Generation of hot electrons and their utilization in photoinduced chemical processes have been the subjects of intense research in recent years mostly exploring hot electrons in plasmonic metal nanostructures created via decay of optically excited plasmon. Here, we present recent progress made in generation and utilization of a different type of hot electrons produced via biphotonic exciton-to-hot electron “upconversion” in Mn-doped semiconductor nanocrystals. Compared to the plasmonic hot electrons, those produced via biphotonic upconversion in Mn-doped semiconductor nanocrystals possess much higher energy, enabling more efficient long-range electron transfer across the high energy barrier. They can even be ejected above the vacuum level creating photoelectrons, which can possibly produce solvated electrons. Despite the biphotonic nature of the upconversion process, hot electrons can be generated with weak cw excitation equivalent to the concentrated solar radiation without requiring intense or high-energy photons. This perspective reviews recent work elucidating the mechanism of generating energetic hot electrons in Mn-doped semiconductor nanocrystals, detection of these hot electrons as photocurrent or photoelectron emission, and their utilization in chemical processes such as photocatalysis. New opportunities that the energetic hot electrons can open by creating solvated electrons, which can be viewed as the longer-lived and mobile version of hot electrons more useful for chemical processes, and the challenges in practical utilization of energetic hot electrons are also discussed.

Semiconductor nanocrystals (NCs) doped with Mn2+ ions have attracted much attention for the last two decades due to the unique optical, electronic, and optomagnetic properties useful for various applications such as light emitting devices, biological imaging, and spintronics.1–8 The Mn-doped semiconductor NCs exhibit distinctively different photophysical properties from their undoped counterparts. For instance, Mn-doped II-VI semiconductor NCs exhibit long-lived (10−4–10−2 s) sensitized phosphorescence near 600 nm. This arises from the spin- and symmetry-forbidden ligand field transition of Mn2+ ions via exciton-to-Mn energy transfer if the exciton transition energy is sufficiently high compared to the energy of the emitting Mn ligand field transition.6,9 Such new sensitized luminescence originates from the exchange coupling between exciton and d electrons of Mn2+ ions. While the exciton-Mn exchange coupling exists also in bulk semiconductor hosts, it is considered stronger in the NCs from quantum confinement of the exciton wavefunction, often exhibiting more prominent photophysical properties with their origin in the exciton-Mn exchange coupling.8,10–13 In addition to functioning as the energy acceptor giving rise to the sensitized Mn luminescence, multiple Mn2+ ions can create optically induced ferromagnetism via the formation of exciton magnetic polaron when the bandgap of the host nanocrystal is sufficiently low to prevent exciton-to-Mn energy transfer.11,14–22 Similarly to the sensitization of Mn luminescence that can benefit from the stronger exciton-Mn coupling in quantum confined semiconductor NCs, the formation of exciton magnetic polaron is also facilitated in the NC host compared to the bulk host.20 

While most of the earlier studies on the properties of Mn-doped semiconductor NCs focused on the sensitized Mn luminescence, more recent studies revealed their ability to generate hot electrons via the process similar to photon upconversion in lanthanide-doped upconversion nanoparticles.23 Instead of producing the higher-energy photons from the absorption of two lower-energy photons, higher-energy electron-hole pair is produced from two photoexcited excitons in Mn-doped semiconductor NCs, where Mn2+ ion provides a long-lived intermediate state. Because of the upconverting nature of the hot electron generation process, the energy of hot electrons from Mn-doped semiconductor NCs is very high. They can be several electronvolts more energetic compared to hot electrons from plasmonic metal NCs produced via one photon process. Therefore, Mn-doped semiconductor NCs can be a unique source of the energetic hot electrons performing the electron transfer over a high and wide barrier useful for the chemical processes requiring efficient electron transfer. In fact, the benefits of enhanced electron transfer kinetics and potentially larger special extent of hot electron transfer from Mn-doped semiconductor NCs are being explored in various photochemical processes.24–26 Another unique aspect of the hot electron generation in Mn-doped semiconductor NCs is that upconversion of exciton to hot electrons can occur at relatively low excitation intensities (e.g., <1 W/cm2) due to the very long lifetime of the intermediate state (e.g., >5 ms), which is particularly important for practical application of hot electrons.

