Engineering Band Gap of Ternary Ag 2 Te x S 1 − x Quantum Dots for Solution-Processed Near-Infrared Photodetectors

: Silver-based chalcogenide semiconductors exhibit low toxicity and near-infrared optical properties and are therefore extensively employed in the field of solar cells, photodetectors


Introduction
Near-infrared (NIR) light has several advantages, including great penetration depth, strong confidentiality, and excellent anti-electromagnetic interference properties [1][2][3][4].Thus, NIR materials have found extensive applications across diverse domains such as bioimaging, optical communication, photodetectors, and solar cells [5][6][7][8][9][10][11]. Currently, NIR materials are mainly concentrated on PbS, PbSe, HgTe QDs, rare-earth nanocrystals, and single-walled carbon nanotubes (SWNTs) [12][13][14][15][16].Among them, PbS, PbSe, and HgTe QDs contain the toxic heavy metals lead and mercury and rare-earth nanocrystals with non-tunable band gaps and narrow absorption windows.For SWNTs, their application is limited by their broad length distribution, spanning hundreds of nanometers.Therefore, it becomes imperative to foster the creation of novel materials with continuously tunable band gap and high biocompatibility.Silver-based chalcogenide (Ag 2 S, Ag 2 Se, and Ag 2 Te) QDs, as classic NIR materials, possess excellent properties such as low toxicity, wide absorption windows, and good biocompatibility [17][18][19].Among these, Ag 2 S QDs are one of the most extensively studied semiconductor materials and have ultralow solubility product constants (Ksp(Ag 2 S) = 6.69 × 10 −50 ), which enable the minimal release of Ag ions in biosystems, thus ensuring their potential application in biomedical, ReRAMs, and optoelectronic devices [20][21][22].However, due to the large band gap of Ag 2 S (1.1 eV), its photoluminescence (PL) emission peak is limited to less than 1200 nm, and photodetectors based on this material cannot respond to light in longer wavelength bands and have poor power conversion efficiencies [23].Therefore, expanding the band gap of Ag 2 S and improving its photoelectric performance is very important for the application of Ag 2 S materials in the field of optoelectronic devices.
Compared to traditional binary QDs, ternary alloy QDs not only retain the quantum size effect of nanomaterials but also enable effective control of the band gap by adjusting the composition [24,25].Smith et al. [26] studied the influence of different reactive anion precursors on the size and composition of PbSe x Te 1−x , PbS x Te 1−x , and PbS x Se 1−x QDs.The results demonstrated that the highly reactive Chalcogenide precursors bis(trimethylsilyl) (TMS 2 ) help to achieve uniform anion incorporation.In addition, it was possible to modulate the excitonic absorption and fluorescence peak of alloy QDs by adjusting the anions ratios.The next generation of electroluminescent displays based on quantum dots requires the development of efficient and stable Cd-free blue emission devices, which remains a challenge due to the poor photophysical properties of blue emission materials.Jang et al. [27] proposed a method by which to synthesize efficient blue-emitting ZnTeSe QDs.They found that the hydrofluoric acid and zinc chloride additives effectively improved the luminescence efficiency by eliminating the layer faults in the ZnSe crystal structure.Moreover, chloride passivation via liquid or solid ligand exchange results in slow radiative recombination, high thermal stability, and efficient charge transport characteristics, and the fluorescence peak was adjusted to 457 nm, while the photoluminescence quantum yield (PLQY) was elevated to a remarkable 100% by controlling the Te doping level.In addition, Ren et al. [28] proposed a water-phase synthesis method for the preparation of NIR CdHgTe alloy quantum dots.CdHgTe QDs are obtained by heating a mixture of CdCl 2 , Hg(ClO 4 ) 2 and NaHTe in the presence of a thiol stabilizer and exhibit PL emission peaks in the range from 600 to 830 nm, which can be adjusted according to size and composition.The PLQY of CdHgTe QDs is about 20-50%, which depends on its emission wavelength and composition.Compared to other reported NIR quantum dots (such as CdTe/CdHgTe and InAs), the prepared CdHgTe alloy quantum dots have a much narrower emission spectrum, with a full width at half-maximum (FWHM) of only 60-80 nm.HRTEM and XRD characterization show that CdHgTe QDs have a good crystal structure and monodispersity.In order to improve the photostability of CdHgTe QDs and reduce their cytotoxicity, CdS nanocrystal shells were added to the surface of CdHgTe QDs.Kim et al. [29] investigated the synthesis of eco-friendly materials AgBiS 2 QDs and the effect of heat treatment on their properties.Increasing the heat treatment temperature reduces the number of surface functional groups, including N (amine) and S (thiol) groups, and there are fewer defects on the particle surface.However, heat treatment at 300 • C reduces PL intensity even when the ligands are fully removed.By measuring the photocurrent response of the AgBiS 2 photodetector to near-infrared light, the photocurrent of the AgBiS 2 photodetector is the highest after heat treatment at 200 • C. Heat treatment removes excessive protective agents and ligands in the inks and improves the photocurrent response of AgBiS 2 .
Herein, for the first time, we develop a facile mixture precursor hot-injection method for the synthesis of Ag 2 Te x S 1−x alloy QDs.The composition of the alloy QDs was regulated by adjusting the ratio of precursor S to precursor Te.Through augmenting the amount of the precursor Te, the fluorescence peak of the alloy QDs redshifted and the absorption spectrum broadened, thereby achieving controllable tuning of the band gap.Furthermore, a photodetector based on Ag 2 Te 0.53 S 0.47 QDs as the photosensitive layer has been constructed.The narrow band gap photosensitive layer enables the device to exhibit photoresponse to 1350 nm illumination.The device maintained a stable response and exhibited excellent optoelectronic performance under multiple light switching cycles.This is also the first report on the preparation of Ag 2 Te x S 1−x QDs and the investigation of their optoelectronic properties.

