We have studied potassium-intercalated bulk ${\mathrm{HfS}}_2$ and ${\mathrm{HfSe}}_2$ by combining transmission electron energy loss spectroscopy, angle-resolved photoemission spectroscopy, and density functional theory calculations. The results reveal insights into (1) the intercalation process itself, (2) its effect on the crystal structures, (3) the induced semiconductor-to-metal transitions, and (4) the accompanying appearance of charge carrier plasmons and their dispersions. Calculations of the formation energies and the evolution of the energies of the charge carrier plasmons as a function of the potassium content show that certain, low potassium concentrations $x$ are thermodynamically unstable. This leads to the coexistence of undoped and doped domains if the provided amount of the alkali metal is insufficient to saturate the whole crystal with the minimum thermodynamically stable potassium stoichiometry. Beyond this threshold concentration the domains disappear, while the alkali metal and charge carrier concentrations increase continuously upon further addition of potassium. At low intercalation levels, electron diffraction patterns indicate a significant degree of disorder in the crystal structure. The initial order in the out-of-plane direction is restored at high $x$ while the crystal layer thicknesses expand by $33–36\%$. Calculations suggest that this expansion reaches its maximum at doping levels of $x\approx 0.25$ before it reverses slightly for higher concentrations. Superstructures emerge parallel to the planes which we attribute to the distribution of the alkali metal rather than structural changes of the host materials. The in-plane lattice parameters change by not more than $1\%$. The introduction of potassium causes the formation of charge carrier plasmons whose nature we confirmed by calculating the loss functions and their intraband and interband contributions. The observation of this semiconductor-to-metal transition is supported by calculations of the density of states (DOS) and band structures as well as angle-resolved photoemission spectroscopy. The calculated DOS hint at the presence of an almost ideal two-dimensional electron gas at the Fermi level for $x<0.6$. The plasmons exhibit quadratic momentum dispersions which is in agreement with the behavior expected for an ideal electron gas.

A multiscale approach is reported to understand the influence of the Ca and Y co-substitution in Bi-rich iron garnet thin films. Recently, it has been demonstrated that site-selective co-substitution of ${\mathrm{Ca}}^{2+}$ and ${\mathrm{Y}}^{3+}$ in bismuth iron garnet (BIG) leads to either $n$- or $p$-type semiconductor behavior. However, the evolution of the structural and magnetic properties of bismuth iron garnet upon doping is still unknown. In the (Ca,Y):BIG, thin films grown by pulsed laser deposition onto ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ substrates, structural investigations confirm the epitaxial growth of doped thin films with high crystallinity and a complete strain relaxation from 20 nm above the film/substrate interface. While x-ray diffraction only evidences a single-phase garnet, the presence of secondary phase nanocrystallites, that are absent in pure BIG grown in similar conditions, is observed by aberration-corrected scanning transmission electron microscopy. These nanocrystallites form in between garnet grains as textured and poorly crystallized hematite. Despite the presence of nanoneedles, Ca and Y co-substitution preserves the giant Faraday rotation of pure BIG and maintains the Curie temperature above 590 K. Only minor energy shifts (80 meV) or small intensity changes (10%) of the Faraday rotation are observed upon doping or annealing and are mainly related to band-gap evolution and cell volume change. Furthermore, the easy magnetization axis rotates towards in the out-of-plane direction upon doping which is also promising for potential applications in spintronics.

Temperature-dependent resistivity $\rho \left(T\right)$ and magnetoresistance are measured in bulk tetragonal phase of antiferromagnetic CuMnAs and are found to be anisotropic both due to structure and magnetic order. We compare these findings to model calculations with chemical disorder and finite-temperature phenomena included. The finite-temperature ab initio calculations are based on the alloy analogy model implemented within the coherent potential approximation and the results are in fair agreement with experimental data. Regarding the anisotropic magnetoresistance (AMR), which reaches a modest magnitude of 0.12%, we phenomenologically employ the Stoner-Wohlfarth model to identify temperature-dependent magnetic anisotropy of our samples and conclude that the field dependence of AMR is more similar to that of antiferromagnets than ferromagnets, suggesting that the origin of AMR is not related to isolated Mn magnetic moments.

