Single-step synthesis of silver sulfide nanocrystals in arsenic trisulfide

Silver sulfide nanocrystals and chalcogenide glasses (ChGs) are two distinct classes of semiconductor materials that have been exploited for new infrared technologies. Each one exhibits particular optoelectronic phenomena, which could be encompassed in a hybrid material. However, the integration of uniformly distributed crystalline phases within an amorphous matrix is not always an easy task. In this paper, we report a single step method to produce Ag2S nanocrystals (NCs) in arsenic trisulfide (As2S3) solution. The preparation is carried out at room temperature, using As2S3, AgCl and propylamine resulting in highly crystalline Ag2S nanoparticles in solution. These solutions are spin-coated on glass and silicon substrates to produce As2S3/Ag2S metamaterials for optoelectronics. ©2015 Optical Society of America OCIS codes: (160.2750) Glass and other amorphous materials; (160.3918) Metamaterials; (310.0310) Thin films. References and links 1. A. L. Rogach, A. Eychmüller, S. G. 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Introduction
Semiconductor nanocrystals (NCs) and quantum dots are of great interest for their use in a wide range of applications from optoelectronics to biological systems [1,2].In contrast to metallic nanoparticles that have a plasmon band in the visible or UV portion of the spectrum, semiconductor nanocrystals exhibit localized surface plasmon resonances in the infrared region [3,4] making them promising for infrared metamaterials.Silver sulfide is a direct bandgap semiconductor (Eg ~1 eV), commonly used as a solid-state electrolyte, presenting both ionic and electronic conduction [5,6].On account of the quantum confinement effect when synthetized at the nanometer scale, indirect transitions have been observed in the range of 0.9 -1.8 eV and direct transitions are blue shifted, to the range 2.7 -4.0 eV [7].Based on these transitions, new applications have been proposed for silver sulfide, such as, NIR emitters for in vivo imaging [8,9], sensitizers for solar cells [10], and substrates for surfaceenhanced Raman scattering (SERS) [11].In addition, the reversible formation of metallic silver over Ag 2 S NCs when exposed to an electric field has enabled the development of atomic switching [12,13], or optical switching materials [14].
Another promising family of semiconductor materials, with interesting optical properties at the infrared region, is chalcogenide glass (ChG).ChGs have high refractive index (n ≈2-3) and high transmittance over to ~11 μm for sulfides, ~15 μm for selenides and beyond ~20 μm for tellurides [15,16].Moreover, they present a variety of photosensitive phenomena, including photocrystallization, photodarkening, and photodiffusion, which have motivated numerous researches for decades [16,17].Since the first observation of metal photodoping in ChGs, many studies have been performed on the diffusion mechanism of silver in amorphous arsenic trisulfide (As 2 S 3 ) [18][19][20].Basically, by shining light on As 2 S 3 , in which a thin metallic layer of silver is deposited, Ag ions can readily dissolve, resulting in a homogeneous doped layer.The mechanism has been explained through the initial formation of Ag−S bond at the silver and ChG interface, followed by the generation of electron-hole pairs and by the mobility of holes toward the silver layer, while Ag + move in the opposite direction [19].Recently, we showed the formation of metallic silver nanoparticles in chalcogenide solution using laser ablation of a silver target [21].Although studies on Ag photodoping in As 2 S 3 have achieved considerable advances, the synthesis of silver sulfide nanocrystals in such material has not been demonstrated yet.
In this paper, we report a one-step in situ synthesis of uniformly dispersed Ag 2 S nanocrystals in As 2 S 3 .The raw materials (As 2 S 3 and AgCl) are diluted in an amine solvent and solid-state As 2 S 3 :NCs films are prepared by spin-coating the solution on glass or silicon substrates.Such approach enables fabricating samples with arbitrary shapes using soft lithographic processes [22], which is an advantage over other conventional methods like vacuum coating or pulse laser deposition.

