Micro‐Raman investigation of transition‐metal‐doped ZnO nanoparticles

We measured the Raman spectra of ZnO nanoparticles (ZnO‐NPs), as well as transition‐metal‐doped (5% Mn(II), Fe(II) or Co(II)) ZnO nanoparticles, with an average size of 9 nm. A typical Raman peak at 436 cm⁻¹ is observed in the ZnO‐NPs, whereas Zn_1−xMn_xO, Zn_1−xFe_xO and Zn_1−xCo_xO presented characteristic peaks at 661, 665 and 675 cm⁻¹, respectively. These peaks can be related to the formation of Mn₃O₄, Fe₃O₄ and Co₃O₄ species in the doped ZnO‐NPs. Moreover, these samples were analyzed at various laser powers. Here, we observed new vibrational modes (512, 571 and 528 cm⁻¹), which are specific to Mn, Fe and Co dopants, respectively, and ZnO‐NPs did not reveal any additional modes. The new peaks were interpreted either as disorder activated phonon modes or as local vibrations of Mn‐, Fe‐ and Co‐related complexes in ZnO. Copyright © 2007 John Wiley & Sons, Ltd.     

Multiphonon resonant Raman scattering in N‐doped ZnO

Multiphonon resonant Raman scattering in N‐doped ZnO films was studied, and an enhancement of the resonant Raman scattering process as well as longitudinal optical (LO) phonon overtones up to the sixth order were observed at room temperature. The resonant Raman scattering intensity of the 1LO phonon in N‐doped ZnO appears three times as strong as that of undoped ZnO, which mainly arises from the defect‐induced Raman scattering caused by N‐doping. The nature of the 1LO phonon at 578 cm⁻¹ is interpreted as a quasimode with mixed A₁ and E₁ symmetry because of the defects formed in the ZnO lattice. In addition, the previously neglected impurity‐induced two‐LO‐phonon scattering process was clearly observed in N‐doped ZnO. Copyright © 2009 John Wiley & Sons, Ltd.     

Role of Fe doping on structural and vibrational properties of ZnO nanostructures

In this report, Raman and Fourier Transform Infrared (FTIR) measurements were carried out to study the phonon modes of pure and Fe doped ZnO nanoparticles. The nanoparticles were prepared by sol–gel technique at room temperature. The X-ray diffraction measurements reveal that the nanoparticles are in hexagonal wurtzite structure and doping makes the shrinkage of the lattice parameters, whereas there is no alteration in the unit cell. Raman measurements show both E₂^lowE_{2}^{\mathrm{low}} and E₂^HighE_{2}^{\mathrm{High}} optical phonon mode is shifted towards lower wave number with Fe incorporation and explained on the basis of force constant variation, stress measurements, respectively. In addition, Fe related local vibrational modes (LVM) were observed for higher concentration of Fe doping. FTIR spectra reveal a band at 444 cm⁻¹ which is specific to E ₁ (TO) mode; a red-shift of this mode in Fe doped samples and some surface phonon modes were observed. Furthermore, the observation of additional IR modes, which is considered to have an origin related to Fe dopant in the ZnO nanostructures, is also reported. These additional mode features can be regarded as an indicator for the incorporation of Fe ions into the lattice position of the ZnO nanostructures.     

Raman investigations of Zn1−xCoxO nanocrystals: role of starting precursors on vibrational properties

The Raman spectra of sol–gel derived Co‐doped ZnO nanoparticles (NPs) in the spectral range 100–1500 cm⁻¹ were investigated. In the sol–gel method, three different series of Co‐doped ZnO particles, i.e. Zn_1−xCo_xO (x = 0.05, 0.10, 0.15, and 0.20), were obtained using three different starting precursors, viz. cobalt chloride hexahydrate, cobalt acetate tetrahydrate, and cobalt nitrate hexahydrate, respectively. It has been observed that cobalt acetate is a better precursor in comparison to cobalt chloride and cobalt nitrate to obtain single‐phase Co‐doped ZnO NPs. As for cobalt acetate‐derived NPs, no hidden secondary phase of Co₃O₄ was observed for the lower (x = 0.05) Co concentration. The Fröhlich interaction associated with the longitudinal modes was found to be destroyed with increasing Co concentration due to structural disorder and defects induced by the dopant. In addition to ZnO and Co₃O₄ vibrational modes, a few additional modes near 550 and 715 cm⁻¹ were also observed in all cases, which could be attributed to the modes due to Co doping in ZnO. Copyright © 2011 John Wiley & Sons, Ltd.     

