Two-dimensional (2D) materials provide a plethora of novel condensed matter physics and are the new playground in materials science, offering potentially vast applications. One of the critical hurdles for many 2D systems is the synthesis of these low-dimensional systems as well as the prediction and identification of new candidates. Herein, a self-assembly of a monolayer tellurene by bonding CdTe wafers is demonstrated for the first time. The conventional applications of wafer-bonding range from the production of microelectromechanical systems to the synthesis of lattice-mismatched multi-junction photovoltaics. Due to the heterogeneous materials that are typically employed, the bond-interface usually contains a thin amorphous layer or arrays of dislocations. Such an interface is thus itself inactive and in many cases has detrimental effects on the device. The new material phase stabilized in this work consists of an undulating monolayer of tellurium atoms covalently bonded to {111} Cd-terminated CdTe wafer surfaces. First-principles calculations and experimentally observed changes in the localized plasmon excitation energy indicate the clear rearrangement of the underlying band-structure suggesting a metallic character, bands showing linear dispersion, and a significant asymmetric spin-band splitting. The I-V characteristics show the presence of a highly conductive pathway that lowers the resistivity by three orders of magnitude, as compared to bulk CdTe, which can be attributed to the tellurium monolayer. The findings indicate that suitably chosen crystallographic wafer surfaces can act as structural templates allowing the production of exotic phases. The presently stabilized monolayer is an addition to the family of tellurene variants, providing new insights into the fundamental properties of this and other emerging 2D materials, while attracting attention to the unusual side of the wafer-bonding technology exemplified in this study.

Efficiency improvements in poly-CdTe can be achieved by fundamental understanding of the role of impurity atoms that can effectively passivate grain boundaries. In the present work, we investigated CdTe grain boundaries using TEM and SMIM, and studied effect of Se and Cu passivation on CdTe grain boundaries using first principles density functional theory (DFT) calculations. Se and Cu segregation to grain boundaries are thermodynamically feasible. Both Cu and Se can effectively reduce midgap states, but most improvement was achieved when Cu and Se are co-doped together, which is predicted to increase the PV efficiency of CdTe. The study we presented can be extended to dopant studies in other solar cell materials.

Journal Article Leveraging First Principles Modeling and Machine Learning for Microscopy Data Inversion Get access Eric Schwenker, Eric Schwenker Center for Nanoscale Materials, Argonne National Laboratory, Lemont IL, USA Search for other works by this author on: Oxford Academic Google Scholar Fatih Sen, Fatih Sen Center for Nanoscale Materials, Argonne National Laboratory, Lemont IL, USA Search for other works by this author on: Oxford Academic Google Scholar Spencer Hills, Spencer Hills Center for Nanoscale Materials, Argonne National Laboratory, Lemont IL, USA Search for other works by this author on: Oxford Academic Google Scholar Tadas Pualauskas, Tadas Pualauskas Department of Physics, University of Illinois at Chicago, Chicago IL, USA Search for other works by this author on: Oxford Academic Google Scholar Ce Sun, Ce Sun Department of Materials Science and Engineering, University of Texas at Dallas, Dallas TX, USA Search for other works by this author on: Oxford Academic Google Scholar Liang Li, Liang Li Center for Nanoscale Materials, Argonne National Laboratory, Lemont IL, USA Search for other works by this author on: Oxford Academic Google Scholar Alper Kinaci, Alper Kinaci Center for Nanoscale Materials, Argonne National Laboratory, Lemont IL, USA Search for other works by this author on: Oxford Academic Google Scholar Kendra Letchworth-Weaver, Kendra Letchworth-Weaver Center for Nanoscale Materials, Argonne National Laboratory, Lemont IL, USA Search for other works by this author on: Oxford Academic Google Scholar Moon Kim, Moon Kim Department of Materials Science and Engineering, University of Texas at Dallas, Dallas TX, USA Search for other works by this author on: Oxford Academic Google Scholar Robert Klie, Robert Klie Department of Physics, University of Illinois at Chicago, Chicago IL, USA Search for other works by this author on: Oxford Academic Google Scholar ... Show more Jianguo Wen, Jianguo Wen Center for Nanoscale Materials, Argonne National Laboratory, Lemont IL, USA Search for other works by this author on: Oxford Academic Google Scholar Maria K Y Chan Maria K Y Chan Center for Nanoscale Materials, Argonne National Laboratory, Lemont IL, USA Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 23, Issue S1, 1 July 2017, Pages 178–179, https://doi.org/10.1017/S143192761700157X Published: 04 August 2017

