Synchrotron X-ray Photoemission Spectroscopy Research Articles

N-Doped rGO-Like Carbon Prepared from Coconut Shell: Structure and Specific Capacitance

An rGO−like carbon compound has been synthesized from biomass, i.e., old coconut shell, by a carbonization process followed by heating at 400°C for 5 h. The nitrogen doping was achieved by adding the urea (CH4N2O) and stirring at 70°C for 14 h. The morphology and structure of the rGO-like carbon were investigated by electron microscopies and Raman spectroscopy. The presence of C-N functional groups was analyzed by Fourier transform infrared and synchrotron X-ray photoemission spectroscopy, while the particle and the specific capacitance were measured by particle sizer and cyclic voltammetry. The highest specific capacitance of 72.78 F/g is achieved by the sample with 20% urea, having the smallest particles size and the largest surface area. The corresponding sample has shown to be constituted by the appropriate amount of C–N pyrrolic and pyridinic defects.

Valence fluctuation in Yb3Si5 probed by synchrotron X-ray photoemission spectroscopy

We report the direct evidence of valence fluctuation in the thermoelectric material Yb3Si5 by using synchrotron X-ray photoemission spectroscopy. The Yb 3d core level photoemission spectrum of a Yb3Si5 single crystal shows clear separation of the Yb2+ peak and Yb3+ multiplet peaks. From the spectral weight of each component, the mean valence of Yb ions is estimated to be +2.67 at 20 K. Furthermore, a Kondo resonance peak just below the Fermi energy is also observed by high-resolution soft X-ray photoemission spectroscopy, suggesting the valence fluctuating state in Yb3Si5.

(Invited) Atomic-Scale Investigations on the Wet Etching of Group IV Semiconductors in Acidic H2O2 Solution: The Case Ge Versus Si-Ge

