High Temperature Tm Research Articles

Solvent‐Free Preparation and Moderate Congruent Melting Temperature of Layered Lead Iodide Perovskites for Thin‐Film Formation

AbstractLead halide 2D hybrid perovskites (HP) have emerged as promising materials for photovoltaic and lighting applications. Solvent‐free preparations offer greener route, but require congruent melting of halide perovskite for thin films, which has been demonstrated only for monolayered HP (n=1 of the (A)2(MA)n−1PbnI3n+1 series) at the quite high temperature Tm (Tmelting) of 171 °C. Here, we report on the solvent‐free preparation (n=2, 4), thermal behaviour and melt‐processed thin films of a series of HP (GABA)2(MA)n−1PbnI3n+1 (n=1, 2, 4; GABA+: 4‐ammoniumbutyric acid cation). The n=1 and n=2 HP have an exceptional low congruent Tm of 126 °C and 136 °C, respectively, and, for n=1, a very good stability in the molten state. The liquid–solid reaction of molten (GABA)2PbI4 or (GABA)2(MA)Pb2I7 with MAPbI3 in 1/1 (T=130 °C) or 1/2 (T=135 °C) ratio leads to pure phases of the n=2 HP and (GABA)2(MA)3Pb4I11 (n=4, Tm=185 °C), respectively. Melt‐processed thin films of n=1, n=2 and n=4 HP have also been prepared.

Stable Ferroelectric Behavior of Nb-Modified Bi0.5K0.5TiO3-Bi(Mg0.5Ti0.5)O3 Lead-Free Relaxor Ferroelectric Ceramics

Crystal structure, dielectric, ferroelectric, piezoelectric, and electric field-induced strain properties of lead-free Nb-modified 0.96Bi0.5K0.5TiO3-0.04Bi(Mg0.5Ti0.5)O3 (BKT-BMT) piezoelectric ceramics were investigated. Crystal structure analysis showed a gradual phase transition from tetragonal to pseudocubic phase with increasing Nb content. The optimal piezoelectric property of small-signal d 33 was enhanced up to ∼ 68 pC/N with a lower coercive field (E c) of ∼ 22 kV/cm and an improved remnant polarization (P r) of ∼ 13 μC/cm2 for x = 0.020. A relaxor-like behavior with a frequency-dependent Curie temperature T m was observed, and a high T m around 320°C was obtained in the investigated system. This study suggests that the ferroelectric properties of BKT-BMT was significantly improved by means of Nb substitution. The possible shift of depolarization temperature T d toward high temperature T m may have triggered the spontaneous relaxor to ferroelectric phase transition with long-range ferroelectric order without any traces of a nonergodic relaxor state in contradiction with Bi0.5Na0.5TiO3-based systems. The possible enhancement in ferroelectric and piezoelectric properties near the critical composition x = 0.020 may be attributed to the increased anharmonicity of lattice vibrations which may facilitate the observed phase transition from a low-symmetry tetragonal to a high-symmetry cubic phase with a decrease in the lattice anisotropy of an undoped sample. This highly flexible (at a unit cell level) narrow compositional range triggers the enhancement of d 33 and P r values.

Design of Ambient Pressure XPS (AP-XPS) Using Bench-Top X-Ray Source and High-Pressure High-Temperature STM and Applications to Operando Studies of Catalysis

Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

Room and high-temperature scanning tunnelling microscopy and spectroscopy (HT-STM/STS) investigations of surface nanomodifications created on the TiO 2(1 1 0) surface

High-temperature scanning tunnelling microscopy, scanning tunnelling spectroscopy and current imaging tunnelling spectroscopy (HT-STM/STS/CITS) were used to study the topographic and electronic structures changes due to surface modifications of the TiO 2(1 1 0) surface caused by the STM tip. In situ high-temperature STM results showed that the created modifications were stable even at elevated temperatures. The STS/CITS results showed the presence of energy gap below the Fermi level on the untreated regions. The disappearance of energy gap below the Fermi level on the modifications created by the tip was observed. It is assumed that the presence of the tip can change the chemical stoichiometry of the surface from TiO 2− x towards Ti 2O 3.

