A Series on TAS - Applications in Semiconductor Nanocrystal and Quantum Dot Systems

Transient absorption spectroscopy, in addition to its application in studying molecular systems, is a critical technical method for exploring the excited state dynamics of semiconductor nanocrystals or quantum dots. In this discussion, we will use semiconductor quantum dots as an example to elucidate the fundamental principle of transient absorption spectroscopy in detecting the dynamic processes of such materials.

01 Semiconductor Nanocrystal System

Semiconductor quantum dots are quantum confinement materials, and their transient spectral characteristics exhibit numerous similarities to those of molecular materials. In this article, the generation of signals corresponding to the characteristics of the transient absorption spectrum will also be expounded upon in accordance with the different electronic transition processes detectable by the transient spectrum.

02 Excited State

Figure 1a presents the steady-state (ground state) absorption spectrum of a typical II-VI quantum dot (such as CdS, CdSe, etc.), where two or more prominent absorption peaks are typically observed. These peaks are attributed to the first (band edge) exciton (exciton at band edge) absorption peak (1) and a higher energy exciton absorption peak (2). The corresponding electronic transition process is depicted in Figure 1b. After the sample is excited (with some quantum dots transitioning to the excited state), its absorption spectrum undergoes several characteristic changes in comparison to that prior to excitation (ground state) (Figure 2).

Firstly, the intensity of absorption peak 1 decreases, accompanied by a certain degree of spectral redshift. Secondly, absorption peak 2 usually also undergoes a certain extent of redshift. Part of this variation is attributable to the quantum dot transitioning to an excited state, as illustrated in Figure 2b. Upon excitation of the quantum dot, an electron occupies the 1Se orbital of the conduction band (temporarily disregarding the hot electron process), thereby entering the excited state. According to the Pauli exclusion principle, at this point, the probability of an electron transition from the valence band 1Sh orbital to the conduction band 1Se orbital of the quantum dot is reduced to half of that in the pre-excitation (ground state), resulting in a decrease in the absorption intensity of the exciton absorption peak 1. Furthermore, due to the influence of the electron-hole Coulomb interaction (such as the Stark effect or biexciton effect) in the excited quantum dots, the energy levels of the 1Se and 1Pe orbitals in the conduction band are lower compared to those in the ground state (as illustrated by the solid and dashedlines in Figure 2b), causing a redshift of the corresponding absorption peaks. The Stark effect in quantum dots will be discussed in greater detail in the subsequent section.

Figure 1. (a) Steady-state absorption spectra of a typical quantum dot and the corresponding 

(b) Electronic transition process.

k_ETk_ET

Figure 2. (a) Variations in the absorption spectra of a typical semiconductor quantum dot sample before and after excitation

(b) The associated changes in energy levels and transitions

(c) Photoinduced absorption transitions generated by conduction band electrons and valence band holes in the excited state of the quantum dot

(d) The characteristic signal of a typical transient absorption spectrum resulting from the changes in electronic transitions in the excited state of the quantum dot, as illustrated in Figures a - c.

Consequently, the transient absorption characteristic spectrum of the sample can be derived from the variation in the absorption spectrum of the quantum dot before and after excitation (Figure 2d). The absorption peak 2 predominantly exhibits a transient spectral profile with derivative-like characteristics due to the redshift of the spectrum. Absorption peak 1 represents the superposition of two spectral modifications: 1) A decrease in absorption intensity caused by electron state filling in the 1Se orbital of the conduction band, leading to the ground state bleaching signal. 2) A derivative-like transient signal resulting from the spectral redshift induced by the Stark effect. Furthermore, the conduction band electrons and holes in the excited quantum dots can also absorb photons and transition to their respective higher energy level orbitals (Figure 2c), generating photoinduced absorption signals in the transient absorption spectrum (Figure 2d). Since this is an intra-band absorption process in quantum dots and the transition range of its energy level is usually relatively small, the resulting transient absorption spectrum signal will occur in a region far from the low-energy side of the bleaching signal (such as near infrared to mid-infrared).

