Tailoring Electronic Properties of Colloidal Quantum Dots for Efficient Optoelectronics

Colloidal quantum dots (CQDs) are nanocrystals synthesized in solution, boasting remarkable optical properties and notable electronic characteristics, such as size‐tunable bandgaps and high photoluminescence quantum yield. These features, coupled with solution processability, position CQDs as potential candidates for cost‐effective and high‐performance optoelectronic devices. However, several technological challenges hinder the full exploitation of CQDs in optoelectronics. Among these is the need for long insulating organic ligands in liquid‐phase synthesis, which restrict efficient charge injection and transport in quantum dot (QD) films. Furthermore, the high surface‐to‐volume ratios and core–shell structures prompt complexities in terms of doping and modifying electronic properties. The colloidal nature of quantum dots (QDs) also raises challenges regarding controlled deposition and patterning, which are critical for device fabrication. In this review, the imperative is outlined to tailor CQDs for optoelectronic applications, the limitations that obstruct the implementation of desired modifications are elaborated on, and the specific hurdles confronting electronic coupling, targeted doping, and precision patterning of CQDs are focused on. Additionally, herein, a summary of the solutions proposed to date is offered, insights are shared on the discussed topics, and areas warranting future investigation are highlighted.

complicate the controlled doping of quantum dots (QDs).Additionally, the solution-processed methods used to deposit CQDs often lack precision in controlling film thickness and surface morphology.Patterning of CQDs is also problematic, as conventional photolithographic methods are less effective with solution-based materials.
19,[26][27][28][29][30][31][32][33][34] Through this review article, we aim at providing a structured summary of efforts carried out to realize electrical conductivity, on-demand doping, and precise patterning of CQD films.We outline the major challenges that hinder achievement of the aforementioned capabilities in CQDs, summarize important works related to these subjects, and provide our perspectives regarding the matters under debate.The important approaches used to enhance electronic transport in CQD films include ligand engineering, creation of supercrystal assemblies, and hybridization of QDs with other materials.Doping in CQDs can be achieved by tuning stoichiometric ratios of the precursors, injecting impurity dopants, exposing to ambient gases, transferring charge from remote dopants, and electrochemical charging of CQDs.Precise patterning of CQDs can be accessed through high-resolution printing, direct photolithography of QDs made sensitive to light, and modified photolithographic approaches resembling to conventional methods.
Figure 1 presents a graphical summary of the key approaches that have been reported in past for electronic coupling, doping, and patterning of CQDs and summarized in this review.Rest of the article is organized as follows.Section 2 and 3 discuss electronic coupling and doping of CQDs respectively, while Section 4 highlights the essential trade-off between electronic functionality and luminescent efficiency of QDs.Section 5 summarizes the available methods to precisely pattern the CQD films, Section 6 extends the debated ideas to sister materials of CQDs, and Section 7 presents summary and perspectives regarding the ideas discussed throughout the text.

Enhancement of Charge Transport in CQD Assemblies
Efficient charge transport is one of the most desired characteristics for all materials intended to be utilized in electronic and optoelectronic applications.[42][43] The electrical conduction in CQD assemblies is primarily challenged by the lack of robust coupling among the individual dots. [4,5]46] The arrays of CQDs are created once these are printed or coated on a substrate and subsequently the carrier solvent is evaporated.The morphology of such arrays depends on the size, shape, and uniformity of individual QDs, as well as the characteristics, especially evaporation rate of the solvent.For instance, a Figure 1.A graphical summary of reported techniques for electronic coupling, controlled doping, and disposition as well as patterning of CQDs.
carrier solvent with rapid evaporation leaves behind a glassy QD solid with only short-range order while a slowly evaporating solvent results in a relatively long-range-ordered supercrystal of QDs. [4,47]Overall, creating electrically conductive CQD assemblies is a tricky mission and requires various factors to be taken into account.[50][51] The coming subsections discuss in detail the effectiveness and limitations of the methods available to improve charge transport in CQD films.

