Result of the Month(ROM)

The Result of the Month (ROM) section is an institution with a long history. Our goal is to present remarkable and interesting measurements and/or papers done by our customers with Scienta Omicron systems and instruments to the scientific community. 

Bandwidth-Control Orbital-Selective Delocalization of 4f Electrons in Epitaxial Ce Films

Result of the Month, October 2021

In this paper, Yang Liu, Yi-feng Yang, Yi Wu et al. present direct spectroscopic evidence for the bandwidth-control OSD of 4f electrons in epitaxial Ce films, by combining high-quality thin film growth by MBE and in situ measurements from ARPES. More specifically, they uncovered a hitherto unreported metastable phase of Ce, where sharp dispersive quasiparticle bands with pure 4f character can be observed near EF, accompanied by an appreciable dispersion of the lower Hubbard band. Ce films were grown on top of epitaxial graphene layers at a temperature of approx. 100 °C and at a rate of 2 Å/min using a high-temperature effusion cell of a Scienta Omicron Lab10 MBE. For ARPES measurements samples were transferred in UHV from the MBE chamber to a connected Scienta Omicron ARPES Lab equipped with a DA30-L analyser and a VUV5k He lamp. 

Energy Funnelling within Multichromophore Architectures Monitored with Subnanometre Resolution

Result of the Month, September 2021

In this work, Guillaume Schull, Fabrice Scheurer et al. have used luminescence induced by scanning tunnelling microscopy to probe model multichromophoric structures assembled on a surface. Mimicking strategies developed by photosynthetic systems, individual molecules were used as ancillary, passive or blocking elements to promote and direct resonant energy transfer between distant donor and acceptor units. As it relies on organic chromophores as the elementary components, this approach constitutes a powerful model to address fundamental physical processes at play in natural light-harvesting complexes. The STM data were acquired with a LT STM (4.5 K) Scienta Omicron set-up that operated in a ultrahigh vacuum, and was configured to detect the light emitted at the tip–sample junction.

A Time-Domain Phase Diagram of Metastable States in a Charge Ordered Quantum Material

Result of the Month, August 2021

In this work, by using time-resolved optical techniques and femtosecond-pulse-excited STM, Dragan Mihailovic, Jan Ravnik, Michele Diego et al. track the evolution of the metastable states in a material that has been wide recent interest, the quasi-two-dimensional dichalcogenide 1T-TaS2. They map out its temporal phase diagram using the photon density and temperature as control parameters on timescales ranging from 10—12 to 103 s. The introduction of the time-domain axis in the phase diagram enables them to follow the evolution of metastable emergent states created by different phase transition mechanisms on different timescale. For STM experiments they use a Scienta Omicron LT STM (LT Nanoprobe Lab) with optical access.

Identifications and Electronic Characterizations of Four Cyclodehydrogenation Products of H2TPP Molecules on Au(111)

Result of the Month, July 2021

C–H bond activation and dehydrogenative coupling reactions have always been significant approaches to construct microscopic nanostructures on surfaces. By using scanning tunneling microscopy/spectroscopy (STM/STS) and non-contact atomic force microscopy (nc-AFM) combined with density functional theory (DFT), we systematically characterized the atomically precise topographies and electronic properties of H2TPP cyclodehydrogenation products on Au(111). Through surface-assisted thermal excitation, four types of cyclodehydrogenation products were obtained and clearly resolved in the nc-AFM images.

Laboratory Based Hard X-ray Photoelectron Spectroscopy for Buried Interface Analysis of Microelectronic Components

Result of the Month, June 2021

In this work, Nick Barrett et al. have carried out a benchmarking of the Scienta Omicron HAXPES Lab with respect to a synchrotron radiation HAXPES set-up for analysis of buried interfaces for microelectronics applications. The synchrotron HAXPES experiments were done at the BL15XU beamline at JASRI (SPring-8) using 8 keV photons and a Scienta Omicron R4000 analyzer. The HAXPES Lab measurements were done using the 9.25 keV Ga Kα1 source and Scienta Omicron EW4000 analyzer.

Photodriven Transient Picosecond Top‐Layer Semiconductor to Metal Phase‐Transition in p‐Doped Molybdenum Disulfide

Result of the Month, May 2021

In this work, Nomi L A N Sorgenfrei, Erika Giangrisostomi, Raphael M Jay et al. establish how visible light itself can create a transient metallic top layer on bulk crystalline p‐doped 2H‐MoS2. Electron–hole pairs created by optical excitation separate in the surface band bending region of p‐doped semiconducting 2H‐MoS2. This causes a transient accumulation of electrons in the surface region, driving the top‐layer within several picoseconds from the p‐doped semiconducting 2H‐MoS2 into a sheet of metallic 1T‐MoS2 at a remarkably low optical fluence threshold. This mechanism has significant implications on how optically illuminated MoS2 surfaces behave and will positively change the catalytic properties for water splitting. This study was enabled due to the very high transmission delivered by Wide Angle ARTOF spectrometer from Scienta Omicron and was performed at the FEMTOSPEX end station Helmholtz-Zentrum Berlin.

