The Formation of Crystals in Gels Harry N. Holmes J. Phys. Chem., 1917, 21 (9), pp 709–733 DOI: 10.1021/j150180a001 Publication Date: January 1916 R. E. Liesegang. [1] observed that if a drop of silver nitrate solution was placed on a film of gelatine gel containing potassium dichromate the precipitated silver dichromate was formed in concentric rings separated by clear intervals. This “rhythmic banding” of “Liesegang’s rings” will be discussed in a later paper. Hatschek’s usual method [1][3] of work was to make a 5-20% gelatine gel or a 1-5% agar gel containing, for example, a small amount of potassium dichromate. This was allowed to solidify in a test-tube and covered with a solution of silver nitrate. The silver ion slowly diffusing down into the gel reacted with the dichromate ion to form silver dichromate in small crystals. In 1911 Hatschek reported these crystals 0.5 mm long in 10 percent gelatine, lead dichromate 0.03 mm long in 2 percent agar; [...] He also prepared a number of other crystalline compounds such as lead iodide, [...]. Later he used silicic acid gels as the media for diffusion and obtained larger crystals. Hatschek held that the concentration and nature of the gel determined the form and size of the crystals. The in- elastic gels like silicic acid, he thought, were superior to the elastic organic gels like agar and gelatine. He wrote of “the practically universal tendency to form spherical bodies in agar and gelatine” although he did obtain some crystals in these media. [...] The water glass used was a commercial grade known as “water white” with a density of 1.375. The ratio of the Na2O to the Si02 was 1:3.5. When diluted to a density of 1.06 and titrated against hydrochloric acid using phenolphthalein as an indicator it was equivalent to 0.51 N acid. With methyl red as indicator the normality was 0.57. This particular water glass was used at the 1.06 density for many experiments because weaker solutions were too slow in the set of the silicic acid. With this concentration an equal volume of N acetic acid was suitable for most of the experiments. Such a gel set over night and contained a small excess of acid. It was reasonably clear. This rather arbi- trary selection really marked the lowest concentration limit for working convenience. Acetic acid 0.75 N serves as well and gives less excess acid. Several other acids were used for special purposes by the time of set varied with the acid. A partial list follows 1.06 density sodium silicate with equal volume N acetic acid set in 6 hours 1.06 density sodium silicate with equal volume N formic acid set in 36 hours 1.06 density sodium silicate with equal volume N hydrochloric acid 10 days 1.06 density sodium silicate with equal volume N sulphuric acid set indefinite 1.06 density sodium silicate with equal volume N nitric acid set in 4 weeks This applies only to the particular water glass described above. Considering convenience in time of set, clearness of the gel and failure to split as the gel contracts, the following table will be found more useful except in respect to acetic: I. 16 density sodium silicate with equal volume 3 N acetic acid set I .08 density sodium silicate with equal volume 3 N formic acid set 6 hours 36 hours set in IO days indefinite 4 weeks [...] Copper - Perfect tetrahedrons of metallic copper were obtained in a 1.06 water glass-N acetic acid made 0.05 N with respect to copper sulphate. The gel was covered with a 1 percent solution of hydroxylamine hydrochloride. In a week or two the tetrahedrons were large enough to observe with the unaided exe. Those formed near the surface were much smaller than those at greater depths. Of course the farther the reducing solution diffused into the gel the more dilute it became. In a beam of sunlight the faces of the tetra- hedrons gleamed with a splendid copper luster. In nearly all experiments one or more groups of overlapping tetrahedrons were noted. These formed in three radiating lines, making angles of 120° or in some instances of 60°, 120° and 180°. This arrangement of copper crystals is found in mineral deposits. All the other reducing solutions tried were inferior to hydroxylamine. Lead Iodide - Lead iodide crystals had been made by other investigators but merited further study. This is one of the easiest