Ligand Engineering
As mentioned earlier, the outer surface of CQDs is decorated with ligands to help stabilize their colloidal synthesis and enable their colloidal suspensions.When CQDs are deposited on a substrate, these ligands introduce gaps between these dots, hindering electronic transport.Moreover, the insulating nature of the typical organic ligands exacerbates this issue. [4,48,52]Ligand exchange is an extensively used approach for altering the surface properties of CQDs and other nanomaterials. [45,53,54]In attempts to enhance the conductivity of CQDs, long chain-like organic ligands are usually replaced with short inorganic conducting ligands, which help reduce inter-dot spacings and improve electrical conduction within the material. [55,56]igure 2a schematically shows the impact of surface ligands on the inter-dot spacing in CQD assembly.Longer ligands induce wider spaces among the dots and vice versa, while ligand-free QDs can expectedly be packed tightly through careful deposition.The phenomenon of enhanced coupling in QDs is evident from the transmission electron microscope (TEM) images of PbSe QDs shown in Figure 2b, which exhibit reduced interparticle spacings to various degrees when topped with an oleate capping and treated with aniline and ethylenediamine. [57]][60][61] Pre-deposition ligand exchange is considered to be more robust as it takes place in the solution phase but often affects the suspension homogeneity of QDs Inter-dot spacing is reduced when long-chain ligands are replaced with short-chain inorganic ligands enhancing the probability of charge transport (middle).Bare QDs, with no ligands at all, can couple together finely, and in turn, conduct better (bottom).b) TEM image of oleate-capped PbSe QDs forming a closely packed 2D assembly with an average inter-dot spacing of ≈1.8 nm (left).Inter-dot spacings in PbSe QDs reduce to ≈0.8 nm when PbSe QDs are treated with aniline (middle).PbSe QDs come closer when treated with ethylenediamine with inter-dot spacings going less than 0.4 nm (right).(b) Reproduced with permission. [57]Copyright 2006, American Chemical Society.c) PL spectra and liquid-phase PL QY of CdSe/ZnS QDs with the original oleic acid ligands, partially exchanged ligands with OH groups, and fully exchanged inorganic ligands.The PL QY of QDs is reduced by the exchange reactions.Figure data adapted from ref. [234].d) A comparison between the EQE of LEDs featuring as-synthesized and chlorinated CQDs.The EQE enchantment in the case of chlorinated QDs is attributed to improved charge transport due to Cl-based short-length conductive ligands.Figure data adapted from ref. [62].
in the carrier solvent.Additionally, commonly tested materials for ligand exchange are highly toxic and difficult to handle, such as acids and halide-based reagents. [45,62]Post-deposition ligand exchange is also a quite commonly adopted method for inducing conductivity in CQD thin films.Such approaches have also been proven beneficial though the exchange is generally inhomogeneous depending on the morphology of the QD film.The ligands of CQDs present below the surface of the film are not exchanged adequately, and this inconsistency intensifies with increasing the film thickness.Either performed before or after deposition, ligand exchange, being a post-synthesis method, mandates additional human effort, chemical reagents, and laboratory resources.Moreover, the emission characteristics of CQDs are often degraded as a result of ligand exchange processes, as apparent from Figure 2c, showing a sharp decrease in PL QY of ligand-exchanged CdSe/ZnS QDs. [57]espite challenges and shortcomings, ligand exchange remains a promising strategy for tailoring the properties of QDs for various purposes including enhancement of electrical conductivity.Figure 2d compares the external quantum efficiency (EQE) of LEDs featuring as-processed and chlorinated CQDs.EQE of the device is enhanced due to improved charge transport as Cl-based short-length conductive ligands replace the original oleic-acid-based ligands. [62]mprovement of the conductivities can also be achieved by the partial or complete removal of surface ligands, that helps create closely packed QD assemblies. [63,64]This removal process is generally performed after deposition as bare QDs cannot make stable dispersions in carrier solvents, and precipitation of dots occurs instead. [65,66]Thus, pre-deposition ligand removal makes it challenging to use usual coating methods, particularly when attempting to create sufficiently thin QD films.[69] While ligand removal stands as an advantageous approach for improving the conductivity of CQD films, taking residual reagents off the QD film in case of postdeposition removal and choice of the deposition method in case of pre-deposition removal remain the key matters of concern.
Different types of ligands have been employed as a means to purposely dope CQD films and colloids, by identifying their doping character.[72][73][74][75] The chemical nature of ligands, especially the reaction tendencies of associated functional groups, is the primary consideration in attempts to achieve desired types and levels of doping in CQDs.Numerous works can be found in literature that identify chemical species helping with intentional doping of specific classes of CQDs.For instance, treatment of PbSe QDs with hydrazine switches their conductivity from p type to n type, by donating electrons. [55]2][83]

Superlattice Formation
The aim of a typical synthesis procedure is to create uniformshaped monodisperse QDs which can be processed as a colloid for device fabrication.Such colloids fail to yield well-coupled QD arrays when coated or printed on a substrate, as described earlier.By modifying the reaction parameters and precancers, QDs can be made to assemble into closely packed superstructures, commonly referred to as superlattices or supercrystals of CQDs.86] Figure 3a shows the schematic structures of individual QDs and their superlattice.][92]  ) can be assembled into hexagon-shaped supercrystals with an average inter-dot spacing of <0.5 nm.The plots shown in Figure 3b display the absorption cross section (C abs ) of said supercrystals whereas the TEM image in Figure 3d shows their surface morphology.The formation of supercrystals can be realized to improve coupling among QDs while preserving their emission characteristics, as evident from Figure 3c. [93]he plots displayed in Figure 3e compare the characteristic electron mobilities of the epitaxially connected supercrystals and 1,2-ethanedithiol (EDT)-bridged assemblies of PbS QDs against QD diameter.The superlattices of PbS QDs exhibit significantly higher electron mobilities compared to the usual suspension of QDs. [85]A comparison between the measured current densityvoltage ( J-V ) characteristics of solar junctions featuring usual QD films and packed QD solids enabled by hybrid organicinorganic cross-linker reagents is shown in Figure 3f.The closely packed QD solids exhibit superior performance in terms of electrical transport as compared to suspended QDs. [59]

Hybridization with Other Materials
Integration of CQDs with charge-spreading layers is a commonly used technique to enhance their conductivity.[96][97][98][99] The charge-spreading layers play the role of a bridge between CQDs and the electrodes in these devices, enabling efficient charge transport and reducing the probability of charge recombination (refer to Figure 4a).The hybridization of CQDs with conductive materials is common in devices that are aimed at enabling large-area emission and flexible architectures.0][101][102][103][104] Figure 4b shows the schematic structure of a device exciting electroluminescence (EL) from QDs with the help of AC voltage and employing a sparse network of CNTs to improve charge spreading in CQDs. [102]he selection of a material for a charge-spreading layer is made according to specific requirements of the targeted device and deposition is usually performed using solution-based methods.In some cases, conductive materials are directly mixed with CQDs to get a composite that conducts better than pure QDs. [33,95,105][108][109][110] Along with that, composites of QDs and conductive materials have been utilized in light-emitting and photodetection devices to access improved performance.Specifically, near-IR LEDs exhibit higher EQE when a composite of PbS QDs and hybrid conductive perovskites is used over pure QDs (refer to Figure 4c).The enhanced EL is attributed to better utilization of input current owing to improved charge transport in the QD-perovskite composite.Figure 4d-f displays the optical characteristics of a QD-graphene composite and the transient photocurrent response of a detector constructed using the same composite.The composite material shown provides detectors with improved absorption (measured for 600-1200 nm window) and photocurrent compared to pure QDs, as evident from Figure 4d,f.However, the PL of QDs is observed to decrease with increased proportions of graphene, possibly due to the masking of the emitted light (refer to Figure 4e). [111]verall, the hybridization of QDs with conductive materials can help improve the performance of optoelectronic devices due to enhanced charge transport.Given that, the preservation of emission characteristics remains an important consideration while adopting this strategy.
Though the utility of techniques like ligand engineering, superlattice formation, and hybridization for enhancing the conductivity of CQDs is well established, the choice of an appropriate approach is highly specific to application circumstances.114] Superlattice formation is a conceptually simple approach to improve charge transport in CQD assemblies but creating well-ordered and scalable superlattices is a challenging task. [93,115]Hybridization of CQDs with other materials may create interfacial issues and requires careful selection of complementing materials, keeping application circumstances in view. [94]

