We report photo-dynamically provoked photoluminescence blue shifts up to ∼8 meV of oleic acid capped 2.5 nm PbS quantum dots in toluene at room temperature. Exposing the solution to pulsed laser (26 ps, 10 Hz) emissions at 532 nm and 1064 nm, the photo-induced band gap increase is evoked by single and two-photon transitions, respectively. The emission peak blue shifts, recorded in reflection and transmission geometries, show a 2/3 power dependence on the optical stimulus gain, rendering the Burstein-Moss shift to be the underlying inherent n-type doping effect in the quantized colloid.
© 2015 Optical Society of America
1. Introduction and motivation for the current work
The ground-breaking works of Efros and Efros , and Brus  in the 1980s initiated an outburst of publications on the optical properties and possible solar energy conversion applications of quantum dots (QDs), as demonstrated for example by [3–9] (and references therein), while recently, due to the envisaged nano-structured device technologies, doping constraints of solution-processed semiconducting QDs moved to the forefront of research efforts [10–13]. The current methodology of doping encompasses various approaches including a modified reaction used for gold growth on QDs , a micelle-assisted wet-chemistry route , precursor ratio variations , and charge transfer from an additive , to mention a few. While devising a robust doping scheme for ambient condition operation remains still the challenge , the mastery of colloidal QD doping would allow for controlled alteration of the electronic properties and, thereby, promoting widespread device evolutions in the nano-regime based on shape adaptable soft matter.
In comparison to the procedures in [10–13], the employment of photo-dynamic optical doping of QDs via the Burstein-Moss shift (BMS), i.e., photo-dynamic Burstein-Moss doping, also known as dynamic band filling [3,14], represents a chemically inert doping method preserving stoichiometry and texture. However, employment of BMS for active device realizations by incorporating photo-dynamically activated junctions in operative circuit platforms requires an ‘out-of-the-box’ nano-regime design. The vision of, for example, photonically activated QD rectifiers is uncharted territory in the quantum field and requires a change in traditional technological approaches. In this paper, we address the basic question upon the intrinsic scaling law of the spectral blue shift of QD photoluminescence (PL) owing to the observation of the BMS of the emission peak of QDs in solution. The BMS was monitored while exciting the QD solution by single and two-photon pumping, whereas the latter study is not addressed in the literature and reveals new avenues for optical diagnostic measuring for the medical field by the employment of tissue penetrating infrared laser sources.
The dynamic BMS takes place in semiconductors due to the excitation of a sufficiently high electron-hole pair density, which fills the energy levels near the band bottoms, causing a band gap increase . The resulting blue shift of the measured spectra is expected to grow with Iex2/3, where Iex is the intensity of the impinging optical stimulus [3,14,15]. In , optical pumping of colloidal CdS QDs pointed toward dynamic BMS of the absorption edge, however, the trend was not convincingly observed. In addition, the BMS was qualitatively mentioned several times to be the factor of blue shifts in QD absorption or emission spectra [11,16–18], but the 2/3 power trend of optically excited quantized matter was not verified before the recently published work in . Hence, the reported  close to linear scaling of the absorption blue shift of Bi-doped n-type PbS QDs caught our attention because, as explicitly noted in , it clearly exceeds the expected BMS trend. Here-in, motivated by the current investigation of the inherent blue shift scaling of n-type doped PbS QDs, we particularly distinguish the effects stemming from purely electronic alterations opposed to influences caused by morphology and crystal structure distortions.
The photo-dynamic BMS in  was observed at 5 K via the blue shift of the PL excited by the 532 nm continuous wave (cw) laser emission coming from a 2 W Coherent Verdi solid state laser. The samples investigated were films composed of 2.0 nm and 4.7 nm PbS QDs deposited on GaAs and glass with supercritical carbon dioxide . The current work shows photo-dynamic doping of PbS QDs in the most original and conserved state possible - as a colloid in toluene. We present two sets of measurements employing ps laser pulses at room temperature (RT): Common single-photon excited emission and, in order to check the inherence of the observed results, PL evoked by two-photon absorption, which ensures uniformly “cold” QD probing. Both experiments confirm the BMS trend.