This perspective focuses on recent progress made in the research of the generation of hot electrons in Mn-doped semiconductor NCs and the enhanced kinetics and spatial extent of hot electrons transfer beneficial in photoinduced chemical processes. Since there are several recent reviews on Mn doping in various semiconductor NCs, we will be very brief in introducing the currently available Mn-doped semiconductor NCs and lead the interested readers to the relevant literature. In the main part of this perspective, we will discuss the photophysical pathways in Mn-doped semiconductor NCs generating hot electron, detection of photocurrent and photoelectron emission from hot electrons, comparison to plasmonic hot electrons, and recent demonstrations of the enhanced electron transfer kinetics relevant to photocatalysis and photovoltaics applications. Finally, we will conclude with discussion on new opportunities and the remaining challenges in utilizing energetic hot electrons from Mn-doped semiconductor NCs for various photoinduced chemical processes.

There are many earlier and recent reviews on doping of metal ions including Mn2+ in various semiconductor nanocrystals (NCs) and the effects of doping on the optical and electronic properties of the host NCs, which will be useful for the interested readers.1,27–30Table I compiles the list of currently available Mn-doped semiconductor NCs of different host materials and morphologies focusing on II-VI and lead halide perovskite semiconductors, while only a subset of these Mn-doped NCs with the bandgap higher than the emitting transition of Mn2+ ions can produce hot electrons. 0-dimensional quantum dots (QDs), 1-dimensional nanorods and nanowires, 2-dimensional nanoplatelets, and small nanoclusters have been successfully doped with Mn2+ ions. Instead of providing an exhaustive list of the literature, a few relevant ones discussing the synthesis or the properties of the listed Mn-doped NCs are also provided in Table I. Presently, hot electron generation has been demonstrated only in a small number of Mn-doped II-VI semiconductor NCs. Efforts to expand the range of the host semiconductor materials and the morphology of the host NCs are continuously made, which will be important for more efficient harvesting and utilization of hot electrons in various chemical processes.

TABLE I.

Mn-doped semiconductor NCs categorized based on the host semiconductor materials and the morphology of the host NCs.

Host nanocrystal morphology Host nanocrystals materials
II-VI semiconductors 
Core QDs  ZnO,31 ZnS,32–35 ZnSe,36–41 CdS,33,41,42 CdSe,20,43–47 CdTe,41 ZnCdS,48 CdSeTe49,50 
Core/shell QDs  CdS/ZnS,51–54 ZnSe/CdS,55 ZnSe/CdSe,56 CdSSe/ZnS,24 ZnSe/ZnCdSe,57  
  ZnSe/ZnS/CdS/ZnS,58 CdS/ZnSe/ZnS59  
Nanorods/nanowires  ZnS,60 ZnSe,61 CdSe,41 ZnS,62 ZnSe/ZnS63  
Nanoribbons/nanoplatelets/nanosheets  CdSe,64,65 ZnS,66 CdSe/CdS,67,68 CdS/ZnS69  
Nanoclusters  CdSe,70 ZnTe71  
Lead halide perovskites 
Nanocubes  CsPbCl3,72–78 CsPb(Cl/Br)3,72,79–81 CsPbBr3,78,82,83 CsPbI3,84,85 CH3NH3PbCl386  
Nanorods  CsPbCl378  
Nanoplatelets  CsPbCl3,87,88 CsPbBr382,83 
Other semiconductors 
Core QDs  PbSe,89 InAs,90 CuInS2,91 AgInS2,91 Zn-Cu-In-S,92 Zn-In-S93  
Core/shell QDs  InP/ZnS,94 Zn-Cu-In-S/ZnS,95 Zn-In-S/ZnS96  
Nanoclusters  Cd6In28S52(SH)497  
Host nanocrystal morphology Host nanocrystals materials
II-VI semiconductors 
Core QDs  ZnO,31 ZnS,32–35 ZnSe,36–41 CdS,33,41,42 CdSe,20,43–47 CdTe,41 ZnCdS,48 CdSeTe49,50 
Core/shell QDs  CdS/ZnS,51–54 ZnSe/CdS,55 ZnSe/CdSe,56 CdSSe/ZnS,24 ZnSe/ZnCdSe,57  
  ZnSe/ZnS/CdS/ZnS,58 CdS/ZnSe/ZnS59  
Nanorods/nanowires  ZnS,60 ZnSe,61 CdSe,41 ZnS,62 ZnSe/ZnS63  
Nanoribbons/nanoplatelets/nanosheets  CdSe,64,65 ZnS,66 CdSe/CdS,67,68 CdS/ZnS69  
Nanoclusters  CdSe,70 ZnTe71  
Lead halide perovskites 
Nanocubes  CsPbCl3,72–78 CsPb(Cl/Br)3,72,79–81 CsPbBr3,78,82,83 CsPbI3,84,85 CH3NH3PbCl386  
Nanorods  CsPbCl378  
Nanoplatelets  CsPbCl3,87,88 CsPbBr382,83 
Other semiconductors 
Core QDs  PbSe,89 InAs,90 CuInS2,91 AgInS2,91 Zn-Cu-In-S,92 Zn-In-S93  
Core/shell QDs  InP/ZnS,94 Zn-Cu-In-S/ZnS,95 Zn-In-S/ZnS96  
Nanoclusters  Cd6In28S52(SH)497  