Preparation of TBP-Te and TBP-S Precursors
The precursors of TBP-Te and TBP-S were prepared in a glove box.TBP-Te: 2.5 mmol (0.32 g) Te powder was dissolved in 5 mL TBP, employing the use of ultrasonic treatment for a duration of 24 h.TBP-S: 2.5 mmol (0.08 g) S powder was dissolved in 5 mL TBP with vigorous magnetic stirring and ultrasonic treatment for 1 h.The total number of moles of Te and S was kept at 0.10 mmol and the two precursors with different atomic ratios.

Synthesis of Ag 2 Te x S 1−x QDs
In the conventional hot-injection method, a blend comprising AgAc (0.067 g, 0.4 mmol) and Oam (16 mL) was introduced into a 100 mL three-neck flask at room temperature.Subsequently, the solution underwent a remove oxygen process through vigorous magnetic stirring while maintaining a vacuum for 30 min.Afterwards, the temperature of the reaction was subsequently elevated to 120 • C in the presence of a nitrogen atmosphere, resulting in the attainment of a transparent solution.Then, 0.2 mL mixture precursor (TBP-S and TBP-Te) was injected into the solution while vigorously stirring at a speed of 1000 rpm.This temperature was maintained for a duration of 5 min to ensure the consistent growth of the QDs.The products were precipitated with ethanol and recovered by centrifugation at a speed of 12,000 rpm for 10 min.The products were dispersed in chloroform (15 mg/mL).A series of Ag 2 Te x S 1−x samples were used, referred to as S/Te-A (S:Te = 1:0), S/Te-B (S:Te = 2:1), S/Te-C (S:Te = 1:1), and S/Te-D (S:Te = 0:1).

Photodetector Fabrication
The photodetector device fabrication and ligand exchange protocol is based on previous research [30].At first, this process begins with cleaning the glass substrates.Then the Ag 2 Te x S 1−x QDs solution was deposited onto the glass substrate by employing the spin-coating technique under a speed of 1500 rpm and kept for 30 s. Subsequently, a volume of 500 µL of ligand solution (consisting of 5 mM EDT dissolved in acetonitrile) was gently applied onto the film for a duration of 30 s, followed by removal of the ligand solution using a spinning method.Afterwards, the QD film was immersed in acetonitrile 3 times to eliminate any remaining unbound ligands.The ligand exchange and spin-coating were iterated 6 times.Finally, 5 nm/100 nm-thick Ti/Au metal electrodes were deposited through an interdigitated shadow mask (the channel width is 50 µm and the size is 18 mm × 18 mm) through a thermal evaporator.

Characterizations
Talos F200X (ThermoFisher, Waltham, MA, USA) was used to test transmission electron microscope (TEM) and high-resolution TEM (HRTEM) images of Ag 2 Te x S 1−x QDs under an acceleration voltage of 200 kV.X-ray diffraction (XRD) patterns were measured by a PANalytical Empyrean diffractometer (Malven PANalytical, Almelo, Holland) (Cu-Kα λ = 1.54056Å) in the range of 20-80 • .An inductively coupled plasma optical emission spectrometer (ICP-OES) was performed on PerkinElmer Avio 500 (PerkinElmer, Waltham, MA, USA).The absorption spectra were recorded with a PerkinElmer Lambda 750 spectrometer (PerkinElmer, Waltham, MA, USA).The NIR fluorescence spectra were executed on an Applied NanoFluorescence Spectrometer (Houston, TX, USA), applying an excitation laser source of 785 nm.The X-ray photoelectron spectroscopy (XPS) were collected on a PHI-5000 VersaProbe instrument (ULVAC-PHI, Chigasaki, Japan) using Al/Kα radiation as excitation source, the electron emission angle is 34 • , the diameter of the X-ray spot is 250 µm, the base pressure is 4.17 × 10 −7 mBar during the analyses, the samples are etched for 60 s with 1 keV Ar + ions before the XPS scan to remove the surface layer which adsorb water and oxygen in the air.All the XPS spectra recorded for Ag 3d, Te 3d, and S 2p were referenced to the C 1s peak, which was calibrated by positioning it to 289.58 eV−Φ.The 289.58 eV is the binding energy position of the C 1s peak with respect to the vacuum level, and Φ is the work function of Ag 2 Te 0.53 S 0.47 (~5.16 eV) [31,32].Thus, the set value of the extraneous polluted carbon is 284.42 eV.The ultraviolet photoelectron spectroscopy (UPS) was measured by using a Thermo ESCALAB 250Xi instrument (Ther-moFisher, Waltham, MA, USA) with He I radiation.The Keithley 4200-SCS semiconductor analyzer equipped with a Signatone S-1160 Probe Station (Signatone, Gilroy, CA, USA) is employed to characterize the optoelectronic performance of devices.

