Nikolai Yakovlev

Degradation and failure of organic light-emitting devices

The degradation and failure of organic light-emitting device are observed via optical microscopy. The “degraded area” has been identified to be made up of three regions: (1) a dark spot at the center, (2) a nonemitting area forming the core, and (3) a weakly emitting area surrounding the core. It is found that due to metal migration, as evidenced from the secondary ion mass spectrometry profiles, the indium tin oxide/polymer interface roughens during operation. The intense local current at sharp points degrades the polymer causing the formation of the dark center. Further current stress caused the central core to carbonize which may lead to short and/or open circuits accompanied by fluctuations in the device current.

Electron Doping of Ultrathin Black Phosphorus with Cu Adatoms

Few-layer black phosphorus is a monatomic two-dimensional crystal with a direct band gap that has high carrier mobility for both holes and electrons. Similarly to other layered atomic crystals, like graphene or layered transition metal dichalcogenides, the transport behavior of few-layer black phosphorus is sensitive to surface impurities, adsorbates, and adatoms. Here we study the effect of Cu adatoms onto few-layer black phosphorus by characterizing few-layer black phosphorus field effect devices and by performing first-principles calculations. We find that the addition of Cu adatoms can be used to controllably n-dope few layer black phosphorus, thereby lowering the threshold voltage for n-type conduction without degrading the transport properties. We demonstrate a scalable 2D material-based complementary inverter which utilizes a boron nitride gate dielectric, a graphite gate, and a single bP crystal for both the p- and n-channels. The inverter operates at matched input and output voltages, exhibits a gain of 46, and does not require different contact metals or local electrostatic gating.

Fluoride layers and superlattices grown by MBE on Si(111): dynamic RHEED and Sm2+ photoluminescence studies

Abstract The oscillations of reflection high-energy electron diffraction intensity during CaF 2 and SrF 2 epitaxial growth on the (111) surface of silicon and the fluorides have been investigated. From these studies the critical thickness of the fluoride pseudomorphic growth mode was measured for different growth conditions. These data allowed us to grow strongly strained coherent fluoride layers and superlattices. Studies of Sm 2+ ion photoluminescence in these fluoride structures at liquid-helium temperature have been shown to be useful for characterization of the crystalline quality of the films and for the measurement of elastic strain. High quantum yield of the luminescence enabled us to explore doped layers as thin as several monolayers.

General One-Step Self-Assembly of Isostructural Intermetallic CoII3LnIII Cubane Aggregates

A new family of Co/rare-earth intermetallic cubane aggregates [Co3Ln(hmp)4(OAc)5H2O] (Ln = Dy, Ho, Er, Tm, Yb, Y) have been synthesized by self-assembly. Single-crystal X-ray diffraction analysis revealed that they are remarkably isostructural in showing a common [Co3Ln] core. Magnetic studies showed that the Dy, Er, Tm, Yb, and Y complexes are ferromagnetic. The Dy complex exhibits the largest magnetocaloric effect (-ΔSm = 12.58 J kg(-1) K(-1)), which can be attributed to the large magnetic density of Dy(III).

Photoluminescence of Eu2+ and Sm2+ ions in CaF2 pseudomorphic layers grown by MBE on Si (111)

Abstract Impurity photoluminescence spectra of MBE-grown CaF2:Eu2+ and CaF2:Sm2+ epitaxial layers on Si (111) have been investigated. The rare earth ions were used as a sensitive photoluminescent probe to measure CaF2 strain. Compressive strain of thin fluorite films indicated their pseudomorphic growth mode. Critical thickness of this growth mode was found to be about 10 nm at 560°C.

Large Area Directed Self-Assembly of Sub-10 nm Particles with Single Particle Positioning Resolution

Directed self-assembly of nanoparticles (DSA-n) holds great potential for device miniaturization in providing patterning resolution and throughput that exceed existing lithographic capabilities. Although nanoparticles excel at assembling into regular close-packed arrays, actual devices on the other hand are often laid out in sparse and complex configurations. Hence, the deterministic positioning of single or few particles at specific positions with low defect density is imperative. Here, we report an approach of DSA-n that satisfies these requirements with less than 1% defect density over micrometer-scale areas and at technologically relevant sub-10 nm dimensions. This technique involves a simple and robust process where a solvent film containing sub-10 nm gold nanoparticles climbs against gravity to coat a prepatterned template. Particles are placed individually into nanoscale cavities, or between nanoposts arranged in varying degrees of geometric complexity. Brownian dynamics simulations suggest a mechanism in which the particles are pushed into the template by a nanomeniscus at the drying front. This process enables particle-based self-assembly to access the sub-10 nm dimension, and for device fabrication to benefit from the wealth of chemically synthesized nanoparticles with unique material properties.

Structural transformations at CaF2Si(111) interfaces

Abstract Transformations of the atomic structure of the CaF 2 Si(111) interface during annealing have been studied by reflection high energy electron diffraction (RHEED) and X-ray crystal truncation rod (CTR) scattering. The surface morphology after annealing has been studied by atomic force microscopy (AFM). A conversion of the epitaxial relation of the film with respect to the substrate, from type-A (nonrotated) to type-B (with the axes of the film rotated by 180°C around the interface normal), is monitored by RHEED during annealing of the films. On the basis of RHEED, CTR and AFM data, a model of the conversion is suggested. In addition, a spontaneous transition of the type-B interface formed during the growth to the interface with another atomic arrangement has been studied with CTR. The possible role of point defect motion in the CaF 2 film during this transition is discussed.

