Photoluminescence Redshift of AgInS₂ Quantum Dots by Employing Shells with Graded Composition

Abstract

Silver indium sulfide (AIS)/gallium sulfide (Ga–S) core/shell QDs exhibit a narrow band-edge photoluminescence (PL) in the yellow color region, and shifting the PL wavelength is crucial for optical applications. In this study, we attempt to redshift the band-edge PL by incorporating indium sulfide (In–S) shells, which have a smaller bandgap than Ga–S and are expected to broaden the exciton wavefunction. When coated with In–S shells instead of Ga–S shells, a redshift of the band-edge PL was attained. However, an increase in defective PL and a reduction in PL quantum yield occurred due to carrier trapping associated with the extended wavefunction. To address these issues, we coated the AIS/In–S cores/shell QDs with Ga–S shells using recently developed procedures, resulting in spectrally narrow PL in the red region. Interestingly, compositional and structural analyses revealed a decrease in the In ratio, which typically leads to blue shift. The observed redshift, reaching up to 40 nm, is discussed in relation to the formation of shells with graded composition, which provide a broader wavefunction in the excited state compared to discrete shells.

1. Introduction

Semiconductor nanoparticles, or quantum dots (QDs), have been intensively studied owing to their distinctive optical and electronic properties that differ from the bulk material.^1–3 Over the past few decades, the group II–VI semiconductor QDs, particularly Cd- and Pb-based chalcogenides like CdSe,^4,5 CdTe,^6,7 and PbS^8,9 have attracted interests for their potential applications in bioimaging and for various optical devices. These interests are primarily owing to their high photoluminescence (PL) quantum yield (QY) that often surpasses 50 %, in addition to absorption characteristics over a wide wavelength range. However, the widespread use of Cd- and Pd-based QDs has been hindered by their pronounced toxicity.^10,11 Recently, group I–III–VI semiconductor QDs such as AgInS₂ (AIS) have gained attention as environmentally benign alternatives.^12,13 These QDs exhibited reasonably high PL intensity, while their PL was composed of spectrally broad defect emission. The narrow-band emission was achieved when the AIS QDs were coated with gallium sulfide (Ga–S) shells.^14,15 Furthermore, a blue-shift of the band-edge emission was achieved by alloying of AIS (bandgap energy, E_g = 1.8 eV) and AgGaS₂ (E_g = 2.6 eV) to increase the bandgap of the core, resulting in PL in the green region.^16–18

In recent years, advances in tailoring the emission properties of QDs have made them a promising candidate for light-emitting devices.^19,20 In particular, fluorophores for the three primary colors, blue, green, and red, are needed for display applications.^21,22 Common strategies for the wavelength tuning of QDs is doping with transition metals^23,24 or changing the elemental composition of alloyed emissive core.^25,26 Alternatively, the redshift of AIS-based QDs can be achieved by controlling the charge delocalization within the core/shell structure by using an electronic structure known as type II, where the valence and conduction-band-edge potentials are offset in the same direction. For example, we fabricate gallium selenide shells on AIS cores to cause redshifted the PL owing to a specific electronic structure, which is known as type II core/shell structure.²⁷ In that study, we controlled the magnitude of redshift by introducing gallium sulfide selenide alloy shells.

The core/graded shell architecture offers an alternative method to achieve PL redshift.²⁸ This design has been used to tune the optical properties of binary semiconductor QDs, like CdSe, by creating shells with a graded composition. This gradient typically results in an average composition represented as Cd_1−xZn_xSe_1−yS_y, with the ratio of ZnS higher at the outer edge.^29–31 When shells has a graded composition, the distribution of electrons and holes becomes broader compared to the QD that have a distinct compositional gap between the core and shell (discrete shell). This effect results in a redshift of the PL spectra owing to an increase in effective size.²⁹ Therefore, changing the compositional gradient and the thickness of the graded part allow systematic PL shifts. In addition, compared to the discrete shell, the graded shell has less lattice strain, resulting in a decrease in the nonradiative transition and an increased PL QY.^30,32

