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The Cavity Enhanced Microphotoluminescence in a CoreShell N-P CdS/CdO Micrometer Wire and Its Efficient Surface Photovoltage Responses in Whole Visible Range Shuangyang Zou, Weichang Zhou, Ruibin Liu, and Bingsuo Zou J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04053 • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 21, 2017
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The Journal of Physical Chemistry
The Cavity Enhanced Microphotoluminescence in a Core-Shell n-p CdS/CdO Micrometer Wire and its Efficient Surface Photovoltage Responses in Whole Visible Range Shuangyang Zou, a# Weichang Zhou, b# Ruibin Liu, a Bingsuo Zou, a*
a.
Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems,
Beijing Institute of Technology, Beijing 100081, China; b.
Key Laboratory of Low-dimensional Quantum Structures and Quantum Control
of Ministry of Education, College of Physics and Information Science, Hunan Normal University, Changsha 410081, China # These authors contributed equally to this work.
Abstract. A one-step growth of yellow and red CdS:O wire have been realized by SnO2 catalyzed CVD technique. The yellow and red CdS:O wires were obtained upon the temperature rising rates in the tube furnace. Yellow nanowire with less oxygen doping shows luminescence behavior from the acceptor trapped excitons near the bandedge
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of CdS nanowire. The XRD and TEM characterizations proved their formation of CdS/CdO core-shell red wire due to O segregations in CdS lattice when the oxygen doping concentration higher than 4%, even up to 20%. Their micro-luminescence profiles are mainly caused by the CdO shell in the red wire, which give temporal localized exciton emissions near the CdO band-edge(600nm) by fs pulse excitation and redshift enhanced WGM cavity mode emissions by 488nm CW laser excitation. The luminescence of CdS/CdO wire reflects the electronic state modification for CdO nanowires with no luminescence due to the p-n junction formation. The photovoltaic (PV) spectra of yellow and red wire aggregates show CdS and CdS/CdO profiles respectively. The CdS/CdO wires have an extended PV responses from 510nm to 800nm as compared with CdS, the photo-carrier type in the 300-510nm range likes n-CdS while that in the 510-800nm range is p-type from CdO shell. Both luminescence and PV responses could be enhanced by the WGM mode and core-shell N-P junction within a wire with dual PV zones. Such wire can find potential applications in the solar cells and nanophotonic devices.
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Introduction The semiconductor nanowires attract much attention due to their 2d exciton confinement and light waveguide, which modulate their optical properties by tuning their structure, composition and morphology, which can find applications
in
lasing,
optical
connect,
sensing,
solar
cell
and
photo-detecting.1-5 Doping techniques by CVD techniques are often used in the semiconductor preparation to tune its properties or response range. Minor amount doping usually did not change its structure, but modify its electrical and optical properties.6 Semiconductor nanostructures with minor transition metal (TM) ion dopants in nanowire can produce clear emission of d-d transition,7 even tunable emissions with TM ion aggregates in matrix. Epitaxial growth of core-shell nanowire in gas phase can modify their optical properties efficiently.8 Heavy or serious doping by CVD may modify the structure and properties significantly by forming alloyed, for example, CdSSe nanowires or belts to lead to their tunable bandgaps, then producing tunable emission and lasing for one wire.9-10 Similar structure can even happen in the solution growth for nanowire11 and QDs12. Depending on the preparation techniques and process for efficient vapor deposition, core-shell and superlattice nanowire by different materials have been obtained for different applications. Dai et al produced the CdS/SnS2 superlattice by CVD, which show an excitonic lattice.13 Yang reported the core-shell silicon P-N junction nanowire14 for solar cell and Si-Ge superlattice15 nanowire by step by step MOCVD growth with periodical luminescence. Liber et al also reported the tunable emission and lasing from the alloyed III-V semiconductor nanowires with triangle cross-section by similar technique.16
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These core-shell heterostructure and core-shell p-n junction nanowire can even enhance or modify their optical and electrical properties, for example to give enhanced LED emission and solar cell efficiency. For II-VI or III-V core-shell structures usually contribute efficient luminescence or lasing. For example, Si/CdS core-shell nanowire works as efficient LED17 and GaAs/GaAsP coaxial core−shell nanowire give lasers18. Chen obtained ZnO P-N Homojunction nanowire array for near UV LED by in situ CVD technique.19 Even more works on the oxide-related core-shell structure for solar cell or energy devices were published.20-24 Therefore a lot of different techniques have been developed for such heterostructure nanowire preparations, especially those with p-n junction has better expectation. II-VI semiconductors often show n-type behavior, but it can be modified by doping. Zhang and Lee et al synthesized n-type CdSe NWs with tunable conductivity and investigated the photoelectrical characteristic with In doped.25 Akimoto et al26 fabricated ZnSe p-n junction by doping ZnSe with O and Ga respectively via molecular beam epitaxy, which give blue electroluminescence band below the ZnSe band-edge. Wu et al27 have prepared CdS:O film on the surface of CdTe film by rf sputtering, which showed an enhanced photovoltaic (PV) response as compared with CdS/CdTe film solar cell. This CdS:O film was identified to contain SOx species in the CdS lattice.28 Meysing et al29 even detected 40% O atom% in the CdSO film for enhanced PV responses. These findings indicate O incorporation may produce p-type phase in II-VI semiconductor film. The above doping examples basically focus on the few-atoms doping in matrix without structural transformation, or two compounds mixed to form alloy with same structures by dynamically
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deposition into a film, only their lattice lengths have small deviations. These two situations happen for the doping element can incorporate into a lattice or form same structures with counterpart element as the initial element does in a crystal. If two substances with independent crystal structures and comparable amounts co-deposit together by evaporation, can one of them stay in another initial semiconductor crystal matrix to form smooth nanowire or belt? What special properties can happen in such alloy structure? There are no definite answers so far. Here we would like to present an example to see the optical property modulations after an alloyed semiconductor CdSO wire form with miscellaneous structures after enough long time annealing if the oxygen concentration is high enough comparable with sulfur. Oxygen, is also VI element as S and Se, CdO is rocksalt structure but CdS is wurtzite structure. It is difficult to codeposit onto a lattice like CdSSe alloy. Earlier reports indicated that CdS:O film was formed by the high energy RF sputtering,26 which is an forced deposition or collision. By careful experiment for CdS nanowire catalyzed growth in an environment with varied oxygen content, different CdS wires can be obtained. The CdS nanowires doped with low O concentration (
