Carrier localization in zero-dimensional and one-dimensional CdSe–CdS heterostructures

Heterostructured colloidal semiconductor nanoparticles, for instance, spherical core–shell quantum dots (QDs) or elongated quantum dots in quantum rods (QRs), are presently entering commercial markets and discussed for different applications, such as lasing or photocatalysis.^1–3 These applications have in common that they require low-defect nanoparticles but differ in their requirements regarding the movement of charge and energy within the materials. While light emission is favored by well-confined electron–hole pairs, the conversion of optical to chemical or electrical energy typically requires the extraction of excited carriers. QDs show outstanding photoluminescence properties, such as a size-dependent and narrow emission band. While QDs consisting of only one material exhibit many trap states on the particles surface, growing an inorganic shell onto QDs passivates the traps, which leads to high quantum yields. CdSe–CdS core-giant-shell QDs are nowadays readily available with near-unity photoluminescence quantum yields (PLQYs), which makes them interesting candidates for lasing or display applications.^1,4 QRs show especially promising photocatalytic properties.^5–7 While fast exciton formation was observed in CdS QRs,⁸ heterostructured CdSe–CdS core–shell QRs exhibit mobile electrons.⁹ Holes are strongly confined in the core (valence band offset $>0.5$ eV) in these structures, whereas they feature a shallow confinement for the electrons (conduction band offset $<0.2$ eV).¹⁰ Fast hole localization in the core leads to a partial charge separation. The conduction band electrons within the shell, therefore, have a macroscopic mobility and contribute to an observable photoconductivity.⁹ The shallow confinement might also allow core electrons to retain some mobility within the shell and to be extracted into surface-adsorbed molecules, for example. Knowledge of which carriers in which states are mobile within the nanostructure would be of benefit for design considerations for heteronanostructure-based photonic and photocatalytic applications.

In this context, we exemplarily investigate state-of-the-art CdSe–CdS core–shell QDs and CdSe–CdS core–shell QRs. By applying a combination of optical and THz pump–probe experiments, we can resolve the dynamics following selective carrier excitation in the shell (CdS) and core material (CdSe). The QDs show a uniform behavior, independent of the excitation. Carriers quickly condense into excitons. In QRs, electrons within the shell contribute to a significant photoconductivity, which is reduced after electron localization within the core, followed by exciton condensation. Despite the modest localization, core electrons show a negligible contribution to the QR photoconductivity.

To resolve differences in the charge carrier mobility within CdSe–CdS core–shell QDs and QRs, we performed optical transient absorption (TA) and optical-pump THz-probe (OPTP) experiments. States populated by excited carriers lead to a reduced optical absorption by Pauli blocking. A TA experiment then allows following the excited state dynamics, for instance, carrier cooling and transfer.^11,12 THz-electric fields oscillate at frequencies small enough to drive mobile carriers into macroscopic movement. When a THz pulse passes through a sample with mobile carriers, the pulse is damped and the analysis of the differential pulse allows to conclude on the properties of the mobile carriers.^13,14 For the optical experiments, carriers were selectively excited in either core or shell of the QDs and QRs by appropriate optical pulses of 400 nm wavelength for shell excitation and 570 nm for core excitation [cf. absorption spectra in Figs. 1(b) and 3(b)].

CdSe/CdS core-giant-shell quantum dots were synthesized following the protocol of Chen et al.¹⁵ The QDs are composed of a CdSe core with a diameter of 4.0 nm and giant CdS-shell accumulating to an average total diameter of 13 nm (±1.5 nm) as determined by transmission electron microscopy [cf. Fig. 1(a)]. Due to the large shell volume, the absorption of the CdS-shell dominates the UV–vis spectrum [Fig. 1(b)], whereas the absorption of the CdSe core is much weaker. Two core related absorption features can be resolved at 558 and 623 nm, which can be assigned to the 1S_e–2S_h and 1S_e–1S_h transitions, respectively.¹⁶ The low defect density and good surface passivation within the heterostructured QDs result in a near-unity photoluminescence quantum yield (PLQY) (98%) after shell excitation, a radiative charge recombination time of 96.9 ns, and a non-radiative recombination time of 4750 ns [cf. Fig. 1(c)]. Figure 1(d) schematically shows the band alignment in spherical CdSe–CdS core–shell QDs, with a large conduction band offset and a small valence band offset.