Results and Discussion
Figure 1 displays the low-magnification TEM and HRTEM images of Ag 2 Te x S 1−x QDs, which exhibit excellent dispersibility, clear lattice patterns, and good crystallinity.Figure 1a shows TEM image of Ag 2 S QDs (denoted as S/Te-A), which appear as uniformly spherical particles.With the increase in the Te content, the morphology of the QDs gradually transformed from uniform spherical morphology to a branch-like structure, as shown in Figure 1b,c (denoted as S/Te-B and S/Te-C, respectively).When all the S elements are replaced by Te, the morphology changes to a branch-like structure composed of interconnected spherical particles (S/Te-D), as depicted in Figure 1d.
MA, USA).The absorption spectra were recorded with a PerkinElmer Lambda 750 spectrometer (PerkinElmer ,Waltham, MA, USA).The NIR fluorescence spectra were executed on an Applied NanoFluorescence Spectrometer (Houston, TX, USA), applying an excitation laser source of 785 nm.The X-ray photoelectron spectroscopy (XPS) were collected on a PHI-5000 VersaProbe instrument (ULVAC-PHI, Chigasaki, Japan) using Al/Kα radiation as excitation source, the electron emission angle is 34°, the diameter of the X-ray spot is 250 µm, the base pressure is 4.17 × 10 −7 mBar during the analyses, the samples are etched for 60 s with 1 keV Ar + ions before the XPS scan to remove the surface layer which adsorb water and oxygen in the air.All the XPS spectra recorded for Ag 3d, Te 3d, and S 2p were referenced to the C 1s peak, which was calibrated by positioning it to 289.58 eV−Φ.The 289.58 eV is the binding energy position of the C 1s peak with respect to the vacuum level, and Φ is the work function of Ag2Te0.53S0.47 (~5.16 eV) [31,32].Thus, the set value of the extraneous polluted carbon is 284.42 eV.The ultraviolet photoelectron spectroscopy (UPS) was measured by using a Thermo ESCALAB 250Xi instrument (ThermoFisher, Waltham, MA, USA) with He I radiation.The Keithley 4200-SCS semiconductor analyzer equipped with a Signatone S-1160 Probe Station (Signatone, Gilroy, CA, USA) is employed to characterize the optoelectronic performance of devices.

