Angle- and Momentum-Resolved Photoelectron Velocity Map Imaging

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Angle- and Momentum-Resolved Photoelectron Velocity Map Imaging Studies of Thin Au Film and Single Supported Au Nanoshells Jacob Pettine, Andrej Grubisic, and David J. Nesbitt J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10846 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on February 8, 2018

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The Journal of Physical Chemistry

Angle- and Momentum-Resolved Photoelectron Velocity Map Imaging Studies of Thin Au Film and Single Supported Au Nanoshells

Jacob Pettine1, Andrej Grubisic† 2, and David J. Nesbitt * 1,2

JILA, University of Colorado Boulder and National Institute of Standard and Technology, Boulder, CO 80309, United States 1

Department of Physics, University of Colorado Boulder, Boulder, CO 80309, United States

2

Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, CO

80309, United States

†

Current address: Goddard Space Flight Center, NASA, Greenbelt, MD 20771, United States

*Author to whom correspondence should be addressed: [email protected]

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ACS Paragon Plus Environment

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ABSTRACT Transverse (2D) photoelectron velocity distributions are directly measured on 10 nm Au film and single Au nanoshells following multiphoton excitation/photoemission. This unique capability is achieved by combining scanning photoemission microscopy with velocity map imaging, yielding photoelectron spectra as a function of diffraction-limited position on a sample. Detailed 3D photoelectron velocity distributions are retrieved for the Au film by fitting the 2D data with a ballistic (three-step) photoemission model, where contributions from two-photon, three-photon, and d-band processes are identified and further characterized as a function of photon energy using a broadly-tunable, visible femtosecond optical parametric oscillator. These techniques are further applied to investigate the more complex behaviors of single plasmonic Au nanoshells with silica cores. The strong plasmonic near- and far-field signatures are first characterized via optical and photoemission measurements, along with theoretical methods (Mie theory and finite element analysis). This is followed by measurements of the transverse photoelectron velocity distributions for single nanoshells, which reveal at least one surprising result. Specifically, nearly perfect azimuthal symmetry is evident in the nanoshell electron momentum distributions, despite linearly-polarized excitation and anisotropic near-electric-field distributions corresponding to the dipolar/quadrupolar plasmon modes. Possible explanations for such azimuthal symmetry are discussed, with future experiments described to address these possibilities.

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I. INTRODUCTION Photoelectron spectroscopy (PES) methods are widely used for probing electronic structure and the corresponding optical properties of atoms, molecules, and bulk materials.1 Over the past few decades, PES has also been an increasingly important tool for studying light-matter interactions and femtosecond electron dynamics in nanoscale systems, which behave very differently from their bulk material counterparts. The most notable effect that emerges for metal nanoparticles with sub-wavelength dimensions is the extraordinarily strong coupling between optical fields and collective conduction electron oscillations, known as localized surface plasmon resonances (LSPRs). This plasmonic behavior depends sensitively on particle material, shape, and local dielectric environment, providing a number of experimental control parameters for continuous LSPR tunability across the optical spectrum (UV, visible, and IR), particularly for noble metal nanoparticles (Cu, Ag, and Au)2-4 and aluminum.5 A cascade of physical processes is set in motion within and around metal nanoparticles following ultrafast optical plasmonic excitation6 which can be investigated in exquisite detail using state-of-the-art PES techniques at the present limits of spatial (nanometer) and temporal (femto/attosecond) resolution.7-9 Plasmonic nanoparticles and nanostructures are optical antennas that concentrate light into deeply sub-wavelength, nanoscopic dimensions. Strong electric near-field enhancements are produced at the nanoparticle surfaces where electrons pile up during LSPR oscillations, which can profoundly influence and thereby elucidate the underlying plasmonic photophysics and electron photoemission dynamics. These strong plasmonic field enhancements have been cleverly exploited in applications such as surface-enhanced Raman spectroscopy10,11 and nearfield assisted photochemistry.12 Of equal importance is the strong plasmon-assisted absorption, which yields high densities of hot electrons that can in principle be extracted for novel 3



