Kinetics of the Thermal Oxidation of Ir(100) toward IrO2 Studied by Ambient-Pressure X-ray Photoelectron Spectroscopy.

Using time-lapsed ambient-pressure X-ray photoelectron spectroscopy, we investigate the thermal oxidation of single-crystalline Ir(100) films toward rutile IrO2(110) in situ. We initially observe the formation of a carbon-free surface covered with a complete monolayer of oxygen, based on the binding energies of the Ir 4f and O 1s core level peaks. During a rather long induction period with nearly constant oxygen coverage, the work function of the surface changes continuously as sensed by the gas phase O 1s signal. The sudden and rapid formation of the IrO2 rutile phase with a thickness above 3 nm, manifested by distinct binding energy changes and substantiated by quantitative XPS analysis, provides direct evidence that the oxide film is formed via an autocatalytic growth mechanism that was previously proposed for PbO and RuO2.

Centimeter-Sized Single-Orientation Monolayer Hexagonal Boron Nitride With or Without Nanovoids.

Large-area hexagonal boron nitride (h-BN) promises many new applications of two-dimensional materials, such as the protective packing of reactive surfaces or as membranes in liquids. However, scalable production beyond exfoliation from bulk single crystals remained a major challenge. Single-orientation monolayer h-BN nanomesh is grown on 4 in. wafer single crystalline rhodium films and transferred on arbitrary substrates such as SiO2, germanium, or transmission electron microscopy grids. The transfer process involves application of tetraoctylammonium bromide before electrochemical hydrogen delamination. The material performance is demonstrated with two applications. First, protective sealing of h-BN is shown by preserving germanium from oxidation in air at high temperatures. Second, the membrane functionality of the single h-BN layer is demonstrated in aqueous solutions. Here, we employ a growth substrate intrinsic preparation scheme to create regular 2 nm holes that serve as ion channels in liquids.

Microstructural Effect on the Enhancement of Field Electron Emission Properties of Nanocrystalline Diamond Films by Li-Ion Implantation and Annealing Processes.

The impact of lithium-ion implantation and postannealing processes on improving the electrical conductivity and field electron emission (FEE) characteristics of nitrogen-doped nanocrystalline diamond (nNCD) films was observed to be distinctly different from those of undoped NCD (uNCD) films. A high-dose Li-ion implantation induced the formation of electron trap centers inside the diamond grains and amorphous carbon (a-C) phases in grain boundaries for both types of NCD films. Postannealing at 1000 °C healed the defects, eliminated the electron trap centers, and converted the a-C into nanographitic phases. The abundant nanographitic phases in the grain boundaries of the nNCD films as compared to the uNCD films made an interconnected path for effectual electron transport and consequently enhanced the FEE characteristics of nNCD films.

Pulse-resolved intensity measurements at a hard X-ray FEL using semi-transparent diamond detectors.

Solid-state ionization chambers are presented based on thin diamond crystals that allow pulse-resolved intensity measurements at a hard X-ray free-electron laser (FEL), up to the 4.5 MHz repetition rate that will become available at the European XFEL. Due to the small X-ray absorption of diamond the thin detectors are semi-transparent which eases their use as non-invasive monitoring devices in the beam. FELs are characterized by strong pulse-to-pulse intensity fluctuations due to the self-amplified spontaneous emission (SASE) process and in many experiments it is mandatory to monitor the intensity of each individual pulse. Two diamond detectors with different electrode materials, beryllium and graphite, were tested as intensity monitors at the XCS endstation of the Linac Coherent Light Source (LCLS) using the pink SASE beam at 9 keV. The performance is compared with LCLS standard monitors that detect X-rays backscattered from thin SiN foils placed in the beam. The graphite detector can also be used as a beam position monitor although with rather coarse resolution.

Ion bombardment induced buried lateral growth: the key mechanism for the synthesis of single crystal diamond wafers.

A detailed mechanism for heteroepitaxial diamond nucleation under ion bombardment in a microwave plasma enhanced chemical vapour deposition setup on the single crystal surface of iridium is presented. The novel mechanism of Ion Bombardment Induced Buried Lateral Growth (IBI-BLG) is based on the ion bombardment induced formation and lateral spread of epitaxial diamond within a ~1 nm thick carbon layer. Starting from one single primary nucleation event the buried epitaxial island can expand laterally over distances of several microns. During this epitaxial lateral growth typically thousands of isolated secondary nuclei are generated continuously. The unique process is so far only observed on iridium surfaces. It is shown that a diamond single crystal with a diameter of ~90 mm and a weight of 155 carat can be grown from such a carbon film which initially consisted of 2 · 1013 individual grains.

Localization of Narrowband Single Photon Emitters in Nanodiamonds.

