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Perspective
Evidence in Support of Exciton to LigandVibration Coupling in Colloidal Quantum Dots Efrat Lifshitz J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.5b01567 • Publication Date (Web): 08 Oct 2015 Downloaded from http://pubs.acs.org on October 12, 2015
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Evidence in Support of Exciton to Ligand-Vibration Coupling in Colloidal Quantum Dots 5 7
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Efrat Lifshitz, Schulich Faculty of Chemistry, Russell Berrie Nanotechnology Institute, Solid State Institute, Technion, Israel Institute of Technology, Haifa 32000, Israel (
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The perspective focuses on the investigation of an unresolved conflict in semiconductor colloidal quantum dots (CQDs) research, concerning the influence of the immediate surrounding on the optical properties of the materials. Today’s advanced synthetic colloidal procedures offer formation of high quality inorganic crystalline, capped with various organic/inorganic molecular ligands. The perspective aims to clarify whether exciton recombination processes in CQDs are influenced by the type of crystalline-ligand bonding and, moreover, whether these excitonic processes experience direct coupling to the ligands' vibrational modes. Most ligands used have redox characteristics, whose functional groups are added on to the CQDs' surface via coordination, covalent or ionic bonding. he surface-ligand bonding introduce electronic states either above or below the intra-band/inter-band energy gap, resulting in electronic passivation or in creation of trapping states that affect intra-band and inter-band relaxation processes. Furthermore, crystalline electronic states may have a direct coupling to molecular vibrational states via direct overlap of electronic wavefunctions or through a long-range energy transfer process. Also, photoejected carriers resulting from an Auger process or ionization processes, may diffuse temporarily onto a ligand site. These scenarios are discussed in the current publication with supporting theoretical and experimental observations. 34
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Colloidal semiconductor quantum dots (CQDs) are characterized by tunable electronic properties with variation of size, shape and composition.1-3 Colloidal techniques facilitate the formation of high-quality CQDs with surface passivation by molecular ligands, hetero-structuring and scalable processing.4,5 Along with the development of the synthetic procedures, numerous investigations have explored the optical and electrical properties of these materials, including the study of fluorescence quantum yields, 6,7 electron-hole exchange interactions, 8,9 excited state lifetime and polarization, 10-15 generation of multiple excitons and hot carriers. 16-30 Some of these properties are critical prerequisites to applications in a variety of opto-electronic applications, including photovoltaic cells,23,31-35 light sources, 36-39 down/up light converters,40 logic gates,41 photodetectors, 42 and fluorescence tags in biological platforms. 43-48 However, a number of other fundamental issues concerning the commonly observed fluorescence blinking 49-54 and spectral diffusion,49-51 as well as carrier intra-band and inter-band relaxations, 19,55-59 remained inconclusive until the current moment. Related studies have demonstrated that these issues are associated with intrinsic properties of the inorganic moiety, , Auger relaxation, 60-64 exciton charging59,65-69 and phonon assisted cooling,70,71 however may have an influence by the surface and environment properties, such as surface-mediated charge trapping72,73 and surface passivation29,74 (and refs. within). A few studies have paid attention to the molecular ligands' degrees of freedom, 57,64,70,75,76 such as the molecular vibrational modes, 57,77,78 location at selective crystalline-facet, 79 dangling bond, 64 concentration and packing. 