Direct Optical Patterning of Semiconductor Quantum Dots: History
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Francesco Antolini

Patterning, stability, and dispersion of the semiconductor quantum dots (scQDs) are three issues strictly interconnected for successful device manufacturing. Recently, several authors adopted direct optical patterning (DOP) as a step forward in photolithography to position the scQDs in a selected area. However, the chemistry behind the stability, dispersion, and patterning has to be carefully integrated to obtain a functional commercial device. 

  • semiconductor quantum dots
  • ligands

1. Introduction

Semiconductor quantum dots (scQDs) closely related to their function, which is in turn linked to their stability and homogeneity. For real exploitation of the scQDs in a commercial device, this material should have three main characteristics, i.e., (i) stability in the conditions of usage, (ii) homogeneity of distribution within the device to guarantee uniform performances, and, for some applications, like in displays, (iii) the possibility to be patterned. These three characteristics, however, cannot be taken alone, because they are interconnected. Indeed, the development of the chemical processes that underlies the stability and dispersion has to be compatible with the adopted patterning technology.
The patterning strategies of scQDs can be divided into three main groups: photolithography [1], contact printing[2], and ink-jet printing [3]. Some recent developments in patterning are exploring the direct use of light as a tool for patterning as in the “classical” photolithography, but to control the position of the object of interest, the scQDs, without the use of masks and repeated steps of layer curing/etching typical of the photolithographic process. If, on one side, the direct optical patterning (DOP) of scQDs simplifies the process of patterning, on the other side this shifts on the material side the critical issues of the scQDs’ dispersion, protection, and patterning itself. Indeed, the role of light is to block the scQDs in a specific position by creating a network that sticks them, by changing the scQDs solubility or by growing them directly.

1.2 The Semiconductor Quantum Dots

Semiconductor quantum dots [4] (scQDs) are among the most studied and utilized nanomaterials because their compositional and morphological tunability modulates their optoelectronic properties that can be adapted for different applications. On the other side, the relatively easy synthesis in a colloidal state further contributes to their real-world use.
There are two main approaches for semiconductor QD fabrication [5]: the physical methods and the wet-chemical methods.
The physical methods include the molecular beam epitaxy (MBE) and the metal-organic chemical vapor deposition (MOCVD) that allow the preparation of thin layers of semiconductor QDs or deposit them over a wafer.
The chemical methods involve the synthesis in the liquid phase (organic solvent or water) at relatively high temperatures (100–350 °C) in the presence of precursors and surfactants. The controlled combination of the precursors, their ratio, the presence of surface ligands, the reaction temperature, and the duration of the heat treatment determines the stoichiometry, size, and shape of the scQDs. The atomic species forming the scQDs belong to the elements from II–VI, III–V, IV–VI, and IV groups, and include the metal halide perovskites (CsPbX3 X = I, Br or Cl) [6][7][8].
The possibility to control the electro-optical properties of the scQDs through all of these parameters without changing the whole chemistry of the system is a key advantage of this class of materials and opens many research paths [9]. However, finding the right combination of them is not an easy task, especially in the case of core/shell systems. For example, a careful selection of precursors and ligands is strictly connected with the temperature of nucleation and growth of the scQDs, and together their optical properties. For example, if the growth and nucleation processes are not well distinguished final scQDs are not monodispersed (monodispersion means that all the scQDs have the same size) and, hence, the FWHM of the preparation is broad. In the same way, the high crystalline quality of the core ensures a high quality of performance [10].
The modulation of the density of states, i.e., the tuning of the scQDs’ electro-optical properties, is another technological aspect that is deeply studied. One strategy is to modify the shape of the nanocrystals, while another research direction is the doping of scQDs as reported by Mocatta et al. [11].
Another critical aspect of the scQDs’ synthesis that influences their application in thin films is the role of the ligands that mediate the interaction with the substrate and with themselves.

1.3 The Quantum Size Effect and Its Role in the Modulation of the Electro-Optical Properties of the scQDs

The importance of this class of nanomaterials lies in the possibility of modulating their optoelectronic properties by their size, composition, and architecture. The modulation of the optical properties by size is the so-called quantum size effect [9]. This effect takes place when the size of the semiconductor becomes smaller than the wavefunction of the exciton (the electron and hole pair formed by Coulomb interaction) [12], typically below 10 nm, depending upon the type of material [9]. In this condition, the bandgap of the semiconductor material becomes quantized and the effective bandgap increases, decreasing the particle size and modifying the absorption and emission properties (Figure 1).
Nanomaterials 13 02008 g001 550
Figure 1. The valence band and the conduction band of the semiconductor bulk material become quantized (black bars) when the size of the QD (white arrow) becomes smaller than the Bohr diameter. The defect surface states and deep traps (red bars) are due to surface defects and crystal defects.
In such a way, the bandgap of the scQDs can be tuned from different energy levels from the ultraviolet to infrared range. The bandgap fine-tuning enables the specific emission wavelength, the size uniformity of the emitting cores causes the narrow emission, while the absence of crystal defects mainly at the surface (Figure 1) ensures a high photoluminescent quantum yield (number of emitted photons per absorbed photons) and also contributes to narrow emission.
The wavelength tunability (color selection), the narrow emission (color purity), and the photoluminescent quantum yield (brightness) are characteristics of paramount importance for the application on a device. The improvement of the knowledge of the scQDs showed that the source of the problems are the defects states from surfaces of the scQDs (surface traps and deep traps) that perturb the band gap structure, changing the emission wavelength and lowering the luminescent quantum yield.

