The effect of dopant and optical micro-cavity on the photoluminescence of Mn-doped ZnSe nanobelts
© Zhou et al.; licensee Springer. 2013
Received: 14 May 2013
Accepted: 7 June 2013
Published: 5 July 2013
Pure and Mn-doped ZnSe nanobelts were synthesized by a convenient thermal evaporation method. Scanning electron microscopy, X-ray powder diffraction, energy dispersive X-ray spectroscopy and corresponding element mapping, and transmission electron microscope were used to examine the morphology, phase structure, crystallinity, composition, and growth direction of as-prepared nanobelts. Raman spectra were used to confirm the effective doping of Mn2+ into ZnSe nanobelts. Micro-photoluminescence (PL) spectra were used to investigate the emission property of as-prepared samples. A dominant trapped-state emission band is observed in single ZnSe Mn nanobelt. However, we cannot observe the transition emission of Mn ion in this ZnSeMn nanobelt, which confirm that Mn powder act as poor dopant. There are weak near-bandgap emission and strong 4T1 → 6A1 transition emission of Mn2+ in single and nanobelt. More interesting, the 4T1 → 6A1 transition emission in nanobelt split into multi-bands. PL mapping of individual splitted sub-bands were carried out to explore the origin of multi-bands. These doped nanobelts with novel multi-bands emission can find application in frequency convertor and wavelength-tunable light emission devices.
Recently, doped one-dimension (1D) semiconductor nanostructures are especially attractive for their excellent and unique optical and optoelectronic properties [1, 2], which were affected greatly by optical micro-cavity and dopant. 1D nanostructures doped with transition metal (such as Cr, Mn, Fe, Co, and Ni), which can find extensive application in spintronics and nanophotonics [3–5], show novel emission and interesting magnetic transport properties. For example, single crystalline Ga0.95Mn0.05As nanowires show temperature-dependent hopping conduction . Cu-doped Cd0.84Zn0.16S nanoribbons show four orders of magnitude larger photocurrent than the undoped ones, demonstrating potential application in photoconductors and chemical sensors . The emission of transition metal ion has specific wavelength, such as the emission of manganese (Mn) ion which is located generally at 585 nm. Moreover, 1D nanostructures can confine the coherent transport or transmission of photon to the definite direction, that is, 1D nanostructures can form optical micro-cavity easily and work as effective optical waveguide within a nanometer scale . Recently, there is an increasing research interest on the optical micro-cavity and corresponding multi-mode emission spectra in doped 1D nanostructures . Zou et al. observed multi-mode emission from doped ZnO nanowires due to F-P cavity effect . Multi-mode emission was also observed in In x Ga1 - xN superlattice . Except for the inorganic semiconductor nanostructures, organic nanofibers can also act as coherent random laser with multi-mode emission . Recent research shows that the formation of multi-intracavities plays an important role in the multi-mode emission . These multi-intracavities can couple to produce coherent emission. These confined cavities and multi-band emission of 1D nanostructures are affected strongly by synthesis parameter and deliberate doping. The optical properties of 1D nanostructures are sensitive to minute change of crystal quality, crystal defect, and dopant. The latter can introduce defect state and is therefore very important. So, it is necessary to investigate the direct correlation between dopant and optical properties within the nanometer scale.
