Gallium incorporation in blue-emitting In_1−xGa_xP alloy quantum dots facilitated by monomeric gallium precursors

Abstract

Demand for environmentally friendly quantum dots (QDs) in wide color-gamut displays has led to successful development of red- and green-emitting InP QDs with outstanding optical properties. While progress in developing blue-emitting variants remains challenging, In_1−xGa_xP alloy QDs have recently garnered attention as blue emitters. However, Ga incorporation in these In_1−xGa_xP QDs is hindered by the limited reactivity of conventional gallium halide-derived precursors having a dimeric molecular structure. Here, we adopt trimethylgallium which yields monomeric gallium carboxylates as a Ga precursor in the colloidal synthesis of In_1−xGa_xP QDs. This approach promotes efficient Ga incorporation into In_1−xGa_xP QDs with narrow size distributions. The use of zinc chloride and oleylamine for ZnS shell growth on the In_1−xGa_xP cores further adjusts the photoluminescence (PL) wavelength to the blue range and enhances PL quantum yield. The resulting In_1−xGa_xP/ZnS core/shell QDs exhibit a peak emission at 470 nm, 67% of photoluminescence quantum yield, and 40 nm of emission linewidth. Successful employment of these QDs into light-emitting diodes demonstrates their potential as a blue electroluminescent emitter for future QD displays.

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1 Introduction

As colloidal quantum dot (QD) emitters gain prominence in wide color-gamut display technology, efforts to develop environmentally friendly QDs have rapidly intensified.^1,2 Consequently, red- and green-emitting Cd-free QDs have achieved remarkable progress,^3–5 driving the successful commercialization of photoluminescence (PL)-based QD display technology.^6,7 QD display technology is set to advance beyond its current photoluminescence-based applications toward electroluminescent devices, underscoring the need for environmentally friendly blue-emitting QDs.^8,9

One prominent outcome of the efforts for developing blue-emitting QDs is the emergence of Zn chalcogenide-based QDs. ZnSe QDs, with a bulk band gap (E_g) of 2.7 eV, typically emit in the near-ultraviolet region, and various strategies have been employed to shift this emission into the blue range. These include controlling nanocrystal growth chemistry to synthesize ZnSe QDs with near-bulk dimensions (∼10 nm),¹⁰ and alloying ZnSe with ZnTe (with a bulk E_g of 2.26 eV) to synthesize ZnSe_1−xTe_x alloy QDs.^11,12 Despite these advances, Zn chalcogenide QDs continue to face critical challenges; ZnSe_1−xTe_x QDs exhibit emissions from localized states caused by inhomogeneous Te doping, and Zn chalcogenide QDs demonstrate relatively low photo-¹³ or electro-chemical stability.^14,15

III–V group QDs with a covalent nature, such as In_1−xGa_xP alloy QDs, are emerging as an alternative composition for blue QD emitters. Single-composition InP (with a bulk E_g of 1.35 eV and an exciton Bohr radius of ∼10 nm) is theoretically available for blue emission with a sub-2 nm diameter¹⁶ at which, the nanocrystals are not only thermodynamically unstable due to a very high surface-to-volume ratio but also energetically competitive with metastable magic size clusters.¹⁷ Uniform alloying of GaP into the InP lattice can address these thermodynamic limitations. The incorporation of GaP with a wider E_g (2.24 eV) successfully shifts the E_g of InP QDs into the blue emission region.¹⁸

However, a key challenge remains in achieving the incorporation of Ga ions within In_1−xGa_xP QDs. Conventional gallium halide-mediated precursors, widely used in bottom-up colloidal synthesis,¹⁹ generally exhibit lower chemical reactivity compared to their In counterparts, leading to very low Ga incorporation^20,21 or nonuniform GaP alloying that results in a core/shell-like structure.²² Wegner et al. pointed out the low reactivity of the gallium halide-mediated precursors due to their dimeric structure, which features bridging halides and requires high activation energy for decomposition.²⁰ However, it remains unclear how the structural characteristics of gallium precursors affect the synthesis of In_1−xGa_xP QDs.

A top-down synthesis of In_1−xGa_xP QDs via cation exchange reaction is considered a potential strategy to address the reactivity differences between In and Ga precursors.^18,23–25 In the cation exchange process, Ga³⁺ ions substitute In³⁺ ions in the pre-synthesized host lattice of InP QDs, which is driven by Pearson's hard–soft acid–base theory.^26–28 However, this method poses challenges in achieving spatial control of guest cations within the host lattice, as the incorporation of the guest cation has to be initiated from the surface of QDs.^29,30 In addition, it is hard to directly integrate this post-synthetic process into the current mass production systems for QDs, based on the bottom-up colloidal synthesis. Such limitations speak volumes about the necessity for appropriate Ga precursors, enabling efficient Ga incorporation in the colloidal synthesis of In_1−xGa_xP QDs.

