Steve_hi
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High temperature stir bars
Does any one know of a good source for eliptical stir bars capable of withstanding a miniimum of 300° C for research in Nano technology
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bfesser
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Seal an iron rod or cylindrical magnet in a borosilicate glass tube.
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Magpie
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I've seen glass encapsulated stir bars in the VWR and Fisher Scientific catalogs. I can't remember if they were eliptical.
The single most important condition for a successful synthesis is good mixing - Nicodem
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zed
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Well, your reaction conditions have ruled out the best materials. Your Neodymium alloys lose their mojo at under 100C. Cobalt Samarium alloys are
better, but not that much better.
The nice thing about the above materials, is the powerful magnetic coupling that can be achieved.
I'm not sure what magnets will function well at 300C. Is overhead stirring an option?
OK, I checked. Apparently, Alnico magnets work pretty well at temperatures over 300C.
Of course, if you eventually need to deal with high viscosity, high temp, high pressure, and high RPM....you have limited options. A pressure
reactor with a sealed stirring unit will probably be required. Generally from Parr or Autoclave Engineers,
[Edited on 3-3-2013 by zed]
[Edited on 3-3-2013 by zed]
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watson.fawkes
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Quote: Originally posted by zed  | | Your Neodymium alloys lose their mojo at under 100C. Cobalt Samarium alloys are better, but not that much better |
Not correct. See the physical properties on this Wikipedia page on neodymium magnets. The Curie point for these is 310 - 400 °C. Sm-Co listed as 720 - 800 °C. Alnico listed as 700 - 860 °C.
I'll concur about not using Nd, but mostly because there's insufficient headroom in the temperature range. Suddenly losing stirring because of
transient heat could be not a good thing. Sm-Co would be fine.
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zed
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Our number don't jib. Things may have changed. New Alloys Etc.. Based on manufacturers recommendations, I have a different take on this.
Recommended working temperatures for these magnets, are not very high.
http://www.themagnetguide.com/neodymium-magnet.html
Also of note; prices of Samarium Cobalt magnets, are quite high. Too much.
http://www.themagnetguide.com/samarium-cobalt-magnet.html
[Edited on 10-3-2013 by zed]
[Edited on 10-3-2013 by zed]
[Edited on 10-3-2013 by zed]
[Edited on 10-3-2013 by zed]
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watson.fawkes
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| Quote: Originally posted by zed | Our number don't jib. Things may have changed. New Alloys Etc.. Based on manufacturers recommendations, I have a different take on this.
Recommended working temperatures for these magnets, are not very high. |
"Recommended working temperature" is
not the same as "Curie point". Putting a magnet in a stir bar doesn't depend very much on the absolute magnetic strength, which does decrease with
temperature. The main difference would be that its ability to stir viscous liquids would drop somewhat.
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zed
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My take, is that magnetic strength decreases dramatically with temperature. At least, in the case of the rare earth magnets we are discussing. Since
I like to be able to crank up the RPMs enough to get really good gas entrapment (for hydrogenations etc.), my personal requirements for magnetic
stir-bars may differ from yours. I need a very strong magnetic coupling; nothing else will do.
Perhaps, the original petitioner will clarify exactly what he is trying to do? What solvents, what pressures, etc...
300C is pretty hot. If the solvent is water....300C water means very high pressure. Since such pressures mean thick vessel walls, magnetic stir
bars, generally aren't going to work well anyway.
So, Stevehi.....Tell us more.
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Steve_hi
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It's not for me its for a friend Feng Shi doing research in nano technology at the University of Sherbrooke he id doing research with rare earth
metals to do imaging.
