Abstract
Photocathodes—materials that convert photons into electrons through a phenomenon known as the photoelectric effect—are important for many modern technologies that rely on light detection or electron-beam generation1,2,3. However, current photocathodes are based on conventional metals and semiconductors that were mostly discovered six decades ago with sound theoretical underpinnings4,5. Progress in this field has been limited to refinements in photocathode performance based on sophisticated materials engineering1,6. Here we report unusual photoemission properties of the reconstructed surface of single crystals of the perovskite oxide SrTiO3(100), which were prepared by simple vacuum annealing. These properties are different from the existing theoretical descriptions4,7,8,9,10. In contrast to other photocathodes with a positive electron affinity, our SrTiO3 surface produces, at room temperature, discrete secondary photoemission spectra, which are characteristic of efficient photocathode materials with a negative electron affinity11,12. At low temperatures, the photoemission peak intensity is enhanced substantially and the electron beam obtained from non-threshold excitations shows longitudinal and transverse coherence that differs from previous results by at least an order of magnitude6,13,14. The observed emergence of coherence in secondary photoemission points to the development of a previously undescribed underlying process in addition to those of the current theoretical photoemission framework. SrTiO3 is an example of a fundamentally new class of photocathode quantum materials that could be used for applications that require intense coherent electron beams, without the need for monochromatic excitations.
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The data that support the findings of this study are available from the corresponding authors upon request.
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Acknowledgements
We thank C. Wang, B. Singh, F. Silly, M. T. Edmonds and J. Luiten for discussions and X. Xie, S. Zhu, T. Zhou, W. Tang, M. Li, H. Ou and B. Xie for technical assistance. The work at Westlake University was supported by the National Natural Science Foundation of China (grant nos. 12274353 and 11874053), Zhejiang Provincial Natural Science Foundation of China (LZ19A040001), National Key R&D Program of China (grant no. 2022YFA1402200), Yecao Tech (2021ORECH020015), the foundation of the Westlake Multidisciplinary Research Initiative Center (MRIC20210101) and the Westlake Instrumentation and Service Center for Physical Sciences. The work at the Shanghai Synchrotron Radiation Facility was conducted under proposal nos. 2019-SSRF-PT-008678 and 2020-SSRF-PT-013048. The work at UVSOR was conducted under proposal nos. 21-668 and 21-859. The work at Northeastern University was supported by the Air Force Office of Scientific Research under award no. FA9550-20-1-0322 and benefited from the computational resources of Northeastern University’s Advanced Scientific Computation Center and the Discovery Cluster.
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R.-H.H. conceived the project, secured funding, guided the investigations with advice from C.Z. for experiments and M.M., W.-C.C., R.S.M., B.B., A.B. and S.L. for the theory and wrote the manuscript with input from C.H., M.M., R.S.M. and A.B.; C.H., W.Z. and P.R. performed the measurements with help from K.T. and analysed the data. All authors contributed to the discussions.
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C.H., P.R. and R.-H.H. are inventors on patent applications CN202211194195.9 and PCT/CN2022/123415 submitted by Yecao Tech in collaboration with Westlake University on technology related to the photocathode processing and performance described in this Article. The other authors declare no competing interests.
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Extended data figures and tables
Extended Data Fig. 1 SPS at θ = 0° measured from various materials in comparison to SrTiO3(100).
a, Comparison of SPS from different materials at 15 K with hν = 21.2 eV in system I for KTaO3, FeSe, TiSex, Au, Ta and with hν = 10.0* eV in system II for SrSe, MoS2. Materials as labelled are in thin film/flake form (uc denotes unit cell) of either single or poly-crystalline (p.c.) nature. Quality of the samples was confirmed in all cases by LEED and/or ARPES measurement of the initial-state band structure; an example is given in the inset. A continuous/incoherent SPS lineshape is seen in sharp contrast to the discrete/coherent lineshape (reproduced from c) from the 2 × 1-reconstructed surface of SrTiO3(100). b–e, Full-range SPS for SrTiO3(100) for TA = 1100 °C with hν = 6.2 eV (in system II), 10.0* eV (system II with setting 2), and 21.2 eV and 40.8 eV (system I), respectively. The upper axis of b gives the kinetic energy for the raw spectrum. Insets of c–e show (from low to high kinetic energy) the work-function cut-off, Peak 2, Sr-4p core level (if available) and valence band (if available), in-gap states and the Fermi cutoff. Sample IDs (13 and 18) and temperatures are as indicated. Dashed lines mark the high-energy ends of the photoemission spectra that are cut off by the Fermi-Dirac function.
