Formation of CdSe quantum dots from single source precursor obtained by thermal and laser treatment

The formation of the CdSe quantum dots (QDs) starting from a single source precursor^1,2 is a known process used in both solution and in solid state to grow QDs and nanocomposites. In the latter case, the single source precursor is first dispersed in a matrix (commonly a polymer), then the film is treated under temperature to form the nanocomposite.^3–5 

The single source precursors made of II-VI atoms are good candidates for the semiconductor QD formation because the preformed bonds between the metal center and the chalcogenide lead to the formation of nanocrystals with fewer defects directly within the matrix. In addition, a precursor synthesizable in few chemical steps, stable under ambient conditions and characterized by a low decomposition temperature, can be employed for industrial applications. Recently, the decomposition of these molecules has been obtained also by laser opening a new methodology for nanoparticle patterning^6–11 for device manufacturing. The combination of the optical performances of the QDs with the relatively simple technical process of laser direct writing can be considered a new method to obtain complex or highly resolved patterns for device manufacturing.¹² One of the potential areas of application of the direct laser patterning (DLP) of the QDs is in display manufacturing. Indeed, the color conversion filters deposited over every single pixel of a display can be formed by the direct action of the laser forming the QDs of the desired color.^10,12 However, this goal asked for a deep study to set up the correct chemistry of the film and the laser structuring itself. Indeed, the laser action should guarantee the correct QD size (color) and its photophysical stability. The first characteristic is, in principle, achievable by tuning the laser wavelength, fluence, and frequency as shown by several authors,^6,11 while the QD stability and also the emission properties are modulated by the ligands bonded over the QD surface and/or the matrix.

The role of the surface of the QDs is extremely important for the determination of their photophysical properties, because in a particle with a size below a few tens of nanometers, 10%–80% of the atoms are located on the surface. The efficient confinement of the exciton (separation of hole and electron) within the core of the QDs and the absence of nonradiative phenomena is achieved only if the surface of the QDs is well passivated by organic¹³ or inorganic ligands.¹⁴ The role of the ligands is not limited to modulate the QDs photophysical properties, but they act as blocking agents to maintain the growth of the material at the nanoscale level during the synthesis and to mediate the interaction of the QDs with solvents or (solubility) or other materials¹⁵ improving their photophysical properties. The typical organic molecules adopted to block defects on the QDs surfaces are the fatty acids, amines, phosphines, thiols, recently metals and polymers. On the other side, inorganic materials, with similar crystallographic characteristics, are deposited over the QD core forming the so-called core/shell systems.¹⁶ 

The objective of this work is to grow the CdSe QDs from a single source precursor by heating a polymer film loaded with the precursor in the presence of two different ligands. The precursor is the 2-(N,N-dimethylamino)ethylselenolate of cadmium¹⁷ (CdDMASe) that is dispersed in the polymethylmethacrylate (PMMA) matrix that after a thermal treatment becomes luminescent. The role of the used ligands, namely, the oleic acid (OA) and oleylamine (OAm) was evaluated on the basis of the photoluminescent (PL) spectra of the annealed film by changing the precursor/ligand ratio within the film. Once the best conditions of the film formation were defined, the preliminary trials by laser patterning were carried out. Indeed, the general approach to evaluating a precursor as a candidate for laser patterning is its capacity to form QDs by thermal decomposition in solution¹⁷ or in solid state.¹⁰ 

The CdDMASe precursor was already studied by the Samuel group both after thermal treatment and laser patterning coupled with the polyfluorene type polymer and 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBI).^10,18 In these studies, CdDMASe was used without the presence of any ligand that modifies its photophysical properties, and the laser patterning was carried out with a 266 nm laser. In the present work, the source type used is a 355 nm laser (10 ns laser pulse) that is a simpler technical apparatus with respect to 266 nm laser in terms of maintenance and costs. That is the reason because this laser system was introduced in this study indeed it is more appealing for the industrial exploitation of the DLP of QDs. The effect of this new laser source is finally evaluated by observing the patterned polymer/precursor film under an epifluorescence microscope.

In the first part of the paper, the effect of the ligand/precursor ratio and annealing time after thermal annealing in terms of the photoluminescent properties of the film is discussed, then the effect of the laser structuring is described.

Oleylamine (OAm, 70%) was purchased from Fluka, Poly(methyl methacrylate) (PMMA), oleic acid (OA, technical grade, 90%), and chloroform (≥99.8%) were purchased from Sigma-Aldrich and used without further purification.