Facile synthesis of b-NaLuF4 : Yb/Tm hexagonal nanoplates with intense
ultraviolet upconversion luminescence†
Feng Shi, Jianshuo Wang, Xuesong Zhai, Dan Zhao and Weiping Qin*
Received 18th January 2011, Accepted 11th March 2011
DOI: 10.1039/c1ce05092c
Monodisperse, hexagonal phase NaLuF4 : Yb,Tm nanoplates (NPs) with uniform size have been
successfully synthesized by a novel solution-based method. The nanoplates have a perfect hexagonal
shape with a diameter of 180 nm. As a chelating agent and shape modifier, oleic acid (OA) was
introduced into the reaction mixture and played a key role in fine-tuning the nanoplates. A possible
growth mechanism was proposed for the formation of b-NaLuF4 nanoplates. Spectral analysis showed
that the b-NaLuF4 : Yb,Tm nanoplates were excellent materials for intense ultraviolet and
blue upconversion luminescence. To our best knowledge, it is the first time such intense 5-photon
upconversion fluorescence from the 1I6 level of Tm3+ ions, which is much stronger than the 4-photon
upconversion fluorescence from the 1D2 level and the 3-photon upconversion fluorescence from the 1G4
level, has been demonstrated. The analysis on temporal evolutions of UC luminescence suggests that
b-NaLuF4 nanocrystals might be a better kind of upconversion material than their b-NaYF4
counterpart. This powerfully demonstrates that b-NaLuF4 is an excellent host lattice for upconversion
luminescence materials. Due to the unique luminescence, these b-NaLuF4 nanoplates may be promising
for further fundamental research and applications in color displays and solid-state lasers.
Introduction
Chemical and physical studies on the nanometer scale have
experienced an enormous development since the 1990s and led to
the appearance of the new interdisciplinary fields of ‘‘nanoscience
and nanotechnology’’.1 In these fields, high-quality dispersible
inorganic nanocrystals allow independent design of their size-/
shape-dependent properties.2–7 In particular, the rational design
of synthetic protocols toward monodisperse colloidal nanocrystals
and the revelation of their underlying chemical principles
are crucial not only for reproducible large-scale fabrication of
high-quality products, but also for the correct interpretation of
the collective physical properties of an ensemble of particles in
terms of individual features.8–10 The process conditions required
for synthesizing monodisperse particles with micrometer size are
relatively well established, and a similar principle could be
applied to the synthesis of uniform-sized nanocrystals. It is
critical for the successful synthesis of monodisperse nanocrystals
to inhibit additional nucleation during their growth process.11
In addition, phase control is also crucially important, as the
properties of materials are determined first by their phases. From
a fundamental point of view, the physical understanding of the
luminescence properties of rare-earth ions in nanocrystals with
changing crystal phase and local structure are very important. It
is already reported that the crystal phase, size, shape, and dopedconcentration
play a key role in emission properties of rare-earth
doped nanocrystals.12,13 Moreover, the phases of crystalline seeds
can induce different (isotropic or anisotropic) growth kinetics in
the growth of nanocrystals, and therefore result in different
shapes on the nanoscale. In general, organic additives functioning
as ‘‘shape modifiers’’ are favorable in controlling the
morphology and size of nanocrystals because they either
promote or inhibit crystal growth through dynamically modifying
their crystal facets.14 Therefore, a cost-effective and environmentally-
friendly synthetic method becomes more popular
for obtaining monodisperse nanocrystals. Nowadays, one of the
challenges facing the chemists in the field is to develop a facile
approach to synthesize nanocrystals with controllable
morphologies.
Recently, much research attention has been paid to the field of
rare earth materials since they have many potential applications
based on their novel electronic and optical properties from their
4f electrons. Controlled synthesis of well-defined lanthanide
compounds with uniform dimensions and shapes is of extraordinary
importance because the electronic structure, bonding,
surface energy, and chemical reactivity are directly related to
their size and morphology.15–17 To date, numerous efforts have
been devoted to the exploration of various convenient and
State Key Laboratory on Integrated Optoelectronics, College of Electronic
Science and Engineering, Jilin University, Changchun, 130012, P. R.