Over the decades, Si-based complementary metal–oxide semiconductor (CMOS) technology has challenged the semiconductor industry in improving device performance while maintaining the scaling requirements. However, the use of the group IV semiconductor as a channel material is reaching its ultimate limit due to physical complications, such as gate leakage, parasitic resistance/capacitance and size channel effects [1]. Some workaround solutions have implemented trigate structures, while novel gate-all-around (GAA) nanowire or nanosheet devices are also explored.[2] One of the core challenges for these devices is the highly selective removal of sacrificial SiGe epitaxial layers to release active parts of the device.[3] This aspect is further complicated by the introduction of Ge as an alternative channel material of higher mobility.[4]Wet-chemical processing remains an essential step for new nanodevice fabrication. With channel widths of only a few tens of atomic layers, etching control up to the atomic-layer-scale is necessary. To achieve such a level of precision, basic insights in semiconductor/electrolyte interactions are indispensable. Unlike for Si, the surface chemistry of Ge in wet-chemical solutions has remained relatively unexplored. One of the factors impeding in-depth surface analysis is the high reactivity of Ge in air. For this reason, the use of an integrated surface preparation apparatus in a surface analytical tool is crucial to rule out controversial effects of atmospheric oxygen and water.In this work we discuss the wet-chemical etching of Ge (100) in acidic H2O2 solutions. Kinetic studies were performed using inductively coupled plasma mass spectrometry (ICP-MS). Under reaction limited conditions, we showed that surface chloride chemistry has a profound impact on the dissolution kinetics and the anisotropy of wet etching, while proton effects were found to be less important. These results served as a starting point for the comparison of Ge versus SiGe. We provide an atomic scale investigation on the influence of the Si bulk concentration versus the kinetics of etching. The results are complimented by synchrotron X-ray photoemission spectroscopy (SXPS) measurements. We highlight the use of an integrated surface preparation chamber directly connected to SXPS apparatus to obtain fundamental insights into the surface chemistry of these technologically relevant group IV semiconductors. The results suggest that, while the release of Ge atoms from surface sites is favorable, the Si oxide chemistry determines the etching kinetics of SiGe. Basic etching schemes help to explain the formation of a (quasi) self-limiting oxide on SiGe. Energy dependent surface composition analysis provided insight into the nature of the oxide layer that showed depletion of Ge components at the outer surface due to pull out effects.[6]In summary, this research provides a new outlook for various applications such as biomedicine [7], photodetectors [8] and for light absorbing and harvesting of solar energy [9]. In addition, it serves as an eye-opener for the potential of semiconductor surface chemical studies using an UHV-integrated chemical cell connected to XPS to study etching systems.[1] R. Pillarisetty, Nature 479 (2011), p. 324-328.[2] J. Ryckaert et al., IEDM San Francisco, CA, USA, pp. 29.4.1-29.4.4 (2019)[3] K. Wostyn, F. Sebaai, J. Rip, H. Mertens, L. Witters, R. Loo, A. Y. Hikavyy, A. Milenin, N. Horiguchi, N. Collaert, A. Thean, P. Mertens, S. De Gendt and F. Holsteyns, ECS Trans., 69, 147 (2015).[4] M. Kobayashi, G. Thareja, M. Ishibashi, Y. Sun, P. Griffin, J. McVittie, P. Pianetta, K. Saraswat, Y. Nishi: J. Appl. Phys. 106, p. 104117-7 (2009).[5] G. H. A. Abrenica, M. V. Lebedev, G. Okorn, D. H. van Dorp and M. Fingerle, Appl. Phys. Lett., 113, 062104 (2018)[6] Graniel Harne A. Abrenica, Mikhail V. Lebedev, Mathias Fingerle, Sophia Arnauts, Nazaninsadat Bazzazian, Wolfram Calvet, Clement Porret, Hugo Bender, Thomas Mayer, Stefan de Gendt and Dennis H. van Dorp, J. Mater. Chem. C, 2020, 8, 10060 (2020)[7] H. Geng, J. Dai, J. Li, Z. Di and X. Liu, Sci. Rep. 6, 37474 (2016)[8] J. Michel, J. Liu and L. C. Kimerling, Nat. Photonics 4, 527534 (2010)[9] S. L. Shinde, T. D. Dao, S. Ishii, L-W. Nien, K. K. Nanda and T. Nagao, ACS Photonics 4, 1722-1729 (2017)

Large positive magnetoresistance with high Hall mobility and intercalation of Fe dopants in the quenched Bi2Te3 single crystals

A study of Fe-ion intercalation in Bi2Te3 probed by the x-ray diffraction, magnetization, Hall measurement, and spectra of synchrotron x-ray photoemission spectroscopy is reported, in which a large positive magnetoresistance (MR) and a high carrier mobility for applications have been achieved and probed in the quenched FexBi2Te3 single crystals with x = 0.01 and 0.15. Large positive MR of ∼470% with a Hall mobility of ∼44000 cm2/Vs at 5 K and 6 T has been observed on a quenched Fe0.01Bi2Te3 sample with a quenching temperature Tq of 400 °C, where the electrical parameters can be tuned by Tq. Larger c-axis lattice constants and electron-dominant carriers in the quenched samples lead to the inference that the doped Fe atoms are mostly located at the intercalated van der Waals gap positions and exist as a metallic state of Fe0, which is confirmed by the synchrotron x-ray photoemission spectroscopy (XPS) spectra. Combining the x-ray diffraction, magnetization, and Hall-coefficient studies, here we pave a simple way for probing into the location of Fe dopants in Bi2Te3. In addition, novel magnetotransports of Fe-doped Bi2Te3 associated with the absence of ferromagnetism have been presented, and satisfy the scenario that the topological surface state persists without the loss of topological insulating characteristics of the linear band dispersion, and the large MR may originate from the surface magnetism, even in the presence of Fe impurities. This study also discusses the huge enhancement of Hall mobility arising from the surface Dirac electrons that are accompanied with a modified band structure in quenched Fe0.01Bi2Te3.

