Blue-emitting copper-doped zinc oxide nanocrystals.
We report the first synthesis of Cu doped in the core region of ZnO nanocrystals and fluorescing in the blue region, establishing the novel possibility of using these as fluorescent probes.
Letter↗
Biomedical subjects
D D Sarma
Publications and source records attributed to D D Sarma.
Electronic phase separation in correlated oxides: the phenomenon, its present status and future prospects.
Many transition metal oxide materials of high chemical purity are not necessarily monophasic. Thus, single crystals of chemically pure rare earth manganites and cobaltates of the general formula Ln(1-x)A(x)MO(3) (Ln=rare earth metal, A=alkaline earth metal, M=Mn, Co) exhibit the phenomenon of electronic phase separation wherein "phases" of different electronic and magnetic properties coexist. Such phase separation, the length scale of which can vary anywhere between a few nanometers to microns, gives distinct signatures in X-ray and neutron diffraction patterns, electrical and magnetic properties, as well as in NMR and other spectroscopies. While the probe one employs to investigate electronic phase separation depends on the length scale, it is noteworthy that direct imaging of the inhomogeneities has been accomplished. Some understanding of this phenomenon has been possible on the basis of some of the theoretical models, but we are far from unraveling the varied aspects of this new phenomenon. Herein, we present the highlights of experimental techniques and theoretical approaches, and comment on the future outlook for this fascinating phenomenon.
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Unraveling internal structures of highly luminescent PbSe nanocrystallites using variable-energy synchrotron radiation photoelectron spectroscopy.
The internal structure of PbSe nanocrystals was deduced using synchrotron X-ray photoemission spectroscopy for three different sizes of nanocrystals. The photoemission data revealed the layered structure of PbSe nanocrystals with the crystalline PbSe core surrounded by a nonstoichiometric Pb(1-x)Se shell, finally passivated by a capping agent in the outermost layer. A detailed analysis of the experimental data yielded quantitative information on the thickness of three different layers, which is unavailable through any other technique; moreover, the overall sizes of the nanocrystals probed by transmission electron microscopy were in agreement with the corresponding quantity obtained in the present experiment. The present results provide a plausible explanation for the strong variation in the photoluminescence intensity with size observed for these nanocrystals.
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Blue emitting polyaniline.
A family of stable polyaniline (PANI) derivatives exhibiting deep blue photoluminescence (PL) has been prepared. The synthetic strategy provides a unique way to functionalize the PANI backbone.
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Angle-resolved photoemission spectroscopy of the insulating NaxWO3: Anderson localization, polaron formation, and remnant Fermi surface.
The electronic structure of the insulating sodium tungsten bronze, Na(0.025)WO(3), is investigated by high-resolution angle-resolved photoemission spectroscopy. We find that near-E(F) states are localized due to the strong disorder arising from random distribution of Na+ ions in the WO(3) lattice, which makes the system insulating. The temperature dependence of photoemission spectra provides direct evidence for polaron formation. The remnant Fermi surface of the insulator is found to be the replica of the real Fermi surface in the metallic system.
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Origin of ferromagnetism and its pressure and doping dependence in Tl2Mn2O7.
Using the Nth order muffin-tin obital downfolding technique, we investigate the origin of ferromagnetism in pyrochlore Tl2Mn2O7. It is found to be driven by a hybridization induced spin polarization of delocalized charge carriers derived from Tl-s and O-p states. The mean-field estimate of the ferromagnetic transition temperature Tc estimated using computed exchange integrals are found to be in good agreement with measurements. We find an enhancement of Tc for moderate doping with nonmagnetic Sb and a suppression of Tc upon application of pressure, both in agreement with experimental findings.
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Study of the growth of capped ZnO nanocrystals: a route to rational synthesis.
We report the study of complex and unexpected dependencies of nanocrystal size as well as nanocrystal-size distribution on various reaction parameters in the synthesis of ZnO nanocrystals using poly(vinyl pyrollidone) (PVP) as a capping agent. This method establishes a qualitatively different growth mechanism to the anticipated Ostwald ripening behavior. The study of size-distribution kinetics and an understanding of the observed non-monotonic behaviors provides a route to rational synthesis. We used a simple, but accurate, approach to estimate the size-distribution function of nanocrystals from the UV-absorption spectrum. Our results demonstrate the accuracy and generality of this approach, and we also illustrate its application to various semiconducting nanocrystals, such as ZnO, ZnS, and CdSe, over a wide size range (1.8-5.3 nm).