At present, the effect of the graded shell on the optical property for AIS-based QDs is not yet fully understood. However, we have occasionally observed an unexpected redshift in the PL of silver indium gallium sulfide (Ag–In–Ga–S)/Ga–S core/shell QDs.¹⁷ That is, the wavelength of the band-edge PL was longer than that of AIS/Ga–S (580–590 nm),^14,15 whereas the bandgap of Ag–In–Ga–S should, theoretically, be larger than that of AIS.^16–18 We hypothesize that the abnormal wavelength shift that we observed previously is due to the graded structure that is spontaneously generated by the synthesis condition. In this work, AIS core is initially coated with an indium sulfide (In–S) shell, which has a narrower bandgap (2.2 eV) than Ga–S (2.8 eV), with the aim of forming an In–Ga–S graded shell. The core/shell QDs are then reacted with gallium (III) diethyldithiocarbamate (Ga(DDTC)₃), followed by a GaCl₃ treatment to form Ga–S outer shell. Whereas the red-color PL was obtained, compositional and X-ray diffraction (XRD) analyses suggested that, within a series of synthesis operations, the QDs underwent complicated structural changes. These changes are discussed in relation to changes in PL properties, with the formation of graded shells in mind.

2. Experimental

2.1 Chemicals

Silver (I) acetate (Ag(OAc), 99.9 %), indium (III) chloride (InCl₃, 99.9 %), and elemental S (99.999 %) were purchased from Mitsuwa Chemical Co., Ltd. Indium (III) acetate (In(OAc)₃, 99.99 %) was purchased from Sigma-Aldrich. Anhydrous gallium (III) chloride (GaCl₃, >98.0 %) and 1,3-dimethyl thiourea (DMTU, >97.0 %) were purchased from TCI (Tokyo Chemical Industry Co., Ltd.). 1-Dodecanethiol (DDT, 98.0 %) and diethyl ammonium N,N-diethyldithiocarbamate (DEA-DDTC, Wako 1st grade) were purchased from FUJIFILM Wako Pure Chemical Industries Ltd. In addition, oleylamine (OLA, >50.0 %, FUJIFILM Wako Pure Chemical Industries Ltd.) was dehydrated and purified by distillation in the presence of calcium hydride (CaH₂, ≧90.0 %, FUJIFILM Wako Pure Chemical Industries Ltd.), which was stored in a glove box before use.

2.2 Synthesis of Ga(DDTC)₃

The synthesis of Ga(DDTC)₃ was performed based on the methodology detailed in a previous study.¹⁶ Initially, in a N₂ glovebox, a solution of GaCl₃ in dehydrated toluene was prepared, to which was added a toluene solution containing three equivalents of DEA-DDTC. The mixture was reacted for 1 h with rigorous stirring. Upon completion of the reaction, the solvent was removed by a rotary evaporator, and the white powder was further dried under vacuum (100 Pa) for an additional hour. The crude product was purified by re-crystallization in chloroform over a period of 12 h. The filtered product was then dried in a vacuum at 60 °C, and a white powder of Ga(DDTC)₃ was obtained in 82 % yield. It was stored in a desiccator cabinet filled with N₂ before use.

2.3 Preparation of AIS core QDs

Highly monodisperse AIS core QDs were prepared according to a previous procedure.¹⁴ Ag(OAc) (0.2 mmol), In(OAc)₃ (0.2 mmol), DDT (300 µL), and OLA (10 mL) were added to a 50-mL round bottom flask. After degassing at 120 °C, the mixture was heated to 140 °C under an Ar atmosphere and OLA containing DMTU (0.08 mol mL⁻¹) was added dropwise to the solution at a rate of 4 mL h⁻¹ for 30 min. The temperature was maintained for another 30 min to grow the QDs. After the reaction was completed, the turbid solution was centrifuged to remove overgrown particles. The red colored supernatant was precipitated with ethanol, centrifuged, and redispersed in chloroform, which was repeated several times for purification. Finally, the AIS QDs were stored in hexane and kept in the freezer before use.

