Abstract

Silver gratings supporting surface plasmons at 532 nm catalyze the formation of light-emitting solid carbon from ${\rm CO}_2$ gas, revealing a low-energy ${\rm CO}_2$ reduction pathway involving hot electrons. Reversibility and spatial control over the formation of carbon quantum emitters are achieved.

© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Harvesting light to drive chemical reactions on noble metal surfaces via the excitation of surface plasmons can open new chemical transformation pathways [1,2]. Visible light can excite carriers with the energy necessary to drive reactions, either by electron transfer from excited states above the Fermi energy in the metal to unoccupied molecular orbitals in adsorbates [3], or by generating localized heating via thermal relaxation processes [4]. Recently, plasmon-assisted transformations involving ${\rm CO}_2$ gas on various arrangements of Ag nanoparticles has gained attention [5,6]. Various carbonaceous products form in these systems, including carbon monoxide and amorphous carbon, depending on the specific nanoparticle used.

An appropriately designed metallic grating supports a well-defined distribution of excited charge carriers over the entire illuminated area through the efficient excitation and decay of resonant surface plasmons, while providing an effective heat sink that minimizes thermal effects. Both benefits are difficult to realize in random arrangements of metal nanoparticles, where differences in size, interparticle spacing, and degrees of surface oxidation can create an intractable mix of optical, thermal, and chemical responses under the illumination spot. Simple, well-understood and atomically clean Ag gratings enable unambiguous interpretation of chemistry experiments involving hot electrons.

We report the reagent-less reduction of high-purity ${\rm CO}_2$ gas to solid carbon deposits, catalyzed by a silver grating supporting surface plasmons excited by 532 nm (2.33 eV) light. The carbon deposits were monitored in real-time and in situ using surface-enhanced Raman spectroscopy and photoluminescence imaging. The deposits formed include highly luminescent carbon nanodots and larger carbon nanostructures appearing in areas where localized fields are the strongest. The carbon deposits were readily removed from the Ag grating surfaces upon exposure to air and 532 nm (2.33 eV) light, rendering the grating surface non-emissive. Control over the spatial distribution of carbon emitters, combined with the rewritable nature of the process, opens what we believe is a new approach to integrate light-emitting carbon dots with plasmonic nanostructures to realize, e.g., reconfigurable light-emitting metasurfaces [7].

Ag gratings were designed with the aid of the finite element method, as implemented in COMSOL commercial software. As shown in Fig. 1(a), an optimized geometry to couple normally incident $ x $-polarized light at a free-space wavelength of 532 nm to surface plasmons propagating along the surface, is a 50/50 duty cycle rectangular grating of pitch $\Lambda = 492.5\;{\rm nm}$ and ridge height 25 nm. The electric field is enhanced over the illuminated area of the grating surface, such that this entire area continuously supplies hot electrons under CW irradiation. The magnitude of the electric field near the surface of the Ag grating increases by up to a factor of 15 upon absorption of 532 nm light, and the strongest electric fields occur near the (rounded) corners of the Ag ridges. These locations will have the highest density of excited charge carriers, so the chemical transformation of an adsorbate, driven by hot electrons, has the highest probability of occurring here. Experimentally, Ag gratings with a pitch of 480 nm and ridge height of ${\sim}{30}\;{\rm nm}$ were found to couple light with an efficiency up to 75%.

 figure: Fig. 1.

Fig. 1. (a) Unit cell for FEM calculation and distribution of $|{E}|/|{E}_0|$. (b) Time evolution SERS measurement. (c) HIM image of Ag grating surface after 30 min ${\rm CO}_2$ exposure. (d) Surface after 20 min exposure to air. (e) PL from carbon deposits on a grating (532 nm filtered). (f) Time evolution of the integrated emission intensity during exposure to ${\rm CO}_2$ followed by air. (g) PL images of a grating evolving from non-emitting, to emitting yellow light, and back to non-emitting (532 nm filtered).

