Self-assembled Islands Research Articles

Structure and Defect Identification at Self-Assembled Islands of CO2 Using Scanning Probe Microscopy.

Understanding how carbon dioxide (CO2) behaves and interacts with surfaces is paramount for the development of sensors and materials to attempt CO2 mitigation and catalysis. Here, we combine simultaneous atomic force microscopy (AFM) and scanning tunneling microscopy (STM) using CO-functionalized probes with density functional theory (DFT)-based simulations to gain fundamental insight into the behavior of physisorbed CO2 molecules on a gold(111) surface that also contains one-dimensional metal-organic chains formed by 1,4-phenylene diisocyanide (PDI) bridged by gold (Au) adatoms. We resolve the structure of self-assembled CO2 islands, both confined between the PDI-Au chains as well as free-standing on the surface and reveal a chiral arrangement of CO2 molecules in a windmill-like structure that encloses a standing-up CO2 molecule and other foreign species existing at the surface. We identify these species by the comparison of height-dependent AFM and STM imaging with DFT-calculated images and clarify the origin of the kagome tiling exhibited by this surface system. Our results show the complementarity of AFM and STM using functionalized probes and their potential, when combined with DFT, to explore greenhouse gas molecules at surface-supported model systems.

Light-emitting diodes with Ge(Si) nanoislands embedded in photonic crystals

Room temperature lateral p+–i–n+ light-emitting diodes (LEDs) with photonic crystals embedded in the i-region were fabricated on structures with Ge(Si) self-assembled islands and their optical properties were investigated. The use of preliminary amorphization and solid phase epitaxy of the implanted p+ and n+ contact regions made it possible to reduce the impurity activation temperature from 800 °С–1100 °С to 600 °С, which corresponds to the growth temperature of Ge(Si) islands. This resulted in a significant reduction of the detrimental effect of the high-temperature annealing used for diode formation on the intensity and spectral position of the luminescence signal from the islands. It was shown that significant enhancement (more than an order of magnitude) of room temperature electroluminescence of Ge(Si) islands in the spectral range of 1.3–1.55 μm can be achieved due to their interaction with different modes of the photonic crystals. The measured radiation power of the obtained diodes in the spectral range of 1.3–1.55 μm exceeds 50 pW at a pump current of 8 mA, which is an order of magnitude higher than the previously achieved values for micro-LEDs with Ge(Si) nanoislands. The obtained results open up new possibilities for the realization of silicon-based light emitting devices operating at telecommunication wavelengths.

Tailoring On-Surface Molecular Reactions and Assembly through Hydrogen-Modified Synthesis: From Triarylamine Monomer to 2D Covalent Organic Framework.

Relative to conventional wet-chemical synthesis techniques, on-surface synthesis of organic networks in ultrahigh vacuum has few control parameters. The molecular deposition rate and substrate temperature are typically the only synthesis variables to be adjusted dynamically. Here we demonstrate that reducing conditions in the vacuum environment can be created and controlled without dedicated sources─relying only on backfilled hydrogen gas and ion gauge filaments─and can dramatically influence the Ullmann-like on-surface reaction used for synthesizing two-dimensional covalent organic frameworks (2D COFs). Using tribromo dimethylmethylene-bridged triphenylamine ((Br3)DTPA) as monomer precursors, we find that atomic hydrogen (H•) blocks aryl-aryl bond formation to such an extent that we suspect this reaction may be a factor in limiting the ultimate size of 2D COFs created through on-surface synthesis. Conversely, we show that control of the relative monomer and hydrogen fluxes can be used to produce large self-assembled islands of monomers, dimers, or macrocycle hexamers, which are of interest in their own right. On-surface synthesis of oligomers, from a single precursor, circumvents potential challenges with their protracted wet-chemical synthesis and with multiple deposition sources. Using scanning tunneling microscopy and spectroscopy (STM/STS), we show that changes in the electronic states through this oligomer sequence provide an insightful view of the 2D COF (synthesized in the absence of atomic hydrogen) as the end point in an evolution of electronic structures from the monomer.

