Energy-filtered Photoemission Electron Microscopy Research Articles

Experimental studies of electron affinity and work function from titanium on oxidised diamond (100) surfaces

Sub-monolayers of titanium were deposited onto oxidised (100) single-crystal diamond surfaces and annealed in vacuo at temperatures up to 1000 °C to find a temperature-stable termination procedure that produces a surface with Negative Electron Affinity (NEA). The samples were analysed by X-ray Photoelectron Spectroscopy, Ultraviolet Photoelectron Spectroscopy and Energy-Filtered Photoemission Electron Microscopy to determine their electron affinity and work function values. NEA values were observed on samples following annealing above 400 °C, with the largest NEA value being –0.9 eV for a sample coated with a half-monolayer of Ti annealed at 400 °C. Work function values were ∼4.5 eV for all samples annealed at temperatures between 400 and 600 °C, then rose at higher temperatures due to the loss of substantial amounts of O from the surface. Work-function maps indicated that the surface was uniform over areas 5700 μm2, suggesting that the deposition and annealing steps used are reliable methods to produce films with homogeneous surface properties.

(Invited) The Effect of Anion Adsorption on Surface-Alloying during Underpotential Deposition

The spontaneous surface alloying during deposition is an unexplored route for developing new materials with a great potential for applications in catalysis, electronics, sensing and photonics. Although the surface-stress driven alloying has been theoretically predicted and explored on a fundamental level in the Ultra High Vacuum (UHV)1, very little of that knowledge has been employed in practice.The electrochemically controlled surface-alloying has not been understood well, although it was reported in many systems. The formation of UPD monolayer in the presence of specifically adsorbing ions is often accompanied by competitive adsorption, changes of the kinetics and energetics of nucleation and growth, and the co-adsorption that could stabilise new stages and phases of growth2,3. The effect of anion adsorption on surface alloying kinetics and energetics can significantly affect alloys’ properties and their (electro)chemical reactivity.Here in this work, we focused on systems, Pb UPD on Au(111) and Pb UPD on Cu(100), in which surface alloying has been reported in the past4-8. Pb UPD on Au(111) in perchlorate solution has been a well-studied system by a range of surface techniques4,5. However, very little is known about the effect of halide adsorption in this system. In the perchlorate solutions, in-situ surface stress relaxation4,5 and the in-situ STM4 surface changes unambiguously demonstrated changes associated with the surface alloying. Pb UPD on Cu(100) is another system where the formation of Pb3Cu4 ordered surface alloy is reported and shown to self-assemble in the nanoribbons pattern in the presence of chloride6-8. The studies on those UPD systems aimed to understand the effect of strong halides adsorption (chloride and bromide) on the electrochemical behaviour and surface alloying. Both chloride and bromide form well-ordered anion adlayers on Au and Cu while they differ in the strength of their interaction with the substrate and UPD metal layer. The studies included the electrochemical methods such as the polarisation at fixed sub-ML coverages of Pb over an extended period and following the changes in electrochemical behaviour. In addition to that, we investigated some of the structural and compositional changes by surface techniques such as X-ray Photoelectron Spectroscopy (XPS), Ultraviolet Photoelectron Spectroscopy (UPS) Energy-Filtered Photoemission Electron Microscopy (EF-PEEM) and Work Function (WF) mapping9.

Surface-Alloying during Pb Underpotential Deposition on Au

Surface-alloys can form spontaneously during the deposition of one metal onto another, especially in systems with a large lattice mismatch that do not have bulk counterparts (so-called immiscible systems). The spontaneous surface alloying has been widely studied in Ultra High Vacuum (UHV) [1], but these materials have not been exploited much in practical applications. Surface-alloying in an electrochemical environment is an ideal route to the design of new functional materials. Electrochemically controlled surface-alloying would allow room-temperature conditions and better scalability making it more viable and versatile for practical applications.The focus of our study was the surface-alloying during underpotential deposition (UPD) of Pb on Au(111) [2,3]. As shown in Fig 1.a, the cyclic voltammetry (CV) of Pb UPD on Au (111) has asymmetric anodic and cathodic peaks (labelled in the order of their associated appearance). The most positive anodic peak A2 is often associated with the dealloying of Pb incorporated in the Au surface. The electrochemical measurements and the in-situ Scanning Tunnelling Microscopy (STM) characterisation during Pb UPD on Au(111) have shown that the Pb ML stripping is followed by a significant Au surface roughening [2]. During Pb ML formation and dissolution, the top-surface Au atoms get displaced, creating highly mobile Au ad-islands and vacancy islands [2]. The in-situ surface stress measurements have shown a tensile stress relaxation caused by the alloying/dealloying of Pb-Au during Pb ML formation and dissolution [2,4-5].In this study, we used two different electrochemical protocols to examine the surface alloying process: 1) repeated potential cycling within the UPD range and 2) polarisation at a fixed potential in the UPD region. By following the changes in electrochemical behaviour, we established the range of potentials and conditions at which the surface-alloys form. In addition to that, we investigated the structural and compositional changes by surface sensitive techniques such as X-ray Photoelectron Spectroscopy (XPS), Ultraviolet Photoelectron Spectroscopy (UPS), Energy-Filtered Photoemission Electron Microscopy (EF-PEEM) and Work Function (WF) mapping.To study the 'dynamic' alloying, the potential was cycled repeatedly between 0.0 V and 0.5 V (vs Pb/Pb2+), as marked in Fig 1.a, over the potential range negative from the A2 anodic peak. The potential was cycled up to 400 times. After each 100th cycle, the potential was extended to the whole Pb UPD region to observe the changes in the cathodic and anodic peaks. For illustration, Fig 1.b and Fig 1.c show the changes of peaks A2 and A3 with the number of cycles. While both peaks show changes of the shape and total charge, we observe a significant shift of the peak A2 suggesting dramatic changes associated with the morphology of the surface and the amount of alloyed Pb in the top surface layer. The Pb-Au surface alloy composition, based on the charge under the A2 peak, increase from 17% up to 25% of Pb (after 400 cycles) as shown in Fig 1.d. Similar changes of the CV peaks have been observed after the polarisation at the fixed potential of 0.2 V (vs Pb/Pb2+) in the Pb UPD region for different times (15 min - 60 min).Furthermore, the Pb UPD on Au(111) has been studied in the presence of chloride. These results show gradual changes in positions and shapes of all characteristic peaks of the Pb UPD leading to complete disappearance of the A2 dealloying peak, which have not been reported before.

