Pb 5d Core Level Research Articles

A method for studying pico to microsecond time-resolved core-level spectroscopy used to investigate electron dynamics in quantum dots

Time-resolved photoelectron spectroscopy can give insights into carrier dynamics and offers the possibility of element and site-specific information through the measurements of core levels. In this paper, we demonstrate that this method can access electrons dynamics in PbS quantum dots over a wide time window spanning from pico- to microseconds in a single experiment carried out at the synchrotron facility BESSY II. The method is sensitive to small changes in core level positions. Fast measurements at low pump fluences are enabled by the use of a pump laser at a lower repetition frequency than the repetition frequency of the X-ray pulses used to probe the core level electrons: Through the use of a time-resolved spectrometer, time-dependent analysis of data from all synchrotron pulses is possible. Furthermore, by picosecond control of the pump laser arrival at the sample relative to the X-ray pulses, a time-resolution limited only by the length of the X-ray pulses is achieved. Using this method, we studied the charge dynamics in thin film samples of PbS quantum dots on n-type MgZnO substrates through time-resolved measurements of the Pb 5d core level. We found a time-resolved core level shift, which we could assign to electron injection and charge accumulation at the MgZnO/PbS quantum dots interface. This assignment was confirmed through the measurement of PbS films with different thicknesses. Our results therefore give insight into the magnitude of the photovoltage generated specifically at the MgZnO/PbS interface and into the timescale of charge transport and electron injection, as well as into the timescale of charge recombination at this interface. It is a unique feature of our method that the timescale of both these processes can be accessed in a single experiment and investigated for a specific interface.

XPS of the surface chemical environment of CsMAFAPbBrI trication-mixed halide perovskite film

Perovskite-based materials have been considered as the most promising materials in several solar-driven processes, especially photovoltaics. Some features such as high sunlight harvesting and improved carrier transport have been highlighted to have an impact on the efficiency of the above topics, but limited studies have pointed out their surface composition, which mainly influence the above abilities. As a starting point to recognize the surface environment and chemical states of these materials, x-ray photoelectron spectroscopy measurements were performed. Trication-mixed halide perovskite films based on Cs0.08MA0.18FA0.78PbBr0.42I2.58 were prepared by a one step process and deposited on an indium tin oxide substrate by spin-coating. Survey spectra, Pb 4f, I 3d, Br 3d, Cs 3d, C 1s, N 1s, Pb 4d, and Pb 5d core levels, and valence band spectra were measured for the semiconductor sample. Results exhibit symmetrical peaks, indicating that a homogenous solid solution was achieved. Main chemical states of this kind of perovskites such as Cs-Br and Pb-I species are also shown. We deduced that the later signals are associated with the [PbI6]4− octahedra existing in the perovskite lattice, while the former one is related to Cs+ bounded to [PbBr6]4− units on the perovskite surface. Interestingly, the NR4+ signal was also identified, associated with the presence of methylammonium and formamidinium cations.

Size-dependent evolution of electronic structure in neutral Pb clusters—As seen by synchrotron-based X-ray photoelectron spectroscopy

Neutral Pb clusters in the size range from a few tens of atoms up to ∼100atoms are studied using synchrotron-based photoelectron spectroscopy. The electronic structure is seen to differ increasingly from that of the solid when the cluster size decreases. The valence band narrows gradually – down to about one fifth of the solid-state band width at the smallest size of few tens of atoms. Simultaneously the Fermi-edge energy shifts further away from the solid value by more than 2eV. The Pb 5d core level binding energy also increases and the spectral features broaden and change in shape. The changes observed for the valence and for the 5d response energies and shapes are discussed in the context of the metal-to-insulator transition observed earlier in Pb clusters below the critical size of 20–30atoms/cluster.

Photon energy dependence of final state screening in a dilute electron gas system: A synchrotron radiation photoemission study of β-PbO 2

Photoemission spectra of thick films of β-PbO 2 have been measured over a range of exciting photon energies between 40 eV and 150 eV. There is evidence for occupation of conduction band states above the top of the main valence band to give a degenerate electron gas and the spectra terminate in a sharp metallic Fermi edge. Structure associated with screening of shallow 5d core holes by the mobile conduction electrons observed in Al Kα excited X-ray photoemission spectra is suppressed in the lower energy spectra and the Pb 5d core level appears as a simple spin-orbit doublet.