80,81 The perspective focuses on the occurrence of exciton-to-ligand coupling, and its effect on the optical properties of the investigated materials. Organic molecular ligands are the most commonly used; 82-86 however, studies from recent years have also presented implementation of inorganic molecules,87-91 chalcogenides,92 halides91,93 or pseudo halides surfactants.91,92,94-96 The perspective deals solely with the influence of organic ligands, although the effects mentioned below may be generalized to nearly all adlayers. The document discusses the influence of the type of surface-to-ligand bonding and the internal architecture of the inorganic moiety (e.g., core or core/shell) on the exciton-to-ligand vibrational mode coupling, and the consequent effects. The following sections underline the functionality of the organic ligands and their influence on the electronic structure and doping of the CQDs. It is followed by a discussion about a few selective theoretical and experimental observations: (a) Evidence for spectral shifts of Raman modes of typical ligands bonded to CQDs, revealing knowledge about the type of surface-to-ligand bonding; (b) Control of the 1Pe-1Se Intra-band relaxation via exchange of ligands, or/and electron-hole spatial separation; (c) Influence of ligands' nuclear spins on the polarization of diluted magnetic spin in CQDs; (d) Surface re-arrangement and creation of voids following ligand bonding; (e) Excitonic spectral diffusion of individual CQDs due to charge trapping at ligand sites; (f) Excitonic energy loss due to induced excitation of ligand vibrational modes, studied via micro-photoluminescence (µ-PL) of individual CQDs. The prior and most recent observations discussed here unambiguously support the crystalline-ligand coupling. 52
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Functionality of the organic ligands 5
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Controlling the structural and surface properties of the CQDs: The role of surface capping ligands in colloidal synthesis has been studied extensively, revealing their impact on a size control, passivation, 6,82,85,86,97-102 surface reconstruction,80,103 rate of growth and molding the shape of the nanostructures. 103-106 The surface-to-ligand bonding and can be divided into the following categories (see Figure. 1(a)): (I) Molecules with single non-bonding pair and σ-bonds (e.g., alkylamines) are suitable donors, that create a coordinating bond with unsaturated surface metal site (e.g., Cd+2 surface site); 64,76,107 (II) Ligands with double bonds (e.g., triocthylphosphineoxide [TOPO]), creating a metal-to-ligand covalent bond (e.g., Cd-O-P), viz., creation of new bonding/antibonding orbitals that are positioned either within or outside the intra-band or interband energy gaps (see Figure 1(b));78 (III) Ionic bonding (e.g., oleic and phosphonic acids) or a complete charge exchange (b) between surface π-bonding ligands (e.g., pyridine), with optional creation of radical anions and cations that induce mid-band gap states (see Figure 1(b)).64,108,109 Readers are directed to a comprehensive description of optional surface-to-ligand bonding 1: (a) Schematic drawing of alkyl-amine (I), TOPO (II) and alkyl-carboxyl (III) in recent reviews or perspectives. Figure molecules, used as typical capping ligands; (b) An energy diagram showing the CQD's 30,74,76,77,80 Although amines, thiols electronic states (black lines), surface bonding (solid red lines) and antibonding red lines) surface-ligand states. The plausible vibrational intra- and interand acids passivate the Cd+2 (dashed band coupling is displayed by schemes of vibration motion. dangling bonds, removing electron traps, the Se-2 dangling bonds remain un passivated, and act as hole traps. Thiols σ- or π-bonded molecules via their S-atom supply three lone pairs, of which one pair passivates the Cd+2 dangling bonds to form surface-S bonding and anti-bonding states above the bandgap; nevertheless, the excess lone pairs tend to trap holes. 110,111 Surface-ligands bonding occasionally induces an electrical doping characteristic. 112,113 For example, oleic acid capped PbSe(S) CQDs generally show a p-type behavior. 114 However, upon exchange of ligands with hydrazine, they become n-type semiconductors. 115 43
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Surface-to-ligand bonding promotes changes on both sides of the inorganic-organic boundary. As shown below, vibrational modes of bonded ligands are either red- or blue-shifted energetically, with respect to correlated modes of the free molecules, due to redistribution of electronic clouds or/and breakage of the molecular symmetry, when distinct changes indicate the type of the inorganic-organic interface. 78 The CQD-ligand bonding characteristic could have a direct influence on the optical properties of the semiconductor crystalline, when surface-ligand states positioned at mid band-gap energy (see Figure 1(b)) may trap photo-generated with a direct impact on intraand inter-band relaxation processes. 53
