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A Plasmon-Mediated Electron Emission Process Yan Shen, Huanjun Chen, Ningsheng Xu, Yang Xing, Hao Wang, Runze Zhan, Li Gong, Jinxiu Wen, Chao Zhuang, Xuexian Chen, Ximiao Wang, Yu Zhang, Fei Liu, Jun Chen, Juncong She, and Shaozhi Deng ACS Nano, Just Accepted Manuscript • Publication Date (Web): 12 Feb 2019 Downloaded from http://pubs.acs.org on February 12, 2019

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A Plasmon-Mediated Electron Emission Process Yan Shen,†,# Huanjun Chen,†,# Ningsheng Xu,† Yang Xing,† Hao Wang,† Runze Zhan,† Li Gong,‡ Jinxiu Wen,† Chao Zhuang,† Xuexian Chen,† Ximiao Wang,† Yu Zhang,† Fei Liu,† Jun Chen,† Juncong She,† and Shaozhi Deng*,† These authors contributed equally to the work.

#

State Key Laboratory of Optoelectronic Materials and Technologies, Guangdong Province Key

†

Laboratory of Display Material and Technology, School of Electronics and Information Technology, Sun Yat-sen University, Guangzhou 510275, P. R. China Instrumental Analysis & Research Center, Sun Yat-sen University, Guangzhou 510275, P. R.

‡

China *E-mail: [email protected]

ABSTRACT: Light-driven electron emission plays an important role in modern optoelectronic devices. However, such a process usually requires a light field either with a high intensity or a high frequency, which is not favor for its implementations and difficult for its integrations. To solve these issues, we propose to combine plasmonic nanostructures with nano-electron-emitters of low work function. In such a heterostructure, hot-electrons generated by plasmon resonances upon light excitation can be directly injected into the adjacent emitter, which can subsequently be emitted into the vacuum. Electron emission of high efficiency can be obtained with light field


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of moderate intensities and visible wavelengths, which is a plasmon-mediated electron emission (PMEE) process. We have demonstrated our proposed design using a gold-on-graphene (Au-onGr) nanostructure, which can have electron emission with light intensity down to 73 mW·cm-2. It should be noted that the field electron emission is not involved in such a PMEE process. This proposal is of interest for applications including cold-cathode electron sources, advanced photocathodes, and micro- and nano-electronic devices relying on free electrons.

KEYWORDS: plasmon-mediated electron emission (PMEE), Au-on-Gr nanostructure, electromagnetic field, hot electrons, vacuum electron emission

Photoemission, referring to electrons released from the surface of materials by an incidence optical field, forms the basis of various applications including sensitive photodetection, surface analysis, electron beam generation, lithography and microscope.1−5 The photoemission effect was firstly explained by Albert Einstein in 1905, indicating that with moderate incidence light intensities, photon energies may be absorbed by electrons inside a solid, which thereafter gain enough energy to overcome the energy barrier at the solid surface and escaped into vacuum.6 Along with the development of laser technology, photoemission processes can be triggered by high-intensity continuous lasers or laser pulses. In such situations, the surface potential barrier may be decreased by strong optical fields. As a result, electrons can directly tunnel through such a barrier into vacuum. This process is known as laser-field-driven photoemission, which is similar to field electron emission driven by static electric fields.7−9 In particular, ultrafast optical pulse has been adopted to actively control the electron emission characteristics,10−13 which forms


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the basis for the state-of-the-art time-resolved electron microscopy and spectroscopy that can access temporal dynamics on a solid surface with high spatial resolution.14−16 In recent years, another type of photoemission process, i.e. the plasmon-photoemission, where plasmonic metal nanostructures are involved during the electron emission process, has gained much attention due to its great potential in studies and applications of nanoscale light–matter interactions driven by optical fields.17−26 Specifically, the plasmonic metal nanostructures can act as light-trapping components to generate extremely large electric field enhancements around nanostructures.27−30 These confined electromagnetic fields can on one hand generate energetic hot electrons31−34 that can directly transit over surface barrier into vacuum, and on the other hand can narrow down the potential barrier so that electron tunneling may occur from solid into vacuum. Previous studies have demonstrated the delicately engineered plasmonic nanostructures as ultrafast electron emitters, which were excited by femtosecond laser pulses with energy density over 1 GW·cm-2.21-26 However, for practical device application, electron emission with high current and high current density under moderate light intensity excitation is preferred. In our present study, we propose to employ the plasmonic nanostructures to mediate electron emission process, i.e. the PMEE process. In such a process, a plasmonic nanostructure acts as nanoantenna to efficiently adsorb incidence light field. The decay of plasmon resonances generates hot electrons, which then transit into an electron emitter that the plasmonic nanostructure is adhered to. The hot electrons can be released into vacuum under a small static electric field applied to the emitter. The heterostructure system of two-dimensional (2D) materials and plasmonic nanostructures may be a possible route to generate such strong interaction.35−37 To experimentally demonstrate our proposed design, we use a gold nanoparticle as a plasmonic nanostructure (or a mediator) onto a vertical few-layer graphene (vFLG), which is


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an excellent electron emitter as shown in recent years.38−44 Electron emission from such Au-onGr nanostructures were studied using a laser excitation in combination with a small pulsed electrostatic field, or laser excitation alone or electric field alone, which was shown to exhibit outstanding performance.

RESULTS AND DISCUSSION Syntheses and characterizations. As schematically illustrated in Figure 1a, the vFLG was employed as substrate for deposition of ultra-thin gold film via an ion sputtering process, and then the gold nanoparticles were formed onto the surface of the vFLG after a thermal annealing treatment. Figure 1b and 1c show typical SEM and TEM images of the Au-on-Gr nanostructures. The main body of a vFLG flake is of micrometer scale in length and width, which are defined as the size of vFLG along directions perpendicular and horizontal to the substrate. The Au nanoparticles distribute uniformly onto the surface of the vFLG flakes, with the diameters ranging from 5 nm to 100 nm. Within a specific vFLG flake, the diameters of the Au nanoparticles are reduced from the top edge to the bottom. Since we concern about the electron emission properties of the nanostructure, only the graphene’s top edges and the corresponding gold nanoparticles adhered on them are of our major interest. Most of the nanoparticles along the top edges of the graphene are about 50 nm in diameter (Figure S1 in the Supporting Information).


