This is the accepted version of the journal article: Roth, Alison N.; Opare-Addo, Jemima; Gi, Eunbyeol; [et al.]. «Solution-phase synthesis and photoluminescence of quaternary chalcohalide semiconductors». Chemistry of materials, Vol. 35, Issue 5 (March 2023), p. 2165-2172. DOI 10.1021/acs.chemmater.3c00011 This version is available at https://ddd.uab.cat/record/274565 under the terms of the license Solution-Phase Synthesis and Photoluminescence of Quaternary Chal- cohalide Semiconductors Alison N. Roth,1,2 Jemima Opare-Addo,1,2 Eunbyeol Gi,1,2 Silvia Mena,3 Gonzalo Guirado,3 Richard D. Schaller,4,5 Emily A. Smith1,2 and Javier Vela*,1,2 1US DOE Ames National Laboratory, Ames, Iowa 50011, United States; 2Department of Chemistry, Iowa State Univer- sity, Ames, Iowa 50011, United States; 3Departament de Química, Universitat Autònoma de Barcelona, Bellaterra (Bar- celona) 0819, Spain; 4Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States; 5Center for Nanoscale Materials, Argonne National Laboratory, Lemont, Illinois 60439, United States. KEYWORDS. Chalcohalides, quaternary semiconductors, solution synthesis, photoluminescence. ABSTRACT: Mixed-metal chalcohalide semiconductors have emerged as promising candidates for photovoltaic applications. However, preparation of these multinary compounds using solution-phase techniques remains particularly challenging com- pared to traditional solid-state methods. To fully harness their potential, it is desirable to develop synthetic methods that enable control over both the phase purity and dimensionality of chalcohalides. Here, we report the solution-phase synthesis of Pb2SbS2I3 and Pb2BiS2I3 quaternary chalcohalides using readily available precursors. Fine tuning of reaction parameters allows for the isolation of rod-like morphologies with tunable diameters and aspect ratios. Remarkably, these quaternary chalcohalides display photoluminescence as an ensemble as well as at the single particle level, as demonstrated using fluo- rescence microscopy. We further evaluate the relative stability and band gap of Pb2SbS2I3 polymorphs and their coloring pat- terns using electronic structure calculations. The synthetic methods developed here will motivate the study of ever more complex chalcohalides and other multinary semiconductors for new technological applications. Introduction different positions at one site (positional disorder), and an- Chalcohalides are a special class of inorganic materials other crystallographic site can be occupied by either Pb or that have emerged as promising semiconductors for energy Sb (substitutional disorder). These types of disorder in conversion devices.1 Whereas chalcogenide semiconduc- Pb2SbS2I3 are resolved at low temperature, resulting in a tors display high absorption coefficients and tunable band monoclinic (P21/c) structure.15 It remains unclear how the gaps but limited efficiency in solar devices,2,3,4 halide perov- crystallographic disorder—or lack thereof—in Pb2SbS2I3 af- skites display higher power conversion efficiencies but fects the electronic properties for its two polymorphs. lower chemical and thermal stability due to their weak ionic The most common route used to synthesize IV2VS2I3 chal- interactions.5,6,7 Occupying a promising middle between cohalides involves solid-state reaction between the direct these two scenarios, mixed-ion chalcohalides are expected elements or between binary sulfides and iodides at rela- to combine the power conversion efficiency of halide perov- tively high (>500 °C) temperatures, which generally results skites with the enhanced stability of chalcogenides.8,9 In- in bulk material.11,15,16,17 Recently, an alternative route deed, previous experimental and computational investiga- based