Nanopatterns at near atomic dimensions with controllable quantum dot states (QDSs) are promising candidates for the continued downscaling of electronic devices. Herein, we report a phase transition-induced QD system achieved on the √3 × √3-Bi/Si(111) surface reconstruction, which points the way to a novel strategy on QDS implementation. Combining scanning tunneling microscopy, scanning tunneling spectroscopy, and density functional theory (DFT) calculations, the structure, energy dispersion, and size effect on band gap of the QDs are measured and verified. As-created QDs can be manipulated with a dot size down to 2 nm via Bi phase transformation, which, in turn, is triggered by thermal annealing at 700 K. The transition mechanism is also supported by our DFT calculations, and an empirical analytical model is developed to predict the transformation kinetics.

Ultra-thin PbBr2 overlayers on Ag (111) surface have been grown using thermal evaporation (physical vapor deposition) in the coverage range of-0.05 monolayers (ML) to 0.22 ML. The surface structure of these films has been studied by scanning tunneling microscopy, verified by low energy electron diffraction, and simulated using density functional theory. PbBr2 is one of the binary constituents for the growth of CsPbBr3 perovskites and therefore, the growth study of PbBr2 is directly relevant to the growth of the more complex perovskite as segregation of different phases can affect the properties of the material. Atomically resolved images of PbBr2 with a coverage of similar to 0.05 ML show randomly scattered areas of Pb adatoms. Increasing PbBr2 to similar to 0.16 ML show additional evidence for the dissociation of PbBr2, with separate structures associated with Pb and Br. similar to 0.22 ML of PbBr2 results in areas of a well ordered (root 3 x root 3) R30 reconstruction that has a structure incorporating both elements. This study provides the reference information for the better understanding of CsPbBr3 perovskite thin films grown under similar conditions.

Ultra-thin CsBr overlayers on the Ag (111) surface have been grown using physical vapor deposition and studied by scanning tunneling microscopy. Atomically resolved images of CsBr show the formation of the NaCl crystal structure rather than the bulk structure of CsBr, i.e., CsCl, in the first two atomic layers. The structure remains the same up to five monolayers, the maximum layer thickness being studied. Mobility of CsBr islands up to 1.1 nm thickness has been observed at room temperature. CsBr is one of the binary constituents for the growth of CsPbBr3 perovskites. Exploring the growth of CsBr on Ag (111) will provide reference information for the better understanding of the growth of CsPbBr3 perovskite thin films under similar growth conditions.

We examined the dynamics of adsorption and the subsequent growth of submonolayered silver on Si(001) from 100 K to 230 K, using scanning tunneling microscopy and density functional theory. The dynamics is demonstrated to depend on substrate temperature, as described in the following three stages: (I) at 100-140 K, silver is adsorbed as isolated aggregates (regular-Ag4, variant-Ag4 and Ag2), in the absence of single silver adatoms. The spontaneous formation of silver aggregates arises from the hot-atom motion upon the initial impingement of individual silver atoms onto the Si(001) substrate. (II) At 140-190 K, the migration of isolated Ag-aggregates is sufficiently activated, leading to the formation of Ag-chains by surface polymerization. (III) At 190-230 K, there is implication that the Ag-chains become mobile on Si(001), en route to forming patches of 2×2 Ag-films by agglomeration.

Quantum well states (QWSs) with sizable Rashba splitting are a promising quantum phase to achieve spin-split current for quantum computing and spintronics due to their controllable band structures. However, most QWSs were achieved upon metallic substrates with strong bulk electron transport. Developing semiconductor-based QWSs is preferable to minimize substrate interference. Here we report a Pb induced surface reconstruction on Bi/Si(111) α phase. Combining scanning tunneling microscopy (STM) and density functional theory (DFT) the atomic structure has been determined. QWSs and a sizable Rashba band splitting are predicted, with the latter comparable to what is found in other semiconductor heterostructures and an order of magnitude higher than that in Pb/Si(111) QWSs.

We investigate the Bi on Si(111) $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$-phase surface reconstruction using scanning tunneling microscopy. Details of the bias-dependent images of both the reconstruction and defects are presented. Combining our experimental data with density-functional theory calculations, it is confirmed that the honeycomb pattern at low-bias empty states and hexagonal closest-packed pattern at high-bias empty states originate from the Bi ${p}_{x}, {p}_{y}$ states and Bi, Si overlapped states outside bulk band gap, respectively. Analysis of the defect images and their associated densities of states provides further insight into the electronic structure of the surface. In particular, we note the presence of a quantum-dotlike localized state associated with an $\ensuremath{\alpha}$-phase defect structure. These results pave the way for further development of surface structures based on Bi on Si(111) surface with unique electronic properties such as sizable spin-orbit coupling, which might be suitable candidates for spintronic applications.

