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The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D–2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger-assisted charge transfer model. Recent advances in understanding multiple exciton generation, decay, and dissociation are also discussed, with an emphasis on multiple carrier transfer. Finally, the applications of nanocrystal-based systems for photocatalysis are reviewed, focusing on the photodriven charge separation and recombination processes that dictate the function and performance of these materials. The Review ends with a summary and outlook of key remaining challenges and promising future directions in the field. [ABSTRACT FROM AUTHOR]

Two-dimensional (2D) organolead halide perovskites are promising for various optoelectronic applications. Here we report a unique spontaneous charge (electron/hole) separation property in multilayered (BA)2(MA)n−1PbnI3n+1 (BA = CH3(CH2)3NH3+, MA = CH3NH3+) 2D perovskite films by studying the charge carrier dynamics using ultrafast transient absorption and photoluminescence spectroscopy. Surprisingly, the 2D perovskite films, although nominally prepared as "n = 4", are found to be mixture of multiple perovskite phases, with n = 2, 3, 4 and ≈ ∞, that naturally align in the order of n along the direction perpendicular to the substrate. Driven by the band alignment between 2D perovskites phases, we observe consecutive photoinduced electron transfer from small-n to large-n phases and hole transfer in the opposite direction on hundreds of picoseconds inside the 2D film of ∼358 nm thickness. This internal charge transfer efficiently separates electrons and holes to the upper and bottom surfaces of the films, which is a unique property beneficial for applications in photovoltaics and other optoelectronics devices. [ABSTRACT FROM AUTHOR]

One-dimensional colloidal multicomponentsemiconductor nanorods,such as CdSe–CdS dot-in-rod, have been extensively studiedas a promising class of new materials for solar energy conversionbecause of the possibilities of using the band alignment of componentmaterials and the rod-diameter-dependent quantum confinement effectto control the location of electrons and holes and to incorporatecatalysts through the growth of Pt tips. Here we used CdS nanorodsas an example to study the effect of nonuniform diameters along therod on the exciton localization and dissociation dynamics in CdS and(platinum tipped) CdS–Pt nanorods. We showed that, in CdS nanorodsprepared by seeded growth, the presence of a bulb with a larger diameteraround the CdS seed resulted in an additional absorption band lowerin energy than the exciton in the CdS rod. As a result, excitons generatedin the CdS rod could undergo ultrafast localization to the bulb regionin addition to trapping on the CdS rod. We observed that the Pt tipledto fast exciton dissociation by electron transfer. However, excitonslocalized on the CdS bulb showed slower average ET rates than thoselocalized in the rod region. Our findings suggested that the effectof rod morphology should be carefully considered in designing multicomponentnanorods for solar energy conversion applications. [ABSTRACT FROM AUTHOR]

The plasmon-excitoninteraction mechanisms in CdS–Au colloidalquantum-confined plexcitonic nanorod heterostructures have been studiedby transient absorption spectroscopy. Optical excitation of plasmonsin the Au tip leads to hot electron injection into the CdS rod witha quantum yield of ∼2.75%. This finding suggests the possibilityof further optimization of plasmon-induced hot electron injectionefficiency through controlling the size and shape of the plasmonicand excitonic domains for potential light harvesting applications. [ABSTRACT FROM AUTHOR]

