Observation of Phonon Cascades in Cu-Doped Colloidal Quantum Wells

Abstract

Electronic doping has endowed colloidal quantum wells (CQWs) with unique optical and electronic properties, holding great potential for future optoelectronic device concepts. Unfortunately, how photogenerated hot carriers interact with phonons in these doped CQWs still remains an open question. Here, through investigating the emission properties, we have observed an efficient phonon cascade process (i.e., up to 27 longitudinal optical phonon replicas are revealed in the broad Cu emission band at room temperature) and identified a giant Huang–Rhys factor (S ≈ 12.4, more than 1 order of magnitude larger than reported values of other inorganic semiconductor nanomaterials) in Cu-doped CQWs. We argue that such an ultrastrong electron–phonon coupling in Cu-doped CQWs is due to the dopant-induced lattice distortion and the dopant-enhanced density of states. These findings break the widely accepted consensus that electron–phonon coupling is typically weak in quantum-confined systems, which are crucial for optoelectronic applications of doped electronic nanomaterials.

Ultrafast Control of the Optical Transition in Type-II Colloidal Quantum Wells

“…13,17 This is another fingerprint of fully type-II band alignment since the spatially separated electrons/ holes are more polarizable and can heavily interact with longitudinal optical phonons through Frolich interactions. 34 In addition, we observe an averaged interdomain exciton lifetime of ∼54.6 ns via the time-resolved PL measurement (see Figure S1), which is an order of magnitude longer than in intradomain excitons, again supporting the effective charge separation since the spatially indirect nature leads to a reduced electron/hole wavefunction overlapping. 25,33 To probe whether excitation-generated carriers can affect optical transitions in type-II CQWs, we have measured transient absorption (TA) spectra using the pump−probe technique.…”

“…Recently, several reports − ,, have investigated CT dynamics at the domain interface of CdSe/CdTe CQWs; although the role of the carrier interaction/excitonic effect on the charge separation process and how carriers across different domains can conserve the momentum are still under debate, a consensus from all reports is that interfacial CT occurs with near-unity efficiency and on an ultrafast sub-picosecond timescale (e.g., 0.48 ps in ref and 70 fs in ref . We have also noticed that the CT emission shows a large Stokes shift (∼16 nm) and broad linewidth (full width at half maximum: ∼54 nm), which are in stark contrast to the nearly vanishing Stokes shift (2–4 nm) and the narrow emission (8–12 nm) in single-domain CQWs with atomic monolayer thickness confinement. , This is another fingerprint of fully type-II band alignment since the spatially separated electrons/holes are more polarizable and can heavily interact with longitudinal optical phonons through Frolich interactions . In addition, we observe an averaged interdomain exciton lifetime of ∼54.6 ns via the time-resolved PL measurement (see Figure S1), which is an order of magnitude longer than in intradomain excitons, again supporting the effective charge separation since the spatially indirect nature leads to a reduced electron/hole wavefunction overlapping. , …”

Intrinsic Self-Trapped Excitons in Graphitic Carbon Nitride

“…Such a broad PL consists of five emission features centered at 470, 490, 524, 565, and 588 nm, respectively (see the band fitting and peak determination process in Figure S3). The large and irregular energy spacings between these five peaks (∼140−220 meV) can rule out their possible vibronic nature, as reported earlier in the case of phonon cascades 40 and Franck−Condon states. 41 Meanwhile, unlike the broad emission profile in carbon dots, 42,43 the PL profile of PhCN is nearly independent of the excitation wavelength (see Figure S4), implying that these emission features should come from analogous intermediate states instead of different emission transitions.…”

Expanding the color palette of bicolor-emitting nanocrystals

“…In this sense, 2D nanoplatelets (NPLs) offer several advantages: (i) their specific growth mechanism 13,14 enables particularly narrow emissions, (ii) their anisotropic shape enables combining both strong confinement within the thickness while presenting a large lateral extension, (iii) their specific colloidal growth allows the design of complex heterostructures involving multiple interfaces [15][16][17][18][19] (iv) the several post-synthetic doping techniques leading toward dopant 20 or trap emissions. [21][22][23][24][25] Thus, NPLs expand the possibilities for emission quantum engineering.…”