Broad photoluminescence in ternary Ag-In-S and quaternary Ag-In-Zn-S nanoparticles: the role of Zn incorporation

The cation precursor ratios of Ag/In/Zn/S are 1:4:0:0.5, 1:4:0.5:0.5, 1:4:1:0.5 and 1:4:2:0.5, which we refer to as AIS, AIZS1, AIZS2 and AIZS3, respectively. These nanoparticles are stable in air for many months. The XRD patterns of shown in figure 1(a) indicates that all nanoparticles exhibit a cubic spinel AgInS phase with diffraction peaks located at 27.7°, 45.8° and 53.9°. And with an increase of ZnS content, the diffraction peaks are slightly shifted to a higher angle, approaching to those of (111), (220) and (311) planes of the sphalerite ZnS phase (PDF#05-0566). It suggests the formation of AIZS nanoparticles. The nanoparticles exhibit torispherical shape due to the well dispersity, which can be seen from the TEM image (figure 1(b)). The size is found to be 4.2 ± 0.8 nm for AIZS2 nanoparticles. The high-resolution TEM image in the inset demonstrates good crystallinity with clear lattice fringes. The elemental composition of nanoparticles are also measured with EDS, which evidences that Zn ions are incorporated into nanoparticles (table S1 (available online at stacks.iop.org/MRX/8/085002/mmedia)). XPS was further carried out to determine the valence states of the ions in nanoparticles. The Ag 3d exhibits two peaks located at 367.1 eV and 373.2 eV, corresponding to Ag 3d5/2 and Ag 3d3/2. The In 3d also shows two peaks centered at 444.2 and 451.5 eV, indexing to In 3d5/2 and In 3d3/2. While the Zn 2p is located at 1021.0 and 1044.6 eV, corresponding to Zn 2p3/2 and Zn 2p1/2. The S 2p shows a peak at 160.8 eV. These results reveal that the valence states of the ions are Ag⁺, In³⁺, Zn²⁺ and S²⁻ in the AIZS2 nanoparticles, which are in agreement with the reported data in the literature [48]. The UV–vis absorption spectra and PL spectra of nanoparticles are shown in figure 2(a). The absorption spectra exhibit a broad absorption band from ∼350 nm to 700 nm and an increase in absorption with increasing the photon energy. Similar to the previous studies, no distinct excitonic peaks can be clearly seen and are hidden in tail absorption due to the intraband gap transitions arising from the defect states. The absorption edge is blue-shifted and the absorption coefficient decreases in the near infrared and visible spectrum range. It is attributed to the Zn incorporation into the lattice of nanoparticles. The nanoparticles exhibit typical broad PL emissions in the visible spectral range. The large Stokes shifts with respect to the Zn incorporating is observed (figure 2(b)). The FWHM of PL emission is around 330 meV. The large stoke shift indicates that the band-edge emission is almost absent and defect-induced emissions are dominant in these alloyed nanoparticles [46].

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Figure 2(c) shows the time-resolved decay curves of integrated PL for AIS and AIZS nanoparticles. The long-time decay of the PL emission indicates the existence of multiple emissive states with multi-exponential decay feature in these nanoparticles. The PL decay traces can be well fitted by a bi-exponential function with a fast component and a slow component as follows

$$\begin{eqnarray}&&I(t)={I}_{0}+{A}_{1}\exp \left(-\displaystyle \frac{t-{t}_{0}}{{\tau }_{1}}\right)+{A}_{2}\exp \left(-\displaystyle \frac{t-{t}_{0}}{{\tau }_{2}}\right)\end{eqnarray} \tag{ 1 }$$

where ${\tau }_{1}$ (${\tau }_{2}$) is the fast (slow) decay time constant, and ${A}_{1}$ (${A}_{2}$) is the corresponding intensity weight factor. Fitting results show that the fast time constant is in the region of 25–40 ns and the slow time constant is in the region of 180–220 ns. The intensity weight factor of fast decay component decreases after the incorporation of Zn. The average lifetimes ${\tau }_{a}$ can be given as intensity weighted times ${\tau }_{a}=\left({A}_{1}{\tau }_{1}^{2}+{A}_{2}{\tau }_{2}^{2}\right)/\left({A}_{1}{\tau }_{1}+{A}_{2}{\tau }_{2}\right).$ It is found that the average times extend into hundreds of nanoseconds and increase with increasing the Zn content. The average PL lifetimes of AIZS nanoparticles are comparable to previously reported AIZS systems [12], but shorter than (AgIn)_xZn_2(1−x)S₂ nanocrystals [46] and other AIZS nanocrystals [14, 27, 35, 43].