We present a detailed first-principles study which explores the configurational space along the relevant reactions and migration paths involving the formation and dissociation of interstitial carbon-oxygen complexes, ${\mathrm{C}}_{\mathrm{i}}{\mathrm{O}}_{\mathrm{i}}$ and ${\mathrm{C}}_{\mathrm{i}}{\mathrm{O}}_{2\mathrm{i}}$, in silicon. The formation/dissociation mechanisms of ${\mathrm{C}}_{\mathrm{i}}{\mathrm{O}}_{\mathrm{i}}$ and ${\mathrm{C}}_{\mathrm{i}}{\mathrm{O}}_{2\mathrm{i}}$ are found as occurring via capture/emission of mobile ${\mathrm{C}}_{\mathrm{i}}$ impurities by/from O complexes anchored to the lattice. The lowest activation energies for dissociation of ${\mathrm{C}}_{\mathrm{i}}{\mathrm{O}}_{\mathrm{i}}$ and ${\mathrm{C}}_{\mathrm{i}}{\mathrm{O}}_{2\mathrm{i}}$ into smaller moieties are 2.3 and 3.1 eV, respectively. The first is compatible with the observed annealing temperature of ${\mathrm{C}}_{\mathrm{i}}{\mathrm{O}}_{\mathrm{i}}$, which occurs at around $400{}^{\circ }\mathrm{C}$, and below the threshold for ${\mathrm{O}}_{\mathrm{i}}$ diffusion. The latter exceeds significantly the measured activation energy for the annealing of ${\mathrm{C}}_{\mathrm{i}}{\mathrm{O}}_{2\mathrm{i}}(E_a=2.55$ eV). We propose that instead of dissociation, the actual annealing mechanism involves the capture of interstitial oxygen by ${\mathrm{C}}_{\mathrm{i}}{\mathrm{O}}_{2\mathrm{i}}$, thus being governed by the migration barrier of ${\mathrm{O}}_{\mathrm{i}}(E_m=2.53$ eV). The study is also accompanied by measurements of hole capture cross sections and capture barriers of ${\mathrm{C}}_{\mathrm{i}}{\mathrm{O}}_{\mathrm{i}}$ and ${\mathrm{C}}_{\mathrm{i}}{\mathrm{O}}_{2\mathrm{i}}$. In combination with previously reported data, we find thermodynamic donor transitions which are directly comparable to the first-principles results. The two levels exhibit close features, conforming to a model where the electronic character of ${\mathrm{C}}_{\mathrm{i}}{\mathrm{O}}_{2\mathrm{i}}$ can be described by that of ${\mathrm{C}}_{\mathrm{i}}{\mathrm{O}}_{\mathrm{i}}$ perturbed by a nearby O atom.

Understanding the nature of light transmission and the photonic band gap in inverse opal photonic crystals is essential for linking their optical characteristics to any application. This is especially important when these structures are examined in liquids or solvents. Knowledge of the true correlation between the nature of the inverse opal (IO) photonic band gap, the structure of the photonic crystals, and the theories that describe their optical spectra is surprisingly limited compared to colloidal opals or more traditional photonic crystal structures. We examined $\mathrm{Ti}{\mathrm{O}}_2$ and $\mathrm{Sn}{\mathrm{O}}_2$ IOs in a range of common solvents to solve the conflict between Bragg-Snell theory and optical and physical measurements by a comprehensive angle-resolved light transmission study coupled to microscopy examination of the IO structure. Tuning the position of the photonic band gap and index contrast by solvent infiltration of each inverse opal requires a modification to the Bragg-Snell theory and the photonic crystal unit cell definition. We also demonstrate experimentally and theoretically that low fill factors are caused by less dense material infilling all interstitial vacancies in the opal template to form an IO. By also including an optical interference condition for inverse opals with an effective refractive index greater than their substrate, and an alternative internal refraction angle in the substrate, angle-resolved transmission spectra for inverse opals are now consistent with microscopy and refractive index measurements. This work now allows an accurate correlation between the true response of an IO to the index contrast with a solvent, how an IO is infilled, and the directionality and bandwidth of the photonic band gap. As control in functional photonic materials becomes more prevalent outside of optics and photonics, such as biosensing and energy storage, for example, a comprehensive and consistent correlation between photonic crystal structures and their primary optical signatures is a fundamental requirement for application.

