Surface Passivation Mechanism Research Articles

Density functional theory study on vanadium selective leaching separation from iron in stone coal via Fe2O3 (001) surface passivation

When vanadium is extracted from stone coal, iron impurities readily precipitate and reduce the purity of the vanadium product. Thus, separating vanadium from iron impurities at source is essential. The mechanism of surface passivation of Fe2O3 (001) for the selective leaching separation of vanadium from iron in stone coal was established in the present work. Analysis of mineral phase transformation and microscopic morphology indicates that Fe2O3 in stone coal can undergo a reaction with HF and F-. Subsequently, the hematite surface was covered with FeF3 passivation layer, resulting in the inhibition of hematite dissolving reaction. The results of density functional theory (DFT) computations demonstrate that the HF and F- can reform the Fe2O3 (001) surface via sedimentary behavior at mineral interface, and the adsorption energy can be down to -99.93kJ/mol for the F- adsorption configuration. During sulfuric acid leaching with CaF2 as an aiding agent, the transfer of electrons from Fe atoms to F atoms can weaken the stable Fe-F bonds and decrease the reactive activation of Fe atoms in Fe2O3. This study demonstrates the separation of vanadium from iron by means of hematite passivation in the attendance of fluoride at the atomic level of analysis.

Utilization of Flotation Wastewater for Separation of Chalcopyrite and Molybdenite by Selective Surface Passivation

In the flotation separation process of a Cu-Mo-W polymetallic ore, the wastewater from the scheelite cleaning flowsheet contains large numbers of residual flocculants and metal ions, and the separation of chalcopyrite and molybdenite requires a large number of environmentally harmful depressants. Therefore, it is necessary to find new methods to reduce the environmental and cost pressures of wastewater treatment and the use of depressants. In this work, the flotation wastewater from the scheelite cleaning flowsheet for the separation of chalcopyrite and molybdenite by selective surface passivation was investigated for the first time. Flotations of single minerals and artificially mixed minerals with or without immersion pretreatment in the presence and absence of aeration were performed. The results showed that pulp pH had no effect on the flotation of either mineral, and a molybdenite recovery of 93.22% with a chalcopyrite recovery of 10.77% was achieved under the conditions of 10 days of immersion pretreatment with aeration, 350 mg/L of kerosene, and 100 mg/L of MIBC. By combining the electrochemical cyclic voltammetry analysis and characterization by XRD and SEM, the selective surface passivation mechanism of chalcopyrite was discussed, which could be due to the coverage of the insoluble oxidation products, especially jarosite. This work has simultaneously achieved the depressant-free flotation separation of molybdenite and chalcopyrite and the reuse of scheelite flotation wastewater, which is of great significance for environmental protection and cost saving.

Inhibition photooxidation of pyrite under illumination via altering photogenerated carrier migration pathways: Role of DTC-TETA surface passivation

The oxidation of pyrite is the main cause of acidic mine drainage (AMD), which is a very serious environmental problem in numerous mining areas around the world. Previous studies have shown that passivation agents create a hydrophobic film on the surface of pyrite, effectively isolating oxygen and water. However, the presence of abundant sulfide minerals in tailings ponds may exacerbate AMD when exposed to solar radiation, due to the semiconductor properties of pyrite. It remains uncertain whether the current surface passivation coating can effectively prevent the oxidation of pyrite under light conditions. This paper is the first to investigate the passivation effect as well as the mechanism of surface passivation coating on pyrite under illumination from the perspective of materials science. The results demonstrated that the triethylenetetramine-bisdithiocarbamate (DTC-TETA) passivation coating on pyrite almost completely suppressed the photooxidation of pyrite under illumination by changing the migration path of photogenerated charge carriers. The formation of NC(S)2-Fe chelating groups provides atomic-level interface channels for DTC-TETA to transfer electrons to pyrite and creates a favorable reduction environment for pyrite. Besides, DTC-TETA coating greatly improves the electron-hole pairs recombination efficiency of pyrite, which significantly inhibits the photogenerated electron reduction of oxygen to generate reactive oxygen species (ROS). Moreover, DTC-TETA coating captures the photogenerated holes, avoiding direct oxidation of pyrite by holes. Density functional theory (DFT) calculations revealed that the DTC-TETA coating increases the adsorption energy barrier for oxygen and water. The results extend the existing knowledge on passivation mechanisms on pyrite and hold significant implications for the future screening, evaluation, and practical application of surface passivating agents.