Recent studies of group II–VIcolloidal semiconductor heterostuctures,such as CdSe/CdS core/shell quantum dots (QDs) or dot-in-rod nanorods,show that type II and quasi-type II band alignment can facilitateelectron transfer and slow down charge recombination in QD–molecularelectron acceptor complexes. To explore the general applicabilityof this wave function engineering approach for controlling chargetransfer properties, we investigate exciton relaxation and dissociationdynamics in InP (a group III–V semiconductor) and InP/CdS core/shell(a heterostructure beween group III–V and II–VI semiconductors)QDs by transient absorption spectroscopy. We show that InP/CdS QDsexhibit a quasi-type II band alignment with the 1S electron delocalizedthroughout the core and shell and the 1S hole confined in the InPcore. In InP–methylviologen (MV2+) complexes, excitonsin the QD can be dissociated by ultrafast electron transfer to MV2+from the 1S electron level (with an average time constantof 11.4 ps) as well as 1P and higher electron levels (with a timeconstant of 0.39 ps), which is followed by charge recombination toregenerate the complex in its ground state (with an average time constantof 47.1 ns). In comparison, InP/CdS-MV2+complexes showsimilar ultrafast charge separation and 5-fold slower charge recombinationrates, consistent with the quasi-type II band alignment in these heterostructures.This result demonstrates that wave function engineering in nanoheterostructuresof group III–V and II–VI semiconductors provides apromising approach for optimizing their light harvesting and chargeseparation for solar energy conversion applications. [ABSTRACT FROM AUTHOR]

Semiconductor quantum dots (QDs)have emerged as a new class oflight harvesting materials for solar energy conversion due to theirunique size-dependent properties. Most recent studies have focusedon II–VI group (such as CdX, X = S, Se, and Te) QDs and leadsalt (such as PbS, PbSe, and PbTe) QDs. In this paper, we investigateexciton dissociation and annihilation dynamics of Cd3P2QDs, a low bulk band gap (0.55 eV) II–V group material,to explore their potential application as a light harvesting componentfor photoreduction systems. For Cd3P2QDs with1S exciton band at 650 nm, a long-lived single exciton state withlifetime of 259 ns and a high emission quantum yield of 65% were observed.In Cd3P2QD–rhodamine B (RhB, an electronacceptor) complexes, excitons in QDs could be dissociated by ultrafastelectron transfer to RhB (6.2 ps), and the charge separated statehad a long lifetime (31 ns). Although the photoinduced electron transferrate in QD–RhB complexes decreased with increasing QD size,electron transfer was observed in QDs with 1S exciton bands at wavelengthas long as 1050 nm. Compared with CdSe and PbS, Cd3P2QDs with both more strongly reducing excited states and broaderabsorption in the visible and near IR region can be readily achieved,making them potential photosensitizers for photodriven water or CO2reduction reactions. [ABSTRACT FROM AUTHOR]

Light-driven valorization conversion of CO 2 is an encouraging carbon-negative pathway that shifts energy-reliance from fossil fuels to renewables. Herein, a hierarchical urchin-like hollow-TiO 2 @CdS/ZnS (HTO@CdS/ZnS) Z-scheme hybrid synthesized by an in situ self-assembly strategy presents superior photocatalytic CO 2 -to-CO activity with nearly 100% selectivity. Specifically, benefitting from the reasonable architectural and interface design, as well as surface modification, this benchmarked visible-light-driven photocatalyst achieves a CO output of 62.2 μmol·h -1 and a record apparent quantum yield of 6.54% with the Co(bpy) 3 2+ (bpy = 2,2'-bipyridine) cocatalyst. It rivals all the incumbent selective photocatalytic conversion of CO 2 to CO in the CH 3 CN/H 2 O/TEOA reaction systems. Specifically, the addition of HTO and stabilized ZnS enables the photocatalyst to effectively upgrade optical and electrical performances, contributing to efficient light-harvesting and photogenerated carrier separation, as well as interfacial charge transfer. The tremendous enhancement of photocatalytic performance reveals the superiority of the Z-scheme heterojunction assembled from HTO and CdS/ZnS, featuring the inner electric field derived from the band bending of HTO@CdS/ZnS make CdS resistant to photocorrosion. This study allows access to inspire studies on rationally modeling and constructing diverse heterostructures for the storage and conversion of renewables and chemicals.