The possible generation of hot electrons in Mn-doped semiconductor NCs by biphotonic “upconversion” and its plausible photophysical pathways were proposed in early 2010s mostly in two research groups. In subsequent years, generation of hot electrons was experimentally verified in several different device platforms using Mn-doped quantum dots (QDs) as the active element generating hot electrons. In the authors’ group, the initial report on the hot electron generation and the possible mechanism was made from the observation of the unusual excitation intensity dependence of Mn luminescence intensity and the comparison of excitation intensity-dependent reduction rate of methylene blue by Mn-doped and undoped CdS/ZnS QDs.98 In contrast to the intensity of bandedge exciton luminescence that saturates with increasing excitation intensity after the initial increase,99 Mn luminescence intensity exhibited an initial increase followed by a decrease as the excitation intensity increased. The hypothesis in this study was that two consecutive exciton-to-Mn energy transfers to the same Mn2+ ion can occur when the excitation rate is faster than the relaxation rate of the emitting excited state (4T1) of Mn2+ ion. This produces highly excited Mn2+ ions that can give up the electron into the conduction band, which subsequently regain an electron from the valence band of the host to recover the Mn2+ oxidation state. The net result of these processes is the generation of hot electron in conduction band and hole near Mn2+ ion as illustrated in Fig. 1(a), which results in the decrease in the intensity of Mn luminescence from 4T16A1 transition. Although the direct resonant excitation of 4T1 to the higher excited state is possible, such process requires very intense excitation (e.g., ∼107 W/cm2)100 due to the very small absorption cross section of the dipole forbidden ligand field transition. In contrast, the exciton-to-Mn energy transfer as fast as several picosecond demonstrated in Mn-doped CdS/ZnS QDs53 suggests the second energy transfer step promoting 4T1 to the higher level could also be rapid since the electronic coupling mediating the energy transfer should not be very different between the two. This renders the two sequential energy transfers to the same Mn2+ ion a viable pathway for generating hot electrons.

FIG. 1.

Photophysical pathways generating hot electrons in Mn-doped semiconductor nanocrystals. (a) Consecutive exciton-to-Mn energy transfer and (b) exciton-to-Mn energy transfer followed by Auger back energy transfer from Mn2+ ion to electron.

FIG. 1.

Photophysical pathways generating hot electrons in Mn-doped semiconductor nanocrystals. (a) Consecutive exciton-to-Mn energy transfer and (b) exciton-to-Mn energy transfer followed by Auger back energy transfer from Mn2+ ion to electron.

