Semiconductor materials with independently controlled electrical and thermal properties have a unique promise for energy-related applications from thermoelectrics and thermophotovoltaics. Here, using nonthermal plasma synthesized, direct-contact zinc oxide (ZnO) nanocrystal (NC) networks infilled with amorphous $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$, and amorphous ZnO-$\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ mixture, it is shown that such independent control of electrical and thermal properties is achievable. In this study, in addition to our early reports on control of the electrical properties in these two-phase nanocomposites by tailoring the contact radius between NCs, we demonstrate that the infill composition has a significant impact on the overall thermal conductivity of the NC network and can be used for thermal control. It is also shown that in these heterogeneous systems, the phonons are the dominant heat carriers, and the NC-NC contact radius has a negligible effect on thermal transport. The work suggests that this paradigm of independently controlling the electrical and thermal properties of NC-based materials through tuning the NC-NC contact radius and infill composition can be exploited even further by varying NC and infill materials with potential applications ranging from solar cells and light emitting diodes to solid-state energy converters.

Density-functional-theory-based simulations are the leading tool for the computational investigation of ferroelectrics and the parametrization of their classical potentials. However, the predictions often depend strongly on the exchange-correlation functional. The most popular choices, the LDA and GGA, tend to underestimate or overestimate some structural, electric, and energy properties. These impede the development of highly accurate classical potentials that extend the reach of first-principles simulations to finite temperatures and realistic sizes. In this work, we investigate the performance of recently developed Minnesota exchange-correlation functionals on the prototypical ferroelectrics ${\mathrm{BaTiO}}_3$ and ${\mathrm{PbTiO}}_3$ in comparison with some popular ones. We find that there exists a strong correlation between predictions for some properties (tetragonality, phase energy difference, and polarization) by different functionals. Along the correlation line, we find a range of functionals (including some from the Minnesota suite) whose predictions fall between those of the LDA and those of the GGA and, therefore, offer an improvement. A way to relatively rank functional performance with respect to chosen benchmarks is proposed and applied to identify the top performers for ${\mathrm{BaTiO}}_3$ and ${\mathrm{PbTiO}}_3$. The performance is found to be material dependent. Therefore, we propose that the performance assessment carried out in this work is employed to “screen” functionals prior to their use on ferroelectrics. Quick and computationally inexpensive, it is likely to lead to improved descriptions, especially for classical potential parametrization. The Minnesota functional suite is found to be suitable for this task.

Germanium monochalcogenides (GeSe and GeS) are promising materials for various optoelectronic applications because of their solar range bandgaps, high carrier mobilities, high stabilities, earth abundance, and anisotropic optical properties. Precise control of germanium monochalcogenide bandgaps is critical to applications in continuously tunable optoelectronics. In this paper, we combine first-principles calculations and experiments to predict and confirm that alloy engineering is a significant strategy for tailoring germanium monochalcogenide (${\mathrm{GeS}}_{1-x}{\mathrm{Se}}_x$) optoelectronic properties. When the Se content $x$ increases from 0.0 to 1.0, the bandgap decreases from 1.23 to 0.89 eV. In addition, there is a direct-indirect bandgap transition when $x$ is approximately 0.3. Tunable ${\mathrm{GeS}}_{1-x}{\mathrm{Se}}_x$ bandgaps can open up exciting opportunities for the development of various electronic and optoelectronic devices.

We present a procedure for simulating solution deposition of organic thin-films on explicitly modeled substrates via solvent evaporation simulations in a molecular dynamics framework. Additionally, we have developed force fields for the family of IDTBR nonfullerene acceptors, which have been widely employed in the literature as $n$-type materials in several types of organic semiconductor devices, and we analyzed their structure-property relationships using a combination of grazing incidence x-ray scattering measurements, atomistic molecular dynamics simulations, and quantum chemical calculations. We find that thermal fluctuations can have a significant impact on calculated electron transfer integrals, and that the $\pi$-stacking interactions of the electron withdrawing benzothiadiazole building blocks are key to high electron coupling in amorphous thin films of $n$-type materials.