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Figure 1. (a) Schematic illustration of synthesis of the Au-on-Gr nanostructure. (b) Typical SEM image of the gold nanoparticles-decorated vFLGs. (c) TEM image of an individual gold nanoparticles-decorated vFLG flake. (d) The selected-area electron diffraction (SAED) analysis of the marked area D in (c). (e) High-magnification TEM image of the top-edge of the nanostructure. (f) High-resolution TEM image of the marked area F in (e).

The detailed structure and composition of the nanostructures were further characterized with selected-area electron diffraction (SAED) and HRTEM. The SAED analysis clearly reveals typical Debye-Scherrer concentric rings (Figure 1d), which correspond to the (002) plane of the graphene, as well as the (111), (200), (220), and (311) facets of the gold. HRTEM image (Figure 1f) of the marked region F (as shown in Figure 1e) in a typical Au-on-Gr nanostructure indicates that the top ends of the vFLGs are composed of flakes of only 3 carbon-atom-thick layers, with a mean lattice spacing of 0.34 nm between the adjacent layers. Such a sharp edge can facilitate the electron emission due to the strongly localized field enhancement. In the field emission process, a structure with high aspect ratio can cause a strong local electric field.45 In the current study, the sharp top ends of the vertical few-layer graphene can lead to a large aspect ratio, which therefore enhance the electron emission. In addition, each gold nanoparticle supported onto the vFLGs is of good crystallinity. Typical lattice spacings of 0.24 nm and 0.20 nm can be observed, which are consistent with the (111) and (200) facets of the gold, respectively. Raman spectra from the nanoparticles-decorated and pristine vFLG samples were then recorded (Figure S2 in the Supporting Information). After the gold decoration, no obvious peak


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shifts are observed for the D (1348 cm-1), G (1581 cm-1) and 2D (2690 cm-1) modes of the graphene. However, there exist spectral broadening for the D and G peaks, with the spectral widths increasing by ~12 cm-1 and ~32 cm-1, respectively. The spectral broadening can be ascribed to the amorphous carbon formed on the surface of the graphene by carbonization of hydrocarbons during the thermal annealing process.46 On the other hand, in order to characterize the electron emission performances of a specific nanostructure, the work function () value is a pivotal parameter that needs to be determined. To this end, KPFM measurements on the gold nanoparticles-decorated and the pristine vFLG samples were performed (Figure S3 in the Supporting Information). The contact potential difference (CPD, ΔU) values are respectively estimated to be about -97.8 mV and -182 mV for the two samples. Thereafter the original work functions () of the vFLGs without and with the nanoparticles are calculated as 4.448 eV and 4.532 eV, respectively, using Equation 1 as follows:

Sample  Tip  e  U

(1)

where Tip=4.35 eV is the work function of the KPFM tip. The work function of the gold nanoparticles-decorated vFLG sample is a bit higher than that of the pristine one, which we think is due to the higher surface work function of the decorated gold nanoparticles (~5.4 eV).47,48

Plasmon-mediated electron emission. The electron emission properties of the Au-on-Gr nanostructures were firstly characterized on large-area samples. Figure 2a and 2b show the effect of the supercontinuum white-light laser intensity (Ilaser) on the electron emission driven by a small pulsed electrostatic field (E). One can observe from Figure 2a that with the laser intensity gradually increased, electron emission current densities (J) increase significantly. Firstly, without field electron emission, laser illumination with a low intensity can stimulate electron emission.


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More specifically, electron emission occurs using laser intensity as low as 0.073 W·cm-2 (with a peak laser intensity Ilaser-peak of approximate 0.146 W·cm-2) and an applied electric field smaller than 6.0 V·μm-1 (see inset of Figure 2a). In addition, the current densities increase approximately linearly with the increasing laser irradiation power. This portends other emission mechanisms rather than field emission. Secondly, upon laser illumination, the electron emission is still governed by the PMEE process even with increased electric field. Specifically, emission current density over 26 mA·cm-2 can be obtained upon a stimulation condition of Ilaser ~ 5 W·cm-2 (with the peak Ilaser ~10 W·cm-2) and E ~ 8.0 V·μm-1 (Figure 2b). In contrast, even at highest electric field, the field emission current density is only about 10 μA·cm-2 without laser illumination (see inset of Figure 2a); that is field electron currents are negligible here. Thirdly, applied electric fields play key role in the PMEE process, although not for field electron emission. The electric field can help to narrow the barrier at the vFLG surface, which is in favor of electron tunneling into vacuum. From Figure 2b, under the conditions E > 8.0 V·μm-1, the J-E curves exhibit an exponential dependence. More importantly, high performance in electron emission may be seen from Figure 2b. Waveforms of input pulsed electrostatic field and maximum current density were also recorded (see inset of Figure 2b), and emission current density of over 155.6 mA·cm-2 (corresponding to a total current of 4.89 mA under the emission area of 0.0314 cm2), more than an order of magnitude higher than the threshold current density of a carbon nanotube field emitter,45 is obtained with low laser intensity of around 5 W·cm-2 (with the peak laser intensity of around 10 W·cm-2), with negligible field emission (see the curve for laser off). Moreover, the current density versus direct-current (DC) electric field (J-E) curves for Au-on-Gr nanostructures and the pristine vFLGs were also compared with each other, under various incident white-light laser


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intensities (Figure S4 in the Supporting Information). The vFLGs after decoration show a maximum current density much higher than the pristine ones (enhancing by about 662.5%), and this highlights the advantage of our designed material. These results clearly demonstrate that the PMEE process is highly effective using Au-on-Gr nanostructures. Such an excellent characteristic is important for application in power vacuum electronic device and its integration. One should be noted that the distinctive feature of the plasmon-mediated electron emission process is that it uses a combination of moderate light intensity and electric field for driving electron emission from a plasmon-decorated nanostructure. In the existing field emission or laser photoemission processes, usually one needs to apply high electrostatic field (~30 V·μm-1)49,50 or laser intensity (~1 TW·cm-2),34 i.e., the field strength should be high enough to narrow down the surface potential for obtaining measurable tunneling current.