The aforementioned transient signals are the typical transient spectral features of II - VI quantum dot materials, such as CdS, CdSe, and PbS. It is particularly important to note here that in the detection of transient absorption spectroscopy of semiconductor materials (quantum dot, nanocrystal or bulk phase), the transient spectral signal represents the combined contributions of electron and hole signals. The proportion of electron and hole contributions varies across different semiconductor materials. In II - VI series semiconductor quantum dots, the transient signal is primarily contributed by electrons in the conduction band. Although the exact reason remains unclear in the academic realm, it is generally acknowledged that this phenomenon is due to the degeneracy of valence band holes or a higher density of states. Conceptually, the relative change from 100 to 101 (in the valence band) is significantly smaller than the relative change from 0 (in the conduction band). Therefore, the dynamics of excited state signals in quantum dots typically reflect the movement of conduction band electrons. This includes processes such as the orbital filling by electrons, electron-hole recombination, electron trapping in defect states, and electron transfer. In previous studies, to more accurately assess the contribution of electrons and holes to the ground state bleaching signal, researchers introduced load electron or hole acceptors onto the surface of quantum dots. By observing the effects of electron and hole transfer on the dynamics of the excited state signal, they were able to determine the relative contribution of electrons and holes.

03 Stark Effect and Hot Electrons

The Stark effect typically refers to the phenomenon where the energy levels and spectra of atoms or molecules split when subjected to an external electric field. In quantum dots, optical excitation generates electron-hole pairs, which create a built-in electric field within the quantum dot due to Coulomb interaction (Figure 3a). This leads to a shift in the transition energy level caused by the Stark effect, typically towards lower energy. In the transient absorption spectrum, this appears as a transient spectrum with derivative-like characteristics, caused by the red shift of the absorption peak, as illustrated by the spectral shift and the change in the transition energy level shown in Figure 3. Consequently, transient absorption spectroscopy can be employed to detect certain carrier dynamic processes by capturing the spectral features induced by the Stark effect.

The continuous distribution characteristic of the absorption spectrum in quantum dots makes it relatively easy to generate hot electrons within them (Figure 3a). When the energy of the excitation light (E_exc, note that it refers to photon energy rather than photon density) exceeds the band gap energy (E_bg) of the quantum dot, electrons are excited to a higher energy level in the conduction band. Since their energy exceeds that of the band-edge electrons, they are termed as hot electrons. Hot electrons rapidly decay (a process known as thermalization) to the band edge (Figure 3a), typically within a few picoseconds, through interactions with phonons (the lattice vibrations in semiconductors). Unlike band-edge electrons, hot electrons produce only transient absorption spectral features (derivative-like signals) resulting from an exciton absorption red shift caused by the Stark effect before relaxation. After relaxing to the band edge, in addition to the Stark signals, they also generate transition bleaching signals (state filling bleach) (Figure 3b), creating a superposition of the two signals. Therefore, we can detect the generation and relaxation processes of hot electrons by analyzing the dynamics of different characteristic peak wavelength positions in the transient absorption light. Figure 4 compares the dynamical processes of spectral positions for different transient features in quantum dots under the conditions of E_exc > E_bg (when hot electrons are generated) and E_exc = E_bg (when no hot electrons are produced). In the spectral region dominated by the bleaching peak signal, when E_exc > E_bg, a rapid signal generation is observed on the kinetic curve, indicating that the hot electrons relax to the band edge, producing the ground state bleaching signal. In the spectral region, dominated by the Stark signal, when E_exc > E_bg, a rapid decay of the Stark signal appears on the kinetic curve, indicating that the hot electrons relax to the band edge. In practical research, it is necessary to simultaneously compare the spectral and dynamical situations (represented by the dashed lines in Figure 4) for E_exc = E_bg (band edge excitation) so as to ensure that the observed rapid kinetic processes are indeed hot electron signals. Moreover, the relaxation process of hot electrons usually occurs within 1 - 2 picoseconds. Therefore, the ability to observe this process depends on the time resolution of the transient absorption spectrum. Our commonly used femtosecond transient absorption spectrum is typically effective in capturing the process of hot electrons.

The relaxation process of hot electrons is an energy-loss process, which limits the efficient use of light energy in photoelectric conversion (such as solar energy conversion). Therefore, current research focuses on extending the relaxation time of hot electrons and enabling their extraction and conversion, making this a key topic in the fields of materials and kinetics.