CuO (cupric oxide) is a well-known $p$-type semiconductor, suitable for solar cell photovoltaic applications. However, due to the easy formation of defects and Cu-rich layers at the copper(II) oxide heterointerface, commercial application is yet to be successfully implemented. Density functional theory calculations have been employed to study the formation of intrinsic defects and their effect on the electronic properties of CuO. Native impurities were observed, depending on the synthesis conditions, to render the conductivity to $p$-type or $n$-type at a low energetic cost, yet with states embedded deep in the electronic band gap. Respective defect pairs, effectively determining the majority charge carriers, were observed to cluster in near proximity of each other, lowering the formation energy substantially. Hydrogen passivation was illustrated to have a positive effect on deep defect states in $p$-type CuO, without affecting the $n$-type counterpart. Outlined results were found to support the experimentally observed low photoresponse of CuO and further illustrate some of the difficulties related with achieving high-performance samples.

Controlling the strain level in nanowire heterostructures is critical for obtaining coherent interfaces of high crystalline quality and for the setting of functional properties such as photon emission, carrier mobility, or piezoelectricity. In a nanowire axial heterostructure featuring a sharp interface, strain is set by the materials lattice mismatch and the nanowire radius. Here we show that introducing a graded interface in nanowire heterostructures offers an additional parameter to control strain. For a given interface length and lattice mismatch, we first derive theoretically the maximum nanowire radius below which coherent growth is possible. We validate these findings by growing and characterizing various In(Ga)As/GaAs nanowire heterostructures with graded interfaces. We perform chemical and structural characterization of the interface by combining energy-dispersive x-ray spectroscopy and high resolution transmission electron microscopy. In the case of coherent growth, we observe that the mismatch strain relaxes elastically on the side walls of the nanowire around the interface area, while the core of the nanowire remains partially strained. Our experimental data show good agreement with finite element calculations. In particular, this analysis confirms that mechanical strain is largely reduced by interface grading. Overall, our work extends the parameter space for the design of nanowire heterostructures, thus opening new opportunities for nanowire optoelectronics.

The crystal growth velocity is one thermodynamic parameter of solidification experiments of undercooled melts under nonequilibrium conditions, which is directly accessible to observation. We applied the electrostatic levitation technique in order to study the crystal growth velocity $v$ as a function of the undercooling $\mathrm{\Delta }T$ for the intermetallic, congruently melting binary alloy NiTi and the glass-forming alloy Cu–Zr, as well as for the Zr-based ternary alloys $\left({\mathrm{Ni}}_x{\mathrm{Cu}}_{1-x}\right)\mathrm{Zr}$ $(x=0.3,0.4)$ and the Ni-based ternary alloy $\mathrm{Ni}\left({\mathrm{Zr}}_{0.5}{\mathrm{Ti}}_{0.5}\right)$. All investigated systems within this work, except the eutectics ${\mathrm{Cu}}_{0.56}{\mathrm{Zr}}_{0.44}$ and ${\mathrm{Cu}}_{0.46}{\mathrm{Zr}}_{0.54}$, exceeded the hypercooling limit $\mathrm{\Delta }{T}_{\mathrm{hyp}}$ and, remarkably, every $v\left(\mathrm{\Delta }T\right)$ relation changed significantly at $\mathrm{\Delta }{T}_{\mathrm{hyp}}$. Our results for glass-forming CuZr indicate that the influence of the diffusion coefficient $D\left(T\right)$ on $v\left(\mathrm{\Delta }T\right)$ at high undercoolings, as claimed in literature, cannot be the sole reason for the existence of a maximum in the $v\left(\mathrm{\Delta }T\right)$ behavior. These observations could make a valuable contribution concerning an extension of growth theories to undercooling temperatures $\mathrm{\Delta }T>\mathrm{\Delta }{T}_{\mathrm{hyp}}$. Nevertheless, our finding has direct consequences to various disciplines, as our earth and all living beings are examples for nonequilibrium systems. The scatter of our velocity data is at least two orders of magnitude smaller than measurements performed by former works due to our experimental setup, which allowed precise contactless triggering at a specific undercooling, and our analysis method, which considered the respective solidification morphologies.

Superconducting (S) thin film superlattices composed of Nb and a normal-metal spacer (N) have been extensively utilized in Josephson junctions given their favorable surface roughness compared to Nb films of comparable thickness. In this work, we characterize the London penetration depth and Ginzburg-Landau coherence lengths of $S/N$ superlattices using polarized neutron reflectometry and electrical transport. Despite the normal-metal spacer layers being only approximately 8% of the total superlattice thickness, we surprisingly find that the introduction of these thin N spacers between S layers leads to a dramatic increase in the measured London penetration depth compared to that of a single Nb film of comparable thickness. Using the measured values for the effective in- and out-of-plane coherence lengths, we quantify the induced anisotropy of the superlattice samples and compare to a single Nb film sample. From these results, we find that the superlattices behave similarly to layered 2D superconductors.