Figure 2. Plasmon-mediated electron emission properties of the large-area gold nanoparticlesdecorated vFLGs’ thin film. (a) Dependence of the current density from the gold nanoparticledecorated vFLG sample on the laser intensities (Ilaser and Ilaser-peak stand for average and peak laser intensities, respectively.). The applied electric fields are 1.0 V·μm-1, 2.0 V·μm-1, 3.0 V·μm-1, 4.0 V·μm-1, 5.0 V·μm-1, 6.0 V·μm-1, 7.0 V·μm-1 and 8.0 V·μm-1, respectively. Inset: J-Ilaser behavior of the sample at low Ilaser region (from 0 to 0.6 W·cm-2). (b) The current density versus pulsed electric field (J-E) curves measured under different incident laser intensities (Ilaser): 0 (laser off), 0.73 W·cm-2, 1.23 W·cm-2, 1.62 W·cm-2, 2.22·W cm-2, 2.65 W·cm-2, 3.01 W·cm-2, 3.55 W·cm-2, 4.07 W·cm-2, 4.56 W·cm-2 and 5.26 W·cm-2, respectively. Inset: waveforms of input pulse


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electric field and maximum current density. (c) Dependence of the emission current density on the incident laser intensities (Ilaser) for Au-on-Gr nanostructures under different excitation wavelengths (λ): 400-468 nm (CWL~434.0 nm), 405-546 nm (CWL~475.5 nm), 532-675 nm (CWL~603.5 nm), 650-765 nm (CWL~707.5 nm) and 804-945 nm (CWL~874.5 nm), respectively. The applied electric field (E) is fixed at 8.0 V·μm-1. Inset: dependence of the emission current density on the incident laser intensities (Ilaser) for the pristine vFLGs under different excitation wavelengths. (d) Dependence of the emission current density on the excitation wavelengths for gold nanoparticles-decorated vFLGs. (e) Dependence of the current density from another gold nanoparticle-decorated vFLG sample on the laser intensities under excitation by a continuous laser of 532 nm (LE-LS-532-100 TA, LEO). The applied electric fields are 1.0 V·μm-1, 2.0 V·μm-1, 3.0 V·μm-1, 4.0 V·μm-1, 5.0 V·μm-1, 6.0 V·μm-1, 7.0 V·μm-1 and 8.0 V·μm-1, respectively. Inset: J-Ilaser behavior of this sample at low Ilaser region (from 0 to 0.5 W·cm-2). The results are compared with the J-Ilaser curves in Figure 2a to further verify that the PMEE process can occur under laser excitation of low-power.

The dependence of the emission current density on the excitation wavelengths was then studied. The J-Ilaser curves of different excitation wavelengths with a fixed E of 8.0 V·μm-1 are shown in Figure 2c. It can be seen that the Au-on-Gr nanostructures have current densities higher than the pristine ones (see inset of Figure 2c). The extracted J-λ relationship of the nanostructures under different Ilaser is shown in Figure 2d, showing the obvious dependence of electron emission of the Au-on-Gr nanostructures on excitation wavelength. Specifically, the


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enhancement of the emission current density is more prominent in the wavelength range from 405 nm to 546 nm (with the central wavelength of 475.5 nm). Accordingly we attribute the observed enhanced electron emission behaviors to the plasmonic resonance of the gold nanoparticles of the nanostructures. To further confirm this, the light absorption spectrum of the Au-on-Gr nanostructures was measured and compared with the pristine ones. Figure 3a shows the absorption spectra of the two samples. The Au-on-Gr nanostructures exhibit an obvious absorption band around 560 nm. Such an absorption maximum is originated from the plasmon resonance of the gold nanoparticles attached to the vFLGs, which is close to the wavelength range where the electron emission is enhanced (Figure 2d). There are many previous works on plasmon resonance of gold nanoparticles (see for example Ref 51), and our present observation is in consistent with them. To further demonstrate the low-laser-intensity driving electron emission based on the PMEE, a continuous monochromatic laser of 532 nm (LE-LS-532-100 TA, LEO) was also employed as the excitation source to measure the J-Ilaser behavior of another Au-on-Gr nanostructures sample. The excitation wavelength is close to the plasmon resonance wavelength of the Au-on-Gr nanostructures, and the laser has a maximum output power of about 100 mW. After determining the area of the focal spot of this monochromatic light, we can obtain a maximum applied laser intensity of approximately 5.62 W·cm-2, which is on the same order of magnitude as the supercontinuum white-light laser can provide (as shown in Figure 2a). From Figure 2e, one can see that without field electron emission, the continuous laser illumination with very low intensity can as well stimulate the electron emission. Specifically, the electron emission already takes place with a laser intensity as low as 0.056 W·cm-2 and an applied static electric field smaller than 6.0 V·μm-1 (see inset of Figure 2e). Although different samples were used, such a result