Results and Discussion
Figure 1 displays the low-magnification TEM and HRTEM images of Ag2TexS1−x QDs, which exhibit excellent dispersibility, clear lattice patterns, and good crystallinity.Figure 1a shows TEM image of Ag2S QDs (denoted as S/Te-A), which appear as uniformly spherical particles.With the increase in the Te content, the morphology of the QDs gradually transformed from uniform spherical morphology to a branch-like structure, as shown in Figure 1b,c (denoted as S/Te-B and S/Te-C, respectively).When all the S elements are replaced by Te, the morphology changes to a branch-like structure composed of interconnected spherical particles (S/Te-D), as depicted in Figure 1d.Similar results have been reported in other studies on Ag2Te QDs, which elucidate that this morphology is mainly attributed to the driving force generated by strong dipoledipole interactions, leading to the formation of a branch-like structure [33].Additionally, we performed a statistical analysis of the size distributions of the different Ag2TexS1−x QDs (100 particles counted in each image, Figure S1).The diameters of S/Te-A, S/Te-B, S/Te-C, Similar results have been reported in other studies on Ag 2 Te QDs, which elucidate that this morphology is mainly attributed to the driving force generated by strong dipoledipole interactions, leading to the formation of a branch-like structure [33].Additionally, we performed a statistical analysis of the size distributions of the different Ag 2 Te x S 1−x QDs (100 particles counted in each image, Figure S1).The diameters of S/Te-A, S/Te-B, S/Te-C, and S/Te-D were measured as 3.92 ± 0.19 nm, 5.45 ± 0.31 nm, 5.32 ± 0.51 nm, and 5.23 ± 0.32 nm, respectively.
To investigate the crystal structure of the Ag 2 Te x S 1−x QDs, we measured the XRD patterns of the samples with different S/Te ratios.Figure 2 shows the S/Te-A sample consisting of Ag 2 S QDs, with diffraction peaks matching those of Ag 2 S crystals (JCPDS:14-0072) and exhibiting a relatively broad peak width, which is attributed to the small size of the QDs.
The characteristic diffraction peaks of Ag 2 S in the Ag 2 Te x S 1−x QDs gradually weaken with increasing Te content, indicating a phase transition from monoclinic Ag 2 S crystals (JCPDS:14-0072) to monoclinic Ag 2 Te crystals (JCPDS:81-1820).Finally, the characteristic diffraction peaks of the S/Te-D sample coincided with those of pure Ag 2 Te crystals (JCPDS:81-1820), exhibiting broadened diffraction peaks.ICP-OES was used to analyze the actual atomic ratios of the S and Te anions in the samples.Figure S2 illustrates the relationship between Te/(Te + S) for the precursors and the products of the four samples.When the molar ratio of the Te source to the S source is 1:2, the Te-to-S ratio in the product is 0.54:1 (Ag 2 Te 0.35 S 0.65 ).When the molar ratio of the Te source to the S source turns to 1:1, the Te-to-S ratio in the product becomes 1.13:1 (Ag 2 Te 0.53 S 0.47 ) owing to the slightly higher reactivity of the Te source compared to that of the S source.These results demonstrate that the manipulation of the anion ratio in the precursor mixture is an effective means of controlling the anion content within the alloy QDs.
To investigate the crystal structure of the Ag2TexS1−x QDs, we measured the XRD patterns of the samples with different S/Te ratios.Figure 2 shows the S/Te-A sample consisting of Ag2S QDs, with diffraction peaks matching those of Ag2S crystals (JCPDS:14-0072) and exhibiting a relatively broad peak width, which is attributed to the small size of the QDs.The characteristic diffraction peaks of Ag2S in the Ag2TexS1−x QDs gradually weaken with increasing Te content, indicating a phase transition from monoclinic Ag2S crystals (JCPDS:14-0072) to monoclinic Ag2Te crystals (JCPDS:81-1820).Finally, the characteristic diffraction peaks of the S/Te-D sample coincided with those of pure Ag2Te crystals (JCPDS:81-1820), exhibiting broadened diffraction peaks.ICP-OES was used to analyze the actual atomic ratios of the S and Te anions in the samples.Figure S2 illustrates the relationship between Te/(Te + S) for the precursors and the products of the four samples.When the molar ratio of the Te source to the S source is 1:2, the Te-to-S ratio in the product is 0.54:1 (Ag2Te0.35S0.65).When the molar ratio of the Te source to the S source turns to 1:1, the Te-to-S ratio in the product becomes 1.13:1 (Ag2Te0.53S0.47)owing to the slightly higher reactivity of the Te source compared to that of the S source.These results demonstrate that the manipulation of the anion ratio in the precursor mixture is an effective means of controlling the anion content within the alloy QDs.XPS was used to characterize the elements and their valence states in the alloyed QDs. Figure 3a shows the low-resolution scan spectrum of the Ag2Te0.53S0.47QDs, which demonstrates the sample contains Ag, Te, O, S, and C elements.Figure 3b-d show the high-resolution scan spectra of Ag 3d, Te 3d, and S 2p.As illustrated in Figure 3b, the high-resolution Ag 3d region spectra of Ag2Te0.53S0.47QDs show two symmetric peaks, Ag 3d5/2 and Ag 3d3/2, which correspond to binding energies of 367.7 eV and 373.7 eV, respectively.This confirms that the oxidation state of silver is univalent (Ag + ) [34].In Figure 3c, the highresolution Te 3d region spectra show two peaks, Te 3d5/2 and Te 3d3/2, which correspond to binding energies of 572.0 eV and 582.4 eV, respectively, suggesting that the valence of telluride is negative bivalence (Te 2− ) [35,36].Furthermore, the spectrum of the S 2p region was characterized and analysed (Figure 3d).The spectrum contains multiple peaks, which must be separated.Fitting the low-binding-energy peaks yields two distinct peaks with binding energies measuring 162.6 eV and 161.3 eV.These peaks can be attributed to the S 2p1/2 and S 2p3/2, respectively, in agreement with the Ag-S binding energies [37].An additional peak appears in the high-energy region (approximately 168.3 eV), indicating that a XPS was used to characterize the elements and their valence states in the alloyed QDs. Figure 3a shows the low-resolution scan spectrum of the Ag 2 Te 0.53 S 0.47 QDs, which demonstrates the sample contains Ag, Te, O, S, and C elements.Figure 3b-d show the high-resolution scan spectra of Ag 3d, Te 3d, and S 2p.As illustrated in Figure 3b, the high-resolution Ag 3d region spectra of Ag 2 Te 0.53 S 0.47 QDs show two symmetric peaks, Ag 3d 5/2 and Ag 3d 3/2 , which correspond to binding energies of 367.7 eV and 373.7 eV, respectively.This confirms that the oxidation state of silver is univalent (Ag + ) [34].In Figure 3c, the high-resolution Te 3d region spectra show two peaks, Te 3d 5/2 and Te 3d 3/2 , which correspond to binding energies of 572.0 eV and 582.4 eV, respectively, suggesting that the valence of telluride is negative bivalence (Te 2− ) [35,36].Furthermore, the spectrum of the S 2p region was characterized and analysed (Figure 3d).The spectrum contains multiple peaks, which must be separated.Fitting the low-binding-energy peaks yields two distinct peaks with binding energies measuring 162.6 eV and 161.3 eV.These peaks can be attributed to the S 2p 1/2 and S 2p 3/2 , respectively, in agreement with the Ag-S binding energies [37].An additional peak appears in the high-energy region (approximately 168.3 eV), indicating that a small fraction of S may have undergone oxidation to form sulfate ions upon exposure to air [38].
Figure 4 shows the PL emission spectra of Ag 2 Te x S 1−x QDs with four different S/Te ratios under 785 nm excitation.The dashed lines represent the spectral regions that could not be detected because of the response range of the spectrometer (900 nm-1700 nm).The fluorescence peak of the Ag 2 Te x S 1−x QDs exhibits a gradual redshift, transitioning from 955 nm to 1255 nm, 1470 nm, and ultimately reaching 1605 nm.When the size of the direct band gap semiconductor decreases, the PL emission should blue-shift.However, the size of Ag 2 Te x S 1−x QDs decreases, and the PL peak redshift contradicts the above theory (S/Te-B to S/Te-D).This anomalous phenomenon can be explained by Vegard's law, which describes the relationship between the band gap of ternary alloy materials and their elemental composition of elements [39].The band gap of Ag 2 Te x S 1−x alloy can be described as follows: where x is the proportion of component t; and E alloy , E t , and E s are the band gap energies of the alloy material, pure t, and pure s, respectively.Because the band gap of Ag 2 Te (0.06 eV) [40] is much smaller than that of Ag 2 S (1.1 eV) [41], as the Te component increases, the value of x increases, leading to a decrease in the band gap of the alloy QDs and a redshift in the PL emission peaks.
