Unusual Spectral Diffusion of Single CuInS2 Quantum Dots Sheds Light on the Mechanism of Radiative Decay.

The luminescence of CuInS2 quantum dots (QDs) is slower and spectrally broader than that of many other types of QDs. The origin of this anomalous behavior is still under debate. Single-QD experiments could help settle this debate, but studies by different groups have yielded conflicting results. Here, we study the photophysics of single core-only CuInS2 and core/shell CuInS2/CdS QDs. Both types of single QDs exhibit broad PL spectra with fluctuating peak position and single-exponential photoluminescence decay with a slow but fluctuating lifetime. Spectral diffusion of CuInS2-based QDs is qualitatively and quantitatively different from CdSe-based QDs. The differences reflect the dipole moment of the CuInS2 excited state and hole localization on a preferred site in the QD. Our results unravel the highly dynamic photophysics of CuInS2 QDs and highlight the power of the analysis of single-QD property fluctuations.

Probing nearby molecular vibrations with lanthanide-doped nanocrystals.

The photoluminescence (PL) of lanthanide-doped nanocrystals can be quenched by energy transfer to vibrations of molecules located within a few nanometers from the dopants. Such short-range electronic-to-vibrational energy transfer (EVET) is often undesired as it reduces the photoluminescence efficiency. On the other hand, EVET may be exploited to extract information about molecular vibrations in the local environment of the nanocrystals. Here, we investigate the influence of solvent and gas environments on the PL properties of NaYF4:Er3+,Yb3+ upconversion nanocrystals. We relate changes in the PL spectrum and excited-state lifetimes in different solvents and their deuterated analogues to quenching of specific lanthanide levels by EVET to molecular vibrations. Similar but weaker changes are induced when we expose a film of nanocrystals to a gas environment with different amounts of H2O or D2O vapor. Quenching of green- and red-emitting levels of Er3+ can be explained in terms of EVET-mediated quenching that involves molecular vibrations with energies resonant with the gap between the energy levels of the lanthanide. Quenching of the near-infrared-emitting level is more complex and may involve EVET to combination-vibrations or defect-mediated quenching. EVET-mediated quenching holds promise as a mechanism to probe the local chemical environment-both for nanocrystals dispersed in a liquid and for nanocrystals exposed to gaseous molecules that adsorb onto the nanocrystal surface.

Finite-Size Effects on Energy Transfer between Dopants in Nanocrystals.

Many phosphor materials rely on energy transfer (ET) between optically active dopant ions. Typically, a donor species absorbs light of one color and transfers the energy to an acceptor species that emits light of a different color. For many applications, it is beneficial, or even crucial, that the phosphor is of nanocrystalline nature. Much unlike the widely recognized finite-size effects on the optical properties of quantum dots, the behavior of optically active ions is generally assumed to be independent of the size or shape of the optically inactive host material. Here, we demonstrate that ET between optically active dopants is also impacted by finite-size effects: Donor ions close to the surface of a nanocrystal (NC) are likely to have fewer acceptors in proximity compared to donors in a bulk-like coordination. As such, the rate and efficiency of ET in nanocrystalline phosphors are low in comparison to that of their bulk counterparts. Surprisingly, these undesired finite-size effects should be considered already for NCs with diameters as large as 12 nm. If we suppress radiative decay of the donor by embedding the NCs in media with low refractive indices, we can compensate for finite-size effects on the ET rate. Experimentally, we demonstrate these finite-size effects and how to compensate for them in YPO4 NCs co-doped with Tb3+ and Yb3+.

Slow Hole Localization and Fast Electron Cooling in Cu-Doped InP/ZnSe Quantum Dots.

Impurity doping of low-dimensional semiconductors is an interesting route towards achieving control over carrier dynamics and energetics, e.g., to improve hot carrier extraction, or to obtain strongly Stokes shifted luminescence. Such studies remain, however, underexplored for the emerging family of III-V colloidal quantum dots (QDs). Here, we show through a detailed global analysis of multiresonant pump-probe spectroscopy that electron cooling in copper-doped InP quantum dot (QDs) proceeds on subpicosecond time scales. Conversely, hole localization on Cu dopants is remarkably slow (1.8 ps), yet still leads to very efficient subgap emission. Due to this slow hole localization, common Auger assisted pathways in electron cooling cannot be blocked by Cu doping III-V systems, in contrast with the case of II-VI QDs. Finally, we argue that the structural relaxation around the Cu dopants, estimated to impart a reorganization energy of 220 meV, most likely proceeds simultaneously with the localization itself leading to efficient luminescence.

Trapping and Detrapping in Colloidal Perovskite Nanoplatelets: Elucidation and Prevention of Nonradiative Processes through Chemical Treatment.

