Subscriber access provided by Fudan University
Article
Solution-Synthesized In4SnSe4 Semiconductor Microwires with a Direct Band Gap Du Sun, Yihuang Xiong, Yifan Sun, Ismaila Dabo, and Raymond E. Schaak Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04216 • Publication Date (Web): 04 Jan 2017 Downloaded from http://pubs.acs.org on January 6, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Chemistry of Materials is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 5
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Chemistry of Materials
Solution-Synthesized In 4 SnSe 4 Semiconductor Microwires with a Direct Band Gap Du Sun,1 Yihuang Xiong,2 Yifan Sun,1 Ismaila Dabo,2,3 and Raymond E. Schaak1,3* 1
Department of Chemistry, 2 Department of Materials Science and Engineering, and 3 Materials Research Institute, The Pennsylvania State University, University Park, PA 16802 ABSTRACT: Semiconductor materials having direct band gaps that overlap well with the solar spectrum are important for a variety of applications in solar energy conversion and optoelectronics. Here, we identify the ternary chalcogenide In4SnSe4 as a direct band gap semiconductor having a band gap of approx. 1.6 eV. In4SnSe4, which contains isolated tetrahedral [SnIn4]8+ clusters embedded in an In-Se framework, was synthesized by precipitation from solution at 300 ºC. The In4SnSe4 product consists of microwires having lengths of approx. 5-20 µm and widths of approx. 100-400 nm. Band structure calculations predict a direct electronic band gap of approx. 2.0 eV. Diffuse reflectance UV-visible spectroscopy qualitatively validates the predicted direct band gap, yielding an observed optical band gap of 1.6 eV.
INTRODUCTION Semiconductor materials that absorb visible-wavelength and nearinfrared light have widespread importance in optoelectronic applications, including photovoltaic devices, light-emitting diodes, and laser diodes.1-3 For example, the ubiquitous elemental semiconductor silicon has an indirect band gap of 1.1 eV, which allows it to absorb incident solar radiation and renders it useful for photovoltaic devices.4,5 Compound semiconductors having direct band gaps exhibit higher absorption cross sections and faster radiative recombination rates than related indirect band gap systems.6,7 As a result, direct band gap semiconductors are useful for applications that include thin film solar cells,8,9 which benefit from strong light absorption, and diodes and lasers,10,11 because of their ability to facilitate strong and efficient light emission. Many direct band gap semiconductors are structurally related to silicon, adopting ordered variants of the diamond structure.12 Depending on the constituent elements and their arrangements in the crystal structure, the direct band gaps can range from less than 0.2 eV to more than 3 eV.1 Compound semiconductors with direct band gaps close to that of Si are particularly attractive, but the diversity of materials with such characteristics is limited. The identification of new semiconductors with direct band gaps that overlap well with the solar spectrum remains an important research goal.13,14 Among potential new materials that meet such criteria, In4SnSe4 is an intriguing target.15 As shown in Figure 1, In4SnSe4 contains isolated, tetrahedral [SnIn4]8+ clusters embedded in an In-Se framework,15 and as such is structurally distinct from other widely studied compound semiconductors. In4SnSe4 was previously accessed as single crystals by direct combination of the constituent elements or by the reaction of SnSe, InSe, and In at 557 °C, as well as through a tin flux reaction.15-17 It was noted, however, that In4SnS4, a related compound with the same crystal structure, begins to decompose slowly at temperatures well below 500 °C.16,17 With that in mind, we approached the synthesis of In4SnSe4 using a solution route that involves direct precipitation from a mixture of soluble reagents in a high-boiling organic solvent. Here, we report the synthesis and characterization of In4SnSe4 microwires and show, both experimentally and through band structure calcu-
lations, that In4SnSe4 is a direct band gap semiconductor having a band gap near 1.6 eV.
Figure 1. In4SnSe4 unit cell and the [SnIn4]8+ cluster that it contains. EXPERIMENTAL SECTION Materials. In(III) Chloride (InCl3, 99.9%, Aldrich), hexamethyldisilazane (HMDS, >99%, Aldrich), diphenyl diselenide (Ph2Se2, >99%, Aldrich), tin(II) chloride (SnCl2, 99%, Alfa Aesar), oleylamine (OLAM, technical, 70%, Aldrich), and 1-octadecene (ODE, >90%, Aldrich) were used as received without purification. All syntheses were carried out under Ar(g) using standard Schlenk techniques, and work-up procedures were performed in air. Synthesis of In4SnSe4 Microwires. In a typical synthesis, 44 mg of InCl3 (0.2 mmol), 10 mg of SnCl2 (0.05 mmol), 32.2 mg of Ph2Se2 (0.1 mmol), 5 mL of ODE, and 10 mL of OLAM were added into a 100-mL 3-neck round-bottom flask with a condenser, thermometer adapter, thermometer, and rubber septum. Magnetic stirring was started and the solution was degassed under vacuum at 120 °C for ~5-10 minutes. The flask was backfilled with Ar and cooled to 90 °C. HMDS (1 mL) was swiftly injected into the flask and the solution was then heated to 280 °C at a rate of 10 °C/min. The solution was kept at 280 °C for 1 h and then cooled rapidly by removing the flask from the heating mantle. The In4SnSe4 microwires were precipitated by adding 20 mL of a 1:1 ethanol/toluene mixture and then centrifuged at 12,000 rpm for 5 min. The precipitate was washed three times using the same solvent mixture (with centrifugation in between washes) and could
ACS Paragon Plus Environment
Chemistry of Materials
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
then be suspended in hexane, toluene, or ethanol for further characterization. Characterization. Powder XRD data were collected using a Bruker D8 Advance X-ray diffractometer equipped with Cu Kα radiation. TEM images were obtained using a JEOL 1200 EX II TEM operating at 80 kV and a FEI Talos F200X operating at 200 kV. HRTEM images and energy dispersive X-ray spectroscopy (EDX) mapping data were acquired using a FEI Talos F200X operating at 200 kV. Reflectance measurements were collected on a Perkin-Elmer Lambda 950 spectrophotometer equipped with a 150 mm integrating sphere. Samples were prepared by drop-casting a concentrated suspension of the microwires onto a glass substrate. XPS experiments were performed using a Physical Electronics VersaProbe II instrument equipped with a monochromatic Al Kα X-ray source (hν = 1,486.7 eV) and a concentric hemispherical analyzer. Charge neutralization was performed using both low energy electrons (