Raman spectroscopy of Cu doping in Zn1−xCoxO diluted magnetic semiconductor

The room‐temperature ferromagnetism and the Raman spectroscopy of the Cu‐doped Zn_1−xCo_xO powders prepared by the sol–gel method are reported. The x‐ray diffraction (XRD) data confirmed that the wurtzite structure of ZnO is maintained for ZnO doped with Co below 10 at%. The magnetization–field curves measured at room temperature demonstrated that all Co‐doped ZnO powders were paramagnetic. Ferromagnetic ordering is observed for the samples doped with Cu in Zn_0.98Co_0.02O and strongly depends on the concentration of Cu. The relative strength of the second‐order LO peak to the first‐order one in the Raman spectra, which is related to the carrier concentration, of the Cu‐doped Zn_0.98Co_0.02O powder is strongly correlated with the saturation magnetic moment of the system. This seems to be in favor of the Ruderman‐Kittel‐Kasuya‐Yosida (RKKY) or double exchange mechanism of the ferromagnetism in this system. Copyright © 2009 John Wiley & Sons, Ltd.     

Low‐temperature synthesis and Raman scattering of Mn‐doped ZnO nanopowders

Nanocrystalline Mn‐doped zinc oxides Zn_1−xMn_xO (x = 0–0.10) were synthesized by the sol–gel technique at low temperature. The calcination temperature of the as‐prepared powder was found at 350 °C using differential thermal analysis. A thermogravimetric analysis showed that there is a mass loss in the as‐prepared powder till 350 °C and an almost constant mass till 800 °C. The X‐ray diffraction patterns of investigated nanopowders calcined at 350 °C correspond to the hexagonal ZnO structure without any foreign impurities. The average grain size of the nanocrystal that was observed around ∼25–40 nm from transmission electron microscopy matched well with the crystallite size calculated from the line shape of X‐ray diffraction. The chemical bonding structure in Zn_1−xMn_xO nanopowders was examined using X‐ray photoelectron spectroscopy techniques, which indicate substitution of Mn²⁺ ions into Zn²⁺ sites in ZnO lattice. Micro Raman spectroscopy confirmed the insertion of Mn ions in the ZnO host matrix, and similar wurtzite structure of Zn_1−xMn_xO (x < 10%) nanocrystals. Temperature‐dependent Raman spectra of the nanocrystals displayed suppression of luminescence and enhancement in full width at half maximum in pure ZnO nanocrystals with increase in temperature, which suggests an enhancement in particle size at elevated temperature. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman study of structural disorder in ZnO nanopowders

Raman scattering spectroscopy has been used for the characterization of zinc oxide nanoparticles obtained by mechanical activation in a high‐energy vibro‐mill and planetary ball mill. Raman modes observed in spectra of nonactivated sample are assigned to Raman spectra of the ZnO monocrystal, while the spectra of mechanically activated samples point out to the structural and stoichiometric changes, depending on the milling time and the choice of equipment. Observed redshift and peak broadening of the E₂^high and E₁ (LO) first‐order Raman modes are attributed to increased disorder induced by mechanical milling, followed by the effects of phonon confinement due to correlation length decrease. The additional modes identified in Raman spectra of activated ZnO samples are related to the surface optical phonon modes, due to the intrinsic surface defects and presence of ZrO₂as extrinsic defects introduced by milling in zirconia vials. Copyright © 2009 John Wiley & Sons, Ltd.     

InGaN薄膜的紫外共振喇曼散射

利用325nm紫外光激发,对不同组分的In_xGa_1-xN薄膜的喇曼散射谱进行了研究.在光子能量大于带隙的情况下,观察到显著增强的二阶A₁(LO)声子散射峰.二阶LO声子峰都从一阶LO声子的二倍处向高能方向移动,移动量随样品In组分的增加而增大,认为是带内Frhlich相互作用决定的多共振效应引起的.分析了一阶LO声子散射频率和峰型与In组分的关系.在喇曼谱中观察到样品存在相分离现象,并与X射线衍射的实验结果进行 关键词： xGa_1-xN合金')" href="#">In_xGa_1-xN合金 紫外共振喇曼散射 二阶声子 相分离     

Absence of room temperature ferromagnetism in transition metal doped ZnO nanocrystalline powders from PAC spectroscopy

Local structural and electronic environment around ¹¹¹In probe atoms in transition metal doped Zn_1???xT_xO (T=Mn, Co, V and Ni; x = 0.01, 0.02, 0.05) and Cu co-doped Zn_1???xCo_xCu_0.01O (x = 0.01–0.04) have been monitored on an atomic scale by Perturbed Angular Correlation (PAC) spectroscopy. Single phase nanocrystalline powders were synthesized at low annealing temperatures by sol-gel method. PAC measurements exhibited the well known quadrupole interaction frequency, $\upnu_{\rm Q} =$ 31 MHz, which have been attributed to the substitutional incorporation of ¹¹¹In in ZnO matrix. PAC results did not reveal any evidence of magnetic ordering down to 77 K in pure and doped ZnO, which is consistent with the recent observation of paramagnetic behavior in transition metal doped ZnO with synchrotron based studies.     