Extended Data Fig. 2 T-dependent evolution of the SPS measured for TA = 1100 °C.
a, d, SPS image plots at T = 15 K and 280 K, respectively, and b, c, SPS at θ = 0° at different T values as labelled, all with hν = 21.2 eV. e–h Corresponding results obtained from the same sample 13 but with hν = 40.8 eV. Note that the energy regions for Peaks 1 and 2 are shown separately in b, c (f, g). and marked in a, d (e, h) by dashed lines. i, FWHM of Peaks 1 and 2 as a function of T for measurements with hν = 6.2 eV, 10.0 eV, 21.2 eV and 40.8 eV using markers in different colors. Error bars mainly reflect uncertainties associated with the determination of the SPS peak due to background subtraction and the calibration of T at the sample location. Colored ribbons are guides to the eye. The shaded area marks the same T window as in Fig. 2h, i across which trend-changes are observed. j, SPS at θ = 0° from sample 18 at T = 5 K with hν = 10.0* eV. Inset: Magnification of the indicated energy region for Peak 1*, which is barely visible on the intensity scale of the main panel. Results in j (a–h) and of 6.2 eV (other hν) in i were obtained from system II (I).
Extended Data Fig. 3 Schematic illustrations related to the photoemission process.
a–c, Band diagrams for three types of semiconductors of different electron affinity: effective NEA, true NEA and PEA systems. Effective electron affinity (χeff) is negative when the bulk conduction band minimum (CBM) lies above the surface vacuum level (Evac), while the true electron affinity (χ) becomes negative only when the surface CBM lies above Evac. Upward arrows indicate direct transitions due to photoexcitation that creates a minority electron (hole) in the conduction (valence) band. Downward arrows indicate minority electrons decreasing in energy due to thermalization while moving towards the surface. Only electrons reaching the surface with energy higher than Evac can escape, as shown by the horizontal arrows (crossed arrows indicate forbidden electron emissions). Insets show schematic SPS in the blue-shaded regions with the portion cut off by Evac indicated by dashed curves. VBM denotes valence band maximum. d, Formation of unbound (upper half) and bound states (lower half) in the continuum (BICs) due to different potential wells. The barrier height far away from the well marks the energy that separates the discrete and continuous portions of the spectrum. Red curves depict the wave functions of the related states. Bound states reside at discrete energies as indicated by the baselines.
Extended Data Fig. 4 TA-dependent evolution of the work function and valence band spectrum.
a, Work function measured at T = 15 K and 280 K on samples 12 and 13. b, Angle-integrated valence band spectra at θ = 0° measured at T = 15 K on sample 12 for different TA values as indicated. Error bars (if larger than the size of symbols) reflect uncertainties in determining the work-function cut-off at the midpoint of the trailing edge of the SPS and those associated with the distribution of TA across the sample surface. All results were obtained from system I.
Extended Data Fig. 5 Different schemes for sample biasing in systems I and II and the experimental energy resolutions.
a, Schematic showing a bias applied through metal pins to a sample that is electrically floated, mounted on sapphire, and partially shielded by a grounded cold shield and sample/stage. b, Schematic showing a bias applied to a sample in electrical connection with the sample holder that is floated and partially shielded by a grounded cold shield. c, d, Photos showing the parts (circled) on the sample manipulator of systems I and II relating to a and b, respectively. e–g, Angle-integrated energy distribution curves (black curves) measured from a grounded polycrystalline gold sample in system I (e) and system II (f, g) with different settings (see Methods). Red curves are fits to the Fermi-Dirac function used to determine the experimental resolutions as indicated.