Figure 3. (a) The generation and relaxation process of hot electrons in semiconductor quantum dots, along with the Stark effect induced by electron-hole pairs through the Coulomb electric field

(b) The transient spectral signal of the Stark effect generated before the relaxation of hot electrons, and the combined signal of bleaching and the Stark effect generated after relaxation to the band edge.

Figure 4. A comparison of the transient absorption kinetic processes at different spectral feature positions (1 and 2) in the case of quantum dot excitation with Eexc > Ebg (when hot electrons are generated) and Eexc = Ebg (when no hot electrons are generated). The relaxation process of hot electrons leads to the rapid generation and decay of the ground state bleaching (2) and Stark signal (1) in their respective regions.

04 Defect State

Since it is impossible to synthesize perfect quantum dots, widely distributed defect states are generated during the preparation process of quantum dots. These defect states are usually caused by factors such as lattice defects, elemental impurities, and surface ligands, and their presence often significantly influences numerous dynamic processes of quantum dot carriers. Transient absorption spectroscopy can be used to determine the presence of certain defect states in quantum dots and the extent of their influence on carrier dynamics.

It should be noted that the defect states (either electron defect states or hole defect states) mentioned here are specifically those located within the band gap. Their presence typically quickly results in the rapid capture of electrons or holes in the excited state of the quantum dot (Figure 5a). When an electron or hole falls into a defect state, it can generate an absorption electron transition as shown in Figure 5a, which in turn produces an excited state absorption signal in the transient spectrum (Figure 5b). The transition energy level of the defect state charge is typically lower than the band gap. Consequently, the excited state absorption transient signal generated by this absorption process will appear on the low-energy side of the ground state bleaching signal (as depicted in Figure 5b).

Figure 5. (a) The electron and hole defect trapping process, along with the absorption transition process of the defect state charge in the excited quantum dot (where Ktrapping represents the charge defect state trapping rate constant and K0 denotes the electron-hole recombination rate constant) 

(b) The photoinduced absorption signal (Signal 4) generated by the defect state charge absorption transition in the transient spectrum.

Regarding kinetics, the defect state charge trapping process gives results in a rapidly decaying kinetic component in the ground state bleaching kinetics curve (Figure 5c). However, in the absence of a comparison with the ground state bleaching decay kinetic parameters of quantum dots under defect-free conditions, the kinetic parameters for charge trapping in the defect state (Ktrapping) can only be determined from the fast component parameters obtained when fitting the ground state bleach decay kinetic curves. Given the rapid component in the ground state bleaching decay kinetics can be induced by various factors such as defect states, Auger recombination, and electron/energy transfer, and may also depend on the excitation wavelength and sample power, it is not straightforward to attribute the rapid component observed in the kinetics solely to the defect capture process of charges during specific studies. Instead, a detailed analysis of the specific circumstances is essential.

Naturally, if certain characteristic signals in the transient absorption spectra can be clearly attributed to defect states, more detailed information regarding the dynamics of the defect states can be obtained by extracting the kinetics of those spectral features. For instance, if a quantum dot defect state is capable of generating a photoinduced absorption signal (4 in Figure 5), its kinetic curve can reflect the defect state trapping process of the charge and its subsequent decay process (Figure 5c). If the defect states in quantum dots capture only electrons or holes, this process will lead to the separation of excitons into electrons and holes within quantum dots. As a result, the separated electrons or holes typically have a longer decay lifetime than excitons, which can significantly influence the photocatalytic activity of quantum dots. Nevertheless, in photoelectric conversion applications such as solar cells, the loss of photogenerated carriers and open-circuit voltage caused by defect states can reduce the conversion efficiency. Furthermore, excitonic defects (simultaneously trapping both electrons and holes) can also exist in quantum dots or nanocrystalline materials, leading to the rapid recombination decay of excitons. Excitons in quantum dots or nanocrystals can also form self-trapped excitons through interaction with lattice phonons, thereby exhibiting some unique photophysical characteristics (such as long lifetime, broad spectrum fluorescence emission, etc.).