Surface plasmon resonance-induced charge separation plays key roles in plasmon-related applications, especially in photocatalysis and photovoltaics. Plasmon coupling nanostructures exhibit extraordinary behaviors in hybrid states, phonon scattering, and ultrafast plasmon dephasing, but plasmon-induced charge separation in these materials remains unknown. Here, we design Schottky-free Au nanoparticle (NP)/NiO/Au nanoparticles-on-a-mirror plasmonic photocatalysts to support plasmon-induced interfacial hole transfer, evidenced by surface photovoltage microscopy at the single-particle level. In particular, we observe a nonlinear increase in charge density and photocatalytic performance with an increase in excitation intensity in plasmonic photocatalysts containing hot spots as a result of varying the geometry. Such charge separation increased the internal quantum efficiency by 14 times at 600 nm in catalytic reactions as compared to that of the Au NP/NiO without a coupling effect. These observations provide an improved understanding of charge transfer management and utilization by geometric engineering and interface electronic structure for plasmonic photocatalysis.

Colloidal semiconductor nanocrystals as triplet photosensitizers are characterized by a negligible intersystem crossing energy loss as compared to that of traditional molecular sensitizers. This property in principle allows for a large apparent anti-Stokes shift in sensitized triplet-triplet annihilation photon upconversion (TTA-UC) for a variety of applications. In previous systems, however, this advantage is largely erased by the energy loss associated with energy transfer from nanocrystals to surface-anchored triplet transmitter molecules. Here we report visible-to-ultraviolet TTA-UC from 473 to 355 nm, corresponding to an apparent anti-Stokes shift of 0.87 eV, with a quantum efficiency that reaches 4.5% (normalized at 100%). The system consists of CsPbBr 3 nanocrystal sensitizers, phenanthrene transmitters, and diphenyloxazole annihilators. Time-resolved spectroscopy reveals that triplet energy transfer from CsPbBr 3 nanocrystals to phenanthrene can be endothermic yet efficient thanks to a sizable entropic gain. This study exemplifies how entropic effects can be harnessed to enhance or control a plethora of applications with nanocrystals as photosensitizers.

Quasi-2D perovskites, composed of self-organized quantum well structures, are emerging as gain materials for laser applications. Here we investigate the influence of domain distribution on the laser emission of CsPbCl 1.5 Br 1.5 -based quasi-2D perovskites. The use of 2,2-diphenylethylammonium bromide (DPEABr) as a ligand enables the formation of quasi-2D film with a large- n -dominated narrow domain distribution. Due to the reduced content of small- n domains, the incomplete energy transfer from small- n to large- n domains can be greatly addressed. Moreover, the photoinduced carriers can be concentrated on most of the large- n domains to reduce the local carrier density, thereby suppressing the Auger recombination. By controlling the domain distribution, we achieve blue amplified spontaneous emission and single-mode vertical-cavity surface-emitting lasing with low thresholds of 6.5 and 9.2 μJ cm -2 , respectively. This work provides a guideline to design the domain distribution to realize low-threshold multicolor perovskite lasers.

Colloidal semiconductor nanoplatelets (NPLs) are chemical versions of well-studied quantum wells (QWs). For QWs, gating and carrier doping are standard tools to manipulate their optical, electric, or magnetic properties. It would be highly desirable to use pure chemical methods to dope extra charge carriers into free-standing colloidal NPLs to achieve a similar level of manipulation. Here we report colloidal n -doped CdSe and CdSe/ZnS NPLs achieved through a photochemical doping method. The extra electrons doped into the conduction band edges are evidenced by exciton absorption bleaches recoverable through dedoping and the appearance of new intersub-band transitions in the near-infrared. A high surface ligand coverage is the key to successful doping; otherwise, the doped electrons can be depleted likely by unpassivated surface cations. Large trion binding energies of 20-30 meV are found for the n -doped CdSe NPLs, which, in contrast, are reduced by 1 order of magnitude in CdSe/ZnS core/shell NPLs due to dielectric screening. Furthermore, we identify a long-lived negative trion with a lifetime of 1.5-1.6 ns that is likely dominated by radiative recombination.