Close modal

Another photophysical pathway that can generate hot electrons is the Auger energy transfer from the 4T1 state of Mn2+ to exciton shown in Fig. 1(b). The Auger energy transfer from Mn2+ to the electron in the conduction band of Mn-doped QD was reported by Gamelin’s group in their study of Mn luminescence in electrochemically reduced Mn-doped CdS QDs. In their study, Mn luminescence was quenched via Auger de-excitation of the 4T1 state of Mn2+ ion when an electron was injected into the conduction band electrochemically.101 Under this mechanism, the energy stored in the 4T1 state of Mn2+ ion created by the exciton-to-Mn energy transfer from the first photon absorption can be given to electron or (and) hole in the host NCs created by the second photon absorption. If the majority of >2 eV of energy from Mn2+ (4T1) goes to the electron, hot electrons will have >2 eV of excess kinetic energy with respect to its lowest confined electron level of the host NCs. In this case, the resultant effect is the production of a hot electron in the conduction band and hole at the bandedge in the host NCs. It is not straightforward to tell which pathway is dominating between (a) and (b) in Fig. 1 for a given Mn-doped nanocrystal system. If the pathway (b) is the dominating pathway, one should be able to have more control over the average hot electron energy by varying the conduction bandedge of the host NC, whereas the energy of hot electrons from pathway (a) is expected to be less sensitive to the host bandedge level. Nevertheless, it is worth noting that both pathways lead to the generation of hot electrons taking advantage of the extremely long lifetime of the Mn ligand field state (4T1) as the intermediate state for the upconversion process. With >5 ms lifetime of the 4T1 state of Mn2+ ions observed in CdS/ZnS QDs, hot electron generation should be possible at the excitation rate of ∼20 s−1 that is achievable with concentrated solar radiation for the common colloidal semiconductor QDs with an absorption cross section of 10−15–10−14 cm2 above the bandgap.24 An interesting question on pathway (b) is whether one can create hot holes in a similar manner as hot electrons are produced. While the majority of energy appears to be transferred to the electron in the case of Mn-doped CdS/ZnS QDs producing hot electrons, one can, in principle, create hot holes if the electronic coupling mediating the Auger back energy transfer is sufficiently strong between Mn2+ and hole. In fact, the production of hot holes and their use in photoinduced oxidation is also an interesting topic complementary to hot electron-induced processes,102–104 which will benefit from the future development of Mn-doped NCs producing hot holes via the “upconversion” process.

Experimental efforts to detect hot electrons in the form of photocurrent and photoelectron emission were made using several different platforms. In the authors’ group, hot electron photocurrent was measured in a photoelectrochemical cell composed of ITO/Al2O3/QD electrode and Pt wire as the anode and cathode, respectively, using methanolic solution of polysulfides as the electrolyte, as shown in Fig. 2(a).25 The Al2O3 layer acted as the tunneling barrier located ∼2 eV above the conduction bandedge of CdS core in Mn-doped CdS/ZnS QDs.105 Under typical low-intensity cw excitation conditions, excitons in undoped CdS/ZnS QDs produce charge carriers only near the bandedge. Therefore, the electrons in the conduction band do not possess enough energy to transfer through the Al2O3 insulating layer, resulting in the complete absence of photocurrent flowing in the direction indicated with an arrow in Fig. 2(a). In contrast, the ITO/Al2O3/QD anode fabricated with Mn-doped CdS/ZnS QDs exhibited hot electron photocurrent that increases superlinearly with excitation intensity, consistent with biphotonic exciton-to-hot electron upconversion discussed in Sec. III A. In this experiment, the electrode deposited with Mn-doped QDs at submonolayer coverage produced photocurrent at the current density of ∼300 nA/cm2 for 5.4 nm thick Al2O3 layers under the excitation intensity of 1.5 W/cm2, as shown in Fig. 2(b). This indicates that hot electrons are sufficiently energetic to exhibit appreciable photocurrent across >5 nm thick insulating barrier layer.

FIG. 2.

Hot electron photocurrent measurement. (a) Photoelectrochemical cell that detects hot electron photocurrent from Mn-doped QDs, (b) comparison of photocurrent measured with anodes deposited with Mn-doped and undoped QDs, (c) structure of the photodiode detecting hot electron photocurrent, and (d) bias-dependent Mn PL intensity and responsivity of photodiode. Reproduced with permission from Dong et al., ChemPhysChem 17, 660 (2016). Copyright 2016 John Wiley and Sons; and Barrows et al., J. Phys. Chem. Lett. 8, 126 (2016). Copyright 2016 American Chemical Society.

