Samples In Ultrahigh Vacuum Research Articles

Reversible oxygen-induced p-doping of mixed-cation halide perovskites

To fully unlock the potential of metal halide perovskites (MHPs) for use in optoelectronic devices, a comprehensive understanding of their electronic properties is in strong demand but presently lacking. This photoelectron spectroscopy study reveals that the thin films of three important mixed-cation/mixed-halide MHPs behave like intrinsic semiconductors with a very low defect concentration. The Fermi level position in the bandgap can be varied by almost 1 eV by choosing substrates of appropriate work function for samples that were handled under inert conditions. Upon oxygen exposure, two organic/inorganic-cation MHPs become strongly p-doped due to oxygen diffusion into the bulk, a process that is fully reversible when storing the samples in ultrahigh vacuum. In contrast, all-inorganic CsPbI1.8Br1.2 exhibits no electronic property changes upon oxygen exposure. Nonetheless, oxygen is found to effectively remove (light-induced) lead-related surface states of CsPbI1.8Br1.2.

Experimental determination of the {111}/{001} surface energy ratio for Pd crystals

Pd crystals grown in ultrahigh vacuum (UHV) on nanostructured SrTiO3(001) and anatase TiO2(001) thin film substrates were studied using scanning tunneling microscopy. The crystals have the equilibrium shape of a truncated octahedron with a {111} top facet, {111} and {001} side facets, and a {111} interface. A consistent crystal shape is reached only after annealing the samples in UHV at 450 °C or above. By measuring the top facet dimensions of the equilibrium crystals, we determine the ratio of the surface energies for Pd to be γ111/γ001 = 0.77 ± 0.02.

Gold Nanoclusters at the Interface Au/GaAs(001): Preparation, Characterization, and Plasmonic Spectroscopy

Local plasmons of gold nanoclusters formed at Au/GaAs interface are observed and investigated. The gold nanoclusters are prepared by thermal annealing of thin Au films deposited on GaAs(001) surface. In order to prevent chemical interaction between Au and GaAs at high temperatures, a thin layer of GaN is formed on GaAs substrate surface by chemical nitridation in hydrazine sulfide solution prior to deposition of Au. Typical shapes and sizes of Au nanoclusters are obtained from STM characterization of samples in ultrahigh vacuum followed by measuring and calculating their optical reflectivity spectra. In general, the reflectivity spectra of Au/GaAs samples reveal two resonant peaks at the energies about 1.55 and 2.15 eV. The former peak dominates for oxidized GaAs(001) substrates, while the latter peak is clearly visible for nitridized GaAs substrates. Theoretical analysis of the optical spectra of Au/GaAs interface using a model of Au nanospheroids allows us to assign the reflectance peak at 2.15 eV to local plasmons of Au nanoclusters which look like oblate islands on a Au/GaAs(001) sample. In turn, the peak at 1.55 eV is assigned to plasmons of Au nanoclusters prolate perpendicularly to the surface of GaAs crystal in whose near-surface region these Au clusters are buried.

Publisher's Note: “Toward printing molecular nanostructures from microstructured samples in ultrahigh vacuum” [J. Vac. Sci. Technol. B 34, 011801 (2016)

Publisher's Note: “Toward printing molecular nanostructures from microstructured samples in ultrahigh vacuum” [J. Vac. Sci. Technol. B <b>34</b>, 011801 (2016)

Toward printing molecular nanostructures from microstructured samples in ultrahigh vacuum

Transferring molecular nanostructures from one surface to another in ultrahigh vacuum (UHV) by mechanical contact might be a possible route to avoid the severe limitations of in situ molecular synthesis on technologically relevant template surfaces. Here, transfer printing in UHV of molecular structures between metal surfaces is investigated by a combination of scanning tunneling microscopy and scanning electron microscopy/energy dispersive x-ray spectroscopy. The authors present the complete procedure of the printing and characterization process. Microstructured Au-coated MoS₂ samples exhibiting a periodic pillar structure are used as stamp surfaces with Au(111) single crystals as target surface. Polymers of 1,3,5-tris(4-bromophenyl)benzene molecules and graphene nanoribbons with an armchair edge structure are grown on the pillars of the stamp surface. After bringing the two surfaces in mechanical contact, the transferred material is found on the target while decapping occurs on the stamp surface. Polymer structures are probably buried under the transferred stamp material, and in rare cases, evidence for molecular structures is found in their vicinity.

