Search

Broadband absorption in the visible spectrum is essential in optoelectronic applications that involve power conversion such as photovoltaics and photocatalysis. Most ultrathin broadband absorbers use parasitic plasmonic structures that maximize absorption using surface plasmons and/or Fabry-Perot cavities, which limits the weight efficiency of the device. Here, we show the theoretical and experimental realization of an unpatterned/planar semiconductor thin-film absorber based on monolayer transition metal dichalcogenides (TMDCs). We experimentally demonstrate an average total absorption in the visible range (450 nm - 700 nm) of > 70% using > 4 nm of semiconductor absorbing materials scalable over large areas with vapor phase growth techniques. Our analysis suggests that a power conversion efficiency (PCE) of 15.54% and a specific power > 300 W g^-1 may be achieved in a photovoltaic cell based on this metamaterial absorber.

Binary kagome compounds TmXn (T = Mn, Fe, Co; X = Sn, Ge; m:n = 3:1, 3:2, 1:1) have garnered recent interest owing to the presence of both topological band crossings and flat bands arising from the geometry of the metal-site kagome lattice. To exploit these electronic features for potential applications in spintronics, the growth of high quality heterostructures is required. Here we report the synthesis of Fe/FeSn and Co/FeSn bilayers on Al2O3 substrates using molecular beam epitaxy to realize heterointerfaces between elemental ferromagnetic metals and antiferromagnetic kagome metals. Structural characterization using high-resolution X-ray diffraction, reflection high-energy electron diffraction, and electron microscopy reveals the FeSn films are flat and epitaxial. Rutherford backscattering spectroscopy was used to confirm the stoichiometric window where the FeSn phase is stabilized, while transport and magnetometry measurements were conducted to verify metallicity and magnetic ordering in the films. Exchange bias was observed, confirming the presence of antiferromagnetic order in the FeSn layers, paving the way for future studies of magnetism in kagome heterostructures and potential integration of these materials into devices., Comment: The following article has been submitted to the Journal of Applied Physics

Biological neurons are electro-mechanical systems, where the generation and propagation of an action potential is coupled to generation and transmission of an acoustic wave. Neuristors, such as VO2, characterized by insulator-metal transition (IMT) and negative differential resistance, can be engineered as self-oscillators, which are good approximations of biological neurons in the domain of electrical signals. In this study, we show that these self-oscillators are coupled electro-opto-mechanical systems, with better energy conversion coefficients than the conventional electromechanical or electrooptical materials. This is due to the significant contrast in the material's resistance, optical refractive index and density across the induced temperature range in a Joule heating driven IMT. We carried out laser interferometry to measure the opto-mechanical response while simultaneously driving the devices electrically into self-oscillations of different kinds. We analyzed films of various thicknesses, engineered device geometry and performed analytical modelling to decouple the effects of refractive index change vis-a-vis mechanical strain in the interferometry signal. We show that the effective piezoelectric coefficient (d13*) for our neuristor devices is 660 pm/V, making them viable alternatives to Pb-based piezoelectrics for MEMS applications. Furthermore, we show that the effective electro-optic coefficient (r13*) is ~22 nm/V, which is much larger than that in thin-film and bulk Pockels materials., Comment: 37 pages, 20 Figures

Lead free, silicon compatible materials showing large electromechanical responses comparable to, or better than conventional relaxor ferroelectrics, are desirable for various nanoelectromechanical devices and applications. Defect-engineered electrostriction has recently been gaining popularity to obtain enhanced electromechanical responses at sub 100 Hz frequencies. Here, we report record values of electrostrictive strain coefficients (M31) at frequencies as large as 5 kHz (1.04 x 10-14 m2 per V2 at 1 kHz, and 3.87 x 10-15 m2 per V2 at 5 kHz) using A-site and oxygen-deficient barium titanate thin-films, epitaxially integrated onto Si. The effect is robust and retained even after cycling the devices >5000 times. Our perovskite films are non-ferroelectric, exhibit a different symmetry compared to stoichiometric BaTiO3 and are characterized by twin boundaries and nano polar-like regions. We show that the dielectric relaxation arising from the defect-induced features correlates very well with the observed giant electrostrictive response. These films show large coefficient of thermal expansion (2.36 x 10-5/K), which along with the giant M31 implies a considerable increase in the lattice anharmonicity induced by the defects. Our work provides a crucial step forward towards formulating guidelines to engineer large electromechanical responses even at higher frequencies in lead-free thin films., Comment: 26 pages, 4 figures, 8 supplementary figures

Correlated electron materials (CEMs) host a rich variety of condensed matter phases. Vanadium dioxide (VO2) is a prototypical CEM with a temperature-dependent metal-to-insulator (MIT) transition with a concomitant crystal symmetry change. External control of MIT in VO2 - especially without inducing structural changes - has been a long-standing challenge. In this work, we design and synthesize modulation-doped VO2-based thin film heterostructures that closely emulate a textbook example of filling control in a correlated electron insulator. Using a combination of charge transport, hard x-ray photoelectron spectroscopy, and structural characterization, we show that the insulating state can be doped to achieve carrier densities greater than 5x10^21 cm^(-3) without inducing any measurable structural changes. We find that the MIT temperature (T_MIT) continuously decreases with increasing carrier concentration. Remarkably, the insulating state is robust even at doping concentrations as high as ~0.2 e-/vanadium. Finally, our work reveals modulation-doping as a viable method for electronic control of phase transitions in correlated electron oxides with the potential for use in future devices based on electric-field controlled phase transitions., Comment: Main paper 21 pages, 5 Figures. Supporting Information, 18 Pages,15 SI Figures and 1 SI Section