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photovoltaic13,14 and photocatalytic15-17 applications. The plasmon-enhanced electric near-field distributions of various metal nanoparticle geometries have been explored in numerous experimental and theoretical studies,18-20 with photoemission electron microscopy (PEEM) studies directly demonstrating the correlation between plasmonic near-field enhancements and local photoemissivity with better than 50 nm spatial resolution.9,21 Predictions of photoelectron momentum distributions can be made based on these electric near-field distributions, but direct momentum-space measurements and correlations with near-field enhancement distributions have yet to be demonstrated on individual nanoparticles. With high-precision nanoparticle synthesis2,22,23 and nanostructure lithography24,25 methods now available, nanoscience is poised to take advantage of designer nanosystems that enable optically-controlled directional photoemission, e.g. from femtosecond polarization-controlled nanoemitters.8,26 Anisotropic electron emission has already been demonstrated with plasmonic Au nanowires embedded in silicon, which yield the largest photocurrents along the dipolar LSPR axis.27 With prospective application goals such as nanoplasmonic circuitry and solar energy conversion, momentumresolved photoemission studies on single nanoparticles are rapidly becoming essential tools for developing and refining next-generation nanoscience and nanotechnologies. This work presents a new PES technique for studying single-particle nanoplasmonics, with photoelectron angle and momentum resolution achieved via the novel combination of scanning photoemission microscopy and velocity map imaging, henceforth referred to as scanning photoelectron imaging microscopy (SPIM). Details of the SPIM technique will be discussed in Section II. In brief, electrodes are strategically configured in a velocity map imaging (VMI) collection scheme to focus photoelectrons onto a spatially-resolved detector with ,

coordinates linearly proportional to their transverse , velocity vector components. 4


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Following the pioneering development of ion imaging techniques in the late 1980’s by Chandler and Houston,28 the VMI electrode configuration was introduced by Eppink and Parker in 1997,29 representing a dramatic improvement in resolution and collection efficiency with respect to the previous grid-based techniques.30 Since then, VMI has been used extensively in molecular spectroscopy and reaction dynamics,31,32 with significant developments in the high-resolution (~0.1 meV) imaging of slow electrons33 and direct 3D detection using slicing or fast timing techniques.34-36 Recently, VMI has been used to study a Si nanotip array,37 as well as in nanoparticle aerosol studies to investigate field penetration depth and electron mean-free path effects,38,39 solvated electron dynamics in nanodroplets,40 and coherent attosecond electron dynamics.41,42 While nanoparticles in aerosols offer the advantage of being relatively unperturbed by their environment, the photoelectron signals are typically integrated over many nanoparticles in a continually-refreshed jet source. This introduces sample heterogeneity and orientational averaging, which are avoided in the present SPIM technique by instead immobilizing nanoparticles on a surface. Although the presence of the substrate must be carefully accounted for in supported-nanoparticle studies, this configuration enables both the extended observation of single nanoparticles, as well as examination of important charge transfer processes and light-trapping effects that may indeed be facilitated by the presence of a substrate or embedding material.43 The present implementation of VMI for studying surfaces and supported nanoparticles is motivated by the enormously successful application of the technique in molecular spectroscopy and dynamics, while also drawing inspiration from conventional angle-resolved photoemission spectroscopy (ARPES) on surfaces. A number of ARPES techniques have been established over the years, including what may be considered “traditional” ARPES using a hemispherical 5



