Correlating Carrier Density and Emergent Plasmonic Features in Cu_2–xSe Nanoparticles

Abstract

Recently, a wide variety of new nanoparticle compositions have been identified as potential plasmonic materials including earth-abundant metals such as aluminum, highly doped semiconductors, as well as metal pnictides. For semiconductor compositions, plasmonic properties may be tuned not only by nanoparticle size and shape, but also by charge carrier density which can be controlled via a variety of intrinsic and extrinsic doping strategies. Current methods to quantitatively determine charge carrier density primarily rely on interpretation of the nanoparticle extinction spectrum. However, interpretation of nanoparticle extinction spectra can be convoluted by factors such as particle ligands, size distribution and/or aggregation state which may impact the charge carrier information extracted. Therefore, alternative methods to quantify charge carrier density may be transformational in the development of these new materials and would facilitate previously inaccessible correlations between particle synthetic routes, crystallographic features, and emergent optoelectronic properties. Here, we report the use of Se solid state nuclear magnetic resonance (NMR) spectroscopy to quantitatively determine charge carrier density in a variety of CuSe nanoparticle compositions and correlate this charge carrier density with particle crystallinity and extinction features. Importantly, we show that significant charge carrier populations are present even in nanoparticles without spectroscopically discernible plasmonic features and with crystal structures indistinguishable from fully reduced CuSe. These results highlight the potential impact of the NMR-based carrier density measurement, especially in the study of plasmon emergence in these systems (i.e., at low dopant concentrations).

Direct Quantification of Cu Vacancies and Spatial Localization of Surface Plasmon Resonances in Copper Phosphide Nanocrystals

“…These, in turn, can be tuned by varying the synthesis conditions or by post-synthesis treatments. 6,7,8 An increasing density of Cu vacancies translates into a higher hole concentration, and consequently into a shift of the LSPRs toward higher frequencies. Pnictogenide Cu 3-x P NCs also feature a band in the absorption spectrum, peaked at about 1500 nm, which has been recognized as a LSPR and attributed to the presence of Cu vacancies.…”

Switching between Plasmonic and Fluorescent Copper Sulfide Nanocrystals

“… 2 , 5 − 8 When the number of Cu vacancies (indicated by x in Cu 2– x S) is between 0 and 0.04, the nanocrystals attain the chalcocite and djurleite crystal structures and E g varies from 1.1 to 1.4 eV, 3 , 7 , 9 with hole densities up to 10 21 cm –3 . 6 , 10 When the amount of Cu is reduced, the bandgap widens (1.5 eV for x = 0.2, digenite crystal structure; 2.0 eV for x = 1, covellite crystal structure), 3 , 7 , 11 and the hole density becomes an order of magnitude higher. The easily tunable crystal structure of Cu 2– x S nanocrystals results in a wide variety of sizes and shapes attainable for Cu 2– x S nanocrystals by a proper choice of reaction conditions during colloidal synthesis.…”

Effect of Surface Ligand on Chemical Interface Damping in Nonstoichiometric Cu2–xS Semiconductor Nanocrystals: A Direct Correlation between Ultrafast Carrier Dynamics and Photoconductivity

“…NCs are p-type degenerate semiconductors, and free charge carriers appear due to the copper vacancy in the valence band of the NCs. Surface plasmon resonance highly depends on the refractive index of the surrounding medium (solvent) of the NCs system. , The plasmon resonance wavelength inter-related with the free carrier density in the semiconductor NCs. The past studies on the noble metals nanoparticle, the plasmon frequency is determine by the complex dielectric function (ε), dielectric constant (ε m ) of environment of the NCs, and effective mass ( m h ) of the carriers .…”