Further analyses of the PL decay dynamics at different wavelengths provide more insight into the optical properties and the effect of Zn doping on defect states of the nanoparticles. Figures 3(a)–(d) show the PL emission decay spectra recorded by a streak camera coupled with a spectrometer for the four samples. Examples of PL emissions at different delay times are fitted with a Gaussian function as shown in figures 3(e)–(h). With increasing the delay time, the significant red shift of PL emission peak can be observed for all four samples, which indicates that PL decay rate is dependent on emission wavelength. The long-wavelength emissions decay slower than the short-wavelength emissions. Compared with AIS nanoparticles, the PL red shift of AIZS nanoparticles is less pronounced with increasing the delay time. It results most probably from the incorporation of Zn ions into the nanoparticle leading to the partial elimination of the lattice defects.

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Such behavior can be primarily described with a donor-acceptor model [13, 34, 36, 42, 49–53], which is a commonly accepted emission mechanism in I−III−VI₂ QDs. In this model, the observed PL emission originates from the radiative recombination involving the electrons trapped by donor and the holes trapped by acceptor. The PL photon energy ${E}_{PL}$ can be expressed as [13, 34, 36, 52]

$$\begin{eqnarray}&&{E}_{PL}={E}_{g}-\left({E}_{a}+{E}_{d}\right)+\displaystyle \frac{{e}^{2}}{\varepsilon r}\end{eqnarray} \tag{ 2 }$$

where ${E}_{g}$ is the optical bandgap (or exciton energy), ${E}_{a}$ and ${E}_{d}$ are the binding energies of the acceptor and donor that trap a hole and an electron, respectively. ${e}^{2}/\varepsilon r$ is the Coulombic electron-hole interaction, which is a function of the distance $r$ between the trapped charge carriers. $e$ is the electron charge, and $\varepsilon$ is the dielectric constant of the material.

Equation (2) shows that the energies of the emitted photons can be affected by at least three factors, which include: (1) a size distribution of nanoparticles, (2) a distribution of binding energy of the acceptor and donor, and (3) a distance distribution of trapped electron and hole in nanoparticles. A size distribution of the nanoparticles in a colloidal ensemble induces a distribution of ${E}_{g}$ since the energy of interband transition is a function of nanoparticle size in particular for the nanoparticles with sizes smaller than the exciton diameter. The contribution of this factor to the energy distribution can be investigated by using the size-selected nanoparticles. A recent study shows that PL spectral width remains almost unchanged for nanoparticles in the size-selected fractions [42], which confirms that the variation in the distribution of nanoparticle size does not significantly affect the PL spectral width of nanoparticles. The acceptor and donor defects responsible for emissions in such nanoparticles result from the various nanoparticle lattice defects and other possible origins of electron and hole traps (e.g., surface defects). Trapping the photo-generated electrons and holes in these defects leads to the distribution of the binding energy ${E}_{a}$ and ${E}_{d}.$ The contribution of this factor to the PL width can be studied by a modification of the defect states either by decreasing the temperature and reducing the possibility of the thermal activation [53] or by passivating the nanoparticle surface with a shell [14, 39–42]. The Coulombic electron-hole interaction depending on the distance between the trapped electron and hole is the third contribution to the PL energy distribution. The radiative recombination of the trapped electrons and holes located closer to each other leads to higher energy photons, while the lower energy photons are generated from the trapped carriers situated further apart [27]. It is challenging to clearly evaluate the contribution of this factor to the PL energy broadening, in which the size variation of the nanoparticles, the temperature and excitation energy should be taken into account. It is commonly accepted that these three factors contribute to the distribution profile of PL energy.

Figures 3(i)–(l) illustrate extracted PL decay curves of nanoparticles at different emission bands. These curves typically show longer lifetimes for emissions at longer wavelengths. Emission bands at shorter wavelengths (555 ± 5 nm) have fast decays of 12.1, 19.4, 24.7, and 25.2 ns for AIS, AIZS1, AIZS2 and AIZS3 nanoparticles, respectively (table S2). The time constants of slow decays vary from around 100 ns for emission band with shorter wavelengths (555 ± 5 nm) to over 250 ns at longer wavelengths (630 ± 5 nm). The ratio of the fast decay component decreases with increasing wavelength from 550–635 nm. The fastest radiative processes are generally attributed to band-edge emission, which can be confirmed when monitoring near the bandgap. The PL lifetime increases with an increase in emission wavelength, which strongly indicates that the dominant radiative mechanism at longer wavelengths in these AIZS nanoparticles is the radiative donor-acceptor recombination. PL decay lifetimes intermediary between the band-edge emission and the donor-acceptor recombination are free-to-bound transitions, which evolve an electron in the conduction band recombining with a hole in an acceptor defect level [27]. Note that ${\tau }_{a}$ monitored at the PL emission maximum has a value close to the average lifetime of all emissions.