China. E-mail: wpqin@jlu.edu.cn; Fax: + 86-431-85168240(1)-8325;
Tel: 86-431-85168240-8325
† Electronic supplementary information (ESI) available: Experimental
details and additional figures. See DOI: 10.1039/c1ce05092c
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efficient approaches for the fabrication of different kinds of
lanthanide compounds. Among the various synthesis techniques,
a solution-based method, as a typical approach, has been proven
to be a facile and general means for the synthesis of nano- and
micro-sized lanthanide compounds with special morphologies
and architectures.18–21
Rare-earth (RE) fluorides, for example, NaREF4 with high
refractive indexes and low phonon energies, have drawn a lot of
attention in recent years due to their potential applications in
a number of fields, such as solid-state lasers,22 three-dimensional
displays,23 low-intensity IR imaging,24,25 and so forth. Compared
to quantum dots and traditional organic dyes that are pumped by
ultraviolet radiation as fluorescent labels, lanthanide-doped
nanomaterials have shown narrow emission linewidth and very
low background emission without autofluorescence. Upconversion
(UC) luminescence of many different materials has been
investigated by using commercial 980 nm diode lasers as excitation
sources. In virtue of UC technique in biolabeling and bioimaging,
the detectable depth can be greatly increased since
980 nm IR light can penetrate several centimeters in biological
tissues. Therefore, it is essential to select appropriate host
materials and synthesize lanthanide-doped nanoparticles with
favorable optical properties such as high upconversion efficiency
and controllable emission profile. In the past decade, much effort
has been devoted to the shape-controlled synthesis of fluoride
micro- and nano-crystals, and especially to the fabrication of
NaYF4 nanocrystals with different morphologies, because
NaYF4 has been generally regarded as the most efficient
upconversion host. It is well-known that there is a higher
upconversion efficiency of the hexagonal phase than the cubic
phase in fluoride nanocrystals (i.e. NaREF4). Therefore, it is very
important to study the effect of the hexagonal phase structure on
the upconversion luminescence properties of RE doped fluoride
nanocrystals.26 Nevertheless, repeated attempts to synthesize
a kind of fluoride nanocrystals with better UC properties, especially
in intense ultraviolet and blue upconversion luminescence,
have never ceased. Accordingly, in this paper, we present a facile
and relatively environmental-benign approach for the synthesis
of monodisperse lanthanide-doped b-NaLuF4 hexagonal nanoplates
with uniform size, assisted by OA as a shape modifier, and
demonstrate that OA plays a critical role in the formation of their
final morphology. The morphological evolution and the growth
mechanism of the synthesized b-NaLuF4 hexagonal nanoplates
have been studied in detail. Moreover, UC emissions of
b-NaLuF4 : 18% Yb, 0.5% Tm were investigated. Compared
with b-NaYF4 : 18% Yb, 0.5% Tm of the same size, the samples
showed intense ultraviolet and blue upconversion emissions with
uniform dimensions. These results demonstrate that b-NaLuF4 is
an excellent host lattice for UC luminescence of optically active
lanthanide ions.
Experimental
Chemicals
All chemicals were of analytical grade and used without further
purification. LuCl3$6H2O (99.999%), YbCl3$6H2O (99.999%),
TmCl3$6H2O (99.999%), NaOH (98%), NH4F (98%) were
supplied by Shanghai Chemical Reagent Company.
1-Octadecene (ODE, 90%) and oleic acid (OA, 90%) were
supplied by Alfa Aesar.
Synthesis of hexagonal phase b-NaLuF4 : 18% Yb, 0.5% Tm
nanoplates
In a typical procedure for the synthesis of b-NaLuF4 : 18% Yb,
0.5% Tm, 1 mmol RECl3$6H2O (RE ¼ Lu, Yb, Tm) were added
to a 100 mL three-neck round-bottom flask containing ODE
(15 mL) and OA (6 mL). The solution was stirred magnetically
and heated to 160 C for 30 min to form the lanthanide oleate
complexes and remove residual water and oxygen. The temperature
was then cooled to room temperature with a gentle flow of
argon gas through the reaction flask. During this time, a solution
of NH4F (4 mmol) and NaOH (2.5 mmol) dissolved in methanol
(10 mL) was added to the flask and the resulting mixture was
stirred for 30 min. The temperature was then increased to 50 C
to evaporate methanol from the reaction mixture; afterwards, the
solution was heated to 300 C in an argon atmosphere for 60 min
and then cooled to room temperature naturally. The resulting
solid state products were precipitated by addition of ethanol,
collected by centrifugation, washed with ethanol three times, and
finally redispersed in cyclohexane for further experiments.