Thin-film Mg2Si was deposited using radio frequency (RF) magnetron sputtering. Al and Sn were incorporated as n-type dopants using co-sputtering to tune the thin-film electrical properties. X-ray diffraction (XRD) analysis confirmed that the deposited films are polycrystalline Mg2Si. The Sn and Al doping concentrations were measured using Rutherford backscattering spectroscopy (RBS) and energy dispersive X-ray spectroscopy (EDS). The charge carrier concentration and the charge carrier type of the Mg2Si films were measured using a Hall bar structure. Hall measurements show that as the doping concentration increases, the carrier concentration of the Al-doped films increases, whereas the carrier concentration of the Sn-doped films decreases. Combined with the resistivity measurements, the mobility of the Al-doped Mg2Si films is found to decrease with increasing doping concentration, whereas the mobility of the Sn-doped Mg2Si films is found to increase.

A fundamental understanding of the role of vacancies, interstitials, dislocations and grain boundaries on the electronic structure of CdTe may lead to efficiency improvements. Atomistic-level characterization, including microscopy and first principles modeling, is crucial in developing such a fundamental understanding. In the present work, we built atomistic grain boundary and dislocation core models directly from the STEM images using image analysis methods and crystallographic information at the interface. Grain boundaries are modeled using first principles density functional theory (DFT) calculations. Electronic structures of large-scale grain models are also computed with an accurate hybrid functional (HSE06). We report the electronic density of states (DOS) and electrostatic potential profiles of different CdTe grain boundaries to understand charge carrier interactions. Thermodynamics of point defects and pairs of point defects that can exist on or near grain boundaries are studied and pertaining changes in electronic structure are reported. The implications of these electronic structure changes at grain boundaries on photovoltaic performance, and corresponding strategies to improve performance, are discussed.

Controlling the nucleation and growth of interfacial Al4C3 carbide to optimize interfacial microstructure is well accepted as a critical factor to improve thermal properties of Al/diamond composites. In this study, the nucleation and growth mechanisms of Al4C3 carbide have been studied. The inhomogeneous nucleation of Al4C3 is revealed on both diamond (100) and (111) surfaces ((100)D and (111)D surfaces). Al4C3 particles nucleate at (111)D facets on both (111)D and (100)D surfaces. Growth of Al4C3 particles is controlled by diffusion and Ostwald ripening. The proposed mechanisms well explain the observation of high density and small flower-like carbide on (100)D surface and less dense larger plate shape carbide on (111)D surface. The orientation relationship between diamond and Al4C3 was identified to be [11¯0]D//[21¯1¯0] Al4C3 and (111)D//(0003)Al4C3 on both diamond surfaces. Our results will provide guidance to the further improvement of properties of Al/diamond composites.

Extended defects are of considerable importance in determining the electronic properties of semiconductors, especially in photovoltaics (PVs), due to their effects on electron-hole recombination. We employ model systems to study the effects of dislocations in CdTe by constructing grain boundaries using wafer bonding. Atomic-resolution scanning transmission electron microscopy (STEM) of a [1–10]/(110) 4.8° tilt grain boundary reveals that the interface is composed of three distinct types of Lomer dislocations. Geometrical phase analysis is used to map strain fields, while STEM and density functional theory (DFT) modeling determine the atomic structure at the interface. The electronic structure of the dislocation cores calculated using DFT shows significant mid-gap states and different charge-channeling tendencies. Cl-doping is shown to reduce the midgap states, while maintaining the charge separation effects. This report offers novel avenues for exploring grain boundary effects in CdTe-based solar cells by fabricating controlled bicrystal interfaces and systematic atomic-scale analysis.

Grain boundaries (GB) in poly-CdTe solar cells play an important role in species diffusion, segregation, defect formation, and carrier recombination. While the creation of specific high-symmetry interfaces can be straight forward, the creation of general GB structures in many material systems is difficult if periodic boundary conditions are to be enforced. Here we describe a novel algorithm and implementation to generate initial general GB structures for CdTe in an automated way, and we investigate some of these structures using density functional theory (DFT). Example structures include those with bi-crystals already fabricated for comparison, and those planning to be investigated in the future.

An improvement in efficiencies of polycrystalline CdTe can possibly be achieved by understanding the role of grain boundaries. Therefore, we systematically studied the atomic and electronic structures of various high angle grain boundaries including asymmetric tilt and twist grain boundaries using empirical potentials and density functional theory (DFT). The density of states analysis revealed that most grain boundaries lead to the formation of midgap states, which can drastically reduce the photovoltaic efficiency. The planar-averaged electrostatic potential analysis indicated attraction for holes around the grain boundary region.