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analyzer,44,45 more recent angle-resolved time-of-flight (AR-TOF) setups using a delay line detector for concurrent 3D detection,45-47 and momentum imaging using a PEEM column and two hemispherical analyzers.48 Time-, position-, and 1D energy-resolved PEEM also provides impressive capabilities for studying single-nanoparticle plasmonics and electron dynamics49,50 and nano-ARPES is under development for high spatial and angular resolution.51 All of these techniques are powerful and offer different advantages, but only recently has a time-of-flight momentum-resolving electron microscope (ToF k-PEEM) been developed for angle-resolved, single-particle PES studies at optical frequencies.52 Here we instead demonstrate some of the most important experimental capabilities for comprehensive angle- and momentum-resolved photoelectron studies of single nanoparticles using the relatively simple, cost-effective, and versatile SPIM system. Some goals for these studies include (i) the measurement of low-energy (near-threshold) photoelectrons produced via nonlinear emission following visible-photon plasmonic excitation, (ii) efficient and reproducible access to large numbers of addressable nanoparticles for correlated studies and statistics, and (iii) the ability to collect and characterize photoemission from the full 2 upper-half space solid angle without sample manipulation (e.g. tilting). Also of crucial importance are high photoelectron collection efficiencies, such that nanoparticles may be investigated with low integrated pulse energies (which for femtosecond pulses may nevertheless correspond to high peak intensities) to minimize heating, melting, and ponderomotive forces, thereby permitting focus on photoemission processes relevant under truly perturbative laser intensity conditions.

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II. EXPERIMENTAL DETAILS Scanning Photoemission Microscopy. As depicted in Figure 1, our scanning photoemission microscopy technique utilizes a tunable femtosecond laser focused down to a diffraction-limited spot (~500 nm diameter) on the sample by a reflective microscope objective (NA = 0.65) under high vacuum conditions (5 × 10 Torr). The sample stage is raster-scanned

via three quartered piezoelectric posts to address a 30 × 30 m2 region on the sample, along

with coarse piezo motor control to address an overall region larger than 3 × 3 mm2. A nanoshell photoemission intensity map is presented in Figure 1, with a zoomed-in scan on a single nanoparticle demonstrating the diffraction-limited spatial resolution. The focused light stimulates nonlinear photoemission from the sample and the photoelectrons are accelerated by the VMI electrostatic lens toward the detector, where each electron is multiplied ~107-fold via a chevron microchannel plate (MCP) pair. These amplified electron pulses are then further accelerated to impact and excite a phosphor screen (P47 phosphor), from which the luminescence is imaged through a vacuum window onto a 20 FPS, 1-megapixel CCD camera. The images of these events are then centroided in real-time by the acquisition software to achieve sub-pixel spatial resolution, currently only limited by the 32 µm pitch of the MCP pores. For sample coverages < 1 NP/µm2, single nanoparticles are studied with sufficiently low probability of finding multiple particles within the diffraction-limited laser spot. Hundreds of individual nanoparticles can thus be resolved in a single scan, enabling the efficient collection of individual-particle statistics reflecting the signal dependence on laser polarization-, intensity-, and frequency.53-55 Additionally, single nanoparticles can be studied continuously for hours at a

time (even days with minor adjustments) via capacitive sensor closed-loop stabilization of the sample stage position. 7



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Laser System. The laser system consists of an optical parametric oscillator (OPO) pumped by the second harmonic of a 75 MHz, 1.7 W Ti:sapphire oscillator (KMLabs Swift) with pulse durations of ∆ ≈ 50 fs, characterized via frequency-resolved optical gating.56 The OPO signal and idler beams are continuously tunable between 515-775 nm and 830-1800 nm, respectively. Additionally, the Ti:sapphire oscillator can be tuned between 700-1000 nm, with corresponding second harmonic ranging from 350-500 nm, thereby providing nearly continuous coverage from 350-1800 nm. When synchronously-pumped57 by 600 mW of the second harmonic at 400 nm, the OPO outputs approximately 100 mW average power, with the frequency tuned via feedback-stabilized automated adjustment of the OPO cavity length. Despite relatively “weak” (≈ 1 nJ) pulse energies out of the OPO, the combination of high-NA focusing

and femtosecond pulse durations results in high peak intensities at the sample (~50 GW⁄cm% ),

which are far higher than needed for these studies and are typically attenuated by 100-fold or more. The simple advantages of this laser design are the broad tuning range and the ability to measure large photoemission rates (> 100 kHz) without space-charge distortions or sample charging effects that can occur in experiments with higher-intensity, lower-repetition lasers.58 The present combination of high repetition rate (75 MHz) with low emission probability (< 10( electrons/pulse) therefore ensures that the physical processes under investigation remain minimally perturbed by the photoemission process itself. Velocity Map Imaging. The VMI electrostatic lens accelerates and images photoelectrons onto the phosphor-MCP detector , coordinates in linear proportion to their initial , velocity components.29 The top copper electrode that separates the acceleration region

from the drift region is held at ground, with the ratio of the extractor (middle) and repeller 8


