TiO2 anatase plays a central role in energy and environmental research. A major bottleneck toward developing artificial photosynthesis with TiO2 is that it only absorbs ultraviolet light, owing to its large bandgap of 3.2 eV. If one could reduce the bandgap of anatase to the visible region, TiO2-based photocatalysis could become a competitive clean energy source. Here, using scanning tunneling microscopy and spectroscopy in conjunction with density functional theory calculations, we report the discovery of a highly reactive titanium-terminated anatase surface with a reduced bandgap of less than 2 eV, stretching into the red portion of the solar spectrum. By tuning the surface preparation conditions, we can reversibly switch between the standard anatase surface and the newly discovered low bandgap surface phase. The identification of a TiO2 anatase surface phase with a bandgap in the visible and high chemical reactivity has important implications for solar energy conversion, photocatalysis, and artificial photosynthesis.

The unusual electronic properties of diamondoids, the nanoscale relatives of diamond, make them attractive for applications ranging from drug delivery to field emission displays. Identifying the fundamental origin of these properties has proven highly challenging, with even the most advanced quantum many-body calculations unable to reproduce measurements of a quantity as ubiquitous as the optical gap. Here, by combining first-principles calculations and Importance Sampling Monte Carlo methods, we show that the quantum dynamics of carbon nuclei is key to understanding the electronic and optical properties of diamondoids. Quantum nuclear effects dramatically modify the absorption lineshapes and renormalize the optical gaps. These findings allow us to formulate a complete theory of optical absorption in diamondoids, and establish the universal role of quantum nuclear dynamics in nanodiamond across the length scales.

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We present calculations of the correlation energies of crystalline solids and isolated systems within the adiabatic-connection fluctuation-dissipation formulation of density-functional theory. We perform a quantitative comparison of a set of model exchange-correlation kernels originally derived for the homogeneous electron gas (HEG), including the recently-introduced renormalized adiabatic local-density approximation (rALDA) and also kernels which (a) satisfy known exact limits of the HEG, (b) carry a frequency dependence or (c) display a 1/k 2 divergence for small wavevectors. After generalizing the kernels to inhomogeneous systems through a reciprocal-space averaging procedure, we calculate the lattice constants and bulk moduli of a test set of 10 solids consisting of tetrahedrally-bonded semiconductors (C, Si, SiC), ionic compounds (MgO, LiCl, LiF) and metals (Al, Na, Cu, Pd). We also consider the atomization energy of the H 2 molecule. We compare the results calculated with different kernels to those obtained from the random-phase approximation (RPA) and to experimental measurements. We demonstrate that the model kernels correct the RPA's tendency to overestimate the magnitude of the correlation energy whilst maintaining a high-accuracy description of structural properties.

We develop a first-principles theory of phonon-assisted optical absorption in semiconductors and insulators which incorporates the temperature dependence of the electronic structure. We show that the Hall-Bardeen-Blatt theory of indirect optical absorption and the Allen-Heine theory of temperature-dependent band structures can be derived from the present formalism by retaining only one-phonon processes. We demonstrate this method by calculating the optical absorption coefficient of silicon using an importance sampling Monte Carlo scheme, and we obtain temperature-dependent lineshapes and band gaps in good agreement with experiment. The present approach opens the way to predictive calculations of the optical properties of solids at finite temperature. In semiconductors and insulators exhibiting indirect band gaps the optical transitions near the fundamental edge require the absorption or emission of phonons in order to fulfill the crystal momentum selection rule. This mechanism is discussed in every introduction to solid state physics [1,2]. The theory of phonon-assisted indirect optical transitions was developed by Hall, Bardeen, and Blatt (HBB) [3,4], and forms the basis for our current understanding of phonon-assisted optical processes.Despite the popularity of the HBB theory, only very recently was this formalism combined successfully with first-principles density-functional theory calculations [5] powered by Wannier interpolation [6,7]. The work of Ref. 5 stands as the most sophisticated calculation of indirect optical absorption available today, yet it is not entirely parameter-free since an empirical shift of the absorption onset at each temperature was needed in order achieve agreement with experiment. This correction was unavoidable because the HBB theory does not take into account the temperature dependence of band structures.A consistent theory of temperature-dependent band structures was developed by Allen and Heine (AH) [8,9]. In recent years this approach was successfully demonstrated and improved within the framework of firstprinciples density-functional theory calculations [10][11][12][13]. Given these recent advances it is natural to ask whether the HBB theory of indirect absorption and the AH theory of temperature-dependent band structures could be combined in a more general formalism, in view of fully predictive calculations of phonon-assisted optical processes at finite temperature.In this manuscript we show that the quasiclassical method introduced by Williams [14] and Lax [15] (WL) provides a unified framework for calculating optical absorption spectra of solids, including phonon-assisted absorption and electron-phonon renormalization on the same footing. Indeed we show that the HBB and AH theories can be derived from the WL formalism by neglecting electron-phonon scattering beyond one-phonon processes. In order to demonstrate the power of the WL approach we calculate from first principles the phononassisted optical absorption spectrum of silicon at different temperatures using a stochasti...