2.4 Preparation of AIS/In–S core/shell QDs

The synthesis of the In–S shell was performed based on the reported procedure for In₂S₃ nanoparticles with optimization.³³ InCl₃ (0.1 mmol) and 0.4 equivalents of elemental S were mixed with 10 mL of OLA. The reaction mixture was then heated to 80 °C under vacuum. After filling with Ar, the solution temperature was increased to 120 °C, and the AIS QD solution (30 nmol in terms of particles) was injected. The temperature of the solution was increased to 180 °C and maintained for 30 min to allow the In–S shell to grow. Upon completion of the reaction, the solution was cooled and then evacuated at 200 °C to remove unreacted S precursors. After cooling to room temperature, the QDs were purified using ethanol and chloroform. Finally, the sample was dissolved in 1 mL of hexane.

2.5 The formation of Ga–S outer shell

0.1 mmol of Ga(DDTC)₃ and 10 mL of OLA were placed in a 50 mL round bottom flask. The mixture was heated under vacuum at 80 °C for 10 min and then filled with Ar. The temperature controller was set to 230 °C. To prevent Ostwald ripening during the temperature rise, the AIS/In–S core/shell QDs were injected when the solution temperature reached 160 °C. The temperature was further increased from 230 to 280 °C at a rate of 2 °C min⁻¹. Separately, 0.1 mmol of GaCl₃ was dissolved in 1 mL of OLA and loaded into a gas-tight syringe. When the temperature reached 280 °C, the GaCl₃ solution was injected. Heating was continued for another 30 min. After the reaction was completed, the same purification process as for In–S-coated QDs was applied. Finally, the QDs were dissolved in 1 mL of hexane and stored in a freezer.

2.6 Material characterization

Electronic absorption spectra were recorded with a UV–vis spectrometer (JASCO, V-670). PL spectra were measured using a spectrofluorometer (JASCO, FP-8600) at an excitation wavelength of 450 nm. The PL QY was obtained as the absolute quantum yield using a spectrofluorometer (Hamamatsu, PMA-12) equipped with an integrating sphere. Chloroform was used as the solvent for samples and a blank, which were placed in a dedicated quartz cuvette. XRD measurement was performed using a powder X-ray diffractometer (Rigaku, Japan, SmartLab). The QD morphologies were examined by a transmission electron microscopy (TEM, Hitachi, Japan, H-7650) at an acceleration voltage of 100 kV. The elemental compositions of the QDs were estimated by inductively coupled plasma atomic emission spectroscopy (ICP–AES; Shimadzu, ICPS-7510). The sample digestion protocol is described in the Supplementary Information (SI). PL decay curves were collected using a time-correlated single-photon counting setup (Hamamatsu, Quantaurus-Tau).

3. Results and Discussion

3.1 Formation of AIS/In–S core/shell QDs

Figures 1a and 1b display the TEM images of the AIS core QDs, both before and after heat treatment with InCl₃ and elemental S. It is noteworthy that the particle size increased from 4.2 ± 0.4 to 5.3 ± 0.7 nm, while maintaining its spherical shape. This observation suggests the formation of In–S shells, as further corroborated by the size histograms shown in Fig. S1 (SI). Prior to the reaction, elemental analysis of the QDs by ICP–AES revealed a compositional ratio of Ag : In : S of approximately equal to 1 : 1 : 2 (see Table 1 for details). However, post-reaction analysis of purified samples showed a significant increase in the proportions of In and S. Specifically, the ratios of In and S relative to Ag increased by 0.73 and 1.48, respectively. This indicates the formation of In–S layer with an excess of S when compared to the stoichiometric ratio of In₂S₃.

Figure 1.

TEM images of (a) AIS core, (b) AIS/In–S core/shell, and AIS/Ag–In–Ga–S/Ga–S core/graded shell QDs (c) before and (d) after GaCl₃ treatment. Size histograms are shown in Fig. S1 (SI).

Table 1. Elemental compositions of AIS core, AIS/In–S core/shell, and AIS/Ag–In–Ga–S/Ga–S core/graded shell QDs before and after GaCl₃ treatment.