Atomic-scale characterization of contact interfaces between thermally self-assembled Au islands and few-layer MoS2 surfaces on SiO2

The interaction between metallic nanoparticles and transition metal chalcogenides (TMDs) can realize new functionalities in thriving technologies such as optoelectronics and nanoengineering. Here we have investigated the self-assembly of triangular-shaped crystalline Au nanoislands on MoS2 flakes mechanically exfoliated or grown by chemical vapor deposition (CVD). The density and size of the islands are determined by substrate temperature, deposition flux, and subsurface morphology. The thickness of the MoS2 layers is measured by Raman spectroscopy, which also enables the evaluation of the strain and doping distributions induced by the Au islands. Top and cross-sectional images of the Au-MoS2 interface are obtained by scanning electron microscopy (SEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Sub-nanometer resolution of the Au, Mo and S layers reveals that the MoS2 flakes follow the corrugation of the SiO2 substrate, with flattening and wrinkling effects induced by the growth of the Au islands on top.

Topological Defects Induced High-Spin Quartet State in Truxene-Based Molecular Graphenoids

Topological Defects Induced High-Spin Quartet State in Truxene-Based Molecular Graphenoids

Electronic energy levels of porphyrins are influenced by the local chemical environment.

Self-assembled islands of 5,10,15,20-tetrakis(pentafluoro-phenyl)porphyrin (2HTFPP) on Au(111) contain two bistable molecular species that differ by shifted electronic energy levels. Interactions with the underlying gold herringbone reconstruction and neighboring 2HTFPP molecules cause approximately 60% of molecules to have shifted electronic energy levels. We observed the packing density decrease from 0.64 ± 0.04 molecules per nm2 to 0.38 ± 0.03 molecules per nm2 after annealing to 200 °C. The molecules with shifted electronic energy levels show longer-range hexagonal packing or are adjacent to molecular vacancies, indicating that molecule–molecule and molecule–substrate interactions contribute to the shifted energies. Multilayers of porphyrins do not exhibit the same shifting of electronic energy levels which strongly suggests that molecule–substrate interactions play a critical role in stabilization of two electronic species of 2HTFPP on Au(111).

Self-assembled gold nano islands for precise electrochemical sensing of trace level of arsenic in water

Trace level detection of Arsenic in groundwater is one of the major step in the detoxification of drinking water. Accumulation of Arsenic in various parts of the human body through drinking water is a serious health hazard across the world including India. The present experiment describes the template free growth of gold nano island(AuNis) on the ITO surface to integrate with 1,4-dithiothreitol(DTT) for the construction of electrochemical sensor (DTT/AuNis/DTH/Auseed/APTMS/ITO) for trace level arsenic detection in water. Unique recognition of As3+offered by DTT, a ligand having –SH and –OH functional groups, and enhanced electrochemical signal originated from AuNis self-assembled on the ITO surface were exclusively harnessed for trace level detection of Arsenic (As3+) in water. The developed DTT/AuNis/DTH/Auseed/APTMS/ITO electrode was characterized using UV–Visible spectroscopy, Field Emission Scanning Electron Microscope (FESEM), Atomic Force Microscopy (AFM), Raman Spectroscopy, Fourier Transform Infrared Spectroscopy (FTIR), and Cyclic Voltammetry (CV). Wide linearity (R2 = 0.97) has been achieved for a gamut of arsenic concentration from 0.1 to 100 ppb with LOD of 0.085 ppb. The developed sensor offers a good sensitivity of 4.39 mA ppb cm−2. Interference studies were successfully performed for contaminated water samples and in presence of assorted heavy metal ions like Hg2+, Pb2+, Ni2+ and, Cr3+ using 2, 4-Pyridinedicarboxylic acid (PDCA), a sequestering agent. The developed sensor was evaluated as a sensitive, rapid, and reliable platform for the detection of As3+ in real samples offering huge potential for commercialization.

Adsorption and self-assembly of porphyrins on ultrathin CoO films on Ir(100).