to make and most spectacular of all the crystal exhibits. A 1.06 water glass-N acetic mixture containing 2 cc. of N lead acetate to every 25 cc was poured into test-tubes. After the silicic acid gel set firmly it was covered with 2 N potassium iodide. A compact layer of lead iodide quickly formed on the surface followed very soo by crystallization below the surface of the gel. In a few days fern-like fronds grew down into the gel mixed with many hexagonal plates. These concentrations may be varied with interesting results and the lead salt may be used above the gel with the potassium iodide in the gel. The first order is much better. Magnificent gold fern-fronds 8 cm in length and nearly perfect hexagons 5 mm in diameter were obtained in some experiments. In one or two tubes six hexagons grouped around a center much like the arrangement in some snow crystals. On standing in direct sunlight for several months the lead iodide crystals blacken somewhat. Using an excess of the lead acetate as compared with the potassium iodide-for example in U-tubes pouring 0.25 N lead acetate in one arm and 0.1 N potassium iodide in the other with a gel separating the solutions, yellow-white needles and blocks of basic lead iodide formed. Some of the needles were 2 cm in length. Watts [4] states that if lead acetate in. excess is mixed with potassium iodide the basic iodide forms unless sufficient acid is present to prevent. With lead nitrate the ordinary lead iodide results. When formed very slowly the yellow-white needles and blocks gleam brilliantly and seem to have a high index of refraction. Star-shaped groups and crossed stick arrangements were common. This is not the white double salt PbI2.KI.2H20, made by Remsen and Herty [5] by the action of excess potassium iodide on lead iodide. [4] Watts: “Dictionary of Chemistry,” 3, 127. [5] Remsen and Herty(1892): Am. Chem. Jour., volume 14, page 107. In U-tube experiments such concentrations as 0.25 N lead acetate and 0.1 N potassium iodide formed both the ordinary yellow hexagons and, in the arm nearer the lead acetate, the white-yellow basic crystals. In fact these con- centrations could be so varied as to give first only the yellow crystals of lead iodide with a later formation of the white- yellow basic iodide. As the lead acetate diffused through the gel and met the potassium iodide these yellow hexagons formed and later as excess lead acetate diffused into this zone the white needles or blocks developed. Often a basic crystal formed touching the edge of a yellow hexagon and apparently “ate it up” as the excess lead acetate reached that point. With such proportions in the U-tfibe as 0.25 lead acetate and N potassium iodide only the yellow hexagons of the normal iodide form. In a slightly basic gel such as 1.06 water glass-0.5 N acetic acid the formation of the white needles of the basic iodide was greatly favored. Dropping acid on these basic crystals instantly turned them yellow, forming the normal iodide. Using 0.25 N solutions of both lead acetate and potassium iodide in a U-tube a 1.06 water glass-0.5 N acetic acid gel developed white needles in two days, growing to a length of 2 cm in one week; a 1.06 water glass-N acetic acid gel developed yellow hexagons in two days and in one week white stars also appeared; a 1.06 water glass-2 N acetic acid gel developed yellow hexagons in two days and in several weeks no white crystals of the basic salt appeared. The last gel was too acid for a basic salt while the first gel had a slight alkaline reaction. A little starch paste mixed with a 1.06 water glass-N acetic acid gel produced a most interesting result. The gel turned blue around the individual crystals, proving the release of free iodine. It cannot be urged that the excess acid formed hydrogen iodide which was oxidized by dissolved oxygen for in that event the gel would have turned a solid blue. It is possible that the starch intimately mixed with the lead iodide as it formed reduced part of it to lead sub-iodide. There is no question about the existence of a sub-iodide of lead as recently proved by Denham.