on annealing films of Sn2SbS2I3 that were solution-de- tions have demonstrated the semiconducting nature of posited from SbCl3, SnI2, and thiourea in DMF was success- IV2VS2I3 (IV = Sn, Pb; V = Sb, Bi) materials, with direct band fully introduced.13 Similar success was achieved in the solu- gaps ranging from ~1.2–2.0 eV.10,11,12 Because these values tion-phase synthesis of ternary semiconductors within the are near the band gap that is ideal for photovoltaics, the first chalcohalide family: Ternary lead chalcohalides such as solar devices with Pb2SbS2I310 and Sn2SbS2I313 were fabri- Pb4S3I2 and Pb4S3Br2 were prepared through solution-phase cated, revealing promising power conversion efficiencies methods at relatively low (<200 °C) temperatures using (PCE) of ~3–4%. The wider band gap compositions such as metal halide and thiocyanate precursors.18,19 Similarly, BiSI Pb2SbS2I3 may also be of use in photocatalysis.8 nanorods were synthesized with control over particle size IV2VS2I3 chalcohalides are known to adopt either ortho- and morphology.20 These findings strongly suggest that thi- rhombic or monoclinic crystal structures (Figure 1).14,15 For ocyanate precursors, in combination with suitable coordi- example, Pb2SbS2I3 can adopt a highly “disordered” ortho- nating organic ligands, may enable the synthetic develop- rhombic (Cmcm) phase where one Pb atom occupies two ment of more complex—and higher order—quaternary semiconductors. 1 from a dark red solution at 160 °C to a bright orange solu- tion between 180–270 °C. Formation of Pb2BiS2I3 also oc- curs with a noticeable color change from light brown at 160 °C to a dark purple solution between 200–300 °C. However, unlike Pb2SbS2I3, XRD indicates that phase-pure Pb2BiS2I3 does not form until 300 °C, as evidenced by the presence of PbI2 impurity peaks in the diffraction pattern below 280 °C. Scheme 1. Solution-phase synthesis of quaternary chalco- halides (Pn = Sb or Bi; see Methods).21,22 T, t 2 Pb(SCN)2 + PnI3 + 6 O2 Pb2PnS2I3 + 2 SO2 + 4 CO2 + 2 N2 Inspection of the XRD patterns of the quaternary chalco- halides show that individual peaks are selectively attenu- ated—indicating preferred orientation—and/or broad- ened—indicating some small single crystalline domains (Figure 2, “heat-up”). For example, the (200) and (020) re- flections are some of the weakest and broadest XRD reflec- tions observed in solution-grown Pb2SbS2I3 and Pb2BiS2I3, which implies they correspond to short (less crystalline) stacking directions—along x(a) and y(b), respectively (see Supporting Information or SI). In contrast, the (004) reflec- tion is the most intense and narrow XRD reflection ob- served in Pb2BiS2I3, which implies it correspond to a long (more crystalline) stacking direction—along y(c). Indeed, scanning and transmission electron microscopy (SEM and Figure 1. Unit cells of (a) orthorhombic (Cmcm) and (b) TEM) reveal the two quaternary chalcohalides are highly monoclinic (P21/c) Pb2SbS2I3, and (c) orthorhombic (Cmcm) anisotropic and crystalline (Figure 2), consistent with early Pb2BiS2I3. (Cmcm Pb2SbS2I3 exhibits both positional and reports of rod-like macroscopic or “bulk” crystals of substitutional disorder, while Pb2BiS2I3 only exhibits substi- Pb2SbS2I3.11,15 High-resolution TEM (HR TEM) clearly shows tutional disorder.) that (040) planes are stacked across the smaller, diameter dimension of the rods, in agreement with these materials Here, we report the solution-phase synthesis of two such having anisotropic growth along the [001] direction. mixed-metal quaternary chalcohalides, Pb2SbS2I3 and Pb2BiS2I3, using commercially available thiocyanate and The defining feature of the quaternary chalcohalide rods halide precursors. These chalcohalides exhibit highly