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(bottom/sample stage) electrode voltages optimized for the 7.5 cm MCP diameter in the 0-5 eV photoelectron kinetic energy range ()*+, ⁄)-*. = 0.82, with )-*. = −4.5 kV). A mu-metal cylinder shields the drift region from stray magnetic fields, and the large VMI acceleration electric field (~1 kV/cm) at the sample surface minimizes the effects from any small patch fields. The VMI mapping corresponds simply to the projection/integration over the longitudinal velocity component (2 ), with the position of the electron on the detector proportional only to the transverse velocity, 34 = , , as demonstrated in Figure 2. In typical molecular beam

applications, the VMI lens reduces image blurring that would otherwise occur due to a spatiallydiffuse ionization source, and also sidesteps the issues that accompany the “uniform” electric fields originally employed for ion imaging,28 such as low transmission and image distortion due to grids.29 While such spatial blurring effects are negligible for the diffraction-limited nanoscale sources in the present studies, the VMI lens arrangement is still extremely valuable for both the high-quality linear mapping it provides and the near-unity transmission of photoelectrons onto the detector, with the overall collection efficiency of the system limited only by the fractional surface coverage of the MCP pores (~50%). Sample Details. As previously demonstrated by Halas, Nordlander, and coworkers, Au nanoshells with silica cores support LSPRs that are continuously tunable throughout the visible and into the IR by controlling the core/shell aspect ratio during synthesis.59-62 For the present studies, Au nanoshells were purchased commercially (nanoComposix, Inc.) with the LSPR in the middle of the OPO tuning range, initially dispersed in water and stabilized with lipoic acid ligands. Sample preparation involves spin-coating the nanoshell dispersion onto a glass coverslip pre-coated with a 10 nm indium tin oxide (ITO) film. The ITO film is both transmissive to 9



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visible light (6 ≈ 90%) and electrically conductive, which is essential for nanoparticle charge neutralization following a photoemission event. As the simplest nanoparticle geometry with the LSPR in the middle of the visible spectral range, Au nanoshells provide an excellent benchmark test platform for VMI studies of isotropic, supported nanoparticles. Despite their relative geometrical simplicity, the nanoscale curvature and plasmonic properties of nanoshells can still result in interestingly complex photoemission behaviors. Therefore, in addition to investigating single nanoshells, we have also performed parallel studies on thin Au films. This is an important initial step, since the analysis of VMI data generated from surfaces is still a frontier area, which may be most valuably explored for the simple photoemission geometry of a smooth, flat surface. Thus, Section IV contains a brief discussion of photoemission results from thin, extended Au films as a benchmark for more complex VMI geometries, as well as to validate the three-step photoemission model presented in Section III. Preparation of the thin Au film involves sputtering 10 nm of gold onto a 2 nm titanium adhesion layer on a borosilicate coverslip. The polycrystalline film is expected to have similar electronic band structure to the nanoshell gold layer63 but exhibits very different optical characteristics in both the near- and far-field. Most notably, surface plasmons are not efficiently excited on the smooth Au film in these experiments due to insufficient momentum-matching between the photons and the traveling polariton modes. Unlike localized surface plasmon resonances, surface plasmon polaritons have well-defined momenta and require either a dielectric interface or a periodic spatial structure (such as a grating or surface roughness) for efficient coupling with light.64,65