FIG. 2.

Hot electron photocurrent measurement. (a) Photoelectrochemical cell that detects hot electron photocurrent from Mn-doped QDs, (b) comparison of photocurrent measured with anodes deposited with Mn-doped and undoped QDs, (c) structure of the photodiode detecting hot electron photocurrent, and (d) bias-dependent Mn PL intensity and responsivity of photodiode. Reproduced with permission from Dong et al., ChemPhysChem 17, 660 (2016). Copyright 2016 John Wiley and Sons; and Barrows et al., J. Phys. Chem. Lett. 8, 126 (2016). Copyright 2016 American Chemical Society.

Close modal

Gamelin’s group reported the detection of hot electrons in the photodiode structure shown in Fig. 2(c).26 In this photodiode, the photocurrent flows with a concomitant decrease in Mn PL intensity only when sufficient bias capable of injecting the electron to the layer of Mn-doped CdS QDs is applied [Fig. 2(d)]. In this photodiode, hot electrons are produced via Auger energy transfer from Mn2+ ions to the electrically injected electron into the conduction band. Since ZnS is the blocking layer, only the hot electrons sufficiently energetic to overcome the barrier contribute to the measured photocurrent. No hot electron photocurrent was observed in the absence of electron injecting bias. As expected, no hot electron photocurrent was observed even under the forward bias when the device was fabricated using undoped QDs instead of Mn-doped QDs.

In both studies described above, hot electrons that passed through the layer with varying barrier height and width were detected as the photocurrent. More recently, the authors’ group performed an experiment that detects the photoelectrons emitted above the vacuum level anticipating a subpopulation of hot electrons to possess sufficient energy resulting in the ionization of Mn-doped QDs.106 In the case of Mn-doped CdS/ZnS core/shell QDs, the quantum confined electron level of the core in the conduction band is 2.5–3 eV below the vacuum level depending on the size of the QDs.107 Therefore, the average energy of hot electrons can be only a fraction of 1 eV below the vacuum level. Considering the generally broad kinetic energy spectrum of hot electrons, it is reasonable to expect part of hot electrons lie above the vacuum level. In order to verify this possibility, a simple device measuring the current from photoelectron emission composed of two parallel electrodes was constructed, as shown in Fig. 3(a). One of the electrodes is an ITO/glass substrate, where QDs are deposited at submonolayer coverage. The other electrode is made of copper plate separated from the QD-coated ITO/glass by 1.5 mm using an insulating spacer with an opening that allows the free-space travel of the hot electrons between the two electrodes. These electrodes were placed in a vacuum chamber, and the current density between the two electrodes was measured either with or without small bias (−1 to 1 V) under photoexcitation at varying intensities. At 0 V bias, the current density increased nearly quadratically with excitation intensity, which is consistent with the biphotonic upconversion process producing hot electrons, as shown in Fig. 3(b). When undoped QDs were deposited on the ITO/glass substrate instead of Mn-doped QDs at the comparable surface density, no measurable photocurrent was observed. Interestingly, Ag NCs capable of producing plasmonic hot electrons under the same excitation condition do not show any measurable current from photoelectron emission, which will be discussed more in detail in Sec. III C.

FIG. 3.

Photoelectron emission of hot electrons. (a) Diagram of electrodes for detecting photoelectron emission from hot electrons, (b) comparison of the current density from photoelectrons between Mn-doped QD, undoped QD, and plasmonic Ag NC, (c) comparison of the current density from three different Mn-doped QDs (sample A, B, C) with different exciton-to-Mn energy transfer times (τA > τB > τC), and (d) Bias-dependent current density at 29 W/cm2 of excitation intensity. Reproduced with permission from Dong et al., Nano Lett. 16, 7270 (2016). Copyright 2016 American Chemical Society.

FIG. 3.