Herein, we addressed the challenge of the low reactivity of gallium halide-based precursors during the bottom-up colloidal synthesis of blue-emitting In_1−xGa_xP QDs by employing monomeric Ga precursors derived from organometallic trimethylgallium (TMGa). Unlike gallium carboxylates produced from gallium iodide, which maintain a dimeric structure with bridging iodides and di-substituted carboxylate groups, gallium carboxylates derived from TMGa adopt a monomeric structure with fully substituted carboxylate groups. These monomeric precursors react with indium carboxylates and phosphorus precursors at 300 °C to produce In_1−xGa_xP alloy QDs with a narrow size distribution and uniform composition. The photoluminescence properties of the In_1−xGa_xP QDs were further enhanced through ZnS shell growth utilizing zinc chloride and oleylamine. The resulting In_1−xGa_xP/ZnS core/shell QDs were successfully integrated into electroluminescent devices, demonstrating their potential to be widely applied in electroluminescent QD displays.

2 Experimental methods

2.1 Materials

Zinc stearate (Zn(St)₂, 99%), lauric acid (LA, 98%), oleylamine (OAm, 70%), 1-dodecanethiol (DDT, ≥98%), 2-ethylhexanoic acid (2-EHA, 99%), sodium hydroxide (NaOH, ≥98%), gallium(III) iodide (GaI₃, 99.99%), zinc acetate dihydrate (Zn(Ac)₂·2H₂O, ≥99%), magnesium acetate tetrahydrate (Mg(Ac)₂·4H₂O, ≥99%), tetramethylammonium hydroxide (TMAH, ≥99%), and molybdenum(VI) oxide (MoO₃, ≥99.9%) were purchased from Sigma-Aldrich. Anhydrous zinc chloride (ZnCl₂, 98%) and oleic acid (OA, 90%) were purchased from Alfa Aesar. Tris(trimethylsilyl)phosphine (TMS₃P, 97%) was purchased from LABCARE. Trimethylgallium (TMGa, 99%) was obtained from ICHEMS. Indium acetate (In(Ac)₃, 99%), zinc acetate (Zn(Ac)₂, ≥99%), 1-octadecene (ODE, 98%), and trioctylphosphine (TOP, ≥97.5%) were supplied by UNIAM. Aluminum (Al, ≥99.9%) and 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP, ≥99.5%) were purchased from Lumtec. All other organic solvents were obtained from Samchun Chemicals. All chemicals were used as received without further purification.

2.2 Ex situ synthesis of gallium carboxylates using GaI₃ and TMGa

To synthesize gallium carboxylates, 2 mmol of GaI₃ were dissolved in 20 mL of anhydrous methanol under stirring at room temperature. Subsequently, desired equivalents of 2-EHA and NaOH relative to GaI₃ were sequentially added into the solutions. The mixture was allowed to react at room temperature for 1 hour, during which the formation of a white precipitate of gallium carboxylates was observed. The precipitate was collected, washed five times with anhydrous methanol, and dried under vacuum for subsequent analysis.

To synthesize gallium carboxylates using TMGa, TMGa dissolved in TOP was reacted with a desired equivalent of 2-EHA, and the reaction was performed at 200 °C for 1 hour. The obtained gallium carboxylates were filtered out using methanol and further washed with methanol ten times. The precipitate was dried under vacuum for subsequent analysis.

2.3 Synthesis of In_1−xGa_xP alloy QDs

All syntheses were conducted using standard Schlenk line equipment. Indium laurate (In(LA)₃) was prepared by reacting 2 mmol of In(Ac)₃ with 6 mmol of LA in 6 mL of ODE under vacuum (≤200 mTorr) for 2 hours. Gallium laurate (Ga(LA)₃) was prepared by reacting 5 mmol of 1.0 M TMGa dissolved in TOP with 15 mmol of LA in 10 mL of ODE at 200 °C for 30 minutes under a N₂ atmosphere.

To synthesize In_1−xGa_xP cores for blue emission, 2 mmol of Zn(St)₂ was dissolved in 16 mL of ODE and degassed at 110 °C for 30 minutes.³¹ Subsequently, 2 mmol each of In(LA)₃ and Ga(LA)₃ were added to the reaction flask and further degassed at 110 °C for 30 minutes. The reaction flask was then heated to 300 °C under a N₂ atmosphere and held for 1 hour to complete the formation of metal carboxylates. After the reaction temperature was lowered to 110 °C, 3 mL of 0.33 M TMS₃P dissolved in TOP was injected and held for 20 minutes. The reaction flask was then reheated to 300 °C, yielding In_1−xGa_xP cores. After the reaction, the flask was rapidly cooled down to room temperature. The solution was purified twice using a precipitation/redispersion method with anhydrous EtOH/toluene. The purified cores were redispersed in toluene and stored in a glove box for shell growth.

