High Density Of Surface Defects Research Articles

Suppressed Thermal Quenching via Tetrafluoroborate-Induced Surface Reconstruction of CsPbBr3 Nanocrystals for Efficient Perovskite Light-Emitting Diodes.

Although metal-halide perovskite nanocrystals (NCs) have garnered significant attention for optoelectronic applications, the presence of electrically insulating organic ligands in CsPbBr3 NCs hinders efficient charge injection and transportation in light-emitting diodes (LEDs). A common approach to address this issue involves ligand exchange with shorter ligands and precise control of the surface ligand density through additional purification steps. Nevertheless, the practical application of these methods has been hindered by their poor structural integrity and high surface-defect density, which remain a challenge. Our investigation reveals that NOBF4 treatment effectively replaces native ligands with BF4- anions, in which BF4- anions are readily coordinated with the positively charged CsPbBr3 surface metal centers, thereby improving the photoluminescence quantum yield (PLQY) and thermal stability. In particular, the presence of BF4- anions coordinated at CsPbBr3 surfaces efficiently suppresses the pathway of excitons toward thermally activated nonradiative recombination, leading to minimal thermal quenching and superior device performance in green-emitting PeLEDs. Notably, PeLEDs based on CsPbBr3 NCs with the reconstructed surface via NOBF4 treatment exhibit an improved current efficiency of 31.12 cd/A and an external quantum efficiency of 11.24%, increased by 2.8 times compared to that of the pristine sample, indicating the enhanced hole-electron injection and transport into the CsPbBr3 NCs. Therefore, our results highlight the potential of NOBF4 as a versatile reagent for the ligand exchange and surface passivation of CsPbBr3 NCs, thereby offering promising prospects for the development of stable, high-performance PeLEDs.

Recent Progress of Thin Crystal Engineering for Perovskite Solar Cells.

Metal halide perovskite single crystals hold promise for photovoltaics with high efficiency and stability due to their superior optoelectronic properties and weak bulk ion migration. The past several years have witnessed rapid development of single-crystal perovskite solar cells (PSCs) with efficiency rocketed from 6.5% to 24.3%, however, which still lags behind their polycrystalline counterparts. Moreover, the poor device stability under light illumination is contrary to the high ion migration barrier of perovskite single crystals. The key limiting factors should be the low crystalline quality and high surface defect density of solution-grown thin single crystals. Under this circumstance, a review paper summarizing the recent progress and challenges will be instructive for future development of this emerging field. In this manuscript, the crystal engineering used to enhance carrier transport and suppress carrier recombination in vertical single-crystal PSCs will be summarized initially, including crystal growth, component control, surface and interface modification. Subsequently, the application of perovskite single crystals in lateral single-crystal PSCs will be discussed and compared with the conventionally vertical structure. Finally, the challenges and proposed strategies for the development of single-crystal PSCs are provided.

Surface Engineering of Perovskite Single Crystals by Atomic Layer Deposited Tin Oxide for Optical Communication.

Perovskite single crystals with excellent physical properties have broad prospects in the field of optoelectronics. However, the presence of dangling bonds, surface dislocations, and chemical impurities results in high surface defect density and sensitivity to humidity. Unfortunately, there are relatively few surface engineering strategies for single perovskite single crystals. We present a strategy utilizing atomic layer deposited SnOx to passivate surface defects in perovskite single crystals. The photodetector prepared based on the modified FAPbBr3 single crystals exhibits a low dark current of 1.89 × 10-9 A at a 5 V bias, close to 4 times lower with respect to the pristine device, a high detectivity of 2.3 × 1010 jones, and a fast response time of 27 μs. Moreover, the photodetectors feature long-term operational stability because the presence of a dense SnOx capping layer hinders the ingress of moisture and diffusion of ions. We further demonstrate the promise of our perovskite single crystal detectors for real-time subaqueous optical communication.

Hematite (α-Fe2O3) with Oxygen Defects: The Effect of Heating Rate for Photocatalytic Performance.