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The generation of hot electrons combined with SERS on Ag gratings enables real-time, in situ monitoring of chemical activity involving ${\rm CO}_2$ gas driven by surface plasmons. Using a custom gas chamber designed for Raman scattering measurements, a grating was exposed to a continuous flow of ${\rm CO}_2$ at a pressure of 1 atm, and excited with an x-polarized 532 nm CW laser beam carrying 10 mW of power. Time-evolution Raman spectra were recorded under these conditions over a duration of 30 min and are shown in Fig. 1(b). A very rapid spectral evolution, from a flat featureless low noise baseline at ${t} = 0\;{\min }$, shows the growth of three peaks: an extremely broad spectral contribution that red-shifts from 552 nm (${682}\;{{\rm cm}^{- 1}}$) to 565 nm (${1068}\;{{\rm cm}^{- 1}}$) with time, assigned to photoluminescence, and two peaks centered at 572 nm (${\sim}{1345}{{\rm cm}^{- 1}}$) and 580 nm (${\sim}{1585}\;{{\rm cm}^{- 1}}$) characteristic of Raman scattering peaks, which are assigned to D (Disorder) and G (Graphite) bands commonly observed from graphite-like carbon [8]. A short integration time of 5 ms was used for these measurements to avoid detector saturation from the extreme emission. This characteristic spectral evolution, indicating that surface chemical transformations are taking place, was always observed over many repeated experiments. Control experiments were performed under z-polarized excitation in a 1 atm ${\rm CO}_2$ environment for 30 min, followed by a single Raman spectral acquisition using $ x $-polarized light to investigate the effects of polarization and roughness. The as-deposited smooth Ag film produces a featureless low-noise SERS baseline independent of the incoming polarization [magenta trace in Fig. 1(b)], and the grating under $ z $-polarization produces a very weak characteristic spectrum [orange trace in Fig. 1(b)], indicating that roughened Ag surfaces can produce the same effect via low-efficiency roughness coupling to surface plasmons.

Examining the surface of the Ag grating after 30 min of exposure to ${\rm CO}_2$ gas and 10 mW of 532 nm CW light, via high-resolution helium ion microscope (HIM) imaging, reveals that material has formed. Inspection of Fig. 1(c) shows the material as rounded deposits, which render as increased roughness primarily along the edges of grating ridges, relative to a pristine grating, as imaged in Fig. 1(d). The inset to Fig. 1(c) shows an ultrahigh magnification HIM image captured at an angle of 54° of a larger isolated deposit formed along the edge of a grating ridge. The composition of such deposits was confirmed independently as carbon via EDS measurements (not shown). Carbon dots (CDs) are known to produce strong luminescence, have dimensions that are consistently reported to be in the range of 2–20 nm, and exhibit Raman scattering spectra consisting of two peaks (D band and G band) [9]. With strong experimental evidence supporting the formation of carbon deposits, together with the presence of D and G Raman bands, the strong emission observed spectrally [Fig. 1(b)] and visually [Fig. 1(e)] is assigned to PL from CDs.

This process is completely reversible; that is, the carbon dots can be removed from the surface under the same laser exposure by replacing the ${\rm CO}_2$ gas in the sample chamber with lab air. The time evolution of the total emission intensity (integrated over the entire spectral range) is shown in Fig. 1(f), and clearly reveals the growth and decay of emission during ${\rm CO}_2$ and air exposure, respectively. The HIM image of Fig. 1(d) shows the surface of the Ag grating void of deposits after the emission intensity has retraced to zero. Thus, an Ag grating can be configured into either an emitting state or a dark state, as demonstrated in the time evolution PL images of Fig. 1(g).

Measurements of rate constants versus laser power (not shown) form linear trends, and taken with the polarization sensitivity [Fig. 1(b)], suggest that (plasmon-induced) hot electron transfer events are responsible for reducing ${\rm CO}_2$ gas to carbon dots. These linear trends and the high thermal conductivity of the sample rule out thermal effects. A two-step ${\rm CO}_2$ reduction pathway is proposed involving catalysis by the Ag surface and transfer of hot electrons. First, physisorbed CO forms via hot electron transfer from the silver surface into unoccupied states in ${\rm CO}_2$, followed by hot electron driven disproportionation of CO into solid carbon.

This new pathway for reagent-less ${\rm CO}_2$ reduction to light-emitting solid carbon, driven by photons of low energy (532 nm, 2.33 eV), will be of interest to researchers involved in the development of plasmon-driven chemical transformations, industrial scale catalytic processes, and reconfigurable light-emitting metasurfaces.