Identification of intermediate linear structure formed during Bi/Si(0 0 1) surface anneal

Bi nanolines form at around 570 °C during the anneal of a Bi-rich surface, which is composed of a mixture of Bi dimers, Si dimers, and missing dimer defects. High-temperature STM observations find that as well as the stable nanolines previously reported, a second Bi-related linear feature forms in the background, which has not previously been identified. Using quench experiments, and by comparison with tightbinding simulations of candidate surface structures, the structure of this linear feature has been identified. It is concluded that the linear defective feature comprises a single missing dimer defect (1DV) decorated with Bi, and forms as a result of cooperative strain relaxation between the Bi dimers and the 1DV.

Real-time STM study of inter-nanowire reactions: GdSi 2 nanowires on Si(1 0 0)

We report the co-existence and growth of two different types of GdSi 2 nanowires (NWs) following Gd deposition on the Si(1 0 0) surface. The first NW type is hexagonal and similar in appearance and orientation to that previously reported in the literature. The structure of the second NW type is unknown, but its orientation, thermodynamic stability and surface appearance are consistent with an orthorhombic structure. Real-time high-temperature STM studies show that these new NWs are thermodynamically preferred and in regions where both wires co-exist, they grow at the expense of their hexagonal counterparts. The implications of these observations for nanoscale interconnects is discussed.

Nano-cluster formation in halogen etching on Cl/Si(1 1 1)-7 × 7

To furnish semiconductor surfaces at device fabrication, control of micro-structures is expected of the most importance in the near-future. Highly sensitive measurement of desorption from Cl-adsorbed Si(1 1 1) surface indicated that a diffusive process plays an important role on the surface. The desorption barrier was estimated to be 2.2 eV from temperature-dependence of time-course in isothermal desorption, suggesting the desorption site contains distortion. Defects, which may be steps or clusters, are the dominant sites where the associative reaction occurs. In STM images, clusters were observed at step edges, as well as growth of pits and defects. From comparison of the desorption with high temperature STM images, a cluster pins the step movement to form step bunches. Interaction between clusters through the step movements makes clusters aligned. Ordered structures can be obtained when velocity of the surface diffusion matches the rate of the chloride desorption.

Metal adsorption induced faceting on a Si( [formula omitted]) surface where m/ h=1.4–1.5

Initial stage of growth of Au on a Si( h h m ) surface where m/ h=1.4–1.5 (a Si(1 1 1) vicinal surface inclined by about 10.5° toward the [ 1 ̄ 1 ̄ 2] direction) was studied by high temperature STM. The Si( h h m ) clean surface is destabilized by a small amount of Au adsorption and transforms into a stepped-(5 5 7) surface. The surface morphology changes by Au adsorption due to change of the surface energy and stabilized facet depend on Au coverage. The (5 5 7) surface transformed into a hill-and-valley structure composed of (3 3 5) and (1 1 1) facets, and it subsequently transformed into another hill-and-valley structure composed of (5 5 1 1) and (1 1 1) facets.

Anisotropic spillover from elongated nanoparticles

Reaction processes have only recently been imaged at the nanoscale, and advances in this direction are being made by the use of ultramicroscopies such as scanning probe methods. In this paper we describe the application of high temperature STM to the imaging of nanoparticles of Pd fabricated on TiO 2. We have imaged ‘spillover’ of a reactant from the Pd particles (which are active for adsorption) onto the surrounding TiO 2 (which is inactive for adsorption). A surprising observation is made with nanoparticles with a high aspect ratio of length to width. In that case spillover occurs preferentially from the ends of the nanoparticle, and it is speculated that this is due to higher reactivity there for O 2 dissociation, which is due in turn to the lower average coordination of surface Pd atoms at the ends of the particle.