Defect states in quantum dots or nanocrystals typically do not produce prominent bleaching signals in transient absorption spectra due to their weak absorption transition processes. However, when there is a large number of defect states, they may lead to the absorption tail phenomenon in the steady-state absorption spectrum (as illustrated in Figure 5d). At this time, the corresponding bleaching signal appears in the transient absorption spectrum (Figure 5d).

In conclusion, defect states in semiconductor quantum dots or nanocrystals and their impact on carrier dynamics constitute a highly complex process. The content presented here merely encompasses some of the most fundamental phenomena and characteristics of defect states and is intended solely as a reference for related research work.

05 Charge Transfer

The interfacial charge and energy transfer processes in semiconductor quantum dots and nanocrystals are the core processes for their applications in photoelectric conversion (such as photocatalysis, solar cells, photodetectors, etc.). Transient absorption spectroscopy effectively detects the dynamic processes of charge and energy transfer at the surface and interface of materials, making it one of the key methods for investigating the dynamic mechanisms of photoelectric conversion in numerous material systems.

Firstly, we will briefly illustrate the charge transfer process at the interface of semiconductor quantum dots. Semiconductor quantum dot materials typically possess a high light absorption coefficient and a relatively long excited state carrier lifetime. In theory, they can serve as excellent light-harvesting materials in photocatalysis, solar cells, photodetectors, and other devices. Figure 6 depicts the basic charge dynamics processes in quantum dot photocatalysis and solar cell systems. In photocatalytic systems, quantum dots are typically combined with co-catalysts (electron or hole acceptors (EA or HA) that possess catalytic properties and meet energy level matching requirements) to facilitate photocatalytic processes. Under optical excitation, interfacial electron and hole transfer (with k_ET and k_HT representing the electron and hole transfer rate constants, respectively) processes take place between quantum dots and charge acceptors. This process competes with the intrinsic electron - hole recombination (where k₀ represents the intrinsic decay rate constant) of the quantum dots. Hence, if the charge transfer rates (k_ET and k_HT) are significantly greater than k₀, the efficiency of photocatalysis can be enhanced in principle. Moreover, the separated electrons and holes (for instance, electrons in EA and holes in quantum dots) can also recombine (with krec representing the recombination rate). This recombination reduces the efficiency of the separated charges in participating in the photocatalytic reaction. Therefore, a longer charge separation lifetime (a smaller krec value) will, in principle, contribute to improved photocatalytic efficiency. It should be noted that photocatalytic reactions are typically multi-electron reactions, which require multiple-step charge transfer processes to accumulate multiple electrons or holes on the charge acceptor (the co-catalyst). Transient absorption spectroscopy, particularly on the ultrafast time scale, typically detects only the first step of the electron or hole transfer process. Consequently, the study of photocatalytic kinetics typically requires combining various transient detection techniques across different time scales (ranging from femtosecond to second) to obtain more comprehensive kinetic information.

A similar interfacial charge transfer process also occurs in solar cells that utilize quantum dots as light-harvesting materials. However, due to the thickness of the light-harvesting material, the overall charge transfer kinetic process involves the migration of photogenerated carriers within the quantum dot layer, followed by the subsequent charge transfer at the interface with the electron or hole transport layer. This aspect will be further elaborated on in the following chapter titled, "Application of Transient Spectroscopy in Solar Cell Research".

Figure 6. Kinetic processes such as charge transfer, separation, and recombination in quantum dot photocatalytic systems. 