Dechlorinated Ullmann Coupling Reaction of Aryl Chlorides on Ag(111): A Combined STM and XPS Study.

Surface-assisted Ullmann coupling was widely applied to construct various molecular nanostructures on surfaces due to its reliability and controllability. By using 4,4''-dichloro-1,1':3',1''-terphenyl (DCTP) as the precursor, covalently bonded zig-zag oligophenylene chains and hexagonal hyperbenzene rings, e. g., [18]-honeycombenes, were successfully fabricated on Ag(111) via dechlorinated Ullmann coupling reaction. Stepwise annealing was applied to investigate the reaction process in detail. Scanning tunneling microscopy and synchrotron X-ray photoemission spectroscopy were utilized to explore the thermal evolution of the DCTP molecules on Ag(111) under ultrahigh vacuum conditions, evidencing the existence of intact DCTP molecules, chemisorbed Cl atoms, covalently bonded DCTP dimers as well as organometallic C-Ag-C-containing intermediates. These results may help understand dechlorinated Ullmann coupling reaction of aryl chlorides on metal surfaces.

Visualizing anisotropy in the surface oxidation of germanium by wet etching of patterned nanowedges: proof of concept.

We study the anisotropy in surface oxidation for Ge(100) and (110) in HCl/H2O2 solution complemented by synchrotron X-ray photoemission spectroscopy (SXPS) measurements integrated with an in situ etching chamber. Visual anisotropic demonstration is confirmed by lithographic Ge nanowedges.

Tuning the Band Bending and Controlling the Surface Reactivity at Polar and Nonpolar Surfaces of ZnO through Phosphonic Acid Binding.

ZnO is a prime candidate for future use in transparent electronics; however, development of practical materials requires attention to factors including control of its unusual surface band bending and surface reactivity. In this work, we have modified the O-polar (0001̅), Zn-polar (0001), and m-plane (101̅0) surfaces of ZnO with phosphonic acid (PA) derivatives and measured the effect on the surface band bending and surface sensitivity to atmospheric oxygen. Core level and valence band synchrotron X-ray photoemission spectroscopy was used to measure the surface band bending introduced by PA modifiers with substituents of opposite polarity dipole moment: octadecylphosphonic acid (ODPA) and 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctylphosphonic acid (F13OPA). Both PAs act as surface electron donors, increasing the downward band bending and the strength of the two-dimensional surface electron accumulation layer on all of the ZnO surfaces investigated. On the O-polar (0001̅) and m-plane (101̅0) surfaces, the ODPA modifier produced the largest increase in downward band bending relative to the hydroxyl-terminated unmodified surface of 0.55 and 0.35 eV, respectively. On the Zn-polar (0001) face, the F13OPA modifier gave the largest increase (by 0.50 eV) producing a total downward band bending of 1.00 eV, representing ∼30% of the ZnO band gap. Ultraviolet (UV) photoinduced surface wettability and photoconductivity measurements demonstrated that the PA modifiers are effective at decreasing the sensitivity of the surface toward atmospheric oxygen. Modification with PA derivatives produced a large increase in the persistence of UV-induced photoconductivity and a large reduction in UV-induced changes in surface wettability.

Oxygen Adsorption and Water Formation on Co(0001)

Oxygen adsorption and removal on flat and defective Co(0001) surfaces have been investigated experimentally using scanning tunneling microscopy, temperature-programmed and isothermal reduction, synchrotron X-ray photoemission spectroscopy, and work function measurements under ultrahigh vacuum conditions and H2/CO pressures in the 10–5 mbar regime. Exposure of the Co(0001) to O2(g) at 250 K leads to the formation of p(2 × 2) islands with a local coverage of 0.25 ML. Oxygen adsorption continues beyond 0.25 ML, reaching a saturation point of ∼0.39 ML Oad, without forming cobalt oxide. Chemisorbed oxygen adlayers can be reduced on both flat and defective Co(0001) surfaces by heating in the presence of ∼2.3 × 10–5 mbar H2(g). The onset of the oxygen removal as water during temperature-programmed reduction experiments (1 K s–1) is at around 450 K on flat Co(0001) and 550 K on defective Co(0001). By evaluation of isothermal reduction experiments using a kinetic model, the activation energy for water formation is...