[6] [6] Denham(1917): Jour. Chem. Soc., volume 111, page 29. A very little gum tragacanth in the silicic acid gel caused the appearance of spherulites of lead iodide and greatly hin- dered crystallization. In scores of experiments lead iodide showed no tendency to band in silicic acid gels although in agar well-marked bands of crystalline aggregate were the rule. Hatschek reports bands of lead iodide in silicic acid gels when he used tap water in preparing solutions attributing the effect to the bicarbonates present in the water. Lead bromide in white twin crystals was prepared by experiments similar to those with lead iodide. [1] E. Hatschek (1906) Zeit. anorg. Chem., volume 48, page 364. [2] R. E. Liesegang (1907) Zeit. phys. Chem., volume 59, page 444. [2] E. Hatschek (1912): Zeit. Kolloidchemie, volume 10, page 77. ---------------------------------------------------------------------- Preparation of crystals of sparingly soluble salts W. Conard Fernelius and Kenneth D. Detling J. Chem. Educ., 1934, 11 (3), p 176 DOI: 10.1021/ed011p176 Publication Date: March 1934 [Puts two beakers with concentrated salt solutions inside a large tub of water, covered with a layer of paraffin. Diffusion slowly brings the reagents in contact and the insoluble precipitate crystallizes in a few weeks.] The more difficultly soluble lead iodide formed very thin golden plates as large as 5 mm on a side. These plates were frequently matted together. Solubility product 1.39 x 10^-8 (25 C) ---------------------------------------------------------------------- Philosophical Magazine Series 6 Volume 24, 1912 - Issue 141 XXXIII. The photo-electric effect of some compounds A.Ll. Hughes D.Sc. B.A. http://dx.doi.org/10.1080/14786440908637340 [Paywall] ---------------------------------------------------------------------- DOI: 10.1039/C4TA05373G (Communication) J. Mater. Chem. A, 2015, 3, 963-967 Enhancing the performance of planar organo-lead halide perovskite solar cells by using a mixed halide source† Minlin Jiang, Jiamin Wu, Fei Lan, Quan Tao, Di Gao and Guangyong Li Methylammonium lead halide perovskites have emerged as promising photovoltaic (PV) materials because of their excellent optical properties such as high absorption coefficients for a broad range of sunlight absorption. [1] First, a PbI2 solution (dissolved in N,N-dimethylformamide (DMF) with a concentration of 460 mg ml−1) solution [... ] The first batch was dipped into a solution containing a single halide source (CH3NH3I (MAI) dissolved in 2-propanol (IPA) with a concentration of 20 mg ml−1). [...] The reaction between PbI2 and MAI occurs efficiently due to the ordered crystal structure of the PbI2 thin film, which facilitates the intercalation of MAI into the lattice to form MAPbI3 ---------------------------------------------------------------------- A. R. Patel and A. Venkateswara Rao (1980): ''An improved design to grow larger and more perfect single crystals in gels'' Journal of Crystal Growth, volume 49, issue 3, July 1980, Pages 589-590 {{doi|10.1016/0022-0248(80)90134-7} [Use a reservoir with gel connected to two large beakers above it with the reagent] Silica gel (acidified sodium silicate) was used as the growth media. The gel (pH 4.5) was prepared at a constant temperature (25°C) by mixing 160 ml of reagent grade sodium silicate (sp. gr. 1.03) and 100 ml of iN CH3COOH. After setting of this gel, an additional amount of 50 ml of gel solution of the same composition as above may be placed in each of the two side beakers on the already set gel, in order to avoid the damage to the previous gel during the addition of feed solutions in the two side beakers. When the gel was set, the feed solutions of KI and Pb(NO3)2 were added into the two one litre capacity beakers and 20 ml of distilled water over the gel in the beaker, and they were covered wit rubber corks. The concentrations of the feed solutions were varied from 0.1 to 1M. In our experiments, the best results, in terms of intercrystalline separation and crystal size, were obtained using 0.5M concentrations of both the feed solutions. [...] Most of the crystals grown by this method had a size as large as 30 mm x 20 mm x 1 mm ---------------------------------------------------------------------- Chemical decomposition of crystals of lead iodide by irradiation in an electron beam A. J. Forty Discuss. Faraday Soc., 1961,31, 247-253 DOI: 10.1039/DF9613100247 [Decomposes under an electron beam