Correlating Josephson Supercurrents and Shiba States in Quantum Spins Unconventionally Coupled to Superconductors

Result of the Month, April 2021

Here, Paolo Sessi et al. use scanning Josephson spectroscopy to directly visualize the effect of magnetic perturbations on Cooper pair tunneling between superconducting electrodes at the atomic scale. By increasing the magnetic impurity orbital occupation by adding one electron at a time, they reveal the existence of a direct correlation between Josephson supercurrent suppression and YSR states. Moreover, in the metallic regime, they detect zero bias anomalies which break the existing framework based on competing Kondo and Cooper pair singlet formation mechanisms. Based on first-principle calculations, these results are rationalized in terms of unconventional spin-excitations induced by the finite magnetic anisotropy energy.

Inelastic Background Modelling Applied to Hard X-ray Photoelectron Spectroscopy of Deeply Buried Layers

Result of the Month, March 2021

In this work, Ben Spencer et al investigate layers of an organic LED material buried deeply below up to 200 nm of organic material using HAXPES (hv > 9 keV). To gain information from deep below the surface using photoelectron spectroscopy, they model the inelastic background originating from Ir 3d photoelectrons at ~ 2000 eV binding energy. The methodology is applied to both synchrotron and Scienta Omicron HAXPES Lab system, where data taken at Diamond Light Source also helped to benchmark the HAXPES Lab system.

One-Dimensional Confinement and Width-Dependent Bandgap Formation in Epitaxial Graphene Nanoribbons

Result of the Month, February 2021

In this work, Hrag Karakachian et al. grow high-quality armchair graphene nanoribbons on the sidewalls of 6H-SiC mesa structures. ARPES and STS measurements reveal the development of a width-dependent semiconducting gap driven by quantum confinement effects. ARPES measurements were carried out at the Bloch beamline of the MAX IV synchrotron facility in Lund, Sweden. High-resolution energy-momentum cuts were measured using a high performance deflector-based DA30 hemispherical analyser from Scienta Omicron.  The STM/STS measurements were performed in UHV (p < 2 × 10−11 mbar) at 80 K using a Scienta Omicron LT-STM system.

Valley Interference and Spin Exchange at the Atomic Scale in Silicon

Result of the Month, January 2021

Researchers from the ARC Centre for Quantum Computation and Communication Technology (CQC2T) working with Silicon Quantum Computing (SQC) have situated the ‘sweet spot’ for positioning qubits in silicon to scale up atom-based quantum processors.

Creating quantum bits, or qubits, by exactly putting phosphorus atoms in silicon is a world-leading approach in the growth of a silicon quantum computer. This method has been pioneered by CQC2T Director Prof. Michelle Simmons.

In their research, published in Nature Communications, precision placement has proven to be essential for developing robust interactions—or coupling—between qubits.

The team found that there is a special angle, or sweet spot, within a particular plane of the silicon crystal where the interaction between the qubits is most resilient to a valley interference effect.

This was located using scanning tunnelling microscope (STM) lithography techniques developed at UNSW, to observe the atomic-scale details of the interactions between the coupled atom qubits, including the valley interference between the atoms and the envelope anisotropy.

Observation of Electrically Tunable van Hove Singularities in Twisted Bilayer Graphene from NanoARPES

Result of the Month, December 2020

In their paper, A J H Jones & S Ulstrup et al. explore engineering properties of quantum materials by placing a singularity of the density of states near the Fermi energy. Using ebeam lithography for structuring and formation of electrical contacts, a device consisting of twisted bilayer graphene (twBG) on a stack of hBN and graphite was formed. The device was wire bonded to a chip package and introduced to the nanoARPES branch of the I05 beamline at the Diamond light source.

The measurements for this paper were acquired at the I05 beamline at the Diamond light source where a Fresnel zone plate focused 60 eV light down to a 690 nm spot on the sample. A piezo electric stage scanned the sample at 250 nm increments relative to the spot while a Scienta Omicron DA30-L acquired the photoemission spectra at each position (E, k). The DA30-L deflector mode was used to obtain (E, kx, ky)-dependent photoemission intensity.

