In‐Plane Targeted Positioning of Silica‐Coated PbS Quantum Dots Based on Size Differences

<p indent=0mm>Colloidal quantum dots (CQDs) are zero-dimensional semiconductor materials with solution-processable properties. These materials have attracted considerable attention in the research and development of new photodetectors, photovoltaic cells, light-emitting diodes, and chemical sensors. The large exciton Bohr radius and Debye length and the considerable quantum size effect make lead sulfide one of the typical hotspots in CQDs. Owing to the large specific surface area and abundant surface dangling bonds, the surface ligand of CQDs greatly influences their physical and chemical properties. Surface ligand engineering can be used to realize the functional design and performance improvement of quantum dot semiconductor devices. This article reviews the research progress in the surface ligand engineering of lead sulfide (PbS) CQDs, focusing on the influence of surface ligands on their conductive properties and chemical activity. To improve the surface passivation and carrier mobility, PbS CQDs are developed from short-chain organic ligands to inorganic ligands represented by metal chalcogenide complexes (MCC), especially by the introduction of monovalent halogen atomic ligands, through surface ligand engineering. We further introduce the liquid-phase ligand-exchange technology. Compared with film-level ligand exchange, liquid-phase ligand exchange favors complete ligand replacement and one-step deposition of quantum dot solids using a colloidal stable nanoparticle ink. With further detailed research, the air stability of PbS CQDs can be improved. This improvement will not only lay a solid foundation for the application of PbS CQDs in optoelectronic devices but also provide opportunities for the development of room-temperature chemical sensors. In the last part, we discuss the chemical activity of PbS CQDs and their application for gas sensing. CQDs are ideal gas-sensitive materials owing to their large surface area and abundant active sites for gas adsorption, and the surface states formed by gas adsorption considerably affect the physical and chemical properties of the CQDs. Meanwhile, the CQDs have excellent film-forming properties at room temperature. Hence, they can be coated on a silicon substrate by simple and controllable methods such as spin coating or spraying. In summary, the surface ligand engineering of CQDs is an important strategy to develop new semiconductor functional devices. The optoelectronic properties and chemical activity of PbS CQDs indicate sufficient scope for design and regulation with various types, components, and introduction methods of the surface ligand. Great breakthroughs have been made with regard to PbS CQDs in both stability research and low-cost mass production over the last decade. Despite various challenges, the basic research on PbS CQDs for photoelectric and chemical sensing is ongoing. To improve the design ideas and fabrication methods of CQD functional devices, it is necessary to use methods based on theoretical calculations and microcharacterization techniques to reveal the effect of surface ligands on CQDs. The understanding of surface science and device physics of CQDs will drive the utilization of the semiconductor quantum effect. In the future, great breakthroughs are expected for the use of PbS quantum dots in the fields of infrared imaging, spectral analysis, gas sensing, and biochemical sensing.

This abstract presents a solution-processed photovoltaic device based on the PbS (lead sulfide) colloidal quantum dots (CQDs). A simple fabrication method at room temperature makes the solution-processed photovoltaic devices be a suitable cost effective alternative. In addition to simplicity in the fabrication method, a key advantage of photovoltaic devices based on PbS colloidal quantum dots is the tunability of the absorption spectrum by controlling the size and shape of the PbS quantum dots [1]. The structure of the photovoltaic device is shown in the figure. Active area is composed of a superlattice of doped PbS quantum dots sandwiched between two electric contact layers. The active area makes a Schottky contact with an electrode having low work function such as aluminum on one side and an Ohmic contact with a transparent electrode such as ITO (Indium Tin Oxide) on the other side. Absorbed photons in the active layer generate neutral excitons in the PbS quantum dots, and subsequently charge dissociation takes places in the neutral region due to the internal electric field at the Schottky contact. Photogenerated electrons and holes flow toward the Al and ITO contacts, respectively.Fabrication process starts with the deposition of doped colloidal PbS quantum dots on an ITO coated glass substrate by a layer by layer (LBL) method in an inert condition at room temperature. The process is followed by aluminum deposition on top by physical vapor deposition. Fabricated photovoltaic device illustrates promising sensitivity to both entire visible and portion of infrared spectrum of sun light. Since both device structure and fabrication process are simple and process can be done in room temperature, fabrication on the flexible substrates is also feasible.

PbS (lead sulfide) colloidal quantum dots consist of crystallites with diameters in the nanometer range with organic molecules on their surfaces, partly with additional metal complexes as ligands. These surface molecules are responsible for solubility and prevent aggregation, but the interface between semiconductor quantum dots and ligands also influences the electronic structure. PbS quantum dots are especially interesting for optoelectronic applications and spectroscopic techniques, including photoluminescence, photodiodes and solar cells. Here we concentrate on the latter, giving an overview of the optical properties of solar cells prepared with PbS colloidal quantum dots, produced by different methods and combined with diverse other materials, to reach high efficiencies and fill factors.

