Sublimation In Ultrahigh Vacuum Research Articles

Formation of diphenyl-bipyridine units by surface assisted cross coupling in Pd-cyclometalled complexes

The Pd cyclometallated complex [(5-bromo-2-phenylpyridine)Pd(μ-Cl)]2 is deposited on Ag(110) at room temperature by sublimation in ultra-high vacuum. The thermal evolution of the system is followed by scanning tunnelling microscopy and X-ray photoemission spectroscopy, while the initial and final configurations are validated by ab-initio calculations. We observe the surface induced dissociation of the molecule and the occurrence of a cross coupling reaction between the two organic fragments, leading to the surface assisted synthesis of diphenyl-bipyridine molecules. Such a process, occurring with low probability at RT, is thermally activated and competes with desorption. At variance with most cross-coupling reactions at surfaces reported in literature, in this case the reactants come from the dissociation of the same compound so that only one precursor is employed, leading to a simplified preparation protocol. The Br and Cl atoms dissociated from the molecule bind to the surface and promote an extended surface reconstruction upon annealing, which was not observed previously upon deposition of halogenated aromatic compounds.

Surface effects on a photochromic spin-crossover iron(ii) molecular switch adsorbed on highly oriented pyrolytic graphite.

Thin films of an iron(ii) complex with a photochromic diarylethene-based ligand and featuring a spin-crossover behaviour have been grown by sublimation in ultra-high vacuum on highly oriented pyrolytic graphite and spectroscopically characterized through high-resolution X-ray and ultraviolet photoemission, as well as via X-ray absorption. Temperature-dependent studies demonstrated that the thermally induced spin-crossover is preserved at a sub-monolayer (0.7 ML) coverage. Although the photochromic ligand ad hoc integrated into the complex allows the photo-switching of the spin state of the complex at room temperature both in bulk and for a thick film on highly oriented pyrolytic graphite, this photomagnetic effect is not observed in sub-monolayer deposits. Ab initio calculations justify this behaviour as the result of specific adsorbate-substrate interactions leading to the stabilization of the photoinactive form of the diarylethene ligand over photoactive one on the surface.

Room temperature control of spin states in a thin film of a photochromic iron(ii) complex

Thin films of a molecular spin crossover iron(ii) complex featuring a photochromic diarylethene-based ligand have been grown by sublimation in ultra-high vacuum on Au(111) and investigated by photoelectron spectroscopies.

Planar Self-Assembly of Submicron and Nanoscale Wires and Grooves on III–V(110) Surfaces

Metallic Ga and In submicron and nanowires (NWs) tens of microns long naturally form via a self-propelled mechanism on the (110) surfaces of GaAs and InAs, respectively, during noncongruent sublimation in ultrahigh vacuum. Under stringent conditions, low-energy electron microscopy uncovers GaAs and InAs(110) surfaces on the brink of decomposition rapidly assemble planar wires of their cations in the ⟨110⟩ direction. For InP(110), wire formation is unfavorable due to a smooth decomposing front but can be assisted by Au nanoparticles (NPs), which sacrifice themselves to form rough pits via solid–liquid–vapor etching. The resulting self-assembled and AuNP-assisted NWs grow crystallographically in a self-sustainable manner, unless they are obstructed and consumed by stationary microdroplets, leaving emptied grooves. The findings reveal a hitherto hidden natural process on the surfaces of binary crystals capable of producing elementary submicron and nanoscale wires without extrinsic materials, paving the way ...

Magnetic bistability in a submonolayer of sublimated Fe4 single-molecule magnets.

We demonstrate that Fe4 molecules can be deposited on gold by thermal sublimation in ultra-high vacuum with retention of single molecule magnet behavior. A magnetic hysteresis comparable to that found in bulk samples is indeed observed when a submonolayer film is studied by X-ray magnetic circular dichroism. Scanning tunneling microscopy evidences that Fe4 molecules are assembled in a two-dimensional lattice with short-range hexagonal order and coexist with a smaller contaminant. The presence of intact Fe4 molecules and the retention of their bistable magnetic behavior on the gold surface are supported by density functional theory calculations.

Synthesis oftrans-A2B2- andtrans-A2BC-Porphyrins with Polar 4′-(Dimethylamino)tolan-4-yl Substituents, and a Screening Protocol for Vapor-Phase Deposition on Metal Surfaces

In the context of exploring the role of the polar 4′-(dimethylamino)tolan-4-yl group in 3D (single crystals) and 2D [on Au(111)] self-assembly of tetraarylporphyrins, we synthesized and fully characterized a series of meso-substituted trans-porphyrins. A reliable protocol was developed to test the suitability of molecules for intact deposition on surfaces by sublimation in ultra-high vacuum.