Photocarrier transport reconstruction and dramatical performance enhancement in ultrawide-bandgap ε-Ga2O3 photodetectors via surface defect passivation

The semiconductor surface is a major platform dictating carrier transport and recombination, in particular for planar photoconductive devices such as metal-semiconductor-metal (MSM) photodetectors. Here we demonstrate the integration of an ultrathin Al2O3 capping layer to effectively passivate the surface defects of epitaxial ε-Ga2O3 films and substantially enhance the photodetector performance. Incorporating an Al2O3 layer is found to significantly reduce the dark current of ε-Ga2O3 MSM photodetectors by more than three orders of magnitude, and simultaneously enhance the photocurrent and the response speed. With an ultrathin Al2O3 surface layer, the Al2O3/ε-Ga2O3 MSM photodetector achieves a superhigh responsivity of >104 A/W, a detectivity of >1016 Jones with a photo-to-dark ratio of >107, an UV–vis rejection ratio of >104 and millisecond response time for continuous-wave illumination. The surface passivation mechanism is analyzed by combined X-ray photoemission spectroscopy, secondary ion mass-spectroscopy and numerical calculations. It is revealed that the Al2O3 capping layer passivates the surface defects that are in the form of reduced tin species and oxygen vacancies, and reconstructs a faster surface transport channel for more efficient photocarrier collection. This study expands the design concept of surface passivation with ultrawide bandgap semiconductors for developing efficient deep ultraviolet photodetectors.

Evaluating Contributions of Pitch-Carbon Coating to Improved Stability of Si Anodes through Voltage-Resolved Multi-Phase Characterization

Silicon nanoparticles have emerged as a promising alternative to graphite to improve the energy density of next-generation lithium-ion battery anodes. Nano-sized Si domains facilitate rapid ion transport and minimize particle-scale mechanical degradation, but also exhibit increased (electro)chemical reactivity with Li-ion electrolyte components due to their high surface area. We have previously demonstrated that surface modification of Si nanoparticles with pitch-carbon is an effective strategy to reduce these parasitic reactions. In the present work, we holistically evaluate the mechanistic contribution of pitch-carbon coating to the observed stability improvement over uncoated Si. We utilize coupled in situ and ex situ methods to probe changes to solid-surface, volatile headspace, and gas-phase chemistry occurring during initial cycling. Measurements taken at targeted potentials associated with electrolyte species reduction enables the decoupling of specific reaction pathways tied to interfacial stability. Further, we demonstrate the non-trivial role of gas reconsumption in dictating the nature of the passivating surface layer evolved on both uncoated and pitch-coated Si. This multi-phase analysis offers insights into the mechanism of effective surface passivation, which may be applied to inform future Si material development. Figure 1

Dual-emission carbonized polymer dots for ratiometric sensing and imaging of l-lysine and pH in live cell and zebrafish