This work presents the development of a light intensity sensor based using colloidal PbS quantum dots. A simple fabrication method at low cost makes the developed sensor suitable for light detection applications. Photodetectors based on quantum dots have various advantages compared to solid-state photodetectors. The reported advantages include ease of fabrication, narrow and tunable spectral response, large sensing area, reduced dark current, and high sensitivity [1, 2]. Solution-based fabrication makes the whole process simple and cost efficient. The structure of the sensor in this work is shown in the figure having a light sensitive composite sandwiched by two electrodes of which one is usually a transparent electrode made of indium tin oxide (ITO). For validation, we employed PbS colloidal quantum dots and poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) or MEH-PPV to form the light sensitive composite [3]. In the light sensitive composite, a charge separation occurs by a photon generating a neutral exciton at the interface of the conjugated polymer and the quantum dots. We started with an ITO coated glass substrate. The light sensitive composite made of MEH-PPV and colloidal quantum dots was then cast on top by spin coating. For the other electric contact, an aluminum layer was deposited by physical vapor deposition. The fabricated photodetector shows a promising sensitivity to incident light, and further characterization is currently underway. Since the entire structure of the photodetector is simple and the entire process can be done with low thermal budget, the photodetector can also be formed on a flexible substrate.

Ligand modulation of the electrical properties and surface solvation plays a crucial role in the development of functionalized colloidal quantum dots (CQDs) inks for solution‐processed optoelectronics. While inorganic halide ionic ligands can facilitate the n‐type doping of PbS CQDs and establish an electrical double layer in polar solvents for stable high‐concentration n‐type CQD inks, a plausible solvation strategy is still lacking to stabilize p‐type PbS CQDs, thereby resulting in a tedious multi‐step deposition involving short‐chain dithiol molecular ligands for advanced heterojunction CQD solar cells. Here an effective hydrogen bonding solvation strategy is proposed using 2‐mercaptoethanol (ME) ligands to realize stable p‐type PbS CQD ink. This strategy enables DMSO to form a dense solvation layer surrounding the ME‐modified PbS CQDs. With this approach, the PbS‐ME CQD ink exhibits a photoluminescence quantum yield of 52.03%, which is the highest record for PbS CQD inks. Finally, one‐step deposition of a p‐type PbS‐ME layer for PbS CQD photovoltaics is successfully achieved with an impressive power conversion efficiency (PCE) of 10.91%. This p‐type solar ink facilitates the fabrication of ready‐to‐use devices, enabling extensive applications in large‐scale and flexible optoelectronic devices.

Understanding the origins of the excessive Stokes shift in the lead chalcogenides family of colloidal quantum dots (CQDs) is of great importance at both the fundamental and applied levels; however, our current understanding is far from satisfactory. Here, utilizing a combination of ab initio computations and UV-vis and photoluminescence measurements, we investigated the contributions to the Stokes shift from polydispersity, ligands, and defects in PbS CQDs. The key results are as follows: (1) The size and energetic disorder of a polydisperse CQD film increase the Stokes shift by 20 to 50 meV compared to that of an isolated CQD; (2) Franck-Condon (FC) shifts increase as the electronegativities of the ligands increase, but the variations are small (<15 meV). (3) Unlike the aforementioned two minor factors, the presence of certain intrinsic defects such as VCl+ (in Cl-passivated CQDs) can cause substantial electron density localization of the band edge states and consequent large FC shifts (100s of meV). This effect arising from defects can explain the excessive Stokes shifts in PbS CQDs and improve our understanding of the optical properties of PbS CQDs.

Controlled waveguide coupling for photon emission from colloidal PbS quantum dot using tunable microcavity made of optical polymer and silicon

Multispectral 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.

Colloidal quantum dots (CQDs) are emerging solution processed materials combining low cost, easy deposition on large and flexible substrates, and bandgap tunability. The latter feature, which allows spectral tuning of the absorption profile of the semiconductor, makes these materials particularly attractive for light detection applications. Lead sulfide (PbS) CQDs, in particular, have shown astonishing performance as a light sensitive material operating at visible and infrared (IR) wavelengths. Early studies of PbS CQDs used as a photosensitive resistor (photoconductor) showed an impressive responsivity - exceeding 1000 A/W - and a detectivity (D*) higher then 10^13 Jones. This impressive D* was preserved in the successive development of the first PbS CQD photodiode, showing the possibility to realize fast - f_3db > 1Mhz - and sensitive IR detectors. Currently, the field is moving toward the development of hybrid devices and phototransitors. PbS CQDs have been combined in field effect transistors (FETs) with graphene and MoS2 channels, showing ultra-high gain (exceeding 10^8 electrons/photons) and high D*. Recently a photo-junction FET (photo-JFET) has been reported that breaks the inherent dark current/gain/bandwidth compromise affecting photoconductive light detectors. With this presentation we offer a broad overview on CQD photodetection highlighting the past achievements, the benefits, the challenges and the prospects for the future research on this field.