Pb induced charge accumulation on InAs(1 1 1)B

The Pb/InAs(1 1 1)B interface has been studied by synchrotron radiation photoelectron spectroscopy (SR-PES) of valence band and In4d, As3d and Pb5d core levels. Room temperature deposition of ∼1 ML of Pb on InAs(1 1 1)B leads to an ordered overlayer that induces a metallic channel at the surface, as seen through a weak emission in the vicinity of the Fermi level. Its narrow localization in reciprocal space supports the formation of a two-dimensional free electron gas (2DEG) in the surface region. It is proposed that the adsorbed metal layer swaps the initial polarisation of the surface and thus pulls electrons back to the surface. This charge re-arrangement increases the charge density in the accumulation layer and reduces the screening length and thus the depth of the potential well at the surface.

Surface alloying of Pb as a surfactant during epitaxial growth on Cu(1 1 1)

As an important surfactant, Pb surface alloying in surfactant-assisted epitaxy growth was studied by synchrotron radiation photoemission. As annealing causes the submonolayer Pb, distributed as two-dimensional islands on Cu(1 1 1), to form Pb–Cu surface alloy, Cu growth is also found to activate the Pb–Cu surface alloy formation on submonolayer Pb covered Cu(1 1 1). Whereas, different from the fact that the Pb–Cu surface alloy is replaced by a Pb overlayer when the Pb coverage increases up to 1.0 ML, Co deposition gives rise to the Pb–Co surface alloy even on the 1.0 ML Pb covered Cu(1 1 1) surface. In the Pb–Cu surface alloy the Pb 5d core level is shifted toward Fermi level by about 20 meV. Heating de-alloys the Pb–Co surface alloy, while the interface intermixture between Co film and Cu substrate is also enhanced at the same time.

Photoemission study of two dimensional phase transitions on the Pb/Si(1 1 1) surface

Abstract We have studied the low coverage, reconstructed phases of the Pb/Si(1 1 1) adsorption system by photoelectron spectroscopy. With decreasing coverage from 1.3 ML, the top of the valence band shows a transition from a metallic surface to a semiconducting surface, which is stable below a coverage of 0.2 ML. The Pb 5d core level was fitted by one symmetric peak for coverage from 0.1 to 0.2 ML without any observable shift. This state corresponds to Pb adatoms in a two dimensional surface alloy with Si where both atoms are located in the T 4 position, and the surface reconstruction is (√3×√3)R30°. The peak energy and profile are independent of the relative Pb and Si adatom concentrations in the surface. For higher coverages we observe a varying width of the Pb 5d core level due to the appearance of an extra state shifted by about 0.4 eV; the results are consistent with information from STM images. Core level shifts of the Si 2p level are also reported.

Ultrathin Pb film growth on Cu(111) studied by photoemission

The valence bands and the Pb 5d, Cu 3p core levels of Pb films evaporated on Cu(111) were measured by synchrotron radiation photoemission and characterized by low-energy electron diffraction (LEED) and Auger electron spectroscopy (AES). The variation of the surface state at the center of the surface Brillouin zone (SBZ) of Cu(111) with Pb coverage shows that the submonolayer Pb grows on Cu(111) at room temperature (RT) as two-dimensional (2D) islands. With the Pb coverage increasing, the Pb 5d 5/2 core level shifts to higher binding energy monotonically. While the Cu 3p 3/2 core level is shifted toward higher binding energy by about 120 meV due to the deposition of 1.0 ML Pb. At low Pb coverage, subsequent annealing at 200°C gives rise to Pb–Cu surface alloy formation in the first layer of Cu(111). The Pb 5d core level is shifted toward Fermi level by 20~30 meV due to the surface alloying. An assumption about electron charge transfer from Cu to Pb was adopted to interpret the observed core level shifts.

