Electron dynamics in films made of transition metal nanograins embedded in SiO[sub 2]: Infrared reflectivity and nanoplasma infrared resonance
Type
ArticleKAUST Department
Core LabsMaterial Science and Engineering Program
Physical Science and Engineering (PSE) Division
Date
2009-06-08Preprint Posting Date
2009-06-09Online Publication Date
2009-06-08Print Publication Date
2009-06Permanent link to this record
http://hdl.handle.net/10754/552737
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We report on near normal infrared reflectivityspectra of ∼550 nm thick films made of cosputtered transition metal nanograins and SiO2 in a wide range of metal fractions. Co0.85(SiO2)0.15,with conductivity well above the percolation threshold has a frequency and temperature behavior according to what it is find in conductingmetal oxides. The electron scattering rate displays a unique relaxation time characteristic of single type of carriers experiencing strong electron-phonon interactions. Using small polaron fits we identify those phonons as glass vibrational modes. Ni0.61(SiO2)0.39, with a metal fraction closer to the percolation threshold, undergoes a metal-nonmetal transition at ∼77 K. Here, as it is suggested by the scattering rate nearly quadratic dependence, we broadly identify two relaxation times (two carrier contributions) associated to a Drude mode and a midinfrared overdamped band, respectively. Disorder induced, the midinfrared contribution drives the phase transition by thermal electron localization. Co0.51(SiO2)0.49 has the reflectivity of an insulator with a distinctive band at ∼1450 cm−1 originating in electron promotion, localization, and defect induced polaron formation. Angle dependent oblique reflectivity of globally insulating Co0.38(SiO2)0.62, Fe0.34(SiO2)0.66, and Ni0.28(SiO2)0.72, reveals a remarkable resonance at that band threshold. We understand this as due to the excitation by normal to the film electric fields of defect localized electrons in the metallic nanoparticles. At higher oblique angles, this localized nanoplasma couples to SiO2 longitudinal optical Berreman phonons resulting in band peak softening reminiscent to the phonon behavior undergoing strong electron-phonon interactions. Singular to a globally insulating phase, we believe that this resonance might be a useful tool for tracking metal-insulator phase transitions in inhomogeneous materials.Citation
Electron dynamics in films made of transition metal nanograins embedded in SiO[sub 2]: Infrared reflectivity and nanoplasma infrared resonance 2009, 105 (11):114306 Journal of Applied PhysicsPublisher
AIP PublishingJournal
Journal of Applied PhysicsarXiv
0906.1805Additional Links
http://scitation.aip.org/content/aip/journal/jap/105/11/10.1063/1.3126485http://arxiv.org/abs/0906.1805
ae974a485f413a2113503eed53cd6c53
10.1063/1.3126485