The design of low dimensional materials through surface assisted self-assembly requires a better understanding of the factors that limit and control surface diffusion. We reveal how substrate surface defects hinder the mobility of submonolayer organic adsorbates on a metal surface with the model CuPc/Cu(111) system. Postdeposition annealing bonds CuPc molecules into dendritelike clusters that are often mobile at room temperature. Surface defects on Cu(111) create energetic barriers that prevent CuPc cluster motion on the metal surface. This phenomenon was unveiled by the motion of small clusters that show rigid-body diffusion solely in the available space in between defects. When clusters are sufficiently surrounded by defects, they become completely pinned in place and become immobilized.

When a silver film is grown on the Si(001) surface at room temperature, it forms a monolayer density film with a (2 X 2) periodicity. Previous models of this (2 X 2) surface, simulated by density functional theory (DFT), are found to be inconsistent with experimental observations. The DFT simulations provide evidence that a new model, the square tetramer model, describes the structure of the observed (2 x 2)-Ag phase and is fully consistent with scanning tunneling microscopy data. Theoretical evidence of a covalent bond shared between the Ag and Si is found that matches previous experiments. Interestingly, the simulations also show that the stoichiometry between Si and Ag changes with metal coverage as adsorbate-adsorbate and adsorbate substrate interactions balance out. At low coverages, when individual Ag adatoms interact solely with Si, a two-to-one Si-Ag-Si interaction scheme is energetically preferred. At 1 monolayer, when Ag-Ag interactions must be considered, a one-to-one Si Ag interaction scheme is preferred, as it maximizes Ag-Ag interactions.

We describe the atomic structure of the silver film grown on Si(001) at room temperature, as studied by low-temperature scanning tunneling microscopy and density functional theory. Experiment and theory agree on a film structure in which Ag tetramers are identified for the first time. Ag tetramers are found to be adsorbed exclusively at the trough between two Si rows, interacting with four adjacent Si dimers via covalent bonding. Consequently, the π bonds of the Si dimers underneath the silver film are eliminated.

A periodically structured surface or grating has been used to demonstrate state-of-the-art high efficiency crystalline silicon photovoltaic cells. However, until now the understanding of the complete relationship between the grating periodicity, silicon thickness, and absorption enhancement in silicon solar cell with an inverted pyramidal texture is still unclear. In this paper, we simulate front surface inverted pyramidal grating texture on 2-400-mu m thick silicon and optimize it to derive maximum photocurrent density from the cell. We identify a "one-size-fits-all" front grating period of 1000 nm that leads to maximum photo absorption of normally incident AM1.5g solar spectrum in silicon (configured with a back surface reflector) irrespective of the thickness of the crystalline silicon absorbing layer. With the identification of such universally optimized periodicity for the case of an inverted pyramidal grating texture, a common fabrication process can be designed to manufacture high-efficiency devices on crystalline silicon regardless of the wafer thickness. The measured reflectance from submicron and wavelength scale periodic textures also verifies simulation results. (C) 2017 Elsevier Ltd. All rights reserved.

In this paper, we simulate a front surface inverted pyramidal grating texture on 2 to 400 micron thick silicon and optimize it to derive maximum photocurrent density from the cell. We identify a one size fits all front grating period of 1000 nm that leads to maximum photo-absorption of normally incident AM1.5g solar spectrum in silicon (configured with a back surface reflector) irrespective of the thickness of the crystalline silicon absorbing layer. With the identification of such universally optimized periodicity for the case of an inverted pyramidal grating texture, a common fabrication process can be designed to manufacture high-efficiency devices on crystalline silicon regardless of wafer thickness. In order to validate the results of the simulation, we fabricated high resolution inverted pyramidal textures on a 400 micron thick silicon wafer with electron beam lithography to compare the reflectance from submicron and wavelength scale periodic textures. The experimental reflectance measurements on textures confirm that a 1000 nm period grating texture performs better than a 500 nm period texture in reducing reflectance, in agreement with the simulations.

Scanning tunneling microscopy (STM) has been used to observe the growth mode and crystal structure of sub-monolayer (ML) to multilayer thin films of copper phthalocyanine (CuPc) molecules on the Cu(111) surface at room temperature (RT). At sub-ML coverage the molecules are mobile on the surface. At approximately one ML coverage the molecules become sterically confined and lying flat on the Cu substrate form an ordered, multi-domained, 2D oblique lattice. As coverage is increased beyond 1 ML the molecule–substrate interaction diminishes in strength while the intermolecular interaction begins to dominate, causing the layer separation to increase, and the crystal domain size and lattice constants to shrink as the crystal structure begins to more closely resemble the bulk α-phase CuPc molecular solid. This trend continues for the layer-by-layer growth of 3 complete ML, eventually giving way to the emergence of large 3D islands at a coverage equivalent to 4 ML.

Growth and thermal behavior of copper‐phthalocyanine (CuPc) and fullerene (C60) organic nanocomposite thin films, grown on SiO2 and Cu(111) surfaces, have been studied using scanning electron microscopy, X‐ray diffraction (XRD) and scanning tunneling microscopy (STM). It is found that the growth of pure CuPc organic thin films follows an island‐type growth mode. The island formation is found to be dramatically suppressed by the inclusion of C60 during deposition. XRD and STM studies reveal that the CuPc molecular packing is altered upon C60 inclusion, producing disordered CuPc–C60 interfaces. The ordered molecular stacking of CuPc is found to be disrupted completely when C60 concentration reaches 30 wt.%. The thermal stability of the CuPc thin film has been significantly improved upon C60 inclusion. It is suggested that the molecular diffusion kinetics rather than thermodynamics are responsible for the improved stability.