Unconventional Zwitterionic State of l-Cysteine

We report the signature of an unconventional zwitterion in thick (>40 angstrom) L-cysteine films grown at low temperature (100 K) by means of sublimation in ultrahigh vacuum. Using X-ray photoelectron spectroscopy (XPS) we find that protonated amino groups (57%) are less prevalent than in films grown at higher temperatures. A large majority of the protonated amino groups receive the hydrogen through deprotonation of the thiol group (45%) and, in contrast, only a small fraction of the carboxylic groups (13%) is deprotonated. This shows, for the first time, the unconventional zwitterion to be the dominant species in L-cysteine. We believe the unusual growth parameters are responsible for this novel observation.

Seeding Atomic Layer Deposition of High-k Dielectrics on Epitaxial Graphene with Organic Self-Assembled Monolayers

The development of high-performance graphene-based nanoelectronics requires the integration of ultrathin and pinhole-free high-k dielectric films with graphene at the wafer scale. Here, we demonstrate that self-assembled monolayers of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA) act as effective organic seeding layers for atomic layer deposition (ALD) of HfO(2) and Al(2)O(3) on epitaxial graphene on SiC(0001). The PTCDA is deposited via sublimation in ultrahigh vacuum and shown to be highly ordered with low defect density by molecular-resolution scanning tunneling microscopy. Whereas identical ALD conditions lead to incomplete and rough dielectric deposition on bare graphene, the chemical functionality provided by the PTCDA seeding layer yields highly uniform and conformal films. The morphology and chemistry of the dielectric films are characterized by atomic force microscopy, ellipsometry, cross-sectional scanning electron microscopy, and X-ray photoelectron spectroscopy, while high-resolution X-ray reflectivity measurements indicate that the underlying graphene remains intact following ALD. Using the PTCDA seeding layer, metal-oxide-graphene capacitors fabricated with a 3 nm Al(2)O(3) and 10 nm HfO(2) dielectric stack show high capacitance values of ∼700 nF/cm(2) and low leakage currents of ∼5 × 10(-9) A/cm(2) at 1 V applied bias. These results demonstrate the viability of sublimated organic self-assembled monolayers as seeding layers for high-k dielectric films in graphene-based nanoelectronics.

Chromium wheels quasi-hexagonal 2D assembling by direct UHV sublimation

Chromium(III)-based wheels close-pack on the Ag(110) surface forming a quasi-hexagonal 2D network following direct sublimation in ultra high vacuum (UHV). Wheels organization and chemical integrity have been proved through in situ Scanning Tunneling Microscopy (STM) and X-ray Photoelectron Spectroscopy (XPS) studies.

Fabrication of Fractal Surfaces by Electron Beam Lithography

We describe a method based on electron beam lithography to fabricate patterns of fractal islands on a surface. The island morphology resembles that of a random deposition of particles in a diffusion-limited aggregation regime in 2-D, which is often encountered in the growth of atoms and molecules upon ultrahigh vacuum sublimation. With our fabrication protocol, the morphological parameters of the fractal islands (correlation length, fractal dimension, coverage, and roughness) can be controlled. The fabricated structures can be used as templates for investigating the interplay of self-affinity on thin film nucleation and growth, the adsorption of functional molecules, and the anchoring of living cells. Also they can be exploited as masters for nanoimprinting lithography and replica molding.

Structural characterisation of polycrystalline $alpha;,$omega;-dihexyl quaterthiophene thin films by transmission electron microscopy

α,ω-dihexyl quaterthiophene thin films have been deposited by ultra-high vacuum sublimation on silica and potassium hydrogen phthalate substrates. The influence of the substrate on the structure of the films are investigated by transmission electron microscopy and the results are compared with the morphological and optical properties of the same samples.

Structural characterisation of polycrystalline $alpha;,$omega;-dihexyl quaterthiophene thin films by transmission electron microscopy

α,ω-dihexyl quaterthiophene thin films have been deposited by ultra-high vacuum sublimation on silica and potassium hydrogen phthalate substrates. The influence of the substrate on the structure of the films are investigated by transmission electron microscopy and the results are compared with the morphological and optical properties of the same samples.

Crystalline C60monolayers at the solid–organic solution interface

Crystalline monolayers of fullerene have been self-assembled at the interface between a 1,2,4-trichlorobenzene solution and an Au(111) surface. In situ scanning tunneling microscopy shows the adsorbate arrangement to be structurally equivalent to that found in films grown by sublimation in ultra-high vacuum. This new kind of ordered monolayer might be useful for performing reactions on fullerenes at the interface between their organic solutions and a solid substrate.

Steady-state motion of silicon islands driven by a DC current

The understanding and control of structures at the surface of crystals is a fashionable topic nowadays. The role of an electric heating current in shaping the morphology of vicinal (111) and (001) silicon surfaces during sublimation in ultra-high vacuum is well known. Less known is the behaviour of surface features when the crystal is in contact with its own saturated vapour, and thus at equilibrium. We report here the observation by reflection electron microscopy of two-dimensional, micrometre-sized silicon islands on a resistively heated Si(001) substrate held at equilibrium (vanishing super- and undersaturation) at temperatures between 1000 and 1100°C. Surprisingly, the islands are seen to perform a gliding motion at a constant velocity in or against the current direction, depending on the island reconstruction. The value of the velocity is, on the contrary, independent of island reconstruction and size. A simple model based on adatom electromigration is discussed, that allows us to account for all observed features.

