WO 3– x NPL samples (as-synthesized and TDPA-treated) and allowed to react for 15 min before the LSPR spectra were recorded (see the Supporting Information for additional details). All spectra were integrated from 500 to 2500 nm to account for peak broadening and peak shifting to lower energies that are known to occur as free carrier density decreases.…”

Controlled Synthesis and Exploration of CuxFeS4 Bornite Nanocrystals

“…The literature value for the dielectric constant of Cu 5 FeS 4 (3.48) and the frequency-independent Drude model were used for these fits. , Since the effective mass of the carriers in these materials remains largely uncertain, one-electron reductant chemical titrations were performed (Figure A) to independently measure the free carrier density, enabling use of the Drude model to calculate m * for each sample. Determining the free carrier concentration independent of the Drude model is critical toward understanding the properties of plasmonic semiconductors as the Drude model has previously been shown to be inaccurate when calculating the carrier density from the LSPR extinction feature alone. − The Drude model- and chemical titration-extracted parameters are listed in Table , while the frequency-independent fits are shown in Figure B.…”

Effective Mass for Holes in Paramagnetic, Plasmonic Cu5FeS4 Semiconductor Nanocrystals

“…Results from Sn:In 2 O 3 and M:ZnO (M = Al, Ga, In) show the flaw in ignoring the environment at high carrier concentrations, as site occupation effects lead to charge compensation and deviation from parabolic band theory. 15,16 In these studies, the role of a dampening term (Γ), as described within the Jung and Peterson model, was proposed to account for the deviation and assumed to be dependent on energy-level parabolicity, in analogy to the recently explored InN system. 17 A change in parabolicity will lead to a change in carrier effective mass.…”

“…Incorporating a p- or n-type dopant into a semiconductor leads to changes in the Fermi level, often accompanied by perturbation of the electron–electron and electron–phonon interactions for a simple valence band (VB)-conduction band (CB) energy structure. , In such systems, the Drude model will produce a linear trend in LSPR frequency with carrier density as long as vacancies or site occupation changes do not perturb the system. Results from Sn:In 2 O 3 and M:ZnO (M = Al, Ga, In) show the flaw in ignoring the environment at high carrier concentrations, as site occupation effects lead to charge compensation and deviation from parabolic band theory. , In these studies, the role of a dampening term (Γ), as described within the Jung and Peterson model, was proposed to account for the deviation and assumed to be dependent on energy-level parabolicity, in analogy to the recently explored InN system . A change in parabolicity will lead to a change in carrier effective mass.…”