Extended Data Fig. 6 Sample-bias dependence of the FWHM in energy of Peak 1 in the SPS at θ = 0°.
a, b, SPS measured in system I and system II (with setting 2) at T = 15 K and 5 K with hν = 10.0 eV and 10.0* eV, respectively, with different sample biases as labelled from a constant-voltage source (Keithley 2230). c, SPS measured in system II (setting 2) with different biases as labelled from batteries in different combinations. Horizontal axes of a–c give kinetic energy relative to the position of Peak 1. Sample IDs (12 and 19) are as indicated. d, Summary of the FWHM in energy of Peak 1 extracted from a–c. Error bars (if larger than the size of symbols) mainly reflect uncertainties associated with the determination of the SPS peak intensity. The diamond symbol marks the FWHM of 7.1 meV for the result shown in Fig. 3i which, along with results in b–c, were obtained from system II (setting 2). Horizontal dashed lines mark the experimental resolutions measured from a grounded polycrystalline gold sample in system I and system II (setting 2) as indicated (Extended Data Fig. 5e and g).
Extended Data Fig. 7 MCP voltage dependence of the SPS at θ = 0° measured at two temperatures with different hν.
a, b, SPS measured with hν = 6.2 eV at 5 K and 280 K. c–h, SPS measured with hν = 10.0 eV, 21.2 eV and 40.8 eV. Sample IDs (13 and 18) are as indicated. The MCP voltage used and the corresponding counts (i.e., integrated intensities; in bracket) within the kinetic energy window of [3.86, 4.20] eV are labelled for each measurement. All MCP voltages in each panel are proper MCP voltages for measuring that particular SPS, for which the largest voltage was typically applied to obtain the final result. All SPS in each panel are normalized to the intensity of Peak 1. Results in c–h (a-b) were obtained from system I (II).
Extended Data Fig. 8 Summary of the SPS results for TA = 1100 °C from various samples measured in this study.
a–e, SPS at θ = 0° measured on samples with sample IDs as indicated in system I (for sample 1~14) with hν = 21.2 eV and in system II (for sample 15~18) with hν = 10.0* eV and setting 1. a, b, SPS measured near room temperature and at low T (15 K for system I and 5 K for system II). Both Peaks 1 and 2 are visible in a, while Peak 2 is barely visible in b. c, Blow-up of the energy region of Peak 2 (if measured) in b. d, e, Peak 1 measured at low T for samples 10 and 18, after annealing at TA = 1100 °C for different times as marked. f, Energy positions of Peaks 1 and 2 measured in two systems near room temperature and at low T on different samples subjected to annealing for different times. Most of the results are extracted from a–e. The horizontal planes are eyeguides to the two peaks near room temperature and at low T. The vertical plane is an eyeguide to separate the results of samples measured in systems I and II. g, h FWHM of Peak 1 extracted from b–e at low T as marked. The diamond symbol in h marks the FWHM of 7.1 meV for the result shown in Fig. 3i, which was obtained from sample 18 and system II with setting 2. Dashed lines with labels mark the experimental resolutions for the measurements performed in/with the respective system/setting (Extended Data Fig. 5e–g).
Extended Data Fig. 9 Band dispersions associated with Peaks 1 and 2 measured on two different samples at T = 5 K.
a–d (e–h), SPS image plots (energy distribution curves) obtained from system II with setting 2 on samples 15 and 18. Red dots in e–h mark peak positions.
Extended Data Fig. 10 Results for TA = 1100 °C from samples prepared using different SrTiO3(100) substrates and after different treatments.
a–c, LEED patterns of three samples prepared using substrates purchased from Shinkosha (a, b) and MTI (c) with (b) or without (a, c) 0.05wt% Nb doping. All samples show 2 × 1 surface reconstructions. d, Corresponding SPS and e, angle-integrated valence band spectra. f, SPS from sample 7 before and after exposure to air. Two consecutive annealings at 600 °C and 1100 °C were applied after exposure to air. g, SPS from sample 7 before and after BHF etching. The latter is followed by annealing for 2 h at 600 °C. Inset: Sr-4p core level spectra showing a slight decrease of the surface Sr content after BHF etching. All measurements were performed at T = 15 K with hν = 21.2 eV. All results were obtained from system I.
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Hong, C., Zou, W., Ran, P. et al. Anomalous intense coherent secondary photoemission from a perovskite oxide. Nature 617, 493–498 (2023). https://doi.org/10.1038/s41586-023-05900-4
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DOI: https://doi.org/10.1038/s41586-023-05900-4
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