Surface Of Colloidal Quantum Dots Research Articles

P‐Type Colloidal InSb Quantum Dot Ink Enables III–V Group Bulk‐Heterojunction Shortwave Infrared (SWIR) Photodetector

AbstractInfrared (IR) optoelectronics have become important owing to their various applications, such as recognition, autonomous driving, and quantum communications. In particular, detection beyond 1400‐nm wavelength in the shortwave IR (SWIR) spectrum (i.e., 1550 nm) is important for eye safety, and long‐range communication. Recently, group III–V (InAs or InSb) colloidal quantum dots (CQDs) have attracted considerable interest due to their broadband optical tunability and toxic‐elements (Pb and Hg)‐free properties. Herein, a new approach is developed to synthesize highly monodispersed InSb CQD by employing the continuous injection method, which enables facile optical bandgap tuning at a SWIR wavelength of up to 0.9 eV. Furthermore, solution ligand exchange using halides and thiolates results in effective passivation of the InSb CQD surface and renders a stable p‐type CQD ink. Finally, bulk heterojunction (BHJ) structure is demonstrated using n‐type InAs:p‐type InSb CQDs, which exhibits broad absorption up to 1600 nm, and a sixfold higher responsivity compared with the pristine InSb CQD device due to the efficient charge transport in BHJ CQD solids.

Colloidal InAs Quantum Dot-Based Infrared Optoelectronics Enabled by Universal Dual-Ligand Passivation.

Solution-processed low-bandgap semiconductors are crucial to next-generation infrared (IR) detection for various applications, such as autonomous driving, virtual reality, recognitions, and quantum communications. In particular, III-V group colloidal quantum dots (CQDs) are interesting as nontoxic bandgap-tunable materials and suitable for IR absorbers; however, the device performance is still lower than that of Pb-based devices. Herein, a universal surface-passivation method of InAs CQDs enabled by intermediate phase transfer (IPT), a preliminary process that exchanges native ligands with aromatic ligands on the CQD surface is presented. IPT yields highly stable CQD ink. In particular, desirable surface ligands with various reactivities can be obtained by dispersing them in green solvents. Furthermore, CQD near-infrared (NIR) photodetectors are demonstrated using solution processes. Careful surface ligand control via IPT is revealed that enables the modulation of surface-mediated photomultiplication, resulting in a notable gain control up to ≈10 with a fast rise/fall response time (≈12/36ns). Considering the figure of merit (FOM), EQE versus response time (or -3dB bandwidth), the optimal CQD photodiode yields one of the highest FOMs among all previously reported solution-processed nontoxic semiconductors comprising organics, perovskites, and CQDs in the NIR wavelength range.

Dicarboxylic Acid‐Assisted Surface Oxide Removal and Passivation of Indium Antimonide Colloidal Quantum Dots for Short‐Wave Infrared Photodetectors

AbstractHeavy‐metal‐free III–V colloidal quantum dots (CQDs) are promising materials for solution‐processed short‐wave infrared (SWIR) photodetectors. Recent progress in the synthesis of indium antimonide (InSb) CQDs with sizes smaller than the Bohr exciton radius enables quantum‐size effect tuning of the band gap. However, it has been challenging to achieve uniform InSb CQDs with band gaps below 0.9 eV, as well as to control the surface chemistry of these large‐diameter CQDs. This has, to date, limited the development of InSb CQD photodetectors that are sensitive to 1400 nm light. Here we adopt solvent engineering to facilitate a diffusion‐limited growth regime, leading to uniform CQDs with a band gap of 0.89 eV. We then develop a CQD surface reconstruction strategy that employs a dicarboxylic acid to selectively remove the native In/Sb oxides, and enables a carboxylate‐halide co‐passivation with the subsequent halide ligand exchange. We find that this strategy reduces trap density by half compared to controls, and enables electronic coupling among CQDs. Photodetectors made using the tailored CQDs achieve an external quantum efficiency of 25 % at 1400 nm, the highest among III–V CQD photodetectors in this spectral region.

Dicarboxylic Acid-Assisted Surface Oxide Removal and Passivation of Indium Antimonide Colloidal Quantum Dots for Short-Wave Infrared Photodetectors.