The synthesis of cadmium selenolate was carried out according to Kedarnath et al.¹⁷ with modifications avoiding the use of liquid ammonia and reducing selenium with NaBH₄ in ethanol.

Photoluminescent films were deposited on a quartz slides (20 × 10 mm²) for optical characterization or glass slides (25 × 10 mm²) prepared by the spin coating technique, from a chloroform solution of polymethylmethacrylate (PMMA), a CdSe precursor, 2-(N,N-dimethylamino)ethylselenolates of cadmium (CdDMASe), oleic acid (OA), and oleylamine (OAm) as organic ligands.

Photoluminescent films were deposited by the spin coating technique on quartz slides (20 × 10 mm²) for the optical characterization. Spin coating depositions (PoloSpin 150i/200i) were performed on substrates by dropping 100 μl of the solution and spinning for 45 s at 1000 rpm.

The spinning solution was prepared by dissolving in chloroform: polymethylmethacrylate (PMMA) as the polymer, 2-(N,N-dimethylamino)ethylselenolates of cadmium (CdDMASe) as the precursor and oleic acid (OA) or oleylamine (OAm) as the organic ligands.

PMMA was used in a concentration of 100 mg/ml in chloroform, while CdDMASe was studied in the concentration range between 20 and 120 mM. OA and OAm were added as 40, 80, and 100 mM.

In the tests of ligand concentrations, the relative amount of reagents used for the solutions is reported in Table I. In these experiments, the amount of the PMMA and CdDMASe was kept constant, while the concentration of the ligands was varied from 0 to 100 mM.

The temperature of annealing was set at 150 °C and the time of annealing was 1 h.

Analogously, in the studies of precursor concentrations, the amount of PMMA and ligand were kept constant, while the concentration of the CdDMASe was varied from 20 to 120 mM, as shown in Table II.

The temperature of annealing was set at 150 °C and the time of annealing was 1 h. In order to promote the CdDMASe decomposition to CdSe QDs, films were treated under dynamic vacuum at 150 °C in a range between 0 and 180 min in a Büchi vacuum dryer. The temperature was selected on the basis of the TG decomposition profile of the precursor,¹⁷ taking care of preserving the PMMA matrix.

The laser treatment was carried out with a Mosquitoo laser at 355 nm and 10 ns pulse, with the beam diameter in the focus of 30 μm, driven by a computer that regulates the pulse frequency (in a range between 20 and 100 kHz), the beam power (in a range between 200 and 1000 mW) and beam position on the substrate (speed in a range of 100–5000 mm/s). The square is patterned with the laser using a frequency of 40 kHz and a power of 0.3 W, the laser fluence is, therefore, 1.06 × 10³ mJ cm^-2. Each square with a side of 1 × 1 mm² is formed by nine lines.

To investigate the optical properties of the produced films, absorption, photoluminescence, and fluorescence microscopy emission spectroscopies were used. Absorption measurements were recorded on a Jasco V750 spectrophotometer in the spectral range of 200–800 nm, the integration time of 0.6 s, and slit widths of 1.5 nm.

Photoluminescence (PL) emission spectra were obtained by using a Fluoromax 4 Plus (Horiba) spectrofluorimeter equipped with Origin program for data acquisition and analysis in the spectral range from 450 to 700 nm. The typical excitation wavelength of λ_exc = 350 nm was used with a spectral bandwidth of 1.5 nm for both the excitations and emission monochromators and a cut-off filter at 399 nm. The curves were automatically corrected for the spectral response of the detector.

The microscopic images were obtained with a Leica DM2700 microscope equipped with a motorized table and recorded with Leica LAS software. The fluorescent images were acquired with a Leica camera DMC2900. The samples were excited with a Hg source (100 W) and analyzed with a 10× objective and with a Leica filter cube (exc filter: 340 400 nm, dichroic mirror: 400 nm, long pass filter: 425 nm).

All the PL and UV-Vis spectra were elaborated with Origin program. The averaged particle size (D) was determined according to the empirical equation (1) derived as shown by Mutavdžić et al.,¹⁹ 

where x is the wavelength of the emission peak. The values of x and D are both expressed in nanaometers.

The interest on semiconductor nanocrystals and, in particular, the ones formed by CdX (X = S, Se, and Te) relies on their possibility to interact with the visible and IR regions of the light spectrum. In particular, the CdSe nanocrystals play an important role in applications like solar cells²⁰ and displays²¹ because of their high quantum efficiency, the possibility to tune their optical properties, and the well-known methods of synthesis. In solar cells, CdSe type QDs are used in combination with polymer²² or as sensitizer²³ replacing the organic dyes. In this work, the interest on CdSe QDs relies in their application as light down converter in display technology. In particular, it is explored the possibility for in situ synthesis of the CdSe QDs to study new patterning strategies by using laser.