2.4 ZnS shell growth on In_1−xGa_xP cores

0.5 M ZnCl₂ solution was prepared by dissolving 5 mmol of ZnCl₂ in 10 mL of TOP or OAm under stirring at 100 °C in a nitrogen-filled glove box. 0.5 M Zn(OA)₂ was prepared by degassing 5 mmol of Zn(Ac)₂ and 10 mmol of OA in 6.8 mL of TOA at 160 °C for 2 hours.

To grow ZnS shells on In_1−xGa_xP QD cores, 0.37 mL of In_1−xGa_xP cores (optical density of 1.55 at 380 nm) were dissolved in 10 mL of ODE in a three-necked round-bottom flask and degassed at 110 °C for 30 minutes. For surface-treatment of the cores, 1 mL of 0.5 M ZnCl₂ dissolved in TOP was injected into the flask and further degassed for 30 minutes. Backfilling N₂, 1.5 mL of ZnCl₂ dissolved in OAm and 0.19 mL of DDT were added. The reaction flask was heated up to 300 °C and held for 2 hours, which yields the ZnS inner shell. Then, for the growth of the ZnS thick shell, 2.4 mL of 0.5 M Zn(OA)₂ and 0.29 mL of DDT were dropwise injected into the flask at 300 °C and the reaction was continued for another 20 minutes. After the shell growth was finished, the solution was rapidly cooled down to room temperature and purified twice using a precipitation/redispersion method with acetone/toluene. The purified In_1−xGa_xP/ZnS QDs were dispersed in n-octane and stored in a glove box for further use.

2.5 Fabrication of QLEDs

Zinc magnesium oxide (ZnMgO) nanoparticles, used as an electron transport layer (ETL), were synthesized with minor modifications based on a literature method.³² Specifically, 5.1 mmol of Zn(Ac)₂·2H₂O and 0.9 mmol of Mg(Ac)₂·4H₂O were dissolved in 60 mL of DMSO at 60 °C until completely dissolved. Then, 10 mmol of 0.5 M TMAH dissolved in EtOH was rapidly injected into the DMSO solution and reacted for 1 hour, yielding ZnMgO nanoparticles. The nanoparticles were collected by washing twice with acetone and redispersed in EtOH.

To fabricate QLEDs, 33 nm-thick ZnMgO nanoparticle ETLs were spin-coated onto indium-tin-oxide (ITO) substrates at 4000 rpm for 30 seconds, followed by annealing at 150 °C for 25 minutes. QD emissive layers of 1.5 monolayers were deposited by spin-coating an In_1−xGa_xP/ZnS QD solution at 4000 rpm for 30 seconds, followed by annealing at 90 °C for 25 minutes. The spin-coated substrates were then transferred to a thermal evaporator, where 60 nm of CBP, 10 nm of MoO₃, and 100 nm of an Al electrode were deposited under a vacuum of 1 × 10⁻⁶ Torr at evaporation rates of 1.5, 0.2, and 2.0 Å s⁻¹, respectively. The fabricated devices were encapsulated with UV-curable resin and glass lids, and then stored in a glove box for further characterization.

2.6 Characterization

Elemental analysis of gallium carboxylates was performed using a Thermo Fisher Scientific FlashSmart elemental analyzer. X-ray photoelectron spectroscopy (XPS) was conducted with Thermo VG Scientific K-alpha. Fourier transform infrared (FTIR) spectroscopy was performed with Thermo Fisher Nicolet iS50. Laser desorption ionization time of flight (LDI-TOF) mass spectrometry was conducted with Bruker autoflex maX. UV-vis absorption spectroscopy was conducted with a Shimadzu UV-2600i spectrophotometer. PL spectroscopy and PL quantum yield (QY) measurements were carried out using a Hamamatsu absolute PL quantum yield spectrometer C11347 at an excitation wavelength of 365 nm. Time-resolved PL measurement was performed using a light conversion femtosecond laser (PHAROS) coupled with a PicoQuant time-correlated single-photon counting system. The laser output was converted to the excitation wavelength of 365 nm using a light conversion collinear optical parametric amplifier (ORPHEUS). Transmission electron microscopy (TEM) was conducted using an FEI Tecnai F20 microscope operating at 200 kV. Inductively coupled plasma optical emission spectrometry (ICP-OES) was performed with an Agilent ICP-OES 5110. For ICP-OES analysis, samples were digested in aqua regia (a mixture of hydrochloric acid and nitric acid in a volume ratio of 3 : 1) and diluted with deionized water. X-ray diffraction (XRD) spectroscopy was conducted with a RIGAKU Ultima IV system under Cu Kα irradiation. Ultraviolet photoelectron spectroscopy (UPS) measurements were performed with a Thermo Scientific Nexsa G2 using a He I photon source (21.2 eV). The electroluminescence characteristics of blue-emitting quantum dot light-emitting diodes (QLEDs) were measured using a Konica Minolta CS-2000 spectroradiometer coupled with a Keithley 2450 SourceMeter.