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Furthermore, surface capping ligands act as a bridge moiety to the surrounding medium and can be exchanged on demand, controlling solvation and inter-particle interactions. 33,116,117 Also, the capping ligands frontier orbitals put a potential barrier at the CQD surface, rendering a slight normalization of the crystalline electronic energy levels. 118 Other studies indicated the ligands 60
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dielectric environment influence the internal polarity, 64,119,120 which stimulate change in the linear and nonlinear optical properties of the CQDs. 121 6
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Intrinsic electronic states-ligands vibrational modes coupling: Direct interaction between intrinsic electronic states of the inorganic moiety with vibrational modes of capping ligands has been suggested only in recently, particularly when exploring the hot-carrier cooling mechanism. After above band-gap optical excitation, the generated electron and hole tends to relax into the bandedge states. It was originally anticipated that in strongly confined CQDs with electronic states that are separated by several multiples of the typical LO phonon energy (~ 30 meV), hot carriers relaxation would be very slow, experiencing the so-called phonon bottleneck. 79,122-125 The mechanism for a conduction band 1Pe-1Se intra-band relaxation was mostly investigated by a variety of time-resolved femtosecond pump/probe spectroscopic techniques, and has been a topic of debate for many years. The pioneering work by the Klimov group63,126 revealed a 1Pe-1Se intra-band relaxation time ~ 1 psec. Earlier work by the Guyot-Sionnest group57,64 detected similar 1Pe-1Se intra-band relaxation times, but also indicated a variability dependence on the type of ligands (e.g., TOPO, thiocresol, pyridine). The mentioned preliminary works support a plausible model involving intra-band relaxation via an Auger process, viz., cooling of hot electrons by an energy transfer to its counter partner—the hole—via Coulomb interaction. 61 For II-VI and III-V semiconductor CQDs, the heavy mass of the hole and the three-fold degeneracy at the valence band-edge lay a ground for a hole relaxation via phonon-mediated route. 126,127 A conflict arises when considering the case of IV-VI semiconductor CQDs, where quantized states in the conduction and valence bands are nearly symmetrically spaces, 128,129 so that hole relaxation via phonon coupling is not an efficient process. A complementary work by the Guyot-Sionnest group indicated that the intra-band relaxation time is nearly the same upon optical and pumping or electrochemical charging, suggesting that the relaxation might not be related to the generation of electron-hole pairs. This study also expended the investigation of the dependence on the type of surface capping, showing a fast relaxation of 8 psec when CQDs are capped by phosphonic and oleic acid, mediate rate of 10 psec when covered by alkylamines and a longer rate of 30 psec when covered with dodecanethiol. In general, faster relaxation was correlated with the larger interfacial polarity. This study proposed for the first time that intra-band relaxation via the ligand-degrees of freedom is a plausible mechanism. 57 Thus, the relaxation can be correlated with the infrared activity of the molecular ligands, when the vibrational modes serve as a sink to the intra-band relaxations. The NH and CH (~3000 cm-1) and CH bending (~1400 cm-1) might be off resonant with the 1Pe-1Se transitions in CdSe (~4-5 nm) CQDs. However, other modes in phosphonic acid and oleic acids (with infrared absorption at 1500 cm-1 to 3500 cm-1) could act an energy sink to hot carrier cooling. 130,131 The coupling between a hot carrier and the ligands degrees of freedom, occurs either through a direct resonance energy transfer into ligand vibration mode,78,132-134 or by a mediated-charge trapping from the CQDs excited state into surface-ligand bonding orbital, if not by a complete charge transfer into a ligand moiety. 29,54,56 More recent work from the Guyot-Sionnest group135 showed an exceptionally slow electron cooling of > 1 nsec. It was achieved in specially designed CQDs composed of CdSe core, covered by a ZnSe. This CdSe/ZnSe pair exhibits a quasi-type II alignment136 allowing delocalization of the hole into the shell, suppressing an Auger relaxation. Then, an additional shell of CdSe was added, to form a CdSe/(ZnS)ZnSe/CdSe heterostructure, which increased the distance of the hole from 60