Inorganics 2023, 11, x FOR PEER REVIEW 6 of 13 small fraction of S may have undergone oxidation to form sulfate ions upon exposure to air [38].Figure 4 shows the PL emission spectra of Ag2TexS1−x QDs with four different S/Te ratios under 785 nm excitation.The dashed lines represent the spectral regions that could not be detected because of the response range of the spectrometer (900 nm-1700 nm).The fluorescence peak of the Ag2TexS1−x QDs exhibits a gradual redshift, transitioning from 955 nm to 1255 nm, 1470 nm, and ultimately reaching 1605 nm.When the size of the direct band gap semiconductor decreases, the PL emission should blue-shift.However, the size of Ag2TexS1−x QDs decreases, and the PL peak redshift contradicts the above theory (S/Te-B to S/Te-D).This anomalous phenomenon can be explained by Vegard's law, which describes the relationship between the band gap of ternary alloy materials and their elemental composition of elements [39].The band gap of Ag2TexS1−x alloy can be described as follows: (1 ) where x is the proportion of component t; and Ealloy, Et, and Es are the band gap energies of the alloy material, pure t, and pure s, respectively.Because the band gap of Ag2Te (0.06 eV) [40] is much smaller than that of Ag2S (1.1 eV) [41], as the Te component increases, the value of x increases, leading to a decrease in the band gap of the alloy QDs and a redshift in the PL emission peaks.
where α is the absorption coefficient; hν is the incident photon energy; A is a proportionality constant; and E g is the optical band gap, respectively.The band gap was calculated to be 1.45 eV, 1.07 eV, 0.89 eV, and 0.84 eV for S/Te-A, S/Te-B, S/Te-C, and S/Te-D, respectively.As the Te content increased, the absorption wavelength ranges of the alloy QDs broadened, and the band gap decreased.This result is consistent with the alterations observed in the PL emission spectra of Ag 2 Te x S 1−x QDs.Furthermore, the result reveals that the adjustment of Te content effectively controls the band gap of Ag 2 Te x S 1−x QDs. Figure 5 shows the absorption spectra of Ag2TexS1−x QDs with four different S/Te ratios.The S/Te-A to S/Te-D samples do not exhibit distinct excitonic absorption peaks, which is similar to previous reports of Ag2S QDs and Ag2Te QDs [42,43].The optical band gap values of Ag2TexS1−x QDs are calculated by the extrapolated energy intercept of the Tauc plot.For direct bandgap materials, the Tauc equation can be described as follows [44]: where α is the absorption coefficient; hν is the incident photon energy; A is a proportionality constant; and Eg is the optical band gap, respectively.The band gap was calculated to be 1.45 eV, 1.07 eV, 0.89 eV, and 0.84 eV for S/Te-A, S/Te-B, S/Te-C, and S/Te-D, respectively.As the Te content increased, the absorption wavelength ranges of the alloy QDs broadened, and the band gap decreased.This result is consistent with the alterations observed in the PL emission spectra of Ag2TexS1−x QDs.Furthermore, the result reveals that the adjustment of Te content effectively controls the band gap of Ag2TexS1−x QDs.UPS was used to measure the kinetic energy of the Ag2Te0.53S0.47QDs, as shown in Figure 6.The test sample was prepared by spin-coating it into a thin film on a glass substrate.Figure 6a shows the survey of UPS spectrum of the Ag2Te0.53S0.47QDs. Figure 6b portrays the region of high binding energy cutoff.The tangent value in the cutoff region is approximately 16.6 eV, indicating that the Fermi level positioning of the alloy QDs is 4.61 eV.Combined with the low binding energy cutoff region shown in Figure 6c (with a tangent value of approximately 0.54 eV), the energy level of the valence band maximum (EVB) was calculated to be 5.15 eV (relative to the vacuum level EVac).Finally, combined with the band gap of Ag2Te0.53S0.47QDs (Eg = 0.89 eV), the energy level of the conduction band minimum (ECB) can be calculated to be 4.26 eV.Therefore, the accurate depiction of UPS was used to measure the kinetic energy of the Ag 2 Te 0.53 S 0.47 QDs, as shown in Figure 6.The test sample was prepared by spin-coating it into a thin film on a glass substrate.Figure 6a shows the survey of UPS spectrum of the Ag 2 Te 0.53 S 0.47 QDs. Figure 6b portrays the region of high binding energy cutoff.The tangent value in the cutoff region is approximately 16.6 eV, indicating that the Fermi level positioning of the alloy QDs is 4.61 eV.Combined with the low binding energy cutoff region shown in Figure 6c (with a tangent value of approximately 0.54 eV), the energy level of the valence band maximum (E VB ) was calculated to be 5.15 eV (relative to the vacuum level E Vac ).Finally, combined with the band gap of Ag 2 Te 0.53 S 0.47 QDs (E g = 0.89 eV), the energy level of the conduction band minimum (E CB ) can be calculated to be 4.26 eV.Therefore, the accurate depiction of the energy band alignment for the Ag 2 Te 0.53 S 0.47 QDs is depicted in Figure S3.As a proof-of-concept application, Ag2TexS1−x QDs were used as the active layer for the photodetector (PD) device.The photoelectric properties of Ag2TexS1−x QD-based PD were investigated (Figure S4). Figure S4a,b show that neither the Ag2S QD-based or Ag2Te0.53S0.47QD-based PD have any response to 1350 nm illumination.This can be attributed to the large band gap of these two QDs, which cannot be excited by 1350 nm light to generate photocurrent.With the increase in Te component, both Ag2Te0.53S0.47QD-based and Ag2Te QD-based PD generate photocurrent under 1350 nm illumination, and the light response of Ag2Te0.53S0.47QDs is more obvious than that of Ag2Te QDs.Thus, composition adjustment can not only expand the band gap of QDs, but also help to improve the performance of photodetectors.
In order to evaluate the performance of Ag2TexS1−x QD-based PDs, the corresponding figures of merit were calculated The two representative parameters (responsivity R and special detectivity D*) were determined as follows [45]: where Iph is the photocurrent; Id is the dark current; P is the radiated power (product of active area and incident light density); A is the active area of the photodetector; and q is the electron charge.At an applied voltage of 0.5 V and an illumination intensity of 2 mW/cm 2 , the D * of the Ag2Te0.53S0.47QD-based PD reaches 0.91 × 10 10 Jones, and the R is 0.48 mA/W.In addition, the response speed of the Ag2Te0.53S0.47QD-based PD was determined using the rise time (τr) and fall time (τf).Figure 7 shows the response speed curve of the detector at a bias voltage of 0.5 V and an illumination intensity of 2 mW/cm 2 .Figure 7a shows the detector still exhibits good repeatability under continuous 10-cycle switching.The fluctuations observed in the light and dark states may be caused by surface defects generated from ligand exchange processes.Figure 7b demonstrates that τr was 2.11 s and τf was 1.04 s within one switching cycle.As a proof-of-concept application, Ag 2 Te x S 1−x QDs were used as the active layer for the photodetector (PD) device.The photoelectric properties of Ag 2 Te x S 1−x QD-based PD were investigated (Figure S4). Figure S4a In order to evaluate the performance of Ag 2 Te x S 1−x QD-based PDs, the corresponding figures of merit were calculated The two representative parameters (responsivity R and special detectivity D*) were determined as follows [45]: where I ph is the photocurrent; I d is the dark current; P is the radiated power (product of active area and incident light density); A is the active area of the photodetector; and q is the electron charge.At an applied voltage of 0.5 V and an illumination intensity of 2 mW/cm 2 , the D * of the Ag 2 Te 0.53 S 0.47 QD-based PD reaches 0.91 × 10 10 Jones, and the R is 0.48 mA/W.In addition, the response speed of the Ag 2 Te 0.53 S 0.47 QD-based PD was determined using the rise time (τ r ) and fall time (τ f ). Figure 7 shows the response speed curve of the detector at a bias voltage of 0.5 V and an illumination intensity of 2 mW/cm 2 .Figure 7a shows the detector still exhibits good repeatability under continuous 10-cycle switching.The fluctuations observed in the light and dark states may be caused by surface defects generated from ligand exchange processes.Figure 7b demonstrates that τ r was 2.11 s and τ f was 1.04 s within one switching cycle.In order to elucidate the electron-hole transport mechanism in the photodetector, the schematic of energy band diagrams of Au/Ag2TexS1−x QDs/Au PD on the glass substrate is shown in Figure 8. Figure 8a shows the energy band diagram of the Ag2TexS1−x QD-based PD under dark and without external bias.The contact between the metal electrode (Au) and Ag2TexS1−x QDs exhibits a slight barrier height (ΦSBH), which proves that the contact between Au and the Ag2TexS1−x QDs is not a simple Ohmic contact but a Schottky contact.This is consistent with the results of the nonlinear I-V curve of the Ag2TexS1−x QD-based PD. Figure 8b shows the energy band diagram of the Ag2TexS1−x QD-based PD under NIR light with a wavelength of 1350 nm illumination and without external bias.The Ag2TexS1−x QDs are excited to produce charge carriers (electron-hole pairs), which are transported between the Ag2TexS1−x QDs due to the quantum tunneling effect.However, only a very small number of carriers can reach the electrode due to the influence of the Schottky barrier.Therefore, no photocurrent can be observed.In addition, under illumination and with external bias, the trap states of Ag2TexS1−x QDs (including surface defect levels and internal defect levels) serve as trapping centers for photogenerated holes [46].This results in a reduction in the depletion region width and ΦSBH, which offers favorable conditions for carriers tunneling from the Ag2TexS1−x QDs to the metal electrode under external bias, causing increased photocurrent (Figure 8c).