Porphyrins represent a versatile class of molecules, the adsorption behavior of which on solid surfaces is of fundamental interest due to a variety of potential applications. We investigate here the molecule–molecule and molecule–substrate interaction of Co-5,15-diphenylporphyrin (Co-DPP) and 2H-tetrakis(p-cyanophenyl)porphyrin (2H-TCNP) on one bilayer (1BL) and two bilayer (2BL) thick cobalt oxide films on Ir(100) by scanning tunneling microscopy (STM) and density functional theory (DFT). The two substrates differ greatly with respect to their structural and potential-energy landscape corrugation with immediate consequences for adsorption and self-assembly of the molecules studied. On both films, an effective electronic decoupling from the metal substrate is achieved. However, on the 1BL film, Co-DPP molecules are sufficiently mobile at 300 K and coalesce to self-assembled molecular islands when cooled to 80 K despite their rather weak intermolecular interaction. In contrast, on the 2BL film, due to the rather flat potential landscape, molecular rotation is thermally activated, which effectively prevents self-assembly. The situation is different for 2H-TCNPP, which, due to the additional functional anchoring groups, does not self-assemble on the 1BL film but forms self-assembled compact islands on the 2BL film. The findings demonstrate the guiding effect of the cobalt oxide films of different thickness and the effect of functional surface anchoring.

Impact of Ge deposition temperature on parameters of c-Si solar cells with surface texture formed by etching of Si using SiGe islands as a mask

In this paper, the Ge self-assembled islands grown at different temperatures were used as an etching mask for selective anisotropic etching of crystalline silicon (c-Si) in order to form a textured Si surface. The effects of Ge deposition temperature on the texture morphology as well as the parameters of c-Si solar cells fabricated on such structures were studied with the range of temperature from 500 °C to 800 °C. The textures on Si surface with low (<1 μm) etching margin were successfully fabricated for all investigated structures. It was revealed that usage of rapid annealing of Si wafer at 800 °C prior to epitaxial growth along with the low Ge deposition temperature of 500 °C allowed to achieve the relatively high effective carrier lifetime as compared to structures formed at higher (≥700 °C) temperatures thanks to the reduced degradation of Si wafer characteristics. Furthermore, the structure obtained using the lowest deposition temperature of 500 °C exhibited the highest light absorption among all studied samples thanks to the most homogeneous and most pyramidal texture morphology. As a result, the calculated short circuit current density JSC for the sample in which Ge islands were grown at 500 °C was as high as 35.89 mA/cm2. The proposed approach of surface texturing meets the requirements for thin c-Si solar cells due to small etching margin and the obtained results pave the way for increase the efficiency of thin c-Si solar cells by engineering the surface morphology to improve light absorption.

Люминесценция пространственно упорядоченных одиночных и групп самоформирующихся Ge(Si) наноостровков, встроенных в фотонные кристаллы

Photoluminescence (PL) properties of arrays of spatially ordered single Ge(Si) self-assembled islands, including embedded in two-dimensional photonic crystals (PhC), was studied. Incorporation of an array of ordered single islands and their groups into PhC significantly increases the intensity of their PL signal. The most pronounced increase in intensity (up to 30 times at nitrogen temperatures) is observed for an ordered array of single islands. The increase in PL intensity is associated with interaction of island emission with the radiation modes of the PhC, which is more efficient for an array of spatially ordered single islands. As result PL from single ordered Ge(Si) islands embedded in PhC was observed up to room temperature.

Well-Ordered Monolayer Growth of Crown-Ether Ring Molecules on Cu(111) in Ultra-High Vacuum: An STM, UPS, and DFT Study

Crown-ether (CR) ring molecules are known as host molecules for capturing guest species inside the ring. So far CR molecular films have only been grown by drop-casting a CR solution on an inert substrate in air which offers little control over the molecular structure. Here we report the successful growth of a well-ordered CR molecular array on an atomically flat and clean noble metal Cu(111) substrate at 300 K in ultra-high vacuum (UHV), using 4,4′,5,5′-tetrabromodibenzo[18] crown-6 ether (Br-CR). The adsorption, self-assembly, and electronic structures of Br-CR were studied by means of UHV low-temperature scanning tunneling microscopy and spectroscopy, low electron energy diffraction, and angle-resolved ultraviolet photoemission spectroscopy. We found that (1) the Br-CR ring, which is bent both in the crystal and gas phase, flattens upon adsorption on Cu(111). Density functional theory reveals that the two benzene groups of the molecule lie flat on the surface such as to maximize the substrate-molecule interaction. (2) The moderate molecule–substrate interaction allows thermal diffusion of the Br-CR molecules, resulting in the formation of self-assembled monolayer islands with 7 × 4 superstructure. (3) While the deposition of 0.05 ML Br-CR forms multidomain islands with disordered defects, a drastic improvement occurred at 0.25 ML, where only atomically flat single-domain islands were grown. This Br-CR flat ring cavity array could become a template for designing novel two-dimensional arrays of desired guest atoms, ions, or functionalized molecules.