aniso- is their large length-to-diameter (l/w) aspect ratio, with tropic growth, giving rise to rod-like particles with aspect Pb2SbS2I3 exhibiting average width and length of 42±14 nm ratios that can be fine-tuned by changing the reaction con- and 1.7±0.2 µm, respectively (l/w = 40) (Figure 2 and Table ditions. We examine their ensemble as well as their single 1). Similarly, Pb2BiS2I3 rods show average width and length particle photoluminescence using a combination of spectro- of 630±190 nm and 8.9±2.1 µm, respectively (l/w = 14). The scopic techniques. Lastly, we compare our experimental re- average Pb2SbS2I3 rod width observed from SEM is con- sults to new electronic structure calculations to better un- sistent with the average single crystalline domain (Scher- derstand the effects of crystallographic disorder and poly- rer) size estimated from XRD peak widths (46 ± 28 nm). En- morphism on the band gap of these materials. ergy dispersive X-ray spectroscopy (EDS) confirms the stoi- chiometry of each quaternary sample closely matches the Results and Discussion theoretical composition of the desired IV2VS2I3 phases alt- Solution-Phase Synthesis and Structural Characterization. hough, in a few of our initial experiments, EDS revealed the We first prepared Pb2SbS2I3 and Pb2BiS2I3 using a heat-up presence of small amounts of an amorphous PbI2 impurity solution-phase synthesis approach starting from Pb(SCN)2, (see SI and further optimization below). PbI2, and either SbI3 or BiI3 (Scheme 1, see Methods). X-ray diffraction (XRD) confirms that Pb2SbS2I3 begins to form at temperatures as low as 180 °C. The reaction mixture goes 2 Figure 2. Solution-grown quaternary chalcohalides: (a, b) Powder XRD patterns of Pb2SbS2I3 and Pb2BiS2I3. SEM images of (c) Pb2SbS2I3 and (d) Pb2BiS2I3 rods prepared using the heat-up approach. Further exploration of the best synthetic conditions both Pb2SbS2I3 and Pb2BiS2I3 have noticeably lower aspect showed that the exact ligand concentration has a signifi- ratios (l/w = 8–9) as compared to when they are made us- cant impact on the formation of phase-pure quaternary ing the heat-up approach (Table 1). chalcohalides. Specifically, higher concentrations of Ensemble and Single Particle Optical Behavior. Pb2SbS2I3 oleylamine lead to the formation of rock salt PbS. In some and Pb2BiS2I3 are believed to be direct band gap semicon- cases, increasing the concentration of both oleylamine and ductors.11,13 Utilizing Tauc plots of the diffuse reflectance oleic acid results in a mixture of semicrystalline quater- spectra (see Methods and SI), the band gap of Pb2SbS2I3 is nary and ternary products. Similarly, increasing only the extrapolated to be ca. 1.9 eV which agrees with its bright concentration of oleic acid leads to the formation of ter- orange color. Pb2BiS2I3 exhibits a slightly narrower band nary Pb5S2I6. The best results are obtained with relatively gap of 1.6 eV, consistent with its dark purple color (Figure small concentrations of oleic acid and oleylamine (both at 3). Interestingly, chalcohalides prepared using the hot-in- 76 mM) in octadecene, leading to the desired quaternary jection approach exhibit slightly red-shifted absorption phases in phase pure form and, in the case of Pb2SbS2I3, the onsets compared to those made using the heat-up ap- smallest rod diameters (Table 1). proach. In the case of Pb2SbS2I3, this could arise from their In addition to these findings, we learned that we can smaller diameter and thus to a degree of quantum confine- synthesize quaternary lead chalcohalides at slightly lower ment. The band gaps of these materials roughly parallel reaction temperatures without oleylamine. This is signifi- those obtained using a combination of X-ray photoelec- cant