High resolution synchrotron radiation PES study of PbPc deposited on Pt(111)

We studied lead phthalocyanine (PbPc) films sublimated under UHV conditions on a clean Pt(111) substrate. We report results from high resolution photoelectron spectra of the Pb 5d, Pb 4f, C 1s, N 1s and Pt 4f core level using synchrotron radiation. Comparison between ultra thin films and thicker films shows that the first molecular adlayer interacts strongly with the substrate and we report evidence of Pb–Pt alloy formation on the surface. For the thick films, the C 1s and N 1s core level spectra show usual characteristics, however in the case of ultra thin films, we note important differences with respect to some previous results.

Two-dimensional electron gas at InAs(100)1×2/1×4 Pb

Upon deposition of one lead monolayer on the clean, In-terminated InAs(100)4×2/c(8×2) surface, we have obtained a new 1×2/1×4 superstructure and have studied its electronic properties by synchrotron radiation photoelectron spectroscopy (valence bands and In 4d plus Pb 5d core levels). The markedly increased emission in the vicinity of the Fermi level for this reconstruction, as compared to the bare surface, and its narrow localization in k space are the signature of the formation of a strong two-dimensional electron gas in the sub-surface region.

Phase transition of submonolayer Pb/Ge(111): α−3×3R30° ↔ 3 × 3at∼ 250 K

We have studied the low 3 × 3 and high (α−3×3R30°) temperature phases of Pb/Ge(111) at Pb coverage 1/3 of monolayer using synchrotron radiation angular resolved photoemission spectroscopy and low-energy-electron-diffraction measurements. The Ge 3d and Pb 5d core levels and valence bands have been recorded above and below the transition temperature (Tc ≈ 250K). We have observed that the 3 × 3 reconstructed surface is a semiconductor, while the α−3×3R30° phase is metallic. As the system undergoes this transition, a band gap opens in precise areas of the Brillouin zone, supporting the charge-density-wave model for the transition.

Structure determination of ordered [formula omitted] Pb on Ge(111) by photoelectron holography

The technique of photoelectron holography has been applied to the (√3 × √3)R30° phase of 1 3 - monolayer Pb on Ge(111) for a determination of the local bonding structure of the Pb adatom. Angle-resolved photoemission from the Pb 5d core level shows intensity oscillations as a function of photon energy due to diffraction effects. These oscillations were measured at various emission angles and Fourier inverted to yield three-dimensional real-space images of the nearest neighbours to the Pb emitter atom, which is bonded at the T 4 site. The use of the small-cone technique to eliminate artifacts is discussed.

Photoelectron holography of [formula omitted

The technique of holographic image reconstruction based on photoelectron detection has been applied to the Pb Si(111) -(√3 × √3) R30°-β phase for a determination of the surface structure. Angle-resolved photoemission from the Pb 5d core level shows intensity oscillations as a function of photon energy. These constant-initial-state spectra were taken at various emission angles and Fourier inverted to yield three-dimensional real-space images of the nearest neighbors to the emitter atom. These images clearly show Pb atoms adsorbed at the T 4 site.

Pb/GaAs(100): Band bending, adatom-induced gap states and surface dipole

The work presented here shows a correlation between the position of Pb-induced gap states, responsible for the initial movement of the Fermi level (EF) at the Pb–GaAs(100) interface, and the position of the Pb 5d core level measured by photoemission spectroscopy. The difference in Pb-induced gap states is obtained by depositing submonolayer amounts of Pb on the As-rich (2×4)–C(2×8) or the Ga-rich (4×2)–C(8×2) GaAs(100) surfaces. These reconstructions are known to produce a large dipole-induced difference in the work function of the clean surface. The correlation suggests that the reconstruction-induced dipole is mostly preserved during the initial stage of Pb deposition and shifts the energy levels of the adsorbate.