The confinement of laser interactions inside transparent materials assisted by tight optical focusing and short-pulsed nonlinear interactions has driven many high-resolution patterning and probing applications in science and technology. In thin transparent films, laser interactions confined to the film/substrate interface have underpinned blistering and ejection processes for nanofluidic channel fabrication, film patterning and cell catapulting. Here, we harness femtosecond lasers to drive nonlinear interactions within Fabry–Perot interference fringes to define narrow nanolength scale zones for highly resolved internal structuring of a film of refractive index, nfilm, at fringe maxima separated by λ/2nfilm. This novel interaction internally cleaves the film to open subwavelength internal cavities and form thin membranes at single or multiple depths from which follow significant opportunities for writing multilevel nanofluidic channels inside the film, as well as ejecting nanodisks at quantized film depths for coloring and three-dimensional surface patterning that promise new compact types of lab-in-film devices. High-resolution structuring of thin transparent films using a femtosecond laser can aid the fabrication of various devices. Kitty Kumar and co-workers from the University of Toronto in Canada say that interference from reflections at the film’s boundaries causes the laser light to create a series of periodic plasma disks in the dielectric film that eject material. The resulting laser-generated nanovoids could be useful for fabricating miniature cavities and fluidic waveguides or performing nanoscale texturing to improve light harvesting in solar cells or light extraction from LEDs. The researchers tested their scheme with a stream of 200 fs pulses from a 522 nm frequency-doubled fibre laser operating at a repetition rate of 100 kHz. They focused the pulses onto an SiNx film measuring 20–1,545 nm thick.

The adsorption of up to one monolayer (ML) of copper phthalocyanine (CuPc) molecules on a room temperature Cu(111) surface has been studied using scanning tunneling microscopy (STM). Below 1 ML the molecules are in a fluid state and are highly mobile on the surface. At 1 ML coverage the molecules coalesce into a highly ordered 2D crystal phase. At sub-ML coverages, chemisorption of individual CuPc molecules can be induced through exposure to tunneling electrons at a tunneling bias voltage exceeding a threshold value. This tunneling electron induced effect has been exploited to perform molecular STM lithography.

We use the wave optical approach to optimize the front surface inverted pyramidal grating texture on 2 to 400 μm thick crystalline silicon in order to derive the maximum photocurrent density from the cell. We identify a “one size fits all” front grating periodicity of 1000 nm for c-Si absorbing layer configured with a back surface reflector that maximizes the absorption of normally incident AM1.5g solar spectrum irrespective of the layer thickness. With the identification of such universal inverted pyramidal grating texture, a common texturing process can be developed for high-efficiency devices on thick to thin c-si. Furthermore, our studies show that the photocurrent decreases by 0.02 mA/cm2 with every nanometer increase in the width of the flat region (mesa) between inverted pyramids in the optimum texture. The decrease in photocurrent due to reflection from the mesas can be recovered with the addition of an antireflective coating of optimum thickness of 80 nm and refractive index ~ 2.1.

This study reports a high-resolution hard-mask laser writing technique to facilitate the selective etching of crystalline silicon (c-Si) into an inverted-pyramidal texture with feature size and periodicity on the order of the wavelength which, thus, provides for both anti-reflection and effective light-trapping of infrared and visible light. The process also enables engineered positional placement of the invertedpyramid thereby providing another parameter for optimal design of an optically efficient pattern. The proposed technique, a non-cleanroom process, is scalable for large area micro-fabrication of high-efficiency thin c-Si photovoltaics. Optical wave simulations suggest the fabricated textured surface with 1.3 μm inverted-pyramids and a single anti-reflective coating increases the relative energy conversion efficiency by 11% compared to the PERL-cell texture with 9 μm inverted pyramids on a 400 μm thick wafer. This efficiency gain is anticipated to improve further for thinner wafers due to enhanced diffractive light trapping effects.

The electronic structure of p (2 × 3) Ag films on Si(100) is studied by using electron energy loss spectroscopy (EELS) and scanning tunneling spectroscopy (STS). We observe three energy loss peaks with their loss energies around 1.28, 1.63, and 4.0 eV, respectively. They are assigned to the interband transition in the p (2×3) islands, the interface plasmon between Si(100) and Ag crystallites formed concomitantly with the p (2 × 3) islands, and a combination of the surface plasmon and interband transition of the Ag crystallites, respectively. STS over the p (2 × 3) surface also reveals a band gap around 1 eV in good agreement with the EELS observation of the interband transition. The present observation of the band gap suggests that the p (2 × 3) surface is semiconducting. Furthermore, we examine two models previously proposed for the p (2 × 3) surface against the present experimental observation by ab initio band structure calculations.