Supporting Information Orf et al. 10.1073/pnas.1101160108 SI Text Device Characterization. The electronic properties of several

metal-semiconductor-metal devices in both sandwich-type planar and fiber geometries were compared. The materials used in each form factor were selected to be as similar as possible. Selenium was selected to be the semiconductor in both geometries because of its high work function (Φ ∼ 5.9 eV), low melting temperature, and ability to easily convert between the amorphous and crystalline states. Planar devices were fabricated by thermal deposition using Zn, In, Sn, Pb, Te, Au, and ITO as electrodes (Φ ∼ 3.7– 5.1 eV) (1), and their current-voltage characteristics were determined. Consistent with previous reports, each of these contacts (with the exception of chemically similar Te) form rectifying barriers with the semiconductor owing to the large work function difference between the metal and selenium (2). Planar devices were tested at room temperature and not subjected to thermal treatments analogous to fiber drawing due to the low sublimation temperature of unconfined selenium and the tendency of the films to dewet when liquid. The screening of metal-selenium-metal combinations in fiber form was done in a similar fashion. However, because thermal drawing necessitates the metal being liquid during processing, low melting temperature metals and eutectic alloys including indium, Sn93.8 Au6.2 , Sn74 Pb26 , and Sn85 Zn15 (Φ ∼ 4–4.5 eV) were selected as electrodes in lieu of their elemental counterparts (3). Macroscopic performs consisting of different combinations of metal electrodes, a Se97 S3 semiconductor film, and insulating polymer were assembled and then thermally drawn into approximately 35 m of continuous fiber containing the device structure. Surprisingly, in contrast to the planar devices in which all combinations resulted in rectifying contacts, fiber devices exhibited non-ohmic behavior only when one electrode was Sn85 Zn15 . The direction of the rectifying behavior and the sign of the applied voltage in the Sn85 Zn15 ∕Se97 S3 ∕metal (where metal ¼ In, Sn93.8 Au6.2 , Sn74 Pb26 ) fiber devices confirmed that rectification occurs at the Sn85 Zn15 electrode and not the counter electrode. X-Ray Diffraction of Selenium and Zinc Mixtures. In order to verify that ZnSe can form in mixtures of zinc and selenium at fiber drawing temperatures, X-ray diffraction (XRD) was performed on zinc shot incubated in molten selenium. Zinc and selenium shot (Alfa Aesar) were mixed in a quartz ampoule under inert atmosphere before evacuating and sealing the ampoule. The charge was then heated to 260 °C and mechanically rocked for 4 d to maximize the signal of any nascent crystal phases. Upon cooling, the zinc shot was extracted as best as possible from the selenium matrix. Fig. S1 shows the two-dimensional XRD pattern of the sample collected with a Bruker D8 Discover diffractometer using a Vantec-2000 detector and Cu-Kα radiation. The two-dimensional detector was used to collect the data in order to increase the signal and reduce the peak intensity errors associated with potential large grain size and preferred orientation (4). The two-dimensional diffraction pattern was integrated into a more conventional linear XRD plot, shown in Fig. S2. Search-match analysis using the software PANalytical X’Pert HighScore Plus v3 determined that the phase composition of the sample was Zn, Se, and ZnSe. 1. Lide DR (2008) CRC Handbook of Chemistry and Physics (CRC, New York), pp 12–114. 2. Champness CH, Chan A (1985) Relation between barrier height and work function in contacts to selenium. J Appl Phys 57:4823–4825. 3. Orf ND, Baikie ID, Shapira O, Fink Y (2009) Work function engineering in low-temperature metals. Appl Phys Lett 94:113504.

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The search-match algorithm readily identified Zn and Se to be the dominant phases, as expected. The relative intensity of Zn diffraction peaks does not perfectly match the reference card because recrystallization produced large grain sizes, evident from the spottiness of the two-dimensional pattern in Fig. S1. All of the spotty debye diffraction rings observed in Fig. S1 correspond to Zn only, and the remaining peaks have diffraction rings that more closely match the signal expected from an ideal polycrystalline phase. Se has only minor mismatch between observed and expected peak intensity, well within experimental error. There are two unambiguous diffraction peaks that are not produced by Zn or Se at 27.29° and 53.70°, and a global search-match readily identified ZnSe as the single best material that simultaneously matched these peaks and did not require the presence of additional peaks unobserved experimentally. Furthermore, the intensity of the peak at 45.349° is greater than would be produced by Se (111) alone based on the relative intensities of the other Se peaks. There is an additional ZnSe peak at 45.31°, and it is consistent with our interpretation that the peak at 45.349° is the product of diffraction from Se and ZnSe. These three peaks (27.29°, 45.31°, and 53.7°) match the strongest diffraction peaks produced by ZnSe and would not be produced by any oxide, carbide, silicide, or silicate of Zn and/or Se. This result confirms that ZnSe can and does form when Zn and Se are mixed at the fiber drawing temperatures, as expected from the ZnSe phase diagram (5). High-Resolution Energy Dispersive Spectroscopy (EDS) Measurements.