We present the photoinduced size/structure transformation of [Au 37- x Ag x (PPh 3 ) 13 Cl 10 ] 3+ (M37) into [Au 25- y Ag y (PPh 3 ) 10 Cl 8 ] + (M25) cluster. Single-crystal X-ray diffraction revealed that M37 has a tri-icosahedron M 36 metal core assembled via the fusion of three Au 7 Ag 6 icosahedrons in a cyclic fashion and that the M 36 core is further protected by phosphine and chloride ligands. The M37 cluster is found to be highly sensitive toward ambient light, and the M37 → M25 transition is observed with 530 nm irradiation, monitored by time-dependent UV-vis spectroscopy, electrospray ionization mass spectrometry (ESI-MS), and femtosecond transient absorption spectroscopy. Linear-response time-dependent DFT calculations indicated that the strong absorption of the M37 cluster close to 500 nm induces an antibonding-type configuration in the induced electron density within the plane of the three 8-electron systems, possibly promoting dissociation of one of the 8-electron superatoms. This theoretical result supports the experimental observation of the sensitivity of the M37 → M25 transition to 530 nm irradiation.

Charge and electronic energy transfer form the basis of many natural and artificial energy transduction systems. The energy landscapes that drive these transfer processes are often constructed from enthalpy changes. In contrast, the entropic effect, although occasionally invoked to explain some excited-state dynamics, has rarely been used to actively control charge/energy flow. Here we derive a generic formula describing how entropy can quantitatively gate the thermally activated delayed emission lifetime in semiconductor nanocrystal-molecular triplet acceptor complexes and experimentally verify the model using highly emissive, quantum-confined CsPbBr 3 nanocrystals surface-functionalized with multiple phenanthrene triplet acceptors. Triplet energy transfer from photoexcited CsPbBr 3 nanocrystals to phenanthrene is followed by thermally activated repopulation of nanocrystal excitons, leading to delayed nanocrystal emission. The lifetime of delayed emission increases with the phenanthrene/nanocrystal ratio, due to lowering of the free energy of the acceptor state by entropic gain. This study points toward a direction of using entropy to artificially design donor-acceptor light-emitting materials with predetermined excited-state lifetimes.

Colloidal CdSe nanoplatelets (NPLs) have substantial potential in light-emitting applications because of their quantum-well-like characteristics. The self-trapped state (STS), originating from strong electron-phonon coupling (EPC), is promising in white light luminance because of its broadband emission. However, achieving STS in CdSe NPLs is extremely challenging because of their intrinsic weak EPC nature. Herein, we developed a strong STS emission in the spectral range of 450-600 nm by building superlattice (SL) structures with colloidal CdSe NPLs. We demonstrated that STS is generated via strong coupling of excitons and zone-folded longitudinal acoustic phonons with formation time of ∼450 fs and localization length of ∼0.56 nm. The Huang-Rhys factor, describing the EPC strength in SL structure, is estimated to be ∼19.9, which is much larger than that (∼0.1) of monodispersed CdSe NPLs. Our results provide an in-depth understanding of STS and a platform for generating and manipulating STS by designing SL structures.

Solution-processed quantum-confined nanocrystals are important building blocks for scalable implementation of quantum information science. Extensive studies on colloidal quantum dots (QDs) have revealed subpicosecond hole spin relaxation, whereas the electron spin dynamics remains difficult to probe. Here we study electron and hole spin dynamics in CdSe colloidal nanoplatelets (also called quantum wells) of varying thicknesses using circularly polarized transient absorption spectroscopy at room temperature. The clear spectroscopic features of transition bands associated with heavy, light, and spin-orbit split-off holes enabled separate probes of electron and hole dynamics. The hole spin-flip occurred within ∼200 fs, arising from strong spin-orbit coupling in the valence band. The electron spin lifetime decreased from 6.2 to 2.2 ps as the platelet thickness is reduced from 6 to 4 monolayers, reflecting an exchange interaction between the electron and the hole and/or surface dangling bond spins enhanced by quantum confinement.

