Core–Shell Quantum Dots and Surface Passivation

Introduction

In a quantum dot, many atoms lie near the surface. An electron–hole excitation created in the interior may reach surface defects before emitting light. Ligands and inorganic shells can suppress these unwanted pathways. The practical aim is often a high photoluminescence quantum yield, but bright emission alone does not prove that every surface state is gone. The shell's band alignment, strain, defects and thickness determine what happens to carriers.

Core explanation

The photoluminescence quantum yield is Φ PL = number of emitted photons / number of absorbed photons , under specified conditions. If every absorbed photon ultimately produces one measured luminescence photon, Φ PL approaches one. Nonradiative pathways, including trapping and energy transfer to the environment, lower this ratio. A simple competition model is Φ PL = k r/(k r + k nr), where k r is the radiative decay rate and k nr is the sum of nonradiative rates for the population being modelled. This expression assumes comparable first-order pathways from that population; blinking and multiple species can make a real sample more complicated.

Uncoordinated surface atoms, vacancies or poorly matched bonds can create states in the gap or near band edges. These may capture an electron or hole, alter emission wavelength, or provide nonradiative recombination routes. Organic ligands bind to surface atoms and also keep dots dispersed in solvents. Ligand choice therefore affects both electronic passivation and colloid stability. Exchanging a ligand to make dots water-compatible may alter quantum yield even if the inorganic core remains unchanged.

An inorganic shell grows a second material around the core. In a common type-I-like alignment, both electron and hole are preferentially confined near the core, reducing their access to surface defects. In a type-II alignment, electron and hole favour different regions; that can extend charge separation but can reduce wavefunction overlap and shift emission. Exact alignment is material- and size-dependent. Shell growth can passivate dangling bonds, yet lattice mismatch may create strain and defects. A thick shell may protect the core better but changes particle size, absorption, solubility and sometimes carrier localisation.

Passivation must be evaluated under the relevant medium and illumination. Oxygen, moisture, intense light or ligand displacement can create new traps over time. Measured Φ PL depends on excitation wavelength, concentration, reabsorption and instrument calibration. An apparent increase in brightness after adding a shell could result from greater absorption rather than a greater fraction of absorbed photons emitted; quantum yield separates those quantities.

Step-by-step reasoning

Start with where electron and hole are generated and whether they can reach an exposed surface. Identify potential trap-forming defects or environmental quenchers. Predict how a ligand or shell changes access and the relevant decay rates. To compare samples, measure absorbed and emitted photon numbers under controlled conditions; check whether aggregation or reabsorption has changed. State whether the result is a quantum yield, lifetime or raw intensity, because these are related but different observables.

Visual explanation

Draw a core sphere with surface defect sites and arrows showing one photon emitted versus one excitation lost nonradiatively. Add a ligand-coated version, then a core–shell version with a boundary and band-edge diagram. For a type-I-like alignment draw electron and hole concentrated in the core. A second diagram with separated preferred regions illustrates why shell composition can change the recombination pathway.

Real-world analogy

Think of an illuminated theater in which people can leave by a visible front door or several unobserved side exits. Adding a shell closes some side exits, increasing the fraction leaving visibly. This captures competing decay routes but does not imply each dot behaves identically: individual dots may blink and have different defects.

Real-world example

A synthesis team may compare a bare semiconductor core with the same cores after growth of a wider-gap shell. They record absorption, integrated emission and calibrated reference measurements at matched optical density. A larger quantum yield after shell growth suggests reduced nonradiative loss. Electron microscopy and elemental analysis check that a shell actually formed, while lifetime measurements help distinguish altered radiative and nonradiative rates.

Why?

Why are surface traps especially important at the nanoscale? A large fraction of the dot's atoms lies close to an interface, and confined carrier wavefunctions can overlap it. Why use an inorganic shell as well as ligands? A shell can physically and electronically separate the active core from the solution, though it introduces its own material interface that must be controlled.

Common misconception

“Brighter” is not synonymous with “higher quantum yield.” More concentrated dots or stronger absorption can increase total light while Φ PL stays constant. A second misconception is that any shell automatically improves emission: poor band alignment, incomplete coverage or interfacial strain may introduce losses instead.

Worked example

Question: A calibrated experiment finds that a dot dispersion absorbs 8.0 × 10⁶ photons during the measurement and emits 5.6 × 10⁶ photoluminescence photons. What is Φ PL?

Reasoning: Divide emitted photon count by absorbed photon count, not incident count. Φ PL = 5.6/8.0 = 0.70. If 10 million photons had arrived but only 8 million were absorbed, using 10 million in the denominator would answer a different efficiency question.

Answer: The photoluminescence quantum yield is 0.70, or 70%.

Quick check

1. Can a sample become brighter at fixed excitation power without increasing its photoluminescence quantum yield? Answer: Yes. It could absorb more photons because its concentration or absorption cross-section increased.

Exam focus

Define quantum yield using absorbed rather than incident photons. Distinguish core, shell, ligand and solvent roles. When discussing type-I and type-II arrangements, describe carrier localisation instead of treating the labels as universal guarantees of brightness.

Advanced insight

A shell changes more than defects. It can alter dielectric screening of the electron–hole interaction, modify strain and shift the effective confined volume. Time-resolved photoluminescence can help separate radiative and nonradiative changes: in a simple single-population model lifetime τ = 1/(k r+k nr), and Φ PL = k rτ. For example, higher yield with a longer lifetime may be consistent with suppressed k nr, but heterogeneous dots and multi-exponential decays require more careful interpretation. Surface engineering is therefore both electronic-structure and colloid-chemistry design.

Summary

Ligands and shells can passivate traps and protect quantum-dot cores. Their effect is measured by the fraction of absorbed photons emitted, not raw brightness. Band alignment, shell quality, solvent and ligand stability all affect the outcome. A useful comparison controls absorption and checks for aggregation, then combines quantum yield with structural and lifetime evidence.

Practice questions

1. In a simple model k r = 2 × 10⁷ s⁻¹ and k nr = 3 × 10⁷ s⁻¹. Find Φ PL. Answer: Φ PL = 2/(2+3) = 0.40, or 40%.

2. What is one way ligand exchange could lower quantum yield without changing core diameter? Answer: It may leave surface atoms insufficiently coordinated, creating trap-mediated nonradiative pathways.

3. Why might a type-II shell favour charge separation? Answer: Its band alignment can favour the electron and hole in different spatial regions, reducing their overlap.

4. A sample emits the same number of photons after treatment but absorbs half as many. What happens to Φ PL? Answer: It doubles, provided the photon counts are valid and the resulting ratio remains at most one for ordinary one-photon photoluminescence.

Sources: NIST, Optical Properties of CdSe/ZnS Nanocrystals; Epitaxial CdSe/CdS core–shell study, JACS.