Characterization
The phase identification was performed by X-ray diffraction
(XRD) (Model Rigaku RU-200b), using nickel-filtered Cu-Ka
radiation (l ¼ 1.5406 A). The step scan covered the angular
range from 10 to 70 in steps of 0.02. Transmission electron
microscopy (TEM) images were obtained (JEOL 3010) with an
acceleration voltage of 300 kV. Upconversion emission spectra of
the samples were recorded with a fluorescence spectrophotometer
(Hitachi F-4500). A power-adjustable laser diode (980 nm,
0–2 W) was employed as the upconversion pump source. A
Raman shift laser running at 953.6 nm was used as the pulsed
excitation source for temporal investigations. A Fourier transform
infrared (FT–IR) spectrometer (JASCOFT/IR-420) was
used to record infrared spectra of the samples by using the KBr
pellet technique. Powder materials were pressed into a tungsten
mesh grid and installed in an in situ FT–IR transmission cell, and
the samples were degassed in a vacuum system with a residual
pressure of less than 3 104 Torr at room temperature. All
measurements were performed at room temperature. Elemental
analysis was performed on a Perkin-Elmer ICP-OES Optima
3300DV.
Results and discussion
Phase identification and morphologies
The crystal structures and the phase purity of the as-prepared
products were examined by XRD. Typical XRD patterns of
them are presented in Fig. 1, and all diffraction peaks of the
samples depicted in Fig. 1 matched very well with the values for
100, 110, 101, 200, 111, 201, 210, 002, 300, 211, 102, 112, 220,
202, and 310 reflections of hexagonal phase structure NaLuF4
(Joint Committee for Powder Diffraction Studies (JCPDS) card
27-0726). No other impurity peak corresponding to other
impurities was detected, which revealed that pure b-NaLuF4 had
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been fabricated. It can also be seen that the diffraction peaks of
the b-NaLuF4 samples are very sharp and strong, indicating that
products with high crystallinity have been obtained at the high
temperature treatment (300 C). High crystallinity is important
for phosphors, because it generally means less traps and stronger
luminescence.
The detailed structure of the b-NaLuF4 phosphors was
obtained by using TEM and high resolution TEM (HRTEM).
Fig. 2 shows the low-magnification and high-magnification TEM
images, and reveals that the b-NaLuF4 phosphors are uniform
hexagonal nanoplates of 180 nm in the leading diagonal. These
trim nanoplates, displaying uniform morphology and high
quality, are likely to be self-assembled on theTEM grid due to the
interaction of their surface hydrophobic surfactants (i.e. OA). The
HRTEM image in Fig. 2c reveals their highly crystalline nature
and structural uniformity. The interplanar distance (d ¼ 0.51 nm)
matches that of the (100) lattice planes of b-NaLuF4. The corresponding
fast Fourier transform (FFT) of the HRTEM image
(inset in Fig. 2c) further demonstrates a perfect hexagonal crystal
structure, which is in good agreement with the XRD results presented
in Fig. 1. The amount of Lu, Yb, and Tm were quantified
using inductively coupled plasma–atomic emission spectrometry
(ICP–OES), and the Lu/Yb/Tm molar ratio were determined to be
80.98 : 18.5 : 0.52, which is close to the stoichiometric ratio for the
chloride reactants used in the experiment.
The functional groups attached on the b-NaLuF4 : 18% Yb,
0.5% Tm nanoplates were identified with FT–IR studies as shown
in Fig. 3. OA exhibits an absorption band around 3448 cm1,
corresponding to the stretching vibration of hydroxyl group
(–OH) of the –COOH of absorbed oleic acid. The 2928 and
2859 cm1 absorption bands are assigned to the asymmetric and
symmetric stretching vibrations of methylene (CH2) in the long
alkyl chain of the OA molecules. The peak at 3009 cm1 arising
from the]C–H stretching vibration can be seen in the spectrum
and the peak at 1712 cm1 is attributed to the C]O stretching
vibration frequency. In addition, the bands at 1567 and 1461 cm1
can also be assigned to the asymmetric and symmetric stretching
vibration of the carboxylic group (–COOH) of the bound oleic
acid, respectively. Due to the presence of the OA overlayer, these
nanoplates can be transparently dispersed into a nonpolar solvent
such as cyclohexane and be aggregated by adding polar solvent
such as ethanol, which enabled their purification and application
both in solid and fluid environments.
