Analysis by TEM of calcium phosphate films obtained by magnetron
sputtering in the initial stages of crystallization
Elvis Lopez1, Alexandre Mello1, Marcos Farina2, Alexandre Rossi1 and André Linhares Rossi1
1
Brazilian Center for Physics Research, Rio de Janeiro, Brazil.
2
Federal University of Rio de Janeiro, Institute of Biomedical Sciences, Brazil.
Hydroxyapatite (HA), Ca10(PO4)6(OH)2, is the principal mineral constituent of bone and teeth.
Because of its excellent biocompatibility and bioactivity proprieties for bone tissue
regeneration, HA biomaterials has drawn increasing attentions and become a topic of
extensive research. The mechanism of nucleation and growth of biological and synthetic HA is
not completely understood [1]. Results obtained from supersaturated solutions indicate that
an amorphous calcium phosphate phase (ACP) may precede HA precipitation. It was suggested
that the ACP phase is composed of clusters (~ 1 nm) known as “Posner´s clusters” which
further aggregate to form larger 30 nm particles [2]. Different structures and chemical
compositions of the Posner´s clusters were suggested by theoretical studies. In this work we
investigate the initial stage of hydroxyapatite formation in thin films deposited by Radio
Frequency Magnetron Sputtering (RFMS). RFMS is considered a potential technique for
orthopedic and dental implant coatings [3]. The sputtering parameters were adjusted to
produce single phase crystalline hydroxyapatite and pore-free thin films for depostion times
longer than 75 minutes (225 nm thick film) [3]. The early stages of HA growth were
investigated for deposition times of 30s, 60s, 120s and 240s leading films with 2.6, 5.2, 10.4,
and 20.8 nm thicknesses, respectively calculated from the X-ray profile at a low angle (1°
<2θ<5°). The films were deposited directly over transmission electron microscopy (TEM) grids
covered by a polymeric film (formvar). TEM and scanning TEM (STEM) images were obtained
from the top view of the calcium phosphate film (direction perpendicular to the film plane).
Calcium phosphate films deposited for 30s were predominantly amorphous. The film was
composed by dense nanoclusters with 2.7 nm in size immersed in a homogenous ACP layer
(figure 1). Black regions in the image are empty spaces (only 55% of the grid was covered by
the film). The absence of nanoclusters in empty spaces strongly suggested that these clusters
have been formed in a second stage of the deposition process at the expense of the ACP. The
higher brightness signal of the clusters when observed by STEM-HAADF (high annular angular
dark field) indicates a structure with higher density than the surrounding film (the thickness of
the film is approximately constant). The size of the clusters increased from 2.6 nm in the 30s
film to 5.3 nm in the 240s film. Lattice planes were observed in clusters from 120 and 240s
films indicating a local atomic order and the beginning of the crystallization process. A
mechanism of hydroxyapatite crystallization is proposed. At the first stage of deposition
process an amorphous calcium phosphate layer was formed from the sputtering of calcium,
phosphor and oxygen ions on the substrate surface. The plasma energy transferred during the
deposition time induced the nucleation of ACP dense nanoclusters (precursor phase of HA).
Afterwards, the ACP clusters grew in size and were transformed into crystalline
hydroxyapatite.
We thank CNPq, CAPES, FAPERJ.
References:
[1] Biological calcium phosphates and Posner’s cluster; X Yin, MJ Stott, The Journal of Chemical
Physics 118, 3717-3723 (2003).
[2] Synthetic amorphous calcium phosphate and its relation to bone mineral structure; AS
Posner, F Betts, Accounts of Chemical Research 8, 273-281 (1975).
[3] Growth of crystalline hydroxyapatite thin films at room temperature by tuning the energy
of the rf-magnetron sputtering plasma; EO López, Mello A, Sendão H, Costa LT, Rossi AL, et al,
Applied Materials & Interfaces 5: 9435-9445 (2013).
Figure 1. STEM-HAADF from 30s calcium phosphate film obtained by radio frequency
magnetron sputtering (RFMS). Bright spots are calcium phosphate clusters, a precursor phase
of hydroxyapatite crystals immersed in a homogenous ACP layer.