Studies on the complex behavior of optical phonon modes in wurtzite (ZnO)1−x (Cr2O3) x

This paper reports on the use of phonon spectra obtained with laser Raman spectroscopy for the uncertainty concerned to the optical phonon modes in pure and composite ZnO_1?x (Cr₂O₃) _x . Particularly, in previous literature, the two modes at 514 and 640 cm^?1 have been assigned to ZnO are not found for pure ZnO in our present study. The systems investigated for the typical behavior of phonon modes with 442 nm as excitation wavelength are the representative semiconductor (ZnO)_1?x (Cr₂O₃) _x (x = 0, 5, 10 and 15 %). Room temperature Raman spectroscopy has been demonstrated polycrystalline wurtzite structure of ZnO with no structural transition from wurtzite to cubic with Cr₂O₃. The incorporation of Cr³⁺ at most likely on the Zn sub-lattice sites is confirmed. The uncertainty of complex phonon bands is explained by disorder-activated Raman scattering due to the relaxation of Raman selection rules produced by the breakdown of translational symmetry of the crystal lattice and dopant material. The energy of the E ₂ (high) peak located at energy 53.90 meV (435 cm^?1) due to phonon–phonon anharmonic interaction increases to 54.55 meV (441 cm^?1). A clear picture of the dopant-induced phonon modes along with the B ₁ silent mode of ZnO is presented and has been explained explicitly. Moreover, anharmonic line width and effect of dislocation density on these phonon modes have also been illustrated for the system. The study will have a significant impact on the application where thermal conductivity and electrical properties of the materials are more pronounced.     

Study of spin–phonon coupling in LiFe1 − xMnxPO4olivines

LiFe_{1 − x}Mn_xPO₄ olivines are promising material for improved performance of Li‐ion batteries. Spin–phonon coupling of LiFe_{1 − x}Mn_xPO₄ (x = 0, 0.3, 0.5) olivines is studied through temperature‐dependent Raman spectroscopy. Among the observed phonon modes, the external mode at ~263 cm⁻¹ is directly correlated with the motions of magnetic Fe²⁺/Mn²⁺ ions. This mode displays anomalous temperature‐dependent behavior near the Néel temperature, indicating a coupling of this mode with spin ordering. As Mn doping increases, the anomalous behavior becomes clearly weaker, indicating the spin–phonon coupling quickly decreases. Our analyses show that the quick decrease of spin–phonon coupling is due to decrease of the strength of spin–phonon coupling, but not change of spin‐ordering feature with Mn doping. Importantly, we suggest that the low electrochemical activity of LiMnPO₄ is correlated with the weak spin–phonon coupling strength, but not with the weak ferromagnetic ground state. Our work would play an important role as a guide in improving the performances of future Li‐ion batteries. Copyright © 2015 John Wiley & Sons, Ltd.     

Influence of phonon confinement,surface stress,and zirconium doping on the Raman vibrational properties of anatase TiO2 nanoparticles

We present a comprehensive analysis of the Raman spectra of pure and zirconium‐doped anatase TiO₂ nanoparticles. To account for the wavenumber shifts of the E_g(ν₆) mode as a function of particle size (L) and dopant concentration (x), a modification of the standard phonon confinement model (PCM) is introduced, which takes into account the contribution of surface stress by means of the Laplace–Young equation. Together with X‐ray diffraction (XRD) and transmission electron microscopy data, our analysis shows that the surface stress contribution to the observed blue shift of the Raman wavenumber is of the same magnitude as the spatial phonon confinement effect. Annealing experiments show that Zr‐doped nanoparticles exhibit retarded grain growth and delayed anatase‐to‐rutile phase transition by up to 200 K compared to pure anatase TiO₂. XRD shows that Zr doping leads to a unit cell expansion of the anatase structure. Applying the modified PCM to the x‐dependent variations of the E_g(ν₆) Raman mode, the mode‐Grüneisen parameter is found to increase abruptly at x > 0.07 with a concomitant mode softening. This coincides with the x range over which the Zr cations are reported to be displaced from their position in the tetrahedral lattice, and where Zr precipitation occurs upon annealing. The results have implications for the interpretation of Raman spectra of ionic metal oxide nanoparticles and how these are modified upon cation doping. Copyright © 2011 John Wiley & Sons, Ltd.     