Materials and methods
Solution-processing of ChGs in amine solvents has been long established, and the dissolution mechanism involves an electrophilic substitution reaction, where As atoms are attacked by the lone pair electron associated with the amine group [22][23][24].The chemical synthesis employed in this study consists of the dissolution of arsenic trisulfide (alfa aesar 99.999%) in npropylamine (C 3 H 9 N Sigma-Aldric >99%), with a concentration of 133g/L.The dissolution was performed at room temperature, and usually takes 24h to be completed for a solutesolvent ratio of 1 g/7.5 ml.In order to produce Ag 2 S NCs in-situ, silver chloride (Alfa Aesar 99.997%) was dissolved in n-propylamine (80g/L), and then, both solutions, arsenic sulfide and silver chloride, were mixed together in a ratio of 1ml of As 2 S 3 to 0.25ml of AgCl.The reaction readily occurs, resulting in the formation of silver sulfide nanocrystals in suspension.Due to the absence of stabilizing agents, the reaction also produces an amorphous precipitate.
The absorption spectra of the solutions were recorded with a Cary-5000 spectrometer and the nanocrystals were investigated with a Philips CM200 transmission electron microscope (TEM), operating at 200kV, also employed for electron diffraction measurements.Sample preparation for TEM analyses consisted of drop coating a diluted solution (60x with propylamine) over copper grids with a carbon film support.Size distribution was investigated by dynamic light scattering (DLS) measurements using the upper portion of As 2 S 3 /AgCl solution.The reaction residue was investigated with a Bruker-D8 x-ray diffractometer, from 30 to 60 ° (2θ), with steps of 0.02 °, using Cu K α1 radiation.In order to avoid contamination with oxygen, the whole synthesis and solution processing were carried out inside a dry-box with H 2 O and O 2 levels below 1 ppm.Thin films of As 2 S 3 and As 2 S 3 :Ag 2 S NCs were also prepared in a dry-box from their respective solutions by spin-coating.The upper portion of As 2 S 3 /AgCl solution was spun at 2000 rpm for 10 -20s, on glass or silicon substrates.For solvent removal, the thin films were vacuum baked at 60 °C for 1h and then post-baked at 110 °C for 7h.After such annealing, no amine group from the solvent is expected to remain in the film structure [23][24][25], while pore formation is avoided, once the onset temperature for pore formation has been reported at ~120 °C [26].Raman spectra were acquired with a LabRAM -Horiba equipment, using a 50 × objective lens, 20s of integration time and excitation at 532 nm from Ar laser.Film thickness is estimated by ellipsometry measurements (M-2000 Woollam) to be approximately 500 nm.

