A Brief Look At Spintronic Biology Essay

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Microprocessors nowadays form the heart of most electronic devices from computers to home appliances. These microprocessors employ circuits that express data as binary digits, represented by the existence or absence of electronic charges. The flow and accumulation of the electron charges within these circuits is what makes logic operations possible. Conventional Si and GaAs based electronics ignores the spin of the electron, as half of the conduction electrons are in a spin-up state and the remainder are spin-down. The new field of spin based electronics or spintronics is focused upon utilizing the spin degree of freedom of the charge carriers and has the potential for various technological advancements [5{9]. New electronic and optic devices such as the spin-FET (field effect transistor), spin-LED (light emitting diode), MRAM (magnetic random access memory), etc. may also offer numerous benefits such as increased processing speed, decreased power consumption and nonvolatility. The best known successful spin based devices at present are the magnetoresistance (MR) sensors made of multilayers containing metal ferromagnets, showing giant magnetoresistance (GMR) or tunneling magnetoresistance (TMR), which are currently used in the read heads of a hard disk drive [5{9].\ For the implementation of semiconductor based spintronic devices several materials challenges such as injection, transport, manipulation and detection of the spin must be overcome. A ferromagnetic material, in which the charge carriers posses a certain degree of spin polarization, is therefore an essential component of such a device. Diluted magnetic semiconductors are promising materials for spintronics because of their structural compatibility with semiconductors used in present (opto-) electronics, as the epitaxial growth of ferromagnet-semiconductor heterostructures with well-ordered interfaces allows controlled spin injection from the ferromagnetic layer into the semiconductor [10,11].

DMS

Diluted magnetic semiconductors (DMS) consist of standard semiconductors in which some subset of the lattice atoms is randomly substituted by a magnetic atom, thus inserting local magnetic moments in the semiconductor matrix (see Fig. 1.1). These magnetic moments can originate from 3d or 4f open shells of transition metals or rare-earth elements, respectively, so that typical examples of DMS are Cd1ô€€€xCoxSe, Hg1ô€€€xFexSe, Zn1ô€€€xCoxS, Ga1ô€€€xMnxAs, In1ô€€€xMnxAs, Pb1ô€€€xEuxTe, and in a sense Si:Er [12]. In most cases Mn is used as the magnetic dopant. The term DMS is usually reserved for single-phase systems to differentiate them from systems where magnetic second phases are incorporated as precipitates. In early works DMS materials have been also referred as "semi-magnetic semiconductors" [13].

Fig

As magnetic semiconductors these materials combine the complementary properties of semiconductors and ferromagnets. DMS can exhibit a wide range of magnetic properties, from paramagnetism and spin-glass behavior to ferromagnetism. They can span the range from highly insulating to metallic, even in the same alloy system. In the 1970's and 1980's research on DMS was mainly focussed on II-VI based systems in which the valence of the cations matches that of the common magnetic ions such as Mn [12, and references therein]. Although such materials are relatively easy to prepare, it is di_cult to add p- or n-type doping to II-VI based DMS. The magnetic interaction in these materials is therefore dominated by the antiferromagnetic superexchange among the Mn spins, which results in paramagnetic, antiferromagnetic, or spin-glass behavior. However ferromagnetic ordering (with TC < 10 K) can be obtained in II-VI DMS with su_cient p-type doping [15{17], when the antiferromagnetic superexchange is overcompensated by ferromagnetic interactions mediated by band holes.

Overeview of doped ZnS nanoparticles

Many approaches have been taken to prepare diluted magnetic semiconductors (DMS). However, before the 90's, all the work that had been done was on the physical properties of these materials. The first report was done by Weakliem (1962) who studied the corresponding transitions in ZnS doped Co as well as some transitions attributed to double final states. Then, a study on the energy levels of Co in ZnS and ZnSe by Baranowski et al (1967) who reported transitions from the ground state 2A2 (F) to excited states 4T2 (F), 4T1 (F). After that, an explosion came out in the 70's made by Busse et al 1970, Wray and Allen 1971 and Radlinski (1977 and 1978). Andrzej et al (1979) discussed the position of the Co level in the wide bandgap II-IV semiconductor. In 1981, Noras et al, studied the energy levels of cobalt in ZnSe and ZnS using the absorption spectrum. After the report of Fuchs and Koidl (1993) on the energy transfer mechanism between different Co2+ centers in structurally pure 4HZnS using Fourier transform photoluminescence excitation spectroscopy, Hoffmann et al (1994) investigated the energy transfer between Fe2+ centers in polymorphic ZnS. Though, before the 90's most of the reports focused on the problems of the theoretical point of view, and no experiment details are available.