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shows the very similar J-Ilaser behavior as that in the case of supercontinuum laser excitation (Figure 2a), and even exhibits lower laser-intensity driving requirements. This is because the laser energy can be concentrated within a very narrow band for the monochromatic light compared to the white light source with a much broad spectral range, and thus promote the PMEE process at low Ilaser region. On the other hand, these two samples under different types of laser illumination exhibit nearly the same magnitude of electron emission current density at higher Ilaser region. For example, the emitters show the current density of 2.44 mA·cm-2 under the monochromatic laser excitation of 532-nm in wavelength with an intensity of around 2.0 W·cm-2 and an applied static electric field of 8.0 V·μm-1 (Figure 2e), and it has the same order of magnitude for the current density obtained from the sample (over 4.00 mA·cm-2, shown in Figure 2c) under illumination of supercontinuum laser within the plasmonic response band (from 405 nm to 546 nm, CWL~475.5 nm) at the same excitation intensities (see the Supporting Information for the detailed description and discussion). To sum up, the results clearly indicate that our proposed PMEE process can also be performed under the excitation of continuous wave (CW) lasers with low intensity (on the order of mW·cm-2). The electron emission yield (ψ), defined as the efficiency of the plasmon-mediated electron emission process, is an important parameter in our study. It can be expressed as ψ=N2/N1×100%, where N1 is the total number of photons absorbed by all of the gold nanoparticles contributing to the electron emission, and N2 is the total number of emitted electrons from the Au-on-Gr nanostructures. To the best of our knowledge, such a parameter was not found to be considered in previous studies of laser-photoemission or plasmon-photoemission. To calculate ψ, the light absorption spectra of the pristine and Au-on-Gr nanostructure samples were firstly calculated numerically through FDTD method. The vFLG flake is assumed to be decorated by one gold


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nanoparticle, which is set in the center of the vFLG’s top emitting edge. The diameter of gold nanoparticle is set to ~50 nm, based on the diameter size statistics of the decorated gold nanoparticles along emitting edges of the vFLGs (Figure S1 in the Supporting Information). The absorption rate is inspected at the interface location of vFLG’s top emitting edge and the decorated gold nanoparticle under different excitation wavelengths, and it is then expressed by the absorption cross section (σa) (Figure 3b), which also reveals the ability of absorbing light in this location, and can be used to calculate the optical power absorbed and the subsequent electron emission yield. For the Au-on-Gr nanostructures, the simulation result gives rise to a curve in shape similar to that of light absorption shown in Figure 3a. One can see that at the plasmon resonance wavelength, the σa of the Au-on-Gr nanostructure is 1.175 times of that of the pristine one. In addition, it is noteworthy that the plasmon-enhanced light absorption is strongly dependent on the number density of the gold nanoparticles. When the nanoparticle number is changed from 1 to 10, the calculated light absorption of the Au-on-Gr nanostructures is greatly enhanced while the resonance wavelength is almost unchanged (Figure S5 in the Supporting Information).

Figure 3. (a) The experimental light absorption spectra of the pristine vFLGs and Au-on-Gr nanostructures, respectively. (b) The simulated light absorption cross section curves for the pristine vFLG and for vFLG assuming to be decorated by one gold nanoparticle, which is of ~50 nm in diameter and set in the center of the vFLG’s top emitting edge. The absorption rate is inspected at the interface location of vFLG’s top emitting edge and the decorated gold


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nanoparticle, and it is expressed by the absorption cross section (σa) in this figure, and also for the further electron emission yield calculation. (c, d) Results of electron emission yield (ψ) calculation of the gold nanoparticles-decorated vFLGs. The calculated (c) ψ-Ilaser curves at fixed E of 8.0 V·μm-1 and the (d) ψ-E curves at fixed Ilaser of 2.15 W·cm-2 were recorded upon different wavelength excitation ranges, respectively. Insets: dependence of the electron emission yield (ψ) on the excitation wavelengths under different excitation intensities.

With the above simulated σa values, the absorbing optical power (P0) of an individual gold nanoparticle under a given laser intensity can be stated as: P0  I laser   a

(2)

and the number of the absorbed photons (N0) of such a gold nanoparticle under laser illumination may be described as: N 0  P0 / h  P0   / hc

(3)

where λ is the estimated central wavelength (CWL) for the excitation bands, which may be determined by filtering the incident supercontinuum white-light source. In the whole area where the electron emission current is measured (with the area of ~3.14 mm2 in the present study), the total number of photons (N1) absorbed by all of the gold nanoparticles contributing to the electron emission should be estimated as: N1  N 0  n1  n2

(4)

where n1 is the number of gold nanoparticles which contribute to the electron emission in each vFLG emitter, n2 is the total number of vFLG flakes in this emitting area. In the present case, we


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only consider the gold nanoparticles on the top edges of the vFLGs. Thus n1 and n2 are statistically estimated as 29 and 5.76×106 (Figure S1 in the Supporting Information). The total number of emitted electrons (N2) upon various excitation conditions for the Au-onGr nanostructures can be calculated according to the J-E and J-Ilaser data under different excitation wavelengths: N2  q / e  (J  J0 )  s / e

(5)

where e is the charge of a single electron, and q is the total charges collected in this area (s) due to the plasmon-mediated electron emission. Thus, in the calculation, the contribution of current density from the pristine field emission process (J0, obtained on the condition that laser is off) should be subtracted. Thereafter, with these parameters determined, the electron emission yield is finally obtained. Figure 3c and d show the calculated ψ-Ilaser and ψ-E curves associated with different excitation bands. The ψ-Ilaser curves were obtained under a fixed E of 8.0 V·μm-1, while the ψ-E curves were recorded at a fixed Ilaser of 2.15 W·cm-2 (with a peak laser intensity of around 4.30 W·cm-2). It can be observed that under moderate driving conditions (E~8.0 V·μm-1, Ilaser~2.15 W·cm-2), the Au-on-Gr nanostructures have the electron emission yield of about 5.66% (see inset of Figure 3c). More interestingly, ψ exhibits an exponential dependence on the excitation intensities without saturation, and the strongest electron emission yield over 40.16% is calculated under a pulsed E of 10.0 V·μm-1 (see inset of Figure 3d). This is much encouraging for applications in vacuum electron emission (e.g. photocathode).

Mechanisms. The above results clearly indicate the strong correlation between the emission current and the plasmon wavelength of the Au-on-Gr nanostructures. The underlying


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mechanisms were then studied in details and are given as follow. Generally, the plasmon resonances can affect the electron emission from three aspects: the electron supply function, temperature, and surface potential of the nanostructure. To scrutinize these contributions in the plasmon-mediated electron emission process, an in situ electron emission study was conducted on a micro-zone of the nanostructure sample placed inside a modified SEM chamber (Figure 4a, Figure S6 in the Supporting Information). Such an in situ characterization can eliminate the average effect in the ensemble measurements.

