Charge-Tunable Quantum Plasmons in Colloidal Semiconductor Nanocrystals

Abstract

Nanomaterials exhibiting plasmonic optical responses are impacting sensing, information processing, catalysis, solar, and photonics technologies. Recent advances have expanded the portfolio of plasmonic nanostructures into doped semiconductor nanocrystals, which allow dynamic manipulation of carrier densities. Once interpreted as intraband single-electron transitions, the infrared absorption of doped semiconductor nanocrystals is now commonly attributed to localized surface plasmon resonances and analyzed using the classical Drude model to determine carrier densities. Here, we show that the experimental plasmon resonance energies of photodoped ZnO nanocrystals with controlled sizes and carrier densities diverge from classical Drude model predictions at small sizes, revealing quantum plasmons in these nanocrystals. A Lorentz oscillator model more adequately describes the data and illustrates a closer link between plasmon resonances and single-electron transitions in semiconductors than in metals, highlighting a fundamental contrast between these two classes of plasmonic materials.

Two-fluid hydrodynamic model for semiconductors

“…Furthermore, intrinsic semiconductors may contain plasmas created either thermally or by external excitations (e.g., from a laser), and here the electron density can be controlled dynamically with the temperature or the excitation energy, respectively. Plasmonics has already been shown in several papers for doped semiconductors [16][17][18][19][20][21][22][23][24][25][26][27][28], biased semiconductors [29][30][31][32], laser excited semiconductors [33], and thermally excited intrinsic semiconductors [34][35][36][37].…”

“…[24][25][26][27][28][29][30] investigated plasmons in nanostructures of semiconductors, but except for Refs. [27,29] they all used the Drude model to describe their results. And just as for metals one would expect that the Drude model only is accurate for semiconductor structures down to a certain size.…”

In situgrowth of Ag nanoparticles onα-Ag2WO4under electron irradiation: probing the physical principles

“…The ability to do this would increase the range of applications of these materials in many fields, ranging from photovoltaics to bioimaging [37][38][39][40]. Recently, this effect has been discussed concerning semiconductor nanocrystals [41,42]. Desirable electrical and optical properties of novel metal nanoparticles can be achieved by tailoring their size, shape, and morphology [43].…”

Colloidal transparent conducting oxide nanocrystals: A new infrared plasmonic material

“…Though Drude model was originally formulated for metals [18], its application was extended to doped semiconductor nanocrystals as well. Though it can explain the free charge carrier densities in the case of larger NCs (where size is much greater than the De Broglie wavelength of the carrier), it cannot quite explain the behaviour of the smaller NCs quantitatively, particularly due to two reasons; (i) scattering of charge carriers by the nanocrystal surface and (ii) carrier localization in the case of NCs having diameters less than the Bohr-exciton radius as reported by Gamelin et al [20]. The deviation from Drude model was attributed to the presence of quantum plasmons that were partially localized [21] and quantitative description of semiconductor NCs thus requires the inclusion of quantum mechanical treatment too which was provided by the Lorentz oscillator model [20].…”

“…LSPR peaks can arise not only from equilibrium steady-state population of the doped semiconductors, but also from photoexcited undoped semiconductors [36,37]. ZnO has been used as a model system to demonstrate that non-equilibrium carrier population can also display LSPR bands.…”