Multielectron photocatalysis requires sequential, multiple charge transfer from the light absorber to the catalytic site. As a result, many-body effects induced by charge accumulation play a fundamental role in these reactions, especially when photocatalysts are miniaturized to the nanoscale. Here, we study sequential two-electron transfer in a state-of-the-art nanophotocatalyst, CdSe@CdS dot-in-rod (DIR) decorated with Pt tips, using pump-pump-probe transient absorption spectroscopy. Following the first electron transfer (ET) from DIR to the Pt tip, the second ET needs to not only compete with Auger recombination of a positively charged exciton but also experience a large Coulomb barrier exerted by two holes. As a result, both the ET rate and efficiency decrease by an order of magnitude. Analysis using a dissociation-limited long-range charge transfer model reveals that the Coulomb barrier of the second ET is ∼60 meV higher than that of the first one. This study not only uncovers the mechanism and efficiency bottleneck of a real multielectron photocatalyst but also provides general guidelines for the design of multielectron photocatalytic systems.

The strong spin-orbital coupling and intense optical transition in lead halide perovskites enable facile optical injection and manipulation of spin states potentially useful for spintronics, one manifestation of which is the optical Stark effect (OSE). A strong OSE would benefit from discrete energy levels and a strong band edge transition with concentrated oscillator strength, which can be realized in three-dimensionally confined quantum dots (QDs). This idea, however, has not been explored yet for perovskite materials. Here we report the observation of strong OSE in perovskite colloidal QDs using circularly polarized transient absorption spectroscopy. The large OSE shifts correspond to transition dipoles as large as 52 D, which compares favorably to those recently reported for higher-dimensional perovskites as well as transition metal dichalcogenide monolayers. Colloidal synthesis also allows for facile tuning of the OSE spectral ranges of perovskite QDs via sizes and compositions, and the impact of these parameters on the OSE is examined, providing fundamental insights into the band edge transition of lead halide perovskites.

Two-dimensional (2D) organic-inorganic hybrid perovskites are promising materials for next-generation optoelectronic devices owning to their structural and functional versatility and enhanced ambient stability. Recent studies have started to focus on engineering the molecular properties of the organic cations to induce inorganic-to-organic energy/charge transfer for new functionalities, yet many puzzles regarding the inorganic-organic interaction mechanisms remain to be resolved. Here we fabricate 2D lead halide perovskites containing naphthalene methylamine (NMA) cations to study naphthalene triplet sensitization by inorganic excitons. We find that triplet sensitization proceeds via a two-step mechanism initiated by subpicosecond hole transfer from the inorganic layer to naphthalene. We also provide spectroscopic evidence for triplet excimer formation, i.e., the association between triplet and ground state molecules. The intensity ratio between the excimer and triplet emissions can be tuned via the percentage of the NMA cations in the organic layer, offering a route to tunable white-light emitters using 2D hybrid perovskites.

Charge transfer and recombination across the inorganic/organic interface in nanocrystal or quantum dot (QD)-molecule hybrid materials have been extensively studied. Principles of controlling charge transfer and recombination via energetics and electronic coupling have been established. However, the use of electron spin to control transfer and recombination pathways in such systems remains relatively underexplored. Here we use CdS QD-alizarin (AZ) as a model system to demonstrate this principle. Using time-resolved spectroscopy, we found that the charge-separated states (QD - -AZ + ) created by selectively exciting AZ molecules mostly recombined to regenerate ground-state complexes, whereas apparently the "same" charge separated states created by exciting QDs recombined to produce AZ molecular triplet states. Such a difference can be traced to the distinct spin configurations between excited QDs (QD*, with an ill-defined spin) and AZ ( 1 AZ*, spin singlet) and the asymmetric electron and hole spin-flip rates in II-VI group QDs. The transferability of such a principle was confirmed by similar observations obtained for CdS QD-tetracene complexes. Opening an avenue for controlling charge transfer and recombination pathways via electron spin is potentially important for applications such as artificial photosynthesis.