Figure 2(a) compares the obtained TA spectra of QDs after excitation of shell and core. Photoinduced absorption bleaches are observed at 622 and 490 nm. These wavelengths coincide with the core and shell absorption features in the UV–vis spectra, and the bleach features can be assigned to the 1S_e–1S_h transition in the core and shell material. Due to the quasi-type II band alignment, core and shell transitions share the same 1S_e level. Therefore, excitation of the CdSe core leads to a bleach at the shell resonance.¹⁷ Besides that, two-photon absorption might excite charge carriers in the shell material. The ratio of TA bleach in core and shell material is nearly identical for both excitations, suggesting similar carrier distributions. For OPTP, the pulse dampening by transmission through an optically excited sample is especially prominent at the peak field [Figs. 2(b) and 2(c)], and the relative field strength change can be related to the amount and mobility of the photogenerated carriers. Performing a Fourier analysis of the differential THz waveform of excited and unexcited sample allows for a more detailed discussion of the photoconductivity.^13,18 Figure 2(d) compares the carrier dynamics obtained by TA with the OPTP peak field dynamics. TA and OPTP share the same dynamics, which follows the long-lived exciton dynamics [cf. Fig. 1(c)]. To obtain complex conductivity spectra, the sheet-photoconductivities can be calculated by evaluating¹⁹ 

where ΔT(ω) and T(ω) are the Fourier transform of the differential THz waveform after photoexcitation and the THz waveform transmitted through the unexcited sample, ɛ₀ is the vacuum permittivity, c is the speed of light, and n_substrate is the refractive index of the substrate [1.44 + 0.00i for polytetrafluoroethylene (PTFE)].¹⁹ Further details on the data extraction procedure can be found in the work of Gorris et al.²⁰ QD complex photoconductivities shortly after photoexcitation and for longer delays are shown in Figs. 2(e) and 2(f). Independent of excitation wavelength and delay, QDs show zero real photoconductivity and linearly increasing imaginary photoconductivity. The response of bound electron–hole pairs, excitons, to THz-electric fields differs from that of free carriers.^21,22 The exciton is polarized by the oscillating field and reacts depending on the exciton-polarizability. As example, Wang et al. reported zero imaginary photoinduced susceptibility and a frequency-independent real photoinduced susceptibility as a signature of exciton polarization in CdSe QDs.²³ These reported susceptibility features correspond to the observed zero real and linearly rising imaginary photoconductivity. It should be noted that the sign of the imaginary part of a Fourier transformation depends on the used Fourier convention. Therefore, positive and negative imaginary photoconductivities of excitonic systems are found in the literature.^8,9 In the QDs, carriers condense into excitons on sub-picosecond timescales, independent of core or shell excitation. Such systems represent optimal systems for light-emission applications.

CdSe–CdS quantum dots in quantum rods were synthesized according to the procedure described by Jochum et al.²⁴ CdSe-cores with an average diameter of 3.3 nm were used. After growth of an elongated CdS-shell, the particles had an average length of 40 nm (±5 nm) with a width of 4.5 nm (±0.5 nm) [cf. Fig. 3(a)]. The CdS absorption onset is at shorter wavelengths for the QRs (∼480 nm) as compared to the QDs since charge carriers exhibit a stronger confinement in the two directions perpendicular to the rod axis. As observed for the QDs, absorption of the CdSe core is relatively weak, showing a well-defined 1S_e–1S_h transition at 600 nm [cf. Fig. 3(b)]. The measured PLQY after shell excitation was 73%. The radiative and non-radiative charge recombination rates were determined to be 15.6 and 42.2 ns [cf. Fig. 3(c)]. A schematic depiction of the band alignment in CdSe–CdS QRs is shown in Fig. 3(d).