2.7 Energy change calculation in the reaction of gallium precursors using density functional theory (DFT)

We performed first-principles density functional theory calculations by using the projector augmented wave method (PAW)³³ as implemented in the Vienna ab initio simulation package (VASP).³⁴ We used an exchange–correlation functional parametrized by Perdew, Burke, and Ernzerhof³⁵ and the DFT-D3 method with Becke–Johnson damping function for a better description of dispersion interaction.³⁶ The plane waves were expanded up to 400 eV. The atomic structures were optimized until the residual forces became less than 0.01 eV Å⁻¹.

3 Results and discussion

3.1 Structures of gallium carboxylates derived from gallium iodide and TMGa

Solid-state gallium halides are predominantly stable as a structure of Ga₂X₆ dimers (X = Cl, Br, or I) rather than GaX₃ monomers in equilibrium (Fig. 1a).^37,38 Our first-principles DFT calculations support that Ga₂I₆ dimers are more preferred than monomeric GaI₃. The energy change (ΔE) in the dimerization of GaI₃ is calculated to be −0.839 eV (Fig. S1a†). Previous studies have suggested that the reaction of these Ga₂X₆ dimers with carboxylic acids results in the partial substitution of only two halogen atoms with carboxylate groups.²⁰ To quantitatively examine the carboxylate substitution, we ex situ synthesized gallium carboxylates derived from gallium iodide by reacting them with 2-EHA in a methanol medium (Fig. 1a, see Experimental methods).³⁹ To ensure complete carboxylate substitution, an excess amount of sodium 2-ethylhexanoate, exceeding three equivalents, was reacted with gallium iodide.

Fig. 1 Structure of gallium carboxylates derived from a gallium iodide. (a) Schematic illustration of the carboxylate substitution reaction of gallium iodide which exists as monomeric GaI₃ or dimeric Ga₂I₆. The bridging halides in Ga₂I₆ are indicated with white arrows. (b) XPS spectrum of the I 3d core-level region of gallium 2-ethylhexanoate derived from gallium iodide (circles). The I 3d_3/2 (red) and I 3d_5/2 (purple) orbitals were deconvoluted. (c) FTIR spectra of gallium 2-ethylhexanoate derived from gallium iodide with varying feed ratios of gallium iodide to 2-ethylhexanoic acid (1 : 4, 1 : 5, and 1 : 6). (d) Mass percentage of carbon in gallium 2-ethylhexanoate (upper panel). The black dashed lines indicate the calculated mass percentages of carbon for the proposed structures of dimeric and monomeric gallium 2-ethylhexanoates (lower panel).

We first conducted XPS measurements for the I 3d core-level to assess the substitution of iodide to the carboxylate group. The XPS spectra demonstrate the I 3d orbitals, revealing that the iodine atoms still remain in gallium carboxylates (Fig. 1b).⁴⁰ The FTIR spectra for gallium carboxylates exhibit distinctive vibrational peaks corresponding to C–H stretching (2990–2850 cm⁻¹), C–O stretching (1600–1500 cm⁻¹ and 1430–1400 cm⁻¹), and C–H bending (1462 cm⁻¹), indicative of carboxylate groups (Fig. 1c).⁴¹ These results suggest that not all the iodide groups in gallium iodide were fully replaced by carboxylate groups, but were only partially substituted.

The elemental analysis result demonstrates that the weight percent of carbon in gallium carboxylates derived from gallium iodide was approximately 36%, regardless of the excess amount of reacting 2-EHA (Fig. 1d and Table S1†). This value is significantly lower than the theoretical carbon content of monomeric gallium tri-2-ethylhexanoate (57.7%, right lower panel in Fig. 1d). In the dimeric structure of gallium halides, the halogen atoms bridging the two gallium centers, known as bridging halides, are strongly bound to the Ga atoms.^37,38 The large activation energy required for the dissociation of bridging halides contributes to the formation of dimeric and partially substituted precursors with a structure of Ga₂I₂(RCOO⁻)₄ (left lower panel in Fig. 1d). This structure theoretically has a carbon mass percentage of 39%, which is close to the 36% carbon content in the analyzed gallium 2-ethylhexanoates derived from gallium iodide. We attribute the discrepancy in the carbon mass percentage to the contribution from a minor portion of mono-substituted gallium carboxylates.

To investigate the carboxylate substitution of a monomeric Ga compound and thereby to circumvent the challenges of bridging halide dissociation in gallium halides, we selected TMGa as a suitable candidate. While various organometallic compounds (e.g., diethylzinc or dimethylcadmium) have been employed in the colloidal synthesis of QDs,^42–44 the application of TMGa in this field remains largely unexplored.

We also performed first-principles DFT calculations to evaluate the energy change in dimerization (Fig. S1b†) and gallium tri-2-ethylhexanoate formation reaction (Fig. S1c†) from monomeric TMGa. The energy change in the dimerization of TMGa is calculated to be 0.934 eV, which is thermodynamically unfavorable (Fig. S1b†). In contrast, the formation of gallium tri-2-ethylhexanonoates is highly favorable, with an energy change of −3.769 eV. These results suggest that monomeric TMGa would enable efficient carboxylate substitution, leading to the formation of gallium tri-carboxylates.