Hematite (α-Fe2O3) emerges as an enticing material for visible-light-driven photocatalysis owing to its remarkable stability, low toxicity, and abundance. However, its inherent shortcomings, such as a short hole diffusion length and high recombination rate, hinder its practical application. Recently, oxygen vacancies (Vo) within hematite have been demonstrated to modulate its photocatalytic attributes. The effects of Vo can be broadly categorized into two opposing aspects: (1) acting as electron donors, enhancing carrier conductivity, and improving photocatalytic performance and (2) acting as surface carrier traps, accelerating excited carrier recombination, and deteriorating performance. Critically, the generation rate, distribution, role, and behavior of Vo significantly differ for synthesis methods due to differences in formation mechanisms and oxygen diffusion. This complexity hampers simplified discussions of Vo, necessitating careful investigation and nuanced discussion tailored to the specific method and conditions employed. Among various approaches, hydrothermal synthesis offers a simple and cost-effective route. Here, we demonstrate a hydrothermal synthesis method for Vo introduction to hematite using a carbon source, where variations in the heating rate have not been previously explored in terms of their influence on Vo generation. The analyses revealed that the concentration of Vo was maximized at a heating rate of 16 °C/min, indicative of a high density of surface defects. With regard to photocatalytic performance, elevated heating rates (16 °C/min) fostered the formation of Vo primarily on the hematite surface. The photocatalytic activity was 7.1 times greater than that of the sample prepared at a low heating rate (2 °C/min). These findings highlight the crucial role of surface defects, as opposed to bulk defects, in promoting hematite photocatalysis. Furthermore, the facile control over Vo concentration achievable via manipulating the heating rate underscores the promising potential of this approach for optimizing hematite photocatalysts.

Effects of Chemical Valences of Sulfur on the Performance of CsFAMA Perovskite Solar Cells.

The low electrical conductivity and the high surface defect density of the TiO2 electron transport layer (ETL) limit the quality of the following perovskite (PVK) layers and the power conversion efficiency (PCE) of corresponding perovskite solar cells (PSCs). Sulfur was reported as an effective element to passivate the TiO2 layer and improve the PCE of PSCs. In this work, we further investigate the effect of chemical valences of sulfur on the performance of TiO2/PVK interfaces, CsFAMA PVK layers, and solar cells using TiO2 ETL layers treated with Na2S, Na2S2O3, and Na2SO4, respectively. Experimental results show that the Na2S and Na2S2O3 interfacial layers can enlarge the grain size of PVK layers, reduce the defect density at the TiO2/PVK interface, and improve the device efficiency and stability. Meanwhile, the Na2SO4 interfacial layer leads to a smaller perovskite grain size and a slightly degraded TiO2/PVK interface and device performance. These results indicate that S2- can obviously improve the quality of TiO2 and PVK layers and TiO2/PVK interfaces, while SO42- has little effects, even negative effects, on PSCs. This work can deepen the understanding of the interaction between sulfur and the PVK layer and may inspire further progress in the surface passivation field.

Growth mechanisms of interfacial carbides in solid-state reaction between single-crystal diamond and chromium

Interfacial bonding is one of the most challenging issues in the fabrication, and hence comprehensively influences the properties of diamond-based metal matrix composites (MMCs) materials. In this work, solid-state (S/S) interface reaction between single-crystal synthetic diamond and chromium (Cr) metal was critically examined with special attention given to unveil the role of crystal orientation in the formation and growth of interfacial products. It has been revealed that catalytically converted carbon (CCC) was formed prior to chromium carbides, which is counterintuitive to previous studies. Cr7C3 was the first carbide formed in the S/S interface reaction, aided by the relaxation of diamond lattices that reduces the interfacial mismatch. Interfacial Cr7C3 and Cr3C2 carbides were formed at 600 and 800 °C, respectively, with the growth preferred on diamond (100) plane, because of its higher density of surface defects than (111) plane. Interfacial strain distribution was quasi-quantitively measured using windowed Fourier Transform-Geometric Phase Analysis (WFT-GPA) analysis and an ameliorated strain concentration was found after the ripening of interfacial carbides. Textured morphologies of Cr3C2 grown on diamond (100) and (111) planes were perceived after S/S interface reaction at 1000 °C, which is reported for the first time. The underlying mechanisms of Cr-induced phase transformation on diamond surface, as well as the crystal orientation dependent growth of interfacial carbides were unveiled using the first-principles calculation. The formation and growth mechanisms of Cr3C2 were elucidated using TEM and XRD analyses. Finally, an approach for tailoring the interfacial microstructure between synthetic diamond and bonding metals was proposed.