Results and discussion
Figure 1 shows the absorption spectra of As 2 S 3 and AgCl dissolved in propylamine individually, and the mixture of both solutions, named As 2 S 3 :AgCl.As 2 S 3 solution has a sharp absorption edge at 510 nm, resulting in the typical yellowish color of As 2 S 3 compounds, while silver chloride solution is transparent throughout the entire visible spectrum.Absorption bands at 915, 1044 and 1200 nm are due to the organic solvent.The resulting solution from the mixture (As 2 S 3 :AgCl) presents a wide absorption band covering the region 600 -1000 nm and an absorption edge at 555 nm, conferring a brownish color to the solution.Such features are indicative of the chemical reaction which occurred between the species in solution.The specific wavelength of this absorption suggests the formation of Ag 2 S in solution as indirect transitions have been reported in this spectral range [7].However in order to check for the formation of nanocrystals, TEM images are obtained from the diluted solution, as shown in Fig. 2(a) along with electron diffraction measurement.As it can be seen, the chemical reaction produces spherical nanoparticles, uniformly dispersed, with an estimated diameter of 8 nm (obtained using DLS measurements).The diffraction pattern confirms the formation of monoclinic silver sulfide (α-Ag 2 S), in agreement to ICDD card #00-014-0072, also represented in Fig. 2   Besides the formation of Ag 2 S NCs in suspension, a dark precipitate was also observed in the bottom of the reaction vial.XRD and EDS measurements of this precipitate reveled an amorphous phase containing Ag (~4 at.%),As (~38 at.%) and S (~58 at.%).This result suggests that the precipitate is predominantly amorphous As 2 S 3 , because the As:S ratio (0.66) is equivalent to the stoichiometric compound.In order to avoid As 2 S 3 precipitation and investigate the nature of the silver portion in the precipitate, the chemical synthesis was performed using a hundred-fold diluted solution of As 2 S 3 .The XRD pattern of the resulting precipitate is displayed in Fig. 3, in which unreacted precursor AgCl and monoclinic Ag 2 S were identified.This confirms the formation and precipitation of silver sulfide crystals.
The stability of As 2 S 3 :AgCl solution was evaluated over time by its absorption spectrum.The variation of the absorption edge (Δλ cutoff ) is displayed in the inset of Fig. 1, where negative values indicate changes towards smaller wavelengths over the time.A blue shift of 45 nm in the absorption edge was observed during the first 3h after preparation.For longer periods no significant change was detected, and the solution kept stable for at least 20 days.The blue shift is related to the precipitation process, in which large particles precipitate leaving smaller particles in suspension and a corresponding increase in the apparent bandgap energy due to the quantum size effect [7].A rough estimative, based on the mass of silver in the precipitated (~2.2 mg), the average diameter of the NP (8 nm) and the density of Ag 2 S monoclinic crystals (7.2 g/cm 3 ), suggest that the amount of nanoparticles in suspension is around 10 16 particles/ml.
The formation of Ag 2 S NCs can be explained based on the sulfidation of silver ions in solution.It is known that the dissolution process of As 2 S 3 results in arsenic sulfide clusters terminated by excess sulfide dangling bonds [22].Thus, sulfur anions spontaneously react with silver ions that originated from AgCl dissociation, producing nanocrystals of silver sulfide through the reaction 2Ag + + S 2-→ Ag 2 S (ΔH = −2199.5kJ/mol) [27].The sulfidation of Ag 0 nanoparticles using H 2 S exposure is a known method to obtain Ag 2 S NCs in several systems [28,29].However, the presence of sulfur atoms in the chalcogenide solution enables the formation of Ag 2 S NCs without any gas exposure, enabling a single-step synthesis.It is important to note that no additional source of energy (as temperature or irradiation) is necessary to promote the chemical reaction, configuring a simple and fast way to prepare in situ Ag 2 S NCs.In addition, this approach can be exploited for the production of other semiconductor sulfide NCs in ChGs to create novel materials for Mid-IR photonics [30,31].To investigate the structure and physical-chemistry properties of solid-state samples, thin films are prepared from As 2 S 3 :AgCl solution (containing Ag 2 S NCs), and also from the As 2 S 3 solution, for comparison purposes.EDS measurements showed that the films are composed of 63 at.% of S and 37 at.% of As.Thus the As:S ratio is 0.59, indicating an arsenic deficiency when compared to initial As 2 S 3 compound (0.67).Such deficiency has been reported for spincoated chalcogenide glass, and it is related to the As 2 S 3 dissolution, which leads to the formation of As 2 S x clusters terminated with excess of negatively charged S ions [23,32].This feature is preserved in the solid phase, resulting in thin films with excess of sulfur atoms.The composition of As 2 S 3 /Ag 2 S NCs films is 3.2 at.% of Ag, 60.7 at.% of S and 36.1 at.% of As.Considering all Ag atoms form Ag 2 S NCs, the doping amount is half of silver content (1.6 at.% of Ag 2 S NCs) and the remaining S atoms (59.1 at.%) along with As provide a matrix with As:S ratio of 0.61.
Raman spectra of As 2 S 3 and As 2 S 3 :NCs films are displayed in Fig. 4. The broad bands indicate the amorphous structure of the films, and are mainly associated with As 2 S 3 and As 4 S 4 structural units, according to the vibrational energy presented in Table 1 [33][34][35].As shown in Fig. 4, the presence Ag 2 S NCs causes minor alterations to the As 2 S 3 structure, indicated by a decreasing shoulder at 297 cm −1 and the vanishing band at 414 cm −1 , while peaks at 225 and 330 cm −1 get stronger.Thus, based on the assignments presented in Table 1, we believe that the addition of Ag 2 S NPs causes a transformation of As 2 S 3 into As 4 S 4 basic units, in agreement with the increase in As content in As 2 S 3 :NCs films, seen in the EDS data.The As:S ratio is 0.59 for the undoped film, increasing to 0.61 for the films containing Ag 2 S NCs.In fact, Iovu et al. described the dissociation 2As 2 S 3 →As 4 S 4 + S 2 due to rare earth and Mn doping of arsenic sulfide [33].The preparation of arsenic sulfide films containing nanocrystals of silver sulfide reported herein presents a promising metamaterial for infrared technologies, in which photoactive phenomena associated with semiconductor nanocrystals can be exploited to improve the overall material performance [36,37].