It is highly desired to accurately and selectively detect and image intracellular l-lysine and pH in biological systems because they could act as the biomarkers in certain abnormal conditions and may give us a warning of the occurrence of diseases. It has been attracted more focuses to design new ratiometric fluorescent probe for monitoring l-lysine and pH to improve detection accuracy. Carbonized polymer dots (CPDs), which possess carbon/polymer hybrid structure rather than pure carbon structure and constitute of a carbon core and large amounts of functional groups/polymer chains on the surface, rise up as a new type of fluorescent nanomaterials and especially display many advantages for bioanalysis. In this study, o-phenylenediamine (o-PD) and poly(styrene-co-maleic anhydride) (PSMA) are used as the precursors to synthesize the desired CPDs through one-step hydrothermal amide method. The prepared CPDs display two well-resolved fluorescence emission bands, i.e., a very weak emission centered at 470 nm in blue region and a strong emission centered at 558 nm in yellow region. It is found that the two emissions are both responsive to l-lysine based on the surface passivation mechanism, whereas, only the yellow emission is responsive to pH due to the protonation/deprotonation process of the amino groups. Based on the different responsive behaviors, ratiometric detection and imaging of l-lysine and pH are achieved. The prepared ratiometric CPDs probe is successfully applied for l-lysine and pH sensing and imaging at two emission channels in live cell and zebrafish with satisfactory results.

The synergistic mechanism between transfer layer and surface passivation of diamond-like carbon film under different gas pressure environments

The low friction behavior and mechanism of diamond-like carbon (DLC) film are studied under different gas pressure environments. The results present that DLC film against Al2O3 and steel balls shows a relatively stable friction coefficient in the air environment, but it shows an unstable and relatively low friction coefficient under gas pressure of 5 Pa. Under gas pressure of 10−3 Pa, the DLC film fails rapidly although it achieves a short superlubricity state. With the decrease of gas pressure, the gas passivation effect on the surface of DLC film will be weaker and weaker while the formation degree of transfer layer will be more and more continuous and dense. The first-principles calculations imply that interfacial adhesion has a significant influence on the formation of tribofilms and surface wear. Further analysis shows that the low friction behavior and mechanism of DLC films in air, low vacuum (5 Pa) and high vacuum (10−3 Pa) environment are not controlled by graphitization mechanism, but are caused by the synergistic action of transfer layer mechanism and surface passivation mechanism.

Excitonic characteristics of blue-emitting quantum dot materials in group II-VI using hybrid time-dependent density functional theory

Colloidal quantum dots (QDs) of group II-VI are key ingredients of next-generation QD light-emitting diodes technology for display and lighting, yet the understanding of their luminescent characteristics are far from being mature. Using a hybrid time-dependent density functional theory, we have studied the electronic and excitonic properties of blue-emitting colloidal QDs within group II-VI containing a thousand atoms or more, including CdSe, CdS, ZnSe, and ZnS QDs, considering both quantum confinement and surface ligand effects. It is found that the calculated optical gaps are in excellent quantitative agreement with experiment, irrespective of the QD nature. Scaling laws of size-dependent energy gaps governed solely by quantum confinement effects have further been explored at both single-particle level and correlated excitonic level for all QDs. With concurrently stoichiometric control and enhancing quantum confinement effects, we have predicted an unusual switching of symmetry character of the highest occupied molecular orbital state from a ${\mathrm{\ensuremath{\Gamma}}}_{3}$ to a ${\mathrm{\ensuremath{\Gamma}}}_{1}$ symmetry at ultrasmall size ($\ensuremath{\sim}1$ nm) for all QDs. After the switching, pronounced linearly polarized band-edge excitonic emission is activated. The radiative exciton decay lifetime is found to increase monotonically with increasing the QD size and tends to saturate at larger sizes. Finally, we have explored the surface passivation mechanism of inorganic chloride ligand, and identified various favorable Cd-Cl bonding configurations which enable an effective surface passivation resembling the commonly applied pseudohydrogen passivation scheme. We find that chloride ligand serves as a hole delocalization ligand and tends to redshift the absorption spectra, reduce the absorption intensity, and significantly enhance the exciton decay lifetime. Our results provide a guideline for spectroscopic studies of excitonic characteristics of colloidal QDs within group II-VI.