Growth mechanism
One of the critical factors responsible for the shape determination
of the nanoplates is the crystallographic phase of the initial
seeds during the nucleation process. The seeds can be of a variety
of crystallographic phases, but the stable phase is highly dependent
on the environment, especially on temperature.27 The crystal
structure of NaLuF4 exhibits two polymorphic forms, they are
cubic (a-) and hexagonal (b-) phases, Thoma and co-workers
have confirmed the crystallographic structures of these phases
and reported that kinetic energy influences the phase change
from cubic to hexagonal.28 In a previous study, a cubic to
hexagonal phase transformation of NaREF4 was performed by
heating treatment, varying the RE3+ : F ratio and the pH of the
solution etc. In our experiment, the RE3+ : F ratio and the pH of
the solution were fixed. That solid state crystal nuclei was formed
Fig. 1 Powder X-ray diffraction pattern of NaLuF4 : Yb/Tm
nanoplates.
Fig. 2 TEM and HRTEM images of NaLuF4 : Yb/Tm hexagonal nanoplates: (a) low-magnification image, (b) high-magnification image, (c) HRTEM
image of a single nanoplate. Inset: Fast Fourier transform (FFT) of the HRTEM image exhibits a hexagonal symmetry.
Fig. 3 FT–IR spectrum of the NaLuF4 : Yb/Tm nanoplates.
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at room temperature and the reaction system temperature
quickly raised to 300 C, which were key factors leading to
formation of hexagonal phase b-NaLuF4 nanoplates. Therefore,
the (b-) phase is apt to form due to the high temperature which
provides enough energy to overcome the energy barrier between
the cubic and hexagonal NaREF4. In the case of a-NaREF4, Na+
and RE3+ cations are randomly distributed in the cationic sublattice,
whereas for b-NaREF4, there are three types of cation
sites: a one-fold site occupied by RE3+, a one-fold site occupied
randomly by 1/2Na+ and 1/2RE3+, and a two-fold site occupied
randomly by Na+ and vacancies. Thus, the process of b-NaREF4
is of a disorder-to-order character with respect to cations.29
According to previous research, b-NaREF4 is a more stable
phase form compared with the a-NaREF4 under the thermodynamic
driving force effect (300 C),30 relatively high energy is
needed to overcome the dynamic energy barrier of a/b phase
transition. In this work, it seems that the temperature such as
300 C can supply sufficient energy to obtain b-NaLuF4 nanoplates,
so our method is facile and effective for obtaining the
desirable pure products of b-NaLuF4.
The inhibition of additional nucleation during growth, in
other words, the complete separation of nucleation and growth,
is critical for the successful synthesis of monodisperse nanocrystals.
11 Our strategy is to convert all the reactive fluoride
reagents into solid state crystal nuclei before the subsequent
crystal growth/ripening, and then the temperature is raised and
small and non-uniform crystal nuclei are merged to form big and
uniform nanoplates via an Ostwald ripening process.31 Thus, we
can synthesize monodisperse nanoplates from the separation of
nucleation and growth processes, which tend to take place at
different temperatures. Our experimental result suggests that the
successful synthesis of monodisperse nanoplates can be attributed
to the effective separation of nucleation and growth
processes, viz., the different temperature dependencies of nucleation
and growth kinetics.
Oleic acid (OA), acting as the chelating agent and shape
modifier, has an important influence on the b-NaLuF4 shape
evolution in our synthesis. Due to b-NaLuF4 nanoplates having
a hexagonal shape, their surfaces are typically (001) top/bottom
planes and six prismatic side planes of the energetically equivalent
(100) family.32 Different chelating agents show diverse effects
on the shape evolution of b-NaLuF4 nanoplates,33 which gives
rise to different growth rates in different crystallographic directions.
In our experimental conditions, OA interacts more
strongly with (001) facets than (100) facets, resulting in growth
predominantly along the [100] direction to form hexagonal
nanoplates. This explanation can be also certified by the
formation of ZnO hexagonal nanoplates, that the growth along
the [001] direction is prohibited by adsorbing additive Cit3 ions
on (001) surfaces of ZnO34,35 and results in the formation of thin
ZnO nanoplates.