The incorporation of indium in GaN (0001) surfaces in dependence of strain is investigated by combining molecular-beam epitaxy (MBE) growth, quantitative transmission electron microscopy, and density-functional theory (DFT) calculations. Growth experiments were conducted on GaN, as well as on $30\%\pm 2\%$ partially relaxed $\mathrm{I}{\mathrm{n}}_{0.19}\mathrm{G}{\mathrm{a}}_{0.81}\mathrm{N}$ buffer layers, serving as substrates. Despite the only $0.6\%$ larger in-plane lattice constant of GaN provided by the buffer layer, our experiments reveal that the In incorporation increases by more than a factor of two for growth on the $\mathrm{I}{\mathrm{n}}_{0.19}\mathrm{G}{\mathrm{a}}_{0.81}\mathrm{N}$ buffer, as compared to growth on GaN. DFT calculations reveal that the decreasing chemical potential due to the reduced lattice mismatch stabilizes the In–N bond at the surface. Depending on the growth conditions (metal rich or N rich), this promotes the incorporation of higher In contents into a coherently strained layer. Nevertheless, the effect of strain is highly nonlinear. As a consequence of the different surface reconstructions, growth on relaxed $\mathrm{I}{\mathrm{n}}_x\mathrm{G}{\mathrm{a}}_{1-x}\mathrm{N}$ buffers appears more suitable for metal-rich MBE growth conditions with regard to achieving higher In compositions.

Merging concepts from the fields of ab initio materials science and nanophotonics, there is now an opportunity to engineer new photonic materials whose optical, transport, and scattering properties are tailored to attain thermodynamic and quantum limits. Here we present first-principles calculations predicting that Argentene, a single-crystalline hexagonal close-packed monolayer of Ag, can dramatically surpass the optical properties and electrical conductivity of conventional plasmonic materials. In the low-frequency limit, we show that the scattering rate and resistivity reduce by a factor of 3 compared to the bulk three-dimensional metal. Most importantly, the low scattering rate extends to optical frequencies in sharp contrast to, e.g., graphene, whose scattering rate increase drastically in the near-infrared range due to optical-phonon scattering. Combined with an intrinsically high carrier density, this facilitates highly confined surface plasmons extending to visible frequencies. We evaluate Argentene across three distinct figures of merit, in each outperforming the state-of-the-art, making it a valuable addition to the two-dimensional heterostructure toolkit for quantum optoelectronics.

A steady-state photocapacitance (SSPC) setup directly connected to the beamline of a MeV ion implanter is utilized to study primary intrinsic defects in $\beta \text{–}{\mathrm{Ga}}_2{\mathrm{O}}_3$ generated by He implantation at cryogenic temperatures (120 K). At low temperatures, the migration of defects is suppressed, and hence the generation of primary intrinsic defects is expected to prevail. SSPC measurements reveal defect-related optical transitions in halide vapor-phase epitaxy (HVPE) -grown $\beta \text{–}{\mathrm{Ga}}_2{\mathrm{O}}_3$ thin films with onset energies at 1.3 $\left(T_1\right),$ 1.7 $\left(T_2\right),$ 1.9 $\left(T_3\right),$ 2.6 $\left(T_4\right),$ 3.7 $\left(T_5\right),$ and 4.2 eV $\left(T_6\right)$. $T_2,$ $T_4,$ $T_5,$ and $T_6$ were observed in as-received HVPE-grown $\beta \text{–}{\mathrm{Ga}}_2{\mathrm{O}}_3$ thin films, whereby $T_2$ is only sporadically observed. The introduction rates for $T_3,$ $T_4,$ as well as $T_6$ indicate an origin related to primary intrinsic defects. Notably, $T_1$ and $T_3$ are only observed after He implantation at cryogenic temperatures. Hybrid-functional calculations were performed to estimate the optical absorption cross-section spectra for the gallium $\left({\mathrm{Ga}}_{\mathrm{i}}\right)$ and oxygen $\left({\mathrm{O}}_{\mathrm{i}}\right)$ interstitials as well as the corresponding vacancies ($V_{\mathrm{Ga}}$ and $V_{\mathrm{O}}$, respectively), and compared with the measured onsets for optical absorption found by SSPC measurements. Indeed, we propose $T_3$ to be associated with ${\mathrm{Ga}}_{\mathrm{i}}^{(+/+2)}$ and/or $V_{\mathrm{GaI}}^{(-3/-2)}$, while $T_4$ is suggested to be related to $V_{\text{O}K}^{(0/+)}$ $(K=\mathrm{I}, \mathrm{II}, \mathrm{III})$ and/or $V_{\mathrm{GaII}}^{(-3/-2)}$. Additionally, several further charge-state transition levels associated with $V_{\mathrm{GaI}}$ and $V_{\mathrm{GaII}}$ may contribute to $T_4$ and $T_6$. We further studied the kinetics of the defects created with He implantation by exposing the sample to room temperature. The kinetics observed for $T_3$ and $T_4$ further support the proposed assignments of the corresponding defect signatures.

