Bogdan Palosz

1
Institute of High Pressure Physics Polish Academy
of Sciences Warsaw, Poland
Bogdan Palosz
Investigation of relaxation of nano-diamond
surface in real and reciprocal
spaces (diffraction studies)
11th International Conference on New Diamond
Science and Technology (ICNDST) and 9th Applied
Diamond Conference (ADC), May 15 - 18, 2006
Research Triangle Park, North Carolina
2
Investigation of relaxation of nano-diamond
surface in real and reciprocal spaces B. Palosz1,
E. Grzanka1, S. Stelmakh1, C. Pantea2, Th.
Proffen2, and W. Zerda3 1Institute of High
Pressure Physics, Polish Academy of Sciences,
Warsaw, Poland 2LANSCE at LANL, Los Alamos, USA 3
TCU, Physics Department, Forth Worth, Texas,
USA Unique physical, chemical and mechanical
properties of nanocrystalline solids are
determined by their specific structure and, in
particular, by special features of the grain
boundaries. In nanocrystalline solids these
effects are exaggerated due to a very large
surface area-to-volume ratio 1,2. The changes
of the chemical state of the surface is reflected
by changes in the atomic structure of the grains.
In this work we present some preliminary results
on the structural analysis of nanocrystalline
diamond powders (U96) annealed in argon under
ambient pressure at temperatures up to 1200oC.
We show that the analysis of both Bragg
scattering and PDF enables examination of
redistribution of strains in the individual
grains. Conventional techniques of structural
analysis of the atomic structure of solids are
not adequate for nanocrystals due to their very
small size comparable to dimensions of the unit
cell, and to their non-uniform structure the
arrangement of atoms in the grain core is
different than that at the surface. For
characterization of the atomic structure of
nanocrystals we use a concept of the apparent
lattice parameters, alp, which correspond to the
"lattice parameter" values calculated for each
individual Bragg reflection. The variation of
alp's with the diffraction vector Q yields
information on strains present in the grain core
and in the surface shell 3. The alp's for our
annealed nano-diamond powders were measured at RT
with NPDF neutron diffractometer at LANSCE, Los
Alamos, USA. The alp values of the starting
powder are larger than the lattice parameter of
the perfect diamond crystal. For powders
annealed above 1000oC all individual alp-values
are smaller than the corresponding alp values
measured for the starting powder. This
difference measured for the powder annealed at
1200oC corresponds approximately to compression
of a perfect diamond lattice under the pressure
of 2 GPa. The decrease of alp can be associated
with rearrangement of the atoms at the surface
and an increase in the internal pressure inside
the grains 4. Analysis of Pair Distribution
Function (PDF provides information on
inter-atomic distances in our grains. Comparison
of the experimental G(r)s with theoretical
interatomic distance functions G(r)s calculated
for diamond and graphite lattices shows that upon
annealing above 1000oC some graphite-type phases
form. The formation of graphite-type bonds
occurrs at the expense of diamond-type bonds.
The analysis of the experimental G(r)s shows,
that intra-sublattice distances in the primary
powder are shorter than in the perfect diamond
lattice, while the inter-sublattice bonds are
longer. As a result of annealing all the
inter-atomic distances decrease, with somewhat a
much "faster" accelerated decrease above 1000oC.
Refs 1 Gleiter, H., Acta Mater. 48, 1
(2000) 2 Gruen, D.M., Ann. Rev. Mater. Sci.
29, 211 (1999) 3 Palosz, B, et. al., Z.
Kristallogr. 217, 497 (2002) 4 Palosz, B, et.
al., J.Phys.Condensed Matter. 16, 353 (2004).
3
how much of crystalline diamond is in
nano-diamond ?
questions
what is the specific atomic structure of the
diamond lattice in nano-diamond ?
in which form is non-diamond carbon present in
nano-diamond ?
what are the characteristic dimensions of
nano-diamond ?
4
tentative model of nano-diamond, UDD
5

• outline
• experimental method large-Q powder neutron
  diffraction
• reciprocal and real space analysis
• evolution of the atomic structure of nano-diamond
  UD96 upon annealing, up to 1200oC
• phase composition
• lattice parameters (Bragg scattering reciprocal
  space)
• interatomic bonds (total scattering real space)
• (modified) model of a nano-diamond NEW !