Figure 4. In situ micro-zoned electron emission properties of the Au-on-Gr nanostructures. (a) Typical SEM image of the in situ electron emission measurement, showing the W anode with a tip diameter of 2 μm was moved to form a fixed gap of 100 nm from the top of the nanostructures. (b) The electron emission current versus electric field (I-E) curves measured under different incident laser intensities (Ilaser). (c) Raman spectra of the sample under laser illumination with different Ilaser values. (d) The G peak positions and corresponding temperatures (T) recorded during the electron emission under excitations with different Ilaser. (e) Comparison of the theoretical I-E curves with the experimental data under a fixed Ilaser of 1.03 W·cm-2. The calculation was based on the field-thermionic electron emission mechanism. Details: the emission current increment (δI, the green line) and total emission current difference (the blue line versus the black line) from the theoretical calculation, which are almost negligible compared to the experimental data.


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As shown in Figure 4b, enhanced electron emission is clearly observed under light illumination with different intensities. The emission current exhibits similar wavelengthdependence to those shown in the macroscopic measurements. Photo-responses of the electron emission at a fixed DC electric field (E=200 V·μm-1) were characterized by cycling the laser illumination with a square waveform. The laser intensity was kept at 1.03 W·cm-2 (with a peak laser intensity of around 2.06 W·cm-2) and an emission sequence with the same period as that of the laser pulses was recorded (in 8 cycles). The current amplitude fluctuation of the sequence is 6.06%, indicating a stable emission of the vFLGs sample. In a period of 10 s, the on/off ratio can reach as high as 12 (Figure S7 in the Supporting Information). We first discuss the influence of the nanostructure temperature on its electron emission, which may induce thermionic emission. During the plasmon-mediated electron emission, the plasmonic nanostructures may be heated up by the photo-thermal conversion effect of the plasmon resonances. If this occurs, the electron emission should be described by the Richardson– Schottky equation:52 J

 s ( E )1/ 2   4 mek 2T 2 exp( ) h3 kT

(6)

where m and e represent the mass and charge of an electron, k is the Boltzmann’s constant, h is the Planck’s constant, T is the real-time temperature of emitters during the electron emission process, and β is the field enhancement factor, which is closely related to the emitter’s aspect ratio. The β can be obtained through the Fowler-Nordheim (F-N) plots. In addition, the constant βs in the Equation 6 can be expressed as (e3/4πεrε0)1/2, where the ε0 and εr are the permittivity of the vacuum and relative dielectric constant of the sample, respectively. To further ascertain the


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contribution from the thermionic emission currents, the real-time temperature of the Au-on-Gr nanostructures under various incident laser intensities should be determined. Because the lattice vibration of the graphene is strongly dependent on its lattice temperature, we utilized the Raman spectroscopy to record the Raman modes of the samples under different laser excitations, whereby the local temperature of the sample can be extracted. To that end, the Raman spectra at different sample temperatures were firstly collected and employed as calibrations for determining the surface temperature of the Au-on-Gr nanostructure during the electron emission process (Figure S8 in the Supporting Information). Figure 4c gives the Raman spectra under laser illumination with different intensities. As shown in Figure 4d, the temperature of the Auon-Gr nanostructures increases by about 50 degrees when the Ilaser is 1.03 W·cm-2, indicating that the temperature increase during the plasmon-mediated emission process is relatively low. With the temperature information, one can ascertain the contribution from the thermionic emission by substituting the T under different laser illuminations into the Equation 6. Figure 4e shows the emission current increment (δI) (see detail in Figure 4e, the green line) integrated from a sample area of about 3.14 μm2 under a fixed Ilaser of 1.03 W·cm-2. One can see that the current difference is almost negligible compared to the experimental data, which clearly indicates that for the plasmon-mediated electron emission, the contribution from the thermionic emission can be neglected. Both of the electron supply function and surface potential of the Au-on-Gr nanostructure can be changed by injection of the laser-induced energetic hot electrons. As change of temperature T is negligible, the emission current density of the electron emission process can be stated in Fowler-Nordheim (F-N) equation as:45

A(  E )2 B 3/ 2 J exp( )  E

(7)


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where A=1.54×10-6 is the first F-N constant with the unit of A eV·V-2, B=6.83 is the second F-N constant with the unit of eV-3/2 V·nm-1. Figure 5a gives the corresponding F-N (Ln (I/V2)-1/V) plots, extracted from the I-E curves shown in Figure 4b. One can observe that all of the F-N plots are linear, while each curve corresponding to different laser intensities (Ilaser) has their respective slope and intercept. The Seppen–Katamuki (SK) chart53−55 was then employed to calculate the effective work function () and effective field enhancement factor (β) of the emitters (Figure 5b), respectively. In the SK chart, the X and Y coordinates refer to the bFN (intercept) and kFN (slope) of the F-N plots presented in Figure 5a, which can be expressed as k FN

bFN

0.95b  

3

2

and

e3   a 2  e 3b , respectively. and b  8 2m are constants, and α is the a   ln   8 h 3he  1.1  4 0 

effective emitting area, which can be described as: (8)

   0 exp(-9.74  107  1/ 2 ) where α0 is the emission area, and γ is the effect constant of emitter’s shape, which can be introduced from Gomer’s semi-empirical assumptions.56 By fitting the F-N plots, the isolines for the work functions and field enhancement factors can be drawn accordingly, as shown in Figure 5b. From these isolines the relationship between the bFN and kFN of the Au-on-Gr nanostructures under laser illumination of different intensities can be obtained. Thereafter, dependences of  and β values on the laser intensity can be extracted (Figure 5c and 5d). When the incident laser intensity increased from 0 to 1.03 W·cm-2 (~2.06 W·cm-2 for the peak laser intensity), the calculated  decreased from 4.532 eV to 2.585 eV, with a reduction over 42.9%. On the other hand, the corresponding field enhancement factor (β) increased from 7.06×108 to 7.4×108, i.e. very slightly with an increment smaller than 4.82%. We suggest that the evolvements of the


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effective  and β stem from the plasmon resonances of the gold nanoparticles supported by the vFLG emitters. Specifically, the obvious changes in  should be due to the transfer of plasmonicgenerated hot electrons from the gold nanoparticles to the adjacent vFLG flake. Injection of the hot electrons into the graphene can raise its Fermi energy level. On the other hand, as revealed by the in situ SEM and Raman characterizations, the morphology, structure, and composition of the nanostructures are almost unchanged after the electron emission. Therefore the very slight changes of β should be caused by plasmonic-induced electromagnetic field enhancements.