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the exterior surfaces. In the presence of a hole-trapping ligand (thiolate) a slow 1Pe-1Se electron cooling was achieved. While hot carriers cooling via phonon- or ligand vibrational modes mediating processes have been discussed, a direct energy transfer or charge transfer into surface states should be taken into consideration. Kambhampati group demonstrated the deterministic role of direct shunting of hot carrier (hole and electron) into surface trap states, via non-adiabatic process based on nonOppenheimer interactions.156, 157 It should be noted that charge trapping and surface charging are similar concepts, being the main mechanism in a blinking process. Surface charging induces changes in polarity of the CQDs and consequently, coupling to phonon and vibrational modes can also be enhanced. Direct resonance energy transfer between inter-band gap and vibrational modes were reported only in individual cases, associated with a transfer from a small band gap energy in the infrared (in IV-VI CQDs).163, 164 Theoretical work showed potential mixing between phonon and ligand vibration modes that can assist relaxation of surface states, with dependence on the ligands' functional group and on the overall mass of the molecule. 147 More recent work55 expanded the study for the investigation of hot hole cooling and revealed that it takes place by a combination of phonon and ligand vibrational mode mediation, when vibrational modes are dominant, in particular at the smallest CQDs (~ 2nm). There is also a fundamental dependence on the type of capping ligands. Furthermore, when displacement of the ligand exterior layer away from the CQD-core center takes place by growth of an additional inorganic shell, e.g., ZnSe, the hole cooling time is elongated. 56 The influence of organic ligands on fluorescence intermittencies: Numerous studies to date have shown the occurrence of photoluminescence intermittencies when monitoring single CQDs under continuous excitation, involving darkening of the luminescence for periods that last seconds to minutes, intersected by the emergence of bright luminescence events.50, 52,53,123,137,138 The probability for the occurrence of both the "on" and "off" periods was found to obey power-law statistics over orders of magnitude in the time domain with a truncation of the power-law statistics at later times. 123,138 Also, it has been suggested that the blinking is correlated with another phenomenon known as spectral diffusion, where the photoluminescence energy exhibits periodic jumps with shifts of tens of meV and/or energy jittering with shifts of ~1 meV. 79,137,139-142 Recent findings indicate that the jump events show a memory effect bouncing back to a prior position. 143 Another work suggested non-biased random walk-like diffusion. 144 Recent study showed that spectral diffusion contributes to the ensemble emission line broadening. 145 The underlying mechanism primary suggested involvement of an Auger process, 146 in which a CQD exciton undergoes photo-ionization ejection of a photo-excited carrier to a remote acceptorlike state, while leaving a charged QD behind. A subsequent photo-excited electron-hole pair is subjected to rapid Auger non-radiative relaxation into the ground state with quenching of the emission intensity. All carriers eventually return to the CQD core and thus neutralize the dot. Thus, the "on"–"off" periods are related to the switching between neutrality and charging of a CQD. 147,148 Variation of the model suggested that the ejected charges diffuse to the CQD surface, a process that obeys power-law dependence. 28,123,149 More recent work proposed that blinking and spectral jumps are associated with the ejection of a charge from the CQD to the ligand surrounding. 150 In parallel to the experimental and theoretical studies, efforts to fabricate CQDs 60
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that mitigate the blinking and spectral diffusion effects have been demonstrated. 21,151-153 Such non-blinking CQDs have been successfully realized in core/shell hetero-structures with a medium21,154 (e.g., 6-7 monolayers) to giant151,155 (~20 monolayers) shell thickness, enabling partial electron-hole separation and/or core-to-ligand separation, or in an alloyed graded shell, 21,152,156 8
