Efficient Directed Energy Transfer through Size‐Gradient Nanocrystal Layers into Silicon Substrates

Abstract

Spectroscopic evidence of directed excitonic energy transfer (ET) is presented through size‐gradient CdSe/ZnS nanocrystal quantum dot (NQD) layers into an underlying Si substrate. NQD monolayers are chemically grafted on hydrogen‐terminated Si surfaces via a self‐assembled monolayer of amine modified carboxy‐alkyl chains. Subsequent NQD monolayers are linked with short alkyldiamines. The linking approach enables accurate positioning and enhanced passivation of the layers. Two different sizes of NQDs (energy donors emitting at 545 nm, and energy acceptors emitting at 585 nm) are used in comparing different monolayer and bilayer samples grafted on SiO2 and oxide‐free Si surfaces via time‐resolved photoluminescence measurements. The overall efficiency of ET from the top‐layer donor NQDs into Si is estimated to approach ≈90% through a combination of different energy relaxation pathways. These include sequential ET through the intermediate acceptor layer realized mainly via the non‐radiative mechanism and direct ET into the Si substrate realized by means of the radiative coupling. The experimental observations are quantitatively rationalized by the theoretical modeling without introducing any extraneous energy scavenging processes. This indicates that the linker‐assisted fabrication enables the construction of defect‐free, bandgap‐gradient multilayer NQD/Si hybrid structures suitable for thin‐film photovoltaic applications.

Engineering Multilayered Nanocrystal Solids with Enhanced Optical Properties Using Metal Oxides for Photonic Applications

“…For multilayer systems, the application of CVD-grown alumina enhances PL intensity beyond the simple addition of NQD layers (of same NQD concentration as the initial layer), and PL lifetimes continue to increase, suggesting improved surface passivation. This finding is in stark contrast with previous observations, for which the PL properties of multilayer films were inferior to those of individual layers. , The variable and tailorable interlayer distance provided by alumina can further be used to control the rate of energy transfer between multilayers of different sized NQDs Figure c,d shows the PL spectra and lifetimes of a bilayer sample composed of two different sized NQDs (emitting at 585 and 545 nm) and the NQD layers being separated by different intermediate alumina layer thickness grown by varying the number of CVD cycles.…”

Nonradiative Energy Transfer between Doped and Undoped Flat Semiconductor Nanocrystals of Colloidal Quasi-2D Nanoplatelets

“…doped NPL–undoped NPL). Previously, the NRET efficiency was reported for the undoped QD–QD donor/acceptor pairs in excess of 90%; , for the undoped QD-NPL pairs up to 90%; and for the undoped NPL–NPL pairs up to 60 and 90% (for a single NPL–NPL pair) at room temperature. Another important point is the saturation molar ratio value of the efficiencies, which is beginning at ∼0.1 and reaching to the saturation value at ∼0.02.…”

Orientation-Controlled Nonradiative Energy Transfer to Colloidal Nanoplatelets: Engineering Dipole Orientation Factor

“…The overall structure is schematically depicted in Figure a for the nonstacked NPLs and in Figure b for the stacked NPLs. There is also a bottom Al 2 O 3 film, which is deposited directly on Si substrate to avoid energy transfer from the QDs to the Si wafer. − NPLs were laid down via the described self-assembly procedure. Using our setup, which is capable of depositing up to 12 1 × 1 cm 2 substrates simultaneously (see Figure S1), a set of NPL films (i.e., fully nonstacked or fully stacked) were deposited at once to ensure all the NPL films are prepared under the same experimental conditions.…”