Diamond nanocrystals that host room temperature narrowband single photon emitters are highly sought after for applications in nanophotonics and bioimaging. However, current understanding of the origin of these emitters is extremely limited. In this work, we demonstrate that the narrowband emitters are point defects localized at extended morphological defects in individual nanodiamonds. In particular, we show that nanocrystals with defects such as twin boundaries and secondary nucleation sites exhibit narrowband emission that is absent from pristine individual nanocrystals grown under the same conditions. Critically, we prove that the narrowband emission lines vanish when extended defects are removed deterministically using highly localized electron beam induced etching. Our results enhance the current understanding of single photon emitters in diamond and are directly relevant to fabrication of novel quantum optics devices and sensors.

Magnetic Nano-skyrmion Lattice Observed in a Si-Wafer-Based Multilayer System.

Growth, electronic properties, and magnetic properties of an Fe monolayer (ML) on an Ir/YSZ/Si(111) multilayer system have been studied using spin-polarized scanning tunneling microscopy. Our experiments reveal a magnetic nano-skyrmion lattice, which is fully equivalent to the magnetic ground state that has previously been observed for the Fe ML on Ir(111) bulk single crystals. In addition, the experiments indicate that the interface-stabilized skyrmion lattice is robust against local atomic lattice distortions induced by multilayer preparation.

Deterministic coupling of a single silicon-vacancy color center to a photonic crystal cavity in diamond.

Deterministic coupling of single solid-state emitters to nanocavities is the key for integrated quantum information devices. We here fabricate a photonic crystal cavity around a preselected single silicon-vacancy color center in diamond and demonstrate modification of the emitters internal population dynamics and radiative quantum efficiency. The controlled, room-temperature cavity coupling gives rise to a resonant Purcell enhancement of the zero-phonon transition by a factor of 19, coming along with a 2.5-fold reduction of the emitter's lifetime.

Graphene from fingerprints: exhausting the performance of liquid precursor deposition.

Epitaxial graphene is expected to be the only way to obtain large-area sheets of this two-dimensional material for applications on an industrial scale. So far, there are different recipes for epitaxial growth of graphene, using either intrinsic carbon, such as the selective desorption of silicon from a SiC surface, or extrinsic carbon, as via the chemical vapor deposition (CVD) of simple hydrocarbons on transition metal surfaces. In addition, even liquid precursor deposition (LPD) provides well-ordered graphene monolayers. It will be shown that graphene formation on transition metal surfaces by LPD synthesis is a very robust mechanism that even works if carbon is provided in a quite undefined way, namely by using a human fingerprint as a liquid precursor. Graphene growth from fingerprints provides well-ordered monolayers with the same quality as LPD grown graphene using ultrapure synthetic single precursors. The reliability of the self-assembly process of graphene growth on transition metals by LPD therefore offers a simple and extremely robust synthesis route for epitaxial graphene and may give access to production pathways for substrates for which the CVD method fails.

Optical signatures of silicon-vacancy spins in diamond.

Colour centres in diamond have emerged as versatile tools for solid-state quantum technologies ranging from quantum information to metrology, where the nitrogen-vacancy centre is the most studied to date. Recently, this toolbox has expanded to include novel colour centres to realize more efficient spin-photon quantum interfaces. Of these, the silicon-vacancy centre stands out with highly desirable photonic properties. The challenge for utilizing this centre is to realize the hitherto elusive optical access to its electronic spin. Here we report spin-tagged resonance fluorescence from the negatively charged silicon-vacancy centre. Our measurements reveal a spin-state purity approaching unity in the excited state, highlighting the potential of the centre as an efficient spin-photon quantum interface.

Tuned NV emission by in-plane Al-Schottky junctions on hydrogen terminated diamond.

The negatively charged nitrogen-vacancy (NV) centre exhibits outstanding optical and spin properties and thus is very attractive for applications in quantum optics. Up to now an active control of the charge state of near-surface NV centres is difficult and the centres switch in an uncontrolled way between different charge states. In this work, we demonstrate an active control of the charge state of NV centres (implanted 7 nm below the surface) by using an in-plane Schottky diode geometry from aluminium on hydrogen terminated diamond in combination with confocal micro-photoluminescence measurements. The partial quenching of NV-photoluminescence caused by the hole accumulation layer of the hydrogen terminated surface can be recovered by applying reverse bias potentials on this diode, i.e. the NV(0) charge state is depleted while the NV(-) charge state is populated. This charge state conversion is caused by the bias voltage affected modulation of the band bending in the depletion region which shifts the Fermi level across the NV charge transition levels.

Increasing the wear resistance by interstitial alloying with boron via chemical vapor deposition.