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restraining the carrier ejection routes (suppression of Auger process). 157-159 Non-blinking behavior of the mentioned heterostructures was followed using intensity autocorrelation function. For example, the Bawendi group showed relatively high quantum yield or long "on" time fraction of biexciton emission.154,160 The Dubertret group displayed a new way for analyzing blinking statistics161 using CdSe/giant-CdS CQDs, showing non-zero emission for single and biexciton and their charged analogs. 162 The Klimov group displayed nearly unity yield of biexciton emission,163 and decent emission intensity (so called "gray" emission) from positive and negatively charged exaction. 59 The Guyot-Sionnest group investigated the emission intensity of alloyed CdSeS/ZnS heterostructure under electrochemical control. 156 17
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The influence of CQDs architecture on the exciton to ligand-vibrational modes coupling: The internal design of the inorganic moiety determines the charge carriers' confinement potential, Coulomb interactions, Auger relaxation, overlap/separation of carrier distribution functions and electronic tunability with respect to the surrounding. Pure core CQDs (type-I) exhibit similar delocalization of the electron and hole radial distribution functions with minor penetration to the surrounding beyond the dot boundary, thus promoting an overlap with the ligand electronic or vibrational states (see Figure 2(a)). 3
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Fig.2: Schematic drawing of core, core/shell and core/alloyedshell (from left to right) CQDs and the anticipated carriers' distribution functions. Quasi-type-II configuration is assumed for the core/shell structures.
Core/shell CQDs of the II-VI show mostly type-II alignment at the core-shell interface, where on carrier has slightly larger delocalization over the entire hetero-structure (so-called quasi-type-II), while the hole is less accessible for direct contact with surface ligands (see Figure 2(b)). Core/alloyed-shell CQDs are characterized by a smooth potential aft the core-shell interface, thus permitting delocalization of both carriers toward the surface, but not to the same extent (see Figure 2(c)), still can promote overlap with the ligands' electronic or vibrational states. Further heterostructuring complexity have been suggested in a few cases, e.g., CdSe/ZnS/ZnSe/CdSe showing relatively long 1Pe-1Se via control of the distance from the exterior surface, 135 or CdSe/giant-CdS/ZnS164 enabling partial charge separation, as well as a distance from the exterior environment. 45
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Recent investigations - supporting crystalline-ligand coupling Vibrational modes of organic ligands are pruned for deviations upon surface-ligand bonding: The influence of the organic adlayers on the physical properties of the CQDs, primarily depend on the surface-to-ligand bonding. Such bonding can be viewed from the perspective of the ligands themselves, viz., following modification of their vibrational mode frequencies when attached to the CQDs surfaces, with respect to that of three native free-standing molecules in diluted solutions. When hydrogen bonds are involved (e.g., C-H,N-H), symmetric and asymmetric stretching is expected at frequencies ranging from 2800-3500 cm-1. However, bending, rocking, twisting, wagging and scissoring motions of the entire skeleton, as well as modes of function groups (e.g, P=O, COO-), can be expected to appear at the 800-2000 cm-1 frame. A recent study reported the infrared and Raman spectra of typical ligands molecules, trimetylphosphine-oxide (TMPO) and methylamine (MA) capping the CdTe core CQDs. Representative spectra are shown in Figure 3. The study was complemented by DFT theoretical calculations and showed a pronounced red-shift in the phosphine-oxide Figure 3: IR spectra of trimethylphosphineoxide stretching mode frequency when bonded to CdSe CQDs, (TMPO) ligands, bonded to CdSe surface (A) or freedue a change in the type of bond from a double bond standing (B), both measured in tetrachloroethylene solution. Predicated theoretical IR spectra of TMPO P=O to a single P-O bond, when an electron donation when bonded to CdSe (C) or as free-standing from the occupied d-orbital of a surface-Cd atom is molecules (D). [Reprint from reference 78, Copyright Year 2012, Publisher ACS] donated to the π* unoccupied orbital of the ligand. Surprisingly, changes are seen also on the stretching modes of bonds not in direct contact with the surface, which may arise from alteration of charge distribution over the entire molecule, as well as symmetry breaking. Bonding of MA to the CdSe CQDs, showed a mild change, while alkylamine molecules