Structural phase transitions in $f$-electron materials have attracted sustained attention both for practical and basic science reasons, including the fact that they offer an environment to directly investigate relationships between structure and the $f$-state. Here we present results for ${\mathrm{UCr}}_2{\mathrm{Si}}_2,$ where structural (tetragonal $\to$ monoclinic) and antiferromagnetic phase transitions are seen at $T_{\mathrm{S}}=205$ K and $T_{\mathrm{N}}=25$ K, respectively. We also provide evidence for an additional second-order phase transition at $T_{\mathrm{X}}=280$ K. We show that $T_{\mathrm{X}},$ $T_{\mathrm{S}},$ and $T_{\mathrm{N}}$ respond in distinct ways to the application of hydrostatic pressure and $\mathrm{Cr}\to \mathrm{Ru}$ chemical substitution. In particular, hydrostatic compression increases the structural ordering temperature, eventually causes it to merge with $T_{\mathrm{X}},$ and destroys the antiferromagnetism. In contrast, chemical substitution in the series ${\mathrm{UCr}}_{2-x}{\mathrm{Ru}}_x{\mathrm{Si}}_2$ suppresses both $T_{\mathrm{S}}$ and $T_{\mathrm{N}}$, causing them to approach zero temperature near $x\approx 0.16$ and 0.08, respectively. The distinct $T\text{−}P$ and $T\text{−}x$ phase diagrams are related to the evolution of the rigid Cr-Si and Si-Si substructures, where applied pressure semiuniformly compresses the unit cell, and $\mathrm{Cr}\to \mathrm{Ru}$ substitution results in uniaxial lattice compression along the tetragonal $c$-axis and an expansion in the $ab$-plane. These results provide insights into an interesting class of strongly correlated quantum materials in which degrees of freedom associated with $f$-electron magnetism, strong electronic correlations, and structural instabilities are readily controlled.

A hyperkagome iridate ${\mathrm{Li}}_3{\mathrm{Ir}}_3{\mathrm{O}}_8$ is synthesized by an ion-exchange reaction from ${\mathrm{Na}}_4{\mathrm{Ir}}_3{\mathrm{O}}_8$. The transport, magnetic, and thermodynamic measurements suggest a metallic state. The electronic structure calculation shows that ${\mathrm{Li}}_3{\mathrm{Ir}}_3{\mathrm{O}}_8$ hosts a semimetallic electronic structure produced by a competition between the formation of a localized molecular orbital of Ir $d$ electrons on the ${\mathrm{Ir}}_3$ triangles and the strong spin-orbit coupling as in the sister compound ${\mathrm{Na}}_3{\mathrm{Ir}}_3{\mathrm{O}}_8$. The semimetallic state induced by spin-orbit coupling is quite robust against the moderate change of lattice distortion associated with the different ionic radii of ${\mathrm{Li}}^{+}$ and ${\mathrm{Na}}^{+}$.

We evaluate theoretically the thermoelectric efficiency of the layered perovskite ${\mathrm{Sr}}_2{\mathrm{Nb}}_2{\mathrm{O}}_7$ via calculations of the electronic structure and transport coefficients within density-functional theory and Bloch-Boltzmann relaxation-time transport theory. The predicted figure-of-merit tensor $ZT$, computed with energy-, chemical potential–, and temperature-dependent relaxation times, has one component increasing monotonically from around 0.4 at room temperature to 2.4 at 1250 K at an optimal carrier density of around $2\times {10}^{20}{\mathrm{cm}}^{-3}$, while the other components are small. The Seebeck coefficient is about 250 to 300 $\mu \mathrm{V}$/K at optimal doping and reaches 800 $\mu \mathrm{V}$/K at lower doping. We provide a python code implementing various approximations to the energy-dependent relaxation-time transport, which can be used to address different systems with an appropriate choice of material parameters.