The FTIR spectra of TMGa and its derivative, gallium 2-ethylhexanoate, both exhibit identical vibrational peaks corresponding to C–H stretching and C–H bending of the methyl and 2-EHA groups (Fig. S2†). However, following the carboxylate substitution reaction, gallium 2-ethylhexanoate exhibits a distinctive C–O stretching peak, indicating that the carboxylate groups have replaced the methyl groups in TMGa. The LDI-TOF analysis of gallium 2-ethylhexanoates derived from TMGa reveals a sharp peak at 497.52 m/z (Fig. S3†) which greatly approximates the molar mass of ∼500 Da of gallium tri-2-ehtylhexanoate. These results suggest that gallium 2-ethylhexanoate derived from TMGa has a monomeric structure, with the methyl groups fully substituted by carboxylates (Fig. 1d, lower right).

3.2 Synthesis of In_1−xGa_xP alloy QDs using monomeric and dimeric gallium carboxylates

We synthesized In_1−xGa_xP QDs using monomeric and dimeric gallium carboxylates derived from TMGa and gallium iodides, respectively (Fig. 2a). The QD synthetic scheme was modified from the well-established bottom-up colloidal synthesis of InP QDs, which utilizes metal carboxylates and TMS₃P as precursors (Fig. 2a, see the Experimental methods).⁴⁵ To investigate the optical properties of the QDs as a function of Ga content, we varied the Ga-to-In feed ratios to 1, 2, and 3 while maintaining the total amount of Ga and In precursors constant.

Fig. 2 Synthesis and characterization of In_1−xGa_xP alloy QDs using gallium carboxylates. (a) Schematic illustration of the synthetic procedure of In_1−xGa_xP alloy QDs using (i) TMGa and (ii) Ga₂I₆. (b) UV-vis absorption spectra of In_1−xGa_xP QDs using Ga₂I₆ (dashed lines, upper panel) and TMGa (solid lines, lower panel) according to the Ga-to-In feed ratio. The inset shows the transmission electron microscopy image of In_1−xGa_xP QDs using TMGa whose Ga-to-In reed ratio is 1. (c) X-ray diffraction spectra of In_1−xGa_xP QDs. The vertical solid lines represent the standard JCPDS peaks for InP (black, JCPDS #70-2902) and GaP (red, JCPDS #72-4949). The vertical dashed lines indicate the peak positions of the QDs for visual guidance.

The absorption spectra demonstrate that the 1S transition peak wavelength of the QDs blue-shifted with increasing Ga-to-In feed ratios, regardless of the type of gallium carboxylate used (Fig. 2b). However, the 1S transition peak wavelength of QDs using monomeric Ga(LA)₃ was 30–40 nm shorter than that of QDs using dimeric Ga₂I₂LA₄. This suggests that for the same Ga-to-In feed ratio, Ga incorporation into QDs was more efficient when using monomeric Ga(LA)₃ compared to that with dimeric Ga₂I₂LA₄. The half-width at half-maximum (HWHM) of the absorption peaks of monomeric precursor-based QDs was significantly narrower (with a smaller valley-to-peak ratio in the absorption spectra) compared to that of dimeric precursor-based QDs with similar 1S transition energies (Fig. S4†). This result indicates that the monomeric gallium precursor produces QDs with a narrow size distribution (inset image in Fig. 2b).

We attribute the superior optical properties of TMGa-mediated In_1−xGa_xP QDs to the monomeric structure of gallium carboxylates. Monomeric Ga(LA)₃ could effectively decompose during the early stages of the nucleation and subsequent growth of QDs, enabling uniform incorporation of Ga. In contrast, the decomposition of bridging halides in the dimeric gallium carboxylate requires high temperatures close to 300 °C,⁴⁶ at which point a vast majority of indium and phosphorus precursors are already consumed by QD formation. The robust bonding configurations of bridging halides exacerbate the reactivity difference between indium and gallium precursors, leading to nonuniform Ga incorporation. To additionally testify whether the monomeric structure in gallium precursors plays a decisive role in efficient Ga incorporation into QDs, we also synthesized In_1−xGa_xP QDs using monomeric gallium acetylacetonate (Ga(acac)₃) as a replacement of TMGa. The resulting optical properties were similar to those of QDs synthesized with TMGa (Fig. S5†).

In order to investigate how the gallium precursors affect the crystal properties of In_1−xGa_xP QDs, we performed XRD analysis on the samples (Fig. 2c). Both samples exhibited distinctive diffraction peaks corresponding to the (111), (220), and (311) planes (Fig. 2c, dashed lines), which were positioned between the standard peak positions of zinc blende InP (black vertical lines in Fig. 2c) and GaP (red vertical lines in Fig. 2c). While the diffraction peaks of gallium iodide-mediated QDs were closer to the standard peak positions of InP, the peaks of TMGa-derived QDs shifted toward higher 2θ, closer to the GaP standard peaks. This shift suggests higher Ga content incorporated into In_1−xGa_xP QDs derived from TMGa, resulting in a reduction in d-spacing.