Lattice dynamics and low‐temperature Raman spectroscopy studies of PMN–PT relaxors

In this work we present a Raman scattering study of a specific region of the morphotropic phase boundary (MPB) of the [Pb(Mg_1/3Nb_2/3)O₃]_1−x (PbTiO₃)_x relaxor system. We performed low‐temperature measurement for the x = 0.4 composition in the 20–300 K temperature range, and a detailed analysis of Raman spectra of x = 0.4 and x = 0.37 compositions at 180 K. The analysis of Raman spectra indicates a structural phase transition at around 170 K for x = 0.4. The comparison of Raman data from x = 0.4 and x = 0.37 compositions suggests different phases for these samples at 180 K. These results are in accordance with the tetragonal to monoclinic structural phase transition observed in the PMN–PT MPB and contribute to improve the knowledge of the MPB of this solid solution. Additionally, we have performed the lattice dynamics phonon calculation of the (1 − x) PMN–xPT relaxor in order to best understand its complex Raman spectral properties. The normal mode analyses (at q ∼ 0) were performed by considering tetragonal symmetry for the (1 − x) PMN–xPT system and using the rigid ion model and mean field approximation. Our calculated wavenumber values are in good agreement with experimental and calculated results reported for PbTiO₃ thus providing a reliable assignment of the various Raman modes. The low wavenumber modes are interpreted as arising from a lifting of the degeneracy of the vibrational modes related to Mg, Nb and Ti sites. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman spectral probe on increased local vibrational modes and phonon lifetimes in Ho3+‐doped Bi2O3 micro‐rods

We report the appearance and enhancement in intensity of impurity related local vibrational modes in Bi₂O₃ : Ho micro‐rods along with normal modes. Pure and Ho‐doped Bi₂O₃ micro‐rods were synthesized by conventional co‐precipitation method at 60 °C. The structural and morphological studies were carried out using powder X‐ray diffraction technique and scanning electron microscopy, respectively. Raman spectroscopic studies reveal the existence of local phonon vibrational modes (LVM) due to the incorporation of Ho³⁺. Harmonic approximation method was employed to find the dopant‐related peak in the Raman spectra. Variation in full width at half maximum for LVM with increase in Ho³⁺ was also investigated. This increase in FWHM indicates the decrease in crystallinity of the doped samples. The phonon lifetime calculation carried out for each samples and the decrease in phonon lifetime with doping concentration make this material a potential candidate for optical and electronic applications. Copyright © 2016 John Wiley & Sons, Ltd.     

Disorder‐induced Raman scattering effects in one‐dimensional ZnO nanostructures by incorporation and anisotropic distribution of Dy and Li codopants

In Dy³⁺ and Li⁺ codoped ZnO nanowires, the additives accumulate preferentially in {0001} planes, resulting in serious breakdown of the translational symmetry in ab plane and modification of the phonon oscillation field. Not only acoustic overtones, silent optical modes, surface optical (SO) phonon modes, and multi‐phonon processes can be effectively observed in the nonresonant Raman scattering (RS) and the Fourier‐transform infrared (FTIR) spectra, but the quasi‐LO and TO modes of mixed A₁ and E₁ symmetry also show a noticeable red shift from E₁ symmetry (in ab plane) to A₁ symmetry (along c axis). The presence of dislocations and internal strain at the surface layer rich in additives, coming from the segregation of additives, forms a quasi‐bilayer system, resulting in the appearance and enhancement of SO phonon modes in RS and FTIR spectra. The Fano interference, originating from the interaction between the discrete scattering from phonons and the continuum scattering from laser‐induced electrons in the doped nanostructures, leads to typical asymmetric lineshapes on the lower wavenumber sides. Copyright © 2010 John Wiley & Sons, Ltd.     

Raman spectroscopy to probe residual stress in ZnO nanowire

Raman spectroscopic studies are performed to probe the stress along the length of a bent ZnO nanowire. The zone‐centre E₂^high optical phonon shows a systematic red shift as the junction point of the two arms of the nanowire is approached. The mechanism of the red shift is discussed on the basis of the tensile strain. From the red shift of the phonon peak position, the strain at different regions on the nanowire is estimated. Stress in the bent nanowire is also investigated using photoluminescence (PL) spectroscopy. Results of both Raman and PL study confirm that the bent nanowire is under tensile strain. Copyright © 2011 John Wiley & Sons, Ltd.     