Conclusions
In summary, we have successfully prepared Ag2TexS1−x ternary QDs via a facile mixture precursor hot-injection method.TEM images and XRD confirmed that the morphology and lattice parameter gradually shifted from Ag2S to Ag2TexS1−x and finally to Ag2Te QDs.The ICP-OES results reveal the relationship between the compositions of the precursor and samples, which is consistent with Vegard's equation.In addition, optical characterization confirms the feasibility of Ag2TexS1−x QDs with tunable PL emission and band gap by alloying engineering.The EVB and ECB values of the Ag2Te0.53S0.47QDs were also calculated using UPS and absorption spectra, and an accurate diagram of the energy band In order to elucidate the electron-hole transport mechanism in the photodetector, the schematic of energy band diagrams of Au/Ag 2 Te x S 1−x QDs/Au PD on the glass substrate is shown in Figure 8. Figure 8a shows the energy band diagram of the Ag 2 Te x S 1−x QD-based PD under dark and without external bias.The contact between the metal electrode (Au) and Ag 2 Te x S 1−x QDs exhibits a slight barrier height (Φ SBH ), which proves that the contact between Au and the Ag 2 Te x S 1−x QDs is not a simple Ohmic contact but a Schottky contact.This is consistent with the results of the nonlinear I-V curve of the Ag 2 Te x S 1−x QD-based PD. Figure 8b shows the energy band diagram of the Ag 2 Te x S 1−x QD-based PD under NIR light with a wavelength of 1350 nm illumination and without external bias.The Ag 2 Te x S 1−x QDs are excited to produce charge carriers (electron-hole pairs), which are transported between the Ag 2 Te x S 1−x QDs due to the quantum tunneling effect.However, only a very small number of carriers can reach the electrode due to the influence of the Schottky barrier.Therefore, no photocurrent can be observed.In addition, under illumination and with external bias, the trap states of Ag 2 Te x S 1−x QDs (including surface defect levels and internal defect levels) serve as trapping centers for photogenerated holes [46].This results in a reduction in the depletion region width and Φ SBH , which offers favorable conditions for carriers tunneling from the Ag 2 Te x S 1−x QDs to the metal electrode under external bias, causing increased photocurrent (Figure 8c).In order to elucidate the electron-hole transport mechanism in the photodetector, the schematic of energy band diagrams of Au/Ag2TexS1−x QDs/Au PD on the glass substrate is shown in Figure 8. Figure 8a shows the energy band diagram of the Ag2TexS1−x QD-based PD under dark and without external bias.The contact between the metal electrode (Au) and Ag2TexS1−x QDs exhibits a slight barrier height (ΦSBH), which proves that the contact between Au and the Ag2TexS1−x QDs is not a simple Ohmic contact but a Schottky contact.This is consistent with the results of the nonlinear I-V curve of the Ag2TexS1−x QD-based PD. Figure 8b shows the energy band diagram of the Ag2TexS1−x QD-based PD under NIR light with a wavelength of 1350 nm illumination and without external bias.The Ag2TexS1−x QDs are excited to produce charge carriers (electron-hole pairs), which are transported between the Ag2TexS1−x QDs due to the quantum tunneling effect.However, only a very small number of carriers can reach the electrode due to the influence of the Schottky barrier.Therefore, no photocurrent can be observed.In addition, under illumination and with external bias, the trap states of Ag2TexS1−x QDs (including surface defect levels and internal defect levels) serve as trapping centers for photogenerated holes [46].This results in a reduction in the depletion region width and ΦSBH, which offers favorable conditions for carriers tunneling from the Ag2TexS1−x QDs to the metal electrode under external bias, causing increased photocurrent (Figure 8c).