Heavy-metal-free III-V colloidal quantum dots (CQDs) are promising materials for solution-processed short-wave infrared (SWIR) photodetectors. Recent progress in the synthesis of indium antimonide (InSb) CQDs with sizes smaller than the Bohr exciton radius enables quantum-size effect tuning of the band gap. However, it has been challenging to achieve uniform InSb CQDs with band gaps below 0.9 eV, as well as to control the surface chemistry of these large-diameter CQDs. This has, to date, limited the development of InSb CQD photodetectors that are sensitive to 1400 nm light. Here we adopt solvent engineering to facilitate a diffusion-limited growth regime, leading to uniform CQDs with a band gap of 0.89 eV. We then develop a CQD surface reconstruction strategy that employs a dicarboxylic acid to selectively remove the native In/Sb oxides, and enables a carboxylate-halide co-passivation with the subsequent halide ligand exchange. We find that this strategy reduces trap density by half compared to controls, and enables electronic coupling among CQDs. Photodetectors made using the tailored CQDs achieve an external quantum efficiency of 25 % at 1400 nm, the highest among III-V CQD photodetectors in this spectral region.

Arresting Ion Migration from the ETL Increases Stability in Infrared Light Detectors Based on III-V Colloidal Quantum Dots.

III-V colloidal quantum dots (CQDs) are of interest in infrared photodetection, and recent developments in CQDs synthesis and surface engineering have improved performance. Here this work investigates photodetector stability, finding that the diffusion of zinc ions from charge transport layers (CTLs) into the CQDs active layer increases trap density therein, leading to rapid and irreversible performance loss during operation. In an effort to prevent this, this work introduces organic blocking layers between the CQDs and ZnO layers; but these negatively impact device performance. The device is then, allowing to use a C60:BCP as top electron-transport layer (ETL) for good morphology and process compatibility, and selecting NiOX as the bottom hole-transport layer (HTL). The first round of NiOX -based devices show efficient light response but suffer from high leakage current and a low open-circuit voltage (Voc) due to pinholes. This work introduces poly[bis(4-phenyl) (2,4,6-trimethylphenyl)amine] (PTAA) with NiOX NC to form a hybrid HTL, an addition that reduces pinhole formation, interfacial trap density, and bimolecular recombination, enhancing carrier harvesting. The photodetectors achieve 53% external quantum efficiency (EQE) at 970nm at 1V applied bias, and they maintain 95% of initial performance after 19 h of continuous illuminated operation. The photodetectors retain over 80% of performance after 80 days of shelf storage.

Molecular surface programming of rectifying junctions between InAs colloidal quantum dot solids

Heavy-metal-free III-V colloidal quantum dots (CQDs) show promise in optoelectronics: Recent advancements in the synthesis of large-diameter indium arsenide (InAs) CQDs provide access to short-wave infrared (IR) wavelengths for three-dimensional ranging and imaging. In early studies, however, we were unable to achieve a rectifying photodiode using CQDs and molybdenum oxide/polymer hole transport layers, as the shallow valence bandedge (5.0 eV) was misaligned with the ionization potentials of the widely used transport layers. This occurred when increasing CQD diameter to decrease the bandgap below 1.1 eV. Here, we develop a rectifying junction among InAs CQD layers, where we use molecular surface modifiers to tune the energy levels of InAs CQDs electrostatically. Previously developed bifunctional dithiol ligands, established for II-VI and IV-VI CQDs, exhibit slow reaction kinetics with III-V surfaces, causing the exchange to fail. We study carboxylate and thiolate binding groups, united with electron-donating free end groups, that shift upward the valence bandedge of InAs CQDs, producing valence band energies as shallow as 4.8 eV. Photophysical studies combined with density functional theory show that carboxylate-based passivants participate in strong bidentate bridging with both In and As on the CQD surface. The tuned CQD layer incorporated into a photodiode structure achieves improved performance with EQE (external quantum efficiency) of 35% (>1 μm) and dark current density < 400 nA cm-2, a >25% increase in EQE and >90% reduced dark current density compared to the reference device. This work represents an advance over previous III-V CQD short-wavelength IR photodetectors (EQE < 5%, dark current > 10,000 nA cm-2).

