Spatially Resolved Quantitative Magnetic Order Measurement in Spinel
CuCr2S4 Nanocrystals
D. S. Negi,1,2 B. Loukya,1,2 K. Ramasamy, 3,4 A. Gupta3,4 and R. Datta1,2*
1
International Centre for Materials Science, Jawaharlal Nehru Centre for Advanced Scientific Research,
Bangalore 560064, India.
2
Chemistry and Physics of Materials Unit, Jawaharlal Nehru Centre for Advanced Scientific Research,
Bangalore 560064, India.
3
Center for Materials for Information Technology, University of Alabama, Tuscaloosa, Alabama 35487, USA.
4
Center for Integrated Nanotechnology, Los Alamos National Laboratory, Albuquerque, New Mexico, 87123,
USA.
Beam damage under converged electron probe:
Beam damage is a well-known problem in transmission electron microscopy. In scanning
transmission mode the damage is more severe due to an increase in current density of the probe
electrons by the mini condenser lens. The damage proceeds via evaporating atomic species from the
specimen area under irradiation either by radiolysis (significant at lower kV) or knock-on damage
(significant at higher kV) and eventual formation of a hole. The kinetics involved in the hole
formation will depend on the material system. Figure S1 shows what happens in the CuCr2S4
nanocrystals if only a spot size 9 beam (in parallel TEM mode) is focused on a sample area in order to
achieve high spatial resolution. Hole formation occurs in less than a minute. In STEM, since the
current density is much higher, the formation of hole will be accelerated. This will prevent acquiring
good signal for energy loss events at higher kV and thereby obtaining reliable data that truly represent
the material properties.
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(a)
(b)
(c)
Figure S1. (a) & (b) Beam damage under converged probe for a dwell time of less than a minute with
spot size 9 in regular TEM mode.The hole regions in the images are marked with white dotted circles.
(c) With STEM probe the damage will be more rapid as a result of the higher current density due to
stronger beam convergence. The hole regions in the images are marked with white dotted circles.
Experimental method:
We have taken a different experimental approach that is simple and can be used by others on a routine
basis. Previously applied spatially resolved EMCD technique is a difficult option and can be used
either in spectroscopy mode or in imaging mode. In spectroscopy mode, objective aperture is placed
sequentially at two opposite position of Thales circle to acquire data, and in imaging mode, an energy
filtered diffraction pattern is obtained for the needed energy range, e.g, Cr L3,2 edge in this case. Both
methods require exposing the sample area for significantly longer time to obtain useful signal.
Another problem with the spectroscopy mode is that the acquisition of data from two different
locations in the diffraction plane is not simultaneous and often two different intensity levels make it
almost impossible to compare the two spectra.
2
In the present method we used high resolution electron energy loss spectroscopy using gun
monochromator attached with FEI 80-300 keV TITAN microscope. Figure S2 shows how the beam
appears after excitation. The resolution achievable is better than 0.18 eV at a particular setting. A
combination of magnification and GIF entrance aperture as selected area aperture can be used to
select a particular area with high spatial resolution. The decrease in signal due to high magnification
can be compensated by de-magnifying the mono-probe on the sample area to increase the current.
This helps us to choose a very small area and at the same time acquire spectra with good signal.
Figure S2. (a) Monochromatic illumination in a FEI TITAN3TM 80-300 kV microscope. (b)
The resolution is better than 0.18eV as measured from the full width half maximum of the
zero loss peak. (c) Shadow image of a 2.5 mm GIF entrance aperture on GIF CCD.
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Figure S3. Images recorded through the GIF camera showing how a combination of
magnification and GIF entrance aperture is used to select a specific area of the nanocrystal.
Synthesis.
The detailed synthesis of CuCr2S4 nanocubes and nanoclusters can be found in Ref. [31]. For
TEM sample preparation the particles were first dispersed in toluene solution by sonication
and then drop cast on a holey carbon grid (300 mesh size). The grid was subsequently dried
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under an infrared lamp for 1 hour to ensure complete evaporation of toluene, which otherwise
would cause problem during TEM imaging with high electron dose.
Characterization.
The two different morphology nanoparticles were previously well characterized by X-ray
diffraction, transmission electron microscopy and magnetometry, both at 300 K and 5 K, and
the results are provided in ref.31. In the present investigation, a FEI TITAN3TM 80-300 keV
aberration corrected transmission electron microscope was used equipped with a gun
monochromator having energy resolution better than 0.18 eV as measured from the full width
half maxima (FWHM) of the zero loss peak. For acquiring Cr L3,2 absorption edge, a
spectrometer dispersion of 0.03 eV/pixel and GIF entrance aperture of 2.5 mm was used to
improve the signal at the expense of resolution (0.18 eV to ~0.25 eV). Exposure time was ~
10 seconds for each acquisition, which is much lower compared to typical EMCD acquisition
time (~10-15 min). Low temperature measurement was performed in a Gatan single tilt liquid
nitrogen-based cryo-holder at 90 K. EFTEM imaging was performed in the same microscope
equipped with imaging filter.The carbon present in 1-DDT, ODA, OLA capping agents
(Carbon edge 284 eV) was mapped with an exposure of 10 seconds.