Figure 5. F-N plots and Seppen-Katamuki (SK) chart of the plasmon-mediated electron emission from the micro-zoned vFLGs decorated with gold nanoparticles. (a) Typical F-N (Ln (I/V2)-1/V) plots at the high electrostatic field region. (b) The Seppen–Katamuki (SK) chart of the electron emission characteristics extracted from (a). The solid and dotted curves represent the effective work function () and effective field enhancement factor (β) isolines, respectively. The symbols stand for slopes (kFN) and intercepts (bFN) of the F-N plots at different incident laser intensities (Ilaser). (c, d) The dependences of the  and β values on the Ilaser.

To further ascertain the role of hot electrons playing in the electron emission of the Au-on-Gr nanostructures, the KPFM was utilized to characterize the surface potential changes of the


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nanostructures under laser illumination. As shown in Figures 6a–g, with increase of the laser power, the contact potential differences (CPDs) increase, while the work function differences (Δ) decrease (Figure 6g). Such behaviors clearly indicate the reduction of the surface potential of the vFLG flake, which may be attributed to the injection of the energetic hot electrons from the gold nanoparticles into the graphene under laser illumination. To reveal the plasmonicinduced electromagnetic field enhancements, near-field optical characterizations were performed on the pristine and gold nanoparticles-decorated vFLG flakes. To that end, the vFLG samples were scratched from the silicon substrate and dispersed onto a second silicon substrate covered with 300-nm SiO2 layer (Figure 6h and 6j). S-SNOM technique was then employed to measure the near-field optical intensities of the vFLG samples. As shown in Figure 6i and 6k, the Au-onGr nanostructure shows much higher SNOM intensity than the pristine one can exhibit. Several regions with particularly high optical intensity, which are termed as the hot-spots, appear on the surface of the gold nanoparticles-decorated vFLG sample. These enhanced electromagnetic fields can be further corroborated by numerical simulations. As shown in Figure 6l, for the pristine vFLG sample, the calculated near-field optical intensities are almost the same as that of the incidence laser, and no enhancement can be observed across the wavelength range inspected. However, at the same location but with the presence of a gold nanoparticle, the near-field optical intensities show strong wavelength-dependence behavior. Specifically, at the plasmon resonance wavelength of the gold nanoparticle (~580 nm), much enhanced electromagnetic field can be observed at the graphene region adjacent to the gold nanoparticle. The optical near-field enhancement can be more than 20 times in comparison with the pristine graphene.


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Figure 6. KPFM and s-SNOM measurements of the Au-on-Gr nanostructures under laser illuminations. (a) Topography of typical Au-on-Gr nanostructures with an area of 5×5 μm2. (b-f) Corresponding contact potential difference (CPD) images upon laser illumination of 532 nm at different powers. (g) Dependences of the CPD and work function difference (Δ) changes on the laser intensities. (h-k) SEM and corresponding s-SNOM images of the pristine and gold nanoparticles-decorated vFLG samples, respectively. (l) Simulated near-field intensity distributions of the pristine and gold nanoparticles-decorated vFLG samples. The diameter of the nanoparticle is set as 50 nm. The distribution is inspected at the interface location of vFLG’s top emitting edge and the decorated gold nanoparticle.

Now, we may propose the physical process of the plasmon-mediated electron emission, with referring to Figure 7. Firstly, decay of the plasmon resonances can transfer the energy to the individual electrons in the gold nanoparticles and generate the hot electrons, as confirmed by the KPFM characterizations. The hot electrons with high energy will have a large opportunity to directly tunnel into vacuum through the surface barrier of the gold nanoparticles under the electric field (I), or injected into the graphene by going through the barrier at the interface between graphene and gold nanoparticle, which may subsequently tunnel into the vacuum from the graphene’s edges (II). Thirdly, the plasmon-enhanced near-field can be strong enough to reduce the barrier width experienced by the electrons within the graphene next to the gold nanoparticles (III). Fourthly, according to the Fermi-Golden law, the optical absorption of a


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specific structure is determined by the localized electromagnetic field it experiences. The aforementioned enhanced near-field induced by the plasmon resonances can therefore accelerate the optical absorption of the adjacent graphene, which consequently increase the electron interband transitions of the graphene. These addition electrons can also tunnel through the graphene edges under the static electric field (IV). All of these four mechanisms have the opportunity to contribute to the enhanced emission current observed in our ex periments (Figure 7a). To ascertain the priority between the processes (I) and (II), energy level diagrams during the electron emission of the Au-on-Gr nanostructure were drawn and compared. As shown in Figure 7b, the work function of the gold nanoparticles (~5.4 eV) is higher than that of the vFLGs (~4.5 eV), and the barrier (about ~0.9 eV) at the gold–graphene interface is much lower than that at the gold surface barrier. Under the dark conditions with static electric field applied, the electrons tend to emit from the edges of graphene rather than the surface of gold nanoparticle. This is due to the relatively low  value and the atomic-scaled top-edge57 of the vFLG flake, which can lead to a lower and narrower electron tunneling barrier. Upon the laser illumination, it is much easier for the hot electrons to transfer from the gold into the graphene than into the vacuum due to the small barrier at the gold-graphene interface (Figure 7c). As a result, the contribution from process II is believed to be larger than that of the process I. On the other hand, as manifested in process (III), the plasmon-enhanced near-field can also narrow the barrier width felt by the electrons within the graphene next to the gold nanoparticles, which consequently lead to improved electron emission upon laser illumination. According to the previous studies,21-26 for such an effect to take place the plasmon-enhanced near-field should be comparable to that of the electrostatic field. However, in our present study, the effective