Amine‐Free, Directing‐Group‐Free and Redox‐Neutral α‐Alkylation of Saturated Cyclic Ketones

AbstractThe activation of the α‐C−H bond of ketones typically requires an amine and a directing group to guide the reaction selectivity in amine‐catalysis carbonyl chemistry. For an α‐C−H bond activation of ketone, directing groups are also required to control the reaction selectivity. Reported herein is the first α‐alkylation of cyclic ketones in the absence of an amine catalyst and directing group. 1H NMR, XPS, EPR studies and DFT calculations indicate that an α‐carbon radical intermediate is formed through direct and selective activation of the inert α‐C−H bond of ketones chelating on the surface of colloidal quantum dots (QDs). Such an interaction is essential for weakening the C−H bond, as exemplified, using CdSe QDs as the sole photocatalyst to execute α‐C−H alkylation of cyclic ketones under visible‐light irradiation. Without an amine catalyst and directing group, the high step‐ and atom‐economy transformation under redox‐neutral condition opens a new way for α‐C−H functionalization of ketones in carbonyl chemistry.

Amine-Free, Directing-Group-Free and Redox-Neutral α-Alkylation of Saturated Cyclic Ketones.

The activation of the α-C-H bond of ketones typically requires an amine and a directing group to guide the reaction selectivity in amine-catalysis carbonyl chemistry. For an α-C-H bond activation of ketone, directing groups are also required to control the reaction selectivity. Reported herein is the first α-alkylation of cyclic ketones in the absence of an amine catalyst and directing group. 1 H NMR, XPS, EPR studies and DFT calculations indicate that an α-carbon radical intermediate is formed through direct and selective activation of the inert α-C-H bond of ketones chelating on the surface of colloidal quantum dots (QDs). Such an interaction is essential for weakening the C-H bond, as exemplified, using CdSe QDs as the sole photocatalyst to execute α-C-H alkylation of cyclic ketones under visible-light irradiation. Without an amine catalyst and directing group, the high step- and atom-economy transformation under redox-neutral condition opens a new way for α-C-H functionalization of ketones in carbonyl chemistry.

Ligand-induced surface reconstruction in Ag2S colloidal quantum dots for highly luminescent infrared fluorescence

Silver sulfide (Ag2S) colloidal quantum dots (CQDs) have attracted attention as potential bio-imaging materials owing to their narrow bandgap and biocompatibility; however, their low photoluminescence quantum yield (PLQY) has limited their applications. Here we introduce ligand-induced surface reconstruction strategy to dramatically improve the PLQY of Ag2S CQDs. We found that the low PLQY of Ag2S CQDs is due to the undesirable formation of a metallic Ag core during the CQD synthesis. By introducing alkyl thiols as oxidizing agents, the metallic Ag core is reoxidized that Ag vacancies in CQDs are refilled. This induces a reduced metallic Ag core and Ag-rich stoichiometry in the CQDs. The CQDs after the surface reconstruction show a 150× enhancement of photoluminescence at 1230 nm, with an absolute PLQY of 9%. This research suggests that simple ligand treatments can help improve the surface and optical properties of silver chalcogenide CQDs, including Ag2S CQDs, which may allow development of advanced optoelectronic devices.

Excess PbBr2 Passivation of Large PbS Colloidal Quantum Dots to Reduce Dark-Current Density for Near-Infrared Detection

Small-size $\mathrm{Pb}\mathrm{S}$ colloidal quantum dots (CQDs) have achieved excellent performance in photoelectric conversion devices through the ligand-exchange method of mixed lead-halide passivation. However, with the increase of $\mathrm{Pb}\mathrm{S}$ CQD diameter, the proportion of (100) facets on the CQD surface increases and the original proportion of mixed lead-halide ligand cannot passivate (100) facets completely, which will introduce deep defects and deteriorate device performance. Here, we demonstrate an excessive ${\mathrm{Pb}\mathrm{Br}}_{2}$ concentration ligand strategy to sufficiently passivate large-size $\mathrm{Pb}\mathrm{S}$ CQDs with an absorption peak at 1300 nm. The first-principles calculation results suggest that ${\mathrm{Br}}^{\ensuremath{-}}$ can passivate (100) facets more efficiently compared with ${\mathrm{I}}^{\ensuremath{-}}$. With the increase of ${\mathrm{Pb}\mathrm{Br}}_{2}$ concentration (0--0.464 mmol/mL), both optical and electrical measurements imply that defects are effectively passivated, while carrier lifetime increases and dark-current density decreases. Finally, a device with specific detectivity of $5.22\ifmmode\times\else\texttimes\fi{}{10}^{12}$ Jones is obtained. This passivation strategy can also be used in other large-size $\mathrm{Pb}\mathrm{S}$ CQDs (diameter &gt;4 nm) to realize a better device performance.