1.4 The Core@shell Systems

The solution for the stabilization of the scQDs structure arrived from the introduction of the core@shell systems [13][14][15]. In this configuration, the surface defects are fixed by growing an inorganic shell over the scQD core. The shell role is two-fold: the passivation of the surface defects and the localization of the exciton into the core [5]. The growth of another type of material over the scQD should be chosen carefully because the shell material should crystallize over the core without introducing any mechanical (crystallographic) stress, which means more surface defects.
Another crucial point for the realization of the desired core/shell system is the reaction methodology. It is possible to summarize three different methods of growth of the core/shell systems; namely, the two (multiple)-steps synthesis, the one-pot synthesis, and the SILAR (successive ion layer adsorption and reaction). The two-steps reaction is the most used approach because it allows the removal of the reaction byproducts after each reaction. Cao et al. [16], for example, prepared a multiple core/shell/shell system like CdSe/CdS/ZnS with this approach. First, they synthesize the core that is used, after the purification, as a reagent for the synthesis of the shell. The CdSe/CdS is then purified and utilized for the growth of the ZnS final layer. In this case, the metal precursors are CdO, Cd(OAc)2, and Zn(OAc)2, while the selenium and sulfur are added slowly to the reaction mixture. The slow addition of the chalcogenides prevents the nucleation of the shell material. The SILAR methodology forecasts the formation of the shell layer-by-layer. Each layer is realized by two different injections in the reaction vessel of the cationic and anionic precursors [17].
The one-pot synthesis approach consists of the formation of the scQDs using a core with a gradient shell. W.K. Bae et al. [18] introduced this approach, suggesting that the growth of this type of material, indicated as Cd1−xZnxSe1−ySy, is due to different reactivity of the reagents (metal and chalcogenides) mixed in the same vessel when reacting at the same temperature (the reaction is carried out at 300 °C). All of these reaction approaches are widely used by researchers that developed many particular variations, always to improve the photoluminescent quantum yield (PLQY), the FWHM, and the scQDs’ stability.
Cao et al. [16] sorted out a CdSe/CdS/ZnS scQD with an electroluminescence efficiency six orders of magnitude higher than the standard ruthenium complex. That can be used for an electrochemical immunoassay for the development of QD-LEDs. Hanifi et al. [19] set up a method of core/shell synthesis that produces CdSe/CdS scQDs having a PLQY approaching 1. These scQDs with this high PLQY are developed for applications in the photovoltaic field.
Other examples of core-shell systems are the so-called giant QDs. They are systems with a very high shell/core volume ratio (shell thickness higher than 1.5 nm) and the absorption is dominated by the shell [20][21]. These systems are particularly advantageous because they exhibit a high distance between the absorption and emission maxima (Stokes-shift) that improves the efficiency of the optoelectronic devices where these systems are included.
A further example of this band engineering for the optimization of optoelectronic properties is the modulation of the shell shape, like the dot in rods [22][23] and nanoplatelets [24]. Both engineered structures show a high Stokes-shift between the absorption band and emission band that ensures the absence of the reabsorption of the emitted light [25]. Both types of structures were described in this section because the ligands play a pivotal role in their preparation.

2 scQD Dispersion: the Ligands and the Surrounding Environment of the scQDs

The ligands mediate the compatibility of the scQDs within the matrix. The best way to enhance the dispersion is to prepare an scQD with a ligand compatible with the host matrix [26]. The dispersion of the scQDs in a matrix is a particularly important factor for the scQDs’ application, especially in a solid state like in a film, because the aggregation phenomena quench the electro-optical properties of the scQDs[27], nullifying all of the efforts made to obtain nanocrystals with excellent optical properties. An elegant example of the function of the ligand as a “tool” to optimize the dispersion and, hence, the optical properties of the scQDs is found in the work reported by Lesnyak’s group, where the CdSe nanoplatelets (NPLs) were functionalized with a ligand bearing a modified end group, improving the NPLs’ dispersion[27] in a polymer. In this report, the authors faced the typical problem of the scQDs’ application; that is, their dispersion at high concentration in a matrix that has specific characteristics. The high dispersion of the NPLs within the polyisobutylene (PIB) polymer was obtained by functionalizing the NPLs with a ligand bearing, as the functional end group, a short tail as in PIB (Figure 3a), and that has the same chemical nature of the PIB polymer. The same chemical nature of the ligand and the polymer (Figure 3b) ensures the complete miscibility of the QDs in the polymer.