Generally, the defects in these nanoparticles can be categorized into two types: intrinsic crystallographic defects and surface defects [27, 43]. Surface defects provide local sites for relaxation of photoexcited electrons and holes, leading to the faster recombination. Intrinsic crystallographic defects are deep trap states, which provide deep donor-acceptor recombination pathway responsible for slower decays. The average time increases with increasing Zn content, which indicates that the addition of Zn eliminates the lattice defects on the nanoparticle surface and the lattice cation vacancies, thereby reducing non-radiative decay pathways. The passivation of surface states is similar to that of ZnS coating, and the elimination of cation vacancies changes the local environments of the deep traps from AIS to AIZS [53]. The size of these nanoparticles also changes with incorporating the Zn. The PL lifetime is found to be dependent on the bandgap of nanoparticles. The smaller the size is, the shorter the lifetime is. As the smaller nanoparticles exhibit the higher surface to volume ratio, the lifetime can be significantly affected by radiative/non-radiative surface-related transitions. Due to larger overlap of carrier wave function, the probabilities for donor-acceptor transition and recombination for smaller particles is higher than those for larger ones.

To further examine the influence of the Zn inclusion on the radiative and non-radiative recombinations, we calculate the radiative and non-radiative decay rates (${k}_{rad}$ and ${k}_{nr}$) by taking into account the absolute photoluminescence quantum yield ($PLQY={k}_{rad}/({k}_{rad}+{k}_{nr})$) and PL lifetimes ${\tau }_{lifetime}$ for all samples [42]. The radiative decay rate can be expressed as ${k}_{rad}=PLQY/{\tau }_{lifetime}=1/{\tau }_{rad}.$ From the total decay rate ${k}_{total}={k}_{rad}+{k}_{nr}=1/{\tau }_{lifetime},$ on can obtain non-radiative decay rate ${k}_{nr}=1/{\tau }_{lifetime}-1/{\tau }_{rad}.$ The calculated values of ${k}_{rad}$ and ${k}_{nr}$ are given in table 1. As shown in figure 4, $PLQY$ increases from 53.1% for AIS to 60.3% for AIZS2, which indicates that the radiative recombination pathway increases with increasing Zn concentration. $PLQY$ decreases to 57.5% for AIZS3, which means that the non-radiative relaxations are enhanced due to the further addition of Zn, resulting probably in an increase in strain of the lattice.

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Previous studies show that the recombination evolving surface defects is mainly non-radiative, via the free-to-bound transition and donor-acceptor transitions [27, 51]. The surface states arising from dangling bonds could be significantly suppressed by forming a shell of ZnS as a capping material for the AIS nanoparticles [40]. The dominant radiative emissions have been attributed to the donor-acceptor recombination resulting from intrinsic crystallographic defects which include vacancies, interstitial atoms and antisite defects in nanoparticles by many theoretical and experimental studies [12, 27, 51–55]. However, the specified intrinsic defects is difficult to be addressed since the nanoparticle composition is complicated. Based on the previous studies and the experimental results in present study, we speculate that silver vacancies V_Ag, zinc atom on silver site Zn_Ag+ and zinc atom on indium site Zn_In- are main intrinsic defects responsible for donor-acceptor radiative emission. More studies on the further determination of main intrinsic defects and their energy level are required.

Based on these observations, we build a sample model to describe the charge carrier dynamics and radiative/nonradiative recombination processes in these nanoparticles as illustrated in figure 5. Upon photoexcitation, the electrons are excited into conduction band. And the holes residing in the valence band can be trapped by the acceptor on the subpicosecond time scales [26, 27]. In AIS nanoparticles (figure 5(a)), the electron moves first into the defect state (process ①) and subsequently into the donor state (process ②) [26, 27]. The trapped electron in donor state recombines radiatively with the hole localized in the acceptor state (process ③), resulting in nanosecond decay dynamics of the excited-state absorption. It is consistent with the long-time PL decay observation. In most of AIZS nanoparticles (figure 5(b)), the electron moves directly into the donor state (process ①') and then recombines radiatively with the hole localized in the acceptor state (process ②') on the nanosecond time scales.

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