Synchrotron x-ray photoemission study of soft x-ray processed ultrathin glycine-water ice films

Ultrathin glycine-water ice films have been prepared in ultrahigh vacuum by condensation of H(2)O and glycine at 90 K on single crystalline alumina surfaces and processed by soft x-ray (610 eV) exposure for up to 60 min. The physicochemical changes in the films were monitored using synchrotron x-ray photoemission spectroscopy. Two films with different amounts of H(2)O have been considered in order to evaluate the influence of the water ice content on the radiation-induced effects. The analysis of C1s, N1s, and O1s spectral regions together with the changes in the valence band spectra indicates that amino acid degradation occurs fast mainly via decarboxylation and deamination of pristine molecules. Enrichment of the x-ray exposed surfaces with fragments with carbon atoms without strong electronegative substituents (C-C and C-H) is documented as well. In the thinner glycine-water ice film (six layers of glycine + six layers of water) the 3D ice suffers strongly from the x-rays and is largely removed from the sample. The rate of photodecomposition of glycine in this film is about 30% higher than for glycine in the thicker film (6 layers of glycine + 60 layers of water). The photoemission results suggest that the destruction of amino acid molecules is caused by the direct interaction with the radiation and that no chemical attack of glycine by the species released by water radiolysis is detected.

Reduction of native oxides on InAs by atomic layer deposited Al2O3 and HfO2

Thin high-κ oxide films on InAs, formed by atomic layer deposition, are the key to achieve high-speed metal-oxide-semiconductor devices. We have studied the native oxide and the interface between InAs and 2 nm thick Al2O3 or HfO2 layers using synchrotron x-ray photoemission spectroscopy. Both films lead to a strong oxide reduction, obtaining less than 10% of the native As-oxides and between 10% and 50% of the native In-oxides, depending on the deposition temperature. The ratio of native In- to As-oxides is determined to be 2:1. The exact composition and the influence of different oxidation states and suboxides is discussed in detail.

Band offsets of ultrathin high-κoxide films with Si

Valence- and conduction-band edges of ultrathin oxides (${\text{SiO}}_{2}$, ${\text{HfO}}_{2}$, ${\text{Hf}}_{0.7}{\text{Si}}_{0.3}{\text{O}}_{2}$, ${\text{ZrO}}_{2}$, and ${\text{Al}}_{2}{\text{O}}_{3}$) grown on a silicon substrate have been measured using ultraviolet photoemission and inverse photoemission spectroscopies in the same UHV chamber. The combination of these two techniques has enabled the direct determination of the oxide energy gaps as well as the offsets of the oxide valence- and conduction-band edges from those of the silicon substrate. These results are supplemented with synchrotron x-ray photoemission spectroscopy measurements allowing further characterization of the oxide composition and the evaluation of the silicon substrate contribution to the spectra. The electron affinity has also been systematically measured on the same samples. We find reasonably good agreement with earlier experiments where assumptions regarding energy-gap values were needed to establish the conduction-band offsets. The systematics of our photoemission and inverse photoemission results on different ultrathin films provide a comprehensive comparison of these related systems.

Electronic structure of semiconductingCeFe4P12: Strong hybridization and relevance of single-impurity Anderson model

Semiconducting skutterudite CeFe$_4$P$_{12}$ is investigated by synchrotron x-ray photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). Ce 3$d$ core-level PES and 3$d-4f$ XAS, in combination with single impurity Anderson model (SIAM) calculations, confirm features due to $f^0$, $f^1$ and $f^2$ configurations. The Ce 4$f$ density of states (DOS) indicates absence of a Kondo resonance at Fermi level, but can still be explained by SIAM with a small gap in non-$f$ DOS. While Ce 4$f$ partial DOS from band structure calculations are also consistent with the main Ce 4$f$ DOS, the importance of SIAM for core and valence spectra indicates Kondo semiconducting mixed valence for CeFe$_4$P$_{12}$, derived from strong hybridization between non-$f$ conduction and Ce 4$f$ DOS.