Photoelectron emission of hot electrons. (a) Diagram of electrodes for detecting photoelectron emission from hot electrons, (b) comparison of the current density from photoelectrons between Mn-doped QD, undoped QD, and plasmonic Ag NC, (c) comparison of the current density from three different Mn-doped QDs (sample A, B, C) with different exciton-to-Mn energy transfer times (τA > τB > τC), and (d) Bias-dependent current density at 29 W/cm2 of excitation intensity. Reproduced with permission from Dong et al., Nano Lett. 16, 7270 (2016). Copyright 2016 American Chemical Society.

Close modal

This study also compared the current densities from Mn-doped QDs of different doping density and doping location that dictate the rate of exciton-to-Mn energy transfer and overall hot electron generation efficiency, as shown in Fig. 3(c). Among the three Mn-doped QD samples compared in Fig. 3(c), the QDs exhibiting the shorter exciton-to-Mn energy transfer time (τA > τB > τC) exhibited the higher current density (JC > JB > JA). Another interesting measurement made in this study is the current density at negative bias that gives information on the detectable upper limit of the kinetic energy of the hot electrons emitted above the vacuum level, as shown in Fig. 3(d). The current density decreased with negatively increasing bias and became negligible at ∼0.4 V. At positive bias, the current increased with increasing bias likely due to the modification of the trajectory of photoelectrons, resulting in more collection of the electrons at the copper electrode through the small opening made in the spacer. This indicates that the hot electrons at the high-energy tail of the energy spectrum possess ∼0.4 eV of kinetic energy. The significance of this observation in long-range photoinduced electron transfer will be discussed in Sec. IV.

Recently, interests in generation of hot electrons and their application in photochemical processes were revived mostly in plasmonic metal NCs due to the large local electric field enhancement that is beneficial in producing hot electrons via plasmon.108–111 In plasmonic metal NCs, nonradiative decay of the plasmon excites hot electron-hole pair through the process called Landau damping.112 Their ability to enhance the photoreduction or photodissociation of molecules has also been demonstrated.113,114 Considering that benefits of hot electrons in electron transfer and photochemistry is the result of the elevated energy level with respect to their ground state, it is informative to compare the energy levels of plasmonic hot electrons in metal NCs and hot electrons in Mn-doped semiconductor NCs.

Figure 4 compares the key difference between the hot electrons in two different systems. Hot electron generation via Landau damping in plasmonic metal NCs is a one-photon process, and hot electrons are created by promoting the electron at the level below the Fermi level of metal NCs.112 Since the Fermi level of Au and Ag is > 5 eV below the vacuum level, the energy level of hot electrons in Au NCs is ∼3 eV below the vacuum level if the plasmon resonance is in the visible spectral range.112,115 Strong optical extinction of plasmon in the visible and near infrared region in commonly used plasmonic metals, such as Au and Ag, is an advantage of the plasmonic metals as the source of hot electrons over the semiconductors that exhibit the weaker light absorption.116 In contrast, hot electrons from Mn-doped semiconductor NCs generated via biphotonic upconversion is obtained by promoting the electron in the quantum confined level in the conduction band that is located >3 eV below the vacuum level.107 Therefore, the average hot electron energy from Mn-doped semiconductor NCs can be several electron volt higher than that of plasmonic hot electrons reaching very close to the vacuum level. The difference in the energy of these two different hot electrons is clearly seen in Fig. 3(b), where the plasmonic hot electrons do not produce any photoelectron emission current in contrast to the hot electrons produced in Mn-doped QDs.

FIG. 4.

Comparison of energy of hot electrons from Mn-doped semiconductor QDs (left) and plasmonic metal NCs (right).

FIG. 4.

Comparison of energy of hot electrons from Mn-doped semiconductor QDs (left) and plasmonic metal NCs (right).

