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First-principles calculations of metal surfaces. II. Properties of low-index platinum surfaces toward understanding electron emission

Physical Review B

Schultz, Peter A.; Hjalmarson, Harold P.; Berg, Morgann B.; Bussmann, Ezra B.; Scrymgeour, David S.; Ohta, Taisuke O.; Moore, Christopher H.

The stability of low-index platinum surfaces and their electronic properties is investigated with density functional theory, toward the goal of understanding the surface structure and electron emission, and identifying precursors to electrical breakdown, on nonideal platinum surfaces. Propensity for electron emission can be related to a local work function, which, in turn, is intimately dependent on the local surface structure. The (1×N) missing row reconstruction of the Pt(110) surface is systematically examined. The (1×3) missing row reconstruction is found to be the lowest in energy, with the (1×2) and (1×4) slightly less stable. In the limit of large (1×N) with wider (111) nanoterraces, the energy accurately approaches the asymptotic limit of the infinite Pt(111) surface. This suggests a local energetic stability of narrow (111) nanoterraces on free Pt surfaces that could be a common structural feature in the complex surface morphologies, leading to work functions consistent with those on thermally grown Pt substrates.

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TYPE Journal Article YEAR 2021
ScopusOSTIDOI

Work function variations in twisted graphene layers

Scientific Reports

Robinson, Jeremy T.; Culbertson, James; Berg, Morgann B.; Ohta, Taisuke O.

By combining optical imaging, Raman spectroscopy, kelvin probe force microscopy (KFPM), and photoemission electron microscopy (PEEM), we show that graphene's layer orientation, as well as layer thickness, measurably changes the surface potential (Φ). Detailed mapping of variable-thickness, rotationally-faulted graphene films allows us to correlate Φ with specific morphological features. Using KPFM and PEEM we measure ΔΦ up to 39 mV for layers with different twist angles, while ΔΦ ranges from 36-129 mV for different layer thicknesses. The surface potential between different twist angles or layer thicknesses is measured at the KPFM instrument resolution of ≤ 200 nm. The PEEM measured work function of 4.4 eV for graphene is consistent with doping levels on the order of 1012cm-2. We find that Φ scales linearly with Raman G-peak wavenumber shift (slope = 22.2 mV/cm-1) for all layers and twist angles, which is consistent with doping-dependent changes to graphene's Fermi energy in the 'high' doping limit. Our results here emphasize that layer orientation is equally important as layer thickness when designing multilayer two-dimensional systems where surface potential is considered.

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TYPE Journal Article YEAR 2018
ScopusOSTIDOI

Detailed Characterization of Surface Structure and Influence on Field Emission

Proceedings - International Symposium on Discharges and Electrical Insulation in Vacuum, ISDEIV

Hopkins, Matthew M.; Smith, Sean S.; Clem, Paul G.; Berg, Morgann B.; Scrymgeour, David S.; Moore, Christopher H.; Bussmann, Ezra B.; Ohta, Taisuke O.

In most models of vacuum breakdown, there is some initial emission of electrons from the cathodic surface, usually employing some form of Fowler-Nordheim emission. While this may be correct for 'textbook' surfaces, it is generally unreliable for real surfaces and fitted parameters are often used. For example, the beta employed is generally unphysical based on usual definitions (e.g., it incorporates more, but unexplained, physics than just a geometry-based field concentration effect). In this work, we describe experimental efforts to better characterize which surface structure parameters influence the vacuum field emission current.

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TYPE Conference Poster YEAR 2018
ScopusOSTIDOI

Local Electronic Structure Changes in Polycrystalline CdTe with CdCl2 Treatment and Air Exposure

ACS Applied Materials and Interfaces

Berg, Morgann B.; Kephart, Jason M.; Munshi, Amit; Sampath, Walajabad S.; Ohta, Taisuke O.; Chan, Calvin C.

Postdeposition CdCl2 treatment of polycrystalline CdTe is known to increase the photovoltaic device efficiency. However, the precise chemical, structural, and electronic changes that underpin this improvement are still debated. In this study, spectroscopic photoemission electron microscopy was used to spatially map the vacuum level and ionization energy of CdTe films, enabling the identification of electronic structure variations between grains and grain boundaries (GBs). In vacuo preparation and inert transfer of oxide-free CdTe surfaces isolated the separate effects of CdCl2 treatment and ambient oxygen exposure. Qualitatively, grain boundaries displayed lower work function and downward band bending relative to grain interiors, but only after air exposure of CdCl2-treated CdTe. Analysis of numerous space charge regions at grain boundaries showed an average depletion width of 290 nm and an average band bending magnitude of 70 meV, corresponding to a GB trap density of 1011 cm-2 and a net carrier density of 1015 cm-3. These results suggest that both CdCl2 treatment and oxygen exposure may be independently tuned to enhance the CdTe photovoltaic performance by engineering the interface and bulk electronic structure.

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TYPE Journal Article YEAR 2018
ScopusOSTIDOI

Experimental Determination of the Ionization Energies of MoSe2, WS2, and MoS2 on SiO2 Using Photoemission Electron Microscopy

ACS Nano

Keyshar, Keyshar; Kunttal, Kunttal; Berg, Morgann B.; Zhang, Zhang; Xiang, Xiang; Vajtai, Vajtai; Robert, Robert; Gupta, Gupta; Gautam, Gautam; Chan, Calvin C.; Beechem, Thomas E.; Ajayan, Ajayan; Pulickel, Pulickel; Mohite, Mohite; D., Aditya D.; Ohta, Taisuke O.

Here, the values of the ionization energies of transition metal dichalcogenides (TMDs) are needed to assess their potential usefulness in semiconductor heterojunctions for high-performance optoelectronics. Here, we report on the systematic determination of ionization energies for three prototypical TMD monolayers (MoSe2, WS2, and MoS2) on SiO2 using photoemission electron microscopy with deep ultraviolet illumination. The ionization energy displays a progressive decrease from MoS2, to WS2, to MoSe2, in agreement with predictions of density functional theory calculations. Combined with the measured energy positions of the valence band edge at the Brillouin zone center, we deduce that, in the absence of interlayer coupling, a vertical heterojunction comprising any of the three TMD monolayers would form a staggered (type-II) band alignment. This band alignment could give rise to long-lived interlayer excitons that are potentially useful for valleytronics or efficient electron–hole separation in photovoltaics.

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TYPE Journal Article YEAR 2017
OSTIDOI
39 Results
39 Results