Improved surface passivation of lead sulfide quantum dots using less sterically crowded alkylammonium iodides

Efficient surface passivation of colloidal quantum dots (CQDs) is crucial for improving the photovoltaic performance of CQD-based solar cell (CQDSC) devices. Although several alkylammonium iodides (AMIs), representatively tetra-n-butylammonium iodide (TBAI), have been reported for iodide passivation of lead sulfide (PbS) CQDs, the factors influencing the passivation are still barely understood. In this research, the impacts of steric crowding, acidity and ionic dissociation of seven AMIs with different lengths and numbers of alkyl substituents on the iodide passivation of the PbS CQDs are studied. The oleate ligands on the pristine CQD surface are more effectively exchanged with iodides by less sterically crowded AMIs. The higher acidity of tertiary AMIs compared with quaternary AMIs also enhances the elimination of oleate ligands and the passivation of iodides. The extent of the ligand exchange consequently influences the photovoltaic properties of CQDSCs, including trap density, built-in potential and charge mobility. Thus, the triethylamine hydroiodide (tri-EAHI)-treated CQDSC demonstrates the highest power conversion efficiency of 5.37% for a single CQD layer-based cell and 10.1% for a PbS-tri-EAHI/PbS-PDT-based bilayer cell.

Zinc Carboxylate Surface Passivation for Enhanced Optical Properties of In(Zn)P Colloidal Quantum Dots.

Indium phosphide (InP) colloidal quantum dots (CQDs) have generated great interest as next-generation light-emitting materials owing to their narrow emission spectra and environment-friendly components. The minimized surface defects is essential to achieve narrow full-width at half-maximum (FWHM) and high photoluminescence quantum yield (PLQY). However, InP CQDs are readily oxidized in ambient condition, which results in formation of oxidation defect states on the surface of InP CQDs. Herein, we introduce a strategy to successfully passivate the surface defects of InP core by zinc complexes. The zinc carboxylates passivation reduces FWHM of InP CQDs from 130 nm to 70 nm and increases PLQY from 1% to 14% without shelling. Furthermore, the photoluminescence (PL) peak has shifted from 670 nm to 510 nm with an increase of zinc carboxylates passivation, which suggests that excessive zinc carboxylates functions as a size-regulating reagent in the synthesis.

Enhancing the photoinduced charge carrier transfer by coupling the InZnP quantum-dots with PbS shell for solution-processed solar cells application

The III-V group colloidal quantum dots (CQDs) have been successfully synthesized with stable and low-cost chemical reagents recently, but they are rarely used for solid state solar cells due to the poor charge carrier transfer in solid films. In this work, the surface of InZnP (Zn doped InP) CQDs has been reconstructed with PbS via a chemical engineering strategy. Comparing with InZnP CQDs, the long carbon chain ligands capped on the InZnP@PbS CQDs surface can be easily exchanged with small molecules. By capping the mercaptopropionic acid (MPA) onto the InZnP@PbS CQDs surface, the electrical coupling between the InZnP@PbS CQDs has been established, and the photoinduced charge carrier transfer has been significantly enhanced. Based on the above CQDs, solid state solar cell devices are assembled, achieving about 1.5% of power conversion efficiency (PCE). The photoelectric performances of the fabricated solar cells almost do not decay after 150 days storage. Current work may provide a new way for III-V group CQDs application in solution-processed optoelectronics.