Zinc-blende (ZB) CdTe has drawn great attention as optoelectronic and solar energy conversion materials since it has a near optimum band gap of 1.6 eV and a absorption coefficient greater than 5x10 5 /cm. CdTe can be either ZB or wurtzite (WZ) structures, resulting in different electronic properties. It has been reported that the twin superlattice with numerous twin boundaries in III-V and II-VI semiconductor nanowires along growth orientation of ZB structures considerably enhance band gap engineering and mechanical behavior in quasi-one-dimensional materials [1]. This opens new possibilities for properties and functionalities at the atomic and quantum scales by controlling the twin boundary and modulating twin densities. Therefore, it is an in-demand and challenging feat to “create” a single twin boundary in a nanowire or bulk material in order to understand the electronic and mechanical characteristics of the III-V and II-VI quantum well or barrier. Wafer bonding, which enables the direct integration of two or more single crystal wafers with controlled surfaces and orientation, is a key technique in creating a single boundary [2]. In this study, we show the creation of a single boundary between two identical CdTe single crystals.

The stacking of two-dimensional layered materials, such as semiconducting transition metal dichalcogenides (TMDs), insulating hexagonal boron nitride (hBN), and semimetallic graphene, has been theorized to produce tunable electronic and optoelectronic properties. Here we demonstrate the direct growth of MoS2, WSe2, and hBN on epitaxial graphene to form large-area van der Waals heterostructures. We reveal that the properties of the underlying graphene dictate properties of the heterostructures, where strain, wrinkling, and defects on the surface of graphene act as nucleation centers for lateral growth of the overlayer. Additionally, we show that the direct synthesis of TMDs on epitaxial graphene exhibits atomically sharp interfaces. Finally, we demonstrate that direct growth of MoS2 on epitaxial graphene can lead to a 10(3) improvement in photoresponse compared to MoS2 alone.

Dislocation cores have long dominated the electronic and optical behaviors of semiconductor devices and detailed atomic characterization is required to further explore their effects. Miniaturization of semiconductor devices to nanometre scale also puts emphasis on a material's mechanical properties to withstand failure due to processing or operational stresses. Sessile junctions of dislocations provide barriers to propagation of mobile dislocations and may lead to work-hardening. The sessile Lomer–Cottrell and Hirth lock dislocations, two stable lowest elastic energy stair-rods, are studied in this paper. More specifically, using atomic resolution high-angle annular dark-field imaging and atomic-column-resolved X-ray spectrum imaging in an aberration-corrected scanning transmission electron microscope, dislocation core structures are examined in zinc-blende CdTe. A procedure is outlined for atomic scale analysis of dislocation junctions which allows determination of their identity with specially tailored Burgers circuits and also formation mechanisms of the polar core structures based on Thompson's tetrahedron adapted to reactions of polar dislocations as they appear in CdTe and other zinc-blende solids. Strain fields associated with the dislocations calculatedviageometric phase analysis are found to be diffuse and free of `hot spots' that reflect compact structures and low elastic energy of the pure-edge stair-rods.

The single twin boundary with crystallographic orientation relationship (1¯1¯1¯)//(111) [01¯1]//[011¯] was created by wafer bonding. Electron diffraction patterns and high-resolution transmission electron microscopy images demonstrated the well control of the rotation angle between the bonded pair. At the twin boundary, one unit of wurtzite structure was found between two zinc-blende matrices. High-angle annular dark-field scanning transmission electron microscopy images showed Cd- and Te-terminated for the two bonded portions, respectively. The I-V curve across the twin boundary showed increasingly nonlinear behavior, indicating a potential barrier at the bonded twin boundary.

The single phase (1−x)PbTiO3–xBi(Ni1/2Ti1/2)O3 thin films were synthesized on Pt/Ti/SiO2/Si substrate at 600°C by a chemical solution deposition route. The present films exhibit homogeneous and crackfree microstructure with low porosity. The surface roughness decreases from 5.56nm to 1.62nm with solubility. The remanent polarization monotonously decreases with the dopant Bi(Ni1/2Ti1/2)O3 increase. The leakage current desity increases when the solubility increases. O K-edge X-ray absorption spectroscopy and valence-band edge X-ray photoelectron spectroscopy were used to study the electronic structure. The results indicated that the change of ferroelectricity might be ascribed to the hybridizations between O 2p and Pb 6s and Ti 3d orbitals. The ferromagnetic behaviors were also observed in the thin films and saturated magnetization raises monotonously with the Ni solubility due to enhanced superexchange interaction. Magnetoelectic effects increases with dopant Bi(Ni1/2Ti1/2)O3 increase.