The formation of the CdSe QDs in solution starting from the CdDMASe precursor was already shown in the work of Kedarnath.¹⁷ In this work, it was shown that the decomposition takes place within an interval of 150–180 °C in a single step, and that it is possible to obtain CdSe QDs in a hot solution of TOPO/HDA. More recently, CdDMASe was used as single source precursor together with electroluminescent polymers for the generation of a hybrid nanocomposite by Bansal et al.^10,18 This author showed the formation of the CdSe QDs under different temperatures both in the presence and in the absence of polyfluorene type or TBPI matrices. Interestingly, in the neat CdDMASe film and in the CdDMASe/polyfluorene film, the typical CdSe QDs PL emission is absent¹⁸ while when in combination of TPBI the CdSe QD PL spectrum is restored.¹⁰ This different behavior of the CdSe QDs as a function of the matrix is due to the correct alignment of the HOMO-LUMO orbitals of the TPBI (or polyfluorene) and QDs and the different dispersion of the QDs within the polyfluorene type polymer or TPBI molecules.^10,18

In the present paper, the selected matrix is not an electroluminescent polymer or small organic molecules like polyfluorene and TPBI but is the PMMA. This greatly simplifies the interpretative framework because the polymer does not interact with the QDs with its electronic orbitals but acts only as optically transparent “box” containing the QDs. Indeed, the final goal of our work is to study the formation of the QDs within an optically transparent matrix for their final application as color converters of blue light for a future application in display manufacturing.¹² 

The scheme of the experimental activity described in this paper is reported in Scheme 1: the QD laser formation preceded in parallel with the study of the CdSe QDs synthesis by the thermal treatment. This step was necessary to get two main objectives: (i) the optimization of the main film components, namely, the precursor, the type of ligand, and their ratio and (ii) the comparison of the process of the QD growth by heating and laser patterning.

The optimization of the type and concentration of ligand as well as the precursor amount was evaluated based on the PL properties of the produced films.

The first set of experiments was performed to investigate the effects produced by two different ligands, namely, OA and OAm (Fig. 1), on the optical features of the films.

The role of the ligand is to bind the free dangling bonds on the QD surface enhancing the PL intensity and enabling the appearance of the band edge emission. OA and OAm were chosen as ligands on the basis of their chemical structure, which consists of the same organic tail exposed to the environment and different functional head groups that bind to the nanocrystal surface^24,25 and because both molecules are widely used in the QD synthesis. Since the head groups adsorb at the nanocrystal surface, the involvement of OA or OAm in the QDs formation within the polymeric matrix can play a crucial role in the control of the size of the particles and in the optical features of the hybrid nanocomposite films. The initial experiments on the effect of the ligands on the PL properties of the films were shaped by maintaining constant to 1 the ratio between the moles of the precursor and ligand (80 mM).

The absorption and PL spectra of the films produced with OA, OAm and without any ligand, as a control, are shown in Fig. 2. The films without any ligand [Fig. 2(a)] or containing OAm [Fig. 2(c)] exhibit the first absorption transition at about 425 nm, while the absorption spectrum of the film with OA shows a transition at about 520 nm with a lower absorption intensity. The difference between the absorption spectra, in terms of first transition peak position and intensity, is due to the growth of QDs with different characteristics modulated by the presence/absence and type of ligand in the films. The difference of the wavelength of emission is a consequence of the well-known quantum size effect,²⁶ while the difference in absorbance intensity can be ascribed to the different concentrations of QDs in the film. Indeed, starting from the Lambert–Beer law [Eq. (2)] and the equation [Eq. (3)] for the determination of the molar extinction coefficient of the QDs, it is possible to evaluate the concentration ratio between the QDs in the presence of OA and OAm,

where Abs_ligand is the absorbance of the film in the presence of OA or OAm determined experimentally, d_ligand is the film thickness considered the same for both films, ɛ_ligand (λ) is the molar extinction coefficient in the presence of OA at 520 nm or OAm at 425 nm as determined in Yu et al.²⁷ [Eq. (3)], and D is the particle size determined from Eq. (1). From this, calculation results that the concentration of the QDs in the OAm film are approximately four times more than that in the OA film.