Doping of CQDs
The electrical characteristics of a material rely on various factors such as the type of the majority charge carrier, carrier concentrations, and mobility of carriers.[118] The electronic and optical characteristics of CQDs are also considerably influenced by doping.However, the control of doping levels in CQDs is particularly challenging due to several factors.For instance, a single CQD generally comprises a few hundred atoms, most of which are located on the dot's surface due to the high surface-to-volume ratios of CQDs. [23,119]The ligands covering the surface of CQDs passivate the dangling bonds and balance out the possible excess charge of the QD core in addition to providing solvent dispersity.][122][123] The synthesis protocols of CQDs are technically complex and require strict control over reaction parameters and conditions, thus the room available for insynthesis doping of CQDs is also limited. [50,124,125]Another hurdle to maintaining the defined doping levels in CQDs is the possibility of unintended doping of CQDs caused by the chemical interactions of ambient reagents (like oxygen and hydroxyl groups) with the QD surface. [70,126,127] lot of research has been conducted on the matter of ondemand doping of CQDs and the efforts to beat the aforementioned challenges have always been continued.In the coming subsections, we summarize the key approaches introduced in efforts to achieve controlled doping of CQDs along with discussing their merits and limitations.The available literature on the subject suggests that CQDs can be doped by exchanging ligands, tuning stoichiometry, introducing heterovalent impurities, exposing to ambient gases, transferring charge remotely, and  [102] Copyright 2020, Nature Publishing Group.c) EQE-current density performance of LEDs incorporating PbS QDs embedded in hybrid perovskite matrix for enhanced charge mobility.The best devices exhibit an EQE of 5.2% AE 0.3% at an emission center of 1,391 nm in near-IR region.Figure data adapted from ref. [236].d,e) Absorption and emission spectra of pure QDs and those hybridized with graphene.The QD-graphene hybrid exhibits enhanced absorption whereas PL of the hybrid is decreased with increased graphene proportion (wt%, percentage by weight).f ) Transient photocurrent response of QD-graphene composite-based near-IR photodetector under an optical power density of 31.8 mW cm À1 with a fixed bias of 10 V.The composite material offers higher photocurrent levels compared to pure QDs owing to better charge transport.(d-f ) Figure data adapted from ref. [111].
charging through electrochemistry.The following text critically reviews each of the aforementioned techniques excluding ligand exchange which has already been conferred in Section 2.1.

Stoichiometric Tuning
Stoichiometry refers to the relative proportions of various chemical species present in a compound or a material.The CQDs can be differentiated on the basis of the number of constituent elements as binary (e.g., CdSe), tertiary (e.g., CuInSe 2 ), and so forth.The Fermi level (E F ) of CQDs can be controlled by tuning the stoichiometric ratios of the precursors involved in the reaction as it relates to the relative ratios of cations/anions and the presence of structural defects. [43,71,128]igure 5a schematically demonstrates that CQDs can be doped n-type or p-type by adjusting the relative proportions of the precursors supplying anions cations, respectively.The stoichiometry of CQDs is very sensitive to the purity and quantity of the surfactant ligands and heavily relies on the synthesis conditions as well as the size of QDs. [74]As the factors affecting the synthesis of CQDs have to be balanced for successful reactions, the room to tune doping levels in QDs through stoichiometric adjustment of the precursors is fairly limited.][130] For instance, carrier statistics in PbSe QDs can be heavily manipulated by the stoichiometric imbalance between Pb and Se, leading to the synthesis of doped QDs.The synthesized PbSe QDs exhibit n-and p-type characteristics when the relative ratios of Pb and Se are increased, respectively (refer to Figure 5b).The output characteristics of PbSe-based field-effect transistors (FETs) shown in Figure 5d,e are depictive of n-and p-type responses corresponding to Pb-and Se-rich QD channels. [43]

Elemental Impurities
Addition of heterovalent elemental impurities to alter the concentration of either charge is the most widely used approach to dope bulk semiconductors.The CQDs can also be doped with elemental impurities, though such methods are not as successful for CQDs as these are for bulk materials, owing to the challenges described earlier in Section 3. Studies have indicated that the doping levels of CQDs can be adjusted by incorporating impurity atoms during the synthesis process, through triggered ion exchange reactions.Figure 5c schematically describes a typical procedure where CQDs can be doped by the impurity reagents injected during their synthesis.
[133][134] The impurity atoms are generally injected after the nucleation of core is triggered, followed by the growth of the shell.[135] A particular report suggests that a thin film of CdSe QDs can be doped n-type as a result of thermal diffusion of In from an alloyed metal contact. [41]The optical characteristics of Al-and In-doped CdSe QDs can be witnessed in Figure 5f. [133]he cation exchange reactions can be used to achieve both pand n-type doping of CdSe QDs.Unlike the trivalent metals, addition of monovalent metal atoms (e.g., Ag and Cu) yields p-doped CdSe QDs as they act as electron acceptors after substituting Cd þ2 atoms. [136,137]The impurity doping often results in distinctive changes to the properties of CQDs along with the modifying their electronic features.40]

