Upconversion luminescence tuning of beta-NaYF(4) nanorods under 980 nm excitation has successfully been achieved by tridoping with Ln(3+) ions with different electronic structures. The effects of Ce(3+) ions on NaYF(4):Yb(3+)/Ho(3+) as well as Gd(3+) ions on NaYF(4):Yb(3+)/Tm(3+)(Er(3+)) have been studied in detail. By tridoping with Ce(3+) ions, not only were unusual (5)G(5)-->(5)I(7) and (5)F(2)/(3)K(8)-->(5)I(8) transitions from Ho(3+) ions and 5d-->4f transitions from Ce(3+) ions observed in NaYF(4):Yb(3+)/Ho(3+) nanorods, but also an increase in the intensity of (5)F(5)-->(5)I(8) relative to (5)S(2)/(5)F(4)-->(5)I(8) with increasing Ce(3+) concentration, which can be attributed to efficient energy transfers of (5)I(6) (Ho)+(2)F(5/2) (Ce)-->(5)I(7) (Ho)+(2)F(7/2) (Ce) and (5)S(2)/(5)F(4) (Ho)+(2)F(5/2) (Ce)-->(5)F(5) (Ho)+(2)F(7/2) (Ce). Interestingly, with increasing pump power density, the luminescence of NaYF(4):Yb(3+)/Ho(3+) nanorods is always dominated by the (5)S(2)/(5)F(4)-->(5)I(8) transition, whereas the luminescence of Ce(3+)-tridoped NaYF(4):Yb(3+)/Ho(3+) nanorods is dominated by the (5)S(2)/(5)F(4)-->(5)I(8) and (5)G(5)-->(5)I(7) transitions in turn. These observations are discussed on the basis of a rate equation model. Furthermore, Gd(3+)-tridoped NaYF(4):Yb(3+)/Tm(3+)(Er(3+)) nanorods can emit multicolor upconversion emissions spanning from the UV to the near-infrared under 980 nm excitation. (6)P(5/2)-->(8)S(7/2) ( approximately 306 nm) and (6)P(7/2)-->(8)S(7/2) ( approximately 311 nm) transitions from Gd(3+) ions were observed. In addition to the aforementioned luminescence properties, these Gd(3+)-tridoped nanorods also exhibit paramagnetic behavior at room temperature and superparamagnetic behavior at 2 or 5 K.

Luminescence tuning of upconversion nanocrystals.

Publisher Summary This chapter discusses low-dimensional structures of II–VI diluted magnetic semiconductors (DMS) with manganese and describes the carrier concentration induced ferromagnetism in Mn-based IV–VI DMS with a particular emphasis on Mn content and conducting hole concentration dependent paramagnetic–ferromagnetic–spin glass transitions in bulk crystals and thin layers of SnMnTe and related materials. The chapter also discusses the transport, magnetic, and optical properties of bulk crystals and thick epitaxial layers; low-dimensional quantum structures of IV–VI DMS with Eu; the magnetic and electrical properties of IV–VI semiconductors with Gd; and the magnetic and optical properties of all semiconductor ferromagnetic multilayers built of ferromagnetic EuS and nonmagnetic IV–VI (PbSe and PbS) semiconductors. The chapter describes DMS containing transition metals other than manganese. These materials never attracted as much attention as the ones containing Mn, mainly because of severe difficulties in their preparation, particularly when large percentages of the magnetic components were involved.

II–VI and IV–VI Diluted Magnetic Semiconductors – New Bulk Materials and Low-Dimensional Quantum Structures

Bi2Se3, Bi2Te3, and Sb2Te3 tend to grow non-stoichiometric, the defects causing high free carrier densities. Here it was achieved, in the case of Bi2Se3, to produce crystals of n-and p-type conductivity showing concentrations down to 4 × 1016 cm−3. The temperature and magnetic field dependences of their galvanomagnetic properties are investigated. At low magnetic fields and low temperatures negative magnetoresistance occurs, at high magnetic fields no saturation of the positive magnetoresistance is observed which is assumed to originate from the influence of an impurity conduction band. The temperature dependence of the Hall constant enables the evaluation of the energy gap for thermal excitations to (160 ± 10) meV for T 0. The temperature dependence of the mobility between about 50 and 200 K supports the assumption of dominating scattering of the free carriers by acoustical phonons in this temperature range.