The wear resistance of a Rh(111) surface can be strongly increased by interstitial alloying with boron atoms via chemical vapor deposition of trimethylborate [B(OCH3)3] at moderate temperatures of about 800 K. The fragmentation of the precursor results in single boron atoms that are incorporated in the fcc lattice of the substrate, as displayed by X-ray photoelectron diffraction. The penetration depth of the boron atoms is in the range of at least 100 nm with the boron distribution displaying a nearly homogeneous depth profile, as examined by combined X-ray photoelectron spectroscopy and Ar ion etching experiments. Compared to the bare Rh(111) surface, the wear resistance of the boron-doped Rh surface is increased to about 400%, as probed by the scratching experiments with atomic force microscopy. The presented synthesis route provides an easy method for case hardening of micro- or nanoelectromechanical devices (MEMS and NEMS, respectively) at moderate temperatures.

Epitaxial growth of graphene on transition metal surfaces: chemical vapor deposition versus liquid phase deposition.

The epitaxial growth of graphene on transition metal surfaces by ex situ deposition of liquid precursors (LPD, liquid phase deposition) is compared to the standard method of chemical vapor deposition (CVD). The performance of LPD strongly depends on the particular transition metal surface. For Pt(111), Ir(111) and Rh(111), the formation of a graphene monolayer is hardly affected by the way the precursor is provided. In the case of Ni(111), the growth of graphene strongly depends on the applied synthesis method. For CVD of propene on Ni(111), a 1 × 1 structure as expected from the vanishing lattice mismatch is observed. However, in spite of the nearly perfect lattice match, a multi-domain structure with 1 × 1 and two additional rotated domains is obtained when an oxygen-containing precursor (acetone) is provided ex situ.

One- and two-dimensional photonic crystal microcavities in single crystal diamond.

Diamond is an attractive material for photonic quantum technologies because its colour centres have a number of outstanding properties, including bright single photon emission and long spin coherence times. To take advantage of these properties it is favourable to directly fabricate optical microcavities in high-quality diamond samples. Such microcavities could be used to control the photons emitted by the colour centres or to couple widely separated spins. Here, we present a method for the fabrication of one- and two-dimensional photonic crystal microcavities with quality factors of up to 700 in single crystal diamond. Using a post-processing etching technique, we tune the cavity modes into resonance with the zero phonon line of an ensemble of silicon-vacancy colour centres, and we measure an intensity enhancement factor of 2.8. The controlled coupling of colour centres to photonic crystal microcavities could pave the way to larger-scale photonic quantum devices based on single crystal diamond.

STM, SECPM, AFM and Electrochemistry on Single Crystalline Surfaces.

Scanning probe microscopy (SPM) techniques have had a great impact on research fields of surface science and nanotechnology during the last decades. They are used to investigate surfaces with scanning ranges between several 100 mm down to atomic resolution. Depending on experimental conditions, and the interaction forces between probe and sample, different SPM techniques allow mapping of different surface properties. In this work, scanning tunneling microscopy (STM) in air and under electrochemical conditions (EC-STM), atomic force microscopy (AFM) in air and scanning electrochemical potential microscopy (SECPM) under electrochemical conditions, were used to study different single crystalline surfaces in electrochemistry. Especially SECPM offers potentially new insights into the solid-liquid interface by providing the possibility to image the potential distribution of the surface, with a resolution that is comparable to STM. In electrocatalysis, nanostructured catalysts supported on different electrode materials often show behavior different from their bulk electrodes. This was experimentally and theoretically shown for several combinations and recently on Pt on Au(111) towards fuel cell relevant reactions. For these investigations single crystals often provide accurate and well defined reference and support systems. We will show heteroepitaxially grown Ru, Ir and Rh single crystalline surface films and bulk Au single crystals with different orientations under electrochemical conditions. Image studies from all three different SPM methods will be presented and compared to electrochemical data obtained by cyclic voltammetry in acidic media. The quality of the single crystalline supports will be verified by the SPM images and the cyclic voltammograms. Furthermore, an outlook will be presented on how such supports can be used in electrocatalytic studies.

How does graphene grow? Easy access to well-ordered graphene films.

The selective formation of large-scale graphene layers on a Rh-YSZ-Si(111) multilayer substrate by a surface-induced chemical growth mechanism is investigated using low-energy electron diffraction, X-ray photoelectron spectroscopy, X-ray photoelectron diffraction, and scanning tunneling microscopy. It is shown that well-ordered graphene layers can be grown using simple and controllable procedures. In addition, temperature-dependent experiments provide insight into the details of the growth mechanisms. A comparison of different precursors shows that a mobile dicarbon species (e.g., C(2)H(2) or C(2)) acts as a common intermediate for graphene formation. These new approaches offer scalable methods for the large-scale production of high-quality graphene layers on silicon-based multilayer substrates.