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Wurtzite ZnTe Nanotrees and Nanowires on Fluorine-Doped Tin Oxide Glass Substrates Man Suk Song, Seon Bin Choi, and Yong Kim Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b01446 • Publication Date (Web): 27 Jun 2017 Downloaded from http://pubs.acs.org on June 27, 2017
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Wurtzite ZnTe Nanotrees and Nanowires on Fluorine-Doped Tin Oxide Glass Substrates Man Suk Song, Seon Bin Choi, and Yong Kim* Department of Physics, Dong-A University, Hadan-2-dong, Sahagu, Busan 49315, Korea KEYWORDS ZnTe, nanowires, Sn catalysts, wurtzite, zinc blende, micro-photoluminescence, band gap ABSTRACT ZnTe nanotrees and nanowires were grown on fluorine-doped tin oxide glass by physical vapor transport. Sn from a fluorine-doped tin oxide layer catalyzed the growth at a growth temperature of 320 ˚C. Both, the stem and branch nanowires, grew along 〈0001〉 in the rarely observed wurtzite structure. SnTe nanostructures were formed in the liquid catalyst and simultaneously ZnTe nanowire grew under Te-limited conditions, which made the formation of the wurtzite structure energetically favorable. Through polarization-dependent and powerdependent micro-photoluminescence measurements from individual wurtzite nanowires at room temperature, we could determine the so far unknown fundamental bandgap of wurtzite ZnTe, which was 2.297 eV and thus, 37 meV higher than that of zinc-blend ZnTe. From the analysis of doublet photoluminescence spectra, the valence band splitting energy between heavy hole and light hole bands is estimated to be 69 meV.
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II–VI semiconductor nanowires, as building blocks for bottom-up syntheses, have unique characteristics applicable for novel optoelectronic devices.1 These material systems generally have a wurtzite (WZ) crystal structure and polar surfaces uniquely dominate morphological varieties at the nanoscale.2,3 In particular, tree-like nanostructures with a stem and several branches offer an extremely large surface area, which provides functional specialties for device, such as photovoltaic devices, sensors, photocatalysts, and supercapacitors.4–6 Among II–VI nanowires, ZnTe is one of the most promising optoelectronic materials even though there is only a relatively limited number of reports on ZnTe nanowires. It is possible to realize applications like pure-green light-emitting diodes, photodetectors,8 and heterostructure p-n diodes9 from this semiconductor material 7 because it has a direct band gap of 2.26 eV at room temperature and a distinctive p-type conductivity, unlike other II–VI semiconductors. We achieved the growth of ZnTe nanotrees and nanowires on fluorine-doped tin oxide (FTO) glass substrates. This particular design of branched nanowires on a conductive transparent oxide glass is highly desirable for the fabrication of optoelectronic devices.10 Previously, we have reported the Sn-catalyzed growth of CdS nanowires11 and branched nanostructures12 without using conventional Au catalysts that may cause deep level defects, and thus deteriorate the electronic and optical properties in semiconductors. Furthermore, the use of a Sn catalyst supplied from the FTO substrate itself resulted in the exceptional zinc-blende (ZB) crystal structure of CdS nanowires. Moreover, we could prove that Sn with its low surface energy leads to morphological and crystallographic changes in the CdS nanowires. For ZnTe considered in this work, Δ , which is the energy difference between the WZ and ZB structure, was calculated to be approximately 6.4 meV/atom, indicating that the cubic ZB structure is more
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stable than the hexagonal WZ structure.13 Because of this energy difference, ZnTe nanostructures have been reported to have a ZB structure in most studies,8,14–20 including our previous works.21– 23
WZ ZnTe has been rarely found and thus, the material properties including the fundamental