Theoretical calculations.
Theoretical calculations of density of states (DOS), magnetic moment, and electron energy
loss near edge spectroscopy (ELNES) were performed using WIEN2K code [40]. It is a full
potential density functional theory calculation with linear augmented plane wave + local
orbital as basis to calculate the crystal properties. The lattice parameter of CuCr2S4 was
optimized with Perdew–Burke–Ernzerhof (PBE) functional [41] within the generalized
gradient approximation (GGA) with self-consistent field cycles (SCF) until the energy and
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charge values converged below 0.0001Ry and 0.001 electrons, respectively. The muffin tin
radius was selected in such a way so that they do not overlap. Core states and valance states
were separated by an energy of -6 Ry. Rmtkmax parameter was set to 7. After volume
minimization the forces were minimized below 1 mRy/a.u. The relaxed lattice parameters
obtained via force minimization are 9.4 Å and 9.2 Å for ferromagnetic (FM) and
nonmagnetic (NM) configurations, respectively. K mesh of 10x10x10 point was used for the
spin polarized self- consistent field cycle calculation. Tetrahedral method was used for the
Brillouin zone integration. Since exchange correlation functional in GGA and LDA
underestimate the band gap of materials and predicts the erroneous states containing
transition metals, we used the modified Becke-Johnson (mBJ) potential as proposed by
F.Tran and P. Blaha [42]. The mBJ method is computationally economical and gives the band
gap close to GW approximation (G stand for Green function and W stand for Coulomb
interaction). In comparison, the mBJLDA method is empirical and no exchange correlation
functional energy is associated with it, so this method should not be used for structural
optimization, force calculation and comparison of total energy [43-46]. The cohesive energy
for FM and NM structures are determined to be 6.229 eV/atom and 6.103 eV/atom,
respectively. Thus, the FM structure is more stable than non-magnetic by 0.1263 eV/atom.
The calculated magnetic moment per unit cell in the ferromagnetic calculation is ~5 µB,
which is consistent with the previously reported experimental value [47]. The EELS spectra
were simulated with TELNES3 package embedded within WIEN2K package. Spin orbit
interaction was introduced for both nonmagnetic (NM) and ferromagnetic (FM) calculations
for simulating ELNES spectra with magnetization along <001> direction for the latter case.
Spin orbit interaction was considered. A collection semiangle of 5 mrad and convergence
semiangle of 0.1 mrad were used for the calculation. Spectrometer broadening of 0.2 eV was
used to mimic the instrumental energy resolution under the dipole selection rule (Δl= +/- 1).
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Weight ratio between capping layer to 20 nm nanocube:
Molar masses are comparable.
Molar Mass of Oleylamine ( OLA for nano cluster capping): 267.49 g.mol-1 C18H37N
Molar Mass of Octadecylamine (ODA for nano cube capping): 269.51 C18H39N
Molar mass of CuCr2S4 : 295.8 g/mol
The lattice parameter of CuCr2S4 is ~ 1 nm.
Now with a simple calculation for a 20 nm nanocube sample the weight of the cube is ~
295.8
𝑁
× 203 gm
Where, N = 6.023×1023
Each unit cell of CuCr2S4 will have 6 numbers of Cr atoms at the surface. Out of these 6 Cr
atoms 4 are located at the edge and 2 on the face. These 4 Cr atoms will be shared by 2 faces
and the remaining 2 will not be shared.
Therefore, total number of unit cell faces (1 sq. nm) for 20 nm nanocube is = 20×20×6 =
2400.
Total number of surface Cr atoms is = (2×2400)+(4/2+2400) = 9600 atoms
This is the total number of surface Cr atoms available to make bond with organic capping
molecule. If all the Cr atoms are attached with at least one capping agents i.e. in the case of
complete saturation (in reality it may be a lower number) then the weight of one monolayer
ODA over 20 nm cube surface will be~
269.51
𝑁
× 9600 gm
Therefore, the mass ratio between one monolayer ODA (under complete saturation of Cr
atoms) to nanoparticle (20 nm) will be ~1.2. This is significant. For more than a monolayer
this number will increase accordingly and lower the magnetization values observed by bulk
magnetometry techniques. This also exposes the problem with bulk magnetization techniques
to determine magnetization values in terms of emu/gm for nanomaterials with capping
agents.
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Figure S4. (a) Schematic representation of CuCr2S4 unit cell with one monolayer
coverage of ODA capping agents to six number of Cr atoms on one of six possible
surface. (b) Showing the dimension of one CuCr2S4 unit cell within the 20 nm
nanocube sample, there will be total 20×20×20 number of such unit cell to fill the
complete space.
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Figure S5. Experimental spectra along with total and component theoretical spectra
with different magnetic order contributions. Kindly not theory and experimental
spectra do not match exactly in terms of broadening parameters but significant peaks
can be identified.
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