Facile Self-Assembly of Intermetallic [Ni2Gd2] Cubane Aggregate for Magnetic Refrigeration

Molecular nanomagnets used as magnet refrigerants at low temperatures are important advanced materials for cryogenic sensors of astronomical instruments and possible alternatives to ultra-low temperature He technologies. This application is based on the magnetocaloric effect (MCE), which follows an adiabatic demagnetization process in which the change of an applied magnetic field (DB0) leads to a change of the magnetic entropy (DSm) and temperature (DTad). [1c] Molecular nanomagnets that exhibit a large MCE usually have a large ground state spin with a large metal-to-ligand ratio and negligible magnetic anisotropy. The isotropic Gd ion, which has the largest number of unpaired electron spins in the 4f orbital with a weak super-exchange interaction between metal centers, is the metal of choice for high-nuclearity metal (Gd) or intermetallic (3d-Gd) metal aggregates or coordination polymers. The most desirable attribute of these materials is their large ground spin states with an associated high DSm, which is a key parameter that determines the MCE of a magnetic system. Many of these complexes showing promising MCE properties are usually obtained from solvothermal methods under harsh conditions, such as [Ni6Gd6P6], [4a] [Gd24], [4e] [Co8Gd8], [4b] [Co6Gd8], [4d] etc. , or slow evaporation of reaction mixtures over a prolonged period, such as [Ln42M10], [4h] [Mn4Ln4], [6] [Gd ACHTUNGTRENNUNG(HCOO) ACHTUNGTRENNUNG(OAc)2ACHTUNGTRENNUNG(H2O)2],[7] etc. g, i, j] This limits the materials to those that are inert and stable in solution. Another common problem is that the large ground state spins of these polynuclear metal aggregates are offset by complicated magnetic exchange interactions within the metal aggregates which could lead to small DSm, [2,6, 8] thus ultimately reducing the atom efficiency in MCE applications. Our objective is therefore to seek a facile synthetic method for structurally distinctive intermetallic molecular materials that can emulate Gd alloys as molecular magnetic coolants and show satisfactory magnetic entropy. A variety of homoand heterometallic metal aggregates using 2-(hydroxymethyl)pyridine (hmpH) as a ligand have been reported, such as [Mn2Mn III 2] , [9] [Mn3Ni], [10] [Mn2Ln2], [11] [Mn4], [12] [Gd4M8], [13] etc. By using hmpH as a ligand, a series of polynuclear metal aggregates such as the [NaNi6] Anderson-type framework, [14] and [Ni4] [15] and [Co3Ln] cubanes [16] have been prepared. Herein, we report a simple one-pot self-assembly of a [Ni2Gd2] heterometallic cubane at room temperature and its X-ray crystallographic analysis. Different from the reported [Ni6Gd6], [4a] [Ni12Gd36], [4f] and [Ni10Gd42] [4h] systems, this molecular aggregate is stable in CH2Cl2 solution and exhibits a pronounced cryogenic MCE with -DSm=34.4 Jkg K 1 at 4.5 K for a field change of DH=7 T. Its isolation fuels the promise of the use of small intermetallic aggregates and clusters as molecular magnetic refrigerants and paves a chemical pathway to engineering devices. A mixture of Ni ACHTUNGTRENNUNG(OAc)2, GdACHTUNGTRENNUNG(OAc)3, and hmpH (1:1:2) in THF at room temperature self-assembles over 12 hours into a neutral product formulated as [Ni2Gd2ACHTUNGTRENNUNG(hmp)4ACHTUNGTRENNUNG(OAc)6] (1) (Scheme 1). The electrospray ionization (ESI) mass spec-

Manganese fluoride epitaxial growth on Si(111)

Abstract Epitaxial MnF2 layers were grown on CaF2/Si(111) substrates using molecular beam epitaxy. Layer-by-layer growth was observed at room temperature. The first three molecular layers of MnF2 have cubic fluorite lattice inherited from CaF2. Thicker layers have tetragonal lattice of rutile with (110) plane parallel to the substarte surface and [001] axis along 〈110〉 directions of CaF2. Thus there are three orientations of rotational domains in the MnF2 layer. At a temperature higher than 400°C, nucleation begins with three-dimensional clusters which are aligned with respect to steps on CaF2 surface. After coalescence of the clusters, single domain film can be obtained.

Deuterium-oxygen exchange on diamond (100)—a study by ERDA, RBS and TOF-SIMS

Abstract The exchange of radio-frequency plasma-excited atomic O with chemisorbed D, and vice versa, on single crystalline diamond (100) 2×1 has been investigated by elastic recoil detection analysis (ERDA), Rutherford backscattering spectrometry (RBS) and time-of-flight secondary ion mass spectrometry (TOF-SIMS). It was found that the O-D, as well as D-O, exchange processes were thermally activated by the diamond substrate. The surface D was only partially exchanged by atomic O at room temperature. At elevated temperatures, the replacement of D by O was greatly enhanced, with 80% substitution by O at 300 °C. It was also observed that the uptake of O proceeded more readily on the pre-deuterated surface compared to the clean surface. Ultra-shallow depth profiling revealed that atomic beam treatment of single crystalline samples at 800 °C resulted in only superficial uptake of D and O, with no surface incorporation within the shallow analysis depth. The replacement of pre-adsorbed O by RF-excited atomic D was also studied by TOF-SIMS. Pre-adsorbed O was relatively stable and resisted removal at 300 °C. D dosed onto the oxygenated surface was found to co-adsorb with O, possibly as surface bound OD species. Mechanisms for the O-D and D-O exchange processes were discussed in connection with the atomic structure of the C(100) surface.