Triplet energy transfer (TET) from semiconductor nanocrystals (NCs) has recently emerged as a new triplet sensitization paradigm. It remains unclear how trap states pervasive in NCs influence TET or whether trapped excitons can undergo efficient TET. Here we partially address this issue by studying TET from CuInS 2 NCs as a model system because their photogenerated excitons are known to be "self-trapped" due to hole localization to intragap Cu states. We found that, thanks to the long lifetime (209 ± 17 ns) of self-trapped excitons, they could be extracted with an efficiency of ∼92.3% by surface-anchored anthracene despite that the TET rate was relatively slow (57.1 ± 1.7 μs -1 ). We further leveraged this efficient sensitization to achieve triplet-triplet-annihilation photon upconversion (TTA-UC) with a quantum yield of 18.6 ± 0.3%. Thus, this study not only demonstrates trapped excitons can undergo efficient TET as well, but also presents the first TTA-UC system sensitized by nontoxic NCs which is important for the real-life application of this technique.

Recent years have seen the revived interests in the triplet states of polycyclic aromatic hydrocarbons (PAHs) due to their potential applications ranging from photon upconversion to photoredox catalysis. Because of their "dark" nature, these triplets have to be generated via triplet energy transfer (TET) from sensitizers. Traditional sensitization schemes introduce a sizable energy loss (ca. ≥ 0.5 eV) resulting from intersystem crossing (ISC) in sensitizers. Consequently, triplets of naphthalene (Nap), the most energetic PAH triplets (∼2.6 eV), have remained relatively underexplored because ultraviolet (UV) photons and wide-energy-gap sensitizers would be required for the sensitization. Here we show that Nap triplets can be efficiently generated using TET from quantum-confined CsPbBr 3 perovskite nanocrystals (NCs). Quantum confinement is essential to meet the energetics and electronic coupling requirements in TET. Because of the vanishingly small bright-dark states energy splitting in NCs, this sensitization process can take place at the limit of energy conservation, driven by visible photons.

The spectral properties of lead halide perovskite nanocrystals (NCs) can be engineered by tuning either their sizes via the quantum confinement effect or their compositions using anion and/or cation exchange. To date, the latter is more frequently adopted, primarily because of the ease of ion exchange for lead halide perovskites, making the quantum confinement effect seemingly redundant for perovskite NCs. Here we report that quantum confinement is required for triplet energy transfer (TET) from perovskite NCs to polycyclic aromatic hydrocarbons (PAHs). Static and transient spectroscopy measurements on CsPbBr 3 NC-pyrene hybrids showed that efficient TET occurred only for small-sized, quantum-confined CsPbBr 3 NCs. The influences of the size-dependent driving force and spectral overlap on the TET rate were found to be negligible. Instead, the TET rate scaled linearly with carrier probability density at the NC surface, consistent with a Dexter-type TET mechanism requiring wave function exchange between the NC donors and pyrene acceptors. Efficient TET funnels the excitation energy generated in strongly light-absorbing perovskite NCs into long-lived triplets in PAHs, which may find broad applications such as photon upconversion and photoredox catalysis.

Aggregation-induced emission luminogens (termed AIEgens) have emerged as an important class of light-emitting materials with many potential applications. As implied by their names, AIEgens typically exhibit strong emission in the aggregated form for which the intramolecular motions that dissipate excited-state energy are inhibited. Here we demonstrate that AIEgens, using carboxylated tetraphenylethylene (TPE-CA) as an example, can exhibit strong emission in the solution after being grafted onto colloidal nanocrystal (NC) surfaces. Using ultrafast transient absorption spectroscopy, we found that the intramolecular motions of TPE-CAs were strongly suppressed, with rates retarded by 2 orders of magnitude, when they were grafted onto ZnS NCs. As a result, the emission quantum yield (QY) of TPE-CA-NC hybrids in solution was also enhanced by 2 orders of magnitude compared to free TPE-CA molecules in solution. This opens a new avenue for constructing multifunctional hybrid materials capitalizing on the emission of AIEgens. In addition, this methodology can be extended to suppress aggregation-caused quenching (ACQ) of ACQphores in the solid-state form.