Sample	Composition ratios (Ag = 1.00)
	Ag	In	S	Ga
AIS	1.00	0.93	1.99	—
AIS/In–S	1.00	1.66	3.47	—
AIS/Ag–In–Ga–S/Ga–S
(Before GaCl3 treatment)	1.00	1.00	4.11	1.54
(After GaCl3 treatment)	1.00	0.75	2.18	0.35

Figure 2 shows the XRD patterns of the QDs, which are the same samples analyzed in the previously presented TEM images. AgInS₂ primarily exhibits two crystal phases: the tetragonal, often referred to as the chalcopyrite structure, and the orthorhombic, similar to the wurtzite structure. The AIS core QDs are biased toward the tetragonal phase, although the signals are weak due to their small crystal size. After coating with In–S, the tetragonal phase persisted, with no discernible peaks associated with In₂S₃. This finding suggests that the In–S shell is amorphous, similar to the attributes of Ga–S shells reported in recent studies.^14,15

Figure 2.

XRD patterns of AIS core, AIS/In–S, and AIS/Ag–In–Ga–S/Ga–S core/graded shell QDs before and after GaCl₃ treatment. The bars at the top and bottom of the graph represent the reference patterns for the orthorhombic (ICDD 075-6150) and tetragonal AgInS₂ (ICDD 077-6632), respectively.

Figure 3 presents the absorption and PL spectra of the AIS core (displayed in the black curve) and AIS/In–S core/shell (the yellow curve) QDs. The relevant optical parameters are listed in Table S1 (SI). After introduction of amorphous In–S shells, an obvious blue shift (by 140 nm) in the absorption tail was observed, indicating a significant reduction in surface defects. On the contrary, a small shoulder around 540 nm appears to be slightly redshifted after In–S shell coating. Although the local maxima of the absorption spectra of AIS QDs are not obvious compared to CdSe and InP QDs, this change seems to reflect the extension of the exciton wavefunction due to the presence of the shell. The PL spectra showed a new PL component at 593 nm after In–S shell coating, probably due to band-edge PL. However, a significant amount of defective PL remained, consistent with our previous research that In–S coating does not completely remove defects.¹⁵ In addition, the position of band-edge PL was approximately 10 nm longer than that of AIS/Ga–S core/shell QDs (ranging between 575–585 nm). This is due to an insufficient core/shell band offset, with such a redshift being characteristic of core/shell QDs possessing a small band offset.³⁴ Concurrently, the PL QY value was only 1 % for the band-edge PL and 23 % over the entire emission spectrum, which is a significant decrease from 76 % before the In–S shell coating. This drastic QY decrease is attributed to the strong tendency of the In–S surface oxidation to increase the nonradiative transition rate.³⁵

Figure 3.

(a) Absorption and (b) PL spectra of AIS core, AIS/In–S core/shell, and AIS/Ag–In–Ga–S/Ga–S core/graded shell QDs before and after GaCl₃ treatment.

Figure 4 presents the PL decay profiles of the AIS core (the black curve) and AIS/In–S core/shell QDs (the yellow curve). These curves were fitted with a one or three exponential equation given by

  

\begin{equation*} I(t) = A_{1}\exp \left(-\frac{t}{\tau_{1}}\right) + A_{2}\exp \left(-\frac{t}{\tau_{2}}\right) + A_{3}\exp \left(-\frac{t}{\tau_{3}}\right) \end{equation*}

where I(t) denotes the PL intensity at time t, A₁, A₂, and A₃ are the amplitudes, and τ₁, τ₂, and τ₃ are the corresponding lifetimes. When fitting the data, we used the minimum number of exponents necessary to achieve a satisfactory fit. The quality of fit was assessed using the χ² value, a measure of error calculated as the sum of the squared differences between the observed and the expected values, each divided by the corresponding expected value. A χ² value below 1.2 was considered acceptable. The fitting results are summarized in Table 2, which also provides the values of intensity average (τ_avg). As previously reported, the PL of AIS core (τ = 918 ns) can be attributed to the donor–acceptor pair recombination from defect states.^13,36 On the other hand, the PL lifetime of the new peak appearing at 593 nm after In–S coating was significantly shorter than that of the core. This change suggests a transformation in the emission mechanism, possibly to the band-edge transition.¹⁵

Figure 4.

PL decay profiles of AIS core, AIS/In–S core/shell, and AIS/Ag–In–Ga–S/Ga–S core/graded shell QDs before and after GaCl₃ treatment.