In summary, there are four key factors that determine the final
shapes of b-NaLuF4 nanoplates. The first one is the crystallographic
phase of the initial seeds. b-NaLuF4 seeds are in a thermodynamically
stable phase, which means that crystal
anisotropy induces the aeolotropic growth along their different
crystallographically reactive directions. The second one is high
temperature (300 C) in the synthesis process, which can supply
efficient energy to overcome the dynamic energy barrier of
a / b phase transition. Accordingly, hexagonal b-NaLuF4
nanoplates are formed. The third factor is the complete separation
of nucleation and growth processes, which arise from the
different temperature dependencies of nucleation and growth
kinetics. Thus, monodisperse particles of nanoplates can be well
established. The last one is the OA acting as the chelating agent
and shape modifier, which plays an important role in stimulating
the formation of b-NaLuF4 nanoplates and the shape evolution.
Scheme 1 shows the possible formation mechanism for the
b-NaLuF4 hexagonal nanoplates.
UC luminescence properties
It is well known that rare earth fluorides are frequently used as
host lattices for the luminescence of various optically active
lanthanide ions. However, lutetium compounds are an exception
and have been rarely studied.15,33,36 Here, we also investigate the
luminescence properties of the b-NaLuF4 : 18% Yb3+, 0.5% Tm3+
hexagonal nanoplates in an effort to reveal whether b-NaLuF4 is
an efficient host lattice or not. It can be seen from XRD patterns
(Fig. 1) and TEM (Fig. 2) that the doped lanthanides alters
neither the crystal structure nor the morphology of b-NaLuF4
hexagonal nanoplates.
Excited state absorption (ESA) and energy transfer (ET) are
efficient UC mechanisms in RE3+-doped UC materials. Particularly,
under 980 nm excitation, the ET from Yb3+ to Tm3+ in Yb3+-
sensitized Tm3+-doped materials plays a key role in UC processes
of Tm3+ ions because Yb3+ ions have a large absorption cross
section at 980 nm. The UC spectra of b-NaLuF4 : 18 %Yb3+,
0.5% Tm3+ hexagonal nanoplates are presented in Fig. 4, which
were recorded under different excitation powers at room
temperature. We can see from spectral analysis that
b-NaLuF4 : 18% Yb3+, 0.5% Tm3+ hexagonal nanoplates show
intense ultraviolet and blue upconversion luminescence.
Fig. 5 shows the UC luminescence mechanism and population
processes in an Yb3+–Tm3+ codoped system. The UC luminescence
of 684 nm and 790 nm occurs via a two-step ET from
Yb3+ to Tm3+. First, the Tm3+ ions in the ground state 3H6 are
excited to the excited state 3H5 via an ET from neighboring Yb3+
ions. Subsequent non-radiative relaxation of 3H5 / 3F4 populates
the 3F4 level. In the second-step excitation, Tm3+ ions in the
3F4 state are excited to the 3F2,3 states via another ET from
excited Yb3+ ions. The populated 3F2 may non-radiatively relax
to the 3F3 and 3H4 levels, and then produces red (684 nm) and
near infrared (790 nm) emissions by the radiative transitions to
the ground state 3H6, respectively. Some of the Tm3+ ions in the
3H4 state can be excited to the 1G4 level via the phonon-assisted
ET: 2F5/2 / 2F7/2 (Yb3+): 3H4 / 1G4 (Tm3+), and then produce
blue (475 nm) and red (649 nm) emissions simultaneously.
The 1D2 level of Tm3+ ions can not be populated directly via an
ET from excited Yb3+ to the Tm3+ in the 1G4 level due to the large
Scheme 1 Possible formation mechanism for b-NaLuF4 hexagonal
nanoplates.