Metal-halide perovskite nanocrystals show promise as the future active material in photovoltaics, lighting, and other optoelectronic applications. The appeal of these materials is largely due to the robustness of the optoelectronic properties to structural defects. The photoluminescence quantum yield (PLQY) of most types of perovskite nanocrystals is nevertheless below unity, evidencing the existence of nonradiative charge-carrier decay channels. In this work, we experimentally elucidate the nonradiative pathways in CsPbBr3 nanoplatelets, before and after chemical treatment with PbBr2 that improves the PLQY. A combination of picosecond streak camera and nanosecond time-correlated single-photon counting measurements is used to probe the excited-state dynamics over 6 orders of magnitude in time. We find that up to 40% of the nanoplatelets from a synthesis batch are entirely nonfluorescent and cannot be turned fluorescent through chemical treatment. The other nanoplatelets show fluorescence, but charge-carrier trapping leads to losses that are prevented by chemical treatment. Interestingly, even without chemical treatment, some losses due to trapping are mitigated because trapped carriers spontaneously detrap on nanosecond-to-microsecond timescales. Our analysis shows that multiple nonradiative pathways are active in perovskite nanoplatelets, which are affected differently by chemical treatment with PbBr2. More generally, our work highlights that in-depth studies using a combination of techniques are necessary to understand nonradiative pathways in fluorescent nanocrystals. Such understanding is essential to optimize synthesis and treatment procedures.

Optoelectronic Properties of Ternary I-III-VI2 Semiconductor Nanocrystals: Bright Prospects with Elusive Origins.

Colloidal nanocrystals of ternary I-III-VI2 semiconductors are emerging as promising alternatives to Cd- and Pb-chalcogenide nanocrystals because of their inherently lower toxicity, while still offering widely tunable photoluminescence. These properties make them promising materials for a variety of applications. However, the realization of their full potential has been hindered by both their underdeveloped synthesis and the poor understanding of their optoelectronic properties, whose origins are still under intense debate. In this Perspective, we provide novel insights on the latter aspect by critically discussing the accumulated body of knowledge on I-III-VI2 nanocrystals. From our analysis, we conclude that the luminescence in these nanomaterials most likely originates from the radiative recombination of a delocalized conduction band electron with a hole localized at the group-I cation, which results in broad bandwidths, large Stokes shifts, and long exciton lifetimes. Finally, we highlight the remaining open questions and propose experiments to address them.

One Precursor but Two Types of Graphene Nanoribbons: On-Surface Transformations of 10,10'-Dichloro-9,9'-bianthryl on Ag(111).

On-surface synthesis has emerged in the last decade as a method to create graphene nanoribbons (GNRs) with atomic precision. The underlying premise of this bottom-up strategy is that precursor molecules undergo a well-defined sequence of inter- and intramolecular reactions, leading to the formation of a single product. As such, the structure of the GNR is encoded in the precursors. However, recent examples have shown that not only the molecule, but also the coinage metal surface on which the reaction takes place, plays a decisive role in dictating the nanoribbon structure. In this work, we use scanning probe microscopy and X-ray photoelectron spectroscopy to investigate the behavior of 10,10'-dichloro-9,9'-bianthryl (DCBA) on Ag(111). Our study shows that Ag(111) can induce the formation of both seven-atom wide armchair GNRs (7-acGNRs) and 3,1-chiral GNRs (3,1-cGNRs), demonstrating that a single molecule on a single surface can react to different nanoribbon products. We additionally show that coadsorbed dibromoperylene can promote surface-assisted dehydrogenative coupling in DCBA, leading to the exclusive formation of 3,1-cGNRs.

Mapping the Conductance of Electronically Decoupled Graphene Nanoribbons.

With the advent of atomically precise synthesis and consequent precise tailoring of their electronic properties, graphene nanoribbons (GNRs) have emerged as promising building blocks for nanoelectronics. Before being applied as such, it is imperative that their charge transport properties are investigated. Recently, formation of a molecular junction through the controlled attachment of nanoribbons to the probe of a scanning tunneling microscope (STM) and subsequent lifting allowed for the first conductance measurements. Drawbacks are the perturbation of the intrinsic electronic properties through interaction with the metal surface, as well as the risk of current-induced defect formation which largely restricts the measurements to low bias voltages. Here, we show that resonant transport-essential for device applications-can be measured by lifting electronically decoupled GNRs from an ultrathin layer of NaCl. By varying the applied voltage and tip-sample distance, we can probe resonant transport through frontier orbitals and its dependence on junction length. This technique is used for two distinct types of GNRs: the 7 atom wide armchair GNR and the 3,1-chiral GNR. The features in the conductance maps can be understood and modeled in terms of the intrinsic electronic properties of the ribbons as well as capacitive coupling to tip and substrate. We demonstrate that we can simultaneously measure the current decay with increasing junction length and bias voltage by using a double modulation spectroscopy technique. The strategy described in this work is widely applicable and will lead to a better understanding of electronic transport through molecular junctions in general.