Selective passivation behavior of galena surface by sulfuric acid and a novel flotation separation method for copper-lead sulfide ore without collector and inhibitor

Here, we present a novel efficient passivation-separation method for galena and chalcopyrite that includes a novel sulfuric acid selective passivation process for galena and a special flotation process in which conventional inhibitors and collectors are not added. The surface passivation mechanism of galena was characterized by surface contact angle measurements, scanning electron and dispersive spectrometry (SEM-EDS) analysis and X-ray photoelectron spectroscopy (XPS) analysis. The contact angle measurements indicated that sulfuric acid passivation increased the surface hydrophobicity difference between galena and chalcopyrite, which led to the large difference in their floatabilities. The results of the passivation-flotation experiments for the galena-chalcopyrite mixture confirmed that the selective inhibition of galena was realized with this new process, and the floatability of galena was drastically reduced while that of chalcopyrite was slightly promoted; thus, the efficient separation of galena and chalcopyrite was achieved only with the addition of frothers. The SEM-EDS and XPS results revealed that the S element on the galena surface was gradually oxidized from S2− to SO42− during the passivation process, which generated a layer of hydrophilic lead sulfate (PbSO4), while a layer of floatable copper sulfides was formed on the surface of chalcopyrite. In this study, a new highly efficient copper-lead sulfide ore separation method was developed, and the interrelatedmechanism was studied in detail.

Spectroelectrochemical and Chemical Evidence of Surface Passivation at Zinc Ferrite (ZnFe2O4) Photoanodes for Solar Water Oxidation

AbstractRecent advances in low‐cost manufacturing as well as in bulk/interface engineering have positioned zinc ferrite (ZnFe2O4, ZFO) in the spotlight as a candidate material for solar water oxidation. However, the severe recombination at the reactive interface remains as the main source of the poor onset potential. Although catalytic overlayers have shown to override, at least partially, the surface recombination, passivating‐only coatings are barely explored despite holding the key to specifically suppress the recombination. Here, a sub‐nanometer Al2O3 layer is conformally deposited onto nanostructured ZFO, leading to a 100 mV shift in the onset potential reaching 0.80 V versus reversible hydrogen electrode (RHE) and a fourfold photocurrent increase at 1.0 V versus RHE. The passivation‐only effect of Al2O3 is confirmed by the slowing down of the surface recombination detected by intensity‐modulated photocurrent spectroscopy and by the transient photovoltage and photoluminescence experiments. Further characterization of the chemical states at the reactive interface reveals that the partial filling of the surface oxygen vacancies and the formation of a Zn2+–Al3+ Lewis adduct are potentially involved in the surface passivation. This study not only demonstrates that Al2O3 improves ZFO's onset potential but also sheds light on the up until now unknown surface passivation mechanism.

Evolution of the surface passivation mechanism during the fabrication of ex-situ doped poly-Si(B)/SiOx passivating contacts for high-efficiency c-Si solar cells

Passivating the contacts of crystalline silicon (c-Si) solar cells (SC) with a poly-crystalline silicon (poly-Si) layer on top of a thin silicon oxide (SiOx) is currently sparking interest for reducing carrier recombination at the interface between the metal electrode and the c-Si substrate. However, due to the interrelation between different mechanisms at play, a comprehensive understanding of the surface passivation provided by the poly-Si/SiOx contact in the final SC has not been achieved yet. In the present work, we report on an original ex-situ doping process of the poly-Si layer through the deposition of a B-rich dielectric layer followed by an annealing step to diffuse B dopants in the layer. We propose an in-depth investigation of the passivation scheme of the resulting B-doped poly-Si/SiOx contact by first comparing the surface passivation provided by ex-situ doped and intrinsic poly-Si/SiOx contacts at different steps of the fabrication process. The excellent surface passivation properties obtained with the ex-situ doped poly-Si(B) contact (iVoc = 733 mV and J0 = 6.1 fA cm−2) attests to the good quality of this contact. We then propose further STEM, ECV and ToF-SIMS characterizations to assess: i) the evolution of the microstructure and B-doping profile through ex-situ doping and ii) the diffusion profile of hydrogen in the poly-Si contact. Our results show a gradual filling of the poly-Si layer with active B dopants with increasing annealing temperature (Ta), which strengthens the field-effect passivation and enables an iVoc increase after annealing up to 800 °C. We also observe a diffusion of O from the SiON:B doping layer to the interfacial SiOx layer during annealing, that likely enhances the passivation stability of our ex-situ doped poly-Si contact with increasing Ta. Finally, we conclude that the mechanism dominating the surface passivation changes during the fabrication process of the poly-Si/SiOx contact from field-effect passivation after annealing (performed for B-diffusion in the contact) to chemical passivation after following hydrogenation of the samples (performed by depositing a H-rich silicon nitride layer).