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electromagnetic field is much lower than the applied static electric field even if plasmonic optical near-field enhancement of more than 20 times is counted. The weak light field is mainly limited by the low laser intensity we used (~W·cm-2). As a result, the contribution from process (III) to the electron emission under laser excitations is very small in our current study. For the process IV, the electrons generated by the inter-band transitions should be rather limited due to the monolayer or few-layer nature of the vFLG flakes. Consequently, such a process will only become significant under strong laser illumination. On the basis of these analyses, we believe that the mechanism responsible for the plasmon-mediated electron emission should be mainly associated with the high-energetic hot electrons transferring from the plasmonic nanoparticles (Au) into the graphene. These additional hot electrons can increase the supply of electrons which experience smaller surface potential width for emission with high efficiency.

Figure 7. (a) Schematic illustration of possible mechanisms responsible for the plasmonmediated electron emission. Four possible mechanisms are marked with Roman number I, II, III and IV, respectively. (b, c) The corresponding energy level diagrams during the electron emission process from the gold nanoparticles-decorated vFLG’s nanostructure under the dark conditions (b) and upon laser illumination (c). The laser intensity (Ilaser) is assumed to be 1.03 W·cm-2.


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CONCLUSIONS We propose and experimentally demonstrate that light can efficiently induce electron emission into vacuum from solid via the plasmon-mediated electron emission (PMEE) process. In principle, the plasmon-mediated electron emission can effectively occur by attaching a plasmon nanomaterial (as an antenna for light receiving and mediator for hot electron providing) to an electron emitter having high aspect ratio and lower work function. Au-on-Gr nanostructures where vFLG flakes were decorated with gold nanoparticles are shown to be an excellent choice and to give rise to attractive electron emission characteristics. Laser intensity as low as 0.073 W·cm-2 (with a peak laser intensity of around 0.146 W·cm-2) can already give rise to electron emission. A maximum emission current density over 155.6 mA·cm-2 can be obtained with low laser intensity of around 5 W·cm-2, without use of light source of intensities as high as GW·cm-2. Electron emission yield as high as 40.16% is observed, higher than most of photocathodes can provide. All of above may be obtained under conditions of low applied electric fields and of low sample temperature, where field or thermal electron emission is negligible. Such enhanced electron emission is strongly dependent on the plasmon resonance wavelength of the nanostructures. The mechanism is revealed using an in situ micro-zoned analytical facility which suggests that the plasmon-generated hot electrons is first injected into the graphene from gold nanoparticle and then emitting into vacuum. With such a proposed design, one can generate electron emission using laser excitation with moderate intensities and low electric fields, which are in favor of free electron devices as well as their integration, including cold cathode electron sources, advanced photocathodes, micro- and nano- electronic devices, and optoelectronic devices.


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METHODS Synthesis. The inductively coupled plasma chemical vapor deposition (ICPCVD) technique was employed to prepare vFLGs onto a silicon substrate. The substrate was pre-heated to 900 ºC under 15-sccm Ar and 15-sccm H2 at 0.04 Torr, for cleaning the surface of the silicon substrate. Then, a RF power of 1100 W was applied to generate plasma with a DC electric field of 200 V, while flows of 60-sccm CH4 and 10-sccm H2 were introduced into the chamber at 0.058 Torr. The vFLGs were obtained after 15-min growth. Next, an ion sputter apparatus (KYKY SBC-12, 2mA) was used to deposit a thin film of gold onto the surface of the vFLGs, with a sputtering time of about 200 s at 0.0375 Torr. Finally, the sample was annealed under 450-sccm Ar at 3.33 Torr for 90 minutes in a tube furnace (OTF-1200 X). The annealing temperature was set at 570 ºC to make the diameters of the gold nanoparticles adhered onto the top edges of the vFLG flakes as uniform as possible. Many Au-on-Gr nanostructures on a sample were thus prepared.

Characterization. The microscopic structure and morphology of the gold nanoparticledecorated vFLGs (Au-on-Gr nanostructures) were characterized by field-emission scanning electron microscope (FE-SEM) (Quanta 450 FEG, FEI, ~10 kV) and high-resolution transmission electron microscope (HRTEM) (Titan G2 60-300, FEI, ~80 kV). The Raman spectra were acquired using a confocal micro-Raman system (Renishaw inVia Reflex) equipped with a dark-field microscope (Leica, 50×objective, numerical aperture: 0.8), with an excitation wavelength of 633 nm. The signal acquisition time for measuring a typical Raman spectrum was set to 10 s. Kelvin probe force microscopic (KPFM) and atomic force microscopic measurements were performed with custom-designed scanning probe microscope (NTEGRA Spectra, NT-


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MDT), using a Co/Cr coated silicon cantilever. Topography and contact potential difference (CPD) images of the samples were line-scanned and recorded with a selected area of 5×5 μm2, which were then utilized to calculate the work function () at room temperature. Furthermore, a 532-nm-laser was illuminated onto the sample with different intensities to obtain the CPD and corresponding work function differences (Δ) upon the laser irradiation. Light absorption spectra were recorded using a UV–vis-NIR spectrophotometer (HITACHI U-4100), with the gold nanoparticles-decorated and pristine vFLG samples well-dispersed into deionized water. The optical near-field characterizations were conducted with a scattering type scanning near-field optical microscope (s-SNOM: NeaSNOM, Neaspec GmbH). A 633-nm laser was focused onto the gold nanoparticles-decorated and pristine vFLG samples through a metal-coated AFM tip (Arrow-IrPt, Nanoworld). During the measurements, the AFM tip was vibrated vertically with a frequency of 280 kHz, and the back-scattered light from the tip was demodulated and detected at a fourth harmonic of the vibration frequency.58−60