Conclusions
In summary, we have successfully prepared Ag2TexS1−x ternary QDs via a facile mixture precursor hot-injection method.TEM images and XRD confirmed that the morphology and lattice parameter gradually shifted from Ag2S to Ag2TexS1−x and finally to Ag2Te QDs.The ICP-OES results reveal the relationship between the compositions of the precursor and samples, which is consistent with Vegard's equation.In addition, optical characterization confirms the feasibility of Ag2TexS1−x QDs with tunable PL emission and band gap by alloying engineering.The EVB and ECB values of the Ag2Te0.53S0.47QDs were also calculated using UPS and absorption spectra, and an accurate diagram of the energy band

Conclusions
In summary, we have successfully prepared Ag 2 Te x S 1−x ternary QDs via a facile mixture precursor hot-injection method.TEM images and XRD confirmed that the morphology and lattice parameter gradually shifted from Ag 2 S to Ag 2 Te x S 1−x and finally to Ag 2 Te QDs.The ICP-OES results reveal the relationship between the compositions of the precursor and samples, which is consistent with Vegard's equation.In addition, optical characterization confirms the feasibility of Ag 2 Te x S 1−x QDs with tunable PL emission and band gap by alloying engineering.The E VB and E CB values of the Ag 2 Te 0.53 S 0.47 QDs were also calculated using UPS and absorption spectra, and an accurate diagram of the energy band alignment was plotted.The Ag 2 Te x S 1−x QD-based PD was fabricated to investigate

Figure 2 .
Figure 2. XRD patterns of four different Ag 2 Te x S 1−x QDs.