3.3 Composition analysis of In_1−xGa_xP QDs according to the extent of carboxylate substitution from TMGa

Previous analyses demonstrate that the monomeric structure of gallium carboxylates facilitates Ga incorporation in In_1−xGa_xP QDs. However, a question remains as to how the under-substitution of carboxylate (e.g., di-substitution) from TMGa affects the Ga incorporation into In_1−xGa_xP QDs. To address this question, we synthesized In_1−xGa_xP QDs using di-substituted gallium laurate [i.e., intentionally reacting 2 equivalents of LA with TMGa to produce Ga(CH₃)(LA)₂]. Interestingly, the absorption spectra reveal that the wavelength at the 1S transition peaks was seldom changed regardless of the Ga-to-In feed ratio, with only the HWHM of the absorption peaks varying (Fig. S6a†). This result implies that the amount of di-substituted gallium laurate has little influence on the extent of the Ga incorporation into In_1−xGa_xP QDs.

To quantitatively characterize the composition in In_1−xGa_xP QDs, we performed ICP-OES analysis of the QD samples synthesized using Ga(LA)₃ and Ga(CH₃)(LA)₂ (Table S2† and Fig. 3). When Ga(LA)₃ was used, the incorporated Ga-to-In ratio in In_1−xGa_xP linearly increased to In_0.6Ga_0.4P, In_0.4Ga_0.6P, and In_0.3Ga_0.7P as the feed Ga-to-In ratio increased by 1, 2, and 3, respectively (red squares in Fig. 3). This tendency corresponds to the widening of the optical E_g observed as the Ga-to-In feed ratio increased (Fig. 2b). The reaction yield of Ga (i.e., the incorporated Ga-to-In ratio divided by the fed Ga-to-In ratio) for Ga(LA)₃ reached ∼71%. This value is significantly higher compared to the ∼40% reaction yield typically achieved with gallium halide-based precursors.⁴⁷

Fig. 3 Incorporated Ga-to-In ratio in In_1−xGa_xP QDs characterized using ICP-OES. The In_1−xGa_xP QDs were synthesized with varying Ga-to-In feed ratios of 1, 2, and 3, using Ga(LA)₃ (red squares) and Ga(CH₃)(LA)₂ (orange triangles). The dashed line serves as a guide representing unity reaction yield.

When Ga(CH₃)(LA)₂ was used with a feed Ga-to-In ratio of 1, In_0.5Ga_0.5P QDs were produced with a reaction yield of Ga approaching 100% (leftmost triangle in Fig. 3). However, as the Ga-to-In feed ratio increased to 2 and 3, the detected Ga-to-In ratios exceeded the fed Ga-to-In ratio. Considering that TMS₃P is the limiting reagent, this result indicates that TMS₃P selectively reacted with Ga(CH₃)(LA)₂ rather than In(LA)₃, producing GaP QDs apart from In_0.5Ga_0.5P QDs. This result is in line with the consistent 1S transition peaks regardless of the amount of Ga(CH₃)(LA)₂ (Fig. S6a†). Notably, we could synthesize GaP QDs using only Ga(CH₃)(LA)₂ and TMS₃P (Fig. S6a†), even though GaP QDs were typically known to require high formation energies.⁴⁸

To sum up, the under-substitution of carboxylates in monomeric gallium precursors exhibits higher reactivity compared to their fully substituted counterparts. While Ga incorporation into In_1−xGa_xP QDs was more efficient at a low Ga-to-In feed ratio (Ga-to-In ratio of 1) using under-substituted precursors, a separate formation of GaP QDs simultaneously occurred at higher Ga-to-In feed ratios (Ga-to-In ratios of 2 and 3).

3.4 Optical enhancement of In_1−xGa_xP/ZnS core/shell QDs using ZnCl₂ and their electroluminescence applications

The as-synthesized In_1−xGa_xP QDs exhibit negligible photoluminescence (PL QY of ∼1%) due to the nonradiative surface defects.^49,50 Undercoordinated and oxidized surface metal and phosphorus atoms have been generally identified as the primary culprits for nonradiative recombination centers in metal phosphide QDs.^51,52 To address the surface issue of In_1−xGa_xP core QDs, we employed a surface treatment method using zinc halides. The zinc halides act as Z-type ligands, effectively passivating the undercoordinated surface atoms prior to the shell growth (Fig. 4a).^53,54

Fig. 4 Schematic illustration of ZnS shell growth on In_1−xGa_xP cores. (a) Surface defect passivation of In_1−xGa_xP cores using ZnCl₂. (b) Double-step growth of the ZnS shell using ZnCl₂–OAm and Zn(OA)₂.