Funding

Ontario Ministry of Research and Innovation; CMC Microsystems; Ministère de la Défense Nationale; Canada Foundation for Innovation; National Research Council Canada; Natural Sciences and Engineering Research Council of Canada.

Disclosures

The authors declare no conflicts of interest.

Data Availability

Data underlying the results may be obtained from the authors upon reasonable request.

REFERENCES

1. E. Cortes, Science 362, 28 (2018). [CrossRef]  

2. W. C. D. Yang, C. Wang, L. A. Fredin, P. A. Lin, L. Shimomoto, H. J. Lezec, and R. Sharma, Nat. Mater. 18, 614 (2019). [CrossRef]  

3. P. Narang, R. Sundararaman, and H. Atwater, Nanophotonics 5, 96 (2016). [CrossRef]  

4. G. Baffou, R. Quidant, and F. J. Garcia de Abajo, ACS Nano 4, 709 (2010). [CrossRef]  

5. G. Kumari, X. Zhang, D. Devasia, J. Heo, and P. K. Jain, ACS Nano 12, 8330 (2018). [CrossRef]  

6. G. Kumari, R. Kamarudheen, E. Zoethout, and A. Baldi, ACS Catal. 11, 3478 (2021). [CrossRef]  

7. A. Vaskin, R. Kolkowski, R. A. F. Koenderink, and I. Staude, Nanophotonics 8, 1151 (2019). [CrossRef]  

8. A. C. Ferrari and J. Robertson, Phys. Rev. B 61, 14095 (2000). [CrossRef]  

9. E. Dervishi, Z. Ji, H. Htoon, M. Sykora, and S. K. Doorn, Nanoscale 11, 16571 (2019). [CrossRef]  

References

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  1. E. Cortes, Science 362, 28 (2018).
    [Crossref]
  2. W. C. D. Yang, C. Wang, L. A. Fredin, P. A. Lin, L. Shimomoto, H. J. Lezec, and R. Sharma, Nat. Mater. 18, 614 (2019).
    [Crossref]
  3. P. Narang, R. Sundararaman, and H. Atwater, Nanophotonics 5, 96 (2016).
    [Crossref]
  4. G. Baffou, R. Quidant, and F. J. Garcia de Abajo, ACS Nano 4, 709 (2010).
    [Crossref]
  5. G. Kumari, X. Zhang, D. Devasia, J. Heo, and P. K. Jain, ACS Nano 12, 8330 (2018).
    [Crossref]
  6. G. Kumari, R. Kamarudheen, E. Zoethout, and A. Baldi, ACS Catal. 11, 3478 (2021).
    [Crossref]
  7. A. Vaskin, R. Kolkowski, R. A. F. Koenderink, and I. Staude, Nanophotonics 8, 1151 (2019).
    [Crossref]
  8. A. C. Ferrari and J. Robertson, Phys. Rev. B 61, 14095 (2000).
    [Crossref]
  9. E. Dervishi, Z. Ji, H. Htoon, M. Sykora, and S. K. Doorn, Nanoscale 11, 16571 (2019).
    [Crossref]

2021 (1)

G. Kumari, R. Kamarudheen, E. Zoethout, and A. Baldi, ACS Catal. 11, 3478 (2021).
[Crossref]

2019 (3)

A. Vaskin, R. Kolkowski, R. A. F. Koenderink, and I. Staude, Nanophotonics 8, 1151 (2019).
[Crossref]

E. Dervishi, Z. Ji, H. Htoon, M. Sykora, and S. K. Doorn, Nanoscale 11, 16571 (2019).
[Crossref]

W. C. D. Yang, C. Wang, L. A. Fredin, P. A. Lin, L. Shimomoto, H. J. Lezec, and R. Sharma, Nat. Mater. 18, 614 (2019).
[Crossref]

2018 (2)

E. Cortes, Science 362, 28 (2018).
[Crossref]

G. Kumari, X. Zhang, D. Devasia, J. Heo, and P. K. Jain, ACS Nano 12, 8330 (2018).
[Crossref]

2016 (1)

P. Narang, R. Sundararaman, and H. Atwater, Nanophotonics 5, 96 (2016).
[Crossref]

2010 (1)

G. Baffou, R. Quidant, and F. J. Garcia de Abajo, ACS Nano 4, 709 (2010).
[Crossref]

2000 (1)

A. C. Ferrari and J. Robertson, Phys. Rev. B 61, 14095 (2000).
[Crossref]

Atwater, H.