Mechanisms of Surface Passivation during Galena Leaching by Hydrogen Peroxide in Acetate and Citrate Solutions at 25–50 °C

Organic solutions are promising lixiviants for fast and environmentally sustainable Pb extraction from ore minerals at low temperatures. However, engineering of novel leaching flowsheets has been h...

Mechanisms of GaAs surface passivation by a one-step dry process using low-frequency plasma enhanced chemical deposition of silicon nitride

III-V materials are known for their high surface states densities which degrade the performances of photonic and electronic devices. Plasma-enhanced chemical vapor deposition of SixNy is commonly used to passivate surface states and to encapsulate III-V devices and circuits. The plasma is generated with an RF excitation source usually set at a high frequency of 13.56 MHz. To obtain a high quality III-V/SixNy interface, many chemical surface treatments are required before deposition, as well as post-deposition thermal annealing. In this work, we demonstrate that a low-frequency plasma allows for an efficient passivation process by a simple deposition of SixNy, without any wet chemical pretreatment nor post-deposition annealing. This dry process is more compatible with the passivation and encapsulation of completed III-V devices and circuits. Metal-insulator-semiconductor capacitors were fabricated by depositing SixNy on GaAs at a frequency of 90 kHz. CV measurements indicated the unpinning of the Fermi level and a low density of surface states in the mid 1011 cm−2 eV−1 range. X-ray photoelectron spectra have shown the complete suppression of native oxides and an important reduction of elemental arsenic from the GaAs surface. The mechanisms of surface passivation by ionic hydrogen bombardment inherent to the low-frequency plasma deposition of SixNy are discussed.

A mechanism of metastable sulfur speciation and the adsorption on a gold surface in the presence of sulfidic ore and lead in cyanide medium

Metallic gold or gold minerals have high affinity towards aqueous sulfur species during gold leaching process in cyanide solutions, thus the gold surface can often become passivated by the reaction between the different sulfur species and gold. During the cyanide leaching of sulfidic gold ores, the sulfide minerals dissolve into the leach solution and produce metastable sulfur species, such as sulfide, bisulfide, polysulfide, elemental sulfur, and thiocyanate. Most of the above sulfur species can directly passivate the gold surface, and significantly reduce the gold leaching efficiency.This paper pertains to studying the sulfur species formation in cyanide solutions and their adsorption chemistry and mechanism on gold surface. It was found that the lead salts, often added to the cyanidation process to improve the gold leaching efficiency, act as a sulfide oxidant and accelerates its oxidation to different metastable sulfur species. It was shown by electrochemical linear sweep voltammetry (LSV) tests that the gold oxidation current density, i.e. oxidation kinetics, was decreased 32%, from 2.20 to 1.50 mA/cm2, in the presence of a high sulfide ore by increasing the amount of lead salt from 0 to 50 mg/L at 300 mg/L free cyanide (300 mg/L CN). Monosulfide, disulfide and polysulfide sulfur species were detected on the gold surface, by X-ray photoelectron spectroscopy (XPS) method, confirming the adsorption of different sulfur species on the gold surface. At high concentration of added lead, 50 mg/L Pb2+, even elemental sulfur was shown to adsorb on the gold surface. The formation of different sulfur species was explored by thermodynamic modeling, which showed very good agreement with the results of the XPS study. This study helps to develop a better insight on the mechanism of gold surface passivation caused by sulfur species and the findings help to reduce gold passivation by better control of the process condition.