The transition from 3C SiC (1 1 1) to graphene captured by Ultra High Vacuum Scanning Tunneling Microscopy

In this paper we clarify the transformation mechanism of 3C-SiC into graphene upon thermal decomposition, by a combination of high resolution Scanning Tunneling Microscopy (STM) images and first principle calculations. We studied the transition from 3C-SiC to graphene by high temperature annealing of C-terminated 3C SiC(111)/Si(111) samples in Ultra High Vacuum. By using STM we were able to observe very clear atomic resolution images of the transition from SiC (√3×√3)R30° to a new intermediate stage SiC 32×√3R30° (very close to the graphene (2×2) reconstruction) after annealing at 1250°C. We also obtained images of the transformation of the intermediate structure into a (1×1) monolayer graphene, caused by further sublimation of atoms in the subsurface layer. We have interpreted the results by using Density Functional Theory – Local Density Approximation calculations, which give full account of the SiC (√3×√3)R30° reconstruction, but fail to describe the SiC 32×√3R30° structure due to its incommensurability with the 3C-SiC(111) lattice.

Reproducible noncontact force spectroscopy for studying molecules

We developed an alternative method to measure force curves reproducibly by noncontact force spectroscopy for studying molecules. In this method, the force curve is collected without disabling the tip–sample distance feedback thereby in a non-destructive manner. The method can be used when the interaction force versus the tip–sample distance is monotonic and it is useful especially for measuring force curves on organic surfaces. We safely obtained the force curves, which are overlapped with those measured by the conventional method on graphite and lead-phthalocyanine samples in ultra high vacuum.

Magnetic field generation with local current injection

We have established point-like contacts between a metallic surface and a scanning tunnelling microscope (STM) tip and generated a localized magnetic field by injecting a current pulse through the contact. The contacts carry current densities on the order of 1013 A m−2. Magnetic imaging before and after current injection reveals domain nucleation and magnetization reversal on the sub-micrometre spatial scale in ultra-thin magnetic elements of Co and Fe epitaxially grown on Cu(1 0 0). This technique offers a way to apply a magnetic field to an ultra-thin sample in ultra-high vacuum (UHV) with nano-scale accuracy.

The Adsorption and Growth of Copper on As-Terminated GaAs(001): Physical Vapour versus Electrochemical Deposition

The adsorption of copper deposited from an effusion cell in ultra high vacuum and electrodeposited from aqueous solution on As-terminated GaAs(001) is investigated by scanning tunneling microscopy, X-ray diffraction, as well as X-ray standing waves in combination with X-ray fluorescence spectroscopy. The analysis of the adsorbate is performed in situ with the samples either in ultra high vacuum or in aqueous electrolyte under potential control. Deposited as Cu ions from sulphuric acid solution, submonolayer coverages of Cu diffuse into the GaAs interface region assumimg predominantly Ga substitutional positions. Different positions with lower symmetry are occupied by Cu if submonolayer are deposited at room temperature in ultra high vacuum. At higher coverage, epitaxial Cu clusters grow by electrodeposition on the As-terminated GaAs(001), but with the Cu lattice of the islands rotated and inclined with respect to the GaAs substrate lattice, leading in total to eight equivalent domains. At comparable coverage, no epitaxial growth was observed for Cu deposited in ultra high vacuum.

