Many types of fluorescent semiconductor nanoparticles are available nowadays such that the whole region of the optical spectrum from deep ultraviolet (UV) to several microns in the infrared (IR) can be covered (depending on the target application). However only a handful of metals showing useful plasmonic properties are available. Among these metals the most exploited are Ag, Au, Cu and Pt and mainly in the visible range and in the UV. Furthermore, systematic investigations of the IR response were reported mainly for Au-based nanostructures.
This year we reported the reversible tuning of the near-infrared plasmonic response of colloidal Cu2-xSe nanocrystals via a post synthetic, gradual oxidation of Cu2-xSe nanocrystals, either by oxygen or by addition of a Ce(IV) complex. The oxidation was responsible for the decrease of the Cu:Se stoichiometry from values close to 2:1 of the nanocrystals “as synthesized” down to 1.6:1, that is, “x” could be varied from 0 to 0.4. During this variation in x the optical response in the near infrared region (NIR) evolved gradually from a band around 1700 nm (Cu1.96Se), to a band at 1100-1150 nm for the most oxidized species. A subsequent, gradual reduction of the resulting copper deficient nanocrystals by the addition of Cu+ ions could restore the optical response of the “as synthesized” samples, that is, x could again be decreased. Further addition of Cu(I) salt led to disappearance of the absorption band (i.e. to a reduction of x to 0). Then again, controlled oxidation could restore the absorption in the IR. Hence, by means of the two reversible oxidation and reduction processes, a control over x in the range from 0 to 0.4 could be achieved, which allowed to tune the NIR absorption band of the nanocrystals in the 1100-1700 nm range. The studies were supported by computations in the framework of the electrostatic approximation.
We also studied the dynamics of the localized plasmon resonance exhibited by a sample of colloidal Cu2-xSe nanocrystals of 13 nm in diameter and with x around 0.15, upon excitation by ultrafast laser pulses via pump-probe experiments in the infrared, with 200 fs resolution time. The experimental results were interpreted according to the two-temperature model and revealed the existence of strong non-linearities in the plasmonic absorption due to the much lower carrier density of Cu2-xSe compared to noble metals, which led to ultra-fast control of the probe signal with modulation depth exceeding 40% in transmission. These findings add another candidate to the class of NIR plasmonic materials, and the spectral range that is accessible by these nanoparticles is highly appealing for IR spectroscopy and imaging applications.
Relevant papers of the Nanochemistry Department on this highlight:
- S. Deka et al., “Phosphine free synthesis of p-type copper (I) selenide nanocrystals in hot coordinating solvents”, J. Amer. Chem. Soc., 2010, 132, 8912-8914
- D. Dorfs et al., “Reversible Tunability of the NIR Valence Band Plasmon Resonance in Cu2-xSe Nanocrystals”, J. Amer. Chem. Soc., 2011, 133 (29), 11175-11180
- F. Scotognella et al., “Plasmon Dynamics in Colloidal Cu2-xSe Nanocrystals”, Nano Lett. 2011, doi: 10.1021/nl202390s