Interestingly, we found that the use of ZnCl₂ not only passivates the surface but also helps adjust the PL peak wavelength of the In_1−xGa_xP/ZnS QDs. Achieving a PL peak within the range of 460 to 475 nm for blue emitters is critical to meet the spectral requirements of the National Television System Committee (NTSC) while avoiding severe damage to the human eye from short-wavelength emissions.^55–57 The use of Zn(OA)₂, as a conventional Zn precursor, during ZnS shell growth resulted in QDs with an emission peak exceeding 480 nm (Fig. S7a†), requiring a blue shift of PL by at least 5 nm. This longer PL peak wavelength could be attributed to the binding behavior of the oleate ligand in Zn(OA)₂, which acts as a quasi-bidentate ligand due to its chelating geometry.⁵⁸ The chelating nature promotes the formation of strong hybrid molecular orbitals with the shell material, leading to enhanced exciton delocalization into the shell.^59,60

To address the chelation issue, we utilized a ZnCl₂–OAm precursor (ZnCl₂ dissolved in OAm) as a Zn precursor for shell growth, replacing Zn(OA)₂ with a monodentate ligand.^61,62 The monodentate OAm ligand minimized the exciton delocalization by chelation, resulting in a hypsochromic shift of approximately 10 nm, which could then align with the NTSC requirements (solid line in Fig. S7b†). However, after the purification process for the integration into QLEDs, the PL QY of the In_1−xGa_xP/ZnS QDs was reduced to nearly half of its initial value (dashed line in Fig. S7b†) due to the detachment of weakly bound OAm ligands on the ZnS shell during the purification process.⁶³ To prevent ligand detachment after purification, an additional ZnS shell was grown using Zn(OA)₂ (Fig. 4b) to ensure effective surface passivation. This modification produced In_1−xGa_xP/ZnS core/shell QDs with an average diameter of 6.34 nm, consisting of 2.3 nm In_1−xGa_xP cores and a 2.0 nm ZnS shell (Fig. 5a). The resulting QDs exhibited remarkable optical properties, with an emission peak at 470 nm, a narrow linewidth of 40 nm, and a PL QY of 67% after purification (Fig. 5b). The XRD spectra of the In_1−xGa_xP/ZnS QDs were compared with those of the In_1−xGa_xP cores (Fig. 5c). The (111), (220), and (311) diffraction peaks of the zinc blende In_1−xGa_xP cores shifted to higher 2θ values after the growth of ZnS shells. This shift indicates the uniform growth of ZnS thick shell on the In_1−xGa_xP cores, which is responsible for the robust PL properties and narrow linewidths of the In_1−xGa_xP/ZnS QDs.

Fig. 5 Characterization of the In_1−xGa_xP/ZnS core/shell QDs and their electroluminescence properties. (a) TEM image of In_1−xGa_xP/ZnS QDs. (b) UV-vis absorption and PL emission spectra of In_1−xGa_xP/ZnS QDs. The PL peak is located at 470 nm, with a FWHM of 40 nm and a PL QY of 67%. The inset photograph demonstrates the In_1−xGa_xP/ZnS QDs irradiated with a UV light of 380 nm. (c) XRD spectra of In_1−xGa_xP cores and In_1−xGa_xP/ZnS core/shell QDs (lower panel). Standard diffraction peaks for InP (JCPDS #70-2902), GaP (JCPDS #72-4949), and ZnS (JCPDS #86-8464) are provided in the upper panels. Dashed lines indicate the peaks corresponding to the (111), (220), and (311) planes of In_1−xGa_xP cores and In_1−xGa_xP/ZnS QDs, respectively. (d) Energy level diagram with respect to a vacuum level for the In_1−xGa_xP/ZnS QD-employed QLEDs. The energy levels of In_1−xGa_xP/ZnS QDs were characterized using UPS analysis (Fig. S8†). (e) Current (J)–voltage (V)–luminance (L) characteristics of In_1−xGa_xP/ZnS QD-employed QLEDs. The inset shows the PL and EL spectra of the QLEDs where the EL spectrum is measured at 6 V.

The resulting QDs exhibited one of the narrowest linewidths reported to date among blue-emitting In_1−xGa_xP QDs (Table S4†), highlighting their potential for blue-color reproduction in display technologies. However, the underlying reason for the limited PL QY (67%) of In_1−xGa_xP/ZnS QDs, even after uniform ZnS shell growth, remains unclear. We attribute the limited PL QY primarily to the increased contribution of indirect E_g transition arising from high Ga content in In_1−xGa_xP/ZnS QDs.²⁵ We synthesized In_1−xGa_xP/ZnS QDs with varying Ga-to-In feed ratios of 1, 2, and 3 during core synthesis and analyzed their PL characteristics (Fig. S8†). As the Ga-to-In feed ratio increases, the PL spectra blue-shifts (Fig. S8a†), indicating a widened E_g due to a higher amount of Ga incorporated. Concurrently, the PL QY becomes lower (Fig. S8b†). To further elucidate the relationship between Ga content and PL characteristics, we measured time-resolved PL of the samples (Fig. S8c–S8e†). Biexponential fitting of the PL decay curves revealed two distinct decay components (Table S3†): a trap-associated recombination component (1.5–1.6 ns) and a radiative recombination component (45.9–58.7 ns). While the former was relatively consistent regardless of the Ga-to-In ratio, the latter became longer as the ratio increased. The prolonged radiative recombination lifetime, accompanied by a decrease of PL QY, is a distinctive feature of enhanced indirect band gap transition in In_1−xGa_xP/ZnS QDs.²⁵ These results emphasize that precise control of Ga incorporation into In_1−xGa_xP QDs is essential for optimizing both PL QY and emission wavelength to attain efficient blue emission.