Stratified Oxygen Vacancies Enhance the Performance of Mesoporous TiO2 Electron Transport Layer in Printable Perovskite Solar Cells.

The low electrical conductivity and the high surface defect density of the TiO2 electron transport layer (ETL) limit the power conversion efficiency (PCE) of corresponding perovskite solar cells (PSCs). Here, the conductivity and defect modulation of the mesoporous TiO2 (mp-TiO2 ) ETL via oxygen vacancy (OV) management by the reduction and oxidation treatment are reported. Reduction treatment via reducing agent introduces abundant OVs into the TiO2 nanocrystalline particles on the surface and at the subsurface. The following oxidation treatment via hydrogen peroxide removes the surface OVs while remains the subsurface OVs, resulting in stratified OVs. The stratified OVs improve the conductivity of TiO2 ETL by increasing carrier donors and decrease nonradiative centers by reducing surface defects. Such synergy ensures the capability of mp-TiO2 as the well-performed ETL with improved energy level alignment, suppressed interface recombination, enhanced carrier extraction, and transport. As a result, printable hole-conductor-free carbon-based mesoscopic PSCs based on the modulated mp-TiO2 ETL demonstrate a highest reported PCE of 18.96%.

Insights into the mechanism of the symmetry dependent SHG properties in low dimensional KNbO3 structures

The second harmonic generation (SHG) response of low-dimensional nanomaterials is potential dependence on structural symmetry and high-density surface defects, e.g., body contribution and surface contribution.

Unraveling the Mechanism of Maskless Nanopatterning of Black Silicon by CF4/H2 Plasma Reactive-Ion Etching.

The process of deep texturization of the crystalline silicon surface is intimately related to its promising diverse applications, such as bactericidal surfaces for integrated lab-on-chip devices and absorptive optical layers (black silicon—BSi). Surface structuring by a maskless texturization appeals as a cost-effective approach, which is up-scalable for large-area production. In the case of silicon, it occurs by means of reactive plasma processes (RIE—reactive-ion etching) using fluorocarbon CF4 and H2 as reaction gases, leading to self-assembled cylindrical and pyramidal nanopillars. The mechanism of silicon erosion has been widely studied and described as it is for the masked RIE process. However, the onset of the erosion and the reaction kinetics leading to defined maskless patterning have not been unraveled to date. In this work, we specifically tackle this issue by analyzing the results of three different RIE recipes, specifically designed for the purpose. The mechanism of surface self-nanopatterning is revealed by deeply investigating the physical chemistry of the etching process at the nanoscale and the evolution of surface morphology. We monitored the progress in surface patterning and the composition of the etching plasma at different times during the RIE process. We confirm that nanopattering issues from a net erosion, as contributed by chemical etching, physical sputtering, and by the synergistic plasma effect. We propose a qualitative model to explain the onset, the evolution, and the stopping of the process. As the RIE process is started, a high density of surface defects is initially created at the free silicon surface by energetic ion sputtering. Contextually, a polymeric overlayer is synthesized on the Si surface, as thick as 5 nm on average, and self-aggregates into nanoclusters. The latter phenomenon can be explained by considering that the initial creation of surface defects increases the activation energy for surface diffusion of deposited CF and CF2 species and prevents them from aggregating into a continuous Volmer–Weber polymeric film. The clusterization of the polymer provides the self-masking effect since the beginning, which eventually triggers surface patterning. Once started, the maskless texturing proceeds in analogy with the masked case, that is, by combined chemical etching and ion sputtering, and ceases because of the loss of ion energy. In the case of CF4/H2 RIE processes at 10% of H2 and by supplying 200 W of RF power for 20 min, nanopillars of 200 nm in height and 100 nm in width were formed. We therefore propose that a precise assessment of surface defect formation and density in dependence on the initial RIE process parameters can be the key to open a full control of outcomes of maskless patterning.