The PL emission spectra also indicate the growth of QDs with different functional characteristics. Here, the main difference relies on the broadness of the PL peaks. Indeed, the PL emission of the films without any ligand or with OAm show a broad band with a maximum at 542 and 557 nm for OAm and any ligand, respectively. The film containing the OA exhibits a sharp band-edge emission at 560 nm, even if its intensity is low with respect to the defect band starting at about 600 nm.

The PL peak maxima of all the three type of films are located in the 540–560 nm interval because, given the same experimental conditions (polymer concentration, precursor concentration, temperature, and time of annealing), the growth conditions of the particles are almost the same in all the three systems. On the other side, the peak’s broadness suggests the presence of a different level of surface defects that is a function of the presence (or absence) and type of ligand.

In this light, the narrow peak in the PL spectrum of the film with OA suggests that the OA molecules can form a shell around the QD surfaces, interacting with the cadmium dangling bonds. Conversely, the broad band in the spectrum of the film with OAm arises from the CdSe QDs with a greater number of surface defects related to the low grade of grafting of the OAm molecules to the nanocrystal's surfaces.

This occurrence is due to the different interactions between the functional head groups of the ligands on the CdSe QDs surface. Indeed, the carboxylate groups of OA molecules bind strongly the cadmium surface atoms through electrostatic interactions,¹³ while amino terminations of the OAm molecules exhibit lower binding affinity to II–VI QD surfaces.²⁸ 

To further investigate the role of both ligands, the second set of experiments was carried out by changing the ligand amount keeping constant the other parameters, as reported in Table I. Indeed, the binding of the surface defects can be also modulated by the ligand concentration. The results of these experiments are shown in Fig. 3.

As shown in Fig. 3, the positive effect of OA on the formation of the direct band emission at around 560 nm is confirmed for each investigated precursor/ligand ratio, such as 2, 1, and 0.8. In the same way, the increase of the OAm concentration does not induce the formation of the direct bandgap and the PL spectra remain similar in terms of wavelength, exhibiting an FWHM of about 200 nm at around 550 nm, as shown in Fig. 3(b).

Another set of experiments was performed by varying the precursor amount, keeping constant the concentration of the ligands at 80 mM. In this case, the ratio between the precursor and the ligand was changed from 0.25 to 1.5 as reported in Table II (Fig. 4).

The increase of the precursor concentration induces in both ligands a redshift in the PL maxima [Figs. 4(a) and 4(b)]. This means that the particle size, as derived from the PL maxima,¹⁹ increases as a function of the precursor concentration for both ligands. This evidence can be reasonably explained by considering that greater amounts of the precursor can produce bigger CdSe QDs.

The last parameter studied to monitor the QD growth was the annealing time at a given precursor/ligand ratio. The films were prepared using an amount of ligand double than that of the precursor, as a good compromise between the employment of small quantities of precursor and good PL properties of the final films as reported in Table III. The PL spectra acquired for the films annealed for different times were normalized and are reported in Fig. 5. The average size of the QDs was calculated from the PL maxima and the trend of size vs annealing time is reported in Fig. 5(d).

As shown in Fig. 5, an annealing process lasting for 10 min does not produce any significant effect in the optical features of the films prepared with OAm as well as without any ligand. Conversely, the involvement of OA in a concentration double than that of CdDMaSe allows obtaining PL signals even after 10 min of annealing. This finding indicates that, in the presence of OA, the formation of CdSe QDs is relatively fast during the first hour. Then, the process becomes slower and the averaged size approaches 3.8 nm [Fig. 5(d), red lines and circles]. This behavior can be explained considering that there is a nucleation burst at the beginning when the precursor is still present. As soon as the precursor is finished, the growth slows down and the Ostwald ripening takes place²⁹ and finally the growth stops because the QDs are surrounded by the ligand forming a shell.

When the OAm is present [Fig. 5(d), green line and triangles] or even absent [Fig. 5(d), black line and squares], the growth of the nanocrystals is linear with time and does not stop. This fact suggests that any shell surrounds the nanocrystals confirming the weak bond of the OAm on the QD surface that can continue to grow. Anyway, this growth is disordered, as indicated by the broadness of the emission band.