because oleylamine is highly corrosive and requires tron spectroscopy (XPS)23,24 and cyclic voltammetry careful handling (see Methods). Furthermore, we can en- (CV)25,26,27,28,29 (see SI). tirely omit PbI2 as a precursor and instead hot-inject an Upon excitation at 450 nm, crude solutions of Pb2SbS2I3 oleic acid slurry of Pb(SCN)2 at 200 °C, which completely emit at ~650 nm (1.91 eV) (Figure 3). Consecutive washes suppresses the formation of binary or ternary impurities. considerably decrease the overall PL intensity. Time-cor- Using this hot-injection approach, we obtain Pb2SbS2I3 and related single photon counting (TCSPC) shows that the Pb2BiS2I3 powder diffraction patterns that closely match Pb2SbS2I3 rods have an average, ensemble PL lifetime of the relative intensities in the orthorhombic standard pat- 1.2 ns and a quantum yield (QY) <1%. terns (Figure 2, “hot-injection”). Electron microscopy im- aging still reveals distinct rod-like morphologies, although 3 Table 1. Representative properties of solution-grown quaternary chalcohalide semiconductors. Pb prec(s). Pnictide Ligandsc (mL) T t Product App. Widthe Lengthe Aspect (mmol)a prec. (°C) (min) gap (Scherrer)f (µm) ratio (mmol)b (eV)d (nm) (l/w) Pb(SCN)2 (0.2), SbI3 (0.1) Oleic acid (0.25), 270 1 Pb2SbS2I3 1.9 42±14 1.7±0.2 40 PbI2 (0.1) oleylNH2 (0.25) (Cmcm) (46±28) Pb(SCN)2 (0.2) SbI3 (0.1) Oleic acid (8) 250 1 Pb2SbS2I3 1.6 400±120 3.2±0.9 8 (Cmcm) (380±120) Pb(SCN)2 (0.2), BiI3 (0.1) Oleic acid (0.25), 300 1 Pb2BiS2I3 1.6 630±190 8.9±2.1 14 PbI2 (0.1) oleylNH2 (0.25) (Cmcm) (427±464) Pb(SCN)2 (0.2), BiI3 (0.1) Oleic acid (8) 250 1 Pb2BiS2I3 1.3 240±60 2.3±0.4 10 PbI2 (0.1) (Cmcm) (260±190) a[Pb]T = 19–29 mM. b[Pn]T = 9.5 mM. cODE volume = 10.5 mL (VT) - oleic acid volume - oleylNH2 volume. dEstimated using Tauc plots (see Methods). eFrom EM. fFrom XRD. to what has been reported for lead halide perovskites.31 Lead-halide interactions are often defect tolerant due to the presence of Pb2+ 6s2 lone pair bonding contribu- tions.32,33 This unique electronic structure results in shal- low defect states in the band gap or within the valence band which can ultimately improve and increase photolu- minescence stability.34 Moreover, similar to binary nano- crystals (CdSe, etc.), reduction and even complete sup- pression of blinking in lead halide perovskites has also been achieved by surface passivation, either with ligands35 or by growing protective shells over the core material.36 These modifications lead to the elimination of nonradia- tive Auger recombination arising from structural defects and charge carrier trap states.37,38 Therefore, our findings bode well for the use of quaternary chalcohalide semicon- ductor nanoparticles in a wide range of optoelectronic ap- plications and devices. Polymorphism and Coloring from Electronic Structure Calculations. To better understand the polymorphism and optoelectronic properties of the quaternary chalcohalides, we turned to relative energy calculations using the Vienna Ab initio Simulation Package (VASP).39 Because the re- ported orthorhombic Cmcm structure exhibits mixed Pb/Sb occupancy sites, we modeled various supercell structures with different arrangements of Pb and Sb at those positions (see Methods).40,41 The different possible Figure 3. (a) Diffuse reflectance (DR, solid red line), pho- “coloring” patterns for the Cmcm Pb2SbS2I3 supercell can toluminescence (PL, red-dashed line), and photolumines- be described by the orientation of Pb and Sb atoms cence excitation (PLE, red-dashed line, λem = 650 nm) stacked along the [010] direction: Specifically, Pb and Sb spectra of solid film of Pb2SbS2I3 nanorods (b) Photolumi- sites can alternate completely (staggered); the same types nescence of a single