J. L. Musfeldt

Photovoltaic effects in BiFeO3

We report a photovoltaic effect in ferroelectric BiFeO3 thin films. The all-oxide heterostructures with SrRuO3 bottom and tin doped indium oxide top electrodes are characterized by open-circuit voltages ∼0.8–0.9 V and external quantum efficiencies up to ∼10% when illuminated with the appropriate light. Efficiencies are at least an order of magnitude larger than the maximum efficiency under sunlight (AM 1.5) thus far reported for ferroelectric-based devices. The dependence of the measured open-circuit voltage on film thickness suggests contributions to the large open-circuit voltage from both the ferroelectric polarization and band offsets at the BiFeO3/tin doped indium oxide interface.

Photoconductivity in BiFeO3 thin films

The optical properties of epitaxial BiFeO3 thin films have been characterized in the visible range. Variable temperature spectra show an absorption onset near 2.17eV, a direct gap (2.667±0.005eV at 300K), and charge transfer excitations at higher energy. Additionally, we report photoconductivity in BiFeO3 films under illumination from a 100mW∕cm2 white light source. A direct correlation is observed between the magnitude of the photoconductivity and postgrowth cooling pressure. Dark conductivities increased by an order of magnitude when comparing films cooled in 760 and 0.1Torr. Large increases in photoconductivity are observed in light.

Optical band gap of BiFeO3 grown by molecular-beam epitaxy

BiFeO3 thin films have been deposited on (001) SrTiO3 substrates by adsorption-controlled reactive molecular-beam epitaxy. For a given bismuth overpressure and oxygen activity, single-phase BiFeO3 films can be grown over a range of deposition temperatures in accordance with thermodynamic calculations. Four-circle x-ray diffraction reveals phase-pure, epitaxial films with ω rocking curve full width at half maximum values as narrow as 29arcsec (0.008°). Multiple-angle spectroscopic ellipsometry reveals a direct optical band gap at 2.74eV for stoichiometric as well as 5% bismuth-deficient single-phase BiFeO3 films.

Linear and nonlinear optical properties of BiFeO3

Using spectroscopic ellipsometry, the refractive index and absorption versus wavelength of the ferroelectric antiferromagnet Bismuth Ferrite, BiFeO_3 is reported. The material has a direct band-gap at 442 nm wavelength (2.81 eV). Using optical second harmonic generation, the nonlinear optical coefficients were determined to be d_15/d_22 = 0.20 +/- 0.01, d_31/d_22 = 0.35 +/- 0.02, d_33/d_22 = -11.4 +/- 0.20 and |d_22| = 298.4 +/- 6.1 pm/V at a fundamental wavelength of 800 nm.

Optical properties of quasi-tetragonal BiFeO3 thin films

Optical transmission spectroscopy and spectroscopic ellipsometry were used to extract the optical properties of an epitaxially grown quasi-tetragonal BiFeO3 thin film in the near infrared to near ultraviolet range. The absorption spectrum is overall blue shifted compared with that of rhombohedral BiFeO3, with an absorption onset near 2.25 eV, a direct 3.1 eV band gap, and charge transfer excitations that are ∼0.4 eV higher than those of the rhombohedral counterpart. We interpret these results in terms of structural strain and local symmetry breaking.

Size-Dependent Infrared Phonon Modes and Ferroelectric Phase Transition in BiFeO3 Nanoparticles

One emergent property of ferroelectric nanoparticles is the sized-induced structural distortion to a high-symmetry paraelectric phase at small particle sizes. Finite length scale effects can thus be advantageously employed to elucidate ferroelectric transition mechanisms. In this work, we combine infrared spectroscopy with group theory and lattice dynamics calculations to reveal the displacive nature of the ferroelectric transition in BiFeO3, a room temperature multiferroic. Systematic intensity and frequency trends in selected vibrational modes show that the paraelectric phase is Pm3m and the lowest frequency A1 feature is the soft mode that drives the first order transition. Finite length scale effects are also evident in the electronic structure with a red-shifted band gap in nanoscale BiFeO3 compared with that of the rhombohedral film, a result that can impact the development of ferroelectric photovoltaics and oxide-based electronics. Taken together, these findings demonstrate the foundational importance of size effects for enhancing the rich functionality and broad utility of transition metal oxides.

Chemical tuning of the optical band gap in spinel ferrites: CoFe2O4 vs NiFe2O4

We measured the optical properties of epitaxial CoFe2O4 thin films and compared our findings with complementary electronic structure calculations and similar studies on the Ni analog. Our work reveals CoFe2O4 to be an indirect band gap material (1.2 eV, X → Γ in the spin-down channel) with a direct gap at 2.7 eV. The latter is robust up to 800 K. Compared to NiFe2O4, the indirect gap is ≈0.5 eV lower, a difference we discuss in terms of size and covalency effects in spinel ferrites.

Optical Absorption, Luminescence, And Redox Switching Properties Of Polyphenylene Derivatives

A series of polyphenylene derivative polymers with a variety of heterocyclic units along the main chain, and various pendant groups, have been synthesized and their optoelectrochemical properties examined. Polyphenylenes containing electronically isolated emitter centers have optical absorption and luminescence characteristics which are a function of the length of the conjugated unit. A water soluble electroactive rigid-rod poly (p-phenylene), specifically Poly[2,5-bis (propoxy-3-sulfonate)-1,4-phenylene- alt ,4-phenylene], has been prepared. Solution cast films of this polymer exhibit optical evidence for bipolarons via both p-type and n-type electrochemical doping. Further poly ( p -phenylene) (PPP) derivatives are being pursued, including PPPs bearing pendant donor molecules for charge transfer complex formation. A series of polymers containing alternating p-phenylene units with bithienylene and bifuranylene units, specifically poly[1,4-bis (2-heterocycle)-2,5-disubstituted-1,4-phenylenes], have been synthesized with both alkyl and alkoxy pendant groups. Steric interactions disrupt the ability of the redox doped polymers to attain planarity, strongly affecting the polymers oxidation potentials and DC conductivities. The substituent length has little effect on the electronic properties of the polymer, but greatly affects the polymers solubility and transport properties during redox switching.

Observation of a Burstein–Moss Shift in Rhenium-Doped MoS2 Nanoparticles

We investigated the optical properties of rhenium-doped MoS2 nanoparticles and compared our findings with the pristine and bulk analogues. Our measurements reveal that confinement softens the exciton positions and reduces spin-orbit coupling, whereas doping has the opposite effect. We model the carrier-induced exciton blue shift in terms of the Burstein-Moss effect. These findings are important for understanding doping and finite length scale effects in low-dimensional nanoscale materials.

Tunable band gap in Bi(Fe1−xMnx)O3 films

In order to investigate band gap tunability in polar oxides, we measured the optical properties of a series of Bi(Fe1−xMnx)O3 thin films. The absorption response of the mixed metal solid solutions is approximately a linear combination of the characteristics of the two end members, a result that demonstrates straightforward band gap tunability in this system.