ZnSe, a direct semiconductor with a bandgap of 2.63 eV at room temperature, shows excellent optical properties and potential application in light emitting diode and laser diode. 1D ZnSe nanostructures possess novel light emission property . Recently, Vugt et al. observed the novel light-matter interaction in ZnSe nanowires, which can be used to tailor waveguide dispersion and speed of propagating light . In this paper, we synthesize three Mn-ZnSe nanobelts using different dopant compounds. Transmission electron microscopy (TEM) and scanning near-field optical microscopy (SNOM) techniques were used to provide simultaneous investigation on the micro-structure and crystallinity, micro-PL spectrum, and mode-selected mapping image. Both near-bandgap emission and trapped-state emission of ZnSe are observed in Mn-ZnSe nanobelts obtained using Mn powder as dopant. However, the Mn ion transition emission cannot be observed in this ZnSeMn nanobelt. Using manganese chloride (MnCl2) as dopant, strong Mn ion transition emission and weak near-bandgap emission are observed. We can also observe the strong Mn ion transition emission and weak near-bandgap emission in the Mn-ZnSe nanobelts obtained using manganese acetate as dopant. More interestingly, the Mn ion transition emission can split into multi-mode emission due to multi-Fabry-Pérot cavity effect in the nanobelt. Raman spectrum was used to confirm the effective doping. These results are helpful in understanding the effect of dopant on the optical micro-cavities and multi-mode emission. These Mn-ZnSe nanostructures can find promising applications in multicolor emitter or wavelength selective photodetector.
The 1D Mn-ZnSe nanobelts were synthesized by a simple thermal evaporation method. Commercial grade mixed powder of ZnSe and Mn or MnCl2 or manganese acetate (Mn(CH3COO)2) with a weight ratio of 5:1 was used as source material. The obtained samples were labeled as ZnSeMn, , , respectively. The other synthesis processes are similar with our previous report . The evaporation temperature, growth temperature, and growth time are set to 900°C, 600°C, and 45 min, respectively. A yellow product deposited on the silicon wafer after the furnace cools down to room temperature. For comparison, the pure ZnSe nanobelts were also synthesized using ZnSe powder as source material.
XRD (D/max-5000, Rigaku Corporation, Tokyo, Japan), E-SEM (QUANTA 200, FEI, Hillsboro, OR, USA), energy dispersive X-ray spectroscopy (EDS; attached to SEM), and TEM (JEM-3010, JEOL Ltd., Tokyo, Japan) were used to examine the phase structure, crystallinity, and composition of the as-prepared nanobelts. Raman spectroscopy was performed in a confocal microscope (LABRAM-010, HORIBA Ltd., Kyoto, Japan) using He-Ne laser (632.8 nm) as excitation light source. The PL and corresponding mapping were obtained by SNOM (alpha 300 series, WITec GmbH, Ulm, Germany) with He-Cd laser (325 nm) as excitation source at room temperature. In all optical experiments, the excitation signal illuminated perpendicularly onto the sample surface.
Results and discussion
We synthesized pure and Mn-doped ZnSe nanobelts successfully using thermal evaporation method. Mn can dope effectively into ZnSe crystal when MnCl2 or Mn(CH3COO)2 were used as dopants in the source material. EDS mapping indicates that the distribution of Mn is inhomogeneous in the nanobelt. All of these doped nanobelts grew along the <111> direction. HRTEM demonstrates that there are a lot of defect states in the nanobelt. Raman spectra confirm that Mn2+ was doped into and nanobelts successfully. The optical properties are affected strongly by the concentration and spatial distribution of the dopant. Optical micro-cavity also plays an important role to the emission property. Nanobelt shows strong 4T1 → 6A1 transition emission of Mn2+. However, the 4T1 → 6A1 transition emission of Mn2+ in nanobelt splits into many narrow sub-bands due to the formation of integrated multi-Fabry-Pérot cavities, which can couple to produce coherent emission with selected wavelength and cavity mode. PL mapping confirms that there are several micro-cavities in the single nanobelt. Such doped nanobelts with integrated multi-micro-cavities and modulated emission wavelength can be optimized to fabricate nanophotonic devices and quantum coherent modulators.
WZ got his PhD degree in 2010. He is an assistant professor now. RL is an associate professor. DT and BZ are professors.
We thank the NSF of China (term nos.: 51102091, 91121010, 90606001, and 20873039), Research Fund for the Doctoral Program of Higher Education of China (no.: 20114306120003), Program for Changjiang Scholars and Innovative Research Team in University (PCSIRT, no.: IRT0964), and Hunan Provincial Natural Science Foundation (11JJ7001) for the financial support.
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