Close modal

Because of the high excess energy of hot electrons from Mn-doped semiconductor NCs, they can be very efficient in performing photoinduced electron transfer. Since the average energy level of hot electrons from the biphotonic exciton-to-hot electron upconversion process can be close to the vacuum level, transfer of hot electrons should be much more effective than the lower-energy bandedge electrons. As demonstrated in the studies described in Sec. III B, hot electrons can benefit from not only the low energy barrier for the electron transfer but also potentially much larger spatial extent of the electron transfer beyond the immediate surface of the NCs. Such advantages in photoinduced chemical processes were demonstrated in the comparative study of photocatalytic H2 production by hot electrons and bandedge electrons from Mn-doped and undoped NCs sharing the same host NC structure. Figure 5 compares the H2 production rate from water at the pH of 12 in the presence of sacrificial hole scavenger as a function of excitation intensity using undoped and Mn-doped CdSSe/ZnS core/shell QDs. Both NCs exhibit nearly the same absorption spectra since doped Mn2+ ions do not significantly affect the absorption from the host NCs. In this comparison, H2 production rate from Mn-doped QDs (RH2,d) was significantly higher than that from undoped QDs (RH2,ud) at all excitation intensities, as shown in Fig. 5(a). Furthermore, the values of RH2,d/RH2,ud increased linearly at the lower excitation intensities (<0.3 W/cm2), while it saturated at the higher intensities [Fig. 5(b)]. Since H2 is produced by one photon-excited exciton in undoped QDs, the linear increase in RH2,d/RH2,ud with the excitation intensity indicates that H2 production by Mn-doped QDs is the biphotonic process as expected from hot electrons. This result is quite interesting since a reduction in the number of hot electrons from upconversion compared to the one photon-excited bandedge electron is more than compensated by the much higher reactivity of hot electrons. Considering that hot electrons’ ability to travel long distance across the insulating layer as discussed in Sec. III B, there is a possibility that hot electrons leave the surface of the NCs and perform the reduction away from the NC surface. Furthermore, the observed photoemission of hot electrons suggests the possibility of electron transfer to the solvent that could potentially form solvated electron that can open an entirely new channel for photoinduced chemistry. The significance of this long-range electron transfer and possible generation of solvated electron will be discussed further in Sec. V.

FIG. 5.

Photocatalytic H2 production with hot electrons from Mn-doped QDs. (a) H2 production rate for doped (RH2,d, red line) and undoped (RH2,ud, green line) CdSSe/ZnS QDs vs excitation light intensity. The inset shows the magnified view of RH2,ud. (b) The ratio of two H2 evolution rates (RH2,d/RH2,ud) vs excitation light intensity. Reproduced with permission from Dong et al., J. Am. Chem. Soc. 137, 5549 (2015). Copyright 2015 American Chemical Society.

FIG. 5.

Photocatalytic H2 production with hot electrons from Mn-doped QDs. (a) H2 production rate for doped (RH2,d, red line) and undoped (RH2,ud, green line) CdSSe/ZnS QDs vs excitation light intensity. The inset shows the magnified view of RH2,ud. (b) The ratio of two H2 evolution rates (RH2,d/RH2,ud) vs excitation light intensity. Reproduced with permission from Dong et al., J. Am. Chem. Soc. 137, 5549 (2015). Copyright 2015 American Chemical Society.

Close modal

Very recently, the possibility of using hot electrons for a direct reduction in CO2 into CO without using other molecular or metallic cocatalyst was tested in the authors’ group. The preliminary result shows that the production of CO from CO2 in CO2-saturated water with was possible with Mn-doped CdSSe/ZnS QDs as the only photocatalyst under cw excitation. This contrasted with the complete absence of CO in the product when undoped QDs were used under the same experimental condition. The mechanism of a direct hot electron-induced reduction in CO2 to CO is not clear yet, requiring further research. Nevertheless, this demonstrates the unique potential of energetic hot electrons from Mn-doped QDs in photochemical conversion through electron transfer that cannot be achieved with electrons from usual bandedge excitons.

We discussed the generation of energetic hot electron in Mn-doped semiconductor NCs via exciton-to hot electron “upconversion” and their utilization in photochemical processes requiring efficient electron transfer such as in photocatalysis. One of the unique characteristics of hot electrons from Mn-doped semiconductor NCs, compared to plasmonic hot electrons, is very high energy that enables even the photoelectron emission of hot electrons due to the biphotonic nature of the upconversion process. The large energy is particularly beneficial in overcoming the high and wide energy barrier for the electron transfer, which enables the photoinduced electron transfer processes that are difficult to initiate with typical one-photon excited electrons or plasmonic hot electrons. In addition, the light intensity required for the biphotonic exciton-to-hot electron upconversion in Mn-doped semiconductor NCs is relatively low due to very long lifetime of the intermediate state provided by Mn dopant. The excitation light with its intensity equivalent to the concentrated solar radiation can be used to generate energetic hot electrons in common colloidal Mn-doped semiconductor QDs, opening the door to more practical utilization of energetic hot electrons for the photochemical process.