Discrete Electronic Subbands due to Bragg Scattering at Molecular Edges.

The discretization of the electronic structure of nanometer-size solid systems due to quantum confinement and the concomitant modification of their physical properties is one of the cornerstones for the development of nanoscience and nanotechnology. In this Letter we demonstrate that the Bragg scattering of Cu(111) surface-state electrons by the periodic arrangement of tetracyanoquinodimethane molecules at the edges of self-assembled molecular islands, along with the dominant contribution of backscattering processes to the electronic density of states, discretizes the possible values of the electron momentum parallel to the island edge. The electronic structure consists thus of a discrete number of subbands which occur in a nonclosed space, and therefore without quantum confinement.

Implementing Functionality in Molecular Self-Assembled Monolayers.

The planar heterocyclic molecules 1,6,7,12-tetraazaperylene on a Ag(111) metal substrate show different charging characteristics depending on their local environment: next to vacancies in self-assembled islands, molecules can be charged by local electric fields, whereas their charge state is fixed otherwise. This enables the activation of selected molecules inside islands by vacancy creation from scanning-probe-based manipulation. This concept allows for combining the precise mutual atomic-scale alignment of molecules by self-assembly, on one hand, and the implementation of specific functionality into otherwise homogeneous monolayers, on the other. Activated molecules in the direct neighborhood influence each other in their charging characteristics, suggesting their use as molecular quantum cellular automata. Surprisingly, only very few interacting molecules exhibit a rich spectroscopic signature, which offers the prospect of implementing complex functionality in such structures in the future.

Predicting the halogen-n (n = 3-6) synthons to form the "windmill" pattern bonding based on the halogen-bonded interactions.

The "windmill" pattern cyclic halogen polymers (XBr)3 (X = Cl, Br, I) and (BrY)n (n = 3-6, Y = Cl, Br, I) have been investigated using the density functional theory. Due to the anisotropic distribution of its electron density, the halogen atom can form halogen-bonded interactions by functioning as both electron donor sites and electron acceptor sites. For (XBr)3 (X = Cl, Br, I) trimers, the Cl···Cl interaction is the weakest, and the I···I interaction is the strongest. For (BrY)n (n = 3-6, Y = Cl, Br, I), the Br···Br halogen bonds are the strongest in (BrY)4 tetramers. We predict that the iodine-4 synthon may allow creation of a self-assembled island during crystal growth. The angle formed by the electron-depleted sigma-hole, the halogen atom and the electron-rich equatorial belt perpendicular to the bond direction, together with the halogen-bond angle, can be used to explain the geometries and strength of the halogen-bond interactions. © 2018 Wiley Periodicals, Inc.

A Self-Assembling Two-Dimensional Protein Array is a Versatile Platform for the Assembly of Multicomponent Nanostructures.

Rationally designed two-dimensional (2D) arrays that support the assembly of nanoscale components are of interest for catalysis, sensing, and biomedical applications. The computational redesign of a protein called TTM that undergoes calcium-induced self-assembly into nanostructured lattices capable of growing to dozens of micrometers are previously reported. The work demonstrates here that the N- and C-termini of the constituent monomers are solvent-accessible and that they can be modified with a hexahistidine extension, a gold-binding peptide, or a biotinylation tag to decorate nickel-nitriloacetic acid beads with self-assembled protein islands, conjugate gold nanoparticles to planar arrays, or control the immobilization density of avidin molecules onto 2D lattices through co-polymerization of biotinylated and wild type TTM monomers. These results showcase the potential of TTM as a versatile 2D scaffold for the fabrication of hierarchical structures comprising a broad range of nanoscale elements.