P. Narang, R. Sundararaman, and H. Atwater, Nanophotonics 5, 96 (2016).
[Crossref]

Baffou, G.

G. Baffou, R. Quidant, and F. J. Garcia de Abajo, ACS Nano 4, 709 (2010).
[Crossref]

Baldi, A.

G. Kumari, R. Kamarudheen, E. Zoethout, and A. Baldi, ACS Catal. 11, 3478 (2021).
[Crossref]

Cortes, E.

E. Cortes, Science 362, 28 (2018).
[Crossref]

Dervishi, E.

E. Dervishi, Z. Ji, H. Htoon, M. Sykora, and S. K. Doorn, Nanoscale 11, 16571 (2019).
[Crossref]

Devasia, D.

G. Kumari, X. Zhang, D. Devasia, J. Heo, and P. K. Jain, ACS Nano 12, 8330 (2018).
[Crossref]

Doorn, S. K.

E. Dervishi, Z. Ji, H. Htoon, M. Sykora, and S. K. Doorn, Nanoscale 11, 16571 (2019).
[Crossref]

Ferrari, A. C.

A. C. Ferrari and J. Robertson, Phys. Rev. B 61, 14095 (2000).
[Crossref]

Fredin, L. A.

W. C. D. Yang, C. Wang, L. A. Fredin, P. A. Lin, L. Shimomoto, H. J. Lezec, and R. Sharma, Nat. Mater. 18, 614 (2019).
[Crossref]

Garcia de Abajo, F. J.

G. Baffou, R. Quidant, and F. J. Garcia de Abajo, ACS Nano 4, 709 (2010).
[Crossref]

Heo, J.

G. Kumari, X. Zhang, D. Devasia, J. Heo, and P. K. Jain, ACS Nano 12, 8330 (2018).
[Crossref]

Htoon, H.

E. Dervishi, Z. Ji, H. Htoon, M. Sykora, and S. K. Doorn, Nanoscale 11, 16571 (2019).
[Crossref]

Jain, P. K.

G. Kumari, X. Zhang, D. Devasia, J. Heo, and P. K. Jain, ACS Nano 12, 8330 (2018).
[Crossref]

Ji, Z.

E. Dervishi, Z. Ji, H. Htoon, M. Sykora, and S. K. Doorn, Nanoscale 11, 16571 (2019).
[Crossref]

Kamarudheen, R.

G. Kumari, R. Kamarudheen, E. Zoethout, and A. Baldi, ACS Catal. 11, 3478 (2021).
[Crossref]

Koenderink, R. A. F.

A. Vaskin, R. Kolkowski, R. A. F. Koenderink, and I. Staude, Nanophotonics 8, 1151 (2019).
[Crossref]

Kolkowski, R.

A. Vaskin, R. Kolkowski, R. A. F. Koenderink, and I. Staude, Nanophotonics 8, 1151 (2019).
[Crossref]

Kumari, G.

G. Kumari, R. Kamarudheen, E. Zoethout, and A. Baldi, ACS Catal. 11, 3478 (2021).
[Crossref]

G. Kumari, X. Zhang, D. Devasia, J. Heo, and P. K. Jain, ACS Nano 12, 8330 (2018).
[Crossref]

Lezec, H. J.

W. C. D. Yang, C. Wang, L. A. Fredin, P. A. Lin, L. Shimomoto, H. J. Lezec, and R. Sharma, Nat. Mater. 18, 614 (2019).
[Crossref]

Lin, P. A.

W. C. D. Yang, C. Wang, L. A. Fredin, P. A. Lin, L. Shimomoto, H. J. Lezec, and R. Sharma, Nat. Mater. 18, 614 (2019).
[Crossref]

Narang, P.

P. Narang, R. Sundararaman, and H. Atwater, Nanophotonics 5, 96 (2016).
[Crossref]

Quidant, R.

G. Baffou, R. Quidant, and F. J. Garcia de Abajo, ACS Nano 4, 709 (2010).
[Crossref]

Robertson, J.