Controlled Exposure to Ambient Gases
With their ultrasensitive surfaces and high surface-to-volume ratios, CQDs exhibit significant variations to their electronic profiles in response to the environmental reagents, particularly moisture, and ambient gases.The process of unintentional doping of CQDs by the ambient gases can be regulated and controlled doping can be attained by methodizing the interaction.For instance, EDT-treated PbSe CQDs can be made electronand hole-rich by systematically exposing them to N 2 and O 2 , respectively.The underlying mechanism is schematically illustrated in Figure 6a.The transfer characteristics associated with the FETs incorporating O 2 -and N 2 -exposed PbSe QDs indicate the respective p-and n-type behavior (refer to Figure 6b,c).The conductivity of QD films is intensively modulated against the gas pressure in a totally reversible fashion, as rendered by Figure 6d. [141]everal other studies provide evidence for the shifting of Fermi level when CQDs interact with oxygen, though the presence of surface ligands is largely thought to evade oxidation. [70,142]][145] To exemplify, oxidation of PbS and PbSe CQDs leads to the formation of lead oxides and sulfates (e.g., PbSO 3 and PbSO 4 ), giving rise to the creation of trap states and modulation of Fermi level. [144]

Remote Charge Transfer
Doping of materials can be realized by allowing remote transfer of charge from a donor species placed in the vicinity of a host material.[153] The concept of doping CQDs through remote charge transfer is visualized schematically in Figure 6e.
An approach enabling the solution-phase transfer of electrons from sodium biphenyl to CdSe, CdS, and ZnO QDs helps achieve n-type doping of these materials.In this particular demonstration, the donor material, i.e., sodium biphenyl provides biphenyl radical anions and the transfer of electrons to CQDs is confirmed through IR and visible absorption spectroscopic studies. [154]Another such work reports doping of PbS QD solids by employing a library of metal-organic complexes with varying electron affinities (EAs) and ionization energies (IEs).Both p-and n-type doping are accomplished where deep EA complexes, being oxidants, shift the Fermi level of the CQDs toward the valence band edge whereas complexes with low effective IEs act as reductants and shift the Fermi level toward the conduction band edge.The J-V characteristics shown in Figure 6f correspond to a solar cell junction constructed using p-doped PbS QDs where Mo(tfd-COCF 3 ) 3 facilitates remote charge transfer. [151]hough the potential of remote doping is quite obvious and several successful demonstrations have been reported, the utility of remote doping in device design is limited as the process is unstable and often reversed after a short duration of time. [154,155]

Electrochemical Charging
Electrochemical doping of CQDs is principally similar to chargetransfer doping, where charge carriers generated by electrochemical splitting of an electrolyte are absorbed by the QDs.][157] To specify, HgS and HgSe CQDs are reversibly doped n-type when subjected to an electrochemical reaction using tetrabutylammonium perchlorate in propylene carbonate as an electrolyte. [157]The injection of both electrons and holes into the quantum confined states of PbSe QDs has been realized using an anhydrous solution of LiClO 4 in acetonitrile as an electrolyte and Ag as an electrode.The injection of holes is observed to be less profound and doping effect to be more visible with growing dot size. [156]The process of electrochemical doping can significantly degrade the emissive response of QDs but such decline is reversible as the original PL QY of CdSe has been reported to have been restored ≈60 min after the electrochemical reaction ends. [155]he usual approaches reported for electrochemical charging of CQDs have very limited scope for optoelectronics as these require continuously operational electrochemical setups to sustain doping.An approach to realize permanent electrochemical doping of CQDs has also been demonstrated.Figure 6g schematically illustrates the process where CQDs are first doped by electrochemically generated charge carriers and then the solvent is frozen before disconnecting the electric potential.In this way, the charges absorbed by the CQDs remain with them and the reversal of doping is effectively avoided. [158]calable doping of CQDs, like other low-dimensional materials, is an active and long-standing challenge.Despite usefulness for certain applications, post-synthesis approaches like ligand engineering, surface charge transfer, and electrochemical charging suffer from deficits including low stability, lack of control, and reversibility. [159]In-synthesis approaches, like stoichiometric tuning, and inclusion of elemental impurities, usually provide relatively stable doping though achieved doping levels are limited and optical properties of the material may be compromised. [159]

Balancing Luminescence Efficiency and Electronic Potency in CQDs
To enable efficient optoelectronic devices, assemblies of CQDs need to retain robust electronic transport characteristics and high luminescence efficiencies.In real scenarios, simultaneous preservation of these features is immensely challenging while constructing devices from CQDs.Most of the strategies aiming at doping, electrical activation, and patterning of CQDs involve some sort of ligand exchange operation or surface modification process.As the recipes for the colloidal synthesis of CQDs are usually optimized for high optical performance, processes like ligand exchange and surface modification mostly cause a sharp decline in PL QY of CQDs.This gives rise to a trade-off between the electrical performance and optical responsivity of CQDs, which establishes a technologically important challenge.
The tradeoff between PL QY and conductivity can be explained by considering the relationship between charge dynamics and recombination pathways in excitonic systems.An excitonic recombination model, primarily formulated for transition-metal dichalcogenides, suggests that such systems exhibit high PL QYs in the absence of excess charges and vice versa. [140]According to the formulation, excess charges trigger the formation of charged trions which reduce PL QY of the material by recombining in a nonradiative fashion.In contrast, excess charge-free excitonic materials feature only neutral excitons which predominantly recombine in radiative manner and help maintain high PL QYs. [140,160]As CQDs are essentially excitonic systems, ligand exchange and surface modification processes possibly disrupt their charge neutrality and hence the material's brightness is affected detrimentally.The strict contradiction between the need for free charges to enable electrical transport and the requirement to eradicate excess charges for high PL QY mandates looking for strategies to find a balance between the two effects.
The idea of integrating charge-transport layers with CQDs can be beneficial in this regard as it does not necessitate surface modification procedures.The choice of a suitable material for this purpose would certainly depend on the specific application scenarios.For instance, a sparse network of CNTs can work adequately as a charge-transport layer in an AC-driven device prompting pulsed EL from CQDs. [102]More of such methods need to be explored to achieve simultaneous maintenance of electrical performance and optical efficiency of CQDs.