DNA-stabilized silver clusters (Ag:DNAs) have attracted much attention due to their unique sequence-dependent fluorescence and many potential applications; however, the understandings of their atomic structures and functional properties are still limited, which hamper the further development of the novel nanoclusters. Using the advanced computational methods, we propose three atomistic models for the Ag:DNAs, which all contain a four-electron silver core with a two-row planar shape stabilized by the nitrogen atoms in the DNA. By comparing experimental and time-dependent density functional theory-simulated circular dichroism spectra, we have confirmed that these core structures should commonly exist in the Ag:DNAs. We then analyze the electronic and optical properties of the clusters and discover that the silver to DNA transitions have essential contributions to the chiroptical properties of the clusters.

Selective area growth is employed in the vapor-liquid-solid molecular beam epitaxy of ZnTe nanowire arrays. Full control over the location of the individual nanowires is achieved by defined positioning of the growth catalyst. This study addresses the influence of substrate material and growth temperature on the yield of vertical nanowires. The optimized procedure provides arrays of single-crystalline free-standing nanowires with a high ensemble uniformity. The nanowires exhibit a uniform shape with a diameter of about 80 nm and reach a length of more than $3\mu \text{m}$, which makes them suitable as substrates for core-shell nanowires of the topological insulator material HgTe.

Cryogenic scanning tunneling microscopy is employed to investigate the stoichiometry and defects of epitaxial FeSe thin films on ${\mathrm{SrTiO}}_3$(001) substrates under various postgrowth annealing conditions. Low-temperature annealing with an excess supply of Se leads to the formation of Fe vacancies and superstructures, accompanied by a superconductivity (metal)-to-insulator transition in FeSe films. By contrast, high-temperature annealing could eliminate the Fe vacancies and superstructures, and thus recover the high-temperature superconducting phase of monolayer FeSe films. We also observe multilayer FeSe during low-temperature annealing, which is revealed to link with Fe vacancy formation and adatom migration. Our results document the very special roles of film stoichiometry and help unravel several controversies in the properties of monolayer FeSe films.

Grain boundary (GB) segregation has a substantial effect on the microstructure evolution and properties of polycrystalline alloys. The mechanism of nanoscale segregation at the various GBs in multicomponent alloys is of great challenge to reveal and remains elusive so far. To address this issue, we studied the GB segregation in a representative equiatomic FeMnNiCoCr high-entropy alloy (HEA) aged at 450 °C. By combining transmission Kikuchi diffraction, atom probe tomography analysis and a density-based thermodynamics modeling, we uncover the nanoscale segregation behavior at a series of well-characterized GBs of different characters. No segregation occurs at coherent twin boundaries; only slight nanoscale segregation of Ni takes place at the low-angle GBs and vicinal $\mathrm{\Sigma }29b$ coincidence site lattice GBs. Ni and Mn show cosegregation of high levels at the general high-angle GBs with a strong depletion in Fe, Cr, and Co. Our density-based thermodynamic model reveals that the highly negative energy of mixing Ni and Mn is the main driving force for nanoscale cosegregation to the GBs. This is further assisted by the opposite segregation of Ni and Cr atoms with a positive enthalpy of mixing. It is also found that GBs of higher interfacial energy, possessing lower atomic densities (higher disorder and free volume), show higher segregation levels. By clarifying the origins of GB segregations in the FeMnNiCoCr HEA, the current work provides fundamental ideas on nanoscale segregation at crystal defects in multicomponent alloys.

We study the stability and electronic properties of intrinsic point defects, vacancy and self-interstitial, in mono and bilayer phosphorene. We calculate the formation energies, quasiparticle defect states, and charge transition levels (CTLs) of these defects using ab initio density functional theory (DFT) and $GW$ approximation to the electron self-energy. Using the $\mathrm{DFT}+GW$ two paths formalism for studying interstitial in monolayer phosphorene, we show that with the inclusion of electrostatic corrections CTLs can be calculated reliably. Our calculations show that all the native point defects have low formation energies 0.9–1.6 eV in neutral state. Furthermore, we find that vacancy in phosphorene behaves as an acceptorlike defect which can explain the $p$-type conductivity in phosphorene. On the other hand, interstitial can show both acceptor- and donorlike behavior.