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energy mismatch (about 3500 cm1) in it. The cross relaxation
processes of 3F2,3 + 3H4 / 3H6 + 1D2 between Tm3+ ions may
alternatively play an important role in populating the 1D2 level.37
The radiative transitions from the populated 1D2 level to the
ground state 3H6 and the intermediate state 3F4 to yield 363 nm
and 451 nm, respectively. On the other hand, the Tm3+ ions in
the 1D2 state can be excited to the 3P2 state via another ET from
excited Yb3+ ions and then relax rapidly to the 1I6 state. The
ultraviolet emissions of 291 nm and 345 nm can be observed
simultaneously via the transitions of 1I6 / 3H6 and 1I6 / 3F4,
respectively. The pumping power dependence of the fluorescent
intensity has been investigated. For an unsaturated UC process,
the emission intensity is proportional to the nth power of the
excitation intensity, and the integer n is the number of the laser
photons absorbed per upconverted photon emitted. Fig. S1
(ESI†) shows the power dependence of the UC emission intensities:
n ¼ 4.60, 3.69, and 2.79 for 291, 363, and 475 nm emissions,
respectively. This means that the population of the states 1I6, 1D2,
and 1G4 came from five-photon, four-photon, and three-photon
UC processes, respectively. It can be seen from Fig. 4 that the
intense ultraviolet upconversion emissions have been recorded.
These intense ultraviolet emissions can be attributed to the efficient
cross relaxation process (3F2,3 + 3H4/3H6 + 1D2) between
Tm3+ ions, which consequently makes the 1D2 level efficiently
populated. The populated 1D2 level of the Tm3+ may relax
radiatively to two levels: 3F4 and 3H6, which cause 363 and 451
nm emissions, respectively. On the other hand, an increase of the
1D2 population also makes the energy transfer of 2F5/2 (Yb3+) +
1D2 (Tm3+)/2F7/2 (Yb3+) + 1I6 (Tm3+) efficient.38 The populated
1I6 level produces 291 and 345 nm emissions simultaneously.
In order to demonstrate that b-NaLuF4 is an excellent host for
Ln3+ doping, we compared it to the same size b-NaYF4 : 18%
Yb, 0.5% Tm by the dynamic analysis on Tm3+ excited states.
The same sized b-NaYF4 : 18% Yb,0.5 %Tm nanoplates was
synthesized by the same method and the characterized results
(XRD, TEM) are presented in Fig. S2 (ESI†). The decay curves
for the representative emissions from the 1I6, 1D2, and 1G4 levels
of Tm3+ ions in b-NaYF4 : 18% Yb, 0.5% Tm and
b-NaLuF4 : 18% Yb, 0.5% Tm nanoplates were recorded under
953.6 nm pulsed Raman shift laser, as shown in Fig. 6. Each of
the decay curves can be fitted well into a single-exponential
function as I ¼ I0exp(t/s) in the detail, where I0 is the initial
emission intensity at t ¼ 0 and s is the lifetime of the monitored
level. The results obtained from least square analyses were listed
in Table 1.
As can be seen from Table 1, all the lifetimes of the 1I6, 1D2,
and 1G4 levels of Tm3+ ions are longer in b-NaLuF4 nanoplates
than those of the counterpart levels in b-NaYF4 nanoplates. For
UC materials, a long lifetime usually means a highly-efficient UC
luminescence, and therefore, from the above comparison on
lifetimes, we can infer that the b-NaLuF4 hexagonal nanoplates
Fig. 4 UC luminescence spectra of NaLuF4 : 18% Yb, 0.5% Tm nanoplates
under different 980 nm excitation powers.
Fig. 5 Schematic diagram of upconversion processes in an Yb3+–Tm3+
codoped system upon the excitation of 980 nm laser.
Fig. 6 Temporal evolutions of UC luminescence from 1I6, 1D2, and 1G4
levels of Tm3+ ions in b-NaYF4 : 18% Yb, 0.5% Tm (a–c) and
b-NaLuF4 : 18% Yb, 0.5% Tm (d–f) nanoplates by monitoring the UC
emissions centered at 345, 451, and 475 nm under the excitation of
a 953.6 nm pulsed Raman shift laser, black circles experimental data;
coloured solid line: fitting by I ¼ I0 exp(t/s).
3786 | CrystEngComm, 2011, 13, 3782–3787 This journal is ª The Royal Society of Chemistry 2011
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are excellent host nanomaterials for upconversion fluorescence.
The significant difference of the lifetimes should be ascribed to
the dissimilar crystal-field surroundings of Yb3+ and Tm3+ ions
embedded in b-NaYF4 and in b-NaLuF4. The Lu3+ and Y3+ ionic
radii are different (Lu3+ ionic radius is 0.85 A, Y3+ ionic radius is
0.89 A), which may result in the more severe modification and
tailor the local environment of the Yb3+ and Tm3+ ions in the
lattice, and therefore enhance the radiative transition rate
favoring the enhancement of the upconversion emission intensity.