Abnormal Raman spectra in Er‐doped BaTiO3 ceramics

Greatly enhanced and abnormal Raman spectra were discovered in the nominal (Ba_{1 − x}Er_x)Ti_{1 − x/4}O₃ (x = 0.01) (BET) ceramic for the first time and investigated in relation to the site occupations of Er³⁺ ions. BaTiO₃ doped with Ti‐site Er³⁺ mainly exhibited the common Raman phonon modes of the tetragonal BaTiO₃. Er³⁺ ions substituted for Ba sites are responsible for the abnormal Raman spectra, but the formation of defect complexes will decrease spectral intensity. A large increase in intensity showed a hundredfold selectivity for Ba‐site Er³⁺ ions over Ti‐site Er³⁺ ions. A strong EPR signal at g = 1.974 associated with ionized Ba vacancy defects appeared in BET, and the defect chemistry study indicated that the real formula of BET is expressed by (Ba_{1 − x}Er_3x/4)(Ti_{1 − x/4}Er_x/4)O₃. These abnormal Raman signals were verified to originate from a fluorescent effect corresponding to ⁴S_3/2→⁴I_15/2 transition of Ba‐site Er³⁺ ions. The fluorescent signals were so intense that they overwhelmed the traditional Raman spectra of BaTiO₃. The significance is that the abnormal Raman spectra may act as a probe for the Ba‐site Er³⁺ occupation in BaTiO₃ co‐doped with Er³⁺ and other dopants. A new broad EPR signal at g = 2.23 was discovered, which originated from Er³⁺ Kramers ions. Copyright © 2014 John Wiley & Sons, Ltd.     

Bi-induced vibrational modes in GaAsBi

We have studied GaAs_1−xBi_x (up to x3%) using Raman scattering with two different polarization configurations. Two Bi-induced phonon modes are observed at 186 cm⁻¹ and 214 cm⁻¹ with increasing Raman intensity as the Bi concentration increases. By comparing Raman selection rules for the observed Bi-induced phonon modes with those for the substitutional N vibrational mode (GaN mode) in GaAsN, the phonon mode at 214 cm⁻¹ is identified as originating from substitutional Bi at the As site in GaAsBi.     

Structure and ferromagnetic properties of Co-doped ZnO powders

Single-phase Zn_1−xCo_xO (x=0.02, 0.04) powders were synthesized by a simple co-precipitation technique. X-ray diffraction analysis reveals that the Co-doped ZnO crystallizes in a wurtzite structure. The lattice constants of Co-doped ZnO powders decrease slightly when Co is doped into ZnO. Optical absorption spectra show a decrease in the bandgap with increasing Co content and also give an evidence of the presence of Co²⁺ ions in tetrahedral sites. Raman spectra indicate that Co doping increased the lattice defects and induced another Raman vibration mode around at 538 cm⁻¹, which is an indicator for the incorporation of Co²⁺ ions into the ZnO host matrix. Magnetic measurement reveals that the Zn_1−xCo_xO (x=0.02, 0.04) powders clearly exhibit room-temperature ferromagnetic behavior, which makes them potentially useful as building components for spintronics.     

Microstructural and magnetic properties of Ax:Zn1-xO (A=Mn,Gd and Mn/Gd) nanocrystals

We report the microstructural and magnetic properties of transition (3d) and rare earth (4f) metal substituted into the A_x:Zn_1?xO (A=Mn, Gd and Mn/Gd) nanocrystal samples synthesized by solgel method. The structural properties and morphology of all samples have been analysed using X-ray diffraction (XRD) method and scanning electron microscopy. The impurity phase in the XRD patterns for all samples is not seen, except (Mn/Gd):ZnO sample where a very weak secondary phase of Gd₂O₃ is observed. Due to the large mismatch of the ionic radii between Mn²⁺ and Gd³⁺ ions, the strain inside the matrix increases, unlike the crystallite size decreases with the substitution of Mn and Gd into ZnO system. A couple of additional vibration modes due to the dopant have been observed in Raman spectrum. The magnetic properties have been studied by vibrating sample magnetometer. The magnetic hysteresis shows that Mn:ZnO and Gd:ZnO have soft ferromagnetic (FM) behaviour, whereas (Mn/Gd):ZnO has strong FM behaviour at room temperature (RT). The enhancement of ferromagnetism (FM) in (Mn/Gd):ZnO sample might be related to short-range FM coupling between Mn²⁺ and Gd³⁺ ions via defects potential and/or strain-induced FM coupling due to the expansion lattice by doping. The experimental results indicate that RTFM can be achieved by co-substitution of 3d and 4f metals in ZnO which can be used in spintronics applications.