K_ET: Electron Transfer Rate Constant

K_HT: Hole Transfer Rate Constant

K_REC: Separate Electron-Hole Interface Recombination Rate Constant

K₀: Quantum Dot Intrinsic Electron-Hole Recombination Rate Constant

As previously mentioned, in the detection of transient absorption spectroscopy for semiconductor materials, the spectral signal originates from the combined contributions of electrons and holes. When the electron signal significantly exceeds the hole signal (such as in the case of CdS, CdSe, and other II-VI semiconductor quantum dots), the transient spectral signal and dynamic characteristics of the interfacial electron transfer process can be illustrated in Figure 7. When the interfacial electron transfer occurs before the quantum dot adsorbs the electron acceptor on its surface, the transient absorption spectrum signal of the quantum dot undergoes a rapid recovery or decay. The rate constant of the kinetic decay curve increases from k₀ to k₀ + k_ET. Thus, by comparing the variations in the kinetic rate between scenarios where the quantum dot has an electron acceptor and where it doesn't, the rate of interfacial charge transfer, k_ET, can be quantitatively determined. It should be noted that if the hole contributes to the transient spectral signal, the spectral signal generated by the hole will remain after the electron transfer occurs. Since the recombination of electrons (in the EA) and holes (in the quantum dots) separated at the interface typically takes a relatively long time, the residual hole signal will generate a kinetic component that decays at a slower rate. The amplitude of this component depends on the extent of the hole's contribution to the transient spectral signal, while the kinetic constant for its decay corresponds to the recombination rate of interface-separated electrons and holes (k_Rec). In photocatalytic or solar cell applications, fast electron transfer (k_ET >> k₀) and long-lived charge separation (where k_Rec is small) can theoretically enhance the utilization and conversion efficiency of photogenerated charges.

Figure 7. Typical transient absorption spectra and dynamics during electron transfer at the interface of quantum dots.

The hole transfer process at the interface of quantum dots is analogous to the electron transfer process. However, the ability to directly detect the hole transfer process in the transient spectrum depends on the magnitude of the hole's contribution to the transient spectrum signal. If such a contribution exists, a rapid decay in the signal will be observable in the transient spectrum (Figure 8), appearing in the kinetic curve as a rapid decay component (k_HT) with the corresponding intensity. The separated electron (in the quantum dot) and hole (in HA) typically have longer recombination times. If k_Rec << k₀, the electron signal remaining in the quantum dot will exhibit an extended decay time, with the corresponding slow decay component appearing in the kinetic curve.

Figure 8. Typical transient absorption spectra and dynamic features during hole transfer at the interface of quantum dots. It is assumed that holes contribute to the transient absorption spectrum and k_Rec << k₀.

Besides using the ground state bleaching signal, the excited state absorption signal of quantum dots can also be utilized to assess the dynamic process of electron transfer. The principle is similar to that of changes in the ground state bleaching signal. From the above introduction, it can be observed that transient absorption spectroscopy can be used to track processes such as electron and hole interface transfer, as well as the recombination of separated charges in the quantum dot system, through variations in the quantum transient absorption signal. However, the occurrence of charge transfer cannot typically be 100% confirmed solely based on the transient spectral changes of quantum dots. For instance, processes such as energy transfer or the introduction of surface defects due to the adsorption of charge acceptors (which transfer charge to the defects) can also cause quantum dots to exhibit similar spectral characteristics. Consequently, if certain charge acceptors also exhibit corresponding transient spectral signals within the spectral detection range, more accurate detection of the interfacial charge transfer dynamics can be achieved by capturing the product signals following charge transfer.

Let's assume that the electron acceptor adsorbed on the surface of the quantum dot is molecule A. Its steady-state absorption spectrogram lies within the spectral detection range and is distinct from the absorption of the quantum dot. Meanwhile, at a specific excitation wavelength, only the quantum dots are excited while molecule A remains unexcited. The spectral evolution characteristics of the electron transfer process between the quantum dots and molecule A are depicted in Figure 9a. When interfacial charge transfer occurs, the excited quantum dots (QD*) transfer electrons to molecule A, resulting in the formation of positively charged quantum dots (QD+) and negatively charged A⁻ anions. If the steady-state absorption spectrum of A⁻ anions differs significantly from that of molecule A, the formation of A⁻ will weaken the absorption of molecule A, generating a ground state bleaching signal for molecule A in the transient absorption spectrum. Simultaneously, if the absorption spectrum of the A⁻ molecules is also within the detection range, the formation of their absorption signal will also be observable in the transient absorption spectrum. Subsequently, the separated electrons and holes will recombine (k_Rec), causing the entire system to return to the ground state. On the kinetic curve, the rapid decay of the quantum dots' ground state bleaching signal is accompanied by the generation of the ground state bleaching signal of A and the absorption signal of A⁻, along with the subsequent decay process brought about by the recombination of the separated electrons and holes (Figure 9b). Therefore, direct detection of the interfacial electron transfer and recombination processes, as well as the determination of the rate constants, can be achieved by capturing the signal of the electron transfer acceptor product and its corresponding kinetic rate. The transient spectra and kinetic processes of interfacial hole transfer are similar to those of electron transfer and will not be elaborated upon here. It should be noted that it is assumed that only the quantum dots are excited at a specific excitation wavelength. If molecule A is excited simultaneously, the transient spectral signal caused by electron transfer may often be obscured by the signal generated by direct laser excitation. Hence, careful analysis and judgment are necessary.

