1. Centro de Investigación y de Estudios Avanzados del Institúto Politécnico Nacional (Cinvestav IPN)
Palladium Nanoparticles Formation in Si Substrates from Pd Nano- Films by Thermal Annealing in air
MRS-2010
CANCUN
García-Serrano O., Peña-Sierra R., Goiz-Amaro O. and Romero-Rubio G.
Department of Electrical Engineering, Solid State Electronic Section (SEES), CINVESTAV IPN, Mexico D.F.
ABSTRACT ___ Pd nanoparticles were prepared from continuous and homogeneous Pd nano-films by thermal annealing in the temperature range from 800 to 1000 C at normal environmental conditions. The nanoparticles formation is explained by the surface stress , the surface energy between the Pd film, the ultra thin oxide layer (SiO2) and the volume change from Pd to PdO at the used conditions. The resulting Pd nanoparticles attain a random nanoparticle distribution with sizes directly related to the initial thickness of the Pd films. Pd nano-films in the range of 5-25 nm in thickness produce nanoparticle sizes in the range of 15 to 100 nm.
I.- INTRODUCTION __ The development of nanoparticle films is driven in, by their attractive properties influenced by the size confinement effects. The new properties of metallic nanoparticles as their catalytic properties have been studied and used in devices since the middle of the past century, such as their application in solid-state gas sensors. Nanometer-size metallic nanoparticles are attractive because of their applications on optical switches [1] and single electron transistors [2], among other applications. Actually there is a lot of literature on the research of metallic nanoparticles related to the particle-size, as their reactivity and selectivity properties [3]. In this work, the spontaneous formation of 3D strained Pd nanostructures on crystalline Si and quartz substrates by thermal annealing of Pd at 800 and 900 C under environmental conditions is reported.
Table I
II.- METHODOLOGY __ The Pd films were deposited at room temperature on (111) silicon and quartz substrates by a variant of the electroless technique, and by DC-Sputtering. The Pd and Pd-PdO nanostructures were synthesized by thermal annealing on the temperature range of 800 to 900 C in a reactor equipped with a quartz chamber under normal air-environmental conditions. The Table I shows the main characteristics of as-deposited Pd as well as the resulted Pd-PdO nanostructures . In order to determine semi-quantitatively the phases of resulted nanoparticles micro-Raman spectra and glazing X-ray diffraction (XRD) were recorder.
Pd film/Thickness
As-deposited
Thermal treatment
Features
Size of resulted nanostructures:
Height / diameter
80 nm
800 C/ 90 min
100 nm/ 150 nm
900 C / 10 min
20 nm
800 C / 30 min
35 nm / 40 nm
900 C / 3 min
4 nm
800 C / 7 min
11 nm / 11 nm
900 C / 2 min
III.- RESULTS AND DISCUSSION __ The figure 1 include the morphology of formed Pd-PdO nanoparticles and the incremental law for the nanostructures formation extracted from experimental data. From the data included in Fig 1 and Table I, it can be pointed out that the size-dimensions of the nanostructures are proportional to the initial thickness of the homogeneous and continuous nanometric Pd films. e g c a b d f h
Fig 1 __ (a, b) AFM and SEM images of as-deposited Pd film , (c, f and g) SEM images in top view, (d, e) 70º lateral view, (h) Experimental size-distribution.
The nature of nanostructure formation is allowed by the compressive stresses introduced by the volume change resulting from the thermal oxidation in air promotes the strained nanostructure formation on Si and quartz substrates. The geometrical shape developed on Si and quartz substrates can be qualitatively explained by the nature of the Pd thermal oxidation by the oxygen content of the normal environmental conditions and by the phase diagrams of the Pd-O system.
Fig 2 __ Typical XRD of resulted nanostructures Fig 3 __ PL spectra of Pd nanoparticles on quartz Fig 4 __ Raman spectra taken on resulted particles.
The XRD and Raman spectra confirms the Pd and Pd-PdO compositions of the nanostructured films produced at 900ºC and 800ºC, respectively. The resulted strained nanostructures formed encounter application in solid state sensors as catalytic agents to improve the sensitivity.
IV CONCLUSIONS AND REFERENCES __ A simple method to produce Pd-PdO nanoparticles on silicon and quartz substrates with controllable size and distributions is described. The Pd strained nanostructures produced at 900ºC and 800ºC are composed by Pd and Pd-PdO. The size of the nanostructures is related to the initial thickness of the homogeneous Pd nanofilms. [1] R.F. Haglund, et al, Nucl. Instr. and Meth, B91 (1994) 493. [2] A. Nakajima, H. Nakao, et al, Appl. Phys. Lett. 73 (1998) [3] ultrafast studies of electron dynamics in semiconductor and metal colloidal nanoparticles: effects of size and surface, doi: /ar960178j.