We use first-principles-based calculations to investigate the interplay between chemical order and the magnetic properties of $\text{L}{1}_0$ FeNi. In particular, we investigate how deviations from perfect chemical order affect the energy difference between the paramagnetic and ferromagnetic states as well as the important magnetocrystalline anisotropy energy. Our calculations demonstrate a strong effect of the magnetic order on the chemical order-disorder transition temperature and, conversely, a strong enhancement of the magnetic transition temperature by the chemical order. Most interestingly, our results indicate that the magnetic anisotropy does not decrease significantly as long as the deviations from perfect order are not too large. Moreover, we find that in certain cases a slight disorder can result in a higher anisotropy than for the fully ordered structure. We further analyze the correlation between the magnetocrystalline anisotropy and the orbital magnetic moment anisotropy, which allows to study the effect of the local chemical environment on both quantities, potentially enabling further optimization of the magnetocrystalline anisotropy with respect to chemical order and stoichiometric composition.

The control of the interaction between materials and biological tissues is a key factor to optimize the overall performance of implants and prostheses integrated into the body. With this objective in mind, biomimetic hierarchical one- and two-dimensional surface patterns textured at the micro and nano scales were fabricated on titanium alloys using femtosecond laser processing. The experimental results show that laser irradiation promotes surface oxidation together with a polarization-dependent nano-ripple formation. Human mesenchymal stem cells were subsequently cultured on different surface patterns aiming at determining their response to the underlying micro and nano structures. The ripple topography was demonstrated to induce a nonfouling behavior, which could be exploited in the fabrication of biomimetic hierarchical surface patterns to develop cell-trapping modules.

Noncontact mode of atomic force microscopy (nc-AFM) employing a CO-functionalized tip is a very powerful tool for studying molecular structures. However, interpreting nc-AFM images for nonplanar molecules can sometimes be problematic. To illustrate and resolve the nature of such problematic systems, we employ real-space pseudopotentials constructed within density functional theory to simulate nc-AFM images. We focus on several representative nonplanar organic molecules (pentacene, naphthanthrone, olympicene, and 6-phenylhexa-1,3,5-triynybenzene (PHTB)) on a typical substrate: the Cu (111) surface. This substrate results in significant distortions in the molecular geometries of pentacene and naphthanthrone. Including these distortions in simulated nc-AFM imaging notably improves the agreement between the simulated and measured images. In naphthanthrone, the relatively large interaction between the O atom and the Cu substrate offers a straightforward explanation for the absence of the $\mathrm{C}=\mathrm{O}$ bond in the measured image. Nonplanar features such as tilting or twisting are also apparent in olympicene and PHTB. A “triangular” bright feature associated with the $-\mathrm{C}{\mathrm{H}}_2$ group in olympicene appears in simulated and measured nc-AFM images. This feature is directly related to the tilting angle of the molecule with respect to the Cu substrate. The “defective” benzene ring feature and the faint ellipsoidal $\mathrm{C}\equiv \mathrm{C}$ feature in PHTB can be ascribed to its twisted nature. The ability to simulate such subatomic images in nc-AFM reflects the accuracy and efficiency of calculating quantum forces in real space.