Figure 9. Transient absorption spectrum evolution characteristics (a) and dynamic curves (b) resulting from interfacial electron transfer between quantum dots and molecules

Figure (a) encompasses the steady-state absorption spectra of QD, A, and A⁻, which correspond to the transient spectral signal.

In the actual detection of quantum dot charge transfer systems, the charge transfer process is typically determined by observing the transient spectral dynamics of the quantum dots. For certain suitable charge acceptors (such as those that can be selectively excited and exhibit detectable spectral characteristics, most of which are molecular acceptors), more precise detection of their kinetic processes can also be achieved by monitoring the products of the charge transfer reactions. It should be noted that transient spectral detection of charge transfer at the quantum dot interface is often influenced by multiple factors, including the quality and type of quantum dots, variations in receptor and interface structures, as well as the excitation wavelength and power. These factors can lead to complex spectral and dynamic characteristics. Hence, it is essential for researchers to consider the specific conditions and conduct a comprehensive analysis using various technical methods.

06 Energy Transfer

Transient absorption spectroscopy can be utilized to detect the dynamic process of resonance energy transfer within quantum dot systems. Consider a system where quantum dots serve as energy donors and A acts as an energy acceptor. For resonance energy transfer to occur between the two, conditions such as the distance between them and the overlap of the donor-acceptor emission (PL) and absorption spectra must be satisfied (Figure 10a). At a specific excitation wavelength, only the quantum dot donor is excited. When significant energy transfer occurs (for instance, when the energy transfer rate k_EnT exceeds the quantum dot excited state intrinsic recombination rate k₀), the transient spectral signal of the excited state quantum dot (including ground state bleaching, excited state absorption, etc.) will decay rapidly. Since energy transfer involves the combined movement of electrons and holes, the transient spectral signal of the quantum dot will not display the contribution of residual electrons or holes, as observed in charge transfer. If the steady-state absorption of acceptors A falls within the spectral detection range, A will transition to the excited state A* after energy transfer, leading to the generation of signals such as ground state bleaching and excited state absorption (Figure 10a). The excited state A* decays at its intrinsic decay rate (k₀(A)). If A is a fluorescent material or molecule, it will emit fluorescence (PL). The dynamic process corresponding to the transient spectral evolution described above is depicted in Figure 10b. During energy transfer, the ground state bleaching dynamics of the quantum dot will accelerate its decay (k₀ + k_EnT). This is accompanied by the generation (k_EnT) of the ground-state bleaching signal corresponding to A* and the subsequent decay dynamics. Moreover, if acceptors A exhibit a fluorescence signal, the time-resolved PL (TRPL) curve of A can also be collected (the time resolution of TRPL needs to be faster than the energy transfer process). Through its rising edge (which reflects the formation process of A*), the energy transfer kinetic process can be directly captured (Figure 10b).

Figure 10. Characteristics of transient absorption spectrum evolution during energy transfer of quantum dots (a) and corresponding kinetic curves (b)

Figure (a) encompasses the steady-state absorption and emission spectra of the quantum dot donors and acceptors A, which meet the requirements for the resonance energy transfer process to occur.

It can be observed that the changes in the transient absorption spectrum resulting from the energy transfer process of quantum dots closely resemble those caused by the charge transfer process. The key distinction lies in the fact that energy transfer involves the simultaneous movement of electrons and holes, whereas charge transfer represents the independent behavior of electrons or holes. Furthermore, if both the quantum dot donor and the A acceptor are excited at a specific excitation wavelength, the transient spectral signal becomes more complex, which thus demands careful analysis and interpretation.