Aspects of the purification of volatile rare earth metals by UHV sublimation: Sm, Eu, Tm, Yb

Purification of rare earth metals with high vapor pressure below melting point (Sm, Eu, Tm, Yb) have been carried out by high and ultra-high vacuum sublimation and distillation, using direct sublimation, sublimation with a porous solid W filter and novel promising liquid metal through distillation (LMTD). Using the last two methods, high purity RE metals with RRR of about 300 were obtained. Aspects of experimental setup and conditions and its influence on behavior and redistribution of impurities are analysed and future perspective of methods are discussed.

Quasiepitaxial growth of the organic molecular semiconductor 3,4,9,10-perylenetetracarboxylic dianhydride.

Thin films of the organic molecular semiconductor 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) were grown by ultrahigh vacuum sublimation on graphite, and on As-saturated c(4\ifmmode\times\else\texttimes\fi{}4), As-rich (2\ifmmode\times\else\texttimes\fi{}4)-c(2\ifmmode\times\else\texttimes\fi{}8), and Se-passivated (2\ifmmode\times\else\texttimes\fi{}1) GaAs (100) surfaces. The Se-passivated GaAs and graphite surfaces resulted in PTCDA films with good crystallinity, whereas deposition under similar conditions on the (2\ifmmode\times\else\texttimes\fi{}4)-c(2\ifmmode\times\else\texttimes\fi{}8) GaAs surface produced films randomly oriented in the plane parallel to the surface. The films grown on the c(4\ifmmode\times\else\texttimes\fi{}4) GaAs surface showed intermediate molecular ordering. These differences are analyzed in terms of interface bonding between the molecules and the substrate, specifically at defects and dangling bonds which pin the molecules at random sites. These interactions are considerably reduced on the inert surfaces of graphite and Se-passivated GaAs. The results presented here provide information on the conditions under which the quasiepitaxy of organic molecular crystals can be achieved on highly mismatched inorganic substrates.

Atomic Force Microscope Studies of Fullerene Films: Highly Stable C 60 fcc (311) Free Surfaces

Atomic force microscopy and x-ray diffractometry were used to study 1500 A-thick films of pure C(60) grown by sublimation in ultrahigh vacuum onto a CaF(2) (111) substrate. Topographs of the films did not reveal the expected close-packed structures, but they showed instead large regions that correspond to a face-centered cubic (311) surface and distortions of this surface. The open (311) structure may have a relatively low free energy because the low packing density contributes to a high entropy of the exposed surface.

Fluorescence of a merocyanine dye adsorbed on ZnO crystals

An organic merocyanine dye is deposited on well-defined ZnO single crystal surfaces by sublimation in ultrahigh vacuum (UHV). Various surface pretreatments such as annealing in UHV or oxygen, argon ion bombardment, or cleavage in UHV are applied before dye deposition. The optical absorption and fluorescence of the dye as well as the spectrally sensitized photoconductivity are measured for dye coverages between 1013 and 1015 cm-2. Solution and vapor phase spectra of the dye are measured for comparison.The variations of absorption and fluorescence bands with increasing dye coverage can be related to a growing degree of association between dye molecules. At submonolayer coverage the derived quantum yields for fluorescence and sensitized photoconductivity depend strongly on the ZnO surface preparation. There is no observable competition between fluorescence and photoconductivity. The yield of both is limited by nonradiative decay processes. A structural model for the dye-surface interaction is proposed, and the rate constants of various processes are estimated.

The growth forms of small Au particles grown on KBr and NaCl substrates having monatomic steps

Monatomic spiral steps were prepared by ultrahigh vacuum sublimation of KBr and NaCl substrates cleaved in ultrahigh vacuum. Growth forms of Au particles deposited onto the well-defined steps and flat areas of these substrates were studied using transmission electron microscopy and electron diffraction. The results showed that the individual particles grown epitaxially on the flat area of KBr were single crystals with a pyramidal shape, whereas those grown along the steps of KBr were mostly composed of multiply twinned particles (MTPs). On the other hand, most of the particles on NaCl grew with a multiply twinned structure not only along the step but in the flat area. Growth mechanisms for the small particles of Au are discussed.

Onset of graphoepitaxy at a monatomic step

Monatomic spiral steps were prepared by ultrahigh vacuum sublimation of NaCl and KCl substrates cleaved in ultrahigh vacuum. Particles of tin deposited onto the spiral step were examined by transmission electron microscopy and electron diffraction. The results indicated that the particles were epitaxially grown on the monatomic step of the KCl substrate, whereas the particles on the NaCl substrate were graphoepitaxially grown even on a monatomic step. The conditions necessary for graphoepitaxy are discussed.