Table 2. PL decay components of AIS core, AIS/In–S, and AIS/Ag–In–Ga–S/Ga–S core/graded shell QDs before and after GaCl₃ treatment.

Sample	τ1/ns	A1	τ2/ns	A2	τ3/ns	A3	χ2	τavga
AIS	918	1.00	—	—	—	—	1.16	918
AIS/In–S	3.67	0.703	26.6	0.255	182	0.042	1.19	92
AIS/Ag–In–Ga–S/Ga–S
(Before GaCl3 treatment)	11.7	0.526	63.2	0.380	337	0.094	1.07	198
(After GaCl3 treatment)	33.5	0.503	140	0.406	684	0.092	1.05	377

^a Intensity average calculated by $\sum\nolimits_{i}A_{i} \tau_{i}^{2}/\sum\nolimits_{i}A_{i} \tau_{i}$.

3.2 Overcoating with Ga–S outer shell

To further investigate into the effect of In–S shell, AIS/In–S core/shell QDs were subsequently coated with Ga–S outer shells. This additional layer is expected to passivate the In–S shell surface and reduce the nonradiative transition path, especially when such a transition is caused by surface oxidation. The process for applying the Ga–S coating was adapted from a recently developed method using Ga(DDTC)₃ as both the Ga and S sources.¹⁶ After heating the AIS/In–S core/shell QDs alongside Ga(DDTC)₃, there was a notable increase in particle size from 5.3 ± 0.7 nm to 7.0 ± 1.0 nm (see Figs. 1c and S1c, SI). This change confirms the formation of the Ga–S outer shells. Whereas an increase in Ga content was observed, the In ratio within the Ga(DDTC)₃-treated QDs decreased and approached the values of before the In–S shell coating. This result, at first glance, suggests that the In–S shell is largely replaced by Ga–S, which is not surprising given the report that In₂S₃ can be the template for AgInS₂ and ZnIn₂S₄ owing to the presence of In vacancies.^37,38 The XRD patterns remained largely unchanged, indicating that the core QDs retained the tetragonal phase (Fig. 2, green curve). There was no shift in the absorption onset, whereas the band-edge PL was slightly blue-shifted to 587 nm and the ratio of defective PL was decreased compared to that before Ga–S coating (Figs. 3a and 3b, green curves). Nevertheless, the increase in PL QY for the band-edge moiety was marginal, moving from 1 % to 3 %. This trend was further supported by a slight extension of the PL decay profile (see Fig. 4, green curve).

Previously, we experienced the lack of PL QY improvement of the band-edge emission after Ga–S shell formation (reaction with Ga(DDTC)₃). This phenomenon has been attributed to the insufficient decomposition of Ga(DDTC)₃, leading to a high organic residue after heating at 280 °C in OLA.¹⁶ Currently, the most effective method to obtain pure inorganic Ga–S shells is to heat the core/shell QDs with GaCl₃. This process not only facilitates the conversion of Ga(DDTC)₃ to Ga–S but also eliminates trap states in the Ga–S shell through atomic rearrangement. After the GaCl₃ treatment, there was a slight particle growth, possibly due to the formation of a more rigid shell (Figs. 1d and S1d, SI). However, the resulting composition and structure differed from our initial expectations. Specifically, compositional analysis showed a further decrease in the In ratio after the treatment, whereas apparent In/Ga ratio rather increased owing to the decrease in Ga.