microscope] ---------------------------------------------------------------------- Theories of Liesegang's Rings J. W. Hill Transactions of the Kansas Academy of Science (1903-) Vol. 34 (Apr. 23-25, 1931), pp. 303-310 Published by: Kansas Academy of Science DOI: 10.2307/3624507 Stable URL: http://www.jstor.org/stable/3624507 [ PbI2 mentioned in passing ] ---------------------------------------------------------------------- Materials Letters Volume 180, 1 October 2016, Pages 59–62 Facile growth and characterization of freestanding single crystal PbI2 film Xinghua Zhu, Peihua Wangyang, Hui Sun, Dingyu Yang, Xiuying Gao, Haibo Tian Lead iodide (PbI2) is a promising semiconductor material hold layered structure with a hexagonal unit cell for room temperature detectors [1]. It can be widely applied in medicine, nondestructive defectoscopy, X-ray and gamma spectroscopy [1], [2], Moreover, highly crystalline PbI2 film, as the parent material, is also vital for two-dimensional PbI2 gained by mechanical exfoliation method due to its nature layered structure [7]. In a typical procedure, 0.1 g of PbI2 powder without further purification (Aldrich Chemistry Reagent, purity: 99.99%) and 15 ml of deionized water were placed in a 30 ml Teflon-lined autoclave. The autoclave was maintained at 200 °C for 6 h and then air cooled to room temperature. With the decrease of the solution temperature from 200 °C to room temperature to induce the saturation of the solute, the yellow FSC-PbI2 film was successfully grown in the solution, as shown in Fig. 1(a). Then the FSC-PbI2 film was taken out of the solution and air dried for characterization [about 6 mm x 3 mm x 5 um from picture] ---------------------------------------------------------------------- Journal of Crystal Growth Volume 311, Issue 14, 1 July 2009, Pages 3557–3562 Study of the influence of the rare-earth elements on the properties of lead iodide M. Matuchova, K. Zdansky, J. Zavadil, A. Danilewsky, F. Riesz, M.A.S. Hassan, D. Alexiew, R. Kral [Synthesis from the elements, then crystallization from the melt.] We report the introduction of rare-earth elements and other elements as admixtures during synthesis to study their influence on the quality of single crystals. Synthesized material as well as single crystals have been characterized by measurements of electrical resistivity and low-temperature photo luminescence and index of refraction. The structural quality with respect to polytypes was analysed by electron back scatter diffraction. Makyoh topography was applied for surface studies. We have developed [9] a method of direct synthesis that has been continuously improved. The technological parameters have been estimated from thermodynamical data and calculation of ΔG. The lowest temperature for the feasibility of the direct synthesis of both elements has been calculated and is at least 500 °C. The rate of chemical reaction increases with the increasing temperature. The upper limit is estimated by the softening of quartz glass that begins at about 900 °C. The preferred temperature for the synthesis has been found to be 700 °C. The reaction of direct synthesis is based on direct synthesis of both elements. The prepared material is later used for subsequent purification and the actual crystal growth. Lead wires of quality 5N have been cleaned by ethanol and cut into small pieces. Also, small balls of iodine with quality p.a. have been inserted into a small iodine ampoule. A special quartz ampoule of 600 mm×30 mm (l×d) has been proposed and divided into two sections. The first section serves to house the iodine ampoule, and the second section to house the lead wires. The direct synthesis process also takes place in the second section. Initially, the quartz ampoule was washed with a cleaner and then with a distilled water, pure alcohol and ultrapure water, which was prepared using a Millipor ion exchanger with purity of 1018 MΩ. Then the quartz tube was dried in an oven at 250 °C for several hours. Iodine charge weight was used without surplus and with a 5% surplus compared to the stoichiometry of the lead charge to reach proper stoichiometry. After loading the iodine and lead charges, the quartz ampoule was evacuated to about 10−6 mbar. The reaction ampoule is placed in the resistance furnace (Fig. 1). Iodine will evaporate in the resistance furnace