Scanning tunneling microscopy of graphene on Ru(0001)

After prolonged annealing of a Ru(0001) sample in ultrahigh vacuum a superstructure with a periodicity of $\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ was observed by scanning tunneling microscopy (STM). Using x-ray photoelectron spectroscopy it was found that the surface is covered by graphitic carbon. Auger electron spectroscopy shows that between 1000 and $1400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ carbon segregates to the surface. STM images recorded after annealing to increasing temperatures display islands of the superstructure, until, after annealing to $T\ensuremath{\geqslant}1400\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, it covers the entire surface. The morphology of the superstructure shows that it consists of a single graphene layer. Atomically resolved STM images and low-energy electron diffraction reveal an $(11\ifmmode\times\else\texttimes\fi{}11)$ structure or incommensurate structure close to this periodicity superimposed by $12\ifmmode\times\else\texttimes\fi{}12$ graphene cells. The lattice mismatch causes a moir\'e pattern. Unlike the common orientational disorder of adsorbed graphene, the graphene layer on Ru(0001) shows a single phase and very good rotational alignment. Misorientations near defects in the overlayer only amount to $\ensuremath{\sim}1\ifmmode^\circ\else\textdegree\fi{}$, and the periodicity of $\ensuremath{\sim}30\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$ is unaffected. In contrast to bulk graphite both carbon atoms in the graphene unit cell were resolved by STM, with varying contrast depending on the position above the Ru atoms. The filled and empty state images of the moir\'e structure differ massively, and electronic states at $\ensuremath{-}0.4$ and $+0.2\phantom{\rule{0.3em}{0ex}}\mathrm{V}$ were detected by scanning tunneling spectroscopy. The data indicate a significantly stronger chemical interaction between graphene and the metal surface than between neighboring layers in bulk graphite. The uniformity of the structure and its stability at high temperatures and in air suggest an application as template for nanostructures.

Structure and composition of the segregated Cu in V 2O 5/Cu system

We have investigated segregation of copper at the surface of V 2O 5 films deposited onto Cu substrate by employing surface analysis techniques. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) confirmed that the Cu is segregated at the surface and its chemical state is Cu 2O. According to secondary ion mass spectroscopy (SIMS) and glow discharge spectroscopy (GDS), the Cu concentration inside the deposited V 2O 5 layer is low. Ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS) revealed the segregation alters the surface local density of states. Surface analysis of deposited samples in ultra high vacuum (UHV) condition verified that the segregation occurs during the deposition. We have extended kinetic tight binding Ising model (KTBIM) to explain the surface segregation during the deposition. Simulation data approve the possibility of surface segregation during room temperature deposition. These results point out that on pure Cu substrate, oxidation occurs during the segregation and low surface energy of Cu 2O is the original cause of the segregation.

Thermal decomposition of Zr 1 − x Al xN thin films deposited by magnetron sputtering

The thermal stability of the metastable fcc Zr 1 − x Al xN thin films has been investigated by heating the samples in ultra high vacuum ( P < 10 − 7 Pa). For this purpose, two films with chemical composition Zr 0.65Al 0.35N and Zr 0.57Al 0.43N were grown by reactive magnetron sputtering. The films were heated for duration of 60 min between 400–850 °C by steps of 50 °C. The structural evolution of the annealed films has been investigated by using X-ray diffraction (XRD), transmission electron spectroscopy (TEM) and by atomic force microscopy (AFM). Results show that up to annealing temperature of 600 °C the pristine structural and mechanical properties of all the films are retained. At annealing temperatures above 600 °C, important structural modification result as deduced from the shift of the XRD peaks towards the low 2θ values indicating changes in the unit cell dimension. Structural modifications are accompanied by hardness enhancement. In Zr 0.57Al 0.43N films annealed at 850 °C, chemical analysis by X-ray energy dispersive spectrometry evidenced Al-rich regions inhomogeneously distributed in the plane of the film. The results are discussed in terms of the transformation of the fcc Zr 1 − x Al xN structure in a two-phase system composed of Al-poor Zr 1 − x Al xN and low crystallized h-AlN via spinodal decomposition.