In XRD pattern (Fig. 2, the blue curve), a partial splitting of the highest peak (∼27°) was observed. As mentioned, nanoparticles of AIS can exist in two distinct crystal phases, tetragonal chalcopyrite and orthorhombic.¹⁴ In bulk systems, the orthorhombic phase is known as the high temperature phase because of its high tolerance for cation disordering, especially the exchange of Ag and In. However, in the nanocrystalline system, the orthorhombic phase can exist at room temperature when synthesized under certain conditions.²⁶ The three characteristic peaks around 27° are specific to the orthorhombic phase, and it is likely that, during the GaCl₃ treatment, a partial phase transition from tetragonal to orthorhombic took place. Since this transformation has not been observed in AIS/Ga–S core/shell QDs under the same GaCl₃ treatment, the presence of the In–S inner shell might be linked to this occurrence. In addition, the peaks appeared at higher angles relative to the orthorhombic AgInS₂ reference (ICDD 075-6150). This result suggested the occurrence of partial In/Ga substitution, which caused an atomic rearrangement within particle. Such alterations in material properties lead to a noticeable shift in the emission spectrum as shown by the blue curve in Figs. 3a and 3b. A distinct band-edge PL appeared at 602 nm, with its defect PL significantly reduced compared to the untreated states. PL decay analysis revealed an increase in lifetimes, consistent with the earlier observations that the gallium chloride treatment reduces the nonradiative transitions (Table 2). Yet, these recorded lifetimes were longer than the previously documented values for AIS^14,15 and Ag–In–Ga–S^16–18 core/shell QDs that exhibited similar or superior PL QY values. This suggests that the sample after GaCl₃ treatment has a distinct electronic state that is different from the previous AIS or Ag–In–Ga–S core/shell QDs synthesized without an In–S interlayer.

3.3 Effect of reaction temperature for In–S shell coating

In order to more clearly discern the effect of In–S interlayer, the reaction temperature for the In–S coating was incrementally raised from 180 °C to 200 °C and 220 °C, with the aim of producing thicker shells. Figures 5a, 5b and S2a, S2b (SI) displayed the TEM images and size histograms of AIS/In–S core/shell QDs prepared with the growth temperature of 200 °C and 220 °C. These QDs had average diameters of 6.2 ± 1.3 nm and 7.7 ± 1.7 nm when In–S shells were coated at 200 °C and 220 °C, respectively. Compositional examinations highlighted an increased In ratio (Table S2, SI), which varied from In/Ag = 1.66 (180 °C) to In/Ag = 1.72 (200 °C) and further to 1.76 (220 °C). However, these values were below the values expected from the volumetric changes of the particles due to the shell formation, which was V_core/shell/V_core = 200 % for 180 °C, 221 % for 200 °C, and 394 % for 220 °C. This discrepancy became more pronounced at elevated reaction temperatures. Given the increased polydispersity of the latter two QDs, the observed increase in particle size can be ascribed not only to In–S shell expansion, but also to Ostwald ripening, which is likely to be more pronounced at higher temperatures. In terms of crystal structure, the shift toward the orthorhombic phase became pronounced as the reaction temperature increased (see Fig. S3a, SI). Such trends starkly contrast with the Ga–S shell formation process where the tetragonal phase remains after the reaction at 280 °C, thereby highlighting the distinctive nature of the In–S shell.

Figure 5.

TEM images of (a, b) AIS/In–S core/shell QDs and (c, d) AIS/Ag–In–Ga–S/Ga–S core/graded shell QDs. The In–S growth temperatures are (a, c) 200 °C and (b, d) 220 °C. Associated size histograms are provided in Fig. S2 (SI).

Figures 6a, 6b, and S4a show the PL spectra, decay curves, and absorption spectra of AIS/In–S core/shell QDs, respectively, recorded at their respective band-edge PL peaks. A summary of the PL properties is provided in Table S1 (SI). The presence of band-edge PL indicates that the type-I core/shell structure remains intact as the In–S shell thickens. However, there was a notable redshift with increasing temperature for the In–S shell coating, which is due to the broadening of the wavefunction. Meanwhile, the QY values consistently decreased, indicating the increase of nonradiative recombination due to the defective nature of the In–S shells. The two contradictory effects might contribute to the irregular variation of the PL lifetimes, which was 92 ns (180 °C), 91 ns (200 °C), 109 ns (220 °C). When these samples were reacted with Ga(DDTC)₃ and subsequently treated with GaCl₃ to complete the Ga–S shell, the band-edge PL became even more pronounced. Figures 6c, 6d, and S4b show the PL spectra, decay curves, and absorption spectra, respectively. The PL peak wavelength was shifted to 612 nm (for 200 °C) and 622 nm (for 220 °C), showing a redshift of approximately 40 nm from the standard AIS/Ga–S core/shell QDs (575–585 nm). Also, the absorption onset redshifted with an increase in the growth temperature. Much like the samples coated with In–S shell at 180 °C, there was an observable increase in particle size and decrease in the In ratio (see in Figs. 5c and 5d and listed in Table S2), accompanied by a subtle transition to the orthorhombic phase (see in Fig. S3b, SI). An extended PL decay time further suggests a reduction in nonradiative transitions.