The remaining part of this section will discuss new opportunities that energetic hot electrons can bring and other challenges in the generation and utilization of hot electrons from Mn-doped semiconductor NCs for the chemical processes. As discussed in Sec. III, energetic hot electrons can be created near or above the vacuum level, suggesting the possibility of hot electron injection into the solvent medium producing solvated electrons. Solvated electron is often pictured as the isolated electron localized inside the cavity formed by solvent molecules although there are debates on the exact structure in the case of solvated electron in water that may vary depending on how it is generated.117 Solvated electron is a strong reducing agent with its lifetime much longer than hot electrons in the conduction band of the semiconductor NCs. The reductive chemistry of solvated electron with various ionic and molecular electron acceptors is well-known118–121 and its potential benefits in photocatalytic reactions, such as the reduction in CO2 and N2, are recently reported.122–124 One can view solvated electron as the “mobile” and “longer-lived” version of hot electron initially created in Mn-doped semiconductor NCs while still maintaining the activity as the strong electron donor. However, the majority of the methods produce solvated electrons via photoexcitation requiring intense and pulsed UV light to ionize the precursors125–127 or via radiolysis using high-energy ionizing radiation.128–130 Although electrochemical generation of solvated electron is also possible,131 a reduction of solvent molecules at highly negative potential limits its wide applicability. In this regard, hot electrons from Mn-doped semiconductor NCs can be more practical alternative source producing solvated electron that does not require high-energy, high intensity photons, or electrodes. If the solvated electrons can be produced efficiently from hot electrons, it will make the solvated electron-induced electron transfer a practical and powerful tool for the photocatalytic reduction. The direct experimental confirmation of the production of solvated electron from hot electrons generated in Mn-doped semiconductor NCs has yet to be made. However, earlier observation of the solvated electron in the aqueous colloidal solution of CdSe QDs ionized via direct two-photon excitation under intense femtosecond laser light indicates that solvated electron formation is possible from the energetic hot electrons from Mn-doped QDs.126 The verification of the production of solvated electron from the injection of hot electrons will be an important next step toward utilization of solvated electrons derived from hot electrons for chemical processes.

Two aspects of hot electron generation are important from the practical point of view. One is the efficiency of hot electron generation (and extraction). The elementary processes leading to the generation of hot electron, which are determined by the (exchange) electronic coupling between exciton and d electrons of Mn, are competing with various other processes, such as recombination of exciton and charge carrier trapping. Therefore, the relative rates of the exciton-to-Mn energy transfer and Auger energy transfer from Mn to electron should be maximized with respect to the other competing processes. To some extent, these can be achieved by controlling the structure of both the host NCs and spatial distribution and density of dopant within the NCs. The second point is further lowering the required excitation intensity for the hot electron generation via the biphotonic upconversion process, which is largely determined by the lifetime of the excited Mn2+ ligand field state and the absorption cross section of the host NCs. While >5 ms lifetime of Mn2+ dopant was observed routinely in CdS/ZnS host, it can become shorter in different host NCs and at the higher doping density, increasing the required intensity for hot electron generation. Finding new materials for host semiconductor NCs providing larger absorption cross section, longer Mn lifetime and stronger quantum confinement will be one of the solutions. In this respect, strongly confined 2-dimensional perovskite nanostructure is an interesting candidate despite its limited structural stability in polar environment.82 One could also create the hybrid structure of plasmonic nanostructure and Mn-doped NCs to further reduce the required light intensity for exciton-to-hot electron upconversion taking advantage of local field enhancement by the plasmonic nanostructure.

This work was supported by the National Science Foundation (Grant Nos. DGE-1252521, D.P. and CBET-1804412, T.Q.) and Institute for Basic Sciences (Grant No. IBS-R026-D1, D.H.S.).

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