Graphene-diamond junction photoemission microscopy and electronic interactions

Polycrystalline graphene was transferred onto differently terminated epitaxial layers of boron-doped diamond deposited onto single crystal substrates. Chemical and electronic characterisation was performed using energy-filtered photoemission electron microscopy and angle-resolved photoemission spectroscopy. Electronic interaction between the diamond and graphene was observed, where doping of the graphene on the hydrogen and oxygen terminated diamond was n-doping of 250 meV and 0 meV respectively. We found that the wide window of achievable graphene doping is effectively determined by the diamond surface dipole, easily tuneable with a varying surface functionalisation. A Schottky junction using the graphene-diamond structure was clearly observed and shown to reduce downward band bending of the hydrogen terminated diamond, producing a Schottky barrier height of 330 meV.

Surface Investigation on Electrochemically Deposited Lead on Gold

Electrodeposition of Pb on Au has been of interest for the variety of surface phenomena such as the UnderPotential Deposition (UPD) and surface alloying. Here, we examined the interface between the electrodeposited Pb film on Au, using surface sensitive techniques such as X-ray Photoelectron Spectroscopy (XPS), Ultraviolet Photoelectron Spectroscopy (UPS), Energy-Filtered Photoemission Electron Microscopy (EF-PEEM) and Work Function (WF) mapping. The initially electrodeposited Pb overlayer (~4 ML equivalent thickness) was transferred from the electrochemical cell to the UHV system. The deposited Pb layer was subjected to Argon sputtering cycles to remove oxide formed due to air exposure and gradually thinned down to a monolayer level. Surface science acquisitions showed the existence of a mixed oxide/metallic Pb overlayer at the monolayer level that transformed to a metallic Pb upon high temperature annealing (380 °C for 1 h) and measured changes of the electronic interaction that can be explained by Pb/Au surface alloy formation. The results show the electronic interaction between metallic Pb and Au is different from the interaction of Au with the PbO and Pb/PbO mixed layer; the oxide interface is less strained so the surface stress driven mixing between Au is not favored. The work illustrates applications of highly surface sensitive methods in the characterization of the surface alloy systems that can be extended to other complex and ultrathin mixed-metallic systems (designed or spontaneously formed).

Surface Proximity Effect, Imprint Memory of Ferroelectric Twins, and Tweed in the Paraelectric Phase of BaTiO3

We have used energy-filtered photoemission electron microscopy (PEEM) at the photoemission threshold to carry out a microscopic scale characterization of the surface charge and domain structure of the (001) surface in BaTiO3. Signatures of ferroelectric and ferroelastic domains, and tweed, dominate the surface structure of BaTiO3 at room temperature. The surface ferroic signatures are maintained on heating to temperature (~550 K), well above the transition temperature (393 K). This surface proximity effect provides the mechanism for memory of the bulk ferroelectric domain arrangement up to 150 K above TC and thus can be considered as a robust fingerprint of the ferroelectric state near the surface. Self-reversal of polarization is observed for the tweed below TC and for the surface domains above TC. Annealing at higher temperature triggers the dynamic tweed which in turn allows a full reorganization of the ferroic domain configuration.

High-temperature 2D Fermi surface of SrTiO3 studied by energy-filtered PEEM

Functional oxides displaying phenomena such as 2D electron gas (2DEG) at oxide interfaces represent potential technological breakthroughs for post‐CMOS (Complementary Metal Oxide Semiconductor) electronics. Noninvasive techniques are required to study the surface chemistry and electronic structure underlying their often unique electrical properties. The sensitivity of photoemission electron microscopy (PEEM) to local potential, chemistry, and electronic structure makes it an invaluable tool for probing the near surface region of microscopic regions and domains of functional materials. In particular, PEEM allows single shot acquisition of the 2D Fermi surface and full angular probing of the symmetry‐induced intensity modulations. We present results demonstrating a 2DEG at the surface of SrTiO3(001) at 140 K. The 2DEG is created by soft X‐ray irradiation and can be reversibly controlled by a combination of soft X‐rays and oxygen partial pressure.