Figure 6.

PL spectra and lifetimes of (a, b) AIS/In–S core/shell and (c, d) AIS/Ag–In–Ga–S/Ga–S core/graded shell QDs, synthesized at In–S shell growth temperatures of (blue) 180 °C, (sky blue) 200 °C, and (grey) 220 °C. Corresponding absorption spectra and decay components are provided in the supporting information: Fig. S4 (SI) for absorption spectra and Table S3 (SI) for decay components. A photograph of the core/graded shell QD solutions under 365-nm UV irradiation can be viewed in Fig. S5 (SI).

3.4 Discussion

Collectively, the data obtained suggest that the application of the In–S shell coating prior to Ga–S redshifts the band-edge PL wavelength by up to 40 nm. A notable structural variation associated with this wavelength change is the phase shift of the core from tetragonal to orthorhombic. However, based on our 2019 publication, the shape and position of the band-edge PL was irrelevant to the crystalline phases.¹⁴ When the PL of QDs redshifted more than expected from their composition, it is often attributed to carrier delocalization that appears when there is minimal potential offset between the core and shell: quasi-type II band alignment.³⁹ However, a straightforward explanation involving an In–S/Ga–S multishell structure, where the In–S inner shell borders between type I and II relative to the AIS core, was inconsistent with the results of the compositional analyses. Boldt et al. observed a redshift during heating CdSe/CdS/ZnS core/multishell QDs at 310 °C, and attributed it to the formation of CdSe/Cd_xZn_1−xS core/graded shell structure due to interfacial alloying.^40,41 While alloying at the type I core/shell interface typically results in an increased bandgap for the core, the graded composition creates a stepless band-edge potential curve, which broadens the electron wavefunction. If this broadening effect is more pronounced than the increase in the core’s bandgap, a redshift would occur. Accompanying this shift, they also reported a 2.0-fold elongation of PL decay lifetimes and a rise in PL QY. These findings resonate well with the optical variations observed for AIS/In–Ga–S/Ga–S core/multishell QDs during the GaCl₃ treatment.

Figure 7a summarizes the structural changes during the shell formation and the subsequent GaCl₃ treatment. The In–S shell, by itself, contributed to the band-edge PL, although its intensity was weak due to the limited band offset at the core/shell interface. The subsequent Ga–S shell coating improved the band-edge PL ratio, which is consistent with the AIS/Ga–S core/shell QDs without an In–S interlayer. After the GaCl₃ treatment performed at 280 °C, the band-edge PL intensified and redshifted even though the In content decreased to below the level of Ag. If we follow the previous idea that GaCl₃ treatment promotes atomic rearrangement, these contradictory observations suggest the formation of a graded shell. Considering the significant decrease in In content, the structure of the QDs after GaCl₃ treatment is closer to AIS/Ag–In–Ga–S/Ga–S rather than AIS/In–Ga–S/Ga–S. While this structure causes an increase in core’s bandgap, the compositional gradient in the shell region stretched the carrier wavefunction to induce the redshift. It is likely that the stretching of the carrier wavefunction exceeded the effect of Ga substitution in the core.

Figure 7.

(a) Structural illustrations and corresponding electronic diagrams of AIS/In–S core/shell QDs during the formation Ga–S shells and their treatment with GaCl₃. (b) (red line) Calculated bandgap variation of AgInS₂ QDs by quantum size effect and (circles) plots of PL peak energies for AIS/Ga–S core/shell and AIS/Ag–In–Ga–S/Ga–S core/graded shell QDs with In–S shell growth temperatures of 180, 200, and 220 °C.