We show that when ${\mathrm{TiO}}_2$ anatase (001) is exposed to hydrogen plasma that the pristine surface termination becomes unfavorable to another, slightly modified, surface. On this modified surface the topmost ${\mathrm{TiO}}_2$ layer is intact but out of registry with the bottom layers. Nevertheless, the modified surface has significantly improved ability to split water under exposure to sunlight. We show by explicit calculation of the water splitting reaction that the energy barrier that exists on a pristine surface is not present on the modified surface. The valence band maximum of the surface is raised relative to the pristine surface, which is a favorable way of adjusting the band gap in ${\mathrm{TiO}}_2$ to the solar spectrum.

Monolayer black and blue phosphorenes possess electronic and optical properties that result in unique features when the two materials are stacked. We devise a low-strain van-der-Waals double layer and investigate its properties with ab initio many-body perturbation theory techniques. A type-II band alignment and optical absorption in the visible range are found. The study demonstrates that spatially indirect excitons with full charge separation can be obtained between two layers with the same elemental composition but different crystalline structure, proving the system interesting for further studies where dipolar excitons are important and for future optoelectronic applications.

$\mathrm{L}{\mathrm{a}}_{0.4}\mathrm{C}{\mathrm{e}}_{0.6}\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2$ represents a borderline case in the range of solid solutions formed in the pseudobinary system $\mathrm{LaC}{\mathrm{o}}_2{\mathrm{P}}_2\text{−}\mathrm{CeC}{\mathrm{o}}_2{\mathrm{P}}_2$. The material undergoes ferromagnetic ordering at ∼225 K followed by a structural collapse at ∼190 K, which leads to a strong suppression of magnetization. The structural phase transition manifests itself in a gradual decrease in the parameter $c$ and a relatively smaller increase of the parameter $a$ of the tetragonal lattice. Interestingly, a combination of magnetic measurements and nonpolarized and polarized neutron scattering experiments suggests that the structural collapse does not lead to an antiferromagnetically ordered state, observed in samples with the higher Ce content. On the contrary, $\mathrm{L}{\mathrm{a}}_{0.4}\mathrm{C}{\mathrm{e}}_{0.6}\mathrm{C}{\mathrm{o}}_2{\mathrm{P}}_2$ appears to enter a disordered, spin glass state, with gradual dissipation of the ferromagnetic ordering taking place simultaneously with the structural collapse, as evidenced by temperature-dependent measurements of the depolarization factor for a polarized neutron beam passing through the sample. The observed behavior is analogous to that reported for so-called reentrant spin glasses. In the present case, however, the appearance of the reentrant spin glass regime is caused not by tuning the chemical composition but by the structural phase transition. Electronic structure calculations confirm that the loss of magnetic ordering is caused by the subtle change to the density of states at the Fermi level due to the variation of the crystal structure of the material.

Germanium selenide (GeSe) is a van der Waals–bonded layered material with promising optoelectronic properties, which has been experimentally synthesized for two-dimensional (2D) semiconductor applications. In the monolayer, due to reduced dimensionality and, thus, screening environment, perturbations such as the presence of defects have a significant impact on its properties. We apply density functional theory and many-body perturbation theory to understand the electronic and optical properties of GeSe containing a single selenium vacancy in the $-2$ charge state. We predict that the vacancy results in midgap “trap states” that strongly localize the electron and hole density and lead to sharp, low-energy optical absorption peaks below the predicted pristine optical gap. Analysis of the exciton wave function reveals that the 2D Wannier-Mott exciton of the pristine material monolayer is strongly modified by the presence of the defect. The lowest-energy exciton wave function is highly localized around the defect, with the Bohr radius reduced by a factor of 4 when compared to the pristine material and a dipole moment along the out-of-plane axis due to the defect-induced symmetry breaking. Overall, these results suggest that the vacancy significantly modifies the excitonic properties of the system, demonstrating the importance of considering defects in the context of material design.