A density functional theory study on the passivation mechanisms of hydrogenated Si/Al2O3 interfaces

Amorphous aluminum oxide (Al2O3) films are known to provide a high-quality passivation on silicon (Si) surfaces which can result in an enhanced efficiency of Si-based solar cells. After deposition of Al2O3 on Si, a certain temperature treatment is needed to activate the highest surface passivation quality. When the applied temperature is exceeded by a certain level, the passivation quality degrades. This behavior is well known in the production of Si-based solar cells. In order to further elucidate the microscopic origin of passivation mechanisms and its interplay with thermal treatments, we investigate four different atomistic Si/Al2O3 interface models by means of density functional theory simulations. As interfacial hydrogen (H) is deemed to play a key role in Si/Al2O3 surface passivation mechanisms and its amount changes during thermal treatments, two of these models contain hydrogen in different amounts; the other two do not contain any hydrogen. The simulations show that both chemical passivation and field-effect passivation depend on the relative amount of hydrogen via partially competing mechanisms. The obtained results provide novel insights into the passivation mechanisms of Si/Al2O3 interfaces. The results are qualitatively compared to the thermally induced activation and degradation of the Si(100)/Al2O3 surface passivation known from experiments.

Elucidating mechanisms of oxide growth and surface passivation on zinc thin film electrodes in alkaline solutions using the electrochemical quartz crystal microbalance

We utilized electrochemical quartz crystal microbalance to understand electrochemical passivation of the zinc electrode in alkaline media and subsequent oxide growth and stripping. The formation of native zinc oxide on a near pristine Zn surface upon contact with 1 M KOH electrolyte was observed. The surface was seen to roughen with time. During Zn dissolution, the measured mass-charge ratio corresponded to the removal of ZnO and not merely Zn. The mechanism of passivation was determined to be caused by electrochemical etching to form pores. The etching leads to a build-up of solid ZnO and Zn(OH)2 due to electrochemical reactions on the surfaces of the pores. These reactions functionally close the pores off from the bulk. The cycle of Zn deposition and dissolution develops a porous network, which may be the origin of mossy Zn deposits that plague reversible Zn metal batteries.

Study of Surface Passivation and Charge Transport Barriers in DASH Solar Cell

Dopant-free asymmetric heterocontact (DASH) solar cells have recently shown the potential for high efficiency with no toxic dopant gases. In this paper, we simulate both the hole [Ag/IOH/MoOx/a-Si:H(i)] and electron (a-Si:H(i)/TiOx/LiFx/Al) heterocontacts and study the surface passivation and charge transport mechanism using Sentaurus TCAD. Our simulation predicts that the intrinsic amorphous silicon layer [a-Si:H(i)] is mainly required to improve the field-effect passivation at the hole contact (>100 mV increase in V oc) and to improve the chemical passivation at the electron contact. However, it comes at the cost of degradation in fill factor (FF). To overcome the FF loss, high work function material (MoOx WF ∼ > 5.5 eV) is required at the hole contact and low work function material (Al/LiFx WF ∼ V oc - iV oc) in addition to FF loss, whereas at the electron contact, affecting only the FF unless there is a strong chemical passivation loss. This is attributed to the strong hole blocking nature of the TiOx film. This feature is used to predict the nature of the internal transport barriers by combining the terminal characteristics, surface photovoltage, and selectivity loss, which we believe will be helpful for further progress in this arena.