Photodetectors Based on Lead Sulfide Quantum Dot and Organic Absorbers for Multispectral Sensing in the Visible to Short‐Wave Infrared Range

AbstractMultispectral imaging in short‐wave infrared (SWIR) is a powerful analytical technique because of the distinctive spectral properties of many materials in this range. However, conventional SWIR image sensors are beyond the reach of many applications due to their high price. Image sensors based on colloidal quantum dots (CQDs) are expected to deliver affordable infrared image sensors to wider application scope. So far, the demonstrated CQD image sensors do not have a multispectral capability. Here, a dual‐band photodetector based on PbS CQDs is presented. By engineering the surface of CQDs, two oppositely facing pn junctions are fabricated in series, which enable sensing in two spectral channels. Furthermore, an optical cavity is designed that reduces the spectral crosstalk between the two channels and simultaneously enables wavelength‐tunability in one channel. Finally, an organic photodiode (OPD) is integrated with a PbS CQD photodiode in a single device, leveraging a high sensitivity in visible and near‐infrared (NIR) characteristics for OPDs. The presented photodetectors exhibit low dark current below 500 nA cm−2 at 1 V bias, a fast response measured in microseconds, as well as high external quantum efficiency, reaching 70% in NIR and 30% in SWIR.

Solution Annealing Induces Surface Chemical Reconstruction for High-Efficiency PbS Quantum Dot Solar Cells.

Colloidal quantum dots (CQDs) have a large specific surface area and a complex surface structure. Their properties in diverse optoelectronic applications are largely determined by their surface chemistry. Therefore, it is essential to investigate the surface chemistry of CQDs for improving device performance. Herein, we realized an efficient surface chemistry optimization of lead sulfide (PbS) CQDs for photovoltaics by annealing the CQD solution with concentrated lead halide ligands after the conventional solution-phase ligand exchange. During the annealing process, the colloidal solution was used to transfer heat and create a secondary reaction environment, promoting the desorption of electrically insulating oleate ligands as well as the trap-related surface groups (Pb-hydroxyl and oxidized Pb species). This was accompanied by the binding of more conductive lead halide ligands on the CQD surface, eventually achieving a more complete ligand exchange. Furthermore, this strategy also minimized CQD polydispersity and decreased aggregation caused by conventional solution-phase ligand exchange, thereby contributing to yielding CQD films with twofold enhanced carrier mobility and twofold reduced trap-state density compared with those of the control. Based on these merits, the fabricated PbS CQD solar cells showed high efficiency of 11% under ambient conditions. Our strategy opens a novel and effective avenue to obtain high-efficiency CQD solar cells with diverse band gaps, providing meaningful guidance for controlling ligand reactivity and realizing subtly purified CQDs.

Guanidinium-Pseudohalide Perovskite Interfaces Enable Surface Reconstruction of Colloidal Quantum Dots for Efficient and Stable Photovoltaics.

Complete surface passivation of colloidal quantum dots (CQDs) and their strong electronic coupling are key factors toward high-performance CQD-based photovoltaics (CQDPVs). Also, the CQD matrices must be protected from oxidative environments, such as ambient air and moisture, to guarantee air-stable operation of the CQDPVs. Herein, we devise a complementary and effective approach to reconstruct the oxidized CQD surface using guanidinium and pseudohalide. Unlike conventional halides, thiocyanate anions provide better surface passivation with effective replacement of surface oxygen species and additional filling of defective sites, whereas guanidinium cations promote the construction of epitaxial perovskite bridges within the CQD matrix and augment electronic coupling. Additionally, we replace a defective 1,2-ethanedithiol-treated CQD hole transport layer (HTL) with robust polymeric HTLs, based on a judicious consideration of the energy level alignment established at the CQD/HTL interface. These efforts collectively result in high-performance and stable CQDPVs with photocurrents over 30 mA cm-2, ∼80% quantum efficiency at excitonic peaks and stable operation under humid and ambient conditions. Elucidation of carrier dynamics further reveals that interfacial recombination associated with band alignment governs both the CQDPV performance and stability.