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Spin-flop ordering from frustrated ferro- and antiferromagnetic interactions: a combined theoretical and experimental study of a Mn/Fe(100) monolayer.
The occurrence of a noncollinear magnetic structure at a Mn monolayer grown epitaxially on Fe(100) is predicted theoretically, using spinor density-functional theory, and observed experimentally, using x-ray magnetic circular dichroism (XMCD) and linear dichroism (XMLD) spectroscopies. The combined use of XMCD and XMLD at the Mn-absorption edge allows us to assess the existence of ferromagnetic and antiferromagnetic order at the interface, and also to determine the moment orientations with element specificity. The experimental results thus obtained are in excellent agreement with the magnetic structure determined theoretically.
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Emission properties of manganese-doped ZnS nanocrystals.
We have performed steady-state and time-resolved fluorescence studies on undoped and Mn-doped ZnS nanocrystals with approximately 16 A diameter. While there is no band-edge emission, the intensity of the steady-state blue fluorescence from ZnS surface states decreases upon Mn incorporation, which gives rise to an orange emission. These results show that Mn incorporation competes very effectively with the donor-acceptor surface states for the energy transfer from the electron-hole pair excited across the band gap. In both undoped and doped samples, the time-resolved fluorescence studies establish the presence of a distribution of decay lifetimes possibly due to a number of emission centers in the nanocrystals. A faster short-time decay of the blue emission in the Mn-doped samples compared to that in the undoped sample suggests an additional decay channel for the surface states via an energy transfer from these states to the dopant levels.
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Magnetic properties of doped II-VI semiconductor nanocrystals.
We investigate the magnetic properties of two typical II-VI semiconducting nanocrystals, namely, ZnS and ZnO, doped with various concentrations of a transition metal ion, Mn. A wide range of dopant concentrations was explored without changing the size of the nanocrystal, thereby allowing us to study the concentration dependence of various properties independent of any size variation of the host. However, only small doping concentrations could be realized in each case. All the studies were thus carried out with ZnS and ZnO nanocrystals with their respective average sizes fixed at about 1.6 and 4.7 nm. We show that the magnetic properties of such doped systems, remaining paramagnetic down to the lowest temperature (<3 K), can provide important and accurate information concerning the dopant level in such samples.
Crystallization↗
Unusual directional dependence of exchange energies in GaAs diluted with Mn: is the RKKY description relevant?
Ferromagnetism in Mn-doped GaAs, the prototypical dilute magnetic semiconductor (DMS), has so far been attributed to hole mediated RKKY-type interactions. First-principles calculations reveal a strong direction dependence of the ferromagnetic (FM) stabilization energy for Mn pairs, a dependence that cannot be explained within RKKY. In the limit of a hostlike hole engineered here where the RKKY model is applicable, the exchange energies are strongly reduced, suggesting that this limit cannot explain the observed ferromagnetism. The dominant contribution stabilizing the FM state is found to be maximal for 110-oriented Mn pairs and minimal for 100-oriented Mn pairs, providing an alternate explanation for magnetism in such materials in terms of energy lowering due to p-d hopping interactions, and offering a new design degree of freedom to enhance FM.
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Direct observation of large electronic domains with memory effect in doped manganites.
We use a spatially resolved, direct spectroscopic probe for electronic structure with an additional sensitivity to chemical compositions to investigate high-quality single crystal samples of La(1/4)Pr(3/8)Ca(3/8)MnO3, establishing the formation of distinct insulating domains embedded in the metallic host at low temperatures. These domains are found to be at least an order of magnitude larger in size compared to previous estimates and exhibit memory effects on temperature cycling in the absence of any perceptible chemical inhomogeneity, suggesting long-range strains as the probable origin.
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Pressure tuning of electron-phonon coupling: the insulator to metal transition in manganites.