Interfacial Polarons in van der Waals Heterojunction of Monolayer SnSe2 on SrTiO3 (001)

Result of the Month, November 2020

Interfacial polarons have been demonstrated to play important roles in heterostructures containing polar substrates. However, most of polarons found so far are diffusive large polarons; the discovery and investigation of small polarons at interfaces are scarce. In this ACS Nano Letters, Aidi Zhao and Bing Wang et al report the emergence of interfacial polarons in monolayer SnSe2 epitaxially grown on Nb-doped SrTiO3 (STO) surface using ARPES and STM. The STM measurements were performed using LT STM and ARPES measurements were performed with 21.21eV He Iα rays using a DA30-L  – 08 energy analyser, both from Scienta Omicron.

Symmetry Breakdown of 4,4″-Diamino-p-Terphenyl on a Cu(111) Surface by Lattice Mismatch

Result of the Month, October 2020

In a symmetric molecule with identical functional groups, selective activation of only one site is challenging. In this Nature Communications Paper, Ebeling D et al show that 4,4″-diamino-p-terphenyl adsorbs asymmetrically to a metal surface, leading to a change in binding affinity of one of its amine groups. The measurements were performed with a commercial combined Low Temperature AFM/STM (Scienta Omicron). All STM/AFM images were acquired at 5 K under ultra-high vacuum (base pressure < 1.0 × 10–10 mbar). 

Flat Bands in Twisted Bilayer Transition Metal Dichalcogenides

Result of the Month, September 2020

In this Nature Physics (2020), Zhang, Z, Wang, Y, Watanabe, K et al show the existence of a flat band in the electronic structure of 3° and 57.5° twisted bilayer WSe2 via Scanning Tunnelling Microscopy (STM) and Scanning Tunnelling Spectroscopy (STS). The featured figure shows the STM topography of the sample, both a graphene/hBN moiré pattern. The STM/STS measurements were performed in the ultrahigh-vacuum LT-STM from Scienta Omicron operating at 4.6 K.

Tomonaga–Luttinger Liquid in the Edge Channels of a Quantum Spin Hall Insulator

Result of the Month, July 2020

In this study, Stühler, R., Reis, F., Müller, T. et al. explore the impact of electronic correlations on highly localized edge states of the unique quantum spin Hall material bismuthene on SiC(0001). Exploiting the advantage of having an accessible monolayer substrate system, they use STM/STS to visualize the close-to-perfect one-dimensional confinement of the edge channels and scrutinize their suppressed density of states at the Fermi level. Based on the observed spectral behavior and its universal scaling with energy and temperature, they demonstrate the correspondence with a (helical) Tomonaga–Luttinger liquid. The STM/STS measurements have been performed with a low-temperature STM from Scienta Omicron under UHV conditions.

Effects of Nitridation on SiC/SiO2 Structures Studied by Hard X-ray Photoelectron Spectroscopy

Result of the Month, June 2020

This research paper investigates device-relevant stacks of SiC and SiO2 using energy-dependent X-ray photoelectron spectroscopy and combines results from both laboratory and synchrotron HAXPES systems to form a complete picture of the depth distribution and chemical states of nitrogen.

This paper contributes to investigating the main challenge that the semiconductor industry is tackling now, the quality of the interface between SiC and its native dielectric SiO2, which currently limits further improvement of device performance.

New Progress in the Basic Research for Topological Quantum Computing: Zero-Energy Bound States in the High-Temperature Superconductors at Two-Dimensional Limit

Result of the Month, May 2020 

In this work, Prof. Jian Wang Group at Peking University detected novel Majorana zero-energy bound state (ZEBS) resembling the characteristics of Majorana zero modes (MZMs) in interstitial Fe adatoms deposited on the high-temperature superconducting thin films at two-dimensional limit.

This research was enabled by a Lab10 MBE and multiprobe LT STM from Scienta Omicron. 

Light Induced Non-Volatile Switching of Superconductivity in Single Layer FeSe on SrTiO3 Substrate

Result of the Month, April 2020

In this study, Ming Yang et al. show that, in FeSe/SrTiO3 heterostructures, the superconducting transition temperature in FeSe monolayer can be effectively raised by the interband photoexcitations in the SrTiO3 substrate. This research was enabled by a Materials Innovation Platform (MIP) from Scienta Omicron combining two MBE chambers, VT STM, LT STM, and a ARPES Lab including a DA30L-8000 and VUV5k.