VUV-photoelectron spectroscopy on lead clusters deposited from the pulsed arc cluster ion source (PACIS)

Lead clusters grown in a pulsed arc cluster ion source (PACIS) are soft-landed under UHV conditions on cooled polycrystalline silver targets. VUV-photoelectron spectroscopy with light from the synchrotron BESSY applied to mass-selected clusters on cold (160 K) substrates shows that the Pb 5d core level binding energies depend on cluster size and-only beyond a critical amount of deposited clusters-on the coverage Θ. A decreasing particle size induces a core level shift towards lower binding energy with respect to the bulk value, the maximum shift being −0.33 eV. The same value is achieved by atom deposition and can be explained by a Born-Haber cycle. The limits for small two-dimensional and large three-dimensional clusters will qualitatively be discussed by the same model. Deposition on a warm substrate, however, leads-at least for small systems-to a gradual line shift with Θ.

Photoemission study of the growth, desorption, Schottky-barrier formation, and atomic structure of Pb on Si(111)

We have utilized high-resolution synchrotron-radiation photoemission spectroscopy and high-energy electron diffraction to study the various reconstructed surfaces of Pb-covered Si(111). The Si 2p and Pb 5d core levels and valence bands are analyzed as a function of Pb coverage and annealing temperature. We confirm the room-temperature (RT) Stranski-Krastanov growth mode with the two-dimensional adlayer completed at \ensuremath{\sim}1.3 monolayers. The formation of the Schottky barrier has been studied. We measure n-type barrier heights of 0.89 and 1.09 eV for the RT epitaxial phase and the (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} incommensurate phase, respectively. Possible mechanisms that may be responsible for the disagreement between barrier heights as measured by core-level photoemission and electrical techniques are discussed. We also report on the observation of surface-shifted core levels and surface states in the valence band for the (\ensuremath{\surd}3 \ifmmode\times\else\texttimes\fi{} \ensuremath{\surd}3 )R30\ifmmode^\circ\else\textdegree\fi{} phases obtained by annealing to higher temperatures in the desorption regime. These results are used for an investigation of the structural models for these surface phases.

Variation of the Pb and Bi 5d binding energies and branching ratios for thin Pb and Bi overlayers on Ni(110).

Thin Pb and Bi overlayers on Ni(110) have been studied by use of variable-energy photoelectron spectroscopy. The overlayers are characterized by use of the Pb and Bi 5d core-level intensities and binding-energy shifts. The Pb 5d cross section (\ensuremath{\sigma}) and 5${d}_{5/2}$:5${d}_{3/2}$ branching-ratio (BR) dependence as a function of photon energy are quite sensitive to the thickness of the overlayer: as the thickness increases, the cross-section maximum shifts to lower photon energy. This shift is reflected by the point at which the BR crosses the statistical 1.5 value: 57.5 eV for 0.5 monolayer to 42.5 eV for bulk Pb. We postulate that this behavior is due to the sensitive nature of the outgoing f-wave potential, as shown recently by theoretical calculations of heavy-metal d cross sections.

Production of Pb+ ions following 5d core photoionization of tetramethyl lead as revealed by a coincidence experiment.

Ionic fragmentation following Pb 5d core-level ionization in tetramethyl lead (${\mathrm{PbMe}}_{4}$) has been studied with synchrotron radiation. In the wavelength range between 45 and 75 nm, photoionization of the Pb 5d core levels is observed in the threshold electron spectrum of ${\mathrm{PbMe}}_{4}$. The efficiency for the production of ${\mathrm{Pb}}^{+}$ ions increases dramatically in this region, while those of other fragment ions decrease. It is shown that the ${\mathrm{Pb}}^{+}$ ion is predominantly produced following 5d photoionization, by use of the threshold electron---photoion coincidence technique.

Coverage-dependence of Pb 5d core level binding energies on Al(111) and Ni(111)

Coverage-dependence of Pb 5d core level binding energies on Al(111) and Ni(111)

Coverage-dependence of Pb 5d core level binding energies on Al(111) and Ni(111)

The Pb 5d binding energies were measured by UPS in films evaporated onto Ni(111) and Al(111) and characterized by LEED, AES and TDS. The measured binding energies are in good agreement with values calculated in a model based on a Born-Haber cycle. The desorption energies needed for this calculation are taken from the TDS measurements.