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III. THEORETICAL MODEL The majority of VMI applications take advantage of the high level of spatial symmetry offered by atomic and molecular photoelectron/photoion velocity distributions in the gas phase, specifically perfect cylindrical symmetry with respect to an axis parallel to the plane of detection. Under these conditions, the full 3D photoelectron/photoion velocity distributions can be efficiently recovered from the 2D VMI projections using inverse Abel transform algorithms.30,32 Unfortunately, photoemission from surfaces and supported nanoparticles may not exhibit the requisite spatial symmetry necessary to infer the 2 projections directly. In the longterm, our solution will be to implement fast timing on the electron arrivals and thereby measure the full 3D velocity distributions directly.66 However, provided one has a sufficiently accurate and physically justified model (vide infra), it is also possible to perform a forward convolution to fit the 2D photoelectron projections and thereby infer the full 3D velocity distributions. We discuss a simple photoemission model below, with the fitting method demonstrated for Au films and then extended to Au nanoshells in Section IV. It should first be clarified that PES methods in the low-energy (visible) regime essential for studying nanoplasmonic phenomena provide qualitatively different insights compared with UV or X-ray PES studies. For example, the band structure of gold67 makes evident that the visible OPO photons (ℏ9 < 2.4 eV) employed herein have insufficient energy to promote direct/vertical transitions (in which total momentum is conserved modulo the reciprocal lattice vector) from below the Fermi level to states above the vacuum level (? − ~

CFG

>~

Q + CFG

Q

` =Q` ~ℏ£¤ /¦§ ¥ _

Q = R`¡¢

+ 1X

` = U ` ~ℏ£¤ X ¥

¨

6

where a VMI calibration factor, ª («/m ), has been introduced explicitly, with precise conversion ¬ from experimental pixels to physical velocity units quantified via fitting the Au film data. If we further impose conservation of energy (_àb Q = _àb % + Φ = _àb % + 500 K demonstrated previously for 30 nm

thick Au film and similar absorbed pulse fluences77 (~200 μJ/cm%). The relative d-band amplitude ratio fits globally to = 0.061, and the exponential constant approximating the d-

band DOS is globally fit to µ = 4.22 eV ¨ , which nicely reproduces the DOS of gold determined via X-ray PES experiments.80

For an even clearer parsing of the 3PPE, d-band 3PPE, and 2PPE contributions to Au thin film photoemission, the overall signals have been measured as a function of OPO photon energy, as reported in Figure 7. On a logarithmic scale, the total MPPE spectrum demonstrates a dramatic increase in the photoemission rate (over 4 orders of magnitude!) with increasing photon energy between 1.63-2.33 eV at constant intensity. This in turn reflects the increasing electron DOS that can be excited above the vacuum level with increasing photon energy, especially once ℏ9 is large enough to promote 3PPE from the d-band states and 2PPE from the occupied conduction band states. The MPPE curve in Figure 7 is fit by integrating the ejected photoelectron distribution (determined via eq 7) over all velocities and employing the parameters determined in the VMI fits. The fits reproduce the behavior well with only the 2PPE and 3PPE amplitude parameters floated, for which the steep onset of 2PPE between 2.1-2.2 eV photon energies is in excellent agreement with the abrupt transition in power law slopes noted previously in Figure 4a. The fit amplitude of the 2PPE term is nearly four orders of magnitude greater than the 3PPE term, which illustrates how lower-order processes in the perturbative MPPE expansion (eq 3) become dominant once energetically above threshold. It is clear that the 2PPE signals (once energetically accessible) will eventually become stronger than the d-band contributions with increasing OPO photon energy, because the d-band emission is still a threephoton (interband absorption + 2PPE) process and therefore involves an additional transition 22


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matrix element with respect to any two-photon process. As expected, however, the d-band signal (i.e. the difference between the solid and dashed lines in Figure 7) does become much stronger than 3PPE from the conduction band for photon energies ≳ 1.9 eV. Despite a few approximations, the excellent agreement between least-squares fits and photoemission data for both the VMIs at many different photon energies (Figure 5) and the integrated distribution (Figure 7) indicates that the ballistic three-step model with d-band contributions captures much of the essential physics underlying MPPE from thin Au films. Au Nanoshells: Single-Nanoparticle PES. With the optical and electronic properties characterized for a relatively simple Au thin film geometry, we now explore the potentially more complex plasmonic behavior of Au nanoshells. Although the geometric cross-section of a single nanoshell represents only a relatively small fraction of the laser focal illumination area (10% at 650 nm), the combination of strong plasmonic near-field enhancements and the |

Angle- and Momentum-Resolved Photoelectron Velocity Map Imaging

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