A breakthrough in doped wide bandgap nanoparticles came in the 90's, by using 3d transition metal ions as dopants. Transition metal ions with open d-shell electronic configurations have various unique physical properties including the combination of magnetic ground state and low-energy excited state that makes them attractive dopants. A favorable material for these studies is Mn2+ doped nanoparticles, since the first report of ZnS: Mn nanoparticles by Bhargava et al (1994), who reported the optical properties of manganese doped nanocrystals of ZnS. Several studies on doped wide bandgap semiconductors have appeared after that report, including new preparation methods, physical properties and potential applications.

Galakhov and Srkova (1997) studied the electronic structure of ZnS: Co and ZnSe: Co crystals. they confirmed the 3d configuration of the ground state of the cobalt from the X-ray photoelectron spectrum. Moreover, the strong covalence of the cobalt-sulfur bond, but somewhat weaker than for CoS.

For the optical investigations, Borse et al (1999) investigate the luminescence quenching in ZnS nanoparticles due to Fe and Ni doping. By a chemical method, mercaptoethanol have been used to passivate the surface of the particles then under certain conditions highly luminescence particles emitting blue light at λ = 425nm. This blue light emission of ZnS was observed to be completely quenched when doped with iron or nickel ions.

Another major category of dopants for semiconductor nanocrystals is that of luminescence activators, interest in the luminescent properties of pure semiconductor nanocrystals has driven much of the research into these materials for the past decades, and manipulation of the luminescence properties of these nanocrystals by doping with ions such as Mn2+ , Cu2+ or Eu2+ has the potential to broaden the range of useful spectroscopic properties that can be achieved of this class of materials. In 2000, Wei et al measured the photoluminescence of the Mn doped ZnS nanoparticles under hydrostatic pressure at room temperature. The pressure coefficient of the Mn2+ in the fluorescence of ZnS nanoparticles was found to be little higher compared to the bulk material, due to the enhancement in the electron-phonon interaction in the nanoparticles. For the photoluminescence excitation and when Mn2+ was incorporated within the nanocrystals, both the λ = 435 nm blue emission of ZnS and the orange Mn2+ emission at λ = 590 nm were observed. However, in the Mn2+ activated ZnS nanocrystals in which the Mn2+ ions were distributed outside the ZnS nanocrystals, no orange emission were observed, but a new peak at λ = 350 nm appeared and the blue emission of ZnS was quenched and shifted to λ = 390 nm.

For the alternating-current thin film electroluminescence devices (ACTFEL), films of ZnS: Mn, (ZnGa)S: Mn and ZnS: CuCl2 were prepared by co-evaporation of ZnS powder. The films were polycrystalline, strongly oriented with high optical transmission in the visible part of spectrum. The bandgap of ZnS: Mn, (ZnGa)S: Mn and ZnS: CuCl2 films was found to be 3.63, 3.86 and 3.56 eV, respectively. The sheet resistance of the ZnS: Mn and (ZnGa)S: Mn films was greater than 100 MΩ. However, for ZnS: CuCl2 was between 1.5 to 80 Ω. For the photoluminescence spectrum, only one band with corresponding peak at λ = 617.5 nm for (ZnGa)S: Mn, and λ = 515 nm for ZnS: CuCl2. In the other hand, for the spectrum of ZnS: Mn, two bands appeared. One in the green region at λ = 569 nm and the other at λ = 642.26 nm corresponding to the red color (Dimitrova and Tate 2000).

A sol-gel method have been used to synthesis ZnS: Mn nanocrystalline embedded in silica matrix by Bhattacharjee et al (2002). Micro-structural, optical and ellipso-metric studies revealed that quantum size effect occurred in the films. ESR study indicated dispersed Mn2+ impurity rather than Mn cluster in the Mn doped ZnS. A blue emission at λ = 250 nm of ZnS and yellow-orange at λ = 400 nm of Mn doped films have been observed.

Experimental studies of X-ray photoelectron and Co L X-ray emission spectra of ZnS: Co semiconductor were carried out by Gridneva et al (2003). The Co 3d states was determined with respect to the valence-band edges by combination of X-ray emission and X-ray photoelectron spectroscopy. Such good coincidence showed that in the case of ZnS: Co the interaction between the 3d impurity and the host valence-band states was negligible and the Co ions were present in iso-valent Co2+ configuration.