The tailoring effect induced the longer lifetime. Thus, we
may say that the upconversion emission intensity are mainly
attributed to modifications of non-radiative relaxation processes
by controlling crystal phase, shape, size etc.39
Conclusion
In conclusion, we successfully synthesized highly uniform and
well-defined b-NaLuF4 : 18% Yb3+, 0.5 Tm3+ nanoplates with
intense ultraviolet and visible blue upconversion emissions by an
efficient and user-friendly approach using oleic acid as
a chelating agent and shape modifier. The crystal phases and
morphologies can be controlled by the described conditions. The
results demonstrated that higher temperature and complete
separation of nucleation and growth facilitated the formation of
more stable hexagonal phase structures. Compared with the
same sized b-NaYF4 : 18% Yb, 0.5% Tm nanoplates, the
b-NaLuF4 : 18% Yb3+, 0.5% Tm3+ nanoplates have longer fluorescent
lifetimes for the 1I6, 1D2, and 1G4 levels of Tm3+ ions. Such
a spectral character has led to intense 5-photon UC emissions. In
the b-NaLuF4 : 18% Yb3+, 0.5% Tm3+ nanoplates, the ultraviolet
UC emissions from the 1I6 level of Tm3+ were much stronger than
the 4-photon upconversion fluorescence from the 1D2 level and
the 3-photon upconversion fluorescence from the 1G4 level. In
fact, it is the first time that such intense 5-photon upconversion
fluorescence of Tm3+ ions has been demonstrated, which further
confirms that the b-NaLuF4 is an excellent UC host, especially
for high-order photon UC. It is therefore expected that these
single-crystal and ideal uniform nanoplates structures with
intense ultraviolet and blue emissions can be used for applications
in ultraviolet and visible solid-state lasers, biolabeling,
bioimaging, and 3D displays.
Acknowledgements
This work was supported by the National High Technology
Research and Development Program of China (863 Program:
2009AA03Z309), the National Natural Science Foundation of
China (NSFC) (grants 51072065, 60908031 and 60908001) and
the Program for New Century Excellent Talents in University
(No: NCET-08-0243).
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Table 1 Lifetimes of 1I6, 1D2 and 1G4 levels of Tm3+ ions in b-
NaYF4 : 18% Yb, 0.5% Tm (Fig. 6a, 6b, 6c) and b-NaLuF4 : 18% Yb,
0.5% Tm (Fig. 6d, 6e, 6f) nanoplates calculated from the temporal
evolutions in Fig. 6
1I6 (ms) 1D2 (ms) 1G4 (ms)
NaYF4 : 18% Yb, 0.5% Tm 156 198 534
NaLuF4 : 18% Yb, 0.5% Tm 198 291 563
This journal is ª The Royal Society of Chemistry 2011 CrystEngComm, 2011, 13, 3782–3787 | 3787
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Steve_hi
Hazard to Others
Posts: 196
Registered: 4-12-2010
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Dont know if this stuff helps you understand what he might need them for but he asked me where he could get them. He is Chinese and isn't too familiar
with where to get the things he wants around here so I sugested to him to join this forum.
But his english isn't very good
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zed
International Hazard
Posts: 2277
Registered: 6-9-2008
Location: Great State of Jefferson, City of Portland
Member Is Offline
Mood: Semi-repentant Sith Lord
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Alnico bars should work fine. Just direct your friend to e-bay. Provided of course, that his government allows him access to e-bay.
http://www.ebay.com/itm/7pcs-magnetic-stirrer-mixer-stir-bar...
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Steve_hi
Hazard to Others
Posts: 196
Registered: 4-12-2010
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Thanks Zed
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killer_lapin
Harmless
Posts: 47
Registered: 23-7-2010
Location: Qc, CAN
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A simple nail piece encapsuled in a borosilicate tube will work fine, It's like when you put a paper clip in your oil bath to have an even heating. I
used tu do that when I didn't had the nice teflon coated ones.
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