Pb2SbS2I3 rod over time (λexc = 450 nm, of atoms can face each other (Pb-Pb/Sb-Sb facing); or the with a 629±56 nm emission filter). (Movies available in opposite types of atoms can face each other (Alt. Pb-Sb fac- the Supporting Information). ing) (Figure 4). After fully relaxing the unit cells, the Pb-Sb Interestingly, Pb2SbS2I3 rods are photoluminescent facing and (Alt.) Pb-Sb facing supercells maintained ortho- even at the single particle level. Unlike the blinking behav- rhombic structures (Pca21 and Pna21, respectively). In ior that is ubiquitous in colloidal CdSe and other nanocrys- contrast, the Pb-Pb/Sb-Sb facing and staggered supercells tal quantum dots and nanowires,30 fluorescence micros- relaxed into monoclinic structures (P2 and P21/c, respec- copy reveals that individual Pb2SbS2I3 rods display steady, tively). Out of the four different coloring patterns mod- shape-correlated photoluminescence over time, with no eled, the staggered supercell structure has the lowest total OFF periods or blinking recorded under continuous pho- energy, closely resembling the relative energy and atomic toexcitation at 450 nm (Figure 3 and SI movie). Moreover, coloring pattern of the P21/c polymorph (see SI). rather than fading or staying constant, the rods' emission intensity appears to gradually increase over time. Inter- estingly, this lack of fluorescence intermittency is similar 4 features compared to the staggered polymorph. For both of the lowest energy Pb2SbS2I3 polymorphs, the valence band region of the DOS is dominated primarily by I-p and S-p orbitals, while Pb-p and Sb-p orbitals significantly con- tribute to the conduction band region. These features are consistent with the DOS features of Pb and Sn-based qua- ternary bismuth chalcohalides,11 as well as with our own control calculation for Pb2BiS2I3. A comparison of the Crys- tal Orbital Hamilton Population (COHP)46 for the two Pb2SbS2I3 polymorphs, however, shows a higher degree of filled Pb-I antibonding states below the Fermi level for the P21/c phase, as reflected in the negative portion of the COHP diagram (see SI). This may suggest that formation of the disordered phase is more favorable overall, despite both Pb2SbS2I3 polymorphs being very close in energy. Considering that the completely ordered P21/c phase has only been isolated at low temperatures (100 K),15 it is plausible that the disordered (Cmcm) polymorph that we observe experimentally is metastable. In this case, our cal- culations agree well with experiment and offer physical insight into the effects of coloring and polymorphism in Pb2SbS2I3 and similar quaternary chalcohalides. Conclusions In summary, we have successfully developed the solu- tion-phase synthesis of quaternary mixed-metal chalco- halides using both heat-up and hot-injection approaches in combination with commercially available halide and thiocyanate precursors. We demonstrate control over the diameter and aspect ratio of their highly anisotropic, rod- like particles by adjusting ligand concentration, resulting in widths as small as ca. 40 nm. Both Pb2SbS2I3 and Pb2BiS2I3 rods exhibit direct band gap values below 2.0 eV, and Pb2SbS2I3 shows photoluminescence behavior both in solution and at the single particle level. Electronic struc- Figure 4. (a) Relative energies of different Cmcm ture calculations reveal that the disordered Pb2SbS2I3 Pb2SbS2I3 supercell structures (Note: for clarity, some phase likely adopts a staggered atomic coloring pattern Pb/Sb-S/I bonds are omitted and the S and I atoms are and, consequently, has nearly identical electronic and faded). (b) Partial density of states (pDOS) (left) and COHP structural features to the P21/c polymorph. (right) of the staggered supercell structure of