Static and Time-Resolved Terahertz Measurements of Photoconductivity in Solution-Deposited Ruthenium Dioxide Nanofilms

Abstract

Thin-film ruthenium dioxide (RuO2) is a promising alternative material as a conductive electrode in electronic applications because its rutile crystalline form is metallic and highly conductive. Herein, a solution-deposition multi-layer technique is employed to fabricate ca. 70 ± 20 nm thick films (nanoskins) and terahertz spectroscopy is used to determine their photoconductive properties. Upon calcining at temperatures ranging from 373 K to 773 K, nanoskins undergo a transformation from insulating (localized charge transport) behavior to metallic behavior. Terahertz time-domain spectroscopy (THz-TDS) indicates that nanoskins attain maximum static conductivity when calcined at 673 K (σ = 1030 ± 330 S·cm−1). Picosecond time-resolved Terahertz spectroscopy (TRTS) using 400 nm and 800 nm excitation reveals a transition to metallic behavior when calcined at 523 K. For calcine temperatures less than 523 K, the conductivity increases following photoexcitation (ΔE < 0) while higher calcine temperatures yield films composed of crystalline, rutile RuO2 and the conductivity decreases (ΔE > 0) following photoexcitation.

Plasma-Deposited Ru-Based Thin Films for Photoelectrochemical Water Splitting

“…This problem is further complicated by the fact that the OER process is significantly influenced by the structure and nanostructure of ruthenium-based catalysts, which largely depends on the preparation conditions of these materials [15,20]. From the electronic structure point of view, catalysts based on RuO 2 , depending on the production procedure and the post-deposition treatment, can behave as a semiconductor [36] that exhibits both p-type and n-type electrical conductivity [37,38], and also, especially after calcination at high temperature (above 523 K), can reveal metallic-like properties [39].…”

Direct comparison of time-resolved terahertz spectroscopy and Hall Van der Pauw methods for measurement of carrier conductivity and mobility in bulk semiconductors

“…For a thin film on an insulating substrate, the THz transmission through the sample, T̂ s ( ω ), relative to that through the bare substrate, T̂ sub ( ω ), is proportional to the thin film conductivity according to equation (1): $$\frac{{\hat{T}}_{s}false(\omega false)}{{\hat{T}}_{\mathit{\text{sub}}}false(\omega false)}=\frac{{\hat{E}}_{s}false(\omega false)}{{\hat{E}}_{\mathit{\text{sub}}}false(\omega false)}=\frac{1+n_{\mathit{\text{sub}}}}{1+n_{\mathit{\text{sub}}}+Z_{0}d\hat{\sigma }false(\omega false)},$$where, Z 0 is the free space impedance, d is the thin film thickness, n sub is the substrate index of refraction, σ̂ (ω) is the complex-valued frequency-dependent conductivity, and Ê s (ω) and Ê sub (ω) are the Fourier Transforms of the time-dependent THz electric field waveforms transmitted through the sample and substrate, respectively. This expression can be rearranged to give an expression for the conductivity and also rewritten for photoexcited samples which display non-zero static conductivity [20]. …”

Balancing Light Absorption and Charge Transport in Vertical SnS2 Nanoflake Photoanodes with Stepped Layers and Large Intrinsic Mobility

“…Real (Δσ 1 ) and imaginary (Δσ 2 ) photoconductivity components are plotted in Figure b,c for single crystals and vertical nanoflakes, respectively, as a function of THz frequency (ω). In general, the complex conductivity () of both of these samples can be described by Equation , (also known as the Drude–Smith model), where N is the charge carrier density, m* is the effective carrier mass, τ DS is the effective scattering time, and the c ‐parameter characterizes the degree of carrier localization due to the presence of boundaries…”

“…Finally, a much slower component that we attribute to free carrier recom bination decays over >250 ps. In general, the complex conductivity (σ ) of both of these samples can be described by Equation (2), (also known as the Drude-Smith model), where N is the charge carrier density, m* is the effective carrier mass, τ DS is the effective scattering time, and the cparameter character izes the degree of carrier localization due to the presence of boundaries [50,[52][53][54][55][56][57][58][59][60] σ ω τ ωτ ωτ However, while ≈250 ps is the lower limit of the photoexcited carrier lifetime, the radiative lifetime of ≈1.3 ns determined by TRPL is the upper limit.…”