Quasi-Shell-Growth Strategy Achieves Stable and Efficient Green InP Quantum Dot Light-Emitting Diodes.

Indium phosphide (InP) based quantum dots (QDs) have been known as an ideal alternative to heavy metals including QDs light emitters, such as cadmium selenium (CdSe) QDs, and show great promise in the next‐generation solid‐state lighting and displays. However, the electroluminescence performance of green InP QDs is still inferior to their red counterparts, due to the higher density of surface defects and the wider particle size distribution. Here, a quasi‐shell‐growth strategy for the growth of highly luminescent green InP/ZnSe/ZnS QDs is reported, in which the zinc and selenium monomers are added at the initial nucleation of InP stage to adsorb on the surface of InP cores that create a quasi‐ZnSe shell rather than a bulk ZnSe shell. The quasi‐ZnSe shell reduces the surface defects of InP core by passivating In‐terminated vacancies, and suppresses the Ostwald ripening of InP core at high temperatures, leading to a photoluminescence quantum yield of 91% with a narrow emission linewidth of 36 nm for the synthesized InP/ZnSe/ZnS QDs. Consequently, the light‐emitting diodes based on the green QDs realize a maximum luminance of 15606 cd m−2, a peak external quantum efficiency of 10.6%, and a long half lifetime of > 5000 h.

Current transport properties of Pt/n-GaN Schottky diodes with ZnO interlayers

In this work, atomic layer deposition (ALD) was used to grow 20-nm thick ZnO interlayers (IL) on a GaN substrate at 150 and 200 °C and the current transport mechanisms of vertical Pt/ZnO/n-GaN Schottky diodes were investigated. A high density of surface state was observed for the ZnO layer grown at 200 °C. An analysis of the forward current–voltage (I–V) data considering various transport mechanisms demonstrated that the thermionic emission and the tunneling components attributed to the total current for the ZnO layer grown at 150 °C. In contrast, it was observed that the tunneling component mainly caused the current conduction in the ZnO layer grown at 200 °C. Additionally, an analysis of the reverse I–V characteristics exhibited a high density of surface defects on the ZnO layer grown at 200 °C. This study demonstrated that the ALD growth temperature of ZnO IL significantly affects the I–V characteristics of GaN Schottky diodes.

Enhanced photoelectric performance of GaN-based Micro-LEDs by ion implantation

As the new display technology, Micro-LEDs are of the advantages of low power consumption, high brightness, high resolution and fast response time. However, the strong surface non-radiative recombination significantly reduces their efficiency. Some techniques such as epitaxial growth or wet sulfide have been introduced to inhibit the surface recombination, but these expensive or unstable methods ignore the operability of industrial production. Herein, we use an ion implantation method to modify the high-density surface defects on the sidewall, thereby improve the luminous efficiency effectively by inhibiting carrier surface non-radiative recombination. The photoelectric performances of GaN-based Micro-LEDs after N ion implantation were studied. The characterization results indicated that the photoluminescence intensity of Micro-LEDs was increased by 7 times after passivation, which was attributed to the significant elimination of sidewall damage. In addition, the removal of the surface dangling bonds and oxidized nitrides (Ga–N or Ga-OH bonds) after the passivation process were further proved by Auger electron spectroscopy characterization. As a result of the N ion implantation, the external quantum efficiency of packaged Micro-LEDs chip increased about 33%. Therefore, this manuscript will provide a new direction for improving the photoelectric performance of Micro-LEDs.