A High-Temperature STM-Study of Iron Silicide Formation on High Index Silicon Surfaces

AbstractThe nucleation and initial growth of iron silicide on Si(113), Si(5 5 12) and Si(114) is studied using high temperature STM. During MBE and gas source deposition (precursor Fe(CO)5 STM-“movies” at temperatures up to 600°C directly show the kinetics of silicide formation on the surface. A complete 2D-wetting layer is formed on all vicinal surfaces for temperatures below 510°C. A chain-like silicide structure with a mixture of different periodicities (4×n), with n ranging from 2-6, is found. Above one monolayer growth proceeds with anisotropic 3D-silicide-islands. The wetting layer was found to be only metastable at lower growth temperature. At temperatures above 510°C the 2D-silicide layer does not wet the substrate anymore. In the equilibrium growth mode, 3D-islands directly grow on the bare Si-substrate. The anisotropy of the 3D-islands is extremly temperature dependent resulting in an aspect ratio up to 50 at 600°C. Especially on Si(114) a one-dimensionally nanostructured silicide layer can be grown in this way. A lateral spreading of the 3D-islands forming an interconnected silicide layer is achieved on Si(113) by a stochiometric codeposition of Fe and Si.

Effect of environmental O 2 on the dynamical process of the Si(111)‘1×1’→7×7 structural phase transition

The effect of environmental O 2 on the dynamical process of the Si(111)‘1×1’→7×7 structural phase transition has been investigated. In a macroscopic view, growth of the 7×7 DAS structure on the quenched surface after pulsed laser irradiation was monitored in-situ with RHEED under different O 2 pressures. In an atomic scale view, the n× n DAS domain growth was observed with high temperature STM. It has been found that the oxides formed in the ‘1×1’ matrix or phase boundary between n× n DAS-1×1 and the n× n domain boundary is responsible for the suppression of the 7×7 growth. Under the higher O 2 pressure, not only suppression of the 7×7 growth, but also erosion of the 7×7 took place. The threshold O 2 pressure, which suppresses the 7×7 domain growth but does not attack the 7×7 domains themselves, was higher for the specimen kept at 600°C than that at 465°C because of fewer phase boundaries in the former.

Growth mechanism of 7×7 domains from the ‘1×1’ phase on a quenched Si(111) surface studied by high temperature STM

Growth processes of 7×7 domains from the ‘1×1’ phase on quenched Si(111) surfaces were studied by high temperature STM at a temperature around 340°C, which is more than 100°C lower than temperatures at which previous studies had been carried out. Several metastable structure units were newly found, which play essential roles in the transformation to the dimer adatom stacking fault (DAS) structures. Basic restructuring processes were found to be shifts of dimer lines, which surround a faulted triangle, along and perpendicular to the line.

Growth and Decay of Germanium Islands on Silicon Studied by High Temperature Stm

AbstractWe use a scanning tunneling microscope (STM) capable of imaging the growing layer at high temperature during molecular beam epitaxy (MBE) to study the epitaxial growth of Germanium on Silicon and the decay of Ge islands. The periodicity of the (2×N) reconstruction of two-dimensional Ge layers on Si(001) is measured as function of the Ge coverage. Strain energy drives the formation of the (2×N) reconstruction and Si/Ge intermixing. A comparison to total energy calculations predicting the periodicity of the (2×N) reconstruction is used to estimate the amount of Si-Ge intermixing near the surface. The evolution of the size and shape of individual “hut clusters” is measured and explained by kinetically self-limiting growth. The relaxation of kinetically a determined morphology towards equilibrium is followed for a Ge layer on Si(111). Strained two-dimensional as well as partially relaxed three-dimensional islands dissolve and are soaked up by larger three-dimensional islands which are dislocated and therefore fully relaxed.

Reactive re-oxidation of reduced TiO2(110) surfacesdemonstrated by high temperature STM movies

The re-oxidation of slightly reduced TiO2(110) surfaces by exposure to an oxygen pressure of ~2 × 10-7 mbar in the temperature range 473-1000 K occurs by re-growth of TiO2 overlayers by diffusion of Ti n+ interstitials from the bulk. Starting with a (1 × 2) reconstructed surface, scanning tunnelling microscope images of the surface reacting under these conditions show that the (1 × 1) islands nucleate within the (1 × 2) layer and grow laterally. As the islands reach a critical size, which is temperature dependent, a new (1 × 2) layer begins to nucleate and grow. At both extremes of the temperature range nucleation of the second layer occurs before coalescence of the (1 × 1) islands, however, at temperatures between 673 and 773 K large areas of (1 × 1) surface form before growth of the second layer. The reaction is cyclic and several layers of TiO2 can be grown in this way.