A. C. Ferrari and J. Robertson, Phys. Rev. B 61, 14095 (2000).
[Crossref]

Sharma, R.

W. C. D. Yang, C. Wang, L. A. Fredin, P. A. Lin, L. Shimomoto, H. J. Lezec, and R. Sharma, Nat. Mater. 18, 614 (2019).
[Crossref]

Shimomoto, L.

W. C. D. Yang, C. Wang, L. A. Fredin, P. A. Lin, L. Shimomoto, H. J. Lezec, and R. Sharma, Nat. Mater. 18, 614 (2019).
[Crossref]

Staude, I.

A. Vaskin, R. Kolkowski, R. A. F. Koenderink, and I. Staude, Nanophotonics 8, 1151 (2019).
[Crossref]

Sundararaman, R.

P. Narang, R. Sundararaman, and H. Atwater, Nanophotonics 5, 96 (2016).
[Crossref]

Sykora, M.

E. Dervishi, Z. Ji, H. Htoon, M. Sykora, and S. K. Doorn, Nanoscale 11, 16571 (2019).
[Crossref]

Vaskin, A.

A. Vaskin, R. Kolkowski, R. A. F. Koenderink, and I. Staude, Nanophotonics 8, 1151 (2019).
[Crossref]

Wang, C.

W. C. D. Yang, C. Wang, L. A. Fredin, P. A. Lin, L. Shimomoto, H. J. Lezec, and R. Sharma, Nat. Mater. 18, 614 (2019).
[Crossref]

Yang, W. C. D.

W. C. D. Yang, C. Wang, L. A. Fredin, P. A. Lin, L. Shimomoto, H. J. Lezec, and R. Sharma, Nat. Mater. 18, 614 (2019).
[Crossref]

Zhang, X.

G. Kumari, X. Zhang, D. Devasia, J. Heo, and P. K. Jain, ACS Nano 12, 8330 (2018).
[Crossref]

Zoethout, E.

G. Kumari, R. Kamarudheen, E. Zoethout, and A. Baldi, ACS Catal. 11, 3478 (2021).
[Crossref]

ACS Catal. (1)

G. Kumari, R. Kamarudheen, E. Zoethout, and A. Baldi, ACS Catal. 11, 3478 (2021).
[Crossref]

ACS Nano (2)

G. Baffou, R. Quidant, and F. J. Garcia de Abajo, ACS Nano 4, 709 (2010).
[Crossref]

G. Kumari, X. Zhang, D. Devasia, J. Heo, and P. K. Jain, ACS Nano 12, 8330 (2018).
[Crossref]

Nanophotonics (2)

A. Vaskin, R. Kolkowski, R. A. F. Koenderink, and I. Staude, Nanophotonics 8, 1151 (2019).
[Crossref]

P. Narang, R. Sundararaman, and H. Atwater, Nanophotonics 5, 96 (2016).
[Crossref]

Nanoscale (1)

E. Dervishi, Z. Ji, H. Htoon, M. Sykora, and S. K. Doorn, Nanoscale 11, 16571 (2019).
[Crossref]

Nat. Mater. (1)

W. C. D. Yang, C. Wang, L. A. Fredin, P. A. Lin, L. Shimomoto, H. J. Lezec, and R. Sharma, Nat. Mater. 18, 614 (2019).
[Crossref]

Phys. Rev. B (1)

A. C. Ferrari and J. Robertson, Phys. Rev. B 61, 14095 (2000).
[Crossref]

Science (1)

E. Cortes, Science 362, 28 (2018).
[Crossref]

Data Availability

Data underlying the results may be obtained from the authors upon reasonable request.

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Figures (1)

Fig. 1.
Fig. 1. (a) Unit cell for FEM calculation and distribution of $|{E}|/|{E}_0|$ . (b) Time evolution SERS measurement. (c) HIM image of Ag grating surface after 30 min ${\rm CO}_2$ exposure. (d) Surface after 20 min exposure to air. (e) PL from carbon deposits on a grating (532 nm filtered). (f) Time evolution of the integrated emission intensity during exposure to ${\rm CO}_2$ followed by air. (g) PL images of a grating evolving from non-emitting, to emitting yellow light, and back to non-emitting (532 nm filtered).

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