Patterning of CQDs
Controlled deposition and precise patterning are extremely important considerations for any material to be used for  [141].e) A schematic diagram showing the general mechanism of remote charge-transfer doping of CQD films.f ) J-V characteristics of solar cell junctions incorporating remotely doped PbS QDs.The doping process is carried out after deposition and Mo(tfd-COCF 3 ) 3 acts as a remote p-type dopant.The numbers shown as insets represent the doping concentration.Adapted with permission. [151]g) Schematic illustration of the process governing permanent electrochemical charging of CQDs.The CQDs are doped by electrochemically generated charge carriers and the solvent is frozen before disconnecting the electric potential to avoid reversal of doping. [158]esigning devices, especially at micro-and nano levels.Epitaxially grown materials can be robustly patterned using standardized techniques and hence the devices made of those materials are usually efficient and scalable.CQDs, being solution-processed, can be deposited predominantly using coating and printing methods and precise patterning of them is also challenging.][163][164][165][166][167][168] Extensive efforts have been carried out to accomplish controlled deposition and accurate patterning, and a set of such techniques can be witnessed being employed quite frequently.[171][172][173][174][175][176] The detailed discussion on each of the aforementioned methods is presented in the following subsections.

High-Resolution Inkjet Printing
Inkjet printing is a widely used technique to create microscale patterns of solution-based materials.This technique has played a pivotal role in the development of several technologies, notably printed electronics, flexible electronics, and organic electronics. [177,178]Inkjet printing primarily relies on the availability of an inked material and a controllable printing mechanism.A wide range of inkjet printers with varied capabilities are available and inks are generally prepared ensuring their compatibility with the specific printer to be used.][181][182] Inkjet-printed QDs have been employed in plenty of applications, most importantly full-color red-bluegreen (RGB) displays, and LEDs of varied sizes including μ-LEDs. [33,183,184]The RGB displays of QDs commonly require microscale patterns of rectangular shapes which can be created conveniently using an appropriate printing method (refer to Figure 7a).Factors like maximum printing resolution and film uniformity provide the ground for competition among different printing methods.Figure 7b displays a luminescent image of an RGB array created using inkjet printing of CdSe/ZnS CQDs with RGB emission. [185]he use of inkjet printing usually enables the creation of goodquality patterns with sub-hundred microns dimensions but a few challenging matters are still there. [180,186,187]The patterns created with inkjet printing methods can be disrupted by the outflow of QD ink from the edges of the pattern.Even if this discrepancy is eradicated, the flow of QD inks toward the edges of the patterns leads to the formation of coffee rings and a nonuniform film profile. [179,188,189]Jiang et al. show that utilizing 1,2-dichlorobenzene with cyclohexylbenzene as a carrier solvent for green-emitting CdSe/ZnS QDs can help eradicate the coffee-ring effect in microscale printed films, as depicted in Figure 7c. [188]

Transfer Printing
Transfer printing, sometimes referred to as contact patterning, is a conceptually simple but effective technique to create patterns of colloidal materials including CQDs.In a typical process, a film of CQDs is deposited on a source substrate, and a patterned stamp is rigorously pressed on it.A thin film of QDs sticks to the stamp which is consequently transferred to the host  [185] Copyright 2020, Royal Society of Chemistry.c) Florescence micrographs of inkjet-printed CQDs with coffee-ring effect (left) and flat surface profile (right).Reproduced with permission. [188]Copyright 2016, American Chemical Society.d) Schematic illustration of the mechanism governing transfer printing of CQDs.Generally, a patterned stamp is used to collect a thin film of CQDs from a substrate and transfer it to the target substrate.e,f ) Fluorescence microimage and SEM profile of transfer-printed RGB QD stripes onto a glass substrate using a nanostructured stamp.(e,f ) Reproduced with permission. [190]Copyright 2011, Nature Publishing Group.
substrate. [171,172,190,191]A schematic depiction of the transfer process is visualized in Figure 7d.A large number of works can be found in the literature reporting the use of transfer printing to achieve patterned QD films for diverse purposes.][194] Figure 7e,f shows the fluorescence microimage and scanning electron microscope (SEM) profile of transfer-printed RGB QD stripes onto a glass substrate using a nanostructured stamp. [190]ransfer printing works well when the feature size is large enough but achieving precise alignment and registration of the transferred pattern becomes challenging with reduced feature sizes, smaller inter-feature distances, and increased design complexities.Additionally, transfer printing is typically a low-yield process and the uniformity of created patterns is another important challenge that limits the usefulness of the technique. [174,195,196]

Direct Lithography of Photosensitive CQDs
Photolithography is one of the key material processing techniques used to create well-defined patterns of a range of materials and plays a vital role in the fabrication of devices and integrated circuits.A typical photolithography process involves exposing a photosensitive material (photoresist, abbreviated as PR) coated on the substrate to light through a photomask, creating a patterned image that is subsequently developed, etched, or otherwise processed to obtain the desired design.The standard photolithography procedures often necessitate using several chemical reagents for the development and liftoff of the PR.The CQDs, being soluble to a range of chemicals, are not compatible with the usual PR-aided photolithography process.
There is another major type of photolithography, referred to as direct photolithography, that enables direct patterning of photosensitive materials without involving a sacrificial PR coating.In a typical process of such kind, a photosensitive material is directly exposed to light and the exposed portions of the material undergo some light-induced chemical change.The photo-assisted modification of the material (e.g., a change in solubility) makes it possible to etch away the unexposed portions and therefore get the desired pattern. [197,198]The concept of direct lithography has recently attracted much attention as a promising strategy to achieve defined patterns of CQDs.2020, Nature Publishing Group.c) Fluorescence microscope images of lead halide perovskite QDs patterned using photoactivation-enabled photolithography.Reproduced with permission. [199]Copyright 2021, American Chemical Society.
Figure 8b schematically describes one of the reported methods to create photosensitive QDs by mixing them with fluorinated phenyl azides.On exposure to UV light, the formation of ligand cross-linkers (referred to as LiXer by the authors of the report) takes place which makes the UV-exposed CQDs stick to the substrate.The mechanism of cross-linking relies on a UV-instigated chemical reaction between the fluorinated phenyl azides and the alkyl chain of QD surface ligands.The unexposed CQDs can finally be stripped off by dipping the substrate into the carrier solvent. [206]The direct photolithography is equally useful for other classes of CQDs as the idea of photoactivation is often realized by exploiting the surface ligands.Figure 8c presents the fluorescence microscope images of various perovskite QDs patterned using the said approach. [199]t is worth noting that the photosensitive ligands achieved using different chemical reagents are normally activated against a specific wavelength range, hence the respective light sources have to be identified and arranged.209] Photoactivation-enabled direct lithography is perhaps the most efficient and scalable patterning strategy for CQDs among several others reported so far.