Lead halide perovskite nanocrystals (NCs) as promising optoelectronic materials are intensively researched. However, instability is one of the biggest challenges needed to overcome before fulfill their practical applications. To improve their stability, we present a postsynthetic controlled phase transformation of CsPbBr 3 toward CsPbBr 3 /Rb 4 PbBr 6 core/shell structure triggered by rubidium oleate treatment. The resulting core/shell NCs show exceptional photostability both in solution and on-chip. The solution of CsPbBr 3 /Rb 4 PbBr 6 NCs can remain over 90% of the initial emission photoluminescence quantum yields after 42 h of intense light-emitting diodes illumination (450 nm, 175 mW/cm 2 ), which is even better than the conventional CdSe/CdS quantum dots whose emission drop to 50% after 18 h under the same condition. We believe that the exceptional photostability should be resulted from the protection of the robust Rb 4 PbBr 6 shell on CsPbBr 3 NCs.

In multielectron photocatalytic reactions, an absorbed photon triggers charge transfer from the light-harvester to the attached catalyst, leaving behind a charge of the opposite sign in the light-harvester. If this charge is not scavenged before the absorption of the following photons, photoexcitation generates not neutral but charged excitons from which the extraction of charges should become more difficult. This is potentially an efficiency-limiting intermediate event in multielectron photocatalysis. To study the charge dynamics in this event, we doped CdS nanocrystal quantum dots (QDs) with an extra electron and measured hole transfer from n-doped QDs to attached acceptors. We find that the Auger decay of charged excitons lowers the charge separation yield to 68.6% from 98.4% for neutral excitons. In addition, the hole transfer rate in the presence of two electrons (1290 ps) is slower than that in the presence one electron (776 ps), and the recombination rate of charge separated states is about 2 times faster in the former case. This model study provides important insights into possible efficiency-limiting intermediate events involved in photocatalysis.

Carrier doping is important for semiconductor nanocrystals (NCs) as it offers a new knob to tune NCs' functionalities, in addition to size and shape control. Also, extensive studies on NC devices have revealed that under operating conditions NCs are often unintentionally doped with electrons or holes. Thus, it is essential to be able to control the doping of NCs and study the carrier dynamics of doped NCs. The extension of previously reported redox-doping methods to chemically sensitive materials, such as recently introduced perovskite NCs, has remained challenging. We introduce an "intact" carrier-doping method by performing pump-pump-probe transient absorption spectroscopy on NC-acceptor complexes. The first pump pulse is used to trigger charge transfer from the NC to the acceptor, leading to NCs doped with a band edge carrier; the following pump-probe pulses measure the dynamics of carrier-doped NCs. We performed this measurement on CsPbBr 3 NCs and deduced positive and negative trion lifetimes of 220 ± 50 and 150 ± 40 ps, respectively, for 10 nm diameter NCs, both dominated by Auger recombination. It also allowed us to identify randomly photocharged excitons in CsPbBr 3 NCs as positive trions.

For the first time, we report that graphene oxide (GO) can be used as a new "dual-role" binder for Si nanoparticles (SiNPs)-based lithium-ion batteries (LIBs). GO not only provides a graphene-like porous 3D framework for accommodating the volume changes of SiNPs during charging/discharging cycles, but also acts as a polymer-like binder that forms strong chemical bonds with SiNPs through its Si-OH functional groups to trap and stabilize SiNPs inside the electrode. Leveraging this unique dual-role of GO binder, we fabricated GO/SiNPs electrodes with remarkably improved performances as compared to using the conventional polyvinylidene fluoride (PVDF) binder. Specifically, the GO/SiNPs electrode showed a specific capacity of 2400 mA h g -1 at the 50th cycle and 2000 mA h g -1 at the 100th cycle, whereas the SiNPs/PVDF electrode only showed 456 mAh g -1 at the 50th cycle and 100 mAh g -1 at 100th cycle. Moreover, the GO/SiNPs film maintained its structural integrity and formed a stable solid-electrolyte interphase (SEI) film after 100 cycles. These results, combined with the well-established facile synthesis of GO, indicate that GO can be an excellent binder for developing high performance Si-based LIBs.