The tests of dispersion and stability which were carried out, comparing the spreading of the NPLs in three types of polymers (namely: the poly(lauryl methacrylate) (PLMA), the PIB, and the PIB block copolymer (SIBS)) showed the formation of very high transparent films[28], which indicates an optimal dispersion.

Despite a huge amount of work conducted on the study of the organic ligands, it is worth describing also the use of the inorganic ligands. Indeed, they can replace the organic ligands to improve the charge transport between the scQDs[27][28].

Talapin’s group recently showed how the native organic ligand, typically the oleic acid, can be replaced by metal-inorganic salts[27] to obtain intensely luminescent all-inorganic nanocrystals (ILANs). The metal-inorganic salts they used for surface passivation of the scQDs include the metal cations of Cd2+, Zn2+, Pb2+, and In3+ with anions like NO3−, BF4− e triftalate (OTf−).

The role of the metal cations is (i) to remove the native organic ligands, and (ii) to bind the non-metal atom, on the scQD surface. In terms of the Lewis acid-base concept, the ligand metal cation, a Lewis acid, coordinates the electron-rich chalcogenide atom, a Lewis base, at the scQD surface. On the other side, the anion acts as a charge balancer rather than a coordinating agent. The main effect of this ligand exchange is the variation of the scQDs’ solubility (dispersion) of the scQDs in solvents. Indeed, the solubility of the scQDs in non-polar solvents (hexane, toluene, etc.) switches to solubility in polar solvents (DMF, NMF, DMSO, etc.).

3. QD Stability: the Effect of Oxygen and Moisture

Answering the question about the stability of the scQDs under ambient conditions in combination with light will help to adopt the necessary countermeasures to improve the life of any device equipped with this material.

Only recently, the group led by Peng clarified the role of oxygen[29] and water[30] by studying systematically their effect on a well-defined system, the CdSe/CdS core/shell scQDs, in defined experimental conditions in terms of atmosphere (only oxygen, only water, or their defined combination) and different phases at single scQD level or as an ensemble of scQDs in thin film and solution.

The study of the role of oxygen[29] shows that, at the functional level, this molecule maintains the bright state both of the single scQDs and when the scQDs are embedded together in a film (photoactivation). When the oxygen is removed, for example with argon, the scQDs enter a dim state (low emission and small PL shift). The proposed mechanism is that during the photoexcitation, there is the possibility that the scQDs form a trion (two electrons and one hole in the scQDs) bringing the scQDs in the dim charged state (off state). The “bright” state is restored with the presence of oxygen that accept one electron forming the superoxide radical (•O2−). The oxygen reduction returns the scQDs to charge neutrality restoring the scQDs’ optical properties. Another interesting conclusion of this work is that the high quality of the shell avoiding any hole and electron surface traps does not allow any effect of corrosion of the scQDs by the oxygen (the redox potential of the oxygen is quite different from the core/shell scQDs). On the other side, the redox potential of the oxygen should be able to oxidize the surface of bare CdSe scQDs (no core/shell scQDs), especially under photoexcitation producing CdO, SeO2, and CdSeOx [31][32]. The authors conclude that pure oxygen helps to maintain the photophysical properties, and it is not responsible for photo-corrosion of high-quality core/shell nanocrystals. The controversial results found in the literature on the role of oxygen may be due to the non-ideal quality of the prepared core/shell scQDs allowing the corrosion, as reported for the bare CdSe QDs. 

Peng’s group studied also the role of water combined with oxygen, showing that this combination is responsible for the corrosion and the loss of the photophysical properties of the scQDs[30]. The complete “story” of the water and oxygen interaction with the scQDs starts with the “ionization by water and deionization by oxygen” step, as reported in Figure 5. In this first step, the excited nanocrystal is negatively ionized (reduced) by water that dissociates, producing a very reactive specie, the hydroxyl radical (•OH) and protons (H+). The negatively charged scQDs are now in the dim state, but the presence of the oxygen, as shown before, brings the scQDs to a neutral state, restoring its bright state and producing as a byproduct the superoxide ion (equilibrium between neutral/bright state and charged/dim state; Figure 5). The presence of the radical species, especially the hydroxyl radical (•OH) formed under the continuous presence of water and irradiation, brings to an acidic pH and a carboxylate ligand detachment from the scQDs’ surfaces. This phenomenon causes the poor solubility of the QDs in the solvent and the precipitation of the bright scQDs (precipitated/bright state, ligand-destructed/bright state; Figure 5). The loss of the surface ligand exposes the inorganic shell to the formation of surface traps bringing further chemical decomposition even of the shell of the scQDs (photo-corrosion)

with irreversible loss of their photophysical properties (etched/bleached state; Figure 5).

It is worth mentioning that when the scQDs are confined in an area with no access to water and oxygen, like in the display applications (the QD’s film is isolated from the environment), the balance between the brightening and dimming states reaches an equilibrium and the decomposition cannot go ahead.

 

This entry is adapted from the peer-reviewed paper 10.3390/nano13132008

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