An extended temperature and pressure-dependent investigation is carried out on a La0.75Ca0.25MnO3 sample exploiting the infrared absorption technique coupled to a diamond anvil cell. The pressure dependence of the insulator to metal transition temperature T(IM) is determined for the first time up to 11.2 GPa. The T(IM)(P) curve we propose to model the present data has an exponential-like behavior with an associated characteristic pressure P* playing the role of a decay constant. It is found that the equivalence between an external and an internal (chemical) pressure holds over a limited range of pressure, namely, P< or =2P*. Moreover, a certain universality character is associated with the proposed model curve in its ability to account for a large class of low-disorder manganites characterized by intermediate electron-phonon coupling.
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In situ photoemission study of the room temperature ferromagnet ZnGeP2:Mn.
The chemical states of the ZnGeP(2):Mn interface which shows ferromagnetism above room temperature have been studied by photoemission spectroscopy. Mn deposition on the ZnGeP2 substrate heated to 400 degrees C induced Mn substitution for Zn and then the formation of metallic Mn-Ge-P compounds. Depth profile studies have shown that Mn 3d electrons changed their character from itinerant to localized along the depth, and in the deep region, dilute divalent Mn species (<5% Mn) was observed with a coexisting metallic Fermi edge of non-Mn 3d character. The possibility of hole doping through Mn substitution for Ge and/or Zn vacancy is discussed.
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Optical and magnetic properties of manganese-doped zinc sulfide nanoclusters.
We report the synthesis of fixed-size ZnS nanoclusters approximately 24 A in diameter with varying manganese concentrations. Various samples of Zn1-x MnxS, with x = 0, 0.02, 0.055, 0.09, and 0.13, have been prepared and characterized using X-ray diffraction, energy-dispersive analysis of X-rays, UV absorption, fluorescence emission and excitation, electron paramagnetic resonance (EPR), and magnetic susceptibility measurements. The manganese ions are found to substitute Zn randomly without giving rise to any clustering of Mn sites, as seen from EPR and magnetic susceptibility results. Our studies reveal that the band gap of the doped nanoclusters passes through a maximum as the manganese concentration is varied. Also, we observe orange emission from Mn2+ ions in the doped ZnS nanoclusters, apart from the blue emission characteristic of the ZnS defect states. The relative intensity of the orange emission compared with the blue varies with the manganese concentration in a nonmonotonic way. The inverse of susceptibility temperature plots can be plotted onto a universal curve by simple multiplicative constants, thus showing that the magnetic interactions between Mn2+ ions are weak.
Crystallization↗
Blue emission from cysteine ester passivated cadmium sulfide nanoclusters.
A one-pot synthesis is reported of water-soluble cadmium sulfide nanoclusters capped with cysteine ester, with an average size of 2.0 nm and fluorescing in the blue region, establishing the possibility of using these as fluorescent biological probes.
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Electronic and magnetic structures of Sr(2)FeMoO(6).
We have investigated the electronic and magnetic structures of Sr(2)FeMoO(6) employing site-specific direct probes, namely x-ray absorption spectroscopy with linearly and circularly polarized photons. In contrast to some previous suggestions, the results clearly establish that Fe is in the formal trivalent state in this compound. With the help of circularly polarized light, it is unambiguously shown that the moment at the Mo sites is below the limit of detection (<0.25 mu(B)), resolving a previous controversy. We also show that the decrease of the observed moment in magnetization measurements from the theoretically expected value is driven by the presence of mis-site disorder between Fe and Mo sites.
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Spin, charge, and orbital ordering in La0.5Sr1.5Mno4.
We have analyzed the experimental evidence of charge and orbital ordering in La0.5Sr1.5MnO4 using first principles band structure calculations. Our results suggest the presence of two types of Mn sites in the system. One of the Mn sites behaves as an Mn3+ ion, favoring a Jahn-Teller distortion of the surrounding oxygen atoms, while the distortion around the other is not a simple breathing mode kind. Band structure effects are found to dominate the experimental spectrum for orbital and charge ordering, providing an alternate explanation for the experimentally observed results.
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