Finally, we incorporated the In_1−xGa_xP/ZnS QDs into QLEDs with an inverted structure. The device architecture consisted of an ITO cathode, a ZnMgO electron transport layer (ETL), an In_1−xGa_xP/ZnS QD emissive layer, a CBP hole transport layer (HTL), and a MoO₃/Al anode (Fig. 5d and Fig. S9†).^18,64,65 Current (J)–voltage (V)–luminance (L) characteristics show that the QLEDs exhibit a low turn-on voltage of 2.6 V which is close to the optical E_g of the QDs (Fig. 5e). The EL peak of the QLEDs exhibits a moderate extent of red shift from 470 nm to 477 nm due to the quantum-confined Stark effect at a high operation voltage of 6 V (Fig. 5c).⁶⁶ These electrical advantages are attributed to the well-matched energy levels of In_1−xGa_xP/ZnS QDs to those of charge transport layers, leading to the minimal accumulation of drift charges with low carrier injection barriers.⁶⁷ Based on the EL properties of our devices (Fig. S10 and Table S5†), we could highlight the potential of In_1−xGa_xP/ZnS QDs as promising electroluminescent emitters for future blue QLEDs.

4 Conclusions

In summary, we revealed the structural effect of Ga precursors on the Ga incorporation into the In_1−xGa_xP QDs. Due to the strong bonding of bridge halides in Ga₂I₆, Ga₂I₂(RCOO⁻)₄ with a dimeric structure is produced after the carboxylate substitution of Ga₂I₆. These dimeric Ga precursors exhibit low reactivity during the synthesis of In_1−xGa_xP QDs, limiting the Ga incorporation. In contrast, monomeric TMGa produced monomeric gallium carboxylates with tri-substituted carboxylates of Ga(RCOO⁻)₃, facilitating Ga incorporation during the synthesis of In_1−xGa_xP QDs. Meanwhile, partially substituted gallium carboxylates from TMGa (i.e., Ga(CH₃)(RCOO⁻)₂) led to the formation of separate GaP QDs due to their higher reactivity compared to that of their indium counterparts.

The use of ZnCl₂ and OAm in the early stage of ZnS shell growth improved the optical properties of In_1−xGa_xP/ZnS core/shell QDs. The ZnCl₂–OAm precursors adjusted the emission wavelength to the blue range due to their monodentate coordination characteristics. Additional ZnS outer shell growth further passivated the In_1−xGa_xP/ZnS QDs, resulting in efficient and robust PL characteristics, including an emission peak at 470 nm, a linewidth of 40 nm, and a PLQY of 67%. Consequently, the resulting In_1−xGa_xP/ZnS QDs were successfully incorporated into the QLEDs. The In_1−xGa_xP/ZnS QD-based QLEDs show efficient charge injection characteristics due to the well-balanced energy levels of the QDs with the charge transport layers.

These results highlight the promising operational performance of these devices and reinforce the potential of In_1−xGa_xP QDs as environmentally friendly materials for optoelectronic applications. While further advancements in QD synthesis and device engineering are necessary for commercialization, we believe that this work establishes a solid foundation for the deployment of In_1−xGa_xP-based QDs in next-generation display technologies.

Author contributions

T. Kim: investigation, conceptualization, methodology, data curation, writing – original draft, and writing – review and editing. S.-Y. Kim: methodology, data curation, and writing – review and editing. S. Lee: conceptualization, funding acquisition, and writing – review and editing. J.-S. Park: methodology, software, and writing – review and editing. H. Lee: supervision, investigation, conceptualization, methodology, data curation, writing – original draft, and writing – review and editing. D. C. Lee: supervision, conceptualization, project administration, funding acquisition, writing – original draft, and writing – review and editing.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korean government (MSIT) (RS-2021-NR057412 and RS-2022-NR070840). Additional support was provided by the Korea Planning & Evaluation Institute of Industrial Technology (KEIT) funded by the Korean government (MOTIE) (20019417 and RS-2024-00440884) and the Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Government (MOTIE) (RS-2023-KI002692, HRD Program for Industrial Innovation). We also acknowledge support from Samsung Display Co., Ltd.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qi00302d

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