The second part of the study is focused on the laser treatment of the film by using a pulsed UV laser (355 nm, 10 ns pulse length). Here, the same type of films containing PMMA, CdDMASe, and ligands was studied to verify the formation of QDs. The test experiment was arranged to have three types of films containing (i) only precursor and polymer (PMMA/CdDMASe), (ii) the polymer, CdDMASe and OA (PMMA/CdDMASe/OA), and (iii) the polymer, CdDMASe and OAm (PMMA/CdDMASe/OAm). In Table IV are reported the film compositions and laser patterning conditions. Before the laser patterning, the absorption spectra of the three types of films were recorded in order to verify if the laser radiation at 355 nm is adsorbed by the film itself. In all the cases, is observed that the CdDMASe absorption is high below 320 nm even if at 355 is not completely zero, as shown in Fig. 6. This should ensure that a small part of the laser beam can be absorbed by the precursor to form the QDs.⁹ 

The results of laser treatments are reported in Fig. 7 showing both bright-field and fluorescence images of the three different films.

The bright-field images show for each sample well-resolved squares [Fig. 7(a)], consisting of nine lines obtained with a laser fluence of 1.06 J/cm². This means that the radiation affects all types of film. The dark lines suggest that the laser induces the decomposition of organic material, such as the precursor because the PMMA is transparent at this wavelength.

The analysis of the same patterns under the UV light [Fig. 7(b)] reveals that, only in the presence of OA, there is a clear signal output. In the case of the film containing the OAm, only a weak signal is achieved, while in the PMMA/CdDMASe film, any PL emission is observed.

These results point out several conclusions that need further investigations to be confirmed: (i) the formation emissive (functional) patterns are indicative of QD formation; (ii) the emissive pattern can be obtained only in the presence of the ligands; and (iii) the orange PL emission color observed by the naked eye under the microscope suggests a disordered growth of QDs (surface defects).

The formation of the QDs under the laser treatment of the same precursor has been already shown in recent literature^10,30 so the formation of a PL signal suggests the presence of QDs. At this stage, further studies are in progress to confirm the presence of the QDs mainly by using the transmission electron microscopy (TEM) and microscopic fluorescence spectroscopy.

The presence of ligands is recognized as an important parameter that enhances the functionality (Photo-Luminescent Quantum Yield—PLQY) of the QDs. In the case of laser patterning in solid state, probably this role is still more crucial. Indeed, the QD formation takes place in few seconds and in solid state, i.e., the atoms have to recombine with limited mobility in a very short time. Therefore, only if the ligands are present, there is the possibility that the nanocrystals are functional (emissive). This fact could also explain why the laser-treated PMMA/CdDMASe does not show any PL as opposed to what occurs under the thermal treatment. The role of the OA as a better ligand with respect to the OAm has also highlighted in the laser patterning experiments. Indeed, a better PL output is achieved when the OA is used for confirming the role of the carboxylate as bound to the cadmium dangling bond at the QD surface.

Finally, the observed orange emission displayed also for other laser fluences and frequencies (data not shown) suggests that the growing crystals are probably disordered with many surface defects (broad and weak PL signal). Further studies are currently in progress to optimize the laser patterning conditions and the chemistry of the film to enhance the PLQY and to obtain a defined color emission.

In this work, we studied the thermal and laser decomposition of the CdDMASe precursor embedded in PMMA in the presence with two types of ligands, namely, oleic acid and oleylamine. After its decomposition induced by thermal treatment, the precursor with and without the ligands gives rise to luminescence. However, only in the presence of OA, it is possible to observe the formation of the direct bandgap, while the amino group of the OAm does not behave as the film without ligands. The time course of the QDs formation during the time (3 h) at 150 °C shows that the QDs in the presence of the AO stop growing after 1 h, while the QDs without any ligand or with OAm always grow. All these lines of evidence suggest that the OA forms a shell around the QDs, while in the other cases, the QD growth is disordered.

The laser treatment of the film incorporating both ligands shows that, only when the ligands are present, it is possible to obtain a PL signal from the film, even if, in this case, the presence of the OA stimulates a higher intensity PL signal.

Further studies are currently in progress to clarify the role of ligands on the QD surface in the PMMA and the effect of the laser treatment both with optical and structural characterization techniques.

This paper is supported by the European Union Horizon 2020 research and innovation programme under Grant Agreement n779373, project MILEDI (MIcro quantum dot Light Emitting diode and organic light emitting diodes DIrect patterning). The authors thank Dr. Rosa Maria Montereali for helpful suggestions during the development of the research activity reported in this work. The authors thank Dr. Anatol Prudnikau and Dr. Vladimir Lesniak (Technical University of Dresden) for their support on chemical synthesis of the CdDMASe precursor and helpful discussion. The authors also thank Priscilla Reale for her support in part of the experimental activity.