Reversible phase transition between metastable structures of Si(111)c2×8 and 1×1 studied by high-temperature STM

We studied metastable reconstructions on quenched Si(111) surfaces by using high-temperature STM. It was found that the c2×8 is most stable in metastable reconstructions of 2×2, c2×4, c2×8 and 3×3 on the quenched Si(111) surface. The reversible phase transition between the c2×8 and 1×1 structures at about 230°C was found on the surface, which is similar to that between the c2×8 and 1×1 structures on Ge(111) surfaces around 300°C.

Reactivity of O 2 with Si(111) surfaces with different surface structures

Reactivity of O 2 with Si(111) surfaces in which both 7×7 dimer–adatom-stacking fault (DAS) domains and unreconstructed `1×1' region coexist, have been investigated. The change in surface morphology due to O 2 exposure has been monitored in-situ with high temperature STM. The acquired sequential images clearly show that the change due to oxidation preferentially takes place at such sites as `1×1' region, 7×7–`1×1' phase boundary and the missing adatom sites in a 7×7 domain, where adatoms have a space to displace upon oxidation.

In-situ study of gold-induced surface structures and step rearrangements on the Si(001) surface by high-temperature STM

The formation of Au-induced surface structures and step arrangements on the Si(001) surface at temperatures between 500 and 700°C were observed in situ by UHV high-temperature STM. At 500–600°C, various kinds of stripe structures with two-fold periodicity along the stripe ([110] direction) were formed after about 1 ML deposition, and their widths depended on the substrate temperature. At 700°C, the stripes were well ordered and a c(18 × 2) structure was formed. Stripes were elongated along the dimer rows of the original substrate surface, independent of the substrate temperature. During the formation of the stripe structure, an array of single-height steps was rearranged to form straight 2 n-height steps parallel to the stripes, and a single-domain structure was formed at high temperatures.

High-temperature STM of the phase transitions of Au(110) and Pt(110)

It is well known that Au(110)(1×2) and Pt(110)(1×2), which are both MR (missing-row) reconstructed at 300 K, undergo two phase transitions upon heating: an Ising transition at which the surfaces deconstruct and a 3D roughening transition. Our real-space investigation with atomic scale resolution by high-temperature scanning tunneling microscopy reveals that, contrary to the present theoretical models, the Ising transition of Au(110)(1×2) is probably due to antiphase domains developing during the 2D roughening of (1×3) steps; the MR configuration of interior terrace regions remains completely stable up to the 3D roughening temperature (at ≈700 K). The Ising transition of Pt(110)(1×2), however, proceeds by the simultaneous formation of 2D island/hole units at the originally flat terraces in accordance with the step-pairing model of Vilfan and Villain [I. Vilfan, J. Villain, Phys. Rev. Lett. 65 (1990) 1830] or the disordered flat (DOF) phase of den Nijs [M. den Nijs, Phys. Rev. Lett. 6 (1991) 907].

Dynamic observation of In adsorption on Si(111) surfaces by UHV high-temperature scanning tunneling microscopy

Indium adsorption and desorption processes on Si(111) surfaces were observed in situ by UHV high-temperature STM. By the deposition of In on the Si(111)7 × 7 surfaces at 380°C, the surface structure changed successively to 3 × 3 , 31 × 31 and 4 × 1. The number densities of silicon atoms in the restructuring layers for the 3 × 3 , 31 × 31 , and 4 × 1 structures were evaluated to be about 0, 1 and 2 ML, respectively. High-resolution STM images were also taken after the deposition.

High-temperature STM for atomic processes on semiconductor surfaces

This paper describes a new ultrahigh-vacuum scanning tunneling microscope (UHV-STM) which allows atomic-level observation as a function of temperature. It also reports on the use of this instrument for some high-temperature (HT) and low-temperature (LT) studies of semiconductor surfaces including various surface changes of the Si(111), Si(110) and Si(100) surfaces to new reconstructed structures at high temperatures, various surface behaviors, ultra-micro-processing; adsorption of different atom species and growth processes and low-temperature buckling structures of Si(100) dimer rows during ultra-low temperature observation.