Strong Coupling of Colloidal Quantum Dots via Self‐Assemble Passivation for Efficient Infrared Solar Cells

AbstractThe surface passivation of colloidal quantum dots (CQD) is critical for the electronic coupling of CQDs, which significantly affects the photovoltaic performance of CQD solar cells (CQDSCs). Herein, a self‐assemble passivation (SAP) strategy of CQDs is introduced to improve CQD coupling. The PbI2 passivation layer prepared using the SAP method can largely improve surface defect passivation of CQDs, diminishing charge recombination induced by the sub‐bandgap traps. Meanwhile, extensive theoretical simulations reveal that the self‐assembled PbI2 passivation layer works as a “charger bridge” for charge transport between the adjacent CQDs, avoiding CQD fusion. The infrared CQDSCs are fabricated and the SAP‐based CQDSC yields an efficiency of up to 12.3%, which is significantly improved compared with that of the conventional CQDSCs with iodide passivating CQD surface. The improved photovoltaic performance in the SAP‐based CQDSCs is attributed to increased charge extraction, resulting from strong CQD coupling within the CQD solid films. This work provides a simple and facile way to improve the electronic coupling of CQDs for high‐performance infrared CQDSCs.

Ultrafast intraband Auger process in self-doped colloidal quantum dots

Summary Investigating the separate dynamics of electrons and holes has been challenging, although it is critical for the fundamental understanding of semiconducting nanomaterials. n-Type self-doped colloidal quantum dots (CQDs) with excess electrons occupying the low-lying state in the conduction band (CB) have attracted a great deal of attention because of not only their potential applications to infrared optoelectronics but also their intrinsic system that offers a platform for investigating electron dynamics without elusive contributions from holes in the valence band. Here, we show an unprecedented ultrafast intraband Auger process, electron relaxation between spin-orbit coupling states, and exciton-to-ligand vibrational energy transfer process that all occur exclusively in the CB of the self-doped β-HgS CQDs. The electron dynamics obtained by femtosecond mid-infrared spectroscopy will pave the way for further understanding of the blinking phenomenon, disproportionate charging in light-emitting diodes, and hot electron dynamics in higher quantum states coupled to surface states of CQDs.

Role of Oxygen in Two-Step Thermal Annealing Processes for Enhancing the Performance of Colloidal Quantum Dot Solar Cells.

Colloidal quantum dots (CQDs) have large surface-to-volume ratios; thus, surface control is critical, especially when CQDs are utilized in optoelectronic devices. Layer-by-layer solid-state ligand exchange is a facile and applicable process for the formation of conductive CQD solids through various ligands; however, achieving complete ligand exchange on the CQD surface without dangling bonds is challenging. Herein, we demonstrate that CQDs can be further passivated through two-step annealing; air annealing forms sulfonate bonding at (111) Pb-rich surfaces, and subsequent N2 annealing removes insulating oxygen layers from the (100) surfaces of CQDs. By subsequently conducting annealing treatment in two different environments, traps on the surface of CQDs could be significantly reduced. We achieved a 40.8% enhancement of the power conversion efficiency by optimizing each two-step annealing process.

Trap states in lead chalcogenide colloidal quantum dots—origin, impact, and remedies

Colloidal quantum dots (CQDs) based on lead chalcogenides (PbX), i.e., lead sulfide, selenide, or telluride, constitute a class of materials with many intriguing properties and potential applications in (opto-)electronics. These nanosized crystals are employed successfully in a broad variety of devices including field-effect transistors, solar cells, and light emitting diodes, and their performance has increased significantly over the last 20 years. Often, such improvements have been associated with the suppression of detrimental recombination of charge carriers via trap states. Historically, traps have been attributed to dangling bonds on the surface of CQDs that needed to be passivated for proper electronic behavior. More recent understanding goes beyond such simplified views. Surfaces can be bare without necessarily evoking traps. On the other hand, imperfect separation of CQDs and their subsequent agglomeration can generate trapping sites without the need of chemical defects. Experimental and computational approaches that have led to a more accurate understanding are here discussed, and rivaling concepts and ideas are highlighted. Although the community established a much improved understanding of carrier trapping, there is still room to further the knowledge about the precise mechanisms, especially with respect to impacts from the environment. With these limitations notwithstanding, PbX CQDs exhibit large potential that we expect to be unlocked through future improvements in control of the surface chemistry and strategies of thin film assembly.