Multiphase strontium molybdate thin films for plasmonic local heating applications

Abstract

In the search for alternative plasmonic materials SrMoO3 has recently been identified as possessing a number of desirable optical properties. Owing to the requirement for many plasmonic devices to operate at elevated temperatures however, it is essential to characterize the degradation of these properties upon heating. Here, SrMoO3 thin films are annealed in air at temperatures ranging from 75 -500° C. Characterizations by AFM, XRD, and spectroscopic ellipsometry after each anneal identify a loss of metallic behaviour after annealing at 500° C, together with the underlying mechanism. Moreover, it is shown that by annealing the films in nitrogen following deposition, an additional crystalline phase of SrMoO4 is induced at the film surface, which suppresses oxidation at elevated temperatures. ExperimentalThe SrMoO3 PLD target material was prepared from SrMoO4 powder (99.9% purity) supplied by Alfa Aesar. The powder was placed in propan-2-ol and ball milled at 300 rpm for 20 hrs before evaporating the propan-2-ol by placing the powder in an oven overnight at 60° C. The powder was then reduced in a furnace under 100 mL min -1 gas flow of 5% H2 / 95% N2 at 1400° C for 10 hrs. The powder was then pressed into a target with a density of approximately 4 g cm -3 before sintering under the same gas flow conditions at 1500° C for 12 hours.The SrMoO3 target material was rotated throughout each pulsed laser deposition process and held 60 mm from the SrTiO3 substrate. A KrF excimer laser (240 nm) was used for the deposition of all samples with a repetition rate of 8 Hz and a 10 s relaxation period after every 20 pulses. A laser fluency of 1.2 J cm -2 was used. Vacuum conditions, approximately 1×10 -7 Torr, were used for the deposition of all samples and the substrate temperature was 650° C. The samples were cooled to room temperature after each deposition process at a rate of 10° C min -1 , either in vacuum or following annealing in 500 Torr N2 (6N purity, supplied by BOC), prior to removal from the vacuum chamber. Single side polished 5x5 mm (100) oriented STO substrates with a thickness of 0.5 mm were used.An IONTOF ToF-SIMS 5 instrument was used for SIMS depth profiling of the samples. An area of 100x100 μm 2 was analysed using a 25 keV Bi+ LMIG in high current bunch mode with a beam current of approximately 1 pA. Only negative secondary ions were collected. For depth profiling, a 1 keV Cs+ ion beam with a current of 75 nA was used, giving a sputter crater area of 300x300 μm 2 .The surfaces of the SMO films before and after annealing were characterised using X-ray photoelectron spectroscopy (XPS). The spectra were recorded on a Thermo Scientific K-Alpha+ spectrometer operating at a base pressure of 2x10 -9 mbar. This system incorporates a monochromated, microfocused Al Kα X-ray source (hν = 1486.6 eV) and a 180° double focusing hemispherical analyser with a 2D detector. The X-ray source was operated a 6 mA emission current and 12 kV anode bias. Data were collected at pass energies of 200 eV for survey, and 20 eV for core le...

Modifying Metastable Sr1–xBO3−δ (B = Nb, Ta, and Mo) Perovskites for Electrode Materials

“…These changes in the absorption profile coincide well with experimental and theoretical reports. 1 , 2 , 34 − 36 The first noticeable color change (from white to light blue, t < 0.5 h) is ascribed to the removal of surface defects that are periodically arranged within the lattice ( Figure 2 a). These defect sites are eliminated during the H 2 /Ar treatment, transforming the initially white powder (insulating) to a light blue (conductive).…”

“…Owing to their metallic responses, , 4d- and 5d - perovskite oxides (ABO 3 , A = alkaline earth metal and B = Nb, Ta, Mo, etc.) have emerged as attractive charge transport materials for photovoltaic devices .…”

“…Owing to their metallic responses, , 4d- and 5d - perovskite oxides (ABO 3 , A = alkaline earth metal and B = Nb, Ta, Mo, etc.) have emerged as attractive charge transport materials for photovoltaic devices . Recently, it was demonstrated that insulator-to-metal transitions (IMT) and confinement effects can be utilized to drive metallic behavior in these materials under ambient conditions.…”

“…The low electrical resistivities observed for these crystals are due to the formation of oxygen vacancies, , which generate midgap states that donate free electrons (n-type conductivity). However, in the case of metastable ABO 3 perovskites, oxygen-reduced environments such as H 2 /Ar inhibit overoxidation of the B-site ion by removing excess oxygen from the crystal lattice. , These multivalent B-site ions act as charge carrier traps (surface) and transport barriers (bulk), motivating the need to prevent their formation during crystallization. As a result of reducing the oxidation state of the B-site ion, the available d states for free carriers dramatically decrease the electrical resistivity (>10 8 Ω cm for insulators to ∼10 –4 Ω cm for metals) of these structures .…”

Electronic structure of the highly conductive perovskite oxide SrMoO$_3$

“…SrMoO 3 stands out among perovskite transition metal oxides because of its exceptionally low room-temperature resistivity of ≈ 5.1 µΩ cm, only about three times that of copper [1]. This remarkably high conductivity has sparked an interest into possible applications of SrMoO 3 in microwave electronics, as plasmonic devices or as electrodes in oxide heterostructures [2][3][4][5][6][7][8]. The high conductivity of SrMoO 3 is particularly remarkable when placed in the context of other 4d perovskite transition metal oxides.…”