Conclusion
We have used a wet chemistry approach to produce silver sulfide nanoparticles in chalcogenide solution.The chemical synthesis consists of independently dissolving As 2 S 3 and AgCl in propylamine, and mixing both solutions using the ratio As 2 S 3 /AgCl = 1:0.25ml.Such a method results in the spontaneous formation of Ag 2 S nanocrystals, where the sulfur ions are provided by the As 2 S 3 in solution.The monoclinic structure of Ag 2 S NCs is confirmed through TEM and XRD analyses.By spin-coating the resulting solution, we are able to produce ~500 nm thick arsenic sulfide films, doped with 1.6 (at.%)Ag 2 S. The glass network of these films differs from that of an undoped film due to a decrease of As 2 S 3 units in favor of As clusters and As 4 S 4 units.
Figure1shows the absorption spectra of As 2 S 3 and AgCl dissolved in propylamine individually, and the mixture of both solutions, named As 2 S 3 :AgCl.As 2 S 3 solution has a sharp absorption edge at 510 nm, resulting in the typical yellowish color of As 2 S 3 compounds, while silver chloride solution is transparent throughout the entire visible spectrum.Absorption bands at 915, 1044 and 1200 nm are due to the organic solvent.The resulting solution from the mixture (As 2 S 3 :AgCl) presents a wide absorption band covering the region 600 -1000 nm and an absorption edge at 555 nm, conferring a brownish color to the solution.Such features are indicative of the chemical reaction which occurred between the species in solution.The specific wavelength of this absorption suggests the formation of Ag 2 S in solution as indirect transitions have been reported in this spectral range[7].However in order to check for the formation of nanocrystals, TEM images are obtained from the diluted solution, as shown in Fig.2(a) along with electron diffraction measurement.As it can be seen, the chemical reaction produces spherical nanoparticles, uniformly dispersed, with an estimated diameter of 8 nm (obtained using DLS measurements).The diffraction pattern confirms the formation of monoclinic silver sulfide (α-Ag 2 S), in agreement to ICDD card #00-014-0072, also represented in Fig.2(a).A representative high-resolution image (HRTEM) is depicted in Fig. 2(b), in which the interplanar distances corresponding to (120), (103) and (031) planes of Ag 2 S NPs are seen.

Fig. 1 .
Fig. 1.Absorption spectra of As 2 S 3 and AgCl dissolved in propylamine, and the resulting solution after mixing As 2 S 3 /AgCl in a ratio of 1/0.25 ml.The inset shows the variation of the absorption edge (Δλ cutoff ) over the time of As 2 S 3 :AgCl solution.

Fig. 2 .
Fig. 2. (a) TEM image of the NCs disperse in As 2 S 3 :AgCl solution and its electron diffraction pattern in which seven crystallographic planes corresponding to monoclinic Ag 2 S were identified.(b) HRTEM of a single particle, with diameter of 12 nm, where the interplanar distances match to (120), ( 103) and (031) planes of Ag 2 S.

Fig. 3 .
Fig. 3. XRD pattern of the precipitate formed by mixing the solutions of As 2 S 3 hundredfold diluted and AgCl (regular concentration) in propylamine.Monoclinic Ag 2 S and cubic AgCl were identified using ICDD.

Fig. 4 .
Fig. 4. Raman shift of As 2 S 3 and As 2 S 3 :NCs thin films, in which the amorphous structure was lightly affected by the presence of Ag 2 S NCs.