Bi2Se3, Bi2Te3 und Sb2Te3 neigen zu einem unstochiometrischen Wachstum, wobei die Defekte hohe Dichten freier Ladungstrager verursachen. Hier gelang es,im Falle von Bi2Se3, Kristalle mit n-undp-Leitungstypzu erzeugen, die Tragerkonzentrationen bis hinab zu 4 × 1016 cm−3 aufwiesen. Die Temperatur- und Magnetfeldabhangigkeiten der galvanomagnetischen Eigenschaften werden untersucht. Bei kleinen magnetischen Flusdichten und niedrigen Temperaturen wird eine negative Magnetowiderstandsanderung gefunden, bei hohen Feldern fehlt jedes Anzeichen einer Sattigung der positiven Magnetowiderstandsanderung, was auf den Einflus eines Storleitungsbandes zuruckgefuhrt wird. Die Tempera-turabhangigkeitder Hallkonstante ermoglicht die Auswertung der kleiusten Bandlucke fur thermische Anregungen zu (160 ± 10) meV fur T 0. Die Temperaturabhangigkeit der Beweglichkeit zwischen etwa 50 und 200 K unterstutzt die Annahme einer uberwiegen-den Streuung der freien Ladungstrager durch akustische Phononen in diesem Temperatur-bereich.

Galvanomagnetic Properties of Bi2Se3with Free Carrier Densities below 5 × 1017 cm−3

An exhaustive experimental investigation of a series of non-crystalline pyrocarbons has been carried out. In addition to the electrical resistivity, the Hall effect and the positive magnetoresistanoe, the negative magnetoresistance has been especially examined. This effect has been measured as a function of several parameters: temperature (300—1·5 K) magnetic field strength (0·25—60 kgauss) and sample orientation in the magnetic field. It has been shown that the negative component disappears progressively when the crystallinity of the sample increases towards the hexagonal graphite structure, that this component is anisotropic and behaves like a tensor quantity, and that there is no additivity law between the positive and the negative components. A tentative interpretation based on the existence of a pseudo energy gap between the valence and the conduction II bands has not been quantitatively successful. From an empirical analysis, a classification of the different magneto - resistance effects ha...

A study of the negative magnetoresistance in pyrocarbons

Influence of temperature on the two band model for negative magnetoresistance in heavily doped semiconductors

Two band model for negative magnetoresistance in heavily doped semiconductors

Electron paramagnetic resonance of Gd3+ in PbTe

Anisotropy in the magnetic susceptibility of 2H-TaSe2

The EPR spectra of manganese and gadolinium doped PbTe and SnTe semiconductors were measured from 4.2K up to 400K for dopant concentrations ranging from 50 to 50000 ppm. Resolved fine, hyperfine and superhyperfine structure as well as forbidden and cluster lines were measured. The combination of Lorentzian absorption and dispersion derivatives fits the recorded spectra well. Nonlinear broadening of linewidths with temperature is characteristic in these semiconductors. A manganese-induced broad maximum in linewidth versus temperature was found in PbTe around 180K for dopant concentrations in excess of 1.5at%.

F.T. Hedgcock

Papers

Two band model for negative magnetoresistance in heavily doped semiconductors

Electron paramagnetic resonance of Gd3+ in PbTe

Anisotropy in the magnetic susceptibility of 2H-TaSe2

EPR of Mn2+ and Gd3+ Ions in PbTe and SnTe Semiconductors