Pb2SbS2I3. Many additional compositional variations are possible Density of states (DOS) calculations provide further in- within this family of mixed-metal chalcohalides, including sight into the more likely coloring pattern of the disor- those that contain different chalcogens (Se, Te), halogens dered Pb2SbS2I3 polymorph. Using the tight-binding linear (Cl, Br), and chalcogen-to-halogen ratios. Because there is muffin-tin orbital atomic sphere approximation (TB- a heightened demand to develop semiconductors that are LMTO-ASA) method,42 we calculated the DOS of each of the lead-free, tin-based analogs such as Sn2BiS2I3 and different supercells and determined their band gap values Sn2SbS2I3 will be particularly attractive alternatives but (SI). The DOS of the lowest energy staggered polymorph their solution-phase synthesis has yet to be thoroughly ex- shows a band gap of 1.30 eV (Figure 4), which underesti- plored.12 The methods developed in this study are a step- mates the experimentally determined gap (1.9 eV) by ca. ping stone toward synthesizing such quaternary materi- 30%. The remaining supercells display much narrower als. Further preparative and computational investigations band gaps, which underestimate the experimental value of these materials will be needed to move them closer to by 50–75%. Considering both the degree of band gap un- real-life application and deployment. derestimation often observed using TB-LMTO,43,44,45 as well as the relative total energies, these findings suggest Methods that the Cmcm Pb2SbS2I3 polymorph most likely adopts the Materials. Oleylamine (oleylNH2, technical grade, 70%), staggered coloring pattern. Because different Pb/Sb color- 1-octadecene (ODE, technical grade, 90%), lead(II) thiocy- ing patterns appear to affect the calculated band gap, it anate (Pb(SCN)2, 99.5%), and lead(II) iodide (PbI2, 99%) may be possible to achieve band gap tunability by control- were purchased from Sigma Aldrich; antimony(III) iodide ling the extent of crystallographic disorder in Pb2SbS2I3. (SbI3, 99.9%) and bismuth(III) iodide (BiI3, 99.999%) Analysis of the DOS of the P21/c Pb2SbS2I3 polymorph from Strem; oleic acid (OA, technical grade, 90%) from shows a nearly identical band gap (1.35 eV) and pDOS Alfa Aesar; hexanes (99.9%) and methanol (99.9%) from 3 Fisher. All chemicals were used as received. Caution: 629±56 nm emission filter were used. Pb2SbS2I3 nanocrys- Oleylamine is a highly corrosive liquid and must be handled tals were diluted in hexanes and sonicated for 90 min be- with extreme care and in the smallest amounts possible. fore depositing 50 µL on a glass microscope coverslip Synthesis. All reactions were carried out under air. Heat- (Fisher Scientific, Pittsburgh, PA). The solvent was re- up Method. Pb(SCN)2 (0.2 mmol), PbI2 (0.1 mmol) and ei- moved under vacuum and the sample was dried after 15 ther SbI3 or BiI3 (0.1 mmol) were stirred together in a 100 min. Single particle PL movies were collected on an Andor mL round bottom flask containing 1-octadecene (10 mL, iXon Ultra EMCCD camera (Oxford Instruments, Abingdon, 31 mmol), oleic acid (0.25 mL, 0.8 mmol) and oleylamine UK) with 40 ms exposure time and 70x EM gain. Each (0.25 mL, 0.8 mmol) at 110 °C until all solids dissolved. movie was 40 s in duration and 5 movies were collected. The solution was heated to 270 °C (Pb2SbS2I3) or 300 °C ImageJ was used to analyze the PL intensity versus time (Pb2BiS2I3) for 1 min and subsequently cooled to room for the selected particles and background. temperature (R.T.). Hot-injection Method. Alternatively, Cyclic Voltammetry. Electrochemical studies were per- quaternary lead chalcohalides are prepared by injecting a formed by cyclic voltammetry (CV) using the potentiostat slurry