Role of dissolved CO in the solution on the origin of CO pre-oxidation on Pt(1 1 1)-Type electrodes

• CO pre-oxidation occurs on (1 1 1) terrace sites of catalyst surfaces defect-rich. • CO in solution determines the magnitude (and the shape of the curve) of CO pre-oxidation. • CO pre-oxidation involves the diffusion-limited bulk CO electro-oxidation at most active sites. • Main CO electro-oxidation peak is a typical behavior of a surface-confined process. In voltammetric CO stripping experiments in acid media, CO pre-oxidation usually takes place on catalysts with a high density of surface defects, obeyed the conditions of full CO ads layer formation at suitable low potentials into the “hydrogen region” and, importantly, CO traces in the bulk of the solution is required. This study aims to interrogate that. The electro-oxidation of CO was voltammetrically conducted applying different scan rates, employing Pt(1 1 1) terraced surfaces and full CO coverage equilibrated with different content of dissolved CO in the bulk of an acid quiescent solution; also experiments were performed in which CO ads was present only on the (1 1 0)-type steps in a CO-free solution. It was found that: (1) slopes of peak potentials of CO pre-oxidation versus the logarithm of scan rates were smaller for higher CO contents in the solution; (2) charge density of CO pre-oxidation was higher for slower scan rate; (3) CO pre-oxidation takes place at low potentials where the (1 1 0)-type step sites were inactive in the catalysis of CO electro-oxidation. We conclude that, in such a quiescent solution, the content of dissolved CO in the bulk of the solution determines the magnitude and the shape of the curve of the CO pre-oxidation. This can be understandable in terms of prompt provision of CO molecules from the solution (diffusion layer) to surface active sites, that is, the most active sites, released during the CO pre-oxidation. In the CO pre-oxidation on Pt(1 1 1) terraced surfaces, the most active sites lie at the bottom of (1 1 1) terraces adjacent to the steps. The results provide evidence that, in presence of CO traces in the bulk of the solution, on one hand, the pre-oxidation of CO involves the diffusion-limited electro-oxidation of bulk CO at most active sites released during the CO pre-oxidation . On the other hand, the main CO electro-oxidation peak is a typical behavior of a surface-confined process. Our proposition applies to Pt electrodes in acidic medium.

Highly efficient Pt catalyst on newly designed CeO2-ZrO2-Al2O3 support for catalytic removal of pollutants from vehicle exhaust

Pt-CeO2 catalysts have been widely studied for the vehicle emission control. Designing novel CeO2 based supports with improved physical-chemical properties has become a research hotspot to further promote the catalytic performance and stability of Pt-CeO2 catalysts. In this work, through utilizing a unique, two-step incipient wetness impregnation (T-IWI) method for ceria-zirconia-alumina (CZA-T) support preparation, a Pt single site catalyst (Pt/CZA-T) with excellent thermal stability was synthesized. Higher oxidation activity and Oxygen storage capacity (OSC) were achieved on activated Pt/CZA-T, comparing to Pt catalysts on regular CeO2/Al2O3 (Pt/CA) and one-step prepared CeZrOx/Al2O3 (Pt/CZA). Via the modification of hydrophilic/hydrophobic properties of γ-Al2O3 by this unique T-IWI method, finer Ce0.9Zr0.1O2 particles with higher density of surface defects were formed on CZA-T, on which a higher Pt dispersion and stronger Pt-O-Ce interaction were achieved. Upon activation, smaller, well-dispersed Pt clusters on CZA-T were generated. It was concluded that the CO oxidation performance and OSC were highly related to the size of Pt clusters on different supports that we have developed. More Pt sites located at Pt cluster-CeZrOx interfaces, which were the real active sites, were responsible for the highest OSC function and CO oxidation activity of activated Pt/CZA-T catalyst.

Research on the leakage current at sidewall of mesa Ge/Si avalanche photodiode

The effects of surface defects at the sidewall of absorption and multiplication layers on the sidewall leakage current in separate-absorption-charge-multiplication Ge/Si avalanche photodiodes with a mesa structure are investigated. It is found that high-density surface defects and a strong electric field at the sidewall are the reasons for the large sidewall leakage current. In addition, the influence of the width of the guard ring on the sidewall leakage current is also studied. It is found that increasing the width of the guard ring is beneficial to the reduction of the sidewall leakage current by reducing the sidewall electric field and the reduction is not significant when the guard-ring width is greater than 2 µm.