The (1-x)PbTiO3-xBiMeO(3) (Me = Fe, In, Sc) solid solutions were prepared and investigated using high-temperature X-ray diffraction for the B-site dopant effect on their thermal expansion behaviors. Compared with PbTiO3, the negative thermal expansion of (1-x)PbTiO3-xBiFeO(3) was enhanced, whereas that of (1-x)PbTiO3-xBiInO(3) was a little weakened and was much weakened in (1-x)PbTiO3-xBiScO(3). An empirical linear correlation between the average effective radius of the B-site cations and the unit cell volumes at Curie point of the solid solutions was concluded. The relationship was also observed in other PbTiO3-BiMeO3-type solid solutions and was supposed to be widely useful in predicting the thermal expansion coefficient of compounds of this sort. Above the Curie point, the unit cell volumes of the compounds were determined by the radii of the doped B-site cations. The weaking and vanishing of the lattice distortion caused by spontaneous polarization displacements was proved by Raman scatting spectrum, which supported the relationship in the lattice dynamic aspect.

Al-doped PbTiO3 solid solutions were synthesized by a solid state method. Since Al does not have bonding d-orbit or d-electrons, and the substitutions of Al(3+) for Ti(4+) in PbTiO3 is aliovalent, the effect of Al on the structure and spontaneous polarization is quite different from that of Hf, Zr, etc. substitutions in PbTiO3. Usually, the spontaneous polarization is weakened with decreased tetragonality in PbZrxTi1-xO3 and PbHfxTi1-xO3 systems; PbTi1-xAlxO3 (0 ≤ x ≤ 0.10) solid solutions exhibit improved spontaneous polarization with decreased tetragonality (c/a). Lattice dynamics and the crystal structure of PbTi1-xAlxO3 with enhanced spontaneous polarization were investigated by FT-IR, Raman scattering technique, and X-Ray Rietveld method. The Al-doping reinforced the covalence of Pb-O(II), which indicated that the Pb-O hybridization was strengthened. The three transverse optical (TO) modes of A1-symmetry in Raman and the "stretching" and "bending" vibration modes in FTIR further verified the increase of spontaneous polarization (PS) in the A- and B-sites.

Single phase Pb(Ti(0.8)Fe(0.2))O(3-δ) thin films with a thickness of 210 nm and 120 nm were fabricated on Pt/Ti/SiO(2)/Si substrate by a chemical solution deposition technique. The thin film with a thickness of 210 nm showed a homogeneous microstructure, low porosity, low oxygen vacancies, and preferred orientation. It had negligible leakage current and well saturated ferroelectric hysteresis loop compared with the Pb(Ti(0.8)Fe(0.2))O(3-δ) bulk sample. Polarization fatigue characteristic indicated that this film has a potential application as a switcher in some electrical devices. The saturation magnetization in the Fe-doped PbTiO(3) film is weaker than that for bulk sample, and its ferromagnetism is correlated to the F-center exchange (FCE) mechanism. The present results revealed the multiferroic nature of the Pb(Ti(0.8)Fe(0.2))O(3-δ) thin film.

Dissociation process of glutathione-gold(I) polymers in aqueous solution resulted in the formation of a class of ~2 nm gold nanoparticles. Different from the same sized but NaBH(4) reduced gold nanoparticles, these nanoparticles exhibit strong luminescence but no surface plasmon absorption. Luminescence lifetimes of the nanoparticles were found strongly dependent on excitation wavelengths, and singlet and triplet excited states involving the emission were found degenerate in energy. X-ray photoelectron spectroscopic studies showed that nearly 40~50% gold atoms in the luminescent nanoparticles were in gold(I) state, which are responsible for the unique optical properties of the luminescent gold nanoparticles. These luminescent nanoparticles can be considered an intermediate state between luminescent gold(I) complexes and reduced nonluminescent gold nanoparticles.

The structures of (1−x)(K0.5Na0.5)NbO3–xLiNbO3 were investigated by the X‐ray Rietveld method at room temperature. It is surprising to find that the spontaneous polarization of orthorhombic phase reaches at the lowest value and the spontaneous polarization of tetragonal phase reaches at highest value at x=0.06. The lattice parameter of (K0.47Na0.47)NbO3–0.06LiNbO3, which was refined by using two phase model, reasonably described the structure change of the (1x)(K0.5Na0.5)NbO3–xLiNbO3. The temperature dependence of Raman spectroscopy study of the phase transition of (K0.47Na0.47)NbO3–0.06LiNbO3 showed that the increasing of temperature introduced a local disorder even above Tc. The degree of cation ordering determines the electrical properties of the ceramics.