section held at the temperature of 150 °C. Resistance furnace section 2 at 700 °C temperature is used for melting the lead. These conditions will facilitate synthesis and enable the production of lead iodide. After the synthesis the ingot has been purified in zone melting apparatus to reduce possible residual impurities. The main part of the apparatus has been preheated to 200 °C, and the zone has been heated to a temperature of 300 °C with the resulted temperature of ~420 °C in the zone. The pure part of the ingot after zone melting has been used for the crystal growth. The Bridgman–Stockbarger vertical apparatus was used for the growth. The furnace is constructed from 4 centric tubes i.e., inner quartz tube and 3 outer glass tubes, which serves as insulation. The inner heated tube is composed of two sections that have separated heat circuits and are controlled by Eurotherm controllers. To facilitate a better temperature profile two aluminum tubes have been hung in the inner quartz tube. The upper section was kept at 435 °C and the lower section at 370 °C. Between both sections is the region of temperature gradient where the crystal starts to grow. The growth ampoule is inserted in the upper part and melted for about 5 h. After that the growth ampoule is dropped down with chosen velocity. In the lower part the crystal must be cooled very slowly to prevent damages (failures or cracking of the crystal). The growth ampoule has a special construction and is supplied with sapphire nucleus (0 0 1) face. ---------------------------------------------------------------------- A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, J. Am. Chem. Soc., 2009, 131, 6050–6051 Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells Akihiro Kojima, Kenjiro Teshima, Yasuo Shirai and Tsutomu Miyasaka J. Am. Chem. Soc., 2009, 131 (17), pp 6050–6051 DOI: 10.1021/ja809598r [ Pioneer use of MAPbI3 (methylammonium lead iodide) with perovskite structure as a sensitizer for TiO2 solar cells. Article reports ~4% efficiency,but now they are at 20% efficiency. Can be formed on bendable thing glass substrate. ] A vivid color change from colorless to yellow occurred for CH 3 NH 3 PbBr 3 and from yellowish to black for CH 3 NH 3 PbI 3 . ---------------------------------------------------------------------- US Patent Lead iodide film Publication number US3764368 A Publication type Grant Publication date Oct 9, 1973 Filing date Feb 22, 1972 Priority date Feb 20, 1970 Inventors J Jacobs, R Corrigan Original Assignee Bell & Howell Co [ A film containing PbI2 microcrystals is not photosensitive, but becomes photosensitive ("ripened") when heated at 125 C for 30 seconds. That is aided by substances in the film that melt at 125 C and act as solvents, incleasing the size of the PbI2 particles. Then an image can be stored by exposing with strong light (600 W halogen lamp at 1 foot) for 60 seconds, which decomposes the PbI2 to the elements. A suitable reducing additive, such as ascorbic acid, must be included to consume the liberated I2. The exposed film is then developed by chemical processing at ~80C with sodium hydroxide or sodium acetate to dissolve the remaining PbI2, and copper ammonium acetate to replace the metallic Pb by Cu for greater stability. ] ---------------------------------------------------------------------- Ultra-Low Thermal Conductivity in Organic–Inorganic Hybrid Perovskite CH3NH3PbI3 Andrea Pisoni, Jaćim Jaćimović, Osor S. Barišić, Massimo Spina, Richard Gaál, László Forró, and Endre Horváth J. Phys. Chem. Lett., 2014, 5 (14), pp 2488–2492 DOI: 10.1021/jz5012109 [ Get 0.5 W/K/m at room temperature. ] ---------------------------------------------------------------------- On a recent case of butter adulteration Charles E. Cassal Analyst, 1892,17, 113-120 DOI: 10.1039/AN8921700113 [ Paywall ] ---------------------------------------------------------------------- Seth Anthony (2014) http://hdl.handle.net/10217/82503 I. Cognitive and instructional factors relating to students' development of personal models of chemical systems in the general chemistry laboratory. II. Solvation in supercritical carbon dioxide/ethanol mixtures studied by molecular dynamics simulation Ph. D. Thesis, Colorado State University [ Uses KI + Pb(NO3)2 to teach