07 Auger Recombination

When the excitation intensity is quite high, multiple excitons (electron - hole pairs) are generated simultaneously within the quantum dot. At this point, due to the spatial confinement effect within the quantum dot, a strong coupling between excitons will occur, leading to the rapid non-radiative Auger recombination process of multiple excitons. Specifically, the rapid recombination of one exciton transfers energy to the electron or hole of another exciton. Subsequently, the latter is excited, transitioning to a higher energy orbital within its band gap, before rapidly decaying back to the band edge, enabling the quantum dot to return to the excited state of a single exciton (Figure 11a). It is evident that the Auger process involves energy dissipation, and efforts should be made to minimize its occurrence as much as possible in applications such as photocatalysis, photoelectric conversion, and luminescence. Studies have also focused on slowing down the Auger process and reducing or avoiding energy losses by altering the chemical and physical structures of quantum dots. Additionally, if the quantum dot itself carries one or more positive or negative charges (for instance, a quantum dot existing in a defect state or doped with ions), the trion state (where an exciton has a positive or negative charge) formed after excitation will also trigger a rapid decay process of the exciton (as depicted in Figure 11b).

Figure 11. Fast Auger recombination in quantum dot materials

(a) Fast Auger recombination in the biexciton state

(b) Fast Auger recombination in the trion state.

The Auger recombination process in quantum dots can be investigated using ultrafast transient absorption spectroscopy. By collecting the transient absorption excited state kinetic curves at different excitation powers, it is observed that when the excitation power reaches a certain level of intensity, a rapidly decaying component appears in the kinetic curve (Figure 12b). Moreover, both the amplitude and the decay rate of this component increase with the increase in the excitation power, indicating the recombination process of multiple excitons. Generally speaking, the stronger the excitation power, the greater the number of excitons generated within a single quantum dot, leading to a faster Auger recombination process. We can normalize and compare the excited state decay dynamics curves under different powers over a long delay time scale. Eventually, these curves under different powers will exhibit an identical dynamics process on the longer time scale (Figure 12a), suggesting that quantum dots, regardless of the excitation power, will ultimately reach the single-exciton state and undergo the same decay process for a single exciton. By comparing these kinetic curves, we can also calculate the Auger recombination rate. Taking the Auger decay rate of the biexciton as an example (Figure 12b), when the Auger process (the fast component in the dynamics) just begins to appear, this fast component can be attributed solely to quantum dots with biexcitons. Then, by subtracting the kinetic curve obtained under conditions without the Auger process (i.e., with low power excitation) from this curve, the resulting difference in dynamics can be regarded as the Auger decay process for biexcitons. By fitting this difference, we can determine the Auger decay rate for biexcitons. When more excitons are present (under strong excitation power), the number of excitons in each quantum dot follows the Poisson distribution equation. In this case, a more complex dynamic model is needed to determine the multi-exciton Auger decay rate.

Figure 12. The detection of Auger recombination process in multiple quantum dot materials by ultrafast transient absorption spectroscopy dynamics. 

(a) Excited-power-dependent transient absorption excited-state dynamics decay curve: As the excitation power increases, a fast decay component appears in the dynamics curve, indicating the occurrence of the Auger process

(b) Estimation of the Auger recombination rate in the biexciton case using the difference method of dynamics curves.

08 Conclusion

In this article, we have primarily introduced a range of carrier-related transient spectroscopy characteristics and dynamic processes in semiconductor quantum dot systems. These include the excited state, defect state, charge transfer, energy transfer, hot electrons, Stark effect, and Auger recombination. Figure 13 summarizes the dynamic processes detectable by transient spectroscopy within the quantum dot material system. There are numerous similarities between the transient spectra of semiconductor quantum dot materials and molecular material systems. However, it is important to emphasize that due to the distinct mechanisms underlying the generation of transient spectral signals in these two types of material systems, certain dynamic processes will exhibit significant differences. For instance, the electron transfer process typically causes the decay (recovery) of the ground state bleaching signal in quantum dot materials. However, the same process does not necessarily lead to the decay of the ground state bleaching signal in molecular materials. This discrepancy arises from the different mechanisms through which the ground state bleaching signals are generated in organic/inorganic molecules and semiconductor materials. Therefore, in research, specific analyses must be conducted for different material systems, as findings from one system cannot be directly applied to another.

Figure 13. The main carrier dynamics processes detectable by transient absorption spectroscopy in quantum dot materials

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