An other approach used by Warad et al (2005) called co-precipitation method. the nanoparticles were controlled using polyphosphates of sodium called STTP and SHMP. It was observed that the particles sizes were dependent on the amounts of the agents SHMP used during the synthesis. The particles size was found to be between D = 60 to 80 nm with zinc blend structure and a crystalline size of d = 2.2 nm. Under UV exposure the power of ZnS: Mn nanoparticles peaked in the orange-red region with λ = 590 nm. Nanoparticles of ZnS synthesized by rf-magnetron sputtering technique prepared by Ghosh et al (2007). He confirmed the nanocrystalline cubic ZnS phase from the X-ray diffraction patterns. From TEM the particle size was found to be D = 6 nm. The UV-Vis-NIR spectrophotometric measurements showed high transparence of the film ~90% in the wavelength range λ = 400 to 2600 nm. He reported that the photoluminescence at λ = 598 nm, is attributed due to 4T1-6A1 transition of Mn 3d.

Recently a fast development came out, using a new preparation methods to characterize doped ZnS. In 2008, a CO2 laser was used to evaporate ZnS: Mn into fused silica substrates (Hergen Eilers). The shortening of the photoluminescence lifetime indicate an amorphous component of the thin film ZnS: Mn, a mixture of cubic and hexagonal structure have been observed. Benedikt el al (2008), coated ZnS: Mn by silica using hydrolysis and condensation of tetramethoxyorthosilicate (TMOS). The silica-coated ZnS and ZnS: Mn showed an improved thermal stability over uncoated particles, with degradation at 400ËšC. A minimum photoluminescence intensity upon annealing at 600ËšC have been shown. At higher temperatures (800ËšC), ZnS coated Si conserved the typical emission in the blue region, whereas ZnS: Mn coated Si showed different intensive emission that was dependent on the annealing atmosphere.

The electrical and mechanical properties of DMS nanoparticles have been explored by Sreekantha Reddy et al (2009). Zn1-xMnxS (0 ≤ x ≤ 0.25) nanostructured films were prepared at room temperature (300K) by resistive thermal evaporation technique. Using the four point probe in the temperature range of 190 K to 450 K, the resistivity was found between 3.4 x 105 to 9.6 x 105 Ω-cm. The films showed a decrease in resistivity with the increase of temperature which called semiconducting behavior. In the other part, the resistivity showed a definite decrease with the increase of Mn concentration.

Overview of ZnS doped Co or Fe

Iron doped (1 to 5%) ZnS nanocrystal were synthesized by wet chemical method (Bhattacharya and Chakravorty 2007). The particle sizes was found to be between 2 to 4 nm using transmission electron microscope (TEM). DC resistivity were curried out on cold compacted specimens over the temperature range 313 to 472 K, and it decreased of about 10 orders of magnitude lowering compared to undoped ZnS samples. Using the analysis of small polaron hopping conduction mechanism indicated that Fe2+/Fe3+ ions were concentrated at the interface of the nanocrystals. Low temperature magnetization studies indicated ferromagnetic behavior arising out of a one dimensional magnetic system with anti-parallel spin alignment. In 2008, Sambasivam et al, investigated ZnS doped Fe nanoparticles synthesized by co-precipitation method, using thiophenol as the conditioning agent. Using TEM the size of the particles were about 7 nm, and XRD revealed the zinc blend cubic structure. With the increase of Fe2+ concentration, the absorption was blue shifted and the magnetization measurements at room temperature showed the ferromagnetic nature of the doped samples. One year later, he reported the quenching effect on the photoluminescence spectrum. For the low concentration of Fe there were less decrease in the luminescence. however, for higher concentration, there were much more decrease in the intensity.

For the wurtzite ZnS, Chong et al (2009), prepared Zn1-xCoxS (0.0025 ≤ x ≤ 0.1) by solution-phase thermal decomposition method at 150˚C. the XRD and HRTEM exhibited pure hexagonal ZnS for all the different Co concentrations, with particle size of 5 nm. The photoluminescence spectra showed a blue peak which was referred to the Co ions, the other peaks were attributed to ZnS lattice. Based on the photoluminescence results, a schematic of the energy levels have been done, to explain the emission mechanism of the surface sulfur, the sulfur and the zinc vacancies.

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