Superconductivity of SrTiO $ \mathsf {_{3-\delta}}$

Superconducting SrTiO_{3-\delta} was obtained by annealing single crystalline SrTiO_3 samples in ultra high vacuum. An analysis of the V(I) characteristics revealed very small critical currents I_c which can be traced back to a unavoidable doping inhomogeneity. R(T) curves were measured for a range of magnetic fields B at I<<I_c, thereby probing only the sample regions with the highest doping level. The resulting curves B_{c2}(T) show upward curvature, both at small and strong doping. These results are discussed in the context of bipolaronic and conventional superconductivity with Fermi surface anisotropy. We conclude that the special superconducting properties of SrTiO_{3-\delta} can be related to its Fermi surface and compare this finding with properties of the recently discovered superconductor MgB_2.

Cerium oxides and cerium-platinum surface alloys on Pt(111) single-crystal surfaces studied by scanning tunneling microscopy

Three-dimensional clusters of metallic Ce are deposited on Pt(111) surfaces by thermal evaporation in ultrahigh vacuum. Different reactions occur upon heating of the sample in ultrahigh vacuum and during the exposure of oxygen that lead to distinct well-ordered surface phases. Their geometric structures are determined by scanning tunneling microscopy (STM). A two-dimensional ordered surface alloy of ${\mathrm{Pt}}_{5}\mathrm{Ce}$ is obtained by annealing at 1000 K. It forms two different incommensurate overlayers on Pt(111) with long-distance spacings in the moir\'e patterns (13.7 and 14.8 \AA{}). Hexagonally shaped two-dimensional islands are formed upon heating the Pt(111) single crystal with a submonolayer coverage of Ce at 900 K. They are localized at step edges. STM reveals a local $2\ifmmode\times\else\texttimes\fi{}2$ structure with respect to Pt(111). It is assigned to a precursor state of alloy formation. Ordered ${\mathrm{CeO}}_{2}$ phases result from annealing the alloy at 1000 K in oxygen. The surface structure is consistent with oxygen-terminated fluorite-type ${\mathrm{CeO}}_{2}(111).$ During decomposition of ${\mathrm{CeO}}_{2}$ at 900 K, an oxide structure is identified which is attributed to an ultrathin ordered phase of surface-${\mathrm{Ce}}_{2}{\mathrm{O}}_{3}.$

Surface structure of GaAs(2 5 11)

GaAs samples with orientations vicinal to (2 5 11) within $1\ifmmode^\circ\else\textdegree\fi{}$ were prepared by molecular beam epitaxy and analyzed in situ by scanning tunneling microscopy, low-energy electron diffraction, and reflection high-energy electron diffraction. In addition, first-principles electronic structure calculations were carried out. GaAs(2 5 11) is a stable surface whose orientation is located within the stereographic triangle. For a wide range of As-rich conditions a $(1\ifmmode\times\else\texttimes\fi{}1)$ reconstruction forms that is characterized by an inclined series of three As dimers and that fulfills the electron counting rule. The terrace size is limited only by the macroscopic off-orientation of the samples. The surface is perturbed by thin stripes of the nearby orientation (3 7 15). While the dangling bond densities of GaAs(2 5 11) and GaAs(3 7 15) are almost equal, GaAs(3 7 15) violates the electron counting rule. The analysis of this perturbation suggests that, in general, on semiconductor surfaces the gain in stability arising from the minimization of the number of dangling bonds is significantly greater than the gain arising from reaching a semiconducting ground state. Upon annealing of the samples in ultrahigh vacuum, a fairly rough surface structure develops whose mean orientation is different from (2 5 11).

Nanoparticle-induced multi-functionalization of silicon: A plug and play approach

Here, we demonstrate a “plug and play” approach to achieve multi-functionalization of Si. In this approach, externally synthesized functional nanoparticles are introduced onto device quality Si wafers and the surface chemical bonds are manipulated. Sonochemically synthesized Fe 2O 3 nanoparticles are introduced onto Si from an alcohol suspension. On annealing this sample in ultra-high vacuum, the oxygen atoms change the bonding partner from Fe to Si and desorb as SiO at ∼750 °C. This results in the formation of nanoparticles of Fe on the surface and exhibits ferromagnetic behavior. Deposition of a thin layer (2 nm) of Si onto the sample containing the metallic Fe nanoparticles followed by annealing at 560 °C leads to optically active Si. Photoluminescence measurements show that this sample emits light at three different wavelengths, namely 1.57, 1.61 and 1.63 μm, when excited by He–Ne or Ar lasers. Oxidation of this material results in the formation of a selective capping layer of SiO 2. Thus we obtain multi-functional Si in an “all in one” form and we believe that this approach is universal.