create a coordination bonding with the surfaces. Still the N-H stretching exhibits a blue-shift with respect to free-standing molecules, while other modes of fragments at a distance nearly stayed intact in comparison with pristine molecules. The theoretical predictions in reference78 are in agreement with the reported experimental Figure 4: Three-pulse electron spin echo envelope observations, establishing the mechanisms for the CQDmodulation decay curves of Zn0.995Mn0.005O CQDs capped with phosphine or amine ligands, dispersed in toluene (tol) ligand interactions. or deuterated toluene (tol-d8) solution. The modulations Spin coupling between Mn-dopant with nuclei spin of are associated with the nuclear coupling between the Mn electron-spins and the external ligand nuclei. [Reprinted organic ligands as persuasive evidence for the CQDfrom reference 165, Copyright Year 2015, Publisher ACS] ligand coupling: A recent interesting and comprehensive study explored the surrounding (ligands or/and 60
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solvents) nuclei spins on Mn+2-dopant spin-dynamics in diluted magnetic semiconductor (DMS) ZnO, ZnSe and CdSe165 CQDs. The study utilized pulsed electron spin resonance spectroscopy (pESR) to follow the spin-lattice (T1) and spin-spin (T2) relaxation times of the Mn+2 ions embedded within the CQDs with extremely low concentration (around one Mn+2 ion per CQD). The study indicated direct coupling mainly to the hydrogen nuclei of the closest surrounding, the ligands. An exchange of the hydrogen atoms by deuterium, as well as extra growth of the shell over the core CQDs, induced elongation of the relaxation times, suggesting mitigation of the Mn+2ligand interactions, which more likely follow a dipole-dipole interaction. This chemical adjustment of the degree of interaction provides a direct tool to investigate the CQD-ligand interactions, and offers control of magnetic properties in CQDs, directly influencing spin-electronic applications. Representative pESR spectra are shown in Figure 4, comparing the T1 decay curves of CQDs with hydrogenated (tol) and deuterated ligands (tol-d8). 165 18
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Another recent study showed the ligands assisting in transferring energy from a surface state to a Mn+2-dopant states in DMS-CQDs. The study investigated Mn+2-doped CdSe/ZnS CQDs capped with octathiol ligands. While thiol ligands generally trap the photo-generated holes and, consequently, quench the CQDs' emission, in the reported case, these thiols enhanced the Mn +2dopant emission intensity by mediating an energy transfer from a trap side to the dopant excited state, via the surface-to-ligand coupling.166 26
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Optically detected magnetic resonance of trapped holes at a surface-ligand interface: Optically detected magnetic resonance (ODMR) spectroscopy is associated with a spin flip of either unpaired electron or hole spins. A plot of the luminescence intensity, induced by a magnetic resonance event at the excited state, versus strength of an external magnetic field, displays a spectrum that resembles a ground-state electron spin resonance spectrum. In a similar manner, ODMR also supplies information about the g-factor of the individual carriers, interaction with local surrounding and electron-hole exchange interaction. ODMR was Figure 5: (a) ODMR experimental (blue-curve) and simulated (orange-curve) spectra of used to detect trapped photo- CdTe CQDs capped by thiol ligands. Resonance I is related to a trapped hole and generated carriers at interfacial resonance II correspond to a band-edge electron. The inset in (a) displays the PL spectra (bold line), while the dotted lines show the best fit or ligand sites; a collection of corresponding components, when the lower energy one is associated with the trapped-hole to bandstudies has been summarized in edge recombination, as detected by the ODMR; (b) Schematic drawing of the hole trapping site, comprised of a metal vacancy, surrounded symmetrically by Te and S review publications.72,167 atoms. [Reprinted from reference 167, Copyright Year 2001, Publisher Israel Chemical 52
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A representative case is Society]. discussed here, 167-171 related to localization of a hole carrier at the surface-ligand interface. The example deals with the optical properties of CdTe CQDs capped with alkylthiols molecule. The corresponding ODMR spectrum is shown in Figure 5(a) by the blue line, while the orange curve is related to a simulated spectrum. The corresponding photoluminescence (PL) spectrum (see inset 60