The Dzyaloshinskii-Moriya (DM) interaction is present in the transition metal dichalcogenides (TMDC) magnets of form $M_{1/3}T{\mathrm{S}}_2$ ($M=3d$ transition metal, $T\in \left\{\mathrm{Nb},\mathrm{Ta}\right\}$), given that the intercalants $M$ form $\sqrt{3}\times \sqrt{3}$ superlattices within the structure of the parent materials $T{\mathrm{S}}_2$ and break the centrosymmetry. Competition between DM and ferromagnetic exchange interactions has been shown to stabilize a topological defect known as a chiral soliton in select intercalated TMDCs, initiating interest both in terms of fundamental physics and the potential for technological applications. In the current article we report on our study of the materials ${\mathrm{V}}_{1/3}{\mathrm{TaS}}_2$ and ${\mathrm{V}}_{1/3}{\mathrm{NbS}}_2$, using a combination of x-ray powder diffraction, magnetization, and single crystal neutron diffraction. Historically identified as ferromagnets, our diffraction results instead reveal that vanadium spins in these compounds are arranged into an A-type antiferromagnetic configuration at low temperatures. Refined moments are 1.37(6) and 1.50(9) ${\mu }_B$ for ${\mathrm{V}}_{1/3}{\mathrm{TaS}}_2$ and ${\mathrm{V}}_{1/3}{\mathrm{NbS}}_2$, respectively. Transition temperatures $T_c=32\mathrm{K}$ for ${\mathrm{V}}_{1/3}{\mathrm{TaS}}_2$ and 50 K for ${\mathrm{V}}_{1/3}{\mathrm{NbS}}_2$ are obtained from the magnetization and neutron diffraction results. We attribute the small net magnetization observed in the low-temperature phases to a subtle ($\sim 2^{\circ }$) canting of XY spins in the out-of-plane direction. These new results are indicative of dominant antiferromagnetic exchange interactions between the vanadium moments in adjacent $ab$ planes, likely eliminating the possibility of identifying stable chiral solitons in the current materials.

Exploiting first-principles simulations and x-ray absorption near edge spectroscopy (XANES) in high magnetic fields, we investigated the magnetic properties of thin films of zirconia doped with Fe impurities. In our ${\mathrm{Zr}}_{1-x}{\mathrm{Fe}}_x{\mathrm{O}}_{2-y}$ samples, grown by atomic layer deposition (ALD), the Fe dopants are uniformly distributed, ranging from diluted ($x\simeq 2\text{–}3\%$) up to high ($x\simeq 25\%$) atomic concentration. By x-ray magnetic circular dichroism (XMCD), we carefully analyzed, for samples having different Fe concentration, the magnetic moments as a function of temperature, in the range from 5 K up to 150 K, studying the best dopant concentration range maximizing the magnetic signal. Surprisingly, the iron magnetic moment measured for diluted concentrations degrades as the concentration of magnetic dopant increases. On the basis of ab initio simulations, we propose that the microscopic mechanisms responsible for the peculiar magnetic properties of this compound can be explained by oxygen-mediated superexchange mechanism between the Fe dopants producing, at high dopant concentration, an antiferromagnetic coupling between two Fe atoms. We identify and discuss the role of O vacancies to control such microscopic mechanisms.

The atomic and electronic structure of the $p$-type transparent amorphous semiconductor CuI is calculated by ab initio molecular dynamics. It is found to consist of a random tetrahedrally bonded network. The hole effective mass is found to be quite low, as in the crystal. The valence-band maximum (VBM) state has a mixed $\mathrm{I}\left(p\right)\text{−}\mathrm{Cu}\left(t_{2g}\right)\text{−}\mathrm{I}\left(p\right)$ character, and its energy is relatively insensitive to disorder. An iodine excess creates holes that move the Fermi level into the valence band, but it does not pin the Fermi level above the VBM mobility edge. Thus the Fermi level can easily enter the valence band if $p$ doped, similar to the behavior of electrons in In-Ga-Zn oxide semiconductors but opposite to that of electrons in a-Si:H. This suggests that amorphous CuI could make an effective $p$-type transparent semiconductor.

The presence of alloy disorder in III-nitride materials has been demonstrated to play a significant role in device performance through effects such as carrier localization and carrier transport. Relative to blue light emitting diodes (LEDs), these effects become more severe at green wavelengths. Because of the potential fluctuations that arise due to alloy disorder, full three-dimensional (3D) simulations are necessary to accurately relate materials properties to device performance. We demonstrate experimentally and through simulation that increased quantum well (QW) number in $c$-plane green LEDs contributes to excess driving voltage, and therefore reduced electrical efficiency. Experimentally, we grew an LED series with the number of QWs varying from one to seven and observed a systematic increase in voltage with the addition of each QW. Trends in LED electrical properties obtained from 3D simulations, which account for the effects of random alloy fluctuations, are in agreement with experimental data. Agreement is achieved without the need for adjusting polarization parameters from their known values. From these results, we propose that the polarization induced barriers at the GaN/InGaN (lower barrier/QW) interfaces and the sequential filling of QWs both contribute significantly to the excess forward voltage in multiple QW $c$-plane green LEDs.