(Invited) Optoelectronic Application of Colloidal Quatum Dots in Infrared Beyond Si

Among electromagnetic wave spectrum, infrared region has many attractive applications such as a LiDAR, optical communication, telecommunications, thermography, bioimaging, photovoltaics, and night visions. However, the expensive cost of epitaxial III-V semiconductors and the fabrication process hinders the widespread of the infrared technology beyond the silicon absorption range. Colloidal quantum dots have a great potential as an alternative materials for infrared detection. Lead chalcogenide QDs not only have tunable bandgap in infrared region but also have process compatibility with conventional Si ROIC in the device fabrication process, which remarkably reduces fabrication costs. The performance of photodetectors incorporating the lead chalcogenide QDs, however, is suboptimal and needs further improvement. One of the main factors limiting the performance of QD-based photodetectors is a high density of surface defects. QDs inherently have large surface-to-volume ratios and dangling bonds on the surface acted as trap sites. Various methods to reduce surface defects, such as thermal and ligand treatment, have been introduced; however, even after these treatments, QD-based photodetectors still show the relatively low detectivity (D*). In this work, we demonstrate the various approaches to improve the performance of infrared photodetectors and their results. We focused on the reducing the trap density by the surface treatment of QDs using thermal annealing and chemical treatment in order to improve the device performance. We developed the chemical treatment method for reducing trap states with maintaining the existing ligands. Thermal annealing also can reduce the trap density; however, it sintered QDs by atomic diffusion. We found optimal condition for reducing the surface traps with suppressing sintering. The effects of these treatments were evaluated by the device performance and trap characterization. These approaches improve the bandwidth and the responsivity compared to reference devices. Moreover, we treated the surface of QD layers to show the interfacial effect on the QD devices.

All‐Inorganic Cesium‐Based Hybrid Perovskites for Efficient and Stable Solar Cells and Modules

AbstractIn the last ten years, organic–inorganic hybrid perovskites have been skyrocketing the field of innovative photovoltaics (PVs) and now represent one of the most promising solution for next‐generation PVs. Within the family of halide perovskites, increasing attention has been focused on the so‐called all‐inorganic group, where the organic cation is replaced by cesium, as in the case of CsPbI3. This subclass of halide perovskites features desirable optoelectronic properties such as easily tunable bandgap, strong defect tolerance, and improved thermal stability compared to the hybrid systems. When integrated in PV cells, they exhibit high power conversion efficiency (PCE) with record values of 19.03%. However, all‐inorganic perovskite solar cells (PCSs) face several challenges such as i) instability of the CsPbI3 photoactive phase in ambient conditions, ii) inhomogeneous film morphology, and iii) high surface defect density. This work focuses on the mentioned challenges with a special attention on discussing the Cs–Pb–X system (X = I, Br). Then, the most recent and effective approaches for increasing both the PCE and the stability of devices are reviewed, which include material doping, interface engineering, and device optimization. Finally, the first efforts toward the upscaling of Cs‐based PSCs, and predicted methods for enabling large‐scale production, are discussed.

Investigating morphology-dependent antibacterial property of ZnO nanoparticles and developing an insight into oxidative stress generation and cellular response

In this work, morphology-dependent antibacterial property of zinc oxide nanoparticles (ZNPs) has been studied and an attempt has been made to investigate the reactive oxygen species generation potential of these nanoparticles. The antibacterial properties of ZnO nanorods with two different sizes have been quantitatively studied and their Minimum Inhibitory Concentration (MIC) has been determined against E.coli, a gram negative bacteria. It has been observed that ZNPs with smaller size exhibit better bacteriostatic and bactericidal properties. Furthermore, optical analysis has revealed that the smaller nanoparticles having greater efficiency to inhibit cell growth have higher density of surface defects and greater availability of free electrons and holes which leads to the generation of reactive oxygen species (ROS). To develop an insight into the oxidative microenvironment and the consequential cellular response, we have done the transcriptional analysis of oxidative stress gene sodA in ZNPs treated bacterial samples. Expression profile of the gene has been found to be ZNPs concentration dependent. An increase in the concentration of nanoparticles till MIC increased the expression level in a concentration dependent fashion. However beyond the MIC, the expression level of the gene is unexpectedly down-regulated reflecting the detrimental effect of higher levels of ROS on cellular components and metabolic activities. Thus through this study, we have tried to investigate the correlation between the surface morphology of ZNPs, production of ROS, growth-inhibiting activity and cellular response in this condition.