The SEM EDS measurements in Fig. 3B (iv) indicate there is an increased concentration of zinc at the Se97 S3 ∕Sn85 Zn15 interface relative to the Sn85 Zn15 bulk electrode. Higher-resolution examination of this interface is desirable to bring greater confidence in its precise composition. Transmission-electron-microscopy-based EDS measurements should make it possible to probe the composition at Se97 S3 ∕Sn85 Zn15 interfaces in much greater detail. Samples were prepared by standard focused ion beam milling techniques and imaged using a high-angle annular dark field (HAADF) detector scanning transmission electron microscope (STEM). Composition measurements were made by EDS, and the results of these measurements in increasing magnification are summarized in Figs. S3–S6. Figs. S3–S5 very clearly show a strong correlation between location of Zn and Se (and sulfur). Recall that prior to drawing, the constructed preform consists of a Se97 S3 film placed next to a bulk Sn85 Zn15 electrode, thus the coincidence of Zn and Se was initially set to be zero. The situation is dramatically different after thermal drawing, and inspection of the Se, Sn, Zn maps suggest that the presence of zinc is more closely correlated to Se than it is Sn. Indeed, elemental quantification of seven points and a line scan through a central grain show several regions (e.g., points 1,4, and 7) where the ratio of Zn and Se is one-to-one (Fig. S6), which is strongly suggestive of formation of stoichiometric ZnSe. Note also that Zn and Se have nearly zero solubility in each other, and mixtures of Zn and Se are known to be alloys of ZnSe and pure elemental phases (5). 4. BB He, U Preckwinkel, KL Smith, (2000) Advances in X-ray analysis, Proceedings of the 48th Annual Denver X-Ray Conference 43:273–280. 5. Sharma RC, Chang YA (1996) The Se-Zn (selenium-zinc) system. J Phase Equilib 17:155–160.

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Fig. S1. XRD ring pattern of zinc shot incubated in selenium.

Fig. S2. The XRD pattern produced by integrating the two-dimensional diffraction pattern in Fig. S1. Reference patterns from the International Center for Diffraction Data are used to identify which phases produced the observed diffraction peaks.

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Fig. S3. EDS composition maps of mixed Se97 S3 ∕Sn85 Zn15 electrode. (A) STEM HAADF image of sample, dashed black box represents area of interest for Fig. S4. (B) Composite map of Zn, Se, Sn, S element maps (C–F, respectively).

Fig. S4. Magnification of Fig. S3. EDS composition maps of mixed Se97 S3 ∕Sn85 Zn15 electrode. (A) STEM HAADF image of sample, dashed black box represents area of interest for Fig. S5. (B) Composite map of Zn, Se, Sn, S element maps (C–F, respectively).

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Fig. S5. Magnification of Fig. S4. EDS composition maps of mixed Se97 S3 ∕Sn85 Zn15 electrode. (A) STEM HAADF image of sample. (B) Composite map of Zn, Se, Sn, S element maps (C–F, respectively).




5 4 1




500 nm

Fig. S6. STEM HAADF images of mixed Se97 S3 ∕Sn85 Zn15 electrode. (A) Locations of point quantifications corresponding to Table S1. (B) Results of composition line scan across crystal grain in center of image.

Table S1. Quantification of composition at seven locations corresponding to Fig. S6A Location 1 2 3 4 5 6 7

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Zn, at. % 34.50 12.45 2.48 22.46 21.13 6.57 39.97

Se, at. % 35.00 48.10 11.92 18.05 48.10 28.29 40.97

Sn, at. % 27.15 38.35 85.10 57.15 29.07 64.28 15.57

S, at. % 3.35 1.10 0.50 2.34 1.70 0.86 3.50

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