molarity. ] ---------------------------------------------------------------------- Pigments, chemistry of Thomson, J M. Journal of the Society of Arts33 (Nov 21, 1884): 995. [ Mentions changes in color of HgI2 when it is heated (red to yellow) and pressed (yellow to red again). Paywall prevents seeing PbI2. ] ---------------------------------------------------------------------- Paramonova, Z. V., Kucheva, N. I., Growth of Lead Iodide Single Crystals by Counter Diffusion, Uch. Zap. Volgod. Gos. Pedagog. Inst., 30,43 (1966). [ Not found; cited by CRYSTALLIZATION—PART III - "Data Concerning Particular System and Product" Gregory D. Botsaris et al. Says they report crystals up to 10 mm in size.] ---------------------------------------------------------------------- Patnaik soluble in alkali iodide solutions. decomposes at 180°C when exposed to green ligh Lead diiodide is used for recording optical images; for making gold spangles and mosaic gold for decorative purposes; in photographic emulsions; in mer- cury-vapor lamps; in asbestos brake linings; in far-infrared filters; in thermal batteries; in printing and recording papers; and in aerosols for cloud seeding. ---------------------------------------------------------------------- Analytica Chimica Acta Volume 6, 1952, Pages 406-411 The extraction of lead iodide by methyl iso-propyl ketone Philip W. West, Jack K. Carlton doi:10.1016/S0003-2670(00)86967-6 [ Nearly complete extraction of lead from lead iodide solutions acidified with HCl and with excess iodide ions, by shaking with methyl iso-propyl ketone and separating the phases. The extracted solution may be colorless. ] The extraction of lead iodide from acidic aqueous solutions has been found to be possible when the proper organic solvent was employed, the most efficient solvents being the ketones, and in particular, methyl iso-propyl ketone. The removal of practically all interferences was accomplished by a preliminary extraction in which ammonium thiocyanate and hydrochloric acid were used to condition the aqueous solution and methyl iso-propyl ketone was used as the solvent. After separation of the phases, the aqueous phase was treated with potassium iodide and again extracted with methyl iso-propyl ketone. Lead iodide, under these conditions, was essentially extracted completely with cadmium and ruthenium constituting the only interferences. A study of the extraction of the metal iodides revealed that a number of them were extractable into organic solvents. Those metals, having iodides which were at least partially extracted into one or more of the organic solvents, employed were bismuth, mercury, iron, lead, copper, palladium, cadmium, rhodium, gold and ruthenium. Among the organic solvents found to be useful in this extraction were butyraldehyde, n-amyl alcohol, methyl n-amyl ketone, methyl isobutyl ketone, methyl n-propyl ketone, methyl iso-propyl ketone and ethyl acetate. After observing the solvent properties of these organic liquids, methyl iso-propyl ketone was found to be the most desirable solvent because it was relatively inexpensive, highly effective in dissolving the metal iodides, did not tend to form emulsions and was at most only partially miscible with water. This miscibility was reduced considerably in the presence of strong electrolytes such as potassium iodide, ammonium thiocyanate and hydrochloric acid, which were eventually used in the extraction. The extraction of lead iodide from neutral solutions into methyl iso-propyl ketone was incomplete as evidenced by flotation of the lead iodide precipitate in the organic phase. On the addition of hydrochloric acid, the precipitate was dissolved completely in the organic phase. It was found that lead iodide was most completely extracted from aqueous solutions containing 5% by volume of concentrated hydrochloric acid. ---------------------------------------------------------------------- Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment Volume 458, Issues 1–2, 1 February 2001, Pages 406–412 In: Proc. 11th Int. Workshop on Room Temperature Semiconductor X- and Gamma-Ray Detectors and Associated Electronics Lead iodide film deposition and characterization L. Fornaro, E. Saucedo, L. Mussio, L. Yerman, X. Ma, A. Burger http://dx.doi.org/10.1016/S0168-9002(00)00933-5 Lead iodide purification