The oleic acid capped PbS QDs (lot number: SCR-0-058_2) in toluene with concentration of 20 mg/ml were provided by the Center for Applied Nanotechnology (CAN) GmbH located in Hamburg, Germany. The diameter of the spherical QDs and band gap is 2.5 ± 0.3 nm and ∼1.42 eV, respectively . During the measurements the solution was kept in a quartz vial (1 cm × 2 cm × 0.1 cm), which was irradiated at 532 nm and 1064 nm with the unfocused beam (diameter ∼1 cm) of a pulsed Nd:YAG laser (26 ps, 10 Hz) from EKSPLA for the single-photon and two-photon excited PL, respectively, SPL and TPL hereafter. The emission was recorded with the Ocean Optics USD 2000 fiber spectrometer detecting the signal in reflection (RE) and transmission (TR) geometries.
3. Results and discussion
Figures 1 and 2 show the SPL and TPL spectra at various Iex levels. The solid and broken lines represent the measurements in RE and TR geometry, respectively, the dotted lines are Gaussian fits, and the oblique arrows are guides for the eyes for visualizing the BMS. The measurements were carried out in both geometries for up to ten different excitation levels and the blue shift of the SPL and TPL peak energies (ESPL and ETPL) vs. Iex is plotted in Figs. 3 and 4. The symbols denote the experimental results and the solid lines are fits using the BMS expression for bulk semiconductors ,21], and E0 is the corresponding peak energy in the low Iex limit. The fit, whose parameters for both geometries are summarized in Table 1, convincingly match the trends. Consequently, effective electronic coupling between the QDs transforms the response of the colloidal solution to that of a three dimensionally responsive material , which is not affected by multiple exciton generation  since the doubled QD band gap energy exceeds the energy of the optical stimuli . We further notice that the C value for SPL[RE] in Table 1 is virtually identical with that of the 2.0 nm QDs in , pointing towards the intrinsic character of the observed BMS being independent from the nature of the exciting laser (cw vs. pulsed).
Comparing the peak energy locations in Figs. 1 and 2, we see that the TPL, which is hardly influenced by the applied geometry, happens to be in close proximity to the SPL[TR], which reveals a red shift with respect to the SPL[RE]. Equivalent results such as these in Figs. 1 and 2 have been observed by SPL and TPL of thin-film CdS, demonstrating that the red shift is subject to self-absorption [22–24]. The nearly free-of-absorption penetration capability of the 1064 nm line  is the reason that the detection geometry hardly influences the energy location of the TPL peaks . The spectra, overall, satisfactorily match the Gaussian shape representing in sum inhomogeneous broadening of the homogeneously broadened emissions of the individual QDs within the QD ensemble hit by the laser. In agreement with our previous results , optical stimulus gain broadens the SPL and TPL spectra following the power law ∝Iexδ with δ∼0.01 and δ∼0.0015, for SPL and TPL, respectively. Spectral broadening is attributed to being a characteristic feature of the dynamic BMS .
A chapter of its own is the emission intensity vs. Iex of quantized soft matter. For Iex<30 MW/cm2 the TPL intensity grows with Iex1.5-2.0, as expected for two-photon absorption initiated emission processes , but exhibits a Iex0.7-0.8 trend for elevated Iex without signs of limitation because the required limitation intensity scales with L/d, where L is the vial depth (i.e., sample thickness) and d the QD diameter, drastically increasing the saturation threshold with respect to a QD thin film for which L/d∼1 . The SPL, on the other hand, shows a square root like limitation tendency, similar to the photocurrent in nanowires .
The work demonstrates photonic doping of a PbS QD solution achieved by single and two-photon laser excitation. The intrinsic band gap increase due to n-type doping was monitored via the PL peak blue shift and scales with the power of 2/3, which is distinctive for the BMS in bulk semiconductors. Conclusively, the study reveals three-dimensional electronic coupling between colloidal QDs even in solution and points out the fundamental scaling law of intrinsic n-type QD doping.
The work was partially supported by the DGAPA-UNAM PAPIIT projects TB100213-RR170213 (Bruno Ullrich) and IT102013 (J. A. Reyes-Esqueda). PB and AKS acknowledge the DGAPA-UNAM program of postdoctoral fellowships. The authors acknowledge the technical support of R. Garcia and U. Amaya.
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25. The solution’s transmittance in the vial used for the BMS experiments is ~0.90 at 1064 nm.
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