Figure 3 .
Figure 3. (a) Survey XPS spectra of Ag2TexS1−x QDs.High-resolution XPS spectra for (b) Ag 3d, (c) Te 3d, and (d) S 2p.The overlapped spectra of S were fitted with the Gaussian function.

Figure 3 .
Figure 3. (a) Survey XPS spectra of Ag 2 Te x S 1−x QDs.High-resolution XPS spectra for (b) Ag 3d, (c) Te 3d, and (d) S 2p.The overlapped spectra of S were fitted with the Gaussian function.

Figure 5
Figure 5 shows the absorption spectra of Ag 2 Te x S 1−x QDs with four different S/Te ratios.The S/Te-A to S/Te-D samples do not exhibit distinct excitonic absorption peaks, which is similar to previous reports of Ag 2 S QDs and Ag 2 Te QDs [42,43].The optical band gap values of Ag 2 Te x S 1−x QDs are calculated by the extrapolated energy intercept of the Tauc plot.For direct bandgap materials, the Tauc equation can be described as follows [44]:

Figure 4 .
Figure 4. PL emission spectra of the obtained four different Ag2TexS1−x QDs under an excitation of 785 nm (the dotted lines are the real raw experimental data, and the solid lines are the corresponding Gaussian fitting data).

Figure 4 .
Figure 4. PL emission spectra of the obtained four different Ag 2 Te x S 1−x QDs under an excitation of 785 nm (the dotted lines are the real raw experimental data, and the solid lines are the corresponding Gaussian fitting data).Inorganics 2023, 11, x FOR PEER REVIEW 8 of 13

Figure 5 .
Figure 5. Absorption spectrum and Tauc plot for estimating E g (insert) of four different Ag 2 Te x S 1−x samples: (a) S/Te-A, (b) S/Te-B, (c) S/Te-C, and (d) S/Te-D.

Inorganics 2023 , 13 Figure 6 .
Figure 6.(a) Survey UPS spectrum of the Ag2Te0.53S0.47QDs.Amplified areas of (b) the high binding energy regions and (c) the low binding energy regions.

Figure 6 .
Figure 6.(a) Survey UPS spectrum of the Ag 2 Te 0.53 S 0.47 QDs.Amplified areas of (b) the high binding energy regions and (c) the low binding energy regions.
,b show that neither the Ag 2 S QD-based or Ag 2 Te 0.53 S 0.47 QD-based PD have any response to 1350 nm illumination.This can be attributed to the large band gap of these two QDs, which cannot be excited by 1350 nm light to generate photocurrent.With the increase in Te component, both Ag 2 Te 0.53 S 0.47 QD-based and Ag 2 Te QD-based PD generate photocurrent under 1350 nm illumination, and the light response of Ag 2 Te 0.53 S 0.47 QDs is more obvious than that of Ag 2 Te QDs.Thus, composition adjustment can not only expand the band gap of QDs, but also help to improve the performance of photodetectors.

Figure 7 .
Figure 7. (a) Time response of Ag2Te0.53S0.47QD−based PD at 0.5 V under 1350 nm NIR illumination with power densities of 2 mW/cm 2 .(b) I−t curve for determining the rise and fall time of Ag2Te0.53S0.47QD−based PD.

Figure 8 .
Figure 8.The schematic of energy band diagrams of Au/Ag2TexS1−x QDs/Au PD on the glass substrate: (a) under dark and without external bias; (b) under NIR light with a wavelength of 1350 nm illumination and without external bias; (c) under illumination and with external bias.

Figure 7 .
Figure 7. (a) Time response of Ag 2 Te 0.53 S 0.47 QD−based PD at 0.5 V under 1350 nm NIR illumination with power densities of 2 mW/cm 2 .(b) I−t curve for determining the rise and fall time of Ag 2 Te 0.53 S 0.47 QD−based PD.

Figure 8 .
Figure 8.The schematic of energy band diagrams of Au/Ag2TexS1−x QDs/Au PD on the glass substrate: (a) under dark and without external bias; (b) under NIR light with a wavelength of 1350 nm illumination and without external bias; (c) under illumination and with external bias.

Figure 8 .
Figure 8.The schematic of energy band diagrams of Au/Ag 2 Te x S 1 − x QDs/Au PD on the glass substrate: (a) under dark and without external bias; (b) under NIR light with a wavelength of 1350 nm illumination and without external bias; (c) under illumination and with external bias.