was performed by repeated sublimation at 390°C. [...] lead iodide films 50 to 500 μm in thickness were deposited [...] by sublimation of the purified material at near 390°C. [...] The X-ray film detection capabilities were checked by light pulses, X and gamma irradiation. Lead iodide has been considered since the 1970s as a suitable material for X-ray and γ-ray room temperature detection. From the theoretical point of view the material has convenient properties for this task. Due to the high-atomic number of its elements (ZPb=82,ZI=53) the mass absorption coefficient for these radiations determines that 7 and 1560 μm are sufficient for 90% absorption of 6 and 120 keV radiation, respectively [1]. The wide band gap (2.3–2.5 eV) [2] and [3] should allow detectors of this material to operate at room temperature and even above. The non-existence of phase transformations below the melting point allows not only to grow lead iodide monocrystals from the melt but also to use sublimation near the melting point for purification and film deposition. Therefore, several items related with the use of lead iodide as radiation detector have been studied. Synthesis and purification by zone refining have been performed [4], [5], [6], [7] and [8] and purity and stoichiometry of products have been determined by Inductively Coupled Plasma (ICP) [5] and [6], emission spark source spectral analysis, Differential Thermal Analysis (DTA), X-ray powder diffraction [4] and Differential Scanning Calorimetry (DSC) [7]. Although suggested by Lund et al. [2], purification by sublimation has not been reported yet. Monocrystal growth by zone melting [8], vapor transport [9], gel [10], Bridgman and Czochralski methods [11] and [12] has been performed. Lately, attempts have been made for growing lead iodide polycrystalline films. An approach has been made to grow such films by iodination of a lead surface [16] and [17] with the objective of measuring lead iodide hole conductivity. Lead iodide films have also been deposited by thermal evaporation for imaging applications [18]. Products [PbI2] were melted at 450°C, under vacuum (5×10−5 mmHg) [with some decomposition] and at 500 mmHg Ar atmosphere [with no decomposition]. Once melted, they were purified by zone refining [19] in a vertical zone refining furnace (Crystal Research Z-20) at 420°C and at a speed of about 3 cm/h down the length of the ingot, sixty passes. Another melted Alfa Aesar sample was purified by two repeated sublimations at 5×10−5 mmHg with the source product at 390°C and the cold extreme of the ampoule at room temperature. In this way, a rate of sublimation of 1 g/day is achieved. 200 mg of PbI2 was dissolved in 7 ml EDTA 0.1 M ---------------------------------------------------------------------- Materials Research Bulletin Volume 27, Issue 5, May 1992, Pages 537-544 High purity lead iodide for crystal growth and its characterization J. Eckstein, B. Erler, K.W. Benz doi:10.1016/0025-5408(92)90141-L [ Prepare PbI2 by the classical rection and refine by zone melting. ] ---------------------------------------------------------------------- Materials Research Bulletin Volume 17, Issue 3, March 1982, Pages 279-286 Preparation of lead iodide films by iodination of chemically deposited lead sulphide films T.K. Chaudhuri, H.N. Acharya doi:10.1016/0025-5408(82)90074-5 [ Prepare thin films of PbI2 by depositing a PbS film, treating with I2 vapor, and washing the S2 with DMSO. ] These films, deposited at room temperature (30°C) for about an hour, have thicknesses of about 100 nm and particle sizes of the order of 10 nm (23). Such films on glass substrates were introduced into a dark glass chamber having iodine crystals at the bottom. The chamber was kept at various temperatures. The time required for the conversion of PbS to PbI2 depended on the temperature of the enclosure, the thickness of the PbS film, humidity and partial pressure of iodine vapours. Surfaces of the iodized PbS films were washed with carbon disulphide in order to remove sulphur. ---------------------------------------------------------------------- Scaife, C. W. J.; Cavoli, S. R.; Blanton, T. N.; Morse, M. D.; Sever, B. R.; Willis, W. S.; Suib, S. L. (1990). "Synthesis and characterization of lead(II) iodide grown in space". Chemistry of Materials. 