Mechanism of surface passivation of methylammonium lead tribromide single crystals by benzylamine

Hybrid organic-inorganic perovskites are semiconductors that have great potential for optoelectronic applications such as light-emitting diodes, photodetectors, and solar cells. In such devices, the surface plays a crucial role in the performance and stability, as it strongly influences the recombination rate of excited charge carriers. It is reported that molecular ligands such as benzylamine are capable of reducing the surface trap state density in thin films. In this work, we aim to clarify the mechanisms that govern the surface passivation of hybrid perovskites by benzylamine. We developed a versatile approach to investigate the influence of benzylamine passivation on the well-defined surface of freshly cleaved hybrid perovskite crystals. We show that benzylamine permanently passivates surface trap states in these single crystals, resulting in enhanced photoluminescence and charge carrier lifetimes. Additionally, we show that exposure to benzylamine leads to the replacement of the methylammonium cations by benzylammonium, thereby creating a thermodynamically more stable two-dimensional (2D) perovskite (BA)2PbBr4 on the surface of the three-dimensional crystal. This conversion to a 2D perovskite drives an anisotropic etching of the crystal surface, with the {100} planes being most prone to etching. Initially, square etching pits appear spread over the surface. As time elapses, these etching pits broaden and merge to yield large flat terraces that are oriented normally to the cleaving plane when they form. A thorough understanding of the mechanisms governing the surface passivation is crucially important to optimize and design novel passivation schemes, with the ultimate goal of further advancing the efficiency of optoelectronic devices based on hybrid perovskites.

Enhancing the efficiency of CsPbX3 (X = Cl, Br, I) nanocrystals via simultaneous surface peeling and surface passivation.

Inorganic CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (PNCs) are promising materials for next-generation optoelectronic applications due to their tunable emission and high color purity. However, there is still room to improve their photoluminescence quantum yields (PLQYs) in order to promote their applications. Herein, the PLQY of blue light emitting CsPb(Cl/Br)3 PNCs was increased to 83% with ammonium hexafluorophosphate by choosing an appropriate treatment time. The salt peeled off the outermost surface of PNCs with halide vacancies and then passivated the surface. This method is effective at improving the PLQYs of different CsPbX3 (X = Cl, Br, I) PNCs covering the entire visible spectrum; the PLQYs were improved to 25% for CsPbCl3 at 398 nm, 83% for CsPb(Cl/Br)3 at 448 nm, 96% for CsPbBr3 at 504 nm, 86% for CsPb(Br/I)3 at 568 nm, and 98% for CsPbI3 at 687 nm.

Design Principles for Trap-Free CsPbX3 Nanocrystals: Enumerating and Eliminating Surface Halide Vacancies with Softer Lewis Bases.

We introduce a general surface passivation mechanism for cesium lead halide perovskite materials (CsPbX3, X = Cl, Br, I) that is supported by a combined experimental and theoretical study of the nanocrystal surface chemistry. A variety of spectroscopic methods are employed together with ab initio calculations to identify surface halide vacancies as the predominant source of charge trapping. The number of surface traps per nanocrystal is quantified by 1H NMR spectroscopy, and that number is consistent with a simple trapping model in which surface halide vacancies create deleterious under-coordinated lead atoms. These halide vacancies exhibit trapping behavior that differs among CsPbCl3, CsPbBr3, and CsPbI3. Ab initio calculations suggest that introduction of anionic X-type ligands can produce trap-free band gaps by altering the energetics of lead-based defect levels. General rules for selecting effective passivating ligand pairs are introduced by considering established principles of coordination chemistry. Introducing softer, anionic, X-type Lewis bases that target under-coordinated lead atoms results in absolute quantum yields approaching unity and monoexponential luminescence decay kinetics, thereby indicating full trap passivation. This work provides a systematic framework for preparing highly luminescent CsPbX3 nanocrystals with variable compositions and dimensionalities, thereby improving the fundamental understanding of these materials and informing future synthetic and post-synthetic efforts toward trap-free CsPbX3 nanocrystals.