First-principles study of charge-transfer-plasmon-enhanced photoemission from a gold-nanoparticle–sodium-atom emitter

The plasmon-enhanced photoemission process significantly improves the quality of electron emission current, making it applicable for next-generation free-electron devices. However, classical descriptions are less suitable for understanding the interactions between plasmonic structures and other materials at the nanoscale. In this study, we investigate the underlying mechanisms and dynamics of a plasmonic emitter composed of a Au nanoparticle and a sodium atom using real-time time-dependent density functional theory. Calculation results reveal three interaction mechanisms, namely, a near-field enhancement effect, orbital hybridization, and charge-transfer plasmon, that strongly affect the emission behavior. Specifically, the presence of the charge-transfer plasmon leads to a substantial deviation of the excitation laser frequency and significantly enhances photoemission currents. The maximum emission current increases by more than twice compared to the excitation at the original resonance frequency of Au nanoparticles. This work provides a guidance for the practical construction and experimentation of plasmon-enhanced photoemission electron sources.

Figure 1

The schematic of an electron emitter consisting of an Au nanoparticle and a sodium atom. A Gaussian laser beam is used for the emitter excitation with the light polarization direction aligned to the $z$ axis. The charge transfer plasmon (CTP) significantly affects the optimal laser excitation frequency and the enhancement of photoemission when distance $d$ is small.

Figure 2

(a) Absorption spectrum of the plasmonic emitter with varying $d$. Each peak in the spectrum corresponds to a resonance mode of the plasmonic emitter. (b) The density of states (DOS) of the plasmonic emitter with varying $d$. Each peak in the curve represents an electronic state and the peak magnitude represents the available state number.

Figure 3

(a) The photoemission current as a function of the distance $d$ between NP and Na. The blue solid curve with square markers, $I_0$, represents the photoemission current when the emitter is excited at the original plasmon resonance frequency of the NP. The red dashed curve with circular markers, $I_{\max }$, represents the maximum photoemission current by varying the excitation frequency. (b) The corresponding incident laser frequency $f_0$ and $f_{\max }$ of exciting the photoemission current in (a). The $f_{\max }$ increases slowly at first, and decreases suddenly at $1.6\AA$ due to the involvement of the CTP.

Figure 4

Calculation of the time-dependent orbital projections, which represent the probability of electron transfer between the NP and Na. (a) Electron transfer from NP $1p$ to Na $3s$, and (b) electron transfer from Na $3s$ to NP $2s$. The black dashed, blue solid, and red dotted curves correspond to the three conditions ($f_0, d=6.4\AA$), ($f_0, d=0.8\AA$), and ($f_{\max }, d=0.8\AA$), respectively.

Figure 5

Calculated energy spectrum of photoemission electrons under three conditions. Incident photon energy and distance between NP and Na: (a) $\hslash \omega =3.72$ eV and $d=6.4\AA$, (b) $\hslash \omega =3.72$ eV and $d=0.8\AA$, and (c) $\hslash \omega =3.16$ eV and $d=0.8\AA$. Insets are the schematic of electron transfer and emission path.

Figure 6

Simulation box is a parallel pipe with absorption boundaries at the end of the $z$ axis. Na is placed at the origin and the NP is placed near Na, forming a plasmonic nanoparticle-metal emitter. The current calculation plane is set perpendicular to the $z$ axis at $20\AA$ across the simulation region.

Figure 7

(a) The electric field of the incident Gaussian-shaped laser pulse. (b) Dipole moment of the NP under different incident laser intensity at the NP resonant frequency. The curve at 0.1 V/Å exhibits significant changes compared to those of other conditions, while the curves at 0.01, 0.001, and 0.0001 V/Å almost coincide.

Figure 8

(a) The density of states of Na clearly displays two orbitals, Na $3s$ and Na $3p$. (b) The density of states of the NP shows several electron orbitals, including NP $1s$, NP $1p$, NP $1d$, NP $2s$, and NP $2p$.

Figure 9

Calculated electric potential with variance distance. At $d=9.6\AA$, there is a significant potential barrier present at $z=-5\AA$; however, at $d=0.8\AA$, the barrier between Na and the NP almost disappears.

Figure 10

Induced charge density of Na as a function of $d$: (a) $\hslash \omega =3.72$ eV and $d=9.6\AA$, (b) $\hslash \omega =3.74$ eV and $d=6.4\AA$, (c) $\hslash \omega =3.78$ eV and $d=4\AA$, (d) $\hslash \omega =3.82$ eV and $d=3.2\AA$, and (e) $\hslash \omega =3.91$ eV and $d=2.4\AA$. The NP and Na are excited by the optimal light frequency $f_{\max }$.

Figure 11

(a) Density of states of NP-Na emitter as distance $d=0.8\AA$. Projected density of states of (b) Na $3s$, (c) NP $1p$, and (d) NP $2s$ as distance $d=9.6\AA$ (marked by red dotted line) and $d=0.8\AA$ (marked by blue solid line), respectively.