Manganese doped CdSe nanosheets: Optical and magnetic properties Oindrila Halder, and Satchidananda Rath
Citation: AIP Conference Proceedings 2005, 030008 (2018); doi: 10.1063/1.5050739 View online: https://doi.org/10.1063/1.5050739 View Table of Contents: http://aip.scitation.org/toc/apc/2005/1 Published by the American Institute of Physics
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ManganeseDoped CdSe Nanosheets: Optical and Magnetic Properties Oindrila Halderb) and Satchidananda Ratha) School of Basic Sciences, Indian Institute of Technology Bhubaneswar, Argul-Jatni -752 050, Khurda, India. a)
Corresponding author:
[email protected] b)
[email protected]
Abstract.Cadmium selenide (CdSe) nanomaterial of different shape and sizes has been considered as a suitable material in optoelectronics, lasing devices and sensoric applications.Moreover, the increasing technological demands propose the improvement of their performance.One of theapproach towards controlling the properties is, through doping transition metals. Interestingly, Manganese (Mn) doping introducesspin degrees of freedom into the electronic structure of the CdSeyielding improvedoptical and magnetic properties for accomplishment of next generationopto-spintronics and memory devices. This coexistence of tunablemagnetic and optical properties aspires to this current work where we have synthesized and optimized the Mn doping in ultrathin (thickness 1.5nm) CdSe layered nanosheets (LNS) by a scaffold mediated solvothermal technique. Themagnetic and photoluminescence propertiesexhibitedspin allowed radiative transitionsin comparison with un-doped CdSe LNSs.The schematic of spin-active states rendering selective optical and magnetic properties are shown in Fig. 1.
Figure1.Schematic of the feasible transitions and properties acquired by Mn doping in CdSenanosheets.
INTRODUCTION Insertion of spin degrees of freedom into semiconductors by doping with transition metal activates magnetic ordering suitable for spintronic[1],[2] and memory storage[3] applications. The growing research on magnetic ion doped semiconductor, e.g. manganese (Mn) doped II-VI[4] and III-V semiconductors[5],[6]aredevoted to scale-up the order parameter of the interaction between charge carriers and magnetic ions within the semiconductor lattice[3]. In case of Mn doped bulk cadmium selenide (CdMnSe), Z. Nabi et al[7] observed antiferromagnetic (AF) ordering due to short range d–d interaction, whereas, S. Ghosh et al[8]using density functional calculation suggests ferromagnetic (FM) ordering.Depending on the ionic coordination and concentration of the Mn dopant many have proposed the presence of intriguing magnetic states in the Mn doped CdSe systems[9]. Even though, there areopen concerns over the solubility of Mn in the lattice, we have accomplished the doping of Mn into CdSe layered nanosheets (LNSs) lattice followed by successive evaluation of their optical and magnetic properties.
EXPERIMENTAL Mn doped CdSe LNSs were prepared by solvothermal process using cadmium and manganese precursors from the respective cadmium chloride and manganese chloride salt solution. The surfactant octylamine was added to the source precursor for functionalization undera vigorous stirring at constant temperature. Then selenium precursor was injected into the source solution for nucleation and heated at 400 K for 24 hours for growth of the layered nanosheets.At the end, trioctylphosphine oxide(TOPO) was used forcleaning process and further characterizations were performed Advanced Materials AIP Conf. Proc. 2005, 030008-1–030008-4; https://doi.org/10.1063/1.5050739 Published by AIP Publishing. 978-0-7354-1721-2/$30.00
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RESULTS AND DISCUSSIONS Raman scattering of the samples were performed using Horiba T64000 Raman set up at 480 nm fromArgon ion excitation source. The spectrum is shown in Fig. 2. As observed from the Raman spectrum, a series of Raman modes are originating at 178.318 cm-1 (P1), 205.160 cm-1 (P2), 235.563 cm-1 (P3), 256.247 cm-1 (P4), 303.763 cm1 (P5) and 414.286 cm-1 (P6). The intense lines of P2and P6 are attributed to the first order (1LO) and second order (2LO) longitudinal optical (1LO) phonon modes of the hexagonal phase CdSe[10]. Furthermore, the peak position of transverse optical (TO) phonon at P1 is observed as the TO line is responsible for the lattice vibration along caxis of wurtzite phase, the blue-shift in TO mode of the CdSe LNSs gives strong compressive strain along c-axis [11]. The peaks at P4 and P5 are referring to the phonon modes of the Manganese Selenide (MnSe) – like zincblend (ZB) and wurtzite (WZ) phase respectively[12]. Such different phases are observed due to the presence of multivalence Mn. The peak, P3 may be arising from the single or double substitution like states forming dimers. This may be visualized by considering the Mn doping mechanism in the CdSe clusters as observed by G. L. Gutsev et al[13]. Following this, the MnSe clusters of different phases are randomly distributed throughout the 2D-crystal to stabilize the lattice of the LNSs.
Figure 2. Raman spectrum of CdMnSe LNS. The excitation line, 488nm excitation at 200mW power was used for the study.