Restructuring of Nanoporous Gold Surfaces During Electrochemical Cycling in Acidic and Alkaline Media

AbstractThe electrochemical behavior of nanoporous gold (NPG) obtained by dealloying a AgAu alloy has been investigated by means of cyclic voltammetry (CV) in 0.1 M H2SO4 and 0.1 M KOH solutions supplemented by X‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) in order to understand different effects of the electrochemical treatment on the development of the surface structure of NPG. In order to reduce the IR drop caused by the high surface area of the bicontinuous network of pores and ligaments in NPG, NPG was transformed to a powder, from which a small portion was filled into a cavity microelectrode (CME). Additionally, this avoided sample‐to‐sample variation from the dealloying process because many fillings could be made from one NPG monolith. The cycling in 0.1 M H2SO4 led to restructuring of the surface to a more faceted one, only after the residual silver on the surface had been removed in the initial scan. The same cycling program in 0.1 M KOH did not cause restructuring. However, a transfer of the sample to 0.1 M H2SO4 could start the process. The ligament size did not change during restructuring. Additionally, it was found that residual Ag in NPG stabilizes the highly curved surfaces of the ligaments containing a high density of surface defects. The dissolution of the residual Ag in acid electrolytes lifts the blockage towards surface restructuring. These findings form a basis for understanding the electrochemical behavior of NPG and to devise appropriate treatments, for instance for their use in electrocatalysis.

Controlling Defect-State Photophysics in Covalently Functionalized Single-Walled Carbon Nanotubes.

ConspectusSingle-walled carbon nanotubes (SWCNTs) show promise as light sources for modern fiber optical communications due to their emission wavelengths tunable via chirality and diameter dependency. However, the emission quantum yields are relatively low owing to the existence of low-lying dark electronic states and fast excitonic diffusion leading to carrier quenching at defects. Covalent functionalization of SWCNTs addresses this problem by brightening their infrared emission. Namely, introduction of sp3-hybridized defects makes the lowest energy transitions optically active for some defect geometries and enables further control of their optical properties. Such functionalized SWCNTs are currently the only material exhibiting room-temperature single photon emission at telecom relevant infrared wavelengths. While this fluorescence is strong and has the right wavelength, functionalization introduces a variety of emission peaks resulting in spectrally broad inhomogeneous photoluminescence that prohibits the use of SWCNTs in practical applications. Consequently, there is a strong need to control the emission diversity in order to render these materials useful for applications. Recent experimental and computational work has attributed the emissive diversity to the presence of multiple localized defect geometries each resulting in distinct emission energy. This Account outlines methods by which the morphology of the defect in functionalized SWCNTs can be controlled to reduce emissive diversity and to tune the fluorescence wavelengths. The chirality-dependent trends of emission energies with respect to individual defect morphologies are explored. It is demonstrated that defect geometries originating from functionalization of SWCNT carbon atoms along bonds with strong π-orbital mismatch are favorable. Furthermore, the effect of controlling the defect itself through use of different chemical groups is also discussed. Such tunability is enabled due to the formation of specific defect geometries in close proximity to other existing defects. This takes advantage of the changes in π-orbital mismatch enforced by existing defects and the resulting changes in reactivities toward formation of specific defect morphologies. Furthermore, the trends in emissive energies are highly dependent on the value of mod(n-m,3) for an (n,m) tube chirality. These powerful concepts allow for a targeted formation of defects that emit at desired energies based on SWCNT single chirality enriched samples. Finally, the impact of functionalization with specific types of defects that enforce certain defect geometries due to steric constraints in bond lengths and angles to the SWCNT are discussed. We further relate to a similar effect that is present in systems where high density of surface defects is formed due to high reactant concentration. The outlined strategies suggested by simulations are instrumental in guiding experimental efforts toward the generation of functionalized SWCNTs with tunable emission energies.