How Trap States Affect Charge Carrier Dynamics of CdSe and InP Quantum Dots: Visualization through Complexation with Viologen

Abstract

The depth of surface trap states in semiconductor quantum dots (QDs) is influenced by the degree of covalency, which in turn affects the charge recombination process in hybrid donor–acceptor systems. By taking relatively ionic cadmium selenide (CdSe) QDs with shallow trap states and covalent indium phosphide (InP) QDs having deep trap states as examples, the charge-transfer dynamics are explored using viologen derivative as an electron acceptor. Light-induced electron transfer in a 1:1 stoichiometric complex of both the donor–acceptor systems occurs in a picosecond time scale. The presence of deep hole trap states in InP QDs retards the charge recombination to a submillisecond time scale, which is 7 orders of magnitude lower than that in CdSe QDs in homogeneous solutions. The immobile quenchers in the quenching sphere of InP further stabilize the electron-transfer products to seconds through charge hopping, which extends the potential of these systems for charge-transfer and transport applications in photovoltaics.

Ultrafast Electron Transfer from CdSe Quantum Dots to a [FeFe]-Hydrogenase Mimic

“…49 Even though HM does not contain any specific moiety for surface binding such as thiols or amines, previous reports already demonstrated that for colloidal semiconductor nanocrystals, static quenching to an adsorbed, not covalently bound quencher occurs on a ps-timescale. [27][28][29][30][31][32]34 Diffusion-controlled quenching, on the other hand, occurs on a μs timescale, much longer than the exciton lifetime in the CdSe QDs investigated.…”

Core-Size-Dependent Trapping and Detrapping Dynamics in CdSe/CdS/ZnS Quantum Dots

“…It is well-established that the electron and hole trap-states in binary QDs (such as II–VI, III–V, and IV–VI systems) are located in their band gap. , A three-state model consisting of a valence band (VB), a conduction band (CB), and trap-states is the most simplistic representation for explaining the photoluminescence (PL) of QDs. , However, the kinetics of (i) direct exciton recombination and (ii) trapping and detrapping of electron to the surface states depend on how deep the trap-state is below the CB . An important factor influencing the trap-depth is the nature of bonding; for example, the trap-states are shallow in II–VI QDs (cadmium chalcogenide) which are relatively ionic in nature, whereas III–V QDs such as InP possess more covalent character with deep trap-states .…”

“…20,21 However, the kinetics of (i) direct exciton recombination and (ii) trapping and detrapping of electron to the surface states depend on how deep the trap-state is below the CB. 22 An important factor influencing the trap-depth is the nature of bonding; for example, the trap-states are shallow in II−VI QDs (cadmium chalcogenide) which are relatively ionic in nature, whereas III−V QDs such as InP possess more covalent character with deep trap-states. 23 These aspects have been extensively explored by measuring frequency-dependent quantum yield as a function of temperature, frequency-dependent luminescence decay curves, and time-gated emission spectra.…”

Ultrafast Electron Transfer from CdSe Quantum Dots to an [FeFe]-Hydrogenase Mimic

“…In particular, it is known that electron transfer from photoexcited cadmium chalcogenide nanoparticles to surface-proximal small organic molecules can occur on a tens of picoseconds or faster timescale. [27][28][29][30][31][32][33][34] However, this timescale has never been probed for a system of small [FeFe]-H2ase mimics and CdSe QDs. This lack of insight into the actual electron transfer step from the QD to the H2ase mimic motivated us to conduct a fundamental study of the light-induced electron transfer step in a model system consisting of CdSe QDs and the simple H2ase mimic 1,3-(μpropanedithiolato)diironhexacarbonyl (HM) (Figure 1A) employing steady-state and ns-timeresolved PL as well as fs-TA spectroscopy.…”

“…It has already been shown for CdSe QDs that an ultrafast (2 ps) electron transfer to an adsorbed organic acceptor is followed by a slower back-electron-transfer to shallow hole traps on a timescale of tens of ps. 32 As our experiments were conducted in the absence of any additional hole scavenger, this back-electron transfer is also feasible here.…”