TA and OPTP measurements of QRs are summarized in Fig. 4. As for the QDs, the main TA bleach features around 600 and 467 nm originate from Pauli blocking within the core and shell 1S_e–1S_h transitions [cf. Fig. 3(b)]. For the QR geometry, shell excitation results in an increased relative carrier concentration within the shell compared to QDs [Fig. 4(a)]. Lupo et al. measured TA on CdSe–CdS QRs with similar dimensions and assigned the shell bleach after core excitation to electron delocalization into the shell,¹² as expected from the band alignment of quasi-type II structures. Two-photon absorption of the shell material might also contribute to the shell bleach in the QRs.²⁵ A comparison of the core and shell TA bleach dynamics [Figs. 4(b)–4(d)] shows a fast decay of the carrier population in the shell and a rise of the carrier population in the core within a few picoseconds. This can partly be reasoned by a fast carrier transfer from the shell to core, evidenced by the relative timescales of the TA contrast rise [Fig. 4(d)]. Core-localized carriers then show a long lifetime decay in similar means as for the QDs. However, shell bleach shows an additional decay component, not resembled by the rise of the core bleach. This might probably be due to multiexcitonic Auger-type processes in some particles or trapping of electrons at the CdS-shell.²⁶ Interestingly, the OPTP contrast follows the carrier dynamics of shell-localized carriers (TA bleach of the shell). When carries are localized within the core, a reduction of mobile carriers leads to decreasing OPTP contrast and a reduction of Pauli blocking of shell states. After 150 ps, TA dynamics of the core and shell and OPTP dynamics resemble. In contrast to QDs, shell-excited carriers are mobile within the QRs. The complex sheet-photoconductivities of the QRs following shell excitation [Fig. 5(a)] show a significant real photoconductivity when carriers are present in the shell (cf. Fig. 4). This can be explained by the fact that holes are quickly localized in the core. This leads to a partial charge carrier separation, leaving mobile electrons in the conduction band of the shell. Hole-trapping at the surface of the CdS-shell may also contribute to a charge separation. However, we do not observe a photoinduced absorption at wavelengths to the red of the bleach in our TA measurements, which is usually ascribed to trapped holes.^27,28 After electron localization at the core or after direct core excitation, sheet conductivities show an excitonic character [cf. Fig. 5(b)]. This demonstrates that only electrons populating conduction band states of the shell are mobile within the QR, while electron localization within the core results in immobile excitons. The latter occurs by charge transfer or direct core excitation.

In summary, we investigated CdSe/CdS core–shell nanocrystals of spherical and elongated shape with high monodispersity and photoluminescence quantum yields. We studied carrier relaxation and localization by TA and OPTP spectroscopy. In QDs, charge carriers condense into excitons on sub-picosecond timescales, independent of core or shell excitation. In QRs, fast hole localization in the core leads to partial charge separation, resulting in electrons within the shell, which are mobile on macroscopic scales. Core-localized carriers form excitons and do not contribute to a photoconductivity. For light-emitting applications, the excitonic properties of charge carriers in QDs are beneficial and reflected in high QYs. For applications such as photocatalysis, which require charge separation and mobile carriers, QRs are the geometry of choice. However, despite the shallow confinement in CdSe–CdS QRs, core-localized carriers do not significantly contribute to a real photoconductivity, which has to be considered in the material design.

Linear absorption spectra were recorded using a Varian Cary 50 spectrometer. TEM images were analyzed to determine size distributions of the samples. A Joel JEM-1011 instrument was used, operating at 100 kV. Time-resolved photoluminescence was measured employing a FluoTime300 PicoQuant with an excitation wavelength of 450 nm.

OPTP and TA setups were described previously.^20,29 Excitation pulses of 400 and 570 nm wavelength were generated by second-harmonic generation of the 800 nm laser fundamental in a BBO-crystal and in an optical parametric amplifier, respectively. For the OPTP experiments, the samples were dropcasted onto a PTFE substrate and the experiments were carried out under a dry nitrogen atmosphere. PTFE has a constant refractive index of 1.44 + 0.0i in the THz frequency range. TA experiments were conducted on samples in solution. Pump fluences were 80 and 600 μJ/cm² for excitation at 400 nm (shell excitation) and 570 nm (core excitation), respectively.

This work was supported by the Cluster of Excellence “Advanced Imaging of Matter” (EXC 2056-Project ID 390715994) and the Graduate School “Nanohybrid” (Grant No. GRK 2536) of the Deutsche Forschungsgemeinschaft (DFG).

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.