Modified Conventional Photolithographic Methods
It has already been established that the patterning of CQDs by means of conventional lithography is inherently challenging due to their colloidal nature.Nevertheless, certain modifications to QDs and careful choice of PRs as well as chemical reagents used for etching and stripping can allow using photolithography to pattern CQDs.From the available literature, two distinctive routes can be identified that have adopted to pattern CQDs using conventional photolithography.The first route involves deposition of the PR on a pre-deposited QD film with appropriate surface characteristics and carrying out the same processes as traditional photolithography.][212][213] The schematic diagrams shown in Figure 9a,b visually describe the photolithography of In such a method, the pattern is transferred to a sacrificial RP layer, QD film is deposited and finally, the PR is lifted off to get a QD pattern.The appropriate choice of chemicals is an important consideration for both of the previously described methods.c) Fluorescence micrograph of CdSe/CdS QDs patterned through conventional lithography.Reproduced with permission. [214]Copyright 2014, IOP Publishing.d) Fluorescence microscope images of dot-shaped patterns of red-emitting CdSe/CdS/CdZnS QDs, achieved using a photolithographic approach aided by atomic layer deposition (ALD)-deposited ZnO films.Reproduced with permission. [175]Copyright 2021, American Chemical Society.e) PL image of a full-color display realized through layer-by-layer assembly of CQDs.Reproduced with permission. [215]Copyright 2016, American Chemical Society.
CQDs using the first and second route, respectively.For convenience, we term the first route as conventional photolithography process and the second route as liftoff throughout rest of the text.
The role of QD surface ligands is crucial for the photolithography of QDs carried out using either route.The conventional process requires QDs to be resistant to the chemicals used for PR development and stripping while the liftoff demands for QDs with an ability to stick to the substrate and resist the chemicals used for PR stripping.The aforementioned characteristics can be developed by appropriately modifying the surface chemistry of QDs, hence photolithographic patterning of CQDs is often accompanied by a suitable ligand exchange process.Perseveration of the optical properties of CQDs during the surface medication and patterning processes is a challenging task as simultaneous balancing of this many considerations is never an easy mission.For instance, the brightness of PbS QDs is observed to decrease significantly when their native ligands are replaced by inorganic ligands that could tolerate the chemicals used for developing and stripping the PR. [214]Insertion of an intermediate layer between CQDs and the PR can help reduce intermixing of the two materials and retain PL QY of QDs to some extent. [175]Some reports suggest that functionalization of the substrate can serve as a tool to enable layer-by-layer assembly of CQDs or improve adhesion of CQDs to the substrate surface. [175,215]Specific examples of CQD patterns generated using conventional photolithography, liftoff, and layer-by-layer assembly have been displayed in Figure 9c-e. [175,214,215]tterning of CQDs is an inherently challenging task due to the colloidal nature of these materials and the efficacy of available methods is limited by certain shortcomings.For instance, the resolution of inkjet-printed QD patterns is limited, transfer printing is associated with issues like lack of accuracy and low resolution while conventional photolithographic approaches need to resolve compatibility problems.Direct photolithography is a potentially better approach to achieve high-resolution patterns of CQDs, though processes carried out to create photosensitivity may result in material degradation. [216]

Colloidal Quantum Wells as Sister Materials of CQDs
The 0D CQDs, discussed throughout the text, are the subset of a larger family of materials known as colloidal nanocrystals.Colloidal quantum wells (CQWs) or nanoplatelets are another important member of the nanocrystal family having similar properties to those of QDs, except for the shape and dimensionality.CQWs are 2D nanocrystals, synthesized using chemical approaches closely resembling those used to synthesize CQDs.219][220][221][222][223][224][225][226] The geometric variants of CQWs mainly include core-only, core/crown, and core/crown/shell nanoplatelets (refer to schematic structures shown in Figure 10a).The primary function  [227] Copyright 2015, American Chemical Society.c) Optical response of CdSe/CdTe core/crown CQWs. Figure data adapted from ref. [227].d) PL spectra associated with pristine and Cu-doped QWs visualizing the effect of doping on the material's optical response.Adapted with permission. [232]e,f ) Output characteristics of an EL device involving CQWs heterostructures and charge-transport layers.CE: conversion efficiency.Data adapted from ref. [233].
of crowns and shells is to sustain the excitonic confinement, as it is of the shell in the case of CQDs.The deposition of CQWs too is generally accomplished using the wet-coating methods and the optical response of their suspensions and films resembles that of QDs, as evident from the TEM profile and optical spectra of CdSe/CdTe core/crown CQWs shown in Figure 10b,c, respectively. [227]eeping the shared features in view, it can be assumed that CQWs could be tailored using techniques similar to those discussed in this review regarding CQDs.Many of the previously reported works support this assumption by highlighting the similarities between the modification approaches used for CQDs and quantum wells (QWs).[230][231] The plots shown in Figure 10d compare the PL spectra of pristine and Cu-doped QWs, indicating extensive shifting and broadening of the material's emission. [232]The characterization results displayed in Figure 10e,f are associated with a typical EL device involving CQWs heterostructures and charge-transport layers. [233]n short, CQWs are dimensionally unique analogs of CQDs sharing a similar set of electronic and optical properties.The approaches introduced to modify CQDs can expectedly be useful for CQWs as well, though diverging outcomes are also probable.Further studies regarding tailoring of CQWs can open new avenues of possibilities and these can play an important role in the technological advancement of optoelectronics as CQDs potentially can.