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in Figure 5(a)) consists of a broad asymmetric band, having a satellite at the red-side of the excitonic transition, while the spectral dependence ODMR scan (not shown here) revealed the association of the magnetic resonance event solely with the satellite emission. The ODMR spectrum consists of two major bands, showing identical behavior upon changes in the experimental conditions (e.g., laser and microwave intensity, polarization) unveiling mutual correlation between them. The spectrum was simulated by using a conventional spin Hamiltonian (see 72), including electron and hole Zeeman interactions, electron-hole exchange interaction and hyperfine interactions. The simulation revealed an average g-factor of 1.6 and 1.4 for resonance ɪ and ɪɪ, respectively, with a slight anisotropy particularly around resonance ɪ, suggesting the existence of asymmetric surrounding. In comparison with previous evaluation of g-factor of bulk CdTe, resonance ɪ is related to a trapped hole (with effective spin of S=1/2) at a Cd +2 vacancy site, while resonance ɪɪ has a typical g-value of an electron at the conduction band. Thus, the satellite emission is related to a trap-to-band recombination. A suggested trapping site is shown schematically in Figure 5(b), displaying a vacancy tetrahedral site at a unit cell corner, with one edge occupied by the thiol function group, the S atom, hence inducing the asymmetric properties. It is worth noting that the thiol group offers three non-bonding electron pairs of the S-atom, one of which donated to an unsaturated orbit of the Cd atom; this reduction process might form Cd0 which is pruned to a diffusion away from the surface, while the remaining non-bonding pair attracts extra holes toward the exterior interface. The simulations supplied additional meaningful parameters; the electron-hole exchange interaction was found to be ~ 0.45 meV, substantially smaller than expected from an exciton, although, emphasizing localization of a hole at the interface and reduction of the electron-hole overlap. In addition, the best fit lines demanded adding an effective broadening, related to a hyperfine interaction, most probably with the alkyl ligands groups. This study suggests that surface defects may mediate the coupling to the ligand moiety. Actually, this suggestion was carefully examined in recent years, via fsec pump-probe measurements, 172 and the results were explained by the classical energy transfer Marcus model, 173 when a delocalized exciton state is transformed to a state in which one charge carrier is localized on the CQDsurface. 50
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Figure 6: The time evolution of emission peak energy (black
Frequent jumps of photo-generated carriers to line) and of the corresponding full-width-half-maximum surface or ligand positions dictates blinking and (FWHM) of a single CdSe/ZnS CQD; (b) A plot of the measured FWHM (black dots) versus the spectral redshift (the red line is jittering behavior: Two randomly occurring a theoretical fit according to a model discussed in reference phenomena, blinking and spectral diffusion 150 [Reprinted from reference 150, Copyright Year, 2013, Publisher APS Physics] (jittering), affect the photo-physics of CQDs, and both are most likely related to the existence of excess charge. The blinking appears as frequent 57
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switching between emitting to non-emitting stages, and can be suppressed by a change in confinement potential (as previously discussed). The jittering is pronounced as accidental spectral diffusion, followed by a quantum Stark effect generated by a diffusing charge in the vicinity of the CQD-surface. An important recent report proposed that the full-width-half-maximum (FWHM) of CQDs emission-band is controlled by intrinsic (phonon replica, lifetime) and extrinsic (size distribution) effect, as well as by a dominant jittering effect of the individual CQDs.137 A complementary study150 experimentally showed the correlation between FWHM and spectral diffusion energy (ΔE) of a single CdSe/ZnS CQD (see Figure 6). The study included a comparison to a theoretical model, including derivation of a mean distance between the center of the dot and the diffusing charge. It led to the conclusion that a charge is positioned way beyond the inorganic crystalline frame, but coincides with the outer boundary of the ligand layer, strongly suggesting that diffusing charge is located at a ligand site. 18