Combined Scanning Tunneling Microscopy and Synchrotron X-Ray Photoemission Spectroscopy Results on the Oxidative CuI Film Formation on Cu(111)

The initial stage of oxidative CuI film formation on Cu(111) has been studied in an electrochemical environment by means of cyclic voltammetry (CV), in situ scanning tunneling microscopy (STM) and ex situ synchrotron X-ray photoemission spectroscopy (SXPS). Cyclic voltammetric studies indicate a significant acceleration of copper oxidation in the presence of iodide. The reason for that is the iodide-mediated stabilization of cuprous species resulting in a downward shift of the onset potential for copper oxidation. Reactive sites for the copper oxidation followed by iodide complexation are exclusively defects such as substrate step edges. It is the surface-confined supersaturation of mobile CuI species that leads to the two-dimensional (2D) CuI film formation via nucleation and growth of a Cu/I bilayer on top of the preadsorbed iodide phase. In an advanced stage of copper oxidation, however, terraces are directly transformed into the 2D CuI film at the reactive boundary between metallic copper and the growing 2D CuI film. Structurally, this 2D CuI film is closely related to the (111) plane of crystalline CuIbulk (zinc blende type). There is no significant passivation of the copper surface against the oxidative dissolution reaction in the presence of the 2D CuI film. Copper dissolution in the presence of the 2D CuI film proceeds also via an inverse step flow mechanism involving the concerted receding of four atomic layers. A model of this process will be discussed on the basis of STM results. The transition from 2D to a three-dimensional (3D) CuI growth mode is observed for an advanced stage of copper oxidation.

Unraveling Internal Structures of Highly Luminescent PbSe Nanocrystallites Using Variable-Energy Synchrotron Radiation Photoelectron Spectroscopy

The internal structure of PbSe nanocrystals was deduced using synchrotron X-ray photoemission spectroscopy for three different sizes of nanocrystals. The photoemission data revealed the layered structure of PbSe nanocrystals with the crystalline PbSe core surrounded by a nonstoichiometric Pb(1-x)Se shell, finally passivated by a capping agent in the outermost layer. A detailed analysis of the experimental data yielded quantitative information on the thickness of three different layers, which is unavailable through any other technique; moreover, the overall sizes of the nanocrystals probed by transmission electron microscopy were in agreement with the corresponding quantity obtained in the present experiment. The present results provide a plausible explanation for the strong variation in the photoluminescence intensity with size observed for these nanocrystals.

Adsorption of O2 on polycrystalline rhenium metal at room temperature studied by synchrotron X-ray photoemission spectroscopy

Abstract Clean rhenium (Re) surfaces were made by sputtering and annealing in UHV. The clean surfaces were then exposed to O2 in the pressure range of 10−8 (1 L) to 5 Torr (∼1.5×107 L) at room temperature. The surface that was exposed at 5 Torr O2 was also annealed at several temperatures. Core level spectra of the Re 4f and O 1s were acquired at photon energies of 200 and 790 eV, respectively, from clean, O2 exposed, and annealed surfaces. At least two components, approximately 1.4 eV apart, can be identified in the O 1s spectra. One is attributed to atomic oxygen, and the other to molecular oxygen. At least two new features appear in the Re 4f spectra after the clean surfaces were exposed to O2, and they may correspond to surface Re atoms surrounded by different numbers of oxygen. Annealing the O2 exposed surfaces results in rearrangement of surface oxygen. Although equilibrium thermodynamic calculations predict that Re2O7 is the most stable phase in the O2 pressure and temperature range used in the exposure experiments, no appreciable Re oxide was observed, suggesting that the oxidation of Re metal is controlled by kinetics rather than thermodynamics.