Computational Discovery and Experimental Demonstration of Boron Phosphide Ultraviolet Nanoresonators

Abstract

resonances have been realized at visible and infrared wavelengths thanks to the mature lithographic processing of suitable materials, [4] such as silicon (Si), [5] gallium phosphide (GaP), [6] and titanium dioxide (TiO 2 ). [7] It would be desirable to extend the operation of these materials to the ultraviolet, but their small direct band gap energies (≲3 eV) lead to significant absorption losses in the ultraviolet. Wide band gap materials, such as niobium pentoxide [8] and hafnium oxide, [9] offer transparency in the ultraviolet but at the cost of a moderate refractive index (n ≈ 2.1−2.3). Diamond has been theoretically suggested as a potential material, [10,11] but comes with significant nanofabrication challenges. [12] The scarcity of available high-index materials with wide band gap energies calls for the identification of new materials which can advance the rich optical properties of Mie resonances observed in the visible to the ultraviolet. Concurrent advances in first-principles methodology and computing power have recently made it possible to design and discover new materials via high-throughput computations. [13][14][15][16][17] The approach has been successfully applied in several domains, including photovoltaics, transparent conductors, and photocatalysis. [18][19][20] However, to the best of our knowledge, computational discovery of new high-index materials remains largely unexplored. Relevant previous work in this direction has been limited to the static response regime [21,22] reflecting the fact that the major materials databases so far has focused on ground state properties.Here we use high-throughput linear response density functional theory (DFT) to screen an initial set of 2743 elementary and binary materials with the aim to identify isotropic highindex, low loss, and broad band optical materials. For the most promising materials, the computed frequency-dependent complex refractive indices are used as input for Mie scattering calculations to evaluate their optical performance. In addition to the already known high-index materials we identify several new compounds. In particular, boron phosphide (BP) offers a refractive index above three with very low absorption losses in a spectral range spanning from the infrared to the ultraviolet. We then prepare BP nanoparticles and show, by means of darkfield optical measurements and electron energy-loss spectroscopy, that they support size-dependent Mie resonances in the visible and ultraviolet. Finally, we demonstrate a laser reshaping Controlling ultraviolet light at the nanoscale using optical Mie resonances holds great promise for a diverse set of applications, such as lithography, sterilization, and biospectroscopy. Access to the ultraviolet requires materials with a high refractive index and wide band gap energy. Here, the authors systematically search for such materials by computing the frequency-dependent optical permittivity of 338 binary semiconductors and insulators from first principles, and evaluate their scattering properties using Mie theor...

Boron Phosphide Films by Reactive Sputtering: Searching for a P‐Type Transparent Conductor

“…First‐principles calculations using the Bethe–Salpeter equation on G 0 W 0 band structures also yield a similar refractive index in the visible part of the spectrum. [ 16 ] At higher photon energies, the refractive indices of our films decrease due to the inflection point in the extinction coefficient (Figure 4d). The slight increase in the refractive index and extinction coefficient with increasing annealing temperature is probably due to film densification upon annealing (see Supporting Information).…”

“…Because the electronegativity difference between B and P is small, BP is a covalent solid and its band structure is closely related to that of Si and C in the diamond structure. The main difference is an intermediate size of the fundamental indirect band gap for BP (≈2.0 eV) [ 14–16 ] mainly due to an intermediate bond length. Although this band gap corresponds to visible light, the direct band gap of BP is much wider and falls in the UV region (≈4.3 eV).…”

“…Although this band gap corresponds to visible light, the direct band gap of BP is much wider and falls in the UV region (≈4.3 eV). [15][16][17] The weakness of indirect transitions predicted for BP at room temperature [15] is the key factor that could make BP thin films sufficiently transparent for many TCM applications. For example, a 100 nm-thick BP film is expected to absorb negligible amounts of red-yellow light and less than 10% of violet light according to first-principles calculations including electron-phonon coupling.…”

Indirect Band Gap Semiconductors for Thin-Film Photovoltaics: High-Throughput Calculation of Phonon-Assisted Absorption

“…More details can be found in the Computational Details section. The data set includes 338 semiconductors for which we evaluate the band gaps using both the PBE and the GLLB-SC xc-functional. Keeping only nonmagnetic compounds with the GLLB-SC band gap between 0.7 and 3.0 eV reduces the data set further down to 127 compounds.…”

Gallium Phosphide Nanoparticles for Low‐Loss Nanoantennas in Visible Range

“…An alternative material of a nanoantenna operating in a short wavelength range is GaP. It has a moderately high refractive index (n > 3) and a small extinction coefficient at >470 nm (Figure 1a) because of the much larger indirect band gap 2.26 eV (≈550 nm) [12][13][14][15][16][17][18] than that of Si (1.12 eV). For example, the extinction coefficient of GaP at 514 nm is 0.004, which is an order of magnitude smaller than that of Si (0.06).…”