Figure 7b shows the calculation of the quantum size effect for AIS QDs based on the finite-depth effective mass approximation; details are provided in the SI.^42–46 Because we do not get the rigorous structure of the graded shell, a model assuming a uniform core (E_{g, bulk} = 1.87 eV) and a surrounding matrix (E_g = 5.00 eV) was adopted.⁴⁷ The bandgap value of 4.2-nm AIS core QDs is expected to be 2.15 eV (576 nm) and is similar to the reported PL peak wavelength for AIS/Ga–S core/shell QDs (582 nm),¹⁴ validating this calculation. When the PL wavelengths of the AIS/Ag–In–Ga–S/Ga–S core/graded shell QDs were plotted as a function of the apparent diameter, the three plots were at ca. 70 meV above the expected bandgap. This difference may be due to the presence of the Ga–S outer shell, meaning that the actual wavefunction distribution is smaller than the apparent size. Because the variation of the bandgap energy is almost parallel to that by the size effect, the redshift in PL is thought to result primarily from the extension of the excited carrier wavefunction within the graded composition particle.

4. Conclusions

In summary, we demonstrate a PL redshift of the AIS QDs by coating In–S prior to Ga–S shells. As previously reported, the In–S shell by itself exhibited only a weak band-edge PL. However, the presence of the In–S as an inner shell altered the PL characteristics after coating the Ga–S shells. This redshift could be attributed to the inner shell, which has a smaller bandgap than Ga–S, broadening the distribution of the wavefunction and reducing the quantum size effect. However, the elemental composition showed a decrease in In content after the heat treatment with GaCl₃, the process essential to complete the Ga–S shell. This indicated the replacement of In in the core with Ga, theoretically increasing the bandgap. These inconsistent results were discussed in relation to the formation of the shells with graded compositions, which was reported to redshift the PL. Also, the increase in PL lifetimes, indicating the decreased overlap of electron and hole wavefunctions, supported these considerations. By raising the reaction temperature during In–S coating, thicker In–S layers were formed. These nanoparticles also experienced a decrease in In content after Ga–S shell coating. However, the redshift in the band-edge emission was significant, shifting 40 nm compared to standard AIS/Ga–S core/shell QDs, and achieving a red emission with a peak at 622 nm without the use of other elements.

Acknowledgments

This work was supported by Grant-in-Aid for Scientific Research (B) (Grant Numbers JP 22H02168 and JP 23H01786). One of the authors (NK) acknowledges financial support from JICA Innovative Asia Program 4th Batch (Course Number 201905897-J023 to N.K.).

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.24197667.

• 1) Silver indium sulfide (AIS)/gallium sulfide (Ga–S) core/shell QDs exhibit a narrow band-edge photoluminescence (PL) in the yellow color region, and shifting the PL wavelength is crucial for optical applications. In this study, we attempt to redshift the band-edge PL by incorporating indium sulfide (In–S) shells, which have a smaller bandgap than Ga–S and are expected to broaden the exciton wavefunction. When coated with In–S shells instead of Ga–S shells, a redshift of the band-edge PL was attained. However, an increase in defective PL and a reduction in PL quantum yield occurred due to carrier trapping associated with the extended wavefunction. To address these issues, we coated the AIS/In–S cores/shell QDs with Ga–S shells using recently developed procedures, resulting in spectrally narrow PL in the red region. Interestingly, compositional and structural analyses revealed a decrease in the In ratio, which typically leads to blue shift. The observed redshift, reaching up to 40 nm, is discussed in relation to the formation of shells with graded composition, which provide a broader wavefunction in the excited state compared to discrete shells.

CRediT Authorship Contribution Statement

Navapat Krobkrong: Conceptualization (Lead), Data curation (Lead), Formal analysis (Lead), Investigation (Lead), Methodology (Lead), Writing – original draft (Lead)

Taro Uematsu: Conceptualization (Lead), Data curation (Equal), Funding acquisition (Equal), Investigation (Equal), Methodology (Equal), Validation (Equal), Writing – review & editing (Lead)

Tsukasa Torimoto: Supervision (Equal)

Susumu Kuwabata: Conceptualization (Equal), Funding acquisition (Equal)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

Japan Society for the Promotion of Science: JP22H02168

Japan Society for the Promotion of Science: 23H01786

JICA Research Institute: 201905897-J023

Footnotes

T. Uematsu: ECSJ Active Member

T. Torimoto and S. Kuwabata: ECSJ Fellows

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