of Pb(SCN)2 (0.2 mmol) in oleic acid (2 mL, 6.3 Workstation CHInstrument and controlled by the soft- mmol) into a solution of SbI3 or BiI3 (0.1 mmol) in oleic ware CHI660e V14.08. A three-electrode system and a acid (6 mL, 19 mmol) and 1-octadecene (2.5 mL, 7.8 thermostatic electrochemical cell was used. Glassy carbon mmol) at 200 °C, followed by heating the mixture to 250 (WEC, 3 mm of diameter) was used as working electrode, °C for 1 min before cooling to R.T. Purification. Crude chal- while a platinum wire and a saturated calomel electrode cohalide solutions were initially centrifuged at 4500 rpm (SCE) were used as counter (CE), and reference (RE) elec- for 5 min followed by discarding the supernatant. The pel- trodes, respectively. Both CE and RE electrodes were sep- let was then resuspended in hexanes and methanol (5 mL arated from the solution by a salt-bridge. The working of each) and centrifuged again to remove excess ligands. electrode was carefully polished with 1 mm diamond Structural Characterization. Powder X-ray diffraction paste (DP-Paste, P) and rinsed with ethanol. Dispersion (XRD) was measured on a Rigaku Ultima IV diffractometer solutions from 0.5 to 1 mg/mL of each chalcohalide semi- (40 kV, 44 mA) using Cu Kα radiation on a zero-back- conductor was prepared in hexanes and sonicated for 1-2 ground quartz sample holder. Transmission electron mi- minutes. Once the dispersions were prepared, 50 μL of croscopy (TEM) imaging was performed on a JEOL 2100 each sample was drop-cast onto the glassy carbon elec- scanning transmission electron microscope. Samples trode surface (d = 3 mm). After the hexanes fully evapo- were prepared by drop casting dilute solution in hexanes rated, CV at different scan rates (v = 0.05 to 0.5 V s-1) was onto a carbon-coated 200 mesh copper grid. Scanning performed using the chalcohalide modified electrode electron microscopy (SEM) images were acquired on a (working electrode, WE) in a pure acetonitrile + 0.1 M of JEOL JSM-IT200 scanning electron microscope. tetrabutylammonium tetrafluoroborate (TBABF4) solu- tion under a nitrogen atmosphere. Prior to recording the Optical Characterization. Diffuse-reflectance spectra cyclic voltammograms, blank experiments were per- were collected using a SL1 Tungsten Halogen lamp (vis– formed to ensure that the glassy carbon surface of the WE IR), a SL3 Deuterium Lamp (UV), and a BLACK-Comet C- was well polished, fresh and clean. Hence, no electrochem- SR-100 spectrometer (200–1080 nm). The band gap val- ical signals were the detected for non-modified working ues were estimated by extrapolating the linear slope of electrodes in the selected potential range (-2.5–1.5 V). Tauc plots by plotting (Ahν)r versus hν (A = absorbance, hν = incident photon energy in eV, r = 1/2 for direct semi- X-ray photoelectron spectroscopy. XPS measurements conductors).47 Photoluminescence (PL) spectra were col- were collected with a Kratos Amicus/ESCA 3400 instru- lected on a Horiba Jobin Yvon Fluorolog-3 spectrofluo- ment. The powder chalcohalide samples were first rometer (Slit width = 5 nm; λexc = 450 nm). The crude sam- pressed into 8 mm (diameter) by 1.2 mm (height) pellets ples were suspended in hexanes for the solution-phase with a cold pellet press. The pellets were irradiated with photoluminescence measurements. Photoluminescence 240 W un-monochromated Mg Kα X-rays, and photoelec- excitation (PLE) spectra of Pb2SbS2I3 were collected by trons emitted at 0° from the surface were energy analyzed monitoring photoluminescence (PL) emission at 650 nm using a DuPont type analyzer. The pass energy was set at while varying the excitation wavelength between 250– 150 eV. The binding energy of C 1s at 284.6 eV was used 635 nm. TSCPC was performed