Zero-Threshold Optical Gain in Electrochemically Doped Nanoplatelets and the Physics Behind It

Abstract

Colloidal nanoplatelets (NPLs) are promising materials for lasing applications. The properties are usually discussed in the framework of 2D materials, where strong excitonic effects dominate the optical properties near the band edge. At the same time, NPLs have finite lateral dimensions such that NPLs are not true extended 2D structures. Here we study the photophysics and gain properties of CdSe/CdS/ZnS core–shell–shell NPLs upon electrochemical n doping and optical excitation. Steady-state absorption and PL spectroscopy show that excitonic effects are weaker in core–shell–shell nanoplatelets due to the decreased exciton binding energy. Transient absorption studies reveal a gain threshold of only one excitation per nanoplatelet. Using electrochemical n doping, we observe the complete bleaching of the band edge exciton transitions. Combining electrochemical doping with transient absorption spectroscopy, we demonstrate that the gain threshold is fully removed over a broad spectral range and gain coefficients of several thousand cm–1 are obtained. These doped NPLs are the best performing colloidal nanomaterial gain medium reported to date, with the lowest gain threshold and broadest gain spectrum and gain coefficients that are 4 times higher than in n-doped colloidal quantum dots. The low exciton binding energy due to the CdS and ZnS shells, in combination with the relatively small lateral size of the NPLs, results in excited states that are effectively delocalized over the entire platelet. Core–shell NPLs are thus on the border between strong confinement in QDs and dominant Coulombic effects in 2D materials. We demonstrate that this limit is in effect ideal for optical gain and that it results in an optimal lateral size of the platelets where the gain threshold per nm2 is minimal.

Spectroelectrochemistry of CdSe/CdxZn1–xS Nanoplatelets

“…Norris et al have proposed electrochemical charging as a means to control the charging of nanoplatelets and related spectral broadening . Only a few studies have explored the effects of electrochemical charging on the spectral and electrical properties of nanoplatelets. ,− …”

Colloidal Semiconductor Nanocrystal Lasers and Laser Diodes

“…As a result, the band-edge gain can reach very large values exceeding 6000 cm –1 (Figure a) . Very large gain coefficients in NPL materials have been observed in multiple studies utilizing both VSL experiments ,,,, and TA measurements. ,,,,, In addition to the high NPL excitonic occupancies needed for gain saturation, large optical gain coefficients result from extremely high packing densities achievable with the NPLs (Figure b) …”

“…2D colloidal NPLs have been under active investigation as optical gain materials for lasing applications. ,− These structures are analogous to epitaxial quantum wells that are widely employed in present-day semiconductor lasers. , Lateral dimensions of NPLs usually exceed the semiconductor Bohr radius. Therefore, carrier motion is restricted only in one dimension, namely, along the NPL thickness (referred here as the “ z -axis”) .…”

“…157 Very large gain coefficients in NPL materials have been observed in multiple studies utilizing both VSL experiments 146,149,150,156,187 and TA measurements. 107,152,154,155,160,189 In addition to the high NPL excitonic occupancies needed for gain saturation, large optical gain coefficients result from extremely high packing densities achievable with the NPLs (Figure 23b). 153 As was suggested in refs 154 and 190, in addition to excitonic states, NPLs may support stable Coulombically bound biexcitons or excitonic molecules.…”

Effects of Pump Photon Energy on Generation and Ultrafast Relaxation of Excitons and Charge Carriers in CdSe Nanoplatelets

“…The NPLs were dispersed in hexane in an airtight Hellma QS cuvette inside a nitrogen-purged glovebox to avoid contact with air . The Fermi level of such samples is near the middle of the NPL band gap, which implies that unintentional doping and the presence of background charge carriers are negligible . The steady-state optical absorbance spectrum, A 0 ( ℏ ω), as a function of photon energy ℏ ω was obtained with a double-beam PerkinElmer Lambda 1050 UV/Vis spectrometer.…”