6
the experimental method neutron powder
diffraction (NPDF, LANL at LANSCE, Los
Alamos) reciprocal versus real space analysis
7
structural information contained in a
diffraction pattern
8
relationship between
reciprocal space
real space
?
unit cell
inter-atomic distances
coherent scattering ONLY ! information on
long-range atomic order
total scattering total structural
information
9
?
?
the effect of annealing of nano-crystalline
diamond, U96 on its atomic structure
300, 600, 800, 1000, 1200oC
?
10
TEMs of nano-diamond U96
11
reciprocal space analysis
phase composition of nano-diamond non-diamond
carbon graphite-like ? gas-like ?
12
neutron powder diffraction patterns of
nano-diamond U96
NPDF, LANSCE, Los Alamos
calculated pattern
graphite appears as a phase only at 1200oC
13
a presence of non-diamond carbon
after annealing at 1200oC there is a strong
increase of Bragg-to-diffuse scattering ratio
the difference corresponds to approximately 8
wt. of non-diamond non-crystalline carbon in the
sample
14
?
?
real space phase analysis inter-atomic distances
?
15
crystal structures of diamond and graphite
reciprocal space
real space
ao 3.5667 Å
ao 2.456 Å
co 6.696 Å
unit cell diagonal cross-section
16
real-space phase analysis of nano-diamond
sp3
sp2
15!
no indication of presence of sp2 bonds in the
sample annealed up to 1000oC
17
?
the atomic structure of the core of
nano-diamond is it a small single crystal ?
?
18
the dimension of a diamond nano-crystal
is only several times larger than that of the
unit cell
19
limitations of the Bragg equation for
nanocrystalline materials
20
experimental alp-Q dependencies of nano-diamond
U96 powder annealed at different temperatures
(?p ? 2.0 GPa ! )
alp of a perfect crystal lattice
21
theoretical alp-Q dependencies of 5 nm
nano-diamond crystals with a compressive strain
in the surface shell
30 vol. atoms in the surface shell !
22
experimental alp-Q dependencies of 5 nm
nano-diamond crystals with a compressive strain
in the surface shell
average alp values at large Q
0.1
Cu
Mo
at very large Q the measured alp values tend to
match the lattice parameter in the core of the
nano-diamond particle
23
?
the atomic structure of nano-diamond how does
it look like in real space ?
?
24
Experimental evolution of inter-atomic
distances in nano-diamond upon annealing
r4
r2
intra-lattice
inter-lattice
25
?
atomic model of nano-diamond ?
?
26
core-shell model of nano-diamond
27
how much of crystalline diamond is in
nano-diamond ?
90 inner core diamond strained surface shell
1 3
questions
answers
what is the specific atomic structure of the
diamond lattice in nano-diamond ?
core shell structure with compressed (3)
surface
in which form non-diamond carbon is present in
nano-diamond ?
characteristic dimensions of nano-diamond ?
gas-like carbon (?10) with sp3 bonds graphite
at 1200oC
diameter appr. 5 nm surface shell so? 0.3 nm
28
Collaboration
B. Palosz1, E. Grzanka1, S. Stelmakh1, C.
Pantea2, Th. Proffen2, T.W. Zerda3, W. Palosz4
1Institute of High Pressure Physics, Polish
Academy of Sciences, Warsaw, Poland 2LANSCE at
LANL, Los Alamos, USA 3TCU, Physics Department,
Forth Worth, Texas, USA 4BAE Systems, NASA/MSFC,
Huntsville, Alabama, USA