Formation of black silicon using SiGe self-assembled islands as a mask for selective anisotropic etching of silicon

Simple technique of formation of “black silicon” using wet chemical etching of crystalline Si wafers with SiGe self-assembled islands is proposed. The main idea consists of the utilization of SiGe islands as a mask for wet anisotropic etching of Si in alkali-based solution at the first etching stage and further removal of SiGe residuals by etching in a HF:H2O2:CH3COOH mixture at the second etching stage. Initial samples were the crystalline Si wafers with SiGe islands formed on them by deposition of 2.5–14nm of Ge at 800°C. After the two above-mentioned etching steps a submicron relief on a Si surface was formed. Investigation of optical properties of fabricated structures revealed significant decrease of reflection (AM 1.5G weighted reflection ~2–3%) and increase of absorption in the wavelength range of 500–1200nm. Due to the very small amount of Si removal (<0.5µm), utilization of standard chemicals for Si-based solar cell technology and potential suitability for usage in large-scale manufacturing the proposed technique is promising for increasing of efficiency of thin wafers crystalline Si solar cell.

Preparation of SiC/Ge/graphene heterostructure on 4H-SiC(0001)

Due to the wide bandgap of SiC, SiC-based optoelectronic devices can not be operated by visible (VIS) and near-infrared (NIR) light sources. A promising way to solve this problem is to adopt a SiC/Ge/Graphene heterostructure. The Ge film can be used as VIS-NIR-absorbing layer. As a electrode, graphene also forms a Schottky junction with Ge on 4H-SiC. To form the SiC/Ge/graphene heterostructures, Ge film was prepared on 4H-SiC(0001) by chemical vapor deposition, and then graphene on Cu foil was wet transferred on Ge films. The Ge film on 4H-SiC(0001) has polycrystalline structure and consists of self-assembled submicron Ge islands. The monolayer graphene with a few cracks and wrinkles was transferred on the Ge rough surface. The SiC/Ge/graphene heterostructure with high light absorption in the 500–1200nm range was prepared on 4H-SiC successfully.

Complex Pattern Formation from Current-Driven Dynamics of Single-Layer Homoepitaxial Islands on Crystalline Conducting Substrates

Nobody likes micromanaging---never mind nanomanaging. Instead of complex processing, it is very handy to have nanostructures form on their own, according to physical law. This presumes that we know how to set them up to succeed, by engineering their circumstances. Systematic simulations elucidate the kinetics responsible for intriguing, symmetrical patterns of self-assembled islands. This insight enables processing strategies for complex nanopatterning by precise control of a $m\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}o\phantom{\rule{0}{0ex}}p\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}c$ force, with potential applications in optoelectronics and data storage.

Self-Assembly of Organic Ferroelectrics by Evaporative Dewetting: A Case of β-Glycine.

Self-assembly of ferroelectric materials attracts significant interest because it offers a promising fabrication route to novel structures useful for microelectronic devices such as nonvolatile memories, integrated sensors/actuators, or energy harvesters. In this work, we demonstrate a novel approach for self-assembly of organic ferroelectrics (as exemplified by ferroelectric β-glycine) using evaporative dewetting, which allows forming quasi-regular arrays of nano- and microislands with preferred orientation of polarization axes. Surprisingly, self-assembled islands are crystallographically oriented in a radial direction from the center of organic "grains" formed during dewetting process. The kinetics of dewetting process follows the t-1/2 law, which is responsible for the observed polygon shape of the grain boundaries and island coverage as a function of radial position. The polarization in ferroelectric islands of β-glycine is parallel to the substrate and switchable under a relatively small dc voltage applied by the conducting tip of piezoresponse force microscope. Significant size effect on polarization is observed and explained within the Landau-Ginzburg-Devonshire phenomenological formalism.

TEM EDS analysis of epitaxially-grown self-assembled indium islands

Epitaxially-grown self-assembled indium nanostructures, or islands, show promise as nanoantennas. The elemental composition and internal structure of indium islands grown on gallium arsenide are explored using Transmission Electron Microscopy (TEM) Energy Dispersive Spectroscopy (EDS). Several sizes of islands are examined, with larger islands exhibiting high (&amp;gt;94%) average indium purity and smaller islands containing inhomogeneous gallium and arsenic contamination. These results enable more accurate predictions of indium nanoantenna behavior as a function of growth parameters.