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Surface states and Ligands are controlling the gain efficiency: The relaxation dynamics of hot exactions is a primary event restraining key processes in optical gain, hot carriers extraction and multiple exciton recombination. Over the years, numerous efforts have been made, in which carriers were either injected or removed via surface treatment, 57,67 electrochemistry148 or optical pumping.28 Although this topic has already been discussed above, noteworthy example show the influence surface nature and ligands' properties on the gain lifetime and efficiency.55,76 Figure 7 represents a few schemes of the possible relaxations prior to a single or biexciton emission. Panel 7(a) displays an immediate initial stage (continuum) occupied by carriers after optical pumping, that relax into the 1Pe state (labeled P). The 1Pe-1Se intra-band relaxation was discussed in length before, populate the absorbing (SAX) single exciton (X) state, when the latter decay into corresponding emitting (SEX) states. SE is spaced below SA by the Stokes shift (δx). Fast cooling of hot carriers enables the formation of threelevel inter-band lasing system, whereas slow cooling enables intra-band lasing. 4
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Panel 7(b) displays the and absorbing and emitting states of single- (X) and bi(XX) excitons. ΔXA and ΔxxE refers to the Figure 7: Schematic illustration of hot carrier cooling processes in CQDs: (a) binding energies of the absorbing and intra-band relaxation from remote states into the absorbing (SA) and emitting states respectively and δxx is emitting (SE) single-exciton states; (b) Schematic illustration of single- (X) and biexciton (XX) absorbing and emitting states, and binding energies and the XX Stokes shift.174 However, Stokes shifts are marked on the panel (the labels are given in the text); (c) depopulation of P and SxE/SxxE might Direct depopulation of hot states into surface trapping states; (d) Multiple exciton generation (MEG), that may occur when the process is faster than takes place directly into surface, ligand the other alternatives shown in (a) to (c). [(a), (c) and (d) are reprinted from or surface-ligand bonding (see Panel reference 55 Copyright Year 2011, (b) from reference 174, Copyright Year 2012, Publisher ACS] 7(c)). Kambhampati group has showed that surface-mediated processes can immediately affect the gain lifetime in CQDs. 55,175 60
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Depopulation of hot states, can also follow a hot exciton fission for the formation of multiple excitons, MX (see Panel 7(d)), but might complete with carriers' trapping at surface or ligand sites, a concern for a design of photovoltaic cells. 7
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Ligand vibrational mode side-bands to an excitonic emission of individual CdTe/CdTexSe1-x CQD: Recent studies have indicated the significance of core/shell CQDs including alloying composition at the core-shell interface, e.g., CdTe/CdTexSe1-x. These dots inherently promote suppression of an Auger process, when boundary smoothing induces elimination of high frequency contributions to a core level envelope function (e.g., 1Se) and thus, blocks charge ejection from a core-to-remote state.158,159 Indeed, recent work, using cryogenic micro-photoluminescence (µ-PL) spectroscopy of individual CdTe/CdTexSe1-x CQDs, showed pronounced appearance of single- and multiple-exciton recombination emission with exceptional spectral stability, when measured under continuous-wave excitation with a variable power density.21 Carriers' radial 17
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29 Figure 8: (a) μ-PL spectra of individual CdTe/CdTexSe1-x CQDs, recorded at various excitation power. The spectra are comprised of single exciton (red band) and a few multiple excitons, as shows schematically at the insets; (c) Representative μ-PL focused on the single-exciton zero phonon line and its side-bands, some of which are recognized as acoustic phonons, while others are unknown at the current moment and plausibly are
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distribution functions of the discussed CQDs were calculated using effective mass approximation, 21 showing small penetration of both electrons and holes over the entire core-shell structure (not shown here). The µ-PL spectra were recorded by spreading CQDs over a glass substrate with a density