bandgap is largely unknown. Despite the lack of available experimental data, there are several theoretical predictions of the fundamental bandgap of WZ ZnTe 24–27. However, these theoretical results are too widely scattered to reach any consensus. WZ ZnTe has been observed only in the nanoscale regime. For example, a WZ ZnTe shell was epitaxially grown on WZ CdSe core nanowires, surrounding the same.28 ZnTe nanorods or nanocrystals fabricated by a solutionbased synthesis were also found to have a WZ structure.29 Further, ultra-thin ZnTe nanowires with a small diameter (∼ 17 nm) were grown at low temperature by molecular beam epitaxy.30 In all these cases, it was not possible to detect photoluminescence (PL) of ZnTe, which may provide an insight on the fundamental bandgap, because it was completely quenched due to a rapid electron-hole pair separation, which hinders radiative recombination. In this paper, we studied WZ ZnTe nanotrees and nanowires grown by a Sn-catalyzed synthesis at a relatively low temperature. Furthermore, we observed a substantial radiative recombination in micro-photoluminescence (µ-PL) measurements of individual WZ ZnTe nanotrees/nanowires conducted at room temperature. Thus, we could provide the evidence that the fundamental bandgap of WZ ZnTe and the splitting energy between heavy hole and light hole band, which has not reported so far. ZnTe nanotrees and nanowires were grown by physical vapor transport employing a conventional single zone furnace with a one inch diameter quartz tube (Lindberg Blue M Mini Mite, Thermo Scientific). A quartz boat containing ZnTe powder (100 mg, Alfa Aesar 99.99%) was placed in the center zone where the ZnTe source temperature was set to 800 ˚C. Two FTO
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glass substrates (10 mm × 12 mm × 1.1 mm, AGC Asahi Glass) were cut and cleaned. Then, one was placed 17–18 cm downstream from the center. The other FTO glass substrate was situated 14–15 cm downstream for additional Sn vapors to be transferred onto the surface of the as-grown ZnTe nanowires and to induce ZnTe nanowire branching at the other substrate. One substrate served as source of Sn while the other was indeed a substrate for nanowire growth. Figure S1a in the Supporting Information illustrates the schematic of the physical vapor transport system and its temperature profile, which was obtained by calibration with a thermocouple. According to the profile, the temperature range of the substrates was 320–430 ˚C, whereas that of the FTO glass which provides Sn vapor for branching was 620–670 ˚C. Prior to the growth, the system was evacuated by a mechanical pump for half an hour (base pressure ~10-2 Torr); then, N2 carrier gas mixed with 10% H2 was introduced at a flow rate of 200 sccm. The ramp-up time to 800 ˚C was 15 min, followed by holding period for a growth time of 1 h while keeping the pressure at 100 Torr. The sample was cooled to room temperature through the flow of the carrier gas. The morphologies of as-grown ZnTe nanotrees and nanowires were observed by a fieldemission scanning electron microscope (FESEM, JEOL JSM-6700F). X-ray diffraction (XRD) patterns were measured by an X-ray diffractometer (Rigaku Ultima IV). Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) patterns were measured by a TEM (JEOL, JEM2010). Energy-dispersive X-ray spectra (EDX) were obtained using an EDX system (Oxford Instruments, INCA) attached to the TEM. EDX mapping images were acquired using another EDX system (Oxford Instruments, X-Max) attached to another TEM (JEOL, JEM-2100F). TEM specimens were prepared by immersing samples in an ethanol-filled Eppendorf tube, which was sonicated for 10 s to separate the ZnTe nanotrees and nanowires
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from their glass substrates; these nanostructures were then dispersed by a micropipette onto a carbon grid. µ-PL measurements of individual ZnTe nanotrees and nanowires were carried out by utilizing a home-built µ-PL system. A bandpass-filtered Ar+ laser (488 nm) was guided into a modified commercial microscope (Olympus BX60M) with a commercial Raman filter cube (Semrock). The laser beam power was attenuated by a variable neutral density filter with optical density from 0 to 4.0 (Thorlabs). The filter cube was composed of an exciter filter (edge steepness