Summary and Perspectives
In summary, a review of techniques enabling electrical conductivity, on-demand doping, and precise patterning of CQDs was presented.The electrical transport in CQD assemblies can mainly be enhanced by replacing long-chain, insulating organic ligands with short-chain inorganic conducting ligands, creating superlattice structures or hybridizing with other materials.The key methods to realize controlled doping in CQDs include misbalancing charges through stoichiometric tuning, addition of elemental impurities during synthesis, exposing to ambient gases, and enabling charge transfer using remote dopants or electrochemical reactions.Defined patterns of QDs can be generated by means of high-resolution printing, transfer printing, direct lithography facilitated by photosensitization, and conventional photolithography methods with explicit amendments to the process.
The matters discussed in this review are few of the most important considerations regarding CQDs from the perspective of efficient device design.The lack of a robust general strategy to enable steady electronic transport in CQD films is one of the major factors hindering realization of efficient optoelectronic devices of these materials.The productivity of existing approaches to serve the task is often limited as the optical characteristics of CQDs are usually degraded in efforts to improve their conductivity.The possible reason behind this discrepancy could be the relation between excess charge and recombination kinetics of excitonic systems where an increased number of excess carriers causes a decrease in PL QY and vice versa.The scenario demands looking for approaches enabling robust electronic coupling among CQDs without disturbing the overall charge neutrality of the material.
Controlling doping levels in CQDs is also a challenging mission due to their large and ultrasensitive surfaces, the role of surface ligands, and rigid synthesis protocols.The doping techniques reported till date suffer from limitations like lack of control, limited stability, and tough processing methodologies.Further in-depth studies are needed in pursuit of techniques capable of providing stable on-demand doping of CQD assemblies.
Patterning of CQDs using direct photolithography is a promising approach provided reliable protocols are developed to photosensitize CQDs.The said approach is known to provide relatively precise and high-resolution patterns of CQDs compared to other methods. [173,174,197]Conventional photolithographic methods exhibit limited value in the context of patterning QDs as those bring in experimental inconvenience by imposing too many considerations.Inkjet printing and transfer printing are useful methods to create patterns of CQDs but the factors like resolution, uniformity, and yield remain the matters of concern.
The synthesis protocols of CQDs are usually optimized to yield materials with the best optical response, especially in terms of emission efficiency.The processes carried out for enhancing conductivity, doping, and patterning usually deteriorate the optical characteristics of QDs, proposedly due to the alteration of optimum charge dynamics.Based on the nature and requirements of targeted applications, a balance has to be found between the emission efficiency and electronic properties of CQDs to enable high-performance optoelectronics.
In conclusion, CQDs are one of the most efficient material platforms in terms of optical characteristics.The discovery of scalable methods for electronic coupling, on-demand doping, and precise patterning of CQDs are a few of the most concerning aspects and potential subjects of future investigations regarding these materials.Once the aforementioned challenges have been overcome, CQDs promise to deliver a class of the most efficient optoelectronic devices ever seen.

Figure 2 .
Figure 2. Ligand engineering for improved electronic transport in CQDs.a) Conventional long-chain organic surface ligands induce larger inter-dot spacings (symbolized as d) in CQD thin films deterring electronic transport (top).Inter-dot spacing is reduced when long-chain ligands are replaced with short-chain inorganic ligands enhancing the probability of charge transport (middle).Bare QDs, with no ligands at all, can couple together finely, and in turn, conduct better (bottom).b) TEM image of oleate-capped PbSe QDs forming a closely packed 2D assembly with an average inter-dot spacing of ≈1.8 nm (left).Inter-dot spacings in PbSe QDs reduce to ≈0.8 nm when PbSe QDs are treated with aniline (middle).PbSe QDs come closer when treated with ethylenediamine with inter-dot spacings going less than 0.4 nm (right).(b) Reproduced with permission.[57]Copyright 2006, American Chemical Society.c) PL spectra and liquid-phase PL QY of CdSe/ZnS QDs with the original oleic acid ligands, partially exchanged ligands with OH groups, and fully exchanged inorganic ligands.The PL QY of QDs is reduced by the exchange reactions.Figure data adapted from ref.[234].d) A comparison between the EQE of LEDs featuring as-synthesized and chlorinated CQDs.The EQE enchantment in the case of chlorinated QDs is attributed to improved charge transport due to Cl-based short-length conductive ligands.Figure data adapted from ref.[62].

Figure 4 .
Figure 4. Enabling charge transport in CQD films through hybridization with other material systems.a) Schematics presenting the idea of hybridization for electrical conductivity in CQDs.A conductive material facilitates charge spreading in a film consisting of mutually spaced QDs.b) Schematic structure of an EL device employing a sparse network of CNTs as a charge-spreading layer for CQDs and several other light-emitting molecular materials.The said device enables pulsed emission from a range of materials with AC-voltage excitation.Reproduced with permission.[102]Copyright 2020, Nature Publishing Group.c) EQE-current density performance of LEDs incorporating PbS QDs embedded in hybrid perovskite matrix for enhanced charge mobility.The best devices exhibit an EQE of 5.2% AE 0.3% at an emission center of 1,391 nm in near-IR region.Figure data adapted from ref.[236].d,e) Absorption and emission spectra of pure QDs and those hybridized with graphene.The QD-graphene hybrid exhibits enhanced absorption whereas PL of the hybrid is decreased with increased graphene proportion (wt%, percentage by weight).f ) Transient photocurrent response of QD-graphene composite-based near-IR photodetector under an optical power density of 31.8 mW cm À1 with a fixed bias of 10 V.The composite material offers higher photocurrent levels compared to pure QDs owing to better charge transport.(d-f ) Figure data adapted from ref.[111].