Electron emission measurement. The electron emission properties of large-area sample (~3.14 mm2, called the electron emission macroscopic measurement) were characterized within an ultrahigh vacuum chamber (7.5×10-9 Torr). A spherical metal anode with a diameter of 2 mm was placed 100 μm away from the samples. To initiate the electron emission, a laser excitation in combination with a small pulsed electrostatic field, or laser excitation alone or electric field alone was applied to the samples. Specifically, during a typical measurement, the samples were continuously illuminated by a focused supercontinuum white-light laser (SC-Pro, OYSL Photonics, 400 nm-2400 nm) of different laser intensities (Ilaser) with a repetition frequency of 4 MHz and a pulse width of around 125 ns in each period, while pulses voltage (with frequency of


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5 kHz and pulse-width of 3 μs) was applied to the anode (Figure S6a in the Supporting Information). Ilaser was calculated through dividing the incidence laser power by the focused spot area (Figure S9 and Table S1 in the Supporting Information), which was measured by combining the optical power meter (UNO, GENTEC-EO) with the spot analyzer (BM-USB-SP907-OSI, Spiricon). Thus the peak laser intensity could be estimated as about twice the average intensity (Ilaser). The emission current was recorded under various laser and electric field excitations, and the corresponding current density (J) was then calculated on the premise that most of the spot size is larger than the cathode emission area. In the frequency-dependence measurement, the incident laser beam was divided into different excitation bands using pairs of long- and shortwave-pass filters (BLP01/FF01/BSP01, Semrock). Further to investigate the electron emission mechanism, the in situ electron emission characterization of the micro-zone of a sample was performed inside a modified SEM (Quanta 450 FEG, FEI, operated at 10 kV, ~6×10−6 Torr), which was equipped with two independent nanomanipulators (miBotTM, Imina Technologies), a laser scanning arm (Raman-SEM coupling module, Rainshaw inVia) and a Raman spectrometer (Figure S6b and c in the Supporting Information). A small piece of sample was attached to a tungsten (W) microprobe using silver paste, and another W microprobe with tip diameter of 2 μm was acted as the anode. Both of them were mounted onto the manipulators and separated by 100 nm from each other. In a few cycles of applying increasing voltages in the gap (from 0 V to 30 V), the current versus static electric field (I-E) curves of the samples were recorded with laser illumination of different laser intensities (Ilaser). The Ilaser inside the SEM chamber was pre-calibrated using a commercial photodiode, which was placed directly under the laser focusing spot within the chamber (Figure S10, Figure S11, and Table S2 in the Supporting Information). In addition, the in situ SEM and


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Raman spectrometer (with an excitation of 785 nm) were used to characterize the morphology, composition, and temperature changes of the micro-zoned samples during the electron emission process.

Numerical simulations. The electromagnetic responses of the gold nanoparticles-decorated and pristine vFLG samples were studied by finite-difference time-domain method (Lumerical FDTD solutions). The vFLG was modeled as a rectangular thin layer of 1 μm in width and length, respectively, and its thickness was set as 3 carbon layers. The decorated gold nanoparticles were set as spheres with 50-nm diameter. For the calculations of the absorption spectra and electromagnetic field enhancements, linearly polarized plane waves with wavelength ranging from 400 to 800 nm were launched onto the vFLG’s structure from its top edge. A mesh size of 2 nm was set around the graphene, and a smaller mesh size of 0.2 nm was utilized for the regions around the gold nanoparticles.

ASSOCIATED CONTENT Supporting Information Available: . The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Schematic illustrations of the diameter size and areal density statistics of the decorated gold nanoparticles and vFLGs, Raman spectra and Kelvin probe force microscopy (KPFM) measurements of the pristine and gold nanoparticles-decorated vFLGs, comparison of electron emission properties between the Au-on-Gr nanostructures and pristine vFLGs, simulated light


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absorption spectra of Au-on-Gr nanostructures decorated with different nanoparticles, plasmonmediated electron emission measurements for large-area and micro-zoned Au-on-Gr nanostructures, temperature coefficient extraction for the G mode of Raman spectra of the sample, calibration and calculation of the incident laser power values, calculation of the incident laser intensities (Ilaser), optical microscopy images of the focused light spots (PDF) The authors declare no competing financial interests.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Tel: (+86)-20-84110916; Fax: (+86)-20-84037855 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. S.D. and N.S.X. conceived the idea, initiated the present study, and propose experimental scheme. Y.S. and H.C. carried out the experiments. S.D., N.S.X., Y.S. and H.C. discussed and interpreted the results. Y.X., R.Z., L.G., J.W., C.Z., X.C. and X.W. assisted in the experiments. H.W., Y.Z., F.L., J.C. and J.S. assisted in the data analysis. Y.S., H.C., N.S.X. and S.D. co-wrote the manuscript. Y.S. and H.C. contributed equally to this work.

#

ACKNOWLEDGMENT


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This work was supported by the National Natural Science Foundation of China (grant no. 51290271 and no.51702372 and no.91833301), the National Key Basic Research Program of China (grant no. 2013CB933601), the Guangdong Natural Science Funds for Distinguished Young Scholars (Grant No. 2014A030306017), the Guangdong Special Support Program (Grant No. 201428004), Pearl River S&T Nova Program of Guangzhou (Grant No. 201610010084), and the Fundamental Research Funds for the Central Universities.

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14 Gulde, M.; Schweda, S.; Storeck, G.; Maiti, M.; Yu, H. K.; Wodtke, A. M.; Schäfer, S.; Ropers,

C.

Ultrafast

Low-Energy

Electron

Diffraction

in

Transmission

Resolves

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A Plasmon-Mediated Electron Emission Process - ACS Nano (ACS

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