The magnetic properties of the samples were investigated using magnetization Physical Property Measurement System at different temperature. Fig 3 (a-d) shows the magnetic moment (M) versus the magnetic field (H) plots at temperature. 1K , 10 K, 150 K and 300 K. As observed from Fig. 3, the M~H curve measured at 300 K varies linearly with low magnetic field upto 5000 Oe and achieve a saturation beyond it. This indicates a presence of high temperature ferromagnetism of weakly dispersed small clusters (superparamagnetism). On the other hand, the M~H curve at low temperature shows a hysteresis loop with finite remanence and coercivity which is much higher than that of the superparamagnetics sample. Again, the inverse susceptibility (1/χ) versus temperature (T) plot shown in Fig. 4 (a) reveals a sharp magnetic ordering at 48 K.Therefore, two kinds of magnetic ordering are observed in our sample: one with critical temperature about 48 K, and the second one with critical temperature well above room temperature. Further, for deeper understanding of the spins orientations we have performed the Curie-Weiss analysis using relation[14], χ = χd +
where θ, C and
χd
C T −θ
are the Curie-Weiss temperature, Curie constant and diamagnetic susceptibility of the host
respectively. As the Curie-Weiss temperature is related to the exchange interaction, J by the relation [15], θ=
2S (S+1) x ∑i Zi Ji 3k β
where S, x ,zi and kβ are the Spin, number of ith nearest neighbors and Boltzman constant, respectively. Considering, the Mn2+ ions S=5/2 and zi =12 and hexagonal CdSe LNS lattice; the exchange interaction J was kβ
estimated to be -10.12 K.
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Figure 3. Magnetization versus temperature plots at (a) 1 K, (b) 10 K, (c) 150 K and (d) 300 K of Mn doped CdSe nanosheets.
The observed value of the effective moment is comparatively higher than the theoretically obained moment [8],[16], [17] of high spin Mn2+ state which may be due to the co-existence of mixed valence states of Mn and its dimeric form. Fig.4 (b) shows the temperature dependent change in the coercivity (Hc). This may be due to the magnetic dipolar interactions among the inter particles due to the change in temperature. The Ms (saturation magnetization) curve shown in Fig. 4 (c) revealed the decrement in Ms upon increasing temperature, which may be due to the surface spin disorder [18].
(a)
(b)
(c)
Figure 4. (a)the inverse susceptibility (1/χ) versus temperatureplot (b)Coercivity versus temperature changes is plotted in red dots and (c) the saturation magnetizationchange of the M~H loops at different temperature represented in pink triangles.
The optical properties of the samples were analyzed using optical absorption and photoluminescence (PL) measurements. The absorption and PL spectra are shown in Fig. 5 indicated by right and left arrows respectively. The absorption spectrum consists of a sharp excitonic peak at 2.72 eV and 2.87 eV which is corresponding to the electronic transitions between heavy hole (1hh) and light hole (1lh) to conduction (1e) band respectively. The features observed at higher energy may be due to the intrinsic higher order states. As observed from Fig. 5, the PL spectrum exhibits three peaks at energy 2.72 eV ± 0.1, 2.30 eV ± 0.1 and 1.89 eV ± 0.3 respectively. In comparison with absorption, the emission line at 2.72 eV may be assigned as a near band edge emission (NBE) of the LNSs. Further, as the Mn2+ (five localized electrons in the d-orbital) ion experience strong exchange interactions [20]–[23] leading to the splitting of d-orbital into two group as excited 4T1 state and the ground 6A1. Therefore, the emission line that appeared at 2.30 eV may be attributed to the spin (S =5/2) forbidden 5T1 → 6 A1transition. Similarly, for the Mn3+ (four localized electrons in the d-orbital) ion which acts as the next neighboring dopant, produces lattice strain due to the anisotropy of the LNSs. Under this environment, the dorbital undergoes a Jahn-Teller consequence and split into excited5E state and the ground5T2 state by a gap of 1.80 eV energy as a combined interaction of the crystal field and the lattice strain [24]. Hence, the emission line observed at 1.89 eV might be arising from the 5E → 5T2 radiative transitions.
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FIGURE 5. Optical absorption spectrum (indicated by right arrow) and photoluminescence spectrum (indicated by left arrow) of Mn doped CdSe nanosheets. The colored spectra are the fitted lines with experimental data using Gaussian function.
CONCLUSION In summary, the doping of Mn into the ultra-thin (thickness ~1.5 nm) CdSe nanolayer lattice has been achieved and confirmed from the Raman scattering measurements. The spin degrees of freedom introduced by multivalent Mn dopant in the CdSe LNSs accomplished the spin active 5T1 → 6A1and 5E → 5T2 radiative transitions yielding luminescence at 2.30 eV and 1.89 eV in addition to the NBE at 2.72 eV. The interactions of localized d-electrons from Mn2+ and Mn3+ with CdSe LNS lattice depicts the magnetic ordering at ~48K.This affirms the successful doping of Mn in CdSe LNS with luminescence covering the visible spectrum suitable for spin active device applications.
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