2 (6): 777–780. doi:10.1021/cm00012a034 ---------------------------------------------------------------------- Xinfeng Liu, Son Tung Ha, Qing Zhang, Maria de la Mata, César Magen, Jordi Arbiol, Tze Chien Sum, and Qihua Xiong (2015): ''Whispering Gallery Mode Lasing from Hexagonal Shaped Layered Lead Iodide Crystals''. ACS Nano, 2015, volume 9, issue 1, pages 687–695. {{doi|10.1021/nn5061207}} We report on the synthesis and optical gain properties of regularly shaped lead iodide (PbI2) platelets with thickness ranging from 10–500 nm synthesized by chemical vapor deposition methods. The as-prepared single crystalline platelets exhibit a near band edge emission of ∼500 nm. Whispering gallery mode (WGM) lasing from individual hexagonal shaped PbI2 platelets is demonstrated in the temperature-range of 77–210 K, where the lasing modes are supported by platelets as thin as 45 nm. The finite-difference time-domain simulation and the edge-length dependent threshold confirm the planar WGM lasing mechanism in such hexagonal shaped PbI2 platelet. Through a comprehensive study of power-dependent photoluminescence (PL) and time-resolved PL spectroscopy, we ascribe the WGM lasing to be biexcitonic in nature. Moreover, for different thicknesses of platelet, the lowest lasing threshold occurs in platelets of ∼120 nm, which attributes to the formation of a good Fabry–Pérot resonance cavity in the vertical direction between the top and bottom platelet surfaces that enhances the reflection. Our present study demonstrates the feasibility of planar light sources based on layered semiconductor materials and that their thickness-dependent threshold characteristic is beneficial for the optimization of layered material based optoelectronic devices. [ Platelets ~25 μm in diameter. Pumping laser 400 nm perperndicular to the platelet. ] Lead iodide, which consists of a repeating unit of a hexagonally closed- packed layer of lead ions sandwiched between two layers of iodide ions (layered material), has some unique optical and electronic properties that are quite di ff erent from traditional semiconductor gain material such as CdS, ZnO, and GaN. 1--5 In this work, we have synthesized regular-shaped PbI2 platelet with thickness ranging from 10--500 nm using a chemical vapor deposition (CVD) method. Lasing modes are supported in PbI2 platelets as thin as 45 nm, which is the thinnest planar laser ever reported. The average surface roughness of these PbI2 platelets is ∼2 nm, which is perfectly flat at optical level. The high-resolution cross-sectional TEM image (see Figure 1h) of the platelet shows that the interlayer space is around 0.703 nm, which is in good agreement with the (0001) plane spacing theoretical value. 25,26 An individual PbI 2 platelet was optically pumped using 400 nm wavelength femtosecond laser pulses qat 77 K. The optical pump con fi guration is schematically shown in Figure 2, panel a. The pump laser was focused to a spot size of ∼ 40 μm using a 20 x objective A broad spontaneous emission band centered at 500 nm with a full width at half-maximum (fwhm) of λ fwhm ≈ 6 nm can be observed under relatively lower pump fluence excitation ( e.g., P < 100 μ J/cm^2). With increased pump fluence (∼200 μ J/cm^2), a relatively sharp peak centered at around 502 nm with a λ fwhm of ∼ 3.5 nm appears at the longer wavelength side of the main spontaneous emission peak. When the pump fluence is further increased (P > 200 μ J/cm^2), the emission peak intensity increases sharply, and the fwhm of the emission peak reduces to ∼1.4 nm, which exhibits lasing action. 27,28 PbI2 Synthesis Process. Lead iodide powder (Aldrich, 99.999%) was the reaction source and placed into a quartz tube, which is amounted on a single zone furnace (Lindberg/Blue M TF55035C-1). Fresh cleaved muscovite mica substrate (1-3cm 2 ) was cleaned by acetone and then placed in the downstream region inside the quartz tube. The quartz tube was evacuated to a base pressure of 2 mTorr and then followed by a 30 sccm flow of high-purity Ar premixed with 5% H 2 gas. The temperature and pressure inside the quartz tube were set and stabilized to desired values for each halide (380  C, 200 Torr). The synthesis process was finished within 20 min, and then the furnace cooled down to room temperature naturally.