Binary thin film exhibits not only the quantum features of the individual building blocks but also novel collective properties through coupling of colloidal quantum dot components. In this paper, lead sulfide (PbS) and cadmium sulfide (CdS) colloidal quantum dots (CQDs) were synthesized by using oleate and oleylamine as ligand. The as-synthesized PbS and CdS CQDs were monodispersity and well passivation. The average diameter of as-synthesized PbS and CdS CQDs were about 3 nm and 6 nm, respectively. By blending PbS with CdS CQDs and utilizing ethanedithiol for ligand passivation, the responsivity and detectivity of PbS CQDs thin film was enhanced with the weight ratio of CdS CQDs increased, the optimum responsivity and detectivity were 21.9 mA/W and 2.1 × 1010 Jones, respectively. The desirable properties of binary colloidal quantum dot thin films have important applications in future electronic and optoelectronic devices.

Colloidal quantum dots (CQDs) are typically decorated with organic molecules that provide surface passivation and colloidal solubility. An alternate but less studied surface functionalization approach via inorganic complexes can produce stable CQDs with attractive transport and optical properties. Further development of such all-inorganic CQD solids is dependent on the deeper understanding of the energetic and dynamic interactions of the new ligands with the CQD excitons. Herein, a series of four metal chalcogenide (MCC) ligands of the KzXS4 type were attached to PbS CQDs. Out of the four MCC complexes studied, we find that only K4GeS4 ligands yield robust PbS CQD films with bright photoluminescence (PL) in the solid state. A systematic spectroscopic investigation of the K4GeS4-capped CQD films provides evidence of the temperature-dependent ligand-mediated exciton delocalization and trapping processes. At low temperatures, efficient trapping at ligand-induced states is found to occur within ∼6 ns after photoexcitation, followed by a considerably slower exciton back transfer to the CQD core. At elevated temperatures, the CQD films become photoconductive, providing evidence of exciton dissociation via carrier transfer within adjacent dots. The addition of a thin CdS shell suppresses the delocalization and trapping of excitons, resulting in brighter emission and significantly slower transient absorption and PL dynamics.

All-optical approaches to change the wavelength of a data signal are considered more energy- and cost-effective than current wavelength conversion schemes that rely on back and forth switching between the electrical and optical domains. However, the lack of cost-effective materials with sufficiently adequate optoelectronic properties hampers the development of this so-called all-optical wavelength conversion. Here, we show that the interplay between intraband and band gap absorption in colloidal quantum dots leads to a very strong and ultrafast modulation of the light absorption after photoexcitation in which slow components linked to exciton recombination are eliminated. This approach enables all-optical wavelength conversion at rates matching state-of-the-art convertors in speed, yet with cost-effective solution-processable materials. Moreover, the stronger light-matter interaction allows for implementation in small-footprint devices with low switching energies. Being a generic property, the demonstrated effect opens a pathway toward low-power integrated photonics based on colloidal quantum dots as the enabling material.

Improved efficiency of ferroelectric Pb(Zr, Ti)O3 (PZT) based photovoltaic device with colloidal quantum dots

Carrier transport in colloidal quantum dot (CQD) films is strongly influenced by the interfacial coupling between CQDs. Currently, the shape of PbS CQDs synthesized using traditional methods results in random orientation relationships between the crystal facets in CQD films, limiting the coupling strength and the final performance of optoelectronic devices. In this study, post-synthesis surface treatment of PbS CQDs was employed to achieve facet control during secondary growth, manipulating the facets of PbS CQDs at the nanoscale to enhance interfacial coupling within CQD films. Additionally, mixed ligands of PbX₂ (X = Br, I) and anhydrous sodium acetate were used to passivate the PbS CQDs, ensuring sufficient passivation. This method combines facet passivation with strong coupling through the (100) facets of CQDs, thereby enhancing carrier mobility and improving device performance. Experimental results showed that, compared to standard PbS CQD films, the electron and hole mobilities of the PbS CQD films subjected to secondary growth were significantly improved, with hole mobility increased by 6 times. Photodetectors fabricated using these films achieved a quantum efficiency of 33% at 1500 nm under 0 V bias, a threefold improvement compared to standard devices.

Solution‐processed PbS colloidal quantum dots (CQDs) are promising optoelectronic materials for next‐generation infrared imagers due to their monolithic integratability with silicon readout circuit and tunable bandgap controlled by CQDs size. However, large‐size PbS CQDs (diameter &gt;4 nm) for longer shortwave‐infrared photodetection consist mainly of {100} facets with incomplete surface passivation and unsatisfied stability. Here, it is reported that perovskite‐bridged PbS CQDs, in which the {100} facets of the CQDs are epitaxially bridged with CsPbI3–xBrx perovskite, can achieve improved passivation and enhanced stability in comparison with the traditional strategies. The resultant infrared CQDs photodiodes exhibit significantly reduced dark current, nearly 50% enhanced photoresponse, and improved work stability. These superior properties synergistically produce the most balanced performance (with a high −3 dB bandwidth of 42 kHz and an impressive specific detectivity of 6.2 × 1012 Jones) among the reported CQDs photodetectors.