The nitrogen vacancy (NV) center in diamond exhibits spin-dependent fluorescence and long spin coherence times under ambient conditions, enabling applications in quantum information processing and sensing 1, 2 . NV centers near the surface can have strong interactions with external materials and spins, enabling new forms of nanoscale spectroscopy 3-6 . However, NV spin coherence degrades within 100 nanometers of the surface, suggesting that diamond surfaces are plagued with ubiquitous defects 7-10 . Prior work on characterizing near-surface noise has primarily relied on using NV centers themselves as probes [7][8][9][10][11][12] ; while this has the advantage of exquisite sensitivity, it provides only indirect information about the origin of the noise. Here we demonstrate that surface spectroscopy methods and single spin measurements can be used as complementary diagnostics to understand sources of noise. We find that surface morphology is crucial for realizing reproducible chemical termination, and use these insights to achieve a highly ordered, oxygen-terminated surface with suppressed noise. We observe NV centers within 10 nm of the surface with coherence times extended by an order of magnitude.Although it is easy to place NV centers near the surface by low-energy ion implantation 8, 9 or delta-doping 7, 8 , the surface itself can host defects that lead to noise that obscures the sensing target ( Fig. 1a). We observe that coherence time degrades with proximity to the surface in numerous samples with different surface conditions ( Fig. 1b), consistent with prior studies 7, 10 , pointing to the need for new techniques to understand and control diamond surfaces. Gaining precise control over diamond surface chemistry is challenging because diamond is a chemically inert material, and also because it is hard to prepare uniform, flat diamond surfaces. Surface morphology is difficult 2 to control because diamond's hardness makes etching and polishing non-trivial. State-of-the-art diamond polishing can achieve surface roughness below 1 nm, but the resulting surface is highly strained. Plasma etching can remove this strained layer 13, 14 , but this process is highly anisotropic and therefore small differences in initial conditions can lead to dramatic differences in final morphology and termination 15, 16 (see Supplementary Information). Therefore, direct characterization of the surface is crucial for establishing that particular protocols reproducibly lead to specific, desired surface terminations.In this work, we characterize the diamond surface by correlating photoelectron spectroscopy, X-ray absorption, atomic force microscopy (AFM), and electron diffraction with measurements of NV spin decoherence and relaxation to identify and eliminate sources of noise at the surface. We find that surface roughness leads to poor NV coherence, and we observe that surface morphology changes the density of electronic defects observed with photoelectron spectroscopy, even for the same nominal chemical termination, implying that it ...

The stability of the ubiquitous hydroxyl termination and downward band bending on the m-plane () and a-plane () faces of ZnO single crystals was investigated using synchrotron and real-time x-ray photoelectron spectroscopy. On these non-polar surfaces, a strong correlation was found between the surface band bending and surface OH coverage, both of which could be modified via heat treatment in ultra high vacuum (UHV). On the m-plane () face, a threshold temperature of ~400 o C was observed, after which there was a sudden increase in OH desorption and upwards movement of the near-surface bands, leading to a metallic-to-semiconductor transition in the electronic nature of the surface, and a change from surface electron accumulation to depletion. This loss of surface metallicity is associated with the disruption of a stable monolayer of chemisorbed hydroxyl groups that form a closed hydrogen-bonded network, across the rows of Zn-O dimers, on the m-plane () face. The downward band bending and surface electron accumulation layers on both the m-plane () and a-plane () faces could be modified and eventually removed by simple UHV heat treatment, with important implications for the processing and electrical performance of ZnO nanostructures and catalytic ZnO nanopowders, which usually contain a high proportion of these non-polar surfaces.

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The 2-dimensional transformation of the diamond (111) surface to graphene has been demonstrated using ultrathin Fe films that catalytically reduce the reaction temperature needed for the conversion of sp3 to sp2 carbon. An epitaxial system is formed, which involves the re-crystallization of carbon at the Fe/vacuum interface and that enables the controlled growth of monolayer and multilayer graphene films. In order to study the initial stages of single and multilayer graphene growth, real time monitoring of the system was preformed within a photoemission and low energy electron microscope. It was found that the initial graphene growth occurred at temperatures as low as 500 C, whilst increasing the temperature to 560 C was required to produce multi-layer graphene of high structural quality. Angle resolved photoelectron spectroscopy was used to study the electronic properties of the grown material, where a graphene-like energy momentum dispersion was observed. The Dirac point for the first layer is located at 2.5 eV below the Fermi level, indicating an n-type doping of the graphene due to substrate interactions, while that of the second graphene layer lies close to the Fermi level. VC 2015 AIP Publishing LLC.authorsversionpublishersversionPeer reviewe

We develop a method for patterning a buried two-dimensional electron gas (2DEG) in silicon using low kinetic energy electron stimulated desorption (LEESD) of a monohydride resist mask. A buried 2DEG forms as a result of placing a dense and narrow profile of phosphorus dopants beneath the silicon surface; a so-called δ-layer. Such 2D dopant profiles have previously been studied theoretically, and by angle-resolved photoemission spectroscopy, and have been shown to host a 2DEG with properties desirable for atomic-scale devices and quantum computation applications. Here we outline a patterning method based on low kinetic energy electron beam lithography, combined with in situ characterization, and demonstrate the formation of patterned features with dopant concentrations sufficient to create localized 2DEG states.

The degradation of the chemotherapy drug 5-Fluorouracil by a non-pristine metal surfaces is studied. Using Density Functional Theory, X-ray Photoelectron Spectroscopy and X-ray Absorption Spectroscopy we show that the drug is entirely degraded by medicalgrade silver surfaces, already at body temperature, and that all of the fluorine has left the molecule, presumably as HF. Remarkably, this degradation is even more severe than that reported previously for 5-Fluorouracil on a pristine monocrystalline silver surface (in which case 80% of the drug reacted at body temperature)[1]. We conclude that that the observed reaction is due to a reaction pathway, driven by H to F attraction between molecules on the surface, which results in the direct formation of HF; a pathway which is favoured when competing pathways involving reactive Ag surface sites are made unavailable by environmental contamination. Our measurements indicate that realistically cleaned, non-pristine silver alloys, which are typically used in medical applications, can result in severe degradation of 5-Fluorouracil, with the release of HF-a finding which may have important implications for the handling of chemotherapy drugs.

This is the author accepted manuscript. The final version is available from AIP Publishing via http://dx.doi.org/10.1063/1.4967996Pure strain-induced electronic structure modulation in ferromagnetic films is critical for developing reliable strain-assisted spintronic devices with low power consumption. For the conventional electricity-controlled strain engineering, it is difficult to reveal the pure strain effect on electronic structure tunability due to the inseparability of pure strain effect and surface charge effect. Here, a non-electrically controlled NiTi shape memory alloy was utilized as a strain output substrate to induce a pure strain on attached Fe films through a thermally controlled shape memory effect. The pure strain induced electronic structure evolution was revealed by in-situ X-ray photoelectron spectroscopy and correlated with first-principles calculations and magnetic anisotropy measurements. A compressive strain enhances the shielding effect for core electrons and significantly tunes their binding energy. Meanwhile, the strain modifies the partial density of states of outer d orbits, which may affect spin-orbit coupling strength and related magnetic anisotropy. This work helps for clarifying the physical nature of the pure strain effect and developing the pure-strain-assisted spintronic devicesauthorsversionPeer reviewe

The temperature-dependence of photoemission from a gold alloy, n-type β-Ga2O3 and p-type diamond reveals reversible and irreversible changes in energy, due to changes in surface chemistry, band-bending, thermal expansion and a surface photovoltage.