A recent study has reported a power‐conversion efficiency of 5.1% for solar cells employing mesoporous TiO2 films sensitized with quantum dots of stibnite (Sb2S3). Here, a first‐principles atomic‐scale investigation of the interface between TiO2 and Sb2S3 is presented. The proposed atomistic interface model is free of defects, and the calculated energy‐level alignment at the interface indicates that the ideal open‐circuit voltage is as high as 1.6 V. Films sensitized with the isostructural compounds bismuthinite (Bi2S3) and antimonselite (Sb2Se3), which exhibit band gaps closer to the ideal Shockley–Queisser value are also examined. In the case of Bi2S3 the calculations indicate that the lowest unoccupied molecular orbital is too low in energy to inject electrons into TiO2, in agreement with experimental data. For antimonselite (Sb2Se3) the calculations predict a type‐II heterojunction with TiO2, and suggest that Sb2Se3 sensitization may lead to higher power conversion efficiencies than found in the TiO2/Sb2S3 system.

We investigate the quasiparticle band structure of anatase TiO(2), a wide gap semiconductor widely employed in photovoltaics and photocatalysis. We obtain GW quasiparticle energies starting from density-functional theory (DFT) calculations including Hubbard U corrections. Using a simple iterative procedure we determine the value of the Hubbard parameter yielding a vanishing quasiparticle correction to the fundamental bandgap of anatase TiO(2). The bandgap (3.3 eV) calculated using this optimal Hubbard parameter is smaller than the value obtained by applying many-body perturbation theory to standard DFT eigenstates and eigenvalues (3.7 eV). We extend our analysis to the rutile polymorph of TiO(2) and reach similar conclusions. Our work highlights the role of the starting non-interacting Hamiltonian in the calculation of GW quasiparticle energies in TiO(2) and suggests an optimal Hubbard parameter for future calculations.

Dye-sensitized solar cells constitute a promising approach to sustainable and low-cost solar energy conversion. Their overall efficiency crucially depends on the effective coupling of the photosensitizers to the photoelectrode and the details of the dye's energy levels at the interface. Despite great efforts, the specific binding of prototypical ruthenium-based dyes to TiO2, their potential supramolecular interaction, and the interrelation between adsorption geometry and electron injection efficiency lack experimental evidence. Here we demonstrate multiconformational adsorption and energy level alignment of single N3 dyes on TiO2 anatase (101) revealed by scanning tunnelling microscopy and spectroscopy. The distinctly bound molecules show significant variations of their excited state levels associated with different driving forces for photoelectron injection. These findings emphasize the critical role of the interfacial coupling and suggest that further designs of dye-sensitized solar cells should target a higher selectivity in the dye-substrate binding conformations in order to ensure efficient electron injection from all photosensitizers.