Wurtzite ZnTe Nanotrees and Nanowires on ... - ACS Publications

Jun 27, 2017 - peaks in the HT region (blue line) agree well with the phase of cubic ZB ..... Gutierrez, H. R.; Redwing, J. M.; Lew Yan Voon, L. C.; L...
0 downloads 0 Views 2MB Size
Subscriber access provided by NEW YORK UNIV

Communication

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

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.

Nano Letters 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 26

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

Nano Letters

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.

ACS Paragon Plus Environment

1

Nano Letters

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

Page 2 of 26

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

ACS Paragon Plus Environment

2

Page 3 of 26

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

Nano Letters

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

ACS Paragon Plus Environment

3

Nano Letters

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

Page 4 of 26

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

ACS Paragon Plus Environment

4

Page 5 of 26

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

Nano Letters

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