Adsorption of Model Adhesive Compounds onto Plasma-Polymerized Silica-Like Films

Plasma-polymerized silica-like primer films on aluminum substrates were prepared and exposed to diethanolamine (DEA), a model adhesive compound, from dilute solution. Reflection-absorption infrared (RAIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were used to investigate the chemical interactions between the adsorbed compound and the substrate. RAIR spectroscopy showed significant changes in the structural chemistry of the silica-like films after exposure to ethanol with and without DEA. Although no evidence of adsorption was observed in the infrared spectra, XPS indicated that a small amount of model compound did adsorb onto the surface of the silica-like film. The N(1s) photoelectron spectra of the adsorbate showed two components; one was typical of the bulk amine, and the other indicated that the amine group of the adsorbed model adhesive had chemically interacted with the substrate. The adsorbate/substrate interaction strength was also examined by heating the sample in ultra-high vacuum (UHV) and observing the temperature at which desorption occurred. The sample was heated to 250°C for several hours and no desorption was detected. It was concluded that protonation of the amine group had occurred forming a quaternary ammonium ion. It was also likely that bonding between the alcohol functionality and the silica surface added to the adsorption strength.

An apparatus for beam-rocking reflection high-energy electron diffraction and total reflection angle x-ray spectroscopy

Characteristic x-ray emission from surface atoms induced by irradiation of an electron beam during reflection high energy electron diffraction (RHEED) depends on the incident glancing angle θg, since distribution of electron current density near surfaces depends on θg. Depth distribution of elements, adatom height, and adatom site can be determined from θg dependence of x-ray emission. This article describes an apparatus for measuring θg dependence of x-ray emission during RHEED. X rays from the surface are sensitively detected at a grazing take-off angle θt close to the critical angle for the total reflection. A small electron gun is rotated around the sample in ultrahigh vacuum to scan θg, during which the sample and x-ray detector are fixed.

Metallic adhesion forces and tunneling between atomically defined tip and sample

Elements of metallic adhesion are discussed in light of issues in non-contact force microscopy. We have measured forces between an atomically defined W(111) tip and a Au(111) sample in ultrahigh vacuum at 150 K. The W tips are manipulated and characterized on an atomic scale both before and after sample approach by field ion microscopy. We observe strong attractive adhesion forces, which turn repulsive upon further approach of the tip towards the Au surface. Unexpected for a metallic system, there is no spontaneous jump-to-contact, although we find such wetting is inducible with undefined polycrystalline W tips. The force vs. tip–sample distance curve shows only modest hysteresis, and the field ion microscopy images reveal an atomically unchanged tip apex. The tunneling current between tip and sample is found to increase exponentially in a regime of strong adhesive forces.

Stable surface reconstructions on 6H–SiC(000 [formula omitted

We used scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES) to identify and investigate stable surface reconstructions on 6H–SiC(000 1 ̄ ). Starting from ex situ prepared (1×1) ordered surfaces heating of the sample in ultra-high vacuum leads to (3×3), (2×2) C and (1×1) graphitic phases with increasing temperature. The stoichiometry of the phases was determined by AES by comparison to the initial, bulk like (1×1) phase: the (3×3) structure is carbon enriched, while the (2×2) C structure reaches the bulk like stoichiometry again. The two superstructures were investigated in detail by STM revealing significant differences in structural complexity. Additional Si flux during heating produces a silicon enriched surface phase, which also exhibits (2×2) periodicity. However, this new phase, denoted as (2×2) Si, has a different surface structure than the (2×2) C phase obtained by mere heating. Apart from the different stoichiometry this is revealed by significantly different LEED intensities.