using a 405 nm Picoquant for reference. laser operating at 2.5 MHz and 50 ps pulsewidth. Photons Calculations. Relative energy calculations were per- produced by the sample were directed into a 0.3 m spec- formed using the Vienna Ab initio Simulation Package trograph with a CCD for spectral measurement or ava- (VASP),39,48 with projected augmented-wave (PAW) pseu- lanche photodiode for dynamics. Each selected emission dopotentials.49 Electronic exchange-correlation was wavelength was collected for 600 s. treated using the Perdew-Burke-Ernzerhof (PBE) func- Single Particle Fluorescence Microscopy. Single particle tional.50 The cut-off energy for the plane wave basis func- photoluminescence (PL) was recorded on an inverted mi- tions was 500 eV. During the structural optimizations the croscope operated in epifluorescence mode (Nikon volume, atomic positions and cell shape were allowed to Eclipse TE2000U Melville, NY) fitted with a 100x (1.49 relax until the convergence energy was less than 1×10-4 NA) oil immersion objective. An Xcite Series 120 PC mer- eV. The relative total energies were calculated using the cury lamp with a 450±50 nm excitation filter and a tetrahedron method over a 16×16×16 k-point grid. Unit 3 cell representations were generated using VESTA.51 To Javier Vela − US DOE Ames National Laboratory, Ames, Iowa deal with the disordered Pb/Sb or Pb/Bi sites, supercell 50011, United States, and Department of Chemistry, Iowa structures of Cmcm Pb2SbS2I3 and Cmcm Pb2BiS2I3 were State University, Ames, Iowa 50011, United States. Email: constructed by doubling the a-lattice parameter and col- vela@iastate.edu oring the unit cell to assign a single atom to the mixed oc- cupation sites. Density of states (DOS) and Crystal orbital ACKNOWLEDGMENT Hamilton populations (COHP)46 were calculated using the tight binding linear muffin-tin orbital atomic sphere ap- This work was supported by the Ames National Laboratory’s Laboratory Directed Research and Development (LDRD) pro- proximation (TB-LMTO-ASA) package.42 The DOS and gram. The Ames Laboratory is operated for the U.S. Depart- COHPs were calculated using the tetrahedron method af- ment of Energy by Iowa State University under Contract DE- ter converging the total energy on a k-mesh of 7×6×12 AC02-07CH11358. G.G. and S.M. thank the Ministerio de Cien- (Cmcm Pb2SbS2I3 and Cmcm Pb2BiS2I3) or 14×6×12 (P21/c cia e Innovación of Spain for financial support though project Pb2SbS2I3) in the irreducible wedge of the Brillouin zone. PID2019-106171RB-I00. S.M. acknowledges support from the Margarita Salas UCM postdoctoral grants funded by the ASSOCIATED CONTENT Spanish Ministry of Universities with European Union funds - Supporting Information. SEM, TEM, EDS, XRD, diffuse re- NextGenerationEU. Work performed at the Center for Na- flectance, PL, TCSPC, single-particle fluorescence microscopy noscale Materials, a U.S. Department of Energy Office of Sci- movies, XPS, cyclic voltammograms, DOS and COHP calcula- ence User Facility, was supported by the U.S. DOE, Office of tions are provided in the supporting information. This mate- Basic Energy Sciences, under Contract No. DE-AC02- rial is available free of charge via the Internet at 06CH11357. We thank Dapeng Jing, Marquix Adamson, and http://pubs.acs.org.” Phil Yox for assistance with XPS, SEM, and COHP calculations, respectively. AUTHOR INFORMATION REFERENCES Corresponding Author 1 Huang, Y. T.; Kavanagh, S. R.; Scanlon, D. O.; Walsh, A.; Hoye, R. L. Z. Perovskite-Inspired Materials for Photovoltaics and Beyond—From Design to Devices. Nanotechnology 2021, 32, 132004-1–132004-60. 2 Moon, M. M. A.; Rahman, M. F.; Kamruzzaman, M.; Hossain, J.; Ismail, I. B. M. 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