Figure 5 .
Figure 5. Doping of CQDs through stoichiometry tuning and heterovalent impurities.a) A schematic showing the generic mechanism to control doping levels in CQDs by tuning the ratios of constituent chemical reagents.Increasing the relative proportion of a hole-or electron-supplying reagent would dope the material p-type or n-type, respectively.b) Fermi level tuning of PbSe QDs in relation to the varied concentrations of Pb (red) or Se (green) atoms added to QD thin films.[43]c) Schematic depiction of the process to dope CQDs with hetero-valent impurities where dopant precursors are added during the growth process.d,e) Output characteristics of FETs with Pb-rich QDs with varying V G from 0 to 50 V (d) and Se-rich QDs with varying V G from 0 to À50 V (e).(d,e) Data adapted from ref.[43].f ) Absorption and PL spectra of In-and Al-doped CdSe QDs, where aluminum acetylacetonate and trimethylindium precursors utilized during the synthesis act as donors for Al and In ions, respectively.Figure data adapted from ref.[133].

Figure 6 .
Figure 6.Doping CQDs with atmospheric gases, through remote charge transfer, and electrochemical charging.a) Schematic portrayal of the mechanism behind doping of EDT-treated PbSe CQDs with atmospheric gases.The treated QD films are reversibly doped p-type and n-type when exposed to O 2 and N 2 , respectively.b,c) Transfer characteristics of FETs incorporating PbSe channels exposed to O 2 (b) and N 2 (c), with a pressure of 1.2 Torr for each gas.d) Variation of PbSe film conductivity as a function of O 2 pressure and exposure time.The plot provides clear evidence of the reversibility of the employed doping approach.(b-d) Figure data adapted from ref.[141].e) A schematic diagram showing the general mechanism of remote charge-transfer doping of CQD films.f ) J-V characteristics of solar cell junctions incorporating remotely doped PbS QDs.The doping process is carried out after deposition and Mo(tfd-COCF 3 ) 3 acts as a remote p-type dopant.The numbers shown as insets represent the doping concentration.Adapted with permission.[151]g) Schematic illustration of the process governing permanent electrochemical charging of CQDs.The CQDs are doped by electrochemically generated charge carriers and the solvent is frozen before disconnecting the electric potential to avoid reversal of doping.[158]

Figure 7 .
Figure 7. Patterning of CQDs through inkjet and transfer printing.a) Schematic depiction of RGB pattern generation with the help of high-resolution inkjet printing of CQDs.b) Fluorescence microscope image of an RGB display panel created using inkjet printing of CdSe/ZnS CQDs.Reproduced with permission.[185]Copyright 2020, Royal Society of Chemistry.c) Florescence micrographs of inkjet-printed CQDs with coffee-ring effect (left) and flat surface profile (right).Reproduced with permission.[188]Copyright 2016, American Chemical Society.d) Schematic illustration of the mechanism governing transfer printing of CQDs.Generally, a patterned stamp is used to collect a thin film of CQDs from a substrate and transfer it to the target substrate.e,f ) Fluorescence microimage and SEM profile of transfer-printed RGB QD stripes onto a glass substrate using a nanostructured stamp.(e,f ) Reproduced with permission.[190]Copyright 2011, Nature Publishing Group.

Figure 8 .
Figure 8. Direct lithographic patterning of photosensitive CQDs.a) Schematic process flow of a typical lithography process for photosensitive QDs.Thin films of CQDs, made sensitive to light someway, are deposited on a substrate, exposed to UV (or appropriate) light to enable photoinduced modification, and finally developed with suitable chemicals to get the final pattern.b) Schematic description of a reported method to create photosensitive QDs by mixing them with fluorinated phenyl azides through light-driven ligand cross-linking (referred to as LiXer by the authors of the report).The mechanism of cross-linking relies on a UV-instigated chemical reaction between the fluorinated phenyl azides and the alkyl chain of QD surface ligands.Reproduced under the terms of the CC-BY 3.0 license.[206]2020, Nature Publishing Group.c) Fluorescence microscope images of lead halide perovskite QDs patterned using photoactivation-enabled photolithography.Reproduced with permission.[199]Copyright 2021, American Chemical Society.

Figure 9 .
Figure 9. Patterning of CQDs using modified conventional photolithographic methods.a) A schematic flow diagram describing a general process to pattern CQDs using conventional lithography.Such approaches involve process steps including creation of a film, PR deposition, exposure to UV light through a patterned mask, PR developing, and finally stripping of PR to get a patterned QD film.b) Schematic illustration of patterning of CQDs with the help of liftoff.In such a method, the pattern is transferred to a sacrificial RP layer, QD film is deposited and finally, the PR is lifted off to get a QD pattern.The appropriate choice of chemicals is an important consideration for both of the previously described methods.c) Fluorescence micrograph of CdSe/CdS QDs patterned through conventional lithography.Reproduced with permission.[214]Copyright 2014, IOP Publishing.d) Fluorescence microscope images of dot-shaped patterns of red-emitting CdSe/CdS/CdZnS QDs, achieved using a photolithographic approach aided by atomic layer deposition (ALD)-deposited ZnO films.Reproduced with permission.[175]Copyright 2021, American Chemical Society.e) PL image of a full-color display realized through layer-by-layer assembly of CQDs.Reproduced with permission.[215]Copyright 2016, American Chemical Society.

Figure 10 .
Figure 10.Colloidal nanoplatelets or QWs as dimensionally different analogs of CQDs.a) Schematic structures of core-only core/crown and core/crown/ shell CQWs.b) TEM images showing the assembly of coated CdSe/CdTe core/crown hetero-nanoplatelets. Reproduced with permission.[227]Copyright 2015, American Chemical Society.c) Optical response of CdSe/CdTe core/crown CQWs. Figure data adapted from ref.[227].d) PL spectra associated with pristine and Cu-doped QWs visualizing the effect of doping on the material's optical response.Adapted with permission.[232]e,f ) Output characteristics of an EL device involving CQWs heterostructures and charge-transport layers.CE: conversion efficiency.Data adapted from ref.[233].
As a specific example, PbS and PbSe CQDs capped with small and conductive metal chalcogenide complex ligands (such as Sn 2 S 6