High Speed Dynamic and Operando Experiments: Thermal Reduction of Titanium Oxides

Result of the Month, March 2020

The thermal reduction of Titanium oxides was observed in-situ by time-resolved XPS using a new type of multi peak monitoring allowing to observe several XPS lines in a timely manner (figure 1&2). The color-coded maps show multiple XPS spectra over time where the x-axis represents the energy axis and y-axis the time. Intensity is shown from dark red (low count rate) to blue (high count rate). The oxygen reduction occurred within ~ 1 minute by applying a rapid temperature ramp and was found to be a step-wise process with an initial transition from 4+ to suboxide and the metallic state of Titanium at intermediate temperatures between 600 K and 725 K before further reduction of the 3+ oxidation state at temperatures > 725 K.

Controlled Growth of Transition Metal Dichalcogenide Monolayers using Knudsen-Type Effusion Cells for the Precursors

Result of the Month, February 2020

In this work, Antony George & Andrey Turchanin et al present a simple method for controlling the precursor flow rates using the Knudsen-type effusion cells. They characterised the grown MoS2 and WS2 monolayers by optical, atomic force and transmission electron microscopies as well as by X-ray photoelectron, Raman and photoluminescence spectroscopies, and by electrical transport measurements showing their high optical and electronic quality based on the single crystalline nature. XPS was performed in a ultra-high vacuum Multiprobe System from Scienta Omicron using a monochromatized x-ray source and an electron analyser Argus CU with a spectral energy resolution of 0.6 eV.

Selective Triplet Exciton Formation in a Single Molecule

Result of the Month, January 2020

In this work, Kimura K. et al report a single-molecule investigation of electroluminescence using a scanning tunneling microscope and demonstrate a simple method of selective formation of T1 excitons that utilizes a charged molecule. All experiments were conducted using a Scienta Omicron LT STM operating at 4.7 K under ultrahigh vacuum.

Electronic Band Dispersion Determination in Azimuthally Disordered Transition-Metal Dichalcogenide Monolayers

Result of the Month, December 2019

In this work, N Koch et al. demonstrate that the ARPES spectra of azimuthally disordered transition metal dichalcogenide monolayers with 2H phase are dominated by their band dispersion along the two high symmetry directions Γ-K and Γ-M. They exemplify this by analysing the ARPES spectra of four prototypical TMDCs within a mathematical framework, which allows to consistently explain the reported observations. The spectra for an azimuthally disordered WSe2 monolayer/highly oriented pyrolytic graphite (HOPG) sample were measured using Scienta Omicron ARPES Lab equipped with a DA30-L analyser and a VUV5k He source.

A Time- and Angle-Resolved Photoemission Spectroscopy with Probe Photon Energy up to 6.7 eV

Result of the Month, November 2019

In this Review, Prof. Wentao Zhang et al. introduce the development of an advanced trARPES system with probe photon energy up to 6.7 eV. The system is based on a Yb-based laser, KBe2BO3F2 (KBBF) and BBO nonlinear optical crystals, and a hemispherical electron analyser DA30-L 8000 from Scienta Omicron.

Evaluation of Sn-Doped Indium Oxide Film and Interface Properties on a-Si Formed by Reactive Plasma Deposition

Result of the Month, October 2019

Aiming to improve the performance of heterojunction Si solar cells, we evaluated the Sn-doped indium oxide (ITO) / a-Si structure using conventional and hard X-ray photoelectron spectroscopy (XPS, HAXPES), and the cause of the solar cell performance Identified deterioration. HAXPES allows non-destructive evaluation of the SiOx layer at the ITO / a-Si interface. The formation of SiOx at the ITO / a-Si interface increases the contact resistance, which can be reduced by post-deposition annealing (PDA). In addition, PDA facilitated Fermi level evaluation, ITO component precipitation in the a-Si layer, and increased interface roughness. Before the PDA, diffusion of a-Si Sn atoms was observed. In addition, PDA confirmed that Si atoms diffused into ITO. These reactions at the ITO / a-Si interface may be part of the deterioration factor of Si solar cells.

Multiple Topological States in Iron-Based Superconductors

Result of the Month, September 2019

In this work, Peng Zhang & Shik Shin et al. identified topological insulator and Dirac semimetal states near the Fermi energy in different iron-based superconducting compounds. More specifically, they 1) observe that TI bands reminiscent of Fe(Te,Se) exist in Li(Fe,Co)As; 2) predict and observe topological Dirac semimetal (TDS) bands in Li(Fe,Co)As and Fe(Te,Se); and 3) discuss phase diagram of these superconducting topological state4